In-rich InGaN films (XIn>0.5) and InN/InGaN multi-quantum wells were grown on Ga- and N-polarity GaN templates by radio-frequency plasma-assisted molecular beam epitaxy. The In-polarity InGaN films grown at 450°C showed superior crystalline quality and smoother surface morphology compared to the N-polarity samples, which were grown at 500∼550°C. By using the In-polarity In0.7Ga0.3N as a barrier layer, the InN/InGaN multi-quantum wells were successfully fabricated on the III-element polarity GaN templates at 450°C. Fine periodic structures and strong photoluminescence emission around optical communication wavelength were obtained from the In-polarity MQWs. These results indicate that the In-polarity growth is preferred to obtain a high quality InGaN film and the InN/InGaN MQWs in spite of its lower growth temperature.

Blue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Current-voltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8° and 75° C. Capacitance-voltage measurements as a function frequency (20 Hz – 1 MHz) and electroluminescence study at room temperature were also performed. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the bandgap, as extracted from logarithmic plots of the forward characteristics, were not influenced by the measurement temperature. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz – 1 kHz), gradually dropping to a lower value over a frequency range between (1 kHz – 100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of gm/ω as a function of frequency, f. The position of the peak in the gm/ω - frequency (f) plot and dC/dω(for f in the range 1 kHz – 100 kHz ), yielded a concentration of deep-states of approximately 2.2 × 1015/cm3, located at 0.39 eV above the valence bandedge.

Ti/Al ohmic contact with an extremely low specific contact resistance has been formed by the deposition of Ti and Al films on Si+ lanted GaN. The ohmic contact formed by annealing at 600 o C of Ti film with a thickness of 50 nm and Al film with a thickness of 200 nm reveals the good smooth surface and uniform structure as compare to those of contacts formed above 700 °C, which is correlated to whether the Al-Ti alloy is melted during the annealing of ohmic contact or not. The specific contact resistance of 2 × 10-6Ω-cm2 is obtained for Si+ implanted GaN with a dose of 5 × 1013 cm-2. As Si ion dose increases to 5 × 1014 /cm2, the specific contact resistance is reduced to 2 × 10-8 Ω-cm2. It is revealed that the selective doping at high impurity concentration in the surface region by Si+ implantation is useful to reduce the contact resistance for Ti/Al contact to GaN.

Knowledge of the carrier recombination, relaxation, and transport processes in InN materials is essential for determining the applicability of this material system in photonic and electronic applications. In this article, we provide a review of time-resolved spectroscopy experimental techniques and our recent results using these techniques to measure transient processes in InN. Specifically, subpicosecond differential transmission experiments were used to determine the carrier recombination lifetime and the carrier thermalization time of InN. In those experiments, we observed a fast initial hot carrier cooling followed by a slower recombination process. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect at room temperature. An inverse proportionality between the carrier lifetime and the free electron concentration was found. This suggests that donor-like defects or impurities may stimulate the formation of non-radiative recombination centers. Furthermore, we report the measurements of in-plane carrier transport and hole mobility of an InN epilayer by time-resolved transient grating spectroscopy using subpicosecond pulses at 800 nm and ∼1900 nm for grating writing and probing, respectively. The ambipolar diffusion coefficient Da = 2.0 cm2/s and hole mobility µh = 39 cm2/Vs at 300 K near the InN surface were determined by monitoring the transient grating kinetics at various grating periods.

The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed.

Time-integrated and time-resolved photoluminescence measurements on InGaN quantum wells grown by MOCVD on two different substrates (sapphire and GaN) show that the lumi-nescence efficiency in these structures strongly depends on the intensity of carrier excitation. While at low excitation densities the recombination of excited carriers is governed by local-ization effects the behavior drastically changes at higher densities. At room temperature a suppression of nonradiative recombination could be observed that leads to an super linear increase of the luminescence.

For further improvements in AlGaN/GaN heterojunction field-effect transistor performance (HFET), it is necessary to reduce the leakage current of the GaN buffer layer. We found a correlation between the leakage current and the intensity of the yellow luminescence of GaN layers taken by UV lamp excitation. The GaN layers were grown by metal organic chemical vapor deposition on SiC substrates. When the samples were excited by a UV (365 nm) lamp, visible yellow luminescence was observed. The leakage current of the GaN buffer layer was measured after deposition of ohmic metal contact. We confirmed clear correlation between the leakage current and the luminescence intensity based from result that the samples with the larger leakage current showed the stronger luminescence intensity. This correlation gives us useful information to understand the drain-source leakage current of AlGaN/GaN HFET.

We investigated the micrometer-scale structure of epitaxially laterally overgrown GaN (ELO-GaN) on a sapphire without a SiO2 mask using the 2×4 μm2 micro-beam x-ray of an 8-GeV storage ring. The GaN (0 0 0 12) rocking curve of a wing region had a sharp peak with a FWHM of 46 arcsec, while that of a seed region had several broad peaks. This peak-narrowing indicates that the density of threading dislocations (TD) is reduced and the grain size is extended in the wing region by the lateral growth. The observed grain size in the wing region was 10 μm × 5 μm. The mapping of the rocking curves shows that the wing regions hang down from the seed region at a tilting angle of 81.5 arcsec, although there is no SiO2 mask which is a cause of c-axis tilting in ELO-GaN. After comparing the results of ELO-GaN with and without coalescence, we conclude that the small c-axis tilting is caused by the compressive stress induced by the difference in the thermal expansion coefficient of the GaN and the sapphire substrate. Because a shrinkage of grain size was observed after the coalescence of the wing regions, it is thought the coalescence is one of the causes of the generation of the few threading dislocations in the wing regions. We believe that the reduction of the threading dislocations and the expansion of gain size in the wing regions are important for the further improvement of the device characteristics of GaN-based LDs grown on ELO-GaN.

AlN and AlGaN have been grown on 4H-SiC (1-100) substrates by rf-plasma molecular beam epitaxy. AlN assumes a metastable 4H structure to match the in-plane stacking arrangement of the substrate. Initial 2D nucleation of 4H-AlN is revealed by reflection high-energy electron diffraction. The epitaxial quality is evidenced by narrow x-ray diffraction ω-scan line widths less than 100 arcsec for symmetric and asymmetric reflections. Structural characterization results for AlGaN/AlN multiple quantum wells are also presented.

We have investigated the design parameters for high-Q photonic-crystal (PC) bandgap modes in the emission wavelengths of InGaN/GaN multiple quantum wells. We demonstrate experimental schemes to realize 2D triangular-lattice PC membrane structures, which is essential to obtain photonic bandgap (PBG) modes, and the optical properties of L7 membrane nanocavities that consist of seven missing holes in the Γ-K direction. L7 cavities show pronounced resonances with Q factors of 300 to 800 in the PBG as well as the enhancement of light extraction of the broad InGaN/GaN multiple-quantum-well emission by the 2D PBG.

The synchrotron radiation technique of x-ray standing waves (XSW), which allows to directly obtain structural and element-specific data, was successfully used for the investigation of the dopant site distribution in Si doped GaN films grown on (0001) sapphire substrates for the first time. The Si dopant concentration was chosen to 5×1018 cm-3 and 5×1019 cm-3. The measurements were performed on 300 nm thin doped films deposited on high-quality thick undoped GaN films. With this sample structure, influences of the Si dopant induced decrease of the crystalline quality on the XSW signal are suppressed. The XSW data are compared to those obtained from thick homogeneously doped GaN films. All XSW measurements were performed in (002) backscattering geometry. Independent of the dopant concentration, the results indicate that Si atoms are solely incorporated on substitutional Ga sites.

In this paper, we report results from seeded AlN PVT growth experiments carried out using SiC seeds. The purpose of the experiments was to understand the morphology and crystalline quality of PVT AlN crystals grown under the crystal growth environments investigated. AlN single crystal films of 120-650μm in thickness were grown and freestanding AlN single crystal pieces up to 4×5mm2 were obtained. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy. Selected AlN single crystals were studied using a high-resolution triple-axis X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Defects identified in AlN crystals are cracks, dislocations, grain boundaries and crystallite inclusions.

We used temperature dependent near-field magneto-photoluminescence spectroscopy to study emission properties of blue-green InGaN quantum dot (QD) structures with spatial resolution of ∼ 100 nm. The QD structures grown by MOCVD are 3 nm thick In0.12Ga0.88N layers, confined by GaN and In0.04Ga0.96N and containing a dense array of few nanometer size In-rich areas. The spectral features related to single QDs of the lateral size ∼3 and ∼30 nm and the QD ensemble have been identified. The regions of localization of the shallow and deep QD were spatially resolved in temperature dependent spectra. The spatial delocalization of the emission of the deep QDs at T<90 K have been observed.

Growth of GaN and AlGaInN nanowires using metalorganic chemical vapor deposition (MOCVD) is investigated. It is determined that surface kinetics play an important role in non-equilibrium synthesis process such as MOCVD, in contrast to near-equilibrium synthesis by hot-wall furnace reactor. Examination of crystallographic properties of GaN nanowires reveals preferential growth directions which are perpendicular to the c-axis. Such a tendency is analyzed by both thermodynamic and kinetic arguments and attributed to the minimization of (side wall) surface energy. Spontaneous formation of Al(Ga)N/GaN coaxial nanowires with distinct emission at 370 nm is observed. It is identified that the interplay between surface kinetics and thermodynamics facilitates the catalytic growth of GaN core while a limited surface diffusion of Al adatoms leads to nonselective, vapor-solid growth of Al(Ga)N sheath. The knowledge of crystallographic alignment is applied to the formation of arrayed GaN nanowires in both vertical and horizontal fashions, resulting in potentially new paradigms for creating nanoscale devices.

It is difficult to obtain p-type conductivity in beryllium-doped gallium nitride. Even when the material exhibits p-type conductivity, it tends to be highly compensated. Beryllium-doped gallium nitride samples grown by molecular beam epitaxy were investigated using deep level optical spectroscopy (DLOS), photoluminescence (PL), and positron annihilation spectroscopy (PAS) in connection with an annealing study in an attempt to correlate compensation and PL features with microscopic defects.

Interestingly, both DAP PL and a DLOS indicate an energy level that if interpreted as an acceptor would yield an optical activation energy of beryllium in gallium nitride of about 100meV. These signatures are missing in all as-grown gallium-polar gallium nitride doped with beryllium at levels below 2×1014 cm-3. Upon annealing in pure nitrogen or forming gas, the samples clearly exhibit the DAP at 3.38 eV associated with a shallow Be acceptor, but the samples remain semi-insulating. Interestingly, all nitrogen-polar as-grown samples exhibit the DAP emission at 3.38eV. We will discuss more about the effect of annealing on the apparent optical activation of beryllium and the shift of the photoluminescence peak.

DLOS and PAS studies suggest that gallium vacancies and/or gallium-related vacancies are related to compensation in beryllium doped gallium nitride samples. For heavy beryllium doped gallium nitride, there is a correlation between PL at 2.3-2.4eV and a beryllium-related deep acceptor complex. This is supported by PAS studies and DLOS studies. Additionally, there is a correlation between donor-acceptor pair (DAP) at 3.38eV, beryllium concentration, and yellow-red photoluminescence at 2.0 or 2.2eV.

The properties of AlGaN/GaN heterostructures have been a subject of great activity because of their application in high frequency, high power, and high temperature devices. Magnetotransport measurements give the possibility to study the properties of a two-dimensional electron gas. Indeed, Shubnikov-de Haas oscillations of a two-dimensional electron gas can be observed at high magnetic fields. Moreover, magnetoresistance measurements close to zero magnetic field give the possibility to investigate the weak localization and weak antilocalization arising from AlGaN/GaN heterostructures. The latter is related to the spin-orbit interaction on the spin of the carriers present in these heterostructures and is a key-feature of the spin-transistor proposed by Datta and Das [1]. As a matter of fact, the spin orientation between the electrodes of this novel device should be manipulated by the controllable strength of the spin-orbit interaction in the two-dimensional electron gas. In order to obtain information on spin-orbit effects in AlGaN/GaN heterostructures we therefore analyzed the weak antilocalization observed in the magnetoresistance.

Polarization-doped AlGaN/GaN heterostructures were grown by metalorganic vapor phase epitaxy (MOVPE) on the (0001) surface of sapphire substrates. First a 3 mm-thick GaN buffer layer was grown, followed by a Al0.3Ga0.7N layer with a thickness of 20 nm. Magnetotransport measurements were performed over a magnetic field range from –50 mT to +50 mT at various temperatures between 0.1 and 18 K. The Hall-bars were prepared by optical lithography and Ar+-ion-beam-etching technique. The metals used for the ohmic contacts were Ti/Al/Ni/Au. The mobility and carrier concentration in the single occupied subband were obtained from the Shubnikov-de Haas oscillations with the respective values of 9100 cm2/Vs and 6.2×1012 cm-2.

In our case, the Shubnikov-de Haas oscillations of the two-dimensional electron gas reveal the occupation of a single subband in the nearly triangular quantum well. At low magnetic field, weak localization as well as weak antilocalization were observed, showing that strong spin-orbit interaction is present in our structures. A previous report [2] explained the weak-antilocalization as being related to the intersubband scattering due to the occupation of a second subband in a modulation-doped quantum well. We show that weak antilocalization is also present in a polarization-doped quantum well with a single subband being occupied. In this perspective, temperature-dependent weak antilocalization measurements will be presented and analyzed using adequate theoretical models. Finally, the relevant scattering times like elastic scattering time, dephasing time as well as spin-orbit scattering time have been extracted. [1] S. Datta and B. Das, Appl. Phys. Lett. Vol. 56, pp. 665-667, 1990. [2] J. Lu et al., Appl. Phys. Lett., Vol. 85, pp. 3125-3127, 2004

We report the results of two studies of the growth and physical properties of AlGaN-based short-period superlattices (SPSLs), each aimed at improving light emission. In the first experiment, we grow structures on bulk AlN substrates. We observe ∼ 3 times higher luminescence efficiency than identically grown structures on sapphire. In the second experiment, we grow structures on sapphire while controlling the growth mode. We observe a significant improvement in the room temperature cathodoluminescence efficiency (at least by factor of 10) of AlGaN quantum wells when the 3D growth mode is induced by reduced flux of ammonia over identically prepared structures grown in the 2D mode.

The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. The initial oxidation rate of nitrogen polar (0001) AlN is approximately 25% faster than on aluminum polar crystals. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.

In an attempt to identify the performance limiting factors of green GaInN/GaN light emitting diodes (LEDs) we analyze a large number of LED dies fabricated from over 160 epitaxy runs covering the wavelength range from 390 – 580 nm on a quantitative scale of the emission power. As a function of drive current, we analyze the external quantum efficiency (EQE) with particular emphasis on the low current range. We observe three very distinct populations of EQE behaviors. We find that the EQE maximum value has a strong correlation with the forward current in this point, while it has weak correspondences with both, the forward voltage in this point, and the dominant wavelength at 20 mA. The very good linear relationship between the forward voltage in green LED dies at 1 mA and at 20 mA is also observed. All those characteristics are meaningful traits to ultimately converge to a physical electronic transport and bandstructure model for the next step enhancement of green and deep green GaInN/GaN LED performance.

Hydride-Metalorganic Vapor Phase Epitaxy (H-MOVPE) was used to grow a series of films on c-Al2O3 substrates. Depending on the growth temperature and HCl/TMIn molar ratio, InN deposited as a continuous film or a collection of micro or nanorods, or no InN growth was observed. A chemical equilibrium analysis of the In-N-H-Cl system predicts both InN growth and etching regimes with the nanorod growth observed near the growth-etching transition. All InN rod structures demonstrated well faceted hexagonal structure with a near random orientation of the rods, while the films were polycrystalline.