Residual donors and compensation in metalorganic chemical vapor depositionas-grown n-GaN

In our recent report, [Xu , Appl. Phys. Lett. 76, 152 (2000)], profile dist ributions of five elements in the GaN/sapphire system have been obtained us ing secondary ion-mass spectroscopy. The results suggested that a thin dege nerate n(+) layer at the interface is the main source of the n-type conduct ivity for the whole film. The further studies in this article show that thi s n(+) conductivity is not only from the contribution of nitride-site oxyge n (O-N), but also from the gallium-site silicon (Si-Ga) donors, with activa tion energies 2 meV (for O-N) and 42 meV (for Si-Ga), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, i ncluding Hall carrier concentration and mobility, has been modeled by separ ating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-su rface thin layer and is temperature dependent. Si-Ga and O-N should also be shallow donors and V-Ga-O or V-Ga-Al should be compensation sites in the b ulk layer. The best fits for the Hall mobility and the Hall concentration i n the bulk layer were obtained by taking the acceptor concentration N-A=1.8 x10(17) cm(-3), the second donor concentration N-D2=1.0x10(18) cm(-3), and the compensation ratio C=N-A/N-D1=0.6, which is consistent with Rode's theo ry. Saturation of carriers and the low value of carrier mobility at low tem perature can also be well explained. (C) 2001 American Institute of Physics .