Dynamics of defects in semiconductors

Hyperfine interaction techniques involving radioactive probe atoms like the perturbed angular correlation technique (PAC) and the Mössbauer effect have, due to their inherent sensitivity, successfully been applied to the study of defects in semiconductors. By probing the characteristic charge distribution around the probe atom interacting with a defect, they contributed to the microscopic understanding of the nature, structure and stability of complexes formed between radioactive dopant atoms and defects present in elemental and compound semiconductors. Moreover, dynamic effects can be studied by hyperfine interaction probe techniques. In this case, dynamics always means the fluctuation of a charge distribution resulting in a time dependent hyperfine interaction within the time scale defined by the lifetime of the isomeric nuclear state used for the measurement. Such fluctuations can either be caused by structural changes like local rearrangements of a defect complex or by electronic transitions in the semiconductor resulting in a change of the charge state of a defect complex. Examples using PAC to monitor such processes will be discussed for the semiconductor silicon.     

Non-intrusive mapping of subsurface defects in semiconductors

We have adapted differential interference contrast Nomarski microscopy in the transmission configuration to the problem of mapping subsurface defects in semiconductors. We have demonstrated the ability to rapidly measure the depth of the precipitate-free-zone in silicon with a reproducibility of ±1 m in whole Si wafers up to 200 nm in diameter, having an extrinsic doping concentration up to 7×10¹⁹ cm^–3 and a nominal, as received, back side roughness. Because our subsurface defect profiler is completely non-destructive, product wafers can be inspected at various stages of processing and immediately returned to the production line.     

Laser-induced defect formation in semiconductors

The electron-deformation-thermal theory of pulsed laser-induced point defect generation in strongly absorbing semiconductors is developed. The theoretical results obtained are in a good agreement with the results of experiments carried out in Ge, GaAs, and GaP.     

In-situ formation of local inhomogeneities in semiconductors

The transport properties of ionic conductors such as α-AgI and Li_0.23La_0.69Fe_0.3Ti_0.7O₃ and of electronically conducting semiconductors such as α-Ag₂S and Ag_xWO₃ as a result of the application of voltages smaller than the decomposition voltage by two ionically blocking electrodes were investigated. The mobile ionic as well as the mobile electronic species are shifted from one electrode side to the other one. The change in the stoichiometric composition within the sample as a result of this transport process causes the formation of local inhomogeneities leading to changes in the electrical and optical properties. The experimental results also show that Nernst's law is not only valid for ionic conductors but also for semiconductors as well when a steady state is reached for ionically blocking electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Direct visualization of structural defects in 2D semiconductors

Direct visualization of the structural defects in two-dimensional (2D) semiconductors at a large scale plays a significant role in understanding their electrical/optical/magnetic properties, but is challenging. Although traditional atomic resolution imaging techniques, such as transmission electron microscopy and scanning tunneling microscopy, can directly image the structural defects, they provide only local-scale information and require complex setups. Here, we develop a simple, non-invasive wet etching method to directly visualize the structural defects in 2D semiconductors at a large scale, including both point defects and grain boundaries. Utilizing this method, we extract successfully the defects density in several different types of monolayer molybdenum disulfide samples, providing key insights into the device functions. Furthermore, the etching method we developed is anisotropic and tunable, opening up opportunities to obtain exotic edge states on demand.     

Electronic structures of impurities and point defects in semiconductors

A brief history of the impurity theories in semiconductors is provided. A bound exciton model is proposed for both donor-and acceptor-like impurities and point defects, which offers a unified understanding for "shallow" and "deep"impurities and point defects. The underlying physics of computational results using different density-functional theorybased approaches are discussed and interpreted in the framework of the bound exciton model.     

Properties of electrically active structural defects in crystalline semiconductors

The internal structures, composite elements, electrophysical properties, and characteristic parameters of impurity clouds, noise supertraps, disordered domains, and stacking defects are compared in crystalline semiconductors. It is shown that the mentioned macrodefects possess common properties: They have a nucleus and an impurity atmosphere, are separated by a potential barrier, are shaped with the participation of Frenkel defects and oxygen ions, have a threshold inclusion, sharply increase the rate of charge carrier generation-recombination as the electrical field grows, etc. It is noted that current channels in non-crystalline semiconductors are characterized by properties close to these after they have been formed. It is assumed that the electrical activity of the structural macrodefects in crystalline semiconductors may be related to the fundamental properties of noncrystalline semiconductors (for instance, the resistance switching effect). Domains adjoining a macrodefect and destroyed by it visibly manifest these properties.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 95–100, April, 1988.     

Photoinduced defect formation in chalcogenide vitreous semiconductors

Journal of Applied Spectroscopy -     

Imaging and spectroscopy of defects in semiconductors using aberration-corrected STEM

The distribution of single dopant or impurity atoms can dramatically alter the properties of semiconductor materials. The sensitivity to detect and localize such single atoms has been greatly improved by the development of aberration correctors for scanning transmission electron microscopes. Today, electron probes with diameters well below 1 Å are available thanks to the improved electron optics. Simultaneous acquisition of image signals and electron energy-loss spectroscopy data provides means of characterization of defect structures in semiconductors with unprecedented detail. In addition to an improvement of the lateral spatial resolution, depth sensitivity is greatly enhanced because of the availability of larger probe forming angles. We report the characterization of an alternate gate dielectric interface structure. Isolated Hf atoms are directly imaged within a SiO₂ thin film formed between an HfO₂ layer and the silicon substrate. Electron energy-loss spectroscopy shows significant changes of the silicon valence state across the interface structure.     

Schottky defects in cubic lattice of SrTiO3

Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO₃ crystal are investigated by means of a quantum-chemical simulation based on the Hartree-Fock methodology. The occurrence of Sr partial Schottky defect (V_Sr+V_O) and two types of Ti partial Schottky defects (V_Ti+2V_O) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies.