B. D. White

Role of Near-Surface States in Ohmic-Schottky Conversion of Au Contacts to ZnO

A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼0.75eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.

Proton irradiation effects on GaN-based high electron-mobility transistors with Si-doped Al/sub x/Ga/sub 1-x/N and thick GaN cap Layers

1.8 MeV proton radiation-induced degradation in high electron mobility transistors with Si-doped Al/sub x/Ga/sub 1-x/N and thick GaN cap layers is studied up to a fluence of 1/spl times/10/sup 15/ protons/cm/sup 2/. The thick GaN cap layer reduces sheet charge modulation induced by the surface states, as it electrostatically separates the active device layers from the surface, thereby enhancing the device performance. The devices exhibit good tolerance up to 10/sup 14/ protons/cm/sup 2/, with displacement damage being the primary degradation mechanism. Charged defect centers introduced by proton radiation in the active device layers degrade carrier mobility and sheet carrier density. Proton radiation alters the barrier height at the Schottky gate and increases the resistance of the thin film structure.

Electrical, spectral, and chemical properties of 1.8 MeV proton irradiated AlGaN/GaN HEMT structures as a function of proton fluence

We have characterized high-electron mobility transistors and corresponding unprocessed material as a function of 1.8 MeV proton fluence. Electrical data shows degradation of the electrical contacts at low fluences (10/sup 11/-10/sup 14/ p/sup +//cm/sup 2/) and degradation of the channel properties for higher fluences. In conjunction with the electrical data, cathodoluminescence and secondary-ion mass spectrometry results suggest mechanisms for the higher fluence degradation.

Characterization of 1.8-MeV proton-irradiated AlGaN/GaN field-effect transistor structures by nanoscale depth-resolved luminescence spectroscopy

We have used depth-resolved cathodoluminescence spectroscopy to examine AlGaN/GaN modulation-doped field-effect transistors that display degraded source-drain current characteristics after 1.8-MeV proton irradiation, along with bulk heterojunction field-effect transistor material after similar proton irradiation. For both cases, we have observed distinct changes in spectral emission features due to decreased internal electric-field strength and new point defects within different layers of the device structure with nanometer-scale depth resolution. These changes can account for the degraded electrical characteristics.

Simultaneous observation of luminescence and dissociation processes of Mg–H complex for Mg-doped GaN

Both luminescence properties and dissociation kinetics of Mg–H complex for as-grown Mg-doped GaN are simultaneously investigated by low-energy electron-excited nanoluminescence (LEEN) spectroscopy. Ultraviolet luminescence at 3.2–3.3 eV and blue luminescence at 2.8–2.9 eV are observed as predominant LEEN emissions. In-depth profiles of LEEN emission show that the blue luminescence is the predominant emission for highly Mg-doped GaN. Electron-beam exposure less than 50 mC/cm2 produces an increase of the ultraviolet luminescence intensity and reduction of the blue luminescence intensity. These characteristics suggest that the blue luminescence is due to a transition from hydrogen-related deep donor to Mg acceptor and that the ultraviolet luminescence is due to transitions from conduction band and/or shallow hydrogen donor to Mg acceptor. We propose a kinetic model for dissociation reactions of Mg–H complex during electron exposure, and the reaction rate is evaluated to be (3.5±0.3)×10−3 s−1 for electron bea...

Proton-induced damage in gallium nitride-based Schottky diodes

Proton irradiation decreases the doping concentration and increases the ideality factor and series resistance, but has very little effect on the Schottky barrier height in n-Gallium nitride Schottky diodes. 1.0-MeV protons cause greater degradation than 1.8-MeV protons because of their higher nonionizing energy loss. The displacement damage recovers during annealing. Comparison between Schottky diodes and high electron-mobility transistors suggests that the degradation in both types of devices is predominantly due to carrier removal and mobility degradation caused by radiation-induced defect centers in the crystal lattice, with interface disorder playing a relatively insignificant part in overall device degradation.

Effects of deep-level defects on ohmic contact and frequency performance of AlGaN/GaN high-electron-mobility transistors

We have characterized AlGaN/GaN high-electron-mobility-transistors on sapphire and silicon carbide substrates with electrical and microcathodoluminescence spectral measurements. Quarter wafer-scale comparisons of spectral features in the GaN attributed to donor–acceptor pair (DAP) transitions and yellow luminescence (YL) from deep acceptors show that the specific contact resistance is related to the ratio of the DAP to YL defect emission intensities. This suggests that these defects interact to change the contact resistance locally on the GaN side of the AlGaN/GaN interface. We show that changes in the frequency response of these transistors can be attributed to these defects at the interface.

Detection of trap activation by ionizing radiation in SiO2 by spatially localized cathodoluminescence spectroscopy

Microcathodoluminescence (CLS) spectroscopy is used to probe the effect of ionizing radiation on defects inside Al gate oxide structures. Micron-scale Al–SiO2–Si capacitors exposed to 10 keV x-ray irradiation exhibit spatially localized CLS emissions characteristic of multiple deep level traps, including positively charged oxygen-deficient centers and nonbridging oxygen hole centers (NBOHC). Irradiation produces both increases and decreases in their relative emission intensities, depending on spatial location within the oxide. These changes result in a gradient of E′ versus NBOHC defect densities across the oxide thickness between Al and Si interfaces. These results demonstrate that x-ray irradiation-induced deep level traps can be monitored spatially in metal-oxide-semiconductor gate structures, that x-ray irradiation produces separate increases or decreases in E′ versus NBOHC defect densities, and that these changes vary with position within the oxides.

Depth-resolved detection and process dependence of traps at ultrathin plasma-oxidized and deposited SiO2/Si interfaces

Low-energy electron-excited nanoluminescence spectroscopy reveals depth-resolved optical emission associated with traps near the interface between ultrathin SiO2 deposited by plasma-enhanced chemical vapor deposition on plasma-oxidized crystalline Si. These near-interface states exhibit a strong dependence on local chemical bonding changes introduced by thermal/gas processing, layer-specific nitridation, or depth-dependent radiation exposure. The depth-dependent results provide a means to test chemical and structural bond models used to develop advanced dielectric-semiconductor junctions.

Low energy electron-excited nanoscale luminescence: a tool to detect trap activation by ionizing radiation

Ultra-thin SiO/sub 2//Si gate dielectric structures exposed to heavy X-ray irradiation exhibit optical emission characteristic of interface traps. Low energy electron-excited luminescence spectroscopy with nanometer-scale depth resolution yields a characteristic spectral energy and excitation depth dependence. Ultra-thin (5 nm) oxide films on Si substrates exposed to 10 keV, 7.6 Mrad(SiO/sub 2/) [13.7 Mrad (Si)] X-ray irradiation introduces trap densities on the order of 10/sup 11/ cm/sup -2/ ev/sup -1/, localized near the intimate SiO/sub 2/-Si interface. This density is consistent with the trapped oxide and interface charge densities expected based on observed capacitance-voltages shifts of thicker oxides, their corresponding charge densities, and the proportionally smaller charge densities expected for the thinner oxide layers in this study.