B21-O-15Strain in Si/Ge Heterojunction Nanowires

Abstract

Axial heterojunction Si/Ge or Si/Ge/Si nanowires are potentially useful for a range of applications, such as tunnel field-effect transistors and thermoelectric devices. The key to get the optimal device performance is the formation of compositionally abrupt and defect-free heterojunction interfaces. Recent nanofabrication methods greatly improve the quality of the heterointerfaces, and sharp interfaces can be formed between two materials with a large lattice mismatch, e.g. the formation of Si/Ge (4.2% lattice mismatch) [1] and GaAs/InAs (7% mismatch) [2]. In such structures, the coherent strain can effectively modify the electronic properties of the materials. Therefore, measuring the strain distribution and probing the physical and electronic properties of heterojunctions with sufficiently perfection are of great importance for future applications. In this study, we use transmission electron microscope to analyze Si/Ge heterojunction nanowires with the interfacial abruptness less than 2 nm. We use the geometrical phase analysis to evaluate the strain distribution near the heterointerface [3]. The strain near the interface on either the Si side or Ge side can be as large as 2%, which is not easily produced in other structure geometry. Electron energy loss spectroscopy with the monochromated electron beam is used to measure the band structure change of the Si and Ge lattices with different degrees of strain near the interface. [1] C-Y Wen et al, Science 326 (2009) 1247-1250. [2] Ohlsson et al, Physica E 13 (2002) 1126-1130. [3] C-Y Wen et al, Nano Letters (2015) 1654-1659. B21-O-14 doi:10.1093/jmicro/dfv144