G. Kipshidze

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.

HfO2 gate dielectric with 0.5 nm equivalent oxide thickness

Hafnium dioxide films have been deposited using reactive electron beam evaporation in oxygen on hydrogenated Si(100) surfaces. The capacitance–voltage curves of as-deposited metal(Ti)–insulator–semiconductor structures exhibited large hysteresis and frequency dispersion. With post-deposition annealing in hydrogen at 300 °C, the frequency dispersion decreased to less than 1%/decade, while the hysteresis was reduced to 20 mV at flatband. An equivalent oxide thickness of 0.5 nm was achieved for HfO2 thickness of 3.0 nm. We attribute this result to a combination of pristine hydrogen saturated silicon surfaces, room temperature dielectric deposition, and low temperature hydrogen annealing.

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.

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.

Solar-blind ultraviolet photodetectors based on superlattices of AlN/AlGa(In)N

We describe solar-blind photodetectors based on superlattices of AlN/AlGa(In)N. The superlattices have a period of 1.4 nm, determined by x-ray diffraction, and an effective band gap of 260 nm measured by optical reflectivity. Using simple mesa diodes, without surface passivation, we obtain low dark leakage currents of 0.2–0.3 pA, corresponding to the leakage current density of ∼0.3 nA/cm2, and high zero-bias resistance of ∼1×1011 Ω. Excellent visible cutoff is obtained for these devices, with six orders of magnitude decrease in responsivity from 260 to 380 nm. These results demonstrate the potential of junctions formed by short-period superlattices in large-band-gap devices.

Deep Ultraviolet AlGaInN-Based Light-Emitting Diodes on Si(111) and Sapphire

Ultraviolet light-emitting diodes (LEDs) with emission wavelength as short as 280 nm, grown by gas source molecular beam epitaxy with ammonia, are described. The typical multi-quantum well (MQW) structure LED consists of an AlN buffer layer deposited on Si(111) or sapphire, followed by a (Al)GaN buffer layer and two superlattice structures, n- and p-type, with the MQW active region placed between them. Room temperature Hall measurements of n- and p-type AlN/AlGaInN superlattice structures show average hole concentrations of 1 x 10 18 cm -3 , with mobility of 3-4 cm 2 /Vs, and electron concentrations of 3 × 10 19 cm -3 , with mobility of 10-20 cm 2 /Vs. Room temperature electroluminescence spectra of mesa-etched devices show predominant emission at 280 nm.

Deep Ultraviolet Light Emitting Diodes Based on Short Period Superlattices of AlN/AlGa(In)N

We report a systematic study of the optical properties of superlattices of AlN/Al0.08Ga0.92(In)N with periods in the range of 1.25–2.25 nm. The superlattices were grown on sapphire substrates using gas source molecular beam epitaxy with ammonia. Effective bandgaps between 4.5 eV (276 nm) and 5.3 eV (234 nm), as determined by optical reflectivity measurements, were obtained by adjusting the barrier and well thickness. These superlattices can be doped n- and p-type. We demonstrate double heterostructure light emitting diodes operating at wavelengths as short as 262±2 nm.