Hydrogen sensing characteristics and mechanism of Pd∕AlGaN∕GaN Schottky diodes subjected to oxygen gettering

Abstract

Hydrogen sensing characteristics in vacuum and in air were investigated on Pd Schottky diodes that were formed on AlGaN / GaN two-dimensional electron gas wafer and subjected to a surface control process for oxygen gettering. By applying the surface control process, leakage currents in Pd/ AlGaN / GaN Schottky diode were greatly reduced. Such diodes showed high hydrogen detection sensitivities and fast turn-on and -off characteristics in air, although they showed very slow turn-off behavior in vacuum. From detailed measurements of current-voltage ͑I-V͒, capacitance-voltage ͑C-V͒, and current transient characteristics, the sensing mechanism was explained in terms of Schottky barrier height reduction caused by formation of interface dipole by atomic hydrogen. It was shown that dipole formation is controlled in air by the Langmuir isotherm type adsorption behavior, including the reaction between atomic hydrogen and oxygen. Discrepancies in Schottky barrier height values deduced from I-V and C-V measurements have indicated that current transport is not by the standard thermionic emission process, but by the thermionic field emission process through the thin surface barrier ͑TSB͒ in accordance with the TSB model.

High temperature sensing characteristics of a high performance Pd/AlGaN/GaN Schottky diode hydrogen sensor obtained by oxygen gettering

“…2-5 can be explained by the sensing mechanism discussed in detail previously [9]. Namely, the current increase is due to reduction of Schottky barrier height (SBH) caused by interface dipole formed by atomic hydrogen that is generated by dissociation of hydrogen molecule on the catalytic Pd …”

“…Schottky barrier height reduction by interface dipole formation through adsorption of atomic hydrogen [9].…”

Hydrogen sensing characteristics of non-polar a-plane GaN Schottky diodes

“…For example, diodes prepared in situ to avoid oxide interfacial layers showed very poor response to hydrogen [28] and the incorporation of a SiO 2 layer to form a metal-oxide-semiconductor structure was found to greatly enhance the sensing response [22]. However, others have reported that when surface oxide concentrations were minimized on AlGaN/GaN Schottky diodes, the hydrogen detection sensitivity increased [19]. In our case, Auger Electron Spectroscopy showed similar amounts of interfacial oxygen in our a-plane diodes compared to the c-plane devices.…”

SiO2 passivation effect on the hydrogen adsorption performance of a Pd/AlGaN-based Schottky diode

“…This property gives the promise for high-power and high-temperature microwave applications. Moreover, a high density of 2DEG is beneficial to the sensitivity of surface condition and thus can be used to improve the hydrogen sensing capability [16]. Recently, on the other hand, the SiO 2 passivation has been employed in the AlGaN/GaN high electron mobility transistor (HEMT) [17,18].…”

“…The adsorbed hydrogen atoms at Pd/SiO 2 interface are polarized to form hydrogen dipoles by the internal electric field. Thus, hydrogen dipoles cause the lowering effect of Schottky barrier height, and substantially increase the response current [16]. The device parameters can be extracted from the thermionic emission mechanism [21]:…”