Highly efficient silicon nanoarray solar cells by a single-step plasma-based process

Highly efficient solar cells (conversion efficiency 11.9%, fill factor 70%) based on the vertically aligned single-crystalline nanostructures are fabricated without any pre-fabricated p-n junctions in a very simple, single-step process of Si nanoarray formation by etching p-type Si(100) wafers in low-temperature environment-friendly plasmas of argon and hydrogen mixtures.

Plasma-made silicon nanograss and related nanostructures

Plasma-made nanostructures show outstanding potential for applications in nanotechnology. This paper provides a concise overview on the progress of plasma-based synthesis and applications of silicon nanograss and related nanostructures. The materials described here include black silicon, Si nanotips produced using a self-masking technique as well as self-organized silicon nanocones and nanograss. The distinctive features of the Si nanograss, two-tier hierarchical and tilted nanograss structures are discussed. Specific applications based on the unique features of the silicon nanograss are also presented.

Plasma-enabled growth of separated, vertically aligned copper-capped carbon nanocones on silicon

The formation of vertically aligned, clearly separated, copper-capped carbon nanocones with a length of up to 500 nm and base diameter of about 150 nm via three-stage process involving magnetron sputtering, N2 plasma treatment, and CH4 + N2 plasma growth is studied. The width of gaps between the nanocones can be controlled by the gas composition. The nanocone formation mechanism is explained in terms of strong passivation of carbon in narrow gaps, where the access of plasma ions is hindered and the formation of large Cn H2n+2 molecules is possible. This plasma-enabled approach can be used to fabricate nanoelectronic, nanofluidic, and optoelectronic components and devices. © 2010 American Institute of Physics.

Kinetics of the initial stage of silicon surface oxidation : Deal–Grove or surface nucleation?

The nucleation-initiated oxidation of a Si surface at very low temperatures in plasmas is demonstrated experimentally, in contrast to the Deal-Grove mechanism, which predicts Si oxidation at a Si/SiO interface and cannot adequately describe the formation of SiO nanodots and oxidation rates at very low (several nanometers) oxide thickness. Based on the experimental results, an alternative oxidation scenario is proposed and supported by multiscale numerical simulations suggesting that saturation of micro- and nanohillocks with oxygen is a trigger mechanism for initiation of Si surface oxidation. This approach is generic and can be applied to describe the kinetics of low-temperature oxidation of other materials. © 2009 American Institute of Physics.

Nanopore processing in dielectric materials and dielectric template-assisted nanoarray synthesis : using pulsed bias to enhance process throughput and precision

An effective technique to improve the precision and throughput of energetic ion condensation through dielectric nanoporous templates and reduce nanopore clogging by using finely tuned pulsed bias is proposed. Multiscale numerical simulations of ion deposition show the possibility of controlling the dynamic charge balance on the upper template's surface to minimize ion deposition on nanopore sidewalls and to deposit ions selectively on the substrate surface in contact with the pore opening. In this way, the shapes of nanodots in template-assisted nanoarray fabrication can be effectively controlled. The results are applicable to various processes involving porous dielectric nanomaterials and dense nanoarrays.

Self-organized vertically aligned single-crystal silicon nanostructures with controlled shape and aspect ratio by reactive plasma etching

The formation of vertically aligned single-crystalline silicon nanostructures via "self-organized" maskless etching in Ar+ H 2 plasmas is studied. The shape and aspect ratio can be effectively controlled by the reactive plasma composition. In the optimum parameter space, single-crystalline pyramid-like nanostructures are produced; otherwise, nanocones and nanodots are formed. This generic nanostructure formation approach does not involve any external material deposition. It is based on a concurrent sputtering, etching, hydrogen termination, and atom/radical redeposition and can be applied to other nanomaterials.

Controlled-bandgap silicon nitride nanomaterials : deterministic nitrogenation in high-density plasmas

To overcome major problems associated with insufficient incorporation of nitrogen in hydrogenated amorphous silicon nitride (a-SiNx:H) nanomaterials, which in turn impedes the development of controlled-bandgap nanodevices, here we demonstrate the possibility to achieve effective bandgap control in a broad range by using high-density inductively coupled plasmas. This achievement is related to the outstanding dissociation ability of such plasmas. It is shown that the compositional, structural, optical, and morphological properties of the synthesized a-SiNx:H nanomaterials can be effectively tailored through the manipulation of the flow rate ratio of the silane to nitrogen gases X. In particular, a wide bandgap of 5.21 eV can be uniquely achieved at a low flow rate ratio of the nitrogen to silane gas of 1.0, whereas typically used values often exceed 20.0. These results are highly-relevant to the development of the next-generation nanodevices that rely on the effective control of the functional nano-layer bandgap energies.

Silicon on silicon : self-organized nanotip arrays formed in reactive Ar+H2 plasmas

The formation of arrays of vertically aligned nanotips on a moderately heated (up to 500 degrees C) Si surface exposed to reactive low-temperature radio frequency (RF) Ar+H(2) plasmas is studied. It is demonstrated that the nanotip surface density, aspect ratio and height dispersion strongly depend on the substrate temperature, discharge power, and gas composition. It is shown that nanotips with aspect ratios from 2.0 to 4.0 can only be produced at a higher RF power density (41.7 mW cm(-3)) and a hydrogen content of about 60%, and that larger aspect ratios can be achieved at substrate temperatures of about 300 degrees C. The use of higher (up to 500 degrees C) temperatures leads to a decrease of the aspect ratio but promotes the formation of more uniform arrays with the height dispersion decreasing to 1.5. At lower (approximately 20 mW cm(-3)) RF power density, only semispherical nanodots can be produced. Based on these experimental results, a nanotip formation scenario is proposed suggesting that sputtering, etching, hydrogen termination, and atom/radical re-deposition are the main concurrent mechanisms for the nanostructure formation. Numerical calculations of the ion flux distribution and hydrogen termination profiles can be used to predict the nanotip shapes and are in a good agreement with the experimental results. This approach can be applied to describe the kinetics of low-temperature formation of other nanoscale materials by plasma treatment.

Structure of the magnetized sheath of a dusty plasma

A three-component fluid model for a dusty plasma-sheath in an oblique magnetic field is presented. The study is carried out for the conditions when the thermophoretic force associated with the electron temperature gradient is one of the most important forces affecting dust grains in the sheath. It is shown that the sheath properties (the sheath size, the electron, ion and dust particle densities and velocities, the electric field potential, and the forces affecting the dust particles) are functions of the neutral gas pressure and ion temperature, the dust size, the dust material density, and the electron temperature gradient. Effects of plasma-dust collisions on the sheath structure are studied. It is shown that an increase in the forces pushing dust particles to the wall is accompanied by a decrease in the sheath width. The results of this work are particularly relevant to low-temperature plasma-enabled technologies, where effective control of nano- and microsized particles near solid or liquid surfaces is required.

Effect of doping with Co and/or Cu on electronic structure and optical properties of ZnO

This paper reports on ab initio numerical simulations of the effect of Co and Cu dopings on the electronic structure and optical properties of ZnO, pursued to develop diluted magnetic semiconductors vitally needed for spintronic applications. The simulations are based upon the Perdew-Burke-Enzerh generalized gradient approximation on the density functional theory. It is revealed that the electrons with energies close to the Fermi level effectively transfer only between Cu and Co ions which substitute Zn atoms, and are located in the neighbor sites connected by an O ion. The simulation results are consistent with the experimental observations that addition of Cu helps achieve stable ferromagnetism of Co-doped ZnO. It is shown that simultaneous insertion of Co and Cu atoms leads to smaller energy band gap, redshift of the optical absorption edge, as well as significant changes in the reflectivity, dielectric function, refractive index, and electron energy loss function of ZnO as compared to the doping with either Co or Cu atoms. These highly unusual optical properties are explained in terms of the computed electronic structure and are promising for the development of the next-generation room-temperature ferromagnetic semiconductors for future spintronic devices on the existing semiconductor micromanufacturing platform.

Pulsed i-PVD of dielectric nanodot arrays using a nanoporous template

The results of multi-scale numerical simulations of pulsed i-PVD template-assisted nanofabrication of ZnO nanodot arrays on a silicon substrate are presented. The ratios and spatial distributions of the ion fluxes deposited on the lateral and bottom surfaces of the nanopores are computed as a function of the external bias and plasma parameters. The results show that the pulsed bias plays a significant role in the ion current distribution inside the nanopores. The results of numerical experiments of this work suggest that by finely adjusting the pulse voltage, the pulse duration and the duty cycle of the external pulsed bias, the nanopore clogging can be successfully avoided during the deposition and the shapes of the deposited ZnO nanodots can be effectively controlled. A figure is presented.

Aluminum-assisted crystallization and p-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH 4+Ar+H2 gas mixture at a low substrate temperature of 300∈°C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si-Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×1018 cm-3. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300∈°C is proposed.

Enhancement of optical absorption in thin-film solar cells through the excitation of higher-order nanoparticle plasmon modes

Recent research in the rapidly emerging field of plasmonics has shown the potential to significantly enhance light trapping inside thin-film solar cells by using metallic nanoparticles. In this article it is demonstrated the plasmon enhancement of optical absorption in amorphous silicon solar cells by using silver nanoparticles. Based on the analysis of the higher-order surface plasmon modes, it is shown how spectral positions of the surface plasmons affect the plasmonic enhancement of thin-film solar cells. By using the predictive 3D modeling, we investigate the effect of the higher-order modes on that enhancement. Finally, we suggest how to maximize the light trapping and optical absorption in the thin-film cell by optimizing the nanoparticle array parameters, which in turn can be used to fine tune the corresponding surface plasmon modes.

Surface plasmon enhancement of optical absorption in thin-film silicon solar cells

Strong electromagnetic field enhancement that occurs under conditions of the surface plasmon excitation in metallic nanoparticles deposited on a semiconductor surface is a very efficient and promising tool for increasing the optical absorption within semiconductor solar cells and, hence, their photocurrent response. The enhancement of the optical absorption in thin-film silicon solar cells via the excitation of localized surface plasmons in spherical silver nanoparticles is investigated. Using the effective medium model, the effect of the nanoparticle size and the surface coverage on that enhancement is analyzed. The optimum configuration and the nanoparticle parameters leading to the maximum enhancement in the optical absorption and the photocurrent response in a single p-n junction silicon cell are obtained. The effect of coupling between the silicon layer and the surface plasmon fields on the efficiency of the above enhancement is quantified as well.

Effective control of nanostructured phases in rapid, room-temperature synthesis of nanocrystalline Si in high-density plasmas

An innovative and effective approach based on low-pressure, low-frequency, thermally nonequilibrium, high-density inductively coupled plasmas is proposed to synthesize device-quality nanocrystalline silicon (nc-Si) thin films at room temperature and with very competitive growth rates. The crystallinity and microstructure properties (including crystal structure, crystal volume fraction, surface morphology, etc.) of this nanostructured phase of Si can be effectively tailored in broad ranges for different device applications by simply varying the inductive rf power density from 25.0 to 41.7 mW/cm3. In particular, at a moderate rf power density of 41.7 mW/cm3, the nc-Si films feature a very high growth rate of 2.37 nm/s, a high crystalline fraction of 86%, a vertically aligned columnar structure with the preferential (111) growth orientation and embedded Si quantum dots, as well as a clean, smooth and defect-free interface. We also propose the formation mechanism of nc-Si thin films which relates the high electron density and other unique properties of the inductively coupled plasmas and the formation of the nanocrystalline phase on the Si surface.