Microstructural Evolution of Ni-Stanogermanides and Sn Segregation during Interfacial Reaction between Ni Film and Ge_1−xSn_x Epilayer Grown on Si Substrate

Abstract

The Ni-stanogermanides were formed via an interfacial reaction between Ni film and a Ge_1−xSn_x (x = 0.083) epilayer grown on a Si substrate driven by thermal treatment, and their microstructural and chemical features were investigated as a function of a rapid thermal annealing (RTA) temperature. The Ni₃(Ge_1−xSn_x) phase was formed at the RTA temperature of 300 °C, above which Ni(Ge_1−xSn_x) was the only phase formed. The fairly uniform Ni(Ge_1−xSn_x) film was formed without unreactive Ni remaining after annealing at 400 °C. However, the Ni(Ge_1−xSn_x) film formed at 500 °C exhibited large surface and interface roughening, followed by the formation of Ni(Ge_1−xSn_x) islands eventually at 600 °C. The Sn concentration in Ni(Ge_1−xSn_x) gradually decreased with increasing RTA temperature, implying the enhancement of Sn out-diffusion from Ni(Ge_1−xSn_x) grains during the Ni-stanogermanidation process at higher temperature. The out-diffused Sn atoms were accumulated on the surface of Ni(Ge_1−xSn_x), which could be associated with the low melting temperature of Sn. On the other hand, the out-diffusion of Sn atoms from Ni(Ge_1−xSn_x) along its interface was dominant during the Ni/Ge_1−xSn_x interfacial reaction, which could be responsible for the segregation of metallic Sn grains that were spatially confined near the edge of Ni(Ge_1−xSn_x) islands.

1. Introduction

Ge is a promising channel material for high-performance complementary metal-oxide-semiconductor (CMOS) devices due to its higher carrier mobility than Si [1,2]. The device performance of Ge channel devices can be enhanced by applying strain to the Ge channel area using a Ge–Sn alloy (Ge_1−xSn_x), which has a larger lattice constant than bulk-Ge. This concept is similar to that of state-of-the-art strained Si channel engineering [3,4,5,6]. Moreover, Ge_1−xSn_x exhibits higher carrier mobility than Ge, which makes it suitable as an alternative channel material in next-generation CMOS devices [7,8]. However, for a successful implementation of Ge_1−xSn_x as a channel material or a source/drain stressor in CMOS devices, the formation of low resistivity source/drain contact regions is of vital importance, thus demanding the need for a thorough understanding of the growth of these contact materials toward performance enhancement. In general, self-aligned silicides or germanides formed by depositing metal film on Si or Ge followed by thermal treatments at elevated temperature have been widely used to reduce the contact resistance in the source/drain regions and the resistance–capacitance (RC) delay for the realization of high-performance Si- or Ge-based CMOS devices. Similarly, stanogermanides are formed in a self-aligned approach by the reaction of a metal with Ge_1−xSn_x. Ni being an excellent candidate for contact formation on Ge, and NiGe being a promising germanide due to its low contact resistivity, low formation temperature, and self-alignment features, NiGe was considered as a candidate for contact material in the Ge_1−xSn_x system, and we carried out a detailed investigation on the phase formation and atomic diffusion during the Ni_x(Ge_1−xSn_x) growth. Previous reports have mostly focused on Ni-monostanogermanide (Ni(Ge_1−xSn_x)) formed using Ni deposition on Ge_1−xSn_x followed by an annealing process [9,10,11,12]. Nishimura et al., investigating the formation of Ni(Ge_1−xSn_x) films on Ge_1−xSn_x layers with Sn contents ranging from 2.0 to 6.5% using a solid-phase reaction, demonstrated that annealing at 350 °C led to the formation of uniform Ni(Ge_1−xSn_x) layers with smooth surface morphology on Ge_1−xSn_x layers [13]. They also reported the critical temperature at which the agglomeration of Ni(Ge_1−xSn_x) occurred, which varied with the Sn content. Such a Sn content-dependent agglomeration behavior was also reported by Vincent et al. [14]. Tong et al. showed that Ni(Ge_1−xSn_x) with low-resistivity was formed on Ge_1−xSn_x using annealing at 350 °C, and the effect of Se or S implantation prior to Ni-stanogermanidation on its phase formation was negligible [15]. Despite these efforts, however, a detailed knowledge of the structural and chemical characteristics of Ni-stanogermanides formed via the interfacial reaction between Ni and Ge_1−xSn_x driven by the annealing process remains unclear. In the present work, Ni-stanogermanides were formed using Ni deposition on a Ge_1−xSn_x epilayer grown on a Si (100) substrate combined with rapid thermal annealing (RTA) process. The dependency of surface/interface morphologies and phase evolutions of Ni-stanogermanides on RTA temperature was investigated using various analysis techniques. In particular, it will be shown that the segregation of metallic Sn grains near the periphery of Ni(Ge_1−xSn_x) islands could be associated with the out-diffusion of Sn atoms from Ni(Ge_1−xSn_x) along its interface during Ni-stanogermanidation process at the elevated RTA temperature.

2. Materials and Methods

The 110 nm-thick Ge and 150 nm-thick Ge_1−xSn_x epilayers were sequentially grown on B-doped p-type Si (100) wafer with the resistivity of 5–15 Ω·cm using rapid thermal chemical vapor deposition (RTCVD). A Ge buffer layer was grown directly on a Si substrate with two-step growth and an in-situ annealing process to reduce the misfit dislocations, followed by the growth of the Ge_1−xSn_x layer. A Ge buffer layer was used to suppress the propagation of dislocations occurring at the Ge and Si interface caused by lattice constant mismatch about 4.2%. Gaseous Ge₂H₆ and liquid SnCl₄ precursors were used for the Ge_1−xSn_x growth process. A Ge buffer layer was grown on a Si substrate at 350 °C with Ge₂H₆ precursor and H₂ carrier gas. The substitutional Sn composition in Ge_1−xSn_x, determined from reciprocal space mappings around the (224) reflection (not shown here), was estimated to be 8.3%. Initially, the grown Ge_1−xSn_x epilayer was cleaned in an ultrasonic bath of acetone and methanol, immersed in a diluted HF solution (H₂O:HF = 100:1) to remove the native oxide, and finally rinsed with de-ionized water. 30 nm-thick Ni films were deposited on cleaned Ge_1−xSn_x epilayers by means of electron beam evaporation. The samples were then rapid-thermal-annealed for 30 s in N₂ atmosphere between 300 and 600 °C to facilitate the interfacial reaction between Ni and Ge_1−xSn_x films.

Glancing incidence angle X-ray diffraction (GIXRD, PANalytical, X’Pert Pro MRD System, Malvern Panalytical, Malvern, UK) was performed to characterize phases formed in the samples. The surface morphology of the samples was examined using a field emission scanning electron microscope (SEM, S-4200, Hitachi Co., Tokyo, Japan) equipped with energy dispersive X-ray spectroscopy (EDX, EX-250, Horiba, Kyoto, Japan). X-ray photoemission spectroscopy (XPS, K-alpha, Thermo Fisher Scientific Inc., Loughborough, UK) was employed to identify the chemical bonding nature of the Ni-stanogermanides. The C 1s peak in air was fixed to 285.0 eV to set the binding energy scale. Microscopic observations of the samples were made using a field emission transmission electron microscope (TEM, JEM-ARM 200F, JEOL, Tokyo, Japan) operating at 200 keV with EDX (X Max, Oxford, UK).

3. Results and Discussion

Figure 1a shows the GIXRD plots of Ni films deposited on Ge_1−xSn_x/Ge epilayers grown on a Si substrate as a function of RTA temperatures ranging from 300 to 600 °C. The GIXRD plots clearly revealed that Ni-stanogermanides were formed as a result of Ni/Ge_1−xSn_x reaction induced by RTA process. Ni₃(Ge_1−xSn_x) and characteristic Ni peaks were found in the sample annealed at 300 °C, although they could not be sharply separated because of very similar XRD peak positions [15,16,17]. The characteristic Ni peak indicates that the deposited Ni film was not wholly reacted with Ge_1−xSn_x during RTA at 300 °C. The low resistivity Ni(Ge_1−xSn_x) was the only phase of Ni-stanogermanides in the samples after annealing above 400 °C, which is usually reported to be formed at annealing temperatures above 350 °C [1,8,17,18]. After RTA process at 600 °C, additional GIXRD peaks corresponding to Sn and SnO₂ were observed, which could be associated with the segregation and oxidation of Sn or Ge_1−xSn_x surface during Ni-stanogermanidation at high annealing temperature. It should be noted that Ni(Ge_1−xSn_x) peaks were gradually shifted toward the higher 2 θ side as the RTA temperature was increased. For instance, an increase in the RTA temperature resulted in higher 2 θ values of Ni(Ge_1−xSn_x) (111) peaks, which were closer to the NiGe (111) peak [19], as shown in Figure 1b. This indicates that the increase in RTA temperature facilitated the decrease in Sn concentration in Ni(Ge_1−xSn_x) grains associated with the out-diffusion of Sn atoms, leading to the reduction of its lattice constant. A similar behavior was also observed by Zhao et al., investigating the interfacial reaction of Ni with relaxed Si_1−xGe_x (x = 0.2, 0.3) films in the low temperature range of 300–500 °C [20]. They showed that a shift of the characteristic peaks of Ni(Si_1−xGe_x) toward a higher 2 θ value with increasing annealing temperature could be attributed to the segregation of Ge at grain boundaries of Ni(Si_1−xGe_x) during the reaction between Ni and Si_1−xGe_x films and the subsequent formation of Ge-rich Si_1−xGe_x. Detailed phase and microstructural evolutions for RTA-induced Ni/Ge_1−xSn_x interfacial reaction will be shown in TEM results later.

Figure 2a and Figure 2b present the XPS core level spectra of Sn 3d_3/2 and Ni 2p_3/2, respectively, taken from the Ni films deposited on Ge_1−xSn_x films after RTA processes at 300–600 °C. The Sn 3d_3/2 core level spectra were deconvoluted by fitting with Shirley background subtraction to investigate the RTA temperature dependency of the chemical binding features. The XPS characterization is surface sensitive with a probing depth of a few nanometers. The surface of the sample annealed at 300 °C seemed to be covered with unreacted Ni films under which the Ni-rich Ni₃(Ge_1−xSn_x) films are formed when considering the corresponding GIXRD data and the TEM/STEM-EDX results that follow, and hence Sn atoms cannot be detected owing to the probing depth limitation. However, the symmetric peaks centered at 494.7 and 492.8 eV were observed in samples annealed above 400 °C, which correspond to the binding energies of Sn⁰ (metallic Sn) and Sn⁴⁺ (SnO₂), respectively [21,22,23]. This appearance of the Sn⁰ and Sn⁴⁺ peaks on RTA above 400 °C is due to the formation of the Ni(Ge_1−xSn_x) film at the surface that consisted of the Sn atoms. The Sn⁰ peak appeared as a weak shoulder with respect to the main Sn⁴⁺ peak, implying that Sn–O bonding was dominant in the samples annealed above 400 °C. The XPS results showed that significant oxidation of the Ni(Ge_1−xSn_x) film occurred during the RTA process. A formation of Sn⁴⁺ (SnO₂) observed after annealing at temperatures above 400 °C, that is, after the complete consumption of the Ni film and formation of the Ni(Ge_1−xSn_x) film at the surface, implies the growth of surface oxides despite annealing in N₂ gas. This Sn⁴⁺ (SnO₂) observed after annealing at 400 °C could be due to the subsequent oxidation of the Sn atoms available at the surface in the Ni(Ge_1−xSn_x) film when the sample was exposed to air and possible surface contamination under annealing N₂ gas treatment conditions. A similar observation was made of the SnO_x formation with increase in intensity despite annealing Ge_0.883Sn_0.117/Ge in N₂ gas at 400 °C [24]. Interestingly, the area ratios of Sn⁰ to Sn⁴⁺ peaks increased with increasing RTA temperature. This indicates that the amount of metallic Sn near the surface increased with increasing annealing temperature, although the RTA at higher temperature had higher driving force for the surface oxidation of Ni(Ge_1−xSn_x) film. The Ni 2p_3/2 XPS core level spectra (Figure 2b) show that an annealing at 400 °C led to a shift in Ni 2p_3/2 peak toward the high binding energy side. This could be due to a phase transition from Ni₃(Ge_1−xSn_x) to Ni(Ge_1−xSn_x), causing a change in the chemical bonding of Ni atoms [25].

Figure 3 presents the plan-view SEM images of Ni-stanogermanide film formed via RTA process at the temperatures of 300–600 °C. The sample annealed at 300 °C showed pyramid-shaped features with a size of 500 nm, exhibiting a similar surface morphology to the bare Ge_1−xSn_x epilayer surface with a pyramid-shaped structure. This implies that the deposited Ni film followed the morphology of the Ge_1−xSn_x epilayer and that there was an insignificant surface modification in the Ni/Ge_1−xSn_x interfacial reaction during RTA at 300 °C. After RTA process at 400 °C (Figure 3b), a similar but less dominant pyramid-shaped structure with reduced size is observed, due to surface smoothening driven by annealing at 400 °C that induced an increased solid-state reaction between Ni and the Ge_1−xSn_x epilayer. In general, a uniform surface promotes good electrical contact and minimizes barriers to current flow, resulting in lower contact resistance [26]. Thus, the precise control of RTA conditions for maintaining a smooth and well-defined surface of Ni(Ge_1−xSn_x) film is essential for achieving optimal device performance. The AFM measurements (not shown here) indicated that the root-mean-square (rms) roughnesses of the samples annealed at 300 and 400 °C were 14.9 and 10.1 nm, respectively. Additionally, the Ge_1−xSn_x surface was fully covered with Ni₃(Ge_1−xSn_x) and Ni(Ge_1−xSn_x) films formed after annealing at 300 and 400 °C, respectively. Namely, there were no exposed Ge_1−xSn_x regions, as shown in SEM-EDX maps (insets of Figure 3a,b). An increase in the RTA temperature to 500 °C resulted in the occurrence of disintegration and agglomeration of the Ni(Ge_1−xSn_x) film (Figure 3c) forming round shape structures, followed by increased grain separation and round-shaped Ni(Ge_1−xSn_x) island formation at 600 °C. It should be noted that after RTA at 600 °C, most Sn atoms were mainly distributed along the periphery of Ni(Ge_1−xSn_x) islands, as shown in the inset of Figure 3d. This indicates that a large amount of Sn atoms were out-diffused from the Ni(Ge_1−xSn_x) grains, and segregated near the periphery of the Ni(Ge_1−xSn_x) islands, which will be further manifested in the TEM combined with STEM-EDX analysis as discussed later.

The cross-sectional bright field TEM images obtained from the Ni-stanogermanide films formed through RTA processes at temperatures of 300–600 °C are shown in Figure 4. The relatively uniform Ni₃(Ge_1−xSn_x) and Ni(Ge_1−xSn_x) layers were formed after RTA processes at the temperatures of 300 and 400 °C, respectively. For the sample with RTA process at 300 °C, the enlarged TEM image (inset of Figure 4a) showed unreacted Ni overlying the Ni₃(Ge_1−xSn_x) layer, which was expected from the GIXRD results (Figure 1), whereas RTA at 400 °C yielded the formation of Ni(Ge_1−xSn_x) without any remaining unreacted Ni. This implies that the entire Ni film reacted with Ge_1−xSn_x, which was also consistent with the GIXRD results (Figure 1). After RTA above 500 °C, the Ni(Ge_1−xSn_x) underwent structural degradation, resulting in a large-scale interface and surface roughening of the Ni(Ge_1−xSn_x). Such a structural degradation observed in the samples annealed at higher temperature is a typical feature referred to as agglomeration. The driving force for agglomeration is the equilibrium among the surface energy of the film, the interface energy of the grain boundary, and the interface energy between the film and substrate, as proposed by Nolan et al. [27]. In general, the agglomeration could be initiated with grain boundary grooves at the film/substrate interface and could eventually cause the formation of individual islands. Since thermal energy promotes the agglomeration, Ni(Ge_1−xSn_x) film becomes more severely agglomerated at higher temperatures. Additionally, TEM images clearly show the presence of dislocations in Ge_1−xSn_x epilayers caused by lattice mismatch between film and substrate. The dislocation density was calculated using the line-intercept method proposed by Martin et al. [28]. The dislocations were found to be most similar in all samples with the density of about 10⁹ cm⁻², implying the variation of dislocation density caused by thermal treatment required for Ni-stanogermanidation process is insignificant. Moreover, this value is comparable to the previously reported dislocation density of the Ge_1−xSn_x epilayer with Sn content of 5.3% [29]. It was reported that the dislocations in the Ge–Sn system facilitate the transportation of elements through pipe diffusion [30,31]. Thus, the presence of dislocation may influence the Ni-stanogermanidation process. Further details of dislocation-dependent solid-state reaction between Ni film and the Ge_1−xSn_x epilayer, as our future work, will be published elsewhere.

Figure 5 shows the STEM-EDX line profiles for Ni, Sn, and Ge atoms obtained from the Ni-stanogermanides formed after RTA processes at temperatures ranging from 300 to 600 °C. STEM-EDX line profiling was performed along the substrate direction as indicated by arrows in the corresponding STEM Z-contrast images (insets of Figure 5). The profiles obtained from the sample with RTA process at 300 °C (Figure 5a) clearly disclosed the presence of unreactive Ni and Ni₃(Ge_1−xSn_x), as depicted in the cross-sectional TEM results (Figure 4a). The intensity of the EDX line profile for Sn atoms in Ni₃(Ge_1−xSn_x) formed at 300 °C was comparable to that in Ge_1−xSn_x, implying an insignificant difference in the Sn concentration between Ni₃(Ge_1−xSn_x) and Ge_1−xSn_x. However, other samples showed lower profile intensities for Sn atoms in Ni(Ge_1−xSn_x) than those in Ge_1−xSn_x. Moreover, the intensity of the Sn profile in Ni(Ge_1−xSn_x) gradually decreased with increasing RTA temperature. In other words, the STEM-EDX line profiling results suggest that an increase in the RTA temperature promoted the out-diffusion of Sn atoms from Ni(Ge_1−xSn_x). During the solid-state reactions between Ni and Ge_1−xSn_x, Sn content would be expected to remain steady in Ni(Ge_1−xSn_x) film. However, the Ge–Sn system is highly metastable, as solid solubility of Sn in the Ge layer is low (less than 1%) [32]. Thus, Sn atoms tend to be spontaneously out-diffused from the Ni(Ge_1−xSn_x) layer when the RTA temperature is close to Ge–Sn eutectic temperature (504 K) or Sn melting point (505 K). Moreover, such an out-diffusion of Sn atoms becomes more pronounced at elevated temperature due to the exponential temperature dependency of the diffusion coefficient. In general, conventional alloy systems frequently show surface and interface segregations that are driven by elemental diffusion associated with the minimization of their surface energy [33,34]. Nevertheless, the accumulation of Sn atoms near the surface region of Ni(Ge_1−xSn_x) was insignificant in present samples. This could be attributed to the low melting temperature of Sn (231.9 °C) [35]. Namely, during the RTA process, Sn atoms were out-diffused from Ni(Ge_1−xSn_x) to the surface where they might evaporate. In addition, for the samples annealed at 400 and 500 °C, Sn segregation at the interface between Ni(Ge_1−xSn_x) and Ge_1−xSn_x was almost negligible. However, Sn atoms were slightly accumulated in the Ni(Ge_1−xSn_x) interface, as indicated by the arrow in Figure 5d. It should be noted that Ni(Ge_1−xSn_x) grains were formed within the Ge_1−xSn_x epilayers after RTA processes at 400 and 500 °C, while the annealing at 600 °C caused the formation of Ni(Ge_1−xSn_x) islands that penetrated into the Ge buffer layer. Since the amount of Sn atoms out-diffused from Ni(Ge_1−xSn_x) toward the interface were much lower than those in the Ge_1−xSn_x layer, they had a limited contribution to the increase in the concentration of Sn atoms in the Ni(Ge_1−xSn_x)/Ge_1−xSn_x interface in the samples with RTA processes at 400 and 500 °C. This could be responsible for a lack of EDX line profile behavior of the Sn atoms reflecting Sn segregation at the interfaces. On the other hand, for the sample annealed at 600 °C, the bottom of the Ni(Ge_1−xSn_x) island directly contacted with the Ge buffer layer, so the EDX line profiling for Sn atoms revealed the accumulation of out-diffused Sn atoms at the interface.

Similar elemental distribution behavior was also observed by STEM-EDX mapping, as shown in Figure 6. The STEM-EDX mapping shows that an increase in the RTA temperature facilitated the out-diffusion of Sn atoms from Ni(Ge_1−xSn_x) without Sn segregation on its surface. It is noteworthy, however, that there was substantial segregation of Sn atoms that were mainly distributed near the edges of the Ni(Ge_1−xSn_x) island in the sample with RTA process at 600 °C, as indicated by arrows (Figure 6o). This is also consistent with the plan-view SEM-EDX results (Figure 3). Additionally, as shown in the enlarged inset of Figure 6m, the segregated region exhibited the brightest contrast in the STEM Z-contrast image, indicating that this region mainly consisted of Sn atoms. Moreover, the XPS and GIXRD results suggest that the segregated Sn grains were a metallic phase with Sn–Sn bonding. Unlike Sn atoms approaching the Ni(Ge_1−xSn_x) surface through out-diffusion, segregated metallic Sn grains were spatially confined near the edge of the Ni(Ge_1−xSn_x) and less exposed to their surface. Thus, they were able to exist stably without evaporation during the RTA process, even at high temperature. Such a formation of segregated metallic Sn grains could be associated with massive diffusion through the Ni(Ge_1−xSn_x) interface. In other words, after Sn atoms were out-diffused from Ni(Ge_1−xSn_x) toward the interface during interfacial reaction between Ni and Ge_1−xSn_x, they were subsequently diffused further along the Ni(Ge_1−xSn_x) interface, resulting in the accumulation of metallic Sn grains near the edges of Ni(Ge_1−xSn_x) islands. It is well known that phase formation and surface morphology strongly affect electronic properties. The agglomeration of Ni-stanogermanide films, along with Sn segregation, is reported to result in an increase in the sheet resistance, thus deteriorating the electrical properties of the contact [36,37]. This increase in sheet resistance leads to higher specific contact resistivity that could result in the degraded performance of the Ni-stanogermanide/Ge_1−xSn_x-based CMOS devices. The addition of an alloying element like Co or Pt delays the agglomeration of Ni(Ge_1−xSn_x) film and Sn segregation, widening the annealing temperature process window in which the sheet resistance remains low.

The microstructural evolution of Ni-stanogermanide films and Sn segregation associated with the out-diffusion of Sn occurring during solid-state reaction between Ni and Ge_1−xSn_x is summarized as following. Overall phase evolution and morphological variation of solid-state reaction between Ni and Ge_1−xSn_x driven by thermal treatment were examined using XRD and SEM, respectively. RTA temperature-dependent surface roughening of Ni(Ge_1−xSn_x) films was quantitatively determined using AFM measurements. In particular, STEM combined with EDX was employed to show redistribution of Sn atoms and Sn segregation locally confined near the edges of Ni(Ge_1−xSn_x) islands. Moreover, the chemical property of segregated Sn and Ni was assessed using XPS. All of these analysis techniques are mutually complementary to reveal the detailed features of the Ni-stanogermanidation process. The Ni-stanogermanidation is initiated by the growth of a thin layer of Ni-rich Ni₃(Ge_1−xSn_x) phase on RTA at 300 °C with the existence of unreacted Ni films at the surface. The Ni₃(Ge_1−xSn_x) phase is soon followed by the formation of Ni(Ge_1−xSn_x) phase with a complete consumption of the deposited Ni films on RTA at 400 °C and the continued simultaneous Ni(Ge_1−xSn_x) phase growth. An increase in the RTA temperature to 500 °C was high enough to cause the agglomeration of the Ni(Ge_1−xSn_x) films, resulting in rough surface and interface morphologies. A further increase in the RTA temperature to 600 °C led to the increased separation of the agglomerated Ni(Ge_1−xSn_x) grains. Due to the metastability of the Ge–Sn system, the Sn atoms are out-diffused from Ni-stanogermanide films toward the film interface during solid-state reaction between Ni and Ge_1−xSn_x. The higher the RTA temperature, the more easily out-diffusion of Sn atoms occurs. In other words, as the temperature increases, the amount of out-diffused Sn atoms increases. The out-diffused Sn atoms move along the Ni(Ge_1−xSn_x)/Ge_1−xSn_x interface toward the surface, and metallic Sn grains eventually accumulate near the edges of Ni(Ge_1−xSn_x) islands at the RTA temperature of 600 °C.

4. Conclusions

We investigated the structural and chemical properties of Ni-stanogermanides formed using Ni deposition on a Ge_1−xSn_x epilayer grown on a Si (100) substrate, followed by RTA process. Rapid thermal treatment at 300 °C yielded the formation of the Ni₃(Ge_1−xSn_x) film with unreacted Ni. Ni(Ge_1−xSn_x) was the only phase that formed after RTA process above 400 °C. The RTA process at 400 °C yielded the formation of Ni(Ge_1−xSn_x) film with uniform surface and interface morphologies. However, the Ni(Ge_1−xSn_x) film became irregular, with a rough interface, at RTA temperatures in excess of 500 °C, and eventually agglomerated into discontinuous Ni(Ge_1−xSn_x) islands. STEM-EDX line profiling results showed that an increase in RTA temperature facilitated the out-diffusion of Sn atoms from Ni(Ge_1−xSn_x) during the interfacial reaction between Ni and Ge_1−xSn_x. This resulted in a decrease in the Sn content in the Ni(Ge_1−xSn_x), which was consistent with the GIXRD results. The STEM-EDX maps combined with XPS spectra revealed the presence of metallic Sn grains that were mostly distributed near the edge of the Ni(Ge_1−xSn_x) island, which could be attributed to the massive out-diffusion of Sn atoms from Ni(Ge_1−xSn_x) along its interface. From the results demonstrated here, it is evident that a Ni-stanogermanidation process at the RTA temperature of 400 °C leads to the formation of low-resistive Ni(Ge_1−xSn_x) film with better surface and interface morphologies. This process condition could result in better contact properties of source/drain regions, which would be essential to realize high-performance Ge_1−xSn_x-based CMOS devices.