Xu-Qiang Shen

Reduction of the threading dislocation density in GaN films grown on vicinal sapphire (0001) substrates

Structural properties of GaN films grown on vicinal sapphire (0001) substrates with various vicinal angles by plasma-assisted molecular beam epitaxy are investigated. High-resolution x-ray diffraction (HRXRD) results reveal the dramatic improvement of both tilting and twisting grain features of the GaN films when the vicinal angle is larger than 0.5° with the formation of multilayer macro-steps on the surface. The threading dislocation density reduces by over an order of magnitude estimated from the HRXRD results. Cross-sectional transmission electron microscopy observations clearly show that the formation and lateral propagation of macro-steps on the GaN surface play an important role in this dislocation reduction. A method for the reduction of threading dislocation density in GaN epilayers is proposed.

Impact of Vicinal Sapphire (0001) Substrates on the High-Quality AlN Films by Plasma-Assisted Molecular Beam Epitaxy

AlN films grown by plasma-assisted molecular beam epitaxy on vicinal sapphire (0001) substrates were investigated. High structural and optical qualities were confirmed by high-resolution X-ray diffraction and 77 K cathodoluminescence measurements. It was found that changing the vicinal angles of sapphire substrates can easily control the surface morphologies of AlN films. Spiral-growth features were greatly suppressed. Furthermore, well-ordered straight monatomic-layer steps and multi-atomic-layer macro-steps were clearly observed by atomic force microscopy. Surface diffusion and step incorporation kinetics during the growth are the key-factors in determining the surface morphologies.

Essential Change in Crystal Qualities of GaN Films by Controlling Lattice Polarity in Molecular Beam Epitaxy

GaN heteroepitaxial growth on sapphire (0001) substrates was carried out by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). A Ga-polarity growth was achieved by using an AlN high-temperature buffer layer. The epilayer polarity was characterized directly by coaxial impact collision ion scattering spectra (CAICISS). It was found that the properties of the GaN films showing Ga-face polarity, including their structural and electrical properties, were dramatically improved compared to those of films with N-face polarity. This important conclusion is considered to be a breakthrough in the realization of high-quality III-nitride films by MBE for device applications.

Characterization of Polarity of Wurtzite GaN Film Grown by Molecular Beam Epitaxy Using NH3

Effects of the initial nitridation of a sapphire(0001) substrate by NH3 on the polarity of GaN{0001} film have been investigated by coaxial impact collision ion scattering spectroscopy. The polarity of ammonia-molecular beam epitaxial (MBE) film grown on the substrate nitrided using NH3 is assigned as (0001). The effect of the initial nitridation of the substrate by NH3 is found to be contrary to that by nitrogen plasma, where the GaN film grown on the nitrided substrate shows the polarity of (0001). The polarity of GaN film grown by rf plasma-assisted MBE on the substrate which is nitrided using NH3 is also (0001). These findings suggest the possibility of polarity control of the grown GaN film by choosing the N source for initial nitridation of the substrate.

Studies of the annihilation mechanism of threading dislocation in AlN films grown on vicinal sapphire (0001) substrates using transmission electron microscopy

Annihilation phenomena of threading dislocations (TDs) in AlN films grown on vicinal sapphire (0001) substrates are investigated by transmission electron microscopy. It is found that the dislocation density is greatly reduced when the AlN is grown on a 2°-cut vicinal sapphire substrate. Two different types of the annihilation mechanisms of TDs were observed: (i) The coalescence of two TDs with opposite Burger’s vectors b to form a dislocation loop with one b, and (ii) the formation of one TD from the combination of two TDs with different Burger’s vectors, such as [1¯21¯0]+[21¯1¯0]→[112¯0]. These interactions between TDs are caused by the formation of inclined TDs (ITDs), where ITDs are locked and dragged by the macrosteps on the surface during the growth.

Stability of N- and Ga-polarity GaN surfaces during the growth interruption studied by reflection high-energy electron diffraction

GaN films with N- and Ga-polarity were grown on sapphire (0001) substrates using different buffer layers by plasma-assisted molecular-beam epitaxy. The surface stability of each lattice-polarity film during the growth interruption was studied by reflection high-energy electron diffraction (RHEED). It was found that the surface of N-polarity film was unstable against the exposure to the nitrogen plasma flux, while that of Ga-polarity one was stable. This provides a method to clarify the lattice polarity by the in situ RHEED observation directly. A model is proposed to explain the observed phenomenon, where the origin of the phenomenon is mainly attributed to the differences in surface dynamics processes and morphologies between the two kinds of lattice-polarity films.

Electrical properties of AlGaN∕GaN heterostructures grown on vicinal sapphire (0001) substrates by molecular beam epitaxy

Electrical properties of AlGaN∕GaN heterostructures grown on vicinal sapphire (0001) substrates by molecular beam epitaxy (MBE) are investigated. It is found that electrical properties of the two-dimensional electron gas (2DEG) in the heterostructures grown on 2.0°-off vicinal substrates are superior to those grown on the 0.5°-off vicinal substrates. Anisotropic phenomenon of the 2DEG mobility in the heterostructures grown on 2.0°-off substrates is demonstrated, which strongly relates to the macrostep structures on the surface. The 2DEG mobility as high as 2018cm2∕Vs is obtained at the room temperature from the authors’ all-MBE-grown sample measured in the direction parallel to the macrostep. It is suggested that the direction effect should be taken into account when designing the device structure.

Nanometric inversion domains in conventional molecular-beam-epitaxy GaN thin films observed by atomic-resolution high-voltage electron microscopy

GaN films grown on sapphire substrates by conventional molecular-beam epitaxy were investigated by means of atomic-resolution high-voltage electron microscopy (ARHVEM). The atomic positions of Ga and N could be directly discriminated by ARHVEM to determine the polarity in GaN. It was revealed that N polarity GaN films possessed a high density of nanometric inversion domains (IDs) with Ga polarity. The ID boundary was constructed by an inversion and a c/2 translation, and formed fourfold and eightfold coordination along the boundary.

Realization of compressively strained GaN films grown on Si(110) substrates by inserting a thin AlN/GaN superlattice interlayer

We investigate the strain properties of GaN films grown by plasma-assisted molecular beam epitaxy on Si(110) substrates. It is found that the strain of the GaN film can be converted from a tensile to a compressive state simply by inserting a thin AlN/GaN superlattice structure (SLs) within the GaN film. The GaN layers seperated by the SLs can have different strain states, which indicates that the SLs plays a key role in the strain modulation during the growth and the cooling down processes. Using this simple technique, we grow a crack-free GaN film exceeding 2-μm-thick. The realization of the compressively strained GaN film makes it possible to grow thick GaN films without crack generation on Si substrates for optic and electronic device applications.

Growth and characterizations of InGaN on N- and Ga-polarity GaN grown by plasma-assisted molecular-beam epitaxy

Abstract Plasma-assisted molecular-beam epitaxial (RF-MBE) growth of InGaN films on both N- and Ga-polarity GaN underlayers was carried out. Clear evidence was obtained for the difference in the film quality between InGaN grown on the N- and Ga-polarity GaN underlayers, to which the Ga-polarity GaN is preferred. Based on this point, high-quality InGaN films were obtained on the Ga-polarity GaN underlayers with an In composition up to 0.36. Intense photoluminescence (PL) emissions from the InGaN films were observed. Comparing the PL peak energy and the In composition determined by X-ray diffraction, it was found that the band-gap bowing parameter of InGaN films depends on the In composition.