By employing photoluminescence(PL) spectroscopy under dual-wavelength optical excitation, we uncover the presence of deep-level hole traps in Ga(In)NP alloys grown by molecular beam epitaxy(MBE). The energy level positions of the traps are determined to be at 0.56 eV and 0.78 eV above the top of the valance band. We show that photo-excitation of the holes from the traps, by a secondary light source with a photonenergy below the bandgapenergy, can lead to a strong enhancement (up to 25%) of the PL emissions from the alloys under a primary optical excitation above the bandgapenergy. We further demonstrate that the same hole traps can be found in various MBE-grown Ga(In)NP alloys, regardless of their growth temperatures, chemical compositions, and strain. The extent of the PL enhancement induced by the hole de-trapping is shown to vary between different alloys, however, likely reflecting their different trap concentrations. The absence of theses traps in the GaNP alloy grown by vapor phase epitaxy suggests that their incorporation could be associated with a contaminant accompanied by the N plasma source employed in the MBEgrowth, possibly a Cu impurity.

Optically detected magnetic resonance and photoluminescence spectroscopy are employed to study grown-in defects in GaAs0.985Bi0.015 epilayers grown by molecular beam epitaxy. The dominant paramagnetic defect is identified as an isolated arsenic antisite, As-Ga, with an electron g-factor of 2.03 +/- 0.01 and an isotropic hyperfine interaction constant A (900 +/- 620) x 10(-4) cm(-1). The defect is found to be preferably incorporated during the growth at the lowest growth temperature of 270 degrees C, but its formation can be suppressed upon increasing growth temperature to 315 degrees C. The As-Ga concentration is also reduced after post-growth rapid thermal annealing at 600 degrees C.

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly( 3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Effect of thermal annealing on paramagnetic centers in post-growth hydrogenated GaN_0.0081P_0.9919 epilayer is examined by means of photoluminescence and optically detected magnetic resonance (ODMR) techniques. In recent studies, several Ga-interstitial (Ga_i) related centers were found to be activated by the presence of hydrogen in the hydrogenated GaNP alloys. These centers compete with near-band edge radiative recombination. Annealing at 400 ºC in Ar-ambient is found to cause quenching of the Ga_i-related ODMR signals that were activated by post-growth hydrogenation. We tentatively ascribe this effect to dissociation of the H-Ga_i complexes and subsequent out-diffusion of H.

Defect properties of quaternary GaNAsP/GaP epilayers grown by vapor phase epitaxy (VPE) are studied by photoluminescence and optically detected magnetic resonance techniques. Incorporation of more than 0.6% of nitrogen is found to facilitate formation of several paramagnetic defects which act as competing carrier recombination centers. One of the defects (labeled as Ga-i-D) is identified as a complex defect that has a Ga interstitial (Ga-i) atom residing inside a Ga tetrahedron as its core. A comparison of Ga-i-D with other Ga-i-related defects known in ternary GaNP and GaNAs alloys suggests that this defect configuration is specific to VPE-grown dilute nitrides.