Characterization and (Electro)Chemical Studies of Halide-Passivated Ge (100) Surfaces in Acidic Solutions
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

The goal towards miniaturization of silicon (Si) - based complementary metal-oxide semiconductor (CMOS) devices has challenged the semiconductor industry in improving the device performance while sustaining the scale requirement. [1-2] In this perspective, Ge appears to be a suitable candidate to replace Si due to its higher electron and hole mobility than Si. To date, a defect-free GeOx/Ge interface has never been achieved so far in terms of uniformity, composition, and reliability [3-5]. Hence, insight in Ge surface chemistry and morphology is crucial for novel device applications. Here, we report on the (electro)chemical etching behavior, surface morphology and composition of n-type Ge (100) in acidic halide solutions using various analytical and spectroscopic techniques. The use of an integrated (electro)chemical etching chamber connected to X-ray photoelectron spectroscopy (XPS) instrument to exclude the effect of oxygen from the atmosphere is highlighted.
Photoelectrochemical current-voltage (j-V) characteristics of n-doped Ge (100) samples were studied in aqueous HCl solutions. Fig. 1A shows j-V plots measured at different light intensities for 1M HCl at a scan rate of 10 mV/s. Region (I) shows that electron-hole recombination dominates due to the weak electric field at the surface. At more positive potential, a plateau feature (region II) can be seen from the curves. In this region of strong band bending, photoexcited holes generated within and near the depletion layer migrate and diffuse to the solution interface where they react to give rise to oxide formation. The photocurrent in this region is independent of the applied potential. The photocurrent in region II (at 0.8V) is plotted as a function of the light intensity for various HCl concentrations (Fig. 1A inset). In this plateau region, the photocurrent increases linearly with increasing light intensity. These results, indicative for a high oxide solubility, show that no passivating oxide layer is formed during electrochemical etching for photocurrent densities up to ~6.0 mA cm-2. Unexpectedly, random pyramids (Fig. 1B), as evidenced by the formation of characteristic (111) facets, were observed for concentrated HCl solutions and were accompanied by an increase in photocurrent and a decrease in reflectance. By patterning the pyramids, a high structure density could be achieved. The resulting lowering of the reflectance shows that light coupling was improved. XPS data reveal (Fig. 1C) that at low HCl concentration, surface chemistry is dominated by Ge-OH. At high HCl concentrations, Ge-OH is effectively converted into Ge-Cl. Upon surface chlorination, back bonds are being stabilized and as a result the etch rate is lowered. Ge (111) planes serve as an etch stop evidenced by the facetted surface.
On the other hand, in Ge chemical etching, HBr effectively removes (vs HCl) GeO2 and suboxides providing an air stable surface as confirmed by the XPS spectra. IPA rinsing maintains the electrostatics and chemical composition after Br-passivation (Figure 1D). The brominated Ge surface reoxidizes during H2O rinsing in Ar atmosphere, resulting in a strong upward shift of the surface Fermi level indicating an e- donating behavior of H2O. [6]
In summary, this research displays new outlooks for various applications like batteries [7], biomedicine [8], photodetectors [9] and for light absorbing and harvesting of solar energy [10]. In addition, it serves as an eye-opener for the potential of semiconductor surface chemical studies using UHV-integrated electrochemical cell connected to XPS to study etching systems.

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Figure 1