Electronic characterisation and computer modelling of thin film materials and devices for optoelectronic applications

lock-in techniques. A comparison was made of the two-beam photogating experiment, with a single beam current-voltage measurement, which is also influenced by trapped space charge, as indicators of defect distributions. It was found that the photogating measurement is a more accurate indicator of the distribution of space charge, and hence defects, within a device. Application of the photogating effect in a colour detector is introduced and detector structure proposed. The simple structure and the thin film technique of a-Si:H deposition suggests the possibility of a low cost photodetector with high colour resolution. Double beam collection efficiency measurements have been carried out on hydrogenated amorphous silicon p-i-n devices. Apparent collection efficiencies higher than unity were observed, and explained by a process identified as photogating, in which a low intensity weakly absorbed probe beam modulates the photocurrent produced by a high intensity strongly absorbed bias beam. Computer simulations were used to gain insight into the photogating phenomenon. It was found that the gating effect operates by the modulation of the internal field profile in a device, via deeply trapped space charge introduced by the probe beam. Conditions for high collection efficiencies were identified by modelling and by experiment. Collection efficiencies of 100 or greater could be achieved, much higher than any previously reported in the literature. The effects of external parameters including bias and probe beam wavelength and intensity, and applied voltage were studied. Additionally, the effects of internal parameters, such as the density and spatial distribution of defects, were investigated. The photogating phenomenon proved a sensitive and potentially useful indicator of defect density. The time response of the photogating effect revealed slow components to the response, associated with the need to involve deeply trapped space charge in the effect. Measurements of this time response explain in part the much lower values of collection efficiency reported earlier, which were made using a

What MEMS Research and Development Can Learn from a Production Environment

The intricate interdependency of device design and fabrication process complicates the development of microelectromechanical systems (MEMS). Commercial pressure has motivated industry to implement various tools and methods to overcome challenges and facilitate volume production. By now, these are only hesitantly being picked up and implemented in academic research. In this perspective, the applicability of these methods to research-focused MEMS development is investigated. It is found that even in the dynamics of a research endeavor, it is beneficial to adapt and apply tools and methods deduced from volume production. The key step is to change the perspective from fabricating devices to developing, maintaining and advancing the fabrication process. Tools and methods are introduced and discussed, using the development of magnetoelectric MEMS sensors within a collaborative research project as an illustrative example. This perspective provides both guidance to newcomers as well as inspiration to the well-versed experts