Development of Ultraviolet Wavelength-specific Photodetectors

Detecting ultraviolet light in the presence of a visible/infrared background is both technologically challenging as well as critical for a number of applications, including high temperature flame sensing, environmental monitoring, and biomedical diagnostics. Furthermore we use UV light in our day-to-day life and applications range from water purification and polymer curing to fluorescence imaging for security, UV treatment of cancerous cells etc. Each of these applications requires dedicated detectors that are sensitive exclusively to a particular radiation. P-I-N detectors based on AlGaN alloys can be made wavelength-specific by incorporating appropriate n-AlGaN filter layers for absorption of higher energy radiation, while the absorption edge of the intrinsic layer determines the low energy cut-off. This method has been extensively used in solar-blind photodetection. Limitation arises from the necessity of p-type doping of AlGaN – a challenging problem - and from the band-tails present which does not allow the sensitivity range to be arbitrarily narrow.
In this talk we discuss the development of wavelength specific photodetectors based on an alternate scheme using AlGaN multiple quantum wells. Growth of these structures was carried out using the Molecular Beam Epitaxy (MBE) process. MSM photodetectors were fabricated on these quantum wells, and lateral photocurrent was measured.
It was observed that while MSM Photodetectors based on MBE-grown Al0.40Ga0.60N films showed the expected absorption edge at ~290nm, the devices based on  Al0.35Ga0.65N/Al0.55Ga0.45N MQWs showed one a single near-Gaussian room temperature peak at 300nm with a width of 17nm. No other significant photoresponse was obtained in the 200-700nm range. Furthermore, these devices based on MQWs were significantly faster in switching than those based on bulk AlGaN films.
Studies carried out at room and low temperatures indicate that the transition is due to formation and splitting of excitons. However, the phenomenon is strongly dependent on the nature and extent of compositional inhomogeneities present in the material.  The MBE growth process was modified to enable the device responses.
This type of wavelength-selective photodetectors does not require p-type doping or use of a transparent substrate. Further studies are necessary to increase their efficiency and reproducibility.