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Influence of an internal electric field in a sample on the secondary electron emission phenomenon
Thin Solid Films, 238 (1994) 271-275
271
Influence of an internal electric field in a sample on the secondary electron emission phenomenon J. Olesik and B. Catusiflski Institute of Physics, Pedagogical University of Cz~tochowa, AI. Armii Krajowej 13/15, 42-201 Czfstochowa (Poland)
(Received April 7, 1993; accepted August 19, 1993)
Abstract The influence of an internal electric field generated in a sample on the value of the secondary electron emission coefficient and energy distributions of secondary electrons were studied. Microscopic glasses covered on both sides with thin conducting films were used as samples. The dependences of the secondary electron emission coefficient on the bias voltage of the sample for primary electron energies ranging from 25 eV to 200 eV were determined. The diagrams obtained are characterized by high non-monotonic behaviour. Some specific effects of the internal field, such as the presence of secondary electrons with energy greater than incident energy, were found.
1. Introduction In 1936 Malter investigated secondary electron emission from aluminium oxidized and activated by a caesium vapour ( A I - A I 2 0 3 - C s 2 0 ) , i.e. from a metal covered with a thin and poorly conducting film. He observed large values of the secondary emission coefficient, which exceeded significantly the coefficients of materials investigated previously [1, 2]. He also observed significant instability of the phenomenon [3]. It is assumed that anomalous values of Malter emission are caused by charging of the surface of a dielectric as a result of bombardment of the surface with a primary electron beam. On the basis of Malter's results, control of the secondary emission by the generation of an inner electric field in a sample is an interesting possibility. Samples in the form of microscopic cover glasses covered with conducting films on both sides were used for this investigation. This form of samples enabled the internal field in a sample to be controlled by the bias voltage; however, its exact value was difficult to determine. It is easy to determine the electric field intensity inside the glass but it is much more difficult to determine its value in the emitting film. This is mainly due to unknown energy states in the film and their distributions.
2. Samples The samples consisted of microscopic glasses (24 x 24 x 0.2 mm 3) and two semiconducting films (Cd doped In203) evaporated onto opposite surfaces of the glass substrate. One film was an emitter of secondary electrons whereas the other was a field electrode. The
0040-6090/94/$7.00
thickness of the front film (bombarded with electrons) varied within the range 4 - 2 5 0 nm. A wide range of thickness was used because the value of the Debye length of screening was not known. The thickness of the rear film was the same for all samples (1000 nm). The layers were deposited by reactive ion sputtering [4, 5]. In203 is a wide band semiconductor with a band gap of about 3 eV. Its electron conductivity is due to donor levels near the lower edge of the conduction band. There are donor levels for oxide vacancies (0.3 eV) and for doping atoms ( 0 . 1 5 - - 0 e V ) . For thick layers the conductivity is of the order of 105 fl -l m -~. For films thinner than 4 nm, an island-like or channel-like structure is observed. The structural and electrical properties of In203-type thin films are descriped in refs. 4 and 6.
3. Instruments The analyses were carried out with the experimental set-up shown in Fig. 1. The primary electron beam was produced by a Riber CL-305 electron gun. The energy of the beam was changed from 25 eV to 200 eV. Energy broadening of primary electrons was of the order of 0.8 eV. The beam current and its cross-section were measured by a Faraday cup. The obtained current intensity values changed from 1.6 x 1 0 - 3 A m -2 at Ep = 25 eV to 18 x 10 -3 A m -2 at Ep = 200 eV. (The filament current was 3.6 A.) The internal electric field in the sample was generated by a high voltage power supply which allowed a positive or negative polarizing voltage to be applied to the rear side of the sample while the front (emitter) conducting film of the sample remained grounded. Secondary electrons from the sample
© 1994-- Elsevier Sequoia. All rights reserved
J. Olesik, B. Catusihski / Influence of internal electric fieM
272
6 I
,
L
,
,
-15
I
I
I
-10
I
L
I
'
'
'
~[-~]
i
-5
• 1Ep= 50 eV
r~xI
G
~220V
(~
I
F
E I
D
c 'L~..
i A
1 Ep=lOOeV
. . . . . .
Fig. l. Diagram of the electronic measuring system: 1 electron gun CL-305, 2 0 P R - 3 0 4 analyser, 3 sample, Z~ constant-voltage regulator of the whole set-up, Z 2 cathode heating supply, Z 3 analysing voltage supply, Z 4 focusing voltage supply, Z~ Upo] voltage supply, Z 6 collector voltage supply, V] analysing voltage meter, V2 accelerating voltage meter, V3 cathode heating current meter, V4 current meter of the voltage divider in the focusing unit of the electron gun, Vs collector current meter, V 6 collector voltage meter. I
were directed to the energy electron analyser. A fourgrid retarding potential analyse, Riber OPR-304 makes it possible to investigate changes in the secondary emission coefficient when all the grids are short circuited and grounded. This allows free flow of secondary electrons to the collector, thus producing the collector current. The energy distributions of secondary electrons at various internal field intensities and incident electron energies were measured with the retarding field analyser by changing the retarding potential. This technique is standard for the determination of secondary electron energy distributions [3, 7]. The measurement of energy distributions were performed on the same sample just after measurement of the dependence on field intensity of the coefficient 6. In this way in both cases the same area on the sample was bombarded by electrons. The analyses were carried out at a residual pressure of 10 -5 Pa vacuum.
4. Results
It is known [3] that by measuring the primary beam current (using a Faraday cup) and the current in the sample, one can determine the secondary emission coefficient 60 at Upol= 0 (front and rear sides of the sample short-circuited). The collector current I~ is proportional to the secondary current Is, thus I~ is proportional to the coefficient 6. It was assumed that the
-3000
-2000
-1000
0
Ep
=150eV +500
Fig. 2. Dependence of the coefficient 6 on the voltage Upo~and on the electric field intensity e for a sample with a 15 nm thick conducting film.
coefficient 6 for a given Upo, # 0 is proportional to the coefficient f0 for Upo, = 0 according to the formula 3 =(Ic/I°)/6o, where Ic is the collector current for Upol # 0 and I ° is the collector current for Upo~= 0. In this way, from the experimental dependence I¢(Upol), the dependence fi(Upol) was obtained. For samples with a front conducting film of thickness 15-250 nm the relation fi(Upoj) did not change significantly with the thickness of the film. Differences noted between particular samples were of the same value as differences between various areas on the same sample (errors are of the order of 2%). Figure 2 shows fi(Upol) curves for samples with a conducting film 15 nm thick. For the samples with conducting films thinner than 15 nm the curves 6(Upol) change significantly with the thickness of films. Figure 3 shows examples of 6(Upo]) curves for samples with a conducting film 4 nm thick. The thinner the conducting film the more "sensitive" to the voltage Upol the sample becomes. The energy distributions F(E) of secondary electrons were measured for samples with various conducting
J. Olesik, B. Cahts#iski / Influence of internal electric field
-15
-10 ,
-5
273
+2
Ep=50eV 'nmxll
A'
'
' :3oo'v
'
'ev' ,
20
_D
_ -600v
80
t
,--~X"~,~"-:'",'~, --i ...... ~=:¢=0,~ 20 /,0 60 80
Ep=1OOeV
~~E -100ov,
F ' I~ 20
IE~p=ISuOeV po=IV ] -3000
-2000
- 1000
0
,. 500
Fig. 3. Dependence of the coefficient 6 on the voltage Upo~ and on the electric field intensity e for a s a m p l e with a 4 n m thick conducting film.
film thicknesses and at various biasing voltages Upol and incident energies Ep. Figure 4 shows F(E) distributions at values of UpoI corresponding to characteristic changes in the coefficient 6 shown in Fig. 2 (curve for Ep = 100 eV). The symbols A - G denote the intervals of Upo, where the characteristic changes in 6 take place. In Fig. 5 similar distributions for a film 4 nm thick are given. At U p o I = 0 (B) for thick films (15-250 nm) the energy spectra are typical of semiconductors [3]. For thinner conducting films (below 15 nm), a splitting of the so-called "secondary peak", characteristic of compound emitters [3, 8], is observed (Fig. 5). In interval C one can observe a decrease in the "secondary peak" for thick films and splitting of the secondary peak which is even more distinct than in region B. In the interval of Upol corresponding to the minimum 6(Upol) (region D), decay of the F(E) function is observed. In region E (maximum I of 6(UpoO) a shift in the primary peak to
40 G ~ ~ : - : : : i 20 ~
-16oov
=[eV]
120 E[eV] 1:20
L
i E[eVI
I/
60 80 ,, -20oov , , . . . . . l. 60 80 1~0
120 EleVl IL 120
Fig. 4. Energy distributions o f secondary electrons for a sample with a 15 n m conducting layer at Ep = 100 eV.
lower energy is observed. In region F (maximum II of 6(Upo0) the primary peak "returns" to its previous position, i.e. at Ep. For all samples secondary electrons of energy greater than the incident energy Ep are, detected (Figs. 4(F) and Fig. 5(F)).
5. Discussion and conclusion
The non-monotonic behaviour of 6(Upol)" diagrams was very surprising. In view of the fact that • thick conducting films (over 15 nm) cannot be pierced by primary electron beams of energies ranging from 25 eV to 200 eV, one can assume that the observed nonmonotonic behaviour is connected with changes in space charge and free electron concentration distributions in the conducting film. The changes are due to the applied voltage Upon. A negative Upojapplied to the rear side of the sample results in the generation of a higher conduction electron concentration zone near the vacuum-film interface, whereas a lower free electron con-
274
Olesik, B. Calusihski / Influence of internal electric fieM
J.
E[eV].--
0
2O
40
60
8O
I~0
,
-
~ ' E[eV}"
II,
EteVJ
rliC
il__ ~-2b
It L~ o
E[eV]
°°
_iooo,
, L . . . . . ' 20 ' 6'o (3-14oov 20 40 ~o
"
~)
i .',
" ~o~o
leo-
]
Et2o~V
so 1oo
Fig. 5. Energy distributionsof secondaryelectrons for a sample with a 4 nm conductinglayer at Ep = 100 eV.
centration~ zone is generated near the surface adjacent to glass, The arrangement of these zones will be the reverse for a positive voltage Upol. The zone separation phenomenon will occur more frequently the thicker the conducting film is. It is known from secondary emission theory [3, 9, 10] that the coefficient 6 decreases as the electron concentration of the conducting film grows. In the case of biased samples it depends on the density of free electrons in the layer which can be pierced by primary electrons. Application of a negative Upol decreases the range of primary electrons and increases the probability of releasing "hot" electrons by collisions. At the same time a division into two zones compensates the electric field generated in a layer (especially in a thick layer) by the bias voltage Upo]. For thick films, in interval D the effect of increased free electron concentration is larger than other effects. In this way the initial decrease in the coefficient 6 with t h e increase in negative Upo~ values can be explained. When the electric field, produced by the voltage Upol, makes the enriched zone become
thinner to such an extent that it is possible to pierce it with primary electrons, an increase in the coefficient 6 should be observed. This phenomenon actually occurs (Fig. 2). For conducting films of decreasing thickness, the space charge generated in the enriched zone will be less dense. The concentration of electrons in the enriched zone will also be lower. This is probably why in this case no decrease in the coefficient 6 is observed, but rather its grown (for low negative values of Upo0. Studies on secondary electron emission influenced by internal electric field were also performed for metallic layers (palladium, thickness 15 nm). As a result of an analysis based on the 6(Upol) dependence, it was found that these samples were much less "sensitive" to internal electric fields than semiconducting layers of the same thickness. Hence the effect of the internal field is stronger the lower the concentration of conduction electrons in the layer. Studies of secondary electron energy distributions at various Upol (and fixed Ep) seem to confirm the above suggestions. As a result of polarization, two zones of different free electron concentrations can be formed in a sample. The thickness and configuration of the zones as well as the electron concentration in them depend on the value and direction of the voltage Upoj. When the polarization is negative, then in the bombarded film the zone in contact with the vacuum is enriched and the zone near the glass substrate is depleted of electrons. At low negative Upo I (for thick films) electron collisions take place in the enriched zone. The free electron concentration in this zone increases with increasing Upov The decrease in 6 is supported by the energy distributions denoted C and D in Fig. 4. For thin films the assumption of electrons piercing the conducting film is confirmed because the energy spectra obtained are typical of compound emitters (Fig. 5(B), (C)). A distinct increase in the coefficient 6 (maximum I) should be observed when primary electrons start to penetrate into the depleted zone and are accelerated to the emitting surface. This is possible if the enriched zone becomes thinner with increasing negative Upov The accelerated secondary electrons, and also primary electrons which are turned back, most probably leave the sample with small energy losses. The shift in the primary peak of curves E in Figs 4 and 5 is related to these losses. At a certain value of Upo~ (which increases with increasing Ep) the enriched zone may be so thin that some electrons penetrate into the depleted zone without energy loss. The transparency of the enriched zone may be related to the possibility of multiple electron collisions in the depleted zone leading to an energy gain greater than Ep [11]. The probability of such collisions is very small however. The occurrence of maximum II on the 6(Upo0 curve and the observation of high energy secondary electrons (curve F in Figs. 4 and 5) could
J. Olesik, B. Calusihski / Influence of internal electric field
possibly be explained by this effect. In interval G the penetration depth of primary electrons into the depleted zone is so small that the secondary emission decays. Recombination of primary electrons with holes in the depleted zone probably also takes place. If the electric field in the depleted were uniform then one would expect a linear dependence between Upol (corresponding to particular characteristic points of 6(Upo0 curves) and the energy Ep. Such dependences occurred not only in the cases presented in Figs. 2 and 3 but also for the entire range of energy Ep investigated and for all samples. At this stage of investigation it is difficult to suggest an exact mechanism of secondary electron emission controlled by an internal electric field. The basis difficulty lies in the lack of knowledge of the electron penetration depth in the emitting and biased by Upol layer. There one can expect a dependence of the electron penetration depth on energy Ep which is different from that given in the literature (e.g, ref. 10) for unbiased samples. It is possible to determine the value of an electric field created by Upol in the layer when it is not bombarded by primary electrons [12].
275
The results of this work can be applied in the production of electron multipliers. It would be particularly interesting to apply them in channeltrons.
References 1 L. Malter, Phys. Rev., 49 (1936) 478. 2 L. Malter, Phys. Rev., 50 (1936) 48. 3 I. M. Bronsztein and B. S. Frajman, Wtorieznaja Elektronnaja Emissija, Nauka, Moscow, 1969. 4 E. Leja, T. Stapifiski and K. Marszalek, Thin Solid Films, 125 (1985) 119. 5 T. Pisarkiewiez, K. Zakrzewska and E. Leja, Thin Solid Films, 153 (1987) 479. 6 T. Pisarkiewicz, K. Zakrezewska and E. Leja, Thin Solid Films, 174 (1989) 217. 7 J. G. Kozl'ow, Sowriemiennyje Problemy Elektronnoj Spektroskopii, Moscow, 1969. 8 I. M. Bronsztein, Radiotech. Elektron., 5 (1960) 1650. 9 A. van der Ziel, Solid State Physical Electronics, Prentice-Hall, Englewood Cliffs, NJ, 1976. 10 H. Seiler, J. Appl. Phys., 54 (1983) 11. 11 H. Ibach, Phys. Rev. Lett., 27(1971) 5. 12 T. Ando, A. Fowler and F. Stem, Rev. Modern Phys., 54 (1982) 2.
271
Influence of an internal electric field in a sample on the secondary electron emission phenomenon J. Olesik and B. Catusiflski Institute of Physics, Pedagogical University of Cz~tochowa, AI. Armii Krajowej 13/15, 42-201 Czfstochowa (Poland)
(Received April 7, 1993; accepted August 19, 1993)
Abstract The influence of an internal electric field generated in a sample on the value of the secondary electron emission coefficient and energy distributions of secondary electrons were studied. Microscopic glasses covered on both sides with thin conducting films were used as samples. The dependences of the secondary electron emission coefficient on the bias voltage of the sample for primary electron energies ranging from 25 eV to 200 eV were determined. The diagrams obtained are characterized by high non-monotonic behaviour. Some specific effects of the internal field, such as the presence of secondary electrons with energy greater than incident energy, were found.
1. Introduction In 1936 Malter investigated secondary electron emission from aluminium oxidized and activated by a caesium vapour ( A I - A I 2 0 3 - C s 2 0 ) , i.e. from a metal covered with a thin and poorly conducting film. He observed large values of the secondary emission coefficient, which exceeded significantly the coefficients of materials investigated previously [1, 2]. He also observed significant instability of the phenomenon [3]. It is assumed that anomalous values of Malter emission are caused by charging of the surface of a dielectric as a result of bombardment of the surface with a primary electron beam. On the basis of Malter's results, control of the secondary emission by the generation of an inner electric field in a sample is an interesting possibility. Samples in the form of microscopic cover glasses covered with conducting films on both sides were used for this investigation. This form of samples enabled the internal field in a sample to be controlled by the bias voltage; however, its exact value was difficult to determine. It is easy to determine the electric field intensity inside the glass but it is much more difficult to determine its value in the emitting film. This is mainly due to unknown energy states in the film and their distributions.
2. Samples The samples consisted of microscopic glasses (24 x 24 x 0.2 mm 3) and two semiconducting films (Cd doped In203) evaporated onto opposite surfaces of the glass substrate. One film was an emitter of secondary electrons whereas the other was a field electrode. The
0040-6090/94/$7.00
thickness of the front film (bombarded with electrons) varied within the range 4 - 2 5 0 nm. A wide range of thickness was used because the value of the Debye length of screening was not known. The thickness of the rear film was the same for all samples (1000 nm). The layers were deposited by reactive ion sputtering [4, 5]. In203 is a wide band semiconductor with a band gap of about 3 eV. Its electron conductivity is due to donor levels near the lower edge of the conduction band. There are donor levels for oxide vacancies (0.3 eV) and for doping atoms ( 0 . 1 5 - - 0 e V ) . For thick layers the conductivity is of the order of 105 fl -l m -~. For films thinner than 4 nm, an island-like or channel-like structure is observed. The structural and electrical properties of In203-type thin films are descriped in refs. 4 and 6.
3. Instruments The analyses were carried out with the experimental set-up shown in Fig. 1. The primary electron beam was produced by a Riber CL-305 electron gun. The energy of the beam was changed from 25 eV to 200 eV. Energy broadening of primary electrons was of the order of 0.8 eV. The beam current and its cross-section were measured by a Faraday cup. The obtained current intensity values changed from 1.6 x 1 0 - 3 A m -2 at Ep = 25 eV to 18 x 10 -3 A m -2 at Ep = 200 eV. (The filament current was 3.6 A.) The internal electric field in the sample was generated by a high voltage power supply which allowed a positive or negative polarizing voltage to be applied to the rear side of the sample while the front (emitter) conducting film of the sample remained grounded. Secondary electrons from the sample
© 1994-- Elsevier Sequoia. All rights reserved
J. Olesik, B. Catusihski / Influence of internal electric fieM
272
6 I
,
L
,
,
-15
I
I
I
-10
I
L
I
'
'
'
~[-~]
i
-5
• 1Ep= 50 eV
r~xI
G
~220V
(~
I
F
E I
D
c 'L~..
i A
1 Ep=lOOeV
. . . . . .
Fig. l. Diagram of the electronic measuring system: 1 electron gun CL-305, 2 0 P R - 3 0 4 analyser, 3 sample, Z~ constant-voltage regulator of the whole set-up, Z 2 cathode heating supply, Z 3 analysing voltage supply, Z 4 focusing voltage supply, Z~ Upo] voltage supply, Z 6 collector voltage supply, V] analysing voltage meter, V2 accelerating voltage meter, V3 cathode heating current meter, V4 current meter of the voltage divider in the focusing unit of the electron gun, Vs collector current meter, V 6 collector voltage meter. I
were directed to the energy electron analyser. A fourgrid retarding potential analyse, Riber OPR-304 makes it possible to investigate changes in the secondary emission coefficient when all the grids are short circuited and grounded. This allows free flow of secondary electrons to the collector, thus producing the collector current. The energy distributions of secondary electrons at various internal field intensities and incident electron energies were measured with the retarding field analyser by changing the retarding potential. This technique is standard for the determination of secondary electron energy distributions [3, 7]. The measurement of energy distributions were performed on the same sample just after measurement of the dependence on field intensity of the coefficient 6. In this way in both cases the same area on the sample was bombarded by electrons. The analyses were carried out at a residual pressure of 10 -5 Pa vacuum.
4. Results
It is known [3] that by measuring the primary beam current (using a Faraday cup) and the current in the sample, one can determine the secondary emission coefficient 60 at Upol= 0 (front and rear sides of the sample short-circuited). The collector current I~ is proportional to the secondary current Is, thus I~ is proportional to the coefficient 6. It was assumed that the
-3000
-2000
-1000
0
Ep
=150eV +500
Fig. 2. Dependence of the coefficient 6 on the voltage Upo~and on the electric field intensity e for a sample with a 15 nm thick conducting film.
coefficient 6 for a given Upo, # 0 is proportional to the coefficient f0 for Upo, = 0 according to the formula 3 =(Ic/I°)/6o, where Ic is the collector current for Upol # 0 and I ° is the collector current for Upo~= 0. In this way, from the experimental dependence I¢(Upol), the dependence fi(Upol) was obtained. For samples with a front conducting film of thickness 15-250 nm the relation fi(Upoj) did not change significantly with the thickness of the film. Differences noted between particular samples were of the same value as differences between various areas on the same sample (errors are of the order of 2%). Figure 2 shows fi(Upol) curves for samples with a conducting film 15 nm thick. For the samples with conducting films thinner than 15 nm the curves 6(Upol) change significantly with the thickness of films. Figure 3 shows examples of 6(Upo]) curves for samples with a conducting film 4 nm thick. The thinner the conducting film the more "sensitive" to the voltage Upol the sample becomes. The energy distributions F(E) of secondary electrons were measured for samples with various conducting
J. Olesik, B. Cahts#iski / Influence of internal electric field
-15
-10 ,
-5
273
+2
Ep=50eV 'nmxll
A'
'
' :3oo'v
'
'ev' ,
20
_D
_ -600v
80
t
,--~X"~,~"-:'",'~, --i ...... ~=:¢=0,~ 20 /,0 60 80
Ep=1OOeV
~~E -100ov,
F ' I~ 20
IE~p=ISuOeV po=IV ] -3000
-2000
- 1000
0
,. 500
Fig. 3. Dependence of the coefficient 6 on the voltage Upo~ and on the electric field intensity e for a s a m p l e with a 4 n m thick conducting film.
film thicknesses and at various biasing voltages Upol and incident energies Ep. Figure 4 shows F(E) distributions at values of UpoI corresponding to characteristic changes in the coefficient 6 shown in Fig. 2 (curve for Ep = 100 eV). The symbols A - G denote the intervals of Upo, where the characteristic changes in 6 take place. In Fig. 5 similar distributions for a film 4 nm thick are given. At U p o I = 0 (B) for thick films (15-250 nm) the energy spectra are typical of semiconductors [3]. For thinner conducting films (below 15 nm), a splitting of the so-called "secondary peak", characteristic of compound emitters [3, 8], is observed (Fig. 5). In interval C one can observe a decrease in the "secondary peak" for thick films and splitting of the secondary peak which is even more distinct than in region B. In the interval of Upol corresponding to the minimum 6(Upol) (region D), decay of the F(E) function is observed. In region E (maximum I of 6(UpoO) a shift in the primary peak to
40 G ~ ~ : - : : : i 20 ~
-16oov
=[eV]
120 E[eV] 1:20
L
i E[eVI
I/
60 80 ,, -20oov , , . . . . . l. 60 80 1~0
120 EleVl IL 120
Fig. 4. Energy distributions o f secondary electrons for a sample with a 15 n m conducting layer at Ep = 100 eV.
lower energy is observed. In region F (maximum II of 6(Upo0) the primary peak "returns" to its previous position, i.e. at Ep. For all samples secondary electrons of energy greater than the incident energy Ep are, detected (Figs. 4(F) and Fig. 5(F)).
5. Discussion and conclusion
The non-monotonic behaviour of 6(Upol)" diagrams was very surprising. In view of the fact that • thick conducting films (over 15 nm) cannot be pierced by primary electron beams of energies ranging from 25 eV to 200 eV, one can assume that the observed nonmonotonic behaviour is connected with changes in space charge and free electron concentration distributions in the conducting film. The changes are due to the applied voltage Upon. A negative Upojapplied to the rear side of the sample results in the generation of a higher conduction electron concentration zone near the vacuum-film interface, whereas a lower free electron con-
274
Olesik, B. Calusihski / Influence of internal electric fieM
J.
E[eV].--
0
2O
40
60
8O
I~0
,
-
~ ' E[eV}"
II,
EteVJ
rliC
il__ ~-2b
It L~ o
E[eV]
°°
_iooo,
, L . . . . . ' 20 ' 6'o (3-14oov 20 40 ~o
"
~)
i .',
" ~o~o
leo-
]
Et2o~V
so 1oo
Fig. 5. Energy distributionsof secondaryelectrons for a sample with a 4 nm conductinglayer at Ep = 100 eV.
centration~ zone is generated near the surface adjacent to glass, The arrangement of these zones will be the reverse for a positive voltage Upol. The zone separation phenomenon will occur more frequently the thicker the conducting film is. It is known from secondary emission theory [3, 9, 10] that the coefficient 6 decreases as the electron concentration of the conducting film grows. In the case of biased samples it depends on the density of free electrons in the layer which can be pierced by primary electrons. Application of a negative Upol decreases the range of primary electrons and increases the probability of releasing "hot" electrons by collisions. At the same time a division into two zones compensates the electric field generated in a layer (especially in a thick layer) by the bias voltage Upo]. For thick films, in interval D the effect of increased free electron concentration is larger than other effects. In this way the initial decrease in the coefficient 6 with t h e increase in negative Upo~ values can be explained. When the electric field, produced by the voltage Upol, makes the enriched zone become
thinner to such an extent that it is possible to pierce it with primary electrons, an increase in the coefficient 6 should be observed. This phenomenon actually occurs (Fig. 2). For conducting films of decreasing thickness, the space charge generated in the enriched zone will be less dense. The concentration of electrons in the enriched zone will also be lower. This is probably why in this case no decrease in the coefficient 6 is observed, but rather its grown (for low negative values of Upo0. Studies on secondary electron emission influenced by internal electric field were also performed for metallic layers (palladium, thickness 15 nm). As a result of an analysis based on the 6(Upol) dependence, it was found that these samples were much less "sensitive" to internal electric fields than semiconducting layers of the same thickness. Hence the effect of the internal field is stronger the lower the concentration of conduction electrons in the layer. Studies of secondary electron energy distributions at various Upol (and fixed Ep) seem to confirm the above suggestions. As a result of polarization, two zones of different free electron concentrations can be formed in a sample. The thickness and configuration of the zones as well as the electron concentration in them depend on the value and direction of the voltage Upoj. When the polarization is negative, then in the bombarded film the zone in contact with the vacuum is enriched and the zone near the glass substrate is depleted of electrons. At low negative Upo I (for thick films) electron collisions take place in the enriched zone. The free electron concentration in this zone increases with increasing Upov The decrease in 6 is supported by the energy distributions denoted C and D in Fig. 4. For thin films the assumption of electrons piercing the conducting film is confirmed because the energy spectra obtained are typical of compound emitters (Fig. 5(B), (C)). A distinct increase in the coefficient 6 (maximum I) should be observed when primary electrons start to penetrate into the depleted zone and are accelerated to the emitting surface. This is possible if the enriched zone becomes thinner with increasing negative Upov The accelerated secondary electrons, and also primary electrons which are turned back, most probably leave the sample with small energy losses. The shift in the primary peak of curves E in Figs 4 and 5 is related to these losses. At a certain value of Upo~ (which increases with increasing Ep) the enriched zone may be so thin that some electrons penetrate into the depleted zone without energy loss. The transparency of the enriched zone may be related to the possibility of multiple electron collisions in the depleted zone leading to an energy gain greater than Ep [11]. The probability of such collisions is very small however. The occurrence of maximum II on the 6(Upo0 curve and the observation of high energy secondary electrons (curve F in Figs. 4 and 5) could
J. Olesik, B. Calusihski / Influence of internal electric field
possibly be explained by this effect. In interval G the penetration depth of primary electrons into the depleted zone is so small that the secondary emission decays. Recombination of primary electrons with holes in the depleted zone probably also takes place. If the electric field in the depleted were uniform then one would expect a linear dependence between Upol (corresponding to particular characteristic points of 6(Upo0 curves) and the energy Ep. Such dependences occurred not only in the cases presented in Figs. 2 and 3 but also for the entire range of energy Ep investigated and for all samples. At this stage of investigation it is difficult to suggest an exact mechanism of secondary electron emission controlled by an internal electric field. The basis difficulty lies in the lack of knowledge of the electron penetration depth in the emitting and biased by Upol layer. There one can expect a dependence of the electron penetration depth on energy Ep which is different from that given in the literature (e.g, ref. 10) for unbiased samples. It is possible to determine the value of an electric field created by Upol in the layer when it is not bombarded by primary electrons [12].
275
The results of this work can be applied in the production of electron multipliers. It would be particularly interesting to apply them in channeltrons.
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