Inversion of wurtzite GaN(0001) by exposure to magnesium

Magnesium incorporation during the molecular beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime. The development of blue light emitting devices requires better understanding of growth and dopant incorporation in GaN. Though Mg is the most commonly used p-type dopant in GaN based diodes, its incorporation in GaN is poorly understood. Early studies on p-type doping of GaN in metallorganic chemical vapor deposition (MOCVD) revealed that Mg dopant atoms were strongly passivated in these films and the carrier concentration was therefore very low [1,2]; dopant activation can be achieved by various methods [2,3,4]. In molecular beam epitaxy (MBE) growth of pdoped GaN, the high vapor pressure of Mg at GaN growth temperatures is an issue and dopant incorporation may be rather inefficient [5,6]. Studies have shown that the Mg concentration decreases from the surface to the interior of the film [7] suggesting dopant incorporation from a surface Mg layer. Some workers have noted Mg-induced changes in the growth rate of GaN on different crystallographic planes [8], pointing to a surfactant effect of Mg on GaN. The presence of Mg atoms during the growth of GaN has also been associated with the appearance of stacking faults [9]. It is clear that understanding the effects of Mg on GaN crystal stacking and polarity is very important. Most devices are built on the polar basal plane of wurtzite GaN, and the characteristics of these devices depend on whether the GaN film is Ga-polar or N-polar [10]. The identification of polarity has been done by several groups using different methods. Seelmann-Eggebert [11] et al. found that etching of the crystal by an alkali solution was sensitive to polarity. Smith et al. [12] used surface reconstruction to identify polarity of wurtzite GaN, and we use the same technique in this work. We monitor the surface reconstruction of our films using reflection high energy electron