[Show abstract][Hide abstract] ABSTRACT: In this study, the effect of quantum barrier thickness in the multi-quantum wells active region on electrical and optical properties of nitride-based light emitting diodes (LEDs) were investigated and demonstrated. The forward voltage decreased as the thickness of quantum barrier decreased owing to the reduction of series resistance. The external quantum efficiency (EQE) and droop effect can be effectively improved by decreasing the barrier thickness which was attributed to the enhancement of the holes injection and uniform distribution in the active region. However, if barrier was too thin, it would get the opposite effect due to the influence of electron overflow. Regarding the hot/cold factor, the thinner quantum barrier of LEDs achieved a better performance. The reason is that the thicker quantum barrier with poor holes distribution resulted in the holes accumulation of a few MQWs near the p-side layer was more easily influenced by thermal effect and escaped from the QWs.
[Show abstract][Hide abstract] ABSTRACT: In this work, the InGaN/AlGaN multiple-quantum-well (MQW) near-ultraviolet (UV) light-emitting diodes (LEDs) with different aluminum compositions in the barrier layers are investigated and fabricated. The aluminum compositions of LED I, II, and III are 0, 2.5, and 3.85%, respectively. The light output powers of LED I, II, and III at an injection current of 350 mA are 95.46, 135.48 mW, and 179.54 mW, respectively. The light output power measured from LED III was 88% larger than that of LED I. The peak external quantum efficiency was enhanced from 9.24 to 17.26%. This result was attributed to the improvement of carrier confinement in the active region by using AlGaN-based barrier layers. The hot-cold effect of LEDs as functions of junction temperature currently is a very important key issue for actual application to LEDs. As the temperature increased, the injected carriers became excited and easily escaped from the active layer. Thus, the light output power of the LED decreased as the temperature increased. Compared with LED I and LED II, the light output power of LED III with better carrier confinement was decreased more slowly with increasing temperature.
Physica Status Solidi (A) Applications and Materials 02/2014; · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Display Technology 04/2013; 9(4):207-211. · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study employed Ar plasma treatment to selectively damage the p-GaN surface under the p-pad electrode as a current-blocking layer (CBL) on nitride-based light-emitting diodes (LEDs). Increasing the resistivity of the p-GaN region under the p-pad electrode can reduce the current flowing vertically downward from the p-pad electrode. At an injection current of 20 mA, the light output power of LEDs with Ar plasma treatment was 13% larger than that of conventional LEDs. At an injection current of 100 mA, the temperature of the p-pad metal on LEDs with Ar plasma treatment is 13 °C lower than that of the LEDs with a SiO2 CBL. However, the electrostatic discharge endurance of LEDs with Ar plasma treatment is the worst due to the surface damage of p-GaN under the p-pad electrode.
Semiconductor Science and Technology 05/2011; 26(8):085005. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nitride-based light-emitting diodes (LEDs) with an n<sup>-</sup> -GaN layer are proposed and fabricated. By providing a larger series resistance in the vertical direction, it was found that the n<sup>-</sup>-GaN layer could enhance LED output intensity due to the enhanced current spreading. It was also found that LEDs with n<sup>-</sup>-GaN layer thicknesses of 0.15, 0.2, and 0.25 μm could endure electrostatic discharge surges up to -1200, - 1800, and -3000 V, respectively.
IEEE Transactions on Device and Materials Reliability 04/2011; · 1.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the growth of very thick (∼400 nm) quaternary AlInGaN layer on GaN/sapphire template by metalorganic chemical vapor deposition. By properly controlling the trimethylindium molar flow rate, we successfully achieved an Al0.89In0.02GaN layer perfectly lattice matched to the underneath GaN buffer. It was found that we can minimize the number of V-defect pits and the linewidth of X-ray (3 0 2) diffraction peak. Other than the AlInGaN-related photoluminescence peak, we also observed a low-energy band which is originated from indium segregation.
Journal of Crystal Growth 06/2010; 312:1920-1924. · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a new method of analyzing the reliability of GaN metal-semiconductor-metal (MSM) photodetectors (PDs). This paper analyzes and characterizes the reliability of GaN MSM PDs with TiW electrodes under different stressing conditions. Controlling the temperature and injection current makes it possible to stress the device and evaluate its characteristics after stressing. Results show that the dark current and responsivity of PDs change with the aging temperature and current. The aging current density is a dominant factor in reliability. This paper also conducts failure analysis to clarify the PD failure mechanisms. Optical microscope inspection shows that burned-fail electrodes are a major cause of failure. Photoluminance analysis shows that the decline of GaN crystal quality is another cause of failure.
IEEE Transactions on Device and Materials Reliability 04/2010; · 1.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The growth, fabrication, and subsequent electroluminescence (EL) characterization of an n-ZnO/p-GaN heterojunction light-emitting diode prepared on c-Al2O3 substrate are presented. The diode-like I–V characteristics and room temperature EL spectrum with an intense broadband emission in the yellow-green spectral region has been observed with forward bias applied. Photoluminescence (PL) and Raman spectra of the n-ZnO and p-GaN films were also measured. By comparing PL and EL spectra, it was concluded that the deep-level defect-related emission mainly originated from the GaN epitaxial layer.
Thin Solid Films 07/2009; 517(17):5054-5056. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An InGaAs buried-heterostructure photodetector (BH-PD) was proposed and fabricated. By introducing etching and refilling with large bandgap and lower concentration semi-insulating InP, it was found that we can reduce the capacitance of P-I-N PDs by 33% without significantly increasing the reverse leakage current. It was also found that we can achieve a 3-dB bandwidth of 11.8 GHz from BH-PD, which was much larger than the 7.1-GHz 3-dB bandwidth observed from conventional InGaAs P-I-N PDs.
IEEE Transactions on Electron Devices 07/2009; · 2.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report the effect of surface HCl treatment on ZnO photoconductive sensors with Ni/Au electrodes after the etching process. Epitaxial ZnO photoconductive sensor film on sapphire substrates was fabricated and then treated with different HCl concentrations. With an incident light wavelength of 370 nm and an applied bias of 10 V, the responsivity of the sensor measured is around 141 mA/W after being treated with 0.7% HCl solution. The result indicates that the a larger surface available for photodetection could be realized by increasing the surface roughness of the ZnO photoconductive sensor. It was also found that the low-frequency and high-frequency noises of the fabricated sensors were dominated by 1/f-type and shot noises, respectively.
Thin Solid Films 07/2009; 517(17):5050-5053. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study successfully proves that the reliability of nitride-based p–i–n photodetectors (PDs) is highly sensitive to the thickness of intrinsic GaN layers. Results are based on i-GaN layers of 0.25 µm, 0.4 µm and 0.5 µm thicknesses. After current ageing, the p–i–n PDs with thin i-layers exhibited poor electrical strength. Increasing the thickness of the i-layer improved the electrical strength and ESD protection capability of PDs. This result is directly related to the impedance and dislocation density of the i-layer. However, the etched sidewall becomes a weak point when adopting a thicker i-layer.
Semiconductor Science and Technology 04/2009; 24(5):055004. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: GaN metal–semiconductor–metal photodetectors (MSM PDs) on silicon substrates and sapphire substrates were fabricated and characterized. We found that the current–voltage (I–V) characteristics of MSM PDs on the silicon substrate could be approximated by the Poole–Frenkel conduction behavior. This phenomenon was attributed to the presence of micro-grain structure in the silicon-substrate epitaxy layer. The voltage-dependent responsivity of GaN MSM PDs on the silicon substrate was also evidence of micro-grains inside the epitaxy layer. At a low frequency, the 1/f-form noise was a main contribution to both PDs. Moreover, the extremely low β (~0.7) extracted from GaN MSM PDs on the silicon substrate was first reported. Based on the current–voltage behavior, the extremely low β was believed to originate from the silicon-substrate-induced micro-grain.
Semiconductor Science and Technology 10/2008; 23(12):125007. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Leakage properties of nitride-based photodetectors (PDs) subjected to inductively coupled plasma (ICP) etching has been investigated by using emission microscopy inspection (EMMI). ICP etching would cause significant damage to GaN metal-semiconductor-metal (MSM) PDs. The damage was proven to induce leakage current via the conductive surface of the device by using emission microscopy inspection. However, the surface damage of MSM PDs could be partially recovered by E-beam SiO<sub>2</sub> passivation. As for the passivation for p-i-n photodetectors, the effect was not significant in the reduction of dark current due to smaller etched area as compared to the whole area of p-i-n PDs. The leakage current path analysis of p-i-n PDs by EMMI technique had also been investigated. Finally, the plasma enhanced chemical vapor deposition (PECVD) SiO<sub>2</sub> passivation was proven to be a potential process to improve the reliability of p-i-n PDs.
[Show abstract][Hide abstract] ABSTRACT: The optical and electrical characteristics of zinc oxide (ZnO) films grown by molecular-beam epitaxy (MBE) on Si substrates were investigated. ZnO epitaxial layer was successfully grown on nitridated Si(100) substrate initially covered with high-temperature GaN and low-temperature ZnO double buffer layers using MBE. X-ray diffraction and photoluminescence results both indicated that a reasonable quality of ZnO epitaxial layer was obtained. As the CV measurement had indicated, the carrier concentration was reduced virtually in a linear fashion from ZnO surface down to GaN buffer layer. A reduction in electron concentration was caused by the carrier depletion due to the presence of the Schottky barrier of Ni/ZnO. The large density of electron accumulated at the ZnO/GaN interface was due to the large conduction band discontinuity and offset.
IEEE Journal of Selected Topics in Quantum Electronics 08/2008; · 3.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By means of 0.25 mum, 0.4 mum and 0.5 mum-thick i-GaN layers, we have successfully proved the reliability of nitride-based p-i-n photodetectors (PDs) was highly sensitive to the thickness of intrinsic GaN layers. After current aging, the p-i-n PDs with thin i-layer exhibited a poor electrical strength.
[Show abstract][Hide abstract] ABSTRACT: ZnO epitaxial layers were successfully grown on nitridated Si(1 0 0) substrate with high-temperature (HT) GaN and low-temperature ZnO double buffer layers by molecular beam epitaxy. It was found that the HT-GaN buffer was crystalline with both hexagonal and cubic phases. It was also found that numerous cone-shaped nano-islands were formed on the ZnO epitaxial layers with density, average diameter and average height of 1.25 Â 10 9 cm À2 , 300 nm and 150 nm, respectively. X-ray diffraction and photoluminescence results both indicate that quality of our ZnO epitaxial layers was good.
Journal of Crystal Growth 01/2008; 31015(81). · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
IEEE Transactions on Nanotechnology 12/2007; · 1.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the fabrication of ZnO photoconductive sensors epitaxially grown on sapphire substrates with interdigitated Ni/Au electrodes. It was found that there exists an electric field-dependent photoconductive gain in the fabricated sensors. With an applied electric field of 500V/cm, it was found that maximum quantum efficiency was around 2.8% while time constant of the decay transient was τ∼0.556ms.
Sensors and Actuators A Physical 10/2007; 140(1):60-64. · 1.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: AlGaN ultraviolet (UV) metal-semiconductor-metal (MSM) photodetectors (PDs) grown on silicon substrates were fabricated and characterized. With 5-V applied bias, it was found that dark current density of Al<sub>0.2</sub>Ga<sub>0.7</sub>N PDs on silicon substrate was only 7.5times10<sup>-9</sup> A/cm<sup>2</sup>. With an applied bias of 7 V, it was found that peak responsivities were 0.09 and 0.11 A/W while UV/visible rejection ratios (i.e., peak wavelength: 420 nm) were 324 and 278 for Al<sub>0.2</sub>Ga<sub>0.8</sub>N and Al<sub>0.3 </sub>Ga<sub>0.7</sub>N MSM PDs, respectively. Moreover, the noise equivalent power of Al<sub>0.2</sub>Ga<sub>0.8</sub>N MSM PDs was estimated to be 3.5times10<sup>-12</sup> W
IEEE Electron Device Letters 05/2007; · 3.02 Impact Factor