Deposition Of Al-doped ZnO Films Research Articles

Plasma characteristics of 355 nm and 532 nm laser deposition of Al-doped ZnO films

Abstract The plasma characteristics of 355 nm and 532 nm laser ablation of Al-doped ZnO (AZO) target were studied by optical emission spectroscopy and ion probe measurements. Zn emission lines were measured detected at above 0.9 J/cm 2 for 355 nm laser and 0.6 J/cm 2 532 nm laser respectively, while Al species were detected only above 2 J/cm 2 . The kinetic energy of the ions was slightly higher for 532 nm ablation as compared to 355 nm ablation. In addition, the ablation of 532 nm laser was affected by the large laser penetration depth. When deposited at 2 and 4 J/cm 2 , AZO films with energy band gap of 3.45–3.6 eV were obtained. Nanostructured AZO films were obtained by 355 nm laser ablation but nano and micro-particulates were formed in 532 nm laser ablation. The large micron-sized particulates were Al-rich thus affecting not only the morphology but also the stoichiometry of the films. It is thus concluded that despite a lower ablation and growth rate, 355 nm generated Zn, O and also Al species at lower threshold fluence that can lead to high quality AZO films at room temperature.

Effects of inclination angle during Al-doped ZnO film deposition and number of bending cycles on electrical, piezoelectric, optical, and mechanical properties and fatigue life

A flat plate with an adjustable inclination angle between the ion beam and the direction normal to a polyethylene terephthalate (PET) substrate is designed for a deposition stage. Five inclination angles are used to prepare PET/aluminum-doped zinc oxide specimens in order to examine the effects of inclination angle and number of bending cycles on several parameters. The O2 peak intensity ratio (IRO2), obtained from x-ray photoelectron spectroscopy, is identified as one of the controlling factors of the electrical, optical, piezoelectric, and mechanical properties. The effect of inclination angle on IRO2 and the effects of IRO2 on the peak intensity ratios, IRZnO (002) and IRZnO (103), obtained from x-ray diffraction analyses, the composite grain size, and the residual stress in the thin film are investigated systematically. An increase in the inclination angle increases the peak intensity ratios of IRO2 and IRAl2O3, and reduces that of IRZnO. An increase in IRO2 decreases the volume fraction of IRZnO (002) but increases the residual stress of the thin film, and is favorable for the reduction of the composite grain size. The carrier mobility and electrical resistance decrease and carrier concentration increases when IRO2 and IRAl2O3 increase or IRZnO decreases. An increase in the residual stress reduces the fatigue life and piezoelectric coefficient of specimens. The application of a nonzero inclination angle increases the transmittance integration (TI) over the wavelengths of visible light and the intensities of the three peaks in the photoluminescence spectrum. A reduction in film thickness leads to increases in the energy gap and electrical resistance and a decrease in TI.

Raman Spectroscopy of Aluminum-Doped Znic Oxide Thin Films Synthesized By Atomic Layer Deposition

Due to various applications in electronic and optical fields, considerable efforts have been made to develop ZnO based semiconducting materials. ZnO has a wide and direct band gap of 3.37 eV, large exciton binding energy (60 meV), high mechanical and thermal stabilities, and good electrical conductivity. Therefore, ZnO thin films are intensively investigated and applied in solar cells, piezoelectrical devices, gas and biological sensors, catalysts, and high-efficiency short-wavelength optoelectronic nanodevices. To further enhance the properties of ZnO, researchers have been synthesizing ZnO films doped with aluminum or indium. Al doped ZnO (AZO) thin films have been highly developed featuring non-toxicity, low material cost, good thermal stability, high optical transmittance, and good electrical conductivity. [1, 2, 3] Many techniques have been used to deposit AZO thin films currently, such as sol gel processing, magnetron sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among these film deposition methods, ALD has been extensively investigated for the deposition of thin films of metals, alloys, semiconductors, and oxides. Compared with other techniques, ALD is considered as a novel technique for the deposition of AZO films due to its characteristic feature of self-limiting surface saturating reactions in each ALD cycle. ALD technology consequently and accurately controls the layer thickness with unprecedented atomic level precision, as well as the stoichiometry, composition, uniformity of large areas and maintains excellent conformity over complex surface nanostructures. The other advantage of ALD technology is to introduce each ALD chemical precursor sequentially separated by time to react on the surface of the structure. The concentration of dopant in the ZnO films can be precisely controlled by the number of ALD cycles, so that the physical properties of AZO films, such as carrier density and conductivity, can be adjusted to meet the requirements of different applications. In this study, a Savannah 100 cross-flow thermal ALD reactor was used for the deposition of Al doped ZnO thin films. The precursors of aluminium, zinc and oxygen used in the ALD process for the synthesis of Al doped ZnO thin films were trimethylaluminium (TMA: Al2(CH3)6), diethylzinc (DEZ: (C2H5)2Zn) and DI water. The growth temperature of AZO thin films was maintained at 250 oC. The flow rate of N2, used as a carrier gas, was accurately controlled at 20 sccm. In order to deposit ALD AZO films with various percentages of aluminium, TMA cycles were introduced once in every 30 pulses of DEZ and H2O injection. After deposition, AZO films were annealed in air at 500 oC, 600 oC, and 700 oC for 30 min, respectively. X-ray diffraction (XRD) was performed to characterize the crystal structure of ALD ZnO and AZO films. Field emission scanning electron microscopy (FE-SEM) was used to inspect the morphology of AZO films before and after annealing. The surface roughness was analyzed by atomic force microscopy (AFM). The compositions of AZO thin films were analyzed by energy dispersive X-ray spectroscopy (EDS). The sheet resistivity of AZO films was determined by using four-point probe measurements. Raman spectroscopy and photoluminescence (PL) were used to investigate the properties and detect structures of ALD AZO films. The band gaps of AZO films prepared by the ALD process were analyzed from PL spectroscopy results. The materials property and defect structure of ALD AZO films were investigated by Raman spectroscopy.

Wet Chemical Etching of Al‐doped ZnO Film Deposited by RF Magnetron Sputtering Method on Textured Glass Substrate for Energy Application#

The etching of glasses in aqueous hydrofluoric acid (HF) solutions is applied in many technological fields. Particularly, the textured transparent conductive oxide materials on the glass substrate etched by HF were used to improve the current density of solar cells. In this study, the textured glass substrate has been etched by solution and the Al‐doped ZnO (AZO) thin films have been prepared on this textured glass substrates by RF magnetron sputtering method. After the AZO film deposition, the surface of AZO has been etched by hydrochloric acid with different concentration and etching time. Etched AZO thin films had higher haze ratio and sheet resistance than bare AZO glass. Increases in the root‐mean‐square surface roughness of AZO films enhanced from 53.78 to 84.46 nm the haze ratio in above 700 nm wavelength. Our process could be applicable in texturing glass and etching AZO surface to fabricate solar cell in industrial scale. We also carried out fabricating an organic solar‐cell device. Energy conversion efficiency improvement of 123% was obtained with textured AZO‐based solar‐cell device compared with that of nontextured solar‐cell device.

Pulsed laser deposition of Al-doped ZnO films on glass and polycarbonate

Al-doped ZnO (AZO) films were deposited on glass and polycarbonate (PC) at room temperature by using pulsed Nd:YAG laser at 355 nm. AZO thin films were obtained for both substrates at laser fluences from 2 to 5 J/cm2 in O2 partial pressure of 2.1 Pa. The effects of laser fluence on the structural, electrical, and optical properties of the films were investigated. The films with lowest resistivity and highest transmittance have been obtained at 2 J/cm2. The resistivities were 2.29×10−3 Ω cm for AZO on glass and 1.49×10−3 Ω cm for AZO on PC. With increasing laser fluence, the deposited films have lower crystallinity, higher resistivity, and smaller optical bandgap.

Optimization of sputtering parameters for deposition of Al-doped ZnO films by rf magnetron sputtering in Ar + H2 ambient at room temperature

The effects of sputtering pressure and power on structural and optical–electrical properties of Al-doped ZnO films were systemically investigated at substrate temperature of room temperature and H2/(Ar+H2) flow ratio of 5%. The results show that carrier concentration and mobility of the films show nonmonotone change due to the evolution of microstructure and lattice defect of the films caused by introduction of H2 with increasing sputtering pressure and power. The transmittance of the films is also found to be related to the introduction of H2 in addition to usually considered surface roughness and crystallinity. Finally, optimized sputtering pressure and power are 0.8Pa and 100W, respectively, and obtained minimum resistivity and highest transmittance are 1.43×10−3Ω·cm and 90.5%, respectively. In addition, it is found that Eg of the films is mainly controlled by the carrier concentration, but crystallite size and stress should also be considered for the films deposited at different powers.

RF 마그네트론 스퍼터링 방법을 사용해 증착된 Al이 도핑 된 ZnO 박막의 H2/(Ar + H2) 가스 비율에 따른 특성

The properties of Al-doped ZnO (AZO) films were investigated as a function of gas ratio using an AZO (2 wt% ) ceramic target in a radio frequency (RF) magnetron sputtering system. The deposition process was done at and in Torr working pressure and with various ratios of gas. During the AZO film deposition process, partial gas affected the AZO film characteristics. The electron resistivity () was lowest and mobility () was highest in AZO films when the gas ratio was 2.5 %. When the gas ratio was increased above 2.5 %, the electron resistivity increased and mobility decreased with increasing gas ratio in AZO films. The carrier concentration increased with increasing gas ratio from 0 % to 7.5 %. This phenomenon was explained by reaction of hydrogen and oxygen and additional formation of oxygen vacancy. The average optical transmission in the visible light wavelength region over 90 % and an orientation of the deposition was [002] orientation for AZO films grown with all gas ratios.

Enhancement of electrical properties in Al-doped ZnO films by tuning dc bias voltage during radio frequency magnetron sputtering

Al-doped ZnO (AZO) thin films were deposited at room temperature on glass substrates by rf magnetron sputtering with simultaneous dc bias through an external inductor coil. The deposition rates of AZO films deposited using simultaneous rf and dc power along with an inductor coil were 20% higher than those deposited using only rf power. The effects of simultaneous rf and dc bias voltage during the deposition of AZO films were investigated in terms of their resistivity and compressive stress. It was observed that the AZO films deposited at 120 W rf power with 600 μH inductor coil exhibit the lowest resistivity of 6.71 × 10−4 Ω⋅cm.

Atomic layer deposition of Al-doped ZnO films using ozone as the oxygen source: A comparison of two methods to deliver aluminum

Aluminum-doped ZnO films were prepared by atomic layer deposition at 250 °C using diethylzinc (DEZ), trimethylaluminum (TMA), and ozone as the precursors. Two deposition methods were compared to assess their impact on the composition, structural, electrical, and optical properties as a function of Al concentration. The first method controlled the Al concentration by changing the relative number of Al to Zn deposition cycles; a process reported in the literature where water was used as the oxygen source. The second method involved coinjection of the DEZ and TMA during each cycle where the partial pressures of the precursors control the aluminum concentration. Depth profiles of the film composition using Auger electron spectroscopy confirmed a layered microstructure for the films prepared by the first method, whereas the second method led to a homogeneous distribution of the aluminum throughout the ZnO film. Beneath the surface layer the carbon concentrations for all of the films were below the detection limit. Comparison of their electrical and optical properties established that films deposited by coinjection of the precursors were superior.

The Growth Mechanism of Al-doped ZnO using Oxygen Controlled Seed Layer in Si based Thin Film Solar Cells

We studied the growth mechanism of Al-doped ZnO (AZO) films using seed layers for enhancing the performance of transparent conducting oxides (TCOs) in the thin film solar cells. We carried out two-step processes for the deposition of AZO films. Seed layers were deposited on glass substrate as a function of Ar/O2 gas flow ratio. We show the results of our investigation on the micro-structural properties of AZO seed layers using transmission electron microscopy (TEM). The elemental composition and electronic structure changes with the deposition conditions were examined using energy dispersive X-ray (EDX) and reflective electron energy loss spectroscopy (REELS). The optical and electrical characteristics of AZO film using the seed layer with Ar/O2 = 9/1 show a high haze value of 88% at 500 nm and a resistivity value of 3.7 × 10 Ω·cm.

Atomic Layer Deposition of Al-doped ZnO Films: Effect of Grain Orientation on Conductivity

Al-doped ZnO (AZO) films were deposited by atomic layer deposition (ALD) on borosilicate glass and sapphire(0001) substrates. The Al composition of the films was varied from 1% to 4% by controlling the ratio of Zn:Al pulses. Film resistivity was measured as a function of Al content and the substrate temperature used for ALD deposition. X-ray diffraction (XRD) was performed on the films, showing a reduction in lattice parameter, as a function of Al concentration, indicating that Al3+ ions occupy substitutional sites in the ZnO lattice. The resistivity of films deposited on sapphire substrates (7.7 × 10−4 Ω cm) was lower than that on glass (3.0 × 10−3 Ω cm), because of the formation of textured grains with the c-axis aligned with respect to the sapphire surface, as confirmed by XRD. The surface morphology of the films on glass and sapphire was compared using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), which showed similar grain sizes on each substrate, suggesting that the dif...

Room temperature deposition of Al-doped ZnO films on quartz substrates by radio-frequency magnetron sputtering and effects of thermal annealing

High-quality Al-doped zinc oxide (AZO) thin films have been deposited on quartz substrates by radio-frequency magnetron sputtering at room temperature for thin film solar cell applications as transparent conductive oxide (TCO) electrode layers. Effects of post-deposition annealing treatment in pure nitrogen and nitrogen/hydrogen atmosphere have been investigated. Annealing treatments were carried out from 300 °C to 600 °C for compatibility with typical optoelectronic device fabrication processes. A series of characterization techniques, including X-ray diffraction, scanning electron microscopy, Hall, optical transmission, and X-ray photoelectron spectroscopy has been employed to study these AZO materials. It was found that there were significant changes in crystallinity of the films, resistivity increased from 4.60 × 10 − 4 to 4.66 × 10 − 3 Ω cm and carrier concentration decreased from 8.68 × 10 20 to 2.77 × 10 20 cm − 3 when annealing in 400 °C pure nitrogen. Whereas there were no significant changes in electrical and optical properties of the AZO films when annealing in 300–500 °C nitrogen/hydrogen atmosphere, the electrical stability of the AZO films during the hydrogen treatment is attributed to both desorption of adsorbed oxygen from the grain boundaries and production of additional oxygen vacancies that act as donor centers in the films by removal of oxygen from the ZnO matrix. These results demonstrated that the AZO films are stably suited for TCO electrodes in display devices and solar cells.

Deposition of Al-doped ZnO films on polyethylene naphthalate substrate with radio frequency magnetron sputtering

100 nm Al-doped ZnO (AZO) thin films were deposited on polyethylene naphthalate (PEN) substrates with radio frequency magnetron sputtering using 2 wt.% Al-doped ZnO target at various deposition conditions including sputtering power, target to substrate distance, working pressure and substrate temperature. When the sputtering power, target to substrate distance and working pressure were decreased, the resistivity was decreased due to the improvement of crystallinity with larger grain size. As the substrate temperature was increased from 25 to 120 °C, AZO films showed lower electrical resistivity and better optical transmittance due to the significant improvement of the crystallinity. 2 wt.% Al-doped ZnO films deposited on glass and PEN substrates at sputtering power of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa and substrate temperature of 120 °C showed the lowest resistivity (5.12 × 10 − 3 Ω cm on PEN substrate, 3.85 × 10 − 3 Ω cm on glass substrate) and high average transmittance (> 90% in both substrates). AZO films deposited on PEN substrate showed similar electrical and optical properties like AZO films deposited on glass substrates.

Sputter deposition of Al-doped ZnO films with various incident angles

Al-doped ZnO (AZO) films were sputter deposited on glass substrates heated at 200 °C under incident angles of sputtered particles at 0° (incidence normal to substrate), 20°, 40°, 60°, and 80°. In the case of normal incidence, x-ray diffraction pole figures show a strong [001] preferred orientation normal to the film surface. In contrast, in the case wherein the incident angles were higher than 60°, the [001] orientation inclined by 25°–35° toward the direction of sputtered particles. Transmission electron microscopy revealed that the tilt angle of the [001] orientation increased with increasing angle of the incident sputtered particles, whereas the columnar structure did not show any sign of inclination with respect to the substrate plane.

Room-temperature deposition of Al-doped ZnO films by oxygen radical-assisted pulsed laser deposition

Transparent and conductive Al-doped ZnO films have been deposited at room temperature by oxygen radical-assisted pulsed laser deposition. An average optical transmittance of more than 86% (0.7 μm film thickness) in the wavelength range 400–800 nm and resistivity as low as 5×10 −4 Ω cm were obtained from films deposited using a radical oxygen source. The surface roughness of the ZnO films measured by means of atomic force microscopy was found to be below 1 nm, suggesting that soft deposition techniques with low kinetic energy sources can make possible the deposition of high-quality transparent conducting ZnO films, even at room temperature.

Wavelength Dependence of Femtosecond Pulsed Laser Deposition of Zinc Oxide Films

We used 130-fs, 790-nm and frequency-doubled 395-nm laser pulses to study the wavelength dependence of femtosecond pulsed laser deposition of Al-doped ZnO films. The short wavelength pulses helped to increase deposition rates, decrease electrical resistivity of the films, and reduce droplets mixed into deposits. The transmission property of the films in the visible range was unchanged by the frequency doubling. The optical emission study showed a reduction of ion emissions and increase of kinetic energy when the 395-nm pulses were used.