Equilibrium critical thickness for Si1−xGex strained layers on (100) Si

Abstract

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 .

Fabrication technology of SiGe hetero-structures and their properties

“…High temperature (900 8C) post-growth annealing for SiGe films grown at 500-600 8C [51,52] and rapid-thermal chemical vapor deposition (RTCVD) at 900 8C [53] lead to excellent agreement of the experimental critical thickness with the predicted values of the Matthews and Blakeslee model, as shown in Fig. 15(a and b), respectively.…”

Role of interfacial oxygen on the quality and strain stability of pseudomorphic silicon-germanium layers grown on Si substrates

“…In turn, these growth-related defects (diamond defects) arise due to the presence of oxygen at the growth interface. This observation has important consequences when analyzing historical observations of the metastable critical thickness and thermal stability of strained SiGe alloys [11][12][13]. In fact, the present data would suggest that any measured relaxation for similarly metastable strained SiGe layers would imply the existence of growth-related defects (such as O-originated diamond defects).…”

Structural characterization of Si0.7Ge0.3 layers grown on Si(001) substrates by molecular beam epitaxy

“…It is known that even if the thickness of alloy layers exceeds the critical thickness, the residual strain is usually present in the alloy layer up to the larger thickness depending on the Ge composition 14,15 and growth temperature. 16,17 According to People and Bean's theory, 13 the critical thickness for Si 1Ϫx Ge x layer on Si͑001͒ is about 1.0 m, 180 and 60 nm for xϭ0.1, 0.2, and 0.3, respectively. Thus, the step buffer with a thickness of 300 nm should grow pseudomorphically on the Si substrate.…”