Fig. 1. (Colour online) The assumed tin profiles (black curves) along the growth direction in individual Pb_1−xSn_xTe graded samples (numbered from #1 to #4). A value of d=0 corresponds to the surface. The red and blue solid lines represent the Sn depth profile as measured by SIMS using negative and positive secondary ions, respectively. Horizontal dashed grey lines correspond to values in cladding layers with constant chemical composition. The differences in x between negative and positive SIMS results may result from a disturbance of the element's abundance in MBE source materials. Calibration curves for x in Pb_1−xSn_xTe are calculated from isotope ions measurements^[17].

Fig. 2. Schematic representation of the energy of the valence band maximum (EVBM), conduction band minimum (ECBM), and Fermi level (EF) relative to the vacuum level (Evac). Here, Am, ϕ, and Im represent the electron affinity of the material, work function, and ionization potential of the material, respectively.

Fig. 3. (Colour online) Dependence of the intrinsic holes concentration on the Sn content of the Pb_1−xSn_xTe grown on BaF₂. The black solid dots are the data points taken directly from the hall effect measurement and the red dashed line represents a fitting function.

Fig. 4. (Colour online) SIMS signal as a function of sample potential corresponding to the energy of the positive (a) and negative (b) Sn and Te secondary ions.

Fig. 5. (Colour online) SIMS signal ratio of the negative ions as a function of x in Pb_1−xSn_xTe. The grey lines indicate the fit of linear functions (in log-scale) for two sets of experimental points for low and high x ranges. The vertical lines mark the intersection points of the fitting functions for Sn⁻/Te⁻ and Pb⁻/Te⁻ in red and blue, respectively.

Fig. 6. (Colour online) Estimated values of the sample potential change (representing the shift in the energy distribution of the ion signal) corresponding to the change in the relative work function for negative secondary ions in graded samples as a function of x in Pb_1−xSn_xTe. The initial points are on the horizontal grey line at ΔV=0. The spans of x value correspond to the change of x in the experimental depth profiles Fig. 1.

Fig. 7. (Colour online) SIMS signal ratio of the positive ions as a function of x in Pb_1−xSn_xTe. The grey lines indicate the fit of linear functions (in log-scale) for two sets of experimental points for low and high x ranges. The vertical lines mark the intersection points of the fitting functions for Sn⁺/Te⁺ and Pb⁺/Te⁺ in red and blue, respectively.

Fig. 8. (Colour online) Estimated values of the sample potential change (representing the shift in the energy distribution of the ion signal) corresponding to the change in the relative electron affinity for positive secondary ions in graded samples as a function of x in Pb_1−xSn_xTe. The initial points are on the horizontal grey line at ΔV=0. The spans of x value correspond to the change of x in the experimental depth profiles Fig. 1.

Fig. 9. Schematic representation of the PbTe/SnTe heterostructure arranged from the highest (A) to the lowest (E) possible band-offsets. The bottom and upper bars reflect the valence and conduction bands, respectively. Dark grey bars correspond to PbTe (reference), and light grey denotes the SnTe. Cases (B−D) show band-offsets in typical quantum wells type Ⅰ and Ⅱ. The VL is a vacuum level.

Fig. 10. (Colour online) Schematic representation of the band structure evolution of the Pb_1−xSn_xTe in cases B and C. The diagram illustrates the changes in the valence band maximum (VBM) and conduction band minimum (CBM) as the composition x varies from 0 (pure PbTe) to 1 (pure SnTe). The red line represents the VBM, while the black line represents the CBM. The grey-shaded areas highlight the energy gaps between the valence and conduction bands for each composition, demonstrating how the band structure evolves with increasing Sn content.