Characteristics and Mechanisms of Electrical Response to Hydrogen in Nitride-based heterostructures

Due to polarization differences between AlxGa1-x N and GaN, a two-dimensional electron gas forms at the AlxGa 1-xN/GaN interface. Upon exposure to hydrogen in the presence of a catalyst, a strong and rapid electrical response is observed. The chemical stability of the semiconductors and the sensitivity to hydrogen makes the AlxGa1-xN/GaN heterostructure a strong candidate for applications requiring sensors in severe environments (e.g. elevated temperatures, corrosive gases). The physiochemical mechanism of interaction between hydrogen and the device is not well understood. The overarching objective of this research described in this thesis has been to understand mechanism of electrical response upon exposure to hydrogen in an AlxGa1-xN/GaN high electron mobility transistor and to demonstrate the feasibility of this device for use in hostile environments.;AlxGa1-xN/GaN thin film heterostructures were grown using metalorganic vapor phase epitaxy and fabricated into Schottky diode and transistor devices containing a Pt gate that acted both as the catalyst for the decomposition of molecular hydrogen into atomic hydrogen and as for the conduit for diffusion of the latter species to the Pt/Alx Ga1-xN interface. The sensitivity of both types of devices to the presence of hydrogen in the surrounding atmosphere within the range of 0.125 vol.% to 20 vol.% was demonstrated. Upon exposure to hydrogen, a change in shape and a parallel shift of the capacitance voltage curve were observed. The former indicates surface state passivation and the latter, the existence of hydrogen dipoles. An initial increase in the carrier density of the two-dimensional electron gas, followed by saturation was observed using capacitance-voltage profiling. Two different chemical binding states were also observed using time-resolved capacitance measurements.;Surface state passivation and hydrogen-dipole formation are proposed as the mechanisms underlying the observed electrical responses. An ionized donor state at approximately mid-gap and an acceptor state at 0.6 eV beneath the conduction band edge exist and correspond to the two different observed binding states noted above. Simulations conducted in this study show that passivation of the states at approximately mid-gap results in an increase in the magnitude of the 2DEG carrier density. Passivation of the states at 0.6 eV beneath the conduction band edge causes no change to the 2DEG carrier density. The magnitude of increase in 2DEG carrier density predicted by the simulations matches well with empirical data. Upon passivation of a surface state, the proton associated with the hydrogen is still bound to the electron. Thus a dipole is formed at the catalyst/semiconductor interface that causes the voltage shift observed in capacitance-voltage measurements.;Additionally, the sensitivity the device to hydrogen in helium, nitrogen, chlorine, and argon were examined. Time-resolved capacitance measurements revealed (1) gas-phase diffusion to be the limiting kinetic step in the sensor response and (2) the flow conditions within the sensing apparatus will drastically affect the sensitivity of the device. Finally, operation and sensitivity to hydrogen and methane of the device at temperatures up to 200°C were examined. The mechanism for sensitivity to methane is similar to that of hydrogen, except that methane does not readily dissociate upon the catalyst surface. The temperature of the device must be above 70°C to observe the sensitivity of the device.