### Session W36: Focus Session: Optical Properties of Nanostructures of Si & GaAs

Chair: Matt Doty, Naval Research Laboratory
Room: Baltimore Convention Center 339

 Thursday, March 16, 2006 2:30PM - 3:06PM W36.00001: Excitonic Effects and the Optical Properties of Silicon Nanowires Invited Speaker: M.Y. Chou Semiconductor nanowires have potential applications in many fields such as optoelectronics, photovoltaic cells, and especially device miniaturization. The excited-state properties are of critical importance in the design of these functional devices. The low dimensionality and reduced size tend to strengthen the effective Coulomb interaction in these nanostructures. In this study, we focus on the correlated electron-hole states in semiconductor nanowires and the influence of this excitonic effect on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire of a diameter of 1.2 nm. Using plane waves and pseudopotentials, the quasiparticle states are calculated within the many-body perturbation theory with the so-called GW approximation. It is found that the fundamental gap depends on both the orientation and size of the wire, and that the gap increases as the diameter decreases in an inverse quadratic fashion [1]. The electron-hole interaction is then evaluated by the Bethe-Salpeter equation (BSE). The enhanced Coulomb interaction in this confined geometry not only gives rise to an unusually large exciton binding energy of more than 1 eV (compared to a value of 14.7 meV in silicon bulk), but also significantly modifies the relative strength of the absorption peaks. The characteristics of these exciton states will be discussed.\break [1] Quantum Confinement and Electronic Properties of Silicon Nanowires,'' X. Zhao, C. M. Wei, L. Yang, and M. Y. Chou, Phys. Rev. Lett. 92, 236805 (2004). Thursday, March 16, 2006 3:06PM - 3:18PM W36.00002: Electronic structures and optical properties of silicon nanowires Jun Li , Arthur Freeman Recent optical spectroscopic\footnote{Holmes, Johnston, Doty, and Korgel, Science 287, 1471 (2000)} and theoretical/computational studies\footnote{Zhao, Wei, Yang, and Chou, Phys. Rev. Lett. 92, 236805 (2004)} challenge the previous consensus on the nature of the optical properties of Si nanowires (SiNW). Here, we present results of precise theoretical FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB 24, 864 (1981)} determinations of the electronic structures and optical properties of (001) and (111) one nm SiNW. The electronic states at the gaps demonstrate a strong orientation dependent parabolic character in the Brillouin zone and a clear entanglement in real space between 1D and 2D dimensions of the wire. The local symmetry imposed by quantum confinement quenches the transitions around the gap, yielding an optically inactive direct gap. The observed (001) photoluminescence is attributed to a transition rooted in an Si$_8$ ring. The optical structure in the experimental range is well reproduced by our first-principles calculations that include the screened exchange-LDA correction to the well-known failure of the LDA. Our predictions about the anisotropy and orientation dependent optical absorption are easily verified experimentally. Work supported by DARPA B529527//W-7405-Eng-48. Thursday, March 16, 2006 3:18PM - 3:30PM W36.00003: Comparison of the optical properties of bare and capped GaAs nanowires L.M. Smith , Thang B. Hoang , L.V. Titova , H.E. Jackson , J.M. Yarrison-Rice , Yong Kim , H.H. Tan , C. Jagadish We study the optical properties of single bare GaAs nanowires and GaAs/AlGaAs core/shell nanowires fabricated by the VLS technique. SEM and AFM images show that the wires are uniform in length (3-4$\mu$m long) and needle-shaped. Low temperature photoluminescence measurements of individual nanowires indicate that the quantum efficiency of the core/shell nanowires is significantly larger compared to the uncapped nanowires. We believe that the reason for the low emission efficiency of the uncapped nanowires is the significant influence of the surface-related non-radiative trap states. We anticipate that time-resolved measurements will show a significant increase of the recombination lifetime in the core/shell nanowires compared to the uncapped nanowires due to the passivation of the surface-related states by the AlGaAs shell. We acknowledge the support of the NSF through grants 0071797, 0216374, and the Australian Research Council. Thursday, March 16, 2006 3:30PM - 3:42PM W36.00004: Linear optical absorption in silicon and GaAs nanocrystals Murilo L. Tiago , James R. Chelikowsky The linear optical spectrum of Si and GaAs nanocrystals containing up to 100 atoms is calculated and discussed. We use two first-principles theories: time-dependent density-functional theory in the local adiabatic approximation (TDLDA), and the many-body solution of the Bethe-Salpeter equation (BSE). Both theories predict a strong blue shift in the energy-resolved polarizability and in the absorption cross section, which is characteristic of confined systems in the nanoscale. When many-body effects are included, in the framework of the BSE, the low-energy range of the spectrum is modified in two ways: the energy of excitation lines increases by almost 1 eV, as a result of self-energy and screening corrections; and the oscillator strength of those lines is enhanced. In bulk semiconductors, many-body effects are known to produce exciton lines and enhanced absorption in the low-energy range of the spectrum. Although the size of the nanocrystals studied is much smaller than the radius of Wannier excitons, the enhancement in oscillator strength is suggested to have the same source as excitonic effects in bulk samples. Thursday, March 16, 2006 3:42PM - 4:18PM W36.00005: Light from Silicon-Based Nanostructures Invited Speaker: Luca Dal Negro Si-nanocrystals (Si-nc) embedded in SiO$_{2}$ glass matrices shows undoubtedly efficient room temperature light emission under optical pumping and sizable optical gain and light amplification have been demonstrated [1]. However, the presence of an insulating SiO$_{2}$ matrix prevents the fabrication of reliable and efficient electrically-driven devices and the efficiency of light emission is severely curtailed by the slow radiative lifetime of Si-nc. An alternative possibility is offered by the nucleation of Si-nc in dielectric hosts with smaller band-gaps. In this talk we will show our results on light-emitting Si-rich silicon nitride films (SRN) and photonic structures obtained by Plasma Enhanced Chemical Vapor Deposition (PE-CVD) followed by low temperature (500-900\r{ }C) thermal annealing[2]. The optical properties of SRN$_{ }$films are studied by micro-Raman and photoluminescence spectroscopy and demonstrate the presence of small Si-clusters with nanosecond recombination time and negligible emission thermal quenching. The electrical transport properties of SRN films are also investigated and efficient charge injection at low bias voltages is demonstrated. Additionally, we show that SRN matrices are suitable for efficient energy sensitization of Er ions emitting at 1.54 $\mu$m. The light emission mechanism in SRN nanostructures is studied by DFT-LDA \textit{first principles} calculations showing that, largely Stokes-shifted, nanosecond-fast and efficient light emission in PE-CVD deposited SRN samples originates from strongly localized excitons transitions at the surface of small Si-nc ($\sim$ 1-2 nm) embedded in Silicon nitride[3]. Additionally, we show that the presence of bridging nitrogen groups at the surface of small Si nanocrystals can explain the origin of the experimentally measured Stokes-shift and the nanosecond relaxation times[3]. \begin{enumerate} \item L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo \textit{Optical gain in Si nanocrystals}'', Nature 408, 440, 23 November 2000. \item L. Dal Negro, J.H. Yi, V. Nguyen, Y. Yi, J. Michel, L.C. Kimerling, \textit{Spectrally enhanced light emission from aperiodic photonic structures}'', Appl. Phys. Lett., \textbf{86}, 261905, (2005) \item L. Dal Negro, J. H. Yi, L. C. Kimerling, S. Hamel, A. Williamson, G. Galli, \textit{Light Emission from Silicon-rich Nitride Nanostructures, }Appl. Phys. Lett., submitted 2005 \end{enumerate} Thursday, March 16, 2006 4:18PM - 4:30PM W36.00006: Optical characterizations of one-dimensional wetting layers in InGaAs/GaAs quantum dot chains Xiaoyong Wang , Chih-Kang Shih , Zhiming Wang , Gregory Salamo We report spectroscopic evidence for the formation of 1D wetting layers (WLs) during the Stransky-Krastanov (SK) growth of multi-layered InGaAs/GaAs quantum dot (QD) chains. The wire-like features of these 1D WLs were demonstrated by their 1D density of states as well as the anisotropic absorption and emission properties. Two groups of QD's were found sitting on these 1D and the traditional 2D WLs, respectively, with size-dependent polarization anisotropies of $\sim$6{\%}-25{\%} due to their elongated shapes. The previously-unexplored new SK growth mode of 1D WLs could be potentially tailored by varying the In content and barrier thickness to yield QD's and 1D WLs with expected energy level separations. This may lead to the efficient carrier transfer between QD's on top of the same 1D WL for quantum technology applications that require quantum information transfer between different nanostructures of controlled positioning. Thursday, March 16, 2006 4:30PM - 4:42PM W36.00007: Electronic and optical fine structure of GaAs nanocrystals: the role of d orbitals in a tight-binding approach J.G. Diaz , Garnett W. Bryant Electronic structure and optical spectra of GaAs nanocrystals for a wide range of sizes are studied by using both $sp^3s^*$ and $sp^3s^*d^5$ nearest-neighbor tight-binding models. Our results show that the inclusion of $d$ orbitals into a minimal basis set is necessary for a proper description of the lowest electron states, especially in the strong confinement regime. For dot sizes below 2.5 nm, the ground electron state is primarily built of L-point bulk band-states, giving the nanocrystals indirect-gap character. Simpler $sp^3s^*$ models yield an incorrect description of electron states made from bulk band- states away from the Brillouin zone center. In contrast, $sp^3s^*d^5$ models are able to provide a consistent picture of the main optical features in agreement with experiments. Thursday, March 16, 2006 4:42PM - 4:54PM W36.00008: Neutral and charged excitons in single GaAs-based interface quantum dots C.J. Meining , V.R. Whiteside , B.D. McCombe , J.G. Tischler , A.S. Bracker , D. Gammon , A.B. Dzyubenko , M. Byszewski , M. Potemski We report circularly polarized photoluminescence (PL) in high magnetic fields ($<$ 28 T) and optically detected resonance (ODR) experiments of interface fluctuation quantum dots (IFQDs) in narrow GaAs/AlGaAs quantum wells (QWs) doped in the barriers with donors to allow creation of both neutral and negatively charged excitons. In the narrowest QW the diamagnetic shift of the trion is smaller than that of the neutral exciton. This is attributed to the larger spatial extent of the trion wavefunction in these ensembles of weakly confined QDs. Along with a careful study of the excitonic Zeeman splitting and complemented by a comparison of ensemble and single dot ODR measurements, this signature can be used to assign the narrow spectral lines observed in single dot PL studies as neutral and charged excitons. The PL of the trion is found to increase under resonant irradiation with far-infrared laser light, opposite to the behavior observed for wide QWs. Lateral carrier redistribution is believed to be the dominant mechanism that gives rise to the ODR signal in QWs with monolayer well width fluctuations.\\ Work supported in part by NSF-DMR \#0203560. Thursday, March 16, 2006 4:54PM - 5:06PM W36.00009: Nanostructured solar cells Ozgur Yavuzcetin , Cheol-Soo Yang , Tom Russell , Mark Tuominen In this work we investigate the use of nanofabrication technique to improve the overall efficiency of silicon solar cells. The efficiency and the durability of silicon solar cells largely depends on the quality of the anti-reflectivecoating. In this work, the change in the index of refraction on the surface of a substrate can be controlled by the amount of porosity, which is well known in effective medium theory. Also by changing the thickness of the porous layer, the medium can be fine tuned to a specific wavelength as an AR coating. We fabricate the nanoporous layer by using a self-assembled P(S-b-MMA) coating as a mask to etch into the silicon substrate using reactive ion etching. The use of different molecular weight diblock copolymer and different etching time allow us to tune the index of refraction. FT-IR and variable angle ellipsometry provide information about the transmission and reflection properties along with the index of refraction and the thickness of the coating. The investigation of the efficiencies are performed by comparing the I-V plots of conventional and nanostructured cells. Additional research is underway in order to apply this technology to other types of substrates. Thursday, March 16, 2006 5:06PM - 5:18PM W36.00010: Effects of high magnetic fields on the scattering rates of GaAs/AlGaAs and GaInAs/AlInAs quantum cascade lasers A. Wade , G. Fedorov , D. Smirnov , A. Leuliet , A. Vasanelli , C. Sirtori Using magneto-spectroscopy, we investigate the influence of a strong magnetic field on the intersubband scattering rates in MIR GaAs/AlGaAs and GaInAs/AlInAs quantum cascade lasers (QCLs). In our experiments, we measured light-current, voltage-current and laser emission spectra as a function of magnetic field up to 40T with the magnetic field perpendicular to the 2DEG. We observed strong oscillations in the intensity and threshold current. From these, the magnetic field dependences of the intersubband lifetime of both structures were derived and compared to their calculated dependence of electron-LO phonon scattering rates. Thursday, March 16, 2006 5:18PM - 5:30PM W36.00011: Ultra-low threshold quantum dot microdisk laser G.S. Solomon , Z.G. Xie , S. Goetzinger , Y. Yamamoto , W. Fang , H. Cao Ultra-low threshold lasers have applications in low-power communications. These lasers are also of fundamental interest, where a full understanding of lasing based on a few discrete emitters is evolving. This is especially true in solid-state systems, for instance those with a quantum dot (QD) gain medium, where a typical spectrum of discrete emission lines observed at lower pump power is often highly modified under higher pump powers. Here we discuss a microcavity laser containing a dilute QD gain medium that has an ultra low, sub-microwatt CW lasing threshold. The structure is based on a high-quality factor microdisk cavity of GaAs with a low density of InAs-based QDs embedded in the microdisk. We estimate 250 QDs in the 1.8 $\mu$m diameter microdisk under investigation. Of these QDs approximately 60 are spatially located within the modal region of the disk, which extends inwards approximately 250 nm from the disk edge. Only a small portion of these QDs couple to the narrow cavity modes, which have a free spectral range of 45 nm and an initial linewidth of 0.06-0.07 nm. Linewidth narrowing and lifetime reduction with increasing pump are both observed. Despite the small number of QDs it is unlikely from our estimates the system lases from a single QD state.