Deposition Of Silicon Thin Films Research Articles

Magnetron sputtered silicon thin films for solar cell applications

One of the important directions of research in photovoltaics is the development of new thin-film technology, which can replace the currently used, more expensive bulk silicon technology. The article discusses the findings from research focused on optimizing the parameters for the deposition of silicon thin films with P-type electrical conductivity for applications in photovoltaics. The growth rate was determined depending on the change in substrate temperature using reflectometry and the influence of deposition time on optical properties was determined using UV/VIS spectroscopy. Photovoltaic structures were made on substrates with an ITO layer and their electrical parameters were measured. The authors applied the magnetron sputtering method to deposit the layers, selecting it over the commercially used the chemical vapor deposition (CVD) method. This replacement could alleviate the necessity for high temperatures and broaden the potential applications of thin-film solar cells.

Effect of He diluted plasma on the deposition of flexible low-temperature polycrystalline silicon thin film

The growth of a low-temperature polycrystalline silicon (LTPS) thin film on a flexible polyethylene terephthalate (PET) substrate under the assistance of helium plasma has been examined in detail in this study. By utilizing the plasma enhanced chemical vapor deposition method, LTPS thin films are directly deposited on a PET substrate at a relatively low temperature (80 °C), and the variations in the morphology, structure, and electrical property of the samples with He flow are systematically characterized by performing a series of tests. The results show that the purely helium-diluted silane plasma has the function of inducing crystallization and fabricating the c-Si network. After optimizing the He flow rate to 600 SCCM, the LTPS thin film with the largest average grain size of ∼204.7 nm can be obtained, while the maximum dark conductivity (4.16 × 10−5 S/cm) is also achieved. Finally, a detailed discussion is presented to illustrate the growth mechanism of the LTPS thin film in He plasma.

A Shunt-Assisted Silicon Electrode for Micro Electrochemical Machining

Abstract Stray current causes undesired material dissolution in micro-electrochemical machining (micro-ECM). The reduction of stray corrosion, caused by stray current, continues to be a major challenge for accuracy improvement. To limit the distribution of stray current, a shunt-assisted silicon electrode, with an auxiliary anode sharing stray current, is proposed in this study. The auxiliary anode is arranged outside the insulating layer of the sidewall-insulated electrode. It is proved in simulation that the auxiliary anode can help reduce the average material removal rate on the machined surface by 55% and improve processing accuracy. A fabrication process of shunt-assisted silicon electrode by bulk silicon process and thin film deposition process are presented. Microgrooves and holes are machined in ECM experiments. The angle between each side-wall and the vertical plane is less than 10 deg. The gap between the sidewall of the machined structures and electrode-outer-contour is about 30 μm±6 μm for the grooves and 45 μm±10 μm for the holes. These long-term experiments and consistent processing results show the shunt-assisted electrode is reliable in ECM process. But due to the stray corrosion induced by DC power supply and conservative feed method, the effect of the shunt-assisted silicon electrode in inhibiting stray corrosion is not significant. In the future, a micro-ECM system with novel power supply and active control methodologies is expected to better utilize the effect of the shunt-assisted silicon electrode.

Deposition and characterization of silicon thin film on stainless steel by electron beam evaporation

Silicon (Si) is a promising material as anode electrode because of its high theoretical capacity of 3579 mAh g−1 and low electrode potential (< 0.4 V vs. Li/Li+). In this paper, the Si thin film electrodes were fabricated by using electron beam evaporation on the stainless steel (SS) foil at room temperature. The structural and electrochemical characterization were investigated for the Si thin film electrodes before and after annealing at 700 ˚C. X-ray photoelectron spectroscopy analysis revealed that the annealed film layer consisted of SiOx containing about 30 at.% oxygen. The annealed Si thin film electrode resulted in improved electrochemical performance compared to as-deposited Si thin film electrode. The Si thin film electrode after annealing exhibited an initial discharge capacity of 2270 mAh g−1, and sustained excellent cycle stability with retention capacity about 95 % for 100 cycles at 0.5 C-rate. The improvement in cyclability for the annealed Si thin film electrode can be attributed to the enhanced adhesion of Si thin film to SS substrate through interdiffuion at the Si/SS interface owing to the post-annealing process.

Amorphous Silicon Thin Film Deposition for Poly-Si/SiO2 Contact Cells to Minimize Parasitic Absorption in the Near-Infrared Region

Tunnel oxide passivated contact (TOPCon) solar cells are key emerging devices in the commercial silicon-solar-cell sector. It is essential to have a suitable bottom cell in perovskite/silicon tandem solar cells for commercial use, given that good candidates boost efficiency through increased voltage. This is due to low recombination loss through the use of polysilicon and tunneling oxides. Here, a thin amorphous silicon layer is proposed to reduce parasitic absorption in the near-infrared region (NIR) in TOPCon solar cells, when used as the bottom cell of a tandem solar-cell system. Lifetime measurements and optical microscopy (OM) revealed that modifying both the timing and temperature of the annealing step to crystalize amorphous silicon to polysilicon can improve solar cell performance. For tandem cell applications, absorption in the NIR was compared using a semitransparent perovskite cell as a filter. Taken together, we confirmed the positive results of thin poly-Si, and expect that this will improve the application of perovskite/silicon tandem solar cells.

Role of H3 + ions in deposition of silicon thin films from SiH4/H2 discharges: modeling and experiments

A 1D fluid model including a detailed chemistry for silane-hydrogen discharges as well as surface reactions to account for deposition and etching processes has been implemented to study the effects of gas pressure (1 to 3.5 Torr) and silane concentration (2 to 10%) on the deposition rate of silicon thin films in a standard RF-PECVD reactor. The thickness of the films and their deposition rate as functions of the process conditions were determined from optical modelling of UV-visible spectroscopic ellipsometry measurements. The experimental values of the deposition rate were compared with results from the 1D fluid model. SiH3 radicals are found to be the main contributor to the computed deposition rates, while H3 + ions play the main role in the etching process. The study reveals that etching by hydrogen ions must be taken into account to reproduce properly the experimental deposition rate. In particular etching by H3 + ions must be taken into account to achieve a good agreement between the experimental and modelled values of the deposition rate as a function of the total gas pressure and the silane fraction in the discharge.

Inductively Coupled Plasma Chemical Vapor Deposition for Silicon‐Based Technology Compatible with Low‐Temperature (≤220 °C) Flexible Substrates

Herein, an inductively coupled plasma chemical vapor deposition (CVD) (ICP‐CVD) technique is adopted for deposition of silicon dioxide and silicon thin films to fabricate metal–oxide–semiconductor (MOS) capacitors and thin film transistors (TFTs). Prior to the fabrication and characterization of the TFTs, C–V measurements and breakdown tests are conducted on MOS capacitors based on silicon dioxide deposited under temperatures as low as 20 °C. The breakdown field of 9 MV cm−1 is validated afterward, which implies the good electrical insulating property for acting as a gate insulator and the feasibility of the TFTs. After going through forming gas and thermal annealing with the highest temperature of 220 °C, the TFTs yield good gate insulating properties, a threshold voltage of 3.7 V, an on/off ratio of 6.4 × 102, field effect mobility of 1.2 cm2 (V s)−1, and a subthreshold slope of 3.9 V dec−1. They show promising electrical properties under such a low‐temperature fabrication process using ICP‐CVD technique compatible for silicon electronics using low‐cost flexible substrates.

High-rate deposition of high-pure silicon thin films for PV-Absorber layers by crucible-free electron beam physical vapor deposition

In photovoltaics (PV) and microelectronics, there is an ongoing need for fabrication of silicon thin films with excellent material properties and with minimum production cost at the same time. To overcome fabrication gaps we present reached results for depositing silicon thin films by crucible-free electron beam physical vapor deposition (EB-PVD) and for plasma pretreatment of the substrates. The crucible-free EB-PVD process could be improved regarding critical aspects, e.g. occurrence of ingot cracks during heating up of monocrystalline Si evaporation material and spill out of the Si melt on the melting pool edges. Deposition rates above 500 nm/s, corresponding to 30 μm/min, have been reached. Relating heat fluxes will be presented and discussed. By using a double ingot arrangement and superposing vapor from multiple sources the relative deviation of layer thickness could be reduced to < ±5% on a substrate width of 200 mm.

Remotely induced high-density hollow-anode plasma and its application to fast deposition of photosensitive microcrystalline silicon thin film with preferential &amp;lt;110&amp;gt; orientation

We investigated the application of a less hydrogen-dilute and low gas pressure hollow-anode plasma to fast chemical-vapor deposition of photosensitive hydrogenated microcrystalline silicon (μc-Si:H). The hollow-anode plasma was remotely induced at a processing space by transferring a hollow-cathode plasma through a nozzle attached to a partition plate, which operated as an anode and separated the processing space from a hollow-cathode discharge space in an ultrahigh-vacuum hollow-electrode-enhanced glow-plasma transportation (HEEPT) system. The hollow-cathode plasma was excited by applying a very-high-frequency (VHF, 105 MHz) power to a cathode in the hollow-cathode discharge space. Through the use of this hollow-anode plasma under a gas flow rate ratio ([H2]/[SiH4]) of 1.25 (30 sccm/24 sccm), pressure of 80 Pa, and VHF power of 150 W (the highest power tested in this work), we fabricated a well-crystallized and photosensitive μc-Si:H thin film with a highly preferred crystal orientation along the &amp;lt;110&amp;gt; direction at a growth rate of 13 nm/s. Electrical analysis on the self-bias voltage of the cathode (Vdc) revealed that hollow-cathode discharges in the HEEPT system were approximately equivalent to symmetric discharge, i.e., Vdc ≒ 0 V. Optical analysis indicated that the hollow-anode plasma produced an enough amount of atomic hydrogen to grow well-crystallized μc-Si:H thin films, even at the lowest [H2]/[SiH4] ratio (1.25). Optical and electrical analyses and computational plasma simulation demonstrated that the hollow-anode plasma had a lower electron temperature and higher plasma space potential compared with those features of a glow discharge plasma enhanced by a conventional parallel-electrodes system.

Synthesis and Exploratory Deposition Studies of Isotetrasilane and Reactive Intermediates for Epitaxial Silicon.

A synthetic method is presented for the production of isotetrasilane, a higher order perhydridosilane, with the purity and volume necessary for use in extensive studies of the chemical vapor deposition (CVD) of epitaxial silicon (e-Si) thin films. The chemical characteristics, thermodynamic properties, and epitaxial film growth of isotetrasilane are compared with those of other perhydridosilanes. A film-growth mechanism distinct from linear perhydridosilanes H(SiH2) nH, where n ≤ 4, is reported. Preliminary findings are summarized for CVD of both unstrained e-Si and strained e-Si doped with germanium (Ge) and carbon (C) employing isotetrasilane as the source precursor at temperatures of 500-550 °C. The results suggest that bis(trihydridosilyl)silylene is the likely deposition intermediate under processing conditions in which gas-phase depletion reactions are not observed.

Numerical Modeling of Plasma Silicon Discharge for Photovoltaic Application

Abstract In this work, we perform a numerical modeling of silicon thin film deposition for a photovoltaic application in a capacitive coupled plasma reactor (CCP) using RF excitation. Plasma equations are solved by finite element numerical method. The deposition is done by plasma of SiH4/Ar/H2 in low pressure and low temperature. The results provide the fundamental features of plasma discharge such as density, temperature and electrical potential. It is shown that the gas mixture used covers a large energy range, which entails a high deposition rate. Furthermore, the use of CCP reactor yields a uniform deposition without surface deterioration.

Run‐to‐run control of PECVD systems: Application to a multiscale three‐dimensional CFD model of silicon thin film deposition

Deposition of amorphous silicon thin films via plasma‐enhanced chemical vapor deposition (PECVD) and batch‐to‐batch operation under run‐to‐run control of the associated chambered reactor are presented in this work using a recently developed multiscale, three‐dimensional in space, computational fluid dynamics model. Macroscopic reactor scale behaviors are linked to the microscopic growth of amorphous silicon thin films using a dynamic boundary which is updated at each time step of the transient in‐batch simulations. This novel workflow is distributed across 64 parallel computation nodes in order to reduce the significant computational demands of batch‐to‐batch operation and to allow for the application and evaluation in both radial and azimuthal directions across the wafer of a benchmark, run‐to‐run based control strategy. Using 10 successive batch deposition cycles, the exponentially weighted moving average algorithm, an industrial standard, is demonstrated to drive all wafer regions to within 1% of the desired thickness set‐point in both radial and azimuthal directions across the wafer surface. This is the first demonstration of run‐to‐run control in reducing azimuthal film nonuniformity. Additionally, thin film uniformity is shown to be improved for poorly optimized PECVD geometries by manipulating the substrate temperature alone, without the need for re‐tooling of the equipment. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16400 2019

Multiscale three-dimensional CFD modeling for PECVD of amorphous silicon thin films

The development of a three-dimensional, multiscale computational fluid dynamics (CFD) model is presented here which aims to capture the deposition of amorphous silicon thin films via plasma-enhanced chemical vapor deposition (PECVD). The macroscopic reactor scale and the microscopic thin film growth domains which define the multiscale model are linked using a dynamic boundary which is updated at the completion of each time step. A novel parallel processing scheme built around a message passing interface (MPI) structure, in conjunction with a distributed collection of kinetic Monte Carlo algorithms, is applied in order to allow for transient simulations to be conducted using a mesh with greater than 1.5 million cells. Due to the frequent issue of thickness non-uniformity in thin film production, an improved PECVD reactor design is proposed. The resulting geometry is shown to reduce the product offset from ∼ 25 nm to less than 13 nm using identical deposition parameters.

SiH4 enhanced dissociation via argon plasma assistance for hydrogenated microcrystalline silicon thin-film deposition and application in tandem solar cells

We investigate the influence of argon plasma assistance on the depletion of SiH4 in the preparation of μc-Si:H films via plasma-enhanced chemical vapor deposition (PECVD). From optical emission spectrometry (OES) data of the μc-Si:H deposition, we infer that the excited Ar+ and metastable Ar* states enhance the dissociation of SiH4 and H2 molecules to generate ultra-depletion of SiH4. The ultra-depletion of SiH4 increases the μc-Si:H thin-film deposition rate from 4.5 Å/s to 23.4 Å/s at 6-sccm flow of SiH4. This ultra-depletion condition facilitates the gradual substitution of H2 by Ar, which leads to reduced crystallinity at low argon fractions (far), preferential crystallographic orientation along the (111) direction, and fluctuation of the hydrogen content and microstructure factor (R). Thus, the properties of the μc-Si:H thin films can be controlled via control of the atomic hydrogen generation and physical sputtering effects. The highest photosensitivity (2.2 × 103) is observed at far = 56%. Furthermore, we fabricate an a-Si:H/μc-Si:H tandem solar cell with a total thickness of 1450 nm. The cell exhibits an initial efficiency of 11.44%, and the efficiency reduces to 10.24% after 1000 h of light soaking. The cell also exhibits a significantly higher Voc × FF stability over that of Jsc.

Electron-enhanced atomic layer deposition of silicon thin films at room temperature

Silicon thin films were deposited at room temperature with electron-enhanced atomic layer deposition (EE-ALD) using sequential exposures of disilane (Si2H6) and electrons. EE-ALD promotes silicon film growth through hydrogen electron stimulated desorption (ESD) that creates reactive dangling bonds and facilitates Si2H6 adsorption at low temperatures. Without hydrogen ESD, silicon growth relies on thermal pathways for H2 desorption and dangling bond formation at much higher temperatures. An electron flood gun was utilized to deposit Si films over areas of ∼1 cm2 on oxide-capped Si(111) substrates. The silicon film thickness was monitored in situ with a multiwavelength ellipsometer. A threshold electron energy of ∼25 eV was observed for the Si film growth. A maximum growth rate of ∼0.3 Å/cycle was measured at electron energies of 100–150 eV. This growth rate is close to the anticipated growth rate assuming dissociative Si2H6 adsorption on dangling bonds on representative single-crystal silicon surfaces. The Si growth rate also displayed self-limiting behavior as expected for an ALD process. The silicon growth rate was self-limiting at larger Si2H6 pressures for a fixed exposure time and at longer electron exposure times. The silicon growth rate versus electron exposure time yielded a hydrogen ESD cross section of σ = 5.8 × 10−17 cm2. Ex situ spectroscopic ellipsometry showed good conformality in thickness across the ∼1 cm2 area of the Si film. Si EE-ALD should be useful for a variety of applications.

High deposition rate nanocrystalline and amorphous silicon thin film production via surface wave plasma source

A 900MHz surface wave antenna was used for plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films in an H2+SiH4 discharge, with an emphasis on photovoltaic applications. Gas mixtures of 0.7–10% SiH4 at medium pressure (~100mTorr) were tested with an optimal substrate temperature of 285±15°C, producing nanocrystalline hydrogenated silicon (nc-Si:H) at rates up to 3nm/s, while amorphous films were grown in excess of 10nm/s. A sharp transition from crystalline to amorphous growth was seen as SiH4 flowrate increased, as is characteristic of silane PECVD. Increasing both substrate temperature and source power served to move this transition to higher flowrates, and by extension, higher deposition rates for the crystalline phase. Grain size also increased with substrate temperature, ranging from 10±2nm at 200°C up to 15±3nm at 400°C. Electron spin resonance showed that a-Si:H films grown via SWP were of acceptable defect density (~1016cm−3) and conductivity (~10−8S/cm). Conversely, nc-Si:H films were poor quality (~1018cm−3 defect density, 10−3–10−2S/cm conductivity) due to low hydrogenation and small grain size.

Investigation of powder dynamics in silane-argon discharge using impedance analyser

We report the growth of powder formation in Argon (Ar) diluted Silane (SiH4) plasma using 27.12 MHz assisted Plasma Enhanced Chemical Vapor Deposition process with the approach of plasma diagnosis. The appearance of powder during processing contaminates the process chamber which further can alter the film properties; hence plasma diagnosis was vital towards detecting this variation. This work presents for the first time a diagnosis of powder in the plasma using Impedance Analyser (V/I probe) at various concentrations of Argon dilution (10%–90%), chamber pressure (0.3 Torr–0.6 Torr), and applied power (4 W–20 W). Efforts were made to understand the different phases of powder formation (i.e., chain and accumulation process, coalescence phase and α → γ′ transition (powder zone)) by monitoring and evaluating the plasma characteristics such as discharge voltage and current (Vrms and Irms), Impedance (Z), phase angle (ϕ), electron density (ne), bulk field (Eb), and sheath width (ds). From the results of plasma characterization, the coalescence phase can be well diagnosed by the low amplitude of Irms, ϕ, ne, and ds in combination with a high amplitude of Vrms and Eb whereas α → γ′ transition regime diagnose by a lower value of Vrms, Z, ϕ, Eb, and ds with a higher value of Irms and ne which signifies the presence of powder in the plasma. It was also observed that with the increase of applied power, the coalescence phase gets shifted towards the lower Ar dilution percentage. Conversely, the phase transition region from amorphous (a-Si:H) to microcrystalline (μc-Si:H) thin film will observe at the onset of powder formation thus on account of plasma results, deposition of silicon thin films was carried out, and the film properties are in good agreement with plasma characteristics.

Low temperature deposition of polycrystalline silicon thin films on a flexible polymer substrate by hot wire chemical vapor deposition

For the applications such as flexible displays and solar cells, the direct deposition of crystalline silicon films on a flexible polymer substrate has been a great issue. Here, we investigated the direct deposition of polycrystalline silicon films on a polyimide film at the substrate temperature of 200°C. The low temperature deposition of crystalline silicon on a flexible substrate has been successfully made based on two ideas. One is that the Si–Cl–H system has a retrograde solubility of silicon in the gas phase near the substrate temperature. The other is the new concept of non-classical crystallization, where films grow by the building block of nanoparticles formed in the gas phase during hot-wire chemical vapor deposition (HWCVD). The total amount of precipitation of silicon nanoparticles decreased with increasing HCl concentration. By adding HCl, the amount and the size of silicon nanoparticles were reduced remarkably, which is related with the low temperature deposition of silicon films of highly crystalline fraction with a very thin amorphous incubation layer. The dark conductivity of the intrinsic film prepared at the flow rate ratio of RHCl=[HCl]/[SiH4]=3.61 was 1.84×10–6Scm−1 at room temperature. The Hall mobility of the n-type silicon film prepared at RHCl=3.61 was 5.72cm2V−1s−1. These electrical properties of silicon films are high enough and could be used in flexible electric devices.

Detection of powder formation in SiH4/H2 glow discharges

Time-resolved self-bias voltage measurements and spatiotemporal resolved optical emission spectroscopy were used in order to monitor particle formation and instabilities in the initial stage of 13.56 MHz SiH4/H2. The effect of total gas pressure on the particles formation and the time required for the plasma to reach steady state was investigated. Both techniques were very sensitive in monitoring the nucleation and coagulation phase of particles formation. The characteristic times required for reaching steady state were similar for both techniques indicating that electrical properties and chemical kinetics are affected in a similar way from particle formation. The increase of pressure resulted in a significant shortening of the instability period and this can be a significant advantage in the deposition of device grade silicon thin films.

Crystalline silicon thin-film solar cells on ceramic substrates

We provide a review and analysis of research on crystalline silicon thin-film solar cells (CSiTFSCs) on ceramic substrates. The use of foreign substrates (non-silicon materials) for the processing of crystalline silicon solar cells could potentially decrease solar-grade silicon consumption and significantly reduce module costs. In order to enhance the efficiency potential of CSiTFSCs on ceramic substrates, high-temperature silicon film deposition is favored. High-quality electronic-grade silicon films are intended to be deposited at higher temperature as it can help increase both deposition rates and grain sizes. The potential low-cost ceramic substrates have some major restrictions in terms of cell processing technology at high temperatures. In this paper, an overview of the research on thin-film solar-cell technologies on ceramic substrates is presented. Major processing steps for CSiTFSC such as substrate/intermediate layer requirements and silicon thin-film deposition at high temperatures will be discussed. So far, devices have been demonstrated with efficiencies up to 13.4% on graphite, 8.2% on mullite, and 9.4% on silicon nitride (Si3N4) ceramic substrates.