This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: ACS Nano 2019, 13, 5833−5840 www.acsnano.org III−V Integration on Si(100): Vertical Nanospades Lucas Güniat,† Sara Martí-Sánchez,‡ Oscar Garcia,¶ Mégane Boscardin,† David Vindice,† Nicolas Tappy,† Martin Friedl,† Wonjong Kim,† Mahdi Zamani,† Luca Francaviglia,† Akshay Balgarkashi,† Jean-Baptiste Leran,† Jordi Arbiol,‡,§ and Anna Fontcuberta i Morral*,∥,⊥ † Laboratoire des Matériaux Semiconducteurs, É cole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ‡ Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Downloaded via UNIV AUTONOMA DE BARCELONA on December 9, 2019 at 14:15:19 (UTC). Catalonia, Spain ¶ UPC - Universitat Politècnica de Catalunya, Calle Jordi Girona, 1-3, 08034 Barcelona, Spain § ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain ∥ Laboratory of Semiconductor Materials, Institute of Materials, É cole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ⊥ Institute of Physics, É cole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ABSTRACT: III−V integration on Si(100) is a challenge: controlled vertical vapor liquid solid nanowire growth on this platform has not been reported so far. Here we demonstrate an atypical GaAs vertical nanostructure on Si(100), coined nanospade, obtained by a nonconventional droplet catalyst pinning. The Ga droplet is positioned at the tip of an ultrathin Si pillar with a radial oxide envelope. The pinning at the Si/oxide interface allows the engineering of the contact angle beyond the Young−Dupré equation and the growth of vertical nanospades. Nanospades exhibit a virtually defect-free bicrystalline nature. Our growth model explains how a pentagonal twinning event at the initial stages of growth provokes the formation of the nanospade. The optical properties of the nanospades are consistent with the high crystal purity, making these structures viable for use in integration of optoelectronics on the Si(100) platform. KEYWORDS: nanospades, GaAs nanowires, nanowires, vertical growth, [100] growth, Si(100) B ecause of their exclusive and intriguing properties, semiconductor nanowires (NWs) have drawn increas- ing interest among researchers for their potential applications in a diverse range of fields, from p−n junction This work aims at implementing vertical, III−V-based self- catalyzed VLS-grown nanostructures on Si(100). Due to their preferential [111]B growth direction,21 NWs spontaneously grow at an angle of 35° with the (100) surface, which would strongly compromise their functionality.17 Several groups and high-performance transistors1,2 to light-emitting diodes,3 solar cells,4−6 quantum computing,7−9 terahertz detection,10 reported [100]-oriented III−V growth on III−V substrates and even biology.11,12 To complete their path toward using Au as a catalyst. This constitutes a limited number of systems, indicating the still open potential.22−25 So far, electronic and optoelectronic applications, their up-scalability controlled, defect-free [100]-oriented growth has not been and their compatibility with modern semiconductor processing demonstrated on the Si platform nor in a Au-free manner. need to be tackled. There is currently a joint effort in the NW Here, we propose an original way of engineering the contact community to bring the technology closer to compatibility angle from the initial stages of growth by patterning Si(100) with CMOS technology. A first step was replacing Au as the substrates into oxidized nanoscale pillars. This is one of the key liquid catalyst in the vapor liquid solid (VLS) mechanism,13 by elements that we propose to obtain Au-free vertically [100]- other compounds, avoiding deep level traps in Si.14−16 Highly oriented flat nanostructures on the (100) Si platform, with a uniform arrays have been successfully obtained by a self- yield of currently 10% (number of structures over the total catalyzed VLS method,17−20 allowing Au-free opportunities for solar cells. Yet, those very promising results were exclusively Received: February 25, 2019 obtained on Si wafers oriented in the [111] crystallographic Accepted: April 30, 2019 direction, which is not the industry standard. Published: April 30, 2019 © 2019 American Chemical Society 5833 DOI: 10.1021/acsnano.9b01546 ACS Nano 2019, 13, 5833−5840 ACS Nano Article Figure 1. Scheme for the fabrication of the SiO2 mask around (a) Si nanopillars. (b) A thermal oxidation is done at 900 °C (c) for nominal planar thicknesses ranging from 10 to 100 nm, as seen in the TEM micrograph (e). (d) A spin-coating of a polymeric protective layer and (f) a reactive ion etching offer a directional etching of the dielectric mask. (g) 5 nm is kept at the top of the pillar to keep the Si surface intact. A final hydrofluoric acid bath (h) permits uncovering the Si nanopillar’s top. The sample is then introduced in the MBE chamber, and the growth is initiated with a liquid catalyst droplet predeposition (j). (k) SEM pictures of a Ga predeposition of 10 min on Si at 640 °C. The first picture depicts one pillar among the array shown in the second picture. number of nucleation sites). These flat NW structures, called the Si lattice.31−33 The thermal mismatch between both nanospades, exhibit high-quality crystalline structure with species is also expected to produce further internal stresses extremely few defects, rendering them suitable for photonic (tensile) during the cooling.34 Walavalkar et al.30 showed by and energy-harvesting applications. finite element simulations that the overall built-in stress in the Several studies suggest that the configuration of the droplet Si is likely to be tensile at room temperature. The nature of the catalyst determines the crystal phase,18,21 growth direction,23 strain in our pillars is not central to this work, and it will be polarity,26,27 and uniformity.28 Here, we engineer the liquid further clarified in follow-up studies. catalyst droplet in terms of size and contact angle before the Figure 1e shows a representative BF TEM (bright-field VLS growth initiation itself to enable growth in the [100] transmission electron microscopy) micrograph of an oxidized direction. In doing so, a positive height-difference Si surface is Si nanopillar. The nominal diameter of this pillar is 75 nm, and crafted in the shape of a nanoscale pillar. Ga is then the oxidation time is set for a nominal thickness of 100 nm, at a constrained at the tip using a SiO2 mask. By varying the Ga temperature of 1000 °C. The contrast between SiO2 and Si is predeposition time we make the droplet swell. This allows us the result of dynamical scattering at the interface and in the to obtain contact angles that are higher than those expected for bulk Si, which is enhanced by the aforementioned stress.30 The Ga on Si at equilibrium, the latter reported to be unstable for a oxide is thicker at the sidewalls than at the top due to the reproducible NW growth.28 increased surface exposed to oxygen during the thermal The Si nanopillar arrays are produced using a process similar process, but also to the radial tensile strain in the bulk oxide, to that of Varricchio et al.29 First, a dotted pattern is defined on easing the early supply of oxidants radially.31 The core Si pillar a 4 in. Si wafer using electron beam lithography. Then the is 5 nm, indicating that the extent of the self-limiting oxide pillars are defined by RIE (reactive ion etching) using a thickness can be reached in our system. The present work will mixture of SF6 and C4F8, followed by a buffered hydrofluoric focus on samples having a 15 nm Si core. acid bath for resist stripping. The bare Si nanopillar heights Several studies show that a deterministic contact angle vary between 150 nm and 1 μm, and their diameters from 300 engineering should serve in crystal phase engineering21,35 and to 30 nm. In this study we keep the pitch distance at 1 μm. The growth direction.23 Therefore, we focused on the mastering of pillars are subsequently oxidized at 900 °C, the oxide providing the wetting angle of the Ga droplets on top of the pillars. In the role of selective growth mask, as depicted in Figure 1. The order to vary the contact angle, we increased the total volume tips of the pillars are uncovered by a second RIE using a of the droplet by changing the Ga predeposition time. The mixture of SF6 and CHF3. Finally, a diluted HF bath is used for resulting contact angles have been measured by cross-sectional an isotropic etching, ensuring the uncovering of the silicon SEM (scanning electron microscopy). Pillars from 35 to 270 pillar tip; see Figure 1. nm in diameter have been fabricated, oxidized, and loaded in Given the geometry of the Si pillar, its oxidation should the MBE (molecular beam epitaxy) chamber, where a Ga reach a self-terminating regime, resulting in a sub-10 nm equivalent flux of 0.15 × 10−6 Torr is kept for 10 min under an diameter.30 This is due to the build-in of internal stresses upon ultrahigh vacuum. The manipulator temperature is set at 740 oxidation, limiting the reaction rate at the interface. The strain °C, which corresponds to a substrate temperature of 650 °C. tends to be compressive due to the incorporation of oxygen in Figure 2a shows SEM images, representative of the 5834 DOI: 10.1021/acsnano.9b01546 ACS Nano 2019, 13, 5833−5840 ACS Nano Article correctly adapting the process parameters and growth conditions. Figure2b to d highlight structures obtained in the range of contact angles marked in gray. They are oriented along the [100] direction. They exhibit a spade-like shape, substantiating their name. They can reach a high aspect ratio in two orthogonal directions. SEM pictures show the presence of a Ga droplet at the top, which confirms that they grow by the self- catalyzed VLS mechanism. However, their shape evolution throughout the growth appears to be a mixed contribution between VLS19 and VS (vapor−solid),37 the former for the vertical elongation and the later for the horizontal elongation. The nanospades (NSPDs) presented here have been grown using a Ga predeposition step of 10 min as mentioned in Figure2a, a Ga equivalent flux of 0.14 × 10−6 Torr, and a V/III ratio of 12. Figure 2e indicates the yield as a function of the pillar diameter. We can reach a maximum yield close to 10% for 35 nm diameter pillars. The other pillars contain [111]B- oriented GaAs NWs and parasitic growth. The yield drastically decreases for pillars larger than 50 nm, plummeting to a value of 1%. The low yield in smaller pillars can be explained by the Ga droplet falling down the side and, in larger pillars, by the energy of formation of the initial seed, as explained further below. NSPDs have been transferred flat on a TEM grid and observed using an Cs-corrected TEM. Figure 3 shows a high- angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) general overview of the studied nanospade in (a), details obtained on the tip in (b) and (c), and a color map showing the structure at the base in (d). Polarities have been resolved by an intensity analysis, as heavier atoms (As) appear brighter. The overall structure exhibits an original crystalline structure. The base is composed of several symmetrical twins and polarities, while the body consists of a bicrystal. Figure 3d shows the presence of a [100]- oriented domain (light blue) that is A-polar, Ga terminated, which is believed to be in epitaxial relation with the Si. This [100]A-oriented domain creates a truncated octahedron with (111)A facets where twin boundaries form upon further growth, showing a multiple-order twinning mechanism.38 A similar mechanism has been observed in [111]-B-oriented nanowire growth. It consists of the formation of successive twinning events in the three dimensions of space, resulting in Figure 2. (a) Table showing the relation between Si pillars’ well-defined modifications in the nanowire orientation with the diameter and the Ga droplets’ contact angle, along with SEM substrate. Interestingly, after the first monolayer, one side images representing certain diameters. (b, c, and d) SEM pictures exhibits a change of polarity (dark blue) with respect to the of GaAs nanospades on Si pillars and their growth range in the other side (red). To the best of our knowledge, this is one of table. (e) Relation between the yield of nanospades (NSPDs) and the first reports of a polarity change during nanostructure pillar diameter for a 20 nm (nominal) oxide mask. growth. Due to its high energetic cost, this polarity change should be very unlikely to happen.39 Its function is the conservation of the overall polarity in both sides of the configurations obtained. We depict also the values of the truncated octahedral seed. This structure is similar to the InAs contact angle as a function of the pillar diameter. We observe V-shaped membranes obtained by Conesa-Boj et al.,40 where angles between 140° and 75°, from smaller to larger diameter an [100]-oriented truncated octahedron is the seed of two pillars. There is a range between 40 and 100 nm where the [111]B-oriented “wings”. contact angle varies almost linearly. For smaller pillars, the In the case of NSPDs, the top of the seed exhibits the droplets fall down the side, which is expected to be detrimental formation of two extra twins forming an angle of 70.53° with for further MBE growth. For higher diameter pillars, the the vertical direction, giving rise to a pentagonal grain contact angle reaches the equilibrium value of liquid Ga on boundary structure. It is important to note that a regular Si.36 This variation of contact angle should translate to higher pentagon would have angles of exactly 72°, while the {111} pillar diameters for longer deposition times, which shows the twinning happens at 70.53°. There is a 7.32° mismatch that is versatility and potential of this platform. It would be possible relaxed by the creation of dislocations in the first few to tune the diameter and contact angle that one needs by nanometers of the vertical twin boundary. This fact is clearly 5835 DOI: 10.1021/acsnano.9b01546 ACS Nano 2019, 13, 5833−5840 ACS Nano Article Figure 3. HAADF STEM study of the GaAs NSPD shown in (a). (b) Growth front. (c) Central vertical twin with the polarity of each side. (d) Color structural map of the penta-twin at the base of the NSPD and the resolved polarities of each grain. A geometrical phase analysis study at the base shows the plane dilatation on the vertical twin (the strain is around 4%), along with dislocations on this boundary. Far away from the base, after a partial relaxation, the remanent strain in the boundary along the [100] direction is shown to be 2.5%. visible in Figure 3d, where a GPA (geometrical phase analysis) located very close to the (100) substrate surface, i.e., at the study shows four misfit dislocations aligned on this domain base of the structure. Despite the fact that our system is mainly boundary, and a plane dilatation of 4% is measured in the kinetically driven, it is worth noting that such asymmetry can upper region. Figure 3c demonstrates that the structure be caused by a difference in the twinning energies due to the succeeds in compensating for this mismatch, forming a polarity inversion. virtually defect-free bicrystal. Interestingly, Figure 3b shows the growth front formed by Twinning with 5-fold symmetry has been observed in a two {111} planes that seem to grow simultaneously. This is variety of systems such as copper bulk41 and NWs,42 known as the twin plane re-entrant (TPRE) mechanism, diamond,43 Au,44,45 but also GaAs in a study by Däweritz et observed recently in the growth of GaAs NWs,48 but also in al.,46 where RHEED (reflection high-energy electron diffrac- other material systems such as ZnSe,49 InSb,50 and Si or tion) examinations of defects on GaAs(001) reveal a structure Ge,51,52 but also in the bulk. Bulk TPRE configuration permits analogous to the nanospade pentagonal seed. L. D. Marks47 a fast expansion of platelets creating re-entrant edges, which at presents modified Wulff constructions for twinned nano- the same time serve as active nucleation sites. As elucidated by particles, including 5-fold symmetry twins. He divides the R. S. Wagner,53 the TPRE mechanism in the bulk needs at twinned particles into different segments bounded by the twin least two twinning events to be stable. Otherwise, a single twin planes. Each segment possesses planes with given surface would rapidly outgrow itself. The platelet growth rate would energies γx. For each twin boundary, the energy is divided into then be limited by the nucleation at the ridge, which is two fractions, αγ and (1 − α)γ, respectively, for the segments significantly smaller. Nonetheless, Gamalski et al.52 showed sharing this twin, where α is defined as the partition coefficient. that the nanoscale geometry allows steady-state growth based The lowest energy crystal shape is then determined by Wulff on a single twin boundary. Here, the growth rate of the construction for each of the defined segments, with the twin structure is completely determined by the nucleation rate at boundaries included as pseudosurface facets. The volume the twin rather than at the triple-phase line. This prevents any partition and geometric constraints are also taken into of the sides of the twin from outgrowing the other. Thus, the considerations. If values of αx can be found where the structure remains symmetrical despite having one B-polar and segments still assemble to a complete particle, a local energy one A-polar orientation, which are known to have different 1 nucleation and growth rates.26,54−56 minimum may be found. The symmetrical case of α = 2 and One should note that contact angle engineering resulted into equal volumes for each segment is always a solution and a maximized yield of 8%, which is relatively low. We believe proven to be a true minimum. In the case of a 5-fold symmetry this is related to the growth mechanism, combining both a twinned particle, a second solution arises by placing two polarity inversion and a 5-fold symmetry twinning. Nonethe- asymmetric twin relationships, pushing the meeting point of less, we think that the yield could be further controlled. In the five segments well away from the particle center. This case particular, we believe the engineering of the pillar surface by resonates with the NSPDs, where the pentagonal twin is nanofaceting could enable the TPRE mechanism without the 5836 DOI: 10.1021/acsnano.9b01546 ACS Nano 2019, 13, 5833−5840 ACS Nano Article need of the pentagonal twin structure. In addition, triple phase boundary, ceasing to be expanded by the VLS introduction of impurities or change in the growth regime mechanism. After the termination of the [100]-oriented may as well increase the probability of 5-fold symmetry domain, a double-twinning event occurs, modifying the twinning and TPRE.57,58 orientation of the growth front. As a consequence, the We now proceed with the proposal of a growth model for bicrystals constituting GaAs NSPD’s main core are formed the NSPDs, combining our observations with other groups’ and grow perpendicularly to the substrate. The growth will results on the formation of penta-twins59 and the in situ TEM proceed by the TPRE/VLS mechanism at the top, pushing the observation of NW growth.52,54,60 Figure 4 depicts the model structure up. Additional VS growth expands the structure laterally, in two [110] directions (as is the case for typical NW growth). The presence of twins and dislocations in semiconductor architectures could potentially reduce their viability for further applications. In order to assess the impact of the crystalline structure on the optoelectronic properties, we characterized them by luminescence. In particular, we were interested in obtaining signal of the defects at the NSPD base. For this, we increased the volume of the NSPDs by vapor−solid growth akin to ref 61. The thick NSPDs are then transferred flat onto a Au-coated Si substrate. Figure 5 shows in (a) the micro- photoluminescence (μPL) spectra at 300 and 8 K and in (b) the cathodoluminescence (CL) spectra line scan. In the RT (room temperature) μPL spectra, an emission at 867 nm, which corresponds to the band gap of GaAs, can be seen, along with a secondary much weaker peak slightly above noise level between 900 and 930 nm. This secondary feature becomes almost invisible at 8 K, where the overall signal corresponding to the free-exciton recombination emission in bulk GaAs at 1.51 eV and to an impurity-related band between 1.48 and 1.5 eV62−64is much higher. We attribute this weaker peak to defects at the base of the nanostructure. These defects exhibit energy levels with a limited density of states, as is usually the case for localized defects. As expected, emission from such states saturates with increasing excitation power, especially at low temperature, where recombination lifetimes are longer. By looking at the CL spectra in (b), the same feature is observed. The line scan along the NSPD unveils its origin at the base, close to the semioctahedron seed, the polarity inversion, and dislocations. It is concluded that those radiative transitions occur at the mentioned defects and are not observed in the rest of the structure. The CL spectra also show a decrease of the 819 nm exciton emission in the upper half of the structure. This is explained by the increase of the surface to volume ratio, quenching the emission by nonradiative recombinations at the GaAs surface. Further CL character- Figure 4. Schematic growth model depicting in (a) the Ga droplet izations using AlGaAs shells and GaAs quantum wells are pinning, (b) followed by the start of the growth with three VLS envisioned, along with a study on self-assembled quantum growth fronts: a [100]-oriented and two [111]-oriented. The dots65,66 for assessing the impact of the NSPD structure on droplet shrinks progressively, leading to a transient mode (c) that their formation. is followed by (d) the penta-twin formation. The growth is further continued by the VLS/twin plane re-entrant (TPRE) on top and CONCLUSION the VS on the sides. The polarity inversion is highlighted by the dark line, the VLS growth by yellow arrows, and the VS growth by To conclude, we have successfully designed and built an green arrows. Dislocations formed by the built-in strain are innovative Si platform for self-catalyzed VLS growth, consisting represented by inverted “T”-shapes. (f) Tilted SEM view of a of etched nanopillars covered by a thermal oxide. The presence generic nanospade. of a self-limiting oxidation effect permits a Si exposure in the chamber that is under 10 nm, enabling nonequilibrium growth we propose. The process starts with the pinning of the Ga conditions. The appearance of [100]-vertically oriented droplet at the Si/oxide interface at the pillar tip. After a brief nanostructures, called nanospades, is a proof of the viability incubation time, the nucleation of GaAs layers starts. Initially, a of this approach. A yield of NSPDs up to 10% has been [100] Ga-oriented pyramid forms (truncated octahedron), obtained. This is expected to increase with further growth with the two opposite (111)A and (111)B facets. Overall, this optimization. Overall, the NSPDs are mainly a bicrystal results in five liquid/solid growth fronts, leading to a decrease nucleated by penta-twins at the base. The optical properties in the size of the droplet due to Ga consumption. The [111]- are clearly consistent with high-quality GaAs. The penta-twin oriented lateral fronts outgrow the (100) plane, reaching the exhibits weak luminescence below the GaAs band gap, spatially 5837 DOI: 10.1021/acsnano.9b01546 ACS Nano 2019, 13, 5833−5840 ACS Nano Article then used for 2 min to remove any trace of resist. A thermal oxidation is then done at 900 °C for a variable amount of time depending on the desired oxide thickness. After the oxide growth, a 20 nm styrene methyl acrylate based resist (ZEP) film is spin coated and heated for 2 min at 180 °C for polymerization. The sample is inserted in an Oxford Plasmalab system 80 PLUS using a mixture of SF6 and CHF3 at 100 W plasma power for 40 s to uncover the pillars. Ten minutes of O2 plasma permits the removal of the resist, and a last cleaning involving a 5 min acetone bath, a 2 min propan-2-ol bath, and a 35 s HF bath at 1% concentration makes the sample ready for growth. The growths are conducted in a DCA MBE chamber under a Ga BEP (beam equivalent pressure) of 0.14 × 10−6 Torr and an As BEP of 1.7 × 10−6 Torr for 45 min. The samples were characterized by SEM using a Zeiss Merlin and by HAADF STEM using a probe- corrected FEI Titan 60-300 equipped with a high-brightness field emission gun (XFEG) and a CETCOR corrector from CEOS. The μPL measurements were done using an in-house built setup using a 532 nm sapphire optically pumped semiconductor laser, and the CL using an Attolight Rosa 4634 SEM-CL microscope at 5 keV. AUTHOR INFORMATION Corresponding Author *E-mail: anna.fontcuberta-morral@epfl.ch. ORCID Lucas Güniat: 0000-0001-7883-4433 Jordi Arbiol: 0000-0002-0695-1726 Anna Fontcuberta i Morral: 0000-0002-5070-2196 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS L.G. thanks the MBE team for the very nice coordination and collaboration. We thank also the SNF and QSIT for funding the projects 200021_169908 and IZLRZ2_163861. S.M.S. acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”. J.A. and S.M.S. acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. Figure 5. (a) Photoluminescence and (b) 10 K cathodolumines- cence characterization of two different NSPDs. The vertical lines program. The HAADF-STEM microscopy was conducted in in the SEM picture show where the CL spectra have been acquired. the Laboratorio de Microscopias Avanzadas at Instituto de Both studies show the presence of low-energy peaks linked Nanociencia de Aragon-Universidad de Zaragoza. 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