The current–voltage characteristics of Schottky barrier diodes formed on GaN(0001) free-standing substrates with net donor concentrations of 7.6×1015–1.4×1017 cm-3 are discussed. The substrates were grown by hydride vapor phase epitaxy. Ni Schottky contacts were directly formed on chemical–mechanical-polished Ga-polar faces of the substrates. Nearly ideal characteristics for both directions were obtained. The ideality factors for forward characteristics are 1.02–1.05, very close to unity. The reverse characteristics agree well with calculations based on thermionic-field emission theory without any fitting parameter.
Time-resolved photoluminescence (TRPL) and positron annihilation measurements, as well as Al0.23Ga0.77N/GaN heterostructure growth by metalorganic vapor phase epitaxy were carried out on very low defect density, polar c-plane and nonpolar m-plane freestanding GaN (FS-GaN) substrates grown by hydride vapor phase epitaxy. Room-temperature photoluminescence (PL) lifetime for the near-band-edge (NBE) excitonic emission of the FS-GaN substrates increases with increasing positron diffusion length (L+); i.e., decreasing gross concentration of charged and neutral point defects and complexes. The best undoped c-plane FS-GaN exhibits record-long L+ being 116 nm. The fast component of the PL lifetime for its NBE emission increases with temperature rise up to 100 K and levels off at approximately 1.1 ns. The result implies a saturation in thermal activation of nonradiative recombination centers. The surface and interface roughnesses for a Si-doped Al0.23Ga0.77N/GaN/Al0.18Ga0.82N/GaN heterostructure are improved by the use of FS-GaN substrates, in comparison with the structure fabricated on a standard GaN template. The emission signals related to the recombination of a two-dimensional electron gas and excited holes are recognized for an Al0.23Ga0.77N/GaN single heterostructure grown on the c-plane FS-GaN substrate.
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