Si-Ge-Sn Alloys Grown by CVD for Applications in Optics, Electronics and Thermoelectrics

The experimental demonstration of an Sn-based optically pumped laser grown on Silicon by chemical vapor deposition (CVD)¹, the research of group-IV Sn-based semiconductors has exponentially increased. Direct bandgap GeSn alloys offers a unique solution for monolithic integration of photonics in the current CMOS technology. The impediment in reaching room temperature electrically pumped laser based on SiGeSn/GeSn alloys is the low capacity of dissipation of heat², reason why new designs are investigated to date. At the same time, the poor heat dissipation has aroused the interest of the scientific community in the field of thermoelectric. Studies have shown that group-IV Sn-based materials can be tailored to offer very low thermal conductivity³. In the field of electronics, the high mobility associated with the direct bandgap of Si-Ge-Sn alloys boosts the present field-effect transistor (FET) devices’ performance⁴.<br/><br/>The content of Sn in the alloy is directly defining the material properties and through this the specific application. While Sn contents above 14 at.% are best used in photonics and thermoelectric, lower Sn contents, &lt;10 at.%, are sufficient to improve nanowire FET devices. However, due to the low solid solubility of Sn of below 1% and the large lattice mismatch between the lattice constants of these elements, such high Sn concentrations are a challenge⁶. Its application to the design of layered heterostructures with multiple Sn contents is even more challenging considering also the low thermal stability of Sn alloys.<br/><br/>In this work, a simple and novel methodology to control the incorporation of Sn in the alloy is discussed. Starting for a reference growth conditions that offer high crystalline quality, keeping the process temperature and the reactor pressure constant, as well as the flow of the gas precursors, the total flow of the carrier gas (N₂ or H₂) is varied. In this way, the gas velocity and the partial pressure of the precursors changes, thus gas phase reactions are modified with impact on the surface kinetics allowing to control the Sn concentration in the films, offering a simple path towards high-quality GeSn/SiGeSn heterostructures. The method is applied for a precise control of the Sn concentration in different heterostructures. Examples of structures used for applications in electronics (MOSFETs,), photonics (Lasers), and thermoelectric (lattice thermal conductivity) are presented and discussed.