Semiconductor-metal Transition Research Articles

A next-generation transistor with low supply voltage operation constructed based on 2D materials' metal-semiconductor phase transition.

Power dissipation, a fundamental limitation for realizing high-performance electronic devices, may be effectively reduced by an external supply voltage. However, a small supply voltage simultaneously brings another serious challenge, that is, a remarkable device inability in transistors. To deal with this issue, we propose a new transistor design based on the metal-semiconductor phase transition in a AsGeC3 monolayer, which provides a switching mechanism of band-to-band tunneling at on- and off-states by gate-voltage modulation. Our first-principles calculations uncover that the monolayer AsGeC3 field-effect transistors (FETs) with gate lengths of 5, 4, and 3 nm may meet well the requirements for on-state current (Ion), power dissipation (PDP), and delay period (τ) as outlined by the International Technology Roadmap for Semiconductors (ITRS) in 2013 to achieve higher performance by the year 2028. Importantly, high performances are achieved only under a very low supply voltage (VDD = 0.05/0.10 V). Significantly, the AsGeC3 FETs exhibit remarkably lower values of both PDP and τ than those of nearly all the transistors reported up to date. These novel 2D metal-semiconductor phase transition-based FETs open up a new door for designing next-generation low-power electronic devices.

A new material built with alternating Cu sulfide and (Al,Mg) hydroxide molecular sheets: hydrothermal synthesis and selected characteristics.

Two-dimensional materials with new physical phenomena are gaining popularity due to their unique properties. In recent years, a new family of layered compounds inspired by the minerals valleriite and tochilinite which are composed of alternating quasi-atomic sheets of transition metal chalcogenides (sulfides and selenides of Fe, Fe-Cu and other metals) and hydroxides of Mg, Al, Fe, Li, etc., assembled via electrostatic interaction, has arisen as a new synthetic platform for 2D materials. In this work, we synthesized a new promising material composed of alternating quasi two-dimensional sulfide Cu4-xS2 (x = 1-1.5) and hydroxide (Mg1-yAly)(OH)2 (y ∼ 0.25) sheets as multilayer flakes with a lateral size of 1-2 μm and a thickness of several tens of nm. The reliable formation of the material was ensured by an excess of aqueous sodium sulfide along with some quantity of Al salt, which was necessary for the self-assembly of the sulfide and hydroxide sheets driven by their opposite electric charges. A series of samples with varying Al/Mg ratios were examined using TEM, EDS, SAED, XRD, XPS, Raman and UV-vis-NIR spectroscopy. Using the Rietveld refinement procedure, the crystal lattice was determined to resemble the space group P3̄m1 with cell parameters a = b = 3.89 Å and a = b = 3.03 Å for sulfide and hydroxide parts, respectively, and the common parameter c = 12.03 Å. XPS and Raman spectra agree with the chalcocite-like sulfide layers containing Cu+ cations, monosulfide anions and few, if any, S-S bonds. UV-vis-NIR spectra show indirect transitions of 0.8 eV and considerable absorption in the near-infrared region. Thermogravimetry and differential scanning calorimetry studies under an Ar atmosphere suggest a slightly endothermic process and a phase transition at 90 °C accompanied, as suggested by conductivity measurements, by a metal-semiconductor transition; the main destruction with a loss up to 15 wt% occurs at ∼510 °C, due to decomposition of hydroxide layers similar to layered valleriite and tochilinite. The (thermo)electrical data show that, contrary to valleriite, which is an n-type semiconductor with mediocre thermoelectric performance, the proposed material is a heavily doped p-type semiconductor exhibiting good thermoelectric efficiency. The new member of the valleriite family of heterolayered quasi-2D materials demonstrates therefore essentially novel promising properties.

Spin Hall effect modulated by an electric field in asymmetric two-dimensional MoSiAs2Se.

Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs2Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs2Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs2Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

Band Gap Mechanism for Armchair Single Walled Carbon Nanotubes and Metal Semiconductor Transition for Symmetry Breaking

We have studied structures of single walled carbon nanotubes and metal semiconductor transition for the symmetric breaking. It was found that band gap existed for metallic armchair single walled carbon nanotubes. Calculations were made to analyse the meta-stability of the corrugated structures of armchair single walled carbon nanotubes. The corrugated single walled carbon nanotube structures are always lower in energy than the non corrugated nanotubes. The curvature effect was that the corrugated structure breaks the local symmetry between different carbon atoms. A true gap is created which does not vanish even when an external magnetic field is swept. The corrugation length and band gap gaps are decaying functions of the nanotubes radius and approached zero for carbon nanotube such as graphene. This was also true for zigzag and chiral single wall carbon nanotubes. The obtained results were found in good agreement with previously obtained results.

Spinovye sostoyaniya ionov Co i perekhod metall–poluprovodnik v sloistykh kobal'titakh PrBaCo2O5+δ (δ = 0.52, 0.74)

The spin states of Co atoms in PrBaCo2O5 + δ with regard to the paramagnetic contribution of Pr3+ ions have been determined from magnetic property studies for δ = 0.52 and 0.74. Results obtained without considering the paramagnetic contribution of Pr3+ ions are inconsistent with available experimental data. With a decrease in temperature, the metal–insulator transition in PrBaCo2O5.52 becomes sharper according to a sharp change in spin states (from HS/LS to LS/IS) of Co3+ ions. In this case, Co3+ ions occupying octahedra pass from the high-spin state (HS, S = 2) to the low-spin one (LS, S = 0) and those occupying pyramids pass from the LS-state to the intermediate spin state (IS, S = 1), as follows from the available structural data. In PrBaCo2O5.74, the metal–semiconductor transition occurs smoothly from HS/LS to the HS/IS state upon the smooth transition of the Co3+ ion state from (LS, S = 0) to the (IS, S = 1) state in pyramides without change in the spin state of ions Co3+ (HS, S = 2) and Co4+ (LS, S = 1/2) in octahedrons.

Magnetic Semiconductors from Ferromagnetic Amorphous Alloys

Utilizing both charge and spin degrees of freedom of electrons simultaneously in magnetic semiconductors promises new device concepts by creating an opportunity to realize data processing, transportation and storage in one single spintronic device. Unlike most of the traditional diluted magnetic semiconductors, which obtain intrinsic ferromagnetism by adding magnetic elements to non-magnetic semiconductors, we attempt to develop room temperature magnetic semiconductors via a metal-semiconductor transition by introducing oxygen into three different ferromagnetic amorphous alloy systems. These magnetic semiconductors show different conduction types determined primarily by the compositions of the selected amorphous ferromagnetic alloy systems. These findings may pave a new way to realize magnetic semiconductor-based spintronic devices that work at room temperature.

First-principles calculation of Si–C–N structures with metallicity and high hardness

Stable hard and conductive ceramics under ambient conditions are vital for many industrial applications. However, maintaining high hardness while achieving semiconducting-metallic transition in Si3N4 materials is greatly challenging both experimentally and theoretically. Herein, two hexagonal Si–C–N structures, denoted SiC6N7-1 and SiC6N7-2, were constructed using α-Si3N4 as a matrix model. Calculations of elastic constants and phonon spectra showed stable structures under ambient pressure. Both SiC6N7-1 and SiC6N7-2 can possibly be synthesized at the pressures attainable by large-volume press technology. Compared to a-Si3N4, both Si–C–N ternary compounds achieved a semiconductive-metallic transition owing to the elemental substitution. Property calculations showed conductive Si–C–N compounds with high hardness values of 52 GPa for SiC6N7-1 and 31 GPa for SiC6N7-2. Therefore, the introduction of C atoms tuned the intrinsic properties of both structures, achieving high hardness along with good electrical conductivity. Overall, these findings look promising for future design and synthesis paths of novel conductive ceramics with prospective applications.

Temperature-dependent conduction mechanism of NiO@Carbon@Polypyrrole nanomaterial with EMI shielding characteristics

A simple hydrothermal technique and in-situ chemical oxidative polymerization of pyrrole monomer yield the functionalized NiO@C@PPy nanomaterial for electromagnetic shielding applications. The crystal structure, morphology, dielectric and electromagnetic shielding (EMI) performance in the X-band (8.2–12.4 GHz) is thoroughly studied. Impedance spectroscopy is utilized to study the electrical response of a NiO@C@PPy pellet. This study focuses on the modulations of relaxation time with frequency at different temperatures. In the NiO@C@PPy composite, a semiconductor-to-metal transition (SMT) is observed, at 328 K. The conduction mechanism of NiO@C@PPy is explained based on the carrier hopping transport model in Ni2+ and Ni3+ ions. It is evident from the activation energy value (Ea ≈ 0.32 eV) determined from impedance, conductivity, and dielectric data that the relaxation and conduction processes correspond to the same electro-active region. Using the variable range hopping (VRH) model localization length of the carrier is calculated to be 1.56 Å. The NiO@C@PPy sample demonstrated enhanced conductivity and low dielectric values which are vital in EMI shielding applications. Consequently, the electromagnetic interference shielding effectiveness is found to be 21.9 dB of NiO@C@PPy in the X-band frequency range. This composite material is a good candidate for high frequency shielding applications.

Purifying VO2 phase synthesized by sol-gel method via annealing process

Vanadium dioxide (VO2) film has significant interesting phase transition properties called metal-semiconductor transition (MST) which occurs at 68 °C. This transition behavior makes it suitable for various applications, including thermochromic materials for smart windows, sensors, and electronics devices. In this study, we aimed to focus on the purification of VO2 through the sol-gel synthesis method from vanadium alkoxide (VO(OC3H7)3. To analyze the effects of annealing temperature (500 °C-600 °C) on the resulting structural, morphology, and electrical properties, x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), temperature dependent resistance, and infrared (IR) thermal image were considered. The resistance as a function of the temperature shows an abrupt change confirming the phase transition. However, XRD shows an impurity of other vanadium oxides such as V6O13. In addition, the sample annealed at 600 °C shows a non-uniform surface filled with island clusters, while the sample annealed at 500 °C has a smooth and uniform surface. Raman spectra further supports the formation of VO2 (M) clusters mixed with V6O13 in smooth area, resulting in a decrease in electrical properties, significant reduced resistance change at the excessive annealing temperature.

Semiconducting-metallic phase transition with tunable optoelectronics and mechanical properties of halide perovskites TlGeX3 (X = F, Cl) under pressure

Semiconducting-metallic phase transition with tunable optoelectronics and mechanical properties of halide perovskites TlGeX3 (X = F, Cl) under pressure

Visible-Light-Driven Semiconductor–Metal Transition in Electron Gas at the (100) Surface of KTaO3

Two-dimensional electron gas (2DEG) at the (100) KTaO3(KTO) surface and interfaces has attracted extensive interest because of its abundant physical properties. Here, light illumination-induced semiconductor–metal transition in the 2DEG at the KTO surface was investigated. 2DEG was formed at the surface of KTO by argon ion bombardment. The 2DEG prepared with a shorter bombardment time (300 s) exhibits semiconducting behavior in the range of 20~300 K in the dark. However, it shows a different resistance behavior, namely, a metallic state above ~55 K and a semiconducting state below ~55 K when exposed to visible light (405 nm) with a giant conductivity increase of about eight orders of magnitude at 20 K. The suppression of the semiconducting behavior is found to be more pronounced with increasing light power. After removing the illumination, the resistance cannot recover quickly, exhibiting persistent photoconductivity. More interestingly, the photoresponse of the 2DEG below 50 K was almost independent of the laser wavelength, although the photon energy is lower than the band gap of KTO. The present results provide experimental support for tuning oxide 2DEG by photoexcitation, suggesting promising applications of KTO-based 2DEG in future electronic and optoelectronic devices.

The influence of screening function on exciton mott transition in an asymmetric double quantum well

The semiconductor-to-metal transition (SMT) due to excitons exciton mott transition (EMT) in GaAs/Al[Formula: see text]Ga[Formula: see text]As asymmetric double quantum well (ADQW) has been studied theoretically by tuning direct (DX) and indirect (IDX) exciton densities using variation technique combined with the Monte Carlo approximation. The interaction of the optically pumped exciton densities is accounted for through the Thomas–Fermi (TF) dielectric screening and compared with Hartree–Fock (HF) screening. The screening effects of IDX at the barrier regime have been accounted for through size-dependent screening. The important characteristics of EMT, such as (i). binding energy and (ii), diamagnetic susceptibility have been studied in both coupled ([Formula: see text]Å) and isolated ([Formula: see text]Å) regimes of ADQW. The effect of asymmetry on the EMT of DX (IDX) is significant only for [Formula: see text] in the case of the coupled regime of ADQW, which may be due to the dynamic tunneling mechanism. The impact of the screening among the exciton densities to bring out the avalanche breakdown of excitons at the critical concentration has been studied for different kinds of excitons.

Formula omitted]-[formula omitted] junction from localized doping of metal-WSe2 contact with N2H4, NH4 and BF4 molecules: A first-principles investigation

Using the density-functional theory, we investigate the possibility of obtaining a p–n junction in transition metal–semiconductor junctions. The systems consist in metal surfaces, M = Ti(0001), Co(0001), Pd(111) or Pt(111) in proximity to the semiconductor WSe2 monolayer. The latter is doped with hydrazine (N2H4), ammonium (NH4), or tetrafuoroborate (BF4). We determine for the M-WSe2 systems the binding energy, work function of metal surfaces, the Schottky barrier energy with and without molecular doping and the spin polarization. Results show a strong binding between the metal and WSe2 which implies a stable M-WSe2 contacts. The Spin-polarization in Ti-WSe2 (35%), Co-WSe2 (37,5%), Pd-WSe2 (13%) and Pt-WSe2 (-0.2%) suggest a significant metal-to-WSe2 spin injection in M-WSe2. Localization of hydrazine or ammonium on the WSe2 in M-WSe2 (M = Pd, Pt) changes its conductivity from p to n-type. The strong binding, the finite spin-injection, the change in conductivity and the reduction in Shottky barrier, raise the potential for a molecule-based p–n junction in metal-WSe2 field-effect transistor.

Strain effects on the adsorption behavior of PtS2 monolayer as anchoring material for lithium-sulfur batteries: A DFT study

The practical implement of lithium-sulfur batteries is significantly impeded by low electronic conductivity of anchoring substrate and the shuttle effect. To address these issues, we employed density functional theory (DFT) to investigate the behavior of lithium polysulfides (LiPSs) adsorbed on PtS2 monolayers in lithium-sulfur batteries. The results demonstrate a moderate binding strength between PtS2 monolayers and LiPSs, effectively suppressing the shuttle effect. Furthermore, the bandgap of PtS2 monolayers decreased remarkably during the lithiation, accompanied with semiconductor-metal transition, leading to a significant improvement in conductivity. The diffusion barriers and decomposition barrier of the adsorbed of Li2S clusters on PtS2 monolayer were calculated to assess the delithiation reaction kinetics. These values are comparable to those observed in other two-dimension (2D) materials, indicating the potential for rapid charge/discharge in Li-S batteries. Especially we considered the impact of strain arise from volume expansion on binding energy and diffusion processes. Our results indicate that under strain, the PtS2 substrate can still effectively anchor LiPSs while facilitating their diffusion. Through a series of calculations, we can conclude that the PtS2 monolayer holds promise as the anchoring material for Li–S batteries.

Dinitrogen bonding induced metal-semiconductor transition leading to ultrastiffening in boron subnitride

Dinitrogen bonding induced metal-semiconductor transition leading to ultrastiffening in boron subnitride

Electronic and magnetic properties of layered M3Si2Te6 (M = alkaline earth and transition metals)

Recently, a new layered material, Mn3Si2Te6, was identified to be a semiconductor with nodal-line topological property and ferrimagnetic ground state. In this work, we propose a series of structures, M3Si2Te6 (M = alkaline earth and transition metals), and systematically investigate their mechanical, magnetic and electronic properties, and the strain effect to enrich the family of the layered materials for practical applications. We find 13 stable M3Si2Te6, including 5 semiconductors (M = Ca, Sr, Fe, Ru and Os) and 8 metals (M = Sc, Ti, Nb, Ta, Cr, Mo, W and Tc). Two structures (M = Ti and Cr) are antiferromagnetic (AFM), while other structures are non-magnetic (NM). Similar to Mn3Si2Te6, the AFM structures exhibit magnetic anisotropy energies (MAEs) and semiconductors have anisotropic electron effective masses. We further show that compressions along the z-axis can effectively tune the electronic and magnetic properties, such as the semiconductor-metal and NM-AFM transition in Fe3Si2Te6, the two-fold degeneracy of the valence band maximums in Sr3Si2Te6, as well as the reduced MAE for all magnetic structures. These results demonstrate the diverse properties of the layered M3Si2Te6 family and provide promising theoretical predictions for the future design of new layered materials.

Stacking order modulated anomalous valley Hall effect in antiferromagnetic MXene

The valley index is a promising degree of freedom for information processing in electronic devices. However, the researches on valley polarization are mainly focused on ferromagnetic order, which breaks the time reversal symmetry simultaneously. Here, a novel paradigm for achieving stacking order modulated anomalous valley Hall (AVH) effect is proposed in antiferromagnetic monolayers. The paradigm involves the introduction and reversal of nonuniform potentials by modulating the position of substrate, to break the combined symmetry of spatial inversion and time reversal (PT symmetry) and achieve stacking-dependent valley spin splitting. Based on first-principles calculations, we discover spontaneous valley polarization in antiferromagnetic Cr2CH2 MXene and stacking-dependent valley spin splitting in Cr2CH2/Sc2CO2 heterostructure. Furthermore, switching the ferroelectric polarization of monolayer Sc2CO2 results in a semiconductor-metal transition in Cr2CH2/Sc2CO2, accompanied by the disappearance of valley physics. Our findings provide an alternative way to develop controllable valleytronics devices based on antiferromagnetic monolayers.

Investigation of charge-carriers dynamics and sub/super-linear response for La0.8Na0.2-x□xMnO3 (x = 0 and 0.1) perovskite ceramics

La0.8Na0.2-x□xMnO3 (x = 0 and 0.1) compounds were prepared by solid-state reaction. XRD patterns were used to check the structural properties. Experimental results were discussed in terms of different theoretical approaches. The temperature dependence of DC-conductivity displays the presence of a metal-semiconductor transition for x = 0 and 0.1. The introduction of 10%-vacancy provokes a significant increase in the conductivity. The electrical conductivity is explained by many kinds of hopping each one is pronounced in a different temperature region. Two deviation temperatures are observed. These temperatures coincide with the ANC temperatures and the change in the conduction process. For x = 0.1, the Nyquist diagrams shows the presence of a high frequency inductive behavior. AC-conductivity spectra obey to the JPL and DJP. Then, SPL and NCL behaviors appear at high frequency range. The power "s" presents the contribution of numerous conduction models. The scaling behavior reveals that the conductivity spectra obey to the TTS principle.

Pressure-induced superconductivity in the nonsymmorphic topological insulator KHgAs

Recently, topological insulators (TIs) KHgX (X = As, Sb, Bi) with hourglass-shaped dispersion have attracted great interest. Different from the TIs protected by either time-reversal or mirror crystal symmorphic symmetry tested in previous experiments, these materials were proposed as the first material class whose band topology relies on nonsymmorphic symmetries. As a result, KHgX shows many exotic properties, such as hourglass-shaped electronic channels and three-dimensional doubled quantum spin Hall effects. To date, high-pressure experimental studies on these nonsymmorphic TIs are minimal. Here, we carried out high-pressure electrical measurements up to 55 GPa, together with high-pressure X-ray diffraction measurements and high-pressure structure prediction on KHgAs. We found a pressure-induced semiconductor-metal transition between ~16 and 20 GPa, followed by the appearance of superconductivity with a Tc of ~3.5 K at approximately 21 GPa. The superconducting transition temperature was enhanced to a maximum of ~6.6 K at 31.8 GPa and then slowly decreased until 55 GPa. Furthermore, three high-pressure phases within 55 GPa were observed, and their crystal structures were established. Our results showed the high-pressure phase diagram of KHgAs and determined the pressure-induced superconductivity in nonsymmorphic TIs. Thus, our study can be used to facilitate further research on superconductivity and topologically nontrivial features protected by nonsymmorphic symmetries.

Static isotropic pressure induced semiconductor–metal transition and its impact on structural, electronic, elastic, mechanical and optical properties of SrCeO3

Static isotropic pressure induced semiconductor–metal transition and its impact on structural, electronic, elastic, mechanical and optical properties of SrCeO3