GaN-based optoelectronic devices for liquid properties detection

Abstract

Measurement of liquid properties holds significant research importance across fields such as environmental protection, the food industry, manufacturing processes, and so on. The investigation of liquid properties detecting is therefore deemed a top priority. The most commonly used liquid sensing methods, electrical and optical methods, offer their own advantages. However, electrical methods cannot guarantee measurement accuracy in electromagnetic environments, while optical methods are limited by the large size external light source. In order to develop compact liquid sensor, in this thesis, monolithically integrated GaN-based optoelectronic devices are adopted. These GaN devices integrate both light-emitting diode (LED) and photodetector (PD) on the same sapphire substrate, which simultaneously enables both light emission and light detection, enabling precise and portable optoelectronic sensing for liquid properties detection. First, the monolithically integrated GaN device is fabricated for droplet flow monitoring. When the droplet slides across the device, the reflectance at the sapphire boundary decreases due to reduced refractive index contrast, thereby decreasing the quantity of light reflected onto the PD. The integrated device provides a fast transient response with rise and fall times of 3.88 μs and 3.53 μs, achieving instantaneous droplet flow sensing. Then, a monolithically integrated optoelectronic device, equipped with an indicator film, is fabricated for enhanced pH detection. The pH sensor showcases a swift reaction time and an extensive linear range, achieving real-time pH detection with a minimal 1 μL sample and a response time of merely 3.8 s. The wide linear range of 5-13 pH of the sensor demonstrates its promise for practical applications. Finally, the GaN optoelectronic device is designed for organic liquid identification facilitated with photonic crystals. Meanwhile, the reflectance spectrum shift of photonic crystals in interaction with organic molecules is explored. The proposed optoelectronic device could identify 7 kinds of organic solvents in less than 30 s with reproducibility, which offers a viable solution for rapid response and decision-making in emergency situations such as chemical spills, replacing complicated laboratory analysis.