J. C. Bean

Calculation of critical layer thickness versus lattice mismatch for GexSi1−x/Si strained‐layer heterostructures

A calculation of the critical layer thickness hc for growth of GexSi1−x strained layers on Si substrates is presented for 0≤x≤1.0. The present results are obtained assuming misfit dislocation generation is determined solely by energy balance. This approach differs therefore from previous theories (e.g., Matthews et al.), in which the absence of mechanical equilibrium for grown‐in threading dislocations determines the onset of the generation of interfacial misfit dislocations. It is assumed that interfacial misfit dislocations will be generated when the areal strain energy density of the film exceeds the energy density associated with the formation of a screw dislocation at a distance from the free surface equal to the film thickness h. For films thicker than this critical value, screw (and edge) dislocations will be generated at the film/substrate interface. Values obtained for the critical thickness versus lattice mismatch are in excellent agreement with measurements of hc for GexSi1−x strained layers on...

GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy

Ge x Si1−x films are grown on Si by molecular beam epitaxy and analyzed by Nomarski optical interferencemicroscopy, Rutherford ion backscattering and channeling, x‐ray diffraction, and transmission electron microscopy. The full range of alloy compositions will grow smoothly on silicon. Ge x Si1−x films with x≤0.5 can be grown free of dislocations by means of strained‐layer epitaxy where lattice mismatch is accommodated by tetragonal strain. Critical thickness and composition values are tabulated for strained‐layer growth. Multiple strained layers are combined to form a Ge x Si1−x /Si strained‐layer superlattice.

Band alignments of coherently strained GexSi1−x/Si heterostructures on 〈001〉 GeySi1−y substrates

The self‐consistent ab initio pseudopotential results of C. G. Van de Walle and R. M. Martin [J. Vac. Sci. Technol. B 3, 1256 (1985)] have been combined with a phenomenological deformation potential theory to estimate the band gap and band offsets for coherently strained multilayers of GexSi1−x/Si for growth on 〈001〉 GeySi1−y substrates. It is found that ΔEc is negligible and the narrower GexSi1−x gap falls within the wider Si gap (type I band alignment) if the Si in the multilayers is cubic, whereas ΔEc can be appreciable and the GexSi1−x conduction‐band edge tends to be higher in energy than the Si conduction‐band edge(type II band alignment) if both the Si and the GexSi1−x are strained. In particular, the present results resolve the seeming paradox which arose from interpretations of modulation doping experiments using heterojunctions grown either on Si〈001〉 substrates or on an unstrained alloy buffer layer.

Measurement of the band gap of GexSi1−x/Si strained‐layer heterostructures

We have used photocurrent spectroscopy to measure the optical absorption spectra of coherently strained layers of GexSi1−x grown on 〈001〉 Si by molecular beam epitaxy. A dramatic lowering of the indirect band gap, relative to that of unstrained bulk Ge‐Si alloys, is observed. Our results for 0≤x≤0.7 are in remarkably good agreement with recent calculations of the effects of misfit strain on the band edges of coherently strained Ge‐Si heterostructures. At x=0.6, the gap is lower than that of pure Ge.

Modulation doping in GexSi1−x/Si strained layer heterostructures

We report the first observation of the modulation doping effect in Si/Ge0.2Si0.8 heterojunctions grown by molecular beam epitaxy. Peak hole mobilities of ∼3300 cm2 V−1 s−1 have been observed at 4.2 K. These values, although nonoptimum, are comparable to the best reported values for holes in Si/SiO2 inversion layers. Low temperature, angular dependent, Shubnikov–de Haas measurements have demonstrated the two‐dimensional nature of the hole gas and yield a surface carrier density of 3.5×1011 cm−2. From the temperature dependence of the Shubnikov–de Haas amplitudes a hole effective mass of 0.30±0.02mo has been derived. Identical measurements on n‐type heterojunctions having the same Ge content (x=0.2) have failed to show a sustained enhancement of mobility at low temperatures, indicating that ΔEv≫ΔEc.

Raman scattering from GexSi1−x/Si strained‐layer superlattices

Raman spectroscopy has been used to determine built‐up deformation in GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy. By comparing peak positions in commensurate superlattices and single layers with those from incommensurate thick layers of the same composition we can obtain a quantitative determination of strain. Linewidths are affected by the presence of inhomogeneous strain, dislocations, and disorder. Lines are always narrower in superlattice samples, indicating better crystalline quality. In particular, the Raman line from the Si layers of the strained‐layer superlattices is indistinguishable from that from single‐crystalline Si in both linewidth and frequency. This is consistent with the expectation that the entire lattice mismatch is accommodated as a homogeneous tetragonal strain in the alloy layers only.

Pseudomorphic growth of GexSi1−x on silicon by molecular beam epitaxy

GexSi1−x layers are grown on Si substrates over the full range of alloy compositions at temperatures from 400–750 °C by means of molecular beam epitaxy. At a given growth temperature films grow in a smooth, two‐dimensional manner up to a critical germanium fraction xc. Beyond xc growth is rough. xc increases from 0.1 at 750 °C to 1.0 at ∼550 °C. Rutherford ion backscattering measurements indicate good crystallinity over a wide range of growth conditions. Transmission electron microscopy reveals that in thin films, the lattice mismatch between the GexSi1−x and Si layers can be accommodated by lattice distortion rather than by misfit dislocation formation. This pseudomorphic growth condition can persist to alloy thicknesses as large as l/4 μm.

Picosecond optoelectronic detection, sampling, and correlation measurements in amorphous semiconductors

The rapid relaxation of the photoresponse of high‐defect‐density amorphous semiconductors such as evaporated a‐Si has been utilized to develop an electronic measurement capability which can generate and sample electronic transients with speeds ≲10 ps.

GexSi1−x strained‐layer superlattice waveguide photodetectors operating near 1.3 μm

Properties of GexSi1−x strained‐layer p‐i‐n detectors, in which the strained‐layer superlattice itself was used as an absorption region, have been studied for the first time. These devices were grown on (100)Si by molecular beam epitaxy. Using waveguide geometry we have obtained internal quantum efficiencies on the order of 40% at 1.3 μm in superlattices with the Ge fraction x=0.6. The superlattice detectors show the frequency response bandwidth of over 1 GHz and uniformly excellent electrical characteristics for values of x as large as 0.8.

Growth of single‐crystal CoSi2 on Si(111)

Single‐crystal CoSi2 films have been grown under ultrahigh vacuum conditions on Si (111) by both standard deposition and molecular beam epitaxy techniques. Films were analyzed by Rutherford backscattering spectroscopy and channeling, transmission electron microscopy, and low energy electron diffraction. The films are free of grain boundaries but are rotated 180° about the normal to the Si surface. The crystalline perfection, as measured by channeling, is the best yet reported for an epitaxial silicide system. The expected hexagonal misfit dislocation arrays, along with a coarser triangular defect structure, are confined to the plane of the interface.