Transistors with a Profiled Active Layer Made by Hot-Wire Cvd
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ABSTRACT In order to obtain stable thin-film silicon devices we are conducting research on the implementation of hot-wire CVD amorphous and polycrystalline silicon in thin-film transistors, TFFs. We present results on TFTs with a profiled active layer (deposited at >9 Afs), and correlate the electrical properties with the structure of the silicon matrix at the insulator/semiconductor interface, as determined with cross-sectional transmission electron microscopy. Devices prepared with an appropriate H2 dilution of SiH 4 show cone-shaped crystalline inclusions. These crystals start at the interface in some cases, and in others exhibit an 80nm incubation layer prior to nucleation. The crystals in the TFTs with the incubation layer are not cone-shaped, but are rounded off. The hot-wire CVD deposited devices exhibit a high fieldeffect mobility up to 1.5 cm 2V-Isl. Also, these devices have superior stability upon continuous gate bias stress, as compared to conventional glow-discharge a-Si:H TFTs. We ascribe this to a combination of enhanced structural order of the silicon and a low hydrogen content. INTRODUCTION The quest for stable amorphous-silicon based devices has instigated the development of lowhydrogen-content material. One way to obtain this type of material is catalytic chemical vapour deposition (cat-CVD), also called hot-wire CVD (HWCVD), in which a silicon containing gas is
dissociated catalytically at a hot filament of typically 1600-2000 0 C. With this technique various types of silicon and silicon alloys have been fabricated, including amorphous silicon (a-Si:H) and amorphous silicon nitride (a-SiNx). Matsumura explored cat-CVD [1]; the term HWCVD for this technique became prominent after the NREL-group [2] succeeded in fabricating device quality low-hydrogen-content a-Si:H which indeed showed a reduced light-induced degradation, or Staebler-Wronski effect [3]. They also included this material in solar cells [4,51. Bottom-gate thin-film transistors (TFTs) incorporating HWCVD a-Si:H as 'the channel material and silicon dioxide as the gate insulator were explored by us [6-8], and resulted in state-of-the-art TFTs with a field-effect mobility exceeding 0.6 cm 2V-lsl and a threshold voltage around 6 V [9,10]. Chu et al. [11 ] reported on HWCVD-TFTs using a-SiNx as the gate insulator and fabricated TFTs with a mobility of 0.1 cm 2 V-ls-1. A major difference between the different types of TF" appears to be the degradation upon prolonged gate bias stress: the SiO 2TFTs are stable, whereas the a-SiNx-TFTs show a threshold voltage shift of 1.3 V after I h [12]. It is unlikely that this is simply due to the different gate insulators that were used: we found that also the properties of the channel region and a proper pretreatment of the insulator prior to deposition of the hot-wire a-Si:H layer are of crucial importance [9].
31 Mat. Res. Soc. Symp. Proc. Vol. 507 01998 Materials Research Society
In previous work we found that HWCVD deposited thick films (Ž_1.5pm) on glass or c-Si, grown with appropriate H2 diluti
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