Electrical Conduction Mechanism through Anodic Aluminium

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Indian Journal of ChemistryVol. 20A. June 1981. pp. 541-543

Electrical Conduction Mechanism through Anodic AluminiumOxide Films

R. K. NIGAM*, R. C. SAINI, R. KAPOOR &. S. C. KATYALtDepartment of Chemistry, Maharshi Dayanand University, Rohtak 124 001

Received 19 March 1980; revised 20 November 1980; accepted 3 January 1981

Electrical conductivities through anodic aluminium oxide films of different thicknesses have been measured.Localized trap states In the band ofinsulating oxide films are responsible for conductivity through these films. Carriertransport in these films has been interpreted in terms of Poole-Frenkel conduction mechanism. Various parameters.viz. activation energy of the carriers. exchange conductivity, Poole-Frenkel coefficient and Poole-Frenkel pre-expo-nential factor have been calculated. The influence of the nature of the counter electrode, film thickness and temperaturehas been discussed.

ANoDIC oxide films from borate electrolyteson aluminium':" were found to be bothbarrier and amorphous types at room tem-

perature. Theories of electronic junctions wereunable to predict the experimentally observed ins-tability in electrical conduction mechanism in suchfilms. Vermilyea" discussed the conduction mecha-nism for electrolytic tantalum rectifier with lowresistance on the basis of flaws in the films but failedto explain the mechanism quantitatively in the caseof vacuum deposited metal electrode. Recently,Scher and Montroll" described an amorphousmaterial as a network of localized sites for electronsand holes. According to them, carrier transportthrough such a materal is characterized by a succes-sion of hops from one site to another, i.e, a randomdistribution of sites. This fits very well with thetransient current behaviour. The object of the pre-sent work is to carry out a detailed study of thenature of electrical conduction mechanism throughanodic aluminium oxide films.

Materials and MethodsThe surfaces of aluminium specimens (2 em" area)

were prepared". Hansen's? e?-capsulation. techniquewas employed for the electrical conductl~n stu~y.The sample was mounted on a perspex piece withsuitable depression and the contact with the bulkaluminium was obtained with a standard thermo-setting silver preparation. Edges of the specimen wereprotected by an epoxy resin to prevent conductionthrough them. Various specimens correspondingto three different thicknesses, i.e. 75. 90 and l20Veach were prepared" anodically at a current densitylmAjcm2 and at 25°C .. Uniform growth~ of oxi~efilms permitted calculation of the film thickness 111

Angstroms employing a conversion factor 12.5 Aper volt'', The contact with a~odic film was made b'yevaporating Cd, Cu and NI metals separately Invacuo (10-6 torr) using vacuum coating unit "HindHivac" model l2A 4. Out of these three metals, thework functions of two metals i.e. Cu and Ni werehigher and that of Cd lower than that of the substrate

tDepartment of Physics, M. D. University, Rohtak 124001

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metal, aluminium. The conductivity measurementsin all the prepared samples in sandwitch geometrywere made using Phillips d-e micro-voltmeter modelPP-9004 and Million megohrnmeter model RM160 MK III. The magnitudes of the lowest measure-able current and conductance were 10-111A and 10-13

ohm:", respectively. The temperature range of thestudy was from 25° to 100°C. Because of the limi-tations of the instruments used, the accuracy of currentmeasurements below the electric field strength El/2=.100 Vjcm was doubtful. Hence these measurementswere discarded. The electrical circuit used in thepresent study has been described elsewhere",

Results and DiscussionFigure 1 presents the plot of current (1) versus the

applied d.c. voltage (V) across 1500 A shick alumi-nium oxide film. Similar plots have been obtainedfor other samples. All the specimens beyond8V d-e supply show an ohmic conduction while belowthis value, non-ohmic conduction is exhibited. Onincreasing the applied voltage in the reverse direc-tion (i.e. increase in voltage by changing the polarityof the electrodes) approximately the same J-Vdependence has been observed for all the specimensstudied.

While in crystals the mode of carrier transport isaffected by free wave propagation in the bands,the electron wave phase coherence remains limitedto the order of interatomic distances in amorphoussolids. Like all amorphous solids, the anodic oxidefilms on aluminium have been found to exhibitstrong non-linear behaviour of current-field depen-dence at high fields of the order of 105 Vjcm ormore. The experimentally observed behaviour insuch films fits very well Eq. (1).

1= A(T) exp (e ~ £1/2kT) .. (1)

Here A(T) is the weakly temperature-dependent pre-exponential factor and ~, the Poole-Frankel coeffi-cient is given by the relation f3 = (elm€o)1/2 where e,is the high frequency dielectric constant of the amor-phous material and £0 is the permittivity of the free

541

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INDIAN J. CHEM., VOL. 20A, JUNE 1981

120

a. 80E<{

o·~Qx

24VOltOg4

16

40

• Cuo Cd

" NI80

120

Fig. J - Plots of current (1) versus applied d.c. voltage across15001\ thick aluminium oxide film at different metal electrodes.

space. The plots of log I vs El/2 are presentedin Fig. 2. As expected, these plots are linear. Thetheoretical value of ~ is 2.196 X 10-4• A value of11.4 for the high frequency dielectric constant isused for its calculation. Its experimental valuechanged from 0.786 x 10-4 to 1.199 X 10-4 fordifferent samples. The magnitudes of the ~theorand ~expl values were found to be of the same order,i.e. 104• The difference in calculated and experi-mental values may be attributed to the presence ofhigh density ionizable centres under high field ordonor and acceptor like sites in the sample. ThisPoole-Frankel conduction mechanism suggests thepresence of high density ionizable centres of limitedenergy range within the forbidden-gap of the amor-phous anodic films. The presence of impurities,

1.3

-910

-.1500 ~,,.1125 "·,935 "'

-910

-1010

13

Fig. 2 - Plots of log I versus Pia across aluminium oxidefilms of different thicknesses at different metal electrodes.

defects or local non-stoichiometry helps in creatingthese centres or sites. On ionization, these sitesexhibit a local coulombic field which enhances theconduction process of the carriers. The absolutevalue of Poole-Frenkel coefficient measures themagnitude of this coulombic field. The calculatedvalues of ~ and A(T) are presented in Table 1.Thus, the conduction mechanism in anodic alumi-nium oxide films is due to field ionization of thedefect centres over the studied range of electric fieldand temperature. At a given temperature the appliedfield perturbs the coulombic potential of the trapsand the levels of the conduction band. Therefore,it causes the lowering of the potential barrier. Thephysical significance of the term ~ Ell'!. in Eq. (1)is that the ionization energy of a single coulombic-potential well decreases by this much amount (~Elf2)in the direction of the electric field. In the amor-phous solids, the current generated by the normal freecarriers is almost negligible as compared to that

TABLE 1 - VARIOUS PARAMIlTERS CALCULATED FROM Eqs (1) AND (2) FOR FILMS OF DIFFERENT TmcKNBssBS UNDER DIFFERENTMETAL ELECTRODES

Electrode Thickness A (T) X IOU ~ X 106 se; 0'0 0'100

(A) (e V) (Q.-1 crrr'") (.Q. -1 cm=)

Cu 1500 8.931 9.443 0.763 4.302 X 10' 2.290 X 10-31125 7.651 9.404 0.760 3.268 X 10' 1.792 X 10-3

935 6.396 9.242 0.758 2.747 X 10' 1.546 X 10-3

Ni 1500 1.187 11.995 0.942 9.930 X 10· 1.906 X 10-31125 1.082 11.243 0.938 3.190 x 10' 6.892 X 10·'935 0.974 10.042 0.934 8.628 x 108 2.078 X 10-4

Cd 1500 11.079 7.924 0.912 1.693 x 1010 8.258 X 10-3

1125 9.970 7.881 0.901 6.451 x 10' 4.356 X 10-3

935 8.813 7.863 0.872 2.006 x 10· 3.393 X 10-3

542

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NIGAM et al. : ELECTRICAL CONDUCTION THROUGH ALUMINIUM OXIDE FILMS

woz~u_=>1~B

Cd..--.

-1010

(1/T)xl~'l?

Fig. 3 - Plots of conductivity (a) versus1/T for aluminium oxide films of different thicknesses at different metal electrodes.

ld'l---~---:s"

I

generated by the ionizable centres at high fields.Hence, the magnitude of the current observed wouldbe due to these ionizable centres. This effect controlsthe log I vs E1/2 characteristics of a series of insu-lators/semiconductors.

In the Cohen et al.lO band model for amorphousmaterials, it is assumed that the localized states formconduction- and valence-band tails which mayoverlap with the Fermi level lying near the centre ofthe mobility gap. Electrons from states at the top ofthe valence band are transferred to the conductionband ensuring that the Fermi level lies in the regionof the overlap. The d.c. conductivity in such typesof materials follows the equation-s

a = ao exp •.......(LEe/kT) .. (2)where LEe is the thermal activation energy forconduction and ao is the intercept of the ordinate atliT = O. On applying the above relation to ourdata on anodic films on aluminium, it is observedthat the magnitude of LEe, the activation energy ofthe carriers, practically remains unchanged over thestudied temperature range for a given sample and agiven counter-electrode. Pre-exponential factor ao,the exchange conductivity, i.e. the conductivity whenabsolute temperature approaches infinity, is foundto be strongly dependent upon the dielectric filmthickness and the nature of the counter electrode.

A linear behaviour is observed when log a isplotted against IIT (Fig. 3). This suggests an intrinsictype of conduction in anodic films. The low valuesof the slopes of these plots imply that the localizedstates near the mobility edges are responsible forelectrical conductivity in the conduction process.The values LEe, Co and a100, the conductivity at100° (from extrapolation) are presented in Table 1.These values vary with the variation of dielectricfilm thickness and the nature of counter electrode.

/

Ni

•• 1500~,•.. 1125- • ,

•• 935 "

3.0 2.6

The high magnitude of ao from the present datacan also be explained on the basis of the presence ofwide range of deep localised trap states in the di-electric anodic films. If the time taken to inducefield ionization of the localized states is higher thanthat for recombination, the carriers produced (everytime) would be pulled out into the extended states.This might be responsible for the sudden increase incurrent at high fields.

AcknowledgementsThe authors are thankful to Dr K. L. Bhatia,

Reader in Physics, M. D. University, Rohtak, fornecessary laboratory facilities. Two of them (R. K.and R. C. S.) thank the CSIR, New Delhi for seniorresearch fellowships.

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