InGaAs Quantum Disks Research Articles

Site-controlled formation of InGaAs quantum nanostructures-Tailoring the dimensionality and the quantum confinement

We report on InGaAs quantum disks (QDks) controllably formed on the top (001) facet of nano-patterned GaAs pyramidal platforms. The QDks exhibit pyramidal shape with special facets and varied dimensions, depending on the GaAs pyramidal buffer and the amount of InGaAs deposited. The formation of QDks is explained by the overgrowth of an InGaAs layer and thereafter coalescence of small InGaAs islands. Photoluminescence (PL) characteristics of ensemble QDks and exciton features of individual QDks together demonstrate that we may achieve a transition from zero-dimensional (0D) to two-dimensional (2D) quantum structure with increasing QDk size. This transition provides the flexibility to continuously tailor the dimensionality and subsequently the quantum confinement of semiconductor nanostructures via site-controlled self-assembled epitaxy for device applications based on single quantum structures.

Lateral electric-field effects on excitonic photoemissions in InGaAs quantum disks

Electric-field effects on excitons in zero-dimensional InGaAs quantum disks have been examined at low temperature. Photoluminescence from a single isolated disk was measured under the application of a lateral electric field by using the microphotoluminescence technique. A redshift of sharp excitonic luminescence and a decrease in its intensity under increasing electric field were observed. These were found to distinctively depend on the lateral extent of the disks: these were much more prominent in the larger disk. The exciton luminescence was found to be highly polarized along the direction of the field in the larger disk.

Spin relaxation of excitons in zero-dimensional InGaAs quantum disks

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.

Self-organized InGaAs strained quantum disks

We discovered a novel growth mode in a strained InGaAs system on a GaAs (311)B substrate. Surprisingly, this growth mode automatically produces a very small confined nanostructure during the growth interruption with a high substrate temperature by metal-organic-vapor phase epitaxy (MOVPE). We call this phenomenon self-organization, because it is accompanied by a pronounced ordering in the mutual arrangements of nanocrystals. The important point is that built-in strained InGaAs quantum disks within the nanocrystals exhibit a strong photoluminescence emission with a narrow linewidth at room temperature. This indicates that self-organization can possibly produce nanostructures of the quality needed for devices.

Effects of Dimensionality on Radiative Recombination Lifetime of Excitons in Thin Quantum Boxes of Intermediate Regime between Zero and Two Dimensions

We report on effects of dimensionality on radiative recombination lifetime in thin quantum boxes of intermediate regime between 0D and 2D. The temperature dependence of the recombination lifetime is calculated using a theoretical analysis that rigorously treats the electron-hole Coulomb interaction. We show how the dependence evolves from 2D to 0D with a decrease in the lateral width of the box. We also examine the effects of exciton localization, which arises from structural defects in the boxes, on the radiative recombination lifetime. These theoretical results are compared with experimental data obtained from InGaAs quantum disks on a (311)B GaAs substrate. Good agreement between theoretical results and the experimental data is obtained.

Magneto-optical properties of self-organized strained InGaAs quantum disks

We have measured the optical properties of self-organized strained InGaAs quantum disks in high magnetic fields. The disks are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metallorganic vapor-phase epitaxy. The magneto-luminescence properties of these InGaAs quantum disks have been studied for various disks sizes with lateral widths from 100 nm down to 20 nm. We have confirmed that when the lateral confinement exists the exciton binding energy increases even when the spatial extent of the effective potential is greater than the exciton in-plane Bohr radius. This enhancement starts being noticeable when the spatial extent of the potential is two, three times larger than the Bohr radius, where the center-of-mass quantization is negligibly small.

Self-organized InGaAs quantum disk lasers

A self-organization phenomenon of a strained InGaAs/AlGaAs system on a GaAs(311)B substrate during metalorganic vapour-phase epitaxial growth is briefly described, and nanoscale confinement lasers with self-organized InGaAs quantum disks as active region are mentioned. Continuous-wave operation of strained InGaAs quantum disk lasers is achieved at room temperature. The threshold current is around 20 mA, which is considerably lower than that of a reference double-quantum-well laser on a GaAs(100) substrate grown side by side. However, the light output vs. the driving current exhibits a pronounced tendency towards saturation compared with that of the (100) quantum well laser.

Self-organization of strained GaInAs microstructures on InP (311) substrates grown by metalorganic vapor-phase epitaxy

The evolution of low-dimensional microstructures in the growth of strained GaInAs/AlInAs and GaInAs/InP heterostructures on planar InP (311)B and (311)A substrates by metalorganic vapor-phase epitaxy is investigated by atomic force microscopy. The surface structures are found to be similar to those previously reported for GaAs (311)B and (311)A substrates. In particular, the appearance of zero-dimensional microstructures in the GaInAs/AlInAs system on InP (311)B substrates is analogous to the self-organizing formation of buried InGaAs quantum disks in the case of GaAs (311)B substrates.

Atomic force microscopy study of strained InGaAs quantum disks self-organizing on GaAs (n11)B substrates

Strained quantum-box structures are naturally formed during the interrupted growth of AlGaAs and InGaAs films on GaAs (n11)B substrates. InGaAs films organize spontaneously into orderly rows of nanoscale disks buried beneath AlGaAs microcrystals. A comparative study by atomic force microscopy shows the alignment and uniformity to be optimum on (311)B surfaces. Both the uniformity and the shape of the microcrystals are not changed for base widths between 220 and 70 nm. Moreover the size and distance can be controlled independently by the In composition and the InGaAs layer thickness, respectively. In contrast, step bunching occurs on GaAs (n11)A substrates to form wirelike microstructures on GaAs (311)A substrates.

Strong photoluminescence emission at room temperature of strained InGaAs quantum disks (200–30 nm diameter) self-organized on GaAs (311)B substrates

We have recently found that quantum-box-like structures are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metalorganic vapor-phase epitaxy into InGaAs islands (disks) buried beneath AlGaAs. The size of the disks is directly controlled by the In content in the range 200–30 nm. Strong photoluminescence (PL) efficiency at room temperature is observed in these strained quantum disks. Even for the 30 nm disk the radiative efficiency is not reduced compared to the reference (100) quantum well. The PL spectra are characterized by narrow linewidth and well resolved exciton resonances in excitation spectroscopy.

Self-organized growth of strained InGaAs quantum disks

LATERAL confinement of electrons or excitons in low-dimensional semiconductor structures (quantum ‘wires’ and ‘boxes’) leads to new electronic properties1, which can be used to improve the performance of optical devices such as semiconductor lasers and nonlinear optical switches2–4. Several state-of-the-art technologies have been applied to fabricate these quantum structures: lateral structure can be defined using high-resolution lithography combined with dry etching5,6, or by crystal growth on masked substrates7,8. Unfortunately, such processes inevitably result in a deterioration of the crystal quality. But recent reports9,10 of self-organized formation of quantum-wire semiconductor structures have attracted considerable interest as a means of overcoming these difficulties. We describe here the self-organized formation of box-like microstructures during the interrupted epitaxial growth of strained InGaAs/AlGaAs multilayer structures on (311)B gallium arsenide substrates. We find that the InGaAs layers organize spontaneously into homogeneous nanoscale disks embedded in an AlGaAs matrix. This phenomenon appears to arise from a complex interplay between the lattice strain, surface energy and surface migration.