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The epitaxial growth of GaN on Si (111) substrates has been performed using plasma-assisted molecular beam epitaxy with a thin (∼19.3 nm) single-crystal layer of Sc 2 O 3 as a template/buffer layer. The structural properties and in situ epitaxial growth were studied using reflection high energy electron diffraction (RHEED), high-resolution transmission electron microscopy, and high-resolution X-ray diffraction. An orientation relationship of GaN(0 0 0 2)IISc 2 O 3 (111)∥Si(111) and GaN[1010]∥Sc 2 O 3 [422] off 60° with Si[422] was determined. The excellent growth of Sc 2 O 3 on Si and GaN on Sc 2 O 3 was achieved, as observed from streaky and bright RHEED patterns. The Sc 2 O 3 template serves as an effective barrier layer preventing diffusion of Si and Ga during the GaN growth at high temperatures under nitrogen plasma. No cracking was observed in the GaN layer with thickness around 0.2 μm when inspected with an optical microscope.

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.

A review of the recent progress in intersubband transitions in GaN-based superlattice structures grown by plasma-assisted molecular beam epitaxy (MBE) is presented. Careful control of the growth parameters resulted in reduced threading dislocation density as well as optimized interfaces for the superlattices. Intersubband absorption has been observed from a variety of configurations involving GaN single or asymmetric double quantum wells with either thick Al x Ga 1-x N barriers or short-period GaN/Al x Ga 1-x N superlattice barriers. The peak wavelength of absorption can be varied between 1.4-4.2 μm by changing the quantum well thickness. Electron scattering times were measured by the pump-probe technique and were found to be 240-330 fs at 1.55 μm. In addition, intersubband transitions have also been observed for GaN/AlN superlattices grown with a non-polar (1120) orientation.

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.

We report the results of the growth by molecular beam epitaxy of GaN/Al x Ga 1-x N superlattice (SL) structures with single or double quantum wells and thick Al x Ga 1-x N or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/AlxGa1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al0.8Ga0.2N quantum wells.

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 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.

Using two-photon excitation, we study the excitation power density dependence and spatial variation of photoluminescence (PL) in GaN films grown by molecular beam epitaxy. Under our experimental conditions, the excitation power density dependence is quadratic for near band-gap emission (NBE) and linear for yellow luminescence (YL), consistent with the YL process being saturated. The PL mapping reveals NBE fluctuations at the domain-size scale while YL is uniform. These results provide strong evidence that the spatial distribution of deep levels associated with YL is uniform; hence, YL is unrelated to dislocations.

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.

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.

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.

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.

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.

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.

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.

This letter addresses the broadening mechanism of the near-band-gap photoluminescence in GaN films doped n type with silicon. The films were produced by plasma assisted molecular beam epitaxy and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He–Cd laser at 77 K. At low carrier concentration ( 1018 cm−3) the full width at half maximum increases monotonically with carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to tailing of the density of states caused by potential fluctuations due to randomly distributed impurities. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [Phys. Rev. 139, A343 (1965)], and the agreement between model and experim...