Electronic structure and magnetic properties of Cr monodoped and (Cr, Al) codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Cr monodoped and (Cr, Al) codoped in ZnO. The results indicate that Cr monodoped in ZnO favors a spin-polarized state with a total magnetic moment of 7.50μ_B per supercell and the magnetic moment mainly comes from the unpaired 3d electrons of Cr atoms. In addition, it was found that the ferromagnetic exchange interaction between Cr atoms is short-ranged in Cr monodoped ZnO. Interestingly, the ferromagnetic stability can be enhanced significantly by codoping Al_Zn. We think that the enhancement of ferromagnetic stability should be attributed to the additional electrons introduced by Al_Zn codoping.     

First-principles calculation on p-type conduction of (Sb,N) codoping in ZnO

Based on first-principles calculations, (Sb, N) codoped ZnO are investigated. We find that Sb_Zn–4N_O have lower formation energy and can form p-type conduction with smaller hole effective mass. In comparation to monodoping of Sb, Sb_Zn–4N_O complex can form better p-type conductivity than Sb_Zn–2V_Zn, which may be strongly compensated by Sb_Zn defect and result in a decrease of p-type conduction. So we inferred that (Sb, N) codoping in ZnO under O-poor condition should be a realizable candidate of p-type conduction.     

The mechanism of Li, N dual-acceptor co-doped p-type ZnO

The formation of single defects and defect complexes are investigated in Li, N co-doped ZnO by the first-principles plane wave method with projector augmented wave (PAW) pseudo-potential technology. We find that: (i) p-type conductivity could be achieved in single Li doped ZnO under an O-rich condition, since the formation energy of Li_Zn acceptor is much lower than the interstitial Li_i; (ii) the dual-acceptor complex Li_Zn-N_O is unlikely to form, and the good p-type conductivity is mainly attributed to the Li_Zn acceptor, even in Li, N co-doped ZnO; (iii) the additional introduction of N may help compensate the single Li_i donor defects under certain growth conditions, but its role in the p-type conductivity in ZnO remains to be clarified. PACS 71.15.Mb; 73.61.Ga; 71.15.Nc; 71.20.Nr; 71.55.Gs     

Theoretical study of the magnetic interaction of Cr-doped ZnO with and without vacancies

First-principles density-functional theory (DFT) calculations have been performed to study the magnetic properties of ZnO:Cr with and without vacancies. The results indicate that the doping of Cr in ZnO induces obvious spin polarization around the Fermi level and a total magnetic moment of 3.77μ_B. The ferromagnetism (FM) exchange interaction between Cr atoms is short-ranged and decreases with increasing Cr separation distance. It is suggested that the FM state is not stable with low concentration of Cr. The presence of O vacancies can make the half-metallic FM state of the system more stable, so that higher Curie temperature ferromagnetism may be expected. Nevertheless, Zn vacancies can result in the FM stability decreasing slightly. The calculated formation energy shows that V_Zn+Cr_Zn complex forms spontaneously under O-rich conditions. However, under Zn-rich conditions, the complex of V_O+Cr_Zn forms more easily. Thus, ZnO doped with Cr may exhibit a concentration of vacancies that influence the magnetic properties.     

Formation of shallow acceptors in ZnO doped by lithium with the addition of nitrogen

We performed first-principle total-energy calculations to investigate the mechanism for the realization of high quality p-type ZnO codoped with lithium and nitrogen. We find that the higher hole concentrations measured in the codoped ZnO is related to decreased ionization energy of acceptors and reduction of compensations. The dual acceptor N_O-Li_Zn complex proposed in experiments is unstable. While in the (Li_I-N_O)-Li_Zn complex, where acceptor Li_Zn binds to the passivated (Li_I-N_O) complex is stable and acts as a single acceptor. The activation energy of this complex is about 60 meV lower than that of Li_Zn in Li-monodoped ZnO. The formation of inactive (Li_I-N_O) complexes creates a fully occupied impurity band just above the valence band maximum of ZnO. Thus Li atoms binding to this complex is activated by the electrons from the complex state rather than from the host states, accounting for decreased activation energy. Besides, Li_I⁺ and N_O⁻ bind tightly through the Coulomb interaction. Such binding will suppress the amount of compensating donor Li_I and limit the compensation for the desired acceptor Li_Zn.     

Half-metallic ferromagnetism in I2-VI compounds with non-magnetic dopants

First-principles calculations based on the tight-binding linear muffin-tin orbital (TB-LMTO) method were performed to investigate the occurrence of spin polarization in the alkali metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite (anti- CaF₂-type) structure with non-magnetic (N, P, As, Sb and Bi) dopants. The calculations reveal that non-magnetic substitutional doping at anion site can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the antifluorite ferromagnetic state is energetically more stable than the antifluorite non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The calculated magnetic moment is found to be 1.00 μ_B per dopant atom. The magnetic moment is mainly contributed by p orbitals of dopant atom.     

Enhanced ferromagnetism in nano-sized Zn0.95Mn0.05O grains

The present work reports ferromagnetism by doping magnetic Mn atoms in the diamagnetic ZnO matrix and the ferromagnetism has been extended up to 640 K in nano-grained Zn_0.95Mn_0.05O samples. The bulk and nano-grained samples were stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples were estimated from room temperature XRD spectrum. Surface morphology of the samples was examined at room temperature using SEM picture and EDX spectrum. The ferromagnetism of the bulk material shows enhancement in nano-grained samples, which was mainly due to the solution of Mn atoms into the lattice sites of ZnO by mechanical milling. The enhancement of magnetic moment and ferromagnetic ordering temperature with reduction in grain size has been understood in terms of the core-shell structure and existing theoretical models. The present work also demonstrated the role of surface spin disorder on the enhancement of ferromagnetism in Zn_0.95Mn_0.05O nanograins.     

溶剂热合成法制备的Zn1-xMnxO和Zn1-2xMnxLixO纳米材料的磁学性质

用溶剂热合成法在160oC制备出Zn_1-xMn_xO纳米棒和Zn_1-2xMn_xLi_xO纳米颗粒. XRD和拉曼测试结果表明Mn离子已很好地掺入ZnO母体中. M-H图中未观察到磁回滞,ESR谱中的精细结构说明掺杂的Mn离子间没有铁磁相互作用. 共掺Li仅仅改变了产物的形貌,并不能改变其磁学性质.     

Ferromagnetism in phosphorus-doped ZnO: First-principles calculation

The electronic structure and magnetic properties of nonmagnetic phosphorus doped ZnO are investigated using first-principles calculation. Both generalized gradient approximation (GGA) and GGA + U calculations show that each substitutional P atom in ZnO induces a magnetic moment of about 1.0 μ_B, which come mainly from the partially filled p orbitals of the substitutional P and its 12 second neighboring O atoms. The magnetic coupling between the moments induced by P doping is ferromagnetic. The calculated electronic structures indicate that the ferromagnetic coupling can be explained in terms of the two band coupling model.     

First-principles study of electronic structure and ferromagnetism in Ti-doped ZnO

We perform first-principles spin polarized calculations of the electronic structure of Ti-doped in ZnO. Ferromagnetism in Ti-doped ZnO is identified, which is in agreement with recent experimental and calculated results. A net magnetic moment of 0.715μ_B is found per Ti. At a Ti concentration of 12.5%, total energy calculations show that the ferromagnetic state is 68 meV lower than the antiferromagnetic state. The electronic states near Fermi energy are dominated by strong hybridization between O 2p and Ti 3d, which is just the origin of impurity band in Ti-doped ZnO and also implies that the Ti-O bond is quite covalent instead of purely ionic. Since there is no magnetic element in this compound, Ti-doped ZnO appears to be an unambiguous dilute magnetic semiconductor.     

First-principles study of single intrinsic vacancy formation and its effect on the electronic density states and magnetic moment of V-doped ZnO

We calculated the formation energy of single vacancy in V-doped ZnO in different conditions (oxygen or zinc rich) by first principles. Effect of an intrinsic vacancy on the electronic density of states and magnetic moment of V-doped ZnO (Zn₁₅VO₁₆) with and without single vacancy was also calculated. Our calculation was performed by the CASTEP program within spin-polarized GGA approximation implemented in materials studio software. The formation energy showed that oxygen vacancy inclined to stay far from vanadium (V) and zinc vacancy preferred to stay at a position near V. The calculated formation energy also showed that a zinc vacancy may automatically occur but an oxygen vacancy may not appear automatically. Vanadium doping introduced spin-polarization around Fermi level. For an energy favorable vacancy, an oxygen vacancy had little effect on the electronic density of states. A zinc vacancy made the spin-polarization peaks around Fermi level broaden and decreased their magnitude. For the magnetic moment in energy favorable configurations, an oxygen vacancy had little effect on the magnetic moment; a zinc vacancy significantly decreased the magnetic moment (as high as 63.7%). Changes in magnetic moments were consistent with electronic density of states. Our calculation may interpret various experimental magnetic moment values. Our work also provided a reference for preparing V-doped ZnO-based dilute magnetic semiconductors.     

Effect of codoping with C or N on electronic structures and magnetic properties of ZnO:Cu

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of ZnO:Cu. The results indicate that Cu-doped ZnO prefers a ferromagnetic ground state and behaves like a half-metallic ferromagnet. The magnetic moment mainly localizes at Cu atom and the rest mainly comes from the spin polarized O atoms. It has been found that the ferromagnetic stability can be enhanced slightly by substituting an oxygen atom with one N atom; while the ferromagnetic stability can be weakened by replacing one O atom with a C atom. Due to absence of magnetic ion and the 100% spin polarization of the carriers in ZnO:Cu, one can expect that Cu-doped ZnO would be a useful half-metallic ferromagnet both in practical application and in theoretical studies.     

Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy

Zn_0.8Li_0.2O ceramics with wurtzite structure have been fabricated by a solid reaction of ZnO and Li₂CO₃. The effects of substitutional Li atoms on the crystal structure and structural phase transition of ZnO are studied by Raman spectroscopy. The enhancement of E₁(LO) mode in Zn_0.8Li_0.2O ceramics reveals the occurrence of Li-induced structural disorder. Temperature dependent Raman spectra strongly indicate that a structural phase transition occurs at about 448 K in Zn_0.8Li_0.2O ceramics.     

表面效应对Li掺杂的ZnO薄膜材料p型电导的影响

利用第一性原理的方法研究了在ZnO非极性表面和极性表面的不同原子层中, 分别用Li原子去替位Zn原子(记为Li_Zn)后的相对稳定性和热离化能. 计算结果表明Li_Zn处于ZnO表面区域时的稳定性优于在ZnO体中时的稳定性, 并且Li_Zn在表面区域的热离化能要比它在体结构中的热离化能大很多, 于是, ZnO表面效应的存在会使Li掺杂的ZnO薄膜材料的p型导电能力大幅度降低. 这个结果对低维ZnO体系p型掺杂有着重要的指导意义. 我们进一步发现, 在不同的ZnO表面区域里Li_Zn的热离化能会表现出很大的差异是源于不同的表面具有不同的静电势分布.     

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.     

Structural, magnetic properties and photoemission study of Ni-doped ZnO

Nickel-doped ZnO (Zn_1−xNi_xO) have been produced using rf magnetron sputtering. X-ray diffraction measurements revealed that nickel atoms were successfully incorporated into ZnO host matrix without forming any detectable secondary phase. Ni 2p core-level photoemission spectroscopy confirmed this result and suggested Ni has a chemical valence of 2+. According to the magnetization measurements, no ferromagnetic but paramagnetic behavior was found for Zn_0.86Ni_0.14O. We studied the electronic structure of Zn_0.86Ni_0.14O by valence-band photoemission spectroscopy. The spectra demonstrate a structure at ∼2 eV below the Fermi energy E_F, which is of Ni 3d origin. No emission was found at E_F, suggesting the insulating nature of the film.     

Microstructure and thermoelectric properties of Zn1−xAlxO ceramics fabricated by spark plasma sintering

Al-doped ZnO powders were synthesized via solid reaction between Zn(OH)₂ and Al(OH)₃ and consolidated by spark plasma sintering (SPS) to fabricate fine-grained Zn_1−xAl_xO ceramics as a thermoelectric material. X-ray diffraction and spectrophotometer experiments revealed that Al doping into ZnO is enhanced by the present process, and consequently the SPS-processed Zn_1−xAl_xO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al₂O₃ oxide powders. Because of the combined effect of Al doping and grain refinement, the present Zn_1−xAl_xO ceramics show much lower thermal conductivity, which also results in an enhanced dimensionless figure of merit (ZT), than un-doped ZnO oxides prepared also by SPS.     

Half-metallic ferromagnetism in (C, Si, Ge, Sn and Pb)-doped I2-VI compounds: An ab initio study

First-principles calculations were performed to investigate the stability, electronic structure and magnetism in Group IV elements-doped alkali-metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite structure using the linear muffin-tin orbital method in its tight-binding representation (TB-LMTO). The calculations reveal that non-magnetic dopants can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the ferromagnetic state is energetically more stable than the non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The magnetic moment is found to be 2.00 μ_B per dopant atom.     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

Study of point defect on the stability and magneto-optical properties of ZnO:Cu by first-principles

ABSTRACT

The magnetic and optical properties of Cu-doped ZnO systems have been widely studied in experimental, but the magnetic sources of the coexistence of Cu replacing Zn and the O vacancy systems are controversial. First-principles can compensate for the experimental deficiencies. The effects of Cu-doping and point defects on the magnetic and optical properties of ZnO were studied using geometry optimisation and energy calculation based on first-principle generalised gradient approximation?+?U method of the density functional theory. Results indicate that the band gaps and absorption spectra of Zn₁₅CuO₁₆, Zn₁₄CuO₁₆, and Zn₁₅Cu_iO₁₆ systems become narrowed and red-shifted, respectively, compared with those of pure ZnO. In addition, the system with Cu replaces Zn, and Zn vacancy coexists in ZnO. The doping system has the relatively largest magnetic moment and can achieve a ferromagnetic long-range order, and the Curie temperature can reach room temperature. As an electron injection source, this system can reach 100% electron spin-polarisation and exhibit half-metallic properties, which are relatively favourable for dilute magnetic semiconductor (DMS). Therefore, this system has certain theoretical reference value in the design and preparation of light-emitting devices or DMS.