Using molecular beam epitaxy (MBE) technology, two 500-nm-thick PbTe bulk films were grown on 2 freshly cleaved BaF
2 (111) substrates, and a 100-nm-thick CdTe layer was then deposited on top of one of the as-grown PbTe films to form a CdTe/PbTe (111) heterojunction. The RT electrical properties of the epitaxially grown PbTe film and CdTe/PbTe heterojunction materials were characterized using the Hall effect based on the Van der Pauw method (see Fig.
S1). The PbTe film exhibits a p-type electrical conductivity with a hole density of 1.2 × 10
13 cm
−2 and carrier mobility of 459 cm
2/V·s, while the CdTe/PbTe heterojunction exhibits the inverse conductivity type with a much higher electron density of 9.8 × 10
14 cm
−2 and a higher carrier mobility of 760 cm
2/V·s. The important transition from film to heterojunction is governed by the formation of a 2DEG at the interface, which largely determines the electrical properties in the heterojunction [
39]. Several characterization technologies were employed to analyze the surface morphology, cross-section, and crystallinity of the grown materials. Figure
1A presents an atomic force microscopy (AFM) image of the PbTe layer surface. The root mean square (RMS) roughness of the film surface is 0.306 nm, which is comparable to the thickness of a single atomic layer (0.373 nm). Moreover, the AFM image further reveals a clear helical structure on the sample surface. This morphology is attributed to dislocation lines at the PbTe/BaF
2 interface spiraling upward with increasing film thickness. This phenomenon is indicative of a unique dislocation-regulated epitaxial growth mode of PbTe on BaF
2, which promotes line dislocation clustering and thus reduces the overall dislocation density. Figure
1B shows a cross-sectional scanning electron microscopy (SEM) image of the CdTe/PbTe heterojunction, confirming the designed thicknesses of the PbTe (500 nm) and CdTe (100 nm) layers. Similarly, Fig.
1C shows a high-resolution transmission electron microscopy (HRTEM) image of the CdTe/PbTe heterojunction interface, clearly revealing the atomic arrangement near the interface. The image suggests that the epitaxially grown heterojunction has a clean and well-defined interface. It can be observed that CdTe and PbTe share a common interface monolayer of Te atoms. The interface Te atoms receive an excess of valence electrons from both the Cd atoms in CdTe and the Pb atoms in PbTe. In the simplest consideration based on electron counting, i.e., assuming full ionicity and no charge redistribution, on average, the Te atom should receive 1.0 electron (6p) from the 3 Pb atoms and 1.5 electron (5s) from the 3 Cd atoms, implying an excess of valence electrons with total 8.5 valence electrons per interface Te atom, thus spontaneously yielding a metallic interfacial layer (2DEG) [
40]. The 2DEG channel forms spontaneously at the interface during the epitaxial growth process rather than through intentional doping, resulting in low scattering and very high electron mobility in the channel. Additionally, x-ray diffraction (XRD) measurement was also performed on the CdTe/PbTe heterojunction using Cu K
α radiation in the reflection configuration, and the XRD pattern is shown in Fig.
1D. The intense diffraction peaks indicate that both PbTe and CdTe layers are oriented along the [111] direction. The lattice constants of the PbTe and CdTe films were calculated to be 6.450 and 6.504 Å, respectively, using the Bragg diffraction formula and the crystal plane spacing formula (see Fig.
S2). This reveals the small lattice mismatch at the interface, which results in the achievement of the perfect interface of the CdTe/PbTe heterojunction as observed in Fig.
1C.