The surface morphology of thin InxGa1-xN layers in uncapped structures and the overgrowth using GaN or InxGa1-xN with a lower In content than the InxGa1-xN layer as the protection layer of the InxGa1-xN layer were investigated in addition to their dependence on growth temperature during the growth by metalorganic vapor phase epitaxy (MOVPE). It was necessary to decrease the growth temperature of the InxGa1-xN layer in order to realize homogeneous dots. In order to obtain a high luminescence efficiency, on the other hand, it was essential to increase the growth temperature of the protection layer. Sharp lines related to the localized electronic states, which are attributed to InGaN quantum dots, were observed by micro-photoluminescence (μ-PL) for the samples with the InxGa1-xN layers protected by the InxGa1-xN layers grown at 700∼740°C.

We have studied the structural and optical properties of InAlN alloys with compositions nearly lattice-matched to GaN. Scanning electron microscopy measurements reveals a good overall surface quality, with some defect structures distributed across the surface whose density increases with the InN concentration. On the other hand, Raman scattering experiments show three peaks in the frequency range between 500 and 900 cm-1, which have been assigned to InN-like and AlN-like E2 modes and A1(LO) mode of the InAlN. These results agree with theoretical calculations previously reported where two-mode and one-mode behavior was predicted for the E and A(LO) modes, respectively. Photoluminescence and photoluminescence excitation allowed us to determine the emission and absorption energies of the InAlN epilayers. Both energies display a redshift as the InN fraction increases. We find a roughly linear increase of the Stokes shift with InN fraction, with Stokes shift values of ≈0.5 eV in the composition range close to the lattice-matched condition.

We present experimental results demonstrating strong-light matter coupling at low and room temperature in bulk GaN microcavities. Angle dependent reflectivity measurements demonstrate strong-coupling with a Rabi-energy of 50meV at room temperature which is well reproduced with transfer matrix simulations. The absence of strong coupling in the photoluminescence is attributed to the low finesse of the microcavity (Q=60) and is confirmed by simulations which indicate a quality factor of 90 is required to observe strong-coupling in the emission.

Using a novel gallium (Ga) vapor transport technique, thick gallium nitride (GaN) layers have been grown using GaN powder as the source material. In this technique, GaN powder decomposes at 1100°C into gallium and nitrogen vapors. The Ga vapors are transported to the seed substrate and reacted with ammonia to form a crystalline GaN layer. The seed was composed of a sapphire substrate with a 2 microns HVPE GaN layer on it. The growth temperature was set at 1180°C. Using this technique, growth rates as high as 500 μm/hr were achieved. The grown GaN layers were single crystal and were characterized by X-ray diffraction and scanning electron microscopy.

The performance of conventional c-plane GaN-based optoelectronic devices suffers from the effects of strong polarization-induced electric fields along the conduction direction, which result in a reduced overlap between electron and hole wavefunctions. These devices consequently demonstrate low radiative recombination rates, and a blue-shift in peak emission wavelength with increasing bias. There have been several recent demonstrations of light emitting diodes (LEDs) fabricated on non-polar a- and m- plane GaN that show greatly reduced to zero blue-shift of peak emission wavelength, and other recent work on non-polar GaN has yielded hole concentrations that are almost an order of magnitude higher than for c-plane GaN.

The effects of the strong polarization-induced electric fields may, conceivably, also be mitigated or potentially eliminated by growing films on so-called ‘semipolar’ planes. A semipolar plane is any plane that may not be classified as a c-, a- or m- plane, and has at least two non-zero h, i, or k Miller indices and a nonzero l Miller index (, and planes, for example). It is expected that devices grown on these semipolar planes should also demonstrate a reduced blue-shift in peak emission wavelength and higher hole concentrations. Further, recent work also suggests that the indium incorporation efficiency for growth on semipolar planes is comparable to that for growth on the c-plane.

We demonstrate the first green InGaN/GaN LED grown on a planar semipolar GaN template. The LED structure is grown by metalorganic chemical vapor deposition (MOCVD), and the 20 μm-thick, specular and optically transparent template is grown by hydride vapor phase epitaxy (HVPE). The fabricated devices have a peak emission wavelength of ∼525 nm and demonstrate rectifying behavior, with a low turn-on voltage of 3.2 V. We observe a small ∼7 nm blue-shift in the peak emission wavelength during electroluminescence measurements, over the range 20 to 250 mA. We also see an almost linear increase in the output power from 5 mA to 200 mA, with no appreciable decrease in the external quantum efficiency over the same range. We also observe evidence of polarization anisotropy in the emission from the semipolar green LEDs.

Ni Schottky-diodes (SDs) 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN and AlxGa1-xN epilayers. Phase-pure cubic GaN and c-Al0.3Ga0.7N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 µm thick free-standing 3C-SiC (100) substrates. The quality of the cubic group III-nitride epilayers was checked by high resolution X-ray diffractometry, atomic force microscopy and photoluminescence at room temperature and at 2 K. Large deviations from the thermionic emission transport were observed in the current voltage (I-V) behavior of these SDs. Detailed analysis of the I-V characteristics at 300 K and at low temperature showed that a thin surface barrier is formed at the Ni semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier and reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SDs is in good agreement with theoretically calculated values and follows the expected trend. From these experimental data a blocking voltage of higher than 600V is extrapolated for c-GaN films with a doping level of ND = 5×1015 cm-3.

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging—in the scanning electron microscope—to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02°, in GaN thin films. As EBSD has a spatial resolution of ≈ 20 nm, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that channeling contrast in electron channeling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films.

Thick low defect AlN and AlGaN layers grown on ultra violet (UV) transparent substrates are considered as promising substrate materials for the UV light emitters and detectors. Electrically insulating thick AlN layers may serve as the substrates for AlGaN/GaN-based high power high electron mobility transistors (HEMTs). In this paper we report on crack-free up to 20 μm thick AlN layers grown by stress control HVPE on 2-inch sapphire substrates. As-grown surface had a characteristic pyramidal morphology. Being thick enough, AlN layers can be polished to improve surface roughness. The minimum full width at half maximum (FWHM) values of AlN ω-scan x-ray (00.2) and (10.2) rocking curves was about 500 and 1000 arcsec, respectively. The XRD analysis was applied for the threading dislocation density evaluation in grown AlN layer. Screw dislocation density was found to be (3-7)×108 cm-2 for the layers from 3 to 12 μm thick.

We present comparative micro-photoluminescence measurements on InGaN/GaN quantum well and quantum dot samples. Single sharp emission lines were investigated for both kinds of samples as a function of temperature and excitation density. For the sharp emission lines of the quantum dots and the strong localization centers in the quantum well samples comparable experimental findings were obtained such as the independence of their spectral position of the excitation density and the observation of binding and antibinding multiexcitonic states giving clear evidence for the quantum dot nature of localization centers.

We have studied the effects of rapid thermal annealing at 1300°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display a stained surface. Scanning electron microscopy and optical confocal images revealed the presence of crater-shaped inhomogeneities that develop around protruding nuclei with tipically 50 micron diameter. Energy dispersive x-ray microanalysis yields a high concentration of Si in the crater regions as well as non-stoichiometric concentrations of Ga and N, with an excess of N.

Micro-Raman spectra obtained within the crater region exhibit Raman peaks associated with Si3N4 but no trace of GaN modes, while focused ion beam milling of this region leads to the accumulation of metallic Ga in the etched area. These results suggest that a substantial migration of Si from the substrate takes place during the annealing, which severely alters the material in the crater regions. Such annealing effects, which are not observed in GaN grown on sapphire, constitute a severe drawback for a widespread use of Si(111) substrates when high-temperature processing is required.

We have studied the evolution of stress and microstructure of compositionally graded Al1-xGaxN (0 ≤ x ≤1) buffer layers on (111) Si substrates with varying thicknesses. In-situ stress measurements reveal a tensile-to-compressive stress transition that occurs near the half-thickness in each buffer layer. Cross-sectional transmission electron microscopy (TEM) shows a significant reduction in threading dislocation (TD) density in the top half of the buffer layer, suggesting that the compressive stress enhances the threading dislocation annihilation. The composition of the buffer layers varies linearly with thickness, as determined by X-ray energy dispersive spectrometry (XEDS). The composition grading-induced compressive stress offsets the tensile stress introduced by microstructure evolution, thus yielding a tensile-to-compressive stress transition at x ≈ 0.5.

In this investigation, Schottky diodes with different device sizes (150μm, 420μm and 700μm) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150μm diameter Schottky diodes. The best on-state resistance was 7.56 mΩcm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias.

The potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 500 µm thick freestanding HVPE grown GaN:Fe crystals with different Fe-concentration levels ranging from 5×1017 to 2×1020 was studied by means of photoluminescence, photoluminescence excitation (PLE) and Fourier transform infrared (FTIR) transmission experiments. The Fe3+/2+ charge transfer (CT) level was determined to be at 2.86 ± 0.01 eV above the valence band maximum considerably lower than the previously reported value of 3.17 ± 0.10 eV. A bound state of the form (Fe2+, hVB) with a binding energy of 50 ± 10 meV has been established as an excited state of Fe3+. FTIR transmission measurements revealed an internal (5E-5T2) transition of Fe2+ around 400 eV which, until now, was believed to be degenerate with the conduction band. Consequently, a second CT band was detected in PLE spectra.

This paper reports the fabrication and characterization of micromachined ultrasonic transducers (MUT) based on piezoelectric aluminum nitride (AlN) thin films. The MUT device is composed of an Al/AlN/Al sandwiched structure overlaid on top of a silicon (Si) diaphragm. X-ray diffraction (XRD) scan shows that highly c-axis oriented AlN (002) thin films have been grown on Al/Si(100) substrates. Electrical impedance of the MUT devices is analyzed as a function of frequency. The fundamental resonant frequencies of the devices are found in the range of 65-70 kHz, which are in approximation to the theoretical calculation. The effective coupling factors of the devices are also derived as 0.18.

Gross well-width fluctuations have been observed in single green-emitting InGaN/GaN quantum wells (QWs) grown by two methods: (i) a “two-temperature” method, where the GaN barrier has been grown at a higher temperature than the InGaN layer, (ii) a single-temperature method, where the InGaN was annealed in a flow of NH3/N2, prior to capping. InGaN epilayers analogous to these QW structures have been grown and characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM reveals a network of interlinking InGaN strips. Compositional analysis across the InGaN strips has shown that the centres of these strips to be indium-rich compared to the strip-edges. Furthermore, we have correlated the position of threading dislocations (TDs) in relation to these InGaN strips and determined that 90 % of the TDs terminate at the channels between the InGaN strips or near the edges of the strips. We propose that confinement of charge carriers occurs in the indium-rich regions at the centre of the InGaN strips, preventing non-radiative recombination losses at dislocation sites.

Based on the wurtzite crystal structure, large (MV/cm) polarization-induced electric fields are known to exist in InGaN single quantum wells (SQWs) grown perpendicular to the GaN c-axis, and these fields may impact optical device performance due to the quantum-confined Stark effect (QCSE). In general, the QCSE has experimentally been found to be smaller than the theoretical value expected for a coherently strained InGaN QW, and subsequently the InGaN/GaN QW polarization field is often under-estimated as well. In this study, we measure the QCSE in modulation-doped, InGaN/GaN SQW LEDs. The well-behaved capacitance-voltage (majority-carrier) characteristics of these devices allow us to unambiguously determine the applied field with bias. With this analysis, we de-couple the QCSE from the QW polarization field and show that although the applied field approaches the opposing QW polarization field theoretical value (i.e., flatband), the QCSE remains too small. We propose a localized-hole picture of the InGaN QW which explains our optical and electrical measurements.

Characterization of operational AlGaInN heterostructure light emitting diodes (LEDs) is critical to their performance optimization and time-to-failure analysis. Typically, device performance data needs to be corroborated with structural information such as layer thicknesses, charge profiles, and the absolute location of the pn-junction. Here, non-destructive testing by capacitance-voltage profiling is being applied to AlGaInN LED structures. Within a large set of samples with different active layer geometry, we observe distinct layers of high mobile charge accumulation. We correlate those with layer thicknesses derived from an x-ray diffraction analysis of the corresponding epiwafers. In this way, we identify the charge maxima as the upper and lower interfaces of the p-type AlGaN electron blocking layer to the neighboring GaN layers. By means of this successful analysis, we now have the opportunity to monitor epi process performance and stability as well as device degradation progress quasi-continuously over the device lifetime in a non-destructive mode.

GaAs/GaN heterostructures were grown by molecular-beam epitaxy using GaN/supphire (0001) templates. In spite of a ∼20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a large mirror-like area. The GaAs films were as thick as 1um. The surface profile was characterized by atomic-force microscopy, which gave an average roughness of 10 nm for a 5×5 μm scan. Micro-Raman characterization and transmission electron microscopy (TEM) showed that the epitaxial GaAs films had zincblende lattice with (111) orientation, whereas the GaN substrates had wurtzite symmetry. The GaAs/GaN interface was found to be flat and abrupt. A large number of defects have been observed which originated from relaxation of the large lattice mismatch. The defects included misfit dislocations and nanocavities at the interface, as well as dislocations and stacking faults in the bulk of the GaAs film. Sharp interference fringes and characteristic behavior were observed for the ψ and Δ parameters of spectroscopic ellipsometry in the range of 0.75-5.3 eV. Simulation of the optical properties of the GaAs/GaN/sapphire heterostructure indicated a reasonably good optical quality of the layers and interfaces. Photoluminescence (PL) spectra recorded at the temperatures from 17 to 300 K revealed wide and weak radiative bands. Non-radiative processes dominated in recombination of non-equilibrium carriers. The observed PL broadening originated from the band tails that were a result of the high density of charged defects.

Time-resolved photoluminescence (TRPL) spectroscopy has been performed in the temperature range 2 K to 300 K on thick (300 μm – 1 mm) nominally undoped bulk HVPE grown GaN layers. The PL spectra are dominated by the free excitons (FEs) and the two neutral donor bound excitons (DBEs) at about 3.4712 eV and 3.4721 eV at 2 K, assigned to the residual O and Si donors, respectively. The zero-phonon decay curves for both FEs and DBEs are nonexponential, with a fast initial decay and a slower tail at longer times. The initial decay time of the FE is about 130 ps at 2 K in such samples, the corresponding initial decay time for the DBEs for O and Si is close to 300 ps. The different lines corresponding to the so called two-electron transitions for the DBE show a different behaviour, with a longer decay time than the corresponding principal DBE states. The LO phonon replicas of the DBEs also have a decay time in excess of 1 ns in lightly doped samples.

Si doped GaN grown by molecular beam epitaxy on sapphire substrates were characterized by capacitance transient spectroscopy. Conventional deep level transient spectroscopy (DLTS) measurements displayed six deep level defects, labeled A1, A, B, C1, C, and D, with activation energy ranging from 0.20 to 0.82 eV below the conduction band. Based on the logarithmic dependence of the DLTS spectral peaks on the filling pulse width, it is deduced that the defects A, B, C, and D are concentrated in the vicinity of line dislocations. Double-correlation DLTS (DDLTS) measurements, on the other hand, showed that only defects A (0.82 eV) and D (0.22 eV) exhibited deep donor-like characteristics. Following a 1.0 MeV electron irradiation of the GaN sample, one radiation-induced peak, E, with activation energy less than 0.20 eV was observed in the DLTS spectrum. However, after annealing at 350 °C, this DLTS peak intensity was found to diminish significantly.