D. C. S. Dumas

Ge/SiGe quantum confined Stark effect electro-absorption modulation with low voltage swing at λ = 1550 nm

Low-voltage swing (≤1.0 V) high-contrast ratio (6 dB) electro-absorption modulation covering 1460 to 1560 nm wavelength has been demonstrated using Ge/SiGe quantum confined Stark effect (QCSE) diodes grown on a silicon substrate. The heterolayers for the devices were designed using an 8-band k.p Poisson-Schrödinger solver which demonstrated excellent agreement with the experimental results. Modelling and experimental results demonstrate that by changing the quantum well width of the device, low power Ge/SiGe QCSE modulators can be designed to cover the S- and C-telecommunications bands.

Mid-infrared light emission > 3 µm wavelength from tensile strained GeSn microdisks

GeSn alloys with Sn contents of 8.4 % and 10.7 % are grown pseudomorphically on Ge buffers on Si (001) substrates. The alloys as-grown are compressively strained, and therefore indirect bandgap. Undercut GeSn on Ge microdisk structures are fabricated and strained by silicon nitride stressor layers, which leads to tensile strain in the alloys, and direct bandgap photoluminescence in the 3-5 µm gas sensing window of the electromagnetic spectrum. The use of pseudomorphic layers and external stress mitigates the need for plastic deformation to obtain direct bandgap alloys. It is demonstrated, that the optically pumped light emission overlaps with the methane absorption lines, suggesting that GeSn alloys are well suited for mid-infrared integrated gas sensors on Si chips.

Silver antimony Ohmic contacts to moderately doped n-type germanium

A self doping contact consisting of a silver/antimony alloy that produces an Ohmic contact to moderately doped n-type germanium (doped to a factor of four above the metal-insulator transition) has been investigated. An evaporation of a mixed alloy of Ag/Sb (99%/1%) onto n-Ge ( ND=1×1018 cm−3) annealed at 400 °C produces an Ohmic contact with a measured specific contact resistivity of (1.1±0.2)×10−5 Ω-cm2. It is proposed that the Ohmic behaviour arises from an increased doping concentration at the Ge surface due to the preferential evaporation of Sb confirmed by transmission electron microscope analysis. It is suggested that the doping concentration has increased to a level where field emission will be the dominate conduction mechanism. This was deduced from the low temperature electrical characterisation of the contact, which exhibits Ohmic behaviour down to a temperature of 6.5 K.

Germanium on silicon single-photon avalanche detectors using silicon-on-insulator substrates (Conference Presentation)

Single photon avalanche detectors (SPADs) operating in gated-Geiger mode at near infrared wavelengths have applications in quantum key distribution (QKD), eye-safe light detection and ranging (LIDAR), 3D image sensing, quantum enhanced imaging and photonic based quantum information processing. Whilst InGaAs SPADs are commercially available, the high cost and lack of integrated SPADs limit the applications. We have previously demonstrated vertical Geiger mode Ge on Si SPADs at 1310 and 1550 nm operating at 100 K where the Ge is used as an absorber and the lower noise Si is used as the avalanche gain region. At 100 K and 1310 nm a single photon detection efficiency of 4% was demonstrated with a dark count rate (DCR) of 5 MHz. Here we present first results on Ge on Si SPADs grown on top of silicon-on-insulator (SOI) substrates. Both vertical photodetectors and waveguide coupled detectors were investigated with designs aimed to reduce the DCR over previous results. Waveguides and avalanche regions were patterned in the top Si of a SOI wafer before being coated with silicon dioxide. Holes were then etched in the oxide to allow selective area growth of Ge inside these windows and on top of the Si waveguides for the waveguide coupled Ge SPADs. This approach reduces the threading dislocation density compared to bulk Ge growths which aids the reduction of the DCR. The fabricated devices have been tested at both 1310 nm and 1550 nm wavelengths and demonstrate improved performance over previous published results.

Tensile strained GeSn mid-infrared light emitters

Compressively strained GeSn alloys grown on Ge buffers on Si (001) substrates were fabricated into microdisks and strained using silicon nitride stressors. The strained disks are measured to be tensile by Raman spectroscopy, and demonstrate direct bandgap emission in the 3–5 μm gas sensing window.

Tuning the electroluminescence of n-Ge LEDs using process induced strain

LEDs are reported from Ge-on-Si in which process induced strain has increased the emission wavelength. The direct bandgap electroluminescence emits up to ~ μW of power between 1.6 μm and ~1.8 μm, significantly larger than previous LEDs.

Direct band-gap electroluminescence from strained n-doped Germanium diodes

The fabrication and characterisation of LED structures made of Ge grown on Si substrates is reported. The structures are circular mesa of strained n-Ge etched down to an undoped buffer of Ge. The electroluminescence exhibit average power levels at 1.7 μm of 10 μW, many orders of magnitude larger than the nW previously reported. 3 individual mechanisms of emission are identified which can be used to interpret the results encountered in other publications. This work potentially opens the route for integrated source of light and photodetectors above 1.6 μm on Si with applications for lab-on-a-chip and healthcare.