Raffaella Calarco

Mechanism of molecular beam epitaxy growth of GaN nanowires on Si(111)

GaN nanowires have been grown without external catalyst on Si(111) substrates by plasma-assisted molecular beam epitaxy. Nanowire aspect ratios (length/diameter) of about 250 have been achieved. During the initial stage of the growth, there is a nucleation process in which the number of wires increases and the most probable nucleation diameter of about 10nm has been observed, which slowly increases with deposition time. For deposition time longer than the nucleation stage, the nanowire length as a function of diameter monotonically decreases. This phenomenon can be explained by adatom diffusion on the nanowire lateral surface towards the tip.

Interface and Wetting Layer Effect on the Catalyst-Free Nucleation and Growth of GaN Nanowires†

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F

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(x)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.

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Raman scattering of phonon-plasmon coupled modes in self-assembled GaN nanowires

We report the determination of free-electron concentration and mobility of free-standing GaN nanowires (NWs) by line shape analysis of the coupled longitudinal optical phonon-plasmon Raman modes (L+). The E2high phonon mode at 566.9 cm−1 with a sharp linewidth of 2.8 cm−1 indicates strain free NWs with high crystalline perfection. The lattice temperature of the NWs was varied between 313 and 472 K by varying the excitation laser beam power. For unintentionally doped samples at room temperature, an average electron concentration and mobility of strain free NWs were found to be ∼2×1017 cm−3 and 460 cm2/V s, respectively. We have shown that the electron concentration does not change significantly over a temperature range between 313 and 472 K. The electron mobility decreases at high temperatures, in agreement with literature data for compact layers. For Si-doped NWs, the L+ phonon peak is strongly upshifted indicating a higher free-carrier concentration of about 1×1018 cm−3. Asymmetric broadening observed at...

Investigation on Localized States in GaN Nanowires

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of nanodevices. The conductivity, photoconductivity, and persistent photoconductivity behavior of GaN nanowires are observed to strongly depend on the wire diameter. In particular, by spectral photoconductivity measurements, three main sub-band-gap optoelectronic transitions were detected, ascribed to the localized states giving rise to the characteristic blue, green, and yellow bands of GaN. Photoconductivity with below-band-gap excitation varies orders of magnitude with the wire diameter, similarly to that observed for near-band-edge excitation. Moreover, yellow-band-related signal shows a superlinear behavior with respect to the band-edge signal, offering new information for the modeling of the carrier recombination mechanism along the nanowires. The photoconductivity results agree well with a model which takes into account a uniform distribution of the localized states inside the wire and their direct recombination with the electrons in the conduction band.

Flux Quantization Effects in InN Nanowires

InN nanowires, grown by plasma-enhanced molecular beam epitaxy, were investigated by means of magnetotransport. By performing temperature-dependent transport measurements and current measurements on a large number of nanowires of different dimensions, it is proven that the carrier transport mainly takes place in a tube-like surface electron gas. Measurements on three representative nanowires under an axially oriented magnetic field revealed pronounced magnetoconductance oscillations with a periodicity corresponding to a single magnetic flux quantum. The periodicity is explained by the effect of the magnetic flux penetrating the coherent circular quantum states in the InN nanowires, rather than by Aharonov-Bohm type interferences. The occurrence of the single magnetic flux quantum periodicity is attributed to the magnetic flux dependence of phase-coherent circular states with different angular momentum quantum numbers forming the one-dimensional transport channels. These phase coherent states can exist because of the almost ideal crystalline properties of the InN nanowires prepared by self-assembled growth.

Current path in light emitting diodes based on nanowire ensembles

Light emitting diodes (LEDs) have been fabricated using ensembles of free-standing (In, Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron-beam-induced current analysis, cathodoluminescence as well as biased μ-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In, Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect.

Influence of the adatom diffusion on selective growth of GaN nanowire regular arrays

Molecular beam epitaxy (MBE) on patterned Si/AlN/Si(111) substrates was used to obtain regular arrays of uniform-size GaN nanowires (NWs). The silicon top layer has been patterned with e-beam lithography, resulting in uniform arrays of holes with different diameters (dh) and periods (P). While the NW length is almost insensitive to the array parameters, the diameter increases significantly with dh and P till it saturates at P values higher than 800 nm. A diffusion induced model was used to explain the experimental results with an effective diffusion length of the adatoms on the Si, estimated to be about 400 nm.

Mirror-symmetric magneto-optical Kerr rotation using visible light in [(GeTe)2(Sb2Te3)1]n topological superlattices.

Interfacial phase change memory (iPCM), that has a structure of a superlattice made of alternating atomically thin GeTe and Sb2Te3 layers, has recently attracted attention not only due to its superior performance compared to the alloy of the same average composition in terms of energy consumption but also due to its strong response to an external magnetic field (giant magnetoresistance) that has been speculated to arise from switching between topological insulator (RESET) and normal insulator (SET) phases. Here we report magneto-optical Kerr rotation loops in the visible range, that have mirror symmetric resonances with respect to the magnetic field polarity at temperatures above 380 K when the material is in the SET phase that has Kramers-pairs in spin-split bands. We further found that this threshold temperature may be controlled if the sample was cooled in a magnetic field. The observed results open new possibilities for use of iPCM beyond phase-change memory applications.