Synthesis and Processing of Nanocrystalline Ge:Si Materials
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ABSTRACT Crystalline nanometer-scale Ge particles have been synthesized by pulsed laser ablation and introduced into a Si host matrix, grown by chemical beam epitaxy from disilane. The proposed structure of the Ge:Si composite films consists of Ge nanocrystals surrounded by a thin epitaxial Si shell that passivates the surface of the Ge nanocrystallite. The Ge nanocrystallites are randomly oriented with respect to each other and are randomly distributed in a polycrystalline or amorphous Si matrix. Ge nanoparticles with and without Si matrix were deposited directly on C-coated TEM grid and imaged by high resolution TEM. Ge:Si composites deposited on Si wafers were characterized by RBS.
INTRODUCTION Nanoscale materials have been the focus of increasing research interest in recent years becuase of their potential for exhibiting quantum confinement effects, [1]. Solution-based techniques for synthesizing nanocrystalline semiconductors have been successful for II-VI compounds such as CdSe, where it is possible to precisely control the size of the clusters by varying the reaction conditions[2]. Solution-based techniques have also been reported recently for synthesizing nanometer-scale silicon[3] and germanium[4] crystals. Ge nanocrystals have also been synthesized by annealing cosputtered Ge and Si0 215], and annealing GexSii-x oxides[6]. Pulsed laser ablation (PLA) is an alternative technique for synthesizing clusters of almost any solid material [7]. We use this method to produce Ge clusters, then codeposit the Ge clusters into a Si host matrix grown by chemical beam epitaxy (CBE). The Si host is chosen to passivate the Ge clusters while being comptaible with existing Si microelectronic technology.
EXPERIMENTAL Our PLA cluster source is shown schematically in Figure 1. Briefly, a rotating Ge (99.999%) rod is ablated by a pulsed Nd:YAG laser. The ablated Ge plasma is cooled by a pulse of He carrier gas released into the 12 mm long growth channel and starts to condense in the growth channel before expanding into vacuum. The supersonic expansion rapidly cools the carrier gas and quenches the cluster condensation reactions, leaving a distribution of cluster sizes, ranging from Angstroms (e.g., Ge2, 263 Mat. Res. Soc. Symp. Proc. Vol. 326. @1994 Materials Research Society
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Figure 1 Schematic diagram of pulsed laser ablation apparatus. A pulsed Nd:YAG laser ablates the Ge target rod, after which the ablated Ge vapor is cooled and condensed by a gust of He released into the growth channel. Ge3, etc.) to thousands of Angstroms in diameter. As shown by Chiu et al. [8], the cluster size distribution can be controlled by varying process parameters such as the length of the growth channel, and the relative timing of the helium and laser pulses. The clusters are collimated by a skimmer before passing into the codeposition chamber where they impinge upon a substrate pl
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