Atomic Layer Epitaxy of GaAs on Si by Mocvd
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ATOMIC LAYER EPITAXY OF GaAs ON Si BY MOCVD
N.H. KARAM,* V.E. HAVEN,* S.M. VERNON,* J.C. TRAN,** and N.A. EL-MASRY** * Spire Corporation, Patriots Park, Bedford, MA01730 ** North Carolina State University, Materials Engineering Department, Raleigh, NC 27695 ABSTRACT Epitaxial GaAs films have been deposited on Si substrates using Atomic Layer Epitaxy (ALE) for the first time. This has been achieved in a SPI-MO CVDTh 450 reactor especially modified foroALE. After an initial high temperature bakeout, a nucleation layer 100-300 A thick was deposited by ALE. Film growth was then resumed by conventional MOCVD to achieve the desired film thickness. The surface morphologies of the deposited films were found to be comparable to current state of the art conventional GaAs on Si films deposited by the two-step growth process in the same reactor. Selective area epitaxy of GaAs on Si has also been achieved on Si02-coated and patterned Si wafers. The standard two-step deposition technique resulted in epitaxial growth in the patterned windows and poly-GaAs on the oxide mask, while ALE growth resulted in deposition only in the etched windows with no poly-growth on the oxide mask. We will report on the potential of this new deposition technique in producing high quality GaAs-on-Si films. INTRODUCTION The epitaxial growth of GaAs and its alloys on Si has attracted strong interest in recent years [1]. This is due to the potential for monolithic integration of GaAs and Si (MGS) devices, thus combining the high speed and/or optical communication capabilities of GaAs with the sophistication of the Si VLSI technology. Recently, exciting progress has been realized in the area of GaAs on Si by Metalorganic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE) [2]. However, two main problems which remain to be overcome before the full potential of this technology is realized are the lattice mismatch (4.1%) and the thermal expansion mismatch between GaAs and Si. The former situation leads to a high density of threading dislocations (typically 10 8 cm- 2 ) which reduces the diffusion length in minority-carrier devices; nevertheless, lasers [31, LED's [4], and solar cells [5] have been successfully fabricated in GaAs-on-Si structures. The performance of these devices can approach those fabricated in GaAs substrates if the defect density is reduced by two to three orders of magnitude. On the other hand, the thermal expansion mismatch between GaAs and Si results in biaxial tensile stresses in the heteroepitaxial film which eventually lead to film cracking and wafer bowing during the cooling down period from the growth temperature. Cracks have been reported in films thicker than a few microns, and wafer bowing has been found to interfere with photolithographic processes [6]. Atomic Layer Epitaxy (ALE) is a relatively new and promising deposition technique that can lead to the ultimate control and perfection of the deposition process of III-V semiconductor films. In a typical ALE process, the substrate is sequentially exposed to fluxe
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