Adrian Avramescu

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces

Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces, which were obtained by HCl etching at elevated temperatures prior to growth, were investigated. The surface morphology and the defect density of AlN films on such surfaces were significantly improved compared to those on as-received SiC surfaces. This is due to enhanced diffusion length and reduced incoherent boundaries at the coalescence regions of the AlN islands. AlN nuclei on the as-received SiC surface were crystallographically misaligned and thus induced incoherent boundaries at the coalescence stage, resulting in the delay of the two-dimensional growth mode transition and defect formation in AlN films.

Luminescence properties of ZnO films grown on GaAs substrates by molecular-beam epitaxy excited by electron–cyclotron resonance oxygen plasma

Abstract Growth of ZnO films on GaAs(0xa00xa01) substrates is demonstrated using ZnS buffer layers by metalorganic molecular-beam epitaxy excited by electron–cyclotron resonance oxygen plasma. Bright near band-edge luminescence was observed at room temperature and the deep-level emission was found to make little contribution to the optical spectrum. The Stokes shift of the luminescence measured at 15xa0K with the photoluminescence excitation spectrum was almost absent. These distinguished features are the clear manifestations of the quite high optical quality of the ZnO films grown on GaAs substrates.

Self-Organized CdSe Quantum Dots on (100)ZnSe/GaAs Surfaces Grown by Metalorganic Molecular Beam Epitaxy

II–VI semiconductor low-dimensional structures, quantum dots, have been grown on GaAs substrates by metalorganic molecular beam epitaxy (MOMBE). Before the heteroepitaxial growth, atomically flat, As-stabilized GaAs surfaces were prepared by high-temperature As cleaning using tris-dimethylamino-arsenic (TDMAAs). CdSe thin films deposited on (100)ZnSe/GaAs surfaces have been investigated with atomic force microscopy (AFM) and were found to form three-dimensional islands with rather uniform size distribution. A large mismatch (~7%) of lattice constants between CdSe and ZnSe pseudomorphically grown on GaAs possibly results in the Stranski-Krastanov growth mode. CdSe quantum dots with a diameter of 97±11 nm were successfully formed at 350°C.

Semiconductor photonic dots: Visible wavelength-sized optical resonators

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160–300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots.

Atomic force microscope lithography on carbonaceous films deposited by electron-beam irradiation

A nanometer-scale mask that can be used for selective growth or selective etching was obtained by two-stage patterning. In the first step, a microscopic patterning was performed using a scanning electron microscope (SEM). In the SEM the mask was deposited by decomposition of residual oil molecules in the vicinity of the semiconductor surface. In the second stage, patterning of the carbonaceous film was made using an atomic force microscope (AFM). Use of the AFM assures not only a precise alignment with the previous marks but also a better resolution. Applying an electric bias between the conductive tip and the substrate surface previously covered with the carbonaceous film gave the local modification of the film composition. Further etching resulted in the pattern formation on the semiconductor surface with a minimum linewidth of ∼22 nm.

Atomic force microscope nanolithography on SiO2/semiconductor surfaces

We report on the fabrication of a nanometer-scale SiO 2 mask on a semiconductor surface using an atomic force microscope (AFM). Layers of SiO 2 5-35nm thick sputtered on GaAs substrates are covered with very thin polymethyl methacrylate (PMMA) films by spin-coating. The AFM tips were used mechanically to modify the polymer layers. Lines with 18-nm width and holes with 20-nm diameter were successfully patterned on the PMMA films. These patterns were transferred into the SiO 2 layers by wet etching in buffered HF. Linewidths as small as 40 nm have been obtained. Arrays of lines and holes with 200 nm periodicity were successfully fabricated.

Growth of AlN-SiC solid solutions by sequential supply epitaxy

Achieving p-type conductivity in the wide-bandgap semiconductors is still a major research challenge. The AlNSiC alloys have the potential for p- and n-type conductivity based on the properties inherited from the easily-to-dope SiC alloy. However, the growth of high quality AlN–SiC alloys was not achieved yet. In this work we propose a new approach for the growth of AlN–SiC alloys by sequential supply epitaxy (SSE) in a low-pressure metalorganic chemical vapor deposition reactor. The alternated supply of trimethyl aluminum and tetraethyl silane during a first sequence followed by the supply of ammonia and ethylene during a second sequence is used to reduce down to almost eliminating the gas-phase reactions. Also, the SSE technique makes it possible to grow the AlN–SiC solid solutions at temperatures rather low, in the range 1200–13001C. Solid solutions with a rich content of AlN were investigated. The electron spectroscopy for chemical analysis (ESCA) measurements show that SiC is incorporated in AlN and solid solution is actually achieved. The alloy compositions as revealed by ESCA were in agreement with those estimated from X-ray diffraction. The full width at half maximum of the AlN–SiC diffraction peak is in the range 200–300 s. The surface of the epilayers was mirror-like and had a roughness with a RMS value o5 nm. r 2002 Elsevier Science B.V. All rights reserved.

Atomic force microscope based patterning of carbonaceous masks for selective area growth on semiconductor surfaces

Carbonaceous masks for selective growth on GaAs substrates were fabricated with high resolution by anodization with an atomic force microscope (AFM). Mask deposition is made by a 15-kV accelerated electron-beam irradiation in a scanning electron microscope. The local anodization of the carbonaceous film under intense electric field is investigated and the main factors for improving resolution and reproducibility are discussed. The “edge effect” of the anodized region, revealed in the electric-field distribution at the tip–water–film interfaces is identified as the main factor responsible for the resolution degradation during patterning. Short forward bias pulse for anodizing the carbonaceous film and the subsequent reverse bias pulse for neutralizing the space charge, locally accumulated during the forward bias, are shown to be effective for the higher pattern resolution and also for deepening the patterning depth. Based on the analysis, a modulated-amplitude pulsed bias mode is proposed and is demonstrat...

Growth mechanism of selectively grown II–VI semiconductor photonic dots for short-wavelength light emitters

Abstract The growth mechanism of the ZnS and CdS semiconductor photonic dots grown by metalorganic molecular-beam epitaxy (MOMBE) was studied. In the initial growth process, the delay time of growth initiation was present for small-area ZnS selective growth. The lateral growth during the selective epitaxy was not linear to the growth time, which was explained by considering the diffusion of the Zn and Cd precursors on the {0xa03xa04} side facets to the (0xa00xa01) top plane during the growth. The calculation of this model was in good agreement with the experimental results. The apex of the grown structures was studied, and the apex of CdS was sharper than those of ZnS and Zn 0.9 Cd 0.1 S. This shows that it is possible to position the CdS layer which has a narrow bandgap only on the apex of the Zn(Cd)S photonic dot as a light-emitting layer.