Angle resolved photoemission study of the alloy scattering effect in Hg_1−xCd_xTe

Angle resolved photoemission spectroscopy (ARPES), a technique which provides information on the k‐resolved density of states, has been used to study the alloy disorder effect in Hg_1−xCd_xTe alloys in order to make comparison with theory. Using off‐normal emission with photon energies from 15 to 30 eV and assuming free‐electron final states, the band dispersions along the [111] and [100] directions were determined for CdTe and HgTe as well as for Hg_0.7Cd_0.3Te. Within the framework of the coherent potential approximation (CPA), the alloy disorder effect is characterized by a self‐energy term which is complex and energy dependent. The real and imaginary parts of the self‐energy term characterize the shifted energy and the damping of a particular virtual crystal eigenstate ‖n,k〉. Because of the k‐resolving power of ARPES, the alloy scattering effect can be studied by comparing the spectra of the alloy Hg_1−xCd_xTe, and the binaries CdTe and HgTe. The deviation of the peak position from the virtual‐crystal approximation (VCA) value and the extra broadening of the line shape reflect the energy shift and the damping of the VCA Bloch states due to the alloy disorder. Our studies show that for the lower valence‐band states (binding energy from 2 to 6 eV) the alloy disorder effect agrees with the CPA calculation using the empirical tight‐binding method by Hass et al. However, the uppermost bands show anomalous behavior: the binding energies of the upper bands of Hg_0.7Cd_0.3Te lie outside the CdTe and HgTe limits, indicating that there are effects not included in the original theoretical approach. A possible explanation in terms of anion disorder due to different Te renormalization, depending on whether a Te is bonded to Cd or Hg atoms, is discussed.