Pressure-Enhanced Interdiffusion in Amorphous Si/Ge Multilayers: Implications for Defect-Mediated Diffusion
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STEVEN D. THEISS, F. SPAEPEN AND M. J. AZIZ Division of Applied Sciences, Harvard University, Cambridge, MA 02138
ABSTRACT
We have studied the pressure-dependence of the interdiffusion rate in amorphous Si/Ge multilayers. Samples were annealed in an externally-heated diamond anvil cell at 693 K at pressures ranging from 0 to 3.1 GPa. Interdiffusion rates were determined by ex situ x-ray diffraction measurements of the decay of the artificial Bragg peaks associated with the multilayer periodicity. Scaling experiments were performed to factor out the effects of concentration and time dependence in the diffusivity. All samples showed a consistent increase in diffusivity with applied pressure, characterized by a negative activation volume ranging from -44±3 to -37±2 percent of the atomic volume of Si for films ranging in average Si composition from 25 to 71 percent, respectively. These results are consistent with a model for diffusion in amorphous Si and Ge based on dangling bond migration.
INTRODUCTION
Point defects control two properties of fundamental importance to amorphous semiconductor device performance: diffusion rates [1) and electrical conductivity [2]. Though the subject of many experiments and much theoretical work, the nature of the point defects in amorphous semiconductors is still not well understood. As new applications for these materials continue to be found, a more complete and accurate picture of their defect structure and properties is needed. Vacancy and interstitial point defects in crystalline materials are well
defined. Point defects in amorphous semiconductors, however, probably exist in a range of states and configurations. Although there is no direct evidence for the presence of interstitiallike defects, Electron Spin Resonance (ESR) experiments have demonstrated the existence of singly-occupied electronic states, which is widely regarded as evidence for the presence of neutral dangling bonds [3]. Viscous flow studies on amorphous Si [4) indicate that the concentration of defects must be much larger than can be accounted for by ESR, suggesting the existence of charged dangling bonds or some other types of defects. The purpose of the present study is to explore the role of defects in mass transport in amorphous semiconductors by investigating the effects of hydrostatic pressure on interdiffusion in a-Si/a-Ge multilayers. If we consider the typical, Arrhenius form of the diffusion equation, we can relate the activation volume to the diffusivity, D, in the following way [5]:
D = Doexp( -AG- kT)
(1)
where
AG* = AE*+ PA V*- TAS*
15 Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society
(2)
AV*-kT(ln%
)T
(
Here, AG* is the Gibbs free energy of activation; and AE*, A V*, and AS* are the activation energy, activation volume and activation entropy for diffusion. From Eq. 3, we see that the diffusivity can either increase or decrease with pressure, depending on the sign of AV *. As is often done with AG*, A V* can be split into two components: (4) AV*=AVO +AV where AV ,
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