Silicon-based thin-film transistors with a high stability

General project discription

Hydrogenated amorphous silicon (a-Si:H) is a semiconductor material that is used in many applications such as thin-film transistors (TFTs) and solar cells. Our group is interested in both the fundamental aspects and the growth of this material as well as in the application of a-Si:H in devices.
Amorphous silicon, commonly deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD) is a metastable material. Silicon dangling-bond defects are created, e.g. in TFTs after prolonged application of a gate voltage or in solar cells after prolonged illumination. In TFTs defect-state creation near the insulator-semiconductor interface leads to a shift of the threshold voltage. Annealing at temperatures above 150^oC recovers the initial state.

We study inverted-staggered TFTs incorporating different kinds of silicon layers deposited either on thermally grown silicon oxide, on PECVD silicon nitride, or on Hot-Wire Chemical Vapor Deposition (HWCVD) deposited silicon nitride. As substrate we use both heavily doped c-Si wafers and glass substrates. Photolithography and etching of the deposited layer stacks is performed at the the University of Twente (MESA Institute). We showed that TFTs incorporating a-Si:H deposited by the HWCVD technique have state-of-the-art properties and are highly stable upon gate-voltage stress. This can be seen in a reduced threshold-voltage shift after prolonged gate-voltage stress compared to PECVD a-Si:H TFTs. Together with the high deposition rate > 1 nm/s as achieved with HWCVD this makes hot-wire TFTs promising for industrial applications. Our aim is to investigate the fundamental metastable mechanism in such devices. We study the influence of the silicon microstructure and hydrogen-bonding on metastability, particularly in the region near the semiconductor-insulator interface.

High lights & latest news

Top-gate TFTs
Top gate TFTs incorporating low temperature poly-Si deposited by HWCVD reached a field-effect mobility of 4.7 cm2/Vs. The work has been done in cooporation with the Princeton University (US). (article)

Hot-Wire silicon nitride
Silicon nitride (a-SiNx) deposited by HWCVD at substrate temperatures of 300-400^oC has been recently developed in our group. We demonstrated the application as gate-dielectric in bottom-gate thin-film transistors (TFT), yielding a field-effect mobility of 0.3 cm2/Vs in a TFT with both a-Si:H and a-SiNx deposited by HWCVD. (article)
Hot-wire silicon nitride is, furthermore, an attractive material to be applied as passivation layer for compound-semiconductor devices and other semiconductor devices (solar cells), as no surface damaging ion bombardement is present during deposition.

State-of-the-art VHF-PECVD and Hot-Wire CVD a-Si:H TFTs
In 1997-1999 we presented HWCVD a-Si:H and het-Si:H TFTs with a high field-effect mobility and an superior stability upon gate bias stress, as compared to PECVD a-Si:H TFTs (Meiling et al., Appl. Phys. Lett 70, p.2681, 1997, Stannowski et al. Appl. Phys. Lett 75, p.3674, 1999).
We now studied a series of TFTs with identical device structure (inverted staggered, W/L = 230), PECVD a-SiNx gate dielectrics, and a-Si:H films deposited with different techniques. Very-High Frequency (VHF) PECVD (13.56 - 70 MHz) and Hot-Wire CVD were used. All TFTs had state-of-the-art characteristics with mobilities of 0.6-0.7 cm2/Vs. In a comparative study we quantified the stability of these TFTs under gate-bias stress. (article)

What determines the stability of PECVD and HWCVD a-Si:H TFTs?
We showed that the stability of PECVD a-Si:H TFTs is correlated with the intrinsic stress in the silicon film. Layers with high compressive stress exhibit a poor stability. Thus, the TFT stability inceases with decreasing compressive stress. This finding suggests that the stability of a-Si:H is correlated with the Si network structure, namely, a high density of short (compressed) Si-Si bonds results in a high defect (dangling bond) creation rate.
However, the results for our HWCVD a-Si:H TFTs deposited at substrate temperatures above 350C do not agree with this concept: Higher substrate temperatures generally increase both the TFT stability and the compressive stress. Thus, another mechanism must be responsible for the superior stabily of hot-wire a-Si:H, which is not yet resolved.

Who is involved?

Experimental techniques

Plasma-Enhance Chemical Vapor Deposition (PECVD) for a-Si:H and a-SiN_x:H in the ASTER
Hot-Wire Chemical Vapor Deposition (HWCVD) for a-Si:H, het-Si, and a-SiN_x:H in the PASTA
Current-Voltage (I/V) and Capacitance-Voltage (C/V) Measurements
Fourier-Transform Infrared Spectroscopy (FTIR)

Publications

B. Stannowski, H. Meiling, A.M. Brockhoff, and R.E.I. Schropp, Thin-film transistors based on hot-wire amorphous silicon on silicon nitride, Materials Research Society Spring Meeting, San Francisco, Apr 5-9, 1999, Symp. Proc. 557 pp. 659-664. (pdf file)
B. Stannowski, R.E.I. Schropp, and A. Nascetti, High energy-barrier for defect creation in thin-film transistors based on hot-wire amorphous silicon, Appl. Phys. Lett. 75 (1999) 3674.
B. Stannowski, A.M. Brockhoff, A. Nascetti, and R.E.I. Schropp, Metastability of hot-wire silicon thin-film transistors, ICAMS 18 (Intern. Conf. on Amorphous and Microcrystalline Semiconductors), Snowbird, Utah, Aug 22-27, 1999, J. Non.-Cryst. Solids 266-269 (2000) 464-468.(pdf-file)
R.E.I. Schropp, B. Stannowski, and J.K. Rath, Optimization of Interfaces in thin-film transistors and thin-film solar cells using hot-wire chemical vapor deposition, 9th Intern. Conf. on Production Engineering, Aug 30 - Sep 1, 1999, Osaka, JSPE (Japan Society for Precision Engineering) Publication Series No. 3 (1999) 521.
B. Stannowski, A.M. Brockhoff, and R.E.I. Schrpp,Stability study of thin-film transistors made by hot-wire chemical vapor deposition, SAFE99 (STW workshop: Semiconductor Advances for Future Electronics, Mierlo, Nov 24-25, 1999. (pdf-file)
R.E.I. Schropp, J.K. Rath, B. Stannowski, C.H.M. van Werf, Y. Chen, S. Wagner, Low temperature poly-Si layers deposited by hot-wire CVD yielding a mobility of 4.0 cm^2/Vs in top gate thin film transistors, Materials Research Society Spring Meeting, San Francisco, Apr 24-28, 2000, Symp. Proc. 609. (pdf file)
R.E.I. Schropp, B. Stannowski, A.M. Brockhoff, P.A.T.T. van Veenendaal, and J.K. Rath, Hot Wire CVD polycrystalline silicon semiconducting thin films for application in thin film transitors and solar cells, Materials Physics and Mechanics 1 (2), 73-82 (2000).
B. Stannowski, C.H.M. van der Werf, and R.E.I. Schropp, Hot-Wire Chemical-Vapour Deposition for low temperature deposition of silicon-nitride layers, Proc. of the 3rd Intern. Conf. on Coatings on Glass, Oct 29 - Nov 2, 2000, Maastricht, 387-394 (2000). (pdf file)
B. Stannowski, J.K. Rath, and R.E.I. Schropp,Hot-Wire Silicon Nitride for Thin-Film Transistors, SAFE2000 (STW-workshop: Semiconductor Advances for Future Electronics, Veldhoven, Nov 29 - Dec 1, 2000. (pdf file)
B. Stannowski, and R.E.I. Schropp, Hot-wire amorphous silicon thin-film transistors on glass, Thin Solid Films 383, 125-128 (2001). (pdf file)
B. Stannowski, M.K. van Veen, R.E.I. Schropp, Towards an All-Hot-Wire TFT: Silicon Nitride and amorphous Silicon deposited by Hot-Wire Chemical Vapor Deposition, Materials Research Society Spring Meeting, San Francisco, Apr 16-20, 2001, Symp. Proc. 664. (pdf file)
B. Stannowski, J.K. Rath, and R.E.I. Schropp, Hot-Wire Silicon Nitride for Thin-Film Transistors, Thin Solid Films 395, 338-341 (2001).(pdf file)
J.K. Rath, B. Stannowski, P.A.T.T. van Veenendaal, M.K. van Veen and R.E.I. Schropp, Application of hot-wire chemical vapor-deposited Si:H films in thin film transistors and solar cells, Thin Solid Films 395, 320-329 (2001).(pdf file)
R.E.I. Schropp, B. Stannowski, J.K. Rath, New challanges in thin film transistors research, ICAMS 19 (19th Intern. Conf. on Amorphous and Microcrystalline Semiconductors Science and Technology), Nice, France 27-31 Aug 2001, J. Non.-Cryst. Solids 299-302, 1304 (2002).(pdf file)
B. Stannowski, R.E.I. Schropp, R.B. Wehrspohn, M.J. Powell, Amorphous-silicon thin-film transistors deposited by VHF PECVD and Hot-wire CVD, ICAMS 19 (Intern. Conf. on Amorphous and Microcrystalline Semiconductors Science and Technology), Nice, France 27-31 Aug 2001, J. Non.-Cryst. Solids 299-302, 1340 (2002).(pdf file)
B. Stannowski and R.E.I. Schropp,Performance of VHF-PECVD and hot-wire CVD amorphous silicon thin-film transistors: A comparative study, SAFE2001 (STW workshop: Semiconductor Advances for Future Electronics, Veldhoven, Nov 28-29, 2001.(pdf file)

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Last update: Wednesday, 14-Aug-2002 14:54:46 CEST

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