AD A138 241 CALCULATIONS OF MISFIT DISLOCATION AND DANGLING BONDDENSITIES IN ABRUPT..(U) AEROSPACE CORP EL SEGUNDO CACHEMISTRY AND PHYSICS LAB R B SCHOOLAR 15 DEC 83

INCASSIFED TRODB841494507)iSDTR-83-88 FG 2012 N(EIUZEEEEEEE

111 11,2

-~o L

MICROCOPY RESOLUTION 'TEST CH4ART

NNA OUA OtMF STAWMDS -9*3-A

00a

L) ric

SELECTFEB27 U84

, WK .1B

~- ~ - ~4 ~am

We~ report va satmitted by h Arospae Orporatioe, 31 Segudo. CA902450 made COstrect OD. M070F43W090S4 with the Spams 01,1.1., P.O. Box02M4O, rldwy NestAl Coter, Los hmploe CA "009* It we rwvied adapproved for ram herpec Corporation by S. Vrersta, 01rector. CheeLatryand Pbyefcs laboratory. ,let Lt Dpbort~l Ve tere. W~IGJ, WO tVe projectofficer ter the Alousi Oriented lwwttion and - Upertmeutation (11)PNogrwe

This report has beem reviewed by the Public Affairs Off ice (PAS) and isreleasable to the National tedibsto i formatlon service (NTIS). At XTIS. Itwill he aveleable to the general pUlcinluding foreign nationals,

Mei :ewheieat report bus bass revtd endito approved for publication,.Publiesoio of ft report dow: t constitute Air. Foce approval of theweportes fiadlege or comclusiose It to, publisbe only for t0e excbmage andstimulation of idess.

1"ojst Of fiee jes Oft bw;, Aft leree

1ps -41s~g Osq

t"A

~-'&~4*~ ~J>442 ~ NO:j

UnclassifiedSECURITY CLASSIFICATION Of THIS PAGE (UM Dme Snow**

READ UISTRUCTIOsREPORT DOCUMENTATION PAGE 337033 COMPLEIDG FORM1. REORNUNIE GOVT ACCESSIN NO: S. RCIPIENT'S CATALOG NIISE

SD-TR-83-88 r~b-, u~ 41 ______________4. TITLE (&" U.*NeI) S. TYPE OF REPORT a PERIOD COVERED

Calculation of Misfit Dislocationsand Dangling Bond Densities In 6. PeRrPaQmixe. ORG. REPORT? NumSERAbrupt Hgi..jxCdxTe Heterojunctions TRm0844945-07)-l

r1. ATNMOXr D. CONTRACTIOR GANT NUNSEWge)Richard B. Schoolar

704701 -83-C-0084

S. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM RIEET POET TASKThe Aerospace Corporation aRE W O~ UNI NUMBE RSEl Segundo, Calif. 90245

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATESpac Divsion15 December 1983

Los Angeles Air Force Station is. NUMBER or PAGESLos ngeesCali. 90096

1.MNITOING AGENCY NANE ADDR230if OEfo..mee fAr CONAWIM0. OW..) WIS SECURITY CLASS. (of 'NommtUnclassified

%So HAMIuICA IOWNGAtING

IS. DISTRIBUTION STATEMENT (of AWS abpm)Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of It.. abotue saftd inMSWe*O 611 Nm MW*pGO)

1S. SUPPLEMENTARY NOTES

SO. KEY WORDS (Comeaw. on side of 880006Wp old ~~P WIWI unS. )

Epit axial growth \Inter acesHgCdT* Mercu cadmium tellurideInfrared detectors

SO AShTRACT t(Aioinme em.. a#* Of aMd W1110111 IV mA.Based on the classical theo of epitaxial c stal groh h iftdsoations and dangling bond densit as of abrupt 1 HjCd!T eeojscinhave been calculated. *for th --! whiere (x2 - 1 Y. the dangling bonddensity Is an the order of 10 em' . Such l1rSe aangling bond densities mayproduce high Interface recombinat ion velocities or band-bending at theinterface

* POEMA UnclassifiedIP*6SN.~0

The mrcury-cadmium-telluride (1gl_Cd.Te) alloy system has become an

important semiconductor material for fabrication of infrared detectors. A

Ulquid-phase-epitaxy (LPE) technique is commonly used to grow this semiconduc-

tor with the desired Cd content on CdTe substrates. This alloy layer-

substrate combination has been referred to as a "lattice matched" system since

the lattice constants of HgTe and CdTe differ by only 0.3%. The high perform-

ance of p-n Junction photovoltaic detectors produced from these epitaxiallayers has been taken as evidence of low surface recombination velocities and

low defect densities at the Ihl.zCd.Te/CdTe interface.1 Consequently, inter-

eat has been shown for using LPE techniques to produce more complex double-

layer heterojunctions for device applications. 2 These structures are com-

prised of an n-type layer of composition A2 on a rtype layer of composition x1and, in theory, should have lower leakage currents than p-n homojunction

photodiodes. However, the defect structure of HglCd1Te heterojunctions has

not been established.

The purpose of this report is to present calculations of the misfit

dislocation and dangling bond densities at abrupt Ug Cd Te/Egl Cd Teheterojunctlons where hx - (x2 - x1 ) is a variable. Thl ag'..xCd Te/&TeInterface Is a special case with the variable z becoming (I-x). The calcula-

tions are made for the (111) plane using the theoretical treatment of Oldham

and Milnes. 3 According to this study, the misfit dislocations for (111)heterojunctlos may lie in the , , and directions.with spacing

h between sets as shown in Fig. 1. For pure edge dislocations, h is given by

1 2

r2 ( 2 -a2

where a, and a2 are the lattice spaciag for x, and z2. respectively, on both

sides of the beterojunction. The Interface dangling bond density Ms is them

Me DTIC TABUnannouncedJusti ication

Distribution/

Availability Codes

Avail and/or

Dist Spsis)

: ... . - ' ,- - ln

(211> DIRECTION (111> DIRECTION

X2

1071

Tigere 1. Misfit Dilcation Lims In the (111) Plowe of anAbrupt g1..3Cd.Te Usatelrojuact ioo. The spacing hIs PIOmted fuss the lattice constants &I and aou both sides of the Interf ace.

2 7'U Pt

where c Is the mean spacing between bonds along the dislocation line and for

(111) orientations is given by

81 j-C a 7

A recent study of HBglCd.Te epitaxial crystal growth4 has shown that

mirror-samooth films can be obtained on 1gl_xCdxTe substrates when Lx < 0.03.

Networks of dislocation lines similar to the ones shown in Fig. I were

observed when Ax > 0.07.

According to the literature, the lattice constants of CdTe and UgTe at

room temperature are 6.4818A and 6.4620A, respectively.5 The lattice para-

meter of Ngl-xCdxTe varies approximately linearly with x across the entire5composition range. The linear thermal expansion coefficients for both mater-

ials Is 5.0 x 10-6C71 above 300 K.

6 ,7

The dislocation line spacing and dangling bond densities for abrupt (Ill)

IRsl-Cd.Te heterojunctions are shom in FiS. 2 as a function of ax. Since the

thermal expansion coefficients of CdTe and UgTe are identical, these calcula-

* tions apply for all LPE growth temperatures.

These dangling bonds should occur in the HSl-zCdxTe epitaxial layer and

not in the CdTe substrate since the alloy has the smaller lattice constant.

In the case of heterojunctions between two alloy semiconductors, the dangling_bonds should occur In the layer wi~th the smaller lattice constant which also

has the smaller energy gap. It Is well established that such dangling bondscan generate either donor-like or acceptor-like interface states, recombina-

tion centers, traps, or In rare instances may remain neutral. The interfacial

barrier height generated by donor or acceptor-like states can be estimated

from As following the method outlined by Many, Goldstein, and Grover 8 for

calculating surface potential. According to these calculations, barrier

heights in excess of 4 KT/q can be generated whenever ax > 0.1.

4!

a 3

- U.:3) N V

- 04

004 4-4J~

'us

04a

01 "0U

1~ 0

914

to U (~

U? ~W1

(W~)L ir

.J

According to this calculation, the Hgl_xCdxTe/CdTe interface is far from

ideal when Ax > 0.1. For values of Ax typically encountered in epitaxial

growth of detector layers, Me ) 01 2 c - 2 . Such high densities of dangling

bonds could easily give rise to high Interface state densities and cause

severe minority carrier recombination. Of course these interface states may

produce band-bonding of the proper sign to screen minority carriers from the

CdTe interface. This screening could reduce the interface recombination

velocity and produce the enhanced "limited volume" diode characteristics

reported for p-n Junctions fabricated in Hgl..Cd.Te epitaxial layers.1

In the case of double layer heterojunctions with coincident metallurgical

and p-n Junctions, the interface states would occur in the depletion region

and could produce high recombination rates and excess leakage currents. Any

band-bending which would screen thermally generated carriers would also repel

optically generated carriers and degrade detector quantum efficiency. Thus,

ftl-xCdxTe heterojunctions with Ax ) 0.1 might be expected to exhibit poor

photodiode characteristics.

I conclude that abrupt Hg1 _xCdxTe heterojunction Interfaces should have

misfit dislocations and dangling bond densities in excess of 1011 ca - 2 when-

ever Ax > 0.10. Such large numbers of dangling bonds may cause severe

minority carrier recombination and/or band-bending at the interface. Thus,

these Interfacial defects may play an important role, or may even dominate

heterojunction device characteristics.

I

References

1. M4. Lanir, C.C. Wang, and A.H.I. Vandervyck, Appi. Phys. Lett. 34, 50

(1979).

2. S.R. Shin, A.H.B. Vandervyck, J.C. Kim, and D.T. Cheung, Appi. Phys. latt.

37. 402 (1980).

3. W.G. Oldhaim and A.G. milnes, Solid-State Electron. 7, 153 (1964).

4. Y. Nemirousky, S. Margalit, E. Finkuan, Y. Shacham - Damand, and 1.

Kidron, J. Electron, Mater. 11, 133 (1982).

5. J.C. Woolley and B. Ray, J. Phys. Chen. Solids 13., 151 (1960).

6. Novikova, Fiz. Tverd. Tela.3, 178 (1961) Eng. Trans.:Soviet Phys. - Solid

State J3, 129 (1961).

7. Novikova, and N. Kh. Abrikosov, Fit. Tverd. Tela 5 218(96)Eg.

Trans.:Soviet Phys. - Solid State Z, 1558 (1964).

8. A. Many, Y. Goldstein, and N.D. Grover, Semiconductor Surfaces, Chapter 4

(North Holland Publishing Company, Amsterdam, 1965).

JlxtPT

i4S ,

02 "\-o

,44 qz 4,

WSW

ATE

LMED