Electron-phonon renormalization of the absorption edge of the cuprous halides

Compared to most tetrahedral semiconductors, the temperature dependence of the absorption edges of the cuprous halides (CuCl, CuBr, CuI) is very small. CuCl and CuBr show a small increase of the gap $E_0$ with increasing temperature, with a change in the slope of $E_0$ vs. $T$ at around 150 K: above this temperature, the variation of $E_0$ with $T$ becomes even smaller. This unusual behavior has been clarified for CuCl by measurements of the low temperature gap vs. the isotopic masses of both constituents, yielding an anomalous negative shift with increasing copper mass. Here we report the isotope effects of Cu and Br on the gap of CuBr, and that of Cu on the gap of CuI. The measured isotope effects allow us to understand the corresponding temperature dependences, which we also report, to our knowledge for the first time, in the case of CuI. These results enable us to develop a more quantitative understanding of the phenomena mentioned for the three halides, and to interpret other anomalies reported for the temperature dependence of the absorption gap in copper and silver chalcogenides; similarities to the behavior observed for the copper chalcopyrites are also pointed out.Comment: 14 pages, 5 figures, submitted to Phys. Rev.

Isotope effects on the electronic critical points of germanium: Ellipsometric investigation of the E

Within the past years the optical excitations of electrons have been measured for semiconductor samples of different isotope compositions. The isotope shift observed have been compared with calculations of the effects of electron-phonon interaction on the electronic band structure. While qualitative agreement has been obtained, some discrepancies remain especially concerning the E1 and $E_{1}+\Delta_{1}$ transitions. We have remeasured the effect of isotope mass on the E1 and $E_{1}+\Delta_{1}$ transitions of germanium with several isotopic compositions. The results, obtained by means of spectroscopic ellipsometry, confirm that the real part of the gap self-energies induced by electron-phonon interaction is larger than found from band structure calculations, while the imaginary part agrees with those calculations, which are based on a pseudopotential band structure and a bond charge model for the lattice dynamics. Our results agree with predictions based on the measured temperature dependence of the gaps. We compare our data for E1 and $E_{1}+\Delta_{1}$ with results for the lowest direct (E0) and indirect (Eg) gaps. The measured values of $\Delta_{0}$ and $\Delta_{1}$ increase noticeably with increasing isotope mass. Similar effects have been observed in the temperature dependence of $\Delta_{1}$ in $\alpha{-}{\rm Sn}$ and $\rm GaSb$. A microscopic explanation for this effect is not available

Optical anisotropy of (001)-GaAs surface quantum wells

We report a reflectance difference spectroscopy (RDS) study of the optical anisotropy of GaAs:(001) Surface quantum wells consisting of a thin GaAs layer (3-30 nm thick) embedded between an arsenic reconstructed surface and an AlAs barrier. The RDS spectra display anisotropic contributions from the free Surface and from the GaAs/AlAs interface. By comparing RDS spectra for the c(4x4) and (2x4) Surface reconstructions, we separate these two contributions, and demonstrate that the anisotropy around the E-1 and E-1 + Delta(1) transitions comprises a component originating from modifications of bulk states near the surface. The latter is attributed to anisotropic strains induced by the surface reconstruction. The experimental data Lire well described by a model for the RDS response of the multilayer structures, which also takes into account the blue energy shifts and the changes in oscillator strength of the E-1 and E-1 + Delta(1) transitions induced by quantum-well confinement