Developing spectroscopic ellipsometry to study II-VI and diluted magnetic semiconductors

Abstract

We have constructed a rotating analyzer spectroscopic ellipsometer (RAE) to study effects of magnetic and nonmagnetic doping on the £1 and £1 + .11 band gap energies in ZnSe-based II-VI semiconductors. To remove the natural surface oxide overlayer which distorts the intrinsic dielectric response of the sample, a chemical etching technique using dilute NI40H solution was developed. The successful removal of the oxide overlayer on ZnSe was confirmed via the XPS technique. For diluted magnetic semiconductors (DMS), we found that the £1 and £1 + .11 band gap energies increase with x for Zn1_xFexSe and Zn1_xCoxSe, and decrease with x for Zn1_xMnxSe. A sp-d direct exchange interaction model which explained the r-point band gap energy of Zn1_xMnxSe was applied. The calculated band gap energies at the Lpoint are only consistent with Zn1_xMnxSe data. We showed that an sp-d hybridization model, which includes the location of the energy levels of the magnetic impurity d-levels can account for the concentration dependence of £1 and £1 + .11 band gap energies of all three materials. For ZnxCd1_xSe systems, all spectral features of CdSe were identified as E0 , E0 + .10 , £1, £1 + .11, E2, and E~ threshold energies from band structure calculations using a nonlocal empirical pseudopotential method. Many-body effect has to be included in the calculation of the dielectric function of CdSe to obtain good agreement with the measured spectrum. Concentration dependent spin-orbit splitting band gap .11 (x) is well explained by the statistical fluctuation of the alloy composition.