First principles study of Mg2X (X=Si,Ge, Sn,Pb): Elastic,optoelectronic and thermoelectric properties

Full potential linearized augmented plane wave method within the framework of density functional theory was applied to calculate the structural, elastic, electronic, thermoelectric and optical properties of Mg₂X (X=Si, Ge, Sn, Pb) compounds. Exchange-correlation effects were treated using generalized gradient approximation and modified Becke–Johnson technique. Calculated structural parameters were found in good agreement to the experimental data. With the pressure application, the lattice constant decreased while the bulk modulus increased. Brittleness and ductility of these compounds were interpreted via the calculated elastic constants. The optical properties like complex dielectric function, refractive index, reflectivity, and optical conductivity were investigated in the pressure range 0–10 GPa. Very high reflectivity in a wide energy range indicates the usefulness of these materials as a shield from high energy radiations. In addition, the thermopower of the materials was calculated as a function of the chemical potential at various temperatures. These materials are suitable for applications in optoelectronic and thermoelectric devices due to their high thermopower and narrow bandgap.     

用第一性原理研究Sn掺杂 Ga1.375In0.625O3的电子结构和导电性

用第一性原理计算了Sn 替位Ga1.375In0.625O3化合物的Ga 原子(Ga1.25In0.625Sn0.125O3)和Sn 替位Ga1.375In0.625O3化合物的In原子(Ga1.375In0.5Sn0.125O3)的结构、电子能带和态密度。Ga1.25In0.625Sn0.125O3半导体材料比Ga1.375In0.5Sn0.125O3材料具有大的晶格参数和强的Sn–O离子键。在Sn掺杂 Ga1.375In0.625O3化合物中,Sn 原子优先取代In 原子。Sn掺杂 Ga1.375In0.625O3化合物显示n型导电性, 杂质能带主要由Sn 5s 态组成。Ga1.375In0.5Sn0.125O3化合物的光学带隙大于Ga1.25In0.625Sn0.125O3化合物的光学带隙。 Ga1.25In0.625Sn0.125O3 具有小的电子有效质量和大的电子迁移率,Ga1.375In0.5Sn0.125O3 具有多的相对电子数和好的导电性。     

A family of flexible two-dimensional semiconductors: MgMX2Y6(M=Ti/Zr/Hf; X=Si/Ge; Y=S/Se/Te)

Inspired by the recently predicted 2D MX₂Y₆(M=metal element;X=Si/Ge/Sn;Y=S/Se/Te),we explore the possible applications of alkaline earth metal(using magnesium as example) in this family based on the idea of element replacement and valence electron balance.Herein,we report a new family of 2D quaternary compounds,namely MgMX₂Y₆(M=Ti/Zr/Hf;X=Si/Ge;Y=S/Se/Te) monolayers,with superior kinetic,thermodynamic and mechanical stability.In addition,our results in...     

N掺杂和氧空位对PbTiO3电子结构和导电性的影响研究

用第一性原理计算了P型N掺杂PbTiO3的电子密度差分、能带结构和态密度,讨论了氧空位对N掺杂PbTiO3性能的影响。在PbTiO3中掺杂N杂质后,PbTiO3的价带向高能级发生移动,费米能级进入价带顶部,带隙变窄,N掺杂PbTiO3表现出典型的P型半导体性能。当N掺杂PbTiO3中含有氧空位时,导带发生下移,受主被完全补偿。计算结果与实验数据相吻合。     

Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties

We investigated the interfacial effects on magnetic properties in Co₂Fe(Al,Si)/Ge (CFAS/Ge) and CFAS/MgO/Ge systems to demonstrate the effects of the interface structure on magnetic properties. CFAS and CFAS/MgO were deposited on the i-Ge(111) substrate. In-situ reflection high energy electron diffraction (RHEED) patterns showed epitaxially grown CFAS and MgO on Ge(111). According to the X-ray diffraction (XRD) ϕ-scan of CFAS(220), we determined that the crystallographic orientation relationships were CFAS(111)<–110>// Ge(111)<–110> and CFAS(111)<–110>//MgO(111)<–110>Ge(111)<–110>. The magnetic properties were measured by the vibrating sample magnetometer (VSM) and the saturation magnetization M_s value of CFAS with 2-nm thick MgO reached the value of L2₁ ordered one. A uniaxial magnetic anisotropy behavior was observed both in CFAS/Ge and CFAS/MgO/Ge structures after annealing. We confirmed the behavior did not only originate from the CFAS/Ge interface but also CFAS/MgO and the ordering structure.     

Structural,mechanical and electronic properties of sodium based fluoroperovskites NaXF3 (X=Mg,Zn) from first-principle calculations

The structural stability, mechanical, electronic and thermodynamic properties of the cubic sodium based fluoro-perovskite NaXF₃ (X=Mg, Zn) have been studied using density functional theory (DFT). The generalized gradient approximation of Perdew–Burke and Ernzerhof (GGA-PBE) is used for modeling exchange-correlation effects. In addition, the alternative form of the GGA proposed by Engel and Vosko (GGA-EV) is also used to improve the electronic band structure calculations. The results show that both compounds are stable in the cubic Pm3m structure. From Poisson׳s ratio, it is inferred that cubic anti-perovskite NaXF₃ are ductile in nature and that bonding is predominantly of ionic in nature. The electronic band structure calculations and bonding properties show that anti-perovskites have an indirect energy band gap (M–Г) with a dominated ionic character. The thermal effects on thermal expansion coefficient, Debye temperature and Grüneisen parameter were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The calculations are found to be in good agreement with other results.     

Mechanical and electronic properties of Si,Ge and their alloys in P42/mnm structure

Structural, mechanical, and electronic properties of Si–Ge alloys in P4₂/mnm structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code. The calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew–Burke–Ernzerhof, PBEsol. The calculated excess mixing enthalpy is positive over the entire germanium composition range. The calculated formation enthalpy shows that the Si–Ge alloys are unstable at 0 K; however, the alloys might exist at specified high temperature scale. The anisotropic calculations show that Si₁₂ in P4₂/mnm structure exhibits the greatest anisotropy in Poisson’s ratio, shear modulus, Young’s modulus and the universal elastic anisotropy index A^U, but Si₈Ge₄ has the smallest anisotropy. The electronic structure calculations reveal that Si₁₂ and Si–Ge alloys in P4₂/mnm structure are indirect band gap semiconductors, but Ge₁₂ in P4₂/mnm structure is a direct semiconductor.     

Zn掺杂和Sn空位对p型SnO2电子结构和光学性能的影响

用第一性原理计算了本征SnO2、Zn掺杂SnO2、带Sn 空位(VSn)的SnO2和Zn-VSn共掺杂 SnO2的电子结构和光学性能。结果表明,Zn 原子替位SnO2中的Sn原子后费米能级进入价带,价带顶的空能态由Zn 3d和O 2p态组成,Zn掺杂SnO2显示p型半导体性能。 带Sn空位的Zn掺杂SnO2的相对空穴数量较Zn掺杂SnO2的相对空穴数量有明显增加,Sn空位有助于增加Zn掺杂SnO2的p型导电性。Zn掺杂SnO2在可见光区域有明显的光吸收,带Sn空位的Zn掺杂SnO2的光吸收较Zn掺杂SnO2明显增强,吸收光谱发生蓝移。     

First-principles calculations of structural,elastic, electronic and optical properties of XO (X=Ca,Sr and Ba) compounds under pressure effect

The structural, elastic, electronic and optical properties of XO (X= Ca, Sr and Ba) compounds were investigated by the density functional theory. A good agreement was found between our calculated results and the available theoretical and experimental data of the lattice constants. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure derivatives are also calculated. SrO and BaO compounds present a transition phase at 39.72 and 27.28 GPa. The SrO compound shows a change from direct band gap (Γ–Γ) to indirect band gap (Γ–X) at about 15 GPa. The top of the valence bands reflects the s electronic character for all structures. We investigate the effective mass of electrons as function of pressure at the Γ point for CaO, SrO and BaO compounds. Calculations of the optical spectra have been performed for the energy range 0–60 eV. The origin of the spectral peaks was interpreted based on the electronic structures. The enhancement of pressure increases the static dielectric function and refractive index of CaO, SrO and BaO.     

Ferroelectric DRAM (FEDRAM) FET with metal/SrBi2Ta2O9/SiN/Si gate structure

N-channel ferroelectric dynamic random access memory (FEDRAM) FETs with SrBi₂Ta₂O₉/SiN/Si structure were fabricated and characterized. The estimated switching time (t_sw ) of the fabricated FET, measured at applied electric field of 376 kV/cm, was less than 50 ns, which could be significantly reduced upon scaling. Its remnant polarization (2P_r) was measured to be about 1.5 μC/cm², which is more than one order of magnitude higher than that required for FEDRAM operation. The stored information retains more than three orders of magnitude of on/off ratio up to three days at room temperature, with little fatigue after 10¹¹ switching cycles     

基于密度泛函理论计算的3d过渡金属掺杂CuGaS2电子结构分析

3d transition metals doped CuGaS2 are considered as possible absorbing material candidates for intermediated band thin film solar cells. The electronic structure and optical properties of 3d transition metals doped CuGaS2 are investigated by using density functional theory calculations with the GGA ＋ U method in the present work. The doping with 3d transition metals does not obviously change the crystal structure, band gap, and optical absorption edge of the CuGaS2 host. However, in the case of CuGa1-χTMχS2 （TM = Ti, V, Cr, Fe, and Ni）, there is at least one distinct isolated impurity energy level in the band gap, and the optical absorption is enhanced in the ultraviolet-light region. Therefore, these materials are band thin film solar ceils. The calculated results are very well better explain them. ideal absorber material candidates for intermediated consistent with experimental observations, and could better explain them.     

New dielectric material system of Nd(Mg1/2Ti1/2)O3–CaTiO3 at microwave frequency

The microwave dielectric properties of (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ (0.1  x  1.0) ceramics prepared by the conventional solid state method have been investigated. The system forms a solid solution throughout the entire compositional range. The dielectric constant decreases from 152 to 27 as x varies from 0.1 to 1.0. In the (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ system, the microwave dielectric properties can be effectively controlled by varying the x value. At 1400 °C, 0.1CaTiO₃–0.9Nd(Mg_1/2Ti_1/2)O₃ has a dielectric constant (ε_r) of 42, a Q × f value of 35 000 GHz and a temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 43 000 GHz for x = 0.9 is achieved at the sintering temperature 1500 °C.     

(Ca,K)(Zn,Mn)2As2: Ferromagnetic semiconductor induced by decoupled charge and spin doping in CaZn2As2

We have successfully synthesized a novel diluted magnetic semiconductor (Ca_1−2xK_2x)(Zn_1−xMn_x)₂As₂ with decoupled charge and spin doping. The substitutions of (Ca²⁺, K⁺) and (Zn²⁺, Mn²⁺) in the parent compound CaZn₂As₂ (space group P\begin{document}${\overline 3}$\end{document}m1 (No. 164)) introduce carriers and magnetic moments, respectively. Doping only Mn into CaZn₂As₂ does not induce any type of long range magnetic ordering. The ferromagnetic ordering arise can only when K⁺ and Mn²⁺ are simultaneously doped. The resulted maximum Curie temperature reaches ~7 K, and the corresponding coercive field is ~60 Oe. The transport measurements confirm that samples with K and Mn co-doping still behave like a semiconductor.     

Structural,Mechanical and Thermodynamic Properties of Cu<Subscript>2</Subscript>CoXS<Subscript>4</Subscript> (X = Si,Ge, Sn) Studied by Density Functional Theory

We have investigated the Structural, mechanical and thermodynamic properties of Cu₂CoXS₄ (X = Si, Ge, Sn) by using the density functional theory method. In this paper, we used GGA-PBE functional to find the equilibrium structural parameters and to calculate the elastic properties. The Mulliken population analysis indicates the bonds between S atoms and other three atoms in Cu₂CoXS₄ (X = Si, Ge, Sn) exhibit the feature of covalent bond. Furthermore, the calculated elastic constants prove the mechanical stability of Cu₂CoXS₄ (X = Si, Ge, Sn) in I\(\bar 4\)2m structure. The results are given for B/G and A ^U reveal Cu₂CoXS₄ (X = Si, Ge, Sn) can behave as a ductile and elastic material. Finally, the heat capacity, thermal expansion, entropy and Debye temperature are also reported at the different pressures (0~50 GPa) and temperatures (0~1000 K).     

Electronic,optical and bonding properties of MgYZ2 (Y=Si,Ge; Z=N,P) chalcopyrites from first principles

The electronic and optical properties of MgYZ₂ (Y=Si, Ge; Z=N, P) compounds are carried out using first-principle calculations within the density functional theory. The calculations show close correspondence to the available experimental data compared to the previous theoretical calculations. Band gap decreases by changing the cations Y from Si to Ge as well as Z from N to P in MgYZ₂. The N/P p-states contribute majorly in the density of states. Bonding nature of the herein studied compounds is predicted from the electron density plots. Optical response of these compounds is noted from the complex refractive index, reflectivity and optical conductivity. The direct band gap and the high reflectivity of these compounds in the visible and ultraviolet regions of electromagnetic energy spectrum ensure their applications in optoelectronic and photonic domains.     

ZnO-B_2O_3玻璃对Mg_2TiO_4微波介质陶瓷结构和性能的影响

利用传统固相烧结法制备了ZnO-B2O3玻璃掺杂的Mg2TiO4微波介质陶瓷,研究了ZnO-B2O3玻璃掺杂对所制陶瓷相成分、微观形貌和微波介电性能的影响。结果表明:ZnO-B2O3玻璃掺杂能使Mg2TiO4陶瓷的致密化温度降低200℃左右。当Mg2TiO4中掺杂质量分数2%的ZnO-B2O3玻璃时,经1 300℃烧结所得陶瓷微波性能较好:εr=13.62、Q.f=101 275 GHz、τf=–51×10–6/℃。     

Prediction of semiconducting behavior in minority spin of Co2CrZ (Z = Ga, Ge, As): LSDA

Volume optimization was performed to obtain the theoretical lattice constants by using the generalized gradient approximation (GGA). The electronic and magnetic properties of Heusler alloys Co₂CrZ (Z = Ga, Ge, As) were investigated by using local spin density approximation (LSDA). Amongst the systems under investigation, Co₂CrGe and Co₂CrGa give 100% spin polarization at the Fermi level (E_F). Co₂CrGe and Co₂CrGa are the most stable half-metallic ferromagnets (HMFs); their E_F lie exactly at the gap of 0.24 eV and 0.38 eV, respectively, in the spin-down channel. Even though Co₂CrAs gives a distinct and bigger gap as compared to Co₂CrGa and Co₂CrGe, its E_F is not located at the middle of the gap in the spin-down channel. We have also found that the total magnetic moments increase as the Z goes from Ga to As. The calculated density of states and band structures show the HMF character for Co₂CrGe and Co₂CrGa.