PHYSICAL REVIEW B 84, 245302 (2011) Polarity of GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si(111) Karine Hestroffer, 1,* C´ edric Leclere, 2 Catherine Bougerol, 3 Hubert Renevier, 2 and Bruno Daudin 1 1 CEA-CNRS group “Nanophysique et Semiconducteurs,” Universit´ e Joseph Fourier and CEA Grenoble, INAC, SP2M, 17 rue des Martyrs, 38 054 Grenoble, France 2 Laboratoire des Mat´ eriaux et du G´ enie Physique, Grenoble INP - MINATEC, 3 parvis L. N´ eel 38016 Grenoble, France 3 CEA-CNRS group “Nanophysique et Semiconducteurs,” Institut N´ eel, CNRS and Universit´ e Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France (Received 11 August 2011; published 7 December 2011) Based on the breakdown of Friedel’s law, resonant x-ray diffraction is shown to be a suitable method to determine polarity of non-centrosymmetrical wurtzite gallium nitride (GaN) nanowires (NWs) at a macroscopic scale. It is demonstrated that such GaN NWs grown by plasma-assisted molecular beam epitaxy on bare Si(111) are N-polar, consistent with results obtained by convergent beam electron diffraction. The N-polarity feature is attributed to the formation of a thin Si x N 1–x layer on the Si surface before growth. The use of a thin AlN buffer layer does not modify the GaN NW polarities, suggesting that NWs actually grow between the AlN grains rather than on top of them. DOI: 10.1103/PhysRevB.84.245302 PACS number(s): 61.46.Km, 61.05.cp I. INTRODUCTION Polarity is an intrinsic property of non-centrosymmetrical crystalline structures such as wurtzite. The lack of center of symmetry may induce a spontaneous polarization within the cell, leading to the presence of an internal electrical field, the amplitude of which directly depends on the relative positioning of the atoms in the unit cell and therefore on the lattice parameters. Furthermore, strains within the structure produce an additional piezoelectric polarization that contributes to the total electric field. In the case of heterostructures, the discontinuity of the electric field raising from the difference of polarization between the two materials may lead to the formation of two-dimensional (2D) electron gases at the interfaces. In the case of quantum wells embedded in a barrier material, this gives rise to a carrier separation responsible for a red shift of the luminescence and a reduction of the oscillator strength. This drawback for optical properties, known as quantum confined stark effect (QCSE), has been extensively investigated in III-N semiconductor heterostructures, which are of great interest for optoelectronic applications in the large range of wavelengths covered by the different III-N alloys spanning over the whole visible spectrum down to UV. Nevertheless, considering the lack of appropriate substrates, the growth of high structural quality, defect-free III-N layered heterostructures is difficult. An alternative to this issue has appeared with the breakthrough of gallium nitride nanowires (GaN NWs) grown by plasma-assisted molecular beam epitaxy (PAMBE) on sapphire or silicon 1,2 since no epitaxial relation to the substrate is necessary. With the recent mastering of selective area growth, 3 GaN NWs moreover appear as an attractive base for localized heterostructures such as InGaN on GaN NWs, 4,5 GaN/AlN core-shell NWs, 6 or GaN/AlN quantum dots (QDs) in GaN NWs. 7 It has been demonstrated that, in the case of such GaN QDs, although the piezoelectric component is reduced because of an efficient strain relaxation allowed by the NW geometry, QCSE is still significant, leading to a red shift of the GaN QD emission. Based on a tight-binding method, it has been shown that such an electric field is screened to a large extent by the charges pulled out from the top of the NW heterostructures. 8 This mechanism, crucial to prediction of the optical properties of NW heterostructures is polarity dependent. In the case of 2D layers, it has been found that polarity influences the morphology of the surface, the crystal quality, 9–11 and, most importantly, the incorporation of impurities and vacancies during growth 12 that itself critically affects the optical properties. It is reasonable to expect similar features in the case of NWs, emphasizing the necessity for an absolute determination of their chemical termination, namely metal or N, to fully understand their optical properties. When speaking of polarity, one usually refers to the direction along which GaN grows by considering the Ga-N bond that is colinear to the c-axis of the wurtzite cell. The vector going from Ga and pointing toward N conventionally defines [0001], the positive direction of the c-axis. A structure is said to be Ga-polar or Ga-terminated when its growth direction is [0001]. Reciprocally, a structure is said to be N-polar when its growth direction is [000 ¯ 1]. A large panel of experimental methods such as ion channeling, hemispherically scanned x-ray photoelectron diffraction, x-ray standing wave, Auger electron spectroscopy or coaxial impact-collision ion scattering spectroscopy may be used to determine the polarity of thick 2D layers. In the case of GaN, it has been already widely investigated 9,11,13–16 and appears to depend on the growth technique (metalorganic chemical vapor deposition or MBE), the substrate [Sapphire, Si(111), SiC(0001)], the use or not of a buffer layer, and the growth conditions, leading to a wide set of sometimes contradictory experimental results in literature. 17 Regarding layers grown by PAMBE on sapphire, polarity is determined by the surface nitridation temperature prior to growth 11 or can be controlled by tuning the polarity of an AlN buffer layer, itself determined by the Al/N ratio during deposition. 18 Interestingly, N-polar GaN layers have also been grown successfully on Si 3 N 4 /Si(111). 19 More recently, studies have been performed on GaN wires with a diameter in the range of 1–5 μm, grown by metalorganic vapor phase epitaxy or amonia-MBE. 20,21 Wires grown on nitrided c-sapphire substrate exhibit a mixture of Ga- and N-polarity with a tendency to be rather N-polar while wires grown on Si(111) appear to be Ga-polar at 90%. However, in what concerns the 245302-1 1098-0121/2011/84(24)/245302(6) ©2011 American Physical Society