Yoshihiro Irokawa

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.

Hydrogen Sensors Using Nitride-Based Semiconductor Diodes: The Role of Metal/Semiconductor Interfaces

In this paper, I review my recent results in investigating hydrogen sensors using nitride-based semiconductor diodes, focusing on the interaction mechanism of hydrogen with the devices. Firstly, effects of interfacial modification in the devices on hydrogen detection sensitivity are discussed. Surface defects of GaN under Schottky electrodes do not play a critical role in hydrogen sensing characteristics. However, dielectric layers inserted in metal/semiconductor interfaces are found to cause dramatic changes in hydrogen sensing performance, implying that chemical selectivity to hydrogen could be realized. The capacitance-voltage (C–V) characteristics reveal that the work function change in the Schottky metal is not responsible mechanism for hydrogen sensitivity. The interface between the metal and the semiconductor plays a critical role in the interaction of hydrogen with semiconductor devises. Secondly, low-frequency C–V characterization is employed to investigate the interaction mechanism of hydrogen with diodes. As a result, it is suggested that the formation of a metal/semiconductor interfacial polarization could be attributed to hydrogen-related dipoles. In addition, using low-frequency C–V characterization leads to clear detection of 100 ppm hydrogen even at room temperature where it is hard to detect hydrogen by using conventional current-voltage (I–V) characterization, suggesting that low-frequency C–V method would be effective in detecting very low hydrogen concentrations.

Photovoltaic Action in Polyaniline/n-GaN Schottky Diodes

Schottky diodes were fabricated on n-GaN films by coating them with an organic polyaniline layer as transparent conducting electrodes. These diodes have a high Schottky barrier height (1.28 eV) and a low reverse leakage current (2.7×10-9 A/cm2 at an applied bias of -1 V). The photovoltaic action of these diodes (VOC = 0.67 V and external quantum efficiency ~30%) was studied under the illumination of an Air Mass 1.5 solar simulator. The polyaniline/n-GaN Schottky contacts were found to be sensitive to shorter wavelengths, indicating their potential for use as solar cells.

Effect of Dielectrics on Hydrogen Detection Sensitivity of Metal–Insulator–Semiconductor Pt–GaN Diodes

The hydrogen responses of metal–insulator–semiconductor (MIS) Pt–GaN diodes are compared. MIS Pt–GaN diodes with a 10 nm SiO2 dielectric, deposited by RF sputtering, show a marked improvement in hydrogen detection sensitivity, which is twice higher than that of conventional Pt–GaN Schottky diodes. In sharp contrast, MIS Pt–GaN diodes with a 10 nm SixNy dielectric, deposited by RF sputtering, do not show any hydrogen response.

Correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures

We report on a correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures, employing Schottky barrier diodes. Photo-capacitance spectroscopy measurements reveal three specific deep levels located at ∼2.07, ∼2.80, and ∼3.23 eV below the conduction band, presumably attributable to Ga vacancies and/or impurity C present in the GaN buffer layer. Additionally, from photo-assisted turn-on current recovery measurements, by using 390 and 370 nm long-pass filters, the recovery time becomes significantly faster due to inactivation of their corresponding deep-level traps. Therefore, the ∼2.80 and ∼3.23 eV levels are probably responsible for the carrier-trapping phenomena in the bulk region.

Interface states in metal-insulator-semiconductor Pt-GaN diode hydrogen sensors

Exposure of Pt-SiO2-GaN metal-insulator-semiconductor (MIS) diodes to hydrogen at room temperature is found not only to shift the flat-band voltage toward negative bias values as compared with that in nitrogen, which results in significant sensitivity of the devices to hydrogen, but also to reduce the interface state density (Dit) dramatically for the first time. Pt-SiO2-GaN MIS diodes in nitrogen yields the Dit of ∼8 × 1011 cm−2 eV−1 at 0.4 eV from the conduction band edge (Ec), and hydrogen exposure reduces the Dit by more than one order of magnitude to the low 1010 cm−2 eV−1 range. In sharp contrast to Pt-SiO2-GaN MIS diodes, neither flat-band voltage shift nor Dit (∼1 × 1013 cm−2 eV−1 at 0.4 eV) reduction is observed for Pt-SixNy-GaN MIS diodes upon hydrogen exposure. These results suggest that atomic hydrogen interacts with MIS interface in Pt-SiO2-GaN MIS diodes even at room temperature. In addition, hydrogen treatment would be a promising method in order to reduce the Dit in GaN metal-oxide-semicon...

Deep-Level Characterization of n-GaN Epitaxial Layers Using Transparent Conductive Polyaniline Schottky Contacts

We have successfully investigated surface-related deep levels in n-GaN epilayers with high carrier concentrations by using transparent conductive polyaniline Schottky contacts. High quality Schottky barrier diodes fabricated showed a typical capacitance dispersion phenomenon at ~10 kHz, which is characteristic of conductive polyaniline films with polarization capacitance and resistance components. Steady-state photocapacitance spectroscopy measurements at over this cutoff frequency revealed five photoemission states with their onsets at ~1.40, ~1.70, ~2.08, ~2.64, and ~2.90 eV below the conduction band, being identical with the deep levels commonly observed in GaN and AlGaN/GaN. Particularly, the concentrations of the ~1.70 and ~2.90 eV levels were found to increase significantly with decreasing their probing depth range to the near-surface region of the n-GaN layers. Therefore, these levels are probably subject to the surface conditions of the n-GaN layers.

Shottky Barrier Diodes on AlN Free-Standing Substrates

Lateral Schottky rectifiers were fabricated on bulk single-crystal free-standing AlN substrates. The unintentionally doped substrates display n-type conductivity. The diode shows a low reverse leakage current of ?0.1 nA at -40 V at room temperature. The ideality factor for forward characteristics is 11.7 at room temperature and shows temperature dependence, suggesting the lateral nonuniformities at the metal/semiconductor interface. The fabricated devices are stably operated even at 573 K, owing to the wide band gap (6.2 eV) of AlN. The reverse leakage current of the device is explained by either a trap-assisted tunneling process or one-dimensional variable-range-hopping conduction along the dislocations.

HETEROINTERFACE PROPERTIES OF NOVEL HYBRID SOLAR CELLS CONSISTING OF TRANSPARENT CONDUCTIVE POLYMERS AND III-NITRIDE SEMICONDUCTOR

We have investigated the heterointerface properties of recently developed hybrid solar cells comprising a Schottky contact made of transparent conductive polymer (TCP) and an underlying GaN semiconductor layer. The heterointerface capacitance induced by the depletion layer under the TCP Schottky contact showed a rapid drop at a specific frequency. An intrinsic capacitance component that was derived from the capacitance–frequency (C–f) characteristics of the heterointerface showed clear correlation with the open circuit voltage. Hence, the C–f characterization using TCP Schottky contacts is indicative of the quality of the heterointerface.

Hydrogen-induced change in the electrical properties of metal-insulator-semiconductor Pt–GaN diodes

Exposure of Pt–SiO2–GaN diodes to hydrogen at room temperature is found to change the conduction mechanisms from Fowler–Nordheim tunneling to Pool–Frenkel emission. The capacitance-voltage (C-V) curve for Pt–SiO2–GaN diodes in hydrogen significantly shifts toward negative bias values as compared with that in nitrogen. In sharp contrast, Pt–SixNy–GaN diodes exhibit Pool–Frenkel emission in nitrogen and do not show any change in the conduction mechanism upon exposure to hydrogen. The C-V curve for Pt–SixNy–GaN diodes also does not show any shifts upon the exposure to hydrogen. These results suggest that the work function change in the Schottky metal is not responsible mechanism for the hydrogen sensitivity. The interface between the metal and the semiconductor plays a critical role in the interaction of hydrogen with semiconductor devices, including diodes and field-effect transistors (FETs).