Two-dimensional III-VIA semiconductors and their applications in optoelectronic devices.

• Autores: Qinghua Zhao
• Directores de la Tesis: Andres Castellanos Gomez (dir. tes.), Riccardo Frisenda (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2020
• Idioma: inglés
• Tribunal Calificador de la Tesis: Herre van der Zant (presid.), Cristina Gómez-Navarro (secret.), Juan Francisco Sánchez Royo (voc.), Thomas Mueller (voc.), Ferry Prins (voc.)
• Programa de doctorado: Programa de Doctorado en Física de la Materia Condensada, Nanociencia y Biofísica por la Universidad Autónoma de Madrid; la Universidad de Murcia y la Universidad de Oviedo
• Materias:
  • Física
  • Ciencias de la vida
    • Biofísica
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • Two-dimensional (2D) semiconductors, initialed by the isolation of graphene in 2004, have drawn a great research interest thanks to their remarkable mechanical, electrical, optical, and optoelectronic properties. Their layered structure, originating from the strong in-plane covalent bonds and weak out-of-plane van der Waals interactions, allows such materials being stable with atomically thin geometry. Thanks to the ultrathin nature and dangling-bond-free surface, various advanced heterostructures-based devices with superior performance have been demonstrated without being hampered by their lattice mismatch, which make them as promising candidates for future nano-electronic and optoelectronic applications. As novel 2D members from III – VIA semiconducting group, Gallium selenide (GaSe) and Indium selenide (InSe), are barely explored but recently both reach exciting promise in theory achievements and application research. Thanks to their unique electron band structures and strong light-matter interactions, they both are sensitive to external stimuli, which can be advantageous for certain applications but also brings drawbacks for others, thus motivates the goal of this thesis that exploring how the environment, light and strain play roles on the properties of these materials.

    Two dimensional materials are especially sensitive to the environmental atmosphere and to external stimuli due to their layered structures and large surface-to-volume ratio. Thus, in the first part of this thesis, the interaction between air species and thin GaSe and InSe flakes has been discussed. Thin GaSe can be degraded completely after being exposed in the air for several days, during which macro- and micro-scope surface morphology evolution, chemical composition variation, and laser-induced degradation, as well as how it leads to the photodetector breakdown as a function of exposure time in air have been fully presented. On the contrary, thin InSe shows a different environmental interaction mechanism: the performance of InSe photodetector can be significantly modified, and even reaching a long-term stable behavior in air, without obviously changing the surface morphology and crystal lattice. Both these observations are related to the interaction between air species (e.g. O2 and H2O) and the defects (e.g. selenium vacancies) in the materials. The failure of GaSe photodetectors can be attributed to a full transformation from crystalline GaSe to amorphous Ga2O3 by air species, while the performance variation of InSe devices may be related to the passivation of selenium vacancies in the system. Based on these understandings, both long-term stable photodetectors based on thin GaSe and InSe have been realized using hexagonal boron nitride (h-BN) encapsulation protection (for GaSe) or controllable air exposure (for InSe).

    Thanks to the dangling-bond-free surface of 2D semiconductors and the deterministic transfer methods, one can fabricate pure van der Waals (vdW) metal-semiconductor interfaces, without direct chemical covalent bonding that typically leads to chemical disorder and Fermi-level pinning (FLP). This kind of vdW metal-semiconductor is thus an ideal system to study the Schottky junctions. The defects passivation effect in InSe introduced by air species not only can modify the material properties but also reduces the fermi level pinning at the metal-InSe contact interface. Various Schottky contact based on thin InSe with different van der Waals electrical contacts have been investigated. The Schottky barrier height at the interfaces of Au-InSe, Pt-InSe and InSe-Gr (graphite) are determined to be approximately 460 meV, 540 meV and less than 100 meV, respectively. Taking advantage of the large contact barrier difference, the transport properties of Schottky diodes based on engineered asymmetric van der Waals contacts of thin InSe, including Au-InSe-Gr and Pt-InSe-Gr, have been investigated.

    The dangling-bond-free nature of the surface of 2D materials leads to high mechanical resilience against mechanical deformation. This has motivated a whole sub-field of research focused on using mechanical deformation to tune the electronic properties of 2D materials. The mechanical properties of GaSe and InSe were barely explored at the moment of the elaboration of this thesis and thus to decide to measure the Young’s modulus of InSe to access its suitability in strain engineering applications. Using buckling metrology method, the Young’s modulus of InSe is experimentally determined to be 23 ± 5 GPa, which makes thin InSe one of the most flexible 2D materials. Subsequently, the biaxial strain tunability of thin InSe, including piezoresistance effect, band gap modulation, and strain engineered optoelectronic devices has been discussed. Interestingly, it is further demonstrated that how the strain tunable band gap can be exploited to tune the spectral response of InSe photodetectors.

    This thesis demonstrates the great promise of 2D III-VIA semiconducting materials, especially GaSe and InSe, for future electrical and optoelectronic applications. These results, on the one hand, reveal the important role of traps induced by defects in tailoring the properties of devices based on 2D materials, on the other hand, show the reliability of electronic and optoelectronic properties of van der Waals Schottky contacts, which are both attractive for different applications, such as strain engineering and flexible electronics.