B. D. Weaver

Multicolor photodetector based on GaAs quantum rings grown by droplet epitaxy

Photoresponse from multicolor photodetector was measured in the spectral range of 0.4–6.0μm as a function of temperature and bias voltage. Devices were fabricated from wafers with an active region of five periods of GaAs quantum rings grown by droplet epitaxy technique on lattice matched Al0.3Ga0.7As barriers. The photoresponse spectra exhibit two broad bands in the visible-near-infrared and midinfrared spectral regions. The visible-near-infrared band, which is due to interband transitions, was observed at temperatures as high as room temperature. On the other hand, the midinfrared band, which is due to intersubband transitions, was observed at temperature lower than 80K.

Substrate-Dependent Effects on the Response of AlGaN/GaN HEMTs to 2-MeV Proton Irradiation

AlGaN/GaN high electron mobility transistors grown on Si, SiC, and sapphire substrates were exposed to 2-MeV proton irradiation in incremental fluences up to 6 × 1014 cm-2. The devices were characterized initially and after each irradiation by Hall and dc I-V measurements to probe the mechanisms associated with radiation-induced degradation and failure. It was determined that defects created at the AlGaN/GaN interface introduce scattering centers near the two-dimensional electron gas (2DEG), which result in degraded mobility. Additionally, charged traps in the structure serve to screen the 2DEG resulting in reduced sheet carrier density. These two effects are responsible for degraded I-V behavior, including reduced saturation current and transconductance, increased ON-resistance, and positive threshold voltage shift. Interestingly, the sample with the most pre-existing defects was the most tolerant of radiation-induced damage.

Proton Radiation-Induced Void Formation in Ni/Au-Gated AlGaN/GaN HEMTs

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 × 1014 H+/cm2. Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance.

Critical‐current enhancement in particle‐irradiated cuprate semiconductors

Detailed measurements have been made of the magnetic field (0<H<6u2009T) and temperature (10 K<T<100 K) dependencies of the critical current density jc in Tl2CaBa2Cu2O8 films before and after irradiation with incremental fluences (0<Φ<3×1016 cm−2) of 2 MeV protons. The results are interpreted quantitatively in terms of radiation‐induced changes in (1) the critical temperature, (2) the rate of thermal flux creep, and (3) local scale superconductivity. Radiation‐induced enhancements in jc are described by an expression which allows the fluence that maximizes jc to be predicted as a function of H, T, pinning energy, and particle type.

Degradation mechanisms of 2 MeV proton irradiated AlGaN/GaN HEMTs

Proton-induced damage in AlGaN/GaN HEMTs was investigated using energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), and simulated using a Monte Carlo technique. The results were correlated to electrical degradation using Hall measurements. It was determined by EDS that the interface between GaN and AlGaN in the irradiated HEMT was broadened by 2.2u2009nm, as estimated by the width of the Al EDS signal compared to the as-grown interface. The simulation results show a similar Al broadening effect. The extent of interfacial roughening was examined using high resolution TEM. At a 2u2009MeV proton fluence of 6 × 1014 H+/cm2, the electrical effects associated with the Al broadening and surface roughening include a degradation of the ON-resistance and a decrease in the electron mobility and 2DEG sheet carrier density by 28.9% and 12.1%, respectively.

Proton radiation effects in microwave cavities and ring resonators fabricated from YBa2Cu3O7−δ

We report the first radiation effects study on a superconducting ring resonator made from thin‐film YBa2Cu3O7−δ. Exposure to 2 MeV protons causes the superconducting transition temperature Tc to decrease predictably with fluence. For temperatures below about 0.9Tc , there is no significant change in the transmission coefficient, the center frequency, or the quality factor Q of the resonator, even for doses in excess of 4×1016 protons/cm2 (∼0.04 displacements per film atom). Similarly, the low‐temperature surface resistance Rs of an unpatterned film does not change with irradiation. We show that this insensitivity to radiation is not predicted by standard theory, and that the dominant part of Rs at low temperature is the residual resistance R0. Thus any viable theory describing the origin of R0 must, as a criterion, explain the origin of its insensitivity to large irradiation doses. This criterion is used to evaluate theories ascribing R0 to weak links, flux pinning, impurities, and lattice imperfections.

Impact of Surface Passivation on the Dynamic ON-Resistance of Proton-Irradiated AlGaN/GaN HEMTs

Radiation tolerance of AlGaN/GaN high-electron mobility transistors (HEMTs) is studied with 2-MeV protons, up to a fluence of 6×1014 H+/cm2 (about 200 times of typical Si MOSFET rating). The increase in dynamic ON-resistance (RONDYN) after radiation is observed to be much more severe than that of static ON-resistance. Radiation-induced donorlike traps located near the two-dimensional electron gas trap electrons, which is responsible for the phenomenon. Compared with the devices passivated by conventional plasma-enhanced chemical vapor deposition (PECVD) SiN, GaN HEMTs with 10 nm of in situ SiN before the PECVD SiN step demonstrate much less increase in RONDYN from 2300% to only 300%. The in situ SiN is believed to reduce the process damage by PECVD, improving radiation tolerance.

Proton-induced disorder in InP-based resonant tunneling diodes

We have fabricated arrays of resonant tunneling diodes based on InP substrates for exposure at room temperature with fluences of 3 MeV protons up to 7×1014u2009H+/cm2. Proton fluences below about 1×1013u2009cm−2 have little effect on the resonant tunneling diode but higher fluences decrease the peak current and increase the valley current. We find that proton-induced changes in the operating parameters are qualitatively similar to the effects of doping the wells.

Order‐of‐magnitude method for estimating the fluence that optimizes the critical current in particle‐irradiated cuprate superconductors

The particle fluence that maximizes the critical current enhancement in cuprate superconductors is shown to be closely related to the nonionizing energy loss (displacement damage) of a given particle. In many cases, only the incident particle type and energy need to be known in order to allow an order‐of‐magnitude prediction of the optimal fluence for enhancing the critical current.

Controllable alteration of the pinning energy in Tl2Ca2BaCu2O8 films by proton irradiation

Irradiation with 2 MeV protons increases the vortex pinning energy in Tl2Ca2BaCu2O8 films. The maximum pinning energy occurs at a defect density that depends on (1) the density of flux pinning centers in the unirradiated film, (2) the ratio of pinning energies of the pre‐ and post‐irradiation defects, and (3) the effectiveness of the incident particle in creating pinning centers. An explicit formula for the pinning energy as a function of fluence is proposed.