Intraband optical absorption in conventional and inverted core–shell quantum dots with a parabolic potential well

The finite difference method and density matrix theory are employed to investigate the intraband optical absorption characteristics of two types of core–shell quantum dot (CSQD) structures with a parabolic potential well: InGaN/GaN CSQDs and GaN/InGaN inverted CSQDs (ICSQDs). The impacts of the mixed crystal and quantum size effects on the optical absorption coefficients (OACs) are analyzed. The results indicate that both effects significantly influence the position and intensity of the absorption peaks. The peaks exhibit a non-monotonic variation with core radius and shell thickness: for In0.2Ga0.8N/GaN CSQDs with 3 nm shell thickness, the peak shifts from ∼120 to 300 meV and then to 290 meV as the core radius increases from 1 to 4 nm under the incident light intensity of 3 × 109 W/m2. A similar trend is observed in GaN/In0.2Ga0.8N ICSQDs. Increasing the In composition from 0.1 to 0.4 induces a monotonic blueshift: in CSQDs, the peak shifts from about 130 to 560 meV, while in ICSQDs, it shifts from ∼210 to 440 meV. CSQDs exhibit a broader tunability of the absorption spectrum (with the peak position varying from about 120 to 560 meV) compared to ICSQDs (about 210–440 meV). A thinner barrier, wider well, and larger In composition can enhance the peak magnitude of both structures. Under the same conditions, the GaN/In0.4Ga0.6N ICSQD structure exhibits a larger OAC, with its peak value being ∼1.6 times that of the In0.4Ga0.6N/GaN CSQD structure. These findings provide quantitative guidelines for optimizing InGaN-based QD devices.