Franz-Keldysh Effect in GeSn Detectors
In this paper we report the FK effect in GeSn PIN diodes with Sn concentrations up to 4.2 %. In order to analyze the absorption of GeSn, vertical PIN diodes with different Sn content were produced. First, the epitaxial layers were grown with a solid source molecular beam epitaxy system  on B-doped Si (100) substrates with a high specific resistance ρ > 1000 Ωcm. Fig. 1(a) shows the vertical layer sequence. The layer stack consists of a vertical PIN structure. Starting with a 400 nm B-doped Ge contact layer (1020 cm-3), the active layer with 300 nm intrinsic Ge1-xSnx and a 10 nm Ge spacer follow. The top contact consist of 100 nm Sb-doped Ge (1020 cm-3) and 100 nm Sb-doped Si (1020 cm-3). The three investigated samples have a Sn content of x = 0 %, 2 % and 4.2 %, respectively.
The GeSn PIN heterojunction photodiodes are produced in a quasi-planar technology. The device processing by double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes . A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in Fig 1(b).
The electrical and optical characterization of the diodes is done with on-wafer measurement of the current-voltage characteristics. As a light source, a super-continuum laser with a tunable acousto-optical wavelength filter is used. The light couples into the diode through a lensed glass fiber. From measuring the dark current and photo current, the responsivity of the GeSn detectors were determined by dividing the photo current by the optical power.
Fig. 2 shows the responsivity of the GeSn PIN diodes with Sn concentrations of x = 0 %, 2 % and 4.2 %. With increasing Sn content, the bandgap shifts to higher wavelengths and therefore, the responsivity increases for higher wavelengths. A voltage dependence of the optical responsivity is only shown near the direct bandgap Eg,Γ. This is due to the FK effect.
From the measurement of the responsivity and reflectivity, the internal quantum efficiency can be calculated and further the absorption is determined. The electro-optical modulation from the FK effect was clearly observed in GeSn material. The maximum absorption ratio Δα/α0 for the 2 % Sn sample at room temperature was 1.5 at a wavelength of 1806 nm and a voltage swing of 3 V.