Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρ_c) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρ_c of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10⁻⁵ Ω−cm ² is measured for the Ag metal contacts formed on the n⁺ silicon surface. This value is much lower than the ρ_c data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρ_c of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

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p-GaN表面制备低阻欧姆接触电极的几个关键问题

优良的光电特性使得GaN材料成为当今半导体器件研究领域的热点,但高功函数和低载流子浓度使p-GaN表面难以制备低阻欧姆接触电极、严重妨害了GaN基器件的热稳定性和输出功率.如何制备具有低阻欧姆接触特性的p-GaN电极已成为一个关键的科学和技术问题.探讨了影响p-GaN欧姆接触特性的几个关键因素,如表面预处理工艺、电极材料的选择和厚度、退火工艺等,对此方面的最新进展进行评述和归纳,并提出自己的创新性研究思路.     

Determination of the most reliable path of a planar flow network

For a planar stochastic flow network with binary states of elements, this paper presents an algorithm for determining the most reliable path (MRP) for transmitting a required amount of flow from a source to a terminal. We first establish a theorem stating that the MRP is one of the minimal upper paths (MUPs). This implies that the criteria, developed in a previous article (Lee & Yum, Reliab. Engng System Safety, 39 (1993) 1–10) to eliminate those subnetworks that cannot be a MUP, are still effective in determining the MRP. In the present article, we develop two additional elimination criteria based upon the inherent property of the MRP. An example is included to illustrate the proposed procedures, and computational results are reported.     

A mathematical optimization methodology for the optimal design of a planar robotic manipulator

A general optimization methodology for the optimal design of robotic manipulators is presented and illustrated by its application to a realistic and practical three‐link revolute‐joint planar manipulator. The end‐effector carries out a prescribed vertical motion for which, respectively, the average torque requirement from electrical driving motors, and the electric input energy to the driving motors are minimized with respect to positional and dimensional design variables. In addition to simple physical bounds placed on the variables, the maximum deliverable torques of the driving motors and the allowable joint angles between successive links represent further constraints on the system. The optimization is carried out via a penalty function formulation of the constrained problem to which a proven robust unconstrained optimization method is applied. The problem of singularities (also known as degeneracy or lock‐up), which may occur for certain choices of design variables, is successfully dealt with by means of a specially proposed procedure in which a high artificial objective function value is computed for such ‘lock‐up trajectories’. Designs are obtained that are feasible and practical with reductions in the objective functions in comparison to that of arbitrarily chosen infeasible initial designs. Copyright © 1999 John Wiley & Sons, Ltd.     

New type of charged-particles detector based on point contacts formed in a GaAs-AlGaAs heterostructure by split-gates

We propose a new principle for a low temperature semiconductor detector of charged particles with possibly submicron lateral resolution based on point contacts formed in the 2-dimensional electron gas (2DEG) of a GaAs-Al_xGa_1-xAs heterostructure using split-gates. The detector operates up to liquid nitrogen temperatures. Impinging particles excite locally extra donors in the doping layer of the heterostructure. By measuring simultaneous increase in the conductances of 3 point contacts due to the impacts of charged particles the position of penetration is calculated using the Thomas-Fermi approximation for screening in 2DEG.     

A reliable in vitro approach to assess the stability of acetabular implants using digital image correlation

The main cause of failure of the hip acetabular component is aseptic loosening. Preclinical test methods currently used to assess the stability of hip acetabular implants rely on crude simplifications. Normally, either one component of motion or bone strains are measured. We developed a test method to measure implant 3D translations and rotations and bone strains using digital image correlation. Hemipelvises were aligned and potted to allow consistent testing. A force was applied in the direction of the load peak during level walking. The force was applied in 100‐cycle packages, each load package being 20% larger than the previous one. A digital image correlation system allowed measuring the cup‐bone relative 3D displacements (permanent migrations and inducible micromotions) and the strain distribution in the periacetabular bone. To assess the test repeatability, the protocol was applied to six composite hemipelvises implanted with very stable cups. To assess the suitability of the method to detect mobilisation, six loose implants were tested. The method was repeatable: the interspecimen variability was 16 μm for the bone/cup relative translations, 0.04° for the rotations. The method was capable of tracking extremely loose implants (translations up to 4.5 mm; rotations up to 30°). The strain distribution in the bone was measured, showing the areas of highest strain. We have shown that it is possible to measure the 3D relative translations and rotations of an acetabular cup inside the pelvis and simultaneously to measure the full‐field strain distribution in the bone surface. This will allow better preclinical testing of the stability of acetabular implants.     

Selective MBE growth of nonalloyed ohmic contacts to 2D electron gas in high-electron-mobility transistors based on GaN/AlGaN heterojunctions

Specific features of how nonalloyed ohmic contacts to the 2D conducting channel of high-electron-mobility transistors based on AlGaN/(AlN)/GaN heterostructures are fabricated via deposition of heavily doped n ⁺-GaN through a SiO₂ mask by ammonia molecular-beam epitaxy have been studied. The technique developed makes it possible to obtain specific resistances of contacts to the 2D gas as low as 0.11 Ω mm on various types of Ga-face nitride heterostructures, which are several times lower than the resistance of conventional alloyed ohmic contacts.     

The effects of Ge content on the microstructure and specific contact resistance of solid-state NiAuGe/ZrB2/Au ohmic contacts to n-InGaAs

The Ge thickness, x, of NiAuGe(5 nm/45 nm/xnm)/ZrB₂(50 nm)/Au(20 nm) ohmic contacts to n-lnGaAs was varied between 0 and 20 nm. The microstructures of these contacts, after annealing at 270°C, were investigated using transmission electron microscopy (TEM) and correlated with the respective specific contact resistances. In the absence of Ge, a Ni-Ga-As phase was formed at the metal-semiconductor interface and the specific contact resistance was high (0.63 mm). When thicknesses of x = 10 nm or x = 15 nm of Ge were added, Ni-Ge-As phases were observed, but they were replaced by AuGeAs and NiGe when x = 20 nm. The specific contact resistance was a minimum (0.11 mm) for this composition. Ge was clearly beneficial for ohmic-contact formation. The low-temperature I–V characteristics of the contact containing the largest amount of Ge (that is, x = 20 nm) indicated that electron tunnelling through degenerately-Ge-doped regions was not the dominant ohmic mechanism in these contacts.     

Influence of thermal treatment on the formation of ohmic contacts based on Ti/Al/Ni/Au metallization to n-type AlGaN/GaN heterostructures

Interfacial reactions between Ti/Al/Ni/Au metallization and GaN(cap)/AlGaN/GaN heterostructures at various annealing temperatures ranging from 715 to 865 °C were studied. Electrical current-voltage (I?CV) characteristics, van der Pauw Hall mobility measurements and surface topography measurement with atomic force microscopy (AFM) were performed. The ohmic metallizations were annealed at various temperatures in a rapid thermal annealing system and the annealing time of 60 seconds was kept for all samples. To study the influence of the parameters of annealing process on the properties of the 2 dimensional electron gas (2DEG) the van der Pauw Hall mobility measurement was used. Interfacial reactions between the contact metals and heterostructures were analyzed through depth profiles of secondary ion mass spectroscopy. It was observed that transition from nonlinear to linear I-V behavior occurred after the annealing at 805 °C. For the studied samples, the most promising results were obtained for the annealing temperature of 805 °C. This temperatue ensured not only low contact resistance but also made possible to preserve the 2DEG.     

An engineering methodology to assess effects of weld strength mismatch on cleavage fracture toughness using the Weibull stress approach

This work describes the development of an engineering approach based upon a toughness scaling methodology incorporating the effects of weld strength mismatch on crack-tip driving forces. The approach adopts a nondimensional Weibull stress, [`(s)]_w{\bar{{\sigma}}_w}, as a the near-tip driving force to correlate cleavage fracture across cracked weld configurations with different mismatch conditions even though the loading parameter (measured by J) may vary widely due to mismatch and constraint variations. Application of the procedure to predict the failure strain for an overmatch girth weld made of an API X80 pipeline steel demonstrates the effectiveness of the micromechanics approach. Overall, the results lend strong support to use a Weibull stress based procedure in defect assessments of structural welds.     

A numerical methodology to assess the quality of the design velocity field computation methods in shape sensitivity analysis

The velocity field in shape sensitivity analysis is not uniquely defined although it must meet numerous theoretical and practical criteria. These practical criteria can be used to compare the existing velocity field computation methods which meet the theoretical criteria, but only in qualitative terms. When the FEM is used in design sensitivity analysis (DSA), due to the FE discretization error, the DSA errors will depend on the design velocity field considered. This paper presents a numerical methodology for quality evaluation of design velocity field computation methods in quantitative terms based on the analysis of the DSA discretization error. The sensitivity of the squared energy norm (χ_m = ?∥u∥2/?a_m, a_m being a design variable) has been taken as the magnitude to measure the error of the DSA. For h‐refinements, the squared error in energy norm (∥ e ( u ) ∥²) and the error in χ_m(e(χ_m)) are theoretically related by a constant which is independent of the refinement degree of the FE model. The quality of the design velocity field computation methods can therefore be assessed in terms of the stability of e(χ_m) /∥e(u) ∥2 in sequences of meshes. An example of the use of this methodology, where six design velocity field computation methods are compared, is presented. Copyright © 2004 John Wiley & Sons, Ltd.     

Thickness dependence of the MoO(3) blocking layers on ZnO nanorod-inverted organic photovoltaic devices

Organic solar cells based on vertically aligned zinc oxide nanorod arrays (ZNR) in an inverted structure of indium tin oxide (ITO)∕ZNR∕poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester(P3HT:PCBM)∕MoO(3)∕aluminum(Al) were studied. We found that the optimum MoO(3) layer thickness condition of 20 nm, the MoO(3) can effectively decrease the probability of bimolecular recombination either at the Al interface or within the active layer itself. For this optimum condition we get a power conversion efficiency of 2.15%, a short-circuit current density of 9.02 mA∕cm(2), an open-circuit voltage of 0.55V, and a fill factor of 0.44 under 100 mW∕cm(2) irradiation. Our investigations also show that the highly crystallized ZNR can create short and continuous pathways for electron transport and increase the contact area between the ZNR and the organic materials.     

Technique for calculation of the propagation constant in an optical planar waveguide with a gaussian profile

We performed analysis of a planar waveguide with arbitrary index variations. We obtained numerical results for the propagation coefficient by using first-order Langer and Liouville transformations. The accuracy of the numerical results is confirmed by a comparison with those obtained by other methods.     

The effect of crystallinity in donor groups on the performance of photovoltaic devices based on an oligothiophene-fullerene dyad

Organic photovoltaic devices based on an oligothiophene-fullerene dyad mixed with either an oligothiophene as donor or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor were fabricated to improve charge carrier transport in the films. The origin of the device performance dependence on the mixing ratios was studied by atomic force microscopy and UV-vis absorption spectra and differential scanning calorimetry measurements. We found that the addition of the oligothiophene molecules improved the crystallinity of the donor groups in the film, resulting in a 20% increase in fill factor, while the addition of PCBM slightly lowered the crystallinity, thereby reducing the fill factor.     

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The effect of ZnO film depositions using various film deposition methods such as magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) on the photovoltaic properties of ZnO–Cu₂O heterojunction solar cells is described in this report. In addition, the relationship between the resulting photovoltaic properties and the film deposition conditions such as supply power and substrate arrangement was investigated in Al-doped ZnO (AZO)–Cu₂O heterojunction devices fabricated using AZO thin films prepared by d.c. magnetron sputtering (d.c.MSP) or r.f. magnetron sputtering (r.f.MSP). The results showed that the measured photovoltaic properties of devices fabricated with films deposited on substrates oriented perpendicular to the target were better than those of devices fabricated with films deposited on substrates oriented parallel to the target. It was also found that ZnO film depositions under conditions where a relatively weaker oxidizing atmosphere was used yield better properties than films derived from MSP, which utilizes a high-density and high-energy plasma. Using VAPE and PLD, for example, high efficiencies of 1.52 and 1.42%, respectively, were obtained under AM2 solar illumination in devices fabricated at a substrate temperature around 200 °C.     

Effect of crystalline quality of diamond film to the propagation loss of surface acoustic wave devices

Diamond films with various crystal qualities were grown by chemical vapor deposition on silicon wafers. Their crystallinity was characterized by Raman scattering and electron backscattering diffraction. By fabricating a device structure for surface acoustic wave (SAW) using these diamond films, the propagation loss was measured at 1.8 GHz and compared with the crystallinity. It was found that the propagation loss was lowered in relatively degraded films having small crystallites, a narrow distribution in the diamond crystallite size, and preferential grain orientation. This experiment clarifies diamond film characteristics required for high-frequency applications in SAW filters.     

An approach to calculation of the viscosity of dissociated gas mixtures formed from oxygen,nitrogen, and carbon

The results of the solution of an optimization problem on coordination of the widely used data on the viscosity of individual substances and air as a whole are presented. The proposed method of correction of literature data on the viscosity of the components of air mixtures is used to estimate the possible alteration of similar data for a number of carbon-containing compounds. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 97–101, September–October, 2007.     

Hazard maps and global probability as a way to transfer standard fracture results to reliable design of real components

Design and lifetime prediction of structural and mechanical components require the assessment of the global probability of failure to be determined from stress and strain distributions obtained by FEM, as well as calculation of hazard maps in order to facilitate redesign and recognition of critical parts to be inspected regularly. The so-called generalized probabilistic local approach (GPLA), developed by the authors, allows the primary failure cumulative distribution function (PFCDF) owning to a certain failure type to be determined for a given material from experimental data and used subsequently for probabilistic design. The approach ensures a realistic safety margin provided that the failure criterion represented by an adequate generalized parameter (GP) and the corresponding failure criterion is properly recognized as a reference variable to be considered in the failure assessment. The way in which the results of such a reliability analysis are interpreted encompasses a variety of concepts under which failure can be understood and may be classified as global probability of failure and hazard maps, the former providing the conclusive failure probability for definitive design, and the latter representing, presumably, a risk of local failure that facilitates the possible component redesign but without providing the global probability of failure. In order to promote the implementation of the methodology proposed, an application is exemplary presented for the particular case of experimental results of glass plates. A finite element subroutine is developed for calculation of hazard maps and global probabilities of failure.     

Investigation of the effect of rare earth doping on the electrical and photovoltaic properties of chromotrope 2R thin film devices

Praseodymium (Pr) doped and Samarium (Sm) doped chromotrope 2R (CHR) were used for the fabrication of Schottky devices, by the spin coating technique. The diode in which doped CHR behaves as an n-type organic semiconductor exhibits rectification behaviour in the dark. Doping with rare earths imparts an accelerated improvement in the n-type conductivity as well as in the rectification effect. The observed rectification effects are explained by n-type semiconductivity of the doped CHR thin films. The formation of a blocking contact (Schottky barrier) indium tin oxide with (ITO) electrode and an ohmic contact with Al or ln, also confirms its n-type behaviour. The position of the Fermi level shifts toward the conduction band edge on rare earth doping. Additionally, the concentration of free carriers and mobility of electrons also increase upon doping, with the simultaneous decrease in trap concentration. Various electrical parameters such as barrier height (b), density of ionized donor (Ns) and depletion layer width (W) were calculated from the detail capacitance–voltage analysis of (C–V) characteristics. The photo-action spectra of the devices and absorption spectra of doped CHR layer reveal the formation of a Schottky barrier at the ITO-doped CHR interface and an Ohmic contact at the Al-doped CHR interface. Photovoltaic measurements of these devices provide parameters such as short circuit photocurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (). The effect of rare earth doping on the electrical and photovoltaic parameters are discussed in detail. © 1998 Chapman & Hall     

Peculiarities of the photovoltaic properties of films based on photoconducting polymer and organic dye in samples with free surfaces and between electric contacts

Composite films based on a carbazolyl-containing oligomer with polymethine dye additives exhibit a change in the magnitude and sign of the photovoltaic response on the passage from the samples with free surfaces to sandwich structures. It is concluded that the photovoltaic effect in the former case is determined by the diffusion of positive charge carriers possessing higher mobility, while in the latter case this effect is controlled by a significant drift of carriers in the electric field created by different work functions of the charge-collecting electrodes. It is shown that the photovoltaic effect also takes place in a nonphotoconducting polymer (polyvinyl ethylal). However, neither the sign nor magnitude of the effect in this case change on the passage from the samples with free surfaces to sandwich structures.