Jayaraj, M K(Electrochemical Society, February 18, 2008)
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Abstract:
The optical and carrier transport properties of amorphous transparent zinc indium tin oxide (ZITO)(a-ZITO) thin films and the
characteristics of the thin-film transistors TFTs were examined as a function of chemical composition. The as-deposited films
were very conductive and showed clear free carrier absorption FCA . The analysis of the FCA gave the effective mass value of
0.53 me and a momentum relaxation time of 3.9 fs for an a-ZITO film with Zn:In:Sn = 0.35:0.35:0.3. TFTs with the as-deposited
channels did not show current modulation due to the high carrier density in the channels. Thermal annealing at 300°C decreased
the carrier density and TFTs fabricated with the annealed channels operated with positive threshold voltages VT when Zn
contents were 25 atom % or larger. VT shifted to larger negative values, and subthreshold voltage swing increased with decreasing
the Zn content, while large on–off current ratios 107–108 were kept for all the Zn contents. The field effect mobilities ranged
from 12.4 to 3.4 cm2 V−1 s−1 for the TFTs with Zn contents varying from 5 to 48 atom %. The role of Zn content is also
discussed in relation to the carrier transport properties and amorphous structures.
Jayaraj, M K(American Vacuum Society, January 8, 2008)
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Abstract:
Structural, electronic, and optical properties of amorphous and transparent zinc tin oxide films
deposited on glass substrates by pulsed laser deposition (PLD) were examined for two chemical
compositions of Zn:Sn=1:1 and 2:1 as a function of oxygen partial pressure PO2 used for the film
deposition and annealing temperature. Different from a previous report on sputter-deposited films
Chiang et al., Appl. Phys. Lett. 86, 013503 2005 , the PLD-deposited films crystallized at a lower
temperature 450 °C to give crystalline ZnO and SnO2 phases. The optical band gaps Tauc gaps
were 2.80−2.85 eV and almost independent of oxygen PO2
, which are smaller than those of the
corresponding crystals 3.35−3.89 eV . Films deposited at low PO2
showed significant subgap
absorptions, which were reduced by postthermal annealing. Hall mobility showed steep increases
when carrier concentration exceeded threshold values and the threshold value depended on the film
chemical composition. The films deposited at low PO2
2 Pa had low carrier concentrations. It is
thought that the low PO2
produced high-density oxygen deficiencies and generated electrons, but
these electrons were trapped in localized states, which would be observed as the subgap absorptions.
Similar effects were observed for 600 °C crystallized films and their resistivities were increased by
formation of subgap states due to the reducing high-temperature condition. High carrier
concentrations and large mobilities were obtained in an intermediate PO2
region for the as-deposited films.
Highly conductive and transparent thin films of amorphous zinc indium tin oxide are prepared at room temperature by co-sputtering of zinc
10 oxide and indium tin oxide. Cationic contents in the films are varied by adjusting the power to the sputtering targets. Optical transmission study of
11 films showed an average transmission greater than 85% across the visible region. Maximum conductivity of 6×102 S cm−1 is obtained for Zn/In/
12 Sn atomic ratio 0.4/0.4/0.2 in the film. Hall mobility strongly depends on carrier concentration and maximum mobility obtained is 18 cm2 V−1 s−1
13 at a carrier concentration of 2.1×1020 cm−3. Optical band gap of films varied from 3.44 eV to 3 eV with the increase of zinc content in the film
14 while the refractive index of the films at 600 nm is about 2.0.
Optical emission spectroscopic studies were carried out on the plasma produced by ablation of zinc
oxide target using the third harmonic 355 nm of Q-switched Nd:YAG laser, in vacuum and at three
different ambient gas oxygen pressures. The spatial variations of electron density Ne and electron
temperature Te were studied up to a distance of 20 mm from the target surface. The kinematics of
the emitted particles and the expansion of the plume edge are discussed. The optimum conditions
favorable for the formation of high quality zinc oxide thin films are thereby suggested.
Stable, OH free zinc oxide (ZnO) nanoparticles were synthesized by hydrothermal method by varying the
growth temperature and concentration of the precursors. The formation of ZnO nanoparticles were confirmed by x-ray
diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The
average particle size have been found to be about 7-24 nm and the compositional analysis is done with inductively
coupled plasma atomic emission spectroscopy (ICP-AES). Diffuse reflectance spectroscopy (DRS) results shows that the
band gap of ZnO nanoparticles is blue shifted with decrease in particle size. Photoluminescence properties of ZnO
nanoparticles at room temperature were studied and the green photoluminescent emission from ZnO nanoparticles can
originate from the oxygen vacancy or ZnO interstitial related defects.
Transparent diode heterojunction on ITO coated glass substrates was fabricated using p-type AgCoO2 and n-type ZnO films by pulsed laser
deposition (PLD). The PLD of AgCoO2 thin films was carried out using the pelletized sintered target of AgCoO2 powder, which was synthesized
in-house by the hydrothermal process. The band gap of these thin films was found to be ~3.89 eV and they had transmission of~55% in the
visible spectral region. Although Hall measurements could only indicate mixed carrier type conduction but thermoelectric power measurements of
Seebeck coefficient confirmed the p-type conductivity of the grown AgCoO2 films. The PLD grown ZnO films showed a band gap of ~3.28 eV,
an average optical transmission of ~85% and n-type carrier density of~4.6×1019 cm− 3. The junction between p-AgCoO2 and n-ZnO was found
to be rectifying. The ratio of forward current to the reverse current was about 7 at 1.5 V. The diode ideality factor was much greater than 2.