Franz-Keldysh Effect in GeSn Detectors

The fundament for silicon based photonics is Ge grown on Si and SOI. A simple photonic integrated circuit (PIC) consist of an integrated light source, a waveguide, a modulator and a detector. Due to the fact that Ge is an indirect semiconductors, the light source is the main challenge for PICs. One concept is to add Sn into the Ge matrix. Theoretical calculations predict that Ge_1-xSn_x becomes a direct semiconductor for a Sn content of x = 10 % and therefore is an interesting material mixture for photonic devices integrated on Si. Recent publications have shown that detectors using GeSn as active material excess 40 GHz bandwidth. For PICs, also the modulation of the light is needed. Beside Mach-Zehnder modulators, the modulator can be realized using the Franz-Keldysh (FK) effect. Due to the FK effect, the absorption in the material can be changed by varying the applied electric field.

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 [1] 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 Ge_1-xSn_x 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 [2]. 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 E_g,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.