Abstract
The influence of thermal annealing on the properties of germanium grown on silicon (Ge-on-Si) has been investigated. Depth dependencies of strain and photoluminescence (PL) were compared for as-grown and annealed Ge-on-Si samples to investigate how intermixing affects the optical properties of Ge-on-Si. The tensile strain on thermally annealed Ge-on-Si increases at the deeper region, while the PL wavelength becomes shorter. This unexpected blue-shift is attributed to Si interdiffusion at the interface, which is confirmed by energy dispersive X-ray spectroscopy and micro-Raman experiments. Not only Γ- and L-valley emissions but also Δ2-valley related emission could be found from the PL spectra, showing a possibility of carrier escape from Γ valley. Temperature-dependent PL analysis reveals that the thermal activation energy of Γ-valley emission increases at the proximity of the Ge/Si interface. By comparing the PL peak energies and their activation energies, both SiGe intermixing and shallow defect levels are found to be responsible for the activation energy increase and consequent efficiency reduction at the Ge/Si interface. These results provide an in-depth understanding of the influence of strain and Si intermixing on the direct-bandgap optical transition in thermally annealed Ge-on-Si.
Highlights
Over the past decade, germanium (Ge), a group IV semiconductor material, has become an attractive light source material for its potential application in Si compatible on-chip laser[1,2,3,4,5,6,7]
Supposing that the Ge layer is thermally influenced by Si interdiffusion, samples are etched with different depth to assess how different regions of the Ge layer are affected
Samples are numbered with two digits, where the first stands for the presence of thermal annealing and the second for depth
Summary
Germanium (Ge), a group IV semiconductor material, has become an attractive light source material for its potential application in Si compatible on-chip laser[1,2,3,4,5,6,7]. The post-growth annealing can induce intermixing of Si and Ge at the interface[9,20], which is expected to change the energy levels of direct and indirect band edges, affecting the emission wavelength and the thermal activation energy for the direct bandgap transition. The roles of thermally-induced strain and intermixing on the activation energy were concurrently considered to deduce the band structure change in Ge under the influence of thermal annealing. The degraded optical properties at the interfacial region of thermally annealed Ge-on-Si were explained by the band structure change, along with the contributions of defect-related levels and carrier escape to Δ2 valley
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