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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.

The effects of negative polarization charge at the n -InGaN/p -GaN interface on the performance of hydride vapour phase epitaxy-deposited single heterostructure n -In0.17Ga0.83N/p -GaN light emitting diodes with p -side down are investigated. The negative polarization charge at the interface leads to the formation of a two-dimensional hole gas within the InGaN near the n -InGaN/p -GaN interface, as well as reducing the barrier for hole injection. In addition, electrons encounter experience a significant barrier for injection across the heterointerface. As a result, superlinear increase in light output as injection current increases below 20 A/cm2 and peak emission wavelength shift from 495 nm to 470 nm are observed. We show that the combination of two-dimensional hole-gas formation on the n -InGaN side of the hetero-interface and enhancement of the electron barrier to transport across this interface may reduce efficiency droop at high current density without the need for an AlGaN electron blocking layer. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we demonstrate new results on controllable HVPE growth of nitride materials with a deposition rate below 0.02 microns per minute and the first quantum size structures fabricated by HVPE. The nm-scale layer thicknesses were verified by transmision electron microscopy (TEM) and photoluminescence (PL). Quantum well (QW) GaN/AlGaN and AlN/AlGaN structures have been grown by HVPE for the first time. Properties of the HVPE grown QWs are reported. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper contains results on InN and InGaN growth by Hydride Vapor Phase Epitaxy (HVPE) on various substrates including sapphire and GaN/sapphire, AlGaN/sapphire, and AlN/sapphire templates. The growth processes are carried out at atmospheric pressure in a hot wall reactor in the temperature range from 500 to 750 oC. Continuous InN layers are grown on GaN/sapphire template substrates. Textured InN layers are deposited on AlN/sapphire and AlGaN/sapphire templates. Arrays of nano-crystalline InN rods with various shapes are grown directly on sapphire substrates. X-ray diffraction rocking curves for the (002) InN reflection have the full width at half maximum (FWHM) as narrow as 270 arcsec for the nano-rods and 460 arcsec for the continuous layers. InxGa1–xN layers with InN content up to 10 mol.% are grown on GaN/sapphire templates. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN epilayers grown by plasma-assisted molecular beam epitaxy and exhibiting high internal quantum efficiency (up to 30%) are incorporated into double-heterostructure devices grown on base layers of varying defect density. Growth of these AlGaN active layers, having increased emission from localization of carriers in regions of nanoscale compositional inhomogeneities, is found to benefit from base layers of reduced defect density, including thick AlGaN templates grown by hydride vapor phase epitaxy. Nonlinear radiative processes are observed at high optical excitation for layers grown on lower defect base layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

This study examines two different forms of thermal stability of gate metallizations on GaN/AlxGa1−xN/GaN heterostructures: long-term anneals at moderate temperatures (accelerated ageing) and high-temperature rapid thermal anneals (commonly used for ohmic contact formation). The metallizations Re, Pt, Pd, Au, Ni, Ni/Au, Ni/Ga/Ni, Co and Co/Au were selected based on criteria associated with thermodynamic predictions and the metal work function. Long-term (200 h) thermal stability experiments for unpassivated samples annealed at 500 °C within a tube furnace revealed very good thermal stability of Pt/Au diodes. In addition, Re and Ni/Ga/Ni Schottky diodes may also provide thermally stable gate metallizations for high-temperature devices. As a result of short-term annealing, all metals tested apart from Au and Re yielded stable diodes when annealed for 60 s up to 400 °C. Re contacts showed increases in reverse current densities after annealing at temperatures between 400 and 600 °C, with a dramatic return to as-deposited values following an anneal at 800 °C for 60 s. Upon longer-term annealing at 800 °C for 10 min, dissolution of Ga into the Re film and complete loss of the thin GaN cap on the heterostructure were observed, resulting in the Re metal and underlying AlxGa1−xN layer making direct contact.

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.

In this paper we report on first InGaN-based light emitting structures grown by hydride vapour phase epitaxy (HVPE). InGaN layers and multi layer InGaN/InGaN epitaxial structures were grown on GaN/sapphire template substrates and characterized. InN content in the InGaN layers was varied from 5 to 35 mol.%. Thickness of InGaN layers was controlled from 10 nm to 2 microns. Density of treading dislocations in the InGaN layers was estimated to be in the 109 cm–2 range. X-ray diffraction measurements and transmission electron microscopy data confirmed a formation of InGaN/InGaN superlattice structures. Light emitting diode epitaxial wafers were fabricated by HVPE deposition of n-type InGaN layers and multi layer structures on p-type GaN template substrates. Depending on InN content in the InGaN light emitting regions, peak electroluminescence wavelength varied from 450 to 510 nm. Results of material characterization are reported. Advantages of the proposed upside down LED configuration and future applications of HVPE to grow InGaN layers and structures are discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We probe the electric field in In-face and N-face InN/InGaN multiple quantum wells (MQWs) using low temperature intensity-dependent photoluminescence (PL) under continuous-wave laser excitation at 900 nm. The In-face structure consists of 30 periods of 2.5 nm thick InN wells and 24 nm thick In0.92Ga0.08N barriers. The N-face sample consists of 25 periods of 1.5 nm InN wells and 15 nm thick In0.88Ga0.12N barriers. At low excitation power, the PL peak energy from both In-face and N-face MQWs is redshifted relative to that from bulk InN. As excitation power increases, we observe a 50 and 44 meV blueshift of the PL peak energy from the In-face and N-face MQWs, respectively. The blueshift is a result of the optically induced screening of the built-in electric field. From the measured blueshift, we calculate a minimum change of the electric field to be ∼0.4 MV/cm and ∼0.6 MV/cm in the In-face and N-face well regions, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the fabrication and evaluation of flip-chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nanoscale compositional inhomogeneities (NCI), deposited by plasma-assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 μm x 300 μm device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm2, suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the Stark energy sublevels of Nd3+ ions in GaN grown by plasma-assisted molecular beam epitaxy as determined by luminescence spectra. The photoluminescence spectra is correlated with transitions from the 4F3/2 doublet state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above and below bandgap excitation, with the strongest emission occurring at 1.12 eV (1106 nm). A splitting of the 4F3/2 excited state is determined to be 4.1 meV. From photoluminescence excitation spectra, we also identify the Stark sub-levels of the excited states 4F5/2, 2H9/2, 4F7/2, 4S3/2, 2G7/2, and 4G5/2. Photoluminescence excitation spectra exhibit an optimal excitation energy of 1.48 eV (836 nm). Site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy imply enhanced substantial doping at the Ga site. In addition, optical loss and gain measurements of a GaN:Nd waveguide excited above bandgap indicate an internal loss coefficient of ∼6.5 +/– 2 cm–1 and a net gain of ∼ –2 +/– 2 cm–1. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the properties of high quality HVPE InN and on successful subsequent MBE growth of InN layers with improved characteristics on HVPE InN template substrates. InN layers were grown by HVPE on GaN/sapphire HVPE templates. The (00.2) XRD rocking curve of the best InN layer (RC) had the FWHM of about 375 arc sec, being the narrowest XRD RCs ever reported for HVPE InN. Transmission Electron Microscopy (TEM) revealed that at the GaN/InN interface, the threading dislocations that come from GaN were transmitted into the InN layer. We estimated the dislocation density in HVPE grown InN to be in the low 109 cm–2 range. Reflection high energy electron diffraction (RHEED) confirmed monocrystalline structure of the InN layers surface. Layers photoluminescence (PL) showed edge emission around 0.8 eV. Hall measured free electron concentration was in the range of 1019 -1020 cm–3 and electron mobility was ∼200 cm2/V s. MBE growth of InN was performed on the HVPE grown InN template substrate demonstrating the improvement of material quality in the case of homo-epitaxial growth of InN. Demonstration of the high quality HVPE InN materials opens a new way for InN substrate development. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.