European Materials Research Society (eMRS) Meeting 2022, Strasbourg, France, 30 May - 03 June 2022, pp.23
Germanium (Ge) is a promising candidate to serve as an infrared light emitter in the fully integrated, miniaturized infrared systems serving to a variety of fields ranging from biosensing to optical communication. Even though it is an indirect bandgap material in its bulk form, its emission efficiency can be enhanced utilizing different methods, such as Sn alloying, n-type doping, and application of tensile strain. Sn incorporation and tensile strain moves the emission wavelength towards the mid-infrared wavelengths of the spectrum making it possible to realize fully integrated systems for biosensing. On the other hand, utilization of the small amount of strain together with doping results in efficient light emission, and even lasing, at optical communication wavelengths. In this work, we developed a simple and CMOS-compatible method to fabricate undoped, tensile strained single crystal Ge microstructures, where the method is also suitable to fabricate n-type Ge, making the proposed fabrication flow to be applicable to develop Ge light emitters for different applications. The fabrication procedure of the structures relies on the deposition of the germanium and dielectric layers, such as silicon nitride (SiNx) and silicon dioxide (SiOx), all by using a physical vapor deposition, namely sputtering, which is a room-temperature-operation, environmentally-friendly deposition technique as compared to the chemical vapor deposition methods. The crystallization of the sputtered amorphous germanium is achieved via liquid phase epitaxy following a single rapid thermal annealing process, after which the top capping layer (SiNx) is converted into a stressor layer gaining an intrinsic tensile stress of around 1 GPa. A subsequent lithography process, which utilizes the underetching of the initially stressed nitride film, allows the strain transfer into the lithographically defined germanium microstructure. The amount of the underetching can easily be controlled by changing the duration of the wet etching process, which in turn determines the amount of the strain introduced into the Ge microstructures, as well as the emission wavelength. The method is suitable for both the fabrication of uniaxially and biaxially tensile-strained germanium microstructures, where uniaxial and biaxial strain levels of around 3% and 1.5%, respectively, has been demonstrated and verified via Raman spectroscopy and micro-photoluminescence (micro-PL) measurements, as well as the finite element method (FEM) simulations. Overall, our results show that strained germanium based light emitters for a variety of infrared applications can be realized on Si chips in a CMOS-compatible fashion utilizing cost-efficient and easy-to-use deposition (sputtering) and crystallization (liquid phase epitaxy) methods.