Thomas Teubner

Ge in-plane nanowires grown by MBE: influence of surface treatment

We present a detailed study of morphological phenomena during molecular beam epitaxy (MBE) of Ge nanowires on Ge substrates by means of the vapor–liquid–solid mechanism. Different wet chemical surface passivation methods were tested for their effect on Ge nanowire growth. Clean, smooth and well-passivated surfaces enable the preferential formation of in-plane nanowires, instead of conventional out-of-plane nanowires. Depending on the type of passivation, different growth directions of the self-aligned in-plane wires were observed: exclusively -grown wires on substrates with a very stable passivation layer, both and growth on substrates with a less stable passivation. The morphology of the wires was studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the -grown in-plane wires, nanofaceting of the top and side walls was observed. Based on the analysis, a coherent hypothesis is formulated to explain the experimental findings.

Germanium nanowire growth controlled by surface diffusion effects

Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 °C and 510 °C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature exhibiting the maximum at T = 430 °C. The temperature dependence of growth rate can be attributed to surface diffusion both along the substrate and nanowire sidewalls. Analyzing the diffusive material transport yields a diffusion length of 126 nm at a substrate temperature of 430 °C.

Silicon on glass grown from indium and tin solutions

A two-step process is used to grow crystalline silicon (c-Si) on glass at low temperatures. In the first step, nanocrystalline seed layers are formed at temperatures in the range of 230 to 400°C by either metal-induced crystallization or by direct deposition on heated substrates. In the second step, c-Si is grown on the seed layer by steady-state liquid phase epitaxy at a temperature range of 580 to 710°C. Microcrystalline Si layers with grain sizes of up to several tens of micrometers are grown from In and Sn solutions. Three-dimensional simulations of heat and convective flow in the crucible have been conducted and give valuable insights into the growth process. The experimental results are promising with regard to the designated use of the material in photovoltaics.

Modelling of solution growth of silicon from small indium droplets : homogeneous nucleation

The nucleation frequency and the size of the critical nucleus of silicon in an indium solution have been described on the basis of the classical theory. Due to a lack of reliable data for the free interface energy of crystalline silicon and liquid indium a simple approximation using the Youngs equation and the Eotvoss rule has been applied. The results of the calculation have been compared with experimental data of melt solidification both of the chemically similar Ge and of a substance with a low melting point. In spite of the remaining uncertainties, the homogeneous nucleation of semiconductors in metallic solutions requires considerable undercooling. The high values of the interface energy are mainly responsible for this behaviour. From the calculated dependence of the size of the critical nucleus on the undercooling one can expect that micro-crystallites are stable in a slightly undercooled solution.

In situ removal of a native oxide layer from an amorphous silicon surface with a UV laser for subsequent layer growth

We have developed an in situ method for removing a native silicon oxide layer from an amorphous silicon (a-Si) surface using a UV laser. The a-Si film containing crystalline silicon seeds is used for the subsequent growth of crystalline Si layers by steady-state liquid phase epitaxy (SSLPE). The main goal of this technique is to grow crystalline silicon layers on low-cost glass substrates which can be used as absorber layers for thin film solar cells. We have investigated the interaction between a-Si and laser pulses as well as the growth results by scanning force microscopy (SFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The heating of the a-Si surface by a laser pulse is modelled by numerical simulations using a finite element approach in COMSOL-Multiphysics. The simulations verify that the laser pulse heats a-Si to temperatures sufficient for the thermal desorption of the native oxide layer but lower than both the crystallization temperature of a-Si and the glass transition temperature.