Energy-dependent tunneling from few-electron dynamic quantum dots

We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale ( approximately a few meV) is comparable to those in static QDs as expected.     

InAs自组装量子点GaAs肖特基二极管中的电流输运特性

设计了含有InAs自组装量子点(SAQDs)的新型金属半导体金属隧穿结构,研究了其直流输运特性,观察到了电流迟滞回路现象.这种回路现象是由于紧邻金属肖特基接触的量子点充电和放电引起的,也可以说是由外加电压控制的量子点的单电子过程引起的.分析了量子点总体的充放电特性,量子点中电子在高电场下隧穿出量子点的概率变化决定了量子点的放电过程,而充电过程是由流过量子点层的二极管正向电流决定.理论拟合结果显示充电过程主要由于量子点基态能级俘获电子照成的,激发态对量子点充放电过程只有微弱影响. 关键词： 迟滞现象 自组装量子点 单电子过程     

Optical imaging of electrical carrier injection into individual InAs quantum dots

We image the micro-electroluminescence (EL) spectra of self-assembled InAs quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and demonstrate optical detection of carrier injection into a single QD. Tunneling of electrons and holes into the QDs at bias voltages below the flat-band condition leads to a spectrum of sharp EL lines from a small number of bright spots on the diode surface, characteristic of emission from individual QDs. We explain this behavior in terms of Coulomb interaction effects and the selective excitation of a small number of QDs within the ensemble due to preferential tunneling paths for carriers.     

In-plane and perpendicular tunneling through InAs quantum dots

A Schottky diode with InAs dots in the intrinsic GaAs region was used to investigate perpendicular tunneling (in growth direction) through InAs quantum dots (QDs). At forward bias conditions electrons tunnel from the ohmic back contact into the metal Schottky gate. Peaks appear in the differential conductance when a QD level comes into resonance with the Fermi-level of the n-doped region. The observed tunneling features are attributed to electron transport through the s- and p-shell of the InAs islands. In our in-plane tunneling experiments the islands were embedded in the channel region of an n-doped GaAs/AlGaAs HEMT-structure. In order to study tunneling through single InAs islands, a quantum point contact was defined by lithography with an atomic force microscope and subsequent wet-chemical etching. In contrast to unpatterned devices sharp peaks appear in the I–V characteristic of our samples reflecting the transport of electrons through the p-shell of a single InAs QD.     

Electric and photoelectric properties of GaAs/ZnSe-Ge/ZnSe/Al structures with Ge quantum dots

Current-voltage characteristics and spectral dependences of photovoltage are investigated at T=4.2 and 300 K in stress-free structures with germanium quantum dots (QDs) in the GaAs/ZnSe/QD-Ge/ZnSe/Al system. The “Coulomb staircase” type features in the current-voltage characteristic observed at room temperature without illumination are due to the Coulomb interaction of electrons in resonant tunneling through intrinsic levels in QDs. The features in the photovoltage spectra are related to the absorption of radiation in the system of discrete levels of QDs. An energy band diagram of the structure is constructed based on the experimental data.     

Manipulating Ge quantum dots on ultrathin SixGe1−x oxide films using scanning tunneling microscope tips

Germanium quantum dots (QDs) were extracted from ultrathin Si_xGe_1−x oxide films using scanning tunneling microscope (STM) tips. The extraction was most efficiently performed at a positive sample bias voltage of +5.0 V. The tunneling current dependence of the extraction efficiency was explained by the electric field evaporation transfer mechanism for positive Ge ions from QDs to STM tips. Ge QDs (∼7 nm) were formed and isolated spatially by extracting the surrounding Ge QDs with an ultrahigh density of >10¹² cm⁻². Scanning tunneling spectroscopy of the spatially-isolated QDs revealed that QDs with an ultrahigh density are electrically-isolated from the adjacent dots.     

Disorder and interaction-induced pairing in the addition spectra of quantum dots

We have numerically investigated the electron addition spectra in quantum dots containing a small number (N相似文献   

Nonstationary effects in the system of coupled quantum dots influenced by Coulomb correlations

We investigate the time evolution of filling numbers of localized electrons in the system of two coupled single-level quantum dots (QDs) connected with the continuous-spectrum states in the presence of Coulomb interaction. We consider correlation functions of all orders for electrons in the QDs by decoupling higher-order correlations between localized and band electrons in the reservoir. We analyze different initial charge configurations and consider Coulomb correlations between localized electrons both within the dots and between the different dots. We reveal the presence of a dynamical charge trapping effect in the first QD in the situation where both dots are occupied at the initial instant. We also find an analytic solution for the time-dependent filling numbers of the localized electrons for a particular configuration of the dots.     

Photoluminescence of InAs Self-Organized Quantum Dots Formation on InP Substrate by MOCVD

In this letter, we present results of photoluminescence (PL) emission from single-layer and multilayer InAs self-organized quantum dots (QDs), which were grown on (001) InP substrate. The room temperature PL peak of the single-layer QDs locates at 1608 nm, and full width at half-maximum (FWHM) of the PL peak is 71 meV. The PL peak of the multilayer QDs locates at 1478 nm, PL intensity of which is stronger than that of single-layer QDs. The single-layer QD PL spectra also display excited state emission and state filling as the excitation intensity is increased. Low temperature PL spectra show a weak peak between the peaks of QDs and wetting layer (WL), which suggests the recombination between electrons in the WL and holes in the dots.     

Multi-color tunneling quantum dot infrared photodetectors operating at room temperature

Quantum dot structures designed for multi-color infrared detection and high temperature (or room temperature) operation are demonstrated. A novel approach, tunneling quantum dot (T-QD), was successfully demonstrated with a detector that can be operated at room temperature due to the reduction of the dark current by blocking barriers incorporated into the structure. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunneling, while the dark current is blocked by AlGaAs/InGaAs tunneling barriers placed in the structure. A two-color tunneling-quantum dot infrared photodetector (T-QDIP) with photoresponse peaks at 6 μm and 17 μm operating at room temperature will be discussed. Furthermore, the idea can be used to develop terahertz T-QD detectors operating at high temperatures. Successful results obtained for a T-QDIP designed for THz operations are presented. Another approach, bi-layer quantum dot, uses two layers of InAs quantum dots (QDs) with different sizes separated by a thin GaAs layer. The detector response was observed at three distinct wavelengths in short-, mid-, and far-infrared regions (5.6, 8.0, and 23.0 μm). Based on theoretical calculations, photoluminescence and infrared spectral measurements, the 5.6 and 23.0 μm peaks are connected to the states in smaller QDs in the structure. The narrow peaks emphasize the uniform size distribution of QDs grown by molecular beam epitaxy. These detectors can be employed in numerous applications such as environmental monitoring, spectroscopy, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Electrodeposition of CdS quantum dots and their optoelectronic characterization by photoelectrochemical and scanning probe spectroscopies

CdS quantum dots (QDs) have been electrodeposited onto textured gold substrates from a nonaqueous electrolyte containing Cd(ClO₄)₂and elemental S. The initial deposit consisted of very small (about 3 nm) nanocrystals of CdS which were partially oriented with the Au substrate. With increasing deposit thickness, the crystal size increased and the degree of orientation decreased. Photocurrent spectroscopy and I–V spectroscopy, using a conducting scanning force microscope tip, were used to measure the CdS bandgap variations due to size quantization. The latter method also revealed room temperature conductivity peaks assigned to Coulomb charging of the QDs and evidence for charge tunneling into higher discrete energy levels.     

Energy Spectra of Three-Electron Quantum Dots

By making use of the method of few-body physics, the energy spectra of three electrons confined by two-dimensional and three-dimensional quantum dots (QDs) are investigated. Using the present results, the size and shape effects of QDs on the spectra are revealed.     

Prediction of ferromagnetic correlations in coupled double-level quantum dots

Numerical results for transport properties of two coupled double-level quantum dots (QDs) strongly suggest that under appropriate conditions the dots develop a novel ferromagnetic (FM) correlation at quarter filling (one electron per dot). In the strong coupling regime (Coulomb repulsion larger than electron hopping) and with interdot tunneling larger than tunneling to the leads, an S=1 Kondo resonance develops in the density of states, leading to a peak in the conductance. A qualitative "phase diagram," incorporating the new FM phase, is presented. In addition, the necessary conditions for the FM regime are less restrictive than naively believed, leading to its possible experimental observation in real QDs.     

Electron transport and shell structures of single InAs quantum dots probed by nanogap electrodes

We have investigated electron transport and electron filling in single InAs quantum dots (QDs) using nanogap electrodes. Elliptic InAs QDs with diameter of 60/80 nm exhibited clear shell filling up to 12 electrons. Shell-dependent charging energies and level quantization energies for the s, p, and d states were determined from the addition energy spectra. Furthermore, it is found that the charging energies and the tunneling conductances strongly depend on the shell, reflecting that the electron wave functions for higher shells are more extended in space.     

Spin relaxation mechanism of hopping transport in a 2D asymmetric quantum dot array

Spin relaxation is studied in the hopping conduction mode in 2D arrays of quantum dots (QDs) with structural asymmetry. It is shown that the absence of the “up-down” symmetry in a QD leads to the emergence of a new spin relaxation mechanism in tunneling in a 2D QD array. The difference in spin relaxation mechanisms for symmetric and asymmetric QDs is demonstrated on the basis of theoretical analysis of an elementary event (jump between two tunnel-coupled dots). It is shown that spin flip during tunneling between QDs is the main spin relaxation mechanism in the transport in dense arrays of QDs in Ge placed in weak (1–10 T) magnetic fields.     

Continuum limit of single-electron tunneling

We consider the continuum limit of the standard model for treating single-electron tunneling (SET) of electrons through a one-dimensional array of tunnel junctions. We show that the formalism reduces to the computation of the motion of overdamped particles undergoing potential gradient flow, with the potential being given by the full interacting free energy of the electrons in the system. We show that the tunneling coefficients in the SET model can be re-interpreted in terms of a diffusion coefficient and a temperature and that therefore the SET problem reduces to a fully self-consistent treatment of overdamped particle diffusion.     

Combined STM/AFM visualization of the local density of states in the InAs/GaAs quantum dots

Combined ultra-high vacuum scanning tunneling/atomic-force microscopy (STM/AFM) has been implemented for the first time for the tunneling spectroscopy of the size-quantized states in the InAs/GaAs(001) surface quantum dots (QDs). The tunneling spectra and current images, which reflect the energy and spatial distribution of the local density of the ground and excited states in the QDs have been obtained.     

Few-particle effects in semiconductor quantum dots: observation of multicharged excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.     

Quantum dot p-i-n structure in an electric field

Time-resolved photoluminescence (PL), steady-state PL, and electroluminescence (EL) techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in self-assembled InAs/GaAs quantum dot (SAQD) p-i-n structures under reverse bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the n  =  1 SAQD decreases with increasing reverse bias from ∼3 ns under flat band condition to∼ 400 ps for a bias of −3 V. The data can be explained by a simple model based on electron recombination in the quantum dots (QDs) or escape out of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of excited electronic levels, thermally assisted tunneling of ground state electrons through the upper excited electronic states or thermionic emission to the wetting layer.