Persistent photoconductivity (PPC) effect has been observed in p-type GaN epilayers grown both by metal-organic chemical vapor deposition (MOCVD) and reactive molecular beam epitaxy (MBE) as well as in a two-dimensional electron gas (2DEG) system formed by an AlGaN/GaN heterostructure grown by MOCVD. Its properties have been investigated at different conditions.

We have made an attempt to obtain electronic and atomic structure information on the residual defects that exist in high-resitivity GaN epitaxial layers from optically-detected magnetic resonance (ODMR) experiments performed on the broad 3.0 and 2.2 eV photoluminescence (PL) bands observed from these films. The first observation of magnetic resonance on this 3.0 eV band reveals two ODMR signals. The first resonance is sharp (FWHM ∼ 3.5 mT) with g ∼ 1.950 and is assigned to effective-mass (EM) donors based on previous studies. The second feature is much broader (FWHM ∼18 mT) with a donor-like g-value of ∼ 1.977. This new resonance may be associated with partially EM-like donor states located ∼ 54 - 57 meV below the conduction band edge proposed recently to be involved in this 3.0 eV PL band.

Optical-isothermal capacitance transient spectroscopy (O-ICTS) was used to distinguish the deep levels which occur in unintentionally doped n-type GaN by means of their characteristic optical cross section. GaN grown by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were compared. Correspondence between optical and thermal emission characteristics of previously discovered levels, E2 (∼Ec-0.55 eV) and E4 (∼EC-1.0 eV), were clearly determined by observing their sequential appearance in the ICTS spectra. Whether by thermal or optical stimulation, the emission from E4 was found to be broad in nature; it is consequently believed to involve a defect. The total measured concentration of deep levels, including a prominent level which photoionizes in the range 2.5 to 3.0 eV below the conduction band, is greater in the GaN grown by MOVPE than by HVPE that was tested.

The structural characteristics of MOCVD AlN films with different carbon doping concentrations grown on sapphire were investigated by XRD (θ–2θ scans, ϕ scans and rocking curves ), HRTEM and Auger spectroscopy. The AlN:C films have very high crystalline quality and low resistivity. The epitaxial relationship between AlN and Sapphire is: (0001)AlN //(0001)Sap, [ 12 10]AlN//[0 110] Sap and [10 10]AlN //[ 2110]Sap. With increasing carbon concentration, the AlN films have higher carrier concentrations, and lower resistivities even though they have higher defect density. The resistivity decreases by 8 orders of magnitude with C doping. When the carbon concentration reaches 11%, an interfacial layer of ∼5nm was observed in HRTEM images. This layer suggests that some C in the film is diffusing into the sapphire substrate. However, optical diffractograms obtained from the negatives of the HRTEM showed no appreciable change in the structure of the interfacial layer compared to the pure substrate.

Modulation Extrinsic photoconductivity spectra between 1.44eV and 1.75eV of unintentionally n-doped high resistance GaN film grown by MOCVD are measured at room temperature by using wavelength adjustable Ti:Sapphire laser. We find that there are two major deep levels in the GaN material in the used photon energy range. The relaxation time of excess carriers controlled by those levels are in the order of 10−4sec. The concentration of localized states are determined as 1.8×108cm−3 and 2.5×109cm−3, respectively. A physical model is developed to explain the results and process the data. Using a new method we have determined the optical absorption cross section of deep levels are 1.5×10−17cm2 and 2.7×10−18cm2, respectively.

Phonon-plasmon scattering in non-resonant Raman spectroscopy is used to determine the free electron concentration in Si doped GaN films. For various doping concentration and variable temperature the correlation with magneto-transport data is established. The freeze-out of the carrier concentration at low temperature is thus observed in a purely optical detection scheme. We observe a very long transient time of several hours for the carrier concentration as a reaction to temperature variation. This indicates an indirect capture and emission process with a very small cross section. The value of the Faust-Henry coefficient is determined.

We present the results of a theoretical study of the 2D electron gas mobility at a AlxGa1−xN/GaN heterointerface. All standard mechanisms, including scattering by acoustic and optical phonons, and remote and background (residual) impurities have been included in our calculation of theoretical mobility limits in a AlxGa1−xN/GaN structure. Comparison of calculations with experimental mobilities obtained from high quality MOCVD grown Al0.15Ga0.85N/GaN heterostructures shows that the low temperature mobility in these samples is dominated by scattering from ionized impurities, with a smaller contribution from acoustic phonons.

Electron Spin Resonance (ESR) and related methods have been used to study defects in GaN and in AlxGa1-xN ternary alloys. In particular, Light-induced ESR of a thick MOCVD grown layer of GaN shows that the ESR signature of the deep defect appears for excitation energies > 2.6 eV and saturates above 2.7 eV. This information provides a direct measure of the energetic level position of the diamagnetic to paramagnetic transition of this center. Furthermore, standard ESR investigations of MBE-grown layers of AlxGa1-xN alloys were performed with emphasis on the effective-mass donor resonance. Composition and temperature-dependent measurements of the resonance position are presented.

As part of the feasibility study of using III-V nitride semiconductors for x-ray and γ-ray detection, the irradiation effects on GaN were investigated. GaN films with very different electrical resistivity and electron concentration were used in the study. The electron mobility, photoconductivity spectra, and photo-luminescence spectra were measured before and after irradiation. An enhanced ημτ product for undoped GaN films and an enhanced blue luminescence for a Zn-doped sample were observed after irradiation.

Deep centers were studied in GaN samples grown by organometallic vapor phase epitaxy (OMVPE). Electron traps 0.2 eV and 0.5 eV below conduction band edge and 0.25 eV and 0.50.85 eV above the valence band edge were detected by means of deep levels transient spectroscopy (DLTS), photoelectron relaxation spectroscopy (PERS) and thermally stimulated current spectroscopy (TSC). The photoconductivity at low temperature is shown to be persistent and the magnitude of photosensitivity is dependent on the way the samples are grown. Microcathodoluminescence (MCL) and electron beam induced current (EBIC) measurements indicate that the density of deep recombination centers near the dislocation walls between the misoriented GaN domains is lower than inside the domains. Spatially resolved PERS measurements show that the concentration of the 0.85 eV level is higher in the low angle grain boundary regions that produce bright contrast in EBIC and MCL.

GaN and All-xGaxN films were grown by the method of ECR-MBE. Absorption constants as a function of wavelength were determined from transmission measurements. Photoconducting detectors were fabricated from these films and characterized in terms of their spectral response and photoconductive gain. Mobility-lifetime products were determined from the measurement of photoconductive gain. The resistivity and mobility-lifetime products of the films were varied from 10-109 ohm-cm and 10-3 -10-8 cm2/V respectively by changing the microwave power in the ECR discharge from 20-60 watts. The change in the mobility-lifetime product is attributed to change in the lifetimes of the photogenerated carriers. This assumption is supported by direct measurement of detector response times. Finally, we report for the first time, the detection of alpha particles using GaN detectors.

AIN films were grown by plasma source Molecular Beam Epitaxy (PSMBE) on n-type Si(111) substrates under various growth parameters. I-V and C-V-f dependencies of AI-AIN-Si(111) MIS structures were measured. Electrical characterization of MIS structures with PSMBE grown textured AIN thin films as insulator shows that low current leakage can be achieved, although some samples have rectifying I-V dependencies. C-V-f characteristics reveal evidence of the presence of different types of traps. Additional investigations are needed in order to determine the nature of these traps.

We derive balance transport equations for the electron mobility and drift velocity, which are applicable at any degeneracy of the electron gas. These equations account for the polar optical phonon scattering and ionized impurity scattering and include the effects of screening. These equations are valid only for very high concentrations (above 1019 cm-3 for GaN). However, the comparison with the results of Monte Carlo simulations shows that they fairly accurately reproduce the field-velocity curves in GaN in moderate electric fields (up to 100 kV/cm). The comparison with the electron mobility calculated using the two-step model [1] shows a much larger difference but allows us to illustrate the trends in mobility dependencies caused by electron-electron collisions. We also derive the balance transport equations accounting for the polar optical phonon scattering in a two-dimensional electron gas. The calculations based on these equations, show that the unscreened polar optical scattering mobility is smaller in the two-dimensional gas than in the bulk intrinsic semiconductor and that the mobility decreases with the decrease of the quantum well thickness.

Temperature dependent Hall-Effect-measurements on unintentionally doped cubic GaN epilayers grown by molecular beam epitaxy (MBE) are reported. The cubic GaN layers have been deposited on semiinsulating (001) GaAs-substrates under N-stabilized growth conditions which were controlled by in-situ reflection high energy electron diffraction (RHEED) measurements. GaN-layers, which were fabricated under N-stabilized conditions have a (2×2) surface reconstruction during growth and show p-type conductivity. At room temperature the measured hole concentrations and mobilities are p = 9.7* 1012 cm-3, μp ≅ 350 cm2/Vs, respectively. Temperature dependent measurements of the carrier concentration yield an acceptor activation energy of EA = 0.445 eV. The nature of these defects will be discussed in view of intrinsic defects proposed by theoretical calculations already published in literature. The temperature dependence of the mobility is dominated by polar optical phonon scattering in the investigated temperature range.

We report the observation of UV photoluminescence (PL) from wurtzite GaN by multiphoton excitation. The dependence of the PL intensity on excitation intensity as well as PL excitation measurements with sub-bandgap photon energies indicate the existence of deep defect states centered at about 1.0 eV above the top of the valence band. This result was confirmed by a sum-frequency excitation spectrum. We correlate these measurements with the omnipresent yellow luminescence in GaN. Our two-photon PL excitation spectrum yields a two-photon absorption coefficient that agrees very well with theoretical predictions.

GaN/Al0.08 Ga0.92 N heterostructure has been grown on(0001) 6H-SiC substrates using low pressure metalorganic chemical vapor deposition(LP-MOCVD). Triethylgallium (TEGa), trimethylaluminum(TMA) and ammonia(NH3) were used as the Ga, Al and N sources, respectively. The carrier gas is hydrogen(H2)and the growth pressure is kept at 76 torr. Five pairs of GaN/Al0.08 Ga0.92N(100Å/100Å)were used as buffer layer, then follow by an 1.3 μ m GaN film. The 500Å AlGaN bulk structure was grown on the GaN and Finally an 100Å GaN cap layer to prevent the oxidation of AlGaN layer. The full width half maxima(FWHM) of x-ray peak is 115 arc second for the thick layer of GaN (1.3 μ m), this value is smaller than we reported for the GaN on sapphire substrate (It was about 300 arc second). The Hall mobility and sheet carrier concentration are 887 cm2/Vs, 1.0 × 1016cm-2 at 300K and 4661 cm2/Vs, 7.2 × 1012cm-2 at 77K. This high electron mobility is an indication of a two-dimensional electron gas(2DEG) formed at the GaN/AlGaN interface.

In this paper we employed the TSC method to investigate the traps in GaN. The measured sample was a M-S-M UV-detector of high-resistance GaN on sapphire grown by LP-MOCVD. The relation of dark conductance to temperature clearly showed three major donor levels at 0.019, 0.13 and 0.74eV respectively. TSC measurements from 60 to 380K indicated that there were at least 11 traps in the GaN material. The active energy of those traps were 0.15, 0.19, 0.25, 0.28, 0.33, 0.39, 0.47, 0.55, 0.60, 0.63 and 0.67eV. The range of trap density is from 6 × l014cm-3 to 2 × l018cm-3. By comparing TSC spectrum to dark current, we consider there are at least 4 hole traps in the measured range with energy of 0.25, 0.28, 0.33 and 0.39eV. The illumination time effect was studied and discussed.

Both spontaneous and stimulated emission processes are essential ingredients for constructing a laser from the nitrides. Based on our picosecond time-resolved photoluminescence studies we show that spontaneous radiative recombination is strongly influenced by excitonic effects, both in bulk GaN and in quantum wells. Particularly in quantum wells, localization of excitons plays an important role. We have studied the optical gain spectra in GaInN/GaN and GaN/AlGaN double heterostructures and quantum wells, grown by LP-MOVPE, using the stripe excitation method. Both room temperature and low temperature measurements were performed. Based on our results, we discuss the physical mechanism of optical gain in the nitrides as well as consequences for laser operation. We show that localization or, equivalently, the formation of quantum dot like structures, governs the optical gain mechanism in the nitrides.

Spontaneous emission mechanisms of InGaN single quantum well (SQW) blue and green light emitting diodes (LEDs) and multiquantum well (MQW) laser diode (LD) structures were investigated. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well (QW). The transmission electron micrographs (TEM) indicated fluctuation of In molar fraction in the QWs. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum-confinement Stark effect and band filling of the localized states by excitons.

We measured the emission of (In,Al) GaN films under high intensity optical pumping both in the direction parallel and perpendicular to the film growth. In the edge emission geometry we determine the gain magnitude from the variable stripe length (VSL) method. We use the spontaneous emission collected perpendicular to the layer plane to calculate the spectral dependence of the gain. Theoretical calculations are in good agreement with those experimentally determined gain spectra. We also show that the observalion of a stimulated emission peak perpendicular to the film is predominantly due to scattering of the in-plane stimulated emission but without ruling out contributions from microstructures in the films.