Thermal equilibrium concentrations and effects of negatively charged Ga vacancies in n-type GaAs

We have calculated the thermal equilibrium concentrations of the various negatively charged Ga vacancy species in GaAs. The triply-negatively-charged Ga vacancy, V _Ga ^3– , has been emphasized, since it dominates Ga self-diffusion and Ga-Al interdiffusion under intrinsic and n-doping conditions, as well as the diffusion of Si donor atoms occupying Ga sites. Under strong n-doping conditions, the thermal equilibrium V _Ga ^3– concentration, , has been found to exhibit a temperature independence or a negative temperature dependence, i.e., the value is either unchanged or increases as the temperature is lowered. This is quite contrary to the normal point defect behavior for which the point defect thermal equilibrium concentration decreases as the temperature is lowered. This property provides explanations to a number of outstanding experimental results, either requiring the interpretation that V _Ga ^3– has attained its thermal equilibrium concentration at the onset of each experiment, or requiring mechanisms involving point defect non-equilibrium phenomena.     

Microstructural study of MBE-grown ZnO film on GaN/sapphire (0001) substrate

Single crystalline ZnO film is grown on GaN/sapphire (0001) substrate by molecular beam epitaxy. Ga₂O₃ is introduced into the ZnO/GaN heterostructure intentionally by oxygen-plasma pre-exposure on the GaN surface prior to ZnO growth. The crystalline orientation and interfacial microstructure are characterized by X-ray diffraction and transmission electron microscopy. X-ray diffraction analysis shows strong c-axis preferred orientation of the ZnO film. Cross-sectional transmission electron microscope images reveal that an additional phase is formed at the interface of ZnO/GaN. Through a comparison of diffraction patterns, we confirm that the interface layer is monoclinic Ga₂O₃ and the main epitaxial relationship should be and .      

Kinetic features of the oxide formation on {111} polar planes upon anode treatment of <Emphasis Type="Italic">n</Emphasis>-GaAs

The mechanism and kinetics of anode destruction of {111} polar planes of n-GaAs and morphological features of forming oxide films in the potentiostatic mode of polarization in weakly acid solutions of electrolytes have been studied. It has been found that anode polarization of the gallium plane (111) Ga provides the formation of a porous structure of both the single-crystal matrix and oxide film, which has a planar topology. In this case, the pore density is always commensurable with the surface dope concentration. In contrast to the gallium plane, the anode polarization of the arsenic plane \(\overline {\left( {111} \right)} \) As provides the tangential mechanism of destruction of the semiconductor matrix and the island-type morphology of the oxide. Equal crystallographic orientation of islands is determined by the directive action of the family of oxidized planes \(\left\{ {1\overline {11} } \right\}\) GaAs. However, regardless of the crystallographic orientation of the polar plane, the forming oxide is represented by polycrystalline As₂O₃ and amorphous Ga₂O₃.     

Study on the design of ZnO/PANI composites and the mechanism of enhanced humidity sensing properties

ZnO/PANI composite humidity sensor was prepared by hydrothermal method. The first principles of density functional theory study the sensing mechanism. The calculation shows that the oxygen vacancy on ZnO surface is beneficial to the adsorption of water molecules. The {0 0 0‾1} crystal plane with the largest lattice oxygen number in ZnO has a strong adsorption capacity for water molecules, which is also conducive to improving the humidity sensitivity. PANI is easy to be combined on {0 1‾1 0} plane of ZnO, and it indirectly promotes the growth of {0 0 0‾1} plane, increasing the adsorption of water molecules and the proportion of H⁺ and H₃O⁺ ions. In addition, the N–H group in ZnO/PANI enhances the H⁺ conduction, which further improves the performance of the sensor. The results concluded that the proportion of lattice oxygen in humidity sensor is an important factor of humidity sensor sensitive detection.     

Helimagnetism in gallium substituted LuMn<Subscript>6</Subscript>Ge<Subscript>6</Subscript> studied by nuclear magnetic resonance

Zero-field nuclear magnetic resonance (NMR) of all NMR isotopes (¹⁷⁵Lu, ⁵⁵Mn, ⁷³Ge, ^69,71Ga) in LuMn₆Ge_6-xGa_x, 0 ≤ x ≤ 1, is used to monitor the variation of the hyperfine interaction with the sequence of antiferromagnetic - helimagnetic - ferromagnetic arrangement of the manganese moments of subsequent Kagomé net planes achieved by variation of the gallium content x. According to the ⁵⁵Mn-NMR results, the local Mn moment varies by less than ±5% in this series. ¹⁷⁵Lu-NMR proves canting of the antiferromagnetic sublattices in LuMn₆Ge₆. The anisotropy of the Ge magnetic hyperfine interaction decreases with increasing separation from the hexagonal Lu plane, whereas the absolute value of its isotropic part is only qualitatively correlated with the average separation of the six nearest Mn neighbours. Due to the anisotropic magnetic and electric hyperfine interaction at Ge and Ga sites, the non-collinear magnetic structures are clearly reflected by the NMR spectra, which are described quantitatively in this contribution. The preferred Mn moment direction rotates away from the c direction with x. The conduction or bonding electron spin polarization at the Ga nuclei is increased by 35–80% compared to the Ge nuclei. We argue that this is related with the variation of the magnetic order with the Ga content.     

Thermal faceting of the rutile TiO2(001) surface

Temperature dependent faceting of rutile TiO₂ surfaces cut to the (001) plane has been reported [Tait and Kasowski, Phys. Rev. B20 (1979) 5178]. By comparing LEED data to beam positions calculated for various sets of facet planes, the facet planes have been identified. The first ordered structure observed on annealing ion bombarded surfaces is composed of {011} facets with the facet planes in a (2 × 1) reconstruction. The high temperature structure produced on annealing above 1300K is best described as {114} facets; however, there are deviations of the observed LEED pattern from that calculated for {114} facets, possibly because of the presence of related planes. LEED data have now been obtained on the behavior of (110), (100), (011), (114), and (001) surfaces in UHV. The observed stability of TiO₂ surfaces can be related to the Ti ion coordination numbers in the surface plane as derived from stoichiometric terminations of the rutile lattice.     

Direct insight into grain boundary reconstruction in polycrystalline Cu(In,Ga)SE2 with atomic resolution

This work presents results from high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy on twin boundaries (TBs) and nontwin grain boundaries (GBs) in Cu(In,Ga)Se(2) thin films. It is shown that the atomic reconstruction is different for different symmetries of the grain boundaries. We are able to confirm the model proposed by Persson and Zunger [Phys. Rev. Lett. 91, 266401 (2003)] for Se-Se-terminated Σ3 {112} TBs, showing Cu depletion and In enrichment in the two atomic planes closest to the TB. On the contrary, Cu depletion without In enrichment is detected for a cation-Se-terminated TB. At nontwin GBs, always a strong anticorrelation of Cu and In signals is detected suggesting that the formation of In(Cu) or Cu(In) antisites within a very confined region of smaller than 1 nm is an essential element in the reconstruction of these GBs.     

Structure,composition and optical properties of ZnO:Ga films electrodeposited on flexible substrates

Thin films of ZnO:Ga are electrodeposited on stainless steel from zinc chloride solutions containing gallium with a Ga/Zn molar ratio in the range 1–10%. The surface morphology, structure, composition, and the optical properties of these films are studied using SEM, XRD, EDX, XPS, photoluminescence and transmittance spectroscopy. Results reveal that films with embedded platelets can be grown with a Ga/Zn molar ratio up to 60%. In as-grown films, Ga is present in the form of oxide, hydroxide or hydroxychloride compounds which decompose by annealing. Films annealed at 400°C are composed only from ZnO and Ga₂O₃ phases. The incorporation of Ga in ZnO films does not make a considerable change in the optical transmittance and the photoluminescence properties of the films.     

Local structure around Zn and Ga in solution‐processed In–Ga–Zn–O and implications for electronic properties

We study by X‐ray absorption spectroscopy the local structure around Zn and Ga in solution‐processed In–Ga–Zn–O thin films as a function of thermal annealing. Zn and Ga environments are amorphous up to 450 °C. At 200 °C and 450 °C, the Ga atoms are in a β‐Ga₂O₃ like structure, mostly tetrahedral gallium oxide phase. Above 300 °C, the Zn atoms are in a tetrahedral ZnO phase for atoms inside the nanoclusters. The observed formation of the inorganic structure above 300 °C may be correlated to the rise of the mobility for IGZO TFTs. The Zn atoms localized at the nanocluster boundary are undercoordinated with O. Such ZnO cluster boundary could be responsible for electronic defect levels. Such defect levels were put in evidence in the upper half of the band gap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Growth of high-quality ZnO thin films on (11\bar{2}0) a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. X-ray diffraction and transmission electron microscopy reveal that the ZnO films have high structural quality and an atomically sharp ZnO/Al₂O₃ interface. The full width at half maximum values of the 0002 and $30\bar{3}2$ ZnO ω-rocking curves are 467.8 and 813.5 arc sec for a 600 nm thick ZnO film. A screw dislocation density of 4.35×10⁸ cm^?2 and an edge dislocation density of 3.38×10⁹ cm^?2 are estimated by X-ray diffraction. The surface of the ZnO epilayers contains hexagonal pits, which can be observed in the Zn-polar ZnO. The films have a resistivity of 0.119 Ω?cm, an electron concentration of 6.85×10¹⁷ cm^?3, and a mobility of 76.5 cm²?V^?1?s^?1 at room temperature. Low temperature photoluminescence measurements show good optical properties comparable to ZnO single crystals.     

Field emission and photoluminescence of ZnO nanocombs

Three kinds of new comb-shape nanostructures of ZnO have been grown on single silicon substrates without catalyst-assisted thermal evaporation of Zn and active carbon powders. The morphology and structure of the prepared nanorods are determined on the basis of field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The growth mechanism of the ZnO nanocombs can be explained on the basis of the vapor–solid (VS) processes. In nanocombs 1 and nanocombs 2, the comb teeth grow along [0001] and the comb stem grows along [ $01\overline{1}0$ ], while in nanocombs 3, nanoteeth grow along [ $01\overline{1}0$ ] and stem grows along [0001]. The photoluminescence and field-emission properties of ZnO nanocombs 1–3 have been investigated. The turn-on electric field of ZnO nanocombs 1–3, which is defined as the field required to producing a current density of 10 μA/cm², is 9, 7.7 and 7.1 V/μm, respectively. The field-emission performance relies not only on the tip’s radius of curvature and field enhancement factor, but also on the factor evaluating the degree of the screening effect.     

ZnO高掺杂Ga的浓度对导电性能和红移效应影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,在相同环境条件下建立了浓度不同的由Ga原子取代Zn原子的Zn_1-xGa_xO模型.对低温高掺杂Ga原子的Zn_1-xGa_xO半导体的能带结构、态密度和吸收光谱进行了计算.结果表明:Ga原子浓度越大,进入导带的相对电子数越多,但是电子迁移率反而减小.通过对掺杂和未掺杂ZnO的电导率以及最小间隙带宽度分别进行了比较 关键词： ZnO高掺杂Ga 电导率 红移 第一性原理     

Preparation and characterization of nanocrystalline ZnO particles from a hydrothermal process

Rod-like ZnO nanoparticles were prepared by the hydrolysis of zinc acetate under heating in diethylene glycol (DEG). Structural characterization of the synthesized powders was investigated by XRD, FT-IR, electron paramagnetic resonance (EPR) and transmission electron microscopy (TEM). The size of the particles increased as the amount of H₂O added increased in the nano size range. The average crystallite size calculated from the XRD patterns varied from 6 to 64 nm corresponding to the amount of H₂O added. The ZnO nanopartilces possess the wurtzite type crystallographic structure. It was found that these ZnO nanoparticles had singly ionized oxygen vacancy defect () and superoxide ions from the EPR investigations. A strong near UV emission of the ZnO nanoparticles at about 380 nm was observed and its intensity decreased as the amount of H₂O increased. This emission of ZnO nanoparticles is found to be particles size dependent due to the confinement effect. A green emission at about 540 nm due to the recombination of electrons trapped at singly ionized oxygen vacancies defect () appeared when the amount of H₂O increased. The intensity of the green emission increases as the concentration of increases.     

Ga填充n型方钴矿化合物的结构及热电性能

用熔融退火结合放电等离子烧结(SPS)技术制备了具有不同Ga填充含量的Ga_xCo₄Sb₁₂方钴矿化合物,研究了不同Ga含量对其热电传输特性的影响规律. Rietveld结构解析表明,Ga占据晶体学2a空洞位置,Ga填充上限约为0.22,当Ga的名义组成x≤0.25时,样品的电导率、室温载流子浓度N_p随Ga含量的增加而增加,Seebeck系数随Ga含量的增加而减小. 室温下霍尔测试表明,每一个Ga授予框架0.9个电子,比Ga的氧化价态Ga³⁺小得多. 由于Ga离子半径相对较小,致使Ga填充方钴矿化合物的热导率κ及晶格热导率κ_L较其他元素填充的方钴矿化合物低. 当x＝0.22时对应的样品在300K时的热导率和晶格热导率分别为3.05Wm^-1·K^-1和 2.86Wm^-1·K^-1.在600K下Ga_0.22Co_4.0Sb_12.0样品晶格热导率达到最小,为1.83Wm^-1·K^-1,最大热电优值Z,在560K处达1.31×10^-3K^-1. 关键词： skutterudite化合物 Ga原子填充 结构 热电性能     

Ga<Subscript>Zn</Subscript>-V<Subscript>Zn</Subscript> acceptor complex defect in Ga-doped ZnO

Gallium (Ga)-doped ZnO is regarded as a promising plasmonic material with a wide range of applications in plasmonics. In this study, zinc self-diffusion experiments are adopted to disclose the nature of the dominant compensating defect in Ga-doped ZnO isotopic heterostructures. The (Ga_Zn-V_Zn)^? complex defect, instead of the isolated V_Zn^2?, is identified as the predominant compensating acceptor center responsible for the low donor doping efficiency. The comparative diffusion experiments operated by the secondary ion mass spectrometry reveal a ~0.78 eV binding energy of this complex defect, which well matches the electrical activation energy derived from the temperature-dependent Hall effect measurements (~(0.82±0.02) eV). These findings contribute to an essential understanding of the (Ga_Zn-V_Zn)^? complex defect and the potential engineering routes of heavily Ga-doped ZnO.     

Synthesis of dimension-tunable ZnO nanostructures via the design of zinc hydroxide precursors

A facile synthesis route is presented to achieve dimension-tunable ZnO nanostructures by the design of zinc hydroxide precursors under the surfactant-free condition. From three types of zinc hydroxide precursors, namely, crystalline Zn(OH)(NO₃)(H₂O) nanobelts, amorphous zinc hydroxides microparticles and soluble Zn(OH)^2-₄\mathrm{Zn}(\mathrm{OH})^{2-}_{4} species, the porous ZnO nanosheets, ZnO nanoparticles and ZnO nanowires can be achieved, respectively. The porous ZnO nanosheets exhibit large polar surface area. Thermal analysis indicates that the crystalline Zn(OH)(NO₃)(H₂O) nanobelts were converted to the porous ZnO nanosheets by in situ lattice reconstruction, which was attributed to the unique fibrous structure of Zn(OH)(NO₃)(H₂O) nanobelts. The as-prepared dimension-tunable ZnO nanostructures have potential applications in solar cells, photocatalysis, novel chemical and biological sensors, etc.     

Morphology and microstructure of all-epitaxial ferroelectric tri-layered (Bi,La)4Ti3O12/Pb(Zr0.4Ti0.6)O3/ (Bi,La)4Ti3O12 thin films on SrTiO3(011)

The morphology and microstructure of all-epitaxial (Bi,La)₄Ti₃O₁₂/Pb(Zr_0.4Ti_0.6)O₃/(Bi,La)₄Ti₃O₁₂ (BLT/PZT/BLT) tri-layered ferroelectric films, grown on (011)-oriented SrTiO₃ (STO) substrates by pulsed laser deposition, are investigated by transmission electron microscopy (TEM). X-ray diffraction and electron diffraction patterns demonstrate that the epitaxial relationship between BLT, PZT and STO can be described as ; . Cross-sectional TEM images show that the growth rate of BLT is nearly two times that for PZT at the same growth conditions, and 90° ferroelectric domain boundaries lying on {110} planes are observed in the PZT layer. The 90° ferroelectric domains in the PZT layer extend up to 600 nm in length. Long domains penetrate into the neighboring columnar grain through the columnar grain boundary, whereas others are nucleating at the columnar grain boundaries. The roughness of the PZT/BLT interfaces appears to depend on the viewing direction, i.e., it is different for different azimuthal directions. Planar TEM investigations show that the grains in the top BLT layer have a rod-like morphology, preferentially growing along the [110]_BLT direction. The grain width is rather constant at about 90 nm, whereas the length of the grains varies from 150 to 625 nm. These morphological details point to the important role the crystal anisotropy of BLT plays for the growth and structure of the tri-layered films. PACS 81.15-z; 68.37.Lp; 77.84.-s     

Cluster generation under pulsed laser ablation of zinc oxide

Neutral and cationic Zn _n O _m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with Zn_nO_n-1⁺\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+} and Zn_nO_n-3⁺\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+} being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric Zn_nO_n⁺\mathrm{Zn}_{n}\mathrm{O}_{n}^{+} clusters (n<11) toward hydrogen is found to be high, while oxygen-deficient species are less reactive. The neutral plume particles are mainly stoichiometric with Zn₄O₄ tetramer being a magic cluster. It is suggested that the Zn₄O₄ loss is the dominant fragmentation channel of large zinc oxide clusters upon electron impact. Plume expansion conditions under ZnO ablation with visible and IR laser pulses are shown to be favorable for stoichiometric cluster formation.     

Effects of Si and Ti impurities on electrical properties of sol–gel-derived amorphous SrTa2O6 thin films by UV/O3 treatment

Sol–gel-derived SrTa₂O₆ thin films were fabricated at a low temperature of 500 °C. To improve their leakage current properties, additional UV/O₃-assisted annealing was performed from room temperature to 290 °C. UV/O₃ treatment at 290 °C gave a very low leakage current that was six orders of magnitude lower than that of an untreated thin film. During UV/O₃-assisted annealing, Si and Ti ions diffused from the substrates into the SrTa₂O₆ thin films and occupied the Ta⁵⁺ sites, subsequently generating Si^? and Ti^?. At a heating temperature of 290 °C, large amounts of Ti ions diffused throughout the SrTa₂O₆ thin film. These Ti ions contributed to the generation of inactive combinations of $(\mathrm{Si}^{-}\mbox{--}\mathrm{V}_{\mathrm{o}}^{+})^{+}\mbox{--}\mathrm{Ti}^{-}$ and $(\mathrm{Ti}^{-}\mbox{--}\mathrm{V}_{\mathrm{o}}^{+})^{+}\mbox{--}\mathrm{Ti}^{-}$ , which greatly reduced oxygen vacancies (V_o). Thus, the leakage current was significantly reduced.