(Al2O3) Thin Films.

International Conference on Electronics, Computer and Communication (ICECC 2008) University of Rajshahi, Bangladesh

Electrical and Optical Properties of Indium doped Alumina (Al2O3) Thin Films. 1,2

Khondakar Sumsul Arefin1, Atowar Rahman2. Department of Applied Physics & Electronic Engineering, University of Rajshahi, Bangladesh. e-mail: [email protected], [email protected]

ABSTRACT In this study all thin films are deposited on glass substrate by electron beam evaporation technique at a pressure of about 1.5 x 10-6 torr. The thickness of Indium doped Al2O3 films (25%w, 30%w and 40% w of In2O3) range from 68 nm to 183 nm. Both Electrical and optical properties of Indium doped Al2O3 films are studied. For 25% In2O3 doped alumina the conductivity at room temperature is 23.403 mho/cm and the average transmittance in the visible region is found 27.22 (in %T), for 30% In2O3 doped alumina this value is 83.26 mho/cm and 21.58(in %T) and for 40% In2O3 doped alumina this is 128.53 mho/cm and 16.88(in %T). For Indium doped Al2O3 films, the resistivity decreases with the increase of doping concentration that means the conductivity increases with doping concentration. . KEY WORDS: Thin film, Indium, Alumina, Conductivity, Transmittance. 1. INTRODUCTION When a thin layer of solid material is formed on a solid substrate and if the layer thickness becomes comparable in magnitude with mean free path of the conduction electrons of solid material then this layer is termed as “Thin Films”. Its value differs from material to material. In most cases a film of thickness below a few micrometers can be regarded as thin. The thin film technology is important special branch of physics in which the characteristics of different metals, semiconductors and insulators are investigated in thin film form. Most of the electronics equipment in the modern world is the contribution of thin film technology. The increasing demands of microelectronics in science and technology have greatly stimulated due to the invention of thin film and the expansion has been made on different kinds of thin films. Systematic study of semi-conducting films has been continued for more than half a century. Initially semiconducting films like Si, Ge etc. were studied. The Si and Ge technology is now well established. Now attention has been concentrated to study the compound semiconductor mainly oxide semiconductors (like Alumina). Thin films are most commonly prepared by thermal evaporation technique where the atoms are condensed from vapor phase onto a substrate. Solid material vaporizes when heated to a sufficiently high

ISBN 984-300-002131-3

temperature. Deposition of films in such a process is achieved by one or more phase transformations and the study of the thermodynamics and kinetics of these phase transformation reveals the formation of thin films. 2. DEPOSITION MECHANISM Vacuum evaporation is a deposition technique used to deposit a variety of materials by heating a source material under vacuum until it evaporates or sublimes. This evaporant is then deposited or condensed onto a substrate surface to form a film. The source material melts into a liquid and subsequently evaporates into a gaseous vapor or sublimes directly into a gaseous state. Evaporation takes place in a vacuum where the mean free path of atoms in the evaporant material is much longer than the distance from the source to the substrate. The rate of deposition can be expressed by the Hertz-Knudsen equation.

∂N 1 α ( p ' '− p ) = ......... (1) ∂t A 2 π mkT ∂N = rate of deposition from a source with surface ∂t area A α = evaporation coefficient m = molecular weight of the evaporant k = Boltzmann’s constant T = temperature p’’ = vapor pressure at the evaporant surface p = hydrostatic pressure acting on the source’s surface. Vacuum evaporation is a low-energy process, because the deposited material condenses onto the substrate with very little kinetic energy. Also, the film deposition is almost strictly line of sight since the vapor condenses onto the exposed surfaces adjacent to it and does not coat edges perpendicular to the source. Film deposition thickness can be controlled by the quantity or rate of generated vapor material as well as the distance from the source to the substrate. Rates depend greatly upon the substrate to source geometry, and the deposition rate can vary across large substrates because of this strong function of distance. The

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variation in deposition rate resulting from position relative to the source is given by Knudsen’s cosine law, cos θ /r2 , where r is the radial distance from the source and θ is the angle between that radial vector and the normal to the receiving surface. This can be used to express the thickness variation on a coated surface centered a distance h below or above a source as

tx x = [(1+ )2]-3/2 ...................(2) t0 h where t0 = thickness at the center of the coated surface directly above or below the source tx = thickness of deposition at some distance x from the center of the coated surface When sources are very small in comparison to the source to substrate distance (h), the source can be treated as a point source and the expression reduces to: tx = t 0 ............(3)

Resistivity in Ω-cm x 10-3


45 40 35 30 25 20 15 10 5 0 20

25

30

35

40

45

Doping concentration in %w

Fig 2(a) Variation of resistivity with doping concentration of In2O3 doped Al2O3 thin film.

140

x

h

Substrate

θ

Conductivity σ in

120 100 80 60 40 20 0 20

r

25

30

35

40

45


Fig 1. Schematic illustration of thickness

Fig 2(b) Variation of conductivity with doping concentration of In2O3 doped Al2O3 thin film.

3. RESULT & DISCUSSION

Effect of doping concentration on resistivity, conductivity and sheet resistance Fig 2(a) and 2(b) shows the variation of resistivity and conductivity with different doping concentration of In2O3 respectively at 307 K. It is found from graph that the resistivity decreases with increasing the doping concentration and the conductivity increases with the increase of doping concentration. The variation of sheet resistance with different doping concentration of In2O3 is shown in Fig 2(c). The sheet resistance also decreases with the increase of doping concentration. All the data are given in table 1.

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Sheet Resistance in Ω

3.1 ELECTRICAL PROPERTIES

4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 20

25

30

35

40

45


Fig 2(c) Variation of Sheet Resistance with doping concentration of In2O3 doped Al2O3 thin film.

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Table 1: Data for variation of resistivity, conductivity and sheet resistance with doping concentration. Doping concentration in mole percent

Resistivity, ρ in(ohmcm)x 10-3

Conductivity σ in mho/cm

Sheet Resistance (ohm)x 103

25

42.73

23.403

3.88

30

12.01

83.26

1.019

40

7.78

128.53

1.144

Activation energy Fig 3 shows the variation lnσ with inverse of temperature at different doping concentration. The activation energy for all samples has been calculated using slope of the curves (lnσ ~ 1/T). The calculated values of the activation energy are given in table 3. From the table is seen that there is no fixed dependency of activation energy on doping concentration.


lnσ

27.22

30

21.58

40

16.88

Table 4: Doping concentration dependence of band gap(Eg). Doping concentration in %w

Direct band gap in eV

Indirect band gap in eV

25

4.25

3.6

30

4.24

3.6

40

4.20

3.6

5 25% 30%

4

25

Optical band gap of Indium doped Al2O3 thin films The variations of band gap with doping concentration are given in table 4. From the table, the direct band gap slightly decreases with increasing doping concentration while the indirect band gap remains unchanged.

5.5

4.5

%T

40%

3.5 3 2.2

2.4

2.6

2.8

3

3.2

3.4

1/T in K-1x 10-3 Fig 3. Variation of lnσ with inverse of temperature at different doping concentration. Table: 2. Data for activation energy Doping concentration in %w 25 30 40

Activation energy in eV 0.011 0.116 0.0165

4. CONCLUSIONS In this present work we found that the conductivity of Al2O3 increases with the increases of Indium but the transmittance decreases. References [1] L.I Maissel and R. Glang., “Hand book of Thin Film Technology”, McGraw-Hill Book company, New York ,1970. [2] K.L.Chopra, “Thin Film Phenomena”, McGrawHill Book Company, New York, 1969. [3] C.G. Granqvist, “Thin Solid Film”, 1963. [4]J.J. Thomson, Proc. Cambridge Phil. Soc., 11:120(1901). [5]

3.2 OPTICAL PROPERTIES Table 3 shows the data of transmittance at different doping concentration. Table 3: Data for variation of transmittance with doping concentration at 307 K.

ISBN 984-300-002131-3

K.

Fuchs,

Proc.

Cambridge

Phil.

Soc.,

34:100(1938) [6] F.H. Sondheimer, Phys. Re., 80:401(1950). [7] S.N. Ghos and S. Deb, ASynosis of Physics, The World Press Ltd, Calcutta, 1937.

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