Ph.D. Dissertation Defense Committee

• Committee Chair: Huabin Zhang
• Ph.D. Advisor: Omar F. Mohammed
• External Examiner: Dongping Zhong
• Committee Member: Shadi Fatayer

 

Abstract

The dynamics of charge carriers in semiconductor materials play a pivotal role in determining the performance of optoelectronic devices, such as

photodetectors and solar cells. Therefore, it is essential to understand the various aspects of charge carrier dynamics, including generation, separation, trapping, diffusion, and recombination at the semiconductor surface. This understanding is vital for advancing surface engineering techniques and ultimately enhancing the efficiency and effectiveness of these devices. 

It is well known that the aforementioned dynamics events special on materials surfaces occur in the ultrafast time-domains, from femtoseconds (1 fs = 10⁻¹⁵second) to picosecond (1 ps = 10⁻¹² second) scale. To effectively investigate these rapid processes, advanced time-resolved techniques with exceptional spatial resolution are essential. Among these techniques are ultrafast transmission electron microscopy (UTEM), transient absorption microscopy (TAM), and transient absorption spectroscopy (TAS). However, these methods predominantly yield bulk information due to the significant penetration depth of the incident beam, which limits their surface sensitivity. Fortunately, a groundbreaking technique known as ultrafast scanning electron microscopy (USEM) has been developed, effectively merging ultrafast time resolution with high surface sensitivity, achieving femtosecond (fs) timing and nanometer (nm) spatial resolution. This innovative approach employs a field emission gun within the microscope to generate a series of ultrashort electron pulses that probe the sample both before and after surface excitation, resulting in time-resolved snapshots of secondary electrons (SEs). In this thesis, we leverage the capabilities of ultrafast scanning electron microscopy (USEM) to explore the charge carrier dynamics at the surfaces of various semiconductor materials.

This thesis employs 4D-USEM to investigate surface charge carrier dynamics across various semiconductor systems. First, the effect of crystallographic orientation in single-crystal MAPbI₃ was examined. Significant differences in surface charge density and diffusion length were observed between the (001) and (110) facets, revealing how orientation influences carrier transport and recombination.

Moreover, mixed-cation perovskites FA_0.6MA_0.4PbI₃ and FA_0.4MA_0.6PbI₃ were also studied using 4U-SEM and density functional theory (DFT). In this work, the FA-rich composition showed longer charge carrier lifetimes and fewer surface defects. DFT calculations confirmed that increased FA content facilitates iodide ion migration toward the surface for defect passivation, revealing a link between composition, ion migration, and surface charge carrier behavior.

In addition to employing perovskites, we utilized combined femtosecond transient absorption spectroscopy (fs-TAS) and four-dimensional ultrafast electron microscopy (4D-USEM) to differentiate surface and bulk carrier dynamics in TeSe nanocomposites with varying Te:Se ratios. While fs-TAS indicated consistent bulk behavior across the samples, 4D-USEM revealed significant variations in surface dynamics. This highlights the sensitivity of surface behavior to compositional changes and defect structures, underscoring the importance of these factors in tailoring the properties of TeSe nanocomposites.

Overall, this thesis highlights the critical role of 4D-USEM in probing charge carrier dynamics at the surfaces of various semiconductor materials. It offers valuable insights into how crystalline structure, surface orientation, and termination affect charge carrier behavior. These findings are instrumental for the design and engineering of high-performance optoelectronic devices, emphasizing the need to consider these factors in advancing semiconductor technologies.