Pai-Chun Chang

ZnO nanowire field-effect transistor and oxygen sensing property

Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor deposition method and configured as field-effect transistors. Electrical transport studies show n-type semiconducting behavior with a carrier concentration of ∼107cm−1 and an electron mobility of ∼17cm2∕Vs. The contact Schottky barrier between the Au/Ni electrode and nanowire is determined from the temperature dependence of the conductance. Thermionic emission is found to dominate the transport mechanism. The effect of oxygen adsorption on electron transport through the nanowires is investigated. The sensitivity to oxygen is demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds high potential for nanoscale sensing applications.

Photoluminescence and polarized photodetection of single ZnO nanowires

Single crystal ZnOnanowires are synthesized and configured as field-effect transistors.Photoluminescence and photoconductivity measurements show defect-related deep electronic states giving rise to green-red emission and absorption. Photocurrent temporal response shows that current decay time is significantly prolonged in vacuum due to a slower oxygen chemisorption process. The photoconductivity of ZnOnanowires is strongly polarization dependent. Collectively, these results demonstrate that ZnOnanowire is a remarkable optoelectronic material for nanoscale device applications.

High-performance ZnO nanowire field effect transistors

ZnO nanowires with high crystalline and optical properties are characterized, showing strong effect of the surface defect states. In order to optimize the performance of devices based on these nanowires, a series of complementary metal-oxide semiconductor compatible surface passivation procedures is employed. Electrical transport measurements demonstrate significantly reduced subthreshold swing, high on/off ratio, and unprecedented field effect mobility.

Finite size effect in ZnO nanowires

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.

Electrical and photoconductive properties of vertical ZnO nanowires in high density arrays

High density vertical zinc oxide nanowire arrays were fabricated using highly ordered channels in anodic alumina membranes via chemical vapor deposition assisted by electrochemical deposition methods. Using conductive atomic force microscopy, the electrical transport and photoconduction of individual vertical nanowires were investigated. A negative photoconductivity was observed as a result of electron trapping in the alumina membrane. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies provide a pathway for constructing highly integrated nanoscale electronic and optoelectronic circuits, such as logic circuits, light emitting diodes, solar cells, and ultrahigh resolution imaging sensors.

β-Ga2O3 nanowires: Synthesis, characterization, and p-channel field-effect transistor

Quasione-dimensional Ga2O3 nanowires are synthesized via catalytic chemical vapor deposition method. Their morphology and crystal structure are characterized by electron microscopy and x-ray diffraction techniques, and their optical property is studied by photoluminescence measurement. To develop their future application in nanoelectronic devices, the as-grown Ga2O3 nanowires are doped with zinc to increase its carrier concentration and subsequently fabricated into field-effect transistors. Electron transport measurements show that the doped nanowires exhibit p-type semiconducting behavior with a significant enhancement of conductivity.

Optical size effects in ultrathin ZnO nanowires

Single crystal ZnO wurtzite nanowires grown along the c-axis with diameters down to 4 nm were synthesized by a catalytic vapor transport technique. Photoluminescence spectra of these wires indicate a blue shift of the free exciton by 19 meV due to confinement. This result was obtained by analyzing the line shape of the blue-shifted LO phonon replica of the free exciton. In addition, a surface-related excitonic luminescence feature centered at 3.366 eV was observed with a strongly elevated thermal activation energy.

Flexible photovoltaic technologies

Flexible photovoltaic (PV) devices have attracted enormous attention from academy and industry as a convenient alternative energy source for indoor and outdoor applications. Flexible PV panels can be easily integrated with infrastructures of various shapes and sizes, meanwhile they are light-weight and thus suitable for applications where weight is important. In this review, we will describe the progress that has been made in the field of flexible PV technologies. In addition, a summary will be provided with perspective on the future development of flexible solar cells and new opportunities offered by these devices.

ZnO Nanowire Field-Effect Transistors

Owing to the extraordinary properties and prominent applications in emerging nanoelectronics, ZnO nanowire has attracted tremendous research effort. This paper provides an introductory overview, covering topics ranging from basic nanowire synthesis and fundamental electrical properties to device characteristics based on field-effect transistor configuration.

Temperature dependent conduction and UV induced metal-to-insulator transition in ZnO nanowires

Thin ZnO nanowires with diameters of less than 50nm are configured as field effect transistors and studied for their transport mechanisms at different temperatures under UV illumination and gate modulation. The conductivity exhibits two regimes: at T>50K, thermally activated transport dominates with activation energy around 30–60meV attributed to the shallow donor states and at T<50K, three dimensional variable range hopping reveals in the conduction. In addition, UV irradiation leads to a metal-to-insulator transition at ∼210K. Furthermore, electrostatic gating results in a band bending giving rise to a change in the activation energy.