Daniel Dagnelund

Semi-metallic polymers

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.

Identification of an isolated arsenic antisite defect in GaAsBi

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, AsGa, with an electron g-factor of 2.03 ± 0.01 and an isotropic hyperfine interaction constant A = (900 ± 20) × 10−4 cm−1. The defect is found to be preferably incorporated during the growth at the lowest growth temperature of 270 °C, but its formation can be suppressed upon increasing growth temperature to 315 °C. The AsGa concentration is also reduced after post-growth rapid thermal annealing at 600 °C.

Formation of grown-in defects in molecular beam epitaxial Ga(In)NP: Effects of growth conditions and postgrowth treatments

Effects of growth conditions and post-growth treatments, such as presence of N ions, N2 flow, growth temperature, In alloying, and postgrowth rapid thermal annealing (RTA), on formation of grown-in defects in Ga(In)NP prepared by molecular beam epitaxy are studied in detail by the optically detected magnetic resonance (ODMR) technique. Several common residual defects, such as two Ga-interstitial defects (i.e., Gai-A and Gai-B) and two unidentified defects with a g factor around 2 (denoted by S1 and S2), are closely monitored. Bombardment of impinging N ions on grown sample surface is found to facilitate formation of these defects. Higher N2 flow is shown to have an even more profound effect than a higher number of ions in introducing these defects. Incorporation of a small amount of In (e.g., 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In; however, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configura...

Optically detected magnetic resonance studies of point defects in quaternary GaNAsP epilayers grown by vapor phase epitaxy

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 Gai-D) is identified as a complex defect that has a Ga interstitial (Gai) atom residing inside a Ga tetrahedron as its core. A comparison of Gai-D with other Gai-related defects known in ternary GaNP and GaNAs alloys suggests that this defect configuration is specific to VPE-grown dilute nitrides.

Effect of nitrogen ion bombardment on defect formation and luminescence efficiency of GaNP epilayers grown by molecular-beam epitaxy

Radiative efficiency of GaNP epilayers grown on GaP substrates by solid-source molecular beam epitaxy is significantly improved by reduced nitrogen ion bombardment during the growth. Based on the results of temperature-dependent photoluminescence (PL) and optically detected magnetic resonance studies (ODMR), the observed improvements are attributed to reduced formation of defects, such as a Ga interstitial related defect and an unidentified defect revealed by ODMR. We demonstrate that these defects act as competing recombination centers, which promote thermal quenching of the PL intensity and result in a substantial (34×) decrease in room-temperature PL intensity.

Room temperature spin filtering effect in GaNAs: Role of hydrogen

Effects of hydrogen on the recently discovered defect-engineered spin filtering in GaNAs are investigated by optical spin orientation and optically detected magnetic resonance. Post-growth hydrogen treatments are shown to lead to nearly complete quenching of the room-temperature spin-filtering effect in both GaNAs epilayers and GaNAs/GaAs multiple quantum wells, accompanied by a reduction in concentrations of Gai interstitial defects. Our finding provides strong evidence for efficient hydrogen passivation of these spin-filtering defects, likely via formation of complexes between Gai defects and hydrogen, as being responsible for the observed strong suppression of the spin-filtering effect after the hydrogen treatments.

Effect of postgrowth hydrogen treatment on defects in GaNP

Effect of postgrowth hydrogen treatment on defects and their role in carrier recombination in GaNP alloys is examined by photoluminescence (PL) and optically detected magnetic resonance. We present direct experimental evidence for effective activation of several defects by low-energy subthreshold hydrogen treatment (≤100 eV H ions). Among them, two defect complexes are identified to contain a Ga interstitial. Possible mechanisms for the H-induced defect activation and creation are discussed. Carrier recombination via these defects is shown to efficiently compete with the near band-edge PL, explaining the observed degraded optical quality of the alloys after the H treatment.

Antiferromagnetic interaction in coupled CdSe/ZnMnSe quantum dot structures

Spin polarization of nonmagnetic CdSe quantum dots (QDs) coupled to adjacent ZnMnSe diluted magnetic semiconductor (DMS) is investigated by CW and time-resolved magneto-optical spectroscopy under tunable laser excitation. Efficient enhancement in the degree of σ− circular polarization of photoluminescence from the CdSe QDs is observed under optical excitation at the σ+-active exciton state of the DMS. The fact that the enhancement persists much longer than the exciton lifetime of the DMS rules out a role of the DMS excitons. A possible explanation is discussed in terms of antiferromagnetic coupling between the excitons in QDs and aligned Mn ions in DMS.

Effects of hydrogenation on non-radiative defects in GaNP and GaNAs alloys: An optically detected magnetic resonance study

Photoluminescence and optically detected magnetic resonance techniques are utilized to study defect properties of GaNP and GaNAs alloys subjected to post-growth hydrogenation by low-energy sub-threshold ion beam irradiation. It is found that in GaNP H incorporation leads to activation of new defects, which has a Ga interstitial (Gai) atom at its core and may also involve a H atom as a partner. The observed activation critically depends on the presence of N in the alloy, as it does not occur in GaP with a low level of N doping. In sharp contrast, in GaNAs hydrogen is found to efficiently passivate Gai-related defects present in the as-grown material. A possible mechanism responsible for the observed difference in the H behavior in GaNP and GaNAs is discussed.

Optically detected cyclotron resonance studies of InxGa1−xNyAs1−y∕GaAs quantum wells sandwiched between type-II AlAs∕GaAs superlattices

We report on our results from a systematic study of layered structures containing an InGaNAs∕GaAs single quantum well (SQW) enclosed between staggered type II AlAs∕GaAs superlattices (SL), by the photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. Besides the ODCR signal known to originate from electrons in GaAs, the predominant ODCR peak is shown to be related to carriers with a two-dimensional character and a cyclotron resonance effective mass of m*≈(0.51–0.56)m0. The responsible carriers are ascribed to electrons on the ellipsoidal equienergy surface at the AlAs X point of the Brillouin zone within the SL, based on results from angular and spectral dependences of the ODCR signal. No ODCR signal related to the InGaNAs SQW was detected, presumably due to low carrier mobility despite the high optical quality. Multiple absorption of photons with energy below the band gap energy of the SL and the GaAs barriers was observed, which bears implication on the efficiency of light-...