Using Spectroscopic Ellipsometry to Probe the

1. Using Spectroscopic Ellipsometry to Probe the Phonon Characteristics of BexZn1-xTe Alloys Frank C. Peiris, Kenyon College, DMR 0521147 The incorporation of beryllium into traditional II-VI semiconductors such as ZnSe and ZnTe has allowed one to address some of the problems associated with these materials that culminate in reducing life times in devices based on them. It is believed that beryllium-incorporated II-VI films rectify some of these conventional problems because they form high degree covalent bonds, resulting in greater lattice hardening and lower degradation rates and as a consequence, realizing the dream of using these materials to fabricate blue lasers and light emitting diodes. A well known technique called ellipsometry determines optical and electronic properties of materials by measuring the difference in polarization between the incident and reflected beams. The other advantage is the fact that one can carve out the vibrational characteristics of a material by analyzing the data, providing information on the mechanical and the thermodynamic properties of the underlying material. In this work, we first obtained far-IR ellipsometric data for a series of BexZn1-xTe ternary films and subsequently found the dielectric function for these alloys, as shown below.

2. Using Spectroscopic Ellipsometry to Probe the Phonon Characteristics of BexZn1-xTe Alloys Frank C. Peiris, Kenyon College, DMR 0521147 In observing the previous figures, one recognizes that both spectra show hardly any visible features from around 600 nm to 15000 nm, conveying the fact that the material has no corresponding transitions in this region. The structure around 20000 nm is associated with vibrational (i.e., phonon) transitions. After further analysis, we recognize that this transition, which is associated with a particular vibrational mode, is related to a Be-like phonon mode as the features in both the real and the imaginary parts of the dielectric function increases as function of the Be concentration. Specifically, the increase in imaginary part of the dielectric function can be easily associated with an augmentation of an oscillator amplitude representing a more pronounced absorption. By modeling the dielectric function as a collection of oscillators, we were able to obtain the longitudinal (LO) and transverse (TO) optical phonons associated with this alloy system. As shown in the figure on the right, this modeling scheme allows us to determine the positions of the two phonon modes of this system, an important piece of information that helps to characterize the material. In addition, the model also enables us to obtain the oscillator strength, a parameter that can be associated with the transition probability. An undergraduate student was primaraly responsible for this project. In addition, several senior students used the instrument as part of their experimental physics course.