Growth of Strain-Free GaAs on Si/Sapphire
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GROWTH OF STRAIN-FREE GaAs ON Si/SAPPHIRE
Hyunchul Sohn,* E.R.Weber,* Jay Tu** and J.S. Smith** *Department of Materials Science and Mineral Engineering, University of California, "**Department of Electric Engineering and Computer Science, University of California, Berkeley. ABSTRACT
GaAs epitaxial layers were successfully grown on Si/Sapphire substrates using Molecular Beam Epitaxy(MBE). Residual compressive strain was found in GaAs films on Si/Sapphire. By Photoluminescence, the magnitude of residual strain in GaAs on Si/Sapphire was estimated to be 5x10-4 which is about one order smaller than that of GaAs on Si. As an effort to achieve further reduction in the residual strain, Indium- doped GaAs films were used as buffer layers in order to compensate compressive thermal strain by tensile misfit strain in the GaAs layer. Using this method, strain- free GaAs layers could be grown with thickness up to 0.4 gm on Si/Sapphire. INTRODUCTION
The GaAs on Si heteroepitaxy has suffered from the high density of threading dislocations and the large residual tensile strain in GaAs films on Si substrates which are attributed to the large lattice mismatch(4.1%) and large difference in thermal expansion between GaAs and Si. Much research has been conducted to reduce threading dislocation density in GaAs films3 and some degree of success was achieved with threading dislocation density of 1016 /cm using thermal cycling [1] and Strained Layers.[2] For reducing the high residual strain, postgrowth patterning[3], growth on patterned substrates[4] and strain compensation[5] have been employed with less degree of success. The residual tensile strain found in GaAs films grown on Si substrates is attributed to the differential thermal contraction between GaAs and Si during cooling-down from growth. For reducing thermal strain, Sapphire is an attractive substrate because thermal expansion 6 coefficient of Sapphire(-7.5xl0-6 oC-l) is comparable to that of GaAs(-6.3xl0- 'C-1). Even though the compressive residual strain is expected from consideration of thermal expansion coefficients, observations of tensile residual strain in GaAs on SOS were reported.[6] In this work, we report the observation of compressive strain in GaAs films grown on Si/Sapphire substrates. EXPERIMENTAL
In this experiment, GaAs epitaxial films were grown on [001]- Si / [1102] Sapphire substrates (SOS), using MBE. In order to increase heating efficiency of substrates, 100 nm Ti and 3 [tm Si films were deposited on the back of Sapphire. After annealing for 15 min. at 970 °C for oxide desorption, GaAs films were grown on Si/Sapphire wafers by two-step growth method. Surface morphologies were studied by polarized optical microscope. Residual strain in GaAs on SOS was estimated from 77K Photoluminescence(PL) and was compared to that in GaAs/Si. As an effort to reduce the residual strain in GaAs films further, Indium-doped GaAs layers were used as buffer layers to compensate the compressive thermal strain by tensile misfit strain in GaAs films. Mat. Res. Soc. Symp. Proc
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