Growth of InAsGaSb strained layer superlattices. II

,. . . . . . . .

CRYSTAL

GROWTH ELSEVIER

Journal of Crystal Growth 146 (1995) 495-502

Growth of InAs/GaSb strained layer superlattices. II G.R. Booker a P.C. Klipstein b M. Lakrimi b S. Lyapin b, N.J. Mason b,* I.J. Murgatroyd a, R.J. Nicholas b, T.-Y. Seong a, D.M. Symons b, p.j. Walker a Department of Materials Science, Universityof Oxford, Oxford OX1 3PH, UK b Clarendon Laboratory, Physics Department University of Oxford, Oxford OX1 3PU, UK

Abstract

InAs/GaSb strained layer superlattices (SLSs) have been grown by metalorganic vapour phase epitaxy (MOVPE) at atmospheric pressure. Initially the interfaces of the SLS have been biased towards pairs of GaAs or InSb using three different gas switching sequences. Room temperature Raman optical modes show that growing the interfaces using an ALE (atomic layer epitaxy) like switching sequence gives interfaces of very high quality probably near the optimum, which is a monolayer. Growing with other switching sequences leads to one of the interfaces being non-uniform. By growing samples with alternating (InSb,GaAs or GaAs,InSb) pairs of interfaces it is possible to unambiguously assign this non-uniformity to one of the two possible interfaces for the first time. Furthermore, the influence of the band overlap on interface type has been studied using optimised SLSs in the semimetallic regime.

1. Introduction I n A s / G a S b is a crossed gap type II system with the conduction band of InAs lying below the valence band of GaSb. Furthermore, the bandgap, defined in terms of the energy difference between the electron confinement energy in InAs and the hole confinement energy in GaSb is found to switch from being positive (semiconductor like) to negative (semimetal like) when the InAs layer thickness exceeds 85 A [1]. Previously, most of our work has been on the transport properties of semimetallic strained layer superlattices (SLSs) [2] which have been extensively characterised in pulsed magnetic fields [3] and under

* Corresponding author.

hydrostatic pressure [4]. These SLSs showed good structural integrity (in transmission electron microscopy (TEM)) and up to 95% of the carriers were of intrinsic origin. In a previous paper we made the first low-temperature Raman observation of GaAs-like and InSb-like interface modes in these SLSs [5]. Because there is a change from one binary (InAs) to another (GaSb) and there are no common elements across the interface, a monolayer of either GaAs or InSb must occur at a perfect interface. This is shown in Fig. 1, where two [110] projections are shown, indicating the possible combinations of binary interfaces in a SLS grown on a (001) substrate. In order to clarify the interfaces in the text the four interfaces will be written In(AsGa)Sb, Sb(GaAs)In, As(InSb)Ga, and Ga(Sbln)As, where the growth direction runs from left to right. As can be seen

0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 4 ) 0 0 5 3 6 - 2

G.R. Booker et aL /Journal of Crystal Growth 146 (1995) 495-502

496

from Fig. 1 under ideal conditions the interface must be a monolayer of either GaAs or InSb depending on how the gas-switching is biased (see below). If the growth is not perfect, either through carry-over of group V species or by poor gas switching control, the interface could be a ternary or quaternary alloy. Because of the difficulty of determining whether one atomic layer or more had grown at the interface, and the difficulty of determining whether the interfaces are true binaries or slightly alloyed, they will be referred to as InSb-like or GaAs-like interfaces. Room temperature Raman spectroscopy has been used as a fast growth assessment technique to optimise the growth of these SLSs. In the past it has been used to distinguish between an InSb and a GaAs interface [6,7], and also proved capa-

Ga~

Sb Ib

11Ga~.

Sb II

ble of detecting an alloyed interface and give some estimate of the degree of alloying [8]. Distinct weak InSb-like (180 cm -1) and strong GaAs-like (250 cm -1) interface modes may be observed, and these lie below and above the bulk optical phonon energy at around 235 cm-1. The GaAs-like mode is strongly localised to the interface [9] and is therefore quite sensitive to the local environment. Fig. 1 is a [110] cross section of a SLS with the growth direction vertical. The GaAs interface consists of chains of gallium and arsenic atoms. Because of the symmetry of the zinc-blende lattice these chains of atoms run in alternating (110) directions (i.e. at right angles to one another). In Fig. la the GaAs chain at the bottom interface (Sb(GaAs)In) runs in the plane of the paper and the (In(AsGa)Sb) at the top

i. G a

Ga~

Sb ~ Ga-~. S b / G a ~ " Sb

Sb j G a ~ II

Sb j G a ~ "

•

II

.iGa

SbLI

$b ~ G a ' ~ Sb II II

I In(AsGa)Sbl II /ksjln~A

~l

As(InSb)Ga

II jln....,~

II

II

II

In .~.As ~ In ~ As "I In

(a)

II

II

As ~ In ~ A s - " I n ' A s II II II In~As.11n~.As~ln II

II 11

t

(b)

II

A s " In ~ A s -J In ~.As II II II In'~Asjln~Asjln

II

II

II

As j In ~ / k s "

Sb(GaAs)ln

II

In ~ As j In ~ As ~ In

II

In ~ As

II

11

Iea(Sbln)As! li

ii j Ga~.

Sb ~ Ga~. Sb Sb II II II Ga~ ./Ga $b " " G a ~ $b ~

H

Sb j G a ~ Sb ~ G a ~ $b II II II Ga~ /. Ga~. Sb t l G a Sb

II

IL

Fig. 1. Two [110] projections of a cross-section through the SLS (growth direction designated by arrow) showing the alternating nature of the chains of (a) GaAs or (b) InSb at a perfect interface. Boxed formulae denote the description of each interface used in the text of the paper. Growth on a (001) substrate.

GaAs

GaAs

InSb

InSb InSb

alloyed

B

C

A

B C

A

B B

1624

1618

1629

1628 1620

1640

1694 1707

alternating alternating

GaAs

A

1625

Interface type

Switching type

RUfl No.

SbGa(SbIn)AsIn GaSb(GaAs)InAs

SbGa(SbAsln)AsIn

SbGa(SbIn)AsIn SbGa(SbIn)AsIn

SbGa(SbIn)AsIn

CiaSb(GaAs)InAs

GaSb(GaAs)InAs

GaSb(GaAs)InAs

Interface growth sequence

AsIn(AsGa)SbGa InAs(InSb)GaSb

InA~lnAsSb~aSb

InAs(InSb)GaSb In~(InSb~aSb

1nAsUnSb)GaSb

AsIn(AsGa)SbGa

AsIdAsGa)SbGa

AsIn(AsGa)SbGa

235

235 235

235

239

239

236

GaSb bulk LO

246

247 242

242

253

254

251

GaAs-like interface

Optical phonons (cm-- ‘1

Raman shows both interfaces to be of high quality with GaAs-like mode in position predicted theoretically and found previously in MBE study Raman shows one interface to be of high quality and one to be non-uniform, the frequency of the bulk optical phonon suggests some strain Raman shows one interface to be of high quality and one to be non-uniform, the frequency of the bulk optical phonon suggests some strain Raman also shows InSb-like mode at 190 cm-‘, GaAslike mode is in agreement with previous MBE study GaAs-like mode suggests some alloying GaAs-like mode is in agreement with prevous MBE study Raman GaAs-like mode shows the effect of deliberately growing an alloyed interface. SampIe shows good periodicity, see Ref. 1151 Sample shows poor periodicity, see Ref. [151

Table 1 Growth details of SLSs grown at 500°C; all the SLS had 20 periods (for details of the interface growth sequence see Fig. 1; for details of the switching types, see Ref. [151)

498

G.R. Booker et al. /Journal of Crystal Growth 146 (1995) 495-502

interface has chains of atoms running in and out of the plane of the paper. As can be seen from Fig. lb the case for InSb interfaces is similar to that for GaAs interfaces but the absolute orientations are reversed in each case. Because these chains of atoms run in orthogonal directions polarised Raman scattering will give specific information about both of the interfaces in each SLS. The band-offset between InAs and GaSb has been found to be dependent on whether the interface is InSb or GaAs-like. The accurate measurement of this is important for the design of antimonide-based devices including tunnel structures and infrared emitters and detectors. We describe here the growth and characterisation of I n A s / G a S b (SLSs) grown by MOVPE.

500 #1625 #1629

I - -

| I

LO

400

300 t-

200 ¢..-

GaAs-like interface modes

100

2. Experimental procedure Growth was carried out in an atmospheric pressure M O V P E reactor as previously described [10,11]. Reactants used were trimethylgallium (TMGa), - 9 ° C ; trimethylantimony (TMSb), 0°C; trimethylindium (TMIn) (solid double bubbler [12]), 15°C; and tertiarybutylarsine (tBAs), 0°C; all supplied by Epichem Ltd. The substrates were on-axis GaAs pre-coated with GaSb [13]. The Raman measurements were performed at 300 K using different Ar + and Kr + laser lines for excitation and a Jobin-Yvon T64000 triple grating spectrometer with a multichannel CCD detector to analyze the scattered light. The typical spectral bandpass was ~ 0.5 cm-1.

3. Results Initially, the effect on the Raman scattering optical modes of biasing the interface between the GaSb and InAs either towards a monolayer of InSb, or towards a monolayer of GaAs, was investigated. In this study we used three different types of switching sequences to achieve either of these interfaces. The simplest (A) is based on atomic layer epitaxy (ALE) where only one species is switched into the reactor at any one time during the growth of the interface, B is a system

0

200

i

i

i

I

220

i

i

i

i

i

240

i

I

i

260

Raman shift (cm -1) Fig. 2. Raman scattering from SLS #1625 (GaAs-like interface) and #1629 (InSb-like interface) showing the bulk LO mode is shifted slightly from that expected in GaSb and the frequency of the GaAs-like interface modes.

where there is always a binary growing and finally C is a combination of A and B. Further details of these switching sequences are given in Ref. [15]. The first six samples had paired interfaces (i.e. either Sb(GaAs)In and In(AsGa)Sb or Ga(Sbln) As and As(InSb)Ga growth from right to left). A SLS sequence of 28 seconds for both InAs and GaSb was repeated six times using switching sequence A, B or C; for pairs of GaAs or pairs of GaAs or pairs of InSb interfaces. These six samples are summarised in the first part of Table 1. Switching sequence A achieves the biasing of the interface by allowing each element in turn to flow over the growing surface. Fig. 2 shows the Raman results from two SLSs grown using switching sequence A. These show a strong GaSb longitudinal optical (LO) phonon mode from the bulk GaSb in the SLS, together with GaAs-like inter-

G.R. Booker et al. /Journal of Crystal Growth 146 (1995) 495-502

face modes above this. In #1625 the GaAs-like interface mode is at 251 cm -1. In #1629, which was biased towards an InSb interface this GaAslike m o d e is shifted as expected to 243 c m - 1 and is much much weaker. These results agree well with a previous molecular b e a m epitaxy (MBE) [8] study which investigated the effect of alloying of the interface (i.e. towards In GaAsSb) on the R a m a n optical modes. The optical modes in both #1625 and #1629 are close to the expected frequency for bulk GaSb. In #1625 the frequency is slightly higher at 237 c m - 1 and in #1629 slightly lower at 235 cm -~. Although these shifts are small their direction is consistent within the other six samples shown in Table 1 and suggests that even in SLSs with high quality interfaces there are still small amounts of residual strain of opposite sign depending on the interface type. It seems likely that in #1625 and #1629 there is very little alloying or strain in either the GaAslike or the InSb-like SLS interfaces grown using

60

i

,

i

499

,

i

,

i

z(x'x')z

\ 50 /

- - - z(y'y')z #1618, T=300K \

,..

GaAs-like interface modes

40

, \ ~=" -=, 30

t

\

20

\

10

I

245

,

I

~

1

250

255

260

Raman shift (cm1) Fig. 4. GaAs-like mode from polarised Raman scattering for SLS #1618 (grown with switching sequence C) the two interface modes are not superimposed on one another suggesting dissimilar, non-uniform interfaces.

60

z(x'x')z - -

#1625, T=300K

50

•~ - -

z(y'y')z

4O

GaAs interface mode

>, 30 t-"

--

20

10 240

,

I 245

i

I 250

h

I 255

i 260

Ramanshift (cm1) Fig. 3. GaAs-like mode from polarised Raman scattering for SLS #1625 (grown with switching sequence A) the two interface modes are superimposed on one another suggesting similar, uniform interfaces.

switching sequence A. This was confirmed by further R a m a n measurements which are given in greater detail elsewhere [14] in which the polarisation was used to characterise one or other of the two interfaces in an independent and selective way. Fig. 3 shows the room temperature spectra taken in each configuration are very similar for #1625 which suggests that both GaAs interfaces are in a similar environment. This, together with the absence of strain mentioned above, implies that both the interfaces are close to a monolayer. SLSs with interfaces biased towards G a A s were also grown using sequence B #1624 and C #1618, and gave R a m a n results which were quite different to those discussed above. The GaSb bulk optical mode is shifted from its unstrained position by 3 cm -1 (236-239 cm -1) indicating approximately 1% strain. In addition the interface modes for #1618 shown in Fig. 4 are quite different for the two polarisation configurations. One spectrum is similar to that in Fig. 2 suggesting it

G.R. Booker et al. /Journal of Crystal Growth 146 (1995) 495-502

500

is uniform but the other shows a double hump and is thus non-uniform in some respect. By comparing these spectra with ones taken from samples with alternating interfaces (#1694 and #1707 below) it is possible to assign the origin of these two modes in such a way that suggests that it is the GaAs chains that run in the [110] direction (i.e. the In(AsGa)Sb) which is of acceptable quality and that the Sb(GaAs)In interface is nonuniform. Since #1625 showed no problems at this interface it is unlikely that there is an intrinsic growth problem but that it is likely to be due to the growth of more than one monolayer of GaAs at this particular interface when using sequences B and C. This suggestion is supported by the fact that TEM images of #1618 showed evidence of dislocations emerging from within the SLS. SLSs with interfaces biased towards InSb were also grown using switching sequence B (#1628) and sequence C (#1620). Again the Raman resuits are not as simple to interpret as #1629. Although #1620 has a GaAs-like mode at the expected frequency of 242 cm -1, in #1628 this

f=l

120

mode is at 247 cm -1 suggesting some alloying according to previous references. In order to test this possibility #1640 was grown with an interface switching sequence that ensured that, by switching the group V species as adjacent events, the interface would be deliberately alloyed. Thus, instead of the usual switching sequence of Ga off, Sb off, In on, As on; which would produce an InSb-like interface, the switching was Ga off, Sb off, As on, In on (and vice versa for the other interface) to produce an interface with the group V elements alloyed. As can be seen from Table 1, #1640 had a GaAs-like mode at 246 cm-1 exactly as predicted. Whilst it is easy to understand this alloying in #1640 it is not clear why #1628 should have an alloyed interface. Further work, on lowtemperature Raman measurements is in progress and will be reported in the future. In order to investigate this interface phenomenon further, two samples were grown with alternating interfaces using switching sequence B. Sample #1694 was grown with Ga(Sbln)As, In(AsGa)Sb and sample #1707 with Sb(GaAs)In,

(a)

iL #1707 T=300K

80

(/) ¢.-~

40

o

f=l

Ll

¢-

#1694

¢-~

50

f=2 LL

25

00

'

110

'

'

70

" - ' 40

50

60 '

R a m a n shift cm -1

Fig. 5. FLAPs (a) and cross-sectional TEM (b) from #1694 and #1707 grown with alternating interface pairs and showing the improved periodicity found when growing with Ga(Sbln)As, In(AsGa)Sb interfaces. Markers represent 30 nm.

G.R. Booker et al. /Journal of Crystal Growth 146 (1995) 495-502

501

Table 2 Electrical details of semimetallic SLSs used for band overlap measurements together with the band overlap modelled from the results Run #

Interface type

ne/layer (1011 cm -2)

nn/layer (1011 cm -2)

P-c (cm2/V • s)

Izh (cm2/V - s)

Band overlap (4 K) (meV)

1687 1690 1691 1692

InSb/InSb InSb/GaAs GaAs/InSb GaAs/GaAs

6.5 5.0 5.5 4.3

5.3 4.5 4.0 2.6

53000 36000 38000 11 000

7000 6000 6000 3 100

144/144 143/103 107/147 105/105

As(InSb)Ga interfaces. A comparison of the folded longitudinal acoustic phonons (FLAPs) in Fig. 5a for #1694 and #1707 shows that the presence of the Sb(GaAs)In interface (mentioned above) has the effect of degrading the structural periodicity of the SLS and this result is confirmed by the TEM shown in Fig. 5b. Why one GaAs interface should introduce this non-uniformity is difficult to explain but it is possible that at the Sb(GaAs)In interface the GaAs grows slightly faster (where it is growing on GaSb) than at the In(AsGa)Sb, interface (where it is growing on InAs). It is also possible that one of the two InSb interfaces is less uniform than the other and this is currently being investigated by low-temperature Raman measurements. The investigation of the band-offsets involved the growth of four semimetallic SLSs with a variety of interface combinations using switching sequence B. The electron and hole densities and mobilities are shown in Table 2. These were measured using field dependent two carrier fits and confirm that the InSb-like interfaces lead to higher mobilities and carrier densities at 4 K as previously shown in our study of double heterostructures. Modelling of these electrical resuits give band offsets also shown in Table 2 from the sum of the electron and hole confinement energies and their respective Fermi energies [4]. This suggests that there is approximately 40 meV difference in band overlap for the two interfaces studied. Such band overlap information will be useful in designing tunnel structures and infrared optical devices in the future. Indeed sample #1694 has already been shown to have a bandgap of around 5/xm [15]. Both the electrical and the optical results confirm that SLSs can be grown

with interfaces biased towards either InSb or GaAs. Initial results suggest that switching sequence A gives the most uniform interface, as measured by room temperature Raman measurements. Further experiments are in hand to see if the electrical and structural properties are also optimised with this switching sequence.

4. Conclusions We have demonstrated the ability of MOVPE to grow InAs/GaSb SLSs with high quality GaAs and InSb interfaces. The effect of the different interfaces on the band overlap has been measured for the first time. Well controlled InSb-like and GaAs-like interfaces have also been observed in the room temperature Raman optical region.

Acknowledgements We would like to thank MCP Ltd. for the provision of substrates under a UK D T I / S E R C LINK initiative. We would also like to thank Keith Belcher and Simon Moulder for technical assistance.

References [ll L.L. Chang, J. Phys. Soc. Jap. 49 (1980) 997, suppl. A. [2] M. Lakrimi, C. Lopez, R.W. Martin, G.M. Summers, G.M. Sundaram, K.S.H. Dalton, N.J. Mason, R.J. Nicholas and P.J. Walker, Surface Sci. 263 (1992) 575. [3] K.S.H. Dalton, M. van der Burgt, M. Lakrimi, R.J. Warburton, M.S. Daly, W. Lubczyfiski, R.W. Martin, D.M. Symons, D.J. Barnes, N. Miura, R.J. Nicholas, N.J. Mason and P.J. Walker, Surface Sci. 305 (1994) 156.

502

G.R. Booker et al. /Journal of Crystal Growth 146 (1995) 495-502

[4] D.M. Symons, M. Lakrimi, R.J. Warburton, R.J. Nicholas, N.J. Mason, P.J. Walker, M.I. Eremets and G. Hill, Phys. Rev. B, in press. [5] C. Lopez, R.J. Springett, R.J. Nicholas, P.J. Walker, N.J. Mason and W. Hayes, Surface Sci. 267 (1992) 176. [6] I. Sela, L.A. Samoska, C.R. Bolognesi, A.C. Gossard and H. Kroemer, Phys. Rev. B 46 (1992) 7200. [7] M. Yano, H. Furuse, Y. Iwai, K. Yoh and M. Inoue, J. Crystal Growth 129 (1993) 807. [8] B.V. Shanabrook, B.R. Bennet and R.J. Wagner, Phys. Rev. B 48 (1993) 17172. [9] A. Fasolino, E. Molinari and J.C. Maan, Superlattice Microstruct. 3 (1987) 117. [10] C. Goodings, N.J. Mason, P.J. Walker and D.P. Jebb, J. Crystal Growth 96 (1989) 13.

[11] N.J. Mason and P.J. Walker, J. Crystal Growth 107 (1991) 181. [12] N.D. Gerrard, L.M. Smith, A.C. Jones and J. Bosnell, J. Crystal Growth 121 (1992) 500. [13] R.M. Graham, A.C. Jones, N.J. Mason, S. Rushworth, L.M. Smith and P.J. Walker, J. Crystal Growth 145 (1994) 363. [14] S. Lyapin and P.C. Klipstein, Superlattice Microstruct. [15] G.R. Booker, P.C. Klipstein, M. Lakrimi, S. Lyapin, N.J. Mason, R.J. Nicholas, T.-Y. Seong, D.M. Symons, T.A. Vaughan and P.J. Walker, J. Crystal Growth 145 (1994) 778.