AlGaN/GaN-Schichtsysteme für piezoelektrisch angeregte Resonatoren

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors receive more and more attention. This work is divided into two main parts in order to pave the way for the realization of GaN-based piezoelectric MEMS: in the first main section, the growth of GaN-based heterostructures on three selectively etchable substrates is introduced. In the first case, the MOCVD growth of GaN on 4H-SiC is demonstrated, which has proven its isotropic etchability recently. Second, the epitaxial growth of GaN is introduced on sputtered AlN sacrificial layers, which enables the processing of GaN-based MEMS on sapphire. In the last case, the epitaxial growth of GaN has been developed on silicon using a thin 3C-SiC buffer layer to prevent meltback etching of the substrate. A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GAN heterostructures is reported in the second main section. Thereby, the lower GaN layer represents the mechanically supporting layer, while the AlGaN film, and in some cases an additional GaN cap layer serves as the piezoelectrically active layers for actuation. The 2DEG (at the lower AlGaN/GaN interface) provides the conducting channel which was used as back electrode for the piezoelectric actuation. Electroreflectance spectroscopy is applied to determine the electric field distribution across the whole structure as function of the applied voltage. Piezoelectric force microscopy yields the field (voltage)-dependent actuation of the layers. By correlating these two techniques it was possible to determine the piezoelectric modulus d33 with considerably improved reliability. A value of 5 pm/V was found for AlGaN with 31% Al. The processing of the first GaN-based resonators is presented on all three etchable substrates introduced above. Finally, the resonant behaviour of thus developed and fabricated MEMS resonators is demonstrated for flexural bending via piezoelectric excitation and vibrometer read-out and for longitudinal bending by a purely electrical actuation and read-out scheme.Auf Grund der gestiegenen Anforderungen für mikroelektromechanische Systeme (MEMS) hinsichtlich Stabilität, Miniaturisier- und Integrierbarkeit, steigt das Interesse an neuen Materialsystemen wie den Halbleitern großer Bandlücke. Die vorliegende Arbeit gliedert sich in zwei Schwerpunkte, die für die Herstellung GaN-basierter MEMS bewältigt werden mussten: zum Einen wurde das Wachstum auf selektiv zu GaN ätzbaren Substraten untersucht. In der vorliegenden Arbeit werden drei Substrate vorgestellt, welche sowohl das epitaktische Wachstum von GaN als auch das Freistellen der Struktur erlauben. Dazu gehört die Verwendung von 4H-SiC als Substrat, welches sich kürzlich als isotrop ätzbar erwiesen hat. Als zweites wird das epitaktische GaN-Wachstum auf nanokristallinen, gesputterten AlN-Opferschichten gezeigt, welches die MEMS-Herstellung auf Saphir ermöglicht. Im letzten Fall erfolgt das Wachstum auf Siliziumsubstraten mit Hilfe einer 3C-SiC-Zwischenschicht. Die piezoelektrischen Eigenschaften von (GaN/)AlGaN/GaN-Heterostrukturen standen im zweiten Schwerpunkt im Fokus. Dabei dient die AlGaN-Schicht und in einigen Fällen eine zusätzliche GaN-Deckschicht als piezoelektrisch aktive Schicht. Das hochleitfähige 2D Elektronengas (2DEG) an der unteren AlGaN/GaN-Grenzfläche stellt dabei die zur Anregung benötige Rückelektrode zur Verfügung. Mit Hilfe der Elektroreflexion konnte die elektrische Feldverteilung in Abhängigkeit von der angelegten elektrischen Spannung bestimmt werden. In Kombination mit der Piezokraftmikroskopie, bei welcher die spannungsabhängige Auslenkung der Schichten untersucht wurde, konnte das piezoelektrische Modul d33 für Al0.31Ga0.69N zuverlässig mit 5 pm/V bestimmt werden. Die Prozessierung der ersten GaN-basierten MEMS wird schließlich auf allen drei zuvor eingeführten Substraten vorgestellt, das Schwingungsverhalten für die piezoelektrische Anregung von Transversalschwingungen mit Hilfe von Vibrometermessungen sowie das rein elektrische Auslesen von Longitudinalschwingungen (ebenfalls für piezoelektrische Anregung) demonstriert

Automated parameter extraction of ScAlN MEMS devices using an extended Euler-Bernoulli beam theory

Magnetoelectric sensors provide the ability to measure magnetic fields down to the pico tesla range and are currently the subject of intense research. Such sensors usually combine a piezoelectric and a magnetostrictive material, so that magnetically induced stresses can be measured electrically. Scandium aluminium nitride gained a lot of attraction in the last few years due to its enhanced piezoelectric properties. Its usage as resonantly driven microelectromechanical system (MEMS) in such sensors is accompanied by a manifold of influences from crystal growth leading to impacts on the electrical and mechanical parameters. Usual investigations via nanoindentation allow a fast determination of mechanical properties with the disadvantage of lacking the access to the anisotropy of specific properties. Such anisotropy effects are investigated in this work in terms of the Young’s modulus and the strain on basis of a MEMS structures through a newly developed fully automated procedure of eigenfrequency fitting based on a new non-Lorentzian fit function and subsequent analysis using an extended Euler–Bernoulli theory. The introduced procedure is able to increase the resolution of the derived parameters compared to the common nanoindentation technique and hence allows detailed investigations of the behavior of magnetoelectric sensors, especially of the magnetic field dependent Young‘s modulus of the magnetostrictive layer

Anisotropy of the ΔE effect in Ni-based magnetoelectric cantilevers: a finite element method analysis

In recent investigations of magnetoelectric sensors based on microelectromechanical cantilevers made of TiN/AlN/Ni, a complex eigenfrequency behavior arising from the anisotropic ΔE effect was demonstrated. Within this work, a FEM simulation model based on this material system is presented to allow an investigation of the vibrational properties of cantilever-based sensors derived from magnetocrystalline anisotropy while avoiding other anisotropic contributions. Using the magnetocrystalline ΔE effect, a magnetic hardening of Nickel is demonstrated for the (110) as well as the (111) orientation. The sensitivity is extracted from the field-dependent eigenfrequency curves. It is found, that the transitions of the individual magnetic domain states in the magnetization process are the dominant influencing factor on the sensitivity for all crystal orientations. It is shown, that Nickel layers in the sensor aligned along the medium or hard axis yield a higher sensitivity than layers along the easy axis. The peak sensitivity was determined to 41.3 T −1 for (110) in-plane-oriented Nickel at a magnetic bias flux of 1.78 mT. The results achieved by FEM simulations are compared to the results calculated by the Euler–Bernoulli theory

Sensing applications of micro- and nanoelectromechanical resonators

The sensitivity of micro- and nanoscale resonator beams for sensing applications in ambient conditions was investigated. Micro-electromechanical (MEMS) and nanoelectromechanical systems (NEMS) were realized using silicon carbide (SiC) and polycrystalline aluminium nitride (AlN) as active layers on silicon substrates. Resonant frequencies and quality factors in vacuum as well as in air were measured. The sensitivity behaviour under ambient conditions with a mass loading in the range of picogram (pg) was verified and measurements with biological mass loading were performed. In addition, the sensitivity to pressure variations was analysed