Material science of Rydberg Excitons in cuprous oxide

Albeladi, Aisha 2024. Material science of Rydberg Excitons in cuprous oxide. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Cu2O is a promising material for quantum technologies due to its ability to host Rydberg excitons with high principal quantum numbers. These excitons are particularly interesting because their large Bohr radius allows them to behave as microscale objects which can be manipulated and transported over longer distances within the material. This is essential for integration of quantum technologies into larger scale system. A key question motivating this research is: why do synthetic Cu2O crystal exhibit poorer excitonic properties compared to natural, especially when high principal quantum number excitons are only observed in natural material? The objective of this thesis is to investigate sub-bandgap defect states, considering them as a factor preventing the creation of high principal quantum number excitons in synthetic Cu2O therefore, limiting its technological use. This is based on one of the hypotheses that high concentrations of charge impurities in synthetic material introduce internal electric field that distort the exciton wave function, limiting the highest observable quantum number excitons. This thesis addresses the defect detection through a combination of electrical and optical techniques of synthetic single crystal Cu2O. These techniques include temperature dependent Hall measurements, room temperature spectrally resolved photoconductive response to pulsed nanosecond infrared laser light and mid-infrared (MIR) and Terahertz (THz) spectroscopy across a wide temperature range. In the electrical characterization phase, Ohmic contacts with low specific contact resistance were successfully optimized and fabricated, allowing precise Hall measurements and photoconductivity measurements and hence assessing the intrinsic properties of Cu2O accurately. Therefore, the specific contact resistance (ρc) was tested between different metallization schemes using a unique CTLM design and Au-Cu2O without adhesion layer was chosen as an optimal contact with specific contact resistance of 1.698×10-2 Ωcm-2. Then the Hall-measurement device was fabricated using a Van der Pauw configuration and Au contacts. The temperature dependence Hall-measurements revealed key insights into material’s conductivity, activation energy, and Hall mobility. The temperature dependent conductivity analysis indicates two different activation energies at 0.2 and 0.1 eV corresponding to different conduction mechanisms across the temperature range which could indicate presence of different defect states in sub-bandgap. Also, carrier concentration measurements confirmed that the material has two activation energies, and the two acceptor states occur at approximately the same value as the previously obtained from conductivity measurements. Hall mobility showed minimal variation across the studied temperature range. Phonon scattering and neutral impurities were found to be the dominated mechanisms limiting Hall mobility. Our photoconductivity study used a novel technique to investigate the sub-bandgap defect states in sub-bandgap. This measurement was performed using pulsed wavelength-tuned laser photoconductivity spectroscopy, at room temperature. This time-resolved measurement was key to distinguishing between different optical process in Cu2O by analysing the time-dependent photoresponse, which showed two different photoconductance recovery timescales, of the order of nanoseconds and microseconds. Different excitation-wavelength dependent resonance photoresponse was observed in the sub-bandgap photoconductivity spectrum at 820nm, 900nm, 930nm, and 980nm which is consistence with known impurity features such as copper and oxygen vacancies. We present a detailed investigation of the infrared and terahertz spectral properties of Cu₂O. By comparing our experimental measurements with the calculated phonon spectrum, we identify and explain, for the first time, the mid-infrared (MIR) absorption features observed between 100–115 meV and 135–155 meV as higher-order replicas of the Reststrahlen bands. Furthermore, no additional absorption features were detected in the MIR or THz spectra of both synthetic and natural samples, suggesting the absence of shallow impurity states in this material. Our findings provide new insights into the fundamental optical properties of Cu₂O.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	Rydberg excitons, Synthetic vs. natural Cu₂O, Sub-bandgap defect states, Hall mobility, Ohmic contacts, Specific contact resistance (ρc), Photoconductivity, Mid-infrared (MIR) spectroscopy, Terahertz (THz) spectroscopy.
Funders:	Self funded
Date of First Compliant Deposit:	17 March 2025
Last Modified:	17 Mar 2025 12:19
URI:	https://orca.cardiff.ac.uk/id/eprint/176907

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