Annealing effect on temperature stability and mechanical stress at the “CdxPb1−xS film – substrate” interface 250 D O I: 1 0. 15 82 6/ ch im te ch .2 02 0. 7. 4. 20 Maskaeva L. N., Kutyavina A. D., Pozdin A. V., Miroshnikov B. N., Miroshnikova I. N., Markov V. F. Chimica Techno Acta. 2020. Vol. 7, no. 4. P. 250–258. ISSN 2409–5613 Annealing effect on temperature stability and mechanical stress at the “CdxPb1−xS film — substrate” interface L. N. Maskaevaab*, A. D. Kutyavinaa, A. V. Pozdina, B. N. Miroshnikovc, I. N. Miroshnikovac, V. F. Markovab a Ural Federal University, 19 Mira st., Ekaterinburg, 620002, Russia b Ural Institute of the State Fire Service of the EMERCOM of Russia 22 Mira st., Ekaterinburg, 620022, Russia c National Research University “Moscow Power Engineering Institute” 14 Krasnokazarmennaya st., Moscow, 111250, Russia *email: larisamaskaeva@yandex.ru Abstract. The article establishes the upper temperature steadiness limit of СdxPb1-xS supersaturated solid solutions obtained by chemical bath deposition. СdxPb1-xS (x = 0.06; 0.122; 0.176) and (x = 0.02–0.05) films remained stable under the heating up to 405– 410 and 450 K, respectively. SEM studies have shown that heating of СdxPb1-xS films (x = 0.02–0.05) to 620 K leads to the structure destruction. Internal mechanical com- pressive stresses at the “СdxPb1-xS film-substrate” interface was calculated in the range of 300–900 K for the first time ever, the highest values reached 2000–2750 kN/m2 for a number of the films compositions. In contrast to solid solutions, the expansion stresses up to 100 kN/m2 were derived for the CdS layer at 900 K. The obtained temperature steadiness boundaries and the mechanical stresses of СdxPb1-xS films must be taken into account in the development of photonic devices based on such materials. Keywords: chemical bath deposition (CBD); thin films; CdxPb1−xS solid solutions; annealing; mechanical stress Received: 12.10.2020. Accepted: 27.12.2020. Published:30.12.2020. © Maskaeva L. N., Kutyavina A. D., Pozdin A. V., Miroshnikov B. N., Miroshnikova I. N., Markov V. F., 2020 Introduction One of the most important plac- es among photosensitive semiconductor compounds is occupied by СdxPb1‑xS sub- stitutional solid solutions, which are used to  register visible and infrared radiation in the range of 0.4–3.0 μm. The most com- mon areas of their practical use are the man- ufacture of IR detectors [1, 2], environmen- tal control devices [3, 4], solar cells [5–7], as well as fast-response flame detectors [8]. The most effective method for the syn- thesis of films of CdxPb1-xS solid solutions is chemical deposition from aqueous media (chemical bath deposition CDB). During this process, layers are formed with a B1 structure that are strongly supersaturated with respect to  the  substituting compo- nent [9]. Expansion of  the  temperature range of  using photonic devices based on CdxPb1-xS films presupposes a  clear 251 knowledge of the temperature boundaries of the solid solutions stability of various compositions, as  well as  their mechani- cal characteristics. Given the  high level of  supersaturation in  chemically depos- ited films of CdxPb1-xS solid solutions, a sig- nificant increase in temperature can lead to  structural, morphological, and phase transformations in the layer [10], as well as the appearance of internal mechanical stresses at  the  “film  — substrate” inter- face [11]. The reason of their occurrence is the difference in the values of the crystal lattice constants, elastic moduli, and ther- mal expansion coefficients of the film and substrate. Since the film and the substrate are rigidly connected to  each other, and the film is much thinner than the substrate, it can experience significant mechanical stress of compression or tension to match the geometry of the substrate [11]. Even an approximate estimate of elastic stresses is important for practical applica- tion. It will make possible to qualitatively determine the  distribution of  stresses in a thin-film layer and evaluate their in- fluence. Thus, annealing of ZnS and ZnSe films obtained by thermal discrete evapora- tion on NaCl cleavages, quartz and silicon substrates, has led, in particular, to the van- ishing of inclusions of the high-tempera- ture wurtzite phase in the sphalerite ma- trix, to improvement of the structure and homogenization of the phase composition. In  this case, the  films heating to  200  °C for an hour has caused to the appearance of mechanical stresses in the layers, reach- ing approximately −170∙109 N/m2 [12, 13]. Moreover, the authors of the works note that the  mechanical stresses in  the  films on the rock salt cleavage represent the ex- pansion ones, and stresses in  the  films sedimented on quartz and silicon sub- strates are the  compressive ones. It was shown in  [14] that internal mechanical compressive stresses in ZnSe films chemi- cally deposited at 353 K are proportional to an increase in their thickness and de- pend on the thermal expansion coefficients of the film and substrate. In this regard, it is necessary to take into account the com- patibility of the thermal expansion coeffi- cients of the used substrates and the semi- conductor layer for creating multilayer functional structures. In [15], we showed that the magnitude of  the  mechanical compressive stresses arising at the “film — substrate” interface are presumably related to various condi- tions of  nucleation and growth during the chemical deposition of CdxPb1-xS layers on silicon, sitall, fused quartz, ITO, slide and porous glasses substrates. In this case, the magnitude of the internal elastic stress- es in the layers is asymbatic to the tempera- ture expansion coefficients for the studied substrate materials. At the same time, there are no data on temperature boundaries of stability and internal mechanical stresses in CdxPb1-xS supersaturated solid solutions films. Thereby, in this work, we intend to es- tablish the effect of annealing on the tem- perature stability and mechanical stresses at the “CdxPb1-xS film — substrate” inter- face for various phase compositions of sol- id solutions. Experimental CdxPb1-xS (х ≤ 0.176) solid solutions films were obtained by chemical bath depo- sition from a  reaction mixture contain- ing fixed concentrations of sodium citrate Na3C6H5O7, an  aqueous solution of  am- monia NH4OH, and thiourea N2H4CS. 252 The  concentration of  cadmium chloride CdCl2 was varied from 0.01 to 0.1 mol/l. The content of lead acetate Pb(CH3COO)2 was 0.04 and 0.06 mol/l. Deposition was carried out in sealed reactors on previous- ly degreased sitall substrates for 120 min in a TS-TB-10 thermostat at 353 K with an accuracy of maintaining the tempera- ture of ± 0.1 K. The thickness of the obtained films was estimated using an interference microscope (Linnik microinterferometer) MII-4M with a measurement error of 20%. The  degradation of  the  synthesized CdxPb1-xS (х = 0.06; 0.122; 0.176) solid so- lutions was studied by isothermal anneal- ing in an argon atmosphere in the tempera- ture range 298–673 K for 1 hour in a SNOL 7,2|1100L muffle furnace. In  parallel, the resistance of the CdxPb1-xS films (х = 0.02–0.05) was continuously monitored at heating from 300 to 900 K in air (rate ~ 12  K/min), followed by  slow cooling to room temperatures. The  crystal structure of  synthesized CdxPb1-xS solid solution films was studied by  X-ray diffraction method using dif- fractometer Dron-4 with copper anode Cu Kα1,2. The structural and morphological char- acteristics of  the  as-deposited CdxPb1-xS films were studied using a MIRA 3 LMU scanning electron microscope at an elec- tron beam accelerating voltage of 10 kV, and the annealed ones were investigated using a  Vega II SBU scanning electron microscope (Tescan, Czech Republic) at  an  electron beam accelerating voltage of 20 kV. An  approximate estimate of  the  me- chanical stresses σΔα in the “CdxPb1-xS solid solution film — substrate” two-layer struc- ture was carried out according to the equa- tion proposed in [16]: ( ) ( ) ( ) Cd Pb S sub Cd Pb Cd Pb S Cd Pb S sub Cd Pb S 1 1 1 1 1 6 1 3 4 x x x x x x x x x x SE h T h h ‑ ‑ ‑ ‑ ‑ Dα ⋅ ⋅ α ‑α ⋅ ⋅D σ = ‑ν ⋅ ‑ (1) where Cd Pb1x x S E ‑  — Young’s modulus of Cdx- Pb1−xS solid solution; αsub, Cd Pb1x x S‑α  — ther- mal expansion coefficient of sitall substrate and film, respectively; ΔT  — setpoint temperature difference; Cd Pb1 x x S‑ ν — film Poisson’s ratio; sub Cd Pb S1 , x x h h ‑  — thickness of substrate and films, respectively, upon condition hsub >> Cd Pb1x x Sh ‑ . Results and discussion The effect of the annealing temperature on the content of the substituent compo- nent (cadmium sulfide) in CdxPb1-xS solid solutions is shown in Fig. 1, which images the equilibrium phase diagram of the CdS– PbS system. CdxPb1-xS films with initial CdS contents of 6.0, 12.2, and 17.6 mol.% were annealed at 403, 473, 573, and 673 K for 1 h. The layers were obtained from a re- action mixture containing 0.04 mol/l lead acetate with varying the cadmium chloride content from 0.02 to 0.1 mol/l. Annealing for 1  hour at  673  K dem- onstrated almost complete correspond- ence of the composition of the annealed films to  the  equilibrium phase diagram of the PbS–CdS system [17]. The obtained results confirm the supersaturated metasta- ble character of the chemically deposited CdxPb1-xS solid solutions films (0.060 < x < 0.176). XRD studies of the annealed layers have revealed that, all samples retain dif- fraction reflections corresponding to the B1 structure. However, they shift to the region of smaller angles, which means an increase in the lattice period of the solid solution. In addition, the background intensity in- creases due to  the  formation of  an  indi- 253 vidual X-ray amorphous phase of  CdS. At the same time, no additional reflections were detected on the XRD patterns. The established changes in the initial compositions of the CdxPb1-xS supersatu- rated solid solutions are a  consequence of  their decomposition into two phases: a  solid solution with a  lower cadmium content (x) and an individual CdS phase. The decay indicator of the studied solid solutions is also a changing their electro- physical properties. For this purpose, we investigated the dependence of the resist- ance R of CdxPb1-xS films in the temperature range from 300 to 900 K (Fig. 2). Layers synthesized from a reaction mixture with a lead acetate concentration of 0.06 mol/l were used as  initial samples. The  initial content of cadmium chloride in the reac- tor was 0.01, 0.02, 0.04, 0.10  mol/l. Ac- cording to  the  XRD data, the  resulting films had the compositions: Cd0.02Pb0.98S, Cd0.03Pb0.97S, Cd0.04Pb0.96S and Cd0.05Pb0.95S. As seen from Fig. 2, as the temperature rises to 450 K, the films resistance decreas- es by about an order of magnitude. The be- havior of the curves of all the films under discussion in the range of 300–450 K is ap- proximately similar. However, a  slightly greater resistance decrease is observed for the  Cd0.02Pb0.98S solid solution (1) com- pared to other samples. It can be assumed that the  solid solution structure partial recrystallization occurs in  this tempera- ture range. Note that all the samples under study have a pronounced photosensitivity to visible and near-IR radiation. A  further temperature increase (up to 450–500 K) is accompanied by an abrupt resistance decrease by more than three or- ders of magnitude. In this case, the other electrophysical properties of the films also transform, the photoresponse disappears, and the  Hall mobility sharply declines. The revealed effect is due to the decompo- sition beginning at Т > 450 K of supersatu- rated solid solutions (B1 structure) with Fig. 1. Effect of the annealing temperature on the composition of CdxPb1–xS supersaturated solid solutions in the range 403–673 K and correspondence of obtained data to the equilibrium phase diagram of the CdS — PbS system. Initial content of CdS in solid solution samples at 298 K, mol.%: 6.0 ( ), 12.2 (•), 17.6 (∇) 17.6. The duration of the annealing was 1 hour. Phase equilibrium diagram of CdS — PbS is given according to [17], dash line denotes extrapolated data [17] 254 separating the  individual B3 cadmium sulfi de phase from the  CdxPb1-xS crystal lattice. Th e degradation of the supersaturated solid solution generally ends at T ≈ 600– 650  K.  Th e  fi lm becomes two-phase, its residual resistance increases (a conductiv- ity decreases) due to additional scattering of electrons at phase boundaries in accord- ance with Nordheim’s rule. In  the  range 650–750 K the observed resistance increase by about an order of magnitude is a conse- quence of the oxidation process, fi rst of all, of lead sulfi de to the formation of oxygen- ated phases (PbO, PbSO4) [18]. For comparison, Fig. 3 shows SEM im- ages of as-deposited fi lms of solid solutions Cd0.02Pb0.98S (a) and Cd0.04Pb0.96S (c) and SEM images of  these fi lms aft er heating in air to 893 K (b) and (d), respectively. It should be noted that the as-deposited fi lms morphology is  approximately the  same. Th ey are diff erent only in the sizes of crys- tallites formed their surface: 0.4–0.6 μm for Cd0.02Pb0.98S and 0.4–0.8  μm for Cd0.04Pb0.96S. Th e  annealing Cd0.02Pb0.98S fi lm consists mainly of 5–12 µm globules, formed by  smaller spherical particles (~ 0.50–1.0 µm). Single ~ 1–2 μm crystallites are observed on its surface. Fig. 3. SEM images of Cd0.02Pb0.98S (a, b) and Cd0.04Pb0.96S (c, d) solid solutions fi lms before (a, c) and aft er annealing at 893 K in air (b, d). SEM images of as-deposited layers were obtained using a MIRA 3 LMU scanning electron microscope at an electron beam accelerating voltage of 10 kV, and ones aft er annealing were received using a Vega II SBU scanning electron microscope (Tescan, Czech Republic) at an electron beam accelerating voltage of 20 kV Fig. 2. Th e resistance dependence of Cd0.02Pb0.98S (1), Cd0.03Pb0.97S (2) Cd0.04Pb0.96S (3), Cd0.05Pb0.95S (4) solid solutions fi lms on the heating temperature in air 255 The Cd0.04Pb0.96S film globules with sizes from 4 to 20 μm are destroyed after heating to 893 K under the temperature and an in- creasing internal stresses. For example, structure of the film at the Fig. 3d exhibits strongly deformed spherical particles or globules with broken edges. As  already noted [12, 13], anneal- ing of  chemically deposited films even at  200  °C leads to  the  appearance of  in- ternal mechanical stresses. Therefore, in  this work, we performed a  quantita- tive assessment of the elastic mechanical stresses arising during annealing at  the “CdxPb1−xS film — substrate” interface ac- cording to Eq. (1). The  main physical characteristics (Young’s modulus E, temperature coeffi- cient of  linear expansion TCLE α, Pois- son’s ratio ν) of the CdxPb1−xS multicom- ponent compound films were determined by  the  additive change properties rule of  PbS and CdS for each composition of  the  solid solution [16]. The  values of  physical characteristics for CdxPb1−xS solid solutions films and individual lead and cadmium sulfides with the indication of their thickness are given in Table 1. As noted by many researchers, TCLE differences between the film and the sub- strate plays decisive role in the occurrence of mechanical stresses in such systems. Note that the temperature expansion coefficients values for the  synthesized compounds (Table  1) change insignificantly: from 18.2·10−6  K−1 to  18.7·10−6  K−1. The  TCLE of the sitall substrate is 5.0·10−6 K−1, and its thickness is 0.51 mm that are used for mechanical stresses calculating arising at the “film — substrate” interface. Fig.  4 plots a  quantitative esti- mate of  elastic mechanical stresses at  the  “CdxPb1−xS solid solution film  — sitall substrate” interface for the tempera- ture range 300–900  K.  The  calculation have taken into account the composition x and the layer thickness h. The thickness altered from 690 to 920 nm. For compari- son, the figure also shows the temperature dependences of mechanical stresses arising in individual sulfides PbS and CdS. As  seen from Fig.  4, mechanical stresses in  the  discussion films increase with increasing temperature. Neverthe- less, the CdS film does not practically has the mechanical stresses at 350–400 K tem- perature range. And upon reaching 900 K, insignificant expansion stresses 100 kN/ m2 are established. The reason of this fact is practically equal the TCLE of substrate Table 1 Young’s modulus Е, linear thermal expansion coefficient α, Poisson’s ratio ν, thickness h of PbS, CdS and CdxPb1−xS solid solution films Parameter Film PbS Cd0.02Pb0.98S Cd0.03Pb0.97S Cd0.04Pb0.96S Cd0.05Pb0.95S CdS Young’s modulus, E. 10−10, Pa 7.02 6.96 6.94 6.91 6.88 4.20 Linear thermal expansion coefficient, α∙106, K−1 19.0 18.7 18.5 18.3 18.2 2.5 Poisson’s ratio, ν 0.280 0.282 0.283 0.284 0.285 0.380 Films thickness, h, nm 400 740 690 920 870 360 256 and of cadmium sulfide (TCLE of substrate is only 1.25 times greater than CdS TCLE). As  for the  film of  individual lead sulfide and solid solutions films based on PbS, elastic compressive stresses are cre- ated in them, that evidenced by the nega- tive temperature dependences σ = f (T) in Fig. 4. The  absolute value of  mechani- cal stresses of  these films increases with an increasing the annealing temperature from 350 to  893  K.  Compressive stress- es rise in absolute terms from about 200 to 1200 kN/m2 for PbS and from 150–250 to 2000–2750 kN/m2 for CdxPb1−xS under these conditions. The ambiguous change of this characteristic with the composition of the solid solution requires attention. It would seem that the mechanical compres- sive stresses of the CdxPb1−xS films should be lower with a cadmium content increas- ing due to  their partial compensation by  the  expansion stresses (the  contribu- tion of CdS). However, a more pronounce compressive stresses growth is  observed in the film of the Cd0.02Pb0.98S solid solu- tion (4) compared with Cd0.04Pb0.96S (6). The reason of that case may be larger layer thickness of film with the CdS content х = 0.04 (920 nm vs 740 nm). This is probably why crystallites just melted with the forma- tion of  globules in  the  Cd0.02Pb0.98S layer at 893 K (Fig. 3b), while there was a radical destruction of similar globular formations in the Cd0.04Pb0.96S film (Fig. 3d). The in- creasing layer thickness by a factor of 1.25 led the rise of internal mechanical stresses by  1.5 times. The  same effect of  the  de- posited film thickness on the magnitude of mechanical stresses was discovered for zinc selenide [14]. The  strong cracking of the ZnSe film occurred due to compres- sive stresses at the site of globules adhesion formed it. Conclusions We determined the upper temperature stability limit of chemically deposited films of СdxPb1-xS. It is in the range of 405–410 K for CdS content (x) in supersaturated solid solutions equal to 0.06, 0.122 and 0.176, and about 450 K for x from 0.02 to 0.05, re- Fig. 4. Elastic mechanical stresses dependence in the systems “film — sitall substrate” on the annealing temperature in air for CdS(1), PbS (2), Cd0.03Pb0.97S (3), Cd0.02Pb0.98S (4), Cd0.05Pb0.95S (5), Cd0.04Pb0.96S (6) films 257 spectively. Solid solutions decompose into two phases at higher temperatures: a solid solution with the  equilibrium cadmium sulfide content at  the  given temperature and X-ray amorphous CdS. The  quanti- tative assessment of internal mechanical stresses caused by annealing of СdxPb1-xS films was carried out for the first time ever in the range of 300–900 K. It is shown that the mechanical stresses at the “СdxPb1-xS film  — sitall substrate” interface in- crease with annealing temperature. Thus, the compressive stresses under these con- ditions increase in  absolute terms from 150–250 to 2000–2750 kN/m2 depending on the composition of the solid solution. 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