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The cation and the anion vacancies in cadmium diphosphide: A NIR-VIS, IR, and Raman study
Accepted Manuscript Title: The cation and the anion vacancies in cadmium diphosphide: a NIR-VIS, IR, and Raman study Author: K.V. Shportko PII: DOI: Reference:
S0924-2031(17)30180-7 http://dx.doi.org/doi:10.1016/j.vibspec.2017.08.004 VIBSPE 2732
To appear in:
VIBSPE
Received date: Revised date: Accepted date:
11-6-2017 7-8-2017 7-8-2017
Please cite this article as: K.V.Shportko, The cation and the anion vacancies in cadmium diphosphide: a NIR-VIS, IR, and Raman study, Vibrational Spectroscopyhttp://dx.doi.org/10.1016/j.vibspec.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The cation and the anion vacancies in cadmium diphosphide: a NIRVIS, IR, and Raman study
K. V. Shportko V.E. Lashkaryov Institute for semiconductor physics of NAS of Ukraine, 41 Nauki av., Kyiv 03028, Ukraine.
Tel.: +380 95 3968060; Fax: +380 44 46 8994; E-mail address: [email protected]
Abstract The present work studies the impact of the cadmium and phosphorus vacancies on the properties of the tetragonal cadmium diphosphide that is promising for the optoelectronic and photovoltaic applications. Obtained results of the NIR-VIS transmittance, IR reflectance, and Raman scattering measurements have shown that phosphorus and cadmium vacancies provide a rich playground for the design of the optical and vibrational properties of cadmium diphosphide. Keywords: Cadmium diphosphide; vacancies; optical bandgap; phonons;
1. Introduction Binary and ternary diphosphides have been the topic of research in recent years due to their varied possible applications and interesting properties.
Cadmium and zinc diphosphides CdP2 and ZnP2 are characterized by a number of unique properties that make it promising for various applications in optoelectronics, such as in intensity stabilizers and pulse stretchers for solid state lasers, optical and thermal sensors and polarizing prisms for thin-film waveguides [1] and photovoltaics [2]. Tetragonal cadmium (-CdP2) and zinc (-ZnP2) diphosphides (Figure 1) belong to the space symmetry group P41212 ( D ). In -CdP2 and -ZnP2 each 4 4
metal (Cd or Zn) atom is surrounded with four phosphorus atoms, and each phosphorus atom is surrounded with two metal and two phosphorus atoms. In CdP2 and -ZnP2 phosphorus atoms build up zigzag chains. These continuous chains of phosphorus atoms can be regarded as covalently bonded molecules, and run through the crystal in the (010) and (100) directions. In the (001) direction all these chains are interrupted by metal atoms. There are four phosphorus chains per unit cell running alternately parallel to (010) or (100) [3]. The metal atoms are at the center of a deformed tetrahedron and link the phosphorus chains in 3D structure [4]. The chemical bonding in the binary tetragonal diphosphides exhibits complex character: phosphorus–phosphorus bond carries covalent character, whereas in metal–phosphorus bonds a proportion of ionic character (from 16 up to 54% by different estimations) is present [5]. The elementary cell consists of four layers revolved from each other by 90°. The sequence of packing these layers is frequently broken, and instead of four it is possible to observe the multiplets of six, less often five layers [3]. To study the vibrational properties of -CdP2 and -ZnP2 we have employed the temperature-dependent IR and Raman spectroscopy in the earlier studies [6– 8]. It has been demonstrated that decreasing the bond distance in the phosphorus chain at low temperatures causes an additional amount of anharmonicity of the corresponding modes, which is manifested in more pronounced temperature dependences of their frequencies and damping coefficients. It was rather an unexpected finding, since in the contrast to metal atoms, which weakly interact
with P atoms and between each other, P atoms within the chain are linked by strong covalent bonds, whose length is shorter then Cd(Zn)-P and Cd(Zn)Cd(Zn) [9]. In [9] considering ZnP2 it was assumed that effect might be attributed to the buckling of the phosphorus chains, in such a way, decreasing the bond distance in the chain, or to promotion of the double bonding between the P atoms in the chain or to a charge transfer take place in a direction which strengthens the P-P bonds and slightly weakens the Zn(Cd)-P bonds. In order to better understand this mechanism we focused on the CdP2 samples doped by ZnP2 nanoclusters, employing IR reflectance, ATR, and Raman spectroscopy in our previous studies [10,11]. Analysis of the impact of the doping on the vibrational properties of CdxZn(1 − x)P2 solid solutions has shown that presence of the ZnP2 nanoclusters
leads to the shift of the phonon
frequencies, as well as impacts 0,inf, and strengths of oscillators used to model the dielectric function of CdxZn(1 − x)P2 in the IR [10]. The doping of CdP2 single crystals by ZnP2 nanoclusters results also in the position and the width of the dispersion branches of the surface polaritons in CdxZn(1 − x)P2. This effect is more pronounced in the low frequency dispersion branches that originated from the phonons which correspond to the motion of the cation sublattice [11]. The vacancies in the cation and anion sublattices can result in the magnitude of the phenomena observed in [8,12]. The present research is aimed to study the influence of the cadmium and phosphorus vacancies on the optical properties of CdP2 at room temperature.
2. Experimental Polycrystalline CdP2 was obtained from the initial elements by twotemperature way. Ampoules with polycrystalline CdP2 were vacuumized till 10-3 Pa, soldered, and put into horizontal and vertical resistance furnace. Single crystals of CdP2 were grown in the conic side of the ampoule. Constancy of the temperature in evaporation zone during all the process of growth was achieved
by moving the ampoule sideways crystallization zone with speed of 0.6– 0.8 mm/hour. XRD confirms that grown single crystals of CdP2 are singlephase. To obtain the samples of CdP2 with cadmium and phosphorus vacancies, single crystals of CdP2 were soldered in vacuumized quartz ampoule and placed into the resistance furnace. Variation of the annealing temperature and the annealing time leads to the evaporation either cadmium or phosphorus. This enabled us to obtain samples of CdP2 with cation and anion vacancies. The stoichiometry has been controlled by XRF. Thus, obtained series of samples CdP1.918 and Cd0.995P2 were compared with CdP2 (Figure 1). In the present work we used samples of single crystals in the shape of plates with a size of 2 mm × 3mm × 1 mm, that were cut along the (001) crystallographic direction, and polished. To study the optical properties we employed NIR-VIS transmittance, IR reflectance, and Raman spectroscopy. To measure NIR-VIS transmittance spectra we used Perkin-Elmer® Lambda 25 spectrometer. VIS transmittance spectra have been obtained in 1-5 eV spectral range using non-polarized radiation. IR reflectance spectra of studied samples were measured in the range from 100 to 500 cm-1 at room temperature, using a Bruker® IFS 88 spectrometer with a globar as the radiation source and polarized radiation using Specac® metal grid polarizer. Spectra were taken for the E c orientation of the electrical vector E of the IR radiation with respect to the crystal. Raman spectra were measured in the spectral range from 85 to 500 cm-1 at room temperature using a FRA-106 Raman attachment applying the diode pump Nd:YAG laser of ca. 200 mW power and liquid nitrogen cooled Ge detector for y(zz)y back scattering configuration, employing the Specac® Glan-Thompson prism polarizer. To analyze the IR reflectance spectra we used the model of the dielectric function which includes following components: inf describes the polarizability of bounded electrons and plays the major role in the higher energy range (i.e. core electrons) [13], phonon contributions were described by Lorentz oscillators
with 3 parameters: the frequency j, and damping coefficient j and Sj is the oscillator strength [14]. Measured Raman spectra were analyzed in CoRa [15], by modelling the Raman bands with Gauss and Lorentz profiles.
3. Results and discussion 3.1 NIR-VIS spectroscopy We start with results of the NIR-VIS transmittance spectroscopy, since they provide the most obvious evidence of the impact of vacancies on the properties of studied samples. Spectra of cadmium diphosphides in the range around the absorption edge are shown in the inset of the Figure 2. Here one can clearly distinguish the shift of the absorption edge in the spectra of cadmium diphosphide that contain cadmium and phosphorus vacancies. The fundamental absorption edge in Cd0.995P2 is more sloping that in CdP1.918 and CdP2, this evidences changes in the electron distribution near the Fermi level.
The
dispersion of the absorption coefficient shown in the Figure 2 was calculated from the transmittance spectra to determine the optical bandgap Eg. Obtained value of the Eg in pure CdP2 is 2.01 eV that agrees with results reported in [16]. In the sample Cd0.995P2 with cadmium vacancies the Eg is 2.02 eV. The sample with phosphorus deficit CdP1.918 exhibits the most pronounced shift of the optical absorption edge, and the corresponding Eg here is 2.09 eV. Vacancies cause the defect levels within the bandgap and this leads to increase of the Eg. Having an evidence of the impact of the phosphorus and cadmium vacancies on the Eg, in the following we focus on their impact on the vibrational properties of cadmium diphosphide. 3.2 Raman spectroscopy The visual inspection of the Raman spectra presented in the Figure 3 shows that obtained Raman spectra of studied samples cadmium diphosphide samples exhibit generic pattern. To perform the quantitative analysis of the obtained data we performed the fit of Raman spectra using set of Gauss profiles and analyzed
trends in their parameters. According to [9] due to the similarity of the structure of tetragonal ZnP2 and CdP2, the low frequency Raman peaks in the spectra of cadmium diphosphide have been attributed to the Cd-P and Cd-Cd modes, whereas the high frequency peaks were assigned to the internal vibrations of the phosphorus chain. Similar approach has already been employed in [8,10]. In [8] the anisotropy of ZnP2 and CdP2 resulted in the difference in the shift
of the Raman peaks in the spectra recorded at different configurations. Effect of nanoclusters of ZnP2 built in the CdP2 matrix has been observed in the shift of the Raman peaks of CdXZn(1-X)P2 in the spectra recorded at room temperature [10]. This effect found to be more pronounced in the peaks that correspond to Cd(Zn)-P and Cd(Zn)-Cd(Zn) modes, i.e. low frequency Raman peaks. To learn more about the effect of the cation and anion vacancies on the vibrational properties of the cadmium diphosphide we focused on the intensity of Raman peaks in the spectra of studied samples. For this purpose for all observed peaks we calculated the relative intensity change:
I
ref
I x I ref
,
(1)
here Ix is the intensity of the Raman peak in the spectrum of studied cadmium diphosphide sample with stoichiometric deviations, Iref is the intensity of the corresponding Raman peak the spectrum of CdP2. Obtained values were plotted vs. corresponding peak position and presented in the inset in the Figure 3 accompanied by linear trend lines. The slope of the trend line enables one to notice the differences in peaks’ intensities in the spectra of studied samples. As one can notice from the inset in the Figure 3, in the case of phosphorus vacancies in the sample CdP1.918, high frequency Raman peaks that
correspond to the internal vibrations of the phosphorus chain exhibit lower intensity in comparison with those of corresponding peaks in CdP2. On the contrary, cadmium vacancies weaken the low frequency Raman bands in Cd0.995P2. Thus, the trends in the behavior of the intensities of Raman peaks
might be used to evaluate the concentration of cation and anion vacancies in cadmium diphoshide. 3.3 IR reflectance spectroscopy To learn more about the impact of the vacancies on the vibrational properties of cadmium diphosphide we now focus on the phonon scattering by the vacancies. For this purpose IR reflectance spectra that are shown in the Figure 4 were collected. Noticeable enough is fact that the same phonon peaks exhibit different heights in spectra of different samples. We performed the same fit procedure of reflectance spectra as described in [6]. To model the contribution of the phonons into the dielectric permittivity we used classical Lorentz oscillator with three parameters: the resonant frequency, the strength, and the damping coefficient. The last one is the measure of the decay time and is mainly responsible for the height phonon peak in the reflectance spectrum. Therefore, we studied the changes in the damping coefficients upon presence of the vacancies in cadmium diphosphide samples. For all observed peaks we calculated the relative change of oscillator damping:
ref
x ref
,
(2)
here x is the damping of the oscillator in the spectrum of studied cadmium diphosphide sample with stoichiometric deviations, ref is the damping of the oscillator in the spectrum of CdP2. The point of our interest is the distribution of this relative change of oscillator damping along the oscillator frequency axis that is plotted in the inset in the Figure 4. Vacancies breaking long-range order in a crystal are an additional source of scattering of phonon modes. Phonon scattering mechanisms by the defects has been considered in [17,18]. In comparison with CdP2 the phosphorus vacancies cause additional damping of the high-frequency modes in CdP1.918, while the cadmium vacancies in Cd0.995P2 influence low-frequency phonons causing increase of their damping.
4. Conclusions Thus, the present work provides a new, remarkable finding regarding the modifying the properties of cadmium diphosphide. By employing the optical techniques which are widely used to characterize the impurities in semiconductors, we have shown that apart from the doping [10], phosphorus and cadmium vacancies provide a rich playground for the optical and vibrational properties of cadmium diphosphide. This can be employed in the various optoelectronic devices. Vacancies influence also the flow of heat [18] and the anharmonicity of the phonon modes in media. Studying these phenomena in cadmium diphosphide may extend the results obtained in [7], and therefore will be in the detailed focus of other studies.
Acknowledgements. The author gratefully acknowledges the support from the DAAD (German Academic Exchange Service). The author thanks Prof. Dr. V. M. Trukhan and Dr. T.V. Shoukavaya for the help in obtaining the samples.
References [1] Marenkin, S.F., Trukhan, V.M., Phosphides and arsenides of Zn and Cd, IP A.N. Varaksin, Minsk, 2010. [2] D. Stepanchikov, S. Shutov, Cadmium phosphide as a new material for infrared converters, Semicond. Physics, Quantum Electron. Optoelectron. 9 (2006) 40–44. [3] J.G. White, The crystal structure of the tetragonal modification of ZnP2, Acta Crystallogr. 18 (1965) 217–220. doi:10.1107/S0365110X6500049X. [4] C. Manolikas, J. van Tendeloo, S. Amelinckx, The “devil’s staircase” in CdP2
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doi:10.1002/pssa.2210970106. [5] I.I. Gorban, I.S., Gorinya, V.A., Dashkovskaya, R.A., Lugovoi, V.I., Makovetskaya, A.P., Tychina, One and two-phonon states in tetragonal ZnP2 and CdP2 crystals, Phys. Stat. Sol. (B). 86 (1978) 419–424. [6] K.V. Shportko, R. Rückamp, V.M. Trukhan, T.V. Shoukavaya, Reststrahlen of CdP2 single crystals at low temperatures, Vib. Spectrosc. 73 (2014) 111–115. doi:10.1016/j.vibspec.2014.05.001. [7] K. V. Shportko, A.D. Izotov, V.M. Trukhan, T. V. Shelkovaya, E.F. Venger, Effect of temperature on the region of residual rays of CdP2 and ZnP2 single
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doi:10.1134/S0036023614090204. [8] K.V. Shportko, R. Rueckamp, T.V. Shoukavaya, V.M. Trukhan, H.M. ElNasser, E.F. Venger, Effect of the low temperatures on the Raman active vibrational modes in ZnP2 and CdP2, Vib. Spectrosc. 87 (2016) 173–181. doi:10.1016/j.vibspec.2016.09.024. [9] T. Jayaraman, A., Maines, R.G., Chattopadhyay, Effect of high pressure on the vibrational modes and the energy gap of ZnP2, Pramana - J. Phys. 27 (1986) 291–297. [10]
K. Shportko, T. Shoukavaya, V. Trukhan, J. Baran, S. Starik, E.
Venger, The Role of ZnP2 Nanoclusters in the Vibrational Properties of Cd x Zn(1 − x)P2 Solid Solutions, Nanoscale Res. Lett. 11 (2016) 423. doi:10.1186/s11671-016-1635-y. [11]
K. V. Shportko, T.R. Barlas, J. Baran, V.M. Trukhan, T. V.
Shoukavaya, E.F. Venger, Spectroscopy of the Surface Polaritons in the CdXZn(1−X)P2 Solid Solutions, Nanoscale Res. Lett. 12 (2017) 87. doi:10.1186/s11671-017-1880-8.
[12]
K.V. Shportko, How the phosphorus chains impact on the
vibrational properties of diphosphides ZnP2 and CdP2 at low temperatures, Semicond. Phys. Quantum Electron. Optoelectron. 19 (2016) 377–383. doi:10.15407/spqeo19.04.377. [13]
J. Reul, L. Fels, N. Qureshi, K. Shportko, M. Braden, M.
Grüninger, Temperature-dependent optical conductivity of layered LaSrFeO4, Phys. Rev. B - Condens. Matter Mater. Phys. 87 (2013). [14]
M. Fox, Optical Properties of Solids, Oxford University Press,
2006. [15]
K. Shportko, R. Ruekamp, H. Klym, CoRa: An Innovative Software
for Raman Spectroscopy, J. NANO- Electron. Phys. 7 (2015) 3005. [16]
W. Żdanowicz, A. Wojakowski, Some Optical Properties of CdP 2,
Phys. Status Solidi. 10 (1965) K93–K97. doi:10.1002/pssb.2220100240. [17]
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Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials, Sci. Rep. 4 (2015) 5085. doi:10.1038/srep05085. [18]
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defects in semiconductors and nanostructures: Phonon trapping, phonon scattering, and heat flow at heterojunctions, J. Appl. Phys. 115 (2014) 12012. doi:10.1063/1.4838059.
Figure captions
Figure 1. Compositional triangle of studied diphosphides. P
CdP1.918 Cd0.9991Zn0.0009P2 CdP2 Cd0.995P2 Cd 0.9997Zn0.0003P2 0.5
ZnP2
0.5 Zn3P2
Cd
0.5
Zn
Figure 2. Transmittance spectra of studied cadmium diphosphide samples. Inset. Spectral dependence of the absorption coefficient. 1– CdP2, 2 – CdP1.918,
2000
Transmittance
Absorption coefficient, cm-1
3 – Cd0.995P2.
1500
0.2
0.1
0
1000
1.9
2
2.1
Photon energy, eV
1 500
2 3
2.01
0 1.9
2
2.02
2.09 2.1
Photon energy, eV
2.2
Figure 3. Raman spectra of studied cadmium diphosphide samples. 1– CdP2, 2 – CdP1.918, 3 – Cd0.995P2. Inset. Dependence of the relative intensity change of
Intensity, a.u.
1
2 3 467 464 450
Relative intensity change
the Raman peaks on the peaks’ position. 0.8 0.6 0.4
91
0.2 0 485
285
308 312 297 330
417
485
385
385
285
185 85 Mode wavenumber, cm-1
117 125
185 Raman shift, cm-1
103
85
Figure 4. IR reflectance spectra of studied cadmium diphosphide samples. 1– CdP2, 2 – CdP1.918, 3 – Cd0.995P2. Inset. Dependence of the relative change of the oscillators’ damping on their positions.
1
0.8
0.6
Relative changeof the oscillator damping
Reflectance
1
2
0.4
3 0.2 0 500
400
-0.2
300 200 100 Mode wavenumber, cm-1
182
116
214
0.4 330 310 465
246
449
0.2
0 500
400
300
200 100 Wavenumber, cm-1
S0924-2031(17)30180-7 http://dx.doi.org/doi:10.1016/j.vibspec.2017.08.004 VIBSPE 2732
To appear in:
VIBSPE
Received date: Revised date: Accepted date:
11-6-2017 7-8-2017 7-8-2017
Please cite this article as: K.V.Shportko, The cation and the anion vacancies in cadmium diphosphide: a NIR-VIS, IR, and Raman study, Vibrational Spectroscopyhttp://dx.doi.org/10.1016/j.vibspec.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The cation and the anion vacancies in cadmium diphosphide: a NIRVIS, IR, and Raman study
K. V. Shportko V.E. Lashkaryov Institute for semiconductor physics of NAS of Ukraine, 41 Nauki av., Kyiv 03028, Ukraine.
Tel.: +380 95 3968060; Fax: +380 44 46 8994; E-mail address: [email protected]
Abstract The present work studies the impact of the cadmium and phosphorus vacancies on the properties of the tetragonal cadmium diphosphide that is promising for the optoelectronic and photovoltaic applications. Obtained results of the NIR-VIS transmittance, IR reflectance, and Raman scattering measurements have shown that phosphorus and cadmium vacancies provide a rich playground for the design of the optical and vibrational properties of cadmium diphosphide. Keywords: Cadmium diphosphide; vacancies; optical bandgap; phonons;
1. Introduction Binary and ternary diphosphides have been the topic of research in recent years due to their varied possible applications and interesting properties.
Cadmium and zinc diphosphides CdP2 and ZnP2 are characterized by a number of unique properties that make it promising for various applications in optoelectronics, such as in intensity stabilizers and pulse stretchers for solid state lasers, optical and thermal sensors and polarizing prisms for thin-film waveguides [1] and photovoltaics [2]. Tetragonal cadmium (-CdP2) and zinc (-ZnP2) diphosphides (Figure 1) belong to the space symmetry group P41212 ( D ). In -CdP2 and -ZnP2 each 4 4
metal (Cd or Zn) atom is surrounded with four phosphorus atoms, and each phosphorus atom is surrounded with two metal and two phosphorus atoms. In CdP2 and -ZnP2 phosphorus atoms build up zigzag chains. These continuous chains of phosphorus atoms can be regarded as covalently bonded molecules, and run through the crystal in the (010) and (100) directions. In the (001) direction all these chains are interrupted by metal atoms. There are four phosphorus chains per unit cell running alternately parallel to (010) or (100) [3]. The metal atoms are at the center of a deformed tetrahedron and link the phosphorus chains in 3D structure [4]. The chemical bonding in the binary tetragonal diphosphides exhibits complex character: phosphorus–phosphorus bond carries covalent character, whereas in metal–phosphorus bonds a proportion of ionic character (from 16 up to 54% by different estimations) is present [5]. The elementary cell consists of four layers revolved from each other by 90°. The sequence of packing these layers is frequently broken, and instead of four it is possible to observe the multiplets of six, less often five layers [3]. To study the vibrational properties of -CdP2 and -ZnP2 we have employed the temperature-dependent IR and Raman spectroscopy in the earlier studies [6– 8]. It has been demonstrated that decreasing the bond distance in the phosphorus chain at low temperatures causes an additional amount of anharmonicity of the corresponding modes, which is manifested in more pronounced temperature dependences of their frequencies and damping coefficients. It was rather an unexpected finding, since in the contrast to metal atoms, which weakly interact
with P atoms and between each other, P atoms within the chain are linked by strong covalent bonds, whose length is shorter then Cd(Zn)-P and Cd(Zn)Cd(Zn) [9]. In [9] considering ZnP2 it was assumed that effect might be attributed to the buckling of the phosphorus chains, in such a way, decreasing the bond distance in the chain, or to promotion of the double bonding between the P atoms in the chain or to a charge transfer take place in a direction which strengthens the P-P bonds and slightly weakens the Zn(Cd)-P bonds. In order to better understand this mechanism we focused on the CdP2 samples doped by ZnP2 nanoclusters, employing IR reflectance, ATR, and Raman spectroscopy in our previous studies [10,11]. Analysis of the impact of the doping on the vibrational properties of CdxZn(1 − x)P2 solid solutions has shown that presence of the ZnP2 nanoclusters
leads to the shift of the phonon
frequencies, as well as impacts 0,inf, and strengths of oscillators used to model the dielectric function of CdxZn(1 − x)P2 in the IR [10]. The doping of CdP2 single crystals by ZnP2 nanoclusters results also in the position and the width of the dispersion branches of the surface polaritons in CdxZn(1 − x)P2. This effect is more pronounced in the low frequency dispersion branches that originated from the phonons which correspond to the motion of the cation sublattice [11]. The vacancies in the cation and anion sublattices can result in the magnitude of the phenomena observed in [8,12]. The present research is aimed to study the influence of the cadmium and phosphorus vacancies on the optical properties of CdP2 at room temperature.
2. Experimental Polycrystalline CdP2 was obtained from the initial elements by twotemperature way. Ampoules with polycrystalline CdP2 were vacuumized till 10-3 Pa, soldered, and put into horizontal and vertical resistance furnace. Single crystals of CdP2 were grown in the conic side of the ampoule. Constancy of the temperature in evaporation zone during all the process of growth was achieved
by moving the ampoule sideways crystallization zone with speed of 0.6– 0.8 mm/hour. XRD confirms that grown single crystals of CdP2 are singlephase. To obtain the samples of CdP2 with cadmium and phosphorus vacancies, single crystals of CdP2 were soldered in vacuumized quartz ampoule and placed into the resistance furnace. Variation of the annealing temperature and the annealing time leads to the evaporation either cadmium or phosphorus. This enabled us to obtain samples of CdP2 with cation and anion vacancies. The stoichiometry has been controlled by XRF. Thus, obtained series of samples CdP1.918 and Cd0.995P2 were compared with CdP2 (Figure 1). In the present work we used samples of single crystals in the shape of plates with a size of 2 mm × 3mm × 1 mm, that were cut along the (001) crystallographic direction, and polished. To study the optical properties we employed NIR-VIS transmittance, IR reflectance, and Raman spectroscopy. To measure NIR-VIS transmittance spectra we used Perkin-Elmer® Lambda 25 spectrometer. VIS transmittance spectra have been obtained in 1-5 eV spectral range using non-polarized radiation. IR reflectance spectra of studied samples were measured in the range from 100 to 500 cm-1 at room temperature, using a Bruker® IFS 88 spectrometer with a globar as the radiation source and polarized radiation using Specac® metal grid polarizer. Spectra were taken for the E c orientation of the electrical vector E of the IR radiation with respect to the crystal. Raman spectra were measured in the spectral range from 85 to 500 cm-1 at room temperature using a FRA-106 Raman attachment applying the diode pump Nd:YAG laser of ca. 200 mW power and liquid nitrogen cooled Ge detector for y(zz)y back scattering configuration, employing the Specac® Glan-Thompson prism polarizer. To analyze the IR reflectance spectra we used the model of the dielectric function which includes following components: inf describes the polarizability of bounded electrons and plays the major role in the higher energy range (i.e. core electrons) [13], phonon contributions were described by Lorentz oscillators
with 3 parameters: the frequency j, and damping coefficient j and Sj is the oscillator strength [14]. Measured Raman spectra were analyzed in CoRa [15], by modelling the Raman bands with Gauss and Lorentz profiles.
3. Results and discussion 3.1 NIR-VIS spectroscopy We start with results of the NIR-VIS transmittance spectroscopy, since they provide the most obvious evidence of the impact of vacancies on the properties of studied samples. Spectra of cadmium diphosphides in the range around the absorption edge are shown in the inset of the Figure 2. Here one can clearly distinguish the shift of the absorption edge in the spectra of cadmium diphosphide that contain cadmium and phosphorus vacancies. The fundamental absorption edge in Cd0.995P2 is more sloping that in CdP1.918 and CdP2, this evidences changes in the electron distribution near the Fermi level.
The
dispersion of the absorption coefficient shown in the Figure 2 was calculated from the transmittance spectra to determine the optical bandgap Eg. Obtained value of the Eg in pure CdP2 is 2.01 eV that agrees with results reported in [16]. In the sample Cd0.995P2 with cadmium vacancies the Eg is 2.02 eV. The sample with phosphorus deficit CdP1.918 exhibits the most pronounced shift of the optical absorption edge, and the corresponding Eg here is 2.09 eV. Vacancies cause the defect levels within the bandgap and this leads to increase of the Eg. Having an evidence of the impact of the phosphorus and cadmium vacancies on the Eg, in the following we focus on their impact on the vibrational properties of cadmium diphosphide. 3.2 Raman spectroscopy The visual inspection of the Raman spectra presented in the Figure 3 shows that obtained Raman spectra of studied samples cadmium diphosphide samples exhibit generic pattern. To perform the quantitative analysis of the obtained data we performed the fit of Raman spectra using set of Gauss profiles and analyzed
trends in their parameters. According to [9] due to the similarity of the structure of tetragonal ZnP2 and CdP2, the low frequency Raman peaks in the spectra of cadmium diphosphide have been attributed to the Cd-P and Cd-Cd modes, whereas the high frequency peaks were assigned to the internal vibrations of the phosphorus chain. Similar approach has already been employed in [8,10]. In [8] the anisotropy of ZnP2 and CdP2 resulted in the difference in the shift
of the Raman peaks in the spectra recorded at different configurations. Effect of nanoclusters of ZnP2 built in the CdP2 matrix has been observed in the shift of the Raman peaks of CdXZn(1-X)P2 in the spectra recorded at room temperature [10]. This effect found to be more pronounced in the peaks that correspond to Cd(Zn)-P and Cd(Zn)-Cd(Zn) modes, i.e. low frequency Raman peaks. To learn more about the effect of the cation and anion vacancies on the vibrational properties of the cadmium diphosphide we focused on the intensity of Raman peaks in the spectra of studied samples. For this purpose for all observed peaks we calculated the relative intensity change:
I
ref
I x I ref
,
(1)
here Ix is the intensity of the Raman peak in the spectrum of studied cadmium diphosphide sample with stoichiometric deviations, Iref is the intensity of the corresponding Raman peak the spectrum of CdP2. Obtained values were plotted vs. corresponding peak position and presented in the inset in the Figure 3 accompanied by linear trend lines. The slope of the trend line enables one to notice the differences in peaks’ intensities in the spectra of studied samples. As one can notice from the inset in the Figure 3, in the case of phosphorus vacancies in the sample CdP1.918, high frequency Raman peaks that
correspond to the internal vibrations of the phosphorus chain exhibit lower intensity in comparison with those of corresponding peaks in CdP2. On the contrary, cadmium vacancies weaken the low frequency Raman bands in Cd0.995P2. Thus, the trends in the behavior of the intensities of Raman peaks
might be used to evaluate the concentration of cation and anion vacancies in cadmium diphoshide. 3.3 IR reflectance spectroscopy To learn more about the impact of the vacancies on the vibrational properties of cadmium diphosphide we now focus on the phonon scattering by the vacancies. For this purpose IR reflectance spectra that are shown in the Figure 4 were collected. Noticeable enough is fact that the same phonon peaks exhibit different heights in spectra of different samples. We performed the same fit procedure of reflectance spectra as described in [6]. To model the contribution of the phonons into the dielectric permittivity we used classical Lorentz oscillator with three parameters: the resonant frequency, the strength, and the damping coefficient. The last one is the measure of the decay time and is mainly responsible for the height phonon peak in the reflectance spectrum. Therefore, we studied the changes in the damping coefficients upon presence of the vacancies in cadmium diphosphide samples. For all observed peaks we calculated the relative change of oscillator damping:
ref
x ref
,
(2)
here x is the damping of the oscillator in the spectrum of studied cadmium diphosphide sample with stoichiometric deviations, ref is the damping of the oscillator in the spectrum of CdP2. The point of our interest is the distribution of this relative change of oscillator damping along the oscillator frequency axis that is plotted in the inset in the Figure 4. Vacancies breaking long-range order in a crystal are an additional source of scattering of phonon modes. Phonon scattering mechanisms by the defects has been considered in [17,18]. In comparison with CdP2 the phosphorus vacancies cause additional damping of the high-frequency modes in CdP1.918, while the cadmium vacancies in Cd0.995P2 influence low-frequency phonons causing increase of their damping.
4. Conclusions Thus, the present work provides a new, remarkable finding regarding the modifying the properties of cadmium diphosphide. By employing the optical techniques which are widely used to characterize the impurities in semiconductors, we have shown that apart from the doping [10], phosphorus and cadmium vacancies provide a rich playground for the optical and vibrational properties of cadmium diphosphide. This can be employed in the various optoelectronic devices. Vacancies influence also the flow of heat [18] and the anharmonicity of the phonon modes in media. Studying these phenomena in cadmium diphosphide may extend the results obtained in [7], and therefore will be in the detailed focus of other studies.
Acknowledgements. The author gratefully acknowledges the support from the DAAD (German Academic Exchange Service). The author thanks Prof. Dr. V. M. Trukhan and Dr. T.V. Shoukavaya for the help in obtaining the samples.
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Figure captions
Figure 1. Compositional triangle of studied diphosphides. P
CdP1.918 Cd0.9991Zn0.0009P2 CdP2 Cd0.995P2 Cd 0.9997Zn0.0003P2 0.5
ZnP2
0.5 Zn3P2
Cd
0.5
Zn
Figure 2. Transmittance spectra of studied cadmium diphosphide samples. Inset. Spectral dependence of the absorption coefficient. 1– CdP2, 2 – CdP1.918,
2000
Transmittance
Absorption coefficient, cm-1
3 – Cd0.995P2.
1500
0.2
0.1
0
1000
1.9
2
2.1
Photon energy, eV
1 500
2 3
2.01
0 1.9
2
2.02
2.09 2.1
Photon energy, eV
2.2
Figure 3. Raman spectra of studied cadmium diphosphide samples. 1– CdP2, 2 – CdP1.918, 3 – Cd0.995P2. Inset. Dependence of the relative intensity change of
Intensity, a.u.
1
2 3 467 464 450
Relative intensity change
the Raman peaks on the peaks’ position. 0.8 0.6 0.4
91
0.2 0 485
285
308 312 297 330
417
485
385
385
285
185 85 Mode wavenumber, cm-1
117 125
185 Raman shift, cm-1
103
85
Figure 4. IR reflectance spectra of studied cadmium diphosphide samples. 1– CdP2, 2 – CdP1.918, 3 – Cd0.995P2. Inset. Dependence of the relative change of the oscillators’ damping on their positions.
1
0.8
0.6
Relative changeof the oscillator damping
Reflectance
1
2
0.4
3 0.2 0 500
400
-0.2
300 200 100 Mode wavenumber, cm-1
182
116
214
0.4 330 310 465
246
449
0.2
0 500
400
300
200 100 Wavenumber, cm-1