Pere Roca i Cabarrocas

Influence of Cu as a catalyst on the properties of silicon nanowires synthesized by the vapour–solid–solid mechanism

Unlike typical Au used as a catalyst for the synthesis of silicon nanowires via the vapour?liquid?solid mechanism, Cu has been found to induce a synthesis process governed by the vapour?solid?solid mechanism. Moreover, the temperature window for obtaining high-quality wires with Cu has been found to be relatively smaller than that shown by the Au: from 600 to 650??C. However, high-resolution transmission electron microscopy analysis reveals significant new properties of the nanowires obtained. They have the peculiarity of successively switching the silicon structure from diamond to the wurtzite phase along the growth direction. This change of the crystalline structure implies that it has an important impact on the transport properties and characteristics of electronic devices. The results will be important for the future integration and application of silicon, where electrical and thermal transport properties play a significant role.

Plasma-enhanced low temperature growth of silicon nanowires and hierarchical structures by using tin and indium catalysts

Plasma-enhanced low temperature growth (<300 degrees C) of silicon nanowires (SiNWs) and hierarchical structures via a vapor-liquid-solid (VLS) mechanism are investigated. The SiNWs were grown using tin and indium as catalysts prepared by in situ H(2) plasma reduction of SnO(2) and ITO substrates, respectively. Effective growth of SiNWs at temperatures as low as 240 degrees C have been achieved, while tin is found to be more ideal than indium in achieving a better size and density control of the SiNWs. Ultra-thin (4-8 nm) silica nanowires, sprouting from the dendritic nucleation patterns on the catalysts surface, were also observed to form during the cooling process. A kinetic growth model has been proposed to account for their formation mechanism. This hierarchical structure combines the advantages of the size and position controllability from the catalyst-on-top VLS-SiNWs and the ultra-thin size from the catalyst-on-bottom VLS-ScNWs.

a-Si:H Deposition from SiH4 and Si2H6 rf-Discharges: Pressure and Temperature Dependence of Film Growth in Relation to α-γ Discharge Transition

We present an interpretation of the pressure and temperature dependence of the growth kinetics of hydrogenated amorphous silicon (a-Si:H) in SiH4 and Si2H6 rf-glow discharges. At constant rf power, the a-Si:H deposition rate increases drastically at a given pressure depending on the nature of the gas and on the wall temperature. The threshold nature of this transition is attributed to the onset of an electron avalanche due to ion-induced secondary electron emission and ionization in the plasma sheaths close to the electrodes. We analyze the effect of various plasma parameters governing this α-γ discharge transition in terms of equivalent circuit of the discharge and power dissipation mechanisms. The ratio of a-Si:H deposition rates from SiH4 and Si2H6 rf discharges at the same rf power and flow rate is strongly dependent on the pressure because the α-γ transition occurs at a lower pressure for Si2H6 than for SiH4. The transition is shifted to higher pressures as the temperature increases primarily because of the reduction of gas density, which explains contradicting results in the literature on the influence of temperature on a-Si:H deposition rate. At a given rf power and substrate temperature, the optical, structural and electrical film properties are correlated with the variation of deposition rate as a function of SiH4 or Si2H6 pressure.

EXPERIMENTAL EVIDENCE FOR NANOPARTICLE DEPOSITION IN CONTINUOUS ARGON-SILANE PLASMAS : EFFECTS OF SILICON NANOPARTICLES ON FILM PROPERTIES

These last few years a great effort has been made to understand the mechanisms of powder formation in silane discharges. It is now well established that powders are negatively charged and thus confined in the plasma. Therefore, one does not expect powders to contribute to the deposition, unless the plasma is switched off. We present here experimental evidence for the deposition of nanoparticles, even in the case of a continuous discharge. Experimental conditions for nanoparticle formation while avoiding powder formation have been determined from light‐scattering and transmission electron microscopy measurements. Nanoparticle deposition has been studied by in situ ellipsometry in silane–argon discharges. From a comparison of the growth kinetics and the optical properties of films obtained under continuous and modulated discharges we conclude that nanoparticle deposition can take place even when the discharge is on. The implications of these discoveries on the properties of hydrogenated amorphous silicon ar...

Plasma production of nanocrystalline silicon particles and polymorphous silicon thin films for large-area electronic devices

Powder formation in silane plasmas has been considered as a technology drawback because it might lead to the formation of macroscopic defects in the deposited layers. Here we summarize our recent efforts in controlling the formation of powder precursors, in particular, nanocrystalline silicon particles, aiming at their incorporation in the films. Indeed, the incorporation of clusters and crystallites along with SiHx radicals allows production of polymorphous silicon films with improved structure and transport properties with respect to standard amorphous silicon films.

Absorbing one-dimensional planar photonic crystal for amorphous silicon solar cell

We report on the absorption of a 100nm thick hydrogenated amorphous silicon layer patterned as a planar photonic crystal (PPC), using laser holography and reactive ion etching. Compared to an unpatterned layer, electromagnetic simulation and optical measurements both show a 50% increase of the absorption over the 0.38-0.75micron spectral range, in the case of a one-dimensional PPC. Such absorbing photonic crystals, combined with transparent and conductive layers, may be at the basis of new photovoltaic solar cells.

Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires

The occurrence of heterostructures of cubic silicon/hexagonal silicon as disks defined along the nanowire ⟨111⟩ growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. We observe that during growth, lamellar twinning on the cubic phase along the ⟨111⟩ direction is generated. Consecutive presence of twins along the ⟨111⟩ growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram.

In situ generation of indium catalysts to grow crystalline silicon nanowires at low temperature on ITO

In situ generation of indium catalyst droplets and subsequent growth of crystalline silicon nanowires on ITO by plasma-enhanced CVD are reported, and the wurtzite (Si–IV) phase is clearly evidenced in some wires.

Observation of Incubation Times in the Nucleation of Silicon Nanowires Obtained by the Vapor-Liquid-Solid Method

We report the observation of a characteristic incubation time in the growth of silicon nanowires using the vapor-liquid-solid growth mechanism. This incubation time manifests itself during the growth process as a characteristic time delay in the range of several seconds to minutes, prior to which no nanowires are formed. The observation is in excellent agreement with a theoretical model based on the diffusion of silicon through the catalyst, which predicts the presence of an incubation time, as determined by diffusion of the growth constituent through the solid catalyst. Furthermore the theoretical dependence of the incubation time on the activation energy is derived, and validated experimentally for the first time by measuring the incubation times of silicon nanowires obtained by chemical vapor deposition for both gold and copper as a catalyst. The experimentally observed incubation times are in excellent agreement with the theoretically predicted incubation times. The reported incubation times are a universal feature of vapor-liquid-solid growth and can be applied to any other metal/ semiconductor system for the synthesis of nanowires and provide a novel route to determine the phase space for nanowiresynthesis.

Polymorphous silicon thin films produced in dusty plasmas: application to solar cells

We summarize our current understanding of the optimization of PIN solar cells produced by plasma enhanced chemical vapour deposition from silane–hydrogen mixtures. To increase the deposition rate, the discharge is operated under plasma conditions close to powder formation, where silicon nanocrystals contribute to the deposition of so-called polymorphous silicon thin films. We show that the increase in deposition rate can be achieved via an accurate control of the plasma parameters. However, this also results in a highly defective interface in the solar cells due to the bombardment of the P-layer by positively charged nanocrystals during the deposition of the I-layer. We show that decreasing the ion energy by increasing the total pressure or by using silane–helium mixtures allows us to increase both the deposition rate and the solar cells efficiency, as required for cost effective thin film photovoltaics.