Poster Spotlight: Orientation Control of MBE Grown SnTe Layers on GaAs Substrates

Masakazu Kobayashi^{1, 2}, Su Nan¹

1. Dept. EE & BS, Waseda University, Shinjuku, Tokyo, Japan.
2. Waseda University, Material Research Technology Institution, Shinjuku, Tokyo, Japan.

Masakazu Kobayashi^{1, 2}, Su Nan¹

1. Dept. EE & BS, Waseda University, Shinjuku, Tokyo, Japan.
2. Waseda University, Material Research Technology Institution, Shinjuku, Tokyo, Japan.

SnTe is one of the attractive material for a topological crystalline insulator (TCI). SnTe films have been grown directly on GaAs (100) substrates by molecular beam epitaxy, and single domain SnTe layers preferentially oriented to (100) were achieved by now. The (111) oriented SnTe domains and Te were easily included in the film. The crystallographic orientation control and surface smoothness improvement was mainly performed in this study.

SnTe layers were grown on (100) and (111)B oriented GaAs substrates by molecular beam epitaxy (MBE). Most of structures were formed on (100) GaAs substrates with (100) oriented ZnTe buffer layers. Elemental sources were used and the molecular beam intensity ratio (J_Te/J_Sn) was varied between 0.6~2.9. The beam intensity ratio was tuned by changing the beam intensity of Te. The substrate temperature was calibrated using the oxide desorption temperature of chemically etched GaAs substrates prior to the film growth, and it was varied from 200 ^oC to 300 ^oC. The film thickness of SnTe was about 100 nm.

The introduction of the ZnTe buffer layer was effective to improve the crystallographic properties of SnTe layers. Atomic force microscopy observation revealed that plateaus with atomically smooth surfaces were obtained for SnTe layers prepared on ZnTe buffer layers. On the other hand, samples prepared without ZnTe buffer layers haven’t exhibited the atomically smooth surfaces. The size of the plateau was increased by increasing the molecular beam intensity ratio up to 1.0.

Since the growth temperature was relatively low, the inclusion of the excess material was not negligible. According to the result of X-ray diffraction (XRD) measurement, a diffraction peaks originated from hexagonal Te was observed with SnTe peaks when samples were grown with Te rich (J_Te/J_Sn was between 2.9 and 1.5) conditions. In the secondary electron microscopy measurements, featureless surface was confirmed when the XRD of the sample exhibited no Te peak while dotted materials were clearly observed on the surface when the hexagonal Te peak was observed. The distribution density of the dot was high when the Te diffraction peak signal intensity was strong. Those dotted materials are presumed to be the residual of Te.

The orientation of the material was affected by the growth temperature. When the substrate temperature was raised, (111) SnTe domains were started to appear. The XRD peak intensities of (100) and (111) domains were comparable when the substrate temperature was 250 ^oC. In order to eliminate the (111) domain inclusion, the substrate temperature needed to be maintained below 240 °C.

In order to enhance the (111) domain formation, (111)B oriented GaAs substrates were employed. (100) domains could be mostly eliminated when the substrate temperature was raised to 270 ^oC.

A standard van der Pauw Hall measurement was performed to characterize the electrical property at room temperature. The (100) sample that didn’t include the Te residual exhibited the resistivity of about ohm*cm. The resistivity has increased as the presence of Te residual became noticeable. Based on the literatures, the room temperature resistivity of SnTe and Te were about ohm*cm ^[1] and 0.3 ohm*cm ^[2], respectively. The higher resistivity obtained for the grown film might be originated from the mixing of Te residual. Although the surface morphology was improved by the introduction of the ZnTe buffer layer, the electrical properties were not improved. Boundaries of plateau probably became the center of carrier scatterings.

Acknowledgement
This work was supported in part by the Waseda University Grant for Special Research Projects.

References
1. Athwal, I. S., et al., Thin solid films, 162 (1988) 1-6.
2. Wang, D., et al., J.Alloys & Compounds, 773 (2019) 571-584.