A. Rizzi

Electrical Conductivity of InN Nanowires and the Influence of the Native Indium Oxide Formed at Their Surface

The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between core bulk (quadratic) and shell sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

Electronic and vibrational properties of semiconducting crystalline FeSi2 layers grown on Si(111)

Semiconducting FeSi2 thin layers have been grown on Si(111) by solid state reaction under ultrahigh vacuum (UHV) conditions. The different reaction steps in the temperature range 380–750u2009°C were followed in situ by Auger electron spectroscopy (AES) and electron energy‐loss spectroscopy (EELS). FeSi2 is formed between 550 and 680u2009°C. At higher temperatures the silicide thin film disrupts forming islands as shown by EELS and ex situ medium‐energy ion scattering (MEIS). The low energy excitation spectra of the thin FeSi2 layers were measured by high‐resolution electron energy‐loss spectroscopy (HREELS). Surface phonon excitations at 50 meV energy are observed and explained in the framework of dielectric theory of surface scattering. This is the first time that Fuchs–Kliewer surface phonons, typical for heteropolar crystals such as GaAs, ZnO, etc. are so clearly observed on a silicide surface by HREELS. Their presence gives evidence of the semiconducting character of the FeSi2 overlayer. A broad loss structur...

AlN and GaN epitaxial heterojunctions on 6H–SiC(0001): Valence band offsets and polarization fields

From a series of in situ photoemission experiments macroscopic electric fields are clearly demonstrated in SiC/AlN, SiC/GaN, and GaN/AlN heterostructures grown by molecular beam epitaxy on 6H–SiC(0001). A significant contribution is due to the spontaneous polarization; the piezoelectric term alone would not explain the sign of the field measured in SiC/AlN. The experimental field has lower intensity as compared to theory: the role of electronic gap states at the surface is pointed out. A self-consistent tight-binding approach which is able to describe polarization fields, dielectric screening, and free carrier screening is applied for a more consistent theoretical discussion of the experimental data. The valence band offset (VBO) has been determined for all heterojunctions under study and the apparent dependence on the overlayer thickness, due to the presence of the strong polarization fields, has been pointed out in view of a correct determination of the VBO. The VBOs at the heterojunctions obtained by e...

Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200kV∕cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720fs in our LT GaN material.

Si(111)/FeSi2 heterostructures: Formation and properties of the low temperature metallic (γ) and semiconducting (β) disilicide phases

Very thin layers (<100 A) of FeSi2 have been grown on Si(111) by solid phase epitaxy (SPE) under ultrahigh vacuum conditions. Two different phases for the iron disilicide layers, γ‐ and β‐FeSi2, can be obtained, depending on the initial Fe coverage, by annealing at temperatures below 600u2009°C. For low coverages (∼3 A Fe) the metallic cubic γ silicide phase is obtained, while for higher Fe coverages (∼ 20 A) the semiconducting orthorhombic β phase is grown. A quantitative Auger analysis shows how the Fe initial coverage dependence can be related to the RT formation of an intermixed FeSi2‐like layer at the interface. The electronic properties and annealing behavior of these ultrathin silicide layers on Si(111) have been studied in situ by high resolution electron energy loss spectroscopy and ultraviolet photoemission spectroscopy. Due to the very small thickness of the metastable γ phase as grown by SPE, no direct phase transformation to the β phase could be observed by thermal treatment.

Tracking defect-induced ferromagnetism in GaN:Gd

We report on the magnetic properties of GaN:Gd layers grown by molecular beam epitaxy (MBE). A poor reproducibility with respect to the magnetic properties is found in these samples. Our results show strong indications that defects with a concentration of the order of 10^19 cm^-3 might play an important role for the magnetic properties. Positron annihilation spectroscopy does not support the suggested connection between the ferromagnetism and the Ga vacancy in GaN:Gd. Oxygen co-doping of GaN:Gd promotes ferromagnetism at room temperature and points to a role of oxygen for mediating ferromagnetic interactions in Gd doped GaN.

Phase Separation in Single InxGa1–xN Nanowires Revealed through a Hard X-ray Synchrotron Nanoprobe

In this work, we report on the composition, short- and long-range structural order of single molecular beam epitaxy grown In(x)Ga(1-x)N nanowires using a hard X-ray synchrotron nanoprobe. Nano-X-ray fluorescence mapping reveals an axial and radial heterogeneous elemental distribution in the single wires with Ga accumulation at their bottom and outer regions. Polarization-dependent nano-X-ray absorption near edge structure demonstrates that despite the elemental modulation, the tetrahedral order around the Ga atoms remains along the nanowires. Nano-X-ray diffraction mapping on single nanowires shows the existence of at least three different phases at their bottom: an In-poor shell and two In-rich phases. The alloy homogenizes toward the top of the wires, where a single In-rich phase is observed. No signatures of In-metallic precipitates are observed in the diffraction spectra. The In-content along the single nanowires estimated from X-ray fluorescence and diffraction data are in good agreement. A rough picture of these phenomena is briefly presented. We anticipate that this methodology will contribute to a greater understanding of the underlying growth concepts not only of nanowires but also of many nanostructures in materials science.

X-ray absorption in GaGdN: A study of local structure

In this study, we report on the incorporation of dilute Gd amounts into GaN films grown by molecular beam epitaxy. A combination of x-ray fluorescence with x-ray absorption spectroscopic techniques enabled us to examine not only the distribution of rare earth atoms in the GaN matrix but also the short-range structural order. Our results show Gd atoms in a trivalent state with tetrahedral coordination, thus substituting Ga in the wurtzite GaN structure.

Comment on influence of crystal polarity on the properties of Pt/GaN Schottky diodes [appl. phys. lett. 77, 2012 (2000)]

n, GaN Because of its uniaxial wurtzite structure GaN exhib new interesting electronic properties as compared with c sical, cubic III–V semiconductors. Many of these propert are due to the built-in strong polarization fields, spontaneo and in addition, piezoelectric ones in the case of strai layers. In this context, Karrer et al. report about differing Schottky-barrier heights of Pt/GaN contacts measured molecular-beam epitaxy grown-GaN layers with Gaand N-face polarity. From capacitance–voltage and curre voltage measurements, the authors derive barrier hei Fexp(Ga2face)51.1 eV and Fexp(N2face)50.9 eV for GaN~0001! and GaN(0001 ̄), respectively. The interpretatio of the authors for this obvious difference is in terms of different band bending on the two polar faces, inferred fr a different screening behavior of the internal polarizat charges at the surfaces. This different band bending claimed to lead to different electron affinities at the two s faces and therefore different Schottky-barrier heights. 1

Optical Emission of Individual GaN Nanocolumns Analyzed with High Spatial Resolution

Selective area growth has been applied to fabricate a homogeneous array of GaN nanocolumns (NC) with high crystal quality. The structural and optical properties of single NCs have been investigated at the nanometer-scale by transmission electron microscopy (TEM) and highly spatially resolved cathodoluminescence (CL) spectroscopy performed in a scanning transmission electron microscope (STEM) at liquid helium temperatures. TEM cross-section analysis reveals excellent structural properties of the GaN NCs. Sporadically, isolated basal plane stacking faults (BSF) can be found resulting in a remarkably low BSF density in the almost entire NC ensemble. Both, defect-free NCs and NCs with few BSFs have been investigated. The low defect density within the NCs allows the characterization of individual BSFs, which is of high interest for studying their optical properties. Direct nanometer-scale correlation of the CL and STEM data clearly exhibits a spatial correlation of the emission at 360.6 nm (3.438 eV) with the location of basal plane stacking faults of type I1.