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Cathodoluminescence excitation spectra of solid rare gases
Solid State Communications, Printed in Great Britain.
Vol. 47, No. 1, pp. 47-50,
CATHODOLUMINESCENCE
0038-1098/83 $3.00 + .OO Pergamon Press Ltd.
1983.
EXCITATION
SPECTRA OF SOLID RARE GASES
F. Coletti and J.M. Debever Groupe de Physique des Etats Condens&,* Faculte des Sciences de Luminy, 70 route uon Case 901, 13288 Marseille CCdex 9, France
Lachamp,
(Received 28 February 1983 by F. Bassani)
We report here experiments on the quantum efficiency of intrinsic luminescence of rare gases excited by electrons with an energy of a few eV to IOOeV. The efficiency spectra is well structured and the thresholds understood in terms of valence bands energy dispersion.
1. INTRODUCTION
and slightly positive in Kr and Xe). (2) the electrons injected in the conduction band are nearly free electron and since there are not optical phonons and since the acoustic phonons have small energies, the only efficient relaxation processes are pairs or excitons creation [2 j, (3) the valence bands, as given by band calculations [3] are quite flat and energy and momentum conversations are decoupled and the energy thresholds are similar to optical thresholds, (4) finally, on the experimental side, there is no problem with IOeV electrons, their energy dispersion (0.3 eV) is small compared to the exciton binding energies and there is no need of energy selection, with a large gain in experimental current. The thermal radiation spectrum of the cathode is completely distinct of the U.V. radiative spectra.
THE STUDY OF EXCITATION spectra of semiconductors or isolating materials gives the luminescence efficiency as a function of the exciting photon energy. Over the last ten years such experiments have given a better understanding of the various processes, radiative or not, which occur between the absorption of the incident photon and the emission, at lower energy, of the photon, characteristic of the radiative recombination of the materials under study. The intermediate steps in de-excitation, usually non-radiative, have an influence on the overall quantum efficiency and sometimes can be identified. Such an experiment with electron excitation (cathodoluminescence excitation spectra: CES) is less easy to interpret: with a well defined photon the excited state is usually easily known; with an electron the situation is not as simple, as the incoming electron has to go in an unknown empty state of the conduction band. Therefore as compared to the optical excitation, with electron, we have (1) multiple excited states, (2) the threshold for pair or exciton creation E, depends not only on the band gap, but also on the momentum dispersion of the bands involved, to conserve energy and momentum, (3) the determination of the energy of the incident electron requires the knowledge of the vacuum level of both the emitting cathode and the crystal under study. The only experiment of CES we know was made on CdS [ I] and shows two weak structures associated with the creation of one and two pairs in the crystal. We report here our CES experiments with solid rare gases. These crystals show advantages when compared to semiconductors: (I) as shown by photoemission, the vacuum level is very close to the bottom of the conduction band (the electron affinity is slightly negative in Ne and Ar
2. EXPERIMENTAL
SET-UP
We introduce the rare gases (99.995% purity) in the vacuum chamber at 10e9 torr and condense them on a gold plated sapphire substrate at about 8 K. At that temperature, the sticking coefficient of rare gas atoms on gold is about one and the thickness of the sample from a few layers to a few microns is controlled by the time duration given in [4] of 10m6 torr partial pressure of rare gX.
Close to the sample, at about I mm in front, we set up a very simple electron gun. It consists of a filament and a wehnelt with a I mm hole. The emitted photons go through that hole and are focused on the entrance slit of a spectrometer by an elliptical mirror. The photons are detected and counted by a photomultiplier in the case of Xe, Kr and Ar; for Ne, we USCa channeltron. We measure at the same time the electron current I(v) through the gold film. The voltage drop through the emitting part of-the filament is about 0.2 eV, which, with the thermal width of the electron energy, gives a 0.5 eV undetermination in the electron energy. We control the optical quality of the samples by comparing
* ERA CNRS 070373. 47
48
CATHODOLUMINESCENCE
EXCITATION
SPECTRA OF SOLID RARE GASES
Vol. 47, No. 1
Condensedf11i-n
Fig. 1. Schematic drawing of the experimental
set-up.
the spectra taken at 100 eV of incident electron energy to the published ones [S]. 3. EXPEKIMENTAL
DATA
For the four rare gusts (1) the luminescence spectra are indcpcndent of the electron energy above a certain threshold,(2) fur thin samples, the threshold current given by gold is not modified by the sample, this remark has already been made on photoemission experiments [6J, but the shape off(v) is modified even for the thinnest samples. On Fig. 2 are given the efficiency spectra and the curves I(v) for the four rare gases in thin samples. For each curve the origin of the energy scale is given by the current threshold (plus the electron affinity in the cases of Xe and Kr). For xenon, the photons are detected at 7.1 eV on the relaxed self-trapped exciton, on the same line at 8.25 eV for Kr. For Ar and Ne we detect the non-relaxed self-trapped exciton at 11.3 and 16.6 eV respectively. For a given rare gas the CES, measured on intrinsic features (relaxed or non-relaxed self-trapped exciton and free exciton in the case of Xe [7]) display the same behaviour, except for a scale factor related to the processes efficiencies. This proves that all states are populated by the same de-excitation cascade. On Fig. 3 we show the recorded data for Xe as a function of the sample thickness for V = 50 V and I = 5 PA. The background noise without excitation is 5 c.p.s. The detectable threshold at 1Oc.p.s. is at about 20 A or 5 layers. For a 600 A sample, we have 5 x 1OS c.p.s. set -’ which leads, as we estimate it (to a factor
E (+_?V)
Fig. 2. Points: lurninesccnt yield (photons/electrons) of Xe, Kr, Ar, Ne vs incident clcctron energy 1L,‘(eV) measured above the bottom of conduction band. FuNfin~: electron current I through the sample vs incident electron energy i:‘. For each caption, the left vertical broken line shows the exciton r(3/2) energy level and the right the band gap energy. The left and right vertical full lines are at fYI and E, energy thresholds respectively.
of IO), to about 5 photons for lOOelectrons. Kr, Ar and Ne give much lower efficiencies in that order (Kr, Ar and Ne). Figure 4 shows the evolution of CES for Xe with samples of 50 A, 400A and 1 pm. The space charge (see further) induces a certain noise and weakens the structure for the 400 A sample, with a shift in energy. For the thickest sample, the structures have disappeared and there is a very large energy shift. The impinging electrons lead to a certain etching of the samples, as reported by Farrell et al. [B] which complicates the experiment as we go from Xe to Ne. The xenon samples are stable, for Kr the detected signal a few percent smaller after one hour of experiment; for Ar it is about 10%. For Ne we are forced to have a permanent gas leak to compensate the desorption rate, and as this rate depends on the electrons energy, the high energy part of the Ne CES should be amplified (Fig. 2).
VoL47,No.1
CATHODOLUMINESCENCEEXCITATIONSPECTRAOFSOLIDRAREGASES
Sample
thickness
(a): curve f i7)
Energy Sample
thickness
of lncldent
electrons
( eV
I
(8) curve (at
Fig. 3. Xc lumincsccnce intensity of the relaxed sclftrapped exciton line vs sample thickness.
For small voltages, a space charge appears, as can be seen on the /(I”) curves of Fig. 2 and Fig. 4. This effect is more important for thick sample. When E’ < I:‘, , the current, through the sample, decreases with time, the decay is stronger for smaller voltage. As soon as L’ reaches I:‘, , the current is stable.
Fig. 4. PUN Iirrc: Xc luminescent yield (photons/ electrons) vs incident electron energy for three sample thicknesses: d = 50 A, 4OOA and I. Hroken line: electron current /(I:‘) through the sample. Table 1. Parumcters of band structure of RGS as obtained by experiment [ 5 ] . Uund Gap E, , energy of first cxciton E, (r 3/Z), electrofl affinity t$, total width of the valence bands WV. Our results, first threshold E, , swoful threshold E2. All energy in e V
Ex (ev) 4. INTERPRETATION The electrons go through the samples when their energy is larger than the vacuum level energy EC + f~‘~. Our choice of the energy scale measures the electron energy from the bottom of the conduction band. On Table 1 we report E, and E, the first and second threshold and different band parameters [S], The precision on E, is much larger than on E2. On Fig. 2, we see a continuous shift of E, with respect to I?~ and Ex, being close to the first in Xe and to the second in Ne. This shift leads us to give the same interpretation of the structures for the different rare gases: E, corresponds to the creation 0: an exciton and the diffusion of the incident electron at the bottom of the conduction band. During the reaction process:
EC (ev) G (ev) WV (eV
El Ez f:‘, -E,
Xe
Kr
8.4 9.3 0.5 3 I1 20 2.6
10.1 1 I.6 0.3 2.3 12.3 22.3 2.2
Ar I’.’ _ _ 14.15 - 0.4 1.7 14 26 I .8
Ne 17.8 2 I .5 - 1.4 I .3 18.8 34 1
initial electron + exciton -i- final electron, almost all the initial electron momentum is carried away by the exciton hole. E, - E, measures the K dispersion of the valence band. The experimental widths, given in Table 1, are compatible with those obtained in photoemission experiments (91 and much larger than those predicated by band calculations. We interpret the first peak of the spectrum in the
50
CATHODOLUMINESCENCE
EXCITATION
following way. The incident electron of energy E is brought into a crystal state. For E < El, the efficiency of energy loss processes (acoustical phonons) is small; the electrons’ mean free path is large and it goes through the crystal and reaches the gold substrate. For E z E, , electron electron scattering occurs, but the mean free path remains still important (a few hundred of is) [2] and the quantum efficiency increases like the involved densities of states. For E > El, the mean free path decreases dramatically and reaches a few A for E 2 E, + 10 eV. The exciton is created at the surface where it decays no radiatively. For E = E,, the final electron has the energy E, and can penetrate the crystal and creates a second exciton, etc. This explanation is confirmed by the dependence of quantum efficiency vs film thickness (Fig. 3) which shows, for very thin samples, the predominant nonradiative decay at interfaces. Although there is, at the present time, no definite view of space charge effects, we are currently working on this field.
SPECTRA OF SOLID RARE GASES
Vol. 47, No. 1
REFERENCES 1. 7
3: 4. 5.
F. Steinrisser, P@s. Rev. Letf. 24,2 I3 (1970). N. Schwentner. Phys Rev. B15,5490 (1976). U. Rtissler, in Rue Gas Solids (Edited by M.L. Klein and J.A. Venables), Vol. 1. p. 505. Academic Press, New York ( 1976). A. Adams & PX. Hansma, Phys. Rev. B22,4258 ( 1980). For a review, see Y.A. Fugol’, Advances in Physics 27, 1 (1978); G. Zimmerer, in Proc. oflnfem. Summer School of Synchrotron Radiation Research (Edited by A.N. Mancini & J.F. Quercia), Vol. I, p. 435. Alghero (1976); B. Sonntag, in Rare Gus Solids (Edited by M.L. Klein & J.A. Venables), Vol. 11, p. 102 1. Academic Press, New York (1977). N. Schwentner & E.E. Koch, Phys. Rev. B14,4687 ( 1976). F. Coletti & A.M. Bonnot, Chem. Phys. Lett. 55, 92 (1978). H.H. Farrell & M. Strangin, Phys. Rev. B6, 4703 ( 1972). N. Schwentner, F.J. Himpsel, V. Saile, M. Skibowski, W. Steinman & E.E. Koch, Phys. Rev. Leff. 34.538 (1975).
Vol. 47, No. 1, pp. 47-50,
CATHODOLUMINESCENCE
0038-1098/83 $3.00 + .OO Pergamon Press Ltd.
1983.
EXCITATION
SPECTRA OF SOLID RARE GASES
F. Coletti and J.M. Debever Groupe de Physique des Etats Condens&,* Faculte des Sciences de Luminy, 70 route uon Case 901, 13288 Marseille CCdex 9, France
Lachamp,
(Received 28 February 1983 by F. Bassani)
We report here experiments on the quantum efficiency of intrinsic luminescence of rare gases excited by electrons with an energy of a few eV to IOOeV. The efficiency spectra is well structured and the thresholds understood in terms of valence bands energy dispersion.
1. INTRODUCTION
and slightly positive in Kr and Xe). (2) the electrons injected in the conduction band are nearly free electron and since there are not optical phonons and since the acoustic phonons have small energies, the only efficient relaxation processes are pairs or excitons creation [2 j, (3) the valence bands, as given by band calculations [3] are quite flat and energy and momentum conversations are decoupled and the energy thresholds are similar to optical thresholds, (4) finally, on the experimental side, there is no problem with IOeV electrons, their energy dispersion (0.3 eV) is small compared to the exciton binding energies and there is no need of energy selection, with a large gain in experimental current. The thermal radiation spectrum of the cathode is completely distinct of the U.V. radiative spectra.
THE STUDY OF EXCITATION spectra of semiconductors or isolating materials gives the luminescence efficiency as a function of the exciting photon energy. Over the last ten years such experiments have given a better understanding of the various processes, radiative or not, which occur between the absorption of the incident photon and the emission, at lower energy, of the photon, characteristic of the radiative recombination of the materials under study. The intermediate steps in de-excitation, usually non-radiative, have an influence on the overall quantum efficiency and sometimes can be identified. Such an experiment with electron excitation (cathodoluminescence excitation spectra: CES) is less easy to interpret: with a well defined photon the excited state is usually easily known; with an electron the situation is not as simple, as the incoming electron has to go in an unknown empty state of the conduction band. Therefore as compared to the optical excitation, with electron, we have (1) multiple excited states, (2) the threshold for pair or exciton creation E, depends not only on the band gap, but also on the momentum dispersion of the bands involved, to conserve energy and momentum, (3) the determination of the energy of the incident electron requires the knowledge of the vacuum level of both the emitting cathode and the crystal under study. The only experiment of CES we know was made on CdS [ I] and shows two weak structures associated with the creation of one and two pairs in the crystal. We report here our CES experiments with solid rare gases. These crystals show advantages when compared to semiconductors: (I) as shown by photoemission, the vacuum level is very close to the bottom of the conduction band (the electron affinity is slightly negative in Ne and Ar
2. EXPERIMENTAL
SET-UP
We introduce the rare gases (99.995% purity) in the vacuum chamber at 10e9 torr and condense them on a gold plated sapphire substrate at about 8 K. At that temperature, the sticking coefficient of rare gas atoms on gold is about one and the thickness of the sample from a few layers to a few microns is controlled by the time duration given in [4] of 10m6 torr partial pressure of rare gX.
Close to the sample, at about I mm in front, we set up a very simple electron gun. It consists of a filament and a wehnelt with a I mm hole. The emitted photons go through that hole and are focused on the entrance slit of a spectrometer by an elliptical mirror. The photons are detected and counted by a photomultiplier in the case of Xe, Kr and Ar; for Ne, we USCa channeltron. We measure at the same time the electron current I(v) through the gold film. The voltage drop through the emitting part of-the filament is about 0.2 eV, which, with the thermal width of the electron energy, gives a 0.5 eV undetermination in the electron energy. We control the optical quality of the samples by comparing
* ERA CNRS 070373. 47
48
CATHODOLUMINESCENCE
EXCITATION
SPECTRA OF SOLID RARE GASES
Vol. 47, No. 1
Condensedf11i-n
Fig. 1. Schematic drawing of the experimental
set-up.
the spectra taken at 100 eV of incident electron energy to the published ones [S]. 3. EXPEKIMENTAL
DATA
For the four rare gusts (1) the luminescence spectra are indcpcndent of the electron energy above a certain threshold,(2) fur thin samples, the threshold current given by gold is not modified by the sample, this remark has already been made on photoemission experiments [6J, but the shape off(v) is modified even for the thinnest samples. On Fig. 2 are given the efficiency spectra and the curves I(v) for the four rare gases in thin samples. For each curve the origin of the energy scale is given by the current threshold (plus the electron affinity in the cases of Xe and Kr). For xenon, the photons are detected at 7.1 eV on the relaxed self-trapped exciton, on the same line at 8.25 eV for Kr. For Ar and Ne we detect the non-relaxed self-trapped exciton at 11.3 and 16.6 eV respectively. For a given rare gas the CES, measured on intrinsic features (relaxed or non-relaxed self-trapped exciton and free exciton in the case of Xe [7]) display the same behaviour, except for a scale factor related to the processes efficiencies. This proves that all states are populated by the same de-excitation cascade. On Fig. 3 we show the recorded data for Xe as a function of the sample thickness for V = 50 V and I = 5 PA. The background noise without excitation is 5 c.p.s. The detectable threshold at 1Oc.p.s. is at about 20 A or 5 layers. For a 600 A sample, we have 5 x 1OS c.p.s. set -’ which leads, as we estimate it (to a factor
E (+_?V)
Fig. 2. Points: lurninesccnt yield (photons/electrons) of Xe, Kr, Ar, Ne vs incident clcctron energy 1L,‘(eV) measured above the bottom of conduction band. FuNfin~: electron current I through the sample vs incident electron energy i:‘. For each caption, the left vertical broken line shows the exciton r(3/2) energy level and the right the band gap energy. The left and right vertical full lines are at fYI and E, energy thresholds respectively.
of IO), to about 5 photons for lOOelectrons. Kr, Ar and Ne give much lower efficiencies in that order (Kr, Ar and Ne). Figure 4 shows the evolution of CES for Xe with samples of 50 A, 400A and 1 pm. The space charge (see further) induces a certain noise and weakens the structure for the 400 A sample, with a shift in energy. For the thickest sample, the structures have disappeared and there is a very large energy shift. The impinging electrons lead to a certain etching of the samples, as reported by Farrell et al. [B] which complicates the experiment as we go from Xe to Ne. The xenon samples are stable, for Kr the detected signal a few percent smaller after one hour of experiment; for Ar it is about 10%. For Ne we are forced to have a permanent gas leak to compensate the desorption rate, and as this rate depends on the electrons energy, the high energy part of the Ne CES should be amplified (Fig. 2).
VoL47,No.1
CATHODOLUMINESCENCEEXCITATIONSPECTRAOFSOLIDRAREGASES
Sample
thickness
(a): curve f i7)
Energy Sample
thickness
of lncldent
electrons
( eV
I
(8) curve (at
Fig. 3. Xc lumincsccnce intensity of the relaxed sclftrapped exciton line vs sample thickness.
For small voltages, a space charge appears, as can be seen on the /(I”) curves of Fig. 2 and Fig. 4. This effect is more important for thick sample. When E’ < I:‘, , the current, through the sample, decreases with time, the decay is stronger for smaller voltage. As soon as L’ reaches I:‘, , the current is stable.
Fig. 4. PUN Iirrc: Xc luminescent yield (photons/ electrons) vs incident electron energy for three sample thicknesses: d = 50 A, 4OOA and I. Hroken line: electron current /(I:‘) through the sample. Table 1. Parumcters of band structure of RGS as obtained by experiment [ 5 ] . Uund Gap E, , energy of first cxciton E, (r 3/Z), electrofl affinity t$, total width of the valence bands WV. Our results, first threshold E, , swoful threshold E2. All energy in e V
Ex (ev) 4. INTERPRETATION The electrons go through the samples when their energy is larger than the vacuum level energy EC + f~‘~. Our choice of the energy scale measures the electron energy from the bottom of the conduction band. On Table 1 we report E, and E, the first and second threshold and different band parameters [S], The precision on E, is much larger than on E2. On Fig. 2, we see a continuous shift of E, with respect to I?~ and Ex, being close to the first in Xe and to the second in Ne. This shift leads us to give the same interpretation of the structures for the different rare gases: E, corresponds to the creation 0: an exciton and the diffusion of the incident electron at the bottom of the conduction band. During the reaction process:
EC (ev) G (ev) WV (eV
El Ez f:‘, -E,
Xe
Kr
8.4 9.3 0.5 3 I1 20 2.6
10.1 1 I.6 0.3 2.3 12.3 22.3 2.2
Ar I’.’ _ _ 14.15 - 0.4 1.7 14 26 I .8
Ne 17.8 2 I .5 - 1.4 I .3 18.8 34 1
initial electron + exciton -i- final electron, almost all the initial electron momentum is carried away by the exciton hole. E, - E, measures the K dispersion of the valence band. The experimental widths, given in Table 1, are compatible with those obtained in photoemission experiments (91 and much larger than those predicated by band calculations. We interpret the first peak of the spectrum in the
50
CATHODOLUMINESCENCE
EXCITATION
following way. The incident electron of energy E is brought into a crystal state. For E < El, the efficiency of energy loss processes (acoustical phonons) is small; the electrons’ mean free path is large and it goes through the crystal and reaches the gold substrate. For E z E, , electron electron scattering occurs, but the mean free path remains still important (a few hundred of is) [2] and the quantum efficiency increases like the involved densities of states. For E > El, the mean free path decreases dramatically and reaches a few A for E 2 E, + 10 eV. The exciton is created at the surface where it decays no radiatively. For E = E,, the final electron has the energy E, and can penetrate the crystal and creates a second exciton, etc. This explanation is confirmed by the dependence of quantum efficiency vs film thickness (Fig. 3) which shows, for very thin samples, the predominant nonradiative decay at interfaces. Although there is, at the present time, no definite view of space charge effects, we are currently working on this field.
SPECTRA OF SOLID RARE GASES
Vol. 47, No. 1
REFERENCES 1. 7
3: 4. 5.
F. Steinrisser, P@s. Rev. Letf. 24,2 I3 (1970). N. Schwentner. Phys Rev. B15,5490 (1976). U. Rtissler, in Rue Gas Solids (Edited by M.L. Klein and J.A. Venables), Vol. 1. p. 505. Academic Press, New York ( 1976). A. Adams & PX. Hansma, Phys. Rev. B22,4258 ( 1980). For a review, see Y.A. Fugol’, Advances in Physics 27, 1 (1978); G. Zimmerer, in Proc. oflnfem. Summer School of Synchrotron Radiation Research (Edited by A.N. Mancini & J.F. Quercia), Vol. I, p. 435. Alghero (1976); B. Sonntag, in Rare Gus Solids (Edited by M.L. Klein & J.A. Venables), Vol. 11, p. 102 1. Academic Press, New York (1977). N. Schwentner & E.E. Koch, Phys. Rev. B14,4687 ( 1976). F. Coletti & A.M. Bonnot, Chem. Phys. Lett. 55, 92 (1978). H.H. Farrell & M. Strangin, Phys. Rev. B6, 4703 ( 1972). N. Schwentner, F.J. Himpsel, V. Saile, M. Skibowski, W. Steinman & E.E. Koch, Phys. Rev. Leff. 34.538 (1975).