The optical constants of amorphous Ge are determined for the photon energies from 0.08 to 1.6 eV. From 0.08 to 0.5 eV, the absorption is due to k-conserving transitions of holes between the valence bands as in p-type crystals; the spin-orbit splitting is found to be 0.20 and 0.21 eV in non-annealed, and annealed samples respectively. The effective masses of the holes in the three bands are 0.49 m (respectively 0.43 m); 0.04 m, and 0.08 m. An absorption band is observed below the main absorption edge (at 300 °K the maximum of this band is at 0.86 eV); the absorption in this band increases with increasing temperature. This band is considered to be due to excitons bound to neutral acceptors, and these are presumably the same ones that play a decisive role in the transport properties and which are considered to be associated with vacancies. The absorption edge has the form: ω2ϵ2∼(hω−Eg)2 (Eg = 0.88 eV at 300 °K). This suggests that the optical transitions conserve energy but not k vector, and that the densities of states near the band extrema have the same energy-dependence as in crystalline Ge. A simple theory describing this situation is proposed, and comparison of it with the experimental results leads to an estimate of the localization of the conduction-band wavefunctions.

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Optical Properties and Electronic Structure of Amorphous Germanium

We report values of pseudodielectric functions $〈\ensuremath{\epsilon}〉=〈{\ensuremath{\epsilon}}_{1}〉+i〈{\ensuremath{\epsilon}}_{2}〉$ measured by spectroscopic ellipsometry and refractive indices $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=n+ik$, reflectivities $R$, and absorption coefficients $\ensuremath{\alpha}$ calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, we have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Our results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among our data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ in optical measurements.

Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV

A numerical Kramers-Kronig analysis is used to predict the refractive-index perturbations produced in crystalline silicon by applied electric fields or by charge carriers. Results are obtained over the 1.0-2.0 \mu m optical wavelength range. The analysis makes use of experimental electroabsorption spectra and impurity-doping spectra taken from the literature. For electrorefraction at the indirect gap, we find \Delta n = 1.3 \times 10^{5} at \lambda = 1.07 \mu m when E = 10^{5} V/cm, while the Kerr effect gives \Delta n = 10^{-6} at that field strength. The charge-carrier effects are larger, and a depletion or injection of 1018carriers/cm3produces an index change of \pm1.5 \times 10^{-3} at \lambda = 1.3 \mu m.

Electrooptical effects in silicon

Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe100-y-zCoyNizOx are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.

https://science.sciencemag.org/content/sci/early/2013/03/27/science.1233638.full.pdf

Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis

The available data on solid solubilities of impurity elements in germanium and silicon are summarized in the form of solidus or solvus curves. New solubility data are presented for the lead-germanium, zinc-germanium, indium-germanium, antimony-silicon, gallium-silicon and aluminum-silicon systems. The correlation of the solid solubilities with the heats of sublimation and the atom sizes of the impurity elements is considered.

Solid solubilities of impurity elements in germanium and silicon

The intrinsic absorption spectra of high-purity single-crystal germanium and silicon have been measured at 77\ifmmode^\circ\else\textdegree\fi{}K and 300\ifmmode^\circ\else\textdegree\fi{}K. The spectral regions studied encompassed a range of absorption coefficient from 0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$ to ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for each material. The germanium data may be interpreted as indicating a threshold for direct transitions at 0.81 ev at 300\ifmmode^\circ\else\textdegree\fi{}K and at 0.88 ev at 77\ifmmode^\circ\else\textdegree\fi{}K. The threshold for indirect transitions was placed at 0.62 ev and 0.72 ev for 300\ifmmode^\circ\else\textdegree\fi{}K and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. For silicon the data were not as readily interpreted However, there is an indication that the threshold for direct transitions should be placed at about 2.5 ev and the threshold for indirect transitions at 1.06 ev and 1.16 ev at 300\ifmmode^\circ\else\textdegree\fi{}K and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively.

Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77°K and 300°K

The absorption spectrum of single crystal NiO has been measured in the 0.1 to 6 ev range and its reflectivity spectrum from 0.025 to 10 ev. The absorption spectrum shows a series of lines from 1 to 3.5 ev arising from internal transitions of the Ni ion. A continuous background absorption occurs in the range from 0.1 to 3.5 ev whose magnitude increases with impurity concentration. The absorption coefficient rises steeply above 3.5 ev and reaches a value of ${10}^{6}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ at and above 4 ev. An absorption line at 0.24 ev is found to be temperature sensitive in both intensity and frequency in the range above 300\ifmmode^\circ\else\textdegree\fi{}K. Its behavior suggests that it is connected with the antiferromagnetic ordering. The reststrahlen spectrum was observed with the following parameters: high- and low-frequency dielectric constants 5.4 and 12, respectively; energies of longitudinal and transverse optical mode vibrations are 0.076 and 0.044 ev, respectively.

Optical Properties of Nickel Oxide

When low-voltage silicon $p\ensuremath{-}n$ junctions are biased in the reverse direction to breakdown, visible light is emitted from the junction region. The effects of surface treatment on the phenomenon are discussed. Two typical light output vs reverse current curves are shown. A typical spectral distribution curve in the 1.8-3.4 ev range is shown. The observations suggest that the light results from a radiative relaxation mechanism involving the high-energy carriers produced in the avalanche breakdown process.

Visible Light from a Silicon p − n Junction

Measurements of the intrinsic absorption edge of $n$-type InP at 77\ifmmode^\circ\else\textdegree\fi{}K and 300\ifmmode^\circ\else\textdegree\fi{}K are reported. Differences are found in the spectra of samples of differing origin. The effects are believed due to impurities. Reflection and absorption spectra in the vicinity of the reststrahlen peak are shown. The reststrahlen wavelength is 30.5 \ensuremath{\mu}, the static and high-frequency dielectric constants are ${\ensuremath{\epsilon}}_{0}=15$ and ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}=10.6$. Freecarrier absorption and reflection spectra are shown. A brief discussion of the implications of the various optical and electrical measurements as regards the conduction band structure is given.

Optical Properties of n-Type InP

The free-hole absorption in $p$-type germanium (gallium-doped) is studied as a function of concentration over the range from ${10}^{15}$ to ${10}^{20}$ acceptors/${\mathrm{cm}}^{3}$. The spectra are modified as the carrier and total impurity concentrations are increased. With increasing concentration the structure in the spectrum becomes less pronounced. The effects observed are consistent with changes in the Fermi level and with nonvertical transitions induced by charged impurity centers.

R. Newman

Papers

Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77°K and 300°K

Optical Properties of Nickel Oxide

Visible Light from a Silicon p − n Junction

Optical Properties of n-Type InP

Effect of Impurities on Free-Hole Infrared Absorption in p-Type Germanium