Structural and Electrical Properties of Molecular Beam Deposited Polycrystalline Silicon
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STRUCTURAL AND ELECTRICAL PROPERTIES OF MOLECULAR BEAM DEPOSITED POLYCRYSTALLINE SILICON.
SYLVAIN L. DELAGE, S.-J. JENG, D. JOUSSE and S.S. IYER IBM Research Division T.J. Watson Research Center P.O. Box 218, Yorktown Heights, NY 10598
ABSTRACT The structural and electrical properties have been investigated of antimony doped polycrystalline silicon films obtained by molecular beam deposition on oxidized silicon substrates. We show that low resistivity films with smooth morphology are obtained by Solid Phase Crystallization of antimony doped amorphous silicon layers deposited at 250'C. A resistivity of 4.3 mn2 cm is obtained by crystallizing the films at temperatures as low as 6500C for 15 minutes. Similar resistivities are typically obtained by Chemical Vapor Deposition at temperature of at least 850 0 C. In-situ crystallization of the amorphous silicon is needed to obtain low resistivity polysilicon. We also show that direct deposition above 650 0 C gives rise to polycrystalline silicon with much higher resistivities. INTRODUCTION Silicon Molecular Beam Epitaxy (MBE) has been used recently in the fabrication of exploratory devices [for instance: 1, 2, 3). This mainly stems from the Ultra High Vacuum (UHV) conditions and the physical deposition of the species, which allow epitaxial growth at moderate temperature, i.e. typically between 550'C and 750*C. Gallium and antimony are the most common dopants used in Si MBE. Until now, little work [4, 5. 6] has been reported on polycrystalline silicon (polysilicon) deposited using the MBE technique. The silicon deposited on any amorphous insulating layer such as silicon dioxide or silicon nitride gives rise to amorphous silicon below 600'C and to polycrystalline silicon above 650*C. This behavior is crucial to device applications. A large part of the exposed surface prior to epitaxy indeed consists of dielectric layers. Furthermore, the cleanliness of the UHV conditions allows us to study the properties of polycrystalline silicon under controlled impurity conditions [7]. The material, therefore, is not altered by contamination, and its study may answer some fundamental questions. The electrical transport and the hydrogenation effects in the molecular beam deposited polysilicon are discussed separately [8]. In this work, we compare the structural and electrical properties of polycrystalline silicon obtained by two different techniques. Polycrystalline silicon may be grown either by Direct Formation (DF) or by Solid Phase Crystallization (SPC). DF occurs if the oxidized substrate is kept above 650°C, and SPC is obtained by crystallizing at temperatures above 650°C an amorphous layer deposited at 250 0 C. We show that SPC samples have a different granular morphology and a much lower resistivity than the DF samples. We also study the thermal stability of the SPC polysilicon and the influence of UHV conditions on the polysilicon formation. SAMPLE PREPARATION A Si MBE system capable of processing up to 150 mm wafers is used [9]. 200 nm thick silicon layers are deposited on 125
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