A. Franciosi

Heterojunction band offset engineering

Abstract Control of band discontinuities in semiconductor heterostructures may introduce a new important degree of freedom in the design of heterojunction devices and allow independent optimization of carrier injection, carrier confinement and ionization thresholds in high speed and optoelectronic devices. We will review recently proposed methods to microscopically control heterojunction parameters by means of local interface dipoles introduced at the heterointerface during growth. A parallel survey of new theoretical models of semiconductor heterojunctions will illustrate our newfound ability to derive from first principles rules of heterojunction behavior. The combination of new empirical methods and theoretical models is establishing the new area of heterojunction engineering in surface and interface science.

Ultrahigh vacuum metalorganic chemical vapor deposition growth and in situ characterization of epitaxial TiO2 films

In a two‐chamber ultrahigh vacuum system, epitaxial TiO2 thin films have been deposited by metalorganic chemical vapor deposition on single crystal oxide substrates over a temperature range of 250–800 °C, using titanium (IV) isopropoxide as the precursor. During the initial stage of epitaxial film deposition, the growing surface quickly planarized and the film’s orientations was determined by the substrate structure. This substrate influence is manifested in the growth of anatase (the low temperature phase of TiO2) on (001) SrTiO3, at high deposition temperatures (800 °C), whereas on either (0001) or (1102) Al2O3 sapphire, epitaxial rutile (the high temperature phase) is formed. In situ Auger electron spectroscopy analyses, before and after growth, revealed a film composition identical to that of a bulk TiO2 standard. No carbon contamination was detected in films grown throughout the deposition temperature range. The decomposition mechanism of this precursor that leads to the absence of incorporated carb...

Low-temperature synthesis of ZnSe nanowires and nanosaws by catalyst- assisted molecular-beam epitaxy

Single-crystal ZnSe nanowires are grown on a prepatterned gold catalyst by molecular-beam epitaxy. Optimum selectivity and maximum nanowire densities are obtained for growth temperatures in the range 400–450°C, but gold-assisted growth is demonstrated for temperatures as low as 300°C. This suggests a diffusion process on/through the catalyst particle in the solid state, in contrast to the commonly assumed liquid phase growth models. Straight wires, as thin as 10nm, nucleate together with thicker and saw-like structures. A gold particle is always found at the tip in both cases.

Electronic structure, surface composition and long-range order in GaN

Abstract We present a study of hexagonal GaN thin films grown by low-pressure chemical vapor deposition on sapphire substrates. We used synchrotron radiation photoemission spectroscopy to probe the electronic density of states and test the validity of existing band structure calculations. Parallel studies of surface composition and atomic structure by means of Auger spectroscopy and reflection high-energy electron diffraction were performed following sputtering/annealing cycles using argon, xenon and nitrogen ions. We found that sputtering with nitrogen ions followed by annealing in vacuum is an effective method to reduce nitrogen surface depletion and produce quasi-stoichiometric ordered GaN(0 0 0 1)1 × 1 surfaces.

Chemical Bonding at the Si-Metal Interface: Si-Ni and Si-Cr,

Chemical bonding at the interface of a near‐noble‐metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.

Photoluminescence of Mn-catalyzed GaAs nanowires grown by molecular beam epitaxy

We present photoluminescence spectra of GaAs nanowires and nanoleaves grown by molecular beam epitaxy using Mn as growth catalyst. At low temperature and low excitation intensity the spectra are characterized by several narrow peaks superimposed onto a broader band. The peak at the highest energy is located at 1.522 eV, i.e. 7 meV above the free exciton position in GaAs, irrespective of growth conditions and of the shape and size of the nanostructures. We suggest that this peak originates from electron–hole recombination in wurtzite-type GaAs.

Microgun‐pumped blue lasers

Microgun‐pumped lasers, in which electron beam pumping is achieved via an array of low‐voltage, field‐emission microtips, were demonstrated in the blue and blue‐green regions of the spectrum. The devices exploit graded index, separate confinement Zn1−xCdxSe/ZnSe heterostructures grown by molecular beam epitaxy on In1−xGaxAs(100) substrates. Lasing thresholds were in the 4–20 kW/cm2 range for temperatures between 83 and 225 K, and the device lifetime exceed several hours at a 12.5% duty cycle for a laser operating at 83 K and a wavelength of 478.4 nm.

ATOMIC DIFFUSION-INDUCED DEEP LEVELS NEAR ZNSE/GAAS(100) INTERFACES

Luminescence spectroscopy measurements of ZnSe/GaAs(100) heterojunctions grown by molecular beam epitaxy reveal the formation of deep levels near ‘‘buried’’ interfaces upon thermal annealing. A pronounced emission at 1.9–2.0 eV appears at temperatures in the 300–400 °C range depending on the ZnSe growth conditions with a constant activation energy of 1.10 eV. X‐ray photoemission spectroscopy indicates a correlation between this deep level and atomic diffusion of Ga and Zn across the heterointerface. Zn‐rich ZnSe growth conditions dramatically reduce this emission, highlighting the importance of local interface composition on thermal stability.

Selective growth of ZnSe and ZnCdSe nanowires by molecular beam epitaxy

Controlled growth of ZnSe and ZnCdSe nanowires is demonstrated by molecular beam epitaxy using Au or Ag catalyst films in the temperature range 400–550 °C. The highest density of small-diameter (10 nm), highly-crystalline ZnSe nanowires is achieved by using Au at 400 °C. Direct growth onto transmission electron microscope grids clearly indicates a tip-growth regime. Pre-patterning of the catalyst film allows highly selective ZnSe deposition as probed by photoluminescence and Raman spectroscopy. In similar conditions, the addition of Cd vapour in the MBE reactor allows the synthesis of ZnCdSe ternary nanowires.

Self-catalyzed GaAs nanowire growth on Si-treated GaAs(100) substrates

Self-catalyzed GaAs nanowire growth was obtained by molecular beam epitaxy on GaAs(001) substrates after predeposition of subnanometer-thick Si layers. Two substrate preparation methods are presented, the first based on the epitaxial growth of Si on GaAs and subsequent exposure to atmosphere, and the second on the direct deposition of Si on epiready GaAs substrates. X-ray photoemission spectroscopy shows that both methods result in a thin Si oxide layer that promotes the growth of GaAs nanowires aligned along the 〈111〉 direction. High densities of nanowires were obtained at substrate temperatures between 620 and 680 °C. Systematic electron microscopy studies indicate that nanowire growth is associated with the formation of Ga nanoparticles on the substrate surface, which act as a catalyst in the vapor-liquid-solid growth mechanism frame. The majority of the nanowires have a pure zinc-blende structure, and their photoluminescence is dominated by a photoluminescence peak 3 to 5 meV in width and centered at ...