Formation of Metastable Solid Solutions in Bi-Ge Films during Low-Temperature Treatment

Abstract

We investigated the mechanism and kinetics of the formation of metastable BiGe solid phases during the amorphous-to-crystalline transformation of Ge films in contact with Bi. Ge/Bi/Ge sandwich films with a Bi film between amorphous Ge films, which were fabricated by sequential deposition of the components in a vacuum, were used in this study. The total thickness and composition of the sandwich films varied in the range from 30 to 400 nm and from 22 to 48 wt% Bi, respectively. Electron diffraction, high-resolution (S)TEM imaging, EDX, and EEL spectroscopy were used for in situ and ex situ characterization of the morphology, composition, and structure of Ge/Bi/Ge films in the temperature range of 20–271 °C. We proved the formation of polycrystalline Ge films containing up to 28 wt% Bi during low-temperature treatment. The interaction process was activated at ≈150 °C, resulting in the crystallization of Ge with the simultaneous formation of a quasi-homogeneous supersaturated solid solution throughout the entire volume of the film at ≈210 °C. We showed that the formation of crystalline Ge films with an extended solid solubility of Bi depended mostly on the overall composition of the tri-layer film. The role of metal-induced crystallization of the amorphous germanium in the formation of the supersaturated solid phases is discussed.

1. Introduction

Natural materials have practically exhausted their possibilities in modern technologies. At the same time, the combination of different components can produce improved or even new unique properties compared to individual substances. Some of the most successful examples include artificially created nanoscale materials with topological, catalytic, or even quantum properties. Layered metal–semiconductor systems have already been successfully introduced into solar cells [1,2,3], ultrasensitive photodetectors [4,5,6], sensors [7,8,9], optical modulators [10,11,12], light emitters [13,14,15], lasers [16,17,18], thermoelectric generators [19,20,21], and components of a new generation of batteries based on magnesium [22,23,24]. Nevertheless, despite the widespread use of nanoscale metal–semiconductor structures in applications, interactions at the metal–semiconductor interface remain one of the key issues in nanoscience and nanotechnology [25].

Germanium is considered a promising element for the replacement of Si in next-generation technologies. Charged particles move through germanium more readily than they do through silicon, making germanium a good material for electronics. For instance, it is a promising material for high-speed thin-film transistors because of its higher carrier mobility [26,27]. The problem of the non-indirect band gap in Ge can be overcome by inducing tensile strain due to the small separation energy of 140 meV between the indirect and direct conduction bands. In Ref. [28], it was shown that the minimum concentration of Sn in the Sn-Ge binary system required for an indirect-to-direct band gap transition is about 6.5 at%, which is significantly higher than the equilibrium concentration of Sn (0.52 at% at room temperature). Such an alloy could be formed only under non-equilibrium conditions. Nevertheless, highly doped semiconductors possess promising features, including a wide range of direct or quasi-direct band gaps that improve the efficiency of photoelectric conversion; smaller volume expansion, which is essential for lithium-battery anode material; and enhanced catalytic and electronic properties [29,30,31]. Moreover, crystalline Ge (c-Ge) films are essential for the development of flexible electronics [32]. However, the condensation of Ge films occurs in an amorphous phase, and various techniques, such as solid-phase crystallization (SPC) [33,34], laser annealing [35,36], metal-induced lateral crystallization (MIC) [37,38], and metal-induced layer-exchange crystallization [39,40], are utilized to achieve crystallization.

Recently, we studied the nucleation and growth of the crystalline phase in amorphous Ge films, which was induced by Ag nanoparticles during in situ heating in TEM [41]. It was found that the crystallization process of a-Ge was implemented in two stages. The first stage, which occurred at low temperatures, resulted in crystallization of the Ge film under the Ag nanoparticles. The lateral crystallization of the a-Ge film was observed in the second stage at a temperature above 400 °C. Typically, the MIC process in layered films is accompanied by the formation and decomposition of metastable solid solutions, as well as mass transfer on a large scale [42,43].

The formation of metastable phases with 30–50 at% Sn has been reported for amorphous Sn-Ge films deposited on glass or beryllium substrates [44]. The abnormal solubility of Sn in polycrystalline Ge (about 22 wt%, equivalent to 32 at%) was reported in recent studies [45,46]. Nevertheless, reports on the formation of metastable solid solutions in Bi-Ge films are virtually absent in the literature. The formation of metastable phases in Bi-Ge amorphous films has only been reported in Refs. [47,48,49].

At the same time, the interface interactions between amorphous Ge and Bi films depend on the configuration of the layers, their thicknesses, and the overall composition of the film system [50,51]. Thus, the layered structure in Ge/Bi/Ge films was almost preserved up to the melting temperature when the ratio of Ge to Bi film thickness was 1:1, despite the crystallization of the Ge layers. The Ge/Bi/Ge sandwich with substantially thicker Ge films did not exhibit the metal-induced crystallization effect, indicating destruction of the layered structure and dissolution of Bi in the Ge films.

Hence, the aim of this work was to investigate supersaturated solid solutions which are formed in Ge/Bi/Ge films with different ratios of Bi to Ge film thicknesses during low-temperature treatment. We aimed to develop an approach enabling the formation of highly doped BiGe alloys with tunable concentrations of Bi via control of the mechanisms of interface interactions in metal–semiconductor films.

2. Materials and Methods

Bi and Ge form a simple eutectic phase diagram with negligible solid-state solubility of the components [52]. The eutectic point lies at a temperature and composition very close to those of pure Bi. Thus, the eutectic temperature is only 0.3 K below the melting point of pure bismuth, which is 271.3 °C.

The Bi-Ge films were formed on a room-temperature substrate by the thermal evaporation of Bi (99.99%, NSC KIPT, Kharkiv, Ukraine) and Ge (99.999%, Able Target Ltd., Nanjing, China) from independent sources in a vacuum of 1∙× 10⁻⁷ Torr. The sandwich-type Ge/Bi/Ge configuration, with a bismuth film confined between two germanium layers, was formed by sequential deposition of the components. The total thickness of the tri-layer films varied in the range of 30–400 nm, with the Bi content varying between 22 and 48 wt%. Freshly cleaved NaCl crystals (Institute for Single crystals, Kharkiv, Ukraine) with a thickness of about 0.5 mm, amorphous SiNx, and quartz crystals were used as substrates. The mass thicknesses of the films were determined using a piezoelectric quartz sensor. The Ge film grew in an amorphous (a-Ge) phase on a room-temperature substrate, whereas bismuth formed a polycrystalline film. The films grown on NaCl were separated from the salt crystals by dissolving them in distilled water and were then placed on a TEM grid.

The morphology, crystalline structure, composition, and phase state of the Ge/Bi/Ge films were studied by a set of highly complementary techniques both in situ and ex situ.

The crystalline structure of the samples was studied in the temperature range of 20–270 °C using a TEM-125C (Selmi, Sumy, Ukraine) transmission electron microscope fitted with an in-house heating holder [53]. Selected area electron diffraction (SAED) patterns were acquired in steps of 10–25 K during the heating and cooling of the films.

HAADF-STEM images, EELS spectra, and EDX chemical element maps were acquired using a probe Cs-corrected FEI Titan3 G2 60−300 transmission electron microscope equipped with ChemiSTEM technology (FEI, now Thermo Fisher Scientific, Waltham, MA, USA) and a Gatan Image Filter (model 966) at 300 kV. The atomic number (Z) of bismuth is two and half times larger than that of germanium (Z_Bi = 83, Z_Ge = 32), which facilitates mass thickness contrast imaging of these elements with a high-angle annular dark-field detector (HAADF) in STEM. A Cliff–Lorimer standard-less method was used for a quantification of the EDX spectra using the Lα lines of Bi and Ge. Low-loss EEL spectrum images 248 × 101 pixels in size were acquired with a pixel size of 4 nm and a dwell time of 10 ms. An EELS dispersion of 0.05 eV per channel and convergence and collection semi-angles of 12 and 22 mrad, respectively, were used.

Quartz resonator Q-factor analysis (the details of the technique are available elsewhere [45]) was used to probe the formation and temperature stability of the liquid phase in Ge/Bi/Ge films with different film thickness ratios. In this case, Ge/Bi/Ge films were deposited on AT-cut quartz crystal with a fundamental frequency of 6 MHz at room temperature. Heating and cooling were performed in the 20–300 °C temperature range at a rate of 2 °C/min. The Q-factor of the quartz oscillator was continuously measured during temperature cycling using an automated system, and the phase transition temperatures were detected by an abrupt change in the Q-factor value [51].

3. Results and Discussion

3.1. Phase Transformations in Ge/Bi/Ge Films Probed by Quartz Resonator

First, we determine the ratio of Ge and Bi film thickness at which melting in Ge/Bi/Ge films does not occur. As an example, Figure 1 shows the temperature dependencies of the Q-factor of the quartz resonator loaded by Ge/Bi/Ge three-layer films with different thicknesses of the Ge layer during the heating–cooling cycle. The thickness of the Bi film was 50 nm in all specimens. As one can see, the temperature dependence of the Q-factor for Ge/Bi/Ge film with a Ge layer thickness of 50 nm shows distinct hysteresis corresponding to the melting and crystallization of Bi between the Ge layers (Figure 1 (a)). This is an expected observation, and it correlates well with our previous studies on melting phenomena in layered films [50,51]. An increase in the Ge thickness to 100 nm in the Ge/Bi/Ge films resulted in a significant decrease in the magnitude of the Q-factor drop, though the melting–crystallization hysteresis is still visible in Figure 1 (b). For thicker Ge films, melting and crystallization were no longer observed. Thus, Figure 1 (c) shows a temperature dependence of the Q-factor of Ge(200 nm)/Bi(50 nm)/Ge(130 nm) film. Please note that no liquid phase was formed. Therefore, we supposed that the dissolution of Bi in Ge layers would be possible before the melting temperature is reached. Consequently, the content of Bi in Ge under low-temperature annealing should reach a value of about 28 wt%.

3.2. In Situ TEM Heating Studies of Ge/Bi/Ge Films

The morphology, structure, and chemical element distribution of Ge/Bi/Ge films with an overall Bi content of about 28 wt% were studied by in situ TEM techniques in the temperature range of 20–205 °C to reveal the mechanisms of interphase interactions in the system.

Figure 2 shows HAADF-STEM images of the morphology of a-Ge(12.5 nm)/Bi(5 nm)/a-Ge(12.5 nm) layered film system at different temperatures. One can see (Figure 2a) that the morphological structure of the as-deposited film consisted of bright areas of irregular shape corresponding to bismuth. This is because the 5 nm thick Bi film was not continuous and had a labyrinth-like morphological structure. The Ge film, grown at room temperature, was amorphous and continuous.

The diffraction patterns of the Bi-Ge layered film system at each studied temperature are shown in the insets in Figure 2, while their profiles are shown in Figure 3. It can be seen that the SAED pattern from the as-deposited film at 20 °C consisted of diffraction maxima of crystalline Bi and a halo of amorphous Ge.

The morphology of the film and its diffraction pattern did not change noticeably during heating to a temperature of about 150 °C (Figure 2b,c), i.e., no interaction happened between the layers. With a further temperature increase, the bright areas corresponding to Bi gradually decreased and almost disappeared at a temperature of about 200 °C.

The weak diffraction maxima of crystalline Ge appeared in the diffraction pattern at a temperature of 150 °C (inset in Figure 2c and Figure 3), pointing out that the morphology evolution in Figure 2d–f was accompanied by crystallization of the a-Ge film. The intensity of the Ge diffraction maxima gradually increased with the temperature increase, while the intensity of the diffraction maxima of Bi decreased (inset in Figure 2d and Figure 3) and completely disappeared at a temperature of about 200 °C (inset in Figure 2e and Figure 3).

It should be noted that 150 °C is the onset temperature of the interaction between Bi and Ge. Thus, the same film sample, which was annealed at a temperature of 150 °C for 120 min (Figure 4), had a similar morphology to the sample heated to a temperature of 180 °C (Figure 2d).

The disappearance of the diffraction peaks of bismuth in the diffraction pattern at about 200 °C (Figure 2f and Figure 3) might be induced by (i) eutectic melting due to the size effect [50,51] or (ii) complete dissolution of bismuth in germanium resulting in a single set of diffraction peaks of the solution, as has been observed, for example, in Au-Ni alloys [54]. Please note that the Bi film was placed between two continuous Ge films with thicknesses significantly larger than that of the Bi, which prevented the sublimation of the Bi during thermal treatment.

To verify the formation of the liquid phase, the film system under study was heated to a temperature of 220 °C, which is higher than the eutectic temperature of Ge/Bi/Ge layered films with thicknesses of layers of 5 nm [50,51], followed by a gradual cooling to room temperature. In this case, the melting and subsequent crystallization of Bi should result in the appearance of diffraction maxima of Bi, as was observed in [50,51]. However, diffraction maxima from Bi did not appear in our films, as revealed by the SAED pattern in Figure 3. Moreover, the diffraction peaks from Bi did not appear during the subsequent heating–cooling cycle of this film, which means that the bismuth did not melt in the system under study.

Hence, we concluded that the dissolution of bismuth in germanium film is responsible for the disappearance of Bi in Figure 2d–f. Unfortunately, Bi and Ge have different crystalline structures, which makes it impossible to determine the composition of their solid solutions from lattice parameter measurements, as was done for the Au-Ni and Ag-Cu systems in Refs. [54,55,56].

Figure 5 shows plain-view HAADF STEM images and EDX chemical element maps of the film system at different temperatures. The bismuth islands were visible at a temperature of 150 °C (Figure 5a–c), while they completely disappeared at 205 °C (Figure 5d–f), resulting in an almost uniform distribution of chemical elements in the film. The quasi-homogeneous distribution of the components remained unchanged after a slow cooling of the film to room temperature, i.e., no separation of the phases occurred. It should be noted that the overall composition of the film was preserved and corresponded to the initial one (Figure 5g–i). Thus, the bismuth was almost completely dissolved in the crystalline germanium.

Figure 6a shows a high-resolution image of the reaction front (yellow dashed curve) of the dissolution of Bi into the Ge film. Fast Fourier transformation analysis of the image revealed that crystalline Bi was observable before the reaction front (Figure 6c), while the crystalline Ge-based phase was identified behind the reaction front (Figure 6b). An enhanced solubility of tin in germanium (22 wt% Sn) was reported in Ref. [45]. The huge solubility of Sn in a-Ge was explained by the effect of electron beam irradiation during a TEM study. In our case, the a-Ge/Bi/a-Ge layered system was annealed at a temperature of 150 °C for 180 min, and the reaction between Bi and a-Ge occurred without the influence of an electron beam. Thus, the enhanced solubility of Bi in a-Ge was not induced by electron beam irradiation, at least in our case.

According to results obtained in Refs. [42,43], it follows that the metal-induced crystallization of a semiconductor can be among the mechanisms governing the solubility of metal in an amorphous semiconductor.

Figure 7a shows a plain-view high-resolution image of the film under study (a-Ge (12.5 nm)/Bi(5 nm)/a-Ge(12.5 nm)) and the corresponding SAED pattern at a temperature of 210 °C. One can see that the SAED pattern contains only diffraction lines of Ge, while the HAADF-STEM image contrast reveals that the film contains both Bi and Ge. Thus, the formed BiGe solid solution has a random alloy structure with a composition of about 28 wt% Bi, as revealed by EDX analysis (Figure 7b).

3.3. Cross-Sectional Characterization of Ge/Bi/Ge Films

For further insight, the dissolution of Bi in Ge films was studied through the thickness of the films annealed at different temperatures. A Ge/Bi/Ge layered system with layer thicknesses of 125 nm for Ge and 50 nm for Bi was used. The overall composition of the film was ca. 28 wt% Ge, which coincided with the composition of the Ge/Bi/Ge films used in the in situ TEM studies (Figure 2). FIB lamellas were cut from films that had been annealed at temperatures of 20 °C, 150 °C, and 271 °C. Figure 8 shows HAADF STEM images of the lamellas (a–c), the corresponding EDX chemical element maps (d–f), and the composition profiles through the film thickness (g–i).

Distinct layers of Bi and Ge are clearly visible in the as-deposited film in Figure 8a,d. The as-deposited thickness of the layered film was 300 nm (Figure 8a). This structure was preserved up to a temperature of 150 °C, which correlates well with the data for the nanoscale film (Figure 2). At this onset temperature, the dissolution of Bi in the a-Ge films was activated. Moreover, the dissolution of bismuth into germanium was accompanied by its crystallization (inset in Figure 8b) and resulted in an increase in the total film thickness to 340 nm, which corresponded to about 13% film swelling (Figure 8b). At the same time, the terminal concentration of Bi in Ge reached about 20 wt% or 8 at% at a temperature of 150 °C, as follows from Figure 8h. The polycrystalline BiGe phase demonstrated excellent temperature stability; the decomposition of the metastable solid solution and segregation of Bi on the surface of the Ge films was observed only at a temperature of about 271 °C. It should be noted that the total thickness of the Bi-Ge layered system shrunk down to 285 nm, which is smaller than the as-deposited film thickness. This is likely due to the sublimation of Bi from the surface of the film system at 271 °C.

The electron structure of BiGe solutions was studied using the valence EELS spectral imaging technique. Valence EELS combined with STEM provides a versatile tool for mapping plasmon energies in materials. The plasmon energy of an alloy varies with its concentration [57], enabling the reliable identification of solid solutions in Bi-Ge films at low electron beam doses. Figure 9 shows the HAADF image and corresponding volume plasmon peak energy map of Ge/Bi/Ge film annealed at a temperature of 150 °C. EELS spectra from the regions marked by squares in Figure 9a are shown in Figure 9c,d. The dissolution of Bi to half the thickness of the Ge layers is revealed by the plasmon peak energy shift in Figure 9b. This observation is in line with the EDX analysis in Figure 8b,e. Importantly, the EELS spectrum of BiGe solution consists of a single symmetrical plasmon peak with an energy of 15.9 eV, which is between those of amorphous Ge (16.2 eV) and Bi (14.4 eV). Hence, our finding proves the formation of a solid solution throughout the thickness of the Ge/Bi/Ge film.

The main finding of this work reveals the formation of polycrystalline Ge films with extended Bi solubility upon low-temperature heating of Ge/Bi/Ge sandwich films. The formation of amorphous BiGe alloys and the electrical and optical properties of the metastable BiGe phase were reported in Refs. [47,48]. Thin bismuth–germanium films were fabricated by vapor co-deposition onto cryogenically cooled glass substrates. It was found that amorphous bismuth–germanium thin films exhibit an n-type majority carrier. The amorphous BiGe alloys are known as low-carrier-mobility materials, with hopping conduction being the most likely transport mechanism [47]. In contrast, the measurements in Ref. [58] indicated p-type conduction for the SnGe metastable phase, which was attributed to acceptor levels of vacancy-related defects. Moreover, a high carrier mobility was found for GeSn polycrystalline thin films.

It was also established in Refs. [47,48] that bismuth is segregated into fine-grained crystallites when its concentration in binary film exceeds 25 wt%. As shown in [48], doping germanium with bismuth to a concentration significantly exceeding the equilibrium one makes it photoactive. We note that the indicated results refer to a mechanical mix of the two components to form an amorphous phase.

In this work, we observed the dissolution of up to 28 wt% of bismuth in crystalline germanium. It was found that the formation of the BiGe metastable phase depends not on the total thickness of the films but on the ratio of components’ thicknesses. Importantly, the concentration of Bi in Ge/Bi/Ge films should be less than ca. 28 wt% to preserve the metastable solid solution. When the terminal concentration is exceeded, eutectic melting is observed in the Bi-Ge films, resulting in the decomposition of metastable phases. These observations are in line with the data of Refs. [47,48].

Our studies point to the key role of the metal-induced crystallization effect in the formation of metastable BiGe solid solutions. However, despite more than 50 years of research [59,60], a fundamental mechanism of the MIC reaction is still under debate in the literature. A prevalent view among researchers posits that MIC is a solid-state reaction, characterized by the dissolution of a semiconductor into a metal matrix, followed by its precipitation in the crystalline phase [61,62,63]. An alternative hypothesis suggests the formation of an intermediate liquid phase within the reaction zone [41,64]. Here, we used a liquid-phase mechanism of the metal-induced crystallization effect to provide a qualitative explanation of the formation of metastable BiGe solid solutions.

A thin film of Bi was located between two films of amorphous Ge in the as-deposited film system. The amorphous phase has excess energy as compared to the crystalline one. The excess energy of a-Ge fueled the contact melting at the Bi-Ge interface at temperatures as low as 150 °C. As a result, nuclei of the liquid phase of the eutectic composition were formed at the interphase boundary. The liquid was substantially undercooled at this temperature, and the Ge dissolved in the liquid phase according to the extension of the liquidus line to the metastable region of the Bi-Ge phase diagram. The progressive dissolution of Ge in the liquid interfered with its temperature stability, leading to a rapid crystallization of the BiGe liquid phase. Equilibrium crystallization of the liquid alloy results in complete phase separation; however, the relatively low temperature of the non-equilibrium crystallization process constrained phase separation and resulted in the formation of metastable BiGe solid solution with a terminal Bi content of about 28 wt%. When the remaining amount of Bi in the reaction zone was sufficient to create a new nucleus of the liquid phase, the reaction front propagated in the volume of the a-Ge layer, leading to its crystallization.

At the same time, the formation of BiGe alloy cannot explain the swelling of the Ge/Bi/Ge film that was observed in Figure 8b. Our assessments showed that the dissolution of 12 at% of Bi in the Ge lattice resulted in a 1% enlargement in its lattice parameter. Therefore, we speculate that the swelling of the films is caused by (i) an increase in the volume of Bi upon solidification and (ii) the formation of many fine, disordered crystallites during the MIC process. For further insight, additional studies of the volumetric changes in layered metal–semiconductor films during MIC are required.

The decomposition of the metastable BiGe phase was activated at pre-melting temperatures, i.e., at about 271 °C, followed by segregation of the Bi atoms to the film surface and their sublimation or evaporation. This process leads to compaction of the Ge polycrystalline film, as was observed in Figure 8f. Importantly, no changes in the morphology or composition of the BiGe alloy were registered during several thermal cycles in a temperature range of 20–250 °C, pointing to the long-term stability of the BiGe alloy.

We expect substantial changes in the electrical and optical properties of the metastable BiGe nanosized alloy, such as (1) a change in the charge conduction mechanism from the classic NNH (nearest-neighbor hopping) to MVRH (Mott variable-range hopping) and (2) a significant shift of the surface plasmon resonance of the BiGe metastable phase toward the region of the visible spectrum.

Moreover, understanding the swelling of amorphous Ge film during metal-mediated crystallization is important for the development and design of ion membranes for new-generation batteries. Understanding the kinetics and mechanism of the formation of metastable phases at the metal–semiconductor interface is essential for the design of epitaxial Ge films on single-crystal Si wafers via surfactant-mediated technology [65,66].

4. Conclusions

The formation of supersaturated solid solutions in Ge/Bi/Ge films during thermal treatment was studied using highly complementary in situ acoustic and electron microscopy techniques. It was found that the melting of Bi between amorphous Ge films occurs only when the overall content of Bi in the system exceeds approximately 28 wt%. In situ TEM annealing of Bi film between amorphous Ge films revealed crystallization of the a-Ge film with the simultaneous formation of supersaturated solid solutions in the temperature range of 150–200 °C. It was shown for the first time that the non-equilibrium solubility of Bi in crystalline Ge film reaches a value of about 28 wt%. Our findings have potential for the design of metastable metal–semiconductor alloys with tunable properties via a simple heat treatment procedure.