Modelling dynamical 3D electron diffraction intensities. II. The role of inelastic scattering

The strong interaction of high‐energy electrons with a crystal results in both dynamical elastic scattering and inelastic events, particularly phonon and plasmon excitation, which have relatively large cross sections. For accurate crystal structure refinement it is therefore important to uncover the impact of inelastic scattering on the Bragg beam intensities. Here a combined Bloch wave–Monte Carlo method is used to simulate phonon and plasmon scattering in crystals. The simulated thermal and plasmon diffuse scattering are consistent with experimental results. The simulations also confirm the empirical observation of a weaker unscattered beam intensity with increasing energy loss in the low‐loss regime, while the Bragg‐diffracted beam intensities do not change significantly. The beam intensities include the diffuse scattered background and have been normalized to adjust for the inelastic scattering cross section. It is speculated that the random azimuthal scattering angle during inelastic events transfers part of the unscattered beam intensity to the inner Bragg reflections. Inelastic scattering should not significantly influence crystal structure refinement, provided there are no artefacts from any background subtraction, since the relative intensity of the diffracted beams (which includes the diffuse scattering) remains approximately constant in the low energy loss regime

Modelling dynamical 3D electron diffraction intensities. I. A scattering cluster algorithm

Three‐dimensional electron diffraction (3D‐ED) is a powerful technique for crystallographic characterization of nanometre‐sized crystals that are too small for X‐ray diffraction. For accurate crystal structure refinement, however, it is important that the Bragg diffracted intensities are treated dynamically. Bloch wave simulations are often used in 3D‐ED, but can be computationally expensive for large unit cell crystals due to the large number of diffracted beams. Proposed here is an alternative method, the `scattering cluster algorithm' (SCA), that replaces the eigen‐decomposition operation in Bloch waves with a simpler matrix multiplication. The underlying principle of SCA is that the intensity of a given Bragg reflection is largely determined by intensity transfer (i.e. `scattering') from a cluster of neighbouring diffracted beams. However, the penalty for using matrix multiplication is that the sample must be divided into a series of thin slices and the diffracted beams calculated iteratively, similar to the multislice approach. Therefore, SCA is more suitable for thin specimens. The accuracy and speed of SCA are demonstrated on tri‐isopropyl silane (TIPS) pentacene and rubrene, two exemplar organic materials with large unit cells

Towards Electron Energy Loss Compton Spectra Free From Dynamical Diffraction Artifacts

The Compton signal in electron energy loss spectroscopy (EELS) is used to determine the projected electron momentum density of states for the solid. A frequent limitation however is the strong dynamical scattering of the incident electron beam within a crystalline specimen, i.e. Bragg diffracted beams can be additional sources of Compton scattering that distort the measured profile from its true shape. The Compton profile is simulated via a multislice method that models dynamical scattering both before and after the Compton energy loss event. Simulations indicate the importance of both the specimen illumination condition and EELS detection geometry. Based on this, a strategy to minimize diffraction artifacts is proposed and verified experimentally. Furthermore, an inversion algorithm to extract the projected momentum density of states from a Compton measurement performed under strong diffraction conditions is demonstrated. The findings enable a new route to more accurate electron Compton data from crystalline specimens

Fully depleted emitter layers: a novel method to improve band alignment in thin-film solar cells

The interface between the emitter and absorber layers in a thin-film solar cell must satisfy two important criteria, namely a small lattice mismatch and electron barrier height. It is shown that the barrier height is lowered when the emitter is fully depleted of free electron carriers by making the layer thinner than its space charge region, thereby enhancing thermionic emission of the photocurrent across the interface. Lattice matching is therefore the only requirement for a fully depleted emitter. The concept is applied to a lattice matched ZnS-Cu2ZnSnS4 (CZTS) interface which has a large intrinsic barrier height. Recent experimental evidence however suggest that ZnS becomes current unblocking when sufficiently thin. The theoretical efficiency for fully depleted ZnS is as high as 16.1%, due to the combination of a large open circuit voltage (1.0 V) from lattice matching and reasonable short circuit current density (24 mA/cm2). Lattice matched GaP and AlP are also potential CZTS emitter layers in the fully depleted configuration. The possibilities for exploring new materials combinations are therefore greater with fully depleted emitters. Furthermore, the concept can in principle be applied to any thin-film solar cell, making it highly versatile