Y.L. Fan

Superior electrical properties of crystalline Er2O3 films epitaxially grown on Si substrates

Crystalline Er2O3 thin films were epitaxially grown on Si (001) substrates. The dielectric constant of the film with an equivalent oxide thickness of 2.0 nm is 14.4. The leakage current density as small as 1.6 X 10(-4) A/cm(2) at a reversed bias voltage of -1 V has been measured. Atomically sharp Er2O3/Si interface, superior electrical properties, and good time stability of the Er2O3 thin film indicate that crystalline Er2O3 thin film can be an ideal candidate of future electronic devices. (c) 2006 American Institute of Physics.

Band offsets of Er2O3 films epitaxially grown on Si substrates

The experimental data on band alignments of high-k Er2O3 films epitaxially grown on Si substrates by molecular beam epitaxy are reported. By using x-ray photoelectron spectroscopy, the valence and the conduction-band offsets of Er2O3 to Si are obtained to be 3.1±0.1 and 3.5±0.3eV, respectively, showing a roughly symmetrical offset at the conduction and the valence band. The energy gap of Er2O3 is determined to be 7.6±0.3eV. From the band offset viewpoint, those obtained numbers indicate that Er2O3 could be a promising candidate for high-k gate dielectrics.The experimental data on band alignments of high-k Er2O3 films epitaxially grown on Si substrates by molecular beam epitaxy are reported. By using x-ray photoelectron spectroscopy, the valence and the conduction-band offsets of Er2O3 to Si are obtained to be 3.1±0.1 and 3.5±0.3eV, respectively, showing a roughly symmetrical offset at the conduction and the valence band. The energy gap of Er2O3 is determined to be 7.6±0.3eV. From the band offset viewpoint, those obtained numbers indicate that Er2O3 could be a promising candidate for high-k gate dielectrics.

Factors influencing epitaxial growth of three-dimensional Ge quantum dot crystals on pit-patterned Si substrate

We investigated the molecular beam epitaxy growth of three-dimensional (3D) Ge quantum dot crystals (QDCs) on periodically pit-patterned Si substrates. A series of factors influencing the growth of QDCs were investigated in detail and the optimized growth conditions were found. The growth of the Si buffer layer and the first quantum dot (QD) layer play a key role in the growth of QDCs. The pit facet inclination angle decreased with increasing buffer layer thickness, and its optimized value was found to be around 21°, ensuring that all the QDs in the first layer nucleate within the pits. A large Ge deposition amount in the first QD layer favors strain build-up by QDs, size uniformity of QDs and hence periodicity of the strain distribution; a thin Si spacer layer favors strain correlation along the growth direction; both effects contribute to the vertical ordering of the QDCs. Results obtained by atomic force microscopy and cross-sectional transmission electron microscopy showed that 3D ordering was achieved in the Ge QDCs with the highest ever areal dot density of 1.2 × 10(10) cm(-2), and that the lateral and the vertical interdot spacing were ~10 and ~2.5 nm, respectively.

Formation of coupled three-dimensional GeSi quantum dot crystals

Coupled three-dimensional GeSi quantum dot crystals (QDCs) are realized by multilayer growth of quantum dots (QDs) on patterned SOI (001) substrates. Photoluminescence spectra of these QDCs show non-phonon (NP) recombination and its transverse-optical (TO) phonon replica of excitons in QDs. With increasing excitation power, peak energies of both the NP and TO peaks remain nearly constant and the width of the TO peak decreases. These anomalous features of the PL peaks are attributed to miniband formation due to strong coupling of the holes and the emergence of quasioptical phonon modes due to periodic scatters in ordered GeSi QDs.

Atomic composition profile change of SiGe islands during Si capping

The 6% Ge isocomposition profile change of individual SiGe islands during Si capping at 640 degrees C is investigated by atomic force microscopy combined with a selective etching procedure. The island shape transforms from a dome to a {103}-faceted pyramid at a Si capping thickness of 0.32 nm, followed by the decreasing of pyramid facet inclination with increasing Si capping layer thickness. The 6% Ge isocomposition profiles show that the island with more highly Si enriched at its one base corner before Si capping becomes to be more highly Si intermixed along pyramid base diagonals during Si capping. This Si enrichment evolution inside an island during Si capping can be attributed to the exchange of capped Si atoms that aggregated to the island by surface diffusion with Ge atoms from inside the island by both atomic surface segregation and interdiffusion rather than to the atomic interdiffusion at the interface between the island and the Si substrate. In addition, the observed Si enrichment along the island base diagonals is attempted to be explained on the basis of the elastic constant anisotropy of the Si and Ge materials in (001) plane. (c) 2006 American Institute of Physics.

Fowler-Nordheim hole tunneling in metal-Er2O3-silicon structures

Fowler-Nordheim (FN) tunneling of holes in metal-Er2O3–Si structures is confirmed. The effective mass of holes in Er2O3 films is estimated ranging from 0.068m to 0.092m, where m is the free electron mass. The film shows a high breakdown electric field of about 70MV∕cm for an Er2O3 film thickness of 8.5nm, implying that the film which is epitaxially grown on Si substrate has smooth interface and surface.

Effect of graphene on photoluminescence properties of graphene/GeSi quantum dot hybrid structures

Graphene has been discovered to have two effects on the photoluminescence (PL) properties of graphene/GeSi quantum dot (QD) hybrid structures, which were formed by covering monolayer graphene sheet on the multilayer ordered GeSi QDs sample surfaces. At the excitation of 488 nm laser line, the hybrid structure had a reduced PL intensity, while at the excitation of 325 nm, it had an enhanced PL intensity. The attenuation in PL intensity can be attributed to the transferring of electrons from the conducting band of GeSi QDs to the graphene sheet. The electron transfer mechanism was confirmed by the time resolved PL measurements. For the PL enhancement, a mechanism called surface-plasmon-polariton (SPP) enhanced absorption mechanism is proposed, in which the excitation of SPP in the graphene is suggested. Due to the resonant excitation of SPP by incident light, the absorption of incident light is much enhanced at the surface region, thus leading to more exciton generation and a PL enhancement in the region. T...

Structural and electrical characterization of ultrathin Er2O3 films grown on Si(001) by reactive evaporation

Stoichiometric, amorphous, uniform Er2O3 films are deposited on Si(001) substrates by reactive evaporation using a metallic Er source at room temperature in an oxygen ambient pressure of 9 ? 10?6?Torr. Transmission electron microscopic measurement shows that the film possesses good thermal stability and forms a sharp interface with its Si substrate after annealing at 700??C for 30?min in ultrahigh vacuum. The effective dielectric constant (k) of the film is measured to be 12.6, and its effective oxide thickness (EOT) can reach 1.4?nm, with a low leakage current density of 8 ? 10?4?A?cm?2 at an electric field of 1?MV?cm?1 after annealing. The characteristics obtained indicate that ultrathin amorphous Er2O3 film could be a promising candidate for a high-k gate dielectric in Si microelectronic devices.

Ordering of low-density Ge quantum dot on patterned Si substrate

We present a study on the growth of a low-density ordered Ge quantum dot (QD) on a nanohole patterned Si (0 0 1) substrate with periods in the order of microns by molecular beam epitaxy. Ordered Ge QDs with different periods were realized, the largest period being 15 µm. From the height-profile evolution of the QD with Ge deposition, it was found that the nanohole filling started at the initial Ge deposition, indicating an immediate nucleation and growth of QDs inside the nanoholes. Such a phenomenon is attributed to a lower surface chemical potential (SCP) inside the nanoholes, which is supported by calculated results on SCP evolution with growth and verified by the observation that the Ge adatoms around the nanoholes exhibited a higher probability of being incorporated into the nanoholes. In this scenario, low density ordered Ge QDs with any large periods could be achieved. The optical properties of the Ge QDs showed a remarkable improvement after a post-growth rapid thermal annealing treatment.

Formation of planar defects over GeSi islands in Si capping layer grown at low temperature

Coherently strained GeSi/Si(001) islands were overgrown with a Si capping layer of different thicknesses at temperature 300 °C. The structures of the islands and the Si capping layer were investigated by high resolution transmission electron microscopy. The shapes of the embedded islands were well preserved, whereas planar defects were observed exactly over the islands in the capping layers. The strain energy in regions over the islands accumulated with increasing thickness of the Si capping layer, resulting in the formation of the planar defects. By means of a two-step deposition in which 20-nm-thick Si capping layer was first deposited at a low temperature of 300 °C followed by 70-nm-thick Si capping layer deposition at a high temperature of 640 °C, the defect-free Si capping layer with flat surface can be obtained.