A class of ratio, product and difference (R.P.D.) estimators in systematic sampling

A class of estimators is proposed for estimating the population parameter (mean) of the study character y, using information on an auxiliary character x, in systematic sampling of which the estimator is proposed by Swain [J. Indian Statist. Ass. 2 (213), 160–164 (1964)], Sukla [Proceedings of the All-India Seminar on Demography and Statistics, pp. 243–248. Varanasi, India (1971)] and difference estimator in systematic sampling are particular cases. The conditions in which this class of estimators performs better than other estimators are derived.     

Comparison of some ratio-type estimators

Some ratio-type estimators are compared, and it is found that the estimator proposed by Sisodia and Dwivedi (Biomet. J.23, (2) 133–139) performs well compared with other estimators, for all situations.     

Bayesian analysis of reliability and hazard rate function of a mixture model

The present article deals with the Bayesian analysis for a modified (general) Mukherji-Islam model [A finite range distribution of failure times, Naval Research Logistics Quarterly, 1983, 30, 487–491] on the basis of failure time data x₁, x₂, …, x_r for a prefixed number of failures. Under the use of different prior densities for the parameter and the squared error loss function (SELF), the Bayesian estimates of hazard rate and the reliability functions are obtained, considering uniform, Gamma and inverted gamma densities as prior failure densities.     

Asymptotic expansion in the limit theorem for the convergence to laplace distribution

Let {ξ_n}₁^∞ be a sequence of non-negative identically distributed random variables with zero mean and finite variance σ², {ν = ν(ε)} a sequence for random variable independent of {ζ_ν}, taking non-negative integer values and having a geometric distribution and with ζ_ν = Σ₁^νζ_ν. Gnedenko and Fraier. [Some notes on a work of Kowa lenka U. N. Letovsk Math. Sb. 1, 181–187 (1969)] proved a convergence theorem for ζ_ν and some other authors proved other convergence theorems. This paper is concerned with some theorems about the asymptotic expansions and their consequences on the extremal properties of the Bernoulli random variable.     

calculations of p-type δ-doped quantum wells in Si

We present the hole subband structure calculation in single and double p-type δ-doped quantum wells in Si based on the 4×4 Luttinger–Kohn Hamiltonian. The valence band bending and the Γ hole states are calculated within the lines of the Thomas–Fermi–Dirac approximation and the effective mass theory at the Brillouin zone center. The obtained zone center eigenstates are then used to diagonalize the k·p Hamiltonian for non-zero k. The hole subband structure is analyzed as a function of the impurity density and the distance between δ wells. It is shown that the application of a 4×4 model to describe the hole ground state in single p-type δ-doped in Si can be misleading.     

Fixed-width confidence interval estimation of the inverse coefficient of variation in a normal population

A sequential procedure is developed in order to construct a confidence interval of “fixed-width and preassigned coverage probability” for the inverse of the coefficient of variation of a normal population. The proposed sequential procedure is proved to be “asymptotically efficient and consistent” in the sense of Chow and Robbins ([1]: Ann. Math. Statist. 36, 457–462 (1965)). Asymptotic distribution of the stopping time is derived.     

A new shrinkage estimator for the exponential scale parameter

This paper proposes a new shrinkage estimator for the scale parameter of the exponential distribution and compares it with the minimum mean squared error estimator and Pandey's estimator [Pandey, IEEE Trans. Reliab. R-32,203–205 (1983)].     

Ruthenium complex-cored dendrimers: Shedding light on efficiency trade-offs in dye-sensitised solar cells

A series of first generation dendrimers provide important insight into the performance of dye-sensitised solar cells (DSSCs). The dendrimers are comprised of a substituted [cis-di(thiocyanato)-bis(2,2′-bipyridyl)ruthenium(II) complex, first generation biphenyl-based dendrons, and either four, eight, or twelve 2-ethylhexyloxy surface groups. The dendrimers were bound to the titanium dioxide of the DSSCs via carboxylate groups on one of the bipyridyl moieties in a similar manner to the ‘gold standard’ [cis-di(thiocyanato)-bis(4,4′-dicarboxylate-2,2′-bipyridyl)]ruthenium(II) 1 (N3). Exchanging one pair of the carboxylate groups on one bipyridyl ligand of N3 with styryl units to give [cis-di(thiocyanato)-(4,4′-dicarboxylate-2,2′-bipyridyl)-(4,4′-distyryl-2,2′-bipyridyl]ruthenium(II) 2 resulted in an improvement in device performance (7.19% ± 0.11% for 2 versus 6.94% ± 0.12% for N3). Devices containing the dendrimers also had good efficiencies but the performance was found to decrease with the increasing number of surface groups, which gives rise to an increase in the molecular volume of the dye. The device containing the dendrimer with four surface groups, 3, had a global efficiency of 6.32% ± 0.13%, which was comparable to N3 (6.94% ± 0.12%) in the same device configuration. In contrast, the dendrimer with twelve surface groups, 5, had an efficiency of 3.69% ± 0.19%. Complex 2 and all three dendrimers have the same core chromophore, which absorbs more light than N3. The decrease in efficiency with increasing molecular volume was therefore determined to be due to less dye being adsorbed. Hence molecular volume and molar extinction coefficient are both first order parameters in achieving high conversion efficiencies and must be taken into account when designing new dyes for DSSCs.     

A Bayesian estimation of reliability model using the linex loss function

In this paper the problem of Bayes estimation of the reliability and the shape parameter p of a finite range failure time model is considered (assuming scale parameter θ is known). Following Zellner [A. Zellner, J. Am. Statist. Assoc. 81, 446–451 (1986)] the asymmetric loss function is used to obtain the Bayes estimators. Efficiencies of the proposed Bayes estimators are obtained with respect to the ordinary Bayes estimators and it was found that the proposed Bayes estimators are better than the ordinary Bayes estimators for quite a wide range of parameters.     

Low band gap vinylene compounds with triphenylamine and benzothiadiazole segments for use in photovoltaic cells

Two new soluble vinylene compounds, TPA-TNP and BTD-TNP, which contained triphenylamine and benzothiadiazole segments, respectively, and terminal p-nitrophenyl units were synthesized and characterized. They showed high thermal stability with decomposition temperature above 450 °C. Their absorption was broad and extended up to about 800 nm with optical band gaps of 1.65–1.67 eV. The long wavelength absorption maximum was located around in between 638 and 650 nm. The presence of the electron-withdrawing nitro groups on the terminal phenyls broadened the absorption of these compounds. The current–voltage characteristics in dark and the IPCE spectra of the devices based on these compounds revealed that these materials behave as p-type organic semiconductors with hole mobility of the order of 10⁻⁵ cm²/V s. The photovoltaic properties of these compounds blended with PCBM were investigated and the power conversion efficiency is 1.13% and 1.32% for TPA-TNP and BTD-TNP, respectively. The device with thermally annealed BTD-TNP photoactive film shows power conversion efficiency of about 2.42%.     

Reliability analysis of a powerloom plant with cold standby for its strategic unit

A powerloom plant consists of two units A and B(n) connected in series. Unit B(n) has n components in parallel redundancy that carry threads of different varieties/colours to unit A. Unit A is the strategic unit that weaves these threads together and produces a long piece of cloth of some standard size. Failure of unit A makes the entire system fail whereas the failure of k components out of n in unit B(n) results in the system working in a degraded state. The present paper deals with the reliability analysis of the powerloom plant with a cold standby for its strategic unit. The common cause failure and critical human errors have also been considered to avoid overestimation of the reliability of the system. Results of Dhillon [Int. J. cyst. Sci. 23, 1277–1287 (1992)] have been deduced as a particular case of our result.     

Efficient iridium(III) based, true-blue emitting phosphorescent OLEDS employing both double emission and double buffer layers

New device architectures and efficient iridium based phosphors were simultaneously developed for fabrication of true-blue phosphorescent organic light-emitting devices (OLEDs). To fully explore the potential of these true-blue-emitting phosphors, we employed a device architecture that incorporates both double-emitting layers (one with hole-transport and the second with electron-transport materials) and double buffer layers for efficient exciton confinement. In addition, the parent, true-blue emitting heteroleptic Ir^III complex Ir1 was synthesized by incorporating one 4,6-difluorophenyl-2-pyridyl cyclometalate (dfppy) together with two 3-(trifluoromethyl)-5-pyridyl pyrazolates (fppz), while others derivatives Ir2–Ir4 were prepared by addition of alkyl substituent at the pyridyl sites. Electrophosphorescence with efficiencies up to 13.7% photon/electron and 20.4 cd/A, and with adequate CIE_x,y color coordinates of (0.157, 0.189) were successfully achieved.     

A new algorithm for reliability evaluation of three-state device complex networks: Minimal cut-minimal path method

In this paper, we have modified W. J. Ruger's method [W. J. Rueger, Microelectron Reliab. 27, 273–277 (1987)], given a new algorithm to compute the two-terminal reliability of three-state device complex networks. It is a fast, simple method by hand and easily used by computers.     

A case study of statistical process control on the prime process using PLASMASK 302U

In this paper, Statistical Process Control (SPC) and statistically designed experiments will be used to optimise a recently developed resist schemes i.e., PRIME (Positive Resist Image by dry Etching). Orthogonal experiments are designed and conducted in order to produce 0.2 μm lines repeatably in PLASMASK 302U resist. Design rules of 0.1 μm are used due to proximity effects. The data is then explored and analysed using surface plots, boxplots, analysis of variance (ANOVA) and linear regression. It was deduced that e-beam dose and 2nd step etch time were the most significant parameters in the process. Linewidth of 0.2 gm and below were achieved with low values of e-beam dose (250–350μC/cm²) and high values of silylation temperature (205–215°C). The optimum range for NUV flood dose was easily found.     

Effects of the electrical stress on the conduction characteristics of metal gate/MgO/InP stacks

The degradation dynamics and post-breakdown current–voltage (I–V) characteristics of magnesium oxide (MgO) layers grown on n and p-type indium phosphide (InP) substrates subjected to electrical stress were investigated. We show that the current–time (I–t) characteristics during degradation can be described by a power-law model I(t) = I₀t^−α, where I₀ and α are constants. It is reported that the leakage current associated with the soft breakdown (SBD) failure mode follows the typical voltage dependence I = aV^b, where a and b are constants, for both injection polarities but in a wider voltage range compared with the SiO₂/Si system. It is also shown that the hard breakdown (HBD) current is remarkably high, involving large ON–OFF fluctuations that resemble the phenomenon of resistive switching previously observed in a wide variety of metal oxides.     

Reliability of a two-way circular consecutively connected system with multistate components

A two-way circular consecutively connected system with multistate components (two-way circular CCSMC) consists of n cyclically ordered components e₁,…,e_n, i.e. e_i+1, succeeds e₁, iε«ng l, …, n − 1ång;, e, succeeds e_n- Each component is capable of sending a signal in left-hand and right-hand directions. All components operate independently. The whole system is functioning if both left-hand and right-hand signals are received by each component. A recursive procedure evaluating the reliability of a two-way circular CCSMC is presented.     

New challenges to the modelling and electrical characterisation of ohmic contacts for ULSI devices

Continuing developments in semiconductor process and materials technology have enabled significant reductions to be achieved in the contact resistance R_c of devices. This reduction is commonly assessed in terms of the specific contact resistance (SCR) parameter ρ_c (Ω cm²) of the metal–semiconductor interface. Such a reduction in SCR is essential, for as device dimensions decrease, then so also must ρ_c and the corresponding contact resistance in order not to compromise the down-scaled ULSI device performance. Thus the ability to accurately model contacts and measure ρ_c is essential to ohmic contact development. The cross kelvin resistor (CKR) test structure is commonly used to experimentally measure the Kelvin resistance of an ohmic contact and obtain the specific contact resistance ρ_c. The error correction curves generated from computer modelling of the CKR test structure are used to compensate for the semiconductor parasitic resistance, thus giving the SCR value. In this paper the increased difficulty in measuring lower ρ_c values, due to trends in technology, is discussed. The challenges presented by the presence of two interfaces in silicided contacts (metal-silicide–silicon) is also discussed. Experimental values of the SCR of an aluminium–titanium silicide interface is determined using multiple CKR test structures.     

Probing stress effects in HfO2 gate stacks with time dependent measurements

Effects of constant voltage stress (CVS) on gate stacks consisting of an ALD HfO₂ dielectric with various interfacial layers were studied with time dependent sensing measurements: DC I–V, pulse I–V, and charge pumping (CP) at different frequencies. The process of injected electron trapping/de-trapping on pre-existing defects in the bulk of the high-κ film was found to constitute the major contribution to the time dependence of the threshold voltage (V_t) shift during stress. The trap generation observed with the low frequency CP measurements is suggested to occur within the interfacial oxide layer or the interfacial layer/high-κ interface, with only a minor effect on V_t.     

Sequential interval estimation procedures for the mean exponential survival time and reliability function

The problems of confidence interval estimation are considered (a) for estimating the mean of a one-parameter exponential distribution and (b) for estimating the reliability function associated with the one-parameter exponential distribution. For the estimation problem (a), the confidence interval of ‘preassigned width and coverage probability’ is considered. For the estimation problem (b), the confidence interval of ‘fixed-ratio width and preassigned coverage probability’ is proposed. The failure of the fixed sample size procedures to deal with these estimation problems is established and sequential procedures are proposed to deal with them. The proposed sequential procedures are proved to be ‘asymptotically efficient and consistent’ in the Chow-Robbins [Chow and Robbins, Ann. Math. Statist. 36,457–462 (1965)] sense. Asymptotic distributions of the stopping times are derived and second-order approximations are obtained for the average sample numbers associated with them.     

Further remarks on sequential estimation of a scale parameter in exponential distributions with an unknown location

The sequential procedure developed by Govindarajulu and Sarkar [Sequential estimation of scale parameter in exponential distributions with unknown location. Utilitas Math.40, 161–178 (1991)] for estimating the scale parameter of an exponential distribution, when the location parameter is unknown, is further analyzed. Generalizing the results of Govindarajulu and Sarkar, the ‘asymptotic risk-efficiency’ of the sequential procedure is established for the general loss function. A simple method of obtaining the asymptotic distribution of the stopping time is given. For the case of quadratic loss function and linear cost of sampling, a much simpler proof for obtaining the second-order approximations for the risk is provided.