Growth Of Si Thin Films Research Articles

Epitaxial growth of Si thin films with hexagonal close-packed structures on metal substrates.

We have studied the epitaxial growth of Si thin films on the Cd(0001) surface using low-temperature scanning tunneling microscopy. When deposited at low temperatures (100K), Si atoms form dendritic islands with triangular shapes, indicating the existence of anisotropic edge diffusion in the process of Si film growth. After annealing to elevated temperatures, the triangular dendritic Si islands become hexagonal compact islands. Moreover, the 2D Si islands located on two different substrate terraces exhibit different heights due to the influence of quantum-well states in Cd(0001) films. Based on high-resolution scanning tunneling microscopy images, it is observed that the first, second, and third Si layers show the pseudomorphic 1 × 1 structure. In particular, the first and second layer islands reveal the opposite triangles, indicating the hexagonal close-packed stacking of Si atoms. These results provide important information for the growth of pristine Si films on metal substrates and the understanding of Si-metal interaction.

Si epitaxy on Cl-Si(100)

Current atomically-precise fabrication methods in Si utilize a scanning tunneling microscope (STM) to pattern a monatomic resist adsorbed on Si(100). Recent interest in the use of Cl-based resists has necessitated further investigations on the impact of the tightly bound Cl to the overall device fabrication process. In particular, impacts to the crystallinity of the Si capping layer used to encapsulate the active device region can have dramatic effects on the functionality of atomic-scale quantum devices. Here, the epitaxial growth of Si films on Cl-Si(100) was studied to identify and characterize the Si thin film growth process. Adatoms diffusion on the surface was simulated based on the experimental results of the number density and distribution of Si adatom chains after <0.10 ML of Si deposition at temperatures ranging from 300–850 K. A diffusion barrier within 0.66–0.88 eV for Si adatoms on Cl-Si(100) was found. SIMS depth profiling of ∼25 nm thick Si films verified that Cl segregated towards the growth surface during Si deposition and was effectively removed below SIMS detection limits from both the buried interface and Si film at 850 K. TEM imaging confirmed good film crystallinity was achieved at temperatures as low as 600 K.

Adaptive Bayesian optimization for epitaxial growth of Si thin films under various constraints

We applied a Bayesian optimization (BO) to optimize the epitaxial growth process of Si thin films. The BO enables us to effectively explore the optimal film growth conditions considering several experimental parameters and their interactions. In this way we reduced the total number of experiments required in the optimization. The epitaxial growth rate was maximized while five quality parameters were maintained within an acceptable range. Additionally, we considered two practical issues: eliminating equipment errors and the time cost of the quality parameter evaluation. To overcome these issues, we adaptively conducted BO with different constraints for different situations. As a result of these optimizations, the crystal growth rate was increased to be approximately twice as high as that under standard conditions, while satisfying the five quality parameter conditions.

Effect of surface tension on crystal growth of Si thin films by a continuous-wave laser lateral crystallization

Effect of surface tension and temperature distribution on the crystal growth of Si thin films by continuous wave laser lateral crystallization is experimentally investigated in comparison with the capped and non-capped samples. It is proposed that the grain-boundary free (100) film is realized by the averaged solid–liquid interface which holds an angle of ∼45° to the substrate. This theory explains well the effects of surface tension and laser power.

Role of electrochemical process parameters on the electrodeposition of silicon from 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid

The electrodeposition from Room Temperature Ionic Liquids (RTILs) has recently emerged as a low cost technique for the growth of group IV thin films or nanostructures giving some promising alternative to classical physical vapour or chemical vapour deposition techniques. As a relatively new field of research, only few studies exist describing the growth mechanism of electrodeposition from RTILs, especially for Si films. In the present work, Cyclic Voltammetry (CV), Electrochemical Quartz Crystal Microbalance (EQCM) and potentiostatic electrodeposition techniques have been used to study the role of the applied potential, the concentration of electroactive species, the temperature, and the use of organic additive on the electrodeposition of Si from 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid. The demonstrated investigation of the influence of these parameters on the purity and morphological features gives a better control over the growth of Si thin films and optimizes the technique to grow Si with structural properties suitable for specific applications.

Balance of optical, structural, and electrical properties of textured liquid phase crystallized Si solar cells

Liquid phase crystallized Si thin-film solar cells on nanoimprint textured glass substrates exhibiting two characteristic, but distinct different surface structures are presented. The impact of the substrate texture on light absorption, the structural Si material properties, and the resulting solar cell performance is analyzed. A pronounced periodic substrate texture with a vertical feature size of about 1 μm enables excellent light scattering and light trapping. However, it also gives rise to an enhanced Si crystal defect formation deteriorating the solar cell performance. In contrast, a random pattern with a low surface roughness of 45 nm allows for the growth of Si thin films being comparable to Si layers on planar reference substrates. Amorphous Si/crystalline Si heterojunction solar cells fabricated on the low-roughness texture exhibit a maximum open circuit voltage of 616 mV and internal quantum efficiency peak values exceeding 90%, resulting in an efficiency potential of 13.2%. This demonstrates that high quality crystalline Si thin films can be realized on nanoimprint patterned glass substrates by liquid phase crystallization inspiring the implementation of tailor-made nanophotonic light harvesting concepts into future liquid phase crystallized Si thin film solar cells on glass.

A Sonochemical Approach to the Fabrication of Laterally Aligned ZnO Nanorod Field Emitter Arrays on a Planar Substrate

A simple and novel sonochemical route has been demonstrated to produce the lateral growth of zinc oxide (ZnO) nanorod arrays on a quartz wafer without the use of metal catalysts. Highly crystalline ZnO nanorods grew laterally on the edge of a Zn thin film seed layer, with preferential growth along the [0001] direction. The average diameter and length of the ZnO nanorods, grown laterally from the edge of the Zn thin film, were 156 and 933 nm, respectively. The vertical growth of ZnO nanorods on the top surface of the seed layer was suppressed by a Si thin film growth barrier. The lateral ZnO nanorod arrays showed excellent field emission properties, 60 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 6.5 V/μm, with a low turn-on field in the range of 3.9-4.0 V/μm. Because of the compatibility of this process with current standard semiconductor microfabrication technologies, this sonochemical approach constitutes a practical technique for the design of state-of-the-art nanodevices based on laterally grown ZnO nanorod arrays on planar substrates.

Real-time study ofa−Si:H∕c−Siheterointerface formation and epitaxial Si growth by spectroscopic ellipsometry, infrared spectroscopy, and second-harmonic generation

The performance of many devices based on Si thin films deposited on crystalline Si $(c\text{\ensuremath{-}}\mathrm{Si})$ is highly governed by interface quality. For many of these applications, only fully epitaxial films or fully amorphous films having an abrupt interface with the substrate are desired. However, the realization of these perfectly sharp interfaces and the mechanisms governing their formation are not fully understood yet. In this study, the interface formation between Si thin films and $c\text{\ensuremath{-}}\mathrm{Si}$ has been investigated by simultaneously applying three complementary optical techniques in real time during low temperature Si film growth. The films were deposited in a hot-wire chemical vapor deposition process by using both native oxide covered and H terminated Si(100) substrates. The formation of hydrogenated amorphous Si $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$, epitaxial Si, and mixed phase Si has been detected with spectroscopic ellipsometry by measuring the optical properties of the growing films. The evolution of the hydrogen content and hydrogen bonding configurations in the films has been monitored by attenuated total reflection infrared spectroscopy. A clear dependence of the hydrogen content on film morphology is observed with the amorphous films containing significantly more hydrogen. The surface and interface sensitive technique of second-harmonic generation (SHG) has been applied both spectroscopically and in real time. The SHG spectra of $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films on Si(100) obtained in the SHG photon energy range of $2.7--3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ revealed a dominant contribution originating from the film/substrate interface related to ${E}_{0}^{\ensuremath{'}}∕{E}_{1}$ critical point (CP) transitions of $c\text{\ensuremath{-}}\mathrm{Si}$. The real-time behavior of the SHG response is shown to strongly depend on differences in initial film morphology, which allows for identification of direct $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}∕c\text{\ensuremath{-}}\mathrm{Si}$ heterointerface formation, nanometer-level epitaxial growth, and fully epitaxial growth at a very early stage of film growth. On the basis of the results obtained by the three optical techniques, the $c\text{\ensuremath{-}}\mathrm{Si}$ surface passivation mechanism by $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ thin films is addressed and it is demonstrated that the combination of the techniques provides a profound method to control processes occurring during Si thin film growth.

The nucleation and growth of silicon thin films on silicate glasses of variable composition using supersonic gas source molecular beam deposition

Supersonic molecular beam techniques have been used to study the nucleation and growth of Si thin films on glass surfaces of variable composition using Si2H6 as the precursor to film growth. We have examined, in particular, the early stages of growth using scanning electron microscopy. Making use of molecular beam techniques to control accurately the precursor exposure we have examined trends in the evolution of the Si island density as a function of the composition of the glass, x, in (2⋅SiO2)1−x(Al2O3⋅CaO)x. The silica composition (1−x) for these samples was varied between 0.25 and 0.75, and comparisons were also made to the nucleation of Si on SiO2 thin films made by thermal oxidation and Corning 1737 display glass. We have found that the incubation time τinc varies only weakly with substrate composition, increasing by only a factor of 3 over the range 1−x=0.25–1.0. Examination of a later stage of nucleation and growth, the time for coalescence, τcoal, indicated a stronger dependence on composition, and this metric varied by a factor of 8 over the same range of composition. These results indicate that the intrinsic reactivity of the surface scales with the silica content of the surface. The maximum island density shows a much stronger, superlinear dependence on silica content, increasing by a factor of 15 as 1−x increased from 0.25 to 1.0. For the silica rich compositions, i.e., SiO2 and 1737, Nmax is essentially independent of substrate temperature and the results can be interpreted by a model for nucleation that is purely heterogeneous, and where surface diffusion plays a minimal role. In contrast, on the most silica dilute glass surface (1−x=0.25), Nmax exhibits an Arrhenius temperature dependence with an apparent activation energy of 1.1 eV. Coupled with the observation of a broader island size distribution on this surface, we conclude that surface diffusion plays a role in nucleation and growth on this silica dilute surface, possibly via Ostwald ripening.

Reaction of Si with HCl to Form Chlorosilanes : Time Dependent Nature and Reaction Model

We propose a chemical vapor deposition (CVD) process with closed gas recycling for making low-cost, crystalline silicon thin films for solar cells, which connects chlorosilane synthesis from Si and HCl with Si thin-film growth by CVD from chlorosilanes. In this work we studied the formation of chlorosilanes by the reaction of Si with HCl at temperatures ranging from 623 to 723 K. The reaction rate is time dependent, and many pores are formed on the surface of particles after reaction. These pores are active sites for chemical reactions, and the reaction rates increase with increasing pore area. The rate can be correlated with the conversion ratio of Si, and the temporal evolution of the reaction rate can be explained by a reaction model called the shrinking-core model with growing pores. By using this model, we estimated the reaction rates per unit area of activated surfaces and converted them into a rate equation that can be used for the reactor design. The incubation time of the reaction can be shortened by pretreating the Si particles in a fluidized bed, which probably creates defects in the native oxide layers on the particles, which in turn become reactive sites. © 2004 The Electrochemical Society. All rights reserved.

Fabrication of resonant-tunneling diodes by Sb surfactant modified growth of Si films on CaF/sub 2/Si

Molecular beam epitaxy growth of Si thin films on CaF/sub 2//Si(111) substrates has been studied. A surfactant-modified solid-phase epitaxy method, where the room temperature Si deposition was followed by annealing under Sb flux, resulted in a continuous, smooth epitaxial crystalline Si film with a sharp (/spl radic/3/spl times//spl radic/3)R30/spl deg/ reconstruction and a surface roughness of 0.15-nm rms for a 2.8-nm Si thin film. This growth technique was used to fabricate CaF/sub 2//Si/CaF/sub 2/ double-barrier resonant tunneling diodes in SiO/sub 2/ windows patterned on Si(111) substrates. A negative differential resistance (NDR) peak was found at /spl sim/0.35 V at 77 K, and the current density at the NDR peak was estimated to be 3-4 orders of magnitude higher than in earlier reports.

Surfactant enhanced growth of thin Si films on CaF 2/Si(1 1 1)

MBE growth of Si thin films on CaF 2/Si(1 1 1) has been studied by various growth techniques with and without surfactant. In both cases high temperature depositions results in a growth of 3D Si clusters and a large discrepancy between the nominally evaporated and the actually deposited amount of Si on the CaF 2 surface. Without surfactant, the deposition at room temperature with a subsequent annealing steps improved the quality of the Si epilayers but was accompanied by the formation of holes and trenches in the Si film. A surfactant-modified epitaxy method, where the room temperature Si deposition was followed by annealing under a flux of the surfactant Sb, resulted in continuous, smooth and epitaxial crystalline Si film on CaF 2, with a sharp ( 3 x 3 )R30° reconstruction. Although the microcrystalline structure is not yet perfect the quality of the layers is much better than that of all films prepared by the other employed growth processes.

Comparison of low temperature growth of Si thin films on amorphous substrates by MBE and PECVD methods

ABSTRACTLow temperature growth of thin crystalline Si films on amorphous glass substrates by Molecular Beam Epitaxy (MBE) as well as by two different plasma enhanced chemical vapour deposition (PECVD) systems both using a DC plasma was carried out and the mictrostructural properties were analysed. In particular, the control over the texture along [220] orientation and the resulting columnar nature of the films were studied. The TEM cross section of Si films grown by MBE shows grains with an average width of 750 nm and a length of 3 microns. Using the low energy PECVD (LEPECVD) technique microcrystalline Si films were obtained with a growth rate of up to 35Å/sec. For high quality Si films with larger grains one may grow Si films by MBE whereas for Si films with passivated grain boundaries and for increased deposition rate, it is preferred to grow Si films by the newly developed methods of DC-PECVD.

Molecular-dynamics simulation of low-temperature growth of silicon films by cluster deposition

Silicon thin-film growth from cluster beams at a substrate temperature of 300 K has been investigated with molecular-dynamics simulations utilizing the Stillinger-Weber two- and three-body interaction potential. The spreading of Si-atom clusters and the structure of grown films have been studied as a function of the incident cluster velocity. Our simulation results show that the films grown at a low substrate temperature of 300 K are amorphous and the substrates suffer heavier damage with an increase in the cluster velocity. As compared with our previous results on Si thin-film growth at a substrate temperature of 1000 K, we found that substrate temperature and cluster velocity had a significant impact in determining the structure of the grown films and the cluster spreading on the substrate.

Excimer-Laser-Induced Lateral-Growth of Silicon Thin-Films

A new excimer-laser crystallization method called the “gradient method", has been developed for large-grain growth of Si thin-films on glass. The method is based on a spatial modulation of an incident light intensity, which triggers the lateral grain growth. Grains of size as large as 5 µm were grown by a single shot irradiation at a substrate temperature of 500°C. By combining a step motion of the sample and the proposed method, the grain could be enlarged drastically.

Low-Temperature (450 °C) Poly-Si Thin Film Deposition on SiO2 and Glass using a Microcrystalline-Si Seed Layer

ABSTRACTA low-temperature (450 °C), remote plasma-assisted CVD process for deposition of poly-Si thin films on SiO2 and Corning 7059 glass in which interface formation is separated from bulk film growth has been developed. This approach is based on first depositing an ultra-thin (&lt;100 Å) microcrystalline-Si seed layer onto the oxide in order to provide nucleation sites at which low-temperature poly-Si film growth can be initiated. Conditions for poly-Si film deposition were optimized by using a low-temperature, remote plasma process that had previously yielded epitaxial growth of Si thin films on crystalline Si substrates. Microstructural characterization was performed on poly-Si films grown with different seed layer thicknesses, and additionally with exposure of this seed layer to a predeposition hydrogen plasma treatment. Results demonstrated that the seed layer thickness and surface morphology played a significant role in promoting crystallinity in the poly-Si overlayer. For example using deposition conditions that yielded epitaxial film growth on Si substrates, films deposited on un-seeded oxide substrates were amorphous, whereas those deposited using a seed layer were polycrystalline. This indicated that interfacial nucleation was the rate limiting step in promoting the low-temperature deposition of poly-Si thin films.

Study of effect of SiH4 gas heating during growth of hydrogenated microcrystalline silicon on SiO2 by plasma-enhanced chemical-vapor deposition

The effect of SiH4 source gas heating on the properties of hydrogenated microcrystalline silicon film grown by plasma-enhanced chemical-vapor deposition was investigated to improve the crystallinity and inhomogeneities at the early stage of growth on an amorphous substrate such as glass. Optimization of the deposition conditions for μc-Si:H film structure and characteristics was carried out for a film around 1000 Å thickness and as a function of the cathode heating temperature Tc. The grazing incidence x-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, and UV-visible spectroscopic ellipsometry results showed that the SiH4 gas heating significantly improved the crystallinity and inhomogeneities from the early stage of Si thin film growth at Tc≳550 °C and Ts of 180 °C condition. The role and effect of the cathode heating in the μc-Si:H growth is discussed.

A 3-D Kinetic Monte Carlo Study of the Growth of Si Thin Films Using Beams With Varying Angle of Incidence

AbstractThree-dimensional Kinetic Monte Carlo simulations were used to model the deposition of a hyperthermal molecular beam at varying angles of incidence. The simulations incorporate incident atom-substrate interactions, many layer growth, and attempt to mimic deposition and growth of Si on a Si(100) substrate. At high angles of incidence, the formation of “flake”-like structures are seen. The growth of the flakes follow the same general trends as previous two-dimensional simulations where non-local shadowing produces a porous thin film and the columns grow into the incident beam. Increased substrate temperature increased the widths of the flakes perpendicular to the path of the beam and allowed the grown film to remain defect-free for a time before columnar structure begins.

A supersonic molecular beam study of the chemisorption of PH3 on the Si(100) surface

Abstract The dissociative chemisorption of phosphine, PH 3 , on the Si(100) surface has been examined with supersonic molecular beam scattering techniques. The effect of phosphorus adatoms, P(a), formed by the dissociative chemisorption of PH 3 , on the reaction of SiH 4 and Si 2 H 6 with Si(100) has also been characterized. All reactions have been examined at substrate temperatures characteristic of steady-state Si thin film growth, i.e. T s ⩾ 450°C. The reaction probability, S R , for PH 3 on Si(100) decreases with increasing substrate temperature and the kinetic energy of the incident molecule, suggesting that chemisorption proceeds via a trapping precursor-mediated dissociation channel. The dependence of PH 3 dissociative chemisorption on the fractional coverage of P(a) and H(a) has been deduced under conditions where the desorption of H 2 and P 2 are finite. The dependence of the dissociative chemisorption of both SiH 4 and Si 2 H 6 on the coverage of P(a) has also been determined. For reaction conditions under which P(a) is the dominant surface species, the reaction probabilities of PH 3 , SiH 4 and Si 2 H 6 are proportional to the quantity 1 − θ 2 P , where θ P is the fractional coverage of the adsorbed phosphorus atoms. In addition, the presence of phosphorus adatoms has also been found to have a significant influence on the reaction pathway of Si 2 H 6 on Si(100). Our results have been employed to formulate a predictive model for the kinetics of Si thin film growth in presence of PH 3 (g).

Preparation of Crystalline Si thin Films by Spontaneous Chemical Deposition

AbstractThe novel technique for the preparation of crystalline Si thin films termed “Spontaneous Chemical Deposition” has been proposed, in which silane decomposes spontaneously by gas phase reactions with fluorine at reduced pressure. The technique provided us the crystalline films in a wide range of the preparation conditions by a choice of the external parameters for the reactions. The films exhibited unique characteristics in the chemical structure and the optical and electrical properties, different from the conventional uc-Si:H thin films by the silane plasma processes.The technique have been successfully applied for the homoepitaxial growth of Si thin films.