Growth of the Bi2Se3 Surface Oxide for Metal–Semiconductor–Metal

Article pubs.acs.org/JPCC

Growth of the Bi2Se3 Surface Oxide for Metal−Semiconductor−Metal Device Applications Yun-Chieh Yeh,† Po-Hsun Ho,† Cheng-Yen Wen,*,†,∥ Guo-Jiun Shu,‡,∥ Raman Sankar,‡,∥ Fang-Cheng Chou,‡,§,∥ and Chun-Wei Chen†,∥ †

Department of Materials Science and Engineering and ‡Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ∥ Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan S Supporting Information *

ABSTRACT: The effect of the surface structure of Bi2Se3 on its interior properties has been well studied recently, but the interfacial structure and electrical properties of the oxidized Bi2Se3 surface are little known. In contrast to the self-limited formation of native oxide on Bi2Se3, the degree of oxidation on the Bi2Se3 surface in oxygen plasma is enhanced. Results of transmission electron microscopy and X-ray photoelectron spectroscopy show that the surface of the oxidized Bi2Se3 is composed of a layer of amorphous bismuth oxide (BiOx), and the thickness of the BiOx layer can be controlled by the length of the plasma process. Electrical measurements of this structure present the Schottky-type transport property at the interface between the oxidized layer and the bulk Bi2Se3 crystal, and the turn-on voltage depends on the thickness of the surface BiOx layer. This study of the structure, formation mechanism, and electrical properties of the surface oxide of Bi2Se3 formed in oxygen plasma provides useful information for future development of electronic devices based on bismuth chalcogenides.

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INTRODUCTION

Although Bi2Se3 has attracted much attention for future topological insulator applications on electronics or spintronics, most of the properties reported so far were measured in vacuum. It is also known that the surface oxidation of Bi2Se3 strongly affects its transport properties. Therefore, understanding growth mechanism of the surface oxide of Bi2Se3 will be essential for future applications of Bi2Se3-based electronic devices, analogous to the important scenario of the development of the SiO2 dielectric layer on Si in the VLSI technology.

Bismuth selenide (Bi2Se3) has recently been extensively studied for applications in topological insulators (TIs).1,2 Its crystal structure is a stack of quintuple layers separated by the van der Waals gap, and each quintuple layer is composed of five atomic planes in the sequence of Se−Bi−Se−Bi−Se.3−5 Intrinsic bismuth selenide (Bi2Se3) has a band gap of ∼0.3 eV,6 but the Bi2Se3 crystal usually has excess charge carriers, which result from the vaporization of selenium during crystal growth.2,7−9 It is also found that, when the surface of Bi2Se3 is covered with its native oxide, an additional n-type doping occurs near the interface.10 The formation of native oxide on Bi2Se3 in the ambient conditions is self-limited to the first two quintuple layers.10 Kinetics analysis for the formation of native oxide on the surface of a similar layered structure, Bi2Te3,11 shows that the oxidation of the top first quintuple layer is relatively fast, but continuous oxidation into deeper layers is restricted by slow jumping of oxygen atoms across the van der Waals gap. It is therefore expected that more energetic oxygen species may cause stronger oxidation effects on the Bi2Se3 surface. Yet, most investigations regarding the oxidation of Bi2Se3 focus on the structure and effects of the native oxide.10,12,13 Extensive oxidation of Bi2Se3 has only been observed from Bi2Se3 crystals annealed in air at 450 °C.14 In this study, we show that the surface oxide on Bi2Se3 can be grown in a controllable manner using oxygen plasma. © 2016 American Chemical Society

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EXPERIMENTAL DETAILS

Growth of Bi2Se3 Single Crystals. The Bi2Se3 single crystals used in this study were grown by the vertical Bridgman method. Bismuth (99.99%) and selenium (99.99%) powder were first mixed in the stoichiometric ratio of Bi2Se3 in argon and then sealed in a quartz tube at vacuum of 3 × 10−2 Torr. The mixture was annealed at 650 °C for 18 h, followed by the growth of the Bi2Se3 single crystal in the vertical Bridgman furnace. The melt was formed and kept at 850 °C for 24 h to ensure the formation of homogeneous Bi2Se3. Solidification of the melt was made by slowly pulling the sample at a rate of 0.5 mm/h off the hot zone, which was maintained around the Received: October 24, 2015 Revised: January 14, 2016 Published: January 22, 2016 3314

DOI: 10.1021/acs.jpcc.5b10425 J. Phys. Chem. C 2016, 120, 3314−3318

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The Journal of Physical Chemistry C solidification temperature, 705 °C, into a cooler zone with a small temperature gradient of 1 °C/cm. Oxygen Plasma Process. One μm thick Bi2Se3 crystals in a size of 2 × 2 mm2 were prepared by exfoliation using Scotch tape. The thin crystals were transferred onto insulator substrates with the c-axis of Bi2Se3 perpendicular to the substrates. The as-cleaved crystals were cleaned in acetone and methanol baths to remove organic residues. Growth of the oxide layer at the surface of pristine Bi2Se3 crystals was in 75 W oxygen plasma for various lengths of time, ranging from 10 s to 5 min. On the other hand, growth of native oxide on Bi2Se3 was in the air of 30% humidity at 25 °C for 2 weeks. Device Fabrication for Electrical Measurements. Devices in the structure of Au/Bi2Se3/oxidized Bi2Se3/Au (from bottom to top) were prepared for the current−voltage measurement. The bottom Au electrode in a dimension of 100 μm × 100 μm and 150 nm thick was thermally evaporated onto an insulator substrate, which was predeposited with a 5 nm thick chromium buffer layer. An exfoliated Bi2Se3 crystal was mounted on the bottom electrode, followed by oxygen plasma treatment to oxidize its surface. A layer of epoxy (EpoxyBond 110, Allied High Tech Products, Inc.) was laid on the crystal surround to prevent direct contact between the top and bottom electrodes. Finally, the Au top electrode was deposited by thermal evaporation. The active area of the device was 0.01 mm2. The current−voltage measurements were performed using the Keithley 2410 source meter. X-ray Photoelectron Spectroscopy Analysis. XPS analysis was conducted in the VG Scientific ESCALAB 250 system. The X-ray source in the system was generated from an aluminum target (photon energy of 1486.8 eV) with the pass energy of 20 eV. The takeoff angle for photoelectron collection was 45° from surface normal. Argon plasma etching at a rate of 3 Å/s was used to sputter the surface for depth profiling. The instrument work function was calibrated to give a metallic gold (Au 4f7/2) binding energy of 84.0 eV. XPS spectra were analyzed by the XPSPEAK 4.1 peak fitting program, and the Shirley function was used for background subtraction. Transmission Electron Microscopy Analysis. Crosssectional samples were prepared by mechanical polishing, followed by 5 kV Ar+ ion beam thinning using the Gatan Precision Ion Polishing System (PIPS II). STEM images and energy-dispersive X-ray spectroscopy (EDS) maps were obtained in a JEOL 2100F 200 kV TEM, equipped with a probe type corrector for the spherical aberration of the objective lens.

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Figure 1. (a) Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the native oxide of Bi2Se3. The arrows label the first quintuple layer on the surface, which is partially amorphized. (b) Cross-sectional HAADFSTEM image of the Bi2Se3 surface after processed in oxygen plasma for 60 s. (c) STEM-EDS compositional maps of Bi and Se in the area labeled in (b). The white dashed line in each figure indicates the boundary between the oxidized region and bulk Bi2Se3 crystal.

RESUTLS AND DISCUSSION The cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1(a)) of the native oxide of Bi2Se3 shows that the first quintuple layer of Bi2Se3 is partially amorphized, while the second quintuple layer remains intact. By contrast, the oxidation of Bi2Se3 formed in 75 W oxygen plasma is more pronounced (Figure 1(b)). The plasma-induced oxide layer is inhomogeneous. It contains an amorphous layer in the top region (labeled as (1) in Figure 2(b)) and a partially crystalline layer in the deeper region (labeled as (2) in Figure 2(b)). The two regions are hereafter named the surface oxide layer and the interfacial layer, respectively. In Figure 1(b), the intensity is sensitively proportional to Z1.7 (Z is the atomic number),15 so that the bulk Bi2Se3 crystal appears brightest in the image. The crystalline nanoparticles in the interfacial layer have similar

intensity to the Bi2Se3 crystal, implying the composition of the crystalline grains is close to that of bulk Bi2Se3. The distribution of Bi and Se elements in the oxidized layer is probed using STEM-EDS. As shown in Figure 1(c), Bi is uniformly distributed in the oxidized layer, while Se remains only in the interfacial layer. The chemical composition of the plasma-induced surface oxide layer is further analyzed using depth-resolved XPS. Figure 2 shows the representative XPS Bi 4f and Se 3d spectra of the 3315

DOI: 10.1021/acs.jpcc.5b10425 J. Phys. Chem. C 2016, 120, 3314−3318

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Figure 2. XPS spectra of (a) Bi 4f and (b) Se 3d measured from different positions in an oxidized Bi2Se3 that has been exposed to oxygen plasma for 60 s. The number labeled by each curve in (b) is the factor by which the intensity is multiplied.

surface oxide layer, the interfacial layer, and the bulk Bi2Se3 crystal. In the spectra of the surface oxide layer, BiOx (indicated by the peaks at 159.2 and 164.5 eV) and a small amount of Se (indicated by the SeOx and Se peaks at 58.8 and 55.3 eV, respectively) are present. The spectra of the interfacial layer contain the peaks of BiOx, a small peak of Se, and those of Bi2Se3 (peaks at 163.4 and 157.9 for Bi 4f; 54.1 and 53.3 for Se 3d), which are due to the crystalline Bi2Se3 nanoparticles in the interfacial layer. With increasing length of the plasma process, the thickness of the interfacial layer remains similar. Only the thickness of the surface BiOx layer increases linearly with the process time (Figure S1 in Supporting Information). In Figure 3(a,b), the current−voltage (I−V) relationships of the devices made of pristine Bi2Se3 and Bi2Se3 crystals with different degrees of surface oxidation are compared. In Figure 3(a), the linear I−V relationship for the device containing pristine Bi2Se3 indicates the formation of ohmic contact at the interface between the Bi2Se3 crystal and the Au electrodes. The native oxide formed on the surface of Bi2Se3 crystal does not strongly change the electrical property, because the I−V relation is still linear. By contrast, the current−voltage relationship of the devices made of the plasma-oxidized Bi2Se3 crystals (Au/bulk Bi2Se3/oxidized Bi2Se3/Au) exhibits the rectifying characteristics of the metal−semiconductor− metal (MSM) device structure, as shown in Figure 3(b). It is worth mentioning that the formation of Se vacancies during

Figure 3. Current density (I) vs voltage (V) curves measured at room temperature from (a) pristine Bi2Se3 crystal and native oxide on Bi2Se3 crystal and (b) Bi2Se3 crystals that have been processed in oxygen plasma for 30 s (blue curve) and 60 s (red curve). (c) I−V curves measured at different temperatures from the Bi2Se3 crystal that has been processed in oxygen plasma for 60 s. The inset in (c) is the plot of I/AT2 as a function of the inverse of temperature (1/T) for the current I retrieved at −2.3 V of each temperature.

Bi 2Se3 crystal growth usually results in high electron concentration in the crystal. (The Hall measurement on the Bi2Se3 crystal used in this study shows that the electron concentration in the crystal is as high as 1019 cm−3.) As a result, the Bi2Se3 crystal has the metallic transport behaviors16,17 and forms ohmic contact with the Au electrode. Therefore, the rectifying properties of the I−V curves in the forward and reverse directions in Figure 3(b) are attributed to the formation 3316

DOI: 10.1021/acs.jpcc.5b10425 J. Phys. Chem. C 2016, 120, 3314−3318

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The Journal of Physical Chemistry C of the carrier conduction barriers respectively at the interface between bulk Bi2Se3 and the oxidized Bi2Se3 layer and at the interface between the oxidized Bi2Se3 layer and the Au electrode. In Figure 3(b), the device made of the Bi2Se3 crystal with a shorter plasma treatment (30 s) exhibits a lower threshold turnon voltage. From this dependence of process time, it is suggested that the semiconducting behavior in the MSM structure is given by the amorphous BiOx surface oxide layer, and its thickness determines the turn-on voltage. The optical absorption measurement and the Tauc plot analysis (Figure S2 in Supporting Information) show that the band gap of the plasma-induced oxide layer on Bi2Se3 is ∼2 eV, supporting the semiconducting property of the BiOx surface oxide layer. The Schottky barrier height at the bulk Bi2Se3/oxidized Bi2Se3 interface is of interest. We therefore measure the temperature-dependent current−voltage relationship of the plasma-oxidized Bi2Se3 crystal in the temperature range between 273 and 373 K (Figure 3(c)). The current density at −2.3 V in the reverse bias direction at each temperature is retrieved to calculate the Schottky barrier height at the interface between bulk Bi2Se3 and the oxidized Bi2Se3 layer. On the basis of the thermionic emission equation I = AT2exp(−Φ/kT),18−20 where Φ is the effective barrier height, k the Boltzmann’s constant, and A the Richardson’s constant, the Schottky barrier height (Φ) is estimated to be 0.31 eV. The measured barrier height is reasonable because the valence band maximum of pristine Bi2Se3 is higher than that of the oxidized Bi2Se3 layer (Figure S3 in Supporting Information). Since the surface BiOx oxide layer makes the semiconducting property in the devices of Figure 3(b), it is of interest to know the formation mechanism of the surface oxide layer in the plasma oxidation processes. In Figure 4, the XPS Bi 4f and Se 3d spectra of the Bi2Se3 surface processed with various plasma durations are compared with those of the native oxide on Bi2Se3 and the pristine Bi2Se3 surface. The spectrum of the native oxide contains small peaks of BiOx, SeOx, and elemental Se. For the sample processed in oxygen plasma for only 10 s, strong BiOx (159.2 and 164.5 eV), SeOx (58.8 eV), and elemental Se (55.3 eV) peaks appear in its spectrum. Note that the peak intensities of SeOx and Se are much weaker than those of BiOx. After a prolonged oxygen plasma process, e.g., 5 min, the surface comprises only BiOx; the ratio of Bi content to O is about 1 to 1.43. The presence of elemental Se implies that selenium is segregated when Bi2Se3 is oxidized. The ratio of peak intensity of Se to SeOx is comparable after a short plasma process (≤30 s) but decreases after 30 s. The vapor pressures of Se and SeOx are high,21,22 so that they are relatively easy to vaporize due to heat generated by oxygen plasma bombardment. Eventually, only the BiOx phase remains on the surface.

Figure 4. XPS (a) Bi 4f and (b) Se 3d spectra of the as-cleaved, airexposed, and the Bi2Se3 surfaces exposed to oxygen plasma with various lengths of time 10 s to 5 min. Since some of the peaks in (b) are weak, the intensity is multiplied by a factor labeled on each curve.

properties on the thickness of the BiOx surface oxide layer suggests that using oxygen plasma to create the surface oxide of Bi2Se3 can potentially be a useful route to monolithically fabricate Bi2Se3-based electronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10425. (i) Estimation of the thickness of the amorphous surface layer of Bi2Se3 from XPS depth analysis (Figure S1), (ii) measurement of the band gap of Bi2Se3 surface oxide from reflective UV−vis absorption spectrum (Figure S2), and (iii) determination of the valence band maximum of pristine Bi2Se3 and Bi2Se3 surface oxide using ultraviolet photoemission spectroscopy (Figure S3) (PDF)

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CONCLUSION In contrast to the formation of native oxide, the thickness of the oxidized layer on Bi2Se3 formed in high-density oxygen plasma is controllable. In plasma-induced surface oxidation, Bi2Se3 is decomposed to form BiOx, followed by the evaporation of Se and SeOx. Finally, an amorphous BiOx layer is on the surface. The current−voltage measurements on the oxidized Bi2Se3 surface present the characteristics of metal−semiconductor− metal devices, indicating the formation of the Schottky barrier between bulk Bi2Se3 and oxidized Bi2Se3. The formation of the Schottky barrier opens the possibility of designing novel devices based on Bi2Se3. In addition, the dependence of the electrical

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: +886-2-3366-1311. Notes

The authors declare no competing financial interest. 3317

DOI: 10.1021/acs.jpcc.5b10425 J. Phys. Chem. C 2016, 120, 3314−3318

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ACKNOWLEDGMENTS We acknowledge Dr. Ching-Ping Chang and Dr. Sz-Chian Liou for technical assistance with the TEM analysis. F.C.C., G.J.S., C.W.C., and C.Y.W. acknowledge the support from the Ministry of Science and Technology in Taiwan under Grants MOST-102-2119-M-002-004, MOST-104-2811-M-002-006, NSC 102-2119-M-002-005, and NSC 100-2112-M-002-019, respectively.

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DOI: 10.1021/acs.jpcc.5b10425 J. Phys. Chem. C 2016, 120, 3314−3318