Study of defect states in GaN films by photoconductivity measurement

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Abstract
Optical absorption by defect states in gallium nitride (GaN) films was studied by photoconductivity (PC) spectroscopy at room temperature. A number of undoped n‐type and Mg‐doped p‐type samples were employed in the present study. The PC response per absorbed photon decreased by more than four orders of magnitude as GaN became p‐type conducting. Furthermore, all the samples exhibit PC response at photon energies far below the band gap energy of GaN. The optical absorption increases with photon energy hν from 1.5 to 3.0 eV approximately as exp(hν/E0). The parameter E0 ranges from 180 to 280 meV, and is considerably smaller for the insulating p‐type sample. For a p‐type conducting sample, the PC response is flat between 0.7 and 1.4 eV. A model for the density of states distribution in the forbidden gap of GaN and the effect of Mg doping is proposed.

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