Pit-formation in germanium homoepitaxial layers

With increasing importance of germanium (Ge) for semiconductor quantum technologies, the necessity of growing defect-free Ge films has become crucial. Here, Ge homoepitaxial growth experiments were conducted using molecular beam epitaxy (MBE). This requires a smooth (${\sigma }_{rms}\le 10$ Å) and contamination-free substrate surface. We have shown that common ex-situ wet chemical cleaning methods are not sufficient. Foreign atoms like carbon stick to the substrate surface and tend to form clusters which results in macroscopic growth defects which are referred here as pits. The density of such pits is in the range of $10^6$ to $10^9$ cm⁻². The formation and characteristics of the pits have been investigated. Pits emerge after a growth at 370 °C with a thickness of above 5 nm . At growth temperatures lower than 300 °C, the density of pits increases, while it decreases at temperatures higher than 400 °C. Pits can be faceted with the main facets being {1 1 3} and {3 15 23}.

Furthermore, a procedure is presented involving a Ge buffer growth at room temperature with a high growth rate (0.02 nm/s). This leads to a coverage of present contamination and the formation of a new substrate surface which prevent pit formation.