Electronic structure of Ga$_{1-x}$Cr$_{x}$N and Si-doping effects   studied by photoemission and X-ray absorption spectroscopy
  The electronic structure of the magnetic semiconductor Ga$_{1-x}$Cr$_{x}$N and the effect of Si doping on it have been investigated by photoemission and soft x-ray absorption spectroscopy. We have confirmed that Cr in GaN is predominantly trivalent substituting for Ga, and that Cr 3$d$ states appear within the band gap of GaN just above the N 2$p$-derived valence-band maximum. As a result of Si doping, downward shifts of the core levels (except for Cr 2$p$) and the formation of new states near the Fermi level were observed, which we attribute to the upward chemical potential shift and the formation of a small amount of Cr$^{2+}$ species caused by the electron doping. Possibility of Cr-rich cluster growth by Si doping are discussed based on the spectroscopic and magnetization data. 
G. S. Song, M. Kobayashi, J. I. Hwang, T. Kataoka, M. Takizawa, A.   Fujimori, T. Ohkouchi, Y. Takeda, T. Okane, Y. Saitoh, ...