Deposition Of Amorphous Silicon Research Articles

A study of activated phosphorus distribution within silicon substrate for polysilicon passivating contacts based on an in-line PVD system

An in-line PVD system consisting of plasma oxidation chamber and phosphorus-doped amorphous silicon deposition chamber was used to fabricate highly-doped polysilicon passivating contact structure for commercial TOPCon solar cells, which can help to achieve simple fabrication process and excellent electrical properties. In this paper, we studied how the doping profiles of ionized phosphorus atoms in the passivating contact formed by this method varies with the deposition conditions, especially the in-diffusion profile in the silicon substrate. Increasing the RF power of plasma oxidation can prevent phosphorus from penetrating through SiOx to form a shallow profile, and reducing the power is conducive to the formation of more pinholes on SiOx, resulting in a wide profile following the shallow profile. The shallow profile effectively inhibits the surface recombination rate and thus achieve good passivation quality, while the emergence of the following wide profile helps to reduce the contact resistivity by providing more majority carrier transport channels. The 100 W condition constructed a shallow and gradually widening doping profile, exhibiting a low recombination current density of 16.5 fA/cm2 and a low contact resistivity of 1.87 mΩ·cm2. However, the SiOx thickness and density obtained at 80 W are insufficient for achieving a shallow-first-then-widen doping profile, even with adjustments to thermal activation conditions. Additionally, we found the engineering of surface microstructure on silicon wafers also has an impact on the doping profile.

Selective rounding for pyramid peaks and valleys improves the performance of SHJ solar cells

AbstractThe first step in the fabrication of SHJ (silicon heterojunction) solar cells is forming random pyramids on the Si wafer surfaces. The pyramid profile would affect the following amorphous silicon deposition. Two CP (chemical polishing) solutions consist of HNO3 + HF and O3 + HF, respectively, are used to modify the pyramid profile. It is found that these two solutions have selective rounding effects on the pyramid profiles. The first one rounds the pyramid valleys and the second one rounds the pyramid peaks. The combination of the two CP methods improves the passivation quality of amorphous silicon and promotes the conversion efficiency of SHJ cells.

Comparative Study of Silicon Nanowires Grown From Ga, In, Sn, and Bi for Energy Harvesting

A high density of silicon nanowires for solar cell applications was fabricated on a single crystalline silicon wafer, using low eutectic temperature metal catalysts, namely, gallium, indium, tin, and bismuth. The use of silicon nanowires is exploited for light trapping with an aim to enhance the efficiency of solar cells. Additionally, we have optimized the deposition parameters so that there is merely deposition of amorphous silicon along with the growth of silicon nanowires. Thus, it may improve the stability of silicon-based solar cells. The different catalysts used are extensively discussed with experimental results indicating stable growth and highly efficient silicon nanowires for photovoltaic applications. To test the stability, we measured the open-circuit voltage for four hours and the change in voltage was ±0.05 V. The fabrication of all-crystalline silicon solar cells was demonstrated using the conventional mature industrial manufacturing process that is presently used for the amorphous silicon solar cells. To summarize, this research compares various post-transition metals as a catalyst for the growth of nanowires discussing their properties, and such silicon nanowires can be utilized in several other applications not only limited to photovoltaic research.

Room-temperature deposition of low H-content SiNx/SiNxOy thin films using a specially designed PECVD system

Low-temperature deposition of amorphous silicon nitride/oxynitride (a-SiNx/a-SiNxOy) is of significance for many industrial applications. In this work, low H-content a-SiNx/a-SiNxOy films were prepared at room temperature by a specially designed plasma enhanced chemical vapor deposition (PECVD) system using Ar-diluted SiH4 and pure N2 as the reactant gases. The plasmas for the deposition of a-SiNx/a-SiNxOy films were studied as a function of discharge power and working pressure. It was found that the density of N atoms in plasma plume was found to have significant impacts on the compositions and optical properties of a-SiNx/a-SiNxOy films and the correlations between the plasma discharge and the as-deposited films were explored. With the increase of N atoms in the plasma, the growth rate of a-SiNx/a-SiNxOy films was increased and the refractive index was gradually reduced, and thus the value of refractive index at 633 nm was tuned from 2.20 to 1.64 due to the increase of the atomic ratio of N to Si in the films. In addition, the O and H contents were increased with the increase of N density in the plasma and have contributions to the variation in the refractive index of a-SiNxOy films as well. The low-temperature deposition of a-SiNx/a-SiNxOy films with tunable refractive index exhibited a great potential in practical applications, especially in flexible electronics.

Effects of tunneling oxide defect density and inter-diffused carrier concentration on carrier selective contact solar cell performance: Illumination and temperature effects

In this study, technology computer-aided design (TCAD) was used to investigate carrier selective contact solar cell performance based on the boundary of tunneling oxide and electron selective contact layer properties. The role of tunneling oxide quality in the passivation and inter-diffusion properties of the plasma-enhanced chemical vapor deposition of phosphorus-doped amorphous silicon as an electron selective contact layer is studied for carrier selective contact solar cells. Tunnel oxide quality varies according to the ratio of Si4+ to Si2+ state. For the Si4+/Si2+ ratio of 4, open-circuit voltage of 730 mV and the lowest interface trap density of 5.3 × 1010 cm−2 eV−1 are achieved. The change in diffusion depth of the doped layer with respect to the annealing temperature is analysed by transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) measurement. Carrier selective contact solar cell parameters are optimized by incorporating the experimental values of interface trap density, inter-diffused impurity concentration, and depth in the Quokka 3 TCAD tool. Solar cell conversion efficiency of 24.31% was obtained. To understand the response of carrier selective solar cell to the environmental changes, the output characteristics of the solar cell were studied by varying the illumination temperature by 10% of the standard test conditions (STC) and temperature from 277 K to 377 K (difference of 100 K).

Transferless Inverted Graphene/Silicon Heterostructures Prepared by Plasma-Enhanced Chemical Vapor Deposition of Amorphous Silicon on CVD Graphene.

The heterostructures of two-dimensional (2D) and three-dimensional (3D) materials represent one of the focal points of current nanotechnology research and development. From an application perspective, the possibility of a direct integration of active 2D layers with exceptional optoelectronic and mechanical properties into the existing semiconductor manufacturing processes is extremely appealing. However, for this purpose, 2D materials should ideally be grown directly on 3D substrates to avoid the transferring step, which induces damage and contamination of the 2D layer. Alternatively, when such an approach is difficult—as is the case of graphene on noncatalytic substrates such as Si—inverted structures can be created, where the 3D material is deposited onto the 2D substrate. In the present work, we investigated the possibility of using plasma-enhanced chemical vapor deposition (PECVD) to deposit amorphous hydrogenated Si (a-Si:H) onto graphene resting on a catalytic copper foil. The resulting stacks created at different Si deposition temperatures were investigated by the combination of Raman spectroscopy (to quantify the damage and to estimate the change in resistivity of graphene), temperature-dependent dark conductivity, and constant photocurrent measurements (to monitor the changes in the electronic properties of a-Si:H). The results indicate that the optimum is 100 °C deposition temperature, where the graphene still retains most of its properties and the a-Si:H layer presents high-quality, device-ready characteristics.

Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes.

Herein, copper silicide (Cu15Si4) nanowires (NWs) grown in high densities from a metallic Cu substrate are utilized as nanostructured hosts for amorphous silicon (aSi) deposition. The conductive Cu15Si4 NW scaffolds offer an increased surface area, versus planar substrates, and enable the preparation of high capacity Li-ion anodes consisting of a nanostructured active material. The formation method involves a two-step process, where Cu15Si4 nanowires are synthesized from a Cu substrate via a solvent vapor growth (SVG) approach followed by the plasma-enhanced chemical vapor deposition (PECVD) of aSi. These binder-free anodes are investigated in half-cell (versus Li-foil) and full-cell (versus LCO) configurations with discharge capacities greater than 2000 mAh/g retained after 200 cycles (half-cell) and reversible capacities of 1870 mAh/g exhibited after 100 cycles (full-cell). A noteworthy rate capability is also attained where capacities of up to 1367 mAh/g and 1520 mAh/g are exhibited at 5C in half-cell and full-cell configurations, respectively, highlighting the active material's promise for fast charging and high power applications. The anode material is characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.

Silicon variation and phytolith morphology in different organs of Dendrocalamus brandisii (Munro) Kurz (Bambusoideae)

Silicon is an essential element for the growth and development of bamboo plants that significantly affects the biochemical and physical properties for industrialized bamboo utilization. Silicon is biomineralized as diverse forms of phytoliths in the bamboo organs. We investigated the silicon variation and phytolith morphology in different organs of Dendrocalamus brandisii (Munro) Kurz. The silicon content in D. brandisii (Munro) Kurz varies and increases in the order of leaf > sheath > branch > culm > shoot > root. The phytolith morphotypes and presence frequency in different D. brandisii organs varied closely interrelated with the cell structure and organ functions. Amorphous silicon deposition and few varieties and numbers of phytolith were found in D. brandisii root and shoots. Rich distribution of the saddle and dumb-bell phytoliths was found in all organs except the root and mature culm and branch. More elongate and rectangular phytoliths were found in mature culm and branch. Round and elliptical phytoliths might be firstly formed followed by dumb-bell and saddle phytoliths. Phytolith sizes increased with increasing culm, branch and leaf age, but did not demonstrate a specific trend with culm height. The result could contribute to enriching phytolith morphology in Gramineae and provide theoretical basis and references for plant taxonomy and large-scale bamboo industrial processing and utilization.

Flash Lamp Annealed Polycrystalline Silicon TFT Dependence on Surface Morphology and the Back-Channel Material Interface

This work studies the operation of polycrystalline silicon p-channel TFTs prepared via Flash Lamp Annealing (FLA). Silicon nitride was investigated as a back-channel interface layer to suppress void formation via dewetting of molten silicon. Predefined polygons of amorphous silicon were crystallized on display glass using single-pulse FLA exposure. Two distinct surface morphologies were realized within the same exposed region as a result of the underlying silicon nitride film and localized variation in the substrate thermal contact during PECVD deposition of amorphous silicon. While both resulting morphologies have polysilicon grains on micron scale, differences in TFT electrical behavior appear to coincide with differences in grain structure and boundary regions. The electrical results also indicate inferior electrical performance in response to the back-channel silicon nitride in comparison to results demonstrated on SiO2. Extracted parameters provide a quantitative assessment of the device operation as related to surface morphology and the back-channel material interface.

Pyramid size control and morphology treatment for high-efficiency silicon heterojunction solar cells

This paper investigates the formation process of surface pyramid and etching characteristics during the texturing process of mono-crystalline silicon wafers. It is found that there is an etch rate transition point in alkaline anisotropic etching when {100} plane-dominated etch turns to {111} plane-dominated etch, and the pyramid size has a strong linear correlation with the etch amount at the transition point. Several techniques were developed to control the pyramid size by monitoring and adjusting the etching amount. A wide range of average pyramid sizes were successfully achieved, from 0.5 to 12 μm. The experiments of the pyramid size on the light reflectance, the minority carrier lifetime (MCLT), and the performance of silicon heterojunction (SHJ) solar cells were carried out and analyzed. A desirable range of pyramid sizes was empirically determined by our investigation. In order to reduce the density states on the texturing surface, the wet-chemical smoothing treatment was also investigated. The smoothing treatment improves the passivation quality and the performance of the solar cells. Through pyramid size control and morphology treatment, together with the amorphous silicon (a-Si:H) deposition improvement, and electrode optimization, high performance of SHJ solar cells has been achieved, up to conversion efficiency 23.6%.

Структурные изменения в пленках Si-CuSi при интеркаляции ионов лития

Abstract: Films of amorphous silicon reinforced with crystalline copper silicide inclusions were tested as anode material in liquid lithium-ion cells. The films were made by the method of layer-by-layer magnetron deposition of amorphous silicon and copper, followed by low-temperature (100 ÷ 200 ° C) annealing of the structure. During the tests, the anodes revealed the effect of long-term intensive growth of their reversible capacity. To describe the effect, a phenomenological model is proposed, based on the counter-migration of silicon and copper atoms in a non-uniform field of elastic mechanical stresses arising during the cycling of a lithium-ion cell.

Dry Passivation Process for Silicon Heterojunction Solar Cells Using Hydrogen Plasma Treatment Followed by <italic>In Situ</italic> a-Si:H Deposition

A fully dry and hydrofluoric-free low-temperature process has been developed to passivate n-type crystalline silicon (c-Si) surfaces. Particularly, the use of a hydrogen (H2) plasma treatment followed by in situ intrinsic hydrogenated amorphous silicon (a-Si:H) deposition has been investigated. The impact of H2 gas flow rate and H2 plasma processing time on the a-Si:H/c-Si interface passivation quality is studied. Optimal H2 plasma processing conditions result in the best effective minority carrier lifetime of up to 2.5 ms at an injection level of 1 × 1015 cm−3, equivalent to the best effective surface recombination velocity of 4 cm/s. The reasons that enable such superior passivation quality are discussed in this paper based on the characterization of the a-Si:H/c-Si interface and c-Si substrate using transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, and deep-level transient spectroscopy.

Effect of oxidation on intrinsic residual stress in amorphous silicon carbide films.

The change in residual stress in plasma enhanced chemical vapor deposition amorphous silicon carbide (a-SiC:H) films exposed to air and wet ambient environments is investigated. A close relationship between stress change and deposition condition is identified from mechanical and chemical characterization of a-SiC:H films. Evidence of amorphous silicon carbide films reacting with oxygen and water vapor in the ambient environment are presented. The effect of deposition parameters on oxidation and stress variation in a-SiC:H film is studied. It is found that the films deposited at low temperature or power are susceptible to oxidation and undergo a notable increase in compressive stress over time. Furthermore, the films deposited at sufficiently high temperature (≥325 C) and power density (≥0.2 W cm-2 ) do not exhibit pronounced oxidation or temporal stress variation. These results serve as the basis for developing amorphous silicon carbide based dielectric encapsulation for implantable medical devices. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1654-1661, 2019.

Enhanced photoresponse of Ge/Si nanostructures by combining amorphous silicon deposition and annealing

In order to inhibit high carrier recombination rates in Ge-on-Si nanostructures, GexSi1–x nanoislands were covered by a thin amorphous silicon layer via a low pressure CVD process. It is demonstrated that the surface photovoltage (SPV) signal in capped GexSi1–x/Si is increased by an order of magnitude compared to that of bare GexSi1–x islands, which can be due to the effective passivation of recombination centers at the a-Si/GexSi1–x interface. The effect is even more enhanced after subsequent annealing at 400 °C in an O2 ambient environment, with the signal increases ranging from 5 to 10 times. The observed increased photovoltage is accompanied by longer time decays in the SPV transients, being most increased after the annealing step. These results show that the photoexcited electron-hole pairs can be efficiently separated by the internal electric field at the a-Si/GexSi1–x/c-Si interfaces and can contribute to the photovoltage with decreasing recombination in GexSi1–x islands or at the interfaces. This work can facilitate the photovoltaic applications of Ge/Si heterostructures.

Single-material Bragg reflectors and their applications in VCSELs

We present single-material distributed Bragg reflectors (DBRs) fabricated by oblique-angle deposition of amorphous silicon (a-Si). The high and low refractive indices of a-Si films were obtained at two incident vapour flux angles (θα) of 0° and 80° by determining the quarter-wavelength thicknesses of 67 nm at 0° and 128 nm at 80°. The fabricated single-material Bragg reflectors with only five pairs provide high reflectivity values of over 95%. The proposed DBRs were applied to intra-cavity contacted vertical-cavity surface-emitting lasers (VCSELs) to reduce fabrication costs and step. For the fabricated five pair a-Si/a-Si DBRs on VCSELs, the normalized stop bandwidth of 31.63% was measured while high reflectivity values over 95% were maintained over a wide wavelength range of 710 - 1020 nm. The fabricated VCSELs with Da = 7 μm obtained output power and threshold current, which are reasonable measured results.

A comparative study on the direct deposition of μc-Si:H and plasma-induced recrystallization of a-Si:H: Insight into Si crystallization in a high-density plasma

Abstract Deep insight into the crystallization mechanism of amorphous silicon is of theoretical and technological significance for the preparation of high-quality microcrystalline/polycrystalline silicon. In this work, we intensively compare the present two plasma-involved routes, i.e., the direct deposition and recrystallization of precursor amorphous silicon (a-Si) films, to fabricate microcrystalline silicon. Both the directly deposited and recrystallized samples show multi-layered structures as revealed by electronic microscopy. High-density hydrogen plasma involved recrystallization process, which is mediated by the hydrogen diffusion into the deep region of the precursor a-Si film, displays significantly different nucleation configuration, interface properties, and crystallite shape. The underlying mechanisms are analyzed in combination with the interplay of high-density plasma and growing or treated surface.

Preparation of poly-Si films by inverted AIC process on graphite substrate

Graphite/a-Si/Al laminated structures were deposited on graphite substrate using magnetron sputtering technology. The substrate temperature of amorphous silicon (a-Si) deposition and the Al/Si thickness ratio have strong influences on a-Si crystallization, various conditions were used to investigate the polycrystalline silicon (poly-Si) thin films grown by inverted aluminum-induced crystallization (AIC) for process optimization. In this paper, we established the AIC model to explain the effects of the two factors in the inverted AIC process. The poly-Si thin films were characterized By means of Scanning electron microscope (SEM), X-ray diffraction (XRD) and Raman spectroscopy (Raman), which showed that the samples with strong preferred (111) orientation and high crystallization quality that were favorable for epitaxially growing poly-Si thick film cells.

A statistics modeling approach for the optimization of thin film photovoltaic devices

The growing interest in exploring thin film technologies to produce low cost devices such as n-i-p silicon solar cells, with outstanding performances and capability to address the highly relevant energy market, turns the optimization of their fabrication process a key area of development. The usual one-dimensional analysis of the involved parameters makes it difficult and time consuming to find the optimal set of conditions. To overcome these difficulties, the combination of experimental design and statistical analysis provides the tools to explore in a multidimensional fashion the interactions between fabrication parameters and expected experimental outputs.Design of Experiment and Multivariate Analysis are demonstrated here for the optimization of: (1) the low temperature deposition (150°C) of high quality intrinsic amorphous silicon (i-a-Si:H); and (2) the matching of the n-, i-, and p-silicon layers thickness to maximize the efficiency of thin film solar cells. The multiple regression method applied, validated through analysis of variance and evaluated against exact numerical simulations, is shown to predict the overall intrinsic layer properties and the devices performance.The results confirm that experimental design and statistical data analysis are effective approaches to improve, within a minimum time frame and high certainty, the properties of silicon thin films, and subsequently the layer structure of solar cells.

Quantitative assessment of molecular dynamics-grown amorphous silicon and germanium films on silicon (111)

Molecular dynamics based on the empirical Tersoff potential was used to simulate the deposition of amorphous silicon and germanium on silicon(111) at various deposition rates and temperatures. The resulting films were analyzed quantitatively by comparing one-dimensional atomic density profiles to experimental measurements. It is found that the simulations are able to capture well the structural features of the deposited films, which exhibit a gradual loss of crystalline order over several monolayers. A simple mechanistic model is used to demonstrate that the simulation temperature may be used to effectively accelerate the surface relaxation processes during deposition, leading to films that are consistent with experimental samples grown at deposition rates many orders-of-magnitude slower than possible in a molecular dynamics simulation.

Limitations of high pressure sputtering for amorphous silicon deposition

Amorphous silicon thin films were deposited using the high pressure sputtering (HPS) technique to study the influence of deposition parameters on film composition, presence of impurities, atomic bonding characteristics and optical properties. An optical emission spectroscopy (OES) system has been used to identify the different species present in the plasma in order to obtain appropriate conditions to deposit high purity films. Composition measurements in agreement with the OES information showed impurities which critically depend on the deposition rate and on the gas pressure. We prove that films deposited at the highest RF power and 3.4 × 10−2 mbar, exhibit properties as good as the ones of the films deposited by other more standard techniques.