Heterojunctions by alloying

Solid-State

Electronics

Pergamon Press 1965. Vol. 8, pp. 1-5.

HETEROJUNCTIONS J. R. DALE

BY ALLOYING and M. J. JOSH

Mullard Research Laboratories, (Received

22 March

Printed in Great Britain

1964; in

Redhill, Surrey

revisedform 27 April 1964)

Abstract--Work is described which shows that layers of GaSbzAsi-z, GQI~~-~As and MnsXs of device quality can be produced on GaAs using a simple alloying technique. The use of these layers in making heterojunctions with GaAs for opto-electronic devices is suggested and some of their optical and electrical properties are discussed. R&sumB-On decrit les travaux qui demontrent que les couches de GaSbzAsrmz, GaInl-zAs et MnsAs ayant desqualites de dispositifs electroniques peuvent Ctre produitcs sur 1’AsGaenemployant une techiiique d’alliage simple. L’emploi de ces couches pour fabriquer des heterojunctions avec 1’AsGa destinees aux dispositifs optico-tlectroniques est suggere et quelques-unes de leurs proprietes optiques et Clectriques sont discutees. Zusammenfassung-Es wurde nachgewiesen, dass durch ein einfaches Legierungsverfahren praktisch verwertbare Schichten van GaSbzAsi_Z, GalInimzAs und MnsAs auf GaAs erzeugt werden konnen. Die Verwendung dieser Schichten zur Herstellung von heterogenen Uberglngen mit GaAs fur optisch-elektronische Gerate wird vorgeschlagen, und einige ihrer optischen und elektrischen Bigenschaften werden erortert. INTRODUCTION

IT HAS been known for many years that electronic devices can be made in which an electrical signal is converted into light and back into electricity.(l) Recent observations of highly efficient light emission from some forward biased p-n junctions in GaAs(sJ) have been made. To make full use of these properties in transistor like devices@-@ an efficient collector is required. In order to collect most of the light in these devices an increased absorption constant is required within the collector depletion layer. This characteristic could be obtained by the deposition of Ge on GaAs, (ANDERSON@) has described some of the theoretical problems associated with such a junction), or by using some other heterojunction where the energy gap of the deposited layer is less than that of GaAs. OLDHAM and MILNE@) have described a method of making n-n InP-GaAs heterojunctions and discussed some of the theoretical aspects of such a junction. Abrupt monocrystalline junctions between two different semiconductor materials have been made by vapour transport techniques. ANDERSON~~)

deposited Ge epitaxially on GaAs using iodine as the transport reagent and OKADA grew layers of GaAs on Ge by a similar process. PIZZARELLO(13) deposited polycrystalline layers of a solid solution, GaAs,PL-,, on GaAs and SAN--MEI transported a range of alloys of GaP and GaAs on to substrates of either GaP or GaAs using iodine as the reagent. These techniques have shown(t4) that device quality solid solution-GaAs heterojunctions can be epitaxially deposited by vapour transport. However most of the reported work on the production of solid solution layers has been concerned with GaAs-GaP alloys which have too high an energy gap to be used in the GaAs transistor like devices suggested by REDIKER and others.+-*) An investigation of the InAs-GaAs system by VAN HOOK and LENDER and WOOLLEY and SMITHY suggests that a material having suitable properties could be obtained with a solid solution of InAs in GaAs with the composition GaL_,In,As where x is greater than O-08. In view of the well-known inherent disadvantages of the vapour transport process it was thought that an alloying technique would offer an

2

J.

Ii.

DXL,E

and

attractive alternative approach to the production of layers of a solid solution of GaAs and InAs on a GaAs substrate. The investigation to be described is based on the observations of DALE and GOSH who showed that Bi based alloys can be used to obtain reproducible monocrystalline epitaxial recrystallisation of GaAs on GaAs substrate. By a modification of these alloys it has been possible to prepare layers of solid solutions on GaiZs. EXPERIMENTAL The possibility of recrystallising layers of a solid solution from an alloy was examined by reacting weighed quantities of BiiInAs-GaAs or BiiGaSb-GaAs with a dopant in sealed vials at 450-750°C and slow cooling. Alloys for making solid solution layers were made from high purity elements to minimize the chance of obtaining ambiguous results from impurities having high diffusion rates. (In all cases the copper and magnesium content n-as less than 0.1 to 0.2 ppm.) The alloys were prepared by reaction of the elements in the desired proportions in evacuated silica vials, the vapour space being kept to a

RI.

Type

Min. pm-wetting temp. “C

1.

90 Bi 10 Gash

P

350

2.

80 Bi 20 GaSb

P

3.50

3.

90 Bi 10 InAs

P

350

4. 5

80 Bi 20 InAs 90 Bi 5 InAs 5 Mn

P

350

p

400

6.

85 Bi 10 InAs

5 Mn

p

400

7.

75 Bi 15 InAs

10 Mn

p

450

8. 9.

85 Bi 10 InAs 98 Bi 2 Mn

5 Cd

10.

90 Bi 10 Mn

JOSH

minimum to prevent loss of 4s from the alloys. The vials were heated to ‘900-1OOO’C for 224 hr in a vibratory furnace to allow complete reaction to take place without segregation of the components. This was followed by rapid quenching into silicone oil or carbon tetrachloride so that the vials shattered under the surface of the quench in such a way that pellets were formed. The composition of the alloys used are shov n in Table 1. GaAs substrates were prepared by lapping dice with 15 p alumina grinding powder and immediately prior to alloying etching in a lo-30 per cent solution of bromine in methanol at O’C. Alloying n-as carried out in evacuated sealed silica tubes but before sealing the pellets were pre-\vet on to the GaAs to facilitate handling. Bi 2 per cent Cd was used as an ohmic contact to P-type GaAs and 45 Bi 55 Sn 5 Pt (parts by. weight) to the n-type. (li) Alloying was normall! carried out at 500-600-C for 30 min to 2 111 dependent on the alloy composition and type of heterojunction required, followed by slow cooling over 1-Y hr. Examination of the electrical characteristics of

Table

Alloy composition (parts bp weight)

J.

I

Xppros. composition of recrystallised layer

Remarks

-1s content dependent on allo>inr: temp; but generall>- less than 10 per cent Good diode characteristics-photosensitive Poor diode characteristics. Alloy 4 more constant than 3

p

350

P

400

Cd doped MnzXs

P

450

Mnzr\s

GaxInl-.rAs

Two layer structure interface layer thought to be Ga,Inl-.A As. 5. Good diode characteristicsphotosensitive Interface layer l/4 p thick hLnn.ls up to 50 p. Good diode characteristics-photosensitive Poor diodes-photosensitive Good diode characteristics. Kot very photosensitive Good diode characteristics. Not very photosensitive

HETEROJUNCTIONS

alloyed heterojunctions consisted of an assessment of the rectification characteristics and a measurement of the capacitance vs. applied voltage. Evaluation of the optical properties consisted mainly of the measurement of the optical absorption, collector efficiency and the optical response time. A comprehensive metallographic examination of the recrystallised layers was made to assess both the crystalline perfection of the layer and the interface and also to determine the approximate composition and homogeneity. The composition of the layers was also assessed by X-ray probe microanalysis and X-ray diffraction techniques. RESULTS

AND

DISCUSSION

The alloying results are briefly summarised in Table 1. It was found possible to recrystallise single phase crystallites of GaAs-InAs and GaAs-GaSb alloys from Bi based alloy solutions. Figure 1 shows single phase crystals of GaSb,Asi-, (where x is approximately 0.1) recrystallized from an alloy of 78% Bi, 20% GaSb, 2% GaAs after reacting at 750°C for 8 hr and slow cooling over a further 8 hr. GaAs-InAs solid solutions could be formed directly by alloying In to GaAs. Figure 2 shows a thick In contaminated recrystallised layer of Ga,Inl_ZAs on a GaAs substrate (where x is small). This type of layer was obtained by alloying indium pellets on to GaAs dice at 500-600°C for a few hours and cooling rapidly. Reproducible quality, thickness and composition of the layers could not be achieved by this technique and contamination by free indium gave poor rectification characteristics. Reproducible solid solution layers of the approximate composition GaSba.sAsc.1 were produced by alloying pellets of 80% Bi, 20% GaSb (by weight) on to a GaAs substrate at 500-600°C. GaAs was dissolved from the substrate by the liquid bismuth and on slow cooling layers of the solid solution recrystallised epitaxially on the substrate. Slow cooling over 2 to 4 hr prevented the substrate cracking. During fast cooling cracking occurred in the GaAs immediately under the alloy layer and was probably due to the strain created by lattice mismatch at the heterojunction. Photosensitive diodes could be made from p-type layers recrystallised on n-type substrates with rectification ratios of 103-104 and breakdown voltages of 3 to 7 V at less than 0.1 mA.

BY

ALLOYING

3

Figure 3 shows a typical heterojunction obtained by alloying a pellet of 80% Bi-20%GaSb on to a (111) GaAs substrate at 500°C. In general more uniform wetting and recrystallisation occurred on --the A( 111) faces than on the B( 111) faces. Recrystallized layers of Ga,Ini-,As were obtained in a similar manner by alloying pellets of Bi-InAs (alloy No. 4) on to GaAs substrates. Figure 4 shows a typical recrystallized solid solution layer of this type. Diode characteristics of these layers were poor but could be improved by the addition of up to 5 per cent of ap-type dopant to the pellet alloy. The most promising results were obtained with the Bi-InAs-Mn series of alloys. Using alloy compositions 5 and 6 (Table 1) two layer structures were generally obtained. Figure 5 shows a typical heterojunction of this type. The light layer adjacent to the GaAs is thought to be a solid solution of Ga,Ini-,As, (this is estimated from available metallographic data such as etching properties, microhardness, colour, etc.). This layer was normally of the order of only Q-p thick making an accurate estimate of its composition unreliable by conventional techniques. The darker layer above it has been shown to be MnzAs by X-ray microprobe and X-ray diffraction analysis. Examination of about 75 of these devices showed them to have rectification ratios of 103-104, (the quoted rectification ratios were obtained by dividing the reverse resistance immediately prior to breakdown by the forward series resistance), series resistances of 25-200 !L!, breakdowns of 2-10 v. Most of these measurements were obtained using a substrate doping of approximately 3 x 101s donors/ems. At higher doping levels (1017 donors/ ems) softer reverse breakdowns were generally observed. Measurements of junction capacity as a function of voltage applied in the reverse direction can be generally fitted by the equation Ctc( I’, + 1-0))l/a-z. The dark characteristics of these junctions were only measured at room temperature so that no further check was available of the diffusion voltage.@) At room temperatures, la exp Q~F/~RT suggesting recombination at a trapping centre(ss) or a p-i-n junction.c2r)

J.

4

R.

DALE

and

Typical optical response curves for these heterojunctions are shown in Fig. 6, these were obtained by illumination through the layer of GaAs. It can be seen that they rise to a peak value of over 95 per cent at 7 V bias with radiation close to that emitted by GaAs diffused junctions. The shape of these response curves will be slightly distorted by the 300 A resolution of the spectrometer used in these measurements. Optical response times for these heterojunctions are shorter than 10-T set, equipment limitations have at present prevented the measurement of faster response times.

temperature

06

07

/ I 06

\ 0.9

Wavelength, FIG.

IO

II

M.

J. JOSH

of the substrate. There is evidence(ls) that the (001) planes of MnsAs which has a tetragonal structure fit the (100) planes of GaAs with only about 5 per cent mismatch. At present little is known of the optical and electrical properties of MnsAs. Using alloy No. 7 MnsAs can be recrystallised in layers up to 50-p thick, with this alloy the solid solution layer cannot be detected by metallographic techniques (limit of detection about l/4 p). However electrical and optical properties appear to be similar to the two layer heterojunctions previously described. From these results it is thought that the solid solution layer is relatively inactive and the MnsAs layer is the major component of heterojunctions. Figure 7 shows a typical heterojunction of this type. The MnaAs recrystallised layer contains trapped islands of a Ga-In rich phase due to a high cooling rate. Polycrystalline layers (see Fig. 8) formed by very rapid cooling still exhibited diode characteristics but the optical and electrical properties were much inferior to the epitaxially recrystallized (i.e. single crystal) layers. Trapped inclusions of a Ga-In phase had no apparent effect. Heterojunctions formed between Ga and Bi contaminated MnsAs, recrystallised from Bi-Mn alloys (No. 9 and 10, Table 1) and GaAs exhibited similar electrical and metallographic properties (see Fig. 9) to the MnsAs layers recrystallised from Bi-InAs-Mn alloys. They were, however, not photosensitive. This suggests that the optical properties of MnsAs-GaAs heterojunctions are determined to a large extent by the method of formation of the MnsAs and also the probable type and concentration of impurities in the layers.

1;

/A

6. Optical response curves of a heterojunction recrystallised from an alloy of Bi-InAs-Mn.

The electrical results obtained seem to depend somewhat on the composition of the alloy pellets used and more strongly on the quality and impurity concentration of the substrate. However further work will have to be carried out to observe any definite trend. There is some indication that the properties of the heterojunctions will depend on the orientation

Acknomledyements-Thanlis are due to Mr. H. C. WRIGHT for helpful discussions, to Mr. A. J. HALL, Mr. R. J. DOWNEY and Mr. J. F. X. THOMPSON for measurements and to Ir. M. CLERK for X-ray probe microanalysis. APPENDIX Etch to reveal grain boundaries solution layers. 1 part H&04 (SG 1.84) 1 part HsOs (100 ~01s.) 8 parts deionised water Etching time 30 set to 2 min

in recrystallised

solid

HETEROJUNCTIONS REFERENCES 1. G. DIEMER and J. G. VAN SANTEN, Philips Res. Rept. 15, 368 (1960). 2. R. J. KEY= and T. M. QUIST, Proc. Inst. Rndio Engrs. 50, 1822 (1962). 3. M. I. NATHAN,W. P. DUMKE, G. BURNS, F. H. DILL and G. LASHER, Appl. Phys. Lett. 1, 62 (1962). 4. R. H. REDIKER, T. M. QUIST and B. LAX, Proc. Inst. Elect. Electron. Engrs. 51,218 (1963). 5. R. F. RUTZ, Proc. Inst. Elect. Electron. Engrs. 51, 470 (1963). 6. F. F. FANG and H. N. Yu, Proc. Inst. Elect. Electron. Engrs. 51, 860 (1963). 7. A. G. FOYT, Proc. Inst. Elect. Electron. Engrs. 51, 852 (1963). 8. J. R. A. BEALE and P. C. NEWMAN, private communication. 9. R. L. ANDERSON, Solid-State Electron. 5, 341 (1962). 10. W. G. OLDHAM and A. G. MILNES, Solid-State Electron. 6. 121 (1963).

BY

5

ALLOYING

11. R. L. ANDERSON,I.B.M. J. Res. Dez.4,283 (1960). 12. T. OKADA. T. KANO and Y. SASAKI. I. Phvs. Sot. Japan, i6, 2591 (1961). 13. F. A. PIZZARELLO, J. Electrochem. Sot. 109, 226 (1962). 14. SAN-MEI Ku, J. Electrochem. Sot. 110, 991 (1963). 15. H. J. VANHOOK and E. S. LENKER, Trans. Sot. Amer. Inst. Metall. Engrs. 227 (1963). 16. J. C. WOOLLEY and B. A. SMITH, Proc. Ph~ls. Sot. 70B, 153 (1957). 17. J. R. DALE and M. J. JOSH, Solid-State Electron. 7, 177 (1964). 18. H. NOWOTNY and F. HALLA, J. Phys. Chem. (B)36, 322 (1937). 19. A. VAN DER ZIEL, Solid State Physical Electronics, p. 282. MacMillan. 20. G. T. SAH, R. N. NOYCE and W. SHOCKLEY, Proc. Inst. Radio Engrs. 45, 1228 (1952). 21. R. N. HALL, Proc. Inst. Radio Engrs. 40, 1512 (1952). .I

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