Paper EN-TuP2
Deposition of Size-controlled Ge Nanoparticle Film by High-pressure rf Magnetron Sputtering for Quantum Dot Solar Cells

Tuesday, October 29, 2013, 6:00 pm, Room Hall B

We here present deposition of Ge nanoparticle films for quantum dot solar cell applications. Semiconductor nanoparticle films have attracted much at tention because of the unique characteristics such as tunable band gap and multiple exciton generation effects of quantum dots [1]. Ge nanoparticles have more outstanding quantum confinement effects than Si nanoparticles due to the larger excitonic Bohr radius of 24.3 nm for bulk Ge than 4.9 nm for bulk Si. In this study, we deposited Ge nanoparticle films by radio frequency (rf) magnetron sputtering in argon and hydrogen gas mixture at 20 % H₂ dilution ratio under a high pressure condition of 1.5 Torr. At such high pressure, Ge nanoparticle formation in gas phase is possible, because the mean free path of Ge atoms is as short as an order of micrometer.

We measured Raman spectra of Ge thin films and bulk Ge crystal. We obtained the full width at half maximum (FWHM) and peak frequency of each peak of Raman spectra, as a function of gas flow rate (R_{gas flow}). The film deposited at R_{gas flow} = 80 sccm shows a peak at 298 cm^-1, and this peak is assigned to Ge crystal. As R_{gas flow} increases, the peak slightly shifts to higher frequency, and FWHM increases from 7.2 cm^-1 at R_{gas flow} = 80 sccm to 11.7 cm^-1 at R_{gas flow} = 250 sccm. FWHM and peak frequency strongly depend on the particle size [2]. Form the dependence, the Ge particle size at R_{gas flow} = 250 sccm is deduced to be 6-7 nm. We also fabricated in Ge quantum-dot sensitized solar cells, and succeeded in carrier generation in Ge nanoparticles.

[1] G. Uchida, et al., “Effect of nitridation of Si nano-particles on the performance of quantum-dot sensitized solar cells ”, Jpn. J. Appl. Phys, Vol. 51, pp. 01AD01-1 – 01AD01-5, 2011.