Molecular beam and solid-phase epitaxies of silicon under ultra-high vacuum

Low temperature growth of silicon epitaxial layers has been performed by two methods: by molecular beam epitaxy and by solid-phase crystallization of amorphous films deposited onto crystalline substrates under ultra-high vacuum.     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Modular design of locally ordered tetrahedral structures of Si,SiO<Subscript>2</Subscript>, and H<Subscript>2</Subscript>O: I. investigation of the possible cooperative local crystallization in amorphous matrices of these compounds

The hypothesis is put forward that the structure of amorphous silicon and other amorphous compounds with a tetrahedral coordination of atoms consists of local regions characterized by an ordered but not crystalline structure and that, owing to the regularity inherent in this structure, solid-phase crystallization of the amorphous compounds under consideration can occur through the cooperative mechanism. The modular design is used to construct a possible structure of similar local regions and to describe the mechanism of its cooperative transformation into a diamond-like structure with the possible formation of numerous microtwins. The difficulties encountered in the course of the cooperative transformation are considered. Algorithms are proposed for constructing one more connected structure of local regions from the same modules (closed hollow loops of bonds), which undergoes a cooperative transformation into crystalline silicon without the aforementioned difficulties.     

Abscheidung dünner Siliziumschichten durch Zersetzung eines Siliziumtetrahlorid-Wasserstoff-Gemisches in einer Hochfrequenz-Glimmentladung

The paper describes a technique for depositing thin films of silicon on several substrate materials by decomposition of a SiCl₄–H₂ mixture in a h.f. discharge. Depending on the growth conditions and on the substrates used the formed silicon layers are amorphous, polycrystalline or well defined monocrystalline. Vapour etching of differently orientated monocrystalline silicon substrates in the apparatus yields crystal surfaces which can be used as base for consecutive epitaxial growth.     

Growth and optical property characterization of textured barium titanate thin films for photonic applications

We have investigated the growth of barium titanate thin films on bulk crystalline and amorphous substrates utilizing biaxially oriented template layers. Ion beam-assisted deposition was used to grow thin, biaxially textured, magnesium oxide template layers on amorphous and silicon substrates. Growth of highly oriented barium titanate films on these template layers was achieved by molecular beam epitaxy using a layer-by-layer growth process. Barium titanate thin films were grown in molecular oxygen and in the presence of oxygen radicals produced by a 300 W radio frequency plasma. We used X-ray and in situ reflection high-energy electron diffraction (RHEED) to analyze the structural properties and show the predominantly c-oriented grains in the films. Variable angle spectroscopic ellipsometry was used to analyze and compare the optical properties of the thin films grown with and without oxygen plasma. We have shown that optical quality barium titanate thin films, which show bulk crystal-like properties, can be grown on any substrate through the use of biaxially oriented magnesium oxide template layers.     

Cw argon laser crystallization of silicon films: Structural properties

This paper deals with the structural characterization of amorphous silicon films deposited on glass in the amorphous state and then post-crystallized using a continuous wave argon laser. In opposite to the excimer laser crystallization method, the processing window is wider. Due to the low cooling rate induced by the continuous irradiation, very large grains are obtained. With an epitaxial growth induced by an adequate overlapping of the laser traces, grains as large as 100 μm can be reached. Electron back-scattered diffraction analysis highlights the single crystalline character of the large size grains crystallized with this kind of laser. The technique is able to produce large area single crystalline regions, suitable to fabricate high speed circuits.     

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

The epitaxial growth of sputtered silicon layers on differently oriented substrates at low temperature - An electron microscopic study

We investigated epitaxial silicon films deposited on differently oriented substrates by pulsed magnetron sputtering at temperatures of 500-550 °C. Our scanning and transmission electron microscopic as well as electron backscattering investigations show that epitaxial films grow not only on (1 0 0)-oriented substrates, but also on (2 1 0)-, (4 1 1)- and (3 1 1)-oriented ones. A change to the (1 0 0) orientation is found for the growth on (1 1 1)- , (3 2 1)- and close to (1 1 0)-oriented substrates. For these orientations transmission electron microscopic investigations show stacking faults, microtwins and small amorphous inclusions in a region starting at the substrate-film interface up to thicknesses of 150-200 nm. With increasing film thickness above 200 nm the crystalline perfection of the epitaxial layers improves.     

Low-temperature atomic assembly of stoichiometric gallium arsenide from equiatomic vapor

The low-temperature atomic assembly of homoepitaxial GaAs thin films on the (0 0 1) surface has been investigated using molecular dynamics with a Stillinger–Weber potential energy function. During equiatomic vapor deposition, crystalline growth was observed for substrate temperatures above 35% of the melting temperature. Below this temperature, the critical epitaxial thickness began to rapidly decrease as defects were increasingly incorporated and eventually nucleated an entirely amorphous structure. The atomic assembly mechanisms of arsenic dimer incorporation, as well as gallium vacancy formation, were studied just above the amorphous/crystalline growth transition temperature. The adsorption of arsenic dimers was found to show dependence upon the orientation of the deposited molecule. Atomic processes responsible for the formation of the gallium vacancy defects were observed, and the influence of growth temperature on defect formation was also identified.     

Study of Ge epitaxial growth on Si substrates by cluster beam deposition

Ge epitaxial layers with reasonable quality were grown on Si (1 1 1) substrates by cluster beam deposition (CBD) process. Molecular dynamics study of the low energy Ge clusters deposition process utilizing the Stillinger–Weber two- and three-body interaction potentials was carried out to compare the experimental results. Both experimental and simulation results prove that the substrate temperature plays a dominant role in the epitaxial growth of Ge films in CBD process. The influence mechanisms of temperature are discussed.     

Investigation of the role of cadmium sulfide in the surface passivation of lead sulfide quantum dots

Surface passivation of PbS nanocrystals (NC), resulting in strong photoluminescence, can be achieved by the introduction of CdS precursors. The role of CdS in the surface passivation of PbS NCs is uncertain, as the crystalline structure of CdS and PbS are different, which should impede effective epitaxial overgrowth. Absorption spectroscopy is used to show that the CdS precursors strongly interact with the PbS NC surface. Electron microscopy reveals that the introduction of CdS precursors results in an increased particle size, consistent with overcoating. However, we also find the process to be highly non-uniform. Nevertheless, evidence for epitaxial growth is found, suggesting that effective surface passivation may be possible.     

The synthesis of aligned silicon nanowires under ambient atmospheric pressure

Highly ordered amorphous silicon nanowires were successfully synthesized from single crystalline silicon wafer at the pyrolysis temperature of 1050 °C under ambient atmospheric pressure. Both poly (phenylcarbyne) and nickel nitrate played important roles in the growth of silicon nanowires. The fabrication of ordered silicon nanowires was controllable and repeatable, confirmed by the experimental results. The morphology and microstructure analysis of the as-obtained samples showed the highly ordered amorphous silicon nanowires were obtained, determined by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and FT-infrared spectroscopy. A solid-liquid-solid growing process was proposed.     

Epitaxial growth of ferroelectric oxide films

This review paper begins with a brief overview of the most common ferroelectric materials, the perovskites and the Aurivillius families. The epitaxial growth of ferroelectric epitaxial films can be a viable approach to improve the ferroelectric properties, in particular for the layered perovskites. Defects related to a polycrystalline structure, which lead to a degradation of ferroelectric properties like remanent polarisation, piezoelectric coefficient, charge retention, and may cause time-dependent fatigue problems, can be prevented. However, it is also to be considered that effects connected with thin films like substrate clamping, strain or finite thickness may limit the film properties. Substitution of elements allows the adjustment of the film characteristics to the device function. Additionally, the orientation of the films can be controlled by the appropriate choice of the substrate, which is important due to the anisotropy of the ferroelectrics.The deposition methods commonly used for ferroelectric oxide layers are reviewed, particularly with regard to epitaxial growth. The conditions under which stoichiometric, crystalline growth can be obtained are described. The paper primarily focuses on the MO-CVD technique.Furthermore whether epitaxial growth of ferroelectric films occurs or not depends on several conditions like lattice mismatch between film and substrate, surface orientation and crystal symmetry of the substrate, thermal expansion of film and substrate. The influence of these parameters on epitaxial growth is discussed. Local epitaxial growth of ferroelectric layers on metallic electrodes is also mentioned due to its importance in device fabrication. The site-engineering concept is shortly reviewed as the substitution of elements constitutes a simple way to modify film properties in thin film technology.     

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN¹ and AlN²) with an epitaxial orientation relationship of [0 0 0 1]_AlN || [0 0 1]_Si and AlN¹ || AlN² || [1 1 0]_Si. The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(0 0 1) substrates with an epitaxial orientation relationship of [0 0 0 1]_AlN parallel to the surface normal and 0 1 1 0_AlN || [1 1 0]_Si.     

Back contacted a-Si:H/c-Si heterostructure solar cells

This paper shows how the amorphous/crystalline silicon technology can be implemented in the interdigitated back contact solar cell design. We have fabricated rear-junction, backside contact cells in which both the emitter and the back contact are formed by amorphous/crystalline silicon heterostructure, and the grid-less textured front surface is passivated by a double layer of amorphous silicon and silicon nitride, which also provides an anti-reflection coating. The entire self-aligned mask and photolithography-free process is performed at temperature below 300 °C with the aid of one metallic mask to create the interdigitated pattern. An open circuit voltage of 687 mV has been measured on a 0.5 Ωcm p-type monocrystalline silicon wafer. On the other hand, several technological aspects that limit the fill factor (50%) and the short circuit current density (32 mA/cm²) still need improvement. We show that the uniformity of the deposited amorphous silicon layers is not influenced by the mask-assisted deposition process and that the alignment is feasible. Moreover, this paper investigates the photocurrent limiting factors by one-dimensional modeling and quantum efficiency measurements.     

Autodoping of Epitaxial Silicon Layers (III). Theoretical Treatment of Lateral Autodoping

Solving dopants from the silicon surface into the volume during epitaxial layer growth is commonly described as a solution equilibrium, which forms an essential part of modelling intentional silicon doping. Above buried layers a so-called redistribution autodoping causes the inversely directed process within an initial layer growth period. When the layer begins to grow, the dopants are not totally buried by the silicon deposited, but are partially swept towards the surface to develop the redistribution equilibrium. Above that part of the layer surface located above buried layers, simultaneously a certain dopant partial pressure is established. It gives rise to a vertical and lateral dopant transport by means of gas diffusion. The flow in lateral direction is considered the source of lateral autodoping. In the present paper a theoretical model of autodoping is developed and the layer deposition parameters are discussed with regard to minimizing autodoping effects.     

Local selective homoepitaxy of silicon at reduced temperatures using a silicon-iodine transport system

Selective deposition of homoepitaxial silicon can be achieved using silicon-iodine transport within a quartz ampoule at temperatures down to nearly 600°C. A segregation of gaseous components due to their density differences and a temperature gradient along the deposition region serve to optimize the silicon deposition. The variation of the epitaxial growth process with total pressure and temperature has also been studied. The process shows basic advantages for silicon device applications.     

Microstructure optical and electrical studies of cadmium selenide thin films

Nucleation and growth of cadmium selenide thin films are of considerable interest because of their direct effect on the optical and electrical properties of this material. Vacuum-deposited layers on amorphous and crystalline substrates showed a polycrystalline structure. The layers were deposited at a pressure of 10⁻⁴ Pa, with two deposition rates of 12 nm/s and 15 nm/s. The electrical resistivity and the optical absorption were studied for a group of layers on quartz substrate. As well the morphology and the microstructure were investigated by X-ray and electron microscopy. Few activation energies were estimated from the optical absorbance and thermo-resistance.