Y. Z. Chiou

Nitride-based LEDs with an insulating SiO2 layer underneath p-pad electrodes

We proposed a simple method to reduce the current crowding effect of nitride-based light emitting diodes (LEDs) without extra dry etching and refill. It was found that we can achieve much better current spreading by inserting an insulating SiO 2 layer between the epitaxial layer and the p-pad electrode. It was also found that we can enhance light output intensity by 22%. Furthermore, it was found that 20 mA forward voltage only increased slightly from 3.32 to 3.37 V with the insertion of the SiO 2 layer. The reliability of the proposed LED is also good.

GaN-based p-i-n sensors with ITO contacts

Nitride-based p-i-n sensors with indium-tin-oxide electrodes on Mg-doped AlGaN/GaN strain layer superlattice structure were fabricated and characterized. It was found that the fabricated sensors exhibit small dark current and large reverse breakdown voltage. With an incident wavelength of 355 nm, we achieved a peak responsivity of 0.17 A/W which corresponds to 59% external quantum efficiency for sensors with 500/spl deg/C annealed ITO(70 nm) p-contacts.

Crabwise ZnO Nanowire UV Photodetector Prepared on ZnO : Ga/Glass Template

Vertical well-aligned and crabwise ZnO nanowires were prepared on patterned ZnO:Ga/glass substrates by reactive evaporation method under different growth conditions. The average length and diameter of vertical well-aligned ZnO nanowires were around 1 mum and 50-100 nm, respectively. In contrast, the average length and diameter of crabwise ZnO nanowires were around 5 mum and 30 nm, respectively. Upon illumination with UV light (lambda = 362 nm), it was found that measured responsivities were 0.015 and 0.03 A/W for the crabwise ZnO nanowire photodetector biased at 10 and 15 V, respectively. Furthermore, a rejection ratio of approximately 10 was obtained for the crabwise ZnO nanowire photodetector with an applied bias of 10 V.

The thickness effect of p-AlGaN blocking Layer in UV-a bandpass photodetectors

By means of 60-, 150-, and 300-nm-thick blocking p-Al/sub 0.1/Ga/sub 0.9/N layers, we have successfully achieved high performance on nitride-based p-i-n bandpass photodetectors. The peak responsivities were estimated to be 0.131, 0.129, and 0.13 A/W at 354, 357, and 356 nm for 60-, 150-, and 300-nm-thick p-Al/sub 0.1/Ga/sub 0.9/N blocking layer, respectively, corresponding to a quantum efficiency of around 46%. Moreover, spectral responsivity for 300-nm-thick blocking p-Al/sub 0.1/Ga/sub 0.9/N layer shows the narrowest bandpass characteristics from 320 to 365 nm (UV-A region). The photodetectors exhibit a 20-V break-down voltage and a small dark current of around 3 pA at 5-V reverse bias. For our 330 /spl times/ 330 /spl mu/m/sup 2/ devices given bias of 0 V, the detectivity D/sup */ limited by Johnson noise are calculated to be 3.43 /spl times/ 10/sup 13/, 6.77 /spl times/ 10/sup 13/, and 8.22/spl times/10/sup 13/ cm/spl middot/Hz/sup 0.5/W/sup -1/, respectively.

Investigating the Effect of Piezoelectric Polarization on GaN-Based LEDs With Different Quantum Barrier Thickness

The effect of temperature-dependent electroluminescence (EL) on nitride-based light-emitting diodes (LEDs) with different thicknesses of quantum barrier are studied and demonstrated. It was found that quantum confined stark effect (QCSE) of 6-nm thick barrier was more slightly than that of 9- and 12-nm thick barrier. The results indicated that the polarization field is independent of ambient temperature due to no clearly change of blue-shift value. The results also pointed out that the polarization field within the active region of 12-nm thick barrier was stronger than the others due to larger variation of the wavelength transition position (i.e. blue-shift change to red-shift) from 300 to 350 K, and thus it needed more injection carriers to complete the screening of QCSE. In this study, we reported a simple method to provide useful comparison of electrostatic fields within active region in nitride-based LEDs, specifically for structures consisting of identical active regions with different barrier thicknesses.

Enhancement in Output Power of Blue Nitride-Based Light-Emitting Diodes With an Electron Retarded Layer

In this study, GaN-based blue light-emitting diodes (LEDs) with an electron retarded layer (ERL) were investigated and demonstrated. The external quantum efficiency (EQE) and efficiency droop effect can be effectively improved by introducing the ERL which was attributed to the retard of the electrons rejected into the multiple quantum wells (MQWs). Therefore, the electron overflow effect can be effectively suppressed and carrier distribution can become more uniform in the MQWs. Regarding the thermal effect, the hot-cold factors of LEDs with ERL can achieve a better performance due to the carrier uniform distribution in the MQWs, which is not easily influenced by the temperature. On the other hand, the temperature dependence of the electroluminescence (EL) of LEDs with ERL also can exhibit a better property especially at lower temperature.

Low Dislocation Densities of Nitride-Based Light-Emitting Diodes with a Preflow of NH3 Source before Growth of AlN Buffer Layer

In this study, the AlN buffer layer grown on c-plane sapphire (Al2O3) substrates by metal–organic chemical vapor deposition (MOCVD) was controlled by a preflow of the NH3 source. With the treatment of NH3 preflow, the monolayer of AlN was formed on the surface of a sapphire substrate by the decomposition of the NH3 source and Al2O3 substrate. It was found that the density of threading dislocations significantly decreased owing to the abatement of lattice mismatch between a GaN layer and a sapphire substrate. At an injection current of 350 mA, the light output intensity of the light-emitting diode (LED) with NH3 preflow was increased by 14.1% compared with that of a conventional LED. On the other hand, LEDs with NH3 preflow treatment could endure electrostatic discharge (ESD) surges up to -3000 V and caused a low rate of light output degradation at a high ambient temperature.

Investigation of optical and electrical properties of GaN-based blue light-emitting diodes with various quantum well thicknesses

Abstract. Optical and electrical properties of gallium nitride (GaN)-based blue light-emitting diodes (LEDs) with various indium gallium nitride (InGaN) quantum well (QW) thicknesses were investigated. As the QW thickness was increased, the light output power of GaN-based LEDs also increased. The increase can be attributed to the increase in the carrier radiative recombination rate in the active region. However, the turn-on voltages of these fabricated LEDs are different. This was attributed to the increase in the polarization field with increasing QW thickness. In regard to the hot/cold factor, LEDs with a thicker QW achieved better performance at a low-injection current owing to the lower defect density. The hot/cold factor at a high-injection current would be mainly influenced by the efficiency droop mechanism.

Ni/Au contacts on homoepitaxial p-ZnSe with surface oxygen plasma treatments

Contact properties of Ni∕Au on homoepitaxial p-ZnSe with oxygen plasma treatments were investigated. The authors found that Zn distribution profiles for p-ZnSe with and without treatments were almost identical. They also found that Se concentration near the surface decreased while O concentration near the surface increased after oxygen plasma treatment. They also observed hillocks, which were related to Se vacancies and/or isoelectronic oxygen impurities, on the surface of 15W oxygen plasma-treated samples. Furthermore, they found that they could achieve the lowest offset voltage from the sample treated with 15W oxygen plasma.