The hall effect reversal in neodymium ditelluride

NOTES

414 References

1. Final Technical Summary Report, 1nwstigation of Integrally Composed Variable Energy Gap Photouoltaic Comerter. Eagle-Picker Research L,abora-

2. B. 3. E. 4. J.

tories, Contract DA36-039-~~-87408 (1962). GOLDSTEIN, Reo. Sci. Instrum. 28, 289 (1957). Solid-State

TANNENBAUM,

(1961). CRANK,

The Mathematics

Press, Oxford (1956).

I?lectron.

ofD$usion,

I

123

2,

Clarendon

I

I

the types MTe, Ma’l’eJ, bIz’l’ea, bITe2, and lVITe3. The electrical properties of polycrystalline specimens of some of these compounds have been reported,(l-4) and the data indicate that they are semimetals or semiconductors, depending on their composition. In an attempt to remove ambiguities in the data on these compounds, WC prepared single crystals of several of the sesqui- and ditellurides and studied their electrical properties.

I

I

I

IO”-

i a *

IO’

9876-

-

5-

5

43-

0

HALL

COEFFICIENT,

n TYPE

.

HALL

COEFFICIENT.

p TYPE

4 3

+ RESISTIVITY 2

I

I

2.0

4.0

I

I

I

6.0

8.0

i 0.0

II

103/T. FIC.

1.

The

electrical

conductivity oxygen

The Hall effect reversal in neodymium (Received 24 August 1962; in reeked 11 September 1962) THE rare earth tellurides

ditelluride

occur as compounds

of

i

2

I

I/ “K

and Hall exposure.

form

1 12.0

coeficient

of Nd’l’cz

after

mild

It was found that the investigated (LaTez, T\SdTez, PrTez) exhibited a the Hall effect with temperature anomalies which emphasize the need

ditellurides reversal in and other for extreme

NOTES

415

0 HALL COEFFICIENT, . HALL COEFFICIENT,

- ,‘o” -8 -7 -6 -5 -4

n TYPE p TYPE

-3

+ RESISIVITY

-2 J

1

I

I

I

I

4.0

6.0

0.0

10.0

12.0

103/T,

I

I/OK

FIG. 2. The electrical conductivity and Hall coefficient of NdTez after more severe oxygen exposure.

care in the interpretation of the electrical characteristics of impure rare earth tellurides. The properties of NdTea are presented below. Single crystals were prepared from the polycrystalline sesquitellurides by the vapor transfer technique@ in evacuated, sealed quartz tubes with iodine as the transfer agent. The hot and cold ends of the tubes were maintained at 900°C and 65O”C, respectively, and the ditellluride crystals deposited between 830°C and 700°C in 2-10 days. The crystals possessed a tetragonal structure,(a) space group D,\-P4/nmm, which contains two non-equivalent tellurium positions.

The electrical measurements were performed at a pressure of 10-s mm Hg with standard potentiometric techniques, using a field of 2000 G to produce the Hall effect. The crystals were etched and polished chemically(7) prior to the measurements. The data on NdTea are shown in Figs. 1 and 2. This compound exhibited n-type conduction at low temperatures andp-type conduction at elevated temperatures with an apparent energy gap of 0.48 eV which was calculated from the high temperature portion of the resistivity curve using the equation p = po exp(E/2kT). From the data

lVOTES

416

in Fig. 1 it was found that the net carrier concentration increased from 4.8 * l@” cIn+ at 83°K to 1.4 - 1016 cm--3 at 200°K; at temperatures above the Hall effect reversal it decreased from 5.0 * 1Oli cm+ at 300°K to 5.4 - 101” ~111-3 at 524°K. Corresponding changes were calculated for the Hall mobility. It was observed that the temperature of the reversal shifted to a lower value after an exposure of the crystal to air for 2 weeks at room temperature which suggested that an impurity contributed to the reversal. Furthermore, the crystals acquired a light tarnish in air at room temperature, and S-ray studies of La’l‘ez had sho\vn a broadening of the ditelluride lines as a result of its exposure to air which suggested that the shift in the reversal might be caused by an oxygen impurity. Since the data could be reproduced after several temperature cyclings to 524°K it was considered that the tellurium content of the crystal remained unchanged during the measurements. although a tellurium loss might be associated with the oxygen uptake. A “shoulder” was observed in the Hall coefficient curve in the high temperature region, and it became more pronounced after the oxygen exposure. Current studies are concerned with an elucidation of the observed phenomena in terms of nonstoichiometry and impurity effects and in terms of the non-equivalence of the tellurium atoms in the neodymium ditelluride lattice.

Avco Research and Advanced Development Division, J. c’. ANDRELLOS Wilmington, P. BRO Massachusetts, U.S.d.

References 1. 1. F. MILLER. I;. J. REID and R. C. HIMES, J. Electvochem. Soc.~lO6, 1013 (1959). 3. 1. F. MILLER ad R. C. HIMES. Paper IV-3 presented at the Rare Earth Conference; Lake A-rvoruhend, California (1960). 3. F. L. CARTER, Scientific Paper 23-929~8902-P7, Westinghouse Research Laboratories, Pittsburgh (1961). 4. R. C. VICKERP and H. M. MUIR, Adzam. Energy Conu. 1, 179 (1961). 5. H. SCHAFER, Chemische Transpovtreaktionen, Verlag Chemie, Weinheim (1962). 6. P. BRO and S. J. WOLNIK, to be published. 7. P. BRO, J. Electvochem. Sot. 109, 750 (1962).

On the measurement of cross-sectional resistivity variation on semiconductor crystals

‘~IIE FOUR point probe test of VALDES(‘) is a usual method for measuring the resistivitp on somiconductor crystals. This method furnishes reliable results for homogeneous crystals. To determine cross-sectional resistivity variations, miniature four point probes are used, but, if the resistivity varies within the probe distances(z) difficulties are encountered. In this paper a simple, accurate method is described for determining the radial distribution of resistivity on slices of rotation symmetrical crystals, layers and diffused sheets. On Fig. 1, a sketch of the arrangement is sho\vn.

I:rc.

1. Rleasuring

arrangement.

NOTATION Potential difference between probes. Current passed through the crystal. Distance between probes. Crystal thickness. Crystal radius.

Two ohmic contacts are prepared on a thin round slice of silicon or germanium crystal. A small area contact ( < 0.5 mm) is applied in the centre and the periphery surface of the crystal is plated by nickel, copper, gold, rhodium or some other suitable metal, respectively, using masking techniques. Direct current is passed through the contacts. Using this arrangement, the distributions of current and potential are rotation-symmetrical and the radial variation of potential can be precisely determined by moving the point probes. The crystal to be tested is mounted on a versatile manipulator stage. The point probes arc rhodium