Photo-assisted electrochemical etching of high-Al-content AlGaN sacrificial layers for UVC applications

Selective etching of AlxGa1−xN with x > 0.5, e.g., for the substrate lift-off of UVC emitters, is extremely challenging due to the very high bonding strength of the compound. Even electrochemical etching (ECE), which has proven very effective in AlxGa1−xN with lower x, leads to porosification rather than complete etching. We show here that by adding illumination during etching, i.e., photo-assisted electrochemical etching (P-ECE), complete etching can be achieved for x up to 65%. We studied the P-ECE of a range of sacrificial layers consisting of a highly Si-doped ([Si] = 3 × 1019 cm−3) multilayer AlGaN sacrificial layer of five periods of 5 nm AlxGa1−xN/5 nm AlyGa1−yN, with Al content (x/y) between (38%/48%) and (65%/75%) to lift-off Al0.76Ga0.24N membranes. The etching behavior was studied by varying the applied bias voltage, illumination intensity, and excitation wavelength of the photoexcitation source. The N-polar surface of a membrane grown on those multilayer sacrificial layers had a root-mean-square surface roughness between 5.9 and 7.8 nm, with a 5 × 5 μm2 scan size. By using a single 10 nm-thin sacrificial layer instead, we could obtain even smoother etched surfaces with a roughness of below 3 nm, comparable to that of the as-grown surface. We, thus, show that P-ECE is a promising technique for lifting off AlGaN membranes with high Al content and obtaining accurate control of the membrane thickness.