Effects of cooling rate and post-heat treatment on properties of ZnO thin films deposited by sol–gel method

doi:10.1016/j.apsusc.2011.05.092

Applied Surface Science

Volume 257, Issue 21, 15 August 2011, Pages 9019-9023

https://doi.org/10.1016/j.apsusc.2011.05.092 Get rights and content

Abstract

Sol–gel spin-coated ZnO thin films are cooled with different rates after the pre-heat treatment. Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman, and photoluminescence (PL) were carried out to investigate the effects of the cooling rate during pre-heat treatment on structural and optical properties of the ZnO thin films. The ZnO thin films cooled slowly exhibit mountain chain structure while the ones cooled rapidly have smooth surface. The ZnO thin films cooled rapidly have higher c-axis orientation compared to the ones cooled slowly. The narrower and the higher near-band-edge emission (NBE) peaks are observed in the ZnO thin films cooled rapidly.

Highlights

► Deposition of precursor solution on Si substrate for the growth of ZnO thin films by sol–gel spin-coating method. ► The solution was cooled with different rates after pre-heat treatment. ► The surface morphology, the residual stress, and the optical properties are affected by the cooling rate. ► The rapid cooling is more suitable than the slow cooling.

Introduction

Zinc oxide (ZnO), with a direct band gap of 3.37 eV at room temperature (RT) and a large exciton binding energy of 60 meV, attracts considerable attention because of its promising applications for ultraviolet light-emitting diodes and laser diodes [1], [2], opto-electronic devices [3], [4], and surface acoustic wave devices [5], [6]. There are various methods for the deposition of ZnO thin films including physical vapor deposition [7], chemical vapor deposition (CVD) [8], molecular beam epitaxy [9], spray pyrolysis [10], sputtering [11], pulsed laser deposition (PLD) [12], ink-jet printing [13], basic erosion [14], and sol–gel methods [15], [16]. The important advantages of sol–gel processing, for the purpose of conventional thin film deposition, are chemical composition control, homogeneity of the sol solution, least expensive, and non-requirement of complicated equipment or dangerous source materials. However, sol–gel method requires thermal annealing in air at high temperature because precursor films prepared by spin-coating of organic zinc compound solution must be oxidized to form ZnO thin films.

In general, the thermal annealing in sol–gel process for the growth of ZnO thin films consists of pre- and post-heat treatment. The pre-heat treatment appears to be a crucial step. Indeed, it governs the orientation of the crystallites during solvent evaporation and removal of the organic compounds. The temperature of pre-heat treatment varies from case to case, but a clear rule can be deduced in its choice. It should be higher than the boiling point of the solvent and the additives and near the crystallization temperature of ZnO. A temperature around 300 °C appears to be the most appropriate [17], [18] in the case of 2-methoxyethanol [CH₃OCH₂CH₂OH] (2MOE) and monoethanolamine [NH₂CH₂CH₂OH] (MEA) as solvent and additive, respectively, to produce (0 0 2) oriented ZnO films. Besides the pre-heat treatment, the post-heat temperature should be carefully chosen. The preferential orientation for instance (0 0 2) was increased with increase in the post-heat temperature. A temperature range of 500–600 °C appears to be the most appropriate one [17], [18]. It is also well known that the properties of ZnO films deposited by sol–gel technique are determined by various parameters such as precursor concentration, additive, aging time, substrate, annealing temperature, and thickness [18], [19], [20], [21], [22]. The heat treatments appear to be one of the most important factors and these factors have been discussed by many studies. However, there are few reports on the effects of cooling rate during thermal annealing process on the properties of ZnO films.

In this study, ZnO thin films were deposited on Si substrates by sol–gel spin-coating method. The deposited ZnO thin films were pre-heated with different cooling rates. The effects of the cooling rate during pre-heat treatment on structural and optical properties of the ZnO thin films were investigated by atomic force microscopy (AFM), X-ray diffraction (XRD), Raman, and photoluminescence (PL). And also, post-heat treatment was carried out to investigate the properties of the ZnO thin films.

Section snippets

Experiments

ZnO thin films were deposited on p-type Si (1 0 0) substrates by sol–gel spin-coating method. The spin-coating solution contained zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] and equivalent molar MEA dissolved in 2MOE. The concentration of zinc acetate was chosen to be 0.6 mol. The resulting solution was then stirred at 60 °C for 120 min to yield a homogeneous and stable colloid solution, which served as the coating solution after being cooled to RT. Prior to growth of the ZnO thin films, the Si

Results and discussion

Fig. 1 shows the AFM 3 μm × 3 μm images of the as-grown and the post-annealed ZnO thin films. The surface of the ZnO thin films with the cooling rate of 5 °C/min [sample (a)] exhibits mountain chain structure with root mean square (RMS) roughness for 23.1 nm. In the previous studies, two possible reasons were reported for the formation of the structure. Zhao et al. [23] reported that the surface morphology can be formed by slow cooling. After the deposition of precursor solution on substrates, Zn²⁺

Conclusion

ZnO thin films were directly grown on Si substrates by using sol–gel spin-coating method. The ZnO thin films are cooled with different rates after the pre-heat treatment. The surface of the ZnO thin films with the cooling rate of 5 °C/min shows mountain chain structure and the ZnO thin films cooled rapidly exhibit smooth surface morphology. The ZnO thin films cooled rapidly have higher c-axis orientation compared to the ones cooled slowly. The tensile stress of the ZnO thin films is increased by

Acknowledgements

This research was supported by Basic Science Research Program Through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0016147).

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Effects of cooling rate and post-heat treatment on properties of ZnO thin films deposited by sol–gel method

Abstract

Highlights

Introduction

Section snippets

Experiments

Results and discussion

Conclusion

Acknowledgements

Mater. Chem. Phys.

Thin Solid Films

Solid State Electron.

Thin Solid Films

Mater. Res. Bull.

J. Cryst. Growth

Nucl. Instrum. Methods: Phys. Res., Sect. B

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Physica B

Thin Solid Films

J. Cryst. Growth

Superlattices Microstruct.

J. Alloys Compd.

Sol. Energy Mater. Sol. Cells

J. Eur. Ceram. Soc.

Sens. Actuators B

Mater. Sci. Eng. B

Opt. Mater.

J. Lumin.