Open Access
Add to BibSonomy
Abstract
Complex chalcogenide systems like PbS1-xSex seem to be promising semiconductors with a great potential for highly sensitive photodetectors of the mid-IR range and thermoelectric working at room temperature. The first group of problems that scientists face is how to synthesize materials with a homogeneous chemical and phase composition and a well-defined stoichiometry. The second is how to avoid contamination of such sensitive materials with residues of unreacted precursors and installation materials. In addition, the technological approach should allow the potential scale-up of the process for commercial applications of the above materials. In this work, we report the applicability of the plasma-enhanced chemical vapor deposition (PECVD) in preparation of PbS1-xSex complex inorganic chalcogenide materials of various stoichiometry and phase composition in function of plasma process conditions. Elemental high-pure lead, sulfur, and selenium were the initial substances. RF (40.68 MHz) non-equilibrium plasma discharge at low pressure (0.01 Torr) was used for the initiation of interactions between the starting materials. The PECVD process was studied by optical emission spectroscopy (OES). Various analytical methods were utilized to characterize the obtained materials.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Complex chalcogenide inorganic materials like Pb-S-Se seem to be promising semiconductors with a great potential for highly sensitive photodetectors of the mid-IR range and thermoelectric working at room temperature. However, judging by the number of comprehensive scientific publications, we may only talk about episodic studies of these materials, especially in thin-film forms.
The main problem is the lack of a reliable method for the synthesis of such multi-element inorganic systems.
Several articles are devoted to the theoretical aspects of studying the structure and properties of these materials. E.g., Bafekry and co-workers [1] studied (Ge, Sn, Pb)(S, Se, Te) alloys by a first-principles calculation. It was found that these compounds feature strong absorption in the visible spectral region, suggesting their potential for applications in optoelectronics, photovoltaics, and nanoelectronics.
A few works are devoted to the synthesis and study of the properties of bulk Pb-Se-S samples. Yamini and co-workers [2] synthesized bulk samples of single-phase ternary Pb-Se-Te, and quaternary Pb-S-Se-Te alloys by traditional thermal method from high purity PbS, PbSe, Pb, and Te in the sealed ampoule. It was established that band-gap energies exhibit nonlinear relationships with compositions that are promising for tuning the band-gap energy and lattice parameters of Pb chalcogenide alloys for optical and electronic applications. Qin and co-workers [3] managed to prepare the bulk samples of Pb-S-Se-Te alloys by the melting method followed by a subsequent spark plasma sintering (SPS) technique of high thermoelectric performance reached through tuning of carrier concentration and manipulating of the electronic band structure. In [4], the bulk samples of PbS1-xSex thermoelectric alloys were made by a conventional melting technique. The optimization of thermoelectric performance was reached via gradual changing of the alloy composition. Androulakis and co-workers [5] reported the study results for Pb-S-Se bulk samples doped with PbCl2, excess of Pb, and Bi aiming to obtain n-type of materials conductivity. The authors used the traditional thermal synthesis in a furnace using the corresponding elements.
Since the 1980s, attempts have been made to synthesize and study thin Pb-S-Se films. E.g., in [6], Pb-S-Se heteroepitaxial films were prepared by hot-wall evaporation and studied. The films possessed good crystalline perfection but had low electron mobilities, probably due to deviations from stoichiometry. Contemporary techniques for thin Pb-S-Se films and nanostructured materials preparation also include chemical vapor deposition (CVD) [7–9], and bath deposition methods [10].
The first group of problems facing the scientists face is how to synthesize the materials with a homogeneous chemical and phase composition and a well-defined stoichiometry. The traditional methods of physical sputtering targets with complex inorganic compositions do not give the desired result. Chalcogenides have the general property of incongruent evaporation, i.e. one with a change in composition. The second group, is how to avoid contamination of such sensitive materials with residues of unreacted precursors and installation materials. This statement is especially actual for CVD and solution-based approaches. In addition, the synthesis method should provide potential process scalability for commercial applications of the above materials. Finally, it is ideal when the technological approach provides the possibility to controllably change the electrophysical characteristics of the materials by doping, especially within one vacuum cycle.
Thus, according to the above, there are a few technological barriers to overcome to drastically improve the quality of contemporary thin Pb-S-Se films.
In this work, we report employment feasibility for plasma-enhanced chemical vapor deposition (PECVD) in preparation of PbS1-xSex complex inorganic chalcogenide materials of various stoichiometry and phase composition in function of plasma process conditions. Elemental high-pure lead, sulfur, and selenium were the initial substances. RF (40.68 MHz) non-equilibrium plasma discharge at low pressure (0.01 Torr) was used for the initiation of interactions between the starting materials.
2. Experimental section
The scheme of a plasma-chemical installation for PbS1-xSex synthesis is presented in Fig. 1. The installation is described in detail in [11]. The initial substances Pb, S, and Se of high purity were loaded in the corresponding reservoirs supplied with the external resistive heaters and thermocouples. The temperatures of Pb, S and Se sources were 700, 130 and 240 °C, respectively. Preliminary experiments have shown that it is more convenient to regulate the ratio of chalcogens in the gas phase not by the source temperature but by the flow ratio of the carrier gas (Ar) blown through the furnaces with sulfur and selenium. The flows of carrier gas through the lead furnace and plasma-forming gas were kept constant in all syntheses to ensure constant pressure in the reactor. The power of the plasma generator was constantly maintained at 70 W.
Fig. 1. The scheme of a plasma-chemical installation for PbS1-xSex synthesis. MFC – mass-flow controller, OES – optical emission spectrometer.
Initially, the entire installation was evacuated to a residual pressure of 1×10−4 Torr and held for 1 hour to remove residual moisture and atmosphere adsorbed on the reactor walls. The pressure was monitored with a combined wide-range vacuum gauge WPH-300 (Chuanbei Vacuum Technology (Beijing) Co., China). During the synthesis, the operating pressure in the system was kept constant at a level of 1×10−1 Torr. C-sapphire (0001) substrates with size 10×10×1 mm3 were used for the deposition. In all experiments, the substrate temperature was constantly kept at 15 °C. Emission spectra of chemically active plasma for the Ar-Pb-S-Se system were obtained and studied in the range of 185÷1080 nm using a setup consisting of three optical emission spectrometers AvaSpec-Mini4096CL (UV + VIS + NIR) (Avantes, the Netherlands) with 1200 lines/mm and 0.12 nm resolution. The investigation of chemical composition of the obtained samples was carried out by X-ray microanalysis using a scanning electron microscope JSM IT-300LV (JEOL Ltd., Japan) with an X-MaxN 20 energy-dispersive elemental analysis detector (Oxford Instruments, UK) in a high vacuum and at an accelerating voltage of 20 kV. The surface morphology features of the thin-film samples were studied by atomic force microscopy (AFM) using a scanning probe microscope SPM-9700 (Shimadzu, Japan). To determine the electrical characteristics of the thin PbS1-xSex films, Hall measurements were performed in the van der Pauw configuration. Changes in resistivity and Hall coefficient (in a constant magnetic field of 0.517 T) were performed at direct current. Contacts on the surface of thin lead chalcogenide films were created by soldering metallic indium.
3. Results and discussion
3.1 Optical emission spectroscopy of the plasma deposition process
Studying the nonequilibrium chemically active plasma with the RF discharge using optical emission spectroscopy allows to determine the presence of the individual metastable intermediate particles in the gas phase, as well as to assume a possible mechanism of the plasma-chemical process as a whole. As a rule, in studying mixtures, the classical mechanisms of electron impact/electron attachment are realized; in this case, both excited states of the initial substance atoms, namely, Pb, S, Se, and Te, can be formed, as well as the positive or negative ions of these elements.
The typical emission spectra of the Pb-S-Se vapor mixture in argon plasma during the plasma-chemical synthesis of the thin PbS1-xSex films in the ranges of 200÷1000 nm and 200÷400 nm are shown in Figs. 2 and 3, respectively (Fig. 3 is a partial extraction from Fig. 2).
Fig. 2. The typical emission spectra of the Pb-S-Se vapor mixture in argon plasma in the range of 200÷1000 nm at the generator power of 70W: (a) Ar, (b) Ar + Pb, (c) Ar + Pb + S, (d) Ar + Pb + S+Se.
Fig. 3. The emission spectra of the Pb-S-Se vapor mixture in argon plasma in the range of 200÷400 nm at the generator power of 70W: (a) Ar, (b) Ar + Pb, (c) Ar + Pb + S, (d) Ar + Pb + S+Se.
The emission spectrum of the pure argon under the conditions of the experiment is shown in Fig. 2(a) for comparison. When the lead source was heated to the temperature of 700 °C, the emission spectrum exhibited the characteristic emission lines of excited lead atoms (Figs. 2(b), 3 (b)), the most intense of that were as follows: 217.0, 224.7, 239.4, 257.7, 261.4, 266.3, 280.2, 283.3, 287.3, 357.3, 364.0, 368.3, and 374.0 nm. Adding the sulfur vapor into the (Ar + Pb) gas mixture led to a sharp decrease in the intensity of the lead atomic lines. Besides, the broad molecular emission bands from excited sulfur fragments (S2-S8) appeared in the spectrum. Earlier in [12,13], it was shown that S8 molecules dominate in the vapor phase in the temperature range of 300÷700 K.
Under the conditions of the plasma discharge, upon interaction with electrons S8 molecules undergo decay with the formation of excited states S2* according to the reaction [13,14]:
In Figs. 2(c) and 3 (c), the excited S2* fragments are represented by the following emission lines: 282.9, 290, 293, 336.9, 394, 419.3, and 447.8 nm. The emission lines of the sulfur atomic states were not found in the spectra, since, under the conditions of the experiment, the electron temperature in the plasma discharge was about 1 eV [15], while the atomization of S2 molecules requires the energy of about 10 eV [14,16].
Adding the selenium vapor to the ternary Ar-Pb-S gas mixture caused the appearance in the spectrum of low-intensity emission lines of the selenium atoms Se (I), namely, 204.0, 207.5, and 216.4 nm, due to atomization of Se2 clusters according to the reaction:
It should be noted that, unlike, for example, the As-S system [17], the formation of the intermediate particles of the [PbS2]* or [PbS]* type, which are responsible for the growth of the solid phase on the substrate surface, did not occur in the gas phase. There were no molecular lines corresponding to such fragments in the spectra.
Thus, the plasma-chemical mechanism as a whole can be a combination of the following chemical reactions.
In the gas phase:
On the substrate:
The plasma-chemical process is schematically depicted in Fig. 4.
The PbS1-xSex solid-phase growth took place on the surface of the substrate due to the formation of the structural [Pb-S] and [Pb-Se] fragments.
3.2 EDXA and SEM results
Scanning electron microscopy (SEM) was used for studying the surface morphology and microstructural features of the thin PbS1-xSex films. Studying the effect of the chalcogens (Se:S) ratio in the gas phase on the chemical composition and morphology of the resulting films was carried out with the remaining process parameters unchanged, such as the lead content in the gas phase, the plasma power in the discharge, the pressure in the reactor, and the substrate temperature, and changing only the ratio of sulfur to selenium in the gas phase. An increase in the selenium content in the gas phase was accompanied by an increase in the selenium content in the films of ternary lead chalcogenides deposited on the substrates. Thus, by changing the ratio of the carrier gas flows through the sulfur and selenium sources, it is possible to obtain the almost continuous series of solid PbS1-xSex solutions with a uniform distribution of elements over the surface, which is an undoubted advantage of the proposed method. All the obtained samples of the thin PbS1-xSex films had a mirror-like surface. The average thickness of the PbS1-xSex films was about 1 µm (the average deposition rate is 15–20 nm/min). The thickness of the samples was estimated by the step formation method. EDX mapping showed a uniform distribution of Pb, S, Se elements over the surface for all obtained samples. In Fig. 5, the distribution map of elements is presented for the PbS0.5Se0.5 sample as an example.
Fig. 5. Distribution maps of elements for the PbS0.5Se0.5 sample. The color spots indicate the element distribution.
SEM images of the sample surfaces and chemical composition of the PbS1-xSex films are presented in Fig. 6. According to the SEM data, all samples had a quite uniform surface morphology. On all films of ternary lead chalcogenides, cubic grains with sharply outlined edges were evenly distributed over the surface. It should be noted that the films were free from defects such as cracks, flaking, or pinholes.
From the SEM images shown in Fig. 6, it follows that an increase in the crystal size is observed with an increase in the selenium content in the films, which also agrees with AFM data (see below).
3.3 AFM results
The effect of the chemical composition for the synthesized samples on the surface properties was studied by atomic force microscopy (AFM). Figure 7 shows AFM images (3×3 µm,) for the surface of the PbS1-xSex films. Histograms near the images show the ranges of changes in the height of irregularities on the surface of the samples. The films with low selenium content exhibited a more uniform surface morphology and almost did not contain large clusters. With an increase in the selenium content, an increase in the surface roughness of the films was observed. According to the surface images, the particles were uniformly distributed on the surface of the film. The typical grain size (effective diameter) was about 100 nm for the films obtained without selenium. With an increase in the selenium content in the films (from x = 0.2 to x = 0.9), an increase in the average surface roughness of the films with Ra = 20.37 nm, Rz = 103.27 nm to Ra = 63.25 nm, Rz = 222.06 nm, respectively, was observed. It can be seen from the AFM images that crystalline particles agglomerated on the surface of the films with an increase in selenium content; in addition, an increase in the typical grain size (an effective diameter) was observed up to about 200÷300 nm.
Fig. 7. AFM images of the PbS1-xSex films with various compositions. (Ra is arithmetical mean roughness, Rz is ten-point mean roughness). The scale bar is 1 µm
3.4 X-ray diffraction study
Diffraction patterns of the PbSxSe1-x films depending on the macrocomposition (x) are shown in Fig. 8. The obtained results allow asserting that the samples are single-phase solid solutions with a cubic lattice corresponding to the structural type of rock salt.
The peak in the region of 29÷30° is related to the Bragg reflection from the plane with indices (200) dominating in almost all curves, which, in turn, indicates the preferred orientation of crystallites in this direction. An increase in the selenium content led to a clearly observed shift of the peak towards lower values of the angle 2θ.
The cubic lattice parameter (a) can be calculated using the following equation [18]:
where λ – x-ray wavelength, (h k l) – Miller indices and θ – Bragg angle.The extracted lattice parameters showed a nearly linear dependence on the composition. The lattice constant increased with increasing the selenium content according to Vegard's law (Fig. 9). Since the ionic radius of S2- (1.75 Å) is smaller than that of Se2- (1.98 Å), and the lengths of Pb-S (0.297 nm) and Pb-Se (0.296 nm) bonds are very close, the replacement of sulfur ions by selenium ions probably led to stretching crystal lattice.
The average crystallite size (D) of the materials was calculated from the intense peak using the Scherer equation [18]:
where K is dimensionless particle shape factor (0.94), β $\beta$ is peak half-width.It was found that the size of the crystallites was approximately 40÷50 nm depending on the plasma-chemical deposition conditions of the films.
The dislocation density (δ) was calculated using the formula given in [18]:
In thin films, deformation occurs due to crystal lattice defects, including vacancies, dislocations, and antisite defects. The values of deformation (ɛ) in our films were obtained using the following relation [18]:
where βs is integral peak width.All the results of calculating the values for the lattice parameter, crystallite size, dislocation density, deformation, as well as the position of the angle 2θ for the (200) orientation, depending on the macrocomposition, are presented in Table 1.
Table 1. Calculated values of the lattice parameter (a), crystallite size (D), dislocation density (δ) and deformation (ɛ) of the PbS1-xSex films.
3.5 Optical study
We have found the absorption coefficients from the transmission and reflection spectra of the PbS1-xSex films. Figure 10 shows the dependence of the absorption coefficient of PbS1-xSex films in Tauc coordinates for a direct-gap material. With an increase in the selenium content in the films, the optical band gap decreases approximately from 0.42 for PbS to 0.33 eV for PbS0.1Se0.9. This is due to a rise in the density of localized states in band gap, which causes its shift towards lower values.
3.6 Hall effect measurements
Studying the current-voltage characteristics revealed a linear dependence of the voltage drop across the contacts on the value of the transmitted current in the range of ± 10 µA; therefore, the contacts were ohmic. The Hall measurement data of the PbS1-xSex films depending on the composition are shown in Table 2. The obtained resistivity value of the PbS film without selenium was 0.625 Ω·cm. The film exhibited p-type conductivity with hole concentration and mobility of 7.8×1018 cm−3 and 12 cm2/V·s, respectively. The hole type of conductivity is characteristic of undoped PbS and is associated with electrically active Pb vacancies [19]. A low mobility value is expected for a polycrystalline film.
Table 2. Resistivity, mobility and concentration of charge carriers in the PbS1-xSex films
According to the data from Table 2, all films exhibited p-type conductivity. The resistivity of the PbS1-xSex films strongly depended on the selenium content. The resistivity of the film with the maximum Se concentration was the lowest. The decrease in resistivity upon partial replacement of sulfur by selenium is associated with the increase in the grain size observed in the SEM and AFM images, as well as a decrease in the band gap of the films. When sulfur is almost completely replaced by selenium (PbS0.1Se0.9), the material with the lowest resistivity is formed, since the carrier density of PbSe is higher than that of PbS [20]. The carrier concentration in the PbS1-xSex films became higher when selenium was replaced by sulfur compared to a binary PbS film. We assume that the primary decrease in concentration at x = 0.2 is explained by effects at the grain boundary. There is a large concentration of defects at grain boundaries, such as stacking faults, surface states resulting from incomplete atomic bonding, etc. These states capture free-charge carriers, thereby reducing their density. A further increase in the carrier concentration in the films upon the substitution of selenium for sulfur occurred due to the higher carrier concentration in PbSe than in PbS, as mentioned above. At x ≤ 0.4, the mobility increased with an increase in the selenium content in the films due to an increase in the mean free path as a result of a decrease in the carrier density. With a further increase in the selenium content (x ≥ 0.5), the mobility decreased, which may be due to an increase in carrier scattering on carriers due to an increase in their concentration, which plays an important role at room temperature.
4. Conclusions
The employment feasibility of the PECVD method for preparation of the triple PbS1-xSex chalcogenide materials with various stoichiometry and phase composition was shown. All the obtained samples possess a mirror surface and a uniform distribution of elements on the surface. It was observed that an increase in the crystal size takes place with an increase in the selenium content in the films. The obtained results allow asserting that the samples are single-phase solid solutions with a cubic lattice corresponding to the structural type of rock salt, which was also confirmed by the AFM and SEM data. Resistivity, carrier concentration, and carriers’ mobility of the PbS1-xSex films strongly depend on the selenium content. The proposed PECVD method for the synthesis of the PbS1-xSex films with adjustable compositions allows tailoring as the optical properties as the band gap of transparent semiconductor material, providing control over the optical absorption spectrum, which is important for the manufacture of IR detectors operating in a defined frequency range.
Funding
Russian Science Foundation (19-79-10124).
Acknowledgments
The SEM studies were performed by the Collective Usage Center “New Materials and Resource-saving Technologies” (Lobachevsky State University of Nizhny Novgorod).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. Bafekry, M. Shahrokhi, A. Shafique, H. R. Jappor, M. M. Fadlallah, C. Stampfl, M. Ghergherehchi, M. Mushtaq, S. A. H. Feghhi, and D. Gogova, “Semiconducting Chalcogenide Alloys Based on the (Ge, Sn, Pb) (S, Se, Te) Formula with Outstanding Properties: A First-Principles Calculation Study,” ACS Omega 6(14), 9433–9441 (2021). [CrossRef]
2. S. Aminorroaya Yamini, V. Patterson, and R. Santos, “Band-Gap Nonlinearity in Lead Chalcogenide (PbQ, Q = Te, Se, S) Alloys,” ACS Omega 2(7), 3417–3423 (2017). [CrossRef]
3. B. Qin, X. Hu, Y. Zhang, H. Wu, S. J. Pennycook, and L. Zhao, “Comprehensive Investigation on the Thermoelectric Properties of p-Type PbTe-PbSe-PbS Alloys,” Adv. Electron. Mater. 5(12), 1900609 (2019). [CrossRef]
4. H. Wang, J. Wang, X. Cao, and G. J. Snyder, “Thermoelectric alloys between PbSe and PbS with effective thermal conductivity reduction and high figure of merit,” J. Mater. Chem. A 2(9), 3169 (2014). [CrossRef]
5. J. Androulakis, I. Todorov, J. He, D.-Y. Chung, V. Dravid, and M. Kanatzidis, “Thermoelectrics from Abundant Chemical Elements: High-Performance Nanostructured PbSe–PbS,” J. Am. Chem. Soc. 133(28), 10920–10927 (2011). [CrossRef]
6. R. Neuelmann, A. Marino, and K. Reichelt, “The growth of epitaxial single crystal PbS1−xSex films by hot wall evaporation,” J. Cryst. Growth 64(3), 609–612 (1983). [CrossRef]
7. M. J. Bierman, Y. K. A. Lau, and S. Jin, “Hyperbranched PbS and PbSe Nanowires and the Effect of Hydrogen Gas on Their Synthesis,” Nano Lett. 7(9), 2907–2912 (2007). [CrossRef]
8. J.-P. Ge, J. Wang, H.-X. Zhang, X. Wang, Q. Peng, and Y.-D. Li, “Orthogonal PbS Nanowire Arrays and Networks and Their Raman Scattering Behavior,” Chem. Eur. J. 11(6), 1889–1894 (2005). [CrossRef]
9. Y. K. A. Lau, D. J. Chernak, M. J. Bierman, and S. Jin, “Formation of PbS Nanowire Pine Trees Driven by Screw Dislocations,” J. Am. Chem. Soc. 131(45), 16461–16471 (2009). [CrossRef]
10. M. A. Malik, M. Afzaal, and P. O’Brien, “Precursor Chemistry for Main Group Elements in Semiconducting Materials,” Chem. Rev. 110(7), 4417–4446 (2010). [CrossRef]
11. L. Mochalov, A. Logunov, and V. Vorotyntsev, “Structural and optical properties of As–Se–Te chalcogenide films prepared by plasma-enhanced chemical vapor deposition,” Mater. Res. Express 6(5), 056407 (2019). [CrossRef]
12. V. M. Vorotyntsev, V. M. Malyshev, L. A. Mochalov, A. N. Petukhov, and M. E. Salnikova, “The capture of nanosized particles by the directional crystallization of sulfur,” Sep. Purif. Technol. 199, 214–221 (2018). [CrossRef]
13. N. M. Erdevdy, O. B. Shpenik, and P. P. Markush, “Electron-Impact Excitation of Gas-Phase Sulfur,” J. Appl. Spectrosc. 82(1), 19–24 (2015). [CrossRef]
14. S. J. Brotton and J. W. McConkey, “Dissociative excitation and fragmentation of S 8 by electron impact,” J. Chem. Phys. 134(20), 204301 (2011). [CrossRef]
15. L. Mochalov, R. Kornev, A. Logunov, M. Kudryashov, A. Mashin, A. Vorotyntsev, and V. Vorotyntsev, “Behavior of Carbon-Containing Impurities in the Process of Plasma-Chemical Distillation of Sulfur,” Plasma Chem. Plasma Process. 38(3), 587–598 (2018). [CrossRef]
16. L. Mochalov, A. Nezhdanov, M. Kudryashov, A. Logunov, A. Strikovskiy, M. Gushchin, G. Chidichimo, G. De Filpo, and A. Mashin, “Influence of Plasma-Enhanced Chemical Vapor Deposition Parameters on Characteristics of As–Te Chalcogenide Films,” Plasma Chem. Plasma Process. 37(5), 1417–1429 (2017). [CrossRef]
17. L. Mochalov, D. Dorosz, M. Kudryashov, A. Nezhdanov, D. Usanov, D. Gogova, S. Zelentsov, A. Boryakov, and A. Mashin, “Infrared and Raman spectroscopy study of As S chalcogenide films prepared by plasma-enhanced chemical vapor deposition,” Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 193, 258–263 (2018). [CrossRef]
18. T. S. Shyju, S. Anandhi, R. Sivakumar, and R. Gopalakrishnan, “Studies on Lead Sulfide (PbS) Semiconducting Thin Films Deposited from Nanoparticles and Its NLO Application,” Int. J. Nanosci. 13(01), 1450001 (2014). [CrossRef]
19. X. Zheng, F. Gao, F. Ji, H. Wu, J. Zhang, X. Hu, and Y. Xiang, “Cu-doped PbS thin films with low resistivity prepared via chemical bath deposition,” Mater. Lett. 167, 128–130 (2016). [CrossRef]
20. R. Kumar, G. Jain, R. Saini, and P. Agarwal, “Compositional Effects on Properties of PbS1-xSex Thin Films,” Chalcogenide Lett. 7(4), 233–240 (2010).
