Oxide Nanowire Materials and Devices
ZnO Nanowire Transistors and Sensors
One of the more interesting semiconducting materials for nanowire fabrication and exploitation is ZnO. This binary oxide is an n-type semiconductor with a direct bandgap of 3.37eV. ZnO is attractive for optical applications, given its large bandgap and large exciton binding energy. Motivation for investigating ZnO nanowires include understanding low-dimensional electron dynamics, development of nano sensors for optical, chemical, biological detection, and exploration of nanoelectronic devices. We report on catalyst-driven molecular beam epitaxy of ZnO nanorods. The process is site-specific, as single crystal ZnO nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300-500°C. The nanorods are uniform cylinders, exhibiting diameters of 15-40 nm and lengths in excess of 1 m. With this approach, nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics.
ZnO Nanowire Transistors
Research activities at UF are examining the use of ZnO nanowires as nano-transistors. Single ZnO nanowire metal-oxide semiconductor field effect transistors (MOSFETs) have been fabricated using nanowires grown by site selective Molecular Beam Epitaxy. When measured in the dark at 25C,the depletion-mode transistors exhibit good saturation behavior, a threshold voltage of -3V and a maximum transconductance of order 0.3 mS/mm .Under ultra-violet (366nm) illumination, the drain-source current increase by approximately a factor of 5 and the maximum transconductance is 5 mS/mm. The channel mobility is estimated to be 3 cm2 /V.s, which is comparable to that reported for thin film ZnO enhancement mode MOSFETs and the on/off ratio was 25 in the dark and 125 under UV illumination.
ZnO Nanowire Sensors
Semiconducting nanorods and nanowires are excellent candidates for chemical sensing, given their large surface-to-volume ratios and low weight. With the emergence of hydrogen as a point-of-use fuel source, the detection of hydrogen gas in low concentrations is increasingly important. The ability to detect hydrogen selectively at room temperature is enabling because it avoids the use of on-chip heaters that add to the power consumption and weight. One method to increase detection hydrogen detection sensitivity is to use a catalytic metal coating or to actually dope the sensor material with the transition metal. This leads to catalytic dissociation of H2 to atomic hydrogen, which produces a sensor response through binding to surface atoms and altering the surface potential. ZnO nanorods are attractive for a wide variety of sensing applications because of the ease of synthesis, ability to readily transfer them to cheap substrates and their bio-safe characteristics Using ZnO nanowires, researchers at UF has develop ultra-sensitive hydrogen sensors. Detection of hydrogen concentrations as low as 10 ppm have been realized in simple devices that employ arrays of nanowires.
Oxide Nanowire Cored Heterostructures:
While the formation of planar semiconductor heterostructures is common for thin films, the synthesis of one-dimensional heterostructures is difficult. Axial heterostructures, in which the chemical modulation is imposed along the length of the nanowire axis, have been reported for a few systems, such as InAs/InP and Si/SiGe nanowires. Little work has addressed the synthesis of nanowire heterostructures in which the chemical modulation extends radially from the wire center. Researchers at UF have investigated the formation and properties of radial heteroepitaxial ZnO/(Mg,Zn)O nanowires in which the (Mg,Zn)O is either wurtzite or cubic. Synthesis is achieved via the catalyst-driven molecular beam epitaxy technique. The nanowires were grown on Ag- or Au-coated substrates at growth temperatures ranging from Tg = 300 to 500 °C, using Zn, Mg, and O3/O2 as the reactive flux. Structural and compositional analyses indicate that the core of nanowire is ZnO possessing the hexagonal wurtzite structure, with the (Mg,Zn)O sheath assumes either the wurtzite or the cubic rock salt structure, depend on the relative flux of Mg. Since (Mg,Zn)O has a larger bandgap energy (up to 7.8 eV) than that of ZnO (3.37 eV), these radial heterostructure nanorods provide an interesting system for quantum confinement and 1-D nanoscale device studies.
Related Papers and Review Articles:
"Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods," Wang, H.T. ; Kang, B.S.; Ren, F.; Tien, L.C.; Sadik, P.W.; Norton, D.P.; Pearton, S.J.; Lin, J., Applied Physics A (Materials Science Processing), Volume A81, Issue 6, 2005, Pages 1117-1119.
"Room-temperature hydrogen-selective sensing using single Pt-coated ZnO nanowires at microwatt power levels," Tien, L.C. ; Wang, H.T.; Kang, B.S.; Ren, F.; Sadik, P.W.; Norton, D.P.; Pearton, S.J.; Lin, J., Electrochemical and Solid-State Letters, Volume 8, Issue 9, 2005, Pages G230-G232.
"Hydrogen-selective sensing at room temperature with ZnO nanorods," Wang, H.T. ; Kang, B.S.; Ren, F.; Tien, L.C.; Sadik, P.W.; Norton, D.P.; Pearton, S.J.; Lin, J., Applied Physics Letters, Volume 86, Issue 24, 2005, Pages 243503-243501-3.
"ZnO/cubic (Mg,Zn)O radial nanowire heterostructures," Y.W. Heo, M. Kaufman, K. Pruessner, K.N. Siebein, D.P. Norton, F. Ren, Applied Physics A: Materials Science & Processing, Volume 80, Issue 2, 2005, Pages 263 – 266
"UV photoresponse of single ZnO nanowires," Y.W. Heo, B.S. Kang, L.C. Tien, D.P. Norton, F. Ren, J.R. La Roche, S.J. Pearton, Applied Physics A: Materials Science & Processing, Volume 80, Issue 3, 2005, Pages 497 – 499
"pH measurements with single ZnO nanorods integrated with a microchannel," Kang, B.S. ; Ren, F.; Heo, Y.W.; Tien, L.C.; Norton, D.P.; Pearton, S.J., Applied Physics Letters, Volume 86, Issue 11, 2005, Pages 112105-112101-3.
"ZnO nanowire growth and devices," Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, S.J. Pearton and J.R. LaRoche, Materials Science and Engineering: R: Reports, Volume 47, Issues 1-2, 20 December 2004, Pages 1-47
Additional information on this and related research can be seen at http://norton.mse.ufl.edu.