Enhanced mobility of Li-doped ZnO thin film transistors fabricated by mist chemical vapor deposition

doi:10.1016/j.apsusc.2014.02.080

Applied Surface Science

Volume 301, 15 May 2014, Pages 358-362

https://doi.org/10.1016/j.apsusc.2014.02.080 Get rights and content

Highlights

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The mist CVD Li-doped ZnO film has changed from hexagonal to tetragonal structure depending on Li amounts.
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The mobility of Li-doped ZnO film has improved about 10² times higher than that of ZnO film by mist CVD.
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The addition of Li into ZnO semiconductors may be the enhanced crystallinity and reduced defect states.

Abstract

Mist chemical vapor deposition (mist-CVD)-processed, lithium (Li)-doped ZnO thin film transistors (TFTs) are investigated. Li doping significantly increases the field-effect mobility in TFTs up to ∼100 times greater than that of undoped ZnO. The addition of Li into mist-CVD-grown ZnO semiconductors leads to improved film quality, which results from the enhanced crystallinity and reduced defect states, including oxygen vacancies. Our results suggest that Li doping of ZnO-based oxide semiconductors could serve as an effective strategy for high-performance, mist-CVD-processed oxide TFTs with low-cost and low-temperature fabrication.

Introduction

ZnO-based amorphous oxide semiconductors have been intensively studied as an active channel material for thin film transistor (TFT) backplanes, particularly for active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light-emitting diodes (AMOLEDs) with ultra-definition, a high frame rate, and large size [1], [2], [3], [4]. These oxide semiconductor materials meet the combined requirements of high optical transparency and low temperature film processing for the development of next-generation applications including flexible electronics and “see-through” displays. However, despite their superior performance, most oxide semiconductor materials have been fabricated by expensive, vacuum-based physical vapor deposition techniques such as sputtering [5], [6], pulsed laser deposition (PLD) [1], [7], ion-assisted deposition [8], and molecular beam epitaxy (MBE) [9]. While these oxide semiconductors are considered to be emerging materials, their reliance on vacuum-based processes results in significantly increased manufacturing costs, which is a major obstacle for the realization of inexpensive, large-area electronics.

On the other hand, if a vacuum atmosphere is not essential, chemical vapor deposition (CVD) possesses significant advantages over other techniques. However, in order to supply zinc in the gas phase, one needs to use source materials that have high vapor pressure, such as dimethylzinc or diethylzinc, but these are generally flammable and expensive. In contrast, if a water or alcohol-based solution of zinc is ultrasonically atomized and the aerosol particles that form are transferred to the substrate for the chemical reaction to form thin films, one can use safe and inexpensive sources such as zinc acetate or zinc nitrate. This simple, safe, and cost-effective deposition technique based on the above mechanism was named mist-CVD and has been applied for the deposition of ZnO [10], [11], [12], [13], [14], [15], [16]. Unfortunately, conventional ZnO films deposited by mist-CVD have exhibited poor electrical performance with high resistivity (>10³ Ω cm) [13] and low mobility (<10⁻³ cm²/V/s) [16], which is unacceptable for applications such as next-generation displays.

To address this issue, we employed an alkali metal dopant, lithium (Li), as an electron donor in the ZnO matrix. Furthermore, we investigated the physical, chemical, and electrical properties of Li-doped ZnO films grown by the mist-CVD process. It was demonstrated that TFTs fabricated using Li-doped ZnO have significantly enhanced field-effect mobility compared to that of undoped ZnO TFTs.

Section snippets

Experimental

Undoped and Li-doped ZnO thin films were deposited using a mist-CVD process on p-type Si with a thermally grown SiO₂ layer (thickness = 100 nm) or on glass substrates. An undoped ZnO precursor solution with a concentration of 0.015 M was prepared by directly dissolving zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] in methanol. Li-doped ZnO precursor solutions were prepared by adding lithium nitrate (LiNO₃) to the ZnO precursor solution with Li doping concentrations of 10, 20, and 30 mol% in relation to

Results and discussion

To determine the possible chemical reactions of the Li-doped ZnO deposition, thermogravimetry and differential thermal analysis (TG-DTA) was performed in an air atmosphere. The TG-DTA results of the Li-doped ZnO precursor solution (Li–30-ZnO) are shown in Fig. 1(a). At temperatures around 55 °C, a small endothermic peak and a gradual weight loss are observed. This behavior is ascribed to the evaporation of the main solvent. A strong endothermic peak and an abrupt weight loss around 85 °C are due

Conclusion

We demonstrated that the mist-CVD process can be used for the deposition of doped ZnO films as well as for the fabrication of high-performance TFTs onto large area substrates under an ambient atmosphere with a low processing temperature (∼250 °C). The alkali metal (lithium) in the ZnO films acted as a shallow donor, which dramatically enhanced the field-effect mobility of the ZnO TFTs. Physical and chemical structure analyses confirmed that the Li doping resulted in reduced oxygen vacancies as

References (28)

J.S. Park et al.
Thin Solid Films
(2012)
Y. Shimura et al.
Thin Solid Films
(2008)
J.G. Lu et al.
J. Cryst. Growth
(2007)
K.-H. Kim et al.
J. Nanoelectron. Optoelectron.
(2010)
Y. Yang et al.
J. Cryst. Growth
(2004)
M.L. Ruiz et al.
Thermochim. Acta
(2010)
H.Y. Jung et al.
Thin Solid Films
(2012)
K.M. Shaju et al.
Electrochim. Acta
(2002)
K. Nomura et al.
Nature (London)
(2004)
J.-S. Park et al.
J. Electroceram.
(2013)

H.-S. Kim et al.

Sci. Rep.

(2013)

H.-S. Kim et al.

ACS Appl. Mater. Interfaces

(2012)

J. Nishi et al.

Jpn. J. Appl. Phys.

(2003)

L. Wang et al.

Nat. Mater.

(2009)

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Enhanced mobility of Li-doped ZnO thin film transistors fabricated by mist chemical vapor deposition

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Thin Solid Films

Thin Solid Films

J. Cryst. Growth

J. Nanoelectron. Optoelectron.

J. Cryst. Growth

Thermochim. Acta

Thin Solid Films

Electrochim. Acta

Nature (London)

J. Electroceram.

Sci. Rep.

ACS Appl. Mater. Interfaces

Jpn. J. Appl. Phys.

Nat. Mater.