A. Konkar

Nature of strained InAs three‐dimensional island formation and distribution on GaAs(100)

The substrate temperature and arsenic pressure dependence of the density of InAs three‐dimensional (3D) islands formed on GaAs(100) is found to exhibit a behavior that cannot be reconciled within the currently popular view of MBE growth. Rather, either an arsenic coverage induced strain enhanced In migration or strain dependent arsenic incorporation at islands, or both, appear to be operative. Plan‐view and cross‐sectional transmission electron microscopy, including the nature of the Moire fringes, are used to obtain cluster size distribution and demarcation between size regime for coherent versus incoherent islands. The results point to the possibility of realizing a regular array of quantum dots made of coherently strained 3D islands of uniform size via growth on prepatterned substrates.

Realization of optically active strained InAs island quantum boxes on GaAs(100) via molecular beam epitaxy and the role of island induced strain fields

Abstract We report here the realization of strained InAs three-dimensional islands on GaAs(100) with optical characteristics that reveal lateral quantum confinement (i.e. quantum box behavior). The importance of the cap layer growth conditions and methodology in achieving optically active InAs islands and the existence, range, and impact of island-induced strain fields on the cap layer growth are uncovered via marker layer experiments. Strong optical emission from the InAs islands is observed in correlation with the transmission electron microscope (TEM) observation of uniform coherent islands under optimized growth conditions. Photoluminescence excitation (PLE) spectroscopy reveals the presence of the energy transitions due to the three-dimensional electronic confinement in such InAs islands. The InAs islands buried under the GaAs were found to be quite stable upon annealing to 100°C higher than the growth temperature.

Stress-engineered spatially selective self-assembly of strained InAs quantum dots on nonplanar patterned GaAs(001) substrates

The lattice-mismatch stress-induced two-dimensional-to-three-dimensional morphology change is combined with interfacet adatom migration to selectively assemble parallel chains of InAs islands on top of [110] oriented stripe mesas of sub-100-nm widths on GaAs(001) substrates. On such mesa stripes, prepared in situ via size-reducing epitaxy, deposition of InAs amounts subcritical for island formation on planar GaAs (001) is shown to allow self-assembly of three, two, and single chains of InAs three-dimensional island quantum dots selectively on the stripe mesa tops for widths decreasing from 100 nm down to 30 nm.

Mechanisms of strained island formation in molecular‐beam epitaxy of InAs on GaAs(100)

Results of a systematic examination of InAs island formation on GaAs(100) as a function of deposition conditions and thickness are presented. A non‐Arrhenius dependence of the island density on substrate temperature and a decrease in the island density with increasing As4 pressure at lower substrate temperatures is observed, indicating that currently popular frameworks of compound semiconductor molecular‐beam epitaxical growth and island formation mechanism(s) need to be enlarged. Plan‐view transmission electron microscopy (TEM) and use of the behavior of Moire fringes provides the island size distribution and demarcation between coherent and incoherent islands. Photoluminescence (PL) behavior is shown to vary significantly with the growth conditions and to correlate to the attendant structural nature revealed by TEM. The issue of lateral quantum confinement (i.e., three‐dimensional islands as quantum boxes) is shown to be subtle and complex, calling for caution in interpreting PL behavior. The results su...

Focused ion beam assisted chemically etched mesas on GaAs(001) and the nature of subsequent molecular beam epitaxial growth

Focused ion beam assisted chemical etching is examined as an ultrahigh vacuum compatible in situ direct-write approach to patterning substrates to create mesas with arbitrary shapes (i.e., sidewall angles) independent of crystallographic orientation. Ga+ ion beam assisted Cl2 etching of GaAs(001) is employed as a vehicle. A phenomenological model is used to predict mesa profiles as a function of ion beam conditions. Mesas created under various conditions are characterized via atomic force microscopy and good agreement is found between the measured and predicted profiles. Examination of the growth profile evolution on such mesas with as-patterned sidewall angles between ∼10° and ∼60° reveal that mesa top size reduction suitable for nanostructure fabrication on stripes oriented along the [110] and [100] directions occurs only for as-patterned mesa sidewall angles greater than ∼19° and ∼45°, respectively, which are the angles subtended by the {114} and {101} facets that emerge during growth and cause mesa t...

Carrier relaxation and recombination in GaAs/AlGaAs quantum heterostructures and nanostructures probed with time-resolved cathodoluminescence

We have examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL). In the cases of QWRs and QBs, the nanostructures were grown via a size-reducing growth approach on pre-patterned GaAs(001) substrates composed of stripes and mesas, respectively. The growth involved deposition of multiple GaAs/AlGaAs layers in order to establish both structural and optical markers which facilitated the identification of important features in transmission electron microscopy (TEM) and CL experiments. In TEM measurements, the lateral dimensions of the top-most GaAs layers in typical stripe and mesa structures comprising the QWRs and QBs delineate GaAs regions expected to exhibit 2D and 3D quantum confinement effects, respectively. Time-delayed CL spectra of all three structures reveal that the initial capture of carriers in the active regions occurs on a time scale less than the temporal resolution of the CL system...

In-situ fabrication of three-dimensionally confined GaAs and InAs volumes via growth on non-planar patterned GaAs(001) substrates

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 〈100〉 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving ∼ {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known ∼ 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, > 5 ML for mesa size ∼ 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.

Realization of three‐dimensionally confined structures via one‐step in situ molecular beam epitaxy on appropriately patterned GaAs(111)B and GaAs(001)

The realization of three‐dimensionally confined GaAs/AlGaAs structures on GaAs (111)B and GaAs (001) substrates via one step in situ molecular beam epitaxy is reported. Growth is carried out on nonplanar patterned substrates with crystallographically equivalent mesa top edges. Equivalent side facets evolve during growth and completely surround the mesa top. Adatom migration from these facets to the mesa top result in shrinkage of the mesa top area leading to mesa pinch‐off. Scanning and transmission electron microscopy provide evidence for the realization of structures with lateral linear dimensions ≲500 A. Cathodoluminescence images from the (111)B structures attest to their high optical quality.

Fabrication of strained InAs island ensembles on nonplanar patterned GaAs(001) substrates

We report on the behavior of InAs deposited on nonplanar GaAs(001) substrates patterned with ≲0.5 μm wide stripe mesas oriented along the [110] and 〈100〉 directions and with square mesas with a lateral size of ≳0.5 μm oriented along the 〈100〉 directions. Interfacet migration of In from the sidewalls to the mesa top leads to an enhanced InAs island density on the stripe as well as square mesa tops compared to that on the planar unpatterned region. Using such interfacet migration and InAs deposition amount less than needed for island formation on planar GaAs(001), we demonstrate complete selectivity in the positioning of InAs islands on the [110] oriented stripe mesas of widths ≲100 nm, with islands forming exclusively on the mesa tops. These islands arrange in mesa-width-dependent parallel chains. They show photoluminescence (PL) comparable to that from the islands on the planar substrates. The polarization dependence of the PL suggests the presence of anisotropy in strain fields and potential elongation...

Time‐resolved cathodoluminescence study of carrier relaxation in GaAs/AlGaAs layers grown on a patterned GaAs(001) substrate

We have examined the kinetics of carrier relaxation in three‐dimensionally confined GaAs/AlGaAs layers obtained by growth on prepatterned GaAs(001) with time‐resolved cathodoluminescence (CL). Time‐delayed CL spectra at 87 K reveal that (i) relaxation of hot carriers into the largest 3D confined regions occurs on a time scale of a few hundred ps during the onset of luminescence, and (ii) the luminescence decay time also increases for these larger confined regions, owing to thermal reemission from QWs, diffusion across AlxGa1−xAs barriers, and carrier feeding from surrounding thinner QWs.