Phase Transformation through Metastable Structures in Atomically

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Phase Transformation through Metastable Structures in Atomically Controlled Se/Sb MultiLayers Ju Heyuck Baeck, Tae Hyeon Kim, Hye Jin Choi, Kwang Ho Jeong, and Mann-Ho Cho* Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea

bS Supporting Information ABSTRACT: Multilayer films composed of individual layers of [Sb(8.84 Å)/Se(12.6 Å)](Sb4Se6), [Sb(8.84 Å)/Se(7.2 Å)](Sb4Se4), and [Sb(15.4 Å)/Se(7.2 Å)](Sb6Se4) were synthesized using effusion cells controlled at the subatomic scale. After an annealing process, the Sb4Se6 multilayered film with an Sb2Se3 orthorhombic structure had a high resistance and a clean valence band edge similar to that for a band shape of a semiconductor, whereas the Sb6Se4 film with an Sb rhombohedral structure and an Sb2Se3 orthorhombic structure had a low resistance and a band tail that originated from their metallic characteristics in the near Fermi level. In the case of Sb4Se4, a metastable Sb4Se4 monoclinic structure was induced at an annealing temperature of 200 °C because of the unstable, local, and anisotropic distribution of each element in the vertical direction of multilayer films with a specific stoichiometry. Moreover, the nonbonding states originating from a band-gap state were generated in the film with a metastable structure. When the annealing process was conducted at 256 °C, the linear diffusion of elements in the film induced the most stable crystal structure with a stable stoichiometry. That is, the multilayered Sb4Se4 film underwent a steplike resistance change through a two-level phase change process. The findings indicate that a multilayered system with an atomically controlled thickness can be utilized to control the electrical resistance, metastable phase formation, and the valence band structure in an Sb
Se alloy system.

’ INTRODUCTION In recent years, chalcogen-based alloys have been actively studied for use in next-generation memory devices and optical applications because of their speed of crystallization (within the nano time scale), good photoconducting properties, and ease of nanoscale device fabrication.1
4 Alloys that undergo a phase change process as the result of the application of thermal energy or laser irradiation show large differences in electrical conductivity and optical reflectivity between amorphous and crystal states. These phenomena are well known in the cases of phase change materials, including Ge
Sb
Te ternary and Te-based binary alloys.5
7 However, the potential of positive attributes of these materials, such as crystallization
amorphization speed and thermal stability of the alloys, are not sufficient for use in next-generation memory devices.8,9 Among the many phase change materials examined, Sb
Se alloy systems have not been extensively studied, and some of the reported results in Sb
Se alloy system have conflicted with those of Ge
Sb
Te alloys.10 On the basis of the orthorhombic crystal structure of Sb2Se3 (lattice parameters: a = 11.62, b = 3.11.77, and c = 3.96), the reported optical and thermal characteristics are limited (in the case of the Sb2Se3 orthorhombic crystal structure, the atoms in the crystal structure are uniquely arranged and the atoms between the layers are bonded by van der Walls forces11
13). In addition, except for Sb2Se3 in the Sb
Se alloy system, the nature of the crystal structures produced as a function of stoichiometry has not been reported. Currently, the reported phase change properties of general Sb
Se alloys indicate that phase change occurs at a temperature between r 2011 American Chemical Society

200 and 250 °C, the speed for phase transition is about 30 ns, which is faster than that for a Ge
Sb
Te alloy, and the change in the ratio of resistance is 104 scales before and after phase change.10,14 The results indicate that Sb
Se alloys have the potential for use in nextgeneration memory devices. In the study, we report on the electrical properties, electronic structure, and conduction mechanism of various Sb
Se films deposited using an atomically controlled multilayer method. The findings indicate that a new crystal structure of a metastable phase is formed in the Sb4Se4 system, which has not been reported previously. Moreover, the crystal structure and its electronic structure are the inherent result of the formation of an atomically controlled film with a layered structure.

’ EXPERIMENTAL SECTION Growth of Atomically Controlled Multilayers. We fabricated multilayer films composed of elemental layers of Sb and Se, in an attempt to produce phase change materials with various electrical properties and to investigate atomic reactions that occur between the elements at the interface. In our experiment, each elemental layer was deposited using a Se and Sb effusion cell source under base pressures of 1  10
9 Torr and each cell shelter was independently controlled by means of a quartz thickness controller. The growth rate of Sb and Received: March 15, 2011 Revised: June 3, 2011 Published: June 04, 2011 13462

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Figure 1. RBS spectra (2 MeV) of multilayer films composed of individual layers of (a) [Sb(8.84 Å)/Se4(12.6 Å)] for Sb4Se6, (b) [Sb(8.84 Å)/ Se(7.2 Å)] for Sb4Se4, and (c) [Sb(15.4 Å)/Se (7.2 Å)] for Sb6Se4.

Se was maintained at 2 Å/s during the growth process. The thickness and density of the elemental layer (Sb or Se) were calibrated by X-ray reflectivity (XRR) measurements, and the total thickness and stoichiometry of the multilayer films were determined from basic information on elemental layers. The values for films synthesized by the multilayer deposition method were consistent with Rutherford back scattering (RBS) data. Through comparing the experimental values with predicted values, we also confirmed the total thickness and stoichiometry of multilayer films of three types using a Rutherford back scattering (RBS) method, as shown in Figure 1. The multilayer films were composed of repeated unit layers of Sb(8.84 Å)/ Se(12.6 Å), Sb(8.84 Å)/Se(7.2 Å), and Sb(15.4 Å)/Se(7.2 Å). The elemental layers were calibrated by XRR, and the repeating numbers of the unit layers were determined to be 25, 32, and 45 for the multilayer films, respectively. The predicted total thickness (stoichiometry) of each film was 536 Å (Sb0.41Se0.59), 513 Å (Sb0.55Se0.45), and 1020 Å (Sb0.68Se0.32), respectively. The RBS experimental data were in reasonable agreement with the fitting data, and the results show that the measured total thickness (stoichiometry) was 550 Å (Sb0.38Se0.62), 515 Å (Sb0.51Se0.49), and 1050 Å (Sb0.63Se0.37) respectively, as shown in Figure 1. When the predicted values were compared with the measured value, the error ranges for thickness and stoichiometry were about 0.4 Å and 0.14% per periodic number, respectively. The results of RBS experiments indicate that samples synthesized by a multilayer method are sufficiently reliable to permit the physical properties to be quantitatively determined. The multilayer films composed of unit layers of Sb(8.84 Å)/Se(12.6 Å), Sb(8.84 Å)/Se(7.2 Å), and Sb(15.4 Å)/Se(7.2 Å) are hereafter referred to as Sb4Se6, Sb4Se4, and Sb6Se4, respectively, in the remainder of this paper. Measurement of RBS (Rutherford Back Scattering) and Sheet Resistance. RBS measurements were carried out to measure the thickness and stoichiometry of each multilayer film. In these experiments, we used an incident He2þ beam with an energy of 2 MeV, and the angle of the detector, with an energy resolution of 14 keV, was fixed at an angle of 170° with respect to the direction of the incident beam. To observe electrical properties, after the deposition of each multilayer film with a thickness of 100 nm on glass, we measured

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the sheet resistance for each annealing temperature without interrupting the vacuum at 1  10
9 Torr by using a four-point probe and a halogen heating lamp. This method can protect films from contaminants, such as oxygen, carbon, and organic particles, and permits the critical temperature point for the phase change that accompanies the change in sheet resistivity to be determined. Experiments for Confirmation of Phase Change. At each critical temperature confirmed from a change in sheet resistance, X-ay diffraction curves, TEM images, and Raman spectra were obtained to permit the change in crystal structure to be examined. The phase change and lattice constants in multilayer films were analyzed from XRD (X-ray diffraction) curves using a monochromatic X-ray at a wavelength of 1.54056 Å. We also confirmed the formation of a metastable phase and a stable phase in the atomically controlled film through cross-sectional TEM (transmission electron microscopy) images. To protect specimens from thermal damage, the samples for TEM measurement were produced using a liquidnitrogen-cooled stage at a temperature of 100 K. Raman spectral data were used to observe short-range ordering, the difference in crystal structure between metastable and stable phases, and the segregation of elements. The wavelength and the power of the light source used were 514.532 nm (Ar-ion laser) and 0.1 mW, respectively, and multilayer films with a thickness of 100 nm were prepared on Si(001) for the Raman experiments. Experiments for Confirmation of Electronic Structure. To investigate changes in the electronic structure through the diffusion process, we fabricated multilayer films composed of unit layers of Sb(8.84 Å)/Se4(12.6 Å), Sb(8.84 Å)/Se(7.2 Å), and Sb(15.4 Å)/Se(7.2 Å) on a Si(001) substrate. The thickness of these multilayer films was 50 nm, and the specimens were transferred at a pressure of 10
8 Torr to permit XPS (X-ray photoemission spectroscopy) and UPS (ultraviolet photoemission spectroscopy) spectra of the films to be collected without interrupting the vacuum. In the XPS and UPS experiments, the light sources used to confirm changes in the chemical bonding state and electron structure of each multilayer film were a monochromatic X-ray (Al KR) and an unfiltered UV (He I) source. The Fermi edge was determined from UPS and XPS spectra collected on a clean Au sample, and the core-level energy was calibrated using clean Au 4f. After transfer to a spectral chamber, the chemical reactions of elements and changes in band structures were investigated using XPS and UPS experiments at the isothermal phase change temperatures for each sample and then returned to room temperature.

’ RESULTS AND DISCUSSION The sheet resistance of the Sb4Se6, Sb4Se4, and Sb6Se4 films changed from high to low states with increasing annealing temperature from room temperature to 300 °C in all cases, as shown in Figure 2. However, it was possible to detect distinguishing differences in resistance change depending on the structure of the multilayer. After the annealing process, the sheet resistance of Sb4Se6 decreased within a comparable order (a change from 500 to 30 MΩ), whereas the corresponding values for Sb4Se4 and Sb6Se4 decreased significantly (a change from 200 and 3 MΩ to 1.7 KΩ and 10 Ω, respectively) when compared with that of the Sb4Se6 film. The resistance change in chalcogenide materials is consistent with phase change characteristics transformed from an amorphous to a crystalline state. In the case of the Sb6Se4 specimen, in which the film has a higher Sb ratio than the other two films, the resistance after phase 13463

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Figure 2. Temperature dependence on sheet resistance for (a) Sb4Se6, (b) Sb4Se4, and (c) Sb6Se4 multilayer films and temperature dependence on sheet resistance of a codeposited SbSe film in the inset.

change was similar to that of the Sb bulk material, as shown in Figure 2, curve c. In the case of the Sb4Se6 film, the sheet resistance of 30 MΩ after the annealing process is in good agreement with the reported resistance value for Sb2Se3 with an orthorhombic crystal structure, as shown in Figure 2, curve a.15 In particular, the Sb4Se4 film shows a distinct change in resistance at two levels (change from 200 to 30 MΩ and, lastly, a decrease to 1.7 KΩ), and the critical temperature for phase change is relatively low (170 °C) in the case of the first level change, compared with the other samples, as shown Figure 2, curve b. In the case of the codeposited SbSe film, a two-level resistance change was not observed and the phase change temperature (235 °C) was similar to that for the second level of the multilayer Sb4Se4 film, as shown in the inset of Figure 2, indicating that resistance change at two levels in the Sb4Se4 film is a unique characteristic that is induced by the presence of a multilayer structure. A resistance change at two levels in multilayer Sb4Se4 films has not been reported yet. Therefore, the focus of this study was on the unique resistance change of an Sb4Se4 multilayer film grown by an alternative molecular beam. To confirm that the phase transition is related to the change in sheet resistance, the crystal structure of the samples was investigated by XRD (X-ray diffraction) measurements. The experiment was performed with films that had been annealed at the temperature designated by the arrow points shown in the sheet resistance data because the sheet resistance drastically drops at a specific temperature region and is closely related to the phase transition, as shown in Figure 2. The XRD curves show that the as-grown multilayer films, initially in an amorphous state, were transformed into a crystalline state after the annealing treatment in all cases. The observed peak for the Sb4Se6 film corresponds

to a single phase of an Sb2Se3 orthorhombic structure with lattice parameters of a = 11.62 Å, b = 11.77 Å, and c = 3.962 Å.16 No phase separation was observed at annealing temperatures of 200 and 276 °C, as shown in Figure 3a. The sheet resistance value in Figure 2, curve a, is also consistent with that reported for an Sb2Se3 orthorhombic structure. The results from sheet resistance and XRD measurements show that the crystal structure of the multilayered Sb4Se6 film was well-formed through interdiffusion between the interface of the Sb and Se layers and reactions between Sb and Se. In addition, the stoichiometry of the crystal structure is consistent with the initial ratio of the elemental layers. In the case of the Sb6Se4 specimen, the diffraction peaks indicate that the amorphous structure crystallized to an Sb2Se3 orthorhombic and an Sb rhombohedral structure with lattice parameters of a = 4.3 Å, c = 11.27 Å, and γ = 120°.17 In particular, the extraction of Sb at annealing temperatures of 235 and 274 °C indicates that phase separation had occurred, as shown in Figure 3c. In comparing the sheet resistance change with the crystal structure, the segregation and crystallization of Sb result in the low resistance in the Sb6Se4 film. The XRD peaks in the Sb4Se4 film are more complicated after the annealing treatment; that is, new peaks can be seen, which were not observed in other annealed multilayer films, as shown in Figure 3b. The peaks (2θ = 32.1, 29.9, and 21.9°) were observed in the film annealed at 200 °C and disappeared after an annealing treatment at 256 °C. Moreover, the peaks corresponding to the Sb2Se3 orthorhombic structure and the Sb rhombohedral structure were enhanced at 256 °C. These results suggest that the Sb4Se4 multilayer film annealed at 200 °C has a metastable phase, which causes a distinct resistance change at two levels. It would be very interesting to 13464

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Figure 3. X-ray diffraction patterns of (a) Sb4Se6, (b) Sb4Se4, and (c) Sb6Se4 multilayer films annealed at a critical phase change temperature and X-ray diffraction patterns of (d) a codeposited SbSe film annealed at the critical phase change temperature.

compare the XRD curve and sheet resistance of codeposited films with those of multilayer films, because the kinetic process for crystallization in an alternatively layer-controlled film could be obtained. The comparison indicates that the metastable phase is very unique and is characteristic of a multilayered structure; that is, a metastable structure was not observed in codeposited SbSe films, as shown in Figure 3d. Moreover, the broad hollow shape between 15° and 25° even in the sample annealed at 256 °C indicates that the crystallization in the codeposited film is very low and the local crystalline Sb2Se3 structure is embedded in an amorphous-like structure. Thus, the stoichiometry and the anisotropic distribution of each element in multilayered structures can induce the formation of unstable bonding structures in the vertical direction.18 Finally, the system reaches a thermodynamic equilibrium state through the diffusion of elements by the thermal energy, that is, resulting in the formation of the most stable crystal structure with a suitable stoichiometry through the formation of a metastable phase. Moreover, the elements that remain after the formation of the most stable stoichiometric structure can undergo a segregating process. Evidence for this can be found in the XPS and TEM measurements described below. First, to define the crystal structure of the metastable phase, we performed density functional theory-generalized gradient approximation (DFT-GGA) calculations using the PBE functional and geometry optimization with respect to the results from the XPS measurement (detailed results of the XPS measurement are discussed below). To model the metastable crystal structure, we used the space group symmetry (P21/n[14]) of monoclinic As4Se4 with lattice parameters of a = 9.55 Å, b = 13.80 Å, c = 6.71 Å, and β = 106.44°19 because the physical and chemical characteristics of As are very similar to those of Sb. In particular, the electronic valence state of Sb in Sb4Se4 is similar to that for As4Se4.20 The calculated metastable crystal structure is identified

Figure 4. Unit cell of (a) an Sb2Se3 orthorhombic structure and (b) an Sb4Se4 monoclinic structure calculated using the PBE functional and geometry optimization.

as an Sb4Se4 monoclinic structure (space group P21/n[14]) with lattice parameters of a = 10.66 Å, b = 15.64 Å, and c = 7.60 Å and a lattice angle of β = 105.23°, as shown in Figure 4b. In light of the d-spacing calculated using the lattice parameters of the modeled structure, peaks located at 32.1, 29.9, and 21.9° in the XRD patterns of Figure 3b can be indexed as Sb4Se4 monoclinic (351), (311), and (221) planes, respectively. Next, to confirm the formation of a metastable structure in the Sb4Se4 film, we also obtained Raman spectra and high-resolution transmission electron microscopy (HRTEM) images. The Raman spectra were normalized and their backgrounds were subtracted from the raw spectra, as shown in Figure 5. The broad 13465

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Figure 5. Raman-scattering spectra of (a) as-deposited Sb, (b) as-deposited Se, (c) Sb4Se4 annealed at 200 °C, (d) Sb4Se4 annealed at 256 °C, and (d) Sb4Se6 annealed at 276 °C and Raman-scattering spectra of as-deposited Sb4Se4 in the inset.

band located in the 118
250 cm
1 region, caused by an amorphous phase, is observed in the as-grown Sb4Se4 film, as shown in the inset of Figure 5, while narrow and strong bands at 157, 166, 191, and 213 cm
1 are observed for the Sb4Se4 film annealed at 200 °C owing to the formation of a highly crystalline structure, as shown in Figure 5, spectrum c. The change from a broad band to a narrow band indicates that the amorphous state of the film is changed to a highly crystalline structure. In single layer films composed of only elemental Se, strong, narrow bands were observed at 143 and 240 cm
1, which are attributed to a
Se
Se
Se
chain structure and a Se8 ring structure.21 In the case of a single Sb layer, Raman shift frequencies at 113 and 151 cm
1 are induced by a polycrystalline structure (rhombohedral structure) with phonon line modes of Eg and A1g, respectively.22 In comparing the peaks caused by single Sb or Se layers with those by multilayered Sb4Se4 films, the structural change shows that the segregation of Sb and its subsequent crystallization are induced and chain and ring structures composed of pure Se are not formed in multilayered Sb4Se4 films because an ordered metallic Sb bonding is only generated, as evidenced by the shoulder peaks at 113 and 151 cm
1. The results showing the absence of segregation of Se and the segregation of Sb in annealed films are consistent with the XRD data. Moreover, the peaks indicate that the multilayered Sb4Se4 film with the amorphous structure is changed into crystalline Sb2Se3 with an orthorhombic structure (Raman shift peaks for the Sb2Se3 orthorhombic structure were located at 191 and 213 cm
1, as shown for the Sb4Se6 film annealed at 276 °C). However, the strong bands at 157 and 166 cm
1 do not correspond to any crystalline structure. After an annealing process at 256 °C, the

strong bands at 157 and 166 cm
1 disappear and a narrow peak at 151 cm
1, caused by the segregation of Sb, appeared, as shown in Figure 5, spectrum d. The changes in the strong bands located at 157 and 166 cm
1 are consistent with the change in the resistance and XRD, indicating the generation and extinction of a metastable phase. The results of the TEM measurement are also consistent with the Raman spectra and XRD curves. The Sb4Se4 sample annealed at 200 °C has various crystal structures (Sb4Se4 monoclinic, Sb2Se3 orthorhombic, and Sb rhombohedral structure). The boundary of the crystal region in the sample annealed at 200 °C is not cleanly divided in Figure 6a, whereas the boundary of the film after an annealing process at 256 °C is distinctly separated into three regions, Sb2Se3 orthorhombic, Sb rhombohedral structure, and an amorphous state in Figure 6b. The TEM results for the Sb4Se4 films annealed at 200 and 256 °C are also in agreement with the d-spacing value for the calculated Sb4Se4 monoclinic structure and the collected XRD data. The above findings show that it is possible to control the metastable phase and metallic segregation process in an Sb
Se alloy system through the fabrication method using an atomically controlled multilayer. We also investigated the chemical bonding characteristics and change in valence band through analyses of core-level spectroscopy and valence band structures, which can explain that the resistance change in Figure 2 is caused by electronic structures in multilayer films. To perform a quantitative analysis of the chemical bonding states, electronic structures, and atomic reactions before and after the annealing process, we obtained core-level spectra for each elemental layer of Sb or Se as references. The binding energies of 13466

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Figure 6. Cross-sectional TEM images and fast Fourier transform (FFT) of Sb4Se4 multilayer films annealed at (a) 200 and (b) 256 °C.

Figure 7. Core-level spectra of Sb 3d5/2, Sb 4d, and Se 3d obtained for each individual film layer for (a) Sb or Se and various multilayer films, such as (b) as-deposited Sb4Se6 and (g) Sb4Se6 annealed at 276 °C, (c) as-deposited, (h) Sb6Se4 annealed at 274 °C, (d) as-deposited Sb4Se4, (e) Sb4Se4 annealed at 200 °C, and (f) Sb4Se4 annealed at 256 °C. The circles are data points, and solid curves are fits. The relative binding energy scale is referred to the Sb 3d5/2, Sb 4d5/2, and Se3d5/2 binding energies for the major homopolar bonding state.

Sb 3d5/2, Sb 4d5/2, and Se 3d5/2 were measured at 32.1, 527.8, and 54.7 eV, respectively, which are consistent with the reported values.23,24 Using the referenced homopolar bonding energy, it is possible to measure the change in the relative binding energy in films with multilayer structures for each annealing temperature, as shown in Figure 7. In Figure 7d, the spectra for Sb 3d5/2 and Sb 4d of asgrown Sb4Se4 films were distinctly deconvoluted with three components, that is, Sb
Sb homopolar bonding (0.0 eV), Sb
Sb
Se bonding (0.6 eV), and Sb
Se bonding state (1.1 eV). Although

only XPS data for films with an amorphous state cannot be directly related to detailed atomic configurations, bonding structures, and bonding numbers, we can provide simple information on species of bonded next-neighbor atoms with respect to the structure of multilayer films with the local and anisotropic distribution of elements. Using the data for peak changes in annealed multilayer films, it is possible to explain the origin of Sb 3d5/2 and Sb 4d peaks that had been deconvoluted with three components, because the change in the crystal structure of the film, confirmed by XRD and TEM 13467

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Figure 8. Valence band photoemission spectra of Sb4Se6, Sb4Se4, and Sb6Se4 in (a) the region from 0 to 6 eV and (b) near the Fermi level. Each spectrum was obtained at the critical temperature for phase change.

)

experiments, is closely related to the chemical bonding state. Thus, comparing the Sb 3d peak for the Sb single layer film in Figure 7a with that of the annealed Sb4Se6 film in Figure 7g, it appears that the peaks located at 0 and 1.1 eV binding energy positions are originated from an Sb
Sb homopolar bonding state in the single Sb layer film and an Sb bonding state in the Sb2Se3 orthorhombic structure, respectively.25 As shown in the crystalline structure of Figure 4, the chemical bonding state in a core-level spectrum is mainly effected by the species bonded with nearest-neighbor atoms, except for a nonbonding state.26,27 In the case of an Sb2Se3 orthorhombic structure in Figure 4a, elemental Sb exists in two states, that is, SbI coordinated by three Se atoms (p-bonding configuration), forming a slightly distorted trigonal pyramidal structure, and Sb coordinated by five Se atoms (a resonance bonding configuration or a hybridization of p and d orbitals28), forming a tetragonal pyramidal structure.16 However, two states bonded only by Se atoms cannot be distinguished in the XPS spectra because of the limit of resolution of the XPS equipment. The results for the chemical binding of Sb indicate that the peak located at 0.6 eV cannot originate from an Sb in the Sb2Se3 orthorhombic structure. A more likely possibility is that the peak located at a binding energy of 0.6 eV is induced by an excess of elemental Sb in the Sb4Se4 film than that of a stoichiometric Sb2Se3 structure in the Sb4Se6 film. The representative crystal structure of V
VI group compounds with a high V group stoichiometry is a V4VI4 monoclinic structure, as shown in Figure 4b. In an Sb4Se4 monoclinic structure, the bonding structure of Sb has one state

coordinated by two Se and one Sb atom (Se2
Sb
Sb), resulting in only p
p bonding.27 On the basis of the fine structure, the peaks of 0, 0.6, and 1.1 eV can be assigned to Sb metallic bonding and Sb4Se4 monoclinic and Sb2Se3 orthorhombic structures, respectively. When the annealing temperature is increased from 200 to 256 °C, the area ratio of components located at 0.0, 0.6, and 1.1 eV change from 0.13, 0.33, and 0.54 to 0.21, 0.20, and 0.59, respectively. The results show that a metastable state with a monoclinic structure is transformed into a metallic Sb bond and a stable Sb2Se3 structure by the applied thermal energy. The Se 3d peak of the Sb4Se4 film at each temperature is not easily deconvoluted, because the Se atom is coordinated only by Sb atoms in Sb4Se4 and Sb2Se3 crystalline structures, as shown in Figure 4. On the other hand, the two states caused by metallic Se and Se
Sb bonding are cleanly distinguished, as shown in the Se 3d spectrum of the Sb4Se6 multilayer film of Figure 7b. In the case of as-grown Sb4Se6, an Sb metallic bonding peak is not observed in the Sb 3d and Sb 4d peaks because the thickness of the Se layer located up- and down-Sb the unit layer in the multilayer film is sufficient, while Se 3d peak is deconvoluted into two components (Se
Se metallic bonds and Se
Sb covalent bonds), as shown in Figure 7b. Through the annealing process, the Se metallic bonding peak disappears and a stable Sb2Se3 structure is formed. The stoichiometry, as determined by XPS, is consistent with the formula Sb2Se3. In the case of the Sb6Se4 multilayer film, Sb segregation is enhanced during the annealing process, as shown in Figure 7c,h, which is consistent with the XRD results. 13468

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The Journal of Physical Chemistry C We also measured the valence band of each multilayer film because the coordination and configuration of atoms better reflect the valence band structure than core-level bonding. In Figure 8, the backgrounds of all spectra were removed and were then normalized. The zero position for binding energy was determined with respect to the valence-band maximum (VBM) because the Femi level of semiconductor materials is not observed in PES (photoemission electron spectroscopy) measurements. Figure 8a shows the valence band structures for the Sb4Se6, Sb4Se4, and Sb6Se4 films at the each critical temperature. Generally, the valence band of V
VI group compounds can be divided into two regions in the near Fermi edge, that is, the nonbonding or lone pair bond state located at one region, 0
2.5 eV, and a p-like bonding state located at the other, 2.5
6.2 eV.20,29 The shape of the valence band shape of the Sb4Se6 film showed no distinguishing change, before and after phase transition. Moreover, the ratio of the lone pair bonding and p-like bonding states was not changed significantly during phase transition, although a small change in the fine structure in the region from 0 to 2.0 eV was observed. The clean shape of the valence band edge in the Sb4Se6 film shows the band structure of a well-defined semiconductor in the spectra of Figure 8b. Using the calculated band structure, it is possible to determine the fine structure in the lone pair bonding region. The findings indicate that this is induced by Se atoms in the welldefined crystal structures (see Figure S1 in the Supporting Information). On the other hand, the clean shape of the valence band for the Sb6Se4 film changes to a tailed band structure, as shown in Figure 8b for the Sb6Se4 film, resulting from the formation of an Sb rhombohedral structure with metallic characteristics (see Figure S2 in the Supporting Information). The results are consistent with the low resistance characteristics that develop after the phase transition, as shown by the resistance data. Finally, the Sb4Se4 multilayer film with a metastable phase at 200 °C shows a gap state between the valence and conduction bands because the nonbonding states are greater than that for the Sb4Se6 film and less than that of the Sb6Se4 film. In comparing the film annealed at 100 °C with that annealed at 200 °C, as shown in the spectra of the Sb4Se4 film of Figure 8a, the results well reflect the imbalanced distribution of p-like bonding and nonbonding states and the lone pair bonding state in the peaks located in 0
6.2 eV.30 However, the imbalanced distribution in the band becomes changed to a more balanced state after an annealing process at 256 °C because a stable Sb2Se3 orthorhombic and Sb rhombohedral structure is formed through the annealing process. The results indicate that the gap state in the Sb4Se4 film is caused by the discontinuous band tail, resulting in a change in resistance of the semiconductor. Therefore, we conclude that the two-level change in resistance is caused by the formation of a metastable phase with the gap state.

’ CONCLUSION We fabricated multilayer films using Sb and Se unit layers, by controlling the thickness at the atomic scale. According to the thickness ratio of Sb and Se, each film showed distinguishing differences, such as the formation of a stable state, a metastable state, and segregation phenomena. After the annealing process, the Sb4Se6 multilayer film with a single phase has a high resistance that originates from an orthorhombic structure and a clean valence band edge, whereas the formation of an Sb rhombohedral structure in the Sb6Se4 multilayer film and the

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structure of the band tail in valence band spectra indicate a low resistance that originates from the characteristic metallic band at the near Fermi level. In the case of the Sb4Se4 multilayer film, an unstable bonding structure is induced in the vertical direction due to the stoichiometry of the metastable phase and the local and anisotropic distribution of each element in films with a multilayer structure. After an annealing process at 256 °C, the unstable bonding structure is transformed to a stable crystal structure through a metastable phase. During the process, the valence band of the film is converted from a clean band edge into a band shape with a gap state, resulting in a resistance change at two levels. Finally, the results of change in the band edge in the near Fermi energy region show that the band structure can be successfully controlled by alternating the thickness of the Sb and Se in Se
Sb alloy systems.

’ ASSOCIATED CONTENT

bS

Supporting Information. The calculated valence band structure of Sb4Se4 monoclinic, Sb2Se3 orthorhombic, and Sb rhombohedral structures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The study for DFT calculations was coadministered with Dr. Y. Yi of KRISS. This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea, and a joint program between Yonsei University and Hynix semiconductor. ’ REFERENCES (1) Ovshinsky, S. R. Phys. Rev. Lett. 1968, 21, 1450–1453. (2) Wuttig, M. Nat. Mater. 2005, 4, 265–266. (3) Jung, Y.; Lee, S. H.; Ko, D. K.; Agarwal, R. J. Am. Chem. Soc. 2006, 128, 14026–14027. (4) Yu, D.; Wu, J. Q.; Gu, Q.; Park, H. K. J. Am. Chem. Soc. 2006, 128, 8148–8149. (5) Yamada, N.; Ohno, E.; Nishiuchi, K.; Akahira, N.; Takao, M. J. Appl. Phys. 1991, 69, 2849–2856. (6) Pirovano, A.; Lacaita, A. L.; Benvenuti, A.; Pellizzer, F.; Bez, R. IEEE Trans. Electron Devices 2004, 51, 452–459. (7) Redaelli, A.; Pirovano, A.; Pellizzer, E.; Lacaita, A. L.; Ielmini, D.; Bez, R. IEEE Electron Device Lett. 2004, 25, 684–686. (8) Nam, S. W.; Kim, C.; Kwon, M. H.; Lee, H. S.; Wi, J. S.; Lee, D.; Lee, T. Y.; Khang, Y.; Kim, K. B. Appl. Phys. Lett. 2008, 92, 111913. (9) Yoon, S. M.; Choi, K. J.; Lee, N. Y.; Lee, S. Y.; Park, Y. S.; Yu, B. G. Jpn. J. Appl. Phys., Part 2 2007, 46, L99–L102. (10) Yoon, S. M.; Lee, N. Y.; Ryu, S. O.; Choi, K. J.; Park, Y. S.; Lee, S. Y.; Yu, B. G.; Kang, M. J.; Choi, S. Y.; Wuttig, M. IEEE Electron Device Lett. 2006, 27, 445–447. (11) Petzelt, J.; Grigas, J. Ferroelectrics 1973, 5, 59–68. (12) Zhai, T. Y.; Ye, M. F.; Li, L.; Fang, X. S.; Liao, M. Y.; Li, Y. F.; Koide, Y.; Bando, Y.; Golberg, D. Adv. Mater. 2010, 22, 4530–4533. (13) Jeon, H.-W.; Ha, H.-P.; Hyun, D.-B.; Shim, J.-D. J. Phys. Chem. Solids 1991, 52, 579–585. (14) Kang, M. J.; Choi, S. Y.; Wamwangi, D.; Wang, K.; Steimer, C.; Wuttig, M. J. Appl. Phys. 2005, 98. 13469

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