Ray-Hua Horng

High-power GaN-mirror-Cu light-emitting diodes for vertical current injection using laser liftoff and electroplating techniques

A large-area (1 /spl times/ 1 mm) vertical conductive GaN-mirror-Cu light-emitting diode (LED) fabricated using the laser liftoff and electroplating techniques is demonstrated. Selective p-GaN top area was first electroplated by the thick copper film, and then an excimer laser was employed to separate the GaN thin film from the sapphire substrate. The luminance intensity of the vertical conductive p-side-down GaN-mirror-Cu LED presented about 2.7 times in magnitude as compared with that of the original GaN-sapphire LED (at 20 mA). The light output power for the GaN-mirror-Cu LED was about twofold stronger (at 500 mA). A more stable peak wavelength shift under high current injection was also observed.

Improvements in the heteroepitaxy of GaAs on Si by incorporating a ZnSe buffer layer

Heteroepitaxy of GaAs on Si with a ZnSe interlayer by low‐pressure metalorganic chemical vapor deposition is reported. The structural and electrical properties of the GaAs epilayers grown on ZnSe/Si substrates were found to be superior to those of the GaAs directly on Si. The surface dislocation density of the GaAs/ZnSe/Si film can be reduced to 2×105 cm−2, which is one order of magnitude lower than that of GaAs/Si. The planar Schottky diode fabricated on the GaAs/ZnSe/Si sample shows a reverse breakdown voltage as high as 30 V, whereas the diode on GaAs/Si has a breakdown voltage of about 12 V. In addition, the residual stress in the GaAs heteroepilayers calculated from photoluminescence peak shifts was 8.2×108 dyn/cm2 for the GaAs/ZnSe/Si structure, as compared to 2.7×109 dyn/cm2 for the GaAs directly on Si. This indicates that the ZnSe intermediate layer is also effective in reducing the residual stress in the GaAs film grown on Si.

A research on the persistent photoconductivity behavior of GaN thin films deposited by r.f. magnetron sputtering

Abstract The persistent photoconductivity (PPC) behavior has been characterized in sputtered GaN thin films using the room illumination with a wavelength of 254 nm under a 5-V bias. It was found that the N 2 partial pressure in the sputtering atmosphere has an evident effect on the PPC behavior. The obtained data show that the nitrogen vacancy is the candidate for PPC effect in the sputtered GaN film. At lower N 2 partial pressures, the nitrogen vacancy can be induced and resulted in GaN films with more donor levels as compared with those of samples deposited at higher N 2 contents. An energy band model that can account for the experimental observation of PPC behavior is proposed.

Tunability of p- and n-channel TiOx thin film transistors

To acquire device-quality TiOx films usually needs high-temperature growth or additional post-thermal treatment. However, both processes make it very difficult to form the p-type TiOx even under oxygen-poor growth condition. With the aid of high energy generated by high power impulse magnetron sputtering (HIPIMS), a highly stable p-type TiOx film with good quality can be achieved. In this research, by varying the oxygen flow rate, p-type γ-TiO and n-type TiO2 films were both prepared by HIPIMS. Furthermore, p- and n-type thin film transistors employing γ-TiO and TiO2 as channel layers possess the field-effect carrier mobilities of 0.2 and 0.7 cm2/Vs, while their on/off current ratios are 1.7 × 104 and 2.5 × 105, respectively. The first presented p-type γ-TiO TFT is a major breakthrough for fabricating the TiOx-based p-n combinational devices. Additionally, our work also confirms HIPIMS offers the possibility of growing both p- and n-type conductive oxides, significantly expanding the practical usage of this technique.

Deep-Ultraviolet Photodetectors Based on Epitaxial ZnGa 2 O 4 Thin Films

A single-crystalline ZnGa2O4 epilayer was successfully grown on c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition. This epilayer was used as a ternary oxide semiconductor for application in high-performance metal–semiconductor–metal photoconductive deep-ultraviolet (DUV) photodetectors (PDs). At a bias of 5 V, the annealed ZnGa2O4 PDs showed better performance with a considerably low dark current of 1 pA, a responsivity of 86.3 A/W, cut-off wavelength of 280 nm, and a high DUV-to-visible discrimination ratio of approximately 107 upon exposure to 230 nm DUV illumination than that of as-grown ZnGa2O4 PDs. The as-grown PDs presented a dark current of 0.5 mA, a responsivity of 2782 A/W at 230 nm, and a photo-to-dark current contrast ratio of approximately one order. The rise time of annealed PDs was 0.5 s, and the relatively quick decay time was 0.7 s. The present results demonstrate that annealing process can reduce the oxygen vacancy defects and be potentially applied in ZnGa2O4 film-based DUV PD devices, which have been rarely reported in previous studies.

AlGaInP Red LEDs with Hollow Hemispherical Polystyrene Arrays

A hollow hemispherical polystyrene (HHPS) was fabricated to reduce total internal reflection in AlGaInP-based LEDs. At an injection current of 350 mA, the external quantum efficiencies of LED-I, LED-II, LED-III, and LED-IV are 20.92%, 24.65%, 27.28%, and 33.77% and the wall-plug efficiencies are 17.11%, 20%, 22.5%, and 27.33%, respectively. The enhanced performance is attributed to the light output power enhancement through the surface roughness, microlens-liked PS hemisphere, and scatter-liked HHPS array. In this paper, the rigorous coupled wave analysis (RCWA) numerical method was also conducted to demonstrate the HHPS array effectively enlarge the effective light cone.

Band alignment of In2O3/β-Ga2O3 interface determined by X-ray photoelectron spectroscopy

The energy band alignment of the atomic-layer-deposited In2O3/β-Ga2O3 ( 2 ¯ 01) interface is evaluated by X-ray photoelectron spectroscopy. The X-ray diffraction pattern reveals that the In2O3 film grown at 160 °C is amorphous, while it becomes polycrystalline at a higher deposition temperature of 200 °C. The bandgaps, determined by reflection electron energy loss spectroscopy, are 4.65, 3.85, and 3.47 eV for β-Ga2O3, polycrystalline In2O3, and amorphous In2O3, respectively. Both amorphous and polycrystalline In2O3/β-Ga2O3 interfaces have Type I alignment. The conduction and valence band offsets at the polycrystalline (amorphous) In2O3/β-Ga2O3 interface are 0.35 and 0.45 eV (0.39 and 0.79 eV), respectively. These observations suggest that polycrystalline In2O3 as an intermediate semiconductor layer is beneficial to the barrier reduction of metal/Ga2O3 contact.The energy band alignment of the atomic-layer-deposited In2O3/β-Ga2O3 ( 2 ¯ 01) interface is evaluated by X-ray photoelectron spectroscopy. The X-ray diffraction pattern reveals that the In2O3 film grown at 160 °C is amorphous, while it becomes polycrystalline at a higher deposition temperature of 200 °C. The bandgaps, determined by reflection electron energy loss spectroscopy, are 4.65, 3.85, and 3.47 eV for β-Ga2O3, polycrystalline In2O3, and amorphous In2O3, respectively. Both amorphous and polycrystalline In2O3/β-Ga2O3 interfaces have Type I alignment. The conduction and valence band offsets at the polycrystalline (amorphous) In2O3/β-Ga2O3 interface are 0.35 and 0.45 eV (0.39 and 0.79 eV), respectively. These observations suggest that polycrystalline In2O3 as an intermediate semiconductor layer is beneficial to the barrier reduction of metal/Ga2O3 contact.

Processing and characterization of high-resolution GaN-based micro-LED display (Conference Presentation)

In this research, the fabrication of Blue-light micro-LED display, high contrast and with resolution is 64  32 pixels, is developed and improved. In traditional micro-LED technique, some parts of LED light may be scattered caused by total reflection effect is produced in the inner structure of sapphire substrate, part of light may be absorbed by epilayer and metal electrodes in the sapphire structure. The contrast of LED is reduced caused by total reflection effect. Black photoresist is used to fill in the gaps between pixels for preventing leakage. And the light shielding property of black photoresist is also be used to separate each pixel LED to raise the contrast of micro-LED display. The luminous efficacy of this new blue-light micro-LED can be raised by below three LED fabrication improvements. Polishing sapphire substrate till it is as transparent, coating metal wires, which are with good reflectivity, are as p-electrodes, and electrodeless design is applied on n-electrodes.Driving mode of this display is multi-electrodes addressable controlling, series resistance is a key factor for the photoelectric characteristics and homogeneity of micro-LED display. There are several different types metal wires and electrodes developed in this study to analyze the photoelectric characteristics of micro-LED display with different wires and electrodes, performance of blue-light micro-LED display can be improved.

Efficiency Enhancement in InGaN Photovoltaic Cells With Inverted Textured Surface

We report on the improvement of conversion efficiency of InGaN photovoltaic (PV) cells with inverted textured surface (ITS). InGaN PV cells with ITS showed short-circuit current density and conversion efficiency of 0.63 mA/cm2 and 1.01%, which are 68.17% and 73.33% higher than the characteristics of conventional InGaN PV cells, respectively. The improvement can be attributed to the reduction of both the shading effect of the electrode contact pad and surface reflection of the incident irradiation under one-sun air-mass 1.5-G illumination (100 mW/cm2).

Enhanced Light Extraction of High-Voltage Light Emitting Diodes Using a Sidewall Chamfer Structure

High-voltage light-emitting diodes (HV-LEDs) were prepared with 4 × 2 microcells. A novel technique for enhancing the light extraction of HV-LEDs by using wet-etched chamfer structures in the sidewalls of each cell is proposed. The thicknesses of the u-GaN layers used were in the range of 3–7 <italic>μ</italic>m. Simulations revealed that the light extraction of the HV-LEDs with chamfer structures would be enhanced by increasing the u-GaN thickness. The output power (@80 mA) of the HV-LEDs without chamfer structures was 538.0–539.1 mW. However, the output powers (@80 mA) of the HV-LEDs with chamfer structures and u-GaN thicknesses of 3, 5, and 7 <italic>μ</italic>m were increased to 555.3, 573.1, and 561.6 mW, respectively. The 7 <italic>μ</italic>m thick u-GaN layer led to an extremely large thickness of the HV-LED epitaxial structure, which caused difficulties during the wet etching process. Because of the apparent decrease in the wet etching rate of GaN, the damages to the epitaxial structures of the HV-LED with the 7 <italic>μ</italic>m thick u-GaN was more obvious. Therefore, the devices light extraction was slightly reduced. The results confirm that chamfer structures are highly useful for improving the performance of HV-LEDs, especially for the u-GaN thickness of 5 <italic>μ</italic>m.