Study on Be and Si Doping of Cubic Boron Nitride Films YE QING

Study on Be and Si Doping of Cubic Boron Nitride Films YE QING MASTER OF PHILOSOPHY CITY UNIVERSITY OF HONG KONG APRIL 2008

CITY UNIVERSITY OF HONG KONG 香港城市大學 Study on Be and Si Doping of Cubic Boron Nitride Films 鈹和矽摻雜立方氮化硼薄膜的研究 Submitted to Department of Physics and Materials Science 物理及材料科學系 In Partial Fulfilment of the Requirements for the Degree of Master of Philosophy by YE QING 葉青 April 2008 二零零八年四月

i Abstract This work presents the recent progress in research of synthesis and doping of cubic boron nitride (cbn) films, prepared by chemical and physical vapor deposition (CVD and PVD) methods. Cubic boron nitride (cbn) is a synthetic material which does not exist in nature. This compound material was sythesized in 1950-s using a high-pressure high-temperature (HPHT) method for the first time. After that, great interest and considering effort was invested into the research of this material due to its outstanding physical and chemical properties and vast potentials for mechanical, electronic and electro-optical applications. In 1980s, synthesis of cbn thin films was demonstrated in a number of laboratories. The films, however, had limited thickness, low crystal quality, and suffered from delamination problems. The research progress of cbn films was stagnated nearly for twenty years until launching several innovative technological approaches to cbn synthesis. The major breakthroughs account the introducting fluorine-assisted growth process, lowering the ion kinetic energies close to thermal energies and employing carbon substrates or carbon buffer layers. City University of Hong Kong has played an important role in this technological advancement. The motivation behind the research of cbn films is in great potential applications of cbn in many industrial fields. Cubic BN has the second highest hardness and thermal conductivity next only to diamond. Some of its properties are even superior to diamond. It is thermally and chemically more stable than diamond particularly in contact with

ii molten ferrous material. cbn is a wide band gap semiconductor, which has wider band gap than diamond (6.3 vs 5.5 ev). Cubic BN can be both p- and n-type doped while n-type doping is still problematic for diamond, which make cbn a more promising material than diamond in electronic applications. The outstanding mechanical, chemical and thermal properties allow using cbn in machining tools, fusion reactors and protective coatings. While HPHT cbn has been commercialized and is available in powder forms with grain size less than 1mm, cbn film has not been implemented into industrial practice. The deposition of high quality, thick cbn film is still a challenge because of the poor adhesion, and limited thickness to a maximum of 200 nm, an unavoidable soft and mechanically weak abn/tbn interlayer formed before cbn film growth on most substrates, the large internal residual stress and nanocrystallinity nature induced by energetic ion bombardment. This work shows the success of synthesizing high quality cbn films with extreme high phase purity and relatively better crystallinity in our laboratory. The cbn films are deposited from a complex plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD). The ECR plama is formed in an environment composed of five precursor gases. Nitrogen (N 2 ) and boron trifluoride (BF 3 ) are gas precursors for the formation of BN. Fluorine acts as a selective etchant which can remove the undesired sp 2 -bonded BN phase preferentially. Hydrogen (H 2 ) gas is used to balance the etching of fluorine species in the plasma and mediate the deposition rate. Inert argon (Ar) ions contribute to the overall momentum transferred to

iii surface atoms of growing films to open surface bonds for incorporation of arriving activated radical and promote the formation of sp 3 -bonds. Helium (He) ions enhance plasma density by penning ionization process, which in return allows reduction of bias voltage and ion kinetic energy of impinging particles. By the introduction of diamond buffer layer, the large lattice mismatch and surface energy problem between cbn films and foreign substrates are fixed. Growth of high-quality phase-pure cbn films directly on diamond layer is achieved and the soft, weak abn/tbn interlayer is skipped. Fourier transform infrared (FTIR) and Raman spectroscopic analyses show only cbn peak, without any abn or hbn signals. The atomic-scale analysis by high resolution transmission electron microscopy (HRTEM) is consistent with spectroscopic data and shows disclosed local heteroepitaxial relationship between poly-crystalline diamond and cbn. The developed synthesis method allows further development of novel cbn-based electronic devices operating in harsh environments. To study the doping effect of cbn by Si element, silicon tetrafluoride (SiF 4 ) is introduced into the cbn synthesis system during cbn growth. The work is supported by the success of growing high-quality cbn films by fluorine-assisted ECR MPCVD and good understanding of its mechanism. Silicon tetrafluoride, with similar structure to BF 3, is expected to promote substitutions of B sites with Si atoms. It is revealed that Si cooperation has improved the conductivity of cbn films by 6 orders of magnitude. The

iv Si doped cbn thin film exhibits n-type conductivity, which is confirmed by Hall-effect measurement. It is also revealed that excessive SiF 4 gas flow rates (Si/B ratio higher than 0.5) suppress the cbn nucleation and growth, resulting in the formation of sp 2 BN phases. Beryllium (Be) ion implantation is also tried on cbn and hbn films in this work. After implantation and thermal annealing treatment, FTIR spectra illustrate that the implanted BN films retained their original phase, and Hall-effect measurements reveal that p-type conduction is induced for all BN samples. The resistivities of hbn and cbn samples reduce by 5 orders and 4~6 orders, respectively, after Be implantation.

vi Table of Contents Abstract Acknowledgement Table of Contents List of Figures List of Tables i v vi ix xi Chapter 1. Structures and Properties of Boron Nitride Polymorphs 1 1.1 Various Structures of Boron Nitrides and Their Properties 3 1.2 Mechanical and Chemical Properties of Cubic Boron Nitride 6 1.3 Electrical and Electronic Properties of Cubic Boron Nitride 10 Chapter 2. Thermodynamics and Kinetics of Boron Nitrides 13 Chapter 3. Synthesis Techniques of Cubic Boron Nitride 17 3.1 Brief Introduction to Synthesis of Cubic Boron Nitride 17 3.2 Mechanisms of Ion-assisted cbn Film Deposition 21 3.2.1 Sputter model 22 3.2.2 Thermal-spike model 22 3.2.3 Compressive stress model 23 3.2.4 Subplantation model 23 3.2.5 Summary of the models 24 3.3 Vapor Phase Deposition Techniques 25 3.3.1 Ion plating 26 3.3.2 Ion-assisted pulsed laser deposition 26 3.3.3 Sputtering deposition 27 3.3.4 Plasma enhanced chemical vapor deposition (PECVD) 28 3.3.5 Other deposition techniques 29

vii Chapter 4. Characterization of Boron Nitride Thin Films 30 4.1 Vibrational Characteristics of Crystalline Boron Nitride 30 4.2 Chemical Composition of Cubic Boron Nitride Films 33 4.3 Microstructures of Cubic Boron Nitride Films 34 Chapter 5. Experimental details 37 5.1 Preparation of Diamond Buffer Layers 37 5.2 Deposition of Intrinsic Cubic Boron Nitride Thin Films 38 5.2.1 Electron cyclotron resonance plasma enhanced chemical vapor deposition 38 5.2.2 Radio-frequency magnetron sputtering 40 5.2.3 Doping of cubic boron nitride thin films 42 5.2.3.1 In situ doping of cubic boron nitride thin films with SiF 4 using ECR MPCVD system 42 5.2.3.2 Doping of cubic boron nitride thin films by beryllium ion implantation 42 5.2.3.3 Electrodes preparation on cubic boron nitride thin films 42 5.3 Characterization and Analysis of Cubic Boron Nitride Films 43 Chapter 6. Synthesis of High-quality Intrinsic Cubic Boron Nitride films by ECR MPCVD system 44 6.1 Deposition of Cubic Boron Nitride Films on Silicon Substrates 44 6.2 Direct Growth of High-quality Cubic Boron Nitride Films on Diamond Layers 48 6.3 Summary of Synthesis of High-quality Cubic Boron Nitride Films 52 Chapter 7. In Situ Doping of Cubic Boron Nitride Thin Films with SiF 4 Gas 54 7.1 Background 55 7.2 Deposition of Silicon-doped BN Thin Films 57 7.3 Electrical Properties of Si-doped cbn Thin Films 62 7.4 Summary of In Situ Si Doping of Cubic Boron Nitride Thin Films 68

viii Chapter 8. Beryllium Implantation of Boron Nitride Thin Films 69 8.1 Brief Introduction to Ion Implantation Doping Technique 69 8.2 Beryllium Ion Implantation Doping of BN Thin Films 70 8.2.1 Beryllium doping of CVD cbn thin films by low energy ion implantation 70 8.2.2 Beryllium doping of PVD cbn thin films by low energy ion implantation 72 8.2.3 Beryllium doping of hbn thin films by low energy ion implantation 74 8.2.4 Discussions of doping effects of BN samples 76 8.3 Summary of Beryllium Doping of BN Thin Films by Ion Implantion 76 Chapter 9. Conclusions 78 References 81 List of Publications 88 Abstract of Publications 90

ix List of Figures Figure 1-1 Structures of the sp 2 -bonded phases hbn and rbn and the sp 3 -bonded phases wbn and cbn. Figure 1-2 Thermogravitic measurement of cbn, hbn, diamond and graphite in a stream of air at variable temperatures. Figure 2-1 The phase diagram of BN proposed by Bundy and Corrigan at 1975 (green line) and refined version by Solozhenko at 1999 (red line). Figure 3-1 Schematic diagram showing the layer structure of cbn films. Figure 3-2 Simplified schematic representations of (a) the sputter model, (b) the thermal spike model, (c) the stress model and (d) the subplantation model of cbn formation. Figure 3-3 Schematic of two basic PVD techniques for cbn deposition; (a) ion beam-assisted PVD, (b) plasma-assisted PVD. Figure 4-1 The transmission infrared spectra of (a) hbn and (c) cbn films and the Raman spectra of bulk (b)hbn and (d) cbn crystallites. Figure 4-2 XPS spectra (vicinity of N(1s) and B(1s) peaks) of typical films: charts (a) and (b) are the spectra for hbn, while (c) and (d) are the spectra for cbn. Figure 4-3 Typical growth sequence of cbn films revealed by HRTEM. Figure 5-1 A schematic diagram of ECR MPCVD system for the deposition of cbn films. Figure 5-2 A schematic drawing of the RF-MS deposition system employed in the synthesis of BN films. Figure 6-1 FTIR spectrum of the cbn films deposited on silicon. Figure 6-2 Raman spectrum of the cbn films deposited on silicon. Figure 6-3 Cross-sectional images of the cbn films with thickness of ~1 µm. Figure 6-4 FTIR spectra of cbn films on (a) diamond coated silicon substrate; and (b) uncoated silicon substrate. Figure 6-5 Cross-sectional SEM image of cbn film epitaxially grown on polyd layer.

x Figure 6-7 Epitaxial growth of cbn on diamond: (a) HRTEM imaging indicates a seamless boundary between the cbn film and the diamond interlayer; (b) The interface between the diamond and cbn is revealed by elemental mapping of boron, carbon and nitrogen using EELS. Figure 7-1 Plain-view SEM images of cbn films at Si/B ratios of (a) 0.05, (b) 0.1, (c) 0.3, and (d) 0.4; and hbn films at at a Si/B ratios of (e) 0.5 and (f) 1 by the ECR MPCVD system. Figure 7-2 FTIR spectra of extrinsic cbn films grown at different Si/B ratios in the ECR MPCVD system. Figure 7-3 I-V characteristic of Si-doped and intrinsic cbn films at room temperature. Figure 7-4 (a) Resistivity of Si-doped cbn film at 0.1 Si/B gas flow ratio, measured at varied temperatures; (b) illustration of Ti/Au electrode pattern on the Si doped cbn sample. Figure 7-5 Illustration of Hall effect measurement of Si-doped cbn film with SiF4 at 0.1 Si/B gas flow ratio, by four-point probe method at room temperature. Figure 7-6 UV Raman spectra of Si-doped cbn thin film with SiF4 at 0.1 Si/B gas flow ratio. Figure 7-7 XPS spectra of Si-doped cbn thin film with SiF4 at 0.1 Si/B gas flow ratio, (a) survey scan, (b) B1s, (c) N1s, (d) Si2p. Figure 8-1 SEM cross-sectional view of as-deposited cbn film by ECR MPCVD system. Figure 8-2 FTIR spectra of as-deposited, and implanted and annealed cbn film by ECR MPCVD system. Figure 8-3 SEM cross-sectional view of as-deposited cbn film by RFMS system. Figure 8-4 FTIR spectra of as-deposited, and implanted and annealed cbn film by RFMS system. Figure 8-5 SEM cross-sectional view of as-deposited hbn film by RFMS system. Figure 8-6 FTIR spectra of as-deposited, and implanted and annealed hbn film by RFMS system.

xi List of Tables Table 1-1 Lattice parameters of BN polytypes Table 1-2 Conductivity and activation energy of extrinsic cbn with different dopants Table 3-1 Some difference between cubic boron nitride and diamond Table 5-1 Process parameters of cbn films prepared by ECR MPCVD Table 5-2 Process parameters of cbn films by RF-MS Table 7-1 Hall effect properties of the Si doped cbn thin film at room temperature