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The optical properties of GaN nanowhiskers with average diameters of 10 to 20 nm formed by photoelectrochemical etching were investigated using photoluminescence (PL) and cathodoluminescence (CL) spectroscopy and imaging. The main donor-bound exciton peak of the etched sample showed a red-shift of 9 meV compared to the unetched sample. CL mapping revealed that the luminescence originated from the surrounding matrix of GaN while the nanowhiskers themselves were non-radiative.

We measured the resonant nonlinear optical absorption and refraction due to intersubband transitions in GaN/AlGaN multiple quantum well structures at 1.5 mum. The measured values are much smaller than that predicted by theoretical studies. This is attributed to a larger than expected dephasing of excited electrons. At high excitation intensity, the absorption can be saturated. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We experimentally demonstrate the formation of GaN nanotip pyramids by selective and anisotropic etching of N-polar GaN in KOH solution. For samples grown with adjacent Ga- and N-polar regions on the same wafer, the KOH solution was found to selectively etch only the N-polar surface while leaving the Ga-polar surface intact. An aggregation of hexagonal pyramids with well defined {10 1 1} facets and very sharp tips with diameters less than ∼20 nm were formed. The density of the pyramids can be controlled by varying the KOH concentration, solution temperature or the etch duration. The GaN etching activation energy is estimated to be Ea≈0.587 eV. Dense GaN pyramids with sharp tips have applications in both electronic and photonic devices.

We use hydrostatic pressure as an instrument to reveal a strong nonlinearity of the electrical polarization in group-III nitride quantum well structures. From the photoluminescence peak energies of the quantum well emission at different applied pressures we obtain the values of the built-in electric field in the wells and the corresponding well-barrier polarization difference. We found that in both the InGaN/GaN and GaN/AlGaN systems the field and the polarization difference increase with pressure much faster than expected from the conventional (linear) model of polarization. This behavior is explained by the dramatic strain dependence of the piezoelectric coefficients of the group-III nitrides, which constitutes the nonlinear piezoelectric effect.

GaN quantum wells were studied, grown by plasma-assisted molecular beam epitaxy with a sub-band spacing of ∼740 meV (λ=1.67 µm). The GaN quantum wells are clad on both sides with short-period superlattice barriers. Using the time-resolved pump-and-probe technique, with 1.55 µm pump and 1.70 µm probe wavelength, an intersub-band electron scattering time of 370 fs was measured.

High-quality superlattice structures of GaN/AlGaN were grown on (0 0 0 1) sapphire substrates by molecular beam epitaxy. The threading dislocation density was reduced by growing low-temperature AlN layers in between the high-temperature GaN. In addition, in situ monitoring of the growth rate was achieved using pyrometric interferometry. Cross-sectional transmission electron microscopy of the superlattice structures revealed abrupt interfaces between GaN/AlGaN and excellent layer uniformity. We observed intersubband absorption at wavelengths as short as 1.52 μm in the GaN/AlGaN material system. A range of intersubband absorption peaks was observed between 1.52 and 4.2 μm by varying the well thickness and barrier Al content. In addition, the distribution of the built-in electric field between the well and barrier layers was also found to affect the intersubband transition wavelength.

We present the rationale, architecture, and implementation of an intelligent room, built upon a dense radio-based network of diverse electronic sensors, and interfaced with a cluster of dedicated computers. This room (an "iRoom") is capable of high-order tasks in event detection, analysis, processing, deduction, and inference. Initial focus has been to provide answers to the "Five W's" regarding a human visitor: Who, What, When, Where, and Why. The foundation technique that underpins this research is Sensor Fusion: the mathematical combination of disparate sensor reports and analyses into a rational result which is more accurate and more useful than its constituent parts.

We describe a method of using a network of thermopile sensors distributed along the walls of a room to locate a person within the room. At any given time, a person would be in the view of at least one of the sensor nodes. An algorithm is used to calculate the distance of a person from each sensor node based on its temperature reading and locate the person based on a combination of the sensor readings from the distributed nodes. This method is immune to lighting changes and works even in total darkness. Furthermore, this method of sensing can also detect events such as a person walking past the doorway to a room, lingering outside, and entering or leaving the room.

We have demonstrated GaN microstructures formed by polarity-selective chemical etching in KOH solution. It was found that KOH selectively etches N-polar GaN but not Ga-polar GaN. The polarity-patterned GaN was grown by plasma-assisted molecular beam epitaxy. For intermediate etching times, hexagonal GaN pyramids were formed in the N-polar regions. With prolonged etching, the N-polar GaN can be completely removed. A one-dimensional array of GaN stripes and a two-dimensional array of hexagonal holes formed in a GaN matrix have been fabricated. Extremely smooth vertical sidewalls have been achieved along with an etch depth of up to 4 µm.

In situ Ga-doped ZnO nanotips were grown on amorphous fused silica substrates using metalorganic chemical vapor deposition. Structural, optical, and electrical properties of as-grown ZnO nanotips are investigated. Despite the amorphous nature of fused silica substrates, Ga-doped ZnO nanotips are found to be single crystalline and oriented along the c-axis. Photoluminescence (PL) spectra of Ga-doped ZnO nanotips are dominated by near-band-edge emission with negligible deep-level emission. The increase in PL intensity from Ga doping has been attributed to the increase of Ga donor-related impurity emission. Current–voltage characteristics of the ZnO nanotips are measured by conductive-tip atomic force microscopy, which shows the conductivity enhancement due to Ga doping.

We report the experimental demonstration of second-harmonic generation in periodically poled GaN by first-order quasiphase matching. The periodically poled structure was grown by plasma-assisted molecular-beam epitaxy. We observed about 9 μW of second-harmonic power from a fundamental input laser wavelength of 1658.6 nm with a normalized conversion efficiency of 12.76% W−1 cm−2. The ability to perform nonlinear wavelength conversion by periodic poling and the fact that GaN has a very wide window of transparency, pave the way for GaN to be used for nonlinear optical devices in telecommunications as well as a nonlinear light source for biochemical detection in the far infrared and deep ultraviolet.

We have grown nonpolar GaN/AlN multiple quantum wells by plasma-assisted molecular-beam epitaxy on R-plane sapphire substrates. X-ray diffraction and selected-area diffraction data show that the III-nitride epilayers are oriented in the [11 20] direction with the [0001] axis lying in the plane of the substrate. The 18-A GaN quantum wells exhibit luminescence at 326 nm, which is in agreement with the transition energy calculated using a flat-band model; that is, without the presence of a built-in electric field.

We measured the resonant nonlinear optical response of the intersubband transitions in GaN/AlGaN multiple quantum well structures. The measured value for the nonlinear susceptibility is found to be much smaller than previous theoretical predictions. This is attributed to a large electron dephasing due to material imperfections. We show that at higher incident intensities, absorption saturation is possible, and measure the saturation intensity for various wavelengths around the transition resonance. We also discuss the prospects of using such structures as building blocks for all-optical nonlinear switches, in light of our experimental findings.

Scanning Kelvin force microscopy is applied to study the charge nature of threading dislocations on GaN surfaces. On the oxidized surface, the surface potential maps show little change near dislocations, indicating that if the dislocations are charged in the bulk, the charges are either screened or depleted due to band bending. After cleaning in hot H3PO4, the potential near dislocations located at domain boundaries and inside domains is found to be lower, consistent with excess local negative fixed charges. Curiously, no contrast was seen for the screw dislocations at the centers of growth spirals even after H3PO4 treatment. Thus, either these screw dislocations have no gap states, or if they do have gap states, the positions are higher in energy (closer to conduction band edge) than the gap states of other dislocations.

We report on a comparative study of room temperature intersubband electron scattering lifetimes in GaN/AlGaN single and coupled double multiple quantum well (QW) samples with peak absorption wavelengths ranging from 1.4 to 1.7 μm. Using time-resolved pump-probe spectroscopy electron scattering times as short as ∼160 fs have been measured for a coupled QW sample and ≲300 fs for single QW samples. While no significant dependence on the excitation power has been observed, a decrease of the scattering times with increasing probe wavelength has been measured and may be attributed to monolayer fluctuations in the samples.

Intersubband optical absorption around 1.55 μm has been measured in GaN/AlGaN multiple quantum wells (MQWs) grown by molecular-beam epitaxy. First, peak absorption wavelengths as short as 1.41 μm are reported for ultranarrow, ⩾11 A wide, well-doped MQWs with high, 85%, AlN mole-fraction barriers. Second, in order to enable modulation doping as well as the use of lower AlN mole-fraction barriers, we designed and fabricated QWs embedded in barriers consisting of a short period superlattice of narrow GaN QWs and only 65% AlN mole-fraction barriers. The resulting electron Bragg confinement allows peak absorption wavelengths as short as 1.52 μm. Furthermore, the structures can now be modulation doped through doping of the narrow superlattice wells and subsequent charge transfer into the active well. We observe a reduction of the absorption linewidth, from ∼200 to ∼130 meV, for these structures.

Intersubband optical absorption in narrow, 15–30 A wide, GaN/AlGaN quantum wells has been measured. The samples were grown by molecular-beam epitaxy on sapphire substrate and the barrier AlN mole fraction was varied from 0.45 to 0.8. Peak absorption wavelengths ranged from 4.2 μm for 30 A wide wells to 1.77 μm for a 15 A wide well. Modeling shows that a large contribution to the considerable spectral width of the absorption of ∼150 meV likely results from monolayer fluctuations. In addition to the composition dependent band offset, the intrinsic electric fields in the wells and barriers are the determining factors for the shortest possible wavelength.

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free.

GaN/SiC heterojunction diodes have been fabricated and characterized. Epitaxial n-type GaN films were grown using metalorganic chemical vapor deposition (MOCVD) and electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) on p-type Si-face 6H-SiC wafers. The I–V characteristics have diode ideality factors and saturation currents as low as 1.2 and 10−32 A/cm2, respectively. The built-in potential in the MOCVD- and ECR-MBE-grown n-p heterojunctions was determined from capacitance–voltage measurements at 2.90±0.08 eV and 2.82±0.08 eV, respectively. From the built-in potential the energy band offsets for GaN/SiC heterostructures are determined at ΔEC=0.11±0.10 eV and ΔEV=0.48±0.10 eV.

We present a study of the optical properties of different GaN/AlGaN multiple quantum well heterostructure samples. In particular, a comparative study of room temperature intersubband electron scattering lifetimes in a variety of single and coupled quantum well samples is presented for different excitation powers and wavelengths. All samples were grown by molecular beam epitaxy on sapphire substrate. Typical quantum well widths of ~10 to 15 a were used together with AlN bulk and superlattice barriers with an AlN-mole fraction of ~0.65 and ~0.9, respectively. In absorption measurements, the single quantum well samples show intersubband absorption peaks ranging from ~1.4 to ~1.7 μm wavelength. Two absorption peaks at ~1.75 and ~2.45 μm were measured for an asymmetric coupled double quantum well structure. The intersubband electron scattering lifetimes were determined with time resolved pump-probe measurements using pump and probe beams with a pulse width of ~130 fs and wavelengths of ~1.55 and ~1.7 μm, respectively. For the single quantum well samples, intersubband scattering lifetimes of ~260 to ~300 fs have been measured. For the upper excited state of the coupled double quantum well, a lifetime of ~230 fs has been found.

In this paper we have systematically studied the chemical etching of Ga and N-polar GaN with KOH, varying as a parameter either the concentration or the temperature of the solution for a fixed etch duration. The growth polarity can be controlled by the deposition of an appropriate buffer layer during molecular beam epitaxy (MBE). Using a patterned template, we grown samples with adjacent Ga-and N-polar regions. The N-polar GaN was found to be selectively etched while the Ga-polar GaN remained intact. We have made preliminary measurements of electron field emission from the GaN nanotip pyramids. Anode was suspended 50-500/spl mu/m above the film and I-V measurements were made at 50/spl mu/m increments.

In this paper, we show that different GaN heterostructures exhibit distinct nonlinear optical response. The response is studied using spectrally integrated pump-probe technique. The intersubband (ISBT) relaxation dynamics of excited carriers in GaN quantum well(QW) structures and their nonlinearities is essential for possible ultrafast optoelectronic applications based on these transitions.

Summary from only given. We report on the first comparative study of room temperature intersubband (IS) scattering lifetimes in a variety of single and coupled double GaN/AlGaN MQW samples at different excitation powers and wavelengths. Furthermore, samples with two types of QW barriers are compared: bulk-like barriers (85% AlN mole fraction) and superlattice (SL) barriers (65% AlN mole fraction). All samples were grown by molecular beam epitaxy on sapphire substrates.

We present measurements of intersubband transitions in the communications wavelength range using GaN/AlGaN heterostructures that have multiple single and coupled quantum wells.

In this work, we carry out TEM analysis on GaN layers grown on sapphire and doped in situ by MBE. In parallel, photoluminescence and electroluminescence experiments are used to determine the possible emission of the grown layers. It has been shown previously that the emission peaks at Er concentrations of about 1% and that their intensity dramatically decreases with increasing Er concentration probably due to compositional quenching. We report on the evolution of the microstructure versus composition and try to explain the quenching effects that can be related to the microstructure.

Electron field emission was measured from GaN nanotip pyramids formed by polarity-selective chemical etching in KOH solution. The GaN samples were grown by plasma-assisted molecular beam epitaxy and consisted of regions of Ga- and N-polar GaN grown at the same time. The pyramids were formed only in the N-polar regions and have extremely sharp tips with diameters estimated to be less than 20 nm. Field emission measurements showed a characteristic Fowler-Nordheim behavior. The average turn-on field was 1.6 V/μm with a corresponding normalized field enhancement factor of about 1500.

Many talks of molecular beam epitaxy as an important technology for the mass production of quantum devices will be presented in this conference. This talk will concentrate on the development and the unique features of quantum cascade lasers such as high-power output, tuning characteristics, and the extension to GaN material. (C) 2001 Elsevier Science B.V. All rights reserved.

Summary form only given. Fueled by the recent success of mid-infrared quantum cascade lasers, optical devices based on intersubband (IS) transitions face a rising interest also in other wavelength ranges. In particular, the intrinsically ultrafast electron dynamics associated with IS-transitions warrants research into the latter for the communications wavelength range (/spl lambda//spl sim/1.55 /spl mu/m). Few material systems, however, provide a sufficiently large band-discontinuity for the quantum wells (QWs) to accommodate the large subband spacing needed for such short wavelengths, Sb-based materials or group-III-nitrides presently being the best candidates. The rapid success of GaN-based interband light emitting diodes and lasers makes this material system particularly attractive. We present the first IS-transitions in the communications wavelength range, with peak absorption wavelengths as short as 1.4 /spl mu/m.

A review is given on insulators (oxides and nitrides) which have been deposited on GaN to form metal-insulator (oxides and nitrides)-semiconductor (MOS or MIS) diodes with a low interfacial density of states (Dit). These insulators include AlN, SiO2, Si3N4, SiO2/Ga2O3, and Ga2O3(Gd2O3). Techniques for depositing these insulators and methods for cleaning GaN surfaces prior to the insulator deposition are discussed. Recent progress on GaN MOSFET's (with SiO2/Ga2O3, and Ga2O3(Gd2O3) as gate dielectrics) and MISFET's (with AlN as a gate dielectric) is also reviewed. When exposed to room air, GaN surface is not as robust as previously thought. Therefore, preparation of a clean GaN surface for deposition of oxides and nitrides is necessary to achieve a low Dit. By heating GaN samples in UHV to clean the surfaces followed by deposition of Ga2O3(Gd2O3) and SiO2, we have achieved a low Ditwith negligible hysteretic loops in the capacitance-voltage curves

The effects of inductively-coupled plasma etching on the quality of ohmic contacts to etched n- GaN are reported. A high density chlorine plasma and a range of rf bias power levels (incident ion energies) are employed to etch n-GaN films with varying doping levels. Resulting plasma damage degrades contact ohmicity in all cases examined. At moderate levels of rf bias power the extent of this degradation for nitride layers with lower doping levels (mid-1017 cm−3) is similar to the degradation for nitride layers with higher doping levels. With increased rf bias, the degradation becomes more severe in films with higher carrier concentration. Annealing at 700 °C rapidly improves the contact quality. Studies of cumulative annealing time, up to 160 seconds, show that the improvement in contact quality takes place within the first twenty seconds of annealing.

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy doped n type with Si to the levels of 10¹⁵-10²⁰cm⁻³ was investigated. The room temperature electron mobility versus carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model of scattering by charged dislocations. The mechanism of this scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. It was found that in the low carrier concentration region (

We investigate the optical properties of GaN grown over SiO2 on SiC substrates by electron cyclotron resonance assisted molecular beam epitaxy. The photoluminescence spectra and refractive index of GaN were compared for GaN/SiO2/SiC and GaN/SiC. Strong band-edge luminescence was observed at 3.40 eV from the GaN on both SiO2/SiC and on SiC. No defect-related yellow luminescence was observed. The refractive index of GaN at 1.96 eV (632.8 nm) was measured at 2.22 and 2.24 for GaN/SiO2/SiC and GaN/SiC, respectively.

Intersubband optical absorption at /spl lambda/ /spl sim/ 2.1 /spl mu/m wavelength in doped 17.5 /spl Aring/ wide GaN quantum wells (QWs) with 51 /spl Aring/ wide intermediate AlN barriers is reported. A /spl sim/600 nm thick GaN template and 15 QWs have been grown by molecular beam epitaxy on R-plane sapphire substrate and display pure A-plane orientation. QWs with A-plane orientation are essentially free from spontaneous polarisation, which is expected to greatly simplify the design of GaN/Al(Ga)N devices employing intersubband transitions.

Heteroepitaxy of single-crystal Gd2O3 on GaN (with a wurtzite hexagonal close-packed (hcp) structure) and single-crystal GaN on Gd2O3 was achieved. In situ reflection high-energy electron diffraction reveals a sixfold symmetry in the in-plane epitaxy of the rare earth oxide on GaN and also in the overgrowth of GaN on the oxide. Single-crystal x-ray diffraction measurements find that these single-crystal oxide films are indeed the high temperature hexagonal phases of the sesquioxides, stabilized by the GaN substrate epitaxy. Despite a large mismatch in the lattice constant, the fully relaxed oxide films are of excellent structural quality. The x-ray diffraction results revealed that the GaN grown on the rare earth oxide is a single-crystal and has the same crystallographic hcp structure as the underlying GaN. The structures of both layers of GaN were also studied by cross section transmission electron microscopy.

GaN/AlGaN multiple quantum wells with a subband spacing of ~740 meV (λ = 1.67 µm) have been studied. Using the time-resolved pump-and-probe technique, with 1.55 µm pump and 1.70 µm probe wavelength, the measurement of an intersubband electron scattering time of 370 fs is reported.

Ga2O3(Gd2O3), electron beam evaporated from a single crystal Ga5Gd3O12 garnet, was ex situ deposited on molecular beam epitaxy grown GaN of Ga-polar surface. Using capacitance–voltage measurement, accumulation and depletion behavior was observed in the Ga2O3(Gd2O3)/GaN metal–oxide–semiconductor diodes, with an interfacial density of states less than 1011 cm−2 eV−1. The Ga2O3(Gd2O3)/GaN interface remains intact with the samples subject to rapid-thermal annealing up to 950 °C, as studied from x-ray reflectivity measurements.

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.