Development of III-Nitride Superjunctions

Abstract

Abstract Body: As silicon technology reaches its practical and theoretical limit, III-nitride materials offer a promising alternative to maintain the growing demand for high power electronic devices. Current GaN devices already show promising results, offering higher breakdown voltages and reduced on resistances compared to their silicon predecessors. However, these devices are still limited by the classical Baliga figure of merit (BFOM). In order to surpass the BFOM, novel device geometries have been introduced such as the superjunction (SJ). The concept of the SJ has been demonstrated in silicon, with devices such as CoolMOS which have surpassed the BFOM. To date, there are no demonstrations of III-nitride SJs in the literature. In order to achieve III-nitride SJs, there are several research barriers to address. First, the necessity for alternating, lateral n- and p-type doping regions with zero net charge. The inherent polar doping selectivity of GaN can be used to achieve the doping scheme for a lateral GaN p/n junction. Oxygen, which unintentionally incorporates into N-polar GaN at levels >1019 cm-3, acts as the n-type dopant, whereas Ga-polar GaN does not readily incorporate oxygen. Accordingly, lateral polarity junctions (LPJs) with alternating domains of O doped N-polar and Mg doped Ga-polar GaN have been fabricated to realize lateral p/n junctions. Second, the proper doping profiles must be attained in the N- and Ga-polar domains for SJ operation. For drift regions, the n-type doping in the N-polar domain (and p-type doping in Ga-polar domain) must be reduced to ~1017 cm-3 for micron wide domains. By implementing the chemical potential control (CPC) framework, MOCVD process conditions were designed in order to decrease the oxygen concentration by increasing the growth supersaturation. This led to a reduction in oxygen from >1019 cm-3 to low 1017 cm-3. Third, due to the difficulty of making reliable Schottky contacts to the N-polar surface, the alternating Schottky/ohmic contacts on a typical superjunction are replaced with alternating p+/n- junction (N-polar) and p+/p- junctions (Ga-polar). In this direction, a N-polar p/n junction with rectifying behavior comparable to a Ga-polar p/n junction was achieved. Finally, the growth of a GaN LPJs must exhibit N-/Ga-polar domains with a smooth surface and equal growth rates. It is understood that supersaturation can be used to control the surface morphology, relative growth rates, and defect incorporation of the domains. However, the high supersaturation required to realize the necessary doping profile for SJ operation contradicts the intermediate supersaturation conditions required to obtain a smooth surface morphology and equal domain growth rates. It must be noted that high supersaturations result in low O N-polar dominated growth, and low supersaturations in Ga-polar dominated growth. To overcome these anticorrelated requirements, we demonstrate a supersaturation modulated growth (SMG) where the V/III ratio, and thus supersaturation, was modulated between low and high values. A GaN LPJ with a smooth surface and equal domain heights, with the necessary doping profile is hence demonstrated.