Co-ZnO films were prepared on oxidised silicon by magnetron sputtering at room temperature both with and without a ZnO buffer-layer. The Co-ZnO films consisted of Co particles dispersed in a semiconductor matrix. The combination of a Co-ZnO layer and a ZnO buffer-layer has a higher magnetoresistance than the Co-ZnO layer alone on an insulating Si substrate. The causes of this effect were investigated using X-ray photoelectron spectroscopy, depth profiling using Auger electron spectroscopy and electrical resistance as well as measurements of the change in the saturation magnetisation, the field cooled- and zero field cooled-magnetisation. This work has shown clearly what criteria are needed to optimise the magnetoresistance and how these conditions may be met by adding a buffer-layer thus making granular films based on ZnO more suitable for applications as field sensors.
The discovery of the giant magnetoresistive effect (GMR) [
Dilute magnetic semiconductors offer an effective way to produce spin-injection into a semiconductor, whereby transition metal (TM) ions are uniformly substituted as cations into the semiconductor host lattice. Homogeneous, dilute magnetic semiconductors may be formed, of which doped ZnO is a typical example. Moreover, ZnO barrier-based magnetic tunnel junctions [
Experimental parameters, including the thicknesses of Co and ZnO layers and also post-annealing process, have been optimised in order to obtain large MR values as previously reported [
Co-ZnO films of varying thickness, with or without a buffer-layer of ZnO, were prepared on thermally oxidised Si(100) by magnetron sputtering at RT. The thickness of the ZnO buffer-layers were chosen to be 5 nm, 10 nm, 15 nm, 20 nm, 35 nm, 50 nm, 75 nm, 100 nm, 150 nm. The nominal structure of Co-ZnO films in all the samples is [Co(0.6 nm)/ZnO(0.7 nm)]10; this was achieved by sequentially depositing an ultra-thin 0.6 nm Co layer and a 0.7 nm ZnO layer for 10 periods. The Co and ZnO deposition rate is 0.041 nm/sec and 0.056 nm/sec, respectively. For these thin layers, granular films (GFs) are formed, rather than a multilayer [
The structures of the samples were investigated by X-ray diffraction, XRD and transmission electron microscopy, TEM. Auger electron spectroscopy, AES depth profiling was performed in order to obtain the composition of the samples and observe the diffusion at the interface between a GF layer and a buffer-layer or a substrate. X-ray photoelectron spectroscopy, XPS was also performed to investigate the composition and the chemical state of Co in the samples. The magnetic field dependence of MR at RT was measured by using a four-probe method with the current in the plane. The maximum applied magnetic field was 18 kOe. Zero-field-cooled and field-cooled (ZFC/FC) magnetic moments of the samples were measured from 2 K to 300 K in 100 Oe using a SQUID magnetometer with a field applied parallel to the film plane. The magnetic properties of the thin films were measured using SQUID magnetometer at 5 K.
The observed MR for GF-based samples in the maximum applied magnetic field of 18 kOe at RT is shown in
The metallic Co fraction in the GF layer was obtained from XPS. The Co 2p core-level XPS spectrum of the GF sample with a ZnO (20 nm) buffer-layer is shown in
tallic Co to Co2+ ions for the GF layer is ~0.5 from a comparison between the areas of their 2p3/2 peaks, and the atom ratio of Zn/Co of ~0.6 was also obtained from the XPS data. Therefore, the atomic concentration of metallic Co in the GF layer is equal to 1/(1 + 0.6) × 0.5/(1 + 0.5) = 20.8%. This is considerably higher than that found in our previous study of Co/ZnAlO samples where the metallic Co atomic concentration in that sample was only 4.09%, resulting from a much lower ratio of metallic Co to Co2+ ions of ∼0.145 [
In order to analyse the interface between the GF layer and the substrate or the ZnO buffer-layer, AES compositional depth-profiles were obtained by Ar+ etching. Results for the GF samples grown without a buffer-layer and with a ZnO (50 nm) buffer-layer are shown in
The composition of the GF layer was obtained by both AES and XPS. The concentrations of the three main elements Zn, Co, and O were obtained by averaging the results from AES and XPS, in consideration of the certain deviation from different methods. It is found that the concentrations of the three main elements Co, Zn, and O in GF layer are 39.3 ± 2.0 at.%, 23.1 ± 1.2 at.%, and 37.6 ± 1.9 at.%, respectively; this also implies that the microstructure of the GF layer may be a mix of metallic Co particles and semiconducting Zn1-xCoxO1-δ grains. The above discussion of the XRD, TEM, XPS, and AES of the samples indicates that the GF layer has a granular structure, in which the Co particles are dispersed in a semiconductor matrix and that Co diffusion may occur at the interface between GF and ZnO buffer-layer.
Further information is also obtained from the dependence of the resistance on the thickness of the buffer-layer which is shown in
ZFC/FC magnetic moment measurements were also performed, in a field of 100 Oe, in order to study the magnetic behavior of the Co particles in the different GF-based samples, as shown in
The difference between the Co particles in the GF grown with and without a buffer-layer is most pronounced in the Curie-Weiss plots made at temperatures well above the Tbs. The susceptibilities follow the Curie-Weiss relation, χ = C/(T + θ), where CGF = 533.51 emu Oe−1deg−1, Cbuffer/GF = 158.52 emu Oe−1deg−1, θGF = 75.85 K and θbuffer/GF = 2.90 K. A positive Curie-Weiss constant, θ value characterises the presence of an antiferromagnetic interaction between the nanoparticles. A good MR material has very little coupling between the magnetic clusters so that the magnetisation of each cluster is free to respond to an external field; hence the reduction in θ caused by the buffer-layer is very beneficial to an increased MR. This can be evidenced from the larger MR of GF with a buffer-layer compared with that of the GF without a buffer-layer.
The change in the Curie constant, C, by a factor of ~3 indicates that the mean size of the nanoparticles has been reduced strongly by the inclusion of a ZnO buffer layer. The Tb and the Ms were smaller in the film grown with a buffer layer by 40% and 20%. This indicates that the inclusion of the buffer layer has resulted in a modest decrease in the mean size of the nanoparticles but a much larger decrease in the average of the mean square size due to a reduction in the width of the size distribution due to the elimination of some of the largest nanoparticles.
According to the above discussion about microstructures and magnetic properties of the GF samples, there are two effects that act together to increase the MR in the films grown with a buffer layer. These are the reduction of the θ from 75.85 K to 2.9 K and the reduction of the size of the nanoparticles which increases the efficiency of the Coulomb blockade [
We now consider the physical processes that may be taking place at the interfaces between the GF and the silicon substrates and with the buffer. We have found that the inclusion of the buffer layer has resulted in a composite structure in which the mean size of the nanoparticles, the width of the particle size distribution, the Ms and the C and θ have all decreased and the MR and the resistance have increased. The inclusion of the buffer layer will relieve strain and also can allow for cobalt atoms to diffuse into the ZnO buffer and may facilitate oxygen atoms to diffuse into the GF layer eliminating some of the oxygen vacancies.
There is evidence for interdiffusion at the interface between the GF and the buffer from the AES data which monitors the width of the interface which increases to ~11 nm for a thick buffer. The MR and the resistance depend strongly on the thickness of the buffer layer for thicknesses less than ~20 nm which is the range over which diffusion is likely to occur. The diffusion of Co out of the GF will reduce both the size of the nanoclusters as well as the amount of Co2+ in the semiconductor. This will reduce the magnetisation of the film and the size of the clusters. The diffusion of oxygen into the GF will reduce the concentration of oxygen vacancies which will increase the resistance and decrease the magnetism [
In this work, the influence of ZnO buffer-layer on microstructures and MR properties of Co-ZnO samples was studied. Co-ZnO GF layer consisted of Co particles dispersed in a semiconductor matrix. Moreover, the interface between the GF and the buffer-layer is more diffuse than the GF on oxidised Si, which may be due to the diffusion of Co and O ions between the GF and the ZnO buffer-layer. The combination of GF and a buffer-layer has a higher RT MR than the GF on an insulating Si substrate. The GF sample without a ZnO buffer-layer shows negative MR ratio of 8.3% at RT. Adding ZnO buffer-layers increases the negative MR ratio of GF-based sample to 11.9%. The similar maximum of their ZFC magnetizations implies the similar average size of the Co particles; the only difference is the wider distribution of Co particle size in Co-ZnO sample without a buffer-layer. Therefore, these phenomena may be related with the difference of Co intergrain interactions, the distribution and the sizes of the Co nanoparticles, the number of oxygen vacancies in GF layer and the thickness of GF layer due to the diffusion at the interface in the sample with and without a ZnO buffer layer.
This paper has shown the combination of GF and a ZnO buffer-layer has a higher MR than the GF on an insulating Si substrate and has shown why this occurs. ZnO-based GF with a ZnO buffer-layer may be more suitable for applications as field sensors.
This work was supported by the National High Technology Research and Development Program of China (863 Program, No. 2014AA032904), the National Science Foundation of China (Nos. 51025101,11274214, 61306109), and the Research Foundation for the Doctoral Program of Higher Education (No. 20101404120002).