SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si₃N₄) deposited over the n⁺-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si₃N₄ layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n⁺-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n⁺-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρ _c (in Ω-cm²) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρ _c  = 2.53 × 10⁻⁵ Ω-cm² is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρ _c is two orders of magnitude lower than the typical value of ρ _c  = 3 × 10⁻³ Ω-cm² reported previously for the Ag contacts of the solar cell. Such low value of ρ _c for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρ _c versus impurity doping level (N _s ) shows that measured value of the ρ _c follows a linear relationship with the N _s as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n⁺-Si surface of the silicon solar cell.

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Structural and electrical properties of swift heavy ion beam irradiated Fe/Si interface

The present work deals with the mixing of iron and silicon by swift heavy ions in high-energy range. The thin film was deposited on a n-Si (111) substrate at 10⁻⁶ torr and at room temperature. Irradiations were undertaken at room temperature using 120 MeV Au⁺⁹ ions at the Fe/Si interface to investigate ion beam mixing at various doses: 5 × 10¹² and 5 × 10¹³ ions/cm². Formation of different phases of iron silicide has been investigated by X-ray diffraction (XRD) technique, which shows enhancement of intermixing and silicide formation as a result of irradiation. I-V measurements for both pristine and irradiated samples have been carried out at room temperature, series resistance and barrier heights for both as deposited and irradiated samples were extracted. The barrier height was found to vary from 0·73–0·54 eV. The series resistance varied from 102·04–38·61 kΩ.     

Electrical characterization of low temperature deposited oxide films on ZnO/<Emphasis Type="Italic">n</Emphasis>-Si substrate

Thin films of silicon dioxide are deposited on ZnO/n-Si substrate at a low temperature using tetra-ethylorthosilicate (TEOS). The ZnO/n-Si films have been characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The border trap density (Q_bt) and fixed oxide charge density (Q_f/q) of the SiO₂/ZnO/n-Si films are found to be 3.9 × 10¹⁰ cm⁻² and 1.048 × 10¹¹ cm⁻², respectively. The trapping characteristics and stress induced leakage current (SILC) have also been studied under Fowler-Nordheim (F-N) constant current stressing.     

Two-stage doping of silicon with phosphorus-32 and sulfur-32 isotopes

We describe a principally new approach to obtaining solid sources of sulfur for the two-stage diffusion doping of silicon, which eliminates the surface erosion of wafers. According to the proposed method, ³²P radionuclide is diffused into a near-surface region of silicon in the first (introduction) stage and then converted into ³²S isotope in the second state. It is shown that the samples of silicon doped by this method with ³²S isotope contain deep levels with the ionization energies E _c − 0.13 eV, E _c − 0.25 eV, E _c − 0.37 eV, and E _c − 0.50 eV, which exhibit a donor character and are related to the sulfur impurity. It is established that the diffusion of ³²S isotope into p-Si leads to a change in the type of conduction, while the diffusion of this isotope into n-Si decreases the resistivity of the initial material.     

<Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis> junctions in ZnO implanted with group V ions

n-Type ZnO〈Ga〉 films were implanted with 150-keV N⁺ (As⁺) ions to a dose of 7 × 10¹⁵ cm⁻² and then annealed in atomic oxygen at different temperatures. p-Type conductivity was obtained at annealing temperatures in the range 770–870 K. The parameters of the p-type layers were determined by photoluminescence spectroscopy, secondary ion mass spectrometry, and Hall effect measurements. According to the Hall data, the p-type layers had a resistivity of ∼30 Ω cm, carrier mobility of ∼2 cm²/(V s), and carrier concentration of ∼10¹⁸ cm⁻³. The electroluminescence spectra of the p-n junctions produced by ion implantation showed a band at 440 nm, due to recombination via donor-acceptor pairs.     

Investigation of the thermal stability of picosecond gallium arsenide dynistor switches

The operation of GaAs n⁺-p-i-n ⁰-p ⁺ dynistor structures has been demonstrated experimentally under conditions of reversible avalanche breakdown at temperatures up to 200 °C with switching times remaining under 140 ps. A numerical simulation refined the influence of various parameters of the semiconductor on the temperature dependence of the switching characteristics. Pis’ma Zh. Tekh. Fiz. 24, 73–78 (August 12, 1998)     

The formation,migration, agglomeration and annealing of vacancy-type defects in self-implanted Si

The evolution of vacancy-type defects has been studied by variable-energy positron annihilation spectroscopy (VEPAS) in samples of high-quality FZ p-type (001) silicon wafers implanted with 4 MeV Si²⁺ ions at room temperature to doses of 10¹²–10¹⁴ cm⁻². The average vacancy concentration increases as (ion dose)^{0.70 ± 0.06}. Progressive isochronal annealing measurements show that open-volume point defects (having a VEPAS signature close to that for divacancies) anneal between 500–600°C. VEPAS with enhanced depth sensitivity (via progressive etching) verified that single 30 min anneals to 550 and 600°C lead to the formation of buried clusters V _N with an average N of 3.5 lying between depths of 2.2 and 3.6 μm (both ± 2 μm), close to the peak of vacancy damage just shallower than the ion range predicted by simulation. The concentration of these clusters increases as (ion dose)^{2.6 ± 0.1}. Single anneals to higher temperatures reduce all open-volume point defect concentrations to below the limit detectable by VEPAS.     

Electrical characteristics and inhomogeneous barrier analysis of aniline green/p-Si heterojunctions

In this study, we identically prepared the aniline green/p-Si organic–inorganic devices (total 27 diodes) formed by direct evaporation of an organic compound solution on to a p-Si semiconductor wafer, and then studied the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of these devices. It was seen that the aniline green organic thin film on the p-Si substrate showed a good rectifying behavior. The barrier heights (BHs) and ideality factors of all devices were extracted from the electrical characteristics. Mean BH and ideality factor were calculated as 0.582 eV and 2.999, respectively from the I–V characteristics. Additionally, the mean barrier height and mean acceptor doping concentration from C–V measurements were calculated as (0.61 ± 0.10) eV and (5.54 ± 0.68) × 10¹⁴ cm⁻³, respectively. The discrepancy in the BH values obtained from I–V and C–V characteristics has been attributed to different nature of the measurements. This can also be due to the existence of the interfacial native oxide and the organic aniline green thin layer between the semiconductor and contacting top metal.     

Test beam results of Current Injected Detectors (CID) irradiated up to 5×10 1 MeV neq/cm

Two full size strip detectors were investigated in this study: one with p⁺ strips (p⁺/n⁻/n⁺) and another with n⁺ strips (n⁺/p⁻/p⁺). Both detectors, are made of magnetic Czochralski silicon (MCz-Si) and irradiated to S-LHC fluencies, were tested with 225 GeV muon beam in the CERN H2 area. The Current Injected Detector (CID) sensors were operated in a cooling box capable of providing a −53 °C temperature. Results indicate a relative charge collection efficiency (CCE) at 5×10¹⁵ n_eq/cm² above 30% in irradiated p⁺/n⁻/n⁺ CID detector at 600 V bias voltage. The signal to noise ratio of this CID module was about eight and a forward current of 30 μA was needed for detector biasing. In standard reverse bias, the same detector could not provide a sufficiently large signal for particle tracking purposes. A p-type (n⁺/p⁻/p⁺) sensor was irradiated to a fluence of 2×10¹⁵ n_eq/cm² and measured under the same test beam conditions. According to the theory of CIDs developed by the CERN RD39 Collaboration, this detector module could be biased up to only 230 V due to the low irradiation fluence. The CCE at 230 V was 35% in CID operation and 20% when reverse biased.     

Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T _c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO₂ plane for cuprates—shows a strong linear correlation to the inverse of T _c in the form (2x)²=m ₁1/T _c with a slope of m ₁=(2.786±0.029)×10⁻¹⁵ m² K. The mercury cuprate homologous series HgBa₂Ca _n−1Cu _n O_2n+2+δ with n=1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ.     

Testing metal-dielectric-semiconductor tunnel structures on p-silicon as nuclear particle detectors

Metal-dielectric-semiconductor (MDS) structures with aluminum nitride (AlN) as a tunnel dielectric based on high-ohmic p-type silicon substrates have been studied. The samples were characterized with respect to the charge collection efficiency and energy resolution on probing with 5.4-MeV α particles. In addition, the nature of noises and the state of the AlN-p-Si interface were investigated. It is established that the parameters of these MDS structures as radiation detectors are close to those of widely used Schottky-barrier detectors based on n-Si (Au-n-Si). A decrease in the concentration of deep centers at the AlN-p-Si interface allows the proposed MDS structures to compete successfully with n-Si based detectors, which is due to a higher purity of the initial material.     

BF2 ion implantation in silicon through surface oxides,behaviour of the fluorine with rapid thermal annealing

BF₂ ion implantation through surface oxides has been investigated to form shallow p⁺/n junctions. BF₂ ion implantation was performed at 25 keV at a dose of 5.4 × 10¹⁴cm⁻² through surface oxides of different thicknesses into crystalline silicon. Rapid thermal annealing (1000°C/10 s) was used for dopant activation and radiation damage removal. Secondary ion mass spectroscopy (SIMS) was used to obtain the boron and fluorine distribution profiles. p⁺/n junctions as shallow as 0.12 μm were formed with reasonable sheet resistance. The study shows that, as expected, dopant loss in the surface oxide during ion implantation results in higher values of sheet resistance. Out-diffusion of fluorine during RTA resulted in a fluorine loss of 50 to 65% from the silicon. Also, fluorine was found to segregate at the oxide/silicon interface.     

Oxidation of carbohydrazide with nitric acid

The kinetics of carbohydrazide oxidation with nitric acid in aqueous solutions was studied. In the range [HNO₃] = 3–7 M, the reaction rate is described by the equation −d[(NH₂NH)₂CO]/dt = k[(NH₂NH)₂CO][HNO₃] ⁿ , where n = 3.6 and 2.9 at 70 and 90°C, respectively. The constant k = (6.65 ± 0.23) × 10⁻⁴ l^2.9 mol^−2.9 h⁻¹ at 90°C, and the activation energy of the reaction in 7 M HNO₃ is 124 kJ mol⁻¹. The Fe(III) and especially Tc(VII) ions considerably accelerate the reaction, whereas the uranyl ions accelerate it insignificantly. The reaction mechanism in which the reactive species oxidizing carbohydrazide is nitronium ion NO₂⁺ was suggested.     

Mg- and Zn-modified calcium phosphates prepared by biomimetic precipitation and subsequent treatment at high temperature

Powders of magnesium-modified as well as zinc-modified calcium phosphates (Me-β-TCP and HA) with a (Ca²⁺+Mg²⁺+Zn²⁺+Na⁺+K⁺)/P ratio of 1.3–1.4 and various Me²⁺/(Me²⁺+Ca²⁺) ratios (from 0.005 to 0.16) were prepared in biomimetic electrolyte systems at pH 8, mother liquid maturation and further syntering at 600–1000°C. Some differences in zinc and magnesium modifications have been prognosed on the basis of thermodynamic modeling of the studied systems and explained by the Mg²⁺ and Zn²⁺ ion chemical behaviour. The temperature as well as the degree of Zn²⁺ and Mg²⁺ ions substitutions were found to stabilize the β-TCP structure and this effect was more prononced for zinc. Thus, zinc-modified β-TCP powders consisting of idiomorphic crystals were obtained through sintering of Zn²⁺ ion substituted calcium phosphates precursors at 800–1000°C. The Mg²⁺ ion substitution leads to obtaining magnesium-modified β-TCP with spherical grains.     

Defect formation in silicon implanted with ∼1 MeV/nucleon ions

Defect formation processes in silicon implanted with ∼1 MeV/nucleon boron, oxygen, and argon ions have been studied using microhardness and Hall effect measurements. The results indicate that ion implantation increases the surface strength of silicon single crystals owing to the formation of electrically inactive interstitials through the diffusion of self-interstitials from the implantation-damaged layer to the silicon surface. The radiation-induced surface hardening depends significantly on the nature of the ion, its energy, and the implant dose. In the case of low-Z (boron) ion implantation, the effect had a maximum at an implant dose of ∼5 × 10¹⁴ cm⁻², whereas that for O⁺ and Ar⁺ ions showed no saturation even at the highest dose reached, 1 × 10¹⁶ cm⁻². When the ion energy was increased to ∼3 MeV/nucleon (210-MeV Kr⁺ ion implantation), we observed an opposite effect, surface strength loss, due to the predominant generation of vacancy-type defects.     

Use of Thermal Ionization Mass Spectrometry for Measuring the Oxygen Isotope Ratio in Uranium Oxides1,2

A method was developed for precise determination of oxygen isotope ratios in uranium oxides. Thermal ionisation mass spectrometry (TIMS) was used for direct measurements of n(U^{1 8}O⁺)/n(U^{1 6}O⁺) using the molecular species UO⁺. Suitable sample handling and filament preparation techniques were developed in order to obtain reproducible results and avoid oxygen contamination. The actual measurements were performed using the total evaporation method. By this technique the achieved precision for n(U^{1 8}O⁺)/n(U^{1 6}O⁺) measurements was in the range of 0.04%. The peak jump techniques was also used for measurements, and the results of both techniques are discussed. The TIMS measurement are verified by comparative measurements using secondary ion mass spectrometry (SIMS).     

Changes in silicon samples bombarded by 900–2300 eV hydrogen ions

We have performed a series of statistically designed experiments to evaluate the changes which occur in silicon samples as a result of hydrogen ion bombardment. Single-crystal silicon and polycrystalline silicon, and also edge-defined film-fed growth (EFG) solar cell devices, were exposed to a hydrogen ion beam produced by a Kaufman ion source. The experimental parameters which were systematically varied include the maximum hydrogen ion energy (900, 1600 and 2300 eV), the energy spread of the ions in the beam, the ion current density (0.8, 1.4 and 2.0 mA cm^?2), the total dose (1 × 10¹⁸, 2 × 10¹⁸ and 4 × 10¹⁸ ions cm^?2) and the bulk sample temperature during bombardment (200, 275 and 350 °C). We observed the changes in the short-circuit current, the open-circuit voltage, the photovoltaic conversion efficiency and the fill factor associated with the EFG devices, the change in the spectral reflectivity of the single-crystal silicon samples and the ratio of the numbers of SiH to SiH₂ groups present in the polycrystalline silicon samples. The results of our study indicate that the properties of hydrogen-ion-bombarded silicon change with both the maximum ion energy and the ion current density with a significant parametric interaction between the current density and the bulk sample temperature.The energy distribution of the ions present in the hydrogen ion beam affects the spectral reflectivity of single-crystal silicon. The changes associated with hydrogen ion bombardment are independent of dose for the conditions studied. The average absolute air mass 1 efficiency of EFG devices without intentionally applied antireflective coatings increased from 8.7% before exposure to 10.2% after exposure to the optimal conditions determined by this study.     

New p-ary sequence family with low correlation and large linear span

In this paper, for an odd prime p and positive integers n, m, and e such that n = me, a new family S{\mathcal{S}} of p-ary sequences of period p ⁿ − 1 with low correlation and large linear span is constructed. It is shown that S{\mathcal{S}} has maximum correlation 1+p^[(n+2e)/2]{1+p^{n+2e\over 2}}, family size p ⁿ , and maximal linear span [((m+3)n)/2]{{(m+3)n\over 2}}. When m is even, the proposed family S{\mathcal{S}} contains Tang, Udaya, and Fan’s construction as a subset. Furthermore, when n is even and e=1, S{e=1, \mathcal{S}} has the same correlation and family size, but larger linear span compared with the construction by Seo, Kim, No, and Shin.     

Macroporous silicon: efficient antireflective layer on crystalline silicon

A macroporous silicon layer (ma-PS) electrochemically grown on crystalline silicon surface can be used as an efficient antireflective layer in optical devices as antireflection coating. In this work, we presented the ma-PS layers fabricated on crystalline silicon (c-Si) n-type and p ⁺-type, obtained by electrochemical etching. The morphology, porosity, thickness of ma-PS layer can be adjusted by controlling the electrochemical formation conditions. The optical behaviour of the antireflective coating over the solar spectrum is determined, resulting in very low values of the normalized reflectivity coefficient (below ~1%). The reflectivity measurements were evaluated at 45° in the different samples of the ma-PS/c-Si.     

Formation of hydrogen donors in proton-implanted epitaxial silicon

The formation of shallow hydrogen donors in epitaxial silicon implanted with 300-keV hydrogen ions has been studied at implant doses from 10¹³ to 6 × 10¹⁵ cm⁻², using commercial Mo-Si Schottky diodes with the active base region made of epitaxial phosphorus-doped silicon 5 μ m in thickness (ρ = 1.05 and 1.8 Ω cm). The results demonstrate that, at sufficiently high implant doses (F ∼ 10¹⁵ cm⁻²) and temperatures from 350 to 475 °C, there are at least two types of donors, one of which exhibits bistable behavior due to the negative correlation energy of a singly ionized doubly charged donor. At ∼475°C, the bistable H donor is fully annealed, whereas the concentration of stable donors remains unchanged at temperatures from 350 to 475°C. Under fixed post-implantation heat-treatment conditions (350°, 20 min), it is the implant dose that determines which type of H donor will form: at doses in the range 10¹³ to 10¹⁴ cm ⁻², H donors of the former type are formed, and the maximum in their profile coincides with the projected ion range; implant doses from 10¹⁴ to 10¹⁵ cm⁻² produce H donors of the latter type, and the peak in their profile is located closer to the irradiated surface, in the region of the highest radiation damage.