Self-Seeded Growth of GaAs Nanowires by MetalâOrganic Chemical

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Self-seeded growth of GaAs nanowires by metal-organic chemical vapor deposition Sema Ermez, Eric J. Jones, Samuel C. Crawford, and Silvija Gradecak Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00131 • Publication Date (Web): 05 May 2015 Downloaded from http://pubs.acs.org on May 10, 2015

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Crystal Growth & Design

Self-seeded growth of GaAs nanowires by metalorganic chemical vapor deposition Sema Ermez, Eric J. Jones, Samuel C. Crawford, Silvija Gradečak* Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

Abstract: Self-seeded growth of semiconducting nanowires offers significant advantages over foreign metal-seeded growth by eliminating seed-associated impurities. However, density and diameter control of self-seeded nanowires has proven challenging although it is required for integration of nanowires into optoelectronic devices. We report the self-seeded growth of GaAs nanowire arrays on GaAs (111)B, (110) and (111)A substrates by metal organic chemical vapor deposition. Our approach involves two steps: the in situ deposition of Ga seed particles and subsequent GaAs nanowire growth. Control of nanowire diameter and array density is achieved via Ga seed deposition temperature and substrate orientation; increased seed deposition temperatures or changing substrate orientation from (111)A to (110) and (111)B yields reduced areal density and larger nanowire diameters. The density and diameter control approaches could be extended to other self-seeded III-V nanowire material systems.

Keywords: GaAs, nanowire, self-assisted growth

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Nanowires combine the intrinsic properties of semiconductors with low dimensionality, which makes them suitable building blocks for electronic and optoelectronic applications,1-3 such as solar cells,4-8 light emitting diodes,9,

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and high electron mobility devices.11,

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The intrinsic

properties of GaAs, including a direct bandgap of 1.42 eV – suitable for realization of solar cells with theoretical efficiencies up to 32.8%13 – and a high electron mobility (8800 cm2/V.s),14 make GaAs as a technologically appealing material system for a number of applications. To utilize the one-dimensionality of GaAs nanowires in optoelectronic devices, the realization of high-purity nanowires with controlled diameter, density, and alignment is critical. A common method to grow one-dimensional nanowires involves the use of foreign seed particles;15 in this approach, a seed particle alloys with the nanowire material(s) and nanowire nucleation occurs preferentially at the seed particle-nanowire interface. Commonly, nanowire growth is achieved using foreign metal seeds such as Au,16-18 Ni,19, 20 Cu21 or Pd,22-24 which alloy with at least one of the nanowire constituent elements. The area of the seed particle-nanowire interface controls the nanowire diameter and one-dimensional growth occurs via layer-by-layer growth.25-27 While foreign metal seed particles can be used to control nanowire morphology,28, 29 incorporation of metal atoms into nanowires during the growth and/or post-processing can affect the nanowire optoelectronic properties.30-32 Incorporation of the seed atoms depends on the exact materials system, such as the combination of seed particle and nanowire elements, and on the growth conditions including temperature and growth rate. Recent electron microscopy and electron tomography studies have provided direct visualization of seed atoms incorporated into nanowires for Au-seeded33 and Al-seeded34 Si nanowires as well as Au-seeded GaAs nanowires.35 Furthermore, the presence of foreign metal seeds may influence the growth of more complex nanowire structures. For example, in the case of core-shell nanowires, Au incorporation


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into the nanowire during a high-temperature shell deposition has been observed affecting n-type shell doping of GaAs nanowires.36 Finally, for many optoelectronic devices, including nanowirebased solar cells, removal of metal seed particles may be necessary prior to device fabrication to reduce reflection and eliminate non-radiative recombination centers.8, 37 Self-seeded growth, in which one of the nanowire constituent elements is used as the seed particle, circumvents the issue of possible contamination by eliminating the use of foreign seeds. In the case of III-V nanowires, the group-III elements form seed droplets on the substrate and drive the nanowire growth. Self-seeded nanowire growth has been demonstrated in several III-V material systems, including InAs,38-40 InP,41,

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GaAs43,

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and GaP.45 To date, the self-seeded

growth of GaAs nanowire arrays has been achieved by molecular beam epitaxy (MBE). The density and diameter control of the MBE-grown self-seeded nanowires is more challenging compared to the foreign metal-seeded growth because nanowire density depends on the in situ nucleation of the group-III seed particles. To achieve nucleation, MBE GaAs nanowire growth has been performed on substrates with a thin SiOx layer44, 46-49 such that Ga droplets nucleate at pinholes in the oxide layer. In this way, the density of nanowires is affected by parameters such as oxide thickness49 and growth temperature,39,

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whereas the diameter of the nanowires is

affected by the precursor flow rates.46, 49 Recently, density and diameter control of self-seeded GaAs nanowires has been demonstrated by combining droplet epitaxy and self-seeded growth, where GaAs islands are formed by droplet epitaxy on oxide-free substrates and nanowire growth occurs on top of these islands.50 This approach separates the nucleation and growth of nanowires providing control over density and diameter during the nucleation step. Despite the recent success in MBE growth of self-seeded GaAs nanowires, self-seeded growth of GaAs nanowire array by metal organic chemical vapor deposition (MOCVD) has shown


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limited progress, although MOCVD is an industrial-scale technique that can yield high growth rates and high nanowire throughput. Early studies have shown the premise of Ga as a seed material51 and GaAs nanowire growth was achieved on SiO252,

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or bare GaAs substrates;54

however alignment, morphology or density-diameter control of nanowire arrays were limited. While diameter and position controlled nanowires has been grown without a seed particle by selective area epitaxy (SAE) in MOCVD, this method requires mask pattern layer.18,

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In

contrast to the previous studies, here we demonstrate self-seeded growth of GaAs nanowire arrays with controlled density and diameter on GaAs substrates without any mask or oxide layer and using MOCVD. The Ga seed nucleation step, which controls the nanowire diameter and density, is demonstrated to be dependent upon Ga adatom chemical potential, surface mobility, and surface energy of GaAs substrates. We show that the growth of nanowires and resulting morphology depend on the balance of incorporation and extraction rates of Ga into and out of the Ga droplets. By decoupling seed deposition and nanowire growth steps, we achieve control over the nanowire density and diameters. Analysis of our results demonstrates that this technique is general and can be applied to other self-seeded III-V nanowires. Experimental procedure. GaAs nanowires were grown in a cold-walled, horizontal-flow MOCVD under atmospheric pressure using trimethylgallium (TMGa) and arsine (AsH3) as group-III and group-V precursors, respectively. Prior to the growth, GaAs (111)B, (111)A, and (110) substrates were cleaned by sonication in acetone, methanol, and deionized water for 5 min each. During the growth, H2 carrier gas flow was kept constant at 15 slpm (standard liters per minute). As-cleaned substrates were annealed at 600°C for 10 min under hydrogen flow prior to seed deposition in the MOCVD. Ga seed deposition was performed by flowing TMGa at temperatures between 500°C − 600°C with an effective flow rate of 1.67 sccm for one minute,


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except when noted otherwise. After Ga seed deposition, substrates were cooled to the growth temperature under hydrogen, and the nanowire growth was performed by flowing TMGa and AsH3 between 420°C – 435°C. Effective flow rates of precursors during growth range between 0.36 – 1.67 sccm for TMGa and 3.35 – 18.6 sccm for AsH3. At the end of the growth, both precursors were turned off and the chamber was cooled under hydrogen ambient, unless noted otherwise. The morphology of nanowires was studied using a FEI Helios NanoLab 600 scanning electron microscope (SEM), operated at 5 kV. The areal density, diameter, and length of nanowires were measured from SEM images. Cross-sectional transmission electron microscopy (TEM) samples of the nanowire base region were prepared using the lift-out method on the FEI Helios 600 focused ion beam (FIB). The structure of individual nanowires was studied using a JEOL 2010F TEM operated at 200 kV. Ga-seeded GaAs nanowire growth. As described above, we performed Ga-seeded GaAs nanowire growth in two steps: in situ deposition of Ga seed particles (Figure S1) and subsequent nanowire growth (Figure 1). Figure 1a shows the resulting morphology of as-grown self-seeded GaAs nanowires grown on GaAs (111)B substrate at 420°C with V/III ratio of 2. The majority of nanowires exhibit an epitaxial relationship with the substrate resulting in vertical nanowires on the substrate and are observed to have two distinct regimes of tapering (diameter change per length), as shown in Figure 1b. At the nanowire base, significant tapering is observed followed by a discrete reduction in tapering that is maintained throughout the nanowire growth. We suggest that the observed morphology evolves through the following steps (Figure 1c): (i) Ga droplet formation starts as a result of effective in situ TMGa deposition at higher temperatures (500°C – 600°C);56 (ii) after engaging both precursors (TMGa and AsH3) at 420°C – 435°C,


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GaAs nanowires start to nucleate preferentially at the Ga seeds and the Ga droplets transform into GaAs bases; (iii) after the base formation, nanowire growth occurs as a result of TMGa and AsH3 supply, and the nanowire growth ceases when Ga droplet is depleted. It is crucial to separate seed deposition temperature and nanowire growth temperature in Ga-seeded growth by MOCVD. In contrast to MBE, MOCVD requires TMGa decomposition to form Ga adatoms. Therefore, higher temperatures are required for effective TMGa decomposition56 during the seed deposition step compared to the nanowire growth step, when the temperature must be lowered to ensure precursor decomposition only at the seed particle.

Figure 1. Self-seeded nanowire growth. (a) SEM image of a GaAs nanowire array showing the alignment of nanowires on the GaAs (111)B substrate. (b) SEM image of a single nanowire taken at 45º tilt showing the nanowire morphology with two regions (base and nanowire) that have distinct tapering rates. (c) Nanowire growth schematic; in situ Ga seed deposition is followed by nanowire nucleation and growth. The balance of Ga incorporation into the seed


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particle and Ga extraction from the seed particle determines the tapering of the nanowire during growth. Nanowire length and diameter is denoted as l and d, respectively. The tapering rate is determined by the balance of Ga incorporation into the seed particle and its extraction out of the seed particle, dictated by the TMGa and AsH3 flow rates, respectively (Figure 1c). When the incorporation rate of Ga is smaller than extraction rate, Ga is depleted from the seed, which shrinks and consequently yields a tapered nanowire morphology. This tapering behavior was evaluated by measuring the nanowire length (l) as a function of starting nanowire diameter (d) for different growths of same V/III ratio (Supporting Figure S2). For a given growth, the tapering rate T’=d/l depends on the growth parameters and is similar irrespective of starting nanowire diameter, which we successfully modeled by a simple incorporation and extraction mechanism (Supporting Information). Furthermore, to investigate different starting diameters, d, we have varied seed deposition conditions among different growths with same V/III ratio and observed similar tapering rate regardless of the nucleation conditions (Figure S2). Nucleation and growth of nanowires. To gain better understanding of nanowire nucleation and growth mechanisms, we investigated the base and nanowire body using cross-sectional TEM structural analysis (Figure 2a). The base grows epitaxially from the substrate and is free of stacking faults and twin planes (Figure 2b). More detailed TEM analysis is shown in Supporting Figure S3. The selected area diffraction (SAD) pattern obtained from the base and substrate region (Figure 2c) shows a zincblende structure with a growth direction and no twinning reflections were observed. In contrast, the SAD pattern from the nanowire region (Figure 2d) exhibits double-spot features, a signature of twin planes in this region.


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Figure 2. Structure of the nanowire base region. (a) Cross-sectional TEM image of the nanowire base and substrate. The red box outlines the base-substrate junction, shown in high magnification in (b). The green and orange boxes outline the regions used to obtain SAD patterns from the base and nanowire, respectively. (b) High resolution TEM image of the interface between the base and substrate, taken along 〈10-1〉 zone axis. Base region is defect-free whereas a defective overgrowth layer can be seen outside of the nanowire base. (c-d) SAD patterns from the base (c) and nanowire (d) regions. Twinning planes in the nanowire cause double spots seen in the nanowire SAD pattern. (e-f) Schematic representation of two possible base formation mechanisms: base formation occurs as a part of the nucleation process (e) or as a result of preferential sidewall deposition at the substrate-nanowire junction (f).


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Two possible mechanisms that can explain the base formation are: (1) base formation occurs during nucleation (Figure 2e), or (2) base formation occurs as a result of preferential sidewall deposition at the nanowire-substrate junction (Figure 2f). These hypotheses can be examined by the extent of the parasitic overgrowth at the base-substrate interface where nucleation occurs. From higher magnification images obtained at the edge of the base-substrate region (as shown in the red box in Figure 2a), we observe that the overgrowth layer – which appears as a darker contrast due to the presence of defects – stops at the base edge. Simultaneously with the nanowire growth, a parasitic thin film overgrowth on the substrate occurs via excess adatoms that do not contribute to the nanowire growth. This thin-film overgrowth layer has a lower growth rate than the nanowire base because of lower nucleation probability on the bare substrate than below the seed particle, and is defective due to non-optimized conditions for thin film growth. If the base formation occurs as a part of the nucleation process (schematically shown in Figure 2e), the parasitic overgrowth layer would be observed on the substrate but not in the base region. However, if the base formation occurs due to preferential sidewall growth at the substrate-nanowire junction during the growth, as depicted in Figure 2f, the defective overgrowth layer would also be observed in the base region. Cross-sectional TEM study in Figure 2a-d shows that overgrowth layer does not extend into the base region, suggesting that the operating mechanism is as depicted in Figure 2e, and that base formation occurs as a part of the nucleation process. In contrast to the defect-free structure of the base, structural analysis of the nanowires above the base (Figure 3) reveals the presence of rotational twin planes perpendicular to the nanowire growth direction. The sidewalls of the self-seeded GaAs nanowires have a zigzag structure when viewed along the [10-1] zone axis (Figure 3a and 3b) due to a change of sidewall direction after


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each twin plane. The SAD pattern (Figure 3c) confirms the zincblende crystal structure of the nanowires and growth along the direction, while the observed double spot structure in the SAD pattern is caused by symmetry in the crystal structure due to twin planes.57,

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This

observation of twin planes along the growth direction is consistent with twinning commonly observed in III-V nanowires.57-61

Figure 3. Structural analysis of the nanowires. (a) Bright field TEM images of GaAs nanowires along the [10-1] zone axis. (b) High resolution TEM image showing rotational twins. (c) SAD pattern obtained from the region shown in (a) with double spot structure as a result of rotational twins.

Density and diameter control. As discussed above, nanowire growth is governed by the base nucleation step. Therefore, the density and base diameter of nanowires can be determined during the Ga seed deposition step and controlled by varying two parameters that affect the Ga seed formation kinetics: Ga seed deposition temperature and substrate orientation. First, we modified the seed deposition temperature (between 500 °C – 600 °C), while keeping all other growth


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parameters the same (the nanowire growth was performed at 435°C on GaAs (111)B substrates with V/III ratio of 5). In these growth series, nanowires were cooled under AsH3 flow down to 350°C to suppress kinking at the tip of the nanowires during cooling. Top-view SEM images show the change in the areal density of nanowires as a function of the seed deposition temperature (Figure 4a – e); as the seed deposition temperature increases, the areal density is reduced, whereas the average nanowire diameter increases (Figure 4f). The overall nanowire morphology and structure remain the same, with the single-crystalline nanowire base and twinned nanowires (insets of Figure 4a – e), as previously described.

Figure 4. Effect of Ga seed deposition temperature on the nanowire density and diameter. (a-e) Top-view SEM images of GaAs nanowires grown with different Ga seed deposition temperatures (indicated in each image). All growths are performed on GaAs (111)B substrates at 435°C. Nanowires appear as dots in the top-view images because they grow vertically on the


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substrate. Insets show the nanowire morphology obtained at a 45º tilt. (f) Areal density and average diameters of nanowire arrays as a function on the seed deposition temperature. In addition to the seed deposition temperature, the density and diameter of nanowires also depend on the substrate orientation, as shown in Figure 5a-c. In these experiments, seed particle deposition temperature (600 °C) and growth conditions were kept the same, and the nanowires were grown on GaAs (111)B, (110), or (111)A substrates at 435°C with V/III ratio of 11. The differences in nanowire diameter and density indicate that these parameters are dictated by the seed formation dynamics on different surfaces. The areal density increases from (111)B to (110) and (111)A, whereas the nanowire diameter shows the opposite trend. In addition, the nanowires grow along the direction on all substrates, resulting in vertical nanowires on GaAs (111)B and (111)A surfaces and inclined nanowires on GaAs (110) surfaces.

Figure 5. Effect of substrate orientation on nanowire density and diameter. (a-c) GaAs nanowire growth on (111)B, (110) and (111)A substrate, respectively. Areal density is given at the bottomleft corner of each image.

Temperature-dependent density and diameter trends (Figure 4) can be explained by the surface diffusion of Ga adatoms and/or seed nucleation kinetics. At lower temperatures, limited mobility of Ga adatoms causes slow surface diffusion and results in a higher density of smaller seed


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particles, as seen in Figure 4f. The density-diameter trend is disrupted when the seed deposition temperature is increased from 575 °C to 600 °C, the average diameter decreases contrary to the density-diameter trend (Figure 4f). This can be due to increased Ga adatom desorption at higher temperatures. Similar temperature-dependent density trends were observed in island growth by the droplet epitaxy, which is a similar process to the nanowire nucleation step described in this paper, and have been attributed to surface diffusion.62 The seed nucleation kinetics takes into account changes in chemical potentials and surface energies, and is described in Supporting Information in more details. We have investigated the effect of seed deposition temperature for a given Ga amount and observed that density and diameter of nanowires are coupled. However, in a recent self-seeded MBE study,50 nanowire density and diameter were controlled by the seed deposition temperature and Ga flow supplied during the seed deposition, respectively, suggesting that a similar approach could be applied in the self-seeded MOCVD. We note that obtaining nanowires with smaller diameter than observed in this work could be possible by further decreasing the seed deposition temperature, but higher TMGa flows would be required to overcome its incomplete decomposition at lower temperatures. We now consider the same parameters (nucleation kinetics and surface diffusion) to understand the differences in density and diameter as a result of substrate orientation. Because the trend of larger diameter and lower density nanowires is similar to the temperature-controlled seed deposition, we first consider the effect of Ga adatom surface diffusion length on the seed formation dynamics. Denser nanowires on the GaAs (110) surface compared to (111)B surface would imply that the surface diffusion length on (110) surface should be shorter, but it has been reported that Ga surface diffusion length on GaAs (110) surface is longer than on GaAs (111)B surface.63 Therefore, surface diffusion cannot explain the observed trends. Rather, we should


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consider the seed nucleation kinetics. According to the nucleation theory, the Gibbs free energy of seed particle nucleation, ∆GN, is related to the chemical potentials of Ga adatom, Ga atom in liquid seed particle, and surface energies (see Supporting Information for more details). The trend of larger diameter and lower density nanowires would be observed when the critical nucleus size of the seed particle (the size above which the growth of the particle is energetically favorable) is larger on (111)B surface compared to (110) surface. Larger critical nucleus size on (111)B suggests that the Ga adatom chemical potential (µsurfGa) or substrate surface energy (γsv) under Ga-rich conditions is lower on (111)B surface compared to (110) surface. The chemical potential of Ga adatoms on a (111)B surface should indeed be lower, because under ideal conditions, GaAs (111)B surface is As-terminated, whereas GaAs (110) surface has an equal distribution of Ga and As atoms on the surface. Same arguments also apply to describe the density-diameter differences on (111)A surface compared to (110) or (111)B surfaces. By understanding the effect of surface diffusion and seed nucleation kinetics, it is possible to predict density and diameter trends of self-seeded III-V nanowires in general. Firstly, the effect of seed deposition temperature on nanowire density and diameter would be similar in other III-V material systems, i.e. increasing density with decreasing seed deposition temperature, as long as the seed deposition temperature range is lower than the desorption-limited region for adsorbed seed adatoms. Secondly, the choice of the substrate is expected to result in similar trends for density and diameter, i.e. the areal density should increase for higher chemical potential of adatoms. Beyond the effect of temperature and substrate orientation, prediction of the trends based on material system selection is more complicated. When the seed particle material changes, the seed surface energy, seed-substrate interface energy and chemical potential of adsorbed seed atoms change as well, and the critical nucleus size of seed particles is a function of


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all of these (Supporting Eqn. S6). If surface energies are dominant, lower surface energy metal seed particles, or higher surface energy substrates would result in increased density and smaller diameters. The effect of chemical potential will be such that higher chemical potential of adsorbed atoms would result in smaller critical nucleus size, therefore higher density would be expected. Conclusions. We have demonstrated controlled density and diameter of self-seeded GaAs nanowire array growth by MOCVD. The growth is achieved in two steps; in situ Ga seed deposition at higher temperatures followed by nanowire nucleation and growth at lower temperatures. Two distinct parts of the nanowire were observed: a defect-free nanowire base with a high degree of tapering followed by the nanowire itself, which has a lower degree of tapering but contains twin planes perpendicular to the nanowire growth direction. The density and diameter of the nanowire arrays were controlled by varying the seed deposition temperature and substrate orientation. These parameters affect the Ga adatom surface diffusion, Ga adatom chemical potential, and substrate surface energies. By understanding the nucleation kinetics, we demonstrated that higher density and smaller diameter nanowire arrays are achieved by lower seed deposition temperatures or by using (111)A or (110) substrates instead of (111)B substrates. Nanowire growth via a self-seeded mechanism eliminates possible impurities from foreign metal seed by using one of the constituent elements of the nanowire as the seed particle. Our findings on the nucleation and growth mechanisms and associated density-diameter control of GaAs nanowire arrays can be readily extended to other III-V material systems and serve as a general approach to self-assisted nanowire growth.


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ASSOCIATED CONTENT Supporting Information Available: Ga seed deposition step, nanowire tapering, growth model, details of nanowire nucleation, investigation of radial shell growth and details of densitydiameter dependence on seed deposition temperature and surface orientation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Science Foundation (NSF) ERC – QESST (EEC1041895) and the MIT Energy Initiative. The authors acknowledge access to Shared Experimental Facilities provided by the MIT Center for Materials Science and Engineering supported in part by MRSEC Program of National Science Foundation under award number DMR-08-19762.


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For Table of Contents Use Only

Self-seeded growth of GaAs nanowires by metal-organic chemical vapor deposition Sema Ermez, Eric J. Jones, Samuel C. Crawford, Silvija Gradečak*

Synopsis: MOCVD self-seeded growth of GaAs nanowires with controlled density and diameter on GaAs substrates was demonstrated. Our approach involves two steps: the in situ deposition of Ga seed particles and subsequent GaAs nanowire growth. Control of nanowire diameter and array density is achieved via Ga seed deposition temperature and substrate orientation.


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Figure 1 83x88mm (300 x 300 DPI)



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Figure 2 83x103mm (300 x 300 DPI)


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Figure 3 83x63mm (300 x 300 DPI)



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Figure 4 176x101mm (300 x 300 DPI)


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Figure 5 177x50mm (300 x 300 DPI)



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TOC 82x44mm (300 x 300 DPI)


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Self-Seeded Growth of GaAs Nanowires by MetalâOrganic Chemical

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Self-Seeded Growth of GaAs Nanowires by MetalâOrganic Chemical