Tsung-Hsi Yang

The Roles of Threading Dislocations on Electrical Properties of AlGaN/GaN Heterostructure Grown by MBE

The role played by different types of threading dislocations (TDs) on the electrical properties of AlGaN/GaN heterostructure grown by plasma-assisted molecular beam epitaxy (MBE) was investigated. Samples with different defect structures and densities were prepared and measurements were taken from the same sample to study the correlative behavior of various TDs. From the Hall measurement, the electron mobility in two-dimensional electron gas channel was mainly controlled by the edge dislocation, which has a dominant amount in the material. The edge TDs acted as Coulomb scattering centers inside the channel and reduces the carrier mobility and increased its resistance. Screw TDs played a much significant role than edge TDs in determining the reverse-bias leakage current of Schottky barrier diodes. Leakage current is affected slightly by the reduction of free carrier density in the channel for samples with a higher edge TD density, but screw TD, which acted as the current leakage path, was more deleterious to the reverse-bias leakage current of AlGaN/GaN structure.

Study of thermal stability of nickel monogermanide on single- and polycrystalline germanium substrates

We have investigated the thermal stability of nickel monogermanide (NiGe) films formed by rapid thermal annealing on both single- and polycrystalline Ge substrates. We found that the NiGe phase is the only one present after nickel germanidation in the temperature range 400–700°C. A fairly uniform NiGe film formed on the single-crystalline Ge; it possessed excellent resistivity (15.6μΩcm) and was thermally stable up to 550°C, but it degraded rapidly at higher temperatures as a result of agglomeration. In contrast, the NiGe film formed on the polycrystalline Ge exhibited much poorer thermal stability, possibly because of polycrystalline Ge grain growth, which resulted in columnar NiGe grains interlaced with Ge grains that had a dramatically increased sheet resistance. As a result, we observed that the sheet resistances of NiGe lines subjected to annealing at 500°C depended strongly on the linewidth when this width was comparable with the grain size of the polycrystalline Ge.

Growth of high-quality Ge epitaxial layers on Si(100)

A method of growing high-quality epitaxial Ge layers on a Si(100) substrate is reported. In this method, a 0.8 mum Si0.1Ge0.9 layer was first grown. Due to the large lattice mismatch between this layer and the Si substrate, many dislocations form near the interface and in the lower part of the Si0.1Ge0.9 layer. A 0.8 mum Si0.05Ge0.95 layer and a 1.0 mum top Ge layer were subsequently grown on the Si0.1Ge0.9 layer. The formed interfaces of Si0.05Ge0.95/Si0.1Ge0.9 and Ge/Si0.05Ge0.95 can bend and terminate the upward-propagated dislocations very effectively. The in situ annealing process was also performed for each individual layer. Experimental results show that the dislocation density in the top Ge layer can be greatly reduced, and the surface is very smooth, while the total thickness of the structure is only 2.6 mum. (Less)

High-speed GaAs metal gate semiconductor field effect transistor structure grown on a composite Ge∕GexSi1−x∕Si substrate

In this study we used a low-pressure metal organic vapor phase epitaxy method to investigate the growth of GaAs metal gate semiconductor field effect transistor (MESFET) structures on a Si substrate. The buffer layer between the Si substrate and the grown GaAs epitaxial layers was a composite Ge∕Si0.05Ge0.95∕Si0.1Ge0.9 metamorphic layer. We used transmission electron microscopy to observe the microstructures formed in the grown GaAs∕Ge∕SixGe1−x∕Si material and atomic force microscopy to analyze the surface morphology and the formation of antiphase domains in the GaAs epitaxial layers. The measured Hall electron mobility in the channel layer of a MESFET structure grown on a 6° misoriented Si substrate was 2015 cm2 V−1 s−1 with a carrier concentration of 5.0×1017 cm−3. The MESFET device fabricated on this sample exhibited good current-voltage characteristics.

Study of nickel silicide contact on Si/Si 1-x Ge x

The properties of nickel silicide formed by depositing nickel on Si/p/sup +/-Si/sub 1-x/Ge/sub x/ layer are compared with that of nickel germanosilicide on p/sup +/-Si/sub 1-x/Ge/sub x/ layer formed by depositing Ni directly on p/sup +/-Si/sub 1-x/Ge/sub x/ layer without silicon consuming layer. After thermal annealing, nickel silicide on Si/p/sup +/-Si/sub 1-x/Ge/sub x/ layer shows lower sheet resistance and specific contact resistivity than that of nickel germanosilicide on p/sup +/-Si/sub 1-x/Ge/sub x/ layer. In addition, small junction leakage current is also observed for nickel silicide on a Si/p/sup +/-Si/sub 1-x/Ge/sub x//n-Si diode. In summary, with a Si consuming layer on top of the Si/sub 1-x/Ge/sub x/, the nickel silicide contact formed demonstrated improved electrical and materials characteristics as compared with the nickel germanosilicide contact which was formed directly on the Si/sub 1-x/Ge/sub x/ layer.

Optical characterization of ZnSe epilayers and ZnCdSe∕ZnSe quantum wells grown on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates

Several approaches have been employed to grow high-quality ZnSe epilayers on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. The ZnSe epilayers were characterized by photoluminescence spectroscopy. Migration enhanced epitaxy and inserting an in situ thermal annealing ZnSe buffer layer effectively reduced the intensity of deep level emissions from the ZnSe epilayer grown on a 6°-tilted Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrate. Optimized conditions for growing high-quality ZnSe were used to deposit ZnCdSe∕ZnSe multiple quantum wells on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. Photoluminescence spectroscopy revealed quantum-confinement effect in the ZnCdSe multiple quantum wells. The evolution of the exciton emission peak energy and the linewidth as a function of temperature indicate a low density of localized sites in the sample with a well width of 1nm. In the high-temperature regime, the thermal quenching of the excitonic emission intensity from ZnCdSe quantum well structures was governe...

Epitaxial Overgrowth of Gallium Nitride Nano-Rods on Silicon (111) Substrates by RF-Plasma-Assisted Molecular Beam Epitaxy

Strain-free gallium nitride (GaN) overgrowth on GaN nano-rods is realized by RF-plasma assisted molecular beam epitaxy (RF-MBE) on silicon (Si) substrate. The strain-free condition was identified by the strong free A exciton (FXA) photoluminescence (PL) peak at 3.478 eV and the E2 high phonon Raman shift of 567 cm-1. It is clearly demonstrated that the critical diameter of GaN nano-rods is around 80 nm for the overgrowth of strain-free GaN. The blue-shift of PL peak energy and phonon Raman energy with decreasing the diameter of nano-rod result from the strain relaxation of overgrowth GaN.

Layout design of high-quality SOI varactor

This paper presents the geometry effect on the characteristics of accumulation type SOI varactor with mesa-isolation technology. Constant gate area varactors with various geometry condition were implemented to investigate the effects of layout design parameters on overall varactor performance. Physical and mathematic analysis based on the measurement results show that parasitic capacitance at the edge of active region seriously degrades the device quality factor at smallest gate length. The optimized SOI varactor has a quality factor Q of about 150/GHz/pF at medium gate length varactor. The experiment result can serve as a design reference for high-quality SOI varactors.

Growth of ZnSe Epilayer on Si Using Ge/GexSi1-x Buffer Structure

The epitaxial growth of ZnSe layers on Si substrates utilizing a Ge/Ge0.95Si0.05/Ge0.9Si0.1 buffer structure is demonstrated. In this study, we examine the structure, optical characteristics and atomic interdiffusion of the ZnSe epilayer grown on Si. In a sample with a 2° off-cut Si substrate, the outdiffusion of Ge into the ZnSe epilayer is suppressed. The low-temperature PL measurements indicate that the sample with a 2° off-cut Si substrate improves its optical characteristic effectively. The X-ray diffraction analysis and transmission electron microscopy (TEM) results indicate that the use of a 2° off-cut Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si substrate markedly improves the crystallinity of and reduced the number of threading dislocations in ZnSe grown on Si.

Flower-Like Distributed Self-Organized Ge Dots on Patterned Si (001) Substrates

Self-organized Ge dots were obtained utilizing ultra high vacuum chemical molecular epitaxial growth of Ge on electron beam lithographically patterned Si (001) substrates. The dimensions of these etched Si mesa are 65/23/200 nm in diameter/height/period. The sizes and arrangement of the Ge dots were characterized by scanning electron microscopy and atomic force microscopy. The Ge dots have an average base width of 10 nm and the size is quite uniform. Due to the energetically favorable sites, the Ge dots tend to form homocentrically along the Si mesa edge, and their distribution is flower-like.