Zuimin Jiang

Band alignments and photon-induced carrier transfer from wetting layers to Ge islands grown on Si(001)

Temperature- and excitation-power-dependent photoluminescence measurements were carried out for the multilayer structure of Ge islands grown on a Si(001) substrate by gas-source molecular-beam epitaxy. When the excitation power increases from 10 to 400 mW, the photoluminescence peak from the Ge islands showed a large linear blueshift of 34 meV while that of the wetting layers did not change much. These two different power dependences are explained in terms of type-II and type-I band alignments for the islands and the wetting layers, respectively. When the sample temperature increased from 8 to 20 K, an anomalous increase of photoluminescence intensity for islands was accompanied by a rapid decrease of that from the wetting layers, implying that a large portion of photon-induced carriers in the wetting layer was transfered to the neighboring islands and the Si layer, respectively, thus resulting in an increase of photoluminescence intensity of the islands.

Thermal stability of multilayer films Pt,Si, W/Si, Mo/Si, and W/C

The thermal stability of multilayer films has been studied by use of small‐angle x‐ray diffraction. The temperatures at which the periodicities of Pt/Si, W/Si, Mo/Si, and W/C multilayers begin to be worse are 200, 300, 400, 900 °C, respectively. The temperatures of total mixing of Pt/Si, W/Si, Mo/Si, and W/C are 600, 600, 700, and >900 °C, respectively. The periods of Mo/Si and W/Si decrease about 5%, 10%, respectively, after annealing at 400 °C for 0.5 h. The period of W/C increases continuously with the increasing temperatures. After annealing at 1000 °C for 0.5 h the increment of the period of W/C is about 20%. The former may be mainly due to the interfacial reaction between metal and Si and the latter may be due to the expansion of C films in W/C.

Defective photonic crystals with greatly enhanced second-harmonic generation.

A one-dimensional defective photonic crystal structure is proposed with the aim of studying its nonlinear optical properties. In such a structure, extremely enhanced second-harmonic generation with an efficiency ~5 orders of magnitude higher than that of ordinary films is demonstrated in a numerical simulation. Extraordinary phase conditions of the process in such a structure were explored, and efficient forward and backward second-harmonic generation could be achieved simultaneously. The mechanism of the enhancement is the high field intensity and efficient wave coupling introduced by light localization in defect states, from which many other nonlinear processes can also benefit.

GERMANIUM DOTS WITH HIGHLY UNIFORM SIZE DISTRIBUTION GROWN ON SI(100) SUBSTRATE BY MOLECULAR BEAM EPITAXY

The growth of very uniform Ge dots on Si(100) is achieved by using molecular beam epitaxy. The atomic force microscopy and the transmission electron microscopic observations illustrate that the size uniformity of the dots is not worse than ±3%, i.e., the base dimension is 100±3 nm. The Raman spectrum reveals a peak downward shift of Ge-Ge mode caused by the phonon confinement in the Ge dots. A very narrow photoluminescence peak with the width of 1.6 meV at the energy of 0.767 eV is observed at the temperature of 16 K. We attribute this peak to the free exciton longitudinal acoustic phonon replica originated from the Ge dots.

Self-assembled SiGe quantum rings grown on Si(001) by molecular beam epitaxy

SiGe quantum rings (QRs) were grown by partially capping on Ge quantum dots (QDs) on Si(001). Atomic force microscopy images show the shape transformation from QDs to QRs. Initial capping, with a Si layer thickness less than 2 nm, will result in the decrease of height of QDs and increase of base diameter of QDs. Capped with a Si layer, QDs will change into QRs. The mechanism of transformation from QDs to QRs is discussed. The strain will redistribute after capping, thus the strain energy relief, together with high Ge surface diffusion and Ge surface segregation at a relative high temperature of 680 °C, play the dominant role.

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.

Composition and its impact on shape evolution in dislocated Ge(Si)/Si islands

The composition distribution of Ge(Si)/Si (001) islands grown at 700 °C by molecular beam epitaxy is investigated using high-spatial resolution x-ray energy dispersive spectrometry in a scanning transmission electron microscope. Island shapes are investigated using cross-section transmission electron microscopy. Results show nonuniformity of the composition distribution in the islands, which affects the evolution of the aspect ratios of height-to-base diameter of dislocated islands.

1.55 μm reflection-type optical waveguide switch based on SiGe/Si plasma dispersion effect

Based on total internal reflection and plasma dispersion effect, a SiGe/Si asymmetric optical waveguide switch with transverse injection structure has been proposed and fabricated. The switch performance is measured at the wavelength of 1.55 μm. A modulation depth of 90% at an injection current of 110 mA is obtained, and the switching time is about 0.2 μs. The device reaches a maximum optical switching at the injection current of 120 mA. The extinction ratio is larger than 34 dB and the crosstalk and insertion loss are less than −18.5 and 2.86 dB, respectively.

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.

Lattice strains and composition of self-organized Ge dots grown on Si(001)

X-ray diffraction measurements at different grazing angles for self-organized Ge dots grown on Si(001) are carried out by using synchrotron radiation as a light source. The lattice parameters parallel and perpendicular to the surface are determined from the grazing angle and ordinary x-ray diffraction spectra. A 1.2% lattice constant expansion parallel to the interface and a 3.1% lattice expansion along the growth direction, as compared with the Si lattice, are found within the Ge dots. Based on the Poisson equation and the Vegard law, the Ge dot should be a partially strain relaxed SiGe alloy with the Ge content of 55%. The composition change in Ge dots is suggested to be caused by the atomic intermixing during the islanding growth. In the small grazing angle x-ray diffraction spectrum, a peak located at the higher angle side of Si(220) is observed. The origin of this peak is attributed to the near surface compressive strain in the peripheral substrate regions surrounding the Ge dots. This compressive strain is induced by the formation of Ge dots and leads to a -0.8% lattice constant change parallel to the interface