Growth Of Silicon Layers Research Articles

In situ spectroscopic ellipsometry study of low-temperature epitaxial silicon growth

Low-temperature growth of doped epitaxial silicon layers is a promising way to reduce the cost of p-n junction formation in c-Si solar cells. In this work, we study process of highly doped epitaxial silicon layer growth using in situ spectroscopic ellipsometry. The film was deposited by plasma-enhanced chemical vapor deposition (PECVD) on a crystalline silicon substrate at a low substrate temperature of 200 °C. In the deposition process, SiF4 was used as a precursor, B2H6 as doping gas, and a hydrogen/argon mixture as carrier gas. A spectroscopic ellipsometer with a wide spectral range was used for in situ spectroscopic measurements. Since the temperature during process is 200 °C, the optical functions of silicon differ from these at room temperature and have to be adjusted. Thickness of the epitaxial silicon layer was fitted on in situ ellipsometric data. As a result we were able to determine the dynamics of epitaxial layer growth, namely initial layer formation time and epitaxial growth rate. This study opens new perspectives in understanding and monitoring the epitaxial silicon deposition processes as the model fitting can be applied directly during the growth.

Microwave plasma assisted process for cleaning and deposition in future semiconductor technology

The epitaxial growth of silicon layers is an important step in the fabrication of semiconductor devices. For conventional silicon epitaxy, high temperatures, up to 900 °C are necessary. However, in future, semiconductor technology epitaxy processes at lower temperatures are required to increase the integration density. The goal of this study was to investigate microwave plasma assisted processes for the selective removing of thin silicon oxide, the cleaning of silicon surfaces and the depositing of high quality silicon films. The main focus was to apply these processes for low temperature epitaxy. All processes, such as oxide removal, cleaning and deposition, were done in one chamber and with microwave plasma assistance. In order to remove silicon dioxide, the etching behavior of hydrogen, fluorine, and hydrogen/fluorine plasma was studied. It was shown, that with hydrogen/fluorine plasma, the best selectivity of oxide to silicon was reached. The deposition process of silicon was studied by growing μc-Si films. The process was characterized and optimized by spectral ellipsometry. After a successful characterization of all process steps, silicon epitaxy layers have been grown with in-situ removal of native oxide and in-situ surface cleaning. The temperature for all process steps was reduced below 450 °C.

Design of periodic nano- and macro-scale textures for high-performance thin-film multi-junction solar cells

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/μc-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si:H/μc-Si:H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 μm and vertical peak-to-peak heights around or above 0.3 μm are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 μm and heights around 0.6 μm lead to a >6% gain in short-circuit current of the bottom cell.

Approaches and challenges in optical modelling and simulation of thin-film solar cells

Optical modelling and simulations present an indispensable tool in the design, analysis and optimisation of thin-film solar cells of different technologies. In this paper highlights and challenges of different numerical modelling approaches are reviewed, from one-dimensional to rigorous two- or three-dimensional optical modelling. A concept of Coupled Modelling Approach (CMA) is proposed to be used, in which different optical models are coupled together to achieve best performance in speed and accuracy of simulations. A Combined Ray-Optics Wave-optics Model (CROWM) is presented as a simple example of the CMA. In the second part two examples of modelling and simulations are presented. To demonstrate the applicability of 3-D rigorous optical modelling, results of optimisation of periodic substrate surface texture in a micromorph silicon solar cell are shown. Furthermore, a model of non-conformal layer growth is employed to determine morphologies of internal interfaces and to make a selection of suitable textures for defect-less thin-film silicon layer growth. The CROWM simulator was employed to demonstrate the usability of coupled modelling on the example of optimisation of macro-textures in organic solar cells.

Tailoring the surface morphology of zinc oxide films for high-performance micromorph solar cells

Self-textured zinc oxide polycrystalline films prepared by metalorganic low-pressure chemical vapor deposition combine excellent transparency, conductivity and light-scattering ability when used as electrodes for high-efficiency thin-film silicon solar cells. However, the growth of silicon layers with low defect density, which are necessary for high-performance solar cells, often requires the rough surface morphology of as-deposited zinc oxide films to be smoothened. This is usually achieved by a post-deposition argon plasma-etching treatment. We investigate here an alternative method to modify the surface morphology by changing the zinc oxide growth conditions over only the last hundreds of nanometers of the total film thickness. We discuss two types of zinc oxide cap layers, one grown with the addition of ethanol and the other with an enhanced diethylzinc (DEZ) precursor flow. We show that the presence of either type of cap layer does not alter the layer׳s electrical properties, but does slightly diminish the layer׳s light-scattering ability. The cap layer grown with ethanol leads to a more pronounced leveling of the film texture, while the high-DEZ-grown layer better preserves the sharp features of the underlying ZnO. Finally, zinc oxide films with cap layer are used as front electrodes for tandem amorphous/microcrystalline silicon solar cells, and are compared to rough films treated with an argon plasma. With rising thickness of the capping layers, the efficiency of tandem cells increases reaching values over 12%, and approaches that of cells on Ar plasma treated ZnO films.

Specificity of mono- and disilane decomposition at silicon surface under conditions of epitaxial growth

Basic kinetic parameters of surface hydrogen desorption and of adsorbed silicon hydrides decomposition has been evaluated by kinetic simulation based on data of the technological experiments on silicon layer growth at 450–700°C. For the molecular epitaxial growth, the silicon surface population with silicon hydrides fragments has been estimated. The relationship between the surface hydrides pyrolysis frequency and the rate of silicon atoms incorporation into the growing crystal has been found.

Sapphire surface preparation for the growth of silicon layers by molecular-beam epitaxy

The structural perfection and surface morphology of sapphire substrates vacuum-annealed at atomic Si flux densities from 5 × 1015 to 1 × 1016 at/(cm2 s) have been studied by electron diffraction, X-ray diffraction, and atomic force microscopy. The results demonstrate that, after annealing in the range 1160–1330°C, the sapphire has a smooth, single-crystal surface, which enables the growth of twin-free silicon epilayers at 600–700°C.

Mechanisms controlling the phase and dislocation density in epitaxial silicon films grown from silane below 800 °C

We construct a phase diagram for silicon layer growth on (001) Si by hot-wire chemical vapor deposition (HWCVD), for rates from 10 to 150 nm/min and for substrate temperatures from 500 to 800 °C. Our results show that a mixed mono and dihydride surface termination during growth causes polycrystalline growth; some H-free sites are needed for epitaxy. For epitaxial films (T>620 °C), the dislocation density decreases with increasing growth temperature because of reduced O contamination of the surface. The best HWCVD epitaxial layers have dislocation densities of 105 cm−2.

Resistive interlayer for improved performance of thin film silicon solar cells on highly textured substrate

The deposition of thin-film silicon solar cells on highly textured substrates results in improved light trapping in the cell. However, the growth of silicon layers on rough substrates can often lead to undesired current drains, degrading performance and reliability of the cells. We show that the use of a silicon oxide interlayer between the active area and the back contact of the cell permits in such cases to improve the electrical properties. Relative increases of up to 7.5% of fill factor and of 6.8% of conversion efficiency are shown for amorphous silicon cells deposited on highly textured substrates, together with improved yield and low-illumination performance.

10.1007/s11454-008-2012-3

The method of ultrahigh-vacuum low-temperature (T = 850°C) purification of silicon single crystals having the (100) and (111) orientation and implanted with low-energy (E = 40 keV) iron ions with various doses (Φ = 1015−1.8×1017 cm−2) and subjected to pulsed ion treatment (PIT) in a silicon atom flow has been tested successfully. The formation of semiconducting iron disilicide (β-FeSi2) near the surface after PIT is confirmed for a Si(100) sample implanted with the highest dose of iron ions. The possibility of obtaining atomically smooth and reconstructed silicon surfaces is demonstrated. Smooth epitaxial silicon films with a roughness on the order of 1 nm and a thickness of up to 1.7 μm are grown on samples with an implantation dose of up to 1016 cm−2. Optical properties of the samples before and after the growth of silicon layers are studied; the results indicate high quality of the grown layers and the absence of iron disilicide on their surface.

Epitaxial growth of silicon on silicon implanted with iron ions and optical properties of resulting structures

The method of ultrahigh-vacuum low-temperature (T = 850°C) purification of silicon single crystals having the (100) and (111) orientation and implanted with low-energy (E = 40 keV) iron ions with various doses (Φ = 1015−1.8×1017 cm−2) and subjected to pulsed ion treatment (PIT) in a silicon atom flow has been tested successfully. The formation of semiconducting iron disilicide (β-FeSi2) near the surface after PIT is confirmed for a Si(100) sample implanted with the highest dose of iron ions. The possibility of obtaining atomically smooth and reconstructed silicon surfaces is demonstrated. Smooth epitaxial silicon films with a roughness on the order of 1 nm and a thickness of up to 1.7 μm are grown on samples with an implantation dose of up to 1016 cm−2. Optical properties of the samples before and after the growth of silicon layers are studied; the results indicate high quality of the grown layers and the absence of iron disilicide on their surface.

Convection-assisted chemical vapor deposition (CoCVD) of silicon on large-area substrates

Results obtained from a recently completed chemical vapor deposition (CVD) system for the epitaxial growth of silicon layers on rectangular, large-area substrates of up to 43 cm×43 cm are reported. In contrast to conventional CVD systems, buoyancy convection is not suppressed but utilized to assist and stabilize the gas flow. The new method is particularly suited for large-area deposition at atmospheric pressure, as in such regimes the influence of buoyancy convection tends to emerge and must be taken into account. In deposition experiments growth rates of up to 1.2 μm/min were obtained. Parasitic deposition on the reactor walls was not observed. Operation at different tilt angles of the system was applied in order to clarify the influence of buoyancy convection. Tilting of the system, with respect to a horizontal substrate alignment, resulted in an increase of growth rates and also changed their spatial distribution. At optimum angle a spatial uniformity of layer thickness of ±10% was obtained, without any substrate movement. On epitaxial layers etch pit densities of 0.6×10 3–5×10 3 cm −2 were obtained. The system was investigated by numerical fluid dynamic simulations. The temperature distribution, the gas flow, and the distribution of growth rates were calculated at different tilt angles. The simulated growth rates are in good agreement with experimental results.

Characterization of stress induced in SOS and Si/γ-Al 2O 3/Si heteroepitaxial thin films by Raman spectroscopy

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si/γ-Al 2O 3/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si/γ-Al 2O 3/Si thin film is much smaller than that of as-grown SOS and SPE upgraded SOS films. The stress decrease for double heteroepitaxial film Si/γ-Al 2O 3/Si mainly arises from the smaller lattice mismatching of 2.4% between silicon top layer and the γ-Al 2O 3/Si epitaxial composite substrate, comparing with the large lattice mismatch of 13% for SOS films. It indicated that γ-Al 2O 3/Si as a silicon-based epitaxial substrate benefits for reducing the residual stress for further growth of silicon layer, compared with on bulk sapphire substrate.

Hydrogen Desorption from the Surface under the Conditions of Epitaxial Growth of Silicon Layers from Monosilane in Vacuum

For the first time, the desorption coefficient of hydrogen and activation energy of desorption are directly determined during the growth of an Si epitaxial layer from silane in vacuum using experimental data on the degree of surface coverage of an Si wafer with hydrogen. The results obtained are compared with earlier results obtained under low-temperature conditions, for example, by thermodesorption spectrometry. The found values are used to calculate the crystallization coefficient and its dependence on the growth temperature and decomposition rate for monosilane at the growth surface.

Porous Silicon Formation under Constant Anodization Conditions: Homogeneous Regime or Transition?

This work studies the behavior of the porous silicon layer growth when varying wafer resistivity (p-type) and current density. It shows that under a constant anodizing treatment the porous layer formation regime could be unstable and a transition from a homogeneous nanoporous layer to a filled macroporous layer could occur. The conditions for which the transition takes place are clearly established and it is proved that a slight experimental change modifies drastically the morphology of the porous layer. A low current density and a high wafer resistivity favor the transition but it influences neither the dissolution valence nor the porosity. The transition behavior is compared with reported results and is discussed on the basis of the proposed models for macropore formation. It demonstrates that a complete interpretation of the transition, which leads to the production of macroporous structure, needs to take into account not only the physical and chemical properties of the system but also the configuration of the preparation cell. © 2003 The Electrochemical Society. All rights reserved.

Quasi-Epitaxial Growth of Silicon Layers by Hydrogen Reactive Magnetron Sputtering at Temperatures as Low as 200 °C

Quasi-Epitaxial Growth of Silicon Layers by Hydrogen Reactive Magnetron Sputtering at Temperatures as Low as 200 °C

Silicon avalanche photodiodes for particle detector:: modelling and fabrication

Large-area arrays (30×30) of metal/resistive layer/silicon (MRS) avalanche photodiodes as 150×150 μm sub-pixels were developed and fabricated to detect short-wavelength scintillator signals for high-energy particle detection. Modelling of MRS photodiodes was performed using McIntyre's approach of local electric field to optimize semiconductor doping profiles and resistive layer parameters and to obtain the minimum value of effective k-factor (the holes to electrons ionization coefficients ratio) less than 0.01 under low excess noise factor and high gain. The resistive layer/silicon surface barrier suppresses minority carriers injection to decrease dark current and effective k-factor. Test samples of silicon avalanche photodiodes arrays have been fabricated using low-rate epitaxial growth of silicon layer, impurity doping, and resistive layer deposition processing. The integral gain for experimental specimens was ∼100.

Amorphous-Porous Silicon Heterojunction for Gas Sensor Application

In this work we studied the compatibility of amorphous thin film technology with the porous silicon in case of large area applications like gas sensors. We found that thin amorphous silicon layer growth on porous silicon substrates preserves the room temperature photoluminescence of porous material. Also amorphous–porous silicon heterojunction shows a good rectifying behaviour. Those properties, correlated with the porous silicon sensitivity to the gas environments, can be effectively used to realise heterojunction diodes whose current voltage characteristics are modified by the gas reactivity on the porous surface. Sensitivity of those devices and response time to different gas exposure have been investigated.

High concentration n-type doping in Si layers epitaxially grown by ultra-high vacuum chemical vapor deposition with cracking heater

A novel growth technique for high concentration n-type doping in silicon layers is demonstrated. An extra heater to decompose source molecules is added to a conventional ultra-high vacuum chemical vapor deposition system. The growth of silicon layers occurs at a low substrate temperature of 500°C, where the segregation of dopant atoms is minimal. A doping concentration as high as 1 × 10 20 atoms/cm 3 was achieved with arsine molecules as the dopant source. An abrupt doping profile of a 3 decade variation in less than 10 nm is also achieved.

Atomic Force Microscopy Investigation of Tensile-Stressed Silicon Grown by Rapid Thermal Chemical Vapour Deposition on Si0.68Ge0.32 Relaxed Pseudo-substrates

Rapid thermal chemical vapour deposition (RTCVD) has been applied to the growth of silicon layers under tensile stress on relaxed Si0.68Ge0.32 buffer layers at 610 and 810°C. Their surface morphology was characterised with atomic height resolution by Tapping-mode atomic force microscopy (TM-AFM). While a uniform isotropic nanoroughness is revealed on pseudo-substrates, a quite different roughness can be observed for silicon films. For high temperature grown films, high surface diffusion rates of adsorbed species enable an extended reconstruction of the Si (100) surface: large domains showing terraces as well as faceted planes appear; the resulting non-uniform silicon film exhibits early mechanical failure. In contrast, low temperature grown Si films exhibit almost the same morphology as pseudo-substrates for thicknesses up to 15 nm with no apparent mechanical failure. For thicknesses beyond 20 nm, terraces form on the steepest part of the slopes, whereas beyond 80 nm <110> slip lines indicate the plastic yield of the film.