Morphology and defect study in low pressure MOCVD grown AlGaAs

A low pressure MOCVD system has been adopted to grow AlGaAs epitaxial layer on Si-doped or S.I. GaAs substrate by using TMA, TEG and AsH₃ as the sources.The effect of temperature on the surface morphology has been studied and a defect free surface has nearly been obtained.From EPMA analysis, at lower growth temperature (<710°C), the “whiskers” which were observed in this study are composed of a region with much less aluminum content than the surrounding normal region, but gallium and arsenic contents are nearly the same. There are few “precipitate hillock” defects on the epilayers especially under low growth temperature (<720°C), in which region, the aluminum content is greater than the surrounding region but gallium and arsenic contents are less.Raising the growth temperature (>750°C), both of “whisker defect” and “precipitate hillock” are eliminated. Very few “twin faults” defects and “bird-fault” defects on the epitaxial layers grown under high growth temperature were observed.     

Monolithic integrated resonant tunneling diode and heterostructure junction field effect transistor circuits

We have developed a simple technology for monolithic integration of resonant tunneling diodes (RTDs) and heterostructure junction-modulated field effect transistors (HJFETs). We have achieved good device performance with this technology: HJFETs had transconductances of 290 mS/mm and current densities of 310 mA/mm for a 1.5 μm gate length; RTDs had room temperature peak to valley ratios greater than 20:1 with current densities of 42 kA/cm². With this technology, we have demonstrated a monolithically integrated RTD + HJFET state holding circuit that can serve as a building block circuit for self-timed logic units. This circuit is resistor-free and operates at room temperature. The state holding circuit showed large noise margins of 1.21 V and 0.71 V, respectively, for input low and input high, for a 1.7 V input voltage swing. We have examined the transient response of the circuit and investigated the effect of circuit design parameters on propagation delay. We identify the RTD valley current as the limiting factor on propagation delay. We discuss the suitability of RTD + HJFET circuits such as our state holding circuit for highly dense integrated circuits.     

Van der Waals epitaxy of ultrathin crystalline PbTe nanosheets with high near-infrared photoelectric response

Nano Research - Lead telluride (PbTe) is one of the reliable candidates for infrared (IR) optoelectronics with optimum band-gap as well as excellent photoelectric properties. Great interests had...     

Simultaneously controlling phase,morphology and up-conversion luminescence via lanthanide doping in Bi2O3: Yb3+/Tm3+ nanoparticles

Bi₂O₃: Yb³⁺/Tm³⁺ nanoparticles with varied phase structures and morphologies were synthesized via the co-precipitation method assisted with a subsequent high-temperature calcination process. The influences of experimental parameters including dosage of urea, dopant concentration, reaction temperature and time on the crystal phase structures and morphologies of the as-prepared nanoparticles were investigated. Results revealed that the morphology, the crystal phase and the up-conversion emission of Bi₂O₃: Yb³⁺/Tm³⁺ could be simultaneously controlled by varying Yb³⁺ doping concentration. With increasing Yb³⁺ doping concentration (0-40%), the dramatic morphological evolution (nano-sheet to nano-flower to nano-sphere) and phase transition (monoclinic to tetragonal to cubic phase) were observed. Under the excitation of CW and pulse 980 nm lasers, Bi₂O₃: Yb³⁺/Tm³⁺ samples exhibited red (693 nm) and NIR (795 nm) double-band up-conversion emissions, where the increase of Yb³⁺ concentration leaded to the decreased emission intensity ratio of NIR to red (I₇₉₅/I₆₉₃) and the suppressed pulse-width-dependent up-conversion output.     

Microcomputer controlled arc oscillator for automated TIG welding

A magnetic arc oscillation system has been developed to control the fusion characteristic of arc welding. A single-pole electromagnet generates a transverse magnetic field parallel to the weld line. The weave pattern of the magnetic field, hence the bead geometry, is controlled by a custom-built computer. The applied magnetic field has a marked influence on the bead width, but less on the bead depth. Experiments also show that bead appearance is improved by magnetically oscillating the arc.     

Thermal peeling stress analysis of thin-film high-TC superconductors

Thermal peeling stress between a thin film and the substrate is caused by the mismatch of thermal expansion coefficients while the film and substrate undergo a temperature change. The thermal peeling stress resulting from the temperature decrease from ambient to operating conditions (cryogenic temperatures) between a thin-film high-temperature superconductor and its substrate is calculated using finite element analysis (FEA). The superconductor thin film is idealized as a long bridge on a large substrate. A two-dimensional FEA model is applied to calculate the tensile (peeling) stress at the thin film/substrate interface. Results are obtained for different geometries and temperature conditions, and these results are compared with analytical predictions. A stress singularity is found at the very edge of the thin film which is not predicted by the analytical prediction. The peeling stress can be very high due to this stress singularity, even if the temperature change is not large. The stress singularity area depends on the local geometry of the edge, suggesting that refining the geometry of the thin-film HTS device is important.     

Field emission properties of diamond and carbon nanotubes

Both diamond and carbon nanotubes are efficient field emitters. Both materials can emit electrons at very low electric fields (3–7 V/μm for a current density of 10 mA/cm²). Moreover, nanotube emitters are capable of delivering very high emission currents densities, with current density routinely exceeding 1 A/cm². Improvements of emission uniformity are critical in realizing the potential of these carbon materials in enabling practically useful cold cathode devices.     

Carbon nanotube growth by rapid thermal processing

A new method for carbon nanotube (CNT) growth by rapid thermal processing (RTP) of amorphous carbon film is reported. This is a two-step method, involving an ion-beam sputtering process followed by an RTP treatment. Amorphous carbon film containing iron was first deposited by ion-beam sputtering. The as-deposited films were then annealed by RTP under an inert environment. High-density multi-wall carbon nanotubes with a length of 1 μm and a diameter of approximately 30 nm were observed on the surface of amorphous carbon film after RTP at 1200°C for 30 s in nitrogen (N₂). Detailed morphology and structure analyses of carbon nanotubes thus obtained using SEM and HR-TEM indicated that the number density and growth rate of the CNT were dependent on the process temperature and ambient gas. The use of RTP provides a straightforward, convenient and cost-effective method to grow carbon nanotubes with controllable length and density.     

A comparison of the mechanical properties of fibers spun from different carbon nanotubes

Spinnable carbon nanotube (CNT) arrays with different CNT structures have been synthesized using different growth methods and carbon sources, and long and stable fibers have been produced. Parameters of the nanotubes such as tube diameter, wall thickness, tube length and level of defects were found to play a more important role in the mechanical properties of the fibers than did the initial tube arrangement. To improve the fiber strength, as well as the modulus, the tubes must be long and have a small diameter and thin walls. The strongest fiber from double- and triple-walled CNTs is 1.23 GPa in strength, and 32% and 221% higher than those from CNTs with ∼6 and ∼15 walls (932 and 383 MPa), respectively. The fiber strength can be improved by 25%, up to 1.54 GPa, after poly(vinyl alcohol) infiltration with volume fraction of ∼20%. Our study also shows that C₂H₄ is superior to C₂H₂ as the carbon source for the growth of mainly double- and triple-walled CNTs, and therefore the spinning of high-strength fibers.