Despite the fact that water is an essential resource, measuring its levels accurately and reliably can be a real challenge, especially in fields like agriculture, oceanography, and engineering. Over the years, techniques like UV light sensing have been developed which is a non-intrusive approach and can provide quick measurements. However, these sensors are often affected by interference from ambient light, which may the measurements drastically. While silicon-based photodetectors have been widely used for detecting visible and near-infrared light, their operation is limited to temperatures below ~200°C, making them unsuitable for many industrial applications.

On the other hand, the use of Gallium Nitride (GaN) UV photodetectors can aid in the creation of quick, continuous, and simultaneous water level readings. Furthermore, GaN can selectively absorb UV wavelengths which allows more accurate sensing of liquid levels. When compared to typically visible light approaches, the GaN UV photodetectors detect incident UV photons refracted at the water-air interface, providing for more steady and accurate measurements.

Ⅰ. Simulation Methodology and Parameters Used

The main aim of the experiment was to develop a liquid-level sensor using a GaN UV photodetector that is encapsulated with a thin layer of polymer (PDMS) to protect it from an electrical short circuit due to water. In order to test the prototype in a real-world scenario, the experiment consisted of a GaN UV photodetector placed below the water level in a cylinder, and 365 nm UV light was illuminated at the center of the cylinder. A schematic of the liquid-level sensor is shown in Figure 1. As the water level increases, more UV rays are refracted and concentrated on the sensing area, increasing the photocurrent excited by UV photons in the GaN film.

Fig 1 Overall water level measurement level in the container.

Figure 2 shows the computational simulation results obtained using the COMSOL Multiphysics software, indicating the operation principle of the liquid-level sensor. The refractive index of the thin PDMS layer for a 365-nm wavelength was set as 1.45 at room temperature. The change in the optical intensity allowed rapid liquid level measurement even though the distance between the UV source and the sensor was fixed.

Fig 2a: Simulation effects for refractive indices for air, water, and PDMS.

Fig 2b: How cylinder dimension affects UV concentration on GaN surface.

Since the refractive index mainly depends on temperature, the device must be designed based on the refractive index of each media at different temperatures. To calculate the width of the circular illuminating area (D) at the bottom of the cylinder, the following formula was derived and used:

Equation 1

From the above equation, H, h, and l represent the distance from the bottom to the UV source, water height, and thickness of PDMS, respectively. As per Figure 2b, D is maximized when the cylinder is empty, depicting the effect of cylinder dimension on sensing.

Ⅱ. Evaluating the Results Obtained

The liquid-level sensor consists of a GaN film of approximately 500 nm thickness on a Si(111) substrate with a resistivity of <5 Ω∙cm. The GaN film was electrically isolated from the Si substrate with buffer layers, and Al wires were bonded to the GaN surface to create multiple metal electrodes. The sensor was operating based on the principle of refraction of UV light through water, where the position of refraction changes with the water's height.

Upon exposure to 365 nm UV illumination, excited electrons generated an ohmic-like photocurrent, increasing the total current level of the sensor. The transient current response recorded was fast, and the sensor exhibits a high signal-to-noise ratio with different water heights and UV emission angles. The PDCR and SNRdB, however, may decrease in the presence of impurities in water, reducing the sensor's sensitivity. The absorbance and reflectance spectra of the GaN sensor show a sharp cutoff wavelength near 370 nm, indicating that the liquid-level sensor only absorbs UV wavelengths, with a high UV-visible rejection ratio.

Fig 3 Photocurrent w.r.t water, where cylinder diameter was 6cm with 1V at an angle of 120°.

Figure 3 depicts the photocurrent response of the liquid-level sensor in relation to the water height when a UV source with an emission angle of 120° was used. The results indicate that as the water level increased, more UV light was directed towards the sensing area, leading to a proportional increase in the photocurrent. But, the response time received was slower and the PDCR was lower compared to cases where an emission angle of 45° was employed. The steady region observed between 3 and 6 cm of water height was attributed to the interference of lights focused on the sensing area by those reflected by the inner wall of the cylinder. Also, the photocurrent did not increase linearly with respect to the water level, and a dead region was observed when the cylinder's diameter decreased to 4 cm.

On the other hand, when an emission angle of 45° was used, the photocurrent response exhibited a linear increase as the water level rose above 6 cm. The sensor utilizing the 120° UV source demonstrated a maximum increase in photocurrent of approximately 34.4%, while it was approximately 25.2% for the sensor employing the 45° UV source. This difference was due to the higher concentration of UV light in the sensing area as the emission angle increased.

Ⅲ. Conclusion

The experiment aimed to develop liquid-level sensors utilizing a GaN UV photodetector. The use of UV light and a corresponding photodetector enhanced the sensitivity of the sensor, enabling reliable and continuous monitoring of liquid levels in various environments, including high-temperature power plants, nuclear reactors, geothermal plants, and chemically corrosive tanks.

The sensors' fabrication and characterization involved varying several parameters, such as cylinder diameter, UV source emission angle, etc. The experimental results confirmed the feasibility of utilizing wide-bandgap semiconductors as a platform for liquid-level sensors, showcasing their potential for continuous and reliable monitoring in harsh environments.