Light Electron Effective Masses Research Articles

Stability and electronic properties of ‘4-8’-type ZnSnN2 thin films free of spontaneous polarization for optoelectronic applications

Ternary nitride ZnSnN2 is a promising photovoltaic absorber material. In this work, we using first-principles calculations investigate the stability and electronic properties of ‘4–8’-type ZnSnN2 thin films. We find that below a certain thickness ‘4–8’-type thin films have lower total energy than polar films in the wurtzite-derived structures. For 4-layer ZnSnN2 thin film, the Pna21/Pmc21 → 4–8 transitions can spontaneously occur at finite temperatures. All ‘4–8’-type thin films studied are semiconducting and free of spontaneous polarization, the bandgap of which can be tuned by the thickness of films, ranging from 1.4 eV to 1.8 eV. Furthermore, these films show light electron effective masses, and octet-rule-preserving disorder has insignificant effects on the electronic properties. Our results provide new insights into the structure of ZnSnN2 in the thin film form and guidance for the experimental investigation.

Metal Sulfide Ion Exchangers: High Acid Stability of Na2xMg2y-xSn4-yS8 (NMS) and Topotactic Conversion to 2D Solid Acids with Semiconducting Character.

Metal sulfide ion exchange materials (MSIEs) are of interest for nuclear waste remediation applications. We report the high stability of two structurally related metal sulfide ion exchange materials, Na2xMg2y-xSn4-yS8 (Mg-NMS) and Na2SnS3 (Na-NMS), in strongly acid media, in addition to the preparation of Na2xNi2y-xSn4-yS8 (Ni-NMS). Their formation progress during synthesis is studied with in-situ methods, with the target phases appearing in <15 min, reaction completion in <12 h, and high yields (75-80%). Upon contact with nitric or hydrochloric acid, these materials topotactically exchange Na+ for H+, proceeding in a stepwise protonation pathway for Na5.33Sn2.67S8. Na-NMS is stable in 2 M HNO3 and Mg-NMS is stable in 4 M HNO3 for up to 4 h, while both NMS materials are stable in 6 M HCl for up to 4 days. However, the treatment of Mg-NMS and Na-NMS with 2-6 M H2SO4 reveals a much slower protonation process since after 4 h of contact both NMS and HMS are present in the solution. The resultant protonated materials, H2xMg2y-xSn4-yS8 and H4x[(HyNay-1)1.33xSn4--1.33x]S8, are themselves solid acids and readily react with and intercalate a variety of organic amines, where the band gap of the resultant adduct is influenced by amine choice and can be tuned within the range of 1.88(5)-2.27(5) eV. The work function energy values for all materials were extracted from photoemission yield spectroscopy in air (PYSA) measurements and range from 5.47 (2) to 5.76 (2) eV, and the relative band alignments of the materials are discussed. DFT calculations suggest that the electronic structure of Na2MgSn3S8 and H2MgSn3S8 makes them indirect gap semiconductors with multi-valley band edges, with carriers confined to the [MgSn3S8]2- layers. Light electron effective masses indicate high electron mobilities.

Investigation of Aluminum Arsenide Honeycomb Monolayer via Density Functional Theory

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.

Oriented-growth Ta3N5/SrTaO2N array heterojunction with extended depletion region for improved water oxidation

Depletion region at solid-solid or solid-liquid interface is mainly responsible for separating charge. Extending depletion region is an effective strategy to promote charge separation, and thus improving photoelectrochemical (PEC) performance. Herein, we designed an array heterojunction photoanode consisting of a-axis oriented-growth Ta3N5 nanorod arrays coated by SrTaO2N shell. This array heterojunction with a contiguous Ta3N5/SrTaO2N solid-solid and SrTaO2N/electrolyte solid-liquid junctions maximizes the charge separation and light absorption in depletion layer, shortens the hole diffusion distance to electrolyte, and effectively transfers electrons along the a-axis (light electron effective mass) of Ta3N5 to conductive substrate. As a result, compared with Ta3N5, Ta3N5/SrTaO2N array heterojunction exhibits more excellent PEC performance. Our results demonstrated a potential crystal oriented growth strategy of nanostructured heterojunctions with extended depletion region for promoting charge separation and overcoming the short carrier diffusion distance.

DFT coupled with NEGF study of the electronic properties and ballistic transport performances of 2D SbSiTe3.

Identifying novel 2D semiconductors with promising electronic properties and transport performances for the development of electronic and optoelectronic applications is of utmost importance. Here, we show a detailed study of the electronic properties and ballistic quantum transport performance of a new 2D semiconductor, SbSiTe3, based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism. Promisingly, monolayer SbSiTe3 owns an indirect band gap of 1.61 eV with a light electron effective mass (0.13m0) and an anisotropic hole effective mass (0.49m0 and 1.34m0). The ballistic performance simulations indicate that the 10 nm monolayer SbSiTe3 n- and p-MOSFETs display a steep subthreshold swing of about 80 mV dec-1 and a high on/off ratio (106), which indicate a good gate-controlling capability. As the channel length of SbSiTe3 decreases to 5 nm, its p-MOSFET also effectively suppresses the intra-band tunneling. Therefore, 2D SbSiTe3 is a potential semiconductor for future nanoelectronics.

Enhanced charge separation by oriented growth of Ta3N5-Cu2O n-p array heterojunction

Tantalum nitride (Ta3N5) is one of the most promising photoresponsive semiconductor materials for efficient solar energy conversion, but its fast carrier recombination has hindered research progress severely. Fabricating a heterojunction structure is an effective strategy to promote the charge separation efficiency and thus enhance solar conversion efficiency. Herein, we designed a p-n heterojunction photoanode consisting of n-type Ta3N5 nanorod arrays (NRAs) that grew along the a-axis (light electron effective mass) and p-type Cu2O nanoparticles. This NRA heterojunction shortens the hole diffusion distance, transfers electrons along the a-axis effectively, and enlarges the space charge region. The heterojunction improves the charge separation efficiency of Ta3N5 NRAs significantly, and Ta3N5-Cu2O exhibits a photocurrent density of 9.19 mA cm−2 at 1.6 V vs normal hydrogen electrode (VNHE, pH = 0), an onset potential of 0.326 VNHE, and a maximum incident photon to current efficiency of 60% at 380 nm. Our results demonstrated a potential crystal oriented growth strategy of nanostructured heterojunctions for overcoming the short carrier diffusion distance and fast carrier recombination.

Intriguing electronic insensitivity and high carrier mobility in monolayer hexagonal YN

A novel two-dimensional h-YN monolayer with high carrier mobility, insensitive electronic responses to strain and light electron effective masses in its few layer structures was predicted to be a promising candidate for future nanoscale electronic devices in high-strain conditions.

Invited; First-Principles Studies of the Defect Formation in III-V FETs Grown by Fin Replacement Method

Introduction III-V fin-shaped field-effect transistors (FinFETs) are considered to be attractive candidates for high-performance n-type FETs, thanks to both the light electron effective mass and to the improved electrostatic control. III-V FinFETs integrated on a Si substrate are advantageous with respect to their cost effectiveness. Aspect Ratio Trapping (ART) is one of the possible integration roads to be followed to achieve the growth of non-silicon channels on a Si substrate by reducing the density of defects obtained in the filled trenches [1]. In the ART process, the III-V fin is grown inside trenches of SiO2by using a metal organic vapor phase epitaxy (MOVPE) process, which implies that the precursors interact not only with the semiconductors but also with the oxides. The reaction of the precursors with the oxides could lead to the formation of defects around the interface which could be responsible for the degradation of the electrical reliability of III-V FinFETs. In this study, we report the evaluation of the formation of defects due to the reactions between the precursors and the oxides using first-principles simulations. Methodology The formation enthalpies [2] of the defects are calculated using the Quantum Espresso package [3] based on density functional theory. The lower the formation enthalpy is, the more likely the reaction is to occur. We used the Perdue-Burke-Ernzerhof exchange-correlation potential [4] combined with ultrasoft pseudopotentials to compute the energetics of the reactions. The charge states of the defects are assumed to be neutral. One of the considered reactions consists into the formation of interstitial defects in SiO2 due to the diffusion of the III-V components and of the doping atoms from the precursors. The list of considered precursors and oxides are summarized in Table 1. Mg and Zn could be used as acceptor doping sources for the III-V compounds. For the SiO2, an amorphous model is used, built from classical molecular dynamics simulations with the LAMMPS package [5]. Results and Discussion Fig. 1 shows the calculated formation enthalpies. For the precursors, we also envisaged some intermediate atomic configurations that correspond to the transition products active during the MOVPE process. Our simulations show that the initial states of the precursors have high formation enthalpies, indicating that there is no driving force to promote the diffusions of the III-V elements and of the dopants into the STI. However, as the growth proceeds and that the precursors begin losing their organic ligands, the formation enthalpies are significantly reduced. Zn and Mg have the lowest formation enthalpies of all of the reactants considered, which suggests that the diffusion of doping sources into SiO2 could be favored. In order to confirm the latter for the Mg case, secondary ion mass spectrometry measurements (SIMS) were performed on a SiO2 blanket exposed to the Mg-precursors (see Fig. 2). The diffusion of Mg is clearly observed near the SiO2surface, which is consistent with the theoretical picture (Fig. 1). In order to prevent these defect-related issues, it will be important to understand a risk of formation of defects using the ab initio approaches. Acknowledgment This work has been supported by the IMEC core partners and IMEC Industrial Affiliation Programs (IIAP).

First-principle study on substitution of Cu or P into Ni–Sn intermetallic compounds

The effect on substitution of Cu or P into the Ni–Sn intermetallic compound (IMC) is examined by analyzing the electronic structure based on first-principle calculation. When the Cu atom is substituted for the Ni one in Ni 3Sn 2, the resulting Ni–Cu–Sn IMC was energetically unstable, and then it had the higher electrical conductivity by the light electron effective mass. Meanwhile, when the P atom is replaced by the Sn one, the Ni–Sn–P IMC had the higher bulk modulus by the strong p–d hybridization between Ni and Sn or P, and also its electrical conductivity is lessen by the heavy electron effective mass.

Effective masses and complex dielectric function of cubic HfO2

The electronic band structure of cubic HfO2 is calculated using an ab initio all-electron self-consistent linear augmented plane-wave method, within the framework of the local-density approximation and taking into account full-relativistic contributions. From the band structure, the carrier effective masses and the complex dielectric function are obtained. The Γ-isotropic heavy and light electron effective masses are shown to be several times heavier than the electron tunneling effective mass measured recently. The imaginary part of the complex dielectric function ϵ2(ω) is in good agreement with experimental data from ultraviolet spectroscopic ellipsometry measurements in bulk yttria-stabilized HfO2 as well as with those performed in films deposited with the tetrakis diethylamido hafnium precursor for energies smaller than 9.5eV.

Electronic structure of ultrathin (GaAs)n(AlAs)n [001] superlattices and the Ga0.5Al0.5As alloy

Using self-consistent electronic structure calculations we contrast the energy levels of the ultrathin (GaAs)n(AlAs)n [001] superlattices (n=1,2) with those of the disordered Ga0.5Al0.5As alloy and a long period (n→∞) superlattice. Conventional Kronig–Penney and effective mass models suggest that, because of the relatively light electron effective masses and small barrier heights, only delocalized superlattice conduction states would exist in the n=1 limit. We find a number of such conventional ‘‘averaging states’’ (delocalized on both sublattices). In addition, we also find states localized on a single sublattice. For small n’s, the latter are divided into two classes: (i) ‘‘repelling states’’ (distinct alloy states which fold in the superlattice into states of identical symmetry, which, in turn, repel each other and tend to localize), and (ii) ‘‘segregating states’’ (a pair of localized states Ψα and Ψβ, where symmetry compels Ψα to have a vanishing angular momentum component l on a subset α of unit cell atoms, whereas the complementary state Ψβ is localized on the other atoms β. These states are split by the potential difference Vβl −Vαl). We analyze new luminescence, reflectance, and Raman data in light of our theoretical model. Studies of the II-VI superlattices (CdTe)1(HgTe)1 shows similar behavior.