K. Tonisch

Two-dimensional electron gas based actuation of piezoelectric AlGaN/GaN microelectromechanical resonators

Free-standing piezoelectric AlGaN/GaN beam resonators have been prepared on silicon substrates. The two-dimensional electron gas at the interface of the III/V heterostructure has been employed to act as back electrode for the piezoelectric active layer. The fundamental mode as well as higher order resonant modes of flexural vibration has been excited piezoelectrically and analyzed using optical laser–Doppler vibrometry. The experimental investigations were carried out under normal ambient conditions. The specific piezoelectric actuation scheme is described and the dependence of the measured resonant frequencies between 0.2 and 8.1 MHz on geometry and material parameters is investigated.

Strain- and pressure-dependent RF response of microelectromechanical resonators for sensing applications*

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. A semiconductor fabrication process has been applied to prepare resonant AlN and SiC beams operating at frequencies between 0.1 and 2.1 MHz. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. For systematic studies of the resonant frequencies and quality factors, the induced voltage was measured using time domain and frequency domain techniques. Resonator geometry, material and ambient pressure were varied to attain a generalized understanding of the RF performance. The dependence of the resonant frequency on tensile axial strain has been derived analytically and extended to include highly strained beams. Based on these formulas, accurate detection of the residual layer strain after fabrication is presented. To describe the quality factor a chain of beads model has been applied successfully. The influences of the beam width and the pressure-dependent viscosity on the model parameters are analyzed.

Pulsed mode operation of strained microelectromechanical resonators in air

A pulsed mode magnetomotive operation of micro- and nanoelectromechanical devices in air is demonstrated, where viscous damping determines the quality factor of the device. An enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed. Internal strain is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates. Comparing unstrained and strained resonators, an increase of the quality factor by one order of magnitude from about 30 to 300 was achieved. This increase will improve the sensing performance of such resonant structures for an operation in ambient environment.

Piezoelectric actuation of (GaN/)AlGaN/GaN heterostructures

A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GaN heterostructures is reported. The electromechanical properties of two types of heterostructures with an Al content of 31% are compared. Only a single two-dimensional electron gas (2DEG) is formed for samples with thin GaN cap layers, while both a 2DEG and a two-dimensional hole gas coexist in the case of thick GaN caps. 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 applied external voltage. Electroreflectance spectroscopy is applied in order to determine the electric field distribution across the whole structure as a function of the applied voltage. It is found that only a part of the modulation voltage drops across the active region. Piezoelectric force microscopy yields the...

Coalescence aspects of III-nitride epitaxy

In this work, coalescence aspects of wurtzite-III-nitride epitaxy are addressed. The coalescence phenomena have been studied in thin epilayers by means of electron and atomic force microscopies, and electron and x-ray diffractions. This study generalizes the growth parameters responsible for the rapid coalescence of III-nitride films, and describes the coalescence qualitatively and, partly, analytically for the case of heteroepitaxy in nonequilibrium conditions. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for GaN and InN depositions. The rate of coalescence has been found to impact on the structure and morphology of III-nitride epilayers. A simple growth model was suggested to explain the formation of domain boundaries and (0001) stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped 2H domains may form a noncoherent boundary explicitly along a {11¯00} plane. We also suggest that the...

High-Frequency Performance of GaN High-Electron Mobility Transistors on 3C-SiC/Si Substrates With Au-Free Ohmic Contacts

As an alternative to the standard Au-based ohmic contacts, in this letter, Ti/TiN contacts were used to fabricate GaN high-electron mobility transistors (HEMTs) with two barrier designs (Al0.2Ga0.8N/AlN and Al0.35Ga0.65N). The ohmic contact resistance for this Au-free metallization scheme with a very smooth surface morphology is 0.13 Qmm and the specific contact resistance is ~10-6 Ωcm2. Our best 100-nm gate transistors show a maximum drain current density of 1.13 A/mm and a peak extrinsic transconductance of 388 mS/mm. The fastest transistors with a gate length of 80-nm achieve cutoff frequencies of 176 GHz, rivaling the fastest GaN HEMTs on silicon and silicon carbide substrate with comparable gate length and Au-based ohmic contacts.

Electric field distribution in GaN∕AlGaN∕GaN heterostructures with two-dimensional electron and hole gas

Ga-face GaN∕AlGaN∕GaN heterostructures with different cap thicknesses are investigated by electroreflectance spectroscopy (ER). The voltage dependent electric field strengths of the barrier and cap layers are determined. The AlGaN electric field amounts of up to −2.6MV∕cm, whereas the GaN electric field is always below 700kV∕cm. The two electric fields have opposite signs. Characteristic features in the voltage maps of the ER spectra are assigned to the formation/depletion of a two-dimensional electron gas below and a two-dimensional hole gas above the AlGaN barrier. Between −6.5 and 0V, both carrier gases coexist.

Suspended nanowire web

A complex three-dimensional, nanowire based nanoarchitecture is presented, which can be processed by high-throughput bottom-up procedures without any high-resolution lithography. It combines the benefits of three self-organization mechanisms to produce nanostructures, i.e., the formation of nanoneedles, the droplet formation out of a thin metal film, and the vapor-liquid-solid growth of nanowires. The principle is demonstrated for a silicon based suspended nanowire web. Cell adherence on this assembly was found to be superior to other nanostructures. The possibility of fluid transport beneath the nanowire web enables improved microcatalyst principles and the realization of novel interfaces for biosensing or bioelectronics.

Smooth ceramic titanium nitride contacts on AlGaN/GaN-heterostructures

Two contact systems on AlGaN/GaN-heterostructures were investigated with respect to their electrical performance and surface morphology. The first system is a standard Ti/Al-based multilayer contact system, generally exhibiting an excellent electrical performance. However strong surface roughening and degradation after annealing due to high diffusion rates of aluminum and gold often occur, which in turn affect the contact stability and reliability. Thus, a contact system based on sputtered titanium nitride was studied. TiN is well known as a thermally stable material with a good conductivity, but is used only indirectly via thermal transformation of thin Ti layers in typical group III nitride circuits. Using the TiN contact system, we were able to achieve a competitive electrical performance with a minimum value of contact resistivity of 1.8 × 10−5 Ω cm2 at an annealing temperature of 850 °C for 45 s. Most importantly, the surface roughness was greatly reduced (maximum rms values of 1.8 nm). The contact formation via rapid thermal processing was studied using Auger electron depth profiling.

Electromechanical resonances of SiC and AlN beams under ambient conditions

MEMS resonators offer great potential for RF sensor and filter applications. A semiconductor process has been used to prepare SiC and AlN beam resonators. The metallised beams are excited by an RF current in a permanent magnetic field. The resonant response is detected by the induced voltage. Despite the weakness of the signal, accurate detection has been achieved in the time domain, under ambient conditions in a magnetic field of about 0.5 T. The resonant response bears valuable information on the structural quality of the material and identifies potential for further improvement. The time domain technique presents an elegant approach to sensing applications.