In the present work, AlGaN/GaN high electron mobility transistors (HEMTs) have been grown with very thin buffer layers on silicon substrates in view of developing nano electromechanical systems (NEMS) for sensors applications. To ensure transducer operation in the MHz range together with low mechanical stiffness, epitaxial structures with thickness below 1 μm have to be developed. We report on the evolution of the material and electrical properties of AlGaN/GaN HEMTs with thicknesses varying from 2 μm to 0.5 μm. The set of parameters obtained includes in-plane Young modulus of 250 GPa in association with carrier density of 6 × 10 12 cm −2 and mobility above 1000 cm 2 V −1 s −1 . The resulting behavior of demonstration transistors validates these epilayers for electromechanical resonators operation.
We present the first results about microelectromechanical (MEMS) resonators fabricated on epitaxial nitride semiconductors with thin buffers engineered for MEMS and NEMS applications. These results assess the use of thin buffers for GaN MEMS fabrication. On a 700 nm thick AlGaN/GaN epilayer, a high tensile stress is observed to increase the resonant frequency. The electromechanical coupling efficiencies of integrated transducers are assessed and compared with previously obtained results on commercially available 2-µm thick epilayers used for power transistor applications. A 28 nm/V actuation efficiency is measured on the 700-nm thick structure which is slightly better than the one measured on the 2-µm buffer. The electrical response of a gate-less detector designed as a piezoresistance was carried out and a gauge factor of 60 was estimated. These results show that material issues can be unlocked to exploit the potentialities of III-nitrides for NEMS applications.
Phone: þ33 4 93 95 42 00, Fax: þ33 4 93 95 83 61The present work is dedicated to the study of the influence of metal-organic vapor phase epitaxy (MOVPE) growth conditions on the properties of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures with thin simple buffer layers on Si(111). In a first series of samples grown on high resistivity silicon, the conditions were varied within the GaN buffer layer while kept unchanged for the AlN nucleation layer and HEMT barrier and cap layers. XRD and AFM revealed some differences in epilayers structural quality especially in case of excessive growth pressure or V/III ratio. Capacitancevoltage (CV) measurements revealed differences in the pinchoff regime of the two-dimensional electron gas (2DEG) located at the AlN/GaN interface. Except in one case, the buffer leakage current between isolated devices correlates with the pinch-off behavior. The majority of these structures exhibited sheet carrier densities of 1 Â 10 13 cm À2 and electron mobility between 1100 and 1400 cm 2 V À1 s À1 depending on the GaN channel growth conditions. The output and transfer characteristics (maximum drain currents and leakage currents) of the transistors are in agreement with the previous electrical characterizations. Thanks to the combination of structural and electrical characterizations we are then able to determine the optimized growth conditions for such HEMT structures. Compared with high resistivity silicon, we obtained better structural and electrical quality on conductive substrates. A 2DEG with an electron mobility of 1700 cm 2 V À1 s À1 has been achieved within a structure with a 0.5 mm thick buffer producing an off-state breakdown voltage of 117 V. This reveals the crucial role of the substrate surface properties on the resulting quality of the heterostructures.
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