Planar Hexagonal Germanium Grown on Cadmium Sulfide Substrate by Low-Energy Plasma-Enhanced Chemical Vapor Deposition.

Germanium (Ge) is a material of significant interest in the field of semiconductor technology due to its electronic and optical properties. The scientific community has recently intensified research efforts to obtain hexagonal Ge-2H which has been shown to exhibit a direct bandgap. The formation of Ge-2H has thus far been confined to nanodomains obtained mainly via core/shell nanowires [1]. This approach inherently restricts the active volume, thereby hindering fundamental property investigations and device fabrication. To overcome this constraint, it is therefore essential to synthesize high-quality planar Ge-2H crystals. The main objective of this work is to grow Ge-2H layers on wurtzite bulk substrates with m-plane orientation. As bulk gallium arsenide does not exist in the wurtzite phase, hexagonal cadmium sulfide (CdS-2H) substrate was selected as an alternative hexagonal template, due to the closely matched lattice parameters with Ge. The Ge deposition was performed by Low Energy Plasma Enhanced CVD at low temperature (low-T) (200 – 300°C) in order to prevent Cd desorption from the substrate, which would lead to a substoichiometric surface and possibly result in the loss of the substrate’s template effect on germanium growth. Prior to the low-T growths on CdS substrates, a preliminary study on low-T epitaxy on silicon substrates was conducted, obtaining crystalline thin Ge epilayers even at ultra-low-T (200°C). The Ge/CdS samples were structurally characterized by HR-XRD, SEM-EDX, STEM, and Raman spectroscopy. In the first growth at 300°C, the resulting film displayed dendritic structures, approximately 40 nm thick, with observable Ge intermixing with CdS, as shown in Fig. (a). This outcome is likely due to the excessive growth temperature. In the second growth, the temperature was reduced to 200°C, leading to the formation of an amorphous but compact Ge layer about 50 nm thick (as depicted in Fig. (b)). For the third growth, the temperature was increased to 250°C while the Ge layer thickness was limited to just 10 nm, specifically to rule out the possibility that excessive elastic energy from lattice mismatch caused the amorphization observed previously. Fig. (c) shows Raman spectra of the 10 nm-thick Ge/CdS sample, performed with rotating sample and fixed polarizer. In the configuration x(y,y)x, according to Porto notation and corresponding to 90° in the image, it is possible to detect the transverse optical E2g mode of Ge-2H (at ~287 cm-1 [2]), whose intensity decreases until it disappears, moving towards the configuration x(z,z)x configuration (0° in the image). Further investigations, including PL measurements, will be performed as soon as possible to possibly confirm the successful planar Ge-2H growth.