Carrier mobility in Cr2O3

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Chtm. .Solid.~, 1977, Vol. 38, pp. 333-334.

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CARRIER MOBILITY IN Cr,03 D. DE COGAN Department of Electronic and Electrical Engineering, University of Birmingham,BirminghamBl5 2TT,England and G. A. LONERGAN Chemical Laboratory,Trinity College,Dublin2, Ireland (Received 2 February 1976;accepted 18June 1976)

Since the electrical resistivity of a material is a function of both the concentration of carriers and their mobility, while the Seebeck coefficient depends on the concentration, a correlation of the variation of these two quantities with temperature should give a measure of the temperature dependence of the mobility. Bosman and Crevecoeur [ I] were able to show by this means that the carrier mobility in Li doped NiO was not thermally activated. We have used a similartechnique on Cr203and have obtained similarresults. Cr203 is an oxygen excess semiconductor[2,3] in which it is found that the addition of M*+ions (e.g. Mg, Ni) as impurities lead to an increase in acceptor states, while the addition of M” ion impurities (e.g. Ti) reduces the number of acceptor states(41. R-Type conduction has not so far been observed in this material. Doped and undoped samples of Johnson Matthey “Specpure” material were pressed into discs and fired for 24 hr at 1450°Cin air. Initial experiments had shown that the influence of the ambient during firing was much less than the influence of added impurities. The samples contained I% atomic impurity ions, added as the oxide. It has been observed by Hauffe and Block[4]for NiO doping and Fischer and Dietrich[S] for MgO doping that the hole concentration is proportional to the impurity concentration up to at least 1%.Rectangular bars were cut from the discs and the Seebeck coefficient and resistivity were measured in the range 18”-4oo”C. Resistivity was measured using the four probe method in both the linear and van der Pauw [6] configurations. Figures 1 and 2 show the results of log,,p and a/(2.3(k/e)) vs l/T”K for nickel and magnesium doped material and indicate clearly that the mobility in these samples does not have an exponential dependence with temperature, as would have been expected with a hopping mechanism. A similar result was also obtained for undoped Cr,O,. A I% at. doping in Cr,O, represents roughly 3.7x 10mimpurity

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Fig. 2. Plots of (I/(2.3(kle)) and log,,,p vs l/T”K for Cr,Ol t 1% Mg. ions/cm’. Values of p at l/T”K = 0 obtained by extrapolation are shown in Table 1. These represent pc,,r.,+o, = (P.., er)-‘, where P,, is the saturated hole concentration. Using arguments similar to those used by Bosman and Crevecoeur[l], an extrapolation of (r/(2.3(k/e)) vs I/T”K to l/T =0 indicates the existence of considerable compensation. If no compensation were present, then the doping concentration would place an upper limit on the value of P.,, since one M1’ impurity ion gives rise to one hole(41. This implies, that in the event of zero compensation, the mobility, given by (p~IIT.I(-O,Pule)-‘,would be greater then or equal to IOcms*/volt set for Cr203+ 1% Mg and 50cms’lvolt set for Cr,O, t 1% Ni. Compensation would make these values even larger. In view of this and the fact that the soft X-ray studies of Fischer171suggest the existence of relatively broad d-bands in Cr,O,, Hall measurements were undertaken as a confirmation. Doped and undoped samples were cemented onto a heat source and supported inside a dewar vessel between the poles of a magnet capable of fields up to 10kOe. The current was measured with a Keithley picoammeter, while a Keithley battery operated floating input electrometer was used to observe voltage. However no Hall voltage was detected in the region 18-4OO”C. This is surprising in view of the fact that both Morin[B]and Bosman and van Daal[9] were able to measure Hall voltages in Fe,O,. Morin’s results seemed to show the correct dependence on donor impurity concentration, although the Seebeck effect indicated an order of magnitude discrepency, which he suggested might be due to the Table 1.

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Fig. 1. Plots of a/(2.3(klc)) and log,, p vs I/T”K for Cr,O, t 1% Ni.

Cr,O, t 1%Mg 1.6x 10-l

Cr*O,+ 1%Ni 3.3 x lo-’

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Technical

“mesh-like network of conducting paths in the polycrystalline sample”. Various methods have been used to determine Hall mobility in such materials as the alkali halides(l0, 111, but these require that charge carriers be photo-activated. There are no reports in the literature of photoconduction in Cr203 and we have been unable to detect any photo-response in evaporated films in the region 27&700 grn, nor have we been able to detect any Hall voltage using these films. However in spite of this, it would appear that it still requires very sensitive measurements, particularly on single crystals to be undertaken, in order to determine whether a true Hall effect exists and whether it will confirm our estimates of mobility in this material.

Notes

REFERENCES I. Bosman A. and Crevecoeur C., Phys. Reo. 144, 763 (1966). 2. Hagel W. C., J. Appl. Phys. 36, 2586 (1965). 3. Crawford J. A.and VestR. W., J. Appl. Phys.35,2413(1%4). 4. Hauffe K. and Block .I., Z. Phys. Chem. (Leipzig) 198, 232

(1951). 5. Fischer W. A. and Dietrich 6.

I. 8. 9. 10. Il.

H., Z. Physik. Chem. N.F. 41,205 (1964). van der Pauw L. J., Philips Res. Repts. 13, 1 (1958). Fischer D. W., J. Phys. Chem. Solids 32, 2455 (1971). Morin F. J., Phys. Reu. 83, IO05 (1951). Bosman A. and van Daal H., Phys. Rev. 158, 736 (1%7). Redlield A. G., Phys. Reu. 94, 526 (1954). Smith G. C., Rev. Scientific Instruments 40, 1454 (1969).