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Anxiety symptoms gradually emerge during childhood and adolescence. Individual differences in behavioral inhibition (BI), an early-childhood temperament, may shape developmental paths through which these symptoms arise. Cross-sectional research suggests that level of early-childhood BI moderates associations between later anxiety symptoms and threat-related amygdala–prefrontal cortex (PFC) circuitry function. However, no study has characterized these associations longitudinally. Here, we tested whether level of early-childhood BI predicts distinct evolving associations between amygdala–PFC function and anxiety symptoms across development.
Eighty-seven children previously assessed for BI level in early childhood provided data at ages 10 and/or 13 years, consisting of assessments of anxiety and an fMRI-based dot-probe task (including threat, happy, and neutral stimuli). Using linear-mixed-effects models, we investigated longitudinal changes in associations between anxiety symptoms and threat-related amygdala–PFC connectivity, as a function of early-childhood BI.
In children with a history of high early-childhood BI, anxiety symptoms became, with age, more negatively associated with right amygdala–left dorsolateral-PFC connectivity when attention was to be maintained on threat. In contrast, with age, low-BI children showed an increasingly positive anxiety–connectivity association during the same task condition. Behaviorally, at age 10, anxiety symptoms did not relate to fluctuations in attention bias (attention bias variability, ABV) in either group; by age 13, low-BI children showed a negative anxiety–ABV association, whereas high-BI children showed a positive anxiety–ABV association.
Early-childhood BI levels predict distinct neurodevelopmental pathways to pediatric anxiety symptoms. These pathways involve distinct relations among brain function, behavior, and anxiety symptoms, which may inform diagnosis and treatment.
In our x-ray calorimetry effort, we have developed several techniques which may be helpful to other groups working in this field. We are studying several different monolithic and composite calorimeter designs. In our readout configuration, the preamplifier circuit employs negative voltage feedback which allows us to accurately measure the temporal profile of the thermal pulse produced by an x-ray absorbed in a micro-calorimeter. Rise times of less than two microseconds have been observed in monolithic devices operating at .3 K. Furthermore, the feedback preamplifier can be configured for either positive or negative electro-thermal feedback. This preamplifier system is followed by an analog pulse shaping amplifier with a frequency response that can be adjusted to yield the maximum signal to noise ratio for a given thermal response of the calorimeter. In addition, we have developed several diagnostic procedures which have been useful in determining the operating and noise characteristics of our devices. These include an infrared light-emitting diode which flashes a discrete amount of energy on to the calorimeter, and a capacitively coupled test input to the preamplifier which allows us to directly determine the total noise in the thermal detection system. Finally, we are developing an adiabatic demagnetization refrigerator with a temperature control system that is designed to stabilize the 0.1 K cold stage to better than 8 μK. This is required for a resistive thermal detector with resolving power of 1000.
We report nuclear magnetic resonance measurements of deuterone diffused into silicon powders. Deuterium spectra show a two-component lineshape corresponding to two distinct hydrogen sites. The component with a narrow Lorentzian lineshape is assigned to D2 molecules. The second component is a quadrupolar doublet powder pattern, and its characteristic splitting provides a value of the electric field gradient at the site of the deuterium atom of 1.45×105 dyn/cm.
Dilute III-Nx-V1-x alloys were successfully synthesized by nitrogen implantation in GaAs and InP. The fundamental band gap energy for the ion beam synthesized III-Nx-V1-x alloys was found to decrease with increasing N implantation dose in a manner similar to that commonly observed in epitaxially grown GaNxAs1-x and InNxP1-x thin films. The fraction of N occupying anion sites (“active” N) in the GaNxAs1-x layers formed by N implantation was thermally unstable and decreased with increasing annealing temperature. In contrast, thermally stable InNxP1-x alloys with N mole fraction as high as 0.012 were synthesized by N implantation in InP. Moreover, the N activation efficiency in InP was at least a factor of two higher than in GaAs under similar processing conditions. The low N activation efficiency (<20%) in GaAs can be improved by co-implanting Ga and N in GaAs.
We present experimental results of impurity and self-diffusion in an isotopically controlled silicon heterostructure extrinsically doped with phosphorus. As a consequence of extrinsic doping, the concentration of singly negatively charged native defects is enhanced and the role of these native defect charge states in the simultaneous phosphorus and Si self-diffusion can be determined. Multilayers of isotopically controlled 28Si and natural silicon enable simultaneous analysis of 30Si self-diffusion into the 28Si enriched layers and phosphorus diffusion throughout the multilayer structure. An amorphous 260 nm thick Si cap layer was deposited on top of the Si isotope heterostructure. The phosphorus ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer, preventing the generation of excess native defects and enabling the determination of the Si self-diffusion coefficient and the phosphorus diffusivity under equilibrium conditions. These samples were annealed at temperatures between 950 and 1100°C to study the diffusion. Detailed analysis of the diffusion process was performed on the basis of a P diffusion model which involves neutral and positively charged mobile P species and neutral and singly negatively charged self-interstitial.
Dilute III-Nx-V1-x alloys were successfully synthesized by nitrogen implantation in
GaAs and InP. The fundamental band gap energy for the ion beam synthesized III-Nx-V1-x alloys was found to decrease with increasing N implantation dose
in a manner similar to that commonly observed in epitaxially grown GaNxAs1-x and InNxP1-xthin films. The fraction of N occupying anion sites ("active" N) in
the GaNxAs1-x layers formed by N implantation was thermally unstable and
decreased with increasing annealing temperature. In contrast, thermally
stable InNxP1-x alloys with N mole fraction as high as 0.012 were synthesized by N
implantation in InP. Moreover, the N activation efficiency in InP was at
least a factor of two higher than in GaAs under similar processing
conditions. The low N activation efficiency (<20%) in GaAs can be
improved by co-implanting Ga and N in GaAs.
Under certain conditions, nucleation and growth can lead to substantial stresses in nanocrystals embedded in a host matrix. These stresses may be relaxed through subsequent annealing treatments. A model is presented for the relaxation of these stresses via diffusive processes within the matrix. The model reflects the effects of surface tension, potential phase transformations at or near the processing temperature, and differential thermal expansion. It is demonstrated that the model describes well the stress relaxation of ion beam synthesized Ge nanocrystals embedded in a silica matrix.
MOCVD growth of As-doped GaN using dimethylhydrazine, triethylgallium and tertiarybutylarsenic has been investigated. A maximum doping concentration of 4.0 × 1019cm−3 at growth temperatures between 600°C and 800°C was obtained. At 1000°C the As doping level dropped below the SIMS detection limit of ∼1.0 × 1017cm−3. The As incorporation depended only weakly on variations of the V/III molar flow ratio between 11 and 61. Raising the As/V molar flow ratio from 0.01 to 0.06 increased the As concentration which then decreased by further increase to 0.11. Different morphologies of the layers were found depending on the growth conditions. A surfactant-like behavior of As was observed leading to smooth GaN films grown on top of the As-doped GaN layer. Two characteristic luminescence peaks at 3.31 eV and 3.425 eV were found for samples doped with As below 900°C. These spectral features are believed to originate at extended lateral defects - presumably stacking faults.
The elemental semiconductors silicon and germanium can be purified to electrically active impurity concentrations as low as ∼1010cm−3. Highly sensitive, energy dispersive analytical techniques have been developed to identify and measure the concentration of the residual elemental impurities. The application of these techniques to very pure materials has also resulted in the discovery of a large number of new levels which are due to impurity/defect complexes. Photothermal ionization spectroscopy using uniaxial stress or a magnetic field, electron paramagnetic resonance, and doping experiments using stable and radioactive elements have been used in combination to identify the composition and the structure of some of the new centers.
We have undertaken a systematic study of the effect of co-implantation on the electrical properties of C implanted in GaAs. Two effects have been studied, the additional damage caused by co-implantation and the stoichiometry in the implanted layer. A series of co-implant ions were used: group III (B, Al, Ga), group V (N, P, As) and noble gases (Ar, Kr). Co-implantation of ions which create an amorphous layer was found to increase the electrical activity of C Once damage was created, maintaining stoichiometric balance by co-implantation of a group III further increased the fraction of electrically active carbon impurities. Co-implantation of Ga and rapid thermal annealing at 950°C for 10 s resulted in carbon activation as high as 68%, the highest value ever reported.
Ion implantation into silica followed by thermal annealing is an established growth method for Si and Ge nanocrystals. We demonstrate that growth of Group IV semiconductor nanocrystals can be suppressed by co-implantation of oxygen prior to annealing. For Si nanocrystals, at low Si/O dose ratios, oxygen co-implantation leads to a reduction of the average nanocrystal size and a blue-shift of the photoluminescence emission energy. For both Si and Ge nanocrystals, at larger Si/O or Ge/O dose ratios, the implanted specie is oxidized and nanocrystals do not form. This chemical deactivation was utilized to achieve patterned growth of Si and Ge nanocrystals. Si was implanted into a thin SiO2 film on a Si substrate followed by oxygen implantation through an electron beam lithographically defined stencil mask. Thermal annealing of the co-implanted structure yields two-dimensionally patterned growth of Si nanocrystals under the masked regions. We applied a previously developed process to obtain exposed nanocrystals by selective HF etching of the silica matrix to these patterned structures. Atomic force microscopy (AFM) of etched structures revealed that exposed nanocrystals are not laterally displaced from their original positions during the etching process. Therefore, this process provides a means of achieving patterned structures of exposed nanocrystals. The possibilities for scaling this chemical-based lithography process to smaller features and for extending it to 3-D patterning is discussed.
Wide bandgap GaN very often shows a high electron concentration. Although several impurities such as O and Si have been identified, the concentration is not high enough to account for the number of free carriers. As a consequence native defects namely the nitrogen vacancies are widely considered to be present at high densities. Several calculations predict different energy levels of this strongly localized defect. We present photoluminescence experiments of wurtzite GaN and AlGaN layers under large hydrostatic pressure to search for localized defects within the questionable energy range of 3 .0 to 3 .8 eV above the valence band edge.
Impurities and defects with masses smaller than the masses of the host semiconductor crystal atoms typically exhibit vibrational frequencies well above the phonon frequency spectrum. These vibrational modes produce sharp spectral absorption features in the infrared. Because of their strong spatial localization these modes are not affected by neighboring impurities and/or defects with concentrations up to 1019 cm−3. This insensitivity is especially advantageous when the free carrier concentration must be reduced through the introduction of electron irradiation defects or when highly doped thin layers must be investigated. LVM spectroscopy with perturbations such as polarization of the probe light, uniaxial and hydrostatic stress, and isotope substitution has been highly successful in identifying the structure and composition of a large number of defect complexes. Hydrogen, in particular, forming a wide variety of complexes in elemental and compound semiconductors has been extensively studied with LVM spectroscopy. For example, it has been shown recently that nitrogen acceptors are hydrogen passivated in MOCVD grown ZnSe. Carbon and oxygen have been investigated in all major semiconductors with LVM spectroscopy. The extreme simplification of the spectrum of bond centered oxygen through isotope enrichment of several Ge crystals has been demonstrated. Additional recent investigations of importance to the currently much studied semiconductors will be reviewed.
A method for wafer annealing which is effective in suppressing defects and raising minority carrier diffusion lengths in n-type bulk GaAs is described. The beneficial effect of the annealing is shown to be associated with the proximity surface with measurements of photoresponse as a function of depth. The concentration of the hole trap HCX (Ev+0.29eV) varies as a function of depth from the surface, qualitatively, as might be expected of the concentration of the dominant recombination center in the material. The impact of improving the material in this manner on the performance of Zn diffused solar cells is demonstrated.
The 0.11 eV Mn acceptor has been investigated using different kinds of FΠR techniques, Zeeman spectroscopy, and photoluminescence. The results clearly fits into the Зd5+ shallow hole model for Mn° and show that the 0.11 eV level originates from the io-nization of a neutral, substitutional Mn acceptor at a Ga-site. The ground state binding energy obtained from the effective-mass like excited states is 112.4 meV.
Extremely narrow far-infrared lines  of OH-donors in ultrapure germanium are used to probe the fundamental dynamic processes of impurities. We examine the 1s-2p transition as a function of laser intensity, using photothermal ionization spectroscopy (PTIS) modified by Zeeman tuning with frequency-fixed lasers. We observe a change of the resonance line shape in the intensity region near 10-4 W/cm2. This effect can be quantitatively understood in a rate equation model which shows that at the critical intensity the ground state becomes depleted and the dependence of the recombination on the degree of ionization becomes important. Therefore the critical intensity depends also on compensation.
Cu diffused GaAs samples have been investigated using different kinds of FΠR techniques and photoluminescence. The results suggest that tne “0.15 eV” level originates from the ioruzation of a neutral, nearly substitutional Cu acceptor at a Ga site. Furthermore, the results indicate a distortion in the  direction. The ground state binding energy obtained from the effective-mass-like excited states is 157.8 meV at 6K.
We present a detailed study of the photoluminescence (PL) properties of a wide range of GaAs material diffused with the group I element Li. The effects of the Li diffusion are investigated through its effects on existing photoluminescence bands in the as-grown material as well as the appearence of new such bands. Among new PL bands resulting from the Li doping of n-type and semi-insulating material the most pronounced ones are a strong deep band at 1.34 eV and shallower bands at 1.45 and 1.48 eV. The origin of these PL bands will be discussed.