In-situ, ultra high vacuum combined scanning tunneling microscope/atomic force microscope (STM/AFM) studies were undertaken to examine the initiation of 3D InAs islands on GaAs (100) and their density and size distribution as a function of growth conditions. A decreasing island density with increasing As4 pressure is observed and points to the significance of strain in affecting In migration and As4 incorporation. A stack of InAs islands separated by GaAs spacer layers exhibit a vertically self-organized growth. Through analysis of a phenomenological model, this is shown to be a consequence of a directional In adatom migration caused by the islandinduced nonuniform strain fields.

The two-dimensional (2D) to three-dimensional (3D) morphology change in the highly strained growth of InAs on GaAs(001) is examined via in-situ, ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and contact-mode atomic force microscopy (C-AFM). The formation of 3D InAs islands (∼2-4nm high) at an InAs delivery, θ∼1.57ML is found to bepreceded by the appearance of small quasi-3D clusters (∼0.6-1.2nm high). The 2D to 3D transition is found to occur over a range of θ from ∼1.45ML to 1.74ML, with a varying and gradual mass transfer from 2D to 3D features with increasing θ. The InAs 3D islands are also examined in this study using non-contact AFM (NC-AFM) in order to assess the usefulness of this technique for imaging 3D features. Unlike the constancy observed in the C-AFM images, the NC-AFM images exhibit a marked imaging condition dependence. The variability observed in the NC-AFM images is qualitatively compared to the outcome of the simplest, force-gradient model of NC-AFM in order to extract a guideline for NC-AFM imaging of 3D features.

A dynamic model for the simulation of thermal chlorine etching of gallium arsenide is developed. The primary motivation for the development of the simulation is the design and testing of real time adaptive feedback controllers which rely upon in-situ optical measurements of etch depth obtained via spectroscopic ellipsometry. The basis for the model is an empirically derived relationship between etch rate, chlorine pressure, and substrate temperature. The chlorine pressure in the chamber is regulated by a throttle valve which determines the effective pumping rate of a turbo-molecular pump which is used to evacuate chlorine pressure dynamics and a second-order damped harmonic oscillator with zero-order hold valve position command inputs is used to model the dynamics of the throttle valve. An output equation is used to model the fact that ellipsometry based etch depth and chamber pressure can be observed at discrete time intervals. Unmeasurable parameters which appear in the model are identified, and the model is validated using experimental data. An adaptive linear-quadratic Gaussian based controller based on our model which forces etching to precede at a desired rate while estimating the often difficult to measure substrate temperature is designed and then tested using our simulation.

A dynamic model for the simulation of thermal chlorine etching of gallium arsenide is developed. The primary motivation for the development of the model is the design and testing of real time feedback controllers which rely upon in-situ optical measurements of etch depth obtained via spectroscopic ellipsometry. Unmeasurable parameters which appear in the model are identified, and the model is validated using experimental data. A linear-quadratic controller based on our model is designed and tested.

We report on studies of excitation transfer processes in vertically self-organized pairs of unequal-sized quantum dots (QDs), created in InAs/GaAs bilayers having differing InAs deposition amounts in the first (seed) and subsequent layer. The former and latter enable independent control, respectively, of the density and the size distribution of the second layer QDs. This approach allows us to enhance the average volume and improve the uniformity of InAs QDs, resulting in low-temperature photoluminescence at 1.028 eV with a linewidth of 25 meV for 1.74 ML (seed)/3.00 ML InAs stacking. The optical properties of the formed pairs of unequal-sized QDs with clearly discernible ground-state transition energy depend on the spacer thickness and composition. Photoluminescence results provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer in such asymmetric QD pairs. Transfer times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness.

Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap layers grown by migration enhanced epitaxy. Photoluminescence excitation (PLE) spectra reveal the excited state spectrum as a function of size for self-assembled InAs QD's in multilayered samples with 36-ML spacers. The observed energy shifts and splittings are consistent with those of hole states numerically calculated for pyramidal QD's supporting assignment to the transition between the electron ground |000〉 and the |001〉 excited hole state. The optical results suggest the island shape uniformity to improve in multilayered samples, which is attributed to the contribution of the buried islands to the surface strain altering the island formation kinetics and energetics that also underlie vertical self-organization. Time-resolved photoluminescence (TRPL) results yield a lifetime of 40 ps for the first excited |001〉 hole state, attributed to multiphonon relaxation processes bridging the approximately 100 meV level separation, and ground-state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, showing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE spectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing recombination processes.

We have examined the optical properties of self-assembled InAs quantum dots (QDs) grown on prepatterned GaAs(001) substrates with polarization sensitive and time-resolved cathodoluminescence (CL) imaging and spectroscopy techniques. The InAs QDs were formed using a novel application in self-assembled molecular beam epitaxial growth, which entailed the growth of InAs on preformed [11̄0]-oriented stripe mesas. Interfacet In adatom migration occurred along the stripe side-walls during growth, enabling the selective formation of linear arrays of InAs QDs on the stripe mesas. The total InAs deposition needed to induce the two-dimensional to three-dimensional morphology change on the stripes is less than that required to initiate QD formation on the unpatterned substrates. The QDs formed on the mesa top were found with a luminescence distribution redshifted relative to QDs in the valley region, indicating that QDs with a larger average size were formed on the mesa top. The lower density of QDs in the valley region led to a weaker emission and sharper δ-like transitions at lower beam currents, relative to emission from QDs on the mesa. CL imaging was employed to study the spatial distribution of luminescence and identified the presence of relatively small QDs situated near the edges of the valley region along the lower surface of the stripe edges. An excitation- and energy-dependent polarization anisotropy relative to the 〈110〉 directions was observed in CL emission. A polarization anisotropy reversal was found between CL from QDs on the mesa and in the valley regions, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical properties. The CL was examined as a function of temperature to evaluate the thermal re-emission of carriers and the associated activation energies. The carrier relaxation kinetics were studied with time-resolved CL to measure differences in the CL onset and decay rates for QDs in the valley and mesa regions.

The nature of the two-dimensional (2D) to three-dimensional (3D) morphological transition in the highly strained epitaxy of InAs on GaAs(001) is discussed, based on in situ scanning tunneling microscope and atomic force microscope studies, combined with photoluminescence (PL) and PL excitation spectroscopy results. A re-entrant 2D–3D morphology change is observed, in which quasi-3D (Q3D) clusters appear, disappear, and reappear well in advance of the formation of 3D islands. We suggest that the Q3D clusters may act as a kinetic pathway to 3D island formation, spreading out the 2D–3D transition over a delivery range of ∼0.3 monolayers. Large (>∼50 nm wide) 2D clusters sitting on top of the wetting layer (WL) undergo morphological changes with increasing strain and ultimately lose their material to 3D islands. Small (<20 nm) 2D clusters decorating the WL appear to contribute to the commonly observed redshift of the InAs WL PL peak prior to 3D island formation. A diffusion-limited formation of 3D islands for our growth conditions is indicated by the behavior of the 3D island density as a function of growth rate.

We report on the behavior of InAs deposited on nonplanar GaAs(001) substrates patterned with ≲0.5 μm wide stripe mesas oriented along the [11̄0] 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 [11̄0] 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 of islands in the [11̄0] direction. The significance of the stripe mesa edge orientation to the island formation is revealed by vastly different island densities and InAs morphology on the [11̄0] oriented versus 〈100〉 oriented stripe mesas. These differences reflect difference in migration from the different sidefacets that surround the (001) mesa top.

We have examined the optical properties of self-assembled InAs quantum dots (QDs) with polarization sensitive and time-resolved cathodoluminescence (CL) techniques. The InAs QDs were formed via self-assembly during molecular beam epitaxial growth of InAs on unpatterned GaAs(001). CL spectra exhibited a two-component line shape whose linewidth, intensity, and peak positions were found to be temperature and excitation dependent. The two components are found to be consistent with state filling of the QDs, resulting in emission involving ground state and excited state excitonic transitions. The luminescence intensities and lineshapes of the QD and wetting layer (WL) excitonic transitions were analyzed with constant excitation and time-resolved CL for various temperatures and excitation levels to study the thermal activation, re-emission, and recombination kinetics of carriers. Thermal quenching of the QD ground state and excited state components in the 105–175 K range is correlated with a rise in the WL emission, showing that the WL carrier distribution is fed partially by thermal reemission of carriers from the QDs. A more rapid thermal quenching of the QD and WL excitonic emissions in the 185–300 K range is consistent with thermal reemission of carriers into the GaAs matrix. Time delayed CL spectra show that carriers in the excited states rapidly feed the ground state during the decay phase of the luminescence, further confirming that the two components are derived from the same QD. The CL decay time for the QD luminescence was found to be wavelength and temperature dependent, owing to thermal reemission into the WL. The CL decay times ranged from 0.1 to 2.0 ns. A polarization anisotropy was found in CL spectroscopy, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical emission.

We report on a remarkable image contrast reversal in noncontact atomic force microscope (NC-AFM) imaging of nanosized three-dimensional (3D) particles. We show that the image contrast of such 3D particles can switch from positive to negative as a function of NC-AFM imaging conditions and this occurs during, both, in situ ultrahigh vacuum imaging and imaging in air. Our results indicate that the contrast reversal can arise from a tip-sample interaction force-gradient-dependent instability of the NC-AFM feedback loop. Exploiting the above instability to induce selective tip-sample contact, we propose and demonstrate a protocol for the controlled nanomanipulation of 5 nm diameter gold particles, in air at room temperature using the NC-AFM. The contrast reversal phenomenon is proposed to be universal, suggesting its potential applicability to nanomanipulation in a variety of materials systems.

Excitation transfer processes within self-organized quantum dot (QD) pairs in bilayer InAs/GaAs QD samples are investigated. QDs in samples with a 1.74-ML InAs seed layer and a 2.00 ML InAs second layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-state transition energy. Photoluminescence (PL) and PL excitation results for such asymmetric QD pairs provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer. Variations in the optical behavior as a function of the spacer thickness and composition are attributed to the barrier-dependent tunnel probability. Tunneling times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness. [S0163-1829(98)52640-9].

Some remarkable recent results of the in situ scanning tunneling microscope (STM) and atomic force microscope (AFM) studies of InAs coherent 3D island initiation and evolution on GaAs(001) are presented in the larger context of the field of strained epitaxy. The role of nano and meso scale mesas in manipulating stress/strain is illustrated through examples of growths on in situ, purely growth control prepared stripe and square mesas with linear dimensions as small as ∼ 40 nm. A set of basic kinetic processes and their strain dependence is identified and suggested to form a good core that has the potential for providing a unified framework for understanding strained epitaxy ranging from low misfits to high misfits.

Excited states and energy relaxation process are studied for self-organized InAs/GaAs quantum dots (QDs). Depending on the sample, excited state transitions or multi-LO-phonon resonances are found in photoluminescence excitation (PLE) spectra, revealing the size-dependent excited state splitting or the carrier relaxation mechanism, respectively. Time-resolved photoluminescence (PL) results indicate sample-dependent non-radiative recombination, leading to a model for the observed PLE behavior, analogous to hot carrier relaxation in higher dimensional systems. Carrier relaxation in the self-organized InAs/GaAs QDs proceeds by multi-10-phonon scattering on a 40 ps time scale, much shorter than radiative (> 500 ps) and non-radiative (> 100 ps) recombination times, accounting for the absence of a phonon bottleneck effect in PL spectra.

In-situ , real-time, spectroscopic ellipsometry (SE) is utilized to study thermal chlorine etching of GaAs in an all ultra-high-vacuum interconnected growth and etching system. In the low temperature (between:_40°C and:_120°C) range, the etch rate is found to exhibit an Arrhenius dependence on substrate temperature with an activation energy of 11.6Kcal/mole and to be proportional to essentially the square root of the chlorine pressure. An SE feedback based real-time etch process control algorithm is developed and successfully implemented on the basis of the above noted input - output relation derived from the experimental data base.

Using in-situ, ultrahigh vacuum scanning tunneling and atomic force microscopy and ex-situ photoluminescence (PL) and PL excitation we show that the growth of highly strained InAs on GaAs(001) proceeds via a re-entrant behavior of the 2D to 3D morphology change. Quasi-3D clusters of heights 0.6–1.2 nm first appear at an InAs delivery (θ) of ∼ 1.25 ML, only to disappear for θ ∼ 1.3 ML and reappear again at θ ∼ 1.45 ML, prior to 3D island initiation at ∼ 1.57 ML. With increasing θ, an observed tendency of the 3D islands towards lateral size equalization is shown to be accompanied by mass exchange between the 2D and 3D surface features. Molecular dynamics simulations reveal a decreasing binding energy for atoms at island edges as a function of increasing island size and island—island interaction, thereby suggesting a role for atom detachment from larger islands in the size equalization process.

We have examined carrier thermalization, recombination, and band filling in GaAs/AlGaAs quantum boxes with low-temperature cathodoluminescence (CL). The temperature dependence of the quantum box CL intensity for T⩽ 90 K exhibits an Arrhenius behavior, as a result of carrier thermalization between the quantum box and surrounding barrier regions. The width of the quantum box luminescence is found to increase rapidly with an increasing excitation density and reveals an enhanced phase-space and real-space filling, in comparison to the behavior observed for quantum wells.

The two-dimensional (2D) to three-dimensional (3D) transition in highly strained growth of InAs of GaAs(001) is investigated using in situ scanning tunneling microscopy and photoluminescence spectroscopy. Remarkably, InAs structural features up to five monolayers (ML) high appear at similar to 1.25 ML: disappear, and reappear prior to the onset of well-developed 3D islands at 1.57 ML, thus manifesting a hitherto unrecognized reentrant behavior in the formation of 3D islands. The results provide new insights into the long-standing problem of the kinetic aspects of 2D to 3D morphology change not embodied in the widely encountered Stranski-Krastanow growth mode.

We report on (a) the effect of growth interruption on the growth profile evolution in growth on non-planar patterned mesa tops via substrate-encoded size-reducing epitaxy (SESRE) and (b) the optical behavior of isolated 3D-confined GaAs volumes as well as 3D-confined GaAs volumes coupled with 1D-confined quantum wells (QWs) fabricated by SESRE. Steady-state excitation and time-resolved cathodoluminescence (CL) are used for these optical studies.