R. S. Wagner

Alpha-, boron-, silicon- and iron-ion-induced current transients in low-capacitance silicon and GaAs diodes

High-speed current transient measurements were made over a large range of linear energy transfer (LET), using a wideband 70-GHz (6-ps-risetime) sampling oscilloscope on high resistivity GaAs diodes and 1-, 3-, and 10- Omega -cm silicon diodes. For 3- and 5-MeV alpha particles, 12-MeV boron, 18-MeV silicon, and 12- and 100-MeV iron ions incident on these devices, risetimes in the range from about 38 ps to 100 ps were produced depending on LET and device resistivity. Results are compared to the productions of various models. >

CMOS VLSI single event transient characterization

The MOSIS VLSI IC process has been characterized for picosecond single-event transients for large-area diode test structures. Three test structures were designed for a CMOS, p-well, 3- mu m process, including two MOS diodes for SEU transient measurements. Devices were irradiated with 5-MeV boron ions and 5-MeV alpha -particles. Bulk CMOS single-event transients are found to exhibit a wide variety of electrical interactions when the devices are irradiated, owing to complex technology profiles and multijunction effects. The transients characterized in this work can be used to simulate single-event upsets in SRAM (static RAM) circuits. >

Grain boundary effects on carrier transport in undoped polycrystalline chemical‐vapor‐deposited diamond

A quantitative measure of grain‐boundary effects on the carrier transport properties in polycrystalline chemical‐vapor‐deposited diamond has been obtained using a 10‐ns hard x‐ray excitation source. Two device geometries were used to gain insight into the extent of grain‐boundary effects: one having the applied electric field normal and the other parallel to the grain orientation. The applied electric field intensity was varied to adjust the mean‐free carrier drift distance. The degradation in the carrier transport properties at an electric field intensity of 10 kV/cm by the grain‐boundary appears to be approximately a factor of two in comparison to the intragrain carrier transport.

Flat response detectors for the vacuum ultraviolet and soft x‐ray region: InP:Fe photoconductors

We have demonstrated that InP:Fe photoconductive detectors are sensitive to radiation between 20 and 3000 eV. Within experimental error the measured response is flat in this energy range, in agreement with theory. This property, coupled with the simplicity and high speed (full width at one half maximum of less than 200 ps) of these devices, makes them very interesting for the measurement of pulsed vacuum ultraviolet and x‐ray radiation and the total instantaneous integrated radiative power emitted from a pulsed source. Low fidelity response can occur in these devices for excitation times that are many times longer than the primary response time of the detector.

An Approach to Measure Ultrafast-Funneling-Current Transients

Funneling-current transients are predicted to occur on a picosecond time scale. We have developed an advanced approach to measure picosecond single-event-upset currents in semiconductor devices with <40 ps time resolution. Our approach utilizes high-bandwidth sampling of repetitively-produced events. In this paper we describe the experimental approach and report characterization studies of the time resolution of the measurement components. We also report first measurements of picosecond single-event-current transients.

Picosecond Photoconductors as Radiation Detectors

We have developed a new class of extremely high-speed radiation detectors. They are simple and inexpensive to fabricate, rugged, and reliable. We have demonstrated their sensitivity to gamma-rays, x-rays, soft x-rays, charged particles, and light and have obtained response speeds of <100 ps. Their current response is proportional to incident-radiation intensity. The detectors are not used for detecting single particles or measuring particle energy. Their current response is due to the modulation of the conductance of the semiconductor crystal by transientradiation events. The devices are fabricated from bulk-semiconductor crystals and achieve picosecond response by fast carrier relaxation in the crystals. Fast carrier relaxation is obtained by intentionally introducing trapping and recombination centers into the semiconductor crystal by impurity doping or radiation damage. Here we discuss the design, fabrication, and characterization of InP:Fe, GaAs, and neutron-damaged InP:Fe and GaAs photoconductive radiation detectors. We also present a model for transient response in InP:Fe photoconductors.

Observed circuit limits to time resolution in correlation measurements with Si‐on‐sapphire, GaAs, and InP picosecond photoconductors

We report cross‐correlation measurements of the response of photoconductor pulsers and sampling gates excited by a femtosecond laser. The photoconductors were fabricated in microstrip transmission line structures on Si‐on‐sapphire, semi‐insulating GaAs, and semi‐insulating InP wafers. The photoconductor sampling gates were ion beam damaged to produce short carrier lifetimes (<3 ps in one case). Damage was introduced with 6 MeV 20Ne on the Si‐on‐sapphire, 2 MeV 2H on the GaAs, and 2 MeV 4He on the InP. Doses in the range 1012–1015 cm−2 were used. Our results show circuit limits to the time resolution in correlation measurements from two sources: (a) RC time constants due to photoconductor gap capacitance and transmission line characteristic impedance and (b) dispersion in microstrip transmission lines.

Excitation and Fe concentration dependences in the impulse photoconductance of InP:Fe

We report impulse response measurements on InP:Fe photoconductors excited by laser and electron beam radiation. Measurements are reported on crystals with Fe concentrations from 2×1015 cm−3 to 4×1016 cm−3 and with excited electron‐hole‐pair densities of ∼1012 cm−3 and 9×1017 cm−3. Measured signal decays are purely exponential in character, and decay times are inversely related to Fe concentration. No long‐lived tails are observed. Decay times show no dependence on excitation level for excited carrier concentrations that are well above and well below the Fe concentrations. The magnitude of the photoresponse indicates that electrons and not holes are the primary current carriers. The data suggest that for impulse excitation photoconductance decay in InP:Fe is due to trap‐assisted recombination of electrons and holes at the Fe sites, with a rate determined by the species with the slower capture rate.

Integrated picosecond photoconductors produced on bulk Si substrates

We report optoelectronic cross‐correlation measurements of the response of photoconductor pulsers and sampling gates formed on Si wafers. These photoconductors were fabricated with standard integrated circuit fabrication techniques followed by shadow‐masked ion beam irradiation. Successful ion beam irradiations were performed with 2 MeV 2H, 6 MeV He, and 30 MeV O with doses of 1015 ion/cm2. Deep damage was necessary to eliminate long‐lived background currents in the cross correlations. Carrier lifetimes of 96, 47, and 29 ps were observed in photoconductors with carrier mobilities of ∼250 cm2/Vs.

Proton recoil detector of fusion neutrons using natural diamond

Diamond, with its high radiation damage resistance, is an attractive alternative to silicon for neutron measurements in next step fusion experiments. A 200-μm-thick type IIa natural diamond with Ti/Au contacts was tested at the LAMPF-WNR facility by time-of-flight neutron energy identification. The crystal, having a carrier lifetime of up to 1 ns, was arranged in a low-energy-resolution, high-sensitivity proton recoil telescope consisting of a polyethylene radiator and a low-energy-proton Teflon filter. This arrangement is similar to the triton burnup monitor of Croft et al. [Rev. Sci. Instrum. 64, 1418 (1993)], where a silicon photodiode was used as a recoil proton detector. The observed sensitivity for 14 MeV neutrons (DT) is (1.25±0.15)×10−3 counts/neutron. However, a high contribution of neutron-induced events in the diamond, mainly carbon (A=12) recoils, was observed. A one-dimensional calculation for the detector response to carbon recoil and proton deposition is compared to the measurements. Poor energy resolution of the diamond detector precludes pulse height discrimination between direct 2.5 MeV neutrons events and proton events corresponding to 14 MeV neutrons. Therefore, an overall DT/DD neutron sensitivity ratio of only ∼6.5 is achieved. This value is much lower than the ratio of 540 reported by Croft et al. in their silicon (A=28) monitor.Diamond, with its high radiation damage resistance, is an attractive alternative to silicon for neutron measurements in next step fusion experiments. A 200-μm-thick type IIa natural diamond with Ti/Au contacts was tested at the LAMPF-WNR facility by time-of-flight neutron energy identification. The crystal, having a carrier lifetime of up to 1 ns, was arranged in a low-energy-resolution, high-sensitivity proton recoil telescope consisting of a polyethylene radiator and a low-energy-proton Teflon filter. This arrangement is similar to the triton burnup monitor of Croft et al. [Rev. Sci. Instrum. 64, 1418 (1993)], where a silicon photodiode was used as a recoil proton detector. The observed sensitivity for 14 MeV neutrons (DT) is (1.25±0.15)×10−3 counts/neutron. However, a high contribution of neutron-induced events in the diamond, mainly carbon (A=12) recoils, was observed. A one-dimensional calculation for the detector response to carbon recoil and proton deposition is compared to the measurements. Poor e...