W. Ye

Mechanisms of nitrogen incorporation in GaAsN alloys

We have investigated nitrogen incorporation mechanisms in dilute nitride GaAsN alloys grown by plasma-assisted molecular-beam epitaxy. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions reveals significant composition-dependent incorporation of N into nonsubstitutional sites, presumably as either N–N or N–As split interstitials. Furthermore, we identify the (2×1) reconstruction as the surface structure which leads to the highest substitutional N incorporation, likely due to the high number of group V sites per unit area available for N–As surface exchange.We have investigated nitrogen incorporation mechanisms in dilute nitride GaAsN alloys grown by plasma-assisted molecular-beam epitaxy. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions reveals significant composition-dependent incorporation of N into nonsubstitutional sites, presumably as either N–N or N–As split interstitials. Furthermore, we identify the (2×1) reconstruction as the surface structure which leads to the highest substitutional N incorporation, likely due to the high number of group V sites per unit area available for N–As surface exchange.

Evolution of structural and optical properties of ion-beam synthesized GaAsN nanostructures

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.

Formation and coarsening of Ga droplets on focused-ion-beam irradiated GaAs surfaces

We have investigated the formation and coarsening of Ga droplets on focused-ion-beam (FIB) irradiated GaAs surfaces. To separately examine formation and coarsening, Ga droplets were fabricated by Ga+ FIB irradiation of GaAs substrates with and without pre-patterned holes. We determined the droplet growth rate and size distribution as a function of FIB energy following irradiation. The data suggest a droplet formation mechanism that involves Ga precipitation from a Ga-rich layer, followed by droplet coarsening via a combination of diffusion and Ostwald ripening or coalescence via droplet migration (dynamic coalescence).

Control of InAs∕GaAs quantum dot density and alignment using modified buffer layers

We have investigated the patterning effects of GaAs buffers during the growth of InAs∕GaAs quantum dot (QD) superlattices (SLs). One-, five-, and ten-period QD SLs were deposited on GaAs buffer layers grown at 580°C and/or 500°C, with various annealing steps. High-temperature-grown buffers consist of relatively flat surfaces, while low-temperature-grown buffers contain “mound-like” features elongated along the [11¯0] direction. Isotropic distributions of QDs are observed for QD growth on flat buffers. Interestingly, QD alignment along the [11¯0] direction is observed for QD SL growth on buffers containing mounds. This anisotropic QD alignment is enhanced as the number of QD SLs increases and is dependent on the density of mounds. For flat buffers, the density of QDs decreases with stacking, consistent with the model of Tersoff [J. Tersoff, C. Teichert, and M. G. Lagally, Phys. Rev. Lett. 76, 1675 (1996)]. However, for buffers containing mounds, this effect is compensated by an increase in QD density. We p...

Mechanisms of GaAsN growth: Surface and step-edge diffusion

We have investigated the mechanisms of GaAs1−xNx film growth by plasma-assisted molecular-beam epitaxy. A comparison of in situ reflection high-energy electron diffraction and scanning tunneling microscopy (STM), with ex situ atomic force microscopy, reveals a temperature-dependent interplay between surface and step-edge diffusion. At low temperatures, layer-by-layer growth is observed, presumably due to limited adatom surface mobility. As the temperature increases, the interplay between surface and step-edge diffusion leads to multilayer growth. For sufficiently high temperatures, adatoms overcome the step-edge diffusion barrier, resulting in layer-by-layer growth once again. The temperature range for multilayer growth is influenced by the Ga flux and may be narrowed by using As2. Using in situ STM, we quantified the activation energies for Ga surface diffusion, Ed, and step-edge diffusion, Ee, during layer-by-layer GaAsN growth. We estimate Ed=0.75 and 0.96 eV for growth using As4 and As2, respectively....

Stress evolution in GaAsN alloy films

We have investigated stress evolution in dilute nitride GaAs1−xNx alloy films grown by plasma-assisted molecular-beam epitaxy. For coherently strained films (x 2.5%, in situ wafer curvature measurements reveal a signature for stress relaxation. Atomic force microscopy and transmission electron microscopy measurements indicate that stress relaxation occurs by a combination of elastic relaxation via island formation and plastic relaxation associated with the formation of stacking faults.

Matrix-seeded growth of nitride semiconductor nanostructures using ion beams

We have examined the matrix-seeded growth of narrow-gap nitride nanostructures in nitrogen ion implanted GaAs and InAs. Low-energy implantation followed by rapid thermal annealing (RTA) results in the formation of 2–3 nm sized amorphous precipitates in a crystalline matrix. On the other hand, high-energy implantation results in an amorphous layer, with or without crystalline remnants. When the ion-beam-synthesized amorphous matrix is a continuous amorphous layer, subsequent RTA leads to the formation of 4–5 nm zinc blende (ZB)-GaN-rich crystallites in an amorphous matrix. When this matrix contains crystalline remnants, subsequent RTA leads to the formation of 2–4 nm ZB-GaN-rich crystallites within the amorphous regions. These results suggest that the matrix plays an important role in the nucleation and growth of narrow-gap nitride nanostructures, and that matrix-seeded growth may provide an opportunity to control the structure and properties of the nanostructures.

Formation and blistering of GaAsN nanostructure layers

We report the formation and blistering of GaAsN nanostructure layers induced by nitrogen ion implantation into GaAs followed by high-temperature rapid thermal annealing. The GaAsN nanostructure layers consist of zincblende GaN-rich GaAsN nanostructures in an amorphous matrix, apparently formed near the depth of maximum ion damage. Cross-sectional transmission electron microscopy reveals a series of nanometer-sized cavities near the depth of maximum nitrogen concentration, suggesting the formation of nitrogen gas bubbles during annealing. The nitrogen bubbles coalesce and push away the GaAsN nanostructure layer, forming micrometer-sized blisters. The simultaneous formation and blistering of GaAsN nanostructure layers may provide an opportunity for the integration of the nanostructure layers with a variety of substrates.

Thermoelectric properties of quantum dot chains

We have studied the thermoelectric properties of quantum dot (QD) chain nanocomposites in which the QDs are aligned to form chains. In both the InAs/GaAs and Ge/Si systems, we use a constant relaxation time approximation to calculate and compare the electrical conductivity, Seebeck coefficient, and thermoelectric power factor for QD chains, three-dimensional ordered QD arrays, random QD arrays, and bulk GaAs or Si. Our calculations suggest that the incorporation of QD chains into a matrix increases its thermoelectric power factor by a factor of 3 (1.5) for the InAs/GaAs (Ge/Si) system.

Formation and evolution of ripples on ion-irradiated semiconductor surfaces

We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga+ FIB irradiation of InSb, we tuned the local beam incidence angle (θeff) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θeff induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θeff leads to island formation on the ripple crests, followed by nanorod (NR) growth. This ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.