MAGNETRON DEPOSITION OF TRANSPARENT AZO FILMS FROM COMPOSITE AND SINGLE-ELEMENT TARGETS

Characterization of the plasma plume produced by laser ablation from Al and Al2O3 targets was carried out on the basis of the line profile analysis of Al(I) (2P°–2S) emission. The spatial distribution and density parameters of electrons and Al atoms in the plume were obtained by comparing observed spectral line profiles with a theoretical calculation. The results showed different behavior for the Al and Al2O3 targets. The Al atoms from the Al2O3 target were populated in a smaller region than those from the Al target.

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

Transparent Conductors are essential components in many opto-electronic devices. Ultrathin Metal Films (UTMFs) represent an effective alternative to the ITO state-of-art. Their potential was demonstrated in organic solar cells with efficiencies comparable to those with ITO.

We report a novel strategy for the front passivation of solar cells via aluminum-doped zinc oxide (AZO) films in the case of CIGS solar cells, leading to the highest efficiency of 15.07% without alkali metal post treatment and anti−reflective layer. The good passivation of CIGS solar cells via AZO films is attributed to the field passivation simulated by the SCAPS−1D software. The AZO films also exhibit high optical transparency both in visible and near infrared wavelength region, high conductivity, and cost−effective fabrication advantage. Importantly, the AZO films are deposited at room temperature via radio−frequency magnetron sputtering, showing that the AZO films are also applicable to other solar cells such as perovskite solar cells. Our work is of significance for advancing the development of CIGS−based photovoltaics devices by the well front passivation of AZO. The wide application of AZO in other solar cells such as perovskite solar cells and related tandem solar cells may also accelerate the development of these solar cells because of potential passivation of AZO, low deposition temperature, and high optical transparency of AZO.

RF sputtered, aluminum-doped zinc oxide (ZnO:Al or AZO) is an attractive candidate material as transparent conductive oxides in the fabrication of opto-electronic devices. High electrical conductivity and optical transparency are two key requirements in such applications. This paper reports on the formation of AZO films on glass substrates in an RF-sputtering chamber modified to facilitate in situ heating during deposition. The influence of chamber pressure, RF power, and deposition temperature has been systematically studied and the electrical parameters such as film resistivity, carrier concentration, carrier mobility as well as optical transmission have been analyzed. Film deposition at 250 °C and a low chamber pressure of 0.5 mT resulted in a very low resistivity of 2.94 × 10−4 ohm cm. The structural properties of the films with the lowest resistivity have been further analyzed by x-ray diffraction (XRD) and PL measurements and are compared with the film deposited at room temperature. The XRD results show dominant peaks along (103) orientation for the AZO films with slightly improved crystal quality at higher temperature. Evolution of near band edge and deep level emission photoluminescence peaks also indicate improvement in crystal structure with increased deposition temperature.

In this paper a tunable optic system is presented for use in various optical systems. In contrast to most of the tunable optical components which are composed of a waveguide, an electro-optical layer, and electrodes of different materials. The new system consists of a TiO₂ waveguide with ZnO as a functional layer on top. The TiO₂ layer acts as a high index waveguide, the ZnO system consists of a ZnO:Al/ZnO/ZnO:Al sandwich structure. The ZnO film is used as an electro-optical cladding for the TiO₂ waveguide while the two ZnO:Al films act as transparent electrodes. Applying a voltage results in a shift of the effective refractive index of the waveguide because of the electro-optical effect of the ZnO. The TiO₂ film is deposited on SiO₂ by a PECVD-process from a metal organic precursor CpTiCh (cyclopentadienyl-cycloheptatrienyl-titanium). ZnO and ZnO:Al are rf-sputtered from a Zn target and ZnO:Al target, respectively. While both ZnO layers are c-oriented polycrystals, the TiO₂ grows in a nanocrystalline formation without any texture. The configuration of the high index material TiO₂ in combination with the transparent and electro-optical ZnO layer allows the use in integrated optical subsystems such as active couplers or active micro ring resonators. The system is designed for a wavelength of 1550 nm.

In this work, we present the effects of the Al2O3:ZnO ratio on the optical and electrical properties of aluminum doped ZnO (AZO) layers deposited by atomic layer deposition, along with AZO application as the anti-reflective coating (ARC) layer and in heterojunction configurations. Here, we report complex refractive indices for AZO layers with different numbers of aluminum atomic cycles (ZnO:Al2O3 = 1:0, 39:1, 19:1, and 9:1) and we confirm their validity by fitting models to experimental data. Furthermore, the most conductive layer (ZnO:Al2O3 = 19:1, conductivity ∼4.6 mΩ cm) is used to fabricate AZO/n+/p-Si thin film solar cells and AZO/p-Si heterojunction devices. The impact of the AZO layer on the photovoltaic properties of these devices is studied by different characterization techniques, resulting in the extraction of recombination and energy band parameters related to the AZO layer. Our results confirm that AZO 19:1 can be used as a low cost and effective conductive ARC layer for solar cells. However, AZO/p-Si heterojunctions suffer from an insufficient depletion region width (∼100 nm) and recombination at the interface states, with an estimated potential barrier of ∼0.6–0.62 eV. The work function of AZO (ZnO:Al2O3 = 19:1) is estimated to be in the range between 4.36 and 4.57 eV. These material properties limit the use of AZO as an emitter in Si solar cells. However, the results imply that AZO based heterojunctions could have applications as low-cost photodetectors or photodiodes, operating under relatively low reverse bias.

The emerging transparent thin-film transistor (TFTs) technology has recently received great attention in the active-matrix organic light-emitting diode (AMOLED) flexible displays. This TFT technology also has the potential to enable enhanced functionality in conventional large-area electronics applications ranging from smart windows and heads-up displays to flexible sensor arrays. During the past few years, amorphous transition-metal oxides have been suggested to be one of the best candidates in this application. Amorphous InGaZnO (IGZO) TFTs are the most promising transparent-metal oxide semiconductor due to its high optical transparency and field-effect mobility (μFE) in TFT devices compared to conventional amorphous silicon (a-Si:H). The amorphous structure is more suitable for large-area processes than other polycrystalline metal-oxide TFTs. These unique advantages have resulted in the rapid development of IGZO TFT technology. Given the relatively poor selectivity to common semiconductor etchants, IGZO TFTs are typically fabricated using lift-off processes to define the electrical contacts. However, the success of the emerging transition-metal oxide materials system for flexible electronics requires compatibility with conventional TFT microfabrication manufacturing processes. Flexible IGZO TFTs having 85% optical transparency in the visible regime were fabricated on polyethylene napthalate substrates using a selective wet etching process to eliminate the need for lift-off processing. The devices were processed directly onto the plastic platform at a maximum temperature of 150°C. The TFT structure consisted of a sputtered 100 nm aluminum doped zinc oxide (AZO) gate electrode, a 300nm SiNx gate dielectric deposited using plasma enhanced chemical vapor deposition (PECVD) method at 150ºC, and a 50nm IGZO layer deposited using RF sputtering at room temperature. A 150 nm AZO source/drain (S/D) electrodes were deposited by using RF sputtering at 150ºC and patterned wet etching process in diluted HCl solution having a selectivity of 11 between AZO and IGZO. The channel width and length of the studied TFTs were 200 and 100 µm, respectively. The fabricated TFTs exhibited a field-effect mobility of ~10 cm2/V.sec, threshold voltage of ~5.0 V, a sub-threshold swing of 0.8 V/decade, and an Ion/Ioff ratio of 107. No current crowding behavior was observed for the TFTs at the low drain-source voltage (VD) regime. The I-V characteristics showed excellent ohmic contacts can be fabricated using AZO S/D electrodes through a wet etching process and promises a more conventional approach to large-area flexible electronics fabrication. The highly selective etching process provides a means to fabricate IGZO based circuits with low processing complexity that will enable system-on-“plastic” integration of flexible transparent flexible displays. In addition, the effect of long-term electrical bias on the electrical stability of the devices will also be presented.

All transparent metal oxide photoelectric device was fabricated with structure of Ag nanowire/NiO/ZnO/AZO/FTO by magnetron sputtering system. In order to achieve p/n junction, p-type NiO was deposited onto the n-type ZnO layer. The AZO (Aluminium-doped zinc oxide) was applied as buffer layer for effective transport and collection of the photo-induced electrons. Under light illumination, electrons and holes are generated in the hetero-junction of p-type NiO and n-type ZnO. The AZO layer between ZnO and NiO layers induces the efficient carrier collection from the ZnO side to the negative electrode, due to the similar structure of AZO to ZnO. In addition, the AZO insertion layer is efficient to suppress the loss of hole carriers, resulting in the low leakage current value. The overall transparency of the Ag nanowire/NiO/ZnO/AZO/FTO device is about 70% for visible light range, and thus which can be applied in the invisible transparent electronics, including photodetectors and solar cells.

To figure out the effects of aluminum-doped ZnO (AZO) and Ag layers for the transparent electrode vertical (TEV) photoconductive semiconductor switch (PCSS) reported in our previous works, several different structures of high power, new vertical, extrinsic-triggered PCSS were designed and their performances are presented. All the PCSSs were fabricated on the vanadium compensated semi-insulating (VCSI) 4H-SiC substrates. The ON-state performance of all device versions was characterized by a test circuit without load resistor. Among all structures, the version with AZO transparent window, AZO subcontact layer, and silver mirror reflector has been testified to be the most reasonable structure, and shows the best results as well. Its minimum ON-state resistance is 6.11 Ω at the optical power density of 2.62 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The factor for the best ON-state performance of this structure is attributed to the improvement of utilization efficiency of laser energy, and good contact between the AZO and SiC substrate.

Aluminum-doped zinc oxide (AZO) was cosputtered from ZnO and Al targets on glass substrates at room temperature. The effect of Al-target power and annealing atmosphere on structural and optoelectronic properties of AZO films was investigated.[Al]/[Zn] ratio increased to 0.258 when the Al-target power increased to 250 W. At a fixed Al-target power of 200 W, the [Al]/[Zn] ratio was 0.104 and the as-deposited AZO film demonstrated a low resistivity of 3.19 × 10−4 Ω-cm and high transmittance of approximately 90% in the visible region. After annealing at 500 °C in forming gas (5% H2 in Ar) atmosphere, the resistivity of the AZO film can be further reduced to 9.38 × 10−5 Ω-cm. Given its low-temperature process and good optoelectronic properties, sputtered AZO films that use ZnO and Al targets have high potential in various optoelectronic devices.

Electrical properties of Al2O3 film deposited at low temperatures

Aluminum doped zinc oxide (AZO) films are prepared by rf magnetron sputtering on glass or Si substrates using specifically designed ZnO targets containing different amount of Al2O3 powder as the Al doping source. The structural, electrical, and optical properties of the AZO films are investigated in terms of the preparation conditions, such as the Al2O3 content in the target, rf power, substrate temperature and working pressure. The crystal structure of the AZO films is hexagonal wurtzite. The orientation, regardless of the Al content, is along the c axis perpendicular to the substrate. The doping concentration in the film is 1.9 at. % for 1 wt % Al2O3 target, 4.0 at. % for 3 wt % Al2O3 target, and 6.2 at. % for 5 wt % Al2O3 target. The resistivity of the AZO film prepared with the 3 wt % Al2O3target is ∼4.7×10−4 Ω cm, and depends mainly on the carrier concentration. The optical transmittance of a 1500-Å-thick film at 550 nm is ∼90%. The optical band gap depends on the Al doping level and on the microstructure of the films, and is in the range of 3.46–3.54 eV. The optical band gap widening is proportional to the one-third power of the carrier concentration.

Influence of magnetic field configuration on plasma characteristics and thin film properties in dual magnetron reactive high power impulse magnetron sputtering discharge with Al in Ar/O2 mixture

The paper will discuss the effect of pulsing single or two unbalanced magnetrons in a closed magnetic field configuration on the structure and properties of tribological Cr-Al-N coatings. Nanocrystalline Cr-Al-N coatings were reactively deposited from Cr and Al elemental targets using two unbalanced magnetrons, which were powered in both dc, pulsing only Al target and asynchronously pulsing both Cr and Al targets at 100 kHz and 50% duty cycle conditions. The ion energy distributions of these deposition and pulsing conditions were characterized using a Hiden Electrostatic QuadruPole Plasma Analyzer. It was found that pulsing two magnetrons asynchronously at 100 kHz and 50% duty cycle produced higher ion energies and significant increased ion fluxes than pulsing none or pulsing only one (Al) target. The structure and properties of Cr-Al-N coatings synthesized under different dc and pulsing conditions were investigated using X-ray diffraction, scanning electron microscopy, nanoindentation and ball-on-disk wear test, and were correlated with the effects of ion energies and ion flux regimes observed in the plasma diagnostics. The advantages of using pulsed magnetron sputtering producing different energetic ion regimes to enhance the ion bombardment on the growing films and therefore achieving the improved density, refinement of grain size and properties are illustrated.