Pixel Resistance Optimization of a Si0.5Ge0.5/Si MQWs Thermistor Based on In-Situ B Doping for Microbolometer Applications

The state-of-the-art microbolometers are mainly based on polycrystalline or amorphous materials, typically Vanadium oxide (VOx) or amorphous-Silicon (a-Si), which only have modest temperature sensitivities and noise characteristics. The properties of single crystalline SiGe/Si multi quantum wells (MQWs) have been proposed as a promising material1. Particularly, SiGe/Si MQWs structure with high Ge concentration is expected to provide very high temperature coefficient of resistance (TCR) values between 6 to 8% 2. Although SiGe/Si MQWs structure as a thermistor material is extremely promising, difficulty of defect free deposition and high sheet resistance of high Ge concentrated SiGe layers are the two main bottlenecks of this approach. In this work, a very high TCR of -5.5 %/K is achieved for SiGe/Si MQWs including 50% Ge with an acceptable noise value of 2.7 x 10-13 V2/Hz at 10 Hz. The initial pixel resistance of 3 period of SiGe/Si MQWs with 50% Ge concentration is measured as 21 MΩ, which might not be compatible with the ROIC design. By the optimization of insitu Boron (B) doping level in SiGe layers of the MQW stack, 210 kΩ for 25 x 25 μm2 pixel size is achieved. The optimized B doping density of ~1 x 1018 cm-3 in SiGe wells did not cause any significant change in the TCR value whereas the 1/f noise performance is even enhanced due to the in-situ doping process and measured as 2.9 x 10-14 V2/Hz at 10 Hz.