- Email: [email protected]
Infrared study of a La0.7Sr0.3MnO3-δ micrometric transistor channel
ARTICLE IN PRESS
Physica B 403 (2008) 1922–1926 www.elsevier.com/locate/physb
Infrared study of a La0:7Sr0:3 MnO3d micrometric transistor channel M. Ortolania,, J.S. Leea, U. Schadea, I. Pallecchib, A. Gadaletab, D. Marre´b a
Berliner Elektronenspeicherringgesellschaft fu¨r Synchrotronstrahlung (BESSY), Albert-Einstein Strasse 15, 12489 Berlin, Germany b CNR-INFM LAMIA and Universita` di Genova, Via Dodecaneso 33, 16146 Genova, Italy Received 1 August 2007; received in revised form 24 October 2007; accepted 25 October 2007
Abstract We studied the polaron conductivity in a field-effect transistor (FET) based on the doped oxide La0:7 Sr0:3 MnO3d , where the electric field penetration depth is enhanced due to suppression of metallic conduction by large oxygen deficiency d, by mid-infrared microspectroscopy (wavelengths from 1.4 to 12 mm) on a 3 mm wide active channel. Synchrotron radiation was used to obtain the midinfrared response at the diffraction limit. We found that bound polarons, although clearly detected, play a minor role in the electric fieldinduced dc conductivity modulation, which may be then attributed to the accumulation/depletion of free carriers. r 2007 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Synchrotron radiation; Field-effect transistor; Oxide electronics; Polarons
Field-effect transistors (FETs) based on manganites like La0:7 Sr0:3 MnO3d (LSMO) display unconventional effects of the electric field on the conduction mechanism, due to the key role played by local charge, spin and orbital degrees of freedom. Indeed, their sensitivity to both electric and magnetic fields opens new perspectives for oxide-based electronics. A variety of behaviors has been reported on different field-effect devices: the shift of the metal-to insulator transition temperature [1,2], the enhancement of the electric field effect in a magnetic field [3,4], the complete suppression of the metallic state by ferroelectric field effect in ultrathin films [5], the tuning of ferromagnetic response [6]. However it is clear that due to the high carrier concentration of metallic manganites exceeding 1021 cm3 , the relative change observed in the dc transport due to the field effect is much smaller than in FETs based on conventional semiconductors or organic polymers. Dramatic effects can only be observed in ultrathin films, whose conducting thickness above the dead layer is of the order of
Corresponding author at: Istituto di Fotonica e Nanotecnologie, Via Cineto, Romano 2, 00156 Rome, Italy. Tel.: +39 06 41522 1; fax: +39 06 41522 220. E-mail address: [email protected] (M. Ortolani).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.375
the electric field penetration depth, i.e. less than 1 unit cell in optimally doped films [5]. Alternatively, field effect may be enhanced if a semiconducting state with lower carrier concentration is obtained in oxygen-deficient manganites, where electron doping by oxygen vacancies compensates hole doping and the double exchange mechanism is inhibited. Most of the doped charges should then occupy bound states, coupled to local lattice distortions (polarons), whose typical binding energy lies in the mid-infrared region (0.1–1 eV), as suggested by infrared studies [7]. A field effect of few percent on the dc resistance has been reported in oxygen-deficient [4] and lightly doped [3] semiconducting manganites. It is then desirable to investigate if the polaronic response can be tuned by shifting the Fermi level with an external electric field, as recently proposed for an organic FET [8]. Furthermore, dc transport techniques did not provide yet a definitive answer to the question whether the resistance modulation is due to homogeneous modulation of carrier concentration, tuning of volume fractions of separated phases [4,9], injection of free carriers or else disruption of the insulating state [2]. Infrared spectroscopy provides a bulk probe of the active channel of FETs, independent on percolation, contact resistance and film thickness effects, and highly sensitive to polaronic carriers [8,10]. In this paper, we report an infrared
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
investigation of the active channel of a FET based on oxygen-deficient LSMO. A 40 nm thick oxygen-deficient LSMO film was grown by pulsed-laser ablation technique on a 500 mm thick SrTiO3 (STO) substrate, as described elsewhere [4]. A FET was designed in a planar geometry by patterning the LSMO film with standard optical lithography and wet etching in HCl. The film is divided in three areas (as shown in Fig. 1d and in the picture in Fig. 2a), with the central area (A) forming the active channel, together with the source and drain pads, and the two other areas forming the side-gate pads (G). Since no top or embedded metallic contact is required, the active channel can be directly inspected by the infrared probe. As we shall explain below, key points of the present device are the small width of the channel ð3 mmÞ, ensuring a high density of the accumulated or depleted charges per unit volume, and the high dielectric constant of the substrate ðr ¼ 300 for STO at room temperature). When a voltage bias V g is applied to the gate pads, the electric field distribution is almost symmetrical with respect to the surface plane, while the magnitude of the electric displacement vector is stronger by a factor r in the substrate than in the air. Being the free surface charge proportional to the normal component of the electric displacement vector, the mobile carriers are mostly accumulated/depleted at the channel-substrate interface and their density is proportional to r . In Fig. 1c, the
1923
calculated profiles of the magnitude of the electric field at the film/substrate interface and of its normal component for V g ¼ 30 V and r ¼ 300 are shown. It can be seen that the electric field is peaked beneath the channel edges; yet, beneath the middle of the channel it is still remarkably large, namely, just a factor six smaller than the peak value. On one hand, the uneven electric field distribution makes channel inhomogeneity effects more important; on the other hand the whole width of the channel undergoes a significant field effect. For V g ¼ 30 V and r ¼ 300, the computed capacitance per unit length in our geometry is 1.2 nF/m and we derive an equivalent density of charge carriers accumulated/ depleted in the channel of Dn 2 1018 cm3 . Experimentally, for V g ¼ 30 V a resistance modulation DZ=Z 3% was observed short after growth at several temperatures (Fig. 1b). If we assume Dn=n DZ=Z, we obtain an estimate of the carrier concentration n of the present deoxygenated film of the order of 1020 cm3 , hence significantly smaller than in fully oxygenated samples. Indeed, starting from the nominal value of 0.3 per Mn site estimated from the chemical formula and assuming no oxygen deficiency, one would obtain n 1021 cm3 . The absence of any metal-insulator transition, typical of fully oxygenated La0:7 Sr0:3 MnO3 , confirms that the carrier concentration is at least one order of magnitude smaller than the optimal value. A more precise estimate of n is prevented by the fact that the external field may cause even
4.8 10 Rchannel (Ω)
T=220 K 4.6
5
0
4.4 0
100 200 Temperature (K)
300
-40
-20
0 20 Gate Bias (V)
40
E (106V/m)
10 |E|
5 0
Ez -5 -20 +/-
-10
y (μm)
10
20 +/-
STO Fig. 1. Channel resistance (a) and field modulation at 220 K (b). (c) Calculated electric field profile for V g ¼ þ30 V. (d) Scheme of the side-gate transistor. Dashed areas are LSMO gate (G) and channel (A) pads.
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
1924
wavelength (μm) 10
7
6
5
4
3
y (μm)
20
R (y) / R (y = +10 μm)
2.0
y = +30 μm -30 μm
0 -20
y = -20 μm
1.5 y = 0 μm ; -2 μm
y = +5 μm; -7 μm 1.0
1000
1500
2000
2500
wavenumbers 1.0 Tfilm
4 Rfilm
3000
(cm-1)
2
0.5
295K 245K 220K
0
295K 245K 220K
0.0 0
2000
4000
0
2000
4000
wavenumbers (cm-1) Fig. 2. (a) Microspectroscopic linescan of the active channel along the transverse direction y at 295 K. Each curve is the average of 64 spectra taken at 4 cm1 resolution (the linescan was completed in less than 10 min). In the inset, visible image of the side-gate transistor illuminated from the back. Dark areas are LSMO, light areas the bare substrate. (b,c) Reflectance and transmittance of the LSMO film. Dashed lines are fits at 295 K with an offset for clarity.
interferometer and then to the microscope. The above described setup allowed us to probe the microscopic infrared response of the sample as a function of temperature for wavelengths l from 1.4 to 11 mm (corresponding to frequencies o of 90027000 cm1 ). In the present experiment, the accessible frequency range is considerably reduced by diffraction since both the sample size ðo3 mmÞ and the aperture size ð5 mmÞ are smaller than the wavelength. In spite of that, the midinfrared response of the active channel of the sample could be detected for lt6 mm thanks to the high brilliance of synchrotron radiation, as demonstrated by the line scan performed through the active channel along the transverse direction with an aperture size of 15 5 mm2 (see for example the clear contrast between the y ¼ 0 mm and the y ¼ 5 mm spectra). We then measured the LSMO film reflectance Rfilm and transmittance T film at several temperatures in the midinfrared region by using the bare substrate reflectance/ transmittance as a reference (Fig. 2b and c). The evident feature around 1100 cm1 in Rfilm is due to an optical phonon of STO, which was also limiting the nonzero transmission region to 1300 cm1 . In order to derive the optical constants of the film in the mid-infrared, we can describe the film response in our confocal arrangement by the Fresnel formulae for a LSMO slab sandwiched between the vacuum and the STO. The reference (free areas of the substrate) is modelled with a single vacuum–STO interface. In doing so, we neglect any effect of the interface between the back surface of the substrate and the vacuum; this is a reasonable assumption, since the back surface of the substrate is unpolished and it is located far away ð500 mmÞ from the focus of the confocal arrangement, which has a focal volume of few mm3 . We therefore use: Rfilm
1 8pk3 oD r12 þ r23 e2in~2 od 2 R 1þe ¼ , 1 þ r12 r23 e2in~2 od 1 þ R2 e8pk3 oD
T film
2 4pk3 oD 1 t12 t23 ein~2 od 2 ð1 RÞ e ¼ , 1 þ r r e2in~2 od 1 þ R2 e8pk3 oD 12 23
larger relative effects on the channel resistance than those probed by transport measurements, since in a side-gate geometry the source and drain contact resistances in series with the channel resistance cannot be completely eliminated. The sample was mounted on the cold finger of a closedcycle nitrogen-gas Joule–Thompson microcryostat with BaF2 windows for infrared transmission/reflection measurements. The cryostat had electrical feedthroughs for the simultaneous measurements of optical and electrical properties, and application of the gate voltage. The cryostat chamber was placed into the optical path of a confocal infrared microscope Nicolet Continuum. The synchrotron beam from the storage ring BESSY-II was fed through the IRIS beamline into a Fourier-transform
where d ¼ 40 nm is the film thickness, D ¼ 500 mm is the substrate thickness, R ¼ jr13 j2 , n~ x ¼ nx þ ikx are the complex indexes of refraction, rxy ðtxy Þ are the reflection (transmission) coefficient at the xy interface and the subscripts x; y ¼ 1, 2, 3 refer to the vacuum, LSMO and STO, respectively. Since rxy and txy are completely determined by the indexes of refraction, we performed a simultaneous fitting of Rfilm and T film by modelling the unknown n~ 2 with Drude–Lorentz oscillators, n~ 1 ¼ 1 and n~ 3 from Ref. [11]. The fit gives unambiguous results between 1000 and 6000 cm1 , shown in Fig. 2b and c. The real part of the optical conductivity of oxygendeficient LSMO sðoÞ can be now calculated from n~2 and the result is plotted in Fig. 3a, together with an estimate of
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
σfilm(Ω-1cm-1)
1000 100 T = 295 K 245 K 220 K
10
T(+Vg) / T(-Vg)
1 1.02
Vg = 30 V
1.00
0.98 0
1000
2000
3000
4000
5000
wavenumbers (cm-1) Fig. 3. (a) Infrared conductivity of the LSMO film as obtained from the Drude–Lorentz fit. (b) Field modulation of the infrared transmission of the active channel, measured with synchrotron radiation through an aperture of 15 5 mm2 and 20 sets of 512 scans. The bar indicates the experimental uncertainty on the absolute level.
the dc conductivity obtained by a geometrical calculation from the resistance data in Fig. 1a. sðoÞ strongly increases with increasing o and it is weakly temperature-dependent, as usually found in non-metallic manganites [12–14]. Since the conductivity in the mid-infrared is three order of magnitude higher than that at zero frequency, we can conclude that most of the doped holes are trapped in bound states. The broad maximum in sðoÞ around 3500 cm1 can then be interpreted as a polaron conductivity band [7,13]. The exact nature of the polaron conductivity band (charge polaron, spin polaron, excitation involving the orbital degree of freedom) is currently under debate, but in all cases some effect of an external electric field on the conductivity may be expected. We then studied the effect of the external field on the transmittance of the active channel. Under a gate bias V g , the accumulated charges will decrease the channel transmission from the equilibrium T film ðoÞ to TðV g ÞðoÞ in a frequency range defined by their optical conductivity sa ðoÞ. It is reasonable to expect sa ðoÞ to have contributions in the mid-infrared range, since the accumulated charges may behave like polarons just like the pre-existing carriers. The exact amount of the transmission change in the mid-infrared is difficult to predict, since it depends both on the density of accumulated charges na and the thickness of the accumulation layer a, but a reasonable guess is that it should be of the order of the dc conductivity modulation, i.e. 3%. An opposite effect is expected for positive gate bias þV g . Fig. 3b shows the channel transmission ratio TðþV g Þ=TðV g Þ, obtained after averaging 20 successive acquisitions of 512 scans each with an aperture size of 15 5 mm2 . No field effect on TðþV g Þ=TðV g Þ is observed
1925
within the experimental uncertainty of 0:3% at any temperature up to V g ¼ 30 V. Note that we made sure that no net current was flowing in the channel within noise levels ðo0:5 nAÞ. The data in Fig. 3 might be explained (i) by a of the order of one unit cell, or (ii) by very small na (much less than 3% of the pre-existing carrier density n), or (iii) by sa ðoÞ=na 5sfilm ðoÞ=n in the mid-infrared. In all cases, the variation of TðþV g Þ=TðV g Þ could be smaller than our noise level. The first case cannot be excluded completely, but we recall that the present film has a reduced n, hence a should be larger than in a film with full oxygen stoichiometry. In the second case, the effect of the electric field in this manganite would be different from carrier accumulation, but still producing dc conductivity modulation (see for example Ref. [4]). In the third case, sa ðoÞ would be significantly different from zero only at low frequencies, indicating that the accumulated carriers behave mainly as free carriers with minor modification of the polaronic states, as recently found in rubrene FETs [10]. Infrared studies at lower frequencies, which have to be performed on differently patterned devices to overcome the diffraction limit, would help to check the latter possibility, in order to understand whether the resistance modulation in manganites is due to simple band filling in a rigid band picture or rather the density of states is rearranged by band filling, as indeed expected for a strongly correlated material. The above reported result is somehow surprising, since in semiconducting manganites the charge transport modulation by an external electric field weaker than those used in the present experiment should be related to large changes in the polaron system [15], which in turn should influence the infrared absorption. We therefore explored all known possibilities for a plausible intrinsic cause for the lack of an observable field effect in the infrared spectrum in the present experiment. The infrared signal should be observable in this experiment thanks to an enhanced electric field penetration depth in the present semiconducting manganite film. However, the latter property is based on the peculiar chemical composition of the present sample, which features an oxygendeficient LSMO layer over a STO substrate which is stoichiometric in oxygen content. One could therefore mention the possibility of oxygen ion migration from the STO substrate to the oxygen-deficient manganite film, whose equilibrium composition is also at full oxygen stoichiometry. Oxygen migration is normally observed at high temperatures, but it might eventually take place at room temperature together with prolonged illumination of tightly focussed synchrotron radiation, at least at the interface layer of the micrometric area of the side-gate transistor channel. If this was the case of our sample, the LSMO layer at the interface with the substrate would have metallic properties instead of semiconducting ones, and the field effect in the infrared spectrum could have been reduced. Since the re-oxygenated metallic
ARTICLE IN PRESS 1926
M. Ortolani et al. / Physica B 403 (2008) 1922–1926
areas might be inhomogeneous or not connected, they would not necessarily lead to metallic-like dc transport in Fig. 1a. Similar studies on semiconducting manganites which are stoichiometric in oxygen content, such as Nd0:7 Sr0:3 MnO3 or La1x Cax MnO3 are in progress to clarify this point. In conclusion, we used infrared synchrotron microspectroscopy to probe a manganite FET with characteristic length scales at the diffraction limit. We exploited the side-gate configuration to determine the optical constants of the active material and hence to derive the presence of bound polarons. A major modification of the occupation of the polaron band, due to the shift of the Fermi level under application of an external electric field to the gate electrode, can be ruled out in the present device. M.O. acknowledges support of the Investment Bank Berlin under the project Terahertz Radar. I.P., A.G. and D.M. acknowledge support from the EU under the project Nanoxide, contract no. 033191.
References [1] I. Pallecchi, L. Pellegrino, E. Bellingeri, A.S. Siri, D. Marre´, Appl. Phys. Lett. 83 (2003) 4435. [2] H. Tanaka, J. Zhang, T. Kawai, Phys. Rev. Lett. 88 (2002) 027204. [3] T. Zhao, S.B. Ogale, S.R. Shinde, R. Ramesh, R. Droopad, J. Yu, K. Eisenbeiser, J. Misevich, Appl. Phys. Lett. B 84 (2004) 750. [4] I. Pallecchi, L. Pellegrino, E. Bellingeri, A.S. Siri, D. Marre´, Phys. Rev. B 71 (2005) 014406. [5] X. Hong, A. Posadas, C.H. Ahn, Appl. Phys. Lett. 86 (2005) 142501. [6] T. Kanki, H. Tanaka, T. Kawai, Appl. Phys. Lett. 89 (2006) 242506. [7] A. Nucara, A. Perucchi, P. Calvani, T. Aselage, D. Emin, Phys. Rev. B 68 (2003) 174432. [8] N. Sai, Z.Q. Li, M.C. Martin, D.N. Basov, M. Di Ventra, Phys. Rev. B 75 (2007) 045307. [9] F.X. Hu, J. Gao, Appl. Phys. Lett. 88 (2006) 132502. [10] M. Fischer, M. Dressel, B. Gompf, A.K. Tripathi, J. Pflaum, Appl. Phys. Lett. 89 (2006) 182103. [11] P. Dore, A. Paolone, R. Trippetti, J. Appl. Phys. 80 (1996) 5270. [12] K. Tobe, T. Kimura, Y. Tokura, Phys. Rev. B 69 (2004) 014407. [13] M.W. Kim, J.H. Jung, K.H. Kim, H.J. Lee, J. Yu, T.W. Noh, Y. Moritomo, Phys. Rev. Lett. 89 (2002) 016403. [14] K. Takenaka, R. Shiozaki, S. Sugai, Phys. Rev. B 65 (2002) 184436. [15] S.B. Ogale, V. Talyansky, C.H. Chen, R. Ramesh, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 77 (1996) 1159.
Physica B 403 (2008) 1922–1926 www.elsevier.com/locate/physb
Infrared study of a La0:7Sr0:3 MnO3d micrometric transistor channel M. Ortolania,, J.S. Leea, U. Schadea, I. Pallecchib, A. Gadaletab, D. Marre´b a
Berliner Elektronenspeicherringgesellschaft fu¨r Synchrotronstrahlung (BESSY), Albert-Einstein Strasse 15, 12489 Berlin, Germany b CNR-INFM LAMIA and Universita` di Genova, Via Dodecaneso 33, 16146 Genova, Italy Received 1 August 2007; received in revised form 24 October 2007; accepted 25 October 2007
Abstract We studied the polaron conductivity in a field-effect transistor (FET) based on the doped oxide La0:7 Sr0:3 MnO3d , where the electric field penetration depth is enhanced due to suppression of metallic conduction by large oxygen deficiency d, by mid-infrared microspectroscopy (wavelengths from 1.4 to 12 mm) on a 3 mm wide active channel. Synchrotron radiation was used to obtain the midinfrared response at the diffraction limit. We found that bound polarons, although clearly detected, play a minor role in the electric fieldinduced dc conductivity modulation, which may be then attributed to the accumulation/depletion of free carriers. r 2007 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Synchrotron radiation; Field-effect transistor; Oxide electronics; Polarons
Field-effect transistors (FETs) based on manganites like La0:7 Sr0:3 MnO3d (LSMO) display unconventional effects of the electric field on the conduction mechanism, due to the key role played by local charge, spin and orbital degrees of freedom. Indeed, their sensitivity to both electric and magnetic fields opens new perspectives for oxide-based electronics. A variety of behaviors has been reported on different field-effect devices: the shift of the metal-to insulator transition temperature [1,2], the enhancement of the electric field effect in a magnetic field [3,4], the complete suppression of the metallic state by ferroelectric field effect in ultrathin films [5], the tuning of ferromagnetic response [6]. However it is clear that due to the high carrier concentration of metallic manganites exceeding 1021 cm3 , the relative change observed in the dc transport due to the field effect is much smaller than in FETs based on conventional semiconductors or organic polymers. Dramatic effects can only be observed in ultrathin films, whose conducting thickness above the dead layer is of the order of
Corresponding author at: Istituto di Fotonica e Nanotecnologie, Via Cineto, Romano 2, 00156 Rome, Italy. Tel.: +39 06 41522 1; fax: +39 06 41522 220. E-mail address: [email protected] (M. Ortolani).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.375
the electric field penetration depth, i.e. less than 1 unit cell in optimally doped films [5]. Alternatively, field effect may be enhanced if a semiconducting state with lower carrier concentration is obtained in oxygen-deficient manganites, where electron doping by oxygen vacancies compensates hole doping and the double exchange mechanism is inhibited. Most of the doped charges should then occupy bound states, coupled to local lattice distortions (polarons), whose typical binding energy lies in the mid-infrared region (0.1–1 eV), as suggested by infrared studies [7]. A field effect of few percent on the dc resistance has been reported in oxygen-deficient [4] and lightly doped [3] semiconducting manganites. It is then desirable to investigate if the polaronic response can be tuned by shifting the Fermi level with an external electric field, as recently proposed for an organic FET [8]. Furthermore, dc transport techniques did not provide yet a definitive answer to the question whether the resistance modulation is due to homogeneous modulation of carrier concentration, tuning of volume fractions of separated phases [4,9], injection of free carriers or else disruption of the insulating state [2]. Infrared spectroscopy provides a bulk probe of the active channel of FETs, independent on percolation, contact resistance and film thickness effects, and highly sensitive to polaronic carriers [8,10]. In this paper, we report an infrared
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
investigation of the active channel of a FET based on oxygen-deficient LSMO. A 40 nm thick oxygen-deficient LSMO film was grown by pulsed-laser ablation technique on a 500 mm thick SrTiO3 (STO) substrate, as described elsewhere [4]. A FET was designed in a planar geometry by patterning the LSMO film with standard optical lithography and wet etching in HCl. The film is divided in three areas (as shown in Fig. 1d and in the picture in Fig. 2a), with the central area (A) forming the active channel, together with the source and drain pads, and the two other areas forming the side-gate pads (G). Since no top or embedded metallic contact is required, the active channel can be directly inspected by the infrared probe. As we shall explain below, key points of the present device are the small width of the channel ð3 mmÞ, ensuring a high density of the accumulated or depleted charges per unit volume, and the high dielectric constant of the substrate ðr ¼ 300 for STO at room temperature). When a voltage bias V g is applied to the gate pads, the electric field distribution is almost symmetrical with respect to the surface plane, while the magnitude of the electric displacement vector is stronger by a factor r in the substrate than in the air. Being the free surface charge proportional to the normal component of the electric displacement vector, the mobile carriers are mostly accumulated/depleted at the channel-substrate interface and their density is proportional to r . In Fig. 1c, the
1923
calculated profiles of the magnitude of the electric field at the film/substrate interface and of its normal component for V g ¼ 30 V and r ¼ 300 are shown. It can be seen that the electric field is peaked beneath the channel edges; yet, beneath the middle of the channel it is still remarkably large, namely, just a factor six smaller than the peak value. On one hand, the uneven electric field distribution makes channel inhomogeneity effects more important; on the other hand the whole width of the channel undergoes a significant field effect. For V g ¼ 30 V and r ¼ 300, the computed capacitance per unit length in our geometry is 1.2 nF/m and we derive an equivalent density of charge carriers accumulated/ depleted in the channel of Dn 2 1018 cm3 . Experimentally, for V g ¼ 30 V a resistance modulation DZ=Z 3% was observed short after growth at several temperatures (Fig. 1b). If we assume Dn=n DZ=Z, we obtain an estimate of the carrier concentration n of the present deoxygenated film of the order of 1020 cm3 , hence significantly smaller than in fully oxygenated samples. Indeed, starting from the nominal value of 0.3 per Mn site estimated from the chemical formula and assuming no oxygen deficiency, one would obtain n 1021 cm3 . The absence of any metal-insulator transition, typical of fully oxygenated La0:7 Sr0:3 MnO3 , confirms that the carrier concentration is at least one order of magnitude smaller than the optimal value. A more precise estimate of n is prevented by the fact that the external field may cause even
4.8 10 Rchannel (Ω)
T=220 K 4.6
5
0
4.4 0
100 200 Temperature (K)
300
-40
-20
0 20 Gate Bias (V)
40
E (106V/m)
10 |E|
5 0
Ez -5 -20 +/-
-10
y (μm)
10
20 +/-
STO Fig. 1. Channel resistance (a) and field modulation at 220 K (b). (c) Calculated electric field profile for V g ¼ þ30 V. (d) Scheme of the side-gate transistor. Dashed areas are LSMO gate (G) and channel (A) pads.
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
1924
wavelength (μm) 10
7
6
5
4
3
y (μm)
20
R (y) / R (y = +10 μm)
2.0
y = +30 μm -30 μm
0 -20
y = -20 μm
1.5 y = 0 μm ; -2 μm
y = +5 μm; -7 μm 1.0
1000
1500
2000
2500
wavenumbers 1.0 Tfilm
4 Rfilm
3000
(cm-1)
2
0.5
295K 245K 220K
0
295K 245K 220K
0.0 0
2000
4000
0
2000
4000
wavenumbers (cm-1) Fig. 2. (a) Microspectroscopic linescan of the active channel along the transverse direction y at 295 K. Each curve is the average of 64 spectra taken at 4 cm1 resolution (the linescan was completed in less than 10 min). In the inset, visible image of the side-gate transistor illuminated from the back. Dark areas are LSMO, light areas the bare substrate. (b,c) Reflectance and transmittance of the LSMO film. Dashed lines are fits at 295 K with an offset for clarity.
interferometer and then to the microscope. The above described setup allowed us to probe the microscopic infrared response of the sample as a function of temperature for wavelengths l from 1.4 to 11 mm (corresponding to frequencies o of 90027000 cm1 ). In the present experiment, the accessible frequency range is considerably reduced by diffraction since both the sample size ðo3 mmÞ and the aperture size ð5 mmÞ are smaller than the wavelength. In spite of that, the midinfrared response of the active channel of the sample could be detected for lt6 mm thanks to the high brilliance of synchrotron radiation, as demonstrated by the line scan performed through the active channel along the transverse direction with an aperture size of 15 5 mm2 (see for example the clear contrast between the y ¼ 0 mm and the y ¼ 5 mm spectra). We then measured the LSMO film reflectance Rfilm and transmittance T film at several temperatures in the midinfrared region by using the bare substrate reflectance/ transmittance as a reference (Fig. 2b and c). The evident feature around 1100 cm1 in Rfilm is due to an optical phonon of STO, which was also limiting the nonzero transmission region to 1300 cm1 . In order to derive the optical constants of the film in the mid-infrared, we can describe the film response in our confocal arrangement by the Fresnel formulae for a LSMO slab sandwiched between the vacuum and the STO. The reference (free areas of the substrate) is modelled with a single vacuum–STO interface. In doing so, we neglect any effect of the interface between the back surface of the substrate and the vacuum; this is a reasonable assumption, since the back surface of the substrate is unpolished and it is located far away ð500 mmÞ from the focus of the confocal arrangement, which has a focal volume of few mm3 . We therefore use: Rfilm
1 8pk3 oD r12 þ r23 e2in~2 od 2 R 1þe ¼ , 1 þ r12 r23 e2in~2 od 1 þ R2 e8pk3 oD
T film
2 4pk3 oD 1 t12 t23 ein~2 od 2 ð1 RÞ e ¼ , 1 þ r r e2in~2 od 1 þ R2 e8pk3 oD 12 23
larger relative effects on the channel resistance than those probed by transport measurements, since in a side-gate geometry the source and drain contact resistances in series with the channel resistance cannot be completely eliminated. The sample was mounted on the cold finger of a closedcycle nitrogen-gas Joule–Thompson microcryostat with BaF2 windows for infrared transmission/reflection measurements. The cryostat had electrical feedthroughs for the simultaneous measurements of optical and electrical properties, and application of the gate voltage. The cryostat chamber was placed into the optical path of a confocal infrared microscope Nicolet Continuum. The synchrotron beam from the storage ring BESSY-II was fed through the IRIS beamline into a Fourier-transform
where d ¼ 40 nm is the film thickness, D ¼ 500 mm is the substrate thickness, R ¼ jr13 j2 , n~ x ¼ nx þ ikx are the complex indexes of refraction, rxy ðtxy Þ are the reflection (transmission) coefficient at the xy interface and the subscripts x; y ¼ 1, 2, 3 refer to the vacuum, LSMO and STO, respectively. Since rxy and txy are completely determined by the indexes of refraction, we performed a simultaneous fitting of Rfilm and T film by modelling the unknown n~ 2 with Drude–Lorentz oscillators, n~ 1 ¼ 1 and n~ 3 from Ref. [11]. The fit gives unambiguous results between 1000 and 6000 cm1 , shown in Fig. 2b and c. The real part of the optical conductivity of oxygendeficient LSMO sðoÞ can be now calculated from n~2 and the result is plotted in Fig. 3a, together with an estimate of
ARTICLE IN PRESS M. Ortolani et al. / Physica B 403 (2008) 1922–1926
σfilm(Ω-1cm-1)
1000 100 T = 295 K 245 K 220 K
10
T(+Vg) / T(-Vg)
1 1.02
Vg = 30 V
1.00
0.98 0
1000
2000
3000
4000
5000
wavenumbers (cm-1) Fig. 3. (a) Infrared conductivity of the LSMO film as obtained from the Drude–Lorentz fit. (b) Field modulation of the infrared transmission of the active channel, measured with synchrotron radiation through an aperture of 15 5 mm2 and 20 sets of 512 scans. The bar indicates the experimental uncertainty on the absolute level.
the dc conductivity obtained by a geometrical calculation from the resistance data in Fig. 1a. sðoÞ strongly increases with increasing o and it is weakly temperature-dependent, as usually found in non-metallic manganites [12–14]. Since the conductivity in the mid-infrared is three order of magnitude higher than that at zero frequency, we can conclude that most of the doped holes are trapped in bound states. The broad maximum in sðoÞ around 3500 cm1 can then be interpreted as a polaron conductivity band [7,13]. The exact nature of the polaron conductivity band (charge polaron, spin polaron, excitation involving the orbital degree of freedom) is currently under debate, but in all cases some effect of an external electric field on the conductivity may be expected. We then studied the effect of the external field on the transmittance of the active channel. Under a gate bias V g , the accumulated charges will decrease the channel transmission from the equilibrium T film ðoÞ to TðV g ÞðoÞ in a frequency range defined by their optical conductivity sa ðoÞ. It is reasonable to expect sa ðoÞ to have contributions in the mid-infrared range, since the accumulated charges may behave like polarons just like the pre-existing carriers. The exact amount of the transmission change in the mid-infrared is difficult to predict, since it depends both on the density of accumulated charges na and the thickness of the accumulation layer a, but a reasonable guess is that it should be of the order of the dc conductivity modulation, i.e. 3%. An opposite effect is expected for positive gate bias þV g . Fig. 3b shows the channel transmission ratio TðþV g Þ=TðV g Þ, obtained after averaging 20 successive acquisitions of 512 scans each with an aperture size of 15 5 mm2 . No field effect on TðþV g Þ=TðV g Þ is observed
1925
within the experimental uncertainty of 0:3% at any temperature up to V g ¼ 30 V. Note that we made sure that no net current was flowing in the channel within noise levels ðo0:5 nAÞ. The data in Fig. 3 might be explained (i) by a of the order of one unit cell, or (ii) by very small na (much less than 3% of the pre-existing carrier density n), or (iii) by sa ðoÞ=na 5sfilm ðoÞ=n in the mid-infrared. In all cases, the variation of TðþV g Þ=TðV g Þ could be smaller than our noise level. The first case cannot be excluded completely, but we recall that the present film has a reduced n, hence a should be larger than in a film with full oxygen stoichiometry. In the second case, the effect of the electric field in this manganite would be different from carrier accumulation, but still producing dc conductivity modulation (see for example Ref. [4]). In the third case, sa ðoÞ would be significantly different from zero only at low frequencies, indicating that the accumulated carriers behave mainly as free carriers with minor modification of the polaronic states, as recently found in rubrene FETs [10]. Infrared studies at lower frequencies, which have to be performed on differently patterned devices to overcome the diffraction limit, would help to check the latter possibility, in order to understand whether the resistance modulation in manganites is due to simple band filling in a rigid band picture or rather the density of states is rearranged by band filling, as indeed expected for a strongly correlated material. The above reported result is somehow surprising, since in semiconducting manganites the charge transport modulation by an external electric field weaker than those used in the present experiment should be related to large changes in the polaron system [15], which in turn should influence the infrared absorption. We therefore explored all known possibilities for a plausible intrinsic cause for the lack of an observable field effect in the infrared spectrum in the present experiment. The infrared signal should be observable in this experiment thanks to an enhanced electric field penetration depth in the present semiconducting manganite film. However, the latter property is based on the peculiar chemical composition of the present sample, which features an oxygendeficient LSMO layer over a STO substrate which is stoichiometric in oxygen content. One could therefore mention the possibility of oxygen ion migration from the STO substrate to the oxygen-deficient manganite film, whose equilibrium composition is also at full oxygen stoichiometry. Oxygen migration is normally observed at high temperatures, but it might eventually take place at room temperature together with prolonged illumination of tightly focussed synchrotron radiation, at least at the interface layer of the micrometric area of the side-gate transistor channel. If this was the case of our sample, the LSMO layer at the interface with the substrate would have metallic properties instead of semiconducting ones, and the field effect in the infrared spectrum could have been reduced. Since the re-oxygenated metallic
ARTICLE IN PRESS 1926
M. Ortolani et al. / Physica B 403 (2008) 1922–1926
areas might be inhomogeneous or not connected, they would not necessarily lead to metallic-like dc transport in Fig. 1a. Similar studies on semiconducting manganites which are stoichiometric in oxygen content, such as Nd0:7 Sr0:3 MnO3 or La1x Cax MnO3 are in progress to clarify this point. In conclusion, we used infrared synchrotron microspectroscopy to probe a manganite FET with characteristic length scales at the diffraction limit. We exploited the side-gate configuration to determine the optical constants of the active material and hence to derive the presence of bound polarons. A major modification of the occupation of the polaron band, due to the shift of the Fermi level under application of an external electric field to the gate electrode, can be ruled out in the present device. M.O. acknowledges support of the Investment Bank Berlin under the project Terahertz Radar. I.P., A.G. and D.M. acknowledge support from the EU under the project Nanoxide, contract no. 033191.
References [1] I. Pallecchi, L. Pellegrino, E. Bellingeri, A.S. Siri, D. Marre´, Appl. Phys. Lett. 83 (2003) 4435. [2] H. Tanaka, J. Zhang, T. Kawai, Phys. Rev. Lett. 88 (2002) 027204. [3] T. Zhao, S.B. Ogale, S.R. Shinde, R. Ramesh, R. Droopad, J. Yu, K. Eisenbeiser, J. Misevich, Appl. Phys. Lett. B 84 (2004) 750. [4] I. Pallecchi, L. Pellegrino, E. Bellingeri, A.S. Siri, D. Marre´, Phys. Rev. B 71 (2005) 014406. [5] X. Hong, A. Posadas, C.H. Ahn, Appl. Phys. Lett. 86 (2005) 142501. [6] T. Kanki, H. Tanaka, T. Kawai, Appl. Phys. Lett. 89 (2006) 242506. [7] A. Nucara, A. Perucchi, P. Calvani, T. Aselage, D. Emin, Phys. Rev. B 68 (2003) 174432. [8] N. Sai, Z.Q. Li, M.C. Martin, D.N. Basov, M. Di Ventra, Phys. Rev. B 75 (2007) 045307. [9] F.X. Hu, J. Gao, Appl. Phys. Lett. 88 (2006) 132502. [10] M. Fischer, M. Dressel, B. Gompf, A.K. Tripathi, J. Pflaum, Appl. Phys. Lett. 89 (2006) 182103. [11] P. Dore, A. Paolone, R. Trippetti, J. Appl. Phys. 80 (1996) 5270. [12] K. Tobe, T. Kimura, Y. Tokura, Phys. Rev. B 69 (2004) 014407. [13] M.W. Kim, J.H. Jung, K.H. Kim, H.J. Lee, J. Yu, T.W. Noh, Y. Moritomo, Phys. Rev. Lett. 89 (2002) 016403. [14] K. Takenaka, R. Shiozaki, S. Sugai, Phys. Rev. B 65 (2002) 184436. [15] S.B. Ogale, V. Talyansky, C.H. Chen, R. Ramesh, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 77 (1996) 1159.