Molecule Clustering Dynamics in the Molecular Doping Process of Si(111) with Diethyl-propyl-phosphonate.

The molecular doping (MD) process is based on the deposition of dopant-containing molecules over the surface of a semiconductor substrate, followed by the thermal diffusion step. Previous studies suggest that, during the deposition, the molecules nucleate clusters, and at prolonged deposition times, they grow into self-assembled layers on the sample to be doped. Little is known about the influence of nucleation kinetics on the final properties of these layers and how they change when we modify the solution properties. In this work, we examine the nucleation rate and the molecular surface coverage kinetics of diethyl-propyl phosphonate on silicon at different solution concentrations and how these conditions influence the final electrical properties of the doped samples. We present a high-resolution morphological characterization of the as-deposited molecules together with the electrical results of the final doped samples. The experimental results show a non-obvious behavior, explained through understanding of the competition between the molecules' physisorption and chemisorption mechanisms. As a consequence, due to the deeper knowledge of the deposition phase, a finer tuning of the conductive properties of MD-doped samples is achieved.

Early Stages of Aluminum-Doped Zinc Oxide Growth on Silicon Nanowires.

Aluminum-doped zinc oxide (AZO) is an electrically conductive and optically transparent material with many applications in optoelectronics and photovoltaics as well as in the new field of plasmonic metamaterials. Most of its applications contemplate the use of complex and nanosized materials as substrates onto which the AZO forms the coating layer. Its morphological characteristics, especially the conformality and crystallographic structure, are crucial because they affect its opto-electrical response. Nevertheless, it was difficult to find literature data on AZO layers deposited on non-planar structures. We studied the AZO growth on silicon-nanowires (SiNWs) to understand its morphological evolution when it is formed on quasi one-dimensional nanostructures. We deposited by sputtering different AZO thicknesses, leading from nanoclusters until complete incorporation of the SiNWs array was achieved. At the early stages, AZO formed crystalline nano-islands. These small clusters unexpectedly contained detectable Al, even in these preliminary phases, and showed a wurtzite crystallographic structure. At higher thickness, they coalesced by forming a conformal polycrystalline shell over the nanostructured substrate. As the deposition time increased, the AZO conformal deposition led to a polycrystalline matrix growing between the SiNWs, until the complete array incorporation and planarization. After the early stages, an interesting phenomenon took place leading to the formation of hook-curved SiNWs covered by AZO. These nanostructures are potentially very promising for optical, electro-optical and plasmonic applications.

Study of the Molecule Adsorption Process during the Molecular Doping.

Molecular Doping (MD) involves the deposition of molecules, containing the dopant atoms and dissolved in liquid solutions, over the surface of a semiconductor before the drive-in step. The control on the characteristics of the final doped samples resides on the in-depth study of the molecule behaviour once deposited. It is already known that the molecules form a self-assembled monolayer over the surface of the sample, but little is known about the role and behaviour of possible multiple layers that could be deposited on it after extended deposition times. In this work, we investigate the molecular surface coverage over time of diethyl-propyl phosphonate on silicon, by employing high-resolution morphological and electrical characterization, and examine the effects of the post-deposition surface treatments on it. We present these data together with density functional theory simulations of the molecules-substrate system and electrical measurements of the doped samples. The results allow us to recognise a difference in the bonding types involved in the formation of the molecular layers and how these influence the final doping profile of the samples. This will improve the control on the electrical properties of MD-based devices, allowing for a finer tuning of their performance.

Chemical Vapor Deposition Growth of Silicon Nanowires with Diameter Smaller Than 5 nm.

Quantum confinement effects in silicon nanowires (SiNWs) are expected when their diameter is less than the size of the free exciton (with a Bohr radius ∼5 nm) in bulk silicon. However, their synthesis represents a considerable technological challenge. The vapor-liquid-solid (VLS) mechanism, mediated by metallic nanoclusters brought to the eutectic liquid state, is most widely used for its simplicity and control on the SiNWs size, shape, orientation, density, and surface smoothness. VLS growth is often performed within high-vacuum physical vapor deposition systems, where the eutectic composition and the pressure conditions define the minimum diameter of the final nanowire usually around 100 nm. In this article, we present and discuss the SiNWs' growth by the VLS method in a plasma-based chemical vapor deposition system, working in the mTorr pressure range. The purpose is to demonstrate that it is possible to obtain nanostructures with sizes well beyond the observed limit by modulating the deposition parameters, like chamber pressure and plasma power, to find the proper thermodynamic conditions for nucleation. The formation of SiNWs with sub-5 nm diameter is demonstrated.

Direct observation of single organic molecules grafted on the surface of a silicon nanowire.

Silicon nanowires inspire since decades a great interest for their fundamental scientific importance and their potential in new technologies. When decorated with organic molecules they form hybrid composites with applications in various fields, from sensors to life science. Specifically the diethyl 1-propylphosphonate/Si combination is considered as a promising alternative to the conventional semiconductor n-type doping methods, thanks to its solution-based processing, which is damage-free and intrinsically conformal. For these characteristics, it is a valid doping process for patterned materials and nanostructures such as the nanowires. Our joined experimental and theoretical study provides insights at atomistic level on the molecular activation, grafting and self-assembling mechanisms during the deposition process. For the first time to the best of our knowledge, by using scanning transmission electron microscopy the direct visualization of the single molecules arranged over the Si nanowire surface is reported. The results demonstrate that the molecules undergo to a sequential decomposition and self-assembling mechanism, finally forming a chemical bond with the silicon atoms. The ability to prepare well-defined molecule decorated Si nanowires opens up new opportunities for fundamental studies and nanodevice applications in diverse fields like physics, chemistry, engineering and life sciences.

Study on the Physico-Chemical Properties of the Si Nanowires Surface.

Silicon nanowires (Si-NWs) have been extensively studied for their numerous applications in nano-electronics. The most common method for their synthesis is the vapor-liquid-solid growth, using gold as catalyst. After the growth, the metal remains on the Si-NW tip, representing an important issue, because Au creates deep traps in the Si band gap that deteriorate the device performance. The methods proposed so far to remove Au offer low efficiency, strongly oxidize the Si-NW sidewalls, or produce structural damage. A physical and chemical characterization of the as-grown Si-NWs is presented. A thin shell covering the Au tip and acting as a barrier is found. The chemical composition of this layer is investigated through high resolution transmission electron microscopy (TEM) coupled with chemical analysis; its formation mechanism is discussed in terms of atomic interdiffusion phenomena, driven by the heating/cooling processes taking place inside the eutectic-Si-NW system. Based on the knowledge acquired, a new efficient etching procedure is developed. The characterization after the chemical etching is also performed to monitor the removal process and the Si-NWs morphological characteristics, demonstrating the efficiency of the proposed method and the absence of modifications in the nanostructure.

Performance of natural-dye-sensitized solar cells by ZnO nanorod and nanowall enhanced photoelectrodes.

In this work, two natural dyes extracted from henna and mallow plants with a maximum absorbance at 665 nm were studied and used as sensitizers in the fabrication of dye-sensitized solar cells (DSSCs). Fourier transform infrared (FTIR) spectra of the extract revealed the presence of anchoring groups and coloring constituents. Two different structures were prepared by chemical bath deposition (CBD) using zinc oxide (ZnO) layers to obtain ZnO nanowall (NW) or nanorod (NR) layers employed as a thin film at the photoanode side of the DSSC. The ZnO layers were annealed at different temperatures under various gas sources. Indeed, the forming gas (FG) (N2/H2 95:5) was found to enhance the conductivity by a factor of 103 compared to nitrogen (N2) or oxygen (O2) annealing gas. The NR width varied between 40 and 100 nm and the length from 500 to 1000 nm, depending on the growth time. The obtained NWs had a length of 850 nm. The properties of the developed ZnO NW and NR layers with different thicknesses and their effect on the photovoltaic parameters were studied. An internal coverage of the ZnO NWs was also applied by the deposition of a thin TiO2 layer by reactive sputtering to improve the cell performance. The application of this layer increased the overall short circuit current Jsc by seven times from 2.45 × 10-3 mA/cm2 to 1.70 × 10-2 mA /cm2.