Substrates preparation

Prior to the introduction of the samples to the growth chamber an ex-situ wet chemical cleaning is carried out with trichlorethylene, acetone and ethanol in sequential ultrasonic baths. The samples are rinsed with deionized water and dried with air. Then an in-situ thermal cleaning inside the growth chamber consists in degassing the sample at 450 °C for 3 h, followed by a biased plasma etching on the substrate at 40 W for 20 min, to remove the native germanium oxide from the surface. Finally, a high temperature annealing at ~750 ° C for a few minutes is performed until a 2×1 substrate reconstruction is observed by means of reflection high-energy electron diffraction (RHEED), which indicates that the substrate surface is atomically clean and flat, which is a necessary condition for epitaxial growth.

Deposition technique and methods

Magnetron-sputtering technique is used for the growth of Ge(001)/Mn₅Ge₃ thin films with a nominal thickness of 50 nm. The Mn₅Ge₃ thin films were grown by codeposition of Mn and Ge at different temperatures using radio-frequency power sources operating at 45 and 30 W, respectively. The Mn₅Ge₃ rate deposition is 2.9 nm/min. The codeposition of Mn and Ge at different substrate temperatures (Ts) is known as the reactive deposition epitaxy (RDE) method as the solid reaction between add-atoms takes place at the moment they reach the substrate surface.

X-ray diffraction

The X-ray diffraction (XRD) patterns, performed in grazing incidence of the samples synthesized at 650, 750 and 850 °C and the C-doped sample grown at 750 °C are shown in Figure 1. The XRD patterns reveal mainly the existence of the Mn₅Ge₃ phase. At temperatures between 650 and 750 °C one can observe the existence of small Mn₁₁Ge₈ clusters, however as shown later in the magnetic M-T measurements, there is no measurable signal attributed to this phase. Additionally, the hexagonal Mn₅Ge₂ phase, which is also antiferromagnetic, is observed for the growth at 850 °C and for the C-doped sample at 750 °C. Matsui et al., also reported that T _s higher than 700 °C favors the growth of the Mn₅Ge₃ phase while the Mn₁₁Ge₈ phase disappears [²³] (as shown in the inset in Figure 1). It can be established that in the sample grown at 850 °C and the C-doped sample grown at 750 °C, a majority Mn₅Ge₃ phase is obtained in form of thin films on Ge(001) with a small amount of Mn₅Ge₂ nanoclusters. Regarding the Mn-Ge phase diagram the Mn₁₁Ge₈ phase, which is antiferromagnetic, is formed between 350 and 750 °C. It can also be noted that at higher temperatures (i.e. 796 °C) Mn₅Ge₃ dominates the phase formation.

Figure 1 XRD patterns in grazing incidence of undoped Mn₅Ge₃ thin films grown on Ge(001) at 650, 750 and 850 °C and the C-doped thin film grown at 750 °C. The inset shows the reduction of the intensity of the (421) peak of Mn₁₁Ge₈. 

Atomic force microscopy

The atomic force microscopy (AFM) images shown in Figures 2(a) to 2(c) reveal the surface topography of the samples synthesized at 650, 750, 850 °C and the topography of the C-doped sample grown at 750 °C is shown in Figures 2(e) and 2(f). The root mean square (RMS) roughness of the samples increases with increasing T_s, RMS = 32, 51 and 96 nm for the growth at 650, 750, 850 °C, respectively. The C-doped sample grown at 750 °C compared to the undoped sample grown at the same temperature shows a RMS roughness of 24 nm, this roughness reduction (i.e. from 51 to 24 nm) is accompanied with a grain size reduction from ~450 nm for the undoped sample to ~100 for the C-doped sample. The line profile of the AFM micrograph of the C-doped samples Figure 2(g) shows a close-packed arrangement of the grains implying a continuous coverage growth obeying to a Volmer-Weber growth mode. On the other hand, the undoped samples obey also to a Volmer -Weber growth mode but as seen on the line profile scan in Figure 2(d) it is clear that Mn₅Ge₃ do not cover homogeneously the Ge substrate, only 3-dimensional islands are observed [Figures 2(a)-2(c) ]. Two diagrams describing the Volmer-Weber growth modes of the undoped and doped samples are shown in Figures 2(h) and 2(i), respectively. Finally, the scanning electron micrograph of a cross section of the C-doped sample confirms the suggestion of the growth mode [Figure 2(j) ]. The surface free energy of the Ge(001) substrate is 1710 erg/cm^-2, the fact that C-doped Mn₅Ge₃ forms a close-packed grain arrangement on the Ge surface implies a higher surface free energy than that of the undoped compound, but still lower than that of the substrate. According to the Young's equation, a continuous film is obtained if y _s ≥ у _int + yf where γ represents the surface energy of the f (film), s (substrate) and int (interface). If y _s < y _int + yf, the growth corresponds to a 3-dimensional island-like growth leading to bare substrate regions surrounding the islands as in the case illustrated in Figures 2(a)-2(c). Thinner films may exhibit a lower surface roughness allowing nanolithography patterning for the fabrication of for example the source and drain of a spin-FET transistor.

Figure 2 Atomic force microscopy images of the films grown at (a) 650 °C, (b) 750 °C, (c) 850 °C and (d) the C-doped sample grown at 750 °C. (e) The Volmer-Weber growth mode representation for the undoped samples and (f) the Stranki-Krastanov growth mode for the C-doped sample. 

Figure 3(a) shows a RHEED pattern of a 2x1 reconstructed Ge(001) surface along the [100] direction indicating that the surface is atomically smooth prior to thin film growth. Figure 3(b) reveals the RHEED pattern taken inmediately after the initial stage of the deposition, the diffusive nature of the pattern indicates that the surface is rough due to the 3-dimensional island growth, confirming the information obtained from the AFM experiments, concluding that the growth obeys a Volmer-Weber growth mode.

Figure 3 RHEED patterns of (a) a Ge(001) 2x1 reconstructed surface, and (b) the initial growth stage of the Mn₅Ge₃ thin film. 

Magnetic properties

Figure 4 shows the M-T plots, under an applied magnetic field of 2 T, ensuring saturation of the films, for all samples. It can be observed that the C-doped sample has the highest Tc = 390 K compared to ~300 K for the undoped samples. The Tc was determined by the fit of the experimental data with the equation:

Figure 4 M-T of all samples. The T _C was obtained from the fit of the experimental data using Eq. 1. The C-doped sample has a T _C of 390 K. 

where m = M _s /M _o and τ = T/Tc are the reduced spontaneous magnetization and reduced temperature, respectively, and s and p are parameters with p > 3/2 and s > 0 [²⁴]. Furthermore, the saturation magnetization obtained in this sample Ms = 250 kAm^-1 is much lower than that of samples grown at 250 °C, grown by magnetron sputtering reported earlier (Ms = 633 kAm¹, Ref. 13) and lower than 1200 kAm^-1 for Mn₅Ge₃ thin films grown on Ge(111). The reduction of Ms may be due to the formation of dead magnetic phases, usually appearing at the interface with the substrate but also at the surface of the film (when high roughness) and at the intergrain boundaries. This achievement is considered a great advantage over the samples grown at lower T _s . STT is a mechanism where a spin-polarized electric pulse switches the magnetic orientation of a FM layer, a requirement to reduce the magnitude of the electric pulse and the required time to achieve the switch is a low Ms of the FM material. Additionally the material has been grown on Ge(001) substrates which are compatible with the Si(001)-complementary metal-oxide-semiconductor (CMOS) technology. This feature combined with the enhancement of the T _c up to 390 K, makes Ge(001)/Mn₅Ge₃C_0.6 grown at 750 °C a perfect candidate for spintronic applications. Hence, materials with low magnetizations are required for the fabrication of magnetic tunnel junctions with low energy consumption. The shape of the curves in the M-T graph corresponds to ferromagnetic materials, as no additional peaks characteristic of the antiferromagnetic-FM transition of Mn₁₁Ge₈ are observed along the decreasing curve.