MBE-Grown SixGe1-x-
ySny Diode and Quantum Well Diode Structures with High Sn Content for Optical Applications
Authors: Schwarz Daniel , Clausen Caterina Johanna , Fischer Inga Anita , Funk Hannes Simon , Oehme Michael , Weißhaupt David , Schulze Joerg 
Published: February 2020

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 Ge1-zSnz laser was demonstrated experimentally [6], this binary alloy presented itself as a direct Group IV semiconductor at high Sn concentrations. However, the growth of Ge1-zSnz 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 SixGe1-x-ySny 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, SixGe1-x-ySny 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 Ge1-zSnz as the well material. We have recently investigated the molecular beam epitaxy (MBE) based growth of SixGe1-x-ySny 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 SixGe1-x-ySny layers by MBE for SixGe1-x-ySny pin diodes. We doped SixGe1-x-ySny 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 SixGe1-x-ySny QW structures, where doped SixGe1-x-ySny layers work as cladding for a Sn-rich Ge1-zSnz quantum well, with the aim of combining the advantages of lattice matched growth on Ge with the direct semiconductor behaviour of Ge1-zSnz. We discuss material and device characterization results of a series of three SixGe1-x-ySny 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 SixGe1-x-ySny layers is calibrated and kept at R = 1 Å/s. The condition of lattice matched growth of SixGe1-x-ySny (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 TSub = 200 °C for the growth of the ternary alloy, which is mainly based on our experience from previous SixGe1-x-ySny growth studies.
In our pin diode device layers, the intrinsic zone is sandwiched between p- and n-doped SixGe1-x-ySny 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-SixGe1-x-ySny 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 SixGe1-x-ySny 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 jD = 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 SixGe1-x-ySny bulk alloys with high Sn content, it can also be used to dope the ternary alloy as a prerequisite for the fabrication of SixGe1-x-ySny diodes that contain single or multiple quantum well structures with high-Sn content Ge1-zSnz. We discuss possible improvements on growth strategies as well as future steps for the investigation of MBE-grown Sn-based light emitters on Si.

Figure 1