Co-doped ZnO Films Research Articles

Induction of p-type conduction in sputtered deposited Al-N codoped ZnO thin films

Al and N codoped ZnO thin films were grown on n-Si (100) substrate by sputtering technique. Hall effect measurements of as-grown films exhibited n-type conduction, however 500 °C Ar annealed codoped films showed p-type conductivity with a hole concentration of 9.9 × 10 16 cm − 3 , resistivity of 15.95 Ω-cm and hole mobility of 3.95 cm 2/Vs, respectively. Codoped ZnO thin films were found to be highly c-axis oriented with good crystal quality. A neutral acceptor-bound exciton and donor–acceptor-pair emissions that appeared at room temperature photoluminescence measurement verify p-type conduction in Al and N codoped ZnO film. The current–voltage characteristics of p–n heterojunction evidently showed a diode like rectifying behaviour.

Effect of substrate temperature on the photoelectrochemical responses of Ga and N co-doped ZnO films

Ga–N co-doped ZnO thin films with reduced bandgaps were deposited on F-doped tin-oxide-coated glass by radio-frequency magnetron sputtering at different substrate temperatures in mixed N2 and O2 gas ambient. We found that Ga–N co-doped ZnO films exhibited enhanced crystallinity when compared to undoped ZnO films grown under the same conditions. Furthermore, Ga–N co-doping ensured enhanced N-incorporation ZnO thin films as the substrate temperature is increased. As a result, Ga–N co-doped ZnO thin films exhibited much improved photoelectrochemical (PEC) response, compared to ZnO thin films. Our results therefore suggest that the passive co-doping approach could be a means to improve PEC response for bandgap-reduced wide-bandgap oxides through impurity incorporation.

Influence of reducing anneal on the ferromagnetism in single crystalline Co-doped ZnO thin films

This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on a-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become more conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16 μB/Co2+) in the Zn0.95Co0.05O single crystalline film with reducing annealing in the absence of Zn vapour, the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism (0.65 μB/Co2+) at room temperature. This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films, and the corresponding ferromagnetic mechanism is discussed.

Hybrid p-type ZnO film and n-type ZnO nanorod p– n homo-junction for efficient photovoltaic applications

Simple hybrid p– n homo-junctions using p-type ZnO thin films and n-type nanorods grown on fluorine tin oxide (FTO) substrates for photovoltaic applications are described. The ZnO nanorods (1.5 μm) were synthesized via an aqueous solution method with zinc nitrate hexahydrate and hexamethylenetetramine on ZnO seed layers. The 10-nm-thick ZnO seed layers showed n-type conductivity on FTO substrates and were deposited with a sputtering-based method. After synthesizing ZnO nanorods, aluminum-nitride co-doped p-type ZnO films (200 nm) were efficiently grown using pre-activated nitrogen (N) plasma sources with an inductively-coupled dual-target co-sputtering system. The structural and electrical properties of hybrid p– n homo-junctions were investigated by scanning electron microscopy, transmittance spectrophotometry, and I– V measurements.

Variation of N acceptor energy induced by Al–N codoping in ZnO films

Recombination mechanisms of Nitrogen-related emissions in undoped and Al–N codoped ZnO films were investigated by low temperature photoluminescence spectra. The peak near 3.312 meV was existed both in these two kinds of ZnO films. This peak was classified as the transition of the acceptor-bound exciton (A 0X) related to N O acceptor. The A 0X transition peak in Al–N codoped ZnO films shifted to lower energy comparing with the position of A 0X in undoped ZnO films. This phenomenon was induced by the incorporation of Al. A photoluminescence recombination possibly due to free-electron-to-acceptor (FA) transition was observed at temperatures higher than 40 K in Al–N doped ZnO films. The acceptor ionization energy was estimated from the energy position of the FA luminescence to be 183.7 meV. Moreover, the excitation intensity-dependent PL spectra which taken at 80 K in Al–N codoped ZnO film was measured. The luminescence band labeled donor–acceptor pair (DAP) shifts to higher energies with increase excitation intensity.

P-type ZnO thin films prepared by in situ oxidation of DC sputtered Zn3N2:Ga

The feasibility of a new fabrication route for N and Ga codoped p-type ZnO thin films on glass substrates, consisting of DC sputtering deposition of Zn3N2:Ga precursors followed by in situ oxidation in high purity oxygen, has been studied. The effects of oxidation temperature on the structural, optical and electrical properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance and Hall effect measurements. The results were compared to a control film without Ga. XRD analyses revealed that the Zn3N2 films entirely transformed into ZnO films after annealing Zn3N2 films in oxygen over 500 °C for 2 h. Hall effect measurements confirmed p-type conduction in N and Ga codoped ZnO films with a low resistivity of 19.8 Ω · cm, a high hole concentration of 4.6 × 1018 cm−3 and a Hall mobility of 0.7 cm2/(V·s). These results demonstrate a promising approach to fabricate low resistivity p-type ZnO with high hole concentration.

Orientation degree dependence of magnetic properties of Co doped ZnO thin films by sol–gel process

Co-doped ZnO films were successfully fabricated on Si substrates by sol–gel process. The effects of Co concentration and preheating temperature on the structure and magnetic properties of the Zn1−xCoxO films were systematically studied. The results revealed that the films were highly c-axis oriented and contained no impurity phase. With preheating temperature increasing, the orientation degree of the films decreased. When the Co concentration and oxygen vacancy of the films are almost changeless, the enhancement of ferromagnetism in films originates from the orientation degree of the films increasing.

Ferromagnetic Co-doped ZnO film and fine patterns prepared using photosensitive sol–gel method

Using zinc acetate and cobalt acetate as raw materials, isopropanol as solvent, and monoethanolamine as stabilizer, photosensitive Co-doped ZnO sol was prepared by chemical modification with benzoylacetone. Chelate rings of benzoylacetone with zinc ions are formed in the Co-doped ZnO sol and corresponding gel films. Irradiation of the gel film by a UV lamp in air leads to decomposition of the chelate ring. Selective irradiation followed by leaching and annealing gave rise to the formation of Co-doped ZnO fine patterns. The Co-doped ZnO film prepared using the photosensitive sol–gel method had a wurtzite structure and showed room-temperature ferromagnetic property.

Microstructure and magnetic properties of Co-doped ZnO films deposited by gas flow sputtering

Abstract Co-doped ZnO films with a Co concentration of 8–20 at.% were fabricated using the low-energy process of gas flow sputtering. X-ray diffraction, X-ray photoelectron spectroscopy, and optical absorption measurements revealed that the Co ions replace Zn ions in the ZnO matrix and that the Co ions have an oxidation state of 2+. The magnetic properties of the film depend on the Co concentration. The plots of magnetization and inverse susceptibility vs. temperature indicate that the film with a high Co concentration (20 at.%) contains a ferromagnetic component, while that with a low Co concentration (8 at.%) contains an antiferromagnetic component. The film with an intermediate Co concentration (10 at.%) contains a ferromagnetic component with a low Curie temperature. Hysteresis was not found in magnetization curves for all the samples, including the sample at 5 K. The films exhibited a high resistivity of 4 × 107–2 × 108 Ω cm at room temperature, and carrier-mediated magnetism is not likely to be applicable for the mechanisms of the magnetism in the films.

ZnO nanowires array p-n homojunction and its application as a visible-blind ultraviolet photodetector

We demonstrated a simple and low-cost fabrication of ZnO p-n homojunction. The junction consists of n-type ZnO nanowires array by a hydrothermal method covered with p-type Al, N co-doped ZnO film by a sol-gel method. The junction exhibits good rectification characteristics, with reverse leakage current and rectification ratio of ∼5 μA and ∼150 at bias of 3 V, respectively. The junction is operated as a photodetector when light radiation is shined on the glass-side of the device. The photodetector shows a peak responsivity at 384 nm with UV-visible responsivity ratio (R384 nm/R550 nm) of ∼70 at an operating bias of −3 V.

Electrical and photovoltaic properties of cobalt doped zinc oxide nanofiber/n-silicon diode

Nanofiber Co-doped ZnO (ZnO:Co) film was deposited on n-type silicon substrate at room temperature by using sol–gel method. Scanning electron microscopy results indicate that cobalt doped zinc oxide film has a nanofiber structure. The electrical and photovoltaic properties of the Au/ZnO:Co/n-Si diode were investigated by current–voltage, capacitance–voltage, transient current and steady state photoconductivity measurements. The Au/ZnO:Co/n-Si diode exhibits a photovoltaic behavior with a maximum open circuit voltage V oc (0.195 V) and short-circuit current I sc (2.63 μA) values under 100 mW cm 2 illumination intensity and the photoconductivity mechanism of the diode is controlled by the presence of continuous distribution of traps. The interface state density for the diode was determined by conductance method under dark and illumination conditions. The interface state density D it and time constant τ values for the diode under dark and 100 mW/cm 2 conditions were found to be 1.26 × 10 10 eV −1 cm −2, 6.02 × 10 −5 s and 9.81 × 10 10 eV −1 cm −2, 7.35 × 10 −7 s, respectively. The obtained photovoltaic and photocapacitance results indicate that the diode can be used both as a photodiode and photocapacitive sensor for visible light sensor applications.

Magnetic anisotropy in nanocrystalline Co-doped ZnO thin films

We report structural, optical and magnetization properties of cobalt-doped zinc oxide thin films prepared on Si substrates by spray pyrolysis technique. Grazing angle X-ray diffraction measurements confirms the wurtzite structure of Zn 1− x Co x O films. Optical reflectance measurements reveal that the optical gap energy increases with increasing Co concentration indicating that Co 2+ ions replaces Zn 2+ ions in the ZnO lattice. Hysteresis measurements demonstrate that the Co-doped ZnO films are magnetically anisotropic and they exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe values are observed for a field applied perpendicular to the sample surface compared to in-plane direction.

Enhanced ferromagnetism in single crystalline Co-doped ZnO thin films by Al codoping

High-quality (Co, Al)-codoped ZnO single crystalline films have been grown on a-plane sapphire substrates using molecular-beam epitaxy. Al codoping yields the Co-doped ZnO thin films to exhibit metallic conducting behavior with high fee carrier concentration. X-ray absorption studies confirm that nearly all Co ions are in divalent state and actually substituted into the ZnO lattice without formation of any detectable secondary phase. Compared with weak ferromagnetism (0.32 μ B/Co 2+) in the Co-doped ZnO single crystalline film, the film with additional Al codoping was found to have much stronger ferromagnetism (0.83 μ B/Co 2+) at room temperature. The magnetic anisotropy was also observed in the (Co, Al)-codoped ZnO thin film, indicating the ferromagnetism is intrinsic. Our experimental observations suggest that additional electron doping is effective to strengthen the ferromagnetism in Co-doped ZnO films, which is well consistent with the recent theoretical description of carrier-mediated magnetism in Co-doped ZnO.

Magnetic, magnetoresistance and electrical transport properties of Ni and Al co-doped ZnO films grown on glass substrates by direct current magnetron co-sputtering

200-nm-thick Zn 0.85Al 0.04Ni 0.11O films were deposited on glass substrates at 300 K and 573 K by co-sputtering ZnO:Al and Ni targets. A temperature dependence of magnetization and resistivity within 50–300 K and a room temperature magnetoresistance (MR) were measured. X-ray photoelectron spectroscopy was used to analyze chemical valences of Ni in the films. The films have the room temperature ferromagnetism. A magnetization of the film grown at 300 K decreases slightly and that of the film grown at 573 K decreases markedly with increasing temperature. The small negative MR is observed for the film deposited at 300 K while the positive MR is obtained for the film prepared at 573 K. For the film grown at 300 K, the carrier transport mechanism is thermally activated band conduction. However, the carrier transport mechanism in the film grown at 573 K is the Mott's variable range hopping in the temperature range below 100 K and the thermally activated band conduction above 140 K. The metallic Ni atoms and Ni 2+ ions exist in the films. The ferromagnetism is mainly attributed to the Ni clusters in the Zn 0.85Al 0.04Ni 0.11O film.

Magnetic and X-ray absorption investigations of Co-doped ZnO films

We present an investigation of magnetic and structural properties of Co-doped ZnO (ZCO) film grown by pulsed laser deposition at different dopant concentrations (cCo). X-ray diffraction patterns show that the films are single phase and exhibit ferromagnetism (FM) above room temperature (RT) with coercive fields up to 700 Oe. X-ray absorption fine-structure spectroscopy (XAFS) at the Co edge suggests that in films grown below 600°C dopant clustering involve less than 10% of the Co atoms in the alloy. In samples grown at higher temperature a larger fraction of Co atoms is involved in the formation of small metallic clusters. The experimental work has been accompanied by preliminary first-principles Density Functional Theory calculations.

Structure and deep ultraviolet emission of Co-doped ZnO films with Co 3O 4 nano-clusters

We have investigated the microstructure and photoluminescent properties of Zn 1− x Co x O ( x ≤ 0.377) films deposited on (0 0 1)-oriented Al 2O 3 substrates using radio-frequency magnetron sputtering. A single phase with a wurtzite-like structure was found in Zn 1− x Co x O films with x up to 0.105. However, a small amount of Co 3O 4 clusters could be observed when the doping level x reached 0.313. The PL spectra showed that Co doping in ZnO films would suppress near band edge (NBE) and violet emissions due to the degradation of material quality and the increase of nonradiative centers. For Zn 1− x Co x O films with higher Co concentration ( x = 0.313 and 0.377, respectively), a broad intense emission band centered at 332 nm (deep ultraviolet emission) was observed. The intense deep ultraviolet emission should be related to the O 2− → Co 2+ charge-transfer process in clusters Co 3O 4 embedded in Zn 1− x Co x O films.

Room temperature ferromagnetism and ferroelectricity behavior of (Cu, Li) co-doped ZnO films deposited by reactive magnetron sputtering

Single-phase Zn 0.95− x Cu x Li 0.05O thin films have been prepared on Pt (1 1 1)/Ti/SiO 2 substrates by reactive magnetron sputtering method. The XRD, XPS and absorption measurements confirmed the polycrystalline nature of the films and the substitution of Zn 2+ by Cu 2+ ions. The sputtered Zn 0.90Cu 0.05Li 0.05O film shows multiferroic properties exhibiting a saturated ferroelectric loop with a remanent polarization of 6 μC/cm 2 and a saturated loop with a saturation magnetization of 0.43 μ B/Cu at room temperature. The origins of the ferromagnetism and ferroelectricity in these films are discussed.

Deposition and properties of B–N codoped p-type ZnO thin films by RF magnetron sputtering

B–N codoped p-type ZnO thin films have been realized by radio frequency (rf) magnetron sputtering using a mixture of argon and oxygen as sputtering gas. Types of conduction and electrical properties in codoped ZnO films were found to be dependent on oxygen partial pressure ratios in the sputtering gas mixture. When oxygen partial pressure ratio was 70%, the codoped ZnO film showed p-type conduction and had the best electrical properties. Additionally, the p-ZnO/ n-Si heterojunction showed a clear p– n diode characteristic. XRD results indicate that the B–N codoped ZnO film prepared in 70% oxygen partial pressure ratio has high crystal quality with (0 0 2) preferential orientation. Meanwhile, the B–N codoped ZnO film has high optical quality and displays the stronger near band edge (NBE) emission in the temperature-dependent photoluminescence spectrum, the acceptor energy level was estimated to be located at 125 meV above the valence band.

Defects related room temperature ferromagnetism in p-type (Mn, Li) co-doped ZnO films deposited by reactive magnetron sputtering

Abstract We report on the defects related room temperature ferromagnetic characteristics of Zn 0.95– x Mn x Li 0.05 O ( x = 0.01, 0.03, 0.05 and 0.08) thin films grown on glass substrates using reactive magnetron sputtering. By increasing the Mn content, the films exhibited increases in the c -axis lattice constant, fundamental band gap energy, coercive field and remanent magnetization. Comparison of the structural and magnetic properties of the as-deposited and annealed films indicates that the hole carriers, together with defects concentrations, play an important role in the ferromagnetic origin of Mn and Li co-doped ZnO thin films. The ferromagnetism in films can be described by bound magnetic polaron models with respect to defect-bound carriers.

Sol–gel derived zinc oxide films alloyed with cobalt and aluminium

ZnO films codoped with 5 at.% Co and 1 at.% Al were prepared by sol–gel technique on corning glass and silicon substrates with precursor sols of different pH values. The pH was varied from 5.4 to 11 by adding varying amounts of monoethanolamine to the sol. Since pH plays an important role in controlling the properties of films, we discuss in detail the effect of pH value on the structural, morphological and optical properties of the grown films. X-ray diffraction and atomic force microscopy images reveal that the size of crystallites increases with pH of the sol. The variation of pH in the reaction system influences the density of homogeneous nucleation and the crystal growth along the c-axis. High quality Co and Al codoped ZnO films annealed at 600 °C have been obtained using a sol with pH = 9. These sol–gel derived films find their suitability to be used as dilute magnetic semiconductors.