Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing

With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34?eV and a weak emission at 0.15?eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.     

X-Ray Spectrometry of Copper: New Results on an Old Subject

We review recent, and some less recent, measurements of several emission spectra of copper. The results are discussed with special emphasis on elucidating the structure of the Kα_1,2 and Kβ_1,3 diagram lines and their underlying transitions. These lines are found to contain ≈30 % contribution from 3d spectator hole transitions. Other multielectronic transitions, the 2p spectator hole (satellites) and 1s spectator hole (hypersatellites) transitions were also measured. They are discussed paying special attention to the evolution of the lineshapes and intensities from the excitation threshold to saturation. Trends in the measured quantities depending on the spectator hole’s shell and subshell are also discussed.     

Stark effect in a multilayer system of coupled InAs/GaAs quantum dots

Electron emission in a system of vertically coupled quantum dots (VCQDs) in InAs/GaAs p-n-heterostructures obtained by molecular beam epitaxy has been studied by means of deep-level transient spectroscopy (DLTS) as a function of the number of quantum dot (QD) rows and the reverse bias voltage. For a GaAs spacer thickness of d _GaAs = 40 Å, the system occurs in a molecular state, irrespective of the number of QD rows. An increase in this number leads to a decrease in the Stark shift, which is probably related to a decrease in the lattice strain potential in the vicinity of VCQDs.     

Growth of InAs Quantum Dots on GaAs (511)A Substrates: The Competition between Thermal Dynamics and Kinetics

The growth process of InAs quantum dots grown on GaAs (511)A substrates has been studied by atomic force microscopy. According to the atomic force microscopy studies for quantum dots grown with varying InAs coverage, a noncoherent nucleation of quantum dots is observed. Moreover, due to the long migration length of In atoms, the Ostwald ripening process is aggravated, resulting in the bad uniformity of InAs quantum dots on GaAs (511)A. In order to improve the uniformity of nucleation, the growth rate is increased. By studying the effects of increased growth rates on the growth of InAs quantum dots, it is found that the uniformity of InAs quantum dots is greatly improved as the growth rates increase to 0.14 ML s^?1. However, as the growth rates increase further, the uniformity of InAs quantum dots becomes dual‐mode, which can be attributed to the competition between Ostwald ripening and strain relaxation processes. The results in this work provide insights regarding the competition between thermal dynamical barriers and the growth kinetics in the growth of InAs quantum dots, and give guidance to improve the size uniformity of InAs quantum dots on (N11)A substrates.     

Effect of high energy proton irradiation on InAs/GaAs quantum dots: Enhancement of photoluminescence efficiency (up to ∼7 times) with minimum spectral signature shift

We demonstrate 7-fold increase of photoluminescence efficiency in GaAs/(InAs/GaAs) quantum dot hetero-structure, employing high energy proton irradiation, without any post-annealing treatment. Protons of energy 3–5 MeV with fluence in the range (1.2–7.04) × 10¹² ions/cm² were used for irradiation. X-ray diffraction analysis revealed crystalline quality of the GaAs cap layer improves on proton irradiation. Photoluminescence study conducted at low temperature and low laser excitation density proved the presence of non-radiative recombination centers in the system which gets eliminated on proton irradiation. Shift in photoluminescence emission towards higher wavelength upon irradiation substantiated the reduction in strain field existed between GaAs cap layer and InAs/GaAs quantum dots. The enhancement in PL efficiency is thus attributed to the annihilation of defects/non-radiative recombination centers present in GaAs cap layer as well as in InAs/GaAs quantum dots induced by proton irradiation.     

Metamorphic self-assembled quantum dot nanostructures

The light-emission energy E of self-assembled semiconductor quantum dots (QDs) is determined by the complex interplay of parameters such as compositions of QDs and confining layers (CLs), strain of QDs (imposed by the QD mismatch to CLs) and sizes and shapes of QDs. In order to have RT emission in the 1.55 μm photonic window from InAs QDs, the QD–CL lattice mismatch should be in the 4–5% range, values much lower than that of pseudomorphic InAs on GaAs (7%). We show that by: i) growing InAs QDs on virtual substrates consisting of metamorphic InGaAs buffers on GaAs and ii) using the thickness-dependent partial relaxation of buffers (acting also as lower CLs, LCLs) and suitable InGaAs compositions, the QD–CL mismatch can be tuned in the 5–7% range. Our experimental results on MBE-grown metamorphic InAs/In_xGa_1−xAs QD structures show that for x and LCL thicknesses d in the 0.09–0.35 and 20 nm–1000 nm ranges, respectively, the band-gap of the QD material and the band-discontinuities that confine carriers are such that the RT emission wavelengths range from 1.3 μm up to values that may exceed 1.55 μm. By using x and d as two degrees-of-freedom, not only that E can be selected but also the barrier energy for confined carriers' thermal escape can be maximised, in order to achieve efficient emission at RT.     

Effect of post-growth rapid thermal annealing on bilayer InAs/GaAs quantum dot heterostructure grown with very thin spacer thickness

We have investigated the effect of post-growth rapid thermal annealing on self-assembled InAs/GaAs bilayer quantum dot samples having very thin barrier thickness (7.5-8.5 nm). In/Ga interdiffusion in the samples due to annealing is presumed to be controlled by the vertical strain coupling from the seed dots in bilayer heterostructure. Strain coupling from embedded seed QD layer maintains a strain relaxed state in active top islands of the bilayer quantum dot sample grown with comparatively thick spacer layer (8.5 nm). This results in minimum In/Ga interdiffusion. However controlled interdiffusion across the interface between dots and GaAs barrier, noticeably enhances the emission efficiency in such bilayer quantum dot heterostructure on annealing up to 700 °C.     

Two-fold emission from the S-shell of PbSe/CdSe core/shell quantum dots

The optical properties of PbSe/CdSe core/shell quantum dots with core sizes smaller than 4 nm in the 5-300 K range are reported. The photoluminescence spectra show two peaks, which become increasingly separated in energy as the core diameter is reduced below 4 nm. It is shown that these peaks are due to intrinsic exciton transitions in each quantum dot, rather than emission from different quantum dot sub-ensembles. Most likely, the energy separation between the peaks is due to inter-valley coupling between the L-points of PbSe. The temperature dependence of the relative intensities of the peaks implies that the two emitting states are not in thermal equilibrium and that dark exciton states must play an important role.     

A Tunneling Dielectric Layer Free Floating Gate Nonvolatile Memory Employing Type‐I Core–Shell Quantum Dots as Discrete Charge‐Trapping/Tunneling Centers

A nonvolatile memory with a floating gate structure is fabricated using ZnSe@ZnS core–shell quantum dots as discrete charge‐trapping/tunneling centers. Systematical investigation reveals that the spontaneous recovery of the trapped charges in the ZnSe core can be effectively avoided by the type‐I energy band structure of the quantum dots. The surface oleic acid ligand surrounding the quantum dots can also play a role of energy barrier to prevent unintentional charge recovery. The device based on the quantum dots demonstrates a large memory window, stable retention, and good endurance. What is more, integrating charge‐trapping and tunneling components into one quantum dot, which is solution synthesizable and processible, can largely simplify the processing of the floating gate nonvolatile memory. This research reveals the promising application potential of type‐I core–shell nanoparticles as the discrete charge‐trapping/tunneling centers in nonvolatile memory in terms of performance, cost, and flexibility.     

Time-resolved detection of single-electron interference

We demonstrate real-time detection of self-interfering electrons in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage, the quantum point contact exerts a back-action on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives noncoherent electronic transitions in the quantum dots.     

Structural and optical properties of GaAs quantum dots formed in SiO2 matrix

A shallow sequential ion implantation of As and Ga ions into pure silica substrates was conducted to form GaAs quantum dots near the substrate surface. The main efforts were made to understand how the post thermal annealing affects the formation, thermal stability, and chemical composition of nanoparticles in a silica host. When the sample was annealed at 600 °C under 96% Ar + 4% H₂, X-ray diffraction (XRD), infrared reflectance (IR reflectance), and tunneling electron microscopy (TEM) confirmed the formation of GaAs quantum dots near the silica surface. X-ray photoemission spectroscopy (XPS) showed 3d electron binding energies of Ga and As at 18.8 eV and 40.75 eV suggesting GaAs nanocrystal formation. An additional band was also observed at ∼ 20.12 eV which was attributed to the presence of GaO_x. At 1000 °C, however, an additional peak is also observed near 44.1 eV indicating As₂O₃ formation. It was argued that the formation of GaO_x at 600 °C and As₂O₃ at 1000 °C was primarily due to the volatile nature of the Ga and its related compounds. High resolution Rutherford back scattering (RBS) using ¹⁴N³⁺ ions showed that 1000 °C annealing resulted in 50% loss of Ga and a 35% position shift toward the surface while As concentration is unchanged with a 25% position shift toward the surface.     

Tuning optical properties of high In content InGaAs/GaAs capped InAs quantum dots by post growth rapid thermal annealing

The effect of rapid thermal annealing on InAs quantum dots (QDs) capped with In_0.4Ga_0.6As/GaAs layer has been investigated by photoluminescence (PL). An unusual red shift of the PL emission peak has been observed for an annealing temperature (T_a) of 650 °C together with a pronounced improvement of the PL from the quantum well like heterocapping layer (QW). This behavior is attributed to the strain induced phase separation of the hetero-capping alloy. However, for T_a = 750 °C, a blue shift of the QDs PL peak has been observed with respect to that of the as-grown sample. For this annealing temperature the PL intensity of the QW exceeds that of the QDs indicating a relatively prominent In/Ga interdiffusion. When annealed at 850 °C, only the PL arising from the QW can be detected in addition to a broadened low energy side band indicating the dissolution of the QDs at that temperature.     

Production of nanometer-size GaAs nanocristals by nanosecond laser ablation in liquid

This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm2 in our experiments) and is as low as 2.5 nm for the highest power used. The absolute value of the average diameter also decreases significantly, from 13.7 to 8.7 nm, as the laser power increases in this interval. Due to the narrow nanoparticle size dispersion achieved at the highest laser powers two absorption band edges are clearly distinguishable at about 1.72 and 3.15 eV which are ascribed to E0 and E1 effective optical transitions, respectively. A comparison of the energies with those known for bulk GaAs allows one to conclude that an average diameter of the investigated GaAs nanoparticles is close to 10 nm, i.e., they are quantum dots. High resolution transmission electron microscopy (HRTEM) images show that the bare GaAs nanoparticles are nanocrystalline, but many of them exhibit single/multiple twin boundary defects or even polycrystallinity. The formation of the GaAs crystalline core capped with a SiO2 shell was demonstrated by HRTEM and energy dispersive X-ray (EDX) spectroscopy. Effective band edges can be better distinguished in SiO2 capped nanoparticles than in bare ones, In both cases the band edges are correlated with size quantum confinement effect.     

Self-assembled InAs/GaAs quantum dots covered by different strain reducing layers exhibiting strong photo- and electroluminescence in 1.3 and 1.55 microm bands

The effect of different InGaAs and GaAsSb strain reducing layers on photoluminescence and electroluminescence from self-assembled InAs/GaAs quantum dots grown by metal-organic vapour phase epitaxy was investigated. The aim was to shift their luminescence maximum towards optical communication wavelengths at 1.3 or 1.55 microm. Results show that covering by InGaAs strain reducing layer provides stronger shift of photoluminescence maximum (up to 1.55 microm) as compared to GaAsSb one with similar strain in the structure. This is caused by the increase of quantum dot size during InGaAs capping and reduction of quantum confinement of the electron wave function which spreads into the cap. Unfortunately, the weaker electron confinement in quantum dots is a reason of a considerable blue shift of electroluminescence from these InGaAs structures since optical transitions move to InGaAs quantum well. Although strong electroluminescence at 1300 nm was achieved from quantum dots covered by both types of strain reducing layers, the GaAsSb strain reducing layer is more suitable for long wavelength electroluminescence due to higher electron confinement potential allowing suppression of thermal carrier escape from quantum dots.     

Ferromagnetism in One Layer of Self-organized InMnAs Quantum Dots

One layer of self-assembled InMnAs quantum dots with InGaAs barrier was grown on high-resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of ferromagnetic structure was confirmed in the InMnAs dilute magnetic quantum dots. The one layer of self-assembled InMnAs quantum dots was found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T _C =80 K. It is likely that the ferromagnetic exchange coupling of sample with T _C =80 K is hole-mediated resulting in Mn substituting Ge. PL emission spectra of InMnAs samples grown at temperature of 210°C and 285°C show that the interband transition peak centered at 1.31 eV comes from the InMnAs quantum dot.     

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.     

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.     

Valence Band Structure of Coupled Diluted Magnetic Quantum Dots

The valence band structure of double InAs/GaAs diluted magnetic vertical quantum dots has been studied in the frame of multiband k⋅p calculation. We find that due to anisotropic effective mass, the ground state can be tuned from the characteristic of heavy- to light-hole state through just changing the parameter of quantum coupling between two QDs. We also show how this crossover manifests itself in the p–d exchange interaction between hole and Mn spins.     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures by submonolayer migration-stimulated epitaxy

Multilayer structures of InGaAs/GaAs quantum dots fabricated by submonolayer migrationstimulated epitaxy have been studied experimentally by scanning tunneling microscopy and results are presented. These results clearly show that in multilayer structures, ordering of nanoobjects into rows occurs in InAs and InGaAs heteroepitaxial layers. Pis#x2019;ma Zh. Tekh. Fiz. 23, 80–84 (November 26, 1997)     

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage (C–V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.