Electrical charge transfer in NiF2 thin films

Thin Sohd Fdms, 88 (1982) 153-162 ELECTRONICS AND OPTICS

153

E L E C T R I C A L C H A R G E T R A N S F E R I N NiF 2 T H I N F I L M S J. SALARDENNE,M. SALAGO'ITY,J. PICHON AND A. S. BARRII~RE Laboratotre de Recherches en Electrotechmque et Physique du Sohde, 351 cours de la Ltb6ratlon, 33405 Talence C6dex (France)

(ReceivedJune 8, 1981, accepted October 9, 1981)

The electrical properties of granular NiF2 thin films are investigated. It is shown that for temperatures in the range 7 7 - 4 0 0 K the conductivity and the polarization mechanisms are the result of variable-range hopping of electrons between localized states. These states are the result of a small fluorine deficiency, which was demonstrated by 0t particle backscatterlng measurements. They are situated in the gap 1.75 eV below the conduction band of the material.

I. INTRODUCTION Nickel coatings are generally used to protect mechanical components in contact with fluorine and its compounds because the fluorination of nickel is relatively difficult 1. For the same reason, nickel is used as an electrode in fluorine electrochemistry. However, nickel can be chemically attacked by fluorine under some circumstances and hence the efficiency of the electrochemical reactions is decreased. Therefore a study of the electrical properties of NiF 2 thin films is of interest. Since single crystals of N i F 2 are usually needle-shaped, it is difficult to carry out electrical measurements on them. Therefore a study of fiat N1F 2 thin films of large area IS also of interest from a fundamental point of view. The results presented were obtained using thin films prepared by sublimation under vacuum of high purity powder which had been re-fluorinated under fluorine just before use. (This treatment was carried out at the Laboratolre de Chlm~e du Solide du C N R S de l'Universit8 de Bordeaux I.) The details of the conditions of preparation and the characterization of the films obtained have been described elsewhere 2 and we give only a brief outline in this paper. The texture and the crystallization state of the films depends mainly on the temperature Ts of the substrate during the condensation of the vapour. For Ts > 400 K the samples are granular. The average diameter of the grains increases with Ts, and they crystalhze in the tetragonal system with the (110) planes parallel to the substrate. The composition of the films was deduced from measurements of ct particle backscattering. If the powder is heated at about 1000 K for 5 min before sublimation at Ts < 650 K, a small fluorine deficiency is produced in the sample bulk. The 0040-6090/82/0000-0000/$02 75

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anionic vacancy content varies from 0.1~o to 5 ~ of the nickel content when the residual pressure durmg the film preparation is Increased from 10-T to 10-5 Torr. The fluorine deficiency is more marked near the film surfaces and increases at the external surface after exposure to air. The corollary of this behaviour is that the oxygen content increases in this part of the film; this cap be explained by hydration of the compound. The fluorine deficiency is not observed when the samples are coated with gold. An appreciable increase in the fluorine vacancy content in the sample bulk is observed/'or Ts > 650 K or after extended anneahng of the films at about 500 K. The optical gap of NiF 2 thin films has been deduced from absorption measurements 3 to be about 9 eV. Furthermore, an absorption threshold correlated with the fluorine vacancy concentration is observed at 1.75 eV. It is necessary to take these results into account in the analysis of the electrical phenomena reported here. 2.

EXPERIMENTAL DETAILS

2.1. Materials and procedures

The NIF 2 films were prepared under a residual pressure P of 10- 7 Torr and at a substrate temperature Ts of 550 K. In these conditions the ratio of the number of fluorine vacancies to the number of nickel atoms was about 10 3 in the sample bulk and the average diameter of the grains was 550/~. The metal/NiF2/metal structures had an area of 4 m m 2, and the thickness of the NiF2 films ranged from 2000 to 8000/~. Gold, aluminium and bismuth electrodes were used, and their resistances, which were controlled during the preparation, were kept below 0.1 f~/R. The electrical studies were performed in a vacuum at temperatures ranging from 77 to 450 K. The a.c. measurements were made using a frequency response analyser operating at 10-3-102 Hz and a resistance-capacitance bridge operating at 10-105 Hz. 2.2. D.e. measurements

When a step voltage was applied the current stabilized in a very short time (t < 1 s). The nature of the electrode metal had no effect on the current obtained, whatever the temperature or the applied electric field. Figure 1 shows the logarithm of the steady state current plotted against the logarithm of the applied voltage for an N i F 2 film 4 0 0 0 ~ thick at various temperatures. When the applied electric field is below 5 x 1 0 3 V cm -~ ohmic behavlour is observed at all temperatures. At higher electric fields the current increases more quickly. Curves of the logarithm of the current versus the reciprocal of the temperature are plotted for the same sample between 100 and 400 K in Fig. 2. The dependence is non-linear, and the conductivity depends only weakly on the temperature. It should be noted that there is an appreciable increase in the current for T > 450 K, and that it does not return to its initial value after cooling the sample (Fig. 3). The new characteristic is stable if the temperature remains lower than 400 K and can be obtained from the previous characteristic by a translation towards higher conductivity values; the translation amplitude increases with the heating temperature and the time.

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2.3. A.c. results

The variations in the parallel conductance Gv and in the parallel capacitance Cp with the frequency are shown in Fig. 4 for various temperatures. In the high frequency range the parallel conductance obeys the classical relation Gp = A(T)og"

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When the temperature increases from 100 to 400 K, n varies from 0.9 to 0.8. Conversely, at low frequencies, Gv tends towards a limiting value close to the d.c.

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4000/~ thick c o n d u c t a n c e Go of the cell. The frequency d o m a i n for which Gp ~ Go increases with the t e m p e r a t u r e . T h e parallel c a p a c i t a n c e Cp d e p e n d s w e a k l y on the t e m p e r a t u r e a n d r e m a i n s n e a r l y c o n s t a n t when the frequency increases from 102 to 105 Hz. The p e r m i t t i v i t y of the N i F 2 film can be d e d u c e d from the value of Cp a n d is f o u n d to be a b o u t 3 at 300 K a n d 10 3 Hz. H o w e v e r , when the frequency is decreased from 102 to 10 ~ H z a n d T IS greater t h a n 300 K, Cp increases slightly. 3. DISCUSSION I o n i c c o n d u c t i v i t y 1s generally c h a r a c t e r i z e d by a s t r o n g a c t i v a t i o n energy. The c u r r e n t - t e m p e r a t u r e v a r i a t i o n s o b s e r v e d for N i F 2 thin films are very weak.

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Fxg 3 The current-temperature charactensUcs of an Au/N1F2/Au structure after anneahng at temperatures greater than 450 K Furthermore, ionic conduction should correspond to an anionic displacement in such a compound and in that case the gold electrode should be blocking. However, the d.c. and a.c. characteristics do not reveal any space charge formation. Finally when a d.c. is passed through a B1/NiF2/Bi structure no variation in the resistance of the electrodes, which would indicate fluorine ion displacement, can be observed. Therefore we conclude that the electrical transport process m this material is electronic 4. As the same current is obtained for the same applied voltage regardless of the electrode material (bismuth, gold or alumlnium), a bulk-limited conductivity mechanism is possible. This is confirmed by the linear variation in the conductance with the insulator thickness. The results obtained for films heated above 450 K allow a better under-

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standing of the conductivity mechanism. In this case, et particle backscattering measurements have shown that partial dissociation of NiF2 takes place at these temperatures and leads to an increase in the concentration of fluorine vacancies; consequently the conductivity increases appreciably. After cooling of the samples, the conductivity remains very high but the dependence of the current on the applied voltage and on the temperature is conserved. Thus it appears that the conduction mechanism is the same as that before annealing. These observations suggest that the electrical charge transport in NiF2 thin films must be analysed on the basis of electron transfer between the localized states corresponding to the fluorine deficiency. It has been shown by optical absorption studies that these states are located in the gap around an energy E d of 1.75 eV below the conduction band. They can be assumed to be coulombic wells. In this case the height of the barrier separating two adjacent sites is given by Eo

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where e Is the electronic charge, ~r is the dielectric permittivity of the material and s is the distance between two sites. When an average value of 50/~,, deduced from nuclear analysis, is assumed for s, Eo is about 1.35 eV. As the observed conductivity is weakly temperature dependent, the electrical charge transfer mechanism must be electron hopping between the localized states 5. Thus, these states must be partially filled with electrons and the Fermi level must be situated near Ed. We have seen (Fig. 2) that the dependence of log I on T-1 is non-linear, and this shows that nearest-neighbour transfers alone are inadequate to describe the conduction mechanism 6. We have attempted to explain the experimental results by using the classical relation t r = a o exp(-~-~) In this case, it is possible to extract the index n from the data and we obtain n ~ 0.24. Curves of log I versus T - 1/4 are plotted in Fig. 5. A linear dependence is observed over a very large temperature range (100-400K) when the applied field is greater than 104 V cm -1. At lower fields the linear relation only holds at lower temperatures. Therefore it appears that electrical charge transfer through NiF2 thin films can be interpreted in terms of variable-range hopping 7 of electrons between localized states corresponding to fluorine vacancies. However, the details of the mechanism, and particularly of the effect of the applied field, are not well understood. Previous results have shown that the fluorine vacancies are randomly distributed and consequently that the heights of the potential barriers separating adjacent sites are also randomly distributed. However, the probability of electron transfer between two sites depends on the applied field F. It should be noted that the decrease e F s in the energy difference AE between two adjacent sites in the field direction is proportional to their separation s. The current-voltage curves of Fig. 1 are in good agreement with the theoretical law I ~ sinh(~V) given by Hill 8, and the applied field acts on the distribution ofthe transfer probabilities to modify the shape of the curves such that log ! is a function of T - 1/4 (Fig. 5).

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J. SALARDENNEet al.

The a.c. results can be interpreted using the same model. Firstly, the quasiconstancy of the capacitance shows that no appreciable space charge is created in the structure and therefore that the currents are bulk limited. However, it should be noted that the parallel conductance Gp is only weakly dependent on the temperature and that no thermally stimulated depolarization has been observed on these samples between 77 and 400 K. These results show that, as in the d.c. case, the polarization mechanism is not thermally activated and must therefore be due to electron hopping. = 35o

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The dependence of Gp on ~ found m the high frequency range (eqn. (1)) can be mterpreted on the basis of the Debye model with a distribution of relaxation times z.

ELECTRICAL CHARGE TRANSFER IN N i F 2 THIN FILMS

161

The relaxation time for hopping conduction with a random site distribution is a function of AE and s: z = Zo t a n h ( ~ - ) e x p ( 2 ~ s )

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where ~ is a constant equal to the reciprocal of the distance over which the wavefunction of an electron in a localized state decays to 1/e of its initial value 6. Pollack and Geballe 9 treated this problem, but in their hypothesis the exponent n is close to 0.8 when eqn. (1) is applicable. Thus the larger values of n observed at low temperatures and high frequencies appear to correspond to the multiple-hopping model 1°. Finally, the fact that Gp tends towards Go at low frequenoes clearly indicates that the charge transfer mechanisms in NiF2 thin films are connected with the same localized electronic states for both d.c. and a.c. applied potentials. It should be noted that the a.c. results presented in this paper are very similar to those obtained for FeF 3 thin films 11. In particular, for both compounds the dependence of the parallel conductance on co in the high frequency range is governed by the classical law Gp -- Aco" and tends towards the d.c. value Go at low frequencies. The analogy between the electrical properties of FeF 3 and N i F 2 is not surprising. The electrical behaviour of both compounds is due to electron transfer between randomly distributed localized states, and the transfer probability, which depends on the distance between the states, is also distributed. The only difference between these two fluorides results from the location in the gap of the localized states corresponding to the observed fluorine vacancies (1.75 eV and 1.1 eV below the conduction energy for NiF2 and for FeF3 respectively). Consequently, the potential barrier heights which separate two adjacent sites are given by eqn. (2) as 1.36 eV for NiF2 and 0.7 eV for FeF 3 for s = 50/~. Therefore the processes of electron transfer between the localized states are different: variable-range hopping occurs at all temperatures for NiF2, whereas for FeF 3 hopping over the potential barrier is possible at high temperatures 4'5. The M o t t - D a v i s model x° appears to be satisfactory for NiF2, whereas an extension of Pikes' model i 2 has been proposed ~ for FeF3. 4. CONCLUSION We have shown that the electrical charge transport in NiF2 thin films IS caused by electronic migration and that the current is bulk limited. The conductivity and the polarization mechanism are related to variable-range hopping between localized states corresponding to fluorine vacancies randomly distributed in the sample bulk. These states are situated 1.75 eV below the conduction band of the material. An increase in the anion deficiency causes an increase in the current but does not change the nature of the conductivity. This result and the fact that the samples peel offwhen their thickness exceeds 1 ~tm (ref. 2) offers an explanation for the observation that in fluorine electrochemistry the reactions can continue after fluorination of the electrodes but with a decreased efficiency. Finally, we have shown that electrical charge transfer in both NiF 2 and FeF 3 thin films can be explained by electron hops between localized states due to fluorine

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vacancies. The only difference between the two c o m p o u n d s is the l o c a t i o n of these states in the forbidden b a n d of the material. I n the case of deep levels (NiF2) the p o t e n t i a l barrier separating adjacent sites is too high to allow activated j u m p s . I n contrast, when the states are located near the c o n d u c t i o n b a n d of the c o m p o u n d (FeF3), the electrons can j u m p over the potential barrier at high t e m p e r a t u r e s 4' 1 This m o d e l can p r o b a b l y also be applied to m a n y i n s u l a t i n g c o m p o u n d s in which impurities or a n i o n vacancies p r o d u c e c o u l o m b i c wells in the forbidden band. REFERENCES

1 2 3 4

M. Jacob, A.M. Andream, B Blanchard, J F FloyandR Stepham, Vtde, Suppl.,189 (1978) l G Gevers, A. Lachter, M Salagolty and A. S. Barri6re, J. Cryst. Growth, 49 (1980) 45 G Gevers, J. Plchon and A S Bam6re, Phys Status Sohdl B, 104 (1981) 166 M Lascaud, A Lachter, J Salardenne and A S. Barn6re, Thin Sohd Fdms, 59 (1979) 363.

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A Miller and E Abrahams, Phys. Rev, 120 (1960) 745. N.F. Mott, Philos. Mag., 19 (1969) 835 R M. Hill, Phtlos Mag., 24 (1971) 1307 M. PollackandT. H Geballe, Phys Rev,122(1961) 1742. N.F. Mott and E A. Davis, Electronw Processes zn Non-Crystalhne Matertals, Clarendon, Oxford, 1971. 11 A Lachter, G Gevers, M Lascaud, J Salardenne and A. S. Barn6re, Thin Sohd Fdms, 67 (1980) 193 12 G.E. Plkes, Phys Rev B, 6(1972) 1572