Effect of Strain and Polarization Grading on Hole Transport across Tunneling Barriers between Metals and Wurtzite Indium
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0994-F08-05
Effect of Strain and Polarization Grading on Hole Transport across Tunneling Barriers between Metals and Wurtzite Indium Gallium Nitride Choudhury Jayant Praharaj formerly with Cornell University, Ithaca, NY, 14853
Abstract We theoretically model the transport of holes across graded wurtzite Indium Gallium Nitride layers with large barriers to metals of the order of 2 electron volts. The effect of continuous strain grading and the resulting piezoelectric grading is explicitly taken into account. As data about critical thicknesses for dislocation creation are scarce for these materials, the grading widths considered for the calculations are deliberately kept small to ensure that the layers are below theoretically predicted critical thickness limits. The spatial variation of spontaneous and piezoelectric polarization creates bulk bound polarization charges that have a strong effect on the electrostatics of the layers, and creates the optimum conditions for efficient tunneling of holes. We also explicitly model the effect of the different hole masses for the valence band. Three orders of magnitude increase of tunneling intensity is seen for split-off holes with effective masses of 0.15 for the case of moderate grading from 30 % Indium to 0% Indium over 30 angstroms, compared to the case without grading. A more aggressive grading from 30 % Indium to 0% Indium over 10 angstroms does not lead to any extra benefits and leads to a decrease in tunneling intensity. The electric field for the latter case dominates the electric field for the former both near the top and the bottom of the barrier. However, the effective barrier width at the valence band edge becomes higher for the case of grading over 10 angstroms, and most of the carriers see a damped tunneling amplitude.
Introduction Strain and polarization grading in group III nitride wide-band gap semiconductor epitaxial layers offer new opportunities for optimization of device performance. The wurtzite group III nitrides have non-vanishing spontaneous polarization and strained layers have piezoelectrically induced polarization. These polarizations are typically of the order of 1013 electrons per cm2 [ 1, 2, 3, 4 ]. These large spontaneous and piezoelectric polarization magnitudes give rise to very high built-in fields. These high fields often have non-trivial effects on device characteristics because of their impact on carrier transport. In this paper, we study the effect of continuous grading on the transport of holes across indium nitride-metal heterojunctions. Transport of holes across wide band-gap heterostructures is of technological significance due to a number of factors. Hole masses tend to be large in these semiconductors. This makes transport of holes sluggish in device structures, leading to low current densities and high contact resistances. Our calculations demonstrate the positive effects of continuous polarization grading on hole transport across Schottky barriers to indium gallium nitride. We also explicitly model the effect of the different h
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