Atomic Layer Epitaxy of GaAs on Ge Substrates
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ATOMIC LAYER EPITAXY OF GaAs ON Ge SUBSTRATES
J. RAMDANI, B.T. MCDERMOTT AND S.M. BEDAIR Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695-7911
ABSTRACT We report on the low temperature growth of GaAs on Ge substrates using Atomic Layer Epitaxy. Low temperature deposition has resulted in substantial reduction of the outdiffusion of Ge into the GaAs epilayer as being indicated from SIMS. The I-V characteristics of the GaAs/Ge heterojunction were thyristor like or near abrupt depending on the growth temperature. We also report on the use of the Atomic Layer Epitaxy self-limiting adsorption process of TMGa to control the diffusion of Ga into Ge substrates at the monolayer level.
INTRODUCTION For over two decades, there has been great interest in the growth of GaAs on Ge. Ge has only .07% lattice and 2% thermal expansion mismatch to GaAs and has the highest hole mobility of Group IV and Il-V semiconductor materials. Several growth techniques have investigated this system, such as Chemical Vapor Deposition (CVD) and Molecular Beam Epitaxy (MBE) techniques. The GaAs/Ge heterojunction has potential applications such as: heterojunction bipolar transistors [1], high efficiency solar cells 121,and the possibility of monolithically integrating 11-V devices with those of Si by using Ge as a buffer layer [3]. However, this combination (GaAs/Ge) suffers from autodoping which leads to the modification of the interfacial electronic structure, especially when a n-GaAs/p-Ge junction is desired. Atomic layer epitaxy (ALE) is a growth technique that can potentially address this current GaAs/Ge interdiffusion problem. For III-V semiconductor compounds, ALE growth proceeds by alternating deposition of group III and group V molecules, resulting in a well controlled stochiometric growth 141. In the present work, the growth of GaAs on Ge has been carried out by ALE as an alternative low temperature growth techniques, to minimize the interdiffusion of different atomic species. In addition we report on the feasibility of insitu Ga doping of Ge by taking advantage of the self-limiting mechanism inherent in the ALE process. EXPERIMENTAL Both n-type and p-type Ge substrates were used. They had 0.1 Qkcm resistivity and were oriented 6' off toward for the n-type and 9° off toward for the p-type. The Ge substrates were first solvent cleaned successively in 1,1,1 Trichloroethane, Acetone and methanol. Surface etching was a two-step procedure; first exposure to HCI: 11202:1120 (1:1:10) then D.I. water rinsed, and finished with Mat. Res. Soc. Symp. Proc. Vol. 163. ',,1990 Materials Research Society
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NH 4 OH:H 2 0 2:H2 0 (1:1:10), D.I. rinsed and dried with N2 gas flow. The substrates were annealed at 650'C for 10 minutes in H2 prior to initiating growth at the desired temperature (4000C - 500'C) for these samples. Details concerning the ALE growth process have been reported elsewhere [5-6]. TMGa [(Ga (CH 3)3 )] (-10%) and AsH3 (10% in H2 ) were used as the sources for Ga and As
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