Al ZnO Research Articles

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd–Scuseria–Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Nanoscale contacts between semiconducting nanowires and metallic graphenes

Here we report on the metal–semiconductor junction characteristics of a semiconducting ZnO nanowire grown directly on a metallic graphene film. The extracted specific contact resistivity of the graphene–ZnO nanowire contact (1.5 × 10−5 Ω·cm2) is comparable to that reported for Al–ZnO contacts. Based on the assumption that thermionic-field emission is the dominant mechanism, we obtained a zero-bias effective barrier height of 0.413 eV for the graphene–ZnO nanowire Schottky contact. We thus demonstrate that as a result of the enhanced tunneling at the contact, the graphene–nanowire contact exhibits near-ohmic current–voltage characteristics with a low contact resistance.

Preparation and characterization of Al doped ZnO thin films by sol–gel process

Transparent conductive ZnO films doped with 0–5mol% of Al3+ (or AZO films) were deposited on the soda–lime glass substrates by sol–gel multilayer dip-coating. The effects of the processing conditions, especially annealing in vacuum, on the structure, optical and electrical properties of the AZO films were investigated by XRD, Raman spectroscopy, TEM and SEM, spectrophotometer, four-point prober and Hall-effect measurements. Al3+-doping led to destruction and reorganization of ZnO structure, but AZO films still had a hexagonal wurtzite structure with a preferential orientation along (002) crystal plane. After annealed in vacuum, the AZO films had better optical transmittance (>90%) and conductivity (smaller by two orders of magnitude), since annealing in vacuum promoted the growth of grains and release of residual stress, and also decreased the defects and disorder in the films. But extending the vacuum annealing time, the grains grew along other orientations except (002) crystal plane, so that the film’s square resistance became bigger. Carrier concentration (∼2.3×1019/cm3) of annealed AZO films increased with increasing Al3+ amount, but Hall mobility in the 1% Al–ZnO film was the biggest, ∼19.1cm2/Vs.

Al and Si doping of sputtered ZnO thin films

In this work the effect of Al and Si content on the electrical, optical and structural characteristics of Radio Frequency (RF) sputtered ZnO thin films (∼ 150 nrn thick) have been investigated. ZnO:Al and ZnO:Si films were deposited on glass through ZnO and Al targets and ZnO and Si targets co-sputtering, respectively. The Al and Si content have been varied between ∼ 1018 cm−3 and ∼ 4 × 1021 cm−3, as determined by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) measurements. The X-ray analysis, in the Θ − 2Θ configuration, revealed a surfactant action of both Si and Al in the films, i.e., a reduction of the absolute stress ⊥ to the c axis from ∼ 0.6 − 0.8 GPa to ∼ 0.4 GPa and ∼ 0.2 GPa for Al and Si doping, respectively. In addition, a corresponding decrease of the root mean square (RMS) of the surface roughness with higher Al and Si content has been observed by Atomic Force Micropscope (AFM) measurements. Finally, the electrical characterization of the ZnO films, performed by room temperature Hall measurements, suggested that the best structural conditions in terms of grain size and absolute stress correspond, as expected, to the minimum resistivity.

Pump-probe third harmonic generation kinetics of gold nanoparticle-attached aluminum-doped zinc oxide substrate

Size-aligned gold nanoparticles (AuNPs) of 5, 10, and 20nm were attached on aluminum-doped zinc oxide (Al–ZnO) substrates chemically using for the first time 3-aminopropyl-trimethoxysilane as a linker molecule. By using these substrates, we present the pump-probe studies of the AuNPs deposited on the Al–ZnO with the different particle sizes and inter-particle distances. The obtained results may be a consequence of interaction of surface Plasmon resonances with the intra-band contributions to the nonlinear optical susceptibilities and to the interactions with the phonons.

Photoluminescence and Transparent Conductive Properties of the Al and Sb Codoped ZnO Thin Films Annealed in Different Atmospheres

Al and Sb codoped ZnO thin films were prepared through a sol-gel spin coating method on glass substrates and annealed in different atmospheres. The XRD results show that the films have hexagonal wurtzite ZnO structure and SEM results reveal that the films annealed in hydrogen consist of hexagonal nanorods with diameters of 84 nm and lengths of 422 nm, however the films annealed in other atmospheres without nanorods. The photoluminescence (PL) spectrum shows that the emission peaks of the films are mostly at 390 and 460 nm, and the film annealed in hydrogen has the strongest intensity of peak at 390 nm and the film annealed in air has the strongest intensity of peak at 460 nm. The electrical properties show that the films annealed in hydrogen have a lowest resistivity of 1.02×10-3 Ω•cm.

Photoluminescence and Transparent Conductive Properties of the Al and Sb Codoped ZnO Thin Films Annealed in Different Atmospheres

Photoluminescence and Transparent Conductive Properties of the Al and Sb Codoped ZnO Thin Films Annealed in Different Atmospheres

Effect of annealing temperature on the quality of Al-doped ZnO thin films prepared by sol–gel method

The different thermal expansion coefficients and lattice mismatch between ZnO and Al may produce residual stress in Al-ZnO (AZO) thin films. Annealing processes can be applied to modulate this residual stress. In this study, three different rapid thermal annealing (RTA) temperatures (350, 450, and 600 °C) were applied to an AZO thin film, prepared using sol–gel method. The mechanical properties, optical properties, and structure of the AZO thin film were investigated experimentally. The results show that increasing the RTA temperature increased the Young’s modulus and hardness of the films. The grain size of the films also increased with increasing RTA temperature. However, the film thickness and shear stress component decreased with increasing RTA temperature. Both compressive and tensile stress decreased gradually with increasing film thickness after RTA treatment. It was demonstrated that the use of a relatively high RTA temperature can effectively relax the residual stress in AZO thin films.

Effects of metal contacts and dopants on the performance of ZnO-based memristive devices

We have performed a detailed investigation of the fabrication and switching characteristics of ZnO-based memristive devices. The effects of doping and various metal contacts have been studied. It is observed that, with the use of Al metal contacts and ZnO:Al layers, relatively high ROFF/RON ratios of 200 can be achieved. Much improved device stability and reproducibility, on the other hand, can be realized by using either nominally undoped or Mg-doped ZnO memristive layers. The experimental results may be well explained by an oxygen vacancy formation and migration model. The present work represents an important step in realizing high performance ZnO-based memristive devices for future nonvolatile memory applications.

Effect of Interface Regions on the Thermoelectric Properties of Alternating ZnO/ZnO:Al Stripe Structures

A series of bar-shaped samples consisting of lateral arrangements of alternating ZnO:Al and ZnO stripes was fabricated by radiofrequency (RF)-sputtering and microfabrication techniques on glass substrates. Throughout the series, the number of interfaces between ZnO and ZnO:Al was varied whilst the material fractions of ZnO:Al and ZnO within the bars were not altered. Lateral thermoelectric transport parameters, i.e., Seebeck effect and electrical resistivity, were measured as a function of temperature for all microstructured samples and two reference samples of ZnO:Al and undoped ZnO. The transport direction through the bar was perpendicular to the stripe direction, such that the electrons and phonons have to pass all interfaces. The transport coefficients of the microstructured samples show clear dependence on the number of interfaces between ZnO and ZnO:Al. Thermoelectric measurements, photoluminescence, and Raman measurements indicate that this is due to diffusion of Al donors along the grain boundaries into the undoped ZnO stripes, which takes place during the fabrication process. Modeling of the dependence of the Seebeck coefficient and the resistivity of the series of samples on the basis of a network model accounting for donor diffusion supports these findings.

Mechanism of forming (2 2 0/2 0 4)-oriented CuInSe 2 film on Al:ZnO substrate using a two-step selenization process

A (2 2 0/2 0 4)-preferred CuInSe 2 (CIS) film formed using a two-step selenization process is reported, and the growth mechanism is explained. The CIS (2 2 0/2 0 4)-oriented film was grown on an Al:ZnO (AZO) coated glass for superstrate-type solar cell applications. The selenization temperature, Se vapor pressure, and reactive mechanisms of each selenization step were investigated. The first-step selenization at 400 °C favors the CIS (1 1 2) growth as the selenization time increases. For the second-step selenization, a high temperature (≧550 °C) and high Se vapor pressure throughout the process have a strong influence in promoting the CIS (2 2 0/2 0 4) growth. The oxygen in the self-formed In 2O 3 layer at the AZO interface can be replaced by selenium, and transforms to an In 2Se 3 (3 0 0)-preferred film, which favors the CIS (2 2 0/2 0 4) formation, in a transient and high Se vapor pressure selenization process. A Cu-rich surface, which is the usual case for selenizing precursor and which favors the CIS (1 1 2) growth, can be optimized to promote the CIS (2 2 0/2 0 4) growth by adding a thin In layer onto a slightly Cu-rich Cu/In precursor.

Variations in electrical and physical properties of Al:ZnO films with preparation conditions

This study examined the effects of the RF magnetron sputtering power and substrate temperature on the electrical and physical properties of Al:ZnO (AZO) thin films deposited on GaAs(011) and Si(001) substrates. The stress on the films, which was estimated by determining the position of the (002) XRD diffraction peaks, varied from 5.28 GPa to 2.29 GPa for the films deposited between 100 and 250 W RF power. A similar trend was observed for the films prepared at substrate temperatures ranging from R.T. to 300 °C. The films prepared at 100 °C showed the least amount of stress and the largest concentration of charge carriers. The concentration of charge carriers produced from the presence of [AlZn] is closely related to the stress on the films. Post-deposition annealing in a reducing atmosphere had not only decreased the compressive stress, but had also formed oxygen vacancies. The increased concentration of charge carriers after annealing was attributed to the ionization of oxygen vacancies with a probability of 0.1±0.03.

Processing Parameters and Property of AZO Thin Film Prepared by Magnetron Sputtering

As we all know, transparent and conductive Al:ZnO (AZO) films are being considered for manufacturing transparent electrodes in flat panel displays, solar cells and organic light emitting diodes due to their high electro optical quality, high material availability and low material cost for large area applications. In this experiment, AZO thin films were prepared on glass substrate by Magnetron Sputtering method with a 2wt % Al -doped ceramic target .The processing parameters on electrical properties of AZO thin film were investigated. When processing pressure is 0.8Pa,the sputtering power is 225W,the temperature of substrate is 225°, the AZO thin films have a lowest resistivity of 9.81×10−4 Ω.cm and visible light transmittance of more than 85%.

Influence of hydrogen on the properties of Al and Ga-doped ZnO films at room temperature

Low resistivity and high transmittance of Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) transparent conductive thin films have been achieved by use of pulsed dc magnetron sputtering in a hydrogen environment at room temperature. The addition of hydrogen to the sputtering gas can reduce the resistivity of the films and improve their electrical properties compared to those prepared without H2, because the hydrogen acts a shallow donor. The average transmittance was over 85% in the visible region, and the lowest resistivity of the AZO and GZO films was 4.01×10(-4) (Ω-cm) and 4.39×10(-4) (Ω-cm), respectively.

Enhanced Optical Output Power of Tunnel Junction GaN-Based Light Emitting Diodes with Transparent Conducting Al and Ga-Codoped ZnO Thin Films

High quality Al and Ga-codoped ZnO (AGZO) thin films were successfully deposited both on sapphire substrates and GaN-based light emitting diodes (LED) with a tunnel junction (TJ) layer by using the radio frequency magnetron sputtering technique at room temperature. The AGZO thin films grown on sapphire substrate showed high transparency (96.3% at 460 nm) and low resistivity (6.8 ×10-4 Ω cm). The AGZO thin films deposited on the GaN-based LED with a TJ layer exhibited weak ohmic behavior although it improved slightly with the annealing. The optical output power of the TJ GaN-based LED with an AGZO transparent conducting layer was about 12.6 mW at 20 mA, and the external quantum efficiency was 23.0%. These values are approximately 1.7 times larger than that of the TJ GaN-based LED with a conventional Ni/Au layer.

Magnetotransport properties of high quality Co:ZnO and Mn:ZnO single crystal pulsed laser deposition films: Pitfalls associated with magnetotransport on high resistivity materials

The electrical resistivity values for a series of pure and doped (Co, Mn, Al) ZnO epitaxial films grown by pulsed laser deposition were measured with equipment designed for determining the direct current resistivity of high resistance samples. Room-temperature resistances ranging from 7 x 10(1) to 4 x 10(8) Omega/sq were measured on vacuum-reduced cobalt-doped ZnO, (Al,Co) co-doped ZnO, pure cobalt-doped ZnO, Mn-doped ZnO, and undoped ZnO. Using a four-point collinear geometry with gold spring-loaded contacts, resistivities were measured from 295 to 5 K for resistances of < approximately 10(12) Omega/sq. In addition, magnetoresistance and Hall effect were measured as a function of temperature for select samples. Throughout the investigation, samples were also measured on commercially available instrumentation with good agreement. The challenges of transport measurements on high resistivity samples are discussed, along with some offered solutions to those challenges.

Transmission electron microscopy of solution‐processed, intrinsic and Al‐doped ZnO nanowires for transparent electrode fabrication

A solution-based chemistry was used to synthesize intrinsic and Al-doped (1% and 5% nominal atomic concentration of Al) ZnO nanostructures. The nanowires were grown at 300 degrees C in trioctylamine by dissolving Zn acetate and Al acetate. Different doping conditions gave rise to different nanoscale morphologies. The effect of a surfactant (oleic acid) was also investigated. An electron microscopy study correlating morphology, aspect ratio and doping of the individual ZnO wires to the electrical properties of the spin coated films is presented. HRTEM revealed single crystalline [0001] wires.

Electrical resistivity and transmittance properties of Al and Ga-codoped ZnO thin films

Films were deposited on glass substrates by sputtering gallium-doped zinc oxide (GZO) (Ga2O3: 90 wt%, ZnO: 10 wt%) and aluminum-doped zinc oxide (AZO) (Al2O3: 3 wt%, ZnO: 97 wt%) targets simultaneously using a direct-current (DC)/radio-frequency (RF) magnetron cosputtering system. The concentration of gallium (Ga) in the film was varied by using different RF powers for sputtering the GZO target with the DC power for sputtering the AZO target fixed. A minimum resistivitywas obtained at an RF sputtering power of 200 W for the GZO target when the DC sputtering power for the AZO target was fixed at 60 W. It was found that the resistivity of AZO thin films (6.40 × 10−3 Ω-cm for 0 W) could be lowered by more than one order by cosputtering AZO and GZO targets to make AGZO thin films (2.14 × 10−3Ω-cm for 200 W) without lowering their transmittance at all. In addition, the origin of the enhancement in the electrical property of the AZO by additional Ga doping is discussed.

Low temperature Al and B-codoped ZnO transparent conductive oxide films deposited as a function of the deposition power

Low temperature Al and B-codoped ZnO transparent conductive oxide films deposited as a function of the deposition power

Hall mobilities of Al- and Ga-doped ZnO polycrystals

This work focuses on the Hall mobilities ( μ) of Al-doped ZnO (Al–ZnO) and Ga-doped ZnO (Ga–ZnO) polycrystals. Their Hall mobilities show opposite behavior as a function of the carrier concentration ( n). The potential barrier of Al–ZnO is responsible for the positive dependence in the plot of μ vs. n, while the negative slope observed in Ga–ZnO suggests scattering dominated by charged defects. The near-band-edge photoluminescence (UV, 375 nm) as a function of carrier concentration shows higher photoluminescence for Ga–ZnO compared with Al–ZnO, which also implies a larger potential-barrier effect for Al–ZnO. Such a distinctive difference in the potential-barrier effect also reasonably explains the dissimilarity in the temperature dependence of the electrical resistivity of the samples, demonstrating the control of carrier transport through the potential barrier by dopant for ZnO polycrystals.