As-deposited Al-doped ZnO Films Research Articles

Influence of oxygen vacancies on surface charge potential and transportation properties of Al-doped ZnO nanostructures produced via atomic layer deposition

Abstract High quality Al-doped ZnO (AZO) films are advantageous for many applications. Their properties can be tuned by controlling the doping concentration and intrinsic defect density. In this work, high-quality AZO films have been synthesized using the atomic layer deposition (ALD) technique. Throughout the doping cycle and post-annealing treatments under various atmospheres, the oxygen-related vacancies were controlled. The effect of oxygen vacancies on the charge transportation and surface potential were studied. The O 1s X-ray photoelectron spectrometry (XPS) spectra of the AZO film were deconvoluted into three components related to the O 2 − species, corresponding to the oxygen in the ZnO lattice ( O L ); oxygen vacancies or defects ( O V ); and chemisorbed or dissociated ( O C ) oxygen species. In the case of the as-deposited films and films annealed under various atmospheres, a co-relationship between the O V and mobility ( μ ) can be determined. In the case of the N 2 -annealed film, the O V fraction is at its maximum value, while the other components are at their minimum values. Among the as-deposited films, the maximum O V fraction is obtained when a Al:Zn ratio ( R Al/Zn ) of 7% is used. When R Al/Zn = 7%, the μ value of the as-deposited AZO film is enhanced from 12.1 cm 2 V −1 s −1 ( R Al/Zn = 3%), to 18.5 cm 2 V −1 s −1 . It shows its potential application as photoanode. The Hall Effect and the XPS analysis of the film reviled a co-relationship between O V and μ with the Al-doping concentration or post annealing atmosphere. Kelvin probe atomic force microscopy (KPFM) was used to evaluate the surface charge potentials of the films. The N 2 -annealed AZO film with R Al/Zn = 3% ( AZO-3-N 2 ) exhibited the maximum negative potential (−115.79 mV); however, the film with R Al/Zn = 7% ( AZO-7-N 2 ) exhibited the maximum positive potential (797.23 mV). Hence, the properties of these films may directly pertain to the bit readout signal and reliability of charge storage and memory applications.

Fabrication of Al-Doped ZnO Film with High Conductivity Induced by Photocatalytic Activity

We have fabricated Al-doped ZnO films by a spin-spray method, achieving high conductivity by Al-ion doping and photocatalytic activity of the ZnO. The surface morphology of the as-deposited films was varied by changing the Al concentration and addition of citrate ions. As-deposited Al-doped ZnO film without citrate ions showed rod array structure with increasing rod width as the Al concentration was increased. Meanwhile, Al-doped ZnO film deposited with addition of citrate ions changed to exhibit dense and continuous surface morphology with high transmittance of 85%. The lowest resistivity recorded for undoped and Al-doped ZnO film was 2.1 × 10−2 Ω cm and 5.9 × 10−3 Ω cm, after ultraviolet (UV) irradiation. The reason for the decreased resistivity is thought to be that Al-ion doping and the photocatalytic activity of ZnO contributed to improve the conductivity.

Bifacial 8.3%/5.4% front/rear efficiency ZnO:Al/n-Si heterojunction solar cell produced by spray pyrolysis

Using an as-deposited Al-doped ZnO (AZO) film synthesized by ultrasonic spray pyrolysis directly onto crystalline silicon (c-Si), we demonstrate a bifacial AZO/n-Si heterojunction solar cell (HJSC) with high efficiency of 8.3%/5.4% under front/rear illumination. To optimize fabrication process, the influence of substrate temperature TD (in the range 310–460°C), annealing, and film thickness d on the film and AZO/c-Si junction properties were studied systematically. SEM, ellipsometry, EDX spectroscopy, transmission, reflection, and external quantum efficiency spectra, resistivity ρ, Hall, Suns–Voc, and light I–V measurements were used for the analysis. Annealed junctions, AZO/n-Si and AZO/p-Si, as well as AZO/p-Si junction with as-deposited films showed small open-circuit voltage Voc (<300mV). The highest Voc (∼480mV) showed AZO/n-Si junction with as-deposited film grown at 410°C. We employed as-deposited AZO films grown at 410°C in AZO/(nn+)Cz-Si/In2O3:F bifacial heterojunction solar cells, which differed only in the AZO film thickness. Increasing d from 260 to 910nm resulted in the following: (1) the photocurrent did not changed; (2) ρ and Rsh of the film, Voc and the series resistance of the HJSCs decreased; (3) the fill factor FF and efficiency η increased (for front illumination, FF: from 29.1 to 57.3%, η: from 3.3 to 8.3%, respectively). At rear illumination, the best cell showed the efficiency of 5.4%. At 1-sun front illumination and 20–50–100% 1-sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 9.1–10.3–12.1% efficiency.

Characterization of Al-Doped ZnO Transparent Conducting Thin Film Prepared by Off-Axis Magnetron Sputtering

The off-axis sputtering technique was used to deposit Al-doped ZnO (AZO) films on glass substrates at room temperature. For the illustration of the sample position in the sputtering chamber, the value ofR/ris introduced. Here,ris the radius of AZO target and R is the distance between the sample and the center of substrate holder. A systematic study for the effect of deposition parameters on structural, optical, and electrical properties of AZO films has been investigated in detail. As the sample position ofR/ris fixed at 1.8, it is found that the as-deposited AZO film has relatively low resistivity of 2.67 × 10−3 Ω-cm and high transmittance above 80% in the visible region. Additionally, after rapid thermal annealing (RTA) at 600°C with N2atmosphere, the resistivity of this AZO film can be further reduced to 1.19 × 10−3 Ω-cm. This indicates the AZO films prepared by off-axis magnetron sputtering and treated via the appropriate RTA process have great potential in optoelectronic applications.

Effect of Post Deposition Annealing Treatments on Properties of AZO Thin Films for Schottky Diode Applications.

High-quality aluminum (Al) doped ZnO (AZO) thin films were deposited on silicon substrates by RF sputtering at room temperature. The deposited films were annealed from the temperatures 350 °C to 650 °C in pure nitrogen (N₂) ambient. The effects of annealing on the microstructural, optical and electrical properties of the AZO films were investigated. A detailed analysis by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Four Probe and Hall measurements was performed to study the properties of these AZO thin films. It was observed that all of the as-deposited and annealed AZO films have homogenous surfaces and hexagonal wurtzite structures with good crystalline quality. The study also suggested that there was an intermediate post annealing temperature (450 °C) at which the deposited ZnO film exhibit best surface characteristics. Pd/AZO Schottky devices were fabricated with 450 °C annealed AZO thin films and the parameters of Schottky devices were extracted from I-V characteristics. These results indicated that the Pd/AZO films were very much suitable for various optoelectronics applications particularly for metal semiconductor metal based UV detector application.

Poole-Frenkel effect on electrical characterization of Al-doped ZnO films deposited on p-type GaN

This paper presents the electrical properties of Al-doped ZnO (AZO) films directly grown on two types of p-type GaN thin films. The low-pressure p-GaN thin films (LP-p-GaN) exhibited structural properties of high-density edge-type threading dislocations (TDs) and compensated defects (i.e., nitrogen vacancy). Compared with high-pressure p-GaN thin films (HP-p-GaN), X-ray photoemission spectroscopy of Ga 3d core levels indicated that the surface Fermi-level shifted toward the higher binding-energy side by approximately 0.7 eV. The high-density edge-type TDs and compensated defects enabled surface Fermi-level shifting above the intrinsic Fermi-level, causing the surface of LP-p-GaN thin films to invert to n-type semiconductor. A highly nonlinear increase in leakage current regarding reverse-bias voltage was observed for AZO/LP-p-GaN. The theoretical fits for the reverse-bias voltage region indicated that the field-assisted thermal ionization of carriers from defect associated traps, which is known as the Poole-Frenkel effect, dominated the I-V behavior of AZO/LP-p-GaN. The fitting result estimated the trap energy level at 0.62 eV below the conduction band edge. In addition, the optical band gap increased from 3.50 eV for as-deposited AZO films to 3.62 eV for 300 °C annealed AZO films because of the increased carrier concentration. The increasing Fermi-level of the 300 °C annealed AZO films enabled the carrier transport to move across the interface into the LP-p-GaN thin films without any thermal activated energy. Thus, the Ohmic behavior of AZO contact can be achieved directly on the low-pressure p-GaN films at room temperature.

Transport characteristics of thermally degraded ZnO films doped with Al

The thermal degradation characteristics of Al doped ZnO (AZO) films, which were prepared by radio frequency magnetron sputtering at room temperature and subsequently annealed in air, were examined by making comparisons with those of the as-deposited and the vacuum-annealed AZO films. AZO films annealed in air at temperature higher than 300 °C exhibited severely degraded conductivity due to the decrease of carrier concentration and Hall mobility, which could be interpreted as grain boundary scattering from the thermally activated mobility behavior. Experimental observations, that vacuum-annealing of the as-deposited AZO film led to substantial reduction in carrier concentration and that subsequent vacuum-annealing of the air-annealed AZO film could not restore the carrier concentration unlike the restoration of Hall mobility up to 80%, led to a perception that substantial amount of free carriers in the as-deposited AZO film were generated from oxygen vacancies. It was proposed that ratios of the barrier height to the Fermi level, which were determined by taking into account the non-parabolic nature of the conduction band, could be used as a measure for the influence of grain boundary scattering on the overall electron transport mechanism in AZO films.

Effects of annealing and deposition temperature on the structural and optical properties of AZO thin films

Al-doped ZnO (AZO) thin films were deposited on p- type Si(100) substrate by r.f magnetron sputtering at 200, 300 and 400 °C substrate temperatures. The deposited films were annealed in air atmosphere for 1 h at temperatures of 700, 800 and 900 °C. The deposition temperature and post-deposition annealing effects on structural and optical properties of the AZO samples were analyzed using X-ray diffraction, atomic force microscope and photoluminescence (PL). After annealing, the value of full width half maximum of the diffraction peaks was decreased as well as, the intensity of visible and strong UV PL emission peaks were increased with temperature. However, the deep-level emission related with zinc point defects was removed by annealing of the samples. Results revealed that all of the as-deposited and annealed AZO films have hexagonal structure along (002) direction and their crystallinity were improved with the increased deposition and post-growth annealing temperatures. In addition, the surface roughness and the particle size of the films were increased with increased deposition and annealing temperatures.

Influence of hydrogen plasma treatment on Al-doped ZnO thin films for amorphous silicon thin film solar cells

This study investigates the effects of H 2 plasma treatment on characteristics of Al-doped ZnO (AZO) thin films prepared by RF magnetron sputter at 200 °C for amorphous silicon (α-Si) thin film solar cell fabrication. Results of X-ray diffraction analysis showed that the structure of the plasma-treated film did not change but its crystallinity deteriorated as compared to that of the as-deposited film. The electrical resistivity of the AZO films decreased after H 2 plasma treatment, regardless of plasma power. The most improvements in the electrical and optical properties of the AZO film were obtained by applying H 2 plasma RF power of 50 W, where the film resistivity decreased from 1.23 × 10 −3 to 8.23 × 10 −4 Ω cm and the average optical transmittance in the wavelength range of 400–700 nm increased slightly from 89.5% to 91.7%. To enhance light trapping in solar cells, surface-textured AZO films were developed using diluted HCl etching and the haze ratio beyond 30% was obtained. Additionally, the α-Si thin film solar cells consisting of the prepared AZO thin film as the transparent electrodes were fabricated. The efficiency of the cell increased form 3.26% for the as-deposited AZO film to 5.14% for the 0.2%-HCl-etched and H 2 plasma-treated film.

Rapid thermal annealed Al-doped ZnO film for a UV detector

A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering method was used in the initial growth of AZO films and a rapid annealing process was performed on the as-deposited AZO film under N 2 atmosphere for 3 min. An as-deposited AZO film had an optical transmittance of 84.78% at 550 nm and a resistivity of 7.8 × 10 − 3 Ω cm. A rapid annealing process significantly improved the optical transmittance and electrical resistivity of the AZO film to 99.67% and 1 × 10 − 3 Ω cm, respectively. The structural changes of the AZO films were investigated by X-ray diffraction and transmission electron microscopy. The high quality AZO film was used to fabricate a metal–semiconductor–metal (MSM) structure for a UV detector. The MSM device provided a stable current of 25 μA at a bias of 2 V in a dark condition. Under UV illumination, the MSM device was highly responsive to UV light uniformly and repeatedly, and it enhanced the current by 80% at 45 μA. This rapid thermal annealing process may provide a useful method to fabricate quality AZO films for photoelectric applications with a low thermal budget.