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Observation of ion desorption from MgO(001) by electron bombardment
Vacuum/volume41/numbers1-3/pages213 to 214/1990 Printed in GreatBritain
0042-207X/9053.00+ .00 © 1990 Pergamon Press plc
Observation of ion desorption from MgO(001 ) by electron b o m b a rd m e n t T Gotoh, S T a k a g i and G T o m i n a g a ,
Department of Physics, Toho University, Funabashi-sh£ 274 Japan
Electron (E < 1 keV) stimulated desorbed positive ions (H +, H2 + and 0 +) from a MgO(O01) surface are observed by means of a quadrupole mass analyser (QMA) and by Time-of-Flight (TOF) methods. The desorbed oxygen ions have two different kinetic energies, 7.8 and 4.6 eV. The origin of the ions with the lower energy is identified as adsorbed species which are ionized by electron bombardment (from examining experimental results). The total yield of oxygen ions rises above energies of 80 eV for the primary electron beam. This threshold energy of 80 eV corresponds to the core-hole creation energy in the 2p state of Mg 2+. The desorption of the faster oxygen ion is explained by the Knotek-Feibelman mechanism of the electronic transitions following core-hole creation in the Mg 2 + ion.
1. Introduction Recently we observed Photon Stimulated Desorption (PSD) of H ÷, Li ÷ and F ÷ from LiF 1 and established that the desorption is initiated by Li ls core-hole creation followed by an inter atomic Auger decay. The desorption of ionic species from lithium fluoride by electron bombardment 2 is explained also by the mechanism proposed by Knotek and Feibelman 3. Desorptions from other alkali halide crystals are also found in many reports 4. The observation of desorbed positive ions from the alkaline earth oxide crystal should be difficult since in this system two electrons must be emitted from the oxygen in the crystal by the K F mechanism. PSD from MgO by high energy photon ( > 500 eV) irradiation is observed and explained by an initial excitation which occurs in K-edge of the oxygen ion 7. However, magnesium oxide is especially stable against irradiation by electrons 5. Nevertheless, we have observed positive ion desorption from the MgO(001) surface. The experimental procedure and the results of ESD from MgO are mentioned in the next section.
2. Experimentalprocedureand results In Figure 1 the experimental apparatus is shown schematically. The specimen of MgO(001) surface is mounted in the uhv chamber (base pressure 2 x 10 -9 torr) immediately after cleavage in air. After a 150°C bake of the whole apparatus, the specimen is heated to the desired temperature by a tungsten wire heater located behind the specimen. The ejected electrons from the oxide cathode electron gun (EG) strike the specimen surface along a (111 ) direction with an incident angle 45 °. The electron beam (100 #A) is focused on the surface to a spot size of about 0.5 mm diameter. The desorbed ions are analysed by a Quadrupole Mass Analyser (QMS) located with its axis perpendicular to the incident direction of the electrons and detected by the channeltron-type detector. Instead of the ionization cage of QMS, four grids are placed in front of the entrance of the QMS for kinetic energy analysis of the ions. The mass scanning of the QMS is controlled by a saw-tooth voltage generated by the programmable power supply (PPS) through the QMS-controller (Q Cont). The saw-tooth voltage is also compared with the
desired voltages corresponding to the initial and the final mass number. The comparaters (C) generate the start and stop pulses to control the multi-channel analyser (MCA) in which the signal of the channeltron-type detector is accumulated through the preamplifier (Pre-A), the linear amplifier and the single channel analyser (SCA). The ion yields of O ÷, H2 ÷ and H ÷ are shown in Figure 2 as a function of the primary electron energy (Ee). The ordinate shows the ion yields of the each ion on arbitrary scale. In the whole region of Ee H ÷ ions are the main desorbed species. However O ÷ and H ÷ curves show a maximum around 150eV; H2 ÷ curve shows no peak but rather an increasing yield with Ep. From these curves we can determine the onset energies 80, 60 and 30 eV for O ÷, H ÷ and H2 ÷ respectively. The result of the kinetic energy analysis of the desorbed oxygen ions with the retarding grids is shown in the upper part of Figure 3. The triangle symbols are measured by bombardment of 400 eV electrons at room temperature from a surface which had stood for a long time in the vacuum. The circles are measured from the same surface area and at the same conditions after a heavy electron dose and showed no
EG
e-/~MgO ,6,*-----
--~QMS
start[~M~S Figure 1. Schematicexperimentalapparatus.QMS:quadrupolemass analyser with four grids.
213
T Gotoh et al:
Ion desorption from MgO(001 ) 500
xl
o
.O"
:t "~ 2 5 0 ¢n
E
. - C Y " ~
'-I
z
x5-a i
i
I
i
i
300
80eV (O÷; A ) 60eV ( H + ; O ) 30eV ( H2* ;I-] ) Electron
500
0
600
Channel
Ep(eV)
1000
Number (Full Scale 2 0 p S )
Figure4. The spectrum of the Time-of-Flight (TOF) of the ions.
Energy
Figure2.
Primary electron energy dependences of the ion yields of O +, H * and H2+. change of the ion yield against electron irradiation. A graphical differentiation of the solid curve in the upper part is shown in the lower part of Figure 3. The desorbed oxygen ions have two different kinetic energy components, 4.6 and 7.8 eV respectively, but before electron irradiation O ÷ ions with the higher kinetic energy are screened by ions of the lower energy. The Time-ofFlight (TOF) spectrum is obtained in another uhv apparatus with pulsed electron irradiation and is shown in Figure 4. The details of the T O F method are found elsewhere 7. The abscissa of the figure shows the flight time of the desorbed ions and the 20/as of the full scale is sectioned in 1024 channels. The ordinate shows the number of the ions. The sharp peak at 1000 ch is due to soft X-rays shifted by the delay line of the detector circuit, and this position is a starting point of the ion flight time. The arrows in the figure show the two oxygen ion peaks with different kinetic energies. These values are consistent with the results of the retarding grid measurement on the QMS.
3. Discussion The measured onset energy of the oxygen ions at 80 eV (Figure 1) corresponds to the binding energy of 2p electrons of Mg 2 ÷ ions. Excited Mg 3÷ ions that are created by this process have many
decay branches. As the mechanism leading to ion desorption, we propose inter atomic Auger decay. After the creation of a 2p hole in Mg 2 ÷ the electron transits from the valence state of O z- of the 2p hole of Mg 2÷ and one or two valence electrons 3 eject from O due to the excess energy between the levels causing the O - ion to become O ° or O+. Coulomb repulsion between O ÷ and Mg 2÷ will lead to desorption. We have never observed any magnesium positive ions in any case. This situation might be due to the neutralization probability of Mg ÷ at the surface being very close to unity because of the long residence time in the vicinity of the surface. The residual gas in the vacuum should adsorb to the surface if it stands for a long time in the vacuum. These adsorbed gas molecules should be desorbed by the initial bombardment of the electron beam. After the desorption of the adsorbed molecules (triangles in Figure 3) one can observe the desorption from the intrinsic MgO surface (circle points in Figure 3). The origin of the desorbed oxygen ions with the lower kinetic energy 4.6 eV, is the adsorbed molecules and the ions with the higher energy 7.8 eV desorb directly from the MgO matrix. The adsorbed molecule had a longer distance from Mg 2+ than the separation of Mg 2 + and 0 2- in the MgO lattice. The longer distance results in the smaller kinetic energy by the Coulomb repulsion. The ionization mechanism of the adsorbed species is not discussed at this time, whether by direct ionization or another ionization mechanism. 4.
Conclusion
The observation of electron stimulated desorption from MgO(001) surface has been made and it is found that the desorbed oxygen ions had two different kinetic energies. The faster oxygen ion is assigned to desorption from the MgO matrix and the slower one originated from an adsorbed species. It is not clear whether the origin of H ÷ and H2 ÷ ions is from the environment or from the MgO crystal.
Returding Voltage (U ;V)
=d
References O
"U
>.. "U
t
0
10
t
20
Kinetic Energy of Ion (Ek;eV)
Figure3. The ion current of O + and the energy distribution of desorbed O
+ '
214
T Yasue, T Gotoh, A lchimiya, Y Kawaguchi, M Kotani, S Ohtani, Y Shigeta, S Takagi, Y Tazawa and G Tominaga, Japan J Appl Phys, 25, L363 (1986). 2 T Yasue, A Ichimiya and S Ohtani, Surface Sci, 186, 191 (1987). 3 M L Knotek and P J Feibelman, Phys Rev Lett, 40, 964 (1978). aT R Pian, M M Traum, J S Kraus, N H Tolk, N G Stoffel and G Margaritondo, Surface Sci, 128, 13 (1983). s H Tokutaka, M Prutton, I Hiffinbotham and T E Gallon, Surface Sci, 21, 223 (1970). 6 R L Kurtz, R Stockbauer, R Nyholm, S A Flodstroem and F Senf, Phys Rev, B35, F7794 (1987). 7 T Gotoh, S Takagi and G Tominaga, Proc DIET IV, Gloggnit- (1989) To be published.
0042-207X/9053.00+ .00 © 1990 Pergamon Press plc
Observation of ion desorption from MgO(001 ) by electron b o m b a rd m e n t T Gotoh, S T a k a g i and G T o m i n a g a ,
Department of Physics, Toho University, Funabashi-sh£ 274 Japan
Electron (E < 1 keV) stimulated desorbed positive ions (H +, H2 + and 0 +) from a MgO(O01) surface are observed by means of a quadrupole mass analyser (QMA) and by Time-of-Flight (TOF) methods. The desorbed oxygen ions have two different kinetic energies, 7.8 and 4.6 eV. The origin of the ions with the lower energy is identified as adsorbed species which are ionized by electron bombardment (from examining experimental results). The total yield of oxygen ions rises above energies of 80 eV for the primary electron beam. This threshold energy of 80 eV corresponds to the core-hole creation energy in the 2p state of Mg 2+. The desorption of the faster oxygen ion is explained by the Knotek-Feibelman mechanism of the electronic transitions following core-hole creation in the Mg 2 + ion.
1. Introduction Recently we observed Photon Stimulated Desorption (PSD) of H ÷, Li ÷ and F ÷ from LiF 1 and established that the desorption is initiated by Li ls core-hole creation followed by an inter atomic Auger decay. The desorption of ionic species from lithium fluoride by electron bombardment 2 is explained also by the mechanism proposed by Knotek and Feibelman 3. Desorptions from other alkali halide crystals are also found in many reports 4. The observation of desorbed positive ions from the alkaline earth oxide crystal should be difficult since in this system two electrons must be emitted from the oxygen in the crystal by the K F mechanism. PSD from MgO by high energy photon ( > 500 eV) irradiation is observed and explained by an initial excitation which occurs in K-edge of the oxygen ion 7. However, magnesium oxide is especially stable against irradiation by electrons 5. Nevertheless, we have observed positive ion desorption from the MgO(001) surface. The experimental procedure and the results of ESD from MgO are mentioned in the next section.
2. Experimentalprocedureand results In Figure 1 the experimental apparatus is shown schematically. The specimen of MgO(001) surface is mounted in the uhv chamber (base pressure 2 x 10 -9 torr) immediately after cleavage in air. After a 150°C bake of the whole apparatus, the specimen is heated to the desired temperature by a tungsten wire heater located behind the specimen. The ejected electrons from the oxide cathode electron gun (EG) strike the specimen surface along a (111 ) direction with an incident angle 45 °. The electron beam (100 #A) is focused on the surface to a spot size of about 0.5 mm diameter. The desorbed ions are analysed by a Quadrupole Mass Analyser (QMS) located with its axis perpendicular to the incident direction of the electrons and detected by the channeltron-type detector. Instead of the ionization cage of QMS, four grids are placed in front of the entrance of the QMS for kinetic energy analysis of the ions. The mass scanning of the QMS is controlled by a saw-tooth voltage generated by the programmable power supply (PPS) through the QMS-controller (Q Cont). The saw-tooth voltage is also compared with the
desired voltages corresponding to the initial and the final mass number. The comparaters (C) generate the start and stop pulses to control the multi-channel analyser (MCA) in which the signal of the channeltron-type detector is accumulated through the preamplifier (Pre-A), the linear amplifier and the single channel analyser (SCA). The ion yields of O ÷, H2 ÷ and H ÷ are shown in Figure 2 as a function of the primary electron energy (Ee). The ordinate shows the ion yields of the each ion on arbitrary scale. In the whole region of Ee H ÷ ions are the main desorbed species. However O ÷ and H ÷ curves show a maximum around 150eV; H2 ÷ curve shows no peak but rather an increasing yield with Ep. From these curves we can determine the onset energies 80, 60 and 30 eV for O ÷, H ÷ and H2 ÷ respectively. The result of the kinetic energy analysis of the desorbed oxygen ions with the retarding grids is shown in the upper part of Figure 3. The triangle symbols are measured by bombardment of 400 eV electrons at room temperature from a surface which had stood for a long time in the vacuum. The circles are measured from the same surface area and at the same conditions after a heavy electron dose and showed no
EG
e-/~MgO ,6,*-----
--~QMS
start[~M~S Figure 1. Schematicexperimentalapparatus.QMS:quadrupolemass analyser with four grids.
213
T Gotoh et al:
Ion desorption from MgO(001 ) 500
xl
o
.O"
:t "~ 2 5 0 ¢n
E
. - C Y " ~
'-I
z
x5-a i
i
I
i
i
300
80eV (O÷; A ) 60eV ( H + ; O ) 30eV ( H2* ;I-] ) Electron
500
0
600
Channel
Ep(eV)
1000
Number (Full Scale 2 0 p S )
Figure4. The spectrum of the Time-of-Flight (TOF) of the ions.
Energy
Figure2.
Primary electron energy dependences of the ion yields of O +, H * and H2+. change of the ion yield against electron irradiation. A graphical differentiation of the solid curve in the upper part is shown in the lower part of Figure 3. The desorbed oxygen ions have two different kinetic energy components, 4.6 and 7.8 eV respectively, but before electron irradiation O ÷ ions with the higher kinetic energy are screened by ions of the lower energy. The Time-ofFlight (TOF) spectrum is obtained in another uhv apparatus with pulsed electron irradiation and is shown in Figure 4. The details of the T O F method are found elsewhere 7. The abscissa of the figure shows the flight time of the desorbed ions and the 20/as of the full scale is sectioned in 1024 channels. The ordinate shows the number of the ions. The sharp peak at 1000 ch is due to soft X-rays shifted by the delay line of the detector circuit, and this position is a starting point of the ion flight time. The arrows in the figure show the two oxygen ion peaks with different kinetic energies. These values are consistent with the results of the retarding grid measurement on the QMS.
3. Discussion The measured onset energy of the oxygen ions at 80 eV (Figure 1) corresponds to the binding energy of 2p electrons of Mg 2 ÷ ions. Excited Mg 3÷ ions that are created by this process have many
decay branches. As the mechanism leading to ion desorption, we propose inter atomic Auger decay. After the creation of a 2p hole in Mg 2 ÷ the electron transits from the valence state of O z- of the 2p hole of Mg 2÷ and one or two valence electrons 3 eject from O due to the excess energy between the levels causing the O - ion to become O ° or O+. Coulomb repulsion between O ÷ and Mg 2÷ will lead to desorption. We have never observed any magnesium positive ions in any case. This situation might be due to the neutralization probability of Mg ÷ at the surface being very close to unity because of the long residence time in the vicinity of the surface. The residual gas in the vacuum should adsorb to the surface if it stands for a long time in the vacuum. These adsorbed gas molecules should be desorbed by the initial bombardment of the electron beam. After the desorption of the adsorbed molecules (triangles in Figure 3) one can observe the desorption from the intrinsic MgO surface (circle points in Figure 3). The origin of the desorbed oxygen ions with the lower kinetic energy 4.6 eV, is the adsorbed molecules and the ions with the higher energy 7.8 eV desorb directly from the MgO matrix. The adsorbed molecule had a longer distance from Mg 2+ than the separation of Mg 2 + and 0 2- in the MgO lattice. The longer distance results in the smaller kinetic energy by the Coulomb repulsion. The ionization mechanism of the adsorbed species is not discussed at this time, whether by direct ionization or another ionization mechanism. 4.
Conclusion
The observation of electron stimulated desorption from MgO(001) surface has been made and it is found that the desorbed oxygen ions had two different kinetic energies. The faster oxygen ion is assigned to desorption from the MgO matrix and the slower one originated from an adsorbed species. It is not clear whether the origin of H ÷ and H2 ÷ ions is from the environment or from the MgO crystal.
Returding Voltage (U ;V)
=d
References O
"U
>.. "U
t
0
10
t
20
Kinetic Energy of Ion (Ek;eV)
Figure3. The ion current of O + and the energy distribution of desorbed O
+ '
214
T Yasue, T Gotoh, A lchimiya, Y Kawaguchi, M Kotani, S Ohtani, Y Shigeta, S Takagi, Y Tazawa and G Tominaga, Japan J Appl Phys, 25, L363 (1986). 2 T Yasue, A Ichimiya and S Ohtani, Surface Sci, 186, 191 (1987). 3 M L Knotek and P J Feibelman, Phys Rev Lett, 40, 964 (1978). aT R Pian, M M Traum, J S Kraus, N H Tolk, N G Stoffel and G Margaritondo, Surface Sci, 128, 13 (1983). s H Tokutaka, M Prutton, I Hiffinbotham and T E Gallon, Surface Sci, 21, 223 (1970). 6 R L Kurtz, R Stockbauer, R Nyholm, S A Flodstroem and F Senf, Phys Rev, B35, F7794 (1987). 7 T Gotoh, S Takagi and G Tominaga, Proc DIET IV, Gloggnit- (1989) To be published.