Structural, optical and electrical study of undoped GaN layers obtained by metalorganic chemical vapor deposition on sapphire substrates

We investigate optical, structural and electrical properties of undoped GaN grown on sapphire. The layers were prepared in a horizontal reactor by low pressure metal organic chemical vapor deposition at temperatures of 900 °C and 950 °C on a low temperature grown (520 °C) GaN buffer layer on (0001) sapphire substrate. The growth pressure was kept at 10,132 Pa. The photoluminescence study of such layers revealed a band-to-band emission around 366 nm and a yellow band around 550 nm. The yellow band intensity decreases with increasing deposition temperature. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies show the formation of hexagonal GaN layers with a thickness of around 1 μm. The electrical study was performed using temperature dependent Hall measurements between 35 and 373 K. Two activation energies are obtained from the temperature dependent conductivity, one smaller than 1 meV and the other one around 20 meV. For the samples grown at 900 °C the mobilities are constant around 10 and 20 cm² V ^-1 s^{- 1}, while for the sample grown at 950 °C the mobility shows a thermally activated behavior with an activation energy of 2.15 meV.