内嵌InAs量子点的场效应晶体管特性研究

研究了内嵌InAs量子点的异质结场效应晶体管在室温和低温下的电学特性,获得了量子点影响下器件的输出特性曲线。在室温下,通过分别测试在近红外光照和量子点充电条件下器件的Ⅰ-Ⅴ特性,证明了量子点通过类似纳米悬浮栅的作用,对邻近沟道的二维电子气施加影响。在低温下观察到器件漏电流出现负微分电导现象。这一现象可由2DEG和量子点之间的共振隧穿来解释。这些结果提供了一种新的操作传统场效应晶体管的方法,并有望制成新型量子点存储器。     

Deep level transient spectroscopy on charge traps in high-k ZrO2

The deep trap properties of high-dielectric-constant (k) ZrO₂ thin films were examined by deep level transient spectroscopy (DLTS). The hole traps of a ZrO₂ dielectric deposited by sputtering were investigated in a MOS structure over the temperature range, 375 K-525 K. The potential depth, cross section and concentration of hole traps were estimated to be ∼ 2.5 eV, ∼ 1.8 × 10^− 16 cm² and ∼ 1.0 × 10¹⁶ cm^− 3, respectively. DLTS of ZrO₂ dielectrics can be used to examine the threshold voltage shift (?V_th) during the operation of SONOS-type flash memory devices, which employ high-k materials.     

Type-II CdSe/CdTe/ZnTe (core-shell-shell) quantum dots with cascade band edges: the separation of electron (at CdSe) and hole (at ZnTe) by the CdTe layer

The rational design and synthesis of CdSe/CdTe/ZnTe (core-shell-shell) type-II quantum dots are reported. Their photophysical properties are investigated via the interband CdSe-->ZnTe emission and its associated relaxation dynamics. In comparison to the strong CdSe (core only) emission (lambda(max) approximately 550 nm, Phi(f) approximately 0.28), a moderate CdSe-->CdTe emission (lambda(max) approximately 1026 nm, Phi(f) approximately 1.2 x 10(-3)) and rather weak CdSe-->ZnTe interband emission (lambda(max) approximately 1415 nm, Phi(f) approximately 1.1 x 10(-5)) are resolved for the CdSe/CdTe/ZnTe structure (3.4/1.8/1.3 nm). Capping CdSe/CdTe with ZnTe results in a distant electron-hole separation between CdSe (electron) and ZnTe (hole) via an intermediate CdTe layer. In the case of the CdSe/CdTe/ZnTe structure, a lifetime as long as 150 ns is observed for the CdSe-->ZnTe (1415 nm) emission. This result further indicates an enormously long radiative lifetime of approximately 10 ms. Upon excitation of the CdSe/CdTe/ZnTe structure, the long-lived charge separation may further serve as an excellent hole carrier for catalyzing the redox oxidation reaction.     

Activation energy and carrier dynamics of CdTe/ZnTe quantum dots on GaAs and Si substrates

We investigate the activation energy and carrier dynamics of CdTe/ZnTe quantum dots (QDs) grown on GaAs and Si substrates. The activation energy of the electrons confined in QDs on the Si substrate, as obtained from the temperature-dependent photoluminescence (PL) spectra, is lower than that of electrons confined in QDs on the GaAs substrate. Time-resolved PL measurements used to study the carrier dynamics show shorter exciton lifetimes for QDs on the Si substrate. This behavior is attributed to the fact that defects and dislocations in the QDs on the Si substrate provide nonradiative channels.     

Molecular ground hole state of vertically coupled GeSi/Si self-assembled quantum dots

We study the ground state of a hole confined in two vertically coupled GeSi/Si quantum dots as a function of the interdot distance and dot composition within the sp(3) tight-binding approach. Both quantum-mechanical tunneling and inhomogeneous strain distribution are included. For pure Ge dots, the strain is found to have two effects on the hole binding energy: (i)?reduction of the binding energy below the value of the single dot with increasing dot separation and (ii)?molecular bond breaking for intermediate interdot distances and posterior bond restoration at larger distance. Both effects are smeared upon Ge-Si intermixing.     

Deep level transient spectroscopy of anisotropic semiconductor GaTe

Deep level transient spectroscopy (DLTS) was carried out on single crystals of the layered chalcogenide p-GaTe using Schottky barriers parallel and perpendicular to the layer planes to study the possible anisotropy of the defect levels. Deep levels with the same energies (0·28 eV and 0·42–0·45eV) have been found in both directions with concentrations ranging from 10¹³cm⁻³ to 10¹⁴ cm⁻³ and capture cross-sections from 10⁻¹⁵cm² to 10⁻¹⁷cm². The difference in the spectra obtained from the two planes and the possible reason for the deep level energies being independent of crystal orientation are discussed.     

Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells.     

Luminescence and resonant energy transfer of two sizes of CdTe quantum dots embedded in gelatin films

We have used two sizes of highly monodisperse colloidal CdTe nanocrystals (or quantum dots) and gelatin to form quantum dots (QDs)-gelatin films. Photoluminescence (PL) results for the mixed systems show that the quenching of the emission of the small dots was accompanied by the enhancement of the emission of the large dots as their average separation decrease. The phenomena were attributed to the occurrence of the long-range resonance transfer (LRRT) between two different sizes of QDs when the concentration of the gelatin is low enough to ensure the close proximity of the dots. Determined by the overlap between the emission spectrum of the donor QDs (small dots) and the absorption spectrum of the acceptor QDs (large dots), FRET provides efficient coupling between two different sizes of QDs via long-range dipole-dipole interaction. The PL spectrum changes were also confirmed by true-color images of the mixed QDs system, which show that the colors change gradually from red to green at the gelatin concentrations varying from 0.01% to 0.25 wt% in their initial solution. The experimental results are expected to open new opportunities to modulate the luminescence of semiconductor QDs in a simple way.     

Synthesis and characterization of CdTe quantum dots embedded gelatin nanoparticles via a two-step desolvation method

Novel CdTe quantum dots (QDs) embedded gelatin nanoparticles (CdTe/gelatin nanoparticles) were synthesized via a two-step desolvation method. The morphology and size distribution of the nanoparticles were characterized by transmission electron microscope (TEM) and laser particle size analyzer. They are presented spherically and relatively uniform with a diameter of 150 nm. The luminescent properties of the nanoparticles were investigated by using fluorescence spectrophotometry and fluorescence microscopy. The fluorescence stability of nanoparticles was tested in vitro. It was found that the nanoparticles were stable in water and phosphate-buffered saline (PBS) solution (pH 7.4) for at least 15 days. A possible formation mechanism of the CdTe/gelatin nanoparticles was also proposed. The inherent stability and biocompatibility indicate that the nanoparticles are expected to be promising candidates for in vivo biological imaging studies.     

Temperature dependence of excitonic radiative decay in CdSe quantum dots: the role of surface hole traps

Using atomistic, semiempirical pseudopotential calculations, we show that if one assumes the simplest form of a surface state in a CdSe nanocrystal--an unpassivated surface anion site--one can explain theoretically several puzzling aspects regarding the observed temperature dependence of the radiative decay of excitons. In particular, our calculations show that the presence of surface states leads to a mixing of the dark and bright exciton states, resulting in a decrease of 3 orders of magnitude of the dark-exciton radiative lifetime. This result explains the persistence of the zero-phonon emission line at low temperature, for which thermal population of higher-energy bright-exciton states is negligible. Thus, we suggest that surface states are the controlling factor of dark-exciton radiative recombination in currently synthesized colloidal CdSe nanocrystals.     

Experiments and modeling of alloying in self-assembled quantum dots

We review the recent advances in the experimental and theoretical investigation of alloy distribution in semiconductor quantum dots (QDs). X-ray diffraction analysis, as well as wet chemical etching, represent two powerful techniques that are able to measure the alloy distribution inside the dots. From a theoretical point of view, determination of the alloy distribution follows from consideration of the thermodynamic quantities involved in the formation and stability of the QD: strain energy, surface energy, internal energy and entropy. Starting from the alloy distribution, the investigation of its role in influencing the electronic and optical properties of QDs is possible. Tight binding and ab initio calculation show the band structure of non-uniform alloyed Ge/Si and InAs/GaAs quantum dots. While for Ge/Si the indirect bandgap does not offer a strong photoluminescence spectra, direct-bandgap materials offer intense light emission, including the range for telecom applications (1.77–1.37 μm). Control of alloying inside the QDs allows for the tailoring of their band structure and photoluminescence spectra, where high alloy gradients induce a blue-shift of the spectra, compared to a more uniform composition.     

Crystalline Ge quantum dots embedded in SiO2 matrix synthesized by plasma immersion ion implantation

A new plasma process, i.e. a combination of plasma immersion ion implantation and deposition (PIII&D) and high power impulse magnetron sputtering (HiPIMS), was developed to implant non-gaseous ions into material surfaces. The new process has the great advantage that thin film deposition and non-gaseous ion implantation can be achieved in a single plasma chamber. In this study, Ge ions were successfully implanted into SiO(2) thin film, which resulted in uniformly and homogeneously distributed crystalline Ge quantum dots (Ge-QDs) embedded in a SiO(2) matrix even without a further annealing process. Broader areas of application of PIII&D technology are envisaged with this newly developed process.     

Electro-elastic tuning of single particles in individual self-assembled quantum dots

We investigate the effect of uniaxial stress on InGaAs quantum dots in a charge tunable device. Using Coulomb blockade and photoluminescence, we observe that significant tuning of single particle energies (≈-0.22 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.9 μeV/MPa) under tensile stress. Modest tuning of the permanent dipole, Coulomb interaction and fine-structure splitting energies is also measured. We exploit the variable exciton response to tune multiple quantum dots on the same chip into resonance.     

Deep levels in GaN studied by deep level transient spectroscopy and Laplace transform deep-level spectroscopy

In this study we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77–350 K. Si-doped GaN layers were grown by Molecular Beam Epitaxy technique (MBE) on sapphire substrates. DLTS signal spectra revealed the presence of four majority traps: two hightemperature and two low-temperature peaks. Using LDLTS method and Arrhenius plots the activation energy and capture cross sections were obtained. For two high-temperature majority traps they are equal to E1 = 0.65 eV, σ₁ = 8.2 × 10^?16cm² and E2 = 0.58 eV, σ₂ = 2.6 × 10^?15 cm² whereas for the two low-temperature majority traps E3 = 0.18 eV, σ₃ = 9.72 × 10^?18 cm² and E4 = 0.13 eV, σ₄ = 9.17 × 10^?18 cm². It was also found that the traps are related to point defects. Possible origin of the traps was discussed and the results were compared with the data found elsewhere [1–5].     

Raman characterization of CdTe nanocrystallites embedded in a glass matrix

Abstracts are not published in this journal     

Admittance spectroscopy of Ge quantum dots in Si

Self-assembled Ge quantum dots (QDs) embedded in Si-Schottky diodes were studied using admittance spectroscopy. A sample with one layer of Ge QDs embedded in p-Si was grown on a p⁺-substrate by molecular beam epitaxy and was processed into a Schottky diode structure. Activation energies have been determined as a function of the external bias voltage, which shifts the Fermi level in the sample. Two discrete activation energies of 318 and 303 meV have been extracted in the region of high bias voltage. They are identified as single and double charged QD ground state which is split up by a Coulomb charging energy. At approximately 40 meV lower activation energies several discrete levels attributed to excited dot states with a smaller energy separation of typical 5 meV are observed.     

Piezoelectric coupling effect on exciton–phonon scattering rates in CdSe quantum dots embedded in glass matrix

The scattering of excitons by acoustic phonons in nanostructures such as quantum dots generally controls relaxation process to the lowest energy states, and is a basis for understanding optical properties and coherence effects in these systems. In our work we have studied theoretically the scattering of excitons via acoustic phonons in a CdSe disk-shaped semiconductor quantum dot under an applied magnetic field. The scattering rate (SR) is calculated considering the exciton–phonon scattering by the piezoelectric coupling potential mechanism. We discuss the influence of the external applied magnetic field, and the quantum dot size on the SR. Our calculations show that the exciton–acoustic phonon scattering rate depends significantly on these parameters.