Self-assembled Quantum Dot Arrays Research Articles

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

Quantum confinement effects in Si/Ge heterostructures with spatially ordered arrays of self-assembled quantum dots

Magnetotunneling spectroscopy was employed to probe the confinement in vertical Si/Ge double-barrier resonant tunneling diodes with regularly distributed Ge quantum dots. Their current-voltage characteristics reveal a steplike behavior in the vicinity of zero bias, indicating resonant tunneling of heavy-holes via three-dimensionally confined unoccupied hole states in Ge quantum dots. Assuming parabolic confinement, we extract the strength of the confinement potential of quantum dots.

Strain Effects and Potential Profiles of Multiple Vertically Stacked InAs/GaAs Self-Assembled Quantum Dot Arrays with Different Sizes

Strain Effects and Potential Profiles of Multiple Vertically Stacked InAs/GaAs Self-Assembled Quantum Dot Arrays with Different Sizes

Predicting and Understanding Order of Heteroepitaxial Quantum Dots

Heteroepitaxial self-assembled quantum dots (SAQDs) will allow breakthroughs in electronics and optoelectronics. SAQDs are a result of Stranski-Krastanow growth whereby a growing planar film becomes unstable after an initial wetting layer is formed. Common systems are Ge$_{x}$Si$_{1-x}$/Si and In$_{x}$Ga$_{1-x}$As/GaAs. For applications, SAQD arrays need to be ordered. The role of crystal anisotropy, random initial conditions and thermal fluctuations in influencing SAQD order during early stages of SAQD formation is studied through a simple stochastic model of surface diffusion. Surface diffusion is analyzed through a linear and perturbatively nonlinear analysis. The role of crystal anisotropy in enhancing SAQD order is elucidated. It is also found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical wetting surface height that marks the onset of non-planar film growth.

Annealing induced transition of flat strained InGaAs epilayers into three-dimensional islands

We report arrays of self-assembled quantum dots through roughening transition of strained but atomically flat layers into three-dimensional (3D) islands. Atomically flat two-dimensional InGaAs epilayers were grown on GaAs(001) substrates below 360°C. When heated higher than 420°C, they were observed to undergo roughening transitions. The morphology, height, and width of the resultant 3D features were found to be a strong function of the annealing time and temperature. Furthermore, at a particular set of parameters, dot chains were observed. The strain field of the flat layer seemed uniform in the roughening stage, but appeared to induce anisotropic diffusion at the subsequent growth stage.

Strain-controlled correlation effects in self-assembled quantum dot stacks

The authors show that elastic interactions of an array of self-assembled quantum dots in a parent material matrix are markedly distinct from the elastic field created by a single point defect and can explain the observed abrupt correlation-anticorrelation transition in semiconductor quantum dot stacks. Finite volume effects of the quantum dots are shown to lead to sharper transitions. Their analysis also predicts the inclination angle under which the alignment in successive quantum dot layers occurs in dependence on the material anisotropy.

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe self-assembled quantum-dot arrays

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe quantum-dot (QD) arrays grown by molecular beam epitaxy were investigated. Transmission electron microscopy showed that vertically stacked self-assembled CdSe QD arrays were embedded in the ZnSe barriers. The results for the photoluminescence (PL) data at 18 K demonstrated clearly that the transition behavior from uncoupled to coupled peaks depended on the ZnSe barrier thickness. The temperature-dependent PL measurements showed that the activation energy of the electrons confined in the CdSe QDs increased dramatically with decreasing ZnSe spacer layer thickness due to the strong coupling between CdSe/ZnSe QD arrays. The present observations can help improve understanding of the dependence of the coupling behavior and activation energy in CdSe/ZnSe QDs on the spacer layer thickness.

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E 1–HH 1) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.

Simulation evidence for lateral excitation transfer in a self-assembled quantum-dot array

Simulations of InAlAs/AlGaAs self-assembled quantum-dot arrays containing as many as 30 individual dots are used to identify a mechanism for lateral excitation transfer through partially delocalized heavy-hole states. Individual hole states exhibit wave-function splitting between several dots in the array, as well as partial confinement in the wetting layer, and have strong overlap with multiple conduction-band electron states in different quantum dots. Electron–hole pair energies involving these partially delocalized hole states correspond well with narrow resonances seen in the experimental photoluminescence excitation spectra taken for similar quantum-dot arrays using low-temperature near-field scanning optical microscopy.

Kinematical X-ray scattering theory of self-assembled quantum dot arrays and superlattices

Kinematical X-ray scattering theory of self-assembled quantum dot arrays and superlattices

Microstructural and interband transition studies of multiple stacked layers of Si-doped InAs self-assembled quantum dot arrays inserted into GaAs barriers

Microstructural and optical properties of Si-doped InAs quantum dot (QD) arrays inserted into undoped GaAs barriers have been investigated by using energy dispersive X-ray fluorescence (EDX), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The EDX pattern, the TEM image, and the selected area electron diffraction pattern showed that self-assembled Si-doped InAs vertically stacked QD arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted to the low-energy side with increasing temperature and that the distribution of carriers in the InAs QDs varied with changing temperature. These results indicate that Si-doped InAs QD arrays inserted into GaAs barriers hold promise for potential applications in optoelectronic devices.

Long-range Coulomb interaction in arrays of self-assembled quantum dots

An array of $3\ifmmode\times\else\texttimes\fi{}{10}^{7}$ Ge self-assembled quantum dots is embedded into the active channel of a Si metal-oxide-field-effect transistor. Conductance oscillations with a gate voltage resulting from a successive loading of holes into the dots are observed. Based on measurements of the temperature dependence of the conductance maxima, the charge-transfer mechanism in the channel is identified as being due to variable-range hopping between the dots, with the typical hopping energy determined by interdot Coulomb interaction. The characteristic spatial dimension of the hole wave functions as well as the charging energies of the dots are determined from the conductance data. The effect of the proximity of a bulk conductor on hopping transport is studied. We find that putting a metal plane close to the dot layer causes a crossover from Efros-Shklovskii variable-range hopping conductance to two-dimensional Mott behavior as the temperature is reduced. At the crossover temperature the hopping activation energy is observed to fall off. The metal plane is shown not to affect the conductance of samples which show Mott hopping. In the Efros-Shklovskii hopping regime, the conductance prefactor was found to be $\ensuremath{\simeq}{e}^{2}/h,$ and the conductance to scale with the temperature. In the fully screened limit, the universal behavior of the prefactor is destroyed, and it begins to depend on the localization length. The experimental results are explained by a screening of long-range Coulomb potentials, and provide evidence for strong electron-electron interaction between dots in the absence of screening.

Electrochemically self-assembled quantum dot arrays

We report some results pertaining to the magnetic, optical and topographic properties of electrochemically self-assembled quantum dots. Some of these dots self-order into two-dimensional hexagonal-close-packed arrays that are among the most periodic reported so far. We will review nonlinear optical properties of self-assembled CdS quantum dots and present new results pertaining to the nanomagnetic properties of cobalt dots. These structures may have important applications in magnetics, electronics and nonlinear optics.

Preferential nucleation of Ge islands at self-organized pits formed during the growth of thin Si buffer layers on Si(110)

The epitaxial growth of thin (∼20–40 nm) Si buffer layer on Si(110) leads to the formation of ∼100-nm-wide, uniformly sized faceted pits. The cause of these rhombohedral pits is revealed to be the overgrowth of a homoepitaxial layer over clusters of coherent contaminant particles, possibly SiC. Deposition of Ge on such “pitted” surfaces shows highly selective nucleation of pairs of coherent islands at the opposite corners of the pits along the 〈110〉 direction. Continued deposition leads to strain relaxation of one or both of the islands within the pit which then rapidly coarsen to form a single Ge island within the pit. Our observations offer insight into heterogeneous nucleation mechanisms important for producing controlled arrays of self-assembled quantum dots.

Single-electron charging and Coulomb interaction in InAs self-assembled quantum dot arrays

Sequential single-electron charging is observed in InAs self-assembled quantum dots using capacitance spectroscopy. In this system, the Coulomb energy is smaller than the interlevel energy spacings due to the quantum confinement and both effects can be separately identified. A theoretical model is proposed for this system and the capacitance experiments were devised in order to experimentally observe the effects of Coulomb interaction between electrons on the dots. The effects of inter- and intradot Coulomb interaction have been observed in the capacitance spectra. A good agreement between the proposed model and experiment is achieved.