Virginie Brandli

Searching for THz Gunn oscillations in GaN planar nanodiodes

A detailed study of GaN-based planar asymmetric nanodiodes, promising devices for the fabrication of room temperature THz Gunn oscillators, is reported. By using Monte Carlo simulations, an analysis of the static I-V curves and the time-domain evolution of the current obtained when varying some simulation parameters in the diodes has been made. Oscillation frequencies of hundreds of GHz are predicted by the simulations in diodes with micrometric channel lengths. Following simulation guidelines, a first batch of diodes was fabricated. It was found that surface charge depletion effects are stronger than expected and inhibit the onset of the oscillations. Indeed, a simple standard constant surface charge model is not able to reproduce experimental measurements and a self-consistent model must be included in the simulations. Using a self-consistent model, it was found that to achieve oscillations, wider channels and improved geometries are necessary.

Experimental demonstration of direct terahertz detection at room-temperature in AlGaN/GaN asymmetric nanochannels

The potentialities of AlGaN/GaN nanodevices as THz detectors are analyzed. Nanochannels with broken symmetry (so called self switching diodes) have been fabricated for the first time in this material system using both recess-etching and ion implantation technologies. The responsivities of both types of devices have been measured and explained using Monte Carlo simulations and non linear analysis. Sensitivities up to 100 V/W are obtained at 0.3 THz with a 280 pW/Hz1/2 noise equivalent power.

Dislocation filtering and polarity in the selective area growth of GaN nanowires by continuous-flow metal organic vapor phase epitaxy

The early growth stages of GaN nanowires on GaN-on-sapphire templates with a patterned dielectric mask have been characterized by using transmission electron microscopy. The dielectric mask aperture (200–800 nm) determines the presence or absence of threading dislocations arising from the underlying template, which results in dislocation-free nanowires for small apertures and dislocation bending for larger apertures, owing to three-dimensional (3D) growth. The Ga polarity of the underlying template is conserved in all nanowires irrespective of the aperture size, even in regions grown laterally above the mask. The pure Ga polarity assures spatially homogeneous optical properties as evidenced by cathodoluminescence.

GaN: A multifunctional material enabling MEMS resonators based on amplified piezoelectric detection

The properties of a new class of electromechanical resonators based on GaN are presented. By using the flexural modes of a doubly clamped beam, the two-dimensional electron gas (2-DEG) present at the AlGaN/GaN interface can be modulated by a field effect arising from the GaN buffer piezoelectricity. This leads to active piezoelectric transducers for which we show experimental results with detailed bias condition studies up to 10 MHz. Associated with modeling of the transduction physics, this allows explaining how the 2-DEG properties lead to the transconductance effect in the electromechanical domain.

Gallium nitride approach for MEMS resonators with highly tunable piezo-amplified transducers

The properties of a new class of electromechanical resonators based on GaN are presented. By using the two-dimensional electron gas (2-DEG) present at the AlGaN/GaN interface and the piezoelectric properties of this heterostructure, we use the R-HEMT (Resonant High Electron Mobility Transistor) as an active piezoelectric transducer up to 5MHz [1]. In addition to the amplification effect of piezoelectric detection, we show that the active piezoelectric transduction has a strong dependence with the channel mobility that is controlled by a top gate. This allows to envision highly tunable sensors with co-integrated HEMT electronics.

Selective area sublimation of GaN for top-down fabrication of nanostructures (Conference Presentation)

A fraction of a SiNx mono-layer is formed on a GaN layer by exposing the surface to a Si flux. When the sample is heated under vacuum at high temperature (900°C), we observe the sublimation of GaN in the regions uncovered by the thermally resistant SiNx mask. This selective area sublimation (SAS) process can be used for the formation of nanopyramids and nanowires with a diameter down to 4 nm. Also, if InGaN quantum wells are included in the structures before sublimation, InGaN quantum disks with quasi identical sizes in the 3 dimensions of space can be formed using SAS.

Gallium nitride MEMS resonators: How residual stress impacts design and performances

Stressed gallium nitride micro-resonators with integrated piezoelectric transducers have been designed, fabricated, and characterized. In resonant beams, it is well known that tensile stress can be used to increase the resonant frequency. Here we additionally calculate the modification of the mode shape functions of out-of-plane flexural modes. We derive an analytical model to predict both the resonant frequency and the piezoelectric actuation factor of our resonators. We show that a good agreement can be obtained and that the actuator must be properly designed to optimize the electromechanical transduction.