1.1 The general background and motivation
Thin-film transistors (TFT), which use a thin semiconductor film on an insulating substrate as the active channel, was first demonstrated in 1961 by Dr. P. K.
Weimer. However the first TFT used CdSe as the active device channel, not now commonly used amorphous Si(a-Si). Until 1979 the first amorphous Si TFT was demonstrated by P. G. le Comber , W. E. Spear. With its simplicity in structure and fabrication, applications of thin-film transistors in image sensors and displays become more and more popular. But for now the great demand information and desire high resolution displays has stimulated higher interest in poly-Si TFT. This is due to a much higher mobility and drive current of poly-Si TFTs, compared to amorphous Si counterparts, which enables the integration of peripheral circuits on the same panel in active matrix liquid crystal displays (AMLCD) manufacturing [1][2].
Generally, AMLCDs used a-Si TFTs as a switching element to control the gray level in liquid crystal display(LCD) [5] In AMLCDs, TFTs play as switching device to turn ON/OFF the current path for charging/discharging the liquid crystal capacitor.
However, a-Si TFTs has poor effect field mobility (0.5-1 cm2/V-s) and higher turned on voltage. In order to improve the TFTs performance, the poly-Si TFT was developed in 1980 by S.W. Depp and A. Juliana. The undoped poly-Si was deposited by low-pressure CVD at 625 OC, and the films were approximately 0.75 m thick.[3]
The major advantage of polycrystalline silicon poly-Si TFT technology is its suitability for multifunctional active-matrix displays, because it enables the integration of driver electronics, sensors, memories, and peripheral circuits on the glass substrate to produce system-on-glass SOG displays. But the temperature in the
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process is too high to use on glass. Therefore in 1991 T.W. Little and K. Takahara developed low-temperature (T≦600 oC) polycrystalline silicon thin film transistor (poly-Si TFTs) which was fabricated by solid phase crystallization (SPC) of amorphous silicon(a-Si).[4]
Over the years, amorphous, polycrystalline, and recently, nano/microcrystalline forms of silicon have gained prominence as low temperature alternatives to crystalline silicon for large-area applications. Amorphous silicon is the current material for most of the thin film transistors used in liquid crystal displays, and a host of other applications. It is a versatile material for limited mobility applications, and can be reliably grown at very low temperatures, but suffers from bias stress and light induced degradation. Polycrystalline silicon on the other hand has much higher mobilities, and hence suitable for high-speed CMOS applications. But it requires processing at much higher temperatures, which is out of scope of inexpensive plastic substrates. Although methods of converting amorphous silicon to polycrystalline silicon exist by laser induced crystallization, it suffers from problems of device uniformity besides being expensive.
For this reason we have been pursuing thin-film transistor technology based on nano/microcrystalline silicon, nc-Si:H/μc-Si:H as an inexpensive alternative. This semiconductor can provide sufficient electron mobility [5–7].Moreover, it can be fabricated at low temperatures which are compatible with the plastic substrates [7].
The table 1. summarizes the low-temperature silicon processes prevalent at present.
Although amorphous silicon-based TFTs are currently used in addressing the pixels of AMLCD. The material is deposited at low temperature compatible with the use of plastics. However, the low field effect mobility in these TFTs limits the panel size.[8]
Then, TFTs with higher mobility are needed. Nano/Microcrystalline silicon (nc-Si) is now the main candidate to reach this goal.It can be deposited by different techniques
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at low temperature compatible with the use of plastics substrates. In this thesis, a-Si:H TFTs and nc-Si:H/μc-Si:H TFTs were demonstrated under low temperature(T≦200
oC) by HWCVD and HDPCVD.
Table 1. Status of silicon materials for TFTs [10]
1.2 TFT Structures
The thin-film transistor usually refers to MOSFET as opposed to other kinds of transistors. The structure is similar to MOSFET. It can be roughly classified into top-gate and bottom-gate types depending upon the placement of the gate dielectric relative to the channel material. In bottom-gate (inverted) devices, the gate dielectric is below the active layer, while in top-gate devices the gate dielectric layer is above the active layer similar to conventional MOSFETS. These can be further classified into coplanar and staggered types depending upon the location of the source and drain contacts relative to the gate. In coplanar TFTs, the source and drain contacts are on the same side of the active region as the gate contact, whereas in staggered structures, the source and drain contacts are on the opposite side of the active region as compared to the gate contact. These device geometries are schematically described in the Fig. 1.
The inverted staggered in currently industry is most commonly used structure for a-Si:H TFT. One of the most important reasons for this is that silicon nitride forms an excellent gate dielectric with amorphous silicon which is currently the material of choice owing to low cost and low temperature fabrication. In this thesis a top-gate
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structure was used to take advantage of the highly crystalline top surface of the film.[11-14].
Fig. 1 Schematic of commonly used TFT structures
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