Stability of hydrogenated nanocrystalline silicon thin-film transistors (original) (raw)
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Stability of hydrogenated amorphous silicon thin film transistors on polyimide substrates
Solid-State Electronics, 2005
The inverted staggered hydrogenated amorphous silicon thin film transistors (a-Si:H TFT) were fabricated on 6 lm thick polyimide substrate at temperatures below 300°C. The a-Si:H TFT off current is below 10 À12 A, the on/off current ratio is 107,thethresholdvoltageis10 7 , the threshold voltage is 107,thethresholdvoltageis2-3 V, field effect mobility is $ 0.5 cm 2 /Vs, and the subthreshold slope is $0.4 V/decade. The stabilities of a-Si:H TFTs were studied and the device parameters determined before and after a bias stress of V GS = 20 V for t = 10, 10 2 , 10 3 and 10 4 s. The threshold voltages shifted to higher values and on/off ratio decreased with the duration of bias stress. The device characteristics were measured in the dark and under the light illumination. Threshold voltages and on/off current ratio both decreased. Temperature dependant measurements of transfer and output characteristic of a-Si:H TFT in the range from 22°C to 125°C were also investigated. When temperature is increased threshold voltages decreased and the field effect mobility increased.
Stability of hydrogenated polymorphous silicon thin-film transistors under DC electrical stress
IET Circuits, Devices & Systems, 2012
The authors fabricated bottom-gate (BG) back-channel etched (BCE) thin-film transistors with hydrogenated polymorphous silicon (pm-Si:H) as the channel material. This material is obtained using the same low-cost plasma-enhanced chemical vapour deposition (PECVD) techniques as amorphous silicon. The authors first show the improvement of the threshold voltage stability of pm-Si:H TFTs under bias stress compared to a-Si:H counterparts. Then, pm-Si:H TFTs degradation is investigated under different gate bias stress conditions. It has been found that the degradation mechanisms are dependent on the gate stress conditions involving state creation in the channel material and charge trapping at the channel/ gate SiN x interface.
Hydrogenated amorphous silicon technology for chemically sensitive thin-film transistors
Sensors and Actuators B-chemical, 1992
Top-gate hydrogenated amorphous silicon thin-film transistors have been fabricated which show electrical characteristics suitable for application in the field of chemical sensors. These devices have been specialized to two different types of sensors: (a) Pd-gate hydrogen sensors; (b) K+ ion sensors. The obtained results show that the present technology can be successfully applied to the fabrication of gas-sensitive and ion-sensitive field-effect transistors.
Hydrogenated nanocrystalline silicon thin film prepared by RF-PECVD at high pressure
Journal of Non-Crystalline Solids, 2010
Hydrogenated nanocrystalline silicon (nc-Si:H) has strong potential to replace the hydrogenated amorphous silicon (a-Si:H) in thin film transistors (TFTs) due to its compatibility with the current industrial a-Si:H processes, and its better threshold voltage stability [1]. In this paper, we present an experimental TFT array backplane for direct conversion X-ray detector, using inverted staggered bottom gate nc-Si:H TFT as switching element. The TFTs employed a nc-Si:H/a-Si:H bilayer as the channel layer and hydrogenated amorphous silicon nitride (a-SiN x) as the gate dielectric; both layers were deposited by plasma enhanced chemical vapor deposition (PECVD) at 280 o C. Each pixel consists of a switching TFT, a charge storage capacitor (C px), and a mushroom electrode which serves as the bottom contact for X-ray detector such as amorphous selenium photoconductor. The chemical composition of the a-SiN x was studied by Fourier transform infrared spectroscopy. Current-voltage measurements of the a-SiN x film demonstrate a breakdown field of 4.3 MV/cm. TFTs in the array exhibit a field effect mobility (μ EF) of 0.15 cm 2 /V•s, a threshold voltage (V Th) of 5.71 V, and a subthreshold leakage current (I sub) of 10-10 A. The fabrication sequence and TFT characteristics will be discussed in details.
Chemically sensitive hydrogenated amorphous silicon thin-film transistors
Journal of Non-Crystalline Solids, 1991
In this work the application of a-Si:H thin film transistor (TFT) technology in the field of chemical sensors is presented. In particular, two different types of devices have been fabricated: a) Pd-gate hydrogen sensors; b) K+-ion sensitive FET (ISFET). The obtained results show that a-Si:H TFT can be succesfully used as gassensitive and ion-sensitive sensors and can represent a promising technology in this area.
Analysis of bias stress on unpassivated hydrogenated amorphous silicon thin-film transistors
IEEE Transactions on Electron Devices, 1998
Both the subthreshold slope and the threshold voltage in inverted-staggered amorphous silicon thin-film transistors (a-Si:H TFT's) are vulnerable to metastable changes in the density of states (DOS) due to Fermi level displacement. In previous work, we have used passivated and unpassivated TFT's to distinguish between the effects of bulk states and interface states at the top passivating nitride interface [1], [2]. Here we report the results of experimental measurements and twodimensional (2-D) simulations on unpassivated TFT's. Since there are no top interface states, all the observed changes are due solely to the bulk DOS. The subthreshold current activation energies in a-Si:H TFT's are compared for n-channel nonpassivated TFT's before and after bias stress. The experimental results agree well with the 2-D simulations, confirming that the dependence of subthreshold current activation energy on gate bias reveals the distribution of the DOS in energy but cannot resolve the magnitude of features in the DOS. This type of analysis is not accurate for TFT's with a top passivating nitride, since the activation energies in such devices are affected by the interface states.
Journal of Display Technology, 2013
The bottom-gate nc-Si based thin-film-transistors (TFTs) grown by using the low-temperature plasma-enhanced chemical vapor deposition (LT-PECVD) system with He diluted SiH are demonstrated. With the RF plasma power increasing from 20 to 100 W, the crystalline volume ratio of the nc-Si inside the a-Si:H film significantly increases from 12.5% to 32%, and its deposition rate is also enhanced from 9.5 to 14.5 nm/min. The faster deposition at higher plasma greatly suppresses the residual oxygen content in nc-Si film to 4% or less, which reduces the flat-band shifted voltage of the MOS diode by 2 volts. The increased crystalline volume with suppressed oxide in nc-Si films contribute to the enhanced Hall mobility and conductivity. The nc:Si TFT decreases its threshold voltage from 3.3 V to 2.7 V, and enlarges its field mobility from 0.3 to 1.3 cm V s. The defect density in the nc-Si TFTs further decrease by one order of magnitude to 7.5 10 cm eV , which causes a shrinkage on the sub-threshold operation range to make easier the operation of the nc-Si TFTs entering into the above-threshold regime at lower voltage. The hydrogen-free He diluted SiH growth has shown its compatibility with the conventional recipe for the high-mobility nc-Si TFT fabrication.
Nanocrystalline Silicon Films Deposited by RF PECVD for Bottom-gate Thin-film Transistors
MRS Proceedings, 2006
Undoped nanocrystalline silicon (nc-Si) films were deposited by 13.56 MHz PECVD at 250 º C by varying RF power, reactor pressure, silane and hydrogen flow rates. To find the process window for device-quality nc-Si, we chose the Taguchi method to optimize the deposition process. The results show that silane and hydrogen flow rates are the dominant factors to film crystallinity. The results also indicate that for highly hydrogen diluted plasma, the reactor pressure does not have a significant effect on the film crystallinity. We have obtained nc-Si films with crystallinity ranging from 50% to 80% at film thicknesses ranging from 20nm to 100nm by optimizing the deposition conditions. A dark conductivity of 10-6 S/cm and conductivity activation energy of 0.46eV were measured. Tri-layer bottom-gate inverted-staggered TFTs were fabricated with different active layers comprising fully nc-Si and nc-Si / amorphous silicon (a-Si) bi-layer. The best TFT exhibits a field-effect mobility ~1 cm 2 /Vs, threshold voltage ~4V, and on/off current ratio ~10 5 .
2005
In this thesis, intrinsic hydrogenated nanocrystalline silicon thin films for solar cells application have been deposited by means of the hot – wire chemical vapour deposition (HWCVD) technique and have been characterised for their performance. It is noticed that hydrogenated nanocrystalline silicon is similar in some aspects (mainly optical) to its counterpart amorphous silicon actually used as the intrinsic layer in the photovoltaic industry. Substantial differences between the two materials have been found however in their respective structural and electronic properties. We show that hydrogenated nanocrystalline silicon retains good absorption coefficients known for amorphous silicon in the visible region. The order improvement and a reduced content of the bonded hydrogen in the films are linked to their good stability. We argue that provided a moderate hydrogen dilution ratio in the monosilane gas and efficient process pressure in the deposition chamber, intrinsic hydrogenated n...