We present a study of the growth of high quality GaN films on Si(111) substrates by MetalOrganic Chemical Vapor Deposition technique. In order to improve the quality of the epitaxial films we introduced different nucleation or buffer layers and combinations of them. Our results obtained on an optimized AlN nucleation layer will serve as reference point. In order to improve the quality of the epitaxial films we introduced different combinations of nucleation and intermediate layers. The first combination consists of an optimized AlN nucleation layer followed by a 1 m-thick GaN film, on which we deposited SixNy/GaN intermediate layers. Based on the optimized AlN nucleation layer, we introduced AlGaN/GaN superlattices or AlN intermediate buffer layers. Additionally, we present results on the modification the Si(111) surface with NH3 to promote nucleation from selective GaN islands. In all experiments the total thickness of the GaN epilayers was 3 m m. X-ray diffraction, photoluminescence, Hall measurements and atomic force microscopy were used in order to elucidate the effectiveness of these growth processes. For the most successful deposition scheme, the one with the SixNy/GaN intermediate layers, the resulting GaN layers are of high quality as compared to the other methods. The donor bound exciton, which dominates the photoluminescence spectrum, showed a full width at half maximum (FWHM) of about 50 meV at room temperature and 10 meV at 4K. The FWHM of the symmetric (0002) rocking curves in w-scan is about 640 arcsec. The root-mean-square roughness, as measured by atomic force microscopy, does not exceed 10 Å.

GaNAs and GaInNAs growths are subjects of considerable interest due to its technological importance in long wavelength lasers emitting within the optical-fiber communication wavelength window (1.31 – 1.55 m m). We study GaNAs and GaInNAs materials growth on (100) semi-insulating GaAs substrate with high nitrogen compositions (>2%) using a solid source molecular beam epitaxy (SSMBE) system in conjunction with a RF plasma source. GaNAs layer with high nitrogen compositions of 4.85% and 6% with good XRD peak intensities were successfully grown. GaInNAs quantum wells (QWs) were then grown with reference to the nitrogen compositions measured in the GaNAs materials to obtain nitrogen compositions > 2%. The photoluminescence (PL) peak positions of the GaInNAs QWs blueshifted after annealing at 840°C and 10min. It was found that the blueshift of PL peaks are highly dependent on nitrogen compositions.

We investigated the behavior of photoluminescence (PL) in a wide range of temperatures and excitation intensities from Zn-doped GaN grown on c-plane of sapphire by hydride vapor phase epitaxy (HVPE). The low-temperature PL spectrum exhibited sharp peaks in the excitonic part with the full width at half maximum (FWHM) of about 5 meV. The peaks were attributed to free excitons and excitons bound to shallow donors and to Zn-related acceptors. A blue band with a maximum at 2.9 eV and FWHM of 0.33 eV dominated in the PL spectrum at low temperatures. We observed a significant UV shift (about 50 meV) of this band when the excitation density was increased from 10-6 to 100 W/cm2 or when the temperature of the sample was increased from 15 to 300 K. With increasing temperature, the blue band intensity quenched by 104 times in the range 180 - 250 K with the activation energy of about 0.64 eV. When the sample was illuminated from the substrate side, the blue band intensity dropped only 30 times in the temperature range of 180 - 340 K, and the activation energy did not exceed 0.25 eV. The PL spectrum of the GaN:Zn samples under investigation also contained broad red and green bands with maxima at about 1.8 and 2.4 eV, respectively. In PL from the substrate side, the blue and yellow (2.2 eV) luminescence dominated in the defect-related region of PL spectrum. Time-resolved PL measurements revealed nonexponential decay of the defect-related bands.

We systematically investigated the optimum growth condition of wide-bandgap quaternary InAlGaN for ultraviolet (UV) light-emitting diodes (LEDs) grown on SiC by metal organic vapor phase epitaxy (MOVPE). We obtained intense UV emission in the wavelength of 315-370 nm from quaternary InAlGaN, which was as high as blue emission from InGaN. We found that the optimized growth temperature of quaternary InAlGaN is shifted from 790 to 870 °C as the emission peak wavelength is changed from 370-315 nm. The strongest PL was obtained with the wavelength around 330 nm. The growth condition for intense PL is found to be decided by two factors, i.e., the carrier confinement in In segregation region and crystalline quality. We also found that the small In mole fraction of 2-5 % is enough to obtain maximally PL intensity for InAlGaN, which is much smaller value than that for InGaN, due to the large carrier confinement with small In fluctuation.

The Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.

A comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation.

We developed the MOCVD apparatus equipped with RHEED system, which enable us to observe in-situ and real time RHEED for GaN film growth in ~100mTorr of pressure. We attempted to grow GaN film with this MOCVD chamber in 100mTorr. The in-situ RHEED was subsequently observed along the film deposition process in order to understand both the role of buffer layer and the mechanism of GaN film growth by MOCVD on highly lattice-mismatched substrate like sapphire. The results indicate that oxygen removed from the sapphire surface was observed during its cleaning in H2 flow at 1100°C. The dependence of re-crystallization and evaporation of the buffer layer on the annealing ambient was also detected. Although the nitrogen was slightly deficient, HT-GaN film with smooth surface was obtained in 100mTorr by adding H2 gas and reducing total flow rate. In preliminary deposition, the RHEED oscillation-like was observed in MOCVD-GaN growth. Thus, our developing deposition system has a potential to understand the growth mechanism with atomic level.

The consequences of the anisotropic resistance in AlGaN/GaN strained layer superlattices for the operation of laser diodes were studied for structures driven in LED mode. A series of laser structures containing different Mg-doped AlGaN bulk and AlGaN/GaN strained layer superlattice cladding layers was compared to estimate the current spreading in the cladding due to the formation of 2D-hole gases. Current-voltage measurements revealed a significant spread of the current path in the superlattices, whereas no spreading was seen for the bulk cladding. In contrast to this, investigations of the electroluminescence showed no significant differences.

AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors (DBRs) with up to 45 periods have been grown on (0001) sapphire substrates by r.f. plasma-assisted molecular beam epitaxy (PAMBE) with the aid of two Al effusion cells. Several samples were grown with an Al mole fraction varying between 0.38 ≤ x ≤ 1 (0 ≤ y ≤ 0.4) at temperatures of up to 890°C. In all samples, an AlxGa1-xN buffer layer was used to prevent cracking of the quarter wave stack and improving surface morphology by choosing the Al content so that strain energy in the DBR structure would be compensated.

X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations were performed to determine the thickness of the quarter wave layer periods and the Al mole fraction of corresponding AlxGa1-xN single layers.

Room-temperature calibrated reflection and transmission (R&T) measurements were performed. Thus stray and self-absorption of the DBRs were extracted from reflectance and transmittance. The thickness of the quarter wave layers was designed such that the measured peak reflectances appeared between 346 nm to 421 nm. The dispersion data, including refractive indices and absorption coefficients, used in the calculation were extracted from R&T measurements done on the above mentioned AlxGa1-xN single layers.

A systematic investigation of the damage accumulation in GaN films induced by 150keV Ar ions as a function of implantation temperature and dose rate has been conducted. The depth distribution of the disorder in the Ga sublattice has been measured by RBS/channeling spectrometry with a glancing angle detector geometry to provide enhanced depth resolution. Two disordered regions were identified in the damage depth distribution: a near-surface peak and a bulk damage peak. These regions exhibit different behavior as a function of implantation temperature. The height of the near-surface peak, as expected, decreased with implantation temperature approaching the magnitude of the unimplanted sample. The displaced atomic density in the bulk damage peak also followed this tendency and decreased in the temperature range from room temperature to 500°C. The implantation at higher temperatures resulted in an increase in the amount of damage, reaching a maximum at about 700°C and displayed a characteristic “reverse annealing” behavior. Further increase of the implantation temperature to 1000oC reduced the disorder. The influence of the dose rate and implantation temperature on the radiation damage accumulation is discussed.

The effect of energetic nitrogen ion bombardment during growth may have a deleterious effect on the material quality. To avoid the bombardment effect of energetic nitrogen ions, a modified mode for GaAsN growth using dispersive nitrogen is reported. High quality GaAsN epilayers and good GaAsN/GaAs interface were achieved using this growth mode. The results suggest that the surface of samples grown using dispersive nitrogen has fewer defects than those grown using direct nitrogen beam. The optical property of GaAsN samples grown using the dispersive nitrogen technique was found to improve, due to the lower ion bombardment effect. This growth technique is expected to be advantageous for growing high quality GaAsN materials for optoelectronic applications.

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.

Properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agrrement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ΔEc=-0.4 eV andΔEv=0.6 eV. Whenchanging GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ΔEc=0.2 eV andΔEv=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.

Magnetotransport properties of Al0.22Ga0.78N/GaN modulation-doped heterostructures have been studied at low temperatures and high magnetic fields. The inter-subband scattering of the two-dimensional electron gas was observed. The inter-subband scattering is very weak and depends weakly on temperature when temperature is between 1.3 K and 10 K and becomes stronger with increasing temperature when temperature is higher than 10 K. The strain relaxation of the Al0.22Ga0.78N layer influences the inter-subband scattering. It is suggested that the inter-subband scattering is dominant by the elastic scattering when temperature is lower than 10 K, and changes to be dominant by the inelastic scattering of the acoustic phonons when temperature is higher than 10 K.

Optical and electrical properties of 340 nm-band bright UV-light emitting diodes (LEDs) were compared between In0.03Al0.20Ga0.77N and other active regions. Single peaked and high efficiency ultraviolet (UV) emission at 345 nm from In0.03Al0.20Ga0.77N LEDs is achieved under continuous current injection conditions. Any significant broadening and peak shift of the electroluminescence (EL) spectrum were not observed. We attribute such a good properties to using of high quality InAlGaN-quaternary active layer. The EL intensity of the InAlGaN quaternary-based LED showed more one order of magnitude higher intensity than that of AlGaN and GaN based LEDs. Additionally, The I-L characteristics of the InAlGaN quaternary-based LED showed a linearly increasing of intensity with increasing of injection current density, that is also observed for InGaN based LEDs. This shows the recombination efficiency of the InAlGaN-based LED is as much as that of InGaN based LEDs. From these results InAlGaN quaternary is expected to be a promising material for UV LEDs and LDs.

The electronic structure of dislocations in GaN is controversial. Several experimental techniques such as carrier mobility studies and cathodoluminescence experiments have indicated that dislocations are charged while theoretical studies point to intrinsic states and/or point defect accumulation along the core as a source of electrical activity. Electron Energy Loss Spectroscopy (EELS) studies have the ability to probe the electronic structure of extended defects. Here we report rst principles calculations of the EELS spectrum applied to edge dislocations in GaN. It is found that the electrostatic potential at N atoms in the vicinity of the dislocation varies by the order of a volt and casts doubt on any simple interpretation of core loss spectroscopy. On the other hand, low loss spectroscopy leads directly to detailed information about any gap states. The low loss spectrum obtained by the theory is in good agreement with recent experimental work and indicates that threading dislocations in p-type GaN possess acceptor levels in the upper half of the gap.

Piezoelectric properties of wurtzite AlN and GaN/AlN are investigated using scanning force microscopy (SFM). The magnitude of the effective longitudinal piezoelectric constant d33 of AlN and GaN/AlN thin films are measured and reported, and the d33 coefficients of these films are verified using an interferometric technique. Simultaneous imaging of the topography, and of the phase and magnitude of the piezoelectric strain is performed. Using a GaN film with patterned polarities, we demonstrate that polarity can be inferred from the phase image of the piezoelectric strain. We report d33=3±1 pm/V for AlN/SiC and 2 1 pm/V for GaN/AlN/SiC. Films grown by organo-metallic vapor phase epitaxy (OMVPE) on SiC, sputtered AlN films and films grown by molecular beam epitaxy (MBE) are characterized and compared.

We compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low density of inversion domains (1x107 cm-2). For N-polar GaN the situation is the reverse: the density of dislocations and inversion domains are 5x108 and ~1x1011 cm-2, respectively. One of the important conclusions derived from the combined PL and TEM study is that inversion domains do not seem to affect the radiative efficiency very adversly, whereas dislocations reduce it significantly.

A novel type of low-resistance ohmic contacts is demonstrated utilizing polarization-induced electric fields in thin p-type InGaN layers on p-type GaN. An increase of the hole tunneling probability through the barrier and a concomitant significant decrease of the specific contact resistance can be attributed to a reduction of the tunneling barrier width in the InGaN capping layers due to the polarization-induced electric fields. The specific contact resistance of Ni (10 nm) / Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2 × 10-2 Ω cm2 and 6 × 10-3 & cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the Fermi level position within the band gap for thin metal overlayers of Au, Ni, and Ti on n-GaN and p-GaN. The Fermi level position was determined with the measured Fermi edge of the metal on the sample in order to correct for the presence of non-equilibrium effects. There are two different behaviors observed for the three metals studied. For Au and Ti, the surface Fermi positions on n-GaN and p-GaN are roughly 0.5 eV apart within the band gap. For Ni, the n-GaN and p-GaN have a Schottky barrier that forms at the same place at the gap.