Optical properties of semimagnetic semiconductors studied by thermoreflectance spectroscopy

Abstract

The effect of introducing the magnetic elements Ni and Co into a II–VI host lattice, and their influence on the fundamental band gap energy, have been studied by means of thermoreflectance (TR) spectroscopy, following the evolution of the E0 transition at the Γ-point of the Brillouin zone, in terms of the magnetic ion concentration, and/or the film crystallite size. Two systems have been studied: (1) Cd1−xNixTe thin films grown by r.f. supttering onto Corning glass substrates with Ni concentrations in the range: 0 < x < 0.15. The films showed a nanocrystalline structure with the grain size decreasing as the film Ni content increases. From the TR spectra it was found that the band gap shifts to higher energies as the nanocrystalline size decreases, giving us evidence of a quantum-size effect due to electron confinement; (2) Cd1−xCoxSe crystals grown by the vapor phase chemical transport technique at a temperature of 950°C, for two concentration values: x = 0.01 and 0.05. As compared to a pure CdSe sample, the crystal with a Co concentration of 1 at% shows a shift of the E0 transition of about 70 meV to lower energies. For both samples, there appear additional oscillations in the low energy part of the TR spectrum which we interpret as due to the levels introduced by the Co-3d orbitals. The shifts of the E0-point are also discussed.