Layer Of Self-assembled Quantum Dots Research Articles

Hysteretic capacitance-voltage characteristics of self-assembled quantum dots far from equilibrium with their environment

Capacitance-voltage measurements on self-assembled quantum dot layers exposed to strong electric fields and with large distances to the reservoirs show a marked hysteretic behavior. It is shown that at low temperatures this hysteresis can be explained quantitatively in terms of state-dependent capture and emission rates that are obtained by a rate equation model, applied to the measured capacitance transients. The occupation dynamics and the steady-state configuration can be extracted from these data via a Markov chain model.

Probing indirect exciton complexes in a quantum dot molecule via capacitance-voltage spectroscopy

Capacitance-voltage spectroscopy has proved to be a very powerful experimental technique towards the investigation of carrier-carrier interactions both qualitatively and quantitatively in complex coupled nanostructures. Here, we exploit this method to observe indirect exciton complexes in a quantum dot molecule and to quantify the electron-hole interactions between two dots in a quantum dot molecule, formed by vertical stacking of self-assembled quantum dot layers. While frequency-dependent measurements distinguish between the $s$- and $p$-charging behavior, under perpendicular magnetic fields, reordering of the quantized states charging sequence is observed along with the formation of a Landau fan in the wetting layer that is used to reconstruct the Fermi energy level.

Tunneling Through Finite Quantum Dot Superlattices

From first principles, we arrive at a generalized Tsu–Esaki formula for current per unit-cell in the case of one-dimensional, time-independent, open-system charge-transport along the growth direction of stacked layers of self-assembled quantum dots with perfect, transverse Bravais-lattice layout. We rigorously derive, using symmetry considerations, great simplifications to the final formula and identify significant computational benefits that would make modeling and simulation for such a complex problem not only feasible but also efficient. We allow an arbitrary sequence of quantum dot layers with or without intervening spacer and wetting layers. While the symmetry does suggest that final expressions for transmission coefficients should have simple form, the split, open boundary conditions as well as lack of longitudinal symmetry cast doubt on this. Employing a multiple sequential scattering view of processes within the heterostructure layers and using a limiting argument, we prove that the device behaves as a diffraction grating which distributes incident-carrier wavefunction-phases into specific patterns of reflected and transmitted phases which forms the basis for our conclusions.

Intensity fluctuations in bimodal micropillar lasers enhanced by quantum-dot gain competition

We investigate correlations between orthogonally polarized cavity modes of a bimodal micropillar laser with a single layer of self-assembled quantum dots in the active region. While one emission mode of the microlaser demonstrates a characteristic S-shaped input-output curve, the output intensity of the second mode saturates and even decreases with increasing injection current above threshold. Measuring the photon autocorrelation function ${g}^{(2)}(\ensuremath{\tau})$ of the light emission confirms the onset of lasing in the first mode with ${g}^{(2)}(0)$ approaching unity above threshold. In contrast, strong photon bunching associated with superthermal values of ${g}^{(2)}(0)$ is detected for the other mode for currents above threshold. This behavior is attributed to gain competition of the two modes induced by the common gain material, which is confirmed by photon cross-correlation measurements revealing a clear anticorrelation between emission events of the two modes. The experimental studies are in qualitative agreement with theoretical studies based on a microscopic semiconductor theory, which we extend to the case of two modes interacting with the common gain medium. Moreover, we treat the problem by a phenomenological birth-death model extended to two interacting modes, which reveals that the photon probability distribution of each mode has a double-peak structure, indicating switching behavior of the modes for pump rates around threshold.

The photocurrent of resonant tunneling diode controlled by the charging effects of quantum dots

We experimentally studied the photocurrent of AlAs/GaAs/AlAs double barrier resonant tunneling diode (RTD), which is composed of an InAs layer of self-assembled quantum-dots on top of AlAs barrier layer. It is found that the charging InAs quantum dots can effectively modulate the carrier transport properties of the RTD. Moreover, we also found that the resonant tunneling current through a single energy level of an individual quantum dot is extremely sensitive to the photo-excited holes bound nearby the dot, and the presence of the holes lowers the electrostatic energy of the quantum dot state. In addition, it is also observed that the photocurrent behaves like step way with the individual photon pulse excitation when the illumination is low enough. The experiment results well demonstrated the quantum amplified characteristics of the device.

Light emitting diodes with InAs/GaAsSb self-assembled quantum dot layer embedded in GaAs

Luminescence properties of metalorganic vapor phase epitaxy grown light emitting diodes (LEDs) with active InAs/GaAs quantum dot (QD) layer covered by GaAsSb strain reducing layer (SRL) were investigated at temperatures from 10 to 400K. Results show that the use of GaAsSb SRL with up to 14% Sb strongly increases luminescence, redshifts the emission maximum up to 1.4μm while keeping the type I transition, narrows the luminescence linewidth and keeps the separation energy between the ground and excited state as in InAs/GaAs QD LEDs without SRL. The strong ground state emission which persists up to 400K is dominated by recombination of electrons and holes confined in QDs while the broad emission from excited states, which dominates below 250K, involves recombination from the states located in QDs and SRL. The ground state electroluminescence shows the typical Stranski–Krastanov dot temperature properties: the thermally induced narrowing of the electroluminescence linewidth and the increased QD gap energy shrinkage with temperature. The electroluminescence intensity of the ground and excited state transitions increases with temperature (up to 80K) due to the thermal escape of electrons from the wetting layer which reduces radiative recombination via wetting layer states. The dominant mechanism responsible for the thermal quenching of electroluminescence at elevated temperatures is the escape of electrons from QDs to the GaAs barrier.

A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients.

Effect of Charge State in Nearby Quantum Dots on Quantum Hall Systems

Transport properties have been measured on two-dimensional electron system (2DES) with a nearby InAs self-assembled quantum dot layer, in order to clarify the anomalous behavior of magnetoresistance, ρ xx , and Hall resistance, ρ xy at $\nu=\frac{1}{3}$ , which was an unsolved problem in our previous paper (Takehana et al.: J. Phys. Soc. Jpn. 75, 114713, 2006). The significant suppression of both ρ xx and ρ xy was found to be reduced in the vicinity of $\nu=\frac{1}{3}$ , which indicates that the fractional quantum Hall effect prevent the spin-flip process between localized electrons and that extended 2DES, according to the discussion of the previous paper.

Improving size distribution of InAs quantum dots for intersubband devices

We present the growth and characterization of InAs quantum dots on Al x Ga 1− x As surfaces for intersubband devices. This requires the quantum dot energy levels in the Al x Ga 1− x As matrix to be above the GaAs bandedge. Using standard As 4 fluxes (beam equivalent pressure 8e−6 Torr), inhomogeneous broadening of the quantum dot size distribution increases with increasing Al content in the Al x Ga 1− x As matrix. Reducing the As 4 overpressure during In deposition is found to greatly improve the size distribution of the quantum dots, while producing slightly larger dots and a reduction in the density of small dots ( h<1.3 nm). Annealing at the higher standard As 4 flux for 30 s, after the reduced As 4 In deposition, produced a negligible change in the quantum dot size distribution. Utilizing surface dots on top of 30 layers of self-assembled quantum dots, the maximum quantum dot height for ground state energies above the GaAs bandedge is determined to be 2 nm for Al 0.30Ga 0.7As and 3 nm for Al 0.45Ga 0.55As.

Influence of Growth Break before Capping on Photoluminescence Dynamics of CdSe/ZnSe Self-Assembled Quantum Dots

Influence of growth breaks before capping of CdSe self-assembled quantum dot layers on photoluminescence dynamics was examined in three samples. Short (5 s) break resulted only in a small blue shift, caused probably by partial strain relaxation and/or Zn interdiffusion. Long (20 min) break induced a strong broadening and red shift of the spectra, combined with a dramatic slow down of the photoluminescence decays. The main result of the long break was identified as introduction of defects (impurities), which generate local electric fields and act as traps of photogenerated carriers.

Enhanced spontaneous emission in a photonic-crystal light-emitting diode

We report direct evidence of enhanced spontaneous emission in a photonic-crystal (PhC) light-emitting diode. The device consists of p-i-n heterojunction embedded in a suspended membrane, comprising a layer of self-assembled quantum dots. Current is injected laterally from the periphery to the center of the PhC. A well-isolated emission peak at 1.3μm from the PhC cavity mode is observed, and the enhancement of the spontaneous emission rate is clearly evidenced by time-resolved electroluminescence measurements, showing that our diode switches off in a time shorter than the bulk radiative and nonradiative lifetimes.

Designing high electron mobility transistor heterostructures with quantum dots for efficient, number-resolving photon detection

We describe the design of the epitaxial layers for an efficient, photon-number-determining detector that utilizes a layer of self-assembled quantum dots as an optically addressable gate in a field-effect transistor. Our design features a dedicated absorption layer where photoexcited holes are produced and directed with tailored electric fields to the quantum dot layer. A barrier layer ensures that the quantum dot layer is located at a two-dimensional potential minimum of the structure for the efficient collection of holes. Using quantum dots as charge traps allows us to contain the photoexcited holes in a well-defined plane. We derive an equation for a uniform size of the photon signal based on this precise geometry. Finally, we show corroborating data with well-resolved signals corresponding to different numbers of photons.

Coulomb‐mediated electron bunching in tunneling through coupled quantum dots

AbstractThe recent observation of super‐Poissonian shot noise in the tunneling current through two layers of selfassembled quantum dots is analyzed and discussed in terms of a sequential tunneling model for a single weakly coupled quantum dot stack.We demonstrate that the phenomenon of bunching in the electron transfer can be explained by the sole effect of Coulomb interaction between electrons inside the stack. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Operational Analysis of a Quantum Dot Optically Gated Field-Effect Transistor as a Single-Photon Detector

We report on the operation of a novel single-photon detector, where a layer of self-assembled quantum dots (QDs) is used as an optically addressable floating gate in a GaAs/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> As delta-doped field-effect transistor. Photogenerated holes charge the QDs, and subsequently, change the amount of current flowing through the channel by screening the internal gate field. The photoconductive gain associated with this process makes the structure extremely sensitive to light of the appropriate wavelength. We investigate the charge storage and resulting persistent photoconductivity by performing time-resolved measurements of the channel current and of the photoluminescence emitted from the QDs under laser illumination. In addition, we characterize the response of the detector, and investigate sources of photogenerated signals by using the Poisson statistics of laser light. The device exhibits time-gated, single-shot, single-photon sensitivity at a temperature of 4 K. It also exhibits a linear response, and detects photons absorbed in its dedicated absorption layer with an internal quantum efficiency (IQE) of up to (68 plusmn18)%. Given the noise of the detection system, the device is shown to operate with an IQE of (53 plusmn 11)% and dark counts of 0.003 counts per shot for a particular discriminator level.

Role of quantum capacitance in coupled low-dimensional electron systems

We have investigated the charging behavior of a layer of self-assembled InAs quantum dots placed in close vicinity to a two-dimensional electron gas (2DEG). As the gate bias is changed, the number of electrons in each system is altered simultaneously. Based on the quantum capacitance of the involved layers we develop a general model to determine the charging state of coupled low-dimensional systems from capacitance-voltage (CV) spectroscopy. The model is then applied to the special case of a layer of self-assembled quantum dots coupled to a 2DEG. As a complementary method, we have employed Hall voltage measurements. We find that the measurement of the two-dimensional carrier density through lateral transport provides a direct insight into the vertical charging process of the quantum dot system. Six individual charging peaks related to the occupation of the $s$ and $p$ shells of the dots can be resolved. In agreement with results from CV spectroscopy, Coulomb blockade and quantization energies can be extracted. Moreover, the Hall measurement offers a higher peak-to-valley ratio and a better estimate for the number of simultaneously charged dots than the capacitance data.

Tuning the cross-gap transition energy of a quantum dot by uniaxial stress

We show that a piezoelectric actuator can be used to apply uniaxial stress to a layer of self-assembled quantum dots. The applied stress leads to a change of the quantum dot's ground state exciton energy by up to a few hundred μ eV . This approach allows the possibility of an in situ and continuous tuning of the stress at temperatures down to 4 K and offers an alternative to tuning by temperature and Stark effect. We measure the relative change in the charging energy to the n-doped back contact by capacitance and the change in the exciton energy by photoluminescence. By tuning the uniaxial stress we are able to perform reflection spectroscopy on a single dot.

Single layer and stacked CdSe self-assembled quantum dots with ZnCdMgSe barriers for visible and white light emitters

We have grown structures with single layers of self-assembled quantum dots (SAQDs) and stacked layers of SAQDs in the II-VI materials systems CdSe∕ZnCdMgSe. The structures were grown on InP substrates by molecular beam epitaxy. Good control of the quantum dot (QD) size by controlling the CdSe deposition time was obtained, giving structures whose emission can be adjusted to be at any wavelength within the visible spectrum range. Stacked QD structures consisting of three QD layers emitting in the red, green and blue (R-G-B) regions of the spectrum, respectively, were grown. Photoluminescence measurements exhibited bright white emission that could be observed by eye, at 77K or at room temperature, as a result of the mixing of the three lines. These results indicate that this material may be an attractive alternative for optical applications in the R-G-B range and may be useful for the fabrication of white light sources.

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.

Multi-functional edge driven nano-scale cellular automata based on semiconductor tunneling nano-structure with a self-assembled quantum dot layer

We present a feasibility study of the semiconductor tunneling nano-structure consisting of multiple layers of two semiconductors along with a quantum dot layer for potential application in a cellular automata logic module. The elementary logic cell of the proposed CA module consists of a couple of tunnel diodes connected in series through a quantum dot. The charge of the quantum dot is considered as a logic variable. The local interconnections of nano-cells are achieved via the in-plane tunneling in the quantum dot layer. On the basis of approximate tunneling characteristics, multiple associative states and state dynamics are simulated. There are two ultimate advantages of the proposed CA scheme: (i) potential realization of a number of logic functions in one module, and (ii) reduced number of cell contacts required for read-in and read-out procedures (only edge cells have individual contacts). Examples of image processing using different logic functions are presented.

Magneto-Optical Study of Multiple Layers of Self-Assembled Quantum Dots Involving Diluted Magnetic Semiconductors

Magneto-optical experiments were carried out on structures comprised of multiple layers of self-assembled quantum dots (QDs) involving diluted magnetic semiconductors (DMSs). Photoluminescence (PL) from interband ground state transitions was clearly observed in these DMS-based QD systems. The PL energy from QD multilayers appears at a lower energy than that emitted by a single QD layer, suggesting that there exists electronic coupling between the QD layers. When an external magnetic field is applied, the PL peaks from QDs both in single-layer and in multilayer form exhibit large Zeeman shifts and a significant enhancement of intensity, a behavior that is typical for many low dimensional systems involving DMSs. In contrast to this behavior, however, we have observed a decrease of the PL intensity as a function of magnetic field in multilayer structures where alternating QW layers contain DMS and non-DMS QDs. We will show evidence that this effect arises from carrier transfer between pairs of QDs from adjacent layers (double QDs) due to the large Zeeman shifts of the conduction and valence bands characteristic of DMS QDs.