ELECTRONIC AND OPTICAL-PROPERTIES OF PBTE PB1-XEUXTE MULTIPLE-QUANTUM-WELL STRUCTURES/

The frequency dependence of the absorption constant alpha(omega) and t he enhancement of the refractive index n(omega) in the region of inter band transitions between confined quasi-two-dimensional states of the valence and conduction bands of PbTe/Pb1-xEuxTe multiple-quantum well (MQW) structures is studied experimentally as well as theoretically. T ransmission, reflectivity, and photoconductivity spectra of several MQ W's with PbTe well widths from 62 to 118 angstrom and Pb1-xEuxTe barri er widths from 486 to 621 angstrom (x almost-equal-to 3%) are compared with calculated spectra. The experimental data are compared to calcul ations of the transmission and reflectivity of the multilayer structur es based on frequency-dependent dielectric functions epsilon(omega) of the buffer and MQW layers. For the buffer layer experimentally determ ined values for epsilon(omega) are used, whereas for the MQW system th e sum of a background dielectric function and an additional term, whic h takes into account the electronic contribution due to interband tran sitions between electric subbands in the valence band (VB) and conduct ion band (CB), are used. For calculating the second term two approache s were followed. The conventional approach is based on treating these interband transitions as originating from confined states within the P bTe wells only. The second, more realistic, approach is based on a com plete calculation of the band structure of the MQW systems using a k.p envelope function approximation which yields proper energy eigenstate s, eigenfunctions, and oscillator strengths. The absorption constant a lpha(omega) is then obtained from an integration in k space. The elect ronic contribution deltaepsilon(VB-CB)(omega) to the total dielectric function is independent of the position z along the growth direction i n the MQW structure, i.e., in this respect the MQW is treated as being optically homogeneous. The calculated transmission and reflectivity d ata fit the experimental data very well. The steplike changes in alpha (omega) result in cusplike changes of the refractive index n(omega). F rom a comparison of the experimentally determined interband transition energies with the calculated values on several samples, the conductio n-band offset is determined to be DELTAE(c)/DELTAE(g) =0.55+/-0.2.