Elsevier

Applied Surface Science

Volume 419, 15 October 2017, Pages 764-769
Applied Surface Science

Full Length Article
Pulsed laser deposition of tin oxide thin films for field emission studies

https://doi.org/10.1016/j.apsusc.2017.05.020Get rights and content

Highlights

  • Pulsed laser deposition was used to deposit SnO2 thin films.

  • SnO2 flakes were observed on film surface at high substrate deposition temperatures.

  • Field emission properties of all the films were studied.

  • Highest emission current density and high enhancement factor were recorded for film deposited at 700 °C substrate temperature.

Abstract

A comparative study of Pulsed Laser Deposition (PLD) based Tin Oxide (SnO2) thin films deposited at various substrate deposition temperature (Ts) has been performed. Surface morphology of the films was studied by Field Emission Scanning Electron Microscopy (FESEM) and surface composition of the films by X-ray PhotoelectronSpectroscopy (XPS) technique. X-ray diffraction (XRD) technique has been used to investigate crystalline nature of the deposited films. Field Emission (FE) properties of the SnO2 films were measured and a significantly low turn on field (2.1 V/μm) (field necessary to draw an emission current density of 10 μA/cm2) for films deposited at high substrate temperature (700 °C) was observed. Field enhancement factor estimated from FE studies was found to strongly depend on the surface morphology of the films. Overall good field emission current stability was observed for all SnO2 films. Dependence of FE properties on surface morphology, surface composition and deposition environment has been observed and analyzed systematically. Significantly low turn on field with high emission current density and field enhancement factor exhibited by films deposited when substrate was maintained at 700 °C has been mainly correlated to surface morphology and surface composition.

Introduction

Tin oxide (SnO2) is a key functional material that has been widely used for optoelectronic devices [1], gas sensors [2], cold cathodes in Field Emission (FE) devices [3], [4]. It is an n-type semiconductor with a wide band gap of ∼3.6 eV at room temperature. Extensive work has been carried out to explore various properties of SnO2 (such as electrical and optical) relevant for different applications. Low work function, good mechanical strength, high melting point and good electrical properties are some of the key features of a material to effectively serve as a good field emitter device. SnO2 nanostructures such as nanorods, nanowires, nanoflowers, nanobelts, and nanoribons have been extensively studied in recent years for FE applications [5], [6], [7], [8], [9]. However, FE properties of SnO2 thin films synthesized by pulsed Laser Deposition (PLD) technique have not been widely studied yet.

A variety of deposition methods have been reported to synthesize SnO2 thin films which includes hydrothermal synthesis, chemical vapor deposition, sputtering and PLD [10], [11], [12]. Among these deposition techniques, PLD stands out on account of its unique features, particularly easy control of deposition parameters (laser fluence, deposition time, distance between substrate and target) which effectively determine the properties of the deposited thin films. PLD technique has been successfully used to deposit SnO2 thin films and the influence of deposition parameters on the film properties has been studied [13], [14], [15]. In these studies, properties of PLD SnO2 films such as microstructure, electrical resistivity and optical transmittance have been observed to depend strongly on deposition parameters, mainly on substrate temperature (Ts) and deposition environment [13], [14], [15]. A strong influence of the deposition temperature on microstructure of SnO2 films prepared by PLD technique has been reported [16].

Field Emission (FE) is the extraction of electrons from conducting/semiconducting materials under a strong applied electric field. FE has technological applications in various micro and nano devices. In FE the emitted current depends on the material composition, as well as, the microstructure of the emitting surface. FE properties such as, turn on field and emission current density have been found to be highly sensitive to variations in the surface morphology and surface composition of the emitter. These properties determine the effective work function of the emitting surface [17]. Hence, surface properties of field emitters play a vital role in determining FE properties. Development of FE based cathodes using various one dimensional (1D) and two dimensional (2D) nanostructured materials such as CNTs, graphene and reduced graphene oxide have been reported [18]. 2D materials are well known in FE applications for their thin planar structure along with high aspect ratio (ratio of lateral size to sheet thickness).

In case of PLD of SnO2, thin films obtained by ablating pure SnO2 target under oxygen atmosphere at different deposition temperatures show remarkable modifications in the microstructure of the film. While, a completely columnar structure in case of films deposited at higher temperature has been reported, films deposited at lower temperature have shown cauliflower-like structure [16]. Therefore, changing the deposition parameters can easily control the surface microstructure and properties in case of PLD based SnO2 films. This enables successful enhancement of FE properties of such PLD SnO2 films.

Here, we report a FE study of SnO2 thin films grown by means of PLD technique at different substrate temperature ranging from room temperature (RT) to 700 °C. Microstructural properties of all the films have been studied using Field Emission Scanning Electron Microscope (FESEM). Crystalline nature of the films has been investigated by Grazing Incidence X-ray diffraction (GIXRD) technique while chemical structure has been studied by X-ray Photoelectron (XPS) Spectroscopy. FE properties, such as, turn on field, current density and enhancement factor have been investigated and analyzed. Our results have established a strong dependence of all salient FE properties on surface microstructure of the film, as well as, on surface composition.

Section snippets

Experimental

Pulsed Laser Deposition was performed using a Q-switched pulsed Nd:YAG laser operating at a repetition rate of 10 Hz delivering laser pulses of 5 ns duration (FWHM) at a second harmonic wavelength of 532 nm. Commercially available SnO2 powder (99% pure) was cold pressed at a pressure of 10 Ton for 1 min to form a circular disk of ∼20 mm diameter and 3–4 mm thickness, to be used as a target for PLD. Ultrasonically cleaned silicon substrates of 5 mm × 5 mm size have been used for all PLD runs. The target

Results and discussion

Fig. 1 shows FESEM micrographs of the SnO2 films deposited at different substrate temperatures. The microstructure of the PLD SnO2 films was found to change significantly with the deposition temperature. Presence of larger particulates on the surface of films deposited at room temperature is clearly visible in Fig. 1a. Diffused as well as separated nanoparticles are also observed on the surface of these large particulates. However, films deposited at higher temperature (400 °C) showed a

Conclusion

PLD technique has been successfully employed for deposition of SnO2 thin films in vacuum at different substrate deposition temperatures. Effect of substrate deposition temperature on the surface morphology and surface composition of the films has been investigated and found to critically depend on deposition temperature. FE study of all films has been carried out. Good FE has been observed for all films whereas highest FE has been recorded for films deposited at high Ts of 700 °C which has been

Acknowledgements

This work was supported by collaboration PhD program between University of Mumbai and Bhabha Atomic Research Centre, Mumbai, India. Authors gratefully acknowledge Prof. S Basu, BARC for experimental support provided for X-ray diffraction measurements and Dr. R Fernandes for X-ray photoelectron spectroscopy measurements.

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