Invited: Nanopartilce Composite Plasma CVD Films and Some Applications

Nanoparticle composite films contains nanoparticles to tune optical, electrical, and mechanical properties of the films. Contribution of nanoparticles nucleated homogenously in reactive plasmas to formation of film structure is commonly discarded, because flux of nanoparticles to film surface is low and heterogeneous reactions on film surfaces are believed to determine predominantly the film structure. Here we overview nanoparticle formation in reactive plasmas [1] and then we show three examples in which nanoparticle flux can modify significantly film structure and hence film properties. The first example is a-Si:H. A-Si:H p-i-n solar cells show a performance degradation under prolonged light illumination. It is widely accepted that this effect is related to the creation of metastable defects. We have succeeded in depositing highly stable a-Si:H films using a multi-hollow discharge plasma CVD method by which the volume fraction of amorphous Si nanoparticles in films deposited in the upstream region is significantly reduced by driving clusters  towards the downstream region [2, 3]. Conventional a-Si:H films contain a few % of amorphous Si nanoparticles in their volume, whereas our stable a-Si:H films contain less than 0.01%. The second example is Ge nanoparticle films. We report electrical power generation of Ge quantum-dot (QD) sensitized solar cells with rectification in a TiO2/Ge/polysulfide electrolyte system. We analyzes the effect of adding Sn in Ge films on device performance, where a band-gap Eg value of Ge1-xSnx alloys is tunable from 0.34 < Eg < 0.61 eV for Sn composition ratio x = 0.06 to 0.15[4,5]. The third example is porous low-k film. We have succeeded in depositing porous films with relatively high young modulus by depositing “isolated nanoparticles” [6]. Nanoparticles can be employed as another tuning knob of film structure control.

Work supported by MEXT, JSPS, JST, NEDO, and PVTEC.

[1] M. Shiratani, et al., J. Phys. D: Appl. Phys., 44, 174038 (2011).

[2] M. Shiratani, et al., MRS Proc. 1426 (2012) DOI:http://dx.doi.org/10.1557/opl.2012.1245.

[3] Y. Kim, et al., Thin Solid Films 523, 29 (2012).

[4] G. Uchida, et al., Jpn. J. Appl. Phys. 51, 01AD01 (2011).

[5] H. Seo, et al., Electrochim. Atca 87, 213 (2013).

[6] S. Nunomura, et al., Jpn. J. Appl. Phys., 44, L1509 (2005).