Direct Growth of Al2O3 on Black Phosphorus by Plasma-Enhanced Atomic Layer Deposition
© The Author(s). 2017
Received: 15 December 2016
Accepted: 20 March 2017
Published: 20 April 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) [10–18], have been widely studied for the potential applications in the next generation devices including field emitters [10–12], 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 [21–25]. 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) [29–33], creating saturated P2O5 on BP surface [34–36], atomic layer deposited dielectric capping [27, 37–40]. Nevertheless, h-BN passivation requires complicated environmental conditions and has extremely low yield [29–33]. 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 [34–36]. 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
The RMS roughness of BP samples before and after PEALD at different temperatures
The average roughness (before/after) (nm)
Standard deviation (before/after) (nm)
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.
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.
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