Kousaku Shimizu

Low-temperature growth of polycrystalline Si and Ge films by redox reactions of Si2H6 and GeF4

Abstract Low-temperature growth of polycrystalline Si and Ge films has been investigated by low-pressure chemical vapor deposition (LP-CVD) featuring a redox reaction of disilane (Si 2 H 6 ) and Germanium tetrafluoride (GeF 4 ). The film growth is established in a wide range of 0.4–100 torr and in low-temperature range of 300–450°C. The Si content significantly depends on the reaction modes in which either Si 2 H 6 or GeF 4 is activated preferentially, and thus the film composition is varied from >90 atm% Si to >90 atm% Ge. It is found that the isolated nuclei are formed directly on the substrate surface and provides a good basis of high crystallinity in a thin film irrespective of the film contents. The mechanism of film growth and the origin of low-temperature crystal growth are discussed from a chemical point of view according to the experimental results.

Low temperature poly-SixGe1−x deposited by reactive thermal CVD for thin film transistor application

Abstract We have prepared a highly silicon-rich polycrystalline silicon–germanium (poly-SixGe1−x, x=0.98) film at 450 °C by the reactive thermal chemical vapor deposition (RTCVD) from a gaseous mixture of disilane (Si2H6) and germanium tetrafluoride (GeF4) under He dilution. We fabricated inverted-staggered bottom-gate n-channel thin-film transistors (TFTs) with films of 200 nm thickness deposited directly on SiO2/Si substrates. The TFT characteristics are influenced by the grain size of the poly-SixGe1−x films and carrier mobility was increased significantly after hydrogenation. The fairly high mobility of 36 cm 2 / V s and a low threshold voltage of 1.8 V have been achieved in the TFT prepared with the film having the largest grain size of 100 nm. We discuss the effect of crystallinity and defects in the films in conjunction with the TFT characteristics.

Ni-seeding effects on the properties of polycrystalline silicon-germanium grown at low temperature

We have investigated the Ni-seeding effects on the low-temperature growth of the polycrystalline silicon–germanium (poly-SixGe1−x) films by reactive thermal chemical vapor deposition with Si2H6 and GeF4. Very thin Ni films (<2 nm) were deposited on the glass substrates by electron beam evaporation in prior to poly-SixGe1−x deposition. The crystallinity and electrical performance of the poly-SixGe1−x films were characterized. p-channel bottom-gate thin-film transistors (TFTs) were also fabricated with these films to evaluate the effect of Ni seeding on device performance. We found that a certain amount of Ni, i.e., less than 0.5 nm, impacts upon the grain size and crystallinity without additional degradation of electrical properties due to the incorporation of Ni in the film. A 0.2 nm equivalent thickness of Ni gives the best crystallinity and the largest grain size (135 nm) and results in the highest mobility (31 cm2/V s) in p-channel TFTs fabricated on SiO2/Si substrates.

Low Temperature growth of poly-crystalline film of Silicon-rich Silicon-Germanium by Reactive Thermal Chemical Vapor Deposition

Low temperature growth of poly-SiGe has been investigated by reactive thermal chemical vapor deposition method, which is a newly developed technique for preparing poly-SiGe by using redox reactions in a set of source materials, i.e., Si 2 H 6 and GeF 4 . In order to prepare silicon-rich poly-SiGe of high mobility, a series of experiment on total pressure, gas flow rates of the source materials and dilution gas of He, and residence time at 450°C has been investigated. At 0.45 Torr, high crystallinity films with high silicon content were prepared, however, homogeneity of film thickness and reproducibility of the film growth was quite low for device application. For overcoming this problem, the growth condition has been studied especially in higher-pressure range of 5-15 Torr. Appropriate choice of the residence time and the gas flow ratios lead to significant improvement in the Si content in the films. Finally, more than 95% of silicon-rich poly-SiGe films, which is p-type, has 7.5 cm 2 /Vs of Hall mobility and (220) orientation, have been prepared at 10 Torr and 450°C within ±2% fluctuation of reproducibility which is enough to fabricate devices.