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A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb, Gd or Cr impurities
Infrared Physics & Technology 62 (2014) 132–135
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb, Gd or Cr impurities G.V. Colibaba ⇑, E.P. Goncearenco, D.D. Nedeoglo, N.D. Nedeoglo Department of Physics, Moldova State University, A. Mateevich Str. 60, MD-2009 Chisinau, Republic of Moldova
h i g h l i g h t s IR luminescence spectra of ZnSe doped with Yb, Gd, or Cr have good coincidence. The good correlation between the component parts of the bands at 1 and 2 lm takes place. Control of the composition of IR luminescence spectra by changing stoichiometric deviation is possible.
a r t i c l e
i n f o
Article history: Received 26 July 2013 Available online 22 November 2013 Keywords: IR luminescence ZnSe Rare-earth impurities Cr impurity
a b s t r a c t Infra-red (IR) photoluminescence (PL) spectra of ZnSe crystals doped with Yb, Gd rare-earth impurities and Cr impurity are investigated. The influence of stoichiometric deviation on the spectra is studied and the structure of complex IR PL bands is analysed. The good coincidence between the structures of IR PL spectra of the samples doped with Yb, Gd, and Cr is shown. Correlation between the component parts of the bands at 1 and 2 lm is found and possibility to control the composition of IR PL spectra by enrichment of the samples with Zn or Se is discussed. The models that explain the formation of complexes based on rare-earth and background Cr and Cu impurities, responsible for IR PL bands, are proposed. Keywords: IR luminescence, ZnSe, Rare-earth impurities, Cr impurity. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental technique
II–VI wide-bandgap semiconductors are widely studied due to their prospects for optoelectronics applications. Investigation of radiative properties of these crystals doped with d- and f-elements is of peculiar interest. Intrashell electron transitions within these impurity ions may be responsible for efficient emission in various ranges of IR spectra [1–4]. Many papers deal with luminescent properties of ZnSe crystals doped with Cr because of its efficient luminescence in mid-IR spectral range. The PL spectra of the majority of ZnSe:Cr crystals consist of two bands localized at 0.95–1 and 1.95–2.47 lm [1–4]. The last band is attributed to radiative transitions between the ground (5T2) and first excited (5E) states of Cr2þ Zn ion and defines the prospects for use of these crystals as active elements for IR lasers [5–7]. However, there are limited data on PL properties of ZnSe crystals doped with rare-earth elements (REEs), such as Yb and Gd. The influence of annealing in various media on PL properties of similar samples is also little studied. All of this motivates the relevance of this research.
ZnSe crystals doped with Yb and Gd were obtained by physical vapour transport method in sealed quartz ampoules evacuated down to 104 torr with the use of YbSe and GdSe vapours as dopant sources. The ZnSe wafers with h1 1 1i orientation were used as substrates. The growth temperature of the crystals was 1050 °C and growth rate was about 1–1.5 mm/day. The duration of crystal growth was 4–6 days. The undercooling was varied in the range of 3–20 °C depending on growth chamber geometry which regulated the vapour flow rate of GdSe, YbSe and ZnSe and allowed to vary the doping level. The growth chambers with the grown crystals were extracted from the furnace within a period of 150 min. The density of twins and subgrain boundaries in the crystals was no more than 0–2 cm1. The average crystal size was 1 cm in diameter and 5–7 cm in height (Fig. 1, inset). ZnSe:Cr crystals were obtained by thermal diffusion of Cr during annealing in CrSe vapours at 1050 °C. The as-grown crystals were cut in plates, which were subsequently treated in brome-methanol and boiled NaOH solutions. Then the samples of 1 mm thickness were annealed in sealed evacuated quarts ampoules in the atmosphere of saturated Zn (1 atm) or Se (3 atm) vapours. The ZnSe:Yb crystals were also annealed in Zn, Se, and Se + Na melts with Na concentration in the melt of
⇑ Corresponding author. Tel.: +373 22 577709. E-mail address: [email protected] (G.V. Colibaba). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.11.009
G.V. Colibaba et al. / Infrared Physics & Technology 62 (2014) 132–135
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Fig. 1. PL spectra of ZnSe:Yb crystals before and after annealing in Zn or Se vapours. T = 100 K. To obtain the real spectrum intensity, the intensity values should be reduced by the numbers attached to the curves. Inset: the image of typical ZnSe:Yb crystal grown by physical vapour transport method.
0.3 at.%. The annealing temperature in Zn or Se vapours and Zn melt was 920 °C, in Se or Se + Na melt was 850 °C. The annealing duration was 50 h. Ampoules were extracted from the furnace within a period of 30 s, without quenching. IR PL in the range from 800 to 3000 nm was excited by Nd3+:YAG laser (532 nm (2.32 eV), 130 mW/mm2, 64 Hz, pulse width of 8 ms) at temperatures of 100 and 300 K and registered using PbS sensor. 3. Experimental results 3.1. Luminescence of ZnSe:Yb crystals A C band at 0.98 lm (1.25 eV) with full width at half maximum (FWHM) about 135 meV is registered in near-IR PL spectra of ZnSe:Yb crystals (Fig. 1). A narrow band at 1.06 lm is caused by PL excitation source. A B band at 1.65 lm (0.75 eV) with FWHM about 90 meV is observed in the long-wavelength range. Annealing in Zn vapours leads to an increase of the C band intensity, a disappearance of the B band and an appearance of a long-wavelength A band at 2.04 lm (0.6 eV) with FWHM about 70 meV. On the contrary, enrichment of these crystals with Se by means of annealing in chalcogen vapours leads to an increase of the B band intensity (Fig. 1). Localization of these IR PL bands is practically unchanged in the temperature range from 100 to 300 K that indicates that the bands are caused by intrashell transitions within impurity centres. It is noteworthy that the similar A and C bands were reported by other authors [1–4,8] for PL spectra of ZnSe:Cr crystals and were also attributed to intrashell transitions. The IR PL bands A and C are also observed in the spectra of undoped ZnSe crystals, however, they are considerably less intensive. The B band is not observed in the spectra of undoped crystals in the investigated temperature range. Contrary to the annealing in Zn vapours, the thermal treatment in Zn melt of the same duration and at the same temperature leads to the total disappearance of all the observed IR PL bands (Fig. 2). A
Fig. 2. PL spectra of ZnSe:Yb crystals after annealing in the melts of Zn, Se or Se + Na. T = 100 K.
comparative analysis of the PL spectra for ZnSe:Yb crystals annealed in pure Se and Se + Na melts allows concluding that additional presence of shallow Na acceptors, which lower the equilibrium Fermi level, favours an increase of the B band intensity due to decreasing intensity of the C and A bands (Fig. 2). 3.2. Luminescence of ZnSe:Gd crystals Gd impurity, as well as Yb, is responsible for IR PL in the ranges of 0.98–1 lm (1.25 eV) and 2.05 lm (0.6 eV) (Fig. 3). Similar to ZnSe:Yb samples, the annealing in Zn vapours reduces the total intensity of emission near 2 lm. At the same time, the A band broadens up to 120 meV that indicates its non-elementary character. The C band also broadens. As in the case of ZnSe:Yb crystals, enrichment of ZnSe:Gd crystals with Se enhances the intensity of IR PL band at 1.68 lm (0.73 eV; B band) due to weakening of the A-band components. The C band intensity also decreases (Fig. 3). A comparative analysis of PL spectra of ZnSe:Gd samples with various stoichiometric deviations allows to resolve complex PL spectra by Alentsev–Fok method [9,10]. The result of decomposition of non-elementary PL bands is shown in Fig. 4. In PL spectra presented as the number of photons per unit energy range, the A band consists of three Gaussian-like components A1, A2 and A3 localized at 2.32 lm (0.53 eV), 2.05 lm (0.60 eV) and 1.88 lm (0.65 eV), respectively. The C band consists of C1, C2 and C3 components localized at 1.10 lm (1.13 eV), 1.00 lm (1.24 eV) and 0.94 lm (1.32 eV), respectively (Fig. 4, inset). Enrichment of ZnSe:Gd crystals with Zn enhances contribution of the A1 and A3 components, as well as C1 and C3 components. It is worthy to note that localization energies of Ci components (i = 1, 2, 3) corresponds to double localization energies of Ai components with high accuracy. A good correlation is also observed for the ratio between the component intensities: Ci/Cj Ai/Aj (i, j = 1, 2, 3). 3.3. Luminescence of ZnSe:Cr crystals The doping of ZnSe crystals with Cr impurity leads to appearance of the bands at 0.96 lm (1.3 eV) and 2 lm (0.6 eV) in low-temperature PL spectra, which are similar by their shapes and localizations to the C and A bands in the spectra of ZnSe:Gd:Zn (Figs. 3 and 5). At higher temperature, the PL spectra of ZnSe:Cr and ZnSe:Gd:Zn samples almost coincide for specific impurity concentrations (Fig. 5, inset). Similar to the ZnSe:Yb and ZnSe:Gd crystals, enrichment of ZnSe:Cr with Zn leads to a decrease of the total mid-IR PL intensity (Fig. 5). The structure of IR PL bands is unchanged with modification of stoichiometric deviation of the samples doped with Cr. Increase of the doping level with Gd, Yb, as well as Cr impurities increases optical density of the crystals in the 500–3000 nm spectral range. However, for Gd and Yb impurities, there are no
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Fig. 3. PL spectra of ZnSe:Gd crystals before and after annealing in Zn or Se vapours. T = 100 K. Inset: normalized IR PL spectra of these samples. T = 100 K.
Fig. 4. Mid-IR PL bands and their components for ZnSe:Gd crystals before and after annealing in Zn or Se vapours, presented as the number of photons per unit energy range. T = 100 K. Inset: composition of near-IR PL band for ZnSe:Gd:Zn samples. T = 100 K.
Fig. 5. PL spectra of ZnSe:Cr crystals before and after annealing in Zn or Se vapours. T = 100 K. Inset: normalized PL spectra of ZnSe:Gd:Zn and ZnSe:Cr samples. T = 300 K.
well-shaped bands of impurity absorption, contrary to the case of ZnSe:Cr crystals, which are characterized by the absorption band attributed to intrashell transitions at 1.7 lm.
4. Discussion The common characteristic for the above-discussed impurities is the decrease of radiative properties of ZnSe crystals in the mid-IR PL spectra due to enrichment of the samples with Zn (Figs. 1–3 and 5). The observed transformations of PL spectra may be associated with rising equilibrium Fermi level due to increasing concentration of native donor defects, such as VSe and Zni. The modifying equilibrium Fermi level changes the charge state of impurity ions responsible for intrashell transitions that determine IR PL spectra of semiconductor materials. This model was previously examined in detail for ZnSe:Cr crystals [11]. Experimental data suggest the correlation between the contributions of the C and A-band components in all the investigated
samples. PL spectra of ZnSe:Cr crystals [8] consist of the similar A and C bands as the spectra of the samples investigated in the present paper. It was shown that the bands at 1 and 2 lm are attributed to intrashell transitions within the same Cr2þ Zn impurity ions [8]. The band at 2 lm was attributed to the transitions between the first excited and ground states of impurity ion, while the band at 1 lm was associated with the transitions between the second excited and ground states. The components of these bands appear to correspond to fine-structure energy levels of impurity ions. The B band is probably attributed to other centres, the density of which is rather high in the materials that show near p-type conductivity and increases with decreasing Fermi level. The Fermi level decreases due to enrichment of the crystals with Se that increases concentration of VZn native acceptor centres, as well as due to incorporation of shallow acceptor impurities, such as Na (Figs. 1–3). It is interesting to note a good correlation between maximum positions, the values of FWHM and behaviour of IR PL bands under
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changing stoichiometric deviation of the samples doped with Yb and Gd. As these impurities have different electron configurations: 4f14 for Yb and 4f75d1 for Gd, a different splitting of their energy levels in a crystal lattice electric field should take place. As a result, the various IR PL bands caused by intrashell transitions between corresponding split levels should be observed. The most interesting fact is the coincidence between maximum positions of the A and C bands in the PL spectra of ZnSe:Yb and ZnSe:Gd crystals with that of ZnSe:Cr crystals. Thereby, it may be assumed that isolated Gd and Yb impurity ions do not form radiative centers in the ZnSe crystals for the investigated spectral and temperature ranges. The observed IR PL bands are attributed to the intrashell transitions within non-controlled background impurity ions from the group of d-elements, energy levels of which are splitted by crystal lattice field. For ZnSe, the most typical non-controlled impurities from the d-elements group are Cr and Cu. In the PL spectra of undoped ZnSe crystals, the bands at 0.96 and 2 lm, which can be attributed to the background Cr impurity, are not registered. If Cr impurity is responsible for some PL bands in the spectra for ZnSe:Yb and ZnSe:Gd crystals, than the following statements can be assumed: (i) the presence of non-radiative centres in the undoped ZnSe crystals, which include in their composition almost all amount of the background chromium impurity; (ii) doping with REEs leads to the formation of the complexes between Cr Cr2þ and REEs, which are responsible for the PL bands localZn ized in the near region of the bands attributed to the isolated Cr2þ Zn ions. According to [12], Cu impurity does not form any centres responsible for IR PL in undoped ZnSe crystals. However, the PL spectra of the Cu-doped sulphides, such as ZnS, CdS and ZnCdS solid solution, consist of the complex bands localized near 1.6– 1.7 lm that is in good correlation with the spectral positions of the A and, especially, B bands in the spectra of ZnSe. In sulphide crystals, Cu impurity is placed in the nodes of crystal lattice with tetrahedral symmetry. Electrical field of the lattice of this symmetry splits the upper energy level of Cu ion with 3d9 electron configuration, as well as Cr ion with 3d4 electron configuration, into two levels with splitting parameter of 10 Dq [12,13]. In the undoped ZnSe crystals, Cu is placed in the nodes of trigonal symmetry that induces another type of level splitting. Thus, some of the IR PL bands in the spectra for ZnSe:Yb and ZnSe:Gd crystals are probably caused by the background Cu impurity that is fixed in the nodes of crystal lattice with tetrahedral symmetry by Yb and Gd impurity ions placed in the nearest nodes. The present model explains, for example, the total disappearance of IR PL bands after the annealing of ZnSe:Yb samples in pure Zn melt, while thermodynamically equivalent annealing in Zn vapours at the same temperature and pressure does not quench this emission (Figs. 1 and 2). As it is known, the annealing in pure Zn is a very efficient method for ZnSe crystal purification from the fast diffusing Cu impurity. At the same time, the annealing in Zn vapours is not an efficient purification method because of low pressure of
Cu vapours. It is established that addition of Cu impurity, with concentration no less than 0.3 at.%, in the Zn melt keeps the IR PL intensity at the same level for the ZnSe crystals doped with the above-discussed REEs. 5. Conclusions Impurities of Yb and Gd f-elements incorporated into ZnSe crystals are responsible for IR PL bands attributed to intrashell transitions and localized at 1.0 lm (1.25 eV), 1.65 lm (0.75 eV) and 2.0 lm (0.6 eV), respectively. The bands at 1 and 2 lm are also observed in the PL spectra of ZnSe:Cr crystals. Surplus of Zn favours the increasing contribution of correlating bands at 1 and 2 lm, which are non-elementary and consist of at least three components. Surplus of Se, as well as incorporation of shallow acceptors, enhances contribution of the band at 1.65 lm. A good coincidence of maximum positions of the IR PL bands, the values of FWHM and behaviour under changing stoichiometric deviation of ZnSe:Yb, ZnSe:Gd and ZnSe:Cr samples may be explained in the frame of the model that considers the formation of complexes based on rare-earth and background Cr or/and Cu impurities fixed in the nodes of crystal lattice with tetrahedral symmetry. Acknowledgement This work was carried 11.817.05.11F Grant.
out
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References [1] A. Gallian et al., Spectroscopic studies of molecular-beam epitaxially grown Cr2+-doped ZnSe thin films, Appl. Phys. Lett. 86 (2005) 091105. [2] V.V. Fedorov, A. Gallian, I. Moskalev, S.B. Mirov, En route to electrically pumped broadly tunable middle infrared lasers based on transition metal doped II–VI semiconductors, J. Lumin. 125 (2007) 184–195. [3] V.E. Kisel et al., Spectral kinetic properties and lasing characteristics, Opt. Spectrosc. 99 (4) (2005) 663–667. [4] V.Y. Ivanov, A.A. Shagov, A. Szczerbakow, M. Godlewski, Excitation mechanism of blue anti-Stokes and 2.4 mm infrared emission in ZnSe:Cr, Physica B 308 (2001) 942–944. [5] N. Vivet et al., Structural and photoluminescence properties of Cr2+:ZnSe films deposited by radiofrequency magnetron co-sputtering for mid-infrared microlaser applications, Mater. Sci. Eng., B 146 (2008) 236–240. [6] I.T. Sorokina, Cr2+-doped II–VI materials for lasers and nonlinear optics, Opt. Mater. 26 (2004) 395–412. [7] A. Sennaroglu et al., Continuous-wave broadly tunable Cr2+:ZnSe laser pumped by a thulium fiber laser, Opt. Commun. 268 (2006) 115–120. [8] M.U. Lehr et al., Identification of near-infrared Cr2+ luminescence in ZnSe, J. Phys.: Condens. Matter 9 (1997) 753–763. [9] M.V. Fok, Separation of composite spectra into individual bands with an Alentsev method, Work Phys. Inst. AN SSSR 59 (1) (1972) 3–24. [10] O.M. Bordun, O.Z. Drobchak, Luminescence centers in dried urine samples containing urate and oxalate salts, Ukr. J. Phys. Opt. 10 (2009) 124–133. [11] G. Colibaba et al., Influence of annealing medium on photoluminescence and optical properties of ZnSe:Cr crystals, J. Lumin. 145 (2014) 237–243. [12] M. Aven, J.S. Prener, Physics and Chemistry of II–VI Compounds, NorthHolland, Amsterdam, 1967 (Chapter 9). [13] S. Trushkin, S.W. Biernacki, Le Van Khoi, A. Suchocki, High pressure study of ZnSe:Cr2+ crystals: the origin of the 1.25 eV luminescence, J. Phys.: Condens. Matter 19 (2007) 096213.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb, Gd or Cr impurities G.V. Colibaba ⇑, E.P. Goncearenco, D.D. Nedeoglo, N.D. Nedeoglo Department of Physics, Moldova State University, A. Mateevich Str. 60, MD-2009 Chisinau, Republic of Moldova
h i g h l i g h t s IR luminescence spectra of ZnSe doped with Yb, Gd, or Cr have good coincidence. The good correlation between the component parts of the bands at 1 and 2 lm takes place. Control of the composition of IR luminescence spectra by changing stoichiometric deviation is possible.
a r t i c l e
i n f o
Article history: Received 26 July 2013 Available online 22 November 2013 Keywords: IR luminescence ZnSe Rare-earth impurities Cr impurity
a b s t r a c t Infra-red (IR) photoluminescence (PL) spectra of ZnSe crystals doped with Yb, Gd rare-earth impurities and Cr impurity are investigated. The influence of stoichiometric deviation on the spectra is studied and the structure of complex IR PL bands is analysed. The good coincidence between the structures of IR PL spectra of the samples doped with Yb, Gd, and Cr is shown. Correlation between the component parts of the bands at 1 and 2 lm is found and possibility to control the composition of IR PL spectra by enrichment of the samples with Zn or Se is discussed. The models that explain the formation of complexes based on rare-earth and background Cr and Cu impurities, responsible for IR PL bands, are proposed. Keywords: IR luminescence, ZnSe, Rare-earth impurities, Cr impurity. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental technique
II–VI wide-bandgap semiconductors are widely studied due to their prospects for optoelectronics applications. Investigation of radiative properties of these crystals doped with d- and f-elements is of peculiar interest. Intrashell electron transitions within these impurity ions may be responsible for efficient emission in various ranges of IR spectra [1–4]. Many papers deal with luminescent properties of ZnSe crystals doped with Cr because of its efficient luminescence in mid-IR spectral range. The PL spectra of the majority of ZnSe:Cr crystals consist of two bands localized at 0.95–1 and 1.95–2.47 lm [1–4]. The last band is attributed to radiative transitions between the ground (5T2) and first excited (5E) states of Cr2þ Zn ion and defines the prospects for use of these crystals as active elements for IR lasers [5–7]. However, there are limited data on PL properties of ZnSe crystals doped with rare-earth elements (REEs), such as Yb and Gd. The influence of annealing in various media on PL properties of similar samples is also little studied. All of this motivates the relevance of this research.
ZnSe crystals doped with Yb and Gd were obtained by physical vapour transport method in sealed quartz ampoules evacuated down to 104 torr with the use of YbSe and GdSe vapours as dopant sources. The ZnSe wafers with h1 1 1i orientation were used as substrates. The growth temperature of the crystals was 1050 °C and growth rate was about 1–1.5 mm/day. The duration of crystal growth was 4–6 days. The undercooling was varied in the range of 3–20 °C depending on growth chamber geometry which regulated the vapour flow rate of GdSe, YbSe and ZnSe and allowed to vary the doping level. The growth chambers with the grown crystals were extracted from the furnace within a period of 150 min. The density of twins and subgrain boundaries in the crystals was no more than 0–2 cm1. The average crystal size was 1 cm in diameter and 5–7 cm in height (Fig. 1, inset). ZnSe:Cr crystals were obtained by thermal diffusion of Cr during annealing in CrSe vapours at 1050 °C. The as-grown crystals were cut in plates, which were subsequently treated in brome-methanol and boiled NaOH solutions. Then the samples of 1 mm thickness were annealed in sealed evacuated quarts ampoules in the atmosphere of saturated Zn (1 atm) or Se (3 atm) vapours. The ZnSe:Yb crystals were also annealed in Zn, Se, and Se + Na melts with Na concentration in the melt of
⇑ Corresponding author. Tel.: +373 22 577709. E-mail address: [email protected] (G.V. Colibaba). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.11.009
G.V. Colibaba et al. / Infrared Physics & Technology 62 (2014) 132–135
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Fig. 1. PL spectra of ZnSe:Yb crystals before and after annealing in Zn or Se vapours. T = 100 K. To obtain the real spectrum intensity, the intensity values should be reduced by the numbers attached to the curves. Inset: the image of typical ZnSe:Yb crystal grown by physical vapour transport method.
0.3 at.%. The annealing temperature in Zn or Se vapours and Zn melt was 920 °C, in Se or Se + Na melt was 850 °C. The annealing duration was 50 h. Ampoules were extracted from the furnace within a period of 30 s, without quenching. IR PL in the range from 800 to 3000 nm was excited by Nd3+:YAG laser (532 nm (2.32 eV), 130 mW/mm2, 64 Hz, pulse width of 8 ms) at temperatures of 100 and 300 K and registered using PbS sensor. 3. Experimental results 3.1. Luminescence of ZnSe:Yb crystals A C band at 0.98 lm (1.25 eV) with full width at half maximum (FWHM) about 135 meV is registered in near-IR PL spectra of ZnSe:Yb crystals (Fig. 1). A narrow band at 1.06 lm is caused by PL excitation source. A B band at 1.65 lm (0.75 eV) with FWHM about 90 meV is observed in the long-wavelength range. Annealing in Zn vapours leads to an increase of the C band intensity, a disappearance of the B band and an appearance of a long-wavelength A band at 2.04 lm (0.6 eV) with FWHM about 70 meV. On the contrary, enrichment of these crystals with Se by means of annealing in chalcogen vapours leads to an increase of the B band intensity (Fig. 1). Localization of these IR PL bands is practically unchanged in the temperature range from 100 to 300 K that indicates that the bands are caused by intrashell transitions within impurity centres. It is noteworthy that the similar A and C bands were reported by other authors [1–4,8] for PL spectra of ZnSe:Cr crystals and were also attributed to intrashell transitions. The IR PL bands A and C are also observed in the spectra of undoped ZnSe crystals, however, they are considerably less intensive. The B band is not observed in the spectra of undoped crystals in the investigated temperature range. Contrary to the annealing in Zn vapours, the thermal treatment in Zn melt of the same duration and at the same temperature leads to the total disappearance of all the observed IR PL bands (Fig. 2). A
Fig. 2. PL spectra of ZnSe:Yb crystals after annealing in the melts of Zn, Se or Se + Na. T = 100 K.
comparative analysis of the PL spectra for ZnSe:Yb crystals annealed in pure Se and Se + Na melts allows concluding that additional presence of shallow Na acceptors, which lower the equilibrium Fermi level, favours an increase of the B band intensity due to decreasing intensity of the C and A bands (Fig. 2). 3.2. Luminescence of ZnSe:Gd crystals Gd impurity, as well as Yb, is responsible for IR PL in the ranges of 0.98–1 lm (1.25 eV) and 2.05 lm (0.6 eV) (Fig. 3). Similar to ZnSe:Yb samples, the annealing in Zn vapours reduces the total intensity of emission near 2 lm. At the same time, the A band broadens up to 120 meV that indicates its non-elementary character. The C band also broadens. As in the case of ZnSe:Yb crystals, enrichment of ZnSe:Gd crystals with Se enhances the intensity of IR PL band at 1.68 lm (0.73 eV; B band) due to weakening of the A-band components. The C band intensity also decreases (Fig. 3). A comparative analysis of PL spectra of ZnSe:Gd samples with various stoichiometric deviations allows to resolve complex PL spectra by Alentsev–Fok method [9,10]. The result of decomposition of non-elementary PL bands is shown in Fig. 4. In PL spectra presented as the number of photons per unit energy range, the A band consists of three Gaussian-like components A1, A2 and A3 localized at 2.32 lm (0.53 eV), 2.05 lm (0.60 eV) and 1.88 lm (0.65 eV), respectively. The C band consists of C1, C2 and C3 components localized at 1.10 lm (1.13 eV), 1.00 lm (1.24 eV) and 0.94 lm (1.32 eV), respectively (Fig. 4, inset). Enrichment of ZnSe:Gd crystals with Zn enhances contribution of the A1 and A3 components, as well as C1 and C3 components. It is worthy to note that localization energies of Ci components (i = 1, 2, 3) corresponds to double localization energies of Ai components with high accuracy. A good correlation is also observed for the ratio between the component intensities: Ci/Cj Ai/Aj (i, j = 1, 2, 3). 3.3. Luminescence of ZnSe:Cr crystals The doping of ZnSe crystals with Cr impurity leads to appearance of the bands at 0.96 lm (1.3 eV) and 2 lm (0.6 eV) in low-temperature PL spectra, which are similar by their shapes and localizations to the C and A bands in the spectra of ZnSe:Gd:Zn (Figs. 3 and 5). At higher temperature, the PL spectra of ZnSe:Cr and ZnSe:Gd:Zn samples almost coincide for specific impurity concentrations (Fig. 5, inset). Similar to the ZnSe:Yb and ZnSe:Gd crystals, enrichment of ZnSe:Cr with Zn leads to a decrease of the total mid-IR PL intensity (Fig. 5). The structure of IR PL bands is unchanged with modification of stoichiometric deviation of the samples doped with Cr. Increase of the doping level with Gd, Yb, as well as Cr impurities increases optical density of the crystals in the 500–3000 nm spectral range. However, for Gd and Yb impurities, there are no
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Fig. 3. PL spectra of ZnSe:Gd crystals before and after annealing in Zn or Se vapours. T = 100 K. Inset: normalized IR PL spectra of these samples. T = 100 K.
Fig. 4. Mid-IR PL bands and their components for ZnSe:Gd crystals before and after annealing in Zn or Se vapours, presented as the number of photons per unit energy range. T = 100 K. Inset: composition of near-IR PL band for ZnSe:Gd:Zn samples. T = 100 K.
Fig. 5. PL spectra of ZnSe:Cr crystals before and after annealing in Zn or Se vapours. T = 100 K. Inset: normalized PL spectra of ZnSe:Gd:Zn and ZnSe:Cr samples. T = 300 K.
well-shaped bands of impurity absorption, contrary to the case of ZnSe:Cr crystals, which are characterized by the absorption band attributed to intrashell transitions at 1.7 lm.
4. Discussion The common characteristic for the above-discussed impurities is the decrease of radiative properties of ZnSe crystals in the mid-IR PL spectra due to enrichment of the samples with Zn (Figs. 1–3 and 5). The observed transformations of PL spectra may be associated with rising equilibrium Fermi level due to increasing concentration of native donor defects, such as VSe and Zni. The modifying equilibrium Fermi level changes the charge state of impurity ions responsible for intrashell transitions that determine IR PL spectra of semiconductor materials. This model was previously examined in detail for ZnSe:Cr crystals [11]. Experimental data suggest the correlation between the contributions of the C and A-band components in all the investigated
samples. PL spectra of ZnSe:Cr crystals [8] consist of the similar A and C bands as the spectra of the samples investigated in the present paper. It was shown that the bands at 1 and 2 lm are attributed to intrashell transitions within the same Cr2þ Zn impurity ions [8]. The band at 2 lm was attributed to the transitions between the first excited and ground states of impurity ion, while the band at 1 lm was associated with the transitions between the second excited and ground states. The components of these bands appear to correspond to fine-structure energy levels of impurity ions. The B band is probably attributed to other centres, the density of which is rather high in the materials that show near p-type conductivity and increases with decreasing Fermi level. The Fermi level decreases due to enrichment of the crystals with Se that increases concentration of VZn native acceptor centres, as well as due to incorporation of shallow acceptor impurities, such as Na (Figs. 1–3). It is interesting to note a good correlation between maximum positions, the values of FWHM and behaviour of IR PL bands under
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changing stoichiometric deviation of the samples doped with Yb and Gd. As these impurities have different electron configurations: 4f14 for Yb and 4f75d1 for Gd, a different splitting of their energy levels in a crystal lattice electric field should take place. As a result, the various IR PL bands caused by intrashell transitions between corresponding split levels should be observed. The most interesting fact is the coincidence between maximum positions of the A and C bands in the PL spectra of ZnSe:Yb and ZnSe:Gd crystals with that of ZnSe:Cr crystals. Thereby, it may be assumed that isolated Gd and Yb impurity ions do not form radiative centers in the ZnSe crystals for the investigated spectral and temperature ranges. The observed IR PL bands are attributed to the intrashell transitions within non-controlled background impurity ions from the group of d-elements, energy levels of which are splitted by crystal lattice field. For ZnSe, the most typical non-controlled impurities from the d-elements group are Cr and Cu. In the PL spectra of undoped ZnSe crystals, the bands at 0.96 and 2 lm, which can be attributed to the background Cr impurity, are not registered. If Cr impurity is responsible for some PL bands in the spectra for ZnSe:Yb and ZnSe:Gd crystals, than the following statements can be assumed: (i) the presence of non-radiative centres in the undoped ZnSe crystals, which include in their composition almost all amount of the background chromium impurity; (ii) doping with REEs leads to the formation of the complexes between Cr Cr2þ and REEs, which are responsible for the PL bands localZn ized in the near region of the bands attributed to the isolated Cr2þ Zn ions. According to [12], Cu impurity does not form any centres responsible for IR PL in undoped ZnSe crystals. However, the PL spectra of the Cu-doped sulphides, such as ZnS, CdS and ZnCdS solid solution, consist of the complex bands localized near 1.6– 1.7 lm that is in good correlation with the spectral positions of the A and, especially, B bands in the spectra of ZnSe. In sulphide crystals, Cu impurity is placed in the nodes of crystal lattice with tetrahedral symmetry. Electrical field of the lattice of this symmetry splits the upper energy level of Cu ion with 3d9 electron configuration, as well as Cr ion with 3d4 electron configuration, into two levels with splitting parameter of 10 Dq [12,13]. In the undoped ZnSe crystals, Cu is placed in the nodes of trigonal symmetry that induces another type of level splitting. Thus, some of the IR PL bands in the spectra for ZnSe:Yb and ZnSe:Gd crystals are probably caused by the background Cu impurity that is fixed in the nodes of crystal lattice with tetrahedral symmetry by Yb and Gd impurity ions placed in the nearest nodes. The present model explains, for example, the total disappearance of IR PL bands after the annealing of ZnSe:Yb samples in pure Zn melt, while thermodynamically equivalent annealing in Zn vapours at the same temperature and pressure does not quench this emission (Figs. 1 and 2). As it is known, the annealing in pure Zn is a very efficient method for ZnSe crystal purification from the fast diffusing Cu impurity. At the same time, the annealing in Zn vapours is not an efficient purification method because of low pressure of
Cu vapours. It is established that addition of Cu impurity, with concentration no less than 0.3 at.%, in the Zn melt keeps the IR PL intensity at the same level for the ZnSe crystals doped with the above-discussed REEs. 5. Conclusions Impurities of Yb and Gd f-elements incorporated into ZnSe crystals are responsible for IR PL bands attributed to intrashell transitions and localized at 1.0 lm (1.25 eV), 1.65 lm (0.75 eV) and 2.0 lm (0.6 eV), respectively. The bands at 1 and 2 lm are also observed in the PL spectra of ZnSe:Cr crystals. Surplus of Zn favours the increasing contribution of correlating bands at 1 and 2 lm, which are non-elementary and consist of at least three components. Surplus of Se, as well as incorporation of shallow acceptors, enhances contribution of the band at 1.65 lm. A good coincidence of maximum positions of the IR PL bands, the values of FWHM and behaviour under changing stoichiometric deviation of ZnSe:Yb, ZnSe:Gd and ZnSe:Cr samples may be explained in the frame of the model that considers the formation of complexes based on rare-earth and background Cr or/and Cu impurities fixed in the nodes of crystal lattice with tetrahedral symmetry. Acknowledgement This work was carried 11.817.05.11F Grant.
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