| Summary: | 碩士 === 國立中興大學 === 精密工程學系所 === 103 === Flexible InGaN-based green light-emitting diodes (LEDs) were fabricated using laser lift-off (LLO) and twice epilayer transferring techniques. However, the piezoelectric polarization field induced by the lattice mismatch between the InGaN and GaN layers, which owns high indium content in the multiple quantum wells (MQWs), causes a band bending in the c-oriented MQWs, which results in electron overflowing out of the active region. Therefore, this study focused on the current-voltage (I-V), light output power, electroluminescence (EL), photoluminescence (PL), Raman, and XRD characteristics of the InGaN-based green LEDs on a flexible polyimide substrate under bending were discussed.
The forward voltages of the flexible thin-film LED, the non-bending flexible LED and substrate LED at an injection current of 20 mA were 2.37, 2.39 and 2.31 V, respectively. The devices on three type LEDs show typical diode characteristics, however the flexible device shows a little higher operating voltage compared to the substrate LED. These differences result from the final design of device structure and the stress in epilayers after the transferring process.
Raman spectra are dominated by the E2 (TO)-high mode phonons of the banding to Lc=15 cm of flexible thin-film LED, the banding to Lc=10 cm of flexible thin-film LED, non-bending flexible LED, and substrate LED at 567.0, 566.4, 567.6 and 568.2 cm-1, respectively. Obviously, the flexible thin-film LED shows more tensile stress because of the external bending using the twice epilayer transferring.
Without being applied stress, tensile stress and compressive stress results of the simulation, the energy gap were 2.31, 2.28 and 2.32 eV, respectively. It was shown that the piezoelectric polarization of LED could be relaxation due to the external stress and direction after the transfer process.
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