Growth and crystallographic feature-dependent characterization of spinel zinc ferrite thin films by RF sputtering
© Liang and Hsia; licensee Springer. 2013
Received: 18 October 2013
Accepted: 9 December 2013
Published: 19 December 2013
ZnFe2O4 (ZFO) thin films exhibiting varying crystallographic features ((222)-epitaxially, (400)-epitaxially, and randomly oriented films) were grown on various substrates by radio-frequency magnetron sputtering. The type of substrate used profoundly affected the surface topography of the resulting ZFO films. The surface of the ZFO (222) epilayer was dense and exhibited small rectangular surface grains; however, the ZFO (400) epilayer exhibited small grooves. The surface of the randomly oriented ZFO thin film exhibited distinct three-dimensional island-like grains that demonstrated considerable surface roughness. Magnetization-temperature curves revealed that the ZFO thin films exhibited a spin-glass transition temperature of approximately 40 K. The crystallographic orientation of the ZFO thin films strongly affected magnetic anisotropy. The ZFO (222) epitaxy exhibited the strongest magnetic anisotropy, whereas the randomly oriented ZFO thin film exhibited no clear magnetic anisotropy.
KeywordsSpinel Oxide Crystallographic feature Sputtering Surface morphology Magnetic property
Recently, spinel-structured ferrite oxides have been intensively investigated because of their versatile physical and chemical properties as well as technological applications in magnetic sensors, biosensors, and photocatalysts [1, 2]. ZnFe2O4 (ZFO) is one of the major ferrite oxides with a spinel structure, and it has remarkable magnetic and electromagnetic properties regarding its state of chemical order and cation site occupancy in lattices . Moreover, it is also a semiconductor, processes light response, has photochemical characteristics, and can be used as a material for supercapacitors [4, 5].
ZFO in various forms, such as powders, films, and various nanostructures, prepared using different methodologies have been reported [6–8]. Many ZFO nanostructures can be used as versatile building blocks for fabricating functional nanodevices; however, integrating the reported methodologies for preparing nanostructured ZFO into Si-based semiconductor device processes remains a challenge. ZFO in thin-film form is promising and is compatible in the fabrication of devices with Si semiconductors. Yamamoto et al. prepared ZFO thin films on a single-crystal sapphire substrate by using pulsed laser deposition and examined the effect of the deposition rate on its magnetic properties . ZFO thin films with a microlevel scale were grown on glass substrates by radio-frequency (RF) sputtering at room temperature, and the magnetic properties of the films were investigated . Ogale et al. used a pulsed laser evaporation method to synthesize ZnO and Zn x Fe3−xO4 mixed-phase thin films on sapphire substrates using ZnFe2O4 pellets; however, this is not an efficient method for obtaining single-phase spinel ZFO thin films . Polycrystalline ZFO films were also prepared by spin-spray deposition; however, controlling the film thickness to be less than several hundred nanometers is challenging . Although several groups have proposed the fabrication of ZFO films using versatile methodologies, the sputtering technique is promising for preparing oxide thin films with excellent crystalline quality and controllable film thickness for device applications because it is a technique that enables large-area deposition and easy process control [13, 14]. It is well known that crystallographic features affect the properties of versatile oxide films [13, 15]. However, the crystallographic feature-dependent properties of sputtering-deposited spinel ZFO thin films are still inadequate. This might obstruct applications of such films in devices. In this study, ZFO thin films were grown on various single-crystal substrates by RF sputtering to fabricate ZFO thin films with varying crystallographic features. The correlation between the crystallographic features and the characterization of the ZFO thin films was investigated.
ZnFe2O4 (ZFO) thin films were grown on yttria-stabilized zirconia (YSZ) (111), SrTiO3 (STO) (100), and Si(100) substrates, using RF magnetron sputtering. The yttria content in YSZ substrates was 15%. The sputtering ceramic target adopted in the experiment was prepared by mixing the precursor oxide powders of ZnO and Fe2O3 to obtain a proportion of Fe/Zn = 2, pressing the powders into a pellet, and sintering the pellet at a high temperature to achieve a high density. The thickness of the ZFO thin films was fixed at approximately 125 nm, and the growth temperature was maintained at 650°C. The gas pressure of deposition was fixed at 30 mTorr, using an Ar/O2 ratio of 2:1 for the films. The atomic percentages of the as-deposited films were calculated based on the X-ray photoelectron spectroscopy (XPS) spectra of the Zn2p, Fe2p, and O1s regions. The chemical binding states of the constituent elements of the ZFO thin films were also investigated.
The crystal structures of the samples were investigated using X-ray diffraction (XRD), applying Cu Kα radiation. The surface morphology of the ZFO films was determined using scanning electron microscopy (SEM) and atomic force microscopy (AFM) at an area of 1 μm2. The detailed microstructures of the as-synthesized samples were characterized using high-resolution transmittance electron microscopy (HRTEM). The composition analysis was performed using an energy-dispersive X-ray spectrometer (EDS) attached to the TEM. Thin slices for cross-sectional TEM analysis were prepared using a dual-beam focused-ion-beam (FIB) instrument. The areas selected for cutting with an ion beam were protected by an amorphous carbon overlayer. Adjust the beam currents to mill initial trenches, thin the central membrane, and polish for electron transparency of membrane. Finally, FIB milling was used to capture a free membrane from trenches for a TEM analysis. The room temperature-dependent photoluminescence (PL) spectra were captured using the 325-nm line of a He-Cd laser. A superconducting quantum-interference device magnetometer was used to measure the magnetic properties of the samples.
Results and discussion
ZFO spinel thin films exhibiting epitaxially and randomly oriented crystallographic features were grown on various substrates by RF magnetron sputtering at 650°C. The XRD and TEM results indicated that growing the ZFO thin films on the YSZ (111) and STO (100) substrates promoted the formation of (222) and (400) epitaxial films, respectively. The film grown on the Si substrate exhibited a polycrystalline structure. The surface morphology of the ZFO thin film substantially depended on its crystallographic features. The SEM and AFM images demonstrated that the surface of the ZFO (222) epitaxial film was flat and smooth; however, the surface of the randomly oriented film was rough and exhibited 3D grains. The visible emission bands of the ZFO thin films were attributed to growth-induced oxygen vacancies. The ZFO thin films demonstrated a spin-glass transition temperature of approximately 40 K. The ZFO (222) epitaxial film exhibited the most marked magnetic anisotropy among the samples.
This work is supported by the National Science Council of Taiwan (grant no.NSC 102-2221-E-019-006-MY3) and National Taiwan Ocean University (grant no. NTOU-RD-AA-2012-104012). The authors thank assistance in SEM examination given by the sophisticated instrument user center of National Taiwan Ocean University.
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