Congratulations to Dr. Adnan for passing his PhD defense exam!

MSE Ph.D. Presentation

Probing Exciton Physics in Wide Bandgap Materials to Understand the Absorption and Photoresponsivity Behavior with Applications

Md Mohsinur Rahman Adnan

Advisors: Profs. Roberto C Myers & Enam Chowdhury

Time: Thursday, June 20th, 2024, 10:00 AM – 12:00 PM
Place: Dreese Lab 260


An exciton is an electrically neutral quasiparticle that consists of an electron and a hole attracted to each other by the Coulombic force of attraction due to the opposite charge of the constituent pair. Exciton can form with the absorption of a photon into a material (insulator or semiconductor) as an intermediate state which can soon dissociate and produce photocarriers i.e., electron and hole under suitable condition. Normally a photon with above bandgap energy would get absorbed into any material of interest, but under an applied electric field a photon with below bandgap energy can be absorbed due to band bending as the wave function of the constituent electron and hole leak respectively from the Conduction Band and Valance Band into the forbidden region. This absorption process is known as the Franz-Keldysh (FK) effect. In Wide Bandgap Material (WBM)s such as Gallium Nitride and Beta-phase of Gallium Oxide the FK effect is dominated by the formation and later dissociation of exciton. The below bandgap photon absorption via intermediate exciton state can be a process of general interest to explain the below bandgap photoresponsivity of the WBM under study. The understanding of exciton physics in this context can be utilized to engineer important applications such as to accurately detect and quantify the local electric field and the onset of breakdown behavior of the material which can help in designing Radio Frequency (RF) and power electronic devices with high reliability and stability. This dissertation looks at the excitonic physics of Wide Bandgap Materials i.e., GaN and Beta Ga2O3 with the objective to explain the experimentally observed photoresponsivity characteristics through eXciton Franz Keldysh (XFK) effect. Application of the exciton mediated below bandgap photon absorption physics in mapping out the electric field variation of GaN p-n diode and Beta Ga2O3  Schottky diode with applied external voltage are also discussed to demonstrate the importance of such studies. In addition to these unusual electric field dependent absorption and photocarrier generation processes, Beta Ga2O3  also displays a striking polarization dependent optical absorption that is a far cry from any previously explored wide bandgap semiconductors. The anisotropy in optical absorption of Beta Ga2O3  has been demonstrated experimentally by varying light’s polarization orientation and measuring the anisotropic photocurrent. The origin of the polarization anisotropy are the non-zero off-diagonal terms within the complex dielectric tensor of this monoclinic crystal. There exist non-negligible off-diagonal components in both the real and imaginary parts of the dielectric tensor, especially in the near bandgap region. As a result, not only is the absorption polarization dependent, but so is the refractive index, resulting in a material that is both dichroic and birefringent. These physical properties have their origin in the energy shift of the excitonic transitions and the orientation anisotropy of the exciton dipole moments that change the absorption behavior and photon flux decay in Beta Ga2O3 . The decay behavior of the incident photon flux in this material is modified away from the well-known isotropic Beer-Lambert law; giving rise to depth dependent absorption behavior which itself arises from a depth dependent polarization rotation as well as optical dispersion. Understanding the highly selective polarization dependent solar-blind behavior of Beta Ga2O3 from the perspective of raw photoresponsivity calculated from Electro-Magnetic wave equation solution can confirm the observed experimental results and provide basis for engineering ultra-sensitive narrowband UV photodetectors.