Al-doped ZnO Layer Research Articles

Selective Void Deposition by Continuous, Ultrathin Ag Film Enabled Stable, High-Performance AgNWs-Based Transparent Heaters.

Metallic nanowire-based transparent conductors (MNTCs) are essential to various technologies, including displays, heat-regulating windows, and photo-communication. Hybrid configurations are primarily adopted to design stable, high-functioning MNTCs. Although hybrid MNTCs enhance electrical performance, they often suffer from optical degradation due to losses associated with the hybrid layers. Highly conductive hybrid MNTCs with minimal reduction in transparency are achieved with AgNWs/Ag(O)/Al-doped ZnO (AZO) design. The design provides a high visible light transmittance of 95.1%, representing a minimized optical loss of 3% compared to pristine AgNWs by optimizing optical interference between the AZO and Ag(O) layers. Furthermore, it allows for enhanced mobility of metallic nanowires by controlling the selective formation of conductive layers in the voids of the nanowire networks. The oxygen additive enables a continuous Ag ultrathin film of 6nm in the macro-voids of AgNWs system, corresponding to 25 times higher mobility for AgNWs/Ag(O)/AZO than that of sole AgNWs. The significant enhancement in the mobility of AgNWs/Ag(O)/AZO induces a reduction of sheet resistance of MNTCs by 73%. The AgNWs/Ag(O)/AZO, with an optimized sheet resistance of 24 Ω sq-1, is explored for transparent heater applications, demonstrating a fast thermal response with reliable stability, as evidenced by consistent high-temperature profiles during prolonged operation.

Highly effective passivation of crystalline silicon surfaces achieved through transparent conductive Al-doped ZnO layers

Highly effective passivation of crystalline silicon surfaces achieved through transparent conductive Al-doped ZnO layers

Influence of AZO buffer layer on characteristics of ZnO film on glass substrates for Schottky UV photodetector applications

An Al-doped ZnO (AZO) thin film was introduced into the structure of Ag/ZnO schottky Photo detectors (PDs) prepared by magnetron sputtering method on glass substrates. Influences of AZO layer on characteristics of ZnO film and performances of Ag/ZnO schottky PDs were discussed in detail. Results investigated by means of the scanning electron microscopy (SEM), X-ray diffraction (XRD) and Photoluminescence (PL) spectra show that the introduction of an AZO buffer layer can effectively decrease the defects and improve the crystallization of the ZnO thin film. Compared with Ag/ZnO schottky PDs, the Ag/ZnO/AZO PDs possess more excellent rectifying characteristics, the photocurrent (Iph) to dark current (Id) ratio are improved from 6.4 to 100.7. Given the bias voltage at -2 V, the photoresponsivity R and detectivity D* are increased from 0.15 A W-1 to 4.08 × 1010 Jones to 0.69AW-1 and 3.78 × 1011 Jones, respectively. The response speed becomes faster, the response and delay times are decreased from 281 to 395 ms to 178 and 300 ms, respectively. The results provide a new idea for the preparation of high performance ZnO film UV PDs on low-cost glass substrates.

Flexible epitaxial LiNbO3 thin film with ferroelectricity and nonlinear optical response

Flexible or wearable devices are of great interests in the modern technologies, and oxide thin films are critical components for such applications. In this work, epitaxial LiNbO3 (LNO) thin film integrated on flexible mica substrate has been achieved. Al-doped ZnO (AZO) layer has been first deposited as buffer layer to obtain the epitaxial LNO film, furthermore AZO serves as bottom electrode for electrical measurement. The as-deposited LNO film exhibits high epitaxial quality with c-direction growth. The transmittance measurement indicates the high transparency of the film, ～85 % in visible range and ～90 % in near infrared region up to 1500 nm. The second harmonic generation (SHG) measurement confirms the nonlinearity of the flexible LNO film. The LNO/AZO/mica film also presents apparent ferroelectric response. Overall, the high-quality LNO integrated on flexible mica could be potentially applied in wearable optical and electrical devices.

Impact of atomic layer deposition temperature on electrical and optical properties of ZnO:Al films

This work highlights the impact of growth temperature on the electrical and optical properties of Al-doped ZnO (AZO) films deposited by the atomic layer deposition (ALD) technique. The ALD process and super-cycle sequence have been optimized, identifying their influence on film resistivity. By using this optimum ALD procedure, the optical and electrical properties of AZO films have been widely analyzed considering the deposition temperature. Results show promising values with film resistivity in the range of 1 mΩcm and average optical absorption below 2% for 50 nm thick AZO layers. Hall effect, X-ray diffraction and ellipsometry measurements point out that these excellent values are related to their high carrier concentration and mobility, crystalline phase and optical band gap, resulting in ALD AZO films with excellent properties to be applied in photovoltaic devices as transparent conductive oxide electrode.

Highly sensitive hybrid silicon carbonitride piezoresistive sensors enabled by coupling piezoelectric effect

Piezoresistive sensors with high working temperature and excellent sensitivity are highly desired for in-situ measuring pressure in various high temperature applications. Polymer-derived silicon carbonitrides (SiCNs) are a novel class of materials with excellent stability and piezoresistive performance at high temperature, yet challenging to build to meet the rapid growth of large-area electronics. This work demonstrates a thin and highly sensitive hybrid SiCN piezoresistive films enabled by coupling piezoelectric effect of Al-doped ZnO (AZO). The hybrid thin films with the optimal nano-domain phase of SiCN features ultrahigh gauge factors of 6518 to 31,195 upon various compressive stresses, which are significantly larger than those of single-layer SiCN and other existing piezoresistive materials. First-principles calculations reveal that the charges from AZO layer accumulate and move toward the carbon clusters in SiCN under compressive stress, resulting in greater tunneling currents between carbon clusters and more significant piezoresistive effect. Moreover, superior piezoresistive performance of the optimal hybrid system can be characterized by atomic-scale binding energy and band gap change, as formation of the hybrid structure facilitates interfacial charge separation and increases density of electron at conduction band. This work opens an avenue for improving sensitivity of piezoresistive sensors by effective coupling of piezoelectric materials.

Enhancing the Efficiency of Inverted Organic Solar Cells with Treatment Techniques: Numerical and Experimental Study

The combination of UV-ozone and Al-doping enhancements in the ZnO ETL layer was studied for fullerene-based (P3HT:PCBM) and nonfullerene-based (J71:m-ITIC) solar cells. The study focused on morphological and interfacial defects and their relation to the electrical and optical properties of different Al-doped ZnO layers. The study also investigated the surface recombination velocity at the interface of the ZnO/active layer, as well as the carrier lifetime and diffusion length of the active layer theoretically. The most effective Al-doping dosage was 3% for P3HT:PCBM and 2% for J71:m-ITIC, attributed to the varying sensitivity of the active layer materials to morphological enhancement.

Fabrication and characterization of an AlGaN light emitting diode with Al-doped ZnO as a current spreading tunnel junction layer

We fabricated an AlGaN light emitting diode (LED) with a heavily Al-doped n-type ZnO layer on a p-type contact layer as a tunnel junction (TJ) to improve carrier injection into the LED. We characterized its electrical and optical properties and compared them to those of an AlGaN LED without ZnO. From the I–V characteristic of the LED with ZnO, we observed a threshold voltage of circa 2 V, which could be due to Zener breakdown of the type II heterostructure of n-ZnO/p-GaN as a TJ. From the electroluminescence measurement, we observed a similar emission peak in both AlGaN LEDs at ultraviolet (UV) wavelengths, but a broad emission band around 365 nm in the LED with ZnO. This emission could be originating from ZnO photoexcited by the UV LED emission. The dependence of these peak intensities on input currents shows that there is a monotonic increase in the light emission intensity for the UV LED emission, but a saturation behavior after the threshold voltage for the emission from the ZnO. This saturation behavior is attributed to an overflow of photoexcited electron–hole pairs into p-GaN, strongly suggesting that n-ZnO/p-GaN works as a TJ. Electroluminescence data also show that the presence of the ZnO film facilitates current spreading, which enables device operation at large input currents. Therefore, ZnO can work as a current spreading TJ layer and improve the performance of the AlGaN LED.

Al-doped ZnO based long range optical fibre sensor for efficient low refractive index detection

In this article, a simulation of surface plasmon resonance (SPR) based Al-doped ZnO (AZO) coated long-range low refractive index detections in infrared range sensor by the finite element method is presented. Plasmonic material Al-doped ZnO is used for SPR conditions in the desired range. The effects of AZO layer thickness on resonance wavelength, confinement loss, and sensitivity of the proposed sensor are examined for different analyte refractive indices. We have optimized coated AZO layer thickness of 90 nm and width of 124.70 µm in our work. The proposed infrared sensor has achieved refractive index sensitivity 2000–16,000 nm/RIU, and the resolution $$5.00\times {10}^{-5}-6.25\times {10}^{-6}\, \mathrm{RIU}$$ for the analyte’s refractive index range from 1.23 to 1.37. The proposed sensor may be utilized for detecting low refractive index organic chemicals, biomedical, and liquid foods and may also be used in other sensing applications.

Improvement of electrical characteristics of flexible AZO/Ag/Cu/AZO transparent conductive films by Cu deposition

Flexible electrodes are prepared by deposition on polyethylene naphthalate substrates, which melt at elevated temperatures, and are, therefore, generally unsuitable for deposition at high temperatures. However, only limited improvement in the conductivity can be achieved for Al-doped ZnO (AZO) films formed at low temperatures. Multilayer transparent conductive films (AZO/Ag/AZO), in which a conductive metal such as Ag is sandwiched between AZO, exhibit superior resistivity and electrical stability against bending compared to AZO films and have attracted considerable attention. In this study, AZO transparent conducting films were investigated as alternatives to indium tin oxide. The electrical characteristics of AZO/Ag/AZO films are not optimal at low temperatures owing to oxidation of Ag and its diffusion into the AZO layer. We, therefore, developed transparent conductive films with an AZO/Ag/Cu/AZO structure in which an intermediate Cu layer suppresses the oxidation of Ag and inhibits its diffusion into the substrate-side AZO layer, changing the deposition conditions of Cu. The optimal characteristics were obtained at a Cu deposition rate of 0.08 nm/s. A further increase in Cu layer thickness suppresses the oxidation of the Ag layer and its diffusion into the substrate-side AZO layer, thereby improving resistivity. Notably, a 5 nm thick Cu layer exhibited exclusive Cu regions, which further prevented the oxidation of Ag and its diffusion into the substrate side of the AZO layer, with a resistivity of 5.12 × 10−5 Ω cm. This resistivity is comparable to that of existing transparent conducting films used in practical applications; however, the transmittance of the AZO/Ag/Cu/AZO film was reduced owing to the low transparency of Cu.

Metafilms for visible and infrared compatible camouflage of high-temperature targets

With the rapid development of multispectral detection, infrared and visible compatible camouflage becomes necessary. Metafilms with dielectric/metal/dielectric (D/M/D) structures can be highly transparent in visible band (380 ∼ 780 nm) and highly reflective in infrared atmospheric windows (3 ∼ 5 μm, 8 ∼ 14 μm). The metafilm can be deposited on the equipment surface, and the high visible transmittance can make the original camouflage coating continue to achieve visible camouflage, while the low infrared emissivity can inhibit the infrared signal to achieve infrared camouflage. Compatible camouflage is urgently needed by high-temperature targets such as exhaust pipes and engine cabins. Therefore, the thermal stability of multilayer structure is very important. In this study, a D/M/D-structured metafilm with improved thermal performance is proposed. Al-doped zinc oxide (AZO) is selected as the material of the dielectric layers due to good thermal stability, and high visible transmittance is realized through the mechanism of admittance matching. Ag is selected as the material of the metal layer to increase infrared reflectance. The metafilm with the structure of AZO/Ag/AZO is rigorously designed and fabricated. The results from Fourier transform infrared spectrometer and spectrophotometer show that the integrated visible transmittance and infrared emissivity at room temperature is higher than 0.87 and lower than 0.05, respectively. The camouflage performance of the metafilm is demonstrated on a flexible polyethylene terephthalate (PET) substrate. The camouflage performance of metafilm samples at 20 ∼ 140 °C is tested on a model cabin. The metafilm does not affect the original camouflage coating, so it can achieve visible camouflage. The radiation temperature of the metafilm is approximately 80 °C lower than that of the control surface, and the infrared signature is significantly attenuated. In order to further investigate the thermal stability and thermal fatigue resistance of the metafilm, metafilm deposited on quartz substrate is continuously heated and periodically heated at different temperatures. It is found that the sample can withstand continuous heating at 450 °C for 4 h or repeated heating for 20 cycles. SEM (scanning electron microscope) and EDS (energy dispersive spectrometer) scanning shows that if heated at higher temperature or for more cycles, the AZO layer becomes blocky, and the proportion of Ag and O changes significantly. This leads to the decrease of visible transmittance and the increase of infrared emissivity of samples.

Impact of coherent core/shell architecture on fast response in InP-based quantum dot photodiodes.

Solution-processed, cadmium-free quantum dot (QD) photodiodes are compatible with printable optoelectronics and are regarded as a potential candidate for wavelength-selective optical sensing. However, a slow response time resulting from low carrier mobility and a poor dissociation of charge carriers in the optically active layer has hampered the development of the QD photodiodes with nontoxic device constituents. Herein, we report the first InP-based photodiode with a multilayer device architecture, working in photovoltaic mode in photodiode circuits. The photodiode showed the fastest response speed with rising and falling times of τ r = 4 ms and τ f = 9 ms at a voltage bias of 0 V at room temperature in ambient air among the Cd-free photodiodes. The single-digit millisecond photo responses were realized by efficient transportation of the photogenerated carriers in the optically active layer resulting from coherent InP/ZnS core/shell QD structure, fast separation of electron and hole pairs at the interface between QD and Al-doped ZnO layers, and optimized conditions for uniform deposition of each thin film. The results suggested the versatility of coherent core/shell QDs as a photosensitive layer, whose structures allow various semiconductor combinations without lattice mismatch considerations, towards fast response, high on/off ratios, and spectrally tunable optical sensing.

All-optical AZO-based modulator topped with Si metasurfaces

All-optical communication systems are under continuous development to address different core elements of inconvenience. Here, we numerically investigate an all-optical modulator, realizing a highly efficient modulation depth of 22 dB and a low insertion loss of 0.32 dB. The tunable optical element of the proposed modulator is a layer of Al-doped Zinc Oxide (AZO), also known as an epsilon-near-zero transparent conductive oxide. Sandwiching the AZO layer between a carefully designed distributed Bragg reflector and a dielectric metasurface—i.e., composed of a two-dimensional periodic array of cubic Si—provides a guided-mode resonance at the OFF state of the modulator, preventing the incident signal reflection at λ = 1310 nm. We demonstrate the required pump fluence for switching between the ON/OFF states of the designed modulator is about a few milli-Joules per cm2. The unique properties of the AZO layer, along with the engineered dielectric metasurface above it, change the reflection from 1 to 93%, helping design better experimental configurations for the next-generation all-optical communication systems.

Photocatalysis performance enhancement of Ag2O/Al-doped ZnO heterojunction by introducing ZnO nanorod array

Fabric-based nano semiconductor photocatalysts have attracted much attention in recent years. In this study, polyester fibers were modified by Ag2O/ZnO nanorod (NR)/Al-ZnO p-type/high resistivity/n-type (P-HR-N) heterojunction arrays. Al-doped ZnO (Al-ZnO) seed layer was reactively sputtered on polyester fibers. Al doping was used to change the work function (WF) of the ZnO film. The ZnO NR arrays were hydrothermally grown on the Al-ZnO seed layer and modified with Ag2O film. The work functions (WFs) of Ag2O and Al-ZnO layers were measured, resulting in 5.53 and 4.81eV, respectively. The WF difference between the two layers was 0.72eV. The electron-hole separation of the P-HR-N heterojunction array was characterized by surface photovoltage (SPV) spectra. The photocatalytic activities of the prepared samples were investigated by removing Rhodamine B (RB) under UV irradiation. The P-HR-N heterojunction sample displayed a better photocatalytic performance than the Ag2O/Al-doped ZnO PN heterojunction sample and ZnO NR/Al-ZnO array-coated sample. The enhanced photocatalytic performance of the P-HR-N heterojunction based sample was mainly attributed to the high and widened built-in electric field (BEF) in the multilayer film.

Performance Enhancement for Indium-Free Metal Oxide Thin-Film Transistors with Double-Active-Layers by Magnetron Sputtering at Room Temperature.

We prepared an indium-free metal oxide thin-film transistor (TFT) using a double-active-layers structure at room temperature. We changed the growth sequence of Al-doped zinc oxide (AZO) and zinc oxide (ZnO) double-active-layers on Si/SiO2 substrates by magnetron sputtering deposition to regulate the field-effect performance of TFTs. According to the analysis of field-effect properties before and after annealing in different atmospheres, the performance of TFT devices with ZnO/AZO/SiO2/Si double-active-layers was obviously better than that with single AZO or ZnO active layer and inverted AZO/ZnO/SiO2/Si double-active-layers in the device structure. The active layer with higher carrier concentration (AZO in this case) was closer to the dielectric layer, which was more favorable for carrier regulation in TFT devices. In addition, the annealed device had a lower on/off ratio (Ion/Ioff), easier-to-reach on-state, and higher mobility. Furthermore, the performance of the devices annealed under vacuum condition was obviously better than that annealed under air atmosphere. The Ion/Ioff could reach 6.8 × 105 and the threshold voltage was only 2.9 V.

Influence of the Al-Doped ZnO Sputter-Deposition Temperature on Cu(In,Ga)Se2 Solar Cell Performance.

Heterojunction Cu(In,Ga)Se2 (CIGS) solar cells comprise a substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al. Here, Al-doped zinc oxide (AZO) films were deposited by magnetron sputtering, and the substrate temperature was optimized for CIGS solar cells with two types of CIGS light absorbers with different material properties fabricated by three-stage co-evaporation and two-step metallization followed by sulfurization after selenization (SAS). The microstructure and optoelectronic properties of the AZO thin films fabricated at different substrate temperatures (150–550 °C) were analyzed along with their effects on the CIGS solar cell performance. X-ray diffraction results confirmed that all the deposited AZO films have a hexagonal wurtzite crystal structure regardless of substrate temperature. The optical and electrical properties of the AZO films improved significantly with increasing substrate temperature. Photovoltaic performances of the two types of CIGS solar cells were influenced by changes in the AZO substrate temperature. For the three-stage co-evaporated CIGS cell, as the sputter-deposition temperature of the AZO layer was raised from 150 °C to 550 °C, the efficiencies of CIGS devices decreased monotonically, which suggests the optimum AZO deposition temperature is 150 °C. In contrast, the cell efficiency of CIGS devices fabricated using the two-step SAS-processed CIGS absorbers improved with increasing the AZO deposition temperature from 150 to 350 °C. However, the rise in AZO deposition temperature to 550 °C decreased the cell efficiency, indicating that the optimum AZO deposition temperature was 350 °C. The findings of this study provide insights for the efficient fabrication of CIGS solar cells considering the correlation between CIGS absorber characteristics and AZO layer deposition temperature.

In situ monitoring atomic layer doping processes for Al-doped ZnO layers: Competitive nature of surface reactions between metal precursors

In this work, surface reactions during the atomic layer doping (ALDp) process of aluminum-doped zinc oxide (AZO) films have been studied. Conventional supercycle and alternative quasi-simultaneous codosing methods are analyzed within the 100–200 °C substrate temperature range. Two quasi-simultaneous codosing cases are investigated: (1) diethylzinc (DEZ) followed by trimethylaluminum (TMA) and (ii) TMA followed by DEZ. Quasi-simultaneous codosing experiments featured back-to-back DEZ/TMA or TMA/DEZ precursor and H2O pulses separated by nitrogen (N2) purge cycles. The grown films were characterized via (i) real-time in situ ellipsometry to monitor the individual surface ligand exchange reactions via variations in the film thickness in each half-cycle; (ii) ex situ ellipsometry to determine the film optical constants; (iii) x-ray photoelectron spectroscopy to measure the elemental composition and chemical bonding structure, and (iv) x-ray diffraction to evaluate the crystal properties. The most significant finding of the study is the dominance of TMA over DEZ: for all of the quasi-simultaneous codosing samples, no matter which precursor is pulsed first and whether there is a time delay between TMA and DEZ pulses or not, zinc (Zn) incorporation within the AZO films is substantially lower than aluminum (Al). This result demonstrates the competitive nature of surface reactions between TMA and DEZ, where the winning side is TMA. Al is effectively incorporating into the film while severely limiting Zn-incorporation and even replacing chemisorbed Zn-groups via conversion surface reactions. As a result, the quasi-simultaneous codosing approach for AZO films using DEZ and TMA precursors leads to minimally (less than 2%) Zn-doped Al2O3 films (ZAO), depicting the advantages of controlled ALDp process via the conventional supercycle method.

Transparent electrode of ITO/AZO DBR-diffuser structure to implement various and vivid colors for BIPV windows

For building-integrated photovoltaic windows, colored transparent hydrogenated amorphous silicon solar cells and modules were fabricated using a transparent electrode structure of a distributed Bragg reflector (DBR) and diffuser. The transparent DBR-diffuser electrode consists of multiple layers of indium tin oxide and Al-doped zinc oxide. This optically functional electrode not only implements various colors but also enhances color clarity. The transparent DBR-diffuser electrodes were designed using optical simulation and exhibited a wide color range of blue, green, and red by simply changing the thicknesses of the indium tin oxide and Al-doped zinc oxide layers. Using only eight pairs of DBRs, the electrode showed vivid colors and suppressed color noise by 23% with the diffuser structure. Because the color can be realized with a small amount of reflected light, the efficiency of transparent solar cells does not differ substantially in color. The developed transparent DBR-diffuser electrode can be widely applied to improve the color quality of both opaque and transparent solar cells and modules composed of various materials.

Influence of Al Doping on the Morphological, Structural and Gas Sensing Properties of Electrochemically Deposited ZnO Films on Quartz Resonators

The detection of hazardous gases at different concentration levels at low and room temperature is still an actual and challenging task. In this paper, Al-doped ZnO thin films are synthesized by the electrochemical deposition method on the gold electrodes of AT-cut quartz resonators, vibrating at 10 MHz. The average roughness, surface morphology and gas sensing properties are investigated. The average roughness of Al-doped ZnO layers strongly depends on the amount of the doping agent Al2(SO4)3 added to the solution. The structural dependence of these films with varying Al concentrations is evident from the scanning electron microscopy images. The sensing properties to ethanol and ammonia analytes were tested in the range of 0–12,800 ppm. In the analysis of the sensitivity to ammonia, a dependence on the concentration of the added Al2(SO4)3 in the electrochemically deposited layers is also observed, as the most sensitive layer is at 3 × 10−5 M. The sensitivity and the detection limit in case of ammonia are, respectively, 0.03 Hz/ppm and 100 ppm for the optimal doping concentration. The sensitivity depends on the active surface area of the layers, with those with a more developed surface being more sensitive. Al-doped ZnO layers showed a good long-term stability and reproducibility towards ammonia and ethanol gases. In the case of ethanol, the sensitivity is an order lower than that for ammonia, as those deposited with Al2(SO4)3 do not practically react to ethanol.

Breakthrough instruments and products: Investigation of atomic layer deposited Al:ZnO layer by spectroscopic ellipsometry from the deep-UV to the mid-IR in one instrument.

The Semilab SE-2000 spectroscopic ellipsometer is a versatile thin film characterization instrument capable of spectroscopic ellipsometry measurements covering a large spectral range from ultraviolet to near infrared within a few seconds and into the mid-infrared in a few minutes. It is suitable for characterizing thin films from monolayers to complex multi-layer laminates and bulk materials. This article demonstrates the unique capabilities of the SE-2000 system by the wide spectral range investigation of Al doped ZnO layers on different substrates and with different layer structures. Using data fits to the Drude dispersion law, the electrical properties of Al:ZnO were determined despite the presence of other conductive layers. The results were corroborated with four-point-probe measurements on a single Al:ZnO layer deposited on a glass substrate.