Two-Step Liquid Phase Crystallized Germanium-Based Photodetector for Near-Infrared Applications

This work presents a wafer-scale two-step liquid phase crystallization process to obtain polycrystalline but epitaxial germanium on 2”-silicon wafers (Ge-on-Si). The crystallinity is confirmed using Raman spectroscopy and X-ray diffraction, with an extracted dislocation density of 108cm
^-2
, an improvement of 10x over the previous report. The scanning electron imaging shows a uniform Ge film with a grain size of up to 12μm. Metal-semiconductor-metal (MSM) near-infrared (IR) photodiodes are fabricated on the epitaxial Ge with two different electrodes and two different surface passivation interlayers. Irrespectiveof the electrode/passivationcombination, all devices exhibit Ohmic characteristics. Contact resistivity varies from 3 mΩcm
²
to 560 mΩ cm
²
. This is contrary to virtually every previous report on MSM detector on n-type Ge. To probe further, kelvin probe force microscopy is used to characterize the surface potentials at grain boundaries and metal-Ge interfaces. The grain-to-grain band bending is very low, ~40 meV, hence, device characteristics are completely dominated by the metalGe band-bending which is 150-300 meV. The MSM devices using amorphous-Ge(30nm)/Al (100nm) and TiO2 (5nm)/Au (80nm) electrodes show an average spectral responsivity (SR) of 0.50 ± 0.16 A/W and 0.35 ± 0.09 A/W at 1550 nm and voltage bias of -3V, respectively. The maximum SR is 0.78 A/W and 0.48 A/W, respectively.