Influence of Sb3+ ions on photoconductive properties of chemically deposited PbS films

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Abstract

PbS films were obtained on glass substrates using the chemical bath deposition (CBD) method. The infra red (IR) photosensitivity increases of about 1000 times when a reducing agent (hydroxylamine hydrochloride NH2OHHCl) is added in the deposition bath. A further enhancement was tried by adding small amounts of antimony chloride (SbCl3) solution in the deposition bath. The result was just opposite. It was found that the photosensitivity decreases with increasing the Sb salt content. The changes in PbS photoconductive properties are correlated with the changes in film morphology due to the two compounds added in the bath — the reducing agent and the Sb salt.

Introduction

Lead sulphide (PbS) is known to have a very good photosensitivity in the infra red (IR) domain (800–3000) nm at room temperature) [1], [2]. The most used method for obtaining PbS films is the chemical bath deposition (CBD) [3], [4], [5], because is inexpensive and easy to handle. IR detectors were successfully manufactured using CBD deposited PbS films. Unfortunately, these detectors were used mainly in military applications, thus, during the cold war practically no literature was published on PbS. That explains why after 1990 the interest for studying PbS properties had reborn [6], [7], [8]. One of the problems that is still under study refers to the influence of deposition bath content on PbS film properties and to the link that appears to exist between the crystallite size and photoconductivity [9], [10]. According to these early studies it appears that there is an optimum crystallite size, around 300 nm, for which a maximum photosensitivity is obtained. The crystallite size is controlled by changing the bath composition. In the present paper, the influence of a reducing agent (hydroxylamine hydrochloride NH2OHHCl) on the photoconductive properties of PbS films is investigated. Early studies had shown that oxidant or reducing agents introduced in the deposition bath significantly enhance the photoconductive properties of PbS [2], [3], [4]. A further enhancement was tried by adding small amounts of trivalent antimony Sb(III), under the form of antimony chloride (SbCl3) solution, in the deposition bath.

Section snippets

Experimental

The method used to obtain PbS films is CBD. The raw materials are lead nitrate (Pb(NO3)2), as Pb2+ ions source, thiourea, as S2− ions source, and sodium hydroxide (NaOH) that is used both as complexing agent for Pb2+ ions and to set the solution pH to 11. Details about the deposition process are given in a previous paper [5]. Small amounts of antimony chloride solution (0.1 g of SbCl3 in 1 l of NaOH 0.69 M) were added in the deposition bath. This, in reaction with sodium hydroxide (NaOH),

Results and discussions

The antimony sulphide (Sb2S3) has a lower solubility product than the PbS (2.9×10−59 compared with 1.1×10−29), thus it will precipitate first. But, Sb2S3 cannot exist in alkali medium, being soluble in NaOH with the formation of natrium tioantimonit (NaSbS2) and natrium tiooxyantimonit (NaSbOS):Sb2S3+2NaOH=NaSbS2+NaSbOS+H2O

These processes are generated by the presence of Sb(III) and consumes the S2− ions released by decomposition of thiourea. This leads to significant changes in the induction

Conclusions

The main conclusions of this paper are.

The reducing agent has a considerable effect on the growth mechanism of PbS films and leads to a considerable enhancement of their photoconductive properties, probably by introducing sensitising centers connected to film morphology. The crystallite size is smaller, the crystallite size dispersion is larger and the film surface is smoother in case of the films deposited using the reduction agent.

The Sb(III) introduced in the deposition bath in conjunction

Acknowledgements

This work was supported through the NATO project SfP-971970-INOWATE.

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