Erik V. Johnson

Synthesis of silicon nanocrystals in silane plasmas for nanoelectronics and large area electronic devices

The synthesis of silicon nanocrystals in standard radio-frequency glow discharge systems is studied with respect to two main objectives: (i) the production of devices based on quantum size effects associated with the small dimensions of silicon nanocrystals and (ii) the synthesis of polymorphous and polycrystalline silicon films in which silicon nanocrystals are the elementary building blocks. In particular we discuss results on the mechanisms of nanocrystal formation and their transport towards the substrate. We found that silicon nanocrystals can contribute to a significant fraction of deposition (50–70%) and that they can be positively charged. This has a strong influence on their deposition because positively charged nanocrystals will be accelerated towards the substrate with energy of the order of the plasma potential. However, the important parameter with respect to the deposition of charged nanocrystals is not the accelerating voltage but the energy per atom and thus a doubling of the diameter will result in a decrease in the energy per atom by a factor of 8. To leverage this geometrical advantage we propose the use of more electronegative gases, which may have a strong effect on the size and charge distribution of the nanocrystals. This is illustrated in the case of deposition from silicon tetrafluoride plasmas in which we observe low-frequency plasma fluctuations, associated with successive generations of nanocrystals. The contribution of larger nanocrystals to deposition results in a lower energy per deposited atom and thus polycrystalline films.

Separate control of the ion flux and ion energy in capacitively coupled radio-frequency discharges using voltage waveform tailoring

We experimentally characterize an argon plasma in a geometrically symmetric, capacitively coupled rf discharge, excited by pulse-type tailored waveforms (generated using multiple voltage harmonics). The results confirm a number of predictions made by recent particle-in-cell simulations of a similar system and demonstrate a unique form of control over the ion flux and ion energy in capacitively coupled plasmas; by increasing the number of applied harmonics (equivalent to decreasing the pulse width), it is possible to increase the plasma density and ion flux (together with the power deposition) while keeping the average ion energy on one of the electrodes low and constant.

Radio-frequency capacitively coupled plasmas excited by tailored voltage waveforms: comparison of experiment and particle-in-cell simulations

Using a range of different diagnostics we have performed a detailed experimental characterization of a capacitively coupled rf plasma discharge excited by tailored voltage waveforms in argon (3–13 Pa). The applied pulse-type tailored waveforms consist of between 1 and 5 harmonics (with a fundamental of 15 MHz), and are used to generate an electrically asymmetric plasma response, manifested by the formation of a strong dc bias in the geometrically symmetric reactor used. Experimental measurements of the dc bias, electron density, ion current density, ion-flux energy distributions at the electrodes and discharge current waveforms, are compared with a one-dimensional particle-in-cell simulation for the same operating conditions. The experimental and simulation results are found to be in good agreement over the range of parameters investigated, and demonstrate a number of unique features present with pulse-type tailored waveforms, including: increased plasma density and ion flux with the number of harmonics, and a broader control range of the ion bombarding energy.

Microcrystalline silicon solar cells deposited using a plasma process excited by tailored voltage waveforms

Thin film solar cells in a p-i-n structure with an absorbing layer of intrinsic hydrogenated microcrystalline silicon (μc-Si:H) deposited through plasma enhanced chemical vapour deposition excited by tailored voltage waveforms have been prepared. The use of an asymmetric voltage waveform decouples the ion-bombardment energy at the growth surface from the injected power and allows the growth of good quality μc-Si:H at reasonable deposition rates (3 A/s) using low pressure, powder-free conditions. Unoptimized photovoltaic devices with an efficiency of 6.1% are demonstrated using an i-layer deposited at 1.3 A/s and a process pressure of 500 mTorr.

Ion flux asymmetry in radiofrequency capacitively-coupled plasmas excited by sawtooth-like waveforms

Using particle-in-cell simulations, we predict that it is possible to obtain a significant difference between the ion flux to the powered electrode and that to the grounded electrode—with about 50% higher ion flux on one electrode—in a geometrically symmetric, radiofrequency capacitively-coupled plasma reactor by applying a non-sinusoidal, ‘Tailored’ voltage waveform. This sawtooth-like waveform presents different rising and falling slopes over one cycle. We show that this effect is due to differing plasma sheath motion in front of each electrode, which induces a higher ionization rate in front of the electrode which has the fastest positive rising voltage. Together with the higher ion flux comes a lower voltage drop across the sheath, and therefore a reduced maximum ion bombardment energy; a result in contrast to typical process control mechanisms.

Growth mechanisms study of microcrystalline silicon deposited by SiH4/H2 plasma using tailored voltage waveforms

The use of Tailored Voltage Waveforms is a technique wherein one uses non-sinusoidal waveforms with a period equivalent to RF frequencies to excite a plasma. It has been shown to be an effective technique to decouple maximum Ion Bombardment Energy (IBE) from the ion flux at the surface of the electrodes. In this paper, we use it for the first time as a way to scan through the IBE in order to study the growth mechanism of hydrogenated microcrystalline silicon using a SiH4/H2 chemistry. We find that at critical energies, a stepwise increase in the amorphous to microcrystalline transition thickness is observed, as detected by Real Time Spectroscopic Ellipsometry. The same energy thresholds (30 eV and 70 eV) are found to be very influential on the final surface morphology of the samples, as observed by Atomic Force Microscopy. These thresholds correspond to SiHx+ bulk displacement (30 eV) and Hx+ (70 eV) surface displacement energies. A model is therefore proposed to account for the impact of these ions on th...

Radio frequency current-voltage probe for impedance and power measurements in multi-frequency unmatched loads

A broad-band, inline current-voltage probe, with a characteristic impedance of 50 Ω, is presented for the measurement of voltage and current waveforms, impedance, and power in rf systems. The probe, which uses capacitive and inductive sensors to determine the voltage and current, respectively, can be used for the measurement of single or multi-frequency signals into both matched and unmatched loads, over a frequency range of about 1-100 MHz. The probe calibration and impedance/power measurement technique are described in detail, and the calibrated probe results are compared with those obtained from a vector network analyzer and other commercial power meters. Use of the probe is demonstrated with the measurement of power into an unmatched capacitively coupled plasma excited by multi-frequency tailored voltage waveforms.

Ion Energy Threshold in Low-Temperature Silicon Epitaxy for Thin-Film Crystalline Photovoltaics

Plasma-enhanced chemical vapor deposition (PECVD) enables epitaxial silicon deposition for up to several micrometers and at low temperatures (as low as 150 °C). We present herein a detailed study of the effect of ion energy at high (above 2 torr) and low (below 1 torr) pressure, where the plasma and surface reactions are expected to be different, i.e., driven, respectively, by high-order and low-order silane precursors. We find a sharp energy threshold at low pressure, above which no epitaxy can be obtained, but this threshold is relaxed at high pressure. The occurrence of epitaxy breakdown is studied and compared in detail for these two different pressure regimes.

Carrier transport and luminescence in composite organic–inorganic light-emitting devices

Abstract A model is presented to explain the light-current–voltage characteristics of composite light-emitting structures. These structures are composed of a conducting polymer matrix impregnated with a sheet of inorganic quantum dot nanocrystals. Such structures were reported to exhibit emission characteristics which were extremely promising but, at the same time, not explicable using available models. We present a generalized model that can describe the salient observed characteristics.

Experiment and modelling of very low frequency oscillations in RF-PECVD: a signature for nanocrystal dynamics

Very low frequency (VLF) oscillations in the DC self-bias voltage and the optical emission spectroscopy signatures of diluted SiH4, GeH4 and SiF4 plasmas are documented. The oscillations occur under conditions where nanoparticles are generated in the plasma, and close to the transition between amorphous and nanocrystalline material growth. The frequency, intensity and waveforms of the oscillations are shown to have a dependence on gas temperature, RF-power, total pressure and flow rate. The observation of VLF oscillations is a positive indication of the formation of a nanocrystalline film, but the converse inference is not valid. A macroscopic, zero-dimensional model for the plasma dynamics is proposed incorporating a feedback mechanism involving nanoparticles to generate the VLF oscillations. The requirements for the model to reproduce the experimental results are (i) long particle residence times (>10?s), (ii) slow particle growth rates (0.4?nm?s?1) and (iii) the rapid onset of nucleation suppression by large particles. These conditions allow us to reproduce the undamped oscillations and oscillation frequencies observed experimentally. The long particle residence times may explain the complete crystallization of the nanoparticles in the plasma.