T. Zabel

1.9% bi-axial tensile strain in thick germanium suspended membranes fabricated in optical germanium-on-insulator substrates for laser applications

High tensile strains in Ge are currently studied for the development of integrated laser sources on Si. In this work, we developed specific Germanium-On-Insulator 200 mm wafer to improve tolerance to high strains induced via shaping of the Ge layers into micro-bridges. Building on the high crystalline quality, we demonstrate bi-axial tensile strain of 1.9%, which is currently the highest reported value measured in thick (350 nm) Ge layer. Since this strain is generally considered as the onset of the direct bandgap in Ge, our realization paves the way towards mid-infrared lasers fully compatible with CMOS fab technology.

Group IV Direct Band Gap Photonics: Methods, Challenges, and Opportunities

The concept of direct band gap group IV materials offers a paradigm change for Si-photonics concerning the monolithic implementation of light emitters: The idea is to integrate fully compatible group IV materials with equally favorable optical properties as the chemically incompatible group III-V-based systems. The concept involves either mechanically applied strain on Ge or alloying of Ge with Sn and permits to drastically improve the insufficient radiative efficiency of Ge. The favorable optical properties result from a modified band structure transformed from an indirect to a direct one. The first demonstration of such a direct band gap laser, accomplished in GeSn, exemplifies the capability of this new concept. These systems may permit a qualitative as well as a quantitative expansion of Si-photonics into traditional but also new areas of applications, provided they can be operated energy efficiently, under ambient conditions and integrated with current Si technologies. This review aims to discuss the challenges along this path in terms of fabrication, characterization and fundamental understanding, and will elaborate on evoking opportunities of this new class of group IV-based laser materials.

Accurate strain measurements in highly strained Ge microbridges

Ge under high strain is predicted to become a direct bandgap semiconductor. Very large deformations can be introduced using microbridge devices. However, at the microscale, strain values are commonly deduced from Raman spectroscopy using empirical linear models only established up to e100 = 1.2% for uniaxial stress. In this work, we calibrate the Raman-strain relation at higher strain using synchrotron based microdiffraction. The Ge microbridges show unprecedented high tensile strain up to 4.9% corresponding to an unexpected Δω = 9.9 cm−1 Raman shift. We demonstrate experimentally and theoretically that the Raman strain relation is not linear and we provide a more accurate expression.

Structural and optical properties of 200 mm germanium-on-insulator (GeOI) substrates for silicon photonics applications

Integrated laser sources compatible with microelectronics represent currently one of the main challenges for silicon photonics. Using the Smart CutTM technology, we have fabricated for the first time 200 mm optical Germanium-On-Insulator (GeOI) substrates which consist of a thick layer of germanium (typically greater than 500 nm) on top of a thick buried oxide layer (around 1 µm). From this, we fabricated suspended microbridges with efficient Bragg mirror cavities. The high crystalline quality of the Ge layer should help to avoid mechanical failure when fabricating suspended membranes with amounts of tensile strain high enough to transform Ge into a direct bandgap material. Optical GeOI process feasibility has successfully been demonstrated, opening the way to waferscale fabrication of new light emitting devices based on highly-tensely strained (thanks to suspended membranes) and/or doped germanium.

Lattice strain and tilt mapping in stressed Ge microstructures using X-ray Laue micro-diffraction and rainbow filtering

Micro-Laue diffraction and simultaneous rainbow-filtered micro-diffraction were used to measure accurately the full strain tensor and the lattice orientation distribution at the sub-micron scale in highly strained, suspended Ge micro-devices. A numerical approach to obtain the full strain tensor from the deviatoric strain measurement alone is also demonstrated and used for faster full strain mapping. We performed the measurements in a series of micro-devices under either uniaxial or biaxial stress and found an excellent agreement with numerical simulations. This shows the superior potential of Laue micro-diffraction for the investigation of highly strained micro-devices.

Correlation between emission intensity of self-assembled germanium islands and quality factor of silicon photonic crystal nanocavities

We present a comparative microphotoluminescence study of the emission intensity of self-assembled germanium islands coupled to the resonator mode of two-dimensional silicon photonic crystal defect nanocavities. The cavity-mode intensity is investigated for L3 and hexapole cavities with a range of different mode quality factors. For each type of cavity, many nominally identical samples are probed to obtain reliable statistics. As the cavity-mode quality factor increases, we observe a clear reduction of the average mode emission intensity under conditions of strong optical pumping. This clear trend is compared with simulations based on a dissipative master-equation approach that describes a cavity weakly coupled to an ensemble of emitters. We obtain direct evidence that reabsorption of photons in the cavity is responsible for the observed trend. When combined with the observation of cavity linewidth broadening in power-dependent measurements, we conclude that free carrier absorption limits the cavity-mediated light enhancement under conditions of strong excitation.

Quantum confinement effects in GeSn/SiGeSn heterostructure lasers

The development of a light source on Si, which can be integrated in photonic circuits together with CMOS electronics, is an outstanding goal in the field of Silicon photonics. This could e.g. help to overcome bandwidth limitations and losses of copper interconnects as the number of high-speed transistors on a chip increases. Here, we discuss direct bandgap group IV materials, GeSn/SiGeSn heterostructures and resulting quantum confinement effects for laser implementation. After material characterization, optical properties, including lasing, are probed via photoluminescence spectrometry. The quantum confinement effect in GeSn wells of different thicknesses is investigated. Theoretical calculations show strong quantum confinement to be undesirable past a certain level, as the very different effective masses of r and L electrons lead to a decrease of the L-to Γ-valley energy difference. A main limiting factor for lasing devices turns out to be the defective region at the interface to the Ge substrate due to the high lattice mismatch to GeSn. The use of buffer technology and subsequent pseudomorphic growth of multi-quantum-wells structures offers confinement of carriers in the active material, far from the misfit dislocations region. Performance is strongly boosted, as a reduction of lasing thresholds from 300 kW/cm2 for bulk devices to below 45 kW/cm2 in multi-quantum-well lasers is observed at low temperatures, with the reduction in threshold far outpacing the reduction in active gain material volume.

GeSn lasers for monolithic integration on Si

Lasing under optical pumping is shown in suspended GeSn microdisks fabricated on a Ge virtual substrate with a lasing threshold below 1 mW at 20K.

GeSn lasers for mid-infrared silicon photonics (Conference Presentation)

At least four groups have demonstrated GeSn direct bandgap material and shown cryogenic temperature lasers under optical pumping. With up to 16% of Sn, our lasers operate up to 180K and lase up to wavelengths of 3.1 um. We will describe our efforts to reduce the threshold, increase the operating temperature, and evolve towards electrical pumping in these lasers.Thes efforts involve improvements of epi growth, electrical passivation, doping, heterostructures, strain control...

Will Ge and GeSn lasers enable Si photonics in the Mid-Infrared?

For the near-infrared (near-IR) optical data communications, silicon photonics became a mature technology. Building on the technological developments associated with datacoms, silicon photonics now expands into the mid-infrared (mid-IR), mostly for the optical gas sensing. Here as well, the development is hampered by the absence of monolithically integrated laser sources compatible with the CMOS fab processing.