D. V. Irzhak

X-ray Bragg diffraction from langasite crystal modulated by surface acoustic wave

X-ray diffraction on the X cut of a langasite crystal (La3Ga5SiO14) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) has been studied at the ESRF synchrotron radiation source. Due to the sinusoidal modulation of the crystal lattice involved by the SAW diffraction satellites appear on the rocking curve, with their number, angular positions, and intensities depending on the amplitude and wavelength of the ultrasonic superlattice. Full extinction of a specific satellite could be performed by adjusting the acoustic amplitude. It is shown that x-ray diffraction can be used to study surface acoustic wave field distributions in crystals.

X-ray topography analysis of acoustic wave fields in the SAW-resonator structures

The formation of fields of standing surface acoustic waves (SAW) in LiNbO/sub 3/ and La/sub 3/Ga/sub 5/SiO/sub 14/ (LGS) crystals was studied by high-resolution topography method on a laboratory X-ray source. The fields of standing SAW were formed using SAW-resonator structures consisting of interdigital transducer (IDT) and reflecting gratings. The SAW amplitudes and power flow angles were measured by X-ray topography, diffraction in acoustic beam was visualized, and the SAW interaction with the crystal structure defects was studied.

Dynamical theory for calculations of X-ray spectra from crystals modulated by surface acoustic waves

The dynamical theory was used to model X-ray diffraction by the YZ-cut of an LiNbO3 crystal modulated by surface acoustic waves (SAWs). It includes both the calculation of the crystal lattice deformation, induced by a surface wave of Rayleigh type, and the simulation of X-ray wavefields in the crystal based on Takagi–Taupin equations. A detailed discussion is included on the effect of peak splitting occurring at high acoustic amplitudes. The developed theory entirely describes this effect and proves to be a powerful tool for understanding X-ray diffractometry and topography measurements by SAW-modulated crystals.

Diffraction of a focused x-ray beam from La3Ga5SiO14 crystal modulated by surface acoustic waves

X-ray Bragg diffraction on an acoustically modulated La3Ga5SiO14 crystal (langasite, LGS) was studied in the saggital geometry where a surface acoustic wave (SAW) serves as diffraction grating of normal incidence. A set of parabolic compound Be refracting lenses was used to focus x-ray radiation in order to obtain good angular and spatial resolution of diffraction satellites. X-ray diffraction spectra analysis provided information on the acoustic properties of the LGS crystal and enabled the determination of SAW amplitudes and wavelengths.

Surface acoustic wave propagation in graphene film

Surface acoustic wave (SAW) propagation in a graphene film on the surface of piezoelectric crystals was studied at the BESSY II synchrotron radiation source. Talbot effect enabled the visualization of the SAW propagation on the crystal surface with the graphene film in a real time mode, and high-resolution x-ray diffraction permitted the determination of the SAW amplitude in the graphene/piezoelectric crystal system. The influence of the SAW on the electrical properties of the graphene film was examined. It was shown that the changing of the SAW amplitude enables controlling the magnitude and direction of current in graphene film on the surface of piezoelectric crystals.

Domain matching epitaxy of GaN films on a novel langasite substrate: an in-plane epitaxial relationship analysis

We report the epitaxial growth of c-plane GaN films on a novel langasite (La3Ga5SiO14, LGS) substrate by plasma-assisted molecular beam epitaxy. The in-plane epitaxial relationship and the structural properties of GaN films on an LGS substrate were investigated using in situ reflective high energy electron diffraction (RHEED), high resolution X-ray diffraction (HR-XRD) and Raman spectroscopy. The in-plane epitaxial relationship between GaN and LGS determined using RHEED pattern was found to be GaN[100]//LGS[210] and GaN[110]//LGS[140]. HR-XRD results confirmed the exact epitaxial relationship, and showed that six reflection peaks of GaN(102) were shifted around 19° from those of LGS(102). Raman analysis revealed that a minute compressive strain still existed in the GaN film due to the very small lattice mismatch between GaN and LGS. The results obtained in this study demonstrate that the nearly lattice-matched LGS can be a promising and futuristic substrate material for the growth of GaN, and it is foreseen that our results could be a reference for the further development of high performance nitride-based devices.

Formation of regular domain structures and peculiarities of switching of the spontaneous polarization in lithium tantalate crystals during discrete electron irradiation

The processes of the formation of domain structures in lithium tantalate crystals (≈300 μm thick) under electron irradiation in a scanning electron microscope were studied. Successive discrete quasi-point irradiations with an electron beam over the —Z-cut surface were used. This method of drawing by an electron beam made it possible to control the implanted charge and to examine the influence of the distances between conditionally point charges on the domain structure formed during switching. The effect of change in the direction of the beam displacement under irradiation on the domain-line width was investigated. The domain lattice with a period of 12 μm was formed on the —z face of the crystal, which transformed into a 2D-type structure deep in the crystal.

Piezoelectric strain coefficients in La3Ga5.3Ta0.5Al0.2O14 and Ca3TaGa3Si2O14 crystals

Independent piezoelectric strain coefficients d11 and d14 in disordered La3Ga5.3Ta0.5Al0.2O14 (LGTA) and ordered Ca3TaGa3Si2O14 (CTGS) crystals of the langasite family were measured by high-resolution X-ray diffraction (HRXRD) under external electric field application which causes changes in the interplanar spacing because of the reverse piezoelectric effect. The experiment showed that the piezoelectric strain coefficients can be precisely determined by measuring changes in the interplanar spacing using the optical scheme of a triple-axis X-ray diffractometer. The measured independent piezoelectric strain coefficients d11 and d14 for LGTA and CTGS crystals are d11(LGTA) = 6.455 × 10−12 C/N, d14(LGTA) = −5.117 × 10−12 C/N; d11(CTGS) = 3.330 × 10−12 C/N, d14(CTGS) = −15.835 × 10−12 C/N.

XBIC Investigation of the Grain Boundaries in Multicrystalline Si on the Laboratory X-Ray Source

It is shown that the X-ray beam induced current method (XBIC) can be realized at the laboratory X-ray source using the polycapillary x-ray optics. The images of iron contaminated grain boundaries in multicrystalline Si are obtained. It is shown that the grain boundary XBIC contrast is 2-3 times smaller than the EBIC one. A simulation of XBIC and EBIC contrast values for two-dimensional defects is carried out and a good correlation between the experimental and calculated values is obtained. The dependence of grain boundary XBIC contrast on the X-ray beam width is calculated.

X-ray beam induced current method at the laboratory x-ray source

The x-ray beam induced current method (XBIC) is realized on the laboratory x-ray source using the polycapillary x-ray optics. It is shown that rather good images of grain boundaries in Si can be obtained by this method. The parameters of x-ray beam are estimated by the simulation of Schottky diode image. A good correlation between the experimental and calculated grain boundary XBIC contrast is obtained. The possibilities of laboratory source based XBIC method are estimated.