A low-temperature method for improving the performance of sputter-deposited ZnO thin-film transistors with supercritical fluid

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A low-temperature method for improving the performance of sputter-deposited ZnO

thin-film transistors with supercritical fluid

Min-Chen Chen, Ting-Chang Chang, Sheng-Yao Huang, Kuan-Chang Chang, Hung-Wei Li, Shih-Ching Chen , Jin Lu, and Yi Shi

Citation: Applied Physics Letters 94, 162111 (2009); doi: 10.1063/1.3124658 View online: http://dx.doi.org/10.1063/1.3124658

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/16?ver=pdfcov Published by the AIP Publishing

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A low-temperature method for improving the performance of

sputter-deposited ZnO thin-film transistors with supercritical fluid

Min-Chen Chen,1Ting-Chang Chang,1,2,a兲Sheng-Yao Huang,1Kuan-Chang Chang,3 Hung-Wei Li,4Shih-Ching Chen,1Jin Lu,1,5and Yi Shi5

1

Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

2

Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

3

Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

4

Department of Photonics and Display Institute, National Chiao Tung University, Hsin-Chu 300, Taiwan

5

Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

共Received 24 February 2009; accepted 5 April 2009; published online 23 April 2009兲

A low-temperature method, supercritical CO2 共SCCO2兲 fluid technology, is employed to improve

the device properties of ZnO TFT at 150 ° C. In this work, the undoped ZnO films were deposited by sputter at room temperature and treated by SCCO2 fluid which is mixed with 5 ml pure H2O.

After SCCO2treatment, the on/off current ratios and threshold voltage of the device were improved

significantly. From x-ray photoelectron spectroscopy analyses, the enhancements were attributed to the stronger Zn–O bonds, the hydrogen-related donors, and the reduction in dangling bonds at the grain boundary by OH passivation. © 2009 American Institute of Physics.

关DOI:10.1063/1.3124658兴

In recent years, ZnO-based thin-film transistors 共TFTs兲 have attracted much attention for the application in flexible displays or transparent active matrix organic light-emitting diode displays,1–3which is due to their excellent uniformity and great transparency to the visible light of a wide band gap 共3.2–3.3 eV兲. Many deposition methods have been used to prepare the ZnO films, such as sputtering,4,5 pulsed laser deposition,6 and chemical vapor deposition.7 The sputter deposition is more favorable among these methods because the fabrication of ZnO TFT is comparable to the conven-tional amorphous-Si TFT. However, the ZnO thin films, de-posited at room temperature, without doping usually exhibit an insulating property.8,9 In order to improve the quality of ZnO films, a high-temperature postdeposition-annealing10or a high deposition temperature8 is necessary. Nevertheless, high-temperature process is not suitable to substrates with low glass transition temperatures 共Tg兲, such as glasses and plastics. Therefore, finding a low-temperature method to en-hance the quality of ZnO thin film is required.

In our previous works, supercritical CO2 共SCCO2兲 fluid

technology has been proposed to improve the dielectric prop-erties of sputter-deposited hafnium oxide films and passivate the defect states in hydrogenated amorphous-silicon TFTs 共a-Si:H TFTs兲.11,12

In this study, we also employed the su-percritical CO2 共SCCO2兲 fluid technology at 150 °C to

en-hance the performance of sputtered ZnO TFT. The SCCO2

fluid exhibits liquidlike property, which has excellent trans-port ability. Furthermore, the SCCO2 fluid has gaslike and

high-pressure properties to diffuse into the nanoscale struc-tures without damage. Hence, the SCCO2fluid can carry the

H2O molecule effectively into the ZnO films at low

tempera-ture and passivate traps by H2O molecule at low

tempera-ture. The experimental results focus on material analyses and

electrical characteristics of the sputter deposited undoped ZnO film to investigate the efficacy of the SCCO2treatment.

The undoped ZnO TFT is the gate and bottom-contact type structures. The channel width 共W兲 and length 共L兲 of the TFTs were 50 and 8 ␮m, respectively. The device was fabricated by the following deposition procedure. First, a 300-nm-thick MoW gate electrode was deposited and pat-terned on the glass substrate. For the gate insulator, a 300-nm-thick silicon nitride共SiNx兲 was deposited by plasma en-hanced chemical vapor deposition. Then, a 100-nm-thick indium tin oxide film was deposited and patterned in forming the source/drain electrodes. Finally, a 100-nm-thick undoped ZnO film was formed by rf magnetron sputtering system at room temperature. The active layer was defined using pho-tolithography and wet etching. To optimize the device char-acteristics, two different post-treatments were performed on the ZnO film. For the first method, device labeled as “H₂O vapor” was placed in a pressure-proof stainless steel cham-ber, and a pure H2O vapor was immersed into ambience at

150 ° C for 1 h. For the second method, device labeled as “SCCO2” was performed on a supercritical fluid system full

of 3000 psi SCCO₂ fluids which was mixed with 5 ml pure H2O at 150 ° C for 1 h. Finally, we define a control sample

without any treatment marked as “standard.” After these dif-ferent treatments, the electrical characteristics of the device were measured in the dark environment at room temperature using Agilent 4156-C semiconductor analyzer. In addition, x-ray photoelectron spectroscopy 共XPS兲 was used to deter-mine the chemical functional bonds of the ZnO films.

Figure 1 shows the typical transfer curves IDS-VGS at

V_DS= 20 V. The threshold voltage 共Vt兲 was defined as the gate voltage at which the drain current reaches 1010 _{A. The}

sub-threshold slope共SS兲 was extracted from drain current in the sub-threshold region 共the drain current from 10−12 to 10−10 _{A兲 under saturation operation. As shown in the figure,}

the characteristics of undoped ZnO TFTs were much affected

a兲_{Electronic mail: tcchang@mail.phys.nsysu.edu.tw.}

APPLIED PHYSICS LETTERS 94, 162111共2009兲

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by the post-treatment. For the standard ZnO TFT, the device characteristics exhibit the largest Vtof 65.81 V and the low-est current. The result indicated that the standard device with undoped ZnO film as an active channel does not show ac-ceptable transistor characteristics. However, because the as-deposited ZnO film is consisted of numbers of ZnO grains, the carriers have to transport from source to drain via the grain boundaries as the device is operated. Therefore, the traps in grain boundaries may cause the potential barrier re-sulting in a low drain current and a high Vt. After H2O vapor

treatment, the device exhibits a slightly improved drain cur-rent 共⌬ Ion⬃1 order兲 and SS 共⌬ SS⬃3.36 V/decade兲.

However, a large Vtstill existed, and it means that only par-tial traps were passivated by the H₂O vapor treatment. Clearly, for the SCCO2treatment, Vt, drain current, and on/ off current ration共Ion/Ioff兲 were significantly enhanced

com-pared to the two devices. It exhibited the Vtof 27.92 V, the SS of 3.93 V/decade, and Ion/Ioff of 106. The field-effect

mobility was extracted by standard method 共IDS^ 0.5 versus

V_GS plot etc.兲, the corresponding mobility values for three devices were 2.82E-5, 1.01E-3, and 2.6E-2. From the above results, SCCO2 treatment is the most effective method to

improve the device property because the most traps at grain boundaries were passivated.

In order to analyze the composition of the ZnO film after post-treatment, the XPS analysis is performed, and the bind-ing energies have been calibrated by takbind-ing the carbon C 1s peak 共285.0 eV兲 as reference. The Zn 2p3/2 peaks of ZnO

thin films after different treatments are shown in Fig.2, and

the energy state of standard sample at about 1021.6 eV cor-responds to Zn–O bonding.13 After the treatment of H2O

vapor and SCCO₂ treatment, the binding energy shifts to 1021.9 and 1022.1 eV, respectively. The stronger Zn–O bonding indicates that Zn atoms in ZnO film have been oxi-dized more completely with the H2O molecules. In addition,

the SCCO2treatment is more efficient than H2O vapor

treat-ment in transporting H₂O molecules into the ZnO film. The O 1s peaks of various treatments ZnO thin films are shown in Fig. 3. The O 1s peaks have a shoulder at higher binding energy, and it increases obviously after the treatment of H2O vapor and SCCO2. The lower energy peak detected at

530.0 eV corresponds to O–Zn bonding, and the higher en-ergy peak detected at 531.6 eV is attributed to O–H bonding, which can be due to the H2O molecules adsorption in the

ZnO films.14However, the standard sample without any post-treatment also exhibit the high-energy peak of O–H bonding, which originated from the surface of ZnO films with ad-sorbed H2O molecules as it is exposed to the air. In addition,

the H2O absorbed in ZnO thin films was dissociated into H

and OH groups, and rapidly diffused into the ZnO film.15As previous study reported, the hydrogen atoms can be incorpo-rated into ZnO to form hydrogen-related donors, such as the FIG. 1. Transfer curves of undoped ZnO TFT after different treatment

conditions.

FIG. 2. XPS of Zn 2p_3/2spectra for the ZnO thin films after different treatment conditions.

FIG. 3. XPS of O 1s spectra with two resolved O bonding components for the ZnO thin films after various treatment conditions, including共a兲 standard, 共b兲 H2O vapor, and共c兲 SCCO2.

162111-2 Chen et al. Appl. Phys. Lett. 94, 162111共2009兲

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substitutional hydrogen bound to zinc atoms and the intersti-tial hydrogen bound to oxygen atoms, and improve the elec-trical conductivity of ZnO film.16 Otherwise, the dangling bonds共or traps兲 in the grain boundaries can be passivated by OH and form Zn–OH bonds. Therefore, the dangling bonds 共or traps兲 induced potential barrier can be suppressed. By using the integrated peak area with the sensitivity factors, the atomic concentrations of Zn and O were listed in Table I. The O–H bonding for the standard sample and the ZnO film after H2O vapor are 15.02% and 16.36%, while the O–H

bonding after SCCO2treatment rapidly increases to 46.13%.

The results of the O 1s spectra indicate that H2O molecules

can be effectively carried into the ZnO film by the highly diffusing SCCO2 fluid and passivate the traps.

In summary, the significant improvement of electrical characteristics for ZnO TFTs by low-temperature SCCO2

treatment was demonstrated. After the treatment of SCCO₂ fluid mixed with pure H₂O, the OH concentration of ZnO films rapidly increased, indicating that the dangling bonds 共or traps兲 within the ZnO films are passivated by forming Zn–OH bonds. It was verified that the SCCO2 fluid is an

effective transporter to carry the H₂O molecules into sputter-deposited ZnO films and enhances the device’s electrical properties.

This work was performed at National Science Council Core Facilities Laboratory for Science and Nano-Technology in Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under Contract Nos. NSC-3114-M-110-001 and NSC 97-2112-M-110-009-MY3.

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TABLE I. Atomic concentrations calculated from XPS of the ZnO films after various treatment conditions.

Zn 2p_3/2 共%兲 O–Zn 共%兲 OH 共%兲 Standard 55.32 29.66 15.02 H2O vapor 53.98 26.83 16.36 SCCO2 36.21 17.66 46.13

162111-3 Chen et al. Appl. Phys. Lett. 94, 162111共2009兲