M. Holtz

High quality GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.

Enhanced Signal-to-Background Ratios in Voltammetric Measurements Made at Diamond Thin-Film Electrochemical Interfaces

Large signal-to-background (S/B) ratios for the Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-) redox couples in KCl have been observed in cyclic voltammetric measurements made at a conductive diamond thin-film electrode without any conventional surface pretreatment. The S/B ratios were a factor of ∼16 and 8 larger at diamond than at freshly polished glassy carbon (GC) for Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-), respectively. The polycrystalline diamond film, grown on a p-Si(100) substrate, possessed significant cubic {100} faceting, as evidenced by AFM images, and was of high quality, as indicated by Raman spectroscopy. The high degree of electrochemical activity without surface pretreatment, the enhanced S/B ratios, and the excellent response stability demonstrate that diamond might be an attractive new electrode material for electroanalysis.

Microfabrication and characterization of teflon AF-coated liquid core waveguide channels in silicon

The fabrication and testing of Teflon AF-coated channels on silicon and bonding of the same to a similarly coated glass wafer are described. With water or aqueous solutions in such channels, the channels exhibit much better light conduction ability than similar uncoated channels. Although the loss is greater than extruded Teflon AF tubes, light throughput is far superior to channels described in the literature consisting of [110] planes in silicon with 45/spl deg/ sidewalls. Absorbance noise levels under actual flow conditions using an LED source, an inexpensive photodiode and a simple operational amplifier circuitry was 1/spl times/ 10/sup -4/ absorbance units over a 10-mm path length (channel 0.17-mm deep /spl times/0.49-mm wide), comparable to many commercially available macroscale flow-through absorbance detectors. Adherence to Beers law was tested over a 50-fold concentration range of an injected dye, with the linear r/sup 2/ relating the concentration to the observed absorbance being 0.9993. Fluorescence detection was tested with fluorescein as the test solute, a high brightness blue LED as the excitation source and an inexpensive miniature PMT. The concentration detection limit was 3 /spl times/ 10/sup -9/ M and the corresponding mass detection limit was estimated to be 5 /spl times/ 10 /sup -16/ mol.

Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT ...

Raman studies of nitrogen incorporation in GaAs1−xNx

We report direct-backscattering Raman studies of GaAs1−xNx alloys, for x⩽0.03, grown on (001) GaAs. The Raman spectra exhibit a two-mode behavior. The allowed GaAs-like longitudinal-optic phonon near 292 cm−1 is found to red shift at a rate of −136±10 cm−1/x. This is well described by the combined effects of strain and alloying. The GaN-like phonon near 470 cm−1 is observed to increase in intensity in direct proportion to x, and to systematically blue shift at a rate of 197±10 cm−1/x. This blue shift is likewise attributed to strain and alloying. The GaAs-like second-order features are also seen to broaden slightly and diminish in intensity with increasing nitrogen concentration. These results are attributed to a weak breakdown in the zincblende-crystal long-range order, possibly related to the presence of ordered domains within the random alloy.

Visible and ultraviolet Raman scattering studies of Si1−xGex alloys

We report Raman studies of the Si–Si phonon band in Si1−xGex alloys, where the excitation is by visible and ultraviolet (351 nm) light. At a wavelength 351 nm, the optical penetration depth is extremely shallow (≈5 nm). By varying the excitation from 351 to 514 nm, the optical penetration depth spans from 5 to 300 nm. Two sets of samples were examined. Thin layers grown using molecular beam epitaxy were coherently strained to match the lattice constant of the silicon substrate. Thick layers grown using organo–metallic chemical vapor deposition were strain relaxed. For the thin, strained layers, visible excitation produces a spectrum, which is a superposition of the substrate and the epilayer phonon bands. Reducing the wavelength (and, consequently, penetration depth) allows us to isolate the epilayer spectrum. Phonon energies obtained using all excitation wavelengths agree. We conclude that Raman scattering from these alloys using 351 nm laser light gives us bulk alloy properties pertinent to the near-sur...

Controlled growth of GaN nanowires by pulsed metalorganic chemical vapor deposition

Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.

Self-heating study of an AlGaN∕GaN-based heterostructure field-effect transistor using ultraviolet micro-Raman scattering

We report micro-Raman studies of self-heating in an AlGaN∕GaN heterostructure field-effect transistor using below (visible 488.0nm) and near (UV 363.8nm) GaN band-gap excitation. The shallow penetration depth of the UV light allows us to measure temperature rise (ΔT) in the two-dimensional electron gas (2DEG) region of the device between drain and source. Visible light gives the average ΔT in the GaN layer, and that of the SiC substrate, at the same lateral position. Combined, we depth profile the self-heating. Measured ΔT in the 2DEG is consistently over twice the average GaN-layer value. Electrical and thermal transport properties are simulated. We identify a hotspot, located at the gate edge in the 2DEG, as the prevailing factor in the self-heating.

Dependence of the stress–temperature coefficient on dislocation density in epitaxial GaN grown on α-Al2O3 and 6H–SiC substrates

We report measurements of stress in GaN epitaxial layers grown on 6H–SiC and α-Al2O3 substrates. Biaxial stresses span +1.0 GPa (tensile) to −1.2 GPa (compressive). Stress determined from curvature measurements, obtained using phase-shift interferometry (PSI) microscopy, compare well with measurements using accepted techniques of x-ray diffraction (XRD) and Raman spectroscopy. Correlation between XRD and Raman measurements of the E22 phonon gives a Raman-stress factor of −3.4±0.3 cm−1/GPa. We apply PSI microscopy for temperature dependent stress measurements of the GaN films. Variations found in the stress–temperature coefficient correlate well with threading dislocation densities. We develop a phenomenological model which describes the thermal stress of the epitaxial GaN as a superposition of that for ideal GaN and the free volume existing in the layers due to the threading dislocations. The model describes well the observed dependence.

AlN/AlGaInN superlattice light-emitting diodes at 280 nm

Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN(1.2 nm thick)/AlGaInN(0.5 nm thick) doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7–1.1)×1018 cm−3, with the mobility of 3–4 cm2/V s and electron concentrations of 3×1019 cm−3, with the mobility of 10–20 cm2/V s, at room temperature. These carrier concentrations are sufficient to form effective p–n junctions needed in UV light sources.