First-principles Density Functional Perturbation Theory Research Articles

Effect of spin-polarization on structural, electronic, and lattice dynamical properties of ‘MnY2Ga’ Full Heusler alloy

In this paper, lattice dynamical properties and other properties such as structural, electronic band structure, and density of states of Full Heusler alloy ‘MnY2Ga’, are studied by using first-principles density functional perturbation theory and density functional theory. The calculated spin-polarization ratio for MnY2Ga is found to be −31.6%, which signifies its false half-metallic character. The phonon dispersion spectra, phonon density of states, and different phonon modes have been determined for the first time using non-spin and spin-polarized calculations and discussed in detail. The lattice dynamical calculations confirm the cubic phase (L21) of MnY2Ga at the ambient conditions of pressure and spin plays a crucial role in vibrations of phonons. The present work reveals that spin-modifies the atomic interactions and structures, and results to give a flexible lattice, which will encourage further applications for the design of functional materials.

Phononic bandgap and phonon anomalies in HfN and HfN/ScN metal/semiconductor superlattices measured with inelastic x-ray scattering

Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector (q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

(Invited) Thermal Transport in 2D Materials from First Principles: The Role of Interface and Substrate

The persistent down-scaling of nanostructures, such as electronic devices, sensors, NEMS, and nanocomposites, increases the surface-to-volume ratio and introduces atomic-scale disorder at boundaries and interfaces. To avoid these issues, the nanoelectronics community has turned to intrinsically two-dimensional (2D) materials platforms. 2D materials have tremendous potential for next-generation nanoelectronics beyond the 5 nm technology node due to their atomic flatness and absence of dangling bonds, which prevents scattering from interface roughness. However, heat dissipation and its removal from hot spots in the monolayer remains a critical concern to the design of 2D-based devices [1]. Thermal currents flowing in an atomic layer can either dissipate through source/drain contacts in a transistor configuration, or through a supporting substrate via van der Waals (vdW) coupling to it. When a 2D material is supported by a substrate, the interfacial area formed between it and the substrate is often far larger than the lateral source/drain contact area. Thus, the majority of waste heat is removed across the 2D-substrate interface and then via the substrate.The thermal boundary conductance (TBC) between the 2D layer and substrate should be well characterized for reliable 2D device performance. Interfaces formed between 2D vdW materials and 3D substrates are fundamentally different than same-dimension 3D-3D and 2D-2D interfaces due to the presence of a vdW gap and the different dimensionalities of the phase spaces on either side of the interface. In this invited talk, I will review the progress in understanding lattice thermal transport, both in-plane and cross-plane, in 2D mono and few-layer materials from first principles. This work builds on vibrational properties of the 2D materials calculated from Density Functional Theory combined with Boltzmann transport equation for phonons. Then I will introduce our recent work aimed to tackle the question of selecting the best substrate for each 2D material from the point of view of heat dissipation and apply these results to 2D devices. Several recent papers measured the TBC between various monolayers and mostly the silicon dioxide (SiO2) substrate, reporting a wide range of values due to inconsistent sample quality. Therefore, it is imperative to build predictive methods for quantifying the TBC between MLs and various substrates. Here, we use a combination of phonon dispersions from first-principles density functional perturbation theory simulations and our 2D-3D TBC model [2, 3]. We investigate the TBC between combinations of six atomic layers (h-BN, graphene, MoS2, MoSe2, WS2, and WSe2) and six substrates (SiO2, AlN, GaN, 6H-SiC, diamond, and Al2O3). We show that TBC is higher for softer substrates with smaller speed of sound, but of the 6 substrates we compared, amorphous SiO2 consistently produced higher TBC than crystalline substrates. Our work helps build a roadmap for quantifying the TBC between various 2D monolayers and their substrates and provides a framework for other 2D-3D interfaces to be studied.Next, we employ our first-principles model to calculate the TBC of several beyond-graphene 2D materials, such as and blue and black phosphorene, on amorphous and crystalline substrates. A trend emerges that 2D materials with lower ZA branch frequencies have higher TBCs when placed on a-SiO2. Our results provide selection criteria for 2D materials that improve interfacial heat transport in 2D devices with amorphous and crystalline substrates.To further probe the role of interfacial phonon transport, we develop a coupled electro-thermal model for FL-WSe2 stacks where we simultaneously solve for the current and Joule heating by treating the stack a resistor network. The resulting rise in temperature is obtained from a FL-TBC model [4] to shed light on self-heating and heat dissipation in such devices. We find that the temperature rise in the top layers is significantly larger than the bottom layers because the bottom layers have higher TBC and conduct heat more efficiently to the substrate [4,5]. The higher temperature of top layers, in turn, significantly reduces their mobility, which is strongly temperature-dependent as they are shielded by the bottom layers from substrate impurity scattering. We also uncover that, unlike monolayer FETs, a significant amount of heat is dissipated laterally through the contacts in FL devices at high VDS due to relatively large thermal healing lengths of the top layers.

(Invited) Self-Heating in Advanced CMOS-Compatible and 2-Dimensional Semiconductor Devices

For many years the computer industry has relied for progress on the stellar rate of scaling of MOSFETs in integrated circuits. The usual expectation, based on Moore's law, is that the number of transistors packed on a chip doubles every eighteen months. Sustaining this pace requires aggressive research into the numerous bottlenecks that threaten to slow it down. In the past decade, an issue has emerged that threatens to impose an absolute limit on how many transistors can be packed onto a die. This is the issue of heat dissipation. The persistent down-scaling of nanostructures, electronic devices, sensors, and NEMS increases the surface-to-volume ratio and introduces atomic-scale disorder at boundaries and interfaces, which scatters heat carriers (phonons) and increases thermal resistance.In this invited talk, I will present my recent work on numerical simulation and modeling of heat dissipation and phonon transport in a broad range of nanostructures, focusing primarily on extrinsic and disorder effects such as grain/sample boundaries, interfaces, edges, and alloy mass disorder. The talk revolves around the phonon Boltzmann transport equation (pBTE), while the salient feature of the work is that it employs a full phonon dispersion computed from first principles and combined with a momentum-dependent model of electron and phonon scattering. I will show that the thermoelectric response of SOI and Si-membrane-based nanostructures can be improved by employing the anisotropy of the lattice thermal conductivity, revealed in ultrathin SOI nanostructures due to the interplay between the anisotropy of the phonon dispersion and the strong boundary scattering. Next, I explore the consequences of nanostructuring on silicon/germanium and Si/Si-Ge alloy superlattices, and show that the drastic reduction and high anisotropy of thermal conductivity comes from the increased interaction of lattice waves with rough interfaces and boundaries. I will close the loop with applications of my phonon transport models to thermal effects in ultrascaled gate-all-around and junctionless MOS devices.Going beyond silicon CMOS, two-dimensional (2D) materials have tremendous potential for next-generation nano- and optoelectronics. However, heat dissipation and its removal from hot spots in the monolayer remains a critical concern to the design of 2D-based devices. When a 2D material is supported by a substrate, the interfacial area formed between it and the substrate is often far larger than the lateral source/drain contact area. Thus, the majority of heat is removed across the 2D-substrate interface and then via the substrate. Interfaces formed between 2D vdW materials and 3D substrates are fundamentally different than same-dimension 3D-3D and 2D-2D interfaces due to the presence of a vdW gap and the different dimensionalities of the phase spaces on either side of the interface. In this invited talk, I will review the progress in understanding lattice thermal transport, both in-plane and cross-plane, in 2D mono and few-layer materials.Several recent papers measured the TBC between various monolayers and mostly the silicon dioxide (SiO2) substrate, reporting a wide range of values due to inconsistent sample quality. Therefore, it is imperative to build predictive methods for quantifying the TBC between MLs and various substrates. Here, we use a combination of phonon dispersions from first-principles density functional perturbation theory simulations and our 2D-3D TBC model. We investigate the TBC between combinations of six atomic layers (h-BN, graphene, MoS2, MoSe2, WS2, and WSe2) and six substrates (SiO2, AlN, GaN, 6H-SiC, diamond, and Al2O3). We show that TBC is higher for softer substrates with smaller speed of sound, but of the 6 substrates we compared, amorphous SiO2 consistently produced higher TBC than crystalline substrates.Another route to improving mobility and boosting drive current in 2D devices is to replace the monolayer with few-layered (FL) TMDs. Despite recent advances on demonstrating improved electrical performance of FL TMD FETs, there has been less attention towards their thermal management, which is crucial for modern nanoelectronic devices. We study heat dissipation in FL-WSe2 stacks using a coupled electro-thermal model and Raman thermometry experiments. The resulting rise in temperature is obtained from a FL-TBC model to shed light on self-heating and heat dissipation in such devices. We find that the temperature rise in the top layers is significantly larger than the bottom layers because the bottom layers have higher TBC and conduct heat more efficiently to the substrate. The higher temperature of top layers, in turn, significantly reduces their mobility, causing the electrical current to reroute toward the bottom layers where the TBC is higher. This rerouting improves heat removal and limits mobility degradation. We conclude that the optimal number of layers is 6-9 and that FL TMD devices are inherently thermally resilient.

First principles study on the stability and thermodynamic properties of N[sbnd]Au co-doped graphene

Based on first-principles density functional perturbation theory and first-principles molecular dynamics (MD) method, the effects of N and Au dopants as well as doping concentration on the structure stability, thermal stability and thermodynamic properties of graphene were systemically studied. It is found that the formation energy, phonon dispersion curve and MD simulation analysis indicate that N-doped graphene is more stable than Au and NAu co-doped graphene at the ground state or room temperature (300 K). The graphene doped with N, Au and NAu pair at different doping concentration can significantly affect its thermodynamic properties. Regardless of low or high concentrations, the specific heat and entropy of the doped graphene system increase as temperature increases, whereas free energy decreases. The results provide a theoretical basis for regulating the thermal conductivity of graphene and for relevant developing device applications.

First-principles analysis of vibrational modes of calcite, magnesite and dolomite

Calcite, magnesite and dolomite are important in Earth's evolutionary history, so they are significant in sediments and biominerals. As we know, the experimental investigations of calcite, dolomite, magnesite with Raman spectra are widespread. In order to show microprocess about Raman spectra, the first-principles vibration calculations deserve further discussion. So, the first-principles density functional perturbation theory is employed to calculate the phonon dispersion, IR spectrum, Raman spectrum, thermodynamic properties and dielectric permittivity. The vibration modes of phonon dispersion curves are investigated through symmetry system. The vibration type from group theory is used to discuss the IR activity and Raman activity, where the minerals space group are R3¯c, R3¯c and R3¯ for clacite, magnesite and dolomite, respectively. The calculated phonon dispersions show that minerals with ionic and covalent bonds are dynamically stable. The vibration analysis of IR modes and Raman modes reveals the strongest covalent bond in (001) plane, and it can produce bigger shear modulus. In addition, the differently cationic masses have a little effect on thermodynamic and dielectric properties for minerals. The investigated results in this paper can promote the analysis of minerals at microscopic level, where the information of vibration can expound the stably combinative direction of electron cloud of minerals. So it can present the macroscopically mechanical property, which would help researchers to research earth mantle.

Quantifying thermal boundary conductance of 2D–3D interfaces

Heat dissipation in next-generation electronics based on two-dimensional (2D) materials is a critical issue in their development and implementation. A potential bottleneck for heat removal in 2D-based devices is the thermal pathway from the 2D layer into its supporting substrate. The choice of substrate, its composition and structure, can strongly impact the thermal boundary conductance (TBC). Here we investigate the temperature-dependent TBC of 42 interfaces formed between a group of six 2D materials and seven crystalline and amorphous substrates. We use first-principles density functional perturbation theory to calculate the full phonon dispersion of the 2D layers and substrates and then input them into our model for phonon transport across the 2D–3D interface. Our results show that the TBC depends on the overlap between the vibrational frequencies and can be varied by nearly two orders of magnitude, from as low as ∼0.6 MW · m K−1 (h-BN on diamond) to ∼40 MW · m K−1 (h-BN on SiO2), for the same 2D layer by changing the substrate material. We find that amorphous materials significantly boost the TBC relative to their crystalline counterparts, assuming the two interfaces have the same adhesion, owing to the low-frequency Boson peak feature in their vibrational density of states (vDOS). For crystalline substrates, we further correlate constituent material properties with the calculated TBCs and find that the TBC strongly depends on a combination of the speed of sound, Debye temperature, and density of the substrate as well as the bandwidth of the flexural branch in the 2D material. We conclude that softer substrates with sharp low-frequency features in their vDOS, such as amorphous materials, polymers, and nanoparticles, could have higher TBC, leading to a trade-off between TBC and the thermal conductivity of the substrate.

Enhancement of thermoelectric figure-of-merit of graphene upon BN-doping and sample length reduction

Using first-principles density functional perturbation theory based calculations of length-dependent lattice thermal conductivity (κL) and using our previously calculated results [R. D’Souza and S. Mukherjee, Phys. Rev. B 95, 085435 (2017)] of electrical transport, we report results of thermoelectric figure-of-merit (ZT) of monolayer and bilayer graphene. We find a nearly tenfold increase in ZT for the graphene sample doped with boron nitride and reduced sample length. We also compare κL calculated using the iterative real space method with conventional analytical Callaway-Klemens method and obtain the flexural (ZA) phonon modes to be dominant in thermal transport unlike in the latter method. Our calculations are in good agreement with available experimental data.

First-principles analysis of a molecular piezoelectric meta-nitroaniline.

The piezoelectric and elastic properties of a molecular piezoelectric meta-nitroaniline (mNA) in its single-crystal form were investigated in the framework of first-principles density functional perturbation theory (DFPT). Results support the recent experimental findings those despite being soft and flexible, mNA's piezoelectric coefficients are an order of magnitude greater than that of ZnO and LiNbO3. A molecular-level insight into the piezoelectric properties of mNA is provided. These results are helpful not only for better understanding mNA, but also for developing new piezoelectric materials.

Local inversion symmetry breaking and spin-phonon coupling in the perovskite GdCrO3

Our detailed temperature dependent synchrotron powder x-ray diffraction studies along with first-principles density functional perturbation theory calculations, enable us to shed light on the origin of ferroelectricity in GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar distortion is driven by local symmetry breaking and by local distortions dominated by Gd off-centering. Our study reveals an intimate analogy between GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less displacive compared to Y, which results in an orthorhombic P na21 structure in GdCrO3 in contrast to monoclinic structure in YCrO3 and consequently, decreases its polar property. This is due to the subtle forces involving Gd-4f electrons either directly or indirectly. A strong magneto-electric coupling is revealed using Raman measurements based analysis in the system below Cr-ordering temperature, indicating their relevance to ferroelectric modulation.

Vibrational modes in the Pmc21 structure of ZnGeN2

Abstract Vibrational normal modes at the Brillouin zone center and related Raman and infrared spectra are calculated from first-principles density functional perturbation theory for the until now hypothetical Pmc21 structure of ZnGeN2 and are compared with those in the known Pnb21 structure. Although there are great similarities in phonon spectra, we show that the distinctions are specific enough to experimentally distinguish the two crystal structures.

Density functional theory study of structural, electronic, and thermal properties of Pt, Pd, Rh, Ir, Os and PtPd X (X = Ir, Os, and Rh) alloys

The structural, electronic, mechanical, and thermal properties of Pt, Pd, Rh, Ir, Os metals and their alloys PtPdX (X = Ir, Os and Rh) are studied systematically using ab initio density functional theory. The groundstate properties such as lattice constant and bulk modulus are calculated to find the equilibrium atomic position for stable alloys. The electronic band structure and density of states are calculated to study the electronic behavior of metals on making their alloys. The electronic properties substantiate the metallic behavior for all studied materials. The firstprinciples density functional perturbation theory as implemented in quasi-harmonic approximation is used for the calculations of thermal properties. We have calculated the thermal properties such as the Debye temperature, vibrational energy, entropy and constant-volume specific heat. The calculated properties are compared with the previously reported experimental and theoretical data for metals and are found to be in good agreement. Calculated results for alloys could not be compared because there is no data available in the literature with such alloy composition.

Dynamical instability and Fermi surface topology inNi2FeGa from first principles

The phonon spectrum of stoichiometric Heusler alloy ${\text{Ni}}_{2}$FeGa is calculated for the high-temperature cubic austenite phase by using first-principles density functional perturbation theory. We also compute the elastic constants of the alloy from the initial slopes of the acoustic phonon branch. The ${\text{TA}}_{2}$ phonon branch along [110] direction shows softening with a minimum dip at $\ensuremath{\zeta}=0.58$ which indicates the possibility of modulated phases prior to martensitic transformation. We also map the Fermi surface of this alloy both in 3D and 2D to check the presence of any nesting vectors. The observed nesting parameter is in good agreement with the above value of the wave vector in the [110] direction where phonon softening occurs.

Lattice dynamics and Raman spectrum of rutile TiO2: The role of soft phonon modes in pressure induced phase transition

Using the plane-wave pseudopotential technique based on the first-principles density functional perturbation theory (DFPT), we have studied the vibrational properties and Raman susceptibility tensor at ambient and high pressure of rutile phase of TiO2. Full phonon dispersion curves and phonon densities of states with projected phonon density of states and Raman tensors at high pressures are calculated and given. It is found that rutile TiO2 shows a pressure induced phase transition, especially when lattice dynamical instabilities are involved, like the soft phonon modes, at a hydrostatic pressure lower than 10 GPa. An analyses of the vibrational displacements is given. The possibility to use Raman line intensities as an additional tool in the study of phase transitions is also discussed.

Effect of Interface Structure on Thermal Boundary Conductance by using First-principles Density Functional Perturbation Theory

We choose a Si/Ge interface as a research object to investigate the influence of interface disorder on thermal boundary conductance. In the calculations, the diffuse mismatch model is used to study thermal boundary conductance between two non-metallic materials, while the phonon dispersion relationship is calculated by the first-principles density functional perturbation theory. The results show that interface disorder limits thermal transport. The increase of atomic spacing at the interface results in weakly coupled interfaces and a decrease in the thermal boundary conductance. This approach shows a simplistic method to investigate the relationship between microstructure and thermal conductivity.

Strong phonon anomalies and Fermi surface nesting of simple cubic Polonium

The unknown lattice dynamics of simple cubic Polonium is calculated using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. We notice several phonon anomalies, in particular along major symmetry directions namely M – R , R – Γ , Γ – M , M – X , and X – Γ . The analysis of the Fermi surface strongly suggests that the observed phonon anomalies are Kohn anomalies arising from strong Fermi surface nesting.

Calculation of phonon spectrum and elastic constants of FePd intermetallics: Performance of LDA and GGA

The phonon spectrum and elastic constants of L1 0 ordered FePd intermetallics have been calculated by first-principles density-functional perturbation theory using LDA and GGA approximations for the exchange-correlation part of the potential. The performances of the two functionals are compared by computing lattice dynamics at different volumes. The results are analyzed using relevant force constants. We find that the GGA at the experimental structure produces best agreement with the available neutron scattering results. This investigation provides a systematic understanding of the connections between the phonon spectrum and the inter-atomic interactions as well as of the ability of the different exchange-correlation functionals used in ab initio calculations in addressing them for FePd intermetallics. A strategy for modeling the inter-atomic interactions in FePd disordered alloys is also discussed based upon these results.

Large phonon band gap inSrTiO3and the vibrational signatures of ferroelectricity inATiO3perovskites: First-principles lattice dynamics and inelastic neutron scattering

We report on first-principles density functional perturbation theory calculations and inelastic neutron scattering measurements of the phonon density of states, dispersion relations, and electromechanical response of $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$. The phonon density of states of the quantum paraelectric $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ is found to be fundamentally distinct from that of ferroelectric $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ and $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ with a large, $70--90\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, phonon band gap. The phonon dispersion and electromechanical response of $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ reveal giant anisotropies. The interplay of covalent bonding and ferroelectricity strongly modulates the electromechanical response and gives rise to spectacular signatures in the phonon spectra. The computed charge densities have been used to study the bonding in these perovskites. Distinct bonding characteristics in the ferroelectric and paraelectric phases give rise to spectacular vibrational signatures. While a large phonon band gap in $A\mathrm{Ti}{\mathrm{O}}_{3}$ perovskites seems to be a characteristic of quantum paraelectrics, anisotropy of the phonon spectra correlates well with ferroelectric strength. These correlations between the phonon spectra and ferroelectricity can guide future efforts at custom designing still more effective piezoelectrics for applications. These results suggest that vibrational spectroscopy can help design novel materials.

Phonon related properties of transition metals, their carbides, and nitrides: A first-principles study

Lattice dynamics of body-centered cubic (bcc) Vb-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated phonon spectra of the bcc metals are in excellent agreement with results of inelastic neutron scattering experiments. We show that the superconductivity of transition metal carbides (TMC) and transition metal nitrides (TMN) is related to peculiarities of the phonon spectra, and the anomalies of the spectra are connected to the number of valence electrons in crystals. The calculated electron-phonon interaction constants for TM, TMC, and TMN are in excellent agreement with experimentally determined values. Phonon spectra for a number of monocarbides and mononitrides of transition metals within the cubic NaCl- and hexagonal WC-type structures are predicted. Ideal stoichiometric B1 crystals of ScC, YC, and VC are predicted to be dynamically stable and superconducting materials. We also conclude that YN is a semiconductor.

Phonon dispersion relations and softening in photoexcited bismuth from first principles

The phonon dispersion relations for equilibrium and photoexcited bismuth are calculated from first-principles density-functional perturbation theory, with constrained occupation of excited electronic states. The dependence of phonon frequency on photoexcited electron-hole plasma density is found for modes throughout the Brillouin zone. The resulting phonon dispersion curves are in good agreement with available neutron-scattering data for the equilibrium occupation of electronic bands. We find the effect of phonon softening by the electron-hole plasma to be substantially larger in the optical modes than in the acoustic modes throughout the Brillouin zone.