- Email: [email protected]
Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals
Thin Solid Films 518 (2010) 2599–2602
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals G. Krishnaiah a,⁎, B.K. Reddy b, N. Madhusudhana Rao c, J. Subrahmanyam b, D. Raja Reddy b, J.L. Rao b, P. Sreedhara Reddy b a b c
Department of Physics, S.V.A. Govt. College (M), Srikalahasti 517644, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India School of Science and Humanities, VIT University, Vellore, India
a r t i c l e
i n f o
Article history: Received 29 June 2008 Received in revised form 31 July 2009 Accepted 31 July 2009 Available online 13 August 2009 Keywords: Zn1 − xCrxTe crystals Resistivity Diffuse reflectance Vapour phase growth
a b s t r a c t The optical and electrical properties of vapour phase grown crystals of diluted magnetic semiconductor Zn1 − xCrxTe were investigated for 0 ≤ x ≤ 0.005. The diffuse reflectance spectra exhibited an increase in the fundamental absorption edge (E0) with composition x and were also dominated by a broad absorption band around 5200 cm− 1 arising from 5T2 → 5E transition. The 5T2 and 5E levels originate from the crystal field splitting of the 5D free ion in the ground state. The electrical resistivity measurements revealed semiconducting behaviour of the samples with p-type conductivity in the temperature range of 200–450 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) or semimagnetic semiconductors are classical semiconductors (such as CdTe, ZnSe or InAs) in which a controlled fraction of a nonmagnetic cation is randomly replaced by magnetic ions of transitional metals or rare earth metals [1–3]. The interest in the study of DMSs is stimulated by their unique electronic, magnetic, and magneto-optical properties [4–6]. For example, their semiconducting and structural properties such as the band gaps and lattice parameters can be varied in a controlled fashion by changing the composition. Their magnetic properties exhibit spinglass transition [7], antiferromagnetic clusters [8], magnetic excitations [9], and other magnetic effects of current interest [10]. Furthermore the existence of exchange interaction between the magnetic ions and band electrons leads to useful physical effects, such as large Faraday rotation [11], giant negative magneto-resistance [12] and the magnetic polaron generation [13]. DMSs offer a unique opportunity for studies, which combine elements of semiconductor physics and magnetism. The possibility of using electron spins in electronic devices known as spintronic devices has attracted a growing interest in DMS. In these devices both charge and spin of the electrons are utilized as carriers of information. The interest greatly increased after the discovery of ferromagnetic DMSs as they can be used in spintronic devices as an effective source of spinpolarized electrons. These unique properties and the potential applications of the DMSs, in particular Zn1 − xCrxTe system exhibiting
⁎ Corresponding author. Tel.: +91 9440866412. E-mail address: [email protected] (G. Krishnaiah). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.194
ferromagnetic properties [14,15] have motivated the authors to undertake the present work. The authors have earlier reported the growth and structural properties [16], EPR and magnetic properties [17] of vapour phase grown Zn1 − xCrxTe crystals. EPR studies revealed that in addition to Cr2+ some Cr3+ also exists in the host lattice [17]. This paper reports on the investigations on diffuse reflectance and electrical properties of Zn1 − xCrxTe crystals. 2. Experimental details Zn1 − xCrxTe crystals with x = 0, 0.001, 0.002, 0.003, 0.004 and 0.005 were grown from ZnTe and CrTe by the modified vapour phase growth technique [18]. 4 N pure ZnTe, Cr and Te (M/s Sigma Aldrich) were used as source materials. Since cubic phase of CrTe is not stable, CrTe in cubic phase is prepared afresh. Appropriate quantities of ZnTe and freshly prepared CrTe were ground thoroughly to obtain homogeneous mixture. This was sintered at 900 °C in a quartz tube in a vacuum at 266 × 10− 5 Pa for 48 h and cooled slowly to room temperature at the rate of 15 °C/h. This sintered material was used as charge for crystal growth. A growth temperature of 1050 °C, a temperature gradient of 30–40 °C/cm and a pulling rate of 0.4– 0.6 mm/h were found to be optimum growth parameters. Single phase crystals were obtained only for rather low Cr concentrations x ≤ 0.005 as reported previously [16]. Attempts to grow crystals with higher x resulted in the formation of CrTe precipitates. Similar observations were reported for (Zn, Cr)-based DMSs by Herbich et al. [19]. Energy dispersive analysis of X-rays was used for the estimation of Cr content in the crystals grown in the present work. The estimated concentrations were found to be less than the target compositions by
2600
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602
Fig. 1. Diffuse reflectance spectrum of pure ZnTe and Zn1 − xCrxTe powder samples at room temperature. Fig. 2. Energy gap as a function of composition (x).
about 0.03 at.% in samples of all the compositions. The X-ray diffraction studies showed that the crystals were in cubic structure [16]. The room temperature diffuse reflectance spectra of the powdered samples of all compositions were recorded using optical Carey 5E-Spectrophotometer in the photon energy region 0.62– 6.2 eV. A specially designed metal vacuum cryostat and a high vacuum pump (M/s AVAC Engineers) were used for the electrical resistance measurement. Samples of about 4 mm long, 3 mm wide, and 2 mm thick cut from the as-grown crystals of Zn1 − xCrxTe were subjected to electrical resistance measurements in the temperature range 200– 450 K. The exact dimensions of samples for estimating the electrical resistivity were measured using a travelling microscope. Keithley electrometer (Model No. 2000DMM) interfaced with a computer measured the electrical resistance of the samples as a function of temperature at regular intervals. 3. Results and discussion 3.1. Optical properties Fig. 1 shows the diffuse reflectance spectra of pure ZnTe and Zn1 − xCrxTe samples of all compositions in the photon energy region 1.55–6.2 eV. In the spectrum of ZnTe, the E0 structure originates from transitions at the fundamental absorption edge. It is very difficult to find (E0 + Δ0) related transitions in the optical spectra of Te-based compounds (ZnTe and CdTe) [20]. Interestingly our spectra suggest the presence of the E0 + Δ0 structure. The pronounced structures found in the 3–6 eV region are due to E0 + Δ0, E1, E1 + Δ1 and E2 transitions. The values of band gaps and the corresponding transitions of E0, E1 and E2 are given in Table 1. The experimental values are in good agreement with the reported values [20]. The figure also shows that, as composition x increases, the fundamental absorption edge E0 (Eg) shifts towards higher energies. The band gap energy varies linearly with x in the present samples and is shown in
Fig. 2. Since the fundamental energy gap of zinc-blende CrTe is larger than that of ZnTe, it is obvious that Eg would increase with the Cr concentration in the Zn1 − xCrxTe system. No data is available in literature on the variation of band gap with concentration x in Zn1 − xCrxTe system, although Vallin et al. [21] reported a linear variation of the optical-absorption coefficient with Cr concentration in the range 0–4 × 1019 atoms/cm3. The present linear dependence of Eg on x can be expressed by Eg(x) = 2.258 + 0.699x. The diffuse reflectance near-infrared spectra for samples of Zn1 − xCrxTe of all compositions except for x = 0 exhibit a broad band at 5200 cm− 1 as shown in Fig. 3. The neutral Cr atom has 24 electrons, with electronic configuration 1s2 2s2 2p6 3s2 3p6 3d5 4s1. Cr atoms incorporated into II–VI crystal lattice occupy cation sites, while two of the outer electrons delocalize into the conduction and valence bands. The divalent chromium (Cr2+) ion incorporated into II–VI semiconductor creates deep impurity levels. The ground state for Cr2+ free ion is 5D with the orbital quantum number L = 2 and the total spin S = 2, yielding 25 fold degeneracy. In tetrahedral symmetry, the degeneracy is partially removed; the 5D term splits into fifteen-fold degenerate orbital triplet 5 T2 ground state and ten-fold degenerate orbital doublet 5E. The corresponding energy level diagram is shown in Fig. 4. The broad band observed at 5200 cm− 1 is assigned to 5T2 → 5E transition. The energy difference between 5T2 and 5E gives the crystal field splitting parameter
Table 1 Band gap energies and the corresponding transitions in ZnTe. Transition
Γ8–Γ6 Γ7–Γ6 L4, 5–L6 L6–L6 X7–X6
Band gap energy (eV) Present values (± 0.02 eV)
Reported values [20]
2.26 3.22 3.60 4.15 5.45
2.28 3.20 3.58 4.14 5.23
(E0) (E0 + Δ0) (E1) (E1 + Δ1) (E2)
Fig. 3. Absorption band observed in the diffuse reflectance near-infrared spectra around 5200 cm− 1.
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602
2601
Fig. 4. Energy level diagram for Cr2+ ion in tetrahedral symmetry.
Δ = 10Dq= 5200 cm− 1. This value lies within the range reported for Cr2+ in tetrahedral symmetry [22,23]. Sadofyev et al. [24] also observed a weak absorption band at1.7 µm (5882 cm− 1) at room temperature in Cr doped ZnTe epilayer with doping level around 1018 atoms/cm3. Hence, it is concluded that in the present Cr doped ZnTe samples the chromium ions substitute for the divalent cation (Zn2+) and exist in the form of Cr2+ in ZnTe lattice.
3.2. Electrical properties At room temperature, the resistivities of the Zn1 − xCrxTe samples of all compositions (x=0.001, 0.002, 0.003, 0.004 and 0.005) lie in the range of 400–1500Ω cm. Hot probe test revealed that all the samples exhibited ptype conductivity. Fig. 5 shows the variation of resistivity ‘ρ’ with temperature ‘T’ for the samples. A decrease in resistivity with increase temperature is observed indicating semiconducting the behaviour of the present samples. Increase in Cr content resulted in a drop in electrical resistivity. Saito et al. [25,26] also observed semiconducting behaviour in thin films of Zn1 − xCrxTe while Ozaki et al. [27] reported a metal– insulator transition in the p-type Zn1 − xCrxTe films co-doped with nitrogen with increase in temperature. The observed decrease in resistivity with Cr addition in the present ZnTe crystals can be explained as follows. The ‘d’ states of magnetic impurity in DMS (here Cr d state in ZnTe) generally appear near the Fermi level. The energy levels of Cr impurity in a II–VI semiconductor [28] are shown in Fig. 6. These states may split, resulting in high-spin configuration of ‘d’ electrons. The ‘d’ states of Cr split under the influence of tetrahedral crystal field of ZnTe, leading to a lower doublet eg state and a higher energy t2g state. The t2g state hybridizes with the ‘p’ orbitals of the valence band further splitting into t-bonding and t-antibonding states. The t-bonding states participate in the bonding and hence are localized. However, the antibonding states have higher energy level and contain itinerant electrons. The energy of the bonding state lies very close to the conduction band and hence with increase in temperature the electrons in this state jump into the conduction band due to thermal
Fig. 6. Levels of Cr impurities on II sites in II–VI semiconductors.
activation. As Cr concentration increases more electrons are promoted to the conduction band, leading to a further decrease in resistivity. The semiconducting behaviour in Zn1 − xCrxTe may be originated from the following three mechanisms. (i) Because of strong ionicity of ZnTe and the spatial inhomogeneous distribution of Cr dopant, holes are in strongly localized states. The hopping conductivity dominates the hole transport process. (ii) In the growth of Zn1 − xCrxTe samples, some Cr atoms stay on the interstitial positions. The interstitial Cr atoms act as donors [29]. The self-compensation effects decrease the hole concentration, forbidding the Fermi level from entering into the valence band. (iii) Antisite Te atoms decrease the hole concentration in Zn1 − xCrxTe. To identify the dominant mechanism responsible for the semiconducting behaviour of Zn1 − xCr xTe further in-depth investigations on both theoretical and experimental aspects are needed. 4. Conclusions The optical and electrical properties of the samples obtained from Zn1 − xCrxTe crystals were investigated. It was concluded that when chromium ions are incorporated into ZnTe lattice, they substitute for the divalent cation sites. The band gap energy varied linearly with composition x. Temperature dependence of electrical resistivity showed semiconducting behaviour with p-type conductivity. Acknowledgements The authors are grateful to the University Grants Commission, New Delhi, Government of India, for financial support. One of the authors Dr. G. Krishnaiah is thankful to UGC (SERO), Hyderabad, for sponsoring him as a teacher fellow under Faculty Improvement Programme. References
Fig. 5. Variation of resistivity with temperature Zn1 − xCrxTe samples.
[1] J.K. Furdyna, J. Kossut (Eds.), Diluted Magnetic Semiconductors, In: Semiconductors and Semimetals vol. 25, Academic Press, London, 1988. [2] M. Balkanski, M. Avernous (Eds.), Diluted Magnetic Semiconductors, Plenum Press, New York, 1991. [3] J. Kossut and W. Dobrowolski, In: K.H.J. Buschow (Ed.), Magnetic Materials, North Holland, Amsterdam, vol.7, 1993, p. 231. [4] R.R. Galazaka, Inst. Phys. Conf. Ser 43 (1979) 133. [5] P.K. Willardson, A.C. Beer (Eds.), Semimetals and Semiconductors, Academic Press, New York, vol.25, 1988. [6] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [7] S.B. Oserof, Phys. Rev. B 25 (1982) 6584. [8] T. Dolling, T.M. Holder, V.F. Sears, J.K. Furdyna, W. Giriat, J. Appl. Phys. 53 (1982) 7644. [9] A.K. Ramadas, J. Appl. Phys. 53 (1982) 7649. [10] H. Ohno, Science. 281 (1998) 951. [11] J.A. Gaj, R.R. Galazaka, M. Nawrocki, Solid State Commun. 25 (1978) 193.
2602 [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602 A. Mycielski, J. Mycielski, J. Phys. Soc. Jpn. 49 (1980) 809. M. Nawrocki, R. Planel, G. Fishman, R.R. Galazaka, Phys. Rev. Lett. 46 (1981) 735. H. Saito, V. Zayets, S. Yamagata, K. Ando, Phys. Rev. B 66 (2002) 081201. N. Ozaki, N. Nishizawa, K.T. Nam, S. Kuroda, K. Takita, Phys. Status Solidi (c) 1 (2004) 957. G. Krishnaiah, N. Madhusudhana Rao, D. Raja Reddy, B.K. Reddy, P. Sreedhara Reddy, J. Cryst. Growth 310 (2008) 26. G. Krishnaiah, N. Madhusudhana Rao, B.K. Reddy, D. Raja Reddy, T. Mohan Babu, S. Sambasivam, P. Sreedhara Reddy, Phys. Lett. A 372 (2008) 6429. P. Chandrasekharm, D. Raja Reddy, B.K. Reddy, J. Cryst. Growth 63 (2) (1983) 304. M. Herbich, W. Mac, A. Twardowski, K. Ando, Y. Shapira, M. Demianiuk, Phys. Rev. B 58 (1998) 1912. K. Sato, S. Adachi, J. Appl. Phys. 73 (1993) 926.
[21] J.T. Vallin, G.A. Slack, S. Roberts, Phys. Rev. B 2 (1970) 4313. [22] J.T. Vallin, G.D. Watkins, Phys. Rev. B 9 (1974) 2051. [23] M. Kaminska, J.M. Baranowski, S.M. Uba, J.T. Vallin, J. Phys. C: Solid State Phys. 12 (1979) 2197. [24] G. Yu, V.F. Sadofyev, E.M. Pevtsov, P.A. Dianov, M.V. Trubenko, J. Korshkov, Vac. Sci. Technol. B 19 (2001) 1483. [25] H. Saito, W. Zaets, S. Yamagata, Y. Suzuki, K. Ando, J. Appl. Phys. 91 (2002) 8085. [26] H. Saito, V. Zayets, S. Yamagata, K. Ando, Phys. Rev. Lett. 90 (2003) 207202. [27] N. Ozaki, I. Okabayashi, T. Kumekawa, N. Nishizawa, S. Marcet, S. Kuroda, K. Takita, Appl Phys Lett. 87 (2005) 192116. [28] P.H. Dederichs, K. Sato, H. Katayama-Yoshida, Phase Transit. 78 (2005) 851. [29] K.M. Yu, W. Walukiewicz, T. Wojtowicz, I. Kuryliszyn, X. Liu, Y. Sasaki, J.K. Furdyna, Phys. Rev. B 65 (2002) 201303.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals G. Krishnaiah a,⁎, B.K. Reddy b, N. Madhusudhana Rao c, J. Subrahmanyam b, D. Raja Reddy b, J.L. Rao b, P. Sreedhara Reddy b a b c
Department of Physics, S.V.A. Govt. College (M), Srikalahasti 517644, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India School of Science and Humanities, VIT University, Vellore, India
a r t i c l e
i n f o
Article history: Received 29 June 2008 Received in revised form 31 July 2009 Accepted 31 July 2009 Available online 13 August 2009 Keywords: Zn1 − xCrxTe crystals Resistivity Diffuse reflectance Vapour phase growth
a b s t r a c t The optical and electrical properties of vapour phase grown crystals of diluted magnetic semiconductor Zn1 − xCrxTe were investigated for 0 ≤ x ≤ 0.005. The diffuse reflectance spectra exhibited an increase in the fundamental absorption edge (E0) with composition x and were also dominated by a broad absorption band around 5200 cm− 1 arising from 5T2 → 5E transition. The 5T2 and 5E levels originate from the crystal field splitting of the 5D free ion in the ground state. The electrical resistivity measurements revealed semiconducting behaviour of the samples with p-type conductivity in the temperature range of 200–450 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) or semimagnetic semiconductors are classical semiconductors (such as CdTe, ZnSe or InAs) in which a controlled fraction of a nonmagnetic cation is randomly replaced by magnetic ions of transitional metals or rare earth metals [1–3]. The interest in the study of DMSs is stimulated by their unique electronic, magnetic, and magneto-optical properties [4–6]. For example, their semiconducting and structural properties such as the band gaps and lattice parameters can be varied in a controlled fashion by changing the composition. Their magnetic properties exhibit spinglass transition [7], antiferromagnetic clusters [8], magnetic excitations [9], and other magnetic effects of current interest [10]. Furthermore the existence of exchange interaction between the magnetic ions and band electrons leads to useful physical effects, such as large Faraday rotation [11], giant negative magneto-resistance [12] and the magnetic polaron generation [13]. DMSs offer a unique opportunity for studies, which combine elements of semiconductor physics and magnetism. The possibility of using electron spins in electronic devices known as spintronic devices has attracted a growing interest in DMS. In these devices both charge and spin of the electrons are utilized as carriers of information. The interest greatly increased after the discovery of ferromagnetic DMSs as they can be used in spintronic devices as an effective source of spinpolarized electrons. These unique properties and the potential applications of the DMSs, in particular Zn1 − xCrxTe system exhibiting
⁎ Corresponding author. Tel.: +91 9440866412. E-mail address: [email protected] (G. Krishnaiah). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.194
ferromagnetic properties [14,15] have motivated the authors to undertake the present work. The authors have earlier reported the growth and structural properties [16], EPR and magnetic properties [17] of vapour phase grown Zn1 − xCrxTe crystals. EPR studies revealed that in addition to Cr2+ some Cr3+ also exists in the host lattice [17]. This paper reports on the investigations on diffuse reflectance and electrical properties of Zn1 − xCrxTe crystals. 2. Experimental details Zn1 − xCrxTe crystals with x = 0, 0.001, 0.002, 0.003, 0.004 and 0.005 were grown from ZnTe and CrTe by the modified vapour phase growth technique [18]. 4 N pure ZnTe, Cr and Te (M/s Sigma Aldrich) were used as source materials. Since cubic phase of CrTe is not stable, CrTe in cubic phase is prepared afresh. Appropriate quantities of ZnTe and freshly prepared CrTe were ground thoroughly to obtain homogeneous mixture. This was sintered at 900 °C in a quartz tube in a vacuum at 266 × 10− 5 Pa for 48 h and cooled slowly to room temperature at the rate of 15 °C/h. This sintered material was used as charge for crystal growth. A growth temperature of 1050 °C, a temperature gradient of 30–40 °C/cm and a pulling rate of 0.4– 0.6 mm/h were found to be optimum growth parameters. Single phase crystals were obtained only for rather low Cr concentrations x ≤ 0.005 as reported previously [16]. Attempts to grow crystals with higher x resulted in the formation of CrTe precipitates. Similar observations were reported for (Zn, Cr)-based DMSs by Herbich et al. [19]. Energy dispersive analysis of X-rays was used for the estimation of Cr content in the crystals grown in the present work. The estimated concentrations were found to be less than the target compositions by
2600
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602
Fig. 1. Diffuse reflectance spectrum of pure ZnTe and Zn1 − xCrxTe powder samples at room temperature. Fig. 2. Energy gap as a function of composition (x).
about 0.03 at.% in samples of all the compositions. The X-ray diffraction studies showed that the crystals were in cubic structure [16]. The room temperature diffuse reflectance spectra of the powdered samples of all compositions were recorded using optical Carey 5E-Spectrophotometer in the photon energy region 0.62– 6.2 eV. A specially designed metal vacuum cryostat and a high vacuum pump (M/s AVAC Engineers) were used for the electrical resistance measurement. Samples of about 4 mm long, 3 mm wide, and 2 mm thick cut from the as-grown crystals of Zn1 − xCrxTe were subjected to electrical resistance measurements in the temperature range 200– 450 K. The exact dimensions of samples for estimating the electrical resistivity were measured using a travelling microscope. Keithley electrometer (Model No. 2000DMM) interfaced with a computer measured the electrical resistance of the samples as a function of temperature at regular intervals. 3. Results and discussion 3.1. Optical properties Fig. 1 shows the diffuse reflectance spectra of pure ZnTe and Zn1 − xCrxTe samples of all compositions in the photon energy region 1.55–6.2 eV. In the spectrum of ZnTe, the E0 structure originates from transitions at the fundamental absorption edge. It is very difficult to find (E0 + Δ0) related transitions in the optical spectra of Te-based compounds (ZnTe and CdTe) [20]. Interestingly our spectra suggest the presence of the E0 + Δ0 structure. The pronounced structures found in the 3–6 eV region are due to E0 + Δ0, E1, E1 + Δ1 and E2 transitions. The values of band gaps and the corresponding transitions of E0, E1 and E2 are given in Table 1. The experimental values are in good agreement with the reported values [20]. The figure also shows that, as composition x increases, the fundamental absorption edge E0 (Eg) shifts towards higher energies. The band gap energy varies linearly with x in the present samples and is shown in
Fig. 2. Since the fundamental energy gap of zinc-blende CrTe is larger than that of ZnTe, it is obvious that Eg would increase with the Cr concentration in the Zn1 − xCrxTe system. No data is available in literature on the variation of band gap with concentration x in Zn1 − xCrxTe system, although Vallin et al. [21] reported a linear variation of the optical-absorption coefficient with Cr concentration in the range 0–4 × 1019 atoms/cm3. The present linear dependence of Eg on x can be expressed by Eg(x) = 2.258 + 0.699x. The diffuse reflectance near-infrared spectra for samples of Zn1 − xCrxTe of all compositions except for x = 0 exhibit a broad band at 5200 cm− 1 as shown in Fig. 3. The neutral Cr atom has 24 electrons, with electronic configuration 1s2 2s2 2p6 3s2 3p6 3d5 4s1. Cr atoms incorporated into II–VI crystal lattice occupy cation sites, while two of the outer electrons delocalize into the conduction and valence bands. The divalent chromium (Cr2+) ion incorporated into II–VI semiconductor creates deep impurity levels. The ground state for Cr2+ free ion is 5D with the orbital quantum number L = 2 and the total spin S = 2, yielding 25 fold degeneracy. In tetrahedral symmetry, the degeneracy is partially removed; the 5D term splits into fifteen-fold degenerate orbital triplet 5 T2 ground state and ten-fold degenerate orbital doublet 5E. The corresponding energy level diagram is shown in Fig. 4. The broad band observed at 5200 cm− 1 is assigned to 5T2 → 5E transition. The energy difference between 5T2 and 5E gives the crystal field splitting parameter
Table 1 Band gap energies and the corresponding transitions in ZnTe. Transition
Γ8–Γ6 Γ7–Γ6 L4, 5–L6 L6–L6 X7–X6
Band gap energy (eV) Present values (± 0.02 eV)
Reported values [20]
2.26 3.22 3.60 4.15 5.45
2.28 3.20 3.58 4.14 5.23
(E0) (E0 + Δ0) (E1) (E1 + Δ1) (E2)
Fig. 3. Absorption band observed in the diffuse reflectance near-infrared spectra around 5200 cm− 1.
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602
2601
Fig. 4. Energy level diagram for Cr2+ ion in tetrahedral symmetry.
Δ = 10Dq= 5200 cm− 1. This value lies within the range reported for Cr2+ in tetrahedral symmetry [22,23]. Sadofyev et al. [24] also observed a weak absorption band at1.7 µm (5882 cm− 1) at room temperature in Cr doped ZnTe epilayer with doping level around 1018 atoms/cm3. Hence, it is concluded that in the present Cr doped ZnTe samples the chromium ions substitute for the divalent cation (Zn2+) and exist in the form of Cr2+ in ZnTe lattice.
3.2. Electrical properties At room temperature, the resistivities of the Zn1 − xCrxTe samples of all compositions (x=0.001, 0.002, 0.003, 0.004 and 0.005) lie in the range of 400–1500Ω cm. Hot probe test revealed that all the samples exhibited ptype conductivity. Fig. 5 shows the variation of resistivity ‘ρ’ with temperature ‘T’ for the samples. A decrease in resistivity with increase temperature is observed indicating semiconducting the behaviour of the present samples. Increase in Cr content resulted in a drop in electrical resistivity. Saito et al. [25,26] also observed semiconducting behaviour in thin films of Zn1 − xCrxTe while Ozaki et al. [27] reported a metal– insulator transition in the p-type Zn1 − xCrxTe films co-doped with nitrogen with increase in temperature. The observed decrease in resistivity with Cr addition in the present ZnTe crystals can be explained as follows. The ‘d’ states of magnetic impurity in DMS (here Cr d state in ZnTe) generally appear near the Fermi level. The energy levels of Cr impurity in a II–VI semiconductor [28] are shown in Fig. 6. These states may split, resulting in high-spin configuration of ‘d’ electrons. The ‘d’ states of Cr split under the influence of tetrahedral crystal field of ZnTe, leading to a lower doublet eg state and a higher energy t2g state. The t2g state hybridizes with the ‘p’ orbitals of the valence band further splitting into t-bonding and t-antibonding states. The t-bonding states participate in the bonding and hence are localized. However, the antibonding states have higher energy level and contain itinerant electrons. The energy of the bonding state lies very close to the conduction band and hence with increase in temperature the electrons in this state jump into the conduction band due to thermal
Fig. 6. Levels of Cr impurities on II sites in II–VI semiconductors.
activation. As Cr concentration increases more electrons are promoted to the conduction band, leading to a further decrease in resistivity. The semiconducting behaviour in Zn1 − xCrxTe may be originated from the following three mechanisms. (i) Because of strong ionicity of ZnTe and the spatial inhomogeneous distribution of Cr dopant, holes are in strongly localized states. The hopping conductivity dominates the hole transport process. (ii) In the growth of Zn1 − xCrxTe samples, some Cr atoms stay on the interstitial positions. The interstitial Cr atoms act as donors [29]. The self-compensation effects decrease the hole concentration, forbidding the Fermi level from entering into the valence band. (iii) Antisite Te atoms decrease the hole concentration in Zn1 − xCrxTe. To identify the dominant mechanism responsible for the semiconducting behaviour of Zn1 − xCr xTe further in-depth investigations on both theoretical and experimental aspects are needed. 4. Conclusions The optical and electrical properties of the samples obtained from Zn1 − xCrxTe crystals were investigated. It was concluded that when chromium ions are incorporated into ZnTe lattice, they substitute for the divalent cation sites. The band gap energy varied linearly with composition x. Temperature dependence of electrical resistivity showed semiconducting behaviour with p-type conductivity. Acknowledgements The authors are grateful to the University Grants Commission, New Delhi, Government of India, for financial support. One of the authors Dr. G. Krishnaiah is thankful to UGC (SERO), Hyderabad, for sponsoring him as a teacher fellow under Faculty Improvement Programme. References
Fig. 5. Variation of resistivity with temperature Zn1 − xCrxTe samples.
[1] J.K. Furdyna, J. Kossut (Eds.), Diluted Magnetic Semiconductors, In: Semiconductors and Semimetals vol. 25, Academic Press, London, 1988. [2] M. Balkanski, M. Avernous (Eds.), Diluted Magnetic Semiconductors, Plenum Press, New York, 1991. [3] J. Kossut and W. Dobrowolski, In: K.H.J. Buschow (Ed.), Magnetic Materials, North Holland, Amsterdam, vol.7, 1993, p. 231. [4] R.R. Galazaka, Inst. Phys. Conf. Ser 43 (1979) 133. [5] P.K. Willardson, A.C. Beer (Eds.), Semimetals and Semiconductors, Academic Press, New York, vol.25, 1988. [6] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [7] S.B. Oserof, Phys. Rev. B 25 (1982) 6584. [8] T. Dolling, T.M. Holder, V.F. Sears, J.K. Furdyna, W. Giriat, J. Appl. Phys. 53 (1982) 7644. [9] A.K. Ramadas, J. Appl. Phys. 53 (1982) 7649. [10] H. Ohno, Science. 281 (1998) 951. [11] J.A. Gaj, R.R. Galazaka, M. Nawrocki, Solid State Commun. 25 (1978) 193.
2602 [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Krishnaiah et al. / Thin Solid Films 518 (2010) 2599–2602 A. Mycielski, J. Mycielski, J. Phys. Soc. Jpn. 49 (1980) 809. M. Nawrocki, R. Planel, G. Fishman, R.R. Galazaka, Phys. Rev. Lett. 46 (1981) 735. H. Saito, V. Zayets, S. Yamagata, K. Ando, Phys. Rev. B 66 (2002) 081201. N. Ozaki, N. Nishizawa, K.T. Nam, S. Kuroda, K. Takita, Phys. Status Solidi (c) 1 (2004) 957. G. Krishnaiah, N. Madhusudhana Rao, D. Raja Reddy, B.K. Reddy, P. Sreedhara Reddy, J. Cryst. Growth 310 (2008) 26. G. Krishnaiah, N. Madhusudhana Rao, B.K. Reddy, D. Raja Reddy, T. Mohan Babu, S. Sambasivam, P. Sreedhara Reddy, Phys. Lett. A 372 (2008) 6429. P. Chandrasekharm, D. Raja Reddy, B.K. Reddy, J. Cryst. Growth 63 (2) (1983) 304. M. Herbich, W. Mac, A. Twardowski, K. Ando, Y. Shapira, M. Demianiuk, Phys. Rev. B 58 (1998) 1912. K. Sato, S. Adachi, J. Appl. Phys. 73 (1993) 926.
[21] J.T. Vallin, G.A. Slack, S. Roberts, Phys. Rev. B 2 (1970) 4313. [22] J.T. Vallin, G.D. Watkins, Phys. Rev. B 9 (1974) 2051. [23] M. Kaminska, J.M. Baranowski, S.M. Uba, J.T. Vallin, J. Phys. C: Solid State Phys. 12 (1979) 2197. [24] G. Yu, V.F. Sadofyev, E.M. Pevtsov, P.A. Dianov, M.V. Trubenko, J. Korshkov, Vac. Sci. Technol. B 19 (2001) 1483. [25] H. Saito, W. Zaets, S. Yamagata, Y. Suzuki, K. Ando, J. Appl. Phys. 91 (2002) 8085. [26] H. Saito, V. Zayets, S. Yamagata, K. Ando, Phys. Rev. Lett. 90 (2003) 207202. [27] N. Ozaki, I. Okabayashi, T. Kumekawa, N. Nishizawa, S. Marcet, S. Kuroda, K. Takita, Appl Phys Lett. 87 (2005) 192116. [28] P.H. Dederichs, K. Sato, H. Katayama-Yoshida, Phase Transit. 78 (2005) 851. [29] K.M. Yu, W. Walukiewicz, T. Wojtowicz, I. Kuryliszyn, X. Liu, Y. Sasaki, J.K. Furdyna, Phys. Rev. B 65 (2002) 201303.