V. Kuryatkov

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.

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.

AlGaInN-based ultraviolet light-emitting diodes grown on Si(111)

Ultraviolet light-emitting diodes grown on Si(111) by gas-source molecular-beam epitaxy with ammonia are described. The layers are composed of superlattices of AlGaN/GaN and AlN/AlGaInN. The layers are doped n and p type with Si and Mg, respectively. Hole concentration of 4×1017 cm−3, with a mobility of 8 cm2/Vs, is measured in Al0.4Ga0.6N/GaN. We demonstrate effective n- and p-type doping of structures based on AlN/AlGaInN. Light-emitting diodes based on these structures show light emission between 290 and 334 nm.

Influence of defects on structural and electrical properties of VO2 thin films

We present the structural and electrical properties of (011) preferred polycrystalline (Poly) and multidomain (020) epitaxial (Epi) VO2 thin films grown at different temperature (Ts) and on different substrates with variable defects. These defects cause variation in strain, metal-insulator transition (MIT) temperature (TMIT), activation energy (ΔEa), and charge carrier type in insulating phase. Both the Poly- and Epi-VO2 behave n-type conductivity when grown at relative low TS. As TS increases, defects related acceptor density increases to alter conductivity from n- to p-type in the Poly-VO2, while in the Epi-VO2 donor density increases to maintain n-type conductivity. Moreover, the strain along monoclinic am axis dramatically reverses from tensile to compressive in both the Poly- (848 K < TS < 873 K) and Epi-VO2 (873 K < TS < 898 K), and eventually approaches to a constant in the Poly-VO2 (TS ≥ 898 K) in particular. TMIT decreases with increasing the carrier density independent of the conductive type in ...

Tunable dual-band terahertz metamaterial bandpass filters

We report metamaterial terahertz (THz) bandpass filters with tunable dual-band selectivity. The shift in the center frequency of the device is achieved by actively modifying the effective length of the resonators. This was realized by introducing vanadium dioxide (VO2) bridges interconnecting specific regions of each resonator. Raising the temperature across the phase transition shifted the resonance frequency by ~32% due to changes in the electrical conductivity of the VO2. Measured THz transmission response of the proposed dual-band filter was in good correspondence with simulations.

Si-doped AlxGa1−xN(0.56⩽×⩽1) layers grown by molecular beam epitaxy with ammonia

We describe experiments on Si doping in AlxGa1−xN grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2×1020cm−3 for the entire range of compositions investigated, from x=0.56 to 1.0. These conditions resulted in donor concentrations of ∼1×1019cm−3 up to x=0.85. Layers of AlxGa1−xN up to x=0.85 show good mobility and low resistivity. In these layers, the activation energy, Ea, of Si stays below ∼25meV and Si can be considered a shallow donor. For AlN content above x=0.85 the donor activation energy increases to Ea∼250meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.

Low-noise photodetectors based on heterojunctions of AlGaN–GaN

We describe detailed current–voltage and noise measurements carried out on AlGaN–GaN heterojunction photodetectors. Dark current densities below 1×10−10 A/cm2 are measured at a bias level of −5 V, at room temperature. In diodes with a diameter of 50 μm, low leakage currents result in the zero-bias noise spectral density as low as 3.6×10−32 A2/Hz. Based on the combined electrical and noise measurements, we calculate room-temperature thermally limited specific detectivity greater than 2.4×1014 cm Hz1/2 W−1. Background-limited specific detectivity exceeds 3.5×1013 cm Hz1/2 W−1.

Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl2/Ar plasma etching

We study the effects of plasma etching on the evolution of surface roughness of GaN and AlN. The etch-induced roughness is investigated using atomic force microscopy by systematically varying plasma power, chamber pressure, and Cl2/Ar mixture gas composition. GaN etches three to four times more rapidly than AlN for identical plasma conditions. For both GaN and AlN, we find that the surface roughness is correlated to etch rate. Induced roughness remains comparable to the as-grown value provided etching is carried out below rates 400 (GaN) and 90 nm/min (AlN). Above these cutoff etch rates, the roughness increases in proportion to etch rate. This result is independent of plasma parameters varied to produce the higher etching rates. By analyzing the surface properties through the power spectral density (PSD), we correlate roughness with the formation of fine-scale features present as a consequence of more aggressive etching. The cutoff etch rates and spatial-frequency dependence of the PSD are interpreted us...

Mg and O codoping in p-type GaN and AlxGa1−xN (0<x<0.08)

We describe Mg and O codoping experiments in gas-source molecular-beam epitaxy of GaN and AlGaN that produce high levels of Mg incorporation and activation. In order to obtain the highest level of Mg incorporation the surface stoichiometry was optimized by adjusting the NH3/Ga and NH3/(Ga+Al) flux ratios. The lowest acceptor activation energy and the highest hole concentration, p=2×1018 cm−3, were measured in samples of p-GaN and p-AlxGa1−xN with well-defined Mg/O ratios determined by secondary ion mass spectrometry. Measurements of the temperature dependence of diffusion current in p–n junctions formed in Al0.08Ga0.92N and GaN show acceptor activation energy of 195±10 and 145±15 meV, respectively. Low activation energies are attributed to successful codoping.

Plasma etching of AlN/AlGaInN superlattices for device fabrication

We report a study of plasma etching of GaN, AlN, and AlN/AlGaN superlattices for the processing of deep ultraviolet light emitting diodes. Etching was carried out using inductively coupled plasma of chlorine diluted with argon under reactive ion etching conditions. Using parameters selected for etch rate, anisotropy, and surface smoothness, we study etching of n- and p-type superlattices. The former etches at a rate of 250 nm/min, which is intermediate to that of AlN and GaN, while the latter exhibits a slower etch rate of 60 nm/min. Based on these studies, we prepare low-leakage p–n junctions and mesa light emitting diodes with peak emission at 280 nm.