ZnO Wurtzite Single Crystals Prepared by Nanorod-Assisted Epitaxial

Oct 19, 2009 - Department of Materials Science and Engineering, Korea Advanced Institute of Science and ... Cryst. Growth Des. , 2010, 10 (1), pp 321â...
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DOI: 10.1021/cg900907d

ZnO Wurtzite Single Crystals Prepared by Nanorod-Assisted Epitaxial Lateral Overgrowth

2010, Vol. 10 321–326

Dong Chan Kim,† Ju Ho Lee,‡ Hyung Koun Cho,*,† Jae Hyun Kim,§ and Jeong Yong Lee‡ †

School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheondong, Jangan-gu, Suwon, Gyeonggi-do, 440-746, Korea, ‡Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea, and §Department of Nano & Bio Technology, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), Daegu 704-230, Korea

Received August 3, 2009; Revised Manuscript Received September 17, 2009

ABSTRACT: We have developed a novel method to grow thick single crystalline ZnO films for the use of supporting layers by employing nanorod-assisted epitaxial lateral overgrowth (NRELO). The NRELO ZnO films were epitaxially grown on vertically arrayed nanorods at reduced temperatures by metalorganic chemical vapor deposition. The resultant films had epitaxial structures similar to the conventional ZnO films on sapphire and relatively low dislocation density. During the growth evolution of the NRELO films, the dominant stress was changed from nearly stress-free in the nanorods to strong in-plane tensile stress in the top region, and the rotation of the (0002) plane clearly disappeared in the NRELO ZnO film. This study shows that ZnO NRELO has the possibility to be used as supporting layers in optoelectronic devices with transparent and vertical stack structures because it exhibited high transparency, electrically semiconducting, and low defect density at the same time.

1. Introduction A considerable amount of recent research activity has focused on the growth and application of high crystalline ZnO layers. ZnO grown via vacuum deposition has garnered significant research interest as an alternative to the established GaN and Si-based devices such as light-emitting-diodes (for solid-state lightening) and thin-film-transistors (for backplanes in the organic LEDs).1-4 Zinc is a cost-effective material that is abundantly available. In addition, the synthesis process for ZnO compounds is quite simple and straightforward.5 Conversely, the fabrication of bulk GaN specimens is difficult and expensive, so freestanding GaN substrates using hydride vapor phase epitaxy are mainly used despite both the time-consuming nature of the GaN fabrication process and the degradation related to crack generation.6,7 While ZnO substrates are a suitable alternative to GaN, a reduction in the fabrication cost and a realization of large sizes must be satisfied for their common use in commercial applications. ZnO active layers are expected to display high radiative recombination efficiency due to their larger exciton binding energies of 60 meV.8 In addition, ZnO-based thin films exhibit both high transparencies and suitable conductivities without structural degradation. The synthesis process for low-dimensional ZnO nanostructures is also relatively simple.9,10 ZnO is similar to GaN with regard to crystal structure, bandgap, and lattice constant. Most ZnO and GaN films are grown on sapphire substrates that possess a large lattice constant and a high thermal expansion coefficient mismatch. As such, fabricated films have high dislocation densities.11 For high efficiency optical and electronic devices, a key requirement common to both ZnO and GaN is to grow epitaxial layers with low defect density, which depends strongly on the

choice of substrates as well as on the crystal quality of intermediate or buffer layers.12-17 Several approaches aimed at reducing the dislocation density in GaN layers have been successfully applied. The representative methods are epitaxial lateral overgrowth (ELO) and pendeoepitaxy (PE), which result in low defect density in selected areas.18-20 However, these techniques are not effective for ZnO films, and the systematic studies on the development of novel methods are lacking. In addition, these regrowth-based techniques commonly require extra masks and a complex lithography process that hinders mass production. As presented in our previous reports, we successfully grew the ZnO thin films on vertical nanorod arrays for the formation of a top contact layer by varying the growth parameters in the metalorganic chemical vapor deposition (MOCVD) process.21-23 In particular, the structures fabricated on sapphire substrates were nearly epitaxial, similar to conventional ZnO thin films on sapphire.21 Thus, it is expected that further growth of the top ZnO films under optimized conditions can produce the thicker epitaxial ZnO layers, which can be used as substrate. In this study, we employed the nanorod-assisted epitaxial overgrowth (NRELO) method to grow thick ZnO layers with low defect density. The possibility of utilizing this technology for the fabrication of supporting layers in the optoelectronic devices was also investigated. 2. Experimental Section

*To whom correspondence should be addressed. E-mail: chohk@skku. edu.

The NRELO ZnO films were deposited on Al2O3 (0001) substrates in a vertical MOCVD reactor operating at low pressure. Diethylzinc (DEZn, purity 99.9995%) and oxygen gas (O2, purity 99.9999%) were used as the precursors, and argon was employed as the carrier gas for the DEZn source. The procedures used to produce the NRELO ZnO films were divided into the first stage of ZnO nanorod production and the second stage of ZnO film manufacture. These stages can be controlled by varying the growth parameters such as temperature and reactor pressure.21-23 In this experiment, the reactor

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pressure was maintained at 1 Torr through the entire process. The nanorods in the first stage and the film in the second stage were continuously grown at 420 and 260 °C, respectively. A detailed growth procedure for these structures is described elsewhere.20 In order to characterize the internal strain behaviors in the first stage for the ZnO nanorods and the second stage for the ZnO film, we performed step growth of the NRELO ZnO films with different growth times (30, 60, 120, and 180 min) on strain free ZnO nanorods. We performed the analysis for internal strain behaviors from the each XRD data of thick NRELO ZnO films grown by step growth. The morphologies of the NRELO ZnO films were examined by field-emission scanning electron microscopy (FESEM, JSM6700F). The structural properties were investigated by X-ray diffractometry (XRD, Bruker AXS D8 Discover) and transmission electron microscopy (TEM, JEOL JEM3100F). The optical transmission spectra and Hall measurements for the NRELO ZnO films were carried out using a UV/vis/NIR spectrometer (UV/vis, Varian Cary 5000) and the van der Pauw configuration, respectively. The optical transmission spectra were obtained by compensating measurement values with respect to sapphire substrate and Hall measurements were carried out by contact method using the 4 point probes. The room temperature photoluminescence (PL) measurements were carried out using a 325 nm line excitation from a He-Cd laser.

3. Results and Discussion In our work, the ZnO nanorods with the c-axis preferential orientation were epitaxially grown on sapphire substrates at g300 °C via an MOCVD method akin to that used for GaN on sapphire substrates and showed perfect vertical alignment. Subsequent to the synthesis of these nanorod arrays, we have also fabricated multidimensional ZnO layers with sequences of 2D (substrate or buffer layer) f 1D (nanorods) f 2D (film) by growing film-like ZnO on vertically arrayed nanorods, which allows the use of the nanorods as active layer components by forming a relatively flat top contact layer.21-23 Dimensional control of the ZnO layers in the MOCVD

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chamber could be achieved by varying such parameters as growth temperature and reactor pressure.23 Interestingly, the film-like ZnO top layers on the well-aligned ZnO nanorods exhibited homoepitaxial growth with an orientation relationship identical to that of the ZnO nanorods, indicating high crystalline quality of a single domain, when the growth temperature and the reactor pressure were decreased and increased, respectively.23 These results demonstrate the possibility of producing thicker homoepitaxial ZnO layers if the growth of the ZnO films is continuous under conditions identical to those of the 2D ZnO top layer. Figure 1 shows the plane-view and cross-sectional FE-SEM images of ZnO layers grown on epitaxial nanorods/sapphire substrates. The top ZnO layers were deposited at a reduced temperature (260 °C) for 20 min [Figure 1a, Sample A] and 180 min [Figure 1b, Sample B], while the other growth parameters were held constant. The thicknesses of the top regions for Samples A and B were 0.6 and 8.3 μm, respectively. Sample A with a thinner top ZnO film showed a relatively flat surface morphology and faint hexagonally shaped grain contrast. A hexagonal disk corresponded to approximately one or two nanorods, based on a comparison of their density (Figure 1a). Therefore, it was assumed that the disks were induced by the lateral growth of a nanorod at a relatively low temperature. The crystal structure for the top ZnO layers was directly ascertained by a XRD phi-scan. As shown in Figure 1c, the XRD phi-scan for the ZnO {1012} Bragg reflection of Sample A shows a regular 60° period, illustrating exact 6-fold symmetry of the ZnO top layer along the in-plane orientation. This implies that the ZnO top layer exhibits homoepitaxial behavior identical to that of the ZnO nanorods. It was found that the ZnO top layer grown on the nanorods had an epitaxial structure with a 30° rotated domain structure with respect to the sapphire substrates. As such, the

Figure 1. Plane-view and cross-sectional FE-SEM images for NRELO ZnO films grown for (a) 20 and (b) 180 min on ZnO nanorods. Phi-scan profiles for the (1012) plane of NRELO ZnO films grown for (c) 20 and (d) 180 min on ZnO nanorods.

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fabricated structures are similar to conventional ZnO films grown directly on sapphire substrates, despite the insertion of the ZnO nanorods. This technique for the growth of epitaxial ZnO layers using nanorod arrays is named nanorod-assisted epitaxial lateral overgrowth (NRELO), and it provides a novel method for fabricating ZnO single crystals with large thickness. A similar concept using nanorods have recently been reported for GaN with the goal of reducing threading dislocations.24 The NRELO ZnO film grown for 20 min does not have sufficient thickness for use as supporting layers. Thus, we extended the growth time of the NRELO ZnO films to 180 min. Figure 1b shows the SEM images obtained from an NRELO ZnO film grown at 260 °C for 180 min on homonanorod arrays. The structure consisted of nanowires (the bottom region, Zone A) and a thick film with a hexagonally shaped surface morphology (the upper region, Zones B and C). The total thickness of the grown NRELO ZnO film was ∼8.3 μm. The enlarged plane-view image in Figure 1b clearly shows that the thick NRELO ZnO film was covered with a high density of micrometer-sized disks. Nearly all of the microdisks have perfect hexagonal shape and the trace lines of the disks are regularly arrayed with rotation angles of 60°, as indicated by the dotted lines in Figure 1b. This is further evidence indicating the epitaxial nature of the NRELO ZnO films. The average diagonal dimension of the hexagonal disks was about 3.3 μm, while the sizes of the microdisks in the Sample A were ∼1 μm. The enlarged disk size was attributed to the coalescence of grains originating from each nanorod in the initial stage, where the density of the microdisks was below 1/100 of the nanorod density. Figure 1d shows the XRD phi-scan for the ZnO {1012} Bragg reflection of Sample B. Compared to Sample A, more intense and narrower

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diffraction peaks with perfect 6-fold periods were revealed. This finding is indicative of a single crystalline structure that demonstrates an enhancement in the overall crystalline quality. These results suggest that rotation domains with different orientations are considerably reduced by grain coalescence during the growth of subsequent NRELO ZnO films. Currently, we are unable to provide separated ZnO layers, because of sample cracks and handling problems after the detachment process. However, based on the above results, a novel process to separate the films from the substrates and to obtain single crystal supporting layers could be realized if further optimization were to be performed. The technique to separate thick and insulating substrates (such as sapphire and glass) can provide many advantages for the fabrication of ZnO-based nanoelectronic/photonic devices. For example, in insulating substrates, a heat sink for heat dissipation is required so that stable operation of the devices may be achieved. Device fabrication using a vertical stack is difficult, because sapphire substrates have inferior thermal and electrical conductivities. However, these problems would be completely resolved, if we were able to separate the film from the sapphire substrate without any degradation in the crystal quality. As shown in Figure 1b, thick NRELO ZnO films are categorized into two regions: (i) the dense epitaxial film after complete coalescence (Zone B) and (ii) the upper film consisting of grains with an inverted pyramidal shape (Zone C). Details on the evolution of the growth process for these layers are discussed in Figures 2 and 3. Figure 2a-d shows the cross-sectional bright-field TEM images of the NRELO ZnO films along the growth direction from the nanorods/sapphire substrate interface. The corresponding selected-area diffraction patterns (DPs) are also

Figure 2. (a-d) Cross-sectional bright-field TEM images of the different positions of Sample B. The corresponding SADPs are shown on the right-hand side of each image. (e) A magnified TEM image in Zone C of (a) for Sample B. (f) The coalescent region of the grains originating from each nanorod showing no discontinuity of the (0002) plane can be seen.

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Figure 3. The c-lattice constant and residual in-plane biaxial stress (a) of ZnO layers grown at various growth temperatures and (b) in different stages of the NRELO ZnO films.

included on the right-hand side of each figure. In the initial stage of nanorod production, the DP exhibited a distinct arcshaped spread, indicating that the nanorods were somewhat rotated about the (0002) axis. The degree of the rotation was continuously reduced by increasing the film thickness. Finally, the NRELO ZnO film exhibited diffraction spots with perfect circular shapes. These DPs confirmed the single crystalline nature of the NRELO ZnO films again. Figure 2e shows a magnified TEM image obtained from Zone C. Previously, we reported that the ZnO film on the nanorod arrays was grown with branch shapes from the {1010} side facets of the nanorods due to enhanced lateral growth at low temperature.21 The primary growth of these branches proceeded along the {1012} planes at the initial stage. Even though Zone C corresponds to complete coalescence by the lateral growth of the nanorods, the traces formed during the coalescence of the grains were observed along the {1012} planes, as shown in Figure 2e,f. Here, the junction region of the grains is the consequence of the competition between the lateral and the vertical growth.25,26 However, the junction region exhibited no defect-related atomic arrangements, such as crystallographic tilting of the (0002) plane or discontinuity of the (0002) lattice fringes. This implies that excellent epitaxial overgrowth was achieved by using the nanorod seeds. The bright contrast observed in the junction region of Figure 2f may be attributed to unfilled atoms at the atomic sites during the coalescence. The surface morphologies of the films strongly correlates with the strain status stored in the film. Stress analysis was performed by investigating the c-axis lattice parameter obtained from XRD characterization and was correlated with the growth evolution of the NRELO ZnO films. First of all, in order to evaluate the relationship between the strain status and the film morphology, the ZnO layers, not the NRELO ZnO films, were deposited by MOCVD at various temperatures and at a pressure of 1 Torr [Figure 3a]. The XRD measurements consisting of Zone A, Zone A þ Zone B, and Zone A þ Zone B þ Zone C for the samples at each step were also carried out for comparison [Figure 3b]. Furthermore, we performed step growth of the NRELO ZnO films with different growth times (30, 60, 120, and 180 min) on strainfree ZnO nanorods (Zone A) to characterize the internal strain

behaviors in the NRELO ZnO film. The XRD measurements consisting of Zone A, Zone A þ Zone B, and Zone A þ Zone B þ Zone C for the samples at each step were also carried out for comparison [Figure 3b]. Thus, we could analyze the internal strain behaviors from the XRD measurements of thick NRELO ZnO films grown by step growth. Here, the sapphire (0006) peak was used to calibrate the positions of the ZnO (0002) peak for a more exact analysis. According to previous reports, the in-plane biaxial stress in these ZnO layers can be obtained from the following equation:27   9 c -c0 ð1Þ σ ¼ -453:6  10 c0 where c and c0 (= 0.52066 nm from JCPDS Card No. 361451) are the c-axis lattice constant evaluated from Bragg’s law (2d sin θ = nλ) and the c-axis lattice constant for strainfree bulk ZnO, respectively. The parameter σ is the residual inplane biaxial stress calculated from the out-of-plane stress. Figure 3a,b shows the c-lattice constants and residual in-plane biaxial stresses for the ZnO layers grown at various temperatures (180-420 °C) and the different stages of the NRELO ZnO films, respectively. The ZnO layers grown at various temperatures showed different in-plane stress behaviors and surface morphologies.28 The changes in morphology with respect to growth temperature are described elsewhere.27 In the temperature window where 2D film-like surface morphology was observed (e210 °C), the in-plane stress changed from tensile to compressive stress as the growth temperature was increased. A growth temperature above 210 °C led to the formation of a rugged surface, indicative of a morphological transition from a flat surface to one composed of nanorods. This evolution induced an enhanced compressive in-plane biaxial stress. On the other hand, a further increase in the growth temperature resulted in the formation of vertically aligned nanorods and a slow decrease in the compressive stress. In particular, the residual in-plane biaxial stress was nearly eliminated at the maximum temperature of nanorod synthesis. Nanorods with a narrow diameter have an absence of structural defects, such as dislocations and grain boundaries, and possess a small contact area with the substrates. These factors lead to stress-free ZnO layers. The ZnO

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Figure 4. (a) The room-temperature PL spectra for NRELO ZnO films grown for 20 and 180 min on nanorods. The insets show the deep-level emission region. (b) Optical transmission spectra for NRELO ZnO films grown on nanorods for 20 and 180 min. The inset displays the (Rhν)2 as a function of the photon energy (hν) for the NRELO ZnO films.

nanorods used for the growth of NRELO ZnO films were deposited at 420 °C, which corresponds to a temperature where a nearly stress-free status is observed. For the growth of 2D ZnO films on the nanorods, the strain behavior of the ZnO layers abruptly changed from compressive to tensile, which coincided with the tensile stress found in the film. Figure 3b reveals that the growth of the NRELO ZnO films on nanorods promoted a high tensile stress as the film thickness increased in Zone C. Therefore, it was expected that an increase in the film thickness accumulates the stress stored by the coalescence of grains and the generation of defects. The top region of the NRELO ZnO films possessed a maximum tensile stress which caused a rugged surface morphology in the final stage, as shown in Figure 1b. NRELO ZnO films for homoepitaxial ZnO-based optoelectronic devices are expected to provide several characteristic functions. They are anticipated to play an important role in achieving improved conductivity and transparency in supporting layers for light-emitting-diodes and transparent electronic devices.8 Therefore, analyses of the electrical conductivity and optical transparency of the NRELO ZnO films are quite important. Figure 4a shows the room-temperature PL spectra of the NRELO ZnO films, which exhibited strong near-band-edge (NBE) emission peak at 3.37 eV with very weak deep-level emission. The presence of strong NBE emission with very weak deep-level emission indicates that the NRELO ZnO films grown at low temperature have good optical properties with few defects, as would be expected from the TEM results. As shown in Figure 4b, the transmission spectra of the NRELO ZnO films with different film thicknesses maintain good optical transmittance of greater than 80% in visible wavelength range, despite considerably large ZnO thicknesses. This finding indicates that the NRELO ZnO films may be suitable for transparent electronics applications. In the ultraviolet region, the obvious absorption was seen and the optical bandgap obtained from the absorption edge was approximately 3.2 eV for both Samples A and B. As shown in Table 1, the NRELO ZnO films exhibited n-type semiconducting behavior in the as-deposited state. An increase in the NRELO film thickness resulted in a continuous increase in both the electrical conductivity and the carrier mobility. The carrier concentration and mobility of a NRELO ZnO film were measured and found to be 3.4  1018 cm-3 and 20 cm2/ V 3 s, respectively. Obviously, the attainment of good electrical characteristics is ideal for a n-type semiconducting supporting layer in devices with a vertical stacking structure. The direct growth of ZnO films on heterosubstrates such as sapphire has generally led to a high dislocation density of ∼109/cm2 and inferior emission and electrical performance.29 However, the

Table 1. Electrical Properties for NRELO ZnO Films Grown on Nanorods for 20 and 180 min growth time of NRELO 20 min 180 min

resistivity (Ω 3 cm) 1.2  10-1 7.2  10-2

mobility (cm2/V 3 s) 2.1 20

carrier density (cm-3) 1.5  1018 3.4  1018

NRELO process assisted by high-quality and defect-free nanorods reduced the dislocation density in the ZnO top region, as shown in Figure 2e. The undesirably high surface roughness of the NRELO ZnO films currently inhibits the use of our process as an alternative fabrication method for ZnO single crystals. However, based on the structural, optical, and electrical properties of the NRELO ZnO films, the potential of this method as a novel fabrication process is unquestionable. Further research to improve the surface roughness is underway. 4. Conclusions Thick, single crystal ZnO layers with low defect density were produced by depositing vertically arrayed nanorods with heteroepitaxial structures onto sapphire substrates. After coalescence was induced by the lateral growth of grains originating from each nanorod, long-term growth in an MOCVD chamber resulted in thicker ZnO single crystals. Our NRELO ZnO films provide a significant possibility for the direct formation of ZnO supporting layers. The single crystalline nature of the NRELO ZnO films was confirmed by XRD phi-scans of the ZnO {1012} reflection and the DPs. Crystallinity was also enhanced by increasing the film thickness. Besides, the thickest NRELO ZnO film had the highest tensile stress which induced a morphological change. The NRELO ZnO films showed good optical transmittances in the visible wavelength range, as well as excellent n-type semiconducting behaviors. These properties were observed despite considerably large film thicknesses, As such, the NRELO films may be employed as supporting conductive layers in transparent electronics. Acknowledgment. This research was supported by the KRF Grant (KRF-2008-314-D00153), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2009-0078876), and the Pioneer Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2009-0083009).

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