Christian D. Poweleit

Observation of large electron drift velocities in InN by ultrafast Raman spectroscopy

Electron transport in an InN film grown on GaN has been studied by transient Raman spectroscopy at T=300K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2×108cm∕s, significantly larger than those observed for other III-V semiconductors, such as GaAs and InP. These experimental results have been compared with ensemble Monte Carlo simulations and good agreement has been obtained.Electron transport in an InN film grown on GaN has been studied by transient Raman spectroscopy at T=300K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2×108cm∕s, significantly larger than those observed for other III-V semiconductors, such as GaAs and InP. These experimental results have been compared with ensemble Monte Carlo simulations and good agreement has been obtained.

Raman imaging of patterned silicon using a solid immersion lens

We show an enhanced spatial resolution using a solid immersion lens by directly imaging the Raman scattered light from silicon masked by periodic metal lines. A glass hemisphere solid immersion lens with an index of refraction n=1.868 at 488 nm is used in conjunction with a 100×0.8 numerical aperture objective to obtain the enhanced spatial resolution. The increased numerical aperture is demonstrated by a direct line scan over the periodic metal lines. Compared with near-field optical microscopy, the solid immersion lens technique overcomes the difficulty of limited excitation power obtainable with tapered fibers, while providing excellent spatial resolution which in principle could be extended to the 0.1 μm range.

Optical spectroscopy of bulk GaN crystals grown from a Na–Ga melt

Colorless transparent platelet and prismatic GaN crystals up to 3–4 mm, grown from a Na–Ga melt (0.6–0.7 mol fraction of Na) at temperatures of 700–800 °C in a modest (5 MPa) pressure of N2, are characterized using Raman scattering, room and low temperature photoluminescence, and reflectance. They exhibit sharp free and bound exciton luminescence features (down to 0.22 meV full width at half maximum), including multiple excited states. Residual Mg and Zn acceptors and a 33.6 meV donor (possibly ON) are identified. Raman spectra suggest free carrier concentrations down to the low to mid 1016 cm−3 range.

Optical characterization of bulk GaN grown by a Na–Ga melt technique

Colorless platelet and prismatic GaN crystals grown from a Na-Ga melt are characterized using room and low temperature photoluminescence (PL), reflectance, and micro-Raman scattering. The largest (∼ 3-4 mm) platelet shows a neutral donor-bound exciton (D 0 ,X) PL peak with a full-width at half-maximum (FWHM) of 2.2meV at 1.7 K. Raman scattering reveals an A 1 (LO) phonon mode in this sample at 739 cm 1 , implying a free electron concentration (n) around 2-3 x 10 17 cm 3 . Smaller platelets grown in a pyrolytic BN crucible show even sharper exciton peaks, down to 0.22 meV FWHM. The stronger A 1 (LO) Raman peak lies at 733 cm 1 in this case, virtually unshifted by plasmon interactions. This observation implies n is in the mid 10 16 cm -3 range or below. Residual Zn acceptors are frequently observed, and two-electron transitions identify the binding energy of the residual donor species as 33.6 meV, which may be ON The 2.2 eV yellow PL band is generally weak or absent. The high purity and excellent optical properties may be due to gettering of donors such as O by the Na in the melt.

Nanosynthesis routes to new tetrahedral crystalline solids: Silicon-like Si3AlP

We introduce a synthetic strategy to access functional semiconductors with general formula A(3)XY (A = IV, X-Y = III-V) representing a new class within the long-sought family of group IV/III-V hybrid compounds. The method is based on molecular precursors that combine purposely designed polar/nonpolar bonding at the nanoscale, potentially allowing precise engineering of structural and optical properties, including lattice dimensions and band structure. In this Article, we demonstrate the feasibility of the proposed strategy by growing a new monocrystalline AlPSi(3) phase on Si substrates via tailored interactions of P(SiH(3))(3) and Al atoms using gas source (GS) MBE. In this case, the high affinity of Al for the P ligands leads to Si(3)AlP bonding arrangements, which then confer their structure and composition to form the corresponding Si(3)AlP target solid via complete elimination of H(2) at ∼500 °C. First principle simulations at the molecular and solid-state level confirm that the Si(3)AlP building blocks can readily interlink with minimal distortion to produce diamond-like structures in which the P atoms are arranged on a common sublattice as third-nearest neighbors in a manner that excludes the formation of unfavorable Al-Al bonds. High-resolution XRD, XTEM, and RBS indicate that all films grown on Si(100) are tetragonally strained and fully coherent with the substrate and possess near-cubic symmetry. The Raman spectra are consistent with a growth mechanism that proceeds via full incorporation of preformed Si(3)AlP tetrahedra with residual orientational disorder. Collectively, the characterization data show that the structuro-chemical compatibility between the epilayer and substrate leads to flawless integration, as expected for pseudohomoepitaxy of an Si-like material grown on a bulk Si platform.

Morphological and optical properties of Si nanostructures imbedded in SiO2 and Si3N4 films grown by single source chemical vapor deposition

Luminescent Si nanocrystals imbedded in amorphous SiO2 and Si3N4 networks have been prepared using an approach based on deposition of Si-rich Si–O and Si–N films by ultrahigh vacuum-chemical vapor deposition reactions of the single-source precursors O(SiH3)2 and N(SiH3)3 respectively. The film growth is conducted on Si (100) at temperatures of 750–850 °C and at extremely high rates of 20–30 nm per min with complete hydrogen elimination. Rapid thermal annealing of the as-deposited films at 1100–1200 °C for 30–60 s generates Si nanocrystals with tunable sizes, discrete shapes, and uniform distributions. The phase, composition, and microstructure of the films are characterized by a variety of analytical techniques including high-resolution electron microscopy. The room temperature photoluminescence (PL) is blueshifted substantially with respect to pure Si and appears to be independent of the Si3N4 and SiO2 dielectric medium. The PL energy increases with decreasing crystal size in accordance with quantum conf...

Studies of inactivation mechanism of non-enveloped icosahedral virus by a visible ultrashort pulsed laser

BackgroundLow-power ultrashort pulsed (USP) lasers operating at wavelengths of 425 nm and near infrared region have been shown to effectively inactivate viruses such as human immunodeficiency virus (HIV), M13 bacteriophage, and murine cytomegalovirus (MCMV). It was shown previously that non-enveloped, helical viruses such as M13 bacteriophage, were inactivated by a USP laser through an impulsive stimulated Raman scattering (ISRS) process. Recently, enveloped virus like MCMV has been shown to be inactivated by a USP laser via protein aggregation induced by an ISRS process. However, the inactivation mechanism for a clinically important class of viruses – non-enveloped, icosahedral viruses remains unknown.Results and discussionsWe have ruled out the following four possible inactivation mechanisms for non-enveloped, icosahedral viruses, namely, (1) inactivation due to ultraviolet C (UVC) photons produced by non-linear optical process of the intense, fundamental laser beam at 425 nm; (2) inactivation caused by thermal heating generated by the direct laser absorption/heating of the virion; (3) inactivation resulting from a one-photon absorption process via chromophores such as porphyrin molecules, or indicator dyes, potentially producing reactive oxygen or other species; (4) inactivation by the USP lasers in which the extremely intense laser pulse produces shock wave-like vibrations upon impact with the viral particle. We present data which support that the inactivation mechanism for non-enveloped, icosahedral viruses is the impulsive stimulated Raman scattering process. Real-time PCR experiments show that, within the amplicon size of 273 bp tested, there is no damage on the genome of MNV-1 caused by the USP laser irradiation.ConclusionWe conclude that our model non-enveloped virus, MNV-1, is inactivated by the ISRS process. These studies provide fundamental knowledge on photon-virus interactions on femtosecond time scales. From the analysis of the transmission electron microscope (TEM) images of viral particles before and after USP laser irradiation, the locations of weak structural links on the capsid of MNV-1 were revealed. This important information will greatly aid our understanding of the structure of non-enveloped, icosahedral viruses. We envision that this non-invasive, efficient viral eradication method will find applications in the disinfection of pharmaceuticals, biologicals and blood products in the near future.

Quantification of degradation in nonceramic housing materials by laser ablation

A laser irradiation technique has been explored to quantify visible as well as invisible degradation of housing materials used for nonceramic insulators. Nonceramic insulators that were removed from service and new samples of the housing that were subjected to accelerated testing in the laboratory have been evaluated. The method uses a constant power laser source for delivering the same energy to the test specimen. The proposed method is fairly quick and could be looked upon as an additional method for characterizing aging of nonceramic insulators. It has been shown that this method provides a better characterization of aging than is obtained from Fourier Transform Infrared Spectroscopy. The housing materials evaluated include silicone rubber, ethylene propylene diene monomer rubber and cycloaliphatic epoxy resin.

Near-field solid immersion lens (SIL) microscope with advanced compact mechanical design

A compact mechanical package is developed for a standard microscope that implements a solid immersion lens (SIL) on a retractable bimorph swing arm. With the compact package mounted on an inverted microscope, far-field and near-field images are obtained at the same location by moving the SIL in place with the swing arm. With white-light incoherent illumination, the resolution of this system for observing digital versatile discs (DVDs) is around 200nm with an effective NA of 1.5. Imaging with the SIL is compared to an atomic force microscopy (AFM) scan.

Degradation Dynamics of Polymeric Housing Materials Used for HV Line and Station Apparatus

A combination of laser irradiation and thermogravimetric analysis has been utilized to understand the degradation dynamics of polymeric materials used for housings of outdoor high voltage (HV) line and apparatus insulators, namely, silicone rubber, ethylene propylene diene monomer rubber and cycloaliphatic epoxy. The induced temperature rise was calculated using laser photo-thermal models and the results were found to be in good agreement with experimental findings. It has been shown that there are significant differences in the degradation dynamics of silicone rubber when compared with ethylene propylene diene monomer and epoxy. It has also been shown that the laser irradiation technique can be used as an efficient method to rank polymers materials for arc tracking and erosion resistance, especially for present day materials that will easily pass the screening tests prescribed in the ASTM and IEC standards.