Light Emission of Sn/Ge MQW pin Diodes on Si
Authors: Oehme Michael , Hack Michael , Starz Leon , Darwish Hussain , Spieth Christian , Schaefer Soeren C. , Wanitzek Maurice , Seidel Lukas , Kasper Erich , Schwarz Daniel 
Published: December 2024

Si-based photonic integrated circuits (PIC) with optically active components monolithically integrated on a Si chip are considered one of the solutions for the innovation of the next generation of information and communication technology. Si-based photonics opens up new possibilities for applications in data communications, sensor technology, biological and environmental sensing systems, but also future technologies including integrated quantum technologies, optical computing, artificial intelligence-based technologies and neuromorphic photonics. Remarkable advances have been made in key components of Si-based PICs, including high-performance Si-based modulators, photodetectors as well as waveguides. An efficient electrically pumped light source on Si still remains a challenge. The first lasers monolithically integrated on Si were demonstrated at low temperatures using the CMOS (Complementary Metal Oxide Semiconductor)-compatible material system SiGeSn.
This paper reports the light emission from multiquantum well (MQW) structures composed of multi-stacked 2 Monolayer (ML) Sn/20 nm Ge layers incorporated into a pin structure. These samples were grown using molecular beam epitaxy. The schematic layer stack is shown in Figure (a). The MQW structure is placed in the intrinsic region of a Ge pin diode on a Si (100) substrate. Figure (b) shows the high-resolution x-ray (HR-XRD) space map of the device structure. The measurement data suggest that the compressively strained 2ML Sn/Ge MQW are grown pseudomorphically on the Ge virtual substrate (VS). Furthermore, the measurement shows clear information about the periodic Sn/Ge structure. In preliminary studies, the wetting layer thickness was determined to be 1.4 ML Sn. Figure (c) shows an atomic force microscopy (AFM) image of Sn dot formation on the Ge (100) surface for 2 ML Sn. The device is processed using a single mesa process with a very low thermal budget.
The electroluminescence measurements are carried out in a cryostat, the temperature of which can be continuously regulated from room temperature (RT) to 12 K. The light is collected via a special indium fluoride multimode-fiber and detected with the NIRquest spectrometer in a wide wavelength range between 1000 nm and 2500 nm. In order to understand the influence of the MQW structure, a reference diode with exactly the same growth parameters but without Sn was grown and processed in parallel. Their temperature-dependent EL spectra are shown in figure (d). At RT we see the typical EL spectrum of a Ge diode with the direct band (Γ-HH) and indirect band (L-HH) transition. Due to the indirect semiconductor properties of Ge, the intensity decreases with lower temperatures. The maxima shift towards the blue because of the temperature dependent band gap. At very low temperatures a new luminescence occurs, which we identify as a defect luminescence of the Ge.
The temperature-dependent EL spectra of the 2 ML/Ge MQW pin diode are shown in figure (e). Four peaks can be observed depending on the temperature. At RT the Ge peak as in the reference sample can be measured, but with a different intensity ratio. At very low temperatures we find a second, sharper peak next to the defect luminescence peak of Ge (see inset). This shows a direct behavior because the EL intensity increases with decreasing temperature. In the paper we discuss these different peaks in detail and give an interpretation based on photogenerated carrier numbers, temperature dependent occupation and quantum confined radiative transmission probabilities.

Figure 1