28 August 2017 from 16:15 to 17:15

PhD defence: Pulsed plasma synthesis of Si and SiGe nanocrystals

This thesis describes the synthesis and characterization of silicon and silicon germanium nanoparticles (NPs) in a dedicated plasma reactor.

We investigate the oscillations found in the optical emission intensity and plasma current to estimate the time scales for dust formation. These measurements showed a specific periodic oscillation in the dusty regime. However these plasma properties showed no fluctuations in the non-dusty regime. We hypothesize that the fluctuations arise from periodic formation and ejection of a dust cloud from the plasma bulk when a critical dust size is reached.

The proof of concept of the operation of the NP reactor is provided by demonstrating a fast and simple technique to synthesize non-embedded, non-agglomerated free standing Si NPs. Two ensembles of particles have been synthesized originating from two different locations, from the bulk of the plasma and from the locally enhanced plasma in the holes of the grounded electrode. They have significantly different characteristics in terms of size, shape, and crystallinity. We highlight that the locally enhanced plasma is beneficial as it contributes to the formation of, among other particles, free standing quantum sized crystalline nanoparticles due to the presence of excess energetic electrons.

Precise size control of NPs can be obtained by pulsing the RF power with alternating ON (tON) and OFF times (tOFF). We study the time evolution of the formation of particles as a function of tON using high resolution electron microscopy and we identify three phases, ultimately leading to cauliflower structured particles. We hypothesize that these complex polycrystalline particles originate from a single nucleus and that gas phase species are subsequently added to this growing nucleus.

The role of tOFF is studied to tailor the size of the silicon NPs. This study infers that when tOFF is less than the gas residence time, complex multi-sized nanostructures form because some NPs which are not pumped out of the reactor stay between the electrodes and their growth continues for various pulses. This situation is overcome for longer tOFF, which allows the source gas to be refreshed, making conditions beneficial for obtaining quantum sized monodisperse NPs.

We investigate the relationship between the various plasma parameters such as power density (P), pressure (p) and inter-electrode distance (d) on the crystallinity of the Si NPs obtained in the gas phase. One of the less studied parameters for NP growth in PECVD reactors is the inter-electrode distance d. We studied the entire p and d process parameter space possible for our reactor and show that the optimum parameter space for the growth of NPs with improved morphology and crystallinity is the regime of higher p and larger d.

We have successfully synthesized highly crystalline and homogeneously alloyed Si1-xGex NCs in continuous and pulsed plasmas. Agglomerated NCs have been produced with remarkable control over their composition by altering the precursor GeH4 gas flow in a continuous plasma. The presence of Si1-xGex alloy particles is confirmed with multiple techniques consistently yielding the same composition. In the pulsed plasma mode, quantum-sized free standing alloy NCs were obtained. The NCs synthesized here have potential applications in tandem solar cells with subcells of which the band gap can be tuned. 

Start date and time
28 August 2017 16:15
End date and time
28 August 2017 17:15
PhD candidate
Mw. A. Akshatha Mohan
Dissertation
Pulsed plasma synthesis of Si and SiGe nanocrystals
PhD supervisor(s)
prof. dr. R.E.I. Schroppprof. dr. J.K. Rath