First-principles investigation of Mn δ-layer doped GaN/AIN/GaN (0001) tunneling junctions

摘要

Highly spin polarized diluted ferromagnetic semiconductors are expected to be widely used as electrodes in spintronic devices. Based on density functional theory calculations, we investigate the feasibility of using Mn-doped wurtzite GaN/AlN/GaN(0001) trilayer junctions for tunnel magnetoresistance (TMR) devices. We address some key issues affecting the degree of spin polarization and spin tunneling transport with the aim of realizing the appealing half-metallicity and large TMR ratio. We propose digital δ-Mn layer doping in GaN, close to the GaN/AlN interfaces for enhanced performance. Layer-resolved band structure and density of states calculations reveal that Mn dopants produce local metallic or half-metallic states surrounded by the host semiconductor materials. Spin polarized electrons can transport across the interfaces, free of the conductivity mismatch problem owing to the strong hybridization between Mn 3d states and the states of surrounding host atoms. The calculated TMR ratio is found to depend sensitively on the dopant concentration. Half-metallicity and large TMR ratios are predicted for "low" dopant concentrations (1/2 and 1/4 monolayers), while a high concentration (1 monolayer) produces metallic states and thus a decreased TMR ratio. Very thin A1N barrier layers are predicted to yield low TMR ratios. We also study the role of two types of structural defects close to the Mn atoms at the interfaces, namely, atomic mixing (Al replaces Ga and vice versa), and N and Ga vacancies. While the studied atomic interdiffusion defects have little effect on the TMR ratio, both N and Ga vacancies are found to destroy the half-metallicity and lead to a substantial reduction of the TMR ratio, and thus should be eliminated for enhanced device performance.