Effects of Cu–Zn phases on electronic properties in ZnO:Cu films

Theory predicts Cu-doped ZnO (ZnO:Cu) has p-conductivity; however, this has only been demonstrated in a small number of experimental and mechanistic studies. In this paper, ZnO:Cu films were grown in situ with varying Cu content, prepared using radiofrequency atomic source–assisted molecular-beam vapor deposition. The results indicate that ZnO:Cu films with dopant of Cu²⁺ only had n-type behavior. As the Cu content increased, Cu⁺ was the major dopant and the ZnO:Cu films had p-type behavior. However, excess Cu dopant resulted in the formation of second phases of Cu₂O and Cu–Zn. The formation of a Cu–Zn phase increased the content of Zn vacancy, thus increasing hole concentration. Stronger alloy scattering decreased carrier mobility. Therefore, Cu⁺ dopant and Zn vacancy give ZnO:Cu films p-conductivity properties.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

Photophysical investigation of the formation of defect levels in P doped ZnO thin films

Zinc oxide (ZnO) offers a major disadvantage of asymmetry doping in terms of reliability, stability, and reproducibility of p-type doping, which is the main hindrance in realization of optoelectronic devices. The problem is even more complicated due to formation of various native defects in unintentionally doped n-type ZnO. The realization of p-type conductivity in doped ZnO requires an in-depth understanding of the formation of an effective shallow acceptor, as well as donor-acceptor compensation. Photophysical properties such as photoconductivity along with photoluminescence (PL) studies have unprecedentedly and effectively been utilized in this work to monitor the evolution of various in-gap defects. Phosphorus (P) doped ZnO thin films have been grown by RF magnetron sputtering under various Ar to O₂ gas ratios to investigate the effect of O₂ on the donor-acceptor compensation by comprehensive photoconductivity measurements supported by the PL studies. Initial elemental analyses indicate presence of abundant zinc vacancies (V_Zn) in O-rich ambience. The results predict that P sits in the zinc (Zn) site rather than the oxygen (O) site causing the formation of P_Zn–2V_Zn acceptor-like defects, which compensates the donor defects in P doped ZnO films. Photocurrent spectra uniquely reveal presence of more oxygen vacancies (V_O) defects states in lower O₂ flow, which gets compensated with an increase in the O₂ flow. Successive photocurrent transients indicate probable presence of more V_O in the films grown with lower O₂ flow and more V_Zn in higher O₂ flow. Overall the photosensitivity measurements clearly present that O-rich ambience expedites the formation of acceptor defects which are compensated, thereby lowering the dark current and enhancing the ultraviolet photosensitivity.     

Intrinsic point defects and charge carrier trapping in monolayer BiOX (X = I,Br, and Cl)

Two-dimensional (2D) layered bismuth oxyhalides, BiOX (X = I, Br, and Cl), have great potential in optoelectronics and photocatalysis applications. The intrinsic point defects are crucial for carrier conductivity and transport. However, the understanding for defect physics of 2D atomic-scale BiOX are still unclear. Herein, through the first-principles calculations, we investigate the formation of intrinsic point defects and their effect on charge carrier trapping in 2D monolayer BiOX. Under a O-poor condition, the donor defects, such as the Bi_ad, Bi_X, V_O, and V_X, can form spontaneously and induce a high n-type conductivity. The V_X shows a shallow transition level and has no defect states. In contrast, the Bi_ad, Bi_X, and V_O display deep transition levels and obvious localized defect states that are responsible for the charge carrier trapping. As O becomes richer, the concentration of acceptor defects increases. Nevertheless, the donor and the acceptor defects can strongly compensate each other, pinning the Fermi energy in the band gap. The dominant acceptor defects, such as the Br_Bi, O_Br, and O_Cl, show the deep transition levels and serve as carrier traps due to the charge localized around the defect sites. Our work gives an insight into the defect physics of atomic-scale 2D BiOX and provides a guidance for their optoelectronics and photocatalysis applications.     

Reverse manipulation of intrinsic point defects in ZnO-based varistor ceramics through Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase

Intrinsic point defect structure plays a crucial role in functional ceramics with a grain-grain boundary microstructure. In the present study, a novel method of reversely manipulating intrinsic point defects (oxygen vacancy, V_o and zinc interstitial, Zn_i) in ZnO-based varistor ceramics is proposed, which makes use of Zr-stabilized high ionic conducting β_III-Bi₂O₃ intergranular phase. It is found that Zr-doping not only modifies the grain growth by the formation of secondary Zr-rich phase, but also influences the intrinsic point defect structure via the stabilized β_III-Bi₂O₃ phase, resulting in reduced V_o density but increased Zn_i density. The reverse manipulation is unambiguously demonstrated by broadband dielectric spectroscopy and further confirmed by the accelerated ageing experiment. The proposed intrinsic point defect dynamics unveil an important but usually neglected function of the dopant that has little solid solubility in ZnO grains, which opens up a promising way to tailor the material property.     

Effects of substrate temperature on thermal stability of Al-doped ZnO thin films capped by AlOx

Effects of substrate temperature on the thermal stability of Al-doped ZnO (AZO) films have been studied. Degradation of electrical properties of AZO films by annealing under flowing N₂ gas depends on their crystallinity controlled by the substrate temperature. A thin AlO_x capping layer was employed to passivate the thermal degradation of the AZO layer. A strong correlation between Zn desorption and reduction in carrier concentration was observed. Thermal desorption of Zn was prevented by the AlO_x layer, retaining carrier concentration. With the AlO_x capping layer, the reduction in Hall mobility was prevented in samples with good c-axis orientation, while the reduction in Hall mobility was still observed in poor c-axis oriented films. However, the reduction was smaller than that in bare AZO films. The dependence of Hall mobility evolution on the substrate temperature, and therefore, on crystallinity, strongly suggests the impact of grain boundary scattering on thermal degradation. An increase in optical mobility, which was evaluated from optical spectra using the Drude model, with annealing temperatures, supports the conclusion that an increase in grain boundary scattering by annealing caused the degradation of Hall mobility. The increase in grain boundary scattering induced by Zn desorption was prevented by the capping layer, while contributions of domain alignment and other segregation of defects to the grain boundary scattering, which depend on the substrate temperature retained, leading to different evolutions of Hall mobility.     

Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering

Al-doped ZnO ceramics were sintered by conventional sintering method and spark plasma sintering (SPS) respectively. Electrical properties and microstructure have been investigated by various measurements. The samples sintered via SPS exhibit a huge electrical conductivity, up to 3.0 × 10⁵ S/m at room temperature, which was much higher than that of the sample sintered via the conventional sintering. Structural and morphorlogical characterizations pointed out that the further incorporation of Al ions and the absence of a secondary phase, contribute to the increase of the carrier concentration. Raman spectroscopy revealed the occurrence of structural distortions and a disorder induced by Al doping. Photoluminescence spectra were interpreted by different electronic active defects such as the defect complexes (Al_Zn-Zn_i) which play a key for the high electrical conductivity. Thus, SPS and Al doping modified the microstructure and the concentration of the electronic active defects to ensure high electrical conductivities in doped ZnO-based ceramics.     

Assorted analytical and spectroscopic techniques for the optimization of the defect-related properties in size-controlled ZnO nanowires

In this article, the important role of the intrinsic defects in size-controlled ZnO nanowires (NWs) which play a critical role in the properties of the NWs, was studied with a combined innovative experimental analysis. The NWs prepared by both the aqueous solution method and chemical vapour deposition process were of increasing length and decreasing size-to-volume (S/V) ratio. The combined approach involved different analytical and spectroscopic techniques and from the correlation between the different measurements, the concentration of the oxygen vacancies jointly with the zinc interstitials defects and the zinc vacancy defects was observed to be positively or negatively correlated, respectively, with the magnitude of the photoluminescence intensity and radiative lifetimes. Furthermore, the experimental results also suggest that the oxygen vacancy defects are not only spatially located on the surface of the NW but an increasing fraction of the total oxygen vacancy defects connected with the green emission is also located in an annulus region beneath the surface as the ZnO NWs elongate. On the other hand, as the donor concentration plays a critical function in the properties of the ZnO NWs, an analytical model was derived for the calculation of the donor concentration of the NWs directly from its reverse-biased current-voltage characteristics obtained from the conductive atomic force microscopy measurements.     

Synergistic optimization of electrical-thermal properties of dual vacancy Bi1−x-yPbyCu1−xSeO by improving mobility and reducing lattice thermal conductivity

We fabricate Bi_1?x-yPb_yCu_1?xSeO (x = 0, 0.03, 0.06, y = 0, 0.10) samples via 4 min-microwave synthesis combined with 5 min-spark plasma sintering. The phase composition, microstructure, valence, and electrical and thermal transport properties of the samples are investigated at 298–873 K. Pb doping provides impurity carriers and increases the concentration to 0.9–3.0 × 10²⁰ cm^-³ . Bi and Cu vacancy could provide a carrier transport channel to reduce carrier scattering probability, leading to improved mobility. Twin crystals, stacking faults, and grain boundary segregation are observed in Bi_0.87Pb_0.10Cu_0.97SeO on scanning transmission electron microscopy. Bi and Cu vacancy increase the sample point defects in Pb-doped or undoped samples which results in a decrease in lattice thermal conductivity. The lattice thermal conductivity of Bi_0.87Pb_0.10Cu_0.97SeO is decreased to an extremely low value of 0.13 Wm^?1 K^?1 and a maximum ZT value of 1.09 is achieved at 873 K.     

Effect of the Cu vacancy on the thermoelectric performance of p-type Cu1−xInTe2 compounds

In this study, the effect of Cu vacancy on the thermoelectric performance of Cu_1-xInTe₂ is reported, where x is 0, 0.04, 0.06, 0.08 and 0.10. Cu vacancy can yield excess holes lifting the carrier density of CuInTe₂, which is an intrinsic p-type semiconductor. Meanwhile, the mass fluctuation caused by Cu vacancy attributes to the enhanced point defects phonon scattering, resulting in a reduced lattice thermal conductivity. The optimum Cu vacancy content is found to be 0.04, attaining a maximum zT value of 0.83 at 820 K. Meanwhile, there is a 50% enhancement compared to that of pure sample which arises from the large power factor and the relatively low thermal conductivity. Our result indicates the great potential of Cu_1-xInTe₂ for thermoelectric application at middle-temperature.     

Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array

A Sb-doped ZnO microrod array was fabricated on an Al-doped ZnO thin film by electrodeposition. Strong violet luminescence, originated from free electron-to-acceptor level transitions, was identified by temperature-dependent photoluminescence measurements. This acceptor-related transition was attributed to substitution of Sb dopants for Zn sites, instead of O sites, to form a complex with two Zn vacancies (V_Zn), the Sb_Zn-2V_Zn complex. This Sb_Zn-2V_Zn complex has a lower formation energy and acts as a shallow acceptor which can induce the observed strong violet luminescence. The photoresponsivity of our ZnO p-n homojunction device under a negative bias demonstrated a nearly 40-fold current gain, illustrating that our device is potentially an excellent candidate for photodetector applications in the ultraviolet wavelength region.     

Boosting the thermoelectric performance of Bi2O2Se by isovalent doping

N‐type Bi₂O₂Se has a bright prospect for mid‐temperature thermoelectric applications on account of the intrinsically low thermal conductivity. However, the low carrier concentration of Bi₂O₂Se (~10¹⁵ cm^?3) severely limits its thermoelectric performance. Herein, the boosting of the carrier concentration to ~10¹⁹ cm^?3 can be realized in our La‐doped Bi₂O₂Se ceramic samples, which could be ascribed to the formation of isoelectronic traps and the narrowing of band gap, and contribute to a marked increase in the electrical conductivity (from 0.03 S cm^?1 to 182 S cm^?1). Our X‐ray absorption near‐edge structure spectra results reveal that a local disordering of oxygen atoms could be an important reason for the intrinsically low thermal conductivity of Bi₂O₂Se, and the point defects can also suppress the lattice thermal conductivity in La‐doped Bi₂O₂Se. The ZT value can be enhanced by a factor of ~4.5 to 0.35 at 823 K for Bi_1.98La_0.02O₂Se as compared to the pristine Bi₂O₂Se. The coordinated optimization of electrical and thermal properties demonstrates an effective method for the rational design of high‐performance thermoelectric materials.     

Electroless deposition of transparent conducting and 〈0 0 0 1〉-oriented ZnO films from aqueous solutions

Electroless ZnO deposition on a glass substrate from dissolved oxygen-free aqueous solutions containing Zn(NO₃)₂ and dimethylamineborane (DMAB) was examined to yield ZnO films applicable to a transparent conducting oxide (TCO). Concentration of Zn(NO₃)₂ was optimized in terms of crystal growth orientation and surface morphology using XRD and AFM, and that ranging from 0.065 to 0.075 M was found to provide well 〈0 0 0 1〉-oriented dense ZnO films. The polycrystalline ZnO films deposited with Zn(NO₃)₂ concentration of 0.07 M had a preferred 〈0 0 0 1〉 growth orientation and exhibited high visible transparency. Top-view and cross-sectional FE-SEM images revealed that hexagonal columnar ZnO grains with 200 nm in diameter and 290 nm in length grew almost vertically from a glass substrate. Heat treatment at 723 K under a reductive atmosphere was performed to increase the intrinsic carrier concentration in the ZnO film, and Hall effect measurements revealed low electrical resistivity of 4.7 × 10⁻³ Ω cm.     

A sandwich structure assisted by defect engineering for higher thermoelectric performance in ZnO-based films

Gallium (Ga) doping together with low dimensionality has been a promising approach to improve thermoelectric performance of zinc oxide (ZnO) materials, due to the increase of carrier concentration and suppression of phonon transport. So far, the highest power factor of Ga-doped ZnO (GZO) thin films has reached 280 μW m⁻¹ K⁻², which is still limited for practical applications. In this work, we have simultaneously optimized the electrical conductivity and Seebeck coefficient of GZO thin films using the combination of oxygen defects and sandwich structure (GZO-ZnO-GZO). Benefiting from energy filtering effect at the interface between GZO and ZnO layers and high oxygen vacancy concentration, the density of states (DOS) effective mass has been increased together with a relatively high carrier concentration. As a result, an improved power factor value of 434 μW m⁻¹ K⁻² at 623 K has been achieved, which is comparable to the best values reported for ZnO-based films. This method of combining defect engineering and sandwich structure design shows great potential in enhancing the thermoelectric performance of ZnO-based thin films or other oxide materials.     

Enhanced CO2 photocatalytic conversion into CH3OH over visible‐light‐driven Pt nanoparticle-decorated mesoporous ZnO–ZnS S-scheme heterostructures

In the present contribution, the design and fabrication of Pt nanoparticle-decorated mesoporous ZnO–ZnS heterostructures were described and used effectively for photocatalytic CO₂ conversion to yield CH₃OH. TEM images of the mesoporous Pt/ZnS–ZnO heterostructure demonstrated spherical ZnO NPs ~20 nm, and Pt NPs ~3 nm were well dispersed on the porous ZnS–ZnO heterostructure. The formation of CH₃OH over the Pt/ZnS–ZnO heterostructure was 78, 39 and 20 times larger than that bare ZnS, ZnO NPs and ZnS–ZnO, respectively. The optimal Pt/ZnO–ZnS heterostructure exhibited a high CH₃OH formation rate of 81.1 μmolg^-1h^-1, which is about 44, 22 and 20 times larger than that of bare ZnS (1.86 μmolg^-1h^-1), ZnO (3.72 μmolg^-1h^-1), and ZnO–ZnS (4.15 μmolg^-1h^-1), respectively. The significantly enhanced reduction of CO₂ was imputed to the synergistic effects of the ZnO–ZnS heterostructure and the incorporation of Pt NPs. The synthesized photocatalyst provides a new transfer route through which carriers can migrate to the outer surface as well as pore channels of the mesoporous ZnO–ZnS, therefore significantly minimizing the transfer distance for carriers, inhibiting photoinduced electron-hole recombination, and diminishing the mobility resistance, as determined using photoluminescence, photocurrent response, and electrochemical impedance spectra measurements.     

Effects of Li doping and point defect on the magnetism of ZnO

The magnetism sources and magnetic mechanism of Li doping and point defect, which coexist in the presence of hexagonal wurtzite ZnO, are controversial. To solve these problems, the effects of Li doping and point defect on the magnetism of ZnO were studied using geometry optimization and energy calculation based on the first-principle generalized gradient approximation + U (GGA + U) method of density functional theory. Results showed that the coexistence of Li doping and Zn vacancy can achieve ferromagnetic long-range order, and the Curie temperature of the doping system can achieve room temperature. In addition, results showed that magnetic moments are significantly different when the structures of the systems are also different with the same doping amount and doping method, which are advantageous for the enhancement of the magnetic properties of dilute magnetic semiconductors. The magnetism source of Zn₁₄LiO₁₆ is the hybrid coupling electron exchange effect between O-2p and Zn-3d orbits. With the coexistence of Li replacing Zn and Zn vacancy, the closest relative distance between doping and vacancy leads to the lowest formation energy and highest stability. In the condition of the highest stability of the ground state, all the doping systems of Li replacing Zn and O vacancy, doping system of interstitial Li and Zn vacancy, and doping system with the coexistence of Li replacing Zn, interstitial Li, and Zn vacancy are non-magnetic, which are considered worthless in the design and preparation of diluted magnetic semiconductors (DMSs).     

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (C_i) along parallel to c-axis facilitates the growth of carbon sp² clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant C_i will be able to enter into V_Zn to form C_Zn or combine with lattice O to form (C_O)_O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm⁻¹. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.     

Role of vanadium ions,oxygen vacancies,and interstitial zinc in room temperature ferromagnetism on ZnO-V2O5 nanoparticles

In this work, we present the role of vanadium ions (V⁺⁵ and V⁺³), oxygen vacancies (V_O), and interstitial zinc (Zn_i) to the contribution of specific magnetization for a mixture of ZnO-V₂O₅ nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10⁻³ to 3.5?×?10⁻³ emu/gr. Pure ZnO powders (1.34?×?10⁻³ emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zn_i, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of V_O concentration. For the ZnO-V₂O₅ system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V₂O₃ and secondary phases containing V⁺³ ions; at the same time, an increase of V_O is observed with an abrupt fall of magnetization to σ?~?0.7?×?10⁻³ emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including V_O and Zn_i depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zn_i at the same time that V₂O₅ NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of V_O and Zn_i was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.     

ZnS与NaCl共混煅烧法制备高效绿色ZnO荧光粉

以氯化钠为助熔剂,在较低温度(约700℃)下的空气气氛中煅烧硫化锌合成了ZnO绿色荧光粉. 测定了不同煅烧温度下合成荧光粉的激发光谱和发射光谱,以及硫、氯元素含量. 结果表明,合成荧光粉的主晶相为六方纤锌矿结构的ZnO,并有少量残存的ZnS相存在. 在350 nm波长光激发下,该荧光粉的发射光谱只有505~510 nm绿色峰,无380 nm激子发射. 根据给体-受体机制,该发射是源于光生电子从局域缺陷中心(给体)向陷阱空穴(受体)的跃迁. 煅烧过程中氯化钠的存在大大提高了产物的荧光发射强度,这归因于荧光粉中硫、氯的共掺杂对缺陷VO+和Zn(1-z)+形成的直接贡献. 该荧光粉能与近紫外发光二极管相匹配,有望应用于白光二极管.     

Evaluation of diffusion mechanism and structural characterizations of ZnS compound during chemical reaction of Zn and S in liquid phase

The present study investigates the diffusion mechanism, morphology and structural characteristics of crystalline zinc sulfide (ZnS) during the reactive diffusion process. The samples with nominal composition of Zn₅₀S₅₀ were prepared via capsulation of high purity of zinc and sulfur followed by the annealing process at various reaction temperatures. The prepared samples were characterized using X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The structural measurements confirm the formation of zinc sulfide with wurtzite structure during the annealing process at 550°C. The wurtzite allotrope of ZnS is not stable at a high annealing temperature and is transformed to cubic zinc-blende structure. During the annealing process, a continuous layer of ZnS compound forms at the Zn/S interface. Both Zn and S diffuse into the formed ZnS layer and the growth of that occurs mainly toward the Zn side. Under this condition, Kirkendall voids form and accumulate near the ZnS/S interface.