Photoelectrochemical Pyramid Texturization of n-Type Ge (100) Probed By Integrated Electrochemical Chamber/X-Ray Photoelectron Spectroscopy
Authors: Abrenica Graniel Harne Abelido , Lebedev Mikhail V. , Le Hy , Hajduk Andreas , Fingerle Mathias , Mayer Thomas , De Gendt Stefan , van Dorp Dennis H. 
Published: February 2020

Recently, Germanium (Ge) has gained a massive attention from different research fields like energy1-2, photonics3-4, biomedicine5, and most importantly, nanoelectronics6. Historically, Ge was the very first semiconductor utilized as a key material for transistors by John Bardeen, William B. Shockley and Walter H. Brattain but was abandoned because of its low purity, narrow band gap and chemically unstable oxide.7 From being left behind, after silicon (Si) found its way to fame, Ge is now enjoying resurgence in the world of CMOS devices.8 Due to the extreme size scaling of Si-based complementary metal-oxide semiconductor (CMOS) devices, performance enhancement can no longer be achieved due to intrinsic mobility issues, which can be compensated by the higher carrier mobility of Ge.9 In this context, Ge is a promising candidate to replace Si. Hence, central understanding in Ge surface and interface chemistry is currently indispensable for the development of novel nanodevice applications. In this work, we report the very first formation of Ge pyramids through electrochemical etching and its surface chemistry in HCl solutions under applied potential conditions.

As a starting point, voltammetric (j-V) measurements were performed for a large concentration range of the acid (Figure 1a). In all cases in the plateau region (at 0.8V), a linear increase in photocurrent density with increasing light intensity was measured (Figure 1a inset). These results are attributed to a high oxide solubility and, consequently, low oxide coverage (supported by XPS results). No characteristic oxide passivation is observed during anodic etching of n-type Ge, even in the case of a very dilute 0.05 M HCl solution and current densities as high as ~6.0 mA cm-2. Morphological studies (AFM and SEM) revealed a striking and unexpected anisotropy in the surface chemistry of etching for high HCl concentrations, evidenced by the formation of random pyramids with characteristic (111) facets (Figure 1b). For Si, patterning can be used to further lower reflectance and hence improve light coupling effects. In order to scrutinize if similar effects can be observed for Ge, we fabricated a honeycomb template with circular openings of 1 mm to expose the Ge via photolithography. After photoanodic etching of the patterned Ge sample at 0.8 V in 8 M HCl solution, highly ordered pyramidal structures were obtained which lowered the reflectance further by 6% as presented in Figure 1c. This shows that light coupling effects are very likely vital. Integrated electrochemical etching chamber connected to X-ray photoelectron spectroscopy (XPS) instrument as displayed in Figure 1d, which excludes the effect of O2 and H2O vapor from the atmosphere, was utilized. Core-level Ge3d spectra suggest that the process of pyramid texturization is induced by the presence of Ge-Cl bonds on the Ge (100) surface during photoanodic dissolution (Figure 1e). A strong decrease in Ge-OH upon increasing the HCl concentration from 1 M to 8 M is accompanied by an increase in chemically bonded Cl, supported by the chemical shift of 0.6 eV. As the Cl content is similar based in Cl 2p (data not shown), it is very likely that for lower HCl concentration, with less Ge-Cl, physisorption of the halide ion is more important. While, at high HCl concentration, chemisorption governs.

Based on these results, we propose surface electrochemical reaction schemes that relate the observed anisotropy in etching to surface chemistry. We believe that these findings are of fundamental interest and of technological importance for new developments in the fields of energy, photonics, biomedicine, and nanoelectronics.

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Figure 1