A model is proposed for the line shape of the optical dielectric function of zinc-blende semiconductors. For comparison with previously proposed models, this model is used primarily with spectroscopic ellipsometry data (but also transmission data below 1.5 eV) to obtain an analytic room-temperature dielectric function for GaAs. It is found to be more generally valid than the harmonic-oscillator model, the critical-point (CP) model, or the model of Adachi. It is applicable over the entire range of photon energies, below and above the lowest band gaps, incorporates the electronic band structure of the medium, and exactly satisfies the Kramers-Kronig transformation.It goes beyond the CP parabolic-band approximation in that it correctly takes into account the full analytic form of the electronic density of states and thus does not require the use of arbitrary cutoff energies. Also, it allows one to go beyond the usual approximation of Lorentzian broadening, which is known to be incorrect for elements and compounds above very low temperatures. For these reasons, it results in excellent quantitative agreement with experimental results for the dielectric function and for its derivatives with respect to photon energy, much better than that given by earlier models. Finally, the parameters of the model are physically significant and are easily determined as functions of composition for semiconductor alloys. Application of the model to the fitting of spectroscopic data on GaAs strongly suggests that spectroscopic ellipsometry does not measure the true bulk dielectric function. It also supports the conclusion that the line-shape broadening in GaAs at room temperature is more nearly Gaussian than Lorentzian.

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A theory is presented to explain the dependence of the superconducting transition temperature T c on the changes in the phonon frequency spectrum and electronic density of states which result from lattice disorder. Numerical calculations of T c are presented for films composed of crystalline granules, for films composed of amorphous granules, and for homogeneous amorphous metals. The calculations are in good agreement with experimental values of T c .Experiments 1 ' 2 on disordered films of a variety of metals and on disordered dilute Sn-Cu alloys 3 have shown that the superconducting transition temperature T c increases with increasing lattice disorder. None of the explanations proposed 4 ' 5 for the enhancement of T c in superconducting films has been applicable to all experimental situations discussed in this paper. Although no calculations are presented for the case of very thin films, 4 the theory proposed here is applicable to films composed of small metallic crystallites, 1 ' 6 films composed of amorphous metallic granules, 1 ' 2 ' 6 and homogeneous amorphous superconducting alloys. In particular, a two-parameter model for granular films of nearly-free-electron metals yields numerical values for the enhancement of T c which are in good agreement with the experimental values found by Buckel and Hilsch 1 and von Minnigerode. 2 The theory proposed here is based on the assumption that the average amplitude of ionic vibrations is larger in a disordered lattice than a perfect crystal. For disordered films which consist of small crystalline granules, 6 this increase in the average amplitude of ionic vibrations results from the many ions which are in lattice positions of reduced symmetry near the surface of the granules. Since these ions are held in place by weaker ionic forces than those found in a bulk crystal, they undergo localized ionic vibrations of larger amplitude and lower frequency than those found in a bulk crystal. As a result, the formation of granules (1) increases the average amplitude of ionic vibrations and decreases the average phonon frequency (^ph)> anc * (2) broadens the peaks in the phonon density of states D(w). For homogeneous amorphous metals, the increased average amplitude of ionic vibrations results from the weakened forces acting on all of the ions. In this case, the first effect is usually more important than the second. Both the effects found in homogeneous amorphous metals and those found in crystalline granular films occur in the case of disordered films which consist of amorphous granules.The increase in the average amplitude of the ionic vibrations increases the average electronphonon coupling constant. This increases both T c and the phonon contribution 7 X to the electron mass renormalization constant Z(0) = (1 + X)Z e . Here, Z e is the Coulomb mass renormalization constant. 8 Note that X is approximately given by 7where N(Q) is the electronic band density of states at the Fermi level, M is the ionic mass, (oOp^2) is the average squared phonon frequency, and (J 2 ) is the aver...

CdTe is one of the leading materials used in solar photovoltaics. However, the maximum reported CdTe cell efficiencies are considerably lower than the theoretically expected efficiencies for the ∼1.48 eV CdTe band gap. We report a class of single crystal CdTe-based solar cells grown epitaxially on crystalline Si that show promise for enhancing the efficiency and greatly lowering the cost per watt of single-junction and multijunction solar cells. The current-voltage results for our CdZnTe on Si solar cells show open-circuit voltages significantly higher than previously reported for any II-VI cells and as close to the thermodynamic limit as the best III-V-based cells.

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High concentration photovoltaic (HCPV) systems offer the highest photovoltaic (PV) conversion efficiencies. Also, as production is beginning to ramp up, HCPV is becoming cost competitive with thin-film poly-CdTe and crystalline Si systems in high solar insolation regions. High solar concentrations, X ∼ 500, are used to increase cell efficiencies and greatly reduce the cell area per unit of incident solar radiation, thereby greatly reducing the cell cost per watt. The monolithic three-junction (3J) solar cells presently used in HCPV systems typically consist of two epitaxial III-V homojunctions, such as GaInP and GaInAs, grown on an active Ge substrate by metal-organic chemical vapor deposition (MOCVD). The III-V bandgaps are chosen to match the currents generated in each junction and minimize the energy lost to thermalization of the electron-hole pairs generated, subject to the constraint of approximate lattice matching. We propose using cells consisting of one or more CdTe-based II-VI homojunctions grown on large-area active Si substrates by high-throughput MBE or a less expensive high-vacuum deposition technique as an alternative to III-V based multijunction cells grown by MOCVD. The bandgap of Si is more optimal than that of Ge for two-junction (2J) or 3J cells, and lattice mismatches affect the efficiencies of such cells only slightly, which allows greater freedom in the choice of bandgaps, and thus the potential for higher efficiencies. Also, such cells could be manufactured at a much lower cost due to the larger area, much lower cost and superior mechanical properties of Si substrates as compared to Ge substrates. The much lower cell cost also would enable medium concentration PV systems that would require more cell area, but with simplified, less expensive tracking and optics, resulting in lower overall system costs. Promising initial results from material-property measurements and single-junction and 2J CdZnTe/Si cell characterization results are given. Both the promise of the proposed technology and the challenges it faces are discussed.

We have used electrolyte electroreflectance (EER) to characterize ZnSe/GaAs and ZnSe/AlAs interfaces. The great sensitivity of EER to interface space-charge regions enabled us to detect both interface crossover transitions and transitions to triangular-well interface states. The observation of these transitions provides the first unambiguous proof that the ZnSe/GaAs interface is type I and allowed us to determine the band offsets and band bendings, the diffusion lengths across each interface, and the amount of interdiffusion.