Ohmic contact to AlN:Si using graded AlGaN contact layer

We formed a graded-AlGaN contact layer to improve the Ohmic characteristics of Si-doped AlN. Linear I-V characteristics were obtained for AlN with the graded-AlGaN layer, and the current was three orders of magnitude larger than that for AlN without the one. The specific contact resistivity decreased with the increasing thickness of the graded-AlGaN layer. This was probably due to a reduction in the three-dimensional negative charge density induced by the polarization charge in the graded AlGaN layer. A minimum contact resistivity of 1.4 ×10−2 Ω cm2 was obtained for a 330-nm-thick graded-AlGaN layer. To obtain the Ohmic contact, the Si-dopant concentration (NSi) should be larger than the negative fixed charge density (ρπ) induced by the polarization charge. However, the heavily doped graded-AlGaN layer (NSi=2.4×1019 cm−3) became semi-insulating due to self-compensation. The results indicated that reducing ρπ by relaxing the compositional slope in the graded layer can improve the Ohmic characteristics.