UV-C AlGaN photodetectors with cut-off wavelength below 220 nm

High-power UV lamps, UV LEDs, and excimer lasers are widely used for UV lithography, medical applications, and disinfection. For most applications it is necessary to monitor the emitted power in a certain wavelength range. We have extended our studies on UV photodetectors to devices that are only sensitive in the UV-C and vacuum UV (< 200 nm) region and have cut-off wavelengths below 220 nm.

As reported earlier, our metal semiconductor metal photodetectors (MSM-PDs) with Al_~0.5Ga_~0.5N absorbers are sensitive only below 280 nm and have very high responsivity due to the use of AlN/sapphire templates with reduced defect density. This defect reduction is achieved by epitaxial lateral overgrowth (ELO). It leads to an increase in responsivity by up to two orders of magnitude compared to photodetectors on standard templates due to internal gain. A strong anisotropy was found depending on the finger orientation on the template.

This approach has been extended to MSM-PDs with Al_0.9Ga_0.1N absorber layer and AlN absorber layer. The spectral characteristics show a sharp cut-off with a decrease by more than two orders of magnitude within 10 nm at 20 nm for AlN and 215 nm for Al_0.9Ga_0.1N (Fig. 1). Devices with fingers perpendicular to the trenches in the template (ELO-O) show up to 50 % higher responsivity than devices with fingers parallel to the trenches (ELO-P) and devices on standard templates (planar). The absolute difference between the template types is much lower than for PDs with Al_~0.5Ga_~0.5N absorbers, and no gain is observed.

Cathodoluminescence investigations show that the facetted growth in Al_0.9Ga_0.1N is much less pronounced than in Al_~0.5Ga_~0.5N. So the band discontinuities between different areas are too small to cause internal gain. The responsivity anisotropy of the photodetectors on ELO templates can be correlated to the defect density distribution in the absorber layer: The plan view panchromatic CL image (Fig. 2) shows stripe-like areas with reduced defect density parallel to the template trenches. In these channel-like areas recombination at defects is reduced and thus the carrier transport is more efficient. Therefore, the responsivity of the ELO-O devices is higher than that of ELO-P PDs with the carrier flow perpendicular to the channels crossing the areas of high defect density. The responsivity of the planar PDs is similar to that of ELO-P PDs or even lower due to the higher density of defects which are homogeneously distributed in this case.

In conclusion, the responsivity of UV MSM photodetectors with cut-off wavelengths below 220 nm can be increased by using defect-reduced templates with stripe pattern.

The AlGaN photodetector activities were funded by the BMBF within the Berlin WideBaSe initiative (03WKBT02C).

Publications:

A. Knigge, M. Brendel, F. Brunner, S. Einfeldt, A. Knauer, V. Kueller, U. Zeimer, M. Weyers, "Solar-blind Al_xGa_1-xN MSM Photodetectors on Patterned AlN/Sapphire Templates with 0.4 < x < 1", accepted for publication in Phys. Status Solidi C 2013.

A. Knigge, M. Brendel, F. Brunner, S. Einfeldt, A. Knauer, V. Kueller, U. Zeimer, M. Weyers, "AlGaN Metal–Semiconductor–Metal Photodetectors on Planar and Epitaxial Laterally Overgrown AlN/Sapphire Templates for the Ultraviolet C Spectral Region", Jpn. J. Appl. Phys., vol. 52, no. 08JF03 (2013).

V. Kueller, A. Knauer, U. Zeimer, M. Kneissl, M. Weyers, "Controlled coalescence of MOVPE grown AlN during lateral overgrowth", J. Cryst. Growth, vol. 368, pp. 83-86 (2013).

FBH research: 06.01.2014