Letter pubs.acs.org/NanoLett
Controlled Modulation of Diameter and Composition along Individual III−V Nitride Nanowires Sung Keun Lim,†,§ Sam Crawford,†,§ Georg Haberfehlner,‡ and Silvija Gradečak*,† †
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ CEA-Leti, MINATEC Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France S Supporting Information *
ABSTRACT: Semiconducting nanowires have unique properties that are distinct from their bulk counterparts, but realization of their full potential will be ultimately dictated by the ability to control nanowire structure, composition, and size with high accuracy. Here, we report a simple, yet versatile, approach to modulate in situ the diameter, length, and composition of individual segments within (In,Ga)N nanowires by tuning the seed particle supersaturation and size via the supply of III and V sources during the growth. By elucidating the underlying mechanisms controlling structural evolution, we demonstrate the synthesis of axial InN/InGaN nanowire heterojunctions in the nonpolar m-direction. Our approach can be applied to other materials systems and provides a foundation for future development of complex nanowire structures with enhanced functionality. KEYWORDS: Nitrides, nanowires, diameter modulation, heterostructure, polarity
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emiconductor nanowires are quasi-one-dimensional single crystals that have emerged as promising materials for the development of photonic and electronic devices with enhanced performance.1−3 Control of the nanowire composition and morphology is an ultimate goal in designing novel nanowire devices with functionalities that are superior to those of current thin film technologies. Spatial variation of the composition forms the basis of many functional devices, including light emitting devices (LEDs),4 lasers,5 high electron mobility transistors,6 and multijunction solar cells.7 Furthermore, diameter modulations along the nanowire axis could be used to enhance device performance, including improved light trapping by minimizing reflection and maximizing absorption,8 efficient thermoelectric conversion through increased phonon scattering in structures with multiple diameter modulations,9 or enhanced field emission from thin nanowire regions with increased curvature. 10 Simultaneous control over both composition and morphology would further expand the realm of possible nanowire architectures, but achieving this goal has so far been challenging or elusive. Particle-mediated nanowire growth, by either the vapor− liquid−solid11 or vapor−solid−solid12 mechanism, is a versatile technique in which the nanowire morphology depends on the size and shape of the metal seed particle. Using this approach, nanowire morphology can be modulated in several ways including self-organized oscillatory motion of the seed particle due to energetic instability at the vapor/liquid/solid (v/l/s) triple-phase boundary,13,14 which can be promoted by impurities15 and results in a high density of stacking faults13 © XXXX American Chemical Society
or by using a template that confines the seed particle and limits the size of the particle/nanowire interface.8 Diameter changes have also been related to changes in nanowire kinetics, but the fluctuations were attributed to the presence of defects.16 In this work, we demonstrate controlled modulation of both the diameter and the composition along individual (In,Ga)N nanowires through the modulation of the seed particle size by tuning the supply of precursors during the growth. Our approach is simple and versatile and does not require a template nor the introduction of impurities or other defects detrimental to the device performance. We applied this idea to demonstrate diameter modulation of binary InN and GaN nanowires and then extended the approach to grow composition- and diameter-modulated InN/InGaN axial heterostructure nanowires in the nonpolar [1−100] mdirection. We concentrate on III−V nitrides because of their superior optoelectronic properties and broad-range bandgap tunability that make them attractive material systems for a variety of heterostructure-based optoelectronic devices.4−7,17 In addition, nonpolar heterojunctions are predicted to play a critical role in realization of high-efficiency nitride-based optoelectronic devices,18−20 as they mitigate effects associated with the quantum-confined Stark effect observed in polar heterojunctions. To date, III−V nitride axial heterostructures have only been reported in the polar [0001] c-direction;21,22 we therefore Received: January 11, 2012
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focus on the synthesis of axial heterostructures in the nonpolar [1−100] m-direction. By analyzing composition, morphology, and growth rate of the (In,Ga)N heterostructures, we elucidate the underlying mechanisms controlling the structural evolution, which can be readily extended to other nanowire materials systems. During particle-mediated nanowire growth, nanowire diameter (d) depends on the seed particle volume (V) and the particle−nanowire contact angle (β) and can be described as ⎛ 3V ⎞1/3 (1 + cos β)1/2 d = 2⎜ ⎟ ⎝ π ⎠ (1 − cos β)1/6 (2 + cos β)1/3
(1)
where V depends on the size of the starting seed and on the amount of the source material incorporated into the seed.23 In the case of III−V nitrides, the group III elements (In, Ga, Al) alloy readily with the metal seed, but the group V source (N) is generally insoluble.24 Instead, N reaches the v/l/s interface and diffuses along the l/s interface, where III−V solidification occurs.25,26 Therefore, we posit that the flow of III and V sources can separately control the rates of III source incorporation into and extraction out of the seed particle, respectively, consequently altering in situ the seed composition, nanowire diameter, and growth rate. To test our hypothesis, we first investigated the unique role of the V source during the growth of binary InN nanowires synthesized by chemical vapor deposition via Au particlemediated growth at 560 °C (see details in Supporting Information, including Figure S1). Scanning electron microscopy (SEM) images (Figure 1a) show that the CVD-grown InN nanowires have either hexagonal or triangular crosssections, while bright field transmission electron microscopy (BF-TEM) images and corresponding selected area diffraction (SAD) patterns confirm that these grow along the [0001] direction (c-InN) and the [1−100] direction (m-InN) of the wurtzite crystal structure, respectively (Figure 1b). The growth direction is determined by the growth substrate as a majority of nanowires grew along the c- and m-directions on c- and a-GaN substrates, respectively. To study the nanowire diameter modulation in InN nanowires, we kept the supply of group III source (In) constant but modulated the flow of the group V precursor (ammonia, NH3) between 300 standard cubic centimeters per minute (sccm) and 50 sccm. Notably, the resulting InN nanowires (Figure 1c and Supporting Information, Figure S2) show distinct regions with larger and smaller diameters (hillocks and valleys, respectively). These diameter variations can be induced in a controlled manner, as we demonstrated by varying the durations of low and high NH3 flow in three different growths (Figure 1d); the lengths of the hillocks and valleys were proportional to the durations of low and high NH3 flow, respectively, indicating that lowering NH3 flow yields nanowire diameter expansion, while resumption of high NH3 flow causes contraction back to their original diameter. Additionally, when the durations of low and high NH3 flow were equal (growth I), the hillocks were 39 ± 7% shorter than the valleys, suggesting that reduced NH3 flow also reduces the growth rate. According to eq 1, the nanowire diameter variations can be related to changes in the seed particle volume, which we estimated from the size of the initial Au seed particle and the In composition in the resulting Au−In alloy particle measured using energy dispersive X-ray spectroscopy (EDS, Figure 2).
Figure 1. (a) SEM images of c- and m-InN nanowires grown with 300 sccm NH3 and transferred to a TEM grid. (b) SAD patterns (left) and BF-TEM images (right) of m- and c-InN nanowires. (c) BF-TEM image (top) and schematic illustration (bottom) of a diametermodulated m-InN nanowire from growth III, described in (d) (corresponding results for nanowires from growths I and II are shown in Supporting Information, Figure S2). The numbers 1−3 refer to the different segments, each consisting of durations of low and high NH3 flow. (d) Growth time and normalized lengths of hillocks (low NH3 flow) and valleys (high NH3 flow) in m-InN nanowires from growths I−III. Lengths were normalized to the length of the valley grown with 300 sccm NH3 for 10 min and are proportional to the corresponding growth times. Insets schematically illustrate the NH3 flow as a function of time in growths I−III. All scale bars represent 100 nm.
(Here, we take into account that N is insoluble in Au.) We note that the in situ composition of the particle during growth can be different from the ex-situ EDS measurements, because some of the supersaturated In can be extracted from the seed particle during the postgrowth cooling step. In our InN nanowires, however, a localized In2O3 layer (bcc, Ia3̅, a = 1.0117 nm27) was observed between the InN nanowire and the Au−In particle (Figure 2a,b), which we attributed to oxidized (through reaction with impurity oxygen or water vapor in the reactor) In extracted during cooling and included in the seed volume calculations. From EDS measurements of multiple nanowires (Figure 2c), we found that the seed particles have greater In composition (71 ± 6%) after lower NH3 flow than after higher NH3 flow (50 ± 7%). Here, the error of the measurements B
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Figure 2. (a) BF-TEM image (left) of a c-InN nanowire grown with 300 sccm NH3 taken along the [11−20] zone axis. White box indicates region of the HR-TEM image (right). Scale bars in the BF- and HRTEM images represent 50 and 5 nm, respectively. (b) Fast Fourier transform (FFT) of the HR-TEM image shown in (a). The FFT consists of two separate sets of spots that can be attributed to InN at the [11−20] zone axis of the wurtzite structure (white) and In2O3 at the [011] zone axis of the bcc structure (yellow). (c) Dark-field scanning transmission electron microscopy (DF-STEM) images of cInN nanowires grown with 300 and 25 sccm (left and right insets, respectively) of NH3, and EDS spectra collected from within the redand blue-dashed regions in the DF-STEM images. Both scale bars represent 50 nm. The regions with In2O3 and Au were defined based on the results of EDS mapping (not shown). EDS spectra were normalized by the Au peak.
Figure 3. (a) BF-TEM image of an m-GaN nanowire taken along the [0001] zone axis. Segments 1−4 correspond to the flows described in (b). (b) Average diameter of each nanowire segment normalized to the diameter of segment 2. The inset shows H2 flow through the Gacoated quartz tube injector during the nanowire growth. (c,d) SEM images (left) of GaN nanowires viewed along the [0001] c-axis (c) and [11−20] a-axis (d), as confirmed by SAD in Supporting Information, Figure S3. Diagrams on the right schematically illustrate the changes in nanowire morphology in cross-sectional (top) and zone axis (bottom) views. Red and blue indicate wide and narrow diameters, respectively. All scale bars represent 100 nm.
includes small compositional differences among different nanowires as well as accuracy in defining the precise interface between the nanowire and the seed particle regions. Taking into account the molar volumes of liquid In and Au at 560 °C,28 this compositional change would correspond to a 93% volume expansion of the seed particle and, assuming constant β, to a 24% diameter increase, consistent with our diameter measurements (see Supporting Information). While measuring in situ seed composition and accounting for changes in wetting angle would yield a more precise calculation, our results clearly show that reduced NH3 flow induces increased seed particle volume and consequent nanowire diameter expansion. Based on the promising results through source V modulations, we next demonstrated the ability to modulate nanowire diameter using the III source. Here we concentrated on m-directional GaN nanowires and varied the source III (Ga) flux by adjusting the H2 flow through a Ga-coated quartz injector during growth at 835 °C (details in Supporting Information). BF-TEM images (Figure 3a) show that brief cessations of the H2 flow generated short, thin-diameter regions in the GaN nanowire. Furthermore, the change of H2 flow from 50 to 10 sccm reduced, reversibly, the nanowire diameter by 13 ± 5% (Figure 3b) and the growth rate by 65 ± 9%. The triangular cross-section of the nanowire is maintained during these diameter changes, but the c-plane facet remains straight along the nanowire (Figure 3c,d and Supporting Information, Figure S4). These anisotropic diameter changes may be due to differences in surface facet energies between the {0001} facet and the {11−22} facets (see Supporting Information).
For the binary nanowires, reduced V flux during nanowire growth decreased the growth rate but increased the diameter (InN nanowires), whereas a reduced III flux reduced both the growth rate and diameter (GaN nanowires). As we initially suggested, these differences reflect the opposite effects that the III and V fluxes have on the balance between the rates of III source incorporation into and extraction out of the seed particle. We propose a model for III source balancing within the seed particle to explain these phenomena (for more details, see Supporting Information). At steady state, the seed particle volume and nanowire diameter are constant, and the rate of IIIsource incorporation into the seed particle is balanced by the rate of extraction through reaction with V source. When the NH3 flow is reduced during the growth of InN nanowires, the extraction rate of In within the seed particle is reduced, as evidenced by the slower growth rate. The imbalance between the fast incorporation and slow extraction rates leads to increased seed particle volume and, consequently, nanowire diameter. As the chemical potential of In in the seed particle increases, the incorporation rate decreases, while the extraction rate increases. These rates eventually become balanced, and a new steady-state diameter is reached. The opposite process occurs upon resumption of high NH3 flow, and the original, smaller diameter is restored. For GaN, on the other hand, a reduced III flux decreases the incorporation rate of Ga into the seed particle, yielding a lower supersaturation of Ga within the seed particle and a consequent decrease in both diameter and growth rate. C
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minima and maxima. EDS measurements (Supporting Information, Figure S6) further show that the seed particles contain 34 ± 4% and 46 ± 6% of group III elements for the InGaN (thin) and InN (thick) segments, respectively, implying that seed particle volume is smaller during the growth of the InGaN segments, consistent with the measured reduction in diameter. Motivated by the unique anisotropic diameter changes observed in m-directional GaN nanowires, we further investigated the Ga-driven diameter changes in axial heterostructures by applying quasi-periodic TMG pulses (Figure 5a) during InN nanowire growth. This process yielded nanowires with corresponding quasi-periodic diameter variations along the nanowire axis, i.e., a “caterpillar” morphology (Figure 5b). The full three-dimensional morphology of the nanowire heterostructure was reconstructed using SEM and electron tomography analyses (Figure 5c−e and Supporting Information, videos S1 and S2) and further confirmed by TEM and SAD measurements (Supporting Information, Figure S7). Interestingly, the volume changes in these nanowires heterostructures are accommodated by changes in relative nanowire facet lengths, which consequently alter the cross-sectional geometry along the nanowire in two distinctive steps (Figure 5f). At position 1, the nanowire has a truncated triangular cross-section consisting of two {11−22} facets and a {0001} facet. Small truncated facets of the same family of planes ({11−22} and {0001}) in the corners of the nanowire become more dominant upon diameter reduction, first yielding a diameter reduction along the ⟨11−22⟩ direction (step I in Figure 5g). Next, the upper {0001} facet broadens in addition to the lower corner {11−22} facets, yielding a highly truncated (almost hexagonal) nanowire cross-section in step II (Figure 5g). Consistent with the aperiodic heterostructure shown in Figure 4, EDS and tomography results show that Ga is localized within the thindiameter regions (Supporting Information, Figure S8), corresponding to step II of the morphology evolution. To explain the two-step cross-section evolution and associated volume changes along the axial heterostructure nanowires, we suggest that step I occurs during Ga incorporation into the seed particle, while step II occurs during InGaN solidification. In step I, TMG is introduced and Ga incorporates into the seed particle, but InGaN is not yet formed. The lower {11−22} facets expand at the expense of the upper {11−22} facets and the lower {0001} facet. Due to the inherent c-directional polarity in nitrides, the polarity of the upper and lower facets is opposite. The relative abundance of III and V species on surface facets affects the relative stability of facets of different polarity within the same family29 as well as the stability of {0001} facets.30 Thus the presence of Ga adatoms on the sidewall facets is presumably responsible for the changes observed in step I. In step II, the formation of InGaN yields abrupt changes in facet energies and is coincident with the broadening of the upper {0001} facet. The most stable {0001} facets for InN and GaN have been observed to be (000−1) and (0001), respectively.31,32 We therefore suggest that the lower facet is (000−1), and the upper (0001) facet is stabilized as Ga is incorporated into the nanowire. While the exact geometry of seed particles and the composition-dependent surface energies should be considered to quantitatively describe the shape evolution of nitride nanowire axial heterostructures, our model is consistent with the two-step cross-section evolution and the volume changes associated with the addition of Ga. Furthermore, these results
Taken together, the results above provide clear evidence that controlled diameter variations can be achieved in binary InN and GaN nanowires. As discussed above, future device applications will require more complex nanowire heterostructures, and we therefore extended our approach to demonstrate synthesis of axial InGaN heterostructures using a similar flow-controlled approach. InN/InGaN heterostructures were grown in the nonpolar m-direction by introducing intermittent trimethylgallium (TMG) pulses during CVD growth of m-InN nanowires (Figure 4a). The SAD pattern
Figure 4. (a) TMG pulses introduced during the growth of m-InN nanowires resulting in InN/InGaN axial heterostructures. (b,c) SAD (b) and BF-TEM (c) images taken along the [0001] zone axis of a nanowire with three thin-diameter regions corresponding to segments A, B, and C in (a). (d) DF-STEM image (top) of a InN/InGaN axial heterostructure taken along the [11−20] zone axis. Profiles of the In and Ga compositions (center) and the nanowire diameter (bottom) were obtained along the black arrow in the DF-STEM image. All scale bars represent 100 nm.
and BF-TEM image (Figure 4b,c) show that the resulting nanowire remains m-directional throughout its entire length and has three distinct regions (A, B, and C) of reduced diameter corresponding to TMG pulses (Supporting Information, Figure S5). Furthermore, EDS line scans (Figure 4d) show that Ga composition anticorrelates with the nanowire diameter variations. Within region C, the Ga composition was measured to be 16 ± 2% and 8 ± 1% at the respective diameter D
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Figure 5. (a) TMG flow during nanowire growth of m-directional, caterpillar-shaped InN/InGaN axial heterostructures. (b) SEM image (left) and corresponding tomography reconstruction (right) of a caterpillar-shaped nanowire lying on a lacey carbon TEM grid. (c−e) SEM images (left) and corresponding tomography reconstruction (right) of caterpillar-shaped InN/InGaN nanowires taken along two ⟨0001⟩ directions and one ⟨11−22⟩ direction, described by insets and supported by diffraction patterns (Supporting Information, Figure S7). Colors represent different facets defined in (i). (f) Reconstructed model of the nanowire morphology using the same facet color scheme as in (c−e). (g) Cross-sections at positions 1−3 extracted from the tomography results at the positions labeled in (b) and illustrated in (f). Colors of cross-sections and facets correspond to those in (c−f). The facet inclination in the upper left corner of the reconstructed cross-sections is an artifact resulting from the limited tilt range of the tomography series. (h) Overlaid cross-sections shown in (g) emphasize the evolution during steps I and II. (i) Identification of the nanowire facets and their two-step evolution. The “upper” {0001} and {11−22} facets are shown in red/purple and orange, respectively. The “lower” {0001} and {11−22} facets are shown in green and blue, respectively. All scale bars represent 50 nm.
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indicate that one of the significant factors controlling shape evolution is the surface energy at each nanowire facet. Relative facet energies could potentially be modified by changes in growth parameters such as temperature and pressure,33 in addition to the flow-controlled approach that we have demonstrated here. In conclusion, our results provide a foundation for the controlled synthesis of complex nanowire architectures. By varying both III and V sources during nanowire growth, we reveal that imbalances between the rates of source incorporation into the seed particle and extraction by solidification can generate changes in nanowire diameter by altering the volume of the seed particle. Studies on binary InN and GaN nanowires demonstrate that the length and width of individual nanowire segments can be continuously and reversibly tuned using this technique. The InN/InGaN heterostructures, in addition to constituting the first demonstration of nonpolar-directional InGaN axial heterostructure nanowires, reveal mechanisms of simultaneous composition and morphology changes, which offer further opportunities to more optimally design nanowire architectures for functional nanowire-based devices. Our approach to modulate nanowire composition and diameter is based on the fundamental processes governing the seedmediated nanowire growth and, therefore, represents a simple and general strategy that is broadly applicable to other nanowire materials systems.
ASSOCIATED CONTENT
S Supporting Information *
Electron tomography videos, methods, morphology of InN nanowires grown with modulated NH3 flow, source-balancing model for binary III−V nitride nanowires, morphology of GaN nanowires with diameter modulated by Ga flux, and composition and morphology of InN/InGaN axial heterostructure nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Corresponding author:
[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the MRSEC Program of the National Science Foundation under award no. DMR0819762, in part by NSF CAREER award no. DMR-0745555. The authors acknowledge access to Shared Experimental Facilities provided by the MIT Center for Materials Science Engineering supported in part by MRSEC Program of National Science Foundation under award number DMR-0213282 and E
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(29) Joyce, H. J.; Gao, Q.; Hoe Tan, H.; Jagadish, C.; Kim, Y.; Zou, J.; Smith, L. M.; Jackson, H. E.; Yarrison-Rice, J. M.; Parkinson, P.; Johnston, M. B. Prog. Quantum Electron. 2011, 35, 23−75. (30) Northrup, J. E.; Neugebauer, J. Phys. Rev. B 1999, 60, R8473− R8476. (31) Jain, A.; Weng, X. J.; Raghavan, S.; VanMil, B. L.; Myers, T.; Redwing, J. M. J. Appl. Phys. 2008, 104, 053112. (32) Gao, Y.; Craven, M. D.; Speck, J. S.; DenBaars, S. P.; Hu, E. L. Appl. Phys. Lett. 2004, 84, 3322−3324. (33) Hiramatsu, K.; Nishiyama, K.; Onishi, M.; Mizutani, H.; Narukawa, M.; Motogaito, A.; Miyake, H.; Iyechika, Y.; Maeda, T. J. Cryst. Growth 2000, 221, 316−326.
to the nanocharacterization platform (PFNC) at MINATEC Campus in Grenoble. S.C. thanks NSF for a Graduate Research Fellowship. We thank Pierre Bleuet for fruitful discussion on electron tomography as well as Narciso Gambacorti and the MIT-France MISTI program.
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