Investigation has continued into the use of SixGe1−x multilayer structures (MLS) as a buffer layer between a Si substrate and a GaAs epitaxial layer in order to accommodate the 4.1% lattice mismatch. SixGe1−x 4-layer and 5-layer structures terminating in pure Ge have been grown using molecular beam epitaxy. Subsequent GaAs heteroepitaxy has allowed evaluation of these various GaAs/SixGe1−xMLS/Si (100) structures. Antiphase domain boundaries have been eliminated using vicinal Si (100) substrates tilted 6° off-axis toward [011], and the etch pit density in GaAs grown on a 5-layer SixGe1−x MLS on vicinal Si (lOO) was measured to be 106 cm−2.

Si1−xGex/Si heterostructures with varying layer-thicknesses have been characterized using photoluminescence and magnetic resonance detected on photoluminescence. Three of the four samples studied exhibit sharp photoluminescence bands at different energies. For a 120 Å Si/40 Å Si1−xGex heterostructure, magnetic resonance of an electron in the Si and of a hole in the Sil. xGex layers were observed. These results indicate cross-interface, or Type II, excitonic recombination. Further, anisotropie magnetic resonance spectra indicate the presence of dangling-bonds defects in the heterostructures.

In this paper we address the problem of producing SiGe buffer layers of acceptable quality for the growth of symmetrically strained SiGe structures. Initially we consider SiGe layers grown to well beyond the metastable critical thickness and examine the degree of residual strain both as - grown and post anneal. The defect levels in metastable SiGe layers following high temperature anneal were also studied. A buffer layer was grown consisting of stacked metastable SiGe layers each of which is annealed in situ prior to the growth of the next layer and terminating with a 0.45 SiGe alloy. This produces nearly fully relaxed 1.15pim thick structures with threading dislocation densities of 4 × 106cm−2. Limited area growth on Si suggests that elastically relaxed material free of both threading and misfit dislocations can be produced.

The objective of this paper is to demonstrate the epitaxial growth of SiGe strained layers using atmospheric-pressure chemical vapor deposition (APCVD). We have grown SiGe layers with various thicknesses and Ge concentrations at temperatures ranging from 800–1000°C. The samples were studied using a variety of methods, including transmission electron microscopy (TEM), high resolution X-ray diffraction (HRXRD) and Raman spectroscopy (RS). Both HRXRD and RS results indicate that samples with about 10% Ge and a thickness of about 1000 Å are almost fully strained. TEM analyses of these samples indicate a film defect density less than 105/cm2. SIMS results indicate that the oxygen concentration in the epitaxial layers is lower than that found in CZ substrates.

Our analyses also indicate that as-grown epitaxial Ge layers several microns thick have a defect density less than 107/cm2. The relatively low defect density in both SiGe and Ge layers grown on Si has been attributed to far higher dislocation glide velocity at the relatively elevated growth temperatures employed in CVD and to very good growth cleanliness.

Undoped thin layer silicon on insulator has been fabricated using wafer bonding and selective etching techniques using an MBE grown Si0.7Ge0.3 layer as an etch stop. Defect free, undoped 200–350 nm silicon layers are routinely fabricated using this procedure. A new selective silicon-germanium etch has been developed that significantly improves the ease of fabrication of the BESOI material.

The prospect of efficient electroluminescence and photodetection in Si-based heterostructures has stimulated considerable interest in recent years. Si-based optoelectronic devices would allow monolithic integration of mature Si technology with optical signal processing. However, Si and epitaxial Si1−xGex/Si (100) alloys are indirect semiconductors and band structure perturbations are necessary to enhance luminescence efficiencies to practical levels. In this review we consider the relative success of the various approaches used to obtain efficient luminescence from Si based heterostructures after first considering the optical properties of Si grown by MBE and doped by several techniques. Exciton binding centers such as isoelectronic defect complexes, zone folding in atomic layer superlattices, e.g. SimGen, and rare earth doping, e.g. Er in Si and Si1−xGex alloys are discussed. The internal quantum efficiencies for the above processes will be compared in both electroluminescence and photoluminescence together with the prospects for room temperature light emission. The twin requirements of band structure engineering and superlative crystal quality extend the challenge to the materials scientist / crystal grower further into the nanoscopic regime. It is a formidable task involving the controlled removal of non-radiative point defects and complexes, controlled introduction of radiative centers, and simultaneous maintenance of coherent interfaces without misfit dislocation injection. Recent progress in our study of intense photoluminescence (wavelength range 1.2 -1.7 μm) from random Si1−xGex alloys (0.53 > × > 0.06) grown by MBE, a 1.5 μm Si0.82Ge0.18/Si LED and a novel Si1−xGex/Si optoelectronic switch will be highlighted.

Si/Ge superlattices offer the possibility to study the influence of various effects on the electronic band structure. Strain, quantization, zone folding, and wave vector mixing effects are expected to create novel optical properties in these Si-based multilayer structures. New optical transitions were observed by various experimental methods like photoluminescence and modulation spectro-scopy. In this article we will review the present state of knowledge of the optical properties of short period strained layer Si/Ge superlattices and discuss the problems and prospects for optical applications.

In MBE Si1−xGex which is grown to thicknesses greater than the critical thickness hc, the dislocation-related luminescence peaks Dl and D2 have energies which are independent of x up to x ≈ 0.3, and then decrease, as observed in LPE Si1−xGex. In MBE Si1−xGex layers grown to thicknesses less than hc, post-growth annealing produces dramatic changes in the luminescence, giving spectra as from relaxed alloy, even though the relaxation determined by X-rays is negligible. These results establish photoluminescence as a sensitive diagnostic tool for detecting dislocations in Si1−xGe*.

Short-period SimGen (m monolayer (ML) Si, n ML Ge. n+m<∼40ML∼5.5nm) strained layer superlattices (SLS) are grown on <100> silicon by low temperature molecular beam epitaxy. Various characterization tools such as X-ray diffraction, transmission electron microscopy, Rutherford backscattering, Raman spectroscopy and photocapacitance measurements are used to analyze the growth quality, stram distribution, periodicity, interface sharpness and optical properties of the SLS. Recent photoluminescence experiments give hints of a direct bandgap transition from a 10 ML Si6Ge4 SLS in the near infrared spectral region. I-U and C-U measurements on mesa diodes (Am=2 10−4 cm2) are performed at various temperatures down to T=35K. Photocapacitance measurements show a Wannier-Stark localization of the superlattice states in a p+-n doped Si4Ge4 SLS diode, for the first time observed in type II superlattices. The observed transitions are believed to be defect or impurity related and are discussed in terms of a Wannier-Stark ladder behaviour.

The effects of strain and confinement on optical phonons in a Si12Ge4 strained layer superlattice grown by MBE on c-Si (001) were studied as a function of hydrostatic pressure (T = 295 K) using Raman scattering. The change of phonon frequency with pressure, dω/dP, for the principal quasi-confined LO mode in the Ge layers is found to be significantly smaller than that for bulk crystalline Ge because the magnitude of biaxial strain decreases in the Ge layers with added pressure and because the Grüneisen parameter of the confined mode is smaller than that of the Γ-point optical phonon. More generally, it is noted that the magnitude of biaxial strain in many strained layer superlattices initially decreases with the application of hydrostatic pressure, making the structures more stable.

Well resolved band-edge luminescence of excitons in silicon-germanium alloy strained layers, quantum wells, and superlattices has been observed in films grown by Rapid Thermal Chemical Vapor Deposition. The signal is due to bound excitons at low temperatures and free excitons at higher temperatures, and has a strong no-phonon signal which is caused by alloy scattering. Bandgaps inferred from photoluminescence agree well with those measured by absorption spectroscopy, inferring that a no-phonon process dominates the band-edge absorption.

We report the growth and characterization of GexSi1−x films for optical waveguiding. GexSi1−x/Si waveguides were grown by rapid thermal processing chemical vapor deposition. An average attenuation of 3.3 dB/cm was achieved for a 1 μm thick Ge0.04Si0.96 layer patterned into rib waveguides 2000 Å deep with widths of 5 μm. Directional couplers were also fabricated. Average coupling efficiencies of 85% were achieved for 1.5 μm interwaveguide separation.

Low-temperature selective epitaxial growth (SEG) of silicon using a dichlorosilane-hydrogen mixture in an LPCVD hot-wall reactor has been discussed with respect to the wafer preparation and the deposition cycle. The surface morphology and the quality of epilayers are strongly affected by residual oxide islands at interface. A reduction of local growth rate near interfacial oxides is attributed to the dissolution of oxide at interface, and this reduction can lead to pits and textured features on the Si epitaxial surface. An ex-situ HF vapor or an HF dip with an in-situ small DCS 900°C prebake step can completely remove surface oxide prior to the deposition and achieve defect-free Si epilayers at the deposition temperatures of 850°C and 800°C. It is also found that fluorine atoms can play a major role in the removal of surface oxide.

Short period Si/Ge superlattices have been grown on Ge (001) and Si (001) substrates by molecular beam epitaxy. The optical properties of the superlattices have been studied with photoreflectance. (PR) and resonant Raman scattering (RRS). With PR we are able to observe new, structural induced transitions for all superlattices which are related to E0-and E1-like gaps. The analysis of PR spectra is complicated by an optical etalon effect if the samples are sufficently thick. The E1-like transitions in the range between 1.9eV and 2.7eV are also studied with RRS. Due to the confinement of the optical phonons in the Ge and Si layers RRS is able to probe the bandstructure in each layer seperately. Localized electronic states in the Ge layers can be observed with RRS for a Si4Ge18 superlattice and are compared with PR measurements.

This paper reports the incorporation of erbium into MBE Si and Si/Ge alloys with substrate temperatures of 500°C and 700°C. Using a solid source MBE system, concentrations of erbium between 1018 and 1022 cm−3 have been studied by photoluminescence, electrical measurements, SIMS and TEM. We find no shallow donors or acceptors attributable to erbium but we observe a high concentration of deep states with an activation energy of ∼360 meV. The photoluminescence output is of greatest magnitude when [Er] =2 × 1018 cm−3. Above this concentration the onset of erbium precipitates can just be observed using TEM and at even higher concentrations structured growths of erbium suicide are apparent. The effect on the optical activity of Si:Er that has subsequently been implanted with oxygen is also reported.

We show that the infinite Si-Ge superlattices predicted to exhibit strong optical transitions across the fundamental gap and between conduction subbands also give rise to significant optical nonlinearities. Efficient optical transitions can also be achieved in finite-length structures grown on Si or Ge substrates. However, the zone folding argument does not apply and the electronic structure strongly reflects the finite length of such systems.

The intersubband absorption of electrons in modulation doped Si/Si1−xGex multiple quantum wells has been observed [1]. Various samples with different well widths and carrier densities have been studied. Narrow absorption lines are observed in waveguide geometry. Self consistent subband calculations are in good agreement with the experimental values.

The extraction of recombination lifetime from the current-vol tage characteristics of diode structures containing Si1−xGex strained layers is discussed. Electrical measurements are used in conjunction with computer simulations to extract minority carrier lifetime in Si1−xGex layers with various oxygen concentrations. The minority carrier lifetime in Si1−xGex increases from several psec at an oxygen concentration of 2×1020 cm−3, to greater than 0.5 μs at concentrations below 3×1017 cm−3. The structures are analyzed for sensitivity of the current characteristics to Si1−xGex minority carrier lifetime. Calculations predict that the maximum lifetime which can be extracted from such structures is greater than 100 μsec. However, due to limitations imposed by perimeter currents, the maximum lifetime which can be extracted from our diode structures is on the order of 3 μsec. Maximizing area to perimeter ratio of the diode structures and moving the Si1−xGex-SiO2 interface away from the active device region is required in order to increase the maximum extracted lifetime from such structures.

Two dimensional hole gases have been investigated in Si/SiGe modulation doped heterostructures grown by RT-CVD for the first time. Single, both normal and inverted, and double heterostructures were studied. The results suggest that any asymmetry due to dopant segregation or autodoping between the normal and inverted structures occurs on a scale of less than 1 nm.

We have investigated magneto-transport properties of differently strained Si/ Si1−xGex resonant tunneling devices. The built-in strain was either put in the Si layers, by means of a thick., relaxed Si1−xGex buffer layer, or in the Si1−xGex layers, in which case all Si1−xGex layers were grown below the critical thickness, and a Si1−xGex spacer layer with graded Ge content was used. Magnetic fields parallel to the interface have been employed to probe the in-plane dispersion in the quantum well. This is used to study the effect of band-mixing in the two strain configurations. A field perpendicular to the interface resolves some Landau level splitting. Most strikingly, however, is the similarity in the spectra with the case when the magnetic field is applied parallel to the interfaces. This indicates broadening of the levels, possibly due to scattering, and the importance of 3-dimensional band structure effects.