arXiv subjects

Felipe Murphy-Armando

Publications and source records attributed to Felipe Murphy-Armando.

5 recordsLinked to original sources

arXiv2025

First-Principles Calculation of Spin-Relaxation Due to Alloy and Electron-Phonon Scattering in Strained GeSn

GeSn has emerged as a promising material for spintronics due to its long spin-lifetime, compatibility with silicon technology, high mobility and tunable electronic properties. Of particular interest is the transition from an indirect to a direct band gap with increasing Sn content, which enhances optical properties, electron transport and we find also affects spin transport behaviour, which is critical for spintronics applications. We use first-principles electronic-structure theory to determine the spin-flip electron-alloy scattering parameters in n-type GeSn alloys. We also calculate the previously undetermined intervalley electron spin-phonon scattering parameters between the $L$ and $\Gamma$ valleys. These parameters are used to determine the electron-alloy and electron-phonon scattering contributions to the n-type spin-relaxation of GeSn, as a function of alloy content and temperature. As in the case of phonon scattering, alloy scattering reduces the spin-relaxation time. However, switching the spin transport from the typical $L$ valley of Ge to the $\Gamma$ valley by sufficient addition of Sn, the relaxation time can be substantially increased. For unstrained, room temperature GeSn, we find a Sn concentration of at least $10\%$ is required to achieve a spin-relaxation time greater than Ge, with $17\%$ Sn needed to increase the spin-relaxation time from the nanosecond range to the microsecond range. At low temperatures (30K), adding $10\%$ Sn can increase the spin-relaxation time from $10^{-7}$s to 0.1s. Applying biaxial tensile strain to GeSn further increases the spin-relaxation time and at a lower Sn content than in unstrained GeSn.

cond-mat.mtrl-sci↗

arXiv2024

First-Principles Calculation of Alloy Scattering and n-type Mobility in Strained GeSn

We use first-principles electronic-structure theory to determine the intra- and inter-valley electron-alloy scattering parameters in n-type GeSn alloys. These parameters are used to determine the alloy scattering contributions to the n-type electron mobility of GeSn at $300K$ and $15K$ using a first iteration of the Boltzmann transport equation in the relaxation time approximation. For unstrained GeSn, we find that a Sn concentration of at least $13.5\%$ is needed to achieve an electron mobility greater than that of Ge. Our results show that the mobility of GeSn can be over $25$ times higher than the mobility of Ge, or $10^5$ cm$^2$/(Vs). At $15K$, less than $6\%$ Sn incorporation into Ge quadruples its mobility, which suggests GeSn has potential applications as a high mobility 2D electron gas. Applying biaxial tensile strain to GeSn further increases the mobility and at a lower Sn content than in unstrained GeSn.

cond-mat.mtrl-sci↗

arXiv2019

Ultrafast relaxation of symmetry-breaking photo-induced atomic forces

We present a first-principles method for the calculation of the temperature-dependent relaxation of symmetry-breaking atomic driving forces in photoexcited systems. We calculate the phonon-assisted decay of the photoexcited force on the low-symmetry $E_g$ mode following absorption of an ultrafast pulse in the prototypical group-V semimetals, Bi, Sb and As. The force decay lifetimes for Bi and Sb are of the order of $10$ fs and in good agreement with recent experiments, demonstrating that electron-phonon scattering is the dominant mechanism relaxing the symmetry-breaking forces. Calculations for a range of absorbed photon energies suggest that larger amplitude, symmetry-breaking atomic motion may be induced by choosing a pump photon energy which maximises the product of the initial $E_g$ force and its lifetime. We also find that the high-symmetry $A_{1g}$ force undergoes a partial decay to a non-zero constant on similar timescales, which has not yet been measured in experiments. We observe that the imaginary part of the electron self-energy, averaged over the photoexcited carrier distribution, provides a reasonable estimate for the decay rate of symmetry-breaking forces.

cond-mat.mtrl-sci↗

arXiv2018

Acoustic Deformation Potentials of $n$-Type PbTe from First Principles

We calculate the uniaxial and dilatation acoustic deformation potentials, $Ξ^{\text{L}}_{u}$ and $Ξ^{\text{L}}_{d}$, of the conduction band L valleys of PbTe from first principles, using the local density approximation (LDA) and hybrid functional (HSE03) exchange-correlation functionals. We find that the choice of a functional does not substantially affect the effective band masses and deformation potentials as long as a physically correct representation of the conduction band states near the band gap has been obtained. Fitting of the electron-phonon matrix elements obtained in density functional perturbation theory (DFPT) with the LDA excluding spin orbit interaction (SOI) gives $Ξ^{\text{L}}_u = 7.0$~eV and $Ξ^{\text{L}}_d = 0.4$~eV. Computing the relative shifts of the L valleys induced by strain with the HSE03 functional including SOI gives $Ξ^{\text{L}}_u = 5.5$~eV and $Ξ^{\text{L}}_d = 0.8$~eV, in good agreement with the DFPT values. Our calculated values of $Ξ^{\text{L}}_u$ agree fairly well with experiment ($\sim 3-4.5$~eV). The computed values of $Ξ^{\text{L}}_d$ are substantially smaller than those obtained by fitting electronic transport measurements ($\sim 17-22$~eV), indicating that intravalley acoustic phonon scattering in PbTe is much weaker than previously thought.

cond-mat.mtrl-sci↗

arXiv2017

Strain-induced effects on band-to-band tunnelling and trap-assisted tunnelling in Si examined by experiment and theory

Strain is commonly used in metal-oxide-semiconductor technologies to boost on-state performance. This booster has been in production for at least a decade. Despite this, a systematic study of the impact of strain on off-state leakage current has been lacking. In this work we use experimental data and ab-initio calculations to refine existing models to account for the impact of strain on band-to-band tunnelling and trap-assisted tunnelling in silicon. We observe that the strain may dramatically increase the leakage current, depending on the type of tunnelling involved. For band-to-band and trap-assisted tunnelling, low uniaxial strains of 0.1% (or 180 MPa) can increase the leakage current by 60% and 10% compared to the unstrained case, respectively. Using our models, we predict that compressive strain on the order of 1% (or 2 GPa) can increase the leakage current by 150 times. Conversely, tensile strain may diminish or at most double the leakage current in all observed cases. Though detrimental in conventional inversion-mode MOSFETs, these processes may be used to boost the performance of Tunnel Field Effect Transistors, where on-state current is defined by band-to-band tunnelling.

cond-mat.mes-hall↗