Co-doped ZnO Films Research Articles

A high-k Cu-doped ZnO film formed via Ga-ion implantation: The acceptor-donor co-doping approach

Dielectric thin films having high permittivity (high-k) and low dielectric loss is essential for developing high performance capacitive devices like metal oxide field effect transistor or thin film transistor. Ga ion implantation performed on Cu-doped ZnO film fabricated by pulsed laser deposition with optimized doping concentrations and post-implantation annealing yielded film having high permittivity and low dielectric loss (ε = 87 and tan δ = 0.17 at the frequency of 1 kHz). Moreover, the permittivity exhibits good stability over a wide range of frequency from 20 Hz to 10 MHz. The high-k film was characterized by detailed dielectric studies, including frequency dependence of permittivity and dielectric loss, complex electrical modulus analysis, impedance spectroscopy and ac conductivity. The enhancement of the permittivity was attributed to the correlated potential barrier hopping of electrons between the neighboring acceptor-donor defect complex states in the band gap created by the co-doping, thus acting as electric dipoles polarizing the film. This work opens up future possibility for ‘dielectric engineering’. The three-dimensional dielectric spatial profile can be controlled via the selective area ion implantation with the depth controlled by the ion implantation energy. • A high-k Ga-Cu co-doped ZnO film formed via pulsed laser deposition and ion implantation method. • Optimized doping concentrations yielded high permittivity and low dielectric loss (ε=87 and tan δ=0.17 at 1 kHz). • Enhancement of permittivity was attributed to the correlated barrier hopping of electrons between neighboring defect complex.

F, Mg and Ga co-doped ZnO transparent conductive thin films by dual-target magnetron sputtering: Fabrication, structure, and characteristics

F, Mg and Ga co-doped ZnO (FMGZO) films are deposited on glass substrates at room temperature by magnetron sputtering using MgF2 and ZnO:Ga targets. The effects of RF sputtering power applied to the MgF2 target on structure, morphology, composition, electrical and optical properties of the films are investigated in detail. The experimental results show that all samples are polycrystalline films with hexagonal wurtzite structure and low surface roughness. The FMGZO film deposited at RF sputtering power of 15 W exhibits the highest figure of merit of 5.66 × 10−2 Ω−1 with the resistivity of 6.5 × 10−4 Ω cm, the carrier concentration of 4.12 × 1020 cm−3 and the Hall mobility of 23.38 cm2/V s, while the average optical transmittance is as high as 95.23% in the visible range. The photoelectric performance of the FMGZO film is significantly higher than that of previously reported co-doped ZnO films, which makes it suitable for various high-efficiency optoelectronic devices.

Large linear electrostrain of acceptor-donor Co-doped ZnO films

• Piezoelectric coefficient of Fe 3+ -Li + co-doped ZnO thin film is significantly enhanced by constructing acceptor-donor Fe 3+ -Li + ionic pairs in ZnO structure. • The Fe 3+ -Li + co-doped ZnO film has a large linear electrostrain. • The high permittivity has been obtained which is about one order of magnitude larger than that of pure ZnO film. • The piezoelectric performance of the films can be further improved by thermo-electrically treatment. In this paper we present the effective strategy of Fe 3+ and Li + co-doping to enhance the electrostrain of ZnO thin films. Zn 0.88 (Fe 0.06 Li 0.06 )O thin film exhibits an excellent d 33 * value of 415 pm/V and large linear electrostrain of 0.68% after thermal-electric treatment. Both the X-ray diffraction and first principle calculation results have indicated that the Fe 3+ and Li + ions are inclined to adopt a preferential alignment along the edges approximately parallel to [001] direction in the zinc-oxygen tetrahedron and constitute a defect dipole (ionic pairs). It is considered that the local lattice distortion and the electric field-triggered polarization extension and rotation generated by preferential distributed Fe 3+ -Li + ionic pairs are responsible for the outstanding piezoelectric properties and large linear electrostrain.

EFFECT OF ANNEALING OF Co-DOPED ZnO THIN FILMS ON STRUCTURAL AND MAGNETIC PROPERTIES DEPOSITED BY SOL–GEL/SPIN-COATING TECHNIQUE

Cobalt-doped ZnO films were grown on the glass substrates using sol–gel/spin-coating technique to investigate the effect of annealing on the structural and magnetic properties. The X-ray diffraction (XRD) patterns of the Co-doped ZnO films are dominated by the (002) peak, suggesting an up-standing array of ZnO structure hexagonal (wurtzite) with a good crystalline quality, however, the secondary phases of Co3O4 and Co are present in the samples. With the annealing temperature increased, the secondary phases tend to disappear completely and the intensity of the (002) peak increased, indicating a high crystallinity of the samples. For the ZnO majority phase, the lattice constant ([Formula: see text] decreases (from 5.232 [Formula: see text] to 5.224 [Formula: see text]), while the crystallite size increases (from 22.040[Formula: see text]nm to 24.018[Formula: see text]nm) as the annealing temperature varies from 380∘C to 600∘C. Significant changes in the dislocation density ([Formula: see text], strain [Formula: see text] and stress [Formula: see text] of the Co-doped ZnO films were also observed, by increasing the annealing temperature. All samples display a ferromagnetic behavior with variations in the saturation magnetization ([Formula: see text], [Formula: see text] and [Formula: see text] emu/cm[Formula: see text] and coercive field ([Formula: see text], 104 and 75 Oe) for the temperatures of 380∘C, 500∘C and 600∘C, respectively. The magnetic behavior of Co-doped ZnO films confirms the exchange interaction between the local spin moments produced by the oxygen vacancy. In addition, the ferromagnetic existence of the samples (380∘C, 500∘C and 600∘C) can be attributed to certain nanoparticles or to the binding of Co[Formula: see text] ions at the Zn[Formula: see text] location in the ZnO lattice. Finally, it appears that the ferromagnetism at room temperature found in these films, is consistent with endogenous defects (oxygen vacancies) and magnetic ions insertion along the same lines.

Effects of electron doping on the d0 magnetism in N-implanted ZnO and ZnAlO films

In this paper, N ions are implanted into ZnO and ZnAlO films with different carrier concentrations prepared by radio frequency reactive magnetron sputtering on sapphire substrates. The structural, electrical, optical and magnetic properties of N-doped and (Al, N) co-doped ZnO films is investigated, and the particular emphasis is placed on the effects of carrier concentration on the defects induced magnetism in the films. Our results show that all the doped ZnO films are ferromagnetic at room temperature. A trace amount of additional Al doping has a significant effect on the improvement of ferromagnetic properties, and a maximum saturation magnetization of 73 emu/cm3 is obtained for (Al, N) co-doped ZnO films. The optical band gaps of ZnO films increase with the increasing Al doping content, which is owing to the combined effect of high carrier concentration and the Burstein Moss effect. The average transmittance of all ZnO films exceeds 80%. Our results confirm that appropriate electron doping can effectively enhance the magnetic moment in N implanted ZnO which may also apply to other ZnO systems with d0 magnetism.

Photovoltaic Properties of ZnO Films Co-Doped with Mn and La to Enhance Solar Cell Efficiency.

In the present investigation, ZnO films co-doped with Mn and La were synthesized by the sol–gel technique. XRD analysis revealed that ZnO had a hexagonal structure. Mixed hexagonal and cubic phases appeared in ZnO containing Mn (1%) and La (1.5%). The grain size, d-spacing, unit cell, lattice parameters, atomic packing fraction, volume, strain, crystallinity, and bond length of co-doped ZnO films were determined as a function of doped ion contents. Through UV analysis, it was found that pristine ZnO had Eg = 3.5 eV, and it decreased when increasing the doping concentration, reaching the minimum value for the sample with 1% Mn and 1% La. The optical parameters of the films, such as absorption, transmittance, dielectric constants, and refractive index, were also analyzed. DSSCs were fabricated using the prepared ZnO films. For pure ZnO film, the values were: efficiency = 0.69%, current density = 2.5 mAcm−2, and open-circuit voltage = 0.56 V. When ZnO was co-doped with Mn and La, the efficiency increased significantly. DSSCs with a ZnO photoanode co-doped with 1% Mn and 1% La exhibited maximum values of Jsc = 4.28 mAcm−2, Voc = 0.6 V, and efficiency = 1.89%, which is 174% better than pristine ZnO-based DSSCs. This material is good for the electrode of perovskite solar cells.

Al and F co-doped ZnO films prepared by the SILAR method: Characterization and performance as active layers in TFTs

Al and F co-doped ZnO thin films were deposited on glass and SiO2/Si substrates by the successive ionic layer adsorption and reaction method, at which AlF3 was used as doping precursor. The resulting films showed high transparency and good adherence to the substrates. The X-ray diffraction analysis revealed that all the films had a wurtzite-type hexagonal structure with preferential orientation along the c-axis. The porous structure of the films, formed by matchstick-shaped structures, was modified as effect of increasing the AlF3 concentration. That is, less porous films were obtained for higher AlF3 concentrations. The optical transparency and bandgap of the film prepared with the highest concentration of the precursor dopant was about 88.57% and 3.27 eV, respectively. Thin film transistors were fabricated with (Al, F):ZnO films as active layers. The interconnectivity of grains was the most influencing factor in the electrical performance of the fabricated devices. Thus, the device with the least porous film had a saturation mobility of 1.362 × 10−2 cm2 V−1 s−1 and a threshold voltage of 7.2 V.

Local Electronic Structure of Lithium Nitrogen Codoped ZnO Films Revealed by X-ray Absorption Fine Structure Spectroscopy

Local Electronic Structure of Lithium Nitrogen Codoped ZnO Films Revealed by X-ray Absorption Fine Structure Spectroscopy

A pathway for ZnO p-type transformation and its performance in solar cells

Aiming at the breakthrough of ZnO in solar cells, we investigate the theoretical rationality and experimental operability of its structural components. In this paper, First Principle Calculation is implemented to achieve the optimum doping ratio for ZnO p-type transformation. Then, Al-N co-doped ZnO films are prepared by magnetron sputtering. The quality of p-type ZnO films are controlled by changing the ratio of N2O to Ar in the sputtering atmosphere. Results shown that the p-type ZnO films with excellent photoelectrical properties can be successfully obtained in the condition of N2O:Ar = 1: 4, where the resistivity is as low asρ=3.678×10-5Ω·cm, the carrier concentration isN=1.3066×1021cm-3, the average transmittance of the visible region is about 90%. In addition, we first explore the feasibility on the fabrication of p-type Al-N co-doped ZnO/n-type Al-doped ZnO homojunction and its application in solar cell.

Nucleation/Growth and Optical Proprieties of Co-doped ZnO Electrodeposited on ITO Substrate

A Co-doped ZnO layer was prepared by electrodeposition method on indium doped tin oxide (ITO) substrate using a cathodic reduction from nitrate medium with different doping percentages of cobalt. The bath temperature was controlled at 70 °C. The films were cathodically electrodeposited in a bath containing 5 mM Zn(NO3)2. 6H2O, while the source of Co is Co(NO3)2.6H2O where 0.1M KNO3 was used as supporting electrolyte. The nucleation and growth mechanism of Co-doped ZnO nuclei have been studied by cyclic voltammetry and chronoamperometry. The cyclic voltammetry shows that the electrodeposition of ZnO and Co-doped ZnO at a negative potential around -1.0 V versus saturated calomel electrode (SCE) is a quasi-reversible reaction controlled by the diffusion process. Comparing current transients curves obtained by the chronoamperometric method with the theoretical curves of current density j versus t ½ allows us to say that the nucleation is 3D instantaneous, as shown in SEM analysis. The presence of Co does not modify the nucleation and growth mechanism. The XRD patterns show that the substitution of zinc by cobalt does not change the würtzite crystal structure, but the crystallite size decreases with the cobalt percentage. The transmittance spectra indicate that the Co-doped ZnO films are transparent in the visible range. The optical gap increases with the doping percentage of cobalt.

Three-zone model for Ti, Al co-doped ZnO films deposited by magnetron sputtering

Al-doped ZnO is well known as an alternative Transparent Conductive Oxide (TCO) to Indium Tin Oxide (ITO). Recently, co-doped ZnO with Ti and Al (TAZO) have been explored to improve the conductivity while maintaining moisture durability and opto-electronic properties. In this context, the structural, morphological, and optical properties of TAZO films deposited by reactive magnetron are investigated by experiments and Density Functional Theory (DFT) simulations. The results show that TiO2 units substitute Zn atoms in the ZnO wurtzite structure and, whatever the concentration of Ti, the formation of Al2O3 is taking place. In addition, a three-regime model as a function of Ti content is proposed to explain the evolution of the properties. First, for Ti content 〈 2.2 at%, TiO2 is incorporated by substitution of Zn by TiO2 units in the ZnO network leading to a more oxidized Zn state and a hybridization between Zn4s and Ti3d orbitals. This opens the bandgap and increases the compressive stress of TAZO films. Between 2.2 at% and 7.0 at% of Ti content, cauliflower shape grains appear due to the compressive stress induced by small clusters of TiO2. For Ti content 〉 7.0 at%, amorphous TAZO films are observed and are composed of TiO2, Al2O3 and ZnO.

Ultrasonic spray pyrolysis-induced growth of highly transparent and conductive F, Cl, Al, and Ga co-doped ZnO films

In this paper, the ultrasonic spray pyrolysis (USP) technology, under normal pressure conditions was adopts in this study to deposited ZnO films. The cheap zinc acetate, ammonium fluoride, and gallium chloride was used as Zn source, F source, Ga source and Cl source, respectively for deposited F, Cl, Al and Ga co-doped ZnO (FCAGZO) films. The influence of substrate temperature (Ts) on the morphology, structure, optical and electrical properties of ZnO thin films was studied. X-ray diffraction analysis shows that the Ts does not change the crystal structure of the ZnO film, and all films have a hexagonal wurtzite structure. With increase of Ts only the preferred orientation of the film changes from (100) to (002). X-ray photoelectron spectroscopy further confirmed the successful doping of ZnO by F, Cl, Al, and Ga, which accounts for the enhancement in carrier concentration in the FCAGZO films over that of pristine ZnO. As Ts increases, the resistivity first decreases, and then gradually increases. When the optimal substrate temperature Ts = 400 °C, the prepared FCAGZO film has a resistivity of 1.47 × 10-3 Ωcm when rapid heat treatment in N2 atmosphere at 750 °C, and the average transmittance exceeds 85% in the range of 400–1400 nm.

Preparation and Characterization of (Al, Fe) Codoped ZnO Films Prepared by Sol–Gel

In this research, Al-doped and (Al, Fe) codoped ZnO films were prepared on glass substrate by the sol–gel method. The surface morphology, structure, and optical property were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-Visible-Near-infrared (UV-Vis-NIR) spectroscopy. The film surface morphologies all exhibited granular characteristics. With the Fe doping concentration increasing, the codoped films had smaller grain size and tended to be smoother. XRD analysis revealed that all films had a hexagonal wurtzite structure. The codoping can contribute to more Al and Fe ions entering the ZnO crystal structure, but result in the crystalline degree of the films decreasing. XPS results showed that the Al and Fe irons in the films exist in the form of trivalent. Moreover, the doped films had higher transmission, especially for codoped (Fe:Al = 3) film, but their absorption edge shifted to the short-wavelength direction.

Effect of oxygen partial pressure on microstructure, optical and electrical property of C-Al co-doped ZnO films

C-Al co-doped ZnO films with superior optical and electrical property have been deposited at oxygen partial pressure range 1–7 Pa by pulse laser deposition. The influences of oxygen pressure on crystalline, optical and electrical property have been investigated. The C-Al co-doped ZnO films exhibit a superior crystallinity and greater grain size in the range of 1 Pa–3 Pa compared to AZO films. Furthermore, carbon doping at various oxygen partial pressure significantly improves the optical and electrical property. C-Al co-doped ZnO film reaches the lowest resistivity of 3.1×10-4 Ω cm at 1 Pa. The average transmittance is higher than 85% and attains the optimal value of 89.9 %.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Influence of post annealing treatments on the luminescence of rare earth ions in ZnO:Tb,Eu/Si heterojunction

(Tb, Eu)–co-doped ZnO films with about 3 at.% total doping rate were grown by magnetron sputtering on Si substrate. Post annealing treatments were performed at 973–1373 K in continuous nitrogen flow to investigate the transformation of microstructural and optical characteristics by means of X-ray diffraction, transmission electron microscopy, photoluminescence and electroluminescence. For annealing temperatures lower than 1073 K, segregation of Eu and Tb was observed mainly at the film/substrate junction. For temperatures higher than 1173 K, additional phases appeared, namely, Zn2SiO4 and rare earth silicates. For the highest temperature investigated (1373 K), only silica and rare earth silicates remained in the film due to Zn evaporation. PL measurements indicated a very intense Eu emission associated with the presence of rare earth silicate inclusions. Energy transfer from Tb towards Eu was evidenced in this secondary phase. At last, based on these preliminary works, a (Tb, Eu)–co-doped ZnO/Si electroluminescent structure was produced and showed very promising results paving the way for very thin ZnO based light emitting diodes.

Wide Band Gap Al and In Co-doped ZnO Films for Near-Infrared Plasmonic Application

In transparent conducting oxide films, tuning of plasmonic resonance is directly controlled by free electron concentration and thus by activated dopants. In this study, large area AlxInyZn1-x-yO thin films at various concentrations were prepared by spray coating using water as a solvent. The effect of Al/In dopant ratio on the structural, electrical, optical, and plasmonic properties was investigated. Tuning of optical response to a well-defined plasmon resonance is correlated to the above properties of AlxInyZn1-x-yO films. Theoretical fitting based on the Drude-Lorentz (D-L) theory was utilized for extracting the dielectric spectra and cross-over wavelength (ωc). The studies revealed plasmonic properties in NIR for the films with Al/In ratios of A5I5, A2.5I7.5, and A0I10, indicating In as the most activated dopant. Surface plasmon mode simulated using the extracted permittivity values showed the influence of mobility of these films on the broadening of the dip. The minimum plasmonic loss suggests the suitability as an alternative plasmonic material in the near infrared.

Rational design of hydrogen and nitrogen co-doped ZnO for high performance thin-film transistors

This work investigates the effect of nitrogen and hydrogen (N/H) co-doping on the performance of ZnO thin-film transistors (TFTs). Optimum N/H co-doped ZnO TFTs showed high field-effect mobility (25.5 cm2 Vs−1) and Ion/Ioff (107) and low sub-threshold slope (0.25 V/dec.) and threshold voltage (1.2 V). X-ray photo-electron spectroscopy (XPS) and low-frequency noise analysis suggest that the observed improved electrical performance may be attributed to the reduction of the defect concentration and the average interface trap density due to the occupation of the NO–H complex on the oxygen vacancy and Zn interstitials. Moreover, density functional theory calculation and XPS band structure results demonstrate that the N/H co-doped ZnO film slightly changed the valence band maximum energy offset, indicating that the N/H co-doping controlled the carrier concentration of the ZnO film due to the formation of neutral complex N–H states. The enhanced electrical performance of the N/H co-doped ZnO TFT shows significant potential for the use of low-cost thin film electronic applications.

Effect of Co-doping level on physical properties of ZnO thin films

Abstract In this work pure and Co doped ZnO thin films have been deposited onto glass substrate at different levels of Co-doping. XRD, SEM, EDX, XPS and optical spectroscopy (linear and nonlinear) techniques have been used to investigate all films. The grain size was found to increase upon Co doping from 26.2 nm to 50.3 nm. The elemental percentage obtained by EDX and XPS confirm the existence of Co in doped films which increases as increasing the percentage of cobalt salt in the starting solution. The optical band gap was estimated and found to decrease in the low Co concentrations and increases again in higher Co concentration. The dispersion was found to change from anomalous to normal upon Co-doping. In Co-doped ZnO films the dispersion changes to be normal again were modelled using the single oscillator model. The plasma frequency, the refractive index at high wavelength, the optical moments of the films were determined. The nonlinear optical properties of all films were studied using nanosecond 532 nm laser, the doped materials exhibit reverse saturable absorption and negative nonlinear refraction in contrary to pure ZnO which has positive nonlinear refraction.

Structure and physical properties of hydrogenated (Co + Al)-doped ZnO films: Comparative study with co-doped ZnO films

(Co + Al)-doped ZnO films have been synthesized by the RF magnetron sputtering. Films of this composition have first been obtained in mixed atmosphere of Ar + H2. High hydrogen concentration of 20–50% has been used together with high enough substrate temperature of 450 °C. The used technological conditions affected the morphology, chemical composition, optical, electric, and magnetic properties of the films to an even more than in the case of Co-doped ZnO films synthesized under the same conditions and studied earlier. The films exhibit ferromagnetic behavior at room temperature with much greater magnetization and magneto-optical activity compared to the Co-doped films. At the same time, the hydrogenated films show an increase in electric conductivity in comparison with samples synthesized in the atmosphere of Ar + O2. The magnetic nature of the hydrogenated films has been associated with the defect-related mechanism.