Analysis of Local Charges at Hetero-interfaces by Electron Holography - A Comparative Study of Different Techniques

Quantitative Determination of High-Frequency Voltage Attenuation in an Electric-Pulse-Excited Stroboscopic Transmission Electron Microscope

High-frequency electric pulse signals are often applied to stimulate functional materials in devices. To investigate the relationship between materials structure and dynamic behavior under high-frequency electric excitation, the stroboscopic imaging mode is widely used in a transmission electron microscope (TEM). From a technical point of view, it is crucial to quantitatively determine high-frequency attenuation in an electric-pulse-excited stroboscopic TEM. Here, we propose the quantitative method to evaluate the voltage attenuation by using magnification variation of defocused bright-field transmission electron microscopy images in a stroboscopic mode when applying high-frequency electric pulse signals onto a model system of two opposite tungsten tips. The negative voltage excitation possesses higher high-frequency voltage attenuation than the positive voltage excitation due to the possible nonreciprocal transmission of the triangle waves within the circuit between the biasing sample holder and the arbitrary waveform generator. Our approach of high-frequency attenuation determination provides the experimental foundation for quantitative analysis on the dynamic evolution of materials structure and functionality under electric pulse stimuli.

Feld-induced modulation of two-dimensional electron gas at LaAlO3/SrTiO3 interface by polar distortion of LaAlO3

Since the discovery of two-dimensional electron gas at the LaAlO3/SrTiO3 interface, its intriguing physical properties have garnered significant interests for device applications. Yet, understanding its response to electrical stimuli remains incomplete. Our in-situ transmission electron microscopy analysis of a LaAlO3/SrTiO3 two-dimensional electron gas device under electrical bias reveals key insights. Inline electron holography visualized the field-induced modulation of two-dimensional electron gas at the interface, while electron energy loss spectroscopy showed negligible electromigration of oxygen vacancies. Instead, atom-resolved imaging indicated that electric fields trigger polar distortion in the LaAlO3 layer, affecting two-dimensional electron gas modulation. This study refutes the previously hypothesized role of oxygen vacancies, underscoring the lattice flexibility of LaAlO3 and its varied polar distortions under electric fields as central to two-dimensional electron gas dynamics. These findings open pathways for advanced oxide nanoelectronics, exploiting the interplay of polar and nonpolar distortions in LaAlO3.

Impact of beam size and diffraction effects in the measurement of long-range electric fields in crystalline samples via 4DSTEM

Measuring long-range electric fields by 4-dimensional scanning transmission electron microscopy (4DSTEM) is on the verge to becoming an established method, though quantifying and understanding all underlying processes remains a challenge. To gain further insight into these processes, experimental studies employing the center-of-mass (COM) method of the model system of a GaAs p-n junction are carried out in which three ranges of the semi-convergence angle α are identified, with an intermediate one where measuring the built-in potential Vbi is not feasible. STEM multislice simulations including both atomic and nm-scale fields prove that this intermediate range begins once diffraction disks start overlapping with the undiffracted beam. The range ends when the diffraction disks' intensities become so low that they do not affect the measurement significantly anymore and when high-intensity diffractions overlap the center disk completely. From simulations without influence of atoms it is concluded that measuring Vbi has advantages over measuring the electric-field strength, as the potential difference does neither show a significant dependence on the beam size, nor on the specimen thickness.

Measuring Spatially-Resolved Potential Drops at Semiconductor Hetero-Interfaces Using 4D-STEM

Characterizing long-range electric fields and built-in potentials in functional materials at nano to micrometer scales is of supreme importance for optimizing devices, e.g., the functionality of semiconductor hetero-structures or battery materials is determined by the electric fields established at interfaces which can also vary spatially. In this study, momentum-resolved four-dimensional scanning transmission electron microscopy (4D-STEM) is proposed for the quantification of these potentials and the optimization steps required to reach quantitative agreement with simulations for the GaAs/AlAs hetero-junction model system are shown. Using STEM the differences in the mean inner potentials (∆MIP) of two materials forming an interface and resulting dynamic diffraction effects have to be considered. This study shows that the measurement quality is significantly improved by precession, energy filtering and a off-zone-axis alignment of the specimen. Complementary simulations yielding a ∆MIP of 1.3 V confirm that the potential drop due to charge transfer at the intrinsic interface is ≈0.1 V, in agreement with experimental and theoretical values found in literture. These results show the feasibility of accurately measuring built-in potentials across hetero-interfaces of real device structures and its promising application for more complex interfaces of other polycrystalline materials on the nanometer scale.

Electronic and Structural Transitions of LaAlO3 /SrTiO3 Heterostructure Driven by Polar Field-Assisted Oxygen Vacancy Formation at the Surface

The origin of 2D electron gas (2DEG) at LaAlO₃ /SrTiO₃ (LAO/STO) interfaces has remained highly controversial since its discovery. Various models are proposed, which include electronic reconstruction via surface-to-interface charge transfer and defect-mediated doping involving cation intermixing or oxygen vacancy (V_O ) formation. It is shown that the polar field-assisted V_O formation at the LAO/STO surface plays critical roles in the 2DEG formation and concurrent structural transition. Comprehensive scanning transmission electron microscopy analyses, in conjunction with density functional theory calculations, demonstrate that V_O forming at the LAO/STO surface above the critical thickness (t_c ) cancels the polar field by doping the interface with 2DEG. The antiferrodistortive (AFD) octahedral rotations in LAO, which are suppressed below the t_c , evolve with the formation of V_O above the t_c . The present study reveals that local symmetry breaking and shallow donor behavior of V_O induce the AFD rotations and relieve the electrical field by electron doping the oxide heterointerface.