Microcrystalline Silicon Thin Films Research Articles

Effect of Large Uniaxial Stress on the Thermoelectric Properties of Microcrystalline Silicon Thin Films

This study reports on the behaviour of the thermoelectric properties of n- and p-type hydrogenated microcrystalline silicon thin films (µc-Si: H) as a function of applied uniaxial stress up to ±1.7%. µc-Si: H thin films were deposited via plasma enhanced chemical vapour deposition and thermoelectric properties were obtained through annealing at 200 °C (350 °C) for n-(p-) type samples, before the bending experiments. Tensile (compressive) stress was effective to increase the electrical conductivity of n-(p-) type samples. Likewise, stress induced changes in the Seebeck coefficient, however, showing an improvement only in electron-doped films under compressive stress. Overall, the addition of elevated temperature to the bending experiments resulted in a decrease in the mechanical stability of the films. These trends did not produce a significant enhancement of the overall thermoelectric power factor, rather it was largely preserved in all cases.

Highly Microcrystalline Phosphorous‐Doped Si:H Very Thin Films Deposited by RF‐PECVD

Fine‐tuning the crystallinity and doping of the hydrogenated microcrystalline silicon thin films are the key points in obtaining high‐quality devices for above‐integrated circuit (above‐IC) image sensors. This study focuses on the structural and electrical properties of radio‐frequency plasma enhanced chemical vapor deposition (RF‐PECVD) deposited microcrystalline silicon (μc‐Si:H) contact layers. Herein, phosphorus‐doped μc‐Si:H is deposited by capacitively coupled plasma RF‐PECVD (13,56 MHz) from a SiH4 + H2 + PH3 gas mixture with varying phosphine concentrations. The influence of phosphine concentration on dopant concentration, active dopant concentration, and crystallinity is studied by secondary ion mass spectrometry, Hall effect measurement, and Raman spectroscopy, respectively. A high doping level is attained, reaching up to 1.7 × 1020 cm−3. Furthermore, “low‐power” and “high‐power” deposition conditions are studied which lead to incubation layers of different nature.

A design of intelligent delivery drone based on BeiDou navigation and visual processing

This article used automatic control, 5G real-time communication, visual processing and other technologies to design a carbon fiber intelligent delivery drone, based on BeiDou satellite navigation and face recognition. And we proposed a semi-autonomous delivery mode to achieve real-time control of the drone. This mode can reduce flight risks and ensure the safety and accuracy of distribution. The drone can be powered by microcrystalline silicon thin film solar cells. The BeiDou satellite navigation system is used for autonomous positioning, combined with binocular stereo vision technology to observe the surroundings, automatically plan the flight path. On the way, personnel can intervene in real time as needed. Finally, when the drone arrives at the destination, facial recognition technology is used to confirm the identity of the recipient to complete the delivery.

Investigation of Microcrystalline Silicon Thin Film Fabricated by Magnetron Sputtering and Copper-Induced Crystallization for Photovoltaic Applications

Microcrystalline silicon, which is widely used in the microelectronics industry, is usually fabricated by chemical vapor deposition techniques. In recent years, magnetron sputtering has been considered as an alternative because it is a simpler, cheaper and more eco-friendly technique. The big drawback of this technique, however, is the need to recrystallize the as-deposited amorphous silicon, which can be done by metal-induced crystallization. Among the different suitable metals, copper has not been extensively investigated for this purpose. Furthermore, the applicability of the microcrystalline film prepared by this method has not been evaluated for photovoltaic device fabrication. Therefore, this paper reports the fabrication of p-type microcrystalline silicon thin film by magnetron sputtering and copper-induced crystallization techniques, and evaluates its appropriateness for solar cell fabrication. In the first step, 60 nm of silicon followed by 10 nm of copper were deposited on n-type silicon wafer and glass substrates, both by the magnetron sputtering technique. Then, the as-deposited samples were annealed at temperatures from 450 °C to 950 °C. The crystal properties of the resulting films were characterized by Raman and X-ray diffraction spectroscopies and optical and secondary emission microscopies, while their electrical characteristics were determined by Hall-effect, J-V curve and external quantum efficiency measurements. These characterizations confirmed the formation of a layer of microcrystalline silicon mostly in the <111> direction with a crystallization ratio of 93% and a largest grain size of 20 nm. The hole concentration and mobility of the fabricated p-type microcrystalline silicon layer were about 1017~1019 cm−3 and 8 cm2/V.s, respectively. By using the fabricated film as the emitter layer of a p-n junction solar cell, a good rectification ratio of 4100 and reverse saturation current density of 85 nA.cm−2 were measured under dark conditions. The highest photovoltaic conversion efficiency, i.e., 2.6%, with an open-circuit voltage of 440 mV and short-circuit current density of 16.7 mA/cm2, were measured under AM1.5G irradiance. These results indicate that microcrystalline silicon created by magnetron sputtering and copper-induced crystallization has considerable potential for photovoltaic device fabrication.

Physical and photo-electrochemical properties of CuO thin film grown on µc-Si:H/glass. Application to solar energy conversion

The Physical and photo-electrochemical properties of CuO grown on microcrystalline silicon thin films (µc-Si:H) by heating copper film were investigated. The Raman spectrum for µc-Si: H exhibits a peak at 520 cm−1 confirming its good crystallinity with a crystalline volume fraction of 91%. The X-ray diffraction pattern shows narrow peaks with a monoclinic symmetry. The indirect optical transition (1.19 eV) is well matched to the solar spectrum and the low transmittance (<7%at 400 nm), indicating a good optical quality. The capacitance measurements, performed in Na2SO4 electrolyte indicate that CuO/µ-Si:H electrode acts as n-type semiconductor with a donor density ND of 4.88 × 1019 cm−3 implying the existence of oxygen vacancies in the film. The potential of its conduction band (−0.563 VSCE) made up of Cu-eg orbital, is more cathodic than the O2/O2 level (−0.3 VSCE), but not enough to yield O2 reduction under solar light. By contrast, the electrons in the Cu2+: 4 s level excited by the UVA of the solar radiation, leading to oxidation of 26% of RhB (10 ppm) mainly by the O2 radicals within 60 min.

Optimized hydrogen concentration within a remotely induced hollow-anode plasma for fast chemical-vapor-deposition of photosensitive and -preferential microcrystalline silicon thin-films

The chemical-vapor-deposition of photosensitive hydrogenated-microcrystalline-silicon (µc-Si:H) thin films with <110>-preferential orientation was optimized by altering the concentration of hydrogen within a high-density hollow-anode plasma. The µc-Si:H thin films were grown using a ratio of hydrogen (H2) to monosilane (SiH4) in the range of 1.25 ≤ [H2]/[SiH4] ≤ 35, with a gas pressure of 80 Pa. The high-density hollow-anode plasma was excited remotely in a processing space by transferring a hollow-cathode plasma via a nozzle on a partition plate, which separated the processing space from a hollow-cathode discharge space and served as an anode in an ultra-high vacuum hollow-electrode-enhanced glow-plasma transportation (HEEPT) system. The hollow-cathode plasma was excited by applying very-high-frequency (VHF, 105 MHz) power to a cathode in the hollow-cathode discharge space. The growth rate, crystalline volume fraction, and <110>-preferential crystal orientation of the films exhibited almost linear correlations with the ratio of the optical emission intensities of hydrogen atoms (Hα: 656 nm) and monosilane radicals (SiH*: 414 nm) (i.e. HαI/SiH*I). Reducing the [H2]/[SiH4] ratio by decreasing [H2] improved the growth rate, crystalline volume fraction, and <110>-preferential crystal orientation of the films. These results indicated that lower concentration of H2 was optimal for the fast deposition of photosensitive µc-Si:H thin-films with <110>-preferential crystal orientation using the HEEPT system. The <110>-preferential crystal orientation was less dependent on the VHF power, whereas the growth rate and crystalline volume fraction increased as the VHF power was increased. This result suggested that there would be a room for faster growth with retaining <110>-preferential crystal orientation.

Remotely induced high-density hollow-anode plasma and its application to fast deposition of photosensitive microcrystalline silicon thin film with preferential &amp;lt;110&amp;gt; orientation

We investigated the application of a less hydrogen-dilute and low gas pressure hollow-anode plasma to fast chemical-vapor deposition of photosensitive hydrogenated microcrystalline silicon (μc-Si:H). The hollow-anode plasma was remotely induced at a processing space by transferring a hollow-cathode plasma through a nozzle attached to a partition plate, which operated as an anode and separated the processing space from a hollow-cathode discharge space in an ultrahigh-vacuum hollow-electrode-enhanced glow-plasma transportation (HEEPT) system. The hollow-cathode plasma was excited by applying a very-high-frequency (VHF, 105 MHz) power to a cathode in the hollow-cathode discharge space. Through the use of this hollow-anode plasma under a gas flow rate ratio ([H2]/[SiH4]) of 1.25 (30 sccm/24 sccm), pressure of 80 Pa, and VHF power of 150 W (the highest power tested in this work), we fabricated a well-crystallized and photosensitive μc-Si:H thin film with a highly preferred crystal orientation along the &amp;lt;110&amp;gt; direction at a growth rate of 13 nm/s. Electrical analysis on the self-bias voltage of the cathode (Vdc) revealed that hollow-cathode discharges in the HEEPT system were approximately equivalent to symmetric discharge, i.e., Vdc ≒ 0 V. Optical analysis indicated that the hollow-anode plasma produced an enough amount of atomic hydrogen to grow well-crystallized μc-Si:H thin films, even at the lowest [H2]/[SiH4] ratio (1.25). Optical and electrical analyses and computational plasma simulation demonstrated that the hollow-anode plasma had a lower electron temperature and higher plasma space potential compared with those features of a glow discharge plasma enhanced by a conventional parallel-electrodes system.

Employing constant photocurrent method for the study of defects in silicon thin films

Different optical characterization techniques have been performed on a series of microcrystalline silicon thin films deposited using very high-frequency-assisted plasma-enhanced chemical vapor deposition process. The constant photocurrent method has been employed to study the defects states in density of states spectra of hydrogenated microcrystalline silicon thin films. The photocurrent measurements demonstrate anisotropy in the optoelectronic properties of the material. We have analyzed the optical absorption coefficient from UV spectroscopy and with the help of constant photocurrent method. The spectra have been analyzed in broad region and are presented for both the cases, i.e., surface and bulk light scatterings. The spectra were interpreted in terms of disorder, resulting defect density, crystalline/amorphous volume fractions and material morphology. The subgap-related parameters such as absorption coefficient, characteristic energy E0 of tail states and density of subgap defect states together with an estimate of the bandgap of silicon films prepared at various crystalline fractions have also been estimated. The density of localized tail states is found to fall exponentially toward the gap with band tail width of about 110 meV.

Effect of deposition rate on the growth mechanism of microcrystalline silicon thin films using very high frequency PECVD

The intrinsic microcrystalline silicon thin films were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Two series of films with different deposition rate 0.30 nm/s and 1.94 nm/s were prepared. The film surface and gas phase reaction growth processes were monitored with real-time spectroscopic ellipsometry and optical emission spectroscopy. The effect of deposition rate on the microcrystalline silicon thin film growth mechanism has been studied. The microcrystalline silicon surface growth was analyzed with KPZ model. The results show that the growth exponent of β is 0.448 for the films with low deposition rate, and the growth exponent of β is 0.302 for the films with high deposition rate. The growth exponent does not increase with deposition rate, but declines. And the reasons for this phenomenon were explained.

Microcrystalline silicon thin film deposition from silicon tetrafluoride: Isolating role of ion energy using tailored voltage waveform plasmas

The energy of ions during plasma enhanced chemical vapor deposition (PECVD) is known to impact material quality, but isolating this effect from other process parameters (plasma density, pressure) in a capacitively coupled plasma (CCP) is not straightforward. In this work, we utilize a novel radio-frequency (RF) excitation technique – tailored voltage waveforms (TVW) – as a solution to achieve ion flux-energy decoupling through the electrical asymmetry effect. This makes it possible to independently study the impact of ion energy on material deposition. We study the impact of ion energy - more precisely the maximum ion bombardment energy (IBEmax) before collisions - on the PECVD of hydrogenated microcrystalline silicon (µc-Si:H) thin films from an SiF4/H2/Ar chemistry. Through structural and electronic analysis, we find that the variation of IBEmax directly translates into material quality, even at relatively high process pressure. Better material properties (crystalline grain features, material density and photoelectronic response) are obtained for films deposited with moderate values of IBEmax around 45–55 eV. Above this range, a deterioration in material quality is observed, presumably due to more effective bulk atomic displacements induced by the silicon-containing ions. These results are consistent with the performance of single-junction µc-Si:H solar cell devices using these materials as active layers. Optimum performance is obtained for devices with an absorber layer deposited using a plasma excitation resulting in IBEmax in the range of ~45–55 eV. The variation in device performance is mainly due to changes in the open circuit voltage.

P-1.9: Characterization of Self-Aligned Top-Gate Microcrystalline Silicon Thin Film Transistors

P-1.9: Characterization of Self-Aligned Top-Gate Microcrystalline Silicon Thin Film Transistors

Effects of helium concentration on microcrystalline silicon thin film solar cells deposited by atmospheric-pressure plasma deposition at 13.3 kPa

An atmospheric-pressure plasma deposition system was developed and used to deposit intrinsic hydrogenated microcrystalline silicon (μc-Si:H) thin films for μc-Si:H thin film solar cells. The helium (He) gas concentration in plasma increased Raman crystallinity (Xc) of μc-Si:H, which was a critical parameter to determine photovoltaic performance. In addition, dependence of Xc on the He concentration is shown to be closely related to the atomic hydrogen flux during film growth, which is evidenced by plasma diagnostics derived from in situ optical emission spectroscopy measurements. Those analysis contributed to achieve impressive photovoltaic performance – namely, a power conversion efficiency of 4.60%, an open-circuit voltage of 0.52 V, a short-circuit current density of 13.62 mA/cm2, and a fill factor of 0.65, which is the first experimental demonstration of μc-Si:H thin film solar cells fabricated with the atmospheric-pressure plasma deposition under very high working-pressure regime at 13.3 kPa.

Numerical study on conical two-dimensional photonic crystal in silicon thin-film solar cells

To further improve the absorption of thin-film silicon solar cells (TFSSCs), it is essential to understand what kind of texture morphology could present the best light trapping effect, or rather, which structural parameter plays the most important role, and offers the required lateral feature size, height or others. In this paper, the influences of structural parameters of conical two-dimensional photonic crystal (2D PC) on each-layer absorption of the microcrystalline silicon thin film solar cells are numerically studied by using the finite-difference time-domain method when 2D PC is introduced into the intrinsic layer. The results show that both the intrinsic absorption and parasitic absorption are significantly enhanced via introduction of 2D PC into the intrinsic layer. The parasitic absorption is mainly caused by the ITO layer, and the intrinsic absorption shows a sinusoidal fluctuation with the increase of period. It is found that the aspect ratio (height/period) of the 2D PC has a decisive influence on the cell intrinsic absorption. When the period of the 2D PC is less than 1m, the intrinsic absorption first increases and then decreases with the increase of the aspect ratio, and reaches a maximum value with an aspect ratio of 1. For the case of period larger than 1m, the aspect ratio needed to obtain the maximum result is smaller than 1. What is more, the larger the period, the smaller the aspect ratio for maximizing the intrinsic absorption will be. The peak intrinsic absorption can be obtained when a 2D PC with a period of 0.5m and an aspect ratio of 1 is introduced. Compared with that of the flat cell, the short-circuited current density of the above optimized 2D PC cell can be significantly enhanced by 5.8 mA/cm2(from 21.9 to 27.8 mA/cm2), corresponding to a relative enhancement of 27%. In order to improve antireflection performance, it is critical to adopt a textured front-surface morphology where the aspect ratio is higher than 1/2. In addition, the intrinsic absorption increases with the increasing fill factor, and reaches a maximum value when the fill factor of the 2D PC is close to 0.9. The research results of this paper break through the traditional viewpoint of light trapping mechanism which points out that the light trapping effect is mainly dependent on the lateral feature size of the texture, and provide an important guide for obtaining optimized random or periodic texture via experiment.

Doped microcrystalline silicon as front surface field layer in bifacial silicon heterojunction solar cells

In silicon heterojunction solar cells with thin intrinsic layers (SHJ) based on n-type silicon wafers, it is common to use a p-doped front emitter and back surface field (n-layer). In this study, SHJ cells were developed and investigated in front surface field (FSF) configuration with a n-doped layer at the front side. The electrical and optical properties of the FSF play a crucial role in cell operation. Ideally, the FSF demonstrates low light absorption, efficient extraction of electrons to contacts, as well as chemical and electrical passivation. The goal of this work is to study the physical properties of n-doped microcrystalline silicon thin films and apply them as the FSF layer in SHJ solar cells. We demonstrate promising results in such SHJ solar cells with short-current density of 2.9 mA/cm² higher compared to reference cells with amorphous FSF.

Substrate‐independent analysis of microcrystalline silicon thin films using UV Raman spectroscopy

Surface‐sensitive UV Raman spectroscopy is used to analyze the crystallinity of silicon films less than 20 nm thick directly on silicon wafers. The 325‐nm excitation has a Raman detection thickness of only 13 nm within the silicon film, thus eliminating signal from the substrate. We demonstrate measured crystallinities of microcrystalline silicon thin films that are consistent with the microstructure observed in transmission electron microscopy. Comparison is also made to ellipsometry, which is less able to accurately determine crystallinity than UV Raman spectroscopy. The UV Raman approach is particularly useful for layers grown on substrates of the same material but with different microstructure, and can be extended to non‐silicon materials.

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.

Enhanced efficiency of ultrathin (∼500 nm)-film microcrystalline silicon photonic crystal solar cells

Enhancing the absorption of thin-film microcrystalline silicon solar cells at 600–1000 nm wavelengths is very important to the improvement of the energy conversion efficiency. This can be achieved by creating a large number of resonant modes utilizing two-dimensional photonic crystal band edges, which exceeds the Lambertian limit of absorption in random textures. We focus on suppressing the parasitic absorption of back-reflector metal and doped layers in photonic crystal microcrystalline silicon solar cells. We achieve a high active-area current density of 22.6 mA cm−2 for an ultrathin (∼500 nm)-film silicon layer and obtain an active-area efficiency of ∼9.1%, as independently confirmed by the CSMT of AIST.

Modeling of triangular-shaped substrates for light trapping in microcrystalline silicon solar cells

The influence of triangular grating used as a light trapping structure on the optical wave propagation within thin-film microcrystalline silicon (µc-Si:H) solar cells is investigated. A finite difference time domain (FDTD) approach is used to rigorously solve the Maxwell's equations in three dimensions. We apply two parameters of mean surface roughness (Sa) and slope (k) to define triangular structure and study their influence on the absorption of µc-Si:H. When Sa and k are set to 400nm and 1, respectively, a largest enhancement of absorption is achieved. The optimum short circuit photocurrent (Jsc) of a 1-µm thick µc-Si:H solar cell made on such a textured substrate can reach 27.0mA/cm2. The carrier generation rate in the µc-Si:H material is also rigorously analyzed. Finally, we identify some key optical losses in µc-Si:H solar cells and propose for further optimizing the device design.

Using MDECR‐PECVD to study the impact of ion bombardment energy on microstructural properties of μc‐Si:H thin film grown from an SiF4/H2 chemistry

AbstractThe matrix‐distributed electron cyclotron resonance plasma‐enhanced chemical vapor deposition (MDECR‐PECVD) technique has been shown to achieve high deposition rates for hydrogenated microcrystalline silicon (μc‐Si:H) thin film. Due to the fact that plasma is sustained by a microwave discharge, by biasing the substrate holder with additional power supply, one can achieve independent control over the plasma density and the maximum ion bombardment energy (IBE). In this work, we present studies of the impact of IBE on the microstructural properties of the μc‐Si:H film deposited by MDECR‐PECVD. Insufficient ion bombardment is found to be responsible for the substantial presence of nano‐porous regions within the material, resulting in significant post‐deposition oxidation. Good agreement between transmission electron microscopy (TEM) Fresnel contrast analysis and the results of infrared absorption and hydrogen effusion measurements for the deposited films suggest that moderate IBE is of vital importance to achieve high quality μc‐Si:H. In doing so, denser films with significantly decreased nano‐porous regions and better stability are obtained, which is of great interest to optimize the process parameters for solar cell applications. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Activation energy study of phosphorus-doped microcrystalline silicon thin films

The phosphorus-doped microcrystalline silicon thin films were deposited by PECVD (Plasma-enhanced chemical vapor deposition), the activation energy of thin films were measured by activation energy testing equipment. The activation energy of samples with different doping concentration and different depositing temperature were studied. The results showed that: the activation energy of phosphorus-doped microcrystalline silicon thin films lesser than intrinsic films. The unit of activation energy is meV. Crystalline volume fractions have little influence on activation energy when thin films have a good crystalline volume fraction, impurity effect plays an important role on the conductivity.