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The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

In this paper we report on the characterization of n - Al 0.51 Ga 0.49 N active regions and the fabrication of ultraviolet LEDs that contain self-assembled, nanometer-scale compositional inhomogeneities ( NCI - AlGaN ) with emission at ~290 nm. These active regions exhibit reduced integrated photoluminescence intensity and PL lifetime relative to 320 nm NCI - AlGaN active regions that have significantly lower AlN mole fraction, despite having more than an order of magnitude fewer threading dislocations, as measured by transmission electron microscopy. This behavior is attributed to nonradiative recombination associated with the presence of a larger density of point defects in the higher Al content samples. The point defects are ameliorated somewhat by the lower density of NCI AlGaN regions in the higher Al content samples, which leads to a larger concentration of carriers in the NCI and concomitant reduced radiative lifetime that may account for the high observed peak IQE (~ 25%). Prototype flip chip double heterostructure-NCI- ultraviolet light emitting diodes operating at 292 nm have been fabricated that employ a 50% NCI - AlGaN active region.

Indium nitride ( InN ) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In - and N -face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN . There is no discernable difference between the In - and N -face InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p -type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters.

Interest in developing ultraviolet emitters using the III-Nitride family of semiconductors has sparked considerable effort in fabricating AlGaN alloys that exhibit enhanced luminescence based on strong carrier localization, similar to their InGaN brethren. In this paper, we report on the growth of such alloys by plasma-assisted molecular beam epitaxy (PA-MBE) without the use of indium. This enhancement is attributed to the presence of nanoscale compositional inhomogeneities (NCIs) in these materials. The emission wavelength in these materials has been tuned between 275 nm and 340 nm by varying growth conditions. The effects of dislocations on double heterostructures (DHs) that employ an NCI AlGaN active region has been investigated, with an internal quantum efficiency as high as 32% obtained for the lowest dislocation density samples (3×1010 cm−2). Prototype DH-ultraviolet light emitting diodes (DH-UVLEDs) emitting at 324 nm were fabricated employing an NCI AlGaN alloy as the active region.

With growing interest in AlGaN for ultraviolet detectors and high-power/high-temperature electronic devices, the problem of forming high-quality Schottky contacts to this semiconductor has become increasingly important. It was shown that wet-chemical surface pretreatments affect the as-deposited diode characteristics for Au/n-AlGaN Schottky diodes. However, these diodes improve over the course of days when exposed to air at room temperature, exhibiting reduced leakage currents, enhanced barrier heights, and reduced ideality factors. Exposure to oxygen, with an enhanced effect in the presence of water vapor, is responsible for the environmental aging. The environmental aging was found to occur regardless of the source of AlGaN, surface preparation, and metal deposition technique. It was determined that high asdeposited reverse currents were due to current transport beneath the contact area, rather than across the semiconductor surface. Two findings further suggested that the change in electrical characteristics was due to a phenomenon occurring at the metal/semiconductor interface. First, metal thickness played a key role in the rate of change of the electrical characteristics, with thicker contacts being more impervious to surrounding gas species at room temperature. Second, a metal that readily forms an oxide, Ni, exhibited little environmental aging, while noble metals, such as Au and Pt, showed dramatic effects. Mild anneals revealed that the environmental change was partially reversible, which suggests the passivation of electrically active defects at the metal/semiconductor interface as the cause of the altered diode behavior. Taken together, the data indicate that oxidizing species diffuse through noble metal contacts to the metal/semiconductor interface and passivate electrically active defects, which may be reactivated upon mild anneals in N2.

Thermal oxidation of GaN was conducted at 700–900°C with O2, N2, and Ar as carrier gases for 525–630 Torr of H2O vapor. Upon oxidation of both GaN powders and n-GaN epilayers, the monoclinic β-Ga2O3 phase was identified using glancing angle x-ray diffraction. The chemical composition of the oxide was verified using x-ray photoelectron spectroscopy. In experiments conducted using GaN powder, the oxide grew most rapidly when O2 was the carrier gas for H2O. The same result was obtained on n-type GaN epilayers. Furthermore, the thickness of the oxide grown in H2O with O2 as the carrier gas was found to be proportional to the oxidation time at all temperatures studied, and an activation energy of 210±10 kJ/mol was obtained. Scanning electron microscopy revealed a smoother surface after wet oxidation than was reported previously for dry oxidation. However, cross-sectional transmission electron microscopy revealed that the wet oxide/GaN interface was irregular and non-ideal for devicefabrication, even more so than the dry oxide/GaN interface. This observation was consistent with poor electrical properties.

Despite the recent progress in the development of III-Nitride semiconductor based ultraviolet light emitting diodes (UVLEDs), commercially available devices emitting at wavelengths shorter than 370 nm still suffer from poor wall plug efficiencies on the order of ~ 1%. One factor limiting the performance of these devices is the presence of a high density of dislocations that arise from the typical heteroepitaxial growth on c-plane sapphire substrates that result in non-radiative recombination. Previously we have reported on the growth of AlGaN active regions containing self-assembled nanometer scale compositional inhomogeneities (NCI-AlGaN) that demonstrate enhanced luminescence efficiency despite the presence of a large dislocation density [1]. This phenomenon is attributed to (1) the high density of NCI regions that improves the probability of carriers recombining radiatively within them rather than non-radiatively at a structural defect and (2) the subsequent concentration of carriers within the narrower band gap NCI regions that suppresses non-radiative recombination and enhances radiative efficiency due to the reduced radiative lifetime at high carrier density [2,3]. Nevertheless, time-resolved photoluminescence (TRPL) studies show that the reduction of dislocation density in these materials further improves the active region performance [4], a phenomenon that may be due to improved transport from the wider band gap matrix to the NCI and concomitant higher carrier concentration therein. Bulk AlN has significant advantages over conventional c-plane sapphire as a substrate for III-Nitride based UVLEDs including a low dislocation density (etch pit density

We present luminescence spectroscopy measurements of in situ Nd doped AlN grown by plasma-assisted molecular beam epitaxy. A Nd concentration as high as 0.08 at. % is incorporated into the host material. The Nd incorporation efficiency within the AlN matrix is found to be highly sensitive to the Al flux but independent of the substrate temperature (between 800 °C to 950 °C). Photoluminescence, photoluminescence excitation, and combined excitation-emission spectroscopy (CEES) spectra are used to identify the Stark sublevels of the following manifolds: 4I9/2, 4I11/2, 4I13/2, 4F3/2, 4F5/2, 2H9/2, 4F7/2, 4S3/2, 4G5/2, and 4G7/2. A main Nd incorporation site and two minority sites are identified using CEES measurements.

The authors report on the excitation wavelength dependence of terahertz emission from n-InN and bulk p-InAs pumped with femtosecond pulses tunable from 800to1500nm. The terahertz amplitude, normalized to pump and probe power, from both narrow band gap semiconductors remains relatively constant over the excitation wavelength range. In addition, terahertz radiation from In- and N-face InN samples with bulk carrier concentrations ranging from 1017to1019cm−3 is also investigated, showing a strong dependence of terahertz emission on bulk carrier concentration. The experimental results agree well with calculations based on drift-diffusion equations incorporating momentum conservation and relaxation.

The authors demonstrate the impact of growth kinetics on the surface and structural properties of N-face InN grown by molecular beam epitaxy. Superior surface morphology with step-flow growth features is achieved consistently under In-rich conditions in a low-temperature region of 500-540 degrees C. Remarkably, off-axis x-ray rocking curve (omega scans) widths are found to be independent of the growth conditions. The band gap determined from optical absorption measurements of optimized InN is 0.651 eV, while photoluminescence peak emission occurs at even lower energies of similar to 0.626 eV. Hall measurements show room temperature peak electron mobilities as high as 2370 cm(2)/V s at a carrier concentration in the low 10(17) cm(-3) region. Analysis of the thickness dependence of the carrier concentration demonstrates a n-type surface accumulation layer with a sheet carrier concentration of similar to 3x10(13) cm(-2). (c) 2006 American Institute of Physics.

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250cm2∕Vs. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11×1013cm−2 is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of ∼0.65eV for the thickest InN films.

Ti/Al/Pt/Au and V/Al/Pt/Au ohmic contacts were studied on n-Al0.44Ga0.56N and n-Al0.6Ga0.4N. Optimised V/Al/Pt/Au contacts provided lower resistance than did Ti/Al/Pt/Au contacts, which are commonly used on less Al-rich n-AlGaN. Specific contact resistances of 1×10−5 Ω · cm2 and 4×10−5 were achieved on n-Al0.44Ga0.56N and n-Al0.6Ga0.4N, respectively.