Thin Solid Films 520 (2012) 6393–6397

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Thin Solid Films

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Morphological, optical and electrical properties of samarium oxide thin films

Catalin Constantinescu a,⁎, Valentin Ion a, Aurelian C. Galca b, Maria Dinescu a

a INFLPR — National Institute for Laser, Plasma and Radiation Physics, PO Box MG-16, Magurele RO-077125, Bucharest, Romaniab INFM — National Institute of Materials Physics, P.O. Box MG-7, Magurele RO-077125, Bucharest, Romania

⁎ Corresponding author. Tel.: +40 740044770; fax: +E-mail address: catalin.constantinescu@inflpr.ro (C.

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.06.049

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 September 2011Received in revised form 12 June 2012Accepted 13 June 2012Available online 20 June 2012

Keywords:Samarium oxideThin filmsRadio-frequency pulsed laser depositionAtomic force microscopySecondary ion mass spectroscopyOptical properties

We present here results on samarium oxide thin films, obtained by pulsed laser deposition and by radio fre-quency assisted pulsed laser deposition. Three different substrate types were used: silicon, platinum coveredsilicon and titanium covered silicon. The influence of the deposition parameters (oxygen pressure and laserfluence) on the structure and morphology of the thin films was studied. The substrate-thin film interfacezone was investigated; the optical and electrical properties (the losses, dielectric constant and leakagecurrents) were also determined.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Considering their high resistivity, high dielectric constants, andlarge band gap characteristics, rare earth oxides have recently beenextensively investigated for applications, including: optoelectronicdevices, switching mechanism for logic devices, and memories.Amorphous SiO2 is the mostly used gate dielectric material in com-plementary metal–oxide–semiconductor (CMOS) processing, becauseof its remarkable properties as a stable (thermodynamically andelectrically), high-quality Si–SiO2 interface and superior electrical iso-lation properties [1]. It is known that for the industry it is necessary toobtain greater integrated circuit functionality and performance atlower cost. The circuit density must be increased, which has translatedinto a higher density of transistors on awafer. In addition, this shrinkingof the transistor feature size has forced the channel length and gatedielectric thickness to decrease rapidly [2]. The dielectric's thicknessin integrated circuits has been downscaled to 1–3 nm in order toachieve smaller physical dimensions, lower driving voltage and higheroperation speed. A further reduction in the oxide thickness would im-pose several problems, including a high level of the leakage current[3,4] and a large degree of dopant (boron) diffusion in the gate oxide.This downscale of dimension will result in the replacement of SiO2 byothermaterials, due also to the excessive tunneling current and reliabil-ity issues.

Some compounds, such as: Ta2O5 [5,6], Hf1−xSixO2, [7], Al2O3

[8–10], and ZrO2 [2,11,12] are potentially good candidates because oftheir high permittivity (high-k), but they exhibit however undesirable

40 318115383.Constantinescu).

rights reserved.

properties (e.g.: strong leakage currents, film morphology and defectsin the film/interface, etc.), which remain a concern regarding thereplacement of SiO2. Recently, lanthanide oxides – samarium oxide inparticular – are becoming an alternative solution for gate dielectricsin metal–oxide–semiconductor (MOS) devices and other high-tech ap-plications [13–19]. Alternative methods for processing of materials,using laser and plasma sources, i.e. pulsed laser deposition (PLD), andradiofrequency assisted pulsed laser deposition (RF-PLD), were usedto obtain thin films of Sm2O3. These techniques are clean and simple,and were demonstrated to be effective for the growth of films with awide range of compositions, structures and properties [19–29]. Here,we analyze Sm2O3 thin films in terms of morphological, optical andelectrical properties; the thin films were grown at room temperature,as this does not influence thin film stoichiometry [14,21–24].

2. Experimental conditions

2.1. Method, target and thin films

The PLD technique consists in material ablation/removal bybombarding the surface of a target with short energetic pulses of afocalized laser beam, of proper wavelength. This process of laser abla-tion takes place in a vacuum chamber, or in a special gas medium thatis held at constant pressure (in our case: oxygen). Due to the highpower density of the beam, plasma having a plume shape perpendic-ular to the target surface is generated at the incident point (Fig. 1).The substrate to be coated is placed a few centimeters apart of the tar-get facing the top of the plasma plume. Detailed information on theprocedure and experimental setup is presented elsewhere [21–26].

Fig. 1. Image of the RF-PLD experimental setup during the deposition of Sm2O3 thinfilms: the plume at the center is the ablation plasma, and the spherical one on theleft side is the RF-generated one, in oxygen gas.

Fig. 2. AFM images of the Sm2O3 thin films, deposited at identical laser fluence, on:silicon (a), platinum covered silicon (b) and titanium covered silicon (c).

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The target was prepared starting from Sm2O3 powder of 99.9%purity, purchased from Sigma-Aldrich. 2 g were weighted and subse-quently compressed at 10 t/cm2 pressure with a hydraulic press, inthe form of a 13 mm in diameter and 3 mm thick disc. The substrates:silicon, platinum covered silicon (500 nm Pt with a 20 nm Ti bufferlayer, produced by Siegert Consulting e.K.) and titanium coveredsilicon (laboratory made, by depositing 50 nm of Ti on silicon sub-strates), were firstly cleaned in an ultrasonic bath for 15 min, using ace-tone and isopropanol as cleaning mediums, then dried under nitrogenpressured gas. The three substrate types were selected as such in orderto investigate Sm2O3 thin film's adhesion, substrate-thin film interfacequality, andmorphological influence on the thin film growth; themetal-lic layers were also used as bottom electrodes in electrical investigations.

A laser beam from a pulsed Nd:YAG laser system (266 nm wave-length, 7 ns duration of the pulse, 10 Hz) was focused on the Sm2O3

target, with the laser spot of 0.8 mm2. Thin films were preparedwith laser fluences in the range of 2–4 J×cm−2, using 20,000 pulses,at 4 cm target-substrate distance, with three different kinds of sub-strates on each round, with and without using oxygen RF-assistedplasma beam. The RF plasma beam system was run by a 13.56 MHzradiofrequency power supply (Caesar 1310, RF maximum power1000 W); the RF power was chosen to be 75 W, based on the reporteddata in the literature [21,24,29,30]. The substrates were kept at roomtemperature during the depositions, while the oxygen pressure rangeduring deposition was set between 0.02 and 0.6 mbar.

2.2. Analysis

The films surface aspect and roughness were checked using atom-ic force microscopy (AFM), on several different areas and dimensions,with a “Park XE-100” AFM setup produced by “Park Systems”. Theseinvestigations were made in non-contact mode using a silicon carbidetip (10 nm radius of curvature). Scanning electron microscopy (SEM)analyses were performed with an Inspect F FEG-SEM; the electronacceleration voltage can be set between 200 V and 30 kV, while thelateral resolution is approximately 2 nm. A depth composition inves-tigation of the samples was performed on a secondary ion massspectrometry/secondary neutral mass spectrometry (SIMS/SNMS)system, produced by Hiden Analytical. The setup is a high currentdensity (up to 1 mA), with a quadrupole mass spectrometer, andhas a depth profile resolution of approximately 1–5 nm; the lowestmass detection threshold is 0–300 amu, and the detection limit isequal to or below 1 ppm. The formation of crystalline phases wasfollowed by X-ray diffraction (XRD); the diffractograms were collect-ed on a “PAN'alytical X'Pert PRO MRD”, in a 2θ range of 10–80°, usinga CuKα X-ray source. Capacitance and dissipation factor of the

samarium oxide thin layer were measured with an Agilent 4294Aimpedance analyzer, at room temperature, in the range of 1 kHz–2.5 MHz; leakage current densities were measured with an AixACCTTF2000 analyzer, with precision in the order of pico-amperes. Opticalmeasurements were performed by using a Woolam Vertical VariableAngle Spectroscopic Ellipsometer (V-VASE), equipped with a highpressure Xe discharge lamp incorporated in an HS-190 monochroma-tor. Spectroscopic-ellipsometry (SE) measurements were performedin the visible and near-UV region of the spectrum at energies be-tween 250 nm and 1350 nm, step of 10 nm, at 60° and 65° angles ofincidence.

Fig. 4. SNMS spectra revealing the sandwich structure, and the interfaces, of a Sm2O3

sample deposited on platinum covered silicon.

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3. Results and discussion

The AFM analysis of the thin film surface revealed smooth surfacewith few droplets covering the structure, as it can be seen in Fig. 2(a, b, c). There is no significant difference between the PLD depositedfilms and the RF-PLD ones, in terms of roughness (as root mean squarevalue, or RMS), or in droplets' presence. Also, the substrate type doesnot influence the morphology. The RMS of the thin films range 5 to10 nm [16,17,19].

The SEM images in Fig. 3 (a,b) also reveal that the thin film's sur-face is covered in droplets, a phenomenon that was also observed in[14,16,18]. In [14], it has been reported that a rare earth silicatelayer, and a silicon oxide layer, will form at the rare earth oxide thinfilm/silicon substrate interface under thermal treatment (annealing);we did not observe this on depositions at room temperature.

The SNMS spectra in Fig. 4 reveal the layer structure and interfacesof a multi-structured sample, i.e. Sm2O3 on platinum covered silicon,with respect to the sputtering time. Although the information fromthe SIMS/SNMS gives information on the quality of the samples, norigorous quantitative information can be drawn due to calibration is-sues (different rates of sputtering, according to each material). Theinterfaces are clearly visible, with very little diffusion between layersand/or substrate; similar results were obtained for the Sm2O3 on sili-con, and on titanium covered silicon.

The XRD revealed that all the films have only an amorphous back-ground with respect to the reference target material (not shown

Fig. 3. SEM images of the Sm2O3 thin films deposited on silicon substrates: few dropletscan be seen at the surface; the thin film is compact, as seen in cross section (a), andcross section focused at the edge (b).

here). Therefore, we consider our thin films to be uncrystallized atroom temperature, consistent with reported literature [13,16,17];this fact may be due to the native oxide present on the silicon surface[16], or the lattice mismatch.

In Fig. 5 a, the SE spectra of a 61.4±0.3 nm thick Sm2O3 on siliconsubstrate are presented; other values are: the roughness of 4.6±0.3 nm, and the dielectric function (expressed in n and k; n=1.94,and k=6×10−3, at 550 nm), as shown in Fig. 4 b. The optical modelwe used consists of 5 layers (for thefilms deposited on silicon): the sub-strate, the Si–SiO2 interface, the native SiO2, the Sm2O3 layer, and arough top layer which is set to be half air and half Sm2O3. The refraction

Fig. 5. Spectroscopic-ellipsometry spectra on a Sm2O3 film: in a), the open symbolsrepresent experimental data, while the solid line (Ψ) and dashed line (Δ) are obtainedfrom a fitting procedure using a five-layer model; in b), the refractive index (solid line)and the extinction coefficient (dashed line), for the Sm2O3 thin films, are presented.

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indexes of SiO2 and Si are from [31–33]. In order to extract the refractiveindices from the data, the extinction coefficients and the thickness ofthe thin films, respectively, experimental data was fitted using Cauchydispersion with Urbach absorption [27,30,31,34–36]. The obtainedSm2O3 thin films are highly transparent down to 400 nm; the bandgap, Eg, cannot be determined accurately. However, Eg value is in linewith the one determined in [13], being over 4.33 eV (~286 nm). The re-fractive index n reveals that Sm2O3 is a high refractive index material[13,19,20,34,37]; results are consistent to previous published data[20]. In [37], it is given a value of 2.09 at a wavelength of 589.3 nm.This value corresponds to monoclinic crystalline phase and it is with0.1 larger than our value. The refractive index of materials in transpar-ent spectral range is dependent on the atomic (weight) density, andtherefore the amorphous materials have a smaller refractive indexthan the crystalline counterpart [31,38–43]. From ellipsometry mea-surements we can conclude that the thin films are mainly amorphous,consistent with the XRD measurements.

In order to investigate the electrical properties of the thin films, theplatinum covered silicon substrates were chosen, to form capacitors[22,24,27]; top side platinum electrodes were deposited on the Sm2O3

thin films by PLD, from a platinum target, using a stainless steel maskwith 0.27 mm2 circular apertures (hole matrix). The depositions tookplace in vacuum (7×10−6 mbar), at a laser fluence of 2 J×cm−2,20,000 pulses, and at 6 cm target-substrate distance. Electrical mea-surements were performed using a laboratory-made test system. Thedependence of the capacitance to the frequencywas found to range be-tween 615 and 532 pF, at room temperature. The dielectric constant εrwas determined from the capacitancemeasurements, using the parallelcapacitor approximation:

C ¼ ε0εrSd;

Fig. 6. The electrical characteristics of the Sm2O3 thin films deposited by RF-PLD: thedielectric constant ε (a), and the leakage current measurements (b).

where d is the thickness of the samarium oxide thin film (from thespectroscopic-ellipsometry measurements), and S is the area of theupper electrode (with dbbS); it ranges between 14 and 16 at roomtemperature. The losses are small at low frequencies (10−2 at 1 kHz)but the errors in measurement are high due to experimental setup.Leakage current measurements show very low values for samariumoxide thin films, in the range of 10−6 A/cm2–10−5A/cm2, and at highelectrical field (800 kV/cm) (Fig. 6). The I–V behavior has a hysteresisbetween 300 and 800 kV/cm, possibly due to Schottky emission effectbetween the platinum electrode and the Sm2O3 film [13]; furtherwork is in process to clarify the origin of this behavior, as well as to im-prove the characteristics of the thin films, in terms of electric losses, asthis may be related to the chemical stability of the materials and inter-face properties.

4. Conclusion

In this paper we demonstrate that good quality, amorphousSm2O3 thin films can be achieved by PLD and RF-PLD, on differenttypes of substrates. The AFM, SEM and SE measurements correlatein the thin film thickness and morphology. The dependence of the di-electric constant to the frequency was found to range between 615and 532 pF, at room temperature. The dielectric constant εwas deter-mined from the capacitance measurements, ranging between 14 and16 at room temperature. Further investigations are in process to es-tablish the influence of laser wavelength, laser fluence, substrate tem-perature and oxygen gas pressure upon the thin film properties.

Acknowledgments

The authors acknowledge the help provided by: Ioana Goldner,Dan Colceag, Antoniu Moldovan, Catalin Luculescu, Ruxandra Birjega.This work was supported by a grant of the Romanian NationalAuthority for Scientific Research, CNCS–UEFISCDI, project numberPN-II-RU-TE-2011-3-0301.

References

[1] G.D. Wilk, R.M. Wallance, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243.[2] D.A. Buchanan, S.H. Lo, Microelectron. Eng. 36 (1997) 13.[3] G. Heiser, A. Schenk, J. Appl. Phys. 81 (1997) 7900.[4] G.B. Alers, D.J. Werder, Y. Chabal, H.C. Lu, E.P. Gusev, E. Garfunkel, T. Gustafsson,

R.S. Urdahl, Appl. Phys. Lett. 73 (1998) 1517.[5] J.V. Grahn, P.E. Hellberg, E. Olsson, J. Appl. Phys. 84 (1998) 1632.[6] G.D. Wilk, R.M. Wallance, J.M. Anthony, J. Appl. Phys. 87 (2000) 484.[7] T.M. Klein, D. Niu, W.S. Epling, W. Li, D.M. Maher, C.C. Hobbs, R.I. Hegde, I.J.R.

Baumwol, G.N. Parsons, Appl. Phys. Lett. 75 (1999) 4001.[8] E.P. Gusev, M. Copel, E. Cartier, I.J.R. Baumwol, C. Krug, M.A. Gribelyuk, Appl. Phys.

Lett. 76 (2000) 176.[9] V.V. Afanas'ev, A. Stesmans, B.J. Mrstik, C. Zhao, Appl. Phys. Lett. 81 (2002) 1678.

[10] W. Qi, R. Nich, B. Lee, L. Kang, Y. Jeon, J.C. Lee, Appl. Phys. Lett. 77 (2000) 3269.[11] K. Sasaki, J. Maier, Solid State Ionics 134 (2000) 303.[12] R.W. Eason, Pulsed Laser Deposition of Thin Films, John Wiley & Sons, New York,

2007.[13] A.A. Dakhel, J. Alloys Compd. 365 (2004) 233.[14] H. Ono, T. Katsumata, Appl. Phys. Lett. 78 (2001) 1832.[15] V.A. Rozhkov, A.Y. Trusova, I.G. Berezhnoy, Thin Solid Films 325 (1998) 151.[16] K. Shalini, S.A. Shivashankar, Bull. Mater. Sci. 28 (1) (2005) 49.[17] J. Paivasaari, M. Putkonen, L. Niinisto, Thin Solid Films 472 (2005) 275.[18] H.H. Huang, H.P. Chang, Y.T. Chien, M.C. Huang, J.S. Wang, J. Cryst. Growth 287

(2006) 458.[19] H. Yang, H. Wang, H.M. Luo, D.M. Feldmann, P.C. Dowden, R.F. DePaula, Q.X. Jia,

Appl. Phys. Lett. 92 (2008) 062905.[20] D. Yang, L. Xue, In: MRS Proceedings, 780, 2003, http://dx.doi.org/10.1557/PROC-

780-Y1.3, (Y1.3).[21] M. Filipescu, P.M. Ossi, M. Dinescu, Appl. Surf. Sci. 254 (2007) 1347.[22] M. Filipescu, N. Scarisoreanu, V. Craciun, B. Mitu, A. Purice, A. Moldovan, V. Ion, O.

Toma, M. Dinescu, Appl. Surf. Sci. 253 (2007) 8184.[23] I. Vrejoiu, D.G. Matei, M. Morar, G. Epurescu, A. Ferrari, M. Balucani, G. Lmedica, G.

Dinescu, C. Grigoriu, M. Dinescu, Mater. Sci. Semicon. Pocess. 5 (2–3) (2003) 253.[24] M. Filipescu, N. Scarisoreanu, D.G. Matei, G. Dinescu, A. Ferrari, M. Balucani, O.

Toma, C. Ghica, L.C. Nistor, M. Dinescu, Mater. Sci. Semicond. Process. 7 (2004)209.

6397C. Constantinescu et al. / Thin Solid Films 520 (2012) 6393–6397

[25] C. Constantinescu, N. Scarisoreanu, A. Moldovan, M. Dinescu, M. Miron, L.Petrescu, In: Proceedings of SPIE “ALT'06: Advanced Laser Tehnologies”, 6606,2007, p. 660619.

[26] C. Gheorghies, S. Condurache-Bota, M. Dinescu, C. Constantinescu, N. Cazacu,Communications 2 (2008) 569.

[27] L. Nedelcu, A. Ioachim, M.I. Toacsan, M.G. Banciu, I. Pasuk, M. Buda, N.Scarisoreanu, V. Ion, M. Dinescu, Appl. Phys. A 93 (2008) 675.

[28] N. Scarisoreanu, G. Dinescu, R. Birjega, M. Dinescu, D. Pantelica, G. Velisa, N.Scintee, A.C. Galca, Appl. Phys. A 93 (2008) 795.

[29] N. Scarisoreanu, F. Craciun, V. Ion, S. Birjega, M. Dinescu, Appl. Surf. Sci. 254(2007) 1292.

[30] T.M. Pan, C.C. Huang, S.X. You, C.C. Yeh, Electrochem. Solid-State Lett. 11 (2008)G62.

[31] V. Ion, A.C. Galca, N. Scarisoreanu, M. Filipescu, M. Dinescu, Phys. Status Solidi C 5(2008) 1180.

[32] C.M. Herzinger, B. Johs, W.A. McGahan, J.A. Woollam, W. Paulson, J. Appl. Phys. 83(1998) 3323.

[33] H.R. Philipp, In: in: E.D. Palik (Ed.), Academic Press, London, 1985, p. 759.

[34] O. Medenbach, D. Dettmar, R.D. Shannon, R.X. Fischer, W.M. Yen, J. Opt. A: PureAppl. Opt. 3 (2001) 174.

[35] L. Cauchy, Bull. Sci. Math. 14 (1830) 6.[36] F. Urbach, Phys. Rev. 92 (1953) 1324.[37] R.M. Douglass, E. Staritzky, Anal. Chem. 28 (1956) 552.[38] A.C. Galca, Ph.D. Thesis; University of Twente, The Netherlands (2006) ISBN:

90-365-2348-6.[39] Spectroscopic Ellipsometry Principles and Applications, Hiroyuki Fujiwara, Published

by Maruzen Co. Ltd, Tokyo, Japan (2007) ISBN-13: 978-0-470-01608-4.[40] C.D. Constantinescu, Ph.D. Thesis; Politehnica University of Bucharest, Romania

(2010).[41] G.E. Stan, I. Pasuk, A.C. Galca, A. Dinescu, Dig. J. Nanomater. Biostruct. 5 (2010)

1041.[42] S. Heiroth, R. Ghisleni, T. Lippert, J. Michler, A. Wokaun, Acta Mater. 59 (2011)

2330.[43] In: Harland G. Tompkins, Eugene A. Irene (Eds.), Handbook of Ellipsometry, William

Andrew, Inc, ISBN: 0-8155-1499-9, 2005.