Si Single Quantum Research Articles

Preparation and Optical Absorption Performance of Si Single Quantum Dots and Si/Ge Double Quantum Dots Doped TiO2 Films.

Si single quantum dots (QDs) and Si/Ge double QDs doped TiO2 films were successfully fabricated via layer-by-layer ion beam sputtering assisted by annealing treatment, and their morphology and optical absorption performance were analyzed in this paper. TEM images show that Si QDs and Si/Ge double QDs prepared in the TiO2 matrix are in a uniform size distribution and high density, especially after annealed at 500 °C. XPS results indicate that Si in the TiO2 films mainly exists in the form of elemental Si and SiO2, and Ge is in elemental form. From the optical absorption spectra, the optical response of the TiO2 films is shifted from ultraviolet to visible and near infrared region when they are doped by the QDs, and the Si/Ge double QDs co-doped TiO2 films show better optical response compared with the Si QDs doped TiO2 films, suggesting a hybrid enhanced effect of double QDs.

Observation of low energy acoustic phonon modes at low temperature from stressed Si nanoclusters

Photoluminescence spectroscopy has been used to study the emission characteristics of stressed Si nanoclusters formed in Si over a wide temperature range of 6 to 300 K. The results show splitting of single narrow peak to four sharp lines with lowering of temperature to 6 K. The four lines could be identified as a zero-phonon line (ZPL), one and higher optical phonon-assisted lower energy lines. The observed ZPL has a linewidth as sharp as ∼ 13 meV at ∼ 6 K. The low temperature measurements further reveal the interaction of ZPL with low energy acoustic phonons as low as 7 ± 1 meV, at low temperature, below 100 K. This has only been seen earlier in the case of single Si quantum dots.

Study of Single Silicon Quantum Dots’ Band Gap and Single-Electron Charging Energies by Room Temperature Scanning Tunneling Microscopy

Scanning tunneling spectroscopy in the shell-filling regime was carried out at room temperature to investigate the size dependence of the band gap and single-electron charging energy of single Si quantum dots (QDs). The results are compared with model calculation. A 12-fold multiple staircase structure was observed for a QD of about 4.3 nm diameter, reflecting the degeneracy of the first energy level, as expected from theoretical calculations. The systematic broadening of the tunneling spectroscopy peaks with decreasing dot diameter is attributed to the reduced barrier height for smaller dot sizes and to the splitting of the first energy level.

Study of the interdiffusion effect on the band structures of Si1−xGex∕Si quantum wells

We investigated the influence of thermal interdiffusion on the band structures of Si1−xGex∕Si single quantum wells (SQWs) with a well width of 125Å and Ge concentration x=0.3. Rapid thermal annealing with the temperatures of 750 and 800°C leads to substantial interdiffusion indicated by a significant photoluminescence (PL) blueshift as large as 46meV. The effect is modeled by a Si–Ge atomic interdiffusion at the heterointerface. The band structures and optical transitions of QW after interdiffusion were calculated based on an error function distribution and the 6+2-band k∙p method. The diffusion lengths of the intermixing process are deduced from the PL shift. The thermal dependence of the interdiffusion coefficients follows the Arrhenius law. An activation energy (Ea) for interdiffusion of 2.75eV is obtained. Our investigation indicates that the 6+2-band k∙p formalism is valid for interdiffused Si1−xGex∕Si QWs.

Triggered luminescence in a strained Si 1−xGe x/Si single quantum well with surface as an electron reservoir

It is demonstrated that strained Si 1− x Ge x /Si single quantum wells (SQWs) are capable of generating light pulses simply by controlling the population of only one type of carriers in the SQW through longitudinal electric fields. The unique band lineup of strained Si 1− x Ge x /Si QWs allows loose confinement of electrons, which makes the electrons easy to be detrapped upon application of electric field. At a weak positive surface bias, an electron reservoir is formed in the near-surface region while holes stay in the QW so that electrons and holes are spatially apart and thereby luminescence signal is diminished. Upon removal of the bias voltage, the electrons that have built up in the near-surface are driven back to the QW, turning on luminescence. Triggered 50 ns light pulse train was generated with alternating bias voltage waveform for a single-shot excitation of electron–hole pairs at the outset.

Two-dimensional electron-hole liquid in single Si quantum wells with large electronic and dielectric confinement.

We report a luminescence study of the electronic properties of the 2D electron-hole liquid in crystalline Si quantum wells with SiO2 dielectric barriers. The Fermi-Dirac condensation of e-h pairs into a metallic liquid is strongly enhanced by spatial localization. We present experimental evidence for the formation of liquid nanodroplets, with size increasing with e-h pair density. The quantum confined regime is observed for well width below 15 nm. The data are analyzed in a confinement model that takes account of the band-gap renormalization by 2D many-body effects and the increase of the Coulomb interactions due to the dielectric mismatch between the Si well and the SiO2 barriers.

Room-Temperature Visible Photoluminescence from Single Crystal Si Quantum Well Structures

Single-crystal Si single quantum well (SQW) structures sandwiched between two SiO2 layers were fabricated using oxygen implantation followed by thermal oxidation. After annealing in hydrogen, the SQW structures emitted two-peak (blue and yellow) photoluminescence (PL) at room temperature. The blue PL (∼2.9 eV) has an invariable peak position regardless of the Si layer thickness or the temperature. The yellow PL changes from 2.0 eV to 2.6 eV with thinning of the Si layer from 5.5 to 1.0 nm. We found that yellow PL disappears at temperatures below 160 K. It is concluded that the blue PL is related to defects in SiO2, and we inferred that the yellow PL is caused by a recombination of the localized electrons at the Si/SiO2 interface and quantum confined holes.

Size Tunable Visible Luminescence from Individual Organic Monolayer Stabilized Silicon Nanocrystal Quantum Dots

Quantum confinement in nanostructured silicon can lead to efficient light emission. However, the photoluminescence (PL) lifetimes in nanostructured silicon are typically very long−approximately 3 orders of magnitude longer than those of direct band gap semiconductors. Herein, we show that organic monolayer coated silicon nanocrystals ranging from 1 to 10 nm in diameter emit with nanosecond-scale lifetimes and high quantum yields, making it possible to measure the PL spectra of single Si quantum dots. The Si quantum dots demonstrate stochastic single-step “blinking” behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature.

Diffusion of single quantum well Si1−xGex/Si layers under vacancy supersaturation

The interdiffusion of a Si/Si0.85Ge0.15/Si single quantum well heterostructure subjected to thermal annealing in a nitriding ambient was investigated as a function of anneal temperature and time. Nitridation of the silicon surface alters equilibrium vacancy point defect populations throughout the structure, which allows the determination of the point defect species important in interdiffusion. Diffusion coefficients of Ge after nitridation of ∼1×10−14 cm2/s for 1100 °C and ∼1×10−13 cm2/s for 1200 °C were extracted. The extent of diffusion in a nitriding ambient was much less than in an inert ambient, which indicates minimal vacancy contribution to interdiffusion. This is in contrast to results from previously published studies performed in an oxidizing ambient.

Diffusion of Ge in Si1-xGex/Si single quantum wells in inert and oxidizing ambients

The interdiffusion of Si/Si0.85Ge0.15/Si single quantum well (SQW) structures subjected to inert- and oxidizing-ambient annealing was investigated as a function of temperature (900–1200 °C) and time. Point defect injection allowed modification of the vacancy and interstitial mediated components of interdiffusion, DV and DI. Diffusion profiles of samples processed in inert and oxidizing ambients were similar, which indicates a vacancy-dominated mechanism. Activation energies of diffusion in inert and oxidizing ambients were found to be 5.8 and 5.0 eV, respectively. A fractional interstitial component fI of ∼0.10 was estimated for the lower temperatures, while a significantly smaller fI of ∼0.02 was estimated for the higher temperatures. Experiments using SQWs with buried boron marker layers showed that dislocations in the Si1-xGex trap point defects and affect interdiffusion behavior.

Photoluminescence fromSi/SiO2single quantum wells by selective excitation

We have studied the photoluminescence (PL) mechanism of crystalline Si single quantum wells sandwiched between ${\mathrm{SiO}}_{2}$ layers by site-selective excitation spectroscopy. The asymmetric PL spectrum is observed in the red spectral region at low temperatures and can be divided into two PL bands. The peak energy of the strong PL band is almost independent of the well thickness and appears near $\ensuremath{\sim}1.65\mathrm{eV}.$ Under selective excitation at energies within the weak PL band, TO-phonon structures are observed in both PL and PL polarization spectra. Steplike TO-phonon structure in the PL spectrum of two-dimensional Si quantum wells is similar to that of zero-dimensional Si nanocrystals. The PL properties of very thin Si quantum wells are discussed.

Quantum confinement and interface effects on photoluminescence from silicon single quantum wells

We have studied photoluminescence (PL) spectrum and dynamics of Si single quantum wells sandwiched between SiO 2 layers. The asymmetric luminescence spectrum consists of two Gaussian bands in the visible and infrared spectral region. From the size-dependence of the PL spectrum and PL decay dynamics, it is concluded that the strong PL band is caused by the radiative recombination in the SiSiO 2 interface states, while the weak PL band in the two-dimensional Si quantum well.

Visible photoluminescence from silicon single quantum wells

We have studied luminescence properties of Si single quantum wells sandwiched between SiO 2 layers. The photoluminescence is ascribed to the radiative recombination both in the SiSiO 2 interface states and in quantum confinement states of the Si well.

Resonant tunneling through a self-assembled Si quantum dot

Nanometer-scale Si quantum dots have been spontaneously fabricated on SiO2 by controlling the early stages of low-pressure chemical vapor deposition from pure silane. The tunneling current through Au/1 nm-SiO2/a single Si quantum dot/1 nm-SiO2/n+-Si(100) double-barrier structures has exhibited the clear current bump or negative conductance at 300 K with a peak current to valley ratio as high as 10.

Analysis of capacitance-voltage characteristics of Si1-xGex/Si quantum-well structures.

The theoretical expressions of the capacitance-voltage (C-V) characteristics of a single quantum well are derived in different bias voltage regions based on solving analytically Poisson's equation. A method to determine the parameters, including the doping concentrations in the well and the barrier, the location of the well, and the thickness of the cap layer, as well as the band offset at the heterointerface from an experimental C-V curve is presented. By carefully constructing a testing sample with the structure of the Al/(thin nitride layer)/Si/${\mathrm{Si}}_{0.67}$${\mathrm{Ge}}_{0.33}$/Si single quantum well, the measurement of a complete C-V curve in a wide voltage range is achieved. The structural parameters of the quantum well derived from the measured C-V curve agree well with the nominal values set by the experimental growth conditions. \textcopyright{} 1996 The American Physical Society.

Efficient carrier blocking by an attractive potential in strained Si1−xGex/Si single quantum well

Efficient carrier blocking by a single ‘‘attractive’’ potential is demonstrated using a strained Si0.85Ge0.15/Si quantum well (QW). The carrier blocking becomes pronounced at increased temperatures as most of the carriers at the QW bound state are thermalized with the barrier band edge. The carrier blocking efficiency at 100 K is almost of the order of unity as compared to insufficient carrier blocking due to a large ‘‘repulsive’’ potential associated with SiO2. The carrier blocking arises from efficient hole reemission from the QW that reflects a highly efficient carrier capture to the strained Si1−xGex/Si QWs from Si barriers. It is further shown that a single QW is the optimized geometry for efficient carrier blocking and that multiple well potentials lead only to a reduced blocking efficiency.

Interface-related defects in a Ge0.4Si0.6/Si single well

Deep-level transient spectroscopy measurements were performed on a Ge0.4Si0.6/Si single quantum well grown by molecular beam epitaxy. The results show hole traps labelled H1 and H2. H1 is a deep level in Si whose activation energy measured from the valence band and capture cross section are 0.3 eV and 3*10-22 cm2 respectively. When an electric field was applied to the quantum well, an anomalous behaviour was observed for H2 only. The activation energy of H2 increased and then saturated as the applied electric field increased. Such behaviour can be explained by identifying H2 as an emission from interface states to a bound state in a quantum well. The depth profile of H2 also showed a spatially confined behaviour which strongly supports the idea that H2 is an interface state. Calculations using a variational method were performed to determine the bound state energy levels of the quantum well in a constant electric field. Comparing the results of experiments with those of the calculations, H2 corresponds to an interface state whose energy level lies at 0.45 eV above the valence band edge of the well; the capture cross section of H2 is 2.1*10-18 cm2.

A Si 1− xGe x/Si single quantum well p-i-n structure grown by solid-source and gas source “hybrid” Si molecular beam epitaxy

By means of “hybrid” Si molecular beam epitaxy (MBE), an n-type Si contact layer for an electroluminescent (EL) p-i-n diode was successfully regrown on a Si 1− x Ge x /Si single quantum well (SQW) layer. The starting undoped SQW layer was grown by gas-source MBE (GSMBE) using disilane (Si 2H 6) and germane (GeH 4), and the n-Si contact layer was regrown by using solid-source MBE after transferring the sample through the air. A (2×1) reconstruction was observed on a GSMBE-prepared Si surface even after the sample was exposed to air for 15 h. Evidence of the excellent quality of the EL p-i-n device was provided by the sharpest emission lines, ≈ 5.5 meV, ever reported in the EL spectra of an SiGe system.

Luminescence study on interdiffusion in strained Si1−xGex/Si single quantum wells grown by molecular beam epitaxy

Optical investigation of interdiffusion at Si1−xGex/Si heterointerfaces has been performed for the first time in strained Si1−xGex/Si single quantum wells (SQWs). Photoluminescence (PL) peak energy blue shift of up to 22 meV due to interdiffusion-induced potential profile modulation was observed after annealing in vacuum. The diffusion coefficients obtained were found to closely follow an Arrhenius behavior with an activation energy of 2.47±0.4 eV. Dramatic increase in the integrated PL intensity was observed in the annealed samples, as a result of the elimination of effective nonradiative centers. Strain relaxation was hardly observed even after 900 °C annealing, indicating the unprecedented structural stability of SQWs in contrast to rather vulnerable thick alloy layers. Anomalous peak red shift, probably due to surface oxidation, was observed by annealing in N2 ambient.

Abrupt compositional transition in luminescent Si1−xGex/Si quantum well structures fabricated by segregant assisted growth using Sb adlayer

Luminescent Si0.8Ge0.2/Si single quantum well structures with abrupt interfaces were successfully grown by segregant assisted growth (SAG) using Sb adlayer. The emission energy shifted to the lower side compared to the sample grown without Sb adlayer due to the suppression of the surface segregation of Ge atoms. Improved abruptness of interfaces was confirmed by sputter depth profile measurement, where no segregation edge was seen in the sample grown by SAG technique. Light emitting p-i-n diode structure can be fabricated by utilizing the incorporation of Sb during SAG.