ARTICLE pubs.acs.org/crystal
Thermally Induced Pore Formation in Epitaxial ZnO Films Grown from Low Temperature Aqueous Solution Jacob J. Richardson,*,† Gregory K. L. Goh,*,‡ Hong Quang Le,‡ Laura-Lynn Liew,‡ Fred F. Lange,†,§ and Steven P. DenBaars† † ‡
Materials Department, University of California Santa Barbara, Santa Barbara, California 93106, United States Institute of Materials Research and Engineering, Agency for Science, Technology, and Research (A*STAR), 3 Research Link, Singapore 117602 ABSTRACT: The formation of pores has been observed in epitaxial ZnO films deposited from low temperature aqueous solution when annealed at 300 °C and higher. The effects of these pores could have a dramatic impact on the ability to utilize low temperature aqueous synthesis to deposit ZnO films for optoelectronic devices. The results of thermogravimetric analysis, evolved-gas mass-spectrometry, and secondary ion mass spectrometry analysis indicate that the formation of pores is related to the expulsion of water vapor from the ZnO crystal. We propose that the pores form via the coalescence of zinc and oxygen vacancies that result from the incorporation, and subsequent removal, of a large concentration of hydroxide ions substituting for oxygen in the ZnO lattice. Despite the large change in solid volume undergone during pore formation, the ZnO films remain single phase and epitaxial and retain good electrical conductivity.
1. INTRODUCTION Many recent publications on ZnO have utilized the fact that it can easily be synthesized from low temperature aqueous solutions. The majority of this work has focused on the synthesis of ZnO nanowires,1 but it has also been demonstrated that epitaxial ZnO films can be produced using low temperature aqueous methods.2,3 Epitaxial films produced this way can have good conductivity and optical transparency and have been used as high quality transparent current spreading layers for GaN lightemitting diodes (LEDs).4 When doped with Ga, such ZnO films can even function as the n type layer in a heterojuction LED formed with p-GaN.5 For use in LEDs and many other electronic device applications, the thermal stability of ZnO films can be important for surviving the subsequent steps of device fabrication. Although ZnO is generally considered to have good thermal stability, the stability of ZnO produced by low temperature aqueous synthesis has not been reported. We have studied the effects of thermally annealing on epitaxial ZnO films produced by a low temperature aqueous method and observed a previously unreported phenomenon of internal pore formation. We show that the formation of pores during annealing can be correlated to a reduction of hydrogen impurities and an expulsion of water vapor from the ZnO. This behavior can be explained through a vacancy coalescence mechanism, similar to what has previously been observed in hydrothermally prepared perovskite materials that contain large concentrations of hydroxide ions substituting for oxygen in the lattice.6 This commonality suggests that the mechanism of pore formation could be relevant to other materials as well. r 2011 American Chemical Society
In the case of epitaxial ZnO films, the presence of pores could be detrimental to the transparency and conductivity, which could pose a significant problem for use in optoelectronic devices. Conversely, the formation of such voids could potentially be used to give beneficial light scattering or create high surface area mesoporous films useful in other applications.7 Therefore, understanding the formation of such pores, so they can be prevented or controllably formed, could be critical for the future utilization of ZnO films synthesized from aqueous solution.
2. EXPERIMENTAL SECTION Zinc oxide films were deposited from aqueous solution using the twostep process described in previous reports.2,4 The first step was used to nucleate a seed layer, and the second step was used to grow a thicker ZnO film. The seed layer deposition was performed by inserting a (111) MgAl2O4 single-crystal substrate into an aqueous solution of 26 mM zinc nitrate (Aldrich) and 300 mM ammonium nitrate (EMD), which had been preheated to 90 °C in a sealed fluoropolymer vessel. Insertion of the substrate was immediately followed by the addition of aqueous ammonia to initiate the precipitation of ZnO. The volume of 1.5 M aqueous ammonia added was determined by the amount needed to increase the pH of a room temperature calibration solution to ∼7.5. After ammonia addition, the vessel was resealed and returned to a 90 °C oven for at least 3 h before removing the substrate. Once removed, the ZnO seeded substrate was thoroughly rinsed with H2O and blown dry Received: April 25, 2011 Revised: June 7, 2011 Published: June 17, 2011 3558
dx.doi.org/10.1021/cg200528e | Cryst. Growth Des. 2011, 11, 3558–3563
Crystal Growth & Design
ARTICLE
Figure 1. XRD 2θ scans of epitaxial ZnO films as-grown and after a 600 °C anneal in air. In both scans, only the MgAl2O4 {111} peaks and the ZnO {0002} peaks are observed. before it was annealed in air at 500 °C for 2 h. The second growth step was performed by inserting the annealed substrates into a room temperature aqueous solution of 26 mM zinc nitrate, 7 mM sodium citrate (Aldrich), and 350 mM aqueous ammonia contained in a sealed PFA vessel. The sealed vessel was then heated in a 90 °C oven for at least 8 h. Longer growth periods for either the first or the second growth step did not appear to lead to any significant changes in film thickness or morphology. Deuterated ZnO films were synthesized due to the enhanced sensitivity for deuterium relative to hydrogen in secondary ion mass spectrometry (SIMS). To synthesize the deuterated ZnO films, the second step growth solution was simply prepared using 99% deuterium oxide (Aldrich) in place of ordinary deionized water. The ZnO powders used for thermogravimetric analysismass spectrometry (TGA-MS) were prepared by essentially the same method used to prepare the films. First, the precipitate formed during the seed layer deposition was separated from the growth solution by centrifugation. The growth solution was decanted away, and the solid was mixed with deionized water to dilute any remaining growth solution. The mixture was again centrifuged to separate the powder, which was then rinsed a second time with deionized water. After a third centrifugation, the water was again decanted away, and the powder was dried in a 40 °C vacuum oven. The resulting ZnO powder was then used to seed the growth of larger particles, via the second step growth process described above, by adding approximately 1 mg to the growth vessel in lieu of a seeded substrate. The resulting powder was separated and dried using the same process described above for the seed powder. The ZnO films were annealed in air using an alumina boat in an openended quartz tube furnace. Anneals were performed at 200, 300, 400, 500, and 600 °C with a heating rate of 1 °C/min and a hold period of 5 h. The thermogravimetric and evolved gas analysis of the ZnO powder samples were performed using a Mettler STARe TGA/sDTA851e coupled to a Balzers ThermoStar Gas Mass Spectrometer. Morphological characterization was performed using a FEI XL30 field emission SEM. X-ray diffraction (XRD) analysis of epitaxial films utilized a Phillips Panalytical X'Pert MRD Pro using a Cu KR source. SIMS depth profiles were performed using a Physical Electronics 6650 Quadrupole System using an oxygen primary ion beam of 100 nA at an accelerating potential of 5 kV.
3. RESULTS The single phase and epitaxial nature of the ZnO films deposited on MgAl2O4 were confirmed by the XRD scans shown in Figure 1. j scans (not shown) determined that the films
Figure 2. SEM images of epitaxial ZnO films (a, g) as deposited, (b, h) after annealing at 200 °C, (c, l) at 300 °C, (d, j) at 400 °C, (e, k) at 500 °C, and (f, l) at 600 °C. The right images are higher magnification from the approximate center of the film's thickness.
displayed the same ZnO [1120]||MgAl2O4 [112] and ZnO [0110]||MgAl2O4 [110] epitaxial relationship that has been reported previously for low temperature aqueous growth of ZnO on (111) spinel.3 Images obtained by scanning electron microscopy (SEM) analysis of the ZnO films after annealing are shown in Figure 2. These cross-sectional images were obtained from samples cleaved subsequent to annealing. The unannealed film and the film annealed at 200 °C, shown in Figure 2a,g and 2b,h, respectively, display no sign of pore formation. However, after 3559
dx.doi.org/10.1021/cg200528e |Cryst. Growth Des. 2011, 11, 3558–3563
Crystal Growth & Design
ARTICLE
Figure 4. TGA (black line) and evolved gas mass spectrometry (gray and dotted lines) results from a ZnO powder sample.
Figure 3. (a) SIMS depth profiles comparing the count ratio of deuterium (m/z = 2) to ZnO (m/z = 84) for an unannealed film and a 600 °C annealed film. (b) The average deuterium to ZnO counts across the depth profile as a function of annealing temperature.
annealing at 300 °C, pores are clearly visible in Figure 2c,i. The higher temperature anneals appear to produce a higher volume fraction of porosity as well as larger diameter pores. Although the films annealed at higher temperatures appear to have more large pores, there is still a high concentration of small pores. This may indicate that the pore nucleation process takes place for long periods of time even at the higher annealing temperatures. There did not appear to be any systematic variation of pore size or density across the thickness of the films. The SIMS depth profiles of the deuterated ZnO films revealed a ratio of deuterium to zinc concentration that varied with depth and annealing conditions. However, without an appropriate standard, it was not possible to use the SIMS to quantitatively analyze the deuterium/hydrogen concentration. Figure 3a shows the count ratio of deuterium (m/z = 2) to Zn68O16 (m/z = 84) as a function of etch time for an unannealed and a 600 °C annealed film. In the unannealed film, the concentration of deuterium relative to zinc appears to decrease linearly from the substrate interface to the top surface. The 600 °C annealed film shows a significantly lower ratio throughout the film thickness but especially at the surface. Figure 3b shows the same ratio, but averaged over the entire thickness of the film, as a function of annealing condition. The deuterium content appears to drop slightly after the 200 °C anneal but much more significantly after a 300 °C anneal. The subsequent decreases after the higher annealing conditions (400, 500, and 600 °C) are then relatively minor. Figure 4 displays the results of the TGA-MS analysis of the ZnO powder, which was performed using a flowing UHP argon
Figure 5. Hall measurement obtained carrier concentration and mobility results for films annealed at different temperatures.
atmosphere and a heating rate of 20 °C/min. The powder begins to lose an appreciable amount of mass above around 200 °C. This corresponds with an increase in the H2O (m/z = 18) mass spectrum signal. The H2O (m/z = 18) mass spectrum signal then has three distinct maxima, at approximately 265, 360, and 600 °C. Each peak in the H2O signal appears to roughly correspond to an increase in the rate of mass loss at the same temperature. However, it should be noted that to obtain a reasonable signalto-noise ratio for the H2O mass spectrum signal, it was necessary to use a large amount of sample and a high heating rate, both of which diminish the accuracy of the measurement with respect to actual sample temperature. Similarly, the TGA measurement also loses accuracy with respect to temperature, so that the mass loss events are smeared over larger temperature ranges. Also shown in Figure 4 is the CO2 (m/z = 44) mass spectrum signal, which is roughly an order of magnitude lower than that of H2O, but still clearly displays peaks at 385 and 545 °C. Figure 5 shows the results of Van der Pauw type Hall measurements performed on ZnO films after different annealing conditions. As compared to the as-grown film, the film annealed at 200 °C showed a higher carrier concentration and mobility, which increased to 2.4 1019 cm3 from 4.7 1018 cm3 and 23 cm2 V1 s1 from 13 cm2 V1 s1, respectively. After the 300 °C anneal, the carrier concentration decreased to 1.8 1019 cm3, but mobility was significantly improved to 3560
dx.doi.org/10.1021/cg200528e |Cryst. Growth Des. 2011, 11, 3558–3563
Crystal Growth & Design
ARTICLE
where n is the free electron concentration. Hydroxide substitution is predicted to act as a shallow donor in ZnO,11 and it has been shown that the n type conductivity observed in films produced from low temperature aqueous solution is related to hydrogen incorporation.12 However, as the Fermi level increases with higher concentrations of hydrogen donors, zinc vacancies are more likely to join free electrons on the right side of eq 1.13 Thus, high incorporation of hydroxide will lead to compensating Zn vacancies, as shown in Figure 6a. On the basis of the SIMS depth profile, which showed a reduction in deuterium concentration after heating, and the detection of H2O evolved from ZnO powder during heating, it appears that the hydrogen in the ZnO films is expelled as water vapor. This is shown schematically in Figure 6b. The formation of water from the hydroxide in the ZnO lattice would be expected to follow the quasi-chemical reaction 2OHO • f H2 OðgÞ + V O •• + OO x
ð2Þ
The resulting oxygen vacancies could then combine with existing zinc vacancies to form free space according to V Zn 00 + V O •• f void Figure 6. Schematic diagram of pore formation via the coalescence of vacancies as water is removed. Zn vacancies are formed to compensate for hydroxide substituting for O (hydrogen interstitial) (a). Oxygen vacancies form when hydrogen is removed from the lattice in the form of water vapor (b). Oxygen vacancies then react with Zn vacancies to form vacancy pairs, which cluster into pore nuclei (c).
62 cm2 V1 s1. Both carrier concentration and mobility then decreased with further annealing, falling, respectively, to 7 1018 cm3 and 18 cm2 V1 s1 after the 600 °C anneal.
4. DISCUSSION The presence of lattice hydroxide has been correlated to the formation of pores in other oxides synthesized from aqueous solution, most notably in BaTiO3 powders produced hydrothermally, where intragranular pores have been observed after heating above 400 °C.810 Hennings et al. have proposed that these pores form as a result of the dehydration of the BaTiO3 powder.6 They propose that a large amount of hydroxide ions are incorporated into the powders during synthesis through substitution on the oxygen site. To maintain charge balance, compensating cation vacancies are also incorporated into the BaTiO3 crystals. As water is driven out during heating, the removal of oxygen results in oxygen vacancies that combine with the existing cation vacancies to form void space. When ZnO is synthesized from aqueous solution, one might expect a similar mechanism to take place. Such a mechanism is shown schematically in Figure 6. Hydroxide substitution on the oxygen site, which can also be represented as a hydrogen interstitial, is believed to have low formation energy,11 and aqueous synthesis clearly presents a source of hydroxide ions. If hydrogen is assumed to be the dominant extrinsic impurity, and self-interstitial and antisite defects are ignored, the relevant charge neutrality expression can be expressed using Kr€oger Vink notation as 1 1 ½OHO • + ½V O •• + ½V O • ¼ ½V Zn 00 + ½V Zn 0 + n 2 2
ð1Þ
ð3Þ
The overall reaction therefore would be 2OHO • + V Zn 00 f H2 OðgÞ + OO x + void
ð4Þ
At the relatively low temperatures used in the aqueous synthesis and the subsequent anneals, the equilibrium concentration of Schottky defects, that is, a zinc and oxygen vacancy pair, is expected to be low.14 As a result, appreciable concentrations of zinc and oxygen vacancies together are unstable, and there should be a large thermodynamic driving force for eq 3 to move forward. However, the low temperature of synthesis would limit diffusion and prevent these vacancies from reaching each other until annealed. In fact, void formation appears to correlate well with the temperature at which first principle calculations predict zinc vacancies to become mobile.15 The surface energy would be minimized by zinc and oxygen vacancies recombining at the surface of the film, but the lower mobility of oxygen vacancies15 may prevent this from occurring. As a result, eq 3 leads to empty unit cells that coalesce to nucleate pores in the bulk of the epitaxial ZnO film. This is shown schematically in Figure 6c and is seen experimentally in the SEM images of Figure 2. The SEM, SIMS, and TGA-MS results all seem to be in agreement that the formation of the pores is correlated with the loss of H2O starting between 200 and 300 °C. If all weight loss is assumed to be due to H2O generation, then estimates can be made for the starting concentration of hydrogen in the as-grown ZnO. If all of the hydrogen present in the ZnO is assumed to be in the form of OH substituting for O, that is, interstitial H, and that charge compensation is accomplished solely by the presence of Zn vacancies, then the relevant reaction can be represented simply as Zn10:5x Hx O f 0:5xH2 O + ð1 0:5xÞZnO
ð5Þ
From the TGA results, the as-grown ZnO powder loses approximately 1.2% of the original mass when heated. Solving for x in eq 5 based on this result gives an initial OH concentration of about 10%. Assuming that the lattice constants of the starting and final ZnO are relatively similar, which they appear to be by XRD, this would correspond to a loss in volume of about 5%. Examining the SEM images in Figure 2, the relative volume of the 3561
dx.doi.org/10.1021/cg200528e |Cryst. Growth Des. 2011, 11, 3558–3563
Crystal Growth & Design pores appears to be at least this high after annealing at the higher temperature conditions. Although the position of the ZnO peaks in the XRD scans does not appear to shift significantly after annealing, a broadening of the peak width is observed between the scan of the 600 °C annealed film to that of the unannealed film in Figure 1. This may be due to increased strain in the film generated by the volume change. A 10% level of OH substitution is out of the range of what is typically considered an impurity and into the range of what is better characterized as an alloy between ZnO and Zn(OH)2. There are numerous phases of Zn(OH)2, and while they are all considered to be thermodynamically unstable with respect to ZnO and water, it is well-known that it is possible to synthesize several of them by precipitation from an aqueous solution of a zinc salt.16 However, on the basis of the XRD and SEM, there is every indication that the as-grown films are single phase and maintain the Wurtzite crystal structure and the lattice parameters characteristic of ZnO. Although Zn(OH)2 is known to form an amorphous phase, which would be invisible to XRD, this will typically convert to more stable crystalline hydroxide phases or ZnO when aged in solution. In air, the dehydration of Zn(OH)2 to ZnO is typically observed around 125 °C,17 much lower in temperature than the weight loss events observed for our ZnO powders. However, Shaporev et al. have reported an additional weight loss event around 200 °C when heating zinc hydroxide prepared using ammonia to cause precipitation from aqueous solution.18 They attributed this weight loss to the decomposition of zinc hydroxycarbonates, but on the basis of the relatively low CO2 concentration in the evolved gas, this explanation seems unlikely to be occurring here. In addition, the peaks in the H2O and CO2 signals occur at different temperatures and thus could not be caused by decomposition of a hydroxycarbonate that would release large amounts of both simultaneously. While the decomposition of carbonaceous species cannot be ruled out as a source of weight loss and pore formation, the combined evidence of the TGA, evolved gas analysis, SEM, XRD, and SIMS indicate that dehydration of single phase Wurtzite material is the primary mechanism. It is also interesting to note that the TGA-MS results clearly show several water evolution events at different temperatures. This may indicate the presence of hydrogen in the form of multiple defect types with different stabilities.19 However, according to eqs 24, the creation water may also be controlled by the ability of zinc and oxygen vacancies to combine with each other. Therefore, it is worth noting the similarities between the temperatures predicted for migration of VZn00 , VO••, and VOx by first principles,15 266, 382, and 636 °C, respectively, and the peaks in water evolution at 265, 360, and 600 °C. The electrical properties of the as-grown and annealed ZnO films also seem to agree with the proposed mechanism of pore formation. The initial Hall carrier concentration of less than 1019 is clearly far below the concentration of hydrogen in the as-grown films. This indicates that most of the hydrogen must be compensated by other defects or exist in the form of electrically inactive defects.11,20 After annealing at 200 °C, both the carrier concentration and the carrier mobility have increased. This may in part be due to the activation of surface conductivity, which has been observed after annealing ZnO.21 The change in carrier concentration could also be due to electrically inactive hydrogen defects transforming into electrically active ones. It is also possible that the 200 °C anneal may provide some degree of structural improvement, by allowing atomic disorder, which has been kinetically trapped by the low synthesis temperature to be
ARTICLE
corrected. This would obviously improve the mobility but could also increase the apparent carrier concentration by improving the homogeneity of the film.22,23 After the 300 °C anneal, the carrier concentration has dropped, as one might expect if the hydroxide being driven from the film were the main electron donors. The electron mobility then also begins to drop above 300 °C, where the increasing volume fraction of the pores must begin to impede the flow of electrons. Currently, it is unclear if the type of pore formation seen in this work is common in ZnO prepared by low temperature aqueous synthesis or specific to the conditions used in these experiments. However, there are several aspects of the nature of most work on the aqueous synthesis of ZnO, and of the pore formation itself, that could have allowed it to be previously unrecognized. For one, there is no phase change accompanying the pore formation that is measurable by XRD. Also, although the pores are obvious in the cross-sectional SEM images, the exposed surface of the films remained smooth and unbroken by any pores until the highest annealing conditions. This is expected since the vacancies near the surface will diffuse to it rather than nucleating a pore, but it can make pores invisible in SEM images unless the ZnO was fractured after annealing. If the pores grow large enough, they will scatter visible light, noticeably transforming a transparent ZnO film into a translucent or opaque one. However, most work on low temperature aqueous synthesis deals with polycrystalline films or nanostructures, which already scatter visible light. In addition, very little work appears to have even been performed looking at the effects of annealing on ZnO synthesized from aqueous solution. With all of that being said, it is also very possible that synthesis conditions could have a strong influence on the incorporation of hydroxide substitutions and vacancies in ZnO prepared from aqueous solution and thus on pore formation. In this case, other aqueous synthesis techniques in the literature may not lead to ZnO crystals that form internal pores upon annealing, but it is certainly worth examining further.
5. CONCLUSION We have shown that large amounts of hydrogen are being incorporated into epitaxial ZnO films produced using a low temperature aqueous synthesis technique and that pores form when this hydrogen is expelled as water vapor. We propose that the pores result from the coalescence of zinc vacancies, present to electrically compensate the hydrogen, and oxygen vacancies, which result from the formation of water vapor. This defectbased mechanism is supported by the fact that there is a large change in solid volume with no evidence of a phase transformation. This phenomenon has a strong effect on both the transparency and the electrical conductivity of the ZnO films and could thus be very important if low temperature aqueous synthesis were to be used to produce ZnO films for optoelectronic applications. The fact that a similar mechanism for pore formation appears to be operating in both ZnO and hydrothermally prepared perovskite powders, two structurally and chemically different oxides that have been prepared under different pH and temperature conditions, may be an indication that this phenomenon is more prevalent in materials prepared from aqueous solution than previously recognized. Further work is needed to address the potential generality of this phenomenon to other oxides prepared from aqueous solution, as well as how different synthesis conditions can affect the formation of pores in ZnO. 3562
dx.doi.org/10.1021/cg200528e |Cryst. Growth Des. 2011, 11, 3558–3563
Crystal Growth & Design
’ AUTHOR INFORMATION Corresponding Author
*Tel: 805-637-1647. Fax: 805-893-8486. E-mail: jakejr@ engineering.ucsb.edu (J.J.R.) or
[email protected] (G.K.L.G.). Notes §
Deceased.
’ ACKNOWLEDGMENT We thank Dr. Anderson Janotti for his valuable input regarding defects in ZnO and Dr. Tom Mates for assisting with the SIMS measurements. This work was supported by the National Science Foundation under Grant No. 095254. This work also made use of the MRL Central Facilities supported by the MRSEC Program of the NSF under Award No. DMR0520415; a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org).
ARTICLE
concentrated zinc sulfate solutions with aqueous ammonia. Russ. J. Appl. Chem. 2010, 83 (11), 1925–1929. (18) Shaporev, A.; Ivanov, V.; Baranchikov, A.; Polezhaeva, O.; Tret'yakov, Y. ZnO formation under hydrothermal conditions from zinc hydroxide compounds with various chemical histories. Russ. J. Inorg. Chem. 2007, 52 (12), 1811–1816. (19) Janotti, A.; Van de Walle, C. G. Hydrogen multicentre bonds. Nat. Mater. 2007, 6 (1), 44–47. (20) Shi, G. A.; Saboktakin, M.; Stavola, M.; Pearton, S. J. “Hidden hydrogen” in as-grown ZnO. Appl. Phys. Lett. 2004, 85 (23), 5601–5603. (21) Look, D. C.; Mosbacker, H. L.; Strzhemechny, Y. M.; Brillson, L. J. Effects of surface conduction on Hall-effect measurements in ZnO. Superlattices Microstruct. 2005, 38 (46), 406–412. (22) Bierwagen, O.; Ive, T.; Van de Walle, C. G.; Speck, J. S. Causes of incorrect carrier-type identification in van der Pauw--Hall measurements. Appl. Phys. Lett. 2008, 93 (24), 242108. (23) Matsumura, T. A Theoretical Study on Van Der Pauw Measurement Values of Inhomogeneous Compound Semiconductor Thin Films. J. Mod. Phys. 2010, 01 (05), 340–347.
’ REFERENCES (1) Sunandan, B.; Joydeep, D. Hydrothermal growth of ZnO nanostructures. Sci. Technol. Adv. Mater. 2009, 10 (1), 013001. (2) Andeen, D.; Kim, J. H.; Lange, F. F.; Goh, G. K. L.; Tripathy, S. Lateral Epitaxial Overgrowth of ZnO in Water at 90 °C. Adv. Funct. Mater. 2006, 16 (6), 799–804. (3) Andeen, D.; Loeffler, L.; Padture, N.; Lange, F. F. Crystal chemistry of epitaxial ZnO on (111) MgAl2O4 produced by hydrothermal synthesis. J. Cryst. Growth 2003, 259 (12), 103–109. (4) Thompson, D. B.; Richardson, J. J.; Denbaars, S. P.; Lange, F. F. Light Emitting Diodes with ZnO Current Spreading Layers Deposited from a Low Temperature Aqueous Solution. Appl. Phys. Express 2009, 2, 042101–042101. (5) Le, H.; Lim, S.; Goh, G.; Chua, S.; Ang, N.; Liu, W. Solution epitaxy of gallium-doped ZnO on p-GaN for heterojunction lightemitting diodes. Appl. Phys. B: Lasers Opt. 2010, 100 (4), 705–710. (6) Hennings, D. F. K.; Metzmacher, C.; Schreinemacher, B. S. Defect chemistry and microstructure of hydrothermal barium titanate. J. Am. Ceram. Soc. 2001, 84 (1), 179–182. (7) Gr€atzel, M. Mesoporous oxide junctions and nanostructured solar cells. Curr. Opin. Colloid Interface Sci. 1999, 4 (4), 314–321. (8) Lee, J. J.; Park, K. J.; Hur, K. H.; Yi, S. C.; Koo, S. M. Synthesis of Ultrafine and Spherical Barium Titanate Powders Using a Titania NanoSol. J. Am. Ceram. Soc. 2006, 89 (10), 3299–3301. (9) Noma, T.; Wada, S.; Yano, M.; Suzuki, T. Analysis of lattice vibration in fine particles of barium titanate single crystal including the lattice hydroxyl group. J. Appl. Phys. 1996, 80 (9), 5223–5233. (10) Nakano, H. Microstructural evolution by in-situ TEM observations of oxides. J. Ceram. Soc. Jpn. 2011, 119 (1385), 1–7. (11) Van de Walle, C. G. Hydrogen as a cause of doping in zinc oxide. Phys. Rev. Lett. 2000, 85 (5), 1012–1015. (12) Zhang, Y. B.; Goh, G. K. L.; Ooi, K. F.; Tripathy, S. Hydrogenrelated n-type conductivity in hydrothermally grown epitaxial ZnO films. J. Appl. Phys. 2010, 108 (8), 083716–083714. (13) Janotti, A.; Van de Walle, C. G. Fundamentals of zinc oxide as a semiconductor. Rep. Prog. Phys. 2009, 72 (12), 126501–126501. (14) Kr€oger, F. A. The Chemistry of Imperfect Crystals; NorthHolland Pub. Co., Interscience Publishers: Amsterdam, New York, 1964. (15) Janotti, A.; Van de Walle, C. G. Native point defects in ZnO. Phys. Rev. B 2007, 76 (16), 1–22. (16) Baes, C. F.; Messmer, R. E. The Hydrolysis of Cations; John Wiley & Sons Inc.: New York, 1976. (17) Voropanova, L.; Baratov, L.; Kovalev, V. Dehydration of zinc hydroxide precipitate produced by neutralization of weakly 3563
dx.doi.org/10.1021/cg200528e |Cryst. Growth Des. 2011, 11, 3558–3563
