Abstract
Tin monosulfide (SnS) usually exhibits p-type conduction due to the low formation enthalpy of acceptor-type defects, and as a result n-type SnS thin films have never been obtained. This study realizes n-type conduction in SnS thin films for the first time by using RF-magnetron sputtering with Cl doping and sulfur plasma source during deposition. N-type SnS thin films are obtained at all the substrate temperatures employed in this study (221-341 C), exhibiting carrier concentrations and Hall mobilities of ~2 x 10 18 cm-3 and 0.1-1 cm V-1s-1, respectively. The films prepared without sulfur plasma source, on the other hand, exhibit p-type conduction despite containing a comparable amount of Cl donors. This is likely due to a significant amount of acceptor-type defects originating from sulfur deficiency in p-type films, which appears as a broad optical absorption within the band gap. The demonstration of n-type SnS thin films in this study is a breakthrough for the realization of SnS homojunction solar cells, which are expected to have a higher conversion efficiency than the conventional heterojunction SnS solar cells.
Highlights
Controlling the electrical properties of semiconductors by impurity doping is one of the most important techniques in semiconductor technologies
The thin film prepared at a substrate temperature (Tsub) of 333 °C is firstly discussed as a representative sample
0.498 4.6 × 1018 +4.8 × 103 +1.4 × 104 p-type 3.1 × 10–4 1.4 × 1015 1.5 × 100 the obtained films are indexed as α-Sn antisites (SnS), and no impurity phases such as polymorphs of SnS, SnS2, and Sn2S3 were observed whether sulfur plasma was supplied or not during deposition
Summary
Controlling the electrical properties of semiconductors by impurity doping is one of the most important techniques in semiconductor technologies. In SnS bulk pellets and single crystals, it was found that substitutional anion doping of S2– with halide ions, such as Cl– and Br–, successfully achieved n-type electrical conduction [7,8,9,10,11]. It was found that using the sulfur plasma source during deposition significantly suppressed the formation of acceptor-type defects in the SnS films, presumably VSn and SnS, as well as ambipolar VS. This is the key to realize n-type conduction in Cl-doped SnS thin films. The fabrication technique demonstrated here for n-type SnS thin films is expected to revolutionize SnS solar cells, from heterojunctions developed over the last two decades to homojunctions with much higher efficiency
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