Microcrystalline Silicon Films Research Articles

Effect of the RF Power of PECVD on the Crystalline Fractions of Microcrystalline Silicon (μc-Si:H) Films and Their Structural, Optical, and Electronic Properties

In this work, we report on the deposition of microcrystalline silicon (µc-Si:H) films produced from silane (SiH4), hydrogen (H2), and argon (Ar) mixtures using the plasma-enhanced chemical vapor deposition (PECVD) technique at 200 °C. Particularly, we studied the effect of RF power on the crystalline fraction (XC) of the deposited films, and we have correlated the XC with their optical, electrical, and structural characteristics. Different types of characterization were performed in the µc-Si:H film series. We used several techniques, such as Raman scattering spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM), among others. Our results show that RF power had a strong effect on the XC of the films, and there is an optimal value for producing films with the largest XC.

Comparative Study on the Quality of Microcrystalline and Epitaxial Silicon Films Produced by PECVD Using Identical SiF4 Based Process Conditions.

Hydrogenated microcrystalline silicon (µc-Si:H) and epitaxial silicon (epi-Si) films have been produced from SiF4, H2 and Ar mixtures by plasma enhanced chemical vapor deposition (PECVD) at 200 °C. Here, both films were produced using identical deposition conditions, to determine if the conditions for producing µc-Si with the largest crystalline fraction (XC), will also result in epi-Si films that encompass the best quality and largest crystalline silicon (c-Si) fraction. Both characteristics are of importance for the development of thin film transistors (TFTs), thin film solar cells and novel 3D devices since epi-Si films can be grown or etched in a selective manner. Therefore, we have distinguished that the H2/SiF4 ratio affects the XC of µc-Si, the c-Si fraction in epi-Si films, and the structure of the epi-Si/c-Si interface. Raman and UV-Vis ellipsometry were used to evaluate the crystalline volume fraction (Xc) and composition of the deposited layers, while the structure of the films were inspected by high resolution transmission electron microscopy (HRTEM). Notably, the conditions for producing µc-Si with the largest XC are different in comparison to the fabrication conditions of epi-Si films with the best quality and largest c-Si fraction.

Improving structural, optical, and electrical properties of μc-Si:H using non-uniform hydrogen dilution treatment on RF-PECVD prepared layers

Hydrogenated microcrystalline silicon (μc-Si:H) has been used in solar cell technology to improve solar conversion efficiency. In this work, the effects of some of the main preparation parameters of μc-Si:H have been studied. The μc-Si:H films were deposited in a standard RF-PECVD at the frequency of 13.56 MHz on glass and silicon substrates without any extra heat treatment. By changing the combination of silane (SiH4) and hydrogen (H2) in the deposition chamber, the proportion of crystalline silicon in the resulting film can be altered. The effects of hydrogen dilution ratio, defined as R=H2SiH4, on the structural, optical, and electrical properties of hydrogenated silicon, have been investigated. Two hydrogen dilution strategies have been used: uniform and non-uniform. The hydrogen dilution ratio was changed from 20 to 33, while other parameters such as RF power, deposition pressure, temperature, and time were remained constant at 30 W (power density of 300 mW/cm2), 0.3 Torr, 300 °C, and 60 min, respectively. Since increasing the number of hydrogen atoms during the deposition process can cause surface damage and consequently, the reduction of crystal growth, the effect of non-uniform dilution ratio has also been considered. The properties of the resulting hydrogenated microcrystalline silicon films were analyzed using X-ray diffraction (XRD), atomic force microscope (AFM), Raman spectroscopy, UV–Visible–near-infrared spectroscopy, and Fourier transform infrared (FTIR) spectroscopy etc. The maximum crystalline ratio of 61%, the crystallite size of 83.3 nm, the energy gap of 1.6 eV, the hydrogen content of 6.7%, and structural coefficient of 0.53, and the highest conductivity have been obtained for the hydrogenated microcrystalline silicon films prepared using the non-uniform dilution ratio of hydrogen. The lower energy gap of hydrogenated microcrystalline silicon with respect to that of the hydrogenated amorphous silicon, results in the absorption of a wider band of the solar spectrum and, therefore, higher efficiency of the related solar cells. In addition, the higher conductivity of hydrogenated microcrystalline silicon compared to that of the hydrogenated amorphous silicon, is of interest for application in photovoltaic and opto-electronic devices.

Studies on optical signal due to oxygen effect on hydrogenated amorphous/crystalline silicon thin films

We have studied the effects of oxygen on hydrogenated amorphous/crystalline silicon films in terms of their structural and optical properties. Different hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated between microcrystalline and amorphous transition region. X-ray diffraction, Raman, FTIR and UV-Vis emission spectrometry have been used to characterize different films. A comparison of the results with those of different types of films like hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon ($\mu$c-Si:H) films reveal their superiority as an excellent substance for solar cell. X-ray diffraction, FTIR and Raman spectral analysis show that difference of the H dilution effect has a major effect on the structure of the film and the optical properties. Photoluminescence analysis of amorphous silicon-oxygen and silicon-hydride alloy films has established their efficient application appropriate as Si based light emitting devices. A large optical band gap of 1.83 eV and appearance of strong photo luminescence at 2.0 eV validates the applicability of a-SiO:H film as a better alternative for the solar cells.

Validation of MEMS Based New MicroLPCVD Technique for Thin Films Deposition

In this work we present and optimized a new microCVD technique to produce thin films for microelectronics devices. The technique was first presented in a recent work and here we successfully utilize to produce microcrystalline Silicon films at 1 Torr and temperatures of 740 oC, utilizing SiH4 as precursor gas. The technique is based on a MEMS micro- heater and presents 3 main advantages: (1) the heating region is very localized, (2) allows to attain easily temperatures as high as 1000 oC) and (3) present very short heating and cooling time. In addition to the fabrication process of MEMS micro-heater, we also present a multiphysics computer simulation study to understand its thermal response of MESM micro-heater and to predicting the final temperatures attainable with the technique.

The effects of air, oxygen and water exposure on the sub-bandgap absorption, the electronic conductivity and the ambipolar diffusion length in highly crystalline microcrystalline silicon films for photovoltaic applications

Reversible and irreversible changes due to long term air and short term de-ionized water (DIW) or pure oxygen exposure were investigated in about 1 µm thick hydrogenated microcrystalline silicon (µc-Si:H) films deposited on rough glass substrates, thereby comparing highly crystalline with compact material. Time and temperature dependent dark conductivity, steady-state photoconductivity, the steady-state photocarrier grating and dual-beam photoconductivity methods have been used to study the effects. Standard measurement procedures defined previously have been carefully applied to record the changes after different treatments using the steady-state methods under light. After long term air exposure of highly crystalline µc-Si:H films, a thermal annealing step leads to an increase in dark conductivity (σD) and steady-state photoconductivity (σph) as well as to a significant increase in the sub-bandgap absorption. These effects are likely due to a reversible recovery from surface adsorbents in a porous microstructure after air exposure resulting in surface charge and Fermi level shifts in agreement with earlier results. Compact µc-Si:H films showed only marginal effects upon an annealing after long term air exposure suggesting much reduced susceptibility to surface adsorbent induced by Fermi level shifts. Five hours exposure to de-ionized water at 80 °C caused more than an order of magnitude increase in σD and σph and a substantial decrease in the sub-bandgap absorption spectrum in highly crystalline as well as in compact µc-Si:H films. In addition, minority carrier diffusion lengths measured by the steady-state photocarrier grating method improved significantly. The changes after exposure to water were not reversible upon our standard annealing procedure. Exposure to high purity oxygen gas at 150 °C resulted in similar effects like the exposure to DIW. Also here the changes in material properties were not reversible upon annealing. Results are discussed in terms of adsorption and chemical reactions on surfaces in the porous highly crystalline material versus the materials with more compact structures. Results are compared to earlier observations and consequences for device application will be indicated.

Deposition of Amorphous and Microcrystalline Films of Silicon by the Gas-Jet Plasma-Chemical Method

Uniform thin films of amorphous and microcrystalline silicon are obtained within the forevacuum range of pressures by the gas-jet plasma-chemical method with the activation of gas by an electron beam. The effect of the flow rate of the carrier gas argon, the concentration of monosilane, the pressure in the reaction chamber, material of the substrate, and value of the current of the activating electron beam on the rate of deposition, photosensitivity, and crystallinity of the layers of silicon is investigated. For the amorphous silicon films, deposition rates of >1 nm/s are achieved, and for layers with a crystallinity exceeding 60%, the rate of deposition exceeded 0.6 nm/s. It is found that the substrate material does not have an effect on the structure of the deposited layers of silicon and the rate of their deposition.

Effects of ratio of hydrogen flow on microstructure of hydrogenated microcrystalline silicon films deposited by magnetron sputtering at 100 °C

Hydrogenated microcrystalline silicon (µc-Si:H) films were prepared on glass and silicon substrates by radio frequency magnetron sputtering at 100 °C using a mixture of argon (Ar) and hydrogen (H2) gasses as precursor gas. The effects of the ratio of hydrogen flow (H2/(Ar+H2)%)) on the microstructure were evaluated. Results show that the microstructure, bonding structure, and surface morphology of the µc-Si:H films can be tailored based on the ratio of hydrogen flow. An amorphous to crystalline phase transition occurred when the ratio of hydrogen flow increased up to 50%. The crystallinity increased and tended to stabilize with the increase in ratio of hydrogen flow from 40% to 70%. The surface roughness of thin films increased, and total hydrogen content decreased as the ratio of hydrogen flow increased. All µc-Si:H films have a preferred (111) orientation, independent of the ratio of hydrogen flow. And the µc-Si:H films had a dense structure, which shows their excellent resistance to post-oxidation.

Осаждение пленок кремния, легированных бором и фосфором газоструйным плазмохимическим методом

AbstractDoped silicon films are fabricated using diborane and phosphine as doping gases by gas-jet plasma-chemical deposition with the application of an electron beam. The influence of the dopant-gas concentration, the addition of a fluorine-containing gas, and the background pressure on the conductivity and crystalline structure of silicon layers is investigated. Boron-doped amorphous films ( a -Si:H) with a conductivity up to 5.2 × 10^–3 (Ω cm)^–1 are fabricated; when doping with phosphorus, microcrystalline silicon films ( mc -Si:H) with a crystallinity up to 70% and conductivity at a level of 1 (Ω cm)^–1 are fabricated.

Deposition of Silicon Films Doped with Boron and Phosphorus by the Gas-Jet Plasma-Chemical Method

Doped silicon films are fabricated using diborane and phosphine as doping gases by gas-jet plasma-chemical deposition with the application of an electron beam. The influence of the dopant-gas concentration, the addition of a fluorine-containing gas, and the background pressure on the conductivity and crystalline structure of silicon layers is investigated. Boron-doped amorphous films (a-Si:H) with a conductivity up to 5.2 × 10–3 (Ω cm)–1 are fabricated; when doping with phosphorus, microcrystalline silicon films (mc-Si:H) with a crystallinity up to 70% and conductivity at a level of 1 (Ω cm)–1 are fabricated.

Осаждение аморфных и микрокристаллических пленок кремния газоструйным плазмохимическим методом

Using gas-jet plasma-chemical method with gas activation by an electron beam, uniform thin films of amorphous and microcrystalline silicon in the forevacuum pressure range were obtained. Effects of argon carrier gas flow rate, monosilane gas concentration, background pressure in the reaction chamber, the substrate material, and the magnitude of the activating electron beam current on the deposition rate, photosensitivity, and crystallinity of silicon layers were studied. Deposition rate above 1 nm/s was achieved for films of amorphous silicon, and for layers with crystallinity about 60% the deposition rate was exceeded 0.6 nm/s. It is established that the substrate material does not affect the structure of the deposited silicon layers and their deposition rate.

Controllability of structural and electrical properties of silicon films grown in atmospheric-pressure very high-frequency plasma

Amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) films were deposited in atmospheric-pressure (AP) He/H2/SiH4 plasma excited by a 150 MHz very high-frequency (VHF) power at a temperature of 220 °C. The variations of thickness and film crystallinity in the gas flow direction were studied using two electrodes (length = 16 and 5 mm). The electrical properties of the deposited Si layers were evaluated by fabricating bottom-gate thin film transistors (TFTs). The results showed that the chemical reactions both in gas phase and on the growing film surface were significantly enhanced in AP-VHF plasma, promoting phase transition from amorphous to microcrystalline on a time scale of the order of less than 0.1 ms. The performance of bottom-gate TFTs showed that a-Si layers formed in the upstream portion of the plasma zone had good enough electrical property (field-effect mobility of 1–1.5 cm2 · V−1 · s−1) despite very high deposition rates (several tens of nm · s−1). Meanwhile, µc-Si layers deposited in the downstream portion of the plasma zone did not acquire their original superiority over a-Si films with reference to electrical properties, although the use of the shorter electrode greatly improved the layer quality. The precise control of gas residence time is primarily important for the deposition of µc-Si films with desired qualities through sufficiently passivating grain boundaries with a-Si tissues.

Boron-doped hydrogenated mixed-phase silicon as thermo-sensing films for infrared detectors

Silicon materials have been widely used as thermo-sensing layers in infrared detectors or uncooled micro-bolometers. Parameters such as a large thermal coefficient of resistance (TCR), low sheet resistance (Rs), and low 1/f noise are important for high performance of these devices. However, there is always a trade-off between these parameters. For example, the crystalline silicon materials typically exhibit low Rs and 1/f noise, and significantly low TCR, while the amorphous silicon materials generally have large TCR, and considerably high Rs and 1/f noise. Consequently, the best trade-off can be achieved by using a mixed-phase structure of silicon materials, i.e. an intermediate form between the crystalline and amorphous structures. Herein we report the important characteristics of hydrogenated mixed-phase silicon films, deposited by the plasma-enhanced chemical vapour deposition process, for infrared detectors. The films in the mixed-phase structure showed high TCR values in the range of 2–3%K–1 and moderate sheet resistances in range of 10–40MΩsq−1. These results indicate that the mixed-phase silicon films are potential alternatives to conventional boron doped hydrogenated amorphous and microcrystalline silicon films for use as thermo-sensing layers in infrared detectors.

Influence of excitation frequency and electrode separation on the growth of microcrystalline silicon films and their application in single junction microcrystalline solar cell

In this paper we present the role of plasma excitation frequency and electsrode separation on the growth of microcrystalline silicon thin films at two different hydrogen dilutions of silane and different power densities. We optimized the process conditions to develop device quality microcrystalline material. Optoelectronic and structural properties of the developed material have been correlated with the solar cell properties. Growth rate ~7 A/s has been achieved using plasma excitation frequency of 27.12 MHz at 15 mm electrode separation. We have noticed the positive effects after reducing the electrode separation in higher frequency (27.12 MHz). Optimized microcrystalline film of activation energy 0.55 eV and grain size of 14.61 nm has been developed and is applied to fabricate single junction microcrystalline solar cell. Solar cell with initial cell efficiency of 7.75% with short circuit current density of 24.98 mA/cm2 and open circuit voltage of 0.47 V and fill factor of 0.66 has been achieved.

Absolute density of precursor SiH3 radicals and H atoms in H2-diluted SiH4 gas plasma for deposition of microcrystalline silicon films

Microcrystalline hydrogenated silicon films were produced at a high deposition rate of about 2 nm/s by using a capacitively coupled plasma under a practical pressure of around 1 kPa. The SiH4 source gas was almost fully dissociated when highly diluted with H2 gas, and the dominant species in the gas phase were found to be SiH3 radicals, which are film-growth precursors, and H atoms. The absolute density of these species was measured as the partial pressure of SiH4 gas was varied. With the increasing SiH4 gas flow rate, the SiH3 radical density, which was on the order of 1012 cm−3, increased linearly, while the H-atom density remained constant at about 1012 cm−3. The film growth mechanism was described in terms of precursors, based on the measured flux of SiH3 radicals and H atoms, and the relative fraction of higher-order radicals.

(Invited) Microcrystalline Silicon Based TFTs and Resistors for Reliable Flexible Electronics

Silicon based flexible electronics is shown here as the right way when considering the reproducibility of the process and the long-term reliability, the only way leading to commercially available products. Microcrystalline silicon films are deposited directly on plastic substrates (PEN and Polyimide) at low temperature. Films are highly crystallized. Crystalline volume is higher than 73% for 50nm thick film as calculated from Raman spectra. Using such film, resistances are fabricated and checked as deformation gauges. The mechanical behaviour of microcrystalline silicon films under longitudinal as well as transversal tensile (Figure 1) and compressive stress is studied. The effect of the nature of the substrate and its thickness on this behaviour is studied also. These studies led to determine the maximum strain that can hold microcrystalline silicon film. This strain limit is reached by bending the film at different curvature radii depending on the thickness of the substrate. Lower curvature radius is reached with thinner substrate. Lower than 1 mm radius, leading to fold in half the film, is reached without breaking it (Figure 2). This high flexibility together with demonstrated reliability led the use of a matrix of microcrystalline silicon based gauges in the determination of any form and in the characterization of human pulse (Figure 3). Thin-film transistors are also fabricated on previous substrates at the same low temperature. Undoped microcrystalline silicon forms the active layer of these transistors. Phosphorus doped microcrystalline silicon is used as source and drain regions. The gate insulator is also deposited at low temperature. TFTs show very high flexibility. TFTs support lower than 1 mm curvature radius leading to compressive as well as tensile strain, without breaking (Figure 4). More than 200 times of such extreme bending are done without changing the TFT’s characteristics (Figure 5). Figure 1

(Invited) Microcrystalline Silicon Based TFTs and Resistors for Reliable Flexible Electronics

Silicon based flexible electronics is shown here as the right way when considering the reproducibility of the process and the long-term reliability, the only way leading to commercially available products. Microcrystalline silicon films are deposited directly on plastic substrates (PEN and Polyimide) at low temperature. Films are highly crystallized. Crystalline volume is higher than 73% for 50nm thick film as calculated from Raman spectra. Resistances and TFTs based on such silicon film are bent at lower radius than 1 mm without breaking. More than 200 times of such extreme bending are done without changing the TFT’s characteristics

Development of open air silicon deposition technology by silane-free atmospheric pressure plasma enhanced chemical transport under local ambient gas control

Open air silicon deposition was performed by combining silane-free atmospheric pressure plasma-enhanced chemical transport and a newly developed local ambient gas control technology. The effect of air contamination on silicon deposition was investigated using a vacuum chamber, and the allowable air contamination level was confirmed to be 3 ppm. The capability of the local ambient gas control head was investigated numerically and experimentally. A safe and clean process environment with air contamination less than 1 ppm was achieved. Combining these technologies, a microcrystalline silicon film was deposited in open air, the properties of which were comparable to those of silicon films deposited in a vacuum chamber.

Boron doping effects on microcrystalline silicon film roughness studied by spectroscopic ellipsometry

Microcrystalline silicon thin films were prepared in the p-type chamber using radio frequency plasma enhanced chemical vapor deposition. The surface roughness evolution of microcrystalline silicon thin films was investigated with spectroscopic ellipsometry (SE). The differences between the real-time SE and ex-situ SE have been studied. The effects of boron doping on the surface roughness have been analyzed using real-time SE. For the intrinsic microcrystalline silicon thin films, with an increase in the deposition time, the surface roughness exhibited the following behavior: (a) quickly increased, (b) gradually increased, (c) slightly increased. In the case of boron-doped microcrystalline silicon thin films, the surface roughness showed different behavior: (a) slightly increased, (b) quickly increased, (c) rapidly dropped, (d) increased again. Based on the KPZ model, the intrinsic silicon thin film growth exponent β is about 0.4, corresponding to limited diffusion model. In contrast, boron-doped silicon thin films behavior cannot be described by the KPZ model. Boron catalysis effects promote the reaction radical BHx aggregation, and result in shadowing. Boron doping changed the film growth process.

On the interplay of interface morphology and microstructure of high-efficiency microcrystalline silicon solar cells

The relationship between light trapping and microstructure of high-efficiency microcrystalline silicon thin-film solar cells on honeycomb textured substrates is investigated by experiments, optical simulations and analytical calculations. The solar cells realized on the honeycomb textured substrates exhibit short circuit current of 30mA/cm2 and energy conversion efficiencies exceeding 11%. The microstructure of the solar cells is limited by the formation of so called “cracks” in the film, which negatively affect the short circuit current, fill factor and open circuit voltage. The formation of cracks is studied by transmission electron microscopy, optoelectrical measurements and simulations of the 3D morphology of the solar cell. Furthermore, a simple analytical model is presented to calculate the critical thickness at which cracks are formed. Both models are compared to experimental results. Guidelines are provided on how to avoid the formation of cracks in microcrystalline silicon films on the textured substrates while maximizing the light trapping properties.