Luminous Efficiency of Axial In_<i>x</i>Ga_1–<i>x</i>N/GaN Nanowire Heterostructures: Interplay of Polarization and Surface Potentials

Using continuum elasticity theory and an eight-band <b>k</b>·<b>p</b> formalism, we study the electronic properties of GaN nanowires with axial In_<i>x</i>Ga_1–<i>x</i>N insertions. The three-dimensional strain distribution in these insertions and the resulting distribution of the polarization fields are fully taken into account. In addition, we consider the presence of a surface potential originating from Fermi level pinning at the sidewall surfaces of the nanowires. Our simulations reveal an in-plane spatial separation of electrons and holes in the case of weak piezoelectric potentials, which correspond to an In content and layer thickness required for emission in the blue and violet spectral range. These results explain the quenching of the photoluminescence intensity experimentally observed for short emission wavelengths. We devise and discuss strategies to overcome this problem

Strain Engineering of Nanowire Multi-Quantum Well Demonstrated by Raman Spectroscopy

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial In_<i>x</i>Ga_1–<i>x</i>N quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the In_<i>x</i>Ga_1–<i>x</i>N/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures

Anomalous Strain Relaxation in Core–Shell Nanowire Heterostructures via Simultaneous Coherent and Incoherent Growth

Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the narrow lattice mismatch range restricting planar heterostructures, owing to misfit strain relaxing at the free surfaces and partitioning throughout the entire nanostructure. In this work, we uncover a novel strain relaxation process in GaAs/In_<i>x</i>Ga_1–<i>x</i>As core–shell nanowires that is a direct result of the nanofaceted nature of these nanostructures. Above a critical lattice mismatch, plastically relaxed mounds form at the edges of the nanowire sidewall facets. The relaxed mounds and a coherent shell grow simultaneously from the beginning of the deposition with higher lattice mismatches increasingly favoring incoherent mound growth. This is in stark contrast to Stranski–Krastanov growth, where above a critical thickness coherent layer growth no longer occurs. This study highlights how understanding strain relaxation in lattice mismatched nanofaceted heterostructures is essential for designing devices based on these nanostructures

Nanowires Bending over Backward from Strain Partitioning in Asymmetric Core–Shell Heterostructures

The flexibility and quasi-one-dimensional nature of nanowires offer wide-ranging possibilities for novel heterostructure design and strain engineering. In this work, we realize arrays of extremely and controllably bent nanowires comprising lattice-mismatched and highly asymmetric core–shell heterostructures. Strain sharing across the nanowire heterostructures is sufficient to bend vertical nanowires over backward to contact either neighboring nanowires or the substrate itself, presenting new possibilities for designing nanowire networks and interconnects. Photoluminescence spectroscopy on bent-nanowire heterostructures reveals that spatially varying strain fields induce charge carrier drift toward the tensile-strained outside of the nanowires, and that the polarization response of absorbed and emitted light is controlled by the bending direction. This unconventional strain field is employed for light emission by placing an active region of quantum dots at the outer side of a bent nanowire to exploit the carrier drift and tensile strain. These results demonstrate how bending in nanoheterostructures opens up new degrees of freedom for strain and device engineering

Photoelectrochemical Properties of (In,Ga)N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy

We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)­N NWs exhibited an anodic photocurrent, however, no O₂ but only N₂ evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H₂. After photodeposition of Pt on such NWs, the photocurrent density was significantly enhanced to 5 mA/cm² at a potential of −0.5 V/NHE under visible light irradiation of ∼40 mW/cm². Also, incident photon-to-current conversion efficiencies of around 40% were obtained at −0.45 V/NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p-(In,Ga)­N NWs are a promising basis for solar hydrogen production

Spontaneous Nucleation and Growth of GaN Nanowires: The Fundamental Role of Crystal Polarity

We experimentally investigate whether crystal polarity affects the growth of GaN nanowires in plasma-assisted molecular beam epitaxy and whether their formation has to be induced by defects. For this purpose, we prepare smooth and coherently strained AlN layers on 6H-SiC(0001) and SiC(0001̅) substrates to ensure a well-defined polarity and an absence of structural and morphological defects. On N-polar AlN, a homogeneous and dense N-polar GaN nanowire array forms, evidencing that GaN nanowires form spontaneously in the absence of defects. On Al-polar AlN, we do not observe the formation of Ga-polar GaN NWs. Instead, sparse N-polar GaN nanowires grow embedded in a Ga-polar GaN layer. These N-polar GaN nanowires are shown to be accidental in that the necessary polarity inversion is induced by the formation of Si_<i>x</i>N. The present findings thus demonstrate that spontaneously formed GaN nanowires are irrevocably N-polar. Due to the strong impact of the polarity on the properties of GaN-based devices, these results are not only essential to understand the spontaneous formation of GaN nanowires but also of high technological relevance

GaAs–Fe₃Si Core–Shell Nanowires: Nanobar Magnets

Semiconductor–ferromagnet GaAs–Fe₃Si core–shell nanowires were grown by molecular beam epitaxy and analyzed by scanning and transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and magnetic force microscopy. We obtained closed and smooth Fe₃Si shells with a crystalline structure that show ferromagnetic properties with magnetizations along the nanowire axis (perpendicular to the substrate). Such nanobar magnets are promising candidates to enable the fabrication of new forward-looking devices in the field of spintronics and magnetic recording

Control over the Number Density and Diameter of GaAs Nanowires on Si(111) Mediated by Droplet Epitaxy

We present a novel approach for the growth of GaAs nanowires (NWs) with controllable number density and diameter, which consists of the combination between droplet epitaxy (DE) and self-assisted NW growth. In our method, GaAs islands are initially formed on Si(111) by DE and, subsequently, GaAs NWs are selectively grown on their top facet, which acts as a nucleation site. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and the same degree of control is transferred to the final GaAs NWs. We show how, by a suitable choice of V/III flux ratio, a single NW can be accommodated on top of each GaAs base island. By transmission electron microscopy, as well as cathodo- and photoluminescence spectroscopy, we confirmed the high structural and optical quality of GaAs NWs grown by our method. We believe that this combined approach can be more generally applied to the fabrication of different homo- or heteroepitaxial NWs, nucleated on the top of predefined islands obtained by DE

Radial Stark Effect in (In,Ga)N Nanowires

We study the luminescence of unintentionally doped and Si-doped In_<i>x</i>Ga_1–<i>x</i>N nanowires with a low In content (<i>x</i> < 0.2) grown by molecular beam epitaxy on Si substrates. The emission band observed at 300 K from the unintentionally doped samples is centered at much lower energies (800 meV) than expected from the In content measured by X-ray diffractometry and energy dispersive X-ray spectroscopy. This discrepancy arises from the pinning of the Fermi level at the sidewalls of the nanowires, which gives rise to strong radial built-in electric fields. The combination of the built-in electric fields with the compositional fluctuations inherent to (In,Ga)N alloys induces a competition between spatially direct and indirect recombination channels. At elevated temperatures, electrons at the core of the nanowire recombine with holes close to the surface, and the emission from unintentionally doped nanowires exhibits a Stark shift of several hundreds of meV. The competition between spatially direct and indirect transitions is analyzed as a function of temperature for samples with various Si concentrations. We propose that the radial Stark effect is responsible for the broadband absorption of (In,Ga)N nanowires across the entire visible range, which makes these nanostructures a promising platform for solar energy applications

Polarity-Induced Selective Area Epitaxy of GaN Nanowires

We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The approach is based on the fact that these nanostructures do not form in plasma-assisted molecular beam epitaxy on structurally and chemically uniform cation-polar substrates. By <i>in situ</i> depositing and nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to induce the selective area epitaxy of N-polar GaN nanowires. We show that the nanowire number density can be controlled over several orders of magnitude by varying the amount of predeposited Si. Using this growth approach, we demonstrate the synthesis of single-crystalline and uncoalesced nanowires with diameters as small as 20 nm. The achievement of nanowire number densities low enough to prevent the shadowing of the nanowire sidewalls from the impinging fluxes paves the way for the realization of homogeneous core-shell heterostructures without the need of using <i>ex situ</i> prepatterned substrates