- Email: [email protected]
Hydrogenated amorphous and polycrystalline silicon TFTs by hot-wire CVD
Journal of Non-Crystalline Solids 227–230 Ž1998. 1202–1206
Hydrogenated amorphous and polycrystalline silicon TFTs by hot-wire CVD H. Meiling ) , A.M. Brockhoff, J.K. Rath, R.E.I. Schropp Debye Institute, Utrecht UniÕersity, P.O. Box 80.000, NL-3508 TA Utrecht, The Netherlands
Abstract We studied the incorporation of hydrogenated amorphous silicon and polycrystalline silicon in thin-film transistors. These semiconductor layers are deposited with hot-wire chemical vapour deposition. The saturation mobility of the amorphous silicon transistors is 0.62 cm2rV s and the activation energy of the OFF-current and ON-current 0.77 eV and 0.12 eV, respectively. The polycrystalline silicon transistor shows ‘amorphous’ behaviour, with properties similar to the amorphous silicon transistor, except for the activation energy of the OFF-current Ž0.45 eV. which is only slightly less than that of bulk polycrystalline silicon. We conclude that the initial stage of the polycrystalline silicon growth is actually amorphous, which was confirmed by Raman spectroscopy. This amorphous incubation layer completely determines the ON-state transistor properties. We show that with hot-wire deposition, amorphous silicon thin-film transistors can be made under different deposition conditions. These transistors are stable upon gate bias stress, unlike conventional plasma-deposited amorphous silicon transistors. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Raman spectroscopy; Amorphous silicon; Polycrystalline silicon; Transistors; Gate biass stress; Hot-wire CVD
1. Introduction Amorphous-silicon thin-film transistors, TFTs, suffer from metastability upon prolonged gate bias stress. A shift of the onset of current conduction—the threshold voltage—is observed when a prolonged positive gate voltage is applied. This effect is reversible, and similar to the light-induced degradation of hydrogenated amorphous silicon, a-Si:H. There is evidence that upon illumination of bulk amorphous silicon, structural changes occur prior to an increase in the defect density, as was pointed out recently by Fritzsche w1x. The role of hydrogen atoms in the )
Corresponding author. Fax: q31-30 254 3165; e-mail: [email protected]
metastable behaviour is yet to be understood, but it is observed that decreasing the hydrogen content, or replacing a-Si:H by microcrystalline or polycrystalline material, reduces the metastability of thin film silicon devices. One deposition technique that combines the two desired effects is hot-wire chemical vapour deposition, HWCVD. In this technique, a silicon containing gas is dissociated catalytically at a hot filament, usually tungsten w2x. The process pressure controls secondary gas phase reactions, through which the growth precursors are formed. By adjusting the process parameters, including the deposition temperature and the gas composition, one can readily obtain hydrogenated amorphous silicon, a-Si:H, or hydrogenated polycrystalline silicon, poly-Si:H. The depo-
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 2 9 8 - 1
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
sition temperature mainly influences the hydrogen content, whereas dilution of silane with hydrogen affects the etching of weak Si–Si bonds. Only recently, a-Si:H films deposited with HWCVD were incorporated in solar cells w2x and in thin-film transistors w3–5x. Also HWCVD poly-Si:H films were recently incorporated in cells and TFTs w6,7x. An additional advantage of the HWCVD technique is the deposition rate that can be achieved. For device-quality a-Si:H a rate of 2.0 nmrs and for poly-Si:H a rate of 0.5 nmrs are obtained, both substantially larger than with the conventional deposition techniques. We report on the application of hot-wire-deposited a-Si:H and poly-Si:H films in TFTs. We present the transfer characteristics of the TFTs before and after prolonged gate bias stress and show that a shift of the threshold voltage is negligible. We also show the transfer characteristics at elevated temperatures and determine the gate-voltage-dependent activation energy of the source-drain current for both types of TFT.
2. Experimental We optimized material properties by analyzing bulk a-Si:H and poly-Si:H films. These films were deposited simultaneously on glass and on crystalline silicon wafers in our multichamber ultra-high-vacuum deposition system, PASTA w8x. One of PASTA’s process chambers is equipped with both an RF electrode assembly and a hot-wire assembly w7x, to accommodate for both plasma-enhanced chemical vapour deposition, PECVD, and HWCVD of material in the same process chamber. This chamber also allows for specific plasma andror atomic hydrogen treatments prior to the hot-wire deposition of a-Si:H and poly-Si:H films. The a-Si:H films are deposited from silane at a process temperature of 4308C, a filament temperature of 18508C, and a process pressure of 20 m bar. The poly-Si:H films are deposited using 10% silane in hydrogen at a pressure of 100 m bar. The pre-deposition temperature of the poly-Si:H films is 4308C. Under the present process conditions the poly-Si:H films do not require a post-hydrogenation step to passivate grain boundary defects.
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The thin-film transistors are fabricated in the same process chamber as described above. As the substrate we used a 0.010–0.018 V cm n-type ²100: crystalline silicon wafer that was thermally oxidized. Since the 200-nm thick oxide is considered to be trap-free, we assume that the observed effects as presented in this paper can entirely be ascribed to phenomena in the semiconductor material. The TFTs are of the inverted–staggered type with an intrinsic layer of 250 nm. The source and drain aluminium contacts are thermally evaporated to a thickness of approximately 50 nm. Prior to the source and drain metallization, heavily phosphorus-doped contact layers are deposited on top of the a-Si:H and poly-Si:H layers. For the a-Si:H TFT we use a 50-nm nqa-Si:H contact layer, for the poly-Si:H TFTs we use a 50 nm nq microcrystalline silicon contact layer, to reduce the misalignment of the electronic band structure between the poly-Si:H and the contact layer. Both doped layers are deposited in a 13.56-MHz glow discharge. Conventional back-channel-etch photolithography and etch procedures are used to create transistors with a channel length L of 100 m m and a channel width W of 500 m m. The TFT properties were measured in the dark at room temperature under ambient conditions using a semiconductor parameter analyzer ŽHP4152A.. In a different setup, we measured the temperature-dependent current–voltage characteristics. The gate-voltage-dependent activation energy EaŽ Vg . of the source-drain current then is determined from the Arrhenius plots.
3. Results Details on the material properties of both a-Si:H and poly-Si:H films can be found in Ref. w7x. Here we will only briefly summarize the main material properties. 3.1. a-Si:H films The amorphous-silicon films are deposited at a rate between 1.7 and 2.1 nmrs, with no detectable 5Si–H 2 bonding configuration in the material. The hydrogen content amounts to 8 at.%, as determined from Fourier transform infrared spectroscopy. The
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H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
minority carrier diffusion length from the steady state photocarrier grating technique, SSPG, is 190 nm. From reflection and transmission experiments with monochromatic light the Tauc optical bandgap is determined to be 1.7 eV. The photoresponse amounts to 1.5 = 10 5 and the density-of-states Žas determined from the thermally stimulated conductivity. is 6 = 10 15 cmy3. 3.2. Poly-Si:H films The poly-Si:H films were deposited at a rate of 0.5 nmrs. The minority carrier diffusion length in the poly-Si:H films as used in the TFT is 538 nm. Other electronic properties reflect the intrinsic nature of the poly-Si:H films: the Tauc optical band gap is 1.1 eV and the dark conductivity activation energy amounts to 0.54 eV. The Hall electron mobility is 4 cm2rV s. From X-ray diffraction measurements, the crystal orientation is determined to be in the ²220: direction only. Atomic force microscopy, AFM, revealed that full coalescence of crystals occurs and that the average crystal size is about 70 nm: no amorphous phase is detected between the crystals. We determined the crystalline volume fraction from Raman scattering to be 95%. The hydrogen content is less than 0.5 at.%. We conclude that the grain boundaries are almost fully reconstructed and that the fraction of non-passivated grain boundary defects is the minor fraction of the material, resulting in the high diffusion length for minority carriers w6x. 3.3. TFTs In Fig. 1 we present the linear and saturation transfer characteristics of the a-Si:H and poly-Si:H TFTs. The circles represent the a-Si:H TFT and the triangles the poly-Si:H TFT. The closed symbols are for the room-temperature properties of the TFTs in the annealed state, the open symbols represent the properties after 1.5 h of continuous q25 V gate bias stress at room temperature. From the linear transfer characteristics in Fig. 1a we observe that the a-Si:H and poly-Si:H TFT behave similarly. The ON-current of the a-Si:H TFT is a factor of two larger Ž5 = 10y7 A vs. 2.5 = 10y7 A. and the OFF-current is somewhat less than for the poly-Si:H TFT. The saturation mobility ms , as calculated from the slope
Fig. 1. Room-temperature linear Ža. and saturation Žb. transfer characteristics of hot-wire-deposited a-Si:H and poly-Si:H TFTs before and after q25 V gate bias stress for 1.5 h.
of the linear part of the saturation transfer characteristics of Fig. 1b, amounts to 0.62 " 0.05 cm2rV s for the a-Si:H TFT and is 0.44 " 0.05 cm2rV s for the poly-Si:H TFT. The threshold voltage Vt is nearly identical, at 6.5 " 0.5 V for the a-Si:H TFT and at 7.0 " 0.5 V for the poly-Si:H TFT. After gate bias stress essentially no shift of Vt or deterioration of ms is observed for both types of TFT: actually, ms of the a-Si:H TFT slightly increases to 0.74 " 0.05 cm2rV s. To investigate the electrical properties of the channel material in more detail we determined the activation energy of the source-drain current Is at each applied gate voltage Vg , by measuring the temperature dependence of the transfer characteristics. Specifically the gate voltages for which the TFT is in the OFF-state Ž Vg < Vt . and in the ON-state, where Vg 4 Vt , are of interest. As an example—the corresponding plots for the a-Si:H TFT show a similar trend—we plot in Fig. 2 the temperature-dependent transfer characteristics for the poly-Si:H TFT, after the TFT was annealed to eliminate residual effects of the bias voltage stress experiments. The transfer characteristics are measured from room temperature up to 1008C. We limited the temperature range in order to prevent thermal stress of the TFTs. At Vg s q20 V ŽON-current. and at Vg s y4 V ŽOFFcurrent. we determined the activation energy Ea of the source-drain current Is . In Fig. 3, the Arrhenius plots are presented for both TFTs at these two gate voltages, and the resulting curves are labeled with
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
1205
a-Si:H TFT has an Ea of 0.77 " 0.02 eV, the Is of the poly-Si:H TFT has an Ea of 0.45 " 0.02 eV.
4. Discussion
Fig. 2. Temperature-dependent transfer characteristics of the hotwire-deposited poly-Si:H TFT. Each curve is labeled by its measurement temperature.
the calculated EaŽ Vg .. The closed symbols Žv . represent the a-Si:H TFT, the open symbols Ž`. the poly-Si:H TFT. In the ON-state we found for both TFTs a nearly identical activation energy of 0.12 " 0.02 eV and 0.14 " 0.02 eV, respectively. In the OFF-state they differ substantially: the Is of the
Fig. 3. Gate-voltage-dependent activation energy Ea for the hotwire-deposited a-Si:H and poly-Si:H TFT. Straight lines are linear fits through the data.
From the linear and saturation transfer characteristics it can be seen that the a-Si:H and poly-Si:H TFT behave similarly, and that they show characteristics that are typical for a-Si:H TFTs. Only the activation energy of the OFF-current is different for the two types of TFT. The observed effects can be explained, if one considers the conduction mechanisms that determine the ON-current and OFF-current. The ON-current in these field-effect devices is determined by the conduction mechanism of the channel material at the SiO 2ra-Si:H interface. The channel typically extends only F 20 nm into the semiconductor. The OFF-current of the TFTs is determined by the properties of the entire semiconductor layer w9x. Therefore, the activation energy of the OFF-current should reflect the nature of the bulk semiconductor material, whereas the activation energy of the ON-current reflects the properties of the channel material. The Ea of the OFF-current for both TFTs confirm the intrinsic nature of both the a-Si:H and poly-Si:H films that are incorporated in the TFTs: the values of 0.77 eV and 0.44 eV are only slightly less than what is found for the bulk of hot-wire deposited a-Si:H and poly-Si:H w7x. From the nearly identical values of Ea of the ON-current, we conclude that the channel material of both TFTs is mostly amorphous. For the growth of poly-Si:H films under the present conditions, apparently the nucleation density on SiO 2 is small, and nucleation only occurs after several tens of nanometers of growth. This effect was recently confirmed by Raman spectroscopy measurements that were performed on poly-Si:H films deposited on glass, where the primary laser beam was incident through the glass. We observed an amorphous phase of the poly-Si:H film at the glass substrate. Also, high-resolution TEM pictures show an initial amorphous phase when these types of poly-Si:H films are deposited on glass w6x. Further research is in progress to investigate possible differences of poly-Si:H de-
1206
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
posited on glass vs. deposited on thermally grown SiO 2 . Especially the nucleation density on these oxides will be the subject of further study.
5. Conclusions State of the art a-Si:H TFTs have been made with hot-wire CVD. These TFTs do not show any shift of the threshold voltage upon prolonged gate bias stress. Also, ‘poly-Si:H’ TFTs have been made with hotwire CVD. The characteristics of the poly-Si:H TFT are similar to those of the a-Si:H TFT. Although the bulk of these layers is indeed polycrystalline silicon, the interface with the oxide insulator is mostly amorphous, as was determined from the activation energy of the OFF-current. We conclude that under the present deposition conditions, the nucleation density on SiO 2 is too low to initiate instantaneous growth of polycrystalline silicon. Also, this poly-Si:H TFT with the amorphous channel did not show any threshold voltage shift upon gate bias stress.
Acknowledgements The research of Dr. Meiling has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences ŽKNAW.. A.M. Brockhoff acknowledges the support of the Founda-
tion for Fundamental Research on Matter ŽFOM.. Thanks are due to K.F. Feenstra for the extensive process development of the hot-wire a-Si:H films, to E.C. Molenbroek for valuable discussions on HWCVD, and to C.H.M. van der Werf for the deposition of all the films and devices used in this study.
References w1x H. Fritzsche, in: S. Wagner, M. Hack, E.A. Schiff, R. Schropp, I. Shimizu ŽEds.., Amorphous and Microcrystalline Silicon Technology—1997, Vol. 467, Materials Research Society, Pittsburgh, 1997, p. 19. w2x A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 Ž1991. 6728. w3x H. Meiling, R.E.I. Schropp, Appl. Phys. Lett. 69 Ž1996. 1062. w4x H. Meiling, R.E.I. Schropp, Appl. Phys. Lett. 70 Ž1997. 2681. w5x V. Chu, J. Jarego, H. Silva, T. Silva, M. Reissner, P. Brogueira, J.P. Conde, Appl. Phys. Lett. 70 Ž1997. 2714. w6x J.K. Rath, A.J.M.M. van Zutphen, H. Meiling, R.E.I. Schropp, in: S. Wagner, M. Hack, E.A. Schiff, R. Schropp, I. Shimizu ŽEds.., Amorphous and Microcrystalline Silicon Technology —1997, Vol. 467, Materials Research Society, Pittsburgh, 1997, p. 445. w7x R.E.I. Schropp, K.F. Feenstra, E.C. Molenbroek, H. Meiling, J.K. Rath, Philos. Mag. B 76 Ž1997. 309. w8x A. Madan, P. Rava, R.E.I. Schropp, B. Von Roedern, Appl. Surf. Sci. 70–71 Ž1993. 716. w9x H.C. Slade, M.S. Shur, S.C. Deane, M. Hack, in: M. Hack, E.A. Schiff, S. Wagner, R. Schropp, A. Matsuda ŽEds.., Amorphous Silicon Technology—1996, Vol. 420, Materials Research Society, Pittsburgh, 1996, p. 257.
Hydrogenated amorphous and polycrystalline silicon TFTs by hot-wire CVD H. Meiling ) , A.M. Brockhoff, J.K. Rath, R.E.I. Schropp Debye Institute, Utrecht UniÕersity, P.O. Box 80.000, NL-3508 TA Utrecht, The Netherlands
Abstract We studied the incorporation of hydrogenated amorphous silicon and polycrystalline silicon in thin-film transistors. These semiconductor layers are deposited with hot-wire chemical vapour deposition. The saturation mobility of the amorphous silicon transistors is 0.62 cm2rV s and the activation energy of the OFF-current and ON-current 0.77 eV and 0.12 eV, respectively. The polycrystalline silicon transistor shows ‘amorphous’ behaviour, with properties similar to the amorphous silicon transistor, except for the activation energy of the OFF-current Ž0.45 eV. which is only slightly less than that of bulk polycrystalline silicon. We conclude that the initial stage of the polycrystalline silicon growth is actually amorphous, which was confirmed by Raman spectroscopy. This amorphous incubation layer completely determines the ON-state transistor properties. We show that with hot-wire deposition, amorphous silicon thin-film transistors can be made under different deposition conditions. These transistors are stable upon gate bias stress, unlike conventional plasma-deposited amorphous silicon transistors. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Raman spectroscopy; Amorphous silicon; Polycrystalline silicon; Transistors; Gate biass stress; Hot-wire CVD
1. Introduction Amorphous-silicon thin-film transistors, TFTs, suffer from metastability upon prolonged gate bias stress. A shift of the onset of current conduction—the threshold voltage—is observed when a prolonged positive gate voltage is applied. This effect is reversible, and similar to the light-induced degradation of hydrogenated amorphous silicon, a-Si:H. There is evidence that upon illumination of bulk amorphous silicon, structural changes occur prior to an increase in the defect density, as was pointed out recently by Fritzsche w1x. The role of hydrogen atoms in the )
Corresponding author. Fax: q31-30 254 3165; e-mail: [email protected]
metastable behaviour is yet to be understood, but it is observed that decreasing the hydrogen content, or replacing a-Si:H by microcrystalline or polycrystalline material, reduces the metastability of thin film silicon devices. One deposition technique that combines the two desired effects is hot-wire chemical vapour deposition, HWCVD. In this technique, a silicon containing gas is dissociated catalytically at a hot filament, usually tungsten w2x. The process pressure controls secondary gas phase reactions, through which the growth precursors are formed. By adjusting the process parameters, including the deposition temperature and the gas composition, one can readily obtain hydrogenated amorphous silicon, a-Si:H, or hydrogenated polycrystalline silicon, poly-Si:H. The depo-
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 2 9 8 - 1
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
sition temperature mainly influences the hydrogen content, whereas dilution of silane with hydrogen affects the etching of weak Si–Si bonds. Only recently, a-Si:H films deposited with HWCVD were incorporated in solar cells w2x and in thin-film transistors w3–5x. Also HWCVD poly-Si:H films were recently incorporated in cells and TFTs w6,7x. An additional advantage of the HWCVD technique is the deposition rate that can be achieved. For device-quality a-Si:H a rate of 2.0 nmrs and for poly-Si:H a rate of 0.5 nmrs are obtained, both substantially larger than with the conventional deposition techniques. We report on the application of hot-wire-deposited a-Si:H and poly-Si:H films in TFTs. We present the transfer characteristics of the TFTs before and after prolonged gate bias stress and show that a shift of the threshold voltage is negligible. We also show the transfer characteristics at elevated temperatures and determine the gate-voltage-dependent activation energy of the source-drain current for both types of TFT.
2. Experimental We optimized material properties by analyzing bulk a-Si:H and poly-Si:H films. These films were deposited simultaneously on glass and on crystalline silicon wafers in our multichamber ultra-high-vacuum deposition system, PASTA w8x. One of PASTA’s process chambers is equipped with both an RF electrode assembly and a hot-wire assembly w7x, to accommodate for both plasma-enhanced chemical vapour deposition, PECVD, and HWCVD of material in the same process chamber. This chamber also allows for specific plasma andror atomic hydrogen treatments prior to the hot-wire deposition of a-Si:H and poly-Si:H films. The a-Si:H films are deposited from silane at a process temperature of 4308C, a filament temperature of 18508C, and a process pressure of 20 m bar. The poly-Si:H films are deposited using 10% silane in hydrogen at a pressure of 100 m bar. The pre-deposition temperature of the poly-Si:H films is 4308C. Under the present process conditions the poly-Si:H films do not require a post-hydrogenation step to passivate grain boundary defects.
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The thin-film transistors are fabricated in the same process chamber as described above. As the substrate we used a 0.010–0.018 V cm n-type ²100: crystalline silicon wafer that was thermally oxidized. Since the 200-nm thick oxide is considered to be trap-free, we assume that the observed effects as presented in this paper can entirely be ascribed to phenomena in the semiconductor material. The TFTs are of the inverted–staggered type with an intrinsic layer of 250 nm. The source and drain aluminium contacts are thermally evaporated to a thickness of approximately 50 nm. Prior to the source and drain metallization, heavily phosphorus-doped contact layers are deposited on top of the a-Si:H and poly-Si:H layers. For the a-Si:H TFT we use a 50-nm nqa-Si:H contact layer, for the poly-Si:H TFTs we use a 50 nm nq microcrystalline silicon contact layer, to reduce the misalignment of the electronic band structure between the poly-Si:H and the contact layer. Both doped layers are deposited in a 13.56-MHz glow discharge. Conventional back-channel-etch photolithography and etch procedures are used to create transistors with a channel length L of 100 m m and a channel width W of 500 m m. The TFT properties were measured in the dark at room temperature under ambient conditions using a semiconductor parameter analyzer ŽHP4152A.. In a different setup, we measured the temperature-dependent current–voltage characteristics. The gate-voltage-dependent activation energy EaŽ Vg . of the source-drain current then is determined from the Arrhenius plots.
3. Results Details on the material properties of both a-Si:H and poly-Si:H films can be found in Ref. w7x. Here we will only briefly summarize the main material properties. 3.1. a-Si:H films The amorphous-silicon films are deposited at a rate between 1.7 and 2.1 nmrs, with no detectable 5Si–H 2 bonding configuration in the material. The hydrogen content amounts to 8 at.%, as determined from Fourier transform infrared spectroscopy. The
1204
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
minority carrier diffusion length from the steady state photocarrier grating technique, SSPG, is 190 nm. From reflection and transmission experiments with monochromatic light the Tauc optical bandgap is determined to be 1.7 eV. The photoresponse amounts to 1.5 = 10 5 and the density-of-states Žas determined from the thermally stimulated conductivity. is 6 = 10 15 cmy3. 3.2. Poly-Si:H films The poly-Si:H films were deposited at a rate of 0.5 nmrs. The minority carrier diffusion length in the poly-Si:H films as used in the TFT is 538 nm. Other electronic properties reflect the intrinsic nature of the poly-Si:H films: the Tauc optical band gap is 1.1 eV and the dark conductivity activation energy amounts to 0.54 eV. The Hall electron mobility is 4 cm2rV s. From X-ray diffraction measurements, the crystal orientation is determined to be in the ²220: direction only. Atomic force microscopy, AFM, revealed that full coalescence of crystals occurs and that the average crystal size is about 70 nm: no amorphous phase is detected between the crystals. We determined the crystalline volume fraction from Raman scattering to be 95%. The hydrogen content is less than 0.5 at.%. We conclude that the grain boundaries are almost fully reconstructed and that the fraction of non-passivated grain boundary defects is the minor fraction of the material, resulting in the high diffusion length for minority carriers w6x. 3.3. TFTs In Fig. 1 we present the linear and saturation transfer characteristics of the a-Si:H and poly-Si:H TFTs. The circles represent the a-Si:H TFT and the triangles the poly-Si:H TFT. The closed symbols are for the room-temperature properties of the TFTs in the annealed state, the open symbols represent the properties after 1.5 h of continuous q25 V gate bias stress at room temperature. From the linear transfer characteristics in Fig. 1a we observe that the a-Si:H and poly-Si:H TFT behave similarly. The ON-current of the a-Si:H TFT is a factor of two larger Ž5 = 10y7 A vs. 2.5 = 10y7 A. and the OFF-current is somewhat less than for the poly-Si:H TFT. The saturation mobility ms , as calculated from the slope
Fig. 1. Room-temperature linear Ža. and saturation Žb. transfer characteristics of hot-wire-deposited a-Si:H and poly-Si:H TFTs before and after q25 V gate bias stress for 1.5 h.
of the linear part of the saturation transfer characteristics of Fig. 1b, amounts to 0.62 " 0.05 cm2rV s for the a-Si:H TFT and is 0.44 " 0.05 cm2rV s for the poly-Si:H TFT. The threshold voltage Vt is nearly identical, at 6.5 " 0.5 V for the a-Si:H TFT and at 7.0 " 0.5 V for the poly-Si:H TFT. After gate bias stress essentially no shift of Vt or deterioration of ms is observed for both types of TFT: actually, ms of the a-Si:H TFT slightly increases to 0.74 " 0.05 cm2rV s. To investigate the electrical properties of the channel material in more detail we determined the activation energy of the source-drain current Is at each applied gate voltage Vg , by measuring the temperature dependence of the transfer characteristics. Specifically the gate voltages for which the TFT is in the OFF-state Ž Vg < Vt . and in the ON-state, where Vg 4 Vt , are of interest. As an example—the corresponding plots for the a-Si:H TFT show a similar trend—we plot in Fig. 2 the temperature-dependent transfer characteristics for the poly-Si:H TFT, after the TFT was annealed to eliminate residual effects of the bias voltage stress experiments. The transfer characteristics are measured from room temperature up to 1008C. We limited the temperature range in order to prevent thermal stress of the TFTs. At Vg s q20 V ŽON-current. and at Vg s y4 V ŽOFFcurrent. we determined the activation energy Ea of the source-drain current Is . In Fig. 3, the Arrhenius plots are presented for both TFTs at these two gate voltages, and the resulting curves are labeled with
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
1205
a-Si:H TFT has an Ea of 0.77 " 0.02 eV, the Is of the poly-Si:H TFT has an Ea of 0.45 " 0.02 eV.
4. Discussion
Fig. 2. Temperature-dependent transfer characteristics of the hotwire-deposited poly-Si:H TFT. Each curve is labeled by its measurement temperature.
the calculated EaŽ Vg .. The closed symbols Žv . represent the a-Si:H TFT, the open symbols Ž`. the poly-Si:H TFT. In the ON-state we found for both TFTs a nearly identical activation energy of 0.12 " 0.02 eV and 0.14 " 0.02 eV, respectively. In the OFF-state they differ substantially: the Is of the
Fig. 3. Gate-voltage-dependent activation energy Ea for the hotwire-deposited a-Si:H and poly-Si:H TFT. Straight lines are linear fits through the data.
From the linear and saturation transfer characteristics it can be seen that the a-Si:H and poly-Si:H TFT behave similarly, and that they show characteristics that are typical for a-Si:H TFTs. Only the activation energy of the OFF-current is different for the two types of TFT. The observed effects can be explained, if one considers the conduction mechanisms that determine the ON-current and OFF-current. The ON-current in these field-effect devices is determined by the conduction mechanism of the channel material at the SiO 2ra-Si:H interface. The channel typically extends only F 20 nm into the semiconductor. The OFF-current of the TFTs is determined by the properties of the entire semiconductor layer w9x. Therefore, the activation energy of the OFF-current should reflect the nature of the bulk semiconductor material, whereas the activation energy of the ON-current reflects the properties of the channel material. The Ea of the OFF-current for both TFTs confirm the intrinsic nature of both the a-Si:H and poly-Si:H films that are incorporated in the TFTs: the values of 0.77 eV and 0.44 eV are only slightly less than what is found for the bulk of hot-wire deposited a-Si:H and poly-Si:H w7x. From the nearly identical values of Ea of the ON-current, we conclude that the channel material of both TFTs is mostly amorphous. For the growth of poly-Si:H films under the present conditions, apparently the nucleation density on SiO 2 is small, and nucleation only occurs after several tens of nanometers of growth. This effect was recently confirmed by Raman spectroscopy measurements that were performed on poly-Si:H films deposited on glass, where the primary laser beam was incident through the glass. We observed an amorphous phase of the poly-Si:H film at the glass substrate. Also, high-resolution TEM pictures show an initial amorphous phase when these types of poly-Si:H films are deposited on glass w6x. Further research is in progress to investigate possible differences of poly-Si:H de-
1206
H. Meiling et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1202–1206
posited on glass vs. deposited on thermally grown SiO 2 . Especially the nucleation density on these oxides will be the subject of further study.
5. Conclusions State of the art a-Si:H TFTs have been made with hot-wire CVD. These TFTs do not show any shift of the threshold voltage upon prolonged gate bias stress. Also, ‘poly-Si:H’ TFTs have been made with hotwire CVD. The characteristics of the poly-Si:H TFT are similar to those of the a-Si:H TFT. Although the bulk of these layers is indeed polycrystalline silicon, the interface with the oxide insulator is mostly amorphous, as was determined from the activation energy of the OFF-current. We conclude that under the present deposition conditions, the nucleation density on SiO 2 is too low to initiate instantaneous growth of polycrystalline silicon. Also, this poly-Si:H TFT with the amorphous channel did not show any threshold voltage shift upon gate bias stress.
Acknowledgements The research of Dr. Meiling has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences ŽKNAW.. A.M. Brockhoff acknowledges the support of the Founda-
tion for Fundamental Research on Matter ŽFOM.. Thanks are due to K.F. Feenstra for the extensive process development of the hot-wire a-Si:H films, to E.C. Molenbroek for valuable discussions on HWCVD, and to C.H.M. van der Werf for the deposition of all the films and devices used in this study.
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