Robert M. Fleming

Effects of clustering on the properties of defects in neutron irradiated silicon

We have exposed silicon bipolar transistors to fast neutrons and characterized the properties of the resulting defects using capacitance-based spectroscopy of the n-type collector. We have performed low-temperature electron capture measurement of the divacancy (=/−) and vacancy-oxygen (−/0) defects after the samples were annealed from 350–500 K. We show from a simple rate equation analysis that one can define an unambiguous test for cluster-induced reductions of defect level occupation due to slow capture. This allows easy identification of deep level transient spectroscopy (DLTS) levels where the capture is inhibited due to band bending. Our measurements show extremely long, temperature-dependent capture times for the doubly charged state of the divacancy. We have modeled the capture dynamics as a function of annealing using a simple electrostatic band-bending approach coupled with a realistic simulation of the cluster size and shape distribution as estimated from computer simulation of the damage cascad...

A bistable divacancylike defect in silicon damage cascades

Two deep level transient spectroscopy (DLTS) electron emission signatures, previously labeled E4 and E5, have been shown to be bistable with respect to minority carrier injection at room temperature. These result from two charge state transitions of the same defect. We have performed DLTS measurements as function of annealing between 350 and 680 K, using minority carrier injection after each annealing stage to make E4 and E5 visible. We show that the E4–E5 pair is associated with defect clusters which dominate after neutron or ion damage with annealing characteristics that closely parallel to those of silicon divacancies found in damage clusters. At annealing temperatures above 500 K, the E4–E5 pair ceases to be bistable and exists after anneals in thermal equilibrium. We show that the stable E4 peak appears to be the same emission signature previously labeled the L center. The transformation of the E4–E5 bistable pair into the stable L center and a stable E5 companion level occurs at the same temperature...

Defect-driven gain bistability in neutron damaged, silicon bipolar transistors

Using deep level transient spectroscopy, the authors have measured the defect spectrum in the collector of a n-p-n bipolar transistor following fast neutron irradiation as well as the gain on the same device. They show that a slow change observed in both the gain and deep level traps in the n-type collector at 300K are bistable. The transistor gain and the defects can be returned to the postirradiation condition by forward bias at room temperature, i.e., by operating the transistor (gain) or injection through the base-collector diode (defect spectrum).

Charge trapping and annealing in high-/spl kappa/ gate dielectrics

We examine the radiation response, annealing characteristics, and long-term reliability of capacitors with Al gates and Al/sub 2/O/sub 3/-SiO/sub x/N/sub y/ gate dielectrics stacks which received a forming gas anneal (FGA) or an O/sub 2/ and FG anneal after high-/spl kappa/ deposition. By comparison to a theoretical capacitance-voltage (CV) curve, the FG annealed devices are found to have a large preirradiation interface trapped charge density of /spl sim/7/spl times/10/sup 11/ cm/sup -2/, whereas devices annealed in O/sub 2/ and FG show a large density (/spl sim/9/spl times/10/sup 11/ cm/sup -2/) of negative bulk charge. The midgap voltage shift (/spl Delta/V/sub mg/) increases monotonically with dose for both sets of devices, but the O/sub 2/ annealed devices exhibit 50% less trapping at a total dose of 2 Mrad(SiO/sub 2/). The radiation-induced voltage shifts are found to recover during long duration biased anneals as a result of tunneling and thermal annealing. For short times and large biases, the annealing response is found to be dominated by tunneling. After 1,000 s of annealing, there is a 50% reduction in /spl Delta/V/sub mg/ for devices annealed at 2.0 MV/cm and a 7.5% recovery for devices annealed at 1.0 MV/cm. For longer times, the annealing response of these devices is dominated by thermal annealing. Accelerated life testing shows these devices have a broad failure distribution with a large population of extrinsic failures. Extrapolation of the reliability data suggests these particular devices would have to be operated at an electric field less than /spl sim/2.5 MV/cm to achieve a ten-year operational lifetime. Improved reliability is, therefore, required before insertion into a manufacturing environment.

Defect annealing in neutron and ion damaged silicon: Influence of defect clusters and doping

We have explored defect annealing in radiation damaged silicon in a regime characterized by defect clusters and higher doping. Several types of pnp and npn Si bipolar transistors have been irradiated with ions and neutrons, then isochronally annealed from 300 to 600 K to study the evolution of deep level transient spectroscopy (DLTS) defect signatures. Variations in these data with radiation environment, Fermi level, annealing temperature, and doping density have been used to separate the contributions of three dominant defects to the DLTS defect spectra. We find that the normal Si divacancy and a divacancylike defect with similar properties make similar contributions to a DLTS peak normally associated with transitions from the single minus charge state of the divacancy. However the latter defect is clearly associated with the presence of defect clusters. The vacancy-donor center can also contribute to this high temperature DLTS signature, and its relative importance can be quantitatively assessed by vary...

Transformation kinetics of an intrinsic bistable defect in damaged silicon

The positions of the electronic levels of an intrinsic bistable defect have been measured using deep level transient spectroscopy (DLTS) in n- and p-type damaged silicon bipolar transistor diodes after minority carrier injection and thermal annealing. The kinetic rates observed during conversion of this defect have been determined using both DLTS and transistor gain measurements on devices irradiated with electrons and neutrons. First order conversion kinetics are observed during both injection and thermal annealing of this defect in electron damaged transistors, but more complicated, stretched-out kinetics are seen in neutron irradiated devices. The latter behavior can be successfully modeled as a small spread in the energy barriers for atomic displacements of this defect probably due to strain or electrostatic variations expected in damage clusters. The measured injection bias dependence of the recombination-driven transition to the bistable state of this defect is unlike that seen for generation-recomb...

Annealing neutron damaged silicon bipolar transistors: Relating gain degradation to specific lattice defects

Isochronal anneal sequences have been carried out on pnp and npn transistors irradiated with fast neutrons at a variety of fluences. The evolution of base and collector currents was utilized to characterize the annealing behavior of defects in both the emitter-base depletion region and the neutral base. Various annealing biases, theoretical modeling, as well as previous deep level transient spectroscopy (DLTS) data, were used to assign the relative magnitude of each of the important defects to the total recombination current. We find that donor-vacancy pairs in the neutral n-type base of our pnp transistors are responsible for about 1/3 of the postdamage lifetime degradation, while the remaining recombination currents can be largely attributed to a cluster-related divacancylike defect which has no shallow state DLTS emission peak. This latter defect anneals gradually from 350 to 590 K. Generation/recombination currents in the base-emitter junctions in both types of devices were found to anneal in a simila...

Performance and Breakdown Characteristics of Irradiated Vertical Power GaN P-i-N Diodes

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence <; 1013 cm-2. The unipolar figure of merit for power devices indicates that while the on-resistance and breakdown voltage degrade with irradiation, vertical GaN P-i-Ns remain superior to the performance of the best available, unirradiated silicon devices and on-par with unirradiated modern SiC-based power devices.

Metrics for Comparison Between Displacement Damage due to Ion Beam and Neutron Irradiation in Silicon BJTs

We present a series of metrics for comparison between displacement damage due to heavy ion and neutron irradiation in silicon bipolar junction transistors. We have compared ion and fast neutron irradiations to determine an ion-to-neutron damage equivalence. We find that a combination of metrics (damage factor, deep level transient spectroscopy (DLTS) and annealing factor) are needed to ensure a comprehensive understanding of the physics involved in the ion-to-neutron conversion. The linearity of the damage factor (primarily probing the base-emitter junction) is not enough to ensure a valid comparison; rather, we must also use additional techniques (DLTS and capacitance measurements) to ensure that collector compensation is not occurring. As a result, care must be taken in choosing the irradiation beam for ion exposures. The displacement damage should peak in the sensitive region of the device to both ensure maximum gain degradation and to minimize collector compensation.

Field dependent emission rates in radiation damaged GaAs

We have measured the temperature and field dependence of emission rates from five traps in electron damaged GaAs. Four of the traps have previously been identified as radiation defects. One of the traps, seen in higher doped diodes, has not been previously identified. We have fit the data to a multiphonon emission theory that allows recombination in GaAs to be characterized over a broad range of temperature and electric field. These results demonstrate an efficient method to calculate field-dependent emission rates in GaAs.