Influence of substrate direct current bias voltage on microcrystalline silicon growth during radio-frequency magnetron sputtering

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared on glass, aluminum-covered glass and Si wafer substrates at various substrate bias voltages (V_sb) between -400 and +50 V, and the influence of V_sb on their structural properties was investigated. The crystallinity (crystalline volume fraction and crystallite size) of the μc-Si:H films deposited on glass remained unchanged with respect to V_sb. For μc-Si:H films deposited on aluminum within the V_sb range of -20 to +50 V, the crystallinity also remained unchanged and showed the same crystallinity as that of the films deposited on glass substrate. However, the crystallinity of the μc-Si:H films deposited on aluminum-covered substrate was reduced as V_sb decreased from -20 to -100 V, and the film at V_sb=-400 V was completely amorphous.     

High quality microcrystalline Si films by hydrogen dilution profile

Novel hydrogen dilution profiling (HDP) technique was developed to improve the uniformity in the growth direction of μc-Si:H thin films prepared by hot wire chemical vapor deposition (HWCVD). It was found that the high H dilution ratio reduces the incubation layer from 30 nm to less than 10 nm. A proper design of hydrogen dilution profiling improves the uniformity of crystalline content, X_c, in the growth direction and restrains the formation of micro-voids as well. As a result the compactness of μc-Si:H films with a high crystalline content is enhanced and the stability of μc-Si:H thin film against the oxygen diffusion is much improved. Meanwhile the HDP μc-Si:H films exhibit the low defect states. The high nucleation density from high hydrogen dilution at early stage is a critical parameter to improve the quality of μc-Si:H films.     

Amorphous silicon carbide thin films (a-SiC:H) deposited by plasma-enhanced chemical vapor deposition as protective coatings for harsh environment applications

We investigated amorphous silicon carbide (a-SiC:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) as protective coatings for harsh environment applications. The influence of the deposition parameters on the film properties was studied. Stoichiometric films with a low tensile stress after annealing (< 50 MPa) were obtained with optimized parameters. The stability of a protective coating consisting of a PECVD amorphous silicon oxide layer (a-SiO_x) and of an a-SiC:H layer was investigated through various aging experiments including annealing at high temperatures, autoclave testing and temperature cycling in air/water vapor environment. A platinum-based high-temperature metallization scheme deposited on oxidized Si substrates was used as a test vehicle. The a-SiO_x/a-SiC:H stack showed the best performance when compared to standard passivation materials as amorphous silicon oxide or silicon nitride coatings.     

Gradient etching of silicon-based thin films for depth-resolved measurements: The example of Raman crystallinity

An etching procedure was applied to microcrystalline silicon (μc-Si:H) thin films in order to obtain a wedge-shaped profile for depth-resolved characterization. A microfluidic flow cell that merges deionized water with a potassium hydroxide solution (KOH_aq) was utilized. The samples consisted of texture-etched ZnO:Al on a Corning Glass substrate, a microcrystalline p-doped layer serving as seed layer and the investigated intrinsic microcrystalline or amorphous silicon (a-Si:H). Along the etched profiles, microscopic Raman spectroscopy was used to estimate the crystalline volume fraction X_c for samples deposited with intentionally varied silane concentration to investigate the a-Si:H/μc-Si:H and μc-Si:H/a-Si:H transition.     

Preparation of high quality polycrystalline silicon thin films by aluminum-induced crystallization

High quality polycrystalline silicon (poly-Si) thin films without Si islands were prepared by using aluminum-induced crystallization on glass substrates. Al and amorphous silicon films were deposited by vacuum thermal evaporation and radio frequency magnetron sputtering, respectively. The samples were annealed at 500 °C for 7 h and then Al was removed by wet etching. Scanning electron microscopy shows that there are two layers in the thin films. After the upper layer was peeled off, the lower poly-Si thin film was found to be of high crystalline quality. It presented a Raman peak at 521 cm^− 1 with full width at a half maximum of 5.23 cm^− 1, which is similar to c-Si wafer.     

Influence of the deposition pressure on the preparation of μc-Si:H thin films in hot-wire-assisted MWECR-CVD system

Considering the important influence of the deposition pressure on the growth of thin films, such as deposition rate, crystalline volume fraction and density, etc., and based on the analysis of the advantages and disadvantages on the mono-pressure method, we proposed a new method of high- and low-pressure combination to prepare hydrogenated microcrystalline silicon (μc-Si:H) films, i.e. at first we used high pressure to deposit film in 2 min in order to minish the thickness of incubation layer from the amorphous phase transition to crystalline phase, and then used low pressure to deposit film in 18 min to improve the density and decrease the oxidation of the film. The experimental results showed that using this new method the thin film with high crystalline volume fraction of 61% and low light-induced degradation ratio of 5.6% at 210 min was obtained, and meanwhile, it also possessed higher density and better photoelectronic properties than mono-pressure method.     

Structural changes in tungsten wire and their effect on the properties of hydrogenated nanocrystalline cubic silicon carbide thin films

We investigated the structural changes in tungsten wire heated to 1800 °C in SiH₄/CH₄/H₂/N₂ atmosphere and the effect of the aging tungsten wire on the properties of N-doped hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films. The aged tungsten wire had two parts: hot parts of the middle of the wire and relatively cold parts connected to clamps. Tungsten carbide (W₂C) layer formed in the wire of the hot parts, while crystalline silicon and cubic silicon carbide (c-Si/3C-SiC) layer deposited on the wire of the cold parts. N-doped nc-3C-SiC:H thin films were deposited for 5 min (thickness: ~ 30 nm) after the tungsten wire was heated under the same condition as during the film deposition for given times (exposure time). No changes in the structural, electrical and optical properties of the nc-3C-SiC:H thin films were observed for the exposure time up to 450 min.     

Effect of thermal coupling on the electronic properties of hydrogenated amorphous silicon thin films deposited by electron cyclotron resonance

In photovoltaic devices, rather thin intrinsic layers of good quality materials are required and high deposition rates are a key point for a cost-effective mass production. In a previous study we have shown that good quality amorphous silicon (a-Si:H) films can be deposited by matrix distributed electron cyclotron resonance (MDECR) plasma CVD at very high deposition rates (∼ 2.5 nm/s). However, only thick films (> 1 μm) exhibited good transport properties. A very poor thermal coupling between the substrate holder and the substrate is the main reason for such a behaviour. We present here experimental data which support this conclusion as well as the improved transport and defect-related properties of new very thin a-Si:H samples (thickness around 0.3 μm) deposited at a higher temperature than the previous ones.     

Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the μc-Si:H films with different H₂/SiH₄ ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of μc-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of μc-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si---H bonds in μc-Si:H and in polycrystalline Si thin films are located at the grain boundaries.     

Interpretation of the hydrogen evolution during deposition of microcrystalline silicon by chemical transport

Hydrogen diffusion is a crucial step in film growth by chemical vapor deposition of both hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µ-Si:H) materials. To gain an insight into the correlation between hydrogen diffusion and the amorphous to microcrystalline transition, we have exposed freshly deposited intrinsic, boron- and phosphorus-doped a-Si:H thin films to hydrogen (or deuterium) plasma in conditions of µc-Si:H deposition by chemical transport. Using both in-situ and ex-situ characterizations techniques, we examined the kinetics of hydrogen excess evolution during the plasma exposure. Solution of the partial differential equation for the diffusion of mobile H atoms with a specific boundary condition that accounts for the reduction of atomic H flux with the growth of the µc-Si:H layer supports the theory that the out-diffusion is a consequence of the growth of the µc-Si:H layer.     

Analysis on the interfacial properties of transparent conducting oxide and hydrogenated p-type amorphous silicon carbide layers in p-i-n amorphous silicon thin film solar cell structure

Quantitative estimation of the specific contact resistivity and energy barrier at the interface between transparent conducting oxide (TCO) and hydrogenated p-type amorphous silicon carbide (a-Si_1 − xC_x:H(p)) was carried out by inserting an interfacial buffer layer of hydrogenated p-type microcrystalline silicon (μc-Si:H(p)) or hydrogenated p-type amorphous silicon (a-Si:H(p)). In addition, superstrate configuration p-i-n hydrogenated amorphous silicon (a-Si:H) solar cells were fabricated by plasma enhanced chemical vapor deposition to investigate the effect of the inserted buffer layer on the solar cell device. Ultraviolet photoelectron spectroscopy was employed to measure the work functions of the TCO and a-Si_1 − xC_x:H(p) layers and to allow direct calculations of the energy barriers at the interfaces. Especially interface structures were compared with/without a buffer which is either highly doped μc-Si:H(p) layer or low doped a-Si:H(p) layer, to improve the contact properties of aluminum-doped zinc oxide and a-Si_1 − xC_x:H(p). Out of the two buffers, the superior contact properties of μc-Si:H(p) buffer could be expected due to its higher conductivity and slightly lower specific contact resistivity. However, the overall solar cell conversion efficiencies were almost the same for both of the buffered structures and the resultant similar efficiencies were attributed to the difference between the fill factors of the solar cells. The effects of the energy barrier heights of the two buffered structures and their influence on solar cell device performances were intensively investigated and discussed with comparisons.     

Large-area SnO2: F thin films by offline APCVD

In this paper, we reported the successful preparation of fluorine-doped tin oxide (FTO) thin films on large-area glass substrates (1245 mm × 635 mm × 3 mm) by self-designed offline atmospheric pressure chemical vapor deposition (APCVD) process. The FTO thin films were achieved through a combinatorial chemistry approach using tin tetrachloride, water and oxygen as precursors and Freon (F-152, C2H4F2) as dopant. The deposited films were characterized for crystallinity, morphology (roughness) and sheet resistance to aid optimization of materials suitable for solar cells. We got the FTO thin films with sheet resistance 8-11 Ω/□ and direct transmittance more than 83%. X-ray diffraction (XRD) characterization suggested that the as-prepared FTO films were composed of multicrystal, with the average crystal size 200-300 nm and good crystallinity. Further more, the field emission scanning electron microscope (FESEM) images showed that the films were produced with good surface morphology (haze). Selected samples were used for manufacturing tandem amorphous silicon (a-Si:H) thin film solar cells and modules by plasma enhanced chemical vapor deposition (PECVD). Compared with commercially available FTO thin films coated by online chemical vapor deposition, our FTO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si:H solar cells and ideal open voltage and short circuit current for a-Si:H solar modules.     

Physical properties of hydrogenated Al-doped ZnO thin layer treated by atmospheric plasma with oxygen gas

Hydrogenated Al-doped ZnO (H:AZO) thin films were deposited on glass substrates at room temperature by radio-frequency magnetron sputtering at various hydrogen flow rates. The addition of hydrogen improved the resistivity of the H:AZO films significantly. A thin insulating layer was produced on H:AZO films by atmospheric pressure plasma with Ar/O₂ reactive gas. The resistivity degenerated and the optical band gap of the oxygen plasma-treated H:AZO films decreased from 3.7 eV to 3.4 eV. This was attributed to a decrease in the hydrogen concentration at the film surface according to elemental depth analysis.     

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E) = ε₁(E) + iε₂(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5-4.5 eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4 nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.     

High band gap nanocrystallite embedded amorphous silicon prepared by hotwire chemical vapour deposition

Hydrogenated silicon films ranging from pure amorphous to biphasic silicon i.e., nanocrystallites embedded amorphous silicon are prepared in an indigenously fabricated hot wire chemical vapor deposition chamber by varying the substrate temperature (T_s) and process pressure (PP). The deposition rates are found to be about 2.5-14 Å/s, which is very much appreciated for the fabrication of cost effective devices. While the films deposited at low T_s are amorphous in nature, those deposited at T_s ≥ 200 °C contain nanocrystallites embedded in the amorphous network. These mixed phase films show high crystalline fraction of 50-56%. All the films deposited at 250 °C, by varying PP, are nanocrystallite embedded with crystalline fraction 60-75%. The optical band gaps of the films (2.0-2.37 eV) are high compared to the regular films, whereas the hydrogen content remains in the reported range of 2.5-5 at.%. We attribute the high optical band gap to the improved order as well as the presence of low density amorphous tissues surrounding the grain boundary regions. The ease of depositing films with tunable band gap is useful for fabrication of tandem solar cells.     

Highly conductive p-type nanocrystalline silicon films deposited by RF-PECVD using silane and trimethylboron mixtures at high pressure

In this paper we present a study of boron-doped nc-Si:H films prepared by PECVD at high deposition pressure (≥4 mbar), high plasma power and low substrate temperature (≤200 °C) using trimethylboron (TMB) as a dopant gas. The influence of deposition parameters on electrical, structural and optical properties is investigated. We determine the deposition conditions that lead to the formation of p-type nanocrystalline silicon thin films with very high crystallinity, high value of dark conductivity (>7 (Ω cm)⁻¹) and high optical band gap (≥1.7 eV). Modeling of ellipsometry spectra reveals that the film growth mechanism should proceed through a sub-surface layer mechanism that leads to silicon crystallization.The obtained films are very good candidates for application in amorphous and nanocrystalline silicon solar cells as a p-type window layer.     

Enhancement of microcrystalline n-i-p solar cell performance via use of pre-covering layers and H2 treatment

We study the effects of a-Si:H and μc-Si:H covering layers and an H₂ treatment on the characteristics of μc-Si:H thin film solar cells deposited in open single chamber very high frequency plasma enhanced chemical vapor deposition systems. Secondary ion mass spectrometry is used to evaluate the phosphor concentration in the μc-Si:H material. Compared to use of an a-Si:H covering layer, use of a μc-Si:H covering layer reduces dopant contamination by a relative 50%, and improves efficiency by a relative 6%, and use of an H₂ treatment reduces dopant contamination by a relative 64%, and improves efficiency by a relative 17%.     

Chemical states of carbon in amorphous boron carbide thin films deposited by radio frequency magnetron sputtering

Boron carbide thin films were deposited by radio frequency (RF) magnetron sputtering and characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high resolution transmission electron microscopy. The results reveal that the structure of thin films deposited at substrate temperatures lower than 350 °C is amorphous. We found that there are four chemical states for carbon in amorphous boron carbide thin films deposited by RF magnetron sputtering. One is the segregated carbon in form of the graphitic inclusions in the thin film identified by Raman spectroscopy and Raman mapping using two strong peaks at ~ 1360 cm^− 1 and ~ 1590 cm^− 1, but the XPS results show that the graphitic inclusions do not connect to the substrate directly. On the surface the carbon forms C=O bonds characterized by the peak of C1s core level at 285.0 eV besides B-C bonds in the boron carbide with the peak of C1s being at 282.8 eV. The detailed analysis of B-C bonds in the boron carbide shows that there are two states for carbon atoms in B-C bonds: in the C-B-C models with C1s peak at 282.3 eV and in the icosahedra with C1s peak at 283.3 eV.     

沉积条件对硅基薄膜非晶转微晶相变、沉积速率及其光电性能的影响(英文)

本文采用HWA-MWECR-CVD系统制备了微晶硅薄膜。研究了氢稀释比、反应压强以及微波功率对微晶硅薄膜非晶转微晶相变及其相关性能的影响。实验结果表明:当氢稀释比为94%、反应压强为1.5Pa以及微波功率为500W时,高质量的微晶硅薄膜可以被获得,如2.86*104的高光敏性,1nm左右的沉积速率以及8.9%的光致衰退速率等。     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.