Germanium on silicon photonic devices

There is presently increased interest in using germanium (Ge) for both electronic and optical devices on top of silicon (Si) substrates to expand the functionality of Si technology. It has been extremely difficult to form an Ohmic contact to n-Ge due to Fermi level pinning just above the Valence band. A low temperature nickel process has been developed that produces Ohmic contacts to n-Ge with a specific contact resistivity of , which to date is a record. The low contact resistivity is attributed to the low resistivity NiGe phase, which was identified using electron diffraction in a transmission electron microscope. Light emission from Ge light emitting diodes (LEDs) was investigated. Ge is an indirect bandgap semiconductor but the difference in energy between the direct and indirect is small (~136 meV), through a combination of n-type doping and tensile strain, the band structure can be engineered to produce a more direct bandgap material. A silicon nitride (Si3N4) process has been developed that imparts tensile strain into the Ge. The stress in the Si3N4 film can be controlled by the RF power used during the plasma enhanced chemical vapour deposition. LEDs covered with Si3N4 stressors were characterised by Fourier transform infrared spectroscopy. Electroluminescence characterisation (EL) revealed that the peak position of the direct and indirect radiative transitions did not vary with the Si3N4 stressors due to the device geometries being too large. Therefore, nanostructures consisting of pillars smaller than a micron were investigated. Photoluminescence characterisation of 100 nm Ge pillars with Si3N4 stressors show emission at much longer wavelengths compared to bulk Ge (> 2.2 μm). In addition, the EL from Ge quantum wells grown on Si was also investigated. EL characterisation demonstrates two peaks around 1.55 and 1.8 μm, which corresponds to the radiative recombination between the direct and indirect transitions, respectively. This result is the first demonstration of EL above 1.45 μm for Ge quantum wells. Finally, the fabrication of Ge-on-Si single-photon avalanche detectors are presented. A single-photon detection efficiency of 4 % at 1310 nm wavelength was measured at low temperature (100 K). The devices have the lowest reported noise equivalent power for a Ge-on-Si single-photon avalanche detector (1×10-14 WHz-1/2)

1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 μm and 1.8 μm, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare

Integrated Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips

Germanium-on-silicon waveguides are designed, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the in-coupling efficiency. On-chip propagation along bends and straight sections up to 0.8 mm is demonstrated around λ = 6 μm

Benchmarking the Use of Heavily-Doped Ge Against Noble Metals for Plasmonics and Sensing in the Mid-Infrared

Despite the recent introduction of heavily-doped semiconductors for mid-infrared plasmonics, it still remains an open point whether such materials can compete with noble metals. We employ a whole set of figures of merit to thoroughly assess the use of heavily-doped Ge on Si as a mid-infrared plasmonic material and benchmark it against standard noble metals such as Au. In doing this, we design and model high-performance, CMOS compatible mid-infrared plasmonic sensors based on experimental material data reaching plasma frequencies up to about 1950 cm−1. We demonstrate that plasmonic Ge sensors can provide signal enhancements for vibrational spectroscopy above 3 orders of magnitude, thus representing a viable alternative to noble metals

Ultra Broadband Mid-Infrared Ge-on-Si Polarization Rotator

The design and modelling of an ultra broadband Ge-on-Si waveguide polarization rotator is presented. The polarization rotator demonstrates high extinction ratio (≥ 18.5 dB) and low insertion loss (≤ 1 dB) over the full operating range of 8 to 11 μm wavelength

Mid-Infrared Plasmonic Platform based on Heavily Doped Epitaxial Ge-on-Si: Retrieving the Optical Constants of Thin Ge Epilayers

The n-type Ge-on-Si epitaxial material platform enables a novel paradigm for plasmonics in the mid-infrared, prompting the future development of lab-on-a-chip and subwavelength vibrational spectroscopic sensors. In order to exploit this material, through proper electrodynamic design, it is mandatory to retrieve the dielectric constants of the thin Ge epilayers with high precision due to the difference from bulk Ge crystals. Here we discuss the procedure we have employed to extract the real and imaginary part of the dielectric constants from normal incidence reflectance measurements, by combining the standard multilayer fitting procedure based on the Drude model with Kramers-Kronig transformations of absolute reflectance data in the zero-transmission range of the thin film.Comment: Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2014 39th International Conference o

Understanding the Sidewall Dependence of Loss for Ge-on-Si Waveguides in the Mid-Infrared

Measurements of sidewall roughness by atomic force microscopy has been used to understand the waveguides losses of Ge-on-Si mid-infrared rib waveguides. Simulations indicate the measured roughness is well below values corresponding to the measured losses indicating sidewall roughness scattering is not the dominant loss mechanism

Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips

Germanium-on-silicon rib waveguides are modelled, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the waveguide losses at 5.8 μm wavelength

Germanium Plasmonic Nanoantennas for Third-Harmonic Generation in the Mid Infrared

We explore the nonlinear optical properties of plasmonic semiconductor antennas resonant in the mid infrared. The nanostructures are fabricated on silicon substrates from heavily doped germanium films with a plasma frequency of 30 THz, equivalent to a wavelength of 10 μm. Illumination with ultrashort pulses at 10.8 μm produces coherent emission at 3.6 μm via third-harmonic generation