Misfit dislocation glide velocities have been measured in Si1−xGex/Si heterostructures. Dislocations were deliberately introduced at sites of crystalline damage, the samples were then annealed, and dislocation propagation distances measured using defect selective chemical etching. A number of different sample configurations were investigated with different layer thicknesses and alloy compositions. The measured velocities were found to depend on a number of factors including anneal temperature, an activation energy (which was found to depend on the Ge mole fraction), the effective misfit stress (which is a function of the Ge mole fraction and layer thickness), and the length of the threading arm of the misfit dislocation. Si/Si1−xGex/Si buried-layer structures typical of the heterojunction bipolar transistor were also studied. Two possible relaxation mechanisms, involving two- and three-segment dislocation configurations, are considered and an evaluation of the most likely mechanism for a range of different structures is presented. A complete quantitative analysis is made of all the results and expressions have been derived for the misfit dislocation glide velocity as a function of layer thickness and alloy concentration for all types of layer configuration.
The critical thickness for Si1−xGex strained layers for the alloy range 0<x<0.15 has been determined from annealed epilayers using mapping techniques which allow single dislocation detection and composition thickness measurements over large areas (∼50 cm2 ). A series of Si1−xGex layers was deposited by molecular beam epitaxy in which the composition (x) and thickness (h) were continuously varied across the substrate to produce a slowly changing strain energy density through the stable/metastable transition. On annealing at either 750 or 900 °C for 30 min, an abrupt transition in relaxation behavior was found at critical values of thickness and composition (hc,xc ). Increasing the anneal temperature or time did not shift the transition giving identical (hc,xc ) values. At strain thicknesses above these critical values a large increase in defect density was observed (>∼104 , cm−2) whereas in thinner strained epilayers, below the thermodynamic stability curve, no misfit dislocations were found. Nomarski microscopy of defect etched surfaces and x-ray topography were used to reveal misfit dislocations formed during the initial stages of relaxation. The appearance of single misfit dislocations at a density ≊1 cm−2 was taken as the criterion for a ‘‘relaxed’’ layer. The critical strain and thickness in the vicinity of these transition points were determined on the as-grown wafer by x-ray diffraction and Rutherford backscattering spectrometry with confirmation of layer thicknesses by cross-sectional transmission electron microscopy. The Matthews–Blakeslee [J. Cryst. Growth 27, 118 (1974)] equilibrium critical thickness he (nm), vs Ge atom fraction curve given by xe =0.55/he ln(4he /b) for 1/2 a0〈110〉, 60° glide dislocations with a Burgers vector b ∼0.4 nm, is an excellent fit to these experimental data, i.e., xc =xe and hc =he .
We present accurate low-temperature results for the thermopower, S, of Si inversion layers for a range of electron densities spanning the metal/insulator transition. The results at metallic densities are in good agreement with t h e calculated phonon drag contribution which clearly dominates S. We also conclude that phonon drag is also important for activated conduction.
A series of Si/Si,-,Ge, strained-layer superlattice structures has been studied by x-ray double-crystal diffractometry, Raman spectroscopy and transmission electron microscopy. The periodicity of the superlattices, the alloy composition and the degree of relaxation have been measured. The precisions of the three techniques are discussed and the results critically compared.
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