J. Phys. Chem. B 2006, 110, 21073-21076
21073
Thermal Stability of Cubic Boron Nitride Films Deposited by Chemical Vapor Deposition J. Yu,*,†,‡ Z. Zheng,‡ H. C. Ong,‡ K. Y. Wong,‡ S. Matsumoto,§ and W. M. Lau‡,| Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, UniVersity Town, Xili, Shenzhen 518055, P. R. China, Department of Physics and Materials Science and Technology Research Center, Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong, People’s Republic of China, AdVanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Surface Science Western, UniVersity of Western Ontario, London, Ontario, Canada ReceiVed: February 20, 2006; In Final Form: July 12, 2006
Thermal stability of well-crystallized cubic boron nitride (cBN) films grown by chemical vapor deposition has been investigated by cathodoluminescence (CL), Raman spectroscopy, and scanning electron microscopy (SEM) with the cBN films annealed at various temperatures up to 1300 °C. The crystallinity of the cBN films further improves, as indicated by a reduction of the relevant Raman line width, when the annealing temperature exceeds 1100 °C. Structural damage or amorphization was observed on the grain boundaries of the cBN crystals when annealing temperature reaches 1300 °C. The CL spectra are found to be unchanged up to 1100 °C after annealing at 500 °C, showing the stability of the cBN films in electronic properties up to this temperature. New features were observed in the CL spectra when annealing temperature reaches 12001300 °C.
1. Introduction Much work has been devoted to cubic boron nitride (cBN) films because of the unique mechanical, chemical, and electronic properties of cBN. For examples, cBN has a hardness only second to diamond, a good chemical stability at high temperature,1-3 the widest band gap (>6.4 eV, indirect) among all IV and III-V compounds,3-5 high-thermal conductivity,3,6 and pand n-type dopability.7 Superior to diamond in chemical stability, cBN can be used as protective coatings of cutting tools for ferrous metals. Because of its wide band gap, cBN can be used to fabricate ultraviolet (UV) detectors and light-emitting diodes. Indeed, UV light-emitting diode has been demonstrated in a p-n junction configuration by using a high pressure-high temperature (HP-HT) grown cBN crystal.5,8 Compared with other III-V compounds such as GaN and GaAs, the luminescence properties of cBN are not thoroughly studied. Earlier researchers reported the deep-level luminescence from HP-HT synthesized cBN crystals,9-15 but the origins of the spectra features have not been completely identified. As for cBN films, the study about the luminescence properties was much less reported.16,17 Taylor et al. investigated the cathodoluminescence (CL) from sputtering deposited BN films.16 They observed the near-band-gap UV emission from hexagonal BN films and found that the deep-level emission from cBN predominant films is dependent on the applied bias voltage. Vetter et al. measured the CL from europium-doped cBN films grown by mass-selected ion beam deposition.17 Although cBN films are technologically important for high-temperature electronic application, the thermal stability of their electronic * To whom correspondence should be addressed. E-mail: msejyu@ hotmail.com. † Harbin Institute of Technology. ‡ The Chinese University of Hong Kong. § National Institute for Materials Science. | University of Western Ontario.
properties is not reported so far. In this work, the thermal stability, both in microstructure and electronic properties, of the well-crystallized cBN films deposited by chemical vapor deposition (CVD) was investigated by annealing the samples at temperatures up to 1300 °C. 2. Experimental Methods The cBN films were deposited on (100)-oriented silicon wafers of 1 mm in thickness by bias-assisted dc jet plasma chemical vapor deposition (CVD) in an Ar-N2-BF3-H2 system. The details of the deposition system have been shown elsewhere.18 The gas flow rates of Ar, N2, H2, and BF3 (diluted in Ar at 10%) were 20 000, 1500, 5-3.5, and 30 sccm, respectively. The growth temperature was about 1100 °C. A negative bias of -85 V was applied to the substrate. The chamber pressure was 50 Torr during growth. The dc jet power was about 5 kW. These growth parameters were optimized by many growth experiments and have been reported in detail.18-21 Deviation from these optimized parameters always induces the decrease of cBN concentration in the films or deterioration of cBN crystallinity. For example, high-quality cBN films could be obtained at -85 V, but cBN deposits could not be obtained while decreasing the bias voltage to -70 V, and the cBN concentration decreased greatly with increasing the bias voltage.19 In this study, only the samples deposited at the optimized conditions were investigated. The samples in this study were grown for 30 min, and the film thickness was measured to be about 7.6 µm from the interference pattern of the infrared spectrum by a method described in ref 20. The grown samples were annealed in a tube furnace in N2 atmosphere at the temperatures of 500, 700, 900, 1100, 1200, and 1300 °C, respectively. The duration time is 3 h for all annealing experiments. A Renishaw RM-1000 Micro Raman Spectrometer was used to characterize the structure of the cBN films. All Raman spectra were measured at room temperature with the
10.1021/jp0610766 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/29/2006
21074 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Yu et al.
Figure 1. Raman spectra of the cBN films as-deposited and annealed at different temperatures.
514.53-nm line of an argon laser. The laser beam was focused at the sample through a microscope with a spot size of approximately 2 µm and a power of 20 mW. The CL measurements were carried out by an Oxford Instrument MonoCL system equipped in a scanning electron microscope (SEM) at room temperature. A CCD detector was used in CL measurements. The accelerating voltage and the probe current were kept at 10 kV and 4.23 nA, respectively. The probe current was calibrated by using a Faraday cup before and after the experiments. 3. Results and Discussion Figure 1 shows the Raman spectra of the cBN films as-deposited and annealed at 1100, 1200, and 1300 °C. The Raman spectra of the films annealed at 500, 700, and 900 °C did not change much in peak position and line width compared with that of the as-deposited film, so we only show the spectra annealed at 1100-1300 °C in Figure 1. Besides the Si peak at about 977 cm-1, all spectra exhibit two main peaks at 1053.6 and 1304.6 cm-1, which are attributed to scattering by the transverse optical (TO) and longitudinal optical (LO) phonon modes of cBN, respectively. The full widths at half-maximum (FWHM) of the respective TO/LO phonon modes are 26.5/16.2, 25.4/15.8, 23.9/13.0, and 19.8/10.0 cm-1 for the as-deposited sample and that annealed at 1100, 1200, and 1300 °C. The FWHM did not change much for the samples annealed below 1100 °C but decreased markedly while increasing the annealing temperature from 1100 °C to 1200 °C, especially to 1300 °C. The FWHM of TO and LO modes decreases for 1.5 and 2.8 cm-1 with increasing annealing temperature from 1100 to 1200 °C and decreases for 4.1 and 3.0 cm-1 with increasing annealing temperature from 1200 to 1300 °C. As has been demonstrated, the decrease of the line width means an improvement of the crystallinity, that is, an increase of the crystal size or decrease of defect density in the films.23,24 Raman spectroscopy is an effective and reliable facility for characterizing the crystallinity of cBN films. Our previous results have repeatedly verified that the changes in FWHM of Raman peaks present identical trends to that of X-ray diffraction (XRD) peaks.20,21 Furthermore, the Raman spectroscopy appears to have higher sensitivity than XRD in detecting defects. So, XRD measurements were not carried out in this work. It is shown that the remarkable structural changes and crystallinity improvement start from 1200 °C during annealing. The improvement of the crystallinity should be because the defects decrease during high-temperature annealing. Interestingly, a new band near LO peak appeared at
Figure 2. SEM images of the cBN films: (a) as-deposited and (b) and (c) annealed at 1300 °C.
about 1270 cm-1 after 1300 °C annealing, the origin of which was not identified clearly. Simulations based on the spatial correlation model and experimental observation show that the LO mode downshifts to lower wavenumber when the crystal size decreases, and the peak shifted to about 1260 cm-1 for amorphous cBN.23,24 So, this band should be related to the presence of nanocrystalline or amorphous cBN. It is inferred that the surface structure of the cBN crystals was damaged and that nanocrystalline or amorphous cBN formed on the crystal surface while the crystallinity of the inner part of the cBN crystals was greatly improved by heat treatment at 1300 °C. No transformation from cBN to hBN was observed for all the annealing experiments up to 1300 °C. Several groups have investigated the effects of annealing on the structure of cBN films.25-27 It was reported that the cBN films are stable up to 1150 K in nitrogen, 1200 K in air, and 1470 K in a vacuum, and higher temperature leads to disintegration of the films.27 The much higher thermal stability in nitrogen here originates from the much improved crystallinity of our films. Consistent with the Raman data showing no crystallinity changes for annealing below 1100 °C, the SEM images also show no noticeable changes in film morphology for annealing up to 1200 °C. However, the film annealed to 1300 °C loses its original coherent film nature and disintegrates into a film of loosely bound powders. A comparison of the SEM images of the as-prepared film (a) with that of the annealed film at 1300 °C (b) is summarized in Figure 2. The as-prepared films are dense and rough with some pits of various sizes on the surface. Great changes in appearance and morphology were observed for the film annealed at 1300 °C, with the original semitransparent, continuous, and well-adhered film changed to the opaque and powderlike film. The film became very soft and can be scraped off the substrate easily after annealing at 1300 °C. As shown in Figure 2b, the annealed film is loose and flat compared
Thermal Stability of Cubic Boron Nitride Films
Figure 3. Normalized CL spectra of the cBN films as-prepared and annealed at different temperatures.
with Figure 2a. It is also observed that the degree of film disintegration appears to be more serious near the film pits than the flat film areas, which suggests the correlation between film disintegration and the presence of defects in the original film. From Figure 2c, it is shown that the annealed film is composed of many particles with the size of hundreds of namometers to over 1 µm. These particles are believed to be single cBN crystals because they fall in the same dimension range with that of the cBN crystals measured by transmission electron microscopy (TEM) in our previous reports.28,29 These particles are not closely bound to each other, which indicates that the structural destruction occurs in the grain boundaries. With these results and the fact that the disintegrated films still show the Raman characteristics of cBN as shown in Figure 1, we postulate that during annealing at 1200-1300 °C, some main defects in the film become mobile with some of them recombining and annihilating and thereby the overall crystallinity improves and sharper Raman peaks appear. At the same time, some of the defects and impurities (such as hydrogen, oxygen, and fluorine) segregate to grain boundaries and cause breakages of intergranular bonds. As such, the original coherent film disintegrates into a film of loosely bound highly crystallized cBN powders with no significant grain growth in reference to the original grain structure. Figure 3 shows the corresponding normalized CL spectra of the as-deposited cBN film and that annealed at different temperatures. The spectrum of the as-prepared film shows three emission bands at 2.07, 2.44, and 3.00 eV, respectively. Since all the peaks were below the band gap of the cBN, they are supposed to arise from the defects. The band at 3.00 eV is close to the reported band UCL (or US-1) luminescence centers,30 which are commonly observed in undoped cBN crystals. The emission band at 2.44 eV was observed previously for HPHT synthesized cBN crystals and was attributed to the multivacancy complexes of boron and nitrogen vacancies of cBN.12 The origin of the band at 2.07 eV is not identified in this work. The intense peak at 2.07 eV of the as-deposited film shifts to 1.96 eV after 500 °C annealing, which stays almost constant during the subsequent annealing up to 1100 °C. No obvious shifts were observed for the bands at 2.44 and 3.00 eV after annealing up to 1100 °C. However, there are only two peaks after annealing above 1100 °C, which are at 2.28 and 3.40 eV for 1200 °C and 2.47 and 3.51 eV for 1300 °C. The reason that the peak shifts after 500 °C annealing is not known clearly. However, it is probably related to the release of the incorporated or absorbed impurities, mainly H originating
J. Phys. Chem. B, Vol. 110, No. 42, 2006 21075 from the reactive gas of H2 during deposition. The film composition has been measured for the sample deposited under similar conditions to the present work by secondary ion mass spectrometry (SIMS).31 It is demonstrated that hydrogen is the main impurity with concentrations as high as 10 at. %, which fluctuates across the film thickness. In addition, small amounts (
