Tensilely strained germanium nanomembranes for infrared light emitting devices
Boztug Yerci, Cicek Hatice
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development of group-IV semiconductor lasers has attracted significant attention in recent years, since it represents the key missing ingredient for the large-scale monolithic integration of electronics and photonics in a CMOS-compatible fashion. The main challenge is to convert the indirect-bandgap group-IV materials into efficient light emitters. Many researchers have focused on improving the light emission efficiency of these materials in the near-infrared (NIR) spectral region, to replace the existing chip-to- chip communication technology with optical links. At the same time, group-IV lasers operating at mid-infrared (MIR) wavelengths also possess many important applications, mainly in the area of chemical and biological sensing, such as trace-gas detection, environmental monitoring, medical diagnostics, and industrial process control. Motivated by these applications, here I focus on improving the light emission efficiency of germanium (Ge). The small energy difference between its direct and indirect bandgaps can be further decreased with the introduction of tensile strain, leading to significantly improved radiative efficiency. At the same time, the bandgap energy shifts into the technologically important 2.1-2.5 µm MIR atmospheric transmission window. At 1.9% tensile strain, Ge even becomes a direct-bandgap semiconductor. In this work, tensile strain is introduced in Ge nanomembranes (NMs), i.e., single-crystal sheets with nanoscale thicknesses, through the application of mechanical stress. Our strain-resolved photoluminescence (PL) measurements performed on these NMs demonstrate a significant red-shift and enhancement in the emission spectra with increasing strain. PL measurement results obtained with a 24-nm-thick NM also reveal that the membrane is converted into direct-bandgap Ge with the application of 2% tensile strain. Furthermore, theoretical analysis of the high-strain PL spectra shows that population inversion can be achieved in these ultrathin NMs with gain values as high as 300 cm−1. Two-dimensional periodic structures fabricated on the top surface of such membranes result in further enhanced light collection through first-order diffraction of the in-plane emitted luminescence. Furthermore, the cavity modes of these periodic structures are also resolved in the strain-dependent PL spectra. These results are promising for the demonstration of Ge NM lasers operating in the technologically important 2.1-2.5 µm spectral region for potential applications in biochemical sensing and spectroscopy.
Thesis (Ph.D.)--Boston University