Eutectic formation in chromium-doped indium phosphide

Journal

of Crystal

Growth 21 (1974) 117-124

EUTECTIC

0 North-Holland

FORMATION

B. W. STRAUGHAN,

K. LLOYD

Great Maluern,

7 May 1973; revised

Co.

IN CHROMIUM-DOPED

D. T. J. HURLE,

Royal Radar Establishment, Received

Publishing

manuscript

Worm. received

INDIUM

PHOSPHIDE

and J. B. MULLIN WRl4

3PS,

13 September

U.K. 1973

The occurrence of second phase separation in chromium-doped indium phosphide is reported. The phenomenon has been studied in crystals grown by the high pressure Liquid Encapsulation Technique. The second phase was examined by infra-red microscopy, X-ray micronanalysis and scanning electron microscopy. X-ray diffraction patterns of the second phase were obtained after the host material had been chemically removed. On this evidence the second phase has been identified as eutectic needles of chromium phosphide (CrP). The needles showed a high degree of short range order and a general alignment almost parallel to the growth axis.

parameters were a pull rate of 2 cm/hr and a rotation rate of 10 rpm. The crystals can be made semi-insulating by doping their melts with N 0.1 wt’/, chromium. Good quality single phase material with resistivity generally in the range 103-lo4 ohm cm occurs in the first 50 y/, to 90 “/: of the grown crystals. However, towards the bottoms of the crystals as the chromium concentration builds up in the melt (k,,, <: I), a second phase separates out in the solid. This effect can be enhanced by increasing the Cr doping level or by growing the crystals at a faster rate.

1. Introduction Chromium-doped indium phosphide when suitably prepared is semi-insulating. In single crystal form it is of considerable value as a substrate material for assessing the electrical properties of device quality layers grown onto it by vapour epitaxial techniques’). However, crystals of the substrate material when grown under certain conditions by the high pressure Liquid Encapsulation Technique’) have been shown to contain a second phase. In this paper we report evidence of this phase separation and conclude that the second phase forms by a eutectic reaction. Eutectic structures involving the formation of heavy metal phases in A”‘B” compounds have been prepared by Miller and Wilhelm3,4) and by Reiss and Renner’). A review paper by Galasso’) covers the preparation and properties of these and other unidirectionally solidified eutectics. Phase separation in Cr-doped semiinsulating gallium arsenide has been reported by Gorelik et al.7) and recently in boat-grown indium phosphide by Starocel’tseva et aL8), but in neither case were eutectic structures reported.

2.2.

METALLOGRAPHIC EXAMINATION

The effects of the second phase were first observed during the polishing stage of slice preparation using 2% Br2 in CH,OH. The regions of the crystal which contain the second phase are shown in fig. 1. Results from this crystal and others indicate that the proportion

I

GROi’fTH

AXIS


P

2. Experimental 2.1.

CRYSTAL GROWTH

The present study has been carried out on indium phosphide crystals pulled using [ 11 I] orientated seeds. Effects similar to those found in this study had also been observed in [IOO] grown crystals. Typical growth

REGION SECOND

CONTAINING PHASE

diagram of crystal showing the Fig. I. Schematic second phase separation and the convex boundary.

117

region

of

118

Fig.

B. W. STRAUGHAN,

2.

Micrograph

second phase (which

of

the

(I

I I)

is responsible

P surface

II. T. J. HURLE.

etched

in 2”,,

for the poor surface

K. LLOYD

bromine/methanol

AND

solution

J. H. MULLIN

showing

the region

uhich

contains

the

finish).

Fig. 3. Infra-red transmission micrographs of the sample in fig. 2. The micrographs are taken at the boundary of the polished and pitted material: (a) the rods emerging at the small angle to the surface: (h) the rods emerging normal to the surface.

of the crystal containing the second phase varied between IO ‘:, and 35”; for Cr doping levels between 0.06”4 and 0.13”,; (wt”; in the melt). Using suitable

chemical etchants”), growth striae were developed and revealed that the growth interface was essentially flat during the growth of the crystal, whereas the boundary

EUTECTIC

FORMATION

IN

CHROMIUM-DOPED

INDIUM

PHOSPHIDE

119

Fig. 4. Infra-red transmission micrographs of a vertical (1 IO) slice from the same crystal as fig. 2. The boundary between material with and without eutectic is quite distinct. The [I 111 growth direction is indicated by the arrow. (a) Shows the “arrowhead” branching; (b) shows an increased density of eutectic rods in one plane and another series of rods out of focus in a lower plane.

delineating the region of phase separation was quite convex (fig. 1). This significant observation suggests that there existed a marked radial concentration gradient of Cr in the melt during growth; this would be in addition to the one normal to the interface. Fig. 2 is a micrograph which shows a typical surface etching feature on a slice cut normal to the growth axis. The slice was taken at a point where 75 % of the crystal had grown. The poorly-etched surface around the periphery marks the boundary of the second phase. Traces of growth striae are visible but there are no obvious effects due to natural growth facets. Slices taken lower down the crystal show a gradual spread of the second phase across the section. An infra-red transmission micrograph of a region in the same slice of InP, is shown in fig. 3. Using IR microscopy it is possible to study the morphology and orientation of the second phase. Fig. 3a shows a rodlike structure emerging at a small angle to the surface normal; in fig. 3b the structure is emerging normal to the surface. Figs. 4a and 4b were obtained from a vertical slice having a (1 IO) surface. The plane of the micrographs contains the growth axis, the direction

being indicated by an arrow. The micrographs reveal that the eutectic rods have a high degree of short range order. The boundary between material with and without eutectic is very distinct and marked banding is evident. The rods clearly act as nucleation sources and give rise to branching during the development of the structure. This gives a characteristic multiple arrowhead-like appearance. The simultaneous termination of many rods occurs presumably as a result of a rising temperature fluctuation. The steeply inclined boundary between eutectic and non-eutetic regions is shown, in fig. 4a, by the large angle which the boundary makes to the plane of the banding (which is the plane of the solid-liquid interface). The eutectic rods were aligned to within 25”-30” of the growth axis in the crystal so far discussed, in similar [ 11 I] and [IOO] crystals and in non-single ingots. The grain boundaries did not affect the rod alignment. However, in another crystal where a cellular structure was formed due to constitutional supercooling, the rod alignment was dominated by the cellular interface morphology. Fig. 5 is an example of an etched (I 11) P surface of a sample cut normal to the growth direction

120

R. W. SI‘RAUGHAN.

1). ‘I-. J. HURL&

Fig. 5. Etched (I I I) P surfacc uith cellular structure. The been removed. The cell walls are aligned parallel to I IO

Fig. 6. (arrowed

An infra-red transmission “a” in fig. 5).

micrograph

hole

of two cell walls

K. LLOYD

in the centre

AND

is wbcrc

J. H. MULLIN

an clcctrical

me3surcments

Fig. 7. An infra-red transmission micrograph section of cell walls (arrowed “b” in fig. 5).

5amplc

of

an

had

intcr-

EUTECTIC

FORMATION

IN

CHROMIUM-DOPED

INDIUM

PHOSPHIDE

121

(cl Fig. 8.

Scanning electron micrographs

of eutectic rods isolated by etching away some of the matrix material: (a) a group of rods; (b) X-ray micrograph of the phosphorus content; (c) X-ray micrograph of the chromium content.

of the crystal (see discussion). The arrangement of the rods in the region of the intersection of a pair of linear boundaries is shown in fig. 7. 2.3.

(b) The cellular structure was developed by increasing the growth rate of the crystal. The boundaries (cell walls) are aligned parallel to (I IO). An IR transmission micrograph of part of two of these boundaries (fig. 6) reveals that the cell walls are in fact composed of eutectic rods. There is no evidence of any other gross microsegregation such as solute trialslo) which would indicate that a liquid such as liquid In had been trapped during the formation of the cell walls. The eutectic rods are inclined away from the centre from

this crystal.

COMPOSITION

OF THE EUTECTIC

An etch of composition 6 H,0/2 HNO,/I HCI was used to extract the eutectic rods from the indium phosphide matrix. This gave sufficient material to produce an X-ray powder diffraction pattern. A number of chromium/phosphorus structures are given in the literature1’-‘3 ) but orthorhombic CrP (a = 5.362 A, b = 3.1 13 A, c = 6.018 A) quoted by Rundqvist14) gave the best fit to the diffraction data obtained from the eutectic rods. In addition a quantity of chromium monophosphide (CrP) was obtained from a commercial source (BDH Ltd., Poole, Dorset, U.K.) and the powder pattern of this material accurately matched the pattern due to the eutectic rods. Further evidence for the composition is shown in fig. 8. These micrographs have been obtained using the scanning electron microscope and the specimen has eutectic rods emerging from the surface after some of the matrix material had been etched away. In the composite picture, fig. 8a is the usual SEM micrograph of a group of rods, fig. 8b is an X-ray micrograph of the

STRAUGHAN,

B. W.

122

D.T.

J. HURLE,

phosphorus content of the same rods and fig. 8c is an X-ray micrograph of the chromium content. These X-ray counts have been calibrated and givea chromium/ phosphorus atomic ratio of approximately I : I. 2.4.

CHROMIUM

The analysis

CONCENTRATION

IN THE CRYSTALS

of the chromium

concentration

in these

crystals was made by a mass spectrometric technique using AEI MS7 equipment. The aim was to determine the segregation coefficient of chromium in InP. The analytical results are summarised in table I. The results TABLE Analysis Crystal No.

1

of Cr-doped

InP crystals

Cr concentration

Specimen position*

(puma)

L97

0.25 (centre) 0.25 (edge) 0.43 (centrc) 0.43 (edge) 0.70 (centre) 0.70 (edge) Initial melt

L298

0.10 (centre) 0.20 (centre) 0.85 (centrc) 0.85 (edge) Initial melt

0.3-0.4 0.3-0.4 0.4 0.4 30 looo”* 2200

Mean value

Effective distribution coefficient?

(I .0-l .4)

10-A lo-+ 1

(1.0~ 1.4)

I .o I .o

10-AI lo-4’ r I I

6.4 2.9

IOVA loyal

10-1

I

I .o 0.5 30 70 I400

1 ’ 4.7

10-i

J

* Slice position represented by mass position in slice. i Assuming normal frecre segregation ** Visual evidence of eutectic.

fraction

of crystal

grown;

( ) specimen

indicate

that

single

phase

formation chromium

behaviour.

the

eflective distribution coefficient for semi-insulating InP is 3 x IO-“. The

of the eutectic produced a sharp rise in the concentration (- 1000 ppma) in the crystal.

3. Discussion The inference drawn in section be a marked radial non-uniformity

2.2. that there must in Cr concentration

is not

mass

wholly

substantiated

by the

spectrographic

results in the table. Whilst a radial variation of 30 : 70 is is evident in L298 at 85”(, grown, no such variation apparent in L97 at 25 ‘I{,and 43 “,; grown. The precision of the measurements is believed to be better than * 50”;; the accuracy was better than f loo”,,. The precision quoted implies that any radial gradient in L97 must be less than 1: 3.

K. LLOYD

AND

J. B. MULLIN

Such radial non-uniformity in chromium concentration as exists is presumably a result of pool mixing conditions in the melt. It arises from the restriction to slow rotation rates imposed by the presence of the boric oxide encapsulant. Examination of table 1 suggests a marked increase in Cr concentration (from 1 ppma to 30-70 ppma) immediately prior to the visible occurrence of a eutectic structure. The reason for this is not clear but we note that a similar, although smaller, increase in /Ccrrfor Cr in GaAs has been observed by Gimel’farb et al.“). Correlating infra-red examination with the mass spectrometric results leads us to conclude that the eutectic forms (in the absence of a cellular structure) when the melt concentration exceeds 0.9 i 0.2 at yb CI (corresponding to a solidus concentration of - 70 ppma). The banding of the eutectic must be due to temperature fluctuations caused either by the crystal rotation or by non-steady convective motion in the meltl”). From fig. 4, the mean spacing of the bands is - 130 pm which, with the growth rate of 3. I cm hr- ‘. corresponds to a time spacing of - I.5 sec. This compares with a crystal rotation time of - 6 set and suggests that the banding is due to convective temperature fluctuations rather than to the crystal rotation. The branching of the eutectic rods occurs during the periods of acceleration of the sol&liquid interface in order to minimise the lateral diffusion path. It is well known”) that in the steady state i2c = constant, where i. is the rod spacing and c is the growth rate. The sudden termination of some rods in fig. 4a gives clear evidence that melting back has occurred during a part of the temperature oscillation cycle. The periodic variation in the thickness of the rods can be seen tig. 8a. The cellular structure shown in fig. 5 is very characteristic of growth. in the [I I I] direction, under conditions of constitutional supercooling. of zincblende and diamond cubic crystals when the growth surface is convex”). The structure is composed of [I I I ) facets. Once a cellular structure has formed all components of the melt which are rejected at the solidliquid interface tend to segregate into the cell boundaries. It is this fact which leads to the formation of the eutectic CrP in the cell boundaries. The cellular structure is due to constitutional supercooling produced

EUTECTIC

FORMATION

IN

CHROMIUM-DOPED

INDIUM

either by the rejected Cr, or by rejected excess In in the melti7). However, the latter would be expected to lead to deep cell-boundary grooves with trapped In droplets at the roots as has been observed in InSb 17). Deep grooves result from the inability of excess In to diffuse back into the bulk melt. By contrast, Cr can readily be removed from the cell boundary by the formation of the CrP eutectic phase so that, in this case, deep cell boundary grooves will not form. The micrographs show no evidence of trapped In or of deep cell boundary grooves strongly suggesting that the cell structure is due to the rejection of the chromium. Similar effects in the InSb-NiSb eutectic have been reported by Hurle and Hunt’ 5). The absence of any evidence of In inclusions is perhaps a little surprising. It is common to find conditions of excess In near to the ends of pulled InP crystals. We must assume that the amount of excess In was sufficiently small under the conditions examined for it to be able to be rejected into the melt. We can estimate the gradient of constitutional supercooling produced by the rejected chromium from the relation18): muC,( I - k)k,,,

ds dx i = (-1

Dk

dT, ( dx ) i’

where m = liquidus slope, C, = melt concentration, D = diffusion coefficient of Cr in the liquid, k, k,,, = distribution and effective distribution coefficients of Cr, and (dT,/dx), = temperature gradient in the liquid at the interface. If (ds/d.u)i > 0 one can expect a cellular structure to form”). The following estimated values are used: D = 5 x IO-’ cm2 set-‘, (dTL/ds)i = 100 K cm--’ and form, the value for an ideal dilute solution II? = - RT,,,/ As, where As, is the entropy of fusion of InP, T,, its melting point and R is the gas constant. With T,, = 1343 K and As, = 14.7 eu 20), k,,,/k - 1, v = 4.5 cm hr-l and C, = 1.6 mole”/, CrP (the concentration in the melt immediately prior to the formation of a cellular structure), one obtains: ds dx i (-1

= 72-100

N 0 K cm-‘.

Taking account of the fact that our ideal solution estimate for IMprobably gives significantly too low a value,

PHOSPHIDE

123

EUTECTIC Rcos

4

(Ill )P MELT

showing the Fig. 9. Eutectic rod growth in cellular boundaries { 111) facets, the cellular structure and the rods inclined away from the centre of the crystal.

the calculation indicates that Cr rejection could well be the cause of the formation of the cellular structure. If the eutectic rods occur only in the cell boundaries, they will no longer by aligned in the growth direction, but would be expected to be aligned approximately along the bisectrix of the pair of facets forming the cellular structure. Thus the rods should make an angle of - 35” to the growth axis on three sides of the hexagonal pattern shown in fig. 5, and an angle of N 55” on the remaining three sides. (See angles 0, and 0, respectively on fig. 9.) The experimental observations are in broad agreement with these expectations. 4. Conclusions A second phase in the form of eutectic rods of composition CrP is present in semi-insulating chromium-doped indium phosphide crystals. In the crystals there are radial and axial chromium concentration gradients and when the level of chromium reaches - 70 ppma the eutectic forms. The eutectic is aligned fairly closely to the crystal growth axis unless the growth rate is sufficiently high to cause constitutional supercooling. The segregation coefficient of Cr in InP is 3 x 10e4. Acknowledgments The authors gratefully acknowledge the assistance of Dr. P. J. Tufton and Mrs. D. S. Kemp on the growth of the crystals and the etching experiments respectively. The valuable suggestions of Dr. W. Bardsley and skilled help of Mr. D. G. Coates are much appreciated. Contributed by permission of the Director of RRE. Copyright controller HBMSO.

124

H. W.

STRAUGHAN.

D. T. J. HURLE,

References I) R. C. Clarke,

B. D. Joyce and W. H. E. Wilgoss. Solid State Commun. 8 (1970) 112.5. 2) J. B. Mullin, R. J. Heritage, C. H. Holliday and B. W. Straughan, J. Crystal Growth 3/4 C1968) 28 I. 3) A. Miiller and M. Wilhelm, J. Phys. Chem. Solids 26 (1965) 2021. 4) A. Miiller and M. Wilhelm. J. Phys. Chem. Solids 22 (1965) 2029. 5) B. Reiss and Th. Renner, Z. Naturforsch. 21a (1966) 546. 6) F. S. Cinlasso, J. Metals 19 (1967) 17; 22 (1970) 40. 7) S. S. Gorelik. V. A. Mitonenko, Yu. M. Litvinov and Yu. M. Ukrdinskii, Soviet Phys.-Cryst. 15 (1971) 952. 8) S. P. Starocel’tseva, T. T. Dedegkaev, V. S. Kulov and S. G. Metreveli, Izv. Akad. Nauk SSSR. Neorg. Mater. 8 (1972) 1491. 9) J. B. Mullin, A. Royle and B. W. Straughan, in: Proc,. /~ltw,r. ~~~/,I/~. 011 Gtrllirrw Ar.w/li&, Aachen, 1970 (Inst. of Physics and Phys. Sot.. London) p. 41. IO) W. Bardsley, J. S. Boulton and D. T. J. Hurle, Solid State Electron. 5 (1962) 395.

K. LLOYD

AND

J. R. MULLIN

I I) M. Hansen. C‘onstitrrtion of Bintrr.~, A//~,~~J (McCrawHill. Neb York. 1958) 2nd ed., p. 548. 12) R. P. Elliot. Comtitutiwt of'Biwrr_r Allo~~s(McGrawHill. New York, 1965) 1st Suppl.. p. 354. 13) W. Jcitschko and P. C. Donohue, Acta Cryst. B 28 (1972) 1893. 14) S. Rundqvist. Acta Chem. Stand. 16 (1962) 187. 15) D. T. J. Hurle and J. D. Hunt, Iron and Steel Inst. Publ. 110, (1968) 162. 16) K. A. Jackson and J. D. Hunt. Trans. AIME 236 (1960) 1129. 17) D. T. J. Hurle, 0. Jones and J. B. Mullin. Solid State Electron. 3 (1961) 317. IX) D. T. J. Hurle, Solid State Electron. 3 (1961) 37. 19) W. Bardsley, J. M. Callan, H. A. Chedzey and D. T. J. Hurle. Solid State Electron. 3 (1961) 142. 20) N. N. Sirota, in: S~~nico~~rl~rctor.s (ltzdSemi-.Mctuls, Vol. 4. Eds. Willardson and Beer (Academic Press, New York, 1968) p. 35. 21) F. A. Gimcl’farb. B. G. Girich, M. G. Mil’vidskii, 0. V. Pelevin and V. I. Fistul, Soviet Phys -Solid State 11 (1970) 1612.