Bandwidth considerations in modulated and transient photoconductivity measurements to determine localized state distributions

This work examines the influence of limited instrumental bandwidth on the accuracy of recovery of the density of localized states in semiconductors from transient and modulated photoconductivity data. Paradoxically, knowledge of the short-time transient photoresponse can be vital in the estimation, via a Fourier transform, of the density of deep-lying states. We demonstrate that retention of the natural response of a bandwidth limited system, although subject to distortion at short times, can lead to much improved accuracy in density of states determination than simple truncation of the short-time response. It is shown that this improvement arises simply from the integrating effect of a bandwidth limited system over short time intervals, which makes it possible to access and exploit information originating at times much shorter than the instrumentation rise time. These concepts are exemplified using computer simulated transient photoconductivity for several model systems including one which mimics the expected density of states in amorphous silicon

The use of areal surface texture parameters to characterize the mechanical bond strength of copper on glass plating applications

This report describe research into the role that surface topography plays in influencing the mechanical bond strength of the electroless copper plating of novel glass substrates. The work considers bespoke laser machining of glass substrates, electroless plating chemistry, areal surface topography analysis using non-contact optical techniques, paramaterization of the surfaces using ISO 25178 areal parameters, and scratch testing of plated copper to measure the adhesive bond strength. By correlating bond strength to appropriate areal parameters, it is anticipated that better mechanical adhesive potential of machined glass surfaces can be achieved

Effects of bandwidth limitations on the localized state distribution calculated from transient photoconductivity data

The possible effects of experimental bandwidth limitation on the accuracy of the energy distribution of the density of localized states (DOS) calculated from transient photoconductivity data by the Fourier transform method is examined. An argument concerning the size of missing contributions to the numerical Fourier integrals is developed. It is shown that the degree of distortion is not necessarily large even for relatively small experimental bandwidths. The density of states calculated from transient photodecay measurements in amorphous arsenic triselenide is validated by comparing with modulated photocurrent data. It is pointed out that DOS distributions calculated from transient photoconductivity data at a high photoexcitation density are valid under certain conditions. This argument is used to probe the conduction band tail in undoped a-Si:H to energies shallower than 0.1 eV below the mobility edge. It is concluded that there is a deviation in the DOS from exponential at about 0.15 eV below the mobility edge

Study of self-alignment of μBGA packages

In this paper, a detailed study of the self-alignment of BGA packages is presented. Complete self-alignment can be achieved even for a misalignment of the package of larger than 50% off the test board pad centres. A small residual displacement of the package from perfect alignment after reflow is observed. The reason for this displacement is the action of gas flow viscous drag on the package during reflow. The use of eutectic SnPb solder paste slightly reduces self-aligning ability, due to the increase in the solder volume, which reduces the restoring force. Exposure of the solder paste to a 25 C and 85% RH humidity environment also has a detrimental effect on the self-alignment of the BGA package, due to solvent evaporation and moisture absorption in the paste causing solderability degradation. The self-alignment of the package is also affected when there is slow spreading of molten solder on the pad surface. This is attributed to the reduction of restoring force due to the decrease in effective wetting surface area of the board pad