Amorphous Hydrogenated Silicon Films Research Articles

Hydrogenated Amorphous Silicon Film Transistor Fabrication with Back-Channel-Oxidized Method

Hydrogenated Amorphous Silicon Film Transistor Fabrication with Back-Channel-Oxidized Method

Observation of electrically detected electron nuclear double resonance in amorphous hydrogenated silicon films

We report on the electrical detection of electron nuclear double resonance (EDENDOR) through spin-dependent tunneling transport in an amorphous hydrogenated silicon thin film. EDENDOR offers a many orders of magnitude improvement over classical ENDOR and is exclusively sensitive to paramagnetic defects involved in electronic transport. We observe hyperfine interactions with 1H nuclei very close to silicon dangling bond defects. These observations substantially extend recent EDENDOR observations involving silicon vacancy defects and 14N hyperfine interactions with fairly distant nitrogen atoms in 4H-SiC bipolar junction transistors. We have improved the detection scheme utilized in the earlier study by combining magnetic field modulation with RF amplitude modulation; this combination significantly improves the operation of the automatic power leveling scheme and the overall sensitivity.

Study on Passivation of Monocrystalline Silicon by Tandem Hydrogenated Amorphous Silicon Film

Study on Passivation of Monocrystalline Silicon by Tandem Hydrogenated Amorphous Silicon Film

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.

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.

Evolution of femtosecond laser-induced periodic structures formed on amorphous silicon surface

The laser-induced periodic surface structures (LIPSS) with period close to the femtosecond laser radiation wavelength (1.25 μm) were formed on amorphous hydrogenated silicon films (a-Si:H). The ripple ridges direction relative to laser radiation polarization was changed from perpendicular to parallel with increasing number of pulses from 50 to 1000. The experimentally observed LIPSS evolution is in a good agreement with the model proposed by J.E. Sipe where so-called efficacy factor depends on the real and imaginary parts of the dielectric constant and defines the LIPSS wave vector on the irradiated surface. In turn, the dielectric constant complex value is varied due to concentration change for the nonequilibrium electrons excited by different number of high-power femtosecond laser pulses. According to the theoretical modelling, the excited nonequilibrium electron concentration required for turning the LIPSS direction is equal to 8.2·1021 cm−3.

Ultrafast Carrier Recombination and Transient Lattice Temperature Changes in 25 nm Thin Hydrogenated Amorphous Silicon Films

Hydrogenated amorphous silicon (a-Si:H) represents an important material for optoelectronic and photovoltaic applications. Fundamental knowledge regarding its charge carrier dynamics and thermalization behavior after photoexcitation is therefore required. Here, we provide a detailed broadband UV–vis–NIR transient absorption study of 25 nm thin a-Si:H films on glass upon pumping at 390, 400, or 650 nm, covering the wavelength range 300–1600 nm. Photoexcited carriers relax in the extended states within a few hundred femtoseconds and recombine very quickly. In contrast to a previously suggested bimolecular recombination mechanism for carrier densities larger than 5 × 1018 cm–3, we unambiguously identify Auger-type third-order recombination with the rate constant k3 = (2.3 ± 0.6) × 10–29 cm6 s–1, which is extracted from the absorption decay in the NIR region. The released recombination energy leads to a fast lattice heating which is indicated by a fast rise of absorption in the UV–vis region. This spectral fe...

Crystallization of submicron amorphous hydrogenated silicon films with different hydrogen concentration by nanosecond ruby laser irradiation

Pulse laser annealing remains an actual problem aimed to crystallization of amorphous films on nonrefractive substrates. But, the most previous works studied laser crystallization of relatively thin (<300 nm) a-Si:H films and mainly used excimer lasers. But, excimer lasers are not suitable for crystallization of submicron a-Si:H films due to low penetration depth in a-Si:H at such wavelengths. The problem can be resolved by using lasers with longer wavelengths. The desirable result of crystallization also depends on the choice of proper laser fluence, which is different for films with different hydrogen concentrations. In this work, the processes of a pulsed ruby laser induced crystallization of submicron (0.7 μm) amorphous hydrogenated silicon films with different hydrogen concentrations (2, 12, and 39 at. %) by different laser fluences were investigated. The films were prepared on glass substrates by plasma enhanced chemical vapor deposition technique followed by isothermal annealing in nitrogen atmosphere. The laser annealing (λ = 694 nm) was carried out at a pulse duration of 80 ns (full width at half-maximum) in the fluence range from 0.6 to 2.1 J/cm2. The laser fluence thresholds for surface area crystallization were found for different hydrogen concentrations in the films. The increase of hydrogen concentration leads to an increase of the threshold energy density (laser fluence) for surface area crystallization due to a decrease of light absorption in the films with a higher hydrogen concentration. Also, it was shown that ruby laser radiation can penetrate and partially crystallize the full depth of the submicron a-Si:H film, but the problem of homogeneity remains.Pulse laser annealing remains an actual problem aimed to crystallization of amorphous films on nonrefractive substrates. But, the most previous works studied laser crystallization of relatively thin (<300 nm) a-Si:H films and mainly used excimer lasers. But, excimer lasers are not suitable for crystallization of submicron a-Si:H films due to low penetration depth in a-Si:H at such wavelengths. The problem can be resolved by using lasers with longer wavelengths. The desirable result of crystallization also depends on the choice of proper laser fluence, which is different for films with different hydrogen concentrations. In this work, the processes of a pulsed ruby laser induced crystallization of submicron (0.7 μm) amorphous hydrogenated silicon films with different hydrogen concentrations (2, 12, and 39 at. %) by different laser fluences were investigated. The films were prepared on glass substrates by plasma enhanced chemical vapor deposition technique followed by isothermal annealing in nitrogen atmosph...

Giant enhancement of free charge carrier concentration in boron-doped amorphous hydrogenated silicon under femtosecond laser crystallization

The modification of boron-doped amorphous hydrogenated silicon films [a-Si:H(B)] with femtosecond laser radiation is studied in this work. It is demonstrated that femtosecond laser crystallization of the a-Si:H(B) film area leads to extremely high values of the free charge carrier (hole) concentration, which is typical for degenerated semiconductors. The free charge carrier concentration is locally determined by measuring the Raman spectra in the modified areas. The shape of Raman spectra is typical for Fano resonance. It is found that the charge carrier concentration in the modified areas may exceed 1020 cm−3 depending on the femtosecond irradiation conditions. The areas with such a high concentration of free charge carriers are also characterized by high volume fraction of crystalline phase (more than 90%). Such a sharp increase in the free charge carrier concentration in the modified areas may be explained by an increase in concentration of electrically active boron atoms. The activation energy of the temperature dependence of the conductivity for laser beam treated areas is in full agreement with the data obtained from the analysis of the Raman scattering spectra.

Femtosecond laser pulse modification of amorphous silicon films: control of surface anisotropy

A one-dimensional surface relief with a 1.20 ± 0.02 µm period was formed in amorphous hydrogenated silicon films as a result of irradiation by femtosecond laser pulses (1.25 µm) with a fluence of 0.15 J cm−2. Orientation of the formed structures was determined by the polarization vector of the radiation and the number of acting pulses. Nanocrystalline silicon phases with volume fractions from 40 to 67% were detected in the irradiated films according to the analysis of Raman spectra. Observed micro- and nanostructuring processes were caused by surface plasmon–polariton excitation and near-surface region nanocrystallization, respectively, in the high-intensity femtosecond laser field. Furthermore, the formation of Si-III and Si-XII silicon polymorphous modifications was observed after laser treatment with a large exposure dose. The conductivity of the film increased by three orders of magnitude at proper conditions after femtosecond laser nanocrystallization compared to the conductivity of the untreated amorphous surface. The conductivity anisotropy of the irradiated regions was also observed due to the depolarizing contribution of the surface structure, and the non-uniform intensity distribution in the cross-section of the laser beam used for modification.

Uniformity control of the deposition rate profile of a-Si:H film by gas velocity and temperature distributions in a capacitively coupled plasma reactor

The effect of neutral transport on the deposition rate profiles of thin films formed by plasma-enhanced chemical vapor deposition (PECVD) is investigated to improve the uniformity of amorphous hydrogenated silicon films. The PECVD reactor with a cylindrical showerhead is numerically simulated with a variation of the gas velocity and temperature in the capacitively coupled plasma with an intermediate-pressure SiH4/He gas mixture. The modulation of the gas velocity distribution results in a noticeable change in the density distributions of neutral molecules such as SiH4, SiH3, H, SiH2, and Si2H6, especially in the vicinity of the electrode edge. With the locally accelerated gas flow, the concomitant increase in Si2H6 density near the electrode edge induces increases in both the electron density and the deposition rate profile near the electrode edge. In addition, it is observed that changing the surface temperature distribution by changing the sidewall temperature can also effectively modulate the plasma density distributions. The simulated deposition rate profile matches the experimental data well, even under non-isothermal wall boundary conditions.

Analysis of intermediate pressure SiH4/He capacitively coupled plasma for deposition of an amorphous hydrogenated silicon film in consideration of thermal diffusion effects

To achieve rapid, uniform deposition of an amorphous hydrogenated silicon (a-Si:H) film, a capacitively coupled plasma (CCP) is often used at an intermediate pressure (>100 Pa), with a silane (SiH4)-based mixture. At these pressures, heavy particle interactions (such as ion–ion, ion–neutral, and neutral–neutral reactions) contribute significantly to the formation of precursor radicals. By adding a consideration of the thermal diffusion effects to the neutral transport equation, the chemical processes have been numerically analyzed with variation in the number fraction of SiH4 and electrode spacing using a two-dimensional fluid model of radio frequency discharges in a cylindrically symmetric CCP reactor. The non-uniformity of the deposition rate profiles increases consistently as electrode spacing increases, although the non-uniformity of the plasma parameters decreases with the increase of electrode spacing. The simulated deposition rate profiles match well with the experimental data for the change of electrode spacing. Based on the validation of our model, we propose predictive designs to potentially improve the reactor and process by modifying the thermal and electrical surface conditions.

Anisotropy of optical, electrical, and photoelectrical properties of amorphous hydrogenated silicon films modified by femtosecond laser irradiation

Two types of independent anisotropic structures have been formed simultaneously in amorphous hydrogenated films by applying a femtosecond laser pulse to them, i.e., a structure with a period of several micrometers to several tens of micrometers and a structure with a period of several hundred nanometers. The formation mechanisms of these strictures are different, which allows us to orient them relative to each other in a desirable way. Both structures independently influence the optical properties of the modified films, which causes the diffraction of transmitted light and making the films polarization-sensitive. The conductivity of the modified films correlates with the mutual orientation of the anisotropic structures, whereas no interrelation between the photoconductivity and optical performance of the modified films has been observed.

Flexible Electronics: High Pressure Chemical Vapor Deposition of Hydrogenated Amorphous Silicon Films and Solar Cells (Adv. Mater. 28/2016).

On page 5939, J. V. Badding and co-workers describe the unrolling of a flexible hydrogenated amorphous silicon solar cell, deposited by high-pressure chemical vapor deposition. The high-pressure deposition process is represented by the molecules of silane infiltrating the small voids between the rolled up substrate, facilitating plasma-free deposition over a very large area. The high-pressure approach is expected to also find application for 3D nanoarchitectures.

High Pressure Chemical Vapor Deposition of Hydrogenated Amorphous Silicon Films and Solar Cells.

Thin films of hydrogenated amorphous silicon can be produced at MPa pressures from silane without the use of plasma at temperatures as low as 345 °C. High pressure chemical vapor deposition may open a new way to low cost deposition of amorphous silicon solar cells and other thin film structures over very large areas in very compact, simple reactors.

Femtosecond Laser Crystallization of Boron-doped Amorphous Hydrogenated Silicon Films

Crystallization of amorphous hydrogenated silicon films with femtosecond laser pulses is one of the promising ways to produce nanocrystalline silicon for photovoltaics. The structure of laser treated films is the most important factor determining materials' electric and photoelectric properties. In this work we investigated the effect of femtosecond laser irradiation of boron doped amorphous hydrogenated silicon films with different fluences on crystalline volume fraction and electrical properties of this material. A sharp increase of conductivity and essential decrease of activation energy of conductivity temperature dependences accompany the crystallization process. The results obtained are explained by increase of boron doping efficiency in crystalline phase of modified silicon film.

Surface Morphology and Structural Modification Induced by Femtosecond Pulses in Hydrogenated Amorphous Silicon Films.

This work investigates the modification, resulting from fs-laser irradiation (150 fs, 775 nm and 1 kHz), on the structure and surface morphology of hydrogenated amorphous silicon (a-Si:H) thin films. The sample morphology was studied by performing a statistical analyzes of atomic force microscopy images, using a specially developed software that identifies and characterizes the domains (spikes) produced by the laser irradiation. For a fluence of 3.1 MJ/m2, we observed formation of spikes with smaller average height distribution, centered at around 15 nm, while for fluencies higher than 3.7 MJ/m2 aggregation of the produced spikes dominates the sample morphology. On the other hand, Raman spectroscopy revealed that a higher crystalline fraction (73%) is obtained for higher fluences (> 3.1 MJ/m2), which is accompanied by a decrease in the size of the produced crystals. Therefore, such results indicate that there is a trade-off between the spike distribution, crystallization fraction and size of the nanocrystals attained by laser irradiation, which has to be taken into account when using such approach for the development of devices.

The Nature and the Kinetics of Light-Induced Defect Creation in Hydrogenated Amorphous Silicon Films and Solar Cells

The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon (a-Si:H) films and solar cells are investigated by means of Doppler broadening positron annihilation spectroscopy, Fourier transform photocurrent spectroscopy, and J-V characterization. There is a strong correlation between the open volume deficiencies in a-Si:H and the Staebler-Wronski effect (SWE). The carrier generation and recombination profiles in the absorber layer are spatially correlated, and the recombination due to defects in the top and bottom parts of the absorber layer is different. Furthermore, the various defect distributions in the bandgap have different defect creation kinetics. It is demonstrated that the SWE defect formation kinetics in a solar cell are very complex and can impossibly be described by one time scaling ~ t <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</sup> as is often claimed.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Trions in silicon nanocrystals in an amorphous hydrogenated silicon matrix

A simple theoretical model of the radiative recombination in amorphous hydrogenated silicon films containing silicon nanocrystals is suggested. This material is used for the fabrication of modern thin-film solar cells. The calculation results are in good agreement with experimental data on the photoluminescence spectra.