AlGaN-based MSM UV-photodetectors with high external quantum efficiency at 0 V
Fig. 1: Design of an a-MSM photodetector, a) cross-sectional sketch of epitaxial layer stack with identical electrodes on the absorber layer surface (MSM) under top or bottom illumination (arrows), b) schematic plan-view image of the a-MSM detector geometry (upper image). Light-microscopic enlargement (bottom image) shows the interdigitated electrode combs with Pt- and Ni-based metallization.
Fig. 2: Spectral EQE of a-MSM photodetector at 0 V under top and bottom illumination. Inset: linear dependence of photocurrent with incident optical power at 240 nm.
The AlxGa1-xN material system is attractive for the detection of ultra-violet (UV) radiation. Owing to the dependence of the large band gap energy on Al mole fraction x (3.4 eV to 6.2 eV) the detectors’ cut-off wavelength is adjustable between 365 nm and 200 nm. Moreover, the material is robust against UV radiation. The simple detector design (see Fig. 1) requires litte effort in epitaxy and processing of such devices. However, when all electrodes form Schottky contacts, photocurrent flows only under biased conditions. Furthermore, the electrodes partly block the radiation incident on the device, reducing the detection efficiency (external quantum efficiency - EQE). Asymmetric metal-semiconductor-metal (a-MSM) photodetectors with different metallization schemes have been developed at the FBH in order to enable zero-bias operation. This was achieved by combining one electrode comb forming a Schottky contact and with the second comb being ohmic. The epitaxial layer stack below the actual detection layer is transparent and illumination from the bottom avoids the shading losses from the contacts.
An only 100 nm thin Al0.5Ga0.5N absorber layer is deposited by metal organic vapor phase epitaxy onto an AlN buffer layer on a sapphire substrate (Fig. 1a). Within an area of 400 µm x 400 µm, on the absorber 2 µm wide electrodes have been processed in a co-planar comb geometry with a distance of 2 µm (Fig. 1b). One of the electrode combs is based on Ni and has been annealed at 800°C. The subsequently deposited Pt-based electrode comb forms the Schottky contact. Because of this asymmetry, photocurrent can flow at zero bias. Upon bottom illumination at 0 V the a-MSM photodetector shows an almost constant EQE of 24% below 250 nm until the AlN buffer layer absorption cuts-off near 210 nm (Fig. 2). The photocurrent linearly depends on the incident optical power (inset). In contrast, the EQE under top illumination is reduced by a factor of two in this spectral region due to the electrode shading.
As a result of the new detector design, linear UV photodetectors with an EQE of 24% at 0 V have been realized on the basis of AlGaN.
Publication
M. Brendel, M. Helbling, A. Knigge, F. Brunner, M. Weyers, "Solar-blind AlGaN MSM photodetectors with 24 % external quantum efficiency at 0 V", Electron. Lett., DOI: 10.1049/el.2015.2364