Abstract
The lack of useful and cost-efficient group-IV direct band gap light emitters still presents the main bottleneck for complementary metal-oxide semiconductor-compatible short-distance data transmission, single-photon emission, and sensing based on silicon photonics. Germanium, a group-IV element like Si, is already widely used in silicon fabs. While the energy band gap of Ge is intrinsically indirect, we predict that the insertion of Ge-Ge split-[110] interstitials into crystalline Ge can open up a direct band gap transmission path. Here, we calculate from first principles the band structure and optical emission properties of Ge, Sb, and Sn split-[110] interstitials in bulk and low-dimensional Ge at different doping concentrations. Two types of electronic states provide the light-emission enhancement below the direct band gap of Ge: a hybridized L-$\mathrm{\ensuremath{\Gamma}}$ state at the Brillouin zone center and a conduction band of $\mathrm{\ensuremath{\Delta}}$ band character that couples to a raised valence band along the $\mathrm{\ensuremath{\Gamma}}$-X direction. Majority carrier introduced to the system through doping can enhance light emission by saturation of nonradiative paths. Ge-Sn split interstitials in Ge shift the top of the valence band towards the $\mathrm{\ensuremath{\Gamma}}$-X direction and increase the $\mathrm{\ensuremath{\Gamma}}$ character of the L-$\mathrm{\ensuremath{\Gamma}}$ state, which results in a shift to longer emission wavelengths. Key spectral regions for datacom and sensing applications can be covered by applying quantum confinement in defect-enhanced Ge quantum dots for an emission wavelength shift from the midinfrared to the telecom regime.
Highlights
Practical monolithic solutions for direct band gap light emitters that can be readily implemented to industrial standards of Si technology are heavily sought after to push optoelectronic data transmission to the inter-and intrachip level [1,2,3,4,5,6,7,8,9,10]
In Ref [14], it is claimed that the dimer distance and the Ge next-nearest neighbor distances (Ge-nnn) are identical, while in Ref. [13], all distances turn out significantly smaller, which is probably due to a very small used lattice parameter
While for the Ge-Sb interstitial, the VB maximum remains at the point, we find that for the Ge-strength g (Sn) split interstitial, the VB states in the X direction are lifted in energy and form the clear new valence-band maximum
Summary
Practical monolithic solutions for direct band gap light emitters that can be readily implemented to industrial standards of Si technology are heavily sought after to push optoelectronic data transmission to the inter-and intrachip level [1,2,3,4,5,6,7,8,9,10] Based on such emitters, the prospect of quantum applications utilizing complementary metal-oxide semiconductor-compatible single-photon sources emitting in the telecom regime would uplift the field of quantum cryptography by the power of Si-based microelectronics [11,12]. We expect that the opening of direct recombination paths in Ge by inserting split-[110] interstitials will be of particular interest regarding applications in future Si photonics devices
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