Investigations on phosphorus doped amorphous/nanocrystalline silicon films deposited by a filtered cathodic vacuum arc technique in the presence of hydrogen gas

Phosphorus doped amorphous/nanocrystalline silicon (a-Si:H/nc-Si:H) thin films have been deposited by a filtered cathodic vacuum arc (FCVA) technique in the presence of hydrogen gas at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 °C. The films have been characterized by using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity (σ_D), activation energy (ΔE), optical band gap (E_g) and secondary ion mass spectroscopy. The XRD patterns show that RT grown film is amorphous in nature but high temperature (225 and 350 °C) deposited films exhibit nanocrystalline structure with (111) and (220) crystal orientations. The crystallite size of higher temperature grown silicon film evaluated was between 13 and 25 nm. Raman spectra reveal the amorphous nature of the film deposited at RT, whereas higher temperature deposited films show crystalline nature. The crystalline volume fraction of the silicon film deposited at higher temperatures (225 and 350 °C) was estimated to be 58 and 72%. With the increase of T_s, the bonding configuration changes from mono-hydride to di-hydride as revealed by the FTIR spectra. The values of σ_D, ΔE and E_g of silicon films deposited at different T_s were found to be in the range of 5.37×10⁻⁴–1.04 Ω⁻¹ cm⁻¹, 0.05–0.45 eV and 1.42–1.83 eV, respectively. Photoconduction of 3.5% has also been observed in n-type nc-Si:H films with the response and recovery times of 9 and 12 s, respectively. A n-type nc-Si:H/p-type c-Si heterojunction diode was fabricated which showed the diode quality factor between 1.6 and 1.8.     

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40 MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiO_x:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E₀₄, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiO_x:H/n-μc-Si:H structure showed an enhancement in short-circuit current density J_sc from 13.32 to 13.60 mA/cm², and in conversion efficiency from 8.53% to 8.61%. The increased J_sc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660 nm, as demonstrated by the external quantum efficiency.     

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40 MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiO_x:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiO_x:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density J_sc. The increased J_sc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680 nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.     

Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition

Biaxially aligned yttria-stabilized zirconia (YSZ) films on Ni-based alloy substrates were realized with high deposition rate of 0.5 μm min⁻¹ by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing effect in the ISD process. To raise I_c values, YBCO thickness was increased up to 5 μm. Thick YBCO films with high J_c values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high I_c value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved.     

Improvement of hydrogenated amorphous silicon germanium thin film solar cells by different p-type contact layer

In this study, we report an appreciably increased efficiency from 6% up to 9.1% of hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells by using a combination of different p-doped window layers, such as boron doped hydrogenated amorphous silicon (p-a-Si:H), amorphous silicon oxide (p-a-SiO_x:H), microcrystalline silicon (p-µc-Si:H), and microcrystalline silicon oxide (p-µc-SiO_x:H). Optoelectronic properties and the role of these p-layers in the enhancement of a-SiGe:H cell efficiency were also examined and discussed. An improvement of 1.62 mA/cm² in the short-circuit current density (J_sc) is attributed to the higher band gap of p-type silicon oxide layers. In addition, an increase in open-circuit voltage (V_oc) by 150 mV and fill factor (FF) by 6.93% is ascribed to significantly improved front TCO/p-layer interface contact.     

Broadband triple-layer SiOx/SiOxNy/SiNx antireflective coatings in textured crystalline silicon solar cells

The purpose of this study is to reduce textured crystalline silicon (TCS) substrate surface-reflectivity over a wide spectral range (300–1100 nm), to improve the step coverage of the textured structure, and to shift the minimal value of reflection from the unabsorbed region to the absorbed region. The TCS solar-cell interface between air and silicon was added to a SiO_x/SiO_xN_y/SiN_x triple-layer anti-reflective coatings (TLARCs) structure using the plasma-enhanced chemical vapor deposition (PECVD) growth method. This paper presents theoretical and practical discussions, as well as the experimental results of fabricating the films and devices. The average reflection of the SiO_x/SiO_xN_y/SiN_x TLARs reduced to 2.01% (300–1100 nm). The minimal value of reflection was shifted from 1370 nm (unabsorbed region) to 968 nm (absorbed region). The SEM images show effective step coverage. In comparison to the untreated TCS solar cells, applying the experimental SiO_x/SiO_xN_y/SiN_x TLARCs to conventional TCS solar cells improved the short-circuit current density (J_sc) by 7.78%, and solar-cell efficiency by 10.95%. This study demonstrates that the SiO_x/SiO_xN_y/SiN_x TLARCs structure provides antireflective properties over a broad range of visible and near-infrared light wavelengths. An effective step coverage and minimal value of reflection from unabsorbed region shift to the absorbed region is demonstrated.     

Fabrication of inverted pyramid structure by Cesium Chloride self-assembly lithography for silicon solar cell

Inverted pyramids were fabricated through a method combining cesium chloride (CsCl) self-assembly technology and anisotropy corrosion of silicon solar cells. Ti film with nanoporous masks was formed by lift-off the CsCl nanoislands for the inverted pyramids. The pyramids were then formed by anisotropy corrosion of alkaline solution. The average diameter and morphology of the pyramids were controlled by varying the average diameter of CsCl nanoislands from 400 nm to 1.5 µm and by varying the etching time of alkaline solution from 2 to 8 min. The inverted-pyramid texture could suppress reflection to below 10% at wavelengths from 400 to 1000 nm, which was much lower than that of planar wafer. A solar cell fabricated from the pyramids had higher short-circuit current density (J_sc) and photovoltaic conversion efficiency (PCE) compared with those of planar solar cells for the good antireflection property. The solar cell showed a PCE of 15.25%, a J_sc of 38.35 mA/cm², and an open-circuit voltage of 555.7 mV.     

Characterization and simulation analysis of laser-induced crystallization of amorphous silicon thin films

The effect of laser energy density on the crystallization of hydrogenated amorphous silicon (a-Si:H) thin films was studied theoretically and experimentally. The thin films were irritated with a frequency-doubled (λ=532 nm) Nd:YAG pulsed nanosecond laser. An effective finite element model was built to predict the melting threshold and the optimized laser energy density for crystallization of intrinsic amorphous silicon. Simulation analysis revealed variations in the temperature distribution with time and melting depth. The highest crystalline fraction measured by Raman spectroscopy (84.5%) agrees well with the optimized laser energy density (1000 mJ/cm²) in the transient-state simulation. The surface morphology of the thin films observed by optical microscopy is in fairly good agreement with the temperature distribution in the steady-state simulation.     

a-SiOx:H passivation layers for Cz-Si wafer deposited by hot wire chemical vapor deposition

In order to get the high photoelectric conversion efficiency a-Si:H/c-Si solar cells, high quality intrinsic hydrogenated passivation layer between the a-Si:H emitter layer and the c-Si wafer is necessary. In this work, hot wire chemical vapor deposition (HWCVD) is used to deposite intrinsic oxygen-doped hydrogenated amorphous silicon (a-SiO_x:H) and hydrogenated amorphous silicon (a-Si:H) films as the intrinsic passivation layer for a-Si:H/c-Si solar cells. The passivation effect of the films on the c-Si surface is shown by the effective lifetime of the samples that bifacial covered by the films with same deposition parameters, tested by QSSPC method. The imaginary part of dielectric constant (ε₂) and bonds structure of the layers are analyzed by Spectroscopic Ellipsometry(SE) and Fourier Transfom Infrared Spectroscopy(FTIR). It is concluded that: (1) HWCVD method can be used to make a-SiO_x:H films as the passivation layer for a-Si:H/c-Si cells and the oxidation of the filament can be overcome by optimizing the deposition parameters. In our experiments, the lowest surface recombination velocity of the c-Si wafer is 3.0 cm/s after a-SiO_x:H films passivation. (2) Oxygen-doping in the amorphous silicon layers can increase H content and the band-gap of films, similar as the phenomenon of the films deposited by PECVD.     

High-resolution DLTS studies of vacancy-related defects in irradiated and in ion-implanted n-type silicon

Using high-resolution Laplace deep-level transient spectroscopy (DLTS), we have compared the electron emission characteristics of vacancy-related defects in silicon. The samples include material irradiated with high-energy protons, material implanted with a heavy ion and silicon irradiated with 2 MeV electrons. We show that in the proton- and electron-irradiated material the DLTS peak in the region of the (- -/-) state of the divacancy at E_c=0.23 eV contains only one feature. The DLTS peak at 250 K which contains the signal derived from the (-/0) state of the divacancy is much larger in ion-implanted silicon than in electron-irradiated silicon. The Laplace DLTS is able to resolve clearly the (-/0) divacancy state and the V–P defect, whereas conventional DLTS shows only a broad peak in that region.     

Investigation of optical and chemical bond properties of hydrogenated amorphous silicon nitride for optoelectronics applications

The aim of this work is to determine optimal deposition parameters of silicon nitride for optical applications. The authors present the investigation of hydrogenated amorphous silicon nitride SiN_x:H deposited by the low temperature PECVD method in high frequency reactors. The study of hydrogen bonds in the SiN_x:H thin films were detailed. The impact of NH₃, SiH₄ and N₂ flow ratio and radio frequency power on optical coefficients in relation to chemical composition and roughness of the film is studied. The correlation between chemical bonds (N–H, Si–H) and refractive index and extinction coefficients is systematically verified. The experimental results show that the films with high refractive indexes superior to 2.05 and low roughness of about 0.35 nm can be achieved for optoelectronics applications by tuning the flow ratio or decreasing the RF power. A variety of processes have been suggested as compatible with low thermal budget (under 350 °C) in order to integrate optical waveguides with lower loss. In particular, the incorporation of N₂ as dilution gas is suited to the fabrication of SiN_x:H films optical waveguide requiring low N–H bonds, low concentration of hydrogen [H] and high refractive index.     

p-Layer bandgap engineering for high efficiency thin film silicon solar cells

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (V_oc) of solar cells and the p-layer optical band gap (E_p). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an E_p 1.95 eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9 nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650 nm. These results have guided us to achieve high V_oc value 1.03 V for thin film silicon based single junction solar cell.     

Effect of substrate temperature on indium tin oxide (ITO) thin films deposited by jet nebulizer spray pyrolysis and solar cell application

This study focused on the effect of substrate temperature (350 °C, 400 °C, and 450 °C) on morphological, optical, and electrical properties of indium tin oxide (ITO) films deposited onto porous silicon/sodalime glass substrates through jet nebulizer spray pyrolysis for use in heterojunction solar cells. X-ray diffraction analysis confirmed the formation of pure and single-phase In₂O₃ for all the deposited films whose crystallinity was enhanced with increasing substrate temperature, as shown by the increasing (222) peak intensity. Morphological observations were conducted using scanning electron microscopy to reveal the formation of continuous dense films composed of nanograins. The UV–vis spectra revealed that the transmittance increased with increasing substrate temperature, reaching a value of over 80% at 450 °C. The photoelectric performance of the solar cell was studied using the I–V curve by illuminating the cell at 100 mW/cm². A high efficiency (η) of 3.325% with I_sc and V_oc values of 14.8 mA/cm² and 0.60 V, respectively, was attained by the ITO solar cell annealed at 450 °C.     

Microstructure evolution of amorphous silicon thin films upon annealing studied by positron annihilation

Amorphous silicon (a-Si) thin films were prepared on glass substrates by plasma enhanced chemical vapor deposition (PECVD). Influence of annealing temperature on the microstructure, surface morphology, and defects evolution of the films were studied by X-ray diffraction (XRD), atomic force microscope (AFM) and positron annihilation Doppler broadening spectroscopy (DBS) based on a slow positron beam, respectively. The S parameter of the as-deposited a-Si thin film is high, indicative of amorphous state of Si film with many defects. The a-Si gradually grows into polycrystalline silicon with increasing temperature to 650 °C. For the films annealed below ~450 °C, positron diffusion lengths are rather small because most positrons are trapped in the defects of the a-Si films and annihilated there. With further rising the temperature to 600 °C, the diffusion length of positrons increases significantly due to the removal of vacancy-type defects upon annealing at a high temperature. The results indicate that the coalescence of small vacancy-type defects in a-Si thin film and the crystallization of a-Si occur around 450 °C and 650 °C, respectively.     

Texturization of silicon wafers with Na2CO3 and Na2CO3/NaHCO3 solutions for heterojunction solar-cell applications

The formation of pyramidal structures by anisotropic etching of 〈1 0 0〉-oriented monocrystalline silicon wafer surfaces is an effective method to reduce reflection losses originating on the front side of conventional silicon solar cells and silicon-heterojunction (SHJ) solar cells. One of the most common methods of texturization used in the solar-cell industry is based on aqueous solutions of NaOH or KOH and isopropyl alcohol (IPA). However, IPA is toxic and relatively expensive, so efforts are being made to replace it. Among the potential alternatives, solutions based on Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures have been proposed. In the present study, solutions of Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures were prepared in order to form pyramidal structures on silicon wafer surfaces. It was not possible to obtain uniform and completely textured surfaces by using aqueous solutions consisting only of Na₂CO₃. NaHCO₃ must be added in order to achieve uniform textured surfaces with low hemispherical reflectance suitable for SHJ solar-cell applications. Textured surfaces with good uniformity and low average hemispherical reflectance (15.4%) were prepared from 〈1 0 0〉 silicon substrates with relatively low etching times (25 min). Good surface passivation (lifetime >600 μs and implicit open-circuit voltage of 690±10 mV) on these p-type textured wafers were achieved.     

Realizing high breakdown voltage for a novel interface charges islands structure based on partial-SOI substrate

A novel interface charge islands partial-SOI (ICI PSOI) high voltage device with a silicon window under the source and its mechanism are studied in this paper. ICI PSOI is characterized by a series of equidistant high concentration n⁺-regions on the bottom interface of top silicon layer. On the condition of high-voltage blocking state, inversion holes located in the spacing of two n⁺-regions effectively enhance the electric field of the buried oxide layer (E_I) and reduce the electric field of the silicon layer (E_S), resulting in a high breakdown voltage (V_B). It is shown by the simulations that the enhanced field ΔE_I and reduced field ΔE_S by the accumulated holes reach to 449 V/μm and 24 V/μm, respectively, which makes V_B of ICI PSOI increase to 663 V from 266 V of the conventional PSOI on 5 μm silicon layer and 1 μm buried oxide layer with the same silicon window length. On-resistance of ICI PSOI is lower than that of the conventional PSOI. Moreover, self-heating-effect is alleviated by the silicon window in comparison with the conventional SOI at the same power of 1 mW/μm.     

Growth and characterization of silicon oxide films formed in the presence of Si,SiC, and Si3N4

In order to comparatively study the growth and characterization of silicon oxide films on Si-based substrates, top-cut solar grade silicon (SOG-Si) containing Si₃N₄ rods and SiC lumps were used as raw materials and respectively heated at 1773 K and 1873 K under Ar gas. The samples were investigated by Focus Ion Beam/Scanning Electron Microscope (FIB/SEM) and Energy Dispersive Spectroscopy (EDS). Results indicated that silicon oxides with different morphologies successfully grew on the substrates via various mechanisms. Passive oxidation was evident in the formation of a dense SiO₂ surface layer on the base material at 1773 K, while active oxidation was evident in the formation of SiO₂ with particle, rod, and nanowire-like morphologies, which was the re-oxidation product of SiO at 1873 K under the active-to-passive transition. Si, SiC, and Si₃N₄ have the similar oxidation tendency to form silicon oxides under either passive or active regimes.     

Mixed phase silicon thin films grown at high rate using 60 MHz assisted VHF-PECVD technique

Mixed phase amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin films were deposited by VHF-PECVD (60 MHz) using Argon (Ar) as the diluent of silane. These amorphous and crystalline silicon thin films were deposited by varying the argon dilution (f_Ar) from 10–97.5% while keeping other process parameters constant. The effects of argon dilution on deposition rate, structural and optical properties of micro/nanocrystalline silicon thin films are studied. It has been observed that the films deposited from f_Ar 10–70% showed the deposition rate >20 Å/s with the highest deposition rate achieved of ~25 Å/s. Structural characterization has been performed by micro-Raman analysis and Atomic force microscopy. Raman shift towards higher wave number (515 cm⁻¹) with increase of f_Ar indicates variation in crystallinity of silicon films. HRTEM studies revealed the distribution of grain size and the degree of crystallinity. Optical absorption spectroscopy confirmed the increase in band gap of the materials from 1.5 to 2.1 eV.     

Formation and characterization of porous silicon films obtained by catalyzed vapor-chemical etching

Porous silicon films obtained by the metal-assisted vapor-chemical etching technique have been characterized. For the film formation, epitaxial (100) N/P⁺, 1–5 Ω cm monocrystalline silicon wafers were used. The vapors of an alcoholic solution of H₂O₂/HF were drawn towards the silicon surface, which was previously covered with a thin layer of gold (~8 nm) for the catalytic etching. For the optical and morphological characterization of porous films, Raman spectroscopy, Ellipsometry, FTIR spectroscopy and SEM images were used. The films thickness kept a linear relationship with etching time. A porosity gradient from the surface towards the interface (65% to 12%) was observed in the films. A large amount of Si–H bonds as related to O–Si–O bonds were observed and the pore size depends on the HF concentration. Irregular morphology was found in films formed with 50% HF.     

Photoluminescence from c-axis oriented ZnO films synthesized by sol-gel with diethanolamine as chelating agent

ZnO films were synthesized on SiO₂/Si substrates through the sol-gel technique using diethanolamine as chelating agent and annealed in Ar+O₂ atmospheres with different O₂ flow-rates in the 10–100 sccm range. Samples were studied by scanning electron microscopy and X-ray diffraction, evidencing a nanostructured morphology with a preferential orientation along the (0 0 2) direction (c-axis orientation), which is uncommon when diethanolamine is used as the chelating agent. The room temperature photoluminescence spectra show strong UV emissions at around 375 and 384 nm from near band-edge transitions and phonon replica, and a broad defect-related band extending from the visible to near infrared (∼500–800 nm). The analysis of the defect-related emission band and its various components as a function of the O₂ flow-rate is discussed in terms of contributions from specific luminescent point defect centers established during annealing.