EuS THIN FILM PROPERTIES IN RELATION TO STOICHIOMETRY AND DEFECTS

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EuS THIN FILM PROPERTIES IN RELATION TO STOICHIOMETRY AND DEFECTS

J. Köhne, G. Mair, N. Rasula, B. Saftic, W. Zinn

To cite this version:

J. Köhne, G. Mair, N. Rasula, B. Saftic, W. Zinn. EuS THIN FILM PROPERTIES IN RELATION

TO STOICHIOMETRY AND DEFECTS. Journal de Physique Colloques, 1980, 41 (C5), pp.C5-127-

C5-129. �10.1051/jphyscol:1980522�. �jpa-00219957�

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JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 6, Tome 41, juin 1980, page C5-127

EuS THIN FILM PROPERTIES IN RELATION TO STOICHI0METRY AND DEFECTS J. Kohne, G. Mair, N. Rasula, B. Saftic and W. Zinn

Institut fuv Festkdvyevforschung der Kernforsohungsanlage Julich, DS170 JUlieh, R.F.A.

Résumé .- Par évaporation de EuS, on a condensé des couches minces polycristallines de EuxSy sur des substrats de verre de quartz à des températures comprises entre

20 K et 1400 K. La composition atomique, y/x, de ces couches minces varie de 0,8 à 1,2. Les changements systématiques observés à la température de Curie, la

magnétisation, le champ coercitif, la mobilité des parois et la résistance électrique sont discutés en relation avec les changements mesurés dans la composition chimique

(stoechiométrie) et avec les différences dans la concentration des défauts de structure .

Abstract.- Polycrystalline E uxSv films with atomic ratios, x/y, ranging between 0.8 and 1.2 have been prepared by vacuum evaporation of EuS on to quartz glass substrates at temperatures ranging between 20 K and 1400 K. The systematic changes observed for the Curie temperature, the magnetization, the coercive field strength, the wall mobility, and the electrical resistivity have been related to the measured changes in chemical composition (stoichiometry) and to differences in the concentration of structural defects .

1. Introduction.- Thin film samples of EuS are required for different purposes and experiments concerned with magnetic semi- conductors . The electric and magnetic properties of vacuum deposited films, however, usually differ from those of bulk and single

crystal samples due to deviations in the chemical composition as well as to a high concentration of lattice defects /1,2,3,4/.

In order to study these effects of unstoichiometry and lattice defects syste- matically, polycrystalline Eu S layers

x y

with atomic ratios, y/x, ranging between 0.8 and 1.2 have been prepared and investi- gated .

2. Sample Preparation .- By vacuum evaporation of EuS-powder from an electron beam heated tungsten crucible we have produced a molecular beam consisting mainly of Eu- and S-atoms besides EuS- and S5-molecules /5/.

At a constant rest gas pressure of 10 torr -9 and with fixed evaporation rates of about 1 nm/s the composition and the properties of thin films have been varied by changing the properties and the temperature of the substrate surface only. T he above mentioned y/x values have been realized for quartz glass substrates by varying the substrate temperature between 10 K and 1400 K. Repro- ducible surface properties have been achie-

ved by proper chemical etching immediately before the evaporation process .

3 . Measurements and Results .- From 50 cps hysteresis measurements the saturation magnetization, M , the Curie temperature, T , the coercive field strengths, H , and the wall mobility, g, have been deduced, (see e.g. ref ./l/ for the principle of the measuring method) .

From 2 MeV helium backscattering spectra / 6 / the y/x ratio has been determined experimentally .

The electrical resistivity, p, has been measured by means of the well-known van der Pauw method using four point con- tacts .

In figure 1 the ratio y/x (stoichiometry) in relation to the observed Curie temperature, T , and the electrical resistivity, p, is plotted as a function of the substrate temperature, T .

Two regions can be distinguished in the bahaviour of y/x v s . T : Below T =

•" s s 800 K Eu-rich films are obtained (y/x<l), while above 800 K the films turn out to be partly S-rich and to approach stoichiometry

(y/x=l) with increasing T .

The Curie temperature, T , of the films is generally found smaller than that of bulk and s .c . EuS as indicated in figure 1.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980522

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JOURNAL DE PHYSIQUE

101 " , , , , , , , , , , , ,

1

X

oS10, +MgF, A St XCo F,

T,IKl

Fig. 1 : Resistivity, log p, Curie temperature, T

,

and stoichiometry, y/x, as a functiog of substrate temperature, T _S

.

Even for the stoichiometric Eu S -films at x Y

T s>l 300 K T is lower by about 2 K indicating a still substantial, but equal density of defects to exist in both EuS sublattices.

An additional overall monotonic decrease of T c to a value as low as 10 K a t T s = 20 K

is found when lowering Ts. This is attribu- ted -co an equivalent increase of the total defect concentration. The broad maximum in the T c vs. T dependence in the Eu-rich

S

region between T s = 300 K and 700 K can be explained by internal oxidation of the Eu- excess from a previously published detailed analysis of the optical transmission and reflection spectra near the absorption edge of the films under discussion here /7/.

Accordingly, the additional Eu-0-Eu exchange interactions are suggested to be responsible for the T c increase of up to

2 K, which in this T s region is superposed to the monotonic decrease of T c due to defects as mentioned before.

This internal oxidation of the Eu- excess after deposition and when the samples are exposed to normal atmosphere, becomes also visible in the log p vs

.

T behaviour

shown in the upper part of figure 1 for comparison. Below 600 K the p-data are not reproducible and unstable, thus indicating an uncontrollable and different degree of internal oxidation. Only above 600 K the resistivity, p, becomes a reproducible, nearly exponentially increasing function of Ts. The values of about lo8 Qcm observed at aboutTs = 900 K are also typical for stoichiometric bulk and s .c. samples.

The explanation of the T c maximum by internal oxidation of the EuS-excess is further supported by the results of the hysteresis measurements shown in fiqure 2.

Again for comparison they all are plotted as a function of T

.

Characteristic minima are

S

found in the dependence of the wall mobility log g vs. Tc, and of the magnetization, Ms vs

.

^T again in the T region between

s t s

300 K and 700 K.

Fig. 2 : Coercive force, H

,

wall mobility, log g, and saturation magn~tizatian, Ms as a function of substrate temperature, Ts.

Ms in figure 2 is the value related to the homogeneously magnetized state of the film obtained by saturation with ac-field ampli- tudes of about 0.0 2 T

.

The minima in g and Ms correspond with the Tc-maximum in figure 1 and with the pronounced increase of the

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coercive field strength below T s = 7 0 0 K as shown in the upper part of figure 2. Inter- nal oxidation of the Eu-excess, in fact, should lead to considerable magnetic inhomo- geneit* due to the different length and electronic character of the Eu-0-Eu chemical bond and, hence, of the crystal fieldeffecks and exchange interactions / 7 / . The magnetic inhomogeneities in turn must lead to an enhancement of the magnetization ripple effect /8/ and, hence, to the observed minima of g and Ms as well as to an increase of Hc. In addition, in this T s region the ma- crostructure of the film becomes apparently coarse-grained, which may cause an additional increase of H _C

.

For comparison we have also included in the data shown on the figures 1 and 2 first results obtained with etched silicon single crystalline substrates, as well as with optically polished substrates of CaF2 and MgF2 single crystals.

While the data for the Eu S -films on x Y

silicon fit well with the systematics shown in figures 1 and 2 for the films on quartz glass, the data for films on CaF2 and MgFZ are different and exhibit in particular a much weaker dependence on T

.

^T^{he T}^c-values

in the latter case approach fairly well the s.c value of 16.5 K.

The reason for these higherT -values C

may be due either to different properties of the substrate surfaces, or to remaininq unstoichiometries, or to a smaller concentration of defects, which has to await further experimental clarification.

4. Conclusion.- Polycrystalline Eu S films x Y obtained by vacuum evaporation of EuS bulk material on to quartz glass substrates only partially approach the magnetic, electric, and optical properties typical for bulk and s .c. EuS samples. First results on films deposited on silicon, MgF2, and CaF2 single crystalline substrates indicate, however, that the film quality can be further impro- ved by proper use of now available molecular beam epitaxy techniques.

In particular, they will enable us to

study further the apparent influence of the substrate surface properties on the epita- xial growth and quality of fairly s a c . EUS films.

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