Spin-dependent direct gap emission in tensile-strained Ge films on Si substrates

Abstract

The circular polarization of direct gap emission of Ge is studied in optically excited tensile-strained Ge-on-Si heterostructures as a function of doping and temperature. Owing to the spin-dependent optical selection rules, the radiative recombinations involving strain-split light (cΓ-LH) and heavy hole (cΓ-HH) bands are unambiguously resolved. The fundamental cΓ-LH transition is found to have a low temperature circular polarization degree of about 85%, despite an off-resonance excitation of more than 300 meV. By photoluminescence (PL) measurements and tight-binding calculations we show that this exceptionally high value is due to the characteristic energy dependence of the optically induced electron spin population. Finally, our observation of a direct gap doublet clarifies that the light hole contribution, previously considered to be negligible, can dominate the room temperature PL even at low tensile strain values of ≈0.2%

Progress towards Spin-Based Light Emission in Group IV Semiconductors

“…In this work, a set of samples characterized by different doping, but the same tensile strain, was addressed, demonstrating a record-high PL polarization degree of 85% at low temperatures for a not-intentionally doped sample. First-neighbor tight-binding (TB) calculations including strain-induced effects clarified that the DSP for CB electrons optically coupled to SO states is 90%, in striking agreement with the experimentally-reported PL data [152]. This further corroborated the physical picture according to which, in the low doping regime, the direct gap recombination is dominated by the small fraction of electrons optically coupled to the SO states, whereas electrons excited from HH and LH VB states possess a large kinetic energy and are more likely to be scattered out of the zone center.…”

“…The impact of biaxial tensile strain on the spin-dependent direct-gap radiative recombination was studied by Vitiello et al utilizing epitaxial Ge-on-Si(001) heterostructures [152]. In this work, a set of samples characterized by different doping, but the same tensile strain, was addressed, demonstrating a record-high PL polarization degree of 85% at low temperatures for a not-intentionally doped sample.…”

Optically reconfigurable polarized emission in Germanium

“…Simultaneously, a few percent of the whole electron ensemble can be promoted at the edge of the Γ valley as |1/2, 1/2〉 spin-up electrons via transitions involving split-off |1/2, −1/2〉 VB states (Fig. 1a,b ) 6 , 40 , 41 .…”

Photonic Band Gap and Light Routing in Self-Assembled Lattices of Epitaxial Ge -on- Si Microstructures

“…To understand the presence of the two distinct contributions in the unpatterned Ge we take into account the tensile strain resulting from the thermal mismatch with the Si substrate. [23] This has a strong effect on the band structure of Ge since it induces a redshift and, most importantly, it breaks the valence band degeneracy at the center of the Brillouin zone, [22] as shown in the calculated band structure reported in Figure 3b. The two PL structures observed in the top panel of Figure 3a can thus be ascribed to radiative pathways at the center Γ of the Brillouin zone from the bottom of the conduction band (cΓ) towards heavy hole (HH) and light hole (LH) bands, respectively.…”

“…[20,21] To verify the presence of the PBG we performed continuous wave PL experiments on the microcrystals, comparing the results with those obtained on the reference unpatterned layer. [22] The comparison is useful to discriminate the effect of the material from that of the microstructure. To ensure that the microcrystals and the reference Ge film have the same material properties, we measured the PL of the epilayer in an unpatterned region of the same chip on which the fabrication process was performed.…”