Electrical and Optical Characterization of ITO Thin Films

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2009 International Students and Young Scientists Workshop "Photonics and Microsysterns"

Electrical and Optical Characterization of ITO Thin Films

Marcin Lapinski1), Jaroslaw Domaradzkii), Eugeniusz L. Prociowi), Karolina Sieradzka1), Barbara Gomicka2) 1) Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology

Janiszewskiego 11117, 50-372 Wroclaw, Poland

2) Electrotechnical Institute Wroclaw Division of Electrotechnology and Materials Science, Wroclaw, Poland Email address:marcin.lapinski@pwr.wroc.pl

Abstract - Rapid development of a new field of knowledge known as transparent electronics requires searching for new transparent materials which could be applied as Transparent Conducting Oxide - TCO and Transparent Oxide Semiconductors - TOS. The most important parameters of these materials are resistivity measured at room temperature and transparency level in the visible range of light spectrum.

In this paper the results of electrical and optical measurements of ITO thin films belong to TCO materials have been shown. The ITO thin films were deposited on glass substrates by magnetron sputtering process. Characterization of ITO films were carried out by means of optical transmission measurements, van der Pauw method and on the basis of Hall effect investigations. From electrical point of view measured films have conducting properties at room temperature and n-type of electrical conduction. The optical results of ITO thin films showed high transparency in the visible range of light spectrum.

I. INTRODUCTION

Transparent conducting oxides are used mainly in optoelectronic applications such as flat panel displays, thin film transistors, electroluminescent devices, solar cells, gas sensors and light emitting diodes [1,2]. The value of electrical conductivity depends on among others a kind and concentration of applied dopants. However, usually the increase of conductivity degrades the transparency level.

Hence it is necessary to find a compromise between high optical transparency and high electrical resistivity [3].

Indium tin oxide (ITO) is one of the most popular TCO material, used particularly for transparent electrodes in LCD and OLED displays [3,4], solar cells [3,5] and also as antistatic coatings [6]. The basic properties of ITO oxides are first of all good electrical conductivity (about 2.5'103 Scm-1 at room temperature [7]) and high optical transparency (about 90% in visible range of light spectrum [7]) and high infrared reflectance.

In this work electrical and optical paramameters of ITO thin films have been presented. Resistivity was measured by van der Pauw method. Mobility was determined by Hall effect measurements.

II. EXPERIMENTAL PROCEDURE

Investigated thin films were prepared from 99.5 % target in purity ITO by magnetron sputtering method. The sputtering process was carried out in argon atmosphere with

978-1-4244-4303-1/09/$25.00 ©2009

IEEE

the working pressure of argon about 10-5 Pa. During fabrication process the films were deposited on 10 ^mmx 10

mm glass (Coming 7059) substrates. The film thickness measured by Taylor Hobson TalySurf CCI Lite optical profiler was 525 ^run.

The main electrical parameters of semiconducting and conducting materials are resistivity (or conductivity), carrier mobility, carrier concentration and type of electrical conduction. Especially important is the resistivity value, because it determines the fundamental criterion by which semiconductors and conducting materials can be distinguished. In the case of electrical measurements investigated films have ohmic contacts located at the sample's comers (Fig. 1.). The contacts based on TiSi were deposited through the metallic mask.

3D

2 1

Fig. 1 Schematic view of measured sample with TiSi metal contacts located at comers.

Resistivity of the ITO films was measured by van der Pauw method. This technique is useful for measuring small samples. Dimensions of the sample and spacing of contacts are unimportant [8]. Current falling and voltage measurement have been assured by Keithley's source measure unit 2611 type. For the resistivity determination we should realize voltage measurement in only 2 configuration [9], but in order to obtain more precise results the measurements in 8 configuration were done and the results were averaged. Resistivity were calculated from dependence:

:....P-,,-A_+ ....:.P....:.B'-.+--,-P-,,-C_+ ....:.P....:.D,,

P= 4 (1)

where PA, PD, Pc, PD are fragmentary resistivity calculated from formula:

_ 1tdfA

R12•34

+

1S3.41

PA ^-

In 2 2 (2)

where: R12,34 is ratio of voltage measured between 1 and 2 contacts and current applied between 3 and 4 contacts, R23,41 is ratio of voltage measured between 2 and 3 contacts

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53

and current applied between 4 and 1 contacts [8,9], fA is geometrical factor [9]. Similarly PB, Pc, and PD, with different resistance values were calculated. For example for PB calculation R12,43 and R23,14 have been used.

Carrier mobility, carrier concentration and type of electrical conduction can be calculated using Hall effect.

Measurements were performed in two opposite direction of magnetic field. For each magnetic field direction the voltage was measured in 4 configurations (Fig. 2) [9].

a) 2

b) 2

c) 2

d) 2

Fig. 2 Configuration of Hall voltages measurements for specified direction of magnetic field. The same measurements should be done for opposite origin of

magnetic field.

For the measurements two magnets with magnetic field of 0.485 and 0.724 T were used. The Hall voltage were determined from formula:

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where: Vl3p, Vl3n, V31p, V31n, V24p, V24n, V42p, V42n -voltages measured in two magnetic field direction for 4 different configurations. For example the voltage V l3p was measured by current passed between 2 and 4 contacts in "positive"

magnetic field. Sign of Hall voltage informs about type of electrical conduction. The type of electrical conductivity was determined as follows: when the sign of voltage is negative (VH < 0) examined thin films has n type of electrical conduction, while the sign of voltage is positive sign (VH ^>0) informs about p type of electrical conduction.

If the Hall voltage and resistivity are known the carrier mobility can be calculated from well known formula [10]:

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where: d - sample thickness, I - current, B - magnetic field.

The optical measurements of manufactured ITO thin film were done by means of optical transmission method in the spectral range from 300 to 1000 nm in "so called"

bright configuration. Sample were illuminated with white light from a halogen lamp at a normal incidence. The transmitted light was collected by QE 65000 CCD spectrometer.

III. RESULTS

Before determination of electrical parameters of ITO thin films, linear behavior of prepared contacts have been confirmed by I-V measurements (Fig. 3). Voltages were measured in van der Pauw configuration. Characteristics for each voltages were linear. The test results have shown that prepared contacts were symmetric. Results of resistivity measurements are shown in Figure 4. For these thin films resistivity has calculated for different current passed thought the sample. It can be seen that the average resistivity value was at 2.8*10.3 Qcm level. It's considered that border between semiconductor and conductor is at about 10-3 Qcm [10]. Therefore the results of resistivity measurements showed that manufactured ITO thin films can be applied as conducting films. It also could be seen that resistivity value was independent from the current.

2.0x10-4,---,

1.0x10-4

�

_0.0

>

-1.0x10-4

. : .

• V,2

a V21

• V'Z3

<> V'Y}.

• v,.

o V43

A V41

A V,4

-2.0x10-4+----�--___r---�---_l

0.0 6.0x10-6 1.2x1 0"

I [A]

Fig. 3 I-V characteristics of ITO thin film measured at room temperature. For example, voltage V 12 was recorded between contacts 1 and 2 when current 134 was passed

thought 3 and 4 contacts.

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54 2009 International Students and Young Scientists Workshop "Photonics and Microsysterns"

5.00x10-3.---,

E

^... ^...

g

^2.50x10-3

a.

10-1

I [mAl

Fig. 4 Resistivity of ITO thin films vs. current measured at room temperature. The films were deposited on glass

substrates.

The Hall voltage and mobility were also calculated for different current bias which was passed through the sample during measurements. Measurements were done for two different magnetic field: 0.485 and 0.724 T. In Fig. 5 the characteristics of Hall voltage vs. current for the ITO thin films measured at room temperature have been presented.

Hall voltage measurements have revealed that tested ITO thin film had n-type electrical conduction (all measurements points were below 0 V).

In Fig. 6 the results of carrier mobility measurements have been presented. It can be seen that the carrier mobility of the ITO was almost independent on the current, and was about 5 cm2V·1s·l•

>

I

3.0x10"

0.0

> -3.0x10"

-6.0x10"

•

1E-3 0.01

I [A]

•

• ^B=0,485T

00II ^{B=0,724 T}

0.1

Fig. 5 Hall voltage of ITO thin films vs. current measured at room temperature.

50

...

I ... 1/) 25

I >

N E ₀

� �

-25

• 00II

I

[mAl

• B=0,485 T

00II ^{B=0,724 T}

10°

Fig. 6 Carrier mobility of ITO thin films vs. current measured at room temperature. The films were deposited on

glass substrates.

The optical transmission spectrum for ITO thin film have been presented in Fig. 7. It can be seen that manufactured ITO film have high transparency. For ITO thin films the transmission coefficient was 69 % at 550 nm^.

It's considered that materials for transparent electronics should have optical transmission in visible light range over 35% [11]. Examined ITO films deposited on Coming 7059 glass can be applied as transparent film. From the optical transmission spectrum also results that the adsorption edge

(A.cutoff)

for manufactured ITO was 354 nm^.

100.---.

80

,...,60

cf2.

...

20

O+--��r_�-_.-�-_.-��

200 400

Acut off 600

A

[nm]

800 1000

Fig 7 Optical transmission spectra of analyzed ITO thin films.

IV. CONCLUSIONS

In this work optical and electrical properties of ITO thin films have been presented. The results of our work have shown that ITO thin films deposited by magnetron sputtering method on glass substrates had low resistivity

(p

= 2.8* 10-3 ncm) and had n type of electrical conduction.

Electrical parameters of manufactured thin film are comparable with literature data. For example Meng et al.

[12] received similar resistivity (1.1 *10-3 ncm) for ITO thin films prepared by magnetron sputtering method as well.

Results of optical properties exhibited that prepared ITO thin films has good transparency (T I,. � 70 % in visible light range). This parameter is also comparable with literature data (TI,. �75 [12]).

The results of work testifies that examined thin films can be applied as TCO films and use in transparent electronics applications, such as transparent electrodes in OLED displays.

ACKNOWLEDGEMENTS

This work was fmanced from the statute sources given by Polish Ministry of Science and Education (No. 343 646) .

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