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Virtual GaN substrates on Si wafers


The atomic structure of the interface between GaN and Sc2O3 was addressed by ab-initio calculations.  In order to keep the interface electrically neutral oxygen vacancies in the oxide layer are filled with nitrogen. It was found that at the GaN/ Sc2O3 interface N-Ga-O-Sc bonding configuration exists. This stacking sequence results in consequence in N-polar GaN films.

Growth characteristics of Sc2O3/Y2O3 buffer

The morphology and structure studies indicate that the Sc2O3/Y2O3 buffers on Si(111) are atomically flat, closed and single crystalline with (111) vertical orientation. The oxides are of type-B stacking with respect to the Si(111) substrate and fully relaxed for Sc2O3 and Y2O3 layer thicknesses as low as a few nanometers. Plastic relaxation of the oxide buffer was found by TEM to occur via formation of misfit dislocations at the Sc2O3/Y2O3 and Y2O3/Si interfaces (Fig. 8 (a)). A strain free Sc2O3 is highly desirable as it reduces the lattice mismatch to GaN to the smallest possible value. Furthermore, diffuse X-ray scattering studies using synchrotron radiation source proved that the density of the extended structural defects (e.g. stacking faults) on the preferred {111} glide planes orientation in the Sc2O3 layer is below the detection limit. Additionally, thermal stability tests show that the Sc2O3 and Y2O3 are stable at least up to 900°C which is sufficient for GaN growth by MBE. Our studies demonstrate that the novel engineered Sc2O3/Y2O3 buffer approach on Si(111) provides a template of high structural quality for GaN overgrowth.


GaN growth mechanism on Sc2O3/Y2O3 buffer

XPS analysis complemented by ab-initio calculations show that the N-Ga-O-Sc interface bonding configuration is energetically favored at the GaN/Sc2O3 interface. This stacking sequence results in N-polar GaN films. Additionally, based on RHEED investigations it was found that such N-Ga-O-Sc wetting "layer" is tensile strained due to the higher lattice constant of Sc2O3 underneath.

The initial growth stage of GaN on Sc2O3 proceeds by the nucleation of 3D islands which are relaxed and relatively large before coalescence. The relaxation occurs by the formation of misfit dislocations at the GaN/Sc2O3 interface. These islands are single crystalline and crystallize in the hexagonal phase. The epitaxial relationship along the growth direction is (Fig. 8 (b)):


and the in-plane alignment of the heterostructure is characterized by:


As the growth of GaN proceeds, the GaN nucleation islands become bigger and form blocks with a smooth surface. The Williamson-Hall analysis reveal that these GaN blocks are tilted and twisted which causes a high content of threading screw and edge dislocations in the coalesced GaN films, respectively. The coalescence process of misaligned GaN islands is known to be decisive for the quality of micrometer-thick GaN layers.


Fig. 8: (a) TEM cross-section of the GaN/Sc2O3/Y2O3/Si(111) heterostructure viewed along <-110> azimuth (b) specular theta-2 theta scan of the system

Properties of micrometer-thick GaN on Sc2O3/Y2O3 buffer

SEM and TEM results reveal that micrometer-thick GaN layers deposited under Ga-rich conditions are composed of 200 nm-wide, coalesced blocks. Threading screw and edge dislocation densities estimated based on the XRD omega scans are both in the range of 1x1010 cm-2. Despite the large amount of threading dislocations, 10K PL spectra show relatively sharp donor bound exciton transitions and low intensity yellow luminescence emission, which indicate that the GaN layers grown on Sc2O3/Y2O3/Si(111) templates are promising for future optoelectronic applications.

The building and the infrastructure of the IHP were funded by the European Regional Development Fund of the European Union, funds of the Federal Government and also funds of the Federal State of Brandenburg.