GaN (0001) and (0001) samples were grown on sapphire substrates under various growth conditions by molecular beam epitaxy using ammonia as a nitrogen precursor. Electrical and luminescent properties were measured after growth by Hall, CV/IV, and cathodoluminescence (CL). GaN(0001) or N-polar films were produced by initiating growth with a low temperature GaN nucleation layer while GaN (0001) or Ga-polar films were produced by initiating growth with an AlN nucleation layer. The polarity of the film was determined with RHEED and/or etching in KOH. Systematic differences were seen that depended on the polarity of the films. GaN(0001) films had relatively poor electrical properties, including an unintentional carrier concentration on the order of 10E18 cm^(-3) n-type and mobility on the order of 100 cm^(2)/Vs. However, the luminescence properties were fairly good -- in most cases, no detectable yellow/green (540~nm) luminescence was observed in the CL spectrum. On the other hand, the (0001) films had much better electrical properties, including an unintentional carrier concentration on the order of 10E16 cm^(-3) and mobility as high as 225 cm^(2)/Vs. However, the CL spectrum often contained a large yellow/green (540 nm) luminescence indicating a large number of mid-gap states. Surprisingly, the surface morphologies of the N-polar films were much smoother than those of the Ga-polar films. This evidence suggests that growth polarity related defects or differences in the surface reactivity of the two polarities is the cause of the unintentional background doping and mid-gap states very commonly observed in GaN.

* Partially supported by the Office of Naval Research and the National Science Foundation (DMR).