Transparent Conductive Al-doped ZnO Research Articles

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

Improvement of the uniformity of structural and electrical properties of transparent conductive Al-doped ZnO thin films by inductively coupled plasma-assisted radio frequency magnetron sputtering

Transparent conductive Al-doped Zinc oxide thin films were deposited on glass substrates by conventional radio frequency magnetron sputtering (C-RFMS) and inductively coupled plasma-assisted radio frequency magnetron sputtering (ICP-RFMS) methods. The structural and electrical properties of films were studied as a function of the radial position (r) of the substrate in both methods. A good thickness uniformity was obtained in ICP-RFMS, while a significant decrease in film thickness was observed at r=30mm (erosion zone) in C-RFMS. The c-axis orientation was achieved in all positions in ICP-RFMS, while crystallinity was degraded at r≥25mm in C-RFMS. Low resistivity ρ of the order of 10−4Ωcm was obtained at all positions in ICP-RFMS, while ρ was rapidly increased at r≥30mm in C-RFMS. In ICP-RFMS, high-energy ion bombardment is suppressed due to the modified plasma density profile and lower space potential. As for transparency, the sample prepared at r=0mm in ICP-RFMS showed an average visible transmittance of 81%, higher than that in C-RFMS. Throughout the study, it was demonstrated that ICP-RFMS method was promising to improve the radial distribution of structural, electrical, and optical properties of Al-doped ZnO thin films.

Ultraviolet and visible photo-response of transparent conductive Al-doped ZnO (AZO)/n-Silicon isotype heterojunction device

In this work, the electrical and photoresponse measurements of a transparent conductive Al-doped ZnO (AZO)/n-Si heterojunction device were conducted in visible light and UV wavelengths. AZO film was deposited by sputtering onto an n-Si wafer and investigated by means of morphological, chemical and electrical characterizations. The AZO/n-Si rectifying device exhibits an excellent reproducibility without noticeable variations after 90 days of measurements. At self-powered mode, the maximum on/off ratios were determined as 3081 for visible light and 4778 for UV light illumination of 365 nm. The responsivity and detectivity of the AZO/n-Si photodetector were 0.128 A W−1 and 1.05 × 1011 Jones for 365 nm, whereas they were 0.055 A W−1 and 4.60 × 1010 Jones for 395 nm, respectively (at −2.0 V). This study demonstrated that the n-AZO/n-Si isotype heterojunction photodetector was fabricated at low cost and it is a potential candidate in both the visible region and the UV region with a good performance, in contrast to the widely studied pn heterojunctions.

High-rate reactive high-power impulse magnetron sputtering of transparent conductive Al-doped ZnO thin films prepared at ambient temperature

Reactive high-power impulse magnetron sputtering was used for high-rate (deposition rate of 60 nm/min) deposition of conductive (resistivity of 3 × 10−3 Ωcm) and optically transparent (extinction coefficient at the wavelength of 550 nm of 0.01) ZnO:Al thin films at ambient temperature (< 40 °C). We used planar Zn:Al target (3.09 at.% of Al) with diameter of 100 mm and thickness of 6 mm mounted on strongly unbalanced magnetron. The films were deposited in argon‑oxygen atmosphere at constant argon and oxygen partial pressure of 2.0 Pa and 0.1 Pa, respectively. An average target power and voltage pulse length were kept constant to 1.9 Wcm−2 and 200 μs, respectively. A pulse-averaged target power density was varied in the range of 190 Wcm−2–940 Wcm−2. Optical emission spectroscopy was used for better understanding of correlations between plasma discharge properties and structural, electrical and optical properties of prepared films.

Erratum: “Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films” [J. Appl. Phys. 124, 065304 (2018)

Erratum: “Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films” [J. Appl. Phys. 124, 065304 (2018)

Effects of Al Diffusion Process on the Electrical, Optical, and Structural Properties of Transparent Conducting Al-doped ZnO

The effects of post-thermal diffusion of Al into heavily Al-doped ZnO (AZO) on its electrical, optical, and structural properties were investigated. Notably, the thermal diffusion of Al led to a significant enhancement of the optical transmittance-e.g., the transmittances at 550 nm were 90.4% and 96.7% before and after Al thermal diffusion, respectively. The structural analysis revealed that the improved optical transmittance was due to the combined effects of improved crystallinity, smoother surface, and the presence of transparent Al-O. However, the electrical properties of the AZO films after Al thermal diffusion were degraded-e.g., the sheet resistance was increased by 24% after thermal diffusion, which might be attributed to the generation of internal defects due to the heavy Al doping.

Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films

We investigated the effects of the erosion zone of magnetron sputtering (MS) targets on the deposition rates of magnetron-sputtered Al-doped ZnO (AZO) polycrystalline films and on the structural and electrical properties of the resulting AZO films. We deposited AZO films on glass substrates at a substrate temperature of 200 °C, which were placed parallel to the target surface, by radio frequency (RF), direct current (DC) or RF-superimposed DC (RF/DC) MS. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. The structural and electrical properties were investigated by X-ray diffraction and Hall-effect measurements, respectively. For RF-magnetron-sputtered AZO films, the results of the electron probe microanalyzer and photo luminescence measurements show that the effects of the erosion zone of the targets on the properties were very small at any given substrate position, resulting in small differences in the structural and electrical properties among the AZO films grown at different substrate positions. In the case of DC-magnetron-sputtered AZO films, residual damage owing to recoiling Ar atoms was found, particularly for the films grown at substrate positions in the area opposite to the erosion zone of the targets. This gives rise to the formation of nonradiative recombination centers, low carrier concentration, and high contribution of grain boundary scattering to carrier transport owing to poor alignments between columnar grains, resulting in low Hall mobility. In this study, we prove that the use of the RF/DC MS technique is effective in reducing the amount of residual damage.

Performance enhancement of Cu-based AZO multilayer thin films via graphene fence engineering for organic solar cells

Performance enhancement of flexible transparent conductive Cu-based Al-doped ZnO (AZO) multilayer thin films has been achieved by fence engineering of graphene film. The multilayers show the excellent performance (figure of merit of 3.38 × 10−2 Ω−1, commutation quality factor of 430): sheet resistance of 4.37 Ω/sq., resistivity of 3.98 × 10−5 Ω cm, and average transmittance of above 82% in the visible range, which are the highest values obtained up to now in the flexible transparent conductive films. The commutation quality factor is about four times larger than that of indium tin oxide, the most widely used flexible transparent conductive films today. To assess the application potentials, flexible organic solar cells (FOSCs) are achieved, whose efficiency is 4.63%, which is increased by 95% and 26% compared with the reference FOSCs used a conventional ITO and Cu-based AZO multilayer electrodes, respectively.

Optimization design of transparent conductive Al-doped ZnO and mechanism for performance-enhancing GaN light emitting diodes

The current crowding effect is one of the key factors that reduce the performance of light emitting diodes (LEDs). The transparent Al-doped ZnO (AZO) film, which was employed as a current spreading layer for a GaN-based blue LED, had been prepared and optimized by adjusting deposition parameters. Experimental results showed an obvious enhancement of the electroluminescence performance of the LEDs by using the highly transparent and low-resistance AZO layer. The mechanisms were discussed on the basis of the optical and electrical characteristics. The ANSYS simulation analysis proved that the optimized AZO layer weakened the current crowding effect in the GaN-based blue LED.

High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition

The effects of growth temperature on the microstructure, transport and optoelectronic properties of a series of Al-doped ZnO (AZO) films with thickness of ∼30 nm deposited on polished silicon-(100) and glass substrates by the atomic layer deposition (ALD) were investigated. By adopting an in-situ doping growth scheme the critical length effect associated with adjacent Al2O3 layers commonly encountered in previous ALD growth schemes was avoided and effective Al-doping was achieved with the growth temperature ranging from 100 °C to 300 °C. Experimental results showed that, in general, increasing the growth temperature would result in much improved film crystallinity and carrier mobility, with the average transmittance in the visible wavelength range being exceeding 95% in all cases. In particular, for AZO films grown at 300 °C, an unprecedented mobility of 136 cm2V−1s−1 was obtained, comparing to the typical values of 50–60 cm2V−1s−1 reported previously. The resistivity of these 300 °C films (ρ ≈ 6 × 10−4 Ω-cm), nevertheless, is slightly higher than that of some highly-doped ZnO (ρ ≈ 2–4 × 10−4 Ω-cm) prepared by sputtering methods. The secondary ion mass spectroscopy (SIMS) analyses revealed that hydrogen incorporation is the key in reducing the charge trap density and, hence, resulting in much enhanced carrier mobility. The present results promise a keen competitiveness of AZO with the indium tin oxide (ITO) film for thin-film-transistor (TFT) as well as in photovoltaic device applications.

Characteristics of Carrier Transport and Crystallographic Orientation Distribution of Transparent Conductive Al-Doped ZnO Polycrystalline Films Deposited by Radio-Frequency, Direct-Current, and Radio-Frequency-Superimposed Direct-Current Magnetron Sputtering.

We investigated the characteristics of carrier transport and crystallographic orientation distribution in 500-nm-thick Al-doped ZnO (AZO) polycrystalline films to achieve high-Hall-mobility AZO films. The AZO films were deposited on glass substrates at 200 °C by direct-current, radio-frequency, or radio-frequency-superimposed direct-current magnetron sputtering at various power ratios. We used sintered AZO targets with an Al2O3 content of 2.0 wt. %. The analysis of the data obtained by X-ray diffraction, Hall-effect, and optical measurements of AZO films at various power ratios showed that the complex orientation texture depending on the growth process enhanced the contribution of grain boundary scattering to carrier transport and of carrier sinks on net carrier concentration, resulting in the reduction in the Hall mobility of polycrystalline AZO films.

Improved performance of transparent-conducting AZO/Cu/AZO multilayer thin films by inserting a metal Ti layer for flexible electronics.

In order to improve the performance of transparent conductive Al-doped ZnO (AZO)/Cu/Al-doped ZnO multilayer thin films, multilayer thin films of titanium-embedded AZO/Cu/AZO structures have been designed and deposited onto flexible polycarbonate substrates to develop an indium-free transparent flexible electrode. The effect of Ti layer on the structural, optical, and electrical properties of multilayer thin films was investigated. The experiments reveal that the AZO/Ti/Cu/AZO fabricated in argon-oxygen mixtures has the best performance (commutation quality factor of 422.0): resistivity of 3.85×10-5 Ω·cm, sheet resistance of 4.31 Ω/sq, carrier concentration of 1.65×1022 cm-3, mobility of 9.87 cm2/Vs, and acceptable luminous transmittance of above 82% in the visible range.

(Invited) Photoluminescence Properties of ZnO Nanostructures Growth Vertically on Transparent and Conductive Al-Doped ZnO Substrates

ZnO nanostructures were fabricated on transparent conductive Al-doped ZnO substrates by a novel multi-annealing process in which the reducing annealing and oxidization annealing were carried out in turn. As the result, the fabricated ZnO nanostructures were aligned vertically on the Al-doped ZnO film under the optimizing annealing parameters during both reduction and oxidization reactions. The photoluminescence properties of ZnO nanostructures were influenced significantly by the annealing processes. An intense blue-green emission while a suppressed UV emission was observed from obtained ZnO nanostructures, as well as the high transmittance over 65%. The growth mechanism of the ZnO nanostructures was revealed in detail.

Biaxial stress and optoelectronic properties of Al-doped ZnO thin films deposited on flexible substrates by radio frequency magnetron sputtering.

Transparent conductive Al-doped ZnO (AZO) thin films were deposited on polyethylene terephthalate (PET) and polycarbonate (PC) substrates using radio frequency (RF) magnetron sputtering. The biaxial stress was measured with a double beam shadow moiré interferometer, and x-ray diffraction (XRD) was used to investigate the crystal orientation of ZnO. The substrate temperature was varied from room temperature to 150°C in steps of 25°C. The experimental results showed that the residual and shearing stresses increased with the increase in substrate temperature. The residual stress can be separated into principle and shearing stresses by Mohr's circle rule, and the shearing stress (tensile stress) was different from the compressive stress of the residual stress. However, the optimal substrate temperatures for PET and PC were 75°C and 100°C, and the shearing stresses were 424.82 and 543.68 MPa, respectively. AZO/PET and AZO/PC thin films cracked at substrate temperatures of 75°C and 100°C, respectively. AZO/PET thin film at a substrate temperature of 100°C had a resistivity low to the order of 10-3 Ω-cm.

(Invited) Photoluminescence Properties of ZnO Nanostructures Growth Vertically on Transparent and Conductive Al-Doped ZnO Substrates

ZnO nanostructures were fabricated on transparent conductive Al-doped ZnO films by a novel multi-annealing process in which the reducing annealing and oxidization annealing were carried out in turn to achieve the re-crystallization of ZnO film on AZO film. As the result, ZnO nanostructures with good alignment were fabricated vertically on the AZO film with the high transmittance over 80%. The photoluminescence properties of ZnO nanostructures were influenced significantly by the annealing processes. An intense blue-green emission centered at 491nm in the visible range was observed from obtained ZnO nanostructures. The growth mechanism of the ZnO nanostructures was investigated in detail.

Correlation between carrier transport and orientation evolution of polycrystalline transparent conductive Al-doped ZnO films

We have been developing a deposition technology to achieve Al-doped ZnO (AZO) polycrystalline films with a well-defined (0001) orientation. We propose the use of very thin critical layers (CLs) made from Ga-doped ZnO (GZO) films deposited by ion plating with direct-current arc discharge, which strongly affect the orientation of AZO films deposited by direct-current magnetron sputtering. The Al2O3 content in the sputtering target was 0.5wt.%. 500-, 200- and 100-nm-thick AZO films with CLs exhibited high-Hall-mobility values of 50.1cm2/Vs, 40.2cm2/Vs and 32.5cm2/Vs, respectively, compared with those of CL-free AZO films. The presence of CLs with a preferential c-axis orientation as interfaces between AZO films and glass substrates plays a critical role in producing AZO films having a textured polycrystalline structure with a well-defined (0001) orientation.

Investigation of Structural, Optical an Electrical Characterization of AZO Films Used for Solar Cells

Transparent conductive Al-doped ZnO (AZO) thin films used as transparent conductive oxide (TCO) electrode layers for thin film solar cells were deposited on quartz substrate by radio frequency (RF) magnetron sputtering. After deposition, the samples were annealed at various temperatures. The structural, electrical, and optical properties of these films have been analyzed as a function of the annealing temperature. All of these samples exhibit strong (002) diffraction peaks and the average transmittance in visible range is about 85%. The crystallinity of films together with some changes of the electrical resistance has been improved after thermal annealing. The best characteristics have been obtained at 400 °C, where the lowest resistivity is 2.4×10-3 Ω cm and the optical band gap is 3.22 eV.

Optimizing transparent conductive Al-doped ZnO thin films for SiNx free crystalline Si solar cells

Al-doped ZnO (AZO) thin films with resistivity of 6.6 × 10−4 Ω cm were prepared by introducing H2 in DC magnetron sputtering atmosphere. The Hall Effect measurement demonstrates that a part of the hydrogen atoms act as shallow donors while others act as neutral scattering impurities. Only proper H2 partial pressure decreases the resistivity to the lowest value. The properties of SiNx free crystalline Si (c-Si) solar cells were improved when AZO thin films were applied. The films served as both anti-reflection (AR) coatings and front electrode assistance. Ensuring an outstanding AR effect and a low sheet resistance, the thickness of the films was optimized in order to reduce the optical absorption, and thus effectively increase the short circuit current density (Jsc). The influence of the films versus different Ag fingers and emitter sheet resistance was investigated. The results showed that the Jsc was improved apparently (at least by 0.7 mA/cm2), while the fill factor was enhanced obviously from 62 to 70 % and from 73 to 75 %, respectively, which depended on the initial series resistance.

Investigation of the biaxial stress of Al-doped ZnO thin films on a flexible substrate with RF magnetron sputtering

Transparent conductive Al-doped ZnO (AZO) thin films were deposited onto poly(ethylene terephthalate) (PET) substrate, using the radio frequency (RF) magnetron sputtering method. The residual stress of flexible electronics was investigated by a double beam shadow moiré interferometer with phase shifting interferometry (PSI). Moreover, the biaxial stress of AZO thin films can be graphically represented by using Mohr’s circle of stress. The residual stress of AZO thin films becomes more compressive with the increase in sputtering power. The maximum residual stress is −1115.74 MPa, and the shearing stress is 490.57 MPa at a sputtering power of 200 W. The trends of residual stress were evidenced by the X-ray diffraction (XRD) patterns and optical properties of AZO thin films. According to the evaluation results of the refractive index and the extinction coefficient, the AZO thin films have better quality when the sputtering power less than 100 W.

Investigation of the thickness effect on material and surface texturing properties of sputtered ZnO:Al films for thin-film Si solar cell applications

Transparent conductive Al-doped ZnO (AZO) layers are widely used as the front electrode for thin-film silicon solar cells. For superstrate configurations, the front AZO layer simultaneously provides both electrical conductance and optical scattering. To improve the device performance, a textured surface is needed to enhance the photogeneration inside the solar cell through better light scattering. One approach is to wet-chemically etch the AZO films using hydrochloric (HCl) acid. In this work, AZO films with different initial layer thicknesses (900, 700 and 500 nm) are deposited onto soda-lime glass via magnetron sputtering and their texturing behaviours are compared. It is found that not only the material properties but also the surface texturing process greatly depends on the as-grown thickness of the AZO layer. The increased resistivity is mainly caused by a deteriorating carrier mobility as the thickness reduces. In terms of morphology, thick AZO films (i.e., 900 and 700 nm) show similar texture features after etching. In contrast, the thin AZO films (i.e., 500 nm) show irregularly shaped textures and over-etching, which leads to limited scattering. Hence the AZO layer thickness must be suitably chosen to achieve both good electrical conductance and optical scattering for high-efficiency thin-film Si solar cell applications.