Thermal Stability of Metastable GeSn Nanowires

The (Si)GeSn semiconductor material system is highly desired as a potential candidate for optical interconnects and monolithic photonic integration on Si. The demonstrated fundamental direct band gap of this complementary metal-oxide-semiconductor (CMOS) compatible semiconductor has sparked great interest due to its unique properties. Unfortunately, the low solid solubility of gray-Sn (α-Sn) in Ge (1 at.%) and the large lattice mismatch between α-Sn and Ge of approximately 14%, make it difficult to maintain its thermal stability. In fact, rapid thermal annealing (RTA) at temperatures above 400 °C leads to Sn segregation and island formation for Ge_0.9Sn_0.1[1]. Moreover, the low temperature growth of GeSn (<i>&lt;</i>350 °C) can promote the formation of many point defects (mostly divacancies) and act as <i>p</i>-type background impurities [2]. An often-observed background <i>p</i>-type doping of ~10¹⁶-10¹⁸cm⁻³ leads to strong electric field leakage in devices due to high parallel conductance. Consequently, it is highly desirable to enhance the thermal stability of this material. Herein, we present a method to counteract annealing-induced surface segregation of Ge core/GeSn shell nanowires grown by previously reported methods [3]. Surface passivation significantly improves the thermal stability of GeSn alloys through the atomic layer deposition (ALD) of a very thin Al₂O₃ oxide layer (&lt; 5 nm). Next, annealing the ALD-coated NWs at different temperatures (300°C, 350°C, 400°C, 450°C, and 520°C), above the growth condition of the NWs (~275°C) was systematically investigated. First, structural characterization (X-ray diffraction and scanning electron microscopy (SEM)) demonstrated the unchanged strain state and morphology after annealing (9±1 at. % Sn composition). Second, low-temperature (80K) infrared photoluminescence spectroscopy was undertaken on the annealed NWs to better quantify the recombination mechanisms after annealing. Interestingly, two key findings were observed: an average of 5-fold increase in the PL integrated intensity after annealing, and a blueshift of the band-to-band optical transition. This blueshift suggests that deep level traps are not a potential mechanism for such bandgap shift, as a bandgap contraction would be expected in that case. Understanding the mechanism behind such a blueshift is still an ongoing work. However, a plausible hypothesis can be linked to the nature of the short-range ordering in these materials, as it has been shown theoretically to be the case [4]. Third, single NW devices were fabricated to measure the resistivity variation before and after annealing. The resistivity increases from 0.28±0.02 Ω-cm for an as-grown NW to 0.42 ± 0.08 Ω-cm, when annealed at 450°C. The resistivity increase suggests a modest decrease in the p-type carrier density of the NW. The effect of annealing on the structural and optoelectronic properties of the GeSn NW are explored in this work. Further results on the optical bandgap and resistivity changes will be reported.<br/><br/>References:<br/>[1] R. Chen, et al., <i>Journal of Crystal Grow</i>th 365, 29 (2013)<br/>[2] S. Gupta <i>et al.</i>, <i>Applied Physics Letters</i>, 113, 022102 (2018)<br/>[3] A Meng, et al., Materials Today 40, 101 (2020).<br/>[4] X. Jin <i>et al.</i>, <i>Communications Materials</i>, 3, 66 (2022)