rus
Issue #1/2025
F.A.Uskov, I.V.Veryuzskii
MORPHOLOGICAL SURFACE ANALYSIS OF SPIN GAPLESS CoFeMnSi SEMICONDUCTOR THIN FILMS GROWN BY PULSED LASER DEPOSITION
MORPHOLOGICAL SURFACE ANALYSIS OF SPIN GAPLESS CoFeMnSi SEMICONDUCTOR THIN FILMS GROWN BY PULSED LASER DEPOSITION
DOI: https://doi.org/10.22184/1993-8578.2025.18.1.48.58
CoFeMnSi spin gapless semiconductor thin films were grown on a (100) oriented MgO substrate by pulsed laser deposition. In this work, we explored the dependence of CoFeMnSi thin film’s surface morphology on different parameters of growth process. It was shown that an island-like CoFeMnSi thin film with an average grain diameter of D50% = 16.48 nm and roughness parameters Ra = 1.29 nm, Rz = 13.06 nm grows on a (100) oriented MgO substrate if a laser pulse frequency is 1–2 Hz and a pulse energy is 150 mJ. Reducing the frequency of laser pulses to 0.5 Hz with the same pulse energy led to a change in the film growth mechanism to a mixed growth. The film initially grows in the layer-by-layer mode and then 3D islands gradually form. Roughness parameters of the films deposited in this mode decrease to Ra = 0.61 nm and Rz = 11.51 nm. It became possible to implement layer-by-layer film deposition mode by introducing time pauses of 1–2 minutes between the depositions of each CoFeMnSi atomic layer. We found out that the layer-by-layer grown films had solid structure, defects and irregularities of their surface microrelief were smoothed out. The roughness parameters of the samples grown in the layer-by-layer mode decreased to Ra = 0.31 nm and Rz = 4.60 nm. The production of CoFeMnSi thin films with high quality of surface opens up opportunities for fabrication of CoFeMnSi-based heterostructures. With the selected technological parameters of growth process we fabricated MgO/CoFeMnSi/Co thin film with average surface roughness of Ra = 0.17 nm using selected above technological parameters of growth process. The results of this work can be used in fabrication of multilayer structures based on CoFeMnSi and their application in spintronic devices.
CoFeMnSi spin gapless semiconductor thin films were grown on a (100) oriented MgO substrate by pulsed laser deposition. In this work, we explored the dependence of CoFeMnSi thin film’s surface morphology on different parameters of growth process. It was shown that an island-like CoFeMnSi thin film with an average grain diameter of D50% = 16.48 nm and roughness parameters Ra = 1.29 nm, Rz = 13.06 nm grows on a (100) oriented MgO substrate if a laser pulse frequency is 1–2 Hz and a pulse energy is 150 mJ. Reducing the frequency of laser pulses to 0.5 Hz with the same pulse energy led to a change in the film growth mechanism to a mixed growth. The film initially grows in the layer-by-layer mode and then 3D islands gradually form. Roughness parameters of the films deposited in this mode decrease to Ra = 0.61 nm and Rz = 11.51 nm. It became possible to implement layer-by-layer film deposition mode by introducing time pauses of 1–2 minutes between the depositions of each CoFeMnSi atomic layer. We found out that the layer-by-layer grown films had solid structure, defects and irregularities of their surface microrelief were smoothed out. The roughness parameters of the samples grown in the layer-by-layer mode decreased to Ra = 0.31 nm and Rz = 4.60 nm. The production of CoFeMnSi thin films with high quality of surface opens up opportunities for fabrication of CoFeMnSi-based heterostructures. With the selected technological parameters of growth process we fabricated MgO/CoFeMnSi/Co thin film with average surface roughness of Ra = 0.17 nm using selected above technological parameters of growth process. The results of this work can be used in fabrication of multilayer structures based on CoFeMnSi and their application in spintronic devices.
Теги: cofemnsi alloy gapless semiconductor laser deposition spintronics surface morphology thin films бесщелевой полупроводник лазерное осаждение морфология поверхности спинтроника сплав cofemnsi тонкие пленки
INTRODUCTION
Spin gapless semiconductors (SGS) are a new class of promising materials used in modern spintronics devices due to the peculiarities of their zone structure and magnetic properties [1]. Among successfully synthesised chemical compounds exhibiting SGS properties, the quaternary Geisler CoFeMnSi (CFMS) alloy is of the greatest interest for spintronics. The manifestation of SGS properties in CFMS alloy was confirmed experimentally in [2]. In addition, compared to conventional semiconductors, CFMS has a high Curie temperature (~ 778 K) and a saturation magnetization Ms of 3.42 µB/f.u. [3], high spin polarisation [4], and low concentration of charge carriers [2, 5].
Nowadays, CFMS films are used to develop various experimental devices, such as magnetic tunnelling transitions, logic elements, magnetic field sensors, magnetoresistive random access memory with spin moment transfer (STT-MRAM) and others [6]. To fabricate such devices, multilayer heterostructures with alternating CFMS layers with a thickness of no more than 20 nm, buffer dielectric layers with a thickness of about 2 nm, and ferromagnetic/paramagnetic metal layers with a thickness of 5 nm or more are used [7]. The electrophysical characteristics of such structures are critically sensitive to the parameters of each deposited layer, as well as to inhomogeneities and defects at the interfaces between the layers [8, 9].
In this work, changes in surface morphology of thin single-crystalline CoFeMnSi films and structures based on them depending on the fabrication modes are presented. Pulsed laser deposition (PLD) method was used to deposit CoFeMnSi films on MgO (100) substrate and MgO/CoFeMnSi/Co structures. Surface morphology of the obtained samples was studied by atomic force microscopy.
Thin single-crystalline CFMS film preparing with an average surface roughness of less than 0.5 nm will allow subsequent layers to be deposited on its surface to form a spin magnetic tunnelling transition.
EXPERIMENTAL EQUIPMENT AND RESEARCH METHODS
Preparation of thin films of CFMS semiconductor was carried out at the PLD facility. The target from which the material was sputtered was a quaternary Geisler CoFeMnSi alloy with atomic ratio Co:Fe:Mn:Si 1:1:1:1:1 with purity of the starting materials 99.9%. A cobalt target with 99.9% purity of starting materials was used to deposit an additional conductive layer.
The target material was evaporated using a pulsed KrF laser with a radiation wavelength of 248 nm and a pulse duration of 20 ns. The structure of thin films during their growth was observed in situ using equipment for fast electron reflection diffraction (FED) studies. The composition of the atmosphere in the growth chamber was monitored by mass spectrometer. Two optical pyrometers located on both sides of the substrate were used to measure temperature. To ensure uniformity of the target surface material spraying, the target was rotated at a speed of 10 rpm with gradual displacement of the laser beam from its centre to a distance within the target diameter of 25 mm. Technology, fabrication parameters and crystal structure features of CoFeMnSi thin films on MgO (100) substrates are described in more detail in previous works [10, 11].
Measurements of the microrelief irregularities of the film surface were carried out on a scanning probe microscope SMM-2000 (No. 46918 in the State Register of Measuring Instruments of the Russian Federation) in the atomic force microscopy (AFM) mode [12] using CSG30 cantilever probes from TipsNano with a tip radius of 10 nm. The values of roughness parameters obtained in the microscope software correspond to the international standard ISO 4287.
FORMATION OF SINGLE CRYSTAL CFMS FILMS
CFMS films were grown in vacuum at a residual pressure in the growth chamber of P = 1 · 10–5 Pa. Before each deposition process the target was "trained", i.e. the target material was pre-sprayed onto a closed flap: the target was irradiated by laser pulses with a frequency of 1 Hz, the number of pulses was equal to 3000.
Chemically pure single-crystal MgO (100) substrates stored in specialised vacuum packaging were used for application of CFMS films. Since MgO is a hygroscopic material, in order to remove hydrates from the surface of the substrate, it was heated to a temperature of 1000 °C in the growth chamber of the unit. The substrate was kept at this temperature for three hours in oxygen atmosphere at pressure P = 1 · 10–2 Pa. The crystalline state of the near-surface region of the substrate was evaluated by reflections on the patterns obtained during its annealing.
The target-substrate distance was set to 55 mm, and temperature was 650 °C. The laser energy was E = 150 mJ, pulse frequency f = 0.5–2 Hz, and the number of pulses was 10 thousand. Thin films were prepared in one stage without subsequent thermal annealing. The thickness value of the obtained SFMS films was not more than 20 nm.
MEASUREMENT RESULTS AND DISCUSSION
For preparing the CFMS films, MgO (100) substrates from the manufacturer MO Sangyo with dimensions of 10 × 10 mm, thickness of 0.5 mm, with one-sided surface polishing and a reported average roughness value of Ra < 0.5 nm were used. Scanning of the MgO surface using an AFM microscope was carried out after removing the substrate from the package and before its annealing in the PLD unit. Figure 1 shows the AFM image and profilogram of the surface of the MgO (100) substrate.
The average roughness value Ra measured on the microscope was 0.13 nm, which is in accordance with the reported Ra values from the wafer manufacturer. Further deposition of CFMS films was carried out on these substrates.
As a result of the studies of deposited CFMS films, it was found that the surface roughness parameters of CFMS films vary depending on the modes and speed of their growth.
Deposition of CFMS material on MgO (100) substrate at laser pulse frequency f = 1–2 Hz and pulse energy E = 150 mJ leads to the growth of CFMS films by the Volmer-Weber mechanism [13, 14]. Presence of islands is due to the high rate of target material arrival on the substrate surface. Atoms evaporated by laser radiation, falling on the substrate, do not have time to distribute evenly on the substrate surface and concentrate around the atoms fixed on the substrate, forming islands. The granulometric analysis of the film showed that the lateral size of the main number of grains on the film surface ranges from 9.6 nm (D10% parameter value) to 25.1 nm (D90% parameter value) (Fig.2a).
The fabrication of an island CFMS film with high inhomogeneity and surface defectivity makes it impossible to deposit subsequent high-quality thin layers necessary for preparing of the tunnelling magnetic transition. If the rate of arrival of new atoms on the substrate is high, the island mode of film growth will be preserved. To increase film homogeneity and reduce the number of defects on its surface, it is necessary to reduce the rate of arrival of sputtered atoms on the substrate. Decreasing the deposition rate by reducing the pulse frequency to f = 0.5 Hz and using low laser pulse energy E = 150 mJ allowed us to observe a change of the film growth mode to the layer-by-layer Stransky-Krastanov mode [15]. In this mode, the first layers of SFMS are obtained solid. Then, after reaching the critical film thickness, the film growth again becomes island-like due to the stresses between the film and the substrate [16].
The change of the growth mechanism from layer-by-layer island growth to layer-by-layer growth by Frank van der Merwe [17] was achieved by introducing additional pauses of 1–2 minutes between the deposition of each new CFMS atomic layer. Such time delays in the film formation process allowed CFMS atoms to fill the gap between the formed islands. When the layer thickness reached more than 2 nm, the islands merged into a homogeneous film. In this mode, formation of each subsequent CFMS atomic layer on the MgO substrate starts after the atoms of the previous film layer are completely filled with atoms.
Figure 3 shows AFM images obtained from the CoFeMnSi thin films surfaces grown on MgO (100) substrate in island, layer-by-layer and layer-by-layer modes.
AFM surface scanning results of CFMS thin films correlate with their growth modes. The film grown in the Volmer-Weber mode consists of islands whose average height reaches Rz = 13,06 nm, its average roughness Ra = 1,29 nm, and the value of Rmax = 14,24 nm (Fig.3a). In the transition from the island mode of growth to the layer-by-layer growth mode, a change in the film microrelief roughness parameters was observed up to the values of Ra = 0,61 nm, Rz = 11,51 nm, and the value of Rmax = 13,25 nm (Fig.3b). In layer-by-layer growth mode, the average surface roughness of the CoFeMnSi thin film was Ra = 0,31 nm, and the values of Rz and Rmax were 4.6 nm and 7.91 nm, respectively (Fig3c). Table 1 summarises the comparative roughness parameters of the thin films obtained by different growth mechanisms.
Analysis of the obtained results shows that reduction of the pulsed laser radiation frequency from 2 Hz to 0.5 Hz leads to significant changes in the film structure and morphology of its surface. Differences in the film surface topography are reflected in the average microrelief roughness of the films, the value of which decreases from 1.29 nm for the island film to 0.31 nm for the CFMS film deposited in the layer-by-layer mode.
The surface profilograms of CFMS thin films shown in Fig.4 show a significant decrease in the height of the film material islands during the transition from the island (Fig.4a) to the layer-by-layer (Fig.4b) growth mode. The average size (S) of the micrograins of the film surface slightly decreases in the transition from island mode of growth to layer-island mode of growth. The transition to the layer-by-layer growth mode of the CFMS thin film (Fig.4c) is accompanied by a significant decrease of the S parameter, smoothing of the surface microrelief and reduction of the film height difference to ~1.6 nm. The surface profile of the film deposited in the layer-by-layer mode practically repeats the surface profile of pure MgO (100) substrate, which is a confirmation of the layer-by-layer deposition of the obtained film. The crystal lattice state of the deep layers of the obtained films is discussed in more detail in [18].
The roughness parameters of the CFMS film deposited in the layer-by-layer mode are commensurate with the parameters of silicon wafers used in modern integrated circuits production, the average roughness of which is <0.5 nm. The obtained results can be used for preparing of thin films of various materials with a highly ordered crystalline structure and with a minimum number of defects and inhomogeneities at the interface between the layers on the surface of the grown CFMS layer. For this purpose, it is necessary to be able to sequentially deposit the required heterostructure layers on the substrate.
In this work, a turret-type mechanism was used on the PLD unit to deposit a thin layer of cobalt on the surface of the CFMS film grown in the layer-by-layer mode. Formation of the CFMS film and Co film took place under the conditions of one technological process in automatic mode, which excludes the deposition of undesirable impurities in the films and at the interface between CFMS and Co. As a result, the MgO/CoFeMnSi/Co structure was fabricated. AFM scanning of the surface of the obtained structure showed that a thin Co layer deposition of on the CoFeMnSi film led to a decrease in the average surface microrelief roughness to Ra = 0,17 nm (Fig.5). The presented data show that the surface topography of the obtained cobalt film practically repeats the surface topography of the CFMS film shown in Fig.4c, which allows us to conclude that the crystal structure of the obtained film is highly ordered and its epitaxial growth.
Thus, it is shown that the selected technological parameters of epitaxial growth of CFMS films and additional layers from other materials allow preparing of multilayer structures for spintronics devices. The electrophysical characteristics of such structures will depend on many factors, the most important of which include: the crystalline perfection of the grown layers, the state of the surface and the interface between the layers, as well as the composition of the resulting films.
CONCLUSIONS
The surface microrelief of thin single-crystalline CoFeMnSi films produced by pulsed laser deposition on MgO (100) substrate has been studied. The surface roughness parameters of the films were measured by atomic force microscopy. It was found that when the frequency of pulsed laser radiation is reduced from 2 to 0.5 Hz and low laser pulse energy of 150 mJ is used, the island mechanism of film growth changes to a layer-by-layer island mechanism. The change of the growth mechanism to layer-by-layer is achieved by introducing time pauses equal to 1–2 minutes during deposition of each new atomic layer. The growth of CFMS films in the layer-by-layer mode resulted in the smoothing of surface irregularities and microrelief defects. The average roughness of CoFeMnSi thin films decreased from 1.29 to 0.31 nm with decreasing frequency of pulsed laser irradiation. The selected optimal thin film fabrication parameters were applied to grow the MgO/CoFeMnSi/Co structure. The average roughness of the cobalt surface layer Ra was 0.17 nm. The results can be used for preparing the CFMS-based multilayer structures and their application in spintronics devices.
ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Education and Science of Russia under the state assignment (Agreement FSMR-2024-0004).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Spin gapless semiconductors (SGS) are a new class of promising materials used in modern spintronics devices due to the peculiarities of their zone structure and magnetic properties [1]. Among successfully synthesised chemical compounds exhibiting SGS properties, the quaternary Geisler CoFeMnSi (CFMS) alloy is of the greatest interest for spintronics. The manifestation of SGS properties in CFMS alloy was confirmed experimentally in [2]. In addition, compared to conventional semiconductors, CFMS has a high Curie temperature (~ 778 K) and a saturation magnetization Ms of 3.42 µB/f.u. [3], high spin polarisation [4], and low concentration of charge carriers [2, 5].
Nowadays, CFMS films are used to develop various experimental devices, such as magnetic tunnelling transitions, logic elements, magnetic field sensors, magnetoresistive random access memory with spin moment transfer (STT-MRAM) and others [6]. To fabricate such devices, multilayer heterostructures with alternating CFMS layers with a thickness of no more than 20 nm, buffer dielectric layers with a thickness of about 2 nm, and ferromagnetic/paramagnetic metal layers with a thickness of 5 nm or more are used [7]. The electrophysical characteristics of such structures are critically sensitive to the parameters of each deposited layer, as well as to inhomogeneities and defects at the interfaces between the layers [8, 9].
In this work, changes in surface morphology of thin single-crystalline CoFeMnSi films and structures based on them depending on the fabrication modes are presented. Pulsed laser deposition (PLD) method was used to deposit CoFeMnSi films on MgO (100) substrate and MgO/CoFeMnSi/Co structures. Surface morphology of the obtained samples was studied by atomic force microscopy.
Thin single-crystalline CFMS film preparing with an average surface roughness of less than 0.5 nm will allow subsequent layers to be deposited on its surface to form a spin magnetic tunnelling transition.
EXPERIMENTAL EQUIPMENT AND RESEARCH METHODS
Preparation of thin films of CFMS semiconductor was carried out at the PLD facility. The target from which the material was sputtered was a quaternary Geisler CoFeMnSi alloy with atomic ratio Co:Fe:Mn:Si 1:1:1:1:1 with purity of the starting materials 99.9%. A cobalt target with 99.9% purity of starting materials was used to deposit an additional conductive layer.
The target material was evaporated using a pulsed KrF laser with a radiation wavelength of 248 nm and a pulse duration of 20 ns. The structure of thin films during their growth was observed in situ using equipment for fast electron reflection diffraction (FED) studies. The composition of the atmosphere in the growth chamber was monitored by mass spectrometer. Two optical pyrometers located on both sides of the substrate were used to measure temperature. To ensure uniformity of the target surface material spraying, the target was rotated at a speed of 10 rpm with gradual displacement of the laser beam from its centre to a distance within the target diameter of 25 mm. Technology, fabrication parameters and crystal structure features of CoFeMnSi thin films on MgO (100) substrates are described in more detail in previous works [10, 11].
Measurements of the microrelief irregularities of the film surface were carried out on a scanning probe microscope SMM-2000 (No. 46918 in the State Register of Measuring Instruments of the Russian Federation) in the atomic force microscopy (AFM) mode [12] using CSG30 cantilever probes from TipsNano with a tip radius of 10 nm. The values of roughness parameters obtained in the microscope software correspond to the international standard ISO 4287.
FORMATION OF SINGLE CRYSTAL CFMS FILMS
CFMS films were grown in vacuum at a residual pressure in the growth chamber of P = 1 · 10–5 Pa. Before each deposition process the target was "trained", i.e. the target material was pre-sprayed onto a closed flap: the target was irradiated by laser pulses with a frequency of 1 Hz, the number of pulses was equal to 3000.
Chemically pure single-crystal MgO (100) substrates stored in specialised vacuum packaging were used for application of CFMS films. Since MgO is a hygroscopic material, in order to remove hydrates from the surface of the substrate, it was heated to a temperature of 1000 °C in the growth chamber of the unit. The substrate was kept at this temperature for three hours in oxygen atmosphere at pressure P = 1 · 10–2 Pa. The crystalline state of the near-surface region of the substrate was evaluated by reflections on the patterns obtained during its annealing.
The target-substrate distance was set to 55 mm, and temperature was 650 °C. The laser energy was E = 150 mJ, pulse frequency f = 0.5–2 Hz, and the number of pulses was 10 thousand. Thin films were prepared in one stage without subsequent thermal annealing. The thickness value of the obtained SFMS films was not more than 20 nm.
MEASUREMENT RESULTS AND DISCUSSION
For preparing the CFMS films, MgO (100) substrates from the manufacturer MO Sangyo with dimensions of 10 × 10 mm, thickness of 0.5 mm, with one-sided surface polishing and a reported average roughness value of Ra < 0.5 nm were used. Scanning of the MgO surface using an AFM microscope was carried out after removing the substrate from the package and before its annealing in the PLD unit. Figure 1 shows the AFM image and profilogram of the surface of the MgO (100) substrate.
The average roughness value Ra measured on the microscope was 0.13 nm, which is in accordance with the reported Ra values from the wafer manufacturer. Further deposition of CFMS films was carried out on these substrates.
As a result of the studies of deposited CFMS films, it was found that the surface roughness parameters of CFMS films vary depending on the modes and speed of their growth.
Deposition of CFMS material on MgO (100) substrate at laser pulse frequency f = 1–2 Hz and pulse energy E = 150 mJ leads to the growth of CFMS films by the Volmer-Weber mechanism [13, 14]. Presence of islands is due to the high rate of target material arrival on the substrate surface. Atoms evaporated by laser radiation, falling on the substrate, do not have time to distribute evenly on the substrate surface and concentrate around the atoms fixed on the substrate, forming islands. The granulometric analysis of the film showed that the lateral size of the main number of grains on the film surface ranges from 9.6 nm (D10% parameter value) to 25.1 nm (D90% parameter value) (Fig.2a).
The fabrication of an island CFMS film with high inhomogeneity and surface defectivity makes it impossible to deposit subsequent high-quality thin layers necessary for preparing of the tunnelling magnetic transition. If the rate of arrival of new atoms on the substrate is high, the island mode of film growth will be preserved. To increase film homogeneity and reduce the number of defects on its surface, it is necessary to reduce the rate of arrival of sputtered atoms on the substrate. Decreasing the deposition rate by reducing the pulse frequency to f = 0.5 Hz and using low laser pulse energy E = 150 mJ allowed us to observe a change of the film growth mode to the layer-by-layer Stransky-Krastanov mode [15]. In this mode, the first layers of SFMS are obtained solid. Then, after reaching the critical film thickness, the film growth again becomes island-like due to the stresses between the film and the substrate [16].
The change of the growth mechanism from layer-by-layer island growth to layer-by-layer growth by Frank van der Merwe [17] was achieved by introducing additional pauses of 1–2 minutes between the deposition of each new CFMS atomic layer. Such time delays in the film formation process allowed CFMS atoms to fill the gap between the formed islands. When the layer thickness reached more than 2 nm, the islands merged into a homogeneous film. In this mode, formation of each subsequent CFMS atomic layer on the MgO substrate starts after the atoms of the previous film layer are completely filled with atoms.
Figure 3 shows AFM images obtained from the CoFeMnSi thin films surfaces grown on MgO (100) substrate in island, layer-by-layer and layer-by-layer modes.
AFM surface scanning results of CFMS thin films correlate with their growth modes. The film grown in the Volmer-Weber mode consists of islands whose average height reaches Rz = 13,06 nm, its average roughness Ra = 1,29 nm, and the value of Rmax = 14,24 nm (Fig.3a). In the transition from the island mode of growth to the layer-by-layer growth mode, a change in the film microrelief roughness parameters was observed up to the values of Ra = 0,61 nm, Rz = 11,51 nm, and the value of Rmax = 13,25 nm (Fig.3b). In layer-by-layer growth mode, the average surface roughness of the CoFeMnSi thin film was Ra = 0,31 nm, and the values of Rz and Rmax were 4.6 nm and 7.91 nm, respectively (Fig3c). Table 1 summarises the comparative roughness parameters of the thin films obtained by different growth mechanisms.
Analysis of the obtained results shows that reduction of the pulsed laser radiation frequency from 2 Hz to 0.5 Hz leads to significant changes in the film structure and morphology of its surface. Differences in the film surface topography are reflected in the average microrelief roughness of the films, the value of which decreases from 1.29 nm for the island film to 0.31 nm for the CFMS film deposited in the layer-by-layer mode.
The surface profilograms of CFMS thin films shown in Fig.4 show a significant decrease in the height of the film material islands during the transition from the island (Fig.4a) to the layer-by-layer (Fig.4b) growth mode. The average size (S) of the micrograins of the film surface slightly decreases in the transition from island mode of growth to layer-island mode of growth. The transition to the layer-by-layer growth mode of the CFMS thin film (Fig.4c) is accompanied by a significant decrease of the S parameter, smoothing of the surface microrelief and reduction of the film height difference to ~1.6 nm. The surface profile of the film deposited in the layer-by-layer mode practically repeats the surface profile of pure MgO (100) substrate, which is a confirmation of the layer-by-layer deposition of the obtained film. The crystal lattice state of the deep layers of the obtained films is discussed in more detail in [18].
The roughness parameters of the CFMS film deposited in the layer-by-layer mode are commensurate with the parameters of silicon wafers used in modern integrated circuits production, the average roughness of which is <0.5 nm. The obtained results can be used for preparing of thin films of various materials with a highly ordered crystalline structure and with a minimum number of defects and inhomogeneities at the interface between the layers on the surface of the grown CFMS layer. For this purpose, it is necessary to be able to sequentially deposit the required heterostructure layers on the substrate.
In this work, a turret-type mechanism was used on the PLD unit to deposit a thin layer of cobalt on the surface of the CFMS film grown in the layer-by-layer mode. Formation of the CFMS film and Co film took place under the conditions of one technological process in automatic mode, which excludes the deposition of undesirable impurities in the films and at the interface between CFMS and Co. As a result, the MgO/CoFeMnSi/Co structure was fabricated. AFM scanning of the surface of the obtained structure showed that a thin Co layer deposition of on the CoFeMnSi film led to a decrease in the average surface microrelief roughness to Ra = 0,17 nm (Fig.5). The presented data show that the surface topography of the obtained cobalt film practically repeats the surface topography of the CFMS film shown in Fig.4c, which allows us to conclude that the crystal structure of the obtained film is highly ordered and its epitaxial growth.
Thus, it is shown that the selected technological parameters of epitaxial growth of CFMS films and additional layers from other materials allow preparing of multilayer structures for spintronics devices. The electrophysical characteristics of such structures will depend on many factors, the most important of which include: the crystalline perfection of the grown layers, the state of the surface and the interface between the layers, as well as the composition of the resulting films.
CONCLUSIONS
The surface microrelief of thin single-crystalline CoFeMnSi films produced by pulsed laser deposition on MgO (100) substrate has been studied. The surface roughness parameters of the films were measured by atomic force microscopy. It was found that when the frequency of pulsed laser radiation is reduced from 2 to 0.5 Hz and low laser pulse energy of 150 mJ is used, the island mechanism of film growth changes to a layer-by-layer island mechanism. The change of the growth mechanism to layer-by-layer is achieved by introducing time pauses equal to 1–2 minutes during deposition of each new atomic layer. The growth of CFMS films in the layer-by-layer mode resulted in the smoothing of surface irregularities and microrelief defects. The average roughness of CoFeMnSi thin films decreased from 1.29 to 0.31 nm with decreasing frequency of pulsed laser irradiation. The selected optimal thin film fabrication parameters were applied to grow the MgO/CoFeMnSi/Co structure. The average roughness of the cobalt surface layer Ra was 0.17 nm. The results can be used for preparing the CFMS-based multilayer structures and their application in spintronics devices.
ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Education and Science of Russia under the state assignment (Agreement FSMR-2024-0004).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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