Design, fabrication and characterization of gallium nitridebased circular schottky diode for hydrogen sensing

摘要

Recent revolutionary progress of the internet and wireless technologies has create a concept of the “ubiquitous network” society for this 21st century. A so called Intelligent Quantum (IQ) chip has been proposed as the promising electronic device for the ubiquitous network society environment. An IQ chip is an III-V semiconductor chip with sizes of millimeter square where not only nanometer scale quantum processors and memories are integrated on this chip but also other devices such as wireless power supply and various sensing devices so that such ideal concept can be realized. This research is carried out to reveal a possibility of utilizing III-V base material as a sensing device, in particular as a hydrogen (H2) gas sensor. High temperature operation and long term stability are important requirements for a H2 sensor, thus an undoped-Alluminium Nitride/Gallium Nitride (AlGaN/GaN) high-electron-mobility-transistor (HEMT) structure is chosen as the base material. The sheet concentration and mobility of epitaxial layers determined by Hall measurement were 6.61×1012 cm-2 and 1860 cm2/Vsec, respectively. The devices fabrication were etched by an inductively-couple-plasma reactive ion etching (ICP-RIE) system for mesa isolation? with a Chlorine (Cl)-based gas system consisting of Boron Trychloride (BCl3) and Chlorine (Cl2) gases. The ohmic contacts are formed by deposition of Titanium/Aluminium/Titanium/Aurum (Ti/Al/Ti/Au) (20/50/35/50 nm) multilayers followed by rapid thermal annealing at 850 °C for 30 s in nitrogen (N2) ambient. The Schottky contact was produced by evaporating 5 nm thick catalytic Platinum (Pt) metal. Finally, Titanium/Aurum (Ti/Au) was evaporated as interconnection contact. Typical I-V characteristics measured in vacuum and high purity H2 ambient at room temperature show that both the forward and reverse currents give only a slight change of current upon exposure to H2 because the diffusion rate for H2 atom through the catalytic metal is very slow at room temperature. Thus, it can be said that the sensitivity of gas sensor is quite low at room temperature. However, a large current change by the same amount of H2 concentration is observed as the temperatures increase up to 200 °C because more effective catalytic dissociation of H2 on the Pt surface can be realized at higher temperature. The time-transient response measured at temperature of 200 °C and forward bias of 1 V shows that there is sufficient cracking of H2 for the diode to be a sensitive gas sensor. A constant speed is obtained at each cycle where the average of increment and decrement speed of current are estimated to be 27.6 nA/sec and 17.6 nA/sec, respectively. The increment speed is much faster than the decrement speed for each cycle meaning that the absorption of H2 is faster than desorption. This is because a desorption process requires thermal energy supply, leading to a longer decrement time. These preliminary results indicate that the proposed sensing devices are capable of detecting H2 gas with acceptable performance.