Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Substrates for gallium nitride epitaxy

The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.

Wet etching of GaN, AlN, and SiC : a review

Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed. The symmetry of such crystals is determined by the arrangement of both the charges and the spins, so their contributions to the crystallographic and the magnetic structures, respectively, must be distinguished. Magnetic SHG is introduced as a probe for magnetic structures and sublattice interactions. The specific degrees of optical experiments - including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism. These include local or hidden phase transitions, interacting magnetized and polarized sublattices and domain walls, and magnetic interfaces. SHG in various centrosymmetric and noncentrosymmetric crystal classes of antiferromagnetic oxides such as Cr2O3, hexagonal RMnO3(R=Sc,Y,In,Ho-Lu), magnetic garnet films, CuB2O4, CoO, and NiO, is discussed.

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Second-harmonic generation as a tool for studying electronic and magnetic structures of crystals: review

In this paper we outline a new direction in the area of photonic crystals (PCs), or photonic band gap materials, i.e. one-, two-, or three-dimensional superstructures with periods that are comparable with the wavelengths of electromagnetic radiation. The main (and principal) characteristic of this new class of PCs is the presence of magnetically ordered components (or external magnetic field). The linear and nonlinear optical properties of such magnetic PCs are discussed.

Magnetic photonic crystals

Homo-epitaxial GaN growth on exact and misoriented single crystals: suppression of hillock formation

We present a study on the material properties of GaN films grown on (111) silicon substrates by low-pressure metalorganic chemical vapour deposition using AlN buffer layers. This buffer layer is optimised with respect to growth temperature and time for the optical and structural properties of the GaN epilayers. The insertion of a Si x N y intermediate layer significantly increases the optical and structural properties. It results in a reduction of the D 0 X FWHM to 10 meV and in a 2.5-fold increase of its luminescence intensity. The FWHM of symmetric and asymmetric ω-scans are reduced from 832 to 669 arcsec and from 702 to 547 arcsec, respectively.

High quality GaN layers on Si(111) substrates: AlN buffer layer optimisation and insertion of a SiN intermediate layer

New possibilities for magnetic domain studies are demonstrated using a combination of nonlinear magneto-optical microscopy and a conventional linear polarizing microscope. The use of an optical response that is governed by a higher rank tensor offers sensitivity to additional combinations of magnetization directions and optical wave vector and polarization, which is demonstrated in magnetic garnet films of different crystallographic orientations. We observed a nontrivial modulated domain structure in a (210) film and a clear domain contrast for a (111) film, where the linear image only indicated simple up–down domains and no domain contrast for these two situations, respectively.

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A combined nonlinear and linear magneto-optical microscopy

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001) GaN substrates. In contrast to broad PL emission in exact (0001) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.

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Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

We have studied the effect of an in situ SiN treatment of sapphire substrates on the optical properties of GaN films grown by metalorganic chemical vapor deposition The SiN deposition, partially covering the substrate, forms a mask for the formation of nanoscale auto-organized GaN islands These islands formed upon an increase of the temperature after deposition of a GaN buffer layer on this mask A photoluminescence study of the GaN epilayers obtained by lateral overgrowth of these islands shows significant enhancement of the luminescence emission intensity of the near band edge peaks and a reduction of the full width at half maximum of the donor bound exciton (D0X) peak by 32% down to 4 meV compared to in our standard process The GaN films grown using SiN treatment are highly stressed as evidenced by a blueshift of 10 meV in the D0X peak energies Photoelectrochemical etching in aqueous solution of KOH was applied to reveal the dislocation density The density of “whisker-like” etch features, which fo

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V. Kirilyuk

Papers

Homo-epitaxial GaN growth on exact and misoriented single crystals: suppression of hillock formation

High quality GaN layers on Si(111) substrates: AlN buffer layer optimisation and insertion of a SiN intermediate layer

A combined nonlinear and linear magneto-optical microscopy

Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

Improvement of the optical properties of metalorganic chemical vapor deposition grown GaN on sapphire by an in situ SiN treatment