Studies of p-GaN grown by MBE on GaAs(111)B

We have investigated the properties of Mg-doped GaN grown by molecular beam epitaxy. Active nitrogen is produced by an Oxford Applied Research CARS25 plasma source. Further details of the methods used to grow the films have been presented elsewhere [1]. Growth rates are typically 0.3 mu m/h. Before growth, the GaAs substrates are heated to 620 degrees C to remove the oxide and then exposed to the nitrogen plasma whilst heating to the growth temperature of approximately 700 degrees C. We have studied the structural properties of the epitaxial layers using a Philips Xpert diffractometer using both theta-2 theta and omega scans. The electrical properties have been determined using Hall effect measurements using a true van der Pauw geometry and the corresponding chemical concentrations have been obtained by SIMS profiles using implanted standards. The optical properties have been obtained using 300 K and low-temperature (8 K) photoluminescence measurements using a He-Cd laser for excitation. The results indicate that Mg is incorporated by rapid diffusion from a surface concentration maintained by the incident flux. It follows that the Mg concentration increases with growth time (sample thickness) and with cell temperature, but saturates above a certain Me cell temperature. This limits the doping level to about 10(18) cm(-3) for the growth temperature of 700 degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.