Molecular Beam Epitaxial Growth of Bi2Te3 and Sb2Te3 Topological Insulators on GaAs (111) Substrates: A Potential Route to Fabricate Topological Insulator p-n Junction

High quality Bi2Te3 and Sb2Te3 topological insulators films were epitaxially grown on GaAs (111) substrate using solid source molecular beam epitaxy. Their growth and behavior on both vicinal and non-vicinal GaAs (111) substrates were investigated by reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy. It is found that non-vicinal GaAs (111) substrate is better than a vicinal substrate to provide high quality Bi2Te3 and Sb2Te3 films. Hall and magnetoresistance measurements indicate that p type Sb2Te3 and n type Bi2Te3 topological insulator films can be directly grown on a GaAs (111) substrate, which may pave a way to fabricate topological insulator p-n junction on the same substrate, compatible with the fabrication process of present semiconductor optoelectronic devices.


Introduction
Q2Te3 (Q = Bi, Sb) is a typical V-VI narrow gap semiconductor, having a rhombohedral unit cell that can be considered as groupings of -Te-Q-Te-Q-Teplanes, referred to as quintuple layers (QLs). Within the QL unit, the chemical bond provides the binding force, while the weak van der Waals (vdW) force acts between adjacent QLs. Q2Te3 is a traditional thermoelectric (TE) material (e.g. ZT ≈ 1 for Bi2Te3 at 300 K), [1] that has caused attention in the past decade due to its superior TE performance. Recently, Q2Te3-based research has increased because of the discovery of a new state of matter known as a three-dimensional (3D) "topological insulator" (TI). This material is insulating in bulk with a finite band gap but possesses a gapless surface state protected by time reversal symmetry (TRS), [2] which has been confirmed by angle-resolved photoemission spectroscopy (ARPES) [3][4][5] and transport measurements. [6][7][8][9][10] The emergence of TIs has brought an increase in the study of exotic quantum physics, such as Majorana fermions and magnetic monopoles, [11][12][13] which may pave the way for quantum computation applications. These investigations are also accelerating the development of spintronics due to the spin helical structure of the TI surface state. [14] Additionally, a very recent prediction shows that TI may be a promising candidate for use in high performance photodetectors in the terahertz (THz) to infrared (IR) frequency range because of high absorbance. [15] With so many potential applications, it has become important to understand how to fabricate these materials. For example, molecular beam epitaxy (MBE) is one way to grow high quality TIs due to the advantages of ultra-high vacuum (UHV) background pressure and precise control of growth parameters. [5] In this work, we have systematically investigated the fabrication of high-quality Q2Te3 films by MBE on GaAs substrates. Up to now, Si, SrTiO3, sapphire, and graphene have already been used as substrates for the growth of Q2Te3 films by MBE. [3,16,17,18] There have also been a few investigations on the MBE growth of Q2Te3 on a GaAs substrate. For example, Liu et al. demonstrated epitaxial growth of both Bi2Te3 and Bi2Se3 films on GaAs (100) directly by van der Waals epitaxy (vdWe). [19] They reported that rotation domains formed in the TIs films due to the lattice symmetry mismatch between the hexagonal lattices of Bi2Te3 and the cubic symmetry of the GaAs (001) surface. [19] The vdWe growth method has also been used for the growth of Bi2Se3 on a vicinal Si (111) surface. [20] [22] In this paper, we investigate the growth and behavior of Q2Te3 on both vicinal and non-vicinal GaAs (111) (V-GaAs (111) and Nv-GaAs(111)) substrates by reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Together with Hall and magnetoresistance (MR) measurements, our observations indicate that p type Sb2Te3 and n type Bi2Te3 topological insulator films can be directly grown on a GaAs (111) substrate, enhancing the opportunity to fabricate topological insulator p-n junctions. [23] 2. Experimental A Riber 32P solid source MBE system was used to grow Q2Te3 films on epi-ready GaAs (111) substrates. The growth chamber is equipped with a RHEED (Staib) and a transmission optical system (kSA BandiT) monitoring the substrate band edge to give accurate growth temperature down to 150 °C. High purity 6N Bi, Sb and Te sources were evaporated to provide the beam fluxes for the film growth with a ratio of about 1:12. The morphology of as-grown films were quenched by turning off the manipulator heater right after the growth, and a Veeco ambient AFM was used to characterize the sample surface. High resolution XRD (HRXRD) was performed on a PANalytical X'Pert MRD system equipped with a parabolic mirror and PIXcelTM detector. The interface was characterized on FEI Titan 80-300 TEM. The Hall bar structure for transport measurement was fabricated using standard photo-lithography method (see Supporting Information for more details). The Q2Te3 films were grown on GaAs (111)A and GaAs (111)B substrates. As a result, two kinds of different RHEED evolutions were observed during growth due to the difference between A and B planes of GaAs (111) (see Supporting Information for more details). As a comparison, both V-GaAs (111) and Nv-GaAs(111) substrates were used as substrates for growth of Q2Te3 films. This is motivated by the fact that a vicinal Si (111) substrate was observed to improve the quality of Bi2Se3 epitaxial films grown by vdWe. [20] After deoxidizing GaAs substrates at 610 °C, a two steps growth method, i.e. a high temperature GaAs buffer layer growth at 590 °C and a low temperature Q2Te3 film growth at 250 °C, was performed. More than ten kinds of different samples were investigated, and here we will show the data from the six kinds of samples as shown in table 1.  Figure 1 shows a set of AFM images, which reveal the different growth modes of Q2Te3 films observed on both V-GaAs (111) and Nv-GaAs(111) substrates. Figure 1a is the AFM image of sample A (200 nm Bi2Te3/V-GaAs (111)A-3°). The big terraces and steps run nearly parallel. In most areas, about 1 µm wide terraces and 35 nm high steps can be found. The root-mean-square (RMS) roughness is about 8 nm for 5×5 µm 2 area. Furthermore, small steps with height of about 1 nm were observed on the flat terrace as seen in Figures 1(b-c) (500×500 nm 2 ), consistent with a single QL thickness for each step. This surface morphology covers the entire sample surface based on more AFM measurements at the different positions on the surface of sample A. Sample B (400 nm Sb2Te3/V-GaAs (111)A-3°) shows the similar surface morphology as shown in Figure 1d. Obviously, this is a typical step-bunched surface, [24] which forms with the big steps and terraces through a step-flow growth mode. Such surface morphology is very different compared to reported result of spiral growth. [20,25] In order to avoid concerns over variation in the thickness of GaAs buffer layer or Q2Te3 film, both samples C (30 nm Bi2Te3/V-GaAs (111)A-3°) and D (30 nm Bi2Te3/Nv-GaAs (111)A) were grown on the same molybdenum block simultaneously using the same thick GaAs buffer layer (3 nm) and Q2Te3 (30 nm) films. As shown in Figure 1e, sample C still shows an initial step-bunched surface. However, spiral growth was observed for sample D as shown in Figures 1 For the as-grown Q2Te3 film, AFM investigations indicate that a V-GaAs (111) substrate is conducive to the formation of a step bunched surface through the tendentious step-flow growth induced by the high density of steps providing the high energy barrier to grow over the steps in its surface. [26] Nv-GaAs (111) substrates sustain a smooth surface during spiral growth or partial step-flow growth due to the lower step density associated with non-vicinal substrate surface (see more AFM investigation results in the Supporting Information).

Structural studies with X-ray diffraction and transmission electron microscopy
To determine the crystal quality and epitaxial orientation of as-grown Q2Te3 films, XRD was performed. distinctly demonstrated the tilted angle of 0.055°, 0.046° and 0.002° between Q2Te3 (00,18) and GaAs (222), for samples F, C and D, respectively, indicating higher c-axis epitaxial orientation than vdWe on a vicinal substrate. [21] For these three samples, Sample D grown on non-vicinal substrate has a better c-axis orientation than samples C and F. The RSM result of non-symmetric planes of GaAs (440) and Bi2Te3 (2 � 2,27) for sample D in Figure 2h presents the in-plane lattice mismatch between Bi2Te3 film and GaAs (111) substrate is around 9.3%, which is consistent with the RHEED observation of small critical thickness. Besides, the peak of sample F grown on vicinal substrate with thick GaAs buffer layer is much broader than that of samples C and D, indicating worse crystalline quality. The substrate miscut dependence of the tilted orientation angles for these three samples proves that as-grown Q2Te3 films are sensitive to the surface lattice orientation of the substrates and V-GaAs (111) substrates can reduce their crystalline quality through the unfavorable step bunching process. To further clarify the effect of V-GaAs (111) substrates on the crystal quality of as-grown Q2Te3 film, we investigated the interface between Q2Te3 films and GaAs buffer layer for these three samples by HRTEM. Figure 3(a) gives the HRTEM images of sample F. It is obvious that the interface is disordered and a lot of defects were formed in both the GaAs buffer layer and Sb2Te3 film. Furthermore, the fast Fourier transform (FFT) analysis near the interface confirmed the RSM result that there is a small tilted angle of about 0.055° between the Sb2Te3 film and GaAs (111) substrate. For sample C, a similar result is observed in Figure 3(b), i.e. disordered interface and tilted orientation, except for that fewer defects were formed due to the thinner GaAs buffer layer. In contrast, both a sharp interface and nearly parallel orientation were found for sample D, as shown in Figures 3(c-d). Based on these observations, we can conclude that an Nv-GaAs (111) substrate is better to use than a vicinal substrate in order to fabricate high quality Q2Te3 films on GaAs (111) substrate.

Transport measurements of Q2Te3 thin films
In order to demonstrate the topological insulator behavior of as-grown Q2Te3 films, transport measurements were also performed. For our samples, all the Sb2Te3 films show p type conducting behavior, whereas, all the Bi2Te3 films show n type conducting behavior. Figure 4 shows the typical transport measurement results of Q2Te3 films on GaAs (111)B (see the transport data for Q2Te3 films on GaAs (111)A in Supporting Information). Based on the linear behavior of Hall resistance vs. magnetic field (Rxy-H) below 30 kOe, the carrier density and mobility can be estimated as 7.81×10 18 cm -3 and 959 cm 2 /Vs, 9.13×10 18 cm -3 and 781 cm 2 /Vs for samples E (200 nm Bi2Te3/V-GaAs (111)B-3°) and F (400 nm Sb2Te3/V-GaAs (111)B-3°), respectively, which are typical parameters in TI films. [3,32] Nonlinear Hall behavior was observed over 30 kOe for sample E, as shown in Figures 4(a), which is likely an indication of more than one transport channel (surface and bulk) in the sample. [28] Magnetoresistance (MR) measurements were performed for three different directional fields up to 80 kOe. In the perpendicular field (H), the R(H)/R(0) increases from 1 to 1.73 and 1.56 for samples E and F at 1.8 K, respectively, over the range of 0 to 80 kOe; linear MR was observed above 20 kOe, which may be attributed to the quantum linear MR (QLMR) of the surface states, [27] as shown in Figures 4(c-d). For the parallel field situation (Figures 4(e-f)), R(H)/R(0) increases to 1.15~1.30 at 80 kOe, which are smaller than that in perpendicular field. In a close-up view as shown in Figure 4g, a clear magnetoconductance peak ( σ ∆ ) was observed for sample E in small perpendicular field regime, which can be attributed to the weak anti-localization effect (WAL). [8,21,[29][30][31][32][33] According to the HLN theory [34] ( ) ( ) . Meanwhile, the temperature dependence of normalized resistance (R/Rmin) for sample F has an upturn below 4 K with field 0 kOe and 20 kOe, as shown in Figure 4h, which is reminiscent of the electron-electron interaction in TI films. [8]

Conclusions
In summary, we have successfully demonstrated the MBE growth of Q2Te3 topological insulator films directly on GaAs (111) substrates. Similar to the results for Bi2Se3 grown on graphene/SiC(0001)reported by Liu et al., [25] spiral growth mode was observed for Q2Te3 films on Nv-GaAs (111)A substrate too. However, an step-bunched surface was observed for Q2Te3 films on V-GaAs (111)A substrates. Both XRD and HRTEM results indicate that the Nv-GaAs (111) substrate is better than a vicinal substrate to provide high quality Q2Te3 films. The tilted angle between Q2Te3 film and GaAs (111) substrate may be related to the misfit dislocation with a nonzero net out-of-plane Burgers vector component since Nagai's steps model is not applicable here due to the large lattice mismatch, [35,36] and more interface investigations need to be done to establish the tilting mechanism in this kind of high-misfit heteroepitaxial systems. The observation of linear MR and WAL provides proofs for the possible TI behavior of as-grown Q2Te3 films. [10,27] Hall effect measurements show that unintentional doping happens to as-grown Q2Te3 films: Sb2Te3 films always show p type conducting behavior, whereas, Bi2Te3 films always show n type conducting behavior, creating the potential to fabricate topological p-n junction on the same GaAs (111) substrate, which may offer a new platform to realize exciton condense in the interface. Further, our study paves a way to integrate TI-based devices with present semiconductor optoelectronic devices, generating brand-new multifunction devices.

Molecular Beam Epitaxial Growth of Bi 2 Te 3 and Sb 2 Te 3 Topological Insulators on GaAs (111) Substrates: A Potential Route to Fabricate Topological Insulator p-n Junction
Zhaoquan Zeng Figure 1S shows the typical RHEED evolution for Bi2Te3 films grown on GaAs (111)A substrates. Figure  1S(a-b) show the RHEED patterns of GaAs (111)A surface after deoxidizing and growth of GaAs buffer layer. After several minutes at deoxidizing temperature of 590 °C, the 2×2 reconstruction gradually appears, and it exists during the whole GaAs buffer layer growth. The clear 2×2 reconstruction and sharp streak indicate that a typical GaAs (111)A clean surface is formed [1]. While the substrate was cooled down to 250 °C, the 2×2 RHEED pattern disappeared, and 1×1 pattern appeared ( figure 1S(c-d)). It can be attributed to the predeposition of Te, providing a Te rich growth condition for Bi2Te3. When the Bi shutter was open, the sharp streak became dim and shrank inside slowly (not shown here), which indicated the epitaxial growth of c-axis oriented Bi2Te3 film. As Bi2Te3 grew, the streaky RHEED patterns became sharp, indicating a flat surface. After epilayer growth for scores of nanometers, a set of sharp streaky pattern with clear Kikuchi lines was observed (figure 1S(e-f)), suggesting good crystallinity. It should be mentioned that the 1×1 surface persisted during the whole epilayer growth. Comparing the RHEED patterns of GaAs (111)A and Bi2Te3 epitaxy film, it can be concluded that relaxed Bi2Te3 (001) film has been grown on GaAs (111)A surface with an overlapped in-plane epitaxial relationship of Bi2Te3  Figure 2S shows the typical RHEED evolution for Bi2Te3 grown on GaAs (111)B substrates. Figure 2Sa shows the RHEED patterns of GaAs (111)B surface right after deoxidization at 590 °C, a typical √19×√19 reconstructed surface of GaAs (111)B at high temperature [1] (the other direction is not shown here). During cooling down to 250 °C, a transition from √19×√19 to 2×2 happened as shown figure 2Sb. Once Te shutter was open, the 1×1 surface appeared as figure 2Sc. After scores of nanometers epitaxial growth of Bi2Te3 film, a set of RHEED pattern with Kikuchi lines appeared as figure 2Sd, indicating good crystal quality.

More AFM Investigations:
In order to further clarify the effect of the GaAs (111) surface on the growth mode of as-grown Q2Te3 films, more samples were grown, as shown in Table 1S. Figure 3S shows additional AFM results on the morphology of thicker Q2Te3 films grown on both V-GaAs (111)A and Nv-GaAs(111)A substrates. Figures 3S(a-c) are the AFM images of sample G, and it is very clear that the QL steps with a height of around 1 nm were formed over the whole surface, indicating that spiral growth mode can be maintained during the epitaxial growth of Q2Te3 film. Figure 3S(d-f) are the AFM images of sample H, and big terraces and steps were formed on the surface due to the step-flow growth mode. Even for samples I and J, with smaller miscut angle (2° and 1°), the step-bunched surface morphology was still kept, as shown in figures 3S(g-l). Figure 3S. AFM images of surface morphology for as-grown Q2Te3 films. a) Sample G in 5×5 µm 2 , b) Sample G in 500 nm×500 nm, c) the line profile corresponding to the line in b), d) Sample H in 5×5 µm 2 , e) Sample H in 2×2 µm 2 , f) the line profile corresponding to the line in e), g) Sample I in 5×5 µm 2 , h) Sample I in 2×2 µm 2 , i) the line profile corresponding to the line of h), j) Sample J in 5×5 µm 2 , k) Sample J in 2×2 µm 2 , l) the line profile corresponding to the line in k).

Hall Bar
All the transport measurements are based on the same Hall bar structure as shown in Figure 4S. Figure 4Sa is the schematic diagram (not proportional), and Figure 4Sb is the corresponding optical microscopy image. The standard photo-lithography method was used for fabricating the Hall bar. [2] Positive photoresist S1813 was spun at 4000 rpm for 45 seconds on Q2Te3 films, followed by 110 ℃ baking for 60 seconds. With a mask of Hall bar pattern, the photoresist-coated sample was exposed to ultraviolet light (365 nm wavelength) with exposure power of 8 mW/cm 2 for 7 seconds. The exposed part of the photoresist was removed after 40 seconds of developing (MF CD-26 developer). Then, bare area of Bi2Te3 film with no photoresist was wet-etched with 1 g of potassium dichromate in 10 ml of sulfuric acid and 520 ml of deionized (DI) water, whereas, diluted nitric acid (HNO3: H2O = 1:1) was used for Sb2Te3 film. The expected etching rate for both Bi2Te3 and Sb2Te3 films is about 10 nm per minute, and DI water rinsing is needed.  Figure 5S shows the typical transport data for Q2Te3 films on GaAs (111)A substrates. Low temperature Hall measurements indicates that samples B and D show p and n type conductivity, respectively, which support the conclusion that all the as-grown Sb2Te3 films show p type conducting behavior and all the as-grown Bi2Te3 films show n type conducting behavior. Similar to the case of Q2Te3 films on GaAs (111)B, nonlinear Hall behavior was also observed over 20 kOe for both samples B and D at 1.8 K, as shown in figures 5S(a-b), which is likely an indication of more than one transport channel (surface and bulk). Based on the linear behavior below 20 kOe, the carrier density can be estimated as 3.81×10 19 cm -3 and 2.04×10 19 cm -3 for samples B and D, respectively. Compared to the result in Figure 4h, a stronger upturn was observed for sample B with fields 20 and 80 kOe, as shown in Figure 5Sc. This is also observed for sample D below 10 K as show in figure 5Sd. Figures 5Sc and5Sd show the normalized resistance upturn, which is enhanced by applying perpendicular magnetic field. The behavior is reminiscent of the electron-electron interaction in TI films. [3] Additionally, the WAL induced magnetoconductance peak at small field was also observed for both sample B and D, as shown in Figures  5S(e-f). [3][4][5][6][7] It should be noted that the magnetoconductance peak was well fitted by HLN function. [3] For Sb2Te3 the fitting yields  Figure 5S e-f respectively, suggesting its quantum origin [3]. The MR for three orientations of magnetic field was also measured at 1.8 K for both sample B and D, and it is obvious that the MR for perpendicular field is much higher than those for parallel fields, as shown in Figures 5Sg-h. A near parabolic MR was observed for sample B in perpendicular magnetic field, and it is suppressed when the field is parallel to the current. However, a linear MR was observed for sample D with perpendicular field above 30 kOe, which may be attributed to the QLMR of the surface states [8]. Figure 5S. Transport properties of as-grown Q2Te3 films on GaAs (111)A substrates. a,b) Hall resistance versus magnetic field (black solid line) at 1.8 K for samples B and D, respectively, and the red solid line is the linear fit, c) Temperature dependence of normalized resistance(R/Rmin) at H = 0 kOe, 20 kOe and 80 kOe for sample B, where Rmin is the minimum value of resistance, d) ) Temperature dependence of normalized resistance(R/Rmin) at H = 0 kOe, 4 kOe and 8 kOe for sample D, e) Normalized magnetoconductance in perpendicular magnetic field at T=1.8K for sample B. The red solid line is the fitting curve with HLN equation, f) Normalized magnetoconductance in perpendicular magnetic field at T=1.8K for sample D. The red solid line is the fitting curve with HLN equation, g,h) MR change in both perpendicular and parallel magnetic fields configuration for samples B and D at 1.8 K, and the violet line is the linear fit. We use three different