Grazing Incidence Angle X-ray Reflectivity Research Articles

Achieve 2-inch-diameter homogeneous GaN films on sapphire substrates by pulsed laser deposition

The 2-inch-diameter homogeneous GaN films have been epitaxially grown on sapphire substrates by pulsed laser deposition (PLD) technique with optimized laser rastering and PLD growth conditions. The as-grown GaN films are characterized by in situ reflection high-energy electron diffraction, white-light interferometry, scanning electron microscopy, atomic force microscopy (AFM), grazing incidence angle X-ray reflectivity, reciprocal space mappings, and micro-Raman spectroscopy for surface morphologies and structural properties. The as-grown 2-inch-diameter single-crystalline GaN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity less than 3.4 % and very smooth surface with a RMS roughness less than 1.3 nm measured by AFM. There is a maximum of 1.2 nm thick interfacial layer existing between the as-grown GaN films and sapphire substrates, and the as-grown 310 nm thick GaN films are almost fully relaxed only with an in-plane compressive strain of 0.044 %. This work demonstrates a possibility for achieving high-quality large-scale GaN films with uniform thickness and atomically abrupt interface by PLD, and is of great interest for the commercial development of GaN-based optoelectronic devices.

Interface roughness of double buffer layer of GaN film grown on Si(1 1 1) substrate using GIXR analysis

A double buffer layer (DBL), interface reaction epitaxy (IRE) AlN/β-Si3N4/Si, grown by an IRE of β-Si3N4 and AlN films on Si, was fabricated to improve the crystalline quality of successively grown 30nm GaN on a 30nm AlN buffer layer using plasma-assisted molecular beam epitaxy (PA-MBE). The DBL was first prepared by surface nitridation of Si and successively prepared by IRE between the deposited Al and N atoms in β-Si3N4. Both the AlN buffer layer on the DBL and GaN film on the AlN buffer layer were grown by activity-modulation migration enhanced epitaxy (AM-MEE). Hetero epitaxial grown films of GaN(30nm)/AlN buffer(30nm)/DBL/Si(111) were prepared for analysis using a three layer model of grazing incidence-angle X-ray reflectivity (GIXR), which consisted of three layers of GaN, AlN buffer and Si and of the three interfaces of the GaN surface, GaN/AlN buffer and AlN buffer/DBL/Si. The nitridation temperature dependence of the interface roughness of the DBL was measured to be 0.5 and 0.6nm, for nitridation temperatures of 780 and 830°C, respectively. The full width at half maximum (FWHM) of rocking curve GaN(0002) measured by X-ray diffraction (XRD) for nitridation temperatures of 780 and 830°C were 58.2 and 55.2arcmin, respectively.

Epitaxial growth of nonpolar AlN films on ZnO substrates using room temperature grown GaN buffer layers

The authors have grown nonpolar AlN layers on m-plane ZnO substrates using pulsed laser deposition and investigated their structural properties. The direct growth of AlN on ZnO substrates at 750°C results in the formation of polycrystalline materials due to significant interfacial reactions between AlN and ZnO. On the other hand, m-plane AlN was grown epitaxially on the ZnO substrates by using a GaN buffer layer prepared at room temperature (RT). The full width at half maximum value for AlN 11¯00 x-ray rocking curve was determined to be 468arcsec. Grazing incidence angle x-ray reflectivity measurements revealed that the heterointerface between AlN and RT GaN is quite abrupt. X-ray diffraction measurements revealed that the in-plane epitaxial relationship is ⟨0001⟩AlN‖⟨0001⟩GaN‖⟨0001⟩ZnO. These results indicate that the use of the RT GaN buffer layer makes it possible to take full advantage of small lattice mismatches and the wurtzite structure of the ZnO substrates.

Epitaxial growth of (1 1 1)ZrN thin films on (1 1 1)Si substrate by reactive sputtering and their surface morphologies

We have investigated the epitaxial growth of (1 1 1)ZrN films on (1 1 1)Si substrate at a relatively low temperature using an ultrahigh vacuum DC magnetron sputtering system. The film quality obtained and the epitaxial relationship were evaluated on the basis of analyses of X-ray diffraction (XRD) patterns, X-ray pole figures, and grazing incidence angle X-ray reflectivity (GIXR), transmission electron microscopy (TEM) and atomic force microscopy (AFM) results. It was found that (1 1 1)ZrN films were grown epitaxially on (1 1 1)Si at a substrate temperature of 500 ∘ C with the directional relationship ZrN(1 1 1)[1 1 0] ∥ Si(1 1 1)[1 1 0]. GIXR and AFM results also revealed that the epitaxial (1 1 1)ZrN film has a high film density comparable to that of bulk ZrN and a fairly flat surface morphology with an average surface roughness of approximately 0.25 nm.

Room-temperature epitaxial growth of AlN on atomically flat MgAl2O4 substrates

The authors have grown AlN films on atomically flat MgAl2O4 (111) substrates at room temperature (RT) by pulsed-laser deposition. The in situ reflection high-energy electron diffraction image exhibits a streaky pattern, and the root-mean-square value of the surface roughness as measured by atomic force microscopy is as low as 0.45nm, which indicates that single-crystalline AlN grows epitaxially on MgAl2O4 with a smooth surface, even at RT. Electron backscattered diffraction and x-ray diffraction measurements reveal that AlN {11−20} shows a clear sixfold symmetry without 30° rotational domains, and the epitaxial relationship is AlN [11−20]‖MgAl2O4 [0−11]. Grazing incidence angle x-ray reflectivity characterization indicates that the interfacial reaction between AlN and MgAl2O4 is completely suppressed in the case of RT growth and the heterointerface for the RT-AlN∕MgAl2O4 structure is abrupt and thermally stable.

Epitaxial growth of AlN on single-crystal Ni(111) substrates

We grew AlN thin films on Ni(111) at room temperature using pulsed laser deposition. In situ reflection high-energy electron diffraction observations showed sharp streak patterns, revealing that the AlN films grew epitaxially and were of high quality, even when grown at room temperature. Electron backscatter diffraction measurements showed that the in-plane epitaxial relationship between the film and Ni was AlN[11-20]∥Ni[01-1] with no 30° rotational domains. Grazing incidence angle x-ray reflectivity measurements determined that the interfacial layer was less than 1.2nm thick, if present. This indicates that the heterointerface between the AlN film and Ni substrate is atomically abrupt and that the AlN films serve as an excellent barrier layer for Ni atoms. Furthermore, the refractive index of a 60-nm-thick AlN film was 2.15 at 3.1eV and 2.08 at 1.6eV. These results demonstrate that the AlN films grown on Ni substrates at room temperature are of high quality from the perspective of their structural and optical properties.

Epitaxial Growth of (001)ZrN Thin Films on (001)Si by Low Temperature Process

We have prepared (001)ZrN films epitaxially grown on (001)Si substrates by a low temperature process using an ultrahigh vacuum magnetron sputtering system, and evaluated their epitaxial relationships and film qualities, using X-ray diffraction (XRD) analysis, X-ray pole figure, grazing incidence angle X-ray reflectivity (GIXR), transmission electron microscopy (TEM) and atomic force microscopy (AFM). It is clarified that (001)ZrN films grow epitaxially on (001)Si even at room temperature with the directional relationship of ZrN(001)[110]∥Si(001)[110], and the lattice mismatch at the interface between (001)ZrN and (001)Si is relaxed due to the existence of a very thin transition layer. It is also revealed from GIXR and AFM that the epitaxial ZrN films have a high film density and a flat surface morphology, irrespective of substrate temperature.

Room-temperature epitaxial growth of AlN films

We have grown AlN films on nearly-lattice-matched (Mn,Zn)Fe2O4 (111) substrates by pulsed-laser deposition at room temperature and investigated their structural properties using reflection high-energy electron diffraction (RHEED), grazing incidence angle x-ray reflectivity (GIXR), grazing incidence angle x-ray diffraction (GIXD), and atomic force microscope. We have found that AlN grows epitaxially even at room temperature. In situ RHEED observations have shown that the room-temperature growth of AlN starts with the two-dimensional mode followed by a transition to the three-dimensional mode at the film thickness of 2 nm. GIXR and GIXD measurements have revealed that the heterointerface between AlN and (Mn,Zn)Fe2O4 is abrupt and approximately 90% of the lattice mismatch is released at the interface due to the introduction of crystalline defects such as misfit dislocations. These results indicate that the present technique solves one of the two major problems with heteroepitaxial growths of group III nitrides (mismatch in the thermal expansion coefficients) and alleviates the other problem (mismatch in the lattice constants).

Growth of GaN on nearly lattice matched MnO substrates by pulsed laser deposition

We have grown GaN on nearly lattice matched MnO(1 1 1) substrates by pulsed laser deposition (PLD) and characterized their structural properties using reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), and grazing incidence-angle X-ray reflectivity (GIXR). RHEED observation has revealed that the single crystal GaN(0 0 0 1) grows on MnO(1 1 1) and its in-plane epitaxial relationship is GaN [ 1 1 2 ̄ 0 ]//MnO [ 1 1 ̄ 0 ]. The lattice mismatch between GaN and MnO for this alignment is calculated to be 1.6%. From GIXR measurements, the roughness of the hetero-interface between GaN and the MnO substrate was estimated to be 2.3 nm.

Characterization of hetero-interfaces between group III nitrides formed by PLD and various substrates

We have grown group III nitride epitaxial films on various substrates by pulsed laser deposition and investigated structural properties of the surfaces and the hetero-interfaces using grazing incidence angle X-ray reflectivity (GIXR) and atomic force microscopy (AFM). It has been found that hetero-interfaces between PLD AlN and conventional substrates such as Al 2O 3 and Si are quite abrupt (about 0.5 nm) probably due to a less reactive growth ambience. However, we observed a thin interfacial layer (less than 10 nm) at the hetero-interface between AlN and (Mn,Zn)Fe 2O 4. The surface roughness of AlN is mainly determined by the extent of the lattice mismatch. It has been also found that the roughness at the hetero-interface between GaN and AlN formed by PLD coincides with the surface roughness of the AlN layer.