Search Results - (Author, Cooperation:C. K. Shih)

2012-07-28

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We describe the design and characterization of a new variable low-temperature scanning tunneling microscope (STM) which has been proven to give atomic resolution at temperatures between 77 K and room temperature but which is also capable of performing experiments as low as 4 K. The STM "head'' itself consists of a unique cold dewar made up of an upper and lower reservoir connected together by two tubes which pass through an 8 in. conflat flange. The STM stage is suspended in between the two reservoirs by three long springs which pass through holes in the upper reservoir and also attach to the top flange. An adjustable cold shroud surrounds the STM stage for radiation shielding while allowing sample and tip transfer when raised. It has the additional advantage of providing a controllable heat leak. By raising the shroud, the temperature can be increased; by lowering it, the temperature can be decreased. The cold reservoirs can be filled with either liquid nitrogen or liquid helium. In the case of liquid helium, an additional liquid nitrogen "jacket'' surrounds the STM head. Everything is operated inside a customized ultrahigh vacuum chamber containing low-energy electron diffraction and various sample preparation facilities. A detailed description of the STM is presented together with performance results. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We present a cross-sectional scanning tunneling microscopy (XSTM) study of the spontaneous ordering of Ga0.48In0.52P and Ga0.52In0.48P grown on (001) GaAs substrates by molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE), respectively. The (111)-type alloy ordering could be seen clearly in the OMVPE-grown alloy region. On the other hand, the MBE-grown region shows a very small degree of ordering as revealed by the STM. Most of the ordered region shows (InP)1(GaP)1-type ordering: alternating InP- and GaP-like (1¯11) planes. In addition to this type of ordering, we also observe another type of ordering consisting of two InP-like (1¯11) planes and one GaP-like (1¯11) plane that we call (InP)2(GaP)1-type ordering. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A scanning near-field microscope design using the reflected light intensity as the feedback mechanism is described. Multiple fibers with high numerical apertures provide a high collection efficiency in a reflection geometry. The performance with regard to its response to large spatial variations has been tested by using a Si-grating sample and with regard to variations of local indices of refraction by using GaAs/AlGaAs heterostructure samples. In addition, spatially resolved spectroscopy on GaAs/AlGaAs heterostructures has been obtained. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The valence-band structures of 50-A(ring)-thick layers of GaAs(001) in tension and In0.2Ga0.8As(001) in compression have been determined using angle-resolved photoemission spectroscopy. Our studies show that the Δ3+Δ4 bands and the Δ1 band respond differently to the strain perturbation. For strained GaAs(001), the Δ3+Δ4 bands (Px+Py-like) are shifted up in energy by a maximum amount of 0.3 eV, while in contrast, the Δ1 band (Pz-like) is shifted down by about 0.1 eV. For strained In0.2Ga0.8As(001), the band shifts are in the opposite direction, consistent with the opposite strain conditions. For both materials, the strain-induced changes cannot be characterized simply by rigid band shifts, but rather exhibit significant wave vector dependence. This results in a reduction of the effective mass of the Δ3+Δ4 bands for both GaAs and In0.2Ga0.8As.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The natural valence-band offset (NVBO) between semiconductors in a common anion alloy system can be determined through photoemission core level measurements. In this work, we tested this method in the InxGa1−xAs system. The NVBO between GaAs and InAs is measured to be 0.11±0.05 eV. This result is in approximate agreement with the experimental value of 0.17±0.07 eV determined by x-ray photoemission spectroscopy measurements.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We report on the development of a new two-dimensional micropositioning device, or walker, which is capable of moving across very large distances (in principle unlimited) and with a very small step size (as small as 100 A(ring)/step) in both directions. Based on a unique tracking design, the motion is extremely orthogonal with very little crosstalk between the two directions. Additionally, there is no detectable backlash in either direction. The walker performance has been extensively tested by using a position-sensitive proximitor probe. Tests have been done between 77 and 300 K. However, we project that the walker will be able to operate at temperatures as low as 4 K. This walker system has shown extremely reliable performance in a UHV environment for use with scanning tunneling microscopy and has been especially useful for cross-sectional scanning tunneling microscopy and spectroscopy studies of semiconductor hetero- and homostructures. We show one example of results on the (AlGa)As/GaAs heterostructure system.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Scanning tunneling microscopy (STM) was used to study the (NH4)2S-passivated (110) cross-sectional surfaces of both doped and undoped Al0.3Ga0.7As/GaAs heterostructures on n+-substrates. The ex situ (NH4)2S treatment of the cross-sectional surfaces of heterostructures was found to be very stable against oxidation. STM images showed no appreciable deterioration of surface quality in vacuum after more than 40 days. The spectroscopic results on the undoped epilayer showed diodelike behavior, confirming that an undoped large band gap region can be imaged by STM through carrier injection from the conductive regions.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report studies of GaAs/AlAs short period superlattices using cross-sectional scanning tunneling microscopy. In particular, we investigate the role of growth interrupt time on the resulting interfacial structure. Superlattices with repeated periods of four layers of GaAs and two layers of AlAs are resolved atom by atom. Superlattices grown using a 30 s growth interrupt time are observed while those grown with a 5 s growth interrupt time are not. We also discuss residual effects of the growth interrupt process on layers grown on top of the short-period superlattice. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Scanning tunneling microscopy and spectroscopy is used to study GaAs multiple pn junction samples cleaved in ultrahigh vacuum. Direct topographic contrast over the pn junctions can be observed in the constant current imaging mode. The topographic height in the p-type regions appears much lower (by about 5 A(ring)) than that in the n-type regions. Tunneling spectroscopy measurements show consistency with the assignment of the p- and n-type regions. We discuss a possible mechanism for the observed contrast.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Cross-sectional scanning tunneling microscopy is used to study dilute AlxGa1−xAs with x=0.05 to investigate the bonding configurations within this ternary alloy. Atomically resolved scanning tunneling microscopy images combined with symmetry considerations provide the assignment of first and second layer aluminum atoms. The Al–Al pair distribution function based on the experimental data is compared with the theoretical pair distribution function of a random alloy. While there exists a qualitative agreement, small deviations from the ideal random distribution are also found. © 1996 American Institute of Physics.

Electronic Resource

0032-3888

Chemistry ; Chemical Engineering

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

The rheological characteristics of a number of polymer blends, including amorphous and semi-crystalline plastic matrices, during melting or softening, are analyzed with a batch mixer. The physical changes accompanying the phase transitions taking place as the slat-and-pepper blends are heated and mixed in the region of their component melt transition temperatures are monitored visually through a transparent window with video recordings. The torque of mixing and the mixture temperature are also recorded continuously. The strong effects of additives such as filler or lower melting resin as well as rotor speed, degree of fill, and ram pressure were observed.

5 Ill.

Electronic Resource

0032-3888

Chemistry ; Chemical Engineering

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Mold flows and melt viscosities of an incompatible bi-component EPDM and “Viton” fluoroelastormer system were examined. A marked reduction in the melt viscosities of either component was observed when a small amount of the other component was present. It was speculated that the phenomenon was a result of a slippage between the polymer and the coated capillary surface due to the presence of a minor amount of the incompatible polymer. Evidence is given to indicate that this effect cannot be accounted for using the “melt structural heterogeneity” mechanism proposed for a different system by Andtrianova.

4 Ill.

Electronic Resource

0021-8995

Chemistry ; Polymer and Materials Science

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

An improved differential thermal analytical technique which permits the rapid, convenient characterization of the thermal behavior of crystalline polymers free of any influence of prior thermal history is presented. Characterization of both crystallization and fusion phenomena is described for ethylene/propylene copolymers subjected to well-controlled thermal scanning techniques. Parameters describing these phenomena are derived. While they are nonequilibrium parameters, they are reproducible and capable of correlation with polymer composition. the crystallization onset temperature determined by this cooling technique was found to relate to the molar ethylene content of the copolymers by an equation similar to the one derived by Flory5 based on equilibrium melting point. The relationship was found to hold true for a number of ethylene copolymers, including samples of linear and branched polyethylene, commercial EPDM, and ethylene/vinyl acetate copolymers.

8 Ill.

Electronic Resource