Iam Keong Sou

Impurity effect on weak antilocalization in the topological insulator Bi2Te3.

We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.

ZnSe nanowires epitaxially grown on GaP(111) substrates by molecular-beam epitaxy

We report molecular-beam epitaxy growth of single crystalline ZnSe nanowires with uniform diameters (∼10 nm) on GaP(111) substrates. The growth process was based on the Au-catalyzed vapor-liquid-solid deposition. As determined by electron microdiffraction and high-resolution transmission electron microscopy, ZnSe nanowires grew generally along the 〈110〉 and 〈112〉 directions with the orientation relationship of (111)ZnSe wire//(111)GaP and 〈110〉ZnSe wire//〈110〉GaP. The dominant defects were found to be twins at the interface between the substrate and the nanowires along the (111) plane.

A ZigBee-Based Wireless Sensor Network Node for Ultraviolet Detection of Flame

This paper describes a ZigBee-based wireless sensor network node for the ultraviolet (UV) detection of flame. The sensor node is composed of a ZnSSe UV photodetector, a current-sensitive front end including a high-gain current-to-voltage amplifier with 120 dB and a logarithm converter, and a transceiver operated at a 2.4-GHz industrial, scientific, and medical band. A passive photodetector is designed to have a cutoff at 360 nm and convert the UV emission of flame into picoamperes. Including mixed signal processing and ZigBee transmission, the speed of flame detection is as fast as 70 ms. The sensor node consumes only an average of 2.3 mW from a 3.3-V supply. The performance of a prototype sensor node was verified when the luminous flame was imaged onto the sensor node with different angles ranging from -30° to 30° and distances of 0.1, 0.2, and 0.3 m enabling effective fire safety applications.

HIGHLY EFFICIENT LIGHT EMISSION FROM ZNS1-XTEX ALLOYS

ZnS1−xTex (0≤x≤1) single‐crystal alloy films were grown on GaAs and Si substrates by molecular beam epitaxy. Strong photoluminescence in the yellow to blue light region, with room‐temperature external quantum efficiencies of 2%–4% at an unoptimized excitation wavelength of 365 nm, was observed. The enhancement of luminescence was attributed to the presence of Te isoelectronic hole traps in the films. Strong bowing of the band‐gap energy as a function of composition x was also observed, with the minimum near x=0.7. The line width as well as the Stokes shift of the luminescence peak from the band edge were found to increase as Te composition decreases.

Two-dimensional superconductivity at the interface of a Bi 2 Te 3 /FeTe heterostructure

The realization of superconductivity at the interface between a topological insulator and an iron-chalcogenide compound is highly attractive for exploring several recent theoretical predictions involving these two new classes of materials. Here we report transport measurements on a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy, which demonstrate superconductivity at the interface, which is induced by the Bi2Te3 epilayer with thickness even down to one quintuple layer, though there is no clear-cut evidence that the observed superconductivity is induced by the topological surface states. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12 K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. With the combination of interface superconductivity and Dirac surface states of Bi2Te3, the heterostructure studied in this work provides a novel platform for realizing Majorana fermions.

Molecular-beam-epitaxy-grown ZnMgS ultraviolet photodetectors

Epitaxial growth of Zn1−xMgxS alloy thin films on GaP(100) substrates was carried out using the molecular-beam-epitaxy technique. In situ reflection high-energy electron diffraction studies show that the alloys can be grown with a stable zinc-blende structure up to x around 30%. For x>30%, a structural transition will occur at a critical thickness which is sensitively dependent on the x composition. A near-band-edge peak with a full width at half maximum of about 10 nm was observed in room-temperature photoluminescence measurements made on as-grown alloy thin films. Several Zn1−xMgxS-based Schottky barrier photodetectors were fabricated. Room-temperature photoresponse measurements were performed on these detectors and abrupt long-wavelength cutoffs covering 325, 305, 295, and 270 nm were achieved for devices with Mg composition of 16%, 44%, 57%, and 75%, respectively. The response curve of the Zn0.43Mg0.57S device offers a close match to the erythemal action spectrum that describes human skin sensitivity ...

Temperature dependence of the responsivity of II–VI ultraviolet photodiodes

High-temperature dependence, up to 150 °C, of the photoresponsivity of ZnS, ZnSTe, and ZnSSe photodiodes was investigated in this study. It was found that, in general, the responsivity at higher temperatures will shift to longer wavelengths because of band-gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density-of-state distribution due to lattice expansion at high temperatures, and a simplified model is used to illustrate this hypothesis.

Photoresponse studies of ZnSSe visible–blind ultraviolet detectors: A comparison to ZnSTe detectors

This work focuses on the investigation of the difference between the photoresponse of ZnS, ZnSSe, and that of ZnSTe Schottky-barrier photodiodes, with a particular aim to reveal the underlying causes of the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky barrier photodiodes. To form the bottom electrode layer for the newly developed ZnSSe diode, n-type doping of ZnSSe by incorporating Al flux during molecular beam epitaxial growth was studied. Excellent-to-good dopant activation is achieved for Se composition up to 50%. The measured photoresponse of the diodes clearly indicates that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky-barrier photodiodes. The results also reveal that the ZnSSe diode, having a much better visible rejection power, is a more suitable choice for high-performance visible–blind ultraviolet detection applications.

Aluminum-doped n-type ZnSTe alloy grown by molecular beam epitaxy

Successful n‐type doping of ZnSTe alloy using elemental aluminum source has been carried out by molecular beam epitaxy. Hall effect measurement (300–77 K) was performed on as‐grown ZnS0.977Te0.023 epilayers with various dopant concentrations. Electron carrier concentration as high as 1.3×1019 cm−3 has been achieved. For carrier concentration higher than 5×1018 cm−3, the carrier concentration is independent of temperature, possibly indicating formation of a very shallow donor level. A group of ZnS1−xTex epilayers with different x values was doped using a constant aluminum beam flux for studying the dependence of the dopant activation on Te composition. Good activation of Al dopant was obtained for x value from 0 to a few percent, but it became poor for larger x value and finally Al became inactive for x values higher than 10%. Room temperature photoluminescence measurements on doped and undoped ZnS and ZnS1−xTex layers indicate that Al dopants form deep‐level radiative centers in addition to a shallow dono...

ZnSTe-based Schottky barrier ultraviolet detectors with nanosecond response time

ZnSTe-based Schottky barrier photovoltaic detector arrays were fabricated on GaP(100) using a two-step molecular beam epitaxy growth approach. These detectors exhibit visible blind and ultraviolet (UV) sensitive response with a peak UV responsivity of 0.13 A/W and 1.2×106 V/W at 320 nm. The built-in potential of these detectors was determined to be 1.7 V. The temporal photocurrent response of a 400×400 μm2 detector was measured to be 1.2 ns, limited apparently by the resistance-capacitance (rc) constant of the detector structure.