Pecvd Grown p-i-n Si and Si,Ge Thin Film Photodetectors For Integrated Oxygen Sensors
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PECVD GROWN p-i-n Si AND Si,Ge THIN FILM PHOTODETECTORS FOR INTEGRATED OXYGEN SENSORS Debju Ghosh1,2, Ruth Shinar2, Vikram L Dalal1,2, Zhaoqun Zhou3, and Joseph Shinar1,3 1 Electrical and Computer Engineering, Iowa State University, Ames, IA, 50011 2 Microelectronics Research Center, Iowa State University, Ames, IA, 50011 3 Ames Laboratory-USDOE and Department of Physics, Iowa State University, Ames, IA, 50011 ABSTRACT Recent efforts to advance photoluminescence (PL)-based oxygen sensors have focused on developing compact, field-deployable devices. This has led to organic light emitting device (OLED)-based sensors with a structurally integrated [OLED excitation source]/[sensing film] module. To additionally integrate a photodetector (PD), PECVD for fabrication of thin-film p-i-n and n-i-p Si- and Si,Ge-based PDs was employed. O2 concentrations are advantageously determined by monitoring the effect of O2 on shortening the PL decay time τ of an oxygensensitive dye, rather than on quenching its PL intensity. This approach, which employs pulsed OLEDs, eliminates the need for frequent sensor calibration, minimizes issues associated with background light, and eliminates the need for optical filters, which lead to bulkier sensors. However, it requires PDs with response times significantly shorter than τ. Therefore, the development of thin-film PDs focused on decreasing their response time, and understanding the factors affecting it. In this paper we show that boron diffusion during growth from the p+ to the i layer increases the response time of PECVD grown p-i-n PDs. Incorporating a SiC buffer layer and fabricating superstrate structures, where the p+ layer is grown last, decrease it. Additionally, ECR fabricated PDs show a slower response in comparison to VHF PECVD-grown PDs. INTRODUCTION Photoluminescence (PL)-based oxygen sensors are attractive due to attributes such as high sensitivity, specificity, fast response, long-term stability, and low-maintenance. Such sensors are based on quenching of the PL intensity I and shortening of the PL decay time τ of an oxygensensitive dye, typically embedded in a thin sol-gel or polymeric film, due to collisions with O2 [1-4]. Oxygen-sensors also serve as a basis for monitoring other analytes such as glucose, lactate, ethanol, and cholesterol, by monitoring oxygen consumption during the oxidation reactions of the above-mentioned analytes in the presence of an appropriate specific oxidase enzyme [5-7]. Monitoring O2 via its effect on τ rather than I is advantageous, as it eliminates the need for frequent sensor calibration, and is unaffected by minor changes in the background light, sensor film, or excitation source [1-4]. Generally, field-deployability of PL-based chemical and biological sensors is limited due to issues related to size, ease of fabrication, and consequently cost, and calibration/maintenance. Additionally, the sensors are often restricted to monitoring a single analyte. The organic light emitting device (OLED)-based sensing platform presents an opportun
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