Jinn-Kong Sheu

White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors

Phosphor-converted light-emitting diodes (LEDs) were fabricated by precoating blue/green/red phosphors onto near ultraviolate (n-UV) LED chips prior to package into LED lamps. With a 20-mA injection current, it was found that the color temperature T/sub c/ was around 5900 K and the color-rendering index R/sub a/ was around 75 for the n-UV+blue/green/red white LED lamps. It was also found that no changes in color temperature T/sub c/ and color-rendering index R/sub a/ could be observed when we increased the injection from 20 to 60 mA. These results indicate that such n-UV+blue/green/red white LEDs are much more optically stable than the conventional blue+yellow LEDs.

n-UV+Blue/Green/Red White Light Emitting Diode Lamps

Phosphor-conversion light-emitting diode (LED) lamps were fabricated by precoating blue/green/red phosphors onto near-ultraviolet (n-UV) LED chips prior to packaging them into LED lamps. With a 20 mA injection current, it was found that the color temperature, Tc, was approximately 5900 K, the color rendering index, Ra, was approximately 75, and the CIE color coordinates were x=0.33 and y=0.35 for the n-UV+blue/green/red white LED lamps. No obvious changes in color temperature, Tc, or color-rendering index, Ra, could be observed when we increased the injection from 20 mA to 60 mA. These results indicate that such n-UV+blue/green/red white LED lamps are much more optically stable than the conventional blue+yellow LED lamps.

White-light emission from InGaN-GaN multiquantum-well light-emitting diodes with Si and Zn codoped active well layer

Si and Zn codoped In/sub x/Ga/sub 1-x/N-GaN multiple-quantum-well (MQW) light-emitting diode (LED) structures were grown by metal-organic vapor phase epitaxy (MOVPE). It was found that we can observe a broad long-wavelength donor-acceptor (D-A) pair related emission at 500 nm/spl sim/560 nm. White light can thus be achieved by the combination of such a long-wavelength D-A pair emission with the InGaN bandedge related blue emission. It was also found that the electroluminescence (EL) spectra of such Si and Zn codoped InGaN-GaN MQW LEDs are very similar to those measured from phosphor-converted white LEDs. That is, we can achieve white light emission without the use of phosphor by properly adjusting the indium composition and the concentrations of the codoped Si and Zn atoms in the active well layers and the amount of injection current.

High-dielectric-constant Ta2O5/n-GaN metal-oxide-semiconductor structure

High-dielectric-constant Ta2O5 has been grown on the n-GaN epifilm by rf magnetron sputtering. Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics. Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×104 cm/s as an upper limit using a modified dead-layer model. A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta2O5 as the gate metal. Capacitance-versus-voltage characteristics have been measured. The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 7.0×1016u2002cm−3. The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 4.1×1012u2002cm−2. Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta2O5, and matches closely with calculated values. Hysteresis is observed a...

In0.23Ga0.77N/GaN MQW LEDs with a low temperature GaN cap layer

Abstract Mg-doped p-GaN epitaxial layers prepared at different temperatures were prepared and characterized. It was found that we could achieve a higher hole concentration and a rough surface by reducing the growth temperature down to 800 °C. In 0.23 Ga 0.77 N/GaN multiquantum well (MQW) light emitting diodes (LEDs) with such a low 800 °C-grown p-GaN cap layer were also fabricated. It was found that we could enhance the LED output intensity by more than 90% with the low 800 °C-grown p-GaN cap layer, as compared to the conventional high 1000 °C-grown p-GaN cap layer.

Low-operation voltage of InGaN/GaN light-emitting diodes by using a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN superlattice

Low-resistivity Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN strained-layer superlattices were grown. In these superlattices, the maximum hole concentration is 3/spl times/10/sup 18//cm/sup 3/ at room temperature. Hall-effect measurements indicate high conductivity of this structure in which the high activation efficiency is attributed to the strain-induced piezoelectric fields. This work also fabricated InGaN/GaN blue LEDs that consist of a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN SLs. Experimental results indicate that the LEDs can achieve a lower operation voltage of around 3 V, i.e., smaller than conventional devices which have an operation voltage of about 3.8 V.

Carrier dynamics in nitride-based light-emitting p-n junction diodes with two active regions emitting at different wavelengths

The carrier transport and recombination dynamics of monolithic InGaN/GaN light-emitting p-n junction structures with two active regions are investigated. Room-temperature and low-temperature photoluminescence and room-temperature electroluminescence measurements show two emission bands originating from the two active regions. In electroluminescence, the intensity ratio of the two emission bands is independent of injection current. In contrast, the intensity ratio depends strongly on the excitation intensity in photoluminescence measurements. The dependency of the emission on excitation is discussed and attributed to carrier transport between the two active regions and to the different carrier injection dynamics in photoluminescence and electroluminescence. The luminous efficacy of a Gaussian dichromatic white-light source is calculated assuming a line broadening ranging from 2kT to 10kT. Luminous efficacies ranging from 380 to 440 lm/W are obtained for broadened dichromatic sources.

Low Temperature Activation of Mg-Doped GaN in O2 Ambient

In this study, Mg-doped GaN epitaxial layers were grown by metalorganic vapor phase epitaxy (MOVPE) and annealed in O2, air and N2. It was found that we could achieve a low-resistive p-type GaN by O2-ambient annealing at a temperature as low as 400°C. The resistivity and hole concentration of the 400°C O2-ambient annealed Mg-doped GaN was 2 Ω-cm and 3×1017 cm-3, respectively. These values are equivalent to those values obtained from Mg-doped GaN annealed in N2 ambient at 700°C.

Vertical High Quality Mirrorlike Facet of GaN-Based Device by Reactive Ion Etching

Reactive ion etching (RIE) of GaN has been performed using BCl3 and additives Ar, CH4, and N2 to BCl3 plasma. The etching rate, surface roughness and the etch profile have been investigated. When BCl3/Ar was used as the RIE plasma source with 200 W RF power and 60 mTorr pressure, the highest etching rates of 505 A/min and 448 A/min were obtained for n- and p-GaN, respectively. It was found that the addition of CH4 and N2 to the BCl3 plasma would result in significant changes of the etching results, such as the etching rate and surface morphology. It was also found that with the proper etching parameter, the mirrorlike facet of GaN can be obtained using BCl3/Ar/CH4/N2 by RIE.

Nitride-based blue LEDs with GaN/SiN double buffer layers

GaN epitaxial layers and nitride-based multiquantum well light emitting diode (LED) structures with conventional single GaN buffer and GaN/SiN double buffers were prepared by metalorganic chemical vapor deposition. It was found that we could reduce defect density and thus improve crystal quality of the GaN epitaxial layers by using GaN/SiN double buffers. It was also found that we could use such a GaN/SiN double buffer to achieve more reliable nitride-based LEDs.