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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.

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO||(0002)GaN and (1010)ZnO||(1010)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni’s equation to study the variation of energy bandgap versus temperature.

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 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.

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.

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 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 present experimental evidence of the nonlinear behavior of the macroscopic polarization in GaN/AlxGa1−xN quantum wells. This behavior is revealed by determining the barrier-well polarization difference as a function of applied hydrostatic pressure. The polarization difference and corresponding built-in electric field in the wells increase with applied pressure at a much higher rate than expected from the linear model of polarization. This result, universally observed in the quantum well structures with different AlN mole fraction in the barriers, is explained by the nonlinear dependence of the piezoelectric polarization in GaN and AlN on the strain generated by pressure.

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.

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.

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.

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.