Yoshitaka Nakano

Deep-level optical spectroscopy investigation of N-doped TiO2 films

N-doped TiO2 films were deposited on n+-GaN∕Al2O3 substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 °C in flowing N2 gas. The N-doping concentration was ∼8.8%, as determined from x-ray photoelectron spectroscopy measurements. Deep-level optical spectroscopy measurements revealed two characteristic deep levels located at ∼1.18 and ∼2.48eV below the conduction band. The 1.18 eV level is probably attributable to the O vacancy state and can be active as an efficient generation-recombination center. Additionally, the 2.48 eV band is newly introduced by the N doping and contributes to band-gap narrowing by mixing with the O2p valence band.

Optical bandgap widening of p-type Cu2O films by nitrogen doping

We have investigated the effect of N doping into Cu2O films deposited by reactive magnetron sputtering. With increasing N-doping concentration up to 3%, the optical bandgap energy is enlarged from ∼2.1 to ∼2.5 eV with retaining p-type conductivity as determined by optical absorption and Hall effect measurements. Additionally, photoelectron spectroscopy in air measurements shows an increase in the valence and conduction band shifts with N doping. These experimental results demonstrate possible optical bandgap widening of p-type N-doped Cu2O films, which is a phenomenon that is probably associated with significant structural changes induced by N doping, as suggested from x-ray diffraction measurements.

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Electrical characterization of p-type N-doped ZnO films prepared by thermal oxidation of sputtered Zn3N2 films

We report on p-type conduction in N-doped ZnO (ZnO:N) films that were prepared by oxidative annealing of sputtered Zn3N2 films at temperatures between 500 and 800°C in flowing O2 gas. Room-temperature Hall-effect measurements show a significant improvement of p-type doping characteristics by the oxidative annealing at temperatures between 700 and 800°C, where more N acceptors are activated and the oxidation state is enhanced, as confirmed by deep-level optical spectroscopy and secondary ion mass spectrometry measurements. Therefore, the high-temperature oxidation of Zn3N2 is effective in moderating the self-compensation effect in ZnO:N from the viewpoint of oxygen vacancy annihilation.

Interface properties of thermally oxidized n-GaN metal–oxide–semiconductor capacitors

We report on the interface properties of thermally oxidized n-GaN metal–oxide–semiconductor capacitors fabricated on sapphire substrates. 100-nm-thick β-Ga2O3 was grown by dry oxidation at 880 °C for 5 h. From secondary ion mass spectrometry measurements, an intermediate Ga oxynitride layer with graded compositions is clearly observed at the β-Ga2O3/GaN interface. Capacitance–voltage measurements show a deep depletion feature and a low interface state density of ∼5.5×1010 eV−1 cm−2. Additionally, no discrete interface traps can be detected by deep-level transient spectroscopic measurements. These results indicate that the surface Fermi level is unpinned at the β-Ga2O3/GaN interface, which may be associated with the presence of the interfacial Ga oxynitride layer.

Electrical characterization of band gap states in C-doped TiO2 films

We report on band gap states in C-doped TiO2 films that were prepared by oxidative annealing of sputtered TiC films at 550°C in flowing O2 gas. Deep-level optical spectroscopy measurements revealed three deep levels located at ∼0.86, ∼1.30, and ∼2.34eV below the conduction band. The first level is probably attributable to the intrinsic nature of TiO2, whereas the latter two levels are newly introduced by the C-doping. In particular, the pronounced 2.34eV band contributes to band gap narrowing by mixing with the O 2p valence band. Additionally, the 0.86 and 1.30eV levels can be active as an efficient generation-recombination center.

Oxygen adsorption and VDR effect in (Sr, Ca)TiO 3−x based ceramics

The relation between the oxygen adsorption and the voltage dependence of the resistor (VDR effect) in (Sr, Ca)TiO 3−x based ceramics has been investigated. The nonlinearity of the voltage-current characteristics increased with increasing the barrier height, which is thought to be generated by the oxygen chemisorption. Acceptor type trap levels were detected by means of a zero biased DLTS technique at high temperatures. These interfacial energy levels changed with reoxidizing temperatures, and the change can be explained by the degradation of the chernisorbed oxygen. The high temperature type of the chemisorbed oxygen as O 2− and O is relatively stable due to the strong pinning effect of trapped electrons, with reoxidizing anneals of grain surfaces above the oxidation temperature, and it contributes greatly to the VDR effect. It is concluded that energy barriers are caused by the interface states generated by the chemisorbed oxygen on grain surfaces and that they determine the VDR effect.

Electrical activation characteristics of silicon-implanted GaN

Electrical activation studies of Si-implanted GaN layers on sapphire were made as a function of annealing temperature (1100–1400°C). For an ion dose of 1.0×1014cm−2, the optimum annealing temperature was 1400°C, exhibiting a nearly 100% electrical activation efficiency and a low sheet resistance of ∼450Ω∕square at room temperature. From variable temperature Hall-effect measurements, Si-implanted GaN films annealed below 1200°C displayed deep ionization levels of ∼280meV, whereas samples annealed above 1300°C had shallow ones of ∼11meV. For lateral Schottky diodes fabricated on Si-implanted GaN layers annealed below 1200°C, capacitance frequency and thermal admittance measurements showed a typical dispersion effect characteristic of a single deep donor with an activation energy of ∼133meV. These results illustrate that deep donor levels created by the Si implantation in GaN layers apparently annihilate and transit to shallow levels produced by the Si ion substitution for Ga in the GaN lattice (SiGa) by ann...

Interface properties of SiO2/n-GaN metal-insulator-semiconductor structures

Electrical characterization of SiO2/n-GaN metal–insulator–semiconductor structures fabricated on sapphire substrates was performed by using high-frequency pulsed capacitance–voltage and capacitance-transient techniques. Fast and slow capacitance transients are clearly seen after applying reverse voltages, reflecting thermal emissions of carriers from the SiO2/GaN interface. The temperature dependence of the capacitance–voltage characteristics shows capacitance saturation in deep depletion (>15 V), which is probably associated with the slow capacitance transient. Deep-level transient spectroscopic measurements reveal two interface traps with activation energies of 0.71 and ∼0.76 eV from the conduction band, corresponding to the fast and slow capacitance transients, respectively. Therefore, the observed capacitance saturation may be due to Fermi-level pinning induced by the latter interface trap.

Deep-Level Optical Spectroscopy Investigation of Band Gap States in AlGaN/GaN Hetero-Interfaces

We have investigated band gap states in AlGaN/GaN hetero-structure grown on sapphire substrate, employing capacitance–voltage and capacitance deep-level optical spectroscopy techniques. Two specific deep levels were revealed to be located at ~1.70 and ~2.08 eV below the conduction band, being clearly different from the deep-level defects observed in GaN. Both deep levels showed a significant increase in their corresponding steady-state photo-capacitance in partial pinch-off mode. It is thought that these levels probably stem from a two-dimensional electron gas (2DEG) region at the AlGaN/GaN hetero-interface. In particular, the 1.70 eV level is likely to act as an efficient generation-recombination center for 2DEG carriers.