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 Mg
_x
Zn
_1-x
O/ZnO bilayer. By varying the Mg
_x
Zn
_1-x
O layer thickness (50-400 nm), we modulated the detection wavelength from lowpass to bandpass. For a thin (50 nm) Mg
_x
Zn
_1-x
O film, most short-wavelength (less than 340 nm) incident photons-more than 69%-passed through the Mg
_x
Zn
_1-x
O 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 Mg
_x
Zn
_1-x
O film increased, the short-wavelength photoresponse was gradually suppressed, and a bandpass PD was achieved. A thicker (400 nm) Mg
_x
Zn
_1-x
O 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 Mg
_x
Zn
_1-x
O, few photogenerated electron-hole pairs contributed to the photoresponse; instead, they were trapped by the defects in Mg
_x
Zn
_1-x
O because of its poor crystalline quality.