Optoelectronics Investigations of Electron Dynamics in 2D-TMD Semiconductor Heterostructure Photocells: From Electron-Hole Pair Multiplication to Phonon Assisted Anti-Stokes Absorption
A remarkable prediction of relativistic quantum mechanics is the ability of particle-antiparticle pairs to be created and annihilated when interacting with other high- energy particles. Efficient electron-hole (e-h) pair generation could lead to highly sensitive photodetectors, electroluminescent emitters, and improved-efficiency photovoltaic devices. In this thesis, using advanced optoelectronic measurements, I discuss the discovery of highly efficient multiplication of interlayer electron-hole pairs at the interface of a tungsten diselenide / molybdenum diselenide integrated into a field-effect heterojunction device. Electronic transport measurements of the interlayer current-voltage characteristics indicate that layer indirect electron-hole pairs are generated by hot electron impact excitation at temperatures near T=300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrated near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels.
Spatial imaging as a function of wavelength and spectral imaging with a narrow resolution of 1nm establishes a high level of understanding on the fundamentals of this system through e-h pair multiplication process and enables us to isolate the interlayer exciton species through a phenomenon called phonon assisted antistokes process which identifies an entirely new light-matter interaction within these states. Under optimized experimental conditions phonon assisted anti-stokes absorption near the interlayer exciton edge of a van der Waals semiconductor heterostructure composed of tungsten diselenide and molybdenum diselenide becomes dominant, such that at low photon energies near 1eV, a strong photocurrent peak with several low energy echoes spaced by 30meV below this fundamental absorption feature is observed. This highly efficient process due to the alignment of the exciton dipole moment to the atomic displacement of the out-of-plane optical phonon modes, marks the first and most critical step toward laser cooling of atomic layer semiconductors. Moreover, it could enhance the efficiency of next generation photovoltaics, since it converts vibrational energy into electronic excitations using photons with energies that are lower than the band gap, manuscript is in preparation.