Dissociation of Al2O3(0001) substrates and the roles of silicon and oxygenin n-type GaN thin solid films grown by gas-source molecular beam epitaxy

Unintentionally doped and silicon doped GaN films prepared by molecular bea m epitaxy using ammonia are investigated. Hall, secondary ion mass spectros copy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whet her the n-type background electron concentration is of impurity origin or n ative defect origin. We identify and quantify an anomalous relationship bet ween the Si doping concentration and free carrier concentration and mobilit y using temperature dependent Hall measurements on a series of 2.0-mu m-thi ck GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen i s positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide int erface is also observed. Finally, SIMS is again utilized to show how Si dop ing can be utilized to suppress the diffusion of the oxygen into the GaN la yer from the sapphire substrate. The mechanism of suppression is believed t o be formation of a Si-O bond and a greatly reduced diffusion coefficient o f the subsequent Si-O complex in GaN. (C) 2000 American Institute of Physic s. [S0021-8979(00)07112-7].