J. M. Campbell

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).

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.

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.

Continuous distribution of defect states and band gap narrowing in neutron irradiated GaAs

We find that fast neutron irradiated n- and p-GaAs diodes both show a broad feature in deep level transient spectroscopy (DLTS) previously studied primarily in n-GaAs and termed the “U-band.” The high temperature edge of the broad DLTS feature cuts off at the same temperature in both n- and p-GaAs suggesting that the cut off is due to the DLTS behavior expected for a continuous density of defect states that spans midgap. The band gap implied by the DLTS midgap cut off is 1.36 eV, as compared to the bulk GaAs band gap 1.52 eV. Band gap narrowing is consistent with previous measurements of lattice expansion in neutron irradiated GaAs. This leads to a model of defect cascades that are regions of narrowed band gap with defect levels that are inhomogeneously broadened. We observe, in addition, that the damage cascades are surrounded by large Coulomb barriers that prevent the complete filling of traps in the damaged regions.

Injection deep level transient spectroscopy: An improved method for measuring capture rates of hot carriers in semiconductors

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.