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The roles of buffer layer thickness on the properties of the ZnO epitaxial films
Accepted Manuscript Title: The roles of buffer layer thickness on the properties of the ZnO epitaxial films Author: Kun Tang Shimin Huang Shulin Gu Shunming Zhu Jiandong Ye Zhonghua Xu Youdou Zheng PII: DOI: Reference:
S0169-4332(15)02542-8 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.123 APSUSC 31604
To appear in:
APSUSC
Received date: Accepted date:
5-8-2015 17-10-2015
Please cite this article as: K. Tang, S. Huang, S. Gu, S. Zhu, J. Ye, Z. Xu, Y. Zheng, The roles of buffer layer thickness on the properties of the ZnO epitaxial films, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Full-length article corresponding to the 1st ICASS Poster No. P1.42 or Abstract Ref. No. ICASS2015_0356
The roles of buffer layer thickness on the properties of the ZnO epitaxial films Kun Tanga*[email protected], Shimin Huanga, Shulin Gua*[email protected], Shunming Zhua, Jiandong Yea,b, Zhonghua Xua, Youdou Zhenga a
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Nanjing National Laboratory of Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China b Nanjing University Institute of Optoelectronics at Yangzhou, Yangzhou 225009, China
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Abstract In this article, the authors have investigated the optimization of the buffer thickness for obtaining high-quality ZnO epi-films on sapphire substrates. The growth mechanism of the buffer with different thickness has been clearly revealed, including the initial nucleation and vertical growth, the subsequent lateral growth with small grain coalescence, and the final vertical growth along the existing larger grains. Overall, the quality of buffer improves with increasing thickness except the deformed surface morphology. However, by a full-scale evaluation of the properties for the epi-layers, the quality of the epi-film is briefly determined by the surface morphology of the buffer, rather than the structural, optical, or electrical properties of it. The best quality epi-layer has been grown on the buffer with a smooth surface and well-coalescent grains. Meanwhile, due to the huge lattice mismatch between sapphire and ZnO, dislocations are inevitably formed during the growth of buffers. More importantly, as the film grows thicker, the dislocations may attracting other smaller dislocations and defects to reduce the total line energy and thus result in the formation of V-shape defects, which is connected with the bottom of the threading dislocations in the buffers. The V-defects appear as deep and large hexagonal pits from top view and they may act as electron traps which would affect the free carrier concentration of the epi-layers. Keywords Zinc oxide, Epitaxy, Sapphire, Buffer layer, MOCVD, Material characterizations 1. Introduction The development of technologies to improve the material quality is always a hot topic in the research of a specific functional material. The optimization is unlimited since defects and dislocations always exist. For the ZnO material studied in this article, the material quality is far from the device-ready level. [1] The control of defects and dislocations is always problematic. Thanks to the similarity between GaN and ZnO in terms of structure and properties, the study on ZnO always uses the successful experience of GaN for reference. For instance, the epitaxial growth of GaN on the lattice-mismatched sapphire substrates has achieved a tremendous success, which have led to the commercialization of GaN-based blue light-emitting diodes (LEDs) and laser diodes (LDs). Therefore, sapphire has been an economic and most popularly used substrate for the hetero-epitaxial growth of ZnO. [2-3] Due to the large lattice mismatch and distinctive thermal expansion coefficient between sapphire and GaN, [4] a low-temperature buffer layer is indispensably employed for the epitaxy of GaN on sapphire in order to buffer the strain and dislocations induced by the lattice mismatch. [5-6] For the epitaxy of 1 Page 1 of 17
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ZnO on sapphire, various buffer layers, like GaN, MgO, ZnS, and ScAlMgO4, have been utilized. [4, 7-9] However, the ZnO homogeneous buffer is always preferred since the epitaxial growth on an optimized ZnO homo-buffer would be quite similar as a homo-epitaxy. [10] The lattice mismatch could be released by strains and forming dislocations. It is easy to understand that, when the mismatch between buffer and substrate is sufficiently small, the buffer that can fully release the mismatch by strains should be relatively thick. On the contrary, when the mismatch is large, the strain energy would quickly reach a critical value wherein dislocations are energetically favorable to be formed. In this case, the buffer with no dislocations should be relatively thin. Therefore, a critical thickness has been defined as the thickness at which the first dislocation is formed. The existence of this critical thickness was first detailed by Van der Merwe [11], and modulated by Matthews [12] and People and Bean [13]. For ZnO buffer deposited on c-plane sapphire, there is a significant lattice mismatch (18.3%) between them with a 30 o domain orientation (ZnO [10-10] // Al2O3 [11-20]) [14]. Utilizing the calculation method reported by Fischer et al, [15-17] the critical thickness for ZnO is estimated to be only 0.63 nm, corresponding to 1~2 layers of atoms. As a result, the formation of dislocations in the buffer is unavoidable. It is obvious that the properties of the buffer layer will influence those of the epi-layer. Especially for the thickness, if the buffer layer is too thin, high density of dislocations and rough surface will be formed which is not ideal for the following epitaxy. While if the buffer layer is too thick, the surface is also rough and the dislocations will grow larger due to the over-matured grains and suppressed horizontal growth. Therefore, an appropriate buffer layer thickness is of great importance for the quality of the epi-layer. In fact, several research groups have done works on the optimization of the buffer thickness for further ZnO epitaxy. [10, 18-25] Table I lists their brief results. It can be noticed that the optimized values for the thickness vary among different reports, which is possibly due to distinctive growth method and conditions. Therefore, the roles of buffer thickness on the properties of the ZnO epi-layers are still unclear and require further investigation. In this article, the morphological, optical, structural, and electrical properties of the buffers with three different thickness and the epi-layers grown on the corresponding buffer have been extensively investigated, respectively. The surface smoothness, rather than the other properties, has been proposed as the dominate factor for the optimized growth for the ZnO epi-layer. Besides, the correlation between the formation of the morphological pits (V-defects) observed on the epi-layers and the existence of threading dislocations in the buffers has been established. The formation and properties of the V-defects have also been proposed. 2. Experiments 2.1. Sample preparation Samples discussed in this article were grown on c-plane sapphire substrate by a home-built metal-organic chemical vapor deposition (MOCVD) system with close coupled-showerhead configuration. The details of the MOCVD system have been reported and can be referenced elsewhere. [26] A four-step process for the high-temperature epitaxial growth of ZnO has been established as follows. (1) The as-received sapphire substrates were pretreated for 5 minute at 1100 ºC in nitrogen and hydrogen environment in order to make the sapphire surface ready for growth. Details regarding the pretreatment of sapphire substrates could be referenced in a previous paper of our group. [27] (2) Low-temperature ZnO buffer layers were grown on the pretreated sapphire substrates at 470 ºC using high-purity DMZn [(CH3)2Zn] as Zn precursor and tert-BuOH [(CH3)3C-OH] as O precursor. The growth duration lasts for 5 min, 15 min, and 30 min, which are marked as samples A1, B1 and C1, 2 Page 2 of 17
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respectively. (3) ZnO epi-layers (marked as samples A2, B2, and C2) were grown on the corresponding buffer layers at 850 ºC for 1 hour with N2O as O precursor.
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2.2. Characterizations The thickness of the above samples was measured by a thin film measurement system (Avantes Co.) with optical interference. The crystalline quality was analyzed by high-resolution x-ray diffraction (HRXRD, Philips X'pert Pro diffractometer equipped with a Cu-ray source, λ ~ 0.15405nm). The surface morphology was observed by atomic force microscopy (AFM, Nanoscope IIIa, Digital Instruments Inc.). The photoluminescence (PL) spectra were recorded at room temperature, excited by a He-Cd laser (λ=325nm) with a spectrum resolution of 0.2 nm. A cyro-genic system was employed for the helium-temperature and temperature-dependent PL measurements. The vibrational properties of the samples were recorded by a JOBIN YVON HR800 UV Raman system in the backscattering geometry excited by an Ar+ laser (λ = 514.5 nm) at room temperature. A Hall-effect system (Keithley Instruments Inc.) was utilized for the electrical property measurement, which were performed at room temperature under van der Pauw’s four-point-probe configuration with indium as electrodes. The capacitance-voltage (C-V) measurement was carried out by profiling the barrier capacitance of the Schottky contact between a mercury probe and the sample surface as a function of an applied DC bias. 3. Results and discussion I: The properties of the ZnO buffers with different thickness The thickness data for samples A1-C1 are 78, 133, and 295 nm, respectively, and the corresponding growth rates are calculated as 13, 7.5, and 9.8 nm per minute. The non-constant growth rate indicates that the growth mode is not unique during the 30 minutes time. In order to reveal the reason for the different growth rate, the surface morphology for the buffer samples are thus measured and shown in Fig. 1(a)-1(c). Fig. 1(a), representing the 5-min growth, shows the morphology just after initial nucleation stage with high density of tiny pits, indicating the start of the coalescence of initial grains. The highest growth rate for this stage (stage I) suggests that the growth mode is dominated by vertical growth. It is easy to understand this due to the huge lattice mismatch between sapphire and ZnO. As calculated in the introduction, the critical thickness of ZnO on sapphire in only 1~2 atomic layer, indicating that the surface energy must be released by forming dislocations as the buffer grows thicker. [28-29] In this case, the initial nucleation should be poor and loose. It is well-acknowledged that the atomic stacking along c-axis on nuclei is much easier than the lateral move of atoms on sapphire and attachment of atoms on the edges of the nuclei. [30] As a result, the vertical growth rate is higher than the lateral coalescence rate, leading to the observed surface morphology of sample A1 with a relatively rough surface (RMS ~ 7.62 nm). During the subsequent 10 min growth, the surface morphology has been improved. It can be seen from Fig. 1(b) that the grains become larger and well coalescent, giving a relatively flat surface (RMS ~ 3.79 nm). Combining the reduced growth rate with the observed morphology, it is suggested that the lateral growth dominates in this stage (stage II). The enhanced lateral growth could be understood as follows. (1) The initial growth have filled the spaces of sapphire surface, making the growing frontier homogeneous. Consequently, the adsorption rate of the ad-atoms on the homogenous growing surface must be higher than that on sapphire surface. (2) As the coalescence of grains continues, the distance between nearby grain edges is shorter, making the distance of atoms moving along the surface to the grain edges much shorter. In this case, the desorption rate of the ad-atoms is reduced and the ad-atoms are thus much easier to grow laterally. 3 Page 3 of 17
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Further increase of the growth time for another 15 min (sample C1) again roughens the surface (RMS ~ 12.18 nm) as shown in Fig. 1(c). The morphology shows larger hexagonal grains as well as larger pits and the growth rate increases to 9.8 nm/min, which indicates that the growth mode is dominated by vertical growth again. It could be understood and explained by the relatively low substrate temperature employed for the buffer growth. As the grains basically finish coalescence at stage II, the resultant lateral growth rate would drop. The low migration rate of ad-atoms on the growing surface caused by the low substrate temperature would then make the new growth preference along the existing grains vertically, rather than laterally, forming the final morphology as shown in Fig. 1(c). The evolution of the surface morphology has been summarized in Fig. 1(e) schematically, which clearly exhibits that in order to obtain a flat surface, the growth duration should be controlled at the time-point when the initial coalescence finishes.
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Although the flattest surface has been obtained by a 15-min buffer growth, the other properties of the buffers do not follow the trend of the roughness. Fig. 2 shows the room-temperature PL spectra for all the buffer samples. Sample A1 consists of two emission components at 380 nm (peak-I) and 390 nm (peak-II), respectively, whereas the emission peak at 380 nm dominates in the spectra of samples B1 and C1. The peak-II is generally thought to relate to the high level of strains and dislocations in the transitional layer at the ZnO/sapphire interface. The peak-I is ascribed to the free exciton recombination from well-aligned ZnO lattice. The detection of peak-II in sample A1 is due to that its thickness is comparable to the penetration depth of the 325-nm exciting laser (~ 60 nm), which can collect the interfacial information. Furthermore, the intensity of peak-I increases monotonically as the buffer grows thicker, indicative of better optical and crystalline quality. In fact, the improved optical and crystalline quality has been double-confirmed by Raman spectra and XRD. Fig. 3 shows the Raman spectra recorded for all the buffer samples. As can be seen from the figure, except the modes from sapphire, ZnO E2 modes are obviously detected. The intensity of the E2(L), E2(H)-E2(L), and E2(H) modes monotonically increases as the buffer grows thicker. Since the E2(L) or E2(H) mode is a standard to evaluate the quality of the zinc or oxygen sub-lattice, [31] the ratio of E2(L)/E2(H), standing for the relative improving degree of zinc and oxygen sub-lattices, has been drawn out and plotted in the inset of Fig. 3 for a deeper understanding of the crystalline quality improvement. From sample A1 to B1, the increased ratio indicates that the improvement of zinc sub-lattice is better than that of oxygen sub-lattice during the lateral coalescence of small grains. From sample B1 to C1, the decreased ratio further indicates that the improvement of oxygen sub-lattice can be enhanced during the growth of larger grains. It is proposed that the difference of the oxygen sub-lattice improvement relative to zinc sub-lattice at different growth stage is related to the different mobility and adsorption/desorption rates when the growth happens laterally or vertically. Nevertheless, the quality of both sub-lattices improves with growing thicker. Fig. 4 shows the XRD and XRC results measured on all the buffer samples. Only ZnO (0002) and (0004) peaks are detectable in the XRD patterns, showing a perfect c-axis preferential orientation. Furthermore, the intensities of the (0002)/(0004) XRD peaks and the (0002) XRC peak increase monotonically as growing the buffer thicker, which are also indications of better crystalline quality. The inset of Fig. 4(b) plots the FWHM values of the XRC (0002) peaks for all samples. From sample A1-B1, a sharp decreases of the FWHM value could be seen, indicating a prominent improvement of the crystalline quality during the grain coalescence stage. However, from sample B1 to C1, there is only a slight decrease of the FWHM 4 Page 4 of 17
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values, indicating that the lateral coalescence of small grains plays a key role in determining the crystalline quality of the buffer. The residual carrier concentration drops with the increment of buffer thickness as shown in Fig. 5. The residual carriers may have two origins: (1) intrinsic donor-like defects; (2) extrinsic dopants, like hydrogen and aluminum. The drop of the electron concentration should be related to the reduction of the intrinsic and extrinsic defects. On one hand, as shown by the above characterizations, the crystalline quality improves as the buffer becomes thicker, leading to reduced concentration of intrinsic defects. On the other hand, as the thickness of the buffer increases, the electrical contribution from the aluminum atoms diffused from the sapphire substrate becomes less. It is therefore proposed that these two aspects have resulted in the observed Hall results.
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4. Results and discussion II: The roles of the buffer thickness on the properties of the ZnO epitaxial films
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Provided the understanding of the properties for the buffers, the roles of buffer thickness on ZnO epi-layer growth could be investigated via characterizing the epitaxial films grown on the different buffers A1-C1. First of all, the thickness of the epi-layers (A2-C2) has been measured and fitted as 492, 880, and 1059 nm, respectively. By subtracting the thickness of the buffers, the corresponding growth rates for the epitaxial stage are calculated as 14.0, 25.0, and 25.3 nm per minute. The growth rate of sample A2 is relatively low, possibly due to that the growth of A2 should finish the lateral coalescence of small grains first before further growth. In the condition of high-temperature epi-growth, the surface diffusivity of ad-atoms has been greatly enhanced, favoring the lateral growth. Just due to the high mobility of ad-atoms, the new nucleation at the space between different small grains is considered to be much more difficult than the direct growth at the edge of the existing grains. Consequently, the lower growth rate for sample A2 is attributed to the lateral coalescence of grains. The saturated growth rate for samples B2 and C2 indicates that when the coalescence of grains finishes, the growth on larger grains dominates the epitaxial growth process, regardless of the surface morphology of the buffers.
The surface morphology of the epi-film samples A2-C2 is shown in Fig. 6(a)-6(c). It is obvious to see the deep hexagonal pits with increased size and decreased density. In fact, similar morphologies have been frequently observed in the GaN/InGaN MQWs structures consisting of a threading dislocation terminated by a hexagonal pit. This kind of structures is nominated as V-shape defects. [32-35] For ZnO, Liu et al have reported that the threading dislocations are not uniformly distributed in the ZnO layer but aggregate to form annular regions around columnar epitaxial cores. [36] Several groups have reported that a threading dislocation is connected with the bottom of the V-defect and causes the formation of the V-defect. [37-38] This kind of defects can extend from the sample surface to a few hundred nanometers (300–800 nm) into the bulk. [39] The hexagonal pits observed on samples A2-C2 accords well with the descriptions of the V-defects stated by previous literatures. The pits are therefore ascribed to the V-defects. Moreover, the pits for the buffers, representing the dislocations, [28-29] also shows an increase in size and decrease in density as shown in Fig. 1(a)-1(c), which is similar to the variation trend of the size and 5 Page 5 of 17
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density for the epitaxial samples. Combined with the abovementioned previous literatures, it is possible that the V-defects might inherit from the dislocations formed during the buffer growth. Weber et al have pointed out that dislocations can be attracted to the V-defects to decrease total line energy. [39] Based on this conclusion, although the V-defects originate from the dislocations in buffers, they may grow larger in size by attracting small dislocations when the film grows thicker, which results in larger sizes of the hexagonal pits after 1-h epitaxial growth as shown in Fig. 6(a)-6(c).
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Fig. 6(d) shows the extracted RMS roughness excluding the pit areas. Sample B2 owns a minimum value of 5.33 nm. Actually, other structural and optical characterizations have confirmed that sample B2 has the best property among the three epitaxial films. Fig. 7 shows the room-temperature PL spectra for samples A2-C2. The NBE emission intensity of B2 is the highest, indicative of best optical quality. Fig. 8 shows the Raman spectra. After 1-h epitaxial growth, the intensity of the E2 modes is generally enhanced to a great extent as compared with Fig. 3. Besides, the intensity of the E2 modes increases as the buffer thickness increases. However, the E2(L)/E2(H) ratio shows an opposite trend with the smallest value for sample B2, which indicates that the improvement of the oxygen sub-lattice on sample B2 is the best compared with that of the zinc sub-lattice. XRD patterns for samples A2-B2 [Fig. 9(a)] show a monotonic increase of the ZnO (0002) and (0004) diffraction peaks, according perfectly with the Raman spectra. Meanwhile, the inset shows the calculated coherent lengths from the Scherrer’s equation. [40] The coherent lengths for the epitaxial films are 2 ~ 4 times longer than those for the buffers (207-282 nm), indicative of conspicuous improvement to the grain size and crystalline quality after 1-h epi-growth. At the same time, sample B2 holds the maximum coherent length of ~ 850 nm, which is a sign of best crystalline quality. It is also consistent with the highest Hall mobility of 72 cm2/Vs measured on sample B2. The XRC results shown in Fig. 9(b) further support that sample B2 is of best quality. Although the intensity of XRC increases as the buffer thickness increases, the FWHM value for sample B2 is the lowest. Considering the properties for the buffers, it can be concluded that an appropriate buffer thickness with a well-coalescent and flat surface, rather than with the best crystalline, optical, or electrical quality, is the decisive factor to the quality of the high-temperature epi-grown films. The electron carrier concentration of the epitaxial films has been measured and shown in Fig. 10. It is intriguingly found that the values are almost unchanged to those of the buffers as shown in Fig. 5. Considering that the crystalline quality for the epitaxial films is much better than the buffers, it is reasonable that the buffers dominate the electrical properties of samples A2-C2. In order to reveal and distinguish the electrical properties of the epi-layers, low-temperature PL spectra as well as C-V measurement have been carried out. Fig. 11(a) shows the PL spectra of samples A2-C2 measured at 10 K, and the temperature-dependence of the PL peaks for sample C2 is depicted in Fig. 11(b). Generally, three peaks could be seen from the PL spectra. Regarding the assignment, the peak at 3.362 eV is commonly ascribed to excitons bound to neutral donors (D0X), the peak at 3.368 eV is ascribed to the excitons bound to ionized donors (D+X), and the peak at 3.377 eV is ascribed to free excitons (FX). [41-42] The assignments are valid through analyzing the temperature dependence of these peaks in Fig. 11(b) that the thermal quenching of D0X and D+X is quite rapid and a clear trend of the transformation from D0X/D+X to FX could be seen. It is clear that sample A2 is dominated by the D0X component whereas the peak for sample B2 slightly shift to higher energy side and much broadened. For sample C2, D+X component dominates the spectrum with a shoulder peak at 3.362 eV. The gradual 6 Page 6 of 17
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emergence of D+X and the disappearance of D0X from sample A2 to C2 indicates that the electron concentration near the surficial region decreases, since the air/surface interface would easily cause a band bending and thus a surficial depletion (D+) when the electron carrier concentration is low. In this case, the D+X is also known as SX (surface exictions).
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The low-temperature PL spectra seem to demonstrate that the electron concentration for the epitaxial films near the surface also decreases with increasing the buffer thickness. In order to double-check this issue, C-V measurement have been employed. Fig. 12 shows the measured C-V curves for samples A2-C2. By extracting the slope of the C-2-V, the doping concentration has been estimated as in the order of 1018, 1017, and 1014 cm-3 for samples A2, B2, and C2, respectively. These values are much smaller as compared with the electron carrier concentration measured by Hall-effect (1*1019, 3*1018, and 8*1017 cm-3), indicating that the electrical contribution from the epi-layers could be neglected and the electrical properties are dominated by the buffers. The reduction of electron concentration from sample A2 to C2 could be partially attributed to the improved crystalline quality, meaning less concentration of intrinsic and extrinsic defects as discussed in SEC. 3. Actually, the larger V-defects might also contribute to the reduction of carrier concentration. Based on the correlation between edge threading dislocations and SCM mapping, Liu et al have concluded that the edge threading dislocations would introduce interface trap densities in the annular region. [36] Besides, Müller et al have analyzed the phase of the transmitted beam by electron holography, indications for a high negative line charge at threading dislocations have been found in ZnO epi-layer. [43] In addition, Cherns and Jiao [44] have reported a negatively charged dislocation core in n-GaN when they applied electron holography in a transmission electron microscope to detect the electrostatic potential in the vicinity of single dislocations in GaN. Yokoyama et al [45] have directly observed the carrier depletion around a dislocation in GaP by scanning spreading resistance microscopy. These prior literatures have suggested that the electrons might be trapped along the threading dislocations. As the gradual formation of threading dislocations and V-defects, the donor-like defects, like interstitial zinc (Zni) and thermally diffused aluminum (Al) might be aggregated to the dislocations. Due to the higher diffusivity of Zni and Al, it is easy to form a high-density distribution of Zni and Al along the threading dislocations and V-defects. However, inside the grains, the concentration of Zni and Al would be lower. In this scheme, as the V-defects grow larger, the defects contributing to electron carriers would be gradually aggregated to the V-defects, and therefore, more free electrons are trapped, resulting in a reduced electron concentration for the epi-layers. The schematic diagram for the proposed structures for the epi-layer and buffer have been drawn and summarized in Fig. 13. The V-defects are formed by inheriting the threading dislocations from the buffer. As the epi-layer grows thicker, the dislocations may attract other smaller dislocations as well as electrical active interstitial zinc and/or aluminum atoms. “V”-shaped defects have thus been formed with deep hexagonal pits observed on the surface of the epi-layer. 5. Summary The morphological, optical, structural, and electrical properties have been shown and discussed for the ZnO buffers with three different thickness and the corresponding epi-layers grown on the buffers. The growth process of the buffers including the initial nucleation, lateral coalescence of grains, and vertical growth along grains, has been clearly explained with the help of the morphological analysis. Via the comprehensive evaluation of the properties, the epi-sample grown on the buffer with a flat surface and 7 Page 7 of 17
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well-coalescent grains has the best quality, indicating that the surface morphology for a buffer, rather than the other properties, is the key factor to influence the properties of the epi-layer. Furthermore, deep pits with different size and density have been observed on the epi-layers. The origin for the pits are thought to be the V-shape defects, which is inherited from the threading dislocations formed in the buffer layers. The V-defects grow larger by attracting smaller dislocations and defects as the epi-layer grows thicker, and they may act as electron traps that reduce the free carrier concentration of the epi-layers. Acknowledgement This research was supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003, National Natural Science Foundation of China (Nos. 61025020, 60990312, 61274058, 61322403, and 61274058), the Natural Science Foundation of Jiangsu Province (Nos. BK2011437 and BK 20130013), the Six Talent Peaks Project in Jiangsu Province (2014XXRJ001), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] C. F. Klingshirn, B. K. Meyer, A. Waag, A. Hoffmann, and J. Geurts, Zinc Oxide: From Fundamental Properties Towards Novel Applications (Springer-Verlag, Berlin/Heidelberg/Germany, 2010), p. 177.
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Fig. 1 (a)-(c) The AFM images for samples A1-C1. The scanning area is 5 µm by 5 µm. (d) The extracted surface roughness for samples A1-C1. (e) The schematic diagram showing the evolution of the surface morphology for the different growth stage of the buffers. Fig. 2 The room-temperature PL spectra for samples A1-C1. Fig. 3 The room-temperature Raman backscattering spectra for samples A1-C1. The inset depicts the ratio of E2(L)/E2(H) for the three buffers. Fig. 4 (a) The XRD ω-2θ scan patterns for samples A1-C1. (b) The XRC ω-scan patterns for samples A1-C1. The inset depicts the extracted intensity and FWHM values for the XRC of different buffers. Fig. 5 The electron carrier concentration measured by Hall-effect measurement for samples A1-C1. Fig. 6 (a)-(c) The AFM images for samples A2-C2. (d) The extracted surface roughness for samples A2-C2. Fig. 7 The room-temperature PL spectra for samples A1-C1. Fig. 8 The room-temperature Raman backscattering spectra for samples A1-C1. The inset depicts the ratio of E2(L)/E2(H) for the three buffers.
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Fig. 9 (a) The XRD ω-2θ scan patterns for samples A2-C2. The inset depicts the coherent lengths for the samples calculated from the XRD patterns. (b) The XRC ω-scan patterns for samples A2-C2. The inset depicts the extracted intensity and FWHM values for the XRC of different buffers. Fig. 10 The electron carrier concentration measured by Hall-effect measurement for samples A2-C2. Fig. 11 (a) The 10-K PL spectra for samples A2-C2. (b) The temperature-dependence of the PL spectra for sample C2 from 10 K to room temperature. Fig. 12 The C-V curves for samples A2-C2. Fig. 13 (a) The schematic diagram of the V-defects formed in the epi-layers which are inherited from the threading dislocations formed in the buffer. (b) The cross-sectional view of the diagram, showing the connection and the inheriting relations between the V-defects and the dislocations at the buffer/epi-layer interface.
Table 1 Summary of the results for the optimization of ZnO buffer thickness grown on sapphire Buffer thickness (nm)
FWHM from XRC for
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Growth method
References
epi-layer (arc sec) 0, 25, 50, 75, 110
PLD
0, 25, 50, 75
Magnetron sputtering
0, 5, 10, 20, 50
MOCVD/sputtering MBE
1400
M
AP-AVPE
Takahashi/2000 [18] Nakamura/2002 [19]
720
Bang/2003 [10]
0, 5, 10, 20, 40
2300
Sato/2004 [20]
8, 15, 22, 30
2160
Jung/2004 [21]
10 – 80
-
Kamada/2008 [22]
Magnetron sputtering
0, 5, 10, 15
2063
Gao/2010 [23]
MBE
0, 1, 3, 5, 10, 20, 30
1740
Yang/2010 [24]
0, 20, 40, 60
720
Lee/2012 [25]
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320
Magnetron sputtering
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S0169-4332(15)02542-8 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.123 APSUSC 31604
To appear in:
APSUSC
Received date: Accepted date:
5-8-2015 17-10-2015
Please cite this article as: K. Tang, S. Huang, S. Gu, S. Zhu, J. Ye, Z. Xu, Y. Zheng, The roles of buffer layer thickness on the properties of the ZnO epitaxial films, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Full-length article corresponding to the 1st ICASS Poster No. P1.42 or Abstract Ref. No. ICASS2015_0356
The roles of buffer layer thickness on the properties of the ZnO epitaxial films Kun Tanga*[email protected], Shimin Huanga, Shulin Gua*[email protected], Shunming Zhua, Jiandong Yea,b, Zhonghua Xua, Youdou Zhenga a
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Nanjing National Laboratory of Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China b Nanjing University Institute of Optoelectronics at Yangzhou, Yangzhou 225009, China
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Abstract In this article, the authors have investigated the optimization of the buffer thickness for obtaining high-quality ZnO epi-films on sapphire substrates. The growth mechanism of the buffer with different thickness has been clearly revealed, including the initial nucleation and vertical growth, the subsequent lateral growth with small grain coalescence, and the final vertical growth along the existing larger grains. Overall, the quality of buffer improves with increasing thickness except the deformed surface morphology. However, by a full-scale evaluation of the properties for the epi-layers, the quality of the epi-film is briefly determined by the surface morphology of the buffer, rather than the structural, optical, or electrical properties of it. The best quality epi-layer has been grown on the buffer with a smooth surface and well-coalescent grains. Meanwhile, due to the huge lattice mismatch between sapphire and ZnO, dislocations are inevitably formed during the growth of buffers. More importantly, as the film grows thicker, the dislocations may attracting other smaller dislocations and defects to reduce the total line energy and thus result in the formation of V-shape defects, which is connected with the bottom of the threading dislocations in the buffers. The V-defects appear as deep and large hexagonal pits from top view and they may act as electron traps which would affect the free carrier concentration of the epi-layers. Keywords Zinc oxide, Epitaxy, Sapphire, Buffer layer, MOCVD, Material characterizations 1. Introduction The development of technologies to improve the material quality is always a hot topic in the research of a specific functional material. The optimization is unlimited since defects and dislocations always exist. For the ZnO material studied in this article, the material quality is far from the device-ready level. [1] The control of defects and dislocations is always problematic. Thanks to the similarity between GaN and ZnO in terms of structure and properties, the study on ZnO always uses the successful experience of GaN for reference. For instance, the epitaxial growth of GaN on the lattice-mismatched sapphire substrates has achieved a tremendous success, which have led to the commercialization of GaN-based blue light-emitting diodes (LEDs) and laser diodes (LDs). Therefore, sapphire has been an economic and most popularly used substrate for the hetero-epitaxial growth of ZnO. [2-3] Due to the large lattice mismatch and distinctive thermal expansion coefficient between sapphire and GaN, [4] a low-temperature buffer layer is indispensably employed for the epitaxy of GaN on sapphire in order to buffer the strain and dislocations induced by the lattice mismatch. [5-6] For the epitaxy of 1 Page 1 of 17
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ZnO on sapphire, various buffer layers, like GaN, MgO, ZnS, and ScAlMgO4, have been utilized. [4, 7-9] However, the ZnO homogeneous buffer is always preferred since the epitaxial growth on an optimized ZnO homo-buffer would be quite similar as a homo-epitaxy. [10] The lattice mismatch could be released by strains and forming dislocations. It is easy to understand that, when the mismatch between buffer and substrate is sufficiently small, the buffer that can fully release the mismatch by strains should be relatively thick. On the contrary, when the mismatch is large, the strain energy would quickly reach a critical value wherein dislocations are energetically favorable to be formed. In this case, the buffer with no dislocations should be relatively thin. Therefore, a critical thickness has been defined as the thickness at which the first dislocation is formed. The existence of this critical thickness was first detailed by Van der Merwe [11], and modulated by Matthews [12] and People and Bean [13]. For ZnO buffer deposited on c-plane sapphire, there is a significant lattice mismatch (18.3%) between them with a 30 o domain orientation (ZnO [10-10] // Al2O3 [11-20]) [14]. Utilizing the calculation method reported by Fischer et al, [15-17] the critical thickness for ZnO is estimated to be only 0.63 nm, corresponding to 1~2 layers of atoms. As a result, the formation of dislocations in the buffer is unavoidable. It is obvious that the properties of the buffer layer will influence those of the epi-layer. Especially for the thickness, if the buffer layer is too thin, high density of dislocations and rough surface will be formed which is not ideal for the following epitaxy. While if the buffer layer is too thick, the surface is also rough and the dislocations will grow larger due to the over-matured grains and suppressed horizontal growth. Therefore, an appropriate buffer layer thickness is of great importance for the quality of the epi-layer. In fact, several research groups have done works on the optimization of the buffer thickness for further ZnO epitaxy. [10, 18-25] Table I lists their brief results. It can be noticed that the optimized values for the thickness vary among different reports, which is possibly due to distinctive growth method and conditions. Therefore, the roles of buffer thickness on the properties of the ZnO epi-layers are still unclear and require further investigation. In this article, the morphological, optical, structural, and electrical properties of the buffers with three different thickness and the epi-layers grown on the corresponding buffer have been extensively investigated, respectively. The surface smoothness, rather than the other properties, has been proposed as the dominate factor for the optimized growth for the ZnO epi-layer. Besides, the correlation between the formation of the morphological pits (V-defects) observed on the epi-layers and the existence of threading dislocations in the buffers has been established. The formation and properties of the V-defects have also been proposed. 2. Experiments 2.1. Sample preparation Samples discussed in this article were grown on c-plane sapphire substrate by a home-built metal-organic chemical vapor deposition (MOCVD) system with close coupled-showerhead configuration. The details of the MOCVD system have been reported and can be referenced elsewhere. [26] A four-step process for the high-temperature epitaxial growth of ZnO has been established as follows. (1) The as-received sapphire substrates were pretreated for 5 minute at 1100 ºC in nitrogen and hydrogen environment in order to make the sapphire surface ready for growth. Details regarding the pretreatment of sapphire substrates could be referenced in a previous paper of our group. [27] (2) Low-temperature ZnO buffer layers were grown on the pretreated sapphire substrates at 470 ºC using high-purity DMZn [(CH3)2Zn] as Zn precursor and tert-BuOH [(CH3)3C-OH] as O precursor. The growth duration lasts for 5 min, 15 min, and 30 min, which are marked as samples A1, B1 and C1, 2 Page 2 of 17
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respectively. (3) ZnO epi-layers (marked as samples A2, B2, and C2) were grown on the corresponding buffer layers at 850 ºC for 1 hour with N2O as O precursor.
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2.2. Characterizations The thickness of the above samples was measured by a thin film measurement system (Avantes Co.) with optical interference. The crystalline quality was analyzed by high-resolution x-ray diffraction (HRXRD, Philips X'pert Pro diffractometer equipped with a Cu-ray source, λ ~ 0.15405nm). The surface morphology was observed by atomic force microscopy (AFM, Nanoscope IIIa, Digital Instruments Inc.). The photoluminescence (PL) spectra were recorded at room temperature, excited by a He-Cd laser (λ=325nm) with a spectrum resolution of 0.2 nm. A cyro-genic system was employed for the helium-temperature and temperature-dependent PL measurements. The vibrational properties of the samples were recorded by a JOBIN YVON HR800 UV Raman system in the backscattering geometry excited by an Ar+ laser (λ = 514.5 nm) at room temperature. A Hall-effect system (Keithley Instruments Inc.) was utilized for the electrical property measurement, which were performed at room temperature under van der Pauw’s four-point-probe configuration with indium as electrodes. The capacitance-voltage (C-V) measurement was carried out by profiling the barrier capacitance of the Schottky contact between a mercury probe and the sample surface as a function of an applied DC bias. 3. Results and discussion I: The properties of the ZnO buffers with different thickness The thickness data for samples A1-C1 are 78, 133, and 295 nm, respectively, and the corresponding growth rates are calculated as 13, 7.5, and 9.8 nm per minute. The non-constant growth rate indicates that the growth mode is not unique during the 30 minutes time. In order to reveal the reason for the different growth rate, the surface morphology for the buffer samples are thus measured and shown in Fig. 1(a)-1(c). Fig. 1(a), representing the 5-min growth, shows the morphology just after initial nucleation stage with high density of tiny pits, indicating the start of the coalescence of initial grains. The highest growth rate for this stage (stage I) suggests that the growth mode is dominated by vertical growth. It is easy to understand this due to the huge lattice mismatch between sapphire and ZnO. As calculated in the introduction, the critical thickness of ZnO on sapphire in only 1~2 atomic layer, indicating that the surface energy must be released by forming dislocations as the buffer grows thicker. [28-29] In this case, the initial nucleation should be poor and loose. It is well-acknowledged that the atomic stacking along c-axis on nuclei is much easier than the lateral move of atoms on sapphire and attachment of atoms on the edges of the nuclei. [30] As a result, the vertical growth rate is higher than the lateral coalescence rate, leading to the observed surface morphology of sample A1 with a relatively rough surface (RMS ~ 7.62 nm). During the subsequent 10 min growth, the surface morphology has been improved. It can be seen from Fig. 1(b) that the grains become larger and well coalescent, giving a relatively flat surface (RMS ~ 3.79 nm). Combining the reduced growth rate with the observed morphology, it is suggested that the lateral growth dominates in this stage (stage II). The enhanced lateral growth could be understood as follows. (1) The initial growth have filled the spaces of sapphire surface, making the growing frontier homogeneous. Consequently, the adsorption rate of the ad-atoms on the homogenous growing surface must be higher than that on sapphire surface. (2) As the coalescence of grains continues, the distance between nearby grain edges is shorter, making the distance of atoms moving along the surface to the grain edges much shorter. In this case, the desorption rate of the ad-atoms is reduced and the ad-atoms are thus much easier to grow laterally. 3 Page 3 of 17
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Further increase of the growth time for another 15 min (sample C1) again roughens the surface (RMS ~ 12.18 nm) as shown in Fig. 1(c). The morphology shows larger hexagonal grains as well as larger pits and the growth rate increases to 9.8 nm/min, which indicates that the growth mode is dominated by vertical growth again. It could be understood and explained by the relatively low substrate temperature employed for the buffer growth. As the grains basically finish coalescence at stage II, the resultant lateral growth rate would drop. The low migration rate of ad-atoms on the growing surface caused by the low substrate temperature would then make the new growth preference along the existing grains vertically, rather than laterally, forming the final morphology as shown in Fig. 1(c). The evolution of the surface morphology has been summarized in Fig. 1(e) schematically, which clearly exhibits that in order to obtain a flat surface, the growth duration should be controlled at the time-point when the initial coalescence finishes.
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Although the flattest surface has been obtained by a 15-min buffer growth, the other properties of the buffers do not follow the trend of the roughness. Fig. 2 shows the room-temperature PL spectra for all the buffer samples. Sample A1 consists of two emission components at 380 nm (peak-I) and 390 nm (peak-II), respectively, whereas the emission peak at 380 nm dominates in the spectra of samples B1 and C1. The peak-II is generally thought to relate to the high level of strains and dislocations in the transitional layer at the ZnO/sapphire interface. The peak-I is ascribed to the free exciton recombination from well-aligned ZnO lattice. The detection of peak-II in sample A1 is due to that its thickness is comparable to the penetration depth of the 325-nm exciting laser (~ 60 nm), which can collect the interfacial information. Furthermore, the intensity of peak-I increases monotonically as the buffer grows thicker, indicative of better optical and crystalline quality. In fact, the improved optical and crystalline quality has been double-confirmed by Raman spectra and XRD. Fig. 3 shows the Raman spectra recorded for all the buffer samples. As can be seen from the figure, except the modes from sapphire, ZnO E2 modes are obviously detected. The intensity of the E2(L), E2(H)-E2(L), and E2(H) modes monotonically increases as the buffer grows thicker. Since the E2(L) or E2(H) mode is a standard to evaluate the quality of the zinc or oxygen sub-lattice, [31] the ratio of E2(L)/E2(H), standing for the relative improving degree of zinc and oxygen sub-lattices, has been drawn out and plotted in the inset of Fig. 3 for a deeper understanding of the crystalline quality improvement. From sample A1 to B1, the increased ratio indicates that the improvement of zinc sub-lattice is better than that of oxygen sub-lattice during the lateral coalescence of small grains. From sample B1 to C1, the decreased ratio further indicates that the improvement of oxygen sub-lattice can be enhanced during the growth of larger grains. It is proposed that the difference of the oxygen sub-lattice improvement relative to zinc sub-lattice at different growth stage is related to the different mobility and adsorption/desorption rates when the growth happens laterally or vertically. Nevertheless, the quality of both sub-lattices improves with growing thicker. Fig. 4 shows the XRD and XRC results measured on all the buffer samples. Only ZnO (0002) and (0004) peaks are detectable in the XRD patterns, showing a perfect c-axis preferential orientation. Furthermore, the intensities of the (0002)/(0004) XRD peaks and the (0002) XRC peak increase monotonically as growing the buffer thicker, which are also indications of better crystalline quality. The inset of Fig. 4(b) plots the FWHM values of the XRC (0002) peaks for all samples. From sample A1-B1, a sharp decreases of the FWHM value could be seen, indicating a prominent improvement of the crystalline quality during the grain coalescence stage. However, from sample B1 to C1, there is only a slight decrease of the FWHM 4 Page 4 of 17
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values, indicating that the lateral coalescence of small grains plays a key role in determining the crystalline quality of the buffer. The residual carrier concentration drops with the increment of buffer thickness as shown in Fig. 5. The residual carriers may have two origins: (1) intrinsic donor-like defects; (2) extrinsic dopants, like hydrogen and aluminum. The drop of the electron concentration should be related to the reduction of the intrinsic and extrinsic defects. On one hand, as shown by the above characterizations, the crystalline quality improves as the buffer becomes thicker, leading to reduced concentration of intrinsic defects. On the other hand, as the thickness of the buffer increases, the electrical contribution from the aluminum atoms diffused from the sapphire substrate becomes less. It is therefore proposed that these two aspects have resulted in the observed Hall results.
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4. Results and discussion II: The roles of the buffer thickness on the properties of the ZnO epitaxial films
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Provided the understanding of the properties for the buffers, the roles of buffer thickness on ZnO epi-layer growth could be investigated via characterizing the epitaxial films grown on the different buffers A1-C1. First of all, the thickness of the epi-layers (A2-C2) has been measured and fitted as 492, 880, and 1059 nm, respectively. By subtracting the thickness of the buffers, the corresponding growth rates for the epitaxial stage are calculated as 14.0, 25.0, and 25.3 nm per minute. The growth rate of sample A2 is relatively low, possibly due to that the growth of A2 should finish the lateral coalescence of small grains first before further growth. In the condition of high-temperature epi-growth, the surface diffusivity of ad-atoms has been greatly enhanced, favoring the lateral growth. Just due to the high mobility of ad-atoms, the new nucleation at the space between different small grains is considered to be much more difficult than the direct growth at the edge of the existing grains. Consequently, the lower growth rate for sample A2 is attributed to the lateral coalescence of grains. The saturated growth rate for samples B2 and C2 indicates that when the coalescence of grains finishes, the growth on larger grains dominates the epitaxial growth process, regardless of the surface morphology of the buffers.
The surface morphology of the epi-film samples A2-C2 is shown in Fig. 6(a)-6(c). It is obvious to see the deep hexagonal pits with increased size and decreased density. In fact, similar morphologies have been frequently observed in the GaN/InGaN MQWs structures consisting of a threading dislocation terminated by a hexagonal pit. This kind of structures is nominated as V-shape defects. [32-35] For ZnO, Liu et al have reported that the threading dislocations are not uniformly distributed in the ZnO layer but aggregate to form annular regions around columnar epitaxial cores. [36] Several groups have reported that a threading dislocation is connected with the bottom of the V-defect and causes the formation of the V-defect. [37-38] This kind of defects can extend from the sample surface to a few hundred nanometers (300–800 nm) into the bulk. [39] The hexagonal pits observed on samples A2-C2 accords well with the descriptions of the V-defects stated by previous literatures. The pits are therefore ascribed to the V-defects. Moreover, the pits for the buffers, representing the dislocations, [28-29] also shows an increase in size and decrease in density as shown in Fig. 1(a)-1(c), which is similar to the variation trend of the size and 5 Page 5 of 17
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density for the epitaxial samples. Combined with the abovementioned previous literatures, it is possible that the V-defects might inherit from the dislocations formed during the buffer growth. Weber et al have pointed out that dislocations can be attracted to the V-defects to decrease total line energy. [39] Based on this conclusion, although the V-defects originate from the dislocations in buffers, they may grow larger in size by attracting small dislocations when the film grows thicker, which results in larger sizes of the hexagonal pits after 1-h epitaxial growth as shown in Fig. 6(a)-6(c).
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Fig. 6(d) shows the extracted RMS roughness excluding the pit areas. Sample B2 owns a minimum value of 5.33 nm. Actually, other structural and optical characterizations have confirmed that sample B2 has the best property among the three epitaxial films. Fig. 7 shows the room-temperature PL spectra for samples A2-C2. The NBE emission intensity of B2 is the highest, indicative of best optical quality. Fig. 8 shows the Raman spectra. After 1-h epitaxial growth, the intensity of the E2 modes is generally enhanced to a great extent as compared with Fig. 3. Besides, the intensity of the E2 modes increases as the buffer thickness increases. However, the E2(L)/E2(H) ratio shows an opposite trend with the smallest value for sample B2, which indicates that the improvement of the oxygen sub-lattice on sample B2 is the best compared with that of the zinc sub-lattice. XRD patterns for samples A2-B2 [Fig. 9(a)] show a monotonic increase of the ZnO (0002) and (0004) diffraction peaks, according perfectly with the Raman spectra. Meanwhile, the inset shows the calculated coherent lengths from the Scherrer’s equation. [40] The coherent lengths for the epitaxial films are 2 ~ 4 times longer than those for the buffers (207-282 nm), indicative of conspicuous improvement to the grain size and crystalline quality after 1-h epi-growth. At the same time, sample B2 holds the maximum coherent length of ~ 850 nm, which is a sign of best crystalline quality. It is also consistent with the highest Hall mobility of 72 cm2/Vs measured on sample B2. The XRC results shown in Fig. 9(b) further support that sample B2 is of best quality. Although the intensity of XRC increases as the buffer thickness increases, the FWHM value for sample B2 is the lowest. Considering the properties for the buffers, it can be concluded that an appropriate buffer thickness with a well-coalescent and flat surface, rather than with the best crystalline, optical, or electrical quality, is the decisive factor to the quality of the high-temperature epi-grown films. The electron carrier concentration of the epitaxial films has been measured and shown in Fig. 10. It is intriguingly found that the values are almost unchanged to those of the buffers as shown in Fig. 5. Considering that the crystalline quality for the epitaxial films is much better than the buffers, it is reasonable that the buffers dominate the electrical properties of samples A2-C2. In order to reveal and distinguish the electrical properties of the epi-layers, low-temperature PL spectra as well as C-V measurement have been carried out. Fig. 11(a) shows the PL spectra of samples A2-C2 measured at 10 K, and the temperature-dependence of the PL peaks for sample C2 is depicted in Fig. 11(b). Generally, three peaks could be seen from the PL spectra. Regarding the assignment, the peak at 3.362 eV is commonly ascribed to excitons bound to neutral donors (D0X), the peak at 3.368 eV is ascribed to the excitons bound to ionized donors (D+X), and the peak at 3.377 eV is ascribed to free excitons (FX). [41-42] The assignments are valid through analyzing the temperature dependence of these peaks in Fig. 11(b) that the thermal quenching of D0X and D+X is quite rapid and a clear trend of the transformation from D0X/D+X to FX could be seen. It is clear that sample A2 is dominated by the D0X component whereas the peak for sample B2 slightly shift to higher energy side and much broadened. For sample C2, D+X component dominates the spectrum with a shoulder peak at 3.362 eV. The gradual 6 Page 6 of 17
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emergence of D+X and the disappearance of D0X from sample A2 to C2 indicates that the electron concentration near the surficial region decreases, since the air/surface interface would easily cause a band bending and thus a surficial depletion (D+) when the electron carrier concentration is low. In this case, the D+X is also known as SX (surface exictions).
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The low-temperature PL spectra seem to demonstrate that the electron concentration for the epitaxial films near the surface also decreases with increasing the buffer thickness. In order to double-check this issue, C-V measurement have been employed. Fig. 12 shows the measured C-V curves for samples A2-C2. By extracting the slope of the C-2-V, the doping concentration has been estimated as in the order of 1018, 1017, and 1014 cm-3 for samples A2, B2, and C2, respectively. These values are much smaller as compared with the electron carrier concentration measured by Hall-effect (1*1019, 3*1018, and 8*1017 cm-3), indicating that the electrical contribution from the epi-layers could be neglected and the electrical properties are dominated by the buffers. The reduction of electron concentration from sample A2 to C2 could be partially attributed to the improved crystalline quality, meaning less concentration of intrinsic and extrinsic defects as discussed in SEC. 3. Actually, the larger V-defects might also contribute to the reduction of carrier concentration. Based on the correlation between edge threading dislocations and SCM mapping, Liu et al have concluded that the edge threading dislocations would introduce interface trap densities in the annular region. [36] Besides, Müller et al have analyzed the phase of the transmitted beam by electron holography, indications for a high negative line charge at threading dislocations have been found in ZnO epi-layer. [43] In addition, Cherns and Jiao [44] have reported a negatively charged dislocation core in n-GaN when they applied electron holography in a transmission electron microscope to detect the electrostatic potential in the vicinity of single dislocations in GaN. Yokoyama et al [45] have directly observed the carrier depletion around a dislocation in GaP by scanning spreading resistance microscopy. These prior literatures have suggested that the electrons might be trapped along the threading dislocations. As the gradual formation of threading dislocations and V-defects, the donor-like defects, like interstitial zinc (Zni) and thermally diffused aluminum (Al) might be aggregated to the dislocations. Due to the higher diffusivity of Zni and Al, it is easy to form a high-density distribution of Zni and Al along the threading dislocations and V-defects. However, inside the grains, the concentration of Zni and Al would be lower. In this scheme, as the V-defects grow larger, the defects contributing to electron carriers would be gradually aggregated to the V-defects, and therefore, more free electrons are trapped, resulting in a reduced electron concentration for the epi-layers. The schematic diagram for the proposed structures for the epi-layer and buffer have been drawn and summarized in Fig. 13. The V-defects are formed by inheriting the threading dislocations from the buffer. As the epi-layer grows thicker, the dislocations may attract other smaller dislocations as well as electrical active interstitial zinc and/or aluminum atoms. “V”-shaped defects have thus been formed with deep hexagonal pits observed on the surface of the epi-layer. 5. Summary The morphological, optical, structural, and electrical properties have been shown and discussed for the ZnO buffers with three different thickness and the corresponding epi-layers grown on the buffers. The growth process of the buffers including the initial nucleation, lateral coalescence of grains, and vertical growth along grains, has been clearly explained with the help of the morphological analysis. Via the comprehensive evaluation of the properties, the epi-sample grown on the buffer with a flat surface and 7 Page 7 of 17
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well-coalescent grains has the best quality, indicating that the surface morphology for a buffer, rather than the other properties, is the key factor to influence the properties of the epi-layer. Furthermore, deep pits with different size and density have been observed on the epi-layers. The origin for the pits are thought to be the V-shape defects, which is inherited from the threading dislocations formed in the buffer layers. The V-defects grow larger by attracting smaller dislocations and defects as the epi-layer grows thicker, and they may act as electron traps that reduce the free carrier concentration of the epi-layers. Acknowledgement This research was supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003, National Natural Science Foundation of China (Nos. 61025020, 60990312, 61274058, 61322403, and 61274058), the Natural Science Foundation of Jiangsu Province (Nos. BK2011437 and BK 20130013), the Six Talent Peaks Project in Jiangsu Province (2014XXRJ001), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] C. F. Klingshirn, B. K. Meyer, A. Waag, A. Hoffmann, and J. Geurts, Zinc Oxide: From Fundamental Properties Towards Novel Applications (Springer-Verlag, Berlin/Heidelberg/Germany, 2010), p. 177.
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Fig. 1 (a)-(c) The AFM images for samples A1-C1. The scanning area is 5 µm by 5 µm. (d) The extracted surface roughness for samples A1-C1. (e) The schematic diagram showing the evolution of the surface morphology for the different growth stage of the buffers. Fig. 2 The room-temperature PL spectra for samples A1-C1. Fig. 3 The room-temperature Raman backscattering spectra for samples A1-C1. The inset depicts the ratio of E2(L)/E2(H) for the three buffers. Fig. 4 (a) The XRD ω-2θ scan patterns for samples A1-C1. (b) The XRC ω-scan patterns for samples A1-C1. The inset depicts the extracted intensity and FWHM values for the XRC of different buffers. Fig. 5 The electron carrier concentration measured by Hall-effect measurement for samples A1-C1. Fig. 6 (a)-(c) The AFM images for samples A2-C2. (d) The extracted surface roughness for samples A2-C2. Fig. 7 The room-temperature PL spectra for samples A1-C1. Fig. 8 The room-temperature Raman backscattering spectra for samples A1-C1. The inset depicts the ratio of E2(L)/E2(H) for the three buffers.
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Fig. 9 (a) The XRD ω-2θ scan patterns for samples A2-C2. The inset depicts the coherent lengths for the samples calculated from the XRD patterns. (b) The XRC ω-scan patterns for samples A2-C2. The inset depicts the extracted intensity and FWHM values for the XRC of different buffers. Fig. 10 The electron carrier concentration measured by Hall-effect measurement for samples A2-C2. Fig. 11 (a) The 10-K PL spectra for samples A2-C2. (b) The temperature-dependence of the PL spectra for sample C2 from 10 K to room temperature. Fig. 12 The C-V curves for samples A2-C2. Fig. 13 (a) The schematic diagram of the V-defects formed in the epi-layers which are inherited from the threading dislocations formed in the buffer. (b) The cross-sectional view of the diagram, showing the connection and the inheriting relations between the V-defects and the dislocations at the buffer/epi-layer interface.
Table 1 Summary of the results for the optimization of ZnO buffer thickness grown on sapphire Buffer thickness (nm)
FWHM from XRC for
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Growth method
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epi-layer (arc sec) 0, 25, 50, 75, 110
PLD
0, 25, 50, 75
Magnetron sputtering
0, 5, 10, 20, 50
MOCVD/sputtering MBE
1400
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AP-AVPE
Takahashi/2000 [18] Nakamura/2002 [19]
720
Bang/2003 [10]
0, 5, 10, 20, 40
2300
Sato/2004 [20]
8, 15, 22, 30
2160
Jung/2004 [21]
10 – 80
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Kamada/2008 [22]
Magnetron sputtering
0, 5, 10, 15
2063
Gao/2010 [23]
MBE
0, 1, 3, 5, 10, 20, 30
1740
Yang/2010 [24]
0, 20, 40, 60
720
Lee/2012 [25]
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320
Magnetron sputtering
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