Vapour epitaxial growth and characterization of InAs1-xPx

Journal of Crystal Growl/i 17 (1972) 173—182

North-Holland Publishing Co.

VAPOUR EPITAXIAL GROWTH AND CHARACTERIZATION OF InAs1 ~ J. HALLAIS, C. SCHEMALI and F. FABRE Lahoratoires d~Electroniqueet de Physique Appliquée, 3, Arenue Descartes, 94

—

Limeil-Brévannes, France

The vapour growth of EnAs1 ~ layers has been carried out by the hydride process. The phosphorus rich part of the system (0.7 + x ~ 1) was especially investigated. Heteroepitaxial deposits of 1nAs1~P~and lnP have been performed on substrates such as InAs, GaAs and GaP. A systematic study of the influence of the substrate Orientation on the quality of the layer has been carried out by growth on hemispherical substrates. Preferential planes have been pointed out: (100) and (lii) A for InAs, (Ill) for GaAs and GaP. The band gap variation as a function of the composition has been determined byphotoluminescence at2 4.24.2 at ~K K. and X-ray diffraction measurements. It fits the equation: E~(x)eV = 0.425+0.722 x+0.273 .v

1. Introduction

increasing interest is appearing for transmission-mode photoemission 3). These semi-transparent devices need a heteroepitaxial deposit of the ternary compound on a substrate which may or may not be a semiconductor. The difficulty with the heteroepitaxial growth comes from the lattice mismatch between the layer and the substrate, which is responsible for a great number of defects at the interface. A systematic investigation of various substrates (InAs, GaAs, GaP) with different

The possibility of obtaining high quantum efficiency for the 111—V semiconductor photocathodes1) has stimulated research activity in the field of low band-gap compounds in order to improve the performance in the near-infrared2). Different ternary systems have been investigated and band-gap values as low as I eV can be expected from the phosphorus rich part (0.7 < x < 1) of the JnAs 1 P. solid solution. On the other hand, an

orientations has been carried out and the crystalline

hydrides cracking —_—~..vent

I

H2

PH3

Zn

reaction

mixing

deposition

chloride cracking

ASH3

L..

thermostated bubbler .~.....

H2

A sC13 Fig. I.

Vapour phase epitaxy reactor for the growth of lnAs1 ~

173

174

J. HALLAIS, C. SCI-IEMALI AND E. FABRE

ii]

[1~i] ~ i]

ln( As, P1/ In As

[iii]

_____

1o 0

____

[iii] I n(As, P)/GaAs

Fig. 2a. Fig. 2.

Growth on hemispherical substrates. (a) InAsandGaAs; (b) GaP. Magnification 300>~.

VAPOUR EPITAXIAL

GROWTH AND CHARACTERIZATION

OF

lnAs15P~

175

Cool]

~[i~i]

/“~~

/ (001)

Fig. 2b.

Scanning electron microscope

quality has been observed by X-ray diffraction and reflection topography, and with a scanning electron microscope. The electrical and been optical properties of these heteroepitaxial layers have compared to the previously reported results,

—

back scattered electrons, In(As,P)/GaP.

2. Experimental

800—820 °C,and the seed temperature is approximately 650—700 ~C.the Thep-type residual doping is n-type 1016 3) and doping is assured by (n hydrogen cm on thermostated zinc. Layers of the pure lnP have flow also been prepared by the phosphorus trichloride method5). The reactor is basically the same as those normally used for GaAs growth6).

2. I.

2.2.

GROWTH TECHNIQUES

The hydride method (arsine and phosphine) used for the growth of InAs ~ has been previously 4). The 1experimental apparatus is described shown in by Tietjen fig. 1. It is possible to do an “in situ” etching prior to the growth. The hydrogen chloride used for transport or etching reactions is supplied by thermal craking of arsenic trichloride. The chemical composition and its reproducibility are monitored by adjustable flowcontrollers. The indium source temperature is about —

‘—j

GROWTH AND PERFECTION OF CRYSTALS

The transmission mode photoemission implies a thin layer, the thickness which must be diffusion about thelength; order of magnitude of the of minority carrier furthermore, this layer must be deposited onto a substrate which is transparent for the wavelength of the incident photon. These two conditions demand heteroepitaxy without a thick graded layer which could make the number of defects lower in the constant composition layer, as for instance in the case of the heteroepitaxial

176

J. HALLAIS

Scanning electron microscope

—

C. SCHEMALI

back scattered electrons

X-ray reflection topography

Scanning electron microscope Fig. 3a. Fig. 3.

AND F. FABRE

—

infrared cathodoluminescence

1nAsi_~P~/InAs(I00).

Growth on large area substrates. (a) InAs (100); (b) InAs (Ill) A; Cc) GaAs (111) A; (d) GaP (Ill) A and B.

VAPOUR EPITAXIAL

—~

GROWTH

~

JI

_i

— ~

OF

-

T~ ~ ~

AND CHARACTERIZATION

lnAs1 ~ /

‘

—

~

‘

-

-

_1- ~

~

177

~

~

.

* _________

-

.

-

-~

C

z~~

-

.

I S

100pm

_____________________ ____________________________

Scanning electron microscope

back scallcred electrons Fig. 3b.

‘‘I

X—ray reflection topography

InAs1_~P.jInAs(l1 l)A.

7). In order to determine growth of Ga(As, P) on GaAs the growth planes giving the best crystalline qualities (e.g. mirror smooth surface), epitaxial layers were grown onto hemispherical substrates. These 5 mm diameter samples are prepared by mechanically polishing cubes to a hemisphere, and are chemically etched before the introduction into the reactor. The (001) plane has been chosen as the basis plane of the hemisphere in order to show up the principal directions of interest: [001], [101], [1111A and [III] B. Facets appear for the slower growth planes, the faster ones having grown out8’9). This way a classification of the directions according to the crystalline state of the deposit is possible. Allen 8) has shown that this classification was nearly independent of the chemical composition in the range 0.7 < x < 1 and of the growth conditions: e.g., the element 111 to the element V gaseous flow ratio. Figs. 2a and 2b show the results obtained on InAs, GaAs and GaP hemispheres. In the case of InAs substrate all the simple directions exhibit facets but only one facet (Ill) was noticeable in the case of GaAs. The polarity of this direction will be determined by growing on plane substrates. In a GaP hemisphere, the (001), (Ill) A and (lii) B poles can be distinguished. The differentiation between the two directions A and B was obtained withtheX-ray anomalousdiffusion method10).

Only the (111) B facet is smooth and gives large area steps; in the [lll]A direction the deposit is made of small triangular steps corresponding to a faster growing plane; in the [001] direction, the facet is not well defined. No other identifiable direction has been seen on a GaP hemisphere. Because of the dimension of the facets, it is not possible in these experiments to make a complete observation of the crystalline quality of the layer and of the interface; they have to be supplemented by large area growth on plane substrates. The slices can then be observed by X-ray reflection topography (X-ray topo.) and scanning electron microscope (S.E.M.) with the different detection modes: secondary emission (s.c.) back scattered electrons (b.s.e.), infrared cathodoluminescence (c.l.). Figs. 3a and 3b show the results obtained on InAs substrates, respectively oriented (100) 3 off in [110] direction, and (111) off in [110] direction. For both planes the surfaces are mirror smooth, but a preferential orientation of the defects appears in the case of fig. 3b. The defects fall into lines parallel to the [101], [110], [011] directions, similar to sets of misfit dislocations7). Epitaxial growth on lightly disoriented (I1I)A and B GaAs have shown that a mirror smooth surface appears only on the (111) A plane (fig. 3c). These results 30

178

Scanning electron microscope

J. HALLAIS,

—

C. SCHEMALI

AND E. FABRE

back scattered electrons

Scanning electron microscope

X-ray reflection topography

Fig. 3c.

—

1nAs

infrared cathodoluminescence

1.~P5/GaAs(l1 l)A.

VAPOUR EPITAXIAL

~

GROWTH AND CHARACTERIZATION

OF

lnAs1~P~

179

______

Gal’i I II IA: scannine electron microscope

-

secondars emission

. Gal I II I I B:

. scan 111119

dccl ron

microscope

hack

sLat lered

electrons

GaI’(l II )B: scanning electron microscope trons; (110) cleavage plane Fig. 3d.

—

hack scattered elec.

lnAs1~P~/GaP.

are in8).agreement previously published data of Infrared with cathodoluminescence photographs Allen and X-ray reflection topographs show a high density of defects, basically 2 x 106 cm2. This value is to be cornpared to that estimated by Abrahams7) for a graded composition layer of Ga(As, P): however it must be noticed that the lattice mismatch between the epitaxial layer and the substrate is more important in the case of In(As, P)/GaAs than in the case of Ga(As, P)/GaAs. Thin layer growth have been performed (0.5 to 3 ~tm). The interface is well-defined. X-ray diffraction investigations show the formation of an intermediate layer at the interface: this variation of the composition appears

as of thethan principal casea broadening of layers thicker I ~tm.diffraction line in the Layers grown on (111) A and (111) B plane substrates of GaP exhibit very different surface aspect: the (111) B plane only grows with mirror-smooth surface (fig. 3d). The defect density is similar to that found for a GaAs substrate. A preferential orientation of these defects can be observed in a direction parallel to the misfit dislocations lines. It is possible to grow 3 or 4 p.m thick continuous layers, but there remain holes in the deposit for the shorter growth times. A variation in the chemical composition is observed at the interface by X-ray diffraction and photoluminescence at 4.2 °K.

180

J. HALLAIS.

C. SCHEMALI

AND E. FABRE

H

___135

I1c~~ 4,

ENERGY(eV) 140

Fig. 4.

143

(a)

________________________

0

xlO

04 C~ 08 Mole Fraction of InP

0.2

1

________________________________

Electron mobility of In(As

1..~P5)as a function of mole

fraction of lnP (room temperature).

As for the growth on GaAs substrates, this intermediate layer may be due to a small quantity of gallium incorporated at the beginning of the growth. This is supported by the observation that the peak broadening occurs only when growth takes place on the Ga-containing substrates. In the case of InAs substrates no broadening was observed. 2.3.

ELECTRICAL PROPERTIES

125

Hall measurements were performed on epitaxial layers grown on semi-insulating GaAs and GaP sub3), strates. Theindependent residual doping is n-type (n 1016 and nearly of the composition x cm~

126

1.27

128

Q9

130

131

Energy(ev) Fig. 5.

Photoluminescence

(b) spectra at 4.2

K. (a) Pure lnP;

(b) lnAs 01P0,.

Fig. 4 gives the variation of the room temperature electron mobility as a function of the chemical comt 1) position for layers grown on GaAs. Results of Allen and Thompson’2) on materials grown under different conditions are plotted on the same figure. Deliberate p-type doping with Zn has been obtained

140

E

7

2 0o425+o.722x+a273x

~

TABLEI

p-type In(As,P) Substrate

-

Doping level

GaAs GaAs

3xlO 5 ~<1016

-

E’ 120

Hall mobility (cm2 V’

(cm3) - ~-

-~

--~---

~—-

65 80

~l)

-

lie 75

Fig. 6. _______________

_______

______

80

85 90 Atom Percent P

95

100

Band-gap variation of EnAs

composition at 4.2 K.

1_.5P~as a function of the

VAPOUR EPITAXIAL GROWTH AND CHARACTERIZATION OF

3). The hole mobility in the range of 1016_S x 1018 cm is smaller for layers grown on GaP than for layers grown on GaAs, and is slightly dependent on the doping level (table 1). 2.4. OPTICAL PROPERTIES The photoluminescence technique is used to study the optical properties at 4.2 °K.The sample is cooled in liquid helium and illuminated by a laser beam at 6.328 A (He/Ne, 5 mW). The luminescent light is analyzed in a grating monochromator and detected by a photomultiplier with a S 1 photocathode. The response of this photocathode restricts the measurements to samples in the range: 0.7 x I. Fig. 5a shows the photoluminescence spectrum of a lightly doped p-type InP layer (p lOb cm3). Four recombination radiation peaks can be seen. The Aband, at 1.415 eV, which is not resolved in the figure, is attributed to the annihilation of free or bound excitons13’14). The C-band, at 1.378 eV, is generally attributed to donor—acceptor pair recombinations’ Si 6); the acceptor involved may be related to silicon or zinc1 7). The C’-band, at 1 .335 eV, corresponds to the same transition as the C-band, but with emission of one longitudinal optical phonon of 43 meV. These three recombination radiation peaks are always observed on the lightly doped p-type layers, but the presence of another peak at 1.400 eV may be noticed on some sampies; the nature of this peak is not yet established. Fig. Sb shows the photoluminescence spectrum of a n-type InAs, ~ epitaxial layer (x = 0.9), not intentionally doped. The near band edge emission is at 1.30 eV. Only one impurity centre is observed at 1.27 eV, and its nature is probably the same as that of the centre at 1 .378 eV in pure InP. The band-gap variation of InAs 1~ as a function of the composition at 4.2 °Kis shown in fig. 6. The chemical composition is determined from the lattice parameter measured by X-ray diffraction, assuming 1 8) The value of thea small deviation from Vegard’s law band-gap of InAs at 4.2 °Kis that given by Varshni’ 9)~ For the investigated range (0.7 x 1), the variation E ‘ ~ . 0~x~ IlLS Lile retailon. 2, E0(x) = A+Bx+Cx where A = 0.425 eV, B

at 4.2 °K.

=

0.722 eV and C

=

1nAs1~P~

181

These results are in good agreement with previously published data20’21). The small difference with the results of Antypas et al.20) may be attributed to the difference of temperature (4.2 °Kinstead of 77 °K)and by taking into account the deviation from Vegard’s law in eq. (1). 3. Conclusion We have shown the possibility of the determination of the best growth direction for heteroepitaxial layers by use of hemispherical substrates. The crystalline quality of the deposit can be compared to that obtained on graded composition layers as in the case of Ga(As, P) for instance7). The preferential orientation of defects has been correlated with the misfit dislocation array. The electron mobility of thin InAs,~P~ layers deposited on GaAs or GaP is found to be higher than the previously published values for heteroepitaxial material, though it remains a factor two, lower than that obtained for bulk material. The very promising results obtained on GaP show a possibility of growing a material for the transmission photoemission. Acknowledgements The authors are indebted to L. Hollan and C. Schiller for very helpful discussions and their interest in this work. They would like to thank D. Beaudet and A. Humbert for their assistance in the luminescence, electrical and X-ray diffraction measurements. This work is supported by the D.R.M.E. funds. References 1) J. J. Scheer and J. van Laar, Solid State Commun. 3 (1965) 189.

2) L. W. James, G. A. Antypas, J. J. Liebbing, T. 0. Yep and R. L. Bell, J. Appi. Phys. 42 (1971) 580. 3) Y Z.Liu,J. L. Moll and W. E. Spicer, App]. Phys. Letters 4) J. J. Tietjen, A. P. Maruska and R. B. Clough, J. ElectroSoc. 116B.(1969) 492.and W. H. F. Wilgoss, Solid State 5) chem. R. C. Clarke, D. 1125. Joyce Commun. 8 (1970) 6) A. Boucher and L. Hollan, Onde Electrique 50 (1970) 165. 7) M. S. Abrahams, L. R. Weisberg, C. J. Buiocchi and J. Blanc, J. Mater. Sci. 4 (1969) 223.

(1)

8) A. Allen, Electrochem. 117 (1970) 9) H. L.Hollan andJ. C. Schiller, J. Soc. Crystal Growth1417. 13/14 (1972)

0.273 eV

10) C. Schiller, Compt. Rend. (Paris) 272 (1971) 764. 11) H. A. Allen and F. W. Mehal, J. Electrochem. Soc. 117 (1970) 1081.

182

J. HALLAIS,

C. SCHEMALI ANI) E. FABRE

12) A. G. Thompson and J. W. Wagner, J. Phys. Chem. Solids 32 (1971) 2613. 13) U. Heim, 0. Roder, H. J. Queisser and M. Pilkuhn, J. Luminescence 1,2 (1970) 542.

14) W. J. Turner and G. D. Petit, AppI. Phys. Letters 3 (1963) 102. 15) R. C. C. Leite, Phys. Rev. 157 (1967) 672. 16) U. 1-leim, Solid State Commun. 7 (1969) 445.

17) B. D. Joyce and E. W. Williams, in: Proc. 1970 S.vmpo.siuiu on GaAs (Aachen) p. 6. 18) A. G. Thompson and J. F. Rowe, J. AppI. Phys. 40 (1969) 3280.

19) Y. P. Varshni, Physica 34 (1967) 149. 20) G. A. Antypas and T. 0. Yep, J. AppI. Phys. 42 (1971) 3201. 21) A. Congiu, Phys. Status Solidi (a) 5 (1971) 131.