MgO Buffer Layer Research Articles

Structural study of MgO and Mg-doped ZnO thin films grown by atomic layer deposition

Pure MgO, 1 and 10 at% Mg-doped ZnO layers were grown by atomic layer deposition technique onto (001)-oriented α-Al2O3 and GaN substrates. The structure and the microstructure of the deposited layers were studied by TEM in detail. The pure MgO layer starts to grow with epitaxy with (100)-type defects that transforms to random orientation. In the 10 at% Mg-doped samples epitaxial cubic MgO buffer layer forms at the interface. This buffer layer helps the ZnO to grow epitaxially in the case of α-Al2O3 substrate forming a ZnO/MgO/c-plane α-Al2O3 hetero-structure showing higher mobility with lower carrier concentration, while in the case on GaN substrate the ZnO is strongly textured. Consequently for higher concentration Mg doping (with the formation of MgO buffer layer) α-Al2O3 is better choice to grow epitaxial ZnO layer. The 1 at% Mg-doping of the epitaxial ZnO layer grown onto GaN substrate was successfully implemented, while in the case of α-Al2O3 substrate a thin cubic MgO buffer layer forms. This shows that the success of low concentration Mg doping in ZnO largely depends on the choice of substrate material as well.

Robust Perpendicular Magnetic Anisotropy in MgO/Co2FeAl/MgO Stacks Induced by MgO over Layer and Annealing Temperature

Perpendicular magnetic anisotropy (PMA) in Co2FeAl (CFA)/MgO stacks deposited on the MgO buffer layer is studied as a function of MgO over‐layer thickness and annealing temperature. It reveals that MgO over‐layer (thickness between 0 and 1.5 nm) improves the PMA of CFA grown on MgO buffer layer. It is identified that MgO over‐layer thickness is a crucial factor that affects the magnetic anisotropy. The maximum effective PMA energy density (Keff) of ≈1.22 × 106 erg cm−3 is observed at 400 °C. Besides, PMA is also realized for room temperature (RT) films. Moreover, enhancement in the perpendicular saturation magnetization (Ms┴) with PMA is effectively induced by the CFA/MgO or CFA/Mo interface and is discussed in detail. The grain distribution and surface roughness are investigated by atomic force microscopy (AFM) for without and with MgO over‐layer in MgO/CFA/tMgO (0 and 0.5 nm)/Mo stacks at higher annealing temperature. Therefore, this MgO/CFA/MgO can be an ideal system for the p‐MTJs‐based new generation low power consuming large density data storage spintronic devices with improved thermal stability.

Surface scaling analysis of textured MgO thin films fabricated by energetic particle self-assisted deposition

In the fabrication of a high-temperature superconducting coated conductor, the surface roughness and texture of buffer layers can significantly affect the epitaxially grown superconductor layer. A biaxially textured MgO buffer layer fabricated by ion beam assisted deposition (IBAD) is widely used in the coated conductor manufacture due to its low thickness requirement. In our previous study, a new method called energetic particle self-assisted deposition (EPSAD), which employed only a sputtering deposition apparatus without an ion source, was proposed for fabricating biaxially textured MgO films on non-textured substrates. In this study, our aim was to investigate the deposition mechanism of EPSAD-MgO thin films. The behavior of the surface roughness (evaluated by Rq) was studied using atomic force microscopy (AFM) measurements with three scan scales, while the in-plane and out-of-plane textures were measured using X-ray diffraction (XRD). It was found that the variations of surface roughness and textures along with the increase in the thickness of EPSAD-MgO samples were very similar to those of IBAD-MgO reported in the literature, revealing the similarity of their deposition mechanisms. Moreover, fractal geometry was utilized to conduct the scaling analysis of EPSAD-MgO film's surface. Different scaling behaviors were found in two scale ranges, and the indications of the fractal properties in different scale ranges were discussed.

Bending stability of GaN grown on a metallic flexible substrate by plasma-assisted molecular beam epitaxy

GaN thin films were grown on flexible metallic substrates by molecular beam epitaxy. MgO buffer layers were deposited by spin coating on Ni–Mo–Cr (Hastelloy C-276) alloy tapes that were used as substrates. The structural characterization of the GaN/MgO/hastelloy samples was performed by x-ray diffraction and Raman spectroscopy. The obtained nanometric films have the stable hexagonal phase (α-GaN) with an average crystallite size of 18 nm. The long and short range order of GaN decrease when the structure is bent. The most significant variations in the structural properties occur between 100 and 250 bending cycles.

High Quality Transferable AlN Thin Film by PLD

AlN thin film was epitaxial grown on c-plane sapphire substrate by pulsed laser deposition. To reduce structural defects from largely lattice mismatched substrate, MgO or ZnO buffer layer was inserted between AlN and sapphire. Crystal structure and surface morphology of as prepared AlN were characterized by XRD, AFM, and SEM. It was found that buffer layers significantly improve crystalline quality of AlN, especially using ZnO. Furthermore, a general and steady wet chemical process was developed to selectively etch away ZnO layer, so that high quality free-standing AlN thin film was obtained. This film could be transferred onto any other host substrates such as Si, quartz, etc. Moreover, with no clamping effect from the substrate, the as-prepared free-standing AlN thin films may find potential applications in high sensitivity piezoelectric devices, flexible wearable detectors and so on.

Pulsed laser deposition of epitaxial MgO buffer layer for proton-conducting ceramic electrolytes

This work studies the deposition of epitaxial MgO thin films with thickness of 40nm deposited on bare Si (100) wafer using pulsed laser deposition (PLD), which can serve as a buffer layer for subsequent growth of textured Y-BaZrO3 with high proton-conductivity. The success of epitaxial deposition were achieved by careful preparation of Si substrate by pre-etching with HF solution, as well as by optimizing deposition parameters including oxygen partial pressure and substrate temperatures. Epitaxial (100) MgO film with cubic lattice structure and the lattice constant of 4.21Å were obtained, which showed highly lattice match with Y-BaZrO3 perovskite of 4.21Å.

Planar current anisotropy and field dependence of Jc in coated conductors assessed by scanning Hall probe microscopy

The local distribution of the critical current density, Jc, of coated conductors and YBa2Cu3O (YBCO) films on single crystalline substrate was investigated by scanning Hall probe microscopy. The high spatial resolution of the measurements enabled an assessment of dependence of the local Jc on the local magnetic induction, B, and electric field, E. The derived Jc(B)-dependence agreed well with the global Jc obtained from magnetization loops and provided values of Jc at very low fields, which are inaccessible to magnetization and transport measurements. The anisotropic current flow within the film plane was investigated in YBCO films on miscut SrTiO3 substrates and a GdBa2Cu3O film on an MgO buffer layer prepared by inclined substrate deposition on a Hastelloy substrate. The c-axis currents calculated from the Hall maps were significantly larger than previously reported data obtained from direct transport measurements. The planar current anisotropy at 77 K was found to be highly influenced by the microstructure which can either deteriorate the current flowing across the ab-planes or cause enhanced pinning and increase the critical current flowing parallel to the ab-planes.

Electron-Induced Secondary Electron Emission Properties of MgO/Au Composite Thin Film Prepared by Magnetron Sputtering

As a type of electron-induced secondary electron emitter, MgO/Au composite thin film was prepared by reactive magnetron sputtering of individual Mg target and Au target, and the effects of key process parameters on its surface morphology and secondary electron emission (SEE) properties were investigated. It is found that to deposit a NiO buffer layer on the substrate is conducive to the subsequent growth of MgO grains owing to the lattice matching. The gold addition can raise the electrical conductivity of MgO film and further suppress the surface charging. However, the gold deposition would interfere with the MgO crystallization and increase the surface roughness of MgO/Au film. Therefore, MgO/Au composite thin film with a NiO buffer layer and proper deposition times of MgO and Au can achieve superior SEE properties due to good MgO crystallization, low surface roughness and reasonable electrical conductivity. The optimized MgO/Au composite thin film has a higher SEE coefficient and a lower 1-h SEE degradation rate under electron beam bombardment in comparison with MgO film.

MgO/Cu2O Superlattices: Growth of Epitaxial Two-Dimensional Nanostructures

Alternated stacking of dissimilar layers can produce novel superlattice materials with multiple functionalities. The majority of such work reported in literature on epitaxial superlattices has been on alternating layers with the same space group (SG) and crystal structure (CS), whereas superlattices with the same CS but different SG have not been studied as much. We have grown superlattices with two well-known oxide materials, viz. cuprite (Cu2O, CS = cubic and SG = Pn $$ \bar{3} $$ m) and magnesium oxide (MgO, CS = cubic, SG = Fm $$ \bar{3} $$ m). An MgO buffer layer grown near 650°C at the film–substrate interface was found to be essential to achieving reasonable long-range atomic order. Grazing-angle x-ray diffraction, x-ray reflectivity, and electron diffraction analyses as well as transmission electron microscopy were used to investigate the interface abruptness, smoothness, and general crystallinity of the individual layers. Interdiffusion between MgO and Cu2O near interfacial regions places a limit of 250°C on the growth temperature for fabrication of superlattices with reasonably sharp interfaces.

Interlayer exchange coupling, dipolar coupling and magnetoresistance in Fe/MgO/Fe trilayers with a subnanometer MgO barrier

Fe/MgO/Fe trilayers with a subnanometer MgO tunnel barrier were grown by molecular beam epitaxy. Longitudinal magnetooptic Kerr effect measurements confirmed the existence of the antiferromagnetic interlayer exchange coupling (IEC) between the Fe layers for 2Å<dMgO<6Å. The coupling was enhanced for the trilayer grown on a homoepitaxial MgO buffer layer, and its IEC constant was estimated to be −3.3erg/cm2 at a MgO thickness of 2.7Å. After magnetic characterization, the sample was patterned into circular-shaped pillars with diameters ranging from 200nm to 520nm. We showed that the dipolar coupling that appeared after the nanofabrication process modified the effective coupling between layers, and we determined dependence of the dipolar coupling on the pillar diameter. Finally, magnetoresistance (MR) was measured as a function of MgO thickness (dMgO), and a non-zero MR was found for the MgO as thin as 3.4Å. Extrapolation of the MR (dMgO) dependence to MR=0 allowed us to determine the length of the pinholes in our sample, which was estimated to be (3.2±0.5)Å.

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.

Reel-to-reel deposition of epitaxial double-sided MgO buffer layers for coated conductors

We have successfully employed a double-sided process to deposit MgO buffer layers on both sides of amorphous Y2O3 surface for double-sided YBa2Cu3O7−δ (YBCO) coated conductors (CCs) for the first time, the structure of which is of great prospect to improve the performance and cut the production cost. The biaxial textures of MgO buffer layer are noticeably affected by the ion energy and film thickness, which is demonstrated by X-ray diffraction. The best biaxial texture of double-sided MgO films shows ω-scan of (002) MgO and Φ-scan of (220) MgO yield full width at half maximum values of 4° and 7.8° for one side, respectively, as well as 3.5° and 6.7° for the other side. The subsequent double-sided YBCO films are deposited on the as-prepared MgO template with entire critical current of over 300A/cm for both sides.

Effects of MgO buffer annealing on optical and electrical quality of P-MBE grown ZnO films on c-sapphire

Zinc oxide (ZnO) has been attracting much attention because of its potential applications in photonic and optoelectronic devices. In this present study, we investigated the effect of MgO buffer annealing on the optical and electrical quality of P-MBE grown ZnO films on c-sapphire with MgO buffer layer. The optical quality was observed by low-temperature PL (photoluminescence) measurement in the near band edge emission region measured at 10K and at 77K. The emission line located at 3.368eV dominates the spectrum in both samples (ZnO with and without MgO buffer annealing) at 10K and 77K. This emission can be divided into two peaks, 3.367eV and 3.363eV and assigned as I2 (ionized donor bound excitons emission) and I4 (Hydrogen donor related emission), respectively. The relative intensity of these donor bound exactions to free exaction emission of the sample without MgO buffer annealing is greater than that of the sample with MgO buffer annealing. Comparison of the PL spectra of ZnO with and without annealing revealed that the intensity of free exciton emission from the sample with MgO buffer annealing is twice of that from the sample without annealing. We also found that the intensity of deep-level broad emission is reduced by about 1/3 by MgO-buffer annealing. Hence, the decrease of deep level emission intensity and the increase of free exciton emission intensity by annealing of MgO buffer corresponds to the reduction of defects of the ZnO film. The PL properties also suggest that there are fewer nonradiative recombination centers in ZnO layers with MgO buffer annealing than those in ZnO layers grown without MgO buffer annealing. The electrical quality was measured by room temperature Hall measurements. We found that the samples have a background n-type carrier concentration. The ZnO samples with MgO buffer annealing has a carrier concentration of 1.17×1017 cm-3 and Hall mobility of 120 cm2/V.s, while the ZnO sample without MgO buffer annealing has a carrier concentration of 2.63 × 1016 cm-3 and Hall mobility of 105 cm2/V.s. The improvement of electron mobility of ZnO films by MgO buffer annealing is due to a decrease in dislocation density. We conclude that annealing of MgO buffer layer increases the optical and electrical quality of the ZnO films. These results agree with the structural quality as observed by HRXRD.

Pinning Centers in ISD-MgO Coated Conductors via EB-PVD

GdBa 2 Cu 3 O 7-x (GdBCO) films doped with up to 13.6 mol.% BaHfO 3 (BHO) have been grown by electron beam physical vapor deposition (EB-PVD). A critical current density J c at 30 K, 3 T of 2.34 MA/cm 2 has been achieved for a doping level of 10.6 mol.%. This is the first report of artificial pinning centers being successfully incorporated into REBa 2 Cu 3 O 7-x (REBCO, RE = Y, Gd, Dy . . .) via EB-PVD. The samples were grown on Hastelloy tapes coated with a MgO buffer layer deposited by inclined substrate deposition. The J c of the samples was characterized at 77 K, 1 T and at 30 K, 3 T. An increase in J c (30 K, 3 T) upon Hf addition has been observed in the whole angular range α. The level of Hf addition was found to heavily influence the kind of nanoprecipitate formed, with low levels of Hf causing a reduction in the anisotropy of J c (77 K, 1 T) and higher levels of Hf resulting in the formation of nanoprecipitates elongated parallel to the GdBCO ab-plane, causing an increase in the anisotropy of J c (77 K, 1 T). The largest J c (77 K, sf) measured was 0.63 MA/cm -2 .

A Method to Fabricate Biaxially Textured MgO Buffer Layer for HTS Coated Conductor

In this paper, a radio-frequency magnetron sputtering system without any assisting ion source was employed to fabricate biaxially textured MgO films on the substrates of amorphous Y 2 O 3 /Al 2 O 3 /Hastelloy stacks. During the deposition process, the growing MgO film was bombarded by an off-normal energetic particle flux mainly composed of oxygen atoms and negative oxygen ions, which originated from the target surface. MgO obtained a biaxial texture due to such bombardment; thus, this method is named energetic particle self-assist deposition (EPSAD). The texture of EPSAD-MgO films was evaluated via the X-ray diffraction measurement of the postdeposited homoepitaxial MgO layers. An out-of-plane orientation of MgO(111) and a three-fold symmetric in-plane alignment were observed. The MgO texture was optimal when the EPSAD-MgO thickness was about 10 nm. The formation of biaxial texture during the EPSAD process was verified by a control study, and the influences of targetinclined angle and target-substrate distance were also investigated. Furthermore, the mechanism of the EPSAD-MgO method was compared with those of the inclined substrate deposition and ion beam assisted deposition. This study proposed a new method to fabricate a biaxially textured MgO buffer layer for high temperature superconducting (HTS) coated conductors. More optimization research will be conducted in our future study.

Photovoltaic properties of Bi2FeCrO6 films epitaxially grown on (100)-oriented silicon substrates.

We demonstrate the promising potential of using perovskite Bi2FeCrO6 (BFCO) for niche applications in photovoltaics (PV) (e.g. self-powered sensors that simultaneously exploit PV conversion and multiferroic properties) or as a complement to mature PV technologies like silicon. BFCO thin films were epitaxially grown on silicon substrates using an MgO buffer layer. Piezoresponse force microscopy measurements revealed that the tensile strained BFCO phase exhibits a polarization predominantly oriented through the in-plane direction. The semiconducting bandgap of the ordered BFCO phase combined with ferroelectric properties, opens the possibility of a ferroelectric PV efficiency above 2% in a thin film device and the use of ferroelectric materials simultaneously as solar absorber layers and carrier separators in PV devices. A large short circuit photocurrent density of 13.8 mA cm(-2) and a photovoltage output of 0.5 V are typically obtained at FF of 38% for BFCO devices fabricated on silicon. We believe that the reduced photovoltage is due to the low diffusion length of photogenerated charge carriers in the BFCO material where the ferroelectric domains are predominately oriented in-plane and thus do not contribute efficiently to the photocharge separation process.

Highly Textured IrMn3(111) Thin Films Grown by Magnetron Sputtering

Non-collinear spin ground states in metallic antiferromagnets can give rise to anomalous Hall effect, which enables magnetic state readout in antiferromagnetic spintronic devices. Here we report growth of highly textured films of a non-collinear antiferromagnet IrMn3 on MgO(111) substrates by magnetron sputtering. The films consist of epitaxial (111) twin domains rotated by 60 degrees within the film plane. The films exhibit partial L12 ordering that supports the non-collinear T1 spin structure predicted to give rise to anomalous Hall effect. An MgO buffer layer evaporated onto the substrate increases the L12 order parameter from 0.4 to 0.75 but destroys the (111) crystallographic texture.

Critical current anisotropy of GdBCO tapes grown on ISD–MgO buffered substrate

The critical current anisotropy of a GdBa2Cu3O7 (GdBCO) coated conductor (CC) was investigated. The MgO buffer layer was grown by the inclined substrate deposition (ISD) technique leading to a tilted c-axis growth of the GdBCO layer by 24° from the tape normal. The Jc was measured in two current directions: longitudinal and transverse to the tape. The self-field Jc in the 2 μm thick GdBCO layer was 1.3 MA cm−2 and 0.88 MA cm−2 at 77 K in the longitudinal and transverse directions, respectively. The 30% lower value in the transverse direction results from the current flow across the weakly superconducting layer of GdBCO. The peak in the Jc-anisotropy corresponding to intrinsic pinning appears at 24° with respect to the tape surface in the longitudinal direction and the anisotropy was highly asymmetric in a wide range of fields. Due to the large crystallographic tilt of the GdBCO with respect to the tape axes, a minimum caused by vortex channeling occurred in the transverse current direction which was observed for the first time at 77 K. Another consequence of the large crystallographic tilt angle is a pronounced competing effect of the crystallographic anisotropy and Lorentz force on Jc in the variable Lorentz force configuration.

Compact magnetooptical isolator with cobalt ferrite on silicon photonic circuits

In the telecom wavelength range, the magnetooptical effect of cobalt ferrites is approximately 10 times larger than that of conventional magnetooptical materials such as yttrium iron garnets. In this study, we focus on an application of cobalt ferrite to a magnetooptical isolator that is to be miniaturized and made suitable for integration. First, we prepare polycrystalline cobalt ferrite films deposited on a silicon substrate using a MgO buffer layer. Next, we fabricate a waveguide optical isolator of silicon waveguides by the partial deposition of the cobalt ferrite films. An optical isolation ratio of 5.5 dB is demonstrated.

Effect of annealing temperature on exchange stiffness of CoFeB thin films

We investigate the exchange stiffness constants of 28-nm-thick CoFeB film using Brillouin light scattering. Series of CoFeB films are prepared on the MgO(001) substrate with or without additional 5-nm thick MgO buffer layer, the effect of the annealing temperature on the exchange stiffness constants are studied. We found that the exchange stiffness constant of 400°C annealed sample with MgO buffer increased by 10% form the 200°C annealed sample (=0.73±0.01×10−11J/m), while the exchange stiffness constant of without MgO buffer layer sample increase by 6% from the as-grown sample (=1.11±0.02×10−11J/m).