Si and Ge based nanowires : growth and optical properties

Figure 1: SEM image of Si NWs grown by CVD on silicon (111)

The growth of semiconducting nanowires is of great interest for nanoelectronics, biosensors, and solar cell technology, due to their particular electrical and optical properties arising from their one-dimensional geometry. In particular, small diameter Silicon, Germanium and Silicon/Germanium nanowires (NWs) are viewed as attractive building blocks for future devices operating in the quantum limit. Below, we shortly describe the main results obtained by SiNaPS team in Silicon and Silicon/Germanium NWs growth using Chemical Vapor Deposition (CVD) and Molecular Beam Epitaxy (MBE). Our effort was concentrated on understanding of growth mechanism and optimization of all growth parameters in order to demonstrate new physical properties which may be achieved in low dimensional NWs structures. One of our successful results was the demonstration of the efficient optical band to band emission from a Silicon NW.

 

Figure 2: SEM image of Ge NWs grown by MBE on silicon (001)

Silicon nanowires (SiNWs) have been actively studied during the past decade, as they hold the promise of becoming key building blocks in future electronics and optoelectronics devices. However their successful integration in devices will depend ultimately on the degree of control that can be obtained over structure and physical properties.Silicon, Germanium and Silicon/Germanium NWs are grown in SiNaPS laboratory using so called Vapor-Liquid-Solid (VLS) mechanism by Chemical Vapor Deposition (CVD) and Molecular Beam Epitaxy (MBE). In VLS approach, the metallic droplets catalyze the growth of one-dimensional structures. We show a typical Scanning Electron Microscopy (SEM) image of Silicon NWs grown by CVD in figure 1 and Germanium NWs grown by MBE on figure 2.

 

Figure 3: SEM images of nanotrees grown by CVD. The branches start from gold droplets present on the trunk sidewall.

Depending on substrate crystallographic orientation and growth parameters, the diameter, length, shape and density of NWs can be considerably modified. The aim of SiNaPS team effort was to optimize the growth parameters and to understand the complex correlations between all parameters which influence the physical properties of NWs.We initially investigated the influence of metallic droplets on NWs growth.  We were able to show that when using gold droplets as a metal catalyst in VLS CVD, the presence of gold at the surface of NWs could be controlled by substrate temperature or Silane partial pressure. By optimizing the growth conditions (temperature and Silane pressure) the presence of gold can be limited to the topmost part of the wire, close to the catalyst (see figure 3). Moreover, the small gold droplets observed on sidewalls of NWs can be used as catalysts for the subsequent growth of NWs.This opens the way to the growth of very special structures which we call nanotrees (see figure 3).

 

We also developed an original way to grow nanowires by depositing gold at the bottom of nanoporous alumina. NWs growth follows VLS mechanism inside nanopores (see figure 4). Optical properties of Silicon NWs were investigated for NWs grown under optimized conditions by CVD. We found that for NWs with average diameter of 50 nm the PL spectra were strongly modified by thermal oxidation which induces the formation of a SiO2 shell around the NW. The spectrum of the core-shell NWs was dominated by an asymmetric band peaking near 1.08 eV. Such a band is commonly observed in ultra-pure bulk Silicon and in two-dimensional SiO2/ Si /SiO2 quantum wells. It results from the LO phonon assisted recombination of electron-hole pairs inside a dense plasma which can be either in a liquid or a gas phase depending on temperature, excitation density, and spatial confinement. Moreover, emission yield is ten times higher after thermal oxidation than for as-grown NWs. Thermal oxidation drastically decreases the relative importance of the trap contribution compared to the near gap contribution

 

Figure 4: SEM image of Si NWs grown inside nanoporous alumina

Maj : 27/09/2016 (286)

 

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