An Electron-cyclotron-resonance Plasma Apparatus For

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AN ELECTRON-CYCLOTRON-RESONANCE PLASMA APPARATUS FOR

HYDROG ENATED AMORPHOU S-SILICON THIN FILM PRODUCTION

by

CRAIG C. YOUNG, B.S. in M. E. , B.S. in E.E.

A THESIS

IN

ELECTRICAL ENGINEERING

Submitted to the Graduate Facultyof Texas Tech University in

Partial Fulfillment ofthe Requirements forthe Degree of

MASTER OF SCIENCE

IN

ELECTRICAL ENGINEERING

Approved

December, 1990



73

^'^^^ ACKNOWLEDGMENTS

I would like to thank my committee members. Dr. Thomas

Trost, Dr. Magne Kristiansen, and Dr. Shubhra Gangopadhyay, for

their time and devo tion towa rds this rese arch . A special heart

felt thanks goes out to my committee chairman. Dr. Thomas Trost,

without whose guidance and perseverance I would not have been

able to attain my goals.

I would also l ike to thank everyone associated with the

Pulse d Pow er La boratory for their coop eration and dilige nce . Aspecial thanks to Kim Zinzmeyer, Chris Hatfield, Ellis Loree, and

John Bridges for the use of their ears and their endless assistance

towards the completion of this research.

To my wife Victoria, I cannot express the gratitude that she

so de se rv es . I am etern al ly inde bted to her for the

encouragement, understanding, and support she gave during this

period. Becau se you have walked beside me along my path, I will

strive to tread upon yours.

11



CONTENTS

ACKNOWLEDGMENTS ii

ABSTRACT v

TABLES vi

nOURES vii

CHAPTER

I. INTRODUCTION 1

II. AMORPHOUS SILICON 3

Introduction 3Amorphous Silicon 6

Hydrogenated Amorphous Silicon Properties . . . . 8

The Role of Hydrogen 8

Bandgap and Carrier Transport 8Doping 11Absorpt ion 12

Hydrogenated Amorphous Silicon SolarCell Structures 1 4

The Need For Photovoltaic Energy Conversion . . 1 4Advantages of Amorphous Silicon Solar Cells . . 1 5Types of Solar Cell Structures 16Solar Cell Conversion Efficiency 2 2Stability 2 4

Con clusion 2 4

III . BASIC ELECTRON-CYCLOTRON-RESONANCE

PLASMA THEORY 2 7

Introdu ction 2 7

Microwave ECR Breakdown 2 8

Microwave ECR Energy Coupling 3 0

Energy Transfer 3 0

• • •111



Power Absorption in a Magnetic Field 3 2

M agnetic Mirror Effects 3 8

Conclusion 4 0

IV . SYSTEMLAYOUT 4 1ECR Apparatus 4 1

Safety Interlocks 4 8

V. RESULTS AND CONCLUSION 5 3

Thin-F ilm Production 5 3

Parameter Effects 5 7

ECR Apparatus Improvements 6 2Conclusion 6 6

REFERENCES 6 7

I V



ABSTRACT

Chlorinated hydrogenated amorphous silicon (a-Si:H,Cl) films

and hydrogenated amorphous sil icon carbide (a-SiC:H) fi lms have

been produced us ing a microwave elect ron-cyclo t ron-resonance

(EC R) plasm a deposition appa ratus. The sophisticated ECR system

has performed reliably, producing very stable plasma discharges

for different gases, microwave powers, flow rates, magnetic field

streng ths, and discharg e press ures. Tw enty-two a-Si:H,Cl fi lms

and nine a-SiC:H films were produced using a silicon tetrachloride

(SiCU) liquid source and a proprietary liquid source, courtesy of

the J. C. Schum acher Com pany. The a-Si:H,Cl films have shown

high photoconductivity to dark conductivity ratios comparable to

good quality glow-discharge-prepared a-Si:H films; they are thus

suitab le for solar cell fabrication . The a-SiC:H films have shown

high carbon contents with optical bandgaps on the order of 2.4 eV.



TABLES

1. Ap plications of Am orphous Silicon 4

2 . Sample Param eters For Silicon Tetrachloride 5 4

3 . Sample Param eters For P roprietary Liquid Source . .5 7

V I



HGURES

1. Average annual cost of photovoltaic modules.

(Reflects large orders (> 1000) of large modules

(> 30 W p)) 6

2 . Energy bands in a semiconductor. The density ofstates (a) N(E)=0 in the bandgap of a crystallinesemiconductor and (b) N(E)7tO in the mobility gapof an amorphous semiconductor 1 0

3 . Schematic diagram of a-Si:H solar cells, (a)Heterojunction structure with boron doped p-layer,and phos ohoru s doped n-layer. (b) p-i-n structure

with doped microcrystall ine (^c) n and p layers . . . 1 9

4 . Stacked p-i-n type heterojun ction structure 2 0

5 . A large-area series-connected hydrogenatedam orph ous silicon solar cell array 2 1

6. M icrowave energy transfer in a discharge volume . . 3 1

7 . (a) Electric field drift for an electron in crossedelectric and magnetic fields, (b) Electron motion

atECR 3 7

8. Plasm a confined between mag netic mirrors 3 8

9. Schem atic diagram of the experime ntal ECR plasmadepo si t ion apparatus 4 2

10 . Magnitude plot of axial magnetic field aligned withmicrow ave ECR deposition system. The peak tominimum ratio is 1.3. M agnitude plot provided by

a magnet coil current ( I B ) of 300 A 4 31 1 . (a) Sample holder showing feedthrough connections.

(b) Sam ple plug showing su bstrate mo unting . . . . 4 6

12. Closed-loop pressure control system 4 9

1 3 . Deposition rate as a function of magnet coil current

at 0T=6 SCCM, P=2.6 mTorr, and Rsici4=16.7% . . . .5 9

V l l



14. Deposition rate as a function of plasma dischargepressure at 0 T = 1 O SCCM, I B = 3 0 0 A, and

RsiCl4=10% 6 1

15 . Deposition rate as a function of plasma discharge

pressure at 0 T = 3 3 SCCM, I B = 3 0 0 A, andRsiCl4= 10% 6 2

16. Deposition rate as a function of SiCU relative flowrate at 0 T = 1 O SCCM, I B = 3 0 0 A, and P=4.7 mTorr . .6 3

17. Deposition rate as a function of SiCU relative flowrate at 0 T = 3 O SCCM, I B = 3 0 0 A, and P=11.4 mTorr . .6 4

V l l l



CHAPTER I

INTRODUCTION

The objective of this research is the construction and testingof a system to produce quality hydrogenated amorphous silicon

(a-Si:H) films using a new processing technique. This technique

utilizes a microwave electron-cyclotron-resonance (ECR) plasma

dep osition apparatus. The system has a number of advantages

over its counterparts (e.g., glow dischage, sputtering, etc.) which

will be discussed in the chapters to com e. The main advantage of

using an ECR system is that dense plasmas can be generated at

low proc essing pressures. The system allow s the user to

manipulate various operating parameters so as to optimize the

dep osition pro cess . It is planned that various gases will be used

in the system. So far, silicon tetrachloride (SiCU) has been used as

the silicon source gas to avoid the risks of toxic/explosive silane

gas. A controlled mixture of silicon tetrachloride and hydrogen

gas is used to deposit chlorinated a-Si:H films (a-S i:H,C l). A

proprietary source, from J. C. Schumacher company, is also used to

deposit carbon alloy a-Si:H films (a-SiC:H).

Since production of amorphous silicon films is the main

thrust for this research, a brief background of the material is

covered in Chapter II. A discussion of amorphous silicon solar

cells is also covered in this chapter.

The next chapter deals with microwave ECR theory where

the topics of ECR breakdown and microwave energy coupling are

1



discu ssed . The power absorption for an ECR plasma is examined

and compared to the power absorption necessary to maintain a

plasma in the absence of a magnetic field.

The system layout is illustrated and described in Chapter IV.

A novel arrangement for the microwave source is shown which

allows the ECR plasma to operate at very low impressed electric

fields. This is important since films produced at low absorbed

powers need to be investigated.

Effects of processing parameters on the plasma and film

dep osition are discu ssed in Chapter V. A number of graphs are

shown to illustrate some of the processing parameter effects upon

the dep osition rate. Con clusions and suggestions towards future

system improvements are also given.



CHAPTER n

AMORPHOUS SILICON

I n t r o d u c t i o n

In the short span of a little more than a decade, the

electronic industry has seen the birth and evolution of a truly

spec tacu lar t echno logy , t he deve lopment o f hydrogenated

am orpho us silicon (a-Si:H) as a new electronic m aterial . The

technology has advanced from almost total obscurity to the point

of having established a viable position in the m arketp lace. The

cumulative substrate area coated per year with amorphous silicon

is more than 100,000 m2/year and may exceed ten times that

amount by century's end.^

As early as 1968, researchers were investigating amorphous

silicon for its photoe lectronic properties. Not until 1974, though,

did the material finally display the capability to be used in

electron ic dev ices. During this year RCA scientists fabricated the

first photovoltaic cell containing a-Si:H material , which was

pa ten ted in 1977.2-3 Since that t ime amo rphous silicon has

become a mature technology covering a wide array of applications

and products, as shown in Table 1.

The photovoltaic cel l is the most important of the

am orph ous silicon dev ices listed in Tab le 1. Forty perc ent of

worldwide photovoltaic product ion is now amorphous si l icon.5

The technology is growing to become one of the most promising

renewable energy resources for the future.



Table 1: App lications of Amorphous Silicon

Products Commercially Available

Device Product

Photovoltaic cell

Photoreceptor

Photoconductor

Image sensor

Solar control layer

Calculators, watches, etc.

Electrophotography, LED printers

Color sensors, light sensors, etc.

Contact-type image sensors

Heating reflecting float glass

An ti-reflecting/antistatic layer Television screensThin-film transistor (TFT) Dis play s, telev ision s

Other Proposed Applications

Image pick-up tubes

Position sensors

FETs for logic circuits

FETs for ambient sensors

Fast detectors and modulators

Diodes

Bipolar transistors

Optical Waveguides

Optical recording

LED's

Passivation layers

Charge-coupled devices

Strain gauges

Photolithographic masks



Hydrogenated amorphous sil icon (a-Si:H) has shown

excellent characteristics for use not only in solar cells, but in

ph oto sen sitiv e de vic es,^ and thin-film tran sis tors. " The world

wide interest, stimulated by the development of the a-Si:H solar

cell, has led to considerable advances in the understanding of a-

Si:H properties and fabrication. The rapid growth of amorphous

semiconductor technology is due to the material's amorphous

nature, and excellent optoelectronic properties.

For amorphous silicon solar cell technology to compete with

conventional energy sources, it must first demonstrate better cell

efficiency ( 10-15% range ), improved stability, and reduced cost

(less than $2 per peak watt (Wp)). Conversion efficiencies of 12%

for single-junction laboratory cells (l-cm2), and 13.7-15.6% for

multi-junction laboratory cells have been obtained.8-ô At

present, commercial solar cell modules stabilize at conversionefficiencies of about 5% after one year of operation.

In the past, photovoltaic power has failed to compete in

traditional power generation markets due to high manufacturing

costs. Only recently has the technology made strides in competing

cost-wise, in some areas, with conventional energy sources.

Figure 1 shows the yearly decline in photovoltaic module cost

which is down from $13AVp in 1980, to $3.80AVp in 1988.5

When the selling price reaches $3AVp, solar cells become

cost effective with diesel motors in remote applications such as

irrigation and villag e power. Grid connected photovoltaic power

generation becomes economically viable for supplying peakpower



when prices decrease to about $2/W p. However, for photovoltaics

to compete with nuclear and coal-fired generating plants, the

average selling price must decrease to less than $0.50/Wp.5

$/Wp

1980 1981 1982 1983 1984 1985 1986 1987 1988

YEAR

Figure 1. Average annual cost of photovoltaic modules.(Reflects large orders (> 1000) of large modules (> 30 Wp)).

Amorphous Silicon

Crystalline silicon is characterized by both short and long-

range order, and an indirect bandgap. Amorphous silicon is



non crysta lline and lacks long-range periodic ordering. There

exists however, short-range ordering up to the third and fourth

nearest neighbors which is approximately the same as that for

crystalline sili con. The short-range order is directly responsible

for observable semiconductor properties such as optical

absorption edges and activated electrical conductivities.

The lack of long-range order in amorphous silicon (a-Si) is

the con sequ ence of a very large density (lO^^ to 1020 cm-3) of

defects that exist primarily from broken bonds between the

silicon atom s. Th ese unsaturated or 'dangling' bonds correspond

to atoms missing neighbors at those sites where periodicity is lost.

As a result, even though the nearest neighbor configuration

is similar to that in crystalline silicon, the effect of disorder in

amorphous silicon has a profound influence on its electrical and

optical properties. The disordered atomic structure of amorphoussilicon causes the material to act more like an insulator than a

semicon ductor. The large defect density of gap states provides

fast nonradiative recombination centers that result in poor

electronic properties.

The high defect density in the gap of pure a-Si films makes

it impossible to change the position of the Fermi level byintroducing donor or acceptor type (e.g., phosphorus or boron,

resp ectively) dopants. This results from the fact that a change in

the position of the Fermi level requires a change of the state of

charge for an extrem ely large number of defect states. This

change in the state of charge cannot be compensated because the



8

defect states act as t rapping centers for charged carr iers.

Therefore, the Fermi level is 'pinned' by the gap states in pure

amorphous si l icon.

Hydrogenated Amorphous Silicon Properties

The Role of Hydrogen

In order to effectively dope amorphous silicon, unpinning of

the Ferm i level is nece ssary. This is done by alloying the a-Si

with a passivating age nt. Hydrogen seems the most logical choice

for this pur pos e. The incorporation of hydrogen passivates bonds

that would otherwise be dangling bonds, thereby dramatically

reducing the density of localized states (DOS) in the gap.

Hydrogenated amorphous silicon (a-Si:H) has a DOS less than 10^6

c m " 3 e V - l , which is a density of defects as low as that found in

good quality single crystal semiconductors.

The low density of localized states in a-Si:H produces

desireable device properties, including a relatively long minority-

carrier diffusion length. W ith passivation of the dangling bonds,

doping of the material can be carried out to produce p-type and

n- type semiconductors .

Bandgap and Carrier Transport

The nature of a-Si :H puts certain l imitat ions on the

elect ronic proper t ies which are not present in crys tal l ine

sem icond uctors. This is due to the disordered atomic structure



inherent in amorphous materials but which is absent in the

periodic array of atoms of a crystal.

Energy band theory successfully predicts the semiconductor

properties of crystall ine solids in terms of the conduction and

vale nce energ y ban ds, Ec and Ey in Figure 2(a). These bands

describe the number of quantum states at each energy that

electron s and holes can occupy. The bands correspond to energies

where the densities of states that the electrons and holes can

occupy are high, and the charge carriers in these 'extended' or

'nonlocal ized ' s tates can move freely through the material .

Between the conduction and valence energy bands, the density of

states drops to zero, so that no electrons or holes with energies

that wo uld be in the band gap can exist. This results in the we ll-

know n sem icondu ctor 'forbidden gap' or 'band gap.' The periodic

nature of crystalline material allows the charge carriers to have

large mean free paths, which results in high minority carrier

mobilities of the electrons and holes.

The atomic disorder inherent in amorphous semiconductors

causes d is t inct d i f ferences between them and crys tal l ine

m ateria ls. The absen ce of long range order introduces many

imperfect ions, so that the carr ier mean free paths becomesignifican tly low er than in the crys talline m ater ials. W hen the

disorder increases to the extent that the mean free path is

approximately equal to the interatomic distance, the charge

car riers bec om e localize d. The energy at which the mean free

path approximately equals the interatomic distance is called the



1 0

>

TBandgap

>

DefectStates

TailStates

(a) Density of States (cm 'êV'^) (b) Density of States (cm 'êV '^)

Fig ure 2. Ene rgy bands in a sem iconductor. The density ofstates (a) N(E)=0 in the bandgap of a crystalline semiconductorand (b) N(E):?tO in the mobili ty gap of an amorphoussemi co n d u c t o r .

crit ical energy, and represents the boundary between localized

and non localized sta tes. The resulting energy bands look similar

to those in crystalline semiconductors and are shown in Figure

2(b) wh ere Ec and Ey a re the critical energ ies for the conduction

and valence bands.

While a crystal l ine semiconductor has no energy states

available within the bandgap, an amorphous semiconductor has a

continuum of localized states. As charge carriers move within the

ban dga p they are trapped by the tail states . This trapping slows

down the mobility of the carriers during conduction, to values

betw een 10-^ and 10-3 cm ^V -^ s" !. W ithin the conduction and



11

valence bands carrier transport occurs via extended states with a

much higher mob ility, 1 to 10 cm ^ V -l s' l, but significantly lower,

by a factor of 100 or more, than those in crystalline

semiconductors. So it is not the absence of states that give s rise to

the bandgap in an amorphous semiconductor but the very low

m obility within the gap. Therefore, the gap is more accurately

referred to as a mobility gap, and the carrier mobility is a trap

con trolled transport property. This low mobility limits the

applications of amorphous semiconductors to areas where speed is

not of importance.

Doping

The ability to dope a-Si:H is the critical component that

allow s it to be used in semiconductor devices. The low density of

midgap states allows the conductivity of hydrogenated amorphous

silicon to be modulated by doping. As in the case of crystalline

silic on , a-Si:H can be made n+ or p+ by the incorporation of

phosphorus or boron into the films. The doping efficiency in a-

Si:H is not as high as in crystalline silicon due to the

accompanyment of defects with the incorporation of dopants, and

the eventual pinning of the Fermi level by the tail states.Doping a-Si:H with boron or phosphorus alters the transport

properties, reduces the minority carrier diffusion length, and

increases the density of states in the gap. In a solar ce ll, heavy

phosphorus doping leads to a decrease in the open-circuit voltage

( V o c ) and to a lower short-circuit current density (Jsc)- The



1 2

decrea se in both VQC and Jsc reduces the overa ll con ver sion

efficie ncy of the ce ll. Heavy boron doping has the deleterious

effect of reducing the photoconductivity, by introducing a series

resistan ce. Here again, the cell conversion efficiency is sacrificed.

Nevertheless, the conductivities of intrinsic (undoped) a-Si:H of

10-8 to 10-12 i2 -l c m -l can be increased to - 10-2 Q -l cm -l thro ug h

doping.

Absorpt ion

The disorder and hydrogen content in a-Si:H makes its

optical properties quite different than those of crystalline silicon.

The absorption coefficient for amorphous silicon, in the

wavelength region corresponding to the solar spectrum, is an

order of magnitude higher than that for crystalline silicon.

The magnitude of the absorption coefficient depends uponwhether the semiconductor material used is amorphous or

crysta lline. For crystalline material, the absorption coefficient also

depends upon whether it is a direct or indirect semiconductor. In

an indirect gap semiconductor, such as crystalline silicon, the

probability of light absorption is much less than for a direct gap

semicondu ctor. An excited electron in an indirect semiconductormust go through a momentum change as well as an energy change

in order to reach the conduction band. For GaAs, which is a direct

gap semiconductor, an electron with energy equal to the bandgap

can make the transition from the valence band to the conduction

band without a necessary change in mom entum. This satisfies the



1 3

conservat ion of momentum requirement , thereby giving direct

gap sem icondu ctors a much higher absorption coefficient. Since

amorphous s i l icon has no long-range order , the momentum

con serv ation rule does not apply. This results in a much larger

absorption coefficient for amorphous silicon.

The disordered structure in a-Si:H produces a larger optical

gap (- 1 .7 eV) than in crystalline silicon (-1 .1 eV). This allows a-

Si:H films to absorb a significantly greater amount of light in a

com parab le thickness. This means that an a-Si:H fllm needs a

layer less than 1 j im thick to abso rb solar rad iat ion at

w av elen gth s less than 0.7 j im , while i ts crystall ine coun terpart

needs a layer more than 20 times as thick. Therefore, amorphous

silicon, unlike crystalline silicon, can be used to make thin-film

solar cells.

The two most important mechanisms for the absorption of

light by a semiconductor are the excitation by a photon of an

electron from the valence band to the conduction band and

trans itions within a band . The first mechanism, referred to as the

fundamental absorption, converts radiation into free electrons and

holes when a photon of ene rgy, equal to the energy gap Eg, i s

abso rbed. Only one electron-ho le pair is generated for a photonwith energ y large r than Eg . The exce ss ene rgy is dissipated as

thermal energy and contributes nothing to the conversion process.

The second process is referred to as free carrier absorption where

the energy of a free carrier is increased and not used in the

photovoltaic conversion process.



1 4

An increase in the hydrogen content causes the optical gap

to increase and the corresponding absorption to decrease.

Therefore, a low hydrogen content is needed to optimize the

optical gap; whereas a rather high hydrogen concentration is

required to eliminate the dangling bonds and to obtain good

electronic properties. The opposite is true for doping. It has been

found that doping, with donors and especially with acceptors,

increases the absorption coefficient and decreases the optical

bandgap .

Hydrogenated Amorphous SiliconSolar Cell Structures

The Need For Photovoltaic Energy Conversion

In the not too distant future, renewable energy sources will

have to play a much larger role in meeting the world's energy

dem an ds. Solar energy is attractive sinc e it represents an

inexhaustible source, unlike depletable fossil fuels such as coal,

gas and oil . However, if solar energy is to be com petitive with

other methods of power generation, the cost must decrease and

the operating conversion efficiency increase.

The importance of solar cells cannot be understated; since

their potential for large-scale power generation, capable of

m eeting terrestrial energy requirements, is imm ense. Alm ost

everywhere on earth a typical house roof covered in solar cell

panels can supply enough electrical power for that household.



1 5

The world's future population growth will occur in the third world

developing count r ies a long wi th an increased demand for

electr ical energ y. The world 's populat ion present ly stands at

approximately 5.5 billion, and will tower well above the 6 billion

m ark by ce ntury 's end. At that t ime , 40% of the wo rld's

population will live in rural villages inside third world countries,

where stand-alone electrical energy generation serves as the most

practical solution to the estimated future lack of centralized

gen eration. Solar energy may prove to be very attractive in such

s i t u a t i o n s .

From an ecological point of view, solar cells can help fulfill

the need for clean renew able energy resources. Clean renewables

can conceivably reduce the global warming effects due to

hydrocarbons , dramat ical ly decrease the conscious onslaught

against the world's forests, and extend the lifetime of the earth's

natural resources for generations to come.

Advantages of Amorphous Silicon Solar Cells

Amorphous sil icon solar cells have a number of distinct

advantages over their crystalline counterparts; these are listed as

fol lows:(i) Amorphous silicon has an absorption coefficient about

one order of magnitude greater than crystall ine sil icon in the

ma ximum solar energy wavelength region near 500 nm. This high

absorption coefficient allows a-Si solar cells to be constructed with



1 6

an optically active region on the order of one micrometer thick,

instead of the necessary 20 jim for crystalline silicon.

(ii) Because less semiconducting material is needed in the

construction of amorphous sil icon solar cells, and the growth

temperature is low (200 to 400 °C), the energy requirements in

the manufacturing process are much smaller. This savings reduces

the energy pay-back period.

(i i i) Ine xp en sive sub strate m aterials such as glass or

stainless steel can be used to deposite amorphous silicon since the

semiconductor has a noncrystall ine latt ice structure.

(iv) The lack of long -rang e period ic order in am orphous

silicon allows greater control of physical constants such as the

energy gap.

(v) The ir fabrication processes are we ll suited to automa ted

large-scale, continuous, mass-production thin-film techniques.

Types of Solar Cell Structures

There are three basic types of amorphous silicon solar cell

structures, the Schottky barrier, the p-i-n, and the heterojunction.

The Schottky barrier solar cell has the simplest structure and

exh ibits reaso nab le con version efficiencies (5 .5 % )ii . It consists ofa transparent Schottky contact with a high work function, such as

platin um , on an intrinsic a-Si:H layer. Below the intrinsic layer is

a thin phosphorus doped a-Si:H layer deposited onto a metal

sub strate . The drawb ack with Schottky barrier cells is that they

severely degrade in the presence of moisture, therefore causing



17

stability problems. Better cel l efficienc ies are realized with p-i-n

junctions and heterojunctions.

Doped hydrogenated amorphous silicon exibits extremely

sm all mino rity carrier lifetim es. This is due to the large

concentration of defects incurred during doping . Therefore, p-n

junctions do not exhibit good conversion efficiencie s. To obtain

better cells, the addition of an intrinsic a-Si:H layer, in a p-i-n

con figura tion, is used . The intrinsic layer is approximately fifty

times as thick as the p and n regions. The low density of gap

states (< 1 0 l6 cm -3) in the intrinsic layer allow s the junction to

have a large depletion width with a high minority carrier lifetime.

Most of the light absorption occurs in the intrinsic layer

generating more carriers to participate in the conversion process.

In p-i-n device construction, a transparent conductive oxide

(TCO) such as indium tin oxide (ITO) is deposited onto a glass

substrate. In order to increase light utilization and thus the cell's

current, the surface of this film is textured. ^ Texturing reduces

the optical reflection of the incoming sunlight and increases the

optical path length in the material. The ITO film is followed by a

thin boron doped layer (10-30 nm), an intrinsic layer (300-600

nm), and a phosphorus doped layer (10 -30 nm ). The structure iscompleted with the deposition of a metal reflector/contact, such as

silver or aluminum. Although p-i-n cel ls exhibit efficien cies at or

above 6%, a boron doped top layer is not very appropriate

beca use of the material's large absorption coe fficien t. The

extremely short minority carrier lifetime inherent in this layer



18

will also de gra de the cells efficiency. The se effects d rastically

reduce the convers ion ef f ic iency, especial ly in the shor t

wavelength range.

The problems with the p-type layer are corrected by

replacing it with a hydrogenated amorphous silicon carbide film

(a-S iC:H ). Since the carbon doped film has a different bandg ap

than intrinsic a-Si:H, a heterojunction is formed at the interface.

Carbon alloyed a-Si:H is a wide bandgap material (1.9 eV) that is

transp aren t to inciden t light. The layer increases the open-circuit

voltage of the solar cell by reducing carrier recombination at the

p-i junction.

Figure 3 illustrates two types of a-Si solar cell structures.

The p-i-n cell is the most common structure used in commercial

pro du cts. Th e heterojunc tion solar cell shown in figure 3(a) is

fabricated by first depositing a thin silicon-dioxide (Si02) buffer

layer on glass, and then depositing a 600 nm thick transparent

con duc ting oxide layer of tin-oxide (S n0 2) . This textured layer

enhances l ight trapping over the visible wavelength range and

red uce s the series resis tanc e of the cell structu re. The p-type

layer (10 nm) is a boron doped wide bandgap a-SiC:H film. The

intrinsic a-Si:H layer is typically 250-500 nm thick with a slightamount of boron doping to allow for better intrinsic behavior

unde r illum inatio n. The n-type layer is a pho spho rus doped a-

Si:H film about 20-30 nm thick. The rear contact is an evaporated

or sputtered alum inum . M anufacturers have achieved conversion



19

h\)

Glass

SiO.SnO,

a-SiC:H

a-Si:H

n

(a)

h\)

\ic SiC:H

a-Si:H

jxc Si:H n

(b)

Figure 3. Schem atic diagram of a-Si:H solar ce lls , (a)Heterojunction structure with boron doped p-layer, andpho sohor us doped n-layer. (b) p-i-n structure with dopedm icrocry stalline (|ic) n and p layers.



2 0

efficiencies of 12% by replacing the rear aluminum contact with

an ITO/silver contact.^

Another commercially used amorphous sil icon structure is

that shown in Figu re 3(b). The substrate is stainless steel foil with

both the n-layer and p-layer doped microcrystall ine ( | ic) Si:H.

This configuration produces a flexible solar cell commonly found

in cred it card -size calc ulato rs. Ano ther flexible a-Si structure is

show n in Fig ure 4. In this ca se, two p-i-n structures are stacked

in tande m . The thicknesses of the a-Si:H layers are adjusted so

that the curre nts from the two cells are similar. For large-area

modules, the fi lms are patterned to series-connect the cells as

shown in Figure 5.

h\)

• • i r r o .^c SiC:H

a-S i :H

îc Si:H

îc SiC:H

a-S i :H

îc Si:H

Is ta inless s tee l !

P

i

n

P

Figu re 4. Stacked p-i-n type heterojunction structure.



2 1

M e t a l

a-Si:H

(p-i-n)

Glass Substrate rro

Figure 5. A large-area series-conn ected hydrogenatedamorphous silicon solar cell array.

Conversion efficiencies of 13.7% and 15.6% have been

obtained in multijunction laboratory cell structures that utilize a

narrow bandgap layer (a-SiGe:H:F and CdS/CuInSe2 layers,

respectively) to increase the collection efficiency in the red and

near infrared region . Since a-Si:H has a bandgap of about 1.7 eV,

most of the incident infrared radiation is passed through this

layer. Varying the concentration of germanium from 0 to 100%,

the optical bandgap of a-SiGe:H can be changed from 1.7 to 1.1 eV.

Unfortunately, only low levels of germanium can be incorporated

before the alloying seriously deteriorates the electronic properties

of the material. This deterioration is also common to carbon alloys

with high levels of doping. As a result, a-Si:H alloy solar cells can



2 2

be fabricated with good electronic properties having optical

bandgaps between 1.45 and 1.9 eV.i3

Solar Cell Conversion Efficiency

There are several conditions that must be satisfied for

amorphous silicon solar ce lls to operate efficien tly. First, the

optical absorption coefficient must be sufficiently large to absorb

a significant fraction of the solar energy in the film. For films on

the order of 1 jim in th ickness, the absorption co efficient, a , must

be greater than 10^ cm-1 over at least the visible portion of the

solar spectrum. Second, the photogenerated electrons and holes

must be efficiently collected by contacting electrodes on both

sides of the semiconductor film. This implies that the minority

carrier diffusion length be comparable to the film thickness.

Third, a large built-in potential is necessary since it determines

the output vo ltage of the ce ll. This potential is generated by the

formation of a semiconductor junction such as a heterojunction, a

p-i-n junc tion , or a Schottky barrier. Fina lly, the total solar cell

series resistance must be kept small so that the IR drop is a small

percen tage of the output vo ltag e. Contributions to the series

resistance come from the bulk resistivity, the contacts, the currentcollection grids, and the electrical wiring.

The conversion efficiency of a solar cell is defined as the

ratio of the maximum electrical power (Pm) to the power of the

incident light (Pi), both measured in Wcm -2. The equation can be

written as



23

- _ J n . V n , _ ( F F ) J s c V o c

^ Pi Pi • ^'^

where Jm and Vm are the output current density and voltage for a

cel l operating under maximum output power co nditions, and Pi is

the total power incident on the cell ( P i» 100 mW cm-2 for the sun

directly overhead on a clear day; AM I cond ition). Jsc is the short

circu it current dens ity of the cel l, and VQC is the open circuit

voltage of the ce ll. The fill factor (FF) is defined by

(FF) = imYm_, (2 )J sr V /'SC ^ OC

with a theoretical achievab le value as high as 0.8 5. However,

series resistance effects will lead to considerably lower fill factors

of less than 0.75.

Several authors have estimated a theoretical limit on the

con versio n efficien cy for a-Si:H solar ce lls.i^ The maximum

con versio n efficien cy is found to be approximately 18%. The

upper limits for the three factors that determine the conversion

efficiency where calculated to be V oc « l V, Jsc=22 m Acm -2, and

F F «0.8 0 . Higher efficienc ies are possible if the density of states is

reduced .



2 4

Stabil i ty

Optical degradation is the most important obstacle to

overcome if a-Si:H solar cells are to become a promising energy

sou rce. This degradation is known as the Staebler-W ronski

e f f ec t . 15 It is a phenomenon, common to all amorphous silicon

allo ys , of l ight- induced photoc ondu ctivity decrease. This

phenomenon seems to originate from the breaking of weak Si-Si

bonds by nonradiat ive recombinat ion of photogenerated

carriers. 16-18 The breaking of weak bonds creates a large number

of metastable dangling bonds that degrade the cells

pho tocon duc tivity. This degradation seems to be self-limiting

though, since the number of created dangling bonds will saturate

and cause the collection width to decrease and saturate to a

smaller value.

Therefore, the degradation problem can be circumvented to

a point by keeping the device thickness approximately equal to

the smallest co llectio n width attained. Another means of reducing

the degradation effects is to keep impurities, such as oxygen and

carbon, below 10+20 cm-3 in the intrinsic layer. Stability problems

tend to improve by increasing the deposition temperature, and

decreasing the hydrogen content, i'*

Conclusion

Thin film a-Si:H solar cell structures are rapidly closing the

gap in becoming a viab le alternative energy source. In the last

decade, the overall cost to manufacture a-Si:H solar cell modules



2 5

has steadily declined (presently les s than $3.80/W p). During this

same period, the efficiencies for small-area laboratory cells (1-

cm 2) have been reaching the theoretical limit. The manufacturing

cost still needs to drop to about $2/Wp in order for utility-scale

power generation to be eco nom ically practical. Even though

laboratory cells have shown high conversion efficiencies, large

power modules (1 m2) have only achieved stable efficiencies of

6%. Therefore, improvem ents in manufacturing techniques are

still needed with future research directed at a better

understanding of the physics underlying film deposition and

growth .

Several key areas have been identified as vital to the

successful development of amorphous silicon thin-film solar cells.

The first area involves improved device quality through careful

material preparation, tight processing control, and state-of-the-arthigh vacuum equipment and techniques to reduce contamination.

The second area is the development of high-quality small bandgap

films using amorphous silicon-germ anium allo ys . A third

important issue is amorphous thin-film deposition kinetics. This

requires an understanding of particle bombardment and

deposition parameters on the film's electronic properties.Improved processing parameters are needed to reduce the

number of localized states in the gap and to decrease the tail-state

effe cts. A fourth is the deve lopm ent of numerical models to

simulate dev ice behavior. This will allow the ability to analyze

thin-film solar ce lls without constructing com plete devic es. The



2 6

final, and major area of concern, is the improvement of long-term

device stability. The fundamental issue here is the light-induced

photocond uctivity degradation or Staebler-Wronski effect. An

improved overall understanding of the physics behind the

material is necessary if amorphous silicon is to become a practical

alternative energy source.



CHAPTER III

BASIC ELECTRON-CYCLOTRON-RESONANCE

PLASMA THEORY

I n t r o d u c t i o n

M i c r o w a v e e l e c t r o n - c y c l o t r o n - r e s o n a n c e ( E C R ) p l a s m a

discharges have become an area of increasing interest due to their

use in many plasm a processing applications. Electron cyclotron

resonance (ECR) plasmas have been utilized for the last 25 years

in vario us fields of plasm a techn ology . This techno logy, borrowed

from fusion!9 and electric propulsion plasma,20 has developed into

an attractive plasma discharge process relevant to semiconductor

m anu factu ring. This technology offers a num ber of desirable

cha racteristics, including high plasma densities (=1 0^^-1 0!^ c m '^ ),

high degree of ionization (10% and higher), electrodeless nature,

low gas pressures (lO'^^-lO'-^ Torr), compatibility with active and

corro sive gas es, and stability of operation. Another imp ortant

feature is the abili ty to produce high-quality fi lms at low

deposi t ion temperatures .

The low-temperature and low-pressure microwave ECR

plasma process i s becoming at t ract ive for use in many

semiconductor applications, and constitutes a possible means of

me eting stringent processing requirem ents. Although microwave

ECR plasma systems are being increasingly used for thin-film

deposition, similar systems, for other uses, have been developed;

e.g., plasm a stream and reactive ion beam etching,2i sputtering -

2 7



2 8

type ECR plasma deposition,22 ion implantation,23 and diamond-

film prod uction .24 The movem ent towards smaller, faster, more

densely packed semiconductor devices has spurred a need for

mo re accu rate processing techniques. In order to improve these

techniques for both etching and deposition, better uniformity,

lower damage levels for thinner structures, and lower deposition

temperatures are needed, while still yielding high quality films at

h igh throughputs .

Various types of films have been deposited using microwave

ECR plasm a disc har ges . The general trend is that films can be

deposited at lower temperatures than for CVD or conventional

plasma-CVD processing, with comparable or better fi lm quality.

Although deposition rates are generally low, progress has been

made in scaling these upward through higher flow rates and

greater microwave power.

Microwave ECR Breakdown

ECR plasma absorption occurs when microwave energy is

coupled to the natural resonant frequency of electrons in the

pre sen ce of a static magn etic field. In any gas there is a small

number of electrons present due to ionization by cosmic rays orsome other phe nom eno n. W hen the electron cyclotron frequency

in a magnetic field, which is defined as

0)ce=eB/me, (3)



2 9

(where e = electron charge, B = static magnetic flux density, and

m c = electron ma ss) equals the m icrowave excitation frequency

(co), reson ance occurs. This resonance efficiently transfers energy

from the electromagnetic field to the electrons. The ECR coupling

takes place within a small, thin volume, commonly known as an

ECR layer . W ithin this layer most of the applied power is

absorbed; producing high-energy electrons necessary to sustain a

discharge at low operating pressure. The accelerated electrons

move out of the ECR layer throughout the discharge volume

collisiona lly d issociating and ionizing the neutral gas. The result is

a low-pressure, almost collisionless, plasma that can be varied

from a weakly to a highly ionized state by changing discharge

pressure, gas flow rates, and input microwave power.25

For a standard 2.45-GHz magnetron the corresponding static

magnetic field needed is 0.087 5 T. Several reasons have made2.45 GHz the frequency utilized in all the ECR processing work

reported to date. The magnetic field required for resonan ce at

this frequency is relatively easy to obtain with ordinary water-

coo led solenoid al electrom agnets. The magnetron, hardware and

power supplies are readily available for this operating frequency.

The densities obtained are high enough to be useful in presentmaterials processing applications.



3 0

Microwave ECR Energy Coupling

Energy Transfer

In an arbitrary microwave plasma discharge, the energy

from the electromagnetic field is absorbed by both the electron

and ion g as. In most cases though, direct energy transfer to the

ion gas can be neglected (except for ion cyclotron resonance) due

to the low m obility of the ions. This is because the work imparted

to a charged particle by an electric field varies inversely with the

particle mass.

Ions can still have an effect on plasma dynam ics. As

electrons are extracted from the discharge region, primarily along

the magnetic field lines, an electrostatic potential is created that

tends to pull positive ions in the same direction. Although direct

energy transfer is negligible in an ion gas, ions do undergo circular

Larmor gyration about the magnetic field line s. However, the

greater mass of the ions causes them to have an orbital radius

much larger than that of the elec trons. For an argon ion having

kinetic energy of 5 eV transverse to a magnetic field of 0.0875

Tesla, the Larmor radius is about 23 mm (as compared to 0.1 mm

for an electron). This large orbital radius will cause the ion gas to

loose most of its energy through collisions with the chamber walls.For an ECR discharge, the electron gas is excited directly by the

microw ave electric field. The electrons continuously gain energy

and are accelerated . Neutral and ion gases acquire energy by way

of co llis io ns with the heated electron gas. This ECR energy

transfer process for a discharge volume is shown in Figure 6.



3 1

RadiationLosses

DischargeContainer

Wall

Joule Heatingand ICRHeating

MICROWAVEELECTRIC

FIELD

Elasticand

InelasticCollisions

RadiationLosses

HeatConduction

andConvection

Figure 6. Microwave energy transfer in a discharge v o l u m e .



3 2

Through the interaction with the electron gas, the neutral

and ion gases interchange energy by way of collisions, and

transfer energy to the walls through heat conduction and

conv ect ion . Therefore, los ses from the discharge have a direct

dependence upon the electron density, pressure, gas type, and

discharge geometry.

Power Absorption in a Magnetic Field

Without the presence of a static magnetic field, a plasma

discha rge is pressure depend ent. There fore, at low pressures

(<100 mTorr), it is difficult to sustain a discharge without high

applied electric field s. With the addition of a static magnetic field

high power absorption, even for very low applied electric fields, is

easily obtained.

The equation of motion of an electron will now be used to

study the absorbed microwave power in both cases, with and

without a static magnetic field. Newton's law along with an added

term that represents the momentum loss of the electron due to

collisions with ions and neutrals is known as the Langevin

equation and is given by

- ^ mv" = -e [E + v" X B] - m\)eV , (4 )dt

where v" is the average directed ve loci ty, e is the electron charge,

and x>c is the effectiv e collis ion frequency for electrons. The



33

effective coll ision frequency refers to the fact that an electron

loses all its directed motion an average of De times per second.

Init ia l ly , the magnetic f ie ld is not applied and the

microwave energy is represented by an oscillatory electric field

E = Eo ei ^ (5 )

Neglecting transients, and solving for the steady-state solution,

wh ere v" has the same time depen dence. Equation (4) beco m es

jco m v = -eE - m-OgV . ( 6 )

R e a r r a n g in g ,

v = -fiE ( 7 )m(\)e-i-jo))

This is the average directed velocity, or drift velocity, under the

influenc e of an electric field. The condu ction current density due

to the electric field is equal to

J = - N e 7 e = ^ ^ ^ E . ( 8 )

m(\)e + jco)

The conductivity of the medium due to the electrons is:

^^ __ N £_ e ^ ^^^

m(\)e + jco)



3 4

The conductivity and the applied electric field is now used to

write the expression for the absorbed power density for the

electron gas as

Pabs = R e ( a ) E ^ ( 10 )

w here R e (a ) is the real part of the conductivity, given by

R e(a) = N ^ ( V " ) ( 1 1 .

This term for the conduct iv i ty shows the expl ic i t

dependence of the absorbed power on vjod. With the density and

electric field held constant. Equation (11) has a maximum when

co^De- Th e effective collision frequency, VQ, varies directly with the

gas pre ssu re. For the case of hyd rogen, \)e ~ 4.9 x lO^p, where p =

gas pres sure in Torr at 300 K. Therefore, maximum microw ave

power absorpt ion is discharge-pressure dependent , and good

microwave energy coupling is possible in a coUisional heating

p r o c e s s .

For very low pressures, <100 mTorr, the mean free path

between electron-neutral and electron-ion collisions becomes very

long, \)e«co, and Equation (10) becomes

Pabs = ^ (VCO) E^ • (^2 )



3 5

In order to sustain a discharge at low operating pressures, and

without the presence of a static magnetic field, high plasma

de nsitie s and high applied electric fields are requ ired. W ith the

inclusion of an ECR static magnetic field the discharge process is

greatly simplified.

With the field included, the complete steady-state Langevin

equation is given as

(De-Hjco)m \r=-e [E + v"x B] . (1 3 )

For the simple case where the electric field is perpendicular to the

static magnetic field, the absorbed power by the electron gas

b e c o m e s

P a b s = ^ ^ f ^ [ ^ - + 1 - ] E ^ . ( 1 4 )2 m ^ 2 + (co-COce) 'Ol + (cO+COce)

If the collision frequency is reasonably small with respect to the

frequency of the applied electric field, a)e<<co, Equation (14)

b e c o m e s

P a b s = ^ ^ ^ [ 1 - ] E ' . (1 5 )2 m ^ 2 + (co-COce)

Equation (15) will peak in a resonant manner if the frequency of

the appl ied electr ic f ield approaches the electron cyclotron



3 6

freq uen cy (co=coce). This indicates a high pow er absorption even

with very low impressed electric fields.

This is the ECR condition under which the motion of an

electron in the magnetic field is in synchronization with the

applied osc illato ry electric field. An elec tron with co=(Oce will feel

the us ual Lo ren tz force ev X B and begin to drift in the direc tion

norm al to E and B . This is the well known electric field drift, V E ,

and is illustrated in Fig ure 7(a). In the first quarter cycle of the

electron's orbit, the electron will gain energy from the increasing

electric field, and accelerate in the plane normal to the magnetic

field. Th e elec tron's La rmo r radiu s is increa sing at this time.

During the second quarter of the orbit, when the electric field is

decreasing, the electron will also accelerate, but at a lower rate

than it did du ring the first qua rter cy cle . Th e final half orbit

produces the same effects only in the opposi te direct ion.

Therefore, at ECR, an electron is subject to continuous acceleration

by the elec tric field. Electron s with the "right" phase will gain

energy; whereas electrons with the "wrong" phase wil l lose

ene rgy. The motion that the electron makes is shown in Figure

7(b). The increasing orbit and speed of the electron is limited by

a collision with another particle, a collision with a wall , or

movement out of the ECR region.

It should be noted, however, that as the discharge pressure

incre ase s, pure ECR heating gives way to collisional heating. As

the pre ssu re is increased the collisional frequency -Oe app roa che s





3 8

the magnetic field has little influence on the heating of the

electron ga s. Therefore, ECR is a coup ling technique for low -

pressure discharges where the electrons can orbit many times

between collisions, or at least Ve^coce-

Magnetic Mirror Effects

Most ECR plasma discharge systems employ a nonuniform

static magnetic field comm only known as a magnetic mirror. The

name refers to the fact that this field configuration can reflect

charged particles entering a high-field region . Therefore, a pair of

co ils that form two magnetic mirrors can confine a plasma. This

effect works both on ions and electron s. Figure 8 illustrates this

scheme where the magnetic field is highest in the throat (Bm) and

wea kest at the midplane ( B Q ) . A magnetic mirror is produced

either by permanent magnets or magnetic coils.

B

Figure 8. Plasma confined between magnetic mirrors.



3 9

The basis for plasma confinement in a magnetic mirror is

the invariance of the magnetic moment, î. The magnetic moment

for a gyrating particle is defined as

i =4 -m v// B . (1 6)

As a particle moves into regions of stronger or weaker

fields, its Larmor radius changes, but ji has to remain constant.

Therefore, the particle's perpendicular velocity Vi must increaseduring periods of increasing magnetic field. Since energy must be

cons erv ed , the para llel ve loc ity v„ has to decrease. If the

magnetic field is high enough in the throat, the parallel velocity

will become zero; and the particle is reflected back to the weak-

field region.

In the absence of an electric field, a charged particle moving

into a converging field will spiral with ever decreasing transverse

orbits until it is reflected . The particle will then reverse direction

and spiral back into the weak-field region with increasing orbit.

When a transverse microwave electric field is present in the

magnetic mirror, acceleration of the electrons takes place when

they pass through the ECR region where co=o)ce- Outside this ECR

region, the electrons experience little or no energy absorption as

indicated from Equation (15); and will experience the usual mirror

force.



4 0

Confinement of the plasma is not perfect. A particle with

Vi=0 will have no magnetic moment and feel no force in the

dire ction of the field . A particle with a small ratio of

perpendicular velocity to parallel velocity will also escape if the

maximum field at the throat is not large enough.

Conclusion

From the previous discussion, an ECR plasma offers a

number of desireable characteristics; the most notable being an

efficient means of producing high-quality films at low deposition

temperatures. The electr ode less and electron-cyclotron -heating

nature of the plasma is suitable for many processing applications.

The ECR plasma is easily maintained, even for very low applied

electric fields.

Because the ECR process accelerates high-energy electrons,

the electron energy distribution will be non -M axw ellian. This

makes the form of the distribution dependent upon the applied

electr ic field, and the size and shape of the ECR layer. Therefore,

the electron energy distribution can be controlled by the applied

electric field and the magnetic mirror configuration. This control

over the ECR plasma helps to slow down the electrons prior tostriking the substrate. This is favorab le since most plasma

processing applications do not desire high-energy electrons.



CHAPTER IV

SYSTEMLAYOUT

ECR Apparatus

A schematic diagram of the ECR plasma deposition apparatus

is show n in Figu re 9. The ultra-high-vacuu m system is made

entirely of stainless steel. The discharge chamber, with an inside

diameter of 13.3 centimeters, is located inside two sets of magnet

coils. The magnet coil current is supplied by a Hobart type M-600

dc m otor-g ene rator arc we lder. The coils are used to provide a

m agn et ic f ield in a dual magn et ic mirror configurat ion; a

magnitude plot of the field along the chamber axis is shown in

Fig ure 10. The axial m agn etic field strength is adjusted so that

the field is greater than the ECR value at the coil locations, and

decrea ses with axial position as shown. Positioning the microwave

input to the chamber at the location of the left coil set, places the

waveguide window in the region of greatest magnetic field

inten sity. Th is pos itions the intense ECR layer away from the

waveguide window.

Microwave power is generated by a 2.45 GHz continuous-

wave power source (with a Matsushita 2M137 magnetron) and

int roduced in to the d ischarge chamber v ia a rectangular

wa vegu ide and quartz vacuum window . The incident microwave

pow er can be continuously varied up to 1 kW . The incident power

Pine and reflected power Pref are both measured using a dual

directional coupler and power meters.

4 1



4 2

os D > ...C/3

3«->

h icd

a.

ao

O

a.

u

c

a.X

e

oCO

ON

(1 1

300



4 3

1000

>^

•-3 C

o QI s

^ X = ^ V - ECR

0

MicrowaveSource

10

MagnetCoi l s

20 3 0 Distance (cm)

Langmuir probes

SampleHolder

Viewpor t

Fig ure 1 0. M agnitud e plot of axial m agnetic field alignedwith m icrowa ve ECR deposition system. The peak to minimumratio is 1.3. M agnitude plot provided by a magnet coil current ( I B )of 300 A.

A four-stub tuner is used to match the ECR discharge load

for different disc harg e co nditio ns. A unique feature of the

microwave system is the use of a three-port circulator and

dummy load as an attenuator instead of an isolator.

A circulator is designed to protect the magnetron from large

reflected power conditions that could damage or shorten the life

of the de vic e. The three-p ort circu lator is norm ally configured



4 4

with the microwave input at port 1, the discharge load at port 2,

and a matched dummy load at port 3 . In this arrangement, all the

power entering port 1 leaves port 2, and all the reflected power

from the nonlinear plasma load exits port 3 into the matched

dumm y load . Th is isola tes the magnetron from the nonlinear

discharge and allows it to operate into a matched load

indepen dent of discharge variations. The drawback for this

configuration occurs when trying to operate the magnetron at

output pow er lev els below 50 W atts. Be low this value, magnetron

operation is unstable with similar consequences for the plasma

discharge. Since the plasma deposition process needs power in

the range of 10 to 50 Watts, some method of providing stability is

n e e d e d .

In the method we have chosen, the circulator and dummy

load are configured as an attenuator, with the connections to ports

2 and 3 interchanged and a stub tuner added with the dummy

load (see illustration in Figure 9). This allows the magnetron to

operate at high powers (5 00 W ) where its output is stable. A

portion of the applied power to the dummy load is reflected by

the stub to the discharge chamber. This allow s low m icrowave

power, in the range of 0-50 Watts, to be applied to the dischargechamber withou t magnetron instability problem s. Adapting the

microwave system in this manner produces a controllable, stable

discharge for different operating conditions.

Numerous flanges and viewports provide locations for the

connection of gas pressure gauges, Langmuir probes, gas input.



4 5

and discharge observat ion windows (viewports) as shown in

Figu re 9. Tw o MK S type-1159B m ass flow controllers are used to

maintain a prescribed flow for the hydrogen and liquid-source

gases in units of standard cubic centimeters per minute (SCCM).

Argon and helium are connected at the other two inputs, and are

used as inert discha rges for cleaning and plasma diagnostics. The

gas f low rates for these inputs are cont ro l led by a

G ran vi l le /P hi l l ips var iab le leak valve. Single and double

Langmuir probes are used to measure radial profiles of the

plasma density and temperature at the two locations shown in

Figure 9.

Th e sam ple holder is located 15 centim eters dow nstream

from the right magne t coil set. Figure 11 (a) shows a detailed

draw ing of the samp le holder. It contains an electric heater that

can regulate the substrate temperature from room temperature to

425 °C . Th e holde r is electrica lly isolated from the cham ber and

can be kept at the floating potential or rf biased. A thermocouple

placed inside the holder is used to monitor the substrate

tem per atu re. A wa ter-cooled M axtek type TM-IOOR thickness

m onitor is located above the sam ple holder. The thickness

monitor provides a direct display, at high update rates, of filmthickn ess and depo sition rate during depo sition. This allows for

improved manual control of the vacuum film deposition process.

A manual shutter is used to cover the sample to prevent

depos i t i on dur ing d i scharge tun ing and af t e r comple ted

pro ces sing . A num ber of pilot holes are drilled in the holder to



4 6

HEATERSHUTTERCaSTTROL \

THERMOCOUH^

BIAS

I • 1 1

SAMPLEPLUGPORT

4-Î I

c

CERAMICSPACERS

TOP VIEW

FRONT VIEW

(a)

SAMPLEPLUG

CRYSTALLINESILICON SUBSTRATE

l/4"xl/2" ea.

(b )

G L A S S

S U B S T R A T E

Figure 11 . (a) Sample holder show ing feedthroughconnections, (b) Sample plug showing substrate mounting.



4 7

prevent outga sing from the internal cavities. Feedthroughs to the

sample holder are introduced through a six-way port (see Figure 9

for de tai ls). Sam ple insertion is performed by way of the load-

lock section and substrate feedrod.

Two substrates, one glass and the other crystalline silicon,

are fastened to the substrate plug illustrated in Figure 11(b). The

procedure for inserting the substrates into position in the

discharge chamber prior to film deposition is as follow s: The plug

containing the substrates is attached to the end of the feedrod

inside the load-lock chamber, the chamber is closed and

evacuated, the gate valve between the load-lock chamber and the

discharge chamber is opened, the plug is pushed into place in the

sample holder by means of the feedrod, the feedrod is withdrawn

to the load-lock, and the gate valve is closed . A Cajon™ 0 - r i n g

fitting provides the vacuum seal for the feedrod.

There are two vacuum pump systems attached to the

apparatus. One, conn ected to the load-lock, is com posed of a

Pfeiffer-Ba lzers turbo-molecular vacuum pump in tandem with a

Kinney type KTC -21 roughing pump. Tw o MDC type pneumatic

gate valves are used to isolate the load lock from the system. This

setup is used to pump down the load-lock during sample insertionand extraction, and to obtain base pressures in the neighborhood

of 5 X 10"^ Torr. The other pump sy stem, connected behind the

sample holder location, is composed of a high throughput Kinney

type M B4 03 roots blower (66 liters/second at 10"^ - 1 Torr) in

tandem with a Kinney type TCS-21 roughing pump. This vacuum



4 8

system is operated during film deposition because of its high flow

rate capability, and because of the contaminant exhaust gases.

An MKS type 253A exhaust control valve is located at the

inpu t of the roo ts blow er. The valve is part of a close d-loop

pre ssu re con trol system as illustrated in Figu re 12. The control

system consists of a MKS type 390 Baratron guage connected to a

MKS type 170M-6C and type 170M-25C electronics unit and

rea do ut. The Ba ratron is also conne cted to an MK S type 252

exh aus t valv e con troller. The control system varies the exhaust

valve opening in order to hold the discharge operating pressure at

a set-point level set by the operator. This arrangemen t creates a

very flexible system in which the microwave power and gas flow

rate are held constant while the discharge operating pressure is

varie d. This allows "mapping" of the plasma discha rge process

over a wide array of discharge parameters.

Safety Interlocks

Safety is an important issue since toxic/corrosive sil icon

tetrach loride gas, and flammable hydrogen gas is used. The ECR

app ara tus c on tains a num ber of safety interlo cks to aid the

ope rator in the even t of a system failure. Th ese interlocks are inplace in case a failure occurs in the gas, coolant, microwave, or

vacuum systems.

Hydrogen is a colorless, odorless, tasteless, nontoxic, and

flammable gas. Because hydrogen is lighter than air, it has a

tendency to accumulate in the upper portions of confined areas.



49

O

coo

<D

3t/i

D

CI.O

I

CO

O

u

(S

(L >

300



5 0

Concentrations of hydrogen between 4% and 75% by volume in air

are relatively easy to ignite by a low-energy spark and may cause

explo sions. The amount of hydrogen gas during operation is well

below the flash-over concentration; but for safety precautions, the

exhaust gas is diluted with nitrogen gas prior to exiting the

roughing pump stage. The ECR apparatus is enclosed in a framed

plastic room which is vented to atmosphere by way of an exhaust

fan through a charcoal filter. A section of the vent is also used to

rem ove exha ust gase s from the vacuum system s. This

arrangement allows for a safe operating environment, where an

operator(s) can quickly exit and seal off the enclosure in case of

an uncontrolled gas leak.

The magnet coils, microwave circulator, and dummy load

require a reliable source of cooling water. To accomplish this, the

inlet water line is run through a filtration unit to prevent

sediment buildup, and a one inch diameter water line provides an

ample supply of coo ling water for the system . Pressure switches

are located at the cooling inlet lines to the magnet coils and the

m icrow ave circulator and dummy load. They are adjusted to

shut-off the microwave power supply and magnet coil current in

case of lost coolant flow, low coolant flow, or failure of theoperator to open the coolant va lves. The microwave source also

has an additional cut-off switch that monitors the magnetron's

operating temperature. The magnetron is cooled by forced air

from a blow er. In the case of blow er failure, the temperature



5 1

switch will shut off the microwave power supply and prevent

overheating and eventual destruction of the magnetron.

An instrument rack houses a series of manual switches that

allow the operator to control the two pneumatic gate valves near

the load-lock, the gas inlet valves at the flow controllers, and the

gas inlet va lve at the chamber. All these switches are in series

with a single manual switch in case of an emergency shut-down.

The roots blower, the turbo-molecular pump, and the two

roughing pumps are operated from a series of relays that are also

controlled by a sing le cut-off switch. This setup allow s the

operator to rapidly isolate the discharge chamber from the gas

sources and vacuum pump systems in the event of some type of

system failure.

An H PS series 145 VA CUU M SENTRY™ safety valve is

installed between the turbo-molecular pump and roughing pump.

The safety valve protects the vacuum system in the event of

power failure by isolating the evacuated chamber and turbo-

molecu lar pump and venting the roughing pump. This keeps air

from the chamber, avoids oil backup, and allows the motor to

restart the roughing pump more easily when power is restored.

The valve operates with atmospheric pressure and activates uponloss of electrical power. A solenoid valve is connected in parallel

with the roughing pump's electrical supply. When the electrical

power is on, the solenoid valve is held closed, keeping the safety

va lve open and the vacuum chamber evacuated . Interruption of

electrical power to the mechanical pump causes the solenoid valve



5 2

to open and admit atmospheric air into a portion of the valve.

This closes the safety valve before vacuum pressure is lost in the

cham ber. The force needed to keep the safety valv e shut is

provided by the pressure differential between the atmosphere

and the vacuum chamber. A series of small orifices admit air into

the inlet of the roughing pump to prevent oil back-flow.





5 4

Table 2: Sample Parameters For Silicon Tetrachloride

Sample RH2:Rsia4

^ ( % )

1

2

3

4

5

6

7

89

10

11

12

13

14

15

1617

18

19

20

21

22

71-29

90-10

83-17

97-3

90-10

90-10

90-10

90-1090-10

90-10

90-10

90-10

90-10

90-10

90-10

90-1090-10

90-10

90-10

90-10

90-10

90-10

1 Courtesy J. C. !

2post-a nnea l ing

0T(seem)

7

10

6

33

10

10

10

1010

10

10

10

10

10

10

1010

10

10

10

10

10

Schuma

Power T«A(W) (^C)

20-40

25

10

25

25

40

25

3015

30

25

20

15

10

25

2525

25

25

25

25

25

285

285

250

285

285

285

285

285285

310

310

310

310

315

285

60100

200

400

285

285

285

cher Companj

(A/s)

1.3

1.6

1.0

0.5

3.0

1.2

2.0

3.74.6

3.9

4.0

2.2

1.7

1.2

2.3

3.03.4

3.4

2.4

3.2

1.6

2.7

f

Press.(mTorr)

1.8

7.6

3.5

11.9

4.3

7.6

7.6

4.84.6

4.3

4.7

4.2

4.0

4.0

4.1

4.84.5

4.3

4.9

4.6

4.6

5.0

IB

(A)

300

300

330

300

300

300

300

300300

300

300

300

300

300

300

270300

300

300

300

240

300

CTph /c J Stability

17

48330'poor film

7poor film

97

354

462111

1869

36

88

9

poor

good

good

good

good

good

good

goodgood

good

good

good

good

fair

good

goodgood

good

fair

good

good

good



5 5

ann ealin g, produ ced a cond uctivity ratio of 48,3 3 0. This opto

electronic property is comparable to good quality glow discharge

a-Si:H films. Th e anne aling process relaxe s the film structure and

improves the si l icon-hydrogen bonding, thereby reducing the

den sity of state s. The cond uctivity ratio prior to annealing was

found to be 140.

Because sample 2 showed a high photoconduct ivi ty, the

relative flow rate for SiCU of 10% was used for all remaining

samples produced. The microwave power was the next parameter

varied for samples 5 through 9. The substrate temp erature (Tsub)

was increased from 285 °C to 310 °C for the next five samples (10

thru 14), with the microwave power again being the parameter

va ried . A second SiCU gas source was used for sam ples 15

through 22 since the first source was suspected of being

co ntam inate d with oxy gen . For samp les 15 through 19 the

substrate temperature was varied from no substrate heating to

400 °C in incre ments of 100 °C. Sam ples 15 and 20 through 22

were produced to investigate the reproducibility of the process.

It is important to note that the deposition rates for the six

samples made under identical conditions (sample's 2,5,7,15,20,and

22) w ere not very consiste nt with each other. It is believed thatthis is caused from film buildup on the chamber walls as the

ove rall proc essin g time incre ase s. The film previous ly deposited

on the walls becomes partially etched during the deposition

proc ess, and contributes to the deposition proce ss. Observations



5 6

have shown that the deposition rates increase for later runs at the

same operat ing condit ions.

The operating parameters, deposition rates, and conductivity

ratios for the samples produced from a proprietary liquid source,

courtesy of the J. C. Schumacher company, and hydrogen gas are

tabulated in Tab le 3 . All samples produced from this gas source

contained a very large amount of carbon which produced poor

qua lity films . Th ese carbon alloyed am orphous silicon films (a-

SiC:H) could possibly be used as a window layer (wide bandgap

m ateria l) in p-i-n type solar cells . The first two samples were the

initial sam ples produ ced in the ECR deposition appa ratus. They

were produced prior to the installation of the circulator and

dummy load; at a t ime when the plasma discharge conditions

we re very poo r. The poor discharge condition was believed to be

caused by unstable magnetron operat ion at the low appl ied

pow ers. The rem aining seven liquid source (L.S.) samples were

produced after the circulator and dummy load were installed, at

which t ime the discharge process improved rem arkably. The

relative flow rates for the liquid source ( R L . S . ) gas and hydrogen

( R H 2 ) g^s were varied so as to investigate the percent carbon

co nte nt in eac h film . The total gas flow rate and substratetem pe ra tu re (Tsub) were held at their respective values for the

seven sam ples. The appl ied pow er, operat ing press ure, and

magnet coil current also stayed relatively constant as shown in

Ta ble 3 . From the table, the depo sition rates (rd) rem ain ed

rela t ive ly co nsta nt for samples 3 through 9, al though the



5 7

Table 3 : Sample Parameters For Proprietary Liquid Source-

Sample#

1

2

3

4

5

6

7

8

'

R H 2 : R L 5 .

(% )

50-50

75-25

93-7

90-10

90-10

90-10

72-28

50-5075-25

0T(seem)

20

40

30

30

30

30

30

3030

Power(W )

55

68

25

25

25

25

25

2520

Tgub

275

275

285

285

285

285

285

285285

(A/s)

5

5

1.5

2.2

2.5

2.5

2.5

2.82.8

Press.(mTorr)

13.3

19.2

11.2

25

25

24

25

2515.5

IB

(A)

300

300

300

300

300

300

270

300300

Stability

poor

poor

good

good

good

good

good

good

good

J. C. Schumacher Company

dep osition ra tes did appear to be on the rise as seen by the slight

increase for samples 8 and 9.

Parameter Effects

Th e o p t o - e l ec t r o n i c p r o p e r t i e s o f e l ec t r o n - cy c l o t r o n -

resonant p lasma discharge f i lms are dependent upon many

deposition parameters such as the discharge pressure, gas flow

r a te , absorbed microwave power , subst rate temperature , and

m agn et ic f ield. The hydrogen conc entrat ion in hydrogenated

amorphous sil icon films depends upon some of these deposition

co nd ition s. In gene ral, the hydrogen content will decrea se as the

su bs trate tem pe ratu re increases.26 The loss in hydrogen content



5 8

as the substrate temperature increases causes the densi ty of

defect states to increase.27

Film s d epos ited at low substrate tem peratures (< 200 °C) do

not exibit good elctronic prop erties. These films contain dihydride

(SiH2) and trihydride (SiHs) chains which act as recombination

cen ters. Ab ove 200 °C the hydrogen exists in mono hydride (SiH)

form. It is this fundam ental chang e to m onoh ydride bonding at

higher substrate temperatures that has a major effect on the

reduction of the density of states and, therefore, an improvement

in the opto-e lectronic properties of the ma terial . When a-Si:H

films are heated to temperatures greater than «350 °C, the

hydrogen evolves from the material , and the density of states

increases, causing a reduction in the opto-electronic properties.

Hydrogen content will also increase with an increase in applied

power, and decreases with an increase in discharge pressure.

In the following discussion the deposition rate is graphically

displaye d as a function of the various processing param eters. In

Figure 13 the deposition rate is plotted as a function of the

magnet coil current at various absorbed microwave powers (Pabs)-

For an absorbed power of 8 watts, the deposition rate does not

seem to be affected by an increase in the magnetic field, as long asit is above the threshold value ( I B = 1 8 0 A, B M « 0 . 0 6 3 T ) . It is not

until the applied power is further increased that an increase in

the magnetic field begins to affect the deposition rate, the rate

then displays a peak.



5 9

TH ( A / S )

2 •

1 -

0

360

IrfA)

« Pabs=8W

• Pabs=10W

° Pabs=15W

A Pabs=50W

Figure 1 3. Deposition rate as a function of magnet coilcurrent at 0 T = 6 SCCM, P=2.6 mTorr, and RsiCU-

For the present ECR apparatus setup, a layer of enhanced

brightness always forms outs ide the magnet ic mirror

configuration (between the second magnet coil set and the

viewport, see Figure 9). We take this to be an ECR layer. At lowmagnetic field settings the ECR layer takes the shape of a thin disc

that cov ers the cross section of the discharge chamber. When the

magnetic field is increased, the ECR disc layer propagates towards

the sample holder, following the 0.0875 Tesla magnitude of the

flux density, and gradually forms into a donut-like shape touching



6 0

the cha m ber wa ll. This change in the ECR layer probably changes

the makeup of the particle flux incident on the substrate and thus

the film dep osition rate. The ma ximu m seen in the depo sition-

rate curves may be due to high energy particles being slowed by

the cha m ber wall . A slowing of the part icles al lows more

dep osi t ion than etching to occur. Ad dit ional ly, the general

increase in deposition rate with absorbed power seen in Figure 13

is probably due to an increase in the ion density caused by the

higher power.

Figures 14 and 15 show the deposition rate plotted versus

the discharge pressure, at various absorbed powers, for total flow

rates ( 0 T ) of 10 SCCM and 33 SCCM, respectively. The relative

flow rate for the SiCU gas is 10% and the magnet coil current is

30 0 A m ps. Th ese two graphs show that the deposition rate, at

f ixed ab sorb ed pow er, is relat ive ly con stan t for different

disch arge pres sure s. The increase in the deposition rate, at a

constant pressure, for higher absorbed power is probably due to a

more efficient discharge produced by a greater ion density.

Figures 16 and 17 illustrate the deposition rate versus the

S iC U relative flow rate (R siC u) at various absorbed powers. Figure

16 is for a total flow rate of 10 SCCM at a discharge pressure of4.7 mTo rr, and a magnet coil current of 30 0 Am ps. From the plots

of Figure 16, for fixed absorbed power, it is easily seen that the

deposition rate increases to a maximum with increasing percent

S iC U gas in the discharge. The maximum deposition rates occur

ap pr ox im ate ly at 50 % SiCU relative flow rate for the higher



6 1

r^CA/s)

30 40P (mTorr)

Pabs=5W

Pabs=10W

Pabs=15W

- Pabs=20W

A Pabs=25W

o

•

•

Fig ure 14. Deposition rate as a function of plasma discharge

pressure at 0 T = 1 O SCCM, I B = 3 0 0 A, and RsiCl4= 10^^-

appl ied pow ers. For conce ntrat ions of SiCU above 50% the

dep osi t ion rate gradu al ly drops unt i l etching occ urs. This

phenomenon is probably due to the increased number of chlorine

ions in the plasma discharge.

Figure 17 is for a total SiCU gas flow rate of 30 SCCM at a

disch arge pressu re of 11.4 mT orr. The curves for the deposition

rate follow a similar pattern as that discussed for Figure 16;

except that the deposition rates for relative flow rates beyond

50% are not shown since the maximum lies at 30%.



Âkls)6 2

8

6 -

4 -

2 -

0

o Pabs=5W

• Pabs = 10W

° Pabs=15W

A Pabs=20W

A Pabs=25W

Fig ure 15. Deposition rate as a function of plasma discharge

pressure at 0 T = 3 3 SCCM, I B = 3 0 0 A, and RsiCl4= 10%-

ECR Apparatus Improvements

There are a number of different areas in which possible

additions and changes to the apparatus can be, and already have

been, made in an attempt to improve the discharge process; and

ultim ately to obtain better quality a-Si:H films. Relatively high

levels of metallic contaminants have been found in ECR deposited

samples which originate from the part of the stainless steel

chamber exposed to the high density ECR plasma discharge.28 It



6 3

r^CA/s)

100

Rsicif'')

• Pabs =

n P .Ko=

» Pabs=5W'abs=10W

" ''abs=15W

A Pabs=20W

A Pabs=25W

• Pabs=30W

• Pabs =35 W

• Pabs=40W

Figure 16. Deposition rate as a function of SiCl4 relative flowrate at 0 T = 1 O SCCM, I B = 3 0 0 A, and P=4.7 mTorr.

was found that these metallic impurity levels can be greatly

reduced by covering the stainless steel chamber wall at the ECR

layer with an anodized aluminum sleeve. W e have had a number

of anodized aluminum sleeves fabricated, and these are ready for

future thin film produ ction. Confinement of the ECR discharge

between the magnetic-mirror producing coil sets can be

accomplished by placing a stainless steel screen to the right of the

second magnet co il set (see Figure 9). This wi ll prevent the

microwaves from propagating beyond the magnet coil and setting



6 4

r.CA/s)

12

8 -

4 -

0

o

•

D

•

A

^ S i C l f )

Pabs =5 W

Pabs=10W

Pabs=15W

Pabs=20W

Pabs=25W

Pabs=30W

Pabs=35W

Pabs=40W

Figu re 17. Deposition rate as function of SiCU relative flowrate at 0 T = 3 O SCCM, I B = 3 0 0 A, and P=11.4 mTorr.

up a d isch arge beyond the ma gnet ic-mirror sect ion . The

acquisition and installation of a stainless-steel screen is currently

being performed for future a-Si:H film production, and to

investigate its effects upon film deposition.

Another poss ib le improvement involves redesigning the

sam ple plug (see Figu re 11(b)). The crystalline silicon and glass

sub strate s are held in plac e by four sma ll screw s. From visual

inspection of the deposited films, it is apparent that the screws

affect the film dep osition near their loca tion. Co nce rning the



6 5

sample feedrod, during sample insertion slight leakage can be

obse rved from the cham ber pressu re reado ut. The leakage occurs

at the sing le 0- rin g that seals the feedrod. Although vacuum

grease and 0-ring replacement minimizes this problem, a different

type of feedrod seal is sugg ested. The addition of a hinged load-

lock door for sample insertion and retrieval is another possible

improvement; current ly, much t ime is wasted removing and

replacin g m any bolts. For the liquid source gas flow rate, a more

sensi t ive mass f low control ler is necessary to improve the

repe atab ility of the flow setting. This improvem ent has just been

accomplished by reducing the range of the mass flow controller

redu ced . A new temperture controller is being fabricated in order

to obtain improved constant substrate temperatures, and also to

extend the heating element lifetime.

The most important improvement needed is to replace the

silicon tetrac hlorid e gas source with silane (SiH4) gas. This will

el iminate the chlorine from the discharge, and produce pure

hydro gena ted amo rphous sil icon films. W ork is presently being

do ne to accom plish this task. Safety precaution s have to be taken

into account when converting over to silane due to the explosive

nature of the gas. Acc om plishing this changeover will give TexasTech University the distinction of having the only microwave ECR

plasma apparatus producing a-Si:H films in the country.

A new toroidal ECR deposition system is in the final

con struc tion s tage . Th e new system will perform the same task as

that of the ECR depos ition system discussed earlier. The magnetic



6 6

field is obtained from electrically insulated, water-cooled, copper

tubing wrapp ed around the exterior of the toriodal cham ber. This

new toroidal chamber will replace the deposition chamber and

m ag net coi l sets shown in Figure 9. How ever, al l other

subsystems will remain the same.

Conclusion

This microwave ECR plasma deposition plasma apparatus has the

abi l i ty to operate with different source gases, f low rates,

microwave powers, magnetic field strengths, operating pressures,

and sam ple tem pera tures and voltag es. The system has also

shown the ability to produce stable, repeatable, and controllable

plasm a discharges for a wide range of processing param eters. The

microwave ECR plasma deposition apparatus has shown promising

results in producing a-Si:H,Cl films that are comparable to a-Si:H,Cl

films prod uce d by the glow discha rge proc ess. W ith the added

improvements mentioned above, and the eventual changeover to

silane gas, the ECR apparatus will be in a position to produce high

quality a-Si:H films.



REFERENCES

1. J. P. M. Schmitt, Thin Solid Films, vol. 174, p. 193, 1989.

2. D. E. Carlson, C. R. Wronski, ADDI. Phvs. Lett., vol. 28, p. 671,1 9 7 6 .

3 . D. E. Carlson, "Semiconductor device having a body of

amorphous sil icon," U.S. Patent 4 064 521, 1977.

4 . A. Madan, M. P. Shaw, "The Physics and Applications of

Amorphous Semiconductors," p. 162, Academic Press, Inc.,

San Diego, CA, 1988.

5 . D. E. Carlson, IEEE Tran. Elec. Dev.. vol. 36, p. 2775,1989.

6. T. Kagawa, N. Matsumoto, K. Kumada, Jpn. J. Appl. Phvs.. vol.

2 1 , p. 251, 1982.

7 . P. G. Lecomber, W. E. Spear, A. Gaith, Electron. Letter, vol. 15, p.

179, 1979.

8. A. Catalano, Proc. 19th IEEE Photovoltaic Specialists Conf..

p .1506 , 1987.

9. J. Yang, Proc. 19th IEEE Photovoltaic Specialists Conf.. p. 241,

1 9 8 8 .

10 . D. L. Morel, Proc. 8th E. C. Photovoltaic Solar Energy Conf.. p. 661,

1 9 8 8 .

1 1 . D. E. Carlson, IEEE Tran. Elec. Dev.. vol. ED-24, p. 449,1977.

12. J. Kane, "Photodetector with enhanced light absorption," U. S.

P a t e n t 4 532 537, 1985.

1 3 . C. R. Wronski, Solid State Tech.. vol. 46, p. 114, 1988.

14 . A. Madan, M. P. Shaw, "The Physics and Applications of

Amorphous Semiconductors," p. 168, Academic Press, Inc.,

San Diego, CA, 1988.

6 7



6 8

15 . D. L. Staebler, C. R. Wronski, Appl. Phvs. Lett., vol. 31, p. 292,1 9 7 7 .

16 . M. Stutzman, W. B. Jackson, C. C. Tsai, Phvs. Rev., vol. B32, p.

22. 1985.

17. H. Ohagi, M. Yamazaki, J. Nakata, J. Shirafuji, K. Fujibayashi, Y.Inuishi, Tech. Digest PVSEC-3. Tokyo, 1987, p. 671, 1987.

18. H. Ohagi, M. Yamazaki, J. Nakata, S. Imao, J. Shirafuji, K.Fujibayashi, Y. Inuishi, Jpn. J. Appl. Phvs.. vol. 27, p. L916,1 9 8 8 .

19. W. B. Ard, M. C. Becker, R. A. Dandl, H. O. Eason, A. C. England,J. R. Kerr, Phvs. Rev. Lett., vol. 10, p. 89, 1963.

2 0 . D. B. Miller, G. W. Bethke, AIAA J., vol. 4, p. 835, 1966.

2 1 . Y. Imai, K. Ohwada, J. Vac. Sci. Technol. B. vol. 5, p. 889, 1987.

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2 3 . N. Sakudo, T. Tokiguchi, H. Koike, I. Kanomata, Rev. Sci. Instrum..vol. 54, p. 681, 1983.

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2 5 . J. Asmussen, J. Vac. Sci. Technol.. vol. A7, p. 883, 1989.

2 6 . A. Madan, P. LeComber, W. Spear, J. Non Crvst. Solids, vol. 20, p.

239, 1976.

2 7 . P. J. Zanzucchi, C. R. Wronski, D. E. Carlson, J. Appl. Phys., vol. 48,p. 5227, 1977.

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P E R M I S S I O N T O C O P Y

In p r e s e n t i n g t h i s t h e s i s I n p a r t i a l f u l f i l l m e nt o f th e

r e q u i r e m e n t s f or a m a s t e r ' s d e g r e e a t T e x a s T e c h U n i v e r s i t y , I a g r e e

t h at t h e L i b r a r y a n d m y o iajor d e p a r t m e n t s h a l l m ake it f r e e l y a v a i l

a b l e f o r r e s e a r c h p u r p o s e s . P e r m i s s i o n t o c o p y th i s t h e s i s f or

s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e D i r e c t o r o f t h e L i b r a r y or

m y m a j o r p r o f e s s o r . It i s u n d e r s t o o d t ha t a n y c o p y i n g o r p u b l i c a t i o n

o f t h i s t h e s i s f or f i n a n c i a l g a i n s h a l l n o t b e a l l o we d wi t h o u t m y

f u r t h e r wr i t t e n p e r m i s s i o n a n d t ha t a n y u s e r m ay b e l i a b l e f o r c o p y

r i g h t i n f r i n g e m e n t .

D i s a g r e e ( P e r m i ss i o n n o t g r a n t e d ) A g r e e ( P e i m i s si o n g r a n t e d )

S t u d e n t 's s i g n a t u r e S t u d e n

///z^/?^D a t e D a t e



o



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An Electron-cyclotron-resonance Plasma Apparatus For