Showing papers by "Solid State Physics Laboratory published in 2012"

Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene

Abstract: Graphene's broad bandwidth makes it promising as a photodetector, but common electronics cannot analyse the currents at high frequencies. Here, using photocurrent measurements, laser-induced carrier generation effects in freely suspended graphene and at graphene–metal interfaces are clarified up to 1 THz.

Direct mapping of the formation of a persistent spin helix

Abstract: Spin–orbit interaction induces spin-polarization decay in semiconductor quantum wells. But this decay can be suppressed in favour of a helical spin mode by tuning the interaction. Optical pump–probe measurements provide direct evidence of the resulting helix—a signature that has so far only been inferred from transport measurements.

Irreversibility on the Level of Single-Electron Tunneling

Abstract: We present a low-temperature experimental test of the fluctuation theorem for electron transport through a double quantum dot. The rare entropy-consuming system trajectories are detected in the form of single charges flowing against the source-drain bias by using time-resolved charge detection with a quantum point contact. We find that these trajectories appear with a frequency that agrees with the theoretical predictions even under strong nonequilibrium conditions, when the finite bandwidth of the charge detection is taken into account. The second law of thermodynamics states that a macroscopic system out of thermal equilibrium will irreversibly move toward equilibrium driven by a steady increase of its entropy. This macroscopic irreversibility occurs despite the time-reversal symmetry of the underlying microscopic equations of motion. Also, a microscopic system will undergo an irreversible evolution on a long time scale, but, over a sufficiently short observation time � , both entropy-producing trajectories as well as their timereversed entropy-consuming counterparts occur. It is only because of the statistics of these occurrences that a longterm irreversible evolution is established. This phenomenon is described by the fluctuation theorem [1,2]. Irrespective of the description of the trajectories being system-specific, the fluctuation theorem (FT) relates the probabilities P� ð� SÞ for processes that change the entropy

Dependence of the Dresselhaus spin-orbit interaction on the quantum well width

Abstract: We measured the Dresselhaus spin-orbit interaction coefficient ${\ensuremath{\beta}}_{1}$ for (001)-grown GaAs/Al${}_{0.3}$Ga${}_{0.7}$As quantum wells for six different well widths $w$ between 6 and 30 nm. The varying size quantization of the electron wave vector $z$-component $\ensuremath{\langle}{k}_{z}^{2}\ensuremath{\rangle}\ensuremath{\sim}{(\ensuremath{\pi}/w)}^{2}$ influences ${\ensuremath{\beta}}_{1}=\ensuremath{-}\ensuremath{\gamma}\ensuremath{\langle}{k}_{z}^{2}\ensuremath{\rangle}$ linearly. The value of the bulk Dresselhaus coefficient $\ensuremath{\gamma}=(\ensuremath{-}11\ifmmode\pm\else\textpm\fi{}2)$ eV\AA{}${}^{3}$ was determined. We discuss the absolute sign of the Land$\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}$ $g$ factors and the effective momentum scattering times.

Oxygen vacancy‐induced microstructural changes of annealed CeO2−x nanocrystals

Abstract: Nanocrystalline ceria (CeO2) is known for its ionic conductivity and oxygen storage properties, which depend on the presence of oxygen ion vacancies. The vacancies cause several important changes in CeO2 involving microstrain, electronic structure, magnetic properties, etc. In this article, we focus our attention to the microstructural changes of nanocrystalline CeO2−x annealed at different temperatures in the range 200–500 °C. Structural and vibrational properties were investigated by X-ray diffraction and Raman spectroscopy. It was observed that the content of oxygen vacancies changed significantly with increasing annealing temperature, which plays an important role in the observed microstructural changes of the annealed samples. We demonstrate that the observed microstrain changes, because of variable defect content, dominate over the crystallite size effect. This finding is opposite to the conclusions made by several other authors. A new mode, classified as a probable surface mode, was observed in the Raman spectra at ∼480 cm−1, the appearance of which can be explained by the large defective structure and disorder in the ceria lattice. Copyright © 2011 John Wiley & Sons, Ltd.

Carbondioxide Gating in Silk Cocoon

Abstract: Silk is the generic name given to the fibrous proteins spun by a number of arthropods. During metamorphosis, the larva of the silk producing arthropods excrete silk-fiber from its mouth and spun it around the body to form a protective structure called cocoon. An adult moth emerges out from the cocoon after the dormant phase (pupal phase) varying from 2 weeks to 9 months. It is intriguing how CO2/O2 and ambient temperature are regulated inside the cocoon during the development of the pupa. Here we show that the cocoon membrane is asymmetric, it allows preferential gating of CO2 from inside to outside and it regulates a physiological temperature inside the cocoon irrespective of the surrounding environment temperature. We demonstrate that under simulating CO2 rich external environment, the CO2 does not diffuse inside the cocoon. Whereas, when CO2 was injected inside the cocoon, it diffuses out in 20 s, indicating gating of CO2 from inside to outside the membrane. Removal of the calcium oxalate hydrate crystals which are naturally present on the outer surface of the cocoon affected the complete blockade of CO2 flow from outside to inside suggesting its role to trap most of the CO2 as hydrogen bonded bicarbonate on the surface. The weaved silk of the cocoon worked as the second barrier to prevent residual CO2 passage. Furthermore, we show that under two extreme natural temperature regime of 5 and 50 °C, a temperature of 25 and 34 °C respectively were maintained inside the cocoons. Our results demonstrate, how CO2 gating and thermoregulation helps in maintaining an ambient atmosphere inside the cocoon for the growth of pupa. Such natural architectural control of gas and temperature regulation could be helpful in developing energy saving structures and gas filters.

Quantum capacitance and density of states of graphene

Abstract: We report capacitance measurements in top-gated graphene sheets as a function of charge carrier density. A measurement method using an LC-circuit provides high sensitivity to small capacitance changes and hence allows the observation of the quantum part of the capacitance. The extracted density of states has a finite value of 1◊10 17 m 2 eV 1 in the vicinity of the Dirac point, which is in contrast to the theoretical prediction for ideal graphene. We attribute this discrepancy to fluctuations of the electrostatic potential with a typical amplitude of 100meV in our device.

Investigations on the origin of mass and elastic loading in the time varying distinct response of ZnO SAW ammonia sensor

Abstract: In our earlier report, we gave an account of an interesting observation of a distinct response of ZnO SAW sensor for liquor ammonia. While the sensor response consisted of a small initial decrease in differential frequency followed by large increase for moist ammonia, it showed decrease alone for dry ammonia, water vapors and other interferants. This paper attempts to investigate the fundamental cause of the observed distinct response, by exploring the contributions of various sensor mechanisms such as mass, elastic and acousto-electric loading. To carry out the investigation, four sensors having ZnO films on SAW resonators with varying structural properties were employed. ZnO thin films of same thickness but differing in crystallite size, surface roughness and stress were achieved by depositing the films using RF sputtering under different pressures (10–40 mTorr). Electrical measurements on ZnO thin film show a very small contribution of acoustoelectric effect toward sensing response for liquor ammonia. Initially the elastic loading and later mass loading are identified as the dominant mechanism. The response has been fitted theoretically to determine individual contributions toward the sensing. A mechanism has been proposed and a correlation between the properties of ZnO film and sensor response has been attempted.

Strongly anisotropic spin relaxation revealed by resonant spin amplification in (110) GaAs quantum wells

Abstract: We have studied spin dephasing in a high-mobility two-dimensional electron system confined in a GaAs/AlGaAs quantum well grown in the [110] direction, using the resonant spin amplification (RSA) technique. From the characteristic shape of the RSA spectra, we are able to extract the spin dephasing times (SDTs) for electron spins aligned along the growth direction or within the sample plane, as well as the $g$ factor. We observe a strong anisotropy in the spin dephasing times. While the in-plane SDT remains almost constant as the temperature is varied between 4 and 50 K, the out-of-plane SDT shows a dramatic increase at a temperature of about 25 K and reaches values of about 100 ns. The SDTs at 4 K can be further increased by additional, weak above-barrier illumination. The origin of this unexpected behavior is discussed. The SDT enhancement is attributed to the redistribution of charge carriers between the electron gas and remote donors.

Synthesis, structure and impedance spectroscopic analysis of [(Pb x Sr 1− x )·OTiO 2 ]–[(2SiO 2 ·B 2 O 3 )]–7[BaO]–3[K 2 O] glass ceramic system doped with La 2 O 3

Abstract: Various compositions of glasses were prepared by melt quenching method in the glass ceramic 64[(Pb x Sr1−x )OTiO2]–25[(2SiO2·B2O3)]–7[BaO]–3[K2O]–1[La2O3] (0.5 ≤ x ≤ 1) system doped with La2O3. Dielectric constant, er versus temperature, T, plots revealed diffused peaks, while dielectric loss, D, versus T plots showed shifting in the peaks toward the higher temperature side similar to relaxor-like ceramics. Curie temperature was found to change systematically with changing the concentration of SrO. Impedance spectroscopy results indicated the contributions of polarization process relaxing in low frequency region attributed to polarizations at the crystal to glass interface and the glassy region.

Interference of electrons in backscattering through a quantum point contact

Abstract: Scanning gate microscopy is used to locally investigate electron transport in a high-mobility two-dimensional electron gas formed in a GaAs/AlGaAs heterostructure. Using quantum point contacts (QPC) we observe branches caused by electron backscattering decorated with interference fringes similar to previous observations by Topinka et al. We investigate the branches at different points of a conductance plateau as well as between plateaus, and demonstrate that the most dramatic changes in branch pattern occur at the low-energy side of the conductance plateaus. The branches disappear at magnetic fields as low as 50 mT demonstrating the importance of backscattering for the observation of the branching effect. The spacing between the interference fringes varies by more than 50% for different branches across scales of microns. Several scenarios are discussed to explain this observation.

Photo-induced electrochemical anodization of p-type silicon: achievement and demonstration of long term surface stability.

Abstract: Surface stability is achieved and demonstrated by porous silicon (PS) fabricated using a wavelength-dependent photo-electrochemical (PEC) anodization technique. During anodization, the photon flux for all wavelengths was kept constant while only the effect of light wavelength on the surface morphology of PS was investigated. PS optical sensors were realized, characterized and tested using a photoluminescence (PL) quenching technique. An aliphatic chain of alcohols (methanol to n-octanol) was detected in the range of 10?200?ppm. Long term surface stability was observed from samples prepared under red (750?620?nm) and green illumination (570?495 nm), where the PL quenching cycles evoke the possibility of using PS for stable sensor device applications. This study provides a route for preparing highly sensitive organic vapour sensors with a precise selection of the fabrication parameters and demonstrating their prolonged performance.

Optimization of slow light one-dimensional Bragg structures forphotocurrent enhancement in solar cells

Abstract: In 1D photonic crystal Bragg structures, strong localization of the light occurs in the high refractive index layers at wavelengths on the red edge of the photonic bandgap. We exploit this slow light effect for thin film solar cells in order to increase the absorption of light in silicon, as the latter has a high refractive index. Amorphous silicon and a transparent conductive oxide are chosen as high-index and low-index materials, respectively. Reference thin film cells have equivalent total thickness of amorphous silicon, plus antireflection coating and optional metallic back mirror. Through transfer-matrix calculations, we demonstrate that the spectrally integrated photon flux absorbed in active layers, hence the photocurrent, is enhanced by at least 10% with respect to reference using only a few periods. The enhancement is robust with respect to the light incidence angle. The key of such an enhancement is the tuning of the red edge to both the solar irradiance spectrum and the intrinsic material absorption spectrum, which is achieved by suitably selecting the layer thicknesses.

Chemical modification of graphene characterized by Raman and transport experiments

Abstract: A chemical approach to modify the electronic transport of graphene is investigated by detailed transport and Raman spectroscopy measurements on Hall bar shaped samples. The functionalization of graphene with nitrobenzene diazonium ions results in a strong p-doping of the graphene samples and only slightly lower mobilities. Comparing Raman and transport data taken after each functionalization step allowed the conclusion that two preferential reactions take place on the graphene surface. In the beginning a few nitrobenzene molecules are directly attached to the graphene atoms creating defects. Afterwards these act as seeds for a polymer like growth not directly connected to the graphene atoms. The effects of solvents were excluded by thorough control measurements.

Microstrip triangular patch antenna fabricated on LiTiZn ferrite substrate and tested in the X band range

Abstract: An experimental study of microstrip triangle patch antenna designed and fabricated on LiTiZn ferrite substrate is presented. This antenna geometry has been optimized with IE3D and lithographed on 2 mm thick LiTiZn ferrite substrate. LiTiZn polycrystalline substrate has been synthesized using solid state reaction technique and further characterized for microwave application. We have computed various important antenna parameters such as return-loss, bandwidth, directivity, gain, radiation power, etc. for synthesized ferrite as well as on RT-duroid substrate. A comparison of these results has also been presented.

Transport in a three-terminal graphene quantum dot in the multi-level regime

Abstract: We investigate transport in a three-terminal graphene quantum dot. All nine elements of the conductance matrix have been independently measured. In the Coulomb blockade regime accurate measurements of individual conductance resonances reveal slightly different resonance energies depending on which pair of leads is used for probing. Rapid changes in the tunneling coupling between the leads and the dot due to localized states in the constrictions has been excluded by tuning the difference in resonance energies using in-plane gates which couple preferentially to individual constrictions. The interpretation of the different resonance energies is then based on the presence of a number of levels in the dot with an energy spacing of the order of the measurement temperature. In this multi-level transport regime the three-terminal device offers the opportunity to sense if the individual levels couple with different strengths to the different leads. This in turn gives qualitative insight into the spatial profile of the corresponding quantum dot wave functions.

Transport in a three-terminal graphene quantum dot in the multi-level regime

Abstract: We investigate transport in a three-terminal graphene quantum dot. All nine elements of the conductance matrix have been independently measured. In the Coulomb blockade regime, accurate measurements of individual conductance resonances reveal slightly different resonance energies depending on which pair of leads is used for probing. Rapid changes in the tunneling coupling between the leads and the dot due to localized states in the constrictions have been excluded by tuning the difference in resonance energies using in-plane gates which couple preferentially to individual constrictions. The interpretation of the different resonance energies is then based on the presence of a number of levels in the dot with an energy spacing of the order of the measurement temperature. In this multi-level transport regime, the three-terminal device offers the opportunity to sense if the individual levels couple with different strengths to the different leads. This in turn gives qualitative insight into the spatial profile of the corresponding quantum dot wave functions.

Mechanism of self-assembled growth of ordered GaAs nanowire arrays by metalorganic vapor phase epitaxy on GaAs vicinal substrates

Abstract: We report the self-catalyzed growth of GaAs nanowire arrays by metalorganic vapor phase epitaxy (MOVPE) on GaAs vicinal substrates. The effect of substrate misorientation on the nanowire growth and the influence of growth parameters such as temperature and input V/III ratio have been studied in detail. Variation in the nanowire growth mechanism and consequential changes in the nanowire growth morphology were observed. A VLS growth mechanism with negligible effect of the vicinal surface gave rise to randomly distributed droplet-terminated GaAs nanowires at 400??C and multiprong root-grown GaAs nanowire clusters at 500??C with low V/III ratio. The substrate misorientation effect was dominant at 500??C with higher V/III ratio, in which case the combined effect of the vicinal surface and the self-catalyzed Ga droplets assisted the realization of self-assembled and crystallographically oriented epitaxial nanowire arrays through the vapor?solid mechanism.

Electron flow in split-gated bilayer graphene

Abstract: We present transport measurements on a bilayer graphene sheet with homogeneous back gate and split top gate. The electronic transport data indicates the capability to direct electron flow through graphene nanostructures purely defined by electrostatic gating. By comparing the transconductance data recorded for different top gate geometries - continuous barrier and split-gate - the observed transport features for the split-gate can be attributed to interference effects inside the narrow opening.

Transmission-grating-photomasked transient spin grating and its application to measurement of electron-spin ambipolar diffusion in (110) GaAs quantum wells

Abstract: A circular dichromatic transient absorption difference spectroscopy of transmission-grating-photomasked transient spin grating is developed and formularized. It is very simple in experimental setup and operation, and has high detection sensitivity. It is applied to measure spin diffusion dynamics and excited electron density dependence of spin ambipolar diffusion coefficient in (110) GaAs quantum wells. It is found that the spin ambipolar diffusion coefficient of (110) and (001) GaAs quantum wells is close to each other, but has an opposite dependence tendency on excited electron density. This spectroscopy is expected to have extensive applicability in the measurement of spin transport.

Effect of gate dielectric thickness on gate leakage in tunnel field effect transistor

Abstract: Gate leakage is one of the important parameter expected to limit the performance of Tunnel FETs. We have simulated the effect of gate dielectric thickness on gate leakage in Tunnel FETs, using two dimensional numerical simulations. It has been observed that gate leakage considerably affects the subthreshold characteristics of TFETs. It was found to be most important component of off-state current and should be considered in future TFET device design. Effects of gate metal workfunction on device characteristics, particularly, gate leakage and origin of reverse tunneling at drain have also been discussed.

Electron flow in split-gated bilayer graphene

Abstract: We present transport measurements on a bilayer graphene sheet with a homogeneous back gate and a split top gate. The electronic transport data indicate the capability of directing electron flow through bilayer graphene nanostructures purely defined by electrostatic gating. Comparing the transconductance data recorded for different top gate geometries—continuous barrier and split gate—the observed transport features for the split gate can be attributed to the interference effects inside the narrow opening.

Characterization deep boron diffused p ++ silicon layer

Abstract: In recent years the boron impurity-based dissolved wafer process has been repeatedly demonstrated as a powerful tool for forming single crystal Si microstructures. However, there is very little report on detailed characterization of the deep boron diffused silicon layer. This paper presents the optimization of deep boron diffused p++ silicon layer (>10 μm thick) of boron concentration above 5 × 1019 atoms/cm3. Detailed characterization of the p++ silicon layers, by using high resolution x-ray diffraction, secondary ion mass spectrometry, secondary electron micrograph are done. The optical behaviour of the p++ layers in mid-IR range is also carried out. The stress generated due to the deep diffusion is estimated to be 822 MPa by Raman spectroscopy.

Evaluation of microlithographic performance of 'deep UV' resists: Synthesis, and 2D NMR studies on alternating 'high ortho' novolak resins

Abstract: Lithographic evaluation of a ‘deep UV’ negative photoresist is discussed along with the synthesis of an alternating ‘high-ortho’ novolak resin. 2-D NMR studies (COSY, NOESY, HSQC, HMBC) on this resin are also discussed.