Realizing n-Type Conduction in Tin Monosulfide Thin Films with Cl Doping and Sulfur Plasma Supply

By Laura ThomsonDec 21 2021

In semiconductor technologies, one of the most crucial techniques is to control the electrical properties of semiconductors by doping them with impurity. This technique could enable the conversion of carrier type from p-type to n-type carriers, thereby forming a pn homojunction. 

Image Credit: Macro photo/Shutterstock.com

In the case of nominally undoped semiconductors, the carrier type is often governed by the defects with a low ionization energy and low formation enthalpy. This generally renders carrier-type conversion challenging.

Tin monosulfide (SnS) is a potential material for developing a solar light absorber for thin-film solar cells. It is made of non-toxic and abundant elements and shows a suitable optical property for solar cells. In general, SnS shows p-type conduction because of the intrinsic defects. The acceptor-type Sn vacancy (V_Sn) easily forms under S-rich conditions.

When it comes to SnS bulk pellets and single crystals, it was identified that substitutional anion doping with S^2– that has halide ions (such as Cl^– and Br^–) enabled successful n-type electrical conduction. This doping technique is more propitious for practical use compared to the solid solutions between PbS and SnS (Sn_1−xPb_xS), where x > 0.2.

It has been proposed that acceptor-type defects such as V_Sn and SnS and ambipolar defects such as V_S—both of which are caused by sulfur deficiencies—compensate the electrons added by halogen doping. This prevents the halogen-doped SnS thin films from showing n-type conduction. Hence, novel thin film deposition methods are required to overcome this issue.

In a study published in Physical Review Materials, the combination of radiofrequency (RF) magnetron sputtering and an RF sulfur plasma source was used to fabricate n-type SnS thin films. The use of the sulfur plasma source for deposition was found to considerably inhibit the formation of acceptor-type defects in the SnS films, possibly V_Sn and SnS, as well as ambipolar V_S.

This is crucial in achieving n-type conduction in Cl-doped SnS thin films. The fabrication method shown here for n-type SnS thin films is predicted to transform SnS solar cells—from heterojunctions created in the past 20 years to homojunctions that exhibit considerably higher efficiency.

Methodology

Fabrication of SnS thin films was achieved using RF-magnetron sputtering. The dependence of the physical properties of SnS thin films on the substrate temperature was analyzed by using a SiO₂ glass substrate measuring 50 × 50 mm with a temperature gradient of 221 °C–341 °C across it. The deposition time was set as 30 minutes. The sputtering target used in the experiment was a piece of undoped SnS with a diameter of 2 inches.

Chlorine (Cl) was supplied to the deposition vacuum chamber before performing thin film deposition. This was done by exposing a Cl-containing SnS target to Ar plasma under conditions identical to that of thin-film deposition for 30 minutes. Here, a temporary substrate was placed on the substrate holder.

The deposition chamber was then evacuated to its base pressure of less than 5 × 10^–4 Pa. Subsequently, the substrate was replaced. Sulfur deficiency in the resulting thin films was prevented by using a sulfur plasma source together with RF sputtering.

Results

Figure 1(a) shows that all x-ray diffraction (XRD) peaks of the resulting films are indexed as α-SnS. Moreover, there were no impurity phases such as polymorphs of SnS, SnS₂, and Sn₂S₃ regardless of whether the sulfur plasma was supplied during deposition.

Figure 1. Structural, compositional, and morphological analysis of Cl-doped SnS thin films deposited with and without a sulfur plasma supply. (a) X-ray diffraction (XRD) profiles of films deposited at T_sub = 333 °C, together with a reference pattern of α-SnS (ICSD#24376). (b) Depth profile of Cl concentration and normalized intensities of secondary ions of Sn⁻, ³⁴S⁻, and Si⁻ in the films deposited at T_sub = 333 °C. (c) Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the surface of SnS thin films deposited at Tsub = 333 °C. The root-mean-square (RMS) roughness is also shown. Image Credit: Suzuki, et al., 2021

As shown in Table 1, the chemical compositions of the films that were deposited in the presence and absence of sulfur plasma supply were assessed by x-ray fluorescence spectroscopy (XRF).

Table 1. Physical properties of SnS thin films prepared at T_sub = 333 °C with and without a sulfur plasma supply. All results were obtained at room temperature, except that the Seebeck coefficients were measured at 25 °C–50 °C. Source: Suzuki, et al., 2021

The depth profile of Cl concentration as well as the intensities of the secondary ions of Sn^–, Si^–, and ³⁴S^– governed by time-of-flight secondary ion mass spectroscopy are illustrated in Figure 1(b).

The atomic force microscopy (AFM) and surface scanning electron microscopy (SEM) observations shown in Figure 1(c) demonstrate that the resulting SnS films are made of nearly circular grains with a 10–200 nm diameter whether or not there was sulfur plasma supply. By contrast, grain size tends was found to be somewhat smaller (<100 nm) in the film deposited in the presence of sulfur plasma supply.

From Figure 2(a), it is evident that the binding energies of the valence band edge, S 2p, and Sn 3d peaks of the film (the n-type film) deposited in the presence of sulfur plasma supply shifted downward by ∼0.3 eV than the film (p-type) deposited in the absence of sulfur plasma. From these findings, it is clear that the carrier type of the thin films is governed by the presence or absence of sulfur plasma supply.

Figure 2. (a) X-ray photoelectron spectroscopy (XPS) spectra (excitation source: Ag Lα) of SnS films deposited at T_sub = 222 °C for the valence band (VB), S 2p, and Sn 3d core levels. Differences in the binding energies between n- and p-type films (ΔBE) are shown in each plot. A total resolution of ∼0.5 eV for the measurement resulted in significant spectral broadening. The displayed spectra were obtained by the moving average of the original results. Other characteristics of this film (T_sub = 222 °C) are discussed in the last half of main text. (b) Absorption spectra of the n- and p-type SnS thin films prepared at T_sub = 333 °C. The broad peak centered at 1.1 eV is significant only for the p-type film. Image Credit: Suzuki, et al., 2021

From Figure 2(b), it can be observed that besides the steep slope from 1.3 eV corresponding to the basic SnS absorption, a broad absorption at ∼1.1 eV was considerably more obvious in the film (p-type film) deposited in the absence of sulfur plasma supply than the film (n-type film) deposited in its presence.

Figure 3(a) is a plot of the temperature-dependent carrier concentration in both the p-type and n-type films.

Figure 3. (a) Temperature dependence of carrier concentration and (b) μ_HallT^1/2 of SnS thin films deposited at T_sub = 333 °C. The activation energy (E_a) and grain boundary potential barriers (Φ_B) were estimated according to the following formulas: N = N₀exp(–E_a/kT), where N₀ is a prefactor and k is the Boltzmann constant, and μ_Hall = Le(1/2πm*kT )^1/2exp(–Φ_B/kT), where L is the grain size, e is elementary charge, and m* is the effective mass. The solid lines were added to guide the eye. Image Credit: Suzuki, et al., 2021

The temperature dependence of the Hall mobility for both films is illustrated in Figure 3(b). For both n-type and p-type SnS thin films, the Hall mobility decreases with a decrease in temperature. This corresponds to the fact that phonon scattering or ionized scattering in these films do not govern carrier transport.

Figure 4(a) shows that there was a shift in the primary peaks corresponding to 400 diffractions toward a lower diffraction angle with a decrease in the substrate temperature. This shows that there was an increase in the lattice constant in the a-axis (a₀) with a decrease in the substrate temperature.

Figure 4. Substrate temperature dependence of the various properties of the obtained SnS thin films. (a) Lattice parameter of a₀ of the obtained thin films determined by 400 diffraction peaks (for the x-ray diffraction (XRD) profiles. (b) Chemical composition determined by x-ray fluorescence spectroscopy (XRF). (c) Carrier concentration and (d) Hall mobility of SnS thin films measured at room temperature. All solid lines in the figures were added to guide the eye. (e) Comparison of electrical properties of n- and p-type SnS thin films obtained in this paper and those of previously reported n- and p-type SnS, and n-type Sn_1−xPb_xS solid solutions. Image Credit: Suzuki, et al., 2021

A plot between the chemical composition of SnS thin films and the substrate temperature is shown in Figure 4(b). Figures 4(c) and 4(d) illustrate the dependence of the electrical properties on the substrate temperature for both n-type and p-type SnS thin films. The n-type SnS thin films fabricated in this study exhibit a low Hall mobility and high carrier concentration that the p-type films and n-type bulk single crystals reported earlier (see Figure 4(e)).

The use of sulfur plasma supply during deposition is advantageous for fabricating high-quality undoped p-type SnS thin films as it effectively inhibits the formation of acceptor-type defects.

The sulfur plasma can be supplied to fabricate high-quality thin films with both n-type and p-type conduction. This technique will certainly be crucial for achieving highly efficient SnS homojunction solar cells.

Conclusion

This study achieved n-type electrical conduction in SnS thin films through sulfur plasma supply at the time of deposition in conjunction with Cl doping. In the absence of sulfur plasma supply, the fabricated SnS thin films showed p-type conduction despite having a comparable amount of Cl donors.

The fabrication technique discussed in this study can be used for an extensive range of sputtering conditions. This is because n-type conduction was achieved irrespective of the substrate temperature. To date, SnS heterojunction solar cells have exhibited a stagnant conversion efficiency at low values (4%–5%).

New techniques to produce n-type SnS thin films will allow the fabrication of SnS pn homojunctions and transform the SnS solar cell technology to a shift from heterojunctions to homojunctions to achieve high conversion efficiency.

Journal Reference:

Suzuki, I., Kawanishi, S., Bauers, S. R., Zakutayev, A., Lin, Z., Tsukuda, S., Shibata, H., Kim, M., Yanagi, H., Omata, T. (2021) n-type electrical conduction in SnS thin films. Physical Review Materials, 5, p. 125405. Available online: https://journals.aps.org/prmaterials/pdf/10.1103/PhysRevMaterials.5.125405.

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