Elsevier

Applied Surface Science

Volume 402, 30 April 2017, Pages 463-468
Applied Surface Science

Full Length Article
Impact of high temperature and short period annealing on SnS films deposited by E-beam evaporation

https://doi.org/10.1016/j.apsusc.2017.01.113Get rights and content

Highlights

  • Preparation SnS films using electron beam evaporation at room temperature.

  • SnS films were annealed at a high temperaure for different short period of times.

  • The films showed highly oriented (111) planes with orthorhombic crystal structure.

  • Surface morphology showed bigger and faceted grains embedded in orthorombic.

  • The TEM confirmed that big orthorombic slabs had single-crystalline nature.

Abstract

Thin films of SnS were deposited on Mo-substrate using electron beam evaporation at room temperature. As-deposited SnS films were annealed at a constant high temperaure of 860 K for different short period of times, 1 min, 3 min, and 5 min. The impact of heat treatment period on the physical properties of SnS films was investigated using appropriate characterization tools. XRD analysis revealed that the films were highly oriented along (111) plane with orthorhombic crystal structure. Surface morphology of as-deposited SnS films showed an identical leaf texture where as the annealed films showed large orthorombic slab shape grains in adidition to the leaf shape grains, which indicates the significance of short period annealing at high temperature. The transmission electron microscopy confirmed that those large orthorombic slabs had single-crystalline nature. The results emphasized that the short period annealing treatment at high temperature stimulated the growth of film towards the single crystallinity.

Introduction

Tin sulfide (SnS) is an emerging novel semiconducting material in the family of metal chalcogenides for photovoltaic applications [1]. It possesses the advantages of being non-toxic to the environment and earth-abundant. It has desired properties for photovoltaic applications such as an optical bandgap in the range of 1.0–1.5 eV, a high absorption coefficient, p-type semiconductor nature and excellent hole mobility [2], [3].

Over the past few decades, various physical and chemical methods were used to deposit SnS thin films [3]. However, the record efficiency of 4.3% for SnS-based solar cells was low compared to the theoretical efficiency 33% [4] because of poor electronic transport properties, low minority-carrier life time, and short diffusion lengths [5]. The impurities, defects, traps and smaller grain size of the SnS films can cause the poor electronic transport properties. In order to decrease the defects to improve the crystallinity of SnS films, high temperature annealing is necessary [6]. Annealing is a heat process, in which a semiconductor material is heated to a specific temperature and then allowed to cool. This process is also known as conventional thermal annealing or post-deposition thermal treatment. The rapid thermal process (RTP) is another type of post-deposition thermal treatment, which has the following advantages. RTP generally requires very short processing time, moderate temperatures, and low cost equipment to obtain high quality SnS thin films. Short-annealing of SnS thin films can promotes the grain growth and reduces electrical resistivity [6]. In addition, it relieves the accumulated strain energy, diminishes defects, and enlarges the grain size owing to the movement of dislocations and other structural defects towards the grain boundaries and adsorb/decompose with the surface [3].

Usually, in the annealing process, temperature, time, pressure and type of atmosphere (i.e. vacuum, air, N2, Ar and H2S + N2 or H2S + Ar) can strongly affect the morphology, crystallinity, and electrical properties of SnS films. For example, the grain size and rms surface roughness of the films decreased with the increase of annealing temperature [7]. Excellent improvement in the crystallinity and surface-smoothness for SnS films is observed by Cheng et al. [6] after annealing. Annealing of SnS films for a short time in open air can change the type of conductivity from p- to n-type [8], whereas the films annealed at longer time oxidized [8], [9], [10]. The films annealed in vacuum for a long time showed a drastic change in their chemical composition and also exhibited a new SnS2 phase by a partial release of tin [8]. Therefore, post-heat treatment can play a crucial role to enhance the physical properties of SnS thin films.

In the present investigation, we studied the effect of annealing time at high temperature of 860 K for electron beam evaporated SnS films. Surface morphology of the annealed films showed larger orthorombic slab shape grains in addition to the leaf shape grains. The transmission electron microscopy confirmed that those larg orthorombic slabs had single-crystalline nature. The results emphasized that the short period annealing treatment at high temperature stimulated the growth.

Section snippets

Experimental details

Tin sulfide flakes (SnS, purity 5N, iTasco) and molybdenum-sputtered glass (300 nm) were used as source and substrate materials. Schematic representation of (a) electron beam evaporation system and (b) tubular furnace system is shown in Fig. 1. SnS thin films of 500 nm thickness were prepared on sputtered Mo substrate using electron beam evaporation system (EBE SNTEK, 12-SN-053). SnS flakes were evaporated from graphite crucible by applying constant emission current of 3 mA for evaporation time of

Results and discussion

The as-prepared and annealed SnS films were uniform, strongly adherent to the surface of the substrate and pinhole free. Fig. 2 shows the XRD patterns of the as-deposited and annealed SnS films. The as-deposited SnS film exhibited peaks at 2θ = 30.55∘, 31.57∘, 39.02∘ and 45.51∘ correspond to the (101), (111), (131) and (002) planes of orthorhombic SnS, respectively (JCPDS card no. 29-0354). The peak at 2θ = 40.50∘ related to the (110) of Mo substrate. As clearly seen in Fig. 2, the intensity of all

Conclusions

Thin films of SnS were deposited on Mo-substrate using electron beam evaporation at room temperature. As-deposited SnS films were annealed at a constant high temperature of 860 K for different short period of times, 1 min, 3 min and 5 min. The XRD analysis revealed that the films were highly oriented along (111) plane with orthorhombic crystal structure. The Raman phonon modes at the 93 cm−1, 192 cm−1, 218 cm−1 and 161 cm−1 were assigned to Ag and B2g modes of SnS phase. Bandgap of annealed SnS films

Acknowledgements

This work was supported by the 2014 Yeungnam University Research Grant. This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200760).

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