ARTICLE pubs.acs.org/JPCC
Adsorption of Growth Species on the c-BN(100) Surface Johan Karlsson* and Karin Larsson Department of Materials Chemistry, Uppsala University, Box 538, Uppsala 751 21, Sweden ABSTRACT: The cubic phase of boron nitride (c-BN) is an extremely promising multifunctional material. However, to exploit all possible applications, a successful route for large area chemical vapor deposition (CVD) of c-BN films is required. Adsorption of gaseous growth species onto the c-BN surface is one of the key elementary reactions in CVD growth of c-BN. In the present work, the ability of BHx, BFx, and NHx species (x = 0, 1, 2, 3) to act as growth species for CVD of c-BN, in an H-, F-, or H/F-saturated gasphase, has been investigated using density functional theory (DFT) calculations. It was found that the most optimal growth species for CVD growth of c-BN are B, BH, BH2, BF, BF2, N, NH, and NH2 in an H/F-saturated gas-phase, i.e., decomposition of the incoming BH3, BF3, and NH3 growth species is very crucial for CVD growth of c-BN. It was also found that it would be most preferable to use a CVD method where the incoming BH3, BF3, and NH3 growth species are separately introduced into the reactor, e.g., by using an atomic layer deposition (ALD) type of method.
1. INTRODUCTION Boron nitride (BN) is a binary chemical compound which is isoelectronic to a similarly structured carbon lattice.1 BN therefore exists in various carbon-like crystalline phases. The two main phases are the diamond-like sp3 hybridized cubic phase (c-BN) and the graphite-like sp2 hybridized hexagonal phase (h-BN) (Figure 1).2,3 The hexagonal phase is the thermodynamically stable phase under normal laboratory conditions. It changes into the cubic phase at 6 GPa and 2000 °C. This cubic phase has several outstanding physical and chemical properties, e.g., extreme hardness (second only to diamond), low density (3.48 g/cm3), high thermal conductivity (13 W/(cm K) at T = 300 K), high electrical resistivity (1016 Ω cm), wide band gap (6 6.4 eV, the largest among all IV and IIIV materials), high chemical stability, and transparency from near-ultraviolet to infrared (η = 2.1 for λ = 600 nm).4,5 Furthermore, it possesses a high thermal stability, both in oxidizing environments (up to 1300 °C) and in contact with Fe, Co, and Ni. It is therefore promising as a tool coating for machining of steel, cast iron, and ferrous alloys. It can also be made as both p- and n-type semiconductors, suitable for p-n junction diodes. These properties make c-BN an extremely promising multifunctional material, which could be tailored for a very large range of advanced mechanical, tribological, thermal, electronic, and optical applications. However, to exploit these applications, a successful route for large area chemical vapor deposition (CVD) of c-BN films is required. In CVD of c-BN, a dilute gaseous mixture of B- and N-containing growth species, in an excess of H2 and/or F2, is being introduced into a reactor (Figure 2).6 In the vicinity of a substrate surface, the reactants will be decomposed by using, for instance, a plasma.7 The decomposed growth species will then deposit as a c-BN film onto the substrate. The role of the H and/or F species r 2011 American Chemical Society
is to maintain the sp3 hybridization of the surface atoms by completely hydrogenate, or fluorinate, the growing surface.8 However, surface radical sites will appear during the growth process as a result of atomic H, or F, abstraction reactions.9 Since the gas-phase is saturated with H and/or F species, such abstractions are usually followed by the recapture of an H, or F, atom.10 However, occasionally a surface radical site will attract a B- or N-containing growth species. The addition of N-containing growth species to B radical sites, and B-containing growth species to N radical sites, will then result in a continuous CVD growth of c-BN.11 The dominant c-BN growth surfaces under CVD conditions are the (100), (110), and (111) surfaces.12 The BHx and NHx species (x = 0, 1, 2, 3) have earlier been found to be promising as growth species for the stoichiometric (110) surface and for the B- or N-terminated (111) surface of c-BN.6,13,14 The purpose with the present work has therefore been to continue this series of investigations by studying the ability of BHx, BFx, and NHx species to act as growth species (in a saturated gas-phase containing either H or F, or a mixture thereof) for the B- and N-terminated c-BN(100) surfaces, using density functional theory (DFT) calculations. For this surface type, the different terminations show characteristically different reconstructions when exposed to H or F species; the 100% H- or F-covered B-terminated c-BN(100) surface will be (2 1) reconstructed, while the 100% H- or F-covered N-terminated c-BN(100) surface will be (1 1) reconstructed (Figure 3).15 Received: April 14, 2011 Revised: June 13, 2011 Published: June 27, 2011 16977
dx.doi.org/10.1021/jp203482v | J. Phys. Chem. C 2011, 115, 16977–16983
The Journal of Physical Chemistry C
ARTICLE
Figure 1. The two main phases of BN: (A) c-BN (vsp3 = 109.5°) and (B) h-BN (vsp2 = 120°). Figure 3. Supercells for modeling the adsorption of BHx, BFx, and NHx species onto the (A) 94% (monoradical) H- or F-covered B-terminated c-BN(100)-(2 1) surface, and the (B) 94% (monoradical) H- or F-covered N-terminated c-BN(100)-(1 1) surface.
The Brillouin zone was sampled with a (2 2 1) Monkhorst-Pack grid.24 The convergence criteria within the calculations were 2 105 Ha for the maximum energy change per atom, 4 103 Ha/Å for the maximum force per atom, and 5 103 Å for the maximum displacement per atom. After optimization, the energies for the adsorption processes (1) were calculated according to eq 2 (More negative adsorption energy signifies that the adsorption reaction is more exothermic.) BNð94% XÞ þ Y f BNð94% XÞ Y
Figure 2. CVD growth of c-BN from BFx and NHx species (x = 0, 1, 2, 3) in an H/F-saturated gas-phase.
ð1Þ
ðX ¼ H or F; Y ¼ BHx , BFx , or NHx ; x ¼ 0, 1, 2, 3Þ
2. COMPUTATIONAL DETAILS The geometrical structures and total energies for the various systems were calculated using all-electron DFT, as implemented in the program package DMol3 (Materials Studio, v. 4.2) from Accelrys, Inc.1620 The geometry was first optimized using the local (spin) density approximation (L(S)DA) with the PWC functional, and thereafter further refined with the generalized gradient (spin density) approximation (GG(S)A) and the PW91 functional.21 All calculations were performed with the double numeric basis set with polarization functions (DNP). The surfaces were modeled as supercells (a = b = 10.22 Å, c = 30.85 Å, R = β = γ = 90°) under periodic boundary conditions (Figure 3). 22 (For consistency, the energies of the BHx , BFx, and NHx species have been calculated by including them in a supercell of the same size as for the c-BN(100) surfaces.) The supercells consisted of 10 atomic layers, with 4 4 B (or N) atoms in each layer. To suppress the artificial charge transfer between the two polar ends of the slabs, the dangling bonds on the lower surfaces of the slabs were saturated with H atoms, and a large vacuum distance between the slabs was used (∼20 Å). The upper surface was covered with 15 H or F atoms and one BH x, BF x, or NHx species. (The surface concentration of adsorbed BH x, BF x, or NH x species was approximately 6%). The positions of the B and N atoms within the bottom two layers (including the dangling bondpassivating H atoms) were fixed to simulate the structure of bulk c-BN. The rest of the atoms were allowed to fully relax using the BroydenFletcherGoldfarbShanno (BFGS) algorithm. 23
ΔEads ¼ EBNð94% XÞY ðEBNð94% XÞ þ EY Þ
ð2Þ
where EBN(94% X)-Y, EBN(94% X), and EY are the total energies for the 94% (monoradical) H- or F-covered surface with an adsorbed growth species, 94% (monoradical) H- or F-covered surface, and an BHx, BFx, or NHx molecule, respectively. A more elaborate description of the computational methods that has been used is presented and discussed in ref 15.
3. RESULTS AND DISCUSSION 3.1. Adsorption of BHx and BFx in an H-Saturated GasPhase. The resulting BHx and BFx (x = 0, 1, 2, 3) adsorption
structures, for chemisorption onto a monoradical surface site on the H-covered N-terminated c-BN(100)-(1 1) surface, are shown in Figures 4 and 5. The corresponding adsorption energies for both the H-covered N-terminated c-BN(100)-(1 1) surface and the H-covered B-terminated c-BN(100)-(2 1) surface, are shown in Figure 6 and Table 1. The following orders of adsorption energies were obtained: BHð 686 kJ=molÞ < Bð642 kJ=molÞ < BH2 ð621 kJ=molÞ < BFð609 kJ=molÞ < BF2 ð585 kJ=molÞ < BH3 ð179 kJ=molÞ < BF3 ð172 kJ=molÞðH-covered N-terminated surfaceÞ Bð627 kJ=molÞ < BH2 ð454 kJ=molÞ < BHð414 kJ=molÞ < BF2 ð390 kJ=molÞ
