Papers by Keyword: Gallium Nitride (GaN)

Abstract: Abstract. Gallium Nitride (GaN) nano and micro structures were grown on different substrates, such as ceramic boat and alumina plate using thermal evaporation method with commercial GaN powder under the flow of Argon (Ar) gas atmosphere. Micro structural studies by scanning electron microscopy (SEM) revealed the role of different substrates in the nucleation of the GaN nano and micro wires and ribbons. Additional structural and optical characterizations were performed using energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. Results indicated that the nanowires and nanoribbons are of single-crystal hexagonal GaN and are more and less orderly in their growth with different substrates. The quality of growth of the GaN nanowires and nanoribbons for different substrates is highly dependent on the lattice mismatch between the nanowires and their substrates and it also depends on the conditions of the growth.

Abstract: Optical monitoring of diffusivity in wide bandgap semiconductors was performed by using a picosecond light-induced transient grating technique. The bandgap renormalization and carrier-carrier scattering manifested itself at room temperature as two-fold decrease of the ambipolar diffusion coefficient Da in cubic SiC and 5-fold decrease of Da in diamond at excess carrier density N > 10¹⁷ cm^-3, while for GaN the impact was observed only at T 19 cm^-3, the plasma degeneracy led to enhanced D_a values in SiC and GaN and compensated the diffusivity decrease in diamond.

Abstract: One normally-on N-channel AlGaN/GaN device and two types of normally-off GaN devices have been studied. The normally-on device with Sapphire substrate shows good I_dsat and breakdown characteristics, but the gate leakage current is quite large. The first normally-off GaN hybrid metal insulator semiconductor – high electron mobility transistor (MIS-HEMT) grown on Si substrate exhibits good performance with positive threshold voltage of 3V and the breakdown voltage of over 1800V. However the second normally-off GaN MOSFET structure is rather difficult to exhibit good blocking characteristic compared to GaN MIS-HEMT device due to inadequate device design.

Abstract: Seeded growth of gallium nitride (GaN) crystals on a spontaneously nucleated small GaN by the Na flux method was performed. In this study, we attempted to control the growth habit by changing the flux composition (Ga/Na) and by introducing a small amount of additives (Ca and Li). Our experiment clarified that a low Ga composition was preferred to grow high-crystallinity prismatic GaN crystals with a high growth rate. Furthermore, the transparent GaN single crystals with prism shape could be grown by the addition of Ca and Li.

Abstract: In order to understand dislocations in GaN grown on silicon using metal organic chemical vapor deposition (MOCVD), two samples with different film thickness were grown. The distribution of dislocations and its influence on epitaxial layer were studied. Wet etching can effectively reveal the dislocations in GaN, and the hexagonal etch pits are the emergence of dislocations. After etching in the aqueous KOH: H2O =3:20 (m:m), there are more pits with longer etch time. SEM shows that there are more hexagonal etch pits near the micro-cracks because many dislocations are induced by micro-cracks. In the experiment, ribbon slopes in the two sides of the pits are observed, and they are paralleled to each other. If the growth time prolonged, the ribbon slopes would disappear and the surface become flat, and this mechanism is discussed in this paper.

Abstract: In this research we introduce an inexpensive method to produce highly crystalline GaN Nanowires (NWs) grown on porous zinc oxide (PZnO) using commercial GaN powder, either in argon gas or combination of nitrogen and Ar gas atmosphere, by thermal evaporation. Morphological structural studies using transmission electron microscope (TEM) and scanning electron microscopy (SEM) measurements showed the role of porosity and different gas flowing, in the alignment and nucleation of these NWs. The NWs grown under flow of mix gases have very different diameters of between 50 and 200 nm, but those which were grown in Ar gas atmosphere, have rather uniform diameter of around 50 nm. The length of the GaN NWs was uniform, (around 10 µm). Optical and structural characterizations were performed by energy-dispersive X-ray spectroscopy (EDX) and high resolution X-ray diffraction (HR-XRD). Results revealed that these NWs are of single-crystal hexagonal GaN with [oooı] and [ıoīı] growth directions for the NWs grown under Ar and mixed gas flow.

Abstract: In this paper, novel active narrowband notch filters with triple-layer composite nanostructures on a GaN-based LED are obtained by mainly adjusting the grating period and duty cycle. The three layers consist of two dielectric thin layers and one metallic / dielectric grating layer. The grating layer composes of subwavelength period and thickness rectangular stripes, which lies between a transition layer and a protecting layer. Line-width and attenuation peak properties of the resonance filters are calculated and investigated by using a full vector implementation of Rigorous Coupled Wave Analysis (RCWA) algorithm. It is shown that the grating period can significantly change the filter peak wavelength, and the grating duty cycle heavily changes the filter line-width. The filter attenuation peak has a red shift with 23.3nm as the grating period increases 18nm. The FWHM (Full Width at Half Maximum) of the filter reduces from 1.9nm to 0.28nm as the duty cycle changes from 0.55 to 0.3, which compressed more than six times. Moreover, thickness of each composite nanostructure layer can also affect the narrowband width and peak wavelength of the filter. The results provide guidance in designing, optimizing and fabricating such an active narrowband filter with highly integrated photonic devices.

Abstract: The antiphase domain structure in Ni3Al and Al3Ti+Cu intermetallic alloys was recognized by conventional transmission electron microscopy and large angle convergent beam electron diffraction methods. In the case of antiphase boundary the superlattice excess line is split into two lines with equal intensity on bright and dark field LACBED pattern. This splitting can be considered as typical and used to identify APBs. The recognition between perfect structure of the defect-free matrix and the screw deviation around the nanopipes in GaN epilayers was performed with high accuracy using Zone Axis LACBED images.

Abstract: We investigated properties of nanolayers electrophoretically deposited (EPD) onto semiconductor indium phosphide (InP) or gallium nitride (GaN) single crystals from colloid solutions of metal palladium (Pd), platinum (Pt) or bimetallic Pd/Pt nanoparticles (NPs) in isooctane. Colloids with metal NPs were prepared by reaction of metal compounds with the reducing agent hydrazine in water confined to reverse micelles of surfactant AOT.. Chopped DC electric voltage was applied for the time period to deposit metal NPs, only partly covering surface of the wafer. The deposits were image-observed by scanning electron microscopy (SEM)..Diodes with porous Schottky contacts were made by printing colloidal graphite on the NPs deposited surface and making ohmic contact on the blank side of the wafer. The diodes showed current-voltage characteristics of excellent rectification ratio and barrier height values close to Schottky-Mott limit, which was an evidence of negligible Fermi level pinning. Large increase of current was observed after switching on a flow of gas blend hydrogen in nitrogen (H₂/N₂). The diodes were measured with various H₂/N₂ in the range from 1000 ppm to 1 ppm of H₂. Current change ratios about 10⁶ and about 10 were achieved with 1000 ppm and 1 ppm H₂/N₂.

Abstract: New MOCVD grown UV (ultra-violet) LEDs using low dislocation density GaN buffer layers on sapphire have been studied. Two different LED characteristics of GaN substrates, i.e. 5um-thick and 20um-thick buffer layers, on sapphire are compared with each other. The enhanced LED characteristics show ~29.5% reduction in current-voltage resistance, ~8.5% reduction in turn-on voltage and output power saturation at higher current. Better GaN buffer quality and heat dissipation due to the lower defect density are believed to the enhanced reason.