Multiple Quantum Dots Research Articles

Intersubband transitions in proton irradiated InGaAs∕GaAs multiple quantum dots

The optical absorbance spectra of the intersubband transition in 3 MeV proton irradiated In0.3Ga0.7As∕GaAs multiple quantum dot samples, cut from the same wafer, were investigated as a function of irradiation dose. The intensity of the intersubband transition is observed to decrease as the irradiation dose is increased. The behavior of the intersubband transition in irradiated In0.3Ga0.7As∕GaAs multiple quantum dot samples is compared to those of intersubband transitions in irradiated GaAs∕AlGaAs and InGaAs∕InAlAs multiple quantum well samples. The intersubband transition absorbance spectrum was completely depleted in a multiple quantum dot sample irradiated with 5×1014cm−2. Post-irradiation thermal annealing was performed on the latter sample where thermal annealing recovery of the depleted intersubband transition was not observed. Instead, a broadband with a peak at 5μm is observed after annealing the sample at 500 °C for 15 min. The absence of the broadband in an unirradiated sample that was subject to the same annealing conditions suggests that this band may be related to irradiation-induced defects.

Band-edge exciton transitions temperature in multiple stacked self-assembled (In 1− xMn x)As quantum dot arrays

Multiple stacked self-assembled (In 1− x Mn x )As quantum-dot (QD) arrays were grown on GaAs (100) substrates by using molecular-beam epitaxy with a goal of producing (In 1− x Mn x )As QDs with a semiconductor phase and a high ferromagnetic transition temperature ( T c). Atomic force microscopy, magnetic force microscopy, high-resolution transmission electron microscopy, and energy dispersive X-ray fluorescence measurements showed that crystalline multiple stacked (In 0.84Mn 0.16)As with symmetric single-domain particle were formed on GaAs substrates. Near-field scanning optical spectroscopy spectra at 10 K for the (In 0.84Mn 0.16)As multiple stacked QDs showed that the band-edge exciton transitions were observed. The magnetization curve as a function of the magnetic field at 5 and 300 K indicated that the multiple stacked (In 0.84Mn 0.16)As QDs were ferromagnetic, and the magnetization curve as a function of the temperature showed that the T c was as high as 400 K. These results provide important information on the optical and magnetic properties for enhancing the T c of (In 1− x Mn x )As-based nanostructures.

Tunable Double Quantum Dots in InAs Nanowires Defined by Local Gate Electrodes

We report on low-temperature transport measurements on single and double quantum dots defined using local gates to electrostatically deplete InAs nanowires grown by chemical beam epitaxy. This technique allows us to define multiple quantum dots along a semiconducting nanowire and tune the coupling between them.

Chirped multiple InAs quantum dot structure for wide spectrum device applications

A chirped multiple quantum dot (QD) structure is introduced as a means to controllably broaden the gain spectral width of InAs QDs. The In composition and thickness of InGaAs strain-reducing layers (SRLs) can be used as tuning parameters to adjust the emission wavelength of each layer in the chirped stack. The spectral linewidth depends on the span of the In composition and thickness of the InGaAs SRL. By varying the In composition of each InGaAs SRL, a ground-state photoluminescence spectral width of 78 nm was achieved. The spectral width can be further increased by Be modulation -doping. The chirped multi-layer QD structure was used as the active region of a laser diode, exhibiting a lasing spectrum as wide as 30 nm.

Conductance quantization and zero bias peak in a gated quantum wire

Conductance measurements are reported for an 0.4 μm wide GaAs/AlGaAs quantum wire with 7 cross-channel gates. The device exhibited integral conductance steps, magnetoconductance plateaus in agreement with the multiprobe formula and a conductance feature at 0.65(2 e 2/ h). Differential conductance measurements down to 50 mK revealed a zero bias conductance peak that vanished with an in-plane field of 1 T. The width of this peak was comparable to that reported in high mobility quantum point contacts (Phys. Rev. Lett. 88 (2002) 226805). At low conductances this device also exhibited single electron charging characteristic of a multiple quantum dot.

Wide emission spectrum from superluminescent diodes with chirped quantum dot multilayers

A superluminescent diode (SLED) using chirped multiple InAs quantum dot (QD) layers as the active region is demonstrated. The fabricated QD SLEDs exhibit a large spectral width up to 121 nm, covering the range 1165–1286 nm.

Exciton Localization in Vertically and Laterally Coupled GaN/AlN Quantum Dots

Near-field and time-resolved photoluminescence measurements show evidence of exciton localization in vertically and laterally coupled GaN quantum dots (QDs). The binding energies in multiple period QDs (MQDs) are observed to be stronger by more than six times compared to single period QDs (SQDs). Excitons in MQDs have a short (450 ps) lifetime and persist at room temperature, while SQDs exhibit extraordinarily long (>5 ns) lifetime at 10 K due to reduced spatial overlap of electron and hole wave functions in strained QDs.

Surface ordering of (In,Ga)As quantum dots controlled by GaAs substrate indexes

Self-organized surface ordering of (In,Ga)As quantum dots in a GaAs matrix was investigated using stacked multiple quantum dot layers prepared by molecular-beam epitaxy. While one-dimensional chain-like ordering is formed on singular and slightly misorientated GaAs(100) surfaces, we report on two-dimensional square-like ordering that appears on GaAs(n11)B, where n is 7, 5, 4, and 3. Using a technique to control surface diffusion, the different ordering patterns are found to result from the competition between anisotropic surface diffusion and anisotropic elastic matrix, a similar mechanism suggested before by Solomon [Appl. Phys. Lett. 84, 2073 (2004)].

InGaN/GaN multi‐quantum dot light‐emitting diodes

It has been demonstrated that InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) were successfully fabricated by metal-organic chemical vapor deposition. We have formed nanoscale InGaN self-assembled QDs in the well layers of the active region with a typical 3-nm height and 10-nm lateral dimension. With a 20-mA DC injection current, the forward voltage was 3.1 and 3.5 V for MQD LED and conventional nitride-based multi-quantum well (MQW) LED with the same structure, respectively. It was also found that EL peak position of the MQD LED is more sensitive to the amount of injection current, as compared to the conventional MQW LEDs. (C) 2003 Elsevier B.V. All rights reserved.

Tuning In0.3Ga0.7As∕GaAs multiple quantum dots for long-wavelength infrared detectors

Optical absorption spectra of intersubband transitions in In0.3Ga0.7As∕GaAs multiple quantum dots were investigated using the optical absorption as a function of the number of In0.3Ga0.7As monolayers deposited using the molecular-beam epitaxy Stranski–Krastanow technique. The peak position energy reached 13.7μm for a sample containing 50 monolayers of In0.3Ga0.7As. The lack of the observation of intersubband transitions in small quantum dots, where the number of the deposited monolayer is less than 15 monolayers, is an indication of the absence of quantum confinement. On the other hand, the presence of high dislocations density in larger quantum dots, where the deposited number of monolayers exceeds 50, could be the reason of why the intersubband transitions are degraded.

Temperature-dependent single-electron tunneling effect in lightly and heavily doped GaN nanowires

We studied the electrical transport properties of GaN nanowires with two different doping levels. Measurements taken at various temperatures demonstrate that the electrical transport depends mainly on the single-electron tunneling effect up to a relatively high temperature of $\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The aperiodic oscillations which we observed were attributed to single-electron tunneling through multiple quantum dots within the nanowire, which originated from various defects and the inhomogeneous distribution of the dopants.

Intersubband Transitions in In0.3 Ga0.7As/GaAs Multiple Quantum Dots of Varying Dot-Sizes

ABSTRACTThe optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In0.3Ga0.7As monolayers deposited, a series of samples with varying dot sizes ranging from 10 – 50 monolayers were obtained. The quantum dots grown with size less than 15 monolayers or more than 50 monolayers did not yield any observable measurements of intersubband transition. This suggests that there exist a critical upper and lower limit of In0.3Ga0.7As quantum dots for infrared detectors. A wavelength range of 8.60 – 13.70 μm is achieved for structures grown with the above monolayers range. The theoretical line-shape of the intersubband transition absorption was compared to the experimental measurements. From the lineshape, it was deduced that bound-to-continuum transtition is present in thick quantum dots and bound-to-bound transition is present in thinly grown quantum dots.

Cryptography based on the absorption/emission features of multicolor semiconductor nanocrystal quantum dots

Further to the optical coding based on fluorescent semiconductor quantum dots (QDs), a concept of using mixtures of multiple single-color QDs for creating highly secret cryptograms based on their absorption/emission properties was demonstrated. The key to readout of the optical codes is a group of excitation lights with the predetermined wavelengths programmed in a secret manner. The cryptograms can be printed on the surfaces of different objects such as valuable documents for security purposes.

Growth and optical properties of InAs/GaAs quantum dot structures

The growth and optical properties of InAs/GaAs(001) quantum dot (QD) structures depending on the deposition parameters are investigated. The epitaxial layers were grown in a Riber 32P MBE system and studied by atomic force microscopy and photoluminescence (PL). For a single QD with 2.7monolayers (ML) of InAs deposited at a rate of 0.25ML/s the dots have a dome shape and with increasing substrate temperature Ts from 460 to 520°C their surface density decreases from 2×1010 to 1.2×1010cm−2 and the mean lateral size increases from 40 to 70nm, the dots height does not exceed 8nm. At low beam equivalent pressure of As (below 3×10−6Torr) and higher Ts the segregation of In occurs. The multiple stacked QD structures (2.7 or 4ML of InAs with 4ML GaAs spacer) with the more uniform morphology in the upper layers providing the intense and narrow PL spectrum are formed at Ts=490°C and the flux ratio As4/In=25. The high-quality modulated Si-doped InAs/GaAs QDs-based multilayer heterostructures N-AlGaAs/GaAs/InAs/GaAs/InAs/GaAs/ … /GaAs with the two-dimensional (2D) electron gas of high-density were grown and studied for the first time and in their low-temperature PL spectra the features associated with quantum confinement effects were observed.

InGaN/GaN blue light-emitting diodes with self-assembled quantum dots

InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) active layers were successfully fabricated by using an interrupted growth method in metal-organic chemical vapour deposition (MOCVD). We have successfully formed nanoscale QDs embedded in quantum wells with a typical 3 nm height and 10 nm lateral dimension. It was found that a huge 68.4 meV blue shift in electroluminescence (EL) peak position as the injection current is increased from 3 to 50 mA for the MQD LED. The large EL blue shift reveals that deep localization of exitons (or carriers) originates from QDs will strengthen the band-filling effect as the injection current increases.

InGaN/GaN multi-quantum dot light-emitting diodes

It has been demonstrated that InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) were successfully fabricated by metal-organic chemical vapor deposition. We have formed nanoscale InGaN self-assembled QDs in the well layers of the active region with a typical 3-nm height and 10-nm lateral dimension. With a 20-mA DC injection current, the forward voltage was 3.1 and 3.5V for MQD LED and conventional nitride-based multi-quantum well (MQW) LED with the same structure, respectively. It was also found that EL peak position of the MQD LED is more sensitive to the amount of injection current, as compared to the conventional MQW LEDs.

Simple one-dimensional model for electronic structure calculation of unbiased and biased silicon quantum dots in Coulomb blockade applications

This article presents a simple one-dimensional (1D) model of electronic structure calculation able to treat quantum dots (QDs) under bias voltage. With a view to investigating complex Coulomb blockade devices with multiple QDs, this model aims at providing accurate information on the QD eigenstates within reasonable and optimized computation time. First, the electronic structure of an unbiased QD is obtained from a self-consistent solution of the coupled Schrödinger/Poisson equations as a function of the dot size and the charging state. By comparison with three-dimensional (3D) calculations of total energy at given QD volume, we found that the 1D spherical approximation appears to be very good for a wide range of dot shapes. We develop two techniques to include the effect of external 3D bias potential that breaks the symmetry: (i) a perturbation method and (ii) an expansion of the wave function on the eigenstates of the unbiased dot. If the validity of the first technique is limited to small dots and/or low bias voltage, the latter gives excellent results over a wide range of dot sizes and bias voltages. The results obtained for a single dot device using this 1D model are carefully and successfully compared with a full 3D calculation.

Single-electron tunneling in silicon-on-insulator nano-wire transistors

The gate bias dependent evolution of the Coulomb oscillations in a silicon-on-insulator nano-wire transistor is reported. Transport data obtained for a wide range of front- and back-gate bias strongly suggest that multiple quantum dots (QDs) with different potential depths are formed in the nano-wire channel. Our data can be clearly interpreted as arising from the turning on or off of one of these QDs as the back-gate bias is varied. Quantitative calculation based on the model of single-electron tunneling through two parallel QDs is in reasonable agreement with the measured data in the back-gate bias range where the third dot is not activated.

Effective interaction for charge carriers confined in quasi-one-dimensional nanostructures

A problem of interacting charge carriers confined in quasi-one-dimensional (1D) semiconductor nanostructures has been studied. We have derived an analytical 1D formula for the effective interaction potential between the confined charge carriers. We have applied both the 1D model with the effective potential and the full three-dimensional (3D) approach to an electron pair confined in a single and double quantum dot as well as to an exciton confined in a quantum wire. Comparing the results of the 1D and 3D approaches we have discussed the applicability of the effective 1D interaction potential to the real 3D nanostructures. We have shown that the present effective interaction leads to accurate results for weakly coupled multiple quantum dots and wire-like nanostructures, i.e., the quantum wires and dots with the lateral confinement much stronger than the longitudinal one.

Structural and magneto‐optical studies on multiple quantum dots containing magnetic semiconductors

We have fabricated single and multi-layer self-assembled quantum dot (QD) systems based on CdSe/ZnSe combination, including magnetic semiconductors (DMSs). Transmission electron microscopy (TEM) images shows that magnetic QDs are formed within the CdMnSe layer, and they are vertically correlated in multi-layer quantum dot systems. Magneto-optical experiments were also carried out on those structures. The PL energy from multi-layer QDs 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 multi-layer form exhibit a large intensity enhancements and energy shifts, a behavior that is typical for many low dimensional systems involving DMSs. The observed weak PL energy dependence in low magnetic fields can be understood in terms of magnetic polaron formation in these DMS QD systems. We also discuss statistical magnetic fluctuations in these systems, as revealed by PL line narrowing in a magnetic field.