Formation of Cd x Pb 1− x S/Cd 1-  S Thin-Film Two-Phase Compositions by Chemical Bath Deposition: Composition, Structure, and Optical Properties

The possibility of forming thin-film two-phase compositions Cd x Pb 1- x S/Cd 1-  S by using the chemical bath deposition from aqueous media with adding various cadmium salts has been demonstrated. The crystal structure, chemical composition, morphology, and the band gap were studied by the X-ray diffraction, scanning electron microscopy, elemental analysis, Auger and Raman spectroscopy, and diffuse reflectance measurements. The formation of a Cd x Pb 1- x S/Cd 1-  S substitutional solid solution phase in well-faceted crystallites on the substrate of an X-ray amorphous CdS phase has been experimentally proved. The found differences in their composition are the result of the effect of the nucleophilicity of the anionic component of the cadmium salt on the kinetics of thiourea decomposition. The results demonstrate the possibility of forming thin-film two-phase compositions or heterostructures on the base of cadmium and lead sulfides in one technological stage by using chemical bath deposition, which can be important for the creation of solar cells.


Introduction
For modern solar energy, the most interesting objects are thin-film solar cells in which absorbing layers are based on CdxPb1−xS substitutional solid solutions that is caused by their semiconducting properties. Significant difference in the band gap value for the end members of the solid solution (Eg=0.41 eV for PbS and Eg=2.4 eV for CdS) provides wide range of spectral sensitivity variation [1][2][3]. For example, a model of the multiple bandgap solar cells was proposed in [4], which were based on CdxPb1−xS alloy nanowires of varying composition and delivered the efficiency higher than 40%. As it is shown in literature, there are different methods for producing CdxPb1-xS films such as vacuum evaporation technique [5] and spray pyrolisis [6], method of ion-exchange substitution [7], sol-gel technology [8] or chemical bath deposition (CBD) from aqueous solutions [9][10][11][12][13][14][15]. CBD method is of particular interest, because it is distinguished by instrumental simplicity and low-temperature conditions of the deposition process; the co-deposition of CdS and PbS makes it possible to obtain films of CdxPb1−xS solid solutions with wide range of compositions having variable photoelectric characteristics. The composition of the films synthesized by the co-deposition of CdS and PbS is determined by the specified conditions of CBD process. For example, single-phase supersaturated substitutional solid solutions of CdxPb1−xS with different composition were obtained in [9][10][11][12]. E. Pentia et al. [13] revealed the presence in the deposited films two independent phases of PbS and CdS in addition to CdxPb1−xS and noted that the experimental data obtained did not allow to conclude unambiguously that the deposited layers were only solid solutions of CdxPb1-xS for the entire range of x from 0 to 1. In [13], forming the solid solutions occurs only when the synthesis conditions are close to the deposition of phases of individual lead and cadmium sulfides; in other cases, a mechanical mixture (CdS)x(PbS)1−x is deposited with different composition depending on the synthesis conditions. The close situation was observed in [16]: deposited films contained a virtually two-phase composition of individual sulfides (PbS)1−x(CdS)x, because there was not found continuous change in the lattice parameter and fundamental optical gap. In turn, Deo et al [17] and Barote et al [18] declared about the preparation of Cd0.5Pb0.5S and Cd0.825Pb0.175S films; however, there was no experimental evidence of the formation of solid solutions at all. At that, there is a discrepancy between the reported composition of the solid solutions and the value of the band gap in [17]; in [18] only the peaks corresponding to CdS and PbS are shown in the Xray diffraction patterns.
The results reported in [14,15] attract particular attention. In [14] it was found that single-phase state in CBD films of Pb1−xCdxS with х ≤ 0.15 (sometimes 0.18) was formed only at relatively low concentration of cadmium salt in the reaction bath. The increase in the cadmium salt concentration resulted in the deposition of X-ray amorphous CdS along with the solid 4 solution. Thus, a two-phase system like Pb1−xCdxS+CdS was formed on the substrate. A similar result was obtained in [15], where two-phase layers containing 2-8 mol% of amorphous CdS phase in addition to CdxPb1−xS solid solution were deposited on various substrates (silicon, sitall, ITO coating, glass). Note that the phase segregation with the formation of two-phase films containing CdxPb1−xS solid solution and X-ray amorphous CdS was established earlier upon isothermal annealing of supersaturated CdxPb1−xS solid solutions when they were heated above 405-410 K [19]. The presented data indicate that upon the co-deposition of lead and cadmium sulfides during the CBD synthesis of Pb-Cd-S films, it is possible to form thin-film compositions or heterostructures in one technological stage. The difference in the type of conductivity and photovoltaic properties of the phases formed in this case indicates the prospects for the creation of solar cells on their basis.
Let us also add that the analysis of works on the co-deposition of PbS and CdS and the phase composition of the obtained films shows that to ensure targeted synthesis it is important to take into account not only the complexing agents used in the system, but also the type of the anionic component of the cadmium salt [11]. Therefore, the aim of the present work was to carry out chemical bath deposition of three-phase films of the CdS-PbS system by using different cadmium salt and to study the composition, structure and optical properties to show the possibility of one-stage forming the compositions promising for using in solar radiation converters.

Experiment
Thin-films on the base of lead and cadmium sulfides were synthesized by chemical bath The deposition of the films was carried out during 120 min at a temperature of 353 K in a TC-TB-10 thermostat in pressurized molybdenum glass reactors in which fat free pre-prepared 5 fused quartz substrates were fixed in specially made fluoroplastic devices. The temperature accuracy was maintained at 0.1°.
The thickness of the films was determined using an interference microscope (Linnik microinterferometer) MII-4M with a measurement error of 20%. The thickness of PbS film was about 600 nm; the thickness of the films deposited from the baths with adding cadmium salt was about 650-700 nm.
The crystal structure of the films was studied by X-ray diffraction (XRD) in the range of angles from 20 to 100 degrees, the step was Δ(2) = 0.02 degrees, the scan time was 10 s at a point. A PANalytical Empyrean Series 2 laboratory diffractometer with CuKα radiation was used. A position-sensitive PIXel3D detector in the parallel beam geometry provided the resolution on 2 scale of at least 0.0016 °. The structural parameters of studied CdxPb1−xS films were improved from experimental XRD patterns by the full-profile Rietveld analysis [20,21] using the Fullprof program [22]. To separate the contributions of grain size and deformations in the studied films into the width of the diffraction peaks, the conventional Williamson -Hall plot equation was used [23]: where D was the average size of the coherent scattering regions (CSR), taken as the average particle size, β was the half-width of the peak in radians,  was the wavelength of the X-ray radiation used,  = d/d was the deformation, d was the interplanar distance in Debye's formula 2dsin = (h 2 +k 2 +l 2 ).
Microstructure of deposited films was studied by using a AMP-9510F Field Emission Auger Microprobe (JEOL). For SEM images tilt angle of the sample relative to the electron beam was 0 degrees; the accelerating voltages of electrons were 10 and 30 keV, the beam current was 0.2 nA, and the electron beam diameter was less than 10 nm. Elemental composition of the films was determined by a JEOL JSM-5900 LV scanning electron microscope with an energydispersive X-ray (EDX) analyzer EDS Inca Energy 250.
Auger spectra of the studied films before and after etching were obtained by using a JEOL Jamp-9510F Auger-electron spectrometer. The energy of the primary electron beam was 10 keV.
The angle of inclination of the sample relative to the normal to the primary electron beam was 30 degrees. The diameter of the electron beam during profiling was more than 100 μm and less than 10 nm when analyzed from a point. An argon ion gun with the energy 2000 eV was used to obtain a depth profile. The etching rate was in the range of 3-12 nm/min.
Raman spectra were recorded on a RENISHAW-1000 spectrometer (Renishaw plc., UK) equipped with Leica DML confocal microscope, the notch filter, a CCD camera, and a LCM-S-6 111 solid-state laser with the radiation wavelength of 532 nm. The spectral resolution was 1-2 cm -1 , the laser beam size was 1 μm, the exposure time of one spectrum varied from 20 to 30 s, the number of scans (the number of signal accumulation cycles) was 2.
The diffuse reflectance study of the films was carried out on a UV-3600 spectrometer (Shimadzu, Japanese) equipped with an ISR-3100 integration sphere. The reflectance spectra R(λ) were recorded in the range of 220-2000 nm with the scanning step of Δλ=1 nm. BaSO4 was used as a reflectance standard. The optical band gap energy (Eg) of the phases forming the studied CdxPb1−xS film was evaluated by the method proposed by Kubelka and Munk-Aussig [24]. The Kubelka-Munk function was calculated by the equation: where R∞ was the diffuse reflectance of an infinitely thick sample relative to the reference, k was the molar absorption coefficient, and s was the scattering coefficient. noted that the diffraction patterns of films obtained from the baths with adding cadmium salt are more complex than that of the individual PbS film (Fig. 1 b, c, d). It is clearly seen that, instead of the isotropic background (Fig.1a), the XRD patterns of the films with cadmium have a strong diffuse halo from the amorphous material, as well as sets of peaks from additional crystalline phase with hexagonal structure B4 (P63mc space group) corresponding to cadmium sulfide.  Table 1). A quantitative analysis of diffraction patterns performed using the FullProf software package makes it possible to interpret the observed decrease in the aB1 period of the cubic lattice as the formation of a CdxPb1−xS solid solution with replacing Pb 2+ ions with a radius rPb= 0.119 nm by smaller Cd 2+ ions with rCd= 0.097 nm [27]. It should be noted that, due to the small thickness of the films and the presence of texture, the cadmium concentration cannot be determined from the calculation of the experimental intensities of the XRD peaks. Therefore, the compositions of the solid solutions were estimated from the change in the lattice parameter using Vegard's rule [28], according to which the molar fraction of cadmium is determined as

Structure
Here aPbS, aCdS, aSS are the parameters of the crystal lattices of lead sulfide, cadmium sulfide, and It is seen, that according to the degree of enrichment of the solid solution with cadmium, the salts used in the reaction mixture can be arranged in the following order: CdSO4 → Cd(NO3)2 → Cd(CH3COO)2. The given sequence of salts accords to the degree of their nucleophilicity manifesting the surface charge density and corresponds to the general lyotropic series of anions in their composition [34]. In [34], 27.810 −4 , respectively); the anisotropy of microstrains becomes more pronounced (Table 1).
This is also clearly seen in Insets in Fig. 1.  Comparing results of EDX analysis for crystallites in two-phase compositions ( Table 2) and chemical composition of CdxPb1−xS solid solutions of these films estimated from XRD data (Table 1), the difference between the concentrations of cadmium is seen. There is a number of the reasons for observed difference. The first one is the fact, that the penetration depth of the electron beam in EDX analysis is greater than the thickness of the films under study, i.e. an amorphous sublayer of cadmium sulfide is analyzed simultaneously with the crystallites. The second reason of the observed excess of the content of the substituting component can be associated with the presence of CdS island formations on faces of crystallites as it was shown in [37] As the last reason of the difference between cadmium content determined by EDX and XRD we assume that part of cadmium ions occupies interstitial positions. If cadmium ions occupy intersects, the lattice parameter should be higher than the parameter in the case when all cadmium ions are in the lead positions, therefore the content of cadmium is underestimated.

Morphology
There are many examples of the appearance of interstitial solutions when PbS is doped with elements with smaller size. For example, in work [38] it was shown that an increase or a decrease in the PbS lattice parameter upon doping with Hg was associated with a change in the distribution of mercury atoms in the lattice which was substitution or insertion, respectively.

Auger spectroscopy
For more detailed study of the elemental composition of the films near the surface and at a certain depth, the Auger electron spectroscopy (AES) was used. The survey Auger spectra recorded from the surface of all the films under study show the presence of oxygen atoms associated with surface oxidation and carbon atoms due to natural surface contamination. To remove carbon impurities and the oxidized layer, the films were etched for 1 min to a depth of 5 nm with argon ions. After ion etching, the survey Auger spectra shown in Fig. 5 indicate the absence of carbon and oxygen atoms in the depth of the studied films (the detection limit of carbon and oxygen is about 1 at.%). Figure 5a shows Auger spectrum of PbS film with a narrow peak in low-energy range (90-94 eV) and a weak peak in the middle energy range (249-267 eV) associated with lead ions [39]. In the spectrum of CdS (Fig.5b) there are peaks at 376 and 382 eV attributed to cadmium ions [39,40]. Besides this, in the spectra the peak at 152 eV is seen, which is connected with sulfur ions.
These spectra of the binary compounds were used for interpretation of the atomic depth profile for two-phase CdxPb1xS/Cd1-S compositions considered in the present work.
As it is seen from SEM images (Fig. 4), the surface of the two-phase compositions has a complex structure with light crystallites lying on a dark amorphous CdS sublayer. Therefore, these regions were analyzed by AES. Figure 6 shows the Auger spectra of the amorphous sublayer (intercrystalline space) (left panels) and crystallites (right panels) in the two-phase For CdxPb1-xS/Cd1-S film deposited from the reaction bath with adding Cd(CH3COO)2, the concentration of lead in crystallites is estimated as 39±6 at.%, content of cadmium is 10±2 at.%, and sulfur concentration is 51±8 at.%. At the same time in the regions of the film between the crystallites the content of cadmium, lead and sulfur is determined as 46 ± 7 at.%, 7 ± 1 at.%, and 47 ± 7 at.%, correspondingly. For the films synthesized from the reaction mixture with Cd(NO3)2 or CdSO4 the content of the elements in crystallites and sublayer differs in comparison with previously mentioned two-phase film (represented in Table 3). The concentration of the cadmium increases in crystallites and decreases in sublayer (the places between crystallites) when going from using Cd(CH3COO)2 to Cd(NO3)2 and then to CdSO4.
Predominant content of lead in crystallites and cadmium in sublayer is confirmed by the AES-map of the distribution of the ions shown in Fig.7, where lead atoms are shown by red color and cadmium atoms are shown by blue one. From comparison with SEM image of the same part of the film it is seen that light large crystallites consist of lead sulfide mainly and the place between them mainly contains cadmium sulfide. One can notice that the concentration of the cadmium in the films determined by Auger spectroscopy is close to that estimated by EDX within error of both methods.

Raman spectroscopy
The Raman spectra of binary PbS and CdS films and, as an example, a two-phase CdxPb1-xS/Cd1-S composition deposited from the reaction bath with cadmium acetate are shown in Fig.  8. As it is known, the position and shape of the lines in the Raman spectra depend on the molecular structure of the compound. Generally, spectrum of a crystalline material shows sharp and intense Raman peaks while amorphous or polycrystalline sample presents broad and less intense Raman peaks [41].  [42][43][44]. For the high-symmetric cubic В1 structure ( Fm m 3 space group) of PbS film, the spectrum contains an intense peak at 133 or 134 cm -1 , which is responsible for the combination of longitudinal and transverse acoustic modes (LA+TA), as it was shown in [45][46][47][48][49][50]. The low-frequency mode at 72 cm -1 can be attributed to transverse optical (TO) mode [48]. A close value of the phonon frequency (73 cm -1 ) in the Raman spectrum was reported in [49], and the peak at about 78 cm -1 , attributed to the A1g mode in PbS film, was observed in [43]. The weak Raman scattering line at a frequency of 179 cm -1 in the spectrum of PbS film belongs to the longitudinal optical (LO) mode, while the lines near 430 and 600 cm -1 originate from the first and second overtones of the main LO phonon modes of PbS, respectively [47]. The weak peak at about 275 cm -1 is attributed to two-phonon processes (2134 = 268 cm -1 ) in PbS and in our case is slightly shifted to the high-frequency region [49,51].
For CdS film, the most intensive mode is observed at a frequency of 302 cm -1 and is associated with LO phonons [51][52][53]. In addition to this peak, a line of overtones of LO phonon with a reduced intensity appears at the doubled frequency 2LO (at ~ 602 cm -1 ), which is in agreement with the literature data [54].
An analysis of the Raman spectra of the two-phase composition synthesized with adding

Diffuse reflectance spectroscopy
As is known, an important characteristic for semiconductor compounds is the band gap; therefore, one of the aims of the present work was to estimate it for the films under study. The Thus, the complex studies carried out using X-ray diffraction, EDX analysis, Auger, Raman, and diffuse reflectance spectroscopies confirmed that our fabrication protocol yields two-phase compositions consisting of the substitutional solid solutions (Cd0.067Pb0.933S, Cd0.071Pb0.929S, or Cd0.076Pb0.924S) and the individual Cd1-S phase, which differ in their semiconducting and photoelectric properties [63,64]. As noted in [63], photodetectors based on p-type CdxPb1-xS solid solutions are characterized by a high photoresponse in the visible and near-infrared ranges and high stability of photoelectric characteristics. Note also that in contrast to the studied substitutional solid solutions, cadmium sulfide is characterized by a pronounced n- 16 type of conductivity. In the future, this makes it possible in one technological stage to form twophase compositions and heterostructures that can be used in the creation of solar radiation converters.

Conclusions
As a result of complex comparative studies using the methods of X-ray diffraction, EDX analysis, Auger, Raman, and diffuse reflectance spectroscopies, the thin-film two-phase CdxPb1xS/Cd1-S compositions chemically deposited using cadmium acetate, nitrate and sulfate were studied. According to estimates made by minimizing X-ray diffraction patterns and by In this case, the former Eg values belong to cadmium sulfide, and the latter ones correspond to the band gap of CdxPb1-xS solid solutions located on the sublayer of the amorphous phase of cadmium sulfide. Thus, the complex studies carried out have confirmed the chemical synthesis of two-phase CdxPb1-xS/Cd1-S films, which differ in their composition depending on the cadmium salts used, as well as in their semiconducting properties. In the future, this gives an opportunity in one technological stage to form compositions and heterostructures that can be used in the creation of solar radiation converters.