Hexagonal Boron Nitride Grown Using High Atomic Boron Emission During Microwave Plasma Chemical Vapor Deposition Kallol Chakrabarty, Ivan Arnold, and Shane A

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Hexagonal boron nitride grown using high atomic boron emission during microwave plasma chemical vapor deposition Kallol Chakrabarty, Ivan Arnold, and Shane A. Catledge

Citation: Journal of Vacuum Science & Technology A 37, 061507 (2019); doi: 10.1116/1.5123210 View online: https://doi.org/10.1116/1.5123210 View Table of Contents: https://avs.scitation.org/toc/jva/37/6 Published by the American Vacuum Society

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Thermodynamic stability of hexagonal and rhombohedral boron nitride under chemical vapor deposition conditions from van der Waals corrected first principles calculations Journal of Vacuum Science & Technology A 37, 040603 (2019); https://doi.org/10.1116/1.5107455 Hexagonal boron nitride grown using high atomic boron emission during microwave plasma chemical vapor deposition Kallol Chakrabarty,1 Ivan Arnold,2 and Shane A. Catledge1,a) 1Department of Physics, Center for Nanomaterials and Biointegration (CNMB), The University of Alabama at Birmingham, 1300 University Blvd., 310 Campbell Hall, Birmingham, Alabama 35394-1170 2Department of Physics, Leach Science Center, Auburn University, Auburn, Alabama 36832 (Received 3 August 2019; accepted 3 October 2019; published 17 October 2019) Boron nitride (BN) is a member of Group III nitrides and continues to spark interest among the scientific community for its mechanical properties, chemical inertness, thermal conductivity, and electrical insulating properties. In this study, microwave plasma chemical vapor deposition is used to synthesize BN on silicon substrates. Feed gas mixtures of H2,NH3, and B2H6 are used for a range of systematically varied power, pressure, and flow rate conditions. Plasma optical emission from atomic boron is shown to increase nonlinearly by nearly a factor of five with decreasing chamber pressure in the range from 100 to 10 Torr. Copious amounts of atomic boron in the plasma may be beneficial under some growth conditions for producing high hardness boron-rich nitrides, such as B13N2,B50N2,orB6N, which, to date, have only been synthesized under high pressure/high temperature conditions. Despite the higher atomic boron emission in the plasma at low pressure, BN coatings grown at 15 Torr result in hexagonal BN (B/N ratio of 1), regardless of the B2H6 flow rate used in the range of 0.6–3.0 sccm. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons. org/licenses/by/4.0/). https://doi.org/10.1116/1.5123210

I. INTRODUCTION copper. While many CVD processes for hBN growth rely on Hexagonal boron nitride (hBN), an insulating analog of decomposition of ammonia borane close to its melting point graphene, has gathered interest among researchers due to its with control of its partial pressure critical for high-quality graphenelike structure and exceptional thermal and mechanical growth, Sharma et al. used atmospheric pressure CVD for – μ properties, chemical stability, and UV emission.1 8 It is a wide growth of hBN single crystals (having larger than 25 m edge bandgap semiconductor (bandgap ∼ 5.9 eV) with very high length) on a copper foil through stepwise control of the thermal and chemical stabilities and is often used in devices decomposition of the ammonia borane precursor. This allowed a controlled supply of borazine gas from the decom- operating under extreme conditions. It is stable up to 900 °C 20 in an oxidizing environment and up to 2000 °C in reducing position products leading to the observed large crystal hBN. and inert environments. Recently, it was found that hBN has Thermal CVD is another method often used to synthesize fi other unique properties including biocompatibility and super- continuous lms of monolayer or few layers hBN at higher ∼ hydrophobicity.9 These properties make hBN a potential can- temperature ( 1000 °C) on metal crystal substrates (Cu, Ru, Ni, Fe, etc.). Ammonia borane was also extensively used as a didate for use in a variety of applications including electrical 21–26 insulators, microelectronic mechanical systems, lubricants, precursor gas in these processes. Hexagonal boron nitride neutron sensing laser devices, and biomedical devices. synthesized by the thermal CVD process is often reported to ∼ Hexagonal boron nitride is not found directly in nature; it have good optical properties with a bandgap of 6.0 eV. Very must be synthesized using boron and nitrogen precursors. recently, microwave assisted surface wave plasma CVD was The structure and properties of hBN are dependent on the used to synthesize hBN layers at a much lower temperature ∼ synthesis methods. Several techniques and experimental con- ( 500 °C). This was made possible by sublimation of the solid precursor inside the chamber followed by decomposition ditions have been used to synthesize BN from a variety of 21 boron and nitrogen precursors.10–15 Preparation of boron to form plasma radicals. fi nitride (BN) was reported from high pressure/high tempera- Deposition time is directly related to the lm thickness ture synthesis using diamond anvil cell/laser heating.16–18 for microwave plasma chemical vapor deposition (MPCVD) However, synthesis by high pressure/high temperature can be growth. The geometry and microwave power afforded in our costly and typically only yields very small volumes of mate- MPCVD system make it possible to grow continuous 2 fi rial. Chemical vapor deposition (CVD) has proven to be a 6×6mm or 8 mm diameter samples. While large area lm low-cost and scalable technology for synthesizing a wide growth is possible in other CVD systems, coating uniformity is range of coating materials including BN with large area uni- a challenge, particularly for a single-crystal hBN. Large area fi formity.19 For example, atmospheric pressure CVD has been hBN lms grown using CVD are predominantly polycrystal- used for growth of a single-crystal hBN on substrates such as line and suffer nonuniformity in thickness and/or morphology. Park et al. used plasma enhanced atomic layer deposition to synthesize large-scale uniform hBNs.27 Very recently, deposi- a)Electronic mail: [email protected] tion of a wafer-scale single-crystal hexagonal boron nitride

061507-1 J. Vac. Sci. Technol. A 37(6), Nov/Dec 2019 0734-2101/2019/37(6)/061507/8 © Author(s) 2019. 061507-1 061507-2 Chakrabarty, Arnold, and Catledge: Hexagonal boron nitride grown using high atomic boron emission 061507-2

film in thermal CVD via self-collimated grain formation was produced under a range of controlled experimental condi- reported. Borazine was used as the precursor and gold was tions were uniform and continuous. Reactant gases for used as the substrate, which was robustly attached on the tung- growth of BN coatings include H2,NH3, and B2H6, but little sten foil. The gold was liquefied at 1100 °C, maintaining high is known of the plasma species and underlying spectroscopic surface tension helpful for strong adhesion of boron and nitro- signatures responsible for BN growth in its various structural gen atoms on the gold. High diffusion of atoms on a smooth forms. In this regard, we have carried out optical emission liquid surface at high temperature promotes the circular hBN spectroscopy (OES) to investigate the excited state plasma domain. The main step was the facile rotation of the circular species that may be important for growth of boron nitride hBN grains on the liquid gold substrate, controlled by electro- coatings using this feed gas mixture. From OES measure- static interaction between B and N atoms at the perimeter of ments, we find that emission from atomic boron is highest at each grain that eventually leads to single crystal growth of low chamber pressure and high microwave power. This sug- hBN coatings on a wafer scale.28 In MPCVD, plasma gener- gests the possibility to grow boron-rich nitrides (so-called ated reactive species of the feed gas interact and deposit on the boron subnitrides), such as rhombohedral B13N2, which is substrate. This is a very complex process and depends on dif- predicted to be superhard (>40 GPa).29 This motivated the ferent parameters including the microwave power, chamber current study such that experiments were performed at lower pressure, flow rate of the feed gas, substrate temperature, and pressure and higher power. A report from over two decades deposition time. It is a known challenge to synthesize large- ago claimed hot-filament CVD synthesis of a rhombohedral area single crystalline hBNs via MPCVD. To date, no one has boron-rich nitride (B4N) with a structure analogous to boron 30 reported the synthesis of large-area single crystalline hBNs carbide B4C but with nitrogen substituted for carbon. using MPCVD. Despite the enhanced atomic boron emission in the plasma, In this work, we synthesize BN coatings using microwave the MPCVD conditions used in the current study do not plasma CVD, with a focus on the influence that pressure and yield higher boron content in the coating. Instead, our results diborane flow has on plasma emission of atomic boron and consistently show the formation of hBN (B/N ratio of 1). In the corresponding coating stoichiometry. The BN coatings addition, our attempts to increase the boron content in the coating by increasing the flow rate of B2H6 (keeping all other parameters fixed, including pressure) still result in a B/ N ratio of 1. While this study did not yield the intended boron subnitride coatings, it demonstrates the wide stability range for hBN growth despite large plasma emission from atomic boron for a range of CVD processing conditions.

II. EXPERIMENTAL DETAILS A. MPCVD apparatus Boron nitride coatings were grown in an MPCVD system as shown in Fig. 1 (Wavemat Inc., Plymouth, MI, USA). The sample surface is heated by direct contact with the plasma. Both the sample stage and outer resonance cavity jacket are water-cooled. A quartz bell jar isolates the low-pressure plasma environment from the resonance cavity. N-type (100)-oriented silicon substrates with 525 μm thickness were placed on the surface of a 0.5 in. diameter molybdenum screw placed along the central axis of the bell jar. The microwave power was 1 kW, and chamber pressure was 15 Torr. For all experiments, feed gas flow rates for H2 and NH3 were held fixed at 500 and 1 sccm, respectively. Four different coating types were grown using diborane flow rates of 0.6, 1.0, 2.0, and 3.0 sccm.

B. In situ OES Before any coatings were grown, the optical emission of atomic B (249 nm) and BH (433 nm) in the plasma was investigated for a range of power (600–1000 W) and pressure (10–100 Torr) conditions, keeping the B2H6 ﬂow constant at 1.0 sccm. The optical emission from the plasma was collected

FIG. 1. Schematic of the 6 kW MPCVD chamber showing plasma conﬁned without focusing optics into a Moritex MSS5-1000S-UV within the quartz bell jar. quartz light guide attached to a quartz viewport and guided

J. Vac. Sci. Technol. A, Vol. 37, No. 6, Nov/Dec 2019 061507-3 Chakrabarty, Arnold, and Catledge: Hexagonal boron nitride grown using high atomic boron emission 061507-3 into the spectrometer. The OES data were taken using an D. Characterization techniques Acton Research SpectraPro 500i spectrograph (Princeton Samples were characterized using x-ray photoelectron Instruments, Trenton, NJ, USA) with a 1200 gr/mm grating spectroscopy (XPS), glancing-angle x-ray diffraction (XRD), μ blazed at 300 nm and the entrance slit set at 20 m. Raman spectroscopy, and scanning electron microscopy (SEM). The XPS instrumentation was a Phi Electronics C. Sample preparation VersaProbe 5000, equipped with a micro-focused Al mono- 2 λ N-type (100)-oriented silicon pieces cut into 6 × 6 mm chromatic source ( = 1486.6 eV) and a dual anode conven- with 525 μm thickness were used as the substrate. The sub- tional x-ray source with a neutralizer. Survey spectra were strates were lightly scratched with diamond powder of 2–4 μm taken with an Mg anode x-ray source with an incident average particle size (GE Superabrasives) on a polishing cloth energy of 1253.6 eV. The XRD pattern was obtained by a in order to enhance nucleation. The substrates were cleaned Panalytical Empyrean X-ray diffractometer (Copper Kα1, ultrasonically before and after the diamond scratching process λ = 1.54059 Å). XRD patterns were acquired using a using acetone, followed by isopropyl alcohol and then distilled glancing-angle 2-theta scan with an angle of incidence of 1°. water. Four samples were made keeping the microwave power The diffraction optics included a hybrid monochromator (1 kW), chamber pressure (15 Torr), flow rate of H2 (500 sccm), with a 1/8° divergence slit and a 1/16° antiscattering slit and and flow rate of NH3 (1.00 sccm) fixed but changing the flow a parallel plate collimator on the diffracted beam path with a rate of B2H6. The four different B2H6 flow rates (and the cor- proportional detector. HighScore Plus (version 4.8) was used responding sample designations) were 0.6 sccm (BN 0.6), to analyze the phase structure. Rietveld refinement was used 1.0 sccm (BN 1.0), 2.0 (BN 2.0), and 3.0 (BN 3.0). The dep- to find the lattice constants of each sample. Raman spectra osition time for all four samples was 6 h. were collected using a micro-Raman spectrometer (Dilor

FIG. 2. (a) Atomic boron line intensity (λ = 249 nm) for a pressure range of 20–100 Torr and a fixed power of 1000 W. (b) BH line intensity (λ = 433 nm) for a pressure range of 10–100 Torr and a fixed power of 1000 W. (c) Atomic boron line intensity for a power range of 600–1000 kW at a fixed pressure of 15 Torr. (d) BH line intensity for a power range of 600–1000 kW at a fixed pressure of 15 Torr . Flow rates of B2H6 and NH3 were both fixed at 1.00 sccm.

JVST A - Vacuum, Surfaces, and Films 061507-4 Chakrabarty, Arnold, and Catledge: Hexagonal boron nitride grown using high atomic boron emission 061507-4

XY, Lille, France) with a 532 nm laser, a 1200 groove/mm changes in plasma emission occur with pressure (at a fixed grating, and a 100× microscope objective. SEM images were power of 1000 W), in contrast to gradual changes observed taken using a FEI QuantaTM 650 FEG scanning electron with power (at a fixed pressure of 15 Torr). The highest microscope (Thermo Fisher Scientific, Hillsboro, OR, USA) atomic boron and BH plasma emission occurs for the lowest at 20 kV beam voltage. chamber pressure and the highest microwave power. Based on these observations and our intent to grow boron-rich subnitrides, coatings were grown using 15 Torr and 1000 W. III. RESULTS AND DISCUSSION A. Optical emission spectroscopy B. X-ray photoelectron spectroscopy Figures 2(a) and 2(b) show that the normalized intensity Following deposition, surface chemistry was analyzed of the atomic boron and BH lines decrease nonlinearly with using XPS. Figure 3 shows that regardless of the flow rate of fi increasing chamber pressure (for a xed microwave power of B H , the atomic percentage of boron and nitrogen yields a fl fl 2 6 1000 W, the ow rate of H2 = 500 sccm, and the ow rate of B/N ratio that remains nearly 1 for all samples. B2H6 = 1.00 sccm). The intensity of the atomic boron line decreases by a factor of 5 with increasing pressure from 20 to 100 Torr. The BH line intensity also follows the same C. X-ray diffraction trend and decreases by a factor of 5 with increasing pressure Figure 4 shows x-ray diffraction patterns from each of the from 20 to 100 Torr. From Figs. 2(c) and 2(d), the normal- four samples grown under different B2H6 flow rates. The ized intensity of the atomic boron and BH lines increase by a most intense peak in the XRD pattern for all four samples is factor of 1.3 with increasing microwave power from 600 to associated with the (002) reflection of hBN. The other weak 1000 W (fixed chamber pressure of 15 Torr, flow rate of and broadened characteristic peaks (100), (101), (004), and H2 = 500 sccm, and flow rate of B2H6 = 1.00 sccm). Rapid (110) of hexagonal boron nitride are also represented in the

FIG. 3. XPS survey scans for samples: (a) BN 0.6, (b) BN 1.0, (c) BN 2.0, and (d) BN 3.0. All samples were grown with a fixed microwave power (1 kW), chamber pressure (15 Torr), flow rate of H2 (500 sccm), and flow rate of NH3 (1.00 sccm).

J. Vac. Sci. Technol. A, Vol. 37, No. 6, Nov/Dec 2019 061507-5 Chakrabarty, Arnold, and Catledge: Hexagonal boron nitride grown using high atomic boron emission 061507-5 spectrum. Low crystallinity and disordered structures are spectroscopy is sensitive to lattice vibration modes arising often associated with weak and broad peaks. Such is also the from B-N stretching and can distinguish between cubic BN case for CVD-synthesized turbostratic BN, characterized as (cBN) and hBN phase structures. The strongest peak hexagon layers stacked roughly parallel to each other with a observed for all four BN samples is attributed to the E2g random rotation and translation about the layer normal.31 in-plane mode for hBN. The reported Raman shift for dif- The single-crystal Si substrate can result in sharp peaks near ferent hBN structures is in the range of 1366–1374 cm−1.32 52° and 56°, often affected by sample orientation during the For cBN, the transverse optical phonon mode is centered at XRD scan. XRD data of the BN 3.0 sample were taken after 1056 cm−1 and the longitudinal optical mode is centered at SEM imaging. The XRD peak at 39° for this sample is asso- 1283 cm−1.33 None of the coatings grown in this study ciated with the gold–palladium alloy sputtered onto the reveal the cBN Raman modes. surface prior to SEM imaging. Table I gives measured lattice constants for each of the E. Scanning electron microscopy four samples after a Rietveld refinement fit to the hBN phase. The c/a ratio varies by no more than 4.14% between Figure 6 shows surface morphology of four different samples, and the variation from the accepted standard c/a BN samples. Figures 6(a), 6(c), 6(e),and6(g) are second- value of hBN (2.657 Å) is no more than 5.95%. ary electron images associated with samples BN 0.6, BN 1.0, BN 2.0, and BN 3.0, respectively. The BN coatings show a uniform, fine-grained microstructure with aggrega- D. Raman spectroscopy tion into a cauliflowerlike morphology. BN 0.6 shows Figure 5 shows Raman spectra collected in order to more agglomeration into spheroidal particles compared to confirm the phase identification of the BN coating. Raman other BN coatings. Figures 6(b), 6(d), 6(f),and6(h) are

FIG. 4. XRD patterns for samples (a) BN 0.6, (b) BN 1, (c) BN 2, and (d) BN 3. Insets show the expanded view of the pattern in the range of 40°–80° 2-theta.

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TABLE I. Measured lattice parameter of the four BN samples in this work. F. Discussion As one would expect, coating properties depend on the Standard valuea particular deposition process and gas phase chemistry of the Sample a (Å) b (Å) c (Å) c/a system. In our MPCVD system, chamber pressure has an ﬁ hBNa 2.498 2.498 6.636 2.657 effect on plasma size. For xed power and high chamber pressure, the plasma size was very small and then gradually Measured value expands with decreasing chamber pressure. The column

Sample a (Å) b (Å) c (Å) c/a density of atomic boron and BH increases with plasma size and is highest at low chamber pressure. The extent of B2H6 BN 0.6 2.422 2.422 6.652 2.746 dissociation into atomic boron and BH is expected to be BN 1.0 2.522 2.522 6.992 2.772 higher for higher microwave power, as is confirmed by OES. BN 2.0 2.480 2.480 6.704 2.703 OES shows higher intensity of atomic boron and BH lines at BN 3.0 2.504 2.504 7.049 2.815 lower pressure and higher power. Despite the high atomic aFrom HighScore database: BN (reference code: 96-201-6171). boron plasma emission at 15 Torr and 1 kW, the coating structure was hexagonal with a B/N ratio near 1. In order to increase the boron content in the coating for higher magnification images from BN 0.6, BN 1.0, BN 2.0, growth of boron subnitrides having high hardness, the flow and BN 3.0. All of the high magnification images in Fig. 6 rate of B2H6 in the feed gas was also increased. However, reveal thin platelike structures, which have been reported for XPS data show that this did not increase the boron content in nanocrystalline hBN.34,35 the coating, and the B/N ratio remained near 1 for all coatings

FIG. 5. Raman spectra of all four-boron nitride coatings grown at different B2H6 ﬂow rates.

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FIG. 6. Scanning electron microscopy (SEM) images of the four hBN coatings grown with different B2H6 flow rates. tested. The XRD pattern for all four samples is indexed only with a B/N ratio near 1. No evidence of cubic (sp3 bonding) to the hBN structure. In addition, Raman spectra reveal only BN phase was observed. the E2g in-plane mode for hBN. Although hexagonal boron No external electric or magnetic field was used to increase nitride is the most thermodynamically stable form of BN, the the plasma or ion current density in this study. However, nonequilibrium nature of CVD growth suggests the possibil- reports have shown that negative DC bias can be imple- ity that new superhard boron-rich BN phases could be made mented in order to obtain sp3-bonded cubic BN.36,37 DC under appropriate deposition conditions. Nevertheless, under bias increases the ion bombardment on the surface with ions the conditions of this study, we find that only hBN is formed of high energy. The additional ion bombardment is expected

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J. Vac. Sci. Technol. A, Vol. 37, No. 6, Nov/Dec 2019