Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

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

This review provides numerical and graphical information about many (but by no means all) of the physical and electronic properties of GaAs that are useful to those engaged in experimental research and development on this material. The emphasis is on properties of GaAs itself, and the host of effects associated with the presence of specific impurities and defects is excluded from coverage. The geometry of the sphalerite lattice and of the first Brillouin zone of reciprocal space are used to pave the way for material concerning elastic moduli, speeds of sound, and phonon dispersion curves. A section on thermal properties includes material on the phase diagram and liquidus curve, thermal expansion coefficient as a function of temperature, specific heat and equivalent Debye temperature behavior, and thermal conduction. The discussion of optical properties focusses on dispersion of the dielectric constant from low frequencies [κ0(300)=12.85] through the reststrahlen range to the intrinsic edge, and on the ass...

Semiconducting and other major properties of gallium arsenide

Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients

We describe the design, construction, alignment, and calibration of a photometric ellipsometer of the rotating-analyzer type Data are obtained by digital sampling of the transmitted flux with an analog-to-digital converter, followed by Fourier transforming of the accumulated data with a dedicated minicomputer With an operating mechanical rotation frequency of 74 Hz, a data acquisition cycle requires less than 7 msec The intrinsic precision attainable is high because precision is limited only by shot noise or intrinsic source instabilities, even when relatively weak continuum lamps are used as light sources Precision may be improved by accumulating the data for consecutive cycles at a fixed wavelength The system allows complex reflectance ratios to be determined as continuous functions of wavelength from the near infrared to the near ultraviolet spectral range Data reduction programs can be modified to calculate complex refractive index or dielectric function spectra, or film thicknesses and refractive indices, as well as the usual ellipsometric parameters tanpsi, cosDelta

High Precision Scanning Ellipsometer

Schottky-Barrier Electroreflectance: Application to GaAs

We perform an accuracy analysis of several possible reflectance–difference (RD) configurations that are compatible with standard molecular‐beam epitaxy (MBE) growth chambers, and describe in detail an optical‐bridge system that can determine relative changes in RD signals as small as 5×10−5 under standard growth conditions. Using this system, we determine the RD response of GaAs for changes in surface conditions at different wavelengths and correlate these to simultaneously measured reflection high‐energy electron diffraction (RHEED) intensities. Maximum anisotropies are found at 2.0–2.5 and 3.5 eV for Ga on GaAs and Al on AlAs, respectively, providing a way of spectrally distinguishing Ga–Ga and Al–Al dimers for surface‐chemical investigations, and suggesting that these photon energies are also optimal for modifying growth by light. At photon energies well removed from these anisotropy maxima, RD signals follow changes in surface structure, as RHEED. Our RD‐RHEED correlations provide insight concerning c...

Application of reflectance difference spectroscopy to molecular‐beam epitaxy growth of GaAs and AlAs

Extremely uniform, microscopically smooth, large-grained polycrystalline Si films saturation doped with phosphorous to a carrier concentration $n=3.3\ifmmode\times\else\texttimes\fi{}{10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ were prepared by a combination of low-pressure chemical-vapor-deposition and heat-treating processes to obtain samples suitable for measuring the small effects of large impurity concentrations on the above-band-gap dielectric properties of Si. From 1.5 to 2.5 eV, the effects of heavy doping are described by the Drude free-carrier model with lifetimes determined by scattering from lattice vibrations. Above 3 eV, the dielectric function spectrum of heavily doped Si is remarkably similar to that of undoped material except that the ${E}_{1}$ and ${E}_{2}$ structures are broadened and shifted to lower energies. The impurity-induced broadening is much larger for ${E}_{1}$ than for ${E}_{2}$, indicating that a metastable excitonic resonance is well developed for the former but not for the latter. Present and previous data of the authors, together with previous data of Vi\~na and Cardona, are consistent with an approximately linear shift of the threshold energy with impurity concentration. This indicates that the dominant mechanism responsible for the shifts is the effect of the random impurities on the crystal potential. While the possibility of electron-electron and electron-ion interactions on the electron and hole self-energies cannot be ruled out, comparison with recent calculations of Berggren and Sernelius show that these effects must be much smaller at higher interband transitions than at the fundamental indirect band gap. Using our data as a reference, we show that the anomalous absorption observed by Jellison and co-workers for As-implanted and laser-annealed $c\ensuremath{-}\mathrm{S}\mathrm{i}$ is due to microscopically rough surface layers on their samples. Consequently, the differences that they observe between P and As dopants must involve differences in recrystallization and regrowth. Dielectric-function spectra for amorphous Si films doped in situ with phosphorus are essentially identical within microstructural effects to those of undoped material, indicating no significant influence from heavy doping. At energies below 3.2 eV, the data expressed as ${(\ensuremath{\alpha}nE)}^{\frac{1}{3}}$ or as ${({\ensuremath{\epsilon}}_{2})}^{\frac{1}{2}}$ both vary nearly linearly with energy, suggesting nondirect transitions between simple parabolic bands with momentum matrix elements linearly proportional to $E$ or with dipole elements independent of $E$.

A. A. Studna

Papers

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

High Precision Scanning Ellipsometer

Schottky-Barrier Electroreflectance: Application to GaAs

Application of reflectance difference spectroscopy to molecular‐beam epitaxy growth of GaAs and AlAs

Dielectric properties of heavily doped crystalline and amorphous silicon from 1.5 to 6.0 eV