Suspended Germanium Research Articles

High-speed mid-infrared graphene electro-optical modulator based on suspended germanium slot waveguides.

The mid-infrared (MIR) region is attracting increasing interest for on-chip synchronous detection and free-space optical (FSO) communications. For such applications, a high-performance electro-optical modulator is a crucial component. In this regard, we propose and investigate a graphene-based electro-absorption modulator (EAM) and microring modulator (MRM) using the suspended germanium waveguide platform. The modulators are designed for the second atmospheric window (8 to 12 µm). The incorporation of double-layer graphene on the suspended slot waveguide structure allows for the significant enhancement of light-graphene interaction, theoretically achieving a 3-dB bandwidth as high as 78 GHz. The EAM shows a calculated modulation depth of 0.022-0.045 dB/µm for the whole operation wavelength range. The MRM exhibits a calculated extinction ratio as high as 68.9 dB and a modulation efficiency of 0.59 V·cm around 9 µm. These modulators hold promise for constructing high-speed FSO communication and on-chip spectroscopic detection systems in the MIR atmospheric window.

Suspended germanium membranes photodetector with tunable biaxial tensile strain and location-determined wavelength-selective photoresponsivity

A divergent microstructure was fabricated by complementary metal–oxide–semiconductor compatible processes on the central region of a Ge p–i–n photodetector to enhance the residual tensile strain. A tunable biaxial tensile strain of ∼0.22%–1.01% was achieved by varying the geometrical factors, and it was confirmed by Raman measurements and finite element method simulations. The suspended germanium membranes enhance the absorption across the C- and L-bands (1528–1560 and 1561–1620 nm) and extend the cutoff wavelength to ∼1700–1937 nm. The Ge absorption coefficient is enhanced by ∼4.2× to 2951 cm−1 at 1630 nm, which is comparable with that of In0.53Ga0.47As. Furthermore, due to the varying strain distribution on the Ge mesa, each photodetector presents the location-determined wavelength-selective photoresponsivity characteristics. This work offers a promising approach for adjusting the absorption spectra of the photodetector by harnessing geometrically amplified biaxial strain.

Suspended germanium waveguides with subwavelength-grating metamaterial cladding for the mid-infrared band.

In recent years, sensing and communication applications have fueled important developments of group-IV photonics in the mid-infrared band. In the long-wave range, most platforms are based on germanium, which is transparent up to ∼15-µm wavelength. However, those platforms are limited by the intrinsic losses of complementary materials or require complex fabrication processes. To overcome these limitations, we propose suspended germanium waveguides with a subwavelength metamaterial lateral cladding that simultaneously provides optical confinement and allows structural suspension. These all-germanium waveguides can be fabricated in one dry and one wet etch step. A propagation loss of 5.3 dB/cm is measured at a wavelength of 7.7 µm. These results open the door for the development of integrated devices that can be fabricated in a simple manner and can potentially cover the mid-infrared band up to ∼15µm.

Near-field mapping of the edge mode of a topological valley slab waveguide at λ = 1.55 μ m

Valley-polarized topological states of light allow for robust waveguiding, which has been demonstrated for transverse-electric modes in THz and near-infrared regions of the spectrum. As the topological protection relies on guiding the light via a highly structured surface, direct imaging of the photonic modes at subunit cell resolution is of high interest but challenging, particularly for transverse-magnetic modes. Here, we report mapping the transverse-magnetic modes in a valley photonic crystal waveguide using scattering-type scanning near-field optical microscopy at the optical telecom C-band wavelength. The waveguide based on a triangular air-hole motif with broken inversion symmetry is fabricated from the suspended germanium layer. We observed the launching and guiding of the transverse-magnetic edge mode along the boundary between topologically distinct domains with opposite valley Chern indices. These results are supported by theoretical simulations and provide insight into the design and use of topological protected states for applications in densely integrated optical telecommunication devices.

Comparison of uniaxial and polyaxial suspended germanium bridges in terms of mechanical stress and thermal management towards a CMOS compatible light source.

Germanium (Ge) is a promising candidate for a CMOS compatible laser diode. This is due to its compatibility with Silicon (Si) and its ability to be converted into a direct band gap material by applying tensile strain. In particular uniaxial suspended Ge bridges have been extensively explored due to their ability to introduce high tensile strain. There have been two recent demonstrations of low-temperature optically-pumped lasing in these bridges but no room temperature operation accredit to insufficient strain and poor thermal management. In this paper we compare uniaxial bridges with polyaxial bridges in terms of mechanical stress and thermal management using finite element modelling (FEM). The stress simulations reveal that polyaxial bridges suffer from extremely large corner stresses which prevent larger strain from being introduced compared with uniaxial bridges. Thermal simulations however reveal that they are much less thermally sensitive than uniaxial bridges which may indicate lower optical losses. Bridges were fabricated and Raman spectroscopy was used to validate the results of the simulations. We postulate that polyaxial bridges could offer many advantages over their uniaxial counterparts as potential laser devices.

Design of a suspended germanium micro-antenna for efficient fiber-chip coupling in the long-wavelength mid-infrared range.

Recent developments of photonic integrated circuits for the mid-infrared band has opened up a new field of attractive applications for group IV photonics. Grating couplers, formed as diffractive structures on the chip surface, are key components for input and output coupling in integrated photonic platforms. While near-infrared optical fibers exhibit large mode field diameters compared to the wavelength, in the long-wave regime commercially available single-mode optical fibers have mode field diameters of the order of the operating wavelength. Consequently, an efficient fiber-chip surface coupler designed for the long-wave infrared range must radiate the power propagating in the waveguide with a higher radiation strength than a conventional grating coupler in the near-infrared range. In this article, we leverage the short electrical length required for long-wave infrared couplers to design a broadband all-dielectric micro-antenna for a suspended germanium platform at 7.67 µm. The design methodology is inspired by fundamental grating coupler equations, which remain valid even when the micro-antenna has only two or three diffractive elements. A simulated coupling efficiency of ~ 40% is achieved with a 1-dB bandwidth broader than 430 nm, which is almost twice the typical fractional bandwidth of a conventional grating coupler. In addition, the proposed design is markedly tolerant to fiber tilt misalignments of ±10°. This all-dielectric micro-antenna design paves the way for efficient fiber-chip coupling in long-wavelength mid-infrared integrated platforms.

Suspended low-loss germanium waveguides for the longwave infrared.

Germanium is a material of high interest for mid-infrared (MIR) integrated photonics due to its complementary metal-oxide-semiconductor (CMOS) compatibility and its wide transparency window covering the 2-15μm spectral region exceeding the 4 and 8μm limit of the silicon-on-insulator platform and Si material, respectively. In this Letter, we report suspended germanium waveguides operating at a wavelength of 7.67μm with a propagation loss of 2.6±0.3 dB/cm. To the best of our knowledge, this is the first demonstration of low-loss suspended germanium waveguides at such a long wavelength. Suspension of the waveguide is achieved by defining holes alongside the core providing access to the buried oxide layer and the underlying Si layer so that they can be wet etched using hydrofluoric acid and tetramethylammonium hydroxide, respectively. Our MIR waveguides create a new path toward long wavelength sensing in the fingerprint region.

Localized thinning for strain concentration in suspended germanium membranes and optical method for precise thickness measurement

We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.

Enhanced light emission from improved homogeneity in biaxially suspended Germanium membranes from curvature optimization.

A silicon compatible light source is crucial to develop a fully monolithic silicon photonics platform. Strain engineering in suspended Germanium membranes has offered a potential route for such a light source. However, biaxial structures have suffered from poor optical properties due to unfavorable strain distributions. Using a novel geometric approach and finite element modelling (FEM) structures with improved strain homogeneity were designed and fabricated. Micro-Raman (μ-Raman) spectroscopy was used to determine central strain values. Micro-photoluminescence (μ-PL) was used to study the effects of the strain profiles on light emission; we report a PL enhancement of up to 3x by optimizing curvature at a strain value of 0.5% biaxial strain. This geometric approach offers opportunity for enhancing the light emission in Germanium towards developing a practical on chip light source.

Suspended germanium cross-shaped microstructures for enhancing biaxial tensile strain

We fabricate a suspended germanium cross-shaped microstructure to biaxially enhance residual tensile strain using a germanium epilayer directly grown on a silicon-on-insulator substrate. Such a suspended germanium system with enhanced biaxial tensile strain will be a promising platform for incorporating optical cavities toward the realization of germanium lasers. We demonstrate systematic control over biaxial tensile strain and photoluminescence peaks by changing structural geometry. The photoluminescence peaks corresponding to the direct recombination between the conduction Γ valley and two strain-induced separated valence bands have been clearly assigned. A maximum biaxial strain of 0.8% has been achieved, which is almost half of that required to transform germanium into a direct band-gap semiconductor.

Strain distribution in single, suspended germanium nanowires studied using nanofocused x-rays

Within the quest for direct band-gap group IV materials, strain engineering in germanium is one promising route. We present a study of the strain distribution in single, suspended germanium nanowires using nanofocused synchrotron radiation. Evaluating the probed Bragg reflection for different illumination positions along the nanowire length results in corresponding strain components as well as the nanowire's tilting and bending. By using these findings we determined the complete strain state with the help of finite element modelling. The resulting information provides us with the possibility of evaluating the validity of the strain investigations following from Raman scattering experiments which are based on the assumption of purely uniaxial strain.

Tensile strain mapping in flat germanium membranes

Scanning X-ray micro-diffraction has been used as a non-destructive probe of the local crystalline quality of a thin suspended germanium (Ge) membrane. A series of reciprocal space maps were obtained with ∼4 μm spatial resolution, from which detailed information on the strain distribution, thickness, and crystalline tilt of the membrane was obtained. We are able to detect a systematic strain variation across the membranes, but show that this is negligible in the context of using the membranes as platforms for further growth. In addition, we show evidence that the interface and surface quality is improved by suspending the Ge.

Direct bandgap germanium-on-silicon inferred from 57% 〈100〉 uniaxial tensile strain [Invited

We report uniaxial tensile strains up to 5.7% along 〈100〉 in suspended germanium (Ge) wires on a silicon substrate, measured using Raman spectroscopy. This strain is sufficient to make Ge a direct bandgap semiconductor. Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states; however, recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized. We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers. These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.

Wide band polarizer with suspended germanium resonant grating

An ultra broad band polarizer that operates in the telecommunication wavelength band is proposed. This device, which consists of a single suspended germanium resonant grating layer, is designed using the inverse mathematical method and the rigorous vector diffraction theory. Calculated results indicate that the ultra broad band polarizer exhibits extremely high reflection (R>99%) for TE polarization light and high transmission (T>99%) for TM polarization at the wavelength range greater than 300 nm, and it has an extinction ratio of approximately 1 000 at the 1 550-nm central wavelength. The results of the rigorous coupled wave analysis indicate that the extremely wide band property of the TE polarization is caused by the excitation of strong modulation guided modes in the design wavelength range.