Hydrogenated 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.

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.

Growth and properties of hydrogenated microcrystalline silicon thin films prepared by magnetron sputtering with different substrate temperatures

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were deposited by an radio frequency (RF) (13.56 MHz) magnetron sputtering at different substrate temperatures (100–300 °C), and the influences of substrate temperature on the growth and properties of μc-Si:H thin films were investigated. Surface roughness and crystallinity of the thin films increase as substrate temperature increases. And all thin films are at the transition region (X c = 49.2 %~61.0 %). The μc-Si:H thin films deposited at lower substrate temperature (≤200 °C) represent a weak (220) preferred orientation, while the thin films deposited at higher substrate temperature (≥250 °C) exhibit a weak (111) preferred orientation. The μc-Si:H thin films have a dense structure, and the structural compactness of the thin films slightly increases with substrate temperature increasing. The Fourier transform infrared spectroscopy (FT-IR) results indicate that the μc-Si:H thin films have a low hydrogen content (3.9 at%–5.6 at%), which is in favor of reducing light-induced degradation effect.

Grown Low‐Temperature Microcrystalline Silicon Thin Film by VHF PECVD for Thin Films Solar Cell

Hydrogenated microcrystalline silicon thin films can be used to fabricate stable thin film solar cell, which were deposited by very high frequency plasma‐enhanced chemical vapor deposition at low temperatures (~200°C). It has been found that the obtained film presented excellent structural and electrical properties, such as high growth rate and good crystallinity. With the decreasing of silane concentration, the optical gap and the dark conductivity increased, whereas the activation energy decreased. A reasonable explanation was presented to elucidate these phenomena. In addition, we fabricated p‐i‐n structure solar cells using the optimum microcrystalline silicon thin films, and preliminary efficiency of 4.6% was obtained for 1 μm thick microcrystalline silicon thin film solar cells with open‐circuits voltage of 0.773 V and short‐circuits current density of 12.28 mA/cm2. Future scope for performance improvement lies mainly in further increasing the short‐circuit current.

Investigation of meta- and in-stability effects in hydrogenated microcrystalline silicon thin films by the steady-state measurement methods

Metastability effects because of atmospheric exposure, high purity gasses, and deionized water in hydrogenated microcrystalline silicon thin films with different crystalline volume fractions were studied using well accepted steady-state characterization methods of dark conductivity, steady-state photoconductivity, steady-state photocarrier grating (SSPG) and dual beam photoconductivity (DBP) methods. A standard measurement procedure has been established before using the steady state methods, in which a steady state condition of dark conductivity was established by monitoring the time dependence of dark conductivity. Samples deposited on smooth glass and rough glass substrates exhibit similar reversible and irreversible changes in the properties of microcrystalline silicon film. A reliable correlation of reversible and irreversible changes indicate that dark conductivity and photoconductivity values increase, sub-bandgap absorption spectrum obtained from DBP method decrease and correspondingly minority carrier diffusion lengths obtained from the SSPG method increase in the metastable state in various amount for microcrystalline films with crystalline volume fraction, [Formula: see text] > 0.30. Amorphous silicon and microcrystalline silicon films with [Formula: see text] < 0.30 do not show detectable metastable changes as samples exposed to atmospheric condition as well as high purity oxygen gas and deionized water.

Investigation of metastability and instability effects on the minority carrier transport properties of microcrystalline silicon thin films by using the steady-state photocarrier grating technique

Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of σDark and σphoto within a factor of two, but they did not affect the minority carrier μτ-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80 °C resulted in more than an order of magnitude increase in both σDark and σphoto and an increase in the diffusion length, LD, by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both σDark and σphoto to their annealed-state values, while LD decreases only slightly. Such increase in the LD values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains.

Low temperature magnetron sputtering deposition of hydrogenated microcrystalline silicon thin films without amorphous incubation layers on glass

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were deposited on glass by magnetron sputtering at the substrate temperature of 100°C and the hydrogen dilution ratio of 65%. The crystallinity and microstructure of thin films were systematically studied using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR). The results showed that the μc-Si:H thin film was amorphous and hydrogen-rich at the initial stage of the growth. This amorphous layer was crystallized during further growth. The crystallinity of thin films is not almost changed with the increase of thickness when the thickness of thin film exceeds 70nm. Finally, the μc-Si:H thin film without amorphous incubation layer was obtained. The possible growth mechanism of fabricating μc-Si:H thin films without amorphous incubation layers was also discussed.

Effects of showerhead hole structure on the deposition of hydrogenated microcrystalline silicon thin films by vhf PECVD

The authors present the characteristics of hydrogenated microcrystalline silicon (μc-Si:H) thin films deposited from an SiH4/H2 in 40 MHz plasma enhanced chemical vapor deposition system equipped with a multihole-array showerhead. The effects of a hole-array structure are analyzed in terms of deposition rate, crystallinity, and electrical conductivity of the μc-Si:H film. The effects of process parameters, such as SiH4 concentration, radio frequency power, and total gas flow rate are also investigated. The deposition rate of a multihole-array electrode is generally lower compared with that of the flat electrode. However, a higher deposition rate is found in the deep and dense hole region when the hole shape and density are varied on the electrode. The crystallinity and conductivity are almost not affected by hole dimension and density. It is demonstrated that the multihole-array electrode can be a useful tool for uniformity improvement in a large-sized very high frequency capacitively couple plasma μc-Si:H deposition system.

Effects of growth temperature on μc-Si:H films prepared by plasma assistant magnetron sputtering

Hydrogenated microcrystalline silicon thin films (μc-Si:H) were deposited by plasma assistant magnetron sputtering in Ar-H2 gas mixture. The effects of growth temperature from 150 to 450 °C on properties of deposited Si films were investigated at two different hydrogen diluted gases with [H2]/([Ar]+[H2]) of 10% and 50% at 3 Pa. The crystallinity of Si films examined by Raman scattering exhibited higher degradation by lowering growth temperature from 250 to 150 °C in low hydrogen diluted gas of 10% than that in high hydrogen diluted gas of 50%. The IR absorption band around 845 and 890 cm−1 as well as calculated concentration of bonded hydrogen showed more obvious decrease of samples deposited in low hydrogen diluted gas of 10% than that in high hydrogen diluted gas of 50%. The optical band gasps of both groups of samples measured by ultraviolet-visible optical absorption were decreased with increasing temperature in both gas conditions.

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σD, photoconductivity, σph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O2 gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.

Hydrogenated microcrystalline silicon thin films deposited by RF-PECVD under low ion bombardment energy using voltage waveform tailoring

We present experimental results for hydrogenated amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) thin films deposited by PECVD while using a voltage waveform tailoring (VWT) technique to create an electrical asymmetry in the reactor. VWT dramatically modifies the mean ion bombardment energy (IBE) during growth, and we show that for a constant peak-to-peak excitation voltage (VPP), waveforms resembling “peaks” or “valleys” result in very different material properties. Using Raman scattering spectroscopy, we show that the crystallinity of the material depends strongly on the IBE, as controlled by VWT. A detailed examination of the Raman scattering spectra reveals that the narrow peak at 520cm−1 is disproportionately enhanced by lowering the IBE through the VWT technique. We examine this effect for a range of process parameters, varying the pressure, hydrogen–silane dilution ratio, and total flow of H2. In addition, the SiHX bonding in silicon thin films deposited using VWT is characterised for the first time, showing that the hydrogen bonding character is changed by the IBE. These results demonstrate the potential for VWT in controlling the IBE during thin film growth, thus ensuring that application-appropriate film densities and crystallinities are achieved, independent of the injected RF power.

Doped Microcrystalline Silicon Layers for Solar Cells by 13.56MHz Plasma-enhanced Chemical Vapour Deposition

Doped hydrogenated microcrystalline silicon thin films play a critical role in multi-junction thin-film silicon solar cells, because their crystallinity has a large influence on the properties of intrinsic microcrystalline silicon absorber layers grown on them. The doping efficiency of the doped layers depends strongly on their crystallinity and hence a high-crystallinity doped layer is desired. In this study, highly crystalline doped microcrystalline silicon films are formed on 300mm × 400mm glass substrates using a conventional parallel-plate PECVD reactor operated at 13.56MHz. Raman spectroscopy is used to analyse the crystallinity of the films. The conductivity of the films is measured using the co-planar electrode method. The effects of the deposition parameters on the Raman crystallinity and conductivity of the doped films are investigated. The RF power is found to play a key role for achieving a high crystallinity in the doped layers, whereby a high crystallinity can only be obtained within a narrow RF power range. The influence of the RF power on the lateral thickness uniformity of the deposited films is also examined. It is found that the RF power has a strong influence on the lateral uniformity of the deposited films, with intermediate power giving the best thickness uniformity.

Hydrogen plasma induced crystallization of Si thin films by remote inductively coupled plasma source assistant pulsed dc twin magnetron sputtering

Hydrogenated microcrystalline silicon thin films (μc-Si:H) were deposited by remote inductively coupled plasma assistant pulsed dc twin magnetron sputtering at temperatures below 300°C. The formation of μc-Si:H was only found in the environment of hydrogen plasma, where Ar and H2 mixed gas was used. In pure argon plasma or without the assistance of ICP in the Ar/H2 gas mixtures, all the samples were amorphous structure. It suggested that ICP hydrogen plasma which enhanced the density and energy of H radicals played the key role in the formation of μc-Si:H films.

Effect of radio frequency power and total mass-flow rate on the properties of microcrystalline silicon films prepared by helium-diluted-silane glow discharge

Hydrogenated microcrystalline silicon thin films have been prepared by plasma-enhanced chemical vapor deposition at relatively low deposition temperatures (180 °C). Helium dilution of silane, instead of the more commonly approach of hydrogen dilution, has been used to promote microcrystalline growth. The effect of the applied radio frequency power (RFP) and the total gas flow on the structural, optical and electrical characteristics have been studied. As observed from the structural measurements, microcrystalline growth is favored as the applied RFP is increased and/or the total gas flow is decreased. Increasing the RFP however, brings associated an increase in the defect density in the amorphous tissue surrounding the crystalline grains and/or an increase in intra-grain defects as deduced from the structural, optical and electrical measurements. Microcrystalline growth and defect formation is rationalize in terms of the He* deexcitation process and high energy He + ions bombardment.

Instability effects in hydrogenated microcrystalline silicon thin films

AbstractInstability effects on the dark conductivity (σDark) and photoconductivity (σphoto) of microcrystalline silicon films with different microstructures deposited on smooth and rough substrates, after exposure to different storage conditions have been investigated. Samples stored in vacuum are less affected by aging than films stored in air and sealed plastic bags filled with N2 gas. The aging is reversible after annealing at 450 K. Thin films deposited on smooth and rough substrates with the same thickness and silane concentrations (SC) during the same deposition run showed similar aging effects. Thick samples exhibited a decrease in σDark, while both decrease and increase in σDark occur in thin samples after aging. For each sample, σDark and σphoto presented the same type of aging, independent of thickness and type of substrate used. The applicability of the steady‐state photocarrier grating technique was successfully tested on the rough substrates. Light‐induced degradation studies confirm the dependence of the Staebler‐Wronski effect on the crystallinity of the material (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of aging on the dark conductivity and 1/f noise in hydrogenated microcrystalline silicon thin films

AbstractAging hydrogenated microcrystalline silicon in air is found to affect both the dark conductivity and the 1/f noise. For a sample with a crystalline volume fraction of 0.39, the conductivity decreased by three orders of magnitude at 20 °C after short‐term (10–20 hrs) exposure to air. The conductivity recovered after annealing to 160 °C. Long‐term exposure (2 years) resulted in a permanent decrease by a factor of 16 at 20 °C even after annealing. Long‐term aging also increased the conductivity activation energy from 0.193 eV to 0.342 eV. After short‐term aging and below the annealing temperature, the conductivity prefactor σ0 and the activation energy Eσ follow a Meyer‐Neldel type of relation. Conductance fluctuations measured for annealed and aged states show all the expected characteristics of 1/f noise. The spectra fit a power law with slope ‐1; the slope is not affected by temperature or aging. The magnitude of the noise decreases with temperature after aging, but by much less after annealing. A simple analysis of the product of conductivity and noise magnitude can be used to estimate the free carrier mobility (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low temperature deposition of boron-doped microcrystalline Si:H thin film and its application in silicon based thin film solar cells

Boron-doped hydrogenated microcrystalline silicon thin films (p-μc-Si:H) have been deposited by RF-PECVD method at different temperature, and the temperature dependence of growth kinetics and optoelectronic properties of p-μc-Si:H thin films have been studied. Both the deposition rate and the dark-conductivity of the p-μc-Si:H thin films drop down when the substrate temperature decreases. XRD and Raman measurements are used to characterize the micro-structure of p-μc-Si:H thin films prepared at different substrate temperature. Grain size of p-μc-Si:H thin films with different thickness as a function of substrate temperature has been investigated. Amorphous silicon thin film solar cells with p–i–n structures were fabricated on deposited boron doped μc-Si:H layers. The best cells performance is obtained for p-layers processed at 90 °C.

N 2 decomposition by hot wire and N 2 post-deposition treatment on hydrogenated microcrystalline silicon thin films

Post-deposition treatment of hydrogenated microcrystalline silicon (µc-Si:H) was carried out using a hot wire in atmospheres of N 2, N 2/H 2 or H 2 and the states of the bonds in the µc-Si:H films were investigated using X-ray photoelectron spectroscopy. For the µc-Si:H film treated in N 2 at the filament temperature ( T f) of 1600 °C, a weak N1s peak was observed. It increased slightly with increasing T f from 1600 to 1900 °C and increased dramatically with increasing T f from 1900 to 2000 °C. The N1s peak of the µc-Si:H film treated in N 2/H 2 at T f = 2000 °C was one order of magnitude lower than that in N 2 at T f = 2000 °C . These findings indicate that N 2 molecules decompose on the heated filament and that the addition of H 2 prevents N 2 decomposition.

Preparation of Phosphorus Doped Hydrogenated Microcrystalline Silicon Thin Films by Inductively Coupled Plasma Chemical Vapor Deposition and Their Characteristics for Solar Cell Applications

Intrinsic and phosphorus-doped hydrogenated microcrystalline silicon (microc-Si:H) films were prepared using inductively coupled plasma chemical vapor deposition (ICP-CVD) method. Structural, electrical and optical properties of these films were studied as a function of silane concentration, ICP source power and PH3/SiH4 gas ratio. Characterization of these films from Raman spectroscopy and X-ray diffraction revealed that the conductive film exists in microcrystalline phase embedded in an amorphous network. The condition of electrical properties (sigma(d): approximately 10(-7) S/cm, sigma(ph): approximately 10(-4) S/cm) and activation energy (0.55 eV), satisfied with properties of intrinsic microc-Si:H, was obtained at 1200 W of ICP power and 2% of silane concentration, respectively. At PH3/SiH4 gas ratio of 0.09%, dark conductivity has a maximum value of approximately 18.5 S/cm and optical bandgap also a maximum value of approximately 2.39 eV. The deposition rate was not satisfactory (4.9 angstroms/s) at same condition.