Electronics method to advance the coincidence time resolution with bismuth germanate.

Exploiting the moderate Cherenkov yield from 511 keV photoelectric interactions in bismuth germanate (BGO) scintillators enables one to achieve a level of coincidence time resolution (CTR) appropriate for time-of-flight positron emission tomography (TOF-PET). For this approach, owing to the low number of promptly emitted light photons, single photon time resolution (SPTR) can have a stronger influence on achievable CTR. We have previously shown readout techniques that reduce effective device capacitance of large area silicon photomultipliers (SiPMs) can yield improvements in single photon response shape that minimize the influence of electronic noise on SPTR. With these techniques, sub-100 ps FWHM SPTR can be achieved with [Formula: see text] mm2 FBK near-ultra-violet high density (NUV-HD) SiPMs. These sensors are also useful for detecting Cherenkov light due to relatively high photon detection efficiency for UV light. In this work, we measured CTR for BGO crystals coupled to FBK NUV-HD SiPMs with a passive bootstrapping readout circuit that effectively reduces the SiPM device capacitance. A range of CTR values between 200 [Formula: see text] 3 and 277 [Formula: see text] 7 ps FWHM were measured for 3 [Formula: see text] 3 [Formula: see text] 3 and 3 [Formula: see text] 3 [Formula: see text] 15 mm3 crystals, respectively. This readout technique provides a relatively simple approach to achieve state-of-the-art CTR performance using BGO crystals for TOF-PET.

Evaluation of 72Se/72As generator and production of 72Se for supplying 72As as a potential PET imaging radionuclide.

Positron-emitting 72As is the PET imaging counterpart for beta-emitting 77As. Its parent, no carrier added (n.c.a.) 72Se, was produced for a 72Se/72As generator by irradiating an enriched 7°Ge metal-graphite target via the 70Ge(α, 2 n)72Se reaction. Target dissolution used a fast, environmentally friendly method with 93% radioactivity recovery. Chromatographic parameters of the 72Se/72As generator were evaluated, the eluted n.c.a. 72As was characterized with a phantom imaging study, and the previously reported trithiol and aryl-dithiol ligand systems were radiolabeled with the separated n.c.a. 72As in high yield.

Production of curie quantities of 68Ga with a medical cyclotron via the 68Zn(p,n)68Ga reaction.

PET imaging with 68Ga-labeled tracers has seen a dramatic increase over the past five years primarily due to the increased accessibility of 68Ge/68Ga generators, the availability of tracers with superb targeting properties for labeling, straightforward labeling procedures and the approval of these tracers by regulatory entities. Available 68Ge/68Ga generators nominally deliver up to 1.85 GBq (50mCi) when fresh limiting production and distribution of 68Ga-labeled tracers to a few daily doses per generator. The focus of this study was to provide a simple and efficient method for 68Ga production in clinically relevant quantities using a low energy medical cyclotron with a solid target.

Implementation of a Portable HPGe for Field Contamination Assay.

Using MCNP to construct a detector model based initially on x-ray images of a portable high purity germanium (HPGe) detector followed by normalizing covering material values to also agree with check source responses, a validation of the model was attained. By calibrating the detector parameters using large count spectra, rigorous reproducibility is attained for high activity measurements but does not prevent deviations from normality in error distributions at the very low count events where spectral peaks are not always identifiable. The resulting model was created to allow operational assay of contamination over large areal distributions that could not otherwise be measured, such as the exhaust shaft at the Waste Isolation Pilot Plant (WIPP). Results indicate that contamination levels of activity in the exhaust shaft can be assayed to within a factor of 2. Detection limits are evaluated to be well below the contamination levels, which would constitute a legal environmental release if unfiltered ventilation of the underground facility were used.

Applying a low energy HPGe detector gamma ray spectrometric technique for the evaluation of Pu/Am ratio in biological samples.

The estimation of Pu/(241)Am ratio in the biological samples is an important input for the assessment of internal dose received by the workers. The radiochemical separation of Pu isotopes and (241)Am in a sample followed by alpha spectrometry is a widely used technique for the determination of Pu/(241)Am ratio. However, this method is time consuming and many times quick estimation is required. In this work, Pu/(241)Am ratio in the biological sample was estimated with HPGe detector based measurements using gamma/X-rays emitted by these radionuclides. These results were compared with those obtained from alpha spectroscopy of sample after radiochemical analysis and found to be in good agreement.

Dissolution chemistry and biocompatibility of silicon- and germanium-based semiconductors for transient electronics.

Semiconducting materials are central to the development of high-performance electronics that are capable of dissolving completely when immersed in aqueous solutions, groundwater, or biofluids, for applications in temporary biomedical implants, environmentally degradable sensors, and other systems. The results reported here include comprehensive studies of the dissolution by hydrolysis of polycrystalline silicon, amorphous silicon, silicon-germanium, and germanium in aqueous solutions of various pH values and temperatures. In vitro cellular toxicity evaluations demonstrate the biocompatibility of the materials and end products of dissolution, thereby supporting their potential for use in biodegradable electronics. A fully dissolvable thin-film solar cell illustrates the ability to integrate these semiconductors into functional systems.

Guided mode resonance enabled ultra-compact Germanium photodetector for 1.55 µm detection.

We propose a novel technique of enhancing the photodetection capabilities of ultrathin Ge films for normally incident light at 1.55 µm through the guided mode resonance (GMR) phenomenon. Specifically, by suitably patterning the surface of a Ge thin film, it is possible to excite guided modes which are subsequently coupled to free space radiative modes, resulting in spectral resonances that possess locally enhanced near fields with a large spatial extent. Absorption is found to be enhanced by over an order of magnitude over a pristine Ge film of equal thickness. Furthermore, attenuation of incident light for such a structure occurs over very few grating periods, resulting in significantly enhanced theoretical 3 dB bandwidth-efficiency products of ~58 GHz. The nature of the enhancement mechanism also produces spectrally narrow resonances (FWHM ~30 nm) that are polarization sensitive and exhibit excellent angular tolerance. Finally, the proposed device architecture is fully compatible with existing Si infrastructure and current CMOS fabrication processes.

GeSn-on-Si normal incidence photodetectors with bandwidths more than 40 GHz.

GeSn (Sn content up to 4.2%) photodiodes with vertical pin structures were grown on thin Ge virtual substrates on Si by a low temperature (160 °C) molecular beam epitaxy. Vertical detectors were fabricated by a double mesa process with mesa radii between 5 µm and 80 µm. The nominal intrinsic absorber contains carrier densities from below 1 · 10(16) cm(-3) to 1 · 10(17) cm(-3) for Ge reference detectors and GeSn detectors with 4.2% Sn, respectively. The photodetectors were investigated with electrical and optoelectrical methods from direct current up to high frequencies (40 GHz). For a laser wavelength of 1550 nm an increasing of the optical responsivities (84 mA/W -218 mA/W) for vertical incidence detectors with thin (300 nm) absorbers as function of the Sn content were found. Most important from an application perspective all detectors had bandwidth above 40 GHz at enough reverse voltage which increased from zero to -5 V within the given Sn range. Increasing carrier densities (up to 1 · 10(17) cm(-3)) with Sn contents caused the depletion of the nominal intrinsic absorber at increasing reverse voltages.

Photoinduced features of energy bandgap in quaternary Cu2CdGeS4 crystals.

The quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using a cw 532 nm green laser with energy density about 0.4 J cm(-2). The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15-0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are also reported.

Photo-induced persistent inversion of germanium in a 200-nm-deep surface region.

The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion with a half-life of ~12 hours at a temperature of 220 K which disappears above 280 K. The photo-induced inversion is absent for a sample with a 20-nm-thick gold capping layer providing a Schottky barrier at the interface. This indicates that charge accumulation at the surface is essential to explain the observed inversion. The contactless change of carrier concentration is potentially interesting for device applications in opto-electronics where the gate electrode and gate oxide could be replaced by the semiconductor surface.

Ge/Si heterojunction photodiodes fabricated by low temperature wafer bonding.

We report on the photoresponse of an asymmetrically doped p(-)-Ge/n(+)-Si heterojunction photodiode fabricated by wafer bonding. Responsivities in excess of 1 A/W at 1.55 µm are measured with a 5.4 µm thick Ge layer under surface-normal illumination. Capacitance-voltage measurements show that the interfacial band structure is dependent on both temperature and light level, moving from depletion of holes at -50 °C to accumulation at 20 °C. Interface traps filled by photo-generated and thermally-generated carriers are shown to play a crucial role. Their filling alters the potential barrier height at the interface leading to increased flow of dark current and the above unity responsivity.

Anti-Stokes luminescence in bismuth-doped silica and germania-based fibers.

Luminescence excitation spectra of active centers in bismuth-doped vitreous SiO(2) and vitreous GeO(2) optical fibers under the two-step excitation have been obtained for the first time. The results revealed only one bismuth-related IR active center formed in each of these fibers. The observed IR luminescence bands at 1430 nm (1650 nm) and 830 nm (950 nm), yellow-orange (red) band at 580 nm (655 nm), violet (blue) band at 420 nm (480 nm) belong to this bismuth-related active center in the vitreous SiO(2) (vitreous GeO(2)), correspondingly.

Fast response in-line gas sensor using C-type fiber and Ge-doped ring defect photonic crystal fiber.

An in-line chemical gas sensor was proposed and experimentally demonstrated using a new C-type fiber and a Ge-doped ring defect photonic crystal fiber (PCF). The C-type fiber segment served as a compact gas inlet/outlet directly spliced to PCF, which overcame previous limitations in packaging and dynamic responses. C-type fiber was prepared by optimizing drawing process for a silica tube with an open slot. Splicing conditions for SMF/C-type fiber and PCF/C-type fiber were experimentally established to provide an all-fiber sensor unit. To enhance the sensitivity and light coupling efficiency we used a special PCF with Ge-doped ring defect to further enhance the sensitivity and gas flow rate. Sensing capability of the proposed sensor was investigated experimentally by detecting acetylene absorption lines.

Investigations of morphology and formation mechanism of laser-induced annular/droplet-like structures on SiGe film.

We describe the fabrication of nanostructures on SiGe film by KrF excimer laser with nanosecond pulse width, and find a more direct and clear relationship between the laser irradiation conditions and the nanoscale structures. Perfect annular nanostructures around scattering points on the SiGe film are firstly obtained after the irradiation of a KrF excimer pulse laser beam (100 mJ/cm(2)) at different incident angles. The different shapes of annular structures are related to different energy distributions due to the optical interference between the scattered light and the incident beam. As laser energy increases, a threshold of pulse energy (230 mJ/cm(2)) is found, above which a droplet-like morphology completely replacing the surface annular structures. And the disorder morphology is mainly caused by the thermal effect of the incident beam.

Nanoscale resonant-cavity-enhanced germanium photodetectors with lithographically defined spectral response for improved performance at telecommunications wavelengths.

We demonstrate the use of a subwavelength planar metal-dielectric resonant cavity to enhance the absorption of germanium photodetectors at wavelengths beyond the material's direct absorption edge, enabling high responsivity across the entire telecommunications C and L bands. The resonant wavelength of the detectors can be tuned linearly by varying the width of the Ge fin, allowing multiple detectors, each resonant at a different wavelength, to be fabricated in a single-step process. This approach is promising for the development of CMOS-compatible devices suitable for integrated, high-speed, and energy-efficient photodetection at telecommunications wavelengths.

Comparison of background in underground HPGe-detectors in different lead shield configurations.

In underground HPGe-detector systems where the cosmic ray induced background is low, it is often difficult to assess the location of background sources. In this study, background counting rates of different HPGe-detectors in different lead shields are reported with the aim of better understanding background sources. To further enhance the understanding of the variations of environmental parameters, the background as a function of time over a long period was also studied.

Tough germanium nanoparticles under electrochemical cycling.

Mechanical degradation of the electrode materials during electrochemical cycling remains a serious issue that critically limits the capacity retention and cyclability of rechargeable lithium-ion batteries. Here we report the highly reversible expansion and contraction of germanium nanoparticles under lithiation-delithiation cycling with in situ transmission electron microscopy (TEM). During multiple cycles to the full capacity, the germanium nanoparticles remained robust without any visible cracking despite ∼260% volume changes, in contrast to the size-dependent fracture of silicon nanoparticles upon the first lithiation. The comparative in situ TEM study of fragile silicon nanoparticles suggests that the tough behavior of germanium nanoparticles can be attributed to the weak anisotropy of the lithiation strain at the reaction front. The tough germanium nanoparticles offer substantial potential for the development of durable, high-capacity, and high-rate anodes for advanced lithium-ion batteries.

Efficiency improvement of organic solar cells by tuning hole transport layer with germanium oxide.

Improving optical property is critical for optimizing the power conversion efficiency of organic solar cells. In the present research, we show that modification of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer with GeO2 leads to 15% improvement of power conversion efficiency in a polymer solar cells through enhancement of short circuit currents. Modified PEDOT:PSS layer with optimized concentration of GeO2 assists active layer absorbing much light by playing a role of optical spacer. Using AFM and grazing incidence X-ray diffraction (GIXD) data, we also present the evidence that an addition of GeO2 does not affect crystallinity of active layer.

Si/Ge uni-traveling carrier photodetector.

We have fabricated and characterized a germanium on silicon uni-traveling carrier photodetector for analog and coherent communications applications. The device has a bandwidth of 20GHz, a large-signal 1dB saturation photocurrent of 20mA at -3V, and a low thermal impedance of 520K/W.

Silicon-Germanium multi-quantum well photodetectors in the near infrared.

Single crystal Silicon-Germanium multi-quantum well layers were epitaxially grown on silicon substrates. Very high quality films were achieved with high level of control utilizing recently developed MHAH epitaxial technique. MHAH growth technique facilitates the monolithic integration of photonic functionality such as modulators and photodetectors with low-cost silicon VLSI technology. Mesa structured p-i-n photodetectors were fabricated with low reverse leakage currents of ~10 mA/cm² and responsivity values exceeding 0.1 A/W. Moreover, the spectral responsivity of fabricated detectors can be tuned by applied voltage.