A review of our recent work on Raman scattering and a tribute to K. S. Krishnan

摘要

Raman scattering was discovered in India by Raman and Krishnan in 1928, After 70 years of the discovery, there is still an intense activity in this field world wide. Though a large body of experimental and theoretical work exists on the subject, there are many experimental situations where it is difficult to interpret the observed Raman spectra, One such example is the Raman scattering from in-homogeneously strained semiconductors, In modern integrated circuit chips and heterostructure devices, the stress varies on a length scale of a few nanometer, Typically, diameter of a focused laser beam is similar to l mu m and penetration depth can vary from a few microns to several millimeters, The laser beam samples a large volume of the strained structure. Each point in this volume scatters light with a Raman frequency characteristic of stress at that point, Because the beam is usually focused, the distribution of light intensity in this volume is very complex, The interpretation of observed Raman spectra is therefore very difficult, Recently we have developed a method to calculate theoretically the Raman spectra from such structures. We used Finite Element Method to calculate the stress and strain tensors, We solved the secular equation numerically to obtain Raman frequencies as a function of stress components. We constructed an expression for the distribution of intensity of light inside the semiconductor taking into account the depth at which the beam is focused. We calculated spectra from closely spaced points in the volume sampled by the laser light. We then superposed all these spectra to obtain the spectrum that will be observed in an experiment. Contrary to the usual belief that light penetrates only up to a depth = I/alpha (where a is the absorption coefficient), we found that the light scattered from much larger depths modifies the Raman spectrum. The calculated spectra agree with the observed spectra from quantum wires, from structures used in local oxidation of Si needed for fabricating IC chips and from other heterostructure devices. A brief review of this work is given in this paper. [References: 26]