Micro-electro-mechanical systems (MEMS)
The main applications of GaN-based MEMS are different kinds of oscillators on Si. Here, usually thin bars based on GaN/AlGaN structures are etched and the Si below them removed. Depending on their geometry, these free-standing bars can vibrate at different frequencies and be used, e.g., for frequency filtering . Apart from these applications, GaN is also interesting for other devices where, e.g., high-temperature operation or aggressive media are present. Such a device is a miniature high- pressure sensor as demonstrated by Zimmermann et al. . This miniature sensor depends on a Si bar with a FET structure, and bending the bar changes the carrier density and with it the resistance of the 2DEG, which is then measured and is in principal suited for pressure measurements at high temperatures.
Another example of a MEMS device which generates energy and can be tuned electrically is the balance spring of a watch (Fig. 3.19). These balance springs are often manufactured from Si instead of metal by a dry etching process. Growing c-axis-oriented AlN on top of such Si spiral spring surfaces yields additional piezoelectric material which can be used to generate energy. Here the difficulty in MOVPE growth lies in a required homogeneous coverage of the whole Si surface with a c-axis-oriented group-III-nitride layer perpendicular to the surface which cannot be achieved, e.g., by sputtering techniques. With this, piezoelectric layer energy in the range of nanowatts can be generated, e.g., to charge electronics which in return can correct the motion of the balance spring by applying a voltage. This correction of the spring’s oscillation frequency by quartz-controlled
Fig. 3.19. SEM images of an AlN-coated Si spiral spring for an application as a balance spring in a wrist watch. AlN was grown with its c-axis-oriented perpendicular to the surface normal by MOVPE (left). A voltage between the inner and outer AlN layers of the spring is induced by the spring motion. This can be used for charging an electronic circuit and a feedback loop to tune the oscillation frequency.
electronics can yield high accuracy in a mechanical watch. One can think of other micro-oscillating devices where energy is gained and used to operate electronics for other functions or oscillation control.
Especially in the field of MEMS, many applications can be thought of which make use of the piezo- and pyroelectric properties of the group-III-nitrides for sensors, filters, or frequency tuning. Here the maturity of the Si technology is of huge benefit when it comes to etching special structures as free-standing group- III-nitride layers or bars or thin Si layers which are subsequently overgrown, as in the case of the spiral spring material.