Influence of post-deposition annealing on the structural,optical and electrical properties of Li and Mg co-doped ZnO thin films deposited by sol–gel technique

Lithium (Li) and magnesium (Mg) co-doped zinc oxide (ZnO) thin films were deposited by sol–gel method using spin coating technique. The films were deposited on glass substrates and annealed at different temperatures. The effects of annealing temperature on the structural, optical and electrical properties of the deposited films were investigated using X-ray diffraction (XRD), Ultraviolet–Visible absorption spectra (UV–VIS), photoluminescence spectra (PL), X-ray photo electron spectroscopy (XPS) and Hall measurements. XRD patterns indicated that the deposited films had a polycrystalline hexagonal wurtzite structure with preferred (0 0 0 2) orientation. All films were found to exhibit a good transparency in the visible range. Analysis of the absorption edge revealed that the optical band gap energies of the films annealed at different temperatures varies between 3.49 eV and 3.69 eV. Room temperature PL spectra of the deposited films annealed at various temperatures consist of a near band edge emission and visible emission due to the electronic defects, which are related to deep level emissions, such as oxide antisite (O_Zn), interstitial zinc (Zn_i), interstitial oxygen (O_i) and zinc vacancy (V_Zn) which are generated during annealing process. The influence of annealing temperature on the chemical state of the dopants in the film was analysed by XPS spectra. Ion beam analysis (Rutherford back scattering) experiments were performed to evaluate the content of Li and Mg in the films. Hall measurements confirmed the p-type nature of the deposited films.     

A new self-activated yellow-emitting phosphor Zn2V2O7 for white LED

A new self-activated yellow-emitting Zn₂V₂O₇ phosphor was synthesized by high temperature solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the sample with monoclinic formation of Zn₂V₂O₇. The excitation and emission spectra indicated the phosphor can be efficiently excited by near ultraviolet (NUV) light in 220–400 nm range and exhibit a bright broad yellow emission with the highest emission intensity at 531 nm. The broad emission band from 400 to 650 nm can be attributed to the charge transfer transition in the VO₄ tetrahedra, which suggests that the phosphor is a promising yellow phosphor applied for white light-emitting diodes (WLED).     

Copper,the dominant acceptor in refined,undoped zinc telluride

Comparative measurements have been made of optical absorption and photoluminescence of refined undoped and Cu in-diffused ZnTe single crystals. Strong increases in a bound exciton BE line near 2.375 eV previously identified with the electrically dominant point defect acceptor ‘a’, with binding energy E_A ? 149 meV, suggests that this acceptor is substitutional Cu_Zn. Similarly strong increases in a relatively broad band at slightly higher energy suggests the simultaneous incorporation of shallow donors, possibly interstital Cu_I. These findings indicate that intrinsic defects such as V_Zn neither control the Fermi level in refined ZnTe nor produce shallow acceptors with E_A ? 250 meV, contrary to much previous speculation.     

Single violet luminescence emitted from ZnO films obtained by oxidation of Zn film on quartz glass

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films were studied. The strong single violet emission centering about 413-424 nm was observed in the room temperature PL spectra of the ZnO films. The intensity of violet emission increased and the peak position shift from 424 to 413 nm with increasing oxygen pressures. The violet emission was attributed to the electron transition from the valence band to interstitial zinc (Zn_i) level under low oxygen pressure conditions (50-500 Pa). Under high oxygen pressure conditions (5000-23,000 Pa), both interstitial zinc (Zn_i) and zinc Vacancy (V_Zn) were thought to be responsible for the violet emission.     

Optically detected magnetic resonance of the zinc vacancy in ZnS

Optical detection of magnetic resonance (ODMR) is reported for the single negative charge state, V_Zn^?, of the isolated zinc vacancy in ZnS. Produced by 2.5 MeV electron irradiation, it is detected in a distant donor-acceptor (DA) pair luminescent band at 570 nm in which the vacancy acts as the acceptor. Excitation and emission spectral dependences of the V_Zn^? ODMR signals are analyzed in terms of a configurational coordinate model. We conclude that the double acceptor level (V_Zn⁼/V_Zn^?) is located ～1.1 eV from the valence band edge and that the trigonal Jahn-Teller relaxation energy for the V_Zn^? state is ～0.5 eV.     

Oxygen deficiency effects on recombination lifetime and photoluminescence characteristics of ZnO thin films; correlation with crystal structure

The photoluminescence (PL), recombination lifetime (RL), and X-ray diffraction (XRD) spectra of the samples grown at various O₂ fractions of 0.290 (Zn-rich), 0.585 (moderate), and 0.836 (O-rich) over the total pressures in the growth chamber were investigated. XRD measurements revealed that all the films show highly preferred (0002) orientation. The PL measurements exhibit different dominant emissions in the ultraviolet (UV), violet and blue regions for Zn-rich, moderate and O-rich samples, respectively. Well-known green emission and high intensity of free exciton (FX) transition has been observed in Zn-rich sample after the sample is annealed at vacuum probably due to the oxygen deficiencies. Annealing the moderate sample gives rise to the UV emission at energy of 3.263 eV similar to the observed PL emission spectrum for the Zn-rich thin film. O-rich thin film exhibits a 338 meV acceptor level above the valance band maximum, most probably related to zinc vacancy (V_Zn). Free exciton RL measurements result in 568.23, 397.65, and 797.46 ps for Zn-rich, moderate, and O-rich thin films, respectively. A good correlation was found between crystallite size and the lifetime values.     

Luminescence anisotropy of ZnO microrods

The local features of light emission from ZnO microrods were studied: it is revealed that ZnO luminescence spectra are significantly influenced by the crystal morphology. It is shown that the near and edge ultraviolet emission occurs primarily from the top (0001) planes of ZnO microrods; while the defect related visible emission was found to occur dominantly from the side facets. The room temperature cathodoluminescence analysis revealed that visible emission consists of a few overlapping peaks, arising due to recombination on common points and surface defects (Zn_i, V_o, V_o⁰/V_o^?*_, V_o^** and surface defects.). While at low temperature, only the luminescence due to neutral donor bound exciton (D⁰X) emission is observed. The data obtained suggest that the light emission spectra of ZnO material of diverse morphology cannot be directly compared, although some common spectral features are present.     

非掺杂ZnO薄膜中紫外与绿色发光中心

用直流反应溅射方法在硅衬底上淀积了ZnO薄膜,测量它们的光致发光(PL)光谱,观察到两个发光峰,峰值能量分别为3.18(紫外峰,UV)和2.38eV(绿峰).样品用不同温度分别在氧气、氮气和空气中热处理后,测量了PL光谱中绿峰和紫外峰强度随热处理温度和气氛的变化,同时比较了用FP-LMT方法计算的ZnO中几种本征缺陷的能级位置.根据实验和能级计算的结果,推测出ZnO薄膜中的紫外峰与ZnO带边激子跃迁有关,而绿色发光主要来源于导带底到氧错位缺陷(O_Zn)能级的跃迁,而不是通常认为的氧空 关键词： ZnO薄膜 热处理 光致发光光谱 缺陷能级     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested As_Zn–2V_Zn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of As_Zn and V_Zn. The core level chemical shift in binding energy of As_Zn indicated the formation of As_Zn–2V_Zn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.     

Long-wavelength optical phonons in the ZnTe/Zn<Subscript>0.8</Subscript>Cd<Subscript>0.2</Subscript>Te superlattice

The lattice IR reflection spectra of a ZnTe/Zn_0.8Cd_0.2Te superlattice measured at temperatures of 300 and 10 K are analyzed. The ZnTe/Zn_0.8Cd_0.2Te superlattice is grown by molecular-beam epitaxy on a GaAs substrate with a ZnTe buffer layer. It is found that the lattice IR reflection spectra of the studied structure exhibit only one reflection band. Dispersion analysis of the experimental spectrum has revealed the presence of one lattice TO mode close in frequency to the mode of pure ZnTe. This result is explained by a shift in the frequency of the lattice modes of the ZnTe and Zn_0.8Cd_0.2Te layers of the superlattice toward each other. In turn, this shift is caused by internal elastic stresses in the superlattice due to a mismatch between the lattice parameters of the materials of these layers.     

p型K:ZnO导电机理的第一性原理研究

基于密度泛函理论,利用局域密度近似的第一性原理平面波赝势方法,对掺K以及含有氢填隙(H_i)、氧空位(V_O)、锌填隙(Zn_i)和锌空位(V_Zn)的K:ZnO电子结构分别进行了研究.结果表明,1) 单独掺K可引入浅受主,但系统总能增高;2) K与H共掺可降低系统总能,提升稳定性;3) V_O在K+H:ZnO中的形成比Zn_i困难得多,二者都是 关键词： 氧化锌 p型 第一性原理 电子结构     

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.     

Emission properties of Mn doped ZnO nanoparticles prepared by mechanochemical processing

Mechanochemical processing was reported to introduce lot of crystal defects which can significantly influence emission properties. Nevertheless, to the best of our knowledge, there are no reports on effect of mechanochemical processing on emission properties of transition metal ion doped ZnO. In this study, Zn_1?xMn_xO nanoparticles with different Mn content (x=0, 0.02, 0.04, 0.06, 0.08, and 0.1) were prepared by mechanochemical processing to study the effect of Mn doping and processing on emission properties. Confirmation of nanoparticles size and nanocrystalline nature of hexagonal wurtzite ZnO structure is carried out using transmission electron microscopy (TEM) and selected area electron diffraction (SAED), respectively. The samples were also characterized using Fluorescence Spectroscope before and after heat-treatment. The emission studies revealed that blue emission intensity is stronger compared to UV and green emission in contrast to the earlier reports, where other synthesis routes were employed for the ZnO nanoparticles' preparation. The blue emission originates from the zinc interstitial (Zn_i) and oxygen interstitial (O_i) defects, which indicate that the mechanochemical route resulted in more interstitial defects compared to oxygen substitution (O_Zn) and oxygen vacancy (V_o) defects which otherwise would give green emission. Mn doping resulted in shifting of near-band-edge (NBE) emission and the reduction in the intensities of NBE, blue and green emissions. The initial red shift at lower Mn content could be due to s–d and p–d exchange interactions as well as band tailing effect where as the blue shift at higher Mn content can be attributed to the Burstein-Moss shift. The reduction in emission intensity could be due to non-radiative recombination processes promoted by Mn ions with increasing Mn content.     

Highly enhanced luminescence of ZnO green phosphors by calcining ZnS with NH4Br as additive

Zinc oxide (ZnO) phosphors with highly efficient green emission have been prepared by calcining ZnS with NH₄Br as additive in air atmosphere. The luminescent properties of as-prepared ZnO phosphors were characterized by X-ray photoelectron spectroscopy and photoluminescence. Our results reveal that the green emission is ascribed to a transition of a photo-generated electron from the localized defect centers (V_o⁺) to a deeply trapped hole (V_Zn⁻) within the band gap. The addition of NH₄Br enhances the luminescent emission of ZnO by promoting the formation of vacancies of both oxygen and zinc.     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

Comparative analysis of emission and absorption spectra of zinc oxide powders

Photoluminescence and optical absorption spectra induced by proton and electron irradiation in zinc oxide powders have been investigated. It has been found that the emission band in a visible region with a maximum of about 2.3 eV is a superposition of three bands with 2.55, 2.34, 2.12 eV, respectively, caused by oxygen vacancies V _O⁺, interstitial oxygen O _i ⁻, and zinc vacancies V _Zn⁻ absorbing in the 3.03-, 2.83-, and 2.64-eV bands.     

The orange luminescence band in MgO crystals

Nominally pure MgO crystals exhibit a bright orange luminescence when excited with u v or visible laser light This luminescence has been characterized using a variety of measurements with pulsed and cw laser sources Its amplitude increases with temperature to about 400 K. while the lifetime of emission decreases from 270 μs at 290 K to 9 μs at 433 K This emission appears to originate in hole recombination with Fe²⁺ ions Hole trapping at V^?_OH and V^?_Al centers may occur as an intermediate step Bleaching with 514 5-nm Ar⁺ laser light is reversible upon exposure to u v radiation Present data are compared with those obtained previously on the thermoluminescence of irradiated MgO crystals.     

Luminescence of Ni doped CaF2

Luminescence measurements of CaF₂ : Ni are reported. Before X-irradiation an emission band at 680 nm is observed with the corresponding excitation band at 255 nm. After RT X-irradiation the emission spectrum consists of three peaks at 290, 375 and 680 nm, all of them with an excitation band at 255 nm. The same emission spectrum together with the electron-hole recombination band is obtained by X-ray excitation.A comparison with previous EPR and optical absorption measurements indicates that the emission bands are due to different kinds of Ni²⁺ centers. The emission processes are similar to those found in CaF₂ : Co and CaF₂ : Mn.     

Fe掺杂对溶胶凝胶法制备的ZnO: Ni薄膜结构及发光特性的影响

采用溶胶凝胶法在玻璃基片上制备了ZnO及Ni, Fe共掺杂的Zn_0.95-xNi_0.05Fe_xO (x=0, 0.005, 0.01, 0.03, 0.05) 薄膜. 通过扫描电镜(SEM) 和X射线衍射(XRD) 研究了薄膜样品的表面形貌和晶体结构. 结果表明所有样品都具有(002) 择优取向, Fe掺杂导致ZnO: Ni薄膜的晶体质量变差, 晶粒尺寸减小, 但适当的Fe掺杂有利于获得致密、 均匀的薄膜. XPS测试结果表明样品中Ni离子的价态为+2价, Fe离子的价态为+2价和+3价.室温光致发光(PL) 测量表明, 所有样品均观察到较强的紫外发光峰, 蓝光双峰和绿光发光峰. ZnO: Ni薄膜的发光强度可以通过Fe掺杂进行有效调节. 进而我们讨论了Ni, Fe共掺杂ZnO样品的发光机理.