arXiv subjects

Dallin O. Nielsen

Publications and source records attributed to Dallin O. Nielsen.

5 recordsLinked to original sources

arXiv2026

On the electronic and vibrational dimensionality of nanometer-scale silicon structures

We discuss the problem of assessing the electronic and vibrational dimensionality of a semiconductor nanostructure: How thin and/or wide must a nanostructure be in order to induce electron and phonon confinement? Clarifying the physical justification for common criteria found in the literature, we view the electron coherence length (defined as the electron and phonon inelastic mean free path) as their `field of view' and argue (or, better yet, `speculate') that this sets the important length scale. Considering the example of Si nanosheets at room temperature, and drawing from results found in the literature, we estimate that the critical length below which electrons are subject to quantum confinement is of the order of (or smaller than) 8 nm, when their coherence length is determined by energy losses to phonons and remote phonons in gated structures. On the contrary, no single length-scale can be given for phonons: Taking their coherence length as determined by scattering with electrons and anharmonic three-phonon processes, short wavelength acoustic and optical phonons may be confined only by structures as small as 10 nm. Long-wavelength acoustic phonons, instead, may exhibit a coherence length of the order of 1 micrometer, so that they may be confined over much larger distances.

cond-mat.mes-hall↗

arXiv2026

Electron transport in a 1.6~nm-thick double-gated (100) silicon nanosheet: A theoretical study accounting for phonon confinement and remote-phonon scattering

We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of phonons and the presence of interface hybrid plasmon-phonon excitations (IPPs or `remote phonons'). The band structure is calculated using local empirical pseudopotentials; an approximated elastic continuum model is used to consider the confinement of acoustic phonons; the dielectric continuum limit is used to deal with the IPPs. We find that the electron mobility is affected significantly by the boundary conditions chosen to deal with phonon confinement. The more realistic assumption of phonons clamped at the SiO2/HfO2 interfaces and optical phonons at the Si/SiO2 interfaces results in a room temperature mobility much smaller than what is obtained using the common assumption of bulk phonons in the elastic, high-temperature approximation. We also find that, as a result of the complicated structure of the primed subbands, the high-field saturated velocity is significantly lower than its bulk value, as it had been measured in the past in the case of Si inversion layers but never explained theoretically. Finally, we find that IPP scattering does depress the low-field mobility but to a small extent, thanks to the presence of the interfacial SiO2 layers and to the proximity of the metal gates. Moreover, by keeping electrons `cooler', IPP scattering results in a higher saturated velocity. Therefore, the presence of high-kappa materials in the gate-insulator stacks should not affect negatively the performance of field effect transistors based on Si nanosheets.

cond-mat.mes-hall↗

arXiv2026

On the static dielectric constant of thin dielectrics in extremely scaled silicon nanosheet transistors

We argue that the static dielectric constant of small (thin and/or narrow) semiconductor and insulator nanostructures depends strongly on the their environment. We do so by considering the electronic response simply reviewing, briefly but critically, the existing literature. Regarding the ionic response, in addition to reviewing the literature, we use a simple model to account for the confinement of optical phonons in thin films and show that the reduction of their density of states has a negligible effect on the dielectric constant, in contrast to some claims found in the literature. In general, we argue that in realistic structures, such as double-gated Si nanosheets, the use of the bulk dielectric constants for both the channel and the gate insulators, is justified.

cond-mat.mes-hall↗

arXiv2023

Thermalization of radiation-induced electrons in wide-bandgap materials: A first-principles approach

The present study is concerned with simulating the thermalization of high-energy charge carriers (electrons and/or electron-hole pairs), generated by ionizing radiation, in diamond and $β$-Ga$_2$O$_3$. Computational tools developed by the nuclear/particle physics and electronic device communities allow for accurate simulation of charge-carrier transport and thermalization in the high-energy (exceeding $\sim100$ eV) and low-energy (below $\sim10$ eV) regimes, respectively. Between these energy regimes, there is an intermediate energy range of about 10-100 eV, which we call the "10-100 eV gap", in which the energy-loss processes are historically not well-studied or understood. To close this "gap", we use a first-principles approach (density functional theory) to calculate the band structure of diamond and $β$-Ga$_2$O$_3$ up to $\sim100$ eV along with the phonon dispersion, carrier-phonon matrix elements, and dynamic dielectric function. Additionally, using first-order perturbation theory (Fermi's Golden Rule/first Born approximation), we calculate the carrier-phonon scattering rates and the carrier energy-loss rates (impact ionization and plasmon scattering). With these data, we simulate the thermalization of 100-eV electrons and the generated electron-hole pairs by solving the semiclassical Boltzmann transport equation using Monte Carlo techniques. We find that electron thermalization is complete within $\sim0.4$ and $\sim1.0$ ps for diamond and $β$-Ga$_2$O$_3$, respectively, while holes thermalize within $\sim0.5$ ps for both. We also calculate electron-hole pair creation energies of 12.87 and 11.24 eV, respectively.

cond-mat.mtrl-sci↗

arXiv2023

A first-principles approach to closing the "10-100 eV gap" for charge-carrier thermalization in semiconductors

The present work is concerned with studying accurately the energy-loss processes that control the thermalization of hot electrons and holes that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover thermalization in the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has studied extensively carrier transport in the low-energy range (below ~10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV (the "10-100 eV gap") are poorly known. The aim of this research is to close this gap, by utilizing density functional theory (DFT) to obtain the band structure and dielectric function of GaN for energies up to about 100 eV. We also calculate charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission), using the DFT results and first-order perturbation theory. With this information, we study the thermalization of electrons starting at 100 eV using the Monte Carlo method to solve the semiclassical Boltzmann transport equation. Full thermalization of electrons and holes is complete within ~1 and 0.5 ps, respectively. Hot electrons dissipate about 90% of their initial kinetic energy to the electron-hole gas (90 eV) during the first ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remaining energy is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, during the thermalization, the maximum electron displacement from its original position is found to be on the order of 100 nm.

cond-mat.mtrl-sci↗