Simultaneous Crystallization and Strain Induction Enable Light-Emitting Germanium Nano/Microbridges for Infrared Lasers

Abstract

Tensilely strained germanium has been considered a suitable material platform for the realization of a monolithically integrated infrared laser that could allow the development of miniaturized photonic integrated circuits. The crystalline quality of germanium is one of the concerns in this regard since it has to be in the high-quality single-crystal form to endure the required amounts of tensile strain so that the material turns into a gain medium. For that purpose, various researchers have developed tensilely strained Ge nano/microstructures fabricated from a high-crystalline-quality germanium-on-insulator substrate or an epitaxially grown germanium film on silicon, where the fabrication of germanium relies on costly processes (i.e., molecular beam epitaxy, metal–organic chemical vapor deposition). Here, we introduce a methodology to fabricate tensilely strained single-crystalline suspended Ge microstructures through a room-temperature-operated, easy-to-use, environmentally friendly physical vapor deposition technique, sputtering. A single rapid thermal annealing process allows both the crystallization of the sputtered Ge microstructures via liquid phase epitaxy and transforms the capping layer into a stressor. The dimensions of the microstructures, as well as the amount of strain transferred from the stressor, can be easily adjusted by varying the duration of the corresponding wet etching processes. Suspended germanium microstructures with lengths varying between 2.5 and 20 μm are fabricated, and uniaxial strain levels as high as 2.4% are transferred to microstructures along the [110] direction as demonstrated via Raman spectroscopy. The fabricated microstructures demonstrate room-temperature light emission in agreement with the strain profile calculated via finite element method simulations. The methods introduced in this work are suitable to fabricate moderately doped Ge, as well, with nanoscale dimensions for high strain transfer, which could enhance the gain coefficient and enable Ge to serve as the gain medium of a fully integrated CMOS-compatible laser.