Semester

Spring

Date of Graduation

2021

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Alan Bristow

Committee Member

Matthew Johnson

Committee Member

Tudor Stanescu

Committee Member

Konstantinos Sierros

Abstract

Chalcopyrite crystals in the II-IV-V2 family have received significant interest due to their high nonlinearity, composition-tunable bandgaps, wide transparency windows, and high damage threshold. These semiconductors have been explored for electromagnetic (EM) screening, spintronic and photovoltaic applications, making them good optical and optoelectronic materials. This thesis uses terahertz spectroscopy to understand optical, electronic, and vibrational dynamical processes in CdGeP2, ZnGeP2 and CdSiP2 chalcopyrite semiconductors.

We have employed Terahertz time-domain spectroscopy to investigate temperature-dependent ground-state properties of bulk chalcopyrite crystals that can be related to electronic transport and electron-lattice interactions. The complex spectra provide refraction and absorption from which electron-phonon coupling and average phonon energies are extracted. AC conductivity spectra provide carrier densities and electron scattering times, the temperature dependence of which are associated with unintentional shallow dopants. Temperature dependence of the scattering time is converted into carrier mobility and modeled with microscopic transport mechanisms such as polar optical phonon, acoustic phonons, deformation potential, ionized impurity, and dislocation scattering. Hence, analysis links the terahertz response to properties that can only be obtained using high-frequency AC Hall-effect measurements, Raman scattering, and conventional continuous-wave spectroscopy.

Ultrafast photoexcited charge-carrier dynamics are investigated using time-resolved terahertz spectroscopy. Analysis of the differential THz transient reveals a two-component exponential relaxation, with decay times that increase with pump fluence due to state filling. Inverting the transients recaptures the canonical rate-equation for the system and from which the recombination dynamics are attributed to the entire excitation range. To support the recombination information, THz photoconductivity determines the excited-state carrier transport as a function of temperature. Both elevated temperatures and increased excitation density decreases excited-state carrier mobility and are attributed to carrier-carrier interactions and scattering from phonons.

These model semiconductors, which have application in photonics, allow for further demonstration of pulsed-THz-based spectroscopy to capture both charge transport and dynamics. Additionally, several charge transport and carrier-lattice properties for CdGeP2, ZnGeP2, and CdSiP2 have been reported that can assist with the design and optimize of devices that comprise them.

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