Investigation of Si-Doping and Impurity Incorporation Dependence on the Polarity of GaN by MBE

The ability to control the doping levels of both n- and p-type dopants in semiconductors is an important issue for device applications. In this work, we have investigated the incorporation of Si, which is an n-type dopant in GaN. Wurtzite GaN lacks inversion symmetry when grown along the c-axis, and can be grown with either N-polarity (000 1 bar) or Ga-polarity (0001). We have grown GaN on c-plane sapphire by radio-frequency plasma-assisted MBE and found that GaN grew with N-polarity when the starting buffer layer consisted of low-temperature (~500 degrees C) GaN. In contrast, GaN films with Ga-polarity were obtained when utilizing a high-temperature (~720 degrees C) AlN buffer layer. The polarity of the films was determined by dipping the samples in a 1M KOH solution heated to 150 degrees C. Consistent with the observation of others in the literature, we found that the surface of N-polarity GaN is readily etched whereas the surface of Ga-polarity GaN remains intact. In addition, when the epilayer was cooled below ~400 degrees C after growth was completed, the RHEED pattern showed a (2x2) and (4x4) reconstruction for Ga- and N-polarity samples, respectively.