C. S. Chang

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

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/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. 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/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.

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.

Nitride-Based Light Emitting Diodes With Textured Sidewalls and Pillar Waveguides

Nitride-based light emitting diodes (LEDs) with sidewall texture and pillar waveguides (STPW) were fabricated using conventional lithography method. With 20-mA injection current, it was found that forward voltages were 3.16 and 3.15 V for the conventional LED and the LED with STPW, respectively. It was also found that 20-mA LED output powers were 8.4 and 10.1 mW for conventional LED and the LED with STPW, respectively. The enhancement is attributed to the out-coupling of lateral waveguide mode in the near horizontal directions

Highly Reliable High-Brightness GaN-Based Flip Chip LEDs

The properties of indium-tin-oxide (ITO)/Ni films as transparent ohmic contacts of nitride-based flip chip (FC) light emitting diodes (LEDs) were studied. It was found that 300degC rapid thermal annealed (RTA) ITO(15 nm)/Ni(1 nm) could provide good electrical and optical properties for FC LED applications. It was also found that 20-mA operation voltage and output power of the 465-nm FC LEDs with ITO/Ni/Ag reflective mirror were 3.16 V and 21 mW, respectively. Furthermore, it was found that output intensity of the proposed LED only decayed by 5% after 1200 h under 30-mA current injection at room temperature.

Rapid thermal annealed InGaN/GaN flip-chip LEDs

Nitride-based flip-chip (FC) light-emitting diodes (LEDs) emitting at 465 nm with Ni transparent ohmic contact layers and Ag reflective mirrors were fabricated. With an incident light wavelength of 465 nm, it was found that transmittance of normalized 300/spl deg/C rapid thermal annealed (RTA) Ni(2.5 nm) was 93% while normalized reflectance of 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that 300/spl deg/C RTA Ni(2.5 nm) formed good ohmic contact on n/sup +/ short-period-superlattice structure with specific contact resistance of 7.8/spl times/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. With 20-mA current injection, it was found that forward voltage and output power were 3.15 V and 16.2 mW for FC LED with 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm). Furthermore, it was found that reliabilities of FC LEDs were good.

Highly Reliable Nitride-Based LEDs With Internal ESD Protection Diodes

Nitride-based light-emitting diodes (LEDs) with internal electrostatic-discharge (ESD) protection diodes emitting at 460 nm were proposed and realized. By building an internal GaN p-n junction diode, a negative ESD-induced pulse current is expected to flow through the protection diode without damaging the major LED. The ESD characteristic of the fabricated LEDs was obviously improved with this design. Furthermore, the dimension of the internal p-n diode would influence the capacity for tolerating the ESD stress. It was found that a negative ESD threshold could be significantly increased from 300-400 to 2000 V. On the other hand, the authors managed to bring down the 20-mA operation voltage to 3.29 V using the n-metal finger, which entails a good current spreading under operation as the result of a reduced current-crowding effect. Since a good current spreading beneficially alleviates the thermal effect under long-term operation, an effective pattern layout design clearly would also prolong the lifetime of the proposed LEDs

Effect of rapid thermal annealing on carrier lifetimes of arsenic-ion-implanted GaAs

Femtosecond time‐resolved reflectivity measurements, structural, and electrical analyses have been performed to investigate the effect of rapid thermal annealing (RTA) on GaAs implanted with 200 keV arsenic ions at 1016 ions/cm2. Ultrashort carrier lifetimes from 0.48 fs to 2.3 ps were observed for samples annealed at temperatures between 600 and 800u2009°C. The time constants are somewhat shorter than those of RTA‐annealed low‐temperature molecular‐beam epitaxy grown material, while following the same trend of longer time constants and recovery of resistance at higher annealing temperatures. Arsenic precipitates were not observed.

Flip-Chip p(GaN)-i(GaN)-n(AlGaN) Narrowband UV-A Photosensors

Flip-chip p(GaN)-i(GaN)-n(AlGaN) photosensors with extremely low dark currents were fabricated and characterized. It was found that the sensor with a 0.5-mum-thick Si-doped n+-Al0.15Ga 0.85N layer could only detect optical signals with wavelength in between 325 and 360 nm. With an incident wavelength of 355 nm, the authors achieved a peak responsivity of 0.16 A/W at zero bias, which corresponds to an external quantum efficiency of 56%

Buildup dynamics of the spectrum and the average output power of a homogeneously broadened continuous-wave Ti:sapphire laser

The buildup dynamics of a homogeneously broadened continuous-wave Ti: sapphire laser is experimentally investigated for the first time to our knowledge. The average output power of the laser exhibits relaxation oscillation behavior initially and approaches the steady-state value in ≈20 μs. Concurrently, the longitudinal-mode spectrum evolves from a multicluster spectral distribution (tentatively attributed to the spectral windowing effect of the Brewster-angle-cut Ti:sapphire crystal as a birefringent filter) to a single cluster at 787.5 nm. The spectral narrowing (which is due to gain competition in the homogeneously broadened medium), however, takes ≈500 μs. Physical mechanisms that explain qualitatively the two distinctive time scales are presented.

A Ku band four-stage PHEMT 1W MMIC power amplifier

A successful development of a very high performance and reliable power PHEMT MMIC technology is reported. In this paper, a Ku-Band 1 W AlGaAs/InGaAs/GaAs PHEMT MMIC power amplifier for VSAT ODU application is demonstrated. This four-stage amplifier is designed to fully match for a 50 Ω input and output impedance. With 7 V and 700 mA DC bias condition, the amplifier has achieved 30 dB small-signal gain, 30.8 dBm 1-dB gain compression power with 24.5% power-added efficiency (PAE) and 31.3 dBm saturation power with 27.5% PAE from 14 to 17 GHz.