M. J. Seong

Band gap of GaAs1−xBix, 0<x<3.6%

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

Size-dependent Raman study of InP quantum dots

Raman spectrum of a quantum dot (QD) is characterized by transverse (TO) and longitudinal (LO) optical modes as well as surface optical modes, occurring between the TO and LO modes. We have studied in detail the size-dependence of the Raman spectrum of InP QD of diameter larger than 35 A. The LO phonon frequency decreases while the TO phonon frequency increases with decreasing QD size. The linewidth of the LO phonon broadens and the broadening becomes increasingly asymmetrical towards the low frequency side as the QD size decreases. By analyzing the Raman intensity ratio of the LO phonon to its overtone, we find that the electron-phonon coupling decreases with decreasing QD size.

Composition dependence of Raman intensity of the nitrogen localized vibrational mode in GaAs1-xNx

We have investigated the nitrogen-localized vibrational mode (LVM) Raman intensity and its frequency (ωLVM) as a function of nitrogen concentration in GaAs1−xNx (x⩽0.04) at 300 K. The normalized Raman intensity of nitrogen LVM with respect to that of GaAs–LO phonon as well as ωLVM exhibits a remarkable linear dependence on the nitrogen concentration for x⩽0.03. This provides excellent calibration to determine nitrogen composition not only in the ternary alloy GaAs1−xNx but also in the quaternaries such as Ga1−yInyAs1−xNx.

Γ–L–X mixed symmetry of nitrogen-induced states in GaAs1−xNx probed by resonant Raman scattering

A resonant Raman scattering study near the nitrogen-induced E+ state in GaAs1−xNx at 80 K with special emphasis on all the zone-boundary phonons is used to investigate the full symmetry of the E+ state. We have observed that various phonons at the L- and X-zone boundaries not only emerge as strong and sharp Raman features for excitations near the E+ transition but also exhibit the same intensity resonance enhancement as observed for the zone center phonons, longitudinal-optical (Γ) and transverse-optical (Γ). Our data provide strong evidence of significant L and X components in the wave function of the nitrogen-induced E+ state in GaAs1−xNx.

GaInP2 overgrowth and passivation of colloidal InP nanocrystals using metalorganic chemical vapor deposition

We have used metalorganic chemical vapor deposition to deposit thin GaInP2 passivating films on both isolated and close-packed arrays of colloidal InP/GaInP2 core-shell nanocrystals. Conformal GaInP2 layers of 10–20 nm were grown on the nanocrystals after organic capping molecule removal by a thermal annealing treatment. We show that the InP nanocrystals retain their crystallinity, shape and luminescence efficiency after being exposed to growth temperatures of 600 °C. The GaInP2 nanocrystal composite showed strong photoluminescences indicating effective passivation of surface states. In close-packed nanocrystal arrays, the emission band is redshifted compared to films of isolated nanocrystals indicating electron coupling between dots embedded in GaInP2.

Photogenerated plasmons in GaAs1- xBix

Light scattering measurements in the dilute isoelectronically doped alloy GaAs1−xBix reveal a large free electron population photogenerated by continuous-wave laser excitation at low temperature. Low-temperature time-resolved photoluminescence of the bismuth related near-band-gap states show carrier lifetimes of several nanoseconds. The authors attribute this to trapping of photoexcited holes at bismuth pair or cluster states located near the valence band maximum.

Phonons in Ordered Semiconductor Alloys

The lattice dynamics of ordered semiconductor alloys is described with respect to studies based on Raman and infrared spectroscopy. The Raman and infrared spectra reflect the structural change and symmetry of these alloys that are induced by ordering. Micro-Raman techniques using several different scattering geometry enable symmetry assignments of all the phonon modes observed in ordered GaInP2. The dominance of the long-range electrostatic forces with respect to the short-range interatomic forces is discussed with respect to the lattice-dynamics of this alloy.