Flexible GE/GESN photodetectors with enhanced performance

• Main Author: An, Shu
• Other Authors: Kim Munho
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering::Microelectronics
• Online Access: https://hdl.handle.net/10356/165574
• Institution: Nanyang Technological University

Flexible optoelectronics have been regarded as one of the most promising candidates for wearable and healthcare applications. Among them, flexible near infrared (NIR) photodetectors (PDs) have attracted much attention due to their unique advantages (e.g., flexibility, lightweight) in optical communication. Ge and GeSn are potential materials for NIR PDs because of their tunable and narrow bandgap, fast carrier mobility, and complementary metal-oxide-semiconductor (CMOS) capability. Furthermore, the Ge/GeSn nanomembranes (NMs) with high absorption capability and strong bendability are beneficial for fabricating flexible devices. However, current flexible Ge/GeSn PDs still face the challenges of low responsivity and limited detection range. This thesis is devoted to the development of flexible Ge/GeSn PDs with high responsivity and extended detection range. Firstly, flexible Ge vertical p-i-n PDs with resonant cavity structures were fabricated by direct flip transfer of Ge NMs on polyethylene terephthalate (PET) substrates. The vertical cavity structure consisting of bottom gold and top SU-8 layers as a reflector and an anti-reflection surface, respectively, could enhance the absorption and responsivity in the NIR region (i.e., 1520-1640 nm). The fabricated PDs exhibited a clear rectifying behavior and low dark current density under flat condition. In addition, tensile and compressive strains were introduced by convex and concave bending fixtures to explore the effect of mechanical bending on device performance. Furthermore, mechanical durability was confirmed after multiple bending cycles. In order to improve responsivity and broaden detection wavelength range, GeSn NMs were adopted to fabricate novel electronic and photonic devices by taking advantages of bandgap modulation and enhanced absorption coefficients. As an essential factor for tuning the energy band structure, the strain state of flexible devicesXVI needs to be determined precisely. Raman spectroscopy is a powerful and convenient tool for characterizing strain. However, the lack of Raman coefficient makes it difficult to determine the accurate values of the uniaxial strain in GeSn alloys. Here, we investigated the Raman-strain function of uniaxial strained GeSn NMs. Flexible GeSn NMs with different Sn compositions were fabricated by the growth of GeSn layers on SOI followed by transfer printing on PET substrate, which was proved to be strain-free by relaxing internal strain between the GeSn layer and handling substrate. Then, external strains were introduced by bending fixtures with various radii, resulting in a series of uniaxial tensile strains. Raman peaks were measured under all strain conditions along <100> and <110> directions. The linear coefficients of Raman-strain for Ge0.96Sn0.04 and Ge0.94Sn0.06 were measured along <100> and <110> directions, respectively. As a result, the experimental ratio of linear coefficient (ROLC) was verified to be Sn composition independent. In addition, the compositional dependence of phonon deformation potentials (PDPs) was analyzed qualitatively. This result contributes to a better understanding of the relationship between Raman shift and uniaxially strained GeSn. Thirdly, flexible GeSn metal–semiconductor–metal (MSM) PDs were demonstrated on GeSn NMs released from GeSn on insulator (GeSn OI) substrates. The effect of mechanical uniaxial strain on optoelectronic properties was also investigated. The PDs were fabricated from transfer-printed GeSn NMs on PET substrates. Uniaxial strains along <1 0 0> direction were introduced into the GeSn PDs under bend-down (uniaxial tensile strain) and bend-up (uniaxial compressive strain) conditions, and their values were measured by Raman spectroscopy. The applied strain can affect the band structure of the GeSn alloy, resulting in modulation of the electrical and optical characteristics of the PDs. Accordingly, dark current characteristics show an increase from 8.1 to 10.3XVII mA under the bend-down conditions and a decrease to 7.2 mA under the bend-up conditions, respectively. The optical responsivity at a wavelength of 2 µm increased by 151% under bend-down conditions, while it decreased by 35% under bend-up conditions. In addition, a theoretical study was performed to explore the mechanism of the responsivity enhancement. To improve the performance of flexible GeSn PDs in terms of photodetection range and photo response magnitude, flexible TiN/GeSn PDs based on sub-bandgap absorption were proposed. Single-crystalline GeSn NMs transfer-printed on PET were integrated with plasmonic TiN to create Schottky contact in TiN/GeSn heterojunction. The low Schottky barrier height (SBH) could extend the coverage wavelength and enhance the light absorption capability. The responsivity of the fabricated TiN/GeSn PDs increased from 30 to 148.5 mA W−1 at 1550 nm. There was also a ∼180 nm extension in the optical absorption wavelength. The mechanism of enhanced performance and the effect of external uniaxial strain were investigated. Our results provide a robust and cost-effective method to extend the NIR photodetection capability of flexible group IV PDs. In summary, three approaches to develop high performance flexible Ge/GeSn PDs and the effect of strain on the performance were investigated. The research demonstrated that flexible Ge/GeSn PDs could improve responsivity and increase the wavelength detection capability to 2 µm.