Calcium Impurities as Nonradiative Recombination Centers in InGaN*

Recent experimental work [1] discovered high concentrations of Ca in the active layer of GaN-based light emitting diodes grown by molecular beam epitaxy. The presence of Ca was correlated with a significant reduction in the efficiency of light emission. We have used first-principles calculations to examine the mechanisms by which Ca causes defect-assisted nonradiative recombination, often called Shockley-Read-Hall (SRH) recombination [2]. Calcium substituted on the cation site turns out to be a strong recombination center. Using hybrid density functional calculations we find that substitutional Ca acts as a deep acceptor with a level ~1 eV above the GaN valence-band maximum. Based on calculated nonradiative recombination rates we find that for Ca concentrations of 10¹⁷ cm^-3, the SRH recombination coefficient A in InGaN exceeds 10⁶ s^-1 for band gaps less than 2.5 eV. A coefficients of this magnitude lead to significant reductions in the efficiency of LEDs.

[1] E. C. Young et al., Appl. Phys. Lett. 109, 212103 (2016).
[2] J. X. Shen et al., Appl. Phys. Express 10, 021001 (2017).