Controlling the magneto-transport properties of magnetic topological insulator thin films from Cr$_x$(Bi$_y\,$Sb$_{1-y}$)$_{2-x}$Te$_3$ via molecular beam epitaxy

TL;DR

This study demonstrates that precise MBE growth control of Cr$_x$(Bi$_y$Sb$_{1-y}$)$_{2-x}$Te$_3$ thin films can markedly improve crystal quality and allow systematic tuning of magneto-transport properties. By optimizing the substrate temperature and growth rate, and by varying Cr content, the authors position the Fermi level within the bulk band gap and explore the transition between electron- and hole-dominated transport, achieving indications of charge neutrality and enhanced magnetic response in thin films. The results indicate that low Cr concentrations can be tuned to favor regimes with smaller magnetic exchange gaps, which is theoretically advantageous for realizing superconducting proximity effects when integrated with conventional superconductors, offering a pathway toward MTI-based quantum devices. Overall, the work provides a detailed growth–structure–transport map for CrBST that informs targeted material design for topological quantum computing applications.

Abstract

In this work we present a systematic in-depth study of how we can alter the magneto-transport properties of magnetic topological insulator thin films by tuning the parameters of the molecular beam epitaxy. First, we show how a varying substrate temperature changes the surface morphology and when chosen properly leads to a high crystal quality. Next, the effect of the chromium concentration on the film roughness and crystal quality is investigated. Finally, both the substrate temperature and the chromium concentration are investigated with respect to their effect on the magneto-transport properties of the magnetic topological insulator thin films. It becomes apparent that the substrate temperature and the chromium concentration can be used to tune the Fermi level of the film which allows to make the material intrinsically charge neutral. A very low chromium concentration furthermore allows to tune the magnetic topological insulator into a regime where strong superconducting correlations can be expected when combining the material with a superconductor.

Figures (8)

• Figure 1: Identification of optimal parameters for the growth of the Cr$_x$(Bi$_y$ Sb$_{1-y}$)$_{2-x}$Te$_3$ thin films via MBE. a) Search for optimal growth temperature $T_{\text{sub}}$ where the layer thicknesses are measured via XRR and b) the crystal quality is assessed by the FWHM value of the rocking curve via XRD measurements. c) The rocking curve FWHM values as a function of growth rate with epilayers prepared at optimal $T_{\text{sub}}$. The green area always denotes the optimum zone of the growth parameters. d) to h) Scanning electron microscopy (SEM) images of Cr$_x$(Bi$_y$ Sb$_{1-y}$)$_{2-x}$Te$_3$ thin films for increasing substrate temperature. In the aforementioned optimum zone the films are visibly the smoothest.
• Figure 2: the effect of increasing chromium concentration on the surface morphology and crystal structure. a) to f) AFM images of Cr$_x$(Bi$_y$ Sb$_{1-y}$)$_{2-x}$Te$_3$ thin films with an increasing x from $1$ % to $15$ %. g) rms surface roughness of the thin films as a function of Cr concentration and h) The FWHM values of the rocking curve, acquired at the Cr$_x$(Bi$_y$ Sb$_{1-y}$)$_{2-x}$Te$_3$ (0015) peak in epilayers with increasing Cr contents.
• Figure 3: Resulting curves from the van der Pauw measurements in an exemplary manner for one of the MTI films with a Cr concentration of 6.5%. a) Hall resistance $R_{xy}$ in units of $h/e^2$ as a function of magnetic field. The arrows indicate the direction of the magnetic field sweep. The dashed boxes highlight the anomalous part and the regular part of the Hall resistance, respectively. b) Zoom into the longitudinal resistance $\rho_{xx}$ for small magnetic fields. $\rho_{xx}$ exhibits a peak at the positive and negative coercive field. c) Zoom into regular part of the Hall resistance. The dashed line is a linear fit from which the two dimensional charge carrier density $n_{2d}$ is extracted. d) Zoom into the anomalous part of the Hall resistance. The dashed line is a sloped error function fit to the data from which the coercive field $B_c$ and the anomalous Hall resistance $R_{\text{AH}}$ can be determined.
• Figure 4: Magneto-transport parameters of the thin ($\sim 8$ nm) Cr$_x$(Bi Sb)$_{2-x}$Te$_3$ films as a function of substrate temperature extracted from measurements at $4$ K. a) Sheet carrier concentration $n_{2d}$ multiplied by the sign of the slope of the Hall signal $q=\pm 1$ for hole and electron dominated transport, respectively. b) Mobility $\mu$ calculated from $n_{2d}$ and $\rho_{xx}(B=0)$. c) Anomalous Hall resistance $R_{\text{AH}}$ and d) coercive field $B_c$ as a function of substrate temperature determined via the error function fit to $R_{xy}$ at small magnetic fields. Note that in all subplots some data points have error bars while others do not. This is because multiple films were grown for some substrate temperatures, so they were combined into one data point.
• Figure 5: Transport parameters of thin ($\sim 8$ nm) and thick ($\sim 20$ nm) Cr$_x$(Bi$_y$Sb$_{2-y}$)$_{2-x}$Te$_3$ films measured at $4$ K as function of Cr-concentration. The values of the thick layers are indicated by blue squares, while the thin samples are represented by red circles. a) Carrier concentration $n_{2d}$ multiplied by the sign of the slope of the Hall signal $q=\pm 1$ for hole and electron dominated transport, respectively. b) Zoom into a smaller range of $n_{2d}$ to highlight the switch from electron to hole transport for the thin films. c) Mobility $\mu$ for thick and thin samples. d) Anomalous Hall resistance $R_{\text{AH}}$ in $h/e^2$ as a function of $x$. The thin samples reach much larger values. e) Zoom into $R_{\text{AH}}$ for the thicker samples. f) Coercive field $B_c$.