MOVPE Strip-Like Self-Organization of InAs Grown on InP Vicinal Surfaces
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ABSTRACT We have investigated the influence of MOVPE growth parameters on the surface morphology of InAs nanostructures grown on 0.2' misoriented (001)InP substrates. Thin layers of nominal thickness of about 3 and 6 ML were deposited at 500'C with V/Ill flux ratios ranging from 50 to 240. The samples were cool down from 500 to 350'C during 6 minutes under either arsine or phophine atmosphere. The influence of this step has been found to greatly determine the surface morphology of the nanostructures observed by atomic force microscopy. Dots self-aligned along the steps and forming a non continuous strip, regularly spaced every 3-4 terraces have been obtained. The morphology of the strips can be varied with the growth conditions (V/III flux ratio). In this work, we will propose a mechanism for the formation of the strips observed during the cooling under phosphine atmosphere taking into account an As -+ P exchange.
INTRODUCTION The growth of nanostructured materials is challenging, in part due to the constrain of heteroepitaxy and in part due to the system eminently useful for the production of high efficiency optoelectronic devices. In that sense, considerable progress has been made to determine the best growth parameters and relaxation mechanism of the compressively-strained InAs/InP (3 % lattice mismatch). Moreover, strain relaxation includes several stages such as formation of an InAs wetting layer and formation of islands by the Stranski-Krastanov growth mode [1]. On the other hand, the normal deposition procedure for InAs/InP by MOVPE involves the use of hydrogen as a carrier gas followed by thermal cooling under arsine flow. Furthermore, optical characterization needs the growth of an InP cap layer acting as a confinement layer. The specific growth conditions used in that study, nitrogen as a carrier gas, V/Ill ratio. are key elements in determining the final shape, size and surface distribution of dots. EXPERIMENTS The experiments were performed in a horizontal LP-MOVPE reactor using a 80% N 2 / 20% H 2 mixture as carrier gas. Epitaxy was obtained onto 0.20 off towards (11 )A InP substrates. We used trimethylindium as the Indium source. Thin layers of InAs, nominal thickness of about 3 and 6 monolayers (ML), were deposited at 500'C with V/ill flux ratios ranged from 50 to 240. The samples were cooled from 500 to 350'C during 6 minutes either in arsine or phosphine atmosphere. The surface morphology was firstly examined with an optical microscope under Nomarski contrast. Then, samples were observed by ex-situ AFM with a Digital Instrument Nanoscope H in constant mode force (about 0.58 N/m), the scale height of analysis ranging between 3 and 200 nm. 59 Mat. Res. Soc. Symp. Proc. Vol. 618 ©2000 Materials Research Society
RESULTS AND DISCUSSION Influence of arsine or phosphine on the InAs surface morphology Firstly, we present the surface morphology of a nominal 3 ML thick InAs/InP deposited with AsH 3/TMIn = 150 cooled under arsine. The surface shown in figurel(a) exhibits dots of large diameter (D) of 600 n
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