High-Field Charge Transport and Fluctuation Phenomena in Semiconductors from First Principles

Citation

Hatanpää, Benjamin Henrik James (2025) High-Field Charge Transport and Fluctuation Phenomena in Semiconductors from First Principles. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fc5a-5276. http://resolver.caltech.edu/CaltechTHESIS:09162024-212439874

Abstract

Charge transport and dynamics in semiconductors determine the limits of contemporary high-performance electronic devices. Previously, in order to understand the microscopic mechanisms underlying charge transport, and to efficiently find novel materials for new applications, computational methods were limited to using parameterized scattering rates and simplistic band structure models as inputs. However, with ab-initio methods, only the atomic identities and lattice vectors are needed as inputs. These methods have the capability of providing insights not possible with methods that rely on empirical data, and predicting properties for not-yet-synthesized materials.

While ab-initio computation of low-field transport properties have become common in recent years, these methods have not been extensively applied to non-equilibrium phenomena. In addition, the ab-initio simulation of fluctuational properties (such as the diffusion coefficient or power spectral density of current fluctuations) is an area that has been minimally explored. In order to approach quantum-limited noise levels in devices, a better understanding of the mechanisms that govern electronic noise away from equilibrium is needed.

Thus, motivated by this, the overarching goal of this work is to develop and use first-principles methods to gain insight into the scattering processes that govern high-field electronic transport and noise in well-known semiconductors, and to use the same approach to make predictions and identify promising device applications for novel materials.

The warm electron tensor is a quantity that describes the quadratic change of conductivity with electric field, which provides a quantitative way to examine the heating of the electron gas. However, this has not been examined from first-principles previously. In this work, we report the warm electron tensor of n-Si computed over a large temperature range, and find that the most commonly used order of perturbation theory only captures the qualitative change of the warm electron tensor with angle. However, by including the next-to-leading order two-phonon scattering term in our approach, we find near-quantitative agreement. This finding indicates that two-phonon scattering has a non-negligible role to play in transport in nonpolar semiconductors.

We continue our investigation of n-Si by examining the diffusion coefficient and its anisotropy by applying our Boltzmann transport framework to fluctuational variables. We find that the qualitative features of the anisotropy are correct, but its magnitude is greatly underestimated in comparison to experimental data, while the onset of the noise is overestimated. While this suggests an incorrect description of f-type scattering in our work, by computing the frequency dependence of the diffusion coefficient as well as the piezoresistivity (two observables sensitive to the balance of f- and g-type scattering), we find that the qualitative agreement of these two observables with experiment shows that such a discrepancy cannot be due to an incorrect description. Instead, we suggest that the experiment contains charge transport phenomena not accounted for by our electron-phonon scattering framework.

Finally, we use the same approach to investigate the high-field transport and noise in the novel ultra-wide-bandgap semiconductor cubic boron nitride (c-BN). While c-BN is known for its excellent mechanical and thermal properties, its high predicted saturation velocity and breakdown field make it a promising candidate in high-power and high-frequency devices. However, very few experimental and theoretical studies have probed its transport properties. Here, we show that c-BN exhibits a negative differential resistance (NDR) region below 140 K, and show that the cause is due to an abrupt valley repopulation effect with applied electric field. We also show that the intervalley time in c-BN is extremely large, on the order of diamond, and that this large intervalley time causes a distinct noise peak, most prominent at low temperatures. We discuss how the NDR region and large intervalley time make c-BN a potential candidate for transferred-electron devices and Gunn oscillators, respectively.