Optical phonons in mechanical alloyed Zn50Se50 mixture

Vibrational Spectroscopy 36 (2004) 117–121

Optical phonons in mechanical alloyed Zn50Se50 mixture C.E.M. Camposa,*, J.C. de Limaa, T.A. Grandia, K.D. Machadoa, P.S. Pizanib Departamento de Fi´sica, Universidade Federal de Santa Catarina, C.P. 476, 88 040-900, Floriano´polis, SC, Brazil b Departamento de Fi´sica, Universidade Federal de Sa˜o Carlos, C.P. 676, 13565-905, Sa˜o Carlos, SP, Brazil

a

Received 11 March 2004; received in revised form 13 April 2004; accepted 15 April 2004 Available online 1 June 2004

Abstract The Raman scattering technique was used to follow the optical phonons in mechanical alloyed Zn50Se50 mixture. The as-milled Zn50Se50, the aged and heat treated Zn50Se50 samples were examined to evaluate the influence of the processing method and aging effects on their vibrational properties. The most important changes in the Raman parameters of the Sen chain phonons of non-reacted Se phase and the longitudinal and transversal optical phonons (LO and TO) of the ZnSe phase with aging were associated with the growing of the crystalline Se phase and with both stress rising and the reduction of the bonds effective-charge of the ZnSe phase. The thermal treatment of the aged Zn50Se50 sample induced the separation of the non-reacted Se and an improvement in the cristallinity of the ZnSe phase, which recovered its Raman parameters. In addition, it was also seen a huge increase of TO phonon intensity. # 2004 Elsevier B.V. All rights reserved. PACS: 61.46.þw; 61.82.d; 61.10.i; 65.50.þm; 63.20.e Keywords: Mechanical alloying; X-ray diffraction (XRD); Differential scanning calorimetry (DSC); Raman spectroscopy; Nanocrystalline materials

1. Introduction Non-equilibrium processing of materials has attracted much attention in the scientific world due to the possibility of producing better improved materials than it is possible by conventional methods [1]. Rapid solidification processing (RSP) and mechanical alloying (MA) are two such processing methods with somewhat similar capabilities. The suitability of a particular technique to develop a material depends on the eventual applications for which the product is intended, one fundamental way of comparing these two methods is to evaluate the departure from equilibrium achieved. Calculations show that the maximum departure from equilibrium is 24 kJ/mol in RSP and 30 kJ/mol in MA suggesting that it is possible to achieve more metastable effects during MA [1]. In spite of the advantages and simplicity of MA [2], the technique suffers from some problems. The contamination of powders particles during MA is a major concern. The small size of the powder particles (10ths of nanometers), availability of large surface area, and formation of new surfaces during milling contribute all to the contamination * Corresponding author. Tel.: þ55-48-331-6832; fax: þ55-48-331-9068. E-mail address: [email protected] (C.E.M. Campos).

0924-2031/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2004.04.003

of the powder. In addition, the milling conditions (grinding medium, grinding vessel, milling time, and intensity, etc.) and the atmosphere under which the powder is being milled also contribute to the contamination level. The most common contaminants are oxygen, nitrogen, and iron, and the highest levels reported are 45, 26, and 60 at.%, respectively [2]. As a consequence of these high levels of contamination binary systems can become ternary, and even, quaternary systems. In the special case of binary semiconductors, as for instance ZnSe, high contamination levels of transition-metal ions can produce important changes in its physical properties. Ternary compound semiconductors have very interesting abilities to tune both band gap and the lattice constant for application in the opto-electric devices as well as for the study of basic physical concepts [3]. Among the ternary compounds, diluted magnetic semiconductors (DMS) are the one of the most interesting types of materials. DMS, also referred as semimagnetic semiconductors, are compound II– VI semiconductor alloys with a fraction of non-magnetic cations replaced by the magnetic transition metal ions. Interesting phenomena such as spin glass transition [4], magnetic ordering in a two-dimensional system [5], spin flip Raman scattering [6], observation of polaron [7], metal insulator transition [8], giant Faraday rotation [9], magnetic

C.E.M. Campos et al. / Vibrational Spectroscopy 36 (2004) 117–121

tuning of quantum well band alignment [10], spin superlattice structures [11], and novel spin relaxation process [12] have been discovered. Zn1xFexSe, crystallized in the zinc-blend structure which is similar to ZnSe in the range of Fe content 0 < x < 0:3 [13], is member of a DMS family [14]. Interesting magnetic as well as magneto-optical properties were investigated on both Zn1xFexSe bulk crystals, thin films, and quantum well [9,13,15,16]. Moreover, pressure induced phase transitions of Zn1xFexSe crystals using energy-dispersive X-ray-diffraction (EDXD) [17] revealed that the existence of Fe in the crystal results in a reduction of the transition pressure; such a reduction was believed to be the result of the hybridization of 3d orbital into the tetrahedral bonds [18]. Recently, the Raman scattering experiment has been applied to study both the ionicity and the pressure effect on Zn1xFexSe [12]. The results showed that both LO and Fe TO local modes disappear, while TO and split TO phonon modes did not change above metalization pressure. These facts were attributed to a thin skin depth, which could only sustain the propagation of transverse EM wave but forbid the longitudinal EM wave [3]. In this paper, the Raman scattering technique was used to follow the optical phonons in mechanical alloyed Zn50Se50 mixture. Recently, both structural and aging effect studies of this system were done by us [19,20]. The results revealed that ZnSe phase is always formed in the range from 20 to 70 at.% Se, but the samples are not free of non-reacted elements, even the stoichiometric (Zn50Se50) mixture. In the special case of Zn50Se50, which will be presented in this paper, after 4 years the growing of non-reacted Se phase finished and the crystalline phases in the sample reached their thermodynamic stability. Based on this interesting observation, the as-milled Zn50Se50, the aged and the heat treated Zn50Se50 samples were re-examined to evaluate the influence of the processing method and aging effects on their vibrational properties.

2. Experimental details The Zn50Se50 sample was prepared by MA and characterized by XRD analysis in 1997 following the procedure described in Ref. [19] (it will be called ZnSe-97 hereafter). This sample was kept in a dry desiccator for 4 years and reanalyzed by XRD and differential scanning calorimetry (DSC) techniques following the procedure described in Ref. [20] (it will be called aged-ZnSe-97). The comparison of the XRD patterns of the as-milled and aged samples showed two phases in common (ZnO and ZnSe) differing only by the peaks of trigonal Se presented in the pattern of the aged-ZnSe-97 sample (see Fig. 1; Ref. [20]). The reproducibility of the process was confirmed by XRD analysis of a new Zn50Se50 sample prepared by MA in 2001 (it will be called ZnSe-01) [20]. The pattern of the ZnSe phase was indexed to a cubic structure with space

3

Intensity (arb. units)

118

ZnSe ZnO

(b)

2

(a)

1 30

40

50

60

70

80

90

2 (degrees) Fig. 1. XRD patterns of aged Zn50Se50 samples after the first (RSP from 350 8C) (a) and second (RSP from 350 8C þ slow cooling from 500 8C) thermal treatments (b).

group F-3m and the lattice parameter refined by the Rietveld ˚ (ZnSe-01) and 5.6408 A ˚ (agedmethod were: 5.6478 A ZnSe-97). The mean crystallite size estimated for the ZnSe phase using the Scherrer formula showed that it is in nanometer scale (10 nm). The aging effect observed was attributed to the migration of Se atoms located at the interfacial components of nanometric grains, which can be reached even with the room temperature energy (0.025 eV). In the same Ref. [20], heat treatments of small amounts of aged-ZnSe-97 showed that the samples are hydrophilic and the non-reacted crystalline Se phase can be transformed to amorphous state by RSP. After this, new annealing was performed using all quenched sample, now at 500 8C (it will be called tt-aged-ZnSe). It was observed a phase separation in the quartz tube, part of the sample remain deposited in the inner-walls of the tube after annealing, and the major part was collected and analyzed by XRD and DSC techniques. Fig. 1 shows that the XRD patterns of both annealed samples are quite identical, except by the linewidth reduction and intensity rising of the peaks of the sample tt-aged-ZnSe. Fig. 2 shows the DSC curves of both annealed samples, as well as, of the material deposited in the inner-walls of the quartz tube (see dashed line in Fig. 2). The results show that there is practically no Se in the tt-agedZnSe and that it was sublimed and deposited in the quartz tube. It is interesting to note that the Se deposited in the quartz tube is in amorphous state, but it has thermal behavior different of those obtained by MA (see dotted line in Fig. 2) and RSP, showing a very high crystallization temperature (190 8C) and slightly low glass transition temperature (45 8C). The Raman spectra of the as-milled Zn50Se50, aged and thermal treated Zn50Se50 samples were performed in December 2001 at room temperature using a triple monochromator coupled to a CCD detector and a optical microscope (Len 100 FL). The 514.5 nm line of an argon ion Laser was used as exciting source, always in backscattering

Heat flow (arb. units)

C.E.M. Campos et al. / Vibrational Spectroscopy 36 (2004) 117–121

119

(a)

100

200

300

400

500

o

Temperature ( C) Fig. 2. DSC curves of aged Zn50Se50 samples after the first (RSP from 350 8C) (a) and second (RSP from 350 8C þ slow cooling from 500 8C) thermal treatments (b). Dashed line represents the DSC curve of the material separated during the second thermal treatment. Dotted line is the DSC curve of a-Se produced by MA.

Raman Intensity (arb. units)

(b)

(c)

(b)

(a)

100

150

200

250

300

350

-1

geometry, with less than 360 mW (power) to avoid the overheating of the samples.

Raman shift (cm ) Fig. 3. Raman spectra of pure elemental Se (a), as-milled (the sample measured here is that produced in 2001) (b), and aged Zn50Se50 (c) samples.

3. Results and discussion Fig. 3 shows the Raman spectra of high-purity crystalline Se (a) and mechanical alloyed Zn50Se50 samples produced in 2001 (b), and in 1997 (c). The spectrum of crystalline Se (cSe) shows three modes located at 142, 233, and 237 cm1, which are Sen chains modes (E0 , E00 , and A1, respectively) of its trigonal structure [21–23]. There are Sen chains in amorphous Se (a-Se) produced by MA [24], but a-Se produced by RSP shows different Se units (Se8 rings) with Raman lines at 112, 135, and 251 cm1 [22,23]. This means that the chain form of Se in the as-milled a-Se exhibits a comparable secondary interactions (inter-chains) to c-Se, and much stronger secondary interactions compared to asquenched a-Se. The spectra of both Zn50Se50 samples shown in Fig. 3 have the same features: one weak line, followed by an intense double line that also presents a shoulder at small frequency values, which are located at about 140, 235, and 250 cm1, with the shoulder at about 200 cm1. The weak line and one of the lines of the doublet shows good agreement with the Sen modes of c-Se or a-Se, and although only the XRD analyses [20] show evidences of pure Se phase in the aged sample, these two lines will be attributed to the nonreacted Se. It is well-known from Raman selection rules that for well-oriented semiconductor alloys with zinc-blend structures longitudinal (LO) and transversal optical (TO) phonons can be detected [25]. In special configurations of the experimental setup and according to the sample orientation, the TO modes are only excited by chemical and/or

structural disordering [26]. Recently, a microstructural study of (1 1 1) ZnSe single crystals produced by solid phase recrystalization revealed that this crystals have only the LO phonon mode Raman-active and some also presented the Sen modes of c-Se precipitates [27]. So, the Raman line at 250 cm1 was attributed to the LO phonons of the ZnSe phase and the shoulder observed at 200 cm1 can be attributed to the TO phonons [27–29]. As the MA samples are polycrystalline and has large quantity of defects it is a difficult task to attribute the origin of this TO mode. Although the XRD analyses have shown ZnO oxide in both Zn50Se50 samples [20], it was observed no strong evidence of its Raman line at 439 cm1 [30], in parts due to the interference of second-order Raman effects. In order to confirm all attributions done for the Raman lines, a fitting procedure of the experimental Raman spectra was done and the results are shown in Table 1. From this table one can see that the mode with symmetry E0 of Sen chains showed the most notable variations with both milling process and aging effect. An expressive down shift followed by huge line broadening of this mode is the main effect of milling and aging. On the other hand, the A1 mode of Sen practically does not change its parameters with milling and aging. The most significant change concerning this mode occurred with aging when its intensity increased 10%, indicating the growing of the c-Se, as suggested in Ref. [20]. There is a good agreement between values given for the ZnSe-LO and ZnSe-TO phonons frequencies observed for the as-milled mixture and those reported in the literature

120

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Table 1 Phonon frequency (O),full width at half maximum (G), and relative intensity (I/Imax) values obtained from the best fitting achieved for the experimental Raman spectra by using Lorentzian functions

0

E c-Se o (cm1) G (cm1) I/IA1 ZnSe-01 o (cm1) G (cm1) I/ILO Aged-ZnSe-97 o (cm1) G (cm1) I/ILO tt-Aged-ZnSe o (cm1) G (cm1) I/ILO

ZnSe

Sen 00

E

A1

TO

LO

142.4 8.9 0.15

233.1 4.6 0.31

237.5 5.5 1.00

– – –

– – –

138.0 15.0 0.07

234.0 17.9 0.13

237.8 6.0 0.20

205.5 24.0 0.10

251.6 17.5 1.00

125.5 36.0 0.10

233.8 11.6 0.16

237.6 10.8 0.32

212.30 21.9 0.12

253.4 23.5 1.00

138.9 26.3 0.10

228.8 52.4 0.20

236.0 7.8 0.20

205.6 6.0 0.67

252.5 19.2 1.00

(b)

Raman Intensity (arb. units)

Sample

LO

TO

A1 E''

400 500 600

E' (b)

(a)

100

150

200

250

300

350

-1

[27–29]. According to pressure-induced phonon frequency shifts measured for the ZnSe phase [28], which show a linear progression of the phonon frequency with pressure, the frequency value obtained for the LO mode of the as-milled sample suggests that the ZnSe phase produced by MA is practically free of stress (<5 kbar) [28]. The main effect of the aging in the ZnSe phase was the frequency up shift of both LO and TO phonons. According to Ref. [28] it means that the ZnSe phase is now submitted to a higher pressure (20 kbar) effect. Moreover, the difference between the quadratic frequencies of LO and TO (o2LO  o2TO ),which is directly proportional to the bond effective charge (ionicity) of the alloy [31], showed a important reduction. This fact suggests that the ZnSe phase produced by MA has an important reduction in the ionic character of its bonds due to aging effects. Fig. 4 shows the Raman spectra of the aged Zn50Se50 samples before (aged-ZnSe-97) and after thermal treatments (tt-aged-ZnSe). The main objective of this thermal treatment was to verify the thermodynamic stability of the ZnSe phase, but it was very important to observe the possibilities to manipulate the non-reacted Se phase. After annealing the Raman spectrum of aged-ZnSe-97 still shows the c-Se and ZnSe features, but now the TO phonons of ZnSe was hugely excited and the c-Se lines were attenuated. In addition, small evidences of ZnO lines were observed at about 440 cm1 as a shoulder of second-order Raman lines (see the arrow in the insert of Fig. 4). The fitting parameters of the annealed sample are given in Table 1. From these values one can see that the ZnSe phase recovered its crystallinity with annealing, while the c-Se one has its smallest relative area and its worst crystallinity. This fact agrees with the phase separation observed by the DSC analysis, indicating that a great part of non-reacted Se was sublimed, and complement this infor-

Raman shift (cm ) Fig. 4. Raman spectra of aged Zn50Se50 samples before (a) and after thermal treatments (b).

mation showing that there still is small amounts of nonreacted Se in the tt-aged-ZnSe sample, but it is in its worst crystallinity stage. It is interesting to note that the Se sublimation occurs at a temperature much smaller than the boiling temperature of pure Se (685 8C). The effective charge and stress of ZnSe phase were also recovered by the annealing, assuming values close to that obtained for the asmilled sample.

4. Conclusions The Raman spectra of mechanical alloyed Zn50Se50 samples produced in 2001 and in 1997 showed the Sen chain modes of Se phase and the longitudinal and transversal optical modes (LO and TO) of the ZnSe phase. The most important changes in the Raman parameters of these phonons with aging were associated with the growing of the Se phase and, with both rising of stress conditions and reduction of the bonds effective-charge of the ZnSe phase. The thermal treatment of the aged Zn50Se50 sample implied in a notable crystallinity recovering of the ZnSe phase and a partial separation of the non-reacted Se at temperature range much smaller than its boiling point.

Acknowledgements We thanks the Brazilian agencies CAPES, CNPq, and FAPESP for financial and technical support.

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References [1] C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. Eng. A 304– 306 (2001) 151. [2] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [3] C.-M. Lin, D.-S. Chuu, W.-C. Chou, J.-N. Xu, E. Huang, J.-Z. Hu, J.H. Pei, Solid State Commun. 107 (1998) 217. [4] A. Twardowski, C.J.M. Denissen, W.J.M. de Jonge, A.T.A.M. de Weale, M. Demianiuk, R. Triboulet, Solid State Commun. 59 (1986) 199. [5] N. Samarth, P. Kosowski, H. Luo, J.K. Furdyna, J.J. Rhyne, B.E. Larson, N. Otsuka, Phys. Rev. B 44 (1991) 4701. [6] D.L. Peterson, D.U. Bartholomew, U. Debska, A.K. Ramdas, S. Rodriguez, Phys. Rev. B 32 (1985) 323. [7] D.D. Awshalom, M.R. Freeman, N. Samarth, H. Luo, J.K. Furdyna, Phys. Rev. Lett. 66 (1991) 1212. [8] S. von Molnar, A. Briggs, J. Flouquet, G. Remenyi, Phys. Rev. Lett. 51 (1983) 706. [9] L.P. Fu, T. Schimiedel, A. Petrou, J. Warnock, B.T. Jonker, Appl. Phys. Lett. 60 (1992) 583. [10] X. Liu, A. Petrou, J. Warnock, B.T. Jonker, G.A. Prinz, J.J. Krebs, Phys. Rev. Lett. 63 (1989) 2280. [11] W.C. Chou, A. Petrou, J. Warnock, B.T. Jonker, Phys. Rev. Lett. 67 (1991) 3820. [12] W.C. Chou, A. Petrou, J. Warnock, B.T. Jonker, Phys. Rev. B 46 (1992) 4316. [13] N. Samarth, J.K. Furdyna, Mater. Res. Soc. Symp. Proc. 161 (1990) 427. [14] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [15] A. Twardowski, P. Gold, P. Pernambuco-Wise, J.E. Crow, M. Demianiuk, Phys. Rev. B 46 (1992) 7537.

121

[16] B.T. Jonker, J.J. Krebs, S.B. Quadri, G.A. Prinz, Appl. Phys. Lett. 50 (1987) 848. [17] S.B. Quadri, E.F. Skelton, A.W. Webb, N. Moulton, J.Z. Hu, J.K. Furdyna, Phys. Rev. B 45 (1992) 5670. [18] P. Mahaswaranathan, R.J. Sladek, U. Debsha, Phys. Rev. B 31 (1985) 5212. [19] J.C. de Lima, V.H.F. dos Santos, T.A. Grandi, NanoStruct. Mater. 11 (1999) 51. [20] K.D. Machado, J.C. de Lima, C.E.M. Campos, T.A. Grandi, A.A.M. Gasperini, Solid State Commun. 127 (2003) 477. [21] J.B. Renucci, Ph.D. thesis, Universite Paul Sabatier, Toulouse, France, 1974. [22] F.Q. Guo, K. Lu, Phys. Rev. B 57 (1998) 10414. [23] S. Kohara, A. Goldbach, N. Koura, M.-L. Saboungi, L.A. Curtiss, Chem. Phys. Lett. 287 (1998) 282. [24] J.C. de Lima, T.A. Grandi, R.S. de Biasi, J. Non-Cryst. Solids 286 (2001) 93. [25] P. Yu, M. Cardona, Fundamentals of Semiconductors, second ed., Springer-Verlag, Berlin, Heidelberg, New York, 1999. [26] P.S. Pizani, C.E.M. Campos, J. Appl. Phys. 84 (1998) 6588. [27] A.C. Wright, J. Crystal Growth 203 (1999) 309. [28] S.S. Mitra, O. Brafman, W.B. Daniels, R.K. Crawford, Phys. Rev. 186 (1969) 942. [29] J.R. Ferraro, S.S. Mitra, C. Postumus, C. Hoskins, E.C. Siwiec, Appl. Spcetrosc. 24 (1970) 187. [30] O. Bockman, T. Ostvold, G.A. Voyiatzis, G.N. Papatherodorou, Hydrometallurgy 55 (2000) 93. [31] S. Ves, K. Stro¨ ssner, M. Cardona, Solid State Commun. 57 (1986) 483.