Initial Nucleation Studies of Heteroepitaxial GaAs films on Si Substrates by Modulated Molecular Beam Epitaxy
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INITIAL NUCLEATION STUDIES OF HETEROEPITAXIAL GaAs FILMS ON Si SUBSTRATES BY MODULATED MOLECULAR BEAM EPITAXY
Henry P. Lee, Xiaoming Lius), and Shyh Wang Department of Electrical Engineering and Computer Sciences University of California, Berkeley, CA.
94720 Thomas George and Eicke R. Weber Department of Materials Science and Mineral Engineering University of Calfornia, Berkeley, CA. 94720
Zuzanna Liliental-Weber Center for Advanced Materials and, Material and Chemical Sciences Division, Lawrence Berkeley Laboratory University of California,Berkeley, CA 94720
ABSTRACT Nominal 100 A and 150A thick GaAs layers were deposited on Si substrates by a modulated molecular beam technique and normal molecular beam epitaxy (MBE) at 300*C and 375*C respectively for plan view and cross-sectional transmission electron microscopy (TEM) examinations. From coverage of Moire fringes, it is found that the nucleated GaAs films grown by the modulated molecular beam technique were thinner, streaker and more two-dimensional than the MBE grown films. The same modulated molecular beam technique was also used for the deposition of the initial buffer layer of 3pm thick GaAs on Si films. Results from the 77k photoluminescence(PL) and double crystal X ray diffraction measurements showed that these films have superior optical and structural quality compared to similar films grown by normal two-step MBE. The improvement is attributed to a more twodimensional nucleation of GaAs films associated with the modulated molecular beam growth technique.
INTRODUCTION The use of migration enhanced molecular beam epitaxy (MEE)[l,2] has opened a new direction for growing high quality GaAs films on Si substrates[3-5] as it offers independent and abrupt temporal controls of the group III and V beam fluxes. In MEE growth, the Ga and As shutters are pulsed to open and close at an interval equal to or approaching the time for a monolayer growth of GaAs. The absence of As beam flux during the opening cycle of the Ga shutter enhances the surface mobility of impinging Ga atoms on the host substrate and allows the deposited GaAs films to achieve proper stoichiometry at a much lower substrate temperature (as low as 2001C) than is possible for conventional MBE growth. Results from reflection high energy electron diffraction (RHEED) oscillation studies on a)onleave from Tsinghua University, Beijing China.
Mat. Res. Soc. Symp. Proc. Vol. 145. @1989 Materials Research Society
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homoepitaxial GaAs growth also indicates that the epitaxial growth in MEE scheme followed a more ideal layer by layer two-dimensional (2-d) pattern than normal MBE[1,2]. For GaAs on Si heteroepitaxy, the use of MEE during the buffer layer growth provides an additional mean to control the initial nucleation and thus influencing the dislocation structures of the deposited GaAs films. Furthermore, epitaxial growth of GaAs on Si at a lower temperature without sacrificing the film quality is extremely beneficial. It is expected to reduce residual thermal stress in the epitaxial GaAs films and
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