Acoustoelectric properties of graphene under the influence of saw and external electric field

The effect of SAW on the electrical properties of the few-layer (2-3 layers) graphene is studied. Under the influence of SAW the appearence of acoustoelectric current IAEC in graphene is observed. The sign and magnitude of the induced IAEC in graphene conditioned by magnitude and direction of the electromagnetic fields induced by SAW and an external electric field. When the direction of SAW and Vbias is the same IAEC amplified, when at the opposite direction - IAEC are reduced in result of the interaction of these fields with each other. As a result of the measurement of the induced acoustoelectric (IAEC) current in graphene under the influence of a surface acoustic wave (SAW) the fluctuation nature of acoustoelectric current in the area of electrical neutrality is established at low voltages of an external bias (Vbias) applied on a graphene. The fluctuation character of the IAEC is manifested in all cases of measurements depending on the action of SAW and Vbias near the point of electrical neutrality. Chaotic fluctuation potential of graphene in the area of electrical neutrality is enhanced by the action of SAW that allows observing it in real conditions of the experiment at room temperature in air. The magnitude of IAEC depends on the power of SAW, while there is a parabolic dependence of the induced IAEC on the amplification current of SAW power (ISAW). The parabolic dependence of IAEC on ISAW explained by specific relaxation of acoustic phonons of the piezocrystallical substrate, which is dominant in the process of electronphonon scattering in graphene and acoustoelectric current induction in it. For large magnitudes of Vbias strict linear dependence of the IAEC on Vbias is observed. Large Vbias effectively suppress the appearance of the fluctuation potential of electrons and holes. The ability to control the magnitude and direction of IAEC induced in graphene by SAW is of practical importance

Correlation of electrical properties and acoustic loss in single crystalline lithium niobate-tantalate solid solutions at elevated temperatures

Electrical conductivity and acoustic loss Q−1 of single crystalline Li(Nb,Ta)O3 solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta ratios are investigated. The obtained results are compared to those previously reported for congruent LiNbO3. The temperature dependent electrical conductivity of LNT and LiNbO3 show similar behavior in air at high temperatures from 400 to 700 °C. Therefore, it is concluded that the dominant transport mechanism in LNT is the same as in LN, which is the Li transport via Li vacancies. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, it is shown that LNT bulk acoustic resonators exhibit significantly lower loss, comparing to that of LiNbO3

SEM imaging of acoustically stimulated charge transport in solids

Applications of surface acoustic waves (SAWs) are of great interest for solar energy for the acoustically stimulated transport of charge carriers generated in semiconductors and dielectrics under the influence of light.1–4 A prospective application of SAWs in solar cells could provide a 90% increase in the cell efficiency. SAWs propagating in piezoelectric crystals (piezoelectric semiconductor GaN and GaAs crystals included) have opposite potential values in the SAW minima and maxima due to the piezoelectric effect. Electrons and holes generated by light in a semiconductor or in the subsurface layer of a piezoelectric crystal are correspondingly distributed between SAW minima and maxima. The charges are then transported by SAWs to the solar cell exit at the acoustic wave velocity. Taking advantage of the SAW presence in solar cells, the area of charge “harvest” from the surface of a semiconductor structure or a piezoelectric crystal can be increased, and correspondingly, the solar cell efficiency can be increased too

Coefficients of Thermal Expansion in La₃Ga₅SiO₁₄ and Ca₃TaGa₃Si₂O₁₄ Crystals

The ordered Ca3TaGa3Si2O14 and disordered La3Ga5SiO14 crystals of the lantangallium silicate family were grown via the Czochralski method. The independent coefficients of thermal expansion of crystals αc and αa were determined using X-ray powder diffraction based on the analysis of X-ray diffraction spectra measured in the temperature range of 25~1000 °C. It is shown that, in the temperature range of 25~800 °C, the thermal expansion coefficients are linear. At temperatures above 800 °C, there is a nonlinear character of the thermal expansion coefficients, associated with a decrease in the Ga content in the crystal lattice

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

INTERACTION OF SURFACE ACOUSTIC WAVES WITH ELECTRON AND X-RAY BEAMS

The work covers the surface acoustic waves (SAW) propagating on the surface of the piezo- and ferroelectric crystals. The mechanism of influencing surface charges on the emission of the secondary electrons and on the scattering of the electrons during electron-ray exposure of the oncoming-pin converter structures has been determined. The mechanism of forming potential contrast of the SAW picture in the scanning electron microscope has been determined. The possibilities of excitating SAW with the domen structure, focussing SAW with the bended domen structures, refracting SAW from domen walls, using domen structures as the acoustic waveguides have been determined. The basic regularities have been specified, and the theories of the X-ray radiation diffraction on the ultrasonic super-lattices in the conditions of the total external refraction and in the conditions of the Bragg diffraction in case of the multi--layer interferention X-ray mirrors modulated by SAW have been created. The time modulation of the X-ray radiation proposed for creation of the single ultrashort pulses of X-ray radiation used in the X-ray spectroscopy with high time resolution is used on the synchrotronic X-ray radiation sources.Available from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio

X-ray Diffuse Scattering from Ca3NbGa3Si2O14 Single Crystal under External Electric Field Application

X-ray diffuse scattering from the Ca3NbGa3Si2O14 (CNGS) crystal was measured with a triple axis X-ray diffractometer under the conditions of an external electric field. It is found that the nature of the intensity distribution of the asymmetrical part of diffuse scattering depends on the value of the applied electric field. This phenomenon is apparently associated with different piezoelectric characteristics of defect regions and the rest of the single crystal

Changes in the Raman Spectrum of Monolayer Graphene under Compression/Stretching Strain in Graphene/Piezoelectric Crystal Structures

Results from studying the effect of an applied electric voltage on the Raman spectrum of graphene deposited on a lithium niobate crystal substrate with a ferroelectric domain structure are presented. The use of the principal component method for data processing in combination with correlation analysis made it possible to reveal the contribution to the change in the spectra associated with the linear deformation of the substrate due to the inverse piezoelectric effect. An effect of the graphene coating peeling was found. Furthermore, bending deformations of the graphene coating associated with the presence of a relief on the substrate were found. An analysis of the change in the spectra of graphene under the application of an electric voltage made it possible to determine the height of this relief