Conductivity Of ZnO Research Articles

Toward p-type conduction in Cs-doped ZnO: an eco-friendly synthesis method

An alcohol-free, eco-friendly technique was adapted for the synthesis of undoped ZnO and Cs-(cesium) doped ZnO nanoparticles (NPs). The effect of annealing and dopant concentration on its structural and optical properties was investigated. X-ray diffraction results confirmed the formation of polycrystalline hexagonal wurtzite structure and enhanced crystallinity was observed for 1 mol%: Cs-doped ZnO NPs. Scanning electron microscopy results revealed triangular-shaped NPs and increase in the crystallite size is noticed with increase in dopant concentration. UV–visible results showed shift in the band edge toward higher wave length side with increasing Cs concentration. Reduction in bandgap was observed for Cs-doped ZnO NPs, due to quantum confinement effect. Transmittance value increased to 86 % with the inclusion of Cs in ZnO lattice. Room temperature photoluminescence analysis of Cs-doped ZnO NPs reveals bandedge emission along with 450 nm emission due to Zn vacancy and Zn interstitial defects. Electrical measurements confirmed the realization of p-type conductivity in Cs-doped ZnO NPs with a carrier concentration of 1.3 × 1018/cm3.

Structural peculiarities and point defects of bulk-ZnO single crystals

ZnO single crystals are related to promising direct wide band gap semiconductor materials belonging to the AIIBVI type of compounds with wurtzite structure. “Unintentional” n-type conductivity in ZnO may be caused by zinc and oxygen vacancies, and interstitial zinc atoms. To date, the comprehensive structural investigation and analysis of point defects in ZnO is absent in literature. Green, light green and almost colorless ZnO single crystals grown by the hydrothermal method in concentrated alkali solutions 4М(КОН)+1М(LiOH)+0.1M(NH4OH) on monohedral seeds [0001] at crystallization temperatures in the range of 330–350°C and pressures in the range of 30–50MPa have been firstly investigated by neutron diffraction. It was revealed the presence of additional reflections (∼12–∼16%) for all the crystals caused by kinetic growth effects that give grounds to assign them to the space group P3 rather than to P63mc. Analysis of the refined compositions together with the color of ZnO crystals does not rule out the relationship between the color and vacancies in the zinc and oxygen positions whose concentration decreases with the discoloration of the samples. The analysis of the photoinduced variation of the total and on-axis transmittance versus the CW laser intensity showed that the colored samples have profound deep defects related to oxygen vacancies.

Scaling of Efros–Shklovskii variable range hopping conduction in ZnO:Cd thin films by sol–gel

Electrical transport properties of sol–gel derived ZnO:Cd thin films were investigated. It was observed that for the thin films with relatively high resistivity, a crossover from the Mott variable range hopping (VRH) conduction to the Efros–Shklovskii (ES) VRH conduction was observed at low temperatures. The temperature dependent resistivity within the whole VRH conduction range from the Mott type to the ES type could be well described by a universal scaling law. The observation of ES VRH conduction in ZnO:Cd thin films suggests that the many-body electron–electron interactions should be considered for the ZnO:Cd thin films with high resistivity.

Na effect on flexible Cu(In,Ga)Se2 photovoltaic cell depending on diffusion barriers (SiOx, i-ZnO) on stainless steel

Cu(In,Ga)Se2 (CIGS) based-photovoltaic (PV) cells with different diffusion barriers of SiOx and i-ZnO were fabricated on stainless steel (STS) substrate and their electrical characteristics were investigated by measuring J–V curves under illuminated and dark conditions. The physical properties of the CIGS film depending on type of diffusion barrier were also analyzed using X-ray diffraction and secondary ion mass spectroscopy. The efficiency of the CIGS-PV cell with i-ZnO barrier was approximately 2% higher than that with the SiOx barrier. Through the analysis of dark J–V curves, we discovered that distinctive defects were formed in the band gap of CIGS based on which diffusion barrier contacted the STS. The diffraction pattern showed a slightly different tendency of the peak intensity ratio of (220/204)/(112) in the PV cell with the i-ZnO barrier, which was slightly higher than that in the PV cell with SiOx barrier. In elemental depth profile, a deficient Ga profile was observed near the surface of the CIGS film with the SiOx barrier, and an abundant Na profile within the CIGS film with the i-ZnO barrier was detected. This is attributed to a difference in thermal conduction through the diffusion barriers during CIGS film growth, originating from the larger thermal conductivity of ZnO compared with SiOx.

First Principles Study on the Effects of Cu Doping on the Magnetic Moment and Electronic Properties of V-Doped ZnO

The CASTEP program was used to study the effects of Cu on the structure, magnetic moment and electronic properties of V-doped ZnO. The calculated enthalpies showed that the Cu atoms were inclined to stay near the V atoms. Cu neutralized the delocalized electrons in V-doped ZnO. Cu also suppressed the magnetic moments of Zn and O more than those of the V ions. The different amounts of suppression caused the moments to concentrate on the V atoms. The magnetic moments decreased in Cu and V co-doped ZnO. The suppressions of the magnetic moments and concentrations of the Cu atoms were verified with density of states calculations. The states showed that Cu increased the conduction in V-doped ZnO. The calculated spin isospheres showed that Cu improved the concentration of the magnetic moments on the V sites. The improved concentration enhanced the performance of diluted magnetic semiconductors based on V-doped ZnO. These results help in understanding the role of Cu in V-doped ZnO. They also provide a reference for preparing diluted magnetic semiconductors based on V-doped ZnO

P-Type conductivity and stability of Ag–N codoped ZnO thin films

Silver and nitrogen codoped ZnO films [ZnO:(Ag, N)] were grown on quartz substrates by radio frequency magnetron sputtering deposition followed by ion-implantation technique. Room-temperature (RT) Hall-effect measurements confirm that Ag–N codoped ZnO film is converted to p-type ZnO under optimum post-annealing conditions. The p-type conductivity of ZnO:(Ag, N) is attributed to the formation of the AgZn–NO pairs and/or NO–AgZn–NO triangles, which can create impurity bands above the VBM of ZnO due to the p–d interaction between accepters and hence offer improved incorporation and activation of acceptors. The sample exhibited a stable conductivity-type over four month period after post-annealing, but apparent degradation of p-type characteristic was observed. It is demonstrated that the Ag aggregation (metallic Ag and/or Ag2O) can be stable in the film and metastable interstitial nitrogen is easy to diffuse and be trapped by acceptor NO to form double-donor (N2)O at RT, resulting in a decrease in the conductivity and stability of p-type ZnO:(Ag, N) film.

Electrical measurements of dielectric properties of molybdenum-doped zinc oxide thin films

Effects of molybdenum element content on electrical conductivity of ZnO sprayed thin films were investigated using the impedance spectroscopy method in the frequency ranging from 5Hz to 13MHz for temperature lying in 300–475°C domain. It is observed that AC conductivity is a power law. The values of dielectric constants ε1 and ε2 were found to decrease with frequency and increase with temperature. The activation energy determined from the plot of both DC conductivity and the hopping frequency with 1000/T shows that the hopping conduction is the dominant mechanism. Also, experimental data of DC conductivity were analyzed using the small polaron hopping model. The impedance analysis of undoped ZnO and Mo-doped ZnO (1% and 2%) shows only one semicircle implying the response originated from a single capacitive element corresponding to the bulk grains. However, the same analysis for ZnO:Mo (3% ) shows two semicircles which proves the existence of grain boundaries. Finally, analyses of polaron hopping mechanism and Urbach tailing allow some explanations of these transport phenomena. This study shows an effective variation of electrical measurements of Mo-doped ZnO films in terms of temperature leading to possible use of such films as gas sensors.

Bistability of hydrogen in ZnO: origin of doping limit and persistent photoconductivity.

Substitutional hydrogen at oxygen site (HO) is well-known to be a robust source of n-type conductivity in ZnO, but a puzzling aspect is that the doping limit by hydrogen is only about 1018 cm−3, even if solubility limit is much higher. Another puzzling aspect of ZnO is persistent photoconductivity, which prevents the wide applications of the ZnO-based thin film transistor. Up to now, there is no satisfactory theory about two puzzles. We report the bistability of HO in ZnO through first-principles electronic structure calculations. We find that as Fermi level is close to conduction bands, the HO can undergo a large lattice relaxation, through which a deep level can be induced, capturing electrons and the deep state can be transformed into shallow donor state by a photon absorption. We suggest that the bistability can give explanations to two puzzling aspects.

A formulation strategy for preparation of ZnO–Propylene glycol–water nanofluids with improved transport properties

Experiments were carried out on the influence of nanoparticle–liquid interactions on transport properties (viscosity and thermal conductivity) of ZnO–Propylene glycol–water nanofluids. Our experiments show that the preparation of ZnO–Propylene glycol–water nanofluid through addition of water to well-dispersed ZnO–Propylene glycol dispersion results in decreased viscosity and increased thermal conductivity, in comparison to that of propylene glycol–water mixture, due to favourable nanoparticle–liquid interaction. Such nanofluids are stable even without addition of any stabilizing agent. The study of influence of nanoparticle concentration (⩽2vol%) and temperature (15–50°C) on thermal conductivity reveal higher thermal conductivity enhancements at lower temperatures. This could be attributed to layering of propylene glycol molecules on surface of ZnO nanoparticles. The presence of liquid layers was ascertained through thermal analysis and infrared spectroscopy.

Stability, transparency, and conductivity of MgxZn1−xO and CdxZn1−xO: Designing optimum transparency conductive oxides

The stability, transparency, and conductivity of ZnO are suggested to be tailored by alloying with MgO or CdO to meet wide applications. Our Monte Carlo simulation based on first-principle cluster expansion methods partially explain the solubility and stability data scattered in extensive experiments and further reveal that CdxZn1−xO has much higher solubility than prevalent MgxZn1−xO in a large range of experimentally achievable temperature (400 K–1200 K). Furthermore, first-principles calculations based on hybrid functional methods show that CdxZn1−xO has better n-type doping properties than MgxZn1−xO. The optical gap of CdxZn1−xO could be ∼1.5 eV higher than its fundamental gap due to large Moss-Burstein shift. We thus predict that CdxZn1−xO has great potential to be a better transparent conducting oxide than MgxZn1−xO.

First-principles calculations on the electronic and optical properties of ZnO codoped with Cu-Co

The electronic structures and optical properties of intrinsic, Cu, Co doped and Cu-Co codoped ZnO compounds are calculated using first-principles plane-wave ultrasoft pseudopotential method based on the the density functional theory. The results show that the conductivity of ZnO can be improved by doping Cu and Co because of the increase of the carrier concentration under the order of magnitude of doping concentration in this paper. Cu-Co codoping leads to the degeneration and makes ZnO metallic. Thses three kinds of dopings can cause light absorption enhancement phenomenon in the visible and near ultrasoft regions, in which Cu-Co codoping greatly increases the absorption of solar light due to the synergistic effect between Cu ions and Co ions, which can be used to prepare the high efficiency solar cells.

Study on the effect of In-2N co-doping at preferential locality on the photoelectric function of ZnO (GGA+U)

Nowadays although the study of In-N co-doping effect on the photoelectric function of ZnO is relatively common, all of the In-N co-doped ZnO are of random doping, and the preferential locality doping using the unpolarized structure of ZnO has not been considered so far. Therefore, in this paper, based on the density functional theory using first-principles plane-wave ultrasoft pseudopotential method, the un-doped and the In-N heavily co-doped Zn1-xInxO1-yNy (x= 0.0625, y=0.125) in different orientations have been set up, and band structures and density of states have been calculated respectively. The calculated results show that the In-N atoms along the c-axis orientation has the advantages of high stability over those in the vertical c-axis direction, the band gap is narrower, the effective mass is smaller, the mobility is greater, and the hole concentration is higher, so that the conductivity of ZnO is higher in the In-N heavily co-doped materials. We believe that these results may be helpful to the design and preparation of the conductivity of In-N heavily co-doped ZnO.

The effect of ball-milling on the dispersion of carbon nanotubes: the electrical conductivity of carbon nanotubes-incorporated ZnO

The effect of ball-milling on the dispersion of carbon nanotubes: the electrical conductivity of carbon nanotubes-incorporated ZnO

Impact of (Al, Ga, In) and 2N preferred orientation heavy co-doping on conducting property of ZnO

At present, although there is some studies about the theoretical calculation studies of Zn1-xTMxO1-yNy(TM=Al, Ga, In) p-type doped have been reported. But, they are random doping and without considering the asymmetry of ZnO preferred orientation to doping. Therefore, Six different supercell models Zn1-xTMxO1-yNy (TM = Al, Ga, In. x = 0.0625, y = 0.125) which proportion is TM:N = 1:2 and preferred orientation to co-doped have been constructed based on the first-principles plane wave ultra-soft pseudo potential method of density function theory, in this study.Then calculate the geometric optimization, State density distribution and Band structure distribution for all models, respectively. Results indicate that with the condition of heavily doped and preferred orientation to co-doped, in the same kind of preferred orientation co-doping systems, the electrical conductivity of the system which TM-N bond along the c-axis direction is greater than it perpendicular to the c-axis. In the different kinds co-doping ZnO systems which TM-N bond along the c-axis direction, The co-doping systems of In-N bond along the c-axis direction has the strongest conductivity and the lowest ionization energy and the largest Bohr radius. It is more favorable for electrical conductivity of p-type ZnO. This study can be a theoretical guidance for improve the electrical conductivity of which design and preparation TM:N=1:2 ratio preferred orientation co-doping ZnO systems.

The synthesis and characterization of Ag–N dual-doped p-type ZnO: experiment and theory

Ag-N dual-doped ZnO films have been fabricated by a chemical bath deposition method. The p-type conductivity of the dual-doped ZnO:(Ag, N) is stable over a long period of time, and the hole concentration in the ZnO:(Ag, N) is much higher than that in mono-doped ZnO:Ag or ZnO:N. We found that this is because AgZn-NO complex acceptors can be formed in ZnO:(Ag, N). First-principles calculations show that the complex acceptors generate a fully occupied band above the valance band maximum, so the acceptor levels become shallower and the hole concentration is increased. Furthermore, the binding energy of the Ag-N complex in ZnO is negative, so ZnO:(Ag, N) can be stable. These results indicate that the Ag-N dual-doping may be expected to be a potential route to achieving high-quality p-type ZnO for use in a variety of devices.

Deep level defects in ZnO

The current understanding on intrinsic and extrinsic defects in ZnO is briefly reviewed. Special attention is given to defects defining the doping asymmetry as well as to approaches and theoretical predictions to control the conductivity of zinc oxide. Silver doping is considered a promising way to achieve hole conductivity in bulk ZnO. Results of defect spectroscopic studies on hydrothermally grown single ZnO crystals with an electron concentration of ≈1017cm−3 and ≈1014cm−3 are presented. Besides several other deep level centers in higher doped materials the Zni related level at Ec– (341±2)meV was found to be the dominating donor level in low doped ZnO. Further thermal post-treatments under inert and oxygen ambient conditions result in electrical intrinsic properties. First experiments on ZnO:Ag gave no hints for a detectable electrical activity of silver.

High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites

In this paper, we report a facile low-cost synthesis of the graphene-ZnO hybrid nanocomposites for solid-state supercapacitors. Structural analysis revealed a homogeneous distribution of ZnO nanorods that are inserted in graphene nanosheets, forming a sandwiched architecture. The material exhibited a high specific capacitance of 156 F g−1 at a scan rate of 5 mV.s−1. The fabricated solid-state supercapacitor device using these graphene-ZnO hybrid nanocomposites exhibits good supercapacitive performance and long-term cycle stability. The improved supercapacitance property of these materials could be ascribed to the increased conductivity of ZnO and better utilization of graphene. These results demonstrate the potential of the graphene-ZnO hybrid nanocomposites as an electrode in high-performance supercapacitors.

Strain‐Engineered Modulation on the Electronic Properties of Phosphorous‐Doped ZnO

The modulation of strain on the electronic properties of ZnO:P is investigated by density functional theory calculations. The variation of formation energy (E(f)) and band structure with strains ranging from -0.1 to 0.1 are considered. Although both the conduction band minimum (CBM) and the valence band maximum of ZnO are antibonding states, the CBM is more sensitive to strain, reducing the band gap with an increase in strain. P-substituted O (PO) defects show poor p-type conductivity due to a smaller E(f) and lower lying acceptor levels as a consequence of lattice expansion. The E(f) of P-substituted Zn (PZn) defects decreases under tension, owing to the release of strong repulsive stress induced by excess electrons from PZn. The donor energy band of PZn broadens under tensile strain, which benefits n-type conductivity. For Zn vacancies (VZn) and PZn-2VZn complexes, the distances between the O atoms around VZn are so large that repulsive and attractive interactions become weak, which results in an easy release of the strain. We herein present for the first time that the E(f) values of VZn and PZn-2VZn complexes decrease under both tension and compression, or in the high-pressure rock-salt phase. Under a strain of 0.1 the PZn-2VZn complex shows the smallest E(f). Under -0.07 strain the wurtzite/rock-salt phase transition occurs and the direct band gap becomes an indirect one. The variation of band structures in the rock-salt phase is similar to that in the wurtzite phase. Consequently, the p-type conductivity of ZnO:P can be improved with an increase in solubility of PZn-2VZn or VZn defects.

Structural and Electrical Properties of ZnO Films Deposited with Low-Temperature Facing Targets Magnetron Sputtering (FTS) System with Changes in H<SUB>2</SUB> and O<SUB>2</SUB> Flow Rate

ZnO has been studied as a strong candidate for high-quality TCO in accordance with increasing demand to replace ITO. The origin of n-doping in ZnO is not clearly understood, but recently, the H2 effect has received attention due to the role it plays in O-rich and O-poor conditions. In spite of recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. To control the electrical conductivity of ZnO, this study was performed using an FTS system with H2 and O2 addition at low processing temperature. The structural and electrical properties of ZnO thin films deposited at various H2 and O2 flow rates were investigated using XRD and a sheet resistance meter. In response to changes in H2 and O2 flow rates, the crystallization and related grain size of the ZnO films were somewhat changed. The sheet resistance increased from approximately 10(-1) to approximately 10(4) M ohm/sq. when the O2 flow rate was increased, and the resistance decreased from approximately 10(-1) to approximately 10(-4) M ohm/sq. when the H2 flow rate was increased. The increase of sheet resistance with O2 flow rates could be explained by decrease of oxygen vacancies. The decrease of sheet resistance with H2 flow rates could be explained by increase of the electrons from interstitial hydrogen atoms. The plasma characteristics were analyzed using optical emission spectroscopy (OES). But, the overall spectrum did not change with the H2 and O2 gas flow rates. So, the dramatic changes in the electrical properties of ZnO thin films could be considered to be a result of changes in chemical composition of the thin films rather than the plasma status.

P-type conduction from Sb-doped ZnO thin films grown by dual ion beam sputtering in the absence of oxygen ambient

Sb-doped ZnO (SZO) thin films were deposited on c-plane sapphire substrates by dual ion beam sputtering deposition system in the absence of oxygen ambient. The electrical, structural, morphological, and elemental properties of SZO thin films were studied for films grown at different substrate temperatures ranging from 200 °C to 600 °C and then annealed in situ at 800 °C under vacuum (pressure ∼5 × 10−8 mbar). Films grown for temperature range of 200–500 °C showed p-type conduction with hole concentration of 1.374 × 1016 to 5.538 × 1016 cm−3, resistivity of 66.733–12.758 Ω cm, and carrier mobility of 4.964–8.846 cm2 V−1 s−1 at room temperature. However, the film grown at 600 °C showed n-type behavior. Additionally, current-voltage (I–V) characteristic of p-ZnO/n-Si heterojunction showed a diode-like behavior, and that further confirmed the p-type conduction in ZnO by Sb doping. X-ray diffraction measurements showed that all SZO films had (002) preferred crystal orientation. X-ray photoelectron spectroscopy analysis confirmed the formation of SbZn–2VZn complex caused acceptor-like behavior in SZO films.