Modulation spectroscopy of III-V and II-VI semiconductors and their heterostructures

The exceptional magnetic behavior of II-VI diluted magnetic semiconductors Cd1−xMnxTe, Cd1−xCoxTe, and Cd1−xFexTe is explored in the context of the huge excitonic Zeeman effect using wavelength modulated reflectivity in magnetic fields up to 60 kG and temperatures down to 1.8 K, employing both Faraday and Voigt geometries. The samples are characterized with their compostions (using microprobe analysis), and their magnetization as a function of magnetic field and temperature. For Cd1−xFexTe, the Zeeman splitting is found to be linear in magnetic fields up to 50 kG; at low temperatures, the splitting is independent of temperature, as expected for the Van Vleck paramagnetism of Fe2+. The paramagnetic behavior of Mn2+ and Co2+ is consistent with the Brillouin functions characteristic of their spin 5/2 and 3/2 ground states, respectively. It is shown that, while the antiferromagnetic interaction between Mn2+ ions is limited to nearest-neighbors, for Co2+ it is much stronger, and extends to more distant neighbors. The exchange constants N0α and N0β for all the three cases are determined. The momentum conserving indirect excitonic transitions, from the Γ 15 valence band maximum to the conduction band minima close to X 1-point in the Brillouin Zone have been measured for GaP in piezo-modulated transmission. At 6 K, excitonic signatures due to phonon emission are observed at Egx + ħω ph for TA(X), LA(X), and TO(X) phonons (Egx = free exciton bandgap), whereas at 120 K signatures for both absorption and emission of LA(X) and TA(X) phonons appear. These observations yield Egx = 2.3301(4) eV at 6 K and the phonon energies. Finally, the suppression of S diffusion from a GaP substrate into a GaP epilayer achieved with an intervening GaP/AlGaP superlattice is demonstrated. The energy of the excitonic signature, Eg( x), in the wavelength modulated reflectivity spectrum of Zn1−xMgxTe shows a monotonic increase with x, in contrast to a large downward bowing of Eg(x) in ZnSxTe1−x and ZnSexTe1−x. Free and bound excitonic signatures in photoluminescence spectra of the same samples provide additional confirmation for these trends. The experimental results are interpreted in terms of the virtual crystal approximation and disorder, which underlie the bowing parameter, b