Ta-Ming Kuan

ZnSe Nanowire Photodetector Prepared on Oxidized Silicon Substrate by Molecular-Beam Epitaxy

We reported the growth of ZnSe nanowires on oxidized Si substrate by molecular-beam epitaxy. It was found that average length, average diameter, and density of the ZnSe nanowires were 1.2 μm, 48 nm, and 1.04 X 10 7 cm -2 , respectively. It was also found that the ZnSe nanowires were structurally uniform and defect-free with a pure zinc blend structure. UV photodetectors were then fabricated by sputtering a thick Au film through an interdigitated shadow mask onto the ZnSe nanowires. It was found that photocurrent to dark current contrast ratio of our ZnSe nanowire photodetector was >90 with 0.1 V applied bias.

InGaN/GaN light emitting diodes with a p-down structure

Nitride-based p-down blue light emitting diodes (LEDs) were successfully fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn-on voltage could be reduced from 15 V to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 mA output power was 1 mW for the p-down LED with an Mg+Si codoped interlayer and a rough p-tunnel layer.

The Effects of Mechanical Uniaxial Stress on Junction Leakage in Nanoscale CMOSFETs

This paper reports the influences of uniaxial mechanical stress on the reverse-biased source/drain to substrate junction leakage of state-of-the-art 65 nm CMOS transistors. For n-channel metal-oxide-semiconductor (NMOS) transistors, the band-to-band tunneling (BTBT) dominates the junction leakage current due to heavily doped junction and pocket implants. However, for p-channel metal-oxide-semiconductor (PMOS) transistors with embedded SiGe source/drain, the leakage current is found to result from both BTBT and generation current due to defects generated in the SiGe layer and at the SiGe/Si interface. A four-point bending technique is used to apply mechanical uniaxial stress on NMOS and PMOS devices along the longitudinal direction. It was found that the leakage current of both devices increases (decreases) with applied uniaxial compressive (tensile) stress, and that the strain sensitivity of the junction leakage of NMOS transistors is much weaker than that of PMOS transistors. By combining the bending technique with process strained Si (PSS) technology, additional stress was applied to NMOS and PMOS with high built-in stress to investigate the characteristics of junction leakage under extremely high uniaxial stress. It is shown that uniaxial tensile stress can both enhance the NMOS device performance and decrease the junction leakage. However, for the PMOS, there exists a tradeoff between boosting the transistor performance and decreasing the junction leakage current, so there is a limit in the amount of compressive stress that can be beneficially applied.

P-Down InGaN/GaN Multiple Quantum Wells Light-Emitting Diode Structure Grown by Metal-Organic Vapor-Phase Epitaxy.

An inverted or p-down InGaN/GaN multiple quantum wells (MQW) light-emitting diode (LED) structure is studied. The crystalline quality of the quantum wells is comparable to that of the n-down structure by using a Si or In co-doped GaN:Mg layer underneath the active layer. It was found that I–V characteristics can be improved by insertion of a tunnel layer, either a 3D growth GaN:Mg layer or an AlGaN/GaN superlattice layer. The feasibility of such a structure for practical application is also evaluated by luminescence measurement.

Flicker noise of GaN-based heterostructure field-effect transistors with Si-doped AlGaN carrier injection layer

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure field effect transistors (HFETs) with and without Si-doped AlGaN layer were fabricated and investigated. HFETs with the Si-doped AlGaN carrier-injection layer show better DC performance, and the transconductance is 150 mS/mm. However, the HFETs with Si-doped AlGaN layer present the deviation from the 1/f noise at low frequency. The Lorentz shape was observed in the noise spectrum. It suggests that traps might be more pronounced in this kind of structure. Therefore, the DC characteristics of HFETs can be improved by the insertion of Si-doped AlGaN layer, but it can result in more low-frequency noise with the carrier-injection layer.

On the Carrier Concentration and Hall Mobility in GaN Epilayers

The dependence of Hall mobility and carrier concentration in GaN epilayers on light illumination was examined. It was found that Hall mobility and electron concentration both increased after illumination with red laser (632.8 nm) and green laser (530 nm). However, no changes in Hall mobility and carrier concentration were found, if IR-laser (850 nm) was used. The results reveal that deep-level defects were excited and hence extra carriers were generated by light illumination. The influence is more pronounced for thinner films. These observations indicate that donor-like defect-related states were located 1.48 to 2.33 eV below the conduction band edge.

High optical-gain AlGaN/GaN 2 dimensional electron gas photodetectors

Nitride-based AlGaN/GaN heterostructure two dimensional electron gas (2DEG) photodetectors have been successfully fabricated. By using such an AlGaN/GaN heterostructure, we could significantly reduce the recombination of photogenerated carriers and thus achieve an extremely high photodetector responsiveness. At an incident light wavelength of 240 nm, it was found that the responsiveness could reach 5.2×109 A/W.

Two-step epitaxial lateral overgrowth of GaN

A two-step epitaxial lateral overgrowth (ELO) method was proposed to improve the quality of GaN-based epitaxial layers. In the first step, we grew a three-dimensional GaN low temperature buffer layer at 520 °C for different amount of time, and the second step is similar to conventional ELO. For two-step ELO GaN samples on SiO2 stripes along 〈1120〉 direction, it was found that an 8 min first step growth time could provide us the highest lateral to vertical growth rate ratio and the largest angle between sidewalls and basal plane. Under such growth conditions, we could achieve a surface root-mean-square (rms) roughness of 0.480 nm and an etch pits density (EPD) of 1.6×107 cm−2 in the stripe regions. The surface morphology of the two-step ELO GaN sample is also much better than that of the one-step ELO GaN sample.

Investigation of Metallized Source/Drain Extension for High-Performance Strained NMOSFETs

Extrinsic source/drain series resistance (R SD) is becoming inevitably dominant in state-of-the-art CMOS technologies as the intrinsic device resistance continues to scale with channel length dictated by the Moores Law. As a result, advanced scaling techniques to achieve a lower intrinsic device resistance become less effective, particularly for NMOSFETs. With an attempt to better understand R SD impacts and identify the next key technology enabler, high-performance strained NMOSFETs featuring metallized (NiSi) source/drain extension (M-SDE) are investigated due to its cost-effective process and good short-channel scalability. The spacing between metallized extension and gate electrode edge is shown to play a very important role in R SD reduction and can significantly affect the electrical characteristics of M-SDE NMOSFETs. Tradeoff between R SD reduction and device integrity like junction leakage and reliability is found when the extension-to-gate edge spacing is modulated. On the other hand, by optimizing the NiSi-to-gate edge spacing, M-SDE NMOSFETs exhibit a higher on-current (I ON) and a higher strain sensitivity while maintaining comparable drain-induced barrier lowering, subthreshold swing, I OFF , and hot-carrier reliability as compared with the conventional SDE devices.

N + -InGaAs/InAlAs recessed gates for InAs/AlSb HFET development

In this work, N⁺-InGaAs/InAlAs recessed gates for InAs/AlSb HFET development are presented. Highly doped N⁺-InGaAs cap layers are used to decrease the parasitic resistances in contact and access regions. As-grown modulation-doped epitaxy materials exhibit a Hall mobility of 14,200 cm²/V s and a sheet density of 6.15 ×10¹² cm⁻², while a mobility of 14,600 cm²/V s and a sheet density of 5.61×10¹² cm⁻² are shown after removal of the N⁺-InGaAs cap. Benefiting the energy band lowering using the highly doped cap layers, a low contact resistance of 0.06 Ω-mm is achieved. DC performances of I<inf>DSS</inf>=862mA/mm and g<inf>m, peak</inf>=927mS/mm and RF performances of f<inf>T</inf>=24GHz and f<inf>max</inf>=51GHz are demonstrated in a 2.1μm-gate-length device. An f<inf>T</inf>-L<inf>g</inf> product is as high as 51 GH-μm.