First principles calculations of the electronic structure and optical properties of (001), (011) and (111) Ga0.5Al0.5As surfaces

Using the first-principles plane-wave pseudo-potential method based on density function theory (DFT), the electronic structure and optical properties of Ga_0.5Al_0.5As (001), (011) and (111) surfaces are calculated. Result shows that (001) surface is reconstructed, (011) surface is not reconstructed but wrinkled, (111) surface is only relaxed. (111) is the most stable surface. (001) surface owns the lowest work function. Absorption coefficient and reflectivity of these surfaces are smaller than bulk, the transmittance of the surfaces are larger than the bulk, which is helpful for the incident light to excite photoelectrons. The decrease of the absorption coefficient and reflectivity at (001) surface are the largest. Calculation of electronic structure and optical properties predict that the (001) surface should have the strongest photoemission.     

The effects of Al doping and post-annealing via intrinsic defects on photoluminescence properties of ZnO:Eu nanosheets

Europium (Eu) and Aluminum (Al) co-doped ZnO nanosheets were synthesized by a hydrothermal method. The effects of Al concentration as a dopant and post-annealing of ZnO:Eu nanosheets on its structural, electrical and optical properties were investigated in detail. Prepared samples were characterized structurally using X-ray diffraction (XRD), morphologically using scanning electron microscopy (SEM) and optically using photoluminescence (PL) spectroscopy analyses. No diffraction peak related to dopants in XRD spectrum along with shift in peaks angles relevant to ZnO proved that Al and Eu ions were doped successfully into ZnO nanosheets. This study recommends that extrinsic doping and intrinsic defects have impressive roles on transferring energy to Eu ions at indirect excitations. Based on photoluminescence observations, intra-4f transitions of Eu³⁺ ions at an excitation wavelength of 390 nm allow a sharp red luminescence. Also the results showed that optical properties of ZnO can be tuned by varying the amount of Al concentration. In comparison with annealed Al doped ZnO:Eu nanostructures, as-grown samples showed the stronger PL peaks which indicated the effective role of intrinsic defects beside of extrinsic doping on energy transfer from ZnO host to Eu³⁺ ions which consequently led to producing the strong red emission from these sites.     

sol-gel法制备的ZnO:(Al,La)透明导电膜光电性能

通过X射线衍射、紫外–可见分光光度计、扫描电镜和四探针仪分析等手段,考察了退火温度对ZnO:(Al,La)薄膜微观结构、光学和电学性能的影响,Al掺杂浓度对电阻率的影响。结果表明:随退火温度的升高,薄膜(002)晶面择优取向生长增强,平均晶粒尺寸增大,电阻率降低,透光率上升。在x(Al)为1%,退火温度550℃时,薄膜最低电阻率为1.78×10–3?·cm,平均透光率超过85%。     

Co、Mn的掺杂形式对低压氧化锌压敏陶瓷电性能的影响

研究了Co、Mn的不同掺杂形式对低压ZnO压敏电阻显微结构和电性能的影响。发现以Co(NO3 ) 2 、Mn (NO3 ) 2 溶液代替CoO、MnO2 掺杂 ,可以降低压敏电压 ,增大非线性系数。这主要与钴的高价态有关 ,利于压敏电阻的低压化。     

溅射功率对Zr,Al共掺杂ZnO薄膜结构和性能的影响

室温下,采用直流磁控溅射法,在载玻片衬底上制备出了Zr,Al共掺杂ZnO(AZZO)透明导电薄膜。研究了溅射功率对薄膜的组织结构、表面形貌和光电学性能的影响。结果表明,制备的AZZO透明导电薄膜为六角纤锌矿结构的多晶薄膜,且具有c轴择优取向。当溅射功率为150W时,薄膜电阻率达到最小值1.66×10–3Ω·cm,在可见光区平均透过率超过93%。     

LiNbO_3/ZnO:Al集成结构的外延生长及电性能研究

采用脉冲激光沉积(PLD)法在蓝宝石衬底上先后外延生长了ZnO:Al(ZAO)和LiNbO3(LN)薄膜。通过X射线衍射分析(XRD)可知二者之间的外延关系为:LN(001)//ZAO(001)、LN[110]//ZAO[110]、LN[100]//ZAO[120]。制备了Au/LN/ZAO和ZAO/LN/ZAO两种电容器结构,对其进行了电流-电压(J-E)测试和铁电(P-E)分析,结果表明:LN/ZAO集成结构具有整流作用,ZAO/LN/ZAO结构表现出较好的绝缘性能,所制备的LN薄膜在室温下的剩余极化强度(Pr)约为1×10–6C/cm2,温度的升高能够促进电畴的翻转,使Pr增加为3×10–6C/cm2。     

Structural,chemical bonding and optoelectronic properties of Mg doped zinc chalcogenides: A first principles study

A first-principal technique is employed to investigate the concentration dependence of the structural, electronic band structure, optical and chemical bonding properties of Zn_1−xMg_xS, Zn_1−xMg_xSe and Zn_1−xMg_xTe alloys. Structural parameters such as lattice constants and bulk moduli are found to vary non-linearly with changing concentration x and deviating from Vegard׳s law. Parent binaries as well as ternary alloys have a direct band gap (Γ–Γ) which increases non-linearly with increment in concentration. Chemical bonding nature changes from strong covalency to partial ionic character in increasing Mg-contents. The direct band gap and high optical activity in visible and ultraviolet range reveal the implication of these alloys in the optoelectronic devices applications.     

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.     

First principles study of structural,electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH_x (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH₂ and from hexagonal to cubic phase in GdH₃. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH₃. The calculated magnetic moment values of GdH_x (x=1, 2, 3) are in accord with the experimental values.     

Growth and characterization of ZnO nanowires grown on the Si(1 1 1) and Si(1 0 0) substrates: Optical properties and biaxial stress of nanowires

In this paper the effects of silicon substrates with different orientations on the morphological and optical properties as well as biaxial stress of ZnO nanowires were investigated. The ZnO nanowires were grown on Si(1 0 0) and Si(1 1 1) substrates by the vapor–solid (VS) method using a physical vapor deposition reactor. In addition ZnO nanowires were grown on Si(1 1 1) substrate by the vapor–liquid–solid (VLS) method using an Au film as catalyst, which were deposited on Si(1 1 1) substrate using a sputtering method, with the same conditions. Room temperature photoluminescence (PL) spectrum showed a stronger ultraviolet (UV) peak at 381 nm for the nanowires that were grown on Si(1 1 1) by the VS method than those that were grown on Si(1 0 0) with the same green emission (deep-level emission (DLE)) intensities at about 520 nm peak. On the other hand, the PL result of the ZnO nanowires, which were grown by the VLS method, showed the same intensities for the both UV and DLE peaks. Furthermore, the effects of silicon substrate orientation and Au catalyst on biaxial stress of the nanowires were studied by Raman spectrometer. It was discussed that Au catalyst was one of the important factors that could affect the biaxial stress value of the ZnO nanowires that were grown on Si substrates.     

Local electronic structure,optical bandgap and photoluminescence (PL) properties of Ba(Zr0.75Ti0.25)O3 powders

Ba(Zr_0.75Ti_0.25)O₃ (BZT-75/25) powders were synthesized by the polymeric precursor method. Samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Their electronic structures were evaluated by first-principle quantum mechanical calculations based on density functional theory at the B3LYP level. Their optical properties were investigated by ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL) measurements at room temperature. XRD patterns and Rietveld refinement data indicate that the samples have a cubic structure. XANES spectra confirm the presence of pyramidal [TiO₅] clusters and octahedral [TiO₆] clusters in the disordered BZT-75/25 powders. EXAFS spectra indicate distortion of Ti–O and Ti–O–Ti bonds the first and second coordination shells, respectively. UV-Vis absorption spectra confirm the presence of different optical bandgap values and the band structure indicates an indirect bandgap for this material. The density of states demonstrates that intermediate energy levels occur between the valence band (VB) and the conduction band (CB). These electronic levels are due to the predominance of 4d orbitals of Zr atoms in relation to 3d orbitals of Ti atoms in the CB, while the VB is dominated by 2p orbitals related to O atoms. There was good correlation between the experimental and theoretical optical bandgap values. When excited at 482 nm at room temperature, BZT-75/25 powder treated at 500 °C for 2 h exhibited broad and intense PL emission with a maximum at 578 nm in the yellow region.     

The interplay of electronic structure,molecular orientation and charge transport in organic semiconductors: Poly(thiophene) and poly(bithiophene)

Ultrathin films of poly(thiophene) (PT) and poly(bithiophene) (PBT) were prepared by electrochemical route using ionic liquid (BFEE) as medium and electrolyte. Distinct morphologies and electrical properties were observed in these materials. To evaluate its response in photovoltaics, these films were used as active layer in bilayer geometry solar cells with the electron acceptor molecule C₆₀. The best performance was observed for PT films. In order to probe the differences in molecular dynamics and structural order, ultrafast electron dynamics in the low-femtosecond regime was evaluated by resonant Auger spectroscopy using the core–hole clock method at the sulfur K absorption edge. Electron delocalization times for the different polymeric films were derived as a function of the excitation energy. Photoabsorption measurements were conducted and molecular orientation derived. These results corroborated with the morphology found for these films and thus the performance of PT and PBT in the devices, and with the proposed conduction mechanism.     

Optical and electrical properties of epitaxial (Mg,Cd)xZn1−xO, ZnO, and ZnO:(Ga,Al) thin films on c-plane sapphire grown by pulsed laser deposition

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0 0 0 1) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy.The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band.The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 × 10¹⁴ to 3 × 10²⁰ cm⁻³, which corresponds to a resistivity from 5 × 10⁻⁴ to 3 × 10³ Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm²/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far.Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wide-band gap material for future electronic devices.     

First-principles study of structural,electronic, elastic and thermal properties of intermetallic ternary compounds (RMn2Si2: R=Ce and Nd)

In this study, the structural, magnetic, electronic, elastic and thermal properties of the ternary intermetallic, RMn₂Si₂ (R=Ce and Nd), compounds are presented. The study is carried out by employing the full-potential (FP) linearized augmented plane wave (LAPW) plus local orbital (lo) approach based on the density functional theory (DFT). To depict the exchange-correlation energy (an important component of total energy calculations), the local-density approximation and the local spin density approximation (LDA/LSDA) are used. Our calculated results for equilibrium lattice parameters are in good agreement with the available experimental measurements. The total energy calculations reveal the strong dependence to the distance between atomic species in these compounds. The analysis of the partial and total densities of states (DOS) of both compounds (CeMn₂Si₂ and NdMn₂Si₂) demonstrates their metallic and magnetic character as well. Whereas the calculated values of Poisson׳s ratio and B/G present their brittle makeup. At the end, using a quasi-harmonic Debye model as implemented in GIBBS code, the thermal properties were calculated.     

Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.