Film growth of high quality microcrystalline germanium by ECR plasma CVD

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We have deposited high quality hydrogenated Ge films using the ECR plasma of hydrogen diluted germane generated by microwaves of a very narrow frequency spectrum in order to well control electrons and ions impinging to growing surfaces at the background pressure of the order of 10^<-8> Torr. The films deposited on insulating substrates showed amorphous-microcrystalline phase transition around at the microwave power of 9-12 W,where Hall voltage of the films changed sign due to Hall effect anomaly in amorphous semiconductor. An optoelectronic parameter etamutau gave a maximum at the phase transition similarly as previously observed in the Ge films deposited on conductive substrates. To investigate surface field effect mobility mu_<FE> in contrast with Hall mobility mu_H, we have fabricated bottom-gate type TFT with depositing Ge films as as active layrs, but gate-leak current was observed in particular on muc-Ge : H TFT.This gate breakdown was found not to be due to charging up of electr … More ons and ions on SiO_2 but due to microwave irradiation, and prevented by shielding the samples from the microwaves during deposition. The surface mobility mu_<FE> was correlated with etamutau in a-Ge : H TFT but not in muc-G : H TFT.Etching Ge on SiO_2 revealed that the Ge-SiO_2 interface was significantly roughened by muc-Ge : H deposition in comparison with a-Ge : H.Moreover, we found mu_<FE> to be correlated with surface roughness of Ge-SiO_2 interface in muc-Ge : H TFT, which suggest that the surface roughness is an important factor in mu_<FE> of muc-Ge : H TFT.Though it was revealed by plasma diagnostic that electron temperature and ion irradiation are key factors to obtain high mu_<FE> and etamutau in a-Ge : H TFT,we could not realize the deposition condition to have such high values of mu_<FE> as mu_H<similar or equal>0.3cm^2/Vs in a-Ge : H and mu_H<similar or equal>2cm^2/Vs in muc-Ge : H in the present work. Thus this has been left to the subject for a future study. We also found the existence of two type light degradation effects in a-Ge : H films ; one is the same as that usually observed in intrinsic a-Si : H and the other gives completely opposite feature of photo- and dark current, during and after irradiation, respectively. The former film has slightly better optoelectronic sensitivity and smaller localized gap-states density. However detailed study including the difference of deposition condition for the two type films should be further continued. Less