(167b) Theoretical and Experimental Study of Germanium Nanoparticle Formation in a Controlled Nucleation Gas Phase Process

Semiconducting nanoparticles out of germanium are promising materials for the application in printable electronics. Dispersed in organic liquids and deposited as thin functional film, e.g. as a thin film transistor (Meric et al., 2015). The synthesis of these nanoparticles (NPs) is challenging because the homogenous pyrolysis of the precursor leads to high nucleation rates with subsequent agglomeration and sintering. One strategy to overcome this problem is the introduction of seed particles to suppress the homogenous nucleation (Alam and Flagan, 1986). Therefore, we introduced an external seeding unit supporting the synthesis of semiconducting nanoparticles in a hot-wall reactor. This external unit allows a detailed investigation of the effect of seed particles on the particle growth. The experimental observations are supported theoretically by population balance modelling.

For the experiments, a hot-wall reactor (HWR) is combined with a hot-wire generator (HWG). The HWG is used to produce small metallic seed particles by condensation from a supersaturated vapor. This vapor is generated by passing an electric current through a metallic wire held between to electrodes. The seed particle stream is directly mixed with the precursor (GeH₄) and injected into the hot-wall reactor. In this stage, the NPs grow on the seed particles at temperatures between 700 °C and 1000 °C. At the reactor exit the aerosol is quenched by nitrogen. The particles are sampled via a low-pressure impactor by directly deposition on a substrate or as powder via a filter. For the theoretical investigation a one-dimensional model, which was already successfully introduced in previous works for the formation of silicon nanoparticles (Körmer at al., 2010), was transferred to the germanium system. The influence of the seed and germane concentration and temperature is studied.

The aerosol properties of the seed particles have a significant influence on the Ge NP morphology and can be controlled for example by the used metal and the applied voltage. The seed concentration can be varied in the range from 10⁵ - 10⁸ 1/cm³ and the size from 3 - 15 nm. An increasing voltage leads to a higher wire temperature and this consequently to a higher amount of evaporated material. The seed particle concentration in the reactor increases and homogeneous nucleation is suppressed at a certain threshold (~ 10^-6 - 10^‑7 1/cm³).

The final seeded particles show a well-defined morphology with a size of around 35 nm (Geometric standard deviation GSD: 1.08). XRD measurements show the typical pattern of Bragg peaks for crystalline germanium. A quantitative Rietveld analysis gave a crystallite size of around 31 nm. This value is in good accordance to PSD evaluation by SEM images. The discrepancy may be due to the amorphous oxide layer, which is 3 to 4 nm thick. With increasing partial pressure, the particle grows larger in size, while the morphology and a narrow size distribution are preserved. Even a six-fold increase in the germane partial pressure only leads to a slightly broadening of the particle size distribution (GSD: 1.12). Therefore, narrow distributed Ge NPs can be produced supported by the HWG with a particle size in the range from 25 to 50 nm. The synthesis temperature has no significant influence on the particle properties. These observations can also be seen in the theoretical results. The model shows that the key parameter is the seed concentration, which has a strong influence on the homogenous nucleation rate and the final particle size. The temperature has no impact on particle formation because already at 700 °C a complete conversion of germane takes place in the presence of seed particles. The predicted particle distributions are also in good accordance to the experimental results.

The progress on the synthesis of Ge NPs supported by external seeding will be demonstrated. The influence of the seeding unit and the process parameters on particle formation and properties will be discussed experimentally and theoretical in detail.

This work was supported by the German Research Council (DFG) and the Cluster of Excellence “Engineering of Advanced Materials” (EAM).

Meric, Z., Mehringer, C., Karpstein, N., Jank, M.P.M., Peukert, W., and Frey, L. (2015). Tunable conduction type of solution-processed germanium nanoparticle based field effect transistors and their inverter integration. Phys. Chem. Chem. Phys., 17(34):22106–22114.

Alam, M.K. and Flagan, R.C. (1986). Controlled nucleation aerosol reactors: production of bulk silicon. Aerosol Sci. Technol., 5(2):237–248.

Körmer, R., Schmid, H.J., and Peukert, W. (2010). Aerosol synthesis of silicon nanoparticles with narrow size distribution-Part 2: Theoretical analysis of the formation mechanism. J. Aerosol Sci., 41(11):1008–1019.