Epitaxial Silicon Films Research Articles

Numerical Analysis of Thickness Uniformity Improvement with Different Inlet Plate Numbers during APCVD Process

As the nanometer line width in the semiconductor industry further shrinks, the surface flatness requirements for atmospheric pressure chemical vapor deposition epitaxial silicon films have further increased. Through a series of numerical simulations, this study analyzes the use of inlet plates to solve the problem of uneven wafer thickness during atmospheric pressure chemical vapor deposition. The results show that without the inlet plate, the deposition rate is lower at the center of the wafer and higher at the edge of the wafer. Wafer epitaxy thickness uniformity can be improved when inlet plates are used. By adjusting the position of the inlet plates, this unevenness can be further improved. As the number of inlet plates increases from 0 to 3, the wafer thickness non-uniformity decreases from 12.5% to 1.8% respectively. When the number of inlet plates continues to increase, the thickness uniformity of the wafer can be further improved. When the number of inlet plates is 5, the thickness unevenness of the wafer reaches 0.7%.

Study of interfacial design for direct-current tribovoltaic generators

As a newly discovered physical phenomenon, tribovoltaic effect has potential applications in various fields. Herein, we systematically study the relationship between interfacial properties of the tribovoltaic generator and its output performance using a conductor-semiconductor (C-S) structure. Based on several similar physical models, we derive an equation to quantify the effects of contact area, load force and the density of surface states of the semiconductor on performance of the tribovoltaic generator. The experimental results are in good agreements with the theoretical analysis. Moreover, the contribution of interfacial electric field and interfacial magnetic field to the tribovoltaic effect are discussed, respectively. By using epitaxial silicon (Si) film, the built-in electric field near the C-S interface can be regulated, which leads to suppression of the recombination effect of the tribovoltaic charges, and the short-circuit current of the tribovoltaic generator can be increased by 66.83%. This work paves the way for electrode design and better output performance of flexible DC tribovoltaic generator in the future.

Comparative Study on the Quality of Microcrystalline and Epitaxial Silicon Films Produced by PECVD Using Identical SiF4 Based Process Conditions.

Hydrogenated microcrystalline silicon (µc-Si:H) and epitaxial silicon (epi-Si) films have been produced from SiF4, H2 and Ar mixtures by plasma enhanced chemical vapor deposition (PECVD) at 200 °C. Here, both films were produced using identical deposition conditions, to determine if the conditions for producing µc-Si with the largest crystalline fraction (XC), will also result in epi-Si films that encompass the best quality and largest crystalline silicon (c-Si) fraction. Both characteristics are of importance for the development of thin film transistors (TFTs), thin film solar cells and novel 3D devices since epi-Si films can be grown or etched in a selective manner. Therefore, we have distinguished that the H2/SiF4 ratio affects the XC of µc-Si, the c-Si fraction in epi-Si films, and the structure of the epi-Si/c-Si interface. Raman and UV-Vis ellipsometry were used to evaluate the crystalline volume fraction (Xc) and composition of the deposited layers, while the structure of the films were inspected by high resolution transmission electron microscopy (HRTEM). Notably, the conditions for producing µc-Si with the largest XC are different in comparison to the fabrication conditions of epi-Si films with the best quality and largest c-Si fraction.

Effects of dopant concentration on microstructure and strain states of in-situ phosphorus-doped epitaxial silicon films during dry oxidation

The adjustability of the incorporation and distribution of P dopants in Si materials has led to their application in various electronic and photovoltaic devices. In particular, P-doped Si films have been investigated for dopant-induced phenomena such as oxidation enhancement and dopant segregation during thermal oxidation. Despite their technical as well as scientific importance of dopant behaviors in P-doped Si films, these were hardly observed due to low P incorporation. In this study, we prepared and oxidized P-doped epitaxial Si (Si:P) films grown on the Si(100) substrate using in-situ P-doped deposition technique to incorporate high P concentration up to 9.6% (4.8 × 1021 cm−3) in the films, exceeding solid solubility limits (~3 × 1020 cm−3). We found that in oxidized samples with P concentration greater than 6.2%, defects were generated in the near-surface region of Si:P layers and segregated P was observed at the defected areas. The chemical bonding states of P at the oxidized Si:P film increased in the P 2p peak intensity at the interface between the oxide and the Si:P layer, indicating P segregation at this interface. In the strain states of Si:P films, the decrease in the out-of-plane lattice parameter was observed, while maintaining the constant in-plane lattice parameter, implying strain release due to decreased P concentration in the Si:P layer after dry oxidation.

Defect Generation Mechanism of Epitaxially Grown In Situ Phosphorus‐Doped Silicon on Silicon (111) Substrate

There are many studies regarding improving the quality of in situ phosphorus‐doped (ISPD) epitaxial silicon films as a source and drain material to achieve low specific contact resistivity. The top surfaces of these films, contact interface, in 3D structure devices such as Fin field‐effect transistor and gate‐all‐around field‐effect transistor are grown from diamond‐shaped source and drain with four edges of the (111) surfaces. Therefore, the physical properties of these sources and drains in a 3D structure device are strongly related to those of ISPD silicon grown on the (111) substrate. Here, ISPD silicon is grown using reduced pressure chemical vapor deposition on a silicon (111) substrate at 700 °C, and this film is analyzed using high‐resolution transmission electron microscopy, atom probe tomography, and energy‐dispersive X‐ray spectroscopy (EDX). The results show that this film exhibits many defects such as twin or stacking fault, and phosphorus exhibits cluster distribution. To confirm the relationship between phosphorus and defect generation, an atomic resolution EDX mapping of phosphorus atoms is used, and the position of the observed defects corresponds with the clustered phosphorus in a mapping image.

Dopant Activation of In Situ Phosphorus‐Doped Silicon Using Multi‐Pulse Nanosecond Laser Annealing

In situ phosphorus‐doped epitaxial silicon films have attracted significant attention as source and drain materials because low specific contact resistivities have been achieved on such films by increasing the active carrier concentration using millisecond laser annealing. However, the active phosphorus concentration that can be achieved using millisecond laser annealing is much less than the incorporated concentration. To increase the activation efficiency, nanosecond laser annealing with a dwell time ≈104 times shorter than that of millisecond laser annealing is investigated and the diffusion, strain, microstructure, and electrical properties of single‐ and multipulse nanosecond laser‐annealed samples are examined. The melting depth simulation classifies the energy density regions and explains the limited diffusion in nanosecond laser annealing. After multipulse nanosecond laser annealing, more phosphorus is activated without diffusion than by millisecond laser annealing. Moreover, almost all the incorporated phosphorus atoms are activated by the nanosecond laser, which melts in situ phosphorus‐doped epitaxial silicon films without major strain loss. The increased active carrier concentration presents an opportunity to achieve low contact resistivity characteristics.

Имплантация ионов кремния в сапфир: малые дозы

As a result of ion implantation of silicon ions in sapphire and subsequent high-temperature annealing the precipitates of silicon and alumosilicates are observed in the subsurface sapphire region. X-ray reciprocal space mapping measurements showed the existence of compression stress with deformation -0.12% in the normal direction and tensile stress with deformation 0.2% in R-plane in that region, which reduces the mismatch with the silicon (100) lattice and can lead to an improvement in the structural perfection of epitaxial silicon films grown on such modified sapphire substrates.

Transmission electron microscopy characterization of low temperature boron doped silicon epitaxial films

Transmission electron microscopy techniques to better understand growth mechanisms and annealing of low temperature silicon epitaxy. HRTEM: thickness measurement, crystal morphology, and defect study. GPA (image processing): strain field analysis.

Chemical bonding states and dopant redistribution of heavily phosphorus-doped epitaxial silicon films: Effects of millisecond laser annealing and doping concentration

We investigated the effect of millisecond (ms) laser annealing and doping concentration on the chemical bonding states and dopant behaviors of P-doped epitaxial Si (Si:P) layers grown on Si (1 0 0) substrates using high-resolution X-ray photoelectron spectroscopy (HR-XPS), secondary-ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) measurements. Our XPS results showed that the intensities of P 2p peaks for Si:P films were increased with P concentration and subsequent laser annealing. From the SIMS and AES measurement results, we found that P atoms were slightly accumulated at the near-surface region of the Si:P film by the laser annealing, while macroscopic P concentration being maintained in the whole Si:P films without significant diffusion of P atoms toward the Si (1 0 0) substrate. In addition, we performed ex-situ HF cleaning on the as-grown and laser-annealed Si:P films in order to precisely measure the change in chemical states and dopant distribution at the near-surface region. The intensities of the P 2p peak in the as-grown Si:P films were increased after the HF cleaning due to the removal of native oxide layers from the Si:P films. In contrast, the decrease in P 2p peak intensities was observed in the laser-annealed Si:P films after the HF cleaning, indicating the dopant loss from the near-surface region with native oxide removal.

Study of the Effect of Technetium as a Component of the Catalysts of the REE Compounds on the Process of Decomposition of Monohydrides of Silicon and Germanium

The effect of technetium as a component of the catalysts of the REE compounds on the decomposition process of monohydrides of silicon and germanium during the growth of epitaxial films of silicon and a silicon-germanium solid solution is found. It is demonstrated that due to the fact that the unit cell radius of technetium is 1.358 A and is similar to the unit cell radius of the platinum-group of elements (1.37–1.38 A), as a component of the catalysts of REE compounds, it delivers the stability of the specific catalytic activity and high level of selectivity of the decomposition of monosilane and monogermane into $${\text{SiH}}_{2}^{{{\text{**}}}}$$ and $${\text{GeH}}_{2}^{{{\text{**}}}}.$$ This in turn allows carrying out the process of growing epitaxial films in two stages and, in this way, improving their quality.

Effect of Bias Applied to the Substrate on the Low Temperature Growth of Silicon Epitaxial Films during RF-PECVD

The deposition behavior of silicon films by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) was studied by nonclassical crystallization, where the building block of deposition is charged nanoparticles generated in the gas phase of the reactor. To confirm the existence of nanoparticles in the RF-CVD reactor, nanoparticles were captured on the membrane of the transmission electron microscope (TEM) grid under the condition of the DC bias applied to the holder. The capturing behavior of nanoparticles depended on the bias and the conductivity of the membrane. Also, to examine the effect of the bias on the epitaxial growth, films were deposited on the silicon wafer substrate under the condition of the biases of 0, +1000, and −1000 V applied to the substrate holder. A fully epitaxial film could be grown on a silicon wafer at 550 °C under the bias of −1000 V.

Silicon epitaxial growth accelerated by parallel Langmuir processes using SiH2Cl2 and SiH3CH3 gases

The silicon epitaxial growth behaviour was studied as an application of the parallel-Langmuir process using SiH2Cl2 gas and SiH3CH3 gas. The SiH3CH3 gas was used for producing the SiHx gas in situ by thermal decomposition in the reactor. With the increasing gas concentration of SiH3CH3, several results were obtained: (i) the silicon epitaxial growth rate increased exceeding the value saturated using the SiH2Cl2, (ii) the gas phase concentrations of the chlorosilanes at the exhaust decreased, (iii) the byproduct deposition at the exhaust decreased, and (iv) the gas phase concentration of HCl at the exhaust decreased. We conclude that the SiHx helped consume the SiH2Cl2 gas for the production of a silicon epitaxial film with reductions in the byproduct deposition. Additionally, SiHx might produce SiH2Cl2 in the gas phase by reaction with HCl gas.

Formation of Epitaxial Heavy-doped Silicon Films by PECVD Method

Plasma-enhanced CVD (PECVD) epitaxy at 200 °C was used to deposit heavy doped n-type silicon films. Post-annealing by rapid thermal processing was applied to improve the properties of the epitaxial layer. By analyzing the Raman spectra and the imaginary part of the dielectric constant spectra of the samples, it was found that high-quality heavy-doped epitaxial n-type silicon layer can be obtained by optimizing the parameters of the PECVD depositing process. Reducing the electrodes distance of the PECVD had a great effect on the crystallzation of the epitaxialed n-type silicon films. Sillicon films with high-crystallization were obtained with the electrodes distance of 18 mm. Post-annealing process can improve the crystallization and reduce the resistance of the epitaxial films. In our research, it was found that the sheet resistance (R□) of the post-annealed films with thickness of about 50 nm has a simple relationship with RPH3/SiH4 (ratio of the flow rate of PH3 and SiH4) of the PECVD processing: R□=-184-125 lg(RPH3/SiH4). In the end, high-quality epitaxial n-type silicon film was obtained with R□ of 15 Ω/□ and thickness of ~50 nm.

A new method for Synthesis of Epitaxial Films of Silicon Carbide on Sapphire Substrates (α-Al2O3)

Abstract A fundamentally new method for synthesis of thick (200-400 nm) epitaxial films of silicon carbide on single-crystal Al2O3 substrates is proposed. The method develops the previously discovered method of topochemical substitution of atoms during the transformation of Si into SiC. In the proposed method, a layer of epitaxial Si of orientation (100) or (111) is being preliminarily deposited on the sapphire via CVD or other technique. Then, using a topochemical reaction between the CO gas and the deposited epitaxial silicon film, a part of Si atoms is being substituted by carbon atoms. As a result, the silicon film transforms into SiC film. As experimental studies have shown, in the process of this transformation a high-quality epitaxial SiC layer can be formed on the sapphire surface under certain synthesis conditions. A detailed investigation of grown SiC layers using the methods of electron diffraction, ellipsometry, Raman spectroscopy, X-ray diffractometry and scanning microscopy is conducted and an analysis of experimental data is given. Formation of both cubic 3C-SiC and hexagonal SiC polytypes during the transformation of Si into SiC was revealed by the XRD method. A detailed analysis of the current state of the problems of growth of epitaxial SiC layers on Al2O3 using various methods is provided. The method developed in this study opens up completely new perspectives not only for synthesis of semiconductors, but also for creation of new classes of composite, heat-resistant and other solid coatings.

Chemical state analysis of heavily phosphorus-doped epitaxial silicon films grown on Si (1 0 0) by X-ray photoelectron spectroscopy

In this work, we investigated the chemical bonding states in highly P-doped Si thin films epitaxially grown on Si (001) substrates using high-resolution X-ray photoelectron spectroscopy (HR-XPS). HR-XPS P 2p core-level spectra clearly show spin-orbital splitting between P 2p1/2 and P 2p3/2 peaks in Si films doped with a high concentration of P. Moreover, the intensities of P 2p1/2 and P 2p3/2 peaks for P-doped Si films increase with P concentrations, while their binding energies remained almost identical. These results indicate that more P atoms are incorporated into the substitutional sites of the Si lattice with the increase of P concentrations. In order to identify the chemical states of P-doped Si films shown in XPS Si 2p spectra, the spectra of bulk Si were subtracted from those of Si:P samples, which enables us to clearly identify the new chemical state related to SiP bonds. We observed that the presence of the two well-resolved new peaks only for the Si:P samples at the binding energy higher than those of a SiSi bond, which is due to the strong electronegativity of P than that of Si. Experimental findings in this study using XPS open up new doors for evaluating the chemical states of P-doped Si materials in fundamental researches as well as in industrial applications.

Reflector Influence on Rapid Heating of Minimal Manufacturing Chemical Vapor Deposition Reactor

A small-sized reactor for producing a silicon epitaxial film on a half-inch silicon wafer was studied, taking into account the heat transport near the wafer. The wafer temperature slowly changed over a long time period when the reflector was made of thick plates. In contrast, when thin plates were employed as the reflector material, the wafer temperature quickly increased and easily reached a steady state. Thus, the reactor parts set near the wafer should be small, slim and thin. With the help of wafer rotation and a highly heat-conductive susceptor, a symmetrical and uniform silicon epitaxial film thickness profile could be obtained.

Porous silicon reorganization: Influence on the structure, surface roughness and strain

Porous silicon and epitaxial thickening is a lift-off approach for silicon foil fabrication to avoid kerf losses and produce foils with thicknesses less than 50µm. The crystal quality of the epitaxial silicon film strongly depends on the porous silicon template, which can be adapted through a reorganization process prior to epitaxy. In this work, we investigated the influence of reorganization on the structure of etched porous silicon layers. The reorganization processes were carried out in a quasi-inline Atmospheric Pressure Chemical Vapor Deposition reactor. Variations on the temperatures and process durations for the reorganization step were examined. The cross-sections showed that porous silicon requires temperatures of approximately 1150°C to produce an excellent template for epitaxy. Atomic Force Microscopy measurements on the samples annealed at different temperatures showed the evolution of the pores from as-etched to a closed surface. These measurements confirm that the surface is not yet closed after 30min of reorganization at 1000°C. Different durations of the reorganization step at a fixed temperature of 1150°C all lead to a closed surface with a comparable roughness of less than 0.5nm. X-ray diffraction measurements show a change in the strain in the porous layer from tensile to compressive when the reorganization temperature is increased from 800°C to 1150°C. A longer reorganization at a fixed temperature of 1150°C leads to a reduction in the strain without reducing the quality of the surface roughness. Defect density measurements on silicon layers deposited on those templates confirm an improvement of the template for longer reorganization times. This study shows that our porous silicon templates achieve lower surface roughness and strain values than those reported in other publications.

A New Electrodeposition Process of Crystalline Silicon Utilizing Water-Soluble KF–KCl Molten Salt

As a new production process of silicon solar cells, electrodeposition in high temperature molten salts is worth investigating. One of the most studied molten salt systems for electrodeposition of silicon is LiF-KF or LiF-NaF-KF containing K2SiF6. Many important works regarding this molten salt were undertaken in 1970s and 1980s [1-4]. Cohen reported that both epitaxial and polycrystalline continuous films of compact, coherent and well-adherent silicon were obtained from LiF-KF containing K2SiF6 at 1023 K [2]. One of the problems in the previous works is the removal of the adhered salt on the deposited silicon. This is due to the very low solubility of LiF and NaF to water. Another problem of the previous studies, when aiming at the solar grade purity, is the availability of high purity and low cost silicon ion sources. Conventionally, either K2SiF6 or anode dissolution of Si rod was utilized. However, it is difficult to prepare K2SiF6and Si rod having solar grade purity with low cost.From this background, we propose a new electrodeposition process of crystalline silicon, as shown in Fig. 1. Here, fluoride–chloride molten salt is used as an electrolyte and SiCl4 as a silicon ion source. Among many kinds of fluoride–chloride molten salts, we especially focus on the eutectic KF–KCl (45:55 mol%) molten salt. Since both KF and KCl are highly soluble to water, the adhered salt can be easily removed by water washing. Moreover, high purity SiCl4, which is produced with low cost, can be used as an ion source. When gaseous SiCl4is introduced into the molten KF–KCl, the following reaction proceeds:SiCl4 + 6F− → SiF6 2− + 4Cl− (1)Thus, after the introduction of SiCl4 gas, the resultant molten salt is same as the molten KF–KCl containing K2SiF6. Another advantage of the use of fluoride–chloride molten salt is the capability of Cl2 gas evolution on the carbon anode, which realizes the fixed composition of molten salt as well as the reuse of Cl2 for the production of SiCl4.Firstly, electrochemical reduction of Si(IV) ion on a silver electrode was investigated by cyclic voltammetry in a molten KF–KCl–K2SiF6 system at 923 K. Then, the optimum conditions were explored for electrodepositing adherent, compact and smooth silicon layer. Galvanostatic electrolysis was conducted for silver electrodes with various current densities and K2SiF6 concentrations. As typical results, Fig. 2 shows cross-sectional SEM images of the samples electrodeposited at (a) 38.8, (b) 155 and (c) 310 mA cm-2 in 2.0 mol% K2SiF6. A compact and smooth Si layer was obtained at an intermediate current density of 155 mA cm-2. At a lower current density of 38.8 mA cm-2, detachment of Si layer from the Ag substrate occurred during the water washing. At a higher current density of 310 mA cm-2, the morphology changed into nodular.AcknowledgementsThis study was financially supported in part by the CREST program by Japan Science and Technology Agency (JST).

Full CVD Reel‐To‐Reel Process to Obtain Perfectly Oriented Silicon Films on Metal Tapes with Oxide Buffer Layers**

Silicon films with a sharp biaxial texture on low‐cost, flexible metal tapes are prominent materials for cost‐effective photovoltaics. The cost of such materials can be further reduced by the application of easily scalable chemical deposition methods. In the present article, we report on the application of CVD to obtain epitaxial silicon films on Ni alloy tapes with metal‐organic (MO)CVD‐produced buffer layers. Two types of buffer layer architecture are presented, which enable textured silicon growth on textured Ni(Cr,W) alloy and on non‐textured Hastelloy tape. The Si film appears highly textured and demonstrates chemical purity, indicating the possibility of application of proposed Si/buffer/metal heterostructures formed by CVD for the photovoltaic industry.

Large thickness-dependent improvement of crystallographic texture of CVD silicon films on R-sapphire

Thin (80–800nm) epitaxial silicon films on sapphire substrates were deposited via CVD technique and studied with XRD, SEM, EDS, EBSD, HRTEM and AFM methods. Increase of grain size, reduction of microtwin concentration and strong sharpening of crystallographic texture with increasing film thickness was observed. To our knowledge, XRD texture analysis in relation to film thickness of silicon on sapphire samples was performed for the first time. Thickness-dependent behavior of texture quality and film microstructure can be explained by the model of evolutionary selection and disclination formation.