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Heteroepitaxial Growth and Spatially Resolved Cathodoluminescence of ZnO/MgZnO Coaxial Nanorod Arrays Weizhen Liu,† Yao Liang,‡ Haiyang Xu,†,‡ Lingling Wang,† Xintong Zhang,† Yichun Liu,*,† and Suikong Hark*,‡ Centre for AdVanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal UniVersity, Changchun, PRC, Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong ReceiVed: March 16, 2010; ReVised Manuscript ReceiVed: August 23, 2010
Vertically aligned, single-crystalline ZnO/MgZnO coaxial nanorod arrays were epitaxially grown on GaN substrates by hydrothermal synthesis combined with pulsed laser deposition. Well-defined core/shell heterostructures with high-quality interfaces and coherent epitaxial relationship were confirmed by Z-contrast scanning transmission electron microscopy and line-scan compositional analyses. It is interesting to note that the MgZnO shell thickness gradually decreases from the tip to the base of the nanorods, due to a shadow effect during the deposition. The nonuniform coating allows us to study the carrier confinement and surface passivation effects on a single nanorod. Spatially resolved cathodoluminescence measurements reveal that the band-edge emission intensity of ZnO cores is variable along their length, and depends strongly on the shell thickness. A model, which involves carrier tunneling, surface trapping, and radiative/nonradiative recombination processes, was developed to understand this phenomenon. However, a high-temperature thermal treatment may lead to the stress relaxation and the formation of interfacial defects, and enhance the interdiffusion of interfacial atoms. These degrade the optical quality of coaxial nanorods. Introduction One-dimensional (1D) semiconductor heterostructures with a modulated composition or doping along the axial or radial direction are attracting increased attention as versatile building blocks for nanoscale photonic and electronic devices.1-13 Compared with their axial counterparts, 1D radial heterostructures, especially coaxial core-shell nanowires, offer certain advantages, including a large heterojunction area, effective surface passivation, strong carrier confinement, and waveguide effects. Moreover, energy band engineering can be realized in such 1D heterostructures, giving rise to some novel lowdimensional physical phenomena. Thus, much effort has recently been devoted to study coaxial nanowire heterostructures.6-19 For example, Lieber’s group have demonstrated high-performance solar cells, field-effect transistors, and light-emitting devices based on Si/Ge and group-III nitride coaxial nanowire heterostructures.6-8 ZnO, with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, has currently become a subject of intense research for short-wavelength optoelectronic applications. Extensive experimental investigations have demonstrated that ZnO probably exhibits the richest variety of nanostructures among all materials.20 However, it is known that the surface nonradiative recombinations and surface-mediated deep level (DL) traps may significantly reduce the exciton emissions of ZnO nanostructures.21-23 By coating them with a wider band gap and lower refractive index material, this difficulty can be overcome, due to the confinement of carriers and photons and passivation of surface defects. MgxZn1-xO ternary alloy has a * Author to whom correspondence should be addressed. E-mail:
[email protected] (Y. L.)
[email protected] (S. H.). † Northeast Normal University. ‡ The Chinese University of Hong Kong.
continuously tunable band gap from 3.37 to 7.8 eV and the same wurtzite structure as ZnO (for x < 0.3) with a small lattice mismatch of less than 1%.24 Heteroepitaxial growth of MgZnO on ZnO is expected to produce an atomically abrupt interface. Yi et al. have employed metalorganic vapor phase epitaxy to synthesize coaxial ZnO/MgZnO quantum-well nanowires and nanotubes, and observed a quantum-effect-induced photoluminescence (PL) blueshift.24-27 However, reports on this aspect are very limited. In the present work, we have grown vertically aligned ZnO/MgZnO coaxial nanorod arrays (NRAs) by hydrothermal synthesis combined with pulsed laser deposition (PLD) and have studied the surface coating effect on their luminescence properties by measuring spatially resolved spectra of individual nanorods (NRs). Experimental Section The ZnO/MgZnO coaxial NRAs were grown on p-type GaN/ Al2O3 (0001) substrates in the following two-step process. (1) ZnO NRAs were synthesized by a hydrothermal method without using any catalysts and seed layers. The GaN substrates were submerged in a solution containing zinc acetate dehydrate and hexamethylenetetramine at an equimolar concentration of 0.02 M, then the mixed solution was placed in a preheated oven at 95 °C for 2.5 h. To remove the residual organics and surface contaminations, the products were annealed in air at 500 °C for 30 min. (2) The synthesized ZnO NRAs, together with a bare GaN substrate used as a reference sample, were transferred into a PLD chamber for the MgZnO shell deposition. A Nd:YAG pulsed laser (355 nm, 5 ns, 10 Hz) was employed to ablate a Mg0.15Zn0.85O ceramic target. Before the deposition, the growth chamber was evacuated to a base pressure below 10-5 Pa with a turbo molecular pump. Then, ultrapure O2 gas was introduced into the chamber. The O2 pressure and substrate temperature were kept at 20 Pa and 500 °C, respectively, and
10.1021/jp102395t 2010 American Chemical Society Published on Web 09/15/2010
Cathodoluminescence of ZnO/MgZnO Nanorod Arrays
Figure 1. XRD pattern in semilogarithmic coordinate of ZnO/MgZnO coaxial NRAs; the inset is the enlargement of (0002) diffraction peak.
the growth time was 20 min. The morphology, structure, and composition of the products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), high-resolution transmission electron microscopy (HRTEM, Philips Tecnai 20), X-ray diffraction (XRD), and line-scan energy dispersive X-ray (EDX) microanalysis. The samples were mounted on a coldfinger in a closed-cycle He cryostat for low-temperature PL measurements. The 325 nm line of a He-Cd laser was employed as the excitation source. Its power is ∼1 mW, and the diameter of the laser spot focused on the samples is ∼50 um. Cathodoluminescence (CL) measurements were performed on single NRs using a MonoCL-II system (Oxford Instrument) attached to an SEM. The acceleration voltage and probe current are 20 kV and 1 nA, respectively. The CL/PL measurement conditions are kept the same for different samples. Results and Discussion Figure 1 shows the XRD pattern of ZnO/MgZnO coaxial NRAs. Except for the diffraction signal from the Al2O3 substrate,
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16149 only (000l) diffraction peaks of wurtzite ZnO and GaN were found. However, it is difficult to distinguish the diffraction peak of MgZnO and GaN, due to their almost same c-axis lattice constants and small volume fraction of the MgZnO shell. This point can be supported by XRD results of the MgZnO/GaN reference film (not shown here). XRD studies suggest that a desired epitaxial relationship with the parallel c-axis has been well established between ZnO NRs and GaN film, and the coaxial NRs have an identical crystal orientation. Figure 2 displays the tilted- and top-view SEM images of ZnO and ZnO/MgZnO coaxial NRAs. It can be seen that they are highly ordered and are vertically aligned on the entire GaN substrate. The bare ZnO NRs have diameters of 70-150 nm and lengths of about 2 µm. They exhibit a hexagonal prismlike morphology with smooth side surfaces, and many of them taper gradually toward the tip. After the MgZnO shell deposition, there are slight changes in the morphology. The diameter of coaxial NRs increases to 120-170 nm, thus the MgZnO shell thickness is estimated to be in the range of 10-25 nm. This wide distribution may be attributed to the shadow effect in the PLD process, as discussed below.19 Moreover, the NR surface becomes somewhat rough, and many flat tips are converted into pyramidal ones (See Figure 2d). The former probably results from an island growth mode,24 which occurs when the MgZnO shell exceeds a critical thickness; while the latter is ascribed to the higher axial-to-radial growth rate ratio of the MgZnO shell. The microstructures of single coaxial NRs are examined by HRTEM and selected area electron diffraction (SAED). Only one set of SAED pattern appears in the inset of Figure 3a, which can be indexed to the [21j1j0] zone axis of the wurtzite structure. The diffraction spots from ZnO and MgZnO are indistinguishable because of their nearly equal lattice constants. SAED studies on many NRs confirm that both ZnO core and MgZnO shell are single crystalline, are wurtzite in structure, and that the core grows along the [0001] direction. No diffraction contrast between them is visible in the TEM image, due to their similar
Figure 2. Tilted-view SEM images of ZnO (a and b) and ZnO/MgZnO (c and d) coaxial NRAs; the scale bars in Figures (a, c) and (b, d) are 4 µm and 500 nm, respectively. The inset in (d) is an enlarged top-view image, showing a hexagonal cross section of coaxial NRs.
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Figure 3. (a) Typical TEM image of a single coaxial NR; the inset shows its SAED pattern along the wurtzite [21j1j0] zone axis. (b) Latticeresolved HRTEM image of the MgZnO shell. (c) HAADF-STEM image of a single coaxial NR; the weak contrast indicates the formation of a ZnO/MgZnO core/shell structure. Panels d and e are the line-scan composition profiles of Mg and Zn elements along the axial and radial direction of the NR. (f) A schematic diagram of the growth process and structure of coaxial NRAs.
structural features. The indistinguishable boundary also implies a low density of interfacial defects. However, a weak contrast can be observed in a high-angle annular dark field scanning TEM (HAADF-STEM) image, which is sensitive to the atomic number of the materials. In Figure 3c, the relatively dark outer region of the NR corresponds to the material with lighter elements, namely, MgZnO alloy, because it is a weaker electron scatterer. Figure 3b exhibits a lattice resolved HRTEM image of the MgZnO shell, which further confirms its single-crystalline nature. No embedded particles are found, suggesting that no phase segregation occurs during the MgZnO alloy growth. The clear lattice fringes, with interplanar spacings of 0.52 and 0.28 nm, correspond to the orthogonal (0001) and (011j0) planes of wurtzite MgZnO. The radial and axial line-scan EDX microanalyses reveal different spatial distributions of Mg and Zn elements in NRs. As shown in Figure 3e, Mg and Zn signals have relatively high intensity at the outer and inner region of the NR, respectively. This provides strong evidence for the formation of ZnO/MgZnO core/shell NRs. From the EDX data collected at the NR tip and MgZnO reference film, the Mg content in the MgZnO shell is determined as ∼20 atomic %. It is interesting to note in the axial line-scan profile in Figure 3d that the Mg signal tends to reduce from the tip to the base of the NR, although the noise level is somewhat high because of a very low Mg content on the entire coaxial NR. We believe that the Mg concentration is homogeneous in the MgZnO shell. Thus, this phenomenon implies that the MgZnO shell thickness gradually decreases from the tip to the base. It is obvious that the changing trends of MgZnO shell thickness and ZnO NR diameter are opposite to each other. Thereby, the resulting coaxial NRs have a nearly uniform diameter along their length (See Figure 3f). The shell thickness variation is attributed to a strong shadow effect of neighboring NRs. This effect is common for PLD techniques, where the preferred directionality of the plasma plume hinders the homogeneous coating of the MgZnO shell. Cao et al. have demonstrated that the undesirable shadow effect can be suppressed by reducing the NR area density.19 However, the nonuniform shell thickness offers an opportunity of using a single coaxial NR to study the surface coating effect on its optical properties.
Figure 4. (a) Low temperature (10 K) PL spectra of ZnO and ZnO/ MgZnO coaxial NRAs. (b) RT CL spectra obtained at the tip and base of a single NR, its inset shows the corresponding SEM and panchromatic CL images. (c) RT CL spectrum obtained at the tip of an asgrown and a 650 °C-annealed NRs.
PL spectra of ZnO and ZnO/MgZnO NRAs were measured at low temperatures. As shown in Figure 4a, a dominant narrow peak at 369.7 nm and a weak peak at ∼358 nm are observed in the 10 K spectrum of the coaxial NRAs. The former has the same energy as that of uncoated ZnO NRAs and is assigned to the ZnO donor-bound exciton emission. Obviously, the MgZnO coating increases its emission intensity. The latter is believed to originate from near-band-edge (NBE) emissions of the MgZnO shell, because it is absent in the spectra of ZnO NRAs, and its energy is close to the band gap of the Mg0.2Zn0.8O alloy.28 Its appearance further confirms the presence of the MgZnO coating layer. However, with increasing temperature, the MgZnO emission decays rapidly and is completely quenched above 150 K. Such a rapid temperature quenching behavior may be related to (1) the increased electron-phonon interaction, (2) the presence of surface nonradiative recombination centers and
Cathodoluminescence of ZnO/MgZnO Nanorod Arrays
Figure 5. (a) Typical structure of a single coaxial NR. (b) A schematic diagram showing the energy band structure, the surface trapping, and the radiative/nonradiative recombination processes in the upper and lower regions of the coaxial NR.
DL trapping centers, and (3) the carrier injection from the MgZnO barrier to the ZnO well. The luminescence properties of single coaxial NRs were studied by SEM-CL measurements. Figure 4b shows some representative CL spectra and panchromatic images. An intense ultraviolet (UV) NBE emission from the ZnO core dominates the spectra. The DL emission is very weak and negligible. Thus, the panchromatic CL image is mainly formed by the ZnO NBE emissions. It is worth noting that in the image the CL intensity significantly decreases from the tip to the base of the nearly uniform-diameter coaxial NR. The CL spectra, which are measured in the spot mode, provide the same observations. The integrated NBE emission intensity at the tip is about 10 times stronger than that at the base of the coaxial NR. Such a variation is related to the difference in the MgZnO shell thickness24 and can be understood in terms of the carrier confinement and surface passivation effects. Figure 5 illustrates the energy band structure, the surface trapping, and the radiative/nonradiative recombination processes in the upper and lower regions of the NR. The ZnO/MgZnO coaxial NR forms a quantum-well-like structure, which confines the carriers in the ZnO core and tends to enhance their radiative recombinations. Moreover, the carrier injection from the MgZnO barrier to the ZnO well contributes partly to the emission enhancement. On the other hand, surface states can considerably affect luminescence properties of nanostructures with large surface-to-volume ratios. Dijken et al. have proposed that surface defects and adsorption on bare ZnO nanostructures can trap photogenerated carriers, which then nonradiatively recombine at the surface or tunnel back to DL centers to produce a DL emission.21 In the tip (upper) region of the NR with a thicker MgZnO shell, the effective coating passivates dangling bonds and defect states on the surface of the ZnO core, thereby blocking the undesirable surface-trapping channel and improving its NBE emission efficiency. In contrast, in the base (lower) region, where the shell is very thin and even absent, the carrier confinement is weakened. Although ZnO surface defects are passivated, the carriers can tunnel through the very thin shell and can be trapped by MgZnO surface defect states. As a result, the surface-mediated recombination processes lead to a relatively low UV CL efficiency therein. On the other hand, the interface quality is also crucial for the luminescence efficiency of NR heterostructures. To demonstrate this point of view, the coaxial NR is annealed in air at 650 °C for 30 min. It is known from our previous work that this temperature can cause the interdiffusion of atoms across the ZnO/MgZnO interface, the stress relaxation, and the formation of interfacial defects. It can be seen in Figure 4c that after the sample
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16151 undergoes a thermal treatment, the NBE emission of ZnO significantly reduces, and the undesirable DL emission dominates the whole spectrum. Interestingly, the annealing also activates the room-temperature (RT) NBE emission of MgZnO shells. To suppress the formation of oxygen vacancies, the sample is also annealed in an oxygen atmosphere at the same temperature and time, and similar luminescence change occurs. We consider that the decrease in ZnO NBE emission intensity may be due to the following two points: (1) the excited carriers can be trapped by these interfacial defect states, and then nonradiatively recombine therein or tunnel back to DL centers; (2) with the help of interfacial states, the excited carriers can tunnel through the less sharp interface more easily, and escape from the ZnO well, thereby leading to the reduction of the carrier density in the ZnO. On the other hand, the escape of carriers, that is, the reduction of carrier injection efficiency from the MgZnO barrier to the ZnO well, may be one of the possible reasons for the appearance of weak MgZnO emission. It is also noted that the UV emission of MgZnO appears at ∼350 nm in the RT CL spectrum of the annealed sample, but red-shifts to ∼358 nm in the 10 K PL spectrum of the as-grown sample. We speculate that a possible reason for the observed energy shift may be the stress effect. That is, for the as-grown sample, due to the smaller lattice constant of MgZnO than that of ZnO and the coherent epitaxial relationship between them, a tensile stress is applied to the MgZnO shell. After the sample was annealed, the tensile stress is relaxed by the introduction of interfacial defects. Thus, for the annealed sample, its MgZnO emission peak shifts to the higher energy side. Very recently, a strain-induced band gap modification has also been theoretically predicted in the CdSe/CdTe core/shell nanowire.29 However, more work is needed to completely understand the luminescence behavior of MgZnO. Conclusions In summary, we have used a combined hydrothermal and PLD method to grow vertically aligned ZnO/MgZnO coaxial NRAs. Their core/shell-type structural features were confirmed by HRTEM and STEM-EDX microanalyses. The growth of MgZnO on ZnO is epitaxial with a high-quality interface. The shell thickness variation results in an inhomogeneous spatial distribution of CL intensity, which is discussed based on the carrier confinement and surface passivation. High UV luminescence efficiency makes ZnO/MgZnO coaxial NRAs suitable for developing light-emitting nanodevices, and their highly aligned orientation also facilitates the “bottom-up” integration of nanodevices. However, more work is needed on the precise control over the structure and composition of these coaxial NRs. Acknowledgment. This work is supported by National Natural Science Foundation of China (Grant No. 50725205 and 60907016), Fund from Jilin Province (Grant No. 20100339, 20080102, FH0009), Cultivation Fund of NENU (Grant No. NENU-STC08001), and grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Nos. 411807 and 417507). References and Notes (1) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.-W.; Alivisatos, A. P. Science 2007, 317, 355. (2) Caroff, P.; Wagner, J. B.; Dick, K. A.; Nilsson, H. A.; Jeppsson, M.; Deppert, K.; Samuelson, L.; Wallenberg, L. R.; Wernersson, L.-E. Small 2008, 4, 878. (3) Lu, M.-Y.; Song, J.; Lu, M.-P.; Lee, C.-Y.; Chen, L.-J.; Wang, Z. L. ACS Nano 2009, 3, 357.
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