High sensitivity ultraviolet (UV) avalanche photodiodes (APDs) are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at longer wavelengths due to weak absorption and at shorter wavelengths due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. Recently, we have demonstrated that heterostructure III-Nitride/SiC diodes have promise for improving the efficiency of SiC based detectors throughout the UV spectrum. However, these devices require growth techniques for initiating two-dimensional growth on lattice mismatch substrates as well as suppressing impurity migration at the hetero-interface.

p- SiC

In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. A series of 30nm AlN films were deposited on i-p epitaxial SiC structures grown on 4 degree miscut 4H-SiC. The in-situ preparation techniques explored include annealing at 740 °C while depositing and desorbing Ga with and without N plasma active and the shutter closed. The AlN films were grown at temperatures of 600 °C and 740 °C using a standard approach without growth interruption under N limited conditions or employ an MEE, or interrupted growth approach. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by depth profiling x-ray photoemission spectroscopy (XPS).

Figure 1 compares the morphology and the composition profile of the AlN film grown by MEE (sample A) and the standard process (sample B) with the N plasma off during the Ga preparation. Sample A exhibits an atomically smooth, 2D surface, and a nearly stoichiometric Al to N composition in the AlN layer that drops off at the heterointerface. In contrast, sample B has a higher roughness on a 2x2 mm scale with an absence of discernable steps on the substrate. XPS results indicate a slightly larger Al to N composition in the film (no visible droplets) and a detectable presence of Al in the SiC layer indicating migration. These results suggest that MEE approach can both promote the nucleation of AlN growth on SiC in a 2D mode as well as suppress the migration of Al into SiC.

Figure 2 demonstrates the impact of an active N plasma source during the Ga preparation process despite closing the source shutter on the morphology and interface composition of the AlN films. Sample C was prepared with the N plasma on and was grown at 600 °C. The film exhibits a rough surface morphology despite the use of MEE, consistent with the observations of Okumura et al. [4]. Sample D, grown under similar conditions at a higher substrate temperature of 740 °C exhibits a 2D morphology characterized by micro-steps and possible step-bunching. Sample E was grown at 600 °C similar to sample D, but with the N plasma off during Ga deposition, exhibits an atomically smooth surface morphology consistent with that of the substrate. XPS studies of sample D indicate the presence of GaN as well as metallic Ga at the hetero-interface in contrast to that of sample E. This suggests that active N that leaks around the closed shutter leads to GaN island formation that prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures. The potential partial nitridation of the SiC surface during this process does not impede the nucleation of AlN in a 2D growth mode based upon studies of films grown on substrates annealed at 740 °C with the N plasma active but the shutter closed (data not shown).