Low-Temperature and High-Speed Fabrication of Nanocrystalline Ge Films on Cu Substrates Using Sub-Torr-Pressure Plasma Sputtering

We fabricated nanocrystalline Ge films using radio-frequency (RF) magnetron plasma sputtering deposition under a high Ar-gas pressure. The Ge nanograins changed from amorphous to crystalline when the distance between the Ge sputtering target and the substrate was decreased to 5 mm and the RF input power was 11.8 W/cm2 (60 W), where the deposition rate was as high as 660 nm/min. In addition, the size of the nanocrystalline grains increased from 100 to 307 nm when the RF input power for plasma production was increased from 11.8 W/cm2 (60 W) to 17.7 W/cm2 (90 W). In the developed narrow-gap plasma process at sub-Torr pressures, nanocrystalline Ge films were successfully fabricated on Cu substrates at low temperatures, without the substrate being heated. However, when annealing was conducted under an N2 atmosphere, which is the conventional method to induce solid-phase crystallization, the amorphous Ge layer on a Cu substrate changed to a Cu3Ge crystal layer through interdiffusion of Ge and Cu atoms at 400–500 °C.


I. INTRODUCTION
Crystal Ge films fabricated on various substrate materials at low temperatures have versatile applications in, for example, high-capacity Li + -ion batteries [1] and high-efficiency tandem solar cells [2]. Because of the limited capacity of carbon anodes in Li + -ion batteries, alternative anode materials that are reactive with Li are being actively developed [3], [4]. Among these materials, Ge is one of the most interesting because it has a high theoretical capacity of 1600 mAh/g, which is much higher than the value of 372 mAh/g for a conventional carbon active material [1]. In general, the active anode layers are fabricated on Cu-foil current collectors because Cu does not react electrochemically with Li at the low operating potential of an anode. The fabrication of Ge-based Li + -ion batteries requires a method that enables a Ge layer to be deposited onto a Cu substrate via a rapid and simple procedure and that also enables precise control of the Ge nanostructure to provide good stability against volume changes of the Ge anode during Li alloying/dealloying. The metal-induced crystallization (MIC) method has been studied to realize crystalline Ge films at low temperatures [5], [6], [7], [8]. In the MIC process, a metal layer such as Al, Ag, or Au is deposited as a catalyst underneath the Ge layer and the resultant Ge/Al, Ge/Ag, or Ge/Au bilayer film is subsequently annealed at a low temperature (<400°C) for Ge crystal growth. Also, the conventional plasma sputtering method at a low gas pressure has been reported to form a crystal Ge film, where the substrate is heated to a temperature less than 280°C for crystallization [9], [10]. These methods require strict temperature control of the sample by annealing after film deposition and by substrate heating during deposition, which leads to a long fabrication process. In the present study, we develop an alternative method to fabricate nanocrystalline Ge films on Cu substrates in a singlestep plasma sputtering process conducted at pressures in the sub-Torr range, which is two orders magnitude higher than the gas pressure used in conventional sputtering. The advantage of our process is that it enables direct fabrication of nanocrystalline Ge films at high speed without heating of the substrate. The gas-phase plasma process enables control of the structure (amorphous or crystal) in the film in a simple single-step procedure [11], [12], [13].

II. METHODS
The Ge films were fabricated on an n-type Si wafer and Cu disk using 13.56 MHz radio-frequency (RF) magnetron sputtering, as schematically shown in Fig. 1 [14]. The diameter and thickness of the Cu disk were 15 mm and 80 μm, respectively. The substrate was cleaned with ethanol using an ultrasonic cleaning bath and was then placed at the center of the substrate holder. The sputtering target was a polycrystalline Ge disk (1 inch diameter) with a purity of 99.99%. An RF power of 60 W (11.8 W/cm 2 ) was supplied to the sputtering target for plasma generation. Ar gas was supplied from the direction of the target to the substrate holder at a flow rate of 80 sccm. The Ar-gas pressure was set to a high value of 0.5 Torr. The distance between the target and the substrate holder (z) was varied from 5 to 30 mm. The substrate holder was not heated or cooled during film deposition. Some Ge films were annealed at 300-500°C under N 2 for 3 h after they were deposited, where the annealing temperature was increased at a rate of 3°C/min. The crystal structure of the Ge films was evaluated by Raman spectroscopy (T64000, HORIBA Jobin Yvon) and X-ray diffraction (XRD) θ −2θ scan analysis (SmartLab, Rigaku). The surface morphology and microstructure were analyzed using atomic force microscopy (AFM; Dimension FastScan, Bruker) and scanning electron microscopy (SEM; SU-8010, Hitachi).   0.5 Torr. As shown in Fig. 2(d), the size of the nanograins increased with decreasing z, and the average grain sizes for z = 5, 10, and 20 mm were 100, 57, and 48 nm, respectively. Here, the average size of the nanograins was estimated from the surface SEM by randomly selecting over 20 grains in the SEM image. This result is reasonable because the deposition rate of the Ge film markedly increased with decreasing z; the deposition rate for z = 20 mm was 90 nm/min, whereas that for z = 5 mm was as high as 660 nm/min. The shorter target-substrate distance leads to the larger amount of highly reactive Ge atoms impinging to the substrate surface, which results in the rapid growth of grains on the substrate and the high deposition rate. On the other hand, at high pressures such as 0.5 Torr, Ge nanoparticles could be synthesized in a gasphase plasma because a higher-density plasma is produced locally in front of the cathode sputtering target [1], [14]. The shorter collision mean free path for Ge species would cause nucleation of nanograins in the gas phase [12], [15]. Fig. 3 shows Raman spectra of the Ge nanostructured films deposited at different z. Interestingly, we observed that the Ge structure markedly changes; the spectra of the films fabricated at z = 10, 20, and 30 mm show a broad peak at ∼270 cm −1 , which is assigned to Ge with an amorphous structure, whereas the spectrum of the z = 5 mm film shows a sharp peak at ∼300 cm −1 , which indicates the formation of crystalline Ge.

III. RESULTS AND DISCUSSION
To confirm the structure inside the Ge nanograins in detail, XRD analyses were conducted (Fig. 4). The XRD patterns clearly show that the nanostructured film fabricated at z = 5 mm contains crystals with (111), (220), (311), (400), and (331) orientations. In the narrow-gap plasma sputtering in the sub-Torr range, nanocrystalline Ge films were fabricated at a high deposition rate without heating the substrate holder.
To analyze the mechanism of crystallization in the plasma sputtering process under the condition of a short targetsubstrate distance, the substrate surface temperature was  roughly measured with a thermocouple probe, where the plasma and the neutral gas directly flowed to the small tip surface of the sensor placed on the substrate. As shown in Fig. 5, the thermocouple temperatures rapidly increased with increasing the deposition time until the deposition time reached 5 min (i.e., 70°C at z = 20 mm and 170°C at z = 5 mm) and then showed an almost constant value after 30 min. A shorter z position led to a higher temperature of the sensor tip, which is attributed to 1) increased heat transfer from the sputtering target at an RF input power of 11.8 W/cm 2 through the flowing Ar gas and 2) an increase in the physical and chemical reactions by impinging ions from plasma on the surface [15]. The increased temperature explains the nanocrystal Ge formation in the z = 5 mm sample from a thermal viewpoint, although the melting point of bulk Ge is as high as 938°C. In general, the melting point of nanoparticles is expected to decrease with decreasing particle size [16]. The lower melting point related to nanosized particles and a thin layer might play important roles in the crystallization of nanostructured Ge films at low temperatures. Fig. 6(a) and (b)-(d) show Raman spectra and surface SEM images, respectively, of films deposited at different RF input  powers for plasma production, where all the Si substrates were positioned at z = 5 mm. The spectra of all the films show a sharp Raman peak at ∼300 cm −1 , which is assigned to crystalline Ge. As shown in Fig. 6(b)-(d) (surface SEM images) and Fig. 6(e) (grain size measurement), the average size of nanograins increased from 100 to 263 and 307 nm when the RF input power was increased from 60 W (11.8 W/cm 2 ) to 75 W (14.8 W/cm 2 ) and 90 W (17.7 W/cm 2 ), respectively. The higher RF input power leads to a higher plasma density, to a higher deposition rate, and to a higher temperature of the grain surface, which would enhance nanocrystal growth both in the gas-phase plasma and on the substrate.
A nanocrystalline Ge film was subsequently deposited onto a thin Cu substrate to a thickness of 80 μm using the developed plasma sputtering method under a sub-Torr pressure of 0.5 Torr and small z of 5 mm. Fig. 7(a) and (b) show images of the Ge film surface and the Raman spectra, respectively, of Ge films deposited at z = 5 and 20 mm. Similar to the Raman spectra of Ge films on Si substrates, the Raman spectra of Ge films on Cu substrates show a transition from an amorphous to a crystalline structure as z is decreased from 20 to 5 mm, accompanied by a change in the color of the film surface from metallic to dark gray. We successfully fabricated nanocrystalline Ge films on a thin Cu substrate at high speed using sub-Torr and narrow-gap plasma sputtering.
To demonstrate the advantages of the plasma process for low-temperature nanocrystal formation, we fabricated crystalline Ge films using a thermal annealing process, which is the conventional method for inducing crystallization of a solid phase. Here, the amorphous Ge films were deposited by sputtering at z = 20 mm and subsequent thermal annealing at 300-500°C under a N 2 atmosphere. Fig. 8(a) and (b) show the Raman spectra obtained for Ge films on Si and Cu substrates, respectively. For Ge films on Si substrates, the amorphous Ge layer crystallized at 500°C, which is similar to the crystallization temperature reported in a previous study [10]. However, for the Ge film on a Cu substrate, Raman signals at 270 and 300 cm −1 , which are related to amorphous and crystalline structures, respectively, were not detected after the sample was annealed at 500°C. Interestingly, the color of  the Ge film markedly changed after the annealing process at 500°C (Fig. 8(b)). To identify the film structure, we analyzed the thermally annealed films by XRD. As shown in Fig. 9, peaks corresponding to Cu-Ge alloy were clearly observed in the patterns of films annealed at 400-500°C [17], [18]. Previous studies have shown that annealing treatments at 400°C are sufficient to induce complete crystallization of Cu 3 Ge films [19], [20], [21], [22], where the Cu 3 Ge stoichiometric compound is generated as a result of annealing-induced interdiffusion of Cu and Ge atoms. Via the same mechanism, the amorphous Ge layer on a thin Cu substrate could be converted to a Cu 3 Ge crystal layer by annealing at 400-500°C under N 2 . In the annealing process, a barrier layer between the Cu and Ge layers might be required to obtain a crystalline Ge film [10]. Our experiments show that the low-temperature plasma process is advantageous for the formation of a nanocrystalline Ge film via a simple single-step procedure. Electrochemical performance of Li ion batteries with the nanocrystalline Ge film on a Cu substrate will be tested in future.
In conclusion, we fabricated nanocrystalline Ge films with particle sizes of 100-307 nm by Ar plasma sputtering with a high Ar gas pressure of 0.5 Torr and a short target-substrate distance z of 5 mm, where the Ge films transformed from amorphous to crystalline with decreasing z. A nanocrystalline Ge film was successfully fabricated on a thin Cu substrate at high speed without heating the substrate; thus, the mixing of Ge and Cu atoms to form Ge-Cu alloy was prevented by the use of a low-temperature process. The formation of a nanocrystalline Ge film on various substrate materials was realized in a single-step procedure using low-temperature plasma.