Growth Of Silicon Thin Films Research Articles

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.

Thermodynamic Theory of Silicon Chemical Vapor Deposition

Silicon chemical vapor deposition via silane is determined with classical thermodynamics. Drastic assumptions are used, such as the hydrogen carrier gas does not interact with the silicon thin film growth and the energy of reaction is adjusted to experiments. As a result, a linear function of temperature akin to a Gibbs free energy of reaction is shown to control silicon growth rates, and the characteristic Arrhenius plot is fully determined. The surface response of the growth rate activation energy is neatly mapped, bringing clarity to the parameter space for which a single value is a valid metric. A meta-analysis of growth rates is carried out for a demonstration of the portability of the linear function of temperature across reactors and to mine out reactor scaling factors. A silane heterogeneous decomposition reaction is also a great model chemistry for chlorosilanes. Silicon deposition is notoriously a complex chemical process, but with thermodynamics, the reaction mechanism is successfully reduced to the essentials.

Nanocrystalline silicon thin film growth and application for silicon heterojunction solar cells: a short review.

Doped nanocrystalline silicon (nc-Si:H) thin films offer improved carrier transport characteristics and reduced parasitic absorption compared to amorphous silicon (a-Si:H) films for silicon heterojunction (SHJ) solar cell application. In this article, we review the growth conditions of nc-Si:H thin films as the carrier-selective layers for SHJ solar cells. Surface and growth zone models are analysed at different stages of incubation, nucleation, and growth of the silicon nanocrystallites within the hydrogenated amorphous silicon matrix. The recent developments in the implementation of nc-Si:H films and oxygen-alloyed nc-SiOx:H films for SHJ cells are highlighted. Furthermore, hydrogen and carbon dioxide plasma treatments are emphasised as the critical process modification steps for augmenting the nc-Si:H films' optoelectronic properties to enhance the SHJ device performance with better carrier-selective interfaces.

Simulation of Growth of Silicon Thin Films on Textured and Non-textured Surface: A Comparative Study

We analyzed the difference in growth of thin film silicon layers when deposited on a textured and a non-textured surface. In this investigation characteristics of two solar cells were compared, where one cell was prepared on a textured surface (Cell-A) while the other prepared on a non-textured surface (Cell-B). The intrinsic film deposited on a textured surface was found more defective (2.4 × 1017 cm−3) than that deposited on a flat surface (3.2 × 1016 cm−3). Lower effective flux density of SiH3 precursors on the textured surface can be one of the reasons for higher defect density in the film deposited on textured surface. An Improved light coupling can be achieved by using a thinner doped window layer. The optimum thicknesses of active layers (d) were estimated for best power conversion efficiency (PCE) of Cell-A & Cell-B, and found to be 135 nm & 300 nm respectively. As higher optical transmission of a wide band gap p-type doped window layer is one of the primary concerns, we estimated the effect of using thinner p-type layer for improving optical transmission. By changing the thickness from 15 nm to 3 nm, we observed that the short circuit current density (Jsc) increased from 16.4 mA/cm2 to 20.96 mA/cm2 and PCE increased from 9.4% to 12.3%. Furthermore, it was also noticed that all other device parameters also increased in this process.

Very low-temperature growth of silicon thin films using chlorinated precursors and optical properties

In this work, the effect of the very low substrate temperature and hydrogen dilution on chemical, structural, and optical properties of polymorphous silicon thin films (pm-Si:H) using dichlorosilane as a silicon gas precursor in the plasma enhanced chemical vapor deposition (PECVD) was analyzed. The films were synthesized at a lower deposition temperature in the range from 60 to 150 °C and two H2 dilutions of 60 and 100 sccm. Hydrogen incorporation in silicon thin films has been studied by Elastic Recoil Detection Analysis (ERDA) and Fourier Transform Infrared Spectroscopy (FTIR). FTIR was also used to verify the chemical stability of the material as a function of oxidation state and hydrogen effusion. The ERDA analysis evidenced that the hydrogen content typically does not exceed 30 at. %, and the lowest value obtained is around 10 at. %. With Raman spectroscopy, the crystalline fraction was obtained in the range of 5–20%, and the average size of the embedded nanocrystals was found to be between 2 and 3 nm, which was cross-checked by High-Resolution Transmission Electron Microscopy (HRTEM). Finally, by UV–Vis spectroscopy, the effective optical band gap of this material has been calculated, and the value was found to be around 1.6 eV (absorber layer) for the samples with 100 sccm of H2 dilution and around 2.2 eV (window layer) for the samples with 60 sccm of H2 dilution. Overall the increase in the substrate temperature resulted a better ordering in the amorphous matrix, whereas, with the increase in the hydrogen dilution an improvement in the structure factor was observed. Suitable properties of the deposited material in the present work could be useful for the development of a thin silicon layer for different silicon solar cell technologies.

Effect of deposition rate on the growth mechanism of microcrystalline silicon thin films using very high frequency PECVD

The intrinsic microcrystalline silicon thin films were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Two series of films with different deposition rate 0.30 nm/s and 1.94 nm/s were prepared. The film surface and gas phase reaction growth processes were monitored with real-time spectroscopic ellipsometry and optical emission spectroscopy. The effect of deposition rate on the microcrystalline silicon thin film growth mechanism has been studied. The microcrystalline silicon surface growth was analyzed with KPZ model. The results show that the growth exponent of β is 0.448 for the films with low deposition rate, and the growth exponent of β is 0.302 for the films with high deposition rate. The growth exponent does not increase with deposition rate, but declines. And the reasons for this phenomenon were explained.

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

Directed Atom-by-Atom Assembly of Dopants in Silicon.

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants.

High mobility crystalline silicon film growth below 600 °C from an Au-Si eutectic melt for TFTs

We report the growth of a crystalline silicon thin film on buffered soda-lime glass below 600 °C from a gold (Au) – silicon (Si) eutectic melt with electron mobility of 188 cm2 V−1 s−1 as measured by the Hall effect measurement. The film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, cross-section Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) all confirming a thin continuous film of highly crystalline silicon grown on buffered soda-lime glass. This is a breakthrough process that can replace low temperature polysilicon (LTPS) in thin-film transistor (TFT) fabrication for driving pixels in large displays.

Lattice kinetic Monte Carlo simulation of epitaxial growth of silicon thin films in H2/SiH4 chemical vapor deposition systems

The use of lattice kinetic Monte Carlo techniques for reproducing epitaxial growth of silicon using H2/SiH4 chemistry is described in this work. The implementation of the surface mechanisms is limited to silane and hydrogen adsorption, surface dissociation of higher order hydrides and recombinative hydrogen desorption. Good agreement between simulation and experimental results available in the literature was obtained for H2 desorption peak positions, equilibrium hydrogen coverages and Si growth rates. The formation of (111) and (113) planes is also reproduced, however no evidence of (131) was found even at 850° C, and (119) planes were not formed at the same position than actual grown structures.

Simple processing of back-contacted silicon heterojunction solar cells using selective-area crystalline growth

For crystalline-silicon solar cells, voltages close to the theoretical limit are nowadays readily achievable when using passivating contacts. Conversely, maximal current generation requires the integration of the electron and hole contacts at the back of the solar cell to liberate its front from any shadowing loss. Recently, the world-record efficiency for crystalline-silicon single-junction solar cells was achieved by merging these two approaches in a single device; however, the complexity of fabricating this class of devices raises concerns about their commercial potential. Here we show a contacting method that substantially simplifies the architecture and fabrication of back-contacted silicon solar cells. We exploit the surface-dependent growth of silicon thin films, deposited by plasma processes, to eliminate the patterning of one of the doped carrier-collecting layers. Then, using only one alignment step for electrode definition, we fabricate a proof-of-concept 9-cm2 tunnel-interdigitated back-contact solar cell with a certified conversion efficiency >22.5%. The most efficient silicon solar cells use interdigitated back-contact silicon heterojunction architectures. Here, the authors fabricate this type of cell via a simpler process, using an interband silicon tunnel junction for the electron contact and reaching a certified efficiency higher than 22%.

Influence of excitation frequency and electrode separation on the growth of microcrystalline silicon films and their application in single junction microcrystalline solar cell

In this paper we present the role of plasma excitation frequency and electsrode separation on the growth of microcrystalline silicon thin films at two different hydrogen dilutions of silane and different power densities. We optimized the process conditions to develop device quality microcrystalline material. Optoelectronic and structural properties of the developed material have been correlated with the solar cell properties. Growth rate ~7 A/s has been achieved using plasma excitation frequency of 27.12 MHz at 15 mm electrode separation. We have noticed the positive effects after reducing the electrode separation in higher frequency (27.12 MHz). Optimized microcrystalline film of activation energy 0.55 eV and grain size of 14.61 nm has been developed and is applied to fabricate single junction microcrystalline solar cell. Solar cell with initial cell efficiency of 7.75% with short circuit current density of 24.98 mA/cm2 and open circuit voltage of 0.47 V and fill factor of 0.66 has been achieved.

Heteroepitaxial Writing of Silicon-on-Sapphire Nanowires.

The heteroepitaxial growth of crystal silicon thin films on sapphire, usually referred to as SoS, has been a key technology for high-speed mixed-signal integrated circuits and processors. Here, we report a novel nanoscale SoS heteroepitaxial growth that resembles the in-plane writing of self-aligned silicon nanowires (SiNWs) on R-plane sapphire. During a low-temperature growth at <350 °C, compared to that required for conventional SoS fabrication at >900 °C, the bottom heterointerface cultivates crystalline Si pyramid seeds within the catalyst droplet, while the vertical SiNW/catalyst interface subsequently threads the seeds into continuous nanowires, producing self-oriented in-plane SiNWs that follow a set of crystallographic directions of the sapphire substrate. Despite the low-temperature fabrication process, the field effect transistors built on the SoS-SiNWs demonstrate a high on/off ratio of >5 × 104 and a peak hole mobility of >50 cm2/V·s. These results indicate the novel potential of deploying in-plane SoS nanowire channels in places that require high-performance nanoelectronics and optoelectronics with a drastically reduced thermal budget and a simplified manufacturing procedure.

Disilane addition versus silane-hydrogen flow rate effect on the PECVD of silicon thin films

The effect of small disilane addition to the silane/hydrogen mixture and of the total silane/hydrogen flow rate on the silicon thin film growth rate and crystallinity were investigated. The study was performed by using simplified gas phase chemistry model along with plasma diagnostics such as electrical and deposition rate measurements. The results showed that even small disilane addition induces an increase in the electron density, silane electron-induced dissociation rate, and film growth rate. The increase in the total flow rate caused a linear increase in the film growth rate despite the negligible effect on the discharge microscopic parameters. Similar deposition rates and crystallinities were achieved with both disilane addition and increase in the flow rate, but the deposition efficiency was much higher in the case of disilane addition. The simplified gas phase chemistry model indicated an increase in the silyl production rate either with the disilane addition or the increase in the flow rate. Almost the same silyl production rates were calculated for both parameters, but for the disilane addition case, this was achieved with much less silicon containing molecules in the gas feed. Finally, the increase in the silyl production rate combined to the almost constant hydrogen atoms production, and consumption rate is estimated as the main reason for the drop in the crystallinity with the increase in disilane fraction in the mixture or the total flow rate.

Boron doping effects on microcrystalline silicon film roughness studied by spectroscopic ellipsometry

Microcrystalline silicon thin films were prepared in the p-type chamber using radio frequency plasma enhanced chemical vapor deposition. The surface roughness evolution of microcrystalline silicon thin films was investigated with spectroscopic ellipsometry (SE). The differences between the real-time SE and ex-situ SE have been studied. The effects of boron doping on the surface roughness have been analyzed using real-time SE. For the intrinsic microcrystalline silicon thin films, with an increase in the deposition time, the surface roughness exhibited the following behavior: (a) quickly increased, (b) gradually increased, (c) slightly increased. In the case of boron-doped microcrystalline silicon thin films, the surface roughness showed different behavior: (a) slightly increased, (b) quickly increased, (c) rapidly dropped, (d) increased again. Based on the KPZ model, the intrinsic silicon thin film growth exponent β is about 0.4, corresponding to limited diffusion model. In contrast, boron-doped silicon thin films behavior cannot be described by the KPZ model. Boron catalysis effects promote the reaction radical BHx aggregation, and result in shadowing. Boron doping changed the film growth process.

Textured (111) crystalline silicon thin film growth on flexible glass by e-beam evaporation

We have demonstrated the growth of a (111) preferentially oriented crystalline silicon thin film on a flexible glass substrate at 725°C by utilizing a Thin Film Vapor Liquid Solid (TF-VLS) method. The silicon (Si) thin film growth was accomplished on a flexible glass substrate from an aluminum–silicon (Al–Si) eutectic melt deposited by electron beam evaporation. The resulting oriented crystalline film was subsequently characterized by symmetrical Bragg Brentano geometry X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Raman Spectroscopy for the detection and characterization of Si crystallization. By avoiding high temperature depositions and secondary annealing steps this process permits a low thermal budget and has the capability for high rate, roll-to-roll depositions without breaking vacuum. Furthermore, this method has the theoretical feasibility to induce single crystalline Si growth on the glass substrate, consequently translating to a highly cost effective process for display manufacturing and with the potential to play a major role in the adoption of thin film silicon solar cell technology.

Features of the growth of amorphous silicon thin films synthesized by magnetron sputtering

Features of the growth of amorphous silicon films synthesized by DC magnetron sputtering are studied. The effect of the working-gas pressure on the film-surface roughness and growth rate is investigated. It is shown that the film-growth rate decreases with increasing gas pressure in the working chamber and film-surface roughness changes in proportion to the pressure variation. The obtained experimental dependences can be used in modifying the technique for growing films with specified characteristics.

Computational Study for Reactions of H Atoms with Adsorbed SiH3 and Si2H5 on H-Covered Si(100)-(2 × 1) Surface

Density functional theory calculations with spin-polarization effect were employed to illustrate the adsorption of SixHy (x = 1–2; y = 1–5) species and the reactions of H atoms with adsorbed SiHx (x = 1–3) and Si2H5 species on the H-covered Si(100)-(2 × 1) surface. The configurations and energies of these adsorbates were elucidated. It was found that H vacancy sites can be easy created by SiH3 and Si2H5 radicals with small barriers (2.1 and 1.6 kcal/mol for SiH3 and Si2H5, respectively). The Si2H5, Si2H4, SiHSiH3, SiH3, SiH2, SiH, and Si radials interact with the surface more forcefully than the Si2H6 and SiH4 molecules. Potential energy surfaces for the reaction mechanisms of H + SiHx(a) and H + Si2H5(a) were mapped by using the nudged elastic band method. The calculation results demonstrate that the most favorable pathway is hydrogen abstraction leading to the production of H2 and SixHy (x = 1–2; y = 0–4) because of their low barriers and high exothermicities. Rate constant calculations were performed to study the kinetic behavior for simulation of silicon thin-film growth by chemical vapor deposition processes.

Plasma–surface interaction during low pressure microcrystalline silicon thin film growth

The role of plasma–surface interactions during microcrystalline silicon thin film growth has been studied under low power–low pressure conditions, and a correlation between the plasma–surface interactions and material properties has been provided. The atomic hydrogen flux, inferred by optical emission spectroscopy measurements, has been investigated. The hydrogen-to-silicon growth flux resulted in a ratio of 25 : 140 for the amorphous-to-mixed phase transition, whereas it increased to 40 : 170 for the mixed phase-to-microcrystalline phase transition. The ion contribution to the plasma–surface interaction has also been investigated. For this purpose, a capacitive probe has been implemented locally for direct measurement of the ion flux. For the amorphous-to-microcrystalline phase transition the ion-to-silicon growth flux ratio has been determined to increase from 0.15 to 1, two orders of magnitude smaller than the atomic hydrogen-to-silicon growth flux ratio. The ion energy has been measured with a retarding field energy analyser, which allowed determination of the ion-energy distribution as a function of the silane flow rate. Average ion energies of 15–20 eV have been found for a decreasing silane flow rate, thereby indicating the limited energy transfer to the surface. The main effect caused by the ion arrival at the surface is a locally induced thermal spike enhancing the radical surface diffusion. No prominent detrimental effect, i.e. Si surface or bulk displacement requiring energies greater than 18 eV and 40 eV respectively, or sputtering occurring above 50 eV, has been observed.

Three-dimensional phase field modeling of silicon thin-film growth during directional solidification: Facet formation and grain competition

Adaptive phase field modeling is applied to simulate a directional solidification for silicon in three dimensions. Several parameters are considered for the highly anisotropic interfacial energy and kinetic coefficient of silicon. The morphological evolution and the thermal field near the interface are presented and compared with experiments. The mechanisms for facet formation and grain competition are further discussed.