- Nano Express
- Open Access
Direct Growth of Al2O3 on Black Phosphorus by Plasma-Enhanced Atomic Layer Deposition
Nanoscale Research Letters volume 12, Article number: 282 (2017)
Growing high-quality and uniform dielectric on black phosphorus is challenging since it is easy to react with O2 or H2O in ambient. In this work, we have directly grown Al2O3 on BP using plasma-enhanced atomic layer deposition (PEALD). The surface roughness of BP with covered Al2O3 film can reduce significantly, which is due to the removal of oxidized bubble in BP surface by oxygen plasma. It was also found there is an interfacial layer of PO x in between amorphous Al2O3 film and crystallized BP, which is verified by both X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. By increasing temperature, the PO x can be converted into fully oxidized P2O5.
Two-dimensional (2D) semiconductor materials, such as graphene [1, 2], MoS2 [3, 4], WSe2 , WS2 , MoTe2 [7, 8], SnSe2 , and black phosphorous (BP) [10,11,12,13,14,15,16,17,18], have been widely studied for the potential applications in the next generation devices including field emitters [10,11,12], gas sensors [13, 14], solar cells , field-effect transistors , optoelectronics , and light-emitting diodes . Among these 2D materials, BP is found to be more thermodynamically stable under the ambient conditions . BP is an anisotropic lamellar semiconductor and has a direct band gap of ~0.3–1.5 eV from the bulk to monolayer structure [21,22,23,24,25]. BP transistors also have high carrier mobility up to 1000 cm2/(V · s) at room temperature, on/off current ratio over 104 [18, 26]. Thus, BP may be a promising candidate for electronic and optoelectronic device applications . However, exfoliated BP films will degrade in the air ambient owing to the possible reactions between BP and the adsorbed water and oxygen, thus leading to a significant reduction of BP carrier mobility and on/off current ratio [27, 28]. Therefore, efficient and reliable isolation/passivation layers are necessary for BP to preserve its inherent structure and property.
So far, many efforts on passivation for BP have been made, such as an encapsulation layer for BP with boron nitride (h-BN) [29,30,31,32,33], creating saturated P2O5 on BP surface [34,35,36], atomic layer deposited dielectric capping [27,38,39,, 37–40]. Nevertheless, h-BN passivation requires complicated environmental conditions and has extremely low yield [29,30,31,32,33]. The P2O5 which was created on BP surface provides only the short-time protection since the oxygen and moisture in the air can erode it slowly [34,35,36]. It is also quite tough for atomic layer deposition (ALD) to form a high-quality and uniform top dielectric film on BP because of no dangling bonds. Therefore, it is important to prevent BP-based devices from degradation in the air ambient by covering a protective oxide dielectric. Moreover, uniform and reliable dielectrics are also essentially needed for the top-gate BP devices.
In this work, uniform Al2O3 was directly grown on BP flakes using plasma-enhanced atomic layer deposition (PEALD) with the help of O2 plasma as an oxygen precursor, instead of H2O, to react with trimethylaluminum (TMA) . The composition and properties of the interfacial layer between Al2O3 and BP have been examined by physical characterizations, and the mechanisms behind are analyzed.
Few-layer BP (purity: 99.998%, Smart Elements) was transferred onto a Si substrate with thermally grown 285 nm SiO2 using a micromechanical method with polydimethylsiloxane (PDMS) [1, 28, 42]. Prior to transfer, the SiO2 surface was ultrasonically cleaned in turn by acetone and isopropyl alcohol (IPA) and piranha solutions for 10 min each, followed by 100% O2 annealing at 500 °C for 3 min using rapid thermal annealing (Annealsys As-One). The optical images of BP were acquired by an optical microscope (BA310Met, Motic) equipped with a camera. Raman spectroscopy measurements were performed using LabRam-1B (the Raman spectral resolution was 1.1 cm−1) with an excitation wavelength of 532 nm at room temperature in the air ambient. The laser power was maintained at around 0.5 mW to prevent any heating-induced damage during the measurement. Al2O3 films on BP were deposited using PEALD with O2 plasma and TMA precursors at different temperatures. The freshly exfoliated BP samples were transfer to Picosun 200R ALD chamber (the vacuum pressure was 12 hPa). PEALD of Al2O3 was carried out with successive cycles of O2 plasma and TMA precursors, with an Ar carrier gas (99.9997%, Airgas) at a flow rate of 300 sccm, 15 s pulse + 10 s Ar purge time for O2 plasma (The O2 plasma RF Power was 2000 W), 0.1 s pulse + 5 s Ar purge time for TMA (the precursor temperature was 18 °C) at a substrate temperature of 200 °C. The surface and interfacial properties of Al2O3 on BP were physically characterized using atomic force microscopy (AFM, Dimension Edge, Bruker), XPS (AXIS ULDLDTRA, Shimadzu), and TEM (Tecnai G2 F20 S-TWIN, FEI) measurements at room temperature.
Results and Discussion
Figure 1a shows the optical image of transferred BP sample prepared by mechanical exfoliation from its bulky crystalline. The Raman spectra of thin-layer BP, as denoted by a red circle in Fig. 1a, were examined as a function of exposure time in the air ambient at room temperature, as shown in Fig. 1b. It is noted that all Raman spectra measured in Fig. 1b are calibrated using a Si peak of 520 cm−1. It can be clearly seen one out-of-plane modes (A1g) and two in-plane modes (A2g and B2g) in thin-layer BP . Both A2g and B2g peak positions keep almost unchanged. While for A1g mode, it has redshifted as the exposure time goes up to 30 min and then seems to be stable up to 20 h. This may be attributed to the oxidation of surficial BP in the initial stage and a relatively saturation of oxidation up to 20 h. This is evidenced by the time evolution of BP surface morphology examined by optical microscopy, as shown in Fig. 2a–f. It was markedly observed that BP flake exposed to the air ambient degrades as the exposure time extended and then exhibited a fare rough BP surface with bubbles, as presented in Fig. 2d–f. Figure 2g–i shows AFM images of exfoliated BP flake exposed to the air ambient for 2, 3, and 4 h, respectively. All three BP samples for AFM measurements were taken from the same batch and their RMS roughness is summarized in Fig. 2j. The RMS roughness of BP surface increases as the exposure time increases, indicating the formation of oxidative phosphorus species.
To evaluate the surface quality of Al2O3 film on thin-layer BP, its roughness of RMS was compared quantitatively at different deposition temperatures, as shown in Fig. 3. The corresponding data were summarized in Table 1. Note that more than five samples were measured for each temperature. It was observed that prior to PEALD, the average RMS of thin BP surface is larger than 6 nm, large scattering is due to sample-to-sample variations; however, the RMS reduces to ~3 nm after 100 cycles Al2O3 deposition. This infers that O2 plama as an oxygen precursor can effectively etch the oxidized bubbles in the top-layer BP thin film, thus leading to a significant reduction of RMS, while H2O as an O source may not have this benefit (discuss later).
To understand the impact of the pretreatment of O2 plasma and different oxygen precursors on Al2O3 growth in freshly exfoliated BP samples, Al2O3 deposition on BP was realized by three approaches: (1) 20 cycles O2 plasma pretrement + 100 cycles TMA/O2 plasma, (2) 100 cycles TMA/O2 plasma, and (3) 100 cycles TMA/H2O, as shown in Fig. 4a, b, and c, respectively. Figure 4a, b depicts AFM images of the 100 cycles Al2O3 grown on BP samples by PEALD with and without an oxygen plasma pretreatment, respectively. Using PEALD for Al2O3 growth in BP flakes, it has demonstrated a highly uniform surface morphology of Al2O3/BP. The average RMS roughness of Al2O3/BP samples prepared by PEALD is only 0.4 nm regardless of an oxygen plasma pretreatment, as shown in Fig. 4a, b. For freshly exfoliated BP samples, PEALD (with and w/o pretreatment) can achieve a good uniformity and coverage of Al2O3 films. While for BP samples exposed to the air ambient for certain time, 4 h for example, PEALD with O2 plasma pretreatment is much preferred. O2 plasma pretreatment can create enough nucleation sites for ALD growth. On the other hand, it also has an “etching” effect for thinning BP samples. O2 plasma may penetrate the PO x layer and oxidize the underlying BP, then increase the thickness of PO x layer . On the contrary, Al2O3 films on freshly exfoliated BP grown by ALD with H2O as an oxygen precursor nucleate to an isolated “island” and exhibit a remarkable nonuniform surface profile, resulting in a large RMS roughness of 0.8 nm, as shown in Fig. 4c. It is attributed to the insufficient dangling bonds or nucleation sites in BP surface for ALD growth with H2O as an oxygen precursor . It is worthwhile to mention that BP flake was covered uniformly by Al2O3 film and can prevent O2 or H2O in air ambient further reacting with BP, thus protected BP from degradation. Otherwise, the uncovered portions of BP surface may react with H2O and O2 to produce many bubbles, as shown in Fig. 4c.
Next, chemical analysis of the interfacial characteristics near BP film was examined by XPS characterizations. Figure 5 shows photoelectron spectroscopy measurements of the P 2p core level at different deposition temperatures. Middle and top panels present the P 2p core level after growth of 100 cycles Al2O3 by PEALD at 250 and 350 °C, respectively. The peaks which labeled as P1a and P1b correspond to P-P bonds. The phosphorus core level contains only the characteristic doublet representing the P 2p3/2 (P1a) and P 2p1/2 (P1b) orbitals with peak positions of 130.06 eV and 130.92 eV, respectively, consistent with the report . Two peaks of P2 and P3 were observed at 134.07 and 135.03 eV, corresponding to the PO x and P2O5, respectively, as shown in the middle panel. It can be seen that the PO x peak locates at 134.07 eV, smaller than that of reported P2O3 (134.2 eV) , which may be caused by less O concentration in the interfacial PO x layer. The P3 represents the most dominant oxide component and appears at 135.03 eV, in good agreement with the reported P2O5 binding energies which are between 135.0 eV  and 135.15 eV . When the temperature goes up to 350 °C, interestingly, P2 peak disappears. This is due to the conversion from PO x to P2O5, with the help of reactivity of O2 plasma at high temperatures. However, there is no P3 peak for natively oxidized BP at room temperature and its peaks of P1a; P1b and P2 locate at 130.06 eV(P 2p3/2), 130.87 eV(P 2p1/2), and 134.05 eV, respectively. The absence of P3 peak is due to low temperature or insufficient exposure time for the formation of fully oxidative top layer, which may prevent PO x from converting into P2O5 film.
Finally, the interface properties of Al2O3/BP samples were also characterized by TEM measurements. It can be clearly seen for Fig. 6a that the interfacial PO x layer between Al2O3 and BP was formed during PEALD process with the 20 cycles O2 plasma pretreatment. Figure 6b shows high-resolution TEM (HRTEM) image of the Al2O3/BP sample after the deposition of 100 cycles Al2O3, same scanned region marked by a red square in Fig. 6a. The thickness of PO x and Al2O3 is 6.1 and 10.7 nm, respectively. It is worth noting that Al2O3 and PO x film is amorphous, while our BP sample is single crystalline which is verified by results of selected area electron diffraction (SAED) pattern, as seen from Fig. 6c. This interfacial layer PO x was evidenced by TEM results, indicative of O2 plasma penetrating into PO x layer and reacting with underlying BP.
In summary, we have demonstrated the direct growth of Al2O3 film on BP by using PEALD. The A1g peak of freshly exfoliated BP sample shifts downwards owing to the formation of PO x in the BP surface. The uniform Al2O3 film on BP can be achieved by PEALD with O2 plasma and TMA precursors, which may be attributed to the etching and reactivity of O2 plasma with BP at high temperatures. The interfacial layer of PO x between Al2O3 and BP was converted into P2O5 as the deposition temperature increases to 350 °C, revealed by XPS characterizations. These findings provide insightful information on passivation and top-gate dielectric integration for future applications in BP devices.
Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV et al (2004) Electric field effect in atomically thin carbon films. Science 306:666–669
Patil UV, Pawbake AS, Machuno LG, Gelamo RV, Jadkar SR, Late DJ et al (2016) Effect of plasma treatment on multilayer graphene: X-ray photoelectron spectroscopy, surface morphology investigations and work function measurements. RSC Adv 6:48843–48850
Radisavljevic B, Radenovic A, Brivio J, Giacometti IV, Kis A (2011) Single-layer MoS2 transistors. Nat Nanotechnol 6:147–150
Late DJ, Liu B, Matte HR, Dravid VP, Rao CNR (2012) Hysteresis in single-layer MoS2 field effect transistors. ACS Nano 6:5635–5641
Liu B, Ma Y, Zhang A, Chen L, Abbas AN, Zhou C et al (2016) High-performance WSe2 field-effect transistors via controlled formation of in-plane heterojunctions. ACS Nano 10:5153–5160
Ovchinnikov D, Allain A, Huang YS, Dumcenco D, Kis A (2014) Electrical transport properties of single-layer WS2. ACS Nano 8:8174–8181
Pradhan NR, Rhodes D, Feng S, Xin Y, Memaran S, Balicas L et al (2014) Field-effect transistors based on few-layered α-MoTe2. ACS Nano 8:5911–5920
Late DJ (2016) Temperature-dependent phonon shifts in atomically thin MoTe2 nanosheets. Appl Mater Today 5:98–102
Pei T, Bao L, Wang G, Ma R, Yang H, Gao HJ et al (2016) Few-layer SnSe2 transistors with high on/off ratios. Appl Phys Lett 108:053506
Erande MB, Suryawanshi SR, More MA, Late DJ (2015) Electrochemically exfoliated black phosphorus nanosheets—prospective field emitters. Eur J Inorg Chem 2015:3102–3107
Suryawanshi SR, More MA, Late DJ (2016) Exfoliated 2D black phosphorus nanosheets: field emission studies. J Vac Sci Technol B: Nanotechnol Microelectron: Mater Process Meas Phenom 34:041803
Suryawanshi SR, More MA, Late DJ (2016) Laser exfoliation of 2D black phosphorus nanosheets and their application as a field emitter. RSC Adv 6:112103–112108
Late DJ (2016) Liquid exfoliation of black phosphorus nanosheets and its application as humidity sensor. Microporous Mesoporous Mater 225:494–503
Erande MB, Pawar MS, Late DJ (2016) Humidity sensing and photodetection behavior of electrochemically exfoliated atomically thin-layered black phosphorus nanosheets. ACS Appl Mater Interfaces 8:11548–11556
Late DJ (2015) Temperature dependent phonon shifts in few-layer black phosphorus. ACS Appl Mater Interfaces 7:5857–5862
Pawbake AS, Erande MB, Jadkar SR, Late DJ (2016) Temperature dependent Raman spectroscopy of electrochemically exfoliated few layer black phosphorus nanosheets. RSC Adv 6:76551–76555
Dai J, Zeng XC (2014) Bilayer phosphorene: effect of stacking order on bandgap and its potential applications in thin-film solar cells. J Phys Chem Lett 5:1289–1293
Li LK, Yu YJ, Ye GJ, Ge QQ, Chen XH, Zhang YB et al (2014) Black phosphorus field-effect transistors. Nat Nanotechnol 9:372–377
Wang QH, Kalantar-Zadeh K, Kis A, Coleman JN, Strano MS (2012) Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 7:699–712
Xia F, Wang H, Xiao D, Dubey M, Ramasubramaniam A (2014) Two-dimensional material nanophotonic. Nat Photonics 8:899–907
Jamieson JC (1963) Crystal structures adopted by black phosphorus at high pressures. Science 139:1291–1292
Xia F, Wang H, Jia Y (2014) Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics. Nat Commun 5:4458–4463
Liu XK, Ang K-W, Yu WJ, He JZ, Feng XW, Liu Q et al (2016) Black phosphorus based field effect transistors with simultaneously achieved near ideal subthreshold swing and high hole mobility at room temperature. Sci Rep 6:24920
Keyes RW (1953) The electrical properties of black phosphorus. Phys Rev 92:580–584
Takao Y, Asahina H, Morita AJ (1981) Electronic structure of black phosphorus in tight binding approach. J Phys Soc Jpn 50:3362–3369
Liu H, Neal AT, Zhu Z, Xu XF, Tomanek D, Ye PD et al (2014) Phosphorene: an unexplored 2D semiconductor with a high hole mobility. ACS Nano 8:4033–4041
Wood JD, Wells SA, Jariwala D, Chen K, Cho E, Sangwan VK et al (2014) Effective passivation of exfoliated black phosphorus transistors against ambient degradation. Nano Lett 14:6964–6970
Castellanos-Gomez A, Vicarelli L, Prada E, Island JO, Blanter SI, Buscema M et al (2014) Isolation and characterization of few-layer black phosphorus. 2D Mater 1:025001
Li LK, Ye GJ, Tran V, Fei RX, Chen XH, Zhang YB et al (2015) Quantum oscillations in a two-dimensional electron gas in black phosphorus thin films. Nat Nanotechnol 10:608–613
Gillgren N, Wickramaratne D, Shi Y, Espiritu T, Yang JW, Hu J et al (2015) Gate tunable quantum oscillations in airstable and high mobility few-layer phosphorene heterostructures. 2D Mater 2:011001
Chen XL, Wu YY, Wu ZF, Han Y, Xu SG, Wang L et al (2015) High-quality sandwiched black phosphorus heterostructure and its quantum oscillation. Nat Commun 6:7315
Cao Y, Mishchenko A, Yu GL, Khestanova E, Rooney AP, Prestat E et al (2015) Quality heterostructures from two dimensional crystals unstable in air by their assembly in inert atmosphere. Nano Lett 15:4914–4921
Avsar A, Vera-Marun IJ, Tan JY, Watanabe K, Taniguchi T, Neto AHC et al (2015) Air-stable transport in graphene-contacted, fully encapsulated ultrathin black phosphorus-based field-effect transistors. ACS Nano 9:4138–4145
Edmonds MT, Tadich A, Carvalho A, Ziletti A, Donnell KMO, Koenig SP et al (2015) Creating a stable oxide at the surface of black phosphorus. ACS Appl Mater Interfaces 7:14557–14562
Pei JJ, Gai X, Yang J, Wang XB, Choi D-K, Lu YR et al (2016) Producing air-stable monolayers of phosphorene and their defect engineering. Nat Commun 7:10450
Zhou QH, Chen Q, Tong YL, Wang JL (2016) Light-induced ambient degradation of Few-layer black phosphorus: mechanism and protection. Angew Chem 128:11609–11613
Zhu H, McDonnell S, Qin XY, Azcatl A, Cheng LX, Wallace RM et al (2015) Al2O3 on black phosphorus by atomic layer deposition: an in situ interface study. ACS Appl Mater Interfaces 7:13038–13043
Liu H, Neal AT, Si M, Du YC, Ye PD (2014) The effect of dielectric capping on few-layer phosphorene transistors: tuning the Schottky barrier heights. IEEE Electron Device Lett 35:795–797
Luo X, Rahbarihagh Y, Hwang James CM, Liu H, Du YC, Ye PD (2014) Temporal and thermal stability of Al2O3-passivated phosphorene MOSFETs. IEEE Electron Device Lett 35:1314–1316
Luo W, Milligan CA, Du Y, Yang L, Wu Y, Ye PD et al (2016) Surface chemistry of black phosphorus under a controlled oxidative environment. Nanotechnology 27:434002
Ha SC, Choi E, Kim SH, Roh JS (2005) Influence of oxidant source on the property of atomic layer deposited Al2O3 on hydrogen-terminated Si substrate. Thin Solid Films 476:252–257
Castellanos-Gomez A, Buscema M, Molenaar R, Singh V, Janssen L, Steele GA et al (2014) Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater 1:011002
Lu WL, Nan HY, Hong JH, Chen YM, Ni ZH, Jin CH et al (2014) Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization. Nano Res 7:853–859
Rokugawa H, Adachi S (2010) Investigation of rapid thermally annealed GaP (001) surfaces in vacuum. Surf Interface Anal 42:88–94
Gaskell KJ, Smith MM, Sherwood PMA (2004) Valence band x-ray photoelectron spectroscopic studies of phosphorus oxides and phosphates. J Vac Sci Technol A 22:1331–1336
This work was supported in part by the start-up program JIH1233003 at Fudan University and also sponsored by Shanghai Pujiang Program (16PJ1400800), China.
BBW and HMZ carried out the BP fabrication and Al2O3 growth and measurements. YQD and WJL supervised the work and drafted the manuscript. HLL, PZ, LC, QQS, SJD, and DWZ helped to analyze the experimental results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Wu, B.B., Zheng, H.M., Ding, Y.Q. et al. Direct Growth of Al2O3 on Black Phosphorus by Plasma-Enhanced Atomic Layer Deposition. Nanoscale Res Lett 12, 282 (2017) doi:10.1186/s11671-017-2016-x
- Black phosphorus
- Plasma-enhanced atomic layer deposition
- Oxygen plasma