Raman studies on spintronics materials based on wide bandgap semiconductors

Structural properties of GaN and ZnO layers doped with magnetic impurities were investigated by Raman scattering. Long-range lattice ordering and local atomic arrangement around magnetic impurities were analysed, and their solubility limit was considered. For this study, GaN layers doped with Mn and Cr, and ZnO layers doped with Co and V were prepared by molecular beam epitaxy and pulsed laser deposition, respectively. ZnO layers codoped with Ga and N were also studied for the purpose of p-type activation. These samples were observed using a Raman microprobe using visible and deep UV lasers for excitation. The main results are as follows: In Ga1-xMnxN layers, a uniform solid solution was formed for Mn concentration up to x = 1-2%. An impurity mode was observed at 585 cm(-1) and assigned to a local vibrational mode of Mn substituting the Ga site. At higher Mn concentrations, rapid deterioration in lattice ordering occurred. Ga1-xCrxN layers showed good lattice ordering up to x = 3-5%. The samples showed resonance enhancement of LO-phonon signals when excited by a UV laser at 266 nm (4.7 eV). This indicates the photo-injection of free carriers to a diluted magnetic semiconductor. ZnO layers codoped with Ga and N showed many impurity modes due to host lattice defects. A strong signal at 580 cm(-1) showed a characteristic broadening at high concentrations of N and Ga. This suggested the formation of complex centres with N or related defects. Zn1-xCoxO and Zn1-xVxO layers formed uniform solid solutions up to x similar to 5%, but precipitation of the secondary phase was observed at x > similar to10%. These samples presented common defect modes as observed in codoped samples. Our result suggests that the impurity modes in ZnO-based materials can be used as a sensitive probe of host lattice defects induced by the impurity incorporation process.