Ioannis Chatzakis
Texas Tech University, Lubbock, Texas, USA _______________________________________
Hexagonal boron nitride (hBN) is a wide, indirect bandgap semiconductor with great promise for optoelectronic devices in the ultraviolet and mid-infrared spectral regimes. The dynamic behavior of photogenerated carriers dominates the efficiency of optoelectronic devices. Here, we report on the dynamics of photoexcited free carriers in exfoliated 10B-enriched (99%) hBN at room temperature. By using ultrafast ultraviolet-pump near infrared-probe transient transmission spectroscopy, we can analyze the response of materials over very short time scales. We identify three characteristic recombination rates described by the so-called ABC model [1]. A slow recombination mechanism independent of the excitation fluence at a rate of A~3.9 × 109𝑠-1 was assigned to the Shockley-Read-Hall (SRH) term which is due to the impurities and defects in the lattice. At free carrier densities in the order of 1016𝑐𝑚-3, induced by the excitation pulse, we found that the dominant recombination mechanism is bimolecular with a characteristic rate constant B as of ~2.0 × 10-7𝑐𝑚3𝑠-1. At higher excitation densities ≥ 1017𝑐𝑚-3, the Auger recombination takes place. Auger recombination is one of the three mechanisms that determine the kinetics of photoexcited carriers, and it strongly contributes to losses in quantum efficiency, known as the droop effect and strongly depends on the excitation fluence. The characteristic constant rate C we deduced here is in the range of 10-24 𝑡𝑜 10-26 𝑐𝑚6𝑠-1 [2]. The recombination coefficients for the SRH and biomolecular mechanisms are consistent with earlier reported measurements on other semiconductors [3,4]. The Auger rate we found here is significantly larger than that in other nitride-based semiconductors [5] and is sufficient to reduce the internal quantum efficiency of hBN-based devices at high operating charge densities. The large values of the Auger coefficient deduced here can potentially be attributed to the localization of charges caused by defects and impurities and the built-in polarization field caused by the strain. The strain value is about -1.8%, induced by lattice mismatch between the sapphire and hBN layers, and found to be as large as ~5.9%, close to the theoretical calculation of 5.2 %.
References
[1] Kioupakis et al. New Journal of Physics 15, 125006 (2013)
[2] S. Sharma et. al., ACS Photonics 2023, 10, 3586
[3] Brendel, M.; Kruse, A.; Jönen, H.; et al. Auger recombination in GaInN/GaN quantum well laser structures. Appl. Phys. Lett. 2011, 99, 031106.
[4] Li, X.; Zhang, K.; Treu, J.; et al. Contactless Optical Characterization of Carrier Dynamics in Free-Standing InAs-InAlAs Core-Shell Nanowires on Silicon. Nano Lett. 2019, 19, 990-996.
[5] Kioupakis, E.; Steiauf, D.; Rinke, P.; Delaney, K. T.; Van De Walle, C. G. First-principles calculations of indirect Auger recombination in nitride semiconductors. Phys. Rev. B 2015, 92, 035207.
Email: ioannis.chatzakis@ttu.edu