Preparation and characterization of ZnO transparent semiconductor thin films by sol–gel method

doi:10.1016/j.jallcom.2010.01.100

Journal of Alloys and Compounds

Volume 495, Issue 1, 9 April 2010, Pages 126-130

https://doi.org/10.1016/j.jallcom.2010.01.100 Get rights and content

Abstract

Transparent semiconductor thin films of zinc oxide (ZnO) were deposited onto alkali-free glass substrates by the sol–gel method and spin-coating technique. In this study, authors investigate the influence of the heating rate of the preheating process (4 or 10 °C/min) on the crystallization, surface morphology, and optical properties of sol–gel derived ZnO thin films. The ZnO sol was synthesized by dissolving zinc acetate dehydrate in ethanol, and then adding monoethanolamine. The as-coated films were preheated at 300 °C for 10 min and annealed at 500 °C for 1 h in air ambiance. Experimental results indicate that the heating rate of the preheating process strongly affected the surface morphology and transparency of ZnO thin film. Specifically, a heating rate of 10 °C/min for the preheating process produces a preferred orientation along the (0 0 2) plane and a high transmittance of 92% at a wavelength of 550 nm. Furthermore, this study reports the fabrication of thin-film transistors (TFTs) with a transparent ZnO active channel layer and evaluates their electrical performance.

Introduction

Zinc oxide (ZnO) is a II–VI group, n-type direct bandgap semiconductor that possess some great characteristics, including a wide energy bandgap (3.3 eV) [1], large free exciton binding energy (60 mV), wide range resistivity (10⁻⁴ to 10¹² Ω cm), high carrier mobility, high transparency at room temperature, and good photoelectric, piezoelectric, and thermoelectric properties. As a result, ZnO has been used in numerous applications, including UV detectors, gas sensors, thin-film transistors, photovoltaic devices, piezoelectric transducers, surface acoustic wave (SAW) devices, and thermoelectric devices [2], [3]. This is primarily due to its chemical stability, high carrier mobility, and lower photosensitivity than that of hydrogenated amorphous silicon (a-Si:H) thin films, which typically serves as the active channel layer in a typical TFT array for driving liquid crystal display (LCD) [4] and organic light-emitting device (OLED) display [5]. Recently, ZnO-based transparent thin films have also attracted attention for applications in transparent electronics, including thin-film solar cells, transparent thin-film transistors [6], [7], and transparent electronic circuits [8], [9].

ZnO-based thin films have been prepared by various vacuum deposition techniques and solution-based deposition processes [3]. Solution-based deposition processes offer a simple, low cost, and large area thin-film coating alternative to vacuum deposition techniques. Using the solution process to form oxide semiconductors may improve the manufacturing throughput of microelectronic and optoelectronic devices since it enable a direct pattern technique. Examples of this technique includes inkjet printing [10], [11], microcontact printing [12], [13], and reel to reel printing [14].

The sol–gel method not only enables easy fabrication of a large area thin film at a low cost, but also easily controls over the film composition and uniformity of thickness. Previous reports indicate that solution processes usually produce ZnO films with preferential crystal orientation, which could be important for efficient charger transport and could profoundly influence the mobility of field effect transistors (FETs) [15], [16]. The polycrystalline ZnO thin films with c-axis orientation prepared by the sol–gel process that depends on the sol concentration [17], [18], [19], heat-treatment conditions [1], [17], [20], [21], [22], [23], [24], substrates used [25], [26], [27], and film thickness [20], [28]. In the present study, transparent and semiconducting ZnO thin films were prepared by the sol–gel method using a spin-coating technique. This study also investigates the effect of the heating rate of the preheating procedure on the crystallization, microstructure, and optical properties of ZnO thin films. Finally, this study reports the fabrication of TFTs with a ZnO active channel layer and evaluates their electrical characteristics.

Section snippets

Experimental procedures

To synthesize the ZnO sol, zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O) was first dissolved in an ethanol solvent, and then monoethanolamine (MEA) was added to the solution as a stabilizer. We used ethanol, a low boiling point and non-toxicity solvent to synthesize un-doped ZnO sols. The molar ratio of zinc ions to MEA in the as-prepared sols was maintained at 1.0, and the zinc ions concentration was controlled at 0.75 mole/L. The complex solution was stirred for 2 h at 60 °C to yield a transparent,

Results and discussion

The crystallinity and crystal structure of sol–gel derived ZnO thin films were identified by X-ray diffraction. Fig. 2 shows the XRD patterns of ZnO thin films prepared at different preheating process heating rates (4 or 10 °C/min). These patterns correspond to six diffraction peaks of polycrystalline ZnO at (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) and (1 0 3) planes (JCPDS 36-1451). This result reveals that ZnO gel films preheated at 300 °C and annealed at 500 °C have a hexagonal wurzite structure. Note

Conclusions

Transparent semiconductor ZnO thin films were successfully prepared by the sol–gel method using a spin-coating technique. The as-deposited films exhibited a hexagonal wurtzite structure after annealing at 500 °C in air ambiance for 1 h. Increasing the heating rate of the preheating process from 4 to 10 °C/min obviously improved transparency in the visible range, decreased the surface roughness, and reduced strain for ZnO thin films. In the present study, ZnO thin films formed at a heating rate of

Acknowledgment

The authors gratefully acknowledge the financial support by the Taiwan TFT-LCD Association (TTLA) under Contract No. A643TT1000-S21.

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Preparation and characterization of ZnO transparent semiconductor thin films by sol–gel method

Abstract

Introduction

Section snippets

Experimental procedures

Results and discussion

Conclusions

Acknowledgment

J. Cryst. Growth

Solid State Electron.

Sens. Actuators A: Phys.

Thin Solid Films

Appl. Surf. Sci.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Vacuum

Appl. Surf. Sci.

Ceram. Int.

Mater. Lett.

Solar Energy Mater. Solar Cells

Thin Solid Films

Appl. Surf. Sci.

Appl. Surf. Sci.

J. Cryst. Growth

J. Cryst. Growth

J. Vac. Sci. Technol. B