Search

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. [ABSTRACT FROM PUBLISHER]

We demonstrated the electro-optical characteristics of gallium nitride (GaN)-based ultraviolet (UV) light emitting diodes (LEDs) with sputtered aluminum nitride (AlN) nucleation layer. The introduction of the ex situ sputtered AlN nucleation layer improved the crystal quality of the GaN and the n-AlGaN layer of the GaN-based UV LEDs. Hence, the 20-mA output power of UV LEDs with ex situ AlN nucleation layers is higher than that of UV LEDs with GaN nucleation layers. In addition, the enhanced power output of UV LEDs with ex situ AlN nucleation could reach around 52% in magnitude at peak emission wavelengths of 370 nm compared with power outputs of UV LEDs with GaN nucleation layers. Furthermore, UV LEDs with ex situ AlN nucleation show improved reliability. The UV LEDs with ex situ AlN nucleation layer revealed a power output drop of around 9% within 168 hours , which is less than the around 14% power drop of UV LEDs with GaN nucleation layer. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

Nitride-based light-emitting diodes (LEDs) with an \n^- \-GaN layer are proposed and fabricated. By providing a larger series resistance in the vertical direction, it was found that the \n^-\-GaN layer could enhance LED output intensity due to the enhanced current spreading. It was also found that LEDs with \n^-\-GaN layer thicknesses of 0.15, 0.2, and 0.25 \mu\m could endure electrostatic discharge surges up to -1200, -1800, and -3000 V, respectively. [ABSTRACT FROM PUBLISHER]

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7 × 10-3 Ω·cm and an effective Schottky barrier height of 0.773 eV on u-CaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D* of our detector were 1.987 × 10-10 W and 6.365 × 109 cmHz0.5W-1, respectively. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones. [ABSTRACT FROM PUBLISHER]

In the following study, we demonstrated linear cascade GaN-based light-emitting-diode (LED) arrays at a wavelength of approximately 520 nm. Experimental LEDs were analyzed with the goal to improve the output power and differential efficiency of a single LED. The study shows that using arrays with up to four LEDs connected in series, we can achieve four times the improvement in output power (differential quantum efficiency) under the same bias current as compared to a single LED apparatus. We have also measured the modulation-speed performance of experimental LEDs, and both devices exhibit similar 3-dB bandwidth (90 MHz) under the same bias currents. Experimental results indicate that the cascade connection offers the advantages of significantly enhanced external differential efficiency and provision of a method to use a constant-voltage power supply. The current crowding problem and resistance-capacitance-limited bandwidth degradation issues in a large active area LED can also be minimized using the connection demonstrated in our experiment. [ABSTRACT FROM PUBLISHER]

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 Al0.2Ga0.7N PDs on silicon substrate was only 7.5 × 10-9A/cm2. 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 Al0.2Ga0.8N and Al0.3Ga0.7N MSM PDs, respectively. Moreover, the noise equivalent power of Al0.2Ga0.8N MSM PDs was estimated to be 3.5 × 10-12 W. [ABSTRACT FROM AUTHOR]

We demonstrate a GaN-based phosphor-free near-white-light light-emitting-diode (LED) structure that operates in the visible wavelengths and offers broadening and flattening optical bandwidth performance. The incorporation of GaN-based dual wavelengths (blue and green) multiple-quantum-wells with a transverse p-n junction produces a device which can directly generate stable and near visible white-light emissions. The shape of the optical spectra (440-560 nm) are invariable from low to very high levels of bias currents. The problems of nonuniform carrier distribution and bias dependent electroluminescence spectra that occur in traditional phosphor-free white-light or near-white-light LEDs (with vertical p-n junctions) are eliminated by the demonstrated structure [ABSTRACT FROM PUBLISHER]

We report the homoepitaxial growth of ZnSe layers on ZnSe substrates by molecular beam epitaxy (MBE). It was found that we can only observe an extremely strong ZnSe (004) x-ray peak with a full-width-at-half-maximum of 21.5 arcsec, which is much smaller than that observed from ZnSe grown on GaAs substrates. Photoluminescence and Hall measurement also indicate that the quality of our homoepitaxial ZnSe layers is good. ZnSe-based homoepitaxial metal-semiconductor-metal photodetectors with transparent indium-tin-oxide (ITO) contact electrodes were also fabricated. It was found that although ITO transparent contact electrodes can result in large photon absorption and large photocurrents, the low Schottky barrier height between ITO and homoepitaxial ZnSe would also result in relatively large dark currents. With an incident wavelength of 450 nm and a 1-V applied bias, it was found that the maximum responsivity is about 0.13 A/W, which corresponds to a quantum efficiency of 35%. Furthermore, it was found that the detector responsivity drops by more than two orders of magnitude across the cutoff region. [ABSTRACT FROM AUTHOR]