Core-shell Ge Nanoparticles on Oxide Surfaces for Enhanced Interface Stability
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0933-G02-07
Core-shell Ge Nanoparticles on Oxide Surfaces for Enhanced Interface Stability Scott K. Stanley, and John G. Ekerdt Chemical Engineering, University of Texas at Austin, 1 University Station, C0400, Austin, TX, 78712
ABSTRACT Germanium nanoparticles are grown on HfO2 substrates by hot-wire chemical vapor deposition (HWCVD). The oxidation and thermal stability of these unmodified Ge nanoparticles are studied with X-ray photoelectron spectroscopy (XPS). Core-shell nanoparticles were then prepared by growing the Ge cores with HWCVD and selectively growing Si or C shell layers on the Ge cores by conventional CVD. The formation of core-shell nanoparticles was monitored with XPS and low energy ion scattering. Large differences are observed in the thermal stability and oxide formation for unmodified Ge and the different core-shell nanoparticles. INTRODUCTION Germanium films and nanoparticles are increasingly used in microelectronics due to the higher carrier mobility of Ge over Si and enhanced carrier confinement in Ge nanocrystals compared to Si nanocrystals. Non-volatile flash memory, for example, is moving towards a nanocrystal-based architecture with Si or Ge nanoparticles on a HfO2 tunnel dielectric layer in place of a continuous Si floating gate. However, a primary problem with Ge integration is the rapid formation of low-quality, unstable germanium oxides at the Ge surface. The formation and stability of germanium oxides has been studied for Ge(100) substrates [1] and there are numerous examples in the literature of attempts to stabilize the Ge surface, primarily through nitridation. A method is needed to stabilize the Ge nanoparticle surface that will: (1) prevent oxidation, (2) provide thermal stability for subsequent processing, (3) be compatible with Ge and the gate dielectric, and (4) create an electrically favorable interface, i.e. low fixed charge, low defect density, and low trap density. In this paper, we present the synthesis and oxidation resistant properties of core-shell nanocrystals prepared by hot-wire chemical vapor deposition (HWCVD) and conventional CVD. HWCVD is used to grow high density Ge nanoparticle cores and selective CVD is used to grow a thin Si or C shell or solely a native oxide shell is formed. Core-shell nanoparticles are characterized with in situ X-ray photoelectron spectroscopy (XPS) and in situ low energy ion scattering (LEIS). The shell layers protect the Ge core from oxidation to differing degrees and pronounced differences in the thermal stability are observed. EXPERIMENT All experiments are carried out in a multi-chamber ultra high vacuum (UHV) system that has been described previously [2,3]. The substrates for nanocrystal growth are 10 nm atomic layer deposition grown HfO2 films on Si(100) substrates provided by SEMATECH. Samples are loaded as provided into the system and degassed at 600 oC for 10 min prior to any analysis or
growth. In situ XPS is used to monitor nanoparticle core and shell layer deposition and to analyze bonding in the surface region with Al Kα
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