J. Phys. Chem. C 2009, 113, 7031–7037
7031
Strain-Driven Growth of Zinc Oxide Nanowires on Sapphire: Transition from Horizontal to Standing Growth Babak Nikoobakht,* Susie Eustis,†,‡ and Andrew Herzing† Surface and Microanalysis Science DiVision, National Institute of Standards and Technology, 100 Bureau DriVe Stop 8372, Gaithersburg, Maryland 20899 ReceiVed: December 09, 2008; ReVised Manuscript ReceiVed: March 11, 2009
Recently, we showed large-scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. Here, in examining the cross sections of such nanodevices, we use high-resolution transmission electron microscopy (HRTEM) and large-angle, convergent-beam electron diffraction (LACBED). We show how horizontally grown ZnO NWs influence their underlying sapphire surface and how substrate influences the growth directionality of the NWs. As a NW grows on sapphire, the substrate experiences a compressive strain of ≈7% in its [0001]sap direction (along the width of a NW) to minimize its lattice mismatch with the ZnO NW. Accordingly, ZnO expands along its width to improve its lattice match with the sapphire. The growth direction of (11j00) is suggested to be the direction that produces a lower lattice strain between ZnO and sapphire. Analyses of NW/sapphire interfaces show that single-crystal NWs grow epitaxially and semicoherently with many fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations at the interface to local relaxation of zinc oxide strain into the sapphire surface. This relaxation is in agreement with the observed deformation of the sapphire underneath the NWs. We also define a critical NW thickness beyond which the growth mode changes from horizontal to standing. Results indicate that below this thickness, gold nanodroplets partially wet both sapphire and ZnO crystals. Above the critical thickness, gold preferentially wets the ZnO nanocrystal, and formation of misfit dislocations at the interface becomes energetically favorable. Combination of these two effects is used to explain the observed change in the growth modes of the NWs. Introduction The processes that guide and control “zero”- and onedimensional (1D) growth of nanocrystals on solid surfaces have been of great interest in the past two decades. The motivation is the possible technological applications of such structures in ongoing miniaturization of optoelectronics1,2 and sensing devices.3 In this direction, 1D nanocrystals or nanowires (NWs) are shown to have potential applications in electrically driven nanodevices.4 NWs are generally formed either in standing5-8 or horizontal modes.9-11 Standing NWs are grown on a variety of substrates, regardless of their epitaxial relationship with the substrate.12-14 On the other hand, growth of horizontal NWs is limited to certain materials and substrates due to the strict requirements for 1D crystal growth. These growth methods typically require dissociation and reconstruction of surfacedeposited atoms on a heated silicon surface to drive the formation of NWs. In these methods, 1D nanocrystal growth is promoted due to the anisotropy in surface energy,9 anisotropy in crystal lattice match,10 and symmetry breakdown of the underlying substrate.11 However, using these growth strategies, both the width and length of 1D islands increase with growth time, and the island locations are random. In our previous study, horizontal ZnO NWs were grown from gold nanodroplets using the vapor-liquid-solid (VLS) mechanism. 15 In this process, despite other in-plane growth methods, gold nanodroplets remain the active site in elongation of NWs. * Corresponding author. E-mail:
[email protected] † NIST-NRC postdoctoral associates. ‡ Present address: Directed Vapor Technologies International, Inc., 2 Boar’s Head Lane, Charlottesville, VA 22903.
10.1021/jp810831z
We have used this effect to selectively grow ZnO NWs at designated locations and developed a method for large-scale fabrication of NW field-effect transistors.16 The in situ NW growth and device assembly was shown to be an advantage of this method for commercial development of nanowire-based devices. This study is part of a larger project to further understand the potential structural changes of the horizontal ZnO NWs due to the aging or electron transport in a device. In the current work, we characterize the ZnO-sapphire interface before electron transport through NWs. We use focused ion beam (FIB) for sample preparation; high-resolution transmission electron microscopy (HRTEM); and large-angle, convergent-beam electron diffraction (LACBED) to characterize NW cross sections, their interface with a-plane sapphire, and the extent of interface coherency in a lattice-mismatched system. We use these results to explain the strained epitaxial growth and its effect on loss of NW growth directionality above a certain thickness. We also propose a mechanism for change in the growth mode from horizontal to standing. Experimental Growth of Horizontal NWs. Semiconductor NWs and whiskers have been prepared using the VLS mechanism in which metal droplets act as growth sites to produce NWs.5-8 Using similar principles, we grow horizontal NWs on a singlecrystal a-plane sapphire surface.15 Briefly, a mixture of ZnO (99.999%, puratonic) and graphite (300 mesh, 99%) (0.15 g, 1:1 mass ratio) was loaded on a Si substrate and positioned at the center of an inner tube (13 cm length, 1.9 cm inner diameter). Subsequently, this tube containing a sapphire substrate was
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 04/02/2009
7032 J. Phys. Chem. C, Vol. 113, No. 17, 2009 inserted into a tube furnace such that the mixed powder was placed at the center of an outer tube (80 cm length, 4.9 cm inner diameter). The tube furnace temperature was set at 900 °C, with a ramp rate of 110 °C/min, for 10 min under 0.6 standard liters per minute (SLPM) flow of 99.99% Ar gas. In this process, NWs grow only from the nanodroplets that are located at the edges of a gold pattern. 16 Gold features with known positions are made from a photolithographically generated pattern. FIB Cross-Sectioning. Using a FEI DualBeam instrument, a combination of a focused Ga ion beam and a scanning electron microscope (SEM), electron-transparent cross sections of horizontally grown NWs on a-plane sapphire were prepared. This technique, which is often called FIB-assisted membrane liftout, is the best available method in terms of resolution and site specificity for preparing TEM specimens.17,18 In this study, the procedure for making the membranes was adapted from that described in ref 19. The cross sections of horizontal NWs were characterized using a FEI-Philips CM 300 TEM with a 300 kV accelerating voltage. Data obtained from HRTEM lattice images were compared with those from the simulation of bulk crystal lattices acquired from the inorganic crystal structure database.20 Through-focus series were obtained for all of the images shown. Images were chosen to highlight the desired feature, but measurements and trends were obtained from multiple images and multiple NWs. All the measured and reported lattice dimensions in this work have an uncertainty of (0.05 Å. It is important to realize that the measurement of lattice plane spacings in HRTEM images is influenced by uncertainty due to contrast changes brought about by effects related to the specimen thickness and microscope defocus. Because of this uncertainty, the results presented here represent a semiquantitative illustration of the strain state of ZnO NW and sapphire substrate. LACBED analysis of the NW cross section sample was carried out following the method described by Tanaka et al. and using a FEI 80-300 Titan operating at 80 kV. To prepare TEM membranes, grown NWs were first coated with a 900 nm silicon oxide layer via plasma-enhanced chemical vapor deposition. Subsequently, the substrate was sputter-coated with about 15 nm of carbon film to reduce the charging effects of the surface during the FIB cross sectioning process. Next, a 1-µm-thick, 2-µm-wide, and 20-µm-long platinum line shown in Figure S1a of the Supporting Information was deposited perpendicular to the NW growth direction using an ion beamassisted chemical vapor deposition process available on the FIB instrument. The Pt deposition was carried out to minimize the extent of Ga ion damage to the area of interest. To make an electron transparent cross section, 8 µm × 20 µm trenches were milled on both sides of the deposited Pt line, as shown in Figure S1b of the Supporting Information. The liftout process was completed by cutting the membrane free from the substrate and retracting the probe as shown in Figure 1b (and Figure S1c of the Supporting Information). Finally, the extracted membrane was attached to a grid for TEM examination. The thickness of the membrane at this stage was about 1.5 µm; this was then thinned down to the electron transparency regime through several ion polishing steps (Figure S1d of the Supporting Information). Results and Discussion Horizontal ZnO NWs are grown on a gold-patterned sapphire substrate (a-plane). The growth, as shown in Figure 1a, is directed and takes place selectively where gold nanodroplets are deposited. Figure 1b shows a membrane containing cross
Nikoobakht et al.
Figure 1. (a) SEM image of directed and epitaxial growth of horizontal NWs. (b) A membrane (cross section of NWs), shown by the arrow, removed from the sapphire substrate and attached to a microprobe. (c, d) ZnO NW cross sections. ZnO NWs with diameters less than ≈16 nm clearly deform the underlying surface, resulting in V-shaped interfaces. In all cross sections, NWs and sapphire are coated with a SiO2 layer.
sections of NWs that is removed from the substrate and ready for attachment to a TEM grid for further treatment. Typical examples of cross sections of NWs after their thinning to an electron transparency regime are shown in Figure 1c and 1d. Profiles of these NWs appear to be semicircular, although in some cases, the presence of facets leads to a semihexagonal appearance. In all the studied cross sections, the sapphire at its interface with ZnO undergoes a deformation that, considering its large elastic modulus (403 GPa), indicates a significant stress in its [0003] direction. In Figure 1c, due to this stress, sapphire crystal is forced to reconstruct, forming a V-shaped interface with the ZnO crystal. The V-shaped interface was more prominent in NWs with diameters
