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Relaxation of thermally induced defects in LPCVD amorphous silicon
Journal of Non-Crystalline Solids 227–230 Ž1998. 328–331
Relaxation of thermally induced defects in LPCVD amorphous silicon P. Agarwal b
a,)
, M. Kostana b, S.C. Agarwal a , S.M. Pietruszko
b
a Department of Physics, Indian Institute of Technology, Kanpur 208 016, India Institute of Microelectronics and Optoelectronics, Warsaw UniÕersity of Technology, IMiO PW, ul. Koszykowa 75, 00-662 Warsaw, Poland
Abstract Thermally induced metastability has been studied on P doped LPCVD a-Si Žlow pressure chemical vapour deposited amorphous silicon. films as a function of the concentration of hydrogen in these films. The hydrogen concentration has been varied over two orders of magnitude, from 0.06 at.% in as-deposited films to 15 at.% in heavily implanted films. We find that thermal metastability effects are present even at the lowest concentrations of hydrogen. These become larger as hydrogen concentration increases. Our studies indicate that hydrogen plays an important role in thermal equilibration of a-Si:H and supports the hydrogen glass model. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Metastability; Hydrogen content; Relaxation of defects; LPCVD amorphous silicon
1. Introduction Hydrogen plays a key role in determining the properties of hydrogenated amorphous silicon films. In glow discharge deposited films a-Si:H, it makes doping possible by passivating large number of dangling bond defects caused by bonding disorder. At the same time, various metastable phenomena, such as thermal w1,2x and light induced metastabilities w3x observed in a-Si:H films, have also been ascribed to the movement of hydrogen in these films. Branz and Iwaniczko w4x reported that amorphous Si films with
)
Corresponding author. Tel.: q48-22 660 7782; fax: q48-22 628 8740; e-mail: [email protected].
an extremely small Ž- 0.1 at.%. amount of hydrogen also exhibit similar thermal induced metastability. However, equilibration temperature, TE , above which the films are in thermal equilibrium, is higher for such films. They proposed that in a-Si:H films, the presence of hydrogen increases the network flexibility and therefore, the materials equilibrate at lower temperatures. In the glow discharge deposited a-Si:H films, it is not possible to vary hydrogen concentration over a large range and study its role on thermal equilibrium as well as relaxation phenomena. On the other hand, implantation of hydrogen in LPCVD a-Si films allow us to vary hydrogen content over a wide range w5x. In this paper, we report our studies on effect of rapid quenching and relaxation of metastable quenched state in LPCVD a-Si films as a function of hydrogen
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 7 4 - 4
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
329
concentration, C H . These films contain 2 at.% of phosphorus and C H is varied from ; 0.06 at.% in as-grown films to 15 at.% in the implanted films. This variation of C H over two orders of magnitude helps us examine the role of hydrogen in relaxation phenomenon.
2. Experimental LPCVD a-Si films Ž0.5-m m thick. are grown on oxidised crystalline silicon substrate at 5608C. Asgrown, undoped films contain ; 0.06 at.% hydrogen Žas confirmed by SIMS.. These films are phosphorus doped Ž; 2 at.%. by ion implantation Ždose of 5 = 10 16 cmy1 at 150 keV.. Films are further implanted with different doses of hydrogen in the range from 6.5 = 10 13 to 10 17 cmy1 Žcorresponding to 0.01 at.% to 15 at.%, respectively. and annealed at 4008C for 20 h in nitrogen atmosphere. The conductivity and isothermal relaxation measurements are done in coplanar geometry using aluminium electrodes 0.1 mm apart. The details of conductivity measurements and rapid cooling are described elsewhere w5x. For isothermal relaxation studies, the films are quenched from 650 K, at the rate of ; 200 Krs, and then heated to a fixed temperature to measure conductivity as a function of time. All the measurements are performed in nitrogen atmosphere at 1 Torr after evacuating the measurement system to 5 = 10y3 Torr.
Fig. 1. Ratio of room temperature conductivity, s Ž300 K., in the FQ and SC states as a function of hydrogen content in LPCVD a-Si films.
increase in activation energy is, however, very small for films with C H - 1 at.%. When these films are rapidly quenched ŽFQ. after annealing for 1 h at 650 K, s Ž300 K. increases with a decrease in EA1 . The conductivity vs. inverse temperature in FQ state meets the corresponding curve in SC state at temperature T E and for T ) T E , s ŽT . is nearly independent of the thermal history of the sample. These observations are similar to those observed for the regular a-Si:H films prepared by the plasma CVD method. Fig. 1 shows the ratio of s Ž300 K. in the FQ and SC states as a function of C H . We observe that these films change Žnearly 1.8 times. in s Ž300 K. even for a C H f 0.06 at.%, which increases to 2 at.%. After that, the increase is rapid and becomes nearly
3. Results As reported earlier w5x, the room temperature conductivity s Ž300 K. of undoped as grown a-Si films is f 10y7 Vy1 cmy1 , and increases to f 10y2 Vy1 cmy1 after phosphorus implantation ŽP dose of 1 = 10 17 cmy2 .. Conductivity, s , in the slowly cooled ŽSC. state follows Arrhenius behaviour. The log vs. 1rT curve is characterised by two activation energies; EA1 in the low temperature regime, T - 450 K and EA2 in the high temperature regime, T ) 450 K. As hydrogen concentration increases, s Ž300 K. for these films decreases and is 2 = 10y4 Vy1 cmy1 for C H s 15 at.%, with an increase in activation energy EA1 from 0.21 eV to 0.33 eV w5x. This
Fig. 2. Difference, D EA1 , between activation energy in the SC and FQ states as a function of hydrogen content.
330
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
Fig. 3. Thermal equilibration temperature, TE , in the FQ state as a function of hydrogen content.
three times for C H f 5 at.%. A decrease in this ratio is observed when C H further increases to ; 15 at.%. A similar trend is observed when the difference between activation energy, D EA1 , in SC and FQ state is plotted as a function of C H ŽFig. 2.. The difference is negligible Ž; 15 meV. for C H - 0.5 at.%, but with C H between 1 and 5 at.%, it is 30 meV. However, a further increase in C H seems to decrease this difference. Fig. 3 shows the equilibration temperature, T E , for these films. It is evident that T E s 2708C and is nearly independent of C H up to ; 2 at.% and then decreases. A lowest value of TE s 2208C is observed for C H s 10 at.%. An increase in T E is observed at C H s 15 at.%.
Fig. 5. Relaxation times, t , from the FQ state as a function of inverse temperature in the films with different hydrogen content.
The FQ state is metastable and approaches the SC state for T - T E . The isothermal relaxation studies show that the normalised conductivity, Y Ž t .: Y Ž t . s s Ž t . y s Ž `. 4 r s Ž 0. y s Ž `. 4
Ž 1.
follows a stretched exponential: Y Ž t . s exp y Ž trt .
b
4
Ž 2.
where t and b are the relaxation parameters and s Ž0. and s Ž`. are the conductivities values in FQ state at t s 0 and in SC state, respectively. In Fig. 4, relaxation time t is plotted as a function of C H for different annealing temperatures. We observe that t is nearly independent of C H for the smaller concentrations. It decreases as C H increases above 1 at.%. Further, for all C H , t is smaller for higher temperatures, decreasing by nearly two orders of magnitude as the annealing temperature increase from 410 K to 490 K. The activation energy for relaxation is f 1.0 eV, independent of C H . ŽFig. 5..
4. Discussion
Fig. 4. Relaxation times, t , from the FQ state as a function of hydrogen content in the film at different annealing temperatures.
An increase in s Ž300 K. by nearly five orders of magnitude after implanting LPCVD a-Si films with phosphorus suggests efficient doping in these films. However, a decrease in s Ž300 K. and increase in activation energy with increasing hydrogen concentration has been attributed to the dopant passivation by hydrogen w5x. To discriminate between the effects of doping and radiation damage caused by implanta-
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
tion, a set of samples was implanted with the same doses of Si atoms as P atoms and annealed in the same manner. It was found that the temperature dependence of such samples after annealing were similar to those characteristic of an undoped film. From Fig. 1, we see that fast quenching ŽFQ. increases s Ž300 K. even when C H is as small as 0.1 at.%. The equilibration temperature, TE , for films with the smaller hydrogen contents is nearly 2708C, which is higher than that observed for glow discharge deposited a-Si:H films, but is less than 3558C, as observed by Branz and Iwaniczko w4x for sputtered amorphous silicon films implanted with phosphorus. Also, the change in s Ž300 K. after FQ is comparable to the changes observed in case of glow discharge deposited films and higher in our films when compared with Branz and Iwaniczko w4x, who observed ; 15% increase in s Ž300 K. after fast quenching. When hydrogen concentration increases from 0.1 at.% to nearly 2 at.%, we observe that the ratio of s Ž300 K. in FQ and SC states as well as D EA1 increases, together with a decrease in TE from 2708C to 2608C. These observations are consistent with the hydrogen glass model w1x. However, the decrease in T E is small Ž; 108C. considering the more than one order of magnitude increase in hydrogen concentration. Branz and Iwaniczko w4x analysed their results for hydrogen diffusion and metastability in phosphorus doped sputtered a-Si films and proposed that at TE , diffusion coefficient of hydrogen, D H , is inversely proportional to the hydrogen concentration, C H . Therefore, our observations suggest a decrease in D H , as hydrogen concentration increases. The dangling bonds facilitate hydrogen movement. When hydrogen concentration is increased from 2 at.% to 10 at.%, we observe an increase in the ratio of s Ž300 K. and D EA1 . TE also decreases to 2208C in these films. The relaxation of the thermally induced metastable state in LPCVD a-Si films is described by a stretched exponential. This relaxation is similar to that in the plasma CVD deposited a-Si:H films. We observe in Fig. 5 that irrespective of the concentration of hydro-
331
gen, t is thermally activated with activation energy Et s 1.0 " 0.05 eV. This value of Et is in agreement with that for plasma CVD films and gives credence to the hydrogen glass model. Further, t is nearly independent of C H to 2 at.%, and then decreases.
5. Conclusions We found that thermal equilibration effects can be observed in LPCVD a-Si films, even when hydrogen is present in small amounts Ž; 0.06 at.%.. The higher hydrogen concentration decreases the equilibration temperature and larger quenching effects are seen together with a shift in the Fermi level. We observed that the thermally quenched state relaxes following a stretched exponential at a fixed temperature and the activation energy for relaxation is 1.0 " 0.05 eV, which is the same as that observed for plasma CVD films, and is in favour of motion of hydrogen in these films. The observed decrease in t with an increase in C H also gives credence to the hydrogen glass model.
Acknowledgements One of us ŽPA. is grateful to Department of Science and Technology, New Delhi, India and Committee for Scientific Research ŽKBN., Warsaw, Poland for providing funds for the visit to IMiO PW under Polish-Indo Programme on Scientific Cooperation.
References w1x R.A. Street, J. Kakalios, C.C. Tsai, T.M. Hayes, Phys. Rev. B 35 Ž1987. 1316. w2x P. Agarwal, S.K. Tripathi, S. Kumar, S.C. Agarwal, J. NonCryst. Solids 201 Ž1996. 163. w3x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 Ž1977. 292. w4x H.M. Branz, E. Iwaniczko, Phys. Rev. B 48 Ž1993. 17114. w5x S.M. Pietruszko, M. Pachocki, J. Jang, J. Non-Cryst. Solids 198–200 Ž1996. 73.
Relaxation of thermally induced defects in LPCVD amorphous silicon P. Agarwal b
a,)
, M. Kostana b, S.C. Agarwal a , S.M. Pietruszko
b
a Department of Physics, Indian Institute of Technology, Kanpur 208 016, India Institute of Microelectronics and Optoelectronics, Warsaw UniÕersity of Technology, IMiO PW, ul. Koszykowa 75, 00-662 Warsaw, Poland
Abstract Thermally induced metastability has been studied on P doped LPCVD a-Si Žlow pressure chemical vapour deposited amorphous silicon. films as a function of the concentration of hydrogen in these films. The hydrogen concentration has been varied over two orders of magnitude, from 0.06 at.% in as-deposited films to 15 at.% in heavily implanted films. We find that thermal metastability effects are present even at the lowest concentrations of hydrogen. These become larger as hydrogen concentration increases. Our studies indicate that hydrogen plays an important role in thermal equilibration of a-Si:H and supports the hydrogen glass model. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Metastability; Hydrogen content; Relaxation of defects; LPCVD amorphous silicon
1. Introduction Hydrogen plays a key role in determining the properties of hydrogenated amorphous silicon films. In glow discharge deposited films a-Si:H, it makes doping possible by passivating large number of dangling bond defects caused by bonding disorder. At the same time, various metastable phenomena, such as thermal w1,2x and light induced metastabilities w3x observed in a-Si:H films, have also been ascribed to the movement of hydrogen in these films. Branz and Iwaniczko w4x reported that amorphous Si films with
)
Corresponding author. Tel.: q48-22 660 7782; fax: q48-22 628 8740; e-mail: [email protected].
an extremely small Ž- 0.1 at.%. amount of hydrogen also exhibit similar thermal induced metastability. However, equilibration temperature, TE , above which the films are in thermal equilibrium, is higher for such films. They proposed that in a-Si:H films, the presence of hydrogen increases the network flexibility and therefore, the materials equilibrate at lower temperatures. In the glow discharge deposited a-Si:H films, it is not possible to vary hydrogen concentration over a large range and study its role on thermal equilibrium as well as relaxation phenomena. On the other hand, implantation of hydrogen in LPCVD a-Si films allow us to vary hydrogen content over a wide range w5x. In this paper, we report our studies on effect of rapid quenching and relaxation of metastable quenched state in LPCVD a-Si films as a function of hydrogen
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 7 4 - 4
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
329
concentration, C H . These films contain 2 at.% of phosphorus and C H is varied from ; 0.06 at.% in as-grown films to 15 at.% in the implanted films. This variation of C H over two orders of magnitude helps us examine the role of hydrogen in relaxation phenomenon.
2. Experimental LPCVD a-Si films Ž0.5-m m thick. are grown on oxidised crystalline silicon substrate at 5608C. Asgrown, undoped films contain ; 0.06 at.% hydrogen Žas confirmed by SIMS.. These films are phosphorus doped Ž; 2 at.%. by ion implantation Ždose of 5 = 10 16 cmy1 at 150 keV.. Films are further implanted with different doses of hydrogen in the range from 6.5 = 10 13 to 10 17 cmy1 Žcorresponding to 0.01 at.% to 15 at.%, respectively. and annealed at 4008C for 20 h in nitrogen atmosphere. The conductivity and isothermal relaxation measurements are done in coplanar geometry using aluminium electrodes 0.1 mm apart. The details of conductivity measurements and rapid cooling are described elsewhere w5x. For isothermal relaxation studies, the films are quenched from 650 K, at the rate of ; 200 Krs, and then heated to a fixed temperature to measure conductivity as a function of time. All the measurements are performed in nitrogen atmosphere at 1 Torr after evacuating the measurement system to 5 = 10y3 Torr.
Fig. 1. Ratio of room temperature conductivity, s Ž300 K., in the FQ and SC states as a function of hydrogen content in LPCVD a-Si films.
increase in activation energy is, however, very small for films with C H - 1 at.%. When these films are rapidly quenched ŽFQ. after annealing for 1 h at 650 K, s Ž300 K. increases with a decrease in EA1 . The conductivity vs. inverse temperature in FQ state meets the corresponding curve in SC state at temperature T E and for T ) T E , s ŽT . is nearly independent of the thermal history of the sample. These observations are similar to those observed for the regular a-Si:H films prepared by the plasma CVD method. Fig. 1 shows the ratio of s Ž300 K. in the FQ and SC states as a function of C H . We observe that these films change Žnearly 1.8 times. in s Ž300 K. even for a C H f 0.06 at.%, which increases to 2 at.%. After that, the increase is rapid and becomes nearly
3. Results As reported earlier w5x, the room temperature conductivity s Ž300 K. of undoped as grown a-Si films is f 10y7 Vy1 cmy1 , and increases to f 10y2 Vy1 cmy1 after phosphorus implantation ŽP dose of 1 = 10 17 cmy2 .. Conductivity, s , in the slowly cooled ŽSC. state follows Arrhenius behaviour. The log vs. 1rT curve is characterised by two activation energies; EA1 in the low temperature regime, T - 450 K and EA2 in the high temperature regime, T ) 450 K. As hydrogen concentration increases, s Ž300 K. for these films decreases and is 2 = 10y4 Vy1 cmy1 for C H s 15 at.%, with an increase in activation energy EA1 from 0.21 eV to 0.33 eV w5x. This
Fig. 2. Difference, D EA1 , between activation energy in the SC and FQ states as a function of hydrogen content.
330
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
Fig. 3. Thermal equilibration temperature, TE , in the FQ state as a function of hydrogen content.
three times for C H f 5 at.%. A decrease in this ratio is observed when C H further increases to ; 15 at.%. A similar trend is observed when the difference between activation energy, D EA1 , in SC and FQ state is plotted as a function of C H ŽFig. 2.. The difference is negligible Ž; 15 meV. for C H - 0.5 at.%, but with C H between 1 and 5 at.%, it is 30 meV. However, a further increase in C H seems to decrease this difference. Fig. 3 shows the equilibration temperature, T E , for these films. It is evident that T E s 2708C and is nearly independent of C H up to ; 2 at.% and then decreases. A lowest value of TE s 2208C is observed for C H s 10 at.%. An increase in T E is observed at C H s 15 at.%.
Fig. 5. Relaxation times, t , from the FQ state as a function of inverse temperature in the films with different hydrogen content.
The FQ state is metastable and approaches the SC state for T - T E . The isothermal relaxation studies show that the normalised conductivity, Y Ž t .: Y Ž t . s s Ž t . y s Ž `. 4 r s Ž 0. y s Ž `. 4
Ž 1.
follows a stretched exponential: Y Ž t . s exp y Ž trt .
b
4
Ž 2.
where t and b are the relaxation parameters and s Ž0. and s Ž`. are the conductivities values in FQ state at t s 0 and in SC state, respectively. In Fig. 4, relaxation time t is plotted as a function of C H for different annealing temperatures. We observe that t is nearly independent of C H for the smaller concentrations. It decreases as C H increases above 1 at.%. Further, for all C H , t is smaller for higher temperatures, decreasing by nearly two orders of magnitude as the annealing temperature increase from 410 K to 490 K. The activation energy for relaxation is f 1.0 eV, independent of C H . ŽFig. 5..
4. Discussion
Fig. 4. Relaxation times, t , from the FQ state as a function of hydrogen content in the film at different annealing temperatures.
An increase in s Ž300 K. by nearly five orders of magnitude after implanting LPCVD a-Si films with phosphorus suggests efficient doping in these films. However, a decrease in s Ž300 K. and increase in activation energy with increasing hydrogen concentration has been attributed to the dopant passivation by hydrogen w5x. To discriminate between the effects of doping and radiation damage caused by implanta-
P. Agarwal et al.r Journal of Non-Crystalline Solids 227–230 (1998) 328–331
tion, a set of samples was implanted with the same doses of Si atoms as P atoms and annealed in the same manner. It was found that the temperature dependence of such samples after annealing were similar to those characteristic of an undoped film. From Fig. 1, we see that fast quenching ŽFQ. increases s Ž300 K. even when C H is as small as 0.1 at.%. The equilibration temperature, TE , for films with the smaller hydrogen contents is nearly 2708C, which is higher than that observed for glow discharge deposited a-Si:H films, but is less than 3558C, as observed by Branz and Iwaniczko w4x for sputtered amorphous silicon films implanted with phosphorus. Also, the change in s Ž300 K. after FQ is comparable to the changes observed in case of glow discharge deposited films and higher in our films when compared with Branz and Iwaniczko w4x, who observed ; 15% increase in s Ž300 K. after fast quenching. When hydrogen concentration increases from 0.1 at.% to nearly 2 at.%, we observe that the ratio of s Ž300 K. in FQ and SC states as well as D EA1 increases, together with a decrease in TE from 2708C to 2608C. These observations are consistent with the hydrogen glass model w1x. However, the decrease in T E is small Ž; 108C. considering the more than one order of magnitude increase in hydrogen concentration. Branz and Iwaniczko w4x analysed their results for hydrogen diffusion and metastability in phosphorus doped sputtered a-Si films and proposed that at TE , diffusion coefficient of hydrogen, D H , is inversely proportional to the hydrogen concentration, C H . Therefore, our observations suggest a decrease in D H , as hydrogen concentration increases. The dangling bonds facilitate hydrogen movement. When hydrogen concentration is increased from 2 at.% to 10 at.%, we observe an increase in the ratio of s Ž300 K. and D EA1 . TE also decreases to 2208C in these films. The relaxation of the thermally induced metastable state in LPCVD a-Si films is described by a stretched exponential. This relaxation is similar to that in the plasma CVD deposited a-Si:H films. We observe in Fig. 5 that irrespective of the concentration of hydro-
331
gen, t is thermally activated with activation energy Et s 1.0 " 0.05 eV. This value of Et is in agreement with that for plasma CVD films and gives credence to the hydrogen glass model. Further, t is nearly independent of C H to 2 at.%, and then decreases.
5. Conclusions We found that thermal equilibration effects can be observed in LPCVD a-Si films, even when hydrogen is present in small amounts Ž; 0.06 at.%.. The higher hydrogen concentration decreases the equilibration temperature and larger quenching effects are seen together with a shift in the Fermi level. We observed that the thermally quenched state relaxes following a stretched exponential at a fixed temperature and the activation energy for relaxation is 1.0 " 0.05 eV, which is the same as that observed for plasma CVD films, and is in favour of motion of hydrogen in these films. The observed decrease in t with an increase in C H also gives credence to the hydrogen glass model.
Acknowledgements One of us ŽPA. is grateful to Department of Science and Technology, New Delhi, India and Committee for Scientific Research ŽKBN., Warsaw, Poland for providing funds for the visit to IMiO PW under Polish-Indo Programme on Scientific Cooperation.
References w1x R.A. Street, J. Kakalios, C.C. Tsai, T.M. Hayes, Phys. Rev. B 35 Ž1987. 1316. w2x P. Agarwal, S.K. Tripathi, S. Kumar, S.C. Agarwal, J. NonCryst. Solids 201 Ž1996. 163. w3x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 Ž1977. 292. w4x H.M. Branz, E. Iwaniczko, Phys. Rev. B 48 Ž1993. 17114. w5x S.M. Pietruszko, M. Pachocki, J. Jang, J. Non-Cryst. Solids 198–200 Ž1996. 73.