Optimization of phase-sensitive transparent detector for length measurements

A phase selective partly transparent detector (PSTD) enables length measurement with nm-accuracy by sampling an optical standing wave. The PSTD consists of two transparent n-i-p photodiodes of amorphous silicon (a-Si:H) which are embedded between three transparent conductive oxide (TCO) layers. The two photodiodes measure the intensity of an optical standing wave by means of absorption layers with thicknesses below 50 nm and thus, provide two photocurrents which are proportional to the intensity at their individual positions. For an optimization of the device performance, simulations based on a standard electromagnetic formalism were performed. The considered thin-film structure is a glass/TCO/n-i-p/TCO/n-i-p/TCO layer sequence. The aim was to design a layer stack which avoids significant distortions of the standing wave while the phase shift between the photocurrents approximately amounts to 90 degrees, since this will minimize the measurement error. The comparison of experimentally determined and,simulated data shows that a further adjustment of the fabricated PSTD into an ideal thickness scheme is necessary to enhance the device performance.