Hwangyou Oh

Schottky diodes based on a single GaN nanowire

On a single GaN nanowire, obtained by chemical vapour deposition, several Schottky-junction diodes were fabricated and their electrical transport properties were studied. Alternately attached metal electrodes of Al and Ti/Au formed a Schottky barrier junction (for Al) or an ohmic contact (for Ti/Au), resulting in several diodes on a single nanowire. The current–voltage measurements exhibited clear rectifying behaviour and no reverse-bias breakdown was observed up to the measured voltage, −5 V. The forward-bias threshold voltage was observed to decrease linearly with temperature, from 0.4 V at 280 K to 1 V at 10 K.

Synthesis, electrical and photoresponse properties of vertically well-aligned and epitaxial ZnO nanorods on GaN-buffered sapphire substrates

Vertically well-aligned and epitaxial ZnO nanorod arrays (NRAs) were synthesized by catalyst-free metalorganic chemical vapor deposition on Al2O3 (0001) substrates particularly using an epitaxially grown GaN buffer layer. ZnO NRAs grown directly on Al2O3 (0001) substrates are aligned vertically, but randomly oriented in the lateral direction to the substrates. In sharp contrast, ZnO NRAs grown with a GaN buffer layer show an excellent vertical and epitaxial alignment with mosaic distributions of 0.11° and 1.28° in the out-of- and the in-plane directions, respectively. The electrical measurements using field effect transistors based on individual ZnO nanorods show a pronounced n-type gate modulation with an electron concentration of ∼7.5×1017cm−3 and an electron mobility of ∼25.1cm2∕Vs at a bias voltage of 1 V, while showing quite a high on/off ratio exceeding ∼105. In addition, their high on/off conductivity ratio of ∼103 with UV light gives a potential of their use in nanoscale UV detectors.

Interface charge induced p-type characteristics of aligned Si(1-x)Gex nanowires.

This study reports the electrical transport characteristics of Si(1-x)Gex (x=0-0.3) nanowires. Nanowires with diameters of 50-100 nm were grown on Si substrates. The valence band spectra from the nanowires indicate that energy band gap modulation is readily achievable using the Ge content. The structural characterization showed that the native oxide of the Si(1-x)Gex nanowires was dominated by SiO2; however, the interfaces between the nanowire and the SiO2 layer consisted of a mixture of Si and Ge oxides. The electrical characterization of a nanowire field effect transistor showed p-type behavior in all Si(1-x)Gex compositions due to the Ge-O and Si-O-Ge bonds at the interface and, accordingly, the accumulation of holes in the level filled with electrons. The interfacial bonds also dominate the mobility and on- and off-current ratio. The large interfacial area of the nanowire, together with the trapped negative interface charge, creates an appearance of p-type characteristics in the Si(1-x)Gex alloy system. Surface or interface structural control, as well as compositional modulation, would be critical in realizing high-performance Si(1-x)Gex nanowire devices.

Fabrication of n-type carbon nanotube field-effect transistors by Al doping

We report the effect of an Al layer, covering the central part of the nanotube channel, on the electrical transport properties of carbon nanotube field-effect transistors (CNFETs). The CNFETs, consisting of single-walled carbon nanotube or double-walled carbon nanotube between two Pd electrodes on top of SiO2 layer, which showed p-type or ambipolar transport behaviors, exhibit clear n-type characteristics after the Al deposition. We ascribe such conversions into n-type behaviors to the electron doping in the Al-covered nanotube region, which results in the bending of the nanotube bands nearby the edges of the Al layer. This technique, Al deposition under a high vacuum, may give rise to a practical fabrication method for the n-type CNFET, which may enable us to develop complementary logic nanotube electronic devices.

Nonlinear transport properties in multiwall carbon nanotube heterojunctions

Electronic transport properties of a heterostructure junction made of two different multiwall carbon nanotubes are studied. Independent measurement of the current–voltage characteristics of each nanotube revealed that both of them were lightly p-doped semiconductors with energy gaps whose magnitudes are widely different from each other. The current–voltage characteristics measured across the heterojunction show reproducible rectifying diode behavior. The forward bias current across the heterojunction increases rapidly with the application of a positive gate bias voltage, implying an n-type gate response.

Observation of hysteretic magnetoresistance in Mn-doped GaN nanowires with the mesoscopic Co and Ti∕Au contacts

We have studied the spin dependent tunneling properties of Mn-doped GaN nanowires with ferromagnetic Co contacts. The magnetoresistances were measured between two ferromagnetic Co electrodes, or Co and Ti∕Au electrodes through Mn-doped GaN nanowires. The magnetoresistances of nanowire with the Co electrode indicate hysteretic behaviors, which are commonly observed in tunnel magnetoresistance devices. The magnetoresistance ratio increases from −0.6% at 20K to −9.4% at 1.74K. It is believed that the hysteretic magnetoresistances originate from the tunnel magnetoresistance effect between the ferromagnetic phases of nanowire and Co electrode.

The temperature-dependent electrical transport mechanism of single ZnO nanorods

The electrical transport properties of high quality ZnO nanorods, synthesized by catalyst-free metalorganic chemical vapour deposition, were studied as a function of temperature by fabricating field effect transistors based on single ZnO nanorods. The thermally activated Schottky emission was found to be a ruling transport mechanism in the temperature regime of 70?293?K over a wide range of electric fields with an effective barrier height of ~120?meV. In contrast, the Fowler?Nordheim tunnelling dominated at low temperatures (<70?K) under very high electric fields.

Rectifying diode made of individual gallium nitride nanowire

Abstract We have fabricated a Schottky-junction diode, utilizing gallium nitride (GaN) nanowire, and studied its electrical transport properties. Two kinds of metal electrodes, Al and Ti/Au, were incorporated into an individual GaN nanowire synthesized by chemical vapor deposition method. A Schottky-barrier junction was formed in the Al electrode while ohmic contact was formed in the Ti/Au electrode. The measured current–voltage characteristic exhibited clear rectifying behavior and no reverse bias breakdown was observed up to the measured voltage, −5 V .