Correlation between Extended Defects and Surface Morphology in MBE Grown InAs/GaAs Heterostructures
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ABSTRACT InAs single layers were grown by Molecular Beam Epitaxy on nominally (001) oriented GaAs substrates at growth temperatures ranging from 350 'C to 500 'C and thicknesses between 1 nm and 6 gim. A systematic study of the influence of growth temperature and thickness on crystal defects and surface morphology is discussed by comparing High Resolution X-Ray Diffraction, Transmission Electron Microscopy and Atomic Force Microscopy investigations. Surface hexagonal shaped holes were observed to develop at the lowest temperatures starting from an heterolayer thickness of 50 nm. Both misfit and threading dislocations were revealed; moreover the correlation between hexagonal shaped surface holes and mixed dislocations, with the component of the Burgers vector (b) along the growth axis larger than the minimum interatomic distance, is discussed. The holes increase in size and decrease in density by increasing the layer thickness. An almost complete surface planarization is observed at a thickness of 6 pim by increasing the growth temperature up to 500 'C. INTRODUCTION In the frame of the increasing interest devoted to the growth and physical characterization of semiconductor lattice-mismatched heterostructures, the InAs/GaAs system is largely studied for its potential application in both microwave and optoelectronic device fabrication. Moreover, because of its large misfit (=7.2%), the system is attractive for the comprehension of strain relaxation mechanisms [1]. It is known that in highly mismatched systems, the very beginning of the growth can follow both the Volmer-Weber or the Stransky-Krastanov model. In the latter case, after a first deposition of a continuous film 1 or 2 monolayer thick, the formation of islands of different size occurs according to the growth conditions [1,2]. The onset of 3D growth can involve a damage for the previously deposited film and even for the substrate, as observed by Zhang et al [2]. Based on a previous thermodynamics model [3,4], predicting the optimum growth temperature (T) range for the InAs alloy formation, this study reports the influence of substrate temperature ana layer thickness on surface morphology. High Resolution X-ray Diffraction (HRXRD) was performed to measure the residual strain and to test the overall crystal quality of the layers. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used to study the samples surface morphology and the nature and distribution of crystal defects respectively. EXPERIMENT InAs heterolayers were grown by molecular beam epitaxy (MBE) on nominally (001) oriented GaAs substrates, following a procedure described elsewhere [5]. The films were deposited at four different (Tg) values (350, 400, 450, 500 °C). The growth rate, obtained by RHEED oscillations as the difference between those of InGaAs and GaAs, was 0.68 gmn/h and the layer thickness (t), expressed in equivalent layer by layer coverage, ranged between lnm and 6 gim. HRXRD experiments were performed by a Philips diffractometer equipped with a two crystal-fo
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