Strained Germanium Nanomembrane Characterization Roberto Paiella , Trustees of Boston University, DMR 0907296

1. Direct-Bandgap Light-Emitting Germanium Pumped Above Transparency in Tensilely Strained NanomembranesRoberto Paiella, Trustees of Boston University, DMR 0907296 Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Genanomembranes (NMs) can be used to overcome this limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that sufficient tensile strain can be introduced in mechanically stressed NMs to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, and capable of supporting population inversion as required for providing optical gain. Fig. 1. (a) Optical micrograph of a Ge NM. (b) Schematic diagram of the sample mount used to strain the NMs. (c) Strain-dependent photoluminescence spectra of a 40-nm-thick Ge NM, showing enhanced light-emission efficiency with increasing strain. (d) Photoluminescence spectrum at 1.78% tensile strain, and theoretical fit demonstrating population inversion.

2. Strained Germanium Nanomembrane Characterization Roberto Paiella, Trustees of Boston University, DMR 0907296 During the past year, the Lagally group at UW – Madison employed several undergraduate research assistants, with one of them, Mike Ritt, a junior in Materials Science and Engineering, specifically working on activities related to this project. Mike is shown in the picture at left along with graduate student Jose Sanchez Perez. Jose is a Fellow in the UW – MadisonGraduate Engineering Research Scholars program (http://gers.engr.wisc.edu/), which aims to attract graduate students from under-represented groups into engineering disciplines. He is the primary UW graduate student involved on this grant in the project described in Slide 1. Two additional GERS scholars in the Lagally group perform research on projects unrelated to this grant. Graduate student Jose Sanchez Perez showing Mike Ritt, an undergraduate laboratory aide, the results of straining Ge nanomembranes that Jose prepared and Mike analyzed.