Showing papers on "Potential well published in 1998"

Silicon nanowhiskers grown on a hydrogen-terminated silicon {111} surface

Abstract: Using a hydrogen-terminated Si {111} surface as a substrate, we have grown Si nanowhiskers along the 〈112〉 direction by the vapor–liquid–solid mechanism. The minimum silicon core diameter was 3 nm and the maximum length was about 2 μm. The minimum silicon core diameter is close to the critical value for visible light emission due to the quantum confinement effect. In contrast to an oxidized Si surface, the hydrogen-terminated surface facilitates the formation of small molten Au–Si catalysts at a lower temperature (500 °C) which is slightly above the eutectic temperature. The formation of catalysts and the subsequent growth at the low temperature yield thin Si nanowhiskers on a Si substrate.

Shallow donor states in GaAs-(Ga, Al)As quantum dots with different potential shapes

Abstract: Energies of the ground and some excited states of on-centre donors in spherical quantum dots are calculated, within the effective mass approximation, as functions of the R dot radius and for different potential shapes. We propose an exact numerical solution for the radial Schrodinger equation in a quantum dot with any arbitrary spherical potential by using a trigonometric sweep method. An evident increase in the binding energy is found as the soft-edge-barrier potential model is considered. It is found that the binding energy increases as the dot size decreases up to a dot radius critical value and then, for R slightly smaller than , the impurity wave function spreads to the barrier region and the 3D character is rapidly restored. The properties of the shallow donors in a quantum dot with a double-step potential barrier and multiple barriers are analysed, and two peaks in the binding energy are found. Our results for the spherical-rectangular potential are in good agreement with previous calculations obtained using other methods.

Quantum confinement effect in self-assembled, nanometer silicon dots

Abstract: The first subband energy at the valence band of self-assembled silicon quantum dots grown by low-pressure chemical vapor deposition on ultrathin SiO2/Si substrates has been measured as an energy shift at the top of the valence band density of states by using high-resolution x-ray photoelectron spectroscopy. The systematic shift of the valence band maximum towards higher binding energy with decreasing the dot size is shown to be consistent with theoretical prediction. The charging effects of the silicon dots and the SiO2 layer by photoelectron emission during the measurements have been taken into account in determining the valence-band-edge energy.

Vertical Quantum Confinement Effect on Optical Properties of Strain-Induced Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices

Abstract: Optical properties of multilayer quantum dots (MQDs) self-formed in the GaP/InP short period superlattice (SL)/InGaP multilayer structures are investigated by changing the SL period (P) and InGaP barrier thickness (B). By decreasing P, photoluminescence (PL) peak energy is shifted toward higher energy due to the quantum size effect along the growth direction (vertical direction). PL linewidth broadening with temperature is reduced by decreasing P and B. This improvement is attributed to the reduction of potential distribution among quantum dots (QDs) and the enhancement of quantum confinement along the vertical direction, and to the enhancement of quantum confinement due to the vertical coupling effect between QDs. Stokes shift for the MQDs structure is observed to be small. Very small temperature variation of PL peak energy is observed in these MQDs, which originates from the existence of the multiaxial strains among the MQDs. Electroluminescence (EL) spectra show split peaks corresponding to the subbands of QDs on increasing injection current.

On the potential surface of the locally enhanced sampling approximation

Abstract: The locally enhanced sampling (LES) methodology is used for simulating molecules consisting of a small subsystem that is of interest and a larger subsystem that interacts with the smaller one. The small subsystem is replicated to enhance its sampling and the large subsystem sees the mean field of the replicated components while the replicas are energetically transparent to one another. In this work we investigate the properties of the LES potential surface. It is shown that every local energy minimum on the potential surface of the real system is also a local energy minimum on the LES potential surface. It is also demonstrated that the converse of this is not true; i.e., there are minima on the LES potential energy surface that do not correspond to any minimum on the potential surface of the real system. A quantitative measure is derived to estimate how far minima on the LES potential surface are from minima on the potential surface of the real system. The implications and potential applications of this w...

Experimental Study of the Effect of the Quantum Well Structures on the Thermoelectric Figure of Merit in Si/Si 1- x Ge x System

Abstract: In bulk form, Si1-xGe x is a promising thermoelectric material for high temperature applications. In this paper, we report results from an experimental study as well as theoretical modeling of the quantum confinement effect on the enhancement of the thermoelectric figure of merit. Si/Si1-xGe x , multiple quantum well structures are fabricated using molecular beam epitaxy (MBE) on SOI (Silicon-on-Insulator) substrates in order to eliminate substrate effects, especially on the Seebeck coefficient. A method to eliminate the influence of the buffer layer on the thermoelectric characterization is presented. An enhancement of the thermoelectric figure of merit within the quantum well over the bulk value is observed.

Energy spectrum of a nonideal quantum well in an electric field

Abstract: The effect of an electric field on the energy spectrum of a quantum well with macroscopic fluctuations is studied. The Stark shift of the quasibound states in a quantum well and three field-dependent broadening mechanisms (field-induced homogeneous broadening and broadening due to well width and depth fluctuations) are calculated in a wide range of electric fields. As an example, the effect of an electric field on the energy spectrum of electrons in a 12-nm-wide GaAs/Al0.3Ga0.7As quantum well with 5% width and depth fluctuations is determined.

Structural and Optical Characterization of Porous 3C‐SiC

Abstract: Porous 3C-SiC prepared with conventional electrochemical anodization has been characterized using various techniques, such as Fourier transform infrared spectroscopy, extended X-ray absorption fine structure, and X-ray absorption near edge structure. Intense photoluminescence with energies above the bandgap of bulk 3C-SiC has been observed. This is consistent with the fact that the size of the minimum feature for this material is less than 10 nm, which is small enough to cause the quantum confinement effect. Changes in the electronic structure of the conduction band and shortened Si-C bond length have been clearly observed.

Quantum Confinement in InSb Microcrystallites Embedded in SiO 2 Thin Films

Abstract: Semiconductor InSb microcrystallites were embedded in SiO2 thin films by rf cosputtering technique. Structures of the thin films were characterized by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. Average size of the microcrystallites, depending on post-annealing temperature and time, is on the order of magnitude of nanometer. Absorption spectra of the films were measured and large blue shifts of absorption edge were observed in a wide range from 300 to 1500 nm. The blue shifts were attributed to the quantum confinement effect and explained in the model of effective-mass approximation.

Controlling the Particle Size of Quantum Dots Incorporated in Hybrid Materials

Abstract: Semiconductor PbS quantum dot-doped Ormocers were successfully prepared by the sol-gel technique. Ormocers preparation was based on the use of trifunctional silane precursors at the solution stage. Formation of PbS particles took place in the pores of the Ormocers through lead precursor reaction with H 2 S gas. It was observed that temperature was an important factor in the reaction leading to the first appearance of PbS particles. The dot size of PbS was controlled through chemical interaction with the non-hydrolyzed groups of the trifunctional silane precursors. These groups prevent uncontrolled nucleation and aggregation processes during the particle formation and growth. The control of particle size was studied at different conditions for nucleation and aggregation. Determination of the average particle size was done by XR-Diffraction. Optical absorption spectra were also measured at the UV-VIS wavelength range. Absorption edge blue shifts show the quantum confinement effect in these materials.

Epitaxial growth and photoluminescence of Si/pure-Ge/Si quantum structures on Si(311) substrates

Abstract: Si/pure-Ge/Si quantum structures were grown on Si(311) substrates by gas-source molecular beam epitaxy at various growth temperatures and their optical properties were investigated by photoluminescence (PL) spectroscopy. At higher than , uniform steps which align towards the direction were found to have formed after growing a 2000 A Si buffer layer. A series of Si/pure-Ge/Si layers on the surface steps with various Ge coverage (Q) exhibited intense PL and showed systematic energy shift due to the quantum confinement effect even with Q less than one monolayer (ML), which indicates the existence of the lateral confinement effect. With increasing incident laser power, the PL energy of samples with Q larger than 1 ML shifted to higher energies as can usually be seen for indirect quantum wells (QWs). On the other hand, below 1 ML, PL peaks did not show any blue shift with increasing laser power. This can be explained by a bunched density of states at the band edge in quantum wires (QWRs) which were realized by preferential growth of Ge atoms at the step edges. At of , Ge QWRs were not realized even with Q less than 1 ML as evidenced by a spectral blue-shift with increasing excitation power. This is presumably due to the insufficient surface migration length and the resultant incorporation of Ge atoms at the terraces, leading to the formation of SiGe alloy QWs with 1 ML thickness. For Q larger than 3.0 ML, the growth mode changed to being three-dimensional and isosceles-triangle-shaped Ge islands were formed. However, PL mainly came from the wetting layer since the sizes of the islands were too large and the crystal quality was probably poor.

Synthesis of II-VI semiconductor nanoparticles by mechanochemical processing

Abstract: The synthesis of II-VI semiconductor nanoparticles by mechanochemical reaction has been reviewed. Solid-state displacement reactions between ZnCl/sub 2/ or CdCl/sub 2/ and alkali or alkaline earth chalcogenides were induced during mechanical milling in a steady-state manner, leading to the formation of II-VI semiconductor nanoparticles in a chloride salt matrix. A simple washing process employed to remove the chloride byproduct resulted in separated particles of less than 10 nm. The resulting particles and crystallite sizes were controlled by changing the milling conditions, starting materials, annealing temperature and the presence of a diluent. With decreasing particle size, the band-gap energy and plasmon peak energy of CdS increased due to the quantum confinement effect.

Novel light emitter and wavelength converter device

Abstract: We present a structure which is capable of being fabricated into two distinct devices, both with considerable potential in the field of optical communications in particular with reference to wavelength domain multiplexing (WDM). The structure is based on two back to back p-i-n Ga x Al 1-x As structures with a single quantum well of GaAs in each intrinsic (i) region. The light emitter device operates by forward biasing either of the p-i-n elements. In forward bias holes flood into the quantum well in the intrinsic region. Electrons are prevented from doing so by a potential barrier. A longitudinal electric field applied along the central n-doped region heats the electrons in this region and gives them sufficient energy to overcome the barrier and flood into the quantum well and hence recombine with holes which are already present. The wavelength converter device operates with one p-i-n structure forward biased and one reverse biased. The forward biased element has a quantum well positioned near the p-doped region. Light of the appropriate wavelength is absorbed in this quantum well. The holes scatter out of the quantum well and drift into the p-doped region. The electrons are scattered out of the quantum well and drift towards the n-doped region, creating additional carriers through impact ionisation, thereby creating gain. The electrons flooding over the n-doped region, must overcome a potential barrier to enter the forward biased element, therefore cold electrons are prevented from entering this region. Electrons which are able to overcome the barrier fall into a quantum well positioned near the barrier, where holes are already waiting, as in the light emitting device.

Quantum confinement effect in electroluminescent porous silicon

Abstract: The voltage-tuned electroluminescence (EL) of n-type porous silicon (PS) in a persulphate solution under cathodic polarization was studied. A blue shift with the increase of cathodic polarization and a time-dependent red shift at constant potential of EL spectra have been observed. To investigate the voltage tuning mechanism and luminescence quenching mechanism for the electroluminescence of PS, FT-IR, AFM and electrochemical methods were used. The results show that the voltage-tuned electroluminescence can be attributed to the selective excitation of different sizes of PS crystallites, which agrees with the “quantum confinement model” in PS luminescence.

Ultrafast coherent oscillations of electron wave packet in a step-doped stepped quantum well structure

Abstract: The temporal development of an electron wave packet initially photoexcited in the region with smaller band gap of a stepped quantum well is investigated numerically by directly solving the time-dependent Schrodinger equation. It is shown that, if the photoexcitation region is about 55% to 65% of the total quantum well width, then coherent ultrafast oscillations will occur between the photoexcitation region and the rest of the well region. The period of these ultrafast oscillations increases exponentially with the quantum well width. In addition, the oscillation period ranges between 15 fs for a 50 A quantum well width and 700 fs for a 500 A quantum well width. The above coherent oscillations are also found to be insensitive to variations in the quantum well depth.

Quantum well semiconductor device

Abstract: of EP0866505A semiconductor device includes a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well relative to electrons and holes and the barrier layer forming a potential barrier relative to electrons and holes. The height of the quantum well layer and the height of the potential barrier of the valence band at the interface between the quantum well layer and the barrier layer are set so that the number of quantum levels relative to holes on the valence-band side of the quantum well layer is two or three in the state where the intensity of an electric field generated in the quantum well layer is zero. The semiconductor device is provided with a means for applying an electric field in the quantum well lamination structure in the thickness direction.

Potential difference and photovoltaic effect arising from distortion of the electron wave function in a GaAs quantum well with a thin AlGaAs barrier

Abstract: This paper discusses changes in the spectrum and distortion of the electron wave function of a GaAs quantum well when a thin AlGaAs barrier is introduced into it. The potential difference generated across the quantum well by distortion of the electron wave function is calculated, along with its dependence on the position of the barrier in the quantum well. The photovoltaic response of the structure to optical intersubband excitations is also calculated, along with the role of wave function and electronic spectrum distortion as well as intersubband nonradiative transitions in generating this response. The suitability of a GaAs quantum well with a thin barrier for use as an infrared detector is considered.