Analysis of thin copper Schottky barrier photodetector operable in the near-infrared region

Abstract

Silicon (Si) has been widely used for electronic and photonic applications including light detection. However, Si has a distinct limitation for near-infrared (NIR) photodetection because of its inherent bandgap of 1.12 eV. Achieving NIR photodetection using the CMOS-compatible Si material might be advantageous in terms of cost and scalability. Schottky barrier photodetectors (SBPDs) are emerging as a strong candidate for Si-based NIR photodetection because the detectable wavelength range is not limited to the intrinsic bandgap of Si, but is governed by a Schottky barrier height between a metal and the semiconductor. There are three important conditions that should be satisfied simultaneously to achieve a high external quantum efficiency (EQE): absorption in the metal layer should be high, the Schottky barrier should be low enough, and there should be a long mean free path (MFP) in the layer of metal. In this work, we choose copper (Cu) as the metal layer because Cu satisfies the above conditions. It is known that the Schottky barrier (Cu/n-type Si) is around 0.5 eV and the MFP is ~50 nm. We fabricate Cu SBPDs with various Cu thicknesses (8, 12, 20, and 40 nm) and other SBPDs (20 nm) composed of various metals (Ni, Ag, Au, Pt) to compare with Cu devices. We show the EQEs of the Cu SBPDs for the NIR wavelengths from 1440 nm to 1600 nm and we find that Cu devices have a larger EQE than other metal devices. As we expected, the thinnest Cu SBPD (8 nm) shows the largest EQE with a maximum EQE of 3.38×10-3 at 1510 nm, which is almost 5 times higher than that of the 40 nm device. Furthermore, at a reverse bias of -3 V, the thinnest Cu SBPD achieves an EQE of more than 10-2 at a wavelength of 1440 nm. By employing a thin Cu layer, our patternless planar SBPDs show EQEs higher than, or comparable to, previously reported nanopatterned plasmonic SBPDs.