Structural and physical properties of mercury–iron selenide layers and quantum wells

Epitaxial layers and single quantum wells (SQW’s) of Fermi-level pinned mercury–iron selenide (HgSe:Fe) have been grown by molecular-beam epitaxy on ZnTe buffer layers and characterized by in situ reflection high-energy electron-diffraction (RHEED) and high-field magnetospectroscopy investigations. The onset of strain relaxation at the critical thickness has been determined by time-dependent intensity-profile analysis of different reflexes in the RHEED pattern. In spite of the small mismatch and the very low growth temperature, a growth-mode transition from a two-dimensional–to–three-dimensional (2D-to-3D) Stranski-Krastanov growth mode has been identified, which coincides exactly with the critical thickness equilibrium value of about 61 nm predicted by the Matthews-Blakeslee theory. Due to this mechanism, the surface roughness transition region is extended and the onset of plastic relaxation is delayed up to a thickness of about 280 nm. Hall-effect measurements have been performed to determine the iron concentration in the HgSe layers below and above the Fermi-level pinning threshold concentration. With increasing iron concentration both a pronounced increase of the mobility and decrease of the Dingle temperature have been found in the layers. This agrees well with the present available data from HgSe:Fe bulk crystals and also with the values predicted by the short-range correlation model.

However, the maximum carrier mobility of about 2.7×${10}^5$ ${\mathrm{cm}}^{\mathrm{-}3}$ measured in a 1.5-μm-thick HgSe:Fe layer indicates that long-range correlations also have to be considered in the transport mechanism of mercury–iron selenide. HgSe:Fe SQW’s grown in the strained-layer region below the equilibrium critical thickness have been analyzed by Shubnikov–de Haas (SdH) measurements and Hall-effect measurements in magnetic fields up to 50 T. The existence of a two-dimensional electron system (Q2D) in the SQW has been confirmed by the cosine dependence of the SdH oscillation period. The subband splitting in the SQW in dependence of the quantum-well width has been investigated by Hall-resistance measurements. One subband has been identified experimentally in a 12-nm HgSe:Fe quantum well, whereas for high magnetic fields at least two subbands are measured in the 25-nm structures. The Landau-level splitting has been simulated using the Pidgeon-Brown model. In this way the subband splitting and the spin splitting observed experimentally can be explained. The broadening of the localized iron level has been determined from simulation curves.