- Email: [email protected]
Photoemission studies of amorphous silicon induced by P+ ion implantation
~ ) Pergamon
Solid State Communications, Vol. 96, No. 11, pp. 919-923, 1995 Copyright © 1995 Elsevier Science Lid Printed in Great Britain. All rights t~erved 0038-1098/95 $9.50 + .00
aoaa- l ~
95)oo4a~ l
PHOTOEMISSION STUDIES OF AMORPHOUS SILICON INDUCED BY P+ ION IMPLANTATION G. Pet6 KFKI Research Institute for Materials Science, H-1525 Budapest, P.O. Box 49, Hungry and J. Kanski Physics Department, Chalmers University of Technology, S-41296 G6teborg, Sweden
(Received 3 March 1995 by C. Calandra) An amorphous Si layer was formed on a Si (1 00) surface by P+ implantation at 80keV. This layer was investigated by means of photoelectron spectroscopy. The resulting spectra are different from earlier spectra on amorphous Si prepared by e-gun evaporation or cathode sputtering. The differences consist of a decreased intensity in the spectral region corresponding to p-states, and appearace of new states at higher binding energy. Qualitativity similar results have been reported for Sb implanted amorphous Ge and the modification seems to be due to the changed short range order. Keywords: A. disordered systems, A. semiconductors, E. photoelectron spectroscopies.
1. INTRODUCTION A M O R P H O U S Si and Ge have been subjected to extensive investigations. The current idea concerning their basic properties is that they are not changed compared to the crystalline counterparts, independently of the preparation method and the amorphous Si and Ge is described in terms of four-fold co-ordinated continuous random network (CNR) [1]. In some recent studies, however, contradictory experimental data were presented for ion implantation induced amorphous Ge indicating deviating structural and electronic properties from those established earlier [2]. In the case of ion implanted a-Si several investigations have been shown that the structure is perturbed and can be relaxed by thermal annealing to the wellknow amorphous state [3]. The relaxation process was detected by the large changes of the structural [4] electrical [5] optical [6, 7] vibrational properties [8]. The pertubations in the ion implantation induced a-Si are characterised by the high concentration of defects, around 1-2 at% [9], and highly distorted SiSi bonds via deviation of the bond angles of the nearest neighbours from the tetrahedral data [10, 11].
The density of states data of ion implanted a-Si are exceptionally concentrated to the gap states at the Fermi level which were found to be very high [12]. There is so far no experimental information regarding valence band density of states, except one photoemission experiment which showed a difference between evaporated and ion implanted a-Si [13]. In spite of that the valence band density of states is highly recommended to detect basic properties in the local order. The intention of this work is to perform this investigation and to find out whether the valence band density of states (as observed in photoemission) is different in ion implantation induced amorphous Si than in evaporated or sputtered normal a-Si as was found previously for Ge [2]. 2. EXPERIMENTAL The Si (1 0 0) crystal mounted on a stainless steel block holder was implanted at room temperature with 80 keV P+ ions at a dose of l015 at cm -2. The dose rate was l #Acm -2, the pressure w a s 10 - 4 Pa obtained by turbomolecular pump. This sample was loaded to a separate ADES 400 photoelectron spectrometer via a
919
920
AMORPHOUS SILICON INDUCED BY P÷ ION IMPLANTATION
load-lock system. UV and X-ray photoemission spectra (UPS, XPS) were excited with a He I resonance lamp (21.2eV) and a MgKc~ source (1254eV). The total experimental resolution was 0.2eV (UPS) and 1 eV (XPS). The sample was cleaned by 2 keV Ar ion bombardment at 30 min and the contamination was controlled by XPS. The pressure during the XPS measurement was 5 x 10-s Pa and during the UPS investigation was 5 × 10-7 Pa because of the He introduced to the system by He resonance lamp. The XPS and UPS measurements were carried out successively on the same sample. After the XPS measurements the sample was cleaned again by a short (0.5 min) Ar ion bombardment. All spectra are referred to the Fermi level, which was determined by photoemission from the metal sample holder. In the annealing experiments the temperature was measured by a thermocouple in contact with the sample holder. The temperatures obtained in this way are estimated to be accurate only within 50°C.
Vol. 96, No. 11
N
93
95
97
99
101
103
105 107
Binding energy [eV]
x6
"i N
3. RESULTS The most crucial point of this work is the effect of the oxygen contamination, since, as will be shown below, some implantation induced feature in the UPS spectra occurs in the same energy range as the O 2p emission. After a completed cycle of UPS measurements which was mostly sensitive for the oxygen, the XPS wide scan revealed an O ls signal with nearly 10% of the Si2p intensity as it is seen on Fig 1. With these signals the surface O concentration was estimated to a few percent [14]. From the evolution of the O Is peak we were able to conclude that, this oxygen signal mainly appeared due to outgassing of X-ray source. Therefore, our UPS data represent essentially cleaner surface. The effect of oxygen contamination is not important for the valence band measurement by XPS because the excitation probability O 2p and MgKa excitation is very small [14]. The effect of oxygen contamination which can be grown by X-ray source should be excluded. Moreover there is not any sign of Si-O bonding given by the lack of the correlating peak at the Si 2p emission (at 102 eV binding energy) (Fig. 1). Other notable contaminants were not detected by XPS. We can conclude from the results given above that the valence band data of P+-implanted a-Si given XPS or UPS of Figs 2 and 3 are free of effect of oxygen. This is true for annealing experiments too, because in these cases O 2p is not detectable on the UPS spectra. In spite of that the excitation cross sections are extremely high [14].
!
!
a
518 520 522 524 526 528 530 532 534 536 ~ 8
Binding energy [eV] Fig. 1. The Si 2p and O ls emmisions of wide XPS spectrum for Ar cleaned P implanted Si(1 0 0) after the UPS measurement given by Fig. 3. The spectrum was recorded with constant pass energy. The XPS spectrum from the implantation induced a-Si (see Fig. 2) was found to be significantly different from that of vacuum deposited a-Si [15-17]. The most pronounced deviation is in the range 510 eV below valence band maximum, where the broad peak (7.5eV FWHM) in the spectrum in [16] is replaced by a significantly narrow peak (3eV FWHM) in the present case. Another striking difference can be seen around the VBM, where the spectrum in [16] has a well-developed peak, whereas the present spectrum has a slow onset• Knowing that the O 2p excitation cross section in the XPS range is relatively low, and furthermore, that the contamination level in the two cases was equivalent, we can conclude that the differences are not contamination induced. Thus, the XPS data prove direct evidence that the valence electron structures of the two amorphous systems are indeed different• The UPS spectrum (Fig. 3.) is rather smooth, with only one well-defined peak at 9.2 eV. This peak is due
Vol. 96, No. 11
-4
'
AMORPHOUS SILICON INDUCED BY P÷ ION IMPLANTATION
b
1"2
h
Binding energy [eV] Fig. 2. XPS valence band spectrum of P+-implanted Si (1 00) after cleaning by Ar + ion sputtering, excited with MgKa radiation. to emission from Ar 3p states from atoms embedded during the surface cleaning process. The structureless curve is very different from spectra of evaporated amorphous Si (and of course of crystalline Si) [15, 17], but qualitatively similar to data on implantation induced amorphous Ge [2]. Apart from the Ar 3p peak the only feature is a hump in the 6-9 eV region. An independent test of our experimental setup is to relate the spectra of the implantation amorphized samples to previous data on evaporated amorphous Si and on crystallised Si. For that purpose we examined the effects of annealing for temperatures well below that required for native oxide desorption, i.e. below 800°C. After the first annealing at 500°C the sample was only cleaned by sputtering to remove a clear outgassing related contamination and reannealed at
921
500°C. The spectra obtained after this treatment and the following annealing are shown in Fig. 4. In Fig. 4 we see that annealing at 500°C results in a reduction of this broad structure but without changing the main character of that. Raising the annealing temperature from 500°C to 600°C leaves the spectrum rather unchanged, but at 650°C some spectral redistribution occurs together the drastic decrease of the Ar3p emission. At 700°C the shape is clearly altered; the broad feature at 6-9 eV binding energy is missing and a weakly structured peak is developed in the 0-5eV energy range. This spectrum is very similar to that obtained for evaporated amorphous Si [15, 17]. Further treatments, e.g. annealing at 800°C resulted in relaxed Si(1 00). 2 x 1 surface from the P implanted Si sample. The normal emmision spectrum of this surface is shown in Fig. 5 (curve a). This spectrum contains the features of the clean single crystal Si(1 00)2 × 1 surface [18, 19], with bulk interband transitions at 2 eV and 4 eV and a surface state peak at 0.8 eV binding energy. The existance of the surface peak is a very strong indication of the oxygen free Si surface [18, 19]. After a new sputtering with 500 eV Ar ions the spectra shape reverted to that of evaporated amorphous Si [15, 17], instead of the P implanted amorphous Si, see Fig. 5 (curve b). This observation is good argument that the Ar ion bombardment should not be responsible for the amorphous state which has been investigated before.
, ,._... 700
~,
• °
~°
t'"
OJ
."
././.."
,..,.y"~",,../600
,;
,.. ,,.,
soo
7 " |
1
3
5
7
~}
11
Binding energy [eV]
Binding energy [eV] Fig. 3. UPS valence band spectrum o f P + implanted Si (100).
Fig. 4. UPS valence band spectra from implanted Si(1 00) after different annealings, 500, 600, 650 and 700°C respectively.
A M O R P H O U S SILICON INDUCED BY P + ION IMPLANTATION
922
Vol. 96, No. 11
found in the UPS spectrum, which in addition reveals a new peak in the region of s-p hybrids.
! ........~" .-'
...."',,.,~'
a
5. CONCLUSIONS
A\.,, / ,-..'"
i
' ....
I 3
5
7
9
11
Binding energy [eV] Fig. 5. UPS valence band spectra of the relaxed P implanted Si(1 0 0) surface (a) and after a new sputtering (b).
4. DISCUSSION Because of the reactivity of silicon, evaluation of the present spectra and their correlation to the properties of the amorphous Si requires caution. However, both the XPS analysis, which showed a surface oxygen content in percent range, and the annealing related behaviour indicate that the data discussed here are indeed representative of adequately clean material. Interestingly enough, the transformation from the "as implanted" state to the crystalline one occurs via a state which is characterised by spectra similar to those of evaporated amorphous Si. We thus conclude that the electronic properties of the P+ implantation damaged Si are different from those of evaporated Si. It appears most natural to ascribe these observations to differences in local structural order and the associated electronic orbital hybridisation. A detailed assignment of the different spectral structures is not attempted here, since it would require a theoretical analysis of the possible s - p hybrids. In the sp 3 hybridised state the valence band is usually divided into three regions. The bottom part, dominated by s-states, top part with primarily p-character, and the intermediate range with sp 3 hybrids, Compared to spectra from evaporated a-Si, the present XPS spectrum deviates in the range of s- and sp 3 states, where the implanted material shows a narrowed and shifted peak, and in the p region, which appears significantly broadened. This broadening in the p-region is also
The photoelectric properties of the ion implantation induced amorphous silicon are markedly different from the well-known previously published data of evaporated or cathode sputtered a-Si. It is concluded here that these differences provide evidence of an electronically modified material, the modification lying in the sp-hybrid structure. This in turn, we believe, reflects an amorphous system with severe local perturbation as compared with the tetrahedrally coordinated structure. It appears that such a metastable structure can be induced by implantation processes. These results are qualitatively the same as found earlier for implanted a-Ge [2]. Acknowledgements - - This work is performed within
the framework of a Scientific Exchange Programme between the Hungarian Academy of Science and The Royal Swedish Academy. The work has been supported by grants from the Swedish Natural Science Research Council and OTKA grant (2963).
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9.
10. 11.
D.E. Polk & D.S. Bodreaux, Phys. Rev. Lett. 31, 92 (1973). G. Petr, J. Kanski & U. Sodervall, Phys. Lett. 124, 510 (1987). J. Fortner & J.S. Lannin, Phys. Rev. B37, 10154 (1988). S. Coffa, F. Priolo, J.M. Poate & S.M. Glarum, Nucl. Instr and Methods B80/81, 603 (1993). S. Roorda, W.C. Sinke, J.M. Poate, D.C. Jackobson, S. Dierker, B.S. Dennis, D.J. Eaglesham, F. Spaepen & F. Fuoss, Phys. Rev. !i44, 702 (1991). J.E. Frederickson, C.N. Wadell, W.G. Spitze & G.H. Hubler, Appl. Phys. Lett. 40, 172 (1982). M. Fried, T. Lohner, W.A.M. Aamik, L.J. Hanekamp & A van Silfhout, J. Appl. Phys 71 5260 (1992). W.C. Sinke, T. Warabisako, M. Miyao, T. Toknyama, S. Roorda & F.W. Saris, J. NonCryst. Solids. 99, 308 (1988). J.M. Poate, S. Coffa, D.C. Jakobson, A. Ploman, J.A. Roth, G.L. Olson, S. Roorda, W. Sinke, J.S. Custer, M.O. Thompson, F. Spaepen, E. Donovan, Nuclear Instr. Methods B55 5339 (1991). Y. Hiroyama, T. Motooka, T. Tokuyama, L. Wei & S. Tanigawa, Nuclear Instr. and Methods na0/sl, 982 (1993). L.P. Avakyants & E.D. Obraztsova, J. Mol. Strut. 219, 141 (1990).
Vol. 96, No. 11 12. 13. 14. 15.
AMORPHOUS SILICON INDUCED BY P+ ION IMPLANTATION
S. Coffa, F. Priola & A. Battaglia, Phys. Rev. Lett. 70, 3756 (1993). G. Pet6 & J. Kanski, Solid State Commun. 38, 377 (1981). J.J. Yeh & I. Lindau, Atomic Data and Nuclear Data Tables 32, (1975). D.T. Pierce & W. Spicer, Phys. Rev. B5, 3017 (1972).
16. 17. 18. 19.
923
L. Ley, S. Kowalczyk, R. PoUak & D.A. Shirley, Phys. Rev. Lett. 29, 1088 (1973). B. Von Roedern, L. Ley & M. Cardona, Phys. Rev. Lett. 39, 1576 (1977). F.J. Himpsel & D.E. Eastman, J. Vac. Sci. Techn. 16, 1297 (1979). L.S.O. Johansson, R.I.G. Uhberg & G.V. Hansson, Surf. Sci. 1891190, 479 (1987).
Solid State Communications, Vol. 96, No. 11, pp. 919-923, 1995 Copyright © 1995 Elsevier Science Lid Printed in Great Britain. All rights t~erved 0038-1098/95 $9.50 + .00
aoaa- l ~
95)oo4a~ l
PHOTOEMISSION STUDIES OF AMORPHOUS SILICON INDUCED BY P+ ION IMPLANTATION G. Pet6 KFKI Research Institute for Materials Science, H-1525 Budapest, P.O. Box 49, Hungry and J. Kanski Physics Department, Chalmers University of Technology, S-41296 G6teborg, Sweden
(Received 3 March 1995 by C. Calandra) An amorphous Si layer was formed on a Si (1 00) surface by P+ implantation at 80keV. This layer was investigated by means of photoelectron spectroscopy. The resulting spectra are different from earlier spectra on amorphous Si prepared by e-gun evaporation or cathode sputtering. The differences consist of a decreased intensity in the spectral region corresponding to p-states, and appearace of new states at higher binding energy. Qualitativity similar results have been reported for Sb implanted amorphous Ge and the modification seems to be due to the changed short range order. Keywords: A. disordered systems, A. semiconductors, E. photoelectron spectroscopies.
1. INTRODUCTION A M O R P H O U S Si and Ge have been subjected to extensive investigations. The current idea concerning their basic properties is that they are not changed compared to the crystalline counterparts, independently of the preparation method and the amorphous Si and Ge is described in terms of four-fold co-ordinated continuous random network (CNR) [1]. In some recent studies, however, contradictory experimental data were presented for ion implantation induced amorphous Ge indicating deviating structural and electronic properties from those established earlier [2]. In the case of ion implanted a-Si several investigations have been shown that the structure is perturbed and can be relaxed by thermal annealing to the wellknow amorphous state [3]. The relaxation process was detected by the large changes of the structural [4] electrical [5] optical [6, 7] vibrational properties [8]. The pertubations in the ion implantation induced a-Si are characterised by the high concentration of defects, around 1-2 at% [9], and highly distorted SiSi bonds via deviation of the bond angles of the nearest neighbours from the tetrahedral data [10, 11].
The density of states data of ion implanted a-Si are exceptionally concentrated to the gap states at the Fermi level which were found to be very high [12]. There is so far no experimental information regarding valence band density of states, except one photoemission experiment which showed a difference between evaporated and ion implanted a-Si [13]. In spite of that the valence band density of states is highly recommended to detect basic properties in the local order. The intention of this work is to perform this investigation and to find out whether the valence band density of states (as observed in photoemission) is different in ion implantation induced amorphous Si than in evaporated or sputtered normal a-Si as was found previously for Ge [2]. 2. EXPERIMENTAL The Si (1 0 0) crystal mounted on a stainless steel block holder was implanted at room temperature with 80 keV P+ ions at a dose of l015 at cm -2. The dose rate was l #Acm -2, the pressure w a s 10 - 4 Pa obtained by turbomolecular pump. This sample was loaded to a separate ADES 400 photoelectron spectrometer via a
919
920
AMORPHOUS SILICON INDUCED BY P÷ ION IMPLANTATION
load-lock system. UV and X-ray photoemission spectra (UPS, XPS) were excited with a He I resonance lamp (21.2eV) and a MgKc~ source (1254eV). The total experimental resolution was 0.2eV (UPS) and 1 eV (XPS). The sample was cleaned by 2 keV Ar ion bombardment at 30 min and the contamination was controlled by XPS. The pressure during the XPS measurement was 5 x 10-s Pa and during the UPS investigation was 5 × 10-7 Pa because of the He introduced to the system by He resonance lamp. The XPS and UPS measurements were carried out successively on the same sample. After the XPS measurements the sample was cleaned again by a short (0.5 min) Ar ion bombardment. All spectra are referred to the Fermi level, which was determined by photoemission from the metal sample holder. In the annealing experiments the temperature was measured by a thermocouple in contact with the sample holder. The temperatures obtained in this way are estimated to be accurate only within 50°C.
Vol. 96, No. 11
N
93
95
97
99
101
103
105 107
Binding energy [eV]
x6
"i N
3. RESULTS The most crucial point of this work is the effect of the oxygen contamination, since, as will be shown below, some implantation induced feature in the UPS spectra occurs in the same energy range as the O 2p emission. After a completed cycle of UPS measurements which was mostly sensitive for the oxygen, the XPS wide scan revealed an O ls signal with nearly 10% of the Si2p intensity as it is seen on Fig 1. With these signals the surface O concentration was estimated to a few percent [14]. From the evolution of the O Is peak we were able to conclude that, this oxygen signal mainly appeared due to outgassing of X-ray source. Therefore, our UPS data represent essentially cleaner surface. The effect of oxygen contamination is not important for the valence band measurement by XPS because the excitation probability O 2p and MgKa excitation is very small [14]. The effect of oxygen contamination which can be grown by X-ray source should be excluded. Moreover there is not any sign of Si-O bonding given by the lack of the correlating peak at the Si 2p emission (at 102 eV binding energy) (Fig. 1). Other notable contaminants were not detected by XPS. We can conclude from the results given above that the valence band data of P+-implanted a-Si given XPS or UPS of Figs 2 and 3 are free of effect of oxygen. This is true for annealing experiments too, because in these cases O 2p is not detectable on the UPS spectra. In spite of that the excitation cross sections are extremely high [14].
!
!
a
518 520 522 524 526 528 530 532 534 536 ~ 8
Binding energy [eV] Fig. 1. The Si 2p and O ls emmisions of wide XPS spectrum for Ar cleaned P implanted Si(1 0 0) after the UPS measurement given by Fig. 3. The spectrum was recorded with constant pass energy. The XPS spectrum from the implantation induced a-Si (see Fig. 2) was found to be significantly different from that of vacuum deposited a-Si [15-17]. The most pronounced deviation is in the range 510 eV below valence band maximum, where the broad peak (7.5eV FWHM) in the spectrum in [16] is replaced by a significantly narrow peak (3eV FWHM) in the present case. Another striking difference can be seen around the VBM, where the spectrum in [16] has a well-developed peak, whereas the present spectrum has a slow onset• Knowing that the O 2p excitation cross section in the XPS range is relatively low, and furthermore, that the contamination level in the two cases was equivalent, we can conclude that the differences are not contamination induced. Thus, the XPS data prove direct evidence that the valence electron structures of the two amorphous systems are indeed different• The UPS spectrum (Fig. 3.) is rather smooth, with only one well-defined peak at 9.2 eV. This peak is due
Vol. 96, No. 11
-4
'
AMORPHOUS SILICON INDUCED BY P÷ ION IMPLANTATION
b
1"2
h
Binding energy [eV] Fig. 2. XPS valence band spectrum of P+-implanted Si (1 00) after cleaning by Ar + ion sputtering, excited with MgKa radiation. to emission from Ar 3p states from atoms embedded during the surface cleaning process. The structureless curve is very different from spectra of evaporated amorphous Si (and of course of crystalline Si) [15, 17], but qualitatively similar to data on implantation induced amorphous Ge [2]. Apart from the Ar 3p peak the only feature is a hump in the 6-9 eV region. An independent test of our experimental setup is to relate the spectra of the implantation amorphized samples to previous data on evaporated amorphous Si and on crystallised Si. For that purpose we examined the effects of annealing for temperatures well below that required for native oxide desorption, i.e. below 800°C. After the first annealing at 500°C the sample was only cleaned by sputtering to remove a clear outgassing related contamination and reannealed at
921
500°C. The spectra obtained after this treatment and the following annealing are shown in Fig. 4. In Fig. 4 we see that annealing at 500°C results in a reduction of this broad structure but without changing the main character of that. Raising the annealing temperature from 500°C to 600°C leaves the spectrum rather unchanged, but at 650°C some spectral redistribution occurs together the drastic decrease of the Ar3p emission. At 700°C the shape is clearly altered; the broad feature at 6-9 eV binding energy is missing and a weakly structured peak is developed in the 0-5eV energy range. This spectrum is very similar to that obtained for evaporated amorphous Si [15, 17]. Further treatments, e.g. annealing at 800°C resulted in relaxed Si(1 00). 2 x 1 surface from the P implanted Si sample. The normal emmision spectrum of this surface is shown in Fig. 5 (curve a). This spectrum contains the features of the clean single crystal Si(1 00)2 × 1 surface [18, 19], with bulk interband transitions at 2 eV and 4 eV and a surface state peak at 0.8 eV binding energy. The existance of the surface peak is a very strong indication of the oxygen free Si surface [18, 19]. After a new sputtering with 500 eV Ar ions the spectra shape reverted to that of evaporated amorphous Si [15, 17], instead of the P implanted amorphous Si, see Fig. 5 (curve b). This observation is good argument that the Ar ion bombardment should not be responsible for the amorphous state which has been investigated before.
, ,._... 700
~,
• °
~°
t'"
OJ
."
././.."
,..,.y"~",,../600
,;
,.. ,,.,
soo
7 " |
1
3
5
7
~}
11
Binding energy [eV]
Binding energy [eV] Fig. 3. UPS valence band spectrum o f P + implanted Si (100).
Fig. 4. UPS valence band spectra from implanted Si(1 00) after different annealings, 500, 600, 650 and 700°C respectively.
A M O R P H O U S SILICON INDUCED BY P + ION IMPLANTATION
922
Vol. 96, No. 11
found in the UPS spectrum, which in addition reveals a new peak in the region of s-p hybrids.
! ........~" .-'
...."',,.,~'
a
5. CONCLUSIONS
A\.,, / ,-..'"
i
' ....
I 3
5
7
9
11
Binding energy [eV] Fig. 5. UPS valence band spectra of the relaxed P implanted Si(1 0 0) surface (a) and after a new sputtering (b).
4. DISCUSSION Because of the reactivity of silicon, evaluation of the present spectra and their correlation to the properties of the amorphous Si requires caution. However, both the XPS analysis, which showed a surface oxygen content in percent range, and the annealing related behaviour indicate that the data discussed here are indeed representative of adequately clean material. Interestingly enough, the transformation from the "as implanted" state to the crystalline one occurs via a state which is characterised by spectra similar to those of evaporated amorphous Si. We thus conclude that the electronic properties of the P+ implantation damaged Si are different from those of evaporated Si. It appears most natural to ascribe these observations to differences in local structural order and the associated electronic orbital hybridisation. A detailed assignment of the different spectral structures is not attempted here, since it would require a theoretical analysis of the possible s - p hybrids. In the sp 3 hybridised state the valence band is usually divided into three regions. The bottom part, dominated by s-states, top part with primarily p-character, and the intermediate range with sp 3 hybrids, Compared to spectra from evaporated a-Si, the present XPS spectrum deviates in the range of s- and sp 3 states, where the implanted material shows a narrowed and shifted peak, and in the p region, which appears significantly broadened. This broadening in the p-region is also
The photoelectric properties of the ion implantation induced amorphous silicon are markedly different from the well-known previously published data of evaporated or cathode sputtered a-Si. It is concluded here that these differences provide evidence of an electronically modified material, the modification lying in the sp-hybrid structure. This in turn, we believe, reflects an amorphous system with severe local perturbation as compared with the tetrahedrally coordinated structure. It appears that such a metastable structure can be induced by implantation processes. These results are qualitatively the same as found earlier for implanted a-Ge [2]. Acknowledgements - - This work is performed within
the framework of a Scientific Exchange Programme between the Hungarian Academy of Science and The Royal Swedish Academy. The work has been supported by grants from the Swedish Natural Science Research Council and OTKA grant (2963).
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9.
10. 11.
D.E. Polk & D.S. Bodreaux, Phys. Rev. Lett. 31, 92 (1973). G. Petr, J. Kanski & U. Sodervall, Phys. Lett. 124, 510 (1987). J. Fortner & J.S. Lannin, Phys. Rev. B37, 10154 (1988). S. Coffa, F. Priolo, J.M. Poate & S.M. Glarum, Nucl. Instr and Methods B80/81, 603 (1993). S. Roorda, W.C. Sinke, J.M. Poate, D.C. Jackobson, S. Dierker, B.S. Dennis, D.J. Eaglesham, F. Spaepen & F. Fuoss, Phys. Rev. !i44, 702 (1991). J.E. Frederickson, C.N. Wadell, W.G. Spitze & G.H. Hubler, Appl. Phys. Lett. 40, 172 (1982). M. Fried, T. Lohner, W.A.M. Aamik, L.J. Hanekamp & A van Silfhout, J. Appl. Phys 71 5260 (1992). W.C. Sinke, T. Warabisako, M. Miyao, T. Toknyama, S. Roorda & F.W. Saris, J. NonCryst. Solids. 99, 308 (1988). J.M. Poate, S. Coffa, D.C. Jakobson, A. Ploman, J.A. Roth, G.L. Olson, S. Roorda, W. Sinke, J.S. Custer, M.O. Thompson, F. Spaepen, E. Donovan, Nuclear Instr. Methods B55 5339 (1991). Y. Hiroyama, T. Motooka, T. Tokuyama, L. Wei & S. Tanigawa, Nuclear Instr. and Methods na0/sl, 982 (1993). L.P. Avakyants & E.D. Obraztsova, J. Mol. Strut. 219, 141 (1990).
Vol. 96, No. 11 12. 13. 14. 15.
AMORPHOUS SILICON INDUCED BY P+ ION IMPLANTATION
S. Coffa, F. Priola & A. Battaglia, Phys. Rev. Lett. 70, 3756 (1993). G. Pet6 & J. Kanski, Solid State Commun. 38, 377 (1981). J.J. Yeh & I. Lindau, Atomic Data and Nuclear Data Tables 32, (1975). D.T. Pierce & W. Spicer, Phys. Rev. B5, 3017 (1972).
16. 17. 18. 19.
923
L. Ley, S. Kowalczyk, R. PoUak & D.A. Shirley, Phys. Rev. Lett. 29, 1088 (1973). B. Von Roedern, L. Ley & M. Cardona, Phys. Rev. Lett. 39, 1576 (1977). F.J. Himpsel & D.E. Eastman, J. Vac. Sci. Techn. 16, 1297 (1979). L.S.O. Johansson, R.I.G. Uhberg & G.V. Hansson, Surf. Sci. 1891190, 479 (1987).