Growth Of ZnO Films Research Articles

Электронные состояния зоны проводимости ультратонких пленок тиофен-фенилен со-олигомера и замещенного бифенила на поверхности послойно выращенного ZnO

The results of studying the electronic states of the conduction band and interface potential barrier during the formation of ultrathin films of thiophene-phenylene co-oligomer CH3-phenylene-thiophene-thiophene-phenylene-CH3 (CH3-PTTP-CH3) on the surface of ZnO and films of biphenyl tetracarboxylic dianhydride (BPDA) on the ZnO surface are presented. A 100 nm thick ZnO layer was prepared by atomic layer deposition (ALD). Organic CH3-PTTP-CH3 films and BPDA films up to 8 nm thick were formed by thermal vacuum deposition. During film deposition, the electronic characteristics of the surface were studied using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above EF. In this energy range, the structure of the maxima of the unoccupied electronic states of CH3-PTTP-CH3 and BPDA films was determined. As a result of the CH3-PTTP-CH3 film deposition, a decrease in the work function to 4.0 eV was found, compared with the value of the work function of 4.2 eV measured from the ALD ZnO substrate. This corresponds to the transfer of a negative charge from the СH3-PTTP-CH3 film to the substrate. The charge transfer at the interface between the BPDA film and the ALD ZnO substrate occurs in the opposite direction, since a 4.7 eV increase of the work function was registered during the formation of this interface. The СH3-PTTP-СH3 and BPDA films studied and the layer-by-layer grown ZnO film represent a continuous coating on sufficiently large surface areas of the order of 10 micrometers x 10 micrometers. The roughness of the ZnO surface does not exceed 4 nm, and the surface roughness of CH3-PTTP-CH3 and BPDA films was 10–15 nm.

Conduction band electronic states of ultrathin thiophene-phenylene co-oligomer and substituted biphenyl films on the surface of layer-by-layer grown ZnO

The results of studying the electronic states of the conduction band and interface potential barrier during the formation of ultrathin films of thiophene-phenylene co-oligomer CH3-phenylene-thiophene-thiophene-phenylene -CH3(CH3-PTTP-CH3) on the surface of ZnO and films of biphenyl tetracarboxylic dianhydride (BPDA) on the ZnO surface are presented. A 100 nm thick ZnO layer was prepared by atomic layer deposition (ALD). Organic CH3-PTTP-CH3 films and BPDA films up to 8 nm thick were formed by thermal vacuum deposition. During film deposition, the electronic characteristics of the surface were studied using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above EF. In this energy range, the structure of the maxima of the unoccupied electronic states of CH3-PTTP-CH3 and BPDA films was determined. As a result of the CH3-PTTP-CH3 film deposition, a decrease in the work function to 4.0 eV was found, compared with the value of the work function of 4.2 eV measured from the ALD ZnO-substrate. This corresponds to the transfer of a negative charge from the CH3-PTTP-CH3 film to the substrate. The charge transfer at the interface between the BPDA film and the ALD ZnO-substrate occurs in the opposite direction, since a 4.7 eV increase of the work function was registered during the formation of this interface. The CH3-PTTP-CH3 and BPDA films studied and the layer-by-layer grown ZnO film represent a continuous coating on sufficiently large surface areas of the order of 10 μmx10 μm. The roughness of the ZnO surface does not exceed 4 nm, and the surface roughness of CH3-PTTP-CH3 and BPDA films was 10-15 nm. Keywords: thiophene-phenylene co-oligomers, biphenyl tetracarboxylic dianhydride, ultrathin films, ZnO, atomic layer deposition method, electronic properties, low-energy electron spectroscopy, interface potential barrier.

Investigation of Structural and Optical Properties of ZnO Thin Films Grown on Different Substrates by Mist-CVD Enhanced with Ozone Gas Produced by Corona Discharge Plasma

This study focuses on the growth and physical properties of ZnO thin films on different substrates grown by mist-CVD enhanced with ozone (O3) gas produced by corona discharge plasma using O2. Here, O3 is used to eliminate the defects related to oxygen in ZnO thin films. ZnO thin films are grown on amorphous soda-lime glass (SLG) and single crystals SiO2/Si (100) and c-plane Al2O3 substrates at 350°C of low growth temperature. All ZnO thin films show dominant (0002) diffraction peaks from X-ray diffraction (XRD). As expected, full width at half maximum (FWHM) of (0002) is decreasing in ZnO thin films on single-crystal substrates, especially c-Al2O3 due to similar crystal structure. It is found that the strain in the films is lowest in ZnO/c-Al2O3. The surface morphologies of the thin films are studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. Grown ZnO films have a hexagonal and triangular nanostructure with different nanostructure sizes depending on the used substrate types. The calculated surface roughness is dramatically decreased in ZnO/c-Al2O3 compared to the other grown structures. The confocal Raman measurements show the E2(H) peak of ZnO thin films at 437 cm−1. It is suggested that O3 gas produced by corona discharge plasma using O2 can be useful to obtain better crystal quality and physical properties in ZnO thin films.

Modified SILAR Grown ZnO Films on p‐Si(100) with Enhanced Charge Separation for UV Light Sensing Application

Herein, the growth of pristine ZnO nanostructures thin film on p‐Si(100) by modified successive ionic layer adsorption‐reduction method, revealing significant improvement in the charge collection for ultraviolet light detection, is reported. The deposited ZnO exhibits spindle‐like and hexagonal structure that grows preferentially along the c‐axis. Two‐probe electrical measurements validate the rectifying nature of the constructed n‐ZnO/p‐Si(100), revealing a substantial 11.3‐fold increase in photocurrent at room temperature under 375 nm irradiation at +3 V working voltage. The drastic increase in photocurrent is linked to efficient charge separation caused by charge transfer and band bending effects, as indicated by microsecond time‐resolved absorption measurements, providing useful information for device development.

A Study on the Effect of Base‐Reactant in Low‐Temperature Atomic Layer Deposition of Zinc Oxide

During the low‐temperature atomic layer deposition (ALD) process, residual carbon from the unreacted ligand of precursors causes deterioration in the properties of grown materials. Herein, a simple approach using a base solution as a reactant to reduce residual carbon and improve the low‐temperature ALD reaction is proposed. NaOH dissolved in H2O is used as the reactant base solution and diethyl zinc is used as a reactant. The effects of the base‐reactant on the ZnO film growth are analyzed using X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), photoluminescence (PL), and thin‐film transistor (TFT) measurements. The crystallinity, grain size, and Zn and O contents of the ZnO thin films gradually increased as the concentration of NaOH increased (0, 10, 20, and 30 wt%). Residual carbon occupying Zn sites is reduced as NaOH concentrations increased. The unintended heavily doped state induced by the residual carbon during the low‐temperature ALD process is improved, and the ZnO TFT device fabricated with the NaOH/H2O solution also shows improvements in its field‐effect mobility and the on/off current ratio. This simple approach, using the base solution as the reactant during the low‐temperature ALD process, effectively improves the quality of the grown film with low carbon impurities.

Morphological modification and UV sensitivity enhancement in ZnO:Fe films with a seed layer

In this paper, the effect of the ZnO seed layer on the growth of Fe-doped ZnO (FZO) film has been investigated. Both the film and seed layer have been deposited by the spray pyrolysis technique. FZO film without the seed layer shows flower-like due to particle agglomeration. The addition of the seed layer (FZO/ZnO) changes the morphology into nanorods. This shape is promising due to the higher surface-to-volume ratio, as compared to film structure. The nanostructures also affect the increase of optical bandgap due to the quantum effect of low dimensional crystallite. Moreover, UV sensitivity increases in the FZO/ZnO sample since nanorods have a larger surface-to-volume ratio. This study is essential to improve the functionality of ZnO-based optoelectronics with a fast and simple deposition technique.

The low temperature growth of stable p-type ZnO films in HiPIMS

In this study, the influence of substrate temperature on properties of Al-N co-doped p-type ZnO films is explored. Benefitting from the high ionization rate in high-power impulsed magnetron sputtering, the concentration of ionized nitrogen N+ and ionized zinc Zn+ were increased, which promoted the formation of ZnO films and lowered the necessary substrate temperature. After optimization, a co-doped p-type ZnO thin film with a resistivity lower than 0.35 Ω cm and a hole concentration higher than 5.34 × 1018 cm−3 is grown at 280 °C. X-ray diffraction results confirm that Al-N co-doping does not destruct the ZnO wurtzite structure. X-ray photoelectron spectroscopy demonstrates that the presence of Al promotes the formation of acceptor (No) defects in ZnO films, and ensures the role of Al in stabilizing p-type ZnO.

ZnO Films Incorporation Study on Macroporous Silicon Structure.

In the present work, we developed hybrid nanostructures based on ZnO films deposited on macroporous silicon substrates using the sol–gel spin coating and ultrasonic spray pyrolysis (USP) techniques. The changes in the growth of ZnO films on macroporous silicon were studied using a UV-visible spectrometer, an X-ray diffractometer (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). XRD analysis revealed the beneficial influence of macroporous silicon on the structural properties of ZnO films. SEM micrographs showed the growth and coverage of ZnO granular and flake-like crystals inside the pores of the substrate. The root mean square roughness (RMS) measured by AFM in the ZnO grown on the macroporous silicon substrate was up to one order of magnitude higher than reference samples. These results prove that the methods used in this work are effective to cover porous and obtain nano-morphologies of ZnO. These morphologies could be useful for making highly sensitive gas sensors.

Characterization of ZnO Films Grown by Chemical Vapor Deposition as Active Layer in Pseudo-MOSFET

Characterization of ZnO Films Grown by Chemical Vapor Deposition as Active Layer in Pseudo-MOSFET

Influence of Temperature on Graphene/ZnO Heterojunction Schottky Diode Characteristics.

The paper reports development of graphene/ZnO heterojunction Schottky diode structure and its structural and electrical characterization. Graphene is grown on copper substrate using chemical vapor deposition (CVD) and transferred on flexible substrate (indium Tin Oxide coated PET). The grown thin layer is characterized using scanning electron microscopy and Raman spectroscopy which confirm uniformity and high-quality graphene layer. The sputtered ZnO is deposited and characterized which confirms c-axis (002) orientation and uniform growth of ZnO film. Silver (Ag) as a top electrode has been deposited and I-V measurement has been done. The effect of operating temperature (300 K to 425 K) on I-V characteristics of the fabricated structure has been measured experimentally. The other diode parameters such as ideality factor and effective barrier height have been derived. The reliability of the heterojunction synthesized is proved by the diode ideality factor of 1.03 attained at 425 K. The excellent C-V characteristics (capacitance of 48pF) of the device prove that the device is an excellent candidate for application as supercapacitors. The fabricated structure can be utilized as an ultraviolet photodetector, solar cell, energy storage devices, etc.

Towards high-performance linear piezoelectrics: Enhancing the piezoelectric response of zinc oxide thin films through epitaxial growth on flexible substrates

Ferroelectrics are popular for sensors, actuators and transducers. However, the energy barrier for reversing the polarization leads to extra energy losses, while the hysteresis causes phase-lags and non-linearities between input and output signals. The high piezoelectric constants of ferroelectrics dictate the figure-of-merit of piezoelectric devices. Enhancing the piezoelectric constant of linear-piezoelectrics potentially leads to next-generation piezoelectric devices free from issues associated with ferroelectrics. By epitaxial growth of undoped ZnO films on flexible Hastelloy substrates, we demonstrated an ~85% improvement in the piezoelectric constant, and more importantly maintained the linearity of the piezoelectric response. Strong out-of-plane and in-plane crystallographic alignments are detected in the epitaxial sample, yet modeling of the effective thin-film piezoelectric constant shows only a limited contribution of the optimized textures. The improvement in the piezoelectric response is ascribed mostly to the in-plane epitaxial strain resulted from the lattice mismatch of ZnO to the substrate. The epitaxial ZnO film on Hastelloy promisingly offer high-precision positioners with nano-/pico-meter resolutions. Enhancing the piezoelectric constant by strain engineering potentially removes the barrier for applications of linear-piezoelectrics and enriches the piezoelectric research community.

Rapid fabrication of ZnO film by electrochemical deposition method from aqueous solution

AbstractRecently, ZnO film paid much attention, for many application such as transparent conductive film and photo devise. In order to use as transparent conductive film, ZnO films need be obtained at high growth rate. Fabrication of oxide films by electrochemical deposition from aqueous solutions is possible to fabrication at low temperature. And thickness and electric characteristics of ZnO film can be easily controlled by electrochemical parameters. Mentioned above, this electrochemical deposition is optimal technique for manufacturing technique for transparent conductive film. We will support that the adjustment of the alkaline electrolyte for ZnO film gives the rapidly growth of ZnO films at low temperature. In this investigation, the influence of the electrical and optical properties for ZnO films with the deposition temperature will be investigated. For optical and electrical results, the growth rate and the carrier concentration of ZnO films increase with an increase in the deposition temperature. And the optical bandgap energy is constant at about 3.37 eV. Thus, the electric property and deposition rate of ZnO film will be controlled by the deposition temperature.

Characterization of a 2D Electron Gas at the Interface of Atomic‐Layer Deposited Al2O3/ZnO Thin Films for a Field‐Effect Transistor

AbstractThe 2D electron gas (2DEG) phenomenon that occurs at the amorphous thin film hetero‐oxide interface attracts great attention since it can avoid the use of a single‐crystal oxide substrate. In this study, the analysis of 2DEG at the interface of amorphous‐Al2O3 (a‐AO)/ZnO is conducted using ZnO as the bottom substrate, where both the oxide films are grown by atomic layer deposition. Having used Al(CH3)3 as the Al‐precursor for the a‐AO film growth on the previously grown ZnO film, its strong reducing power induces the 2DEG formation at the interface. As a result of the Hall measurement, the 2DEG at the a‐AO/ZnO interface shows sheet resistance of 2.7 × 104 Ω ▫−1 and Hall mobility of 8.4 cm2 V−1 s−1. Using angle‐resolved X‐ray photoelectron spectroscopy, the thickness of the 2DEG layer is calculated as 0.62 nm, which is ≈120% of the c‐axis of the wurtzite ZnO unit cell. The field‐effect transistor fabricated exhibits a threshold voltage of −2.4 V, sub‐threshold swing of 0.33 V dec−1, and on/off ratio of 9.4 × 106, which significantly outperforms similar devices from previous works. The outstanding operation of 2DEG at the interface of AO/ZnO as a channel presents a possibility for application to a 2D‐based integrated circuit.

Hydrogen in As‐Grown and Annealed ZnO Films Grown by Atomic Layer Deposition

The origin of unintentional hydrogen incorporation in ZnO films grown by thermal atomic layer deposition is investigated by comparing layers deposited using H2O and D2O as oxygen precursors. Secondary ion mass spectroscopy measurements of as‐grown and 800 °C annealed layers provide evidence that the hydrogen contribution originating from the oxygen precursor is weakly chemically bound and is mostly removed by rapid thermal annealing conducted at 800 °C for 3 min in oxygen atmosphere. On the other hand, the remaining hydrogen introduced by ethyl groups originating from the metalorganic zinc precursor withstands such treatment.

Growth and Characterization of ZnO and Al-Doped ZnO Thin Films by a Homemade Spray Pyrolysis

In this work, we have prepared the undoped and Al-doped ZnO thin films by a homemade spray pyrolysis method at 450°C onto glass substrates. The X-ray diffraction patterns of undoped ZnO and aluminized zinc-oxide (AZO) thin films exhibit hexagonal wurtzite crystal structure with high crystalline quality, the crystallite size is nanometric. The morphology of the undoped and Al-doped ZnO thin films also indicate that all samples have a nanoscale grain size around 50 nm, and the microstructure of ZnO films is highly influenced by the aluminum doping. The two films are characterized by UV-visible spectrophotometry showing that the films have a whole optical transmission above 85% in the visible range. The composition of our films is obtained by energy dispersive spectrum, confirmed by Auger electron spectroscopy (AES) and by Rutherford back-scattering spectrometry (RBS) techniques.

Air Annealing Effect on Oxygen Vacancy Defects in Al-doped ZnO Films Grown by High-Speed Atmospheric Atomic Layer Deposition.

In this study, aluminum-doped zinc oxide (Al:ZnO) thin films were grown by high-speed atmospheric atomic layer deposition (AALD), and the effects of air annealing on film properties are investigated. The experimental results show that the thermal annealing can significantly reduce the amount of oxygen vacancies defects as evidenced by X-ray photoelectron spectroscopy spectra due to the in-diffusion of oxygen from air to the films. As shown by X-ray diffraction, the annealing repairs the crystalline structure and releases the stress. The absorption coefficient of the films increases with the annealing temperature due to the increased density. The annealing temperature reaching 600 °C leads to relatively significant changes in grain size and band gap. From the results of band gap and Hall-effect measurements, the annealing temperature lower than 600 °C reduces the oxygen vacancies defects acting as shallow donors, while it is suspected that the annealing temperature higher than 600 °C can further remove the oxygen defects introduced mid-gap states.

Growth and Raman spectroscopy of ultrathin ZnO(0001) films on Ag(001)

The growth and vibrational properties of ultrathin ZnO(0001) films on Ag(001) were investigated by X-ray and ultraviolet photoemission spectroscopy (XPS/UPS), low-energy electron diffraction (LEED), and Raman spectroscopy. Our results indicate the formation of well-defined stoichiometry and crystalline phase ~5 ML thick ZnO(0001) films characterized by two domains rotated by 30°. Further film analysis using Raman spectroscopy showed that the ultrathin films display substantial intensity enhancement of their Raman combination modes. Our findings suggest a significant role of the finite-size film electronic and structural properties on its vibrational properties. These observations may serve as a pathway toward further investigations dependent on film thickness and orientation. More important, the modification of the vibrational properties is expected to be relevant to understanding general phenomena in ZnO nanostructures (i.e. thin films, nanoparticles, nanorods).

Self-compensation reduction as first step of p-type ZnO synthesis

Abstract In this work, we used a new strategy, based on the reduction of self-compensation, to synthesize p-type ZnO thin films. This new protocol is based on the control of intrinsic defects especially the vacancies and the interstitials of both zinc and oxygen networks. It consists of using zinc nitrate as a precursor and varying its concentration on the one hand, and the addition of acetic acid on the other. Hall Effect measurements have shown a decrease in electron concentration as the nitrate concentration increases. A concrete transition in the conductivity of ZnO from n-type to p-type has been observed for concentration around 0,4M. Thermal Analyses (DTA-TGA), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), UV–visible spectrophotometry were used to study the structural, morphological and optical properties of the synthesized layers. All these measures confirm the appearance of a change in behavior when nitrate concentration is approaching 0,4M. This transition in conductivity-type in ZnO layers has been linked to a significant increase in the oxygen content as confirmed by an EDX analysis during films growth. The growth of p-type ZnO films based on zinc nitrate can potentially lead to the development of new transparent devices applicable in optoelectronics and photovoltaic fields.

Emission and opto-dielectric nonlinearity in 2D Cd–ZnO–Na nanostructures: an effect of Na doping

Cd–ZnO–Na alloy nanostructured thin films were synthesized via sol–gel spin coating method on glass substrates and the effect of Na (1, 2 and 3 wt%) doping variation on linear, nonlinear optical, opto-dielectric, and emission properties of the films was investigated. The variations in physical properties with different Na doping concentrations were analyzed using X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), FT-Raman, UV–Vis, and photoluminescence spectroscopy. From XRD patterns, it was observed that the growth of the films occurs along (002) plane with hexagonal wurtzite structure. High percentage of transmittance (viz. 85 to 90%) was recorded for all as grown films. However, the estimated bandgap energy of the films was found to decrease from 3.37 to 3.30 eV with increasing Na doping concentration from 1 to 3 wt%. Emission spectra of the films show an intense and sharp peak near band emission (NBE) at 389 nm whereas a low intense peak was observed at 475 nm. The intensity of NBE peak specifies the significant enhancement in photoluminescence properties of the grown films with increasing Na doping concentrations. Nonlinear optical parameters of the Cd–ZnO–Na films such as χ3 and n2 showed substantial improvements, which were deduced and obtained in the range 1.11 × 10–14–1.91 × 10–12 esu and 5.20 × 10–13–3.19 × 10–11 esu, respectively. The achieved improvement in the grown ZnO films via co-doping with Cd and Na makes them highly suitable candidates for optoelectronics devices applications.

ZnO Nanowalls/Si Substrate Heterojunction Assembly: Morphological, Optical and Electrical Properties

In this paper, a film of n-ZnO nanowalls was deposited over p-silicon (Si) substrate by thermally evaporating metallic zinc powder in oxygen gas environment. Several techniques were used to examine various properties of prepared ZnO nanowalls. The morphological study confirmed that the grown ZnO film possesses nanowalls shaped morphologies containing three dimensional (3D) interconnected nanowalls forming large voids with irregular shapes. Interestingly, whole Si substrate was covered with nanowalls shaped morphologies. The structural characterizations examined by x-ray diffraction (XRD) confirmed that the deposited nanowalls possess wurtzite hexagonal crystal structure. The presence of a strong UV and suppressed green emissions at room-temperature photoluminescence (PL) spectrum exhibited good optical properties of the deposited nanowalls. The electrical properties of the deposited n-ZnO nanowalls over p-silicon substrate assembly was examined in both forward and reverse bias conditions at room-temperature. The fabricated heterojunction device exhibits a standard rectifying behavior which manifests itself in an exponential increase of the current with increase in voltage at forward bias condition. Also, it has a good quality factor similar to an ideal diode.