Solution Processing of GaAs Thin Films for Photovoltaic Applications

Jul 21, 2014 - Christopher S. Blackman,. † and Claire J. Carmalt* .... precursor, [Me2GaAs(H)tBu]2, and a solution-based route to. GaAs thin films v...
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Solution Processing of GaAs Thin Films for Photovoltaic Applications Sanjayan Sathasivam,† Ranga Rao Arnepalli,‡ Bhaskar Kumar,‡ Kaushal K. Singh,‡ Robert J. Visser,*,‡ Christopher S. Blackman,† and Claire J. Carmalt*,† †

Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K. Applied Materials, Inc., 3225 Oakmead Village Drive, Santa Clara, California 95052-8039, United States

‡

ABSTRACT: In this article we present a novel route to high quality GaAs thin films via a solution processing technique (aerosol assisted chemical vapor deposition) using a novel single source precursor [Me2GaAs(H)tBu]2. The thin films, grown on inexpensive glass substrates, were polycrystalline in nature with a Ga to As ratio of 1:1. The morphology studied via SEM showed the films to be smooth and consisting of compact domes. High-resolution transmission electron microscopy (HRTEM) revealed the films to have columnar growth and an average crystallite size of 90 nm. The films also contained low levels of contaminants as determined via energy dispersive X-ray spectroscopy (EDX) mapping, X-ray photoelectron spectroscopy (XPS) depth profiling, and secondary ion mass spectrometry (SIMS).

1. INTRODUCTION Gallium arsenide (GaAs) is a semiconductor that is widely used in photovoltaic and optoelectronic devices.1,2 It displays better theoretical and experimental properties than the ubiquitous silicon-based devices. For example, GaAs thin film solar devices show an efficiency of 28.8% compared to 20.1% that is achieved by amorphous silicon (crystalline silicon devices show an efficiency of 25.0%).3 This is owed to GaAs having a direct bandgap of 1.43 eV, which is close to the 1.34 eV bandgap that is optimum for solar conversion for a single junction solar cell at AM1.5.4,5 GaAs also has high electron mobility that make for better device performance,1,6,7 and is more resistant to heat and radiation damage compared to Si due to the higher threshold energy under high energy radiation.7 However, the high cost and difficulty associated with fabricating GaAs devices has so far limited the use of GaAs photovoltaics to space and military applications. The current methods of fabrication involve epitaxial methods such as metal−organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on expensive single crystal substrates, such as GaAs and germanium, involving dual source precursors. The precursors involved in such depositions are usually trimethylgallium (GaMe3), which is a pyrophoric liquid, and arsine (AsH3), which is a highly toxic gas.8 Liquid tert-butyl arsine (tBuAsH2) has also been used as an alternative to AsH3, but it is still highly toxic.9 Hence these precursors when used on an industrial scale present handling and safety concerns. Single-source precursors can overcome these concerns as they are much less toxic, less pyrophoric and are easier to handle if in the solid state.10 Furthermore, they are very useful in the growth of binary semiconductor film, such as GaAs, as they contain preformed Ga−As bonds with the required elements of the film in the correct ratio. Hence allowing the formation of films at reduced © 2014 American Chemical Society

temperatures and with the correct stoichiometry. The 1:1 ratio of Ga:As in the single-source precursor is advantageous as current dual source techniques such as MOCVD require an excess of arsine (up to 10:1) to obtain stoichiometric GaAs films. In the majority of cases in the literature, GaAs is grown on single crystal substrates such as GaAs and Ge.6 There are very few examples of deposition on amorphous substrates such as glass. Imaizumi et al. reported the use of chemical beam epitaxy to deposit polycrystalline GaAs on glass using arsine and trimethylgallium at 500 °C.11 The deposition required a precracking step of 1000 °C to enable pyrolysis of the arsine precursor. GaAs has also been grown on glass substrates via flash evaporation at 2.7 × 10−6 mbar by Campomanes et al., the as-deposited films required a thermal annealing step to obtain crystallinity.12 Deposition via MBE and low pressure (LP) CVD have also been reported using arsine and metallic sources of Ga on glass substrates. In the literature there are many examples of molecular GaAs precursors. Ranging from simple Lewis acid−base adducts such as [GaR3{AsR′3}] (R = Et, tBu; R′= SiMe3, iPr)13 to more complex dimeric and trimeric compounds such as [R2GaAstBu2]2 (R = Me, Et, nBu)14,15 and [Me2GaAsiPr2]3.16 Though the synthetic routes to GaAs precursors are well documented, the deposition of GaAs films from such precursors is not as well studied, as these precursors are not all compatible with traditional deposition techniques. For example, the dimers [nBu2Ga(μ-EtBu2)2GanBu2]2 (E = P, As) were shown to be effective precursors for deposition of Received: April 10, 2014 Revised: July 10, 2014 Published: July 21, 2014 4419

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crystalline GaE films via LPCVD, but using supercritical chemical fluid deposition in CO2/hexane did not yield GaE and significant carbon deposition occurred.17 Here we present the synthesis of a novel single-source precursor, [Me2GaAs(H)tBu]2, and a solution-based route to GaAs thin films via aerosol assisted chemical vapor deposition (AACVD) on glass substrates. AACVD is a simple, low cost, and scalable technique that has been used to produce high quality thin films for many optical and electrical applications.18−24 In AACVD, the precursor is dissolved in a suitable solvent and, the resultant solution is atomized using a piezoelectric device. The aerosol particles are carried to the deposition chamber using a carrier gas. It can operate at atmospheric pressure and therefore does not require expensive equipment to reach low pressures like LPCVD and MBE. Furthermore, precursors for AACVD need not be volatile.19,25 The major requirement necessary is solubility.

Polycrystalline thin films of GaAs were deposited on glass substrates using the single-source precursor [Me2GaAs(H)tBu]2 via AACVD at 550 °C. The precursor was dissolved in 15 mL of dry toluene (transport medium) giving a precursor concentration of 0.033 g mL−1. The deposited films were smooth and continuous and appeared gray/blue in color under reflected light. They were adherent to the substrate, passing the Scotch Tape test, but were scratched by stainless steel and brass stylus as expected for GaAs films. Crude electrical measurements carried out using a two-point probe showed the films to be insulating with electrical resistance in the MΩ region as expected for undoped GaAs.29 X-ray diffraction (XRD) carried out on the films confirms the presence of cubic polycrystalline GaAs (Figure 1). Peaks corresponding to cubic GaAs (111), (220), (311), (400), and (331) were observed at 27.3°, 45.4°, 53.7°, 66.0°, and 72.9° 2θ values, respectively.

2. RESULTS AND DISCUSSION The single-source precursor, [Me2GaAs(H)tBu]2, was synthesized from the reaction of GaMe3 with tBuAsH2 in a 1 to 1.5 ratio in toluene as shown below (eq 1). The slight excess of the

arsine precursor was required to ensure the reaction went to completion. After 21 h of stirring at 80 °C the solvent was removed in vacuo to produce a yellow solid that was recrystallized in toluene at 3−5 °C. The product was highly soluble in toluene and hexane. The 1H spectrum of [Me2GaAs(H)tBu]2 showed resonances at 0.024 and 0.084 ppm corresponding to the two inequivalent methyl groups on the gallium center, both of these integrate to 3 protons with respect to As−H resonance, which was observed at 2.56 ppm. The resonance for the tertiary butyl groups appeared at 1.43 ppm in the 1H NMR spectrum and corresponds to 9 protons as expected. Mass spectroscopy was unable to determine the molecular ion; however, some molecular fragments such as [MeGaAs(H)tBu]2 at m/z 437 were observed, suggesting that a dimeric molecule is the most likely product although monomers and trimers are possible. Also, elemental analysis carried out on the yellow solid corresponded with the expected values for [Me2GaAs(H)tBu]2. Attempts to grow X-ray quality crystals were unsuccessful. The thermal decomposition of [Me2GaAs(H)tBu]2 was studied using thermogravimetric analysis (TGA) between room temperature and 600 °C. With increasing temperature, there were three major mass losses observed corresponding to the loss of the two tertiary butyl groups along with two hydrogens and the removal of the methyl groups. The precursor fully decomposed at 300 °C with 47% reduction in the mass of the precursor, a relatively low temperature mainly due to the presence of β-hydride groups on the precursor that facilitates facile and clean decomposition. Above 300 °C, there was residual mass loss until 600 °C. Decomposition studies using mass spectrometry on similar dimeric precursors in the literature indicate that the breakdown does occur via a βhydride mechanism on the substrate surface.26−28 Methane and 2-methylpropene are the waste products.23−25

Figure 1. Powder XRD pattern for the GaAs standard and the GaAs films at 550 °C showing the polycrystalline nature of the films on the glass substrates.

The crystallographic preferred orientation was calculated with respect to a GaAs standard pattern. The films showed preferential growth in the (111) direction and a lack of preference in the (400) orientation. This is most likely due to the influence of the amorphous substrate on the crystalline film that has been previously observed for other semiconductor systems.12,30 This interaction with the amorphous substrate was also the main reason for the 0.2% contraction in the cubic unit cell observed in the films. The unit cell parameters were determined by fitting the XRD pattern (Figure 1) to a Le Bail model using GSAS and EXPGUI programs. The Scherrer equation31,32 was applied to the XRD data to estimate the crystallite size in the films, and the average crystallite size was determined to be ca. 30 nm. Raman spectroscopy was carried out on the films, using a 514.5 nm laser, show the two bands expected for cubic GaAs (Figure 2). These are a longitudinal optical phonon mode at 286.8 cm−1 and a doubly degenerate transverse optical TO phonon mode at 263.8 cm−1, which corresponds with literature values for bulk GaAs, although there is a red shift. The small deviation from the literature values could be due to strain in the GaAs lattice arising from growth on glass substrates and possibly due to the small crystallite size of the films (Figure 5). Energy dispersive X-ray spectroscopy (EDX) was used to determine the purity and elemental composition of the films. 4420

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Figure 4. SEM images showing the morphology of the GaAs films on glass substrates grown via the AACVD using the single source precursor [Me2GaAs(H)tBu]2. Figure 2. Raman spectrum observed for the GaAs films deposited at 550 °C via AACVD.

observed between grain boundaries as expected for polycrystalline films (Figure 5c). The crystallite size distribution was calculated from the HRTEM data for both the top and bottom layers of the films (Figure 5d,e). The top layer of the films have an average crystallite size of 98 nm in the top layer and 85 nm in the bottom layer. This trend can be as expected as the bottom layers act as a template and provide a more crystalline surface for film growth. The dark field (Figure 5f) and light field (Figure 5g) images show that the films have columnar growth. This is suitable for polycrystalline films intended for solar applications as it minimizes grain boundaries in the horizontal directions. These side-on images also reveal the film thickness to be 1.5 μm. X-ray photoelectron spectroscopy (XPS) carried out on the films showed the presence of Ga and As as expected also with the O and C due to surface contamination. The Ga 3d peaks were deconvoluted to show the presence of two Ga 3+ environments corresponding to GaAs at 3d5/2 binding energy 19.0 eV and Ga2O3 3d5/2 at binding energy 20.4 eV. These values match well with literature values.34−36 The 3d peaks for As were also resolved to two different As3− environments corresponding to (predominantly) GaAs at a 3d5/2 binding energy of 40.3 eV and As2O3 at 43.7 eV.37,38 XPS analysis also showed the films contained a Ga to As ratio of one to one and matched that of the GaAs standard. An XPS depth profiling study (Figure 6) concluded that as suggested by the EDX mapping (Figure 3) and consistent with literature reports, the oxygen and carbon contamination was mainly surface bound.26 After 100 s of etching, the films were typically low in oxygen, and after 30 s of etching, very low in carbon. After etching for 200 s the O 1s peak disappears, whereas the peak for carbon disappears after only 30 s, confirming that both contaminants are mainly only on the surface. The valence band spectrum for the films (Figure 7) match well with literature findings and that of a GaAs standard carried out under the same conditions.39 The band edge appears near 0 eV for GaAs as like all conventional n-type semiconductors the Fermi level on the surface is pinned close to the center of the band gap by negatively charged surface states. As a result the Fermi level at the surface lies lower than compared to the band edges.40 Secondary ion mass spectrometry (SIMS) measurements were carried out on films to determine any low level carbon or oxygen contamination within the film. The results showed that

EDX analysis showed equal amounts of Ga and As were present in the films and that only Ga, As, and oxygen were present. The O detected was due to contamination on the surface of the films from the formation of a native oxide layer consisting of gallium and arsenic oxides. This is a common occurrence on all GaAs surfaces.33 EDX mapping (Figure 3) carried out on a

Figure 3. EDX mapping of the AACVD grown films at 550 °C. The lack of color (dark blue for As, blue for Ga, orange for C, yellow for O, and green for Si) indicates the lack of that element in that region.

region of a film from the surface through to the substrate confirmed that oxygen was indeed present mainly on the surface of the films as indicated by the lack of color (orange) in the graph. Furthermore, the Ga to As ratio was uniform throughout the film indicating that Ga is only bound to As in the form of GaAs. Carbon contamination is also only predominant on the surface of the films as shown by the lack of red color in the EDX graph in Figure 3. The morphology of the films was studied using SEM (Figure 4). The surface of the films appeared to be composed of compact domes that are uniform across the regions analyzed. The domes are large, defined, and about 1 μm in size. It is important to note that the domes that make up the surface of the films are not made up of single crystals but a collection of crystallites. This is more evident from high-resolution transmission electron microscopy (HRTEM) images and data shown in Figure 5. HRTEM was used as a means to directly determine the grain size distribution and polycrystallinity within the films (Figure 5). The HRTEM images (Figure 5a−c) show distinct and welldefined lattice fringes. Some amorphous regions were also 4421

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Figure 5. HRTEM data for films at 550 °C. Top down high-resolution image (a) shows the presence of grain boundaries in the regions analyzed. The magnified image (b) shows that the lattice fringes are well spaced and defined indicating good crystallinity. Magnified image (c) shows grain boundaries within the film. The graphs showing grain size distributions in the top (d) and bottom layers (e) for the films show that crystallites are larger in the top layer. The dark field (f) and light field (g) show that the both films have columnar structure.

Figure 6. Typical XPS depth profiling study looking for Ga 3d, As 3d, O 1s, and C 1s.

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4. EXPERIMENTAL SECTION Caution! Trimethylgallium and tertbutylarsine are pyrophoric substances that ignite spontaneously in air and therefore must be handled under an inert atmosphere. tert-Buylarsine is highly toxic and must be handled with care. All experiments must be carried out in a f ume cupboard. Postsynthesis, any unreacted, reactive species are removed under vacuum. Postdeposition, the f ilms are air/moisture stable and safe to handle, and all reactive species leave via the exhaust during the AACVD process. Trimethylgallium (99.999%) was used as received from SAFC Hitech Ltd. tert-Butylarsine (99.999%) was purchased from and used as received from Dockweiler Chemicals. Toluene was purchased from Alfa Aesar and stored under alumina columns and dried with Anhydrous Engineering equipment. All reactions were carried out under a nitrogen atmosphere using standard Schlenk and glovebox techniques. 1H and 13C{1H} NMR spectra were obtained on a Bruker AMX-400 spectrometer, operating at 295 K and 400.12 MHz (1H). Signals are reported relative to SiMe4 (δ = 0.00 ppm), and the following abbreviations are used: s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad). Deuterated solvents were obtained from Goss Scientific and were dried and degassed over molecular sieves prior to use. Mass spectra were obtained using Micromass 70-SE spectrometer using chemical ionization (CI) with methane regent gas. Elemental analysis was obtained at UCL. FT-IR was carried out on a PerkinElmer instrument. t BuAsH2 (1 g, 7.50 mmol) in toluene (15 mL) was cooled to −78 °C. This was then added dropwise to a solution of GaMe3 (0.86 g, 7.5 mmol) in toluene (15 mL) also cooled to −78 °C. The colorless solution was allowed to reach room temperature and then heated to 80 °C for 21 h. The solvent was removed in vacuo to obtain a yellow solid. The solid was dissolved in minimum toluene and reduced to below room temperature to yield colorless crystals (72% yield). Anal. Calc. for C12H32As2Ga2: C, 30.93; H, 6.93. Found: C, 29.90; H, 6.60% 1 H NMR (CDCl3): δ 0.024 (s, 6 H, GaMe2), 0.084 (s, 6 H, GaMe2), 1.43 (s, 18 H, AstBu), 2.56 (s, 2 H, AsH). 13C{H} NMR (CDCl3): δ 33.4 (tBuAs), 1.17 (GaMe2). IR (Neat, cm−1) 2947.9 (s), 2924.6 (s), 2883.9 (s), 2854.9 (s), 2123.2 (m), 1454.7 (s), 1390 (w), 1363.1 (s), 1260.4 (m), 1209 (vw), 1188 (w), 1154.8 (s), 1097.5 (m), 1018.4 (m), 935.5 (vw), 860.1 (vw), 715.7 (vs). Mass spec (CI): (m/z) 451 [M − Me], 437 [M − 2Me], 435 [M − 2Me − 2H], 379 [M − 2Me − t Bu], 377 [M − 2Me − tBu − 2H]. Depositions were carried out under nitrogen in a cold wall reactor. Precursor was placed in a glass bubbler and an aerosol mist was created using a piezoelectric device (Vicks ultrasonic humidifier, model number: 4022167500175) placed below the bubbler. [Me2GaAs(H)tBu]2 (0.5 g, 0.95 mmol) was dissolved in dry toluene (15 mL). The resultant solution was stirred to form a homogeneous solution and then atomized. The precursor flow was kept at 0.5 L·min−1. The glass substrate was Corning glass (size 15 cm × 4 cm × 0.1 cm). A top plate was suspended 0.5 cm above the glass substrate to ensure a laminar flow. The substrate temperature was kept at 550 °C. Deposition time was 60 min. After the deposition the bubblers were closed, and the substrates were cooled under a flow of nitrogen. At the end of the deposition the nitrogen flow through the aerosol was diverted and only nitrogen passed over the substrate. The glass substrate was allowed to cool with the graphite block to room temperature before it was removed. Coated substrates were handled and stored in air. X-ray diffraction (XRD) was carried out using a microfocus Bruker GAADS powder X-ray diffractometer with a monochromated Cu Kα (1.5406 Å) source. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific K-Alpha instrument with monochromatic Al−Kα source to identify the oxidation state and chemical constituents. High-resolution scans were done for the Ga (3d), As (3d), O (1s), and C (1s) at a pass energy of 40 eV. The Ga atom % was derived from the Ga−Kα line (9242.9 eV), and the As atom % was derived from the As-Lα (10532.0 eV). The peaks were modeled using CasaXPS software with binding energies adjusted to adventitious carbon (284.5 eV). Additional XPS measurements were carried out by Applied Materials, Santa Clara, Ca, USA. SEM images

Figure 7. XPS valence band measurements for a GaAs standard and the GaAs film grown via AACVD at 550 °C.

for films grown via AACVD using the single source precursor, the carbon concentration near the surface of the films was 6 × 1020 atoms cm−3. This drops gradually and reduces to 7 × 1019 atoms cm−3 after 200 nm into the film, it remains at this level until close to the substrate where the concentration increases again. The oxygen contamination also measured via SIMS showed that the oxygen concentration near the surface is 3.6 × 1021 atoms cm−3. This decreases to 1.3 × 1021 atoms cm−3 after 20 nm into the film and remains close to this level until near the substrate where the oxygen concentration increases due to diffusion from the substrate. The SIMS results reveal that although O and C contamination in the GaAs films are low, the levels are still above what is required for photovoltaic devices. Currently work is being carried out to further reduce C and O contamination in the bulk of the film. The results show that via AACVD it is possible to produce high quality thin films of GaAs from single-source precursors at atmospheric pressure. Films grown at 550 °C on glass substrates showed good crystallinity and were stoichiometric.

3. CONCLUSIONS In this article, we have shown the deposition of high quality polycrystalline GaAs thin films on glass substrates from a novel single-source precursor, [Me2GaAs(H)tBu]2. Depositions were carried out using AACVD, a low cost, scalable chemical vapor deposition technique that operates at atmospheric pressure. Films were analyzed using a range of techniques to show the correct stoichiometry, morphology, and crystallinity, making the films and the AACVD technique suitable for implementation in photovoltaic devices. The single-source precursor, [Me2GaAs(H)tBu]2, containing facile β-hydride groups was an ideal source of Ga and As as it enabled the production of films with low contamination as shown by EDX mapping, XPS depth profiling, and SIMS. Furthermore, the use of the novel precursor allowed the production of films with Ga to As ratio of 1:1 without the requirement of a large excess of arsenic source as is typical in the dual source deposition techniques currently used for GaAs deposition. 4423

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(21) Bhachu, D. S.; Sankar, G.; Parkin, I. P. Chem. Mater. 2012, 24, 4704. (22) Marchand, P.; Hassan, I. A.; Parkin, I. P.; Carmalt, C. J. Dalton Trans. 2013, 42, 9406. (23) Bloor, L. G.; Manzi, J.; Binions, R.; Parkin, I. P.; Pugh, D.; Afonja, A.; Blackman, C. S.; Sathasivam, S.; Carmalt, C. J. Chem. Mater. 2012, 24, 2864. (24) Bhachu, D. S.; Sathasivam, S.; Sankar, G.; Scanlon, D. O.; Cabin, G.; Carmalt, C. J.; Parkin, I. P.; Watson, G. W.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N. Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201400338. (25) Knapp, C. E.; Hyett, G.; Parkin, I. P.; Carmalt, C. J. Chem. Mater. 2011, 23, 1719. (26) Miller, J. E.; Kidd, K. B.; Cowley, A. H.; Jones, R. A.; Ekerdt, J. G.; Gysling, H. J.; Wernberg, A. A.; Blanton, T. N. Chem. Mater. 1990, 2, 589. (27) Cowley, A. H.; Benac, B. L.; Ekerdt, J. G.; Jones, R. A.; Kidd, K. B.; Lee, J. Y.; Miller, J. E. J. Am. Chem. Soc. 1988, 110, 6248. (28) Miller, J. E.; Mardones, M. A.; Nail, J. W.; Cowley, A. H.; Jones, R. A.; Ekerdt, J. G. Chem. Mater. 1992, 4, 447. (29) Synowiec, Z.; Radziewicz, D.; Zborowska-Lindert, I. Advanced Semiconductor Devices and Microsystems. ASDAM 2000 Third International Euro Conference, 2000; Vol. 289, p 293. (30) Ghosh, R.; Basak, D.; Fujihara, S. J. Appl. Phys. 2004, 96, 2689. (31) Scherrer, P. Gottinger Nachrichten Gesell. 1918, 2, 98. (32) Qazi, S. J. S.; Rennie, A. R.; Cockcroft, J. K.; Vickers, M. J. Colloid Interface Sci. 2009, 338, 105. (33) Adams, A. C.; Pruniaux, B. R. J. Electrochem. Soc. 1973, 120, 408. (34) Bertrand, J. Vac. Sci. Technol. 1981, 18, 28. (35) Leonhardt, G.; Berndtsson, A.; Hedman, J.; Klasson, L.; Nilsson, R. Phys. Status Solidi B 1973, 60, 241. (36) Basharat, S.; Carmalt, C. J.; Binnions, R.; Palgrave, R.; Parkin, I. P. Dalton Trans. 2008, 5, 591. (37) Kuhr, H. J.; Ranke, W.; Finster, J. Surf. Sci. 1986, 178, 171. (38) Lindau, I.; Pianetta, P.; Garner, C. M.; Chye, P. E.; Gregory, P. E.; Spicer, W. E. Surf. Sci. 1977, 63, 45. (39) Pollak, R. A.; Ley, L.; Kowalezyk, S.; Shirley, D. A.; Joannopoulos, J. D.; Chadi, D. J.; Cohen, M. L. Phys. Rev. Lett. 1972, 29, 1103. (40) King, P. D. C.; Veal, T. D. J. Phys.: Condens. Matter 2011, 23, 334214.

were taken on a JEOL JSM-6301F Field Emission instrument with acceleration voltage of 5 kV. Images were captured using SEMAfore software. EDX was measured on a JEOL JSM-6301F Field Emission instrument with acceleration voltage of 20 kV. The Ga atom % was derived from Ga−Kα line (9243 eV), and the As atom % derived from the As Kα line (1053 eV). For both SEM and EDX, samples were cut to 10 mm × 10 mm coupons and coated with a fine layer of gold (SEM) and carbon (EDX) to avoid charging. HRTEM and EDX mapping was carried out on Titan 80-300 TEM with EDX at CAMCOR service at the University of Oregon. SIMS was carried out by Evans analytical group, Santa Clara, California.

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AUTHOR INFORMATION

Corresponding Authors

*(C.J.C.) E-mail: [email protected]. *(R.J.V.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Applied Materials Inc., USA for funding the project. Dr. Ben Schmiege and Dr. Davinder S. Bhachu for useful discussion, Mr. Kevin Reeves for assistance with SEM imaging, and Dr. Ghazel Saheli for XPS depth profiling.

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REFERENCES

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