We report on deliberate O doping of GaN epitaxial layers during MOCVD growth, using H2O and CO2 as precursors. The photoluminescence spectra show a dominant 3.27 eV emission at 2 K known to be a donor-acceptor pair (DAP) transition. In our samples the intensity of this DAP spectrum correlates with the commonly observed 3.466 eV acceptor bound exciton (ABE) peak, suggesting these spectra are related to the same acceptor. The general correlation of these acceptor spectra with O concentration (as established with SIMS data) suggest that the acceptor is O-related, most likely a VGa-O complex. The concentration was measured with positron annihilation spectroscopy and found to be in the 1016 cm−3 -1017 cm−3 range in different samples. Considering previous results the identity of this residual acceptor is suggested to be a VGa-O-H complex. Previous suggestions that this acceptor is related to Mg, Si or C are discussed and found to be less likely.

The effect of high temperature, high pressure annealing on morphology, optical and structural properties of free-standing GaN films grown by hydride vapor phase epitaxy is studied. The annealing is found to change the intensities of the photoluminescence peaks as a result of a redistribution of the impurities and native defects in the thick GaN films. A positron annihilation study shows a decrease of the Ga vacancy-related defects below the detection limit after the annealing. The defect redistribution is correlated with a flattening of the stress distribution across the thickness, as revealed by micro Raman study, and with a decrease of the curvature of the annealed free-standing films.

We report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E┴c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.

Moderately and heavily Mg-doped GaN were studied by a combination of post-growth annealing processes and electron beam irradiation techniques during cathodoluminescence (CL) to elucidate the chemical origin of the recombination centers responsible for the main optical emission lines. The shallow donor at 20–30 meV below the conduction band, which is involved in the donor-acceptor-pair (DAP) emission at 3.27 eV, was attributed to a hydrogen-related center, presumably a (VN-H) complex. Due to the small dissociation energy (<2 eV) of the (VN-H) complex, this emission line was strongly reduced by low-energy electron irradiation. CL investigations of the DAP at a similar energetic position in Si-doped (n-type) GaN indicated that this emission line is of different chemical origin than the 3.27 eV DAP in Mg-doped GaN. A slightly deeper DAP emission centered at 3.14 eV was observed following low-energy electron irradiation, indicating the appearance of an additional donor level with a binding energy of 100–200 meV, which was tentatively attributed to a VN-related center. The blue band (2.8–3.0 eV) in heavily Mg-doped GaN was found to consist of at least two different deep donor levels at 350±30 meV and 440±40 meV. The donor level at 350±30 meV was strongly affected by electron irradiation and attributed to a H-related defect.

AlN/GaN superlattices (SLs) with different periods grown on GaN buffer layers were studied by infrared spectroscopic ellipsometry (IRSE), Raman scattering (RS) and highresolution reciprocal space mapping (RSM). The lattice parameters and the degree of strain in the GaN buffer and the SL constituents were determined. Phonon modes originating from the buffer layer and the SL sublayers were identified and their frequency shifts were correlated with the strain state of the films.

In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers, grown with Metal Organic Vapor Phase Epitaxy (MOVPE) at about 8000C, have been studied in detail with optical spectroscopy. Such structures are shown to be very sensitive to a near surface depletion field, and if no additional layer is grown on top of the MQW structure the optical spectra from the individual QWs are expected to be drastically different. For a sample with 3 near surface QWs and Si-doped barriers, only the QW most distant from the surface is observed in photoluminescence (PL). The strong surface depletion field is suggested to explain these results, so that the QWs closer to the surface cannot hold the photo-excited carriers. A similar effect of the strong depletion field is found in an LED structure where the MQW is positioned at the highly doped n-side of the pn-junction. The internal polarization induced electric field in the QWs is also rather strong, and incompletely screened by carriers transferred from the doped barriers. The observed PL emission for this QW is of localized exciton character, consistent with the temperature dependence of peak position and PL decay time. The excitonic lineshape of 35-40 meV in the QW PL is explained as caused by a combination of random alloy fluctuations and interface roughness; the corresponding localization potentials are also responsible for the localization of the excitons in the low temperature range (<150 K). These samples show no evidence of localization due to nanoscale In fluctuations, these commonly observed problems are concluded to be not present in our samples. A second PL feature at lower energy, observed at low temperatures, is shown to be related to an electron pocket at the interface to the underlying n-GaN buffer layer in these samples.

A brief review on the present knowledge of the electronic properties of the Ga(In)NAs ternary and quaternary alloys is given mainly from an experimental perspective. The discussion is focused on Ga(In)NAs with low N composition (< 10 %), where a large amount of experimental work has been done. Important fundamental electronic properties of the material system are analyzed with the emphasis on the nature of the giant band gap bowing in the alloy and nitrogen-induced modifications of the electronic structure of the conduction band. The current knowledge of the key material parameters, relevant for the device applications, such as electron effective mass, recombination processes and band alignment in Ga(In)NAs/GaAs heterostructures, is also reviewed.

Magneto-optical spectroscopy in combination with tunable laser excitation spectroscopy is employed to carry out a detailed study of spin alignment and spin injection in II-VI wide-bandgap semiconductor heterostructures, aiming at optimization of structural design for nano-scale spintronic applications. The use of tunable excitation is shown to provide a valuable opportunity to monitor separately spin relaxation and spin injection processes in the structures. Efficient spin alignment is achieved by using a diluted magnetic semiconductor (DMS) (a layer of ZnMnSe or a ZnMnSe/CdSe superlattice) as thin as 10 nm. The spin alignment efficiency is shown to depend critically on the ratio between the rates of spin relaxation and spin transport within the DMS layer. This allows the realization of spin alignment and spin switching functions by varying the structural design.

In0.11Ga0.89N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers are shown to be very sensitive to a near surface depletion field. For a sample with 3 QWs, similar to what is often used in LEDs, only the QW most distant from the surface is observed in photoluminescence (PL). The appearance of a second lower energy PL peak below the ordinary QW exciton peak is a proof of a substantial band bending across the MQW structure. A similar effect seems to occur in pn-junctions having an MQW in the depletion region of a highly doped n-side. The strong depletion field is suggested to explain these results. The apparent absence of PL from the QWs closer to the surface (pn-junction) is tentatively ascribed to a loss of hole confinement in the strong depletion field.

We report characterization of the red emission band in hydride vapor phase epitaxial GaN using cathodoluminescence spectroscopy and imaging and time-resolved photoluminescence. The observed properties of the emission are consistent with recombination of excitons bound at close donor-acceptor pairs. The time evolution of the emission signal during electron beam irradiation supports the association of the red emission with charged centres.

Experimental results obtained in two different HVPE reactors are analyzed and compared to the theoretical curves, taking into account the surface kinetics, the mass transfer, and parasitic deposition on the glass walls before the substrate. Unexpected high growth rate values, up to 50νm/h, were measured in conditions of expected substrate etching by HCl. A systematic experimental study of this new phenomenon is presented together with a theoretical analysis. This analysis suggests a new mixed general mechanism of growth.

We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN ‘template’ layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN ‘template’. Additionally the effect of Si doping of the GaN buffer layers on the HVPEGaN properties was analysed.

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied. The piezoelectric field as well as short range potential fluctuations are screened via different mechanisms by donor electrons and excited electron-hole pairs. These effects account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence (PL) peak up to 0.2 eV is observed for a Si donor density of 2 × 1018 cm−3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). It appears like 2-dimensional screening of short range potential fluctuations is needed to fully explain the data. We suggest that excitons as well as shallow donors are at least partly impact ionized by electrons in the rather strong lateral potential fluctuations.

A detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional OP donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 °C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 × 1018 cm−3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Å) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed

We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN ‘template’ layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN ‘template’. Additionally the effect of Si doping of the GaN buffer layers on the HVPEGaN properties was analysed.

Photoluminescence spectra were measured for 100 Å wurtzite GaN AlGaN modulation doped quantum wells. Three well-resolved peaks originate from the quantum well. The theoretically calculated confinement energies have been compared to the experimental energy positions and found to be in good agreement with the data, assuming that the piezoelectric field is largely screened by the electron gas. The highest energy transition may originate from the Fermi edge, consistent with the temperature dependence of the photoluminescence. Decay times for the different transitions indicate that the photoexcited holes are localized.

The crystal structure and surface morphology of hydride vapour phase epitaxy grown thick (12-105 μm) GaN layers have been investigated as a function of growth rate using several structure sensitive techniques like atomic force microscopy (AFM), x-ray diffraction (XRD) in ω-2θ and ω-rocking curve measurements as well as low temperature photoluminescence (PL). PL and XRD measurements reveal rather narrow lines: full width at half maximum (FWHM) values of the strongest donor-bound exciton line are in the range from 6.0 to1.8 meV and ω-2θ FWHM values are between 80 and 23 arcsec indicating good structural quality of the films. The ω-rocking curves show a single peak for the thinnest films with a FWHM of 250 arcsec and multiple peaks with FWHM of about 250-350 arcsec in thicker films indicating the formation of several high-quality domains when increasing either thickness or growth rate. Optical microscopy and AFM images reveal a domain type of morphology and also show an appearance of spiral hillocks in layers grown at growth rates exceeding a critical value. We interpret these results as dominating 2D multilayer growth at low growth rates, and competing 2D multilayer and spiral growth mechanisms at high growth rates.

We have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.

We have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.