Effect of rapid thermal annealing and hydrogen plasma treatment on the microstructure and light-emission of silicon-rich oxide film

Silicon-rich silicon oxide (SRSO) films are prepared by plasma-enhanced chemical vapor deposition method at the substrate temperature of 200degreesC. The effect of rapid thermal annealing and hydrogen plasma treatment on tire microstructure and light-emission of SRSO films are investigated in detail using micro-Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and photoluminescence (PL) spectra. It is found that the phase-separation degree of the films decreases with increasing annealing temperature from 300 to 600degreesC, while it increases with increasing annealing temperature from 600 to 900degreesC. The light-emission of the films are enhanced with increasing annealing temperature up to 500degreesC, while it is rapidly reduced when the annealing temperature exceeds 600degreesC. The peak position of the PL spectrum blueshifts by annealing at the temperature of 300degreesC, then it red-shifts with further raising annealing temperature. The following hydrogen plasma treatment results in a disproportionate increase of the PL intensity and a blueshift or redshift of the peak positions, depending on the pristine annealing temperature. It is thought that the size of amorphous silicon clusters, surface structure of the clusters and the distribution of hydrogen in the films can be changed during the annealing procedure. The results indicate that not only cluster size but also surface state of the clusters plays an important role in the determination of electronic structure of the amorphous silicon cluster and recombination process of light-generated carriers.

In-plane optical anisotropy in asymmetric Si1-xGex/Si/Si1-yGey superlattices

A trilayer asymmetric superlattice, Si/Si1-xGex/Si1-yGey, is proposed, in which the broken inversion symmetry makes the microstructure optically biaxial; in particular, inequivalent interfaces in this heterostructure may cause a polarization ratio as large as about 2.5% in the absence of an external field. The electronic structure and absorption spectra for two types of trilayer superlattice with different parameters are calculated by use of the tight-binding model; the findings indicate the importance of the carrier confinement for the anisotropy value. The effect of external electric field on the optical anisotropy for such structures has also been discussed, and a Pockels coefficient of 10-9 cm V-1 estimated.

Properties and turning of intraband optical absorption in InxGa1-xAs/GaAs self-assembled quantum dot superlattice

We investigated properties of intraband absorption in In-x Ga1-xAs quantum dots (QDs) superlattice. Energy levels in conduction band in QDs were calculated for a cone-shaped quantum dot associated with coupling between QDs in the framework of the effective-mass envelope-function theory. Theoretical results demonstrated that energy levels in conduction band were greatly affected by the vertical coupling between quantum dots, which can be used to modify transition wavelength by adjusting the space layer thickness. Intraband transition is really sensitive to normal incidence and the absorption peak intensity is dependent on the polarization. A satisfying agreement is found between theoretical and experimental values. This result opens up prospects for the fabrication of QDs infrared detectors, which work at atmospheric windows.

Optical spectra of CdSe nanocrystals under hydrostatic pressure

Optical spectra of CdSe nanocrystals are measured at room temperature under pressure ranging from 0 to 5.2 GPa. The exciton energies shift linearly with pressure below 5.2 GPa. The pressure coefficient is 27 meV GPa(-1) for small CdSe nanocrystals with the radius of 2.4 nm. With the approximation of a rigid-atomic pseudopotential, the pressure coefficients of the energy band are calculated. By using the hole effective-mass Hamiltonian for the semiconductors with wurtzite structure under various pressures, we study the exciton states and optical spectra for CdSe nanocrystals under hydrostatic pressure in detail. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbit coupling on the hole states are investigated. The Coulomb interaction of the exciton states is also taken into account. It is found that the theoretical results are in good agreement with the experimental values.

A density-functional study of small titanium clusters

The atomic structures and electronic properties of small Ti-N (N=2-10) clusters have been studied by using the density-functional theory with a local spin density approximation. We find that the inner-shells (3s3p) of the titanium atom plays an important role in the formation of the small clusters. We have obtained the ground state of titanium clusters, Ti-7 is found to be a magic cluster, which is in good agreement with the experimental results. Starting with Ti-8 cluster some features of the electronic structure of the titanium bulk have been developed. The ionization potentials and magnetic moments for these small titanium clusters are also presented. (C) 2000 American Institute of Physics. [S0021- 9606(00)30544-X].

X-ray diffraction and optical characterization of interdiffusion in self-assembled InAs/GaAs quantum-dot superlattices

We report on the characterization of thermally induced interdiffusion in InAs/GaAs quantum-dot superlattices with high-resolution x-ray diffraction and photoluminescence techniques. The dynamical theory is employed to simulate the measured x-ray diffraction rocking curves of the InAs/GaAs quantum-dot superlattices annealed at different temperatures. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness, and stress variations caused by interdiffusion are taken in account. It is found that the significant In-Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The diffusion coefficients at different temperatures are estimated. (C) 2000 American Institute of Physics. [S0003-6951(00)02440-2].

Exciton states and optical spectra in CdSe nanocrystallite quantum dots

By using the hole effective-mass Hamiltonian for semiconductors with the wurtzite structure, we have studied the exciton states and optical spectra in CdSe nanocrystallite quantum dots. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbital coupling (SOC) on the hole states are investigated. It is found that the strong SOC limit is a good approximation for hole states. The selection rules and oscillator strengths for optical transitions between the conduction- and valence-band states are obtained. The Coulomb interaction of exciton states is also taken into account. In order to identify the exciton states, we use the approximation of eliminating the coupling of Gamma(6)(X, Y) with Gamma(1)(Z) states. The results are found to account for most of the important features of the experimental photoluminescence excitation spectra of Norris ct nl. However, if the interaction between Gamma(6)(X, Y) and Gamma(1)(Z) states is ignored, the optically passive P-x state cannot become the ground hole state for small CdSe quantum dots of radius less than 30 Angstrom. It is suggested that the intrinsic asymmetry of the hexagonal lattice structure and the coupling of Gamma(6)(X,Y) with Gamma(1)(Z) states are important for understanding the "dark exciton" effect.

Blue-violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers

We report on the strong blue-violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470-390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)04427-2].

Photoluminescence study of InAs/GaAs self-organized quantum dots with bimodal size distribution

We have investigated the temperature dependence of the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy image for uncapped sample. The power-dependent PL study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. Due to lacking of the couple between quantum dots, an unusual temperature dependence of the linewidth and peak energy of the dot ensemble photoluminescence has not been observed. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature.

Experimental determination of local Strain effect on InAs/GaAs self-organized quantum dots

The energy barrier at InAs/GaAs interface due to the built-in strain in self-organized system has been determined experimentally. Such a barrier has been predicted by previous theories. From the deep-level transient spectroscopy (DLTS) measurements, we have obtained the electron and hole energy levels of quantum dots E-e(QD-->GaAs) = 0.13 eV and E-h(QD-->GaAs) = 0.09 eV relative to the bulk unstrained GaAs band edges E-c and E-v. DLTS measurements have also provided evidence to the existence of the capture barriers of quantum dots for electron E-eB = 0.30 eV and hole E-hB = 0.26 eV. The barriers can be explained by the apexes appearing in the interface between InAs and GaAs caused by strain. Combining the photoluminescence results, the band structures of InAs and GaAs have been determined.

Temperature dependence of the optical properties of InAs/GaAs self-organized quantum dots with bimodal size distribution

In this work we report the optical and microscopic properties of self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy (STM) image for uncapped sample. The power-dependent photoluminescence (PL) study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. It is shown that the coupling between quantum dots plays a key role in unusual temperature dependence of QD photoluminescence. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature. (C) 2000 Elsevier Science B.V. All rights reserved.

Evidence of multimodal patterns of self-organized quantum dots

We have used Deep Level Transient Spectroscopy to investigate self-organized InAs/GaAs quantum dots. The existence of different dot families is confirmed by the deconvolution of the spectra in Gaussian components with full width at half maximum of 60-70meV. The strain of quantum dots is responsible for the relaxation of large quantum dots leading to generation of dislocations. (C) 1998 Academic Press.

Quantum confinement effect in silicon quantum-well layers

The electronic states and optical transition properties of silicon quantum-well layers embedded by SiO2 layers are studied by the empirical pseudopotential homojunction model. The energy bands, wave functions, and the optical transition matrix elements are obtained for layers of thickness from 1 to 6 nm, and three oriented directions (001), (110), and (111). It is found that for Si layers in the (001) direction the energy gap is pseudodirect, for these in the (111) direction the energy gap is indirect, while for those in the (110) direction the energy gap is pseudodirect or indirect for a thickness smaller or larger than 3 nm, respectively. The optical transition matrix elements are smaller than that of diner transition, and increase with decreasing layer thickness. When the thickness of a layer is smaller than 2 nm, the Si QW layers have larger transition matrix elements. It is caused by mixing of bulk X states with the Gamma(1) state. The calculated results are compared with experimental results.

ELECTRON-STATES OF A STACKING-FAULT RIBBON IN SILICON

The electronic structure of a bounded intrinsic stacking fault in silicon is calculated. The method used is an LCAO-scheme (Linear Combinations of Atomic Orbitals) taking ten atomic orbitals of s-, p-, and d-type into account. The levels in the band gap are extracted using Lanczos' algorithm and a continued fraction representation of the local density of states. We find occupied states located up to 0.3 eV above the valence band maximum (E(v)). This significantly differs from the result obtained for the ideal infinite fault for which the interface state is located at E(v)+ 0.1 eV.