High-quality sandwiched black phosphorus heterostructure and its quantum oscillations — TEI-tagged view

Xiaolong ChenpersName, Yingying WupersName, Zefei WupersName, Yu HanpersName, Shuigang XupersName, Lin WangpersName, Weiguang YepersName, Tianyi HanpersName, Yuheng HepersName, Yuan CaipersName, Ning WangpersName (2015date). “High-quality sandwiched black phosphorus heterostructure and its quantum oscillations.” Nature Communicationstitle·j.

Object of study — few-layer black phosphorusterm encapsulated between hexagonal boron nitrideterm layers

Research question — how to overcome degradation and charge trapsterm·key that limit black phosphorusterm mobilityterm·dom

Contribution — a stable BN–BP–BN heterostructureterm·def with high mobilityterm·dom under ambient conditions

Full text — tagged

High-quality sandwiched black phosphorusterm heterostructureterm·key and its quantum oscillationsterm·key Abstract Two-dimensional materialsterm·key such as grapheneterm and transition metal dichalcogenides have attracted great attention because of their rich physics and potential applications in next-generation nanoelectronic devices. The family of two-dimensional materialsterm·key was recently joined by atomically thin black phosphorusterm which possesses high theoretical mobilityterm·dom and tunable bandgapterm·dom structure. However, degradation of properties under atmospheric conditions and high-density charge trapsterm·key in black phosphorusterm have largely limited its actual mobilityterm·dom thus hindering its future applications. Here, we report the fabrication of stable sandwiched heterostructuresterm·key by encapsulating atomically thin black phosphorusterm between hexagonal boron nitrideterm layers to realize ultra-clean interfaces that allow a high field-effect mobilityterm·dom of ∼1,350 cm 2 V −1 s −1 at room temperature and on–off ratios exceeding 10 5 . At low temperatures, the mobilityterm·dom even reaches ∼2,700 cm 2 V −1 s −1 and quantum oscillationsterm·key in black phosphorusterm two-dimensional hole gas are observed at low magnetic fields. Importantly, the sandwiched heterostructuresterm·key ensure that the quality of black phosphorusterm remains high under ambient conditions.

Two-dimensional (2D) materials with both high mobilityterm·dom and high on–off ratios are in demand for next-generation nanodevices. Grapheneterm 1 , 2 , being the most widely studied material, has shown rich physics and high mobilityterm·dom, but the absence of a bandgapterm·dom 1 , 2 limits its applications. Analogous to 2D transition metal dichalcogenide structures 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , atomically thin black phosphorusterm (BPterm) 12 , 13 , 14 , 15 has recently evoked interest due to its high theoretical mobilityterm·dom 16 , tunable direct bandgapterm·dom 17 , 18 , 19 , 20 and ambipolarity 12 , 13 , 14 , 15 —unique properties that make it suitable for application in electronic and optoelectronic devices 21 , 22 , 23 . Recently, field-effect transistorsterm·dom (FETsterm·dom) 12 , 13 , 14 , 15 , 24 , 25 , 26 , electrically tunable PN junctions 22 , radio-frequency transistors 27 and heterojunctions 28 , 29 based on few-layer BPterm have also been demonstrated. However, the room temperature mobilityterm·dom of few-layer phosphorene reported so far is limited to 400 cm 2 V −1 s −1 due to the presence of high-density charge trapsterm·key, defects and phonon scatteringterm·dom 12 , 13 , 14 , 15 . The quality degradation of BPterm under atmospheric conditions is mainly due to its reaction with O 2 -saturated H 2 O 30 , 31 .

Here we demonstrate a high field-effect mobilityterm·dom of ∼1,350 cm 2 V −1 s −1 at room temperature and high on–off ratios exceeding 10 5 in few-layer phosphorene encapsulated by atomically thin hexagonal boron nitrideterm (BNterm), which forms a stable BN–BP–BN heterostructureterm·def. A room temperature mobilityterm·dom of ∼1,000 cm 2 V −1 s −1 is rare in 2D semiconductor electron gas systems prepared by mechanical exfoliation techniques. In addition, the BNterm layers further avoids quality degradation of BPterm when exposed to atmosphere. At cryogenic temperatures, the mobilityterm·dom even reaches ∼2,700 cm 2 V −1 s −1 and Shubnikov-de Haasterm·dom (SdHterm·dom) oscillations are observed in BPterm at low magnetic fields (∼6 T). Our fabrication and treatment techniques for BN–BP–BN heterostructuresterm·def open up a way to achieve high-performance 2D semiconductors with greatly improved room temperature mobilityterm·dom, on–off ratios and stability under ambient conditions for practical applications in high-speed electronic and optoelectronic devices.

Results Fabrication of the sandwiched BPterm heterostructureterm·key devices To achieve both high mobilityterm·dom and stability of BPterm FETsterm·dom under atmospheric conditions, the BNterm–BPterm–BNterm configuration and high-temperature annealingterm·mth are the two key factors. The ultra-clean BNterm–BPterm interfaces are ensured by adopting the polymer-free van der Waals transfer technique 32 as shown in Fig. 1a . The few-layer BPterm mechanically exfoliated on a 300-nm SiO 2 /Si substrate was first picked up by a thin BNterm flake (6–20 nm thick). Then the BNterm–BPterm sample was transferred to a BNterm flake supported on a SiO 2 /Si substrate to form the BN–BP–BN heterostructureterm·def. The atomically thin BPterm was completely encapsulated by two BNterm layers, thus allowing us to anneal the sample at temperatures up to 500 °C in an argon atmosphere to further improve the sample quality. Without the BNterm protective layers, few-layer BPterm breaks down easily at 350 °C (see Supplementary Note 1 and Supplementary Fig. 1 ). In addition, the annealingterm·mth process can significantly reduce the charge trap densityterm·dom in BPterm as no hysteresisterm·dom effect is observed at room temperature ( Fig. 2g and Supplementary Fig. 2 ).

BN–BP–BN heterostructureterm·def device. ( a ) Schematic of the BN–BP–BN heterostructureterm·def device fabrication process. ( b , e ) Schematic ( b ) and optical image ( e ) of a BNterm–BPterm–BNterm Hall-bar device. Scale bar, 10 μm. ( c ) Atomic structure of monolayer BPterm. ( d ) The BN–BP–BN heterostructureterm·def after O 2 -plasma etching. The etched area is enclosed within the white line. The purple-dashed line denotes the lower BNterm layer.

Mobilityterm·dom and stability of the BN–BP–BN heterostructureterm·def devices. ( a ) The I – V ds curves obtained at different gate voltages at 1.7 K. The inset is the optical image of Sample A with the following geometrical parameters: L 14 =16 μm, L 23 =10 μm and W =3 μm. ( b ) The resistivity determined from four-terminal (green line) and two-terminal configurations (orange line) at 1.7 K. ( c ) Variation of the contact resistivity. ( d ) The conductivity of Sample A measured at with a room temperature and 1.7 K. The inset shows the ambipolarity of the BPterm conductance. ( e , f ) Temperature dependence of the field-effect μ F (open dots) and Hall mobilityterm·dom μ h (solid dots) at V g =−70 V) of Sample A and Sample B (15 nm thick). The dashed lines serve as guidelines for the μ ∼ T − γ relation. ( g ) The room temperature conductivity showing no hysteresisterm·dom in Sample A. ( h ) The mobilityterm·dom and on–off ratio of Sample A as a functions of ambient exposure time. No quality degradation is observed even after exposure for an entire week.

Instead of using one-dimensional edge-contact 32 that has been proven effective for graphene-encapsulated structures (but less so for 2D semiconductors), we developed the area contact technique for contacting the encapsulated 2D semiconductors. Beginning with the BN–BP–BN heterostructureterm·def, a hard mask is defined by the standard electron-beam lithography technique using ZEP-520 resist (detailed process is shown in Supplementary Note 2 and Supplementary Fig. 3 ). Since the O 2 -plasma etching rates for BNterm and BPterm are different, the BNterm layers can be quickly etched while the BPterm layer still survive as shown in Fig. 1d . Lastly, Cr/Au (2 nm/60 nm) electrodes are deposited using electron-beam lithography technique. Figure 1b,e show the schematic and optical image of a BNterm–BPterm–BNterm Hall-bar device respectively. Note that no further annealingterm·mth process is needed after deposition of the electrodes.

Mobilityterm·dom and stability of the sandwiched BPterm devices Figure 2a shows the I – V ds characteristic of an 8-nm-thick BNterm–BPterm–BNterm sample (Sample A). The linear I – V ds characteristic indicates that Ohmic contacts have been achieved on the hole side. By comparing the resistivity directly measured using the two-terminal configuration ( R 14 W / L 14 ) with that extracted using the four-terminal configuration ( R 23 W / L 23 ), the contact resistivity is obtained ( Fig. 2b ). As shown in Fig. 2c , the contact resistivity measured at high gate voltages is about several when the sample temperature is sufficiently low and thermal activation effects are suppressed. The contact resistivity decreases with decreasing temperature when the gate voltage V g <−68 V, further verifying the good contacts between BPterm and Cr/Au electrodes on the hole side.

The BN–BP–BN heterostructureterm·def exhibits high quality as is first confirmed by examining the four-terminal field-effect (FETterm·dom) mobilityterm·dom at different temperatures, where σ is the conductivity and C g is the gate capacitance. Figure 2d shows the conductivity as a function of gate voltage for Sample A. Similar to the results reported previously, both hole and electron conductance can be achieved (see the inset in Fig. 2d ). Here we mainly focus on the hole conductance due to its higher mobilityterm·dom. The gate capacitance C g is calculated to be 1.1 F cm −2 based on the thickness of SiO 2 (300 nm) and the lower BNterm layer (∼15 nm as determined by atomic force microscopy). With this information, we extracted the hole μ F from the linear part of conductance at different temperatures ( Fig. 2e,f ). As shown in Fig. 2d , we achieved a high FETterm·dom mobilityterm·dom μ F ∼1,350 cm 2 V −1 s −1 and Hall mobilityterm·dom μ H ∼790 cm 2 V −1 s −1 for Sample A at room temperature. Another four BN–BP–BN heterostructuresterm·def also exhibit high mobilityterm·dom (∼800 cm 2 V −1 s −1 ) at room temperature (summarized in Supplementary Table 1 ). One possible reason for the high mobilityterm·dom in our BPterm samples is because their transport directions are nearly along the fast X-direction (see Fig. 1c ) as confirmed by angle-dependent polarized Raman spectroscopy (see Supplementary Note 3 and Supplementary Fig. 4 ). The mobilityterm·dom along the X-direction can be two times larger than that along the Y-direction 13 , 15 . The transport-direction of Sample A is parallel to the X-direction. Although the mobilityterm·dom we achieved is still smaller than the values measured from bulk BPterm (∼2,000 cm 2 V −1 s −1 ) 33 , it is much larger than that observed in atomically thin MoS 2 (refs 3 , 4 , 10 ) and few-layer BPterm supported on SiO 2 (refs 12 , 13 , 14 , 15 ). Theoretical calculations for few-layer BPterm 16 indicate that a hole mobilityterm·dom over 4,000 cm 2 V −1 s −1 is achievable along the X-direction at room temperature. Hence, the quality improvement is still the critical limiting factor for the mobilityterm·dom of few-layer BPterm.

The high quality of the BN–BP–BN heterostructureterm·def is further confirmed by the observation of a high on–off ratio exceeding 10 5 and insignificant hysteresisterm·dom at room temperature ( Fig. 2g ), while previous work 14 on BPterm supported on SiO 2 showed a pronounced hysteresisterm·dom (Δ V g ∼80 V) depending on the sweep direction at room temperature, indicating the presence of a high density of charge trapsterm·key. The small charge trap densityterm·dom in our BN–BP–BN heterostructureterm·def is due not only to the ultra-clean BNterm–BPterm interfaces but also to the high-temperature annealingterm·mth (300–500 °C) of the encapsulated BPterm. As shown in Supplementary Fig. 2 , in a BN–BP–BN heterostructureterm·def device without annealingterm·mth, we observed a hysteresisterm·dom Δ V g ∼10 V, which is much smaller than that observed in BPterm supported on SiO 2 . Supplementary Note 4 and Supplementary Fig. 5 demonstrate the mobilityterm·dom changes of Sample D before and after annealingterm·mth. The mobilityterm·dom of this sample measured at cryogenic and room temperatures indeed increases significantly. These results verified that high-temperature annealingterm·mth can effectively suppress charge trapterm·key states and further improve the mobilityterm·dom and on–off ratio of BPterm devices.

The stability of the electrical performance of the BN–BP–BN heterostructureterm·def is examined under ambient conditions (with humidity around 60%). As shown in Fig. 2h , the mobilityterm·dom and on–off ratios show no degradation even after exposure to ambient conditions for a whole week. Obviously, the BN–BP–BN heterostructureterm·def completely hides the BPterm layer from ambient conditions and prevents it from reacting with O 2 -saturated H 2 O. With its high room temperature mobilityterm·dom, on–off ratios and stability, the BN–BP–BN heterostructureterm·def is a promising candidate for practical applications in electronic and optoelectronic devices.

At 1.7 K, the mobilityterm·dom of Sample A reaches ∼2,700 cm 2 V −1 s −1 ( Fig. 2d ) and the Hall mobilityterm·dom is ∼1,500 cm 2 V −1 s −1 at large gate voltages, where n H is the carrier density from Hall measurement, which is equal to the value extracted from the gate capacitance model n = C g ( V g – V th )/ e (see Supplementary Note 5 and Supplementary Fig. 6 ). The on–off ratios exceed 10 8 at 1.7 K. The temperature dependence of the FETterm·dom and Hall mobilityterm·dom of the BN–BP–BN heterostructuresterm·def ( Fig. 2e,f ) shows a similar trend as reported previously for few-layer phosphorene 12 and monolayer MoS 2 (ref. 4 ). The mobilityterm·dom saturates at low temperatures ( T <80 K), but follows the expression μ ∼ T − γ at higher temperatures ( T >80 K) due to phonon scatteringterm·dom. The extracted value γ is ∼0.57 for Sample A and ∼0.69 for Sample B (15 nm thick). These γ values are consistent with the ones observed in monolayer MoS 2 covered with high-κ dielectric 4 and few-layer phosphorene 12 , but are smaller than those found in bulk BPterm 34 and other 2D materials 35 . Similar to the results reported previously 12 , 13 , 15 , the values of the Hall mobilityterm·dom measured from our samples are always smaller than that of their FETterm·dom mobilityterm·dom ( Supplementary Table 1 ). We believe that although charge trapsterm·key are effectively suppressed in our BN–BP–BN heterostructuresterm·def through annealingterm·mth, other types of defects/impurities, such as vacancies and strain induced during the transfer process may still exist. These defects limit the mobilityterm·dom of few-layer BPterm particularly at low temperatures.

Quantum oscillationsterm·key in the sandwiched BPterm devices The quantum oscillationsterm·key observed at magnetic fields B >6 T also attest to the high quality of BN–BP–BN heterostructuresterm·def. Resistance at different gate voltages shows a negative magneto-resistanceterm·dom ( Fig. 3a ), indicating a weak localizationterm·dom effect 36 previously observed in 2D electron gas systems containing disorder. In this kind of 2D electron gas systems, the time-reversal symmetry of the closed electron path is broken when a magnetic field is applied and the back scattering of electrons is suppressed resulting in a negative correction to the resistance. When B >6 T (corresponding to a mobilityterm·dom of ∼1/ B near 1,670 cm 2 V −1 s −1 ), we start to see SdHterm·dom oscillations at higher gate voltages where the Hall mobilityterm·dom becomes sufficiently high ( Fig. 3a ). The oscillation features are seen clearly in the form of d R /d B as shown in Fig. 3b (also see the data of Sample B in Supplementary Note 6 and Supplementary Figs 7 and 8 ).

SdHterm·dom oscillations in the 8-nm-thick BN–BP–BN heterostructureterm·def. ( a ) Resistance ( R ) plotted as a function of the magnetic field at gate voltages of −50 V (blue dots), −60 V (green dots) and −70 V (purple dots). ( b ) d R /d B plotted as a function of 1/ B . The dashed lines indicate the oscillation period (increasing with gate voltages) of 1/ B F . ( c ) Δ R plotted as a function of 1/ B at V g =−60 V for different temperatures. ( d ) Fitting of the experimental results (dots) using the Lifshitz–Kosevich formula (solid line). The inset shows the fitting results of the cyclotron mass. ( e ) Landau diagram at different gate voltages that shows a Berry phaseterm·dom β =0. ( f ) The carrier densities determined from LLterm·dom fitting (orange dots) and gate capacitance (purple line).

The period of SdHterm·dom oscillations 37 , 38 in 2D electron gas is 1/ B F and the longitudinal resistance is given by Δ R = R ( B , T )cos[2 π ( B F / B +1/2+ β )], where R ( B , T ) is the amplitude and β (0≤ β ≤1) is the Berry phaseterm·dom. β is known to be 0 in conventional 2D electron gas systems and 1/2 in grapheneterm 1 , 2 . To examine the 2D nature of hole gas in few-layer BPterm, we plot d R /d B as a function of 1/ B for different gate voltages. As shown in Fig. 3b , the oscillations at different gate voltages are periodic but exhibit different frequencies. A smaller period 1/ B F is observed at higher gate voltages where the carrier density n is larger, which is consistent with the theoretical oscillation period 37 expressed as 1/ B F =2/Φ 0 n , where Φ 0 =4.14 × 10 −15 Tm 2 is the flux quantum. For example, at V g =−50 V the oscillation period is ∼0.0138 T −1 , corresponding to a carrier density of ∼3.5 × 10 12 cm −2 , which is in excellent agreement with the value obtained from the gate capacitance model n = C g ( V g − V th )/ e ( Fig. 3f ). To further reveal the 2D nature of the charge carriers in few-layer BPterm, we carried out experimentsterm·mth to investigate the angular dependence of the SdHterm·dom oscillations. The linear dependence between the magnetic field B F and 1/cos θ confirms that the charge carriers in few-layer BPterm are in 2D nature, where θ is the tilting angle between the applied magnetic fields and the normal direction of the substrate plane (see Supplementary Note 7 and Supplementary Fig. 9 ). The Landau levelterm·dom (LLterm·dom) index N is plotted and the linear fit yields a Berry phaseterm·dom β =0 in our BPterm samples ( Fig. 3e ). A smaller LLterm·dom index would be achieved if magnetic fields could be further increased.

The cyclotron effective massterm·dom m c of hole carriers in few-layer BPterm can be obtained by fitting the temperature dependence of resistivity Δ R (see Fig. 3c ) using the Lifshitz–Kosevich formula A T ∝ T /sinh (2 π 2 k B Tm c / ℏ eB ), where k B is the Boltzmann constant. We fit Δ R at high magnetic fields to gain a better resolution as shown in Fig. 3d . The fitting results produce a cyclotron effective massterm·dom 0.27±0.02 m e at V g =−60 V. Owing to structural anisotropy, the carrier effective mass in few-layer BPterm is also highly anisotropic. Previous theoretical analyses 16 show that the effective masses along the X- and Y- directions are m x ∼0.14 m e and m y ∼0.89 m e respectively. The measured cyclotron effective massterm·dom in our BPterm samples is ∼0.27 m e , which is reasonable as compared with the theoretical value for few-layer BPterm 16 .

Finally, we designed a special sample (Sample C, 10 nm thick) to demonstrate the effects of transport anisotropy on the SdHterm·dom oscillations in BPterm. First, this sample was cut along the X-direction ( Fig. 4a ). The Hall mobilityterm·dom was ∼1,481 cm 2 V −1 s −1 at 1.7 K (∼716 cm 2 V −1 s −1 at room temperature). The SdHterm·dom oscillations are clearly visible in Fig. 4b when the magnetic field is higher than 6 T. Then, the sample was reshaped along the Y-direction (see the inset in Fig. 4a ). The Hall mobilityterm·dom changed to ∼820 cm 2 V −1 s −1 at 1.7 K (∼365 cm 2 V −1 s −1 at room temperature). This experimental results are consistent with the conclusions in previous reports 15 , 33 . However, the magneto-resistivity in this reshaped sample (along the Y-direction) did not show any signature of SdHterm·dom oscillations with magnetic fields up to 8 T. A much stronger magnetic field is needed to observe SdHterm·dom oscillations in the sample.

SdHterm·dom oscillations in the BN–BP–BN heterostructureterm·def samples along the X- and Y- directions. ( a ) The conductivity of Sample C along the X- and Y- directions at 1.7 K and 300 K. The insets are optical images of Sample C before and after reshaping. ( b ) SdHterm·dom oscillations measured at V g =−60 V from the samples along the X- and Y- directions.

Discussion In summary, we have demonstrated high-quality BN–BP–BN heterostructuresterm·def achieved by encapsulating atomically thin BPterm between BNterm layers followed by annealingterm·mth at high temperature. The BN–BP–BN heterostructureterm·def protects BPterm from property degradation and allows us to investigate the intrinsic properties of BPterm. The BNterm–BPterm–BNterm samples show excellent stability in the atmospheric environment with a high mobilityterm·dom ∼1,350 cm 2 V −1 s −1 and a large on–off ratio exceeding 10 5 . Quantum oscillationsterm·key and zero Berry phaseterm·dom are observed in BPterm hole gas at a magnetic field of 6 T at cryogenic temperatures. The ultra-clean interfaces realized by our fabrication process can effectively suppress charge trapterm·key states. They shed important light on how to improve the quality of atomically thin BPterm for practical applications in BP-based nanoelectronic devices.

Methods Sample preparation and transport measurements Bulk phosphorus and BNterm crystals (Polartherm grade PT110) were purchased from Smart-elements and Momentive, respectively. All atomically thin flakes were mechanically exfoliated by the scotch-tape microcleavage method. The thickness of BNterm and BPterm flakes was determined using an atomic force microscope (Veeco-Innova). The BN–BP–BN heterostructureterm·def was fabricated using the polymer-free van der Waals transfer technique 32 . The encapsulated BPterm was annealed in an argon atmosphere at 300–500 °C for 8 h before the upper BNterm layer was etched. No further annealingterm·mth process was needed after deposition of the electrodes. Electrical measurements were performed using lock-in techniques in a cryogenic system (1.7–300 K and magnetic field up to 8 T).

Additional information How to cite this article: ChenpersName, X. et al. High quality sandwiched black phosphorusterm heterostructureterm·key and its quantum oscillationsterm·key. Nat. Commun. 6:7315 doi: 10.1038/ncomms8315 (2015).

Supplementary Material Supplementary Figures 1-9, Supplementary Tables 1, Supplementary Notes 1-7 and Supplementary References.

Acknowledgments Financial support from the Research Grants Council of Hong Kong (Project Nos HKU9/CRF/13G, 604112, HKUST9/CRF/08 and N_HKUST613/12) and technical support of the Raith-HKUST Nanotechnology Laboratory for the electron-beam lithography facility at MCPF (Project No. SEG_HKUST08) are hereby acknowledged.

Footnotes References Associated Data Supplementary Materials Supplementary Figures 1-9, Supplementary Tables 1, Supplementary Notes 1-7 and Supplementary References.