Exciton Absorption Spectrum of Thin CsPbI 3 and Cs 4 PbI 6 Films

Transcription

1 SSN -X, Optics and Spectroscopy,, Vol., No., pp Pleiades Publishing, Ltd.,. Original Russian Text O.N. Yunakova, V.K. Miloslavskii, E.N. Kovalenko,, published in Optika i Spektroskopiya,, Vol., No., pp. 95. CONENSE-MATTER SPECTROSCOPY Exciton Absorption Spectrum of Thin CsPb and Cs Pb 6 Films O. N. Yunakova a, V. K. Miloslavskii a, and E. N. Kovalenko b a Kharkiv National University, Kharkiv, 677 Ukraine b Kharkiv National University of Radioelectronics, Kharkiv, 666 Ukraine Vladimir.K.Miloslavsky@univer.kharkov.ua, Olga.N.Yunakova@univer.kharkov.ua, kovalenko.elena@bk.ru Received May 9, Abstract A method for preparing thin films of CsPb and Cs Pb 6 complex compounds has been developed. Their absorption spectrum is investigated in the energy range of 6 ev at temperatures from 9 to 5 K. t is found that the CsPb compound is unstable and passes to the Cs Pb 6 phase upon heating at T K. O:./SX9 n contrast to the Cs x Cd x system [], which yields only the Cs Cd complex compound with a change in the molar concentration, solid solutions formed in the Cs Pb system include two ternary compounds: CsPb and Cs Pb 6. According to the thermographic measurements [], these compounds have similar melting temperatures (76 and 68 С, respectively). Studies of the crystal structure of CsPb [, ] showed that it is crystallized into the orthorhombic lattice with four molecules per unit cell; the unitcell parameters are а =.6 Å, b =.8 Å, and с = 7.78 Å (sp. gr. Pnma) at C. The structural elements of the CsPb lattice are double chains composed of (Pb 6 ) octahedra and oriented along the short b axis. The orthorhombic lattice is retained up to temperatures of 56 6 K; at higher temperatures, it passes to the monoclinic phase (according to earlier data []) or to the cubic phase with а = 6.9 Å (according to later detailed studies []). The Cs Pb 6 crystal structure has been less investigated [5, 6]. t was indicated that Cs Pb 6 has a hexagonal lattice with the unit-cell parameters а =.5 Å and с = 8. Å and the number of molecules z = 6 per unit cell. The existence of two complex compounds in the Cs x Pb x system called for studying their electron absorption and reflection spectra. The reflection spectra of CsPb single crystals were measured for the first time in [7]; their analysis revealed the s exciton band at. ev (. K) and a weak shift of this band for polarized light (the light field E oriented perpendicularly or parallel to the c axis): ΔЕ. ev. However, the later study [8] revealed a Cs Pb 6 impurity in CsPb single crystals. Somma et al. [9], who investigated the absorption spectra of films obtained by evaporation of CsPb powder and its deposition on a quartz substrate, revealed not only an exciton band at nm (. ev), but also a narrow strong band at 65 nm (. ev). This band was attributed to the Cs Pb 6 compound in [9], and it was concluded that CsPb is unstable in the gas phase and decomposes to form Cs Pb 6. The results of [7, 9] encouraged Japanese physicists [, ] to develop methods for preparing pure CsPb and Cs Pb 6 phases to measure their absorption spectra. Kondo et al. [, ] proposed to deposit successively thin Cs and Pb layers of a certain thickness on a transparent substrate cooled to 77 K. The ratio of the thicknesses of the two layers was chosen according to the molar composition of the desired compound. Fast deposition of layers on a cold substrate conserved the specified thickness ratio in the two-layer compound. Then the latter was annealed at different temperatures, after which the absorption spectra were measured at 77 K. This procedure made it possible to obtain for the first time a pure CsPb spectrum at an annealing temperature of K and the absorption spectrum of a Cs Pb 6 film annealed at 5 K. The method that was developed in [, ] to obtain pure CsPb and Cs Pb 6 phases is rather laborintensive and does not always lead to the desired result. We proposed an alternative and simpler method for preparing thin films of impurity-free compounds, which is based on the difference between the melting temperatures of CsPb and Cs Pb 6. The thus-prepared samples were used to measure the exciton absorption spectra in the range of photon energies of 6 ev and in the temperature range of 9 5 K. The data obtained were used to determine the main parameters of the exciton bands (spectral position, half-width, etc.) and their temperature dependences. 9

2 9 YUNAKOVA et al A A A A 5 E, ev Fig.. Absorption spectra of thin (Cs) x (Pb ) x films (x =., T = 9 K) formed under different conditions: (, ) the films deposited on a substrate at a temperature T s = 6 C, with subsequent annealing at T ann = () 6 and () C, () the film deposited on a substrate at T s = C, and () the same film annealed at T ann = C. EXPERMENTAL Thin CsPb and Cs Pb 6 films were prepared by evaporating a mixture of pure Cs and c powders with a specified molar composition and depositing it on heated quartz substrates. The substrate temperature was varied from 6 to C. The powder mixture was previously melted under a screen located between the evaporator and substrate. This method was previously applied to obtain thin films of M Cd and M Zn ferroelastics (M = Cs, Rb, or K) [] and is based on the fact that the melting temperature of ternary compounds is generally much lower than those of the initial binary compounds. Note that, upon heating in vacuum, the mixture of powders (Cs) x (Pb ) x forms a melt in the concentration range of. х.; at other х values, the compound is sublimated from the powder, and, in this case, the initial components are present in the films as impurities. When the mixture is explosively evaporated, the films contain also the initial components as impurities. Therefore, we used a (Cs) x (Pb ) x mixture with х =. to prepare CsPb films and a stoichiometric mixture to form Cs Pb 6 films. The phase composition of the films was controlled by measuring the absorption spectra at T = 9 K. This control is based on the significant difference in the spectral positions of the long-wavelength exciton bands in CsPb (. ev), Cs Pb 6 (. ev), Pb (.5 ev), and Cs (5.8 ev). t was found that the mixture evaporation from the melt with subsequent deposition on quartz substrates heated to Т s 8 С form films, the absorption spectra of which contain a wide absorption band; it takes an intermediate position between the long-wavelength exciton bands A and A in the CsPb and Cs Pb 6 spectra (Fig., curve ); apparently, this intermediate band corresponds to the solid solutions of two compounds. At the substrate temperature Т s > С, twophase films are formed, the absorption spectra of which contain bands typical of both CsPb and Cs Pb 6 (Fig., curve ). These phases are transformed into the Cs Pb 6 phase, with a typical spectrum [], several days later or after their annealing at T = K for h (Fig., curve ). The aforementioned films pass also to the Cs Pb 6 phase after annealing at temperatures above K (Fig., curve ). Therefore, Cs Pb 6 is the more stable of the two compounds formed in the Cs Pb system. We will consider the temperature transformations in the films in more detail below. Thin Cs Pb 6 films can also be obtained by evaporating a molten mixture of stoichiometric powders in vacuum and deposing it on quartz substrates heated to C, with their subsequent annealing for h at the same temperature. t is more difficult to form a CsPb film without Cs Pb 6 impurity by evaporation from a melt, because these compounds have similar melting temperatures []. Since the CsPb melting temperature (T m = 76 C) is somewhat higher than that of Cs Pb 6 (T m = 68 C) [], one can obtain a CsPb film without Cs Pb 6 impurity by partial evaporation of the melt on a screen, with subsequent evaporation of the rest on the substrate at a higher temperature (Fig. a). The absorption spectra were measured with an SF- 6 spectrophotometer on films 5 nm thick in the spectral range of 6 ev at T = 9 and 9 K. n the narrower spectral range of..7 ev (within the long-wavelength exciton band), the absorption spectrum was measured in a wide temperature range of 9 59 K, including the temperature of possible phase transition. The parameters of the long-wavelength band A (position Е m, half-width Γ, and the imaginary part of permittivity in the maximum ε m = ε (E m )) were determined according to the technique of [], using an approximation of the band A by a single-oscillator symmetric profile (which is a linear combination of Lorentzian and Gaussian profiles). The parameters of the exciton band (Е m, Γ, and ε m ) were chosen so as to provide the best agreement between the calculated and experimental profiles on the long-wavelength side of the band. OPTCS AN SPECTROSCOPY Vol. No.

3 EXCTON ABSORPTON SPECTRUM 9 EXCTON SPECTRA OF CsPb AN Cs Pb 6 The absorption spectrum of thin CsPb films (Fig. a) contains a narrow strong band A at. ev (9 K) at the long-wavelength edge of the intrinsic absorption band and four shorter wavelength bands: C (.69 ev), C (. ev), (. ev), and (.66 ev); these bands are located against the interband absorption background. The absorption edge of Cs Pb 6 is blue-shifted by. ev in comparison with CsPb (Fig. b). The spectral positions of the Cs Pb 6 bands are. ( A ),.9 ( C ),.6 ( C ),.7 ( ), and 5. ev ( ). With an increase in temperature, the A and C bands in the spectra of CsPb and Cs Pb 6 become redshifted, broaden, and weaken due to the exciton phonon interaction (EP), which indicates their relationship with exciton excitations. Splitting off the A band by a symmetric profile on the long-wavelength side of the C band in CsPb reveals a shoulder A at. ev (Fig. a); we associate this feature with the exciton excitation to the state characterized by the principal quantum number n = (s exciton). On the assumption that the A and A bands belong to the exciton series with the main band A, we used the spectral position of these bands to estimate the exciton binding energy R ex = / (E A E A ) =.57 ev and the band gap E g = E A + R ex =.7 ev. The thus-obtained R ex and E g values are close to those for single crystals (within the determination errors): R ex =.5 ev and E g =. ev [7]. A weak A band at.5 ev (Fig. b) arises in the Cs Pb 6 spectrum after splitting off the A band by a symmetric profile; this band is likely to correspond to the s-exciton excitation. We determined the exciton binding energy R ex =.9 ev and the band gap E g =.56 ev in Cs Pb 6 from the spectral positions of the A and A bands. n the sequence of Pb, CsPb, and Cs Pb 6 compounds the long-wavelength exciton bands are blueshifted (Fig., inset) according to the law Е А (х) = Е х + Е ( х), () where х is the molar concentration of Pb in the compounds and Е is the position of the long-wavelength exciton band in Pb. Extrapolation of dependence () for х yields Е =.6 ev, a value much smaller than the spectral position of the exciton band in Cs (5.8 ev) but close to the position of the Pb + impurity bands in Cs, Rb, and K [, ]. Linear dependence () and its convergence to the spectral position of the Pb + impurity bands in Cs indicate, according.6.. E m, E g, ev Cs Pb 6.5 CsPb..5 Cs x.5. Pb C CsPb A A A A C C Cs Pb 6 5 E, ev to [5], the excitation and localization of excitons in the cationic sublattice of ternary compounds containing lead ions. n the case of this localization, the conduction band in the compounds under study is formed (as well as in Pb ) by the Pb 6р states, while the valence band is formed by the Pb 6s states with an admixture of 5р states. With allowance for the fact that the exciton excitations in Pb [6] and other Pb-containing multicomponent compounds (for example, CsPbCl and CsPbBr [7]) are of cationic type, the exciton bands in the CsPb and Cs Pb 6 spectra are assigned to the electronic transitions in Pb + ions, which are octahedrally coordinated by ions (Pb 6 complexes). Pb + impurity ions in a lattice of symmetry d exhibit the following transitions: А Ag Tu ( S P in a free Pb + ion) (A band), A T or E ( S P ) g u C u Fig.. Absorption spectra of thin CsPb and Cs Pb 6 films recorded at T = () 9 and () 9 K; the dotted line indicates the absorption edge after splitting off the А band by a symmetric profile. The concentration dependences of () the spectral position E m and () the band gap E g in the series of Pb, CsPb, and Cs Pb 6 compounds are shown in the inset. OPTCS AN SPECTROSCOPY Vol. No.

4 9 YUNAKOVA et al...5 E, ev.5..5 (а) (b) 9 K 6 K A 9 K 67 K K A C C C C band), and A g Tu( S P ) (C band) []. The structure of the CsPb and Cs Pb 6 spectra is similar to that of the impurity spectra of Pb + in Cs []; the А and С exciton bands are likely to correspond to the same direct allowed transitions in the Pb 6 octahedron. The weak B band of Cs:Pb +, which is due to the forbidden S P transition in free Pb + ions, was not observed in our spectra. The blue shift of the absorption bands with a decrease in the Pb content in the compounds under study indicates a decrease in the widths of allowed bands and an increase in the band gap E g. The latter linearly increases in the series of Pb, CsPb, and Cs Pb 6 compounds (Fig., inset), which is additional proof of the localization of exciton excitations in (Pb 6 ) octahedra. K 5 E, ev Fig.. (a) Long-wavelength exciton band in the spectra of a 5-nm-thick CsPb film, recorded at different temperatures upon heating. (b) The absorption spectrum of the CsPb film (T = 9 K) () before and () after heating above K. TEMPERATURE EPENENCE OF THE PARAMETERS OF THE LONG- WAVELENGTH EXCTON ABSORPTON BANS N CsPb AN Cs Pb 6 The absorption spectrum of a thin CsPb film in the range of..7 ev was measured at temperatures from 9 to 6 K upon heating and cooling; the spectrum of a thin Cs Pb 6 film was measured only upon heating in the temperature range of 9 5 K. At temperatures from 9 to 67 K, the A band of CsPb is linearly red-shifted (de m /dt =. ev/k) upon heating (Figs., ); the half-width of the A band increases nonlinearly in this temperature range. n the relatively narrow range of 67 K, the absorption spectrum of the thin film changes as follows: the absorption edge is blue-shifted, and the long-wavelength exciton band becomes narrower and sharper (Figs. a, ). Upon cooling the film to Т = 9 K, its absorption spectrum is not recovered. The spectrum of the cooled film (Fig. b, curve ), measured in a wide energy range ( 6 ev) at T = 9 K, is typical (in both the structure and the position of the fundamental absorption bands) of Cs Pb 6. Therefore, we can suggest that heating a CsPb film above K leads to the formation of Cs Pb 6 compound; this suggestion is in agreement with the results of [, ]. However, the question of excess Pb must be solved in this case. The excess Pb phase does not manifest itself in the spectrum of the cooled film (Fig. b, curve ), as is evidenced by the absence of additional absorption at.5 ev (which corresponds to exciton absorption in Pb ) in the transparency region. Apparently, the excess lead manifests itself in the film spectrum as Pb + impurity bands in Cs, which are imposed on the Cs Pb 6 absorption spectrum. According to [], the longest wavelength band А in the spectrum of Cs:Pb + is located at.8 ev (i.e., it is imposed on the A band in the spectrum of Cs Pb 6 (. ev)); the shorter wavelength B and C bands are in the range of. 5. ev []; correspondingly, they are also imposed on the C bands in the Cs Pb 6 spectrum. Apparently, this is the reason for the much higher intensity of the Cs Pb 6 exciton bands in the film with a higher Pb content ( x Pb =.) (Fig. b, curve ) in comparison with the stoichiometric composition ( x Pb =.) (Fig. b). This suggestion is substantiated by the much higher oscillator strength of the A band in the cooled film ( x =.) f = 9. Pb (a value calculated for the volume occupied by the Pb + ion), in comparison with the oscillator strength of the corresponding band in Cs Pb 6 stoichiometric films: f = 9.. Thus, the temperature dependence of the parameters of the long-wavelength exciton band recorded upon heating the CsPb sample is typical of this com- OPTCS AN SPECTROSCOPY Vol. No.

5 EXCTON ABSORPTON SPECTRUM 95 pound, whereas the same dependence recorded upon cooling corresponds apparently to a mixture of Cs Pb 6 and Cs:Pb +. Let us consider in more detail the temperature dependences of the spectral position and half-width of the long-wavelength A exciton band obtained upon heating and cooling the CsPb sample and the tem- A perature dependence of the parameters of the band in the Cs Pb 6 spectrum (Fig. ). As was noted above, the long-wavelength exciton band A in the spectrum of CsPb is linearly redshifted with de m /dt =.5 and.5 ev/k upon heating and cooling, respectively. The A band in the Cs Pb 6 spectrum is also linearly redshifted (de m /dt =. ev/k) with an increase in T (Fig. a). This shift is typical (in order of magnitude) of most ionic crystals, including the compounds under study. The interaction of excitons and longitudinal optical (LO) phonons dominates in ionic crystals, and the largest temperature changes in the parameters of exciton bands occur at ω LO kt. We do not know the ω LO values for CsPb and Cs Pb 6. They can be estimated based on the known parameters ω LO =.7 mev for Pb [8] and ω LO = mev in Cs [9]. With allowance for the molar contents of the compositions, ω LO.5 mev for CsPb and.5 mev for Cs Pb 6. The de m /dt values for CsPb and Cs Pb 6 are slightly larger than de m /dt =.6 ev/k for thin Pb films [], but are much smaller than those for other ionic crystals []. The weak temperature shift of the exciton bands in Pb is related to the compensation of the EP-induced red shift by the blue shift caused by the thermal lattice expansion []. Apparently, the small de m /dt values for the compounds under study are also caused (as for Pb ) by the compensation of the EP-induced negative shift by the positive shift related to the lattice expansion. The temperature behavior of the half-width Γ of the long-wavelength exciton bands of both compounds is nonlinear (Fig. ). According to the theory [], the temperature dependence Γ(T) for excitons of different dimensions d (d =,, or ) is determined as ( ) d ΓT π, () d / γ( d/ )( πb) where γ(d/) is the gamma function (which depends on d), В is the exciton-band width, and =.5C ω LO coth ( ω LO /kt) (C / is the lattice relaxation energy upon exciton excitation). While processing the experimental dependence Γ(Т), we took into account the shape of the exciton band and the contribution of the residual broadening of Γ() to Γ due to E, ev.. (а).6. T, K Γ, ev Γ, ev... (b) the lattice defects. n the spectrum of CsPb recorded at liquid-nitrogen temperature, the shape of the band found according to the technique [] is a linear combination of Lorentzian and Gaussian profiles; the Gaussian component increases upon heating. At the same time, the band in the spectrum of Cs Pb 6 is a Gaussian in the entire temperature range under study. n the case of Gaussian profile the total half-width is determined by the relation / Γ= Γ () +Γ( T), E, ev T, K Fig.. Temperature dependences of (a) the spectral position E m (Т) and (b) the half-width Γ(Т) of the long-wavelength exciton band A in the spectrum of CsPb, obtained upon () heating and () cooling the sample, and () the A band in the spectrum of a -nm-thick Cs Pb 6 film. () OPTCS AN SPECTROSCOPY Vol. No.

6 96 YUNAKOVA et al. where Γ(Т) is described by () with an unknown factor А, which is independent of Т. Processing the experimental dependence Γ(Т) using () for different d showed that the best agreement of the calculation based on () with the experimental data can be obtained at d =. n this case, Γ(Т) = A coth ( ω LO /kt), () and the dependence Γ(Т) in the Γ coth ( ω LO /kt coordinates is linear. Processing this dependence by the least-squares method yields Γ () =.5 ±. ev and А = (. ±.5) ev upon heating and Γ () =. ±. ev and А =(.±.7) 5 ev upon cooling the CsPb sample and Γ () =.6 ±. ev and А = (. ±.7) 5 ev for Cs Pb 6. A calculation (based on () and ()) of the temperature dependence of the half-width with the obtained Γ() and А values yields good agreement with the experimental dependence Γ(Т) (Fig. b). Thus, as follows from the analysis of the temperature dependence Γ(Т), the excitons in CsPb and Cs Pb 6 are of type. CONCLUSONS The technique developed for preparing thin CsPb and Cs Pb 6 films made it possible to study the exciton absorption spectra in the energy range of. 6. ev and the temperature range of 9 5 K. Based on these data, we drew certain conclusions about the temperature dependence of the main exciton-band parameters (spectral position and half-width). t was shown that the long-wavelength exciton excitations in the crystals under study are localized in the sublattices containing Pb + ions coordinated by ions. However, the significant difference between the oscillator strengths of the exciton bands in the spectra of Cs Pb 6 films prepared by two different methods (direct deposition and annealing of CsPb films) should be explained in more detail. To this end, it is necessary to investigate thoroughly the lattice of Cs Pb 6 films prepared in different ways. REFERENCES. O. N. Yunakova, V. K. Miloslavskii, and E. N. Kovalenko, Fiz. Nizk. Temp. (Kiev) 9, 9 ()... N. Belyaev, E. A. Shurginov, and N. S. Kudryashov, Zh. Neorg. Khim. 7, 8 (97).. C. K. Müller, Nature 8, 6 (958)... M. Trots and S. V. Myagkota, J. Phys. Chem. Solids 69, 5 (8). 5. C. K. Müller, Mat.-Fus. Medd. K. an. Vidensk. Selsk. (), (96). 6. B.. Stepin, G. M. Serebrennikova, G. P. Chicherina, V. K. Trunov, and Yu. V. Oboznenko, Zh. Neorg. Khim., 8 (976). 7. N. S. Pidzyrailo, A. S. Voloshinovskii, and S. V. Myagkota, Opt. Spectrosc. 6 (6), 78 (988). 8. S. V. Myagkota, Opt. Spectrosc. 87 (), 9 (999). 9. F. Somma, M. Nikl, K. Nitsch, C. Giampaolo, and A. R. Santucci. S. Phani, Superfic. Vac. 9, 6 (999).. S. Kondo, A. Masaki, T. Saito, and H. Asada, Solid State Commun., ().. S. Kondo, K. Amaya, and T. Saito, J. Phys.: Condens. Matter 5, 97 ().. O. N. Yunakova, V. K. Miloslavskii, and E. N. Kovalenko, Opt. Spectrosc. (), 55 (8).. S. Radhakrishna and K. P. Pande, Phys. Rev. B 7, (97).. S. B. S. Sastry and K. Balasubramanyam, J. Phys. C: Solid State Phys., (978). 5. Y. Onodera and Y. Toyasawa, J. Phys. Soc. Jpn., 8 (967). 6.. Ch. Schlüter and M. Schlüter, Phys. Rev. B 9, 65 (97). 7. K. Heidrich, H. Künzel, and J. Treusch, Solid State Commun. 5, 887 (978). 8. G. Lukovsky, R. M. White, J. A. Benda, W. Y. Liang, R. Zallem, and P. H. Schmidt, Solid State Commun. 8, 8 (976). 9. F. Beerwerth,. Frohlich, and V. Leinweber, Phys. Status Solidi (b) 5, 95 (988).. V. V. Mussil, V. K. Miloslavskii, and V. V. Karmazin, Sov. Phys. Solid State 7, 55 (975).. M. Schreiber and Y. Toyasawa, J. Phys. Soc. Jpn. 5, 58 (98). Translated by Yu. Sin kov SPELL: OK OPTCS AN SPECTROSCOPY Vol. No.