ga2te3 interface formation

An ordered Ga2Te3 phase in the ZnTe/GaSb interfaceC. T. Chou, J. L. Hutchison, D. Cherns, M.J. Casanove, J. W. Steeds, R. Vincent, B. Lunn, and D. A.Ashenford

Citation: Journal of Applied Physics 74, 6566 (1993); doi: 10.1063/1.355118 View online: http://dx.doi.org/10.1063/1.355118 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/74/11?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in Molecular beam epitaxial growth of high-reflectivity and broad-bandwidth ZnTe/GaSb distributed Braggreflectors J. Vac. Sci. Technol. B 31, 03C109 (2013); 10.1116/1.4793475

Optical measurements of single CdTe self-assembled quantum dots grown on ZnTe/GaSb J. Vac. Sci. Technol. B 29, 03C119 (2011); 10.1116/1.3562182

High resolution x-ray diffraction and scattering measurement of the interfacial structure of ZnTe/GaSbepilayers J. Appl. Phys. 82, 2281 (1997); 10.1063/1.366035

Ga2Te3 and tellurium interfacial layers in ZnTe/GaSb heterostructures studied by Raman scattering Appl. Phys. Lett. 60, 2129 (1992); 10.1063/1.107085

Valenceband offset and interface formation in ZnTe/GaSb(110) studied by photoemission usingsynchrotron radiation J. Vac. Sci. Technol. B 6, 1211 (1988); 10.1116/1.584281

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:148.224.6.86 On: Thu, 09 Jul 2015 22:43:44

An ordered GasTea hase in the ZnTe/GaSb interface C. T. Chou H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1 TL, United Kingdom

J. L. Hutchison Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom

D. Cherns H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 I TL, United Kingdom

M. J. Casanove Department of Materials, University of Oxford, Parks Road, Oxford OXI 3PH. United Kingdom

J. W. Steeds and R. Vincent H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 I TL, United Kingdom

B. Lunn and D. A. Ashenford Department of Engineering Design and Manufacture, University of Hull, Hull HU6 7Rx, United Kingdom

(Received 18 May 1993; accepted for publication 12 August 1993)

A Ga2Te3 interfacial phase has been observed in a ZnTe/( OO1)GaSb heterostructure by high resolution electron microscopy under special imaging conditions. This phase exists in domains 5-10 nm in size on the ZnTe side of, and usually 24 nm away from, the interface. A structural model has been proposed for this phase that is derived from the sphalerite cell with cation sites occupied either fully (occupancy 1) or partially (occupancy J/9) by Ga atoms. The fully occupied Ga sites form a regular array of uninterrupted chains along the [ 1 lo] direction of the sphalerite unit cell. The partially occupied Ga sites can also be considered as forming chains containing both Ga atoms and vacancies along the [ 1 lo] direction. Within these chains vacancies are highly mobile, resulting in an average Ga occupancy of 5/9. The unit cell of Ga,Te, is orthorhombic with the space group Amm2. The lattice parameters of the unit cell have been derived from electron diffraction data.

INTRODUCTION

Many wide band-gap II-VI semiconductor thin layers are now epitaxially grown on lattice matching III-V substrates. In the interfaces of the II-VI/III-V heterostructures, formation of thin interfacial layers, consisting mainly of III-VI compounds, has been reported in CdTe/InSb,2 ZnSe/GaAs,3 and ZnTe/GaSb.45 It has also been reported that an interfacial GazSe3 layer can improve the crystallinity of an epitaxially grown ZnSe layer on the GaAs substrate.6 In addition, III-VI compounds are them- selves wide band-gap semiconductors and have the poten- tial to become materials for blue-emitting devices.7 Ef- forts have been made to deliberately grow these III-VI epitaxial layers onto suitable III-V substrates.7 Interest in these III-VI phases is increasing both for control of the interface quality of the II-VI/III-V heterostructures and for exploring the properties of new semiconductor materials.

The properties of bulk GalSej, In2Te3, Ga,S3, and GazTe3 have been reported in several recent papers.289 The authors of these works all suggest that III-VI compounds have structures based on the sphalerite unit cell (for Ga2S3 it is a structure based on the wurzite unit cell) with 2/3 of cation sites occupied and l/3 of sites vacant, whereas the anion sites are fully occupied. Evidence has been presented that, under a slow cooling condition, vacancy ordering develops in these compounds. The com-

pound GazTeJ has been reported to have an ordered vacancy structure based on the sphalerite unit cell. However, the exact ordering pattern is unknown. When the cooling rate is high, then the vacancies are likely to be disordered. There are structural models for vacancy ordering in GazSe3 (Refs. 7, 11, and 12) suggesting that cation sites are either fully occupied or completely vacant, resulting in supercells of three times the size of the sphalerite unit cell along certain directions. As a consequence, the symmetry of the cubic sphalerite is lost, and a monoclinic or orthorhombic structure is formed.

Raman scattering experiments5 have shown the existence of GazTes layers in ZnTe/GaSb heterostructures grown by molecular-beam epitaxy (MBE). In photolumi- nescence (PL) studies of similar specimens, Duddles et al. I3 reported a blue shift of the heavy-hole exciton (X&I emission of 7 meV that could not be explained by the mismatch of 0.1% between ZnTe and GaSb and corresponding to a shift of 2 meV in the heavy-hole band edge of the PL spectrum when the growth was pseudomorphic. They suggested that formation of an interface layer involv- ing GazTe, may contribute to this additional strain.

A direct imaging method would clearly be of great advantage for revealing the structure, the distribution of Ga,Te3, and the strain situation in the interfacial area of ZnTe/GaSb. In this article we show the images of this phase obtained by a new technique of high resolution electron microscopy (HREM), discuss the strain condition in

6566 J. Appl. Phys. 74 (1 I), 1 December 1993 0021-8979/93/74(11)/6566/5/$6.00 0 1993 American Institute of Physics 6566


the interfacial area caused by the distribution of this phase, and derive a structure model based on the domain structure of the HREM images.

EXPERIMENT

A 2.2~pm-thick ZnTe layer was grown on a [OOl] GaSb substrate in a VG V80H MBE system. l3 The growth rate was 0.5 ,um/h at 300 C with the ratio of the Zn/Te jet flow rates J&J, = 1.2.

Cross-section samples of [llO] orientation were pre- pared by preliminary mechanical grinding and then iodine ion-beam (I) milling at 6 kV and at liquid nitrogen tem- perature, until perforation. In the final stage of the ion- beam milling, 4-5 kV was used in order to further decrease the surface damage of the thin foil samples. Samples were then examined at 400 kV in a high resolution JEOL 4000EX II electron microscope with an objective lens spherical aberration coefficient C,=O.9 mm.

RESULTS

It was found that at certain unusual defocus conditions, a superlattice structure could be seen in the ZnTe side of the interface. In Fig. 1, micrographs of the same inter-facial area are shown. Fig. 1 (a) was taken at defocus Af =-47 nm (Scherzer defocus condition); ordinary [llO] sphalerite lattices were observed with no sign of any GazTe3 superlattice. Figures 1 (b) and 1 (c) were taken at defocus A f = 60 and 130 nm, respectively, demonstrating a superlattice of double periodicity in the ZnTe side of the interface. It should be emphasized that these high overfo- GUS settings are not normally used for HREM imaging, in view of the very rapid oscillations of the contrast transfer function (CTF) and the strong attenuation in the chro- matic damping envelope. However, a careful analysis of the CTFs indicates that it is possible to image the superlattice by tuning a crossover of the CTF to the position of dom- inant {l 113, reflections (s indicates sphalerite reflections) while simultaneously enhancing the superlattice reflections (l/2 l/2 l/2),. A detailed discussion of this novel HREM technique is presented elsewhere. l4

Selected area electron diffraction patterns also reveal the existence of a superlattice with about twice the periodicity (see Fig. 2) of the sphalerite structure. The superlattice reflections overlap with the fundamental reflections along directions within the growth plane, showing that this interface phase lattice-matches with ZnTe in the growth plane. On the other hand, the superlattice reflections deviate significantly from the fundamental ones aZong the growth direction, as shown by the arrows in Fig. 2. This leads to different lengths for the axes of the sphalerite unit cell of the superlattice in the interface. If the sphalerite lattice parameter parallel to the growth plane is written as

all and that along the growth direction a, , we find all =0.6089 nm, and al =0.5580 nm.

This results in a unit cell volume of 0.2065 nm3. Pre- vious x-ray diffraction measurement for bulk GazTe3 gave a sphalerite lattice parameter of 0.5896-0.5898 nm.9v5 Thus the unit cell volume is 0.2050 nm3, in reasonable

J?IG. 1. [110] orientation HREM micrographs of the interface area of ZnTe/GaSb. (a) Af=-47 nm, (b) Af=60 nm, and (c) Af=130 MI. Insets show image simulations based on our structure model.

agreement with that of the interface phase (error of less than 0.8%). In comparison, the unit cell volume of ZnTe (0.2256 nm3) is about 9% greater than that of the interface phase. The interfacial phase can thus be considered as having a tetragonally distorted sphalerite cell of bulk Ga2Te3, compressed along the growth direction and ex- panded in the growth plane with a tetragonal distortion of 8%. The strain caused by the coherent existence of GazTe3

6567 J. Appl. Phys., Vol. 74, No. 11, 1 December 1993 Chou et al. 6567 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

148.224.6.86 On: Thu, 09 Jul 2015 22:43:44

FIG. 2 A selected area [IlO], diffraction pattern. Double periodicity superlattice is clearly demonstrated. Arrows show that superlattice reflex tions along the growth direction deviate from the corresponding fundamental reflections.

in ZnTe is sufficient for the 7 meV blue shift of the X, emission in ZnTe, which can be ascribed to a biaxial com- pressive strain of the ZnTe of the order of 0.3%.13 We note from Fig. 3 that the distribution of GazTe3 in the interfacial area is nonuniform; the strain distribution in ZnTe is therefore complicated and further study is required to quantify the effects.

X-ray energy dispersive spectra from small areas have been acquired on a Hitachi HF2000 transmission electron microscope fitted with a field emission gun. These gave Ga,

FIG. 3. Low magnitlcation [l lo] orientation HREM micrograph showing the distribution of the ordered interfacial phase in ZnTe/GaSb.

RIG. 4. [l lo] HREM micrograph taken at Af = - 10 nm showing domain structure of the interfacial phase. Insets demonstrate image simulations for two different variants.

Te and Zn signals from the region of the superlattice, but no quantitative analysis of the composition has yet been carried out.

The superlattice areas appear to be in domains along the interface on the ZnTe side, frequently several nanom- eters away from the interface, although sometimes they can be in direct contact with the GaSb substrate as seen in Fig. 3. It is not clear at this stage why this phase can be some distance away from the interface between ZnTe and GaSb. A structural model of the inter-facial phase has been derived from the HREM observations. In Fig. 4 a micrograph taken at Af = - 10 nm is shown. Domains of different variants (indicated by A, B, and C) separated by a variant boundary (indicated by VB in Fig. 4) and antiphase domains of the same variant (B) separated by an antiphase boundary (APB) can be found in this figure.

STRUCTURE DETERMlNATlON

A distribution of strong and weak spots on a [llO], projection image in domain A is also clearly seen in Fig. 4. It is not unreasonable to assume that this distribution orig- inates from the different occupancies of Ga atoms in the cation sublattice. From this assumption, we conclude that the strong spots correspond to the cation sublattice sites with a high occupancy of Ga atoms, whereas the weak spots are related to the sites of low Ga occupancy. Assum- ing the high occupancy sites to be fully occupied by Ga atoms (occupancy 1 ), then the stoichiometry requires the low occupancy sites of a Ga occupancy of S/9. A great difference between the two occupancies is required by the strong contrast demonstrated in Fig. 4, and other possible combinations of two occupancies will inevitably result in smaller contrast. We would like to point out that certain cation sites in this compound may be occupied by Zn at-


148.224.6.86 On: Thu, 09 Jul 2015 22:43:44

a c1101s

0 o-

.ft

0

0 0 --*-.

c 11Olls, IOTlls

b ~oilisI riolis

d

FIG. 5. Cation sublattice projections along different ( 1 lo), . Closed circles represent Ga positions of occupancy 1, open circles represent Ga positions of occupancy 5/9, and hatched circles represent the alternate arrangement of the above two positions.

oms, forming an alloy of Ga,Te3 and ZnTe. When this situation arises, the occupancies of the cation sites would vary accordingly, however, in any,case the assumption that an ordered distribution of fully and partially occupied cation sites exists is still required. Based on this ordered distribution of Ga on the cation sublattice sites of the crystal in the [l lo] projection [shown in Fig. 5(a)], a threedimensional distribution of the Ga atoms in the cation sublattice can be constructed and is shown in Fig. 6. From the three-dimensional distribution, we can then derive all of the different ( 1 lo), projections illustrated in Figs. 5 (b), 5(c), and 5(d).

The relationship between the unit cell for the ordered structure and that of the sphalerite is as follows (the lattice parameter of which is ao, ignoring the difference between alI anda,):

FIG. 6. Threedimensional view of the cation sites. Atomic positions fully occupied by Ga atoms are represented by large spheres and those with 5/9 Ga occupancy are shown by smaller spheres.

IrIG. 7. Unit cell of the GarTes ordered interfacial phase projected along the c axis. Atom types, z coordinates, and occupancies of different sites are given in the attached table.

all [ W ,,bll [iloi,,cll wi,, [al =(42/2)ao, lb1 =J2ao, 14 =2ao.

The projection of this new unit cell along its c axis, including both cation and anion sites, is shown in Fig. 7. In the attached table the Z coordinates of atoms, atom spe- cies, and the occupancies are given. Related symmetrical elements can be derived from this unit cell. It can be con- cluded that this structure belongs to the space group Amm2. The unit cell volume would be 2ai. There are 8 Te atoms and an average of 5f Ga atoms in the unit cell, giving the average composition 2Ga: 3Te as required. Multislice image simulations based on this structure model have been performed, and the results confirm this model (see insets in Figs. 1 and 4).

DISCUSSION

A new phase, likely to be GazTe3, has been imaged in MBE grown ZnTe/GaSb by HREM under special imaging conditions. This phase is distributed nonuniformly in the ZnTe side of, and frequently 2-4 nm away from, the interface. It has a domain structure with a domain size of 5-10 nm. The thickness of this interface phase varies from 5 to 45 mn. Antiphase domain boundaries and variant domain boundaries have been observed. We recognize, of course, that the ordering may be influenced to some extent by factors such as growth conditions, cooling rate after growth, and residual strain.

The phase exists coherently in the ZnTe layer, with a significantly shortened lattice parameter along the growth direction (a1 =0.5580 run). This provides an additional source of strain in the ZnTe/GaSb heterostructure and may be responsible for a blue shift of X, emission of 7 meV in the PL spectra.


148.224.6.86 On: Thu, 09 Jul 2015 22:43:44

This analysis is based on two samples grown under rather similar conditions and Zn rich in order to suppress the formation of Ga2Te3 at the interface. Raman signals from these specimens indicated thin layers of Ga2Te3, but no Te compared to the earlier data.5 Furthermore, an ap- parent discrepancy with Raman data from eadier samples, in which the presence of unstrained Ga2Te3 and Te at the interface was inferred, could be due to local area varia- tions or the strain in the thin layers, described in the present work, being sufficiently large to severely broaden the Raman lines.

CONCLUSIONS

A structure model has been proposed for Ga2Te3 based on the contrast distribution and the domain structure that matches image simulations well. This model suggests that vacancies in this structure are highly mobile in the cation lattice, resulting in lattice sites with an average 5/9 Ga occupancy, whereas the fully occupied Ga sites form con- tinuous and regularly arranged chains along the [l lo], direction. Our model disagrees with previous x-ray and eleo tron diffraction studies of bulk III-VI compounds that suggest superlattice structures based on either fully occupied or completely vacant cation sites in the sphalerite cell.

ACKNOWLEDGMENTS

We are grateful to the Science and Engineering Re- search Council for its support of this work, and to the

Materials Modelling Laboratory at Oxford University for provision of computing facilities. We also thank the Royal Society and the Centre National de la Recherche Scienti- fique for a visiting fellowship for one of us (M.J.C.).

R. L. Gunshor, A. V. Nurmikko, L. A. Kolodziejski, M. Kobayashi, and N. Otsuka, J. Cryst. Growth 101, 14 (1990).

D. R. T. Zahn, K. J. Mackey, R. H. Williams, H. Milnder, J. Gemts, and W. Richter, Appl. Phys. Lett. 50, 742 (1987).

3A. C Wright, J. 0. Williams, A. Krost, W. Richter, and D. R. T. Zahn, J. Cry& Growth 121, 111 (1992); A. C. Wright and 3.0. Williams, J. Cryst. Growth 114, 99 (1991).

4W. G. Wilke, R. Seedorf, and K. Horn, J. Crvst. Growth 101, 620 (1990).

_

M. P. Halsall, D. Wolverson, J. J. Davies, B. Lunn, and D. A. Ashen- ford, Appl. Phys. L&t. 60, 2129 (1992).

6D. Li, J. M. Gonsalves, N. Otsuka, J. Qiu, M Kobayashi, and R. L. Gunshor, Appl. Phys. Lett. 57,449 (1990).

N. Teraguchi, M. Konagai, F. Kato, and K. Takahashi, J. Cryst. Growth 115, 798 (1991).

*E. Fmkman, J. Taut, R. Kershaw, and A. Wood, Phys. Rev. B 11,3785 (1975).

M. Guymont, A. Tomas, and M. Guittard, Philos. Mag. 66, 133 (1992).

OP. C. Newman and J. A. Cundall, Nature 200, 876 (1963). D. Lubbers and V. Leute, J. Solid State Chem. 43, 339 (1982). z J C. Wooly, B. R. Pamplin, and P. J. Holmes, J. Less-Common Met. 1,

361 (1959). 13N. J. Duddles, J. E. Nicholls, T. J. Gregory, W. E. Hagston, B. Lunn,

and D. A. Ashenford, J. Vat. Sci. Technol. B 10, 912 (1992). 14J. L. Hutchison, C. T. Chou, M.-J. Casanove, D. Chems, J. W. Steeds,

B. Lunn, and D. A. Ashenford, Ultramicroscopy (in press). A. Mukherjee, Bull. Mater. Sci. (India) 2, 55 (1980).

6570 J. Appl. Phys., Vol. 74, No. 11, 1 December 1993 Chou et al. 6570


ga2te3 interface formation

Documents

university of bristol

united kingdom

university of oxford

bristol bs8

unit cell of ga

university of hull

ga atoms

occupied ga sites