Semiconductor Heterojunction Topics: Introduction and Overview

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SEMICONDUCTOR HETEROJUNCTION TOPICS:

INTRODUCTION AND OVERVIEW

A. G. MILNES

Carnegie-Mellon University, Pittsburgh, PA 15213, U.S.A.

(Received 17

June

1985; in

reuisedform

31

Ju v

1985)

Abstract-Semiconductor heterojunctions with ideal lattice matching, well-controlled in fabrication, yield

devices that cannot be achieved in any other way. These devices include modulated-doped high-speed

field-effect transistors, ultra-high-gain and high-speed bipolar transistors, efficient injection lasers and

light-emitting diodes and sensitive photo-detecting structures. Atomic reconstructions take place at

heterojunction interfaces and are process-fabrication-dependent and not adequately understood. The

barrier discontinuities observed are therefore scattered in value and also somewhat dependent on the

determination method. Many papers in this Special Issue contain review aspects of these matters. Others,

however, are specific contributions on very particular heterojunction topics. Not all aspects of heterojunc-

tions are dealt with by the papers that follow, and the present article is intended for newcomers to the

field as a brief commentary on topics that are not adequately represented.

1. INTRODUCTION

Developments in

semiconductor heterojunction

structures up to fifteen years ago are described in the

book,

“Heterojunctions and Metal-Semiconductor

Junctions,” by Milnes and Feucht[l], and in “Semi-

conductor Heterojunctions,” by Sharma and

Purohit[2]. Various aspects have also been reviewed

in other volumes [3,4].

Up to the early 1970’s, the only heterojunction

successes that had been achieved were those with

injection lasers [3]. The GaAs/Al,Ga, _-xAs double-

heterojunction laser fabricated by liquid-phase epi-

taxy was exhibiting threshold-current densities of a

few hundred amperes/cm* and ability to function

above room temperature because of the carrier inver-

sion-confinement and optical confinement made pos-

sible by the band discontinuities. Other devices in-

volving heterojunctions were of marginal value, such

as the nCdS/pCu,S solar cell[5], or even existed

only as concepts that had not been realized in any

meaningful way.

With the development of epitaxy methods, particu-

larly organometallic CVD and molecular-beam epi-

taxy in the mid 1970’s, growth technology improved

in precision and ability to handle ternary and

quatemary III-V semiconductors[4] and a new era

began for semiconductor heterojunction devices.

Control became possible in compositionOand in dop-

ing over distances as small as 50 or 100 A and lattice

matching was improved to 10~3-10~4. The devices

that followed included high-quality lasers and optical

detectors, negative-electron-affinity structures, mod-

ulated-doped FET’s and high-gain high-speed bi-

polar transistors. Also in laboratories across the world

many structures are being studied involving hetero-

junctions that are yet to find significant applications.

The present limitations in the experimental control

of heterojunction spike barriers and in the ability of

theoretical treatments to match these barriers are

briefly discussed in the section that follows. This

issue contains papers by Margaritondo and by Wang

dealing with these matters in detail. Section 2, also,

touches on lattice matching and factors determining

interface states. Three important uses of heterojunc-

tions namely, FETs, bipolar transistors and light

detectors and emitters are then mentioned and fin-

ally, there is a brief discussion of quantum-well

structures and strained-layer superlattices and

speculative bandgap-engineering concepts.

2. ENERGY BAND STEPS CREATED BY

HETJZROJUNCTIONS

In a homojunction the energy gap does not vary

across the junction (except for possible bandgap

narrowing produced by heavy doping) and so con-

duction and valence-band-edge steps are similar and

controlled by Poisson’s equation. However, in a het-

erojunction between two different semiconductors

the variations can be quite different since they may

include energy steps AE, and AE,. that are abrupt

to within a few tens of angstroms. These variations

allow the designer a degree of freedom in indepen-

dent control of majority- and minority-carrier flow

(1).

In metal-semiconductor junctions, the simple

work-function electron-affinity model does not usu-

ally predict successfully the observed Schottky-bar-

rier heights[7-91. The failure to do so has been

attributed to interface-state pinning and more re-

cently the role of interface reactions has been

evoked[lO, 111. Attempts to devise new simple mod-

els, such as the common-anion model[7], useful

though this is, have not stood the test of detailed

experimental examination. In heterojunctions there

is a similar failure of simple models. The first-order

primitive model is that of Anderson-Shockley in

99


2/23

100

A. G. I

’

1I.Nt.S

which AE, is postulated to be the difference of the

semiconductor electron affinities [l, 21. This intuitive

model is useful as an introductory concept for stu-

dents, but the model is not well representative of

experimental results. Quantum-mechanical modeling

has been attempted with various degrees of success

as discussed by Margaritondo and by Wang in this

issue. The problem of obtaining such models in a

form matching experimental results is frustrated by

atomic reconstructions that take place at heterojunc-

tion interfaces, even the most abrupt and carefully

prepared [12]. Complicated situations cannot be ex-

pected to lead to very simple models. The Ander-

son-Shockley model, and the recent dipole-minimi-

zation model of Tersoff[l4] will probably remain the

main vehicles for introducing students to the subject.

Some measured values of AE,, for a number of

binary semiconductors with Si or Ge applied[l5] are

presented in Table 1 and compared with the values

expected from the electron-affinity model (EA). The

EA predictions are often far from these measured

values (16). Departures of i0.15 eV, or even more,

are common.

Tersoffs model postulates that each semiconductor

has states in the bandgap at or near the interface and

at some energy level EB - E,,, often near the mid-

gap, the states change from acceptor to donor in

nature. Tersoff proposes that when a heterojunction

is formed the EB values on the two sides of the

junction line up and this determines the band offset

A E,,. If the band-diagram did not line up in this way

then charge transfer would occur between the states

on either side of the interface and this would create a

dipole field: so from energy considerations the

system favors dipole minimization and therefore

equalization of the

EB

levels. Values of

EB

may be

estimated from A E,, measurements for a set of het-

erojunction pairs and a transitivity table prepared,

something like the table presented by Margaritondo

in this issue. The state-transition-energy EB may be

Table 1. Experimental valence-band discontinuitiea

compared with the values expected from the

electron-affinity model[l5]

Substrate

Gc

Si

GAS

GaP

GaSb

InAs

InP

InSb

CdS

CdSe

CdTe

ZnSe

ZnTe

Electron-Affinity Model

Experimental

Predictions

(eW

(eV)

Si

Ge

Si

Ge

-0.17 -0.31

0.17

0.31

0.05 0.35 0.27 0.70

0.95 0.80 0.33 0.64

0.05 0.20 -0.37

~ 0.07

0.15 0.33

0.15 0.46

0.57 0.64 0.55

0 85

0.00 0.00 - 0.34

~ 0.03

1.55 1.75 1.30

1.61

1.20 1.30 0.49 0.80

0.75 0.85

0.64 0.94

1.25

1.40 1.68

1.99

0.85 0.95 0.64

0.96

expected to play a similar role in Schottky-barrier

formation if E, is a fairly intrinsic property of the

semiconductor. Therefore the model provides a

bridge between Schottky-barriers and heterojunc-

tions since it implies that if Schottky barrier heights

are known for two semiconductors for a common

reference material such as Au, these results may

perhaps be used directly to construct the heterojunc-

tion barrier diagram for these two semiconductors.

Further discussion of the Tersoff model appears in

the papers by Margaritondo and Wang in this issue.

The precise value of the model is something that

time and further experimentation will establish, but

at the very least it is useful as an academic tool in

developing heterojunction concepts for students.

Literature searches of the 2000 or more papers that

have been published on heterojunction topics in the

last decade show that experimental values of band

discontinuities are scattered widely by perhaps f 0.1

eV. Not all measurement techniques yield the same

band discontinuities for specimens of the same semi-

conductor compositions[17], and the nature of the

preparation method may contribute to the values of

discontinuities that are present. For instance. the

A E,, values for Ge grown on (111) Ga. (100) Ga.

(110,100) As and (111) As (111) As surfaces of GaAs

have been reported as 0.48, 0.55, 0.56, 0.60. and 0.66

eV, respectively[lS]. On the other hand, other

workers have not seen orientation dependencies in

AlGaAs/GaAs heterojunctions[l9] and the weight

of the evidence is for no orientation dependence in

well prepared junctions.

Methods for measuring heterojunction barriers in-

clude: (a) photo-emission spectroscopy relative to

core bands (this involves taking the difference be-

tween two large numbers that have corrections that

are of order iO.l eV and are imperfectly known);

(b) photoluminescence of superlattices with para-

bolic wells (this rather critically depends on knowl-

edge of effective mass); (c) CV apparent doping

profiles (this is only really useful for nh’ and pP

structures); and (d) measurement of A E,, by study of

thermal emission of holes in a PIP structure [a tech-

nique developed by S. L. Wright and J. Batey. J.

Appl. Phys. 57(2), 484(1985)].

The conclusions of such studies with the

GaAs/AlGaAs/Ge system are that barriers of

GaAs/AlGaAs are near-enough commutative,

namely A E,?/’ = A EIfjA so the order of growth has

relatively little effect. Secondly it is concluded that in

this three-component system the barriers are fairly

closely transitive, namely A

E;:‘/’ +

A

E,f’< +

A

E,“’ 4

= 0. The values are typically, according to Katnani

and Bauer (1985 Electronic Materials Conference,

Boulder, CO) AlAs/Ge A E,, = 0.78 k .07, AlAs/

GaAs A E, = 0.39 of: 07 and GaAs/Ge A E,, = 0.44 f

.06 eV.

The variation of the barrier A

E,,

as a function of

Al content (x) in the system Al,Ga, ~, As/GaAs

has been reported as a linear relationship over the

whole range x = O-l and is given by A E,, = 0.551(Al)


3/23

Introduction and overview 101

eV in PIP structures. This is not a fixed fraction of

either the direct or indirect bandgap difference. In

another study (M. Heiblum, 1985 IEEE Device Re-

search Conference) structures that were metal

(molybdenum)/iGaAs/nAl,Ga,_,As were ex-

amined by internal photoemission for x in the range

O-0.4, and it was concluded that A E,/A E, was

about 0.60-0.64 in agreement with other work [20,21]

and that A E,, was about 0.47x(Al) eV. For a AER of

about 0.5 eV at x about 0.4, the 0.64 factor corre-

sponds to a barrier for electrons of 0.32 eV.

Several of the papers in this issue discuss hetero-

junction-band-discontinuities. All measured values

however are not presented and a transitivity table

must be used or the general literature must be

searched for AE,., AE,, values once a potential user

has selected the heterojunction-pair of interest. As a

guide to heterojunction-pair selection the following

rules may be suggested.

(a) Select the prime semiconductor of the active

region with due regard for the bandgap and the

mobilities required.

(b) Select the paired semiconductor(s) on the basis

of very close lattice match (say 10e3) and probable

A E, , A E,, values from the literature.

(c) This will commonly involve a binary substrate

and a transition to ternary or quaternary com-

pounds [22]. Select a fabrication process that is com-

patible with this and is not likely to have severe

cross-doping problems (such as may occur in

Ge-GaAs structures if As enters the Ge). Further-

more, thermal-expansion coefficient differences must

be considered if a substrate such as Si is used. For

lattice-mismatched heterojunctions, with mismatch

greater that 0.5%, the odds are strongly against

achieving useful performance if minority-carrier ac-

tions are involved or if trapping actions are likely to

adversely affect the performance of the device that is

under consideration. Similar remarks apply to het-

erojunctions involving polycrystalline layers.

Strained-layer superlattices are presently under

study as discussed briefly in Section 8 of this intro-

ductory paper, as a way of tailoring effective lattice

dimensions and bandgap modulation.

3. LATTICE MATCHING AND INTERFACE STATES

Lattice matching is a very important factor in

determining the quality of epilayers in heterojunc-

tion devices. A diagram of energy gaps versus lattice

constants for a wide range of ternary three-five and

two-six semiconductors is given as Fig. 1. Available

substrates are normally binary in nature, such as

GaAs or InP, since direct attempts to melt-grow

ingots of ternaries usually result in phase-separation

effects. The heterojunction pairs GaAs/Al ,.Ga, _ , As

are naturally close in lattice constants, but can be

further refined in lattice matching by the addition of

small amounts of an element such as phosphorus.

Growth can be by liquid phase epitaxy (LPE),

molecular-beam epitaxy (MBE) or chemical-vapor-

deposition (CVD). There is evidence that with LPE

the interfaces are graded over 100 A or more because

of back-etching of the melt[23] and may not be

uniform on a scale of lo-50 pm in composition

along the plane of the junction[24]. The LPE tech-

nology is less frequently used these days, partly for

this reason but mainly because CVD technology

appears more interesting as a manufacturing process.

In

0 S,Ga, 47A~ grown on InP is another techno-

logically important pair since the devices made pos-

sible by this heterojunction are of use in optical-fiber

communications. The conduction band discontinuity

between these materials has been reported as about

40% of the bandgap difference[2.5]. Stress and dislo-

cations in such structures have been measured by

Chu et a1.(26).

Other alloy semiconductors may be grown lattice-

matched to InP, as may be seen from Fig. 2 the

quaternary alloy diagram for In, _.,Ga,P, _zAsz [27].

Further data of this nature useful for planning

lattice-matched bandgap-varying structures are found

in Kressel and Butler[3]. Other useful references

include Nakajima et al. [28] on In, ,Ga r As, )’PYon

InP; Williams et al.[29] also on III-V quatemary

alloys: Neuse 1301 on (AlGa) (AsSb) quatemaries;

Osamara

et al.

[31] on Al-Ga-Sb; and Masu

et al. [32] on (Al YGa, _ ,)In,,, As.

Films of GaSb, SAs, 5

and Al ,Ga, .~Sb,.As ~,

have been grown by MBE, lattice-matched to InP[33]

but with some evidence of compositional modulation

due to spinoidal decomposition. CdS is also ap-

proximately a lattice match to InP and a A E, of 0.56

eV has been reported[34].

In general if the device requires that a lattice

mismatched film be grown, it is customary to provide

a lattice-graded interface layer. For instance,

In,, zGa,,

As

might be grown on GaAs by providing

a sequence of thin layers stepped in composition

between 0 and 20% In. The stepped composition of

the buffer layer is cooled between each step to allow

strain relief. By this process threading dislocations

tend to be turned into the surface plane instead of

continuing into subsequent layers.

This kind of approach has been applied to the

growth of GaAs on Si with the aid of a thin buffer

layer of Ge. The Ge nucleation layer grown on the Si

is heavily dislocated (1012 cme2) because of the 4%

lattice mismatch, and the first layer of GaAs grown

is similarly dislocated. Growth of the GaAs by a

sequence of 10 growth interrupts, and thermal cycles,

reduces the dislocation density by one or two orders

of magnitude relative to GaAs grown without inter-

ruption, which shows a 10’ cme2 surface dislocation

density[35,36]. It must be noted however that the

thermal-expansion coefficient of Si is less than that

of GaAs and the grown layer is in tension and liable

to cracking and there also may be bending of the

wafer.

Recently, however, it has been concluded that the

provision of a thin buffer layer of Ge is not an

advantage and that surprisingly good quality GaAs

can be grown directly on (100) or 4” off (100) Si


4/23

102

A. G. MILNES

3.6

2.8

2

w” 2.4

Y-

8 2.0

%

$ 1.6

”

&

[L 1.2

:

0.8

I

I

I

I

I

I

I I I I

I

I , I ,

I ,

I

1

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5.40 5.46 5.56 5.64 5.72 5.00 5.88 5.96 6.04 6.12 6.20 6.28 6.36 6.44 6.52

LATTICE CONSTANT, a0 % (300 K)

Fig. 1. Energy gap versus lattice constant for three-t&z and two-six semiconductors.

GaAs

.Y

GaP

120

1 10

220ev

210

100

200

090

1.90

180

f

1.70

160

Fig. 2. Lattice constant and energy gap (77 K) as a function

of composition for the quaternary alloy In, _ ,Ga,P, ;As,.

The solid lines are constant energy gap curves; the dashed

lines are lattice constant curves obtained by application of

Vegard’s law. The intersection of the energy gap curves with

the lattice constant curves makes it apparent that the energy

gap may be varied while a fixed lattice constant is main-

tained, indicating a variety of heterojunction possibili ties

from the infrared to the yellow for the quaternary alloy.

(After Coleman er al. Reprinted with permission from J.

App. Phvs. 47, 2016 1976.j

wafers by molecular-beam epitaxy. The Si wafers are

subjected to a cleaning process at 900 or 1000” C and

growth is begun at a low temperature such as

400-450°C and at a slow rate. This presumably

results in small closely spaced nucleation sites that

interact with each other in such a way that by the

time 1200-1500 A of layer has been grown the

surface tends to be single crystal that is not exces-

sively dislocated and a further thickness can be grown

at a more normal temperature 570°C or higher, and

at a normal growth rate (- 1 pm/hr). Quite good

MESFET and MODFET performances have been

achieved on such material (R. Fischer

et cd.,

1985,

IEEE Device Research Conference, Boulder, CO).

Furthermore it has been shown that GaAs grown by

MOCVD on Si gives similar device performance (T.

Nonaka et al., 1985, IEEE Device Research Con-

ference, Boulder, CO).

This is a promising technology since it may allow a

mixture of III-V and Si devices on large commer-

cially convenient Si substrates. The III-V devices

offer optoelectronic performance possibilities that the

Si devices cannot deliver. The GaAs layers on Si

wafers have not been widely available for enough

examination to determine their limitations. It may be

surmised that they will be subject to having strain

effects that might influence long-term stability and

that the minority-carrier performance will be unim-

pressive.

Time will tell whether this technology

survives as viable and worthy of transfer from the

laboratory to production.

In another approach GaAs of FET quality has

been grown by MBE on Ge by the provision of a 1

pm quantum-well-structure interface of GaAs-

(AlGa)As [37].

The quality of the interface region in a high-grade

heterojunction such as Al rGa, _ 1As/GaAs is of spe-


5/23

Introduction and overview

103

cial interest.

When grown by MBE the interface

width (lo-90% Alp-p height) has been measured as

about 15 A by Auger profiling[23]. In general the

interface of Al.Ga,-,As grown on GaAs (termed

the “normal” interface) is superior in electrical prop-

erties to the interface of GaAs grown on Al,Ga, _ .As

(termed the “inverted” interface) [38]. The valence-

band discontinuities have also been found to exhibit

dependence on the growth sequence by some

workers[39] and not by others. The reason for this is

not properly known and the effect is usually dismiss-

ed as a consequence of greater interface roughness

and possibly greater contamination. In a quantum-

well structure, photoluminescence possibly associ-

ated with carbon has been detected in the first few

tens of A of GaAs grown (40).

Transport of electrons and holes across an ideal-

ized (100) interface of GaA-GaAlAs has been mod-

eled by Osbourn and Smith[41] who predict, for

example, that an electron near the X minimum

normal to the interface in Ga, _

\

Al,

As

should

transmit into the X valley of GaAs with much

greater probability than it transmits into the r

minimum of GaAs.

Continuing the discussion of the perfection of a

heterojunction, consider now the interface states at,

for instance, an n-GaAs/N-Al, 25Ga,,,, As inter-

face (the large N is by convention associated with

the material of larger bandgap). Measurement is

usually made by causing a depletion region to spread

through the interface as reverse bias voltage is ap-

plied with the aid of a Schottky barrier on the

nGaAs or in a

p+

nN

structure (42). If there is a A

E,

at the interface and no interface electron traps, there

will usually be an accumulation region and depletion

region on the two sides of the inteiface. If in ad-

dition to a A EC there is an interfacial sheet of traps

the C-V profile of apparent concentration versus

depletion width (42) becomes as shown in Fig. 3.

Kroemer et

al

[43,44] have shown how to calculate

the interface trap density and the band discontinuity

A

E,

from such data and the method is valid even if

the junction is graded. From Fig. 3 the apparent

sheet density of electron traps may be determined to

be 3 X 10” cmm2. Application of deep-level transient

spectroscopy (DLTS) suggests that

~

he traps are

spread over a distance of about 140 A on the GaAs

side of the interface (the GaAs having been grown

after the AlGaAs in this specimen) with a concentra-

tion of about 2 X 1016 cmm3. The trap-level average

energy was estimated as about E,, - 0.66 eV. From

the threshold-current density (about 3 KA/cm2) for

an injection laser containing this interface a non-

radiative interfacial recombination velocity S( =

auN,) of about lo4 cm/s was inferred. For 3 x

10”

states cm-’

this would represent a capture cross

section u of 8 X lo-l5 cm2 in rough agreement with

an estimate of capture cross sections from the DLTS

measurements. The physical nature of the states

is

not known.

18

“; lo -

P-n JUNCTION

I

I

n-N INTERFACE

2 ,+n-GoAs-----

Y- -

0 _

F

E

Y

s

Y ‘7

G 10

FOR PROFILE

g

i 1 k, ( Jg,,21

0

1000

2000

3000

DEPLETION WIDTH,W(V)(i)

Fig. 3. C-V doping profile of P+- n-N DH laser structure

used to study interface states. The position of the n-N

interface at 0.17 pm was determined by direct measure-

ments in a scanning electron microscope. The tic marks on

the profile indicate the values of the reverse bias voltage in

the C-V measurement. The Debye length of 100 A indi-

cates the lower limit on spatial resolution[42].

Consider other measurements of this nature. In a

study of an Al,,,,Ga, xsAs/A10,4, Ga,,, As double

heterostructure as a function of active-region thick-

ness the value of S obtained was close to lo3

cm/s[45]. Such a low value lends to have little

adverse affect on device performance.

Since 1.55 pm is the wavelength of minimum at-

tenuation in optical fibers, attention has focussed on

In

o,sXGao,4,As which has a suitable bandgap (equiv-

alent to 1.62 pm) and is a lattice match to InP. A

confining material is needed for laser action and

In

o 52Al, 4RAs (bandgap 1.47 eV and a match to InP)

has been used. Capacitance profiling applied to a

Au/Ti N-In,,,Al,,,,As/n-In,,,Ga,,,As/n

’

InP

structure yielded 0.50 eV for the conduction band

discontinuity (corresponding to about 70% of AE,)

[46]. An interface charge density of 4 X 10” cm-*

was obtained which is high enough to suggest that

the junction was somewhat imperfect.

A study has been made of the recombination prop-

erties of LPE grown In,Ga,_,P/GaAs heterojunc-

tions as a function of the degree of lattice

mismatch[47]. This and other related studies have

been discussed by Aspnes[48] with the results pre-

sented in Fig. 4. The measured interface surface

recombination velocity is seen to vary linearly with

the lattice mismatch. The density of dangling bonds

at the interface may be expected to vary as 8Aa,/a,‘,

and so to be about 2.5 x 1013 cm-2 for a 1% lattice

mismatch [49] and 2.3 x 10” cm- 2 for a 0.01% match.

The relationship between the dangling-bond density

and the number of misfit dislocations in the plane of

the interface and threading dislocations developing

from the interface depends on strain effects and

impurity-related defects perhaps present at the inter-

face. The relationship between the interface recombi-


6/23

104

A. G. MILNES

Fig. 4. Interface recombination velocities for lattice-mis-

matched heterojunctions as a function of relative mismatch

c = Au,,/u,,. Data are as follows:

a,~),

In,Ga, _ ,P/GaAs.

determined from measurements of photoinduced short-cir-

cuit currents in p-n junctions; (O,O), In.Ga, ,P/GaAs.

from heterojunction solar cells; (m) Pb,

,Sn

,Te/PbTe,

minority-carrier lifetime measurements on double hetero-

structure lasers; (+), Al,,,Ga,,As/GaAs. Filled and open

symbols represent epitaxial layers in compression and ten-

sion, respectively (after Aspnes (481).

nation density NR and the dangling-bond N,, and

dislocation densities is unclear. The recombination

velocity is given by uv,NR where u is the recombina-

tion cross section and U, is the thermal velocity

- lo7 cm s-‘. If NR is taken equal to NDB, as

assumed by Kressel[49], then

s = olo7SAa,/a;

= 2.5 x 10220Aa,/a,i.

This leads to the theoretical lines shown in Fig. 4 for

two assumed values lo-l4 and lo-l5 cm2 of capture

cross section.

If a film of thickness d is cladded on both faces by

a heterojunction interface of recombination velocity

S, and if the film thickness is much less than the

minority-carrier diffusion length as in a laser, then

the relationship that applies for the effective lifetime

7 is

where 7R is the bulk lifetime. If 7B is lo-’ corre-

sponding to a GaAs minority-carrier diffusion length

of about 10 pm and if

d

is 10m4 cm, then it is seen

that a value of recombination velocity of lo3 cm/s

or less makes 2S/d an acceptably small term in its

effect on 7.

If the results of Fig. 4 for

In ,Ga, ,

P/GaAs are generally valid, then lattice

matching to closer than 10m4 may be desirable in

most heterojunctions. However, the need for lattice

matching may not be quite as severe as suggested by

Fig. 4. There are studies in progress with lattice-

mismatched heterojunctions that are not thin enough

to be pseudomorphic that suggest that other effects

enter, and that equating of recombination centers to

the dangling-bond density of the simple mismatch

model is open to question, perhaps by as much as

two orders of magnitude. However, there is no doubt

that the better the lattice match, the better the chance

of the device being good.

4. HETEROJ UNCTION FIELD EFFEfl

TRANSISTORS

This Special Issue on Heterojunctions has five

papers dealing with field-effect transistors but these

involve special topics. Therefore, some of the funda-

mentals are discussed below and some key references

given. The May 1986 issue of IEEE Transactions on

Electron evices

is to be devoted entirely to hetero-

junction FETs so the present discussion is brief.

GaAs because of its high electron mobility has the

potential of outperforming Si FET’s. GaAs FET

principles are discussed in a volume edited by

DiLorenzo and Khandelwal[50]. There is further dis-

cussion in the papers by Baliga

et

u/.[Sl] and by

Wieder [52]. A major problem with GaAs is that it is

not easy to obtain a low enough density of states

at the interface between GaAs and an insulator

(SiO,, Si,N,, Al,O,

or native oxide) to make

MISFETs that are effective. Hence CMOS operation

is not easy to achieve.

For low-power-dissipation circuits, where normally

off devices are needed, it is necessary to use MESFET

structures that are fully depleted by the inherent

built-in Schottky barrier voltage and to bias these

with a voltage of about 0.5 V to achieve turn-on.

With these enhancement MESFETs the GaAs logic-

gate-delay versus power-dissipation parameters tend

to be in the region of the performance graph shown

in Fig. 5. Thus the GaAs MESFET technology is

superior in performance to Si technologies (except at

present in yield and price). GaAs-heterojunction bi-

Fig. 5. Power-delay regiona of various transistor technolo-

gies. The diagonal lines are lines of constant power-delak

product (from IEEE Spectrrtv~. p 2X

(Feb. 19X4))


7/23


105

polar transistors are seen to be significantly higher

in

power dissipation, with a slight edge

in speed, but

are capable of delivering larger output currents or

powers for heavier loads. The Josephson-junction

technology has reliability and ultra-low temperature

system-integration problems and seems unlikely to

be pursued at present.

The technology labeled MODFET 77 K in Fig. 5 is

of special interest. Modulation-doped field-effect

transistors have very high electron mobility in the

GaAs channel since this is maintained relatively free

of ionized-impurity scattering because the dopants

that supply the channel electrons are confined to an

(AlGa)As region parallel to the channel that is sep-

arated from it by a heterojunction conduction-band

barrier. This effect is useful even at 300” K but

becomes impressive at 77 K. The structures are also

termed selectively doped FETs (SD FETs) or

high-electron-mobility transistors (HEMT) or two-

dimensional electron-gas FETs (TEGFETs). The

concept dates from Bell Laboratories work in 1978

and the history to 1984 has been reviewed by Morkoc

and Solomon [53] and the materials aspects discussed

by DiLorenzo et al. [54].

A typical MODFET energy diagram is that shown

in Fig. 6. The A1,,,SGa,,,,As layer (0.1 pm) is doped

1.5

x

101scm~3. Thus electrons pass over a 100 A

undoped region into a heterojunction notch in a 0.2

pm GaAs layer to form an almost two-dimensional

surface-channel layer of lo’* electrons cm * [55].

The Hall mobility in the surface layer is about 9000

cm*/Vs at 300 K and perhaps as high as 2

x

10’

cm*/Vs at 77 K (56). Typical transconductances are

100~200 mS/mm at 300 K and 300 mS/mm at 77 K

for devices of gate lengths l-3 pm[57-591. Matters

that have been of concern in the fabrication of

MODFETs include the following.

N

5

= 1.14 X td2cm-2 (300K)

Depleted

I

I

Fig. 6. Typical MODFET (AlGa)As-Ga.As energy diagram

(after Morkoc).

(a) Imperfections of the interfaces, particularly the

inverted (GaAs grown on AlGaAs) interface [60-621.

(b) The proportioning of the device, including the

role of the undoped (AlGa)As region, in improving

the channel mobility and aPfecting the available sheet

charge and so the device transconductance[63]. The

selective placing of the Si doping impurities in the

(AlGa)As effectively makes the devices insulated-gate

field-effect transistors with the undoped (AlGa)As

acting as the gate insulator[65,66].

(c) The discovery that Si doping of the (AlGa)As

causes deep donor vacancy complexes (termed DX

centers) to develop in the doped layer and the

MODFETs then exhibit collapse of I V characteris-

tics in the dark and threshold voltage Vr instabilities

associated with change of channel electron con-

centration with light illumination and temper-

ature [59,64]. The effect may be studied by examina-

tion of a persistent-photoconductivity effect [68] at

77 K and is the subject of papers in this issue.

Si doping of GaAs does not produce deep donors

and GaAs does not normally exhibit persistent pho-

toconductivity. For improved threshold stability the

doped (AlGa)As layer in a MODFET may be re-

placed by an AlAs/n-GaAs superlattice (for in-

stance an active 0.5 pm undoped GaAs layer fol-

lowed by a 450 k superlattice of alternating 20 A

layers of undoped AlAs and Si doped GaAs with

an equivalent superlattice doping level of 2

x

10IXcm ’ [59].

(d) Understanding the noise-level-determining fac-

tors in MODFETs. Laviron et d. [69] find room-tem-

perature noise figures of 1.07 dB with associated

gains of 10.5 dB at 10 GHz for 0.5 pm gate length

structures. At 100 K and 17.5 GHz the noise figure

becomes 0.34 dB with associated gain 9.6 dB.

(e) Attention must be given to minimizing the

parasitic resistances and capacitances of the slruc-

tures if the full advantages of potential high-speed

performance are to be obtained[70-721. Dual-

gate-heterojunction FETs of master-slave flip-flops

formed by four cross-coupled

AND/NOR

gates where

each four-input AND/ NOR gate consists of two dual

gate SDHTs of 1 pm gate length[73] have operated

as a divider at 5.5 GHz at room temperature and

10 GHz at 77 K. One-kilobit RAMS based on

MODFETs have been demonstrated with access

times of less than 1 ns.

(f) Another aspect of MODFETs that has been

explored with some success is the feasibility of p-

channel devices. Sheet-carrier concentrations of 1 x

10’” cm’ have been obtained with 77 K hole mobili-

ties of 3650-5000 cm’ V--’ s ’ in Be-doped

(AlGa)As/GaAs structures[74]. Transconductances

of about 30 mS/mm at 77 K have been obtained for

1.5-2 pm gate lengths[75] and p-channel devices

exhibit very little shift of threshold voltage with

light [76].

Structures that consist of (1nGa)As grown on InP

have been giving interesting performances. n-

In

o &a,, 47

As doped 10” cm

’ exhibits a 300 K


8/23

1 6

A. G. MILNES

electron mobility of 8000 cm*/Vs, which is almost

twice that for GaAs of comparable doping. The gap

between the r central valley and the L satellite

valley is larger than for GaAs and therefore an

electron overshoot velocity that is higher by a factor

of two is perhaps achievable[77].

A number of junction or insulated gate FET

structures have been made in (InGa)As[78-821

and selectively doped FETs have been made

with Ga,,,In, 53As/Alo4XIn,,,,As [83]. Recently,

Rosenberg et al. (1985 IEEE Device Research Con-

ference, Boulder, CO) have demonstrated a high

electron mobility transistor that has a GaAs sub-

strate and a thin (300 A) pseudomorphic-strained

active layer of In,,,Ga, ssAs followed by a GaAs

spacer layer and a GaAs

(

n = 2-8 x 10’%mP

s

layer

to provide the carriers for the 6 X 10” cm -’ electron

gas. For 1 pm gate lengths the g,, observed was 170

mS/mm at room temperature and the 77” K elec-

tron mobility was 40,000 cm*/Vs. An important

feature of the device is that it does not exhibit

collapse of the IV characteristics in the absence of

light since AlGaAs and its D-X center is not part of

the structure.

n+

Substrate

b Collector

(a)

n+

1

,i

Substrote

Emitter

(b)

Much remains to be studied in high speed FET

GaAs structures[84-881 including the performance

of various vertical- and permeable-gate structures

[89-921.

Fig. 7. The two single heterojunction bipolar transistor

structures discussed in the text. (a) The conventional

emitter-up/collector-down configuration. (b) The inverted,

emitter-down/collector-up configuration. Note the relative

positions of the narrow and wide bandgap layers in the two

structures; note also the position of the heterojunction. The

devices illustrated are single heterojunction devices but the

5. BIPOLAR HETEROJUNCIION TRANSISTORS

In conventional (homojunction) n+pn bipolar

concept is equally valid for double heterojunction tran

sistors[95].

transistors the emitter is more heavily doped than

the base to control the reverse injection from base to

emitter that otherwise would reduce the current gain.

The valence-band barrier AE,, for heterojunction

transistors, such as the N(AlGa)As/pGaAs/nGaAs

structure, eliminates this problem and the base re-

gion may be more heavily doped than the emitter[l].

This results in a base resistance that is reasonably

low even if the base is made very thin to lower the

base transit time. The low base resistance therefore

reduces the base-emitter and base-collector RC

time-constant terms that also limit the frequency

response of the device provided the associated capa-

citances can be held small by emitter and collector

doping control.

One of the important applications of heterojunc-

tion bipolar transistors is likely to be in analog to

digital converter circuits where very high sampling

rates are desirable and the circuits require the high

driving power (large transconductance) of an HBT.

use of an inverted (collector-up) design in which the

current flow is restricted to the small collector region

of the structure by the barriers of the wide-gap npi

junction near the base in Fig. 7(b) if this is of low

leakage current. With such a structure the relative

frequency performance of bipolar heterojunction and

homojunction-transistors[95] has been estimated to

be about 1.7:1 although this is yet to be demon-

strated. An (AlGa)As/GaAs heterojunction tran-

sistor should have a much higher current gain than a

GaAs homojunction transistor if the emitter-base

interface is well prepared[96] but this is not always

seen for reasons that are not fully examined, never-

theless serviceable current gains of lo-100 are com-

mon. The modeling of (AlGa)As/GaAs devices pre-

sents special problems that have not yet been fully

resolved[97]. The papers by Ankri et al. and Fischer

et al.

in this issue discuss matters relevant to

emitter-injection action in the (AlGa)As system.

The achievement of very-high-frequency perfor-

In a typical heterojunction bipolar transistor the

mance in a bipolar heterojunction transistor depends emitter may be nAl,,,Ga,,As, the base pGaAs (0.1

critically on minimizing device parasitic time pm 10” cm 3, and the collector nGaAs on a semi-

delays[93]. In a normal transistor configuration the

insulating substrate. The two emitter fingers may be

collector area is larger than the emitter area, as 4.5 pm wide and 10 pm long. In such a design[98]

shown in Fig. 7(a), so the collector capacitance-base

the current gain

i,/i,,

observed was 90 and the

resistance time constant in a heterojunction tran-

transconductance per emitter width was 4750

sistor may be a factor limiting the frequency re-

mS/mm. The common-emitter current-gain cut-off

sponse unless this is addressed in the design.

frequency fr was 25 GHz corresponding to an

Krocmer [94] has shown how this may be reduced by

emitter-to-collector transit time of 6.4 ps of which


9/23


5.2 ps was emitter charging time. It was considered

that this could be reduced to obtain an fr of 50

GHz. Other research groups have obtained compar-

able performances [99-1021, so heterojunction bi-

polar transistors are promising devices for ultra-

high-speed integrated circuits. Propagation delay

times of about 50 ps per gate have been achieved in

ring oscillators. This issue of

Solid-State Electronics

contains a study of current-mode-logic HBT circuit

performance by Katoh et al. that predicts switching

times between 30 and 10 ps. The d.c. characteristics

of double heterojunction bipolar transistors are dis-

cussed in the paper by Ankri et al.

Grading of the base region of an HBT to improve

base transit time by provision of a quasi-electric-field

in the base has been examined by Hayes et al.[103].

Heterojunction bipolar transistors tend to exhibit a

significant collector/emitter offset voltage because of

the difference in base-emitter and base-collector

turn-on voltages unless a second heterojunction is

provided at the base-collector junction. Composi-

tional grading of the emitter to base region however

smooths out the conduction band discontinuity and

so reduces the emitter-base junction turn-on

voltage[l04] and reduces the need for a double het-

erojunction structure.

Numerical modeling of heterojunction bipolar

transistors with graded bases suggest that a current-

gain cut-off frequency of 100 GHz should be ob-

tainable [105]. Monte Carlo simulations [106,107]

suggest that in a base region of 1000 A with a

quasi-electric field of about 20 kV/cm there is near-

ballistic transport of the electrons and a current-gain

cut-off frequency of even 150 GHz might be ex-

pected. Ballistic transport has also been considered

in 0.25 pm gate-length FETs[lOS] with prediction of

fr up to 160 GHz. Hot carriers may cross barriers in

real-space transfer and become trapped [109], as in Si

MOSFETs, so such predictions must be viewed with

caution.

The basic concept that electrons in short devices

may attain high velocities and result in fast charge

transfer is however a matter of continued inter-

est [llO-1121. Ballistic transport implies collision-free

transit over distances shorter than a mean free

path for the particular electron energy. The presence

of a conduction band spike (energy discontinuity) at

a heterojunction emitter has some influence on the

energy distribution of the electrons injected in the

base and the mean free paths may be affected by this

and contribute to the degree of ballistic transport in

narrow base transistors. A GaAs planar-doped ver-

sion of this has been reported by Hollis et al. but

with limited performance[ll3].

Although the discussion has centered around

(AlGa)As/GaAs structures, heterojunction bipolar

transistors have been fabricated in the (AlIn)As/

(GaIn)As/InP system [114,115]. More studies with

this and other materials systems are in progress. It is

noteworthy that

nA1

a 35Ga, 65P/pGaP/n GaP

bi-

polar transistors have demonstrated useful transistor

operation at temperatures up to 550” C and might be

of value for geothermal and other specialized appli-

cations[ll7,118].

In recent years Si bipolar transistors have been

studied with emitters of SIPOS or n doped hydro-

genated amorphous silicon [119,120]. The low mobili-

ties and localized energy states that exist in such

emitters are performance-limiting factors. Recently

Sasaki et al. (1985 IEEE Device Research Con-

ference, Boulder, CO) have reported an amorphous

SiC:H emitter for a Si base-collector heterobipolar

transistor. The AE . and AE values observed are

0.16 and 0.54 eV, respectively, with the 1.8 eV emitter

material.

Also recently there has been progress with the

growth of strained Ge,,,Si,, layers on Si and bipolar

transistor action may be expected from these.

6. LIGHT DETECI ION WITH HJXEROJUNCIION

DIODES AND TRANSISTORS

Silica fibers are of low loss in the 1.3-1.6 pm

wavelength range and therefore there have been ex-

tensive studies of light-detecting diodes and tran-

sistors in this range involving ternary or quaternary

III-V semiconductor heterojunction structures. The

ability to sense low light levels with fast response

time and high gains is of prime importance in optical

communication systems.

6.1.

Diode Detectors

Conventional photodiode structures have been well

reviewed by Melchior [121] and others [122]. If a PIN

form is used the photons are absorbed in a depleted

region of high field and the gain is increased by

avalanche multiplication and nanosecond electron

transit times are readily achieved. However care in

design and fabrication is needed to obtain uniform

avalanche action [123,124] and low dark currents.

The performance of a heterojunction avalanche

photodiode with nIn, s,Ga,,,,As as the light absorb-

ing layer and npInP as the avalanche multiplication

junction is shown in Fig. 8. Separate absorption and

multiplication regions tend to reduce tunneling com-

ponents of dark current and in this structure 1 x 10m4

A cmm2 at 0.9V, is achieved, where V, is the junc-

tion breakdown voltage[l23]. When illuminated with

1.15 pm light, the diode has a maximum multiplica-

tion gain of 880 and an external quantum efficiency

of 40%.

Structures of this kind tend not to perform well at

high bit rates because the bandwidth is restricted by

hole trapping at the valence-band discontinuity at

the InP/InGaAs interface. These holes must escape

by thermal emission in the recovery phase and this is

a relatively slow process. The incorporation of a 0.3

pm intermediate layer of n In, ,Ga,, As,, 65PO 5 be-

tween the

nIn

o.ssGa,47As (10 pm) absorbing layer

and the npInP multiplication junction solves this

problem and allows a gain of 40 or 50 at 0.9 VB and

a dark current density of 5

x

10m4 Acm’ (13 nA)


10/23

108

A. G.

MILNES

10 -I-

/l\zn-InGaAc

. .

- AuSn

-11

KI

L I 1 , I

0 40 20 30 40 50

REVERSE BIAS VOLTAGE V,(V)

I O3

1 4

J

IO’

si

5 -E

IO

&

5

g -7

5 10

II

-E

10

-E

10

-1c

10

(a)

I

I

I

I

I

I

I

I I

I

I

10 20 30 40 50 60

REVERSE BIAS VOLTAGE (VI

(b)

Fig. X. A hetcrojunction avalanche photodiode with

In

.)

57Cra,,47As absorbing layer and nInP avalanche multi-

plication layer and window layer [123]. (a) Dark currents as

a function of reverse bias voltage for several diodes made

from the same wafer. Inset shows a schematic cross section

of the HAPD with buffer layer. (b) Typical

against reverse bias voltage as a parameter

length of incident light.

coupled with excellent performance at

420 Mb/s and 1 Crb/s[125].

photocurrent

of the wave-

bit rates of

In other studies with separate absorber and multi-

plication regions, Al, 48 n, 52 n/Ga, 47In, 53As

avalanche photodiodes have exhibited good perfor-

mance [126]. A typical structure and the correspond-

ing energy diagram are shown in Fig. 9. The ad-

vantage claimed for this particular version of an

SAM APD is that the A

E

value is low. This reduces

the pile-up effect of holes at the notch and so the

detector response time is shorter than for other

material combinations.

Hole trapping at a heterojunction interface is the

essential feature of operation of the modulated bar-

rier photodiode of Fig. 10. The device although not

superfast is sensitive to low light powers and may

have a d.c. optical gain of 1000 in the nanowatt

illumination range[127,128]. The gain is caused by

barrier lowering produced by the accumulation of

the photogenerated holes in the valence band notch

region at the source-gate interface. The rise time is

about 50 ps and the fall time about 600 ps for 0.83

pm pulse illumination. This may be compared with 2

ns for a phototransistor in the low-gain base-con-

trolled condition or lo-100 ns in the high-gain Roat-

ing-base condition.

A typical avalanche photodiode of 50-100 ps re-

sponse time needs a bias voltage of 20 V or more

A

+ *‘0.48I”0.52 As

GY : : % ’ ’ ’ . _’ ’

(a)

nEc =0.55ev

(b)

Fig. 9. Avalanche photodiode of (GaIn)As/(AlIn)As. (a)

Schematic of the 1.3 Frn avalanche detector with separate

absorber and multiplication regions. (b) Energy-band di-

. 1.

agram unaer reverse-mas conaltlom


11/23


109

m Ge-Au CONTACT

(a)

v

(b)

Fig. 10. An (AlGa)As/GaAs low-light photodetector struc-

ture. (a) Schematic diagram of the majority-electron photo-

detector. Incident photons are absorbed partly in the gate

but most in the drain region. (b) Schematic representation

of the energy band diagram of the majority-electron photo-

detector under normal bias condition.

although if avalanche gain is sacrificed a few volts is

adequate.

A low

voltage may be generated by trans-

mission of light energy through a fiber to what is

essentially a miniature solar cell structure providing

milliwatts of power

[129].

In a few applications there may be interest in

high-speed photodetectors that require no external

bias voltage. Such a structure is shown in Fig. 11.

The light pulse passes through the (AlGa)As window

layer and generates electron-hole pairs in the

p- GaAs layer where they are separated by the built-

in electric field of the junction region[l30]. Electrons

drift towards the heterojunction interface of the

selectively doped Al,,,Ga,,,As/GaAs structure and

are collected by the Au-Ge electrodes. The photo-

generated holes drift towards the pm GaAs and semi-

insulating GaAs substrate and induce electrons in

the conductive epoxy and a transient signal is cou-

pled into the stripline. Pulses with rise times of 30

ps, and 60 ps width at half maximum, have been

detected. This detector appears suitable for integra-

tion with modulation-doped field-effect transistors.

Photoconductive detectors can in some respects be

comparable to photodiode PIN detectors in high

performance in high-data-rate long-wavelength

lightwave communications systems. Photoconductive

detectors can exhibit gains of a few hundred at low

frequencies and gains of the order of 10 at high bit

rates. The gain is less sensitive to temperature than

for an avalanche photodiode. On the other hand,

the detectors with such photoconductive gain show

long (nanosecond) fall times. In a study of a

Ga,, ,,Ine 53

As photoconductive detector grown on

an Fe doped semiinsulating InP, the received optical

(b)

To

SAMPLING SCOPE

2 DEG

(cl

Fig.

11. Bias-free Al,Ga, _xAs/GaAs photodetector. (a)

Cross-sectional view of the bias-free photodetector (not

drawn to scale). The dashed line indicates the existence of

the two-dimensional electron gas. Note that the Ge-Au

contacts penetrate the n-Al,,,Ga,,As layer. (b) Schematic

diagram showing the mounting scheme of the detector. (c)

Energy-band diagram of the selectively doped

Al, aGa, ,As-GaAs structure. The built-in electric field

separates the photogenerated electron-hole pairs, as in-

dicated by the straight arrows. The inserted circuit diagram

illustrates the electron flows (indicated by arrows). The

capacitance C is associated with the semiinsulating GaAs

substrate. Resistance R, (or

R2)

represents the series resis-

tance associated with the electrode No. 1 (or No. 2).

power necessary for a bit error rate of lo-’ at 1.3

pm was - 34.4 dBm at 1 Gb/s[l31,132].

6.2. New

Photodetector Device Concepts

Although avalanche multiplication provides desir-

able gain it also contributes to signal degeneration

and noise. If the ionization coefficient for holes is


12/23

110 A. G.

MILNES

comparable with that for electrons the holes created

in the first ionizing action travel backwards and

create further hole-electron pairs that have the effect

of deteriorating the response to a pulse of light.

Ideally then in avalanche photodiodes a high ioniza-

tion rate for electrons (a) and low rate for holes

(p) is desirable. In Si the ratio a/p is about 20

and reasonably acceptable excess noise factors are

obtained for Si avalanche photodiodes (APDs) oper-

ating at photon energies above the Si bandgap (cor-

responding to wavelengths shorter than 1.1 pm).

However for smaller bandgap materials the ratio of

a/b is not so favorable. Ideas have been proposed

for overcoming this problem that involve gradedLgap .,

APDs, superlattice and staircase APDs and channel-

ing APD’s[133-1371.

(a)

The concept of the graded-staircase avalanche-

multiplier photodiode is illustrated in Fig. 12. At

E,

each A E step the electric field is high and the

electrons ionize hole electron pairs as suggested by

the arrows in Fig. 12(b). The holes do not ionize

L

in their reverse flow to the cathode because the

A E steps are small. In progress towards construc-

tion of such a photodiode a superlattice of

Al

o 45Ga,, 55

As/GaAs has been studied and an effec-

tive a//3 ratio of 8 demonstrated. However graded

structures are needed to eliminate electron trapping

at AE< notches and there are problems still to be

overcome. (b)

The concept for a channeling avalanche photodi-

Fig. 13. A proposed channeling structure

for

an avalanche

ode is shown in Fig.

13. The structure consists of

photodiode. (a) Schematic of the channeling APD. (b) Band

alternate widegap p-

and low-gap n-layers and

diagram of the channeling APD ( ER1 > E 2). c is the paral-

lel field causing carriers to ionize. AE,. ias been assumed

negligible with respect to A

E,

Fig. 12. Proposed staircase avalanche photodiode. (a) Un-

biased graded multilayer region. (b) The complete staircase

detector under bias. The arrows in the valence band indi-

cate that holes do not impact ionize.

voltage is applied so that all the layers are depleted.

The explanation of device operation that follows is

taken directly from the Capasso-Tsang-Williams

paper [134].

“Suppose that radiation of suitable

wavelength is absorbed in the lower gap

layers thus creating electron-hole pairs.

The two p-n heterojunctions formed at

the interfaces between the relatively nar-

row bandgap and the surrounding higher

bandgap layers serve to confine elec-

trons to the narrow bandgap layers while

sweeping holes out into the contiguous

wider bandgap p-layers where they are

confined by the potential. The parallel

electric field c causes electrons confined

to the narrow bandgap layers to impact

ionize. Holes generated in this way are

swept out in the surrounding higher gap

layers before undergoing ionizing colli-

sions in the narrower gap layers since

the layer thickness is made much smaller

than the average hole ionization distance

l/p. In conclusion, electrons and holes

impact ionize in spatially separated re-


13/23


111

gions of different bandgap. Holes in the

wider gap layers impact ionize at a much

smaller rate compared to the electron

ionization rate in the relatively lowgap

material, due to the exponential depen-

dence of

a /3

on the bandgap, so that

a/@ ca n

be made extremely large. Note

that this structure has the advantage of

providing a high

a/P

ratio at very high

gains (> 100) because electrons and

holes impact ionize in different materi-

als. A typical design for 1.3-1.6 pm

detectors would have

p-

and n-layer

thicknesses of 0.5-1.0 pm and doping

levels p = n = 10’6/cms. The

p

layers

could be of InP or Al,,,In, 52A~ and

the n layers of In,,,Ga,,,As. These

materials can be grown lattice matched

to a semiinsulating Fe doped InP sub-

strate. The estimated a//3 ratio is = 350

for a parallel field of = 2

X

lo5 V/cm

at a gain of = 150 for a layer length of

= 25 pm.”

There are technological difficulties in fabricating

such structures and it remains to be seen whether

good performance can be achieved.

6.3. Phototransistors

Bipolar-junction and field-effect transistors may be

used as light detectors. In the bipolar device the

transistor is designed so that illumination creates

carriers in the base region. The supply voltage is

applied between the collector and the emitter, and

the base floats at a potential VEB that suits the

current flow and photoinduced carrier conditions as

shown in Fig. 14. There is a buildup of excess holes

in the base and so development of the voltage I’,,

that allows a small hole current related to the photon

flux to flow into the emitter. However, the voltage

&.a causes a much larger current of electrons to flow

from emitter to collector. Thus the photon-induced

carriers are multiplied by the injection gain of the

transistor, provided the base is free to find its own

potential. A heterojunction transistor with wide-gap

emitter provides an optically transparent emitter re-

gion for photons within a certain energy range, and

also the high gain associated with the A E,, barrier at

the heterojunction interface. Interface recombination

may reduce this gain and it is not unusual to find

that the transistor gain is low at low currents or low

light levels. At nW power levels the current gain

(h,,) or optical gain may be 30 or 40 and rise to

many hundreds at high signal power levels. The

performance of heterojunction phototransistors in

the material system InGaAs/InP has been studied

by Chand et al. [138] who conclude that many of the

recent reports of very high optical and current gains

may involve avalanche multiplication as in Fig. 14(c)

enhancing the gain of the HPTs. The gain depen-

dence (on the base-collector voltage) reported by

(b)

Cc)

Fig. 14. Heterojunction phototransistor action. (a) Oper-

ation with floating base. (b) Energy-band diagram with hole

accumulation in the base causing emitter-base bias and

electron injection and collection. (c) Optical-input power

and voltage dependence of optical gain.

Chand et al. and attributed to avalanche multiplica-

tion is not generally observed to such a degree. If

holes are being generated by avalanche at a high rate

and must escape from the base by emission into the

emitter, then an unstable switching or looping-action

might be expected. Since this is not seen then it is

possible that heavy recombination is taking place in

the base of Chand’s structure or at the emitter-base

interface. Narrowing of the base width with increase

of &.a is another factor that could contribute to the

dependence of the gain on I&. There is more to be

examined here in respect of HPT gain and frequency

response. The response time of a graded-base photo-

transistor may be a few tens of picoseconds[l39].

Wavelength-selective photodetectors are of poten-

tial interest in wavelength-division multiplexing

transmission systems and in heterojunction tran-


14/23

112

A. Ci. bfII.NES

sisters some degree of selectivity can be achieved by

choice of a suitable absorbing layer in the optical

path as shown in Fig. 15 [140]. The power dependent

gain-bandwidth performance of heteroJunction bi-

polar phototransistors for communication systema

has been considered by Milano et ul.1141].

Modulation-doped FETs may be used as photodc-

SOURCE

SEMI-INSULATING

n+-GoAs12008,2x,d* cn?r

UNDOPED Al, 3 Gao7As( 75iil

tectors. The structure shown in Fig. 16(a) has a

gate-drain spacing of > X pm and a gate length of

>

20 pm, and in spite of these large dimensions it

exhibited (for GaAs laser pulses) a rise time of 12 pb

with a full width at half maximum of 27 ps[140].

The a.c.

(> 20

MHz) external quantum efficiency

was nine times more than for a PIN diode.

MODFET detectors therefore offer comiderable pcr-

formance promise.

In general a PIN detector must be followed by a

0 Ge-AU

m Al

(a)

A”FeN,,Au

Ln DlFFlJSlON

IllP

InGaAs

tl+-IllP

transistor to provide increased gain. Fig. 16(b) shows

s I

IrlP

the integration of a PIN InGaAs detector with an

(b)

InP MISFET transistor [143]. PIN-FET integration

has also been based on the AlGaAs/GaAs

Fig. 16.

FET photodetection. (a) With modulated-doped

FET[142]. (b) With PIN detection and integrated InP MIS

system[144].

FET[143].

6.4. Photocwltuic Solur Cells

There is extensive literature on heterojunctions

applied to solar power generation [145~~147]. The May

1984 IEEE Trunsuctlons on Electron Dec$ices is an

issue devoted entirely to photovoltaics and a broader

review appears in IEEE Spectrum for March 19X4.

In general polycrystalline heterojunction solar cells

show degradation effects and do not exhibit

3

AU-5

hL

, OHMIC CONTACT

I

SUbSTFcATt

I

\

Au-9

OHMIC CC,NTA:CT

(3)

the efficiencies needed to be cost effective (perhaps

> lo-12%’ for air-mass-one conditions is needed in

large arrays).

Single-crystal solar cells based on the III-V system

are capable of performances that exceed those of Si

cells. Single junction efficiencies of 20% or more are

possible with well designed and fabricated heteroface

cells of AlGaAs/GaAs at high solar-concentration

conditions. In multijunction cascade structures

(tandem-cells) the predictions are for 35% efficiency,

but for various technological reasons the present

performance is below that of single crystal cells.

7. HE~‘ER~.JUNCTIONLIGH~‘ EMI~IN~;OIODES

AND

INJEffION LASERS

Heterojunctions have made significant contribu-

tions to light-emitting diodes by allowing ternary

and quaternary structures to be grown effectively,

and by providing carrier confinement and low inter-

facial-recombination and window action. Typical

structures are shown in Fig. 17(a),(b),(c). The GaAs

structure of Fig. 17(b) provides a CW output of 5.X

mW, or a radiance of 92 W/sr cm’, at 150 mA [14X].

The InGaAsP structure has a total hemispherical

light output of 1.2 mW at 200 mA (10 KA/cm’) and

is capable of coupling 40 PW of optical power into a

63 PW core 0.21 NA optical fiber. The proJected

lifetime at room temperature is estimated to be over

10” h[149]. This issue of Solid-State Electronics con-

tains a paper by Komiya et ul. that is concerned with

luminescence from InGaAsP.

Very often an LED has a hemispherical emitting

structure to focus the light into an approximately

I .,,

Fig.

15. A wacclength

xlective heterojuncrion

parallel beam by minimizing total internal reflection.

InGaAaP/InP phototransistor-. (a) Structure \*ith selective

However another approach by Thornton et (11.(1985

ahsorbing layx (h) Band diagram.

IEEE Device Research Conference, Boulder, CO) is


15/23


113

LIGHT EMITTING REGION METAL CONTACT

GaAs SUBST

GoAs(

1225

GoAs

(~ ) CONTACT

GoAs SUBSTRATEtn

I

GaAs (n

GaAs(p

-2pm

‘Zn DIFFUSION

(b)

n-lnGaAsP(Eg =lOZ/~rn)

(Eg =l 2711”)

p-lnGoAsP(Eg =l 02pm)

(c)

’

AuSn

Fig. 17. Typical light emitting diode structures. (a) Double

heterojunction structure with GaAs light emitting region

and Burrus-type geometry for coupling to an optical

fiber.

(b) Structure with Zn diffusion to reduce series resistance.

(c) Structure with InGaAsP active layer that emits at 1.27

to take a GaAs laser structure and almost entirely

eliminate the reflection at one emitting surface by

providing an antireflection coating of ZrO,. This

suppresses laser action at high current densities (per-

haps even

to 6-8

times the current density typical

for laser action in a similar structure with a complete

FabryPerot cavity). In this way 200-350 mW of

CW light has been obtained from an edge-emitting

device at room temperature with no active cooling

and power conversion efficiencies obtained of about

3%. The structureless far-field pattern so obtained,

with absence of laser speckle, is of value in certain

applications.

The general principles of light-emitting diodes have

been discussed by Bergh and Dean [150] and by

Pilkuhn [151]. Extensive use of visible-light-emitting

diodes already exists and may be expected to grow as

high-brightness high-efficiency low-cost diodes are

developed [152]. Diodes are available in red, orange,

yellow and green. Blue emitting diodes of SIC are

expected to come to market soon.

Turning now to semiconductor injection-lasers, the

role of heterojunctions has been of the essence al-

most from their first inception. No attempt will be

made here to review or highlight this role[3,153].

The June 1983 and June 1985 issues of the IEEE

Journul of Quuntum Electronics are given to semicon-

ductor lasers. The

IEEE Journal of Quuntum Elec-

tronics also publishes many of the papers given at

the International Semiconductor Laser Conferences.

The IEEE Spectrum for December 1983 contains a

review of single-frequency semiconductor lasers for

fiber-optic systems. These references are sufficient to

introduce newcomers to the field. Contributions on

injection lasers were not invited for this issue of

Solid-State Electronics in view of the very specialist

nature of present developments and the coverage

available elsewhere.

8. QUANTUM-WELL STRUCTURES AND

STRAINED LAYER SUPERLATTICES AND

BANDGAP ENGINEERING

Semiconductor superlattice and quantum-well

structure studies were initiated by Esaki and co-

workers about 1970. One kind of superlattice con-

sists of periodic layers, a few hundred angstroms

thick, of a homogeneous single-crystal semiconduc-

tor with large doping swings to form n-i-p-i struc-

tures. Such structures may be expected to have un-

usual conductive, capacitance and optical properties.

More usually quantum-well superlattice structures

involve periodic thin layers of two (or occasionally

more) different semiconductors and the semiconduc-

tors are often selected to be closely lattice-matched.

Sometimes they may be lattice-mismatched so that

alternate layers are in elastic tension or compression

and average in lattice constant to the lattice of the

substrate on which they are grown. A journal Super-

luttices and Micro-structures, published by Academic

Press began in 1985.

In a recent review of superlattices[l54] Esaki char-

acterized them as types I-III as shown in Fig. 18.

Type I occurs for systems such as GaA-AlAs and

GaSb-AlSb, or the strained layer structure of

GaA-Gap. The sum of A E, and A E,, is seen to be

equal to the bandgap difference

Eg2 - Eg,

of the

two semiconductors. The type II staggered structure

is found in certain superlattices of ternary and

quatemary IIIIV’s. Here it is seen that AE, - AE,,

ELECTRONS

Ec2

TYPE

in

5

Ev2

Fig. 1X. Discontinuities of bandedge energies at four kinds

of superlattice heterointerfaces: band offsets (left), band

bending and carrier confinement (middle), and superlattices

(right) [154].


16/23

114

A. G.

MILNES

equals the bandgap difference Egz - Eg,. The type

II misaligned structure is an extension of this in

which the conduction band states of semiconductor 1

overlap the valence band states of semiconductor 2.

This has been established as occurring for

InA-GaSb. Electrons from the GaSb valence band

enter the InAs conduction band and produce a di-

pole layer of electron and hole gas shown in Fig.

18(c). The zero gap HgTe/CdTe interface also has

unusual properties since interactions between the

hole bands of the CdTe and the HgTe occur.

Semiconductor superlattices offer several perfor-

mance features of interest for device physicists and

engineers. The value of selective doping for

MODFETs is one property that has already been

amply demonstrated. A second property of value is

in injection-laser structures where the provision of a

quantum-well active region results in modification of

the density of states from a parabolic distribution as

in bulk material to a staircase distribution. This

results in fewer electrons being needed to achieve the

same cavity gain and therefore a lower threshold

current density[l55] as illustrated in Fig. 19.

Another feature of interest of superlattices is that

defect and dislocation densities tend to be reduced in

layers grown above superlattices [156,157]. Impuri-

ties coming from the substrate are intercepted by the

superlattice [158,159]. Device quality GaAs and

AlGaAs can be grown on Ge or Ge/Si substrates for

Al,Ga,_, As BARRIERS

(a)

BARRIER HEIGHT OF THE

MULTILAYERS L m&l

GaAs/Al,Ga,_,As

AlAs MOLE FRACTION X IN Al,Ga,_,

BARRIER LAYERS

(b)

Fig. 19. Quantum well injection laser structure and perfor-

mance[155]. (a) The schematic energy band diagram from

the modified multi-quantum well laser. (b) Shows the varia-

tion of the averaged Jlh of several wafers as a function of

their respective AlAs composition x (and barrier height) in

the Al ,Ga,

_ ,

As barrier layers.

MODFETs with the aid of an intermediate super-

lattice layer. Interdiffusion at Ge/GaAs interfaces

without a quantum-well superlattice has been studied

by Sarma et al.[160]. Certain alloy systems such as

Ga(As, Sb) have miscibility gaps but superlattices of

them can be grown and matched to InP[161].

The Ge-GaAs superlattice has excellent lattice

match but may exhibit planar defects in the GaAs

layers and these have been attributed to the anti-

phase boundaries expected from localized nucleation

of the GaAs on the Ge[162]. Antiphase disorder may

be reduced by careful control of nucleation condi-

tions as found for GaAs and AlGaAs grown on

Si[163]. Control of nucleation has been studied by

Beam

et al.

[164,165] in the growth of Ge, Si,

,/Si

strained-layer superlattices and conditions for avoid-

ance of island nucleation established. Modulation

doping resulting in a two-dimensional hole gas has

been demonstrated for such structures at 4.2 K with

a h& of about 0.1 eV[165,166]. The combination of

(GeSi) and Si is expected to be of device interest in

the next few years.

One feature of superlattices that is usually unde-

sirable is that disordering is produced by impurities

such as Zn at quite moderate temperatures

(5755615°C) [167,168] and by Si at 850°C [169]. This

tends to limit the formation of junctions to in situ

doping during growth. The intermixing phenomenon

induced by diffusion in superlattices has been dis-

cussed by Van Vechten [170].

Tunable below-gap radiation can be obtained from

staggered line-up heterojunctions and quantum-well

structures[l71]. Perhaps of greater device interest is

the demonstration that a long wavelength multi-

quantum-well laser with Ga,,,In,, 53A~ wells and

Al

o.4x n,, s2

As barriers can be made to cover the

range from 1.7 to 1.5 p,rn by adjustment to the well

width from 1000 to 80 A[172].

The thickness of the layers and the composition of

alternate layers may be graded in a superlattice, as

shown in Fig. 20 for a transition from InP to

Ga,, ,,In, 53

As. Such structures have been used in

the fabrication of avalanche photodiodes[l73].

A 50 period multiple-quantum-well superlattice of

GaAs/AlGaAs in a PIN diode structure as shown in

Fig. 21 has an optical absorption edge that is abrupt

because of excitation resonances. Application of an

electric held causes changes of carrier confinement in

the wells and shifts the absorption edge to longer

wavelengths. For 857 nm applied light a factor of 2

change of transmission can be achieved with an 8 V

bias on the diode and the switching time is suitable

for high speed, ns fast modulation[l74]. Optically

bistable light transmission has been demonstrated in

such a structure[l75]. Waveguide action in super-

lattices is discussed in the paper by Bhattacharya

et al. in this issue.

Another bandgap engineering idea that is emerging

is the concept of selective mass tunneling filters.

Tunneling probability depends exponentially on the

barrier E, and carrier mass m as exp( - nr’/“E~/’ )


17/23


I

I

Goo.47’“0.53As

I

I

I

DISTANCE

Fig. 20.

Energy band gap of a graded gap psuedo-quaternary

GaInAsP. The thicknesses of the InP and of the

Gap ,,Inc 5~

As are gradually varied between 5 and 55 A

while keeping constant the period of the superlattice (= 60

A). The dashed lines in (a) represent the average band gap.

(b) Schematics of the HI-LO heterojunction avalanche pho-

todiode incorporating the superlattice. (c) Electric field

pro -

tile[173].

Fig. 21. Schematic view of an AlGaAs/GaAs superlattice

optical modulator. The semiconductor layers are grown via

MBE on a GaAs substrate, and then the diode is defined

lithographically. The lower portion of the figure shows the

calculated electric field strength, S, as a function of position

within the device for two applied voltages[l74].

Xl

GoAS

r3

2

x3

hAS

=1

a)

b)

Fig. 22. Concept for a heterostructure light demultiplexing

device[l76]. (a) Structure. (b) Energy diagram.

Thus it is possible to consider a superlattice in which

there will be selective tunneling of electrons but not

of holes and this may have high photosensitive gain

because the holes are essentially trapped while elec-

trons make many transits of the device. Furthermore

the electron band state formation in the quantum

well insures that tunneling action selects electrons of

particular energies hence giving an energy-resonant

tunneling action. So it is possible to envisage an npn

bipolar transistor that contains quantum wells in the

base and that transmits to the collector only elec-

trons in selected energy ranges that correspond to

certain values of emitter base voltage. So the device

when ultimately refined and developed to have an

undulating output as the emitter-base voltage is

increased may have potential as a logic device with

multiple logic levels or possibly as providing a crude

analog to digital action.

Optical communication systems may in some

applications be multiplexed with light of several

wavelengths. Fig. 22 shows a heterostructure demul-

tiplexer concept in which a detector has a well-struc-

ture graded from GaAs to InAs. Selected absorption

of light of different wavelengths in sequential wells

may allow signal separation [176].

For another potential application it has been sug-

gested that strained layer superlattices in the InAsSb

system may be suitable for long wavelength detector

applications [177]. Suitable absorption must be

achievable in a superlattice of reasonable length and

this may be a problem.

Finally in this examination of bandgap-engineer-

ing a sawtooth superlattice structure proposed by

Capasso et ~I.[1781 is shown in Fig. 23. The action

of the device depends on the lack of reflection sym-

metry and it is suggested that the structure may lead

to high-speed displacement-current photodetectors.

The explanation of the action given by the authors

follows:

“

. electron-hole pairs are excited by

a very short pulse as shown in Fig. 23a.

Electrons experience a high quasielectric

field (typically > lo5 V/cm) due to the

grading whereas the total force acting on

holes is virtually negligible because of

the valence band-edge lineup in p-type

materials. Therefore electrons separate


18/23

116

A. G. ibflLNtS

Fig. 23. Illustration of the formation and decay of the

macroscopic electrical polarization in a superlattice strut-

ture [178].

from holes and reach the low-gap side in

a sub-picosecond time (< 10 ” s).

This sets up an electrical polarization in

the sawtooth structure which results in

the appearance of a voltage across the

device terminals [Fig 23(b)]. This mac-

roscopic dipole moment and its associ-

ated voltage subsequently decay in time

by a combination of (a) dielectric re-

laxation and (b) hole drift under the

action of the internal electric field pro-

duced by the separation of electrons and

holes.

The excess hole density decays by

dielectric relaxation to restore a flat

valence band (equipotential) condition,

as illustrated in Fig. 23(c). Note that in

this final configuration holes have redis-

tributed to neutralize the electrons at

the bottom of the wells. Thus also the

net negative charge density on the low-

gap side of the wells has decreased with

the same time constant as the positive

charge packet (the dielectric relaxation

time).”

When excited with 4 ps laser pulses (X = 6400 A)

at 86 MHz repetition rates, a structure graded from

GaAs to Al,,Ga,,As exhibited a sharp rise-time

output pulse with a 200 ps decay tail. The structure

does not respond to a constant dc light signal. Un-

like conventional semiconductor photodiode and

photoconductive detectors, the current carried in this

photodetector is of displacement rather than conduc-

tion nature since it is associated with a time-varying

polarization.

9. CONCLUSIONS AND OVERVIEW

Heterojunction devices envisaged many years ago

have reached new levels of performance because of

improvements in growth technologies and in physical

understanding and examination of interfaces. This

has led to revitalization of the concept of bandgap

engineering and the free-thinking produced by this

has suggested a number of interesting new device

concepts. Some will survive and others no doubt will

fall by the wayside, but progress is certainly being

made.

The preceding review has attempted to set the

stage for the papers that follow in this special issue.

Basic heterojunction barrier studies are represented

by the papers of Margaritondo and Wang. Bipolar

transistor studies follow with the papers of Ankri

et al. and Katoh et al. Modulated-doped FET struc-

tures are represented by the papers of Look and

Norris, and Schubert

et al.

and Nathan. The first

deals with channel mobility and the others with

undesirable trapping effects. Contacts to such struc-

tures are considered by Mukhergee et 01. and by

Christou and Papanicolaou.

Graded heterojunctions are discussed in the contri-

butions of Fischer et al. and Petrosyan. Then quan-

tum-well effects are examined in the papers by

Masselink

et al.

and Bajaj

et ul.

and superlattices by

Bhattacharya et al.

Photoeffects in InGaAsP are studied in the contri-

butions of Diadiuk and Groves, and Komiya et ul.

and photoeffects in ZnSe-GaAs junctions by Zhuk

et al. Trap levels in heterojunctions are often im-

portant and a comparative study of admittance and

DLTS spectroscopy for CdTe-ZnTe heterojunctions

is offered by Khan and Saji. The growth of

CdTe-InSb heterostructures is discussed by Blat

et

II.

The issue concludes with a paper by the Morkoc

group at the University of Illinois on the very prom-

ising performances achieved for GaAs grown directly

on Si.

Ack~lo,l’led~~emerzts-Contributors are thanked for their

papers and for allowing their manuscripts to be held while

the complete set could be assembled for this special issue.

NSF Grant ECS 82-14859 is acknowledged for partial

support during the preparation of my contribution.

1.

2.

3

4.

5.

6.

I.

8.

REFERENCES

A. G. Mimes and D. L. Feucht, Hetero~u,rc~rro~zs crtrd

Mad Semrconductor Junctrons,

Academic, New York

(1972).

B. L. Sharma and R. K. Purohit, Senrrconducror Iler-

eropnctions,

Documents

Semiconductor Heterojunction Topics: Introduction and Overview