The E3 transition in irradiated GaAs observed in deep level transient spectroscopy (DLTS) was recently discovered in Laplace-DLTS to encompass three distinct components. The component designated E3c was found to be metastable, reversibly bleached under minority carrier (hole) injection, with an introduction rate dependent upon Si doping density. It is shown through first-principles modeling that the E3c must be the intimate Si-vacancy pair, best described as a Si sitting in a divacancy Sivv. The bleached metastable state is enabled by a doubly site-shifting mechanism: Upon recharging, the defect undergoes a second site shift rather returning to its original E3c-active configuration via reversing the first site shift. Identification of this defect offers insights into the short-time annealing kinetics in irradiated GaAs.
Structural disorder causes materials’ surface electronic properties, e.g., work function ([Formula: see text]), to vary spatially, yet it is challenging to prove exact causal relationships to underlying ensemble disorder, e.g., roughness or granularity. For polycrystalline Pt, nanoscale resolution photoemission threshold mapping reveals a spatially varying [Formula: see text] eV over a distribution of (111) vicinal grain surfaces prepared by sputter deposition and annealing. With regard to field emission and related phenomena, e.g., vacuum arc initiation, a salient feature of the [Formula: see text] distribution is that it is skewed with a long tail to values down to 5.4 eV, i.e., far below the mean, which is exponentially impactful to field emission via the Fowler–Nordheim relation. We show that the [Formula: see text] spatial variation and distribution can be explained by ensemble variations of granular tilts and surface slopes via a Smoluchowski smoothing model wherein local [Formula: see text] variations result from spatially varying densities of electric dipole moments, intrinsic to atomic steps, that locally modify [Formula: see text]. Atomic step-terrace structure is confirmed with scanning tunneling microscopy (STM) at several locations on our surfaces, and prior works showed STM evidence for atomic step dipoles at various metal surfaces. From our model, we find an atomic step edge dipole [Formula: see text] D/edge atom, which is comparable to values reported in studies that utilized other methods and materials. Our results elucidate a connection between macroscopic [Formula: see text] and the nanostructure that may contribute to the spread of reported [Formula: see text] for Pt and other surfaces and may be useful toward more complete descriptions of polycrystalline metals in the models of field emission and other related vacuum electronics phenomena, e.g., arc initiation.
The stability of low-index platinum surfaces and their electronic properties is investigated with density functional theory, toward the goal of understanding the surface structure and electron emission, and identifying precursors to electrical breakdown, on nonideal platinum surfaces. Propensity for electron emission can be related to a local work function, which, in turn, is intimately dependent on the local surface structure. The (1×N) missing row reconstruction of the Pt(110) surface is systematically examined. The (1×3) missing row reconstruction is found to be the lowest in energy, with the (1×2) and (1×4) slightly less stable. In the limit of large (1×N) with wider (111) nanoterraces, the energy accurately approaches the asymptotic limit of the infinite Pt(111) surface. This suggests a local energetic stability of narrow (111) nanoterraces on free Pt surfaces that could be a common structural feature in the complex surface morphologies, leading to work functions consistent with those on thermally grown Pt substrates.
This report describes the development of ultra-short pulse laser (USPL) induced terahertz (THz) radiation to image electronic plasmas during electrical breakdown. The technique uses three pulses from two USPLs to (1) trigger the breakdown, (2) create a 2 picosecond (ps, 10 -12 s), THz pulse to illuminate the breakdown, and (3) record the THz image of the breakdown. During this three year internal research program, sub-picosecond jitter timing for the lasers, THz generation, high bandwidth (BW) diagnostics, and THz image acquisition was demonstrated. High intensity THz radiation was optically-induced in a pulse-charged gallium arsenide photoconductive switch. The radiation was collected, transported, concentrated, and co-propagated through an electro-optic crystal with an 800 nm USPL pulse whose polarization was rotated due to the spatially varying electric field of the THz image. The polarization modulated USPL pulse was then passed through a polarizer and the resulting spatially varying intensity was detected in a high resolution digital camera. Single shot images had a signal to noise of %7E3:1. Signal to noise was improved to %7E30:1 with several experimental techniques and by averaging the THz images from %7E4000 laser pulses internally and externally with the camera and the acquisition system (40 pulses per readout). THz shadows of metallic films and objects were also recorded with this system to demonstrate free-carrier absorption of the THz radiation and improve image contrast and resolution. These 2 ps THz pulses were created and resolved with 100 femtosecond (fs, 10 -15 s) long USPL pulses. Thus this technology has the capability to time-resolve extremely fast repetitive or single shot phenomena, such as those that occur during the initiation of electrical breakdown. The goal of imaging electrical breakdown was not reached during this three year project. However, plans to achieve this goal as part of a follow-on project are described in this document. Further modifications to improve the THz image contrast and resolution are proposed, and after they are made, images of photo-induced carriers in gallium arsenide and silicon will be acquired to evaluate image sensitivity versus carrier density. Finally electrical breakdown will be induced with the first USPL pulse, illuminated with THz radiation produced with the second USPL pulse and recorded with the third USPL pulse.
This paper investigates the effects of high dose rate ionizing radiation and total ionizing dose (TID) on tantalum oxide (TaOx) memristors. Transient data were obtained during the pulsed exposures for dose rates ranging from approximately 5.0 × 107rad(Si)/s to 4.7 × 108rad(Si)/s and for pulse widths ranging from 50 ns to 50 μs. The cumulative dose in these tests did not appear to impact the observed dose rate response. Static dose rate upset tests were also performed at a dose rate of ∼3.0 × 108rad(Si)/s. This is the first dose rate study on any type of memristive memory technology. In addition to assessing the tolerance of TaOx memristors to high dose rate ionizing radiation, we also evaluated their susceptibility to TID. The data indicate that it is possible for the devices to switch from a high resistance off-state to a low resistance on-state in both dose rate and TID environments. The observed radiation-induced switching is dependent on the irradiation conditions and bias configuration. Furthermore, the dose rate or ionizing dose level at which a device switches resistance states varies from device to device; the enhanced susceptibility observed in some devices is still under investigation. Numerical simulations are used to qualitatively capture the observed transient radiation response and provide insight into the physics of the induced current/voltages.
At sufficiently high energies, the wavelengths of electrons and photons are short enough to only interact with one atom at time, leading to the popular %E2%80%9Cindependent-atom approximation%E2%80%9D. We attempted to incorporate atomic structure in the generation of cross sections (which embody the modeled physics) to improve transport at lower energies. We document our successes and failures. This was a three-year LDRD project. The core team consisted of a radiation-transport expert, a solid-state physicist, and two DFT experts.
This report documents work conducted in FY13 on electrical discharge experiments performed to develop predictive computational models of the fundamental processes of surface breakdown in the vicinity of high-permittivity material interfaces. Further, experiments were conducted to determine if free carrier electrons could be excited into the conduction band thus lowering the effective breakdown voltage when UV photons (4.66 eV) from a high energy pulsed laser were incident on the rutile sample. This report documents the numerical approach, the experimental setup, and summarizes the data and simulations. Lastly, it describes the path forward and challenges that must be overcome in order to improve future experiments for characterizing the breakdown behavior for rutile.
This document summarizes the work done in our three-year LDRD project titled 'Physics of Intense, High Energy Radiation Effects.' This LDRD is focused on electrical effects of ionizing radiation at high dose-rates. One major thrust throughout the project has been the radiation-induced conductivity (RIC) produced by the ionizing radiation. Another important consideration has been the electrical effect of dose-enhanced radiation. This transient effect can produce an electromagnetic pulse (EMP). The unifying theme of the project has been the dielectric function. This quantity contains much of the physics covered in this project. For example, the work on transient electrical effects in radiation-induced conductivity (RIC) has been a key focus for the work on the EMP effects. This physics in contained in the dielectric function, which can also be expressed as a conductivity. The transient defects created during a radiation event are also contained, in principle. The energy loss lead the hot electrons and holes is given by the stopping power of ionizing radiation. This information is given by the inverse dielectric function. Finally, the short time atomistic phenomena caused by ionizing radiation can also be considered to be contained within the dielectric function. During the LDRD, meetings about the work were held every week. These discussions involved theorists, experimentalists and engineers. These discussions branched out into the work done in other projects. For example, the work on EMP effects had influence on another project focused on such phenomena in gases. Furthermore, the physics of radiation detectors and radiation dosimeters was often discussed, and these discussions had impact on related projects. Some LDRD-related documents are now stored on a sharepoint site (https://sharepoint.sandia.gov/sites/LDRD-REMS/default.aspx). In the remainder of this document the work is described in catergories but there is much overlap between the atomistic calculations, the continuum calculations and the experiments.
Negative bias temperature instability is an issue of critical importance as the space electronics industry evolves because it may dominate the reliability lifetime. Understanding its physical origin is therefore essential in determining how best to search for methods of mitigation. It has been suggested that the magnitude of the effect is strongly dependent on circuit operation conditions (static or dynamic modes). In the present work, we examine the time constants related to the charging and recovery of trapped charged induced by NBTI in HfSiON gate dielectric devices. In previous work, we avoided the issue of charge relaxation during acquisition of the I{sub ds}(V{sub gs}) curve by invoking a continuous stressing technique whereby {Delta}V{sub th} was extracted from a series of single point I{sub ds} measurements. This method relied heavily on determination of the initial value of the source-drain current (I{sub ds}{sup o}) prior to application of gate-source stress. In the present work we have used a new pulsed measurement system (Keithley SCS 4200-PIV) which not only removes this uncertainty but also permits dynamic measurements in which devices are AC stressed (Fig. 1a) or subjected to cycles of continued DC stresses followed by relaxation (Fig. 1b). We can now examine the charging and recovery characteristics of NBTI with higher precision than previously possible. We have performed NBTI stress experiments at room temperature on p-channel MOSFETs made with HfSiON gate dielectrics. In all cases the devices were stressed in the linear regime with V{sub ds}=-0.1V. We have defined two separate waveforms/pulse trains as illustrated in Fig 1. These were applied to the gate of the MOSFET. Firstly we examined the charging characteristics by applying an AC stress at 2.5MHz or 10Hz for different times. For a 50% duty cycle this corresponded to V{sub gs} = - 2V pulses for 200ns or 500ms followed by V{sub gs} = 0V pulses for 200ns or 500ms recovery respectively. In between 'bursts' of AC stress cycles, the I{sub ds}(V{sub gs}) characteristic in the range (-0.6V, -1.3V) was measured in 10.2 {micro}s. V{sub th} was extracted directly from this curve, or from a single I{sub ds} point normalized to the initial I{sub ds}{sup o} using our previous method. The resulting I{sub ds}/I{sub ds}{sup o} curves are compared; in Fig 2, the continuous stress results are included. In the second method, we examined the recovery dynamic by holding V{sub gs} = 0V for a finite amount of time (range 100 ns to 100 ms) following stress at V{sub gs} = - 2V for various times. In Fig 3 we compare |{Delta}V{sub th}(t)| results for recovery times of 100ms, 1ms, 100{micro}s, 50{micro}s, 25{micro}s, 10{micro}s, 100ns, and DC (i.e. no recovery) The data in Fig 2 shows that with a high frequency stress (2.5MHz) devices undergo significantly less (but finite) current degradation than devices stressed at 10Hz. This appears to be limited by charging and not by recovery. Fig 3 supports this hypothesis since for 100ns recovery periods, only a small percentage of the trapped charge relaxes. Detailed explanation of these experiments will be presented at the conference.
Electronic components such as bipolar junction transistors (BJTs) are damaged when they are exposed to radiation and, as a result, their performance can significantly degrade. In certain environments the radiation consists of short, high flux pulses of neutrons. Electronics components have traditionally been tested against short neutron pulses in pulsed nuclear reactors. These reactors are becoming less and less available; many of them were shut down permanently in the past few years. Therefore, new methods using radiation sources other than pulsed nuclear reactors needed to be developed. Neutrons affect semiconductors such as Si by causing atomic displacements of Si atoms. The recoiled Si atom creates a collision cascade which leads to displacements in Si. Since heavy ions create similar cascades in Si we can use them to create similar damage to what neutrons create. This LDRD successfully developed a new technique using easily available particle accelerators to provide an alternative to pulsed nuclear reactors to study the displacement damage and subsequent transient annealing that occurs in various transistor devices and potentially qualify them against radiation effects caused by pulsed neutrons.
Germanium telluride undergoes rapid transition between polycrystalline and amorphous states under either optical or electrical excitation. While the crystalline phases are predicted to be semiconductors, polycrystalline germanium telluride always exhibits p -type metallic conductivity. We present a study of the electronic structure and formation energies of the vacancy and antisite defects in both known crystalline phases. We show that these intrinsic defects determine the nature of free-carrier transport in crystalline germanium telluride. Germanium vacancies require roughly one-third the energy of the other three defects to form, making this by far the most favorable intrinsic defect. While the tellurium antisite and vacancy induce gap states, the germanium counterparts do not. A simple counting argument, reinforced by integration over the density of states, predicts that the germanium vacancy leads to empty states at the top of the valence band, thus giving a complete explanation of the observed p -type metallic conduction.
This report describes an LDRD-supported experimental-theoretical collaboration on the enhanced low-dose-rate sensitivity (ELDRS) problem. The experimental work led to a method for elimination of ELDRS, and the theoretical work led to a suite of bimolecular mechanisms that explain ELDRS and is in good agreement with various ELDRS experiments. The model shows that the radiation effects are linear in the limit of very low dose rates. In this limit, the regime of most concern, the model provides a good estimate of the worst-case effects of low dose rate ionizing radiation.
Mechanisms for enhanced low-dose-rate sensitivity are described. In these mechanisms, bimolecular reactions dominate the kinetics at high dose rates thereby causing a sub-linear dependence on total dose, and this leads to a dose-rate dependence. These bimolecular mechanisms include electron-hole recombination, hydrogen recapture at hydrogen source sites, and hydrogen dimerization to form hydrogen molecules. The essence of each of these mechanisms is the dominance of the bimolecular reactions over the radiolysis reaction at high dose rates. However, at low dose rates, the radiolysis reaction dominates leading to a maximum effect of the radiation.
In this paper, we describe a rate equation approach that leads to new insights about electrical breakdown in insulating and semiconducting materials. In this approach, the competition between carrier generation by impact ionization and carrier recombination by Auger and defect recombination leads to steady state solutions for the carrier generation rate, and it is the accessibility of these steady state solutions, for a given electric field, that governs whether breakdown does or does not occur. This approach leads to theoretical definitions for not only the intrinsic breakdown field but also other characteristic quantities. Results obtained for GaAs using a carrier distribution function calculated by both a Maxwellian approximation and an ensemble Monte Carlo method will be discussed.
We present a series of electronic structure calculations that demonstrate a mechanism for spontaneous ionization of hydrogen at the Si-SiO{sub 2} interface. Specifically, we show that an isolated neutral hydrogen atom will spontaneously give up its charge and bond to a threefold coordinated oxygen atom. We refer to this entity as a proton. We have calculated the potential surface and found it to be entirely attractive. In contrast, hydrogen molecules will not undergo an analogous reaction. We relate these calculations both to proton generation experiments and to hydrogen plasma experiments.
This LDRD project has involved the development and application of Sandia's massively parallel materials modeling software to several significant biophysical systems. They have been successful in applying the molecular dynamics code LAMMPS to modeling DNA, unstructured proteins, and lipid membranes. They have developed and applied a coupled transport-molecular theory code (Tramonto) to study ion channel proteins with gramicidin A as a prototype. they have used the Towhee configurational bias Monte-Carlo code to perform rigorous tests of biological force fields. they have also applied the MP-Sala reacting-diffusion code to model cellular systems. Electroporation of cell membranes has also been studied, and detailed quantum mechanical studies of ion solvation have been performed. In addition, new molecular theory algorithms have been developed (in FasTram) that may ultimately make protein solvation calculations feasible on workstations. Finally, they have begun implementation of a combined molecular theory and configurational bias Monte-Carlo code. They note that this LDRD has provided a basis for several new internal (e.g. several new LDRD) and external (e.g. 4 NIH proposals and a DOE/Genomes to Life) proposals.
Collective impact ionization has been used to explain lock-on in semi-insulating GaAs under high-voltage bias. We have used this theory to study some of the steady-state properties of lock-on current filaments. In steady state, the heat gained from the field is exactly compensated by the cooling due to phonon scattering. In the simplest approximation, the carrier distribution approaches a quasi-equilibrium Maxwell-Boltzmann distribution. In this report, we examine the validity of this approximation. We find that this approximation leads to a filament carrier density that is much lower than the high density needed to achieve a quasi-equilibrium distribution. Further work on this subject is in progress.
A new type of GaAs laser is based on the electron-hole plasma in a current filament and is not limited in size by p-n junctions. High energy, electrically controlled, compact, short-pulse lasers are useful for: active optical sensors (LADAR, range imaging, imaging through clouds, dust, smoke, or turbid water), direct optical ignition of fuels and explosives, optical recording, and micro-machining. The authors present a new class of semiconductor laser that can potentially produce much more short pulse energy than conventional (injection-pumped) semiconductor lasers (CSL) because this new laser is not limited in volume or aspect ratio by the depth of a p-n junction. They have tested current filament semiconductor lasers (CFSL) that have produced 75nJ of 890nm radiation in 1.5ns (50W peak), approximately ten times more energy than ISL. These lasers are created from current filaments in semi-insulating GaAs and, in contrast to CSL, are not based on current injection. Instead, low-field avalanche carrier generation produces a high-density, charge-neutral plasma channel with the required carrier density distribution for lasing. They have observed filaments as long as 3.4cm and several hundred microns in diameter in the high gain GaAs photoconductive switches. Their smallest dimension can be more than 100 times the carrier diffusion length in GaAs. This paper will report spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies which imply lasing for several configurations of CFSL. It will also discuss active volume scaling based on recent high current tests.
To optimally design circuits for operation at high intensities of ionizing radiation, and to accurately predict their a behavior under radiation, precise device models are needed that include both stationary and dynamic effects of such radiation. Depending on the type and intensity of the ionizing radiation, different degradation mechanisms, such as photoelectric effect, total dose effect, or single even upset might be dominant. In this paper, the authors consider the photoelectric effect associated with the generation of electron-hole pairs in the semiconductor. The effects of low radiation intensity on p-II diodes and bipolar junction transistors (BJTs) were described by low-injection theory in the classical paper by Wirth and Rogers. However, in BJTs compatible with modem integrated circuit technology, high-resistivity regions are often used to enhance device performance, either as a substrate or as an epitaxial layer such as the low-doped n-type collector region of the device. Using low-injection theory, the transient response of epitaxial BJTs was discussed by Florian et al., who mainly concentrated on the effects of the Hi-Lo (high doping - low doping) epilayer/substrate junction of the collector, and on geometrical effects of realistic devices. For devices with highly resistive regions, the assumption of low-level injection is often inappropriate, even at moderate radiation intensities, and a more complete theory for high-injection levels was needed. In the dynamic photocurrent model by Enlow and Alexander. p-n junctions exposed to high-intensity radiation were considered. In their work, the variation of the minority carrier lifetime with excess carrier density, and the effects of the ohmic electric field in the quasi-neutral (q-n) regions were included in a simplified manner. Later, Wunsch and Axness presented a more comprehensive model for the transient radiation response of p-n and p-i-n diode geometries. A stationary model for high-level injection in p-n junctions was developed by Isaque et al. They used a more complete ambipolar transport equation, which included the dependencies of the transport parameters (ambipolar diffusion constant, mobility, and recombination rate) on the excess minority carrier concentration. The expression used for the recombination rate was that of Shockley-Reed-Hall (SRH) recombination which is dominant for low to mid-level radiation intensities. However, at higher intensities, Auger recombination becomes important eventually dominant. The complete ambipolar transport equation including the complicated dependence of transport parameters on the radiation intensity, cannot be solved analytically. This solution is obtained for each of the regimes where a given recombination mechanism dominates, and then by joining these solutions using appropriate smoothing functions. This approach allows them to develop a BJT model accounting for the photoelectric effect of the ionizing radiation that can be implemented in SPICE.
The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses at 23A, and over 100 pulses at 1kA. This is achieved by improving the ohmic contacts by doping the semi-insulating GaAs underneath the metal, and by achieving a more uniform distribution of contact wear across the entire switch by distributing the trigger light to form multiple filaments. This paper will compare various approaches to doping the contacts, including ion implantation, thermal diffusion, and epitaxial growth. The device characterization also includes examination of the filament behavior using open-shutter, infra-red imaging during high gain switching. These techniques provide information on the filament carrier densities as well as the influence that the different contact structures and trigger light distributions have on the distribution of the current in the devices. This information is guiding the continuing refinement of contact structures and geometries for further improvements in switch longevity.
The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses. This was achieved by improving the ohmic contacts through the incorporation of a doped layer that is very effective in the suppression of filament formation, alleviating current crowding. Damage-free operation is now possible at much higher current levels than before. The inherent damage-free current capacity of the bulk GaAs itself depends on the thickness of the doped layers and is at least 100 A for a dopant diffusion depth of 4 μm. This current could be increased by connecting and triggering parallel switches. The contact metal has a different damage mechanism and the threshold for damage (approximately 40 A) is not further improved beyond a dopant diffusion depth of about 2 μm. In a diffusion-doped contact switch, the switching performance is not degraded at the onset of contact metal erosion, unlike a switch with conventional contacts. For fireset applications operating at 1 kV/1 kA levels and higher, doped contacts have not yet resulted in improved longevity. We employ multi-filament operation and InPb solder/Au ribbon wirebonding to demonstrate >100 shot lifetime at 1 kV/1 kA.