Status of Amorphous Silicon and Related Alloys Prepared by Photochemical Vapor Deposition
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STATUS
OF
AMORPHOUS SILICON AND RELATED PHOTOCHEMICAL VAPOR DEPOSITION
ALLOYS
PREPARED
BY
MAKOTO KONAGAI Department of Physical Electronics,Tokyo Institute of Technology, 2-12-1,O-okayama,Meguro-kuTokyo 152,JAPAN ABSTRACT A review is given of the current status of amorphous Si(a-Si) and related alloys prepared by photo-CVD. Preparation techniques and film quality of the following materials will be reviewed; "*undoped a-Si films, "*highly conductive n-type and p-type c(:-Si films, " a-SiGe films. *a-SiC films, *superlattice structures. Finally,the present status of a-Si solar cell performance prepared by photo-CVD will be introduced and the merit of photo-CVD will be discussed. INTRODUCTION Up untill now, high quality amorphous Si films have been fabricated by a glow discharge process. However, it is assumed that radiationinduced damage from high energy charged partices ,which might exist to some extent in conventional glow discharge plasma,may limit the film quality and interface properties of devices, such as solar cells. In photochemical vapor deposition(photo-CVD), the photon energy is not high enough to ionize molecules. Therefore, in photo-CVD, the effect of high energy charged particles is considerably reduced. Thus, ion-damage-free, high quality a-Si films and high performance devices can be expected by using photo-CVD. Photo-CVD processes are classified as follows; l)Mercury sensitized photo-CVD, 2)Direct photo-CVD, -Lamp process(usually low pressure Hg lamp), -Laser process(excimer laser). In this paper ,a review is given of the current status of a-Si and related alloys prepared by both Hg sensitized photo-CVD and direct photolytic decomposition precesses. MERCURY SENSITIZED PHOTO-CVD Mercury photosensitization The mercury photosensitization of monosilane and disilane investigated in detail earlier by Pollock et al.[11],and the primary is Hg* + Si H6 > Hg + H + Si H5 , Hg* + Si-4
b
was step
Hg + H + Si 35
where Hg* indicates the excited state of a mercury atom due to the irradiation by UV light. The reaction probability between Hg* and the gas sources is expressed by a quenching cross section. Table 1 shows quenching cross sections for
Mat. Res. Soc. Symp. Proc. Vol. TO. 0 1986 Materials Research Society
258
02 several compounds. The quenching cross section of H2 is 6(A ). This value is high enough to control the amount of hydrogen radicals in the reaction The hydrogen radical plays a significant role for the chamber. microcrystallization of a-Si films and also for the reduction of defect densities in a-SiGe films as described in the following section.
Undoped a-Si by mercury sensitized photo-CVD Since Saitoh et al.[2] showed that high quality a-Si films could be prepared by a photo-CVD in 1982 ,many works have been done to enhance the Table 2 shows the film quality and to clarify the deposition mechanism. typical activities performed up to now in the field of photo-CVD. Tarui et al. investigated on the high-rate deposition of amorphous Si films using mercury sensitized photo-CVD and they fo
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