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Angular dependence and vectorial effect of the photoemission from EuO
Solid State Communications, Vol. 17, pp. 627—630, 1975
Pergamon Press.
Printed in Great Britain
ANGULAR DEPENDENCE AND VECTORIAL EFFECT OF THE PI-JOTOEMISSION FROM EuO P. Munz Laboratorium für Festkorperphysik, ETHZ, Honggerberg, CH 8049 Zurich, Switzerland (Received 25 March 1975 by J.L. Olsen)
M extremely strong dependence of the photoemission on incident angle and polarization of the exciting light is observed for EuO. At an incident angle of 453 the vector ratio reaches values as high as 50. A strong spectral dependence is detected. The effects can be explained by a simple model which is based on the volume photoeffect
1. INTRODUCTION
observed with other pure emitter surfaces. Figure 1 shows the dependences of the photoelectric currents J~and I~on the angle of incidence of the exciting light which is polarized respectively parallel or perpendicular to the plane of incidence. The photon energy is 1.96 eV and the flux is kept constant. 1o is the current for normal incidence. When the electric vector of the illumination is perpendicular to the plane of incidence there is nearly no dependence on angle of incidence. However a strong increase in current I~with increasing incident angle is
USUALLY photoelectric emissions is investigated by light falling perpendicularly on the sample. In this work, the angular and polarization selectivity of the emission is considered. Actually observations in this area date back to the very early experiments on photoelectric emission conducted by Elster and Geitel.1 But there are only a few experiments dealing with pure sample surfaces under ultra-high vacuum conditions. The motivations for such measurements has been (1) to differentiate between surface and volume effect and (2) in the case of volume effect, to get some information about the escape depth of photoelectrons. In short, our aim is to get more knowledge about the mechanism of photoemission. 2. MEASUREMENTS AND RESULTS The measurements have been performed on vacuum cleaved samples of EuO (100) in a vacuum of 1 x 10_b torr at room temperature as described in a former paper about photoemission from EuO.2’3 EuO* shows an extremely pronounced vectorial effect of a size which, it seems, has so far never been _______________
*
Unintentionally doped samples as well as samples which contain 0.5% Gd show approximately the same angle Lnd polarization dependence. 627
observed in the case where the electric vector lies in the plane of incidence. Consequently the vector ratio which is defined as I~/I~ shows an angular dependence which is determined practically exclusively by the angular dependence of I~.This dependency is plotted in Fig. 2. Figure 3 shows the spectral response of the vector ratio. It increases strongly when the photon energy approaches the threshold energy which is about 1.6 eV for the sample considered.3 For photon energies higher than 2.7 eV the effect hasppractically disappeared. This strong dependence on photon energy differs markedly from the results reported on other substances.41°A consequenci of the selective angle- and polarization dependent emission is the fact that the dependence of the quantum yield y on photon energy in the usual for for semiconductors: Y (1w E)3 —
is only fullfi.lled for normal incidence or at oblique
628
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
Vol. 17, No.5
14
L
‘3
f
IaI
l~ 12
with oorrec~oo I-R/
~
0
1
2
-
20
-
EuO.05%Gd
a
~
10 ~~0(100) hiool,96eV
3
30
1.7
20
2,5
(eV)
PHOTON ENERGY I
____________
0
FIG. 3. Spectral dependence of the vector ratio.
~.
0° 10° 20° 30° 40°50° ANGIE OF INCIDENCE
FIG. 1. Angular dependence of the photocurrents. (Some measurements which are corrected for reflexion losses are indicated by triangles.) ____________________________________
/
// /
EuO • 0.5% Gd hi’ - I 63eV
__________________________________________ ~100
_50
o°
50
100
15°
20° 25° 300 350 ANGLE INCIDENCE
~
400
Previous phQtoelectron spectroscopy measurements on Eu03 show that in the very spectral region between threshold and 2.7 eVwhere the vector effect is strong, the emission is dominated by the excitation of impurity and imperfection states. With samples of EuSe and EuTe hardly any impurities and imperfections could be detected by photoemission and as reported above, there was hardly any angle and polarization dependence observed.
1/
I
3. DISCUSSIONAND INTERPRETATION
It seems that none of the Interpretations proposed for other substances4’’°can explain the vectorial effect in EuO. The large escape depth for the electron energies considered, the spectral selectivity, and the exclusive appearance of the effect in EuO samples and not in other calcogenides renders an explanation in terms of
450
FIG. 2. Angular dependence of the vector ratio. incidence for perpendicularpolarized light. This has to be considered when determining the threshold energy E, otherwise a false value may result.
-
For energetic reasons, effects caused by plasmons can be excluded. Also the vector ratio observed seems *
-
Two other europium chalcogenides, EuSe and EuTe were tested fqr the vectorial effect as well. Neither shows any appreciable angle or polarization selectivity,
-
the surface effect improbable. In addition only a relative tively weak surface sensitivity of the angular and polarization dependence is observed*
One monolayer of Cs only decreases the angular and polarization dependence of photoemission whereas for instance in the case of Si5 an initially strong dependence completely disappears aftercovering the surface with much less than one monolayer of oxygen.
Vol. 17, No.5
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
The interplay of this anisotropic angular distribution a and the angular dependent escape probability which is described in the usual way be an energy dependent leads to a strong angular and polarization escape cone11 selectivity. Some calculated and measured curves are compared in Figs. 4 and 5.
90 Eu 0
0
~
80
hv—163eV — calculated 0 measured
70
o ~
629
60 0
The following additional assumptions have been used.
50. 0
40
(1) The energy distribution of the excited electrons in the sample is described by:
30
n(E) D(E hv)°D(E) whereD(E hv) and D(E) denote densities of states. (ii) The density of the impurity and imperfection states D(E —liv) is approximated by a constant.
20
~
—
—
10
___________________________
0
10°
00
20°
30°
40°
50°
ANGLE OF INCIDENCE
(iii) The conduction band with lower threshold
E
1 has a quadratic dispersion law:
FIG. 4. Comparison of the calculated and measured angular dependence of the vector ratio.
2. E E~ k (iv) The work function is set at ~ = 1.6 eV as measured. —
70
excited electrons is arbitrarily described by a cos4a law. a is the angle between the k-vector of the excited
a- 45° calculated
0
0 measured
electron field.(v) The and inner the direction angular distribution of the oscifiating function electric of the Whereas the quantum yield (number of emitted electrons per absorbed photon) can be calculated ex-
40 60 w >
30
0
20 10
0
ceptcompletely be for a constant determined. scaling factor, the vector ratio can
Oo
0
i
1.7
2.0
9 9
c
c
2.5
PHOTON EF4ERGY
i
I 3.0
h,~(eV)
FIG. 5. Comparison of the calculated and measured spectral dependence of the vector ratio.
Although the assumptions are very crude, the calculated curves compare favorably with the measured ones. In accordance with the conclusions of other photoemission measurements3 the agreement suggests that in the spectral range 1.6 ~ hi.’ °~2.7 eV the emission is mainly due to excitation of localized states.
too big to be explained purely by optical effects. But the angular and polarization dependence can at least qualitatively be explained by a very simple, partly ad hoc model which is based on the volume effect in a two step treatment. The most important of the underlying assumptions is the concept of an inner angular dependent distribution function of the excited electrons, whose maximum is parallel to the exciting electric field.
Acknowledgements I would like to express my sincere gratitude to Prof. Dr. G. Busch for his efficient support of this work. I also would like to thank Dr. P. for many valuable discussions andsupport Dr. E. Kaldis forCotti supplying the EuO crystals. Financial by the schweizerischer Nationalfond zur Fordung der wissenschaftlichen Forschung is gratefully acknowledged. —
630
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
1.
ELSTER J. & GEITEL H.,Ann. Phys. 52,433(1894).
2.
COTTI P. & MUNZ P., Phys. Cond. Matter 17,307 (1974).
3.
MIJNZ P. (to be published).
4.
JUENKER D.W., WALDRON J2. & JACCODINE R.J.,J.O.S.A. 54,216(1963).
5. 6.
BROUDY R.M.,Phys. Rev. B3, 3641 (1971). GARTLAND P.O., BERGE S. & SLAGSFOLD B.J.,Phys. Rev. Lett. 30,916 (1973).
7.
MONIN J. & BOUTRY G.A.,Phys. Rev. B9, 1309 (1974).
8.
GRANT D. & CUTLER P.H., Phys. Rev. Lett. 31, 1171(1973).
9. 10.
ARAKAWA E.T., HAMM R.N. & WILLIAMS M.W.,J.O.S.A. 63, 1131(1973). JAZZAR M.S. & FISCHER T.E., Surf Sci. 42,565 (1974).
11.
BERGLUND C.N. & SPICER W.E., Phys. Rev. 136, A1030 (1964).
Vol. 17, No.5
Pergamon Press.
Printed in Great Britain
ANGULAR DEPENDENCE AND VECTORIAL EFFECT OF THE PI-JOTOEMISSION FROM EuO P. Munz Laboratorium für Festkorperphysik, ETHZ, Honggerberg, CH 8049 Zurich, Switzerland (Received 25 March 1975 by J.L. Olsen)
M extremely strong dependence of the photoemission on incident angle and polarization of the exciting light is observed for EuO. At an incident angle of 453 the vector ratio reaches values as high as 50. A strong spectral dependence is detected. The effects can be explained by a simple model which is based on the volume photoeffect
1. INTRODUCTION
observed with other pure emitter surfaces. Figure 1 shows the dependences of the photoelectric currents J~and I~on the angle of incidence of the exciting light which is polarized respectively parallel or perpendicular to the plane of incidence. The photon energy is 1.96 eV and the flux is kept constant. 1o is the current for normal incidence. When the electric vector of the illumination is perpendicular to the plane of incidence there is nearly no dependence on angle of incidence. However a strong increase in current I~with increasing incident angle is
USUALLY photoelectric emissions is investigated by light falling perpendicularly on the sample. In this work, the angular and polarization selectivity of the emission is considered. Actually observations in this area date back to the very early experiments on photoelectric emission conducted by Elster and Geitel.1 But there are only a few experiments dealing with pure sample surfaces under ultra-high vacuum conditions. The motivations for such measurements has been (1) to differentiate between surface and volume effect and (2) in the case of volume effect, to get some information about the escape depth of photoelectrons. In short, our aim is to get more knowledge about the mechanism of photoemission. 2. MEASUREMENTS AND RESULTS The measurements have been performed on vacuum cleaved samples of EuO (100) in a vacuum of 1 x 10_b torr at room temperature as described in a former paper about photoemission from EuO.2’3 EuO* shows an extremely pronounced vectorial effect of a size which, it seems, has so far never been _______________
*
Unintentionally doped samples as well as samples which contain 0.5% Gd show approximately the same angle Lnd polarization dependence. 627
observed in the case where the electric vector lies in the plane of incidence. Consequently the vector ratio which is defined as I~/I~ shows an angular dependence which is determined practically exclusively by the angular dependence of I~.This dependency is plotted in Fig. 2. Figure 3 shows the spectral response of the vector ratio. It increases strongly when the photon energy approaches the threshold energy which is about 1.6 eV for the sample considered.3 For photon energies higher than 2.7 eV the effect hasppractically disappeared. This strong dependence on photon energy differs markedly from the results reported on other substances.41°A consequenci of the selective angle- and polarization dependent emission is the fact that the dependence of the quantum yield y on photon energy in the usual for for semiconductors: Y (1w E)3 —
is only fullfi.lled for normal incidence or at oblique
628
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
Vol. 17, No.5
14
L
‘3
f
IaI
l~ 12
with oorrec~oo I-R/
~
0
1
2
-
20
-
EuO.05%Gd
a
~
10 ~~0(100) hiool,96eV
3
30
1.7
20
2,5
(eV)
PHOTON ENERGY I
____________
0
FIG. 3. Spectral dependence of the vector ratio.
~.
0° 10° 20° 30° 40°50° ANGIE OF INCIDENCE
FIG. 1. Angular dependence of the photocurrents. (Some measurements which are corrected for reflexion losses are indicated by triangles.) ____________________________________
/
// /
EuO • 0.5% Gd hi’ - I 63eV
__________________________________________ ~100
_50
o°
50
100
15°
20° 25° 300 350 ANGLE INCIDENCE
~
400
Previous phQtoelectron spectroscopy measurements on Eu03 show that in the very spectral region between threshold and 2.7 eVwhere the vector effect is strong, the emission is dominated by the excitation of impurity and imperfection states. With samples of EuSe and EuTe hardly any impurities and imperfections could be detected by photoemission and as reported above, there was hardly any angle and polarization dependence observed.
1/
I
3. DISCUSSIONAND INTERPRETATION
It seems that none of the Interpretations proposed for other substances4’’°can explain the vectorial effect in EuO. The large escape depth for the electron energies considered, the spectral selectivity, and the exclusive appearance of the effect in EuO samples and not in other calcogenides renders an explanation in terms of
450
FIG. 2. Angular dependence of the vector ratio. incidence for perpendicularpolarized light. This has to be considered when determining the threshold energy E, otherwise a false value may result.
-
For energetic reasons, effects caused by plasmons can be excluded. Also the vector ratio observed seems *
-
Two other europium chalcogenides, EuSe and EuTe were tested fqr the vectorial effect as well. Neither shows any appreciable angle or polarization selectivity,
-
the surface effect improbable. In addition only a relative tively weak surface sensitivity of the angular and polarization dependence is observed*
One monolayer of Cs only decreases the angular and polarization dependence of photoemission whereas for instance in the case of Si5 an initially strong dependence completely disappears aftercovering the surface with much less than one monolayer of oxygen.
Vol. 17, No.5
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
The interplay of this anisotropic angular distribution a and the angular dependent escape probability which is described in the usual way be an energy dependent leads to a strong angular and polarization escape cone11 selectivity. Some calculated and measured curves are compared in Figs. 4 and 5.
90 Eu 0
0
~
80
hv—163eV — calculated 0 measured
70
o ~
629
60 0
The following additional assumptions have been used.
50. 0
40
(1) The energy distribution of the excited electrons in the sample is described by:
30
n(E) D(E hv)°D(E) whereD(E hv) and D(E) denote densities of states. (ii) The density of the impurity and imperfection states D(E —liv) is approximated by a constant.
20
~
—
—
10
___________________________
0
10°
00
20°
30°
40°
50°
ANGLE OF INCIDENCE
(iii) The conduction band with lower threshold
E
1 has a quadratic dispersion law:
FIG. 4. Comparison of the calculated and measured angular dependence of the vector ratio.
2. E E~ k (iv) The work function is set at ~ = 1.6 eV as measured. —
70
excited electrons is arbitrarily described by a cos4a law. a is the angle between the k-vector of the excited
a- 45° calculated
0
0 measured
electron field.(v) The and inner the direction angular distribution of the oscifiating function electric of the Whereas the quantum yield (number of emitted electrons per absorbed photon) can be calculated ex-
40 60 w >
30
0
20 10
0
ceptcompletely be for a constant determined. scaling factor, the vector ratio can
Oo
0
i
1.7
2.0
9 9
c
c
2.5
PHOTON EF4ERGY
i
I 3.0
h,~(eV)
FIG. 5. Comparison of the calculated and measured spectral dependence of the vector ratio.
Although the assumptions are very crude, the calculated curves compare favorably with the measured ones. In accordance with the conclusions of other photoemission measurements3 the agreement suggests that in the spectral range 1.6 ~ hi.’ °~2.7 eV the emission is mainly due to excitation of localized states.
too big to be explained purely by optical effects. But the angular and polarization dependence can at least qualitatively be explained by a very simple, partly ad hoc model which is based on the volume effect in a two step treatment. The most important of the underlying assumptions is the concept of an inner angular dependent distribution function of the excited electrons, whose maximum is parallel to the exciting electric field.
Acknowledgements I would like to express my sincere gratitude to Prof. Dr. G. Busch for his efficient support of this work. I also would like to thank Dr. P. for many valuable discussions andsupport Dr. E. Kaldis forCotti supplying the EuO crystals. Financial by the schweizerischer Nationalfond zur Fordung der wissenschaftlichen Forschung is gratefully acknowledged. —
630
VECTORIAL EFFECT OF THE PHOTOEMISSION FROM EuO
1.
ELSTER J. & GEITEL H.,Ann. Phys. 52,433(1894).
2.
COTTI P. & MUNZ P., Phys. Cond. Matter 17,307 (1974).
3.
MIJNZ P. (to be published).
4.
JUENKER D.W., WALDRON J2. & JACCODINE R.J.,J.O.S.A. 54,216(1963).
5. 6.
BROUDY R.M.,Phys. Rev. B3, 3641 (1971). GARTLAND P.O., BERGE S. & SLAGSFOLD B.J.,Phys. Rev. Lett. 30,916 (1973).
7.
MONIN J. & BOUTRY G.A.,Phys. Rev. B9, 1309 (1974).
8.
GRANT D. & CUTLER P.H., Phys. Rev. Lett. 31, 1171(1973).
9. 10.
ARAKAWA E.T., HAMM R.N. & WILLIAMS M.W.,J.O.S.A. 63, 1131(1973). JAZZAR M.S. & FISCHER T.E., Surf Sci. 42,565 (1974).
11.
BERGLUND C.N. & SPICER W.E., Phys. Rev. 136, A1030 (1964).
Vol. 17, No.5