Stable ohmic contacts to Zinc Telluride

Stable ohmic contacts to Zinc Telluride

Mlcroelectron. ReUab.,Vol.27, No. 4, pp. 677-684,1987. 0026-2714/8753.00+.00 © 1987PergamonJournalsLtd. Printed in Great Britain. STABLE OHMIC CONT...

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Mlcroelectron. ReUab.,Vol.27, No. 4, pp. 677-684,1987.

0026-2714/8753.00+.00 © 1987PergamonJournalsLtd.

Printed in Great Britain.

STABLE OHMIC CONTACTS TO ZINC TELLURIDE H. S. HAJGHASSEM, W. D. BROWN* Department of Electrical Englneerlng, University of Arkansas, Fay etteville, AR 72701, U.S.A and M. M. LU(~qAN School of Engineering, U n i v e r s i t y of Portland, Portland, OR 97203, U.S.A. (Received for publication 10 April 1987)

ABSTRACT

Zinc Tellurlde is a wide, direct bandgap, II-VI compound semiconductor which, because of Its high density, has potential as a g--,---ray detector. Ohmic contacts to thls material using Au-Cu, Ag-Pt, Pt and Cu-Pt have been fabricated

and tested

at elevated

temperatures.

Specific

contact

resisttvi-

tles ranged from 4.9 x 10 -2 ol~m-cm2 to 1.0 ohm-cm 2 depending on the annealing p r o c e s s

end/or

operating

resistivity

following

temperature.

stabilization

Platinum

at an operating

yielded

the

temperature

lowest of 150°C.

INTRODUCTION One area o f emphasis in recent semiconductor device research has been the investigation of wlde bandgap semiconductor materials.

These materials

are of interest for hlgh operating temperature devices and efficient lightemitting diodes in the visible spectrum.

Silicon and germanium are the most

common semiconductor materials used, respectively, for X-ray and g,mm, ray spectrometry.

However, because of their relatively small bandgap, they must

be cooled and in most cases operated near liquid nitrogen temperature to avoid excessive thermal currents.

On the other hand, compound semiconduc-

tors can operate at much higher temperatures because of their larger bandgap energies. Zinc Telluride (ZnTe) Is a wide-gap, II-VI compound semiconductor on which little effort has been expended in terms of device applications.

ZnTe

has a direct bandgap of 2.25 eV at room temperature, is nQrm~lly P-type due to Zinc vacancies and has a density of about 5.5 g/cm 3.

To whom r e p r i n t

requests

should be addressed. 677

Because of the wide

678

H.S. HAJGHASSEMet al. bandgap and h i g h d e n s i t y ,

ZnTe h a s t h e p o t e n t i a l

e c t r o n i c s end n u c l e a r d e t e c t i o n . tors,

i t has l i m i t a t i o n s

for applications

in optoel-

However, l i k e o t h e r compound s e m i c o n d u c -

end p r o b l e m s s u c h a s l a c k o f q u a l i t y c r y s t a l

t e c h n o l o g y end t h e a b s e n c e o f d e v i c e f a b r i c a t i o n

growth

technology.

One p r o b l e m w i t h c o n t a c t i n g wide bandgap s e m i c o n d u c t o r s i s t h a t o f t e n it

is.difficult,

nately,

if not impossible, to find a suitable metal.

s u c h i s t h e c a s e f o r ZnTe.

Unfortu-

Ohmic c o n t a c t t o P - t y p e ZnTe r e q u i r e s a

m e t a l w i t h a work f u n c t i o n o f 5.75 eV o r g r e a t e r and no m e t a l has s u c h a work f u n c t i o n .

Thus, m e t a l s w i t h l a r g e work f u n c t i o n s must be s e l e c t e d and

v e r y narrow b a r r i e r s rent-voltage

formed i n o r d e r t o a c h i e v e ohmic b e h a v i o r .

characteristic

The c u r -

i s f u r t h e r enhanced i f t h e m e t a l ( s ) used c r e a t e

a h i g h l y doped P - r e g i o n when d i f f u s e d i n t o t h e s e m i c o n d u c t o r . Tupanevich and Kononenco s t u d i e d c h e m i c a l l y d e p o s i t e d s i l v e r c o n t a c t s and a c h i e v e d s p e c i f i c e l e c t r i c a l

resistance

and g o l d

v a l u e s as low a s 3 x

10 -3 ohm-cm2 [ 1 ] . Baker and M i l n e s examined a l a r g e number o f m e t a l s and two m e t a l c o m b i n a t i o n s and d e t e r m i n e d t h a t many o f t h e ~ were u n s a t i s f a c t o r y because of high resistance

end/or nonlinearity

were formed u s i n g a t i n / a r s e n i c ium.

Electroplated

alloy,

[2].

electroless

Acceptable contacts g o l d and d i f f u s e d l i t h -

c o p p e r was u s e d by G r i b k o u s k i i end Beyaevs t o form good

c o n t a c t s w i t h t h e n e e d f o r an a n n e a l i n g s t e p [ 3 ] .

Saji et al diffused lith-

ium end t h e n n i c k e l p l a t e d t h e s u r f a c e t o o b t a i n ohmic b e h a v i o r [ 4 ] . More recently,

silver,

s o l d , end c o p p e r - g o l d c o n t a c t s were s t u d i e d a t e l e v a t e d

t e m p e r a t u r e s [ 5 ] . C o p p e r - g o l d c o n t a c t s were found t o s t a b i l i z e

after

a high

temperature anneal. The work p r e s e n t e d i n t h i s p a p e r i s i n t e n d e d t o p r o v i d e i n f o r m a t i o n a b o u t ohmic c o n t a c t s t o ZnTe a t room t e m p e r a t u r e and a b o v e . formed u s i n g g o l d - c o p p e r , p l a t i n u m , s i l v e r - p l a t i n u m were t e s t e d f o r s t a b i l i t y

at elevated temperatures.

d i s c u s s ZnTe sample p r e p a r a t i o n ,

C o n t a c t s were

and c o p p e r - p l a t i n u m and The f o l l o w i n g s e c t i o n s

c o n t a c t f o r m a t i o n , and e l e c t r i c a l

perfor-

mance of the contacts at room temperature, at elevated temperatures,and at room temperature following annealing at elevated temperatures.

ZITrE SAMPLE PREPARATION

The ZnTe m a t e r i a l u s e d i n t h i s Industries

o f Miami, Oklahoma.

polycrystalltne

s t u d y was p r o v i d e d by E a g l e - P l c h e r

The m a t e r l a l was grown from a m a l t and was

o f t h i c k n e s s 1.25 m a w t t h s i n g l e c r y s t a l

a r e a s o f 5-7

nmn

Ohmic contacts to zinc telluride diameter.

Single crystal

b a r s a few m i l l i m e t e r s

s a m p l e s were c u t from t h i s

ticle

to polishing,

s i z e was g r a d u a l l y

i s h i n g damage and c r e a t e formation and carrier Two e t c h a n t s , tested

initially

contact

e t c h i n g and c l e a n i n g pro-

r e d u c e d f r o m 10 m i c r o n s t o 0 . 3 m i c r o n .

Following

t h e s a m p l e s were c h e m i c a l l y e t c h e d t o remove p o l e tellurium-rich

s u r f a c e which promotes t h i n b a r -

tunneling.

t o d e t e r m i n e w h i c h one c o n s i s t e n t l y F i g u r e 1 shows t y p i c a l

Similar results

produced the lowest

I-V c h a r a c t e r i s t i c s

f o r m e d u s i n g Au-Cu a s t h e c o n t a c t m e t a l on s u r f a c e s

bromine solution

in which the p a r -

0.5X b r o m i n e - m e t h a n o l a n d p o t a s s i u m d i c h r o m a t e , were

resistance.

etchants.

in t h e form of

of approximately 1.0

P o l i s h i n g was a c c o m p l i s h e d u s i n g an a l u m i n a s l u r r y

the mechanical polishing,

rier

material

long and w i t h a c r o s s s e c t i o n

. . , 2 . The s a m p l e s w e r e t h e n s u b j e c t e d cesses.

679

of samples

p r e p a r e d u s i n g t h e two

were o b t a i n e d u s i n g o t h e r c o n t a c t m e t a l s s o t h e

was u s e d i n a l l

further

sample fabrication.

CONTACT FORMATION

The p r i m a r y g o a l o f t h i s which exhibit tures

work was t h e f a b r i c a t i o n

ohmic b e h a v i o r a n d a r e s t a b l e

of contacts

at elevated operating

t o ZnTe tempera-

as noted previously. Electroless

and w a t e r .

Au-Cu c o n t a c t s

The r a t i o

were formed from a s o l u t i o n

of gold to copper in the solution

o f HAuCI 4, CuSO4

was v a r i e d by d i s -

solving different amounts of CuSO 4 in I ml of a solution consisting of 2 gms

0.8

'

e/e

06 0.4

m

0.2

~

0 -0.2



Dichromate

.

Bromine

-0.4 -0.6

-0.8

-40O

-200

0

I

200

400

Vol.tage (mV)

i-z

F i g u r e 1 - I-V c h a r a c t e r i s t i c s f o r Au-Cu c o n t a c t s surface etching solution.

t o ZnTe a s a f u n c t i o n o f

680

H.S. HAJGHASSEMet al. of hlgh purity HAuC14 In one lltar of water.

Contacts were painted onto the

ZnTe samples by multlple applications of the solution.

Between applica-

tions, the samples ware allowed to dry and as a final step, were washed In D.I. water and blown dry with nitrogen. Platinum contacts were made by painting a solution of platinum chloride (H2PtC14-6H20).

Silver-dlffused contacts were formed by painting a solutlon

of 2 Rm of ARNO 3 In 1 llter of water followed by a heating cycle at 400 °C for 2 minutes to diffuse the AE into the ZnTe.

Electroless Pt was deposited

on top of the diffused AE region to provide the metal contact. Cu-Pt contacts were made by painting a solution of 75 mR CuSO 4 dissolved in 1.0 ml of the platinum chloride solution described previously.

All contacts

fabricated using this material exhibited extremely large resistance values. Therefore, further testing was not performed.

RESULTS AND DISCUSSION

All contacts fabricated during thls investigation were tested by measuring their current-voltage characteristics. mal stage and contacted wlth gold probes.

Samples were placed on a ther-

Measurements were made at several

temperatures and at room temperature both before and after hlgh temperature annealing for different periods of time.

The resistivity of the ZnTe

samples was determined to be of the order of 2.2 ohm-cm 2 [5].

However,

sample resistivity increases as temperature increases as shown by the data of Figure 2.

Therefore, observed changes in I-V characteristics with tem-

perature variation are not due entirely to the contact resistance.

To

determine the contact resistance at a given temperature, the resistance of the sample at that temperature must be subtracted from the total measured

4

e~

'6

3--

°~O~o

2--

2

I

l

I

25

3

35

Temperature (K x 103) Figure 2 - Resistivity of single crystal

ZnTe versus temperature.

681

Ohmiccontactsto ~nctellufide resistance.

All specific contact reslstlvltles reported here were deter-

mined In this manner.

Gold-Copper Contacts In a previous investigation of ohmic contacts to ZnTe, gold-copper contacts were shown to exhibit a low resistance and it was suggested that the contact resistance might be a function of the gold-to-copper ratio [5]. Figure 3 shows the I-V characteristics of four different Au-Cu contacts to a ZnTe sample of 75 ohms resistance.

Contact resistance is seen to be minimum

for 98.6Z Cu and 1.4Z Au and increases as the gold-to-copper ratio varies from this value.

Specific contact resistance for the two extremes in the

figure (l.e. 98.6Z Cu and 96Z Cu) are 4.5 x 10 "2 ohm-cm 2 and 3.9 x 10 -1

ohm-cm2, respectively, Several other solutions with copper concentrations smaller and larger than 98.6Z were made and tested In an effort to determine an optimum goldto-copper ratio.

Figure 4 shows plots of specific contact resistance versus

percent copper for three different samples.

The minimum contact resistivity

for all three samples occurs at approximately 98.6Z copper.

The sllght dif-

ferences in minimum value of resistivity and general shape of the curves is probably due to slight variations in the ZnTe samples and/or sample preparation. Specific contact resistance of 98.5Z Cu-I~4Z Au contacts decreases sllghtly (approximately 20Z) as the sample temperature is increased from 25°C to 150°C. Annealing at 75°C for I hour produces a similar decrease

l

0.8 0.6

l

m

0.4

Y

0.2

E ---

~ U

0 -0.2 -0.4 -0.6 -0.8 -I -

160

• =

J -120

-80

o



-4O

0

9 8 . 6 % Cu 9 8 . 9 6 % Cu 9 7 . 9 5 % Cu 96%Cu

I

I

I

40

80

120

160

Vottoge (mV) Figure 3 - I-V characteristics of Au-Cu contacts to ZnTe at room temperature.

682

H.S. HAJGHA,~EMet al.

'6 5 •

4•>¢

3

L

e

I

">



vv

t

:t

),







|

0

U



t

0 97.95





I

98.41



I

98.6

I

98.78

I

98.96

I

99.16

99.33

Copper content (%) Figure 4 - Specific contact resistivity o f Au-Cu c o n t a c t s t o ZnTe v e r s u s p e r cent copper for three different devices.

( f r o m 8.7 x 10 "1 t o 6.9 x 10 -1 ohm-cm2). (24 h o u r s ) a t 150°C r e s u l t

in the specific

Although the contact resistance large for practical

However, e x t e n d e d a n n e a l i n g t i m e s contact resistance

is stable after

device application.

cause of t h e i n c r e a s e in r e s i s t a n c e ,

t h e lSO°C a n n e a l ,

No e f f o r t

s u r f a c e o f t h e ZnTn o f e x c e s s t e l l u r i u m s h o u l d be a n t i c i p a t e d

cuprous t e l l u r i d n .

it

is too

was made t o d e t e r m i n e t h e

but i t p o s s i b l y involves the r e a c t i o n

of copper with tellurium forming cuprous telluride

resistance

doubling.

after

[6J.

thereby depleting the

Stabilization

of the c o n t a c t

the excess tellurium is converted to

Anneal t i m e s o f 24 t o 48 h o u r s a t 150°C w e r e , in f a c t ,

performed with little

change in r e s i s t a n c e .

Platinum Contacts Since the literature

c o n t a i n s no i n f o r m a t i o n a b o u t p l a t i n u m c o n t a c t s t o

ZnTe, p l a t t n u m was s e l e c t e d a s a m e t a l t o be s t u d i e d .

The s p e c i f i c

resisti-

v i t y o f " a s - d e p o s i t e d " c o n t a c t s was o f t h e o r d e r o f 5 . 5 x IU "1 o | ~ - c m 2. typical resulted

I-V c u r v e i s shown i n F i g u r e 5.

A n n e a l i n g a t 75°C f o r 60 m i n u t e s

i n a d e c r e a s e t o 8 . 5 x 10 "2 ohm-cm2.

p r o d u c e d no f u r t h e r The s t a b i l i t y

A

Longer a n n e a l t i m e s a t 75°C

decrease. o f t h e c o n t a c t was t e s t e d by m a i n t a i n i n g t h e t e m p e r a t u r e

o f t h e sample a t 150°C f o r an e x t e n d e d p e r i o d o f t i m e . v i t y d e c r e a s e d t o 1.2 x 10 -2 ohm-cm2 a f t e r 4 . 9 x 10 -2 ohm-cm2 two h o u r s .

The c o n t a c t r e s i s t i -

5 m i n u t e s and t h e n i n c r e a s e d t o

Operation at this

h o u r s p r o d u c e d no f u r t h e r m e a s u r a b l e c h a n g e .

t e m p e r a t u r e f o r up t o 35

Ohmic contacts to zinc tclluridc

683

2 1.5I

j

--

0.5--

E

/-

0

"E

-0,5 -I

U

--





111"

J-

,o2/./ __2.,o°

1

0

I00

200

Vottoge tmV) Fisura 5

-

l-V characteristics of platlunum contact to ZnTe at 27°C before ~annealing.

Silver-Platinum Contacts Silver is known to act as an acceptor in ZnTe [7].

Since ohmic behav-

ior of contacts to ZnTe can only be realized by creating a very thin tunneling barrier as noted prevlously, silver-platinum contacts were studied. Specific contact resistance at room temperature of the "as-deposlted" contact is of the order of 3.5 x 10 .2 ohm-cm 2.

Following an anneal at 50°C for

two hours, the value is 1.5 x 10 .2 . Operation at 150°C results in an increase to 3.8 x lO -2 ohm-cm 2 after 5 minutes and to 2.2 x 10 -I ohm-cm 2 after 40 hours. resistance.

Longer anneal times produce no further change in contact

Although this contact does stabilize at an operating tempera-

ture of 150°C, the resistivity is again too high for practical applications.

SUHHAR¥ AND CONCLUSIONS C o n t a c t s t o ZnTe u s i n g Au-Cu, P t , Ag-Pt and Cu-Pt have been f a b r i c a t e d and

studied as f u n c t i o n s o f high temperature annealing and operation.

Cu-Pt contact resistivity was unacceptably high as formed and could not be improved by annealing. The contact resistivity exhibited by Au-Cu contacts was shown to be a function of the gold-to-copper ratio with the lowest resistivity occuring for 98.6% Cu and 1.4% Au.

However, even the lowest

value of resistivity was too large and 150°C operation caused the value to double. The resistivity o f platinum contacts was o f the order of 0.55 ohm-cm 2 "as-deposlted" and decreased with annealing at both 75°C and 150°C.

The

H S. HMGHASSEMetal.

684 contact resistivity ation

stabilized

at 4.9

a t 150°C. S i l v e r - p l a t i n e m

x

10 -2 ohm-cm2 a f t e r

contacts have an "as-deposited"

of 3 . 5 x 10 -2 ol~n-cm2. However, t h e r e s i s t i v i t y ohm-cm2 a f t e r

2 hours of oper-

increases

resistivity

t o 2 . 2 x 10 -1

40 h o u r s a t 150°C.

Although stable,

the contact resistivity

~s t o o l a r g e .

In conclusion,

ku-Cu, Ag-Pt, and Pt can be used to form ohmic, stable contacts to ZnTe. However, only Pt contacts yield acceptable values of specific contact resistivity for extended operation at temperatures up to 150°C.

REFERENCES 1.

P. A. T u p e n e v i c h and V. K. Kononenko, J . Appl. S p e c t r o s c o p y ,

28, 592, 1978.

2.

W. D. Baker and A. G. M i l n e s , J . E l e c t r o c h e m . S o c . ,

3.

V. P. G r l b k o s k l l , A. K. B e l y a e v a , V. V. Z u b r l t s k i i , V. A. I v a n o v and I . A. K a s h l n a , S o v i e t P h y s i c s : S e m i c o n d u c t o r s , 548, 1981.

4.

H. S a j i , K. Matsumoto and H. F u j i m o t o , E l e c t r i c a l 98, 1, 1978.

5.

M. N. Luqman, "An E v a l u a t i o n o f Z i n c T e l l u r i d e a s a Gaalaa D e t e c t o r f o r High T e m p e r a t u r e A p p l i c a t i o n s " , Ph.D. D i s s e r t a t i o n , U n i v e r s i t y o f A r k a n s a s , 1985.

6.

O. K i r k , E n c y c l o p e d i a o f Chemical T e c h n o l o g y , 2nd E d . , 6 , 197, 1965.

7.

H. H. Woodberry, " P h y s i c s and C h e m i s t r y o f A I I Bv I C o m p o u n d s " , [ R u s s i a n Translation], Mlr, 178, 1970.

119, 1269, 1972.

Engineering in Japan,