Polarity-Dependent High Electrical Conductivity of ZnO Nanorods and

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C: Physical Processes in Nanomaterials and Nanostructures

Polarity-Dependent High Electrical Conductivity of ZnO Nanorods and its Relation to Hydrogen Thomas Cossuet, Fabrice Donatini, Alex M. Lord, Estelle Appert, Julien Pernot, and Vincent Consonni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07388 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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The Journal of Physical Chemistry

Polarity-Dependent High Electrical Conductivity of ZnO Nanorods and its Relation to Hydrogen

Thomas Cossuet,1 Fabrice Donatini,2 Alex M. Lord,3 Estelle Appert,1 Julien Pernot,2,4 and Vincent Consonni.1,* 1

Univ. Grenoble Alpes, CNRS, Grenoble INP, LMGP, F-38016 Grenoble, France.

2

Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut NEEL, F-38000 Grenoble, France.

3

Centre for Nanohealth, College of Engineering, University of Swansea, Singleton Park, SA2

8PP Swansea, United Kingdom. 4

Institut Universitaire de France, 103 Boulevard Saint-Michel, F-75005 Paris, France.

Corresponding author: [email protected]

ABSTRACT A statistical analysis of the electrical properties of selective area grown O- and Zn-polar ZnO nanorods by chemical bath deposition is performed by four-point probe resistivity measurements in patterned metal contact and multi-probe scanning tunneling microscopy configurations. We show that ZnO nanorods with either polarity exhibit a bulk-like electrical conduction in their core and are highly conductive. O-polar ZnO nanorods with a smaller mean electrical conductivity have a non-metallic or metallic electrical conduction, depending on the nano-object considered, while all Zn-polar ZnO nanorods with a larger mean electrical conductivity present a metallic electrical conduction. We reveal, from Raman scattering and spatially-resolved 5 K cathodoluminescence measurements, that the resulting high carrier density of ZnO nanorods with O- or Zn-polarity is due to the massive incorporation of hydrogen in the

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form of interstitial hydrogen in bond-centered sites (HBC), substitutional hydrogen on the oxygen lattice site (HO), and multiple O-H bonds in a zinc vacancy (VZn,Hn). While HBC is largely incorporated in ZnO nanorods with either polarity, HO and (VZn,Hn) defect complexes appear as the dominant hydrogen-related species in O- and Zn-polar ZnO nanorods, respectively. The present findings reveal that polarity greatly affects the electrical and optical properties of ZnO nanorods. They further cast a light on the dominant role of hydrogen when ZnO nanorods are grown by the widely used chemical bath deposition technique. The present work should be considered for any strategy to thoroughly control their physical properties as a prerequisite for their efficient integration into nanoscale engineering devices.


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1. INTRODUCTION ZnO nanorods (NRs) grown by chemical bath deposition (CBD) offer a large number of assets originating from their high aspect ratio at nanoscale dimensions and from their applicative potentiality.1,2 They are thus expected to act as building blocks in a wide variety of nanoscale engineering devices in the fields of electronics, optoelectronics, photovoltaics, and sensors.3 The structural morphology of ZnO NRs grown by CBD has intensively been optimized by revealing the effects of the ZnO seed layer4,5 and of the chemical precursors6,7 as well as by using prepatterned nucleation surfaces in the framework of selective area growth (SAG).2,8 In contrast, very little is known regarding the electrical properties of ZnO NRs grown by CBD especially with regards to the polarity of the crystal and their correlated optical properties,9,10 despite their critical role on the performances of the related nanoscale engineering devices. Owing to the characteristics of the wurtzite phase belonging to the 6mm point group, ZnO exhibits a non-zero dipole moment per unit volume, i.e. a spontaneous polarization, along the caxis.11,12 The [0001] and [0001ത] directions should thus be distinguished. By convention, the cplane is defined as O- (resp. Zn-) polar, when the vector of the Zn-O bond collinear to the c-axis of the wurtzite cell goes from the O (resp. Zn) atom and points toward the Zn (resp. O) atom. Over the last two decades, it has been stated that polarity greatly affects surface configuration, stability and reactivity,13-16 nucleation and growth mechanisms,17-19 impurity and dopant incorporation,20 electro-optical properties,20-25 as well as related device performances.12 The present effects have widely been investigated in the case of ZnO single crystals and epitaxial films, but they are still open, to a very large extent, in the case of ZnO nanostructures26 including NRs.27-29 Consonni et al. showed that ZnO NRs with a controllable O- or Zn-polarity are formed by CBD using the polarity transfer from the respective O- and Zn-polar ZnO single crystals prepatterned by electron beam lithography and acting as nucleation surfaces.30 The polarity transfer


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has subsequently been found to operate from the ZnO polycrystalline seed layers comprising caxis oriented ZnO nanoparticles.31 The same approach was further developed on Ga- and N-polar GaN single crystals as nucleation surfaces to form Zn- and O-polar ZnO NRs by CBD, respectively.32 In contrast, only Zn-polar ZnO NRs are formed by vapor phase deposition techniques owing to the preferential nucleation of inversion domain boundaries.12,28 The possibility to carefully monitor the polarity of selective area grown ZnO NRs by combining advanced lithography with CBD opens the way for more deeply investigating its effects on the nucleation, growth, and properties of the resulting nano-objects. Recently, it was shown that the axial growth rate is much larger for Zn-polar ZnO NRs than for O-polar ZnO NRs.33 The present finding was assigned to the larger surface reaction rate constant for Zn-polar ZnO NRs. This was further correlated with the higher surface dangling bond density required for the zinc incorporation as the limited species in the total chemical reaction. The strong difference in the axial growth rate of ZnO NRs is expected to influence their resulting physical properties. This may have drastic consequences on the properties and performances of any future nanoscale engineering devices. The elucidation of the present polarity related physical properties is an essential prerequisite for real world integration of these ZnO NRs. It is the aim of that paper to carefully investigate the electrical and optical properties of well-ordered O- and Zn-polar ZnO NRs as well as their polarity dependence by using four-point probe resistivity measurements,34,35,36 Raman scattering, and spatially-resolved low temperature cathodoluminescence (CL) measurements. It is shown that O- and Zn-polar ZnO NRs are both highly conductive, which is found to originate from the massive incorporation of hydrogen. The dependence of the properties on their polarity is further revealed and discussed in the light of the fundamental processes at work during the growth.

2. EXPERIMENTAL METHODS


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A polymethyl methacrylate (PMMA) layer with a thickness of 100 nm was spin coated on the O- and Zn-polar ZnO single crystals (CrysTec) and subsequently patterned identically by electron beam lithography (EBL). The hole circular diameter and period of 143 ± 8 nm and 1004 ± 6 nm were used on the domain. The residual PMMA layer inside the holes was removed with an Evactron RF plasma cleaning system using an O2 plasma. Well-ordered O- and Zn-polar ZnO NRs were grown on the patterned O- and Zn-polar ZnO single crystals, respectively, by using CBD under identical conditions and during the same run of experiment in the framework of the SAG approach. The solution was composed of a 30 mM equimolar concentration ratio of zinc nitrate hexahydrate Zn(NO3)2 and hexamethylenetetramine (C6H12N4) from Sigma-Aldrich, which were dissolved and mixed in deionized water. The patterned O- and Zn-polar ZnO single crystals were placed face down in separate beakers, which were heated up to 90 °C for 3 h in a regular oven. The pH was fixed to the standard value around 5.5, as the growth temperature of 90°C was reached. The structural morphology of the ZnO NRs was assessed by field-emission gun scanning electron microscopy (FESEM) using a FEI Quanta 250 FESEM. EDS spectra were recorded with a Bruker detector incorporated in the FEI Quanta 250 FESEM. The ZnO NRs of both polarities were subsequently dispersed in a flat lying configuration on dedicated silicon wafers covered with a 100 nm-thick SiO2 layer grown by thermal oxidation. The four-terminal single ZnO NR devices with ohmic contacts using Ti/Au (175nm/50nm) metallization were fabricated by using an hybrid process combining EBL and CL imaging as reported in Ref. 34 with an additional 375 nm-thick planarization step. The latter was obtained after 5 mn of RF reactive ion etching using an O2 plasma on a spin coated 1.96 µm-thick LOR7A resist. The electrical access was provided by four probe nano-manipulators located in a FEI Inspect F50 FESEM. Multi-probe scanning tunneling microscopy (STM) four-probe resistivity measurements were additionally performed on the dispersed ZnO NRs for direct comparison and confirmation. The samples were loaded into the UHV chamber (base pressure 1x10-11 mbar) and measurements were performed at least 24 hours


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later. Four-point probe measurements were carried out with an Omicron LT Nanoprobe with in situ SEM using a Keithley 2636B sourcemeter. The tungsten STM tips were direct current annealed in UHV to remove probe oxide using the method described by Cobley et al..35 The tips were manually lowered on to the single ZnO NRs in 1 nm steps until a current above the noise was observed to prevent strain on the crystal that may affect the resistance measurements.9,36 Raman scattering spectra were recorded from arrays of polarity controlled ZnO NRs using a Horiba/Jobin Yvon Labram spectrometer equipped with a liquid nitrogen cooled CCD detector. The 488 nm line of an Ar+ laser with a power on the sample surface close to 2 mW was focused to a spot size smaller than 1 µm2 using a 50 times long working distance objective. The spectra were calibrated using a silicon reference sample at room temperature by fixing the theoretical position of the silicon Raman line to 520.7 cm-1. Temperature-dependent Raman scattering measurements were performed from room temperature to 500 °C under oxygen atmosphere. The temperature was monitored with a commercial Linkam heating stage (THMS600) placed under the Raman microscope. 5 K CL measurements of single polarity controlled ZnO NRs were achieved with a FEI Inspect F50 FESEM equipped with a liquid helium cooled stage. The CL signal was collected through a parabolic mirror and analyzed with a 550 mm focal length monochromator equipped with 1800 grooves/mm diffraction grating. CL spectra were recorded with a thermoelectric cooled silicon CCD detector. The low acceleration voltage of 5 kV and small spot size (i.e., less than 10 nm) were used to create the CL signal at the center of single ZnO NWs.

3. RESULTS 3.1. SEM and four-point probe resistivity measurements The morphology of selective area grown O- and Zn-polar ZnO NRs are presented in Figure 1 by FESEM images. The arrays are well ordered with a great structural uniformity. Oand Zn- polar ZnO NRs have a mean length of 3242 ± 85 nm and 3963 ± 60 nm, respectively.


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The difference in the axial and radial growth rates is due to the different surface reaction rate constants between both polar surfaces.33 Both ZnO NRs are basically free of extended structural defects including stacking faults or twins, as presented by conventional and high-resolution transmission electron microscopy imaging in Ref. 30.

Figure 1 Tilted-view FESEM images of selective area grown O- and Zn-polar ZnO NRs on Oand Zn-polar ZnO single crystals pre-patterned using electron beam lithography. The top (i.e. red) and bottom (i.e. blue) triangular areas correspond to the growth on O- and Zn-polar ZnO single crystals, respectively. The insets are the corresponding top-view FESEM images. The electrical measurements were recorded in patterned metal contact and multi-probe STM configurations:37 i) the four-point nanomanipulator probes were placed on the EBL patterned Ti/Au local ohmic contacts and the current injected from the two outer contacts was varied in the range of a few µA while the voltage was measured across the two inner contacts, as revealed in Figure 2a, ii) the four-point STM probes were directly put on the sidewalls of ZnO NRs and the distance between the inner probes from one another was varied while the resistance was measured at each step, as shown in Figure 2b.


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Figure 2 (a) Four-point probe resistivity measurements of single polarity controlled ZnO NRs in patterned metal contact configuration where the current injected from the two outer contacts is varied while the voltage is measured from the two inner contacts. (b) FESEM image illustrating the corresponding configuration. (c) Four-point probe resistivity measurements of single polarity controlled ZnO NRs in multi-probe STM configurations where the inner voltage sensing probe distance is varied and the resistance measured at each step. (d) FESEM image illustrating the corresponding configurations. Both procedures were applied to a population of twenty-three O-polar ZnO NRs and twenty Zn-polar ZnO NRs to make a statistical analysis of their electrical resistivity, as presented in Figure 3. In the metal contact configuration, the O-polar ZnO NRs exhibit a mean electrical resistivity of about 9.8 x 10-2 Ω.cm, which is significantly larger than the mean electrical resistivity of Zn-polar ZnO NRs around 1.1 x 10-2 Ω.cm. Furthermore, the values of the electrical resistivity spread in a much broader range in the case of O-polar ZnO NRs. The corresponding mean electrical conductivity of O- and Zn-polar ZnO NRs are thus of 10.2 and 87.2 S/cm. As a result, the Zn-polar ZnO NRs are, on average, more than eight times more conductive than the Opolar ZnO NRs. The same trend is observed in the multi-probe STM configuration that was used to confirm the measurements in metal contact configuration in the high electrical conductivity range. The mean electrical resistivity of 9.1 x 10-3 Ω.cm for O-polar ZnO NRs is significantly larger than the mean electrical resistivity around 5.6 x 10-3 Ω.cm for Zn-polar ZnO NRs. The deviation in the statistical analysis between both configurations originate from the different


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abilities of the techniques (i.e., multi-probe STM configuration being able to measure the more conductive ZnO NRs due to the direct contact between the tip and the NR surface, in contrast to metal contact configuration) as well as from slightly different conditions used, notably electron beam dose. Also, it is revealed in Figure 3 that the resistivity of ZnO NRs is not dependent upon their radius. Indeed, by considering the high electrical conductivity of ZnO NRs together with their large radius, the transport properties are here governed by the bulk-like conduction in their core. Surface effects originating from the electron accumulation/depletion layer near the surface and modulated by adsorbing species are negligible here and typically arises significantly at much smaller radius below 25 nm in ZnO nanowires9,38-40 and 50 nm in ZnO nanoparticles.41 By assuming that the electron mobility in O- and Zn-polar ZnO NRs is around 50 - 100 cm2/V.s,42 the carrier density approximately lies in the range of 6.4 x 1017 to 1.3 x 1018 cm-3 for O-polar ZnO NRs and of 5.4 x 1018 to 1.1 x 1019 cm-3 for Zn-polar ZnO NRs. The present values are in good agreement with scanning capacitance measurements of CBD-grown ZnO NRs with unknown polarity.10 Also, they typically appear smaller than those of ZnO NRs grown by electrodeposition43 and larger than those of ZnO NRs grown by vapor phase deposition techniques.38,44,45 Interestingly, the Mott transition separating the non-metal / insulator from metal regimes in terms of electrical conductivity operates at an effective critical concentration nc of 4.2 x 1018 cm-3.46 Consequently, some of the O-polar ZnO NRs composing the arrays have a nonmetallic electrical conduction (i.e., a carrier density lower than nc) while other have a metallic electrical conduction (i.e., a carrier density higher than nc). In contrast, all the Zn-polar ZnO NRs composing the arrays have a metallic electrical conduction and thus a carrier density higher than nc. The present statement shows the high electrical conductivity of ZnO NRs grown by CBD as well as the strong effect of their polarity. This suggests a massive incorporation of residual donors during the CBD of ZnO NRs that is further dependent upon their polarity driving the surface terminations.


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Figure 3 Four-point probe resistivity measurements of O- and Zn-polar ZnO NRs as a function of their radius for the patterned metal contact and multi-probe STM configurations.

3.2. Raman scattering measurements The Raman scattering spectra of O- and Zn-polar ZnO NRs are presented in Figure 4. For ସ the wurtzite phase belonging to the ‫଺ܥ‬ఔ (P63mc) space group, the optical modes in the Brillouin

zone center are expressed in the irreducible representations as: Γopt = A1 + E1 + 2E2 + 2B1.47,48 B1 are silent modes. E2low and E2high defined as the respective low and high frequency branches of the E2 mode, are Raman-active non-polar modes, which are attributed to the vibrations of the Zn and O sub-lattices, respectively. A1 and E1 are Raman- and infrared-active polar modes splitting into longitudinal optical (LO) and transverse optical (TO) phonon components owing to macroscopic electrostatic forces within the polar crystal. A1 phonons are polarized along the c-axis, whereas E1 phonons are polarized in the ab-plane.


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Figure 4 Raman scattering spectra of O- and Zn-polar ZnO NR arrays dispersed over a glass substrate and collected at room temperature over the (a) 50 – 900 cm-1 and (b) 2400 – 3700 cm-1 range. In the wavenumber range of 50 to 900 cm-1 as presented in Figure 4a, the Raman scattering spectra show the occurrence of the typical optical phonon modes that are characteristic of the wurtzite structure. The Raman lines at 100, 410, 438, 575, and 581 cm-1 are assigned to E2low, E1(TO), E2high, A1(LO), and E1(LO) modes, respectively.48 The E1(LO) mode has been reported to be hydrogen sensitive and its Raman line position is related to the carrier density owing to Fano resonances involving the coupling of LO phonons with plasmon vibrations.49 However, no significant shift towards high wavenumber appears between O- and Zn-polar ZnO NRs. The Raman line at 332 cm-1 is a second-order line involving the E2high-E2low mode48 and has also been reported to be due to the 1s → 2p donor state transition involving interstitial hydrogen


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in bond-centered sites (HBC).49 Additionally, a weak Raman shoulder at 273 cm-1 also occurs and may be due to the 1s → 2p donor transition involving substitutional hydrogen on the oxygen lattice site (HO).50 Alternatively, the B1low silent mode originating from the relaxation of the Raman selection rules produced by the breakdown of the translational symmetry of the crystal related to hydrogen incorporation may play a role51 and its Raman line position has theoretically been calculated in the range of 261 to 277 cm-1.50 Other second-order Raman lines around 205, 540, and 664 cm-1 are assigned to 2TA / 2E2low, 2B1low / 2LA, and TA+LO modes, respectively.48 In the range of 2400 to 3700 cm-1 as presented in Figure 4b, two broad Raman lines arise at 3390 and 3575 cm-1. The more intense Raman line centered at 3575 cm-1 is unambiguously attributed to HBC as reported in Refs. 52 and 53. The less intense Raman line centered at 3390 cm1

involves the contribution of O-H bonds52,54 on the surface of ZnO NRs. Correlated O-H bending

modes also occur in the range of 1300 to 1650 cm-1 (see Figure S1 as supporting material).55 Interstitial hydrogen in anti-bonding sites (HABO) may further contribute with a theoretical Raman line position at 3352 cm-1,52 but its higher formation energy as compared to the formation energy of HBC is unfavorable.56 More importantly, defect complexes involving multiple O-H bonds in a zinc vacancy in the form of (VZn-Hn), where n lies in the range of 2 to 4, are also expected to play a significant role as reported in Refs. 52,57 and 58. They give rise to infra-red absorption lines in the same wavenumber range pointing at 3312 and 3349 cm-1 for (VZn-H2) defect complexes52 and at 3303 and 3321 cm-1 for (VZn-H3) defect complexes.58 Very interestingly, all these modes vanish after annealing at 500°C (see Figure S2 as Supporting Information), confirming the assignment to hydrogen via HBC, O-H bonds, and likely (VZn-Hn) defect complexes. During annealing, HBC can migrate owing to the low diffusion activation energy of 0.5 eV and hence form HO and electrically inactive H2 molecules.50,59,60 The formation and dissociation of H2 molecules strongly depend on the annealing temperature. A significant part of HBC close to the surface is also expected to diffuse out from the ZnO NRs. The numerous Raman lines in the range of 2700 to 3000 cm-1 are attributed to antisymmetric and symmetric stretching modes of CHx groups.61


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Correlated C-H bonds also occur via the C-H modes in the range of 1300 to 1500 cm-1 (see Figure S1 as Supporting Information).62 These modes are related to residual HMTA molecules, which are adsorbed on the m-plane sidewalls of ZnO NRs and thus inhibit their development and enhance their aspect ratio.7 An additional Raman line at 3078 cm-1 can be attributed to N-H bonds54,61 also related to residual HMTA molecules.7 Importantly, no other additional mode occurs, showing no significant incorporation of residual extrinsic impurities in the lattice, including aluminium, gallium, iron, antimony, or nitrogen.63,64 This somehow excludes the possible role of these impurities in the low electrical resistivity of O- and Zn-polar ZnO NRs. The present statement was further confirmed, to some extent, by EDS spectra collected in a FEGSEM, indicating no aluminium, gallium, iron, antimony, or nitrogen in the O- and Zn-polar ZnO NRs (see Figure S3 as Supporting Information). Instead, the present Raman scattering spectra of O- and Zn-polar ZnO NRs strongly support the contribution of hydrogen as a residual donor to account for their high carrier density.

3.3. 5K cathodoluminescence measurements The spatially-resolved 5K CL spectra of O- and Zn-polar ZnO NRs are presented in Figure 5. The CL measurements were recorded at the center of one single polarity controlled ZnO NR by using a small spot size less than 10 nm and a low acceleration voltage of 5 kV. The near-band edge (NBE) emission is marked by a large number of radiative transitions involving free Aexcitons (FXA), neutral donor-bound B-excitons (D°XB), neutral donor-bound A-excitons (D°XA), and related two-electron satellites (TES), which are labeled in the I nomenclature.65,66


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Figure 5 5 K CL spectra with an 1800 grooves/mm grating of single (a) O- and (b) Zn-polar ZnO NRs highlighting the near-band edge emissions. The CL spectra were recorded under 5 kV acceleration voltage and with a spot size less than 10 nm at the center of single O- and Zn-polar ZnO NRs. Both insets are 5 K CL spectra with a 600 grooves/nm grating of respective single Oand Zn-polar ZnO NRs over a broader energy range. In O-polar ZnO NRs as presented in Figure 5a, the NBE emission is clearly dominated by the intense doublet lines at 3.3619 and 3.3627 eV, respectively. The 3.3619 eV line is associated with the X1 line, which is typical of O-polar ZnO.20 The present line has not been attributed yet, but may involve intrinsic defect complexes. More importantly, the 3.3627 eV line is assigned to the I4 line, which is attributed to neutral donor-bound A-excitons involving HO.49,66,67 The line centered at about 3.332 eV is related to the TES of the I4 line.66 A broad shoulder further lies


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around 3.360 eV and very likely involves neutral donor-bound A-excitons related to HBC.49 Correlatively, the 3.364 and 3.366 eV line are expected to be due to neutral donor-bound Bexcitons involving HBC and HO, respectively.67 The energy separation of about 4.4 meV is in accordance with the A-B valence band splitting.66 Additionally, radiative transitions related to free A-excitons are resolved at 3.377 and 3.376 eV. They are attributed to the longitudinal (AL) and transversal (AT) free A excitons, respectively.66 Alternative assignments involve the lower (PA) and upper (PA) A-polariton branches, respectively.68 The 3.311, 3.236, and 3.166 eV lines, as shown in the inset of Figure 5a, are further associated with first, second, and third LO phonon replica of that emission, each of them being separated by a phonon energy of 72 meV in ZnO.66 In Zn-polar ZnO NRs as presented in Figure 5b, the NBE emission is clearly dominated by the intense line at 3.3616 eV. The present line is assigned to the I5 line, which has recently been attributed to neutral donor-bound A-excitons related to (VZn-Hn) defect complexes.57,67,69,70 The 3.292, 3.218, and 3.144 eV lines, as shown in the inset of Figure 5b, are further associated with first, second, and third LO phonon replica of the I5 line, respectively. The I5 line also exhibits a significant broadening and an asymmetric shape, which are indicative of several additional contributions. A contribution around 3.360 eV very likely involves neutral donor-bound Aexcitons related to HBC.49 The broad shoulder around 3.364 - 3.366 eV is expected to be due to neutral donor-bound B-excitons involving HBC and (VZn-Hn) defect complexes, respectively.67 The 3.311, 3.236, and 3.166 eV lines, as revealed in the inset of Figure 5b, are additionally related to first, second, and third LO phonon replica of free-A excitons. The present spatially-resolved 5K CL spectra of O- and Zn-polar ZnO NRs also support the contribution of hydrogen as a residual donor to explain their high carrier density. Some differences related to the nature of the involved dominant hydrogen-related species are shown between O- and Zn-polar ZnO NRs in agreement with the resistivity and Raman scattering analysis.


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4. DISCUSSION The high electrical conductivity related to the high carrier density of O- and Zn-polar ZnO NRs reveals the formation and/or incorporation of a large concentration of intrinsic defects and/or extrinsic impurities, respectively. Both oxygen vacancies and zinc interstitials were proposed to be at the origin of the n-type of non-intentionally doped ZnO films.71,72 However, oxygen vacancies are deep donors that cannot account for the present high carrier density,73 while zinc interstitials are shallow donors with high formation energy.74 They are further not expected to predominantly form in the present conditions where zinc species are typically the limiting reactants.33,75 Instead, zinc vacancies are expected to be the predominant intrinsic defects, but act as acceptors.71,74,76 The present high carrier density of O- and Zn-polar ZnO NRs consequently originates from the large concentration of extrinsic impurities incorporated. The chemical precursors basically contain some residual impurities with a non-negligible low concentration, such as aluminium or gallium. However, most of these residual impurities form positively charged individual ions at the low pH used.77 They thus undergo electrostatic repulsions with the positively charged c-planes at the top of ZnO NRs. The present considerations are not in favor of their significant incorporation in the bulk of ZnO NRs and their low concentration cannot support the high carrier density either. In contrast, both the Raman scattering and CL measurements strongly support the massive incorporation of hydrogen as a shallow residual donor in ZnO NRs to account for the high carrier density. Previous studies also reported the significant role of hydrogen in ZnO quantum dots formed by wet chemistry using electron paramagnetic resonance and electron-nuclear double resonance spectroscopy.78,79 A schematic of the different hydrogen-related species involved in the present case is presented in Figure 6.


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Figure 6 Schematic of the physical structures of the hydrogen-related species in ZnO NRs, including (a) HBC with the O-H bond parallel to the c-axis (b) HO, and (c) (VZn-H3) defect complexes. From theoretical calculations, it was shown that H+ is the most stable charge state over all Fermi level positions and have sufficient low formation energy owing to the large strength of the O-H bond to induce its high solubility in ZnO.56 Moreover, the crystallization process of ZnO NRs by CBD involves the dehydration of a zinc hydroxide phase80 or of zinc hexahydrate molecules,6,75 depending on the conditions used.81 Hydrogen is, as a result, mainly involved in the crystallization process and, furthermore, saturates the polar c-planes at the top of ZnO NRs at any time of their elongation. Hydrogen in the form of HBC, HO, and (VZn-Hn) defect complexes are incorporated in O- and Zn-polar ZnO NRs. Since the Zn-polar ZnO NRs are, on average, more conductive, the amount of each hydrogen-related species is expected to depend on their polarity. HBC with a low ionization energy of 53 meV49 is massively incorporated in both O- and Zn-polar ZnO NRs. From the present CL measurements, HO with a low ionization energy of 47 meV49 and (VZn-Hn) defect complexes are expected to be the dominant hydrogen-related species in O- and Zn-polar ZnO NRs, respectively. In addition to thermodynamic considerations, the larger growth rate of Zn-polar ZnO NRs may be favorable for the formation of zinc vacancies leading to the formation of (VZn-Hn) defect complexes. The number of involved hydrogen is still open and may directly depend on the concentration of zinc vacancies.58 First theoretical calculations reported that (VZn-H2) defect complexes have the lowest formation energy.82 However, more recent theoretical calculations instead stated that (VZn-H3) and (VZn-H4) defect complexes may


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energetically be favorable as well .83,84 Given that (VZn-H2) defect complexes are neutral, (VZnH3,4) defect complexes as donors are expected to be predominant to account for the high carrier density of Zn-polar ZnO NRs. The present findings are critical for the integration of ZnO NRs grown by CBD into nanoscale engineering devices. While highly conductive ZnO NRs as electron transporting materials are required for nanostructured solar cells, the development of resistive ZnO NRs is also needed for piezoelectric devices, such as nano-generators or pressure-based sensors.1-3 High carrier density can screen the generated piezoelectric potential, limiting in turn its magnitude. This may account for the relatively small piezoelectric potential of 50 - 90 mV in the case of single ZnO NRs grown by CBD85 as compared to the piezoelectric potential of 443 mV in the case of single GaN NWs grown by molecular beam epitaxy.86 Furthermore, most of the ZnO NR arrays grown by CBD are formed on top of a ZnO polycrystalline seed layer with a mixed polarity and thus have a mixed polarity as well.32 Any variation of the polarity in the arrays is expected to introduce detrimental inhomogeneities in the electrical and optical properties by mixing for instance individual ZnO NRs exhibiting a different electrical conductivity magnitude as well as different non-metallic / metallic electrical conduction regimes. The present findings eventually show that any strategy to control the carrier density of ZnO NRs grown by CBD should take into account the massive incorporation of hydrogen.

5. CONCLUSIONS In summary, a statistical analysis of the electrical properties of well-ordered O- and Znpolar ZnO NRs grown by CBD using standard conditions within the SAG approach has been performed over a population of forty-two nano-objects using four-point probe resistivity measurements in patterned metal contact and multi-probe STM configurations. It is shown that Oand Zn-polar ZnO NRs exhibit a high mean electrical conductivity of 10.2 and 87.2 S/cm, respectively, corresponding to a respective carrier density in the range of 6.4 x 1017 to 1.3 x 1018


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cm-3 and of 5.4 x 1018 to 1.1 x 1019 cm-3. Owing to their mean large radius and high electrical conductivity, the nature of the electrical conduction is bulk-like in their core. O-polar ZnO NRs composing the array have a non-metallic or metallic electrical conduction, depending on the individual nano-object considered. In contrast, the electrical conduction in all Zn-polar ZnO NRs is metallic. From Raman scattering spectra and spatially-resolved 5 K CL measurements, the high carrier density is directly attributed to the massive incorporation of hydrogen in the form of HBC, HO, and (VZn,Hn) defect complexes. HBC is largely incorporated in both O- and Zn-polar ZnO NRs. In contrast, HO and (VZn,Hn) defect complexes appear as the dominant hydrogen-related species in O- and Zn-polar ZnO NRs, respectively. The presence of hydrogen is certainly due to the nature of the crystallization process of ZnO NRs grown by CBD in a medium further saturated by hydrogen. The present findings reveal that polarity greatly affects the electrical and optical properties of ZnO NRs. Any strategy to control the carrier density of ZnO NRs grown by CBD as a pre-requisite for real world integration into nanoscale engineering devices should take into account the drastic role of hydrogen.

SUPPORTING INFORMATION Raman scattering spectra of O- and Zn-polar ZnO NR arrays dispersed over a glass substrate and collected at room temperature over the 1200 – 2400 cm-1 range (Figure S1); Raman scattering spectra of O- and Zn-polar ZnO NR arrays dispersed over a glass substrate and collected over the 50 – 3700 cm-1 range after annealing at 500 °C and cooling down to room temperature (Figure S2); EDS spectra collected in a FEG-SEM of single O- and Zn-polar ZnO NRs.

ACKNOWLEDGMENTS Funding by the French Research National Agency through the projects ROLLER (ANR17-CE09-0033) and DOSETTE (ANR-17-CE24-0003) as well as by the Carnot Institute Energies du Futur through the projects CLAPE and ECOLED is also acknowledged. This work was partly supported by the French RENATECH network through the CIME-Nanotech and PTA


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technological platforms in a cleanroom environment. The authors thank Jean-Luc Thomassin from CEA-INAC, Grenoble, France, for his assistance in the EBL experiments. AML would like to thank the support of the Sêr Cymru II fellowship scheme part-funded by the European Regional Development Fund through the Welsh Government. This work was supported by the Centre for Nanohealth, Swansea University, UK.

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