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We report on photoluminescence (PL) spectroscopy and electron microscopy studies of an AlGaN quantum well (QW) structure grown by molecular beam epitaxy under metal-rich conditions with substrate rotation. Both techniques reveal unintentional formation within the AlGaN barriers of a quasiperiodic structure of thin Ga-rich layers, whose period is controlled by both the substrate rotation rate and the AlGaN growth rate. These compositional modulations act as 1-3 monolayer thick QWs emitting below 250 nm with an internal quantum efficiency (IQE) as high as ∼30% at room temperature under weak excitation. Variational calculations of the QW exciton properties indicate that the observed high IQE can result from strong three-dimensional localization of the excitons confined in the narrow QWs. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, growth of InN quantum dots (QDs) on GaN nanowires (NWs) by metal-organic vapour phase epitaxy is demonstrated, illustrating the feasibility to combine 0D and 1D structures for nitride semiconductors. Selective area growth was used to generate arrays of c-oriented GaN NWs using Si3N4 as the mask material. In general, InN QDs tend to form at the NW edges between the m-plane side facets, but the QD growth can also be tuned to the side facets by controlling the growth temperature and the growth rate. TEM characterization reveals that I1-type stacking faults are formed in the QDs and originate from the misfit dislocations at the InN/GaN interface. Photoluminescence measurement at 4 K shows that the peak shifts to high energy with reduced dot size. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we study the optical properties of two high quality fifty-periods of In-polarity InN/In0.73Ga0.27N MQWs samples, grown by radio-frequency plasma-assisted molecular beam epitaxy, with different well (0.5-1 nm) and barrier thicknesses (3-4 nm). We employ spectroscopic ellipsometry at room temperature in the energy range from 0.6 to 6 eV, and incidence angles of 60 and 70°. Ellipsometric data were successfully modelled using the model dielectric function approach and a multilayer model assuming the MQWs as a homogeneous layer. The E0, A and E1 MQWs transition energies were determined and found to exhibit a blueshift with decreasing the well thickness.

GaN substrates with low dislocation densities were prepared by halide vapor-phase epitaxy (HVPE) on c-plane sapphire and by means of a post-growth laser-induced lift-off or natural stress-induced (self-) separation process. The HVPE growth on InGaN/GaN buffer layers and subsequent self-separation method was seen as advantageous, in comparison with the laser-induced lift-off one, in terms of lower cost and better crystalline quality of the GaN material obtained. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

New electron paramagnetic resonance (EPR) measurements in hexagonal and cubic GaN intentionally doped with silicon are presented. In both type of samples the well-known EPR resonance of the dominant shallow donor is observed, whereby the g-tensors are determined to g(parallel to) = 1.9512, g(perpendicular to) = 1.9485 (free-standing hexagonal GaN) and g = 1.9533 (cubic GaN layer grown on 3C-SiC substrate). The spectra show an exceptionally small line width below 0.4 mT and contain no further signature. As a result, beside the line width itself, the EPR line is characterized by its g-tensor exclusively. With the help of a qualitative analysis of the Si donor wave function within effective mass theory (EMT) and a followed up calculation of the hyperfine (HF) splittings in the framework of density functional theory (DFT) the characteristic shape of the EPR lines can be explained by an enhanced delocalization of the unpaired electrons of shallow Si donors at the gallium sublattice due to overlapping impurity and conduction bands.

We have demonstrated optically pumped room-temperature pulse lasing at 300.4 nm from an AlGaN-based multiple-quantum-well (MQW) structure grown by plasma-assisted molecular beam epitaxy on a c-sapphire substrate. The lasing was achieved at the threshold peak power of similar to 12 MW/cm(2). The MQW structure involved AlGaN/AlN short-period super-lattices to decrease the threading dislocation densities from 10(11) down to 10(9)-10(10) cm(-2). Studies of time-resolved photoluminescence (TRPL) spectra and cw PL temperature dependences (10-300K) of different MQW structures, as well as numerical calculations of the optical gain and confinement in the laser structure allowed us to conclude about the optimum design of AlGaN-based MQW structures for the lower threshold UV lasing.

We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to

We have studied composition and strain in Al1–xInxN films with 0.128≤ x≤ 0.22 grown on GaN-buffered sapphire substrates by metalorganic vapor phase epitaxy. A good agreement between the In contents determined by Rutherford backscattering spectrometry (RBS) and Xray diffraction (XRD) is found for x≤ 18, suggesting applicability of Vegard's rule in the narrow compositional range around the lattice matching to GaN. The increase of the In content up to x = 0.22 leads to a formation of sub-layers with a higher composition, accompanied by deviations from Vegard's rule. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonpolar a- and m-plane bulk GaN sliced from boules : structural and optical characteristics

We review a selection of recent research work on III-nitride materials, limiting the scope to bulk properties and quantum well structures. The different stages of development of the compounds AlN, ...

We report a detailed study of the recombination processes of the excitons bound to O and Si shallow donors in bulk GaN. Photoluminescence (PL) and time-resolved PL measurement at different polarization and temperatures were carried out on a high quality 1-mm thick sample grown by Hydride Vapor Phase Epitaxy. Together with the principal donor-bound exciton (DBE) emission lines a large number of two-electron transitions (TETs) related to the ground and excited states of both DBE were observed allowing an accurate determination of the donor binding energies. Polarization properties and optical selection rules as well as the recombination dynamics of different PL lines were also studied and discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To obtain a deeper insight into the mechanism of defect formation in a-plane GaN the defect nature in such films grown by metal-organic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) was investigated by means of transmission electron microscopy. The films showed, beside the frequently found basal plane stacking faults and threading dislocations, prismatic stacking faults originating at the film/substrate interface and facetted voids. The density of basal plane stacking fault was about 105 cm–1 and the density of partial dislocations was in the range of 1010 cm–2, accordingly. Using selected area diffraction the epitaxial relationship between film and layer was determined. Based on these findings the impact of lattice mismatch on the observed defect characteristic is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the effects of film and substrate thicknesses on the bending and strain of thick GaN films grown by HVPE. Both experimental and simulation data, being in a very good agreement, show the highest bending at a critical film thickness slightly smaller than the substrate thickness, while the strain remains decreasing with only a small increase at much higher film thicknesses. Temperature dependent measurements allows a separation of the thermally induced component and an extrapolation of the bending to the growth temperature. The latter was found to be dependent on both the film and substrate thickness and to have a strong effect on the remaining bending of freestanding films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of non-stoichiometry and compensation on fundamental parameters of heavily-doped InN

A detailed observation of discrete well width fluctuations via localized excitons in the photoluminescence (PL) spectra of MOCVD-grown undoped GaN/Al 0.07 Ga 0.93 N multiple quantum wells (MQWs) has been reported. Doublet excitonic features with a distance varying between 10 and 25 meV for different well widths (1.5 to 4.5 nm) are observed in the PL spectra. They are explained in terms of discrete well width variations by one c-lattice parameter, i.e. two GaN monolayers. By mapping the PL measurements across the samples with different excitation spot size, it is shown that the extension of areas with a constant well width is less than 1 μm 2 . TEM pictures give evidence of interface roughness, although the contrast is weak at this low Al composition. In addition we observe a long-range variation of the PL peak position across the sample, interpreted as a fluctuation in Al composition in the barriers. The residual broadening of an excitonic peak (apart from the splitting related to well width fluctuations) is about 10 meV, somewhat larger for larger well widths, and is mainly ascribed to hole localisation potentials in the QWs. Additional broadening occurs in the MQWs due to inequivalent properties of each QW within the excitation spot.

Phonons in strained AlGaN/GaN superlattices (SLs) with constant periods but different Al composition have been studied using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The following SL modes were identified: i) AlGaN localized E1(TO) modes identified for the first time and AlGaN localized A1 (LO) phonons; ii) GaN localized E2, E1(TO) and A1(LO) phonons; iii) delocalized E1(LO) phonons; iv) A1(TO) phonon; v) two modes around 660 cm-1 and 594-625 cm-1, respectively, not predicted by theory. The effects of strain and composition on the phonon frequencies were established and discussed.

As an effort to investigate new techniques to reduce the effect of the strong internal polarization fields in (In,Ga)N/GaN quantum well (QW) structures we have studied the influence of inserting a ...

Radiative recombination mechanism in highly modulation doped GaN/AlGaN multiple quantum wells

We have studied the optical signatures of the Mg acceptor in GaN, using samples that are doped with Mg during MOCVD growth. In order to reduce the defect density in the material and thus achieve narrow linewidths in optical spectra we have used thick HVPE grown GaN layers as templates in the MOCVD growth. The photoluminescence (PL) spectra show two acceptor-related bound exciton peaks at 3.466 eV and 3.455 eV respectively. In the lower photon energy range the 3.27 eV emission with its LO-phonon replicas is dominant, riding on a broad background emission peaking at about 3.1 eV. These results, together with previous data in the literature, indicate that there are two acceptors in Mg-doped GaN, one dominating the optical spectra (the 3.466 eV and the 3.27 eV emissions) and another related to the 3.455 eV and the 3.1 eV emissions. We suggest that the latter is related to the Mg acceptor, while the former is a H-related complex, not necessarily involving Mg.

We report on the emission properties of nonpolar a -plane GaN layers grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). Together with the typical band edge exciton emission, the low-temperature photoluminescence spectra reveal several emission bands in the spectral range 3.29-3.42 eV. These emissions appear with different intensities in layers grown by different techniques and show different thermal quenching and recombination dynamics. The 3.42 eV and 3.35 eV emissions are found to be of an intrinsic origin and are associated with carriers localized at stacking faults. The emission at 3.29 eV, which is present only in MOCVD layers, shows a donor-acceptor pair behavior suggesting that impurities attached to structural defects are involved. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonequilibrium carrier dynamics has been investigated in ELO and HYPE grown GaN layers in a wide temperature and excitation range by using the time-resolved picosecond FWM technique. Carrier lifeti ...

Bending in HVPE GaN free-standing films : effects of laser lift-off, polishing and high temperature annealing

Assessment of phonon mode characteristics via infrared spectroscopic ellipsometry on a-plane GaN

We have used positron annihilation spectroscopy to study the high-pressure annealing induced thermal recovery of vacancy defects in free-standing GaN grown by hydride vapor phase epitaxy (HVPE). The results show that the in-grown Ga vacancy complexes recover after annealing at 1500-1700 K. Comparison of the experimental positron data with ab-initio calculations indicates that the Doppler broadening measurement of the electron momentum distribution is sensitive enough to distinguish between the N and O atoms surrounding the Ga vacancy. We show that the difference between the isolated V Ga in electron irradiated GaN and the V Ga -O N complexes in highly O-doped GaN is clear, and the Ga vacancy related defect complexes that start dissociating at 1500 K can be identified as V Ga -O N pairs.

The strain in a-plane GaN layers of different thickness grown on r-plane sapphire substrates by hydride vapor phase epitaxy was studied by X-ray diffraction. The layers are found to be under compression in the growth plane and under tension in the growth direction. Therefore, the symmetry of the GaN unit cell is no longer hexagonal but orthorhombic. With increasing layer thickness the strain relaxes and the curvature of the wafer increases. Wafer bending is proposed to be the major strain relaxation mechanism. The anisotropic in-plane strain relaxation is attributed to the elastic and thermal anisotropy of GaN and sapphire. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

We have studied depth distributions of the electrical parameters in MBE grown InN films with two types of AlN and GaN buffers. Using independently determined Hall effect electron concentration and mobility profiles, as well as electron concentration profile by photoluminescence measurements, we model the real depth profile of carrier mobility, assuming graded inhomogeneity of the sample. The obtained profiles follow power dependences of the same order for layers grown on the two buffers with a small difference in the function coefficients attributed to a contribution of the interface charge in layers grown on AlN buffers. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

We report on design, fabrication by molecular beam epitaxy, and photoluminescence (PL) studies of III-V/II-VI heterovalent structures containing a GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum well (QW) ...

We report results from detailed optical spectroscopy from MOCVD grown GaN/Al 0.07 Ga 0.93 N multiple quantum wells (MQWs). Effects of Si doping position on the emission energy and recombination dynamics were studied by means of photoluminescence (PL) and time-resolved PL measurements. The samples were Si doped with the same level but different position of the dopant layer. Only the sample doped in the well shows the MQW emission redshifted compare to the GaN bandgap. The redshift is attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. At low temperature the PL decay time of the sample doped in the well by a factor of two is longer than for the barrier doped case. The difference is explained by the effect of interplay of free carriers and ions on the screening of the polarization field in these doped structures.

We report on the complex nature of infrared luminescence and absorption in InN films, which cannot be entirely explained by the concept of a conventional narrow-gap semiconductor. In particular, it concerns the detection of peaks near absorption edges by both thermally detected optical absorption and photoluminescence excitation spectroscopy and the observation of extraordinarily strong resonant enhancement of emission. To describe the experimental data a model is proposed, which takes into account surface plasmons in metal-like inclusions, modifying the optical properties of InN.

We report on the emission properties of GaN/AlN superlattice (SL) structures grown by MOCVD. A set of samples with AlN thickness below 3 nm and different well/barrier thickness ratios has been studied. The SLs with a period between 3 and 8 nm show a photoluminescence (PL) peak in the range 3.3–3.9 eV with a full width at half maximum of about 100 meV. The difference in the transition energies, linewidths and recombination dynamics is described in terms of varying polarization field and electron filling in the wells. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is shown that the wide variation of apparent band-gap observed for thin films nominally referred to as InN is strongly influenced by variations in the nitrogen:indium stoichiometry. InN samples grown by remote plasma enhanced chemical vapour deposition show a change in band-gap between 1.8 and 1.0 eV that is not due to the Moss-Burstein effect, oxygen inclusion or quantum size effects, but for which changes in the growth temperature result in a strong change in stoichiometry. Material non-homogenity and non-stoichiometry appear to be general problems for InN growth. Excess nitrogen can be present at very high levels and indium rich material is also found. This work shows that the extent of the Moss-Burstein effect will have to be reassessed for InN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonpolar GaN thick films with [11-20] orientation were grown on [1-102] oriented sapphire by hydride vapour phase epitaxy (HVPE) utilizing reactively sputtered AlN buffers. Growth rate and microstructure of such films were investigated and compared to those in HVPE [0001] oriented GaN thick films. The structural parameters show an angular azimuth dependence implying in-plane non-homogeneity. The lattice parameters and strain components were determined by using plane and edge symmetric measurement geometries. A red shift observed in the near band edge photoluminescence is explained by the specific strain distribution studied independently by X-ray diffraction. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-crystalline GaN layers with a thickness up to 330 μm were grown by hydride vapor phase epitaxy on basal plane sapphire at gallium stable conditions in a bottom-fed vertical reactor at atmospheric pressure. Positron annihilation spectroscopy experiments were implemented in order to identify native point defects in the as-grown non-intentionally doped n-type GaN. Comparatively low concentrations of Ga vacancy related defects in the order of 10 16 to 10 17 cm -3 were extracted from the positron annihilation spectroscopy data. The Ga vacancy defect concentration was related to the intensity of the yellow photoluminescence band centered at 2.2 eV. The influence of the growth rate on the Ga vacancy related defect concentration was investigated. A trend of decreasing of the defect concentration with increasing of layer thickness is observed, which correlates with improving crystalline quality with the thickness.

We demonstrate that nonstoichiometry is one of the main reason of strong deviation of the InN optical gap in the 0.7 – 2 eV range, with N/In 1 corresponding to the lower and higher energies, respectively. The phenomenon is discussed in terms of atomic orbital energies, which are strongly different for indium and nitrogen, therefore both excess atom incorporation and elimination could change the optical gap. We estimate such trends using the approximation of the empirical nearest-neighbor tight binding theory. It is also demonstrated that resonant absorption in In-enriched regions is an additional factor lowering an effective absorption edge. InN is an artificial material, whose thin films are formed by modern high-purity technologies. No bulk crystals of reasonable optical quality exist so far. In spite of recent breakthroughs in its study, debates on the basic properties of InN continue. Among these, is the exceptional variation of its experimental gap energy over a huge spectral range from ~0.7 up to 2 eV. The theoretical band-gap values, being strongly dependent on the model used, are also varied in the wide range. In particular, 2 eV [1], 1.55 eV [2] and ~0.7 eV [3] had been reported. At present the 0.7 eV value is commonly accepted, although the narrowgap InN possesses some unusual properties, such as disagreement with the common cation/anion rule [4], extraordinary position of the universal hydrogen acceptor/donor crossover level [5], superconductivity [6], high carrier mobility in layers with a huge defect density >10

We report on the studies of linearly polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film, grown by metal organic chemical vapor deposition on a GaN template. The emission of mixed longitudinal-transverse exciton polariton modes was observed up to 130 K that evidences polaritonic nature of the excitonic spectrum up to this elevated temperature.

Performance of III-nitride epitaxy in a low V-to-III gas-flow ratio range under nitrogen ambient in a hot-wall MOCVD system

Application of picosecond four-wave mixing and photoluminescence techniques for investigation of carrier dynamics in bulk crystals and heterostructures of GaN

Free-standing GaN of -330 μm thickness with low defect density was prepared by hydride vapor-phase epitaxy (HVPE) on sapphire in a vertical atmospheric-pressure reactor and a subsequent laser-induced lift-off process. The structural and optical properties of the material were assessed by various characterization techniques, like X-ray diffraction, photo- and cathodoluminescence, spectroscopic ellipsometry, positron annihilation spectroscopy, and transmission electron microscopy. Here, we focus on p-Raman scattering profiling studies providing the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness. Profiles of the free-carrier concentration are obtained from monitoring the LO-phonon plasmon coupled mode. Comparative investigations are performed on the material before and after separation of the sapphire substrate. The GaN material presented here is well capable of serving as a substrate for further homoepitaxial strain-relaxed and crack-free growth needed for fabrication of high-quality III-nitride device heterostructures.

We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.

We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 × 10 18 cm -3 free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.

The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation doped GaN/Al0.07Ga 0.93N multiple-quantum-well (MQW) structures were studied by mea ...

We present a time-resolved experimental study of excitonic magnetic polaron (EMP) dynamics in type II quantum wells ZnMnSe/ZnSSe. Two photoluminescence peaks were observed, which reflects coexistence of localized excitons and EMPs in the same sample. This was possible due to the competition of two localization mechanisms - magnetic localization of heavy holes in the ZnMnSe layers and non-magnetic localization of electrons in the ZnSSe layers.

We report on the polarization properties of the exciton-polariton modes in GaN. The dispersion curves and the expected emission lineshape of polaritons for all polarization configurations are calculated taking into account the spatial dispersion and the simultaneous exciton-photon coupling of all optically active states. An experimental study of the exciton-polariton luminescence in a free-standing GaN layer is also performed. The spectra reveal a clear difference between the emissions polarized perpendicular and parallel to the c-axis of the crystal. The experimental results are discussed in terms of optical selection rules and population of the polariton states.

Compensation processes in magnesium doped GaN epilayers and bulk samples are studied. We demonstrate enhancement of potential fluctuations in Mg doped samples, from Kelvin probe atomic force microscopy measurements. Large- and small-scale light emission fluctuations are also demonstrated. Micro-photoluminescence (PL) study indicates an unusual anti-correlation between the intensities of excitonic and defect-related emission processes in p-type doped structures and also the presence of the so-called hot-PL. Hot-PL observed in compensated p-type samples, we relate to the presence of strong potential fluctuations. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Presented at: 5th International Conference on Nitride Semiconductors (ICNS-5), Nara, Japan, 25-30 May, 2003

In this work we report a study on thick HVPE-GaN layers grown on templates performed by mask-free selective lateral epitaxial growth by PENDEO-epitaxy. The defect distribution relative to the different growth modes is investigated. The emission spatial profile at different wavelengths is examined and correlated to the different character of the growth, the defect distribution and impurity incorporation. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have grown GaN layers with a thickness up to 340 μm in an rf-heated vertical HVPE reactor with a bottom-fed design. The GaN layers were separated from the sapphire substrate by a LLO process. Th ...

We study longitudinal electron transport in InGaN/GaN multiple quantum well (MQW) structures at moderate magnetic field. A step-wise behaviour in magnetic field dependences of both the Hall coefficient and the magnetoresistivity is explained by a magnetic induced localization of electrons in a two-dimensional (2D) potential relief due to the composition fluctuations in the wells. To describe the results we extend the model for a magnetic localization of electrons by treating every quantum well like a quasi-2D system with a cylindrical potential relief. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InN films have been grown by plasma-assisted MBE on Al2O3(0001) substrates using different buffer fabrication techniques: thermo-activated In diffusion into nitridated sapphire during InN buffer annealing and deposition of 20 nm AlxIn1−xN buffer layers with x = 0.1–0.5. Different In/N flux ratio conditions are employed. The InN epilayers demonstrate IR photoluminescence in the 0.6–1.0 μm range, with an ambiguous dependence of the PL intensity on structural quality of the films. The highest quality 0.5 μm thick InN layer having (0002) XRD Θ-rocking curve width below 400 arcsec and smooth continuous interface with the substrate does not show any PL. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)