0 Elsevier, Paris Thin films of indium oxide Ann. Chim. Sci. Mat, 1999,24, pp. 395-398

EFFECTS OF ANNEALING ON THE STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES OF REACTIVELY EVAPORATED THIN FILMS

OF INDIUM OXIDE

A. HAKAM, Y. SAIDI, M. BANOUQ, R. DOUNIA, L. OUBOUMALK

Laboratoire de Spectronomie, Equipe Couches Minces, DBpartement de Physique, Facultb des Sciences, BP. 1014, Rabat-Maroc.

Abstract - Thin films of indium oxide were prepared by thermal reactive evaporation of a mixture of indium oxide and metallic indium. This work is an experimental study of the modifications induced by an annealing treatment, on the structural, electrical and optical properties of indium oxide (In203). The results show important changes of different parameters determined after annealing. The films obtained after annealing at 350 “C for 3 hours under oxygen atmosphere have a good cristallinity These films showed a transmittance of more than 80% in the visible region and a conductivity > 103 (R.cm)-1

R&urn6 - Effet du recuit sur les propriCtQ structurales, Clectriques et optiques de films minces d’oxyde d’indium prkparb par haporation kactive. Des films minces d’oxyde d’indium sont obtenus par &vaporation thermique rkactive d’un melange d’oxyde d’indium et d’indium mktallique. NOW mettons en Cvidence par I’Ctude exp&imentale suivante les changements des propriCtCs structurales, Clecttiques et optiques des films par traitement thermique. Les films ayant subi un recuit ?I 350°C pendant 3 heures sous atmosphbre d’oxygbne prdsentent une bonne cristallisation et manifestent une transmission optique dans le visible sup&ieure 2 80% et une conductivitt? electrique > I O3 (R.cm)-l.

1. 1NTRODUCTlON

Thin films of conducting transparent oxides have found extensive use in many applications, such as alphanumeric displays [l] or transparent contacts to various solar cells [2-31. It is reported that the structural, electrical and optical properties of the indium oxide films strongly depend on preparation methods and conditions [4-71. In this paper we present experimental results concerning the influence of an annealing treatment on the structural, electrical and optical properties of reactively evaporated thin films of indium oxide.

2. EXPERIMENTAL DETAILS

Thin films of In203 were prepared by evaporating a mixture of indium oxide and metallic indium in an oxygen atmosphere of 2. 10e4 Torr [8]. The evaporation boat consisted of an alumina crucible heated by a tungsten heater. The evaporation rate and thickness were controlled

Reprints: A. Hakam, Laboratoire de Spectronomie, Equipe Couches Minces, mpartement de Physique, Facult6 des Sciences, BP. 1014, Rabat, Maroc.

396 A. Hakam et al.

during deposition using a quartz crystal oscillator. The deposition rate for this series was kept constant at 2 8, s-l [9]. The film thickness was about 4000 A. After deposition the films were annealed under the same oxygen atmosphere as during film deposition, the temperature ranging from 200°C to 400°C . The structure of the films was determined using a “CGR (T&a 60)” X-ray diffractometer. The films were characterized optically and electrically. The electrical resistivity of the films was studied using Van der Pauw measurements [ lO].The optical transmission of the films in the wavelength range 300-1200 nm, was recorded using a “CARY 17” beam spectrophotometer.

3. RESULTS AND DISCUSSION

3.1 Structural properties

The structure of the films was revealed by X-ray diffraction. Figure 1 shows X-ray diffraction spectra of non annealed and annealed at 350°C for 3 hours indium oxide films. Every film showed the In203 “bixbyite” structure, with a,, = 1.0118 nm as the dominant phase. Comparing the X spectra with those of In203 powder, it appears that the as grown films have a light tendency to crystallize with a preferential orientation the (222) planes being parallel to the substrate. This tendency is clearly increased by annealing which improves the film cristallinity.

The results of the analysis of the films by R.B.S is illustrated in figure 2. Nearly

stoichiometric indium-oxygen ratio ( [InI [ol ) is obtained for films prepared with a optimum condition

of deposition.

J I

62 52 48 42 36 30 62 52 46 42 96 30

Figure 1. X-ray diffraction patterns of In203 films. a) before annealing, b) after annealing at 350°C for 3 hours.

Indium

- B 1

loooo- i B

_ tl ‘2

6wo- jj E

- 5

2000 - Carbon y Oxygen A. Channels , J, L

20(1 400 600 800 1 Figure 2. Backscattering spectra of In203 films.

Thin films of indium oxide 397

3.2 Electrical prouerties

The films were characterized electrically using Van der Pauw measurements. Figure 3 shows the variation of resistivity of the films of indium oxide as a function of substrate temperature. As the temperature of the substrate increases, the resistivity of the films decreases. A minimum of resistivity ( 1.3 10-J R.cm) is obtained for a temperature about 350°C. The variation of resistivity with heat treatment time is shown in figure 4 . The minimum value of resistivity is obtained for a film heat treated for 3 hours.

10-3 p 200 250 300 350 400 0 50 loo 150 200 Substrate tempera- CC) Heating time (mn)

Figure 3. Resistivity versus substrate Figure 4. Resistivity versus heat temperature treatment time at 350°C.

3.3 Ontical pronerties

Figure 5 shows typical transmission curves of an In203 thin film, prepared at the optimum conditions of deposition, in the wavelength range from 300 to 1200 nm. As can be seen, the transmission is high (more than 80%) in the visible region. The sharp decrease in the transmission spectrum at short wavelengths is due to the fundamental absorption edge and the other observed on the IR side is the result of the increasing reflectivity due to free electrons for frequencies below the plasma oscillation frequency. The variation of transmission of these films observed for k= 600 nm as a function of heating time is shown infigure 6. It appears that for a heating times higher than 3 hours, we obtained a film with a high transmission (more than 80%) in the visible range. The effect of the annealing upon optical properties can be understood on the basis of the following arguments: a) the observed improvement of the cristallinity of the films during annealing increases the mobility of the electrons. It also increases the carrier concentration due to a lowering of the delocalisation level in the conduction band. b) As a consequence, the conductivity increases and the transmission in visible is also improved due to the increase of mobility of free carriers [1 11. The increase of carrier concentration has just for effect concerning transmission spectra to shift the plasma oscillation frequency towards higher values. narrowing the transmission window.

4. CONCLUSION

The experimental study of the modifications induced by an annealing treatment, on the structuralt electrical and optical properties of indium oxide (ln203) shows important changes of different parameters determined after annealing. At the optimum conditions of deposition and after annealing at 350 “C for 3 hours the film exhibits a low resistivity value (1.5 lo-4Q.cm) and a transmittance of more than 80% in the visible region.

398 A. Hakam et al.

90 t

8 80 6 70 .4 y;1 60

.sl 'Z 50 .; 40

40 k 50 30 40 20 1 oun) Heating tirre (II) 10 0 30 &,,I,~~~r,,,,

200 400 600 800 1000 1200 01234567

Figure 5. Optical Transmission of In203 films. Figure 6. Optical Transmission versus heating time at 350°C for h = 600 nm.

AcknowledPments

The authors are indebted to Prof. Dr. J. SALARDENNE (Bordeaux I University-France) for encouraging this work and for valuable criticism during the preparation of the manuscript.

5. REFERENCES

Cl1

121

PI 141 PI PI 171

@I

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(Article rec;u le 12/05/98, sous forme dCfinitive le 08/l 2/98.)