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Studies on structural and optical properties of ZnSe and ZnSSe single crystals grown by CVT method
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Studies on structural and optical properties of ZnSe and ZnSSe single crystals grown by CVT method P. Kannappan, R. Dhanasekaran n Crystal Growth Centre, Anna University, Chennai 600025, India
art ic l e i nf o
Keywords: A1. Optical properties A1. Surface morphology A1. X-ray diffraction A2. Growth from vapour A2. Single crystal growth B1. Semiconducting II–VI materials
a b s t r a c t The ZnSe and ZnSSe single crystals have been grown by chemical vapour transport (CVT) with iodine as a transporting agent. The source and growth temperatures of the reactor are maintained at 850 1C and 900 1C respectively using the two zone horizontal resistive heating furnace. The grown crystals are characterised by structural, surface morphological and optical properties. The XRD pattern shows that the peaks due to reflection from (111), (220) and (311) planes belong to the cubic zinc-blend structure. The composition of grown ZnSe crystal has been determined by EDXRF analysis as Zn (52.5%) and Se (47.5%). The XPS study of grown ZnSe single crystal has been studied. The HRSEM images reveal the step, spiral and triangular patterns. The optical absorption study reveals the absorption cut off wavelength as 483 nm (2.57 eV) for ZnSe and 441 nm (2.81 eV) for ZnSSe crystals. The photoluminescence studies reveal the band edge emission and defect level emissions of ZnSe and ZnSSe single crystals. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been a great deal of interest in wide band gap II–VI compound semiconductors [1]. In particular, ZnSe is a wide and direct transition type compound semiconductor with band gap energy of 2.67 eV at room temperature. The ZnSe based II–VI semiconductors have been extensively used for the fabrication of blue-light emitting diodes (LEDs), laser diodes (LDs) and non-linear optoelectronic devices and so on [2]. These applications seriously depend on the preparation of high quality single crystals and great efforts have been devoted to grow ZnSe single crystals [3]. ZnSxSe1 x is a solid solution of two binary compound semiconductors of ZnS (3.67 eV) and ZnSe (2.67), where the band gap energy changes continuously with the composition x [4]. The ZnSSe makes it suitable for light emitting diodes and laser diodes in the visible region [5,6]. The band gap tuning of ZnSSe is the process of altering the band gap by controlling the composition of ZnS and ZnSe. ZnSSe is an advantage over ZnSe due to the presence of sulphur that widens the band gap, which leads to an increase in the blue response of the device [7]. Therefore, the variation of band gap energy with the composition makes ZnSSe a highly suitable and promising material for wavelength tunable UV photodetector, visible laser diodes (LDs), light emitting diodes (LEDs) and light emitters [8].
n
Corresponding author. Tel.: þ 91 44 2235 8317. E-mail addresses: [email protected], [email protected] (R. Dhanasekaran).
Bulk crystals of ZnSe and ZnSSe are grown by different methods, such as vertical Bridgman method (VB) [9], solid-phase recrystallisation (SPR) [10], physical vapour transport (PVT) [11], chemical vapour transport (CVT), etc. [12]. Among these methods, chemical vapour transport (CVT) method can be designed to grow ZnSe single crystals at comparatively low temperatures (below 1000 1C) [13]. The chemical vapour transport (CVT) system with a chemical transport agent affords possibilities to grow ZnSe and ZnSSe single crystals. In the CVT method, the gaseous molecules like iodine, hydrogen chloride, hydrogen and ammonium chloride can be used as the transporting agent [2]. Among these transporting chemical agents iodine was proved thermodynamically favourable for the growth of ZnSe and ZnSSe single crystals [14]. In the present investigation, the ZnSe and ZnSSe single crystals were grown by chemical vapour transport with iodine as a transporting agent using two zone horizontal resistive heating furnace. The grown crystals were studied for their structural, compositional, surface morphological and optical properties. 2. Experimental procedure 2.1. Synthesis of ZnSe and ZnSSe polycrystalline samples ZnSe and ZnSSe polycrystalline samples have been synthesised in a silica glass ampoule of length 10 cm and diameter 1.4 cm. Before loading the starting materials, the ampoules were cleaned with acetone and then rinsed with double distilled water three times. 40% Hydrofluoric (HF) acid was poured to clean the ampoule. The nitric acid (HNO3) was poured and cleaned well two to three
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times with millipore water. They were then dried and kept in a furnace maintained at the temperature of 100 1C for 24 h. The 99.9% purity Zn and Se were taken in the stoichiometric 1:1 ratio for the synthesis of ZnSe and 1:0.5:0.5 of Zn, S and Se for the synthesis of ZnSSe in a silica glass ampoule. The mixture was heat treated at 1100 1C at the pressure of 1 10 5 mbar for about 24 h. The synthesised polycrystalline samples were used for the growth of single crystals.
resistive heating furnace connected with the Eurotherm temperature controllers with the accuracy of 7 0.1 1C. A reverse temperature profile was maintained across the ampoule with growth zone
2.2. Growth of ZnSe and ZnSSe single crystals The following chemical reactions take place at the source and growth zone regions during the growth of ZnSe and ZnSSe single crystals by CVT method: ZnSeðsÞ þ 2IðsÞ ) ZnI2ðgÞ þ 0:5Se2ðgÞ ) ZnSeðcÞ þ 0:5I2ðsÞ Source zoneð9001CÞ
Growth zoneð8501CÞ
ZnSSeðsÞ þ 2IðsÞ ) ZnI2ðgÞ þ 0:5S2ðgÞ þ 0:5Se2ðgÞ ) ZnSSeðcÞ þ 0:5I2ðsÞ About 3 g of polycrystalline ZnSe sample was loaded in a silica glass ampoule of length 15 cm with 1.4 cm diameter along with 2 mg/cm3 of iodine as the transporting agent. A pressure of 1 10 5 mbar was achieved and the ampoule was vacuum sealed. The sealed ampoule was placed inside a two zone horizontal
Fig. 2. Powder XRD patterns of grown (a) ZnSe and (b) ZnSSe crystals.
Fig. 1. (a and b) As grown, and (d and e) polished ZnSe single crystals and (c) ZnSSe single crystal using the source temperature at 900 1C and growth temperature at 850 1C.
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at a higher temperature for 24 h to remove the sticking powders from the growth zone of the ampoule and to diminish the active sites. During the growth, the temperature difference between the two zones was maintained at 50 1C with the source temperature of 900 1C and the growth temperature of 850 1C. The growth period was about 15 days for the ZnSe and 10 days for ZnSSe single crystals. Then, the furnace was cooled to room temperature at the rate of 30 1C/h. The grown crystals are of different dimensions and morphology. The as grown ZnSe and ZnSSe single crystals are shown in Fig. 1.
3. Results and discussion 3.1. X-ray diffraction (XRD) studies The as grown ZnSe and ZnSSe crystals have been studied by X-ray diffraction using BRUKER D8 Advance X-ray diffractometer. Fig. 2(a) and (b) shows the powder XRD patterns of as grown ZnSe and ZnSSe crystals respectively. The diffraction peaks corresponding to (111), (220) and (311) planes confirm the formation of ZnSe and ZnSSe crystalline phase. The ZnSe and ZnSSe crystals belong to cubic zinc blend structure. The calculated unit cell parameter values for ZnSe are a¼ b¼c ¼5.642 Å; and for ZnSSe, the values are a ¼b¼ c¼5.567 Å. The reported values of lattice parameters for the ZnSe and ZnSSe single crystals are 5.660 Å and 5.550 Å respectively [15,16]. The XRD pattern of the ZnSSe crystal shows a slight peak shift towards the higher diffraction angle due to the addition of S in ZnSe which results in decrease of the lattice parameter. The interplanar distances of ZnSSe crystal decrease for all the XRD planes. The high intensity of XRD pattern clearly shows the good crystalline nature of ZnSe and ZnSSe crystals. The calculated interplanar distances (dhkl) and lattice parameter (a) values are tabulated in Table 1.
Fig. 3. EDXRF studies of ZnSe single crystal.
3.2. EDXRF studies Fig. 3 shows the energy dispersive X-ray fluorescence (EDXRF) spectrum of ZnSe single crystal. The EDXRF measurements were carried in fluorescence mode using Si detector array and X-ray energy source ranges from 1504.7 eV to 12538.9 eV. The chemical compositions were obtained from the X-ray fluorescence spectra measured at the energy of 8675.3 eV and 11253.2 eV of Zn and Se elements respectively. The Zn and Se elements present in the crystal are 52.5% and 47.5% respectively. The slight changes in the composition of ZnSe crystal is due to the high vapour pressure at the growth interface of chemical transport reactions inside the CVT reactor [16]. The grown ZnSSe crystal has the composition of Zn (43.56%), S (29.86%) and Se (26.58%), which has already been reported [17].
Fig. 4. XPS studies of ZnSe single crystal.
constituent elements in ZnSe single crystal was identified with reference to C 1s at 284.60 eV. The BE of Zn and Se elements in 3d states are 10 eV and 55.5 eV respectively. The BE values of 140 eV and 163 eV are corresponding to the Zn 3s and Se 3p states in the ZnSe single crystal. The higher BE values at 1022.5 eV and 1045.5 eV are corresponding to the sublevels of 2p3/2 and 2p1/2 of the Zn element. The binding energies at 530 eV (LMM), 495 eV (LMM) and 180 eV (LMM) reveal that the auger electrons ejected from the Zn and Se atoms are due to the function of the X-ray source [18].
3.3. XPS studies 3.4. Surface morphological studies Fig. 4 shows the X-ray photoelectron spectroscopy (XPS) studies of ZnSe single crystal and provides information about the oxidation states of Zn and Se elements. The binding energy (BE) of Table 1 Calculated interplanar distances (d(hkl)) and lattice parameter values (a) of ZnSe and ZnSSe crystals. Samples
ZnSe ZnSSe
Interplanar distance (Å)
Lattice parameter
d(111)
d(220)
d(311)
d(400)
d(331)
a (Å)
3.2481 3.1934
1.9937 1.9600
1.7018 1.6682
1.4131 1.3845
1.2963 1.2701
5.642 5.567
Fig. 5(a–f) shows the high resolution scanning electron microscopy (HR-SEM) images of different types of morphological patterns on the surface of grown ZnSe and ZnSSe single crystals. The surface morphology of CVT grown single crystals strongly depends on the experimental variables such as supersaturation of vapour, temperature difference between the source and growth zones, concentration of the transporting agent, heat and mass transport processes, kinetics of chemical reaction, vapour–solid interface and the dimensions of the CVT reactor [13]. The different growth runs were carried out for the temperature difference between the source (900 1C) and growth (850 1C) zones of 50 1C using the same growth parameters and composition of the starting materials. The
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Fig. 5. HRSEM images (a) triangular, (b) spiral and (c and d) step growth pattern of ZnSe single crystals; (e and f) layers in the form of step growth pattern of ZnSSe crystals.
as grown ZnSe crystals at the end as well as at approximately middle part of the quartz ampoule are shown in Fig. 1(a). The HRSEM studies have been done on the ZnSe single crystals grown at the end and at approximately middle part of the ampoule. The temperature starts to decrease from source to growth zone with respect to distance in the CVT reactor. The surface morphology of grown ZnSe single crystals shows the step, spiral and triangular pattern whereas the ZnSSe single crystal shows the layer pattern in the form of step growth morphology. Fig. 5(a) and (b) shows the triangular and spiral growth morphology which represent the crystals grown at the end of the ampoule with different experimental runs where the temperature difference is maintained at 50 1C from the source to growth zones. The formation of triangular pattern is due to rough surface produced as a result of higher temperature difference (50 1C) [15]. The formation of spiral growth pattern is due to the variation of temperature at the vapour-solid interface in the CVT reactor. Fig. 5(c) and (d) shows the formation of step growth morphology which represents the crystals grown at approximately
Fig. 6. Optical absorption spectra of grown (a) ZnSe and (b) ZnSSe single crystals.
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the middle part of the ampoule, where the lower temperature difference ( 30 1C) is approximately maintained. The formation of step growth pattern may be due to the formation of defects during ZnSe crystal growth. The step growth pattern of grown crystals reveal the two dimensional nucleation mechanism. Fig. 5(e) and (f) shows the layers in the bunch steps with regular morphologies of ZnSSe single crystals. The layer pattern in the form of step growth morphology of ZnSSe represents the crystals grown at the end of the ampoule where the temperature difference is maintained at 50 1C from the source to growth zone of the ampoule. 3.5. Optical absorption studies Fig. 6(a) and (b) shows the optical absorption spectrum of grown ZnSe and ZnSSe single crystals respectively using CARY 5E UV–vis– NIR spectrophotometer. The optical absorption study reveals the cut off wavelength as 483 nm for ZnSe and 441 nm for ZnSSe single crystals. The band gap energy of grown ZnSe and ZnSSe crystals were calculated to be 2.57 eV and 2.81 eV respectively. The band gap energy of ZnSSe depends on the composition of the sulphur atom present in the crystals. The reported value for the band gap energy of ZnS0.6Se0.4 is 2.85 eV [4]. 3.6. Photoluminescence studies Fig. 7 shows the photoluminescence (PL) spectrum of ZnSe single crystals. The PL spectrum was recorded using LS 55 luminescence spectrometer at room temperature. The samples were excited by using UV laser at 244 nm. The emission peaks for ZnSe and ZnSe single crystals are in the blue, yellow and red regions. The PL spectrum of grown ZnSe single crystal shows the peaks at 420 nm (2.95 eV), 440 nm (2.81 eV), 574 nm (2.16 eV), 634 nm (1.95 eV) and 745 nm (1.66 eV). The emissions at 420 (2.95 eV) and 440 nm (2.81 eV) are attributed to the near band edge emission (NBE) of ZnSe single crystals [19]. The emission at 574 nm and 745 nm are attributed to the defect level emission [20]. Fig. 8 shows the PL spectrum of ZnSSe single crystal. The PL emissions at 353 nm, 420–457 nm and broad range 500–700 nm were observed. The emissions at 353 nm (3.51 eV) and 420– 457 nm (2.95–2.71 eV) are attributed to the near band edge emission (NBE) of ZnSSe single crystal. The near band emission is generally known as donor–acceptor pair (DAP) emission whereas the deep level emission is due to crystal defects such as vacancies, interstitials and dislocations in the crystals [21]. The PL spectrum of ZnSSe is dominated by broad defect level emission centred at 565–595 nm (2.19–2.08 eV) in the yellow-orange region [22].
Fig. 8. Photoluminescence spectrum of ZnSSe single crystal.
The intensity of defect level emission in ZnSSe is higher than the near band edge (NBE) emission. The red emission at 657 nm (1.88 eV) is due to the defect level. The decrease of intensity of near band edge emission in ZnSSe reveals that the band edge recombination is quenched by the defects [23].
4. Conclusions The ZnSe and ZnSSe single crystals were grown by chemical vapour transport (CVT) method with iodine as a transporting agent using two zone horizontal resistive heating furnace. The powder XRD patterns of grown crystals reveal the cubic zinc-blend structure. The compositional variation of ZnSe and ZnSSe is due to the difference in the vapour pressure of molecules inside the CVT reactor during crystal growth. The oxidation state of Zn and Se atoms in ZnSe single crystal has been identified by XPS studies. The HRSEM study reveals the triangular, spiral and step growth patterns on the surface of grown ZnSe and ZnSSe single crystals. The optical absorption cut off wavelength and band gap energy of ZnSe and ZnSSe single crystals were calculated. The photoluminescence studies of ZnSe and ZnSSe single crystals revealed that the near band edge (NBE) emission in blue and defect level emission in the yellow and red regions.
Acknowledgements The authors gratefully acknowledge UGC-DAE Consortium for Scientific Research, Kolkata, India for providing financial support. The authors are thankful to Dr. S. Soundiraraj and Dr. A.R.S. Jayanth, Department of English, Anna University for their support in correcting the manuscript. References
Fig. 7. Photoluminescence spectrum of ZnSe single crystal.
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[10] A. Urbieta, P. Fernandez, J. Piqueras, V. Munoz, Mater. Sci. Eng. B 78 (2000) 105–108. [11] N. Sankar, K. Ramachandran, C. Sanjeeviraja, J. Cryst. Growth 235 (2002) 195–200. [12] O. Senthil Kumar, S. Soundeswaran, R. Dhanasekaran, Cryst. Growth Des. 2 (2002) 585–589. [13] M.J. Tafreshi, K. Balakrishnan, R. Dhanasekaran, J. Mater. Sci. 32 (1997) 3517–3521. [14] K. Bottcher, H. Hartmann, J. Mater. Growth 146 (1995) 53–58. [15] O. Senthil Kumar, S. Soundeswaran, D. Kabiraj, D.K. Avasthi, R. Dhanasekaran, J. Mater. Growth 275 (2005) 567–570. [16] O. Senthil Kumar, S. Soundeswaran, R. Dhanasekaran, Mater. Chem. Phys. 87 (2004) 75–80.
[17] P. Kannappan, K. Asokan, J.B.M. Krishna, R. Dhanasekaran, J. Alloys Compd. 580 (2013) 284–289. [18] M. Achimovicova, P. Balaz, T. Ohtani, N. Kostova, G. Tyuliev, A. Feldhoff, V. Sepelak, Solid State Ion. 192 (2011) 632–637. [19] A.N. Georgobiani, U.A. Aminov, V.A. Dravin, Z.P. Ilyukhina, J. Inorg. Mater. 36 (2000) 119. [20] Huanyong Li, Wanqi Jie, J. Cryst. Growth 257 (2003) 110–115. [21] C.S. Fang, Q.T. Gu, J.Q. Wei, Q.W. Pan, W. Shi, Y.J. Wang, J. Cryst. Growth 209 (2000) 542–546. [22] P. Kannappan, J.B.M. Krishna, G.S. Taki, R. Dhanasekaran, J. Cryst. Growth 362 (2013) 211–215. [23] Li Huan-Yong, Jie Wanqi Qi, Zhang Shi-An, Sun Zhen-Rong, Xu Ke-Wei, Chin. Phys. 15 (2006) 2407–2414.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Studies on structural and optical properties of ZnSe and ZnSSe single crystals grown by CVT method P. Kannappan, R. Dhanasekaran n Crystal Growth Centre, Anna University, Chennai 600025, India
art ic l e i nf o
Keywords: A1. Optical properties A1. Surface morphology A1. X-ray diffraction A2. Growth from vapour A2. Single crystal growth B1. Semiconducting II–VI materials
a b s t r a c t The ZnSe and ZnSSe single crystals have been grown by chemical vapour transport (CVT) with iodine as a transporting agent. The source and growth temperatures of the reactor are maintained at 850 1C and 900 1C respectively using the two zone horizontal resistive heating furnace. The grown crystals are characterised by structural, surface morphological and optical properties. The XRD pattern shows that the peaks due to reflection from (111), (220) and (311) planes belong to the cubic zinc-blend structure. The composition of grown ZnSe crystal has been determined by EDXRF analysis as Zn (52.5%) and Se (47.5%). The XPS study of grown ZnSe single crystal has been studied. The HRSEM images reveal the step, spiral and triangular patterns. The optical absorption study reveals the absorption cut off wavelength as 483 nm (2.57 eV) for ZnSe and 441 nm (2.81 eV) for ZnSSe crystals. The photoluminescence studies reveal the band edge emission and defect level emissions of ZnSe and ZnSSe single crystals. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been a great deal of interest in wide band gap II–VI compound semiconductors [1]. In particular, ZnSe is a wide and direct transition type compound semiconductor with band gap energy of 2.67 eV at room temperature. The ZnSe based II–VI semiconductors have been extensively used for the fabrication of blue-light emitting diodes (LEDs), laser diodes (LDs) and non-linear optoelectronic devices and so on [2]. These applications seriously depend on the preparation of high quality single crystals and great efforts have been devoted to grow ZnSe single crystals [3]. ZnSxSe1 x is a solid solution of two binary compound semiconductors of ZnS (3.67 eV) and ZnSe (2.67), where the band gap energy changes continuously with the composition x [4]. The ZnSSe makes it suitable for light emitting diodes and laser diodes in the visible region [5,6]. The band gap tuning of ZnSSe is the process of altering the band gap by controlling the composition of ZnS and ZnSe. ZnSSe is an advantage over ZnSe due to the presence of sulphur that widens the band gap, which leads to an increase in the blue response of the device [7]. Therefore, the variation of band gap energy with the composition makes ZnSSe a highly suitable and promising material for wavelength tunable UV photodetector, visible laser diodes (LDs), light emitting diodes (LEDs) and light emitters [8].
n
Corresponding author. Tel.: þ 91 44 2235 8317. E-mail addresses: [email protected], [email protected] (R. Dhanasekaran).
Bulk crystals of ZnSe and ZnSSe are grown by different methods, such as vertical Bridgman method (VB) [9], solid-phase recrystallisation (SPR) [10], physical vapour transport (PVT) [11], chemical vapour transport (CVT), etc. [12]. Among these methods, chemical vapour transport (CVT) method can be designed to grow ZnSe single crystals at comparatively low temperatures (below 1000 1C) [13]. The chemical vapour transport (CVT) system with a chemical transport agent affords possibilities to grow ZnSe and ZnSSe single crystals. In the CVT method, the gaseous molecules like iodine, hydrogen chloride, hydrogen and ammonium chloride can be used as the transporting agent [2]. Among these transporting chemical agents iodine was proved thermodynamically favourable for the growth of ZnSe and ZnSSe single crystals [14]. In the present investigation, the ZnSe and ZnSSe single crystals were grown by chemical vapour transport with iodine as a transporting agent using two zone horizontal resistive heating furnace. The grown crystals were studied for their structural, compositional, surface morphological and optical properties. 2. Experimental procedure 2.1. Synthesis of ZnSe and ZnSSe polycrystalline samples ZnSe and ZnSSe polycrystalline samples have been synthesised in a silica glass ampoule of length 10 cm and diameter 1.4 cm. Before loading the starting materials, the ampoules were cleaned with acetone and then rinsed with double distilled water three times. 40% Hydrofluoric (HF) acid was poured to clean the ampoule. The nitric acid (HNO3) was poured and cleaned well two to three
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times with millipore water. They were then dried and kept in a furnace maintained at the temperature of 100 1C for 24 h. The 99.9% purity Zn and Se were taken in the stoichiometric 1:1 ratio for the synthesis of ZnSe and 1:0.5:0.5 of Zn, S and Se for the synthesis of ZnSSe in a silica glass ampoule. The mixture was heat treated at 1100 1C at the pressure of 1 10 5 mbar for about 24 h. The synthesised polycrystalline samples were used for the growth of single crystals.
resistive heating furnace connected with the Eurotherm temperature controllers with the accuracy of 7 0.1 1C. A reverse temperature profile was maintained across the ampoule with growth zone
2.2. Growth of ZnSe and ZnSSe single crystals The following chemical reactions take place at the source and growth zone regions during the growth of ZnSe and ZnSSe single crystals by CVT method: ZnSeðsÞ þ 2IðsÞ ) ZnI2ðgÞ þ 0:5Se2ðgÞ ) ZnSeðcÞ þ 0:5I2ðsÞ Source zoneð9001CÞ
Growth zoneð8501CÞ
ZnSSeðsÞ þ 2IðsÞ ) ZnI2ðgÞ þ 0:5S2ðgÞ þ 0:5Se2ðgÞ ) ZnSSeðcÞ þ 0:5I2ðsÞ About 3 g of polycrystalline ZnSe sample was loaded in a silica glass ampoule of length 15 cm with 1.4 cm diameter along with 2 mg/cm3 of iodine as the transporting agent. A pressure of 1 10 5 mbar was achieved and the ampoule was vacuum sealed. The sealed ampoule was placed inside a two zone horizontal
Fig. 2. Powder XRD patterns of grown (a) ZnSe and (b) ZnSSe crystals.
Fig. 1. (a and b) As grown, and (d and e) polished ZnSe single crystals and (c) ZnSSe single crystal using the source temperature at 900 1C and growth temperature at 850 1C.
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at a higher temperature for 24 h to remove the sticking powders from the growth zone of the ampoule and to diminish the active sites. During the growth, the temperature difference between the two zones was maintained at 50 1C with the source temperature of 900 1C and the growth temperature of 850 1C. The growth period was about 15 days for the ZnSe and 10 days for ZnSSe single crystals. Then, the furnace was cooled to room temperature at the rate of 30 1C/h. The grown crystals are of different dimensions and morphology. The as grown ZnSe and ZnSSe single crystals are shown in Fig. 1.
3. Results and discussion 3.1. X-ray diffraction (XRD) studies The as grown ZnSe and ZnSSe crystals have been studied by X-ray diffraction using BRUKER D8 Advance X-ray diffractometer. Fig. 2(a) and (b) shows the powder XRD patterns of as grown ZnSe and ZnSSe crystals respectively. The diffraction peaks corresponding to (111), (220) and (311) planes confirm the formation of ZnSe and ZnSSe crystalline phase. The ZnSe and ZnSSe crystals belong to cubic zinc blend structure. The calculated unit cell parameter values for ZnSe are a¼ b¼c ¼5.642 Å; and for ZnSSe, the values are a ¼b¼ c¼5.567 Å. The reported values of lattice parameters for the ZnSe and ZnSSe single crystals are 5.660 Å and 5.550 Å respectively [15,16]. The XRD pattern of the ZnSSe crystal shows a slight peak shift towards the higher diffraction angle due to the addition of S in ZnSe which results in decrease of the lattice parameter. The interplanar distances of ZnSSe crystal decrease for all the XRD planes. The high intensity of XRD pattern clearly shows the good crystalline nature of ZnSe and ZnSSe crystals. The calculated interplanar distances (dhkl) and lattice parameter (a) values are tabulated in Table 1.
Fig. 3. EDXRF studies of ZnSe single crystal.
3.2. EDXRF studies Fig. 3 shows the energy dispersive X-ray fluorescence (EDXRF) spectrum of ZnSe single crystal. The EDXRF measurements were carried in fluorescence mode using Si detector array and X-ray energy source ranges from 1504.7 eV to 12538.9 eV. The chemical compositions were obtained from the X-ray fluorescence spectra measured at the energy of 8675.3 eV and 11253.2 eV of Zn and Se elements respectively. The Zn and Se elements present in the crystal are 52.5% and 47.5% respectively. The slight changes in the composition of ZnSe crystal is due to the high vapour pressure at the growth interface of chemical transport reactions inside the CVT reactor [16]. The grown ZnSSe crystal has the composition of Zn (43.56%), S (29.86%) and Se (26.58%), which has already been reported [17].
Fig. 4. XPS studies of ZnSe single crystal.
constituent elements in ZnSe single crystal was identified with reference to C 1s at 284.60 eV. The BE of Zn and Se elements in 3d states are 10 eV and 55.5 eV respectively. The BE values of 140 eV and 163 eV are corresponding to the Zn 3s and Se 3p states in the ZnSe single crystal. The higher BE values at 1022.5 eV and 1045.5 eV are corresponding to the sublevels of 2p3/2 and 2p1/2 of the Zn element. The binding energies at 530 eV (LMM), 495 eV (LMM) and 180 eV (LMM) reveal that the auger electrons ejected from the Zn and Se atoms are due to the function of the X-ray source [18].
3.3. XPS studies 3.4. Surface morphological studies Fig. 4 shows the X-ray photoelectron spectroscopy (XPS) studies of ZnSe single crystal and provides information about the oxidation states of Zn and Se elements. The binding energy (BE) of Table 1 Calculated interplanar distances (d(hkl)) and lattice parameter values (a) of ZnSe and ZnSSe crystals. Samples
ZnSe ZnSSe
Interplanar distance (Å)
Lattice parameter
d(111)
d(220)
d(311)
d(400)
d(331)
a (Å)
3.2481 3.1934
1.9937 1.9600
1.7018 1.6682
1.4131 1.3845
1.2963 1.2701
5.642 5.567
Fig. 5(a–f) shows the high resolution scanning electron microscopy (HR-SEM) images of different types of morphological patterns on the surface of grown ZnSe and ZnSSe single crystals. The surface morphology of CVT grown single crystals strongly depends on the experimental variables such as supersaturation of vapour, temperature difference between the source and growth zones, concentration of the transporting agent, heat and mass transport processes, kinetics of chemical reaction, vapour–solid interface and the dimensions of the CVT reactor [13]. The different growth runs were carried out for the temperature difference between the source (900 1C) and growth (850 1C) zones of 50 1C using the same growth parameters and composition of the starting materials. The
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Fig. 5. HRSEM images (a) triangular, (b) spiral and (c and d) step growth pattern of ZnSe single crystals; (e and f) layers in the form of step growth pattern of ZnSSe crystals.
as grown ZnSe crystals at the end as well as at approximately middle part of the quartz ampoule are shown in Fig. 1(a). The HRSEM studies have been done on the ZnSe single crystals grown at the end and at approximately middle part of the ampoule. The temperature starts to decrease from source to growth zone with respect to distance in the CVT reactor. The surface morphology of grown ZnSe single crystals shows the step, spiral and triangular pattern whereas the ZnSSe single crystal shows the layer pattern in the form of step growth morphology. Fig. 5(a) and (b) shows the triangular and spiral growth morphology which represent the crystals grown at the end of the ampoule with different experimental runs where the temperature difference is maintained at 50 1C from the source to growth zones. The formation of triangular pattern is due to rough surface produced as a result of higher temperature difference (50 1C) [15]. The formation of spiral growth pattern is due to the variation of temperature at the vapour-solid interface in the CVT reactor. Fig. 5(c) and (d) shows the formation of step growth morphology which represents the crystals grown at approximately
Fig. 6. Optical absorption spectra of grown (a) ZnSe and (b) ZnSSe single crystals.
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the middle part of the ampoule, where the lower temperature difference ( 30 1C) is approximately maintained. The formation of step growth pattern may be due to the formation of defects during ZnSe crystal growth. The step growth pattern of grown crystals reveal the two dimensional nucleation mechanism. Fig. 5(e) and (f) shows the layers in the bunch steps with regular morphologies of ZnSSe single crystals. The layer pattern in the form of step growth morphology of ZnSSe represents the crystals grown at the end of the ampoule where the temperature difference is maintained at 50 1C from the source to growth zone of the ampoule. 3.5. Optical absorption studies Fig. 6(a) and (b) shows the optical absorption spectrum of grown ZnSe and ZnSSe single crystals respectively using CARY 5E UV–vis– NIR spectrophotometer. The optical absorption study reveals the cut off wavelength as 483 nm for ZnSe and 441 nm for ZnSSe single crystals. The band gap energy of grown ZnSe and ZnSSe crystals were calculated to be 2.57 eV and 2.81 eV respectively. The band gap energy of ZnSSe depends on the composition of the sulphur atom present in the crystals. The reported value for the band gap energy of ZnS0.6Se0.4 is 2.85 eV [4]. 3.6. Photoluminescence studies Fig. 7 shows the photoluminescence (PL) spectrum of ZnSe single crystals. The PL spectrum was recorded using LS 55 luminescence spectrometer at room temperature. The samples were excited by using UV laser at 244 nm. The emission peaks for ZnSe and ZnSe single crystals are in the blue, yellow and red regions. The PL spectrum of grown ZnSe single crystal shows the peaks at 420 nm (2.95 eV), 440 nm (2.81 eV), 574 nm (2.16 eV), 634 nm (1.95 eV) and 745 nm (1.66 eV). The emissions at 420 (2.95 eV) and 440 nm (2.81 eV) are attributed to the near band edge emission (NBE) of ZnSe single crystals [19]. The emission at 574 nm and 745 nm are attributed to the defect level emission [20]. Fig. 8 shows the PL spectrum of ZnSSe single crystal. The PL emissions at 353 nm, 420–457 nm and broad range 500–700 nm were observed. The emissions at 353 nm (3.51 eV) and 420– 457 nm (2.95–2.71 eV) are attributed to the near band edge emission (NBE) of ZnSSe single crystal. The near band emission is generally known as donor–acceptor pair (DAP) emission whereas the deep level emission is due to crystal defects such as vacancies, interstitials and dislocations in the crystals [21]. The PL spectrum of ZnSSe is dominated by broad defect level emission centred at 565–595 nm (2.19–2.08 eV) in the yellow-orange region [22].
Fig. 8. Photoluminescence spectrum of ZnSSe single crystal.
The intensity of defect level emission in ZnSSe is higher than the near band edge (NBE) emission. The red emission at 657 nm (1.88 eV) is due to the defect level. The decrease of intensity of near band edge emission in ZnSSe reveals that the band edge recombination is quenched by the defects [23].
4. Conclusions The ZnSe and ZnSSe single crystals were grown by chemical vapour transport (CVT) method with iodine as a transporting agent using two zone horizontal resistive heating furnace. The powder XRD patterns of grown crystals reveal the cubic zinc-blend structure. The compositional variation of ZnSe and ZnSSe is due to the difference in the vapour pressure of molecules inside the CVT reactor during crystal growth. The oxidation state of Zn and Se atoms in ZnSe single crystal has been identified by XPS studies. The HRSEM study reveals the triangular, spiral and step growth patterns on the surface of grown ZnSe and ZnSSe single crystals. The optical absorption cut off wavelength and band gap energy of ZnSe and ZnSSe single crystals were calculated. The photoluminescence studies of ZnSe and ZnSSe single crystals revealed that the near band edge (NBE) emission in blue and defect level emission in the yellow and red regions.
Acknowledgements The authors gratefully acknowledge UGC-DAE Consortium for Scientific Research, Kolkata, India for providing financial support. The authors are thankful to Dr. S. Soundiraraj and Dr. A.R.S. Jayanth, Department of English, Anna University for their support in correcting the manuscript. References
Fig. 7. Photoluminescence spectrum of ZnSe single crystal.
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