Gallium-doped Zinc Oxide Research Articles

Optical and photoluminescence studies of precursor stabilised Aluminium-Gallium Zinc oxide thin films

A low cost, versatile, and cost-effective ultrasonic spray pyrolysis approach was used to generate aluminium and gallium doped Zinc Oxide thin films on stretchable corning glass substrates at 400 °C temperature. Ammonium acetate is employed as a stabilizing ingredient in the precursor solution. According to X-ray diffraction patterns, high quality polycrystalline films on glass substrates grew successfully. As Ga doping levels rose, photoluminescence spectra showed an increase in exciton peak emission. As time passes, photoluminescence spectra show a strong emission peak centred at 3.24 eV that is progressively pushed toward higher wavelength. When Aluminum and Gallium were added into the Zinc Oxide lattice, the crystalline size was reduced and the thin film residual stress was raised. In the visible area, all films were extremely clear, with an average transparency of 80%. The optical energy band gap increased from 3.12 eV to 3.3 eV when the doping concentration increased.

Optical and photoluminescence studies of precursor stabilized gallium zinc oxide thin films

Precursor stabilized Gallium doped ZnO thin films were grown using spray pyrolysis on glass substrates. UV–vis spectrophotometry and an X-ray diffractometer were used to characterise the GZO thin films. Gallium is doped with a concentration ranging from 1 at% to 4 at% and its effects on structural and optical features were studied. XRD studies found the thin films to be polycrystalline with a hexagonal wurtzite structure. In the visible range of wavelength (400–800 nm), all GZO films exhibit average transmittance above 90%, with a pronounced absorption edge in the UV area. The PL emission spectra show the Near-band-edge emissions at about 389 nm and strong deep-level emissions from oxygen defect states at about 545 nm are observed in all the films. With increase in concentration of Ga, the intensity of these two bands decreased.

Facile preparation of hazy Ga-doped ZnO electrodes by atmospheric pressure plasma jet

Increasing the optical path length without degrading optoelectronic properties of front electrodes of solar cells is promising to enhance the power conversion efficiency. This can be achieved by increasing the haze (H) of the electrode through texturing, while retaining high transmittance (Tt) and low sheet resistance (Rs). However, it is challenging to balance these properties simultaneously since they are mutually influenced. Here, we present a facile, low-cost, and single-run method of preparing gallium-doped zinc oxide (GZO) with high H (>25%), high Tt (>90% in the visible range, referenced to glass), and low Rs (6.62 Ω sq−1) using atmospheric pressure plasma jet. First, we increase the nozzle-substrate distance, or working distance (WD), to deposit a thin, rough GZO as a haze enhance (HE) layer and immediately deposit a standard GZO layer using the same apparatus. By tailoring the roughness of the HE layer using WD and scanning pitch, we successfully produce high-quality hazy GZO that have competitive figures of merit of 6.3 × 10−2 Ω−1 (referenced to the glass) and 2.1 × 10−2 Ω−1 (overall), and H = 25%. Also, unlike conventional methods, our approach does not require vacuum, etching, additional materials, or change of machine/tooling, which can significantly reduce the operational complexity.

Using Modified-Intake Plasma-Enhanced Metal-Organic Chemical Vapor Deposition System to Grow Gallium Doped Zinc Oxide.

We have used a modified-intake plasma-enhanced metal–organic chemical vapor deposition (MIPEMOCVD) system to fabricate gallium-doped zinc oxide (GZO) thin films with varied Ga content. The MIPEMOCVD system contains a modified intake system of a mixed tank and a spraying terminal to deliver the metal–organic (MO) precursors and a radio-frequency (RF) system parallel to the substrate normal, which can achieve a uniform distribution of organic precursors in the reaction chamber and reduce the bombardment damage. We examined the substitute and interstitial mechanisms of Ga atoms in zinc oxide (ZnO) matrix in MIPEMOCVD-grown GZO thin films through crystalline analyses and Hall measurements. The optimal Ga content of MIPEMOCVD-grown GZO thin film is 3.01 at%, which shows the highest conductivity and transmittance. Finally, the optimal MIPEMOCVD-grown GZO thin film was applied to n-ZnO/p-GaN LED as a window layer. As compared with the indium–tin–oxide (ITO) window layer, the n-ZnO/p-GaN LED with the MIPEMOCVD-grown GZO window layer of the rougher surface and higher transmittance at near UV range exhibits an enhanced light output power owing to the improved light extraction efficiency (LEE).

Robust Stability of Optical and Electronic Properties of Gallium‐Doped Zinc Oxide Thin Films to Gamma Ray Irradiation

Through combined measurements of broadband optical spectroscopy (10 meV to 6 eV), electrical resistivity, and Hall effect, the effects of gamma ray irradiation on electronic and optical properties of gallium-doped ZnO (GZO) thin films, deposited by ion plating direct-current arc discharge, are investigated. A significant number of films, deposited at various discharge currents (I D = 100–200 A) and oxygen gas flow rates (OFR = 0–25 sccm), exposed to doses of 15 and 30 kGy of gamma rays, are studied. The results indicate strong resilience of films to irradiation: visible range transparency is reduced by 10–12% and the optical bandgap shifts to lower energies by less than 3%, while electrical resistivity, carrier concentration, and electron mobility remain nearly unchanged.

Spray deposited gallium doped zinc oxide (GZO) thin film as the electron transport layer in inverted organic solar cells

The electric field-assisted spray deposition of highly conducting and transparent gallium doped zinc oxide (GZO) films, with noticeable improvement in optical, structural, and electrical properties, is reported for application in organic photovoltaics with superior device performance. The GZO films deposited with applied voltages of 0 and 2000V to the nozzle exhibited an improved electrical resistivity of ∼1.8 × 10−3 Ω-cm and a transmittance (in the visible region) of over 80% for the films deposited under electric field, which can be used as electron transport layer (ETL) in inverted organic solar cells (IOSCs- ITO/GZO/PTB7:PC71BM/MoO3/Ag). Improved photovoltaic performance (power conversion efficiency: 6.5%) is obtained for the IOSC devices fabricated GZO ETL with the applied voltage of 2000V in comparison to the device fabricated using GZO ETL with the applied voltage = 0V (power conversion efficiency: 5.8%).

Band offset and electrical properties of ErZO/β-Ga2O3 and GZO/β-Ga2O3 heterojunctions

The fabrication of ohmic contacts on wide bandgap semiconductors, such as β-Ga2O3, is difficult as the scarcity of metals with low-enough work function. The insertion of n+-doped metal oxide layer between metal and the β-Ga2O3 layers are effective for improving the ohmic behaviour. β-Ga2O3, Erbium doped ZnO (ErZO), and Gallium doped ZnO (GZO) films with high crystallinity were deposited by pulsed laser deposition. The band offsets and electrical properties of ErZO/β-Ga2O3 and GZO/β-Ga2O3 heterojunctions were investigated, which revealed the nested gap (type I) structures. The conduction band offsets were determined to be 0.88 eV for ErZO and 0.96 eV for GZO, and their corresponding valance band offsets were found to be 0.46 eV and 0.16 eV. In addition, the difference of the work functions between ErZO and β-Ga2O3 was 0.05 eV; and 0.16 eV between GZO and β-Ga2O3. Solar blind UV detector having fork electrode structure of as-grown β-Ga2O3/ErZO/Ti/Au structure showed good ohmic performance and have superior detector performance comparing with the other two electrode structures, which was ascribed to ErZO’s low band offset and small work function difference with β-Ga2O3. The present result provides an option for fabricating ohmic contact on β-Ga2O3 without the need for post-growth annealing.

A New Miniaturized Gas Sensor Based on Zener Diode Network Covered by Metal Oxide.

The development of “portable, low cost and low consumption” gas microsensors is one of the strong needs for embedded portable devices in many fields such as public domain. In this paper, a new approach is presented on making, on the same chip, a network of head-to-tail facing PN junctions in order to miniaturize the sensor network and considerably reduce the required power for heating each cell independently. This paper is about recognizing a device that integrates both sensing and self-heating. This first study aims to evaluate the possibilities of this type of diode network for use as a gas sensor. The first part concerns the description of the technological process that is based on a doped polysilicon wafer in which a thin layer of metal oxide (a gallium-doped zinc oxide in our case) is deposited by RF sputtering. An electrical model will be proposed to explain the operation and advantage of this approach. We will show the two types of tests that have been carried out (static and dynamic) as well as the first encouraging results of these electrical characterizations under variable atmospheres.

Surface Plasmon Resonance from Gallium-Doped Zinc Oxide Nanoparticles and Their Electromagnetic Enhancement Contribution to Surface-Enhanced Raman Scattering

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga3+ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Gallium doped zinc oxide thin films as transparent conducting oxide for thin-film heaters

The addition of suitable metallic dopants into the indium or zinc oxide matrix is essential to obtain transparent conducting oxide (TCO) thin films for high-performance optoelectronics devices. However, scarcity of indium is one of the major challenges for the common use of indium doped tin oxide (ITO) as a TCO material for future state-of-the-art devices. To overcome the challenge, doped zinc oxide is used an alternative material for traditional ITO and retains both high transparency and electrical conductivity. One such potential material is gallium-doped zinc oxide (GZO). GZO thin films were deposited onto glass as well as Kapton substrates using the pulsed laser deposition technique. Structural, optical, and electro-thermal properties of these films were studied to assess the performance of the films as thin-film transparent heaters. The samples show a good transmittance value greater than 85% in the visible range of the electromagnetic spectrum. At room temperature, the electrical resistivity of GZO films showed a value of 110.46 × 10−4 Ω cm on glass and 2.90 × 10−4 Ω cm on the Kapton substrate, followed by a Joule heating effect, with temperatures reaching more than 120 °C at an applied voltage of ∼12 V. This high transparency, cost-effectiveness, low sheet resistance, and small surface roughness make GZO a unique and potential candidate for various practical applications not only as a transparent electrode but also as an indium free thin-film transparent heater and an affordable transparent conducting oxide in displays.

Fabrication Of p-NiO/n-ZnO:Ga heterostructures for a rectifier diode and a UV photodetector via RF magnetron sputtering and spray pyrolysis synthesis

In this paper, a p–n thin film NiO/ZnO heterojunction for a rectifier diode and a UV photodetector is prepared and characterized. Nickel oxide (NiO) and gallium-doped zinc oxide (ZnO:Ga) thin films are grown by RF magnetron sputtering and spray pyrolysis techniques, respectively. The crystal structure of the thin films is studied by the X-ray diffraction (XRD) method. The transmittance and reflectance are studied by UV–VIS spectroscopy. The p–n electrical parameters are estimated from current–voltage characteristics. The effects of duration of thermal annealing at 450oC on the characteristics of the NiO/ZnO:Ga device are evaluated. The non-annealed diode shows the best rectification coefficient of 105 at ±1 V. The p–n photodetection capability is studied under UV illumination. At a reverse bias of –3 V under 365-nm UV illumination, the device shows a current intensity of ~6.2 × 10-12 A. The observed increase in the reverse current intensity by about two orders of magnitude under a UV lamp with a spectral irradiance of 10 W m-2 μ m-1 indicates a promising application in UV light detection.

Structural and optical properties of ZnO:Ga thin films deposited on ito/glass substrates for optoelectronic applications

Doped (with GaCl3), undoped ZnO and ITO/ZnO:Ga nanostructured thin films are synthesized using the spray pyrolysis method. The doped ZnO thin films are synthesized at the atomic ratio of Ga/Zn added in the starting solution fixed at 1, 2, 3, and 5. Gallium-doped ZnO films synthesized on glass/ITO substrates are annealed at 4500C in different environments: vacuum, oxygen, and hydrogen. X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and current–voltage (I–V) measurements are applied to characterize the structural properties, composition, surface morphology, and electrical properties of ZnO:Ga nanostructured thin films. X-ray diffraction analysis shows that ZnO:Ga films deposited on glass substrates have a dense and homogeneous surface with a hexagonal structure. The ZnO:Ga films deposited on glass/ITO substrates are composed of two phases, namely, hexagonal ZnO and cubic ITO. The I–V characteristics show the presence of good ohmic contacts between Al and In metals and ZnO:Ga thin films regardless of the nature of the substrate and the annealing atmosphere.

Structural and photoluminescence properties of zinc oxide nanorods grown on various transparent conducting oxide seed layers by chemical bath deposition

Zinc oxide (ZnO) nanorods (NRs) were grown directly on gallium doped ZnO (GZO), tin doped indium (III) oxide (ITO) and fluorine doped tin (IV) oxide (FTO) seed layers by chemical bath deposition using the mixed aqueous solutions of Zn(NO3)2•6H2O and C6H12N4 for the different growth times (tgs). Almost all of the NRs grown on the GZO seed layers, denoted by “NRs/GZO”, were vertically aligned to the substrate surface independently of tg. Some of the NRs grown on the ITO and FTO seed layers, denoted by “NRs/ITO” and “NRs/FTO”, respectively, were vertically aligned to the substrate surface and others were inclined. For both the NRs/ITO and NRs/FTO, moreover, the relative portion of the inclined NRs tended to increase with increasing tg. Both the average width (Wav) and average length (Lav) became larger in the order of the NRs/GZO, NRs/ITO and NRs/FTO, corresponding to the ascending order of the average diameter of the grains forming the seed layer. For the NRs/GZO, the Wav, Lav and density of NRs (nNRs) were almost unchanged at tg > 60 min. For the NRs/ITO and NRs/FTO, however, the increase in tg resulted in the increases in Wav and Lav and the decrease in nNRs. The opposite tg dependences between Wav and nNRs observed for the NRs/ITO and NRs/FTO indicated that the coalescence among the neighboring NRs contributed to the enhancement of Wav. For the NRs/GZO, the strains acting along thea- andc-axis directions were compressive and tensile strains, respectively. For both the NRs/ITO and NRs/FTO, however, both the strains along thea- andc-axis directions were compressive strains. All the photoluminescence (PL) spectra of the NRs exhibited a near-band-edge (NBE) emission at ~380 nm and an orange band (OB) emission at ~600 nm. Regardless of the kind of the seed layer, PL intensity ratio of the OB emission to the NBE emission (IOB/INBE) became smaller with the increase in Wav. The decrease inIOB/INBEwith increasingWavis probably due to the decrease in the volume ratio of the surface region with high concentration of interstitial oxygen atoms (Ois) to the bulk region with few Ois and the decrease in the band bending in the surface region.

Lensing for optical communication by diffraction from Peano curve-based fractal with alternative plasmonic material

Focusing performance of Peano space filling curve-based fractal construction is studied at communication wavelength of 1550 nm. Lensing by diffraction from these finite-sized 2D devices of the first three iterated fractal lattice system constructed with gallium-doped zinc oxide is computationally demonstrated in a systematical manner. In this regard, focusing performance parameters such as efficiency, spot size, focal length, focal depth, and fill-factor are determined on the basis of geometrical parameter sweep and fractal generation through finite difference time-domain simulations. Focusing efficiency from 48% up to 60%, absolute efficiency from 14% up to 33.98%, numerical aperture values from 0.127 up to 0.96, spot sizes from 1.02λSP up to 11.25λSP, and focal depth larger than focal length are introduced by all computational samples. A conformal Talbot effect is assigned when appropriate geometric values are provided. A novel perspective is offered by a newly adapted fractal lattice to optics via an alternative plasmonic material perspective, especially for an optical fiber component.

Annealing Effects on Gas Sensing Response of Ga-Doped ZnO Thin Films.

The high thermal conductivity, high electron mobility, the direct wide band gap, and large exciton binding energy of zinc oxide (ZnO) make it appropriate for a wide range of device applications like light-emitting diodes, photodetectors, laser diodes, transparent thin-film transistors, and so forth. Among the semiconductor metal oxides, zinc oxide (ZnO) is one of the most commonly used gas-sensing materials. The gas sensor made of nanocomposite ZnO and Ga-doped ZnO (ZnO:Ga) thin films was developed by the sol–gel spin coating method. The gas sensitivity of gallium-doped ZnO thin films annealed at 400, 700, and 900 °C was studied for ethanol and acetone gases. The variation of electrical resistance of gallium-doped ZnO thin films with exposure of ethanol and acetone vapors at different concentrations was estimated. Ga:ZnO thin films annealed at 700 °C show the highest sensitivity and shortest response and recovery time for both ethanol and acetone gases. This study reveals that the 5 at. % Ga-doped ZnO thin film annealed at 700 °C has the best sensing property in comparison to the film annealed at 400 and 900 °C. The sensing response of ZnO:Ga thin films was found higher for ethanol gas in comparison to acetone gas.

Time-of-flight secondary ion mass spectrometry fragment regularity in gallium-doped zinc oxide thin films

Time-of-flight secondary ion mass spectrometry fragment analysis remains a challenging task. The fragment appearance regularity (FAR) rule is particularly useful for two-element compounds such as ZnO. Ion fragments appearing in the form of ZnxOy obey the rule 2x ge 2y + 1 in the positive secondary ion spectrum and 2x le 2y + 1 in the negative spectrum where the valence of Zn is + 2 and that of O is − 2. Fragment analysis in gallium-doped ZnO (GZO) films can give insights into the bonding of the elements in this important semiconductor. Fragment analysis of 1 and 7 wt% GZO films shows that only the negative ion fragments obey the FAR rule where ZnO‒, 66ZnO‒, 68ZnO‒ and ZnO2‒ ion fragments appear. In the positive polarity, subdued peaks from out-of-the-rule ZnO+, 66ZnO+ and 68ZnO+ ion fragments are observed. The Ga ion peaks are present in both the positive and negative spectra. The secondary ion spectra of undoped ZnO also shows consistency with the FAR rule. This implies that Ga doping even in amounts that exceed the ZnO lattice limit of solubility does not affect the compliance with the FAR rule.

Comparing Thermal Durability and Effects of Annealing Temperature on Characteristics of Hydrogen-Doped ZnO, AZO, and GZO Thin Films

In this work, undoped, aluminum-, and gallium-doped ZnO thin films (ZnO-H, AZO-H, and GZO-H, respectively) deposited on soda-lime glass substrates by magnetron sputtering method in a gas mixture of hydrogen and argon are annealed at various temperatures in the range of 200–500°C in air to evaluate the durability of those films under annealing temperature. From photoluminescence spectra, formation of point defects, especially oxygen vacancies, when hydrogen diffuses out of the films at high annealing temperature is exhibited via a significant increase of visible emissions. We find out that carrier concentration and Hall mobility of AZO-H and ZnO-H films dramatically decrease, while those of GZO-H film are still stable as the annealing temperature increased from 200°C to 300°C. We proposed a model for interpreting the thermal durability of GZO-H film that, at an annealing temperature of 300°C, Ga3+ ions located at adjacent Zn sites can push hydrogen atoms, which are broken out of the antibonding sites which are perpendicular to the c -axis (AB┴), into bond center sites paralleled to the c -axis (BC//). The movement of hydrogen from AB┴ to BC// site also gives rise to the durability of electrical properties of GZO-H films at the high annealing temperature.

Controlling conduction band alignment and carrier concentration in gallium-doped magnesium zinc oxide by reactive cosputtering

Gallium-doped magnesium zinc oxide (GMZO) holds promise as a UV transparent conducting oxide with tunable bandgap and conductivity, though there has been relatively limited exploration of the broad compositional space available. Conductive GMZO films were deposited by reactive cosputtering at room temperature followed by annealing. The contributions of alloying and the Burstein–Moss effect to the optical bandgap were decoupled through comparisons of as-deposited and annealed films. Compositional analysis in conjunction with electrical characterization was used to quantify the activation of Ga doping in MZO. Combinatorial synthesis was used to explore the optoelectronic performance over a broad composition space. Reactive cosputtering can be used to tailor GMZO properties for optoelectronic applications over a wide range of bandgaps (3.3–4 eV) and resistivity (10−3–>100 Ω cm) through appropriate control of the Mg/Zn ratio and Ga content, respectively.

Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Ga-Doped ZnO Coating—A Suitable Tool for Tuning the Electrode Properties in the Solar Cells with CdS/ZnS Core-Shell Quantum Dots

Two layer system from sputtered indium tin oxide (ITO) and gallium doped zinc oxide (Ga:ZnO, GZO) were studied for transparency in the visible electromagnetic range, reflectivity in the near infrared range, conductivity and valent band for a solar cells with quantum dots. The bi-layer coatings produced at optimized oxygen partial pressure, films thickness and surface roughness exhibit improved optical properties without worsening the electrical parameters, even if additional oxygen introduction during the reactive sputtering of the GZO. With an average optical transmittance of 91.3% in the visible range, average reflection and resistivity lower than 0.4 × 10−2 Ω.cm, these coatings are suitable for top electrode in the solar cells. The obtained results reveal that multilayered stacks of transparent ITO/Ga-doped ZnO coatings possess relatively low surface roughness (7–9 nm) and appropriate refractive index. The additional oxidation of GZO films induces modification of the film thickness and respectively of their optical performances.