Properties Of group-III Nitrides Research Articles

Remote epitaxy using graphene enables growth of stress-free GaN

The properties of group III-Nitrides (III-N) such as a large direct bandgap, high melting point, and high breakdown voltage make them very attractive for optoelectronic applications. However, conventional epitaxy on SiC and sapphire substrates results in strained and defective films with consequently poor device performance. In this work, by studying the nucleation of GaN on graphene/SiC by MOVPE, we unambiguously demonstrate the possibility of remote van der Waals epitaxy. By choosing the appropriate growth conditions, GaN crystals can grow either in-plane misoriented or fully epitaxial to the substrate. The adhesion forces across the GaN and graphene interface are very weak and the micron-scale nuclei can be easily moved around. The combined use of x-ray diffraction and transmission electron microscopy demonstrate the growth of stress-free and dislocation-free crystals. The high quality of the crystals was further confirmed by photoluminescence measurements. First principles calculations additionally highlighted the importance of the polarity of the underlying substrate. This work lays the first brick towards the synthesis of high quality III-N thin films grown via van der Waals epitaxy.

AlGaN/GaN Based Heterostructures for MEMS and NEMS Applications

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are receiving more attention. The outstanding properties of group III-nitrides offer many more possibilities for the implementation of new functionalities and a variety of technologies are available to realize group III-nitride based MEMS. In this work we demonstrate the application of these techniques for the fabrication of full-nitride MEMS. It includes a novel actuation and sensing principle based on the piezoelectric effect and employing a two-dimensional electron gas confined in AlGaN/GaN heterostructures as integrated back electrode. Furthermore, the actuation of flexural and longitudinal vibration modes in resonator bridges are demonstrated as well as their sensing properties.

Point defects in group-III nitride semiconductors studied by positron annihilation

Positron annihilation is an established technique for investigating vacancy-type defects in semiconductors. When a positron is implanted into solids, it annihilates with an electron and emits two 511 keV γ quanta. From measurements of Doppler broadening spectra of the annihilation radiation and the positron lifetimes, one can detect vacancy-type defects such as monovacancies and divacancies. We used monoenergetic positron beams to study vacancies in ion-implanted and rare-earth-doped GaN. We identified the defect species and estimated their concentrations from a comparison between the Doppler broadening spectra obtained through the experiments and those calculated using first-principles calculation (projector augmented-wave method). We have thus shown that positron annihilation is a useful tool for studying relationships between vacancy-type defects and properties of group-III nitrides.

Probing the local dielectric/optical properties of group III-Nitrides by spatially resolved EELS on the nanometer scale

Usually, information on the dielectric-optical properties of semiconductors is obtained from synchrotron spectroscopic ellipsometry to cover the desired spectral range between 2 and 25 eV; but such measurements lack spatial resolution. In this work the local electronic structure of (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy. Using subnanometer electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself, typically in the range of nanometers. The low-loss region of the energy spectra reveals information on plasmon excitations and transitions across the band gap and the characteristic shape of the joint density of states. From these results the local dielectric/optical properties can be deduced via Kramers–Kronig transformation. The results obtained are in excellent agreement with theoretical studies and ellipsometry measurements.

EXAFS studies of plasma-enhanced MBE grown Group III-Nitrides

We have been studying the structural properties of Group III-Nitrides on Si substrates. We have shown from X-ray data that alloys of (AlGa)N and (InGa)N can be grown by plasma enhanced molecular beam epitaxy on oxidised (100) Si substrates with good control of the composition, over the entire range from InN to AlN. The composition of the alloys deduced from electron probe microanalysis data agree well with those from X-ray measurements assuming Vegard's law is valid for both alloy systems. SIMS studies show that the film composition is uniform in depth. EXAFS studies show no evidence for spinodal decomposition over the entire composition range for the (InGa)N alloys and indicate a monotonic variation in lattice parameter with increasing In mole fraction.