Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.

Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.

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Graphene nanoribbons ͑GNRs͒ were fabricated by metal mask lithography and plasma etching. GNRs with width ϳ20 nm show field-effect conductance modulation of ϳ12 at room temperature and Ͼ10 6 at 4.2 K. Conductance quantization due to quantum confinement in low field transport was observed. Landauer formula was utilized to fit the experimental data and excellent agreement was obtained. The extracted subband energy separation was found to deviate from the predicted values of perfect armchair GNRs. Transmission probability is much smaller than unity due to scattering by GNR edge/bulk disorder and impurities, indicating a mean free path ϳ40 nm. High field family I-Vs exhibited current saturation tendency and current density as high as 2 A/mm has been measured at low temperature.

The role of spontaneous and piezoelectric polarization in III-V nitride heterostructure devices is discussed. Problems as well as opportunities in incorporating polarization in abrupt and graded heterojunctions composed of binary, ternary, and quaternary nitrides are outlined.