A comparison of the Hall-effect and secondary ion mass spectroscopy on theshallow oxygen donor in unintentionally doped GaN films

We report on temperature-dependent Hall-effect measurements and secondary i on mass spectroscopy on unintentionally doped, n-type conducting GaN epitax ial films. Over a wide range of free carrier concentrations we find a good correlation between the Hall measurements and the atomic oxygen concentrati on. We observe an increase of the oxygen concentration close to the interfa ce between the film and the sapphire substrate, which is typical for the gr owth technique used (synthesis from galliumtrichloride and ammonia). It pro duces a degenerate n-type layer of approximate to 1.5 mu m thickness and re sults in a temperature-independent mobility and Hall concentration at low t emperatures (< 50 K). The gradient in free carrier concentration can also b e seen in spatially resolved Raman and cathodoluminescence experiments. Bas ed on the temperature dependence of the Hall-effect, Fourier transform infr ared absorption experiments, and photoluminescence we come to the conclusio n that oxygen produces a shallow donor level with a binding energy comparab le to the shallow Si donor. (C) 2000 American Institute of Physics. [S0021- 8979(00)00616-2].