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

Using Modified-Intake Plasma-Enhanced Metal-Organic Chemical Vapor Deposition System to Grow Gallium Doped Zinc Oxide

We have used a modified-intake plasma-enhanced metal–organic chemical vapor deposition (MIPEMOCVD) system to fabricate gallium-doped zinc oxide (GZO) thin films with varied Ga content. The MIPEMOCVD system contains a modified intake system of a mixed tank and a spraying terminal to deliver the metal–organic (MO) precursors and a radio-frequency (RF) system parallel to the substrate normal, which can achieve a uniform distribution of organic precursors in the reaction chamber and reduce the bombardment damage. We examined the substitute and interstitial mechanisms of Ga atoms in zinc oxide (ZnO) matrix in MIPEMOCVD-grown GZO thin films through crystalline analyses and Hall measurements. The optimal Ga content of MIPEMOCVD-grown GZO thin film is 3.01 at%, which shows the highest conductivity and transmittance. Finally, the optimal MIPEMOCVD-grown GZO thin film was applied to n-ZnO/p-GaN LED as a window layer. As compared with the indium–tin–oxide (ITO) window layer, the n-ZnO/p-GaN LED with the MIPEMOCVD-grown GZO window layer of the rougher surface and higher transmittance at near UV range exhibits an enhanced light output power owing to the improved light extraction efficiency (LEE).

Effects of B2O3 doping on the crystalline structure and performance of DC-magnetron-sputtered, transparent ZnO thin films

The transparent-conducting performance is estimated through figure-of-merit (FOM) value. To improve poor FOM value of pure ZnO thin films, boron (B) as a donor impurity was doped into the films. Direct-current magnetron sputtering was used to prepare B-doped ZnO (BZO) thin films from sintered ZnO targets with variable B₂O₃ content changing from 0 to 2 wt. %. The x ray diffraction analysis confirmed the preferably c-axis-oriented structure of hexagonal wurtzite ZnO host. The results also showed variation in the film structure versus the B₂O₃ content through calculations of crystal size and residual stress. Depending on the B₂O₃ content, a competition of interstitial and substitutional B³⁺ ions induced more stress or relaxation in lattice structure of the films. At 1% B₂O₃, the BZO thin film had the best crystalline characterization with the lowest stress and large crystal size. In consequence, the BZO 1% film obtained the lowest resistivity of 2.7×10^-3Ωcm, average transmittance of 82.1%, and the best FOM value of 18.8×10²Ω^-1cm^-1. The transparent-conducting performance of the ZnO thin films deposited by direct-current (DC) magnetron sputtering was significantly enhanced through B doping. The good-performance BZO film at 1% B₂O₃ is believed to be of use as electrodes in thin-film solar cells.

Growth of c-plane and m-plane aluminium-doped zinc oxide thin films: epitaxy on flexible substrates with cubic-structure seeds

Manufacturing high-quality zinc oxide (ZnO) devices demands control of the orientation of ZnO materials due to the spontaneous and piezoelectric polarity perpendicular to the c-plane. However, flexible electronic and optoelectronic devices are mostly built on polymers or glass substrates which lack suitable epitaxy seeds for the orientation control. Applying cubic-structure seeds, it was possible to fabricate polar c-plane and nonpolar m-plane aluminium-doped zinc oxide (AZO) films epitaxially on flexible Hastelloy substrates through minimizing the lattice mismatch. The growth is predicted of c-plane and m-plane AZO on cubic buffers with lattice parameters of 3.94-4.63 Å and 5.20-5.60 Å, respectively. The ∼80 nm-thick m-plane AZO film has a resistivity of ∼11.43 ± 0.01 × 10^-4 Ω cm, while the c-plane AZO film shows a resistivity of ∼2.68 ± 0.02 × 10^-4 Ω cm comparable to commercial indium tin oxide films. An abnormally higher carrier concentration in the c-plane than in the m-plane AZO film results from the electrical polarity along the c-axis. The resistivity of the c-plane AZO film drops to the order of 10^-5 Ω cm at 500 K owing to the semiconducting behaviour. Epitaxial AZO films with low resistivities and controllable orientations on flexible substrates offer optimal transparent electrodes and epitaxy seeds for high-performance flexible ZnO devices.

Optoelectronic properties and anisotropic stress of Mo:ZnO thin films deposited on flexible substrates by radio frequency magnetron sputtering

This research investigated the optoelectronic properties and anisotropic stress of Mo-doped ZnO (MZO) films, which were deposited on polyethylene terephthalate and polycarbonate flexible substrates with radio frequency magnetron sputtering. The optical properties, x-ray diffraction (XRD) spectra, Hall effect measurements, and self-made phase-shift shadow moiré interferometer readings were utilized to evaluate the performances of the MZO films. Based on the results, the transmittance and (002) peak size of the XRD spectra decreased when the substrate temperature increased. However, this took place especially when the oxygen flow was on the increase. Also, carrier mobility, carrier concentration, and anisotropic stresses increased at higher substrate temperatures, but this was not the case when the oxygen flow increased. The energy gap (E_g) of the MZO films showed a blueshift with an increase in the substrate temperatures, but this rather changed to a redshift when the oxygen flow was observed to be on the rise.