A. Madhukar

Onset of incoherency and defect introduction in the initial stages of molecular beam epitaxical growth of highly strained InxGa1−xAs on GaAs(100)

Direct evidence for interplanar relaxation in islands at the initial stages of strained epitaxy is presented using molecular beam epitaxically deposited In0.5Ga0.5As on GaAs(100). Concomitant existence of atomic displacements in the substrates to unexpectedly large depths of ∼150 A is found. In incoherent islands, defects are found to be introduced symmetrically near the island edges.

Local atomic and electronic structure of oxide/GaAs and SiO2/Si interfaces using high‐resolution XPS

The chemical structures of thin SiO2 films, thin native oxides of GaAs (20–30 A), and the respective oxide–semiconductor interfaces, have been investigated using high‐resolution x‐ray photoelectron spectroscopy. Depth profiles of these structures have been obtained using both argon ion bombardment and wet chemical etching techniques. The chemical destruction induced by the ion profiling method is shown by direct comparison of these methods for identical samples. Fourier transform data‐reduction methods based on linear prediction with maximum entropy constraints are used to analyze the discrete structure in oxides and substrates. This discrete structure is interpreted by means of a structure‐induced charge‐transfer model (SICT).

In situ, atomic force microscope studies of the evolution of InAs three‐dimensional islands on GaAs(001)

The size evolution of molecular beam epitaxy‐grown strained InAs three‐dimensional (3D) islands on GaAs(001) is examined using in situ ultrahigh vacuum atomic force microscopy. Remarkably, just after the initiation of well‐formed 3D islands at ∼1.57 ML InAs deposition, the lateral size dispersion and average value are found to first increase drastically with the smallest amount (∼0.05 ML) of additional InAs deposition and then decrease and saturate, indicating the onset of a natural tendency for size equalization, including through loss of material from the initially formed largest islands. These observations are found to be consistent with the previously suggested island‐separation dependent influence of the evolving island‐induced substrate strain fields on the adatom migration and incorporation/detachment kinetics that control the evolution of the islands.

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.

High detectivity InAs quantum dot infrared photodetectors

We report a high detectivity of 3×1011 cm Hz1/2/W at 78 K for normal-incidence quantum dot infrared photodetectors with ten layers of undoped InAs/InGaAs/GaAs quantum dot active regions. The high detectivity seen at 1.4 V corresponds to photoresponse peaks at 9.3 and 8.7 μm for positive and negative bias, respectively.

InAs island‐induced‐strain driven adatom migration during GaAs overlayer growth

The impact of the strain fields associated with partially strain relaxed InAs islands on GaAs (100) on the evolution of the growth front profile during subsequent GaAs capping layer growth as a function of the growth temperature is examined via placement of very thin AlGaAs marker layers. Transmission electron microscope studies reveal the presence of strain dominated atom migration away from the islands over dynamically evolving length scales of ∼100–400 A at higher growth temperature whereas at lower growth temperature such an effect is minimal. Anisotropy in the length scale of impact between the [011] and [011] directions is observed. Estimates based upon a suitably adapted formulation of the classical theory of grain growth shows the mass transport to be dominantly strain rather than surface curvature driven.

Independent manipulation of density and size of stress-driven self-assembled quantum dots

A method to independently manipulate the density and size of stress-driven self-assembled quantum dots (QDs) is demonstrated in the InAs/GaAs material system. In bilayer stacks, different InAs deposition amounts in the initial (seed) and second layer are shown to enable independent control, respectively, of the density and the size distribution of the second layer QDs. The approach allows enhancing the average volume and improving the uniformity of InAs QDs, resulting in, respectively, low and room temperature photoluminescence at 1.028 eV (∼1.2 μm) and 0.955 eV (∼1.3 μm) with remarkably narrow linewidths of 25 and 29 meV for 1.74 ML (seed)/3.00 ML InAs stacking.

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.

Punctuated island growth: An approach to examination and control of quantum dot density, size, and shape evolution

The later stages of the evolution of epitaxical island quantum dots are examined systematically for InAs depositions on GaAs(001) following the conventional continuous deposition mode and an approach introduced here called punctuated island growth (PIG). The comparative study provides clear structural and optical evidence for a change in InAs island shape at a self-limiting lateral size, first reached for depositions ∼2 ML. The PIG approach has also allowed realization of the narrowest reported inhomogeneous linewidth of 23 meV for low temperature photoluminescence from a single layer of binary InAs/GaAs quantum dots.

Nanoparticle manipulation by mechanical pushing: underlying phenomena and real-time monitoring

Experimental results that provide new insights into nanomanipulation phenomena are presented. Reliable and accurate positioning of colloidal nanoparticles on a surface is achieved by pushing them with the tip of an atomic force microscope under control of software that compensates for instrument errors. Mechanical pushing operations can be monitored in real time by acquiring simultaneously the cantilever deflection and the feedback signal (cantilever non-contact vibration amplitude). Understanding of the underlying phenomena and real-time monitoring of the operations are important for the design of strategies and control software to manipulate nanoparticles automatically. Manipulation by pushing can be accomplished in a variety of environments and materials. The resulting patterns of nanoparticles have many potential applications, from high-density data storage to single-electron electronics, and prototyping and fabrication of nanoelectromechanical systems.