I. Ahmad

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.

Self-heating in a GaN based heterostructure field effect transistor: Ultraviolet and visible Raman measurements and simulations

We report direct self-heating measurements for AlGaN∕GaN heterostructure field effect transistor grown on SiC. Measurements are carried out using micro-Raman scattering excited by above band gap ultraviolet and below band gap visible laser light. Ultraviolet excitation probes the GaN near the AlGaN∕GaN interface region of the device where the two-dimensional electron gas carries the source-drain current. The visible excitation probes the entire ∼1μm thick GaN layer and the SiC substrate near the interface with GaN. These results thus provide a measure of the average temperature throughout the GaN and of the substrate. Results are backed by combined electrical and thermal simulations. We find that the immediate hot spot region of the device, at the edge of the gate electrode, rises by up to ∼240°C over ambient under the most aggressive drive conditions examined.

Depth dependence of defect density and stress in GaN grown on SiC

We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29to30μm. High level of residual elastic strain was found in thin (0.29to0.73μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

Optical properties of a nanoporous array in silicon

We demonstrate an approach for producing an array of nanopores on a silicon surface. The methods used combine nonlithographic pattern transfer and chlorine plasma etching to produce ∼60nm diam holes up to 1μm in depth. The near-normal specular optical reflectance of these systematically modified surfaces is found to decrease dramatically with pore depth across the entire 2.0–6.0eV photon energy range studied. We adapt an effective medium approximation to model specular reflectance taking into account diffuse scattering by the nanopatterned surface. Micro-Raman measurements show a systematic intensity increase with pore depth. The observed dependence suggests that both insertion and extraction are enhanced by the nanopatterning.

Optical Properties of AlN/AlGa(In)N Short Period Superlattices – Deep UV Light Emitting Diodes

We report optical properties of deep UV light emitting diodes (LEDs). Devices are based on short period superlattices of AlN/Al x Ga 1-x (In)N (x ∼ 0.08) grown by gas source molecular beam epitaxy with ammonia. Structures consist of a 50-nm thick AlN nucleation/buffer layer deposited on sapphire. This is followed by a 1-micron thick Si-doped buffer layer of AlGaN or AlN/AlGa(In)N designed to be transparent for wavelengths longer than 240 nm. The design thickness of the superlattice well layers is systematically varied from 0.50 nm to 1.25 nm and the thickness of the barrier is varied from 0.75 nm to 2.00 nm. The n- and p-type SPSLs were doped with Si derived from silane and Mg evaporated from an effusion cell, respectively. We investigate device structures as well as superlattices which are nominally undoped, p-type, and n-type. Optical properties are investigated using reflectance, cathodoluminescence, and, in the case of LEDs, using electroluminescence. By controlling the properties of the superlattice, we obtain optical gaps ranging from 4.5 eV (276 nm) and 5.3 eV (234 nm). A systematic shift between the optical gap and the CL peak emission energy is discussed. Electrical properties are studied using I-V, C-V, and Hall effect. LEDs based on these superlattices and operating in the range of 260 to 280 nm exhibit turn-on voltages in the range of 4 to 6 V and support dc current densities in excess of 500 A/cm 2 at room temperature. We present results on the electrical and optical properties of our LEDs designed using these studies.