Effects of MgxZn1-xO Thickness on the Bandwidth of Metal-Semiconductor-Metal Bandpass Photodetectors

A metal-semiconductor-metal photodetector (PD) whose detection wavelength can be modulated was fabricated using a MgxZn1-xO/ZnO bilayer. By varying the MgxZn1-xO layer thickness (50-400 nm), we modulated the detection wavelength from lowpass to bandpass. For a thin (50 nm) MgxZn1-xO film, most short-wavelength (less than 340 nm) incident photons-more than 69%-passed through the MgxZn1-xO and were absorbed by ZnO, causing a higher photoresponse in the short-wavelength region and thus a low-pass PD. As the thickness of the MgxZn1-xO film increased, the short-wavelength photoresponse was gradually suppressed, and a bandpass PD was achieved. A thicker (400 nm) MgxZn1-xO film absorbed more short-wavelength (less than 340 nm) incident photons, leaving only 5.5% of the incident photons to be absorbed by the ZnO layer underneath. This reduced the short-wavelength photoresponse, yielding a bandpass PD with a detection wavelength of 325-360 nm. Although the short-wavelength (less than 340 nm) incident photons were absorbed by MgxZn1-xO, few photogenerated electron-hole pairs contributed to the photoresponse; instead, they were trapped by the defects in MgxZn1-xO because of its poor crystalline quality.