Papers by Keyword: Gallium Nitride (GaN)

Abstract: We report on simple techniques for extracting the electrical properties of 1-dimensional semiconductor nanowires using standard ultraviolet (UV) photo-lithography instead of e-beam lithography (EBL), which is a commonly used technique for the fabrication of nanoscale electrical devices. For electrical transport measurement the gallium nitride nanowires (GaN NWs) were prepared by a horizontal hot-wall chemical vapor deposition (CVD) with metallic Ga and NH3 gas for Ga and N sources, and GaN nanowire field effect transistor (FET) structures on a 8×8 mm2 silicon wafer were fabricated by ordinary 2-mask photo-lithography processes. The estimated carrier mobility from the gate-modulation characteristics is on the order of 60 ∼ 70 cm2/V⋅s. We found that our approach is a powerful and simple technique to extract the electrical properties of semiconductor nanowires. The material characteristics of GaN nanowires are also discussed.

Abstract: Silicon carbide (SiC) has become the substrate of choice for III-N epilayers applied to electronic devices due to the lack of a native III-N substrate. This is particularly true for high power applications, since the thermal conductivity of the substrate enhances device performance. Although the GaN lattice match is slightly better for SiC than for sapphire, the dislocation densities that result are still very high (generally in the high 108 cm-2 range) and often deleterious to device performance. Screw-component dislocations are especially critical since they serve as leakage paths in vertically conducting III-N devices. In this paper efforts to reduce the extended defect density in III-N films grown on SiC will be reviewed. Details on recent efforts to use step-free SiC mesa surfaces arrayed on commercial 4HSiC substrates will then be highlighted showing dramatic reductions in extended defect densities and the virtual elimination of critical defects for vertically conducting devices. In these experiments, SiC surfaces that are homoepitaxially grown step-free or of very low step density have been used as growth templates for thin (<3 μm) GaN films deposited on a novel 1000 Å AlN nucleation layer characterized by a total dislocation density two orders of magnitude lower than the previous state-of-the-art, and with no evidence of screw-component dislocations.

Abstract: Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.

Abstract: Amorphous/pseudoamorphous GaN was prepared by pulsed-laser ablation at room temperature without any heat treatment. The structure and chemical composition of the specimens were systematically investigated. Laser ablation at low Ar pressure (<50Pa) led to deposition of smooth Ga-rich films, which is independent with laser energy. Under same pressures, as laser energy increased, the film stoichiometry changed from Ga-rich to near stoichiometric composition. Varying background Ar pressure strongly affected the product structure showing little effect on the chemical composition. Under higher pressure than 100 Pa, fine nanoparticles with a size of 5 nm rather than films were deposited on substrate due to the increased collision by plume confining. The optical band-gap of the deposited a-GaN is 2.8 eV for thin films and 3.9 eV for nanoparticles.

Abstract: This study shows the effect of thermal annealing on GaN/ZnO/Si structures prepared by rf magnetron sputtering. Thermal annealing tended to induce a different crystalline orientation from the c-axis orientation observed with as-deposited films. The sample annealed at 900 oC under excitation at 325 nm showed two emission bands centered at approximately 380 and 550 nm.

Abstract: A comparative study of the structural and electrical properties of GaN films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) at 700°C, with and without AlN buffer layer is presented . Hydrogen plasma was used in addition to nitrogen plasma to produce GaN. The introduction of H-plasma is found to influence the properties of the films. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall Effect measurements show that the sample with AlN buffer layer possesses a smoother and more homogenous morphological characteristics as well as a lower background electron and higher Hall mobility as compared to the sample without buffer layer. X-ray diffraction (XRD) reveals that hydrogenation is capable of producing the epitaxial GaN films at reduced temperatures with the full width at half maximum (FWHM) of the x-ray rocking (XRC) of GaN (0002) reflection was found to be 54.8 and 256 arcmin for samples with buffer layer and without buffer layer, respectively.

Abstract: The performance of InGaN quantum well based Light Emitting Diodes; (LEDs) had been numerically investigated by using standard industrial software, Silvaco. In this work, we found that InGaN single quantum well (SQW) LEDs gives better performance than InGaN triple quantum wells LEDs. The simulation results suggest that the inhomogeneity of electron and hole distributions in quantum wells active region plays an important role in the LEDs performance. The threshold current per μm also increases as the number of quantum well is increased.

Abstract: This paper presents an analysis of the characteristics of two gallium nitride (GaN) films grown on (0001) plane sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD) is presented. The GaN films were characterized by a variety of methods, including scanning electron microscopy (SEM), x-ray diffraction (XRD), photoluminescence (PL), and Raman scattering. SEM micrographs revealed that different growth conditions will lead to different surface morphology of the films. XRD measurements indicated that both films were highly oriented and mono crystalline. PL spectra for both samples exhibited an intense and sharp band edge peak at 3.42 eV with full width at half maximum (FWHM) of 15 and 35 meV respectively. Raman scattering showed that the peaks of E2(high) phonon mode were observed at 568.1 and 570.1 cm-1 respectively. The different growth mode of these films were linked to the growth conditions, in which the growth mechanism could be correlated with the shift of E2(high) phonon mode in Raman scattering.

Abstract: The Current-Voltage-Temperature (I-V-T) characteristics of single layer deposition, consisting of Zr, Ti, or Cr/p-GaN Schottky diodes were determined in the temperature range 27- 100oC. Sputtering method was used for deposition of these metals on p-GaN. Analysis of the measured characteristics at room temperature allows the determination of the electrical parameters, the saturation current Io and the ideality factorη. The barrier heights and effective Richardson coefficients were determined through activation energy plot. It was found that pinning of Fermi level occurred for these metal contacts on p-GaN with the carrier concentration of 5.6x 1017 cm-3, where the Schottky barrier heights of Zr, Ti, or Cr/p-GaN are determined to be in the same range (~0.87eV).

Abstract: In this paper, we report on the characterization of a set of MOCVD grown GaN samples with a variety of structural or crystalline quality. X-ray diffraction (XRD) was used to observe the change of the crystalline structure with deposition temperature. All results show that the structure type of the GaN deposited films is sensitive to the growth temperature. Our results also revealed that a good crystalline structure of GaN films could be grown at temperatures higher than 600°C. Finally, a general picture on the correlations between the growth temperature and the GaN deposited films crystalline is reported.