Article:Cechavicius, B., Kavaliauskas, J., Krivaite, G. et We present an optical study of beryllium ␦-doped GaAs/ AlAs multiple quantum well ͑QW͒ structures designed for sensing terahertz ͑THz͒ radiation. Photoreflectance ͑PR͒, surface photovoltage ͑SPV͒, and wavelength-modulated differential surface photovoltage ͑DSPV͒ spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2 ϫ 10 10 to 5 ϫ 10 12 cm −2 at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ϳ20 kV/ cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures.

The electroreflectance and thermoreflectance spectra of SnSe crystals are investigated in polarized light with E 11 u and E 11 b. The direct energy gaps are 1.05 and 1.24 eV for light polarizations E I I b and E I I a, respectively. The polarization anisotropy of modulation spectra of SnSe is strong near the gap and becomes weak at higher energies. The structures of investigated spectra in the vicinity of the forbidden energy gap are interpreted by direct optical transitions on the high symmetry line A-I?-V in the calculated energy band structure.

The reflectance spectra of orthorhombic GeSe crystals are studied in the region 0.5 to 12 eV in polarized light with E 11 a and E (1 b a t 290 K and a Kramers-Kronig analysis is performed. The thermoreflectance spectra are investigated in the region 1 to 5 eV for E 11 a and E 11 b a t 300 and 90K. The energy band structure calculations of GeSe are carried out by the pseudopotential method, and the spectra of t.he imaginary part of the dielectric constant are calculated by the k * p method. A classification and interpretation of main structures of optical spectra of GeSe by direct interband transitions a t critical points I', U, T and high symmetry lines B' V, A is presented on the basis of calculated band structure and selection rules for optical dipole transitions of orthorhombic A4B6 crystals. I/ZCcJIenOBaHbI CneKTpbI OTPaHEeHHfl OpTOpOM6H~IeCKOrO HpHCTanna GeSe B CneKTpNIb€IO# o 6 n a c~~ OT 0,5 A 0 12 ev B nOJIflpH30BaHHOM CBeTC C E I] a A E I] b npPi TeMnepaTypC! 290 K A nposeneH ~H~J I A~ HpaMepca-Kpomira. kI3yqe~b1 cnewpbr TepMooTpameHufl B AHTepsane OT 1 no 5 eV nns non~pn3a1@ E 11 a A E 11 b n p~ 300 EI 90 K. nposeHeHbI paClieTb1 30HHOfi CTPYKTYPbI &Se MeTOnOM IICeBAOnOTeHqHana H CneKTpOB MHHMOfi gaCTH AH3JIeKTPAYeCKOfi IIPOHH4aeMOCTki k * P-MeTOllOM. Ha OCHOBe PaCCWITaHHOfi ~O H H O~~ CTPYKTYPH H npamn o~6opa n n~ nnnonbHbIx OnTmecwix nepexonoB B OPTO-P O M~M V~C K H X HpHcTannax A4B6 nposeneHa xnaccH@Kaqax H AHTepnpeTaqufl OCHOBHHS CTPYKTJ'P OIITHqeCKAX CneKTpOB GeSe IlpIIMbIMH MeHU(y3OHHbIMH nepeXO&3MH B KPATII-qeCKHX TOqKaX r, u H T, a TaKHEe Ha JIHHHIIX BbICOKOfi CAMMeTpAA B', v H A.