Growth of InAs by MOVPE: A comparative study using arsine, tertiarybutylarsine and phenylarsine

Journal of Crystal Growth 97 (1989) 489—496 North-Holland, Amsterdam

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GROWTH OF InAs BY MOVPE: A COMPARATIVE STUDY USING ARSINE, TERTIARYBUTYLARSINE AND PHENYLARSINE S.K. HAYWOOD, R.W. MARTIN, N.J. MASON and P.J. WALKER

*

Clarendon Laborato,y, Parks Road. Oxford OXI 3PU, UK

Received 9 May 1989

The growth of hulk heteroepitaxial layers of InAs on GaAs substrates (and in some cases on lnP substrates) by atmospheric pressure MOVPE is described. The indium source used was trimethylindium and we present a comparative study of the use of arsine. tertiarybutylarsine (TBAs) or phenylarsine (PhAs). The quality of the epitaxial layers was established from electrical and morphology measurements and showed a marked improvement with TBAs as a result of improved pyrolysis at lower temperatures. The 77 K 2/Vs for samples grown from arsine, to nearly 30,000 cm2/Vs for those grown mobility was found to increase from 11,000 cm allows low growth temperatures close to those used in MBE to be used in from tertiarybutylarsine. The use of about tertiarybutylarsine MOVPE. PhAs was found to pyrolyse at a temperature very similar to arsine and grown layers were very similar to those obtained from arsine.

1. Introduction The heteroepitaxial growth of InAs on GaAs has some interesting potential uses in the fields of infrared detectors and optoelectronic devices [1—61. The large lattice (7.2%) mismatch between the InAs epitaxial layer and the GaAs substrate is a major problem in trying to obtain material of good electrical quality. A high dislocation density occurs at the interface and some of these dislocations continue throughout the epilayer. It is important to try and minimise these if material of good quality is to be produced and layers of mismatched material generally show an improvement in the mobility measurements as the layer thickness increases. It is therefore important that measurements are recorded on suitable layer thicknesses (generally a few urn thick). We have previously reported [7] the first growth by MOYPE of a GaSb/InAs heterojunction. Characterisation by magnetotransport measurements in high magnetic fields has demonstrated the presence of a 2D electron gas in this structure.

*

To whom correspondence should be addressed,

There has been very little work reported on this system and previous samples have been grown by MBE [8]. It was apparent from the data [7] that the quality of the heterojunction was being cornpromised by the relatively poor quality of the thin InAs layer and a thorough study of the bulk growth of this material by MOVPE was needed to establish the best growth parameters. The growth of InAs on GaAs by MBE is fairly well documented (e.g., refs. [9—13]),including a number of studies on the initial stages of the epitaxy [14—17]. By comparison, growth by MOVPE is poorly represented. There is only one systematic survey of the growth and subsequent electrical quality [18,19], a further early paper with brief electrical data [20], and a few other papers which do not deal directly with electrical quality [21—24]. In addition, InAs on GaAs has been grown by plasma assisted epitaxy (PAE) [25,26] with reasonable electrical quality, and by atomic layer epitaxy [27,26]. Recently, strained multilayers of InAs/GaAs have been grown by MOVPE [21,29.30]. The electrical quality [18] of MOVPE grown layers (from triethylindium and arsine) was poor compared to MBE material and one of the major

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factors for this could be the higher temperatures needed in MOVPE growth. The lowest temperature for growth by MOVPE appears to be about

has given promising initial results is phenylarsine (PhAs). This compound might be expected to dissociate into AsH2 and benzene (which would be

550—575°C [18], and this is associated with the poor pyrolysis efficiency of the commonly used Group V precursor, arsine. Below 5500 C, material with very poor surface quality was obtained. The percentage of the arsine pyrolysed at these ternperatures is only of the order of 50% [31]. Therefore, growth at temperatures of the order of 500 C or below, as used in MBE, will be expected to yield material of very poor electrical quality by MOVPE when realistic flow rates are used, Recently, there has been a considerable effort put into producing alternative precursors for the Group V sources used in MOVPE. Arsine has a number of serious drawbacks which have been known for some time. As well as the toxicity problem, it has proved remarkably difficult for the manufacturers to supply gas of a suitably high standard. Oxygen and water remain the main problems, usually associated with poor cylinder preparation. Although the situation has improved somewhat over the last few years, the supply of good gas cannot be guaranteed. Also, the problem of poor pyrolysis of the arsine at low temperatures will still remain. Several alternative sources have been investigated for the growth of GaAs (reviewed in detail in reference [32]), and one of the most promising alternatives appears to be tertiarybutylarsine (TBAs) [32,33]. This material has a suitable vapour pressure for MOVPE work, allowing the use of the material in a stainless steel bubbler rather than a pressurised cylinder, and it pyrolyses at a much lower temperature than arsine. Pyrolysis is 50% complete at 425°C [33]. Additional encouraging signs are that GaAs can be grown from this source at much lower V/Ill ratios than usual, with good surfaces and no increase in the carbon content of the layers as judged from PL data [33]. Also, the electrical quality was promising, given that further increases in the quality of the precursor are likely. It appears that the presence of 2 hydrogen atoms attached to the Group V atom is a major advantage in a suitable precursor, but this is only a tentative conclusion [33]. Another alternative Group V precursor which

stable). Another important point is that PhAs is a volatile liquid at room temperature, with a vapour pressure which is applicable to MOVPE work [34] We report here the results of a study of the growth of InAs on GaAs, comparing the results achieved by the use of either arsine, tertiarybutylarsine or phenylarsine. Mobility data determined from variable temperature Hall experiments is used as an indication of the quality of the epitaxial layers.

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2. Experimental The MOVPE reactor has been described before [35,36]. An 8 cm diameter outer silica cell was used with a removable liner insert. Although the main details are as before, a new inlet shape was used for the cell linear in these experiments. This has been described elsewhere [37]; the main advantage with this is the lack of recirculation at the front end of the cell which means that the entrance region remains clean throughout a growth run. This can also be useful for assessing the presence or absence of any pre-reaction between the materials. Pre-reaction between triethylindium and arsine has been reported recently [38]. The alkyl sources used were TMIn (Epichem, 20°C). TBAs (Cyanamid, 20°C), and PhAs (Epichem, 200 C). The TMIn was a diphos purified sample: this material has given excellent results in the growth of InP [39]. The arsine used was from Union Carbide (Phoenix Grade, 10% or 100%); BOC Ltd. supplied the phosphine (10%). Palladium diffused hydrogen was used as the carrier gas with high purity nitrogen to transport the trimethylindium. Typical flow rates were: TMIn 100—300 SCCM; TBAs 1—jO SCCM; PhAs 300 SCCM; 100% arsine 10—30 SCCM; 10% arsine 100—300 SCCM; 10% phosphine 200—300 SCCM. Substrates used were GaAs: Si, GaAs: UD and GaAs : Cr. All these were of (100) 2° [110] orientation. In a few cases, InP substrates were used, of (100) orientation. All the substrates were degreased using 1,1,1 trichlorethane, methanol and

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isopropanol. The substrates were etched at room temperature in a 5 : 1: 1 mixture of H2S04, H202 and H20, washed in high purity water, dried, loaded into the reactor liner and positioned on a 50 mm molybdenum disc. The initial bakeout consisted of 2 h at 250—300°C under hydrogen followed by a heat treatment to 800°C with arsine, or to 600°C if tertiarybutylarsine or phenylarsine was used. A small amount of TBAs or PhAs was found to be as effective as arsine in suppressing the surface decomposition of the substrate. However, attempts to stabilise InP substrates under arsine were generally unsuccessful and in these cases bakeout took place under phosphine, followed by the growth of a thin (1000 A) buffer layer of InP. Growth times were from 1—4 h with growth rates of the order of 1—3 um/h. A number of layers were grown at various temperatures between 450 and 650°C at a fixed pressure of 1100 mbar. After growth, either the arsine, tertiarybutylarsine, or phenylarsine were left running until the susceptor temperature had reached 250°C. The carrier concentrations and mobilities in the grown layers were determined using variable-ternperature Hall measurements on Van der Pauw structures. An Oxford Instruments continuousflow cryostat, using liquid helium, was used over the temperature region from 4—300 K. The fourcontact resistance and Hall voltages were mea-

sured using a Keithley 220 constant voltage source and Keithley 195A multimeter. A resistive magnet provided a field of up to 0.5 T. Thickness measurements were made by electrochemical profiling of the layers grown on GaAs: Si substrates. These measurements were crosschecked by cleaving the epilayers and viewing under a Nomarski interference contrast microscope. This instrument was also used to examine the morphology of the grown layers. SEM micrographs were obtained using a Cambridge Stereoscan 250 Mk I (located at the SERC Central Facility in Sheffield).

3. Results 3.1. Growth using arsine

Growth runs were at fixed temperatures in the region from 500 to 650 °C and at various V/Ill ratios from 10: 1 to 48: 1. Some representative results of the electrical data of the layers are shown in table 1 and fig. 1. All the layers in table 1 were at least 3 urn thick (see later) and all the layers showed n-type conduction. Material produced at 500°Cwas sometimes difficult to characterise and this agrees with the data of ref. [18] where poor layers were obtained below 550°C.

Table 1 Electrical data for IriAs layers (all data from material grown on GaAs substrates) V/Ill Growth temperature RT 2/V mobility ‘ s) 77 K mobility (cm2/V s) ratio (°C)data for InAs layers grownfrom (cm (a) Electrical arsine

RT carrier (cm _3) concentration

77 K_3) (cm carrier concentration

53:1 42:1 42:1 28:1 28:1

2.8x10’6 1.0X10’7 2.2x10’7 6.0X10’7 2.7x10’7

2.4x1016 8.8X1016 1.8x1017 5.4x1016 2.2x1017

(b) Electrical data for InAs layers grown from tertiarybutylarsine 5:1 475 3288 4290 5:1 500 13940 29640 5:1 525 8121 17490 5:1 550 4639 7048

1.8x1017 2.0x1016 2.1X1016 4.8x1016

1.5x10’7 1.OxlO’6 7.9X10’5 4.0x10’6

(c) Electrical data for InAs layers grown from phenylarsine 1.3:1 500 9326 12100 1.3:1 550 6319 6760

4.1X1016 1.0X1O’7

3.2x10’6 8.9x10’6

500 550 600 600 600

7631 7088 4144 6534 4389

9651 10885 5504 8101 5068

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layers and morphology generally improves with increasing layer thickness. All appeared dull to the naked eye with the best result being obtained at a growth temperature of 550°C. One major prob1cm in the growthofof the InAselectrical from arsine has been irreproducibility quality from

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Growth of InAs hr MO VPE

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sample to sample, even with a fixed V/Ill ratio. This is illustrated in fig. 1. The best material

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V

y V

2

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1

25

50

v/Ill ratio

Fig. 1. Mobility data for InAs grown from arsine: (v) GaAs substrate at 600°C; (v) GaAs substrates at 550°C; (•) InP

substrates at 600°C.

The surface quality of the layers was rough, especially at the higher growth temperatures used. Fig. 2 is an SEM micrograph of an InAs layer grown onto GaAs using arsine at a growth temperature of 600°C. The poor quality is evident with a large number of cracks and voids going deep into the epilayer. Clearly, 2D growth is not occurring over any appreciable distance as the arsenic is incongruently evaporating. All of the surfaces show cross-hatching which is typical of mismatched

obtained was that on InP substrates (as expected from the improved lattice mismatch compared to GaAs); on GaAs substrates the best material was obtained at 550°C and higher V/Ill ratios were generally more successful. The best mobility recorded (see table 1) was nearly 11,000 cm2/V s. with a carrier concentration of I x 1017 cm ~ (at 77 K). The variation of the electrical quality with a change in the V/Ill ratio, is however, much less noticeable than in the case of the growth of homoepitaxial GaAs by MOVPE. The InAs layers were always n-type, with no sign of the p to n conversion with a change in the V/Ill ratio as commonly found for GaAs growth. ‘

3.2. Growth from tertiarybutylarsine

As it was already appreciated that this precursor would be more extensively pyrolysed at

Fig. 2. SliM of .i cro~s—~ection of an I n.\s l.ivcr grown froni ar~inc.

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220

~20 a

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E16

~16

0 12 E

12 N N

N N

8

8

• U

4 450

•

4 500

550

600

Subatrate temperaiure/’C

Fig. 3. 77 K mobility data for InAs grown from TEAs at different temperatures.

lower temperatures, a temperature range of 475—550°Cwas investigated. For the first time, smooth layers could be obtained. The region over which satisfactory growth could be achieved was, however, very narrow. As shown in table 1 and fig. 3, polycrystalline material was obtained at 475 °C and the mobility of the material drops

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10

V/Ill ratio

Fig. 5. Change in 77 K mobibty with V/Ill ratio using TEAs.

quickly between the peak at 500°C and 550°C. Again, all the layers grown had n-type conduction. For the first time, surfaces which appeared shiny to the naked eye were obtained at 500—525 °C. The surface becomes hazy at 550°C and noticeably rough at 475 °C. Fig. 4 is an SEM micrograph of a layer grown at 500°C and shows the great improvement over that represented in fig. 2. There

Fig. 4. SEM of a cross-section of an InAs layer grown from tertiarybutylarsine.

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amount of pyrolysis of the alkyl from an observation of the appearance of an arsenic deposit in the rear portion of a clean reactor tube, indicated a very similar behaviour to arsine. The degree of pyrolysis at 5000 C will therefore be expected to be small and this was confirmed from the mobilities offorlayers at this temperature. The resuits these grown were very similar to those obtained

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with pure arsine. I

~12

2

U

I 4. Discussion

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I 4 0

100

200

300

Temperature/K

Fig. 6. Variability of Hall mobility with temperature: grown from tertiarybutylarsine; (•) grown from arsine.

(E)

is a complete absence of the defects found in the previous case, Fig. 5 illustrated that there is also a very rapid change in the mobility with a change in the V/Ill ratio. A value around 5: 1 is the optimum at a growth temperature of 500 °C. The best mobility at 77 K was nearly 30,000 cm2/V’ s, with a carrier concentration of I x 3, which is noticea10i6 cmfrom material grown bly better than was achieved from arsine at this temperature. At 550°C, the difference in the material becomes less apparent. Fig. 6 is a temperature versus Hall mobility plot for InAs layers grown onto GaAs substrates using (a) TBAs and (b) AsH 3. The plot is very similar to data from MBE layers [40] in that the mobility does not improve below 30 K. It should be noted that the mobility and general quality of the best TBAs grown material is far better than the InAs grown from arsine, and begins to compare with MBE grown InAs. 3.3. Growth from phenylarsine

A few trial experiments were tried with this alternative Group V source, using an early preproduction sample provided by Epichem Ltd. This precursor proved to be somewhat disappointing for the growth of InAs. Visual observations of the

As indicated above, analysis of electrical data of heteroepitaxial layers can be complicated for very thin layers [39] so the majority of layers mentioned above are at least 3 ~tm thick. An increase in the thickness is well known to produce an increase in the measured mobility (see, for example. for MBE data ref. [91and for PAE data ref. [25]). This is presumed to be as a result of a reduced effect of the dislocations as the thickness is increased [14]. Our data agree with this: for samples grown from arsine onto InP at 600 °C with a fixed V/Ill ratio of 28: 1 showed an improvement in the mobility at 77 K from 6670 to 10,876 cm2/V s for an increase of layer thickness from 0.8 to 1.5 ~tm. The much improved electrical results recorded on the samples grown from TBAs show that this precursor is potentially capable of producing material which is closer to that grown by MBE in terms of the mobility data. In fact, the layers are slightly better than some recently reported MBE data for heteroepitaxial layers of InAs grown onto GaAs substrates [9] and it is interesting to see that these workers achieved a great improvement in the material by utilising a strained layer superlattice of InGaAs/GaAs at the interface, presumably by reducing the threading dislocations into the epilayer. We intend to see if this is the case in MOVPE material in the near future. The ability to grow good material with a V/Ill ratio as low as 5 : I indicates the much greater pyrolysis efficiency of the TBAs over the temperature range considered. The rapid change in electrical quality (although remaining n-type) with a change in the V/Ill ratio is indicative that this

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variable must be very carefully controlled as well, It should be noted that we have seen no cvidence of any prereaction of the trimethylindium with either tertiarybutylarsine or phenylarsine as the entrance to the reactor has remained clean throughout growth runs. However, in the case of trimethylindium and arsine, a light brown deposit is apparent during the growth in this inlet region. As we have eliminated recirculation in the entrance region in this reactor design [37] it is assumed that this deposit is a result of the formation of an adduct between the reactants. A similar situation has recently been reported in the reaction between triethylindium and arsine [38]. It is probable that the much bulkier tertiarybutylarsine or phenylarsine cannot form a stable adduct with the trimethylindium and therefore no deposit is observed. This indicates that the use of TBAs or PhAs could have further advantage in avoiding parasitic side reactions during growth.

therefore extremely encouraging to note that we have recently grown a number of InAs samples from a different, later, batch of TBAs and the results are broadly similar. The maximum mobility recorded at 77 K has increased slightly to 2/V s and the optimum V/Ill ratio > 35,000 cm has reduced to closer to 1: I at 500 °C. We are presently investigating the growth of the GaSb/InAs heterojunction with TBAs and the results will be reported elsewhere.

5. Conclusions

References

The data presented above show that a considerable improvement in the quality of heteroepitaxial InAs on GaAs can be achieved with the use of the new Group V precursor, tertiarybutylarsine. The improved pyrolysis of this compound at lower temperatures allows the growth of good InAs at 500 °C, both from the point of view of electrical and surface quality. It would appear that this compound shows great promise, particularly as it is expected that the background impurity levels can be further reduced in future samples. The material used here was known to contain sulphur as an impurity and steps have been taken to improve future batches of TBAs from the impurity point of view [41]. The data show that the InAs appears more reproducible than when grown with arsine sources. Phenylarsine proved to have no advantages over arsine for relatively low temperature growth. Another possibility, trimethylarsenic, was not investigated here as this would be cxpected to be even less pyrolysed at a low temperature due to the extra stability of this molecule. It should be noted that all the TBAs data reported above was from one sample batch. It is

Acknowledgements The authors wish to acknowledge the financial support of the SERC, technical assistance from Mr. D. Morris, the provision of a trial sample of phenylarsine from Epichem Ltd., and the provision of the SEM data from Dr. G. Hill, SERC Central Facility (University of Sheffield).

[1] DI.

Smith and C. Mailhiot, J. Appi. Phys. 62 (1987) 2545. [2] C.G. M.Y.Letters Yen, B.F. Levine, K.K. Choi and A.Y. Cho. Bethea, AppI Phys. 51(1987)1431. [31D.T. Cheung, AM. Andrews. ER. Gertner. G.M. Wilhams, J.E. Clarke, J.G. Pasko and J.T. Longo, AppI. Phys. Letters 30 (1977) 587. [4] H. Sakaki. L.L. Chang, R. Ludeke, C. Chang, GA. Halasz and L. Esaki. AppI. Phys. Letters 31(1977) 211. [51L.O. Bubulac, A.M Andrews, E.R Gertner and D.T Cheung, AppI. Phys. Letters 36 (1980) 734. [6] R.K. Reich and D.K. Ferry, IEEE Trans. Electron. Dcvices ED-27 (1980) 1062. [71S.K. Haywood. A.B. Henriques, D.F. Howell, N.J. Mason, R.J. Nicholas and P.J. Walker. in: Proc. 14th Intern. Symp. on GaAs and Related Compounds, Herakhion, Crete, 1987. Inst. Phys. Conf. Ser. 91, Eds. A. Christou and H.S. Rupprecht (Inst. Phys., London—Bristol, 1988) p. 271. [81L.L. Chang and L. Esaki, Surface Sci. 98 (1980) 70. [91S. Kalem, J. Chyi, H. Morkoc, R. Bean and K. Zanio, AppI. Phys. Letters 53 (1988) 1647. [10] M. Yano, M. Nogami, Y. Matsushima and M. Kimata, Japan. J. Appi. Phys. 16 (1977) 2131. [11] B.T. Meggitt, E.C.H. Parker and R.M. King, Appl. Phys. Letters 33 (1978) 528. [12] J.D. Grange, E.H.C. Parker and R.M. King, J. Phys. D12 (1979) 1601 [13] CA. Chang, CM. Cerrano, L.L. Chang and L. Esaki, J. Vacuum Sci. Technol. 17 (1979) 603.

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S. K. Haywood et at.

/ Growth of JnA.s by MO VPE

[14] CA. Chang, CM. Serrano, L.L. Chang and L. Esaki. Appi. Phys. Letters 37 (1980) 538. [15] H. Munekata, A. Segmuller and L.L. Chang, Appl. Phys. Letters 51(1987) 587. [16] H. Munekata, L.L. Chang, S.C. Woronick and Y.H. Kao, J. Crystal Growth 81(1987) 237. [17] F. Houzay, C. Guille, J.M. Moison, P. Hence and F. Barthe, J. Crystal Growth 81(1987) 67. [18] B.J. Bahiga and S.K. Ghandhi, J. Electrochem. Soc. 121 (1643) 1974. [19] B.J. Bahiga and S.K. Ghandhi. J. Electrochem. Soc. 121 (1946) 1974. [20] H.M. Manasevit and WI. Simpson. J. Electrochem. Soc. 120 (1973) 135. [21] K.J. Montserrat, J.N. Tonhill, J. Haigh, RH. Moss, CS. Baxter and W.M. Stobbs. J. Crystal Growth 93 (1988) 466. [22] T. Fukui and Y. Horikoshi, Japan. J. Appl. Phys. 18 (1979) 2157. [23] T. Fukui and H. Saito, Japan. J. AppI. Phys. 24 (1985) L774. [24] T. Fukui, J. Crystal Growth 93 (1988) 301. [25] T. Hariu, S.F. Fang. K. Shida and K. Matsushita, in: Proc. 13th Intern. Symp. on GaAs and Related Cornpounds, Las Vegas, NV, 1986, Inst. Phys. Conf. Ser. 83, Ed. W.T. Lindley (Inst. Phys., London—Bristol, 1987) p. 165. [26] S.F. Fang, K. Matsushita and T. Hariu, Appl. Phys. Letters 54 (1989) 1338.

[27] MA. Tischler and SM. Bedair. AppI. Phys. Letters 49 (1986) 274. [28] H. Ohno. S. Ohtsuka. A. Ohuchi, T. Matsuhara and H. Hasegawa, J. Crystal Growth 93 (1988) 342. [29] P.K. York. C.J. Kiely, G.E. Fernandez. J.N. Baillargeon and J.J. Coleman, J. Crystal Growth 93 (1988) 512. [30] M. Sato and Y. Horikoshi. J. Crystal Growth 93 (1988) 936. [31] S.P. DenBaars. BY. Maa. P.D. Dapkus, AD. Danner and H.C. Lee. J. Crystal Growth 77 (1986) 188. [32] GB. Stringfellow, J. Electron. Mater. 17 (1988) 327. [33] C.H. Chang, CA. Larsen and GB. Stringfellow, AppI. Phys. Letters 50 (1987) 218. [34] A. Brauers, 0. Kayser, R. KahI, H. Heinecke, P. Balk and H. Hofmann. J. Crystal Growth 93 (1988) 7. [35] S.K. Haywood. A.B. Henriques, N.J. Mason, R.J. Nicholas and P.J. Walker, Semicond. Sci. Technol. 3 (1988) 315. [36] S.K. Haywood, N.J. Mason and P.J. Walker, J. Crystal Growth 93 (1988) 56. [37] C. Goodings, N.J. Mason, P.J. Salker and D. Jebh, 1. Crystal Growth 96 (1989) 13. [38] PD. Agnello and S.K. Ghandhi, J. Electrochem. Soc. 135 (1988) 1530. [39] A.H. Moore, M.D. Scott. J.I. Davies, D.C. Bradley, MM. Faktor and H. Chudzynska. J. Crystal Growth 77 (1986) 19. [40] B.T. Meggitt, E.C.H. Parker, R.M. King and J.D. Grange. J. Crystal Growth 50 (1980) 538. [41] Cyanamid (UK), personal communication.