T. K. Lin

Photo-CVD SiO/sub 2/ layers on AlGaN and AlGaN-GaN MOSHFET

High-quality SiO/sub 2/ was successfully deposited onto AlGaN by photochemical vapor deposition (photo-CVD) using a D/sub 2/ lamp as the excitation source. The resulting interface state density was only 1.1 /spl times/ 10/sup 11/ cm/sup -2/eV/sup -1/, and the oxide leakage current was dominated by Poole-Frenkel emission. Compared with AlGaN-GaN metal-semiconductor HFET (MESHFETs) with similar structure, the gate leakage current is reduced by more than four orders of magnitude by using the photo-CVD oxide layer as gate oxide in AlGaN-GaN metal-oxide-semiconductor heterojunction field-effect transistors (MOSHFETs). With a 2-/spl mu/m gate, the saturated I/sub ds/, maximum g/sub m/ and gate voltage swing (GVS) of the fabricated nitride-based MOSHFET were 572 mA/mm, 68 mS/mm, and 8 V, respectively.

High detectivity GaN metal–semiconductor–metal UV photodetectors with transparent tungsten electrodes

GaN metal–semiconductor–metal (MSM) ultraviolet photodetectors with transparent tungsten (W) electrodes were fabricated and characterized. It was found that the 10 nm thick W film deposited with a 250 W RF power could provide a reasonably high transmittance of 68.3% at 360 nm, a low resistivity of 1.5 × 10−3 Ω cm and an effective Schottky barrier height of 0.777 eV on u-GaN. We also achieved a peak responsivity of 0.15 A W−1 and a quantum efficiency of 51.8% at 360 nm from the GaN MSM UV photodetector with W electrodes. With a 2 V applied bias, it was found that the minimum noise equivalent power (NEP) and the maximum D* of our detector were 1.745 × 10−10 W and 7.245 × 109 cm Hz0.5 W−1, respectively.

ZnSe homoepitaxial MSM photodetectors with transparent ITO contact electrodes

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.

Investigating the Effect of Piezoelectric Polarization on GaN-Based LEDs With Different Quantum Barrier Thickness

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.

High Temperature Performance and Low Frequency Noise Characteristics of AlGaN/GaN/AlGaN Double Heterostructure Metal-Oxide-Semiconductor Heterostructure Field-Effect-Transistors with Photochemical Vapor Deposition SiO2 Layer

High quality SiO2 films were successfully deposited onto AlGaN using photochemical vapor deposition (photo-CVD). The interface state density, Dit, of photo-CVD SiO2 was estimated to be only 1.1×1011 cm-2eV-1 at room temperature and still only 3.5×1012 cm-2eV-1 even at 175°C. With a 1 µm gate length, it was found that the maximum saturated drain-source current (Ids), maximum transconductance (gm) and gate voltage swing (GVS) of the AlGaN/GaN/AlGaN double heterostructure metal-oxide-semiconductor heterostructure field-effect-transistors (MOS-HFETs) fabricated were 755 mA/mm, 95 mS/mm and 8 V, respectively. Even at 300°C, the maximum saturated Ids and maximum gm of the MOS-HFETs fabricated were still kept at 527 mA/mm and 77 mS/mm, respectively. Furthermore, from the low frequency noise power spectrum, it was found that noise power density of the AlGaN/GaN/AlGaN double heterostructure was lower and presented pure 1/f noise with smaller trapping effects than conventional structures.

GaN-Based LEDs With Double Strain Releasing MQWs and Si Delta-Doping Layers

In this letter, we report the fabrication of GaN-based light-emitting diodes (LEDs) with double strain releasing multiquantum wells and Si delta-doping (Si-DD) layers. We find that Si-DD can enhance current spreading in the in-plane direction and also suppress dislocation in the epitaxial layers. By inserting the Si-DD layers, we find that we can achieve a smaller forward voltage. We also find that we can significantly increase the reverse breakdown voltage from 35 to 125 V by introducing Si-DD layers. Furthermore, we find that the output power of the LED with Si-DD is more than 10% larger than that of the LED without Si-DD.

Improved Optical and ESD Characteristics for GaN-Based LEDs With an

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 μm could endure electrostatic discharge surges up to -1200, - 1800, and -3000 V, respectively.

\hbox{n}^{-}\hbox{-GaN}

100 nm thick Ru films were deposited onto n-ZnO epitaxial layers by radio frequency sputtering. It was found that highly transparent RuO x was formed after O 2 annealing. With an incident wavelength of 460 nm, it was found that transmittances of as-grown, 500°C-annealed, 600°C-annealed, and 700°C-annealed Ru films were 56.8, 73.5, 79.6, and 86.8%, respectively. It was also found that as-deposited Ru formed Schottky contact on n-ZnO. However, good ohmic contacts were formed between the annealed Ru films and the underneath ZnO. With 650°C annealing, we achieved a specific contact resistance of only 2.72 X 10 -4 Ω cm 2 . Such a low specific contact resistance should be attributed to the formation of RuO x and the dissociation of oxygen atoms in ZnO during annealing.

Layer

High quality SiO2 was successfully deposited on GaN by photochemical vapour deposition (photo-CVD) using a D2 lamp as the excitation source. It was found that the interface state density was only 8.4 × 1011 cm−2 eV−1 for photo-CVD SiO2 layers on GaN prepared at 300 °C. AlGaN/GaN metal–oxide–semiconductor heterojunction field effect transistors were fabricated with photo-CVD oxide as the insulating layer. It was found that room temperature saturation Ids, maximum gm and gate voltage swing of the devices were 1220 mA mm−1, 240 mS mm−1 and 4.5 V, respectively. Even at an evaluated temperature of 300 °C, the device still exhibits the maximum transconductance of 180 mS mm−1.

Transparent RuOx contacts on n-ZnO

The optical property and electrostatic discharge (ESD) endurance of GaN-based light-emitting diodes (LEDs) with varied undoped GaN thickness are studied and demonstrated. As the undoped GaN thickness increased, the generation of V-shaped defects in the active region was suppressed. Therefore, the output power for a 6-μ.m-thick undoped GaN layer would be enhanced remarkably due to the reduction of nonradiative recombination ratio within the active region. On the other hand, under a reverse ESD pulse voltage of 5.5 kV, the survival rate of the LEDs with an undoped GaN layer thickness of 1.5, 4, and 6 μm were 75%, 65%, and 55%, respectively. It was found that the ESD endurance for a 1.5-μ.m-thick undoped GaN layer with higher internal capacitance was obviously better than others. This could be related to the built-in electric field of LEDs induced by spontaneous polarization field.