D.P. Webb

Carrier transport in thin films of organic electroluminescent materials

Abstract Using the time of flight technique, the carrier mobilities of organic electroluminescent materials, N , N ′-bis-(1-naphhyl)- N , N ′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), tris(8-hydroxyquinoline) aluminum (Alq), and Tb(AcA) 3 phen (AcA: acetylacetone, phen: phenanthroline) (Tb-complex), were measured. At room temperature and an electric field of 2.3×10 5 V/cm, the effective mobility of holes in NPB, electrons in Alq and Tb-complex were (8.8±2)×10 −4 , (6.7±2)×10 −5 and (3±2)×10 −4 cm 2 V −1 S −1 , respectively. The hole mobility of NPB shows an exponential root field dependence as commonly seen in some organic materials.

Determination of gap-state distributions in amorphous semiconductors from transient photocurrents using a fourier transform technique

Abstract We present a novel spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay exhibiting either anomalous or conventional dispersion. A numerical Fourier Transform procedure is used to convert the impulse response time decay data to phase and a]plitude frequency spectra. The DOS is then computed using a procedure developed for analysis of modulated photocurrent (MPC) data in a multiple-trapping context. The method avoids many of the distortions and practical difficulties associated with other TPC analytical techniques. Computer generated TPC and MPC data are used to evaluate the transform technique in comparison with a ‘direct’ interpretive method for model systems of discrete and distributed states.

An experimental evaluation of transient and modulated photocurrent density-of-states spectroscopies

Abstract An evaluation of transient photocurrent (TPC) and modulated photocurrent (MPC) spectroscopies as a means of studying the density and capture properties of localized states in amorphous semiconductors is presented. Freauency-domain analysis of TPC data via the discrete Fourier transform (TPC FT) permits a direct comparison with MPC data obtained using conventional lock-in techniaues to be made. Results obtained from undoped hydrogenated amorphous silicon over a wide range of temperatures and optical excitations are used to explore the limits of resolution and applicability, and to highlight the relative merits, of each approach. It is shown that TPC spectroscopy offers significant practical advantages over MPC spectroscopy from the viewpoint of signal-to-noise performance. Discrepancies between TPC FT and MPC data obtained from the same sample under equivalent conditions suggest that the Fourier transform pairing of these methods is not exact, even when the reauirements of low excitation are met. Capture coefficients of defect states calculated from the temperature dependence of the TPC data and the optical excitation dependence of the MPC data disagree, having values of 7 × 10−9 and 5 × 10−7 cm3 s−1 respectively. Density-of-states profiles agree well for tail states (measured at low temperatures) but are less satisfactory at deeper energies.

Application of Weibull distribution analysis to the dielectric failure of multilayer ceramic capacitors

Abstract The Weibull distribution has been previously applied to the mechanical and dielectric failures of ceramics. In this paper, it is confirmed by experiment that this data treatment method is also valid for application in the dielectric failure of multilayer ceramic capacitors (MLCs) which have undergone screening. The Weibull modulus is found to be a useful parameter indicating the sharpness of the distribution of breakdown voltages and mechanical strength of MLCs. By Weibull analysis, the lately developed relaxor-based MLCs are found to compare favorably with traditional barium-titanate-based MLCs, implying that relaxor MLCs are more attractive than generally realised.

Modulated and transient photoconductivity in a-As2 Se3 *

Modulated photocurrent phase shift measurements (MPC) have been used to probe the valence band tail density of states (DOS) in a-As2 Se3. In the absence of obvious structure, use of optical bias to define a ‘thermalisation energy limit’ allows MPC to be used to determine the attempt-to-escape frequency. Computer modelling of steady state and transient photoconductivity reveals inconsistencies in a charged defect interpretation.

Time and frequency domain studies of photoconductivity in amorphous semiconductors

Abstract We present a general spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay (TPC). The technique assumes trap-limited and is otherwise model-independent. It is valid whether the TPC exhibits anomalous or conventional dispersion, and also works without modification for pre- and post-recombination regions of the decay. A numerical Fourier integral procedure is used to convert the TPC i ( t ) data to frequency domain spectra I ( ω ). The DOS is then computed using a procedure developed by the authors [1] for analysis of modulated photocurrent (MPC) data. The method avoids distortions and computational difficulties associated with other TPC analytical techniques. We report on the application of the method to experimental data on a-Si:H, demonstrating the wide energy range of states accessed, and highlighting the observation that the observed long-time power-law TPC decay, normally associated with a featureless exponential state distribution is consistent with structure in the DOS.

Packaging of Microfluidic Devices for Fluid Interconnection Using Thermoplastics

A new packaging method for microfluidic devices is proposed of polymer over-molding to form a fluidic manifold integrated with the device in a single step. The anticipated advantages of the proposed method of packaging are ease of assembly and low part count, making it suitable for low cost and high volume manufacturing. This paper reports the results of a preliminary investigation into this concept. Glass and silicon inserts of 25 times 20 mm in size, used to represent microfluidic devices, were over-molded in an injection molding process with a range of polymers. The inserts were found to survive the molding process intact. The adhesion between overmold and insert was investigated by subjecting the interface between the overmold and insert surface to a hydrostatic pressure of up to 100 lbf/in2 (6.9 bar). The durability of the interfacial adhesion to hydrolysis was investigated by immersion in water at 50degC for 24 h before testing. Direct measurements of adhesion strength between polymer and glass were also attempted by tensile tests on lap-jointed samples. The best and most durable adhesion for glass and silicon inserts was found for polyamide (PA) 12, which is a low hygroscopicity PA. The ranking of polymers by their performances in the pressurization tests was consistent with the ranking by the calculated work-of-adhesion values for polymer/glass and polymer/silicon joints.

A substrateless process for sustainable manufacture of electronic assemblies

The exponential growth in worldwide production and consumption of electronics, and the short operational lifespan of many products, has resulted in increasing amounts of electronics waste. There is enormous pressure on electronic product manufacturers to reduce the consumption of materials and their subsequent impact on the environment, especially at the end-of-life, through such measures as the EU Directive on Waste Electrical and Electronic Equipment (WEEE). Ideally any product should be separable into its constituent parts at end of life for subsequent reuse, recycling or disposal. However separation of a typical electronic assembly into its constituent parts is problematic because of the intimate nature of the bonding between the glass fibre/thermoset composite laminate, the laminated and embedded copper conductor layers and the soldered electronic components. To address these problems, an alternative processing route for manufacture of electronics assemblies is proposed, in which the electronic components and metal content can be easily separated out from the organic content at end-of-life. No separate printed circuit board is used to interconnect the components so the process may be termed as ldquosubstratelessrdquo. The route has the additional advantage that standard electronic assembly equipment can be used. In this work the process route is described and the implications of adoption for the electronics manufacturing industry considered. The results of initial proof of principle trials are described, and conclusions are drawn as to the development work required to allow adoption of the process by the industry.

Nondestructive methodology for standoff height measurement of flip chip on flex (FCOF) by SAM

Flip chip technology is the emerging interconnect technology for the next generation of high performance electronics. One of the important criteria for reliability is the width of the gap between the die and the substrate, i.e., the standoff height. A nondestructive technique using scanning acoustic microscopy (SAM) for the standoff height measurement of flip chip assemblies is demonstrated. The method, by means of the implementation of a pulse separation technique, time difference of the representative signals of the die bottom and water interface and water and substrate surface interface from the A-scan image can he found. Then, the corresponding standoff height can be calculated. When compared to the traditional destructive measurement method (SEM analysis on sectioned sample), this nondestructive technique yields reliable results.

Evidence for deviations from a single-exponential distribution of conduction band tail states in hydrogenated amorphous silicon: A transient photocurrent analysis

We employ transient photocurrent decay measurements to determine the distribution of conduction band tail states in hydrogenated amorphous silicon. It is found that these experimental results suggest the possibility of deviations from a single-exponential functional form. This is consistent with other more recent experimental determinations of the distribution of conduction band tail states in hydrogenated amorphous silicon.