Chun-Ju Tun

Enhanced efficiency of GaN-based light-emitting diodes with periodic textured Ga-doped ZnO transparent contact layer

GaN-based light-emitting diodes (LEDs) with indium tin oxide (ITO)/Ga-doped ZnO (GZO) composite oxide films serving as a transparent contact layer (TCL) were demonstrated. In this study, the wall-plug efficiency of LEDs (LED-III) with textured ITO/GZO composite TCL can be markedly improved by 200% and 45% of magnitude as compared to conventional LEDs with Ni∕Au TCL(LED-II) and planar ITO/GZO TCL(LED-I), respectively. Compared to LED-II, this enhancement is due to the enhanced light extraction efficiency of ITO/GZO composite TCL with high transparency. Compared to LED-I, ZnO-based TCL with a higher refractive index (n∼2.0) allows further enhancement of light extraction through the creation of a textured structure on transparent conductive oxide TCL deposited on the top surface of LEDs. In addition, the ITO/GZO composite TCL with a thickness of 550nm is far larger than that of Ni∕Au TCL with a thickness of approximately 15nm. Therefore, in addition to the effect of high transparency, the thicker ITO/GZO TCL...

Enhanced light output of GaN-based power LEDs with transparent Al-doped ZnO current spreading layer

In this study, Al-doped ZnO (AZO) Ni-AZO and NiO/sub x/--AZO films were deposited on p-type GaN films. The depositions were followed by thermal annealing to form Ohmic contacts. The p-GaN-AZO contacts exhibited a non-Ohmic electrical characteristic. However, electrical characteristic could be greatly improved by insertion of Ni or NiO/sub x/ between AZO film and p-GaN layer. In case of 1/spl times/1 mm/sup 2/ ultraviolet light-emitting diodes (LEDs) with Ni-AZO contacts, the light output approached saturation point when the injection current was about 400 mA. However, the saturation point was as high as 500 mA for the LEDs with NiO/sub x/--AZO contacts. The lower saturation point could be due to the fact that the resistivity of Ni-AZO films was higher than that of NiO/sub x/--AZO films, thus leading to a severe current crowding effect. The increased resistivity of the Ni-AZO films could be attributed to the interdiffusion between Ni and AZO films. When compared to the LEDs with Ni-Au Ohmic contacts, the light output of the LEDs with Ni-AZO and NiO/sub x/--AZO contacts was higher by 38.2% and 60.6% at 350 mA, respectively.

n ¿ -GaN formed by Si implantation into p-GaN

28Si+ implantation into Mg-doped GaN, followed by thermal annealing in N2 was performed to achieve n+-GaN layers. Multiple implantation was used to form a uniform Si implanted region. It was found that the carrier concentration of the films changed from 3×1017 cm−3 (p-type) to 5×1019 cm−3 (n-type) when the samples were annealed in N2 ambient at 1000 °C. The activation efficiency of Si in Mg-doped GaN was as high as 27%. In addition, planar GaN n+–p junctions formed by Si-implanted GaN:Mg were also achieved.

Planar GaN n(+)-p photodetectors formed by Si implantation into p-GaN

GaN n+–p junction diodes were fabricated by implanting Si atoms into p-GaN. It was shown that we could use these diodes as GaN-based planar photodetectors. It was found that the dark current density of the diodes was around 1.5 μA/cm2 and 50 nA/cm2 at reverse biases of 3 and 1 V, respectively. Spectra response measurements revealed a cutoff wavelength at around 365 nm and a peak responsivity of 0.33 mA/W at 365 nm for the GaN planar n+–p photodetectors. It was also found that the visible rejection ratio was around 260. Furthermore, temporal response measurements revealed that the fall times of these GaN planar n+–p photodetectors were found to be shorter than 0.4 μs.

Characterization of GaN Schottky barrier photodetectors with a low-temperature GaN cap layer

By using organometallic vapor phase epitaxy we have prepared i-GaN/low temperature (LT) GaN/Ni/Au (sample A) and i-GaN/Ni/Au (sample B) Schottky barrier ultraviolet (UV) photodiodes (PDs). It was found that we could significantly reduce leakage current and achieve a much larger photocurrent to dark current contrast ratio by introducing a LT GaN on top of the conventional nitride-based UV PDs. With an incident light wavelength of 350 nm and a −1 V reverse bias, it was found that the measured responsivity was around 0.1 and 0.37 A/W for samples A and B, respectively. Furthermore, it was found that the operation speed of sample A is slower than that of sample B due to the highly resistive LT GaN layer induced large RC time constant.

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperature

The 400-nm near-ultraviolet InGaN-GaN multiple quantum well light-emitting diodes (LEDs) with Mg-doped AlGaN electron-blocking (EB) layers of various configurations and grown under various conditions, were grown on sapphire substrates by metal-organic vapor phase epitaxy system. LEDs with AlGaN EB layers grown at low temperature (LT) were found more effectively to prevent electron overflow than conventional LEDs with an AlGaN one grown at high temperature (HT). The electroluminescent intensity of LEDs with an LT-grown AlGaN layer was nearly three times greater than that of LEDs with an HT-grown AlGaN. Additionally, the LEDs with an LT-grown AlGaN layer in H/sub 2/ ambient were found to increase the leakage current by three orders of magnitude and reduce the efficiency of emission.

Characterization of Si implants in p-type GaN

Si ion implantation into p-type GaN followed by rapid thermal annealing (RTA) in N/sub 2/ has been performed. X-ray diffraction analyses indicate that ion-implanted damage remains even with 1050/spl deg/C, 60 s RTA. By varying implantation and postimplantation annealing conditions, we could convert carrier concentration from p-type 3 /spl times/ 10/sup 17/ cm/sup -3/ into n-type 2 /spl times/ 10/sup 17/ cm/sup -3/ /spl sim/2 /spl times/ 10/sup 19/ cm/sup -3/. It was found that typical activation energies of Si implants in p-GaN are lower than 10 meV. Such activation energies are smaller than those observed from epitaxially grown Si-doped GaN films. A deep donor level with activation energy of 60 meV was also found from some samples. Photoluminescence studies show that the peak appears at 372 nm might be related to implantation-induced defects. It was also found that a green emission band could be observed from Si-implanted GaN. It was shown that such a green emission is related to the yellow band observed from epitaxially grown Si-doped GaN. The transport properties of these Si-implanted samples were also studied.

Effects of Thermal Annealing on Al-Doped ZnO Films Deposited on p-Type Gallium Nitride

In this study, Al-doped ZnO (AZO) and Ni/AZO films were deposited on p-type GaN films followed by thermal annealing to form ohmic contacts. After thermal annealing, the resistivities of AZO films reduced from 5 X 10 -3 to 4-6 X 10 -4 Ω cm. Both as-deposited AZO and Ni/AZO contacts on p-GaN displayed a non-ohmic characteristic. Only the 800°C-annealed Ni/AZO contacts exhibited a linear current-voltage characteristic, showing a specific contact resistance of around 1.2 X 10 -2 Ω cm 2 . After undergoing the annealing in nitrogen ambience, the light transmittance of the Ni/AZO films increased from 70% to higher than 90% in the visible range. These results revealed that the Ni/AZO contact can serve as a suitable transparent current spreading layer for the fabrication of GaN-based light-emitting devices.

Ultra-high-density InGaN quantum dots grown by metalorganic chemical vapor deposition

This study examined how the duration of SiNx treatment on an underlying GaN layer affects the optical property, surface morphology and density of following InGaN quantum dots (QDs). InGaN QDs with extremely high density of near 3×1011 cm-2 exhibited strong photoluminescence (PL) emission at room temperature (RT). Increasing the duration of the SiNx treatment of the underlying GaN layer, the RT-PL peak of the following InGaN nano-islands and QDs was found to be red-shifted from the violet to the greenish region, and the spectrum was broadened. Additionally, the average height of InGaN nano-islands and QDs increased with the duration of SiNx treatment, explaining the redshift of the RT-PL peak.

Low Operation Voltage of Nitride-Based LEDs with Al-Doped ZnO Transparent Contact Layer

We have developed nitride-based multiquantum well light-emitting diodes (LEDs) with E-beam evaporated Al-doped ZnO (AZO) transparent contact layers (TCLs). With 20 mA injection current, it was found that forward voltages were 3.32, 3.33, and 4.91 V, while output powers were 10.1, 11.8, and 6.0 mW for the indium-tin-oxide LED, E-beam evaporated AZO LED, and sputter-evaporated AZO LED, respectively. The low operation voltage of the E-beam evaporated AZO LED is attributed to the deposition of low-resistivity TCLs, and the elimination of plasma damage in the p-GaN layer is attributed to use of the E-beam evaporated AZO. The high output power of the E-beam evaporated AZO LED is due to the enhancement of light extraction resulting from the high refractive index of AZO TCL.