Recent years have seen an increasing interest in Group IV photonics approach for monolithic integration of optoelectronic devices on the Si platform. While passive and active devices such as photodetectors [1,2], modulators [3–5], waveguides and splitters have been successfully realized on-chip, light sources are the last missing key component.

Ever since the optically pumped Ge_1-zSn_z laser was demonstrated experimentally [6], this binary alloy presented itself as a direct Group IV semiconductor at high Sn concentrations. However, the growth of Ge_1-zSn_z is challenging due to the lattice mismatch to Si and Ge, which limits the layer thickness as well as the crystal quality. Furthermore, solid solubility of Sn in Ge is limited, making it difficult to achieve high Sn concentrations.

The ternary alloy Si_xGe_1-x-ySn_y is particularly interesting in this context because it allows to decouple its bandgap from its lattice constant, which enables e.g. the lattice matched growth on Ge at a given Si to Sn ratio. Moreover, Si_xGe_1-x-ySn_y can have a larger bandgap than Ge, which makes it interesting as cladding material. The ternary alloy, therefore, is interesting for the growth e.g. of double hetero (DHS) as well as quantum well structures (QW) with Ge_1-zSn_z as the well material. We have recently investigated the molecular beam epitaxy (MBE) based growth of Si_xGe_1-x-ySn_y bulk alloys lattice matched to Ge with very high Sn content up to 12,5 % [7,8]

Here, we will highlight the opportunities and difficulties of lattice matched growth of Sn-rich Si_xGe_1-x-ySn_y layers by MBE for Si_xGe_1-x-ySn_y pin diodes. We doped Si_xGe_1-x-ySn_y with B as well as Sb, achieving a total layer thickness of up to d = 900nm on a Ge virtual substrate (VS). Furthermore, we developed a growth strategy for Si_xGe_1-x-ySn_y QW structures, where doped Si_xGe_1-x-ySn_y layers work as cladding for a Sn-rich Ge_1-zSn_z quantum well, with the aim of combining the advantages of lattice matched growth on Ge with the direct semiconductor behaviour of Ge_1-zSn_z. We discuss material and device characterization results of a series of three Si_xGe_1-x-ySn_y pin diodes with variable intrinsic zone and a total layer thickness of 900 nm (Fig. 1 (a)).

The growth process takes place in a 6” MBE system, where Si, Ge and Sn are used as matrix materials and B and Sb as dopants respectively. The growth rate of the Si_xGe_1-x-ySn_y layers is calibrated and kept at R = 1 Å/s. The condition of lattice matched growth of Si_xGe_1-x-ySn_y (i.e. x/y = 3.67) requires highly precise calibration of the alloy compound fluxes, which is done in a complex multi-step method. The most critical growth parameter is the growth temperature, which is therefore measured and controlled in-situ by a thermocouple as well as two pyrometers. While a lower temperature allows better suppression of the Sn segregation, it can also result in a reduced crystal quality. For our samples, we chose to keep the substrate temperature at T_Sub = 200 °C for the growth of the ternary alloy, which is mainly based on our experience from previous Si_xGe_1-x-ySn_y growth studies.

In our pin diode device layers, the intrinsic zone is sandwiched between p- and n-doped Si_xGe_1-x-ySn_y layers. While simple co-evaporation of B was used for p-type doping, the n-doped top contact layer was grown as follows: after the growth of the intrinsic zone with a thickness of 300 nm the growth was interrupted to deposit a pre-built up of Sb (≈ 10¹² cm^-2) for the following n-Si_xGe_1-x-ySn_y contact in order to counterbalance the segregation of Sb [9,10].

In order to obtain detailed information of the composition as well as the crystal quality, the samples were analysed by X-Ray diffraction (XRD) and Raman spectroscopy. The reciprocal space map (RSM) of sample A (Fig. 1 (b)) shows that very good lattice matching was achieved for the Si_xGe_1-x-ySn_y layers.

After MBE growth the samples were processed to devices in a CMOS compatible single mesa process. Our samples clearly show diode behaviour with dark current densities as low as j_D = 0.1 A/cm² for sample A (Fig. 1 (c)). We present detailed results on the electrical and optical characterization of our devices.

Our results show that MBE based growth not only enables the fabrication of Si_xGe_1-x-ySn_y bulk alloys with high Sn content, it can also be used to dope the ternary alloy as a prerequisite for the fabrication of Si_xGe_1-x-ySn_y diodes that contain single or multiple quantum well structures with high-Sn content Ge_1-zSn_z. We discuss possible improvements on growth strategies as well as future steps for the investigation of MBE-grown Sn-based light emitters on Si.