Gary Harris

Growth of GaN nanowires by direct reaction of Ga with NH3

Semiconducting, single crystal wurtzite GaN nanowires have been grown by direct reaction of metal Ga with NH3 in a tube furnace. This paper discusses the growth mechanism. Nanowires grow only between 8251C and 925 o C. Their diameters vary between 20 and 150 nm and depend directly on temperature and NH3 flow rate. Wires as long as 500mm have been fabricated; once wires have formed, their length increases directly with time in the reactor. There are three different stages in the process, each of which has its own mechanism. First, a nearly amorphous GaN matrix forms, followed by growth of hillocks of thin GaN platelets. Finally, nanowires emerge from the edges of the platelets in characteristic directions. This analysis can be used as a guide for controlling GaN wire diameters and lengths. Strategies for growth of thinner and thicker nanowires are suggested. Thicker cylindrical structures denoted as rods grow from the face of the platelets. Description of their growth mechanism requires further study. r 2001 Elsevier Science B.V. All rights reserved.

Amorphous and Crystalline Silicon Carbide and Related Materials

Although silicon carbide has been used for more than half a century, its potential as a high-temperature, corrosion-resistant semiconductor has only recently begun to be exploited. Both crystalline and amorphous forms of SiC offer several advantages over Si, GaAs, and InP for high-frequency, high-power, and high-speed circuits. This volume contains reports on high-temperature SiC MOSFETs and MESFETs, secondary harmonic generation in SiC, a-SiC emitter heterojunction bipolar transistors, and bulk crystal growth of 6H-SiC. For newcomers to the field it provides an up-to-date review of technological developments in SiC and related materials, while specialists will find here recent references and new insights into materials for high-temperature, high-power, and high-speed circuit applications.

An investigation of 3C-SiC photoconductive power switching devices

Abstract Photoconductive semiconductor switches (PCSS) have unique advantages such as high power, high speed, negligible time jitter and long lifetime. Silicon carbide (SiC), due to its high dielectric strength and other desired physical properties, is an excellent material for PCSS. However, no result has been reported on cubic silicon carbide (3C-SiC) PCSS. In this work, PCSS were fabricated on the following three types of 3C-S1C material; (i) boron doped, (ii) unintentionally doped single crystals, and (iii) polycrystalline. The PCSS were investigated using ArF and XeCl excimer lasers. Practical switches with many potential applications were successfully fabricated. The best results were obtained from the PCSS made from polycrystalline material. The dark resistivity of the material was as high as 10 6 Ω cm. The operating breakdown field was 250 kV cm −1 , which is the highest reported for all lateral geometry PCSS and was limited by the surface flashover effect. The highest peak photocurrent density through the PCSS was greater than 10 kA cm −2 . The ratio of the off-state resistance on the on-state resistance, R off / R on , was ~ 10 5 , and the lowest on-state resistance was 45 Ω. The width of the photocurrent pulse was 15–30 ns, which was limited by the laser pulse width, indicating that the PCSS can operate in the megahertz range. The trigger gain of the polycrystalline 3C-SiC PCSS was 4.7 and the switching efficiency was 52%.

Biphasic GaN nanowires: Growth mechanism and properties

Catalyst-free vapor-solid nanowire growth has been used to produce novel multiphase zinc-blende/wurtzite gallium nitride nanowires. Multiphase nanowire growth occurred at nanoscale nucleation sites on platelets of gallium nitride. Growth temperature has been shown to exert a strong influence on nucleation site formation. Scanning electron microscopy (SEM) was used to characterize the matrix from which the nanowires grew. Nanowires were characterized with transmission electron microscopy (TEM).

Fast fabrication of NiO@graphene composites for supercapacitor electrodes: Combination of reduction and deposition

Graphene-based inorganic composites have been attracting more and more attention since the attachment of inorganic nanoparticles instead of conducting polymeric materials to graphene sheets turns out higher capacitances and good capacity retention. Here we report a fast fabrication method to prepare NiO@graphene composite modified electrodes for supercapacitors. By this method, preparation of electrochemical active materials of NiO/graphene and modification of the electrode can be simultaneously performed, which is achieved separately by traditional method. Moreover, the problem of poor adhesion of active materials on the surface of the electrode can be well solved. The NiO particles introduced to the films exhibit pseudocapacitive behavior arising from the reversible Faradaic transitions of Ni(II)/Ni(III) and greatly improve the capacitance of the electrodes. With the increase in NiO content, highly reduced graphene can be obtained during cyclic voltammetry sweeping, leading to the increase in the electrode capacitance. The highest specific capacitance of the constructed electrodes can reach 1258 F/g at a current density of 5 A/g.

Polycrystalline cubic silicon carbide photoconductive switch

The first cubic silicon carbide (3C-SiC) photoconductive switches were fabricated from polycrystalline 3C-SiC. The switches had a dark resistivity of 10/sup 6/ /spl Omega//cm. A breakdown field of 250 kV/cm and a peak photocurrent density of 10 kA/cm/sup 2/ through the switch were obtained. The ratio of off-resistance to on-resistance of the switch reached up to 10/sup 5/. The photocurrent had a pulse width as narrow as 15 ns. The trigger gain of the switch was 4.7.

Anodic etching of p-type cubic silicon carbide

Abstractp-Type cubic silicon carbide was anodically etched using an electrolyte of HF:HCl:H2O. The etching depth was determined versus time with a fixed current density of 96.4 mA cm−2. It was found that the etching was very smooth and very uniform. An etch rate of 22.7 nm s−1 was obtained in a 1:1:50 HF:HCl:H2O electrolyte.

Topographic guidance based on microgrooved electroactive composite films for neural interface.

Topographical features are essential to neural interface for better neuron attachment and growth. This paper presents a facile and feasible route to fabricate an electroactive and biocompatible micro-patterned Single-walled carbon nanotube/poly(3,4-ethylenedioxythiophene) composite films (SWNT/PEDOT) for interface of neural electrodes. The uniform SWNT/PEDOT composite films with nanoscale pores and microscale grooves significantly enlarged the electrode-electrolyte interface, facilitated ion transfer within the bulk film, and more importantly, provided topology cues for the proliferation and differentiation of neural cells. Electrochemical analyses indicated that the introduction of PEDOT greatly improved the stability of the SWNT/PEDOT composite film and decreased the electrode/electrolyte interfacial impedance. Further, in vitro culture of rat pheochromocytoma (PC12) cells and MTT testing showed that the grooved SWNT/PEDOT composite film was non-toxic and favorable to guide the growth and extension of neurite. Our results demonstrated that the fabricated microscale groove patterns were not only beneficial in the development of models for nervous system biology, but also in creating therapeutic approaches for nerve injuries.

Nitrogen-activated bowing of dilute InyGa1-yAs1-xNx based on photoreflectance studies

The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga1−yInyAs1−xNx epilayers on nitrogen mole fraction (x), for x⩽0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, EG, was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of EG on x) was found to become less negative with increasing x; the value of b changed from −50 eV, at very low nitrogen fraction, to −20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga1−yInyAs1−xNx alloys.The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga1−yInyAs1−xNx epilayers on nitrogen mole fraction (x), for x⩽0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, EG, was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of EG on x) was found to become less negative with increasing x; the value of b changed from −50 eV, at very low nitrogen fraction, to −20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga1−yInyAs1−xNx alloys.

Semiconductor and photoconductive GaN nanowires and nanotubes

Summary form only given. It is difficult to control carbon nanotube chirality, which directly affects electronic performance. Preliminary results show that unlike carbon nanotubes, conductive properties of GaN are more or less independent of chirality and diameter. They are single crystal and not single or multi-wall. Further, both p and n-type GaN can be made, and its melting temperature is >1400/spl deg/C. Thus GaN nanotubes could be tuned for use in molecular electronic devices.A significant increase in current at the same bias voltage when UV light is shined on the device may be the first demonstration of photoconductivity in a nanowire, and suggests the possibility of using them as opto-electric switches and transistors.