C. K. Wang

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.

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.

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.

The δ-Doped In0.25Ga0.75As/GaAs Pseudomorphic High Electron Mobility Transistor Structures Prepared by Low-Pressure Metal Organic Chemical Vapor Deposition*

The (800 A)GaAs/In0.25Ga0.75As/(0.5 µm) GaAs strain layers with δ-doping in the GaAs cap layer were fabricated without using AlGaAs layers. Triethylgallium (TEG), trimethylindium (TMI), arsine (AsH3) and silane (SiH4) were used as the sources and dopants in these structures grown by the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique for the first time to study these structures. The δ-doping was obtained by a stop-growth process by which a very thin and heavily doped layer (1.86×1013 cm-2) can be realized. For the (δ-doped) GaAs/In0.25Ga0.75As/GaAs high electron mobility transistor (HEMT) structures, the concentration of two-dimensional electron gas (2-DEG) and the Hall mobilities were investigated. Experimental results show that a structure with (80 A) In0.25Ga0.75As as the active channel and with a 70 A spacer layer revealed the highest 2-DEG concentration and mobility product of 5.5times1016 V-1s-1 at a temperature of 77 K. These structures are easy to achieve by the LP-MOCVD mechod and are promising for high-performance field effect transistors (FETs).

Two‐dimensional electron gases in delta‐doped GaAs/In0.25Ga0.75As/GaAs heterostructures

Delta‐doped (δ‐doped)GaAs/In0.25Ga0.75As/GaAs strained‐layer modulation‐doped field‐effect transistor (δ‐SMODFET) structures grown by the low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) technique have been studied for the first time. The δ‐doped GaAs, adopted as the electron supplier, was obtained by a stop‐growth process so that a very thin and heavily doped layer (1.9×1013 cm−2) can be realized. Experimental results show that a structure with an 80 A In0.25Ga0.75As layer as the active channel and an 80 A spacer layer demonstrated the highest two‐dimensional electron gases mobility of 26 800 cm2/V s. This structure is easy to achieve by the LP‐MOCVD method because the growth of AlGaAs is avoided and is promising for high performance FETs.

GaN MIS Capacitors with Photo-CVD SiN x O y Insulating Layers

We report the deposition of high-quality SiO 2 and SiN x O y layers onto GaN/sapphire templates by photochemical vapor deposition (photo-CVD). It was found that the 0.845 nm root-mean-square roughness observed from the photo-CVD SiN x O y layer was much smaller than that observed from photo-CVD SiO 2 layer with the same thickness. GaN metal-insulator-semiconductor (MIS) capacitors with these insulating layers were also fabricated. With an applied electric field of 4 MV/cm, it was found that the leakage current densities were 1 X 10 - 8 and 6 X 10 - 7 A/cm 2 for the capacitors with photo-CVD SiN x O y , and photo-CVD SiO 2 insulating layers, respectively. It was also found that the breakdown field of the capacitors with photo-CVD SiN x O y could reach 13 MV/cm. The interface state density at the SiN x O y /GaN interface was also found to be reasonably low.

GaN MSM UV Photodetector With Sputtered AlN Nucleation Layer

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors (PDs) with ex situ sputtered AlN nucleation layer were investigated and demonstrated. The crystal quality, electrical, and optical properties of GaN PDs were improved using ex situ sputtered AlN nucleation layer. Compared with in situ AlN nucleation layer, it was found that the X-ray rock curve widths and yellow or blue bands of cathodoluminescence spectra of the PDs prepared by ex situ sputtered AlN nucleation layer were significantly reduced and smaller due to the improved crystal quality. It was also found that the dark current and responsivity of PDs with ex situ sputtered AlN nucleation layer were more effectively reduced and enhanced. Moreover, GaN MSM PDs with ex situ sputtered AlN nucleation layer could achieve the higher quantum efficiency and detectivity.

Interactive multimedia synchronisation in the distributed environment using the formal approach

Several synchronisation problems should be solved in order to develop interactive multimedia applications in the distributed environment. These problems include intra-medium synchronisation, inter-media synchronisation, and interactive synchronisation. The dynamic extended finite state machine (DEFSM) model, and the corresponding control schemes to handle four VCR-like user interactions, i.e. reverse, skip, freeze-restart, and scale, in distributed interactive multimedia presentations are proposed. The development of a distributed interactive multimedia synchronization specification and execution system is also described. The synchronisation specification and execution system is based on the proposed DEFSM model and the corresponding synchronisation control schemes. The DEFSM model and the control schemes can be incorporated as the synchronisation control kernel of distributed interactive multimedia systems. In this way, system developers do not need to deal with the details of multimedia synchronisation.

A quantum well δ-doped GaAs FET fabricated by low-pressure metal organic chemical vapor deposition

Abstract A quantum well δ-doped n -type GaAs layer with Ga source open has been grown successfully by the low-pressure metal organic chemical vapor deposition (LP-MOCVD). The measured capacitance-voltage profile shows that a sheet-doping concentration up to 5 × 10 12 cm −2 for the δ-doped GaAs layer can be easily achieved. The full-width at half-maximum (FWHM) is quite narrow. From the Hall measurement, the electron mobility increases inversely proportional to the δ-doping concentration. An enhanced mobiity can be obtained more than 2300 and 4300 cm 2 /Vs with doping concentration of 5.0 × 10 18 cm −3 at 300 and 77 K, respectively. Based on this technique, a quantum well δ-doped GaAs FET has been fabricated and demonstrated. With a gate geometry of 5 × 250 μm 2 and doping concentration of 5.9 × 10 18 cm −3 , the estimated transconductance of the δ-doped FET is 64 mS/mm. Since there is an undoped GaAs layer grown on the top of the δ-doped sheet, the breakdown voltage can be increased significantly (> 17 V). Furthermore, the saturation current density can be obtained higher than 110 mA/mm.

InGaN Metal-Semiconductor-Metal Photodiodes with Nanostructures

III–V nitride semiconductor materials have a wurtzite crystal structure and a direct energy band-gap. At room temperature, the energy band-gap of AlInGaN varies from 0.7 to 6.2 eV depending on its composition. Therefore, III–V nitride semiconductors are particularly useful for lightemitting devices and photodetectors in this wavelength region. Indeed, III–V nitride-based blue and green high brightness light-emitting diodes (LEDs) 1,2) fabricated using InGaN/GaN quantum well (QW) structures are now commercially available for traffic light sources and full color displays. However, relatively few nitride-based blue/ultraviolet (UV) photodiodes could be found in the literature as compared with nitride-based LEDs. Blue/ultraviolet (UV) photodiodes are important devices that can be used in various commercial and military applications. For example, these devices can be applied in space, medical and environmental fields. Currently, light detection in the blue/UV region still uses Si photodiodes. However, since the roomtemperature energy band-gap of Si is only 1.2 eV, the responsivity of Si photodiodes is low in the blue/UV region. With the advent of optoelectronic devices fabricated on wide-direct-band-gap materials, it becomes possible to produce high-performance solid-state photodiode arrays that are sensitive in the blue/UV region. Depending on device structure, nitride-based p–n junction diodes, 3) p–i–n diodes, 4,5) p-n diodes, 6) Schottky barrier detectors, 7) and metalsemiconductor-metal (MSM) photodiodes 8–10) could all be used to detect blue/UV signals. Among these devices, MSM photodiodes have an ultralow intrinsic capacitance and their fabrication process is also compatible with field-effecttransistor (FET)-based electronics. Thus, one can easily integrate GaN MSM photodetectors with GaN-based FET to realize a nitride-based optoelectronic integrated circuit (OEIC). Low-dimensional carrier confinement nanostructures such as quantum wires and dots (or islands) are quite attractive for application to high-performance electronic and optical devices. Recently, it has been shown that nitride nanostructures can be self-organized using the strain-induced Stranski–Krastanov (S–K) growth mode. 11–13) It has also been shown that nitride nanostructures can be self-organized using growth interruption during the metal-organic chemical vapor deposition (MOCVD) growth. 14) Although the successful fabrication of InGaN nanostructures has been reported by many research groups, very few reports on optoelectronic devices with InGaN nanostructures could be found in the literature. 15,16) In this study, we use MOCVD growth interruption to form self-assembled nitride nanostructures; MSM photodiodes with InGaN nanostructures were subsequently fabricated. The photoluminescence (PL) properties of InGaN nanostructures and the characteristics of the fabricated MSM photodiodes will be discussed. 2. Experiments