Article Cite This: ACS Appl. Nano Mater. 2019, 2, 4313−4322
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Passivation of Germanium by Graphene for Stable Graphene/ Germanium Heterostructure Devices Robert M. Jacobberger, Matthew J. Dodd, Marziyeh Zamiri, Austin J. Way, Michael S. Arnold, and Max G. Lagally* Department of Materials Science and Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States
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ABSTRACT: Graphene grown directly on Ge via chemical vapor deposition (CVD) can passivate the underlying Ge surface, preventing its oxidation in ambient air for at least months. However, the factors that govern oxidation of Ge coated with graphene have not been elucidated. We investigate the effect of graphene synthesis parameters and Ge surface orientation on passivation of Ge and correlate these data with the density and type of defects in graphene. Oxidation of Ge can be reduced by increasing the H2:CH4 flux ratio or decreasing the growth rate, which decrease the density of atomic-scale defects, such as point defects and grain boundaries, in graphene. Oxidation of graphene is concomitant with oxidation of Ge and occurs more readily when the density of atomic-scale defects is relatively high. Passivation of Ge, however, depends more strongly on Ge surface orientation, as Ge(110) oxidizes significantly less than Ge(001) or Ge(111), even at the same graphene defect density. These results provide a pathway for engineering high-quality graphene films on Ge, which may enable improved passivation of Ge and direct integration of graphene-based or hybrid graphene/Ge heterostructure devices on conventional semiconductor platforms. KEYWORDS: oxidation, chemical vapor deposition, pinholes, defects, X-ray photoelectron spectroscopy, Raman spectroscopy
1. INTRODUCTION The scalable integration of graphene onto semiconductor platforms is an important step toward utilizing the exceptional structural, electronic, thermal, and mechanical properties of graphene in conventional semiconductor technologies. In particular, graphene/Ge heterostructures have emerged as an exciting materials system for state-of-the-art electronics for several reasons: (1) graphene supported on Ge can simultaneously exhibit high charge carrier mobility and concentration;1 (2) hybrid photodetectors based on graphene/Ge Schottky barrier junctions can exhibit high responsivity;2,3 (3) the insertion of graphene between metals and Ge can alleviate Fermi-level pinning to improve contact resistance;4 (4) the insertion of graphene between dielectrics and Ge can suppress formation of interfacial defect states to reduce hysteresis and leakage current in Ge-based devices;5 (5) graphene can decouple interlayer bonding to grow nonlatticematched III−V semiconductors directly on Ge;6 and (6) chemical templates consisting of alternating stripes of graphene and Ge can direct the assembly of block copolymers into nanoscale patterns.7 These applications, however, are sensitive to the structural, electronic, and chemical nature of the graphene/Ge interface. For example, the charge carrier concentration in graphene supported on Ge surfaces is directly related to the density of interfacial states.1 Furthermore, oxidation of Ge can modify the Schottky barrier and Fermi-level pinning at the graphene/Ge © 2019 American Chemical Society
junction, limiting the performance of graphene/Ge photodetectors and increasing contact resistance.8,9 Oxidation can also vary the graphene/Ge interfacial energy, which can alter heteroepitaxial growth and directed assembly on graphene/Ge templates.10 Therefore, to realize the full potential of applications based on graphene/Ge heterostructures, it is critical to achieve a reproducible, stable, pristine graphene/Ge interface. This is not trivial, however, as the Ge surface forms a native oxide consisting of GeO2 and substoichiometric GeOx in air, resulting in a nonuniform, disordered surface with a high density of interfacial defect states.11−14 A variety of methods to reduce oxidation has been reported, such as termination of the Ge surface with hydride,15−17 chloride,16−18 sulfide,16,17,19 or organic16−18 groups. While organic groups can offer extended stability of Ge for days17,18 or even weeks,16 hydride,16,17 chloride,16−18 and sulfide16 groups only offer relatively shortterm passivation, resulting in oxidation of Ge within minutes, hours, and days, respectively. Moreover, these functional groups are not stable at the temperatures typically used to synthesize graphene on Ge surfaces. Alternately, we recently reported that graphene grown directly on Ge via chemical vapor deposition (CVD) can Received: April 24, 2019 Accepted: June 25, 2019 Published: June 25, 2019 4313
DOI: 10.1021/acsanm.9b00766 ACS Appl. Nano Mater. 2019, 2, 4313−4322
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ACS Applied Nano Materials
Figure 1. (a) Schematic diagram before and after CVD of graphene on a Ge(001) substrate by using a flow of CH4 and H2 at 910 °C. After CVD, the samples are exposed to air for 1 month and are then characterized by using XPS. (b) Plot of intensity of the XPS Ge 2p3/2 peak against binding energy for graphene grown on Ge(001) (left column), Ge(110) (center column), and Ge(111) (right column) with an H2:CH4 ratio of 8.7 (black, bottom), 13 (red), 17 (blue), and 20 (green, top) at 910 °C for 6 h after exposure to atmospheric air at room temperature for 1 month. (c) Plot of the area of the GeO2 peak normalized to the total area of the Ge and GeO2 peaks against H2:CH4 ratio for Ge(001) (blue triangles), Ge(110) (red circles), and Ge(111) (black squares) surfaces covered with graphene.
oxidation covering the entire surface.38,39 Provided that oxidants can reach the graphene/metal interface, corrosion of metals below graphene is actually enhanced by electrochemical reaction between the cathodic graphene and anodic metal.30,40,41 During graphene growth via CVD, defects such as point defects (e.g., Stone−Wales and vacancies), line defects (e.g., grain boundaries and edge dislocations), and pinholes can become trapped in the graphene lattice.42−44 For example, point defects and edge dislocations can incorporate into the lattice as hydrocarbon growth species attach to the edges of growing graphene crystals. Grain boundaries can form when isolated graphene domains that have lattices that are rotated or translated with respect to each other merge, creating a meandering line of defects. Pinholes can also form when isolated graphene domains do not completely coalesce at the atomic scale, resulting in regions of bare substrate that are not covered by graphene.26 Throughout this paper, relatively large voids that vary in size from multiple coalesced vacancies to larger holes are termed “pinholes”, and relatively continuous defects, such as point defects, edge dislocations, and grain boundaries, that generally consist of rings comprising only 5−9 C atoms42−44 are termed “atomic-scale defects”. To optimize passivation of the Ge surface, it is critical to understand how CVD growth parameters affect the formation of both atomic-scale and pinhole defects in graphene. Here, we investigate the effect of the graphene synthesis parameters on passivation of Ge covered by graphene. We find that oxidation of Ge can be minimized by increasing the H2:CH4 flux ratio, decreasing the growth rate, or conducting growth on Ge(110).
passivate the Ge surface, preventing its oxidation for months if optimum growth conditions are used.20 The graphene/Ge interface can be so pristine and protected from oxidation that Ge surface reconstructions are preserved below graphene, even after exposure to air.21−24 The duration of suppressed oxidation lasts longer than or favorably compares to that afforded by other passivation methods reported in the literature.15−18 However, the factors that govern passivation or oxidation of Ge below graphene are not understood. Previous studies on passivation of metal surfaces coated with graphene have shown that to reduce oxidation, it is critical to achieve a strong graphene/substrate interaction and to minimize defects in the graphene film. While the pristine sp2 graphene lattice is impermeable to species as small as He atoms,25 defects can be selectively permeable sites for oxidants to access the underlying substrate.26−28 Consequently, oxidation of metal surfaces preferentially occurs at point defects, grain boundaries, and pinholes (i.e., relatively large voids that are at least several angstroms in diameter) in graphene.29−36 Furthermore, strong coupling between graphene and the metal substrate is critical to reduce intercalation and lateral diffusion of oxidants at the graphene/substrate interface, preventing oxidation from spreading to regions under pristine graphene.37 For example, on metal surfaces that interact strongly with graphene, oxidation can be localized near defects in graphene, and thus, most of the surface can be passivated for several weeks even if graphene is relatively defective or discontinuous. In contrast, on metal surfaces that interact weakly with graphene, oxidants can diffuse laterally along the decoupled graphene/substrate interface, resulting in 4314
DOI: 10.1021/acsanm.9b00766 ACS Appl. Nano Mater. 2019, 2, 4313−4322
Article
ACS Applied Nano Materials
Figure 2. (a−d) SEM images (a, b) and schematic diagrams (c, d) of continuous graphene films grown on Ge(110) before (a, c) and after (b, d) the sample is exposed to FeCl3 to create etch pits selectively in the Ge surface below pinholes in graphene. Scale bars in (a, b) are 2 μm. (e) Plot of pinhole density, ρpinhole, against H2:CH4 ratio for graphene grown on Ge(001) (blue triangles), Ge(110) (red circles), and Ge(111) (black squares) at 910 °C for 6 h. Error bars indicate one standard deviation.
Ge 2p3/2 peak, from which the relative concentrations of Ge in the chemical state of Ge and GeO2 are determined. While substoichiometric GeOx oxide may also be present, we only quantify Ge and GeO2 in this analysis because the XPS data are fit well when only these two species are considered. Immediately after graphene growth, GeO2 is not detected on any of the samples (Figure S1). After exposure to atmospheric air at room temperature for 1 month, however, the samples display varying degrees of oxidation, depending on the Ge surface orientation and H2:CH4 ratio (Figure 1b,c). For Ge(001) and Ge(111), the relative concentration of GeO2 decreases with increasing H2:CH4 ratio. For example, with increasing H2:CH4 ratio from 8.7 to 20, the relative concentration of GeO2 on Ge(001) and Ge(111) decreases from 0.36 to 0.12 and 0.52 to 0.41, respectively. Passivation can be further improved by using even higher H2:CH4 ratios, although growth time also needs to be increased to ensure high graphene coverage (Figure S2). In contrast, on Ge(110), GeO2 is not detected by using any H2:CH4 ratio between 8.7 and 20, indicating that Ge(110) is more readily passivated by graphene than Ge(001) or Ge(111) over a wide range of growth conditions. Regardless of the Ge surface orientation or H2:CH4 ratio, oxidation of Ge is much slower when graphene covers the surface than when the surface is bare (Figure S3), indicating that graphene acts as a barrier that limits access of oxidants to the Ge surface. Figure 1 indicates that passivation of Ge by graphene can be enhanced by growing graphene with a high H2:CH4 ratio, and thus low growth rate, and that passivation is most effective on Ge(110). It has been shown that oxidation of metal surfaces preferentially occurs at defects in graphene29−36 and critically depends on the graphene/substrate interaction strength.37 We therefore hypothesize that the oxidation behavior in Figure 1 can be explained by the effect of H2:CH4 and Ge surface orientation on the (1) density of pinhole defects in graphene, (2) density of atomic-scale defects in graphene, (3) graphene/ Ge interaction strength, or (4) degradation of graphene after exposure to air. To test these hypotheses, we study the effect of the H2:CH4 flux ratio and Ge surface orientation on the density of pinholes, density of atomic-scale defects, and functionalization in graphene films grown on Ge via CVD. After synthesis, graphene appears to be completely continuous for all surface
We then investigate the effect of growth parameters on the formation of defects in graphene to elucidate the oxidation behavior. Passivation is enhanced when the density of atomicscale defects is reduced, despite there being a relatively high density of pinhole defects. The Ge surface orientation affects oxidation more strongly than defect density, as graphene films with the same defect density can passivate Ge(110) longer than Ge(001) or Ge(111). Furthermore, functionalization of graphene with C−O and CO accompanies oxidation of the Ge surface, which may indicate that formation of Ge oxides is mediated via oxidation of graphene. These results provide insight into the factors that control oxidation of Ge below graphene and provide a scalable path toward engineering graphene films on Ge with minimal defects and exceptional passivation properties for next-generation electronic applications.
2. RESULTS AND DISCUSSION 2.1. Effect of H2:CH4 Flux Ratio and Ge Surface Orientation on Oxidation of the Ge Surface. First, we study the effect of the H2:CH4 flux ratio during CVD graphene growth as well as Ge surface orientation on passivation of Ge covered with graphene. Previous work has qualitatively shown that increasing the H2:CH4 ratio can reduce defects in graphene grown on Ge(001).45−47 However, the type of defects formed has not been characterized and the defect density has not been quantified. Furthermore, the Ge surface orientation can affect the graphene/substrate interaction strength.21 Surface orientation and H2:CH4 ratio can also affect the shape and size of graphene crystals that comprise the continuous film, factors that impact the density and structure of grain boundaries.24,48−51 As passivation of metal substrates is strongly influenced by the graphene/substrate interaction37 and defects in graphene,29−36 we hypothesize that the H2:CH4 ratio and Ge surface orientation can be tuned to modify oxidation of the Ge surface. To explore this hypothesis, graphene is grown on Ge(001), (110), and (111) at 910 °C for 6 h by using a CH4 flow rate of 4.6 sccm while varying the H2 flow rate to be 40, 60, 80, and 90 sccm, yielding H2:CH4 flux ratios of 8.7, 13, 17, and 20, respectively (Figure 1a). The Ar flow rate is adjusted so that the total H2 plus Ar flow rate is constant at 300 sccm. X-ray photoelectron spectroscopy (XPS) is used to characterize the 4315
DOI: 10.1021/acsanm.9b00766 ACS Appl. Nano Mater. 2019, 2, 4313−4322
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ACS Applied Nano Materials
Figure 3. (a−c) Raman spectra of graphene grown on Ge(001) (a), Ge(110) (b), and Ge(111) (c) by using an H2:CH4 ratio of 8.7 (black, bottom), 13 (red), 17 (blue), 20 (green), and 22 (purple, top) at 910 °C for 6 h. (d) Schematic diagram of graphene with grain boundaries and no point defects. Each grain is a different color, and the armchair direction of each grain is indicated with an arrow. (e) Plot of extracted grain density, ρgrain, as determined by eq 2 from Cancado et al.55 against the H2:CH4 ratio. (f) Schematic diagram of graphene with point defects (simplified as black circles) and no grain boundaries. (g) Plot of extracted point defect density, ρpoint defect, as determined by eq 4 from Cancado et al.56 against the H2:CH4 ratio. Green lines in (e) and (g) indicate the minimum detectable densities due to the minimum detectable Raman D:G ratio of ∼0.1. The ρgrain and ρpoint defect for graphene grown on Ge(110) with H2:CH4 ≥ 17 are below the detectable limit. Point-to-point variation in Raman D:G ratio is at least 15%, as determined from Raman spectra of graphene grown on Ge(110) with an H2:CH4 ratio of 8.7.
10−3 to 6.0 × 10−2 μm−2, 4.6 × 10−3 to 3.9 × 10−1 μm−2, and 6.0 × 10−2 to 2.6 × 10−1 μm−2, respectively. We attribute the increase in ρpinhole to an increase in the number of graphene islands that have not fully merged at the atomic scale to form a continuous film. With increasing H2:CH4, the nucleation density (see below) and graphene growth rate both decrease.24 Thus, for a constant growth time, individual graphene crystals that comprise the continuous film are less likely to coalesce fully if they are grown with higher H2:CH4. The reduced ρpinhole on Ge(110) compared to Ge(001) and Ge(111) at a given H2:CH4 can possibly be attributed to a faster growth rate on Ge(110) (Figure S6), for example, as a result of a lower energy barrier for (1) decomposition of CH4 to form intermediate CxHy growth species, (2) surface diffusion of these growth species, or (3) attachment of these species to the edge of graphene. As described above, pinholes are relatively large discontinuities in the graphene lattice through which etchant ions can diffuse. Thus, pinholes result in regions of bare Ge that are not covered by graphene. Any bare Ge surface, which may be terminated with hydrogen after growth,53 likely oxidizes within several minutes.16,17 Consequently, it is surprising that passivation of Ge actually improves (Figure 1) as pinhole density increases (Figure 2e). We conclude that while oxidation of Ge must certainly occur at pinholes, this is not the primary route of Ge oxidation. 2.3. Determination of the Density of Atomic-Scale Defects in Graphene. While the FeCl3 etch can quantify the density of relatively large pinholes, this technique does not provide information about relatively continuous atomic-scale defects. The etchant cannot permeate these relatively small
orientations and H2:CH4 ratios when viewed with scanning electron microscopy (SEM) (Figure 2a and Figure S4), as regions of exposed Ge, which display different contrast than regions of Ge that are covered by graphene,24 are not observed. Therefore, any defects that exist in these films are smaller than the resolution of SEM (i.e., 50 Ω·cm) substrates are loaded into a horizontal furnace with a quartz tube inner diameter of 34 mm. Surface roughness of the as-purchased wafers, characterized by AFM, is shown in Figure S12. The samples are annealed at atmospheric pressure at 910 °C in a flow of H2 and Ar for 30 min. The H2 flow rate is varied from 40 to 100 sccm, and the Ar flow rate is adjusted so that the total H2 and Ar flow rate is constant at 300 sccm. After annealing, CH4 is introduced to initiate graphene growth. The furnace is slid away from the samples to terminate growth under the same atmosphere used during graphene synthesis. 4.2. Ge Etching for Quantification of Graphene Pinhole Density. Samples are exposed to 0.02 M FeCl3 for 0.5−5 min, rinsed in deionized H2O, and dried with N2, resulting in etch pits in Ge that are localized below pinholes in graphene. 4.3. Characterization via SEM, Raman, XPS, and AFM. SEM (Zeiss LEO 1550VP and Zeiss LEO 1530) is conducted using an accelerating voltage of 3−5 keV. Images are analyzed by using ImageJ to determine ρpinhole (Figure 2 and Figure S5), multilayer coverage (Figure S8), nucleation density (Figure 4), and graphene growth rate (Figure S6). Raman spectroscopy (Thermo Scientific DXRxi) is conducted by using a laser with wavelength of 532 nm (Figure 3a−c, Figures S2 and S7). The D and G bands are fit with Lorentzian peaks in Matlab. XPS (Thermo Scientific K-alpha) is conducted with a spot size of 400 μm and an analyzer pass energy of 50 eV by using monochromatic Al Kα radiation (1486.7 eV) (Figures 1 and 5, Figures S1−S3 and S9−S11). The Ge 2p3/2 and C 1s XPS peaks are fit by using Voigt curves in CasaXPS or Fityk software. The sampling depth (i.e., 3λ, where λ is the inelastic mean free path) for the Ge 2p3/2 and C 1s XPS peaks is roughly 2.7 and 5.6 nm, respectively.66 Surface morphology is characterized by using AFM (Bruker MultiMode 8) in tapping mode (Figures S9 and S12).
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