P-I-N amorphous silicon for thin-film light-addressable potentiometric sensors

Amorphous silicon (a-Si) has been approved for use as the semiconductor layer in a light-addressable potentiometric sensor (LAPS). A new device structure of niobium oxide (NbOx)/P-I-N amorphous silicon (a-Si) on ITO/glass is proposed for a thin film LAPS with the advantage of a high photovoltage. P-type, intrinsic and N-type a-Si layers are fabricated by plasma enhanced chemical vapor deposition (PECVD) with selective gas dopants on ITO/glass. A single NbOx layer is directly sputtered onto an a-Si substrate and is used as the pH sensing membrane. The temperatures for all of the processes used to manufacture this device are lower than 300 °C, including the rapid thermal anneal (RTA) treatment, which is compatible with conventional fabrication processes for glass substrates in solar cells and thin film transistors. Based on the photovoltage versus gate bias characteristics in standard buffer solutions, the calculated pH sensitivity and linearity from pH 2 to 10 is 40 mV/pH and 99.4% for the P-I-N a-Si LAPS with RTA treatment at 300 °C. Compared with an intrinsic a-Si structure, a 50% higher photovoltage is achieved in the P-I-N a-Si LAPS. Furthermore, XRD and SIMS are used to verify the properties of the a-Si film. A high frequency ac laser signal of up to 20 kHz was obtained with the P-I-N a-Si LAPS while maintaining the same spatial resolution, which is important for two-dimensional images acquired by high-speed scanning with an analog micromirror.