(poster) Efficient surface passivation of germanium by porous Ge layer

Germanium (Ge) is considered a promising semiconductor material for optoelectronic applications in near-infrared (NIR) range. Since most of the applications are based on minority carriers, the amount of recombination active defects both in the bulk of the material and at the surfaces should be minimised. While the Ge bulk can be made of high quality, its surface passivation is more complicated. Indeed, plenty of efforts have been done to minimize surface recombination activity both on planar and textured Ge surfaces. [1-3] This research started with a surprising observation: our nanotextured Ge surface prepared by metal-assisted chemical etching (MACE) without any intentional passivation layer exhibited passivation comparable to state-of-the-art atomic layer deposited (ALD) Al2O3 [2] (Fig.1a). This result triggered us to study whether such MACE solution could be used for surface passivation. Investigation of the influence of the MACE solution components on Ge showed that HF:H2O2 mixture diluted with water clearly formed a film on the surface, which, based on the minority-carrier lifetime measurements, provided excellent passivation (Fig 1a). To study the composition and structure of this film, we carried out X-ray photoelectron
spectroscopy (XPS) and scanning electron microscopy (SEM). In case of original MACE solution, XPS revealed the presence of GeO2 (Fig. 1b) known to provide low interface defect density but simultaneously being unstable and water-soluble [1, 3]. Meanwhile, XPS for the modified solution surprisingly indicated the presence of GeOx which has tendency to form defective GeOx/Ge interface [5, 6]. Based on SEM the film possessed a sponge-like structure, which was possibly composed of porous germanium (PGe) [4] and GeOx. Finally, the structure was shown to retain stable passivation performance for at least several days (Fig. 2). Thus, the modified MACE solution could be considered as a potential method for Ge surface passivation, although the exact mechanisms require further studies.