Innovative Developments in GaN-based Technology

GaN-based materials have been intensively investigated in recent years because of their potential applications for optoelectronic devices operating in the short wavelength spectral range and in high power, and high temperature electronic devices. The III-nitride semiconductors form a continuous alloy system with direct band gaps ranging from 0.7 eV for InN, to 6.2 eV for AlN with 3.4 eV for GaN. This energy range is suitable for band-to-band light generation with colors ranging from infra-red to ultraviolet (UV) wavelengths. GaN-based materials are also ideal for the fabrication of high responsivity and visible blind UV detectors because of their unique properties such as wide and direct band gap, high absorption coefficients, and sharper cutoff of the wavelength detection. The high breakdown voltage of these materials makes them ideal for high power applications and their high saturation velocity is for high speed device operation. Applications include power amplifiers for wireless base stations, low noise amplifiers, and high power switches. The large band gap means that the performance of GaN transistors is maintained up to higher temperatures than silicon transistors. Sensing devices are another important application of nitride materials especially for harsh environments as these materials are thermally and chemically stable. Intensive material studies and simulation work have been carried out on III-nitride semiconductors (GaN and related alloys). Amorphous, microcrystalline, nanocrystalline, and epitaxial III-nitrides have been investigated for a wide variety of applications. Novel growth technology, processing techniques, treatments and optimization of device fabrication parameters have been carried out to enhance material quality and device performance. High quality III-nitrides thin films were successfully grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si substrates. LEDs, photodetectors (light sensors), MOS capacitors, pressure sensors, and gas sensors based on these materials have been fabricated and characterized in our lab. Alongside the experimental work, theoretical work has also been carried out to investigate the performance characteristics of GaN-based light emitting diode (LED), multi-quantum well (MQW) laser structures, and vertical cavity surface emitting laser (VCSEL). Besides that, study on transparent conducting oxides as electrodes for optoelectronic devices has also been carried out to enhance the performance of LED. Porous semiconductor materials have attracted special interest since the discovery of strong photoluminescence in porous Si. Another attractive application of porous semiconductor materials is epitaxial growth, as they retain the crystalline structure of the original material and yet is relaxed and can serve as a template for heteroepitaxial growth of lattice-mismatch materials. Nanostructured porous GaN, ternary, and quaternary III-nitrides (InGaN and InAlGaN) with different mole fractions have been fabricated by three different techniques, namely, metal-assisted UV electroless etching, laser induced etching, and DC as well as novel AC-assisted photo-electrochemical etching. Sensors fabricated from these films have showed significant improvement of performance. Novel growth technique such as low temperature microwave-assisted chemical bath deposition (MA-CBD) has been employed to grow ZnO/GaN heterojunction LED. Also, a novel and low cost "NORLED" (NanO-Rod LED) device which emits in the UV region was fabricated using ultra long ZnO nanorods that were grown on the p-GaN substrate, by a direct heat substrate-modified chemical bath deposition (DHSMCBD) growth process. These methods seem to be promising for fabricating p–n heterojunction nanorod devices. The UV emission is expected to provide high energy to excite phosphor to produce white LEDs for solid state lighting applications. The prospects for III-nitrides are enormous, and these materials are expected to dominate the optoelectronics and electronics market in the years to come

Fabrication and Characterization of Copper Doped Zinc Oxide on p-type and ntype Gallium Nitride by Sputtering

Zinc oxide (ZnO) is a wide band gap semiconductor (3.3 7ev) with promising appli cations in optoelectroni c devices and flat panel display. Copper (Cu) doping changes the properti es of ZnO. Pure and Cu doped ZnO (CZO) were deposited on p-type and 11-type ga llium nitride (GaN) using radio fi·equency (RF) magnetron sputtering of Cu!ZnO alloy target with ratio I 0/90 at room temperature. The effect of different deposition thickness of the thin film on the crysta l structure, surface morphology and surface roughnesswere investigated intensively using X-ray diffraction (XRD), field em ission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) andatomic force microscopy (AFM)respectively. Results indicate th at the films with hexago nal wurtzite structure and preferential c ax is orientation were fabricated. Crystalli zation of thin film is dependent on the deposition thickness. As the thickness of the film increases, the grain size increases and surface roughness decreases

Improved Optoelectronic Characteristics of Post-Annealed Ti/Al/ITO Transparent Conducting Electrodes Deposited on n-GaN

We report on the improved structural. electrical and optical properties of the Ti/AI/ITO transparent conducting electrodes (TCEs) deposited on n-GaN. The TCEs were deposited by RF/DC magnetron sputtering under Ar ambient at room temperature. The as-deposited TCEs were annealed at 600 oc in N2 ambient for 15 min. The structural, electrical and optical properties of the TCEs were characterized by 2-Theta X-ray diffraction (XRD), atomic force microscope (AFM). Hall effects, 1-V and UV-visible measurement system. From XRD measurement, significant ITO (222) and (411) peaks were observed after the sample was post-annealed at 600 oc. Morphological analysis by AFM shows that surface roughness Rq of the post-annealed sample is smoother as compared to the as-deposited sample. The electrical resistivity of the TCEs layer decreases to 8.607x10-5n-cm after the post-annealing process. Further analysis on the 1-V characteristics reveals that the post-annealed samples have better Ohmicbehavior than the as-deposited sample. The post-annealed sample shows high optical transmittance characteristics in visible spectrum of -95%. The figure of merit (FOM) of the as deposited and post-annealed samples are 2.39x1 0-40-1 and 5.91 x1 o-2Q-1, respectively. Therefore, the post-annealed TCEs show the best electrical and optical quality due to the improved structural and morphological characteristics

Influence of Annealing Temperature on InN Thin Films Grown by RF Magnetron Sputtering

This paper presents the study and characterization of indium nitride (InN) films grown on quartz glass and p-Si (111) substrates by RF magnetron sputtering method using pure indium target in argon (Ar) and nitrogen (N 2) environment. The characterization was carried out by high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). XRD results show the growth of polycrystalline wurtzite films with varying peak intensities. The deposited films were annealed in nitrogen environment at different temperatures ranging from 1 00°C to 400°C. The annealing was carried out for four hours and the results were compared with pre-annealing samples

Improvement of Porous GaAs (100) Structure through Electrochemical Etching Based on DMF Solution

We report on the fabrication of porous GaAs (100) using three different acids, H2SO4, HF, and HCl, diluted in DMF based solutions. The mixture of H2SO4 with DMF showed the best porous structures in comparison to other acids. The concentration of the DMF solution was then varied for a fixed concentration of H2SO4. It was apparent that the different concentration of the DMF solvent gave different types of morphology of the porous GaAs. Furthermore, a higher current density improved the uniformity of the pores distribution. The best porous GaAs exhibited well-defined circular shaped pores with high uniformity. To the best of our knowledge, such structure produced in such manner has never been reported so far. Finally, the optimum etching conditions of the pores were proposed

Effect of deposition time on the characteristics of nanocrystalline cds thin films and photodetection properties

Nanocrystalline CdS thin films were grown on silicon substrates using microwave-assisted chemical bath deposition. Aqueous solutions of cadmium chloride (CdCl2) and thiourea [SC(NH2)2], which served as cadmium Cd2+ and sulfur S2– ions sources, respectively were used to synthesize the nanocrystalline CdS thin films. Morphological, structural, and optical analyses revealed that the deposition time have significantly influenced the properties of the prepared thin films. CdS thin film prepared at a deposition time of 20 min showed the optimum characteristics. The photoluminescence measurements showed enhanced structural quality as suggested by the reduction in the intensity of the defect-related emissions. Prolonging the deposition time beyond 20 min does not improve the properties of the grown thin films. Metal-semiconductor-metal photodetectors were fabricated based on the synthesized CdS thin films. Current–voltage measurements showed superior characteristics of the fabricated photodetectors based on CdS thin films grown at deposition times of 10 and 20 min compared to those based on CdS thin film grown at a deposition time of 30 min. Photodetector based on CdS thin film grown at a deposition time of 20 min exhibited enhanced photodetection properties. The device showed the fastest response time of 9 ms with a photosensitivity of 1440% to 500 nm chopped light at an applied bias of –1 V. Whereas, photodetector based on CdS thin film grown at a deposition time of 30 min showed the longest response and recovery times of 35 and 42 ms, respectively with a photosensitivity of 51.70%. Photodetection measurements revealed that schottky contact based devices have a superior performance than those with ohmic contact-based devices

ZnO Nanostructures Assisted Growth By Different NH4F Concentrations For Potential Photovoltaic Applications

In this study, different morphologies of ZnO nanostructures (NSs) were synthesized by adding ammonium fluoride (NH4F) at different concentrations using the chemical bath deposition (CBD) method for exploration of photovoltaic applications. Morphology varies from well aligned hexagonal nanorods (NRs), to nanotetrods (NTs) and nanoflowers (NFs) as observed and revealed by field emission scanning microscopy (FESEM). X-ray diffraction spectrum (XRD) analysis confirmed good crystal quality of ZnO nanostructures (NSs) along (002). Ultraviolet-visible (UV-Vis) analysis confirms strong reflection suppression in the ultraviolet-near infrared (UV-NIR) range, giving a high green signal for solar cell applications. Finally, photoluminescence (PL) emission has shown both strong near band edge (NBE) and deep level emission (DLE) peaks indicating a promising signal for different applications such as for photo sensors and photo catalytic

Effect Of Annealing Temperature On Cerium Oxide Thin Films Grown By DC Sputtering Method

The cerium thin films were deposited on n-type Si (100) substrate by direct current (DC) sputtering followed by post-annealing at different temperature (400ᵒC and 600ᵒC, 800ᵒC, 1000ᵒC) in an oxygen ambient. In this study, the effect of annealing temperature on the crystallized CeO2 thin films was characterized by using grazing incidence X-ray diffraction (GIXRD). The surface topology and surface morphology of the CeO2 were analyzed by using atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM). The energy bandgap was calculated from the ultraviolet-visible spectroscopy (UV-Vis) measurement. GIXRD result shows (111) plane has the highest peak intensity, therefore (111) plane was selected as the preferred orientation for CeO2 thin films. AFM results reveal the root-mean-square (RMS) roughness of the CeO2 thin films decreased as annealing temperature increased from 400ᵒC to 1000ᵒ

Effect Of Annealing Temperature On Cerium Oxide Thin Films Grown By Dc Sputtering Method

The cerium thin films were deposited on n-type Si (100) substrate by direct current (DC) sputtering followed by post-annealing at different temperature (400ᵒ C and 600ᵒ C, 800ᵒ C, 1000ᵒ C) in an oxygen ambient. In this study, the effect of annealing temperature on the crystallized CeO2 thin films was characterized by using grazing incidence X-ray diffraction (GIXRD). The surface topology and surface morphology of the CeO2 were analyzed by using atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM). The energy bandgap was calculated from the ultraviolet-visible spectroscopy (UV-Vis) measurement. GIXRD result shows (111) plane has the highest peak intensity, therefore (111) plane was selected as the preferred orientation for CeO2 thin films. AFM results reveal the root-mean-square (RMS) roughness of the CeO2 thin films decreased as annealing temperature increased from 400ᵒC to 1000ᵒC