Shining Light on Short-Range Ordering in Group-IV Semiconductor Alloys

The (Si)GeSn semiconductor material system is a potential enabler of chip-to-chip optical interconnection and monolithic photonic integration on Si. The demonstrated direct band gap of this complementary metal-oxide-semiconductor (CMOS) compatible semiconductor alloy has sparked great interest. 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 thermal stability of metastable alloys in this system during post-deposition fabrication processes. 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 (<350 °C) can promote the incorporation of point defects (e.g., vacancies, divacancies and clusters) that, among other effects, act as p-type background impurities [2]. Consequently, it is highly desirable to enhance the thermal stability of this material. Herein, we develop a method to counteract annealing-induced surface segregation of Ge core/GeSn shell nanowires grown by reduced-pressure chemical vapor deposition [3]. Surface passivation significantly improves the thermal stability of GeSn alloys through the atomic layer deposition (ALD) of a very thin Al₂O₃ oxide layer (< 3 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, composition 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: up-to to 5-fold increase in the PL integrated intensity after annealing, and a blueshift of the band-to-band optical transition. This blueshift suggests that neither strain variation, nor Sn composition reduction, nor defect-mediated emission, nor deep level traps are a viable mechanism for such a bandgap blueshift. To understand the mechanism behind the blueshift, extended X-ray absorption fine structure (EXAFS) measurements were undertaken to quantify the degree of short-range ordering in GeSn after annealing. Previous ab-initio density field theory (DFT) calculation demonstrated that, as the degree of short-range order (SRO) increases in GeSn, the magnitude of the direct bandgap energy is expected to increase [4]. Exploring the relationship between light emission and local atomic structure in group-IV semiconductors shed lights on the fundamental properties of SRO in semiconductor alloys, which has hitherto been largely unexplored.
Acknowledgement: “This work is supported by the U.S. Department of Energy, under Award No. DEAC02-76SF00515, FWP100786, and Award No. DE-SC0023412, SubAward No. UA2023-351. This research is also supported by the U.S. National Science Foundation, Grant No. DMR-2003266. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.”

References:
[1] R. Chen, et al., Journal of Crystal Growth 365, 29 (2013)
[2] S. Gupta et al., Applied Physics Letters, 113, 022102 (2018)
[3] A Meng, et al., Materials Today 40, 101 (2020).
[4] B. Cao et al., ACS Appl. Mater. Interfaces 12, 57245–57253 (2020)