Photoluminescence Studies of Two-Dimensional Electron Gas in Modulation Doped A1xGa1-xN/GaN Structures Grown on SI 4H-SiC by MOCVD

摘要

Electron devices with ever-increasing operating speed are used for applications such microwave switches or amplifiers. As the physical dimensions of these devices (e.g., field effect transistors) become smaller, thinner channel layers and higher electron concentrations are required to provide high performance operation. This is because current for faster discharge of capacitance in these microwave device structures is proportional to the carrier velocity as well as the carrier density. This requirement of large concentration of electrons can be met by novel heterojunctions. In the case of aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs), the junctions can be aligned so that the energy of the electrons and thus, the donors, is introduced only to the larger band gap AlGaAs material 1,2. The electrons in the AlGaAs layer diffuse into the lower energy GaAs layer where they are confined because of the energy barrier. This modulation doping, i.e., doping of the barrier layer at hetero-interface, thus causes a redistribution of electrical charge across the interface. In the case of n-doped barrier layer, the region in the barrier closer to the interface will be depleted and the corresponding electrons will accumulate in a potential well in the active layer closer to the interface as a 2DEG 3. Having the electrons thus confined at the hetero-interface in the 2DEG very close to the gate and an optimum interface leads to very high mobilities and large electron velocities at very small values of drain voltage. The 2DEG can be investigated by photo excitation, and the resulting radiative recombination between the 2DEG and photo-excited holes in heterostructures observed.