On the electronic transport properties of bismuth oxide thin films

Journal of Non-Crystalline Solids 352 (2006) 1475–1478 www.elsevier.com/locate/jnoncrysol

On the electronic transport properties of bismuth oxide thin films L. Leontie *, G.I. Rusu ‘Al.I. Cuza’ University, Faculty of Physics, Department of Solid State and Theoretical Physics, 11 Carol I Boulevard, 70506 Iasi, Romania Available online 20 March 2006

Abstract The study of the electronic transport properties in polycrystalline Bi2O3 thin films is reported. The samples were prepared by thermal dry oxidation of Bi evaporated films. As prepared films are polycrystalline and b-Bi2O3 phase is prevailing. The mechanism of electrical conduction in studied films is explained by means of Seto model. Some characteristic model parameters were determined: energy barrier Eb = (0.06–0.22) eV, energy Et = (0.17–0.46) eV and density Nt = (2.30 · 1013–1.54 · 1014) cm2 of trapping states.  2006 Elsevier B.V. All rights reserved. PACS: 73.50.Bk; 73.50.Dn; 73.61.Le Keywords: X-ray diffraction; Conductivity; Films and coatings; Absorption

1. Introduction

2. Experimental

In the last years, the investigations on the electronic transport mechanisms and optical properties of bismuth oxide (Bi2O3) in thin films have been much intensified [1– 6]. This fact is due to its interesting characteristics and potential applications such as solar cells, MIS capacitors, microwave integrated circuits, etc. [4,6–9]. The study of the charge transfer is rather complicated in polycrystalline thin films, in which the electrical conduction mechanism is dominated by grain (intracrystalline) and intercrystalline boundary characteristics [10,11]. In a series of previous papers [12–16], we have investigated electrical, optical and photoelectric properties of bismuth oxide thin films, prepared by thermal dry oxidation of vacuum evaporated bismuth films. In present paper, the electronic transport properties in bismuth oxide films are explained using some models proposed for thin films with polycrystalline structure.

Bismuth oxide films were prepared by dry thermal oxidation (in air) of bismuth thin films, deposited onto unheated glass substrates by physical vapor deposition under vacuum. The deposition equipment is in detail described in [17]. Generally, the oxidation procedure consists of following successive runs: a heating (in the temperature range of 300– 775 K); an annealing for a determined time (usually, 0–60 min), at a constant temperature (680–775 K); and a cooling from annealing temperature to the room temperature. The temperature rate was about 3 K/min (slow heating and cooling), and 10 K/min (fast heating and cooling), respectively. The thickness of bismuth oxide films was determined by an interferometric method [18]. The film crystalline structure was examined by X-ray diffraction (XRD) technique. The surface morphology was studied by means of atomic force microscopy (AFM). For a series of samples, we studied the temperature dependence of the electrical conductivity and Seebeck coefficient. The experimental arrangements used for these measurements are described in detail in [19].

*

Corresponding author. Tel.: +40 232 201168; fax: +40 232 201150. E-mail address: [email protected] (L. Leontie).

0022-3093/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.104

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The reflection and transmission spectra (in the spectral range from 280 to 1260 nm) were recorded using a SPECORD UV–VIS M 40 (C. Zeiss, Jena) double-beam spectrophotometer. 3. Results and discussion The correlations between structural characteristics of the films and their oxidation conditions are in detail presented in [12–17]. Thus, a fast heating (heating rate, Rh ’ 10 K/min), until a temperature about 530 K (annealing time ta = 0) and a fast cooling (Rc ’ 10 K/min) to room temperature permit to obtain pure b-Bi2O3 phase (tetragonal structure), for films with lower thickness (d < 0.60 lm). With increasing in the annealing temperature (550 K < Ta < 750 K) and annealing time (0 < ta < 60 min), the films (with thickness d < 0.60 lm) are characterized by a mixture of a-Bi2O3 (monoclinic) and b-Bi2O3, but b phase is predominant. In the case of slow heatings (Rh ’ 3 K/min) the preponderant phase is a-Bi2O3. Our experiments lead to conclusion that values of cooling rate are little important for determination of film structure. In the present paper we are proposed to study the mechanism of electrical conduction in Bi2O3 films. We investigated the polycrystalline films with small thickness (d < 0.60 lm), prepared in conditions that favour a mixed (b-, a-Bi2O3, BiO, as well as other phase components of the Bi–O system) composition, but b-Bi2O3 is predominant. In order to obtain the Bi2O3 films with stable structure and reproducible electronic transport properties, all samples were subjected, after preparation, to a heat treatment consisting of some heating/cooling cycles within a certain temperature range, DT, characteristic for each sample. It was experimentally shown that, after heat treatment, the temperature dependence of the electrical conductivity becomes reversible. This fact indicates the stabilization of film structure and composition. We observed that the lnr = f (103/T) curves for heattreated samples present almost three linear portions. The first one, in the lower temperature range (320 K < T1 < 370 K), is characterized by a smaller slope. The next two portions are characterized by larger slopes. We suppose that in all investigation temperature range, the temperature dependence of the electrical conductivity can be described by [20,21] r ¼ r0  expðEa =kT Þ;

ð1Þ

where r0 denotes a parameter depending on sample characteristics, Ea is the thermal activation energy of electrical conduction, and k is Boltzmann’s constant. In these conditions, for investigated heat-treated samples, Ea changes from 0.06 eV to 0.22 eV for the first portion (T < T1) of lnr = f (103/T) curves, and between 0.52 eV and 0.84 eV for the second portion (T1 < T < T2). In the higher temperature range (T > T2), the values of thermal activation energy are approximately equal to

Eg/2, where Eg is the energy bandgap of bismuth oxide crystals. Undoubtedly, the mechanism of electrical conduction in studied samples can be explained by taking into account the models elaborated for films with polycrystalline (discrete) structure [22–24]. Therefore, the conduction mechanism is dominated by the inherent intercrystallite boundaries [10,11,20]. Generally, the study of electronic transport properties in polycrystalline thin films is based upon the consideration that crystallite boundaries have a space charge region due to interface processes. Consequently, band bending occurs and the potential barriers to the electronic transport result. This fact determines a reduction of carrier mobility and electrical conductivity of a certain material in thin films as compared to single crystals of respective material. A great number of models (Volger [22], Petritz [23], Berger, Seto [25,26], etc.) is currently used for explaining the mechanism of electrical conduction in polycrystalline semiconductor films. These are summarized in some good reviews or monographs [27,28]. In [29,30] we showed that the Seto’s model [25,26] with several modifications, proposed by Baccarani et al. [11,31,32], could explain the mechanism of electrical conduction in studied Bi2O3 films. We take into the account the lnr = f (103/T) curves both for heat-treated samples and for curves obtained at cooling, during heat treatments. We have considered films with thickness d < 0.60 lm, which have a homogenous structure (b-Bi2O3). In the Seto’s model the following assumptions are made: the width of the crystallite boundary is negligible in respect to crystallite size; there is only one type of monovalent impurities at the crystallite boundary, uniformly distributed with a concentration ND; crystallite boundary contains Nt traps located at energy Et with respect to Fermi level (in intrinsic material) at the interface; the traps are initially neutral and become charged by trapping a free carrier. Seto considered that crystallites have similar size and shape and also neglected the mobile carriers in intercrystalline boundaries (boundary width). According to this model, for given values of l, Nt and Et, there is an impurity concentration N D , which indicates the degree of depletion in the film crystallites. Two conditions for impurity concentrations are possible: (a) For N D > N D , the crystallites are partially depleted and the temperature dependence of the electrical conductivity can be determined corresponding to two energy domains [25,26,31,32]. If EF  Et  Eb  kT ;

ð2Þ

the electrical conductivity is given by [31,32] rf ¼ ðe2 lN 2c vn0 =kT Þ  expðEa1 =kT Þ;

ð3Þ

and Ea1 ¼ Eb ;

ð4Þ

L. Leontie, G.I. Rusu / Journal of Non-Crystalline Solids 352 (2006) 1475–1478

where

4  1=2

v ¼ ðkT =2pm Þ

ð5Þ

ð6Þ

If [31] Et þ Eb  EF  kT ;

-10

3

12

-12

4

14

-14 -16

2

18

-18 (a) 20 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60

ð8Þ

103/T (K-1)

where

-6

4

(b) For N D < N D , the electrical conductivity can be written as

8

2 2

rf ¼ ½e l N c N D v=2kT ðN t  lN D Þ  expðEa2 =kT Þ;

ð10Þ

Aa 1=2 ðhm  Eg Þ ; hm

1

-8 -10 -12

10

-14

12

-16

3

14

-18

4

16

-20

2

18

-22 (b) 20 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60

103/T (K-1)

Fig. 1. Temperature dependence of the electrical conductivity. (a) Sample BO/34, (b) sample BO/46.

where hm is photon energy, Eg denotes the optical bandgap, and Aa is a characteristic parameter. According to Eq. (11), Eg can be determined by extrapolating linear portions of (ahm)2 = f(hm) dependences to (ahm)2 = 0 (Fig. 2 and Table 1). These values of Eg were used for estimation of the activation energy in Seto model.

3.0 2.5 2.0

1.60

2.00

2.40

2.80

3.20

3.60

4.00 1.5

BO/34 d=0.34 µm Eg=3.02 eV BO/46 d=0.46 µm Eg=3.04 eV

1.0

1.5 0.5

1.0 0.5 3.02 eV

0.0

0.0 1.50

2.00

2.50

3.00

3.50

hν (eV)

ð11Þ

Fig. 2. Absorption spectra of samples from Fig. 1.

(αhν)2 (1011 cm-2eV2)

where Ea2 is expressed analogous to Ea1 (Eq. (9)). In Eqs. (2)–(10) the following notations have been used: e – electron charge, EF – energy of Fermi level, Nc – effective state density for the conduction band, n0 – electron concentration in the neutral domain of crystallites, k – Boltzmann’s constant, T – absolute temperature, m* – scalar effective mass of charge carriers, er – low frequency permittivity of crystallites, l – average size of crystallites, Eg – energy of the forbidden band, Et – energy of trapping states with respect to Fermi level at interface. Figs. 1 and 2 show the temperature dependence of the electrical conductivity during heat treatment and the absorption spectra of respective samples, respectively. The lnr = f (103/T) curves are plotted at coolings of the samples. By assuming that in the lower temperature range (T < 375 K) the Eq. (3) is valid, from the slopes of these curves the values of the energy barrier, Eb, have been calculated for respective samples (Table 1). The values of impurity concentration, ND, have been determined from Eq. (6). The crystallite size have been determined by peaks of XRD patterns (recorded after each heating/cooling cycle). The relative permittivity was considered to be er = 8.5–8.8 [17]. The values of ND are also indicated in Table 1. Substituting the values of ND and rf (at T = 400 K) into Eq. (10) the values of Nt were calculated (the effective mass is considered to be m* ’ 0.15m0–0.20m0, m0 is free electron mass). For the studied films the energy bandgap was determined from the absorption spectra. For allowed direct transitions, the absorption coefficient is expressed by [11,33]

-ln[σ(Ω-1cm-1)]

6

BO/46 d=0.46 µm 1: 1st Heat.; 2: 1st Cool. 4: 3rd Cool. 3: 2nd Cool.

-ln[σ(Ω-1cm-1)]

ð9Þ

(αhν)2 (1011 cm-2eV2)

Ea1 ¼ Eg =2  Et .

a¼

-8

10

16

ð7Þ

the conductivity becomes 1=2 rf ¼ eN 2c vð2er N 1  ðkTN t Þ1  expðEa1 =kT Þ; D Eb Þ

-6

-ln[σ(Ω-1cm-1)]

Eb ¼ ðe2 l2 N D =8er Þ.

-ln[σ(Ω-1cm-1)]

8

and

-4

BO/34 d=0.34 µm 1: 1st Heat; 2: 1st Cool. 4: 3rd Cool. 3: 2nd Cool;

1

6

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L. Leontie, G.I. Rusu / Journal of Non-Crystalline Solids 352 (2006) 1475–1478

Table 1 Characteristic parameters of some Bi2O3 studied films Sample

Eb (eV)

Ea1 (eV)

Ea2 (eV)

Eg (eV)

n0 (cm3)

Nt (cm2)

17

13

Et (eV)

BO/34

0.06 0.22 0.10

0.79 0.80 0.84

1.10 1.12 1.26

3.02

8.30 · 10 1.25 · 1018 2.33 · 1018

5.24 · 10 1.54 · 1014 7.30 · 1013

0.41 0.39 0.25

BO/46

0.12 0.07 0.10

0.60 0.52 0.44

1.06 1.28 1.35

3.04

1.65 · 1018 8.20 · 1018 2.25 · 1018

8.12 · 1013 6.50 · 1013 2.30 · 1013

0.46 0.24 0.17

4. Conclusions The thermal dry oxidation (high Rh) of Bi vacuum evaporated films (d < 0.60 lm) onto glass leads to polycrystalline Bi2O3 films in b phase. The mechanism of electrical conduction in examined Bi2O3 films can be explained in the frame of the Seto model. By using lnr = f (103/T) experimental curves, the values of some characteristic parameters (barrier energy, energy and density of trapping states), as well as the activation energy of the electrical conduction can be estimated. References [1] A.J. Francklin, A.V. Chadwick, J.W. Couves, Solid State Ionics 70/71 (1994) 215. [2] F. Krok, I. Abrahams, D.G. Bangobango, W. Bogusz, J.A.G. Nelstrop, Solid State Ionics 86–88 (1996) 261. [3] G. Mairesse, in: B. Scrosati (Ed.), Fast Ion Transport in Solids, Kluver, Amsterdam, 1993, p. 271. [4] J. George, B. Pradeep, K.S. Joseph, Phys. Stat. Sol. (a) 100 (1987) 513; J. George, B. Pradeep, K.S. Joseph, Phys. Stat. Sol. (a) 108 (1987) 607; J. George, B. Pradeep, K.S. Joseph, Thin Solid Films 148 (1987) 181. [5] V. Dolocan, F. Iova, Phys. Stat. Sol. (a) 64 (1981) 755. [6] A.B. Kuz’menko, E.A. Tischenko, V.G. Orlov, J. Phys.: Condens. Matter. 8 (1996) 6199. [7] G. Bandoli, D. Barecca, E. Brescacin, G.A. Rizzi, E. Tondello, Chem. Vap. Depos. 2/9 (1996) 238. [8] L. Armelao, P. Colombo, M. Fabrizio, J. Sol–Gel Sci. Techn. 13 (1998) 213. [9] J. Fu, Mat. Sci. Lett. 16 (1997) 1433.

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