A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown on c-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations.

Local high current densities in areas around dislocations with a screw component might be responsible for the observed high leakage currents in GaN-based electronic devices. Using ballistic electron emission microscopy, threading dislocations with a screw component are found to be accompanied by high current densities and low effective Schottky barrier heights. The electronic states responsible for this extremely nonuniform behavior of GaN films are metastable trap states. The experimental results show that acceptor- and donor-like charge traps coexist in the vicinity of dislocations with a screw component.

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We report an extensive investigation of semiconductor band-structure effects in single-barrier Al x Ga 1Ϫx As/GaAs heterostructures using ballistic-electron-emission spectroscopy ͑BEES͒. The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0рxр1). The initial ͑⌫͒ BEES thresholds for Al x Ga 1Ϫx As single barriers with 0рxр0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements at 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime ͑xϾ0.45͒, spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through Al x Ga 1Ϫx As X valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/Al x Ga 1Ϫx As interface produces a significant fraction of this X valley current. A comparison of the BEES thresholds with the expected composition dependence of the Al x Ga 1Ϫx As ⌫, L, and X points yields good agreement over the entire composition range. ͓S0163-1829͑97͒04827-3͔

Ballistic electron emission microscopy (BEEM) measurements on GaN grown on sapphire substrates reveal a second conduction band minimum ∼340 meV above the absolute band minimum at the zone center (Γ point). A significant lateral variation of the energy difference between the two band minima, ±50 meV, was observed which may result from nonuniform strain in the material. The existence of two conduction bands in close proximity may affect device applications, i.e., GaN based lasers and electronic devices.