Buffer layer ZnO-assistant fabrication of c-axis GaN films by using pulsed laser deposition on Si(111) substrate: annealing effects in ammonia ambience

ZnO buffer layers have been used to fabricate GaN thin films by using pulsed laser deposition on Si (111) substrates. c-axis GaN thin films were obtained by annealing in NH₃ atmosphere at 950°C for 15 min. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM) have been used for the characterizations of the crystalline quality, composition, and surface morphology of the films. The annealing in ammonia (NH₃) atmosphere markedly affects the preparation of GaN films and the least annealing time is 15 min under our experimental conditions. The mechanism of the effects of the ZnO buffer layers was studied. In the beginning, Zn–O bonds are destroyed in the interface of the films; a few O and Zn atoms depart from their positions, while N and Ga atoms fill in the empty positions and form a hexagonal structure of a special component. Many bonds (such as Ga–O bonds, Zn–N bonds) existed then. The number of Zn–O bonds decreases and the number of Ga–N bonds increases in the films with increasing of the annealing time. Many other bonds (such as Ga–O bonds, Zn–N bonds) also decreased and more Ga–N bonds formed with annealing time increasing. After having been annealed for 15 min under our experimental conditions, the quality of the hexagonal structure GaN films was markedly improved by the destroying of the Zn–O bonds during high-temperature annealing.     

Influence of optical-damage-resistant dopants on the nonlinear optical properties of lithium niobate

Using the chemical-bond method, nonlinear optical properties of lithium niobate containing different dopants are calculated. In crystals with stoichiometric composition the second order nonlinear susceptibility decreases approximately linearly with increasing dopant concentration. Among the dopants studied – Mg, Zn and In – this behaviour is most highly expressed for In doping. In contrast to that, congruently grown crystals show a different behaviour; only a weak dependence on the dopant concentration is found for, for example, Mg-doped material. Received: 24 October 2000 / Published online: 21 March 2001     

Synthesis and characterization of Mg-doped ZnO hollow spheres

Mg-doped ZnO nanoparticles were synthesized by a simple chemical method at low temperature with Mg:Zn atomic ratio from 0 to 7%. The synthesis process is based on the hydrolysis of zinc acetate dihydrate and magnesium acetate tetrahydrate were heated under refluxing at 65 °C using methanol as a solvent. X-ray diffraction analysis reveals that the Mg-doped ZnO crystallizes in a wurtzite structure with crystal size of 5–12 nm. These nanocrystals self-aggregated themselves into hollow spheres of size of 800–1100 nm. High resolution transmission electron microscopy images show that each sphere is made up of numerous nanoparticles of average diameter 5–11 nm. The XRD patterns, SEM and TEM micrographs of doping of Mg in ZnO confirmed the formation of hollow spheres indicating that the Mg²⁺ is successfully substituted into the ZnO host structure of the Zn²⁺ site. Furthermore, the UV–Vis spectra and photoluminescence (PL) spectra of the ZnO nanoparticles were also investigated. The band gap of the nanoparticles can be tuned in the range of 3.36–3.55 eV by the use of the dopants.     

Novel model for the optical function of GaN

In this work we propose an analytical expression for the complex dielectric function which includes both discrete and continuum exciton effects. We have introduced Lorentzian broadening into Elliott’s formula. The introduction of broadening leads to equations for the dielectric function containing only elementary functions. We have applied the proposed model to the dielectric function of wurtzite GaN in the spectral region 1–10 eV. Excellent agreement with the experimental data has been obtained. We show that the Lorentzian-broadened dielectric function decays more slowly than the experimental data for hexagonal GaN at the low-energy side. This indicates that the broadening of the absorption edge in GaN is not purely Lorentzian. The agreement with the experimental data can be improved using adjustable broadening modification. Received: 29 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Bulk GaN single crystals: a reinvestigation of growth mechanism using Li3N flux

The GaN growth mechanism using Li₃N flux was reinvestigated by designing a new experiment that allowed us to grow GaN simultaneously under varying Ga/Li₃N molar ratios. The results confirm the two-step reactions involved in the Ga–Li₃N system: Li₃N+Ga→Li₃GaN₂+Li (1) and Li₃GaN₂+Ga→GaN+Li (2). It is found that reaction (2) is the main cause leading to small GaN crystals in the products. Larger GaN crystals, however, can grow by a different pathway. The growing process is concerned with the formation of Li–Ga–N melt by dissolving Li₃GaN₂ in Li–Ga melt after reaction (1). GaN crystals up to 3 mm grow from the Li–Ga–N melt upon cooling on the GaN particles by reaction (2). Our results suggest that it is necessary to restrain reaction (2) by choosing proper Ga/Li₃N molar ratios so as to obtain GaN crystals in larger size. PACS 81.05.Ea; 81.10.Dn     

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.     

Impact of dopants in GaN on the formation of two-dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors

The two-dimensional electron gas distribution in AlGaN/GaN high electron mobility transistors is determined from the solution of the coupled Schr?dinger’s and Poisson’s equations. Considering the piezoelectric effect, the two-dimensional electron gas concentration is calculated to be as high as 7.7×10¹⁹ cm^-3. In order to obtain an understanding of how the two-dimensional electron gas distribution is influenced by dopants in GaN, we observed the two-dimensional electron gas concentration and occupation of sub-bands versus dopant concentration in the GaN layer of an AlGaN/GaN heterostructure. Our results show that the two-dimensional electron gas concentration is slightly increased at higher doping levels in GaN, while the quantum confinement in the AlGaN/GaN heterostructure is weakened with the increase of donor concentration in the GaN layer. Received: 26 May 2001 / Accepted: 23 July 2001 / Published online: 23 January 2002     

Synthesis and Raman scattering of GaN nanorings, nanoribbons and nanowires

Low-dimensional GaN materials, including nanorings, nanoribbons and smooth nanowires have been synthesized by reacting gallium and ammonia using Ag particles as a catalyst on the substrate of MgO single crystals. They were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). EDX, XRD indicated that the low-dimensional nanomaterials were wurtzite GaN. New features are found in Raman scatterings for these low-dimensional GaN materials, which are different from the previous observations of GaN materials. Received: 16 November 2000 / Accepted: 17 November 2000 / Published online: 21 March 2001     

Elemental and phase composition analysis of hydrogen-sensitive Pd/Mg–Ni films

A magnesium–nickel (Mg–Ni) film with a palladium (Pd) overcoat is switchable between a conducting metallic state and a semiconducting state through hydridation and dehydridation processes. The Pd overcoat is added to suppress possible oxidation of the alloy layer. However, some Pd and oxygen (O) atoms are still able to diffuse into the alloy layer, such that the distributions of elemental and phase compositions along the direction of depth are quite complicated. In this work, we propose a method to obtain more detailed information on the elemental and phase compositions in a Pd/Mg–Ni film by means of analyzing Pd 3d_5/2, Pd 3p_3/2, Ni 2p_3/2, Mg 1s and O 1s photoelectron spectra. The method was applied to analyze a typical Pd/Mg–Ni film sample. Results verified that Pd can diffuse into the alloy layer. An Mg–O phase is formed near the surface. An Mg–Ni alloy phase dominates at deeper regions. The Mg–Ni phase contains 80% of the total number of atoms in the film, and is non-stoichiometric and highly disordered. Nevertheless, it can give a remarkable change of electrical resistivity during hydridation–dehydridation processes. PACS 79.60.-i; 61.05.C-; 79.60.Ht     

Fabrication of bamboo-shaped GaN nanorods

Bamboo-shaped GaN nanorods were formed through a simple sublimation method. They were characterized by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The TEM image showed that the nanorods were bamboo-like. XRD, HRTEM and SAED patterns indicated that the nanorods were single-crystal wurtzite GaN. Received: 8 January 2001 / Accepted: 28 April 2001 / Published online: 20 December 2001     

An experimental exploration of chemical bond characteristic,bulk modulus and phase stability in ZnO:Cu nanocrystals under high pressure

The high pressure induced phase transitions in Zn_1−x Cu _x O (x=0.005 and 0.011) are investigated by angle-dispersive synchrotron radiation X-ray diffraction. As the pressure increases, phase transformations from the wurtzite structure to the rocksalt structure are observed in both samples, with the transition pressures at 9.8 GPa and 7.9 GPa, respectively. With the increasing of the Cu-doping concentration in ZnO, crystalline parameters, the bulk moduli, and the Zn–O bond lengths all increased, meanwhile, the transition pressures decreased. The results could be explained in terms of the reduction of phase transformation barriers and the lowering of bond energy.     

Effects of composition and cooling rate on the microstructure of?Sn–3.7Ag–0.9Zn–Bi solders

The effects of Bi addition, of less than 3 wt.%, and applied cooling rate on the solidified microstructure of the eutectic Sn–3.7Ag–0.9Zn (weight percent, hereafter) solder were investigated. As observed by microstructural analysis, the increase of Bi content favors the separation of the β-Sn and AgZn intermetallic compounds (IMCs) in the eutectic Sn–Ag–Zn solder. And there are some Bi precipitates formed along with the primary β-Sn dendrites as the concentration of Bi exceeds 2%. As the applied cooling rate increases, the microstructure of the Sn–3.7Ag–0.9Zn–Bi solder is refined, and the segregation of Bi is restrained. By increasing the amount of Bi, the microhardness of the solder increases.     

Electronic properties of a zinc oxide nanotube under uniaxial tensile strain: a density functional theory study

In this article, density functional theory calculations were employed to investigate the electronic properties of (4,4) armchair zinc oxide single-walled nanotubes (ZNONTs) under uniaxial mechanical deformations. It was found that the highest-occupied molecular orbital and the lowest-unoccupied molecular orbital gap and the value of radial buckling will both decrease linearly with the increase of axial strain. The elongation of the ZNONT mainly originates from the decrease and increase of two characteristic bond angles rather than Zn–O ionic bond elongation. This mechanical behavior is very different from the uniaxial tensional processes of carbon nanotubes and silicon carbide nanotubes formed by covalent bonds. The partial densities of states of the Zn atom and O atom show that the unoccupied states are gradually left-shifted as ZNONT elongates from 0 to 15%. Neither Mulliken charge nor deformation density clearly changes with the different tension strains. Bond order analysis also indicates the bonding strength will decrease as the strain increases from 0 to 15%.     

Hydrogen bonds of anti-HIV active aminophenols

Analysis of IR-Fourier spectra from solutions and crystals of antiviral sulfo-containing aminophenols has shown that various types of intramolecular and intermolecular interactions can occur in molecules of these compounds. Three types of intramolecular hydrogen bonds (O–H⋅⋅⋅N, O–H⋅⋅⋅O=S=O, and N–H⋅⋅⋅O=S=O) are formed in CCl₄ solutions of the sulfo-containing aminophenols. The formation of intermolecular H-bonds involving the NH- and OH-groups and the preservation of the intramolecular O–H⋅⋅⋅O=S=O H-bond are characteristic of the anti-HIV active aminophenol crystals. Spectral attributes are determined in order to distinguish between the anti-HIV active and inactive sulfo-containing aminophenols.     

EXAFS Studies of the Local Structure Around Zn in Cd1−x Zn x Te

X-ray absorption spectroscopy has been used to obtain information on the local structure around Zn atoms in CdTe. The Zn–Te distance is 5% smaller than the Cd–Te bond-length in the bulk, but slightly larger than in pure ZnTe. A similar effect is predicted by FLAPW calculations. This revised version was published online in September 2006 with corrections to the Cover Date.     

Novel technique for hetero-epitaxial diamond film growth on cubic boron nitride from iron carbide at high temperature and high pressure

Cubic boron nitride (c-BN) crystals about 0.1–0.3 mmin dimension were treated with iron carbide powders (high purity 99%) with size of 80–100 mesh at a high temperature of 1620 K and a high pressure of 5.2 GPa. It was found that hetero-epitaxial diamond films have been grown on the c-BN from iron carbide. The formation of dia-mond films on the cubic boron nitride can be confirmed by laser Raman spectra, face scan of elements and reflective high-energy electron diffraction. It was suggested that diamond films could be epitaxially formed on the c-BN through decomposition of iron carbide. This approach provides a possible and very effective way to realize hetero-epitaxial growth of homogeneous and large-area diamond films on c-BN, which is different from the conventional technique using a chemical vapor deposition method. Received: 20 December 2000 / Accepted: 9 January 2001 / Published online: 28 February 2001     

c-axis-textured LiNbO3 thin films on Si (111) substrates

We have successfully prepared highly c-axis-textured LiNbO₃ films on hydrogen-terminated Si (111) substrate using sol-gel spin-coating and rapid thermal annealing. These highly c-axis-textured films were obtained with a preheating at 300 °C for 15 min followed by a rapid thermal annealing at 500–700 °C for 120 s. The c-axis orientation of the LiNbO₃ film is due to a weak effect caused by the 3-fold symmetry match between the film and the Si (111) substrate. The c-axis orientation of LiNbO₃ films is very useful in integrated optics devices and metal–ferroelectric–semiconductor nonvolatile memory applications. Received: 15 September / Accepted: 4 December / Published online: 3 April 2001     

Intramolecular hydrogen bonds and antioxidant activity of aminophenols

We have used IR Fourier spectroscopy to study intramolecular interactions in solutions of aminophenols in n-hexane. When the hydroxyl group in the molecule is ortho to the amino group, O-H⋯N and N-H⋯O intramolecular hydrogen bonds are formed in the aminophenols. Adding two tert-butyl groups to the benzene ring of ortho-aminophenols strengthens the O-H⋯N bond in the molecules, and prevents formation of an N-H⋯O bond. Additional acylation of the amino group in ortho-aminophenols leads to formation of an O-H⋯O=C intramolecular hydrogen bond. Formation of the above-indicated intramolecular hydrogen bonds in aminophenols affects the course of radiation-induced reactions occurring in n-hexane with participation of these compounds. The antioxidant properties of the aminophenols are enhanced when the hydroxyl groups in the molecules are found in the free state, and are diminished when strong O-H⋯N or O-H⋯O=C intramolecular hydrogen bonds are formed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 577–582, September–October, 2007.     

Synthesis and characterization of ZnO:Co<Superscript>2+</Superscript> nanoparticles

This work investigates cobalt doped ZnO nanoparticles prepared by using wet chemical methods. The nanoparticles have a typical size of 3–8 nm. The electronic structure as well as the optical and magnetic properties of Co²⁺ have been characterized. X-ray diffraction spectra of the powder show wurtzite ZnO with no secondary Co phases. In the energy range below the bandgap, the optical absorption spectra show the internal d–d transitions related to Co²⁺ incorporated on the Zn lattice site in ZnO. Low temperature photoluminescence measurements confirm these results. Based on the analysis of the g-valuesfor bulk ZnO:Co., electron paramagnetic resonance measurements coincide with the simulation of Co-doped ZnO powder. Thus far, no evidence for ferromagnetism has been obtained. PACS 61.46.Df; 76.30.Fc; 78.67.Bf     

Stranski–Krastanov growth of InGaN quantum dots emitting in green spectra

Self-assembled InGaN quantum dots (QDs) were grown on GaN templates by metalorganic chemical vapor deposition. 2D–3D growth mode transition through Stranski–Krastanov mode was observed via atomic force microscopy. The critical thickness for In_0.67Ga_0.33N QDs was determined to be four monolayers. The effects of growth temperature, deposition thickness, and V/III ratio on QD formation were examined. The capping of InGaN QDs with GaN was analyzed. Optimized InGaN quantum dots emitted in green spectra at room temperature.