CHAPTER V

STRUCTURAL STUDIES ON INDIUM OXIDE, TIN OXIDE AND INDIUM

T IN OXIDE THIN FILMS

5.1 Introduction

The applications of indiun oxide, tin oxide and indium tin oxide have

been remarkable in the field of gas sensors, '" where structure and particle

size are important parameters for high sensitivity and rapid response. In this

chapter the structural properties of indium oxide, tin oxide and indium tin

oxide films are dealt with.

It is known that indium oxide crystallizes in two different structures

as body centered cubic and hexagonal. The body centered cubic structure

is a bixbyite Mn203 type, also called C-type rare earth oxide structure. The

unit cell consists of 16 mol~?cules, i.e., 80 atoms, resulting in a rather

complex structure. This phase is obtained at a low or normal pressure.

The hexagonal type is a high-pressure type phase obtained at 68 Kbar

and at a temperature 800- l f i 00~~ . The structure is of the corundum, or

CCAl203 type, and has 6 molecules per unit cell.

There are two crystallographically non-equivalent indium atoms but

only one type of oxygen atom. The indium is six-fold coordinated and

oxygen is four-fold coordinated. The oxygen atoms around In (1) are

equidistant (In-0 = 0.21 8 nm) and lie nearly at the comers of a cube with

two body diagonally opposite corners unoccupied. In (2) is also

surrounded by six oxygen atoms nearly at the corners of the cube, but

with two face-diagonally opposite corners unoccupied. In this polyhedron

the six In-0 distances have, three different values: 0.213nm, 0.219 nm and

0.223 nm.

Tin oxide (Sn02) single crystal has tetragonal rutile structure with

space group ~ ' ~ ~ t , (P42/mnm). It is a non-symmorphic crystal with two

formula units per unit cell. The unit cell contains six atoms, two tin and

four oxygen. The coordination is 6:3. The oxygen atoms are placed

approximately at the corners of a regular octahedron, and tin atoms are

located approximately at the colFners of an equilateral triang~e.~ The lattice

parameters are a0 = 0.473 nm and co= 0.3186 nm.

The structure of IT0 is essentially tin doped indium oxide, which is

known to crystallize in body centered cubic and hexagonal structures.

When small amount of tin is added to indium oxide all the tin atoms

enter substitutionally in the cation sub lattice. The tin doping has been

observed to induce a change in the lattice constant. Some authors 6-8

obtained 'a' higher than 'a6 of undoped indium oxide, which may suggest

3+ . the incorporation of larger ~ n " ions at either In s~te substitutionally or at

interstitial positions. Agnihotri et a/..' observed a reduction of the lattice

constant with increase in tin concentration in IT0 films. Such a lattice

contraction indicates the replacement of ln3+ ion by sn4' ions that are

smaller. Mossbauer results "' have shown that tin in good quality IT0

films are either in sn4+ state or in Sn '+ state. Thilakan et a/." observed a

change in preferred orientation between (222) and (400) planes when

deposition rate is increased.

5.2 Indium Oxide

Indium oxide thin films have been prepared using the vacuum

coating unit and a tantalum boat as described in Chapter 4 (Section 4.4).

The samples are prepared in an oxygen partial pressure of 3x10'~ mbar at

substrate temperatures 125, .75, 200 and 225OC. The thickness of the

films are 500nm each. XRD spectrum of these samples are recorded

using X-ray diffractometer (Philips - PW 3710) as described in chapter 2

(section 2.23). The radiation used is CuKa and the operating potential is

25KV and current 10mA. The scanning rate is 1°/minute.

Fig 5.1 to 5.4 gives the XRD pattern of the indium oxide thin films

deposited at 125O~, 175OC, 2 0 0 ' ~ and 2 2 5 ' ~ and annealed for 30

minutes at 250'~. At 175'C, the peak obtained at 20 = 30.5' is examined

and identified as the reflection from (222) plane of indium oxide using

ASTM (American Society fcr Testing Materials) data card. The other

peaks are identified and are given in Table 5.1.

Fig. 5.1 XRD pattern of indiurnoxide thin film deposited at substrate temperature 125'~.

LR -t

Fig. 5.2 XRD pattern of indium oxide thin film deposited at substrate temperature 175'~.

28 -, Fig. 5.3 XRD pattern indium oxide thin film deposited at a substrate

temperature200~C.

Fig. 5.4 XRD pattern of indiurn oxide thin film deposited at a substrate temperature 225OC.

Table 5.1 The calculated values of d and the ASTM standard values of d for indium oxide film deposited at 1 7 5 ' ~ and annealed at 2 5 0 ' ~ for 30 minutes

The grain size of the films are determined from the Full Width at

20

21.5

30.5

35.46

45.7

50.98

Half Maximum (FWHM) intensity of the X-ray peaks using Scherrer's

formu~a.'~

Where h = 0.154056 nm, 1 6 ~ = B-b, where B is the width of peak at an 2

angle 28 at half maximum intensity and b is the effect of the instrument as

Inter planar spacing (d)

determined from the broadening of the monocrystalline silicon diffraction

ulo

14

100

30

0

35

C:alculated (nm)

12.92 4.12

100 2.92

23.7 2.53

6.0 1.98

29.5 1.79

line. The values of the grain :size are given in Table 5.2.

hkl

21 1

222

400

431

440

ASTM (nm)

4.13

2.921

2.529

1.984

1.788

Table 5.2 The grain size of Indium oxide thin films deposited at various substrate temperatures

Grain size (nm) 1 -+r

The grain size is found to increase with increase in substrate

temperature. Goswami and 3oswami l3 and Burns l4 report that the

vacuum evaporated indium oxide films may dissociate to species like InO,

In or 02.

As observed, a preferential orientation (222) for indium oxide has

been obtained by Sundaram ttnd Bhagavat l5 and Weng et a1.I6 It is also

reported that the preferred olentation depends on film thickness17 and

deposition conditions. 18

5.3 Tin Oxide

Thin films of tin oxidct have been prepared using the thermal

evaporation technique. The procedure for the preparation is described in

Chapter 4 (Section 4.5). Thickness of the sample is 500 nm each. XRD

spectra of the samples have heen recorded using the Philips (PW -3710)

X-ray diffractometer.

Figures 5.5 to 5.8 show the X- ray diffraction pattern of tin oxide

films deposited at 1 0 0 ~ ~ in an oxygen partial pressure of 1x10.~ mbar and

annealed in air at temperatures 200°c, 225'~, 250% and 275OC

respectively for 30 minutes. When annealed, prominent peak is observed

at 28 = 44.82' for annealing temperature of 200% and 225Oc, which

corresponds to reflection from (21 1) plane. For samples annealed at

250°C and 275% main peak is observed at 29 = 29.76', which

corresponds to reflection from (200) plane. The values of the grain size

are evaluated using Scherrer's formula and are given in Table 5.3.

Table 5.3 The grain size of tin oxide thin films deposited at 1 0 0 ~ ~ in an oxygen partial pressure of lo4 mbar and annealed at various temperatures

Annealing temperature (OC) Grain size (nm)

In tin oxide thin films crystallization to stoichiometric Sn02 from

amorphous phase takes place through Sn203 or Sn304 state. Films of

lower substrate temperature easily crystallize to Sn02 phase. 19

Yamazaki 20 from Rutherford back scattering studies reports the

decrease in the value of x in SnO, as substrate temperature is increased.

So at lower substrate temperature the sample has an initial higher value

Fig. 5.5 XRD pattern of tin oxide thin film annealed at 200'~ for 30 minutes.

a 3 F; b B a .- : g n -

20 30 40 50 60

20 -f

Fig.5.6 XRD pattern of tin oxide thin film annealed at 225'~ for 30 minutes.

Fig.5.7 XRD pattern of tin oxide thin film annealed at 2 5 0 ' ~ for 30 minutes.

Fig. 5.8 XRD pattem of tin oxide thin film annealed at 275'~ for 30 minutes.

of x, which takes the crystalline structure of Sn02 more easily, on

annealing, than does a film dept~sited at a high substrate temperature.

The composition of the reactively evaporated films are between

stoichiometric SnOx and SnO:, where x ranges from 1.25 to 1.57 and

corresponds to intermediate oxide Sn304 with x = 1 .33.2' The structure of

amorphous SnO films can be assumed to consist of slightly distorted,

corner connected oxygen octahedral having central sn4' ions and strained

octahedra. These strained octahedra possess sn2' ions and oxygen

vacancies. The presence of diqerent oxidation states is mainly influenced

by the production procedure and post-deposition annealing conditions.

5.4 Indium Tin Oxide

Indium tin oxide thin films have been prepared by reactively

evaporating different alloys of indium and tin (Indium: Tin = 80: 20, 85: 15,

90: 10 and 95: 5 by weight percentage) from a tantalum boat in a vacuum

coating unit. The method of making alloy is described in chapter 3 section

3.5.The deposition conditions are also described in section 3.5 .The films

are prepared at a substrate temperature of 2 0 0 ' ~ in an oxygen

atmosphere with various tin concentrations. The thickness of the films are

500 nm each. X ray spectra are recorded using the X ray diffractometer

(Philips- PW3710). The radiation used is Cu Ka and the operating

potential is 25 kV at 10 mA.

Fig. 5.9 to 5.13 show the XRD spectra of the films deposited at

substrate temperature 200% alid annealed at 3 0 0 ' ~ for 30 minutes and

having tin concentrations of O'/O, 574, lo%, 15% and 20% respectively.

The most prominent peak i!; found to be at 28 = 30.54', which

corresponds to reflection from 1222) plane. The other peaks are identified

and are given in figure 5.10.

The grain sizes of the samples are determined. The grain size is

found to decrease with increase in concentration of tin. The values are

given in Table 5.4. The present results are in good agreement with the

observations of Mizuhashi. 22

Table 5.4 Grain size of IT0 thin films with various tin concentrations

Tin concentration (%) I Grain size (nm)

The d values of the indium tin oxide thin film deposited with 5% tin

concentration are calculated and are given in Table 5.5.

Fig. 5.9 XRD pattern of indium oxide thin film deposited at 200'~

Fig.5.10 XRD pattern of IT0 thin film with 5% tin concentration deposited at 200°C

Fig.5.11 XRD pattern of 11.0 thin film with 10% tin concentration deposited at 200°C

--. ,.. a . . a . . . . , . . . . . _ I 0 U B

20 + Fig. 5.12 XRD pattern of IT0 rhin film with 15% tin concentration

deposited at 200%

5.13 XRD pattern of IT0 thin film with 20% tin concentration deposited at 200°C

Table 5.5 The calculated d values for Ts = 2 0 0 ' ~ along with the standard d values from ASTM data for IT0 thin film with 5% tin concentration

Calculated

1.789

ASTM

It is found that the c;ilculated values agree well with the ASTM

data. The film exhibits the reflection from the (222) planes as the most

predominant peak in the X- ray diffraction pattern. None of the spectra

indicated any characteristic peaks of Sn, SnO or Sn02, which indicates

that the tin atoms are doped substitutionally into the indium oxide lattice.

For all the three materials, the peaks obtained in the diffraction

spectra are sharp indicating (he highly crystalline nature of the films.

Studies on the grain size on indium oxide thin films show that the

grain size increases with increase in substrate temperature. Grain size of

tin oxide thin films increases with increase in annealing temperature.

Grain size of IT0 thin films is found to decrease with increase in tin

concentration.

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