Session Tu-B2

Nano- and Microcrystalline Silicon: Growth and Characterization II

Chair: Kunji Chen, Nanjing University

Tu-B2.1 10:40–11:00

Microcrystalline Silicon Deposited from SiF4/H2/Ar Gas Mixtures: Material Properties and Growth Mechanisms Studies

Jean-Christophe Dornstetter (1,2), Bastien Bruneau (2), Erik Johnson (2), and Pere Roca i Cabarrocas (2)

1. Total RM – Energies Nouvelles, 24 cours Michelet – La Défense 10, 92069 PARIS LA DEFENSE Cedex, France

2. LPICM-CNRS – Ecole Polytechnique, 91128 PALAISEAU, France

The world record efficiency for single junction hydrogenated microcrystalline silicon (μc-Si:H) thin film solar cells is 10.7%—achieved by EPFL's Institute of Micro-engineering Technology [1]—where a two micron thick μc-Si:H active layer is deposited from SiH4/H2 gas mixtures in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system. One important parameter is the crystalline fraction of the intrinsic μc-Si:H layer; a high crystalline fraction favors a high short-circuit current (Jsc) while a high amorphous fraction favors a high open-circuit voltage (Voc). In practice, the bulk lifetime of the carriers in the i-layer limits its thickness, and the amorphous fraction leads to Light-Induced Degradation (LID).

We have shown that by using SiF4/H2/Ar gas mixtures at high pressure, one can obtain fully crystallized μc-Si:H active layers with high Voc, without LID, and without thickness limitation [2]. We here focus our efforts on the characterization of this material and on understanding its growth mechanisms. In addition to the good device properties (such as a high Voc of 0.54V and good external quantum efficiency) previously shown, recent material investigations by Fourier Transform Photocurrent Spectroscopy (FTPS) have demonstrated a low defect density (absorption coefficient of 3x10–3 cm–1 at 0.8 eV). Further characterization will be presented to better the comprehension of this material. In parallel, parametric studies are performed to deepen the understanding of the growth mechanisms. In particular, plasma generated nanoparticles are required to deposit our μc-Si:H films. One can manipulate the gas mixture or the temperature gradient in the plasma, both of which have a direct impact on nanocrystal formation, to control the film properties. In this work we present results of material properties as a function of process parameters and correlate them with the properties of the plasma, characterized by several techniques such as Optical Emission Spectroscopy (OES), impedance measurements, and Residual Gas Analysis (RGA).

[1] Hänni, et al., IEEE Journal of Photovolatics, 1 (2013) 11–16

[2] J.-C. Dornstetter, S. Kasouit, and P. Roca i Cabarrocas, IEEE Journal of Photovoltaics, 1 (2013) 581–586

Keywords: microcrystalline silicon, SiF4, nanoparticles

Tu-B2.2 11:00–11:20

Tailored Voltage Waveform Deposition of Microcrystalline Silicon-Carbon Alloys from Hydrogen-Diluted Silane and Methane Gas Mixtures

Sofia Gaiaschi (1,2), Rosa Ruggeri (3), Erik Johnson (2), Marie-Estelle Gueunier-Farret (1), Christophe Longeaud (1), Pavel Bulkin (2), Patrick Chapon (4), Giovanni Mannino (3), and Jean-Paul Kleider (1)

1. LGEP - CNRS/SUPELEC, 11, rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, France

2. LPICM, CNRS, Ecole Polytechnique, 91128 Palaiseau, France

3. CNR-IMM, strada VIII n°5, zona industriale, 95121 Catania, Italy

4. HORIBA Jobin Yvon, 16-18, rue du Canal, 91165 Longjumeau CEDEX, France

Despite all advancements, the major limiting factor for thin-film silicon tandem solar cells remains the light-induced degradation of hydrogenated amorphous silicon (a-Si:H). For thin film silicon devices to fully benefit from the ease with which multi-bandgap PV devices can be assembled, stable materials with electronic bandgaps between that of hydrogenated microcrystalline silicon (μc-Si:H, 1.1eV) and that of a-Si:H (1.7 eV) are needed. Thus, a significant breakthrough for this technology would be the replacement of a-Si:H by a stable hydrogenated microcrystalline silicon carbon alloy (μc-Si1–xCx:H). μc-Si1–xCx:H samples have been deposited by standard Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) from a silane (SiH4) and methane (CH4) gas mixture, highly diluted in hydrogen (H2). However, the use of tailored voltage waveforms (TVW's) to excite the plasma for the deposition of this kind of material could lead to a significant evolution of deposition parameters, since it has been shown that this is an effective technique to decouple mean ion bombardment energy (IBE) from injected power. The growth and the structure of such an alloy are complex and need to be better understood. Thus, this work aims at understanding the role of intentionally added carbon during the PECVD of hydrogenated microcrystalline silicon (μc-Si:H), both in slowing the nucleation of μc material, and in changing the basic material properties of the deposited films. The goal is to expand the toolbox of useful materials for thin-film photovoltaics to include non-stoichiometric, multiphasic hydrogenated silicon-carbon alloys. Thin films were first deposited by standard RF-PECVD, fixing the RF power density at 226 mW/cm2 and varying the CH4 flow rate from 1.1 sccm to 1.4 sccm, which leads to a transition from a microcrystalline to an amorphous phase. Then, keeping the same variation of precursors, the effect of IBE was studied using different voltage waveforms: a ''Peaks'' voltage waveform (to reduce the IBE) and a "Valleys" waveform (to increase the IBE). Even upon initial visual inspection, the high IBE conditions produced by the Valleys waveform showed optically very rough, "milky" films, confirming that the change in IBE alone significantly modifies the material structure. Moreover, the crystalline volume fraction determined from the deconvolution of the Ramana spectra shows that by using the Peaks waveform we are able to obtain samples with an higher crystalline content and that the transition to the amorphous phase is shifted to a higher CH4 flow rate. Films were further studied by ellipsometry and Fourier Transform Infrared Spectroscopy to identify their main structure and chemical composition. Surface roughness was studied by Atomic Force Microscopy, while the carbon content was determined by Radio Frequency Glow Discharge Optical Emission Spectroscopy (RF-GDOES). Cross sectional TEM analyses were performed on few samples to ascertain if any carbon was incorporated into the μc-Si grains embedded in the a-Si1–xCx:H matrix, and to study in detail the effect of the variation of IBE on the microstructure. Defect-related and transport properties were studied by modulated and steady state photocurrent techniques.

Keywords: RF-PECVD, tailored voltage waveforms, hydrogenated microcrystalline silicon carbon alloys, RF-GDOES

Tu-B2.3 11:20–11:40

Formation of Nanocrystalline Silicon Thin Film at Low Temperature by Inductively Coupled Plasma (ICP) Assisted CVD Techique and Its Electrical Characterization

Gizem Nogay (1,2), Engin Ozkol (2,3), Zaki Selah (4), Mehmet Güneş (5), and Rasit Turan (1,2)

1. Department of Physics, Middle East Technical University (METU), 06531 Ankara-Turkey

2. Center of Solar Energy Research and Application (GUNAM), Middle East Technical University, 06531 Ankara-Turkey

3. Department of Chemical Engineering, Middle East Technical University (METU), 06531 Ankara-Turkey

4. Department of Physics, Arab American University - Jenin (AAUJ)

5. Department of Physics, Mugla Sitki Kocaman University, 48000 Mugla-Turkey

Hydrogenated nano/micro-cystalline silicon (nc-/μc-Si:H) thin films are the key materials for Si-based thin film solar cell technology. These films have attracted great attention due to their applications in thin film solar cells. In recent years, extensive research efforts have been made to fabricate nano-crystalline or micro-crystalline silicon thin films at low temperatures for flexible polymeric substrates which are not suitable for film deposition at elevated temperature. In the conventional Plasma Enhanced Chemical Vapor Deposition (PECVD) systems relatively high substrate temperature is needed for n-/mc-Si deposition. In addition, it suffers from a number of problems such as low deposition rate, powder formation and peeling off the film. Use of high density plasma source, having richer source of ions and radicals, is proposed as a solution to those problems. One of the most promising high density plasma source is the Inductively Coupled Plasma (ICP) and it is shown to be a promising technique for the synthesis of nc-/μc-Si:H. ICP assisted Chemical Vapor Deposition (CVD) technique also provides high dissociation capacity which leads to low temperature crystallization.

Here we have reported the structural and electrical properties of hydrogenated silicon (Si:H) films deposited by ICP-CVD technique at low substrate temperature using H2 diluted SiH4 as a source gas. Hydrogenated amorphous silicon (a-Si) and nano-crystalline silicon (nc-Si) thin films are studied as a function of power density, total pressure and hydrogen flow ratio. Structure of the films is investigated with both high resolution transmission electron microscopy (HR-TEM) and high resolution field emission scanning electron microscopy (SEM) in detail. We have observed a columnar and porous-like growth in all deposited samples whether they are nc-Si:H or a-Si:H. On the other hand, crystallinity of the deposited films have been analyzed using Raman Spectroscopy (RS) and Grazing Incidence X-ray Diffraction Spectroscopy (GI-XRD). It is found that, crystallinity of the films decreases with increasing pressure. Fourier Transform Infra-Red (FTIR) spectroscopy is performed to explore the bond configurations of the deposited thin films. Interestingly, we observed the dominant Si-O-Si stretching mode with transverse optical (TO) and longitudinal optical (LO) modes in FTIR spectra. We have also confirmed that result with x-ray photoelectron spectroscopy (XPS). In addition to those results, room temperature dark conductivity and temperature dependent dark conductivity measurements are carried out in a cryostat to obtain the activation energies and conductivity prefactor of the samples. We have observed that dark conductivity of the samples are increasing with crystalline volume fraction. We have also observed that there is a reasonable correlation between crystalline fraction and activation energy.

Keywords: micro-/nano-crystalline silicon, ICP-CVD, crystallinity, activation energy

Tu-B2.4 11:40–12:00

Plasma-Surface Interaction during μc-Si:H Thin Film Growth in Low and High Pressure Regimes

Jurgen Palmans (1), Erwin Kessels (1,2), and Mariadriana Creatore (1,2)

1. Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

2. Solliance, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands

Thin-film tandem solar cell manufacturing employing hydrogenated microcrystalline silicon (μc-Si:H) films requires large area uniformity and high processing rate at industrial scale. To meet these key requirements, parallel plate capacitively coupled plasma (CCP) reactors are used operating under H2/SiH4 plasmas in combination with high pressure and power conditions (~10 Torr, ~0.5 W/cm2) leading to optimal SiH4 consumption and, therefore, meeting the demand of high deposition rate. In this contribution the plasma-surface interaction during μc-Si:H thin film growth is addressed for two pressure regimes (~0.45 Torr, 0.1 W/cm2 and ~10.5 Torr, 0.25 W/cm2 resp.) by the implementation of complementary plasma (-surface) diagnostics and material characterization for the a-Si:H to μc-Si:H phase transition. A recently developed CCP reactor, with grounded upper electrode and powered lower showerhead electrode, has been applied therefore. The phase transition has been identified by varying the SiH4 flow rate (with SiH4 content in the range 0.1%–10%) while preserving H2 flow rate, pressure and power density. By means of Raman and Fourier Transform Infrared spectroscopy the film structure has been characterized showing a narrow transition region. The correlation with the plasma-surface interaction is demonstrated by the implementation of a capacitive probe and retarding field energy analyzer, both built in the grounded substrate holder, delivering information on the ion flux and ion energy distribution of ions arriving at the growth surface. Typical ion fluxes in the order of 1014–1015 cm–2s–1 have been obtained depending on the operating pressure. Ion energies have been found to be limited to ~20 eV for both depositing and non-depositing plasmas, demonstrating the limited impact of ions on the growth surface. In combination with the silicon growth flux, determined from RBS/ERD measurements and film thickness, the contribution of ions to the film growth is not larger than 12 eV/Si atom. This result suggests that ions transfer a limited energy to the surface upon impact, most certainly resulting in a thermal spike enhancing diffusion process, and not Si surface or bulk displacement phenomena, which require energies well above 20 eV. On the other hand, the atomic hydrogen flux determined by optical emission spectroscopy has been shown to reach values up to ~160x the silicon growth flux; H flux is therefore confirmed to have a primary role in the growth process of μc-Si:H.

Keywords: microcrystalline silicon, plasma-surface interaction, atomic hydrogen flux

Tu-B2.5 (invited) 12:00–12:30

Silane Plasmas: A Wonderful Toolbox for Silicon Thin Films and Nanostructured Materials

P. Roca i Cabarrocas, S. Abolmasov, R. Cariou, S. Misra, Zh. Fan, L. Yu, and E. Johnson

LPICM, CNRS, Ecole Polytechnique, Palaiseau, France

Hydrogenated amorphous and microcrystalline silicon thin films are routinely produced using silane plasmas. While SiH3 is often considered as the main radical for the obtaining of such films, we will show that moving the process to conditions where silicon clusters and nanocrystals are produced in the plasma can lead to high deposition rates and improved materials, such as hydrogenated polymorphous silicon and polycrystalline silicon [1,2]. Moreover, by changing the substrate from glass to crystalline silicon, it is possible to produce epitaxial thin crystalline silicon films which can be transferred to foreign substrates for flexible electronic devices [3]. Even more interesting, this low temperature epitaxial process can be extended to germanium and to the production of Si/Ge/Si quantum well structures [4]. Last but not least, combining the low temperature plasma process with low melting temperature metal particles such as indium and tin, opens the way to the growth of silicon nanowires, either in-plane [5] or vertical [6] depending on the detailed process conditions. These results show that beyond SiH3, silane plasmas have much more to offer. The ICANS conference started with amorphous materials 30 years ago. Over the years we have demonstrated unexpected possibilities of depositing materials ranging from fully disordered to fully crystallized, which can be easily combined to cover an even larger range of optoelectronic applications.

[1] Y. M. Soro, A. Abramov, M. E. Guenier-Farret, E. V. Johnson, C. Longeaud, P. Roca i Cabarrocas, and J. P. Kleider, J. Non Cryst. Solids 354 (2008) 2092

[2] Y. Djeridane, A. Abramov, and P. Roca i Cabarrocas, Thin Solid Films 515 (2008) 7451

[3] M. Moreno and P. Roca i Cabarrocas, EPJ Photovoltaics 1, 10301 (2010)

[4] M. Labrune, X. Bril, G. Patriarche, L. Largeau, O. Mauguin, and P. Roca i Cabarrocas, EPJ Photovoltaics 3, 30303 (2012)

[5] L. Yu and P. Roca i Cabarrocas, Phys. Rev. B 80 (2009) 085313

[6] Linwei Yu, Benedict O'Donnell, Martin Foldyna, and Pere Roca i Cabarrocas, Nanotechnology 23 (2012) 194011

ICANS 25