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We have performed a detailed contactless electroreflectance study of the interband excitonic transitions on both the Ga and N faces of a 200-μm-thick freestanding hydride-vapor-phase-epitaxy grown wurtzite GaN sample with low defect concentration in the temperature range between 20 and 300 K. The transition energies of the A, B, and C excitons and broadening parameters of the A and B excitons have been determined by least-square fits to the first derivative of a Lorentzian line shape. The energy positions and separations of the excitonic transitions in the sample reveal the existence of residual strain. At 20 K the broadening parameter of A exciton deduced for the Ga (5×10[SUP5] dislocation cm[SUP-2] and N (1×10[SUP7] dislocation cm[SUP-2]) faces are 3 and 7 meV, respectively, indicating a lower defect concentration on the former face. The parameters that describe the temperature dependence of the interband transition energies of the A, B, and C excitons as well as the broadening function of the A and B features are evaluated. The results from an analysis of the temperature dependence of the broadening function of excitons A and B indicate that GaN exhibits a very large exciton-phonon coupling. [ABSTRACT FROM AUTHOR]

Edge-terminated Schottky rectifiers fabricated on quasibulk GaN substrates showed a strong dependence of reverse breakdown voltage V[sub B] on contact dimension and on rectifier geometry (lateral versus vertical). For small diameter (75 μm) Schottky contacts, V[sub B] measured in the vertical geometry was ∼700 V, with an on-state resistance (R[sub ON]) of 3 mΩ cm2, producing a figure-of-merit V[sub B][sup 2]/R[sub ON] of 162.8 MW cm-2. Measured in the lateral geometry, these same rectifiers had V[sub B] of ∼250 V, R[sub ON] of 1.7 mΩ cm2 and figure-of-merit 36.5 MW cm-2. The forward turn-on voltage (V[sub F]) was ∼1.8 V (defined at a current density of 100 A cm-2), producing V[sub B]/V[sub F] ratios of 139–389. In very large diameter (∼5 mm) rectifiers, V[sub B] dropped to ∼6 V, but forward currents up to 500 mA were obtained in dc measurements. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]