Study of GeSn based heterostructures: towards optimized group IV MQW LEDs
Silicon-based lasers have long been desired owing to the possibility of monolithic integration of Si-based photonics with high-speed Si-based electronics. An optically pumped laser based on direct bandgap GeSn grown on Si was successfully demonstrated by D. Buca’s group in early 2015 with a pulsed operation at a cryogenic temperature. Therefore, the development of GeSn-based lasers has been naturally directed towards obtaining a low injection threshold and high operating temperature, which require the use of quantum wells as the active region. In this Optics Express article, the same team provides a careful study of some advanced device structures, such as double heterostructures and multiple quantum wells (MQW), to understand if there is sufficient carrier confinement for them to be used in high performance GeSn lasers. The authors characterized MQW light emitting diodes grown via industry available reactive gas source epitaxy and concluded that the indirect bandgap of GeSn and the lack of type-I alignment are responsible for the relative low emission efficiency. The authors conducted further in-depth modeling work for different structures and eventually discovered that employing ternary SiGeSn as barriers for direct bandgap GeSn wells could offer sufficient band offset for carrier confinement. The proposed SiGeSn/GeSn/SiGeSn MQW structures with 20 nm Si0.08Ge0.81Sn0.11 barriers and 10 nm Ge0.88Sn0.12 wells feature a direct bandgap Ge0.88Sn0.12 and a type-I alignment with a band offset larger than 25 meV (≈1 kT at room temperature). Detailed growth study indicates that the proposed structure can be grown with high optical quality, showing its potential in future light emitting device applications.