Direct Growth of Al2O3 on Black Phosphorus by Plasma-Enhanced Atomic Layer Deposition

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 POx 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 POx can be converted into fully oxidized P2O5.

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 P 2 O 5 on BP surface [34][35][36], atomic layer deposited dielectric capping [27,[37][38][39][40]. Nevertheless, h-BN passivation requires complicated environmental conditions and has extremely low yield [29][30][31][32][33]. The P 2 O 5 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 Al 2 O 3 was directly grown on BP flakes using plasma-enhanced atomic layer deposition (PEALD) with the help of O 2 plasma as an oxygen Optical images of accelerated BP degradation on SiO 2 /Si exposed to air for different time. g-i AFM images of BP flake exposed to the air ambient for 2, 3, and 4 h. All three BP samples for AFM measurements were taken from the same batch. The average thickness of BP in g-i was 130 nm. j The average RMS roughness of BP samples versus the exposure time, the same samples as shown in g-i precursor, instead of H 2 O, to react with trimethylaluminum (TMA) [41]. The composition and properties of the interfacial layer between Al 2 O 3 and BP have been examined by physical characterizations, and the mechanisms behind are analyzed.

Methods
Few-layer BP (purity: 99.998%, Smart Elements) was transferred onto a Si substrate with thermally grown 285 nm SiO 2 using a micromechanical method with polydimethylsiloxane (PDMS) [1,28,42]. Prior to transfer, the SiO 2 surface was ultrasonically cleaned in turn by acetone and isopropyl alcohol (IPA) and piranha solutions for 10 min each, followed by 100% O 2 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. 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-ofplane modes (A 1g ) and two in-plane modes (A 2g and B 2g ) in thin-layer BP [43]. Both A 2g and B 2g peak positions keep almost unchanged. While for A 1g 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 Al 2 O 3 film on thinlayer 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    Fig. 4a, b, and c, respectively. Figure 4a, b depicts AFM images of the 100 cycles Al 2 O 3 grown on BP samples by PEALD with and without an oxygen plasma pretreatment, respectively. Using PEALD for Al 2 O 3 growth in BP flakes, it has demonstrated a highly uniform surface morphology of Al 2 O 3 / BP. The average RMS roughness of Al 2 O 3 /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 Al 2 O 3 films. While for BP samples exposed to the air ambient for certain time, 4 h for example, PEALD with O 2 plasma pretreatment is much preferred. O 2 plasma pretreatment can create enough nucleation sites for ALD growth. On the other hand, it also has an "etching" effect for thinning BP samples. O 2 plasma may penetrate the PO x layer and oxidize the underlying BP, then increase the thickness of PO x layer [35]. On the contrary, Al 2 O 3 films on freshly exfoliated BP grown by ALD with H 2 O 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 H 2 O as an oxygen precursor [37]. It is worthwhile to mention that BP flake was covered uniformly by Al 2 O 3 film and can prevent O 2 or H 2 O in air ambient further reacting with BP, thus protected BP from degradation. Otherwise, the uncovered portions of BP surface may react with H 2 O and O 2 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   [37]. When the temperature goes up to 350°C, interestingly, P2 peak disappears. This is due to the conversion from PO x to P 2 O 5 , with the help of reactivity of O 2 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 2p 3/2 ), 130.87 eV(P 2p 1/2 ), 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 P 2 O 5 film. Finally, the interface properties of Al 2 O 3 /BP samples were also characterized by TEM measurements. It can be clearly seen for Fig. 6a that the interfacial PO x layer between Al 2 O 3 and BP was formed during PEALD process with the 20 cycles O 2 plasma pretreatment. Figure 6b shows high-resolution TEM (HRTEM) image of the Al 2 O 3 /BP sample after the deposition of 100 cycles Al 2 O 3 , same scanned region marked by a red square in Fig. 6a. The thickness of PO x and Al 2 O 3 is 6.1 and 10.7 nm, respectively. It is worth noting that Al 2 O 3 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 O 2 plasma penetrating into PO x layer and reacting with underlying BP.

Conclusions
In summary, we have demonstrated the direct growth of Al 2 O 3 film on BP by using PEALD. The A 1g peak of freshly exfoliated BP sample shifts downwards owing to the formation of PO x in the BP surface. The uniform Al 2 O 3 film on BP can be achieved by PEALD with O 2 plasma and TMA precursors, which may be attributed to the etching and reactivity of O 2 plasma with BP at high temperatures. The interfacial layer of PO x between Al 2 O 3 and BP was converted into P 2 O 5 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.