Alan Howling

Bio: Alan Howling is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Plasma & Silane. The author has an hindex of 32, co-authored 127 publications receiving 3070 citations. Previous affiliations of Alan Howling include École Polytechnique.

Frequency effects in silane plasmas for plasma enhanced chemical vapor deposition

Abstract: It is now generally recognized that the excitation frequency is an important parameter in radio‐frequency (rf) plasma‐assisted deposition. Very‐high‐frequency (VHF) silane plasmas (50–100 MHz) have been shown to produce high quality amorphous silicon films up to 20 A/s [H. Curtins, N. Wyrsch, M. Favre, and A. V. Shah, Plasma Chem. Plasma Processing 7, 267 (1987)], and therefore the aim of this work is to compare the VHF range with the 13.56 MHz industrial frequency in the same reactor. The principal diagnostics used are electrical measurements and a charge coupled device camera for spatially resolved plasma‐induced emission with Abel inversion of the plasma image. We present a comparative study of key discharge parameters such as deposition rates, plasma uniformity, ion impact energy, power transfer efficiency, and powder formation for the rf range 13–70 MHz.

Time-resolved measurements of highly polymerized negative ions in radio frequency silane plasma deposition experiments

Abstract: The time‐resolved fluxes of negative polysilicon hydride ions from a power‐modulated rf silane plasma have been measured by quadrupole mass spectrometry and modeled using a simple polymerization scheme. Experiments were performed with plasma parameters suitable for high‐quality amorphous silicon deposition. Polysilicon hydride anions diffuse from the plasma with low energy (approximately 0.5 eV) during the afterglow after the electron density has decayed and the sheath fields have collapsed. The mass dependence of the temporal behavior of the anion loss flux demonstrates that the plasma composition is influenced by the modulation frequency. The negative species attain much higher masses than the positive or neutral species and anions containing as many as sixteen silicon atoms have been observed, corresponding to the 500 amu limit of the mass spectrometer. This suggests that negative ions could be the precursors to particle formation. Ion–molecule and ion–ion reactions are discussed and a simple negative ion polymerization scheme is proposed which qualitatively reproduces the experimental results. The model shows that the densities of high mass negative ions in the plasma are strongly reduced by modulation frequencies near 1 kHz. Each plasma period is then too short for the polymerization chain to propagate to high masses before the elementary anions are lost in each subsequent afterglow period. This explains why modulation of the rf power can reduce particle contamination. We conclude that for the case of silane rf plasmas, the initiation steps which ultimately lead to particle contamination proceed by negative ion polymerization.

Improving plasma uniformity using lens-shaped electrodes in a large area very high frequency reactor

Abstract: Experiments using a lens-shaped circular electrode are described to measure the correction of plasma nonuniformity due to the standing wave effect in a large area very high frequency plasma reactor. This work is the experimental verification of the theoretical reactor design in cylindrical geometry recently presented by L. Sansonnens and J. Schmitt, Appl. Phys. Lett. 82, 182 (2003). It is found that the lens-shaped electrode effectively compensates the standing wave effects by creating a uniform rf vertical electric field in the plasma volume. The plasma is uniform, except for edge effects, for a wide range of parameters and consequently the design is suitable for plasma processing.

Plasma silane concentration as a determining factor for the transition from amorphous to microcrystalline silicon in SiH4/H2 discharges

Abstract: In this work, the microstructure transition from amorphous to microcrystalline silicon is defined in terms of the silane concentration in the plasma as opposed to the silane concentration in the input gas flow. In situ Fourier transform infrared absorption spectroscopy combined with ex situ Raman spectroscopy has been used to calibrate and validate this approach. Results show that a relevant parameter to obtain mu c-Si : H from SiH4/H-2 mixtures is the plasma composition, which is determined not only by the gas dilution ratio but also by the silane depletion fraction. It is also shown that mu c-Si : H can only be deposited efficiently, in terms of gas utilization, at a high rate by using high input concentration and depletion of silane.

Negative-Ion Mass-Spectra and Particulate Formation in Radio-Frequency Silane Plasma Deposition Experiments

Abstract: Negative ions have been clearly identified in silane rf plasmas used for the deposition of amorphous silicon. Mass spectra were measured for monosilicon up to pentasilicon negative ion radical groups in power‐modulated plasmas by means of a mass spectrometer mounted just outside the glow region. Negative ions were only observed over a limited range of power modulation frequency which corresponds to particle‐free plasma conditions. The importance of negative ions regarding particulate formation is demonstrated and commented upon.

Plasma{material interactions in current tokamaks and their implications for next step fusion reactors

Abstract: The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

Abstract: Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Thin‐film silicon solar cell technology

Abstract: This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. These properties are linked to the microstructure and hence to the i-layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single-junction amorphous and microcrystalline p–i–n-type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc-Si: H/a-Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass-production of thin-film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin-film silicon PV modules, especially in building-integrated PV (BIPV) are shown. In this context, the energy yields of thin-film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.