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Photoinduced effects in RF and VHF a-Si:H films deposited with different ion bombardment
Thin Solid Films 383 Ž2001. 178᎐180
Photoinduced effects in RF and VHF a-Si:H films deposited with different ion bombardment A.S. Abramov a , A.I. Kosarev a,U , P. Roca i Cabarrocas b , M.V. Shutov a , A.J. Vinogradov a a
Department of Solid State Electronics, A.F. Ioffe PTI, St. Petersburg, 194021, Russia b LPICM, UMR 7647 CNRS, Ecole Polytechnique, Palaiseau Cedex, 91128, France
Abstract Hydrogenated amorphous silicon Ža-Si:H. films were deposited in a diode-type reactor in RF and VHF discharge under controlled ion bombardment. Defect concentration in the films was studied as a function of the energy of ions impinging on the growing film in as-grown ŽA., light-soaked ŽB., and annealed states ŽC.. Significant changes in defect concentration, and Fermi-level position with ion energy were observed in the samples in A-and C-states, while ion bombardment caused less changes in the properties in the B-state. The data obtained are discussed in terms of the generation of D 0 and Dy defect states, which are controlled by ion bombardment of the films during growth. 䊚 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon; Ion bombardment; Photo-induced effects
1. Introduction Ion bombardment during plasma deposition is an important factor, affecting the growth and electronic properties of a-Si:H films, and is still being debated. It is recognized that the main growth precursors are the radicals generated in the plasma due to electron᎐molecule collisions. Ions impinging on the growing film surface can have considerable kinetic energy Ž10᎐100 eV. because of acceleration in the electric field near the electrode sheath. Therefore, despite a much lower concentration of ions compared to neutrals, it is expected that ion bombardment plays a significant role. However, how the bombardment ‘works’ and how it is related to deposition variables, such as pressure, frequency, type of ions, flux, etc., is still unclear. The effect of ion bombardment on electronic properties has been reported w1᎐6x. In these studies, film
U
Corresponding author. Tel.: q7-812-328-5701; fax: q7-812-2471017. E-mail address: [email protected] ŽA.I. Kosarev..
properties were characterized in the as-grown state. To our knowledge only in w6x have properties in the lightsoaked state been characterized in films grown under intensive ion bombardment. The latter was achieved by special design of an asymmetric reactor, in which the grounded electrode, where the samples were clamped, was smaller in area than the cap-shaped powered electrode. However, the authors of this work mainly studied the correlation between electronic properties and the growth rate rather than ion bombardment, and ion characteristics were not studied. Previously w3,4x we reported on the effect of ion bombardment on electronic properties of a-Si:H films in the as-grown state for both RF and VHF discharge. In this paper we study the ion bombardment effect on photoinduced changes of electronic properties of a-Si:H films. 2. Experimental The a-Si:H films were deposited in a capacitive-type reactor with both RF and VHF discharge in pure silane. Ion parameters were measured by the retarding
0040-6090r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 2 9 - 1
A.S. Abramo¨ et al. r Thin Solid Films 383 (2001) 178᎐180
field energy-analyzer clamped on the grounded electrode near the substrates for the films. Mean ² Ei : and maximal Ei ion energy values were determined from the measured ion-energy distribution functions. These measurements were carried out under the conditions used during growth of the films. The discharge power and DC bias to the powered electrode were varied to change the parameters of ions impinging the surface of the growing films. Thus, we deposited a-Si:H samples under different and well-characterized ion bombardment. A more detailed description of the deposition system, conditions, and also data on ion characteristics, were reported in w3,4x. Dark conductivity, photoconductivity, density of states, and diffusion lengths were measured in three states: as grown ŽA., light-soaked ŽB. and annealed ŽC. states. Density of states ŽDOS. was measured by the constant photocurrent method ŽCPM.. To achieve the saturated light-soaked B-state, the films were illuminated by a xenon lamp with a water filter, providing light intensity of ; 1 W cmy2 , until the measured characteristics reached the saturated values. Annealing
179
Fig. 2. Photoinduced shift of Fermi level Ef ŽB. y Ef ŽA. in a-Si:H films versus maximal energy of ions Ž Ei . in Ža. RF Žsolid squares. and Žb. VHF Žopen squares.. Solid lines are given as guides for the eyes.
of the samples to achieve the C-state was performed at a temperature of 250⬚C for approximately 1 h under vacuum. 3. Results and discussion
Fig. 1. Defect concentration Ž Nd . as a function of the maximal energy of ions Ž Ei . in a-Si:H films deposited at a discharge frequency of Ža. f s 13.56 and Žb. f s 56 MHz in as-grown ŽA., light-soaked ŽB. and annealed ŽC. states. Solid lines are given as guides for the eyes.
Fig. 1 shows the defect concentration Nd measured by CPM as a function of the ion energy Ei in a-Si:H films deposited in RF and VHF plasma under different ion bombardment conditions. Nd was measured in A-, B-, and C-states. A defect peak was observed at 0.9᎐1 eV from the conductivity band edge. As can be seen, the RF-deposited samples in the B-state had defect concentrations independent of the ion energy, Nd s 1.1᎐1.4= 10 16 cmy3 . In the VHF samples Nd dependence on Ei was observed, with a maximum Nd at Ei s 22 eV. At higher ion energy, some decrease in Nd with Ei was observed. The VHF-deposited a-Si:H films with an ion energy in the range 22᎐26 eV had higher values of Nd ; 5 = 10 16 cmy3 than RF a-Si:H films. It is interesting to note that the largest relative Nd changes between A-, C- and B-states were found in RF and VHF a-Si:H samples deposited under ion bombardment with ion energy Ei f 30 eV. At this ion energy, both RF and VHF samples demonstrated a Nd minimum in A- and C-states. This Nd minimum can be attributed to removal of some structural defects during growth, e.g. one or two Si atoms located at longer or shorter distance than normal bonding. A further increase in Ei probably caused the creation of additional defects. The difference of the Fermi level in A- and B-states, Ef ŽB. y Ef ŽA., as a function of Ei is plotted in Fig. 2.
180
A.S. Abramo¨ et al. r Thin Solid Films 383 (2001) 178᎐180
shown in Fig. 3. This correlation is rather evident, while independence of Nd in the B-state from Ef suggests the creation of Dy states during light soaking, which were not detected by CPM. 4. Conclusions.
Fig. 3. Diagram of correlation between the defect concentration Nd and Fermi level position Ef in the a-Si:H films in A-, B- and C-states.
In both types of sample, Ef ŽB. y Ef ŽA. decreases with increasing Ei . In the VHF samples, this dependence starts with higher values of Ef ŽB. y Ef ŽA.. The Nd values determined from CPM data is related to occupied states, in our case they are the neutral danglingbond ŽD 0 . states. The position of the negatively charged defects ŽDy . is shifted up by positive correlation energy ŽUf 0.3 eV.. In the case of such defects, the Fermi level is controlled by the D 0rDy ratio. Hence, in the RF a-Si:H films ŽFig. 2a., the reducing difference of Fermi levels in A- and B-states, Ef ŽB. y Ef ŽA., with Ei can be attributed to reducing Dy defects in the lightsoaked state ŽB-state. because D 0 is not changed with Ei Žas can be seen in Fig. 2a.. On the other hand, in the VHF samples both D 0 and Dy states are changed with Ei ŽFig. 2b.. The defect concentration, Nd , in a-Si:H films, prepared under different ion bombardment in Aand C-states correlates with the Fermi-level position, as
We have studied photoinduced changes in the a-Si:H samples deposited under different ion bombardment in both RF and VHF discharges. The defect concentration depended on the energy of ions impinging the films during growth. A minimum defect concentration was found at an ion energy of Ei f 30 eV in A-and C-states, while smaller changes in defect concentration with ion energy were observed in light-soaked B-states. The difference between Fermi levels in A-and B-states reduced with ion energy. The samples studied demonstrated no correlation between defect concentration and Fermi level in B-states, in contrast to those for Aand C-states. The data obtained are interpreted in terms of the relationship between Dy and D 0 defects, the creation of which are controlled by ion bombardment during film growth. References w1x P. Roca i Cabarrocas, P. Morrin, V. Chu et al., J. Appl. Phys. 69 Ž1991. 2942. w2x M. Heitze, R. Zedlitz, J. Non-Cryst. Solids 198 Ž1996. 1038. w3x A.I. Kosarev, A.S. Smirnov, A.S. Abramov, A.J. Vinogradov, A.Y. Ustavschikov, M.V. Shutov, J. Vac. Sci. Technol. A 15 Ž1997. 298. w4x A.S. Abramov, A.Y. Vinogradov, A.I. Kosarev, M.V. Shutov, Techn. Phys. 43 Ž2. Ž1998. 180᎐187. w5x E.A.G. Hammers, W.G.J.H.M. van Sark, J. Bezemer, H. Meiling, W.F. van der Weg, Non-Cryst. Solids 226 Ž1998. 205᎐216. w6x S. Will, H. Mell, M. Poschenrieder, W. Fuhs, J. Non-Cryst. Solids 227᎐230 Ž1998. 29.
Photoinduced effects in RF and VHF a-Si:H films deposited with different ion bombardment A.S. Abramov a , A.I. Kosarev a,U , P. Roca i Cabarrocas b , M.V. Shutov a , A.J. Vinogradov a a
Department of Solid State Electronics, A.F. Ioffe PTI, St. Petersburg, 194021, Russia b LPICM, UMR 7647 CNRS, Ecole Polytechnique, Palaiseau Cedex, 91128, France
Abstract Hydrogenated amorphous silicon Ža-Si:H. films were deposited in a diode-type reactor in RF and VHF discharge under controlled ion bombardment. Defect concentration in the films was studied as a function of the energy of ions impinging on the growing film in as-grown ŽA., light-soaked ŽB., and annealed states ŽC.. Significant changes in defect concentration, and Fermi-level position with ion energy were observed in the samples in A-and C-states, while ion bombardment caused less changes in the properties in the B-state. The data obtained are discussed in terms of the generation of D 0 and Dy defect states, which are controlled by ion bombardment of the films during growth. 䊚 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon; Ion bombardment; Photo-induced effects
1. Introduction Ion bombardment during plasma deposition is an important factor, affecting the growth and electronic properties of a-Si:H films, and is still being debated. It is recognized that the main growth precursors are the radicals generated in the plasma due to electron᎐molecule collisions. Ions impinging on the growing film surface can have considerable kinetic energy Ž10᎐100 eV. because of acceleration in the electric field near the electrode sheath. Therefore, despite a much lower concentration of ions compared to neutrals, it is expected that ion bombardment plays a significant role. However, how the bombardment ‘works’ and how it is related to deposition variables, such as pressure, frequency, type of ions, flux, etc., is still unclear. The effect of ion bombardment on electronic properties has been reported w1᎐6x. In these studies, film
U
Corresponding author. Tel.: q7-812-328-5701; fax: q7-812-2471017. E-mail address: [email protected] ŽA.I. Kosarev..
properties were characterized in the as-grown state. To our knowledge only in w6x have properties in the lightsoaked state been characterized in films grown under intensive ion bombardment. The latter was achieved by special design of an asymmetric reactor, in which the grounded electrode, where the samples were clamped, was smaller in area than the cap-shaped powered electrode. However, the authors of this work mainly studied the correlation between electronic properties and the growth rate rather than ion bombardment, and ion characteristics were not studied. Previously w3,4x we reported on the effect of ion bombardment on electronic properties of a-Si:H films in the as-grown state for both RF and VHF discharge. In this paper we study the ion bombardment effect on photoinduced changes of electronic properties of a-Si:H films. 2. Experimental The a-Si:H films were deposited in a capacitive-type reactor with both RF and VHF discharge in pure silane. Ion parameters were measured by the retarding
0040-6090r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 2 9 - 1
A.S. Abramo¨ et al. r Thin Solid Films 383 (2001) 178᎐180
field energy-analyzer clamped on the grounded electrode near the substrates for the films. Mean ² Ei : and maximal Ei ion energy values were determined from the measured ion-energy distribution functions. These measurements were carried out under the conditions used during growth of the films. The discharge power and DC bias to the powered electrode were varied to change the parameters of ions impinging the surface of the growing films. Thus, we deposited a-Si:H samples under different and well-characterized ion bombardment. A more detailed description of the deposition system, conditions, and also data on ion characteristics, were reported in w3,4x. Dark conductivity, photoconductivity, density of states, and diffusion lengths were measured in three states: as grown ŽA., light-soaked ŽB. and annealed ŽC. states. Density of states ŽDOS. was measured by the constant photocurrent method ŽCPM.. To achieve the saturated light-soaked B-state, the films were illuminated by a xenon lamp with a water filter, providing light intensity of ; 1 W cmy2 , until the measured characteristics reached the saturated values. Annealing
179
Fig. 2. Photoinduced shift of Fermi level Ef ŽB. y Ef ŽA. in a-Si:H films versus maximal energy of ions Ž Ei . in Ža. RF Žsolid squares. and Žb. VHF Žopen squares.. Solid lines are given as guides for the eyes.
of the samples to achieve the C-state was performed at a temperature of 250⬚C for approximately 1 h under vacuum. 3. Results and discussion
Fig. 1. Defect concentration Ž Nd . as a function of the maximal energy of ions Ž Ei . in a-Si:H films deposited at a discharge frequency of Ža. f s 13.56 and Žb. f s 56 MHz in as-grown ŽA., light-soaked ŽB. and annealed ŽC. states. Solid lines are given as guides for the eyes.
Fig. 1 shows the defect concentration Nd measured by CPM as a function of the ion energy Ei in a-Si:H films deposited in RF and VHF plasma under different ion bombardment conditions. Nd was measured in A-, B-, and C-states. A defect peak was observed at 0.9᎐1 eV from the conductivity band edge. As can be seen, the RF-deposited samples in the B-state had defect concentrations independent of the ion energy, Nd s 1.1᎐1.4= 10 16 cmy3 . In the VHF samples Nd dependence on Ei was observed, with a maximum Nd at Ei s 22 eV. At higher ion energy, some decrease in Nd with Ei was observed. The VHF-deposited a-Si:H films with an ion energy in the range 22᎐26 eV had higher values of Nd ; 5 = 10 16 cmy3 than RF a-Si:H films. It is interesting to note that the largest relative Nd changes between A-, C- and B-states were found in RF and VHF a-Si:H samples deposited under ion bombardment with ion energy Ei f 30 eV. At this ion energy, both RF and VHF samples demonstrated a Nd minimum in A- and C-states. This Nd minimum can be attributed to removal of some structural defects during growth, e.g. one or two Si atoms located at longer or shorter distance than normal bonding. A further increase in Ei probably caused the creation of additional defects. The difference of the Fermi level in A- and B-states, Ef ŽB. y Ef ŽA., as a function of Ei is plotted in Fig. 2.
180
A.S. Abramo¨ et al. r Thin Solid Films 383 (2001) 178᎐180
shown in Fig. 3. This correlation is rather evident, while independence of Nd in the B-state from Ef suggests the creation of Dy states during light soaking, which were not detected by CPM. 4. Conclusions.
Fig. 3. Diagram of correlation between the defect concentration Nd and Fermi level position Ef in the a-Si:H films in A-, B- and C-states.
In both types of sample, Ef ŽB. y Ef ŽA. decreases with increasing Ei . In the VHF samples, this dependence starts with higher values of Ef ŽB. y Ef ŽA.. The Nd values determined from CPM data is related to occupied states, in our case they are the neutral danglingbond ŽD 0 . states. The position of the negatively charged defects ŽDy . is shifted up by positive correlation energy ŽUf 0.3 eV.. In the case of such defects, the Fermi level is controlled by the D 0rDy ratio. Hence, in the RF a-Si:H films ŽFig. 2a., the reducing difference of Fermi levels in A- and B-states, Ef ŽB. y Ef ŽA., with Ei can be attributed to reducing Dy defects in the lightsoaked state ŽB-state. because D 0 is not changed with Ei Žas can be seen in Fig. 2a.. On the other hand, in the VHF samples both D 0 and Dy states are changed with Ei ŽFig. 2b.. The defect concentration, Nd , in a-Si:H films, prepared under different ion bombardment in Aand C-states correlates with the Fermi-level position, as
We have studied photoinduced changes in the a-Si:H samples deposited under different ion bombardment in both RF and VHF discharges. The defect concentration depended on the energy of ions impinging the films during growth. A minimum defect concentration was found at an ion energy of Ei f 30 eV in A-and C-states, while smaller changes in defect concentration with ion energy were observed in light-soaked B-states. The difference between Fermi levels in A-and B-states reduced with ion energy. The samples studied demonstrated no correlation between defect concentration and Fermi level in B-states, in contrast to those for Aand C-states. The data obtained are interpreted in terms of the relationship between Dy and D 0 defects, the creation of which are controlled by ion bombardment during film growth. References w1x P. Roca i Cabarrocas, P. Morrin, V. Chu et al., J. Appl. Phys. 69 Ž1991. 2942. w2x M. Heitze, R. Zedlitz, J. Non-Cryst. Solids 198 Ž1996. 1038. w3x A.I. Kosarev, A.S. Smirnov, A.S. Abramov, A.J. Vinogradov, A.Y. Ustavschikov, M.V. Shutov, J. Vac. Sci. Technol. A 15 Ž1997. 298. w4x A.S. Abramov, A.Y. Vinogradov, A.I. Kosarev, M.V. Shutov, Techn. Phys. 43 Ž2. Ž1998. 180᎐187. w5x E.A.G. Hammers, W.G.J.H.M. van Sark, J. Bezemer, H. Meiling, W.F. van der Weg, Non-Cryst. Solids 226 Ž1998. 205᎐216. w6x S. Will, H. Mell, M. Poschenrieder, W. Fuhs, J. Non-Cryst. Solids 227᎐230 Ž1998. 29.