Highly sensitive III–V nitride based piezoresistive microcantilever using embedded AlGaN/GaN HFET as ultrasonic detector

Summary form only given.We report, for the first time, an ultra high gauge factor of more than 3500 observed using AlGaN/GaN Heterostructure Field Effect Transistor (HFET) embedded GaN piezoresistive microcantilever. In addition, the deflection transduction signal from the HFET was utilized to determine dynamic bending as well as AC frequency response of the cantilever. Finally, the piezoresistive microcantilver was used to detect very small acoustic pressure waves generated by a piezo chip oscillated at sub nm amplitude at the resonance frequency of the cantilever positioned 1 cm away, highlighting the utility of these cantilevers as highly sensitive ultrasonic transducers. FET embedded microcantilevers are ideal for developing integrated electronic detection platform for biological and chemical analytes. GaN microcantilever with integrated AlGaN/GaN HFET deflection transducer offers very high mechanical, thermal, and chemical stability, in addition to extraordinary deflection sensitivity due to its strong piezoelectric properties. The piezoelectric property of III-V Nitrides causes a highly mobile (>;1500 cm2/Vs) two dimensional electron gas (2DEG) to form at the AlGaN/GaN interface, which gets strongly affected by the deflection induced strain. In addition, the electron mobility also changes due to the change in effective mass. The combined changes in 2DEG and mobility offer very high deflection sensitivity, verified through COMSOL finite element simulations and experimental observations. The effect of mechanical strain caused by microcantilever bending on the 2DEG and the AlGaN/GaN HFET characteristics has been reported experimentally [1] and theoretically [2] earlier, but this for the first time we have obtained such a high Gauge Factor. Microcantilevers were fabricated using III-V Nitride layers on Si(111). The layer structure consisted of i-GaN (2 nm)/ AlGaN (17.5 nm, 26% Al)/i-GaN (1 μm)/Transition layer (1.1 μm)/Si (111) substrate (500 μm). Fig. 1 (a) shows the SEM image of the fabricated device with the HFET shown in the inset. The HFET was fabricated with initial 200 nm mesa etching, followed by Ti(20 nm)/Al(100 nm)/Ti(45 nm)/Au(55 nm) metal stack deposition and rapid thermal annealing for ohmic contact formation. For gate contact, Ni(25 nm)/Au(375 nm) Schottky barrier was used. The fabricated microcantilever dimension is 350×50×2 μm. The GaN cantilever pattern was etched down using Ch based inductively coupled plasma etch process. Fig. 1 (b) shows the schematics of the experimental setup using our wire bonded device (shown as inset in Fig. 2) and Nanopositioner's (PI-611 Z). Fig. 2 shows the Id-V d characteristics of one of our best devices for different gate bias. In Fig. 3 the static bending performance is shown where the drain current is found to change by 6.3 % in magnitude, which gives a gauge factor of 3532. Both the downward and upward bending of cantilever exhibited similar changes. The movement of the nanopositioner was controlled using a Labview program, which was modified to also perform low frequency dynamic bending (up to 40 Hz). Fig. 4 shows the low frequency (0.5 Hz) response of a more typical device when the bending magnitude (both downward and upward) was 25 μm. We found that the low frequency upward and downward bending does not alter the gauge factor, and the response up to 40 Hz is also quite similar. We previously reported [3] a gauge factor of -38 (at Vg=0 V) and -860 in steady state and transient conditions, respectively. But our second generation devices consistently exhibit much higher gauge factor in both static and dynamic bending conditions at zero gate bias. We also extracted the AC response of the cantilevers using a miniature peizoactuator (PL055.31 from PI) and a lock-in amplifIer (SR 850). The ac response of the microcantilever, determined from direct contact oscillation of the piezo chip revealed a resonant peak of the cantilever at 45 KHz (Fig. 5) and a high quality factor of more than 200. The piezo chip was also used as an ultrasonic source and our microcantilever sensor was able to detect sub nm vibrational amplitude of the piezo chip from a distance of I cm. These results highlight the possibility of using III-V Nitride based piezoresistive microcantilevers as highly sensitive ultrasonic sensor for harsh environment, with wide ranging applications in acoustic spectroscopy and imaging.