Material Analyses

The scanning electron microscopy (SEM) images of solid-phase crystallized (SPC) poly-Si films for the Argon-implanted and control samples after secco etching are shown in Figs. 2.2(a) and 2.2(b), respectively. The SEM images apparently reveal the difference in their grian size between the Argon-implanted and control samples. The average grain sizes of poly-Si for the Argon-implanted and control poly-Si samples are approximately 100 nm and 20 nm, respectively. The enhancement on the silicon grain size is descried as follows.

Because the silicon atoms are bounded to underlying thermal SiO2, the rearrangement and volume contraction of silicon atoms at the beginning of crystallization process would produce a large magnitude of tensile stress at the α-Si/SiO2 interface in the control poly-Si film [4.16]. Many silicon nucleation sites related crystalline defects including microtwins and stacking faults are introduced in order to relieve the tensile stress. Therefore, the grain size of poly-Si obtained from many silicon nucleation sites is rather small. In contrast, in the case of the Argon-implanted poly-Si film, because the interface-nucleation rate is almost suppressed, the stress generated from the surface nucleation is easily relieved from the top free surface. Consequently, the silicon nucleation sites associated with crystalline defects are reduced, resulting in larger silicon grain size and better grain crystallinity in the Argon-implanted poly-Si film.

The x-ray diffraction (XRD) patterns of the Argon-implanted and control poly-Si films after SPC annealing are shown in Fig. 4.3. The SPC poly-Si film has two preferred orientations, the dominant orientation of Si (111) and the other orientation of Si (110),

reported by Aoyama et al. [4.20]. The intensity of the preferred orientation of Si (111) and Si (110) in the Argon-implanted poly-Si film is apparently sharper and higher than that in the control poly-Si film. Therefore, the sharper and higher intensity of XRD peaks proved the crystallinity of poly-Si film in the Argon-implanted sample can be improved compared to that in the control sample. The reason why the Argon-implanted poly-Si film has better crystallinity could be ascribed as following. When heavy Argon ions are implanted through the α-Si film with the projected range located beyond the α-Si/SiO2 interface, many recoiled-oxygen atoms from the SiO2 substrate will accumulate at the α-Si/SiO2 interface.

The presence of recoiled-oxygen atoms is believed to reduce the nucleation sites at the α-Si/SiO2 interface [4.16]- [4.19], which suppresses the interface-nucleation rate of Si atoms, and thereby to result in the nucleation of silicon grain from the top free surface of α-Si layer, called surface-nucleation scheme.

To prove the recoiled-oxygen existing at the α-Si/SiO2 interface, the secondary ion mass spectroscopy (SIMS) analysis is performed. Fig. 4.4 shows the SIMS depth profiles of oxygen atoms for the Argon-implanted and control α-Si films. The SIMS depth profile shows that an oxygen-rich region is observed near at the α-Si/SiO2 interface after the Argon implantation. Consequently, when the interface-nucleation rate is suppressed but the surface-nucleation scheme is dominated, the large silicon grains could be formed after Argon implantation treatment.

4.3.2 Device Characteristics

Typical transfer characteristics of the Argon-implanted and control poly-Si TFTs are shown in Fig. 4.5. The measurements were performed at two drain voltages of VDS = 0.5 V and 5 V, and the drawn channel length (L) and channel width (W) are 10 µm and 10 µm, respectively. The electrical parameters of these devices, including threshold voltage (VTH), field-effect mobility (µFE), and subthreshold swing (S.S.) were extracted at VDS = 0.5 V,

whereas the maximum on current (ION), minimum off current (IOFF), and ON/OFF current ratio (ION/IOFF) were defined at VDS = 5 V. The threshold voltage is defined as the gate voltage required to yield a normalized drain current of IDS = (W/L)×100 nA. The major electrical parameters of these poly-Si TFTs are summarized in Table 4.1. Obvious performance improvements are achieved for the Argon-implanted poly-Si TFTs. The threshold voltage and subthreshold swing of the Argon-implanted poly-Si TFT are 1.73 V and 750 mV/dec, whereas the control poly-Si TFT has the value of 5.75 V and 1290 mV/dec, respectively. The threshold voltage and subthreshold swing of the Argon-implanted poly-Si TFT are found to be superior to those of the control one. Some studies reported that the deep trap states originated from the Si dangling bonds, which have energy states near the middle of the silicon bandgap, would greatly influence the threshold voltage and subthreshold swing [4.21]. The poly-Si film with surface-nucleated scheme can improve the crystallinity of silicon, resulting in decreasing the grain boundaries with decreasing the Si dangling bonds in the poly-Si film. In addition, the leakage current of the Argon-implanted poly-Si TFT was smaller than that of the control one. As is well-known, the traps assisted band-to-band tunneling resulted from the high electric field near the drain junction results in the leakage current [4.22]. The result suggests that fewer grain boundaries exist in the Argon-implanted poly-Si film, and thus the leakage current under a high electric field can be reduced.

Fig. 4.5 also shows the field-effect mobility as a function of gate voltage for the Argon-implanted and control poly-Si TFTs. The field-effect mobility is calculated from the transconductance at VDS = 0.5 V. In Fig. 4.5, the maximum field-effect mobility of the Argon-implanted poly-Si TFT has approximately 74 % improvement compared to that of the control one. Because Argon ion is a noble gas which could not react with Si dangling bonds or contribute any dopant species, the improved performances cannot be ascribed to the Argon passivation effect on the grain-boundary trap states and dopant-induced threshold voltage variation. Therefore, the main reason for the field-effect mobility improvement may be

further attributed to the enhancement of grain size and the reduction of grain boundaries, thereby leading to a better microstructure crystallinity of silicon grain in the Argon-implanted poly-Si film.