Shunji Ozaki

Optical constants of ZnSxSe1−x ternary alloys

The methods for calculation of the various optical constants in ZnSxSe1−x ternary alloys are presented. The model used is based on an interpolation scheme, and the effects of alloy composition are properly taken into account in the calculation. The present model reveals distinct structures in the optical spectra at energies of the E0, E0+Δ0, E1, and E1+Δ1 gaps. The optical constants and properties considered here are the complex dielectric constant e=e1+ie2, complex refractive index n*=n+ik, absorption coefficient α, and normal‐incidence reflectivity R. The refractive indices in the transparent region are also presented for a variety of waveguiding device applications.

OBSERVATION OF RESONANT OPTICAL-PHONON ASSISTED TUNNELING IN ASYMMETRIC DOUBLE QUANTUM WELLS

Laser Raman microscope measurements in asymmetric double quantum wells with coupled narrow and wide quantum wells were performed to observe the nonequilibrium longitudinal-optical (LO) phonons that are generated by electrons during the phonon assisted tunneling. Both the Stokes and the anti-Stokes intensities show maxima at a certain applied voltage, where the calculated subband spacing between the wide and the narrow quantum well states is found to be equal to the LO phonon energy. This fact indicates that the population of nonequilibrium LO phonons becomes maximum when resonant LO phonon scattering occurs. A strong reduction in the photoluminescence intensity for the narrow quantum well is also observed at the same bias condition.

Resonant optical-phonon assisted tunnelling in an asymmetric double-quantum-well structure

Electroreflectance (ER) and photoluminescence (PL) experiments have been performed at 10 K on an asymmetric double-quantum-well (ADQW) structure with coupled narrow and wide quantum wells to investigate optical-phonon assisted tunnelling. At a certain external applied voltage, a distinct quenching of the PL intensity for the narrow-quantum-well luminescence is observed. By calibrating the internal electric field through the ER measurements and calculating subband levels in the ADQW structure, the quenching is attributed to resonant optical-phonon assisted tunnelling of electrons.

Electroreflectance study of (AlxGa1−x)0.5In0.5P alloys

Electroreflectance spectroscopy has been applied to the study of (Alx Ga1-x )0.5In0.5P quaternary systems lattice-matched to GaAs. The measurements are made at room temperature in the photon-energy range of 1.7–5.8 eV. The composition dependence of the E0, E0+Δ0, E1, E2 and E2+δ gaps has been determined. The lowest direct band gap is found to be given by E0(x)=1.90+0.57x+0.11×2 eV. The E0+Δ0, and E1 gaps are written as similar quadratic equations. On the other hand, the variation of the E2 and E2+δ gaps is found to be approximately linear. The lowest direct-to-indirect gap crossover is also estimated to occur at x~0.63.

Optical‐Phonon Assisted Tunneling in an Asymmetric Double‐Quantum‐Well Structure

Electroreflectance (ER), time-integrated photoluminescence (PL) and time-resolved PL experiments were performed on an Al0.36Ga0.64As/GaAs asymmetric double-quantum well (ADQWs) structure with coupled narrow and wide quantum wells in order to investigate the optical-phonon assisted tunneling. At a certain external applied voltage, a distinct quenching of the time-integrated PL intensity for the narrow quantum well is observed. By calibrating the internal electric field through the ER measurements and calculating subband levels in the ADQWs structure, the quenching is attributed to the resonant optical-phonon assisted tunneling of electrons between the narrow and wide quantum wells. The decay time of electrons in the narrow quantum well was also found to become minima at the same applied voltage.

Optical absorption and photoreflectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe2

Optical absorption and photoreflectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe2 for light polarization perpendicular (Eu2009⊥u2009c) and parallel to the c-axis (Eu2009‖u2009c) at Tu2009=u200915–300 K. Optical absorption measurements suggest that AgGaSe2 is a direct-gap semiconductor having an optical band gap of E0 ∼ 1.8u2009eV at Tu2009=u200915–300 K. The temperature-dependent PR spectra are obtained at Tu2009=u200920–300 K in the 1.8–2.5u2009eV spectral ranges. The lowest band-gap energy E0 of AgGaSe2 shows unusual temperature dependence at Tu2009≤u200980u2009K. The resultant temperature coefficients dE0/dT are positive at Tu2009≤u200970u2009K and negative above 70u2009K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Δsou2009=u2009327u2009meV and Δcru2009=u2009−288 meV at Tu2009=u200920u2009K, respectively.

Optical constants and electronic energy-band structure of AgGaSe2

The complex dielectric-function spectra, e(E) = e1(E) + ie2(E), of the AgGaSe2 chalcopyrite semiconductor have been measured by spectroscopic ellipsometry (SE) for light polarizations perpendicular (E ⊥ c) and parallel to the c-axis (E ∥ c) in the 1.6–5.3 eV photon-energy range at room temperature. The measured e(E) spectra reveal distinct structures of critical points in the Brillouin zone. Analysis of the numerically derived e(E) spectra facilitates the precise determination of critical-point energies. By performing the band-structure and dielectric-function calculations, these critical points are successfully assigned to specific points in the Brillouin zone. The dielectric-related optical constants of AgGaSe2, such as the complex refractive index n* = n + ik, the absorption coefficient α, and the normal-incidence reflectivity R, are presented.

Photomodulated transmittance spectroscopy of vacuum-evaporated AgGaTe2 films

AgGaTe2 films were deposited on glass substrates by vacuum evaporation. Thermal annealing in dry N2 atmosphere at 400–500 °C changes the deposited film into a single phase of chalcopyrite AgGaTe2. The photomodulated transmittance measurements were carried out for the AgGaTe2 film at temperatures T from 10 to 300 K. The optical band-gap energies were determined to be E0B ~ 1.3 eV and E0A ~ 1.4 eV (T = 10−300 K). The data of E0 vs T were analyzed using an analytical formula for the explanation of the band-gap shrinkage effect in semiconductors.

Positive temperature variation of the bandgap energy in the single-crystalline chalcopyrite semiconductor AgInS2

Optical absorption spectra have been measured on the single-crystalline chalcopyrite semiconductor AgInS2 using polarized light at Tu2009=u200910–300u2009K. The bandgap energy Eg of AgInS2 shows unusual temperature dependence at low temperatures. The resultant temperature coefficient ∂Eg/∂T is found to be positive at Tu2009<u2009130u2009K and negative above 130u2009K. This result has been successfully explained by considering the effects of thermal expansion and electron–phonon interaction. The free-exciton emission of photoluminescence spectra also indicates positive temperature dependence of the peak energies at low temperatures. The exciton binding energy of AgInS2 is determined to be 26.4u2009meV.