James C. Petrosky

Identification of electron and hole traps in lithium tetraborate (Li2B4O7) crystals: Oxygen vacancies and lithium vacancies

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize electrons trapped by oxygen vacancies and holes trapped by lithium vacancies in lithium tetraborate (Li2B4O7) crystals. Our study includes a crystal with the natural abundances of B10 and B11 and a crystal highly enriched with B10. The as-grown crystals contain isolated oxygen vacancies, lithium vacancies, and copper impurities, all in nonparamagnetic charge states. During an irradiation at 77 K with 60 kV x-rays, doubly ionized oxygen vacancies trap electrons while singly ionized lithium vacancies and monovalent copper impurities trap holes. The vacancies return to their preirradiation charge states when the temperature of the sample is increased to approximately 90 K. Hyperfine interactions with B10 and B11 nuclei, observed between 13 and 40 K in the radiation-induced EPR and ENDOR spectra, provide models for the two vacancy-related defects. The electron trapped by an oxygen vacancy ...

Electron and hole traps in Ag-doped lithium tetraborate (Li2B4O7) crystals

Electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and thermoluminescence (TL) are used to characterize the primary electron and hole trapping centers in a lithium tetraborate (Li2B4O7) crystal doped with Ag. Three defects, two holelike and one electronlike, are observed after exposure at room temperature to 60 kV x-rays. The as-grown crystal contains both interstitial Ag+ ions and Ag+ ions substituting for Li+ ions. During the irradiation, substitutional Ag+ ions (4d10) trap holes and two distinct Ag2+ centers (4d9) are formed. These Ag2+ EPR spectra consist of doublets (i.e., the individual 107Ag and 109Ag hyperfine lines are not resolved). One of these hole centers is an isolated unperturbed Ag2+ ion and the other is a Ag2+ ion with a nearby perturbing defect. EPR and ENDOR angular-dependence data provide the g matrix and the 107Ag and 109Ag hyperfine matrices for the more intense isolated hole center. In contrast, the electronlike EPR spectrum produced during the irradia...

The Electronic Structure and Secondary Pyroelectric Properties of Lithium Tetraborate

We review the pyroelectric properties and electronic structure of Li2B4O7(110) and Li2B4O7(100) surfaces. There is evidence for a pyroelectric current along the [110] direction of stoichiometric Li2B4O7 so that the pyroelectric coefficient is nonzero but roughly 103 smaller than along the [001] direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the [110] direction can be correlated with anomalies in the elastic stiffness C33D contributing to the concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that surface charging is dependent on crystal orientation and doping, as well as temperature.

Radiation-induced electron traps in Al0.14Ga0.86N by 1 MeV electron radiation

Electrical properties of defects induced in n-type molecular-beam-epitaxial-grown Al0.14Ga0.86N are studied using deep-level transient spectroscopy (DLTS) to explore the radiation tolerance of AlGaN-based electronic and optoelectronic devices. It has been found that four electron traps labeled R1(0.15±0.02eV), R2(0.21±0.02eV), R3(0.26±0.02eV), and R4(0.33±0.03eV) are created in the electron irradiated Al0.14Ga0.86N. The electron trap R4 is the most prominent radiation-induced defect in the DLTS spectrum and appears to be unique to AlGaN. Although the other radiation-induced traps anneal significantly at or below 400 K, this R4 trap is thermally stable up to 450 K, and could significantly affect the performance of AlGaN-based devices.

Non-charge related mechanism affecting capacitive MEMS switch lifetime

Capacitive microelectromechanical system switches were operated for extended periods of time while tracking the voltages where the beam pulls-in and releases from the insulator surface. Changes in pull-in voltage are explained with charges tunneling from the beam into the insulator. However, charging cannot account for changes in the release voltage which result in switches sticking to the insulator. Pull-in and release voltage measurements provide valuable information on the processes limiting switch lifetime

Temperature Dependent Electrical Characteristics of Neutron Irradiated AlGaN/GaN HFETs

Low temperature neutron irradiated Al0.27Ga0.73N/GaN heterostructures reveal a complex temperature dependent displacement damage formation process. This process results in differences in drain currents at low (80 K) versus high (294 K) temperatures. Irradiation increases the gate and drain currents at 80 K, and decreases the drain current at room temperature. These effects saturate at ~ 3 times 1010 n/cm2 indicating complexing with a native impurity. After a room temperature anneal, the effect on the gate current persists and the drain current partially recovers. A two-step persistent interface trap formation model is presented that explains these results. This model is further supported by CV measurements at 80 K and 294 K after annealing.

An Analysis of the Effects of Low-Energy Electron Irradiation of AlGaN/GaN HFETs

The effects of low energy (0.45 MeV) electron radiation on the gate and drain currents of Al0.27Ga0.73N/GaN HFETs are investigated using IV and CV measurements. Following irradiation, the gate and drain currents increase at low temperatures and reach a saturation level. The gate leakage currents do not fully account for the drain current increase. Following a room temperature anneal, the gate and drain currents return to pre-irradiation levels. These results are explained by the buildup of positive charge in the AlGaN layer at low temperature and traps formed via a complexing precursor in the AlGaN layer near the interface. The positive charge increases the carrier concentration in the 2DEG and hence the drain current. The traps act as trap-assisted-tunneling centers that increase the gate leakage current.

Rare Earth Dopant (Nd, Gd, Dy, and Er) Hybridization in Lithium Tetraborate

The four dopants (Nd, Gd, Dy, and Er) substitutionally occupy the Li+ sites in lithium tetraborate (Li2B4O7: RE) glasses as determined by analysis of the extended X-ray absorption fine structure. The dopants are coordinated by 6-8 oxygen at a distance of 2.3 to 2.5 A, depending on the rare earth. The inverse relationship between the RE¬ O coordination distance and rare earth (RE) atomic number is consistent with the expected lanthanide atomic radial contraction with increased atomic number. Through analysis of the X-ray absorption near edge structure, the rare earth dopants adopt the RE3+ valence state. There are indications of strong rare earth 5d hybridization with the trigonal and tetrahedral formations of BO3 and BO4 based on the determination of the rare earth substitutional Li+ site occupancy from the X-ray absorption near edge structure data. The local oxygen disorder around the RE3+ luminescence centers evident in the structural determination of the various glasses, and the hybridization of the RE3+ dopants with the host may contribute to the asymmetry evident in the luminescence emission spectral lines. The luminescence emission spectra are indeed characteristic of the expected f-to-f transitions; however, there is an observed asymmetry in some emission lines.

The K-shell Auger electron spectrum of gadolinium obtained using neutron capture in a solid state device

Highly doped or alloyed Gd2O3 in HfO2 films form heterojunction diodes with silicon. Single neutron capture events can be identified with a Hf0.85Gd0.15O1.93 to n-type silicon heterojunction. With long pulse integration times and suppression of the smaller pulses, there is agreement between the key pulse height spectral features and those predicted by Monte Carlo simulations. The latter align very well with the decay channels of the Gd following neutron capture, particularly those involving the Gd K-shell Auger electron resonances.

Toward Simultaneous 2D ACAR and 2D DBAR: Sub-Pixel Spatial Characterization of a Segmented HPGe Detector Using Transient Charges

Sub-pixel spatial characterization of an Ortec HPGe double-sided strip detector was measured in preparation for its planned use in a positron annihilation spectrometer (PAS) to simultaneously measure the two-dimensional Angular Correlation (2D ACAR) and Doppler Broadening (2D DBAR) of Annihilation Radiation. Sr-85 514-keV photons were finely collimated onto the center of 25, equally spaced sub-pixels within a single 5-mm by 5-mm intrinsic pixel, using a collimator with an aperture diameter of 0.15 ± 0.05 mm. The location of each full-energy recorded event was determined by analyzing the relative sizes of coincident transient charges on charge collection strips adjacent to the strip recording full-energy events. Interpolation, using ratios of the coincident transient charge sizes, was used to determine two-dimensional locations of full-energy events. Radial location data was fit to a function which describes a Gaussian point spread function uniformly distributed over a circular aperture. The standard deviation of the point spread function, 0.22 mm, is a measure of the spatial resolution of this detector system. The relative efficiency for detection of 514-keV photons across the intrinsic pixel was also measured.