A large variation in the direct band gap of wurtzite AlGaN alloys is very suitable for light detecting devices which are sensitive in the 220 - 365 nm spectral range and have an ability to detect a signal in a narrow spectral band with photon energies close to a high background radiation.
In this work we present studies on AlGaN photoconductors and high performance AlGaN photodetectors based on a combination of three epitaxial AlGaN layers with different alloy composition acting as optical filter, isolator, and detector layer, respectively. A peak responsivity for this device has been achieved at 280 nm with a range of sensitivity between 240 and 300 nm. An AlN layer has been used as an isolating barrier between the AlGaN optical filter and the AlGaN photodetector top layer.
Samples were grown by plasma assisted molecular beam epitaxy on c-plane sapphire substrates. For the accurate measurement of the sub-band gap absorption, spectral photocurrent measurement (SPC) and photothermal deflection spectroscopy (PDS) have been applied to characterise the defect structure of the epilayers. The advantage of CPM and PDS, in comparison to other optical techniques (particularly transmission) is their high sensitivity down to ad ~ 10-7, where a is the absorption coefficient and d the thickness of the film. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities in the films are provided as a measure of composition and structural quality of epitaxial layers. A difference of more than one order of magnitude in the SPC above the band gap has been measured in back- and top-illuminated UV-photodetector heterostructures demonstrating the functionality of the integrated filter (cut-off energy ~ 4.85 eV). The UV/visible contrast of more than four orders of magnitude and in addition a difference in the PDS and SPS absorption spectra of more than one order of magnitude has been observed.

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