Photoluminescence inner core excitation in semiconductor quantum structures

We introduce a photoluminescence inner core excitation (PLICE) for the studies of semiconductor quantum structures. This novel method, in which we use synchrotron radiation as tunable excitation source, is expected to facilitate us to obtain electronic and compositional information about buried quantum structures. Here we report experimental results on quantum dots (QDs) and quantum wires (QWRs), in order to demonstrate potential applicability of the method to the semiconductor nanostructure studies.     

Study on the coupled multiple nanocrystal quantum-dot system

We have studied excess electron filling rule in the coupled multiple nanocrystal quantum-dot systems, i.e. quantum chain and quantum pattern, by the unrestricted Hartree–Fock–Roothaan method. Assuming each quantum dot of quantum pattern to be confined in a three-dimensional spherical potential well of finite depth, we have studied the intradot and interdot electron Coulomb and exchange interactions. By varying the center distance d between the coupled quantum dots, the transition from the strong- to weak-coupling situation is realized. For the systems in question, our results show that, with the filling of excess electrons into the quantum pattern, the corresponding chemical potentials form quasi-band structure, which is similar to the energy-band structure of crystal material. In each chemical-potential band of quantum pattern, the number of chemical-potential curves is equal to the number of quantum dots, and the distributions of them depend strongly on the quantum-dot arrangement structure of quantum pattern.     

Investigations of backscattering peaks and of the nature of the confining potential in open quantum dots

The properties of open quantum dots are examined in magneto-transport. The quantum dots are prepared from a two-dimensional electron system (2DES) in AlGaAs/GaAs by lateral gate structures. These quantum dots are open, i.e. they are still connected to the surrounding 2DES regions. The low magnetic field magnetoresistance shows peak structures. These structures can be related to semi-classical ballistic trajectories in the confining potential of a dot. The calculations of different confining potentials (abrupt “hard-wall” and parabolic “soft-wall”) are compared with the experimental results. The experiments are better described by a soft-wall potential.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

Antiresonance of electron tunneling through a quantum dot array

Electronic transport through a one-dimensional quantum dot array is theoretically studied. In such a system both electron reservoirs of continuum states couple with the individual component quantum dots of the array arbitrarily. When there are some dangling quantum dots in the array outside the dot(s) contacting the leads, the electron tunneling through the quantum dot array is wholly forbidden if the electron energy is just equal to the molecular energy levels of the dangling quantum dots, which is called as antiresonance of electron tunneling. Accordingly, when the chemical potential of the reservoir electrons is aligned with the electron levels of all quantum dots, the linear conductance at zero temperature vanishes if there are odd number dangling quantum dots; Otherwise, it is equal to 2e²/h due to resonant tunneling if the total number of quantum dots in the array is odd. This odd–even parity is independent of the interdot and the lead–dot coupling strength.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Experimental demonstration of coherent coupling of two quantum dots

During the recent years semiconductor nanostructures have attracted considerable interest with respect to potential applications in quantum information processing. In particular, quantum dot molecules have been suggested to provide the building block of a quantum computer: forming quantum gates due to coherent coupling of two dots. The characteristic dependence of the splitting of ‘bonding’ and ‘anti-bonding’ states suggests coherent coupling of two InAs/GaAs quantum dots. Anti-crossings in the fine structure of excitons due to mixing of optically bright and dark states have been observed in Faraday configuration. In Voigt configuration the diamagnetic shift of the quantum dot molecule is enhanced compared to a single quantum dot. These findings altogether demonstrate the coherent coupling of exciton states in quantum dot molecules.     

Electron–phonon interaction in CdS/ZnS quantum dot

Electron–phonon effects on the two first electronic states in both CdS and GaAs quantum dots are investigated. Both confined longitudinal optical (LO) and surface optical (SO) phonons are considered. We use the intermediate-type variational approach. We find that, shifts caused by phonon contribution on electronic energies are more significant for CdS quantum dot. We find, also, that, contrary to GaAs based quantum dots, we shouldn’t neglect the SO phonon contribution for CdS based ones, especially for small dots.     

Properties of narrow gap quantum dots and wells in the InAs/InSb/GaSb systems

The properties of InSb quantum dots grown by metal organic vapour phase epitaxy are summarised as deduced from photoluminescence, magneto-photoluminescence, and far-infrared modulated photoluminescence experiments. A technique is described for shifting the emission of these dots to lower energy by coupling them with a narrow InAs quantum well, leading to the demonstration of electroluminescence at 2.3 μm.     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Controlling emission wavelength from InAs self-assembled quantum dots on InP (0 0 1) during MOCVD

We studied the growth of InAs quantum dots on InP (0 0 1) substrates in a low-pressure metalorganic chemical vapor deposition by using a so-called InP ‘double-cap’ procedure. With double-capping, a photoluminescence spectrum is modified into a series of multiple peaks, where the emission peaks arise from several quantum dot families with different heights changing in a step of integer number of an InAs monolayer. Cross-sectional transmission electron micrograph observations revealed that the shape of double-capped dots is dramatically changed into a thin plate-like shape with extremely flat upper and lower interfaces, being consistent with our interpretation of the photoluminescence spectrum. We showed that the procedure was extremely useful for controlling the emission wavelength from quantum dots in an InAs/InP (0 0 1) system.     

Electronic structure of the p-shell in single, site-selected InAs/InP quantum dots

The spectroscopy of single InAs/InP quantum dots emitting close to 1.55 μm is described. The dots are produced using a nanotemplate deposition technique that allows precise, a priori control of quantum dot position and electronic configuration. The experimentally observed luminescence signal from the p-shell is composed of several lines. Using exact diagonalization calculations of the emission spectra we interpret the splittings between these lines in terms of Coulomb induced, many-body renormalization of the excitonic states and a template-induced shape asymmetry of the quantum dot.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

The temperature dependence of the photoluminescence from overgrown GaAs/Al0.3Ga0.7As quantum dots and wires

Photoluminescence measurements on GaAs/Al_0.3Ga_0.7As quantum dots and wires fabricated using electron bears lithography and reactive ion etching are reported both before and after regrowth with a layer of Al_0.4Ga_0.6As. Dots exhibit little change in luminescence efficiency from the bulk with a reduction in diameter either before or after regrowth. Surface recombination therefore appears to be suppressed. In wires, however, luminescence intensity is very sensitive to wire width, decreasing rapidly with this parameter, but recovers and becomes independent of size after overgrowth. The temperature dependence of the photoluminescence from the dots and wires showed that dots and wires less than 150nm in width luminesced to higher temperatures than the larger diameter structures and dots liminesced to higher temperatures than wires of comparable width. This suggests that there is a finite coherence area effect which increases the radiative lifetimes of excitons in the quantum structures due to the geometric constraint, in the lateral direction in the wires and in all three directions in the dots. Below 20K bound exciton luminescence dominates in the dots but not in the wires. In wires it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. Regrowth at 750°C causes migration of aluminium into the quantum well and causes the shape of the well to become parabolic resulting shifts in the exciton emission to shorter wavelengths, making it difficult to separate the effect of processing from those due to quantum confinement.     

0.7 structure in long quantum wires

While quantized conductance steps in short quantum wires are understood through a single electron picture, additional structure often observed in high-quality one-dimensional systems near g=0.7×(2e²/h) is commonly interpreted as arising due to many-body interactions. Most studies of conductance structure below 2e²/h use short one-dimensional wires where transport is known to be ballistic. We report transport measurements for both short (0.5 μm) and long (5 μm) quantum wires, and use both conductance and nonlinear transport to explore the behavior of one-dimensional wires.     

Pressure-induced modulation of the confinement in self-organized quantum dots produced and detected by a near-field optical probe

Near-field optical probing, or nanoprobing, achieves spatial resolution that surpasses the diffraction limit of light and makes possible the luminescence imaging and spectroscopy of single quantum dots in dense arrays of dots. We use optical nanoprobing to study self-organized InGaAs quantum dots grown on (3 1 1)B oriented GaAs substrates. Here, we emphasize a new feature of nanoprobing: pressure-induced strain modulation near the surface. Operating in near-field optical excitation–collection mode, the probe makes contact with the surface and exerts direct pressure whose main effect is a compressive uniaxial strain under the probe. By adjusting the applied pressure, we modulate the local strain environment in and around a quantum dot, but still preserve the capability to capture its near-field luminescence. Nanoprobe pressure effects modify the confinement potential and radiative emission of single quantum dots, and the coupling strength between dots. This opens new possibilities for the study and control of the optical and electronic properties of single- and coupled-quantum dots.     

Low temperature measurements of the energy loss rate in quantum wires and 2DEGs in an InAlAs/InGaAs/InAlAs heterostructure

Electron heating measurements have been carried out in etched quantum wires of various widths and in two-dimensional electron gases at low temperature in InGaAs quantum wells. The value of the temperature exponent of the energy loss rate, an indicator of the type of predominant energy loss scattering, is found to be n=3, indicative of piezoelectric scattering. At a lattice temperature <1 K, our wires show an exponential behavior expected for deviations from equipartition. Further departure is found at still lower temperatures to a width-dependent loss rate, which is thought to be due to many-body effects in the one-dimensional wires.     

Stable biexcitonic lasing of CuCl quantum dots under two-photon resonant excitation

We have observed ultraviolet lasing of CuCl quantum dots embedded in a NaCl single crystalline matrix. Stable ultraviolet lasing of biexciton (two e–h pairs) luminescence has been achieved below 70 K under the two-photon direct excitation of biexcitons of CuCl quantum dots. Laser action occurs with parallely cleaved surfaces of the NaCl crystalline matrix acting as an optical cavity, which shows a very high efficiency of lasing.     

Raman study of Si–Ge intermixing in Ge quantum rings and dots

The Ge/Si (1 0 0) nanostructures have been studied by atomic force microscopy (AFM) and Micro Raman optical spectroscopy. Two layers of Ge of total thickness 0.75 nm and Si cap with thickness 2.5 nm were deposited by the method of molecular beam epitaxy at the temperature range 640–700 °C. AFM shows both quantum dots and ring-shape Ge nanostructures. From the analysis of the intensity and energy shift of the Raman signal we have found that the average concentration of Ge decreases considerably from 44% to 27%, when the growth temperature increases, whereas the degree of strain relaxation remains roughly the same. This allows us to conclude that intermixing is a dominating mechanism for strain relaxation in processes of transformation of Ge quantum dots to quantum rings.