Characterization of Fe-doped In-Sb-Te ( Fe : 10 at . % ) material with individual electrical-phase-change and magnetic properties

We propose a new electrical-phase-change magnetic material, namely Fe-doped In-Sb-Te (FIST), for possible non-volatile multi-bit memory applications. FIST was formed by typical co-sputter method with Fe 10 at.% doping in In3Sb1Te2. FIST offers the electrical-phase-change and magnetic properties by way of the change of In 4d chemical bonding density and embedded Fe nanoclusters with the size of 4∼5 nm, respectively. It maintained the amorphous phase on the electrical-phase-change. Chemical state of In was only changed to increase the density of In-In chemical bonding during the electrical-phase-change without Fe nanoclusters contribution. Also, the magnetic property by Fe nanoclusters was not changed by the electrical-phase-change. On this basis, we propose the FIST material with the individual electrical-phase-change and magnetic properties for the multi-bit nonvolatile memory materials.


I. INTRODUCTION
2][3][4][5] The main and key properties of these phase-change materials are the extreme changes of both optical reflectivity and electrical resistance during the amorphous-to-crystalline phase-change occurring within the 100 ∼ 400 o C temperature range. 4,5  2 Sb 2 Te 5 ternary alloy (GST) is used especially widely as the main material in both applications, owing to its proper phase-change temperature of 180 o C and its high phase-change speed. 4,5 ][11] Samsung Electronics Co. Ltd. officially announced several years ago a GST-based 512 Mb PRAM. 12Recently as well, In-Sb-Tebased chalcogenide materials were found to have a multi-level structural phase-change property. 13un Tae Kim et  three times as increasing the temperature.Studies such as these have demonstrated the highly promising applications of phase-change materials to multi-level optical media and multi-bit memory devices.
W.-D. Song et al. have commended the utility of Fe-doped Ge 2 Sb 2 Te 5 (FGST) phase-change magnetic material with multi-functional properties. 14They highlighted the variable properties in one material, along with a new degree of freedom associated with the spin carrier and resistance change. 14It can be associated with a growing list of possible and promising applications of both PRAM and spintronics.If this potential is to be realized to any significant degree, the research on phase-change magnetic materials will have to focus on key materials with multi-functional properties.
We here propose a new electrical-phase-change magnetic material, namely Fe-doped IST (FIST), for possible non-volatile multi-bit memory material applications.This material offers resistance change and magnetic information properties by way of electrical-phase-change and embedded Fe nanoclusters.In the present study, we observed this new electrical-phase-change system without the structural phase-change.

II. EXPERIMENTAL DETAILS
Fe-doped In 3 Sb 1 Te 2 (FIST) was formed by co-sputtering with Fe and In 3 Sb 1 Te 2 (IST) targets onto a Si substrate.The deposition rates of Fe and IST were 0.8 and 9.5 Å/sec, respectively.(Fe: 10 at.% in IST) The deposition power, time, and pressure were 500 W, 120 sec, and 10 mTorr, respectively.The thickness of the formed thin film was 100 nm.We measured the resistance as increasing temperature by using a halogen lamp in vacuum chamber with Ar gas under the pressure of 1 mTorr.The ratio of temperature in time was 3 o C/min.Prior to the synchrotron experiment, the FIST thin film was fabricated by Ne + ion mild sputtering in an ultra-high-vacuum chamber for 1 hour at the beam energy of 1 kV.]10 Utilizing XPS measurement, we confirmed a clean FIST thin film lacking the O 1s core-level peak. 19In order to confirm the phases of the samples, their high-resolution synchrotron x-ray powder diffraction data were measured at 8C2 beamline of Pohang Accelerator Laboratory (PLS).The incident x-rays were vertically collimated by a mirror, and monochromatized to the wavelength of 1.5490 Å using a double-crystal Si(111) monochromator.The Fe L-edge absorption and core-level spectra were obtained by NEXAFS and HRXPS, respectively, at the 8A1 beamline of PLS.The magnetic properties were obtained using a superconducting quantum interface device (SQUID) magnetometer.The x-ray magnetic circular dichroism (XMCD) measurements were carried out at the 2A beamline of PLS.The energy resolution of incident light was set to 0.3 eV and the circular polarization ratio was higher than 95%.The spectra were obtained in total electron yield mode with an ∼100 angstrom probe depth.All of the measurements were performed in a UHV environment so as to exclude surface contamination.An electromagnet was used to flip the 0.9 T magnetic field at every photon energy point.All of the spectra were normalized by the intensities of the incident photon beam, which were measured by means of an Au grid positioned in front of the XMCD chamber.

A. Resistance change with XRD and TEM observation
The resistance change was clearly shown by the annealing process at the temperature of 49 o C (Fig. 1(a)).The resistance was changed from 1.0×10 3 to 2.0×10 2 /sq.Also, the resistance was almost unchanged with a temperature increase to 100 o C, and there was stable resistance of 2.0× 10 2 /sq.Before and after annealing, both of the A and B structural phases in Fig. 1(a) were amorphous measured by x-ray diffraction (XRD) with synchrotron radiation.Both the as-received (a-FIST_1) and resistance change samples (a-FIST_2) showed that the resistance change was not due to the amorphous-to-crystalline phase-change.entailed heating at over 300 o C for 10 seconds.However, after the high temperature process, the nanocluster size was observed to increase to 22 ∼ 26 nm, as shown in Fig. 1(c).We assumed that nanoclusters had been made to combine with each other at the high temperature.Also, we confirmed that nanoclusters were not structurally stable at high temperature.

B. Atomic structure of nanoclusters
The nanoclusters' atomic and chemical information was analyzed by near-edge x-ray absorption of fine structure (NEXAFS) and extended x-ray absorption of fine structure (EXAFS) experiments using synchrotron radiation.The x-ray absorption spectra of the Fe L-edge for both the a-FIST_1 and a-FIST_2 are plotted in Fig. 2(a).These show that the two peaks (L 2 and L 3 ) shared the same shape and energy position, indicating thus that the chemical state of Fe is only-metallic Fe (i.e.Fe-Fe bonding).The EXAFS data in Figs.2(b)-2(d) constitutes additional evidence of only-metallic Fe bonding.We obtained the Fe K-edge absorption spectra to confirm the points of difference among Fe foil, a-FIST_1, and a-FIST_2.However, the absorption spectra of a-FIST_1 and a-FIST_2, as shown in Figs.2(b) and 2(c), were identical.As shown in Fig. 2(d) and as obtained by the Fourier transformation from the absorption spectra, we did not observe the other bondings except Fe-Fe bonding with 2.2 Å.In these TEM image results, we assumed that the nanoclusters were composed of only-Fe atoms.We confirmed that the formed thin film was the Fe-nanoclusters-embedded IST system.

C. Magnetic property of Fe nanoclusters
We investigated the magnetic properties of the samples using a superconducting quantum interference device (SQUID; Quantum Design) magnetometer.Figures 3(a) and 3(b) show the temperature-dependent magnetization (M-T) data under an applied magnetic field of 1000 Oe along with the magnetization hysteresis loops measured at the selected temperatures of 10, 110, 200, 250 and 300 K, respectively.The magnetization monotonically decreased with temperature, from 150 emu/cm 3 at 10 K to 45 emu/cm 3 at 300 K.At all of the measured temperatures, the film showed hysteric M vs. H curves with small coercive fields, which is characteristics of ferromagnetic ordering of Fe atoms.The spectra in Figs.3(c) and 3(d) clearly show that the ferromagnetism of these samples originated from the Fe clusters.Even though there were several fine structures on the higher-energy side of the x-ray absorption spectra (XAS), which suggested the existence of non-metallically bonded Fe ions, they did not contribute to the x-ray magnetic circular dichroism (XMCD) spectra.Indeed, Figs. 3(c) and 3(d) show nearly identical line shapes for both, confirming that the magnetic origin was the Fe metal.We confirmed that the magnetic information appeared by Fe nanoclusters was not changed during the resistance change.Therefore, we assumed that the resistance change and magnetic properties were not correlated in this system.

D. Analysis of chemical states for the electrical-phase-change
The core-level spectra of Fe 3p, Te 4d, Sb 4d, and In 4d during the resistance change are plotted in Figs.4(a)-4(d).In the Fe 3p core-level spectra, we observed that the binding energies were 52.7 eV for both a-FIST_1 and a-FIST_2.This shows the typical metallic Fe-Fe bonding, corroborating the TEM, NEXAFS and EXAFS results. 15As shown in Fig. 4(b), the Te 4d core-levels did not change the peak shape or the 4d 5/2 binding energy of 40.1 eV on the a-FIST_1 and a-FIST_2 phases.These peaks were similar to those for the Ge 2 Sb 2 Te 5 , GeTe, and N-doped Ge 2 Sb 2 Te 5 systems. 10owever, we observed changes of the Sb and In 4d core-level spectra during the resistance change.The chemical shift of the Sb 4d 5/2 core-level between a-FIST_1 (31.6 eV) and a-FIST_2 (31.7 eV) was 0.1 eV.However, the peak shape and full-width at the half maximum (FWHM) of the Sb 4d core-level was not altered during the resistance change.The energy shift at the higher binding energy signalled that the Sb atoms were located in a more chemically tight environment. 16In the case of the In 4d core-levels, we observed the new shoulder at around 16.8 eV after the resistance change.In these analyses of the core-level data, we assumed that the chemical states of the embedded Fe nanoclusters had not been changed by the chemical bonding to the other atoms during the resistance change.In fact, this bonding had not contributed to the resistance change.The contribution of the resistance change was assumed to be only the change of the chemical bonding states of the Sb and In 4d core-levels.In order to analyse the spectra in detail, the In 4d core-level spectra of both a-FIST_1 and a-FIST_2 were curve-fitted using Doniach-Sȗnjić curves convoluted as shown in Fig. 5(a). 17The background due to inelastic scattering was subtracted by the Shirley (or integral) method. 18We found that the three peaks were convoluted in the In 4d core-level spectra of both a-FIST_1 and a-FIST_2.The 4d 5/2 core-level binding energies of In 4d-1, In 4d-2, and In 4d-3 were 16.9, 17.5 and 17.9 eV, respectively.Also, the chemical states of In 4d-1, In 4d-2 and In 4d-3 were In-In, In-Sb and In-Te bondings, respectively.We performed a relative intensity area calculation of the curve fittings to find the chemical state concentrations during the resistance change.As Fig. 5(b) illustrates, the number of In-Te and In-Sb bondings during the resistance change was decreased by 2 and 8%, respectively.However, the number of In-In bondings was increased by 10%.In these results, we assumed that the origin of the chemical phase-change (resistance change) without structural phase-change was the increase of the number of In-In metallic bondings.

IV. CONCLUSIONS
In summary, we proposed a new electrical-phase-change magnetic material formed by cosputtering, Fe-doped IST, and measured its structural, magnetic and chemical properties by using synchrotron radiation.We observed the Fe-nanoclusters-embedded IST and the lack of change of Fe-Fe chemical bonding during the electrical-phase-change at the temperature of 49 o C.However, the size of the Fe nanoclusters was increased from 4 ∼ 5 nm to 22 ∼ 26 nm in the high temperature process at the temperature of 300 o C over the duration of 10 sec.We assumed that the origin of  the phase-change with the change of resistance was the increase of In-In chemical state without the effect of both the Fe nanoclusters and the structural amorphous-to-crystalline phase-change.From these results, we assumed that the magnetic and electrical-phase-change properties were individual origins with the Fe nanoclusters and the increase of the In-In chemical state, respectively.On this basis, we assumed that this FIST material with multi-functional properties showed a possibility of non-volatile multi-bit memory material applications.

Figures 1 (
Figures 1(b) and 1(c) show transmission electron microscopy (TEM) images of a-FIST_2 and high temperature process, respectively.In Fig. 1(b), we found nanoclusters with the size of 4∼5 nm.Nanoclusters were randomly distributed in the a-FIST_2 sample with no change the size of a-FIST_1.It is very stable during the resistance change.We performed the high temperature annealing process to confirm the structural stability of nanoclusters.The high temperature process

FIG. 4 .
FIG.4.Core-level spectra of (a) Fe 3p, (b) Te 4d, (c) Sb 4d, and (d) In 4d as measured by HRXPS.During the phase-change, the core-level peaks of Fe and Te did not change with the binding energy and shape.However, the Sb 4d core-level peaks shifted at the higher binding energy of 0.1 eV, and In 4d core-level spectrum appeared a new shoulder at the binding energy of 16.8 eV.

2 FIG. 5 .
FIG. 5. (a) Curve fittings of In 4d core-level spectra for both a-FIST_1 and a-FIST_2.(b) plots the relative area intensity as calculated by the curve fittings.