M. Reason

Observation of surface-avoiding waves: a new class of extended states in periodic media.

Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal the existence of an unusually long-lived acoustic mode at approximately 0.6 THz which couples weakly to the environment by evading the sample boundaries. Classical as well as quantum states that steer clear of surfaces are generally shown to occur in the spectrum of periodic structures, for most boundary conditions. These surface-avoiding waves are associated with frequencies outside forbidden gaps and wave vectors in the vicinity of the center and edge of the Brillouin zone. Possible consequences for surface science and resonant-cavity applications are discussed.

Influence of N on the electronic properties of GaAsN alloy films and heterostructures

We have investigated the effects of N on the electronic properties of Si-doped GaAs1−xNx alloy films and AlGaAs∕GaAsN modulation-doped heterostructures. For bulk-like alloy films, the electron mobility is independent of free carrier concentration and arsenic species, and decreases with increasing N composition. Thus, N-related defects are the main source of scattering in the dilute nitride alloys. For AlGaAs∕GaAsN heterostructures, gated and illuminated magnetoresistance measurements reveal a two-dimensional electron gas mobility which increases with carrier concentration to a constant value. Thus, in contrast to the long-range ionized scattering sources which are dominant in N-free heterostructures, N-induced neutral scattering sources are the dominant source of scattering in AlGaAs∕GaAsN heterostructures. Finally, a decrease in free carrier concentration with increasing N composition is apparent for bulk-like films, while the free carrier concentration is independent of N composition in modulation-doped...

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....

Ultrafast optical generation and remote detection of terahertz sound using semiconductor superlattices

The authors introduce an all-optical approach to study the propagation of high frequency acoustic phonons in which the generation and detection involves two spatially separated superlattices ∼1μm apart. Propagating modes of frequencies up to ∼1THz escape from the superlattice where they are generated and reach the second superlattice where they are detected and spectrally resolved using pump-probe techniques. The measured frequency spectrum reveals finite size effects, which can be accounted for by a continuum elastic model.

Influence of Si–N complexes on the electronic properties of GaAsN alloys

We have investigated the influence of Si–N complexes on the electronic properties of GaAsN alloys. The presence of Si–N complexes is suggested by a decrease in carrier concentration, n, with increasing N-composition, observed in GaAsN:Si films but not in modulation-doped heterostructures. In addition, for GaAsN:Te (GaAsN:Si), n increases substantially (minimally) with annealing-T, suggesting a competition between annealing-induced Si–N complex formation and a reduced concentration of N-related traps. Since Si–N complex formation is enhanced for GaAsN:Si growth with the (2×4) reconstruction, which has limited group V sites for As–N exchange, the (Si–N)As interstitial pair is identified as the dominant Si–N complex.

Localized surface-avoiding modes: a new approach to resonant cavities

We observe a new kind of confined acoustic mode in a superlattice slab. This mode avoids the surfaces and exhibits a maximum at the center of the superlattice which behaves as a resonant cavity.

Generation and Remote Detection of Coherent Folded Acoustic Phonons

Coherent acoustic modes with frequencies in the THz range were generated in a superlattice using femtosecond optical pulses and detected, with little attenuation, after traversing 1.2 μm of bulk GaAs.

Controlled fabrication of electrodes with a few nanometer spacing by selective etching of a GaAs/AlGaAs heterostructure

We present a nanofabrication technique to make electrodes with a separation below 10 nm. This technique takes advantage of the atomic precision of molecular beam epitaxial growth. The key step in the fabrication is the formation of a nanotemplate by selective etching on the cleaved surface of a GaAs/AlGaAs heterostructure. Electrodes with a spacing less than 10 nm are formed by angle evaporation of Au on such a template. The technique can also be extended to fabricate three electrodes that are all less than 10 nm from each other.

Stress evolution and nitrogen incorporation in GaAsN films

We have investigated stress evolution in GaAsN films, using a combination of in situ and ex situ measurements. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions suggests significant composition dependent incorporation of N into nonsubstitutional sites.