Reflectance-difference spectroscopy (RDS) is a recently developed optical technique that allows to monitor chemical and structural changes at a growing semiconductor surface, in-situ and in real-time. This technique was applied recently to organometallic chemical vapor deposition (OMCVD) on a [100] GaAs growth surface. The results show that submonolayer coverage of reacted species can be followed by this technique, which provided unique insights into the microscopic growth mechanisms. The time, temperature and pressure dependences of surface coverage show that OMCVD growth is controlled by two basic processes with distinct activation energies, i.e. reversible chemisorption (at -26 kcal/mole), and decomposition (at 39 kcal/mole) of trimethylgallium (TMG) at surface lattice sites. The importance of reversible chemisorption, which is of an excluded-volume type, due to the large size of the TMG molecule, had been overlooked until now in the literature, where only one activation energy was used to describe growth kinetics.

The self-ordering mechanism of GaAsAlGaAs quantum wires grown by MOCVD on V-grooved substrates was studied using cross-sectional transmission electron microscopy. Structures with barriers composed of different AlGaAs alloys as well as short period AlGaAsGaAs superlattices were investigated for different growth temperatures. The boundaries of these crescent shaped wires can be approximated by hyperbolic profiles. The radius of curvature at the bottom of the grooves increases with increasing growth temperature and decreases with increasing Al mole fraction in the AlGaAs barriers. Deposition of the GaAs wire induces a linear increase of the radius. while subsequent deposition of AlGaAs barriers leads to exponential recovery to its self-limiting value. The self limiting nature of the growth allows the reproducible formation of wires with dimensions in the 10 nm range and less than ±5% variations in shape. Vertically stacked quantum wire arrays formed in this way show quasi-one-dimensional subband structure with 40–50 meV subband separation and strong polarization anisotropy in photoluminescence excitation spectra.

Self-ordering of GaAs/AlGaAs quantum wires grown by organometallic chemical vapor deposition on grooved substrates was studied. The evolution of the surface profile at the corner between two quasi-{111}A planes was evaluated using cross-sectional transmission electron microscopy. The radius of curvature at the corner exhibits a reproducible, self-limiting value of 7.7 ± 0.7 nm, which increases linearly during subsequent growth of GaAs layers and decreases exponentially to its self-limiting value during further growth of AlGaAs layers. This provides the basis for the self-ordering of periodic, vertically stacked arrays of quantum wires with virtually identical shape, size and composition.

AlGaAs/GaAs undoped quantum wires (QWRs) grown by organometallic chemical vapour deposition on V-grooved substrates have been studied by photoluminescence (PL) and photoluminescence excitation (PLE) in zero and finite magnetic fields. From the study of the diamagnetic shift of the peaks as a function of wire width and orientation with respect to the magnetic field, we have deduced one-dimensional effective masses for the lowest subband transitions.

Summary form only given. Epitaxial growth on nonplanar substrates provides a powerful approach for producing high quality laterally-confined semiconductor nanostructures. In this technique, nonplanar surface features prepared using lithography serve as seeds for the formation of self-limiting nonplanar profiles, whose details depend only on the growth parameters. Lower bandgap layers deposited on these nonplanar, self-limiting surfaces can then yield a variety of laterally-confined structures such as quantum wires, quantum dots, vertical quantum wells and vertical quantum wires. This talk reviews recent progress achieved with such nanostructures grown by organometallic chemical vapor deposition, including the understanding of the self-ordering process involved, their electronic and optical properties, and their applications in optoelectronic devices.

Small optical mode size (≤2 μm) is essential for high-performance waveguide-based devices [1] and lasers on III-V semiconductors. High efficiency fiber coupling to small modes, however, requires additional mode-matching elements and submicron alignment tolerances,[2] resulting in costly packages and reliability concems. These problems can be alleviated by monolithically tapering small modes to larger dimensions at chip edges, for which several techniques have been proposed.[2-8] Selective epitaxy, in which locally enhanced epitaxial growth rates are achieved in regions adjacent to dielectric masks, is particularly attractive because of its process simplicity, which requires no epitaxial regrowth nor complicated etch procedures. While selectively grown tapers were previously suggested,[7, 8] actual spot-size tapering using this technique has not been reported. Here we describe selectively grown InGaAsP/lnP tapers with low-loss and threefold spot-size increase.

A far-infrared absorption study of electrons in lightly-doped GaAs/Al0.3Ga0.7As superlattices is presented. Both weakly and strongly coupled superlattices are investigated, and the difference between intersubband transitions and transitions between extended minibands is demonstrated. At low temperatures, the absorption spectra are dominated by donor transitions. The 1s-2p. transition, which is intimately related to the intersubband transition, is observed. All experimental data are compared to an envelope function calculation for the miniband structure and a variational calculation for the donor energies. Excellent agreement between experiment and theory is achieved.

The structure and luminescence properties of GaAs/AlGaAs quantum wires (QWRs) grown by organometallic chemical vapor deposition on nonplanar substrates are described. These crescent-shaped QWRs, as narrow as 10nm in effective width, are formed at the bottom of channels etched into the substrate prior to epitaxy. The wires exhibit high luminescence efficiency and carrier lifetimes comparable to those measured in quantum wells (QWLs) owing to their in situ formation, which minimizes interface defects. Quasi-one dimensional subbands are observed in photoluminescence (PL), PL excitation and amplified spontaneous emission spectra of these QWRs. PL and time-resolved cathodoluminescence studies reveal efficient carrier capture into the QWRs via QWL layers connected to the wires. Application of these structures in efficient QWR lasers is also discussed.

Some of the earliest theoretical speculation, stimulated by the growth of semiconductor superlattices, focused on novel devices based on vertical transport through engineered band structures; Esaki and Tsu promised Bloch oscillators in narrow mini-band systems and Kazarinov and Suris contemplated electrically stimulated intersubband transitions as sources of infrared radiation. Nearly twenty years later these material systems have been perfected, characterized and understood and experiments are emerging that test some of these original concepts for novel submillimetre wave electronics. The authors describe recent experiments on intersubband emission in quantum wells stimulated by resonant tunnelling currents. A critical issue at this time is devising a way to achieve population inversion. Other experiments explore 'saturation' effects in narrow miniband transport. Thermal saturation may be viewed as a precursor to Bloch oscillation if the same effects can be induced with an applied electric field.

Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as ∼10nm have been achieved, corresponding to electron subband separations greater than KBT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated.

Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V-grooved substrates are reported. A model of the crescent-shaped wires yields parabolic QWR potential wells with subbands separated by 21.7, 3.9, and 16.7 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state. Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions. Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model.

We report the first direct observation of reconstructions of semiconductor surfaces in atmospheric pressure (AP) environments. We use reflectance difference spectroscopy (RDS), a surface-sensitive optical probe, to bridge the gap between ultrahigh vacuum (UHV) and AP, and show that the primary surface reconstructions that occur on (001)GaAs surfaces in UHV also occur in the AP environments used with organometallic chemical vapor deposition (OMCVD). Our results justify the applicability of the results of UHV surface science to understanding surfaces under non-UHV environments. Our results also show that during OMCVD growth conditions the surface is terminated with multilayers of As, contrary to generally accepted models. We also apply our newly developed approach, multi-transient spectroscopy (MTS), to the study of atomic layer epitaxy (ALE). Using MTS, we observe surface spectra within a time resolution of 100 ms during actual ALE growth cycles, thus allowing the dynamics of surface reactions to be investigated.

Arsenic dimers and multilayers are shown to exist on (001)GaAs surfaces under atmospheric pressure (AP) organometallic chemical vapor deposition (OMCVD) conditions. We obtained reflectance-difference spectra from surfaces in AP H2 that are equivalent to those obtained from the (2×4) and disordered-c(4×4) reconstructions prepared in ultrahigh vacuum by molecular beam epitaxy. Implications for models of OMCVD growth.

We demonstrate low-loss optical mode-size tapers in InGaAsP/InP optical waveguides fabricated by selective organometallic chemical vapor deposition. The tapers increase the vertical mode size by ≊300%. Taper losses as low as 0.4 dB (on-chip, excluding coupling) were achieved, with 2.6 dB coupling loss to a flat-end, single-mode fiber at 1540 nm wavelength.