Stabilizing the Fermi Level of Cr-Doped Magnetic Topological

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C: Physical Processes in Nanomaterials and Nanostructures

Stabilizing the Fermi Level of Cr Doped Magnetic Topological Insulators by Al Passivation Yu Wu, Qixun Guo, Qi Zheng, Xiulan Xu, Tao Liu, Yang Liu, Yu Yan, Dongwei Wang, Shibing Long, Lijin Wang, Shanwu Yang, Jiao Teng, Shixuan Du, and Guanghua Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09661 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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The Journal of Physical Chemistry

Figure 1. Aging effect on room temperature transport properties of 8 nm CBST films without and with Al capping layers. (a, b) Hall resistance as a function of magnetic field for the devices without (a) and with (b) surface passivation, which were exposed to air for different times (as-grown, 7 days, 14 days, 21 days and 180 days). (c) and (d) are the curves of the carrier densities versus time for the devices without and with surface passivation, respectively. 161x129mm (300 x 300 DPI)

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Figure 2. Electrical transport of the sample without Al passivation at T=2 K. (a) and (b) are Rxy-H and RxxH curves for the uncapped CBST thin film at t=0 day at different back gate voltages (Vg), respectively; (c)(g) The anomalous resistance RAH (left axis open circles) and longitudinal resistance Rxx (right axis solid squares); immediately after growth, 7, 14, 21 and 180 days in air; (h) Schematic band structure and corresponding Fermi levels over time, where CB, VB and SS are bulk conduction band, bulk valance band and surface state, respectively; (i) The maximum of RAH (yellow) and its corresponding Rxx (blue) during 180 days. 249x191mm (300 x 300 DPI)


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Figure 3. Electrical transport of the Al capped sample at T=2 K. (a) and (b) are Rxy-H and Rxx-H curves for the capped CBST thin film at t=0 day at different back gate voltages (Vg), respectively; (c)-(g) The anomalous resistance RAH (left axis open circles) and longitudinal resistance Rxx (right axis solid squares); immediately after growth, 7, 14, 21 and 180 days in air; (h) Schematic band structure and corresponding Fermi levels over time, where CB, VB and SS are bulk conduction band, bulk valance band and surface state, respectively; (i) The maximum of RAH (yellow) and its corresponding Rxx (blue) during 180 days. 249x191mm (300 x 300 DPI)



Figure 4. Gate-dependent coercive field Hc of the samples without Al passivation (a) and with Al passivation (b) at T=2 K, measured at different times after growth. 161x66mm (300 x 300 DPI)


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Figure 5. XPS spectra of Bi 4f (a) and Te 3d (b), for CBST samples without Al passivation at deposition, aged for 180 days, and samples with Al passivation aged for 180 days. 144x109mm (300 x 300 DPI)



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Stabilizing the Fermi Level of Cr Doped Magnetic Topological Insulators by Al Passivation Yu Wu1∥,Qixun Guo1∥, Qi Zheng2, Xiulan Xu1, Tao Liu3, Yang Liu1,2, Yu Yan4, Dongwei Wang5, Shibing Long6, Lijin Wang7, Shanwu Yang8, Jiao Teng1*, Shixuan Du2, Guanghua Yu1 1

Department of Material Physics and Chemistry, University of Science and Technology Beijing,

Beijing 100083, P. R. China 2

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China

3

Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA

4

Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), Institute

of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, P. R. China 5

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center

for Nanoscience and Technology, Beijing 100190, P. R. China 6

School of Microelectronics, University of Science and Technology of China, Hefei, 230026, P.

R. China 7

Experimental and Testing Center, Institute of Advanced Materials and Technology, University

of Science and Technology Beijing, Beijing 100083, P. R. China 8

Collaborative Innovation Center of Advanced Steel Technology, University of Science and

Technology Beijing, Beijing 100083, P. R. China


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∥

These authors contributed equally to this work.

KEYWORDS: magnetically doped topological insulators, degradation in the air, magnetron sputtering, aluminum passivation, anomalous Hall effect

ABSTRACT

The quantum anomalous Hall effect (QAHE) observed in magnetically doped topological insulators is not only fundamentally interesting but also has great potential applications such as quantum computation. To experimentally realize the QAHE, the Fermi level (EF) needs to be stabilized in the tiny surface state gap. However, for topological insulators, even a very short time exposure to the atmosphere can induce a large EF shift. In this work, magnetic topological insulator Cry(BixSb1-x)2-yTe3 (CBST) thin films are successfully prepared via magnetron sputtering, which is a more universal, efficient and affordable method for application. Then the evolution of the EF position has been investigated as a function of the exposure time in the air through gate-dependent transport measurements. This study reveals that the EF position can be stabilized by in situ Al passivation with carrier doping significantly reduced, and the oxidations of Bi and Te atoms can also be suppressed. The availability of wide compositions of magnetron sputtering as well as the EF position stability of the in situ Al passivation may be a key for further investigations of the topological insulator toward the achievement of the QAHE in the sputtered CBST thin-film systems. Topological insulators (TIs) are a new class of quantum matter featuring an energy gap in their bulk band structure and unique Dirac-like metallic states on the surface protected by timereversal symmetry.1 Introducing long-range ferromagnetic order in these materials can lead to a


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gap in the surface states and the seminal discovery of quantum anomalous Hall effect (QAHE) when the Fermi level (EF) is tuned inside both the surface and bulk gaps. For QAHE, the anomalous Hall resistance RAH is rigorously quantized (RAH ~±h/e2 ≈±25.8 kΩ, characterized by a Chern number C = 1 or -1, where h is Planck’s constant and e is the electron charge) even if the external magnetic field H is 0, and the existence of dissipationless edge state leads to a nearly vanishing longitudinal resistance Rxx, which is promising for the applications of low powerconsumption electronic devices.2-3 Until now, QAHE has already been experimentally realized in many magnetically doped TIs, which include Cr-doped 3-6, V-doped 7-8 or co-doped 9-10 (Bi,Sb)2Te3 thin films. However, all of these TI thin films were prepared by molecular beam epitaxy (MBE). From an application point of view, it is really worthwhile to figure out how to grow these thin films by a method that is more compatible with the semiconductor processes, for example magnetron sputtering. Besides, it is widely reported that the surface degradations have strong doping effect and can cause a large EF shift in TIs.11-19 For example, the oxygen or water absorbed on Bi2Se3 surface were found to be served as a p-type 13, 20 or n-type dopant21-24, and finally pin the EF in the bulk conduction band or valence band respectively. In order to address this problem, capping layers such as Al 23, PMMA 21, Se 21, Te 22, F4TCNQ 25 and ZnO 26 have been used in previous studies to protect binary TIs (Bi2Se3, Bi2Te3) or ternary TI ((BixSb1-x)Te3) from degradation and charge carrier doping in the environment. On the contrary, similar studies on the magnetically doped quaternary TI thin films, which turn out to be actually more important since stabilizing the EF of magnetic doped TI in the tiny surface state gap is more difficult but essential for the realization of QAHE, are still lacking.


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In this work, Cry(BixSb1-x)2-yTe3 (CBST) thin films with a thickness of about 8 nm were grown by magnetron sputtering. Samples with a 5-nm aluminum (Al) capping layer were grown under the same conditions for comparison purposes. We studied how their properties changed as time under exposure to environment. Electrical transport measurements suggest that the EF has changed when the bared CBST thin films were exposed to the atmosphere. X-ray photoelectron spectroscopy (XPS) was used for surface elemental characterisation, and the oxidations of Bi and Te elements were observed. Our results show that a thin Al capping layer can protect CBST well from being oxidized and stabilize EF position, which may serve as a very important guideline in the future on the way to realize QAHE in magnetron sputtering grown TIs. RESULTS AND DISCUSSION Figure 1a and b show the Hall curves for CBST thin films without and with Al passivation exposed to air. The samples were all grown on doped Si covered with 300 nm SiO2 by magnetron sputtering with a thickness of about 8 nm. For the uncapped as-grown CBST thin films, the Hall curve displays negative slope verifying that electrons are the majority carriers. After being exposed in the atmosphere for a week, the Hall slope is changed from negative to positive (Figure 1a), indicating that the type of carriers is changed from n-type to p-type. All Hall curves in Figure 1a are S-shape, indicating that there may be contributions of two carriers with different but comparable mobilities.27 The results of the sample with Al capping layer are shown in Figure 1b. Same as the uncapped sample, its carrier type at as-deposited state is also n-type; However, even after being exposed in the air for 180 days, the carrier type of the capped sample is still ntype, and the slope of the Rxy-H curve is only slightly changed and no positive slope appears. The 2D carrier densities n2D=1/(eRH) (where RH is the Hall coefficient) are shown in Figure 1c and d for uncapped and capped CBST thin films, respectively. The n2D of uncapped CBST thin film ACS Paragon Plus Environment

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reverses its sign in a short time (7 days), changing from the original n-type conductor to a p-type conductor, while it changes slowly after ~14 days. However, for the sample with a 5-nm Al layer, the charge carrier doping effect is greatly reduced, and its carrier type and density remain stable over a long period of time. Further electrical transport measurements are performed at low temperatures to determine the anomalous Hall resistance and to evaluate the position of Fermi level in the surface degradation process. Generally, the Hall resistance of the anomalous Hall effect is expressed as: 28-29 Rxy = R0H + RAH(M)

(1)

where R0 is the slope of the ordinary Hall background, M is the magnetisation component in the perpendicular direction, and RAH can be estimated by the intercept of the linear background at a high magnetic field. Figure 2a–b are the Rxy-H and Rxx-H curves for the as-deposited state of the sample without Al passivation. It can be seen that the thin film has good long-range ferromagnetic order and the coercive field is about 326 Oe. Both Rxy and Rxx can be effectively tuned by back gate voltages. When RAH takes a maximum value, it corresponds to adjusting the Fermi level near the Dirac point (DP).3 The maximum RAH of the uncapped sample at the asdeposited state corresponds to Vg=-60 V, indicating that the EF of the sample is above the DP. From Figure 2c–d, it can be seen that with short exposure time in air, RAH (Vg) peak moves to the positive direction, from -60 V to +60 V, showing a p-type doping process. This result is consistent with the fact that the carrier type changed from n-type to p-type in Figure 1a and c. Although the shapes of RAH-Vg and Rxx-Vg curves in Figure 2d-g are similar, revealing that the position of EF is unchanged for long time, the values of RAH and Rxx have changed significantly.


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For ease of understanding, Figure 2h shows that the EF moves in the energy band with the increase of exposure time in the air. The EF is located near the DP at t=0 day and moves down in the direction of the valence band. When the sample is aged for one week, the EF moves below the Dirac point and is close to the valence band. As shown in Figure 2i, for long-term (from the 7th to the 180th day), the maximum of RAH decreases while the corresponding Rxx increases, indicating that surface degradation of uncapped CBST sample may hinder the observation of QAHE. The anomalous Hall resistances of the sample with Al passivation at low temperatures are shown in Figure 3. Similar to the uncapped sample, this sample also has good ferromagnetic order and the negative magnetoresistance effect. The Rxy and Rxx can be effectively tuned when Vg changes from −120 V to 120 V. Within 180 days, Vg corresponding to the maximum value of Rxx is always negative and moves slightly to the positive direction (Figure 3c-g). This shows that the EF is above the DP in the as-deposited state, then moves slightly close to the DP, and is stable after one week (Figure 3h). For the capped CBST sample, the long-term behavior of the maximum of RAH and its corresponding Rxx is quite different from that of the uncapped CBST sample; the maximum of RAH and its corresponding Rxx of the uncapped CBST sample degrade over time, whereas they are more stable of the capped CBST sample upon exposure to ambient conditions (Figure 3i). Additionally, we study the influence of surface degradation in the air on magnetism. For the uncapped sample at t=0 day, when the carrier type of the sample is n-type, the coercive field (Hc) is shown in the red curve in Figure 4a as a function of the gate voltage. The variation of Hc is small, within a range of 323 Oe and 363 Oe, when the gate voltage increases from −120 V to 120 V, showing a gate-independent ferromagnetism. This gate-independent ferromagnetism can be


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attributed to the van Vleck mechanism.10, 29-32 When the sample is aged in the air for one week and the carrier type has changed to p-type, an increase from 329 Oe to 675 Oe occurs in the Hc as the EF is tuned toward the bulk valence band (the black curve in Figure 4a), indicating that the magnetic mechanism is dominated by the hole-modulated Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism.29-30 Similar trends with the black curve but larger value of Hc can be observed for the sample aged for 14, 21 and 180 days (the blue, pink and green curves in Figure 4a, respectively). In comparison, for the capped sample with n-type carrier over 180 days, the Hc is nearly independent on Vg (Figure 4b), indicating the magnetic mechanism is dominated by van Vleck mechanism. Two observations are evident from the data: firstly, it is further proved that the Al capping layer can protect the sputtering grown CBST thin film very well; secondly, the mechanism for the long-range ferromagnetic order in this magnetron sputtering grown CBST magnetically doped TI thin films seems strongly affected by the carrier type and turns out to be bulk-electron-mediated van Vleck paramagnetism dominant and RKKY interaction dominant as the carrier type is n and p respectively. This may be potentially used to control the long range magnetic order mechanism in this magnetically doped TI thin films, 30, 33-34 which is worth of further studies. The chemical or electronic states of each element of uncapped CBST samples with different exposure time in the air were characterised by ex situ XPS. Two peaks at 157.7 eV (dotted line) and 159.0 eV in Figure 5a are consistent with the binding energies of Bi 4f7/2 in Bi2Te3 and Bi2O3 respectively.35 Two peaks at 572.3 eV (dotted line) and 576.2 eV in Figure 5b are consistent with the binding energies of Te 3d5/2 in Bi2Te3 and TeO2 respectively. The oxidations of Bi and Te generated in one hour, when the sample was taken out of the high vacuum growth chamber and transferred to the XPS sample chamber, showing that the CBST thin films are extremely


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sensitive to air. In addition, the contents of Bi2O3 and TeO2 in the sample aged for six months were significantly increased compared with those exposed to air for one hour, which corresponded to the increasing coverage of Bi2O3 and TeO2. Whereas the samples with Al capped have no obvious oxidation peaks in both Bi and Te even if the samples were exposed in air for 180 days. It shows that the Al capping layer makes the CBST surface much more stable and free from oxidation. It has been shown in this study that exposure to air induces a p-type doping for CBST thin films, and that an Al capping layer helps prevent surface oxidation and stabilize the position of EF. We have also confirmed the rapid surface oxidation of CBST thin films by XPS. We can safely conclude that the surface oxidation is most probably the origin of the charge carrier doping as the uncapped CBST exposed to air. However, the details are still not clear. On one hand, in previous studies, pure oxygen serves as a p-type dopant for binary topological insulator Bi2Se3, 13, 20 while the effect of water vapour in the air is shown to cause n-type doping.11, 24, 36 In this study, while the effects of oxygen and moisture on the electrical transport properties of CBST samples cannot be strictly distinguished, surface oxidation is still one of the factors that cannot be ignored in the p-type doping process. On the other hand, for Bi2Se3, it has been shown that oxygen impurity on Se terminated surfaces exhibits an electron doping scenario, while oxygen on Bi terminated surfaces corresponds to a hole-doping scenario.37 Therefore, besides the competition of oxygen and water in the atmosphere, other variables, such as the type of surface termination (e.g. Bi-terminated, Sb-terminated or Te-terminated surface for the CBST samples) or other unknown variables may contribute to this p-type doping process of the CBST samples exposed to air. In addition, more precise and detailed transport measurements or material


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characterisations than those used here would be necessary to confirm the main origin of the ptype doping for the CBST thin films aged in air. CONCLUSION In summary, this study shows that surface passivation with Al is an effective way to stabilize the position of Fermi level for CBST thin films. The Al capping layer contributes to reduce holedoping effect, enhance the anomalous Hall resistance, maintain the van Vleck magnetic mechanism and suppress the oxidation of Bi and Te. Since the realization of QAHE requires fine-tuning the position of EF into the surface state gap, reducing carrier doping effect and stabilizing EF in the magnetron sputtering grown TI thin films may open routes to realize the QAHE in these thin films. EXPERIMENTAL SECTION Material Growth and Sample Preparation. CBST (8 nm) thin films were deposited on 22×22 mm2 silicon (100) substrates (300 nm SiO2/silicon) by magnetron sputtering system (ATC1800F). The co-sputtering method with high purity (99.99%) Bi2Te3 alloying target, Sb2Te3 alloying target, Cr target and Te target was adopted to control the films’ composition. The substrates were kept at 195

during growth. The base pressure of the deposition chamber was

below 3×10-7 Torr and the working argon pressure was set at 2×10-3 Torr. After growth, the thin films were annealed for 25 minutes at the same temperature as the deposition. When the substrates were cooled to room temperature, a 5-nm thick Al thin film was sputtered in situ on the topological insulator thin films, and the Al thin film was naturally oxidized to form AlOx after the sample was taken out of the chamber and exposed to air.


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Transport Measurements. Since the topological insulator films are soft, the films can be manually drawn with fine needles, and an ohmic contact is formed with silver paste at each electrode. The back of the Si substrates are coated with silver as the back gate electrode, and the films are gated with a voltage source Keithley 2410. The Rxx and Rxy data are symmetrised and anti-symmetrised respectively to account for the misalignment of electrodes. The transport measurements are carried out using a 9 T Physical Properties Measurement System (PPMS, Quantum Design) with a base temperature of 2 K. Characterization Methods. XPS characterization of CBST thin films was performed using a ThermoFisher Scientific ESCALAB250X system with monochromatic Al Kα source (hν=1486.6eV).

Supporting Information. The Hall angle as a function of back gate Vg, the temperature dependence of Rxx, Hall curves at different temperatures, the Arrott plot and gate-dependent resistances at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Jiao Teng. E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the National key R & D plan program of China (Grant No. 2017YFF0206104, 2014GB120000), the National Key Scientific Research Projects of China


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(Grants No.2015CB921502), the Natural Science Foundation of China (Grant Nos. 61574169, 61474007, 51331002), Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, the Opening Project of Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences. REFERENCES 1.

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Stabilizing the Fermi Level of Cr-Doped Magnetic Topological

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