Mobility enhancements in AlGaN/GaN/SiC with stair-step and graded heterostructures

In this letter, we report the observation of the enhanced electron mobility in three different Al${}_{\mathrm{0.08}}$ Ga${}_{\mathrm{0.92}}$ N/GaN heterostructures. These structures were deposited on (0001) 6H-SiC substrates by using the low-pressure metalorganic chemical vapor deposition method. The structure was composed of 500 Å AlGaN compositional stair-step layer deposited onto 1.3 $\mu$ m GaN epitaxial layer. There is a 100 Å GaN Cap layer on top of the AlGaN layer to prevent the oxidation of the AlGaN layer. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility for Al${}_{\mathrm{0.08}}$ Ga${}_{\mathrm{0.92}}$ N/GaN heterostructure are 6.6$\times$ 10${}^{\mathrm{12}}$ cm${}^{\mathrm{-2}}$ and 5413 cm${}^{\mathrm{2}}$ /Vs at 77 K, respectively. This high value of mobility is, to the best of our knowledge, the first observed two-dimensional electron gas (2DEG) at the AlGaN/GaN stair-step type of heterostructure. Other interface structures such as graded composition and bulk composition structures were also prepared and 2DEG properties were observed.