Papers by Keyword: Gallium Nitride (GaN)

Abstract: Gallium trichloride (GaCl₃) and azidotrimethylsilane (CH₃)₃SiN₃ were employed as alternatives gallium and nitrogen precursors respectively in the growth of GaN nanowires via a metal-organic chemical vapor deposition (MOCVD) system. Au pre-deposition on Si (100) substrate was using as catalysis seed to grown of GaN nanowires. X-Ray, FE-SEM and AFM analyses reveal that nanowires grown at temperature 1050 C present morphology characteristic to model VLS. Scanning electron microscopy reveal a surface morphology made up of wurzite that suggests that wires growth involve a melting process. A nucleation and growth mechanism, involving the congruent melting clusters of precursor molecules on the hot substrate surface, is therefore invoked to explain these observations. We attributed the improved growth behavior to the nearer-to-equilibrium growth and may be close to local thermodynamic equilibrium.

Abstract: In this article, GaN thin films were successfully grown on p-type silicon (p-Si) substrates with orientation (100) through spin coating method followed by nitridation in ammonia ambient at various temperatures (750 °C, 850 °C, and 950 °C). The morphology of the GaN thin films were performed by using field-emission scanning electron microscopy. The results showed that the grain size increases with increasing nitridation temperature from 750 °C to 950 °C. Optical analysis of the GaN thin films was performed using Fourier transform infrared spectroscopy. It was confirmed from the results that the reflectance intensity of the transverse optical and longitudinal optical phonon modes of wurtzite GaN increases with increasing nitridation temperature. All the measured results show that nitridation temperature plays a very important role in improving the quality of the GaN thin films. Finally, the results revealed that the 950 °C was the optimal growth nitridation temperature for synthesizing GaN thin film.

Abstract: The decay characteristics of the originally-activated GaN photocathode are explored by testing the change of quantum efficiency (QE).The QE after degradation for 9 hours can still keep more than 42% of the original value, and after reactivated with cesium, the QEs are almost recovered. The restorations of both the band bending and the ideal Cs/O ratio on the surface, along with the double dipole model are proposed to explain the QE variation of the GaN photocathode. Further exploration should include the roles played by the achievement of the maximum band bending of the GaN surface when deposited with Cs and its relationship with the surface Mg doping concentration.

Abstract: We developed heterojunction-based Schottky solar cells consisting of π-conjugated polymers and n-type GaN. PEDOT: PSS was used as the transparent Schottky contact material. In order to improve the performance of solar cells, the effects of surface treatment on the electrical performance of PEDOT: PSS/n-GaN Schottky contacts were investigated. The Voc increased from 0.52V to 0.62V,0.54V and 0.54V and Isc from 0.33 mA/cm² to 0.45mA/cm²,0.40mA/cm² and 0.35mA/cm² after HCl, HF solution and oxygen plasma treatments. The I-V and the XPS measurements indicated that the barrier height of PEDOT:PSS/n-GaN was increased from 0.62eV to 0.76eV, 0.72eV and 0.70eV and the ideality factor improved from 1.81 to 1.63, 1.67 and 1.73 respectively, which induced the variation of the solar cells characteristics..

Abstract: The unique optical properties of nanostructured GaN basically, turn it as a very important part of many electronic and optoelectronic devices such as high power transistors, UV detectors, solar cells, lasers and blue LED. The aim of the current study is GaN nanoparticle deposition at low temperature in preferred direction. In this work, GaN nanoparticles were prepared using direct current plasma enhanced chemical vapor deposition (DC-PECVD) method on Si (100) wafer as a substrate at 700°C. Gallium metal and nitrogen plasma were used as precursors. GaN nanoparticles were grown based on the direct reaction between gallium atoms and excited nitrogen species in the plasma. Structural and morphological characterizations of GaN nanoparticles were carried out using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and field emissions electron microscopy (FE-SEM). Preferred (100) direction of GaN nanostructures which obtained by careful control of processing parameters, were revealed by XRD. FE-SEM images show the average diameter of nanoparticles is 37 nm. The EDS results show the Ga to N ratio in the sample was 8.8 to 1.2 by weight which is very close to the Ga to N ratio of prefect GaN crystal. The deviance is related to the nitrogen vacancy of the sample. These results demonstrate a simple inexpensive method for GaN nanoparticle deposition at low temperature which is critical for many of applications.

Abstract: By measuring the current-voltage (I-V) characteristics in the temperature range of 100 K to 300 K, mechanisms of the forward tunneling current and the reverse leakage current of GaN-based blue light emitting diodes are analyzed. For the forward current, both the temperature-independent current slope and an ideality factor larger than 2 are typical features of the defect-assisted tunneling mechanism. For the reverse leakage current, the linear relationship between I and (V+V_bi)^1/2 indicates a hopping conduction mechanism at low bias, while the power law I-V relationship suggests that the space charge limited current dominates the reverse leakage current at high bias.

Abstract: In this paper, two ku band SSPAs using GaAs and GaN MMICs respectively were developed. Both of all have a same 8-way combining architecture and a same packaging module. The two SSPAs were measured on a same pulse width and duty cycle with different drain voltages. Compared with the GaAs SSPA, the GaN SSPA is more compact and efficient.

Abstract: We have developed a chemical process for atomic planarization of gallium nitride (GaN) using a platinum catalyst and ultraviolet (UV) light irradiation. The process is mediated by a hydrolysis reaction catalyzed by platinum as a solid catalyst. Because the reaction occurs selectively from the step edges, a flat surface composed of a straight step-and-terrace structure is obtained. In the absence of UV light, owing to the low step edge density, the removal rate is quite slow, approximately 1 nm/h. In contrast, under UV light, etch pits are formed on the terraces by photo-electrochemical etching causing an increase in the step edge density. We achieved surface planarization with a removal rate of 9.6 nm/h assisted by irradiation with UV light.

Abstract: The dislocations in GaN thin film with GaN/AlN multilayer (ML) as the buffer layer were evaluated using transmission electron microscopy. A high density of dislocations parallel to the GaN/ML interface and a sudden decrease in the dislocation density at the GaN/ML interface were found. Dislocation propagation in the direction parallel to the GaN/ML interface by turning horizontally on the GaN/ML interface is considered to be effective in decreasing the dislocation density at the top layer of GaN.

Abstract: (0001) GaN bulk crystals with a thickness of 3.4 mm and a density of in-grown dislocations of 3.5·10⁶ cm^-2 have been deformed at room temperature using a Vickers indenter at two different sample orientations in relation to the indenter. Dislocations and cracks at the indentations were investigated by means of optical microscopy and scanning electron microscopy in secondary electron contrast and cathodoluminescence imaging. The arrangement of indentation-induced dislocations and cracks is described and the orientation effect is discussed.