Susumu Toko

Effects of gas flow rate on deposition rate and number of Si clusters incorporated into a-Si: H films

The suppression of cluster incorporation into a-Si:H films is the key to better film stability, because incorporated clusters contribute to the formation of SiH2 bonds and hence lead to light-induced degradation of the films. To deposit stable a-Si:H solar cells at a high deposition rate (DR), we studied the effects of the gas flow rate on DR and the number of Si clusters incorporated into a-Si:H films with discharge power as a parameter, using a multihollow discharge-plasma chemical vapor deposition method. We succeeded in depositing high-quality a-Si:H films with the incorporation of few clusters at DR of 0.1 nm/s. We also found that, under a low gas flow rate and a high discharge power, high-density clusters exist in plasma and hence DR is reduced as a result of radical loss to the clusters.

Effects of filter gap of cluster-eliminating filter on cluster eliminating efficiency

We have developed a cluster-eliminating filter which reduces amount of amorphous silicon nanoparticles (clusters) incorporated into a-Si:H films. We have measured film thickness dependence of a ratio of a hydrogen content associated with Si-H2 bonds (CSiH2) to that with Si-H bonds (CSiH) as a parameter of the filter gap (dgap). The cluster removal efficiency increases with decreasing dgap. CSiH2 and the ratio of CSiH2 to CSiH decrease with increasing the film thickness, suggesting the amount of incorporated clusters is high near the interface between the film and the substrate.

Correlation between SiH2/SiH and light-induced degradation of p–i–n hydrogenated amorphous silicon solar cells

We have measured the hydrogen content ratio I SiH2/I SiH associated with Si–H2 and Si–H bonds in p–i–n (PIN) a-Si:H solar cells by Raman spectroscopy. With decreasing I SiH2/I SiH, the efficiency, short-circuit current density, open-circuit voltage, and fill factor of PIN a-Si:H solar cells after light soaking tend to increase. Namely, I SiH2/I SiH correlates well with light-induced degradation of the cells. While a single I-layer has a low I SiH2/I SiH of 0.03–0.09, a PIN cell has I SiH2/I SiH = 0.18 because many Si–H2 bonds exist in the P-layer and at the P/I interface of the PIN solar cells. To realize PIN solar cells with higher stability, we must suppress Si–H2 bond formation in the P-layer and at the P/I interface.

Contribution of ionic precursors to deposition rate of a-Si:H films fabricated by plasma CVD

We have studied contribution of ionic precursors to deposition rate of a-Si:H films in the downstream region of a multi-hollow discharge plasma CVD reactor using a DC bias grid and QCMs. The deposition rate decreases from 1.1 to 0.93 by applying negative bias voltage to the bias grid. The ionic precursors contribute to 7% of the total deposition rate and the dominant ionic precursors are considered to be negatively charged clusters.

Effects of Grid DC Bias on Incorporation of Si Clusters into Amorphous Silicon Thin Films in Multi-Hollow Discharge Plasma CVD

Si clusters formed in silane discharge plasmas are mainly responsible for light induced degradation of hydrogenated amorphous Si (a-Si:H) thin films deposited by the plasmas. Here we have investigated effects of grid DC bias on incorporation amount of Si clusters into a-Si:H films in multi-hollow discharge plasma CVD reactor using quartz crystal microbalances, by which volume fraction of Si clusters in deposited films is quantitatively measured. When the grid potential is lower than plasma potential, the negatively charged clusters are repelled away from the grid by electrostatic force, resulting in lower volume fraction.