Electrical and optical properties of molybdenum trioxide thin films

Transcription

1 Bull. Mater. Sci., Vl. 16, N. 3, June 1993, pp ~ Printed in India. Electrical and ptical prperties f mlybdenum trixide thin films 1. Intrductin S KRISHNAKUMAR and S MENON Schl f Pure and Applied Physics, Mahatma Gandhi University, Ettumanr, Kttayam , India MS received 27 April 1992; revised 12 March t993 Abstract. Infrared spectra f vacuum-depsited mlybdenum trixide thin films have been studied. The variatin f electrical cnductivity with temperature fr different thicknesses f films has been investigated. Electrical cnductivity f the films as a functin f time f UV irradiatin was fund t increase initially, then decreased rapidly and reached a steady value. It increased and reached a steady value with time when irradiatin was cut-ff. Keywrds. Mlybdenum trixide; thin films; IR spectra; activatin energy; UV irradiatin. Mlybdenum trixide (MOO3) is a transitin metal xide having a number f electrical and ptical prperties. Lampert (1984) reviewed numerus inrganic and rganic electrchrmic materials in the cntext f develping a film-based ptical shutter fr energy-efficient windws. Thin films f MO3 can be used in electrnic infrmatin displays and clur memry devices. The last decade saw extensive electrical and ptical investigatin f electrchrmic materials (Yshimura et al 1983; Dnnadieu and Davazglu 1986; Davazglu and Dnnadieu 1987; Davazglu et al 1987). lured centres in thin films f MO 3 can be frmed by UV light irradiatin. Deb and hprian (1966) reprted that the clur centres frmed as a result f free electrns being trapped at xygen in vacancies. The mechanism behind this cluratin is nt yet well understd. In this paper we reprt IR spectra, activatin energy f MO 3 films and variatin f its electrical cnductivity during and after UV irradiatin. 2. Experimental The films f MO 3 were prepared by physical vapur depsitin using a thermal evapratin plant (Hind Hivac, mdel: 12 A4) which prvided a base pressure f Pa. MO 3 (99.9~ pure) (Gdfellw, UK) was evaprated frm a tantalum bat n t the mylar and glass substrates kept abve the surce. A cnstant rate f evapratin was maintained which was mnitred by a quartz crystal thickness mnitr having an accuracy f 1 nm. The value thus btained by thickness mnitr was cunterchecked by using the Fizeau fringe technique (hpra 1969). MO 3 films n mylar substrate was used fr IR studies (IR spectrphtmeter, Shimadzu IR 47). Keithley 617 prgrammable electrmeter was used fr studying d.c. electrical cnductivity by varying temperature f the films n glass substrate having a fixed bias vltage f.8 V. The d.c. electrical cnductivity f the film n a glass substrate was als measured using an electrmeter by applying a fixed bias vltage f.8 V t the film at rm temperature during and after UV irradiatin. 187

2 188 S Krishnakumar and S Menn 3. Results and Discussin The transmissin spectra f MO 3 film frmed n mylar (plyethylene terephthalate) substrate were recrded in IR (4 t 4 cm- 1) regin by an infrared spectrphtmeter with a similar uncated mylar as a reference. Mst f the mlecular vibratinal frequencies were present in this regin. Figure 1 gives the IR spectra f MO 3 film f thickness 134 nm n a mylar sheet. The ntable features f the MO 3 spectra are the weak transmissin bands bserved at 58 and 78cm-1 and the strng transmissin bands at 468, 89 and 1 cm-1. These values are in agreement with the reprted peaks at 468, 62, 81, 89 and 12cm -1 (Eda 1989; Anwar et al 199). Figure 2 shws the temperature dependence f d.c. electrical cnductivity (a) f the films when a fixed bias vltage f.8 V is applied fr three different thicknesses 165, 187 and 28 nm respectively. Assuming an Arrhenius-type relatin, a = aexp(- AE/kT), where a is the pre-expnential factr and k, the Bltzmann cnstant and AE, the thermal activatin energy. The plt In a vs (13/T) exhibits tw straight line regins f different activatin energies f cnductivity fr the same MO 3 sample. The value f activatin energy AE 1 in the lwer temperature regin as well as activatin energy AE 2 in the higher temperature regin shw slight change with thickness f the film. The variatin f d.c. electrical cnductivity as a functin f irradiatin time f MO 3 thin film f thickness 28 nm is shwn in figure 3. Here the d.c. electrical cnductivity was measured while increasing the irradiatin time. It was seen that the cnductivity increased initially, and then decreased much faster. After 3min, it reached almst a steady value. Due t cluratin, the films changed frm greenishblue t dark blue in clur. Figure 4 gives the variatin f cnductivity as a functin 1 8 SI. N {3 E O I- 4 2 w I I Wve number [cm-i) Figure 1. Infrared spectra f MO 3 thin films f thickness 134nm.

3 Prperties f mlybdenum trixide films SI.N A7 (e) SI.N B8 (b) ,xx~ SLN.9~122-3 ( c ) -5. E b -6' I I I I 2.8 2,9 3' 3'I 3'2 3'3 I3/T (K -I ) Figure 2. Temperature variatin f d.c. electrical cnductivity f MO 3 films f thicknesses (a) 165nm, (b) 187nm and (e) 28nm. Table 1. Activatin energies AEI and AE2 btained f6r different thicknesses and MO 3 films. AE1 AE2 Sample Thickness ( K) ( K) N. (nm) (ev) (ev) A B8 187 "43 1" "29 f time after terminating the irradiatin. It was fund that cnductivity increased much faster at the beginning and reached a steady value. The cnductivity f transitin metal xides was due t the hpping f small plarns (Austin and Mtt 1969). When we irradiated MO3 thin film with UV light, initially its cnductivity increased. As a result f irradiatin f the film, the phnns tk the energy which resulted in the electrn-phnn interactin, s that the effective mass f the plarn increased. Since the effective mass increased, the resistivity als

4 19 S Krishnakumar and S Menn 1.3 ~ 1.1 SI. N I 7 E 7~ I X v b q~ q~.5 I I I I I Time (rain) Figure 3. Variatin f d.c. electrical cnductivity as a functin f time while UV irradiating MO3 films f thickness 28 nm. increased, which in turn decreased its cnductivity. After sme time the electrnphnn interactin reached an equilibrium. As a cnsequence, a steady value f cnductivity was reached. Frm figure 3 it is seen that cutting-ff the irradiatin terminated electrn-phnn interactin. The cnductivity rapidly increased and reached a steady value. This suggests that the electrical prperties f MO 3 are sensitive t high energy electrmagnetic radiatin and time f expsure. 4. nclusin Frm IR spectrum it is clear that the film frmed was MO 3. The film was capable f cluring by UV irradiatin. Activatin energy f cnductivity fr MO 3 thin film varied with thickness f the film. The d.c. electrical cnductivity f MO 3 film

5 Prperties f mlybdenum trixide films 191 SI, NO, I -1'I X ~~ -.9 'E q5 ~ -,7 b "1 I I I I I / Time (rain) Figure 4. Variatin f d.c. electrical cnductivity as a functin f time after terminating UV irradiatin f MO 3 film f thickness 28 nm. initially increased and then decreased and finally reached a steady value during UV irradiatin. It increased rapidly and reached a steady value when the irradiatin was stpped. References Anwar M, Hgarth A and Thecharis R 199 d. Mater. Sci Austin I G and Mtt N F 1969 Adv. Phys hpra K L 1969 Thin film phenmena (New Yrk: McGraw Hill) p 13 Davazglu D, Dnnadieu A and Bhke O 1987 Sl. Energy Mater Davazglu D and Dnnadieu A 1987 Thin slid films Deb S K and hprian J A 1966 J. Appl. Phys Dnnadieu A and Davazglu D 1986 Prc. Third SPIE Syrup., Innsbruck Eda K 1989 J. Slid State hem Lampert M 1984 Sl. Enery Mat. II 1 Yshimura T, Watanabe M, Kike Y, Kiya K and Tanaka M 1983 Thin Slid Films 1Ol 141