Amorphous Silicon Films Research Articles

Direct observation of ultra-rapid solid phase crystallization of amorphous silicon films irradiated by micro-thermal-plasma-jet

This paper presents a new method to directly observe the transient evolution of amorphous silicon temperature and the solid phase crystallization during micro-thermal-plasma-jet irradiation. Using the time-resolved reflectivity analysis, nucleation temperature and characteristic crystallization time were determined. When the heating rate increases from 4.45×105to 2.28×106K/s, the nucleation temperature increases from 985 to 1071 °C. The characteristic crystallization time decreases from 95 to 20 μs with an increase in heating rate. Crystallization time as a function of temperature corresponds to the activation energy of 2.9 eV. This energy relates to both the nucleation and growth process. Micro-Raman spectroscopy results indicate that the crystalline volume fraction of solid–phase crystallized Si film is higher than 65 %, and it increases to 80 % with decreasing scanning speed. Both high resolution transmission electron microscopy results and theoretical calculation indicate that there is a distribution of final crystal size. The average grain size is smaller than 50 nm and depends on the annealing conditions, namely the faster heating rate is, the smaller grain size is.

The Carrier Transport Properties of B-Doped Si Nanocrystal Films with Various Doping Concentrations

B-doped hydrogenated amorphous silicon (a-Si:H) films with various doping concentrations were prepared by a plasma-enhanced chemical vapor deposition (PECVD) technique. After thermal annealing, the as-deposited samples, B-doped silicon nanocrystals (Si NCs), were obtained in the films. The electronic properties of B-doped Si NC films with various doping concentrations combined with the microstructural characterization were investigated. A significant improvement of Hall mobility rising to the maximum of 17.8 cm2/V·s was achieved in the Si NC film after B doping, which is due to the reduction of grain boundary (GB) scattering in the B-doped samples. With increasing the doping concentration, it was interesting to find that a metal-insulator transition (MIT) took place in the B-doped Si NC films with high doping concentrations. The different carrier transport properties in the B-doped Si NC films with various doping concentrations were investigated and further discussed with emphasis on the scattering mechanisms in the transport process.

Structural changes in thin amorphous silicon film during electron irradiation

It is shown by methods of Raman spectroscopy that irradiation of amorphous silicon film 50 nm thick by focused electrons with energies of 5–10 keV results in its crystallization at temperatures much less, than the temperatures of melting and annealing of silicon. Analyses have shown that crystallization is caused by the formation of negative charge area on a film surface, by the action of secondary electrons on chemical bonds and mutual location of silicon atoms, and also by the ionic field emission of silicon ions and their field migration to the irradiated zone from surrounding regions. These processes result in the change of equilibrium location of silicon atoms, typical for amorphous phase. The increase of electron energy leads to the increase of crystallization process efficiency.

A Correlative Study of Film Lifetime, Hydrogen Content, and Surface Passivation Quality of Amorphous Silicon Films on Silicon Wafers

We examine correlations between the recombination lifetime and hydrogen content of hydrogenated amorphous silicon films (a-Si:H) and the surface passivation afforded by such films when deposited on crystalline silicon wafers, during annealing at 350–500 °C. Our results show that, as the annealing duration increases, both the a-Si:H recombination lifetime and the surface recombination velocity evolve at a similar rate to the hydrogen concentration. This suggests that the loss of hydrogen during annealing is the direct cause of the reduction in the a-Si:H film lifetime, and that the loss of hydrogen occurs both at the a-Si:H/c-Si interface as well as in the bulk of a-Si:H film. We calculated the activation energy of the surface depassivation reaction during annealing to be 0.62 ± 0.1 eV, which suggests that the depassivation reaction is limited by the migration of hydrogen within the film, without significant hydrogen trapping. Secondary-ion mass spectrometry further demonstrates the loss of hydrogen across the film thickness during annealing.

Spectroscopic ellipsometry study of non-hydrogenated fully amorphous silicon films deposited by room-temperature radio-frequency magnetron sputtering on glass: Influence of the argon pressure

The complex dielectric functions of unhydrogenated amorphous silicon (a-Si) thin films, grown by the radio-frequency magnetron-sputtering (RFMS) technique, at room temperature, upon glass substrates, have been accurately determined by using ex-situ variable-angle spectroscopic ellipsometry, in the photon energy range from 0.73 to 4.96 eV (i.e. 1700 to 250 nm). Mass density of sputter-deposited amorphous silicon films was changed by a simple argon-working-gas pressure control: higher Ar gas pressure results in reduced film mass density, or increased porosity. The optical response of the hydrogen-less amorphous silicon films under study can be successfully parameterized employing a single, Kramers-Kronig (K-K)-consistent Cody-Lorentz (CL) oscillator dispersion model; the Urbach absorption tail included in the CL model, is certainly needed in the present case of hydrogen-free sputtered a-Si films. It has been unambiguously demonstrated that the obtained values of CL-model parameters are reasonably correlated with the particular argon-gas sputtering pressure utilized for each deposition. The behavior of these best-fit CL-model parameters with varying argon pressure from 0.13Pa up to 4.4Pa, can be clearly understood in terms of the effect of the existing micro-voids, within the as-deposited a-Si film material.

Impact of bilayer structures on the surface passivation quality of high‐rate‐sputtered hydrogenated amorphous silicon for silicon heterojunction solar cells

AbstractCrystalline silicon surface passivation effect of intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) films deposited by radio‐frequency facing target sputtering (RF‐FTS) using a two‐step deposition technique was investigated. In the two‐step deposition technique, an i‐a‐Si:H layer was deposited at a high sputtering power condition after the deposition of i‐a‐Si:H at a low sputtering power condition. The two‐step deposition technique drastically improved the passivation quality of i‐a‐Si:H compared with a conventional single‐step deposition technique. Only 0.5‐nm‐thick i‐a‐Si:H deposited at a low sputtering power suppresses the initial sputtering damage to the crystalline silicon surface. A high average deposition rate of 14.1 nm/min was also achieved. A non‐textured silicon heterojunction solar cell using an i‐a‐Si:H passivation layer deposited by the two‐step method shows a conversion efficiency of 17.4% (Voc = 0.679 V, Jsc = 35.0 mA/cm2, FF = 0.732).

Laser-Power Dependence of Poly-Silicon Crystallization Using 355-nm Nanosecond Laser Annealing

We report the laser-power dependence of a poly-silicon thin film fabricated using a 355-nm nanosecond laser annealing. The temperature distribution in the annealed thin film was investigated using thermal simulations as the laser power was varied to obtain the optimum laser conditions for crystalizing an amorphous silicon film. Based on the simulation results, laser annealing experiments were conducted for a 100-nm-thick amorphous silicon film deposited on a SiO2/Si wafer by using a Q-switched 355-nm nanosecond diode-pumped solid-state laser. The characteristics of the annealed silicon film were investigated using Raman spectroscopy and atomic force microscopy, which showed that both the crystal quality and the roughness of the annealed film increased as the laser was power increased. The experimentally obtained optimum laser power condition was found to be consistent with the simulation results. The demonstrated 355-nm nanosecond laser annealing is expected to provide a versatile solution for low-temperature poly-silicon processes.

Occurrence of Sharp Hydrogen Effusion Peaks of Hydrogenated Amorphous Silicon Film and Its Connection to Void Structures

The amount of hydrogen released from plasma‐enhanced chemical vapor (PECVD) deposited hydrogenated amorphous silicon (a‐Si:H) layers is determined by gas effusion measurements. A sharp peak (SP) is observed in the effusion spectra of samples with substrate temperature T S ≥ 200 °C. Light microscopic images indicate the formation of bubbles after deposition for all samples and film deterioration after effusion measurement in correlation with the presence of the hydrogen SPs. Change in substrate temperature varies with the microstructure of the film, the hydrogen concentration, and the density. A low T S leads to a porous structure with large number of interface bubbles, and therefore no hydrogen‐induced SP appears during hydrogen effusion. Whereas high T S causes a compact a‐Si:H film in which the hydrogen effusion is limited by the longer diffusion, while the number of interface bubbles decreases. The storage of near substrate hydrogen in the bubbles in compact material leads to a local explosion by increase in excessive pressure. The difference between the low temperature peak and the position of the SP in the effusion spectra indicates the time required to fill the interface bubbles with hydrogen, which decreases with increasing film density, suggesting that the volume of the interface bubble decreases.

Room-temperature crystallization of amorphous silicon by near-UV femtosecond pulses

Near ultraviolet (λ ≈ 400 nm) femtosecond laser annealing (400 nm-FLA) in a scanning mode was employed to crystallize amorphous silicon (a-Si) films at room temperature. The average grain size of polycrystalline silicon annealed was studied as a function of the incident laser fluence and beam overlap or the number of laser shots irradiated. In general, the grain size can be enlarged by either increasing the beam overlap at a fixed laser fluence or increasing the laser fluence for a fixed number of laser shots. An apparent threshold for the onset of rapid enlargement of grain size was observed for processing at ∼90% overlap and fluences above 25 mJ/cm2. A maximum grain size of ∼280 nm was attained at a laser fluence of 30 mJ/cm2 and overlap of 93.75%, beyond which the grain size attained was smaller, and eventually, ablation was observed at an overlap of 97.5% and higher. These trends and observed surface morphology of annealed samples suggest that the crystallization mechanism is like sequential lateral solidification, similar to 800 nm-FLA and excimer laser annealing. Raman spectroscopic studies show that the degree of crystallization achieved with 400 nm-FLA is even higher than that of 800 nm-FLA. Cross-sectional scanning electron microscopic images indicate that the 100 nm-thick a-Si film is not fully crystallized. This can be explained by the much shorter penetration depth of 400 nm light than that of 800 nm light in a-Si.

Comparative performance evaluation of four photovoltaic technologies in saharan climates of Algeria: ghardaïa pilot station

The aim of this paper is to present an evaluation of the performancerate of four different photovoltaic techniques in the Saharan environment. The purpose of this study is to investigate, analyse, discuss and illustrate the most effective of the different photovoltaic cell technologies (monocrystalline , amorphous silicon , poly-crystalline silicon and cadmium telluridethin film ) installed in Ghardaia which is located in southern of Algeria’s Sahara desert. In order to choose the most suitable technology in the Saharan climate conditions, the energy values produced by the plant were compared to those found by the PVSYST sizing software. The results show that thin-film and amorphous silicon panels produce low illumination, so they are the best choice for the Saharan environment.

Determination of optimum deposition conditions amorphous silicon thin films

The aim of the study is to increase the efficiency of solar cell elements on the basis of thin films of amorphous-nanocrystalline silicon, by incorporating silicon nanocrystals (Si-NPs) into a film of hydrogenated amorphous silicon (a-Si:H). The purpose of the operation is to work out optimum deposition modes of amorphous silicon films. An obstacle here is the occurrence of dust particles affecting the properties of the resulting films. As well as increasing the overall efficiency of photocells due to the creation of single-crystal quantum dots (nanocristalites), which will play the role of centers of flow of parasitic currents. The obtained samples were studied using physical methods of analysis (Raman light scattering, study of the band gap).

(Invited) Fabrication and Characterization of B- or P- Doped Silicon Nanoparticles Embedded in SiOxNy Transparent Matrix

Silicon nanocrystals (Si-Ncs)-based devices are some of the most studied nanomaterials and could be of great interest in many fields such as electronics1, optical sensing2, photovoltaics3 and biomedicine4. First of all, it is important to control their size, shape and aggregation which are parameters related to their potential applications. In our case, these Si-Ncs are embedded in an insulating dielectric matrix of SiOxNy, resulting in a composite material with a tunable effective bandgap. Doping Si-Ncs can enlarge their applicability by modifying their electrical properties5, the effective bandgap5, the light absorption range and the nanoparticles photoluminescence.In this study, we used a magnetron sputtering deposition system to deposit silicon rich silicon oxynitride (SiOxNy) thin films. Then, n- and p-type dopants such as boron, arsenic and phosphorus were introduced in the thin films using ion implantation which was carried out at different doses and accelerating energies5. Thanks to a thermal treatment at high temperature, silicon atoms diffuse and nucleate to form silicon nanostructures with size varying between 1.5 nm and several dozen nanometers. This step is necessary for the phase separation leading to the nanoparticles nucleation.Atom probe tomography (APT) and energy dispersive X-ray mapping in scanning TEM mode, coupled with spectrum imaging on silicon plasmon, were performed to localize the dopant impurities versus the Si-Ncs. The results have showed that n-type (P and As) dopants were energetically favored to be placed into the Si-Ncs core, whereas p-type dopants are preferentially located at the Si-NCs/SiOxNy interface.The effect of composition and post-deposition annealing temperature on the Si-Ncs light-emitting properties was investigated by the examination of photoluminescence (PL) evolution. The dopants distribution profiles within the host matrix were determined by Rutherford Backscattering Spectroscopy (RBS). EFTEM combined with X Rays mapping analysis have exhibited that the incorporation of arsenic at a high doping level (dose of 5x 1016 As/cm2) leads to an increase of the Si-Ncs size by a factor around 4, precisely from 4 nm to more than 20 nm (Figure 1). On the other hand, the EFTEM-X Rays analysis was unable to detect boron dopants in the SiOxNy films. At this doping level, it could be possible to observe a localized surface plasmon resonance (LSPR) 7.For both n- and p-type dopants, the silicon nanoparticles crystalline fraction was found to be much higher than the undoped structures. The dopants facilitate the nucleation and growth of crystalline nanoparticles. This behavior is quite similar to that observed for thin amorphous silicon films. The results have showed that the nanostructures shape have different behaviors depending on the dopants.Finally Schottky diodes were prepared. The I-V characteristic curves show that the electrical conduction in the -doped structures increases with the dopant doses and we obtained a rectifier behavior. The results have showed that the heavily As-doped silicon structures are limited by the tunnel oxide thickness between the large Si-Ncs. These results are very encouraging when aiming at implementing such nanocomposite materials in photovoltaic solar cells.

Direct and High-Throughput Fabrication of Mie-Resonant Metasurfaces via Single-Pulse Laser Interference.

High-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances have been of great interest in a variety of applications such as imaging, sensing, photovoltaics, and others, which led to the necessity of an efficient large-scale fabrication technique. To address this, here we demonstrate the use of single-pulse laser interference for direct patterning of an amorphous silicon film into an array of Mie resonators a few hundred nanometers in diameter. The proposed technique is based on laser-interference-induced dewetting. A precise control of the laser pulse energy enables the fabrication of ordered dielectric metasurfaces in areas spanning tens of micrometers and consisting of thousands of hemispherical nanoparticles with a single laser shot. The fabricated nanoparticles exhibit a wavelength-dependent optical response with a strong electric dipole signature. Variation of the predeposited silicon film thickness allows tailoring of the resonances in the targeted visible and infrared spectral ranges. Such direct and high-throughput fabrication is a step toward a simple realization of spatially invariant metasurface-based devices.

Modification of the Photoelectric Properties of Undoped Hydrogenated Amorphous Silicon Films under Preliminary Illumination at Elevated Temperatures

The effect of preliminary weak illumination at elevated temperatures on the photoelectric properties of undoped α-Si:H films is investigated. It is found that the dark conductivity and photoconductivity of films increases with the preliminary-illumination intensity, and the parameter γ determining the dependence of the photoconductivity on the illumination intensity decreases due to an increasing fraction of the bimolecular recombination of electrons at energy levels of the tail of the density of states of the conduction band. It is assumed that this effect may be induced by the presence of an uncontrolled oxygen impurity and an increase in the concentration of electrically active oxygen as a result of preliminary illumination of the films at elevated temperatures.

Damp-Heat-Stable, High-Efficiency, Industrial-Size Silicon Heterojunction Solar Cells

Summary Silicon heterojunction (SHJ) solar cells employ nanometer-thin stacks of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) films as carrier-selective contacts. To achieve excellent carrier selectivity, the a-Si:H must be carefully optimized to guarantee an atomically sharp a-Si:H/c-Si interface. In this work, by combining experiments with molecular dynamics and ab initio calculations, we unveil that H atoms bonded to internal-void surfaces in a-Si:H broaden its optical band gap via a filamentary effect near the valence-band maximum. The photovoltaic performance of rear-emitter SHJ solar cells can be significantly improved by tailoring the Si−H bonding state in the front a-Si:H passivation layer, resulting in a power conversion efficiency (PCE) of 23.4% on a 6-in cell. By implementing double antireflection coatings (ARCs) of SiNx and SiOx, the PCE is further improved to 23.9%. More importantly, the ARC devices show prominently improved damp-heat stability without encapsulation in 1,000-h aging at 85°C, 85% relative humidity.

Formation of Nanocrystalline Silicon in Tin-Doped Amorphous Silicon Films

The process of crystalline silicon phase formation in tin-doped amorphous silicon (a-SiSn) films has been studied. The inclusions of metallic tin are shown to play a key role in the crystallization of researched a-SiSn specimens with Sn contents of 1–10 at% at temperatures of 300–500 ∘C. The crystallization process can conditionally be divided into two stages. At the first stage, the formation of metallic tin inclusions occurs in the bulk of as-precipitated films owing to the diffusion of tin atoms in the amorphous silicon matrix. At the second stage, the formation of the nanocrystalline phase of silicon occurs as a result of the motion of silicon atoms from the amorphous phase to the crystalline one through the formed metallic tin inclusions. The presence of the latter ensures the formation of silicon crystallites at a much lower temperature than the solid-phase recrystallization temperature (about 750 ∘C). A possibility for a relation to exist between the sizes of growing silicon nanocrystallites and metallic tin inclusions favoring the formation of nanocrystallites has been analyzed.

Impact of Laser Treatment on Hydrogenated Amorphous Silicon Properties

Herein, the application of laser radiation to locally modify the hydrogen distribution within hydrogenated amorphous silicon films on a short time scale is studied. The impact of laser power and irradiation time on the temperature of the silicon layer during the laser treatment and the hydrogen outdiffusion is analyzed. Moreover, the resulting optoelectronic properties of the amorphous silicon are examined. On a timescale of a few seconds or less, the hydrogen concentration in the near‐surface region of the silicon layer can be successfully decreased without major impact on the optoelectronic properties.

Suppression of Parasitic Epitaxy Growth and Realization of High-Quality Wafer Surface Passivation of Silicon Heterojunction Solar Cells

Intrinsic hydrogenated amorphous silicon (i-a-Si:H) films lead to excellent surface passivation of crystalline silicon wafers. However, a-Si:H deposition on a crystalline silicon wafer often results in undesired epitaxy growth, which deteriorates the passivation property, In this paper, we studied the influence of varying plasma enhanced chemical vapor deposition (PECVD) parameters, such as the product of the gas pressure (P) and the electrode distance (Di) and the hydrogen dilution ratio (P = SiH4/(H2 + SiH4) × 100), on the passivation quality and the properties of silicon heterojunction (SHJ) solar cells. Measurements showed that proper combinations of high P × Di and large P values can yield high minority carrier lifetimes (MCLTs) of passivated silicon wafers. Also, the tendency of MCLTs measured from passivated wafers is the same as that for open circuit voltages (Voc) of fabricated SHJ solar cells. A high Voc is obtained from SHJ solar cells when unwanted epitaxial growth is minimized at the wafer surface.

Alternating current impedance spectroscopic investigation of an a-Si:H/c-Si heterojunction with porous silicon multilayers

In this paper, we study structures contained thin hydrogenated amorphous silicon (a-Si:H) films deposited on porous silicon multilayers (Distributed Bragg Reflector DBR) formed by electrochemical etching of crystalline silicon (c-Si) wafers. The a-Si:H thin films were deposited by the Direct-current magnetron sputtering technique in a mixture of argon and hydrogen atmosphere. It is shown in our previous work that the Bragg mirrors formed on crystalline silicon play an important role in the light trapping in a-Si:H. The structural features of porous silicon, in terms of porosity, thickness and surface roughness, were determined using spectroscopic ellipsometry. The elaborated structures were analysed by impedance spectroscopy. The fit of data obtained from the impedance spectroscopy allowed us to determine the electrical equivalent circuit. This permits the identification of the electronic behaviour of the structures and allows us to explain the role of their different components, including the interfaces. For this purpose, we have proceeded successively. First, by the characterisation of porous silicon monolayers, afterward structures formed by porous silicon multilayers as DBR and structures of a-Si:H/c-Si and a-Si:H/PSi(DBR)/c-Si are characterised. A negative capacitance was found for the heterojunction with DBR, which was attributed to the existence of defect states at the interfaces between porous and amorphous silicon.

On the crystallization behavior of sputter-deposited a-Si films on 4H-SiC

The crystallization behavior of sputter-deposited amorphous silicon (a-Si) films on 4H-SiC by post-deposition annealing (PDA) is investigated. Film thicknesses between 100 and 1500 nm were selected. PDA was carried out at temperatures between 800 and 1100 °C. In addition, the influence of the annealing time and the heating rate is evaluated. Furthermore, the impact of high boron doping and the face of the SiC substrate was determined. A low-pressure chemical vapor deposition (LPCVD) Si film was deposited on the same substrate for reasons of comparison. The crystallinity of the Si films was investigated using X-ray diffractometry and transmission electron microscopy, whereas atomic force microscopy was applied to determine the surface roughness. The amorphous phase of the as-deposited Si thin films was crystallized into a polycrystalline microstructure applying different PDA conditions. A sharp and distinct interface to the 4H-SiC substrate without the presence of structural defects, amorphous regions or any indication for alloy formation was observed. No influence of the underlying SiC to stimulate the texture in the Si grain orientation could be found. Different PDA parameters showed an influence on the average grain size and the stress inside the grains. Highly boron-doped samples exhibit mean grain size values up to several hundreds of nanometers. Although starting with a poor film quality represented by an amorphous microstructure due to sputter deposition at low substrate temperatures, polycrystalline Si layers on 4H-SiC substrates are realized exhibiting a more than twenty times lower surface roughness compared to those synthesized with LPCVD.