Thickness Of MgO Buffer Layer Research Articles

Surface polarity control in ZnO films deposited by pulsed laser deposition

We demonstrate a simple and inexpensive method of surface polarity control of ZnO grown by pulsed laser deposition (PLD). The polarity control is achieved in a straightforward way by changing the thickness of MgO buffer layer. The Zn- and O-polar ZnO films possess very distinct growth rate, electron concentration and mobility as well as different defect structures. These different structural and electronic properties result in significant differences in surface reactivity and device performance. For example, Pd Schottky diodes fabricated onto the O-polar ZnO film exhibit lower barrier height and ideality factor compared with the equivalent Zn-polar devices, while methylammonium lead iodide perovskite films are readily formed on O-terminated and rapidly decompose on Zn-terminated surfaces. This can be attributed to higher photocatalytic activity of Zn-terminated surface, as well as higher surface coverage of adsorbed hydroxyl groups. Consequently, our results indicate that polarity engineering to obtain favorable O-terminated surface can result in improved performance of ZnO-containing optoelectronic devices, while Zn-terminated surfaces could be of interest for photocatalytic and sensing applications.

The effect of the MgO buffer layer thickness on magnetic anisotropy in MgO/Fe/Cr/MgO buffer/MgO(001)

The relationship between the magnetic properties and MgO buffer layer thickness d was studied in epitaxial MgO/Fe(t)/Cr/MgO(d) layers grown on MgO(001) substrate in which the Fe thickness t ranged from 0.4 nm to 1.1 nm. For 0.4 nm ≤ t ≤ 0.7 nm, a non-monotonic coercivity dependence on the MgO buffer thickness was shown by perpendicular magneto-optic Kerr effect magnetometry. For thicker Fe films, an increase in the buffer layer thickness resulted in a spin reorientation transition from perpendicular to the in-plane magnetization direction. Possible origins of these unusual behaviors were discussed in terms of the suppression of carbon contamination at the Fe surface and changes in the magnetoelastic anisotropy in the system. These results illustrate a method to control magnetic anisotropy in MgO/Fe/Cr/MgO(d) via an appropriate choice of MgO buffer layer thickness d.

Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures

We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.

Effect of MgO buffer layer thickness on the electrical properties of MgZnO thin film transistors fabricated by plasma assisted molecular beam epitaxy

We report the fabrication and electrical characteristics of thin film transistors based on MgZnO thin films with different thicknesses of MgO buffer layer. The MgZnO thin films with MgO buffer layers were grown on SiO 2/p-Si substrates by plasma assisted molecular beam epitaxy. The effects of the buffer layer thickness on the structural properties of MgZnO films are investigated by X-ray diffraction, and the results show that the crystal quality of the MgZnO film is enhanced with 4 nm MgO buffer layer. The MgZnO TFT with 4 nm MgO buffer layer exhibits an n-type enhancement mode characteristics with a field effect mobility of 1.85 cm 2/V s, a threshold voltage of 27.6 V and an on/off ratio of above 10 6.

Effects of MgO-Buffer Layer on the Structural and Optical Properties of Polycrystalline ZnO Films Grown on Glass Substrate

Characteristics of ZnO films grown on glass substrates with various MgO buffer layer thicknesses have been investigated. All the layers are deposited by plasma-assisted molecular beam epitaxy. Without the MgO buffer layer, surface cracks due to the difference in the thermal expansion coefficients between ZnO and the quartz substrate are easily observed. With the MgO buffer layer, the cracks drastically decrease. The surface morphology changes with different MgO buffer layer thicknesses due to the compressive-tensile stress balance. The ZnO films with a medium MgO buffer layer thickness show a dotlike surface. All samples except those with the dotlike surface exhibit c-axis oriented polycrystalline structures. The photoluminescence emission intensity is also improved by increasing the buffer layer thickness until the optimum MgO buffer layer thickness of approximately 150 nm is reached. Few cracks appear with the reduction of the photoluminescence (PL) intensity when the buffer layer thickness is more than the optimum value. All of the as-grown ZnO films without cracks exhibit n-type conductivity.

Selective growth of Zn- and O-polar ZnO layers by plasma-assisted molecular beam epitaxy

The polarity of ZnO films grown on MgO∕c-Al2O3 templates are successfully controlled by only varying MgO buffer layer thickness. The crystal structure of the MgO buffer on c-Al2O3 is of wurtzite (WZ), when the MgO buffer thickness is less than 2.7nm. As the MgO layer thickness exceeds, the crystal structure of MgO changes to rock salt (RS) structure. A ZnO layer grown on WZ-MgO results in O-polar ZnO, while the polarity of the ZnO layer grown on RS-MgO changes to Zn polar. Possible atomic configurations of the ZnO∕MgO∕Al2O3 interface structure are suggested.

Polarity control of ZnO on sapphire by varying the MgO buffer layer thickness

Polarity-controlled ZnO films with an MgO buffer layer were grown on c-plane sapphire by plasma-assisted molecular-beam epitaxy. Convergent beam electron diffraction results showed that Zn-polarity (+c) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (−c) growth occurred when the layer was less than 2 nm. Reflection high-energy electron diffraction results revealed that MgO growth was Stranski–Krastanov mode, and that the growth mode transition from two- to three-dimensional occurred when the layer was thicker than 1 nm. In conclusion, polarity conversion apparently occurs due to the different atomic structure between the wetting layer and islands of MgO.