Research Of Free-standing Tensilely Strained Germanium Nanomembrane Light-emitting Diodes

Over the past years, active photonic devices for Si-based optoelectronic integrated circuits(OEIC) based on Ge are extensively studied. Ge hasbecome an interesting candidate for active photonic devices on OEIC due to its pseudo-directgap behavior and compatibility with Si complementary metal oxide semiconductor(CMOS) technology. An efficient Si-based light source is one of themost important components of OEIC.In this paper, the band structure of strained Ge is studied using the simulation software DFTB+(density functional tight binding). The result of the simulation shows that biaxial tensile strain makes Ge more direct bandgap like because it shrinks the direct bandgap more than the indirect bandgap of germanium as a result of the differencein deformation potentials of the direct ? valley and the indirect L valleys. Moreover, the mechanism of high stress in silicon nitride thin film is studied systematically in this paper. The effects of the various process parameters on the stress in silicon nitride thin film deposited by PECVD are analyzed and discussed. The silicon nitride thin film with high compressive and tensile stress has been deposited on the optimized process parameters and the compressive and tensile stress are up to-1.38 GPa and 866 MPa, respectively. A new LED based on Ge freestanding NM has been design in this paper. Simulation of this tensile strained Ge free-standing NM LED is presented in this paper. It shows that the strained Ge LED works well with the optimized device parameters. The influence of P/N region doping concentration, intrinsic layer, and strain on the performance of this new LED has also been studied. It is necessary to keep the designed strained Ge LED with heavy P/N doping concentration, appropriate intrinsic layer thickness, and about 1.9% levels of biaxial tensile strain.Based the simulation of Ge free-standing NM LED, we use highly strained silicon nitride thin film as the external stressor to fabricate tensilely strained Ge free-standing NM LED. This approach is quite flexible, both the highly compressive and tensile stress in Ge NM can be obtained with the adjustable process parameters. Here, biaxially tensile strain is introduced by highly strained silicon nitride thin film to transform Ge into a direct-bandgap material with strongly enhanced radiative efficiency. We have obtained up to 1.92% biaxial tensile strained Ge NMs with a clear electroluminescence(EL) spectra. The EL intensity of Ge NMs LEDs increases with higher tensile strain, resulting from the shrinking of bandgap caused by tensile strain. Room temperature EL shows a dramatic intensity increase for the 1.92% tensile strained Ge, confirming the existence of direct bandgap transition in Ge NMs.