Planar Quantum Dots Research Articles

Influence of delta-doping on the performance of Ge/Si quantum-dot mid-infrared photodetectors

The effect of delta-doping on the performance of ten-period Ge/Si quantum-dot (QD) mid-infrared photodetectors (λmax≃3.4 μm) was investigated. Ge QDs fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at 600 °C. Each Si barrier contains a boron delta-doping layer located near the QD plane to provide holes to the dots. Within the sample set, we examined devices with different positions of the δ-doping layer with respect to the QD plane, different distances between the δ-doping layer and the QD plane, and different doping densities. All detectors show pronounced photovoltaic behavior implying the presence of an internal inversion asymmetry. We observed a reversal of the voltage dependence of responsivity with respect to zero bias when the δ-doping plane is carried from the bottom to the top of the dot layer. This result indicates that the main reason for the asymmetric photoresponse is the existence of a built-in electric field due to the placing dopants in the barriers. Devices with a lower doping density (pB=4×1011 cm−2) or with a shorter distance between the doping layer and QDs (d = 2 nm) are found to operate better in a photoconductive mode with the highest peak detectivity of about 6×1010 cm Hz1/2/W at T = 90 K and 0.2 V bias. The best performance is achieved for the device with pB=12×1011 cm−2 and d = 5 nm in a photovoltaic regime. At a sample temperature of 90 K and no applied bias, a responsivity of 0.83 mA/W and detectivity of 8×1010 cm Hz1/2/W at λ=3.4 μm were measured under normal incidence infrared radiation.

Imaging Schottky Barriers and Ohmic Contacts in PbS Quantum Dot Devices

We fabricated planar PbS quantum dot devices with ohmic and Schottky type electrodes and characterized them using scanning photocurrent and photovoltage microscopies. The microscopy techniques used in this investigation allow for interrogation of the lateral depletion width and related photovoltaic properties in the planar Schottky type contacts. Titanium/QD contacts exhibited depletion widths that varied over a wide range as a function of bias voltage, while the gold/QD contacts showed ohmic behavior over the same voltage range.

Theory of fine structure of correlated exciton states in self-assembled semiconductor quantum dots in a magnetic field

A theory of the fine structure of correlated exciton states in self-assembled parabolic semiconductor quantum dots in a magnetic field perpendicular to the quantum dot plane is presented. The correlated exciton wave function is expanded in configurations consisting of products of electron and heavy-hole 2D harmonic oscillator states (HO) in a magnetic field and the electron spin ${S}_{z}=\ifmmode\pm\else\textpm\fi{}1/2$ and a heavy-hole spin ${\ensuremath{\tau}}_{z}=\ifmmode\pm\else\textpm\fi{}3/2$ states. Analytical expressions for the short- and long-range electron-hole exchange Coulomb interaction matrix elements are derived in the HO and spin basis for arbitrary magnetic field. This allows the incorporation of short- and long-range electron-hole exchange, direct electron-hole interaction, and quantum dot anisotropy in the exact diagonalization of the exciton Hamiltonian. The fine structure of ground and excited correlated exciton states as a function of a number of confined shells, quantum dot anisotropy, and magnetic field is obtained using exact diagonalization of the many-body Hamiltonian. The effects of correlations are shown to significantly affect the energy splitting of the two bright exciton states.

Optical properties of multi-layer type II InP/GaAs quantum dots studied by surface photovoltage spectroscopy

We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details.

ELECTRON HEATING IN QUANTUM-DOT STRUCTURES WITH COLLECTIVE POTENTIAL BARRIERS

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, dot occupation, and electric field. Results of our simulations demonstrate that the capture processes are substantially suppressed by the potential barriers and enhanced in strong electric fields. Detailed analysis shows that the effects of the electric field can be explained by electron heating, i.e. field effects become significant, when the shift of the electron temperature due to electron heating reaches the barrier height. Optimized photoelectron kinetics in quantum-dot structures with collective barriers allows for significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors based on these structures.

Clustering in a self‐assembled CdTe/ZnTe quantum dot plane revealed by inter‐dot coupling

AbstractWe study probability of coupling between randomly distributed quantum dots (QDs) in self‐assembled CdTe/ZnTe heterostructures grown with different thicknesses of CdTe formation layer. Photoluminescence excitation spectra reveal a number of QD pairs involved in inter‐dot energy transfer. The statistics of dot pairs was collected at numerous spots of samples. It shows that the observed coupling is more frequent than that expected for a uniform random distribution of QDs.

Electrical properties of self-assembled InAs/InAlAs quantum dots on InP

The characteristics of InAs self-assembled quantum dots (QDs) grown on InAlAs/InP (0 0 1) have been investigated by capacitance–voltage (C–V) measurements and deep-level transient spectroscopy (DLTS). The depth profile of the apparent electron concentration obtained by C–V measurements shows significant carrier accumulation around the position of the InAs QDs plane. In addition to the D1–D5 traps, which are commonly detected in InAlAs layers grown on InP by molecular beam epitaxy (MBE), DLTS investigations show three InAs-related levels located at about 76, 202 and 246 meV below the InAlAs conduction band edge. The applied field is found to significantly enhance tunnel emission processes as seen in the broadening of the peaks with increasing reverse bias. The results suggest that defects in the material barrier can enhance electron tunnelling through barriers.

Highly stacked InGaAs quantum dot structures grown with two species of As

The authors describe successful formation of highly stacked InGaAs quantum dot (QD) structures grown with molecular beam epitaxy. 100-stack InGaAs QDs are grown without using any strain compensation technique or any degradation in crystal quality. InGaAs QDs are aligned in the growth direction and tend to align in the QD plane. As2-grown multistack InGaAs QD structures have superior optical properties to As4-grown structures at a high growth rate of 1μm∕h, whereas the opposite is true at a lower growth rate. The highest and narrowest photoluminescence spectrum is observed in a highly stacked InGaAs QD structure grown with an As2 source and a high growth rate.

Classification of Energy Levels in Quantum Dot Structures by Depleted Layer Spectroscopy

The coexistence of quantum confined energy levels and defect energy levels in quantum dot (QD) structures may cause difficulties in distinguishing between their different origin when using deep-level transient spectroscopy (DLTS). Using InAs/GaAs QDs as demonstration vehicles, we present methodologies to obtain such a classification by DLTS. QD-related spectra measured as a function of repetition frequency of electrical pulses, f, or temperature, T, and reverse voltage, V (R), are depicted as contour plots on (f, V (R)) and (T, V (R)) planes, thus reflecting the complex thermal and tunneling emission of electrons from the ground and excited states. Defect-related levels give rise to different contour patterns and undergo modification, exhibiting double-peak structured emission when defects are agglomerated in the vicinity of the QD plane. This effect is interpreted in terms of an interaction between electron states in traps and the confined QD states.

Dynamics of a Tightly Bound Planar Quantum Dot Subject to Landau Quantization

We have examined the quantum dynamics of a tightly bound planar (2-D) quantum dot subject to Landau quantization in a high magnetic field. The dot is taken to be set on a 2-D host sheet in a perpendicular magnetic field, and it is represented by a 2-D Dirac delta-function potential, which would support just a single subband state in the absence of the magnetic field. The integral equation for the Schro¿dinger Green's function of this system is solved in closed form analytically, and we have studied the dispersion relation for the single-particle subband energy spectrum. We find that it has a plethora of single-particle subband modes induced by Landau quantization.

Luminescence spectra of CdSe/ZnSe double layers of quantum dots

AbstractWe have studied the emission spectra and structural properties of double CdSe/ZnSe quantum dot (QD) sheet structures grown by molecular beam epitaxy in order to elucidate the mechanisms of the electronic and strain field interaction between the QD planes. The thickness of the ZnSe barrier separating the CdSe sheets was in the range of 10‐60 monolayers (ML) in the set of samples studied. We have found that coupling between dots in adjacent layers becomes relatively strong in CdSe/ZnSe double layers structures with 25‐27 ML barrier, while it is rather weak when the barrier thickness exceeds 30 ML. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectrum of the graphene-based planar quantum dot

The dependence of energy levels of the planar narrow-gap semiconductor quantum dot on its radius, energy gap, and potential barrier height was calculated in the two-band approximation. Based on the results obtained, the energy spectrum of the graphene-based planar quantum dot was studied in detail.

Anomalous spin splitting of electrons in type-II InSb quantum dots in InAs

Magnetooptical studies of heterostructures with type-II InSb quantum dots in the InAs matrix grown by molecular beam epitaxy are carried out. Unusual behavior of magnetophotoluminescence from quantum dots measured in the Faraday geometry in the samples with multiple planes of quantum dots is found. Specifically, the peak with σ− polarization, which corresponds to transitions of electrons with s = +1/2, has a higher energy than the σ+ peak corresponding to s = −1/2, which contradicts the negative value of the electron g factor both in the InAs matrix and in the InSb quantum dot. The effect can be interpreted as the result of competition of two channels of radiative recombination, which differ by the initial states of electrons belonging either to the InSb quantum dots or to shallow-level donors in the InAs matrix.

Elaboration and optical properties of type-II ZnTe on ZnSe heterostructures

Special growth conditions are presented in this work, in order to produce ZnTe/ZnSe type-II quantum dots and preserve them during the capping stage. A detailed study emphasizes the high sensitivity of the sample structure to Se/Zn ratio as opposed to other growth parameters. It is shown that nominally identical samples can evolve into two-dimensional quantum well or quantum dot plane, depending on which element is in excess. Transmission electron microscopy, atomic force microscopy and optical characterizations evidence this phenomenon.

Gate-defined quantum-dot devices realized in InGaAs/InP by incorporating a HfO2 layer as gate dielectric

Gate-defined quantum dots in an InGaAs/InP heterostructure are realized by incorporating a high-κ HfO2 material as a gate dielectric using atomic layer deposition. The fabricated quantum-dot devices show Coulomb blockade effect at low temperature. The Coulomb blockade current peaks are found to shift in pairs with the magnetic field applied perpendicular to the quantum-dot plane, due to the filling of electrons into spin-degenerate orbital states. When the magnetic field is applied parallel to the quantum-dot plane, spin splittings of orbital states are observed and the extracted effective g-factors are found to be different for different orbital states.

Tunable ground-state degeneracies in double quantum dots

We have studied the effect of electron–electron interactions on planar double quantum dots subjected to an external magnetic field. Contrary to all other earlier works on similar systems, our accurate results for the energy levels and the dipole-allowed transition energies reveal the absence of singlet–triplet crossing (due to electron correlations only) in the case when the dots occupy only two electrons. For a larger number of electrons, we find the expected crossing of the energy levels. The corresponding far-infrared optical absorption spectra show the effect of electronic interactions as well as the underlying symmetries of these systems. The hyper-exponential increase of the exchange energy, as derived in our model, would facilitate the coherent control of the spin states in a double dot, as proposed in recent experiments.

Effects of the quantum dot ripening in high-coverage InAs∕GaAs nanostructures

We report a detailed study of InAs∕GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15nm and 2×1011cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25nm and densities ranging from 2×108to1.5×109cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects.

Effect ofsp−dexchange interaction on excitonic states inCdSe∕ZnSe∕Zn1−xMnxSequantum dots

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral $\mathrm{CdSe}∕\mathrm{ZnSe}∕\mathrm{ZnMnSe}$ quantum dots (QDs) with various magnitudes of $sp\text{\ensuremath{-}}d$ exchange interaction has been investigated in magnetic fields $B$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnitude of the $sp\text{\ensuremath{-}}d$ interaction is varied by changing the penetration of the exciton wave function ${\ensuremath{\psi}}_{X}$ into the diluted magnetic semiconductor (DMS) barrier, $\ensuremath{\eta}$, by variation of the nonmagnetic $\mathrm{ZnSe}$ barrier layer thickness about a value of $1.75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The small penetration of ${\ensuremath{\psi}}_{X}$ into the DMS barrier allowed a decrease of the X and XX emission line broadening caused by magnetic fluctuations and resolution of the fine structure of X states in a single QD. In contrast to bulk DMSs, the exciton line broadening when $B$ is normal to the QD plane was found to be a nonmonotonic function of magnetic field: at high $B$ it is suppressed due to Mn ion spin alignment, whereas at low $B$ it is decreased due to the mixing of ${J}_{z}=+1$ and $\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{X}$ states in low-symmetry QDs by the electron-hole $(e\text{\ensuremath{-}}h)$ exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the $e\text{\ensuremath{-}}h$ exchange interaction at $B=0$ and (ii) strong enhancement of spin relaxation between ``bright'' $J=1$ exciton states in magnetic fields normal to the QD plane already at $\ensuremath{\eta}\ensuremath{\sim}1%$. The spin relaxation from bright to ``dark'' exciton states is negligible up to $\ensuremath{\eta}\ensuremath{\sim}4%$. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at $B=0$ already at $\ensuremath{\eta}\ensuremath{\sim}4%$.

Demonstration of a Low Threshold Current in 1.54 µm InAs/InP(311)B Quantum Dot Laser with Reduced Quantum Dot Stacks

This article reports the improvement of broad area lasers epitaxially grown on InP(311)B substrate. Thanks to optimized growth techniques, a high density of uniformly sized InAs quantum dots (QDs) up to 1011 cm-2 is obtained. The device, which contains only two stacks of QDs, exhibits a ground state laser emission at 1.54 µm at room temperature associated with a threshold current density as low as 170 A/cm2. Experimental results also demonstrate a modal gain greater than 8 cm-1 per QD plane.

Magnetic field dependence of quantum dot ground states

We study the ground states of a planar quantum dot with N = 5 , 6 , 7 electrons, in the presence of a perpendicular magnetic field. Using a spatially unrestricted Hartree Fock technique followed by spin and angular momentum symmetry restoration, chemical potentials are calculated and transitions between different ground states are identified. A spin blockade in the 6 → 7 transition is found. The structure of the quantum dot wave functions is illustrated by their electron densities.