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We have studied radiative and nonradiative recombination at surface states in GaN, including as-grown samples and those treated with acids or bases. The surface states manifested themselves in two ways: (i) a reversible increase of the photoluminescence (PL) intensity after ultraviolet (UV) illumination in vacuum and (ii) appearance of new PL bands after treatment with acid or base and subsequent exposure to air. It has been established that the GaN surface physi-sorbs species from air (presumably oxygen) which induce surface states acting as nonradiative recombination centers. It has been found that nonradiative recombination of photogenerated carriers via surface states comprises more than 70% of the recombination in some GaN samples. Another type of the surface state, which participates in radiative recombination, has been found in GaN samples with Ga polarity after brief etching of the surface with hot acid or base and subsequent exposure to air. In such samples, a broad PL band emerges in the blue region of the spectrum at low temperatures. The blue band has been attributed to transitions of photogenerated electrons from donors in the near-surface depletion region to the surface states introduced by the above-mentioned procedure. The changes in the GaN surface caused by etching were examined by atomic force microscopy. In some samples the blue band appeared even when no evidence of the layer etching was found except for quite shallow etch pits formed at dislocation sites. The emerging blue band can be related to the surface states formed on the a-planes of etch pits.

A free-standing GaN template grown by hydride vapor phase epitaxy has been characterized by transmission electron microscopy ~TEM!. The TEM investigation was augmented by x-ray diffraction, defect delineation etching process followed by imaging with atomic force microscopy and variable temperature photoluminescence. The density of dislocations near the N face was determined to be, in order, 3613107, 4613107, and about 13107 cm22 by cross-sectional TEM, plan-view TEM, and a defect revealing etch, respectively. The same methods on the Ga face revealed the defect concentration to be, in order, less than 13107 cm22 by plan-view TEM, less than 53106 cm22 by cross-sectional TEM, and 53105 cm22 by defect revealing hot H3PO4 acid, respectively. The full width at half maximum of the symmetric ~0002! x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (101I4) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth was about 1meV each at 10 K, and a green band centered at about 2.44 eV was observed.

We have fabricated GaN quantum dots (QDs) in AlN confined layer structures by molecular beam epitaxy. The size distribution and density of the QDs have been estimated from an atomic force microscopy study. Very high quantum efficiency of photoluminescence (PL) has been obtained in some samples with QDs. Compared to the GaN bulk samples, it increased by orders of magnitude. In some samples the quantum size effect dominated, resulting in the blue-shift of the QD related PL peak, whereas in the samples with larger dots a red-shift up to 0.8 eV has been observed, which is related to strong polarization effects. We have observed a blue-shift of the PL peak with excitation intensity in the samples with large dots due to screening effect. The temperature-induced quenching of PL occurs at higher temperatures compared to bulk GaN due to the confinement of nonequilibrium carriers in the QDs. An excited state has been observed in some samples.

Structural, electrical and optical properties of free-standing 200-µm thick GaN films grown by hydride vapor phase epitaxy (HVPE) have been investigated. After laser lift-off, the GaN substrates were mechanically polished on both Ga and N-sides and dry etched only on the Ga side to obtain a smooth epi-ready surface. Hot H3PO4 chemical etching on both surfaces was used to reveal the defect sites, which appeared as hexagonal pits. The etched surfaces were then examined by atomic force microscopy.

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.