#
Revisiting the van der Waals Epitaxy in the Case of (Bi_{0.4}Sb_{0.6})_{2}Te_{3} Thin Films on Dissimilar Substrates

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^{†}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

## 3. Results and Discussion

#### 3.1. Substrate and Material Choice

#### 3.2. High Degree of Crystallinity, Consistent with vdW Epitaxy

#### 3.3. Morphological Differences, Indicating Quasi-vdW Epitaxy

#### 3.4. Employing Quasi-vdW Epitaxy to Enhance Single Domain Rotational Alignment with the Substrate

#### 3.5. Revealing the Nature of the Film-Substrate Interaction in Quasi-vdW Epitaxy

## 4. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**–

**c**) RHEED patterns and (

**d**) symmetric 2$\theta -\omega $ diffractograms of 10 nm BST on InP (111), Al${}_{2}$O${}_{3}$ (001), and SrTiO${}_{3}$ (111) substrates, respectively. The presented RHEED patterns are recorded along $\langle 2\overline{1}\overline{1}0\rangle $ and are representative for other BST films deposited on these substrates. The XRD data have been given an offset for clarity. The solid (semi-transparent) lines present the smoothed (real) data. The Miller indices of the different crystallographic planes $(hk\ell )$ are indicated. The diffractograms and the sharp streaks observed in the RHEED patterns confirm the crystallinity of all three BST films in the out-of-plane direction.

**Figure 2.**RSMs of 10 nm BST films deposited on (

**a**) InP (111)A; (

**b**) Al${}_{2}$O${}_{3}$ (001); and (

**c**) SrTiO${}_{3}$ (111). All RSMs map out the same region of reciprocal space chosen to cover the three nearest substrate reflections and the reciprocal space position of the BST (0120) and (1019) reflections. The RSMs indicate that there is no significant strain imposed on the film.

**Figure 3.**AFM images revealing the surface morphology of the used substrates (

**a**–

**c**) with a dimension of 1 $\mathsf{\mu}\mathrm{m}$ × 3 $\mathsf{\mu}\mathrm{m}$. Surface morphology of 5 and 10 nm BST films deposited on InP (

**d**,

**g**); Al${}_{2}$O${}_{3}$ (

**e**,

**h**); and SrTiO${}_{3}$ (

**f**,

**i**), with a lateral dimension of 1 $\mathsf{\mu}\mathrm{m}$ × 1 $\mathsf{\mu}\mathrm{m}$ and 3 $\mathsf{\mu}\mathrm{m}$ × 3 $\mathsf{\mu}\mathrm{m}$, respectively. The BST films exhibit randomly inclined crystals when deposited on SrTiO${}_{3}$ and Al${}_{2}$O${}_{3}$, whereas it grows relatively smooth on InP. The observed defects are expected to arise from defects in the nucleation layer of the film due to the relatively large lattice mismatch between the film and the SrTiO${}_{3}$ and Al${}_{2}$O${}_{3}$ substrates.

**Figure 4.**Pole figures taken in symmetric $2\theta -\omega $ configuration at $2\theta =38.06{}^{\circ}$ on 10 nm BST films deposited on (

**a**) InP (111)A; (

**b**) Al${}_{2}$O${}_{3}$ (001); and (

**c**) SrTiO${}_{3}$ (111). $\phi $ and $\chi $ present the azimuthal and tilt angle, respectively. Mapping the BST{1010} of the film on SrTiO${}_{3}$ (

**a**) and InP (

**c**) clearly shows the existence of the twin domains while a domain suppression is achieved in the film on Al${}_{2}$O${}_{3}$ (

**b**).

**Figure 5.**STEM images of the substrate/film interface (

**a**) ADF mapping performed on a 10 nm BST film deposited on InP. STEM cross-sectional view along the $\left[2\overline{1}\overline{1}0\right]$ (in-plane) direction, presenting a regular stacking of the QLs and revealing a well-defined substrate/film interface; (

**b**) HAADF mapping performed on a 10 nm BST film deposited on Al${}_{2}$O${}_{3}$ presenting a cross-sectional view along the $\left[01\overline{1}0\right]$ (in-plane) direction; (

**c**) HAADF STEM cross-sectional view along the $\left[2\overline{1}\overline{1}0\right]$ (in-plane) direction on a 10 nm BST film deposited on SrTiO${}_{3}$.

**Figure 6.**STEM HAADF image (

**a**) and EDX maps (

**b**,

**c**) of lamella presented in Figure 5c, presenting a cross-sectional view along the $\left[2\overline{1}\overline{1}0\right]$ (in-plane) direction on a 10 nm BST film deposited on SrTiO${}_{3}$. The dotted line marks the bottom of the first QL. (

**a**) HAADF image of the area on which the EDX mapping is performed; (

**b**) overlay of elemental maps; (

**c**) individual elemental maps of Bi (red), Sb (green) and Te (blue). The analysis reveals the ordering of Sb in the interfacial layer.

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**MDPI and ACS Style**

Mulder, L.; Wielens, D.H.; Birkhölzer, Y.A.; Brinkman, A.; Concepción, O.
Revisiting the van der Waals Epitaxy in the Case of (Bi_{0.4}Sb_{0.6})_{2}Te_{3} Thin Films on Dissimilar Substrates. *Nanomaterials* **2022**, *12*, 1790.
https://doi.org/10.3390/nano12111790

**AMA Style**

Mulder L, Wielens DH, Birkhölzer YA, Brinkman A, Concepción O.
Revisiting the van der Waals Epitaxy in the Case of (Bi_{0.4}Sb_{0.6})_{2}Te_{3} Thin Films on Dissimilar Substrates. *Nanomaterials*. 2022; 12(11):1790.
https://doi.org/10.3390/nano12111790

**Chicago/Turabian Style**

Mulder, Liesbeth, Daan H. Wielens, Yorick A. Birkhölzer, Alexander Brinkman, and Omar Concepción.
2022. "Revisiting the van der Waals Epitaxy in the Case of (Bi_{0.4}Sb_{0.6})_{2}Te_{3} Thin Films on Dissimilar Substrates" *Nanomaterials* 12, no. 11: 1790.
https://doi.org/10.3390/nano12111790