11 Ohmic Contacts

8/13/2019 11 Ohmic Contacts

1/19

Slide # 1

Ohmic contacts

Common techniques to make ohmiccontacts Choose metal so that its work function

Fmetal is close to that of semiconductorsFsemi (thermal ionic)

Insert thin layer of narrow bandgapmaterial between metal andsemiconductor

Increase the doping level near thesemiconductor surface as high aspossible (tunneling assisted)

Ohmic contacts should be

Low contact resistance (< 10-6 cm2) Thermally stable (does not degrade at

elevated temperature or react withoxygen), which requires no phasechange or no phase change leading tohigh resistance

Smooth morphology

Compatibility with the whole deviceprocess

Both semiconductors and metalsources should be CLEAN!

Michaelson, IBM J. of R&D, 1978


2/19

Slide # 2

Common Techniques for Ohmic contacts

Usually for compound semiconductorsthe ohmic contact by band alignment ishard to realize due to surface states and

Fermi pinning. For p-type, the problemis caused by unavailability of metalswith large enough work function

High n-type doping required for ohmiccontacts to n-type semiconductors,

which can also be realized by interfaciallayer reaction chemistry

M

Evac Evac

EF

M < +Ec-EF = SFor n-typesemiconductor.

Reverse for p-type

(ii) Ohmic contact by high doping

Electrons from

conduction band

can move veryeasily to the

metal and vice

versa by

tunneling

n+ doped

(i) Ohmic contact by band alignment

n doped

B = S -MEF

B = band bending

n-doped


3/19

Slide # 3

Ohmic on n-GaN Possible metals: Ag, Nb, Ti, Al, In, Ta, Cr

Ag: poor adhesion Nb: extremely easy to oxidize thus difficult to

process Ti: formation of TiN (intermetallic) and high N

vacancies in GaN -> good! But easy tooxidize need a stable cap like Au

Al: formation of AlN (not intermetallic) and highN vacancies in GaN -> ok! Also easy tooxidize Au cap is necessary

In: most popularly used for quick contacts Ta: studied by Qiao et al. 5.7e10-7 cm2 on

AlGaN/GaN (2001); but others could notreproduce the results

Others: also studied but not as good as theone below

As deposited or alloy: generally alloyed unlessdoping near the surface is very high!

Popular schemes: Ti/Al/Ni/Au

Ti/Al bilayer: formation of N vacancies, TiN,Al3Ti (thermally very stable ); but ratio of Ti/Alhas to be carefully controlled (~1/2.5)

Add high conductive and protective layer of Au,but Au diffuses easily

Add Ni as diffusion barrier (decent, othermetals were tried, Pd and Pt were worse)

State-of-art: 0.1-0.2 cm2 (~ 10-8 cm2 )

Lim et al, APL 78, 3797(2001)

Liu et al, Solid State Electronics, 42, 677(1998)


4/19

Slide # 4

Schottky contacts

Schottky contacts areformed whenDoping in the

semiconductor is not veryhigh i.e. > ~5x1018 cm-3

The metal work function isgreater than the n- typesemiconductor workfunction

The metal work function islower than p-typesemiconductor work

functionVery high density of

surface states pinning theFermi level at the surface

w.r.t. the conduction band(Example: GaAs)

Schottky contact

n doped

Electrons from

conduction band

or in the metalfaces barrier to

free movement,

and tunneling is

also not easy

M

Evac Evac

EF

M > +Ec-EF = SFor n-type semiconductor andreverse for p-type

s

Bn =

M -


5/19

Slide # 5

Conduction mechanisms in schottky contacts

Thermionic emissionElectrons emit over the barrier

Low probability of direct tunneling

Valid for low doping (ND < ~ 1017 cm-3)

Thermionic-field emissionElectrons use thermal energy to tunnel trough the

thin barrier in the upper end of the conduction

band

Valid for intermediate doping (~ 1017 cm-3 < ND ~ 1018 cm-3); almostohmic

Leakage currentHigh probability of defect-assisted tunneling and

simple conduction

Occurs in poor material/interface quality; dislocations


6/19

Slide # 6

Thermionic emission current: Schottky diode

I-V characteristics

Schottky diode I-V equation:J = J0 (e

qV / kT 1), where J0 is the

saturation current density given by

Forward bias Reverse biasTypical I-V

characteristics

= kT

qTAJ Bno

exp2* T = temperature, A* = effective Richardsons constant


7/19

Slide # 7

Schottky on n-GaN

Experimentally shownvery weak surfacepinning

Surface cleanness hasbeen heavilyinvestigated, however External cleaning is

generally sufficient toachieve decentSchottky

Leakage is largely due

to dislocations Thermal stability is

IMPORTANT Ni does not react with

GaN below ~ 600 C

Pd reacts with GaN at ~

400-500 C W and Rd ~ 600 C Liu et al, Solid State Electronics, 42, 677(1998)

The higher the schottky barrier, the lowerthe leakage current

Using polarization in nitrides i.e.

GaN/AlGaN/GaN structure, the schottkybarrier can be made larger


8/19

Slide # 8

Electrical properties of dislocations in MBE-grown n-GaN(Ed Yu --- UCSD)

1m

topography:

1m

current:

Pure screw dislocations can be highly conductive in MBE-grown n-GaN:

Screw: conducting, uncharged

Edge: nonconducting , e chargeMixed: nonconducting, e charge

Edge and mixed dislocations typically contain negative charge in dislocation core:

high current leakagein Schottky contacts

scattering, localcarrier depletion

[B.S. Simpkins, E.T. Yu, P. Waltereit, J.S. Speck, J. Appl. Phys. 94, 1448 (2003).]

[E. J. Miller, D. M. Schaadt, E. T. Yu, C. Poblenz, C. Elsass, J. Speck, J. Appl. Phys. 91, 9821 (2002).]


9/19

Slide # 9

Mitigation of dislocation-induced leakagecurrents in MBE n-GaN (Ed Yu, UCSD)

1m

AFM

1m

AFM

1m

current

NaOH solution

pH = 13.1 T= 30C

AFM

2m

current

~10A

AFM

2m

current

~100pA

b= 0.800.02V

n= 1.740.01

b= 0.860.02V

n= 1.130.02

unmodified

with electrochemicalprocess

area = 1.2310-4cm2

V = 30V

I ~ 1-10mA t= 1000s

[E. J. Miller, D. M. Schaadt, E. T. Yu, P. Waltereit, C. Poblenz, and J. S. Speck, Appl. Phys. Lett. 82, 1293 (2003).]


10/19

Slide # 10

Ohmic to p-GaN

Similar techniques likeohmic to n-GaN havebeen tried, but:

rC~ 10-3 cm2

P-GaN/Ni/Au annealed inair (N2/O2) proved to beone of the best:

rC~ 10-6 cm2

Why?

After annealing, newphases form: NiO, Ni-Ga-O with Au particles,GaN

NiO is p-semiconductorwith high Ni vacancies

Continuing challenges: Transparency to visible

and UV

Ohmic to p-AlGaN

Tunneling junctioncontacts

Ho et al. JAP 85, 4491 (1999)


11/19

Slide # 11

Another Contact Metal for p-GaN The absence of a metal with a sufficiently high work function. The band gap of

GaN is 3.4 eV, and the electron affinity is 4.1 eV, but metal work functions are

typically ~ 5 eV

The relatively low hole concentrations in p-GaN due to the deep ionization level of

the Mg acceptor ~170 meV The tendency for the preferential loss of nitrogen from the GaN surface during

processing, which may produce surface conversion to n-type conductivity.

Palladium gallide creates Ga vacancies that

reduce contact resistances

Temperature and time of anneal also important

TEM

image


12/19

Slide # 12

Schottky to p-GaN

Schottky (Ni) on as grown GaN:Mg (MOCVD) --- quasi-ohmic (higher Mg near the surface?)

Schottky (Ni) on etched GaN:Mg --- rectifying (tunneling and defect-assisted tunneling still

significant thus it is difficult to extract barrier height and Richardson constant from I-V)


13/19

Slide # 13

Schottky contact characterization

Current-Voltage (IV) measurements

Capacitance-Voltage (CV) measurements

So the intercept of 1/C2 vs. V gives the barrier height

Photoelectric measurements (by photon incidenton the schottky contact; this is very accurate)

Photocurrent R is related to the barrier height as

( )VqNW BnDs

= 2

( )V

Nq

CBn

Ds

=

2 ( )VC Bn 21

BnqhvR ~ So the intercept gives the barrier height

kT

q Bn

eTAJ

= 2*0

=

0

2*

ln

J

TA

q

kTBn. VF vs. J intercept gives J0 and


14/19

Slide # 14

Evaporation systems

Contact Metallization (Ti, Al, Ni, Au etc)

Metal Electron-Beam Evaporation System

Rapid Thermal Annealing System

from 20 oC to 1000 oC in seconds

Target Metal Sourcewith e-beam

Sample


15/19

Slide # 15

Ohmic contacts: n-type or undoped nitride

Standard recipe for ohmic contact:

Ti/Al/Ti/Au or Ti/Al/Ni/Au deposition. Ti/Al thickness ratio is important

Annealing at 800 900 C for about 1 min for alloying. Alloying temperature

and alloying time are important factors controlling contact resistance.

Ti/Al/Ni/Au

Since TiN and AlN are formed by reaction

between the nitride layers and Ti or Al, N-

vacancies are created, which can dope thecontact region and create ohmic contact


16/19

Slide # 16

Specific contact resistivity and sheet resistance

Product of contact resistance Rc and area

A is called specific contact resistivityc:

0

1

=

=

V

c

V

J ( . cm2)

Semiconductor layer resistivity:

ne

1= ( . cm)

tZ

ddx

A

R

d

s

==

0

1()

d

Z

t

Sometimes semiconductor resistance is

expressed in terms of sheet resistance sh

For any semiconductor device there are two main resistances: Contact resistance

Semiconductor resistance

( ) tnetsh ==

1(Can also be expressed in terms of . mm)

The total semiconductor resistance is thengiven by

(/)


17/19

Slide # 17

Ohmic contact characterization:

Transmission line method (TLM)

( )TT LxLx BeAexI // +=

c

ZxV

dx

dI

)(=

Z

I

tZ

I

dx

dV shs ==sh

cTL

=,)(

22

2

TLxI

dxId =

shx/Z

c/(Zx)

I

I(x)

V(x)

I(x+x)

V(x +x)

where

The solution forI(x) is given as:

Now putting the boundary conditionI(x = L) = 0, and finding the

solution for V(x), we can find the contact resistance as the ratio of the

input voltage and input current as:

L

( ) ( )00 === xIxVRC

is called the transfer length.


18/19

Slide # 18

Transmission line method (TLM) II

T

CC

ZLR

=

Z

dRRRR

shcscTot

+=+= 22

sh = /t

d

Z

t

When the following conditions are further

satisfied, d LT , we have,

=

TT

CC

LL

ZLR coth

,ZLR Tcc=Putting Rtot = 0, and using the relation

.2)0( TT LRd ==

we have,

So, the transfer length can be found from the

intercept of the total resistance on the x-axis.

Note that the contact resistivity is not given by the product of the

contact resistance and the total contact area, but by the product of

contact resistance, width Z, and transfer length LT.

OhmicsL


19/19

Slide # 19

Measurement technique

-2 -1 0 1 2-100

-50

0

50

100

B1205 UV LED

n-TLM

Curr

ent,mA

Voltage, V

4um

6um

8um10um

12um

14um

16um

0 5 10 15 200

10

20

30

40

B1205 UV LED

n-TLM

Y =14.51607+1.13839 XR

c=7.258

LT=6.373um

c=6.93*10-5-cm2

Rsh

=170.7/sq

Resistanc

e,

Ohm

gap, um

Typical measurement set up

Slope =

sh/Z

What is wrong in

this measurement?

Plot of total resistance

vs. distance

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11 Ohmic Contacts