M. Kaschel

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80u2002μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55u2002μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

GeSn p-i-n detectors integrated on Si with up to 4% Sn

GeSn heterojunction photodetectors on Si substrates were grown with Sn concentration up to 4%, fabricated for vertical light incidence, and characterized. The complete layer structure was grown by means of ultra low temperature (100u2009°C) molecular beam epitaxy. The Sn content shifts the responsivity into the infrared, about 310u2009nm for the 4% Sn sample. An increase of the optical responsivity for wavelengths higher than 1550u2009nm can be observed with increasing Sn content. At 1600u2009nm, the optical responsivity is increased by more than a factor of 10 for the GeSn diode with 4% Sn in comparison to the Ge reference diode.

Ge-on-Si p-i-n Photodiodes With a 3-dB Bandwidth of 49 GHz

Fast Ge-on-Si p-i-n photodiodes are fabricated and their frequency response is measured up to 67 GHz at a wavelength of 1550 nm. At a bias voltage of -2 V, a 3-dB bandwidth (BW) of 49 GHz is achieved. This is to the best of the authors knowledge the highest BW ever published for Ge photodiodes.

Room-Temperature Electroluminescence From GeSn Light-Emitting Pin Diodes on Si

In this letter, a GeSn light-emitting pin diode integrated on Si via a Ge buffer is demonstrated and it is compared with a light-emitting pin diode made from pure, unstrained Ge on Si. The diode layer structures are grown with a special low-temperature molecular beam epitaxy process. The pseudomorphic GeSn layers (1.1% Sn content) on the Ge buffer are compressively strained. Both light-emitting pin diodes clearly show direct bandgap electroluminescence emission at room temperature. The electroluminescence peak of the GeSn light-emitting pin diode is shifted by 20 meV into the infrared region compared to the electroluminescence peak of the unstrained Ge light-emitting pin diode. The shift is due to the lower bandgap of GeSn and the influence of strain.

Germanium on Silicon Photodetectors with Broad Spectral Range

Vertical germanium on silicon p-i-n photodetectors were grown by molecular beam epitaxy and further processed by a quasi-planar mesa process. In this paper the influence of the n + -top contact layer on the optical responsivity was investigated. The n + -top contact was realized as highly antimony-doped silicon on a germanium heterostructure. With this additional heterostructure on top (relaxed silicon on relaxed germanium), the optical responsivity of the photodetector was broadened. The visible wavelength cutoff was shifted to 650 nm, whereas the long wavelength cutoff remained at 1550 nm.

Franz-Keldysh effect in GeSn pin photodetectors

The optical properties and the Franz-Keldysh effect at the direct band gap of GeSn alloys with Sn concentrations up to 4.2% at room temperature were investigated. The GeSn material was embedded in the intrinsic region of a Ge heterojunction photodetector on Si substrates. The layer structure was grown by means of ultra-low temperature molecular beam epitaxy. The absorption coefficient as function of photon energy and the direct bandgap energies were determined. In all investigated samples, the Franz-Keldysh effect can be observed. A maximum absorption ratio of 1.5 was determined for 2% Sn for a voltage swing of 3u2009V.

Si Esaki diodes with high peak to valley current ratios

We report room temperature current voltage characteristics of Si p+-i-n+ Esaki diodes integrated on silicon substrates. The diodes were fabricated by low-temperature molecular beam epitaxy. Very high and abrupt p- and n-type dopant transitions into the 1020u2002cm−3 ranges are achieved by boron and antimony, respectively. The integrated devices are realized without a postgrowth annealing step. The silicon Esaki diodes show negative differential resistance at room temperature with excellent peak to valley current ratios up to 3.94. A variation in the thickness of the silicon tunneling barrier changes the peak current density over three orders of magnitude.

Franz-Keldysh effect in germanium p-i-n photodetectors on silicon

Ge on Si p-i-n photodetectors were grown by molecular beam epitaxy minimizing tensile strain. The slope of the absorption curve changes by a factor of 2 under varying voltages due to the clearly observable Franz-Keldysh-Effect.

Design of an Integrated Dual-Mode Interferometeron 250 nm Silicon-on-Insulator

The design of a highly efficient integrated dual-mode interferometer with single-mode inputs and outputs and simple fabrication steps in a silicon-on-insulator platform with CMOS compatible technology is presented. Consisting of only one waveguide and two special mode converters, the device offers high sensitivity and immunity to input power fluctuations and temperature changes. Special attention is paid to the design of low-loss mode converters with two single-mode outputs for total power tracking. Considerations on temperature influence, fabrication tolerances, and dual mode waveguide sensitivity conclude the theoretical part. Experimental results of a single-port dual-mode interferometer show extinction ratios up to 31 dB and insertion losses of less than 1 dB at a wavelength of 1550 nm. With an additional functional polymer layer, the selective detection of various concentrations of L-Boc-phenylalanine anilid in an ethanol-water solution is demonstrated.

Integrated dual-mode waveguide interferometer

A novel type of interferometer with an ultra-small footprint is presented. The functional principle is based on the excitation of the first and the second order modes in an integrated dual-mode strip waveguide. The light is coupled from a single mode fiber to the dual-mode waveguide by a conventional grating coupler followed by a taper. The difference in the mode propagation velocity leads to a phase difference between the fundamental mode and the second order mode. A proof of concept in the silicon on insulator technology shows promising extinction ratios of around 30 dB.