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Figure 1c shows the smooth surface morphology of the etched surface after the ELO process, which reveals the absence of the dome-shaped profile. In contrast to the linear grading, a newly optimized abrupt In xAl 1– xAs graded buffer was grown carefully by increasing the In flux from 0.1 to 1.8 Å/s while the Al flux was fixed at a low growth rate of 0.1 Å/s. To address this issue, an abruptly graded In xAl 1– xAs buffer was introduced for Sample B by varying the composition of the graded buffer layer as shown in Figure 1b. This is not acceptable for highly performing PD fabrication because the inhomogeneous thickness of the ELO InAs thin film would have unpredictable device performance with a different infrared absorption depth. Etching the AlAs sacrificial layer was tested using the HF-based solution, and it was found that the linearly graded In xAl 1– xAs buffer was only partially etched and the interface of the InAs/InAlAs layers was quite uneven as shown in Figure 1c. We have increased the In flux from 0.1 to 1.8 Å/s, while the Al flux was decreased from 1.8 to 0.1 Å/s simultaneously. (22) Sample A was grown with a typical linearly graded In xAl 1– xAs buffer on an AlAs sacrificial layer as shown in Figure 1b. In contrast to the GaAs/AlAs/GaAs ELO structure, an In xAl 1– xAs graded buffer was needed to bridge the large lattice mismatch (∼7.2%) between InAs and GaAs as well as to reduce the TDD in the InAs device layer.
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The schematic diagram in Figure 1a illustrates the heteroepitaxial lattice-mismatched InAs device structure on the GaAs substrate. Therefore, the growth of sophisticated buffer layers to reduce the TDD needs to be done for maintaining high performance while simultaneously allowing the ELO process via HF solution for flexible infrared III–V-based PD arrays for various emerging applications. For example, the heteroepitaxial growth of InAs ( E g = 0.36 eV) or InSb ( E g = 0.17 eV) on GaAs suffers from the extremely large lattice mismatch to GaAs, ∼7.2 and 14.6%, and this results in high threading dislocation density (TDD). (18,19) To date, previous flexible III–V PD works have been performed for only “visible” and “near-infrared” wavelengths using the lattice-matched GaAs/AlAs on GaAs wafer or InGaAs/InAlAs on InP wafer, (14,20) but no studies have been reported in the mid-wavelength (3–5 μm) infrared spectral region mainly due to the difficulties of heteroepitaxial growth and ELO process of low-band-gap materials. (8,15−17) In contrast, epitaxial lift-off (ELO) of III–V compound semiconductor thin films enabled flexible and reliable optoelectronic devices such as flexible GaAs solar cells and lasers. However, organic-based PDs still showed lower efficiency, inferior reliability, and slow operation speed compared to inorganic semiconductor-based PDs. Organic-based semiconducting materials have been extensively studied for flexible PDs due to their intrinsic material property.
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This flexible InAs photodetector enabled by the heteroepitaxial lift-off method shows promise for next-generation thermal image sensors. Also, our flexible InAs photodetectors showed excellent optical performance with high mechanical robustness, a peak room-temperature specific detectivity of 1.21 × 10 9 cm-Hz 1/2/W at 3.4 μm, and excellent device reliability. An abruptly graded In xAl 1– xAs (0.5 < x < 1) buffer was found to drastically improve the lift-off interface morphology and reduce the threading dislocation density twice, compared to the conventional linear grading method. Here, we demonstrate high-detectivity flexible InAs thin-film mid-infrared photodetector arrays through high-yield wafer bonding and a heteroepitaxial lift-off process. For wearable health monitoring and implanted biomedical sensing, transfer of active device layers onto a flexible substrate is required while controlling the high-quality crystalline interface. Current infrared thermal image sensors are mainly based on planar firm substrates, but the rigid form factor appears to restrain the versatility of their applications.