掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

On the effectiveness of dislocation loops as sites for the adsorption of implanted ions

It is commonly found that when conventional dopant ions are implanted into a semiconductor, only a fraction of the total number implanted will exhibit the electrical behavior that would be expected for an atom of that type in a regular substitutional position. In this paper we present semi-quantitative calculations which suggest that many of the implanted ions can be adsorbed on dislocation loops that are produced when the implanted layer is annealed. The effect is particularly large when there is a significant size mismatch between the implanted ion and the host atom.

The theory gives a reasonable fit to experimental results if it is assumed that the adsorbed ions are either electrically inactive or else characterized by relatively large carrier ionization energies.     

Ceria: Relation among thermodynamic,electronic hole and proton properties

The proton solubility and the hole conductivity of the rare earth doped ceria have been examined in their relations to the thermodynamic properties of doped ceria under the assumption that the hypothetical species, LnOOH and LnOO (Ln = Rare earth), can be regarded as constituents for representing protons and holes in the fluorite lattice. Focus is made on the dopant dependence, the host dependence and the temperature dependence in the rare earth doped zirconia(or ceria) fluorite lattice. The chemical potentials of the rare earth dopant are less stabilized in the ceria-based oxides than in the zirconia-based ones. The proton solubility in the ceria-based, zirconia-based, and ceria–zirconia solid solutions has been well interpreted in terms mainly of the hydroxidation energy and the stabilization energy of LnO_1.5 in the fluorite lattice. Since the dopant dependence of the stabilization energy of LnO_1.5 is stronger than the hydroxidation energy, the proton solubility becomes high in the smaller dopants. To account for less dopant-dependent behavior in the hole conduction, the peroxidation energy is assumed to have about the same dopant dependence as the stabilization energy. The calculated temperature dependences of proton solubility and hole concentration were compared with available experimental data; it has been suggested that holes and protons in ceria reach to saturation levels with lowering temperature. Some discussions are made on the possible explanation on recently observed anomalous hole conductivity in nano-size Ce_0.8Gd_0.2O_1.9 in terms of plausible effects of miscibility gap, associated Gd enrichment, and simultaneous formation of Ce³⁺ and holes.     

Influence of Tb incorporation on the structural and the optical properties of ZnO nanoparticles

Structural and optical properties of the Tb doped ZnO nanoparticles are systematically studied as a function of the Tb mole-fraction. Our study suggests that the Tb incorporates mostly on the surface and affects the optical properties of the ZnO nanoparticles by influencing the attachment of certain adsorbed groups, which are found to be responsible for the appearance of a broad green luminescence (GL) band in the photoluminescence spectra recorded for these nanoparticles. It has been found that the accumulation of Tb on the surface of the nanoparticles not only enhances the band edge to green luminescence intensity ratio under the vacuum condition but also increases the band gap energy by introducing a hydrostatic compressive strain in individual nanoparticles, which provides a unique opportunity to study the pressure dependence of the optical properties of nanoparticles without applying any external pressure. The hydrostatic compressive strain is explained in terms of the increase of the surface strain energy as a result of the Tb accumulation on the surface of the nanoparticles. The average value of the surface energy density for the particles has been estimated as a function of Tb mole-fraction. The pressure coefficient of the band gap which is obtained from the variation of the band gap energy with the hydrostatic strain has been found to decrease significantly with the particle size for the ZnO nanoparticles.     

Effect of Ir(pq)2acac doping on CBP in phosphorescence organic light-emitting diodes

Doping is one of the most powerful methods amongst the various performance improvement ways. Doping affects the energy levels of the host layer by the energy level of the dopant. This allows the energy bandgap to be adjusted to a desired level and thus generates light corresponding to that energy level. Alternatively, it can act as an energy barrier between the interfaces to change the flow of carriers. In this study, the voltage dependences of undoped and doped devices were observed. Bis(2-phenylquinoline) iridium(III) (acetylacetonate) (Ir(pq)₂acac) was doped in 4,4′-N, N′-dicarbazole-biphenyl (CBP) as the emission layer. The light intensity changes with the doping concentration, and the efficiency was also studied. When a high voltage was applied, the effect of triplet-triplet annihilation (TTA) adversely affected the electron-hole recombination. We analyzed the optimal operating conditions and the effect of doping concentration on OLEDs.     

Strain Directed Assembly of Nanoparticle Arrays Within a Semiconductor

The use of strain to direct the assembly of nanoparticle arrays in a semiconductor is investigated experimentally and theoretically. The process uses crystal strain produced by a surface structure and variations in layer composition to guide the formation of arsenic precipitates in a GaAs-based structure grown at low temperature by molecular beam epitaxy. Remarkable patterning effects, including the formation of single and double one-dimensional arrays with completely clear fields are achieved for particles in the 10-nm size regime at a depth of about 50-nm from the semiconductor surface. Experimental results on the time dependence of the strain patterning indicates that strain controls the late stage of the coarsening process, rather than the precipitate nucleation. Comparison of the observed particle distributions with theoretical calculations of the stress and strain distributions reveals that the precipitates form in regions of maximum strain energy, rather than near extremum points of hydrostatic stress or dilatation strain. It is therefore concluded that the patterning results from modulus differences between the particle and matrix materials rather than from other strain related effects. The results presented here should be useful for extending strain directed assembly to other materials systems and to other configurations of particles.     

Efficient,long-life and Lambertian source of top-emitting white OLEDs using low-reflectivity molybdenum anode and co-doping technology

A top-emitting white organic electroluminescent device (TWOLED) established on low-reflectivity molybdenum (Mo) anode (R ∼ 58% in visible range) for alleviating the undesirable microcavity effect and achieving broad as well as similar emission spectra at different viewing angles has been demonstrated based on a dual-layer emitter composed of doped blue and yellow dopant in the same blue host. The electroluminescent (EL) efficiency and operational lifetime of TWOLED can be further enhanced by a factor of 1.2 and 3.4, respectively, when a co-doping technology of adding another blue dopant into the yellow emitter was incorporated. The enhanced efficiency is attributed to the improved energy transfer from blue host to yellow dopant by cascade process.     

Influence of Dopants in ZnO Films on Defects

The influence of dopants in ZnO films on defects is investigated by slow positron annihilation technique. The results show S that parameters meet S_Al>S_un>S_Ag for Al-doped ZnO films, undoped and Ag-doped ZnO films. Zinc vacancies are found in all ZnO films with different dopants. According to S parameter and the same defect type, it can be induced that the zinc vacancy concentration is the highest in the Al-doped ZnO film, and it is the least in the Ag-doped ZnO film. When Al atoms are doped in the ZnO films grown on silicon substrates, Zn vacancies increase as compared to the undoped and Ag-doped ZnO films. The dopant concentration could determine the position of Fermi level in materials, while defect formation energy of zinc vacancy strongly depends on the position of Fermi level, so its concentration varies with dopant element and dopant concentration.     

Ge/Si半导体量子点的应变分布与平衡形态

基于各向异性弹性理论的有限元方法，研究了金字塔形自组织Ge/Si半导体量子点应变能随高宽比变化的规律：系统的应变能随着高宽比的增大而逐渐减小.并通过自由能(应变能与表面能之和)讨论了量子点的平衡形态.结果表明，对于固定体积的量子点，存在一个高宽比值，称之为平衡高宽比，使得系统的自由能最低.同时，还给出了量子点的应力、应变、流体静应变及双轴应变分布.这些可以作为阐明应变自组织量子点实验的理论基础. 关键词： 量子点 应变分布 自由能 平衡形态     

B、N单掺杂3C-SiC电子结构和光学性质的第一性原理研究

采用第一性原理的密度泛函理论赝势平面波方法,计算了未掺杂与B、N单掺杂3C-SiC的电子结构和光学性质.结果表明:掺杂改变了3C-SiC费米面附近的电子结构;B掺杂使得禁带宽度减小,价带顶上移,费米能级进入价带,形成p型半导体;N掺杂使得禁带宽度减小,导带底下移,费米能级进入导带,形成n型半导体.B、N掺杂均提高了3C-SiC在低能区的折射率、消光系数和吸收系数,增强了对红外光谱的吸收.     

First-Principles Calculation of Electronic Structure and Optical Properties of Sb-Doped ZnO

The geometric structure, electronic structure, optical properties and the formation energy of Sb-doped ZnO with the wurtzite structure are investigated using the first-principles ultra-soft pseudo-potential approach of plane wave based upon the density functional theory. The calculated results indicate that the volume of ZnO doped with Sb becomes larger, and the doping system yields the lowest formation energy of Sb on the interstitial site and the oxygen site. Furthermore, Sb dopant first occupies the octahedral oxygen sites of the wurtzite structure. It is found that Sb substituting on oxygen site behaves as a deep acceptor and shows the p-type degenerate semiconductor character. After doping, the electron density difference demonstrates the considerable electron charge density redistribution, which induces the effect of Sb-doped ZnO to increase the charge overlap between atoms. The density of states move towards lower energy and the optical band gap is broadened. Our culated results are in agreement with other experimental results and could make more precise monitoring and controlling possible during the growth of ZnO p-type materials.     

First-principles study of ferromagnetism in N doped TiO2 and TiO

N doped TiO is nonmagnetic, in which spin-split impurity states are not induced near the Fermi energy (E_F) by N dopant. N doped TiO₂ along with transition-metal (TM) doped TiO is magnetic, in which spin-split impurity states are induced across E_F. The magnetic moment is determined by the 3d4s electron configurations and the valence states of TM-dopant ions when they substitute Ti. Hence, the origin of ferromagnetism of N doped TiO₂ and TiO is not closely related to the width of the band gaps of host oxides, but would be crucially related to that if the dopant can induce spin-split impurity states near E_F.     

Electronic and magnetic properties of V- and Cr-doped zinc-blende AlN

The electronic and magnetic properties of the zinc-blende aluminum nitride doped with V and Cr are studied using the density functional theory (DFT), namely the KKR-CPA-PBE method. Pure AlN is found to be a wide band gap semiconductor, and doping V and Cr single impurities generate ferromagnetic half-metallic behavior. Moreover, the values of the formation energy reveal that these compounds are stable systems for all dopant concentrations. A self-consistent energy minimization scheme determines the ferromagnetic state as the stable magnetic state for V- and Cr-doping AlN. A double exchange mechanism is identified as the mechanism responsible for magnetism in our systems. When increasing doping impurities, the total magnetic moments increase linearly and the Curie temperature T_C, calculated using the mean-field approximation, shows a significant change. The present findings reveal Cr- and V-doped zinc-blende AlN as potential candidates for high Curie temperature ferromagnetic materials.     

有机半导体的物理掺杂理论

基于最低未被占据分子轨道（LUMO）和最高被占据分子轨道（HOMO）的高斯态密度分布与载流子在允许量子态中的费米-狄拉克（Fermi-Dirac）分布,提出有机半导体中物理掺杂的理论模型;研究了掺杂浓度、温度和禁带宽度对载流子浓度的影响,并与一些报道的实验结果做了比较.研究发现无论是否掺杂,温度升高时,有机半导体中的载流子浓度都会增大,并且随温度倒数的线性减小而指数增大;对于本征有机半导体,载流子浓度随禁带宽度的增大而指数下降,随高斯分布宽度的平方指数增加;对杂质和主体不同能级关系的掺杂情形下掺杂浓度对载 关键词： 有机半导体 掺杂 高斯态密度 载流子浓度     

关於固体的现实应力空间(续)

前文[1]综合四理论[2],[3],[4],[5]构成固体现实应力空间之一初步理论,大体反映固态静力学性质,对金属较对非金属固体反映得当,后者受范形变曲面有异于弥氏圆柱。总起来看,前文仅涉及原则概念,未触及具体问题。为使此理论对金属压力加工及材料试验研究有所帮助,本文进一步研究几个问题:1)由应力空间图形比较不同金属的静力学性质;2)受范形变效率及其计算;3)形变过程之轨迹;并得到一定数量或质量上的结论。同时,附带对前文[1]中一个实验记录图的错误作修正,包括在附录内。     

A study of oxygen ion conductivity in doped non-stoichiometric oxides

In doped non-stoichiometric oxides that show oxygen ion conductivity, association commonly takes place between the dopant cations and the compensating oxygen vacancies. The activation energy thus comprises two parts, a migration enthalpy and an association enthalpy. We have determined the effects of structure and host cation type on the migration enthalpies, and the effect of dopant cation size on the association enthalpies. This we did by a variety of methods including theoretical calculations and experiment. We have further reviewed the literature in order to verify our calculations and we conclude that size terms in the association enthalpies are the most important factor in the determination of the magnitude of oxygen ion conduction.     

Highly efficient and bright doped organic electroluminescent diodes using an aluminum electrode

Remarkable improvement in efficiency and electroluminescence (EL) has been observed in an organic EL device, which consists of a hole-transport layer and a luminescent layer. The hole-transport layer is an N,N-bis(3-methyphenyl)-N,N-diphenylbenzidine film. The doped emitting layer consists of 8-(quinolinolate)-aluminum as the host and rubrene as the emission dopant. The doped cell with aluminum cathode demonstrated a luminance in excess of 20,000 cd/m² and an external quantum efficiency of 2.7%, which is about four times and three times, respectively, greater than those of the undoped cell. The EL emission from the device shows spectral narrowing and a shift to higher energy.     

5,6,11,12-四苯基四苯并掺杂8-羟基喹啉铝薄膜的荧光谱及发光机理

用高荧光染料的5，6，11，12－四苯基四苯并对8－羟基喹啉铝进行掺杂，测量其光致发光和电致发光谱。结果表明：在低掺杂时，主发光体是Alq，掺入的Rubrene作为客发光体只是在Alq带隙中引入了分立能级；随着掺入的Rubrene浓度增加，Rubrene成了主发光体，Alq变成了客发光体，出现了发光体的互换现象。由于Rubrene的吸收光谱与Alq的发射谱重叠较大，在光致发光中存在从Alq向Rubrene的能量传递和电荷转移过程，而电致发光则是由于Rubrene导带中电子浓度远大于注入到Alq导带中电子浓度，造成Rubrene导带电子与价带空穴复合的几率比Alq中的复合几率大得多，其EL主要是Rubrene的发光。     

Synthesis and optical properties of phosphorus-doped ZnO nanocombs

Phosphorus-doped ZnO nanocombs with double-sided symmetry were prepared by a modified thermal-evaporation process using phosphorus pentoxide as the dopant source. Scanning electron microscopy and transmission electron microscopy reveal that the single-crystal teeth in the doped nanocombs grow along [0002] direction and that the formation of the tapered tip should be related to the lattice strain caused by the incorporation of phosphorous. Raman spectra further reveal the influence of phosphorus doping on the lattice dynamics of ZnO, and display the amphoteric nature of phosphorus dopant. The p-type doping is confirmed by the temperature-dependent photoluminescence spectra, and the acceptor binding energy is estimated to be about 165 meV.     

Excitations in doped quantum dot insisted by discontinuous reversals of static electric field: Interplay between pulse and dopant site

We explore the excitation profile of a repulsive impurity doped quantum dot. The quantum dot is subject to a discontinuously reversing static electric field. The dopant impurity potential chosen assumes Gaussian form. The investigation reveals the key role played by the dopant location and the number of pulses offered by the external field to the dot in controlling the excitation rate. Time-dependent Hellman-Feynman theorem has been invoked to understand the extent of energy transfer between field direction and the direction where no field is applied. The combined transition rate from ground to other excited states is also determined to support the findings.