3 dimensional polymorphous silicon based metal-insulator-semiconductor position sensitive detectors

In this work we investigate the properties of a polymorphous silicon (pm-Si:H) metal-insulator-semiconductor (MIS) structure used in 3D position sensitive detectors (PSD). For the first time a 3D sensor made-up by pm-Si:H/SiO₂/Au layers is presented. MIS structures present several advantages over p-i-n structures, such as easier fabrication, fast response time and higher resolution. The 1D MIS PSD that constitute the array were extensively studied aiming its application in 3D pattern recognition. The results obtained show that MIS PSD can achieve non-linearities below 2% and sensitivities of 3.2 μA/cm over 6 mm length sensors. The miniaturization of the sensors length to arrays of 6 and 16 mm, respectively showed average non-linearities of about 1.9% for the 16 mm sensor which proved to be the best solution for this MIS structure.     

Amorphous Si1−xCx:H films prepared by hot-wire CVD using SiH3CH3 and SiH4 mixture gas and its application to window layer for silicon thin film solar cells

B-doped hydrogenated amorphous silicon carbon (a-Si_1−xC_x:H) films have been prepared by hot-wire CVD (HWCVD) using SiH₃CH₃ as the carbon source gas. The optical bandgap energy and dark conductivity of the film are about 1.94 eV and 2 × 10^− 9 S/cm, respectively. Using this film as a window layer, we have demonstrated the fabrication of solar cells having a structure of the textured SnO₂(Asahi-U)/a-Si_1−xC_x:H(p)/a-Si_1−xC_x:H(buffer)/a-Si:H(i)/μc-Si:H(n)/Al. The conversion efficiency of the cell is found to be 7.0%.     

Effect of oxygen pressure of SiOx buffer layer on the electrical properties of GZO film deposited on PET substrate

The present work was made to investigate the effect of oxygen pressure of SiO_x layer on the electrical properties of Ga-doped ZnO (GZO) films deposited on poly-ethylene telephthalate (PET) substrate by utilizing the pulsed-laser deposition at ambient temperature. For this purpose, the SiO_x buffer layers were deposited at various oxygen pressures ranging from 13.3 to 46.7 Pa. With increasing oxygen pressure during the deposition of SiO_x layer as a buffer, the electrical resistivity of GZO/SiO_x/PET films gradually decreased from 7.6 × 10^− 3 to 6.8 × 10^− 4 Ω·cm, due to the enhanced mobility of GZO films. It was mainly due to the grain size of GZO films related to the roughened surface of the SiO_x buffer layers. In addition, the average optical transmittance of GZO/SiO_x/PET films in a visible regime was estimated to be ~ 90% comparable to that of GZO deposited onto a glass substrate.     

Characterization of the Polarity Dependent Memory and Switching effect in sandwiches of metal/a-Si/ITO

The Polarity Dependent Memory and Switching effect (PDMS) has been observed in sandwiches of metal/a-Si/ITO layers and metal/a-Si/In. The film composition has been analysed by Auger Electron Spectroscopy and peak heights of the metals, Si and SiO_x have been measured as a function of film depth. No penetration of electrode material into the a-Si film was observed and the interfaces were abrupt even after many cycles of switching. A mechanism based on dendrite formation does not therefore operate in this type of PDMS. Traces of SiO_x have been found at the ITO/a-Si interface which are attributed to the presence of ITO or the deposition of In after exposure of a-Si to air. The SiO_x layer appears only in the OFF state and not in the ON state of the device. Hence a model based on voltage formation and destruction of an approximately 500 Å thick SiO_x layer at the a-Si/ITO interface is given.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Rapid thermal-plasma annealing of ZnO:Al films for silicon thin-film solar cells

Rapid thermal annealing of sputter-deposited ZnO and Al-doped ZnO (AZO) films with and without an amorphous silicon (a-Si) capping layer was investigated using a radio-frequency (rf) argon thermal plasma jet at atmospheric pressure. The resistivity of bare ZnO films on glass decreased drastically from 10⁶ to 10³ Ω·cm at maximum surface temperatures T_max above 650 °C, whereas the resistivity increased from 10^− 4 to 10^− 3-10^− 2 Ω·cm for bare AZO films. On the other hand, the resistivity of AZO films with a 30-nm-thick a-Si capping layer remained below 10^− 4 Ω·cm, even after TPJ annealing at a T_max of 825 °C. X-ray diffraction and X-ray photoemission electron studies revealed that the film crystallization of both AZO and a-Si layers was promoted without the formation of an intermixing layer. Additionally, the crystallization of phosphorous- and boron-doped a-Si layers at the sample surface was promoted, compared to that of intrinsic a-Si under identical plasma annealing conditions. The role of the a-Si capping layer on sputter-deposited AZO and ZnO films during TPJ annealing is demonstrated. The effects of the mixing of phosphorous and boron impurities in a-Si:H during TPJ annealing of flat and textured AZOs are also discussed.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Investigation of brittle failure in transparent conductive oxide and permeation barrier oxide multilayers on flexible polymers

An oxide multilayer structure—consisting of an indium zinc oxide (IZO) conductive layer, a silicon oxide (SiO_x, x = 1.8) water vapor permeation barrier, and an aluminum oxide (Al₂O₃) interlayer—coated on polyethylene terephthalate (PET) is proposed as a transparent flexible substrate for display and photovoltaic applications. Vital properties of the multilayer, such as the low water vapor impermeability of the SiO_x barrier and the high conductance of the IZO film, degraded considerably because of the crack formation in bend geometries, attributed to the large difference between elastic properties of the oxide films and polymers. In order to suppress the crack formation, a 10-nm-thick Al₂O₃ interlayer was sputtered on Ar ion-beam treated PET surfaces prior to a SiO_x plasma-enhanced chemical vapor deposition (PECVD) process. Changes in the conductance and water vapor impermeability were investigated at different bending radii and bending cycles. It was found that the increases in resistance and water vapor transmission rate (WVTR) were significantly suppressed by the ion-beam PET pretreatment and by the sputtered Al₂O₃ interlayer. The resistance and WVTR of IZO/SiO_x/Al₂O₃/PET systems could be kept low and invariable even in severely bent states by choosing the SiO_x thickness properly. The IZO (135 nm)/SiO_x (90 nm)/Al₂O₃ (10 nm)/PET system maintained a resistance of 3.2 × 10^− 4 Ω cm and a WVTR of < 5 × 10^− 3 g m² d^− 1 after 1000 bending cycles at a bending radius of 35 mm.     

Fatigue limit and crack arrest in alkali-containing silicate glasses

Crack growth, including fatigue limit and crack arrest, have been investigated for glasses of the systemsxNa₂O-11Al₂O₃-(89-x)SiO₂,xNa₂O-(100-x)SiO₂ andxNa₂O-7CaO-(93−x)SiO₂ in water as well as in acid and alkaline solutions. From studies of the dependence of the crack arrest on the alkali content of the glass, the kind of alkali (K⁺, Na⁺, Li⁺), the pH of the corrosive medium, the ageing time and ageing loading in conjunction with measuring the alkali leaching behaviour, the basic mechanism of crack arrest and fatigue limit can be concluded. Owing to load- and medium-dependent diffusion processes, a crack-growth retarding leached layer at the crack is generated with modified strength and crack growth properties compared to the bulk properties. In high alkali-containing glasses the process is additionally stimulated by stresses produced in the leached layer at the crack tip and at the crack surfaces.     

Assortment of technological terms for silicon-germanium thin films in gradient configuration

Phase diagrams have been studied to describe the RF PECVD process for intrinsic-hydrogenated silicon Si:H and silicon-low germanium alloy a-Si_1−xGe_x:H thin films using textured Al substrates that have been overdeposited with n-type amorphous Si:H (n⁺ a-Si:H). UV, vis, IR, atomic force microscopy (AFM), Raman spectroscopy, small angle X-ray and cross-section transmission electron microscopy (TEM) are used to establish the phase diagram. The a-Si:H, a-Si_1−xGe_x and μc-Si:H processes are applied for optimization of triple-junction thin silicon-based n-i-p solar cells.     

Structural investigations of RTA boron-doped thin a-Si layers

The structural changes in as- sputtered thin a-Si layer, and after boron doping with rapid thermal annealing are investigated by transmission electron microscopy. Stable hexagonal amorphous/crystalline series of SiO₂ structures, as signed as SiO₂ (SnO₂-V), not revealed in high temperature SiO₂ layers, are observed in all films investigated. Different types of crystalline and high ordered SiO₂ structures are obtained in the BSG film, used for doping. Boron penetration in the a-Si layer starts the crystallization at B/Si ratios lower than 10⁻³. RTA process leads to inhomogeneous disordered polycrystalline silicon layer, with large areas of poly-and monocrystalline silicon, coexisting with various crystalline SiO₂ structures. Faster crystallization and larger monocrystalline silicon regions are observed at higher temperatures and longest durations of the annealing process.     

Characterization of SiNx/a-Si:H crystalline silicon surface passivation under UV light exposure

One of the most promising solution for crystalline silicon surface passivation in solar cell fabrication consists in a low temperature (< 400 °C) Plasma Enhanced Chemical Vapor Deposition of a double layer composed by intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (SiN_x). Due to the high amount of hydrogen in the gas mixture during the double layer deposition, the passivation process results particularly useful in case of multi-crystalline silicon substrates in which hydrogenation of grain boundaries is very needed. In turn the presence of hydrogen inside both amorphous layers can induce metastability effects. To evaluate these effects we have investigated the stability of the silicon surface passivation obtained by the double layer under ultraviolet light exposure. In particular we have verified that this double layer is effective to passivate both p- and n-type crystalline silicon surface by measuring minority carrier high lifetime, via photoconductance-decay. To get better inside the passivation mechanisms, strongly connected to the charge laying inside the SiN_x layer, we have collected the Infrared spectra of the SiN_x/a-Si:H/c-Si structures and we have monitored the capacitance-voltage profiles of Al/SiN_x/a-Si:H/c-Si Metal Insulator Semiconductor structures at different stages of UltraViolet (UV) light exposure. Finally we have verified the stability of the double passivation layer applied to the front side of solar cell devices by measuring their photovoltaic parameters during the UV light exposure.     

Plasma deposition of n-SiOx nanocrystalline thin film for enhancing the performance of silicon thin film solar cells

In this paper, we firstly optimized the properties of n-SiO_x nanocrystalline thin film through tuning deposition parameters by plasma enhanced chemical vapor deposition, so that we can actively control the properties of materials obtained. Secondly, we proposed using n-SiO_x/Al as back reflector for amorphous silicon (a-Si:H) solar cells. Compared to Al single-layer as back reflector, adding an n-SiO_x layer into the back reflector could improve the solar cell performance, which not only enhances the short circuit current density by an improvement of spectral response in the wavelength range of 550-750 nm, but also improves the open circuit voltage. With an optimized n-SiO_x/Al back reflector, a-Si:H solar cells with an intrinsic layer thickness of 270 nm show 13.1% enhancement in efficiency. In addition, a-Si:H/μc-Si:H tandem solar cells with n-SiO_x as intermediate reflector were also researched. As a result, it evidently balanced the current matching between top and bottom cell.     

Identification and tackling of a parasitic surface compound in SiC and Si-rich carbide films

Silicon carbide and silicon rich carbide (SiC and SRC) thin films were prepared by PECVD and annealed at 1100 °C. Such a treatment, when applied to SiC/SRC multilayers, aimed at the formation of silicon nanocrystals, that have attracted considerable attention as tunable band-gap materials for photovoltaic applications. Optical and structural techniques (X-ray photoelectron spectroscopy, Reflectance and Transmittance, Fourier Transformed Infrared Spectroscopy) were used to evidence the formation, during the annealing treatment in nominally inert atmosphere, of a parasitic ternary SiO_xC_y surface compound, that consumed part of the originally deposited material and behaved as a preferential conductive path with respect to the nanocrystal layer in horizontal electrical conductivity measurements. The SiO_xC_y compound was HF-resistant, with composition dependent on the underlying matrix. It gave rise to a Si-O related vibration in FTIR analysis, that may be misinterpreted as due to silicon oxide. The compound, if neglected, can affect the structural and electrical characterization of the material.To overcome this problem, a procedure is analyzed, based on the deposition of a sacrificial capping a-Si:H layer that partially oxidizes, and is removed by tetra methyl ammonium hydroxide (TMAH) after annealing. XPS analysis revealed that the resulting surface is mainly made up of SiC regardless of the composition of the underlying SRC layer. Subsequent SF₆:O₂ dry etching results in a porous SiC-rich surface layer. The proposed method is effective in controlling the SRC surface configuration, and allows the performance of reliable optical and electrical characterization.     

Improvement of the short-circuit current density and efficiency in micromorph tandem solar cells by an anti-reflection layer

An anti-reflection layer has been fabricated and applied in micromorph tandem (a-Si:H/μc-Si:H) solar cells. In this work, the porous anti-reflection layers are produced on glass substrates by plasma enhanced chemical vapor deposition using a CF₄ and O₂ gas mixture. The process is simple and easily controlled. The tandem solar cells with the anti-reflection layer show the increased short-circuit current density of the solar cells due to increased light transmittance from air/glass interface. With the anti-reflection layer, the short-circuit current density of the tandem cell increases by 0.29 mA/cm². Meanwhile, the solar cell efficiency increases from 11.15% to 11.55% (3.5% in relative) which allows us to develop more efficient a-Si based solar cells.     

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.     

Luminescent properties and excitation mechanism of ZnSe quantum dots embedded in ZnS Matrix

In this paper, we report alternative-current thin film electroluminescence structure with ZnSe quantum dots embedded in ZnS matrix as light-emitting center, i.e., ITO/SiO_x (100 nm)/[ZnS (10 nm)/ZnSe (1 nm)]₃₀/SiO_x (100 nm)/Al. Blue emissions at 390 and 477 nm are obtained in its alternative-current electroluminescent spectra. By studying its luminescent spectroscopy and brightness oscillogram of the device, we found that blue emission came from defect states at ZnSe/ZnS interface and the excitation mechanism was hot-electron impact.     

A Novel Ultra-Low Work Function TbFx for High Efficiency Dopant-Free Silicon Solar Cells

The ability of carrier selective contact is mainly determined by the surface passivation and work function for dopant-free materials applied in crystalline silicon (c-Si) solar cells, which have received considerable attention in recent years. In this contribution, a novel electron-selective material, lanthanide terbium trifluoride (TbF_x), with an ultra-low work function of 2.4 eV characteristic, is presented, allowing a low contact resistivity (ρ_c) of ≈3 mΩ cm². Additionally, the insertion of ultrathin passivated SiO_x layer deposited by PECVD between TbF_x and n-Si resulted in ρ_c only increase slightly. SiO_x/TbF_x stack eliminated fermi pinning between aluminum and n-type c-Si (n-Si), which further enhanced the electron selectivity of TbF_x on full-area contacts to n-Si. Last, SiO_x/TbF_x/Al electron-selective contacts significantly improves the open circuit voltage (V_oc) for silicon solar cells, but rarely impacts the short circuit current (J_sc) and fill factor (FF), thus champion efficiency cell achieved approaching 22% power conversion efficiency (PCE). This study indicates a great potential for using lanthanide fluorides as electron-selective material in photovoltaic devices.     

Effect of TiOx seed layer on the texture and electric properties in La and Ca modified PbTiO3 thin films

Lanthanum-and calcium-modified PbTiO₃ (PLCT) ferroelectric thin films were successfully prepared on Pt(111)/Ti/SiO₂/Si substrates by pulsed laser deposition. Influence of TiO_x seed layer on texture and electric properties of PLCT films was investigated. It is found the PLCT films without seed layer exhibited highly (100)-textured, while using about 9 nm TiO_x as seed layer lead to highly (301)-textured. The PLCT film with TiO_x seed layer possess higher remnant polarization (Pr = 26 µC/cm²), better pyroelectric coefficient and figure of merit at room temperature (p = 370 µC/m²k, F_d = 190 × 10^− 5 Pa^− 1/2) than that of film without seed layer. The mechanism of the enhanced electric properties was also discussed.     

Optical properties of SiO2/nanocrystalline Si multilayers studied using spectroscopic ellipsometry

Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO₂/SiO_x multilayers. The SiO₂(2 nm)/SiO_x(2 nm) multilayers are prepared under various deposition temperatures by ion beam sputtering. Annealing at temperatures above 1100 °C leads to the formation of Si nanocrystals (nc-Si) in the SiO_x layer of multilayers. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentz lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO₂/nc-Si multilayers as a mixture of nc-Si and SiO₂. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiO_x layer. This result is compared to expected nc-Si volume fraction, which is estimated from the stoichiometry of SiO_x.