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1Voronezh State University, Universitetskaya pl.,1, 394006, Voronezh, Russia. 2 Voronezh State University of Architecture and Civil Engineering, 20-Letiya ...
Materials Science Forum Vol. 700 (2012) pp 53-57 Online available since 2011/Sep/27 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.700.53

Effect of Thermal Annealing on Electrical Properties of Si-LiNbO3 Ievlev V.M.1,a, Sumets M.P.2,b, Kostyuchenko A.V.3,a 1

Voronezh State University, Universitetskaya pl.,1, 394006, Voronezh, Russia

2

Voronezh State University of Architecture and Civil Engineering, 20-Letiya Oktyabrya st., 84, 394006,Voronezh, Russia a

[email protected], b [email protected]

Keywords: thin films of LiNbO3, structural analysis, ferroelectrics, electrical properties.

Abstract. The substructure and electrical properties of the films with the thickness up to 2.0 µm deposited on Si by the methods of the radio frequency magnetron and ion-beam sputtering of a LiNbO3 target have been investigated. It has been established that in thermally annealed samples an activated conduction mechanism with variable jump distance takes place. As a result of thermal annealing a decrease in the charge localization centers (CLC) in the LiNbO3 films from Nt=3·1018cm-3 to Nt=3·1016cm-3 occurs. Introduction The investigations of the synthesis regularities for the LiNbO3 films, their structure and properties remain the problem of current concern due to a relatively wide range of their potential and practical applications in heterostructures for storage devices, piezoelectric transducers, acoustooptical devices, optical and acoustic wave conductors, etc. In vacuum technologies, RF magnetron sputtering (RFMS) is the most efficient method for growing the films of complex oxides while preserving the initial elemental composition. Possibilities of acquiring LiNbO3 films with the use of this method are described in a series of papers (for example [1, 2]); the possibility to growth fine epitaxial films is described in [3]. The aim of the present work is to study the nature of the changes in the electrical properties of the Si-LiNbO3 heterostructures as a result of the thermal annealing (TA). The films were deposited during the process of radio frequency magnetron sputtering (RFMS) and ion-beam sputtering (IBS) of the single-crystal target LiNbO3 in an Ar (1⋅10-1 Pa) atmosphere. The sputtering was performed at 100 Wt and 70 Wt for the magnetron and the source, respectively. Si(100) and Si(111) plates with n-type conductivity (ρ=4,5Ω·cm) were used as substrates. Some heterostructures were treated by thermal annealing (TA) or pulse photon irradiation (PPI) by xenon lamp emission. The substructure, phase composition and morphology of the film surface were studied by the method of transmission electron microscopy (EMV-100BR), electron diffraction investigation (EG-100M), X-ray diffraction using Cu Kα radiation (ARL X’TRA Thermo Techno) and scanning probe microscopy (Solver P47). The electrical properties of the heterostructures were investigated by the methods based on the I-V characteristic and the frequency dependences of conductivity (Solartron 1260). Results and discussion During the RFMS and IBS processes on the unheated substrates (001), (111)Si the amorphous films are formed. During the ion assisted RFMS process on the heated substrates (550°C) (due to the effect of plasma magnetron discharge) the single-phase films of LiNbO3 with the - axis preferred orientation (Fig.1a,b) are formed. During the IBS and RFMS processes in the absence of ion assistance, single-phase nanostructured films of LiNbO3 with the arbitrary orientation of the grains are formed. With the increase of the films thickness the phase composition All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 152.14.136.96-13/10/12,15:54:32)

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and the grain orientation remain unchanged. The average crystallite size for the films of variable thickness (t) increases with the thickness increasing from 5nm at t~20nm to 35nm at t~100nm. The PPI of the heterostructures ‘Si – amorphous film’ in air (the energy density of irradiation 160 J·cm2 ) leads to crystallization with the formation of a single-phase nanocrystalline structure LiNbO3. The PPI (160 J·cm-2) of the heterostructures ‘Si – nanocrystalline LiNbO3 film’ leads to recrystallization and equilibrium structure formation, with the - axis preferred orientation remaining. As a result of TA (700°C, 60 min) of the initial amorphous films on Si in air the crystallization with the formation of a single-phase nanocrystalline LiNbO3 film takes place. As a result of TA of the initial nanocrystalline films on Si in air the recrystallization with the formation of the block crystalline structure (the block size up to 200 nm) composed of the LiNbO3 subgrains (the average subgrain size 80nm) takes place (Fig. 1c,d).

Fig. 1 TEM images and electron diffraction patterns of the films of about 0,1µm in thickness deposited on the heated (550°C) substrate Si(001) by RFMS method. (a-b) as-grown, (c-d) after TA (700°C, 60 min).

Intensity

The X-ray diffraction patterns (Fig.2) show that as a result of TA of the nanocrystalline film 0,1µm thick, deposited by the IBS method, the average grain size increases two times (from 50nm to 95nm according to Debye – Scherrer method), the phase composition of the films is not changed and the arbitrary orientation of the grains remains.

deg. Fig. 2. The X-ray diffraction patterns of the heterostructure Si(001)-LiNbO3 film 1,0 µm thick deposited by the IBS method: 1- the initial heterostructure, 2- after TA (700°C, 60 min).

As a result of the average roughness the film surface is considerably increased: within the limits of the scanned area 1,5×1,5 µm2 from 45 to 105 nm for the initial nanocrystalline films and from 3 to 26nm for the initial amorphous films 1,0 µm thick.

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The current-voltage characteristics of the heterostructures Si(001)-LiNbO3-Al were taken in range of the temperatures T=20-120°C. The analysis shows that in the heterostructures studied the mechanism of hopping conduction on the charge localization centers (CLC) in the LiNbO3 layer with the variable jump distance is probably realized [4,5].

Leakage current density (A/m2)

1,E+04 1,E+03 1,E+02 2 1

1,E+01 1,E+00 0,0

1,0

2,0

3,0

4,0

5,0

1,E-01 1,E-02 Voltage (V)

Fig. 3. I-V characteristic for the (001)Si-LiNbO3-Al heterostructures at T=20°C (1 – the initial heterostructures, 2 – the heterostructures after annealing).

In the initial heterostructures the hopping conduction mechanism of a non-activated character and may be written in the form: 1    E0  4   J = J 0 exp −     E    

(1)

Where E – the electric field, Jo – the constant independent on the electric field, E0 – parameter of the Mott model. After the TA in the heterostructures the two mechanisms of hopping conduction are realized (Fig. 4): the non-activated mechanism (in the temperature range T=60-120°C) and the thermally activated one (in the temperature range T=20-60°C), which has the form [4]: 1   4 T   0 J (T ) = 1 exp  −     T   T2  

J0

Where T0 – characteristic temperature.

(2)

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Advanced Materials and Nanotechnology

8 7 6

lnJ

5 2 1

4 3 2 1 0 2,5

2,7

2,9

3,1 3

3

3,3

3,5

-1

10 /T, 10 K

Fig. 4. The temperature dependences of the heterostructure conductivity (1 - as-grown, 2 – after TA).

The characteristic property of the hopping conduction mechanism is its dependence on the frequency of the electric field: σ ∝ ω α , where α is changed in the interval α=0,4-0,8 [4,5]. In our case α=0,7-0,9 for initial heterostructures and α=0,8 for the heterostructures after TA. Currentvoltage characteristics and the frequency dependences of conductivity are given in the Table 1.

Type of heterostructure as-grown after annealing 700°C

Table 1. The parameters of hopping conduction mechanism for the LiNbO3 films. Resistivity Average Localized states density CLC concentration, jump distance D(E), [eV-1cm-3] Nt, [cm-3] ρ, [Ω·cm] R, [Ǻ] 9 3·10 55 1,2·1020 3·1018 8·1010 250 2,1·1018 3·1016

Summary 1. 2.

By the methods of the RF sputtering and ion-beam sputtering the single-phase nanocrystalline films of LiNbO3 and of LiNbO3 with - axis preferred orientation have been obtained. The thermal annealing of the initial nanocrystalline films of LiNbO3 on Si leads to the change of the non-activated hopping conduction mechanism to the activated one with the variable jump distance. It has been shown the decrease in the concentration of the charge localization centers in the LiNbO3 film, which give rise to the hopping conduction mechanism.

This work has been supported by an RFBR (Russian Foundation for Basic Research) grant (09-0300189а) and by the grant of the President of the Russian Federation “The Support of the Leading Scientific Schools” (SS-4828.2010.3).

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References [1] T.-H. Lee, F.-T. Hwang, C.-T. Lee, H.-Y Lee, Investigation of LiNbO3 thin films grown on Si substrate using magnetron sputter, Mater. Sci. & Eng. 136 (2007) 92-95. [2] S.-W. Choi, Y.-S. Choi, D.-G. Lim, S.-I. Moon, S.-H. Kim, B.-S. Jang, Y Junsin, Effect of RTF treatment on LiNbO3 MFS memory capacitors, The Korean J. of Ceram. 6 (2000) 138-142. [3] P.J. Hansen, Y. Terao, Y. Wu, A. Robert, J.S. Speck, LiNbO3 thin film growth on (0001)GaN, J. Vac. Sci. Technol. 23 (2005) 162-167. [4] N. F. Mott, E. A. Davis. Electronic Processes in Non-Crystalline Materials, ClarendonPress, Oxford 1971. N. Easwaran, C. Balasubramanian , S. A. K. Narayandass , D. Mangalaraj Dielectric and AC [5] Conduction Properties of Thermally Evaporated Lithium Niobate Thin Films, Phys. Status Solidi 129 (2006) 443-351.

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Effect of Thermal Annealing on Electrical Properties of Si-LiNbO3 10.4028/www.scientific.net/MSF.700.53 DOI References [1] T. -H. Lee, F. -T. Hwang, C. -T. Lee, H. -Y Lee, Investigation of LiNbO3 thin films grown on Si substrate using magnetron sputter, Mater. Sci. & Eng. 136 (2007) 92-95. doi:10.1016/j.mseb.2006.09.001