Search

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 future technologies of telecommunication and laser techniques there is a high demand on microoptical components having aspherical or fresnel- and grid-shaped surfaces. For many of these components the strong requirements concerning optical quality and long term stability can only be reached if high quality inorganic glasses, instead of plastic materials, are applied. A key for enabling the fabrication of these components by low cost moulding and hot embossing processes are coating materials for the forming tools which prevent the sticking between tool and glass. These coatings, however, must not impede the reproducing of the submicron scale-structures prepared, prior to deposition, in the substrates. In the present investigation composite coatings from the systems of ceramic materials were deposited on different kinds of tool materials. Rf-magnetron sputter and ion beam deposition techniques were applied to prepare coatings in nanoscale composite or superlattice structures. The properties of the coatings and the behaviour of coated toolsmoulds were investigated and hot embossing experiments were performed at temperatures of 600°C and higher. Results obtained will be discussed and examples for the hot embossing of components like micro lens arrays and diffractive beam shapers will be presented.

The effect of polarity on the optical and microstructural properties of GaN is presented. A sample with adjacent domains of Ga- and N-polarity material was grown by varying the nucleation layer. This allows a unique opportunity to study the two different polarities under controlled conditions. We found that the N-polarity material has a much lower dislocation density than the Ga-polarity material. The N-polarity material contains voids that are not present in the Ga-polarity region. The surface roughness of the N-polarity material appears to be caused by Ga-polarity inversion domains which lead the growth. A cathodoluminescence study showed that the N-polarity material is much brighter than the Ga-polarity material, suggesting a higher donor concentration, probably due to increased impurity incorporation in the [0001] growth direction or possibly due to an increase in intrinsic point defects. There is also evidence that the N-polarity region contains two types of material, one is flat and can be etched, the other has an inclined facet and does not etch.

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.

In Zukunftstechnologien der Mikrooptik, Telekommunikation, Mikrotechnik und Lasertechnik besteht ein hoher Bedarf an speziellen optischen Komponenten, bei denen die Oberflächen zum Erzielen diffraktiver bzw. asphärisch refraktiver optischer Wirkungen fresnel- und gitterstrukturartige Konturen aufweisen müssen. Ziel ist es, solche Komponenten aus anorganischen Gläsern durch Heißpreß- und -prägeverfahren herzustellen. Hierzu müssen auf Preß- und Prägewerkzeuge Beschichtungen aufgebracht werden, die das Verkleben von Werkzeugoberflächen mit den geformten Gläsern wirksam unterbinden. Darüber hinaus sind an die Schichtmaterialien und die Abscheidetechnik weitere sehr hohe Anforderungen gestellt. So muß an den Werkzeugen eine hinreichend gute Schichthaftung und ausreichende Schichtstabilität bis in den Bereich der Formgebungstemperaturen, die über 600øC liegen, gewährleistet sein. Auch müssen die Beschichtungen an ihrer Oberfläche die gewünschte optische Glätte und eine konturgetreue Nachbildung von Gitterkanten und Vertiefungen aufweisen. Im Vortrag wurden Wege aufgezeigt, wie durch neue Schichtmaterialien und Abscheidetechniken hohe Belastbarkeiten der Formenoberflächen und optisch wirksame Strukturdimensionen im Submikrometerbereich mit Strukturbreiten und Strukturhöhen im Verhältnis 1:1 und höher erreicht werden.

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.

Recent interest in intersubband (IS) transitions in semiconductor heterostructures with large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around ~ 1.55 μm. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy. Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy on sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 μm and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anti-crossing. As expected, asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller – as a result of the comparatively large effective electron mass – than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.

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 (

A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.