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Optical activity in []ZnGa2S4
Materials Letters 61 (2007) 1926 – 1928 www.elsevier.com/locate/matlet
Optical activity in []ZnGa2S4 N.V. Joshi a,⁎, Jorge Luengo b , Fatima Vera b a
b
Departmento de Fisiologia, Universidad de Los Andes, Merida, Venezuela Centro de Optica, Facultad de Ciencias, Universidad de Los Andes, Merida, Venezuela Received 24 February 2006; accepted 27 July 2006 Available online 18 August 2006
Abstract Recently, it has been established that dangling bonds in ordered vacancy compounds play a crucial role in optical activity. With this view a spectroscopic investigation of []ZnGaS4, where [] stands for ordered vacancy, has been carried out at 10 K. The optical absorption spectra were recorded under polarized radiation. Absorption maxima (in perpendicular direction of polarization absorption minima) were obtained at 2.51 and 1.9 eV and they were associated with the transitions from A1 state originated from the vacancy (in D2d symmetry) to the mid-band gap states of the ordered vacancy compounds. These transitions are optically active because the bond bending near the vacancy creates a situation where the component of the magnetic dipole moment is parallel to the electric dipole moment and hence their scalar product is non-zero. © 2006 Elsevier B.V. All rights reserved.
1. Introduction AIIBIII2CVI4 are wide band gap semiconductors [1] and posses several interesting optical properties such as high photosensitivity [2], strong photoluminescence [3] and optical activity [4,5]. The last property has been detected in ordered vacancy compound [] CdGa2S4 ([] stands for ordered vacancy) pure and doped with manganese [4,6]. Its alloy with indium also exhibits optical activity [5] and therefore it is interesting to inspect if optical activity exists in a similar compound []ZnGa2S4. This is very important because this material is transparent in the visible region and it does not have abundant mid-gap defect states or localized defect states which participate in the absorption process. Those states, which are present, might contribute in chiral absorption. In comparison with the other semiconducting compounds of AIIBIII2CVI4 family, []ZnGa2S4 has been little studied particularly its optical properties. It is known that this is an ordered vacancy compound [7] like []CdGa2S4. Because of the considerable similarities between them, optical properties are expected to be similar. However, a detailed analysis performed by X-ray diffraction studies show that the observed c/a ratio for []ZnGa2S4 is 1.97 indicating that there is a cation disorder in the sample [8].
⁎ Corresponding author. E-mail address: [email protected] (N.V. Joshi). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.177
Meanwhile, for the related compound []CdGa2S4, the situation is different. Cadmium and gallium atoms are ordered at different sites and Ga and vacancy are arranged orderly in a different layer. Vacancy ordering plays a significant role not only in the presence of vacancy induced defect states in the mid-gap region but also in their role in optical activity. This is expected to make a difference in optical spectroscopy (both absorption and photoluminescence) as the degree of the ordering of vacancy is much less in [] ZnGa2S4. Therefore, it is very interesting to examine chiral optical properties in this material. An earlier study [9] of absorption spectra shows that undoped []ZnGa2S4 is transparent in the range of 400 nm to 800 nm and it is ideal for window material. This means that the density of mid-gap states is either very low or their oscillatory strength for the corresponding transitions is very small. A careful study in this direction, particularly absorption under polarized radiation has not been carried out so far. The purpose of the present investigation, therefore, is to carry out such investigation in the visible range. 2. Experimental Ternary compound ZnGa2S4 was synthesized from high purity (6 N) elements obtained from Sigma chemical, USA. These elements were mixed in stoichiometric proportions in an evacuated quartz ampoule and heated at 800 °C for 48 h. Then the
N.V. Joshi et al. / Materials Letters 61 (2007) 1926–1928
temperature was increased up to 1100 °C and kept for 12 h. The polycrystalline samples were annealed for 5 days to achieve homogeneity and to reduce intrinsic defects. Good quality oriented polycrystalline samples were obtained which were polished and prepared for optical absorption measurements. The thickness of the selected sample was 0.67 mm. The optical absorption spectra were recorded at 10 K by using a Spex computer controlled monochromator. A GaAs cathode cooled photomultiplier was used in this investigation because of its flat photoresponse in the range from 400 nm to 800 nm. Polarized radiation was incident on the sample. The angle of polarization was adjusted to obtain a maximum intensity which was rotated by 90° and exactly in the same condition as the second spectrum was recorded. Both spectra are shown in Fig. 1. The other experimental details were given in our earlier publication [10]. 3. Results and discussion Fig. 1 shows two absorption spectra (curves a and b) recorded at 10 K obtained with polarized incident radiation and the direction of the electric vector was perpendicular to each other. Recorded spectra differ from the one which was reported with unpolarized radiation and show two well marked absorption bands instead of a complete transparent region [9] between 500 nm and 800 nm. When the material is optically active, the absorption maximum (in curve a) corresponds to the absorption minimum when the electric vector is perpendicular to the previous (curve b). Fig. 1 illustrates this tendency even though it is not so prominent as it is observed in []CdGa2S4 or its alloy with indium [] CdGa2(1−x)In2xS4 [5]. A broad peak A is located at 652.9 nm which has not been reported before by optical absorption spectroscopy probably because the measurement under polarized radiation has not been carried out; however this peak has been observed with photoluminescence studies by Derid et al. [3]. It is clear that the observed band A has neither Lorentzian nor Gaussian form but has a dispersive nature. In the transparent region, the rotating dispersive curve varies smoothly and decreases monotonically towards the longer wave length side. In chiral dispersive curves, it is not easy to locate and separate closely spaced energy states because of excessive broadening. In such circumstances,
Fig. 1. Optical absorption spectra recorded at 10 K. The plane of polarization of incident radiation is perpendicular to each other for curves a and b.
1927
the criterion for the half width at half maximum height also has loose meaning and therefore we will address the broadening of a band by giving its range. The peak A is very broad and is in the range starting from 2.25 eV to 1.65 eV. This broadening also is in close agreement with the peak reported in the photoluminescence spectrum by Derid et al. [3]. The origin for this broad band is attributed to the transition from quasi-continuously distributed states originated from the disordering of cations in the lattice to the acceptor states closely located on the top of the valence band. The source for acceptor states was not discussed before. In []ZnGa2S4 vacancies are ordered [8]. In the present context, vacancy has two fold consequences. Firstly, it creates bent bonds near the vacancies [11]. This means the charge distribution is completely altered and it is not coplanar anymore. A substantial amount of charge is projected out of the plane. It is found that [5] in such circumstances, the transitions originated from these states have the component of the magnetic dipole moment parallel to the electric dipole moment, i.e. these transitions are optically active. The other consequence of the vacancy in these types of compounds, is that it creates localized energy states in the mid-gap region. In Td symmetry, the T2 state corresponding to dangling bonds have three fold symmetry but in a distorted system, the symmetry is reduced to D2d system and it is found that the energy state is split into A1 + B1 + 2E. [5]. Thus in a system like this, there are three mid-gap states originated from dangling bonds. The exact magnitude of the splitting depends upon several parameters; among them is the amount of distortion, misfit in the size of the atoms etc. Therefore, the amount of the splitting is not possible to estimate, however it could be nearly the same order of []CdGa2S4 or its alloy []CdGa2(1−x)In2xS4 for small x. It is found that in this material, A1 state is located above the top of the valence band at 0.1 eV and a similar value is expected for this material. The E state lies in the mid-gap region and it is located from the top of the valence band by an amount 1.79 eV. Therefore, the broad band A, whose maximum located at 653 nm (1.89 eV) can be attributed to the transition from vacancy induced defect state in D2d symmetry and the top of valence band to E state originated also from the vacancy. This assignment differs from the earlier reported where the transition from the acceptor state to the quasi-continuum states is considered. That possibility can be over ruled as it is a chiral absorption band. On the higher energy side there is a well marked peak B located at 2.58 eV. This peak was not reported before because the sample thickness was not adequate or the polarized radiation was not employed. In fact, a systematic theoretical study has been carried out to determine the origin of states located below the bottom of the conduction band in []ZnGa2S4 crystal. This is because there is not enough experimental data available by optical absorption or photoluminescence spectroscopy. However, a systematic study has been carried out for []CdGa2S4. Now, it is known that in this material, there are sets of energy states which give rise to a broad band near 2.5 eV and hence known as “Blue band” which has been reported earlier by Georgobiani and Donu [12]. The exact position, the broadening and the form of “Blue band” varies from sample to sample because these features are sensitive to growth conditions. It consists of two sub-bands located at about 2.6 eV and 2.9 eV separated from the top of the valence band. For convenience, they are called BB1 (blue band 1) and BB2 (blue band 2) [5]. Each subband contains two or more closely separated states which are not easy to separate. The origin for these bands is attributed to the disordering at the cation sites. As the amount of disordering varies from sample to sample, the position and density of these states also vary. A similar behavior is also expected to be valid for the system []ZnGa2S4 as both of them lie in the same family.
1928
N.V. Joshi et al. / Materials Letters 61 (2007) 1926–1928
An earlier investigation of chiral absorption bands in []CdGa2(1−x) In2xS4 (for x − 0.03) reveals that at 2.6 eV there is an absorption maximum and is attributed to the transition from the top of the valence band to the lower level of BB2 [5]. The close agreement between the observed (2.58 eV) and the earlier reported peak (2.6 eV) suggest that the origin for this band can be attribute to the transition from the top of the valence band and A1 state to the lower state of BB2. As mentioned earlier, in a rotatory dispersive curve, it is not possible to separate the contribution from the transitions A1 − N E and top of the valence band to E as they merged in a broad band.
4. Conclusion Optical absorption study of []ZnGa2S4 has been carried out and chiral absorption bands have been detected. The origin for these bands is associated with mid-gap vacancy induced defect states and BB1 states originated from disordering at the cation sites. Acknowledgement The authors are thankful to Consejo de desarrrollo, Cientifico, Humanistico y Tecnologico (CDCHT) of the University of Los Andes for financial assistance.
References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrits Semiconductors: Growth, Electronic Properties and Applications, Pergman Press, 1975. [2] N.V. Joshi, Photoconductivity: Arts, Science and Technology, Marcel Dekker, 1990. [3] Y.O. Derid, E.I. Kozhachenko, S.I. Radaustan, I.M. Tiginyanu, Phys. Status Solidi, A Appl. Res. 113 (1989) K265. [4] N.V. Joshi, J. Luengo, Adv. Mater. Opt. Electron. 9 (1999) 145. [5] N.V. Joshi, J. Luengo, Appl. Phys. Lett. 87 (2005) 111909. [6] N.V. Joshi, J. Luengo, S. Alvarez, J. Martin, J. Phys. Chem. Solids 66 (2005) 2011. [7] M. Fuentes-Cabrera, J. Phys., Condens. Matter 13 (2001) 10117. [8] K. Charlotte, M. Lowe, T.A. Vanderah, Acta Crystallogr. C47 (1991) 919. [9] H.G. Kim, W.T. Kim, Phys. Rev., B 41 (1990) 8541. [10] N.V. Joshi, J. Luengo, S. Alvarez, J. Martin, Mater. Lett. 59 (2005) 345. [11] M. Lanno, J. Bourgoin, Points Defects in Semiconductors, Springer Verlag, 1981. [12] A.N. Georgobiani, V.S. Donu, Phys. Status Solidi, A Appl. Res. 82 (1984) 207.
Optical activity in []ZnGa2S4 N.V. Joshi a,⁎, Jorge Luengo b , Fatima Vera b a
b
Departmento de Fisiologia, Universidad de Los Andes, Merida, Venezuela Centro de Optica, Facultad de Ciencias, Universidad de Los Andes, Merida, Venezuela Received 24 February 2006; accepted 27 July 2006 Available online 18 August 2006
Abstract Recently, it has been established that dangling bonds in ordered vacancy compounds play a crucial role in optical activity. With this view a spectroscopic investigation of []ZnGaS4, where [] stands for ordered vacancy, has been carried out at 10 K. The optical absorption spectra were recorded under polarized radiation. Absorption maxima (in perpendicular direction of polarization absorption minima) were obtained at 2.51 and 1.9 eV and they were associated with the transitions from A1 state originated from the vacancy (in D2d symmetry) to the mid-band gap states of the ordered vacancy compounds. These transitions are optically active because the bond bending near the vacancy creates a situation where the component of the magnetic dipole moment is parallel to the electric dipole moment and hence their scalar product is non-zero. © 2006 Elsevier B.V. All rights reserved.
1. Introduction AIIBIII2CVI4 are wide band gap semiconductors [1] and posses several interesting optical properties such as high photosensitivity [2], strong photoluminescence [3] and optical activity [4,5]. The last property has been detected in ordered vacancy compound [] CdGa2S4 ([] stands for ordered vacancy) pure and doped with manganese [4,6]. Its alloy with indium also exhibits optical activity [5] and therefore it is interesting to inspect if optical activity exists in a similar compound []ZnGa2S4. This is very important because this material is transparent in the visible region and it does not have abundant mid-gap defect states or localized defect states which participate in the absorption process. Those states, which are present, might contribute in chiral absorption. In comparison with the other semiconducting compounds of AIIBIII2CVI4 family, []ZnGa2S4 has been little studied particularly its optical properties. It is known that this is an ordered vacancy compound [7] like []CdGa2S4. Because of the considerable similarities between them, optical properties are expected to be similar. However, a detailed analysis performed by X-ray diffraction studies show that the observed c/a ratio for []ZnGa2S4 is 1.97 indicating that there is a cation disorder in the sample [8].
⁎ Corresponding author. E-mail address: [email protected] (N.V. Joshi). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.177
Meanwhile, for the related compound []CdGa2S4, the situation is different. Cadmium and gallium atoms are ordered at different sites and Ga and vacancy are arranged orderly in a different layer. Vacancy ordering plays a significant role not only in the presence of vacancy induced defect states in the mid-gap region but also in their role in optical activity. This is expected to make a difference in optical spectroscopy (both absorption and photoluminescence) as the degree of the ordering of vacancy is much less in [] ZnGa2S4. Therefore, it is very interesting to examine chiral optical properties in this material. An earlier study [9] of absorption spectra shows that undoped []ZnGa2S4 is transparent in the range of 400 nm to 800 nm and it is ideal for window material. This means that the density of mid-gap states is either very low or their oscillatory strength for the corresponding transitions is very small. A careful study in this direction, particularly absorption under polarized radiation has not been carried out so far. The purpose of the present investigation, therefore, is to carry out such investigation in the visible range. 2. Experimental Ternary compound ZnGa2S4 was synthesized from high purity (6 N) elements obtained from Sigma chemical, USA. These elements were mixed in stoichiometric proportions in an evacuated quartz ampoule and heated at 800 °C for 48 h. Then the
N.V. Joshi et al. / Materials Letters 61 (2007) 1926–1928
temperature was increased up to 1100 °C and kept for 12 h. The polycrystalline samples were annealed for 5 days to achieve homogeneity and to reduce intrinsic defects. Good quality oriented polycrystalline samples were obtained which were polished and prepared for optical absorption measurements. The thickness of the selected sample was 0.67 mm. The optical absorption spectra were recorded at 10 K by using a Spex computer controlled monochromator. A GaAs cathode cooled photomultiplier was used in this investigation because of its flat photoresponse in the range from 400 nm to 800 nm. Polarized radiation was incident on the sample. The angle of polarization was adjusted to obtain a maximum intensity which was rotated by 90° and exactly in the same condition as the second spectrum was recorded. Both spectra are shown in Fig. 1. The other experimental details were given in our earlier publication [10]. 3. Results and discussion Fig. 1 shows two absorption spectra (curves a and b) recorded at 10 K obtained with polarized incident radiation and the direction of the electric vector was perpendicular to each other. Recorded spectra differ from the one which was reported with unpolarized radiation and show two well marked absorption bands instead of a complete transparent region [9] between 500 nm and 800 nm. When the material is optically active, the absorption maximum (in curve a) corresponds to the absorption minimum when the electric vector is perpendicular to the previous (curve b). Fig. 1 illustrates this tendency even though it is not so prominent as it is observed in []CdGa2S4 or its alloy with indium [] CdGa2(1−x)In2xS4 [5]. A broad peak A is located at 652.9 nm which has not been reported before by optical absorption spectroscopy probably because the measurement under polarized radiation has not been carried out; however this peak has been observed with photoluminescence studies by Derid et al. [3]. It is clear that the observed band A has neither Lorentzian nor Gaussian form but has a dispersive nature. In the transparent region, the rotating dispersive curve varies smoothly and decreases monotonically towards the longer wave length side. In chiral dispersive curves, it is not easy to locate and separate closely spaced energy states because of excessive broadening. In such circumstances,
Fig. 1. Optical absorption spectra recorded at 10 K. The plane of polarization of incident radiation is perpendicular to each other for curves a and b.
1927
the criterion for the half width at half maximum height also has loose meaning and therefore we will address the broadening of a band by giving its range. The peak A is very broad and is in the range starting from 2.25 eV to 1.65 eV. This broadening also is in close agreement with the peak reported in the photoluminescence spectrum by Derid et al. [3]. The origin for this broad band is attributed to the transition from quasi-continuously distributed states originated from the disordering of cations in the lattice to the acceptor states closely located on the top of the valence band. The source for acceptor states was not discussed before. In []ZnGa2S4 vacancies are ordered [8]. In the present context, vacancy has two fold consequences. Firstly, it creates bent bonds near the vacancies [11]. This means the charge distribution is completely altered and it is not coplanar anymore. A substantial amount of charge is projected out of the plane. It is found that [5] in such circumstances, the transitions originated from these states have the component of the magnetic dipole moment parallel to the electric dipole moment, i.e. these transitions are optically active. The other consequence of the vacancy in these types of compounds, is that it creates localized energy states in the mid-gap region. In Td symmetry, the T2 state corresponding to dangling bonds have three fold symmetry but in a distorted system, the symmetry is reduced to D2d system and it is found that the energy state is split into A1 + B1 + 2E. [5]. Thus in a system like this, there are three mid-gap states originated from dangling bonds. The exact magnitude of the splitting depends upon several parameters; among them is the amount of distortion, misfit in the size of the atoms etc. Therefore, the amount of the splitting is not possible to estimate, however it could be nearly the same order of []CdGa2S4 or its alloy []CdGa2(1−x)In2xS4 for small x. It is found that in this material, A1 state is located above the top of the valence band at 0.1 eV and a similar value is expected for this material. The E state lies in the mid-gap region and it is located from the top of the valence band by an amount 1.79 eV. Therefore, the broad band A, whose maximum located at 653 nm (1.89 eV) can be attributed to the transition from vacancy induced defect state in D2d symmetry and the top of valence band to E state originated also from the vacancy. This assignment differs from the earlier reported where the transition from the acceptor state to the quasi-continuum states is considered. That possibility can be over ruled as it is a chiral absorption band. On the higher energy side there is a well marked peak B located at 2.58 eV. This peak was not reported before because the sample thickness was not adequate or the polarized radiation was not employed. In fact, a systematic theoretical study has been carried out to determine the origin of states located below the bottom of the conduction band in []ZnGa2S4 crystal. This is because there is not enough experimental data available by optical absorption or photoluminescence spectroscopy. However, a systematic study has been carried out for []CdGa2S4. Now, it is known that in this material, there are sets of energy states which give rise to a broad band near 2.5 eV and hence known as “Blue band” which has been reported earlier by Georgobiani and Donu [12]. The exact position, the broadening and the form of “Blue band” varies from sample to sample because these features are sensitive to growth conditions. It consists of two sub-bands located at about 2.6 eV and 2.9 eV separated from the top of the valence band. For convenience, they are called BB1 (blue band 1) and BB2 (blue band 2) [5]. Each subband contains two or more closely separated states which are not easy to separate. The origin for these bands is attributed to the disordering at the cation sites. As the amount of disordering varies from sample to sample, the position and density of these states also vary. A similar behavior is also expected to be valid for the system []ZnGa2S4 as both of them lie in the same family.
1928
N.V. Joshi et al. / Materials Letters 61 (2007) 1926–1928
An earlier investigation of chiral absorption bands in []CdGa2(1−x) In2xS4 (for x − 0.03) reveals that at 2.6 eV there is an absorption maximum and is attributed to the transition from the top of the valence band to the lower level of BB2 [5]. The close agreement between the observed (2.58 eV) and the earlier reported peak (2.6 eV) suggest that the origin for this band can be attribute to the transition from the top of the valence band and A1 state to the lower state of BB2. As mentioned earlier, in a rotatory dispersive curve, it is not possible to separate the contribution from the transitions A1 − N E and top of the valence band to E as they merged in a broad band.
4. Conclusion Optical absorption study of []ZnGa2S4 has been carried out and chiral absorption bands have been detected. The origin for these bands is associated with mid-gap vacancy induced defect states and BB1 states originated from disordering at the cation sites. Acknowledgement The authors are thankful to Consejo de desarrrollo, Cientifico, Humanistico y Tecnologico (CDCHT) of the University of Los Andes for financial assistance.
References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrits Semiconductors: Growth, Electronic Properties and Applications, Pergman Press, 1975. [2] N.V. Joshi, Photoconductivity: Arts, Science and Technology, Marcel Dekker, 1990. [3] Y.O. Derid, E.I. Kozhachenko, S.I. Radaustan, I.M. Tiginyanu, Phys. Status Solidi, A Appl. Res. 113 (1989) K265. [4] N.V. Joshi, J. Luengo, Adv. Mater. Opt. Electron. 9 (1999) 145. [5] N.V. Joshi, J. Luengo, Appl. Phys. Lett. 87 (2005) 111909. [6] N.V. Joshi, J. Luengo, S. Alvarez, J. Martin, J. Phys. Chem. Solids 66 (2005) 2011. [7] M. Fuentes-Cabrera, J. Phys., Condens. Matter 13 (2001) 10117. [8] K. Charlotte, M. Lowe, T.A. Vanderah, Acta Crystallogr. C47 (1991) 919. [9] H.G. Kim, W.T. Kim, Phys. Rev., B 41 (1990) 8541. [10] N.V. Joshi, J. Luengo, S. Alvarez, J. Martin, Mater. Lett. 59 (2005) 345. [11] M. Lanno, J. Bourgoin, Points Defects in Semiconductors, Springer Verlag, 1981. [12] A.N. Georgobiani, V.S. Donu, Phys. Status Solidi, A Appl. Res. 82 (1984) 207.