Diffusion in strained Si(Ge)

Abstract

Experiments on Si-rich SiGe layers show an exponential increase in Ge difFusion and an exponential decrease in B difFusion as a function of compressive strain, indicating a linear dependence of activation energy on strain. The efFect arises &om the structural relaxation of the lattice around the defect mediating difFusion (inward for a vacancy, outward for an interstitial). We infer the mechanisms of Ge and B difFusion in strain-f'ree and compressively strained Si(Ge) at T ( 1030'C, and draw some general conclusions on strain-modified diffusion in cryst&»ne solids. PACS numbers: 66.30.Jt Strained SiGe/Si heterostructures and superlattices are an essential component of many advanced Si-based devices, but the kinetic mechanisms of SiGe layer relaxation during thermal annealing are still poorly understood. Most previous work on strained-layer relaxation has focused on the nucleation, growth, and multiplication of dislocation loops during growth and subsequent thermal annealing. However, some workers have reported an alternative, diffusive relaxation process [1,2). In particular, Iyer and LeGoues have reported enhanced Si-Ge interdiffusion which is quenched on formation of a high density of dislocations [2]. This observation was attributed to strain-assisted difFusion, based on the thermodynamic analysis of spinodal decomposition by Cahn and Hilliard [3], but no specific physical mechanism for the enhanced Si-Ge interdifFusion has so far been proposed. This remains a significant challenge for our understanding of difFusion in Si and related materials. Recently, enhanced As difFusion [4] and retarded B diffusion [5,6] have been reported in compressively strained Si-rich SiGe layers. In particular, Moriya et al. presented extensive data showing a large reduction (up to a factor of 10) in the intrinsic difFusivity of B in Si(Ge) under compressive strain. By making the critical assumption that B difFusion is mediated by positively charged point defects, Moriya et al. were able to explain their result in terms of band-gap narrowing [6]. Although this assumption is consistent with early diffusion data [7], it appears to be incorrect. More extensive diffusion studies, using isoconcentration p-type and n-type backgrounds, have shown that the contributions of charged and neutral point defects to intrinsic B difFusion are of similar magnitude [ 8,9]. This conclusion rules out a strong reduction in intrinsic B difFusion due to band-gap narrowing, and points to a more drastic strain-related phenomenon.A hint as to the nature of this phenomenon can be found in recent total-energy calculations [10,11). Antonelli and Bernholc computed the formation energies for self-interstitials (b, Eyl) and vacancies (EEyv) in Si as a function of hydrostatic pressure. A linear increase in AEfl and decrease in bEyv were found with increasing pressure, corresponding to an outward relaxation of the lattice around the interstitial, and an inward relaxation of the vacancy. More recently, the same authors computed the effect of tensile strain in a Si l...

Stress effects on defects and dopant diffusion in Si

“…The influence of biaxial stress on the diffusivity has been characterized experimentally [2][3][4][5][6][7][8] by the apparent change in activation energy with biaxial (tensile) strain, ε biax , at constant composition: Additionally, one must take care to isolate experimentally the stress effect from the composition effect;…”

“…heterojunction bipolar transistors for high-power and high-speed applications such as wireless communications. The study of stress effects on diffusion, which have only recently been characterized experimentally [2][3][4][5][6][7][8], is an important part of the study of the stability of such strainedlayer epitaxial materials. Additionally, although bulk wafers cannot sustain significant nonhydrostatic stresses at diffusion temperatures, such stresses are sustained near interfaces with patterned films and in the films themselves.…”

A Novel Model for Boron Diffusion in SiGe Strained Layers Based on a Kinetics Driven Ge-B Pairing Mechanism

“…The simulations were performed by implementing equation [3] in the process simulator DIOS included in the ISE TCAD tools package (4). Figure 2 shows the simulated B profile after a 40sec anneal at 1 100°C.…”

“…In this way, the enhanced B pile-up at high Ge concentrations is well reproduced. The parameters of equation [3] have been determined by fitting various experimental SIMS profiles.…”

Annealing effects on the microstructure of Ge/Si(001) quantum dots

“…Second, strain between islands and the surrounding area can play a role in diffusion. 19 The strain between top islands and the substrate is much smaller than the strain around and within buried islands, and therefore the force pushing the system to reduce the strain energy through elemental redistribution is much smaller in top islands than in buried islands. Third, the surface of unburied islands might have been oxidized and this would affect elemental redistribution during the annealing process.…”