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ABSTRACT: Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied
before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures,
microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13eV compared with bulk ZnO films,
indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of
shallow uncompensated donors from ~1017cm−3 to ~1016cm−3 and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are
the well-known traps with activation energies of 0.3eV and 0.8eV. After annealing, the electron traps observed in the structure
have activation energies of 0.14eV, 0.33eV, and 0.57eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of
band-edge luminescence and produce a deep luminescence defect band at 2.2eV. In addition, a defect vibrational band becomes
visible in Raman spectra near 650cm−1. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.
KeywordsZnO-quantum wells-MgZnO
Journal of Electronic Materials 05/2012; 39(5):601-607. · 1.47 Impact Factor
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ABSTRACT: The properties of phosphorus-doped (Zn,Mg)O polycrystalline and epitaxial thin films are described. The as-deposited (Zn,Mg)O:P
films are n type with high electron carrier density. High resistivity is induced in the films with moderate temperature annealing,
which is consistent with suppression of the donor state and activation of the deep acceptor. The resistivity of the as-deposited
and annealed film is an order of magnitude higher than similar samples with no Mg, consistent with a shift in the conduction
band edge relative to the defect-related donor state. The capacitance-voltage characteristics of annealed metal/insulator/P-doped
(Zn,Mg)O structures in which the (Zn,Mg)O is polycrystalline exhibit p-type polarity. In addition, multiple polycrystalline
devices comprising n-type ZnO/P-doped (Zn,Mg)O thin-film junctions display asymmetric I–V characteristics that are consistent
with the formation of a p-n junction at the interface, although the ideality factor is anomalously high.
Journal of Electronic Materials 04/2012; 34(4):409-415. · 1.47 Impact Factor
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ABSTRACT: The transport and optical properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied.
The carrier type of as-deposited (Zn,Mg)O:P films converts from n-type to p-type with increasing oxygen partial pressure.
All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of oxidation conditions
in realizing p-type (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show a strong dependence on the deposition ambient
at different growth temperatures. The resistivity of the samples deposited in O3/O2 mixture is two orders of magnitude higher than the films grown in oxygen and O2/Ar/H2 mixture. The room-temperature photoluminescence (PL) of the as-deposited films has been shown that growing in the O2/Ar/H2 mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The enhanced ultraviolet
(UV) emission in the films grown in O2/Ar/H2 mixture may result from hydrogen passivation of the deep level emission centers. The annealed ZnO:P films are n-type with
nonlinear dependence of resistivity on annealing temperature. The resistivity increases in the films with annealing at 800°C
while decreasing with further increasing annealing temperature. Strong visible light emission is observed from the ZnO:P films
annealed in oxygen.
Journal of Electronic Materials 04/2012; 35(4):530-537. · 1.47 Impact Factor
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ABSTRACT: In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized,
time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound
excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin
relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain
field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed
fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important
factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.
Journal of Superconductivity and Novel Magnetism 04/2012; 23(1):161-165. · 0.65 Impact Factor
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ABSTRACT: Tin oxide (SnO2) nanorods were grown by high-pressure pulsed laser deposition (PLD). The nanorods were grown without the use of a catalyst
but required high background pressure growth in order to realize small grain columnar growth and nanorod formation, with nanorod
formation most favored on non-epitaxial substrates. The structures and morphology were characterized by field emission scanning
electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). X-ray diffraction and HRTEM analysis
indicate that the as-grown SnO2 nanorods are single crystals with a rutile structure. The nanorods are approximately 50–90nm in diameters and 1.5μm in
length. This method provides an approach for large area synthesis of one dimensional SnO2 nanostructure materials.
Applied Physics A 04/2012; 91(1):29-32. · 1.63 Impact Factor
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ABSTRACT: We investigated the size-dependent electrical conductivities of indium zinc oxide stripes with different widths from 50 nm to 4 microm and with the same thickness of 50 nm deposited by RF magnetron sputtering. The size of the indium zinc oxide stripes was controlled by e-beam lithography. The distance of the two Ti/Au Ohmic electrodes along the indium zinc oxide stripes was kept constant at 25 microm. The electrical conductivity decreased as the size of the indium zinc oxide stripes decreased below a critical width (80 nm). The activation energy, derived from the electric conductivity versus temperature measurement, was dependent on the dimensions of indium zinc oxide stripes. These results can be understood as stemming from surface charge trapping from the absorption of oxygen and/or water vapor, which leads to an increase in the energy difference between the conduction energy band and the Fermi energy.
Journal of Nanoscience and Nanotechnology 04/2012; 12(4):3264-7. · 1.56 Impact Factor
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ABSTRACT: ZnO nanowires were grown on indium tin oxide (ITO) coated glass substrates at a low temperature of 90 degrees C using an aqueous solution method. The ZnO seeds were coated on the ITO thin films by using a spin coater. ZnO nanowires were formed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 x 6H2O) and hexamethylenetetramine (C6H12N4). The pH value and concentration of the solution play an important role in the growth and morphologies of ZnO nanowires. The size of ZnO naonowires increased as the concentration of the solution increased. It was formed with a top surface of hexagonal and tapered shape at low and high pH values respectively. Additionally, the single crystalline structure and optical property of the ZnO nanowires were investigated using high-resolution transmission electron microscopy and photoluminescence spectroscopy.
Journal of Nanoscience and Nanotechnology 02/2012; 12(2):1415-20. · 1.56 Impact Factor
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ABSTRACT: The authors report on the fabrication of thin-film transistors (TFTs) using indium-tin-zinc oxides (ITZOs) as active channel layers. Transparent amorphous ITZO semiconductors were deposited at room temperature by rf-magnetron sputtering, followed by an annealing treatment at 100 ° C . The electrical properties of the ITZO channel layers deposited at room temperature using rf-magnetron sputtering were investigated by controlling the oxygen partial pressure during deposition and introducing postannealing treatments. The devices operated in an n -type enhancement mode exhibited a clear pinch-off behavior and had an on-to-off ratio of ∼10<sup>8</sup> with a low off current of 3×10<sup>-13</sup> A . A field-effect mobility of 17 cm <sup>2</sup>/ V s and a subthreshold slope of 0.5 V/decade were extracted from the device characteristics. These results suggest that ITZO semiconductors show potential as channel materials that are applicable in flexible transparent TFTs.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/2011; · 1.34 Impact Factor
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ABSTRACT: Low resistance Ohmic contacts using Ti/Au metallization on n -type amorphous indium zinc oxide (IZO) ( n∼5×10<sup>20</sup> cm <sup>-3</sup> ) deposited on paper substrates are reported. The minimum specific contact resistivity of 8×10<sup>-7</sup> Ω cm <sup>2</sup> was achieved on IZO films both as-deposited and for annealing temperatures of up to 125 ° C . The contact resistance increased to 4×10<sup>-6</sup> Ω cm <sup>2</sup> at 175 ° C . The sheet resistance was found to vary from 24 to 17 Ω/ sq , and the transfer resistance was ∼0.045 Ω mm for the as-deposited and low temperature annealed samples. The contact resistance was independent of measurement temperature, indicating that field emission plays a dominant role in the current transport. Such Ohmic contacts achieved with little or no annealing are important for paper based electronics requiring low temperature processing (<200 ° C ) .
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 10/2010; · 1.34 Impact Factor
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ABSTRACT: Well-aligned ZnO nanowires were directly grown on indium tin oxide (ITO) glass substrates coated with ZnO seed layers via an aqueous solution route. During the growth process, the ZnO seed layer played the role of providing nucleation sites in the selective growth of the well-aligned ZnO nanowires. The ZnO seed layer was deposited on the substrate using pulsed laser deposition (PLD) at room temperature. The uniform ZnO nanowires were grown in aqueous solution containing zinc nitrate hexahydrate and hexamethylenetetramine at 90 °C. The diameter and length of ZnO nanowires were approximately 80 nm and 2.2 μm, respectively. The selectively patterned ZnO nanowires were fabricated on the ITO-coated substrates by combining this simple solution method and a conventional photolithography process.
Journal of Nanoelectronics and Optoelectronics 07/2010; 5(2):186-190. · 0.56 Impact Factor
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ABSTRACT: Oxide semiconductors are attractive materials for thin-film electronics and optoelectronics due to compatibility with synthesis on large-area, inexpensive glass and flexible plastic substrate. However, development of thin-film electronics has been hampered by the limited number of semiconducting oxides that are p-type. Here, we report on the properties of zinc-cobalt-oxide (Zn–Co–O) films, deposited at room temperature using pulsed laser deposition, that exhibit p-type conduction. Films are deposited at room temperature in a background of oxygen using a polycrystalline ZnCo <sub>2</sub> O <sub>4</sub> ablation target. The p-type conduction is confirmed by positive Seebeck coefficient and positive Hall coefficient. Both electrical resistivity and carrier density are dependent on oxygen background pressure used during deposition. Zn–Co–O films deposited in 50 mTorr oxygen pressure appear to be amorphous based on x-ray diffraction, and show an electrical conductivity as high as 21 S cm <sup>-1</sup> . Distinct rectifying current-voltage characteristics are observed for junctions between Zn–Co–O and n-type InGaZnO films, exhibiting a threshold voltage of ∼2.5 V . P-type Zn–Co–O appears promising for thin-film electronic device technology.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: We investigated the transfer characteristics and the gate-bias stability of amorphous indium-gallium-zinc oxide thin-film transistors when the channel layer was exposed to hydrogen, oxygen, air, or vacuum at room temperature during measurements. The threshold voltage and the drain current were changed by the ambient atmospheres. The threshold voltage shift (ΔVth) under gate-bias stress was faster in hydrogen than in oxygen and vacuum. It is suggested that hydrogen exposure degrades the gate-bias stress stability due to surface accumulation layer creation. The characteristic trapping times, τ, in H2, O2, air, and vacuum were 5×103, 1.5×104, 2×104, and 6.3×104 s, respectively.
Applied Physics Letters 03/2010; 96(10):102107-102107-3. · 3.84 Impact Factor
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ABSTRACT: We have fabricated bottom-gate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature ( ≤ 100 °C). As a water and solvent barrier layer, cyclotene (BCB 3022–35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2 V−1 s−1, threshold voltage (VTH) of 1.9 V, subthreshold gate-voltage swing (S) of 0.65 V decade−1, and drain current on-to-off ratio (ION/IOFF) of ∼ 104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat ∼ 2.1 cm2 V−1 s−1, VTH ∼ 0 V, S ∼ 0.74 V decade−1, and ION/IOFF = 105–106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. The ability to achieve InGaZnO TFTs on cyclotene-coated paper substrates demonstrates the enormous potential for applications such as low-cost and large area electronics.
Applied Physics Letters 02/2010; 96(5):053510-053510-3. · 3.84 Impact Factor
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ABSTRACT: In this paper, we discuss the progress of nitride and oxide semiconductor nanostructures for hydrogen gas sensing. The use of catalyst metal coatings on GaN, InN and ZnO nanowires is found to greatly enhance the detection sensitivity. Pt- and Pd-coated GaN nanowires biased at small voltages show large changes in currents upon exposure to H2 gas at concentrations in the ppm range. Improvements in growth techniques for InN nanostructures have produced nanobelts and nanorods capable of hydrogen detection down to 20 ppm after catalyst coating. Functionalized ZnO nanorods were also investigated for hydrogen detection, but did not generate a relative response as high as that for the nitride-based sensors. All sensors tested exhibited no response at room temperature upon exposure to various other gases including O2, C2H5, N2O and CO2. The high surface-to-volume ratio of nanowires and the ability to use simple contact fabrication schemes make them attractive for hydrogen sensing applications.
Semiconductor Science and Technology 01/2010; 25(2):024002. · 1.72 Impact Factor
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ABSTRACT: We report on measurements and calculations of the ultrafast exciton relaxation dynamics in ZnO. Time-resolved differential reflectivity measurements of bulk ZnO were performed as a function of excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with a fast (∼2-5 ps scale) and a slower (∼50-100 ps scale) component, which depend strongly on excitation wavelength. Theoretical calculations based on a multi-state, coupled rate equation model were directly compared with the experiments to account for the rapid scattering between the A and B valence bands. Results show that the inter-valence band scattering is most likely not responsible for the fast initial relaxation. Instead our results show that carrier diffusion can play an important role in explaining the initial fast relaxation.
Proceedings of SPIE - The International Society for Optical Engineering; 01/2010
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ABSTRACT: We report on the fabrication and the electrical properties of amorphous (α-)InGaZnO4 thin-film transistors deposited on cellulose paper by sputtering at room temperature. The 150-μm-thick paper was used as both a gate dielectric and a substrate for device structural support. The transistors on paper were patterned by lithography and operated in enhancement mode with a threshold voltage of 3.75 V, and exhibited saturation mobility, subthreshold gate-voltage swing, and drain current on-to-off ratio of ∼ 35 cm2 V−1 s−1, 2.4 V decade−1, and ∼ 104, respectively. These results verify that simple cellulose paper is a good gate dielectric as well as a low-cost substrate for flexible electronic devices such as paper-based displays.
Applied Physics Letters 02/2009; 94(7):072103-072103-3. · 3.84 Impact Factor
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ABSTRACT: Amorphous (α-)InGaZnO4 thin film transistors (TFTs) were fabricated on polyimide clean-room tape at low temperature (<100 °C). The α-InGaZnO4 films with an n-type carrier concentration of ∼ 1016 cm−3 were deposited by rf-magnetron sputtering in a mixed ambient of Ar/O2. The bottom-gate-type TFTs showed good saturation mobility ( ∼ 5.3 cm2 V−1 s−1), drain current on-to-off ratio of approximately 105, threshold voltage of 1.1 V, and subthreshold gate-voltage swing of 0.55 V decade−1. These results were comparable to those of the same oxide TFTs that we have fabricated on either glass or polyethylene terephthalate substrates. The results demonstrate that even polyimide clean-room tape can be an appropriate substrate for inexpensive-flexible-adhesive-transparent electronic devices.
Applied Physics Letters 12/2008; 93(25):252103-252103-3. · 3.84 Impact Factor
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ABSTRACT: ZnO is attracting attention for application in transparent nanowire (NW) transistors because of the ease of synthesis of ZnO nanostructures, their good transport properties, the availability of heterostructures, and the possibility for optoelectronic integration. A variety of both top and bottom gate n-type ZnO NW transistors have been reported, showing generally high on/off ratios (10<sup>4</sup> - 10<sup>7</sup>), subthreshold voltage swings of 130-300 mV/dec, and excellent drain-current saturation. Much higher electron mobilities and improved device stability are found when surface passivation is employed, pointing to the importance of controlling surface charge density. Simulations show that defects such as grain boundaries lead to a decrease of drain current. While the dc characteristics of such devices are generally reasonable, there have been no reports of the RF or high-speed switching performance. Additional work is needed on optimized gate dielectrics, reliability, and functionality of ZnO NW transistors.
IEEE Transactions on Electron Devices 12/2008; · 2.32 Impact Factor
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ABSTRACT: The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template
by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although
an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen
background pressures of 500-mTorr results in nanowire diameters as small as 50–90nm, with diameters largely determined by
growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The
deep-level emission is believed caused by oxygen vancancies formed during growth.
Journal of Materials Science 10/2008; 43(21):6925-6932. · 2.02 Impact Factor
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S.J. Pearton,
W.T. Lim,
J.S. Wright,
L.C. Tien,
H.S. Kim, D.P. Norton,
H.T. Wang,
B.S. Kang,
F. Ren,
J. Jun,
J. Lin,
A. Osinsky
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ABSTRACT: ZnO is an attractive material for ultraviolet (UV) light emitters and detectors, and transparent thin-film transistors. It
is also readily synthesized in the form of nanostructures. In this paper we discuss the fabrication of a number of ZnO thin-film
and nanowire devices, including transistors, diodes, and UV and pH sensors. ZnO has been effectively used as a gas sensor
material based on the near-surface modification of charge distribution with surface-absorbed species. The large surface area
of the nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes
them candidates for high-density sensor arrays.
Journal of Electronic Materials 08/2008; 37(9):1426-1432. · 1.47 Impact Factor
