Investigation of Nucleation and Intermixing at Hetero-Interface in III-Nitride-4H-SiC Structures

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 su