Ching-Ting Lee

Long-term thermal stability of Ti/Al/Pt/Au Ohmic contacts to n-type GaN

We present a study of the long-term thermal stability and low specific contact resistance of Ti/Al/Pt/Au multilayer contacts to n-type GaN. The Ohmic performance can be maintained up to 60 and 540 min for thermal annealing at 950 and 850 °C, respectively, and even longer than 600 min for thermal annealing at 750 °C. By comparing the specific contact resistances for Ti/Al/Pt/Au and Ti/Al/Au Ohmic contacts to n-type GaN, the long-term thermal stability may be deduced as due to the contribution of the Pt barrier for preventing Au penetration.

ZnO-on-GaN heterojunction light-emitting diode grown by vapor cooling condensation technique

The growth of ZnO-on-GaN heterostructures was implemented using the vapor cooling condensation system. The technique thus developed was employed to fabricate both the p-GaN∕n-ZnO:In (p-n) and p-GaN∕i-ZnO∕n-ZnO:In (p-i-n) heterojunction light-emitting diodes (LEDs). A rectifying diodelike behavior was clearly observed from both the p-n and p-i-n heterojunction LEDs, with the forward turn-on voltage of 3V and the reverse breakdown voltage of −15V determined for the p-n heterojunction LEDs, compared to 7 and −23V, respectively, for the p-i-n heterojunction LEDs. Based on the results of photoluminescence and electroluminescence studies conducted on these LED structures, the ZnO layer responsible for the peak emission wavelength of 385nm were also verified experimentally.

Metal–oxide–semiconductor devices using Ga2O3 dielectrics on n-type GaN

Using a photoelectrochemical method involving a He–Cd laser, Ga2O3 oxide layers were directly grown on n-type GaN. We demonstrated the performance of the resultant metal–oxide–semiconductor devices based on the grown Ga2O3 layer. An extremely low reverse leakage current of 200 pA was achieved when devices operated at −20 V. Furthermore, high forward and reverse breakdown electric fields of 2.80 MV/cm and 5.70 MV/cm, respectively, were obtained. Using a photoassisted current–voltage method, a low interface state density of 2.53×1011 cm−2 eV−1 was estimated. The varactor devices permit formation of inversion layers, so that they may be applied for the fabrication of metal–oxide–semiconductor field-effect transistors.

Versatile insensitive current-mode universal biquad implementation using current conveyors

We present a versatile multi-input-multi-output biquad configuration. Either a three-input single-output or a single-input three-output universal filter can be realized. The proposed construction is comprised of only three current conveyors and four grounded passive elements, is multifunctional and convenient for monolithic integration, has low sensitivity, and is simple in structure. Its high impedance outputs enable easy cascading in current-mode operation. After slightly modifying an original configuration, another novel single-input three-output universal filter with extra advantageous features has been obtained. The effects of current conveyor nonidealities are discussed and simulation results are provided.

Nitride-based LEDs with 800/spl deg/C grown p-AlInGaN-GaN double-cap layers

GaN-based light-emitting diodes (LEDs) with various p-cap layers were prepared. It was found that surface morphologies of the LEDs with 800/spl deg/C grown cap layers were rough due to the low lateral growth rate of GaN. It was also found that 20-mA forward voltage of the LED with 800/spl deg/C grown p-AlInGaN-GaN double-cap layer was only 3.05 V. Furthermore, it was found that we could achieve a high output power and a long lifetime by using the 800/spl deg/C grown p-AlInGaN-GaN double-cap layer.

Optical properties of ZnO/GaN heterostructure and its near-ultraviolet light-emitting diode

We report on photoluminescence in a ZnO/GaN heterostructure, which showed a donor–acceptor pair emission band at 3.270 eV and the longitudinal optical phonon replicas at 12 K. In comparison with p-type GaN, the positions of the peaks are redshifted. This may be associated with the variation of the residual strain in the GaN layer of the heterostructure. Using this heterostructure, near-ultraviolet light-emitting diodes were fabricated and their electroluminescence properties were characterized.

Ultraviolet photodetectors with ZnO nanowires prepared on ZnO:Ga/glass templates

Vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors were fabricated by spin-on-glass technology on ZnO:Ga/glass templates. With 2V applied bias, it was found that dark current density of the fabricated device was only 2.0×10−7A∕cm2. It was also found that UV-to-visible rejection ratio and quantum efficiency of the fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.

White Light Emission of Monolithic Carbon-Implanted InGaN–GaN Light-Emitting Diodes

We presented white emission of carbon-implanted InGaN-GaN light-emitting diodes. By using the blue light emitting from the InGaN-GaN multiple quantum well to excite the carbon-implanted Mg-doped GaN layer, the yellow-green light emission was obtained. To mix the blue light and the generated yellow-green light, white emission can be obtained in the monolithic InGaN-GaN light-emitting diodes

A bilayer Ti/Ag ohmic contact for highly doped n-type GaN films

Ohmic contacts with low resistance are fabricated on n‐type GaN films using Ti/Ag bilayer metallization. The GaN films are grown by low pressure metalorganic chemical vapor deposition (LP‐MOCVD) with Si as the dopant. Ohmic characteristics are studied for films with carrier concentration range from 1.5×1017 to 1.7×1019 cm−3. The lowest value for the specific contact resistivity of 6.5×10−5 Ω cm2 is obtained without annealing. The barrier height of Ti on GaN is calculated to be 0.067 eV.

Nitride-based LEDs with textured side walls

Nitride-based light-emitting diodes (LEDs) with textured side walls were fabricated. By using plasma-enhanced chemical vapor deposition SiO/sub 2/ layer as the etching mask, we successfully etched the nitride epitaxial layers to achieve wavelike side walls. It was found that such wavelike side walls could mainly enhance the light output at the horizontal directions. With a 20-mA current injection, it was found that the output powers of the LED with textured side walls and normal LED were 9.3 and 8.4 mW, respectively. Furthermore, it was found that such textured side walls will not result in a higher operation voltage.