Electrochemical Deposition of Germanium Sulfide from Room

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Letter pubs.acs.org/Langmuir

Electrochemical Deposition of Germanium Sulfide from RoomTemperature Ionic Liquids and Subsequent Ag Doping in an Aqueous Solution Sankaran Murugesan, Patrick Kearns, and Keith J. Stevenson* Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: A facile room-temperature electrochemical deposition process for germanium sulfide (GeSx) has been developed with the use of an ionic liquid as an electrolyte. The electrodeposition mechanism follows the induced codeposition of Ge and S precursors in ionic liquids generating GeSx films. The electrodeposited GeSx films were characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and Raman and X-ray photoelectron spectroscopy (XPS). An aqueous-based Ag doping method was used to dope electrochemically grown GeSx films with controlled doping compared to the conventional process, which can be used in next-generation solid-state memory devices.

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INTRODUCTION This letter reports a simple electrochemical-based method for the synthesis of germanium sulfides (GeSx) using roomtemperature ionic liquids (RTIL) and an aqueous-based electrochemical method of doping Ag+ into electrodeposited GeSx films, which offers promise for reducing and/or eliminating several energy-intensive processing steps currently required for the fabrication of solid-state battery and conductive bridging random access memory (CBRAM) devices. Chalcogenide glasses, such as GeSx, have been incorporated into optical materials (lasers, fiber optics, and optical lenses for infrared transmission), rewritable discs, and nonvolatile memory devices.1−4 Recently, they have been extensively used as solid electrolytes in CBRAM devices. CBRAM consists of integrated devices made by sandwiching the chalcogenide glasses as solid electrolytes between an inert cathode and a redox-active anode. Several chalcogenide glasses have shown promise in this application, including GeSx doped with metal ions such as Ag+, Li+, and Cu2+.5−7 GeSx doped with Ag+ as the electrolyte and the Ag anode appears to be the most promising and widely used combination because it is more robust at the high temperatures in the back end of the line processing used in integrated circuit manufacturing.5 GeSx glasses can be prepared by different methods, including sol−gel synthesis, chemical vapor deposition, and laser-assisted chemical vapor deposition.1,3,8,9 However, these methods are limited because they require high temperatures, corrosive gases, and/or long processing time frames. For example, GeSx formation by sol− gel synthesis involves the use of H2S gas with specialized equipment (stainless steel high-pressure reactors) to keep the © 2012 American Chemical Society

material away from air, and the reaction must proceed for several days to a month.3,8,9 Chemical vapor deposition involves the use of specialized equipment at high temperatures (400−700 °C), as well as H2S gas with a slow deposition rate of about 12 μm/h,1 thereby limiting the speed, cost, and scale at which thin films can be produced. In general, the synthesized products are not pure or uniform and are dependent upon the precursor materials used in the synthesis. Furthermore, preparing thin films of GeSx with the use of other techniques such as evaporation, sputtering, and ablation suffers from difficulties associated with the incorporation of impurities or nonstoichiometry in sulfur and germanium, which degrade the properties of the GeSx glass.10,11 Additionally, metal ion doping of GeSx glasses can be performed by conventional methods such as chemical vapor deposition and photochemical doping methods, but it is difficult to estimate the concentration of metal doping for Ag+.5,12,13 Therefore, it is desirable to have a low-cost, facile method of synthesizing GeSx in a specific stoichiometry and in thin film architecture amenable to simple doping procedures. RTILs are a family of nonconventional molten salts that can act as templates and precursors to inorganic materials and as solvents. RTILs have been extensively used in solvent extraction and organic catalysis, but there are few reports on their use in inorganic synthesis with hydrothermal/solvothermal methods.14 Additionally, they offer a unique solvent environment for the reduction of reactive Received: February 6, 2012 Revised: March 9, 2012 Published: March 13, 2012 5513

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metals such as Si, Ge, and Sn because of their wide electrochemical window, negligible vapor pressure, and high ionic conductivity.15,16 Recently, Al-Salman et al. have shown the electrodeposition of Si, Ge, and the SiGe alloy with the 1butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([Py1,4]Tf2N) ionic liquid in polycarbonate membrane templates.15 However, the electrodeposition of germanium chalcogenides with RTIL has not yet been fully exploited. Here, we used PP13-TFSI as the electrolyte for the electrochemical deposition of Ge and GeSx. The PP13-TFSI ionic liquid used here consists of a N-methyl-N-propylpiperidinium (PP13) cation and a bis(trifluoromethanesulfonyl) imide (TFSI) anion, yielding a wide electrochemical potential window that has been used as a solvent for lithium ion batteries.17,18

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EXPERIMENTAL METHODS

The PP13-TFSI RTIL was synthesized via a published procedure.17 Briefly, PP13-Br was prepared by mixing propylbromide (SigmaAldrich, 99% assay) with N-methylpiperidine (Sigma-Aldrich, 99% assay) in a 1:1 molar ratio in acetonitrile and was stirred at 70 °C for 24 h. A white precipitate was formed in the solvent. The precipitate was washed in acetonitrile to remove unreacted reagents and dried under vacuum. The RTIL was prepared by reacting PP13-Br and LiTFSI (Sigma-Aldrich, 99.95% trace metal basis) in a 1:1 molar ratio in aqueous solution; when the solution is stirred for 12 h, an organic phase separation occurs. The organic phase was extracted with CH2Cl2. The extract was washed three times with DI water (18 MΩ cm), and the final organic extract was dried in vacuum at 105 °C for 24 h. The vacuum-dried, thick, viscous, colorless liquid was stored in a glovebox over lithium for further electrochemical reactions. GeSx films were deposited from a 0.3 M GeCl4 (Alfa Aesar, 99.9999% (metals basis)) and 0.3 M 1,4-butanedithiol (Sigma-Aldrich, 97% assay) mixture in RTIL using a three-electrode cell assembly. Glassy carbon (GC) (Alfa Aesar, type 2) or indium-coated tin oxide glass sheets (ITO, Delta Technologies, Ltd., 15 Ω/□) were used as working electrodes. Pt wire or graphite was used as a counter electrode, and a Ag wire was used as a quasi-reference electrode (QRE). The synthesized GeSx films were washed with acetone and stored in a desiccator. Ag doping of the synthesized GeSx films was performed using a solution containing 1 mM AgNO3 in 0.2 M H2SO4 in a three-electrode cell. Potentiodynamic experiments were performed between 0.6 and −1.2 V versus a Pt wire QRE, a platinum counter electrode, and a GeSx film on either ITO or GC as the working electrode. To study different levels of Ag doping, a constant potential deposition was performed at −0.4 V versus a Pt wire QRE.

Figure 1. (A) Potentiodynamic deposition of Ge on a GC working electrode immersed in 0.3 M GeCl4 and PP13-TFSI. Scan rate = 10 mV/s. (B) First (a), second (b), and third (c) cycles of potentiodynamic deposition of GeSx on a GC electrode from equimolar concentrations (0.3 M) of GeCl4 and 1,4-butanedithiol in PP13-TFSI. (C) Chronoamperometric GeSx film deposition at a GC electrode at different potentials. (D) Cottrell plot (i vs t−1/2) demonstrating the nonlinear behavior of the GeSx deposition process with the involvement of multiple deposition steps.

germanium precursors, respectively. Figure 1B shows the potentiodynamic deposition of GeSx onto GC. A reduction peak at approximately −1.8 V versus Ag (QRE) was observed and is attributed to the complexation of sulfur and germanium sources with the ionic liquid (Figure S2). Once the complex forms, the reaction proceeds through an induced codeposition process generating GeSx films. The reduction in current and a broad peak formed in the subsequent cycles (Figure 1B) represent the deposition of GeSx onto the GC surface. During the deposition, visually the solution shows the formation of a white colloidal suspension. After the deposition, the white colloidal suspension was washed with acetone, resulting in a white precipitate. Monothiols were also assessed as the sulfur precursor, but we did not observe the formation of GeSx films in their presence, which suggests that dithiols are essential for complexation (Figure S2) and codeposition processes. To confirm the mechanism of GeSx deposition, current transient measurements (chronoamperometry) were performed at constant potential for 600 s (Figure 1C). At several potentials (−1.0, −1.5, −2.0, −2.5, and −2.7 V versus Ag (QRE), the deposition after 100 s converges to a linear deposition rate. However, the initial deposition period for 50 s shows nonlinear behavior, especially at −2.7 V where three different steps are observed (inset of Figure 1C). The step formation from the nonlinear behavior of the deposition process was further demonstrated by the Cottrell plot (Figure 1D), where each change in slope represents a different process. We propose that the deposition process occurs via the initial complexation of Ge and S precursors with PP13-TFSI (I), the deposition of a 2D thin film (II) and an increase in thickness with time (III). Finally, film growth follows on top of the predeposited surface (IV), following the Stranski−Krastanov mechanism19,20 where

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RESULTS AND DISCUSSION To understand the most basic reduction process, we first performed Ge deposition on GC electrodes with PP13-TFSI and 0.3 M GeCl4 as a Ge precursor. Figure 1A shows the potentiodynamic deposition of Ge films on a GC electrode. Two irreversible diffusion-controlled reduction peaks are observed at −1.3 and −2.25 V, which correspond to the reduction of Ge(VI) to Ge(II) and the reduction of Ge(II) to Ge(0), respectively, which is supported by a previous report by Endres et al.18 To test the versatility of this method, Ge films were deposited on different conducting substrates such as glassy carbon, stainless steel, ITO, and a Cu-backed Si wafer (Supporting Information). All of the substrates showed the two reduction peaks of Ge, but the reduction potential of Ge varied with the substrate composition and conductivity (Figure S1). GeSx deposition was performed similarly with potentiodynamic deposition from 0 to −3 V versus Ag (QRE) on GC in RTIL with 0.3 M each of 1,4-butanedithiol and GeCl4 as sulfur and 5514

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Ag was carried out on the electrodeposited GeSx films/GC at −0.4 V versus Pt (QRE) for 600 s. Raman spectroscopy was used to characterize the Ag-doped and undoped GeSx. Bulk crystalline Ge shows a strong Raman cross section at 300 cm−1 for the Ge−Ge optical phonon vibrational mode, but in the electrodeposited Ge films, this band shifts to 280 cm−1 (Figure 3A) and is attributed to the amorphous nature of the film.22,23

the growth process is initiated with a uniform layer and then starts to form islands of GeSx. Similar growth processes have been observed for the deposition of Si, Ge, and SiGe nanostructures.21 The electrodeposited GeSx films were characterized by SEMEDS. Figure 2 shows the GeSx film deposited on GC at −2.7 V

Figure 3. Raman characterization of electrodeposited films. (A) Amorphous Ge showing νs(Ge−Ge) at 280 cm−1. (B) GeSx film showing the νs(Ge−S−Ge) A1 edge shared at 365 cm−1 as the predominant peak and the νs(Ge−S−) satellite peak at 405 cm−1 confirms the deposition process. (C) Ag-doped GeSx films show the formation of a Ge-rich (νs(Ge−Ge) at 278 cm−1) compound by the doping process.

GeSx films (Figure 3B) do not have the 280 cm−1 band, which confirms the absence of a-Ge. Various Raman modes were deconvoluted and identified at 342.7, 365, 380, 405, 430, and 465 cm−1 (Figure S6). The bands at 342.7 cm−1 correspond to the A1 mode of tetrahedral GeS4,3,24 and the bands at 365 and 380 cm−1 are F2 modes of GeS4 tetrahedra.3,24 The 405, 430, and 465 cm−1 bands are due to Ge−S corner sharing, edgesharing GeS4 tetrahedra, and S3Ge−S−GeS3 stretching, respectively.3,24 The presence of these peaks confirms the formation of GeSx in the electrodeposited samples. Raman of the white precipitate that formed in the GeCl 4 /1,4butanedithiol solution shows that it is also GeSx (Figure S7). Therefore, this synthesis is advantageous because it produces GeSx films and particles in a single-step room-temperature process.9 Raman of Ag-doped GeSx films showed a new band at 278 cm−1 along with the stretching frequencies for GeSx (Figure 3C). This is due to the formation of a Ge-rich (νs(Ge−Ge) at 278 cm−1) compound by the Ag doping process. This may be attributed to the bridges connecting the meta-thiogermanate tetrahedral.25 The deconvoluted spectrum (Figure S8) shows bands at 415 and 400 cm−1 attributed to the di- and metathiogermanate tetrahedral modes, respectively.25Ag-doped GeSx also showed a shift in the A1 mode of GeS4 tetrahedra from 365 to 360 cm−1. This shift may be due to the formation of thio-germanate units as well as Ag2S segregation that reduces the stress of the backbone or another possibility of Ag atoms attaching to GeS4 tetrahedra inducing more bond disorder because of the larger size of Ag.26 GeSx formation was further examined and quantified by XPS. Our electrodeposited GeSx films displayed the Ge 3d peak at 30 eV (Figure 4A), which suggest a nonstoichiometric composition of Ge(II) (29.5 eV) and Ge(IV) (30.4 eV) oxidation states as well as the amorphous character of Ge.8,10,27 The absence of

Figure 2. SEM-EDS analysis of a GeSx film deposited on GC by a chronoamperometric method at −2.7 V vs Ag (QRE) for 600 s. (A) Large-area coverage with a smooth, porous structure. (B) The smooth surface contains small particles. The particles present on the films are confirmed to be GeSx by EDS elemental mapping analysis. (C) Secondary electron (SE) image. Secondary electron (SE) image with (D) Ge and S, (E) Ge, and (F) S.

versus Ag (QRE) for 600 s. The wide-field image shows the smooth, porous structure (Figure 2A), whereas the enlarged image shows that the smooth surface contains small particles (Figure 2B), which are confirmed to be GeSx by EDS elemental mapping analysis (Figure 2C−F). This is further established by EDS point analysis (Figure S3) showing the presence of Ge and S with a trace amount of Cl impurities from the precursors. The presence of small spherical particles over the surface further supports the Stranski−Krastanov mechanism of electrodeposition taking place in the GeSx deposition.19,20 The deposition of GeSx over different electrode substrates resulted in an appreciable difference in the quality of the films (Figure S4). The overall quality of the films is better when deposited on glassy carbon compared to stainless steel and Si. Ag doping of electrodeposited GeSx films was performed by electrochemical Ag deposition in aqueous 1 mM AgNO3 in 0.2 M H2SO4. Spectroelectrochemical studies were performed with ITO and GeSx/ITO as the working electrodes (Supporting Information and Figure S5) to determine that Ag was deposited at −0.4 V versus Pt (QRE). A constant potential deposition of 5515

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splitting, respectively, thus confirming a nonmetallic chemically bonded state (Figure 4C).29,30 The atomic concentrations (Table 1) of Ag, Ge, and S confirm that Ag was successfully doped into the GeSx films with 31.45% Ag, which is similar to the maximum amount of Ag (1− 35%) that has been doped into GeSx films by other methods.5,12,13 The XRD of electrodeposited GeSx films shows a broad peak that can be attributed to both the amorphous and glassy character of GeSx (Figure 4D). The broad peak at 15° 2θ may be assigned to monoclinic GeS2.3 This peak-broadening behavior has been previously observed in both CVD films and sol−gel-synthesized amorphous GeS2.1,8 This is further confirmed by HR-TEM measurements of the films with small particles to see the lattice pattern of the small particles (Figure S9). However, they were amorphous in character and did not diffract. The XRD of Ag-doped GeSx films (Figure 4D) also showed peak-broadening behavior similar to that of GeSx, and the absence of a peak for metallic Ag confirms that the amorphous character is retained after the doping process and that doped Ag consists of small particles concealed in the Ge−S matrix.

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Figure 4. XPS analysis of electrodeposited GeSx and Ag doped GeSx films. (A) XPS analysis of Ge 3d in undoped films (red) at 30 eV and Ag-doped GeSx films (blue) showing the splitting behavior of Ge 3d in the presence of Ag. (B) XPS analysis of S in undoped films (red) and Ag-doped GeSx films (blue). (C) XPS analysis of Ag in Ag-doped GeSx films showing the 3d5/2 and 3d3/2 orbital splitting. (D) XRD of the GeSx film (red) and Ag-doped GeSx films (black). The blue lines show the standard JCPDS (no. 89-3722) pattern for Ag.

CONCLUSIONS A simple electrochemical method for the deposition of GeSx films using ionic liquids at room temperature has been demonstrated, and the mechanism has been determined. The deposited films were smooth, porous, and had an amorphous glassy character. An alternative electrochemical method for Ag doping has been developed and characterized, resulting in a doping level of 31% Ag. This is especially useful for nextgeneration solid-state memory devices such as conductive bridge random access memory (CBRAM). Future studies will explore these materials as solid-state electrolytes.

a peak at 32.5 eV confirms the absence of GeO2 in the films.27 Sulfur was observed at 160.9 eV with a shoulder at 161.9 eV (Figure 4B) and is attributed to the Ge−S−Ge and S−S−S structural fragments, respectively.28 Quantitative elemental analysis from high-resolution XPS peak integration is presented in Table 1. On the basis of the atomic concentration, the

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Table 1. Integrated Ge, S, and Ag XPS Data for Undoped and Ag-Doped GeSx element

position BE (eV)

Ge 3d S 2p

29.96 160.9

Ge 3d S 2p Ag 3d

26.6 162.8 368.4

fwhm (eV)

1.42 1.85 Ag-Doped GeSx 2.01 1.81 0.94

ASSOCIATED CONTENT

S Supporting Information *

Material characterization and spectroelectrochemical study of Ag doping on GeSx. This material is available free of charge via the Internet at http://pubs.acs.org.

atomic conc (%)

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39.39 60.61

AUTHOR INFORMATION

Corresponding Author

28.72 39.83 31.45

*E-mail: [email protected]. Tel: +1-512- 2329160. Fax: +1-512-471-8696. Notes

The authors declare no competing financial interest.

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electrodeposited GeSx films are composed of GeS1.54. This further supports the observation of Ge(II) and Ge(IV) oxidation state coexistence at 57−62 atom % of S.10 In the Ag-doped GeSx films (Figure 4A), the Ge 3d peak is split into peaks at 26.7 and 31.7 eV with reduced peak intensities due to the Ge enrichment in GeSx films caused by doping. This germanium-rich GeSx has the tendency to undergo oxidation. Therefore, the peak at 31.7 eV is likely due to surface Ge−O, where GeO2 is 32.5 eV.29,30 The intensity of the S 2p peak is also reduced and shifted from 160.9 to 162.8 eV (Figure 4B). This shift in the S peaks is due to the formation of a strong ionic interaction between Ge−S−Ag components. Similar observations were noted by Kovalskiy et al., who showed that with As sulfides and Ge selenides after Ag doping the As and Ge peaks split.30 The Ag peaks in doped GeSx occur at 368 and 374 eV, which correspond to the 3d5/2 and 3d3/2 orbital

ACKNOWLEDGMENTS Financial support of this work was provided by the R. A. Welch Foundation (grant F-1529). We thank Jaclyn D. WigginsCamacho for assistance with the XPS measurements and E. Kate Walker for helpful comments. The Kratos XPS was funded by the National Science Foundation under grant CHE0618242.

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REFERENCES

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