OPTICAL-PROPERTIES OF SEMIMAGNETIC SEMICONDUCTORS STUDIED BY THERMOREFLECTANCE SPECTROSCOPY

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, fo llowing the evolution of the E-0 transition at the Gamma-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. sputtering onto Corning glass substrates with Ni concentrations in the range: 0 < x < 0.15. These films showed a na nocrystalline 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, givin g us evidence of a quantum-size effect due to electron confinement; (2 ) Cd1-xCoxSe crystals grown by the vapor phase chemical transport tech nique at a temperature of 950 degrees C, for two concentration values: x = 0.01 and 0.05. As compared to a pure CdSe sample, the crystal wit h a Co concentration of 1 at% shows a shift of the E-0 transition of a bout 70 meV to lower energies. For both samples, there appear addition al oscillations in the low energy part of the TR spectrum which we int erpret as due to the levels introduced by the Co-3d orbitals. The shif ts of the E-0-point are also discussed. (C) 1998 Elsevier Science B.V.