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We present resonant fs pump-probe reflectance measurements of excitons in wurtzite GaN epilayers at different lattice temperatures. At 4 K we find that the exciton dynamics is dominated by trapping at defects via acoustic-phonon emission on a time scale of 16 ps. At temperatures above 60 K we observe a much longer relaxation component of 375 ps, which is due to radiative recombination of free excitons. The results are in good agreement with theoretical predictions.

Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

Dislocation-free and strain-free GaN nanopillars, grown on Si by molecular beam epitaxy, were used as nanoseeds for a new form of epitaxial lateral overgrowth (ELO) by metalorganic vapour phase epitaxy (MOVPE) until full coalescence. Such overgrown GaN films are almost relaxed and were used as templates for producing thick GaN layers by halide vapour phase epitaxy (HVPE). The final GaN film is easily separated from the starting Si substrate. This is henceforth a new technology to produce freestanding GaN. The GaN crystal quality was assessed by transmission electron microscopy (TEM), photo- and cathodoluminescence (PL, CL). It was seen that the pillar-ELO is produced from a limited number of nanopillars. Some dislocations and basal stacking faults are formed during the coalescence. However, those that propagate parallel to the substrate do not replicate in the top layer and it is expected that the thickened material present a reduced defect density.

The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from ∼10 10 to 10 6 cm -2 ) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10-300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III-nitride layers.

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 ...

A detailed investigation on p–n junction diodes of GaN using deep level transient Fourier spectroscopy (DLTFS) has been carried out. The typical deep level spectra on the various diodes on the same wafer demonstrate three electron levels labelled as E1, E2 and E3 and a hole trap H1 together with a broad band constituting three new hole levels H2, H3 and H4 therein. The electrical parameters like activation energy, trap concentration and capture cross section due to the observed levels have been measured for the comparison with the literature. The hole levels H1–H4 are found to be potentially involved in the radiative recombination and thereby, the luminescence role of the levels in the device is discussed.

International audience; In order to get semi‐insulating GaN layers with a low dislocation density on sapphire, two kinds of Fe doping were explored by low pressure metalorganic vapour phase epitaxy (LP‐MOVPE): the modulation doping (MD) and the continuous doping (CD). The high crystalline quality and the semi‐insulating character are obtained in the case of the Fe‐MD layers. The effect of Fe doping on the strain was investigated in both kind of layers (MD and CD) by X‐ray diffraction, photoluminescence and reflectivity. The lattice parameters are functions of the iron doping level, whatever the doping mode.

Time-resolved photo luminescence (TRPL) data for temperatures 2-150 K are presented for two thick HVPE samples grown in two different laboratories. The samples both have residual O and Si shallow donor concentrations in the 10(16)cm(-3) range. The radiative decay time for neutral donor-bound excitons (DBEs) related to these donors is found to be about 300 ps. The decay of the DBEs at longer decay times is found to be related to feeding from the free exciton-polariton states. At elevated temperatures the decay of the DBE is very similar to the free exciton decay. (c) 2006 Elsevier B.V. All rights reserved.

Al x Ga 1− x N solar blind photoconductors are fabricated and characterized. The cutoff wavelength of these detectors is as low as 275 and 271 nm with aluminium fraction of 49.6 and 54.1%, respectively. The used AlGaN active layers were grown on (0001) sapphire substrates by low pressure metalorganic chemical vapour deposition (LP-MOCVD). The full width at half maximum (FWHM) of X-ray rocking curve from (0002) diffraction indicates the good quality of these samples. Optical properties are investigated with photoluminescence and absorption measurements. The variation of the spectral response with applied voltage and modulation frequency is investigated. Better results are obtained with 12 Hz and 20 V. Compared to other researches, a high rejection ratio is obtained. The simulation of the photoresponse using the voltage dependent responsivity allows the determination of the carrier lifetime. We obtained a value of 0.15 and 0.13 ms for x =0.49 and 0.54, respectively.

In this work AlGaN/GaN high electron mobility transistors have been grown on Silicon (111), Silicon Carbide and GaN templates on Sapphire. Both the structural and the electrical properties of these layers have been studied in order to determine the impact of substrate choice on dislocation density, strain state, roughness and electron mobility.

In this work AlGaN/GaN high electron mobility transistors have been grown by ammonia source molecular beam epitaxy on Silicon (111), Silicon Carbide and GaN templates on Sapphire. Both the structural and the electrical properties of these layers have been studied in order to determine the impact of substrate choice and buffer layer on active layer quality. Furthermore, an intercalated AlN layer grown on a GaN template is shown to enhance the insulating properties of the buffer. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN layers grown on sapphire by micro-ELO (Epitaxial Lateral Overgrowth) technique are characterised by TEM (Transmission Electron Microscopy). The technique means that randomly distributed, amorphous SiNx islands are deposited onto sapphire in a “Si/N treatment”. Then a GaN nucleation layer (NL) is deposited at relatively low temperature, which is turned to high density of 3D GaN islands. During the overgrowth of such a nucleation layer by a thick GaN layer the dislocations are bent, because the amorphous SiNx islands act like the regular pads of the classical ELO technique. Bending of dislocations and reduced dislocation density values (in the range of 107 cm–2) are proved by TEM. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Free energy and capture cross section of the E2 trap in n-type GaN are investigated by deep level transient spectroscopy with the help of an experimental method, relying on space charge depth modulation [D. Pons, J. Appl. Phys. 55, 3644 (1984)]. This technique is applied with a large range of filling pulse durations (up to 6 orders of magnitude) and for temperature varying between 260 K and 330 K. Entropy and capture barrier are found to be negligible. Analysis of our results makes N in Ga site (N Ga antisite) as the best candidate for this trap.

Room temperature time-resolved four-wave mixing has been performed in MOCVD GaN epilayers, grown on sapphire substrates by using either conventional or a 3D-growth mode with Si/N treatment technique (micro-ELO). Picosecond pulses at 355 nm were used to create light interference pattern and record a spatially modulated carrier distribution by interband transitions, while a delayed probe beam at 1064 nm monitored the carrier dynamics far from the bandgap. The determined values of the bipolar diffusion coefficient and the carrier lifetime were found equal to D a = 1.7-1.8 cm 2 /s and τ R 1.1 ns for standard grown epilayers, while in the samples with ultralow dislocation density they were found equal to 2.4 cm 2 /s and 2.7 ns. In the interface region of the epilayers with high dislocation density, a dependence of D and τ R values on excitation intensity was found. The latter effect was attributed to a screening of potencial barriers around the charged dislocations by free carriers and an ability of the carriers to become less localized and thus avoid the nonradiative recombination at dislocations.

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

The growth of high-quality GaN layers on a wafer size appropriate for device applications is based on heteroepitaxy on foreign substrates. Heteroepitaxial GaN layers with low densities (below 106 cm−2) of extended structural defects can be achieved by lateral overgrowth of mask-patterned templates or by the growth of extremely thick GaN layers as a route towards free-standing GaN-pseudosubstrates. We present the microscopic analysis of such low-dislocation-density GaN layers by means of scanning cathodoluminescence microscopy (CL). Several state-of-the-art concepts of lateral overgrowth are compared, including two-step epitaxial lateral overgrowth of stripe masks (ELO), multi-stack ELO comprising several mask layers as well as an alternative approach involving in situ SiN nano-masks. The self-organized formation of typical microscopic growth domains with characteristic optical properties is evidenced by CL for all lateral overgrowth techniques. This behavior directly fingerprints the different growth rates and the specific impurity incorporation on non-equivalent GaN facets, e.g. (0001), { 1 1 01 } or { 11 2 2 } , always present during lateral overgrowth and closely related to the mask geometry. Accordingly, characteristic CL line shapes found in ELO on periodic, micrometer scale mask patterns are also detected for GaN on in situ SiN nano-masks and clearly reveal the individual facet structure during overgrowth. For thick GaN layers, CL is used to detect the spontaneous appearance of inclined facets inside inverted pyramidal defects. Optimized, thick GaN layers exclusively formed by (0001)-growth are proven to be laterally homogeneous despite periodically varying residual stress and dislocation density of the underlying ELO template.

The performance of nitride-based photodetectors is investigated beyond the usual near-UV (400–300 nm) and mid-UV (300–200 nm) operation ranges. The responses of metal–semiconductor–metal (MSM) photodiodes were analyzed in the vacuum–UV and soft X-ray regions. To interpret the results, the absorption properties and the attributes of each of the photons with energies for producing multiple electron–hole pairs were considered. The soft X-ray characterization showed that in-plane MSMs worked efficiently up to photon energies of 600 eV. Above this value, the absorption decrease makes the diffusion length and layer thickness become critical parameters for the detector behavior. To perform detection in the violet and near-UV, InGaN-based photoconductors were fabricated and spectrally characterized. The devices presented abrupt wavelength cut-offs, demonstrating that the InGaN compositional fluctuations were tolerable up to In contents of 10% for fabricating selective photodetectors. Back-face illumination allowed us to obtain bandpass detectors for these spectral ranges.

We have used thermally stimulated capacitance (TSCAP) and optical deep level transient spectroscopy (O-DLTS) to detect and characterise hole traps in n-type GaN grown by epitaxial lateral overgrowth (ELOG) organo-metallic vapour phase epitaxy (OMVPE). Semi-transparent Ni Schottky barrier diodes (SBDs) were used to probe the space charge region of the GaN layer. Optical excitation was achieved using an ultraviolet light emitting diode with sub-bandgap energy photons at a wavelength of 380 nm. Two hole traps with activation enthalpies of 0.25 eV and 0.85 eV with respect to the valence band were found to be present in concentrations of 5 × 10 14 and 2 × 10 15 cm -3 . This is almost two orders of magnitude higher than the concentration of the electron traps present in this material. We have also found that 1.8 MeV proton implantation introduced additional shallow level hole traps with activation enthalpies of 0.19 eV and 0.23 eV, respectively.

A detailed transmission electron microscopy study is performed on the pyramidal inversion domains that appear in highly Mg-doped GaN grown by metalorganics vapor phase epitaxy or by the highpressure, high-temperature method. From a comparison between high resolution images of the inversion domain boundaries and simulations using different atomic models, we conclude that both basal and inclined domain boundaries are likely formed of a monomolecular layer of the definite compound Mg 3 N 2 . We show that, due to their high concentration, the formation of these defects may account for auto-compensation in Mg-doped GaN. We also show that the local band bending induced by the polarity inversion due to these defects can be at the origin of the blue luminescence of highly Mg-doped GaN, always observed when nanometric pyramidal inversion domains are also present.

The spectral response of metal-semiconductor-metal (MSM) and Schottky barrier photodiodes have been studied in the near- and vacuum ultraviolet (VUV). Devices were fabricated on micro-epitaxial lateral overgrowth GaN layers, which presented dislocation densities as low as 7×107 cm−2. Experimental results indicate that the surface properties become critical for the optical response at short wavelengths. Schottky barrier photodiodes showed a lower VUV sensitivity than MSMs as a result of the radiation absorption in the semitransparent Au top-layer. However, Schottky photodiodes yielded a better time stability operating in photovoltaic mode. For photon energies above 10.5 eV, the quantum efficiency of the MSM photodiodes was enhanced as a consequence of the different nature of light-semiconductor interactions, which provoke a decrease of the absorption coefficient and the generation of multiple electron-hole pairs for each impinging photon. The ionization energy for GaN has been also estimated.

Gallium nitride (GaN) is an extremely promising wide band gap semiconductor material for optoelectronics and high temperature, high power electronics. Actually, GaN is probably the most important semiconductor since silicon. However, achievement of its full potential has still been limited by a dramatic lack of suitable GaN bulk single crystals. GaN has a high melting temperature and a very high decomposition pressure; therefore it cannot be grown using conventional methods used for GaAs or Si like Czochraslski or Bridgman growths.Since there is no GaN bulk single crystal commercially available, all technological development of GaN-based devices relies on heteroepitaxy. Most of the current device structures are grown on sapphire or 6H-SiC. However, since their lattice parameters and thermal expansion coefficients are not well-matched to GaN, the epitaxial growth generates huge densities of defects, with threading dislocations (TDs) being the most prevalent (109–1011 cm−2). As a comparison, homoepitaxially grown GaAs exhibits ~102–104 dislocation cm−2, and homoepitaxial Si almost 0. Actually this large density of TDs in GaN drastically limits the performance and operating lifetime of nitride-based devices. Therefore, there is currently a tremendous technological effort to reduce these defects.Metal organic vapour phase epitaxy (MOVPE) is currently the most widely used technology. Actually, all optoelectronic commercial device structures are fabricated using MOVPE. In MOVPE, the most appropriate precursor for nitrogen is ammonia (NH3), whereas either trimethyl or triethylgallium may be used as a gallium source. MOVPE of GaN requires a high partial pressure of NH3, high growth temperatures (~1000–1100°C) and a growth chamber specially designed to avoid premature reactions between the ammonia and gallium alkyls. Since sapphire (or 6H-SiC) and GaN are highly mismatched, direct growth of GaN is impossible. Therefore, the growth of GaN on any substrate first requires the deposition of a buffer layer, which, to some extent, accommodates the mismatch. Using appropriate nucleation layers allows a reduction of the dislocation density to the low 108 cm−2 range.Though laser diodes (LDs) were demonstrated in the late 1990s with such defect layers, the real breakthrough in laser technology was the dramatic improvement of the LD lifetime at the end of 1997, with the lifetime reaching 10 000 h. This was made possible by implementation of epitaxial lateral overgrowth (ELO) technology, which significantly reduces the dislocation density to below 107 cm−2.In ELO technology, parts of the highly dislocated starting GaN are masked with a dielectric mask, after which growth is restarted. At the beginning of the second growth step, deposition only occurs within the openings, with no deposition observed on the mask. This is referred to as selective area epitaxy (SAE). The TDs are prevented from propagating into the overlayer by the dielectric mask, whereas GaN grown above the opening (coherent growth) keeps the same TD density as the template, at least during the early stages of growth.Currently, two main ELO technologies exist: the simpler one involves a single growth step on striped openings. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas the GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN, above the openings, and low dislocation density GaN, above the masks. With this technique, devices have to be fabricated on the wings. Conversely, in the two-step-ELO (2S-ELO) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the TDs arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this 2S-ELO technology, only the coalescence boundaries are defective. ELO technology produces high quality GaN, with TD densities in the mid 106 cm−2, line widths of the low temperature photoluminescence near band gap recombination peaks below 1 meV, and deep electron trap concentration below 1014 cm−3 (compared with mid 1015 cm−3 in standard GaN). Numerous modifications of the ELO process have been proposed either to avoid technological steps (maskless ELO) or to improve it (pendeoepitaxy, PE). To further reduce the TD density, multiple-step-ELO and pendeo have also been implemented.However, even ELO quality GaN is not good enough for the next generation of LDs. ELO samples do not yet offer a full surface suitable for laser technology. What is needed for LDs with at least 30 mW output power is high quality freestanding GaN with TDs close to or even below 106 cm−2. To reach this crystalline perfection, elaborate technologies are currently being implemented. They, at some stage, involve TD reduction mechanisms occurring in the ELO process.Self-supported GaN with at least ELO quality at an affordable cost is believed to be the next breakthrough in GaN technology.

We report on a comparative study of defect and emission distributions in thick hydride vapor phase epitaxial (HVPE) GaN films grown on two different patterned template structures separately produced by multi-step procedures using metalorganic vapor phase epitaxy (MOVPE). The observed differences in the microstructures and emission distributions at the early stages of the growth in both cases were related to the change of the dominating growth mode sequence and point defects incorporation. Both template structures were found to favor formation of voids in the coalescence regions, which leads to a partial strain relaxation and allows overgrowth of thicker films without cracks.

Hydrogen-ion implantation was studied for Mg-doped hexagonal GaN grown on sapphire. Low temperature photoluminescence spectroscopy (PL) shows two significant features; the implantation-annealing induced yellow band (YL) and a remarkable sharp excitonic peak. In the region 1.73–1.79 eV well resolved optical transitions were observed, which resemble the well known R1 and R2 emission bands from Cr3+ in Al2O3 (ruby). Structural and electronic changes were monitored by inelastic light scattering (ILS) spectroscopy. At high implantation dose and high annealing temperature we observed well resolved bands at 320, 380 and 640 cm−1. The latter band ‘splits’ into 645 and 672 bands at the highest implantation dose. Additionally, implantation-annealing induced band was observed at 430 cm−1. This band was not observed before. Besides these, four of the six Raman allowed modes are present in the spectra: 2A1, E1 and E2. Second order Raman spectroscopy yields several bands in the region 860–1470 cm−1. On the high energy side of the spectra, we monitor luminescence bands at 1.878, 1.85, 1.836 eV and for the first time observed a 1.8 eV band. Using a group-theoretical approach we assign symmetries of the first order phonons at k=0 as well as some experimentally observed second order symmetry allowed modes.

The use of high growth rate metalorganic vapor phase epitaxial GaN on epitaxial lateral overgrown GaN, laser lifted-off, for the fabrication of thick free-standing GaN substrates is explored. Analysis of the spatially resolved photoluminescence spectra as a function of sample thickness corroborates the improvement in material quality in the epitaxial lateral overgrown region. Luminescence quenching and the emergence of an additional donor bound exciton transition characterize the thick GaN layer grown at high growth rate. Stress in the sample, which was overall small due to the separation of the GaN from the substrate by laser lift-off, was found to be compressive near the GaN substrate top surface. Differences in stress between top and back surface of the GaN substrate were found to be as small as 0.1–0.2 GPa.

GaN layers were grown onto Si(1 1 1) by MOCVD with the insertion of AlN/GaN superlattices in order to reduce the strain. The microstructure of the grown layers has been revealed by transmission electron microscopy. Some peculiar features have been found, like three dimensionally grown truncated pyramids. All of the grown layers are single crystalline, hexagonal GaN, and are epitaxial to the silicon substrate. The top surface of the layers is flat with a dislocation density in the range of 10 9 dislocations/cm 2 .

Completely coalesced epitaxial lateral overgrowth (ELO) of GaN on silicon (111) is presented for ELO layer thicknesses below 3 μm. Fast lateral expansion of the ELO-GaN was achieved by metalorganic vapor phase epitaxy at high growth temperature (1120 °C), low pressure (100 mbar), and a high V/III ratio of 8000. Thus full coalescence and a smooth surface (roughness of 5 nm across 100 μm2) are accomplished for wide SixNy masks along 〈11_00〉GaN with a 10 μm period and 3 μm openings. Atomic force microscopy and transmission electron microscopy are used to assess the quality of the layers. The density of dislocations is reduced from 8×109 cm−2 in the GaN template down to 8×108 cm−2 above the mask openings, and finally to 5×107 cm−2 in the laterally overgrown regions. The corresponding strong improvement of the optical properties and the stress present within the epilayer are evidenced by scanning cathodoluminescence microscopy.

A transmission electron microscopy of pyramidal inversion domains induced by Mg doping in MOVPE and bulk GaN is presented. Based on high resolution observations and EDX analysis, two atomic models are proposed for the Mg-rich (0001) inversion domain boundaries. These structural defects appearing for Mg concentrations in the 10(19) cm(-3) range are shown to be possible origins for the auto-compensation and the blue luminescence.

A RCLED structure, composed of InGaN multi-quantum wells inserted in a 3λ GaN based micro-cavity, was grown by metalorganic vapor phase epitaxy. The bottom mirror was a GaN/AIGaN distributed Bragg reflector (DBR). The growth process was followed by in-situ laser reflectometry, thus allowing an accurate control of the optical thickness (OT) of the various layers in the RCLED structure. We used the 543 nm line of a HeNe laser, and both the DBR and 3λ microcavity were designed at this wavelength. Taking into account the OT decrease after cooling (induced by the thermal lattice contraction and the index decrease), a 3λ cavity at 504 nm (green) was obtained. The emission of the InGaN QWs was found to be detuned with respect to the resonance wavelength. Nevertheless, we observed a resonance effect with an enhanced EL emission near the reflectance dip within the stop-band, the width of the EL peak being as low as 17 nm. The resonance effect was further confirmed by angle-resolved electroluminescence measurements. This work was developed within the framework of the European AGETHA project.

Studies carried out on gallium nitride showed that they have a high defect density in their band gap. These defects have direct influence on the electrical characteristics of the devices, by contributing leakage currents and the trap assisted tunnel currents. In this article, we carried out numerical calculations based on the experimental data, in order to study the contribution of each component of the current. A comparison has been made between the devices based on GaN and on AlGaN. We study also the capacitance–frequency characteristics, which enable us to locate defects in the band gap. We discuss the effect of the series resistance on the current–voltage characteristics, as well as the influence of defects on the photodetector response time and on the rejection ratio.

The large frequency shift displayed by the longitudinal optical (LO) phonons A1(LO) and E1(LO) when going from GaN to AlN promises an accurate determination of the composition in AlxGa1−xN bulk layers by Raman spectroscopy. However, this determination is affected by a large uncertainty for low Al mole fractions (x

We have implanted Ni/n-GaN Schottky contacts with 25 keV hydrogen ions (protons). The defects, thus, introduced were studied using deep level transient spectroscopy. We have found that 25 keV proton implantation introduces a complex set of electron traps in GaN, of which most are different to the defects observed after high-energy (MeV) electron and proton implantation. Two prominent defects that could clearly be distinguished from each other have energy levels at 0.22 and 0.30 eV below the conduction band. At least three of the defects detected after 25 keV proton implantation exhibit a metastable character in which they can be reproducibly removed and re-introduced during reverse and zero bias anneal cycles.

Unintentionally doped AlGaN/GaN heterostructures were grown by plasma-assisted molecular beam epitaxy on GaN/Al2O3 templates. An optical, structural, and electrical characterization has been performed to determine and correlate the main factors that limit the low temperature mobility of the 2DEG. Hall effect measurements reveal a high electron density as well as differences in the Hall mobility between samples. Cross-section transmission electron microscopy images exhibit V-shaped pits in the AlGaN top layer of all samples, linked to threading dislocations. Results from cathodoluminescence suggest that the V-shaped pits and the regions close around may have different Al composition than the regions between pits. The AlGaN low-temperature photoluminescence spectra is dominated by nothing donor–acceptor pairs emission in samples having the lowest mobility. The enhanced electric field by the 2DEG charge in samples with high donor–acceptor densities, together with the alloy disorder and surface roughness arising from the pit regions, are most likely the dominant factors that limit the mobility to very low values.

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band. (C) 2002 Elsevier Science B.V. All rights reserved.

Optoelectronic modulation spectroscopy (OEMS) has been used to explore the sub band-gap responses of deep energy states in GaN prepared by MOVPE. Modulation of the channel current of an AlGaN HEMT revealed a replica structure centred on 2.16 eV which cannot be attributed to Fabry-Perot oscillations. This was superimposed on a broad background with an anti-phase response, suggesting a continuous spectrum of states to which electrons were excited from the valence band. The phase of the replica responses confirmed that each peak was due to a transition to the conduction band. The average peak separation was 89 meV, close to the LO phonon energy in GaN, 92 meV, suggesting that charge in the state at 2.16 eV was strongly coupled to the lattice. This appears to be the first report of the phonon replica structure in an absorption spectrum. The observations are compared with a previously published luminescence replica spectrum. It is demonstrated using configuration coordinate diagrams that there should be a coincidence of lines, and this was confirmed experimentally. These observations provide confirmation that the electron state concerned is located 2.16 eV below the conduction band. The geometry of the configuration coordinate diagram leads to the conclusion that the Frank-Condon energy of this defect was between 0.40 and 0.55 eV with a corresponding Stokes shift between 0.80 and 1.10 eV.

Minority carrier transient spectroscopy is performed in Schottky diodes fabricated on hexagonal n-type GaN grown by metalorganic chemical vapor deposition, either doped with two concentrations of Si or unintentionally doped. Capacitance transients are measured after a light pulse sent through the semitransparent contact which generates electron–hole pairs in the depletion zone. They display the characteristic sign of hole emission. The same deep level is detected in all the samples, independent of the doping level and doping species, with a concentration of some 1015 cm−3, even in the sample prepared by epitaxial lateral overgrowth. The ionization energy and capture cross section deduced from Fourier Transform transient spectroscopy are respectively 0.81±0.03 eV and 2×10−14 cm2. Such a capture cross section for holes indicates an attractive potential and hence a negatively charged center before the hole capture. Hole emission is suppressed by electron–hole recombination when a sufficiently long majority carrier pulse is applied after the light pulse. A single recombination time constant is measured and an electron capture cross section near 10−21 cm2, independent of temperature, is deduced. These facts demonstrate that this deep center is a point defect, still negatively charged after a hole has been captured, since it repels electrons, and hence it is a deep acceptor. All these properties fit very well the theoretical predictions previously published about the isolated gallium vacancy in n-type GaN.

BOLD (Blind to the Optical Light Detectors) is an international initiative dedicated to the development of novel imaging detectors for UV solar observations. It relies on the diamond and nitride materials that have lately undergone key advances. The investigation is proposed in view of Solar Orbiter UV instruments, for which the expected properties of the new sensors—visible blindness and radiation hardness—will be highly beneficial. Solar Orbiter is a selected Flexi mission of the European Space Agency (ESA). Despite various improvements over the last few decades, the present UV detectors exhibit limitations inherent to their actual technology. Yet the utmost spatial resolution, temporal cadence, sensitivity, and photometric accuracy will be decisive for the forthcoming space solar missions. The advent of imagers made of a large bandgap semiconductor would surmount many weaknesses, thus opening up new prospects and making the instruments cheaper. As for the ESA Solar Orbiter, the aspiration for wide bandgap semiconductor-based UV detectors is still more sensible, for the spacecraft will approach the Sun where the heat and the radiation fluxes are high. We depict motivations and present activities and programme to achieve revolutionary flight cameras within the Solar Orbiter schedule.

We report the electronic properties, obtained by deep level transient spectroscopy (DLTS), of electron trap defects in n-GaN, grown by hydride vapour phase epitaxy (HVPE), metal-organic vapour phase epitaxy (MOVPE), reactive molecular beam epitaxy (RMBE) and, for the first time, epitaxial lateral overgrowth (ELO) MOVPE. Two prominent traps with energy levels at EC − 0.27 eV and EC − 0.61 eV, respectively, are introduced during all four methods. Their filling kinetics suggest that they are extended defects. On the other hand, the levels at EC − 0.65 eV and EC − 0.21 eV are only detected in HVPE and RMBE grown GaN, respectively, and appear to be characteristic of these methods.

We have investigated stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy. An increased compressive stress was found at the coalescence boundary of two adjacent wings of ELO GaN using confocal micro-Raman spectroscopy. Voids present at the coalescence boundary were identified as major source for this stress concentration. Stress concentration in the vicinity of voids was illustrated using finite element analysis. Differences in stress and crystalline quality between wing and window areas of ELO GaN were also investigated.

Epitaxial lateral overgrowth (ELOG) of GaN was achieved on silicon (111) by metalorganic vapor phase epitaxy. Lateral expansion is obtained at high growth temperature (1120°C), low pressure (100 mbar) and with high V/III ratio (8000). Growth rate of the (0001) c-plane and the lateral facets was investigated for stripes along the [1010] GaN and the [1120] GaN crystallographic directions. Although full coalescence is more difficult on Si (111) substrate than on sapphire, ELOG process with large mask is possible with stripes oriented along the [1010] GaN axis. Coalescence can be achieved with a 10 μm period mask with an overgrown layer thickness lower than 3 μm. Atomic force microscopy and transmission electron microscopy are used to characterize the quality of the layers. The density of dislocations can be decreased from 8 x 10 9 cm -2 in the underlying layer of GaN down to 5 x 10 7 cm 2 in the laterally overgrown region, and 8 x 10 8 cm -2 in the opening of the mask. Cathodoluminescence was used to determine the stress in the FLOG layer before and after coalescence.

The tensile strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy was investigated. Thick (0.9-2.5 μm) GaN layers without any crack were deposited on Si(111) using AIN/GaN superlattices as templates. X-ray diffraction and photoluminescence measurements were used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layer. Evolution of strain as a function of the GaN layer thickness is also determined. Dislocation reduction (from 10 10 to 2.5 x 10 9 cm -2 ) is observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed, with full width at half maximum of the I 2 line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) X-ray rocking curve attests the high crystalline quality of GaN on Si(111 ), when AlN/GaN superlattices are used.

A series of selective (Al,Ga)N quarter-wave reflectors has been grown on sapphire and silicon substrates by metalorganic vapor phase epitaxy. The microstructure of the mirror structures, consisting of GaN quarter-wave layers alternating with AlN, AlGaN or AlN/GaN short-period superlattices, has been assessed in terms of a joint X-ray diffraction (XRD) and transmission electron microscopy (TEM) study.

Impact ionization of exciton states in epitaxial GaN films and GaN/AlGaN quantum-well structures was studied. The study was done using an optical method based on the observation of exciton photoluminescence quenching under application of an electric field. It was established that electron scattering on impurities dominates over that from acoustic phonons in electron relaxation in energy and momentum. The mean free path of the hot electrons was estimated. The hot-electron mean free path in GaN/AlGaN quantum wells was found to be an order of magnitude larger than that in epitaxial GaN films, which is due to the electron scattering probability being lower in the two-dimensional case. (C) 2001 MAIK "Nauka/Interperiodica".

The polarization dependence of biexcitonic signals and quantum beats between A-excitons (X A ) and A-biexcitons (X A X A ) in a high-quality GaN epilayer is measured by spectrally-resolved and time-integrated four-wave mixing. With cross-linear polarised light, mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between X A and X A X A , and agrees well with the calculated X A X A binding energy; while the second beating period corresponds to that between X A and donor bound excitons (D°X). The temperature-dependent homogeneous linewidth shows that the D°X has a larger acoustic phonon coupling coefficient than the X A X A . We also measured the polarization dependent B-biexciton (X B X B ) signal. The effective masses for the A- and B-hole were deduced from the binding energy.

Crack-free GaN/Si(1 1 1) thin layers (0.5 μm) were grown by metal organic vapour phase epitaxy, using either an AlN buffer layer or (AlN/GaN) strained superlattices. High-resolution X-ray diffraction exhibited a full-width at half-maximum as low as 630 arcsec for rocking curve scan on (0 0 0 2) line. Low temperature PL spectra of GaN/Si(1 1 1) show unambiguously a state of tensile biaxial strain for all of the layers. These GaN layers were then lifted-off by wet chemical etching from their original silicon substrate and Van der Waals bonded on sapphire or quartz substrates. The ability of growing thick GaN (15–100 μm) by halide vapour phase epitaxy on such templates has been successfully achieved, marking a step towards self-supported GaN substrates.

Al x Ga 1− x N alloys are very attractive materials for application to ultraviolet photodetection. AlGaN photoconductors, Schottky photodiodes, metal–semiconductor–metal photodiodes, p–n junction photodetectors and phototransistors have been recently developed. In this work we analyse the performance of AlGaN-based photodetectors, discussing present achievements, and comparing the characteristics of the various photodetector structures developed to date.

Since there is no GaN bulk single crystal available, the whole technological development of GaN based devices relies on heteroepitaxy. Numerous defects are generated in the heteroepitaxy of GaN on sapphire or 6H-SiC, mainly threading dislocations (TDs). Three types of TDs are currently observed, a type (with Burgers vector 1/3〈〉); c type (with 〈0001〉) and mixed a+c (1/3〈〉). The Epitaxial Lateral Overgrowth (ELO) technology produces high quality GaN with TD densities in the mid 106 cm—2, linewidth of the low-temperature photoluminescence (PL) near-bandgap recombination peaks

III nitrides have become the most exciting challenge in optoelectronic materials in the last decade. Their intrinsic properties and an intense technological effort have made possible the fabrication of reliable and versatile detectors for short wavelengths. In this work, materials and devices issues are considered to provide a full picture of the advances in nitride UV photodetection. First, basic structures like photoconductors, Schottky, p-i-n and metal-semiconductor-metal photodiodes and phototransistors are compared, with emphasis on their specific properties and performance limitations. The efforts in the design and fabrication of more advanced detectors, in the search for higher quantum efficiency, contrast, signal-to-noise or speed operation, are reviewed afterwards. Metal-insulator-semiconductor diodes, avalanche photodetectors and GaN array detectors for UV imaging are also described. Further device optimization is linked with present materials issues, mainly due to the nitride quality, which is a direct result of the substrate used. The influence of substrates and dislocations on detector behaviour is discussed in detail. As an example of AlGaN photodetector applications, monitoring of the solar UV-B radiation to prevent erythema and skin cancer is presented.

The impact ionization of excitonic states is studied in epitaxial GaN films and GaN/AlGaN quantum well structures. The investigation is carried out by optical methods involving the observation of quenching of the exciton photoluminescence under an applied electric field. It is found that impurity scattering rules the momentum and energy relaxation, rather than the acoustic phonon scattering. The effective mean free path of hot electrons is estimated. In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to be an order of magnitude larger than those of GaN films due to the decrease in scattering probability of the electron in the two-dimensional case.

Deep level transient spectroscopy (DLTS) is performed in MOCVD α-GaN either doped with two different concentrations of Si or unintentionally doped. Capacitance transients are measured in Schottky diodes made of an Au or Ni rectifying contact and an Al/Ti ohmic contact, both on the top of the samples. Only two peaks are detected in each sample in the energy range from the conduction band edge down to 1.1 eV below it, respectively close to 0.50 and 0.92–1.05 eV, by Fourier Transform DLTS (FTDLTS) with concentrations not exceeding 3×1015 cm−3. These two results testify the high crystalline quality of the samples. The deeper level characteristics depend on the shallow impurity, either Si or the unintentional shallow donor, in deep states which comprise in fact a fine structure not evidenced by FTDLTS. A high resolution DLTS (HRDLTS) method is implemented to resolve this fine structure into several sub-levels which cannot be related to distinct chemical environments. The study of emission and capture kinetics confirms that at least three charge states (+,0,-) are involved. It is concluded that MOCVD α-GaN comprises deep centers which are stabilized in such a form with a concentration in the range of a few 1014–1015 cm−3.