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Detection of d–d* transitions of vanadium ions in Zn0.97V0.03Se monocrystal by optical absorption spectroscopy
Materials Letters 59 (2005) 345 – 348 www.elsevier.com/locate/matlet
Detection of d–d* transitions of vanadium ions in Zn 0.97V0.03Se monocrystal by optical absorption spectroscopy N.V. Joshia,*, J. Luengob, Silvana Alvarezb, J. Martinb a
Department of Physiology, Facultad de Medicina, University of Los Andes Merida, Venezuela b Centro de optica, Facultad de Ciencias, University of Los Andes Merida, Venezuela
Received 8 April 2004; received in revised form 5 September 2004; accepted 3 October 2004 Available online 22 October 2004
Abstract By using optical absorption spectroscopy (both polarized and non-polarized radiation), we have detected for the first time the transitions corresponding to 2A2(F), 2E(D), 2T1(H), 2T2(F) and the doublet 2E, 2T2(D) of excited states of V2+ (d3) ions. In addition to this, we have observed structural details which were originated in the charge transfer process. The transition from the ground state of V2+ ions to the bottom of the conduction band was also revealed in the absorption spectrum. Detection of d*–d transitions can be useful to recalculate the parameters of the crystal field splitting (e.g. Racahs parameters) in this system. D 2004 Elsevier B.V. All rights reserved. Keywords: Zn0.97V0.03Se; Transition metal ions; d*–d transitions
1. Introduction Zn0.97V0.03Se is a wide band gap semimagnetic semiconductor and known for potential applications in electronic and photonic devices [1–3]. Recently, the interest has been renewed because of the possible applications in the field of spintronics to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors [4]. Transition metal ions (TM), when introduced in II–VI compounds, play an important role in optical properties of materials and very often they are also used to add coloration in glasses. This is because TM has a series of rather sharp lines corresponding to internal d*–d transitions [5]. In some cases, these lines are closely spaced and they cannot be resolved in normal conditions and appear as an intense band or merged in the absorption background of the sample. The resolution of these lines is an important issue not only from the technical point of view but also for estimating the precise values of crystal field parameters [6]. * Corresponding author. Department of Physiology, University of Los Andes, Merida, Venezuela. Tel.: +58 274 2403111; fax: +58 274 2403045. E-mail address: [email protected] (N.V. Joshi). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.016
The optical absorption studies of ZnSe with vanadium impurities have been studied extensively in the last decade [8]. The identification of specific transitions in optical absorption spectra of vanadium in II–VI compounds is a rather complex issue because even at a moderate concentration, three ionization states namely V+, V++ and V+++ are activated [7]. The excited states of these species are very closely separated and very often they are superimposed. This is exactly the energy region where transitions associated with excited states 3T1(P) are expected. The situation is more complicated as the charge transfer processes are also reflected in the optical spectra and V2+– V3+ also lie in the same region (550–700 nm). One of the typical regions is between wave lengths 600–800 nm where there lies 2A1 , 2E(D) , 2T1(H), 2T2(F) and the doublet 2E(H), 2 T2(D) and even though some attempts have been carried out, the expected lines could not be observed or could not be not well resolved. The purpose of the present investigation, therefore, is to examine this region carefully giving importance to the thickness, the concentration of vanadium atoms and the direction of polarization of the incident radiation as some of the energy levels might have different state of polarization.
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N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
Earlier studies [7,8] show that the possibility to coexist vanadium atoms in several ionization states is rather high and therefore, Zn0.97V0.03Se could be a good candidate for examining closely separated excited states . Our earlier experimental results confirm that the detection of d*–d transitions by optical absorption spectroscopy are not always easy as it is very sensitive to the thickness of the sample [6,9,10]. To detect weak lines, or to resolve closely separated lines the thickness of the sample becomes very critical [9,10]. Taking these features into account, we have focused more attention on the experimental details.
ing, smoothing, etc., were not carried out in order to the maintain authenticity of the spectrum. The spectra recorded clearly shows that there might be a structure in the region 750–800 nm which is not disclosed even with the proper thickness. In this region, one expects d–d* transitions. The energy levels contributing in the optical absorption process might have different polarization states originating from vibronic coupling and can be resolved by using a polarizer. We have, therefore, selected the direction of the electric field so as to activate the absorption process selectively. The spectrum obtained in this way is also included in the figure for comparison purpose.
2. Experimental Good-quality Zn0.97V0.03Se single crystals were grown by chemical vapor transport (CVT) technique. The high purity vanadium (3% of Zn) obtained from Alfa Chemicals (USA) was incorporated in the synthesis. Iodine was used as a transport agent. Diameter and length of the ampoule were 11 and 100 mm, respectively. The growth temperature was 8008 C. The growth time was varied from 10 to 12 h. Reasonable size crystals with good facets were obtained when growth time was 12 h. The quality of the crystals was controlled by X-ray diffraction technique. Good-quality crystals were selected and prepared for optical measurements. Several thicknesses were tried to resolve the details in the range 600–800 nm. The optimum thickness for this spectral range and for this concentration of vanadium was found to be 0.412 mm. The optical absorption spectra were recorded at 50 K by using a Spex computer-controlled spectrometer. GaAs cathode cooled photomultiplier was employed for this investigation. This detector has a flat photo response and hence ideal for resolution of closely spaced weak lines. Photon counting electronics provided by Spex was incorporated. The other experimental details are given in our earlier publications [6]. The spectra recorded at 50 K are shown in Fig. 1. It is worth mentioning that mathematical manipulation such as averag-
Fig. 1. Optical absorption spectra recorded at 50 K: (a) for non-polarized radiation and (b) for polarized radiation.
3. Results and discussion Experimental understanding of the locations of the excited states of the transition metal ions with respect to the bottom of the conduction band has become an important issue in III–V and II–VI semiconducting compounds (DMS) because not only it helps to build the model for spin-orbit interaction and crystal field splitting ( Racahs parameters) but also to understand the luminescence properties of these materials which could be significant for light emitting devices. Moreover, recently it has been found that magnetic impurities in these compounds make them more interesting for spintronics materials. Excited states of vanadium in wide gap II–VI compounds, particularly in CdS, CdTe and ZnSe, have been examined earlier both theoretically and experimentally [7,8]. In all these cases, vanadium occupies cation site and it has Td symmetry. Therefore, ordering of the excited states and the spectral features in these compounds are essentially maintained the same, however, relative positions are shifted due to the variations in the crystal field parameters. Fig. 1 shows the optical absorption spectra recorded with non-polarized (curve a) and polarized radiation (curve b). The main structure of the spectra located between 500 and 725 nm is exactly the same for polarized and non-polarized incident radiation. However, the details of the features are revealed between 725 and 800 nm only for polarized radiation. Optical band gap of ZnSe is 2.77 eV at 50 K and we assume that the band gap of Zn0.97V0.03Se is not altered significantly with respect to ZnSe. On the basis of empirical and theoretical facts and using one electron configuration model, Biernacki et al. [11] have estimated the transition energy from the ground state of V2+ to the bottom of the conduction band and was found to be 550 nm (2.25 eV) for ZnS. Considering the close resemblance between ZnS and ZnSe, a similar value is expected for ZnSe. A careful examination of the form of the absorption edge reveals that at about 545 nm, a sudden change in the behavior of the absorption coefficient is observed indicating that two mechanisms are contributing simultaneously in the absorp-
N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
tion process. The first one is due to the charge transfer process of a doubly ionized vanadium ions and the other is the absorption due to the band gap (direct and phonon assisted) transitions. Because of the superposition of the two processes, the optical absorption edge expected to show a change in the slope in this region and hence this shoulder on the optical absorption edge is attributed to the transition from V2+ state to the bottom of the conduction band. Because of the overlapping, no attempt has been made to determine the band gap by optical absorption spectroscopy. The spectrum reveals three well-resolved absorption peaks and a weak shoulder A at 636 nm. The peaks B, C and D are located at 651, 686 and 696 nm, respectively. Not only the presence of these peaks are common in optical spectra obtained by polarized and non-polarized radiation but also their positions and relative intensities are exactly the same. However, in polarized radiation, three extra peaks are observed on the lower energy side. Peaks E, F and G are located at 735, 750 and 800 nm, respectively. Such clear structures have not been reported before by optical absorption or photoluminescence techniques. However, their presence is expected on the basis of theoretical calculation and in some cases weak and broad bands have been detected in the expected positions. On the higher energy side of peak B, a weak shoulder A is observed at 636 nm. A careful study of the charge transfer process has been carried out for vanadium in II–VI DMS compounds by optical, electrical and also by sensitization spectroscopy [8]. The last one deals with normal excitation spectroscopy with an additional irradiation from the other source. The wavelength range was selected according to the region of interest. This permits to examine the effects of pumping at different wavelengths by sensitizing the centers of vanadium. Such study has been carried out earlier by Peka et al. [8] and reported a broad hump in this region [7]. A charge transfer process for vanadium is given by [7] V2þ þ 15; 500 cm1 ZV3þ þ e c:b: However, the above mentioned photoionization energy was examined by Dieleman [12] by electrical methods and found that the exact energy is slightly higher than the calculated and is approximately 16,100 cm1 (625 nm). Therefore, this weak shoulder, A, is attributed to the photoionization energy required for vanadium doubly charged ions. The weak intensity of this hump is understandable as the concentration of vanadium ions is very low in the investigated samples. In the narrow region, between 640 and 750 nm, we have recorded two sharp and one weak lines. A sharp peak B is located at 651 nm. Earlier work does show the presence of a relatively broad structure located at 660 nm. In fact, according to the earlier theoretical calculation, two closely separated lines are expected [7]. In the present investigation, we did resolve the earlier observed broad structure in two peaks located at 651 and 686 nm corresponding to 2E(D)
347
and 2A2(F) transitions. This is the first time that the structure located at 660 nm has been resolved. On the lower energy side of peak C, a well-marked peak D is observed. In this range, no excited d level of vanadium ion is situated. Such peak has not been detected by photoluminescence technique and earlier experimental data do not show the presence of energy state in the mid-gap region where peak D is observed. However, a careful analysis does show that a noticeable shoulder has been observed in the absorption spectrum by Peka et al. [8]. The rise of the broad band is started at 711 nm very close to the observed D peak. The origin for this structure is associated with the charge transfer process from V+ to V2+ states. The charge transfer equation is given by [8] Vþ þ 14; 050 cm1 ZV2þ þ e c:b: Therefore, peak D can be tentatively associated with the charge transfer process of singly ionized vanadium ions. On the lower energy region of the spectrum, i.e. 700–900 nm some strong transitions originated from 2E, 2T2, 4A2(F), etc., are expected, instead a broad absorption band is observed. The form of the band suggests that there might be super positions of some lines. Indeed, the absorption spectrum recorded with the polarized radiation reveals expected transitions that originated from lower excited states. This is not an uncommon situation where some of the states are very sensitive for a certain direction of polarization of incident radiation. The important parameters which decide this behavior are the symmetry of the electronic states and coupling with phonon modes. An earlier reported study [7] does show a strong and a rather broad peak in the region 740 nm. Fig. 1 shows that exactly in the same region, there are two peaks, E and F. Because of the close separation, peak F appears as a shoulder but its presence is without doubt. On the basis of the energy consideration and comparison with the earlier work suggest that these peaks can be attributed to the transition from 4 T1(F) to 2 T 1(H) and 2T2(F) states, respectively. Near 800 nm, a doublet (2E and 2T2(D)) is expected [7]. A weak broad line with a plateau like configuration has been reported earlier suggesting that this might be an unresolved doublet. In the present experimental investigation, we did observe a strong line located at 800 nm. Because of the presence of noise at this level of detection, the doublet nature has not been confirmed. However, because of the close agreement between the energy values, we can safely conclude that peak G originated from 4T1(F) to 2T2(F). The transition corresponding to 2E is difficult to identify as in the expected position the signal is rather noisy.
4. Conclusions By selecting the proper thickness of the sample for a given concentration of vanadium, and also by using
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N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
polarized radiation, we were able to resolve several d*–d transitions in the range 1.55–1.90 eV. In addition to this, the structures originated from the charge transfer processes have also been detected.
Acknowledgement We are thankful to CONICIT (Consejo Nacional de Investigacio´n Cientı´fica y Tecnolo´gica de Venezuela) for financing the research project under which the present work was carried out.
References [1] J.K. Furdyna, Diluted Magnetic Semiconductors. Semiconductors and Semimetals, vol. 25, Academic Press, New York, 1998.
[2] N.V. Joshi, L. Mogollon, in: B. Pamplin, N.V. Joshi, C. Schawb (Eds.), vol. 10, 1985, p. 65. [3] M. Averous, M. Balkanski, Semimagnetic semiconductors and diluted magnetic semiconductors, Ed, Majorana Int. Sci. Ser., Phys. Sci., vol. 55, 1991. [4] M. Ziese, M.J. Thornton (Eds.), Spin Electronics, Springer, Berlin, 2001. [5] B. Figgis, M.A. Hitchman, Ligand Field Theory and Applications, Wiley-VHC, New York, 2000. [6] N.V. Joshi, J. Luengo, J. Phys. Chem. Solids 64 (2003) 1681. [7] G. Goetz, U.W. Pohl, H.J. Schultz, J. Phys., Condens. Matter 4 (1992) 8253. [8] P. Peka, M.U. Lehr, H.J. Schultz, U.W. Pohl, J. Kreissel, K. Irmscher, Phys. Rev., B 53 (1996) 1907. [9] J. Luengo, N.V. Joshi, Proceeding of the 11th International Conference on Ternary and Multinary compounds, Institute of Physics, Bristol, U.K., 1998, p. 633. [10] J. Luengo, N.V. Joshi, J. Cryst. Growth Technol. 3 (1996) 93. [11] S.W. Biernacki, G. Roussos, H.J. Schulz, J. Phys. C. Solid State Phys. 21 (1988) 5615. [12] J. Dieleman, in: G.D. Thomas (Ed.), II–VI Semiconducting Compounds, Benjamin press, New York, 1967, p. 199.
Detection of d–d* transitions of vanadium ions in Zn 0.97V0.03Se monocrystal by optical absorption spectroscopy N.V. Joshia,*, J. Luengob, Silvana Alvarezb, J. Martinb a
Department of Physiology, Facultad de Medicina, University of Los Andes Merida, Venezuela b Centro de optica, Facultad de Ciencias, University of Los Andes Merida, Venezuela
Received 8 April 2004; received in revised form 5 September 2004; accepted 3 October 2004 Available online 22 October 2004
Abstract By using optical absorption spectroscopy (both polarized and non-polarized radiation), we have detected for the first time the transitions corresponding to 2A2(F), 2E(D), 2T1(H), 2T2(F) and the doublet 2E, 2T2(D) of excited states of V2+ (d3) ions. In addition to this, we have observed structural details which were originated in the charge transfer process. The transition from the ground state of V2+ ions to the bottom of the conduction band was also revealed in the absorption spectrum. Detection of d*–d transitions can be useful to recalculate the parameters of the crystal field splitting (e.g. Racahs parameters) in this system. D 2004 Elsevier B.V. All rights reserved. Keywords: Zn0.97V0.03Se; Transition metal ions; d*–d transitions
1. Introduction Zn0.97V0.03Se is a wide band gap semimagnetic semiconductor and known for potential applications in electronic and photonic devices [1–3]. Recently, the interest has been renewed because of the possible applications in the field of spintronics to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors [4]. Transition metal ions (TM), when introduced in II–VI compounds, play an important role in optical properties of materials and very often they are also used to add coloration in glasses. This is because TM has a series of rather sharp lines corresponding to internal d*–d transitions [5]. In some cases, these lines are closely spaced and they cannot be resolved in normal conditions and appear as an intense band or merged in the absorption background of the sample. The resolution of these lines is an important issue not only from the technical point of view but also for estimating the precise values of crystal field parameters [6]. * Corresponding author. Department of Physiology, University of Los Andes, Merida, Venezuela. Tel.: +58 274 2403111; fax: +58 274 2403045. E-mail address: [email protected] (N.V. Joshi). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.016
The optical absorption studies of ZnSe with vanadium impurities have been studied extensively in the last decade [8]. The identification of specific transitions in optical absorption spectra of vanadium in II–VI compounds is a rather complex issue because even at a moderate concentration, three ionization states namely V+, V++ and V+++ are activated [7]. The excited states of these species are very closely separated and very often they are superimposed. This is exactly the energy region where transitions associated with excited states 3T1(P) are expected. The situation is more complicated as the charge transfer processes are also reflected in the optical spectra and V2+– V3+ also lie in the same region (550–700 nm). One of the typical regions is between wave lengths 600–800 nm where there lies 2A1 , 2E(D) , 2T1(H), 2T2(F) and the doublet 2E(H), 2 T2(D) and even though some attempts have been carried out, the expected lines could not be observed or could not be not well resolved. The purpose of the present investigation, therefore, is to examine this region carefully giving importance to the thickness, the concentration of vanadium atoms and the direction of polarization of the incident radiation as some of the energy levels might have different state of polarization.
346
N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
Earlier studies [7,8] show that the possibility to coexist vanadium atoms in several ionization states is rather high and therefore, Zn0.97V0.03Se could be a good candidate for examining closely separated excited states . Our earlier experimental results confirm that the detection of d*–d transitions by optical absorption spectroscopy are not always easy as it is very sensitive to the thickness of the sample [6,9,10]. To detect weak lines, or to resolve closely separated lines the thickness of the sample becomes very critical [9,10]. Taking these features into account, we have focused more attention on the experimental details.
ing, smoothing, etc., were not carried out in order to the maintain authenticity of the spectrum. The spectra recorded clearly shows that there might be a structure in the region 750–800 nm which is not disclosed even with the proper thickness. In this region, one expects d–d* transitions. The energy levels contributing in the optical absorption process might have different polarization states originating from vibronic coupling and can be resolved by using a polarizer. We have, therefore, selected the direction of the electric field so as to activate the absorption process selectively. The spectrum obtained in this way is also included in the figure for comparison purpose.
2. Experimental Good-quality Zn0.97V0.03Se single crystals were grown by chemical vapor transport (CVT) technique. The high purity vanadium (3% of Zn) obtained from Alfa Chemicals (USA) was incorporated in the synthesis. Iodine was used as a transport agent. Diameter and length of the ampoule were 11 and 100 mm, respectively. The growth temperature was 8008 C. The growth time was varied from 10 to 12 h. Reasonable size crystals with good facets were obtained when growth time was 12 h. The quality of the crystals was controlled by X-ray diffraction technique. Good-quality crystals were selected and prepared for optical measurements. Several thicknesses were tried to resolve the details in the range 600–800 nm. The optimum thickness for this spectral range and for this concentration of vanadium was found to be 0.412 mm. The optical absorption spectra were recorded at 50 K by using a Spex computer-controlled spectrometer. GaAs cathode cooled photomultiplier was employed for this investigation. This detector has a flat photo response and hence ideal for resolution of closely spaced weak lines. Photon counting electronics provided by Spex was incorporated. The other experimental details are given in our earlier publications [6]. The spectra recorded at 50 K are shown in Fig. 1. It is worth mentioning that mathematical manipulation such as averag-
Fig. 1. Optical absorption spectra recorded at 50 K: (a) for non-polarized radiation and (b) for polarized radiation.
3. Results and discussion Experimental understanding of the locations of the excited states of the transition metal ions with respect to the bottom of the conduction band has become an important issue in III–V and II–VI semiconducting compounds (DMS) because not only it helps to build the model for spin-orbit interaction and crystal field splitting ( Racahs parameters) but also to understand the luminescence properties of these materials which could be significant for light emitting devices. Moreover, recently it has been found that magnetic impurities in these compounds make them more interesting for spintronics materials. Excited states of vanadium in wide gap II–VI compounds, particularly in CdS, CdTe and ZnSe, have been examined earlier both theoretically and experimentally [7,8]. In all these cases, vanadium occupies cation site and it has Td symmetry. Therefore, ordering of the excited states and the spectral features in these compounds are essentially maintained the same, however, relative positions are shifted due to the variations in the crystal field parameters. Fig. 1 shows the optical absorption spectra recorded with non-polarized (curve a) and polarized radiation (curve b). The main structure of the spectra located between 500 and 725 nm is exactly the same for polarized and non-polarized incident radiation. However, the details of the features are revealed between 725 and 800 nm only for polarized radiation. Optical band gap of ZnSe is 2.77 eV at 50 K and we assume that the band gap of Zn0.97V0.03Se is not altered significantly with respect to ZnSe. On the basis of empirical and theoretical facts and using one electron configuration model, Biernacki et al. [11] have estimated the transition energy from the ground state of V2+ to the bottom of the conduction band and was found to be 550 nm (2.25 eV) for ZnS. Considering the close resemblance between ZnS and ZnSe, a similar value is expected for ZnSe. A careful examination of the form of the absorption edge reveals that at about 545 nm, a sudden change in the behavior of the absorption coefficient is observed indicating that two mechanisms are contributing simultaneously in the absorp-
N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
tion process. The first one is due to the charge transfer process of a doubly ionized vanadium ions and the other is the absorption due to the band gap (direct and phonon assisted) transitions. Because of the superposition of the two processes, the optical absorption edge expected to show a change in the slope in this region and hence this shoulder on the optical absorption edge is attributed to the transition from V2+ state to the bottom of the conduction band. Because of the overlapping, no attempt has been made to determine the band gap by optical absorption spectroscopy. The spectrum reveals three well-resolved absorption peaks and a weak shoulder A at 636 nm. The peaks B, C and D are located at 651, 686 and 696 nm, respectively. Not only the presence of these peaks are common in optical spectra obtained by polarized and non-polarized radiation but also their positions and relative intensities are exactly the same. However, in polarized radiation, three extra peaks are observed on the lower energy side. Peaks E, F and G are located at 735, 750 and 800 nm, respectively. Such clear structures have not been reported before by optical absorption or photoluminescence techniques. However, their presence is expected on the basis of theoretical calculation and in some cases weak and broad bands have been detected in the expected positions. On the higher energy side of peak B, a weak shoulder A is observed at 636 nm. A careful study of the charge transfer process has been carried out for vanadium in II–VI DMS compounds by optical, electrical and also by sensitization spectroscopy [8]. The last one deals with normal excitation spectroscopy with an additional irradiation from the other source. The wavelength range was selected according to the region of interest. This permits to examine the effects of pumping at different wavelengths by sensitizing the centers of vanadium. Such study has been carried out earlier by Peka et al. [8] and reported a broad hump in this region [7]. A charge transfer process for vanadium is given by [7] V2þ þ 15; 500 cm1 ZV3þ þ e c:b: However, the above mentioned photoionization energy was examined by Dieleman [12] by electrical methods and found that the exact energy is slightly higher than the calculated and is approximately 16,100 cm1 (625 nm). Therefore, this weak shoulder, A, is attributed to the photoionization energy required for vanadium doubly charged ions. The weak intensity of this hump is understandable as the concentration of vanadium ions is very low in the investigated samples. In the narrow region, between 640 and 750 nm, we have recorded two sharp and one weak lines. A sharp peak B is located at 651 nm. Earlier work does show the presence of a relatively broad structure located at 660 nm. In fact, according to the earlier theoretical calculation, two closely separated lines are expected [7]. In the present investigation, we did resolve the earlier observed broad structure in two peaks located at 651 and 686 nm corresponding to 2E(D)
347
and 2A2(F) transitions. This is the first time that the structure located at 660 nm has been resolved. On the lower energy side of peak C, a well-marked peak D is observed. In this range, no excited d level of vanadium ion is situated. Such peak has not been detected by photoluminescence technique and earlier experimental data do not show the presence of energy state in the mid-gap region where peak D is observed. However, a careful analysis does show that a noticeable shoulder has been observed in the absorption spectrum by Peka et al. [8]. The rise of the broad band is started at 711 nm very close to the observed D peak. The origin for this structure is associated with the charge transfer process from V+ to V2+ states. The charge transfer equation is given by [8] Vþ þ 14; 050 cm1 ZV2þ þ e c:b: Therefore, peak D can be tentatively associated with the charge transfer process of singly ionized vanadium ions. On the lower energy region of the spectrum, i.e. 700–900 nm some strong transitions originated from 2E, 2T2, 4A2(F), etc., are expected, instead a broad absorption band is observed. The form of the band suggests that there might be super positions of some lines. Indeed, the absorption spectrum recorded with the polarized radiation reveals expected transitions that originated from lower excited states. This is not an uncommon situation where some of the states are very sensitive for a certain direction of polarization of incident radiation. The important parameters which decide this behavior are the symmetry of the electronic states and coupling with phonon modes. An earlier reported study [7] does show a strong and a rather broad peak in the region 740 nm. Fig. 1 shows that exactly in the same region, there are two peaks, E and F. Because of the close separation, peak F appears as a shoulder but its presence is without doubt. On the basis of the energy consideration and comparison with the earlier work suggest that these peaks can be attributed to the transition from 4 T1(F) to 2 T 1(H) and 2T2(F) states, respectively. Near 800 nm, a doublet (2E and 2T2(D)) is expected [7]. A weak broad line with a plateau like configuration has been reported earlier suggesting that this might be an unresolved doublet. In the present experimental investigation, we did observe a strong line located at 800 nm. Because of the presence of noise at this level of detection, the doublet nature has not been confirmed. However, because of the close agreement between the energy values, we can safely conclude that peak G originated from 4T1(F) to 2T2(F). The transition corresponding to 2E is difficult to identify as in the expected position the signal is rather noisy.
4. Conclusions By selecting the proper thickness of the sample for a given concentration of vanadium, and also by using
348
N.V. Joshi et al. / Materials Letters 59 (2005) 345–348
polarized radiation, we were able to resolve several d*–d transitions in the range 1.55–1.90 eV. In addition to this, the structures originated from the charge transfer processes have also been detected.
Acknowledgement We are thankful to CONICIT (Consejo Nacional de Investigacio´n Cientı´fica y Tecnolo´gica de Venezuela) for financing the research project under which the present work was carried out.
References [1] J.K. Furdyna, Diluted Magnetic Semiconductors. Semiconductors and Semimetals, vol. 25, Academic Press, New York, 1998.
[2] N.V. Joshi, L. Mogollon, in: B. Pamplin, N.V. Joshi, C. Schawb (Eds.), vol. 10, 1985, p. 65. [3] M. Averous, M. Balkanski, Semimagnetic semiconductors and diluted magnetic semiconductors, Ed, Majorana Int. Sci. Ser., Phys. Sci., vol. 55, 1991. [4] M. Ziese, M.J. Thornton (Eds.), Spin Electronics, Springer, Berlin, 2001. [5] B. Figgis, M.A. Hitchman, Ligand Field Theory and Applications, Wiley-VHC, New York, 2000. [6] N.V. Joshi, J. Luengo, J. Phys. Chem. Solids 64 (2003) 1681. [7] G. Goetz, U.W. Pohl, H.J. Schultz, J. Phys., Condens. Matter 4 (1992) 8253. [8] P. Peka, M.U. Lehr, H.J. Schultz, U.W. Pohl, J. Kreissel, K. Irmscher, Phys. Rev., B 53 (1996) 1907. [9] J. Luengo, N.V. Joshi, Proceeding of the 11th International Conference on Ternary and Multinary compounds, Institute of Physics, Bristol, U.K., 1998, p. 633. [10] J. Luengo, N.V. Joshi, J. Cryst. Growth Technol. 3 (1996) 93. [11] S.W. Biernacki, G. Roussos, H.J. Schulz, J. Phys. C. Solid State Phys. 21 (1988) 5615. [12] J. Dieleman, in: G.D. Thomas (Ed.), II–VI Semiconducting Compounds, Benjamin press, New York, 1967, p. 199.