Effect of ambient temperature on electrical properties of nanostructure n-ZnO/p-Si heterojunction diode

The nanostructure n-ZnO/p-Si heterojunction diode was fabricated by sol–gel method. The structural and morphological properties of the nanostructure ZnO film have been investigated. The X-ray diffraction spectra indicated that the films are of polycrystalline nature. The scanning electron microscopy images indicate that the surface morphology of ZnO film is almost homogeneous and the ZnO film is consisted of the circular formed with coming together of the nanoparticles. The electrical characterization of nanostructure n-ZnO/p-Si heterojunction diode has been investigated by current–voltage characteristics. The ideality factor (n) of the diode was found for different ambient temperatures and the obtained 6.40 value for 296 K is higher than unity due to the interface states between the two semiconductor materials and series resistance. The values of n increased with decreasing ambient temperature. The reverse current of the diode increased with illumination intensity of 100 mW cm⁻² and the diode gave a maximum open circuit voltage V_oc of 0.19 V and short-circuits current I_sc of 8.03 × 10⁻⁸ A.     

Fabrication and photoluminescence of P doped ZnO nanobelts by thermal evaporation method

Uniform and flat single crystal ZnO:P nanobelts (NBs) were fabricated on Si (1 0 0) substrates by the thermal evaporation method. The growth process, free-catalyst self-assembly vapor-solid (V-S) mechanism, was described and investigated deeply in terms of thermodynamics and kinetics. Then, the photoluminescence (PL) properties of ZnO NBs were studied in a temperature range from 10 to 270 K. At 10 K the recombination of acceptor-bound exciton (A⁰X) was predominant in the PL spectrum, and was attributed to the transition of P_Zn−2V_Zn complex bound exciton. The active energy of A⁰X and acceptor binding energy were calculated to be 17.2 and 172 meV, respectively. The calculated acceptor binding energy of P doped ZnO nanostructure is in good agreement with that of P doped ZnO film.     

Low-temperature growth of ZnO nanorods on PET fabrics with two-step hydrothermal method

An effective low-temperature growth method to fabricate hexagonally oriented ZnO nanorod arrays onto PET fabrics is reported. The effect of substrate pre-treatment and C₆H₁₂N₄ concentration on the structure of ZnO nanorod arrays were investigated in details by X-ray diffraction (XRD), FE-SEM and ultraviolet protection factor (UPF). The results show that substrate pre-treatment, C₆H₁₂N₄ concentration indeed have great influence on the growth of ZnO nanorod arrays. It is indispensable to introduce a ZnO seed layer on the substrate and under growth condition of n(C₆H₁₂N₄):n[Zn(NO₃)₂] = 1:1, T = 90 °C, t = 3 h, the well-aligned ZnO nanorod arrays with 40-50 nm in diameter and 300-400 nm in length were achieved on the pre-treated PET fabrics. The ZnO nanorods grown on PET fabrics possessed an ultrahigh ultraviolet protection factor of 480.52 in this study, indicating an excellent protection against ultraviolet radiation in comparison with the untreated PET fabrics.     

Formation of two ripple modes on Si by ion erosion with simultaneous Fe incorporation

This report focuses on the self organized nanostructure formation on Si (0 0 1) by erosion with low energy Kr⁺ ions with simultaneous incorporation of metallic atoms, in particular Fe. The incorporation of Fe is thought to play an important role in the formation of some features. In the experimental set-up used here the Fe atoms come from the sputtering of a cylindrical stainless steel target situated between the source and the sample holder. It is demonstrated how the Fe flux can be regulated by operational parameters of the ion source. It is shown that two different ripple modes, one perpendicular to the ion beam projection on the surface and the other parallel, were formed at near normal incidence (α = 20°) with ion energy between 300 eV and 2000 eV and a fluence of 6.7 × 10¹⁸ cm⁻². The perpendicular mode ripples dominated the topography when E_ion = 2000 eV, while the parallel mode ripples were the main features observed when E_ion = 300 eV. The correlation of Fe concentration with ion sources parameters and resulting topography is analyzed. It is demonstrated that a certain Fe concentration is necessary for the formation of ripples that are oriented perpendicular to the ion beam and that the Fe concentration alone does not determine the evolving topography.     

Well-aligned ZnO rod arrays grown on glass substrate from aqueous solution

Well-aligned ZnO rod arrays have been successfully synthesized on glass substrate from the aqueous solution of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ (HMT). Some critical issues such as seed layers, concentration and reaction time were investigated. The results show that ZnO seed layers were pre-requisite for the aligned growth of ZnO rod arrays. The length of rods is tunable in a range from 2 μm to 3 μm by varying the solution concentration and reaction time. X-ray diffraction results demonstrate that ZnO rods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. Microstructure observation by scanning electron microscope confirms that ZnO rods grew up perpendicular to the substrate. Room-temperature photoluminescence (PL) spectrum of rod arrays shows a strong emission band at about 396 nm.     

Studies of structural and morphological characteristics of flower-like ZnO thin film and its application as photovoltaic material

In this work a new method has been employed to synthesize nanocrystalline ZnO powder under hydrothermal conditions at 80 °C using aqueous Zn(NO₃)₂·6H₂O solution and diethylamine (DEA) as the starting materials. The ZnO powder prepared by this novel method was characterized by XRD, energy dispersive X-ray spectroscopy (EDX), FTIR and UV–vis techniques. Calculation based on XRD data revealed ZnO particles to be of nanometer size (∼33 nm). The ZnO powder was subsequently used to make its thin film which exhibited flower like morphology when examined by SEM. Thin ZnO films were sensitized with N719 dye, (Bu₄N)₂[Ru(dcbpyH)₂(NCS)₂], and used as photo-anode to construct sandwich type dye-sensitized solar cell (DSSC). With such cells, V_OC = 0.680 V, J_SC = 0.61 mA cm⁻², fill factor = 0.43 and overall conversion efficiency η = 0.23% were achieved on illumination with visible light (80 mW cm⁻²).     

Lactic acid aided electrochemical deposition of c-axis preferred orientation of zinc oxide thin films: Structural and morphological features

Compact and homogeneous c-axis preferred orientation of zinc oxide (ZnO) films on indium tin oxide (ITO) coated glass have been prepared electrochemically at −1.2 V vs. Ag|AgCl in a weak acidic condition from 0.06 M Zn(NO₃)₂ with 3 mM lactic acid (LA) added. LA was found having strong influence on the electrodeposition of c-axis preferred orientation of zinc oxide films. Other experimental variables such as deposition temperature, potential, and precursor concentration were also conducted in this article. Among these variables, it was found that precursor concentration of zinc nitrate influenced significantly on growth direction and crystal diameter of zinc oxide. Cyclic voltammetry was used to observe the electrochemistry of the deposition. Crystallinities of the films were examined by X-ray diffractometer. The morphologies of zinc oxide films were observed with a field emitting scanning electron microscope. Optical characteristics of zinc oxide layers were measured with UV-vis spectrophotometer. The band gap of the deposited zinc oxide thin films was evaluated from the Tauc relationship of (αhν)² vs. hν, which was found to be 3.31 eV.     

Influence of sputter-etching of substrate on the microstructural and optical properties of ZnO films deposited by RF magnetron sputtering

ZnO films were prepared using radio frequency magnetron sputtering on Si(1 1 1) substrates that were sputter-etched for different times ranging from 10 to 30 min. As the sputter-etching time of the substrate increases, both the size of ZnO grains and the root-mean-square (RMS) roughness decrease while the thickness of the ZnO films shows no obvious change. Meanwhile, the crystallinity and c-axis orientation are improved by increasing the sputter-etching time of the substrate. The major peaks at 99 and 438 cm⁻¹ are observed in Raman spectra of all prepared films and are identified as E₂(low) and E₂(high) modes, respectively. The Raman peak at 583 cm⁻¹ appears only in the films whose substrates were sputter-etched for 20 min and is assigned to E₁(LO) mode. Typical ZnO infrared vibration peak located at 410 cm⁻¹ is found in all FTIR spectra and is attributed to E₁(TO) phonon mode. The shoulder at about 382 cm⁻¹ appearing in the films whose substrates were sputter-etched for shorter time (10-20 min) originates from A₁(TO) phonon mode. The results of photoluminescence (PL) spectra reveal that the optical band gap (Eg) of the ZnO films increases from 3.10 eV to 3.23 eV with the increase of the sputter-etching time of the substrate.     

Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition

Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.40°. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm⁻¹, whereas the E₂ mode of ZnO at 436 cm⁻¹ can barely be identified. Annealing at 500-800 °C causes decrease of 275, 582, 640 and 720 cm⁻¹ and increase of 436 cm⁻¹ intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of ∼10²¹/cm³ was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of Zn_I-N_O or (N₂)_O that act as donor/double donors.     

Hydrothermal growth of ZnO on annealed electrodeposited titanate film: Influence of zinc nitrate and methenamine

Hydrothermal growth of hexagonal ZnO nanorods on the annealed titanate nanotube films is reported as a function of molar ratio of Zn(NO₃)₂ and methenamine (1:1-1:4). The molar ratio of 1:4 results in a dense and thinner rod in comparison with other molar ratios. Corn-like structures of the rods are believed to be due to the higher amine concentration. Raman peaks at 437 and 331 cm⁻¹ are assigned to E₂ and E_2H-E_2L modes. Near band gap edge and green photoluminescence emission indicates the structural and oxygen vacancy. O 1s peak is found built-up of sub-peaks at 530.62, 531.8 and 532.84 eV corresponding to O²⁻ on normal wurtzite structure and OH and oxygen vacancies of ZnO, respectively.     

Effect of Co-doping content on hydrothermal derived ZnO array films

Cobalt doped ZnO films are synthesised using a hydrothermal process. The effect of Co²⁺ concentration on morphology, phase composition, crystallisation and spectroscopic characteristics of ZnO films is investigated. The results indicate that both the structure and morphology of the ZnO films evolve with the concentration of cobalt ions incorporated into the lattice. In the presence of a small amount of Co²⁺ ions, films are formed that comprise hexagonal ZnO nanorods, oriented with the c-axis perpendicular to the substrate. With increasing amount of Co²⁺, cracks in the ZnO nanorods can be observed and growth in the [0 0 1] direction is significantly inhibited. When the Co²⁺ concentration exceeds 0.010 M, ZnO rods with the typical hexagonal structure are no longer observed and instead, ZnO films comprising close-packed grains with an irregular polygonal structure are formed. The epitaxial growth of ZnO films is nearly completely inhibited when the concentration of Co²⁺ is increased above 0.050 M. This behaviour can be explained by the selective adsorption of the organic substances in the solution onto the (0 0 1) ZnO crystal face, thus inhibiting growth in the [0 0 1] direction and disrupting the crystallisation of ZnO films. Increasing the Co content deteriorates the crystallisation of ZnO rods and increases tensile stresses present in the ZnO films.     

Structural, optical and electronic properties of P doped p-type ZnO thin film

P doped ZnO films were grown on quartz by radio frequency-magnetron sputtering method using a ZnO target mixed with 1.5 at% P₂O₅ in the atmosphere of Ar and O₂ mixing gas. The as-grown P doped ZnO film showed n-type conductivity, which was converted to p-type after 800 °C annealing in Ar gas. The P doped ZnO has a resistivity of 20.5 Ω cm (p∼2.0×10¹⁷ cm⁻³) and a Hall mobility of 2.1 cm² V⁻¹ s⁻¹. XRD measurement indicated that both the as-grown and the annealed P doped ZnO films had a preferred (0 0 2) orientation. XPS study agreed with the model that the P_Zn-2V_Zn acceptor complex was responsible for the p-type conductivity as found in the annealed P-doped ZnO. Temperature-dependent photoluminescence (PL) spectrum showed that the dominant band is located at 3.312 eV, which was attributed to the free electronic radiative transition to neutral acceptor level (FA) in ZnO. The P_Zn-2V_Zn acceptor complex level was estimated to be at E_V=122 meV.     

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO₃ (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42μ_B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21μ_B/Co and 0.65μ_B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.     

Growth of carbon nanofibers on aligned zinc oxide nanorods and their field emission properties

Carbon nanofibers were grown by electrodeposition technique onto aligned zinc oxide (ZnO) nanorods deposited by hybrid wet chemical route on glass substrates. X-ray diffraction traces indicated very strong peak for reflections from (0 0 2) planes of ZnO. The Raman spectra were dominated by the presence of G band at about 1597 cm⁻¹ corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band at about 1350 cm⁻¹ originating from disordered carbon. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peak at ∼511 cm⁻¹ for Zn-O stretching mode. Photoluminescence spectra indicated band edge luminescence of ZnO at ∼3.146 eV along with a low intensity peak at ∼0.877 eV arising out of carbon nanofibers. Field emission properties of these films and their dependence on the CNF coverage on ZnO nanorods are reported here. The average field enhancement factor as determined from the slope of the FN plot was found to vary between 1 × 10³ and 3 × 10³. Both the values of turn-on field and threshold field for CNF/ZnO were lower than pure ZnO nanorods.     

Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films

Al doped ZnO thin films are prepared by pulsed laser deposition on quartz substrate at substrate temperature 873 K under a background oxygen pressure of 0.02 mbar. The films are systematically analyzed using X-ray diffraction, atomic force microscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, z-scan and temperature-dependent electrical resistivity measurements in the temperature range 70-300 K. XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. Particle size calculations based on XRD analysis show that all the films are nanocrystalline in nature with the size of the quantum dots ranging from 8 to 17 nm. The presence of high frequency E₂ mode and longitudinal optical A₁ (LO) modes in the Raman spectra suggest a hexagonal wurtzite structure for the films. AFM analysis reveals the agglomerated growth mode in the doped films and it reduces the nucleation barrier of ZnO by Al doping. The 1% Al doped ZnO film presents high transmittance of ∼75% in the visible and near infrared region and low dc electrical resistivity of 5.94 × 10⁻⁶ Ω m. PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. Nonlinear optical measurements using the z-scan technique shows optical limiting behavior for the 5% Al doped ZnO film.     

Electrical and optical properties of ZnO films prepared by sputtering of ZnO targets containing AlN

ZnO films prepared from the ZnO target containing 2% AlN are transparent irrespective of radio frequency (RF) power. The obtained ZnO films have the carrier density of 3.8 × 10²⁰ cm⁻³ or less and the low mobility of 5.3-7.8 cm²/(V s). In the case of 5% AlN target, ZnO films prepared at 40, 60 and 80 W are transparent, whereas ZnO films prepared at 100 and 120 W are colored. As RF power increases from 40 to 120 W, the carrier density increases straightforwardly up to 5.5 × 10²⁰ cm⁻³ at 100 W and is oppositely reduced to 3.2 × 10²⁰ cm⁻³ at 120 W. In the case of 10% AlN target, ZnO films prepared at 60 W or more are colored, and have the carrier density of 4 × 10²⁰ cm⁻³ or less. The N-concentration in these colored films is estimated to be 1% or less. The Al-concentration in the ZnO films prepared from the 5 and 10% AlN targets is higher than 2%. The carrier density of the ZnO films containing Al and N atoms is nearly equal to that of ZnO films doped with Al atoms alone. There is no evidence in supporting the enhancement of the carrier density via the formation of N-Al_xZn_4−x clusters (4 ≥ x ≥ 2).     

Effects of rf power on surface-morphological, structural and electrical properties of aluminium-doped zinc oxide films by magnetron sputtering

Aluminium-doped zinc oxide (ZnO:Al) films were prepared by magnetron sputtering at different radio-frequency powers (P_rf) of 50, 100, 150 and 200 W. The properties of the films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman microscopy, and spectrophotometry with the emphasis on the evolution of compositional, surface-morphological, optical, electrical and microstructural properties. XPS spectra showed that within the detection limit the films are chemically identical to near-stoichiometric ZnO. AFM revealed that root-mean-square roughness of the films has almost linear increase with increasing P_rf. Optical band gap E_gopt of the films increases from 3.31 to 3.51 eV when P_rf increases from 50 to 200 W. A widening E_gopt of the ZnO:Al films compared to the band gap (∼3.29 eV) of undoped ZnO films is attributed to a net result of the competition between the Burstein-Moss effect and many-body effects. An electron concentration in the films was calculated in the range of 3.73 × 10¹⁹ to 2.12 × 10²⁰ cm⁻³. Raman spectroscopy analysis indicated that well-identified peaks appear at around 439 cm⁻¹ for all samples, corresponding to the band characteristics of the wurtzite phase. Raman peaks in the range 573-579 cm⁻¹ are also observed, corresponding to the A₁ (LO) mode of ZnO.     

Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

Fundamental aspects of NOx adsorption on BaO

The nature of NO₂ and nitrite/nitrate pairs adsorbed on BaO has been studied within the density functional theory. Size effects are investigated by comparing results for the BaO molecule, BaO clusters [(BaO)_x, x = 4, 6, 9, 12] and BaO(1 0 0). The adsorption energies show weak dependence on size, which is a manifestation of the fast size convergence of the BaO electronic structure and the local character of the NO₂ adsorption bond. Nitrite/nitrate pair formation is associated with a significant energy gain. For BaO(1 0 0), the stability of the pair is insensitive to the nitrite-nitrate separation, a finding that demonstrates a surface mediated non-local mechanism of molecular pair formation on oxide surfaces. The results have implications for the understanding of NO_x storage and reduction catalysts.     

Determination of electrical types in the P-doped ZnO thin films by the control of ambient gas flow

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5 × 10¹⁷ cm⁻³ and 2.5 cm²/V s, respectively. X-ray diffraction showed that the ZnO (0 0 0 2) peak shifted to lower angle due to the positioning of P³⁻ ions with a larger ionic radius in the O²⁻ sites. This indicates that a p-type mechanism was due to the substitutional P_O. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.