ARTICLE pubs.acs.org/crystal
Design of Epitaxially Strained Ag Film for Durable Ag-Based Contact to p-Type GaN Jun Ho Son, Hak Ki Yu, Yang Hee Song, Buem Joon Kim, and Jong-Lam Lee* Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, 790-784, Korea ABSTRACT: We demonstrated a novel contact structure of a silver (Ag) reflector that had an enhanced thermal stability at an elevated temperature. The Ag film was epitaxially grown on GaN by the insertion of an ultrathin Ni contact layer (∼10 Å). The Ag film in the Ni/Ag structure, which was tensilely strained due to the expansion of the lattice constant by oxidation of Ni (3.532 Å) to NiO (4.176 Å), could compensate for the thermal compressive stress of 0.83 GPa that developed at the interface of Ag with GaN at 500 C. The tensile stress, as determined by high resolution X-ray diffraction, was 0.24 GPa for a single period of the Ni/Ag contact, but it increased to 0.65 GPa for triple periods of the Ni/Ag contact, resulting in further suppression of Ag agglomeration and improved long-term thermal stability.
’ INTRODUCTION GaN-based light-emitting diodes (LEDs) are allowing a new era in solid-state lighting due to their high efficiency, long lifetime, and environmental friendliness.1 3 For general illumination applications, the light conversion efficiency of LEDs should be further improved. The vertical-structure design based on the laser lift-off technique is promising for high-efficiency and high-power LEDs.4,5 In vertical-LEDs (V-LEDs), Ag-based reflective ohmic contacts to p-type GaN have been widely used to improve light extraction efficiency and reduce energy consumption.6,7 However, Ag agglomeration and/or diffusion caused by annealing and/or heat generated during device operation at high current densities leads to degradation in both electrical and optical properties of LEDs.8,9 Hence, to prevent agglomeration and/or diffusion of the Ag film during the fabrication process and device operation must be a key factor to improve the device reliability and performance. There are two representative mechanisms that describe Ag agglomeration: bulk diffusion and surface diffusion of Ag atoms. The bulk diffusion of Ag atoms is enhanced with increases in vacancies in the Ag film.10 Annealing in ambient air increases the vacancy concentration because the vacancies uptake the substitutional oxygen atoms into interstitial sites due to the strong interaction between oxygen atoms and vacancies, leaving vacancies behind. Therefore, by adding dopant atoms with high oxygen reactivities to Ag films, the thermal stability was significantly enhanced due to the suppression of oxygen vacancies and the subsequent bulk diffusion.11,12 The surface diffusion of Ag atoms is driven by surface energy and surface stress.13,14 For the Ag film, the (111)-oriented grains are energetically the most stable. Therefore, the thermal stability of the Ag film could be enhanced r 2011 American Chemical Society
with an epitaxially grown Ag film with the (111) orientation15,16 due to the reduction of surface energy. However, Ag agglomeration has still been found with this configuration because the thermal compressive stress, which is generated during annealing at high temperature due to the difference of the coefficients of thermal expansion (CTE) between the film and substrate, is quite large,17 and this is another driving force for surface diffusion, resulting in the excessive diffusion of Ag atoms and severe subsequent agglomeration. Here, we demonstrate the Ni/Ag multilayer structure to enhance the thermal stability of a Ag-based ohmic contact to p-type GaN by reducing the thermal compressive stress. The thermal compressive stress could be compensated by the residual tensile stress in the Ag layer. The microstructural properties of the contact were investigated by high-resolution transmission electron microscopy (HR-TEM) and high-resolution X-ray diffraction (XRD). Combining the microstructural analysis with experimental data of the surface morphology, contact resistivity, and light reflectance, it could be concluded that the reduction of thermal compressive stress in the Ag film plays a critical role in enhancing the thermal stability of the Ag-based ohmic contact to p-GaN.
’ EXPERIMENTAL SECTION Mg-doped p-type GaN films grown by metal organic chemical vapor deposition (MOCVD) were used in this work. Details on the wafer are described in previous works.16 For measurements of specific contact Received: July 4, 2011 Revised: September 20, 2011 Published: September 21, 2011 4943
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Table 1. Changes of Contact Resistivity and Light Reflectance at 460 nm Wavelength for the Ag, Ni/Ag Single-Layer, and Ni/Ag Multi-Layer Contacts As Functions of Annealing Temperature Ag
Ni/Ag/Ni (Ni/Ag single-layer)
annealing temperature
contact resistivity
reflectance
(C)
(Ω cm2)
(%)
as-deposited
3.2 10
1
300 350
2.4 10 2.2 10
5
400
3.7 10
3
450
5.1 10
2
500
3.9 10
1
4
contact resistivity (Ω cm2)
Ni/Ag/Ni/Ag/Ni/Ag/Ni (Ni/Ag multilayer)
reflectance
contact resistivity
reflectance
(%)
(Ω cm2)
(%)
95
6.2 10
1
86.8
4.8 10
1
87
82.9 76.1
2.1 10 3.4 10
3
85.7 85.2
5.1 10 5.3 10
3
86.5 85.9
65.4
1.9 10
5
84.6
1.2 10
5
85.2
51.3
8.2 10
6
84.2
6.2 10
6
84.8
31.6
2.8 10
5
82.8
1.2 10
5
84.6
5
5
45 was collimated to a photomultiplier tube. A Ag mirror with the certified reflectance of ∼95% in the wavelength range of interest was used as the reflectance standard. A PHILIPS XL30S scanning electron microscope was used to examine the surface of contacts after annealing. An HR-TEM image was collected using a Cs-corrected JEM 2200FS operated at 200 kV. The TEM specimen was prepared by the focused ion beam (FIB) method. The thickness of the Ni contact layer for the TEM specimen was 100 Å, because a thickness of 10 Å is too low for obtaining clear interface images. High-resolution X-ray diffraction (HRXRD) and reciprocal space mapping (RSM) were performed using the highresolution four-axis X-ray diffractometer (Philips X’Pert PRO) with a 4-bounce crystal and a 3-bounce Ge (220) analyzer crystal at the Korea Basic Science Institute (KBSI).
Figure 1. Changes of (a) ratios, Ft/Fo, and (b) light reflectance at 460 nm for the Ag, Ni/Ag single-layer, and Ni/Ag multilayer contacts as functions of annealing time at 500 C in ambient air. resistivity using the transmission line method (TLM), active regions were defined by inductively coupled plasma of Cl2/BCl3 gas, followed by dipping samples into a boiling aqua regia solution of HCl/HNO3 (3:1) to remove surface oxides.18 TLM test structures with 100 50 μm2 pads were patterned on the surface-treated samples using a photoresist. Prior to metal deposition, all the samples were dipped in a HCl/DI (1:1) solution for 2 min. After the HCl treatment, Ag (1800 Å), Ni/Ag/Ni (10/1800/ 20 Å, Ni/Ag single-layer), and Ni/Ag/Ni/Ag/Ni/Ag/Ni (10/600/10/ 600/10/600/20 Å, Ni/Ag multilayer) metals were deposited by electron beam evaporation under a base pressure of 2 10 7 Torr. After the metals deposited on the photoresist were removed, the samples were annealed at temperatures ranging from 300 to 500 C for 1 min in ambient air. Current voltage (I V) characteristics of the contacts were examined using a HP4156 semiconductor parameter analyzer. The reflectance values of the contacts were measured using a tungsten-halogen lamp and a monochromator. The reflected beam at an incidence angle of
’ RESULTS AND DISCUSSION To estimate the feasibility of applying the Ni/Ag multilayer contact to GaN-based V-LEDs, the contact resistivity and light reflectance at 460 nm for Ag, Ni/Ag single-layer, and Ni/Ag multilayer contacts were measured, and the results are listed in Table 1. The Ni/Ag multilayer contact exhibited the lowest contact resistivity, 6.2 10 6 Ω cm2, after annealing at 450 C. Furthermore, the Ni/Ag multilayer contact maintained a high light reflectance compared to the Ag and Ni/Ag single-layer contacts at annealing temperatures beyond 400 C. Figure 1a,b shows the change of specific contact resistivity and light reflectance at 460 nm as functions of annealing time at 500 C to evaluate the thermal stability of the contacts. The contact resistivity of each sample, Ft, was normalized with that after annealing for 1 min, F0, and plotted as Ft/F0. For the Ag contact, the value of Ft/F0 drastically increased by a factor of 440 after annealing for 30 min. Moreover, the Ag contact exhibited nonlinear I V behavior after annealing for 3 h. However, for the Ni/Ag single-layer contact, the value of Ft/F0 increased only by a factor of 133, while it was further reduced to 45 in the Ni/Ag multilayer contact after annealing for 24 h. The light reflectance of the Ag and Ni/Ag single-layer contacts degraded to 21.6% and 72.9%, respectively, after annealing for 24 h. However, the Ni/Ag multilayer contact exhibited a high reflectance of 81%. These results demonstrate that the Ni/Ag multilayer structure plays a role in suppressing degradation of the contact. Scanning electron microscopy (SEM) images showing the surface morphologies of Ag, Ni/Ag single-layer, and Ni/Ag multilayer contacts after annealing are presented in Figure 2. After annealing for 1 min, the surface of the Ag contact was very irregular. Furthermore, the Ag film was fully isolated due to severe agglomeration after annealing for 24 h. On the other hand, the Ni/Ag 4944
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Figure 2. SEM micrographs for the (a) Ag, (b) Ni/Ag single-layer, and (c) Ni/Ag multilayer after annealing for 1 min, and (d) Ag, (e) Ni/Ag singlelayer, and (f) Ni/Ag multilayer after annealing for 24 h. Annealing was carried out at 500 C in ambient air.
Figure 3. Off-axis phi scans of the Ag, Ni/Ag single-layer, and Ni/Ag multilayer contacts for the GaN (102), Ni (220), and Ag (311) reflections before and after annealing at 500 C for 1 min in ambient air.
single-layer and Ni/Ag multilayer contacts similarly exhibited smooth surface morphology after annealing for 1 min. However, after annealing for 24 h, the Ni/Ag multilayer contact exhibited a smoother morphology compared to that of the Ni/Ag single-layer contact. These results agree well with the contact resistivity and light reflectance results. After annealing for 1 min, the Ni/Ag singlelayer and Ni/Ag multilayer contacts exhibited similar contact resisivities and the difference in the light reflectances was only about 2%. However, after annealing for 24 h, the Ni/Ag multilayer contact clearly exhibited a lower contact resistivity and higher light reflectance than the Ni/Ag single-layer contact.
Figure 3 shows the off-axis phi scans, performed for the GaN (102), Ni (220), and Ag (311) reflection before and after annealing at 500 C. For the as-deposited state, no defined symmetry is shown in the Ag contact. However, for the Ni/Ag single-layer and multilayer contacts, 6-fold symmetries that are the same as the hexagonal GaN epilayer are shown and maintained after annealing, indicating the epitaxial growth of the Ag film on the p-type GaN with a thin Ni contact layer.16,19,20 After annealing, the Ag contact also exhibits a weak 6-fold symmetry due to the diffusion of Ag atoms to the (111) plane, aligning to the hexagonal GaN substrate. It is noteworthy that the 6-fold 4945
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Figure 4. (a) HAADF image and EDS composition mappings of Ag, Ni, and O for the Ni/Ag multilayer contact after annealing at 500 C for 1 min in ambient air. HR-TEM micrograph near the interface between (b c) p-type GaN and NiO layer (area 1 and 2) and (d) NiO and Ag layer (area 3). Insets are the fast Fourier transformed patterns corresponding to the selected areas.
)
the reciprocal space map (RSM) technique. Figure 5 shows outof-plane RSM patterns for the three contacts before and after annealing at 500 C for 1 min in ambient air. Compared to the Ag contact, the Ag (111) peak of the Ni/Ag single-layer contact exhibited a small fwhm in the qx direction, indicating the increased crystallinity.21,22 However, for the Ni/Ag multilayer contact, the crystallinity decreased a little compared to the Ni/Ag single-layer contact, and this could be attributed to the thin Ni interlayer. Moreover, the fwhm in the qz direction significantly increased in the Ni/Ag multilayer contact due to a smaller crystallite size and a higher nonuniform in-plane strain.23 The nonuniform strain means that the interplanar spacing, d, on the top and bottom side is different.24 The peak broadening in the d-spacing observed from the RSM is associated with a decrease of the crystallite size or an increase of the nonuniform strain. In general, the crystallite size increases during annealing due to grain growth, resulting in peak sharpening. Therefore, an increase of fwhm in the qz direction could be attributed to the increase of nonuniform strain, which is accompanied by the formation of an NiO layer during oxidation annealing. The in-plane strain of the Ag layer could be calculated from the out-of-plane strain using the equation, ε = (1 υ)/(2υ)ε^, where υ is Poisson’s ratio, ε is the in-plane strain, and ε^ is the out-of-plane strain. However, this method is only suitable for the case of a fully strained film. Therefore, we performed symmetric and asymmetric XRD for the Ni/Ag single-layer and Ni/Ag multilayer contacts as functions of annealing temperature to quantitatively measure the in-plane strain. Asymmetric (200) reflection was selected, because the {111} planes normal to the substrate cause the (200) diffraction at a tilt angle (ψ) of 54.74. The symmetric/asymmetric XRD profiles of the Ni/Ag singlelayer and Ni/Ag multilayer contacts are shown in Figure 6, panels a and 6b, respectively. The calculated in-plane stress of the )
symmetry of the Ni (220) reflection was not shown after annealing. Figure 4a shows the high-angle annular dark-field (HAADF) scanning transmission electron microscopy atomic contrast images and energy dispersive spectrometer (EDS) composition mappings of the cross section of the Ni/Ag multilayer contact after annealing at 500 C in ambient air. EDS results showed that the distributions of the nickel atoms coincide with the oxygen atoms, indicating the formation of NiO in the Ni/Ag multilayer contact because the formation of NiO is thermodynamically favorable, as previously reported.16 The disappearance of 6-fold symmetry of the Ni (220) reflection in Figure 3 could be caused by oxidation of Ni into NiO. The 2θ peak positions of Ni (220) and NiO (220) are 76.37 and 75.442, respectively. Therefore, after annealing, in-plane symmetry of Ni (220) for the Ni/Ag single-layer and multilayer contacts was not shown due to the large difference in peak position between Ni (220) and NiO (220) planes. Furthermore, the layer-by-layer structure of the Ni/Ag multilayer contact was maintained after annealing. Figure 4b,c shows the HR-TEM micrographs near the interface between the p-type GaN and NiO layers (area 1 and 2), respectively. Some regions of the NiO layer exhibit singlecrystalline structures, as shown in Figure 4b, and others exhibit partial polycrystalline structures (Figure 4c). This could be due to the relaxation of the strain induced by lattice expansion of Ni (3.523 Å) into NiO (4.176 Å) during the annealing process. However, the Ag layer exhibits a single-crystalline structure over the entire Ag layer, as shown in Figure 4d. These results (Figures 3 and 4) clearly indicate that the Ag film is epitaxially grown on the p-type GaN with crystallographic alignments of NiO[111]//Ag [111]//GaN[0002]. In order to examine the crystallinity and strain distribution of the contacts, three metallization schemes were investigated using
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Figure 5. Out-of-plane reciprocal space map (RSM) for the as-deposited (a) Ag, (b) Ni/Ag single-layer, and (c) Ni/Ag multilayer contacts and annealed (d) Ag, (e) Ni/Ag single-layer, and (f) Ni/Ag multilayer contacts at 500 C for 1 min in ambient air.
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)
)
)
compressive stress during annealing. It is well-known that a metal film deposited on a substrate with a different thermal expansion coefficient develops thermal stress during annealing.17 The thermal stress, σth, induced by the different CTE between the film and substrate is given by σth = (αf αs)/(1 υ)EΔT, where αf and αs are the linear CTE of the film and the substrate, respectively, E is Young’s modulus of the film material, υ is Poisson’s ratio of the film, and ΔT is the temperature difference. In the case of Ag films deposited on GaN, a thermal stress of 0.83 GPa is generated during annealing at 500 C due to the large difference in CTE between GaN (5.6 10 6/C) and Ag (18.9 10 6/C), and this stress is compressive in nature. This thermal stress is a driving force for grain boundary grooving, hillock formation, and, finally, agglomeration of the Ag film.25 For the Ni/Ag single-layer and multilayer contacts, the in-plane stresses were calculated to be 0.24 and 0.65 GPa, respectively (Figure 6), using the equation, σ = E/(1 υ)ε , where E is Young’s modulus of Ag (83 GPa), υ is Poisson’s ratio of Ag (0.367), ε is the in-plane strain, σ is the in-plane stress, and these stresses are tensile. The lattice constant of NiO (4.176 Å) is larger than that of Ni (3.523 Å), and the formation of NiO is thermodynamically favorable. Therefore, the tensile stress to the Ag layer could be induced by oxidation annealing. Thus, the large residual tensile stress in the Ni/Ag multilayer contact could compensate for the thermal compressive stress at high temperatures, resulting in enhancement of the thermal stability (Figure 1) in an Ag-based ohmic contact to p-type GaN and finally thermal reliability of V-LEDs, as shown in Figure 7. )
Ni/Ag single-layer and Ni/Ag multilayer contact from the XRD results are shown in Figure 6c. As the annealing temperature increases, the in-plane tensile stress significantly increased in the Ni/Ag multilayer contact. As a result, after annealing at 500 C, the tensile stress of the Ag layer in the Ni/Ag multilayer contact increased to 0.65 GPa compared to the Ni/Ag single-layer contact (0.24 GPa). This large tensile stress in the Ag layer could be due to the formation of the NiO interlayer during annealing, because the lattice constant of NiO (4.176 Å) is larger than that of Ni (3.523 Å). To investigate the effect of the reflective p-contact on the thermal reliability of V-LEDs, the changes of the forward voltage (Figure 7a) and electroluminescence (EL) spectra (Figure 7b) for the V-LEDs with Ni/Ag single-layer and multilayer contacts as functions of annealing time were measured at a 350 mA injection current. Before thermal stress, the V-LEDs with the Ni/ Ag multilayer contact exhibited lower forward voltage and higher light output power than those of V-LEDs with a Ni/Ag singlelayer contact due to the lower contact resistivity and the higher light reflectance (Table 1). As the annealing time increased, V-LEDs with the Ni/Ag multilayer contact exhibited less thermal degradation in forward voltage compared to those with the single-layer contact. Furthermore, the decrease of light output power in V-LEDs with the Ni/Ag multilayer contact was smaller than that of V-LEDs with the Ni/Ag single-layer contact after annealing for 24 h. The significant improvement in thermal stability of the Ni/Ag multilayer contact is attributed to relaxation of the thermal
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Figure 6. High-resolution symmetric and asymmetric XRD profiles for the (a) Ni/Ag single-layer and (b) Ni/Ag multilayer contacts as functions of annealing temperature. (c) Changes of in-plane stress for the contacts as a function of annealing temperature.
Figure 7. Changes of (a) forward voltage and (b) electroluminescence (EL) spectra for the V-LEDs with Ni/Ag single-layer and multilayer contacts at a 350 mA injection current as functions of annealing time. Annealing was carried out at 250 C in ambient air.
’ CONCLUSION In conclusion, we demonstrated the modulation of the residual stress in a Ag film using a Ni/Ag multilayer structure to improve the Ag agglomeration resistance and the thermal stability of the film at high temperatures. Because of the increased lattice constant of 19% by oxidation of Ni into a NiO layer, the Ag layer in the Ni/Ag multilayer structure exhibited tensile residual stress (0.65 GPa) after annealing. This residual tensile stress reduces the thermal compressive stress originating from the difference in the CTE between GaN and Ag. Ultimately, the Ag film on the Ni/Ag multilayer has a smaller total compressive stress (0.18 GPa)
compared to a Ag film’s total compressive stress (0.83 GPa) on a pure Ag layer because the thermal compressive stress was compensated by the residual in-plane tensile stress of 0.65 GPa. Because of the reduced total stress on the Ni/Ag multilayer, the surface migration of Ag atoms is suppressed, resulting in significant improvement of the thermal stability and device reliability.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +82(0)54-279-2152. Fax: +82(0)54-279-5242. E-mail: jllee@ postech.ac.kr. 4948
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’ ACKNOWLEDGMENT This work was supported in part by the Industrial Technology Development Program funded by the Ministry of Knowledge Economy (MKE, Korea) and in part by the World Class University (WCU) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (Project No. R31-10059). ’ REFERENCES (1) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Nat. Photonics 2009, 3, 180. (2) Guo, W.; Zhang, M.; Bhattachatya, P.; Heo, J. Nano Lett. 2011, 11, 1434. (3) Kuo, M.-L.; Kim, Y.-S.; Hsieh, M.-L.; Lin, S. Y. Nano Lett. 2011, 11, 476. (4) Wierer, J. J.; David, A.; Megens, M. M. Nat. Photonics 2009, 3, 163. (5) Kim, S.; Jang, J.-H.; Lee, J.-S. J. Electrochem. Soc. 2007, 154, H973–H976. (6) Song, J.-O.; Ha, J.-S.; Seong, T.-Y. IEEE Trans. Electron Devices 2010, 57, 42. (7) Chang, L.-B.; Shiue, C.-C.; Jeng, M.-J. Appl. Surf. Sci. 2009, 255, 6155–6158. (8) Song, J.-O.; Kwak, J. S.; Park, Y.; Seong, T.-Y. Appl. Phys. Lett. 2005, 86, 062104. (9) Chae, S. W.; Kim, D. H.; Kim, T. G.; Ko, K. Y.; Sung, Y. M. Appl. Phys. Lett. 2007, 90, 201113. (10) Mathieu, G.; Gas, P.; Combe-Brun, A.; Bernardini, J. Acta Metall. 1983, 31, 1661. (11) Song, Y. H.; Son, J. H.; Jung, G. H.; Lee, J.-L. Electrochem. SolidState Lett. 2010, 13, H173. (12) Son, J. H.; Jung, G. H.; Lee, J.-L. Appl. Phy. Lett. 2008, 93, 012102. (13) Mullins, W. W. J. Appl. Phys. 1957, 28, 333. (14) Srolovitz, D. J. Acta Metall. 1989, 36, 621. (15) Song, Y. H.; Son, J. H.; Yu, H. K.; Lee, J.-H.; Jung, G. H.; Lee, J. Y.; Lee, J.-L. Cryst. Growth Des. 2011, 11, 2559. (16) Son, J. H.; Song, Y. H.; Yu, H. K.; Lee, J.-L. Appl. Phys. Lett. 2009, 95, 062108. (17) Malgas, G. F.; Adams, D.; Nguyen, P.; Wang, Y.; Alford, T. L.; Mayer, J. W. J. Appl. Phys. 2001, 90, 5591. (18) Kim, J. K.; Lee, J.-L.; Lee, J. W.; Shin, H. E.; Park, Y. J.; Kim, T. Appl. Phys. Lett. 1998, 73, 2953. (19) Liu, Q. Z.; Lau, S. S.; Perkins, N. R.; Kuech, T. F. Appl. Phys. Lett. 1996, 69, 1722–1724. (20) Kim, J. K.; Jang, H. W.; Kim, C. C.; Je, J. H.; Rickert, K. A.; Kuech, T. F.; Lee, J.-L. J. Vac. Sci. Technol. B 2003, 21, 87–90. (21) Yang, S.; Lin, B. H.; Liu, W.-R.; Lin, J.-H.; Chang, C.-S.; Hsu, C.-H.; Hsieh, W. F. Cryst. Growth Des. 2009, 9, 5184–5189. (22) Li, G.; Yang, H. Cryst. Growth Des. 2011, 11, 664–667. (23) An, S. J.; Hong, Y. J.; Yi, G.-C.; Kim, Y.-J.; Lee, D. K. Adv. Mater. 2006, 18, 2833. (24) Cullity, B. D. in Elements of X-Ray Diffraction, 2nd ed.; Addison Wesley Publishing Co.: Reading MA, 1978; Chapter 9. (25) Kim, H. C.; Theodore, N. D.; Alford, T. L. J. Appl. Phys. 2004, 96, 5180.
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