Three-dimensional charge transport mapping by two-photon absorption edge transient-current technique in synthetic single-crystalline diamond

We demonstrate the application of two-photon absorption transient current technique to wide bandgap semiconductors. We utilize it to probe charge transport properties of single-crystal Chemical Vapor Deposition (scCVD) diamond. The charge carriers, inside the scCVD diamond sample, are excited by a femtosecond laser through simultaneous absorption of two photons. Due to the nature of two-photon absorption, the generation of charge carriers is confined in space (3-D) around the focal point of the laser. Such localized charge injection allows to probe the charge transport properties of the semiconductor bulk with a fine-grained 3-D resolution. Exploiting spatial confinement of the generated charge, the electrical field of the diamond bulk was mapped at different depths and compared to an X-ray diffraction topograph of the sample. Measurements utilizing this method provide a unique way of exploring spatial variations of charge transport properties in transparent wide-bandgap semiconductors.Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Applied Physics Letters and may be found at https://doi.org/10.1063/1.509085

Bias dependence and bistability of radiation defects in silicon

Influence of bias on effective dopant concentration in neutron and pion irradiated$p^+ - n - n^+$ diodes has been measured. Detailed studies of annealing of the bias-induced damagehave revealed three components, with introduction rates from 0.005 to 0.008 cm$^{-1}$ andannealing time constants ranging from 5 to 1000 hours at 20$^\circ$C. Variation of annealing temperatures yielded activation energies around 1 eV for all the three components. Bistable behavior of radiation damage under bias has been observed and its activation and annealingstudied. The bistable damage was associated to the fastest annealing component of bias-induced damage.Using the parameterization obtained, a prediction for ATLAS SCT operation was made.Bias-induced damage is shown to require an additional 80 V to fully deplete detectors at the end of LHC operation

Analysis of edge and surface TCTs for irradiated 3D silicon strip detectors

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Radiation Hardness of Thin Low Gain Avalanche Detectors

Low Gain Avalanche Detectors (LGAD) are based on a n++-p+-p-p++ structure where an appropriate doping of the multiplication layer (p+) leads to high enough electric fields for impact ionization. Gain factors of few tens in charge significantly improve the resolution of timing measurements, particularly for thin detectors, where the timing performance was shown to be limited by Landau fluctuations. The main obstacle for their operation is the decrease of gain with irradiation, attributed to effective acceptor removal in the gain layer. Sets of thin sensors were produced by two different producers on different substrates, with different gain layer doping profiles and thicknesses (45, 50 and 80 um). Their performance in terms of gain/collected charge and leakage current was compared before and after irradiation with neutrons and pions up to the equivalent fluences of 5e15 cm-2. Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. The thin LGAD sensors were shown to perform much better than sensors of standard thickness (~300 um) and offer larger charge collection with respect to detectors without gain layer for fluences <2e15 cm-2. Larger initial gain prolongs the beneficial performance of LGADs. Pions were found to be more damaging than neutrons at the same equivalent fluence, while no significant difference was found between different producers. At very high fluences and bias voltages the gain appears due to deep acceptors in the bulk, hence also in thin standard detectors

Gain and time resolution of 45 μ\mum thin Low Gain Avalanche Detectors before and after irradiation up to a fluence of 101510^{15} neq_{eq}/cm2^2

Low Gain Avalanche Detectors (LGADs) are silicon sensors with a built-in charge multiplication layer providing a gain of typically 10 to 50. Due to the combination of high signal-to-noise ratio and short rise time, thin LGADs provide good time resolutions. LGADs with an active thickness of about 45 $\mu$m were produced at CNM Barcelona. Their gains and time resolutions were studied in beam tests for two different multiplication layer implantation doses, as well as before and after irradiation with neutrons up to $10^{15}$ n$_{eq}$/cm$^2$. The gain showed the expected decrease at a fixed voltage for a lower initial implantation dose, as well as for a higher fluence due to effective acceptor removal in the multiplication layer. Time resolutions below 30 ps were obtained at the highest applied voltages for both implantation doses before irradiation. Also after an intermediate fluence of $3\times10^{14}$ n$_{eq}$/cm$^2$, similar values were measured since a higher applicable reverse bias voltage could recover most of the pre-irradiation gain. At $10^{15}$ n$_{eq}$/cm$^2$, the time resolution at the maximum applicable voltage of 620 V during the beam test was measured to be 57 ps since the voltage stability was not good enough to compensate for the gain layer loss. The time resolutions were found to follow approximately a universal function of gain for all implantation doses and fluences.Comment: 17 page

Charge collection properties of irradiated depleted CMOS pixel test structures

Edge-TCT and charge collection measurements with passive test structures made in LFoundry 150 nm CMOS process on p-type substrate with initial resistivity of over 3 k$\Omega$cm are presented. Measurements were made before and after irradiation with reactor neutrons up to 2$\cdot$10$^{15}$ n$_{\mathrm{eq}}$/cm$^2$. Two sets of devices were investigated: unthinned (700 $\mu$m) with substrate biased through the implant on top and thinned (200 $\mu$m) with processed and metallised back plane. Depleted depth was estimated with Edge-TCT and collected charge was measured with $^{90}$Sr source using an external amplifier with 25 ns shaping time. Depleted depth at given bias voltage decreased with increasing neutron fluence but it was still larger than 70 $\mu$m at 250 V after the highest fluence. After irradiation much higher collected charge was measured with thinned detectors with processed back plane although the same depleted depth was observed with Edge-TCT. Most probable value of collected charge of over 5000 electrons was measured also after irradiation to 2$\cdot$10$^{15}$ n$_{\mathrm{eq}}$/cm$^2$. This is sufficient to ensure successful operation of these detectors at the outer layer of the pixel detector in the ATLAS experiment at the upgraded HL-LHC

Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene

We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion