Electrical Characterization of Heat-Treated Tin Monosulfide Thin Films

and Introduction: Tin monosulfide (SnS) thin films have generated much interest in recent years due to their potential application as an absorber layer in thin film photovoltaic cells [1]. Thus, the goal of this project was to improve the quality of SnS films through various methods of heat-treatment by means of grain growth, a reduction in bulk defects, and increased carrier mobility, which can increase the probability of charge collection and thus potentially improve cell efficiency. The effects of the annealing atmosphere (inert nitrogen (N2) versus hydrogen sulfide (H2S) gas), temperature (250-540°C), and annealing time (10 s versus 60 min) on the film’s electrical properties were investigated. Scanning electron microscopy (SEM) and Hall measurement results revealed that annealing the films in H2S atmosphere produced larger, columnar, and uniform grains with greater mobility as compared to the asdeposited and N2 annealed SnS. While the heat-treatment of SnS films in H2S for 10 s and 60 min both produced films of comparable mobility, the film that underwent the 10 s process had a relatively lower carrier concentration, which is an indicator of the amount of bulk defects present. Therefore, the most desirable SnS films were found to be produced through annealing in a H2S atmosphere at 400°C for 10 s. Experimental Procedure: SnS films were deposited using atomic layer deposition (ALD), and annealed at various atmospheres (N2 or H2S), temperatures (between 250-540°C) and times (10 s or 60 min). These films were heated at an average ramp rate of 0.17°C/s before being held at the annealing temperature for the specified time. The heat-treated films were then studied using the SEM and x-ray diffraction (XRD). Square Hall samples of the heat-treated films 1 cm wide were then prepared through deposition of 200 nm of gold, and removal of excess film using a reactive ion etching (RIE). The various electrical properties of each film were then calculated from the measured Hall voltage of each prepared sample. Figure 1: SEM comparison of SnS grains of (a) as-deposited and (b) heat-treated in H2S for 60 min. Results and Conclusions: Cross-sectional SEM images of the heat-treated films demonstrated that when all other conditions were held constant, larger, more columnar grains were produced by annealing in H2S as compared to N2 and as-deposited films. Increasing the annealing temperature also produced an increased grain size. Figure 1 demonstrates the significant change in grain size produced by such an annealing process, with the as-deposited film shown on the left in (a) and heat-treated film in H2S at 400°C on the right in (b). Such grain growth is beneficial in the development of SnS films for use in photovoltaic devices, since it reduces the probability of charge scattering at the grain boundaries, potentially allowing for more efficient charge collection. The results of characterization of the electrical properties are shown in Figure 2 and Figure 3 respectively. The as-deposited films are indicated in both graphs by the point at 200°C, which is the deposition temperature.