Al-doped ZnO Research Articles

Realization of highly rectifying Schottky barrier diodes and pn heterojunctions on κ-Ga2O3 by overcoming the conductivity anisotropy

Novel devices based on orthorhombic κ-Ga2O3 could enable solar blind infrared detection or high-electron mobility transistors with large two-dimensional electron gas densities. Here, we report on the current transport parallel to the growth direction of κ-Ga2O3 layers grown by pulsed-laser deposition on highly conductive Al-doped ZnO back contact layers. Besides ohmic Ti/Al/Au contact layer stacks, vertical Pt/PtOx/κ-Ga2O3 and Pd/PdOx/κ-Ga2O3 Schottky barrier diodes and NiO/κ-Ga2O3 and ZnCo2O4/κ-Ga2O3 pn-heterodiodes are investigated by current–voltage measurements. While a lateral current transport is severely suppressed to less than 10−9Acm−2 due to rotational domains, we record a significant current flow through the ohmic contacts in the vertical direction of >0.1Acm−2. The Schottky barrier diodes and the pn-heterojunctions exhibit rectification ratios of up to seven orders of magnitude. Room temperature current–voltage characteristics of diode ensembles as well as temperature-dependent measurements for selected Pt-based diodes reveal a mean barrier height of ϕBm≈2.1eV and ideality factors down to η≈1.3.

Fast detection and discriminative analysis of volatile organic compounds using Al-doped ZnO thin films

Fast detection and discriminative analysis of volatile organic compounds using Al-doped ZnO thin films

High gain GaN ultraviolet Schottky photodetector with Al-doped ZnO interlayer

We fabricated an GaN Schottky photodiode (PD) with Al-doped ZnO (AZO) interlayer for ultraviolet (UV) photodetection, and its optoelectrical properties with a comparison to the GaN Schottky PD without AZO interlayer. The AZO-interlayered GaN Schottky PD was highly sensitive to 350 nm laser irradiation, having a photoresponsivity of 11.1 AW–1 and detectivity of 1.04 × 1012 cm·Hz1/2·W–1 at -2 V as against 0.28 AW−1 and 2.23 × 1010 cm·Hz1/2·W–1 for the GaN Schottky PD without AZO interlayer. The transient photoresponse revealed a rise and decay time of 203 and 293 ms, respectively for the AZO-interlayered GaN Schottky PD. The introduction of AZO interlayer resulted in a reduction in the noise spectral density, leading to an increase in the detectivity by more than an order, as compared to GaN Schottky PD without AZO interlayer. An improved photodetection behavior of the AZO-interlayered GaN Schottky PD could be ascribed to effective GaN surface passivation and reduction of the surface states by the AZO interlayer. The utilization of the AZO in GaN-based UV photodetectors can provide a potential and simplistic approach towards the fabrication of high performance GaN-based UV photodetectors.

Structural and optical characterization of sol–gel processed Al-doped ZnO waveguide films for integrated optical devices

Structural and optical characterization of sol–gel processed Al-doped ZnO waveguide films for integrated optical devices

Theoretical and experimental studies of Al-doped ZnO thin films: optical and structural properties

Theoretical and experimental studies of Al-doped ZnO thin films: optical and structural properties

Doping efficiency and electron transport in Al-doped ZnO films grown by atomic layer deposition

Transparent conducting Al-doped ZnO films were grown by atomic layer deposition (ALD). Al-doping was introduced by inserting 1 Al2O3 cycle per 28 ZnO cycles. The x-ray photoelectron spectroscopy showed that the density of the Al donors is 2×1021–3×1021 cm−3, while the Hall-effect measurements showed a ten times lower electron density. This low doping efficiency is a well-known inherent problem of the ALD method, and we wanted to explain its origin. We have found that the electron density is reduced by electron traps at the grain surface; however, the effect was too weak to explain the low doping efficiency. Therefore, the mechanism of the Al2O3 doping was analyzed. We have proposed that each Al2O3 molecule ideally provides two single-electron Al donors accompanied by one Zn vacancy, which acts as a two-electron acceptor. This would cause a perfect compensation; however, the compensation is in reality not perfect, which results in weakly efficient doping. Calculations also showed that each Zn vacancy creates a bound pair with an Al donor. To verify our doping model experimentally, it was inserted into the metallic transport theory and compared with the electron transport measurements. A good agreement was found for a broad range of experimental conditions. In the regime of weak localization, the conductivity showed the temperature dependence σ(T)=a+bT3/4, which is a signature of weak localization and electron–electron scattering in a 3D dirty metal.

Optimization of thermoelectric properties of Al-doped Zn1−xAlxO under high pressure and high temperature

ZnO has excellent electrical properties such as high conductivity (σ) and Seebeck coefficient (S) that are important for its thermoelectric properties and have provided the basis for obtaining improved ZnO-based thermoelectric materials by doping. In particular, the most representative Al-doped ZnO (AZO) has excellent thermoelectric performance. Although the Al doping is beneficial for reducing the lattice thermal conductivity of ZnO, the solubility of Al2O3 in ZnO is limited and excessive doping of Al gives rise to the generation of a large amount of the ZnAl2O4 secondary phase that limits the further improvement of the electrical properties. Thus, it is crucial to develop approaches to reduce thermal conductivity while still maintaining high electrical properties. In this work, high pressure and high temperature (HPHT) synthesis was used to adjust the ZnO optical band gap by modifying the lattice parameters of ZnO, and thereby improving the electrical properties of the sample. It was found that the optimal doping content for the samples synthesized at high pressure is different from that of the samples synthesized at normal pressure, providing a new approach for the further improvement of the thermoelectric properties of ZnO. Additionally, high pressure can promote grain refinement that is conducive to the generation of the multi-scale hierarchical structure of the sample that introduces a variety of phonon scattering mechanisms to reduce the thermal conductivity while maintaining a high power factor. The Zn0.96Al0.04O material obtained by sintering at a pressure of 3 GPa and a temperature of 1053 K for 30 min reached the highest power factor (7.8 µW cm−1 K−2) and the highest zT (0.16) at 973 K.

UV random laser in aluminum-doped ZnO nanorods

Vertically aligned Al-doped ZnO nanorods (AZO-NRs) were grown on glass substrate using a chemical bath deposition (CBD) method at various temperatures between 80°C and 130°C. The results showed the Al content in the AZO-NRs strongly depends on the growth temperature. The optimum doping level was attained at 110°C. The morphology was maintained in each sample, and the lasing properties were investigated against the Al-doped variation. The sample with a high doping level exhibited superior random lasing, with high intensity and spectral width of less than 0.08 nm. The same sample also had the lowest pumping threshold of 0.192 mW. More importantly, this study showed the possibility of utilizing doping as a tuning parameter for random lasing, whereby a 7.3 nm redshift in the lasing peak was observed with increasing doping concentration. This study also placed an emphasis on AZO-NRs as potential candidates for tunable random laser devices.

Tunable ultraviolet sensing performance of Al-modified ZnO nanoparticles

The potential of polycrystalline Al-doped ZnO nanoparticles as an active material for UV photodetectors has been investigated. The Rietveld refinement of powder X-ray diffraction data revealed a single hexagonal phase of the nanoparticles. A slight deviation in the lattice cell constants from pristine ZnO was observed, associated with defect creation and strain generated due to Al3+ substitution. High-resolution transmission electron microscopy image reveals a spherical morphology of both the doped and undoped ZnO nanoparticles. Stacking faults observed in the Al-doped samples is an indication of a proper Al-doping and is a signature of high density of crystal defects. The bandgap was found to reduce due to the delocalization of impurity energy states as a result of Al3+ substitution. Consequently, conductivity was improved in doped samples. Photoluminescence spectroscopy revealed a strong dependence of the emissions from defect sites on dopant content. Further, a correlation of the FWHM of the E2high Raman mode to the Urbach energy was observed. The UV sensing analysis demonstrates the enhancement of the photocurrent and improved sensitivity. Thus, this work provides a simple, cost-effective, and tunable processing strategy for synthesizing and applying ZnO-based nanomaterials for high-performance UV photodetectors.

Property approach of Si based ZnO films under thermal shock

ZnO and Al-doped ZnO (AZO) films were deposited on p-Si(100) wafers using magnetron sputtering, the effect of thermal shock on Si based films and its interface were evaluated by various characterization methods such as in-situ nanoscratch, atomic force microscope (AFM), X-ray diffraction (XRD), fourier transform infrared (FTIR) spectra and ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS). The results show the adhesion strength of Si/ZnO interface is relatively stable under thermal shock, the surface grains of Si coated ZnO form dense clusters then refine them. XRD and FTIR show Si based films exhibit (002) orientation with good structural stability related to Zn–O bond, the interfacial reactions of Si based films could experience strong interaction through Si, Zn and O atoms located at Si/ZnO interface during thermal shock. Moreover, some comparatively prominent peaks are observed by UV-DRS, which caused by Si interacted with ZnO, the AZO films deposited on quartz glass substrates illustrate the optical properties are obviously influenced and degraded after thermal shock.

High doping efficiency Al-doped ZnO films prepared by co-injection spatial atomic layer deposition

Aluminum-doped ZnO (AZO) films are prepared by using spatial atomic layer deposition (ALD). The precursors of trimethylaluminum (TMA) and diethylzinc (DEZ) are concurrently introduced into the deposition region. The amount ratio of TMA to DEZ vapor molecules is controlled by varying the temperature of the precursor bubblers from 5° to 77 °C. It is found that the Al doping level can be predicted using the amount ratio of TMA to DEZ output vapors. The film prepared at the bubbler temperature of 41 °C exhibits an Al content of ~1% and a low resistivity of 3.5 × 10−4 Ω-cm comparable to indium tin oxide films. Furthermore, the Al-doping efficiency is as high as 73%. This study is useful in applying the ALD technique to films with multicomponent oxides.

Research on piezo-phototronic effect in ZnO/AZO heterojunction flexible ultraviolet photodetectors

A novel Al-doped (3 wt%) ZnO (AZO) UV photodetector (PD) and ZnO/AZO heterojunction double-layer UV PD were prepared on a polyethylene terephthalate (PET) substrate by utilizing the radio frequency (RF) sputtering technology and conventional photolithography. Because the ZnO/AZO heterojunction possesses the ability to improve the separation of the photogenerated electron-hole pairs, the responsivity of the ZnO/AZO PD is higher than that of the pure AZO PD under the same bias voltage. Moreover, the responsivity of the obtained ZnO/AZO PD showed a 1.13 fold increase from 5.91 to 6.70 A/W when the tensile variable ɛ was 0.16. These findings also provide a new alternative for the preparation of flexible UV PDs with piezo-phototronic effect, which is low-cost and offers environmental sustainability.

Tuning of Optical Band Gap: Genesis of Thickness Regulated Al Doped ZnO Nano-Crystalline Thin Films Formulated by Sol–Gel Spin Coating Approach

Tuning of Optical Band Gap: Genesis of Thickness Regulated Al Doped ZnO Nano-Crystalline Thin Films Formulated by Sol–Gel Spin Coating Approach

CNT nanoengineering for thermally stable selective solar absorption

Solar thermal electricity generation is an appealing green concept, which the efficiency enhancement requires the development of appropriate solar absorbers using an up-scalable processing technique. Here, we report on the nanoengineering of carbon nanotubes (CNT) as an approach to design high-performance solar absorbers using oxide materials in a core/shell architecture. Gas-phase synthesis is deployed for the growth of modified CNT cores with infra-red reflecting and low refractive index shell layers. The designed coating fully benefits from the high absorbance of CNTs and the infrared-reflecting properties of Al-doped ZnO irrespective of the nature of the underlying substrate. The nano-engineered CNT-metal oxide design withstands extended exposure to 1000 K.

Solution processed Al-doped ZnO and its performance in dye sensitized solar cells

Al-doped ZnO rods of nanometer to sub-micrometer size range have been successfully synthesized by a simple yet cost-effective solution processed sonochemical technique. Systematic XRD analysis established the solid solubility limit for Al in the ZnO lattice to be ca. 3 mol% at an elevated annealing temperature of 800 °C. The secondary ZnAl2O4 phase appears with increasing dopant concentrations and at lower annealing temperatures. Significant variations in the optoelectronic properties are induced by modifications in the surface defects of ZnO rods as a result of Al doping. As a consequence, an improved fill factor (FF) of 74.78 and 75.76% with a conversion efficiency (η) of 1.59 and 1.79% have been achieved for the fabricated DSSC devices made of the 800 °C annealed ZnO rods doped by 1 and 3 mol% Al, respectively.

Polar Kerr effect and perpendicular magnetic anisotropy in Fabry–Pérot cavity containing CoPt/AZO magneto-optical interference layer

We investigate the magneto-optical (MO) polar Kerr enhancement of Fabry–Pérot cavities with perpendicular magnetic anisotropy for chemical and biological sensing applications. The MO cavities consist of stacked films with a half-mirrored surface layer, a dielectric interference layer, a total-reflection mirror, and a magnetic metal layer. Co80Pt20 and Al-doped ZnO (AZO) thin films were utilized as the magnetic and interference layers, respectively. The cavities produce two types of MO enhancement depending on the thickness of the magnetic layer. The sample with a thick magnetic layer as a total-reflection mirror generates a single peak in the polar Kerr spectrum. Contrarily, inserting a several-nanometer-thick CoPt layer into the AZO layer produces a resonant-type spectrum. Although both samples generate large MO enhancement factors (>50), the magnetic properties significantly differ. The AZO/CoPt/AZO interference layer controls the crystalline and interface conditions and produces a square-shaped out-of-plane hysteresis loop with a large Kerr rotation angle. Moreover, the use of an ultrathin CoPt film significantly reduces the saturation magnetization field. Thus, Fabry–Pérot cavities with a CoPt/AZO interference layer may serve as high-sensitivity, low-energy-consumption sensor elements.

Modulation of wall thickness of Al‐doped ZnO nanotubes by nanolamination of atomic layer deposition for oxygen sensing

AbstractThe high surface‐to‐volume ratio and feature dimensions of the gas sensors are the key factors for improving the gas response. In this study, a novel method to prepare an Al‐doped ZnO (AZO) nanotube oxygen sensor with tunable wall thickness is reported via the ZnO–Al2O3 nanolamination of atomic layer deposition (ALD) using tris(8‐hydroxyquinoline) gallium nanowire (GaQ3NW) as a template. The ALD of Al2O3 significantly enhances wall uniformity and decreases the wall thickness of the AZO nanotubes. In addition, the incorporation of Al2O3 allows full coverage of AZO on GaQ3NWs. With an increase in the Al2O3 fraction, the carrier concentration increases, but the depth of the depletion layer and gas response of the nanotube sensor are reduced. The gas response of the nanotubes is inversely proportional to wall thickness, suggesting that it is a function of the surface‐to‐volume ratio. When the wall thickness is decreased to 12 nm, the gas response of AZO nanotubes with 2% Al increases significantly to 7. This can be explained by the grain control model, because thin wall leads to the formation of fully charge‐depleted nanotubes.

Effect of oxygen annealing temperature on properties of spatial atomic layer deposited aluminum-doped zinc oxide films

In this work, spatial atomic layer deposited (sALD) Al-doped ZnO (AZO) films are prepared, followed by an oxygen post annealing process. Effects of annealing temperature on structural, electrical and optical properties are systematically investigated. The experimental results show that the band gap of the films is not much affected by the oxygen annealing temperature. However, the oxygen annealing is able to remove the oxygen vacancies and improve crystalline structure. Annealing at too high temperatures leads to oxygen desorption from the film. The electrical characterization reveals that the resistivity of the sALD AZO films can be tuned in a range of 1.1 × 10−3 to 1.7 × 10−2 Ω-cm via oxygen annealing.

Light‐Assisted Resistive Switching Memory Device with Reduced SET/RESET Voltage Using Sol–Gel TiO2 on Al‐Doped ZnO Electrode

Herein, light‐assisted resistive switching (RS) characteristic of low‐cost sol–gel TiO2 film‐based metal–insulator–metal (MIM) configuration using Al‐doped ZnO (AZO) as the bottom electrode is demonstrated. Unprecedentedly, instead of the costly transparent conducting electrode such as tin‐doped indium oxide, AZO electrode is exploited in this study due to its dual role as a transparent and an additional oxygen vacancy (VO)‐rich electrode. The Al/TiO2/AZO device shows electroforming free bipolar RS characteristics, which can be explained by the formation (rupture) of conducting filaments composed of VO. The memory window is increased three times under the blue light illumination compared to dark conditions. Interestingly, the SET and RESET voltages are decreased from 5.2 to 4.5 V and −3.9 to −3.3 V, respectively, under the blue light illumination with respect to the dark condition. The enhancement in the RS effect under the blue light illumination is explained in terms of the photoresponsive properties of TiO2. The MIM device exhibits good switching repeatability with stable retention characteristics, making them attractive for next‐generation nonvolatile resistive random access memory devices.

Highly transparent and conducting Al-doped ZnO as a promising material for optoelectronic applications

Highly transparent and conducting Al-doped ZnO as a promising material for optoelectronic applications