Session JA: Condensed Matter and Nanoscience V

Suppression of free carrier absorption in silicon using multislot SiO$_{2}$/nc-Si waveguide

Nanocrystalline silicon has been proposed as a promising candidate for future silicon CMOS-compatible light emitting device. To achieve Si-based light sources at the standard telecommunication wavelength (1535 nm) Si nanostructures can be doped with rare earth elements. However, free carrier absorption (FCA) has been recognized as a major obstacle towards achieving net optical gain from a material containing Si nanostructures. Thus it is critical to develop approaches that suppress FCA to achieve optical gain in this material system. In this talk, experimental results of pump-induced loss for TE and TM polarization in multislot Er doped SiO$_{2}$/nc-Si waveguides will be presented. Continuous wavelength and ultrafast studies of carriers excited in the nc-Si multilayers reveal strong suppression of transmission loss related to free carrier absorption in Si nanostructures for TM polarized probe light. We demonstrate theoretically and experimentally that free carrier absorption may be reduced under TM polarization as much as 9 times compared to TE polarization. This approach may remove a major obstacle for future Si-based light emission devices, as free carrier absorption may no longer dominate over the optical gain.   

Topology and Geometry Perspectives of Functional Materials

In this talk we address the key concept of topology that impacts materials science in a major way, and to the recent significant advances in our understanding of the important topological notions in a wide class of materials with potential technologies. A paradigm of topology/geometry $\to $ property $\to $ functionality is emerging which goes beyond the traditional paradigm of microscopic structure $\to $ property $\to $ functionality relation. It delineates the active roles of topology and geometry in designing, fabrication, characterization and predictive modeling of novel materials properties and multi-functionalities. After introducing the basic essentials of topology and geometry, we elucidate these concepts through a gamut of nanocarbon allotropes of de novo carbons, hierarchical self-assembled soft- and bio-materials, supramolecular assemblies, nanoporous materials and so forth. The applications of these topological materials range from sensing, energy storage/conversion, catalysis to nanomedicine, to name a few. We illustrate these concepts through resonance Raman spectroscopy and related experiments on nanocarbons as well as a slew of soft- and bio-materials with an emphasis on topological metrology. We also discuss topological defects (local versus extended) such as Stone-Wales and mitosis, domain walls, vortices, boojums, skyrmions, magnetic monopoles in artificial spin ice and Hopf fibrations as well as other (complex and network) topologies in materials and quantum systems.   

Effects of annealing and doping concentration on the critical current density and superconducting properties of $Ba(Fe_{1-x}Co_x)_2As_2$ and $Ba_{0.5}Sr_{0.5}(Fe_{1-y}Co_y)_2As_2$

We report on the study we have made in order to better understand the intrinsic nature of superconductivity and pinning mechanisms in $Ba(Fe_{1-x}Co_x)_2As_2$ with various cobalt concentration and in $Ba_{0.5}Sr_{0.5}(Fe_{1-y}Co_{y})_2$. The critical current density $J_c$(as functions of temperature, and applied magnetic field) for different x values of 0.04, 0.08, and 0.15 (underdoped, optimally doped and overdoped respectively) is determined magnetically. From the $J_c (H)$ analysis, we see that $J_c$ slowly decays with respect to H in all the samples. We also found that $J_c$ in the annealed compared to the as-grown sample is enhanced in the underdoped and the optimally doped $Ba(Fe_{1-x}Co_x)_2As_2$ while there is no difference in the overdoped $Ba(Fe_{1-x}Co_x)_2As_2$ and the $Ba_{0.5}Sr_{0.5}(Fe_{1-y}Co_{y})_2$. Analysis of the $J_c$(T) data shows a weak pinning feature rather than correlated pinning. The pinning force $F_p$ as a function of the irreversible field $H_{irr}$ is also analyzed to gain further insights into the pinning mechanism.   

Comparison of the Structural, Magnetic and Piezoelectric Properties of BiFeO$_{3}$ and BiCrO$_{3}$ Thin Films

Multiferroic BiCrO$_{3}$ is a perovskite-structured oxide that is antiferroelectric and antiferromagnetic at low temperature [1,2] and is structurally very similar to the better studied BiFeO$_{3}$. Here we present a comparative study of the structural, magnetic and piezoelectric properties of BiFeO$_{3}$ and BiCrO$_{3}$ thin films. Highly strained BiFeO$_{3}$ films grown on LaAlO$_{3}$ substrates adopt the nearly tetragonal T' polymorph (c/a $\sim$ 1.25), however, they are rarely structurally uniform, instead tilted T' and tilted S'-polymorphs (c/a $\sim$ 1.09) coexist in the form of stripe patterns [3,4]. More importantly we find that the presence of S' is a prerequisite for ferroelectric switching [4,5]. Here, we extend this work to BiCrO$_{3}$ films grown on various substrates. Synchrotron and neutron measurements allow us to carefully characterize both the structural and magnetic properties of BiCrO$_{3}$ thin films. This research was supported in part by the U.S. DOE-BES, Materials Sciences and Engineering Division, and performed in part at the CNMS, ShaRE, and APS DOE-BES user facilities. [1] F. Sugawara et al., J. Phys. Soc. Jpn. \textbf{25}, 1553 (1968) [2] D.H. Kim et al., Appl. Phys. Lett. \textbf{89}, 162904 (2006) [3] R.J. Zeches et al., Science \textbf{326}, (2009) [4] C. Beekman et al., Adv. Mater (in press) [5] W. Siemons et al., J Phys. D (in press)   

Electron irradiation effects in n-type GaN studied with EPR spectroscopy

Characterizing defects in GaN under a wide range of conditions is critical to improving device performance. Bulk, UID, n-type GaN samples were studied using electron paramagnetic resonance (EPR) spectroscopy before and after irradiation with electrons. Sample A was irradiated with 0.5 MeV electrons (below threshold for Ga displacements, but above that for N), and Sample B was irradiated with 1 MeV electrons (above threshold for both Ga and N displacements). Irradiation was double-sided and the electron dose was 10$^{17}$ cm$^{-2}$. EPR measurements at 3.5 K showed the number of donors per unit area in Sample A decreased with irradiation by 2.1 $\times$ 10$^{15}$ cm$^{-2}$ and in Sample B by 5.3 $\times$ 10$^{15}$ cm$^{-2}$. This difference can be accounted for, considering that the penetration depth of 0.5 MeV electrons was 100 $\mu$m, and that of 1 MeV electrons was 300 $\mu$m. No new EPR lines appeared in the samples after irradiation. The samples were then annealed in dry N$_{2}$, and the donor signal intensity increased. An interpretation of the data is that N interstitials, reportedly forming after irradiation, act as compensating acceptors, lowering the donor signal intensity. N interstitials are also claimed to recombine with N vacancies during annealing, increasing the donor signal intensity.   

Atomic Force Microscopy (AFM) of Ion -- Irradiated Cadmium Zinc Telluride Crystals

Cadmium Zinc Telluride (CZT) is considered a good candidate material for radiation detectors. However, current technology has difficulties in growing large dimension single crystals. Also, leakage (dark) currents along grain boundaries and device surface are still a big issue. In this study, we used AFM to study the changes to the surface topography of CZT crystals irradiated with Pt ions at 180 keV and 1.6x10$^{11}$ ions/cm$^{2}$. The initial CZT sample is prepared by polishing and baseline AFM topographic images are taken, monitoring the values for the average roughness and root mean square. The CZT sample is then treated by etching with 2\% bromine methanol (BM) followed by average roughness and root mean square AFM measurements. Next, the sample is irradiated with Pt ions and measured with AFM. The results show that the surface after irradiation has a smaller roughness and fewer morphology features than before irradiation. Current-voltage electrical measurements were also taken at each preparation stage and correlated with the AFM results. Finally, radiation detectors based on the CZT samples were fabricated and tested for response to radiation. The effects of the Pt irradiation on the radiation detector performance is discussed.