Deep bandtail states picosecond intensity-dependent carrier dynamics of GaN epilayer under high excitation

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionally n-doped GaN epilayer at room temperature under high excitation densities (i.e., N ₀ = 1.0× 10¹⁹– 1.1× 10²⁰ cm^–3) have been investigated with nondegenerate femtosecond pump–probe (267/400 nm) reflectance ( R/R ₀). All R/R ₀ traces possess a 2 ps buildup time that represents an overall time for the initial non-thermal carrier population to relax towards the continuum extremes and then into the probed tail states. We observe a saturation of R/R ₀ initial (first 10 ps) recovery rate _i at a density of 5– 6×10¹⁹ cm^–3 close to the Mott transition threshold obtained from time-integrated PL measurements. Such a saturation phenomenon has been identified as the trap-bottleneck due to the bandtail states and deep traps. As N ₀ is further increased, _i accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N ₀ in the range of the mid-10¹⁹–10²⁰ cm^–3 yields an Auger coefficient of C _a 5.0× 10^–30 cm⁶ s^–1.     

Diffusion of silicon in titanium nitride films. Efficiency of TiN barrier layers

Two kinds of reactively evaporated titanium nitride films with columnar (B ₀ films) and fine-grained film structure (B ₊ films) have been examined as diffusion barriers, preventing the silicon diffusion in silicon devices. The silicon diffusion profiles have been investigated by 2 MeV ⁴He⁺ Rutherford backscattering spectrometry (RBS) after annealing at temperatures up to 900° C, in view of application of high-temperature processes. The diffusivity from 400 to 900° C: D (m² s^–1)=2.5×10^–18 exp[–31 kJ/mol/(RT)] in B ₀ layers and D (m² s^–1)=3×10^–19 exp[–26 kJ/mol/(RT) in B ₊ TiN layers. The diffusivities determined correspond to grain boundary diffusion, the difference being due to the different microstructure. The very low diffusivity of silicon in B ₊ TiN layer makes it an excellent high-temperature barrier preventing silicon diffusion.     

Au diffusion in α-Ti

The solubility and diffusion of Au in -Ti have been studied in a 823–1023 K temperature range using the Rutherford backscattering technique. For this purpose we have implanted Au into -Ti samples. Our results show that the solubility of Au varies between 0.2 and 0.35 at.%. In addition, we found that the diffusion coefficients follow a normal Arrhenius behavior with Q=260 kJ/mol and D _o=1.9×10^–5 m²/s¹. These values are typical for a substitutional diffusion mechanism.     

Delayed muon induced fission of209Bi and the role of meson-exchange currents

The probability for delayed muon induced fission of²⁰⁹Bi has been determined from a ( ^–,f ₁ f ₂) measurement. The measured fission probability P _f =(4.2±0.7)×10^–5 is compared with theoretical predictions. The high fission threshold reaction seems well suited for studying the influence of two-body meson-exchange currents in nuclear muon capture.We are indebted to the following institutes or organizations for financial support: Bundesministerium für Forschung und Technologie der Bundesrepublik Deutschland contract number 06 BN 271 (HP, PD, HH, FR, CR), Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization of the Advancement of Pure Research (NWO) (JK, CTAMdL, WL, AT) and the Schweizer Nationalfonds (LS).     

Enhancement of the electron mobility with infrared photoexcitation in Si: Te at low temperatures

Hall measurements on Te-doped silicon (N _Te 10¹⁶ cm^–3) have been performed in the temperature range between 10 K and 300 K with infrared photoexcitation of electrons into the conduction band. The samples exhibit electron Hall mobilities which are increased by approximately 50% compared to measurements in the dark. The increased electron mobility can be correlated with an increased electron population of shallow donor levels by photoexcitation. Coulomb scattering due to charged shallow donor centers is converted into less efficient dipole scattering (Te-acceptor pairs) by the light-induced redistribution of electrons.     

Non-intrusive mapping of subsurface defects in semiconductors

We have adapted differential interference contrast Nomarski microscopy in the transmission configuration to the problem of mapping subsurface defects in semiconductors. We have demonstrated the ability to rapidly measure the depth of the precipitate-free-zone in silicon with a reproducibility of ±1 m in whole Si wafers up to 200 nm in diameter, having an extrinsic doping concentration up to 7×10¹⁹ cm^–3 and a nominal, as received, back side roughness. Because our subsurface defect profiler is completely non-destructive, product wafers can be inspected at various stages of processing and immediately returned to the production line.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Collisional broadening of Ba I line (553.5 nm) by He or Ar

The Doppler-free line shape of the 6s ^{2 1} S ₀ – 6s ¹6p ^{1 1} P ₁ transition at 553.5 nm in natural Ba in the presence of an argon or helium buffer gas has been measured at 744 K for gas pressures from 0.2 to 100 Torr. Using the measured data for pressures above 5 Torr, the broadening rate coefficients for the half width at half maximum (HWHM) are determined to be (4.9±0.5)10^–9 and (5.0±0.5)10^–9 s^–1 cm^–3 for helium and argon respectively.     

Transient response of infrared photoconductors: The roles of contacts and space charge

The transient response of an extrinsic photoconductor, with two implanted ohmic contacts, has been calculated by solving the full continuity equation using a variable finite difference technique. For a step function in photon flux under constant voltage bias, transient times ranging from 10^–4 to 10^–2 s have been determined for 20 to 200 m thick detectors under the low background fluxes typical of infrared astronomy. The transient response consists of a fast and a slow component, with their relative magnitudes dependent on the ratio of diffusion and drift lengths to the device length. The characteristic time for the fast component is determined, as expected, by the carrier lifetime, but a slower transient response is also present which is controlled by out-diffusion and sweep-out and the establishment of a counteracting electric field barrier. The effects of material and operating parameters have been investigated, and an analytical model is presented for estimating transient response times for any extrinsic photoconductor. Contact-limited transient response will be most limiting for operation of thin device structures under very low photon backgrounds.     

The physics of space charge instabilities and temporal chaos in extrinsic photoconductors

This paper reviews experimental and theoretical work carried out on space charge instabilities and temporal chaotic behavior in cooled extrinsic p-type Germanium photoconductors. Measured dc current-voltage (I–V) characteristics of these devices are strongly nonlinear for moderate electric fields 0.1 V/cm due to field dependence of the rates of free hole capture and impurity impact ionization. Below the threshold field for impurity breakdown, Ge samples behave like damped nonlinear oscillators, exhibiting characteristic chaotic response when driven by a time-periodic voltage. Above impurity breakdown, we observe voltage-controlled negative differential resistance (NDR) in the I–V curves accompanied by spontaneous current oscillations due to moving space charge domains with velocities 10³ to 10⁴ cm/s. Measurements are well explained by a simple rate equation model in which negative differential behavior in the impact ionization rate plays a crucial role. Related work on semiconductor chaos and possible future directions for research are also mentioned.     

Nearest neighbor effect on capture cross sections measured in IR phototransients on Si:In

The extrinsic photoconductive decay at T=20–100 K is analyzed in FZ-grown Si: In material after pulsed irradiation by a PbSSe infrared laser (=4 m). Trapping time constants (=10 ns-100 s) are resolved for the prevalent In acceptor (N _In=10¹⁶–10¹⁷ cm^–3) and for additional shallow acceptors B, Al, and the X(In)-center present at low concentrations (N=10¹²–10¹⁴ cm^–3). Hole capture cross sections determined for the acceptor levels show a large scatter over up to 4 orders of magnitude. It is shown that the capture cross section is dependent on all the dopant concentrations present in the sample due to nearest neighbor interaction. Due to the formation of donor-acceptor dipoles, the capture cross section assumes low values. A model calculation of the interaction based on only fundamental parameters of Si is in accordance with the experimental data within the experimental error. The hole capture cross sections for isolated acceptors are _p=1×10^–12, 1×10^–14, 1×10^–13, 2.5×10^–13 cm² for indium, X-center, aluminum, and boron at the temperatures T=95 K, 100 K, 70 K, 45 K, respectively.     

NMR self-diffusion study of organic glasses: COANP,MBANP, PNP,NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of organic glasses COANP, MBANP, PNP and NPP in their liquid and weakly supercooled states was performed. The NMR phase diagrams, based on the proton NMR transverse relaxation time (T ₂) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phase on cooling the isotropic liquids below their respective melting temperatures. The self-diffusion data exhibit in the supercooled glassy state a non-Arrhenius behaviour and can be described in terms of the Vogel-Fulcher modification of the Arrhenius law,D=D exp{–E _a /[k _B (T–T _VF )]}. The activation energiesE _a and Vogel-Fulcher temperaturesT _VF are 83.2 meV and 239 K for COANP, 66 meV and 249 K for MBANP, and 85 meV and 245 K for PNP, respectively. The flow viscosity data obtained for COANP in the same temperature region as well conform to the Vogel-Fulcher behaviour, exp{E _a ⁽⁾ /[k _B (T–T _VF )]}, withE _a ⁽⁾ =80.4 meV andT _VF =239 K. In case of COANPD was found to increase with decreasing diffusion time in the supercooled (glassy) melt just belowT _M whereas no such behaviour was found aboveT _M .     

Nuclearg-factors of the 1229 and 2911 keV isomers in143Nd

Nuclearg-factors of the 1229 keV 13⁺/2 and 2911 keV 21⁺/2 level and the half-life of the 1229 keV level have been measured to beg(1229)=0.058(5),g(2911)=0.69(12) andT _1/2 (1229)=6.79(2) ns. The experimental g-factors show that the 1229 keV level has a dominant configuration of (f _7/23^–), and the 2911 keV level has an almost pure (f _7/27^–), configuration.The authors would like to thank the crew of CYRIC for the operation of the cyclotron.     

Primary Matter Creation in a Weyl–Dirac Cosmological Model

In the framework of an integrable Weyl–Dirac (W–D) theory a cosmological model is proposed. It describes a universe that began its expansion from a primary pre-Planckian geometric entity containing no matter. During the pre-Planckian period, from R ₀ =5.58×10 ^–36 cm to R_I=5.58×10 ^–34 cm, this embryonic universe has undergone a very rapid expansion and cosmic matter was created by geometry. At R_I the universe was already filled with matter having the Planckian density _P and being in the state of prematter (P=–), while the Weylian geometric elements were insignificant. This state is the Planckian egg that has served as the initial state of the singularity-free cosmological model ⁽¹⁾ considered in the framework of Einstein's general theory of relativity. The W–D character of the geometry and the cosmological constant are significant in the pre-Planckian period during the matter creation. In the dust-dominated period a relic of the W–D geometry causes a global dark matter effect. In between the pre-Planckian and dust period one has Einstein's framework and is negligible.     

Pion production in heavy ion reactions induced by16O and84Kr beams at 60 MeV per nucleon

Low energy ⁺ and ^– production cross sections have been measured in¹⁶O at 65 MeV/u and⁸⁴Kr at 60 MeV/u induced reactions on⁷Li,¹²C,²⁷Al,¹⁰⁸Ag and¹⁹⁷Au. The results are compared with a previous measurement obtained with¹⁶O at 93 MeV/u. Large deviations are observed between light and heavy ion data. At variance with¹⁶O, the ratios ^–/ ⁺ are independent of the target in the Krypton case. A change in the regime of the projectile-target mass scaling of cross-sections is observed with the heaviest projectile.Experiments performed at GANIL facility, F-14032 Caen, France     

Neutron scattering study of the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2

We have carried out a neutron scattering investigation of the static structure factorS(q _2D ) (q _2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr₂CuO₂Cl₂. For the spin correlation length we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr₂CuO₂Cl₂-Monte Carlo data, which cover the length scale from 1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0) ² for the reduced temperature range 0.16<T/2 _s <0.36, whereas current theories predict that at low temperaturesS(0)T ^{2 2}. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ _XY /J=1.4×10^–4. From measurements of the sublattice mangetization we obtain =0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La₂CuO₄, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K₂NiF₄, the gap energy in Sr₂CuO₂Cl₂ scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K₂NiF₄ using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffness _s reduced by 20% from the spin-wave value. As in Sr₂CuO₂Cl₂ we find thatS(0) ² for the reduced temperature range 0.22<T/2 _s <0.47.     

Study of interaction between ZrO2 (YSZ) substrates and YBa2Cu3O7−δ superconducting films

The interactions between YBa₂Cu₃O_7– superconducting films and ZrO₂ substrates with different crystal orientations were investigated. Our results show that the crystal orientation of the substrate influences the orientations of the YBa₂Cu₃O_7– crystals, whose c-axis have preferential orientations parallel to the surface of 100 ZrO₂ substrates. The copper segregation in Y-Ba-Cu-O/ZrO₂ films results from substrate reactions. Y, Ba, and Cu atoms diffusing into substrates were detected by AES and RBS techniques. A thin silver film evaporated on ZrO₂ substrate as a buffer layer effectively slows down or eliminates the substrate interactions. The strong preferential c-axis orientation perpendicular to the substrate surface has been observed in the YBa₂Cu₃O_7– /Ag/ZrO₂ samples.This work has been supported by Science Foundation of Academia Sinica     

Electrooptical Kerr constant and third-order susceptibility measurements in nematic liquid crystals and their mixtures

We report for the first time the determination of the real part of the third order nonlinear susceptibilities ⁽³⁾ (–, , 0, 0) in the isotropic phase of two nematic liquid crystalsp-(ethoxybenzylidine)-p-butylaniline (EBBA) andp-(methoxybenzylidene)-p-butylaniline (MBBA) from electrooptic Kerr effect experiments. The highest value of ⁽³⁾ observed near nematic-to-isotropic transition temperature, at 632.8 nm is found to be 1.067×10^–20 m²V^–2 for EBBA and 6.602×10^–20 m²V^–2 for MBBA.     

Fluorescence Study on Translational Mobility in Polystyrene–Polyethyleneglycol Microbeads

Polyethyleneglycol (PEG) chain mobility in gelatineous microbeads is investigated by means of dynamic excimer formation. Pyrenebutyric acid (PYB) is covalently linked to the chain ends as probe molecule. Excimer formation is monitored by steady-state and time-resolved fluorescence spectroscopy in the presence of a series of liquid phases and in the dry state. PYB concentration in the beads is varied over three orders of magnitude up to c = 6·10^–2 M. The concentration is derived from absorption measurements in stirred bead suspensions, considering the deviations from Lambert–Beer's law in heterogeneous systems. Excimer formation is found to be a dynamic process in the presence of liquid phases which solvate both the polymer and the fluorophore. The collisional rate constant, k _DM, is of the order of k _DM-values of PYB in homogeneous solutions, indicating a high translational mobility. Excimer-to-monomer intensity ratios are in general accordance with the solvation capacity of the liquid phase. In the dry state excimer formation is found only at high PYB concentrations, c 3·10^–2 M. It is concluded that this excimer emission arises from aggregated PYB, since corresponding fluorescence response curves show no rise time.