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Current-voltage characteristics of Li-doped MgO oxidized at elevated temperatures
Y. Chen, J. Boldu, V. Orera
To cite this version:
Y. Chen, J. Boldu, V. Orera. Current-voltage characteristics of Li-doped MgO oxidized at elevated temperatures. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-398-C6-400.
�10.1051/jphyscol:19806101�. �jpa-00220007�
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 7 , Tome 41, Juillet 1980, page C6-398
Current-voltage characteristics of Li-doped MgO oxidized at elevated temperatures
Y: Chen, M. M. Abraham, J . L. Boldu (*) and V. M. Orera (**)
Solid Sate Division, Oak Ridge National Laboratory (***) Oak Ridge, Tennessee 37830, U.S.A.
Abstract. - The Current density has been measured as a function of the electric field ( I vs. E ) at 295 K on MgO single crystals containing stable [ L ~ ] O centers (Lit 0- complexes), produced by heat treatment in flowing oxygen. Whereas the treatment temperature has a pronounced influence on the [LiI0 concentration, electrical conductivity and the non-linearity of the I-E curve, thermal aging has considerably less effect.
During growth of lithium-doped MgO crystals by the arc-fusion technique [I], the crystals are gene- rally subjected to a reducing environment under the electric arc of graphite rods. As a result, most of the lithium coagulates into LizO precipitates [2].
Upon oxidation of these crystals at elevated tempe- ratures, broad optical absorption bands at 1.8 and 5.3 eV are produced [3, 41. The former is due t o the [LiI0 center (a substitutional Li+ ion with an adjaceht 0- ion) and the latter is attributed to an unidentified defect associated with lithium. The stability of these thermochemically generated [LiI0 centers, as oppos- ed to unstable ones induced by ionizing radiation at low temperatures [5, 61, is described as due t o hopping of the holes in a region of high [LiI0 con- centration (called a microgalaxy), which is caused by the dissolution from a Li20 precipitate [4]. The- refore, these regions should be p-type semiconduct- ing. Indeed, the presence of these centers enhances the electrical conductivity ; the greater the [LiI0 con- centration, the higher the conductivity [7].
A characteristic feature exhibited by crystals con- taining [LiI0 defects is that the current density vs.
electric field (I-@ curves are non-linear, but are symmetric for both positive and negative bias.
There are two regimes to the curve. At small electric fields, the effective conductivity is small and the current density is due primarily to polarization effects. At higher fields, the curves become more linear and the current is attributed to field-assisted
(*) Guest scientist from Instituto de Fisica, UNAM, Mexico, D.F.
(**) Guest scientist from University of Zaragoza, Spain.
(***) Operated by Union Carbide Corporation under contract W-7405-eng-26 with the U.S. Department of Energy.
sweeping (tunneling) of holes between neighboring microgalaxies. If these microgalaxies occur in the vicinity of dislocations, as polarization studies [8]
and [LiI0 decoration along slip planes [9] seem to suggest, then the internal electric fields (between microgalaxies) can reach
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lo5 V/cm for external fields of l o 2 - I d V/cm. The purpose of the present work is (1) to investigate how the incubation tempe- rature affects the non-linearity of the I - E curves and (2) to determine the effects of thermal aging on the absorption coefficient of the [L~]O band and the I-E relationship.The [LiI0 concentration and the electrical conduc- tivity increases with incubating temperature up to
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1 600 K when heated isochronally in oxygen [8].The measured I-E characteristics [lo] are different for different incubating temperatures. Typical current-field dependence of the first quadrant for three different temperatures is shown in figure 1. In order to elucidate the contrasting shape of the cur- ves rather than the magnitude of the current den- sity, the normalized current I/Io, where I. is the cur- rent at 2 500 V/cm, is plotted ; 2 500 V/cm was chosen because it is in the linear region of all three curves. It is clear that as the incubation temperature increases, the slope begins to rise at lower fields.
Alternately, the linear portion of the 1 250, 1 450 and 1 550 K curves extrapolates t o zero current at
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800, 400 and 300 V/cm respectively. These observations suggest that for high incubating tempe- ratures, hole tunneling proceeds at lower fields.Therefore, we conclude that the effective dimen- sions of the dielectric medium between the semicon- ducting inclusions (microgalaxies) diminishes with increasing temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806101
CURRENT-VOLTAGE CHARACTERISTICS O F Li-DOPED MgO OXIDIZED C6-399
As the incubating temperature increases above 1 600 K, both the [L~]O concentration and the elec- trical conductivity decreases monotonically [7].
The decrease in the [LiI0 concentrations is attended by the increase of the 5.3 eV band, which is pro- bably due to a configuration energetically more favorable than [LiI0 centers at high temperatures.
The defect responsible for this band is believed to be an anion vacancy charge-compensated by two Li+ ions, possibly with the linear configuration : Li+ [oxygen vacancy] Li+. The normalized curve shape for several samples oxidized at 1 700 K did not differ significantly from the 1 550 K curve shown in figure 1.
Fig. 1. - Normalized current density vs. electric field for sam- ples heated in flowing oxygen for 5 min.
The time dependence of the [LiI0 absorption coef- ficient (a-t curve), produced by heat treatment at 1 308 K in flowing oxygen is shown in figure 2. At such a low temperature for [LiI0 formation, satura- tion is not attained until abbut lo3 s. However, if the sample is placed in an environment of static air, such as in a horizontal tube, the coloration will reach a peak and then diminish, as shown in the lower figure.
The maximum absorption produced in static air is substantially less than that produced in flowing oxy- gen. Therefore, time dependence measurements are meaningful only for flowing gases.
The current density vs. electric field curves for two samples heated at 1 550 K in flowing oxygen for 5 min. and 71 h is shown in figure 3. At this temperature, saturation of the [L~]O concentration is obtained after 100 s. The absorption coefficient of the [LiI0 band was similar for both samples, 155 and 160 cm-I respectively. No significant difference in the I-E characteristics was noted. The implica- tion is that the effective dimensions of the semicon-
- FLOWING OXYGEN
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-
//
t, TIME (s
Fig. 2. - Optical absorption coefficient of the [Lila band vs.
incubation time a t T = 1 308 K in flowing oxygen (top curve) and in static air (bottom curve).
-3000 -2000 -6000 0 1000 2000 3000
E. ELECTRIC FIELD (V/cm)
Fig. 3 . - I vs. E curves for MgO : Li samples oxidized at 1 550 K for (a) 5 min. and (b) 71 h. The absorption coefficients of the [Li]O band were 155 and 160 cm-' respectively.
ducting inclusions d o not change substantially with prolonged incubation in flowing oxygen.
It is concluded that whereas the oxidizing tempe- rature of MgO : Li crystals has a pronounced effect on the [L~]O concentration, electrical conductivity, and I-E curve shape, thermal aging has no major effects on these characteristics.
Acknowledgement. - We appreciate stimulating discussions with J. H. Crawford, Jr., and D. J. Eisen- berg.
C6-400 Y . CHEN, M. M. ABRAHAM, J . L. BOLDU AND V. M. ORERA
DISCUSSION
Question. - T. E. MITCHELL.
My question relates to the MgO-Li,O phase dia- gram. When you anneal, the Li,O precipitates decrease in size, presumably because the phase boundary is being approached. If you anneal for a long time, should not the Microgalaxies around the precipitates disperse ?
Reply. - Y. CHEN
For the temperature range we are dealing with,
< 1 600 K, the evidence is that for prolonged anneal-
ing the inclusions of stable [LiI0 centres (micro- galaxies) do not expand appreciably. The work of Crawford et al. and our own unpublished work on the positive decoration of slip planes indicate that dislocations play a major role, directly or indirectly, in the rapid initial formation of stable [LiI0 centres.
The microgalaxy model mainly accounts for the stability of [LiI0 centres by concentrating substitu- tional Li' ions in selected regions due to the loss of lithium ions from Li,O precipitates.
References
[l] ABRAHAM, M. M., BUTLER, C. T. and CHEN, Y., J. Chem.
Phys. 55 (1971) 3757.
[2] NARAYAN, J., ABRAHAM, M. M., CHEN, Y. and TOHVER, H. T., Philos. Mag. 38 (1978) 247.
[3] ABRAHAM, M. M., CHEN, Y., BOATNER, L. A. and REYNOLDS, R. W . , Phys. Rev. Lett. 37 (1976) 849.
(41 CHEN, Y., TOHVER, H. T., NARAYAN, J. and ABRAHAM, M. M., Phys. Rev. B 16 (1977) 5535.
[5] RIUS, G., Cox, R., PICARD, R. and SANTIER, C., C.R. Hebd.
Sean. Acad. Sci. 271 (1970) 724.
[6] ABRAHAM, M. M., UNRUH, W. P. and CHEN, Y., PIzys. Rev. B 10 (1974) 3540.
[7] CHEN, Y., KERNOHAN, R. H., BOLDU, J. L., ABRAHAM, M. M., EISENBERG, D. J. and CRAWFORD, J. H. Jr. (to be publish- ed).
[8] EISENBERG, D. J., CAIN, L. S., LEE, K. H. and CRAWFORD, J. H. Jr., Appl. Phys. Lett. 33 (1978) 479.
[9] ORERA, V. M., CHEN, Y. and ABRAHAM, M. M., Bull. Am.
Phys. Soc. 24 (1979) 412.
[lo] Guard rings are used in order to prevent contributions due to surface leakage current. Our procedures followed those described by ASTM, 1976 Book of ASTM Standards, Part 38 ASTM Standard D257-76.
