Amorphous SiGe Alloys Research Articles

Raman scattering studies of ion beam induced mixing at the amorphous germanium/crystalline silicon interface

Thin layers of amorphous germanium vacuum deposited onto a 〈111〉 surface of crystalline silicon were irradiated with a Kr+ ion beam to produce an amorphous Si-Ge alloy at the interface. Raman scattering measurements were performed on these films both before and after the ion irradiation. The vanishing of the strong Si-Si lattice mode near 521 cm−1 and an appearance of the localized Si-Ge vibrational mode near 375 cm−1 in the Raman spectra of ion-irradiated films are correlated to the formation of an amorphous alloy of probable composition of Si0.2Ge0.8 at the interface.

Characterization of highly photoconductive amorphous silicon germanium alloys prepared by DC glow discharge

Amorphous SiGe alloys have been deposited in different dc glow discharges (diode, triode configuration) from SiH 4/GeH 4 mixtures with and without H 2 dilution of the neutral gas. Triode reactor and/or H 2 dilution has brought about excellent AMI photoconductivities in the range 1.7eV – 1.4eV of (2×10 −4 − 10 −5)1/ωcm. The incorporation of Ge causes only small increases in Urbach energies as well as some rise in midgap density of states (PDS and CPM). Analyses of transport behaviour from temp. dependent stationary and transient photoconduction experiments (TOF and TPC) however point towards a considerable increase in conduction band tailing and therefore a reduced effective mobility of photoexcited electrons in comparison to a-Si:H.

Glow-discharge amorphous silicon: Growth process and structure

Hydrogenated amorphous silicon (a-Si:H) is the first “structure-sensitive” amorphous semiconductor, and its conduction type is controlled by impurity doping using the glow-discharge technique. However, in contrast to crystalline counterparts, the network structure of amorphous materials takes a wide variety depending on their growth process, and therefore, electronic properties are not unique even in undoped (intrinsic) a-Si:H. In this paper, we are concerned with the growth process of a-Si:H films via the glow-discharge decomposition of SiH 4 , and its relationship to structural, optical and electronic properties of the deposited films. Emphasis is placed on the understanding of the film growth mechanism as well as the microscopic characterization of the film structures. New plasma diagnostic tools such as optical emission spectroscopy and ion mass spectrometry are employed for describing the SiH 4 -glow-discharge plasma, and dominant species responsible for the film deposition is suggested. Structural characterization of a-Si:H includes TEM observation (morphology), infrared absorption (bonded hydrogen), 1 H NMR (spatial distribution of hydrogens), Raman-scattering spectroscopy (local structural order) and ESR (defect density), being discussed in relation with optical and electronic properties. Hydrogenated amorphous SiGe and SiC alloys as well as amorphous superlattice structures are also described as recent important topics.

Comparison of Properties of a-Si1−x Gex:H and a-Si1−x Gex:H:F

AbstractThe structural, electrical and optical properties of amorphous Si-Ge alloys prepared either from mixtures of hydrides or of fluorides will be compared. It will be shown that the majority of many properties studied are essentially the same, the major exceptions being the structure of thin films used for TEM investigation, and the photocurrents developed between contacts in a coplanar configuration. Possible explanations for the result of including fluorine in the preparation plasma will be discussed.

Preparation of High Quality Amorphous Si-Ge Alloys by Radical-Control Techniques

Preparation of High Quality Amorphous Si-Ge Alloys by Radical-Control Techniques

Measurements of Light-Induced Degradation in A-Si, Ge:H, F Alloys,

ABSTRACTThe effects of illumination on the optical and electronic properties of narrow gap hydrogenated and fluorinated amorphous Si-Ge (a-Si1-xGex:H, F) alloys have been evaluated. A series of alloys with optical gaps ranging from 1.30 eV to 1.64 eV has been light soaked at ∼1 sun intensity for 354 hours. Measurements of sub-gap absorption, photo- and dark conductivities and dark conductivity activation energy were made on alloys in the annealed and the light-soaked states. The results indicate that samples with optical gaps ≳ 1.4 eV degrade significantly. The 1.3 eV sample shows no degradation in its optical or electronic properties except for a factor of 5 increase in the dark conductivity.

Preparation of highly photosensitive hydrogenated amorphous Si-Ge alloys using a triode plasma reactor

Highly photoconductive hydrogenated amorphous Si-Ge alloys have been prepared from a SiH4/GeH4 gas mixture using a triode glow-discharge reactor. High photoconductivity (Δσp≂10−4 Ω−1 cm−1) under the AM1 (100 mW/cm2) illumination and low dark conductivity (σd=10−8–10−9 Ω−1 cm−1) have been obtained for the optical gap in the range between 1.5 and 1.7 eV.

NMR and ESR studies on a-Si 1− xGe x:H prepared by magnetron sputtering

NMR, IR and ESR measurements have been performed for hydrogenated amorphous Si-Ge alloy films prepared by magnetron sputtering. NMR signals similar to those for a-Si:H are observed with no motional narrowing effect. The content of dispersed hydrogen atoms estimated from the narrow component of the NMR signal is independent of the Ge content, although the total hydrogen content decreases with increasing the Ge content. Such NMR results are discussed in conjunction with the results of ESR.

Composition of amorphous (Si,Ge):H films from nuclear elastic scattering of 12 MeV protons

Nuclear elastic scattering of 12 MeV protons was employed for the first time to study the composition of thin films of hydrogenated amorphous Si-Ge alloys. The concentrations of H, Si, and Ge were measured simultaneously. The detection limit is about 25 ppm for a 10-min run. Free-standing films or films on a flat substrate with a total thickness of up to 20 μm can be analyzed.

Electron-spin-resonance study of boron-doped amorphousSixGe1−x: H alloys

We report on ESR and Raman spectroscopy in $a\ensuremath{-}{\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$: H prepared by glow discharge with a 1-vol% boron-doping level. For most silicon contents $x$, a preferential deposition of germanium into the solid film is found. The Raman spectra show that the samples are amorphous and that no phase separation is present. Four different ESR resonances can be observed over the compositional range investigated ($0<x<0.7$) at 20 K: the known resonances of the Ge dangling bonds and of localized holes in Ge---Ge bonding states, a signal due to carbon impurities, and a new resonance with strong negative $g$-value shift ($g=1.925$) that seems to be related to surface states. A model for the density of states in the amorphous Si-Ge alloys is proposed that can qualitatively account for the experimental results.

The compositional dependence of the optical and electrical properties of hydrogenated amorphous Si-Ge films prepared by co-sputtering

Hydrogenated amorphous Si-Ge alloy films were prepared by co-sputtering from two half-disc targets. The films are about 1 μm thick and 12.5 cm long. The composition along the length of the film varies continuously from 70 to 5 at.% Si. The optical absorption and photoconductivity as a function of wavelength were measured. The energy gap as evaluated form these measurements decreases linearly from 1.65 eV for 70 at.% Si to 1.1 eV for 7 at.% Si. The photoconductance response ημτ varies markedly with composition, where η is the quantum efficiency for photocarrier generation, μ the mobility and τ the lifetime of the photocarriers. At the silicon-rich end ημτ = 2 × 10 -6 cm 2 V -1, decreases to 2 × 10 -9 cm 2 V -1 at around 40 at.% Si and increases again by one order of magnitude at the germanium-rich end. On the whole the characteristics of our sputtered films are comparable with those reported for similar alloys prepared by glow discharge. An important advantage of the sputtered films, however, is that it is possible to prepare a whole range of alloy concentrations in one run.

Structural studies of crystalline and amorphous SiGe alloys using EXAFS and Raman scattering

Local order of crystalline and amorphous SiGe alloys has been studied by measurements of Ge K-edge EXAFS and Raman scattering. The analysis of the EXAFS data shows the structural disorder with deviations of bond lengths. The effects of alloying and pressure on the Raman spectra which relate to the TO phonons of SiSi, SiGe and GeGe pairs are dscussed in terms of the structural disorder and the charge transfer from Ge to Si.

Effects of annealing and crystallization process in amorphous SiGe alloy films

The change of the structure and the process of crystallization after annealing are investigated by ESR, electrical measurement and X-ray microanalysis for amorphous Si 57Ge 43 thin films prepared by rf-sputtering. It is found that the lorentzian ESR lineshape and linewidth are independent of annealing temperature, while the g-value approaches that for a-Ge with annealing, and the temperature dependence of conductivity obeys the Mott relation. After annealing at rather higher temperature, the non-uniformity of the spatial distribution of Si and Ge atoms is observed. As a result it is suggested that unpaired electrons responsible for ESR and conductivity] exist mainly in Ge sites after annealing and the wave function is most likely to extend over several atomic sites in which the ratio of the number of Ge atoms to that of Si atoms increases with annealing.