A New Method for Preparing Ge Nano-Crystallites Embedded in SiN Y Matrices
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INTRODUCTION Nanometer-size semiconductor crystallites, also termed quantum crystallites and quantum dots, have attracted much attention, because they exhibit the unusual properties and have potential for applying in the new generation photonic devices.[1, 2] Recently, a great deal of research effort is focused on low-dimensional nanostructures made from indirect-gap semiconductors such as Si [3, 4] or Ge[5]. For synthetic methods, in Si or Ge system with the covalent, strongly bonded diamond lattice structures, a quite high temperature will be required to anneal an aggregate of thousands atoms into the stable thermodynamic isomer. Previous works about the preparations of Si and Ge nano-crystallites include mainly two approaches. One is for the synthesis of nanometer-sized Si or Ge crystalline particles using physical vapor deposition and laser ablation[6, 7]. The other is for the nucleation of Si or Ge nano-crystallites by thermal annealing from a solid solution phase[8]. The latter approach also involves laser annealing treatment in amorphous multiquantum well structures made by the plasma enhanced chemical vapor deposition (PECVD) method [9, 10]. The motivation of this work is to explore a new approach for the synthesis of Ge nanocrystallites by using the PECVD method. It is well known that if one is to study the nature of internal confined states in the crystallites, the surface states must be either passivated by chemical derivatization or encapsulated in an insulating phase. Over the past several years, we have successfully prepared Si- or Ge- based multiquantum wells and superlattices such as a-Si:H/a-SiNx:H, a-Si:H/a-SiC,:H, a-Ge:H/a-Si:H and a-Ge:H/a-SiNx:H, etc. The classical and quantum electronic phenomena in these multiquantum well structures 111
Mat. Res. Soc. Symp. Proc. Vol. 358 ©1995 Materials Research Society
have been studied. These compositional modulated multilayers were deposited by the PECVD method with periodically alternated reactant gases, which involved SiH 4 or GeH 4 and NH3 and formed the sublayers of Si or Ge and SiNg respectively. However, with reactant gases of GeH4 and NH 3 mixed in the hydrogen plasma, we did not successfully get GeNx sublayer. These experimental results imply that the Ge-N chemical bond can not be formed and the Ge-Ge or Ge-H bonds can be preferentially bonded under the regular conditions in the hydrogen plasma CVD system. We were much enlightened by this discovery to consider a new approach, termed the preferential chemical bonding formation model, to synthesize Ge nanocrystallites embedded in SiNy matrices by using the PECVD technique with the reactant mixture gases of SiH 4, GeH 4 and NH 3. In this paper, we report the experimental details about this new approach for synthesizing Ge nanocrystallites embedded in a-SiNy matrices. The direct evidences from infrared (IR) absorption spectra, X-ray diffraction (XRD), Raman scattering, and transmission electron microscopy (TEM), will be presented to support our new idea. Finally we discuss briefly the synthesis mec
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