M. G. Spencer

Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

Surface potential measurements on GaN and AlGaN/GaN heterostructures by scanning Kelvin probe microscopy

Surface potentials on GaN epilayers and Al0.35Ga0.65N/GaN heterostructures have been studied by scanning Kelvin probe microscopy (SKPM) in conjunction with noncontact atomic force microscopy. The dependence of the surface potential on doping in GaN films, as well as the variation of surface potential with Al0.35Ga0.65N barrier layer thickness has been investigated. The bare surface barrier height (BSBH), as measured by SKPM, is observed to decrease from ∼1. 40±0.1 eV to ∼0.60±0.1 eV with increasing doping in the GaN epilayers. Schottky barrier height calculated from the measurements of BSBH on n-GaN agrees very well with results from previous studies. We have also estimated the surface state density for GaN based on the measured values of BSBH. The semiconductor “work function” at the Al0.35Ga0.65N surface (in heterostructure samples) is observed to decrease by ∼0.60 eV with increase in barrier layer thickness from ∼50 to ∼440 A. A simple model considering the presence of a uniform density of charged acce...

Cantilever effects on the measurement of electrostatic potentials by scanning Kelvin probe microscopy

Scanning Kelvin probe microscopy (SKPM) is a unique way to measure electrostatic potentials for small geometries. It has numerous applications including characterization of integrated circuits and nanoscale devices. SKPM is attractive because of the quantitative nature of the measurements. In this work, we have examined one of the principal sources of measurement error, the cantilever (which holds the probe tip). The accuracy of measurements of electrostatic potentials on closely spaced regions biased differently is reduced due to a large capacitance gradient associated with the cantilever. However, it is observed that the accuracy of measurements increases as the tip–sample distance is decreased because the capacitance gradient of the tip becomes proportionally larger relative to that of the cantilever. It is further observed that longer tips with smaller cantilever areas measure the electrostatic potentials more accurately as the capacitance gradient of the cantilever is reduced. Scanning probe tips are parametrized by a factor R, which indicates their suitability for SKPM measurements.

Scanning Kelvin probe microscopy characterization of dislocations in III-nitrides grown by metalorganic chemical vapor deposition

Scanning Kelvin probe microscopy has been used in conjunction with noncontact atomic force microscopy for characterizing dislocations in n-GaN and Al0.35Ga0.65N/GaN heterostructures. The surface potential variations around the dislocations present in the Al0.35Ga0.65N/GaN heterostructure have been observed to be 0.1–0.2 V with full width at half maximums (FWHMs) of 100–200 nm. On the other hand, n-GaN shows potential variations of 0.3–0.5 V having FWHMs of 20–50 nm. The dislocations (present in densities of ∼109 cm−2) have been found to be negatively charged for both n-GaN and Al0.35Ga0.65N/GaN heterostructure samples.

Reduced trapping effects and improved electrical performance in buried-gate 4H-SiC MESFETs

Surface effects on the current instability of 4H-SiC MESFETs were studied by comparing different surface structures. The current instability phenomenon was illustrated by bias sweeping methods and current recovery time measurements. A reduction in the current instability was observed for gate-recessed and buried-gate devices compared to the nonrecessed and channel-recessed devices. In addition, the buried-gate devices were found to have higher current density and breakdown voltage compared to the gate-recessed devices, resulting from their shorter effective gate length and lower electric field distribution under the gate, respectively. With high saturation current, high breakdown voltage, and much reduced surface effects, the buried-gate structure is a candidate for high-power SiC MESFETs.

Linear electro-optic effect in cubic silicon carbide

We report the first observation of the electro‐optic effect of cubic silicon carbide (β‐SiC) grown by a low‐pressure chemical vapor deposition reactor using the hydrogen, silane, and propane gas system. At a wavelength of 633 nm, the value of the electro‐optic coefficient r41 in β‐SiC is determined to be 2.7±0.5×10−12 m/V, which is 1.7 times larger than that in gallium arsenide measured at 10.6 μm. Also a half‐wave voltage of 6.4 kV for β‐SiC is obtained. Because of this favorable value of electro‐optic coefficient we believe that silicon carbide may be a promising candidate in electro‐optic applications for high optical intensity in the visible region.

AlN as an encapsulate for annealing SiC

AlN films grown by either organometallic vapor phase epitaxy (OMVPE) or pulsed laser deposition (PLD) can be used to encapsulate SiC when heated in an argon atmosphere at temperatures at least as high as 1600 °C for times at least as long as 30 min. The coverage of the AlN remains complete and the AlN/SiC interface remains abrupt as determined by Auger electron spectroscopy. However, considerable atomic movement occurs in the AlN at 1600 °C, and holes can form in it as the film agglomerates if there are large variations in the film thickness. Also, the SiC polytype near the surface can in some instances be changed possibly by the stress generated by the epitaxial AlN film. Using x-ray diffraction measurements, we also found that, during the 1600 °C anneal, grains with nonbasal plane orientations tended to grow at the expense of those with basal plane orientations in the OMVPE films, whereas grains with only the basal plane orientation tended to grow in the PLD films. However, there is no indication that t...

Electrical characterization of grain boundaries in GaAs

Grain‐boundary capacitance versus voltage, current versus voltage, and capacitance transient measurements are reported on lightly doped n‐type GaAs bicrystals. The measurements are analyzed in terms of the abrupt depletion edge model in which the dominant current mechanism across the potential barrier formed by trapped charge at the interface is thermionic emission. The steady‐state measurements are completely consistent with this model and provide grain‐boundary parameters (barrier height, net carrier concentration in adjacent grains, and interface charge) necessary for interpreting capacitance transient measurements. The latter, made on a bicrystal specially selected for uniformity of the grain‐boundary barrier, reveal the presence of two closely spaced levels at 0.62 and 0.74 eV below the conduction band edge. The capture cross section associated with the dominant lower level is measured directly by two methods and falls in the range 10−14–10−13 cm2.

Lobular capillary haemangioma of the nasal vestibule: a rare entity.

The authors report a case of an 8-year-old boy with lobular capillary haemangioma (LCH) of the nasal vestibule presenting with nasal blockage, nasal discharge and epistaxis. LCH of the nasal vestibule is rarely reported in the English literature, and the authors feel that it should be considered in the differential diagnosis of lesion of the nasal vestibule. Unnecessary investigations could be avoided if its diagnosis is considered, especially in children.

Cavernous haemangioma of the nasal bones : an alternative management option

The authors present a case of bilateral cavernous haemangiomas affecting the posterior ends of both inferior turbinates of the nose. The condition was treated by angiographically controlled embolization. Review of the literature back to 1967 has revealed no other report of embolization being used specifically for this condition. All previous treatments have involved surgery; we describe an alternative therapeutic option.