arXiv subjects

Leah Bergman

Publications and source records attributed to Leah Bergman.

3 recordsLinked to original sources

arXiv2026

Optical Properties of Indium-Gallium-Oxide Microcrystalline Alloy Films: From the Visible to the Deep-UV

The tailored optical properties of $(In_xGa_{1-x})_2O_3$ microcrystalline films were studied as a function of composition x via transmission, Urbach energy analysis, and spatial photoluminescence (PL) mapping of the self-trapped hole (STH) emission, with the objective of addressing material characteristics specific to this alloy system. Up to x = 0.46, the optical gap exhibited a redshift of 1 eV from the deep to the near-UV range, while the STH PL was redshifted by 0.5 eV in the visible range. For higher composition, x = 0.63, the transmission spectra indicated the co-existence of two optical gaps attributed to Ga-rich and to In-rich domains, implying that this sample is phase-separated. However, the saturation behavior of the optical gap and that of the STH PL showed that incipient phase separation occurs at a lower composition: x ~ 0.3. This is consistent with the compositional trend found for Urbach energy, implying that phase segregation in the alloys is a major defect even at its incipient stages. Additionally, Urbach analysis of $(In_xGa_{1-x})_2O_3$ was compared to that of $Mg_xZn_{1-x}O$. Both systems were found to have similar compositional dependence: at lower range, Urbach energies exhibited a negligible increase, while at the higher range a significant dependence on the composition was found. The main difference between the two alloy systems is in their Urbach energy: those for $(In_xGa_{1-x})_2O_3$ were significantly larger than those of $Mg_xZn_{1-x}O$. This stems from the strong hole coupling to phonons of $(In_xGa_{1-x})_2O_3$, which provides a dynamic transition additionally to that of defect-type.

cond-mat.mtrl-sci↗

arXiv2023

Ultra-Wide Bandgap Gallium Oxide Films: UV-Luminescence and Phonon Dynamics at Extreme Temperatures

$β$-Ga$_2$O$_3$ is a semiconductor with bandgap in the deep-UV ~ 5 eV. Due to its strong phonon-hole coupling, holes are self-trapped inhibiting bandgap luminescence at the deep-UV. In contrast, the self-trapped holes (STH) can exhibit a strong luminescence at ~ 3.5 eV. This research addresses the thermal response of the STH photoluminescence (PL), and the role of phonon interactions at temperatures 77 K - 622 K in nanocrystalline films. It was found that the PL intensity strongly diminishes as a function of increasing temperature with activation energy ~ 72 meV. A study of the Raman modes revealed that the intensity of the high frequency modes of the Ga$_I$O$_4$ site decrease with temperature, implying a phonon annihilation process. These modes, which have comparable energy to the STH activation energy, thus can couple to the STH and transition them from a radiative to a non-radiative regime in accordance with the configurational coordinate model at the strong phonon coupling limit. The significantly broad Gaussian linewidth of the PL is also a manifestation of a strong STH-phonon coupling. Furthermore, the peak position of the STH exhibited a negligible temperature response, in contrast to the redshift of ~ 220 meV of the bandedge of the film. In contrast to the intensity behavior of the high frequency Raman modes, the low frequency ones were found to follow the thermal Bose-Einstein phonon population.

cond-mat.mtrl-sci↗

arXiv2005

An Accurate Method for Measuring Activation Energy

In this letter, we present an accurate method for the measurement of activation energy. This method combined the excitation power dependent photoluminescence and temperature dependent photoluminescence together to obtain activation energy. We found with increasing temperature, there is a step transition from one emission mechanism to another. This step transition gives us an accurate measurement of activation energy. Using this new method we found the activation energy of the free exciton A in a GaN thin film is 24 meV. Our result also gives a reasonable explanation of the debate of the origin of the light emission in GaN at room temperature.

cond-mat.mtrl-sci↗