Characterization of morphology and defects in silicon-germanium virtual substrates

Silicon-germanium heterostructures incorporating virtual substrates are successfully used for both microelectronic and optoelectronic applications. However, their use is limited by their surface morphology (e.g. roughness) and defect (e.g. threading dislocations) density. High quality silicon-germanium heterostructures incorporating virtual substrates have been grown epitaxially using different methods. This study reports the effects of the growth parameters on the morphology and defects in different silicon-germanium heterostructures incorporating virtual substrates grown in the Southampton University Microelectronics center (SUMC) by low pressure chemical vapor deposition (LPCVD). Two types of structures: one with a linear and the other with a step variation of the germanium concentration in the virtual substrate, were grown and characterized. Results obtained were in good agreement with others already reported in the literature for similar structures grown using different epitaxial techniques.     

Characterization of laser annealed polycrystalline silicon films on various substrates

The structure and property of eximer laser annealed poly-Si films on various substrate materials such as MoW, Cr, were studied. It was found that the crystallinity of the film depended on the recrystallization energy density and substrate materials. The crystallinity of the film on substrate was the highest quality, and the higher the thermal conductivity of the substrate, the poorer the crystallinity of the poly-Si films at the same laser energy density. In the lower laser energy region than optimum, the grain size and surface roughness were increased with increasing laser power due to the increase of the crystallinity and decreased intrinsic stress. On the other hand, in the higher power region than optimum, with the increase of laser power, X-ray intensity and grain size were decreased due to the fast solidification velocity. There was no metal diffusion into poly-Si film but small amount of Si, less than 3 atomic percent, diffused into the metal film during the recrystallization process.     

Special issue on silicon-germanium alloys

Editorial

Special issue on silicon-germanium alloys     

Further evidence on the observation of compositional fluctuation in silicon-germanium alloy nanocrystals prepared in anodized porous silicon-germanium films

The microstructure of porous anodized silicon-germanium (Si_1−xGe_x) films was characterized using Raman spectroscopy and electron microscopy. Different porosity samples were obtained from undoped (high-resistivity) epitaxial Si_0.87Ge_0.13 films and from single-crystalline and low-resistivity Si_0.90Ge_0.10 wafers. Supporting the earlier observations (on the former type of porous samples) we reported recently, the Raman results presented in this study convincingly demonstrate that as the film porosity increases, the sizes of Si_1−xGe_x nanocrystals decrease and the film composition modifies in favor of Ge. Structure and the scale of the SiGe nanoparticles in these films were shown and examined by high-resolution (HR) electron microscopy.     

Preparation of hot-pressed silicon-germanium ingots: Part I — Chill casting of silicon-germanium alloys

Large chill cast ingots of silicon-germanium produced from melts that were not stirred contained two major alloys, one rich in silicon and one rich in germanium, in addition to a composition continuum. While stirring produced little change in distribution of Si/Ge ratios, inductive stirring did result in ingots that varied in composition but in a more uniform manner. In all cases, dopants were uniformly distributed throughout the cast ingots.     

Bulk growth of silicon-germanium solid solutions

This paper provides an overview of the technology for growing bulk silicon-germanium solid solutions and of the structural properties of the solidified materials. It is an attempt to summarize and value the methods and efforts applied to the controlled crystallization of silicon-germanium melts which were employed during the last four decades. The especially high degree of segregation makes the system sensitive to small changes of growth conditions, which leads to inhomogeneities and strain. The future availability of homogeneous, low-defect Si-Ge crystals through the whole composition range is briefly discussed.     

Epitaxial growth of silicon-germanium single crystals

The problems associated with obtaining single crystals of silicon-germanium with comparable concentrations of the two elements, are discussed. Epitaxial growth from the vapour phase was found to be much more suitable than growth from the melt, which yielded crystals with gross inhomogeneities, even when only 3% of Ge was included. By using a modified epitaxial system which is described, Si-Ge single crystal layers of up to 32% Ge were grown succesfully. It was found that growth temperature must be reduced with increased Ge content for optimum results. The measured growth rate was a function of both the temperature and the molar ratio of Si to Ge in the gas phase.     

Growth of poly-crystalline silicon-germanium on silicon by aluminum-induced crystallization

The formation of poly-crystalline silicon-germanium films on single-crystalline silicon substrates by the method of aluminum-induced crystallization was investigated. The aluminum and germanium films were evaporated onto the single-crystalline silicon substrate to form an amorphous-germanium/aluminum/single-crystalline silicon structure that was annealed at 450 °C-550 °C for 0-3 h. The structural properties of the films were examined using x-ray diffraction, Raman spectroscopy and Auger electron spectroscopy. The x-ray diffraction patterns confirmed that the initial transition from an amorphous to a poly-crystalline structure occurs after 20 min of aluminum-induced crystallization annealing process at 450 °C. The micro-Raman spectral analysis showed that the aluminum-induced crystallization process yields a better poly-crystalline SiGe film when the film is annealed at 450 °C for 40 min. The growth mechanism of the poly-crystalline silicon-germanium by aluminum-induced crystallization was also studied and is discussed.     

Characterization of strain in annealed Cu—Ni multilayers using X-ray diffraction

The strain profile of annealed Cu–Ni multilayers was analysed using an X-ray diffraction (XRD) theory. The annealing times of the multilayers ranged from 0 to 20 h. The strain in each layer was found by fitting the theoretical peak intensities with the experimental ones by iteration and using a kinematical/dynamical theory of XRD. It was found that for increasing annealing times, there was a decrease in the strain profile due to increased interdiffusion between the Cu and Ni layers. The increase in diffusion changed the composition modulation of the multilayers progressively from a trapezoidal wave for the 0 h annealed sample to a sinusoidal wave for the 20 h annealed sample.     

Characterization and photovoltaic device application of dip-coated and rapid thermal annealed CdS films

The structure, composition, electronic and optical properties of dip-coated CdS in the as-deposited condition and following rapid thermal annealing have been investigated. It has been shown that oxygen incorporated in the CdS film can be leached out following rapid thermal annealing. Strongly oriented CdS films with resistivity = 0.16 cm and free electron concentration = 2.65 × 10¹⁷ cm^–3 have been grown. Thin film heterojunction devices fabricated by non-aqueous electrodeposition of CdTe on a glass-ITO-CdS cathode have been shown to exhibit good rectification behaviour and photovoltaic activity.     

Characterization of laser and laser/thermal annealed semiconducting iron silicide thin films

-FeSi₂ is an important semiconducting silicide which is being studied extensively. In this paper, we report our results of the effect of laser and laser-thermal annealing on the properties of -FeSi₂. 5N purity Fe was deposited on Si substrate and was subsequently irradiated by CW and pulsed laser separately followed by thermal annealing to reduce the laser induced damage. The samples were then characterized by sheet resistance, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical reflectance and absorption studies. Lastly, -FeSi₂/n-Si heterojunctions were fabricated and the effect of laser treatment on the junction ideality factor was investigated. All these characterizations indicated the formation of good quality -FeSi₂, particularly after pulsed laser followed by thermal treatment.     

Photoluminescence of CdHgTe epilayers grown on silicon substrates

We have studied the photoluminescence (PL) spectra of thin layers of mercury cadmium telluride (CdHgTe) solid solutions grown by molecular beam epitaxy on silicon substrates. It is established that a disorder in the solid solution structure in these layers does not exceed that in the layers grown by the same method on GaAs substrates. The PL spectra of CdHgTe/Si samples exhibit emission lines characteristic of the structurally perfect material, in particular, the lines due to donor-acceptor recombination and the recombination of excitons bound to impurities.     

The near-infrared photoluminescence of GaAs epilayers grown on Si

In heteroepitaxial GaAs grown on Si (GaAs/Si) there exist deep levels caused by various charged states of defects because of large lattice misfit and thermal expansion mismatch between GaAs and Si. The temperature- and excitation intensity-dependent near-infrared photoluminescence spectra related to the deep levels present in GaAs/Si grown by metal–organic chemical vapour deposition with different ratios of [As]/[Ga] were studied. In terms of configuration coordinate model, the Franck–Condon shifts of near-infrared emission in GaAs/Si were obtained by measuring the variation in full width at half-maximum with temperature. The band-gap shifts with temperature and with mismatch strain in GaAs/Si were considered. Taking Franck–Condon and band-gap shifts into account, the energy relations for the transitions from donor to acceptor, from conduction band to acceptor and from donor to valence band were revised. According to these transition-energy relations and the emission characteristics of GaAs/Si epilayers, three emissions were interpreted as the recombination luminescence of donor–acceptor pairs and two emissions were caused by As interstitial–Ga vacancy complex centres. This revised version was published online in November 2006 with corrections to the Cover Date.     

Multilayer model of indium arsenide epilayers

The charge carrier transport coefficients of an inhomogeneous thin semiconductor in a MOS structure were obtained using a multilayer model. The expression for the Hall coefficient of a three-layer system extended to arbitrary strength magnetic fields was used to separate the bulk transport parameters from the parameters describing transport at the two surfaces. Experimentally, a gate voltage was used to vary the surface under the oxide from depletion to accumulation, and the Hall coefficient was measured as a function of magnetic field. The characteristics of the back surface were obtained with the front surface held at the flat-band condition. The variation of the front surface parameters with gate voltage was obtained with the front surface in accumulation. The measurements were made on a MOS structure consisting of an InAs epilayer deposited by vapor phase epitaxy procedures on a semi-insulating GaAs substrate covered by a pyrolytic silicon dioxide insulating layer and an aluminum gate.     

Characterization of annealed, proton-exchanged optical waveguides in ZnO:LiNbO(3) crystal

We report a systematic study of the annealing process in proton-exchanged ZnO:LiNbO(3) optical waveguides. A z-cut multimode waveguide was subjected to annealing for different durations of time. A two-stage change in index profiles with annealing time was observed, which was consistent with the change in the m-line spectrum. A power-law relationship was established to correlate the optical parameters with annealing time. Annealed Li(+) concentration in the waveguide was solved based on the thermal diffusion equation. An analytical function was used to model the annealed-index profile of single-mode, proton-exchanged waveguides. Good agreement between the theoretical analysis and the experimental result was obtained.     

CdSe band-splitting on thermal annealed films

Semiconducting polycrystalline CdSe thin films were prepared on glass substrates by chemical bath at 65 °C. As-deposited films grew in the metastable cubic sphalerite (S) crystalline structure with good stoichiometry. Upon thermal annealing (TA) in Ar+Se₂ atmosphere at different temperatures in the range 200–500 °C, the gradual phase transformation from cubic modification to hexagonal wurtzite (W) stable phase could be observed. From optical absorption measurements the fundamental energy band gap (E_g) and the second electronic transition (E_g+ΔE_g) were calculated for as-deposited and thermal annealed films. For TA350 °C, S-phase dominates the crystalline structure and only the spin orbit (ΔE_so) contribution to ΔE_g is present. Above 350 °C, the W-phase dominates and the energy splitting (ΔE_cf), owed to crystal field contribution and originated by the loss of lattice symmetry, should be added to ΔE_so in order to complete ΔE_g in the W-phase. The values ΔE_so=0.389±0.011 eV and ΔE_cf=0.048±0.018 eV were found from our analysis, and T_c=350 °C was here defined as the critical point of the phase transformation.     

Ellipsometric investigation of rough zinc arsenide epilayers

We report single-wavelength ellipsometric measurements of the complex index of refraction of rough Zn3As2 films on InP substrates. What we believe to be a novel technique, based on surface roughness measurements by atomic-force microscopy, is discussed to extract useful information from the ellipsometry results. The anticipated presence of a thin oxide layer is confirmed by Auger electron spectroscopy.     

Galvanomagnetic properties of 3C-SiC epilayers grown on hexagonal SiC substrates

Epitaxial 3C-SiC films have been grown on 6H-SiC substrates by sublimation epitaxy in vacuum. The Hall effect in these heterostructures and their magnetoresistance have been measured in a temperature range from 1.4 to 300 K. At liquid-helium temperatures, the samples are characterized by low resistance and exhibit negative magnetoresistance in weak fields (～1 T). Analysis of the experimental results suggests that the low resistance of samples is most probably due to the metal-insulator transition in the epitaxial 3C-SiC films.