Phosphors for LED-based Solid-State Lighting by Anant A. Setlur

he efficacy of solid-state lighting saturation-based quenching. Finally, the discuss these various LED phosphors (SSL) based upon InGaN LEDs phosphor temperature in LED packages with some of their advantages and T has improved by >10x over the can be >150oC, and many LFL/CFL and drawbacks. past decade: the efficacy of cool white CRT phosphors have strong quenching LEDs surpasses linear fluorescent lamp at these temperatures. These additional Phosphors for (LFLs) efficacies (>100 lm/W) and warm requirements make it necessary to white 1W LEDs surpasses compact develop new phosphors specifically for LED-based Lighting fluorescent lamps (CFLs) efficacies pcLEDs. 3+ (>60-70 lm/W). The U.S. DOE has set The challenges and additional Ce -doped garnets.—The main a 2015 efficacy target of 138 lm/W requirements for LED phosphors are yellow phosphor (with compositional for warm white packages, a significant balanced by larger potential composition modifications) used in pcLEDs, 3+ 10 technical achievement that would lead spaces for LED phosphors versus that for Y3Al5O12:Ce (YAG:Ce), was reported 11 to SSL market penetration in many LFL/CFL phosphors. For example, many in 1967 with its primary use (prior to lighting product segments. In LED- silicate phosphors darken in the Hg- pcLEDs) in CRTs. In pcLEDs, YAG:Ce based SSL, violet, blue, and green LEDs plasma due to Hg-adsorption,9 generally absorbs blue LED radiation through the 1 1 are based upon InGaN semiconductors, preventing their use unless they are allowed 4f → 5d transition and emits 1 1 while the red and amber LEDs are coated with a protective layer to prevent yellow light via the reverse 5d → 4f based upon AlInGaP semiconductors.1-4 these reactions. These reactions are not transition (Fig. 2). The emission from 1 Both of these semiconductor systems present in LEDs, opening up many the lowest excited 5d level is to the 1 tend to have much lower efficiencies in potential phosphor compositions. In spin–orbit split 4f ground states, the green, yellow, and amber spectral addition, fluorescent lamp phosphor leading to an extremely broad emission regions. Many aspects regarding the suspensions are water-based, preventing band with a FWHM > 100 nm. The progress in LED chip efficiency, such as the use of phosphors that decompose yellow emission from YAG:Ce and the fundamental causes for the lower green in water. Again, these restrictions are blue radiation that “bleeds” through and amber LED efficiency, and the impact less of an issue in LEDs since many a YAG:Ce coating combines to give of LED-based SSL on lighting energy packaging protocols avoid aqueous white light with a daylight-like color consumption have been discussed processing conditions. However, while temperature (CCT > 4000 K) and elsewhere2-5 and will not be addressed processing issues might be alleviated, reasonable color rendering (CRI ~70- 80), here. The limitations in InGaN and there are potential issues with phosphor enabling pcLEDs to be used in many AlInGaP efficiency make it necessary stability at high temperatures and high applications where color quality is not to use phosphor downconversion (in humidity conditions (e.g. 85oC and 85% a key requirement, including backlights spite of the inherent Stokes losses) to relative humidity) since pcLEDs are not for portable displays and indicators. generate green and yellow light for necessarily hermetically sealed. Since these markets dominate the total high efficacy LED packages, lamps, Many of the needs for new LED pcLED market, current phosphor usage and fixtures. In addition, since the phosphors have been met by the in pcLEDs is heavily skewed toward maximum efficiency of blue and violet discovery and development of new YAG:Ce. The properties of YAG:Ce InGaN LEDs appears to be higher phosphors over the past 10 years. also represent a benchmark for other than the maximum efficiency for red During this time, the field of LED LED phosphors. First, the absorption 2 and emission transitions are parity and (lmax = 600-630 nm) AlInGaP LEDs, the phosphors has moved from a single potential efficacy for a system that uses family of phosphor compositions—the spin allowed, giving strong absorption phosphor downconversion of InGaN Ce3+-doped aluminate garnets—to a of blue LEDs and a fast decay time that LEDs (termed pcLEDs in this article) variety of silicate, aluminate, nitride, prevents saturation quenching. The for the entire white spectrum could oxynitride, sulfide, and fluoride quantum efficiency (QE) of YAG:Ce be greater than systems using AlInGaP compositions, leading to commercial under blue LED excitation is >85%, o 12 LEDs to generate red light, further LEDs that cover a full range of white even at 200 C, and there are no motivating the development of LED CCTs (Fig. 1). This article will briefly indications that YAG:Ce degrades under phosphors across the visible spectrum. blue LED excitation or moisture. Also, There has been extensive research and development for phosphors in LFLs/CFLs, cathode-ray tubes (CRTs), and X-ray films,6 but most of these traditional phosphors are not suitable for pcLEDs. This is usually because these phosphors do not strongly absorb violet or blue LED radiation, leading to LED package losses from scattering. Also, many traditional phosphors use Eu3+, Tb3+, or Mn2+ activators whose transitions are forbidden with long decay times (>1 ms), causing phosphor quenching due to saturation from the high LED radiation flux on the phosphor.7,8 Using Ce3+ and Eu2+ phosphors with 5d1 → 4f1 Ce3+ or 4f65d1 → 4f7 Eu2+ emission transitions that have decay times of <100 ns and <3 ms, respectively, prevents this Fig. 1. Picture of GE Lumination VioTM LED package that is based upon phosphor downconversion of 405 nm LEDs with phosphor powders.

32 The Electrochemical Society Interface • Winter 2009 2+ 22 the synthesis of YAG:Ce is relatively in Ca -a-SiAlON and M2Si5N8 new phosphors. In many respects, straightforward and uses high-purity (M = Ca2+, Sr2+, Ba2+)23 (Fig. 3) that could the discovery of new (oxy)nitrides precursors (Y2O3, Al2O3, CeO2) that have be excited with violet or blue LEDs. Many required some degree of guesswork and been qualified for use in traditional of these new phosphors match the room perseverance since the formation of CFL/LFL/CRT phosphors. and high temperature QE of YAG:Ce. condensed tetrahedral networks with One deficiency for pcLEDs that In addition, these new nitrides do not N3- makes it difficult to draw analogies use only YAG:Ce is that they are degrade under high temperature/high to silicate or aluminate solid-state limited to high CCTs and lower humidity conditions due to the ability chemistry. Investigation of nitride and 3- CRIs, due to a lack of a red spectral of N -containing Si(O,N)4 tetrahedra oxynitride compositional spaces has led component. It is possible to redshift to form condensed and cross-linked to the discovery of more new nitride the Ce3+ 5d1 → 4f1 emission through tetrahedral networks.24 The potential phosphors (and hosts) such as the green 3+ 3+ 13 2+ 4+ 2+ 30 Gd substitution of Y , or Mg - Si for high efficiency and stable pcLED Sr5Al5+xSi21-xN35-xO2+x:Eu (with x~0), 3+ 2+ 31 substitution for Al (octahedral)- phosphors within nitride compositional the orange-red SrAlSi4N7:Eu , the 3+ 14 2+ 32 Al (tetrahedral). This redshift comes spaces has led to extensive investigation yellow Ba2AlSi5N9:Eu , and the green 2+ 33 at the cost of efficiency, especially at of a variety of (oxy)nitride compositions Ba3Si6O12N2:Eu . Further work will high temperatures.14,15 The limits for for potential pcLED phosphors. One likely give more new materials that also red Ce3+-doped aluminate garnets (as family of efficient phosphors are the could be of interest in pcLEDs. 2+ 2+ 2+ 2+ well as intellectual property concerns MSi2O2N2:Eu (M = Ca , Sr , Ba ) The efficient yellow, orange, and in using Ce3+-doped aluminate compositions whose emission ranges red Eu2+ emission from these nitride 2+ garnets) led to further investigation from ~498 nm for BaSi2O2N2:Eu to phosphors can be combined with 2+ 25 of alternate garnet compositions. One ~560 nm to CaSi2O2N2:Eu , with YAG:Ce in warm white LEDs for lamps route was to investigate silicate garnets QE > 85% at T >200oC for the green and fixtures that would normally 26 with the caveat that the composition (lmax~540 nm) SrSi2O2N2 phosphor. use incandescent lamps or CFLs. For space for silicate garnet synthesis at Eu2+-doped b-SiAlON has also been example, phosphor blends of YAG:Ce 2+ ambient pressure is smaller than that reported to be an efficient green and CaAlSiN3:Eu combined with blue for aluminate garnets. In spite of these phosphor.27 For orange and red LEDs can be used for high CRI, warm limitations, there has been progress phosphors, another important family white lamps. In addition, it is also finding silicate garnet phosphors with of materials are based upon Eu2+-doped possible to use all-nitride phosphor 3+ Ce emission maxima ranging from CaAlSiN3 hosts with a distorted wurtzite blends to make pcLEDs that cover a full 2+ 34,35 505-605 nm with room temperature structure. CaAlSiN3:Eu phosphors range of CCTs at CRI > 90. However, QEs that are comparable to YAG:Ce have a ~650 nm emission peak and QEs while (oxy)nitrides have significant phosphors16-18 that also give initial > 85% beyond 200oC (Fig. 3),28 and potential, there are some possible structure-property relationships for the emission maximum can be tuned drawbacks to (oxy)nitride phosphors. Ce3+ emission in garnets.19 It has down to 620 nm by Sr2+ substitution by First, the synthesis of these materials can also been shown that replacing making phosphors using high pressure be more difficult than for typical oxide Al3+(tetrahedral)-O2- with Si4+-N3- in nitriding of arc-melted alloys.29 phosphors. The refractory nature of the garnets leads to an additional red While many of these nitride Si3N4 often requires high temperature component in the emission spectra phosphors were based on known reactions (>1500oC), and many of the due to Ce3+ ions coordinated by N3- .20 (oxy)nitride compositions, there has precursor materials, such as alkaline- 24 However, any orange–red garnet also been investigation of nitride and earth nitrides or Si(NH2)2, require still has stronger high temperature oxynitride phase diagrams to discover glove-box handling due to reactions quenching compared to YAG:Ce. Since this additional quenching is relatively independent of composition, it may be intrinsic and points to a limitation for Ce3+-doped garnets for orange-red phosphors. Excitation Emission Nitride and oxynitride phosphors.—The λ =560 nm λ =450 nm energy position of Eu2+/Ce3+ 4fN-15d1 level em ex 4fN → 4fN-15d1 transitions in inorganic hosts is modified by the covalency 2+ 3+

and polarizability of Eu /Ce -ligand .) bonds.21 Therefore, ligands with a .u lower electronegativity compared to O2- (c(O)~3.4), such as S2- (c(S)~2.6) (a and N3- (c(N)~3.0) will lower the y it energy of the 4fN-15d1 levels making it more likely for Ce3+/Eu2+ absorption ens of InGaN LED radiation. There are t 3+ 2+ efficient Ce /Eu -doped sulfide In phosphors, but there are drawbacks that lead many pcLED manufacturers to move away from sulfide phosphors. First, sulfide phosphor synthesis could require toxic H2S atmospheres or may create H2S as a by-product. Second, many sulfide phosphors are moisture sensitive, degrading in high humidity 300 400 500 600 700 conditions unless phosphor particles are coated with a moisture barrier. Unlike sulfides, little work was done Wavelength (nm) on the luminescence of Eu2+/Ce3+ in nitrides until Hintzen and co-workers Fig. 2. Emission (l = 460 nm) and excitation (l = 560 nm) spectra of Y Al O :Ce3+(3%) 2+ ex em 3 5 12 discovered efficient Eu luminescence phosphor typically used in LED-based lighting.

The Electrochemical Society Interface • Winter 2009 33 Setlur (continued from previous page) with air or moisture. There has been progress in using air-stable precursors Ca2Si5N8 Sr2Si5N8 to make nitride phosphors,29,36 so this issue could be alleviated. However, there have also been reports in the patent Ca-αSiAlON literature that some of these Eu2+-doped nitride phosphors may lose efficiency 37 CaAlSiN over time in pcLED packages. As with .) 3

traditional CRT/LFL/CFL phosphors, .u

it is possible that improved synthesis (a

processes will improve the stability of y some nitride phosphors in pcLEDs. it

Oxide, oxyhalide, and halide phosphors.—

Apart from the newer nitride phosphors, ens there has also been progress in the t development of LED phosphors based In upon more traditional oxide and oxyhalide host lattices. The main set of non-garnet, oxide phosphors for pcLEDs the alkaline earth silicates, 2+ 2+ 2+ 2+ M2SiO4:Eu (M = Ba , Sr , Ca ) that were originally studied in the 1960s.38,39 2+ The emission of Ba2SiO4:Eu peaks at ~505 nm, and Sr2+/Ca2+ substitution 450 500 550 600 650 700 750 leads to a redshift in the emission until ~585-590 nm with the excitation Wavelength (nm) generally peaking in the violet and deep blue (Fig. 4). The excitation of the yellow phosphors in this family extend into ig l 2+ the blue spectral region, making these F . 3. Emission spectra ( ex = 405 nm) of Eu -doped nitride and oxynitride phosphors. phosphors an alternative to YAG:Ce. However, while these phosphors are Eu3+-pcLED phosphors where the only Summary and Future Outlook efficient at room temperature, their excitation bands overlapping with violet main drawback is the relatively strong or blue LED radiation are the forbidden This article is a brief summary of the quenching at elevated temperature Eu3+ 4f-4f transitions. Since these different pcLED phosphors that have o where QE(150 C)/QE(RT) = 60-65%. transitions have low oscillator strengths been developed over the past ten years. Another potentially interesting system (<10-6), the absorption of InGaN LED The discovery and optimization of these is based upon hosts with the Cs3CoCl5 radiation will be weak and micron- materials has involved significant work structure or variants of that structure. sized powders will strongly scatter by the various chemists, physicists, and 2+ Eu -doped materials in this family, incident LED radiation. In addition, materials scientists that are developing 2+ such as (Sr,Ca,Ba)3SiO5:Eu , have a these 4f-4f absorption transitions are these materials. However, while there 40 3+ yellow-orange emission, while Ce - sharp, making the phosphor excitation has been progress, there are still many 3+ doped materials, such as Sr2LaAlO5:Ce , sensitive to the LED wavelength. challenges and new phosphors to be 3+ 41 Sr3SiO5:Ce , or their solid solutions, There has been recent progress in discovered. It is also important to note have a yellow-green emission similar developing high efficiency, Eu3+-doped that the discovery of new phosphor to YAG:Ce. As is the case for the nanophosphors that could significantly compositions is one step toward pcLEDs 2+ M2SiO4:Eu phosphors, many of reduce phosphor scattering,44 so it will implementation with extensive process these phosphors have strong high be interesting to see their impact on optimization required to maximize temperature quenching, but perhaps the pcLED SSL. Mn4+-phosphors have the conversion efficiencies. These aspects compositional flexibility in this system 4 4 spin-allowed, parity forbidden A2 → T2 include various powder processing can lead to better pcLED phosphors. transition that has good absorption techniques and/or other ceramic There also have been reports of various of blue and violet LED radiation.45 For processing methods such as sintering46 2+ 42 oxyhalide phosphors doped with Eu , example, the commercial LFL phosphor or glass-ceramic formation47 to make but further work is also necessary to Mg-fluorogermanate:Mn4+ can improve uniform phosphor parts and plates. determine their utility. As more oxide the color rendering of pcLEDs based These aspects are not covered here but systems are shown to be suitable for on violet LEDs, and there are various are necessary to fully optimize pcLED pcLEDs, there could be more emphasis Mn4+-doped fluoride phosphors that efficacy. on (re)searching these compositional have high efficiency under blue LED spaces for new phosphors. excitation.45 It is important to note that Acknowledgments We close this discussion by noting Eu3+ and Mn4+ have slow decay times that the phosphors discussed to this (~1 ms for Eu3+, >3 ms for Mn4+), so 2+ 3+ This work was partially supported by point are Eu or Ce broadband emitters they are best used in remote phosphor (FWHM > 70 nm). There are advantages the U.S. Department of Energy through packages that reduce the incident flux contract # DE-FC26-06NT42934. This to use narrow-line phosphors (FWHM on the phosphor.43 Whether this limits < 10 nm) in the red spectral region since report was prepared as an account of the potential market for lamps and work sponsored by an agency of the this can simultaneously optimize color packages that use these phosphors is 2 United States Government. Neither rendering and lumens. Within LFL/ also an open question. CFL phosphors, there are two main the United States Government nor activators used to give efficient, red-line any agency thereof, nor any of their emission, Mn4+ and Eu3+. There have employees, makes any warranty, been many literature reports of potential express or implied, or assumes any

34 The Electrochemical Society Interface • Winter 2009 15. V. M. Bachmann, PhD Thesis, Utrecht Universiteit (2007). 16. A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandran, Excitation for Emission and M. V. Shankar, Chem. Mater., 2+ λ 18, 3314 (2006). (Sr,Ca) SiO :Eu ex=405 nm 2 4 17. Y. Shimomura, T. Honma, M. λ em=580 nm Shigeiwa, T. Akai, K. Okamoto, and N. Kijima, J. Electrochem. Soc., 154, J35 (2007).

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y 19. J. L. Wu, G. Gundiah, and A. it Cheetham, Chem. Phys. Lett., 441, 250 (2007). ns 20. A. A. Setlur, W. J. Heward, M. E. te Hannah, and U. Happek, Chem. In Mater., 20, 6277 (2008). 21. P. Dorenbos, Phys.Rev. B, 64, 125117 (2001); P. Dorenbos, Phys. Rev. B, 65, 235110 (2002). 22. J. W. H. van Krevel, J. W. T. van Rutten, H. Mandal, H. T. Hintzen, and R. Metselaar, J. Sol. St. Chem., 165, 19 (2002). 300 400 500 600 700 23. G. Botty, H. T. Hintzen, J. W. H. van Krevel, U.S. Patent # 6,682,663 Wavelength (nm) (2004); Y. Q. Li, J. E. J. van Steen, J. W. H. van Krevel, G. Botty, A. C. A. Delsing, F. J. DiSalvo, G. de 2+ 2+ 2+ 2+ With, and H. T. Hintzen, J. Alloys Fig. 4. Emission spectra (lex = 405 nm) of Eu -doped M2SiO4 (M = Ca , Sr , and/or Ba ) 2+ and Comp., 417, 273 (2006). phosphors and excitation spectra (lem = 580 nm) for (Sr,Ca)2SiO4:Eu . 24. W. Schnick, R. Bettenhausen, B. legal liability or responsibility for the 2. J. M. Phillips, M. E. Coltrin, M. Götze, H. A. Höppe, H. Huppertz, accuracy, completeness, or usefulness H. Crawford, A. J. Fischer, M. R. E. Irran, K. Köllisch, R. Lauterbach, of any information, apparatus, product, Krames, R. Mueller-Mach, G. O. M. Orth, S. Rannabauer, T. or process disclosed, or represents that Mueller, Y. Ohno, L. E. S. Rohwer, J. Schlieper, B. Schwarze, and F. its use would not infringe privately A. Simmons, and J. Y. Tsao, Laser & Wester, Z. Anorg. Allg. Chem., 629, owned rights. The views and opinions Photon. Rev., 1, 307 (2007). 902 (2003). of authors expressed herein do not 3. M. R. Krames, O. B. Shchekin, 25. Y. Q. Li, A. C. A. Delsing, G. de necessarily state or reflect those of R. Mueller-Mach, G. O. Mueller, With, and H. T. Hintzen, Chem. the United States Government or any L. Zhou, G. Harbers, and M. G. Mater., 17, 3242 (2005). agency thereof. Craford, J. Disp. Techn., 3, 160 26. V. Bachmann, T. Jüstel, A. (2007). Meijerink, C. Ronda, and P. J. About the Author 4. E. F. Schubert and J. K. Kim, Science, Schmidt, J. Lumin., 121, 441 308, 1274 (2005). (2006). 5. L. S. Rohwer and A. M. Srivastava, 27. N. Hirosaki, R.-J. Xie, K. Kimoto, Anant A. Setlur is a materials scientist T. Sekiguchi, Y. Yamamoto, T. at GE Global Research in the Ceramics Electrochem. Soc. Interface, 12(2), 36 (2003). Suehiro, and M. Mitomo, Appl. and Metallurgy Laboratory. His current Phys. Lett., 86, 211905 (2005). research interests involve the invention, 6. G. Blasse and B. C. Grabmaier, Luminescent Materials; Springer- 28. K. Uheda, N. Hirosaki, Y. development, and optimization of Yamamoto, A. Naito, T. Nakajima, luminescent materials for various lighting Verlag: Berlin (1994). 7. A. A. Setlur, J. J. Shiang, and U. and H. Yamamoto, Electrochem. and medical imaging applications with Happek, Appl. Phys. Lett., 92, Solid-State Lett., 9, H22 (2006). a primary focus on solid-state lighting 081104 (2008). 29. H. Watanabe, M. Imai, and N. phosphors. Dr. Setlur was previously the 8. T.-G. Kim, Y. S. Kim, and S.-J. Im, J. Kijima, J. Am. Ceram. Soc., 92, 641 Chair of the Luminescence and Display Electrochem. Soc., 156, J203 (2009). (2009). Division of ECS. He may be reached at 9. T. Jüstel, H. Nikol, and C. Ronda, 30. O. Oeckler, J. A. Kechele, H. Koss, P. [email protected]. Angew. Chem. Int. Ed., 37, 3084 J. Schmidt, and W. Schnick, Chem. (1998). Eur. J., 15, 5311 (2009). 10. Y. Shimizu, K. Sakano, Y. Noguchi, 31. C. Hecht, F. Stadler, P. J. Schmidt, References T. Moriguchi, U.S. Patent # J. Schmedt auf der Günne, V. 5,998,925 (1999). Baumann, and W. Schnick, Chem. 1. Multi-Year Program Plan FY’09- 11. G. Blasse and A. Bril, Appl. Phys. Mater., 21, 1595 (2009). FY’15: Solid-State Lighting Research Lett., 11, 53 (1967). 32. J. A. Kechele, C. Hecht, O. Oeckler, and Development, Lighting Research 12. V. Bachmann, C. Ronda, and A. J. Schmedt auf der Günne, P. J. and Development, Building Meijerink, Chem. Mater., 21, 2077 Schmidt, and W. Schnick, Chem. Technologies Program, Office of (2009). Mater., 21, 1288 (2009) Energy Efficiency and Renewable 13. W. W. Holloway and M. Kestigian, 33. K. Uheda, S. Shimooka, M. Energy, U.S. Department of Energy J. Opt. Soc. Am., 59, 60 (1969). Mikami, H. Imura, and N. Kijima, (2009). 14. J. M. Robertson, M. W. van Tol, W. Paper 3195, presented at PRiME H. Smits, J. P. H., Heynen, Philips J. 2008 Meeting, Honolulu, Hawaii, Res., 36, 15 (1981). October 12-17, 2008.

The Electrochemical Society Interface • Winter 2009 35 Setlur (continued from previous page) THE ELECTROCHEMICAL SOCIETY Monograph Series 34. R. Mueller-Mach, G. Mueller, M. R. Krames, H. A. Höppe, F. Stadler, W. The following volumes are sponsored by ECS, and published Schnick, T. Juestel and P. Schmidt, by John Wiley & Sons, Inc. They should be ordered from: ECS, Phys. Status Solidi A, 202, 1727 65 South Main St., Pennington, NJ 08534-2839, USA or (2005). www.electrochem.org/dl/bookstore.htm 35. N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki and R.-J. Xie, Appl. Phys. Lett., 90, 051109 (2007). 36. R.-J. Xie, N. Hirosaki, T. Suehiro, NEW! F.-F. Xu, M. Mitomo, Chem. Mater., Fuel Cells: Problems and Solutions 18, 5578 (2006). by V. Bagotsky 37. T. Jüstel, P. Huppertz, D. U. Wiechert, and D. Uhlich, World 320 pages. ISBN 978-0-470-23289-7 Patent Application # 2007/080541 (2007). 38. T. L. Barry, J. Electrochem. Soc., 115, Electrochemical Impedance Spectroscopy 1181 (1968). by M. E. Orazem and B. Tribollet (2008) 39. G. Blasse, W. L. Wanmaker, J. W. 524 pages. ISBN 978-0-470-04140-6 Ter Vrugt, and A. Bril, Philips Res. Rept., 23, 189 (1968). Fundamentals of Electrochemical Deposition 40. J. K Park, K. J. Choi, J. H. Yeon, S. (2nd Edition) J. Lee, and C. H. Kim, Appl. Phys. by M. Paunovic and M. Schlesinger (2006) Lett., 84, 1647 (2004). 41. H. S. Jang and D. Y. Jeon, Appl. Phys. 373 pages. ISBN 978-0-471-71221-3 Lett., 90, 041906 (2007); W. B. Im, N. N. Fellows, S. P. DenBaars, R. Fundamentals of Electrochemistry (2nd Edition) Seshadri, J. Mater. Chem., 19, 1325 Edited by V. S. Bagotsky (2005) (2009). 722 pages. ISBN 978-0-471-70058-6 42. J. Liu, H. Lian, C. Shi, and J. Sun, J. Electrochem. Soc., 152, G880 Electrochemical Systems (3rd edition) (2005). by John Newman and Karen E. Thomas-Alyea (2004) 43. Y. Zhu and N. Narendran, J. Light & 647 pages. ISBN 978-0-471-47756-3 Vis. Env., 32, 115 (2008). 44. M. Nyman, M. A. Rodriguez, L. E. Modern Electroplating (4th edition) Shea-Rohwer, J. E. Martin and P. P. Edited by M. Schlesinger and M. Paunovic (2000) Provencio, J. Am. Chem. Soc., 131, 888 pages. ISBN 978-0-471-16824-9 11652 (2009). 45. E. V. Radkov, L. S. Grigorov, A. A. Atmospheric Corrosion Setlur, and A. M. Srivastava, U.S. Patent # 7,497,973 (2009). by C. Leygraf and T. Graedel (2000) 46. H. Bechtel, P. Schmidt, W. Busselt, 3684 pages. ISBN 978-0-471-37219-6 B. S. Schreinemacher, Proc. SPIE, 7058, 70580E (2008); R. Mueller- Uhlig’s Corrosion Handbook (2nd edition) Mach, G. O. Mueller, M. R. Krames, by R. Winston Revie (2000). paperback O. B. Shchekin, P. J. Schmidt, 1340 pages. ISBN 978-0-471-78494-4 H. Bechtel, C.-H. Chen, and O. Steigelmann, Phys. Status Solidi Semiconductor Wafer Bonding RRL, 3, 215 (2009). by Q. -Y. Tong and U. Gösele (1999) 47. S. Fujita, A. Sakamoto, and S. 297 pages. ISBN 978-0-471-57481-1 Tanabe, IEEE J. Sel.Top. Quant. Elect., 14, 1387 (2008). Corrosion of Stainless Steels (2nd edition) by A. J. Sedriks (1996) 437 pages. ISBN 978-0-471-00792-0

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