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Low temperature Raman study on the site symmetry of ZnSe:P
Solid State Communications 140 (2006) 1–3 www.elsevier.com/locate/ssc
Low temperature Raman study on the site symmetry of ZnSe:P A.K. Balasubramanian a,∗ , T.R. Ravindran b , R. Kesavamoorthy b , K. Ramachandran c a Department of Physics, Sourashtra College, Madurai-625 004, India b Material Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India c School of Physics, Madurai Kamaraj University, Madurai-625 021, India
Received 7 May 2006; received in revised form 21 July 2006; accepted 29 July 2006 by R. Merlin Available online 15 August 2006
Abstract Raman spectra of phosphorous doped ZnSe are recorded from 9 to 300 K. In addition to TO mode at 208 cm−1 and LO mode at 253 cm−1 , a new mode around 240 cm−1 is observed between 55 and 270 K. The spectra at lower and higher temperatures do not show the new mode. This new mode confirms that there is a reduction of (Se site) symmetry from Td to C3v when P substitutes for Se. This is due to Jahn Teller distortion. c 2006 Elsevier Ltd. All rights reserved.
PACS: 81.10.Bk; 78.30.-j; 81.05.Dz Keywords: A. ZnSe:P; D. Physical vapour transport (PVT); E. Raman spectra
1. Introduction It is known that II–VI compounds when doped with phosphorous will have molecular symmetry Td as these systems have tetrahedral bonding. But as early as 1971, Watts et al. [1] proposed an interesting site symmetry for Se in ZnSe, a II–VI compound semiconductor. When ZnSe is doped with phosphorous, at fairly low temperatures, this phosphorous is found to form a deep acceptor at about 0.7 eV and has a site symmetry of C3v . Since this is unusual as only Td symmetry is expected, they analysed the electron spin resonance (ESR) spectrum of ZnSe:P at 1.3 K and found a C3v symmetry. The ESR lines broaden and disappear as the temperature is increased and no resonance was observed above 10 K. When ZnSe is doped with phosphorous, the P atoms move ˚ towards one of the four neighbouring Se atoms. about 0.2 A This distorts the molecular symmetry of ZnSe and reduces it from Td to C3v symmetry. Since any additional impurity or defect will introduce such deformation leading to Jahn–Teller (JT) distortion, JT is expected for ZnSe:P. Nicholls et al. [2] performed optically detected magnetic resonance (ODMR) experiments on ZnSe crystal doped with phosphorous and ∗ Corresponding address: Department of Physics, Sourashtra College, Plot No 17, Sambandar Street (end), 625 003 Madurai, Tamil Nadu, India. E-mail address: [email protected] (A.K. Balasubramanian).
c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.037
gallium at a temperature of 2 K. They analysed the ODMR spectrum and concluded that the intense 1.91 eV red emission is due to the recombination between shallow donors and deep acceptors in ZnSe. Since Ga ions are presumed to form the donor centres, phosphorous ions substituted at selenium sites are shown to be isolated deep acceptor centres in ZnSe at about 0.7 eV from their ODMR results. Thus, Watts et al. [1] state that with phosphorous, C3v symmetry is possible whereas Nicholls et al. [2] conclude that this C3v is found when both Ga and P are present. Though Watts et al. [1] and Nicholls et al. [2] from their ESR and ODMR results confirm the deep acceptor PSe centre, Nakano et al. [3] carried out a Raman study on ZnSe crystal doped with phosphorous and gallium to observe localized vibrational modes, as these modes are direct evidence for the site symmetry. They varied the temperature of the sample from 10 to 200 K and the Raman spectra at 10 and 77 K were analysed. Two weak modes were seen at 220 cm−1 and 375 cm−1 at 10 K along with strong TO (210 cm−1 ) and LO (255 cm−1 ) phonon modes corresponding to the host ZnSe lattice. The defect mode at 375 cm−1 corresponds to the T2 vibrational mode of Td symmetry. This is identified as being phosphorous substituted at the selenium site. The other mode at 220 cm−1 corresponds to the A1 mode and was presumed to belong to either the C3v or the Td symmetry. Sankar et al. [4] made an attempt to observe the same in a
2
A.K. Balasubramanian et al. / Solid State Communications 140 (2006) 1–3
pure ZnSe single crystal and phosphorous doped ZnSe crystal. The mass spectra of the ZnSe:P crystal do not show any trace of Ga [4]. They made IR, Raman and photoacoustic (PA) measurements at 300 K and in all the above experiments observed the defect mode at 220 cm−1 which is ascribed to JT distortion [4]. In their lattice dynamical calculations based on the molecular model and Green’s function, they confirmed the C3v symmetry with ZnSe:P when interstitial P site is considered. This reduction of symmetry from Td to C3v might be due to JT distortion. But they performed the experiment and calculations only at room temperature. They showed from their calculations that ZnSe:P can have C3v symmetry provided JT is taken into account. Kwak et al. [5, 6] have also carried out theoretical investigations on structural and electronic properties of substitutional phosphorous in ZnSe using pseudo potential calculations. They found that phosphorous forms shallow acceptors in their neutral states and the symmetry is reduced from Td to C3v by JT distortion. Since P is an amphoteric impurity, the substitutional possibility is also considered at room temperature. The mode 220 cm−1 appeared in the A1 representation of C3v symmetry due to JT distortion. Therefore, a phosphorous substituting selenium site may not be possible at room temperature. Similarly, Davies et al. [7], from their spin flip Raman spectroscopic experiments, confirm that phosphorous would act as a shallow acceptor in ZnSe forming a trigonal symmetry. However, a complete study of the site symmetry is possible only when it is analysed in the low-temperature region. At low temperatures, the hole is trapped on the acceptor and hence the study of a reduction of site symmetry at low temperature due to JT can be easily visualized. 2. Experimental details ZnSe single crystal doped with phosphorus was grown by physical vapour transport (PVT) [8]. Here the silica tube was kept stationary in the furnace without any disturbance. No transporting agent was used in order to obtain the pure ZnSe single crystal. Though the size of the single crystal grown by this technique was small compared to those obtained from the melt method, it showed relatively high crystallinity. The pure ZnSe single crystal thus formed was then introduced at one end of the ampoule in the furnace. With the phosphorous at the other end, the vapours of both ZnSe and P were mixed at 1100 ◦ C. Then the mixture of the constituent vapours was cooled to obtain ZnSe:P single crystals. The single crystalline nature of the as-grown crystal ZnSe:P was confirmed by XRD [8]. Wafers of thickness of about 1 mm were sliced from the grown samples and polished with 10% bromine in ethyl alcohol for approximately 2 min. The crystals were cleaved along the (110) face and used for Raman measurements. Raman measurements from 9 to 300 K were carried out using an APD closed-cycle refrigerator. Temperature was controlled by a Lakeshore Model 330 Autotuning temperature controller employing separate silicon diode thermometers for control and sample temperature measurement. Raman spectra were recorded in the backscattering geometry using the 488 nm line of an argon ion laser. A 20 mW laser beam was focused to
Fig. 1. Raman spectra of ZnSe:P at various temperatures. Symbols are experimental data, thin solid lines are the fit to the data. Thick solid lines are additional Raman mode due to Jahn–Teller distortion, dashed lines are LO and TO modes.
approximately 50 µm diameter spot on the sample. Scattered light from the sample was analysed by a SPEX double monochromator and detected with a cooled HAMAMATSU photomultiplier tube operated in the photon counting mode. Scanning of the spectra and data acquisition were carried out using a home-built microprocessor-based data acquisition-cumcontrol system. 3. Results and discussion Raman spectra of ZnSe:P were recorded in the range 150–300 cm−1 at various temperatures from 9 to 300 K and analysed. Fig. 1 shows the Raman spectra at six temperatures. TO and LO modes of ZnSe [9] are clearly seen in these figures at about 210 cm−1 and 255 cm−1 , respectively. Up to 30 K, only these modes were seen. In addition to TO and LO modes, a broad and weak mode at about 235 cm−1 appeared at 55 K. This mode became more intense and blue-shifted to 245 cm−1 as the temperature was increased. With a further increase of temperature, it again became weaker, red-shifted and disappeared at 300 K. Table 1 lists the peak frequency, relative intensity and full width at half maximum (FWHM) of the new Raman mode at various temperatures. However, lattice dynamical calculations of Sankar et al. [4] for interstitial P in ZnSe:P predicted a 220 cm−1 mode at 300 K due to JT distortion leading to a reduction of interstitial site symmetry from Td to C3v . We did not observe any mode around 220 cm−1 at 300 K. Hence the additional mode observed by us
3
A.K. Balasubramanian et al. / Solid State Communications 140 (2006) 1–3 Table 1 The new mode at various temperatures in ZnSe:P Temperature (K)
Peak position (cm−1 )
Area (arb. units)
FWHM (cm−1 )
9 30 55 90 110 130 200 233 270 300
– – 235 (2) 245 (3) 242 (4) 236 (2) 242 (3) 238 (3) 236 (3) –
– – 134 229 146 148 437 276 63 –
– – 20 20 18 17 25 23 17 –
from 55 to 270 K at 235 cm−1 might not be due to interstitial P in ZnSe:P. Nakano et al. [3] observed additional modes at 220 and 375 cm−1 at 10 K owing to P substituting Se in ZnSe:P, Ga. Among these two defect modes, the 375 cm−1 mode has been assigned to the T2 vibrational mode of Td symmetry, while the 220 cm−1 mode has been assigned to JT distortion. It may be appropriate to assign the present observation of 235 cm−1 mode to JT distortion in our sample. In accord with the prediction of Nakano et al. [3] that the 220 cm−1 mode at 10 K was due to substitutional P at Se site, we propose that the 235 cm−1 in ZnSe:P is due to substitutional P at Se site. JT distortion at this site is prevalent only in a narrow temperature range of 55 to 270 K. The observation of this mode in a different temperature range (from 10 to 77 K) by Nakano et al. [3] might be as a result of the presence of Ga in their sample and the purity of the sample. It is therefore concluded from the present measurements that C3v symmetry in ZnSe:P cannot be ruled out, when P acts as a substitutional defect.
Acknowledgement One of the authors (AKB) thanks Dr. N. Sankar of Yadava College, Madurai for providing the sample. References [1] R.K. Watts, W.C. Holton, M. de Witt, Phys. Rev. B 3 (1971) 404. [2] J.E. Nicholls, J.J. Davies, J. Phys. C (Solid State Phys.) 12 (1979) 1917. [3] K. Nakano, P.J. Boyce, J.J. Davies, D. Wolverson, J. Cryst. Growth 117 (1992) 331. [4] N. Sankar, K. Ramachandran, Bull. Mater. Sci. 25 (2002) 329–334. [5] K.W. Kwak, D. Vanderbilt, R.D. King Smith, Phys. Rev. B 48 (1993) 17827. [6] K.W. Kwak, D. Vanderbilt, R.D. King Smith, Phys. Rev. B 50 (1994) 2711. [7] J.J. Davies et al., Phys. Rev. B 64 (2001) 205–206. [8] N. Sankar, K. Ramachandran, C. Sanjeeviraja, J. Cryst. Growth 235 (2002) 195. [9] W. Taylor, Phys. Lett. A 24 (1967) 556.
Low temperature Raman study on the site symmetry of ZnSe:P A.K. Balasubramanian a,∗ , T.R. Ravindran b , R. Kesavamoorthy b , K. Ramachandran c a Department of Physics, Sourashtra College, Madurai-625 004, India b Material Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India c School of Physics, Madurai Kamaraj University, Madurai-625 021, India
Received 7 May 2006; received in revised form 21 July 2006; accepted 29 July 2006 by R. Merlin Available online 15 August 2006
Abstract Raman spectra of phosphorous doped ZnSe are recorded from 9 to 300 K. In addition to TO mode at 208 cm−1 and LO mode at 253 cm−1 , a new mode around 240 cm−1 is observed between 55 and 270 K. The spectra at lower and higher temperatures do not show the new mode. This new mode confirms that there is a reduction of (Se site) symmetry from Td to C3v when P substitutes for Se. This is due to Jahn Teller distortion. c 2006 Elsevier Ltd. All rights reserved.
PACS: 81.10.Bk; 78.30.-j; 81.05.Dz Keywords: A. ZnSe:P; D. Physical vapour transport (PVT); E. Raman spectra
1. Introduction It is known that II–VI compounds when doped with phosphorous will have molecular symmetry Td as these systems have tetrahedral bonding. But as early as 1971, Watts et al. [1] proposed an interesting site symmetry for Se in ZnSe, a II–VI compound semiconductor. When ZnSe is doped with phosphorous, at fairly low temperatures, this phosphorous is found to form a deep acceptor at about 0.7 eV and has a site symmetry of C3v . Since this is unusual as only Td symmetry is expected, they analysed the electron spin resonance (ESR) spectrum of ZnSe:P at 1.3 K and found a C3v symmetry. The ESR lines broaden and disappear as the temperature is increased and no resonance was observed above 10 K. When ZnSe is doped with phosphorous, the P atoms move ˚ towards one of the four neighbouring Se atoms. about 0.2 A This distorts the molecular symmetry of ZnSe and reduces it from Td to C3v symmetry. Since any additional impurity or defect will introduce such deformation leading to Jahn–Teller (JT) distortion, JT is expected for ZnSe:P. Nicholls et al. [2] performed optically detected magnetic resonance (ODMR) experiments on ZnSe crystal doped with phosphorous and ∗ Corresponding address: Department of Physics, Sourashtra College, Plot No 17, Sambandar Street (end), 625 003 Madurai, Tamil Nadu, India. E-mail address: [email protected] (A.K. Balasubramanian).
c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.037
gallium at a temperature of 2 K. They analysed the ODMR spectrum and concluded that the intense 1.91 eV red emission is due to the recombination between shallow donors and deep acceptors in ZnSe. Since Ga ions are presumed to form the donor centres, phosphorous ions substituted at selenium sites are shown to be isolated deep acceptor centres in ZnSe at about 0.7 eV from their ODMR results. Thus, Watts et al. [1] state that with phosphorous, C3v symmetry is possible whereas Nicholls et al. [2] conclude that this C3v is found when both Ga and P are present. Though Watts et al. [1] and Nicholls et al. [2] from their ESR and ODMR results confirm the deep acceptor PSe centre, Nakano et al. [3] carried out a Raman study on ZnSe crystal doped with phosphorous and gallium to observe localized vibrational modes, as these modes are direct evidence for the site symmetry. They varied the temperature of the sample from 10 to 200 K and the Raman spectra at 10 and 77 K were analysed. Two weak modes were seen at 220 cm−1 and 375 cm−1 at 10 K along with strong TO (210 cm−1 ) and LO (255 cm−1 ) phonon modes corresponding to the host ZnSe lattice. The defect mode at 375 cm−1 corresponds to the T2 vibrational mode of Td symmetry. This is identified as being phosphorous substituted at the selenium site. The other mode at 220 cm−1 corresponds to the A1 mode and was presumed to belong to either the C3v or the Td symmetry. Sankar et al. [4] made an attempt to observe the same in a
2
A.K. Balasubramanian et al. / Solid State Communications 140 (2006) 1–3
pure ZnSe single crystal and phosphorous doped ZnSe crystal. The mass spectra of the ZnSe:P crystal do not show any trace of Ga [4]. They made IR, Raman and photoacoustic (PA) measurements at 300 K and in all the above experiments observed the defect mode at 220 cm−1 which is ascribed to JT distortion [4]. In their lattice dynamical calculations based on the molecular model and Green’s function, they confirmed the C3v symmetry with ZnSe:P when interstitial P site is considered. This reduction of symmetry from Td to C3v might be due to JT distortion. But they performed the experiment and calculations only at room temperature. They showed from their calculations that ZnSe:P can have C3v symmetry provided JT is taken into account. Kwak et al. [5, 6] have also carried out theoretical investigations on structural and electronic properties of substitutional phosphorous in ZnSe using pseudo potential calculations. They found that phosphorous forms shallow acceptors in their neutral states and the symmetry is reduced from Td to C3v by JT distortion. Since P is an amphoteric impurity, the substitutional possibility is also considered at room temperature. The mode 220 cm−1 appeared in the A1 representation of C3v symmetry due to JT distortion. Therefore, a phosphorous substituting selenium site may not be possible at room temperature. Similarly, Davies et al. [7], from their spin flip Raman spectroscopic experiments, confirm that phosphorous would act as a shallow acceptor in ZnSe forming a trigonal symmetry. However, a complete study of the site symmetry is possible only when it is analysed in the low-temperature region. At low temperatures, the hole is trapped on the acceptor and hence the study of a reduction of site symmetry at low temperature due to JT can be easily visualized. 2. Experimental details ZnSe single crystal doped with phosphorus was grown by physical vapour transport (PVT) [8]. Here the silica tube was kept stationary in the furnace without any disturbance. No transporting agent was used in order to obtain the pure ZnSe single crystal. Though the size of the single crystal grown by this technique was small compared to those obtained from the melt method, it showed relatively high crystallinity. The pure ZnSe single crystal thus formed was then introduced at one end of the ampoule in the furnace. With the phosphorous at the other end, the vapours of both ZnSe and P were mixed at 1100 ◦ C. Then the mixture of the constituent vapours was cooled to obtain ZnSe:P single crystals. The single crystalline nature of the as-grown crystal ZnSe:P was confirmed by XRD [8]. Wafers of thickness of about 1 mm were sliced from the grown samples and polished with 10% bromine in ethyl alcohol for approximately 2 min. The crystals were cleaved along the (110) face and used for Raman measurements. Raman measurements from 9 to 300 K were carried out using an APD closed-cycle refrigerator. Temperature was controlled by a Lakeshore Model 330 Autotuning temperature controller employing separate silicon diode thermometers for control and sample temperature measurement. Raman spectra were recorded in the backscattering geometry using the 488 nm line of an argon ion laser. A 20 mW laser beam was focused to
Fig. 1. Raman spectra of ZnSe:P at various temperatures. Symbols are experimental data, thin solid lines are the fit to the data. Thick solid lines are additional Raman mode due to Jahn–Teller distortion, dashed lines are LO and TO modes.
approximately 50 µm diameter spot on the sample. Scattered light from the sample was analysed by a SPEX double monochromator and detected with a cooled HAMAMATSU photomultiplier tube operated in the photon counting mode. Scanning of the spectra and data acquisition were carried out using a home-built microprocessor-based data acquisition-cumcontrol system. 3. Results and discussion Raman spectra of ZnSe:P were recorded in the range 150–300 cm−1 at various temperatures from 9 to 300 K and analysed. Fig. 1 shows the Raman spectra at six temperatures. TO and LO modes of ZnSe [9] are clearly seen in these figures at about 210 cm−1 and 255 cm−1 , respectively. Up to 30 K, only these modes were seen. In addition to TO and LO modes, a broad and weak mode at about 235 cm−1 appeared at 55 K. This mode became more intense and blue-shifted to 245 cm−1 as the temperature was increased. With a further increase of temperature, it again became weaker, red-shifted and disappeared at 300 K. Table 1 lists the peak frequency, relative intensity and full width at half maximum (FWHM) of the new Raman mode at various temperatures. However, lattice dynamical calculations of Sankar et al. [4] for interstitial P in ZnSe:P predicted a 220 cm−1 mode at 300 K due to JT distortion leading to a reduction of interstitial site symmetry from Td to C3v . We did not observe any mode around 220 cm−1 at 300 K. Hence the additional mode observed by us
3
A.K. Balasubramanian et al. / Solid State Communications 140 (2006) 1–3 Table 1 The new mode at various temperatures in ZnSe:P Temperature (K)
Peak position (cm−1 )
Area (arb. units)
FWHM (cm−1 )
9 30 55 90 110 130 200 233 270 300
– – 235 (2) 245 (3) 242 (4) 236 (2) 242 (3) 238 (3) 236 (3) –
– – 134 229 146 148 437 276 63 –
– – 20 20 18 17 25 23 17 –
from 55 to 270 K at 235 cm−1 might not be due to interstitial P in ZnSe:P. Nakano et al. [3] observed additional modes at 220 and 375 cm−1 at 10 K owing to P substituting Se in ZnSe:P, Ga. Among these two defect modes, the 375 cm−1 mode has been assigned to the T2 vibrational mode of Td symmetry, while the 220 cm−1 mode has been assigned to JT distortion. It may be appropriate to assign the present observation of 235 cm−1 mode to JT distortion in our sample. In accord with the prediction of Nakano et al. [3] that the 220 cm−1 mode at 10 K was due to substitutional P at Se site, we propose that the 235 cm−1 in ZnSe:P is due to substitutional P at Se site. JT distortion at this site is prevalent only in a narrow temperature range of 55 to 270 K. The observation of this mode in a different temperature range (from 10 to 77 K) by Nakano et al. [3] might be as a result of the presence of Ga in their sample and the purity of the sample. It is therefore concluded from the present measurements that C3v symmetry in ZnSe:P cannot be ruled out, when P acts as a substitutional defect.
Acknowledgement One of the authors (AKB) thanks Dr. N. Sankar of Yadava College, Madurai for providing the sample. References [1] R.K. Watts, W.C. Holton, M. de Witt, Phys. Rev. B 3 (1971) 404. [2] J.E. Nicholls, J.J. Davies, J. Phys. C (Solid State Phys.) 12 (1979) 1917. [3] K. Nakano, P.J. Boyce, J.J. Davies, D. Wolverson, J. Cryst. Growth 117 (1992) 331. [4] N. Sankar, K. Ramachandran, Bull. Mater. Sci. 25 (2002) 329–334. [5] K.W. Kwak, D. Vanderbilt, R.D. King Smith, Phys. Rev. B 48 (1993) 17827. [6] K.W. Kwak, D. Vanderbilt, R.D. King Smith, Phys. Rev. B 50 (1994) 2711. [7] J.J. Davies et al., Phys. Rev. B 64 (2001) 205–206. [8] N. Sankar, K. Ramachandran, C. Sanjeeviraja, J. Cryst. Growth 235 (2002) 195. [9] W. Taylor, Phys. Lett. A 24 (1967) 556.