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Accepted Manuscript Effect of Ge Layer Thickness on the Formation of Mn5Ge3 Thin Film on Ge/Si (111) Burcu Toydemir Yasasun, Aykut Can Onel, Ilknur Gunduz Aykac, Mehmet Ali Gulgun, Leyla Colakerol Arslan PII: DOI: Reference:
S0304-8853(18)31149-1 https://doi.org/10.1016/j.jmmm.2018.10.096 MAGMA 64507
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Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
16 April 2018 19 October 2018 19 October 2018
Please cite this article as: B.T. Yasasun, A.C. Onel, I.G. Aykac, M. Ali Gulgun, L.C. Arslan, Effect of Ge Layer Thickness on the Formation of Mn5Ge3 Thin Film on Ge/Si (111), Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.096
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Effect of Ge Layer Thickness on the Formation of Mn5Ge3 Thin Film on Ge/Si (111) Burcu Toydemir Yasasun1,2, Aykut Can Onel1,2, Ilknur Gunduz Aykac1,2, Mehmet Ali Gulgun3, Leyla Colakerol Arslan1,2* 1
Department of Physics, Gebze Institute of Technology, Kocaeli, 41400 Turkey Nanotechnology Institute, Gebze Technical University, Kocaeli, 41400 Turkey 3 Faculty of Engn. & Nat. Sci., Sabanci Univiversity, TR-34956 Istanbul, Turkey 2
The effect of Ge buffer layer thickness on the formation of epitaxial Mn5Ge3 films and their magnetic properties has been investigated. Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (111) substrates using solid phase epitaxy. The crystalline quality, chemical composition and magnetic properties of the films were investigated by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, electron spin resonance. Increasing the thickness of the Ge layer significantly enhanced the crystallinity and magnetic homogeneity of Mn5Ge3 films. Both structural and magnetic investigations show substantial diffusion of Mn atoms through Ge buffer layer which results in the formation of Mn5Si3 and a delay in Mn5Ge3 formation until the thickness of the Ge layer reaches to 70nm. The observed magnetic behavior for the films grown on different Ge thicknesses is interpreted in terms of changes in the magnetic phases and surface properties. These results show that Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ~8% lattice mismatch between Mn5Ge3 and Si (111). INTRODUCTION Silicon in particular is an ideal material for spintronics because it preserves the spin orientation of the electrons much longer than many other semiconductors [1]. There are a lot of efforts to inject spin into silicon, however, due to the scattering of the spin-polarized carriers at the metal semiconductor interface, only up to 10% of the spin signal is transmitted from ferromagnetic metals into silicon through complex tunnel barriers [2-5]. An interesting
solution is the formation of an interlayer made out of a magnetic semiconductor between silicon and ferromagnetic metal which eliminates the conductivity mismatch and reduces significant material differences between a ferromagnet and silicon. The challenge is to find a magnetic semiconductor that exhibits ferromagnetism at room temperature (RT). Mn-doped Ge seems to be a good candidate for a spin-polarized carrier source at room temperature. However, both theoretical and experimental investigations indicate that Mn atoms are usually inhomogenously distributed in Ge depending on the growth parameters and observed RT ferromagnetism is due to thermodynamically favorable ferromagnetic Mn5Ge3 precipitates embedded inside Ge [6-8]. Due to the recent efforts devoted to increasing the Curie temperature (TC) of Mn5Ge3 significantly above RT [9-11] and also its sufficiently high spin polarization [12, 13], the interest has shifted primarily towards synthesis of thin films of Mn5Ge3 to allow it be directly incorporated into spintronic based devices [14]. For practical use of Mn5Ge3 in electronics, the formation of Mn5Ge3 on Si substrates would be by far the most promising. However, 8% lattice mismatch between silicon and Mn5Ge3 will prevent growth of defect-free epitaxial film unless thickness of the film is below certain critical thickness. In fact, Mn5Ge3 films grown on highly lattice mismatched GaAs and GaSb showed magnetic properties quite different from those of bulk samples due to the epitaxial strain [15]. One way of forming strain free Mn5Ge3 films is solid phase epitaxial growth on Ge(111) substrates [16, 17]. In this method, Ge atoms from substrate and Mn atoms grown on Ge layer recrystallize upon subsequent thermal annealing at elevated temperatures. A considerable number of investigations have been carried out on the fabrication of epitaxial Mn5Ge3 films directly grown on Ge. Experimental studies performed on solid phase epitaxial growth of Mn5Ge3 on Ge (111) substrates showed that growth conditions play a significant role in the magnetic and structural properties of this material [17]. Spiesser et al. reported that magnetic anisotropy of the Mn5Ge3 films grown on Ge (111)
depends on film thickness [17]. A systematic investigation of the growth of thin Mn5Ge3 film on the Si substrate from the viewpoint of epitaxy has not been carried out so far, although such information is crucial for such a system to be combined with the existing Si technology. The growth of polycrystalline Mn5Ge3 films on SiO2 has been reported recently by growing a thick Ge layer buffer layer [18]. If ferromagnetic Mn5Ge3 films are synthesized on a thin crystalline Ge layer formed on the silicon substrate, electrical spin injection into Si across a Schottky tunnel barrier will be realized. This study is essentially needed to get a clear perspective of the influence of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films on Si substrates and their magnetic properties and thus establish the controllability of the magnetization. In the present paper, we report the thickness dependence of Ge buffer layers on the crystallinity and the magnetic properties of Mn5Ge3 films grown on Si (111) substrates by molecular beam solid phase epitaxial growth. Surface morphology could be modified by varying the Ge layer thickness which would affect the diffusion mechanism of Mn atoms in Ge layer. The phase and crystallinity of the ferromagnetic layers were identified by x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) measurements, whereas the magnetic properties are characterized by electron spin resonance (ESR). EXPERIMENTAL Si substrates were rinsed in a 1:10 solution of concentrated HF and deionized water firstly and then annealed in ultrahigh vacuum at 650 °C for several hours and monitored by means of low energy electron diffraction (LEED) until a well-ordered 7x7 reconstruction appeared. Sample growth was performed in a system that is coupled to XPS analysis chamber. The samples are prepared in situ by first depositing a Ge layer with varying thicknesses at 480 °C using
molecular beam epitaxy. Next, Mn layer of 3 nm was deposited at room temperature. Subsequently, the films were annealed for 20 min at 400 ºC. Deposition rates of Ge and Mn were monitored by a quartz-crystal microbalance and were held constant at 0.08 A/s and 0.05 A/s, respectively. The selected Ge films with the following thicknesses were considered: 35 nm (S35), 70 nm (S70), 140 nm (S140), 210 nm (S210) and 280 nm (S280). The crystal structure of the deposited films were examined by a Rigaku Smart-Lab x-ray diffractometer (XRD) which uses a Cu Ka radiation (λ=1.54178Å). Trasmission electron microscope was used to confirm the film thickness and structure. The cross-sectional TEM specimen of the film is prepared using focused ion beam (FIB) micromachining. XPS spectra of the films were collected using a SPECS PHABIOS 100 hemispherical analyser under a base vacuum of less than 1x10−10 Torr, where a monochromatic Al Kα x-ray (hν = 1486.6 eV) was used as the exciting source. The energy scale of the spectrometer was calibrated by setting the measured Au 4f7/2 binding energy to 84eV with regard to Fermi energy. Baseline corrections were made using a Shirley type background correction. The magnetic measurement was carried out using vibrating sample magnetometer (VSM) in the temperature range from 10 to 350K. To obtain information on the microscopic nature of the Mn ions in Ge, electron spin resonance (ESR) was carried out at room temperature using X-band JEOL spectrometer equipped by an electromagnet which provides a dc magnetic field up to 2 kG in the horizontal plane. RESULTS The structural properties of the films and the epitaxial relationship between the Mn5Ge3 film and the Ge layer grown on Si (111) were analyzed by XRD. Figure 1 shows the x-ray diffraction pattern of Mn5Ge3 films formed on Ge layers of thickness from 35 to 280 nm in a semi logarithmic plot. Besides the mean peaks of Ge and Si, the XRD spectra for the films with Ge layer thicknesses of above 140 nm show a pronounced diffraction peak at 35.5° and 75.5º, which corresponds to (002) and (004) plane of Mn5Ge3. When the thickness of the Ge
layer is decreased down to 70 nm, the Mn5Ge3 peaks became broader and their intensity decreased. Furthermore, an additional peak has been observed around 2θ=37.4°. That peak became more pronounced in the XRD spectra of the film with Ge thickness of 35 nm, where the Mn5Ge3 peak disappeared and a higher order peak corresponding to (004) plane of Mn5Si3 phase emerged.
(hkl) planes
Mn5Si3
observed in the films. It is apparent that both Mn5Ge3 and Mn5Si3
grains are uniformly oriented in these samples. The presence of these (00l) peaks in S35 confirm that Mn atoms penetrate in the Si substrate due to diffusion of Mn through thin Ge layer during annealing and form a single phase structure. This indicates that the formation of Mn5Ge3 was delayed until the Ge layer reaches to a certain thickness such that Mn atoms can not diffuse into Si.
In order to investigate the effect of crystal quality of Ge layers on the formation of Mn5Ge3 films, the crystallinity of the Ge layers were evaluated on the basis of the full-width at halfmaximum (FWHM) of XRD rocking curves of Ge (111) peak. The FWHM values of the Ge (111) diffraction peak are displayed as a function of Ge layer thickness in the inset of figure 1. It is seen that the rocking curve profiles of Ge (111) peak appear significantly narrower for thick 280 nm films compared to thin 35 nm samples, leading to the conclusion that the degree of crystallinity increases as the Ge layer thickness increases due to strain relaxation. The Mn5Ge3 (002) signal intensifies as the Ge layer thickness increases. It appears that both Si and Ge actively compete to form bonds with Mn in the formation of their respective Mn phase, depending on the thickness of the Ge layer. It is understood that the Ge buffer layer thickness is quite effective on the formation dynamics of the magnetic phases. XRD results indicate that
there is a critical thickness of the Ge layer on Si to form Mn5Ge3 due to the tendency of favorable for Mn atoms to diffuse into the Si substrate.
For further investigation on the Mn diffusion mechanism, quantitative analyzes of XPS measurements were taken after Mn deposition and subsequent annealing. Mn 2p XPS data (not shown here) showed peaks at 638.7 eV and 650 eV corresponding to the 2p3/2 and 2p1/2 states, respectively. The peaks are separated in energy by 11.3 eV. The energy separation and binding energy of the peaks are consistent with the metallic Mn state. The previous works on Mn5Ge3 show that the line shape of the Mn 2p XPS spectra have a Mn-metal-like shape, similar to our observations [19, 20]The observation of metallic line shape in Mn5Ge3 and Mn5Si3, as it also seen in some Heusler compounds and MnSb alloy, is explained by delocalized magnetic moments on Mn lattice sites [21]. Atomic concentration of the evaporated films has been determined using the ratio of the area of the Mn 2p and Ge 2p core level peaks using the atomic sensitivity factors for the instrument settings. Although the same amount of Mn was deposited on each film, which was annealed at the same temperature, the Mn concentration on the sample surfaces and the phases formed in the films vary depending on the Ge layer thickness. From the measured Mn 2p and Ge 2p core-level peak areas and relative sensitivity factors, we calculated the relative Mn content of all samples. Films showed approximately the same Mn concentration prior to annealing. However, as seen in Fig 2, after the annealing process Mn concentration varied from 20% for S35 to 47% for S280 giving direct macroscopic evidence of Mn subsurface diffusion. The variation in Mn concentrations after the annealing process can be explained by the diffusion mechanism of Mn atoms into the substrate and the differences in surface morphologies. The migration of Mn atoms underneath Ge layer at low Ge layer thicknesses is likely to be responsible for the low manganese
concentration. When the Ge layer thickness is low and domain size is very small, most of the initially deposited Mn must have diffused into the Ge subsurface. The effect of Ge layer thickness on the morphological properties of Mn5Ge3 films grown on Si substrates was presented using atomic force microscopy (AFM). In the light of information obtained from the XRD analyzes, it was considered appropriate to focus on the first three thicknesses. Fig. 3 shows AFM images of the surface of the films grown on Ge buffer layer with thickness of (a) 35, (b) 70 and (c) 140 nm. It can be seen from the images that the surface morphology transforms from a granular form shape to relatively flat surface as the Ge thickness increases. The rms roughness values of 35, S70 and S140 are 7.63 nm, 4.22 nm and 1.77 nm, respectively. Monotonous decrease of surface roughness with increasing Ge layer thickness is related to heteroepitaxial growth process. The MnGe film with tGe= 35 nm on Si(111) exhibits a surface crowded by densely packed and evenly distributed large islands, as seen in Fig. 3(a). As the Ge thickness increases, coalescence of clusters continues and isolated islands evolve into irregularly distributed particles with significantly larger size which initiates two dimensional (2D) growth process with improved surface roughness. The image of the S140 shows a relatively flat surface with mild roughness (Fig 3(c)). As the Ge film thickness increases, reduced stress leads to improvement in crystallographic texture of Ge film. There is a good agreement between the film roughness values determined from AFM and the corresponding FWHM values obtained from XRD scans. When Mn is deposited on S35, we only observe the formation of Mn5Si3 because of the diffusion of Mn atoms into Si. It can clearly be seen that the Mn subsurface diffusion reaction is affected by the degree of crystallinity of Ge layer and this diffusion mechanism plays a critical role in the formation of Mn5Ge3. This idea is supported by TEM analysis.
In Fig 4(a), cross-sectional micrograph of S35 shows dome islands of different sizes formed on Si due to heteroepitaxial growth and nanocluster that clearly extends into the Si subsurface region. In order to investigate those nanoclusters and the distribution of Mn atoms, we also study the elemental distribution of the thin film using energy-dispersive X-ray spectroscopy
(EDX)-STEM mapping. As can be seen in Fig. 4(b-c) Ge forms a dome shaped islands on Si. Mn elements penetrate into the Si substrate either through the exposed regions between the islands or through the center of Ge dome and forming Mn5Si3 phase. It is consistent with the previous results on Mn with Ge quantum dots (QD) where silicide phases preferentially form over germanides during the annealing process [22]. The migration of Mn atoms into Si may be connected with lower diffusion rate of Mn in Ge [22]. The phenomenon of Mn diffusion in the Ge was reported by Zhu et al. and showed that the diffusion paths are strongly related to the crystallographic directions of Ge, Mn easily diffuses into the Ge(111) while on Ge(001) it can act as a surfactant [23]. In agreement with the predictions that Mn atoms diffuse through the bulk Ge and find its path to underneath Si layer. In order to study the influence of the Ge thickness on the magnetic properties, the cases where tGe= 35 nm, 70 nm and 140 nm, were chosen. Above 140 nm, continuous Mn5Ge3 film was formed and it shows similar magnetic characteristics as observed from Mn5Ge3 films directly grown on Ge(111) [24]. We first investigated the temperature dependence of the magnetization under 0.1 T. As can be seen in Figure 5, the film with 140 nm Ge layer undergoes a sharp paramagnetic to ferromagnetic transition at around 300 K which is comparable to Tc of about 296 K for bulk Mn5Ge3 [25], and no obvious change was observed in Tc when Ge layer thickness decreases to 70 nm. When reducing the thickness of Ge layer to 35 nm, a broad PM to FM transition is observed at Tc around 240 K. The substitution of Si for Ge in Mn5Ge3 leads to reduction of crystal size and Curie temperature without changing the crystal structure. Previous studies have shown that the Curie temperature of polycrystalline Mn5Ge3-xSix can range from 298-252 K when x = 0-1.5 [26-28]. Observed Curie temperature is referenced to a value of x=~1.65 for the Mn5GexSi3-x compound [28]. However, from the XRD pattern, the stoichiometry was estimated to be Mn5Si3, without the existence of other phases. On the other hand, recent studies show a ferromagnetic ordering in Mn5Si3 nanoparticles with a high Curie temperature of Tc=590 K due to the size-dependent increase
of surface contribution and quantum confinement effects [29]. It seems that the Curie temperature of Mn5Si3 arises due to the complex effects of magnetic disorder and quantum confinement effects in magnetic nanostructures. Figure 6 shows the magnetic hysteresis loops of the films with varying Ge layer thicknesses recorded at 10 K when the applied field was parallel to the film plane. Because of the low magnetic moment of such thin magnetic films, it was not possible to accurately measure the magnetic hysteresis perpendicular to the film plane using VSM. The normalized hysteresis loops for all samples show a clear ferromagnetic contribution and non-negligible remanent magnetization at 10 K. S140 and S70 present almost the same ferromagnetic contribution with a coercive field strength of Hc= 500 Oe, in agreement with the results of 30 nm thick Mn5Ge3 film [30]. The M-H loop of S35 and S70 do not saturate easily under applied field of up to 10 kOe, compared to samples with thicker buffer layers. The nonsaturating behavior reveals the presence of a superparamagnetic phase due to finite size effects and surface spin disorder of magnetic nanostructures. Since these samples have low moments, the nonsaturation can also be related to dominant paramagnetic contributions arising from the thin buffer layer and the substrate. The slight departure from typical square hysteresis curve can be accounted for with the consideration of exchange and dipolar interactions between the magnetic nanostructures. Mn5Ge3 nanosturcures with the same crystal structure and alignment along the c-axis, but slightly different orientations in the plane of the film, exhibits different effective anisotropies due the magnetostatic interactions between these Mn5Ge3 nanosturcures. The overall magnetic moment and retentivity of films increases with increasing Ge layer thickness. If all Mn atoms would contribute equally to the magnetization, the average magnetic moments per Mn atom for samples S35, S70 and S140 at 10 K are calculated to be 1.12 μB, 1.35 μB, and 2.05 μB, respectively. Comparing with the average magnetic moment values of 2.6 μB per Mn atoms in Mn5Ge3 [31], most of the Mn atoms ferromagnetically interact with each other.
Decrease of magnetization with the decrease of Ge buffer layer thickness is explained by the formation of Mn5Si3 phase which have lower magnetic moment due to reduction of the magnetic moment with the substitution of Si for Ge[32]. Although Mn5Si3 and Mn5Ge3 have the same crystal structure, Mn5Ge3 has much higher unit size therefore higher Mn-Mn atomic separation, leading to higher magnetic moment. The results are in very good agreement with earlier published data for Mn5Ge3-xSix [27].
ESR spectra of S35, S70 and S140 were measured as a function of temperature in order to elucidate the microscopic states of magnetic signals. Figure 7(a) and (b) shows the temperature dependence of the resonance positions of ESR measured with the external magnetic field applied parallel and perpendicular to the film plane, respectively. Consistent with VSM results, ESR signals are observed below the corresponding Curie temperature in both geometries. When an applied field is oriented along the in-plane axis of the film, resonance field and the intensity of signals for all the samples decreased and line width of the signals increased with decreasing temperature. The line broadening with decreasing temperature indicates the magnetic disorder and variation in contribution from exchange interactions. It should be noted that the presence of the Mn5Si3 domains in S70 does not indicate any effect of temperature-dependent magnetic behavior because it shows the same trend as S140 which contains only Mn5Ge3 layer.
The direction of the shift of ESR signal in the out-of-plane direction follows the opposite trend to that for the in-plane alignement. A weak and broad ESR signal appeared in S35 and the resonant field of the signal is shifted from ~360 mT for 240 K up to 1300 for 150 K, as the temperature is decreased. Line width of the signal (not shown here) increased with decreasing temperature as well. Such a weak and broad magnetic signal indicates the existence of
randomly oriented magnetic domains. The ESR spectra for S70 and S140 show a similar evolution with temperature. A single FMR signal is present around Tc. Below Tc, however, the signal splits into two components. The line splitting grows with decreasing the temperature and stays constant below 240 K. It is likely that the multiple resonance occurs due to the emergence of two magnetically nonequivalent Mn atoms which experience different local internal fields.
Fig 8 a-c shows the contour map of resonance signal as a function of magnetic field and polar angle with the color scale relating to the amplitude of the first derivative of absorbed microwave power for S35, S70 and S140, respectively. The angular dependence of FMR spectra was measured at 243K. The ESR spectra of all the films also show a similar angular dependence exhibiting uniaxial magnetic anisotropy along the film plane. The FMR signals of S70 and S140 consist of symmetric resonance lines which can be fitted to a Lorentzian-line shape in parallel orientation. Supplementary Figure S1 shows the angular dependence of H res for S70 and S140. The signals become slightly asymmetric and progressively splits into two peaks as the angle approaches to 90°. The separation between the two signals also increases and the intensity of the FMR signal decreases by approaching the film normal. This may be due to the prominent demagnetization effects in the perpendicular configuration. The splitting of the peak reaches the highest value of 130 Oe when the magnetic field is aligned perpendicular to the film plane. The relative intensity and resonance field positions of these signals depend on the direction of the applied field. The splitting of the FMR spectra of the samples S70 and S140 in the out-of-plane direction is shown in Supplementary Figure S2. This line splitting has never been observed in Mn5Ge3 thin films with thicknesses above 15nm [30, 33]. We believe that the the thickness is the most dominant effect in the observation of line splitting. Angular dependence observed here shows some similarities to spin waves of
GaMnAs [34]. However, spin waves were only present in GaMnAs films with thicknesses in the range of 100 nm to 200 nm due to the pinning of surface modes. Similarly, the occurrence of multiple resonance may be attributed to the existence of two magnetic phases pertaining to bulk and surface modes. Since the Mn atoms at the bulk and surface region experience different local internal fields, there exist a non-uniform magnetization in the film. Difference between the surface and bulk resonance modes decreases with the decrease in magnetization. The signal on the low-field side is identified as the bulk uniform resonance mode, while the higher field signal is acoustic surface mode. The resonance spectrum recorded at perpendicular geometry from the samples gives the evidence for conclusion of surface character of the second signal. As the thickness of the Mn5Ge3 layer increases, the signal intensity increases because uniform bulk mode of Mn5Ge3 increases but the intensity of the second signal stays almost the same. Note that, a single FMR signal can be detected for S35 showing similar angular dependence. However, S35 shows a much less pronounced and broader (ΔH~100 mT) resonance signal indicating large magnetic inhomogeneity. CONCLUSION Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (111) substrates using solid phase epitaxy. The role of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films and their magnetic properties was examined. Both structural and magnetic investigation results indicate that Mn5Ge3 thin film is formed above a critical Ge layer thickness and it shows perpendicular magnetic anisotropy. On the other hand, below the critical thickness of 70nm, Mn atoms directly diffuse into Si layer and form a ferromagnetic Mn5Si3 phase with a Curie temperature of about 250 K. XPS measurements also confirm the diffusion of Mn atoms from Ge towards Si. These results show that highly crystalline Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ~8% lattice mismatch between Mn5Ge3 and Si (111).
ACNOWLEDGEMENT
This work was supported in part by European Union FP7 Marie Curie Program IFMGeDMS Grant. *Corresponding Author: [email protected] CAPTIONS: Figure 1: XRD patterns of the MnGe films deposited on Si (111) substrates using Ge buffer layers of different thicknesses. The inset shows the plot of the FWHM of Ge (111) peak rocking curve vs Ge layer thickness. Figure 2: Atomic concentration of Mn 2p component in MnGe films as a function of Ge film thickness, determined from XPS measurements. Figure 3: AFM images of MnGe films deposited on (a) 35, (b) 70, and (c) 140 nm thick Ge layers illustrating the buffer layer thickness-dependence of the film’s morphology. All images are 10 × 10 μm2 and all AFM images have a height scale of 10 nm. Figure 4: (a) STEM image of MnGe/Si (111) sample showing dome shaped Ge randomly dispersed on Si; (b) and (c) are the corresponding elemental mapping of the Mn and Ge elements, respectively.
Figure 5: Field cooled magnetization vs temperature curve at magnetic field H = 1000 Oe for S35, S70 and S140. Figure 6: Magnetic hysteresis loops at 10 K for the samples with different Ge buffer layer thicknesses. Figure 7: Temperature dependence of e resonance positions for samples S35, S70 and S140 obtained at 9.5 GHz along (a) H//a and H//c. Figure 8: The contour map of resonance signals as a function of magnetic field orientation for (a) S35, (b)S70 and (c) S140.
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Highlights The role of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films and their magnetic properties were examined. For thicknesses below 70nm, Mn atoms directly diffuse into Si layer and form a ferromagnetic Mn5Si3 phase with a Curie temperature of about 250 K. Structural and magnetic properties of Mn5Ge3 were enhanced by increasing Ge buffer film thickness. Mn diffusion into Si confirmed from XRD, ESR and XPS data.
S0304-8853(18)31149-1 https://doi.org/10.1016/j.jmmm.2018.10.096 MAGMA 64507
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
16 April 2018 19 October 2018 19 October 2018
Please cite this article as: B.T. Yasasun, A.C. Onel, I.G. Aykac, M. Ali Gulgun, L.C. Arslan, Effect of Ge Layer Thickness on the Formation of Mn5Ge3 Thin Film on Ge/Si (111), Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.096
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Effect of Ge Layer Thickness on the Formation of Mn5Ge3 Thin Film on Ge/Si (111) Burcu Toydemir Yasasun1,2, Aykut Can Onel1,2, Ilknur Gunduz Aykac1,2, Mehmet Ali Gulgun3, Leyla Colakerol Arslan1,2* 1
Department of Physics, Gebze Institute of Technology, Kocaeli, 41400 Turkey Nanotechnology Institute, Gebze Technical University, Kocaeli, 41400 Turkey 3 Faculty of Engn. & Nat. Sci., Sabanci Univiversity, TR-34956 Istanbul, Turkey 2
The effect of Ge buffer layer thickness on the formation of epitaxial Mn5Ge3 films and their magnetic properties has been investigated. Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (111) substrates using solid phase epitaxy. The crystalline quality, chemical composition and magnetic properties of the films were investigated by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, electron spin resonance. Increasing the thickness of the Ge layer significantly enhanced the crystallinity and magnetic homogeneity of Mn5Ge3 films. Both structural and magnetic investigations show substantial diffusion of Mn atoms through Ge buffer layer which results in the formation of Mn5Si3 and a delay in Mn5Ge3 formation until the thickness of the Ge layer reaches to 70nm. The observed magnetic behavior for the films grown on different Ge thicknesses is interpreted in terms of changes in the magnetic phases and surface properties. These results show that Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ~8% lattice mismatch between Mn5Ge3 and Si (111). INTRODUCTION Silicon in particular is an ideal material for spintronics because it preserves the spin orientation of the electrons much longer than many other semiconductors [1]. There are a lot of efforts to inject spin into silicon, however, due to the scattering of the spin-polarized carriers at the metal semiconductor interface, only up to 10% of the spin signal is transmitted from ferromagnetic metals into silicon through complex tunnel barriers [2-5]. An interesting
solution is the formation of an interlayer made out of a magnetic semiconductor between silicon and ferromagnetic metal which eliminates the conductivity mismatch and reduces significant material differences between a ferromagnet and silicon. The challenge is to find a magnetic semiconductor that exhibits ferromagnetism at room temperature (RT). Mn-doped Ge seems to be a good candidate for a spin-polarized carrier source at room temperature. However, both theoretical and experimental investigations indicate that Mn atoms are usually inhomogenously distributed in Ge depending on the growth parameters and observed RT ferromagnetism is due to thermodynamically favorable ferromagnetic Mn5Ge3 precipitates embedded inside Ge [6-8]. Due to the recent efforts devoted to increasing the Curie temperature (TC) of Mn5Ge3 significantly above RT [9-11] and also its sufficiently high spin polarization [12, 13], the interest has shifted primarily towards synthesis of thin films of Mn5Ge3 to allow it be directly incorporated into spintronic based devices [14]. For practical use of Mn5Ge3 in electronics, the formation of Mn5Ge3 on Si substrates would be by far the most promising. However, 8% lattice mismatch between silicon and Mn5Ge3 will prevent growth of defect-free epitaxial film unless thickness of the film is below certain critical thickness. In fact, Mn5Ge3 films grown on highly lattice mismatched GaAs and GaSb showed magnetic properties quite different from those of bulk samples due to the epitaxial strain [15]. One way of forming strain free Mn5Ge3 films is solid phase epitaxial growth on Ge(111) substrates [16, 17]. In this method, Ge atoms from substrate and Mn atoms grown on Ge layer recrystallize upon subsequent thermal annealing at elevated temperatures. A considerable number of investigations have been carried out on the fabrication of epitaxial Mn5Ge3 films directly grown on Ge. Experimental studies performed on solid phase epitaxial growth of Mn5Ge3 on Ge (111) substrates showed that growth conditions play a significant role in the magnetic and structural properties of this material [17]. Spiesser et al. reported that magnetic anisotropy of the Mn5Ge3 films grown on Ge (111)
depends on film thickness [17]. A systematic investigation of the growth of thin Mn5Ge3 film on the Si substrate from the viewpoint of epitaxy has not been carried out so far, although such information is crucial for such a system to be combined with the existing Si technology. The growth of polycrystalline Mn5Ge3 films on SiO2 has been reported recently by growing a thick Ge layer buffer layer [18]. If ferromagnetic Mn5Ge3 films are synthesized on a thin crystalline Ge layer formed on the silicon substrate, electrical spin injection into Si across a Schottky tunnel barrier will be realized. This study is essentially needed to get a clear perspective of the influence of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films on Si substrates and their magnetic properties and thus establish the controllability of the magnetization. In the present paper, we report the thickness dependence of Ge buffer layers on the crystallinity and the magnetic properties of Mn5Ge3 films grown on Si (111) substrates by molecular beam solid phase epitaxial growth. Surface morphology could be modified by varying the Ge layer thickness which would affect the diffusion mechanism of Mn atoms in Ge layer. The phase and crystallinity of the ferromagnetic layers were identified by x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) measurements, whereas the magnetic properties are characterized by electron spin resonance (ESR). EXPERIMENTAL Si substrates were rinsed in a 1:10 solution of concentrated HF and deionized water firstly and then annealed in ultrahigh vacuum at 650 °C for several hours and monitored by means of low energy electron diffraction (LEED) until a well-ordered 7x7 reconstruction appeared. Sample growth was performed in a system that is coupled to XPS analysis chamber. The samples are prepared in situ by first depositing a Ge layer with varying thicknesses at 480 °C using
molecular beam epitaxy. Next, Mn layer of 3 nm was deposited at room temperature. Subsequently, the films were annealed for 20 min at 400 ºC. Deposition rates of Ge and Mn were monitored by a quartz-crystal microbalance and were held constant at 0.08 A/s and 0.05 A/s, respectively. The selected Ge films with the following thicknesses were considered: 35 nm (S35), 70 nm (S70), 140 nm (S140), 210 nm (S210) and 280 nm (S280). The crystal structure of the deposited films were examined by a Rigaku Smart-Lab x-ray diffractometer (XRD) which uses a Cu Ka radiation (λ=1.54178Å). Trasmission electron microscope was used to confirm the film thickness and structure. The cross-sectional TEM specimen of the film is prepared using focused ion beam (FIB) micromachining. XPS spectra of the films were collected using a SPECS PHABIOS 100 hemispherical analyser under a base vacuum of less than 1x10−10 Torr, where a monochromatic Al Kα x-ray (hν = 1486.6 eV) was used as the exciting source. The energy scale of the spectrometer was calibrated by setting the measured Au 4f7/2 binding energy to 84eV with regard to Fermi energy. Baseline corrections were made using a Shirley type background correction. The magnetic measurement was carried out using vibrating sample magnetometer (VSM) in the temperature range from 10 to 350K. To obtain information on the microscopic nature of the Mn ions in Ge, electron spin resonance (ESR) was carried out at room temperature using X-band JEOL spectrometer equipped by an electromagnet which provides a dc magnetic field up to 2 kG in the horizontal plane. RESULTS The structural properties of the films and the epitaxial relationship between the Mn5Ge3 film and the Ge layer grown on Si (111) were analyzed by XRD. Figure 1 shows the x-ray diffraction pattern of Mn5Ge3 films formed on Ge layers of thickness from 35 to 280 nm in a semi logarithmic plot. Besides the mean peaks of Ge and Si, the XRD spectra for the films with Ge layer thicknesses of above 140 nm show a pronounced diffraction peak at 35.5° and 75.5º, which corresponds to (002) and (004) plane of Mn5Ge3. When the thickness of the Ge
layer is decreased down to 70 nm, the Mn5Ge3 peaks became broader and their intensity decreased. Furthermore, an additional peak has been observed around 2θ=37.4°. That peak became more pronounced in the XRD spectra of the film with Ge thickness of 35 nm, where the Mn5Ge3 peak disappeared and a higher order peak corresponding to (004) plane of Mn5Si3 phase emerged.
(hkl) planes
Mn5Si3
observed in the films. It is apparent that both Mn5Ge3 and Mn5Si3
grains are uniformly oriented in these samples. The presence of these (00l) peaks in S35 confirm that Mn atoms penetrate in the Si substrate due to diffusion of Mn through thin Ge layer during annealing and form a single phase structure. This indicates that the formation of Mn5Ge3 was delayed until the Ge layer reaches to a certain thickness such that Mn atoms can not diffuse into Si.
In order to investigate the effect of crystal quality of Ge layers on the formation of Mn5Ge3 films, the crystallinity of the Ge layers were evaluated on the basis of the full-width at halfmaximum (FWHM) of XRD rocking curves of Ge (111) peak. The FWHM values of the Ge (111) diffraction peak are displayed as a function of Ge layer thickness in the inset of figure 1. It is seen that the rocking curve profiles of Ge (111) peak appear significantly narrower for thick 280 nm films compared to thin 35 nm samples, leading to the conclusion that the degree of crystallinity increases as the Ge layer thickness increases due to strain relaxation. The Mn5Ge3 (002) signal intensifies as the Ge layer thickness increases. It appears that both Si and Ge actively compete to form bonds with Mn in the formation of their respective Mn phase, depending on the thickness of the Ge layer. It is understood that the Ge buffer layer thickness is quite effective on the formation dynamics of the magnetic phases. XRD results indicate that
there is a critical thickness of the Ge layer on Si to form Mn5Ge3 due to the tendency of favorable for Mn atoms to diffuse into the Si substrate.
For further investigation on the Mn diffusion mechanism, quantitative analyzes of XPS measurements were taken after Mn deposition and subsequent annealing. Mn 2p XPS data (not shown here) showed peaks at 638.7 eV and 650 eV corresponding to the 2p3/2 and 2p1/2 states, respectively. The peaks are separated in energy by 11.3 eV. The energy separation and binding energy of the peaks are consistent with the metallic Mn state. The previous works on Mn5Ge3 show that the line shape of the Mn 2p XPS spectra have a Mn-metal-like shape, similar to our observations [19, 20]The observation of metallic line shape in Mn5Ge3 and Mn5Si3, as it also seen in some Heusler compounds and MnSb alloy, is explained by delocalized magnetic moments on Mn lattice sites [21]. Atomic concentration of the evaporated films has been determined using the ratio of the area of the Mn 2p and Ge 2p core level peaks using the atomic sensitivity factors for the instrument settings. Although the same amount of Mn was deposited on each film, which was annealed at the same temperature, the Mn concentration on the sample surfaces and the phases formed in the films vary depending on the Ge layer thickness. From the measured Mn 2p and Ge 2p core-level peak areas and relative sensitivity factors, we calculated the relative Mn content of all samples. Films showed approximately the same Mn concentration prior to annealing. However, as seen in Fig 2, after the annealing process Mn concentration varied from 20% for S35 to 47% for S280 giving direct macroscopic evidence of Mn subsurface diffusion. The variation in Mn concentrations after the annealing process can be explained by the diffusion mechanism of Mn atoms into the substrate and the differences in surface morphologies. The migration of Mn atoms underneath Ge layer at low Ge layer thicknesses is likely to be responsible for the low manganese
concentration. When the Ge layer thickness is low and domain size is very small, most of the initially deposited Mn must have diffused into the Ge subsurface. The effect of Ge layer thickness on the morphological properties of Mn5Ge3 films grown on Si substrates was presented using atomic force microscopy (AFM). In the light of information obtained from the XRD analyzes, it was considered appropriate to focus on the first three thicknesses. Fig. 3 shows AFM images of the surface of the films grown on Ge buffer layer with thickness of (a) 35, (b) 70 and (c) 140 nm. It can be seen from the images that the surface morphology transforms from a granular form shape to relatively flat surface as the Ge thickness increases. The rms roughness values of 35, S70 and S140 are 7.63 nm, 4.22 nm and 1.77 nm, respectively. Monotonous decrease of surface roughness with increasing Ge layer thickness is related to heteroepitaxial growth process. The MnGe film with tGe= 35 nm on Si(111) exhibits a surface crowded by densely packed and evenly distributed large islands, as seen in Fig. 3(a). As the Ge thickness increases, coalescence of clusters continues and isolated islands evolve into irregularly distributed particles with significantly larger size which initiates two dimensional (2D) growth process with improved surface roughness. The image of the S140 shows a relatively flat surface with mild roughness (Fig 3(c)). As the Ge film thickness increases, reduced stress leads to improvement in crystallographic texture of Ge film. There is a good agreement between the film roughness values determined from AFM and the corresponding FWHM values obtained from XRD scans. When Mn is deposited on S35, we only observe the formation of Mn5Si3 because of the diffusion of Mn atoms into Si. It can clearly be seen that the Mn subsurface diffusion reaction is affected by the degree of crystallinity of Ge layer and this diffusion mechanism plays a critical role in the formation of Mn5Ge3. This idea is supported by TEM analysis.
In Fig 4(a), cross-sectional micrograph of S35 shows dome islands of different sizes formed on Si due to heteroepitaxial growth and nanocluster that clearly extends into the Si subsurface region. In order to investigate those nanoclusters and the distribution of Mn atoms, we also study the elemental distribution of the thin film using energy-dispersive X-ray spectroscopy
(EDX)-STEM mapping. As can be seen in Fig. 4(b-c) Ge forms a dome shaped islands on Si. Mn elements penetrate into the Si substrate either through the exposed regions between the islands or through the center of Ge dome and forming Mn5Si3 phase. It is consistent with the previous results on Mn with Ge quantum dots (QD) where silicide phases preferentially form over germanides during the annealing process [22]. The migration of Mn atoms into Si may be connected with lower diffusion rate of Mn in Ge [22]. The phenomenon of Mn diffusion in the Ge was reported by Zhu et al. and showed that the diffusion paths are strongly related to the crystallographic directions of Ge, Mn easily diffuses into the Ge(111) while on Ge(001) it can act as a surfactant [23]. In agreement with the predictions that Mn atoms diffuse through the bulk Ge and find its path to underneath Si layer. In order to study the influence of the Ge thickness on the magnetic properties, the cases where tGe= 35 nm, 70 nm and 140 nm, were chosen. Above 140 nm, continuous Mn5Ge3 film was formed and it shows similar magnetic characteristics as observed from Mn5Ge3 films directly grown on Ge(111) [24]. We first investigated the temperature dependence of the magnetization under 0.1 T. As can be seen in Figure 5, the film with 140 nm Ge layer undergoes a sharp paramagnetic to ferromagnetic transition at around 300 K which is comparable to Tc of about 296 K for bulk Mn5Ge3 [25], and no obvious change was observed in Tc when Ge layer thickness decreases to 70 nm. When reducing the thickness of Ge layer to 35 nm, a broad PM to FM transition is observed at Tc around 240 K. The substitution of Si for Ge in Mn5Ge3 leads to reduction of crystal size and Curie temperature without changing the crystal structure. Previous studies have shown that the Curie temperature of polycrystalline Mn5Ge3-xSix can range from 298-252 K when x = 0-1.5 [26-28]. Observed Curie temperature is referenced to a value of x=~1.65 for the Mn5GexSi3-x compound [28]. However, from the XRD pattern, the stoichiometry was estimated to be Mn5Si3, without the existence of other phases. On the other hand, recent studies show a ferromagnetic ordering in Mn5Si3 nanoparticles with a high Curie temperature of Tc=590 K due to the size-dependent increase
of surface contribution and quantum confinement effects [29]. It seems that the Curie temperature of Mn5Si3 arises due to the complex effects of magnetic disorder and quantum confinement effects in magnetic nanostructures. Figure 6 shows the magnetic hysteresis loops of the films with varying Ge layer thicknesses recorded at 10 K when the applied field was parallel to the film plane. Because of the low magnetic moment of such thin magnetic films, it was not possible to accurately measure the magnetic hysteresis perpendicular to the film plane using VSM. The normalized hysteresis loops for all samples show a clear ferromagnetic contribution and non-negligible remanent magnetization at 10 K. S140 and S70 present almost the same ferromagnetic contribution with a coercive field strength of Hc= 500 Oe, in agreement with the results of 30 nm thick Mn5Ge3 film [30]. The M-H loop of S35 and S70 do not saturate easily under applied field of up to 10 kOe, compared to samples with thicker buffer layers. The nonsaturating behavior reveals the presence of a superparamagnetic phase due to finite size effects and surface spin disorder of magnetic nanostructures. Since these samples have low moments, the nonsaturation can also be related to dominant paramagnetic contributions arising from the thin buffer layer and the substrate. The slight departure from typical square hysteresis curve can be accounted for with the consideration of exchange and dipolar interactions between the magnetic nanostructures. Mn5Ge3 nanosturcures with the same crystal structure and alignment along the c-axis, but slightly different orientations in the plane of the film, exhibits different effective anisotropies due the magnetostatic interactions between these Mn5Ge3 nanosturcures. The overall magnetic moment and retentivity of films increases with increasing Ge layer thickness. If all Mn atoms would contribute equally to the magnetization, the average magnetic moments per Mn atom for samples S35, S70 and S140 at 10 K are calculated to be 1.12 μB, 1.35 μB, and 2.05 μB, respectively. Comparing with the average magnetic moment values of 2.6 μB per Mn atoms in Mn5Ge3 [31], most of the Mn atoms ferromagnetically interact with each other.
Decrease of magnetization with the decrease of Ge buffer layer thickness is explained by the formation of Mn5Si3 phase which have lower magnetic moment due to reduction of the magnetic moment with the substitution of Si for Ge[32]. Although Mn5Si3 and Mn5Ge3 have the same crystal structure, Mn5Ge3 has much higher unit size therefore higher Mn-Mn atomic separation, leading to higher magnetic moment. The results are in very good agreement with earlier published data for Mn5Ge3-xSix [27].
ESR spectra of S35, S70 and S140 were measured as a function of temperature in order to elucidate the microscopic states of magnetic signals. Figure 7(a) and (b) shows the temperature dependence of the resonance positions of ESR measured with the external magnetic field applied parallel and perpendicular to the film plane, respectively. Consistent with VSM results, ESR signals are observed below the corresponding Curie temperature in both geometries. When an applied field is oriented along the in-plane axis of the film, resonance field and the intensity of signals for all the samples decreased and line width of the signals increased with decreasing temperature. The line broadening with decreasing temperature indicates the magnetic disorder and variation in contribution from exchange interactions. It should be noted that the presence of the Mn5Si3 domains in S70 does not indicate any effect of temperature-dependent magnetic behavior because it shows the same trend as S140 which contains only Mn5Ge3 layer.
The direction of the shift of ESR signal in the out-of-plane direction follows the opposite trend to that for the in-plane alignement. A weak and broad ESR signal appeared in S35 and the resonant field of the signal is shifted from ~360 mT for 240 K up to 1300 for 150 K, as the temperature is decreased. Line width of the signal (not shown here) increased with decreasing temperature as well. Such a weak and broad magnetic signal indicates the existence of
randomly oriented magnetic domains. The ESR spectra for S70 and S140 show a similar evolution with temperature. A single FMR signal is present around Tc. Below Tc, however, the signal splits into two components. The line splitting grows with decreasing the temperature and stays constant below 240 K. It is likely that the multiple resonance occurs due to the emergence of two magnetically nonequivalent Mn atoms which experience different local internal fields.
Fig 8 a-c shows the contour map of resonance signal as a function of magnetic field and polar angle with the color scale relating to the amplitude of the first derivative of absorbed microwave power for S35, S70 and S140, respectively. The angular dependence of FMR spectra was measured at 243K. The ESR spectra of all the films also show a similar angular dependence exhibiting uniaxial magnetic anisotropy along the film plane. The FMR signals of S70 and S140 consist of symmetric resonance lines which can be fitted to a Lorentzian-line shape in parallel orientation. Supplementary Figure S1 shows the angular dependence of H res for S70 and S140. The signals become slightly asymmetric and progressively splits into two peaks as the angle approaches to 90°. The separation between the two signals also increases and the intensity of the FMR signal decreases by approaching the film normal. This may be due to the prominent demagnetization effects in the perpendicular configuration. The splitting of the peak reaches the highest value of 130 Oe when the magnetic field is aligned perpendicular to the film plane. The relative intensity and resonance field positions of these signals depend on the direction of the applied field. The splitting of the FMR spectra of the samples S70 and S140 in the out-of-plane direction is shown in Supplementary Figure S2. This line splitting has never been observed in Mn5Ge3 thin films with thicknesses above 15nm [30, 33]. We believe that the the thickness is the most dominant effect in the observation of line splitting. Angular dependence observed here shows some similarities to spin waves of
GaMnAs [34]. However, spin waves were only present in GaMnAs films with thicknesses in the range of 100 nm to 200 nm due to the pinning of surface modes. Similarly, the occurrence of multiple resonance may be attributed to the existence of two magnetic phases pertaining to bulk and surface modes. Since the Mn atoms at the bulk and surface region experience different local internal fields, there exist a non-uniform magnetization in the film. Difference between the surface and bulk resonance modes decreases with the decrease in magnetization. The signal on the low-field side is identified as the bulk uniform resonance mode, while the higher field signal is acoustic surface mode. The resonance spectrum recorded at perpendicular geometry from the samples gives the evidence for conclusion of surface character of the second signal. As the thickness of the Mn5Ge3 layer increases, the signal intensity increases because uniform bulk mode of Mn5Ge3 increases but the intensity of the second signal stays almost the same. Note that, a single FMR signal can be detected for S35 showing similar angular dependence. However, S35 shows a much less pronounced and broader (ΔH~100 mT) resonance signal indicating large magnetic inhomogeneity. CONCLUSION Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (111) substrates using solid phase epitaxy. The role of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films and their magnetic properties was examined. Both structural and magnetic investigation results indicate that Mn5Ge3 thin film is formed above a critical Ge layer thickness and it shows perpendicular magnetic anisotropy. On the other hand, below the critical thickness of 70nm, Mn atoms directly diffuse into Si layer and form a ferromagnetic Mn5Si3 phase with a Curie temperature of about 250 K. XPS measurements also confirm the diffusion of Mn atoms from Ge towards Si. These results show that highly crystalline Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ~8% lattice mismatch between Mn5Ge3 and Si (111).
ACNOWLEDGEMENT
This work was supported in part by European Union FP7 Marie Curie Program IFMGeDMS Grant. *Corresponding Author: [email protected] CAPTIONS: Figure 1: XRD patterns of the MnGe films deposited on Si (111) substrates using Ge buffer layers of different thicknesses. The inset shows the plot of the FWHM of Ge (111) peak rocking curve vs Ge layer thickness. Figure 2: Atomic concentration of Mn 2p component in MnGe films as a function of Ge film thickness, determined from XPS measurements. Figure 3: AFM images of MnGe films deposited on (a) 35, (b) 70, and (c) 140 nm thick Ge layers illustrating the buffer layer thickness-dependence of the film’s morphology. All images are 10 × 10 μm2 and all AFM images have a height scale of 10 nm. Figure 4: (a) STEM image of MnGe/Si (111) sample showing dome shaped Ge randomly dispersed on Si; (b) and (c) are the corresponding elemental mapping of the Mn and Ge elements, respectively.
Figure 5: Field cooled magnetization vs temperature curve at magnetic field H = 1000 Oe for S35, S70 and S140. Figure 6: Magnetic hysteresis loops at 10 K for the samples with different Ge buffer layer thicknesses. Figure 7: Temperature dependence of e resonance positions for samples S35, S70 and S140 obtained at 9.5 GHz along (a) H//a and H//c. Figure 8: The contour map of resonance signals as a function of magnetic field orientation for (a) S35, (b)S70 and (c) S140.
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Highlights The role of Ge layer thickness and surface morphology on the formation of epitaxial Mn5Ge3 films and their magnetic properties were examined. For thicknesses below 70nm, Mn atoms directly diffuse into Si layer and form a ferromagnetic Mn5Si3 phase with a Curie temperature of about 250 K. Structural and magnetic properties of Mn5Ge3 were enhanced by increasing Ge buffer film thickness. Mn diffusion into Si confirmed from XRD, ESR and XPS data.