Photoluminescence characterization of polycrystalline n-type Cu2O films

Electrodeposition was used to deposit Cu₂O thin films on ITO substrates. Photoresponse of the film clearly indicated n-type behavior of Cu₂O in photoelectrochemical cells. The temperature dependence of photoluminescence (PL) revealed that the spectra consist of donor-acceptor pair emissions and the recombination between electrons bound to donors and free holes. We observed that the dominant intrinsic defect, oxygen vacancies, creates a donor energy level at 0.38 eV below the bottom of the conduction band. As a result, this donor level acts as a center for both PL emissions and to produce n-type conductivity in the electrodeposited Cu₂O films. In addition, an acceptor energy level at 0.16 eV from the top of the valence band was observed.     

第一原理研究Sn(O1-xNx)2材料的光电磁性质

基于第一原理的密度泛函理论, 以量子化学从头计算软件 为平台研究了Sn(O_1-xN_x)₂材料的光电磁性能, 分析了体系的态密度、 能带结构、 磁性、 介电虚部及折射率. 计算结果表明, N替代O后, 随着掺杂浓度的增加, 体系的带隙先减小后增大, 掺杂量为12.50%时带隙最窄. 由于N 2p轨道电子的贡献, 在0.55-1.05 eV范围内产生了浅受主能级, 价带和导带处的能级均出现了劈裂及轨道的重叠现象, Sn-O键的键强大于N-O键的键强. 从磁性来看, N原子决定了磁矩的大小. 从介电虚部可知, 掺杂后体系的光学吸收边增宽, 主跃迁峰发生红移, 反射率和介电谱相对应, 各峰值与电子的跃迁吸收有关.     

SrTiO3中形成级联能级的理论分析

以Bi和Cu掺杂为例,通过基于密度泛函理论的电子结构的计算,分析了SrTiO₃体系中形成级联能级的可能性.结果表明,Bi掺杂和Cu掺杂都可以在SrTiO₃的禁带中引入杂质能带,Bi和Cu的共同掺杂可引入两条杂质能带.通过在两条杂质能带之间级联激发,价带顶的电子可以受激跃迁到导带底.采用无辐射跃迁的简单模型,分析指出电子通过级联激发从价带顶受激跃迁到导带底的概率远远高于直接从价带顶跃迁到导带底的概率.这种级联激发可以有效提高导带中的载流子浓度. 关键词： 级联能级 密度泛函 掺杂     

Electrical properties and emission mechanisms of Zn-doped β-Ga2O3 films

We study the electrical properties and emission mechanisms of Zn-doped β-Ga₂O₃ film grown by pulsed laser deposition through Hall effect and cathodoluminescence which consist of ultraviolet luminescence (UV), blue luminescence (BL) and green luminescence (GL) bands. The Hall effect measurements indicate that the carrier concentration increases from 7.16×10¹¹ to 6.35×10¹² cm⁻³ with increasing a nominal Zn content from 3 to 7 at%. The UV band at 272 nm is not attributed to Zn dopants and ascribed as radiative electron transition from conduction band to a self-trapped hole while the BL band is attributable to defect level related to Zn dopant. The BL band has two emission peaks at 415 and 455 nm, which are ascribed to the radiative electron transition from oxygen vacancy (V_O) to valence band and recombination of a donor–acceptor pair (DAP) between V_O donor and Zn on Ga site (Zn_Ga) acceptor, respectively. The GL band is attributed to the phonon replicas’ emission of the DAP. The acceptor level of Zn_Ga is estimated to be 0.26 eV above the valence band maximum. The transmittance and absorption spectra prove that the Zn-doped β-Ga₂O₃ film is a dominantly direct bandgap material. The results of Hall and cathodoluminescence measurements imply that the Zn dopant in β-Ga₂O₃ film will form an acceptor Zn_Ga to produce p-type conductivity.     

Atomic and electronic structures of Mg-doped perfect SrTiO3 and crystals containing oxygen vacancies from first principles

The electronic properties of Mg-doped perfect SrTiO₃ and crystals containing oxygen vacancies systems are investigated by first principles calculation. Dopant formation energy results show that the Mg atoms preferentially enter the Sr site in SrTiO₃. Substitution of Ti by Mg brings some acceptor levels, which introduces the p-type conductivity of SrTiO₃. Creation of oxygen vacancies in SrTiO₃ introduces donor levels, which can contribute to the n-type conductivity. In SrTiO₃ containing oxygen vacancies system, a self-compensation effect will occur when Ti is substituted by Mg, and the system undergoes n-type to p-type transition.     

Electronic structure of TiH2

The electronic structure of TiH₂ has been studied using the augmented-plane-wave method and the LCAO interpolation. The density of states and its orbital components show that the conduction band is Ti d-like and that the valence band is largely derived from the hydrogen orbitals with small Ti 3d hybridization. The electronic charges on the hydrogen atom are ～ 1.5 as compared to 1.6–1.7 of the rare-earth metal hydrides.     

The electronic structure of the valence bands of solid NH3 and H2O studied by ultraviolet photoelectron spectroscopy

The results of a photoelectron study using ultraviolet 40.81 eV photons (UPS) of the outermost bands of the molecular solids NH₃ and H₂O are reported. The binding energies, the energy separation, the band widths and the branching ratio of the two outermost bands of solid NH₃ are found not to be significantly different from the 3a_l and 1e molecular orbital states of the gaseous NH₃ UPS spectrum. This implies that hydrogen bonding has not produced any significant change in the electronic structure of the valence bands of solid NH₃. Because of a much smaller intermolecular hydrogen bond length in solid H₂O compared to that in solid NH₃, the hydrogen bond does, however, produce a significant change in the valence bands of H₂O on solidification, and because of the orbital geometry it predominantly affects the 3a_l molecular orbital state.     

Electronic structure of the valence band for perovskite-type titanium double oxides studied by XPS and DV-Xα cluster calculations

XPS spectra of the valence bands for the perovskite-type titanium double oxides BaTiO₃, SrTiO₃ and CaTiO₃ have been measured and analyzed by means of DV-Xα calculations for the TiO₆ embedded cluster model. The theoretical photoelectron spectra modulated by the photoionization cross-sections are in good agreement with experiment. The XPS results show that the O 2p valence band is constructed of two peaks whose spacing becomes larger in the order BaTiO₃ < SrTiO₃ < CaTiO₃. The DV-Xα results indicate that the greater part of the lower-energy peak is attributed to the levels which have O 2p orbitals pointing to Ti cations and are thus stabilized by the electrostatic potential concomitant with a decreased TiO bond distance. The electrostatic potential also reduces the O 2p-Ti 3d mixing and, together with the repulsion of the electron cloud between the Ti and O ions, makes the TiO bond more ionic.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

Y掺杂SrTiO3晶体材料的电子结构计算

采用基于第一性原理的密度泛函理论平面波超软赝势法, 研究了Y掺杂SrTiO₃体系的空间结构和电子结构性质, 得到了优化后体系的结构参数, 掺杂形成能, 能带结构和电子态密度. 对比掺杂浓度为0125, 025, 033时,Sr_1-xY_xTiO₃和SrTi_1-xY_xO₃的掺杂形成能,发现Y替代Sr能形成更稳定的结构. 对Sr_1-xY_xTiO₃（x=0, 0125, 025, 033） 的结构进行了优化,结果表明Y替代Sr后, 随着掺杂浓度增大, 体系的晶格常数逐渐减小, 稳定性逐渐增强. 对不同掺杂浓度的Sr_1-xY_xTiO₃能带结构的计算结果表明：纯净的SrTiO₃是绝缘体, 价带顶在R点, 导带底在Γ点, 费米能级处于价带顶; 掺杂Y后, 费米能级进入到导带底中, 体系呈金属性;掺杂浓度越大,费米能级进入导带的位置越深,禁带宽度也近似变宽. 关键词： 3')" href="#">SrTiO₃ 电子结构 掺杂 VASP     

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi₂O₅ glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.     

Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical function and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO₃ films deposited on KTaO₃ (100) single-crystal substrates. Optical functions in the 200–1200 nm spectral range were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO₃ films and for homoepitaxial strain-free SrTiO₃: Cr (0.01 at %) films deposited on SrTiO₃ (100) single crystalline substrates showed that this “blue” shift of the band gap could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of the polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO₃ films.     

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb₁₆(OH)₁₆(NO₃)₁₆ single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70 eV in close agreement to the measured one (3.78 eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbO_n ployhedra and N–O of [NO₃]⁻ triangle. The calculated bond lengths and angles show good agreement with the experimental data.     

Energy levels of rare-earth ions in Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The energies of the ground 4f ⁿ levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd₂O₂S crystal has been determined. It is shown that the Pr³⁺, Tb³⁺, and Eu³⁺ ions can be luminescence centers in Gd₂O₂S. The levels of the Nd³⁺, Dy³⁺, Er³⁺, Tm³⁺, Sm³⁺, and Ho³⁺ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce³⁺ ion is near the midgap, due to which Ce³⁺ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd₂O₂S:Pr and Gd₂O₂S:Tb phosphors.     

First-principles study of ferromagnetism in Ti-doped ZnO with oxygen vacancy

Using first-principles density functional calculations, we have investigated the electronic structures of Ti-doped ZnO (Ti is in 4+ oxidation state) with and without oxygen vacancy. The ferromagnetic property is identified in the presence of oxygen vacancy despite Ti being nonmagnetic in its natural phase. The ferromagnetism originates from the charge transferring from donor derived-defect band to unoccupied Ti-3d states and the hybridization between Ti-3d and O-2p band near the Fermi level. On increasing the oxygen vacancy concentration, a transition from a long-ranged magnetic order to a short-ranged interaction is found and the oxygen vacancies prefer to distribute non-uniformly in Ti-doped ZnO.     

DFT study on electronic structures and optical absorption properties of C, S cation- doped SrTiO3

The effects of C cation and S cation doping on the electronic structures and optical properties of SrTiO₃ are investigated by density function theory (DFT) calculations. The calculated results reveal that the top of the valence band is predominately made up of the O 2p states for the pure SrTiO₃. When SrTiO₃ was doped with C cation and S cation, the top of the valence bands consists mainly of O 2p+C 2s hybrid orbitals and O 2p+S 3s hybrid orbitals, respectively. The band gap of SrTiO₃ is narrowed by the doping with C cation and S cation, especially for the C and S-codoped SrTiO₃. Moreover, the red shifts of the absorption edge are found by the calculated optical properties, which is consistent with reported experiment results. It is the explanation for their visible light respondency by the presence of C 2s and S 3s states on the upper edge of the valence band. All of these results can explain the good photocatalytic properties of C, S cation-codoped SrTiO₃ under visible light irradiation.      

The gold donor and acceptor level in p-type silicon

DLTS measurements on the gold donoe and acceptor level in p-type silicon and reported. From the experimental data it must be concluded that the gold donor and the acceptor level are caused by the same defect, that the acceptor level lying 0.546eV below the conduction band is temperature independently pinned to the conduction band and that the hole capture cross section of the acceptor level

_P^A is temperature independent in the measured range of 215 to 345 K.     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd 关键词： 密度泛函理论 电子结构 Cd掺杂ZnO