xAs Quantum Dots Research Articles

Magnetic Field Dependence of Optical Anisotropy in InxGa1xAs Quantum Dots

We present a mechanism that gives rise to a strong magnetic field dependence of the optical properties in quantum dots even when the dots are too small to have the orbital wavefunction significantly influenced by the field. For large indium contents x ∼ 1, top valence band levels in InxGa1—xAs quantum dots exhibit strong entanglement of the states of which the envelope wavefunction is characterized by s- and p-type symmetries. Although the optical anisotropy associated with each hole level is nearly independent of the magnetic field, the total anisotropy is enhanced when a level crossing is induced by the field.

Normal incidence InAs/AlxGa1−xAs quantum dot infrared photodetectors with undoped active region

We have performed a comprehensive investigation of n-type quantum dot infrared photodetectors (QDIPs) based on InAs/GaAs epitaxical island quantum dots (QDs) grown via the innovative punctuated island growth technique. The structural properties of the QDs were investigated with cross-sectional transmission electron microscopy and atomic force microscopy. The electronic properties of the QDs inserted in QDIP devices were investigated with photoluminescence (PL), PL excitation, and intra- and inter-band photocurrent spectroscopy. The influence of AlGaAs layers inserted into the QDIP active regions on the performance of dark current and inter- and intra-band photocurrent was examined. Initial results on intra-band responsivity and detectivity of these QDIPs at 77 K with undoped active region show promise for application.

Wavelength control from 1.25 to 1.4 μm in InxGa1−xAs quantum dot structures grown by metal organic chemical vapor deposition

This letter reports on the realization of long-wavelength InGaAs quantum dots (QDs) fabricated by metal organic chemical vapor deposition. By controlling the In incorporation in the QD layers and/or in the barrier embedding the QDs, we are able to tune the wavelength emission continuously from 1.25 to 1.4 μm at room temperature. Efficient stacking of dots emitting at 1.3 μm is also demonstrated.

Structural anisotropy and optical properties of In xGa 1− xAs quantum dots on GaAs(0 0 1)

InAs and In x Ga 1− x As ( x=0.2 and 0.5) self-organized quantum dots (QDs) were fabricated on GaAs(0 0 1) by molecular beam epitaxy (MBE) and characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence polarization spectrum (PLP). Both structural and optical properties of In x Ga 1− x As QD layer are apparently different from those of InAs QD layer. AFM shows that In x Ga 1− x As QDs tend to be aligned along the [1 1 ̄ 0] direction, while InAs QDs are distributed randomly. TEM demonstrates that there is strain modulation along [1 1 0] in the In x Ga 1− x As QD layers. PLP shows that In 0.5Ga 0.5As islands present optical anisotropy along [1 1 0] and [1 1 ̄ 0] due to structural and strain field anisotropy for the islands.

An optical study of self-assembled InxGa1−xAs/GaAs quantum dots embedded in a two-dimensional electron gas

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1−xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (EF≃13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average quantum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter.

Influence of indium composition on the surface morphology of self-organized InxGa1−xAs quantum dots on GaAs substrates

In x Ga 1−x As self-organized quantum dots with x=1.0, 0.5, and 0.35 have been grown by molecular beam epitaxy. The areal density, distribution, and shapes have been found to be dependent on x. The dot shape changes from a round shape for x=1.0 to an elliptical shape for x⩽0.5. The major axis and minor axis of the elliptical InxGa1−xAs dots are along the [1̄10] and [110] directions, respectively. The ordering phenomenon is also discussed. It is suggested that the dot–dot interaction may play important roles in the self-organization process.

Two-dimensional ordering of self-assembled In xGa 1− xAs quantum dots grown on GaAs(3 1 1)B surfaces

Two-dimensional (2D) ordering of self-assembled In x Ga 1− x As quantum dots (QDs) fabricated on GaAs(3 1 1)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows deferring from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of In x Ga 1− x As solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [0 1 1̄] and [2̄ 3 3], respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between neighbouring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. Photoluminescence (PL) result demonstrates that QDs grown on (3 1 1)B have the narrowest linewidth and the strongest integrated intensity, compared to those grown on (1 0 0) and other high-index planes under the same condition.

Two-dimensional ordering of self-assembled In xGa 1− xAs quantum dots grown on GaAs(3 1 1)B surfaces

The two-dimensional (2D) ordering of self-assembled In x Ga 1− x As quantum dots (QDs) fabricated on GaAs(3 1 1)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows differing from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of In x Ga 1− x As solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [0 1 1 ̄ ] and [ 2 ̄ 3 3] , respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between the neighboring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. The photoluminescence (PL) result demonstrates that QDs grown on (3 1 1)B have the narrowest linewidth and the strongest integrated intensity, compared to those on (1 0 0) and other high-index planes under the same condition.

Spatial confinement of self-organized MBE-grown InxGa1−xAs quantum dots

Several samples with InxGa1−xAs quantum dots have been grown by molecular beam epitaxy using the self-organization method. The substrate temperature was varied from sample to sample and atomic-force-microscopy measurements were carried out to investigate the influence of this parameter on the morphological characteristics of the dots. We observed that, under certain conditions related to the initial cleaning process, the nanostructures could be confined within some regions of the sample.

Exchange interactions in strained quantum dot arrays and possibility of engineering their magnetic properties

Exchange interactions between the holes confined in arrays of strained quantum dots subject to compressive and tensile strain are shown to lead to a ferrimagnetic arrangement of magnetic moments. Using the example of strained InxGa1−xAs quantum dots on InP substrate it is shown how the magnetic properties of semiconductor material can be engineered without resorting to doping with magnetic ions. The transition temperature is estimated to be of the order of 25 K. Potential applications in information storage and processing are considered.

Optical properties of InAlAs quantum dots in an AlGaAs matrix

Structural and optical properties of InxAl1−xAs quantum dots (QD) embedded in an Al0.30Ga0.70As matrix are studied as a function of InAs mole fraction (x). Decreasing of x results in a sequential disappearance of bound electron and hole states in the QDs. There is a range of indium compositions were photoluminescence is determined by type-II optical transitions between electrons localized in a Al0.3Ga0.7As layer and holes localized in the QDs.

Exciton Spin-Splitting in InxGa1—xAs Quantum Wires and Dots

Exciton Spin-Splitting in InxGa1—xAs Quantum Wires and Dots

Universal fluctuations of Coulomb blockade peaks in quantum dots

We describe recent measurements of the statistics and parametric correlations of Coulomb blockade peak heights in shape-deformable GaAs/A1xGa1−xAs quantum dots. We find good agreement between the measured distributions of peak heights and probability distributions predicted by random matrix theory for bothB= 0 andB≠ 0. The measured magnetic flux correlation lengthϕcis of the order of but less than one flux quantum through the dot,ϕc∼ 0.8ϕ0, inconsistent withϕcpredicted by universal scaling theory for isolated chaotic quantum dots.

Radiative recombination in GaAs-AlxGa1−xAs quantum dots

We present experimental and theoretical results on the low-temperature luminescence intensity of dry-etched GaAs-AlxGa1−xAs quantum dots. The luminescence intensity was found to decrease by two orders of magnitude with the decrease of dot sizes from 1 μm to 60 nm. Our intrinsic model of the emission yield invokes slower momentum and energy relaxation mechanisms as the lateral dimensions decrease. The additional extrinsic effect considered involves carrier diffusion with a surface nonradiative recombination velocity. Combining intrinsic and extrinsic effects and using a surface recombination velocity of ∼105 cm/s for GaAs, we can obtain a good fit to the data.