Morphology and Defect Structures of GaSb Islands on GaAs Grown by Metalorganic Vapor Phase Epitaxy

Journal of Electronic Materials, May 1998 by Kim, Joon-Hyung, Seong, Tae-Yeon, Mason, N J, Walker, P J

JOON-HYUNG KIM,1 TAE-YEON SEONG,1* N.J. MASON,2 and P.J. WALKER2

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520 deg C, the islands are longer along the [110] direction; at 540 deg C, they are nearly square, and at 560 deg C, they are longer along the [ 110] direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520 deg C, the lattice mismatch strain was accommodated mainly by 90 deg dislocations; for the sample grown at 540 deg C, the misfit strain was relieved mostly by 90 deg dislocations with some of 60 deg dislocations, and for the sample grown at 560 deg C, the strain was accommodated mainly by 60 deg dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.

Key words: GaSb, high resolution electron microscopy (HREM), metalorganic vapor phase epitaxy (MOVPE), misfit dislocation, transmission electron microscopy (TEM)

INTRODUCTION

GaSb based heterostructures such as Ga(In)Sb and GaSb grown onto GaAs/AlAs mirrors are of increasing research interest because of their potential technological applications in the areas of optoelectronic devices, e.g., resonant cavity enhanced photodetector for gas-sensing.1,2 There is, however, a large lattice mismatch (-7.54%) between GaSb and GaAs, which is expected to be relieved by the introduction of a number of defects, e.g. probably by the formation of interfacial misfit dislocations. Misfit dislocations usually serve as a source for threading dislocations which propagate through the epitaxial layer. Such threading dislocations are known to deteriorate device performance by forming nonradiative recombination centers. In order to control mismatch-induced threading dislocations, Nishimura et al.3 employed a low temperature growth of GaAs buffer layer on Si substrate prior to the layer growth. Kang et al.4 used transmission electron microscopy (TEM) to investigate lattice mismatch relaxation and the generation mechanism of threading dislocations in molecular beam epitaxial (MBE) GaSb films grown at temperature ranging from 420 to 520 deg C. In our previous work,5 we investigated the initial stages of metalorganic vapor phase epitaxy (MOVPE) GaSb films and showed that there is a density of two different-sized islands with crystallographic facets and that the lattice misfit is accommodated by a regular orthogonal array of 90 deg misfit dislocations. We have also studied these islands by atomic force microscope (AFM) and found that the shape of the islands was temperature-dependent.

In this work, we present TEM, transmission electron diffraction (TED), and high resolution electron microscope (HREM) examination of GaSb islands on (001) GaAs substrate grown at temperature ranging from 520 to 560 deg C. It is shown that the morphology and defect behavior of GaSb islands are dependent upon the growth temperature. Possible mechanisms are proposed to describe these features.

EXPERIMENTAL

Details of our growth reactor setup have been described elsewhere.6,7 GaSb films were grown by atmospheric pressure MOVPE in a horizontal silica cell reactor with a radio frequency (rf)-heated graphite susceptor using trimethylgallium (TMGa) and trimethylantimony (TMSb). The substrates were Sidoped semi-insulating (001)-oriented GaAs. The growth temperature varied from 520 to 560 deg C and the growth rate was - 1 mu m/h. Cross-sectional TEM thin films were prepared by mechanical polishing and Ar ion milling using a liquid N^sub 2^ cold stage. [001] planview thin foils were prepared by chemical etching using a solution of HCl/H^sub 2^O^sub 2^/H^sub 2^O. TEM and TED examination was performed in a JEM 2010 instrument operated at 200 kV. The convergent beam electron diffraction (CBED) technique8 was used to determine the polarity between the [110] and [ 110] directions. The polarity of the films was assumed to be the same as that of the underlying GaAs substrate. The diffraction patterns were indexed in respect to a unit cell with basis Ga at (000) and As/Sb at ( 1/4, 1/4,1/4 ). Following the treatment of Tafto and Spence,8 the CBED patterns were obtained from the substrate under dynamic two beam conditions. In Figs. la and lb, the Bragg conditions were fulfilled for the (00-2), (-1,1,9) and (-1,1,-il; 11) reflections, and the (002), (-1,1,11), and (-1,1,-9) reflections, respectively. The occurrence of the dark cross contrast inside the (00-2) reflection (Fig. la) and the bright cross contrast inside the (002) reflection (Fig. lb) shows that the pattern corresponds to [110] cross section.