UV photodetectors based on AlGaN/GaN heterostructures for optical switching
Fig. 1: Schematic diagram of the Al0.25Ga0.75N/GaN-based UV photodetector (a) top view, (b) side view.
Fig. 2: I-V characteristics in a bias voltage range up to 100 V under illumination with different optical power densities between 4 μW/cm² and 100 μW/cm² at a wavelength of 355 nm, inset: corresponding dark current.
Fig. 3: Spectral response of the UV photodetector measured with an incident optical power in the range of 100 μW/cm² and at bias voltages between 5 V and 100 V.
Due to their blindness in the visible region (solar blind) and the radiation resistance, GaN-based UV photodetectors are highly demanded for many application. They are suitable, for example, to control high-power UV lamps, UV LEDs and excimer lasers in a simple, exact and robust way. These are used in UV lithography, for disinfection (sterilization of air and water) and medical applications like treatment of psoriasis.
Depending on the aluminum mole fraction in the AlGaN absorber layers, the cut-off wavelength of the device can be tuned between 200 nm and 365 nm to fit the requirements of the specific application. Alternatively, UV photodetectors can be manufactured directly from AlGaN-GaN heterostructures of transistors. Thus, electronics circuits can be optically switched. Only recently, scientists from the FBH, in cooperation with the TU Berlin, developed a UV photodetector based on an AlGaN/GaN heterostructure for transistors. The detector has an MSM structure with two Ohmic contacts around the photosensitive area (Fig. 1). The two-dimensional electron gas at the interface AlGaN/GaN provides the built-in supply line. Due to the internal gain, this detector is interesting for applications with high photocurrent.
The photodetector presented here reached photocurrents in the mA region at voltages of 100 V (Fig. 2). With dark currents below 100 nA the on/off ratio is higher than 104. Fig. 3 shows responsivity spectra of this detector whose maximum responsivity is as high as 70 A/mW. The cut-off-wavelength is 365 nm, and the contrast between responsivity in the UV and the visible region is 104.
Publication:
M. Martens, J. Schlegel, P. Vogt, F. Brunner, R. Lossy, J. Würfl, M. Weyers, M. Kneissl, "High gain UV photodetectors based on AlGaN/GaN heterostructures for optical switching" (accepted for publication in Applied Physics Letters)
Patent application: DE 10 2011 075 103.3.
FBH research: 20.05.2011