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. Mini Review Cite This: Nano Lett. 2019, 19, 3396−3408 pubs.acs.org/NanoLett The Role of Polarity in Nonplanar Semiconductor Nanostructures María de la Mata,*,†,◆ Reza R. Zamani,‡ Sara Martí-Sánchez,† Martin Eickhoff,§ Qihua Xiong,∥ Anna Fontcuberta i Morral,⊥,# Philippe Caroff,∇ and Jordi Arbiol*,†,○ † Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain ‡ Interdisciplinary Center for Electron Microscopy, CIME, É cole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland § Institute of Solid State Physics, University of Bremen, 28359 Bremen, Germany ∥ School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore ⊥ Laboratory of Semiconductor Materials, Institute of Materials, School of Engineering and #Institute of Physics, School of Basic Sciences, É cole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland ∇ Microsoft Quantum Lab Delft, Delft University of Technology, 2600 GA Delft, The Netherlands ○ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain Downloaded by 81.33.18.64 at 15:39:17:620 on June 12, 2019 * S Supporting Information from https://pubs.acs.org/doi/10.1021/acs.nanolett.9b00459. ABSTRACT: The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III−V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II−VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity- driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures. KEYWORDS: Polarity, semiconductor, III−V, II−VI, growth mechanisms, nanostructures, nanowires The charge transfer between constituents in ionic crystals leads Most known and employed semiconductor materials to distortions of their coordination geometry; namely, crystallize either in the cubic zinc-blende (ZB) structure or in tetrahedral, which translates into two-dimensional (2D) the hexagonal wurtzite (WZ)2 with F43m and P63mc space projected atomic couples where the ions are oppositely charged. groups, respectively. An equivalence can be established when As a consequence, internal electric fields emerge, strongly comparing the growth along the {111} and {0001} directions in related with many physical properties such as the electronic the ZB and WZ structures, as then, both structures are only structure, local charge carrier distributions, and related electro- distinguishable by the bilayer stacking sequence, being ...aAbBc- magnetic features.1 CaAbBcC... and ...aAbBaAbBaAbB... for ZB and WZ, respec- This charge transfer promotes atomic rearrangements leading tively; as displayed in Figure 2 (more details on these crystalline to the observed atomic couples oppositely charged in 2D structures are given in the Supporting Information) and in refs projected images of the crystal structure along certain directions, 3−5. known as dumbbells. In binary compounds, that is, AB, One should note that it is the lack of mirror symmetry in depending on the dumbbell orientation along a certain direction binary compounds which turns them into polar materials. (usually referred to the growth direction) the material is said to Interestingly, the {111} planes in ZB and {0001} in WZ, both have A- (cationic) or B- (anionic) polarity, influencing their growth and physical properties (see Figure 1, showing the Received: January 31, 2019 dumbbell orientation in III−V and II−V semiconductor binary Revised: April 23, 2019 NWs along their growth direction). Published: April 30, 2019 © 2019 American Chemical Society 3396 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Figure 1. Main polarities observed on different semiconductor NW compounds. II−VIs (ZnO, ZnTe, CdS, CdSe, CdTe) and III−Vs (i.e., GaX and InX (X = N, P, As, Sb)) materials analyzed by aberration-corrected scanning transmission electron microscopy (STEM) techniques, as shown in the sketch. Low-magnification and atomic resolution HAADF(/ABF for the nitrides and ZnO) images of the analyzed NWs are displayed, along with intensity profiles along the dumbbells, following the arrow direction shown in the images. In the cases shown above, only ZnO grows along the A-polar direction while the rest do it along B-polar. All the images are vertically oriented along the NWs growth directions. polar planes, are the typical growth fronts in semiconductor formation of transverse twins (perpendicular to the growth nanostructures. Within this context, two opposite polar axis)17 are common in B-polar NWs,3 while A-polar NWs (e.g.. orientations of the same crystal plane experience different InP and GaAs) are free of transverse twins, although they might growth rates,6,7 determining for instance, the symmetry of core− present a low density of lateral twins in B-polar directions (not multishell NWs,8 which can be exploited for the formation of perpendicular to the growth axis).13,18 Twins indeed preferen- novel self-assembled quantum structures with advanced func- tially occur along B-polar directions for InP and GaAs, although tional properties.9−11 Additionally, opposite polar growth they are preferred along A-polar directions in Sn-seeded GaSb directions not only may alter the internal crystal structure nanowires.19 It is important to point out that in nonpolar features but also the overall nanostructure of the system. A- systems such as group IV semiconductors (e.g., Si, Ge) a versus B-polar directed growths show distinctive defect combination of twins can coexist in the same structure, concentrations6,12,13 and impurity/doping incorporation rates, transverse and lateral,17 due to the lack of polarity. The presence leading to nonuniform composition in ternary or quaternary of twin boundaries is closely related to the formation of crystal systems, such as InGaAs or AlGaInAs NWs,14,15 or changing the polytypes, thus a high twin density increases the probability of doping nature from n- to p-type when polarity is reversed.16 The obtaining the WZ structure.3,20 The presence of ZB or WZ 3397 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Figure 2. Schematics of the ZB and WZ structures. (a) Unit cells for the cubic ZB (top) and hexagonal WZ (bottom) phases. (b) The 2D projections of the ZB (top) and WZ (bottom) along the [110] and [1120] zone axes for the ZB and WZ, respectively. (c) The 2D crystal projections along the [112] and [1100] for ZB (top) and WZ (bottom). (d) Pristine ZB structure (left-hand) and twinned ZB (right-hand), where the twin boundary is equivalent to a half monolayer of WZ. (e) A-polar and B-polar dumbbells, referring to the polarity along the arrow direction drawn between both dumbbells. In III−V and II-VIs compounds, A (cyan) is the III or II element, while B (purple) is the V or VI. crystal phases in a nanostructure is strongly influenced by techniques available to produce NWs, including chemical and polarity.13,18,19 In terms of the growing system geometry, the physical methods, each of them requiring the proper environ- formation of different free-standing nanostructures such as mental conditions for the growth to succeed. Thus, different NWs, tripods, tetrapods, nanomembranes, and so on,21−25 as substrates, temperatures, fluxes, precursors, doping, catalyst well as their orientation,26 can be explained attending to polarity seeds, and so forth are involved, and relating their influence to arguments. It is important to note that the term free-standing the material polarity is not straightforward. Although polarity may refer to different situations, however, when used along the has been studied in several semiconductor binary systems, the present work we refer to systems involving nanostructures reasons determining the final polarity of free-standing growing vertically outward from a substrate in a nonplanar architectures are not yet well understood. configuration with only a small portion of the nanostructure In order to find the underlying mechanism ruling the polar contacting the substrate. growth, we have analyzed the most commonly III−V and II−VI Differences in the chemical and physical behavior of polar- binary compounds synthesized as NWs. We have chosen oriented systems are remarkably relevant in the final aberration-corrected STEM to determine the polarity, which applications. It is known that the same surface planes with provides a reliable tool for this purpose.16,30,35,36 Combining opposite polarities exhibit different chemical behaviors and high-angle annular dark-field (HAADF) and annular bright-field optoelectronic properties.27,28 For instance, it has been shown (ABF) STEM techniques, we have been able to easily that in InGaN/GaN quantum wells (QWs), the carrier differentiate and identify the dumbbell constituents, even localization and transport are dependent on the polarity.29 when dealing with compounds containing light elements, such A further related issue regards the polarity preservation or inversion within a nanostructure. Although polarity inversions in oxides (O) and nitrides (N),30 to address the polarity of the are thermodynamically unlikely in unidirectionally grown binary growth plane (see Supporting Information for more details). compounds, more complex morphologies or heterostructured The obtained results are compared to previously published data systems may allow polarity inversions with dramatic implica- on the topic, opening the discussion on how different intrinsic tions on the transport and optical properties.30,31 material properties and extrinsic growth parameters may (or The above-mentioned scenario shows that polarity imposes a may not) induce a preferential polarity, paying special attention further degree of freedom to functionalize binary semiconductor to those compounds following both opposite polar directions. nanostructures, highlighting the importance to control the Subsequently, we exemplify the relevance of the polarity-driven polarity along certain directions. Most binary compounds grow growth mechanisms that allow for the development of a wide easily along the B-polar direction and A-polar growth is hardly variety of morphologies, such as free-standing branched/ achieved.26 Still, there are a few controversial materials,32−34 polypod architectures or 2D-like nanostructures. Finally, we namely, InP, GaN, and ZnO, reported to follow either A- or B- devote the final section of the present work to illustrate possible polar directions. So far, systematic comparison between the polarity inversions in nanostructured semiconductor materials, different systems is challenging due to the many factors affecting discussing a few examples of inversion boundaries, which the growth. This is partially due to the variety of growth eventually allow a smart manifold functionalization of the 3398 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Table 1. Reported Polar Directions for InP, GaN, InN, and ZnO Materials, Indicating the Type of the Growth Mechanism (Spontaneous, Selective Area, or Particle-Assisted Growths Are Distinguished) material polarity references InP A (In) Ikejiri et al.,48 Poole et al.,49 Gao et al.50 → SAG B (P) Algra et al.,51 Dalacu et al.,52 Calahorra et al.53 → particle-assisted GaN A (Ga) Schuster et al.,33 Bengoechea-Encabo et al.54 → SAG B (N) Fernández-Garrido et al.,55 Bertness et al.,56 Schuster et al.57 → particle-assisted Muβ̈ener et al.58 → spontaneous growth InN A (In) Wang et al.59 → spontaneous growth B (N) Stoica et al.,60 Chang et al.,61 Wang et al.59 → spontaneous growth ZnO A (Zn) Baxter et al.,62 Sallet et al.,63 Utama et al.,64 Guillemin et al.,65 Sun et al.,66 Nicholls et al.67 → spontaneous growth Scrymgeour et al.,68 Perillat-Merceroz et al.,69 Consonni et al.70 → SAG Jasinski et al.71 → particle-assisted B (O) Guillemin et al.72 → spontaneous growth Consonni et al.70 → SAG Sallet et al.63 → particle-assisted Figure 3. Intrinsic parameters of the binary compounds analyzed, such as the ratio between the constituent sizes (rb/ra), including the cases of ionic (a), covalent (b), and atomic radius, and the ionic character of the compound showing the electronegativity for each constituent, χ, along with the difference in electronegativity for every considered compound (d), calculated following Pauling’s expression (more details provided in the Supporting Information). optoelectronic properties of the nanostructures (polarity Polarity-Driven Growth of Vertical Nanowires. While nanoengineering). semiconductor NWs have extensively been studied systems over 3399 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review the last years, the available research works usually focus on only Interestingly, among the chalcogenides, we find out that ZnO one material system or a family of them (e.g., nitrides, arsenides, growth is directed by the cationic [0001] A-polar direction, and so forth). Therefore, although there is a huge amount of whereas the other analyzed II−VI compounds follow the most literature on the growth of NWs and the polar growth, there are commonly observed anionic growth, B-polar (top panel in not well-established connections rendering a universal picture. Figure 1). Indeed, ZnO with both A- and B-polarities has been In the following, we present our results in III−V and II−VI NWs reported, analogous to the case of GaN in the III−V family (see and explore the possible effect of intrinsic crystal properties on Table 1). the polar growth. Then, we discuss the role of the growth Some intrinsic parameters of the constituent atoms of the mechanism on the polar growth through few examples of material are known to affect its crystallinity. For instance, the systems reported to grow along two opposite polar directions. ionicity is critical in the resulting atomic package, which is a Atomic Polarity Measurements. III−V Compounds. Among property of the compounds related to the relative electro- all possible III−V combinations, we have characterized GaX and negativity between the constituents. Also, the atomic sizes of the InX NWs, where X = N, P, As, or Sb. Low-magnification STEM elements conforming to the material have an important role on images for the different NWs, atomic resolution HAADF/ABF the resulting phase. STEM images of their structure, and intensity profiles along Polarity in binary compounds is an extra variable on its dumbbells in each binary combination are included in Figure 1. crystallinity. We have tried to correlate some atomic parameters Further details on the characterization methodology can be of the constituents and geometrical factors of the phases with the found in the Supporting Information. All the III−V NWs experimental observed polarity. analyzed (bottom panel in Figure 1) grow along a B-polar Intrinsic Parameters That May Influence the Polar Growth. direction, regardless of their crystalline phase. In order to understand the polarity-driven growth mechanism, Hence, all the NWs here studied belonging to any III−V we have first evaluated a variety of intrinsic properties of the combination grow along either [0001] in hexagonal WZ systems compound constituents, such as the relative radii or the or any ⟨111⟩ B if the material has cubic ZB structure, in good compound ionicity. Starting by the relative atomic size, no agreement with most reported cases. In fact, there is a general trend is observed explaining the experimental observations. As belief supporting the preferential growth of III−V NWs some compounds are more covalent while others have a stronger following anionic polar directions.37 This is, for instance, the ionic character, ionic and covalent radii are also explored (see most common trend observed in GaAs, which grows easily on B- Figure 3a−c and tables provided in the Supporting Informa- polar substrates following B-polar directions. In contrast, it is tion). Interestingly, the ratios calculated for ZnO lie always usually observed that the use of A-polar substrates prevents the within the lower values range, but the overall trends vary formation of vertical NWs for the growth conditions employed, depending on the radii considered. Regrettably, we could not whereas other substrate orientations lead to the formation of find a direct connection between the radii relationships and the tilted NWs along the available ⟨111⟩B directions.26 By growing most common polarities observed for those compounds in the NWs on top of nonpolar substrates, such as Si, GaAs free- nonplanar semiconductor nanostructures. standing nanostructures have been found to easily form along B- Another relevant parameter to consider regards the dangling polar directions. In some cases, complex growth paths involving bonds of the material constituents considered by some the well-established 3D multiple-order twinning mechanism authors.45 Imposed by the tetrahedral coordination of the allow the NW to reach B-polar growth directions after n constituents within the WZ and ZB crystallographic phases, A twinning events.38 However, for very specific growth regimes the and B species at the polar {111} growing planes will have one formation of III−V A-polar NWs can be successfully dangling bond each. In the case of III−V compounds, III-group achieved.6,13,18,19 Indeed, few III−V combinations, such as InP species (A) have such dangling bonds empty and V-group and GaN, may grow along A-polar directions in addition to B- elements (B) have them occupied by unshared electrons. On polar ones. GaN is among the most controversial material in this one hand, the presence of unshared electrons preserves the sense and, actually, there are researchers supporting A-polar tetrahedral environment (tetrahedral bond geometry) around directed growth of GaN elongated nanostructures,39−41 whereas the B species. On the other hand, the existence of empty some works claim the irrevocably B-polar nature of sponta- dangling bonds around A constituents results in a coordination neously formed NWs42 (see Table 1). Even mixed polarities configuration in between planar and tetrahedral. On the basis of have been reported within the same batch of grown GaN NWs,16 these arguments, A-polar surfaces should be less favored due to although with the majority (90%) of the NWs growing along the the surface distortion involved and, thus, B-polar directions [0001] B-polar direction. Interestingly, B-polar growth remains would be preferred. In the case of II−VI materials, these a preference on Si(111) for most planar films of InAs, GaAs, and arguments would be invalidated due to their stronger ionic GaSb as well. Although in some cases, the authors claimed a character, as explained elsewhere.46 Although useful for the control over polarity with interface engineering.43,44 authors to correlate the etching behavior of oppositely polarized II−VI Compounds. Turning now to II−VI combinations, we surfaces, the prevalence of the ionic character over the atomic have determined the polarity in several Zn- and Cd-based configuration imposed by the dangling bonds does not rule the chalcogenides presenting ZB-WZ polytypism. Compounds with unidirectional polar growth of II−VI compounds, accordingly to other nonpolar phases are excluded from the study (as it is the our observations. case of CdO which usually shows rock-salt structure). The Related to the previous point analyzed, we try to extract some analyzed II−VI NWs comprise ZnO, ZnTe, CdS, CdSe, and meaningful information by focusing on the ionic character of the CdTe. ZnO, CdS, and CdSe crystallize as pure WZ; CdTe NWs considered compounds, which might affect the polarity. We use show polytypism from the base to the tip with short WZ and ZB the differences in electronegativities between atomic constitu- axial segments of variable thicknesses, whereas ZnTe NWs have ents, Δδ, as well as Pauling’s ionicities (expressed in percent) for a twinned ZB structure (ZB with highly spaced twin the binary compounds studied (see Figure 3d and Supporting boundaries). Information for further details). Unfortunately, the Δδ or related 3400 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review ionicity does not explain the observed polarity on the NWs. VS-Related Growths. Whenever the use of a seed droplet is Some of the cases reported to grow under A-polarity belong to avoided, for example, by selective area growth (SAG), the the combinations with the highest ionicities, as ZnO or GaN, preferred growth direction of InP is along an A-polar with 55 and 31% of ionic character on their bonding according direction.32,78 Note that the SAG procedure referred is governed to the calculations. However, the InP case, also reported to easily by a VS mechanism (more details are provided in the Supporting follow cationic directions while growing,47 cannot be explained Information), where the gas precursors directly impinge the under this consideration, because its covalent bonds hardly solid substrate for the growth to proceed. In the case of ZnO reach a 4% of ionicity, invalidating any correlation of a favored NWs, they show Zn-polarity if they are spontaneously formed79 polarity dependent on the ionic character of the compound. (more details are provided in the Supporting Information). So far, we could not find any consistent parameter that would In the case of GaN, using SAG conditions leads to the relate the type of polarity with a group of binary semiconductors. formation of A-polar GaN NWs,33 as shown in Figure 4 (see also This points out to the hypothesis that polarity might be mostly determined by the synthesis method and growth parameters used (i.e., temperature, pressure, III/V ratio, phases present during growth, nature of the catalyst seed etc.). Then, based on our results and previously reported data, it appears reasonable thinking that the observed polarity in free-standing nanostruc- tures is a consequence of the parameters (conditions) used during the growth. Unfortunately, a complete comprehension of the dynamics and kinetics has not been achieved yet. Nevertheless, the growth parameters can be divided in two separated groups, depending on whether the growth is particle- assisted or not. This is further elaborated in the following. Growth along A- and B-Polar Directions and the Effect of the Growth Mechanism. As commented above, few systems have been already reported to grow along two opposite polar directions. The most popular cases are InP, GaN, InN, and ZnO (see Table 1), discussed in the following attending to the underlying growth mechanism, that is, vapor−liquid−solid (VLS) or vapor−solid (VS).73 VLS-Related Growths. The VLS mechanism is a metal Figure 4. N- and Ga-polar GaN NWs. GaN NWs grown on Si substrate catalyst-assisted growth method (see further details in the by PAMBE (a−c) and grown by SAG-PAMBE on diamond/Ti- Supporting Information) commonly employed to obtain substrate/mask (d−f). (a,d) Low-magnification HAADF image of the analyzed GaN NWs. (b,e) Atomic-resolution ABF image of the GaN semiconductor NWs. To the best of our knowledge, there is phase shown by the NW included in (a,d), evidencing the N-/Ga-polar not a universal atomistic model that explains all types of particle- directed growth. False-colored images are displayed in (c,f). assisted NW growth. The growth is usually seeded by Au droplets or by group-III elements (self-catalyzed processes). Apart from our observation of B-polar InP NWs, both, A- and Tables 1 and 2). By coating the diamond substrate with a B- polarities are reported in the literature (see Table 1). patterned Ti mask the selective growth results in A-polar NWs,33 Interestingly, the reported B-polar InP NWs are grown by using contrary to their spontaneous formation on bare diamond metal seeds (particle-assisted growth).74,75 Similarly, opposite substrates.57 Morphological differences in NWs with opposite polar directed growth of ZnO NWs has been reported polar directions exist. A-polar NWs exhibit a much lower aspect depending on whether the growth is particle-assisted or not. ratio than B-polar NWs (see Figure 4). Such morphological Among other techniques, ZnO can be synthesized by metal− differences may be indicative of the different dynamics/kinetics organic chemical vapor deposition (MOCVD) in both ways, underlying the self-assembly and SAG procedures (more details particle-assisted and spontaneous growth modes, by tuning the in the Supporting Information). In some cases, we find in the growth parameters (mainly the O/Zn ratio). The resulting NWs literature other examples of SAG NWs with A-polarity, where show O-polarity whenever the synthesis is particle-assisted or the formation of A-polar nanowires is most probably not related VLS (see Table 1). Changing the synthesis from the chemical to the SAG process but to the A-polarity of the substrate.80 vapor transport to metal−organic vapor phase epitaxy The GaN NWs studied here are self-assembled on Si (MOVPE), the growth of ZnO NWs on nonpolar and B-polar substrates, following a catalyst-free synthesis, following the substrates has been achieved. Nevertheless, the spontaneously vapor−solid (VS) mechanism instead of the VLS one, because formed (catalyst free) NWs also follow the Zn-polar [0001] there is not liquid droplet mediating the growth. Investigations direction.76 Another example of this preferential cationic polar on the growth of self-assembled GaN NWs within this regime growth occurs when the ZnO grows on top of GaN NWs, claim the irrevocably B-polar nature of the NWs, irrespectively forming ZnO/GaN heterostructures. Even in this latter case, of the substrate polarity,42 whereas B-polar substrates mostly when growing the ZnO axially on top of the B(N)-polar GaN lead to the formation of dense B-polar NW arrays; using A-polar first grown stems, the ZnO shows A(Zn)-polarity.77 substrates, sparse B-polar NWs are created on top of A-polar Hence, although it turns out that particle-assisted procedures pedestals. A similar kind of growth is observed on nonpolar tend to favor the B-polar directed growth, in agreement with our substrates, such as diamond.81 The substrate orientation will observations, the opposite A-polar direction is still possible define the growth angle (inclination) of the NWs. However, as under selected fine-tuned growth conditions. long as the mismatch between the substrate and the NW 3401 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Table 2. GaN Reported Growth Conditions and Polarity Found reference polarity growth method substrate substrate coverage buffer/mask temperature (°C) V/III ratio BEPGa(mbar) Furtmayr et al.83 N PAMBE Si SixNy 775 3.9 × 10−7 Schuster et al.57 N PAMBE C-diamond 930 Schuster et al.57 Ga SAG-PAMBE C-diamond Ti mask 890 71−500 4 × 10−7 Cherns et al.84 Ga PAMBE sapphire AlN buffer 700−800 Alloing et al.85 N (∼73%) MOVPE sapphire 1100 20−200 N (mainly) ammonia-MBE sapphire 840 Ga (90%) ammonia-MBE Si AlN 700 Brubaker et al.86 Gaa PAMBE Si AlN buffer 820 and 800b 1.0−1.03 Lin et al.87 Ga SAG-MOCVD Si 1050−1175 12.6−2.1 Carnevale et al.88 N (90%) PAMBE GaN(0001) not mentioned 720 and 780c Carnevale et al.89 Ga PAMBE Si not mentioned 720 and 780c Kong et al.90 N PAMBE Si Si 700−800 Gad PAMBE Si AlN buffer 700−800 Hestroffer et al.91 N PAMBE Si SixNy and AlN buffer 800 2.5 den Hertog et al.92 N PAMBE Si 790 Urban et al.93 Ga SAG-PAMBE GaN(0001) Mo mask 780 9 Fernández-Garrido et al.55 N PAMBE SiC(0001) AlN buffer 815−825 N PAMBE SiC(0001) AlN buffer 815−825 a Mg-doped GaN NWs. bTwo hours at 820 °C followed by 12 h at 800 °C. cTemperature of 720 °C for the nucleation and 780 °C for the growth. d AlxGa1−xXN NWs material ensures a Volmer−Weber nucleation, the B-polarity conditions. It should be therefore possible to find the proper will be kept.82 synthesis conditions favoring growth along cationic polar A summary table gathering different growth procedures for directions for any compound (A-polar), in addition to the GaN NWs and the resulting polarity is provided (see Table 2). A most commonly observed anionic-polar growth, as it has been more extensive review on GaN and ZnO polarity can be found in already observed for few systems, as InP, GaN (and InN), ZnO, ref 94. or GaAs. The case of InN is fairly similar to GaN, although reports on Polarity-Driven Morphological Transitions. From the the actual unidirectional polar growth of InN are scarce for the previous observation of NWs growing with a preferred polarity, InN case. InN NWs are also mainly synthesized catalyst-free by which is conserved along the whole length, one can state that PAMBE under N-rich conditions, as GaN NWs, but at lower nucleation at the substrate with a favored polarity may regulate temperatures to prevent InN decomposition. MOVPE growth the yield of NWs. However, the synthesis of more complex through self-catalyzed procedure has been recently reported to morphologies is also possible and their formation can be lead to N-polar InN NWs, too.95 The parallelism between these understood also attending to polarity arguments, along with III-nitrides is obvious, suggesting that they share the growth polytypism and twinning phenomena. mechanism as expected from the similar growth procedures Branched Nanostructures. Among the different morpholo- employed and thus the same polar behavior. gies attainable, branched nanostructures with different number In the case of ZnO, the growth NWs is highly favored on A- of legs are commonly reported in binary (AB) semiconductor polar substrates, as can be inferred from reported studies using materials, as for instance tripods and tetrapods. When obtained periodically polarity-inverted substrates,96−98 where the ZnO epitaxially on top of a substrate, these nanostructures are NW growth only succeeds on cationic-polar substrate regions. perfectly aligned with each other22,33 even in the case of van der Such studies, included in refs 96−98, report Zn-polar NWs Waals incommensurate epitaxy (see Supporting Informa- grown by chemical vapor transport and condensation from ZnO tion).100 Insights on their crystalline structure and polarity powder using Au particles as nucleation seeds96 or by a catalyst- provide key information on the growth mechanisms. free procedure (more details in the Supporting Informa- The proposed growth mechanism for tripods is similar for the tion).97,98 different binary compounds.21,32,101 The central part of the In contrast to what is reported elsewhere,76 Consonni et al.99 tripods (namely the core), where the legs or ramifications merge have shown that ZnO NWs follow the substrate polarity if they together, crystallizes under pristine ZB structure, while the legs are selectively grown by chemical bath deposition (CBD), usually show a polytypic mixture with predominating existence meaning that the Zn- or O- substrate polarity is transferred to of WZ (Figure 5a). This mechanism is known as polytypic the NWs. As the growth proceeds in solution phase, the main branching.102,103 Despite the changes on the crystal phase interactions to be considered during the growth are quite between the different parts (core and legs), they are coherently different from those on vapor phase. More details on the attached, as it can be seen when properly orienting the tripods solution growth of ZnO NWs are provided in the Supporting along the [101]core|| [110]legs or [1120]legs (depending on Information. the considered phase for the legs, ZB or WZ). Therefore, the Taking into account our experimental observations and system evolves by the creation of different legs growing along the previous studies, it is likely that the unidirectional polarity- ⟨0001⟩ direction of the WZ phase, initiated at {111} planes of driven growth at the nanoscale is a consequence of the the ZB core following the most favored polar-direction for the underlying mechanism promoting the growth, which is employed growth conditions. This means that, for instance, in dependent on the synthesis procedure and experimental the case of ZnTe or GaN tripods growing at similar conditions 3402 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Figure 5. Atomic 3D models for the growth of tripods and tetrapods. (a) ZnTe tripods and (b) tetrapods. In both cases, the growth starts by the nucleation of octahedral seeds, either Zn-polar (a) or truncated by nonpolar planes (b), allowing the elongation of the legs along the available B-polar directions (favored polarity for the used growth conditions). Adapted from ref 22. Copyright (2013) WILEY-VCH. than Te/Ga-polar NWs, they lengthen also along Te/Ga-polar Additionally, branched structures with multiple legs can be directions, determined by the nucleation seeds. It is important to obtained in liquid solution by following similar polarity rules as note that as there is a preferred polar elongation direction, NWs polytypic branching occurs105 (more details and a further and tripods necessarily grow from oppositely polarized discussion can be found in the Supporting Information). nucleation seeds, keeping the polarity at the core/leg inter- Free-Standing 2D Nanostructures. Further related cases of face22,33 for the growth to succeed (further details on the growth polarity driven-growth mechanisms can be found in vertical 2D mechanism are available in the Supporting Information). belts, flakes, or nanomembranes. The anisotropic growth Slightly different considerations may be taken into account to behavior along opposite (0001) polar directions has been explain the formation of tetrapods (Figure 5b), whose legs also noticed when growing ZnO,106 ZnS,107 CdS,108 or CdSe109 grow following the favored polarity (growth mechanism nanobelts or nanoribbons by thermal evaporation. In all those studies, the nanobelts grow laterally constricted by {000 ± 1} described at the Supporting Information. planes, which show comblike or sawtooth morphologies Driven by the most stable polarity under the growth asymmetrically developed. The (0001) (A-polar) side tends to conditions employed, the formation of NWs and tripods of show saw/comb-teeth shape while the (0001) (B-polar) side of different materials can be explained based on similar models the belts is more prone to remain flat. This fact is mainly than the one proposed.21,32,101,104 Another example illustrating attributed to the higher reactivity known for the A-polar surfaces such fact can be found in GaN nanostructures. Growth compared to the B-polar ones. In contrast, under different conditions propitious to A-polar growth, lead to A-polar NWs, growth conditions Te-terminated (B-polar) surfaces experience tripods, and nanotubes.33 This is the case of growth on diamond higher growth rates explaining the asymmetric development of by SAG. Similarly, InP tripods grown following selective-area some other ZnTe nanobelts.110 On the basis of the observation metalorganic vapor-phase epitaxy (SA-MOVPE) procedures of Zn enrichment on the growth front, the teeth formation on A- where the growth along A-polar directions is favored versus B- polar (0001) surfaces is believed to follow a self-catalytic polar ones.32 process. For the ZnO and CdS cases, formation of nanofingers 3403 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review Figure 6. Polarity inversion. (a−c) Top panel: InAs V-shaped membrane growing from the center outward, developing the shape shown in (a). Atomic-resolution HAADF measurements evidence the dumbbell orientation in the central part of the architecture both at the top region where both branches merge together, leading to the inversion domain boundary (b) and at the base (c). (d,e) Middle panel: DFT calculations on the conductivity and electrostatic potential for GaAs. (d) Conductivity at the band edges for pristine GaAs (black), GaAs with two orthotwins (blue), and on GaAs with the presence of both paratwin configurations (red). (e) Electrostatic potential as perpendicular to the twin plane for presence of orthotwins (black) and paratwins (red). The arrows point at the twin location. Inset: magnification of the electrostatic potential in the presence of orthotwins. Figure adapted from ref 30. (f) Bottom panel: Polarity inversion experimentally observed at a GaN/ZnO heterostructured NW. The corresponding calculated 1D band structure (left axis) and electric field (right axis) for two different polar heterointerfaces (N-polar/Zn-polar and Ga-polar/O-polar) are also included, adapted with permission from ref 31. Copyright (2014) American Chemical Society. on the anionic polar side can take place depending on the growth However, after switching from InAs to InSb, the catalyst droplet temperature.106 The nanofingers are shorter and thinner than may eventually wet a NW sidewall, allowing the nucleation of a the sawteeth and are only formed at the lower-temperature lateral twin boundary at the transition interface. This twin regime. Although some of the nanobelts were synthesized by the boundary breaks the symmetry of the structure, creating a new particle-assisted procedure, the teeth formation occurs apart {111}B lateral facet that will allow the lateral expansion of the from the droplet, thus, through the vapor−solid mechanism. nanostructure while keep on growing axially. Another example of the asymmetric development of nanobelts is Therefore, under certain growth conditions a specific polarity found in ZnTe extended sheets synthesized by vapor transport, drives the growth direction and, consequently, different which experience different growth rates at their lateral facets morphologies can be obtained. As the fine-tuning of the growth related to oppositely polar orientations.110 Polarity arguments conditions may switch the preferential polar direction, inducing along with the observation of systematic particular defects have such polarity at will in the growing material offers a powerful been used to explain the shape transition observed in InAs/InSb nanoengineering tool for the design of multiple morphologies. system.24 While most of the developed architectures are axial So far, all the examples here reviewed deal with architectures heterostructured NWs, following B-polar directions in both where the polarity is never reversed. However, polarity phases, a non-negligible part of the nanostructures evolve inversions may be possible in certain systems, as we address in toward vertical membrane-like shapes, also named nanosails. the following, with strong implications on the physical InSb ZB NWs usually grow on top the InAs WZ stems. The InSb properties. NW section is limited by lateral {110} nonpolar facets, Polarity Inversions. WZ and ZB structures are prone to the preventing the lateral expansion or overgrowth of the NW. presence of twin boundaries (see Supporting Information for 3404 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review more details), which are planar defects involving a lattice to that reported for the inversion promoted by a paratwin within rotation of 180°, either perpendicular or around its normal a GaAs NW.30 Plain differences are predicted for the electron vector.111 Such crystallographic defects may reverse the polarity transport in the presence of nonpolar and polar twins. The in addition to alter the atomic stacking sequence giving rise to theoretical calculations accounting for a paratwin (polar defect) intermediate polytypes.112 It should be mentioned that to the and an orthotwin (nonpolar defect) by taking GaAs as the model best of our knowledge, no polarity inversions within pure binary system under three different considerations: occurrence of NWs induced by twin defects have been reported. However, the orthotwins and two possible paratwin configurations, that is, presence of polar defects has been observed in other scenarios, mirror twin creating a Ga−Ga bilayer and another for the As−As as a consequence of doping, branching, or material combina- bilayer, as reported elsewhere.30 The performed first-principles tions, as described below. electron transport calculations in the ballistic regime by density InAs V-shaped nanomembranes, which grow vertically on the functional theory (DFT) using the Siesta code 113 are substrate, are a typical example of an inversion domain boundary summarized in the middle panel of Figure 6. (IDB).23 The membranes show a pyramidal nucleus of pure ZB It is easily observed that the conductivity (Figure 6d) at the at the central part of the base and two extended wings, of band edges is expected to dramatically drop if a polar paratwin polytypic InAs phase growing outward the nucleus. The InAs ZB (red curve) occurs compared with the pristine material (black triangular projection of the nucleus grows along the [001] curve), whereas the effect of the nonpolar orthotwin (blue direction following the underlying Si substrate orientation, while curve) would almost not perturb the conductivity. Differences standing on its (001) basal plane in contact with the Si surface between polar and nonpolar twins are even more pronounced in and being laterally faceted by As-polarized {111} planes (Figure the electrostatic potential (Figure 6e), which remains almost 6c). Thus, the membrane expansion proceeds along the anionic unchanged in the presence of orthotwins but is strongly polar [0001] directions lying on the two lateral {111}B facets of modified if the twin inverts the polarity. Moreover, clear the prismatic nucleus, inducing the branching of the structures differences for the electron and hole conductivity can be seen as into two “wings”, both with B-polarity pointing the tips. During the As−As paratwin imposes a barrier for the holes, while the their lateral growth, the membranes keep on growing vertically sharp barrier exhibited by the Ga−Ga paratwin scatters the to the substrate, too. As a consequence, above the top vertex of electrons. This is true for defective homostructures, but even the prismatic triangular nuclei, there is an interaction region more for heterostructures where the different polarization leads where the two wings merge together (Figure 6b). The growth of to interface charges which strongly modify the band alignments the wings along B-polar directions results in the convergence of depending on the polarity at both sides of the heterointerface.31 two A-polar domains facing each other, as shown in Figure 6b,c. The unavoidable discontinuity on the band structures when Therefore, an antiphase boundary defect propagates perpendic- switching the material, that is, at the interface, can be exploited ularly to the substrate along the membranes, starting from the to create two-dimensional electron gases, two-dimensional hole top vertex of the ZB triangular seed, and dividing the membranes gases (see Figure 5f), or tunnel diodes, for instance, by properly into two polar domains. The configuration or atomic arrange- adjusting with the polarity at both sides of the interface. In the ment of the IDB is dependent on the polytypic stacking particular case of GaN/ZnO NWs, it has been experimentally sequences of the regions merging at both sides of the defect. The found that A-polar ZnO may grow on top of B-polar GaN stems, presence of pure WZ segments at both sides of the boundary resulting in an inversion domain heterojunction, where the N leads to the formation of straight defects, while twinning and and O anions face each other. polytypic mixtures results in local deviations of the IDB In conclusion, we have found that NWs growing at the triple trajectory. There are three possible dumbbell orientations in phase line, involving solid, liquid, and vapor phases (VLS the structures, derived from the branching and the polytypism growth), tend to follow mainly B-polar directions in both III−V phenomena. The boundary acts as a mirror defect with and II−VI compounds. The liquid phase is usually intentionally dumbbells oppositely oriented at both sides, and vertically present in the medium by using metallic catalyst particles which aligned dumbbells are allowed in all the different regions, usually will act as nucleation seeds. In addition, the metallic species connecting both wings (Figure 6c). When the two matching supplied during the growth, that is, III- and II-group species, can regions are purely WZ, the atomic arrangements at the boundary result on the formation of liquid droplets with the same effects locations consist of ring-like formations composed by six atoms, on the overall procedure (self-catalytic mode). These metallic alternating As and In, conforming the IDB. Again, depending on droplets collect the supplied vapor species which will arrive by the polytypic sequence, other atomic rearrangements are diffusion on the substrate and NW sidewalls in the case of metals observed. (III and II species) or by impinging the droplet and diffusing It is important to note that the IDB does not extend through through the interface with the substrate (during the the ZB nucleus at the base of the membranes. There, the polarity nucleation)/NW (alternatively nonmetallic species can be reversal is stepped, mediating the presence of the ZB, where all directly incorporated at the triple phase line, as in the case of the dumbbells are vertically aligned. Such situation might induce oxygen), for the V and VI elements. different physical behavior on the different areas of the On the contrary, under certain growth conditions it is possible membranes, that is, the base containing the ZB nucleus or the to force the A-polar growth by using A-polar substrates, and/or upper merging area between the wings. Therefore, in addition to increasing the III/V ratios during the growth, even on unpolar the polarity inversion among the wings of the membranes, one substrates. In this way, a delicate control on the experimental should account for the vertical asymmetry of the system. conditions and on how they rule the growth is essential, since Notice that, as a consequence of the preferential unidirec- their modification can switch the growth regime resulting in a tional polar growth, anionic in this case, the branched different mechanism, and thus, losing the control over the nanostructures develop an inversion domain boundary defect polarity. The known possibility for some materials to grow under not observed in impurity free binary NWs. The effect of a both polarities is encouraging for the general case to be polarity reversal on the electronic properties may be comparable accessible to polarity engineering by finding the proper growth 3405 DOI: 10.1021/acs.nanolett.9b00459 Nano Lett. 2019, 19, 3396−3408 Nano Letters Mini Review conditions. It is worth keeping in mind that the requirement to ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa be fulfilled is enhancing one polar growth over the other, making program from Spanish MINECO (Grant SEV-2017-0706) and the desirable polar direction being developed faster than the is funded by the CERCA Programme/Generalitat de Catalunya. opposite and hence, driving the obtained morphology. More Part of the present work has been performed in the framework of work should be done to reach the desirable understanding on the Universitat Autònoma de Barcelona Materials Science Ph.D. growth, because even for the widely employed VLS procedure a program. S.M.S. acknowledges funding from “Programa complete comprehension of the process has not been achieved Internacional de Becas “la Caixa”-Severo Ochoa”. The yet. HAADF-STEM experiments were conducted in the Laboratorio Outlook. Aiming to shed light over the mechanisms de Microscopias Avanzadas at Instituto de Nanociencia de determining the growth polarity at the nanoscale, we have Aragon-Universidad de Zaragoza. A.F.i.M. thanks SNSF for extensively employed atomic resolution STEM-related method- funding through the NCCR QSIT. This work has received ologies in order to determine the polarity in the most commonly funding from the European Union’s Horizon 2020 Research and synthesized and used III-Vs and II-VIs nanostructures, creating a Innovation Programme under grant agreement No. 654360 complete mapping of the resulting preferential polarities. NFFA-Europe. This project has received funding from the Despite the finding that the vast majority of the nonplanar European Union’s Horizon 2020 research and innovation semiconductor nanostructures follow anionic or B-polar programme under grant agreement No. 823717 − ESTEEM3. ■ directions, few materials show the versatility in growing in both A- and B-polar directions (e.g., GaN, InP, ZnO) and hence REFERENCES open the discussion on the feasibility of predetermining the desirable polarity during growth for any compound material. (1) Nakamura, S. MRS Bull. 2009, 34, 101−107. (2) Arbiol, J.; Fontcubertai Morral, A.; Estrade, S.; Peiro, F.; Kalache, It is noteworthy that the growth procedures leading to B.; Rocai Cabarrocas, P.; Morante, J. R. J. Appl. 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