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Electron emission from amorphous silicon
JOURNALOF NON-CRYSTALLINE SOLIDSg-10 (1972) 965-970
0 North-Holland
Publishing Co.
ELECTRON EMISSION FROM AMORPHOUS M. ERBUDAK
SILICON *
and T. E. FISCHER
Becton Center, Yale University, New Haven, Connecticut 06520, U.S.A.
Amorphous films were prepared by vapor quenching in ultrahigh vacuum. n- and p-type silicon crystals were used as substrates and vapor sources. The measurements (performed on clean and cesium covered surfaces) included photoelectric yield, energy distributions of photoelectrons, surface photovoltage, secondary emission with emphasis on elastic reflections, plasmon excitations, and Auger spectra. The latter were used to control the purity of the samples. Crystallization of the samples in situ was monitored by low energy electron diffraction. Amorphous structures were determined by conventional electron diffraction. Information about densities of states, optical transitions, and position of Fermi level was obtained by direct comparison of photoelectric emission from the amorphous film, a silicon crystal, and a metal measured simultaneously. The results were found to depend on the mode of preparation and annealing of disordered films.
This paper presents the results of measurements of electron emission from amorphous silicon. The data are interpreted in terms of the electronic structure of amorphous semiconductors. All experiments reported below are performed in the same vacuum chamber (p< lo-” torr). The substrate for the amorphous films is a single crystal of Si. The sample under study could be replaced at any time by a metallic emitter to calibrate the photoemission energy analyzer l). Prior to deposition of the film, the substrate is cleaned by heating; its cleanliness is established by well defined LEED patterns, Auger spectra and photoemission characteristics for crystalline silicon. These precautions are taken to exclude the possibility of impurity diffusion during heat treatment of amorphous films. Amorphous silicon films of a few thousand A thickness are deposited at the rate of 1 A/set by sublimating silicon bars (1 Qcm n- or p-type) held at a distance of 6 cm to the substrate which is at room temperature. The pressure in the chamber rises to 5 x 10m9 torr during deposition and rapidly decreases to p c IO-” torr afterwards. The amorphous films were studied with LEED, which showed no diffraction pattern and with secondary emission, which yielded the same bulk plasmon peak and the same Auger peaks as crystalline silicons). No Auger peaks due to impurities could be detected on amorphous films. These ob* Work supported number 69-1742.
by the Air Force Office of Scientific Research, under AFOSR grant
966
M. E R B U D A K A N D T. E. F I S C H E R
servations show that the amorphous films are indeed pure silicon. They also support the idea that chemical binding in amorphous silicon is the same as in crystalline materials: the total density of electrons in the valence band is the same and there is no measurable chemical shift of the valence band with respect to the core states. An earlier investigation by Peterson, Dinan and Fischer 3) has shown that photoelectric emission from amorphous silicon films freshly deposited on cold substrates is not reproducible and is modified by annealing. The same observations were made in the course of this work. The results to be presented below were thus obtained after the films were subjected to moderate heat treatment (typically 5 min at 150 °C) in order to approximate the "ideal glass" 4). After such treatment, all films showed very similar photoemission properties that would, however, change further with annealing at higher temperatures. Fig. 1 shows normalized energy distributions from crystalline and amorphous silicon. Note the strong emission from the top of the valence band at hv=3.38 eV and the sharp high-energy cut-off characteristic of crystalline materials in fig. la. At higher photon-energies, transitions from the top of the valence band are forbidden by k-conserving selection rules s). No selection rules based on k-conservation are effective in the amorphous material, (fig. l b); transitions from any energy level are equally probable. Thus the high-energy end of the distributions in fig. lb are an image of the density of states. We observe the existence of a tail that extends to energies very close to the Fermi level. It has been argued 8, 7) that the high-energy tail of the distributions shown in fig. 2 is due to the imperfect energy resolution of the analyzer. The effects of the latter are clearly visible in the distribution from the metal. At their high-energy end, these distributions should have the abrupt shape of the Fermi-Dirac function: in reality, the "Fermi-drop" of the distribution is broadened by the resolution. We see from fig. I that the tail in amorphous material is much more extended than the experimental broadening. An even more definite proof of the reality of a tail in initial states is given by the distribution obtained at hv =3.02 eV (fig. lb); it is made up entirely of electrons excited from the tail. Its existence cannot be explained in terms of experimental broadening 8). Similar measurements have been performed on amorphous films with clean surfaces 3, 8). Thus the high energy tail is a property of amorphous silicon and not of the adsorbed cesium. One may wish to determine the position of the transition from extended states to localized states from fig. 1b. If we assume that the density of extended states is similar to that of crystals, i.e., p(E),,,(Ev-E) ~, we would place the highest energy for extended states, i.e., Ev, at the extrapolated intersection
967
ELECTRON EMISSION
CESIATED CRYSTALLINE SILICON 3.9
. 8
-O
5.37 Ta
-4.0
--3.0
--2.0 E-EF-hv (eV)
--1.0
0
fig. la.
CESIATED AMORPHOUS SILICON
"o
6.
5
I~.~1
/ ~
~.o~f'\ \
I / 480/4.36/ X ~5i
/
/ --4
Jl /' 3"~9/ - ~3'"a8/33°2~ --3
--2 E--EF--hv
--I
\
\
~'l" 0
(eV)
fig. lb. Fig. I. Energy distribution of photoemitted electrons from silicon. The distributions are normalized, i.e. the area under each curve is proportional to the corresponding photoelectric yield. The abscissa shows the energy of electrons prior to excitation. The curve marked Ta is the high-energy end of an energy distribution from Tantalum. Curves are labeled by their photon energy. (a) crystalline silicon, (b) amorphous silicon.
968
M.ERBUDAK AND T . E . FISCHER
of the convex part of the distributions with the zero line. This occurs I. 1 eV below the Fermi-level. Notice that this extrapolation is identical to a fit 9) of the absorption coefficient to a curve x / ~ ( h v - E g ) . Our data then agrees with the finding that the mobility gap is larger than 1.1 eV and disagrees with the energy gap of 0.6 eV of Fischer and Donovanl°). Fig. 2 shows how photoemission from silicon films is changed by gradual annealing and crystallization. Curve 1 was obtained from the film as deposited. Annealing at 200 < T < 400 °C causes a shift of curve 1 towards higher energies. This shift indicates that the Fermi-level moves closer to the valence band. E F - E , , reduces gradually from 1.1 eV to 0.5 eV. (E v is defined somewhat arbitrarily as the upper "edge" of extended states.) This behavior is in agreement with the decrease in conductivity upon annealing reported by Brodsky and G a m b i n o 9). According to our result then, freshly deposited films of amorphous silicon are n-type; they become intrinsic upon annealing. We have observed this behavior when the source material was 1 f~ cm p-type and n-type: it is not due to chemical impurities, but apparently to structural
(1) FRESH AMORPHOUS FILM (2) ANNEALED,LEED ~'~ ~ SHOWSFAINT lxl SPOTS (3) FURTHERANNEALED, SATELLITES AROUND DISTINCTlxl SPOTS ~ ( 4 ) ANNEALED TO GIVE \ 7~7 SPOTS
hv=8.44 eV
>-0
I,I D'SPLACE0 BY O.6OV
I -4.0
--3.0
I --2.0
I --1.0
I o
E-EF-hV (eV} Fig. 2. Effect of annealing and crystallization of amorphous silicon on energy distributions of photoelectrons at hv = 8.44 eV. Curve 1 freshly deposited. Curve 2, after 5 rain at 550°C, LEED shows faint 1 x 1 spots. Curve 3, 10 min at 750°C, sharp 1 x 1 LEED pattern. Curve 4, 10 min at 1000°C, sharp 7 x 7 LEED pattern.
969
ELECTRON EMI~ION
disorder. Annealing to T < 500°C also causes a decrease in the high-energy tails. Obviously, those tails are less extended since the whole distributions move closer to E F. Annealing to 550°C for 15 min causes the appearance of very faint 1 × 1 spots in LEED which signify incipient epitaxial crystallization. At this point, energy distributions of photoelectrons show the structure of curve 2. Further annealing to 750°C and 1000°C produces curves 3 and 4. The films are epitaxial single crystals with different surface structures. Thus, annealing of amorphous material brings about a shift of the Fermilevel, crystallization causes a reduction in photoemission and structure that are probably due to k-conserving selection rules. Notice that the position of the Fermi-level in the bandgap (0.5 eV in fig. 1) of crystalline material is governed by surface properties 5). It is impossible to tell at this time whether the electronic properties of amorphous films are different at the surface and in the bulk. The only evidence we have is a surface photo-voltage indicating band-bending in crystalline materials and its absence in amorphous films. This evidence suggests flat bands, but is by no means definite proof. Fig. 3 shows energy distributions obtained at 6.35, 8.44 and 10 eV from cesiated amorphous silicon. There is definitely some structure in these curves, corresponding to structure in the density of states in the valence band of amorphous silicon. These curves give the correct energy and width of peaks in the density of states but not their amplitude. The reason is that energy distributions (fig. 3) contain electrons emitted after inelastic collisions and that selection rules based on short range symmetry can still influence transi-
CESIATED AMORPHOUS SILICON
-0
I --8.0
I --7.0
I -6.0
I
I
I
--5.0 --4.0 --3.0 E--E - h v (eV)
I --2.0
I --1.0
I 0
Fig. 3. Energy distributions from cesiated amorphous silicon at hv=6.35, 8.44 and 10 eV. Structure reveals maxima and minima in the density of valence states.
970
M. ERBUDAK AND T. E. FISCHER
tion probabilities. Further work, especially at higher photon energies, wilt be necessary to obtain more information on the density of states. Measurements of electron emission from amorphous silicon films have established beyond reasonable doubt the existence of a high-energy tail in the density of occupied states of samples prepared in the manner described in this paper. These measurements do not, however, give any direct information on the nature of these states. The fact that this tail depends on annealing suggests that they are related to defects. We have also found that the position of the Fermi-level in the bandgap depends on annealing. It is close to the conduction band in freshly deposited samples and moves towards the middle of the gap upon annealing. Our measurements indicate that the gap between extended states is larger than 1.1 eV. Photemission gives indication of structure in the density of states of the valence band; these results are not yet at the point of allowing comparison with theoretical predictions.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
J. H. Dinan, L. K. Galbraith and T. E. Fischer, Surface Sci. 26 (1971) 587. C. C. Chang, Surface Sci. 25 (1971) 53. C. W. Peterson, J. H. Dinah and T. E. Fischer, Phys. Rev. Letters 25 (1970) 861. H. H. Cohen, in: Proc. Tenth Intern. Conf. on the Physics of Semiconductors, Cambridge, Mass. 1970, Eds. S. P. Keller, J. C. Hensel and F. Stern, CONF-700801 (U.S. AEC Division of Technical Information, Springfield, Va., 1970). These results are in excellent agreement with those of F. G. Allen and G. W. Gobeli, Phys. Rev. 144 (1966) 558. D. T. Pierce, C. G. Ribbing and W. E. Spicer, J. Non-Crystalline Solids 8-10 (1972)959. D. T. Pierce and W. E. Spicer, Phys. Rev. Letters 27 (1971) 1217. T. E. Fischer and M. Erbudak, Phys. Rev. Letters 27 (1971) 1220. M. H. Brodsky and R. J. Gambino, J. Non-Crystalline Solids 8-10 (1972) 739. J. E. Fischer and T. M. Donovan, J. Non-Crystalline Solids 8-10 (1972) 202.
0 North-Holland
Publishing Co.
ELECTRON EMISSION FROM AMORPHOUS M. ERBUDAK
SILICON *
and T. E. FISCHER
Becton Center, Yale University, New Haven, Connecticut 06520, U.S.A.
Amorphous films were prepared by vapor quenching in ultrahigh vacuum. n- and p-type silicon crystals were used as substrates and vapor sources. The measurements (performed on clean and cesium covered surfaces) included photoelectric yield, energy distributions of photoelectrons, surface photovoltage, secondary emission with emphasis on elastic reflections, plasmon excitations, and Auger spectra. The latter were used to control the purity of the samples. Crystallization of the samples in situ was monitored by low energy electron diffraction. Amorphous structures were determined by conventional electron diffraction. Information about densities of states, optical transitions, and position of Fermi level was obtained by direct comparison of photoelectric emission from the amorphous film, a silicon crystal, and a metal measured simultaneously. The results were found to depend on the mode of preparation and annealing of disordered films.
This paper presents the results of measurements of electron emission from amorphous silicon. The data are interpreted in terms of the electronic structure of amorphous semiconductors. All experiments reported below are performed in the same vacuum chamber (p< lo-” torr). The substrate for the amorphous films is a single crystal of Si. The sample under study could be replaced at any time by a metallic emitter to calibrate the photoemission energy analyzer l). Prior to deposition of the film, the substrate is cleaned by heating; its cleanliness is established by well defined LEED patterns, Auger spectra and photoemission characteristics for crystalline silicon. These precautions are taken to exclude the possibility of impurity diffusion during heat treatment of amorphous films. Amorphous silicon films of a few thousand A thickness are deposited at the rate of 1 A/set by sublimating silicon bars (1 Qcm n- or p-type) held at a distance of 6 cm to the substrate which is at room temperature. The pressure in the chamber rises to 5 x 10m9 torr during deposition and rapidly decreases to p c IO-” torr afterwards. The amorphous films were studied with LEED, which showed no diffraction pattern and with secondary emission, which yielded the same bulk plasmon peak and the same Auger peaks as crystalline silicons). No Auger peaks due to impurities could be detected on amorphous films. These ob* Work supported number 69-1742.
by the Air Force Office of Scientific Research, under AFOSR grant
966
M. E R B U D A K A N D T. E. F I S C H E R
servations show that the amorphous films are indeed pure silicon. They also support the idea that chemical binding in amorphous silicon is the same as in crystalline materials: the total density of electrons in the valence band is the same and there is no measurable chemical shift of the valence band with respect to the core states. An earlier investigation by Peterson, Dinan and Fischer 3) has shown that photoelectric emission from amorphous silicon films freshly deposited on cold substrates is not reproducible and is modified by annealing. The same observations were made in the course of this work. The results to be presented below were thus obtained after the films were subjected to moderate heat treatment (typically 5 min at 150 °C) in order to approximate the "ideal glass" 4). After such treatment, all films showed very similar photoemission properties that would, however, change further with annealing at higher temperatures. Fig. 1 shows normalized energy distributions from crystalline and amorphous silicon. Note the strong emission from the top of the valence band at hv=3.38 eV and the sharp high-energy cut-off characteristic of crystalline materials in fig. la. At higher photon-energies, transitions from the top of the valence band are forbidden by k-conserving selection rules s). No selection rules based on k-conservation are effective in the amorphous material, (fig. l b); transitions from any energy level are equally probable. Thus the high-energy end of the distributions in fig. lb are an image of the density of states. We observe the existence of a tail that extends to energies very close to the Fermi level. It has been argued 8, 7) that the high-energy tail of the distributions shown in fig. 2 is due to the imperfect energy resolution of the analyzer. The effects of the latter are clearly visible in the distribution from the metal. At their high-energy end, these distributions should have the abrupt shape of the Fermi-Dirac function: in reality, the "Fermi-drop" of the distribution is broadened by the resolution. We see from fig. I that the tail in amorphous material is much more extended than the experimental broadening. An even more definite proof of the reality of a tail in initial states is given by the distribution obtained at hv =3.02 eV (fig. lb); it is made up entirely of electrons excited from the tail. Its existence cannot be explained in terms of experimental broadening 8). Similar measurements have been performed on amorphous films with clean surfaces 3, 8). Thus the high energy tail is a property of amorphous silicon and not of the adsorbed cesium. One may wish to determine the position of the transition from extended states to localized states from fig. 1b. If we assume that the density of extended states is similar to that of crystals, i.e., p(E),,,(Ev-E) ~, we would place the highest energy for extended states, i.e., Ev, at the extrapolated intersection
967
ELECTRON EMISSION
CESIATED CRYSTALLINE SILICON 3.9
. 8
-O
5.37 Ta
-4.0
--3.0
--2.0 E-EF-hv (eV)
--1.0
0
fig. la.
CESIATED AMORPHOUS SILICON
"o
6.
5
I~.~1
/ ~
~.o~f'\ \
I / 480/4.36/ X ~5i
/
/ --4
Jl /' 3"~9/ - ~3'"a8/33°2~ --3
--2 E--EF--hv
--I
\
\
~'l" 0
(eV)
fig. lb. Fig. I. Energy distribution of photoemitted electrons from silicon. The distributions are normalized, i.e. the area under each curve is proportional to the corresponding photoelectric yield. The abscissa shows the energy of electrons prior to excitation. The curve marked Ta is the high-energy end of an energy distribution from Tantalum. Curves are labeled by their photon energy. (a) crystalline silicon, (b) amorphous silicon.
968
M.ERBUDAK AND T . E . FISCHER
of the convex part of the distributions with the zero line. This occurs I. 1 eV below the Fermi-level. Notice that this extrapolation is identical to a fit 9) of the absorption coefficient to a curve x / ~ ( h v - E g ) . Our data then agrees with the finding that the mobility gap is larger than 1.1 eV and disagrees with the energy gap of 0.6 eV of Fischer and Donovanl°). Fig. 2 shows how photoemission from silicon films is changed by gradual annealing and crystallization. Curve 1 was obtained from the film as deposited. Annealing at 200 < T < 400 °C causes a shift of curve 1 towards higher energies. This shift indicates that the Fermi-level moves closer to the valence band. E F - E , , reduces gradually from 1.1 eV to 0.5 eV. (E v is defined somewhat arbitrarily as the upper "edge" of extended states.) This behavior is in agreement with the decrease in conductivity upon annealing reported by Brodsky and G a m b i n o 9). According to our result then, freshly deposited films of amorphous silicon are n-type; they become intrinsic upon annealing. We have observed this behavior when the source material was 1 f~ cm p-type and n-type: it is not due to chemical impurities, but apparently to structural
(1) FRESH AMORPHOUS FILM (2) ANNEALED,LEED ~'~ ~ SHOWSFAINT lxl SPOTS (3) FURTHERANNEALED, SATELLITES AROUND DISTINCTlxl SPOTS ~ ( 4 ) ANNEALED TO GIVE \ 7~7 SPOTS
hv=8.44 eV
>-0
I,I D'SPLACE0 BY O.6OV
I -4.0
--3.0
I --2.0
I --1.0
I o
E-EF-hV (eV} Fig. 2. Effect of annealing and crystallization of amorphous silicon on energy distributions of photoelectrons at hv = 8.44 eV. Curve 1 freshly deposited. Curve 2, after 5 rain at 550°C, LEED shows faint 1 x 1 spots. Curve 3, 10 min at 750°C, sharp 1 x 1 LEED pattern. Curve 4, 10 min at 1000°C, sharp 7 x 7 LEED pattern.
969
ELECTRON EMI~ION
disorder. Annealing to T < 500°C also causes a decrease in the high-energy tails. Obviously, those tails are less extended since the whole distributions move closer to E F. Annealing to 550°C for 15 min causes the appearance of very faint 1 × 1 spots in LEED which signify incipient epitaxial crystallization. At this point, energy distributions of photoelectrons show the structure of curve 2. Further annealing to 750°C and 1000°C produces curves 3 and 4. The films are epitaxial single crystals with different surface structures. Thus, annealing of amorphous material brings about a shift of the Fermilevel, crystallization causes a reduction in photoemission and structure that are probably due to k-conserving selection rules. Notice that the position of the Fermi-level in the bandgap (0.5 eV in fig. 1) of crystalline material is governed by surface properties 5). It is impossible to tell at this time whether the electronic properties of amorphous films are different at the surface and in the bulk. The only evidence we have is a surface photo-voltage indicating band-bending in crystalline materials and its absence in amorphous films. This evidence suggests flat bands, but is by no means definite proof. Fig. 3 shows energy distributions obtained at 6.35, 8.44 and 10 eV from cesiated amorphous silicon. There is definitely some structure in these curves, corresponding to structure in the density of states in the valence band of amorphous silicon. These curves give the correct energy and width of peaks in the density of states but not their amplitude. The reason is that energy distributions (fig. 3) contain electrons emitted after inelastic collisions and that selection rules based on short range symmetry can still influence transi-
CESIATED AMORPHOUS SILICON
-0
I --8.0
I --7.0
I -6.0
I
I
I
--5.0 --4.0 --3.0 E--E - h v (eV)
I --2.0
I --1.0
I 0
Fig. 3. Energy distributions from cesiated amorphous silicon at hv=6.35, 8.44 and 10 eV. Structure reveals maxima and minima in the density of valence states.
970
M. ERBUDAK AND T. E. FISCHER
tion probabilities. Further work, especially at higher photon energies, wilt be necessary to obtain more information on the density of states. Measurements of electron emission from amorphous silicon films have established beyond reasonable doubt the existence of a high-energy tail in the density of occupied states of samples prepared in the manner described in this paper. These measurements do not, however, give any direct information on the nature of these states. The fact that this tail depends on annealing suggests that they are related to defects. We have also found that the position of the Fermi-level in the bandgap depends on annealing. It is close to the conduction band in freshly deposited samples and moves towards the middle of the gap upon annealing. Our measurements indicate that the gap between extended states is larger than 1.1 eV. Photemission gives indication of structure in the density of states of the valence band; these results are not yet at the point of allowing comparison with theoretical predictions.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
J. H. Dinan, L. K. Galbraith and T. E. Fischer, Surface Sci. 26 (1971) 587. C. C. Chang, Surface Sci. 25 (1971) 53. C. W. Peterson, J. H. Dinah and T. E. Fischer, Phys. Rev. Letters 25 (1970) 861. H. H. Cohen, in: Proc. Tenth Intern. Conf. on the Physics of Semiconductors, Cambridge, Mass. 1970, Eds. S. P. Keller, J. C. Hensel and F. Stern, CONF-700801 (U.S. AEC Division of Technical Information, Springfield, Va., 1970). These results are in excellent agreement with those of F. G. Allen and G. W. Gobeli, Phys. Rev. 144 (1966) 558. D. T. Pierce, C. G. Ribbing and W. E. Spicer, J. Non-Crystalline Solids 8-10 (1972)959. D. T. Pierce and W. E. Spicer, Phys. Rev. Letters 27 (1971) 1217. T. E. Fischer and M. Erbudak, Phys. Rev. Letters 27 (1971) 1220. M. H. Brodsky and R. J. Gambino, J. Non-Crystalline Solids 8-10 (1972) 739. J. E. Fischer and T. M. Donovan, J. Non-Crystalline Solids 8-10 (1972) 202.