Photoluminescence and local structure of Ge nanoclusters on Si without a wetting layer

Photoluminescence (PL) originating from single layers of Ge nanoclusters grown on Si(100) via a buffer layer-assisted growth method is investigated. The nanoclusters are characterized by the absence of a wetting layer. They are quasi-zero-dimensional with tunable sizes and exhibit a high cluster density compared to Ge nanoclusters that are formed from the standard Stranski-Krastanov growth. Samples with different cluster sizes show strong and sharp PL in the near infrared. The excitation power and temperature dependencies of PL spectra indicate that the optical transitions are bound-to-bound in nature, suggesting that defects rather than the band structures are associated with the luminescence centers. High-resolution transmission electron microscopy (TEM) reveals that these nanoclusters are amorphous, which could explain the strong defect-related emission.