Influence of nitrogen on the band structure of GaP: a ballistic electron emission spectroscopic investigation

Abstract

Recently, it has been discovered in III-V semiconductors such as GaAs and GaP, that a partial replacement of the column-V elements with nitrogen leads to large changes in the material properties. Specifically, there is a strong influence on the band structure of these semiconductors with a large reduction in the fundamental bandgap. With a 4% nitrogen incorporation, there is a 400meV reduction in the GaAs bandgap. The most striking feature is that the GaAs semiconductor still retains the direct bandgap characteristics. On the GaP front, it has been reported that the incorporation of nitrogen in GaN_xP_1-x alloys for x ≥ 3%, leads to the direct bandgap behavior. This phenomenon has been explained in terms of an interaction between the localized nitrogen states and the extended states at the gamma valley of the conduction band. However, these conclusions are based on the absorption and photoluminescence measurements, which are of indirect nature in fingerprinting the independent contributions of the gamma, X and L valleys. To shed some more light on the transformation of the GaP semiconductor from indirect to direct bandgap regime, in this presentation, we report our ballistic electron emission spectroscopic (BEES) investigations on GaN_xP_1-x (x ≥ 0.032) alloys. BEES is a modified version of scanning tunneling microscopy/spectroscopy, in that a third terminal added on the semiconductor substrate, besides the two common electrodes; STM tip and the Schottky metal contact on the GaNP epilayer. By measuring the current flow through the third contact as function of the tip bias, the I-V data virtually provides the information on the current transport through the semiconductor. Thus, the second derivative BEES provides a direct spectroscopic signature for electron scattering through the different valleys of the semiconductor. Our systematic investigations on the evolution of the GaNP alloy band structure for nitrogen concentrations exceeding 3%, are discussed with in the scope of second-derivative BEES data. GaNP epilayers were grown on a GaP substrate by using gas-source molecular beam epitaxy (MBE).