This invention details a method and system for fabricating an Ultra-Violet (UV) photo detector using a process called Chemical Vapor Deposition (CVD). It involves growing a Zinc Oxide (ZnO) layer on a Gallium Nitride (p-GaN) substrate. The junction formed between these two materials acts as an effective UV photodetector, offering improved performance over traditional methods.
Figure 1. A system for fabricating an Ultra-Violet (UV) photodetector
Commercially available photodetectors are silicon-based, with an optical band gap of 1.1 eV. They require costly optical filters to block visible light and function effectively in the UV spectrum. Moreover, they have significant leakage current, poor efficiency, and reduced sensitivity to detect faint optical signals.
Traditional UV photodetectors rely on external power sources, which often pose risks to public health and environment. Moreover, in a smart sensing system, the management of a vast network of UV photodetectors supported by batteries becomes challenging, necessitating self powered photodetectors.
A wide band gap (3.37 eV), strong chemical stability and other factors make ZnO a potential candidate for UV photodetectors. However, stable p-type doping is not yet possible in ZnO. Hence, this patent describes a novel method using ZnO and GaN heterojunctions to make self-powered UV photodetectors.
- Improved structural quality: The use of a Chemical Vapor Deposition (CVD) process for growing epitaxial ZnO layers on a p-GaN template results in high-quality crystalline films with minimal dislocations.
- Responsivity and low leakage current: The UV photodetector demonstrates a responsivity of 0.4 mA/W to UV light, particularly at 366 nm, with a significant enhancement in photocurrent. Additionally, the device shows very low leakage currents under dark conditions, indicating efficient photodetection with minimal background noise.
- Rapid response time: The invention boasts quick response times, with rise and decay times of 2.5 milliseconds and 3.5 milliseconds respectively, unlike conventional UV photodetectors which can range from 5 to 20 milliseconds.
- Enhanced on-off ratio: The on-off ratio of the photocurrent to dark current is significantly high, reaching a maximum of 1.6 x 104 at a forward bias of 0.25V, compared to existing technologies with a range of 102 to 103.
The prototype detector of 6 mm size has shown peak responsivity of 0.4mA/W at 366 nm and response time of 2.5 ms. It can detect signals down to 9 nW.
An early prototype has been developed, and testing has been done. The current size of the detector is around 6 mm, and it has a response time in the millisecond range. Further reducing the size would help get even better response times.
Apart from CVD, the PLD technique has also been used to fabricate UV photodetectors, and it has shown even better responsivity of 11 mA/W and response time of 1 ms in self powered mode.
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This UV photodetector is sustainable, self powered and maintenance free. It can detect faint signals quickly, making it useful for military applications like missile detection at high speeds. It can be used for space exploration to detect signals coming from celestial bodies. It also has multiple biomedical applications for diagnostics and treatment. Air/water purification, flame sensing, forest fire detection, submarine leakage monitoring are some other applications.
UV photodetectors have many applications in various areas such as engine control, solar UV monitoring, astronomy – space exploration to detect signals coming from celestial bodies, secure space-to-space communications; military – detection of missiles at high speeds; submarine leakage monitoring; lithography aligners; environmental monitoring; biomedical applications for diagnostics and treatment; optoelectronics; air/water purification; flame sensing; forest fire detection; and agriculture.
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