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CITY UNIVERSITY OF HONG KONG

DEPARTMENT OF

PHYSICS AND MATERIALS SCIENCE



 BACHELOR OF ENGINEERING (HONS) IN MATERIALS ENGINEERING 2008-2009

DISSERTATION



Investigation of semiconducting properties of cubic boron nitride



by



CHEUK Anthony



March 2009

Investigation of semiconducting properties of cubic boron nitride



By



CHEUK Anthony



Submitted in partial fulfilment of the

requirements for the degree of

BACHELOR OF ENGINEERING (HONS)

IN

MATERIALS ENGINEERING

from

City University of Hong Kong


March 2009


Project Supervisor :

Prof. Igor Bello


Table of Contents
Page


  1. List of Figures V

  2. List of Tables VI

  3. Acknowledgement VII

  4. Abstract VIII

  1. Background of Cubic Boron Nitride 1

    1. Boron nitride structure 1

    2. Extreme Properties of cubic boron nitride in comparison with diamond 3

    3. Superiority of cubic boron nitride over diamond 4

    4. Doping of cubic boron nitride 5

  2. Synthesis of Cubic Boron Nitride 6

    1. Cubic boron nitride synthesized by a high pressure high temperature method 6

    2. Synthesis of cubic boron nitride by low pressure methods 6

    3. Synthesis of cubic boron nitride by Plasma enhanced chemical vapor
      deposition (PECVD) 10

  3. Experimental 12

    1. Fabrication of cubic boron nitride thin film by electron cyclotron
      microwave plasma CVD 12

    2. Identification and phase analysis of boron nitride films 13

    3. Deposition of Ti/Au contact on the measured boron nitride films 14

    4. Measurement of Semiconducting property of boron nitride films 15

    5. Determination of activation energy of impurities in cubic boron nitride 15

  4. Characterization Techniques 17

    1. Fourier transform infrared spectroscopy 17

    2. Raman spectroscopy 17

    3. Characterization of Doped Cubic Boron Nitride 18

  5. Electrical Properties of Doped Cubic Boron Nitride 19

    1. cBN film sample: CW781 19

    2. cBN film sample: CW785 22

    3. cBN film sample: CW790 25

    4. cBN film sample: CW793 27

    5. Nano diamond sample: W941 29

    6. Be-implanted polycrystalline cubic boron nitride films 32

  6. Conclusions 36

  7. References 37




  1. List of Figures

Fig 1 Crystal structures of hBN, wBN, rBN and cBN.

Fig 2 Characteristic diffraction pattern of BN lattice

Fig 3 (a) PVD reactive sputtering technique, (b) PVD reactive ion beam mixing; (c) Plasma enhanced chemical etching (PECVD)

Fig. 4(a) High resolution transmission electron microscopic (HRTEM) image of cubic BN deposited by a PVD technique (reactive magnetron sputtering) shows amorphous layer on the top of cBN; (b) HRTEM image illustrate that cBN is void of amorphous and hexagonal

Fig.5 FTIR spectra of CBN films

Fig.6 I-V Characteristic of CW781

Fig.7 I-V Characteristic of CW781 under temp. changes from room temp. to 500℃

Fig.8 The temperature dependence of the resistance of cBN films CW781

Fig.9 I-V Characteristic of CW785

Fig.10 I-V Characteristic of CW785 in 100℃

Fig.11I-V characteristic of CW790

Fig.12 I-V Characteristic of CW790 under temp. changes from room temp. to 500℃

Fig.13 The temperature dependence of the resistance of cBN films CW790

Fig.14 I-V Characteristic of CW793

Fig.15 I-V Characteristic of CW793 under temp. changes from room temp. to 400℃

Fig.16 The temperature dependence of the resistance of cBN films CW793

Fig.17 I-V Characteristic of W941

Fig.18 I-V Characteristic of W941 under temp. changes from room temp. to 500℃

Fig.19 The temperature dependence of the resistance of Nano diamond films W941

Fig.20 I-V Characteristic of Be-doped cBN under temp. changes from room temp. to 200℃

Fig.21 The temperature dependence of the resistance of Be-doped cBN

  1. List of Tables

Table 1 Electrical and electronic properties of cBN.

Table 2 Electrical and electronic properties of samples



  1. Acknowledgement

In the end of this year long continuous assessment, I would like express my gratitude to my supervisor Prof. Igor Bello for fully and unlimited support. He has prepared and provided full of knowledge and information for this dissertation. He gave direction and idea which broke the wall of difficulty during the final year project. Out of Prof. Igor, I would like to express my gratitude to Dr. B He and Mr. Yat Ming Chong for their supporting, teaching and assisting in my project.



  1. Abstract

Cubic boron nitride (cBN) has its own extreme property. It makes up by two elements next to carbon and has diamond like structure [1]. Cubic BN shows its extreme property of mechanical, thermal and chemical resistance [4, 7] which are interested by scientists and manufacturers for inviolate and fabricate production. Cubic BN has high oxidation and chemical resistance even in high temperature [7]. It can be used as material in hash environment. In electric property, cubic BN has high band-gap energy and isoelectronic to diamond and unlike diamond it can be p-type and n-type semiconductor by difference dopant [19-27]. Ion bombardment is needed for the film synthesis because it can evidently promote the formation of cubic phase, sp3 bonding. The film cannot be grow in thick because the structure has high-level of stress which may be self destructive when it is exposed to the ambient. Cubic BN is not practically applied in products because we faced in the film synthesized in poor quality, for example; there are low phase purity, poor crystallinity, poor adhesion to substrates, small film areas of deposition and small film thickness. Study of the film fabrication process is one of important parts of this project.


The aim of this project is investigation of semiconducting properties of cubic boron nitride. Intrinsic and doped thick cubic BN films was fabricated by electron cyclotron microwave plasma enhanced deposition (ECR PECVD) using complex plasma induced in mixtures of nitrogen (N2), boron trifluoride (BF3), hydrogen (H2), argon (Ar) and (helium). There was a solid source of Magnesium impurities during growth of cBN films. The doping of Mg cause cBN films became a p-type semiconductor. Another doping method which called ion implantation was used. Beryllium was used as impurities and it was also a p-type semiconducting films. The films were implanted at different ion energies and a relatively large cumulative ion doses. The implanted films were annealed leaded the impurities jump into the cBN lattice sites. Sheet resistance and resistivity of cBN films were measured. The result was shown that the Mg doped cBN films did not work as prefer compared to the intrinsic cBN. On the other hand, the implanted Beryllium cBN film had reduced its resistance with one order of magnitudes and six orders of magnitudes after annealing. Temperature dependent of resistivity of cBN samples were measured. From the date, activation energy can be calculated. Activation energy of implanted beryllium impurities has been found to be 0.2 eV. [40]



  1. Background of Cubic Boron Nitride

    1. Boron nitride structure

Boron nitride (BN) is a synthetic material which appears in several crystallographic structures and phases that include hexagonal (hBN), rhombohedral (rBN), cubic (cBN) wurtzite (wBN), turbostratic (tBN) and amorphous (BN) phases. Hexagonal BN and rBN crystalline phases with sp2 hybridization bonding while cBN and wBN are crystalline phases characteristic with sp3 bonding configuration.



Fig 1 Crystal structures of hBN, wBN, rBN and cBN.


Hexagonal BN is the most encountered phase with a sp2-bonded layered structure. It comprises basal planes with two dimensional six-membered rings of alternating boron and nitrogen atoms. The basal planes are stack like in graphite. However in contrast to graphite the basal planes are stack in an alternating AA'AA'...sequence with a rotion of 30° for each successive ring basal plane (Fig 1a). [1]


Amorphous BN and turbostratic BN (tBN) are often found in hexagonal BN(hBN).

Amorphous BN is the phase with a very short range of crystallinity being on atomic scale. Few atoms are bonded in very small clusters which are mutually randomly oriented. Therefore this phase appears on a microscopic scale as disordered. Electron diffraction yields much diffused rings as result of the disorder nature of this phase. [2]


Turbostratic BN is fundamentally phase with sp2 bonding hybridization at which basal planes are randomly rotated around the c-axis of the hBN as an accidental stacking. Its lattice curvature is extended. Compare to the basal planes of the ordered hBN, tBN has a larger interplanar spacing by about 15%. Characteristic diffraction pattern of tubostratic BN is presented in the transmission electron diffraction pattern, in Fig.2, as an alongated diffraction spot which originates in partly oriented tuborstratic basal planes being in this are nearly parallel to the surface normal. [2]


Rhombohedral BN is hexagonal phase with threefold (ABCABC...) stacking sequence. It is not stacked directly each other. It can be studied as a graphite-like modification oh hBN.



Fig 2 Characteristic diffraction pattern of BN lattice


Starting from hBN and rBN (Fig 1c), diamond-like tetrahedral structure can be produced by cooperative lattice transformation by direct compression also the hexagonal axis. The graphite-like phase becomes two sub-layers which means B and N atoms are displaced in the opposite direction along the c-axis by splitting the (0002) planes. Strong chemical bonding formed between the (0002) basal planes after the compression of hBN and rBN lattices by splitting, and the result is tetrahedral bonding coordination of atoms. Due to the different stacking sequences, rBN transformed to cBN while hBN transformed to wBN (Fig 1b). Both the resulting structures are sp3-bonded. The two sub-lattices can be considered as cBN structures (Fig 1d). They are two interpenetrating face centre cubic (FCC), each of them containing one type of atoms with more than 1/4 the lattice shifting along the diagonal direction. The cBN's bonding is covalent complemented with slightly ionic bonding. It gives rise to longitudinal optical (LO) and transverse optical (TO) phonon modes in the infrared absorption and Raman scattering spectra. The metastable wBN is hexagonal in structures which is similar to lonsdaleite form of diamond. [3]


    1. Extreme Properties of cubic boron nitride in comparison with diamond

Both cubic BN and diamond are materials with extreme properties which originate in extremely high densities of atoms and strong covalent bonding. Diamond has cubic Fd3 m lattice structure with a unit cells of 3.567 Å, while cubic BN has cubic F43m lattice structure with a unit cell of 3.6145 Å. Mass densities of diamond and cubic BN (3.51 and 3.48 g cm-3) are comparable. Diamond the hardest materials (~100 GPa) of all known materials. Cubic BN is the second hardest materials just next to diamond with hardness ~ 70GPa. Obviously the hardness of these materials depends on crystallographic direction. Elastic modulus of these materials is numerically about one order of magnitude higher than their hardness. Both diamond and cubic BN also have the highest thermal conductivities which are 20 and 13W cm-1 K-1, respectively. [4]



    1. Superiority of cubic boron nitride over diamond

The band gap of cubic BN (6.2 eV) is wider than that of Diamond (5.51 eV). Cubic BN can be doped for both p- and n-type conductivities; however n-type semiconducting diamond is still unavailable. [5, 6]


Cubic BN surpasses diamond in chemical resistance. Up to 1300℃, it is chemical inert to various metals particularly molten ferrous materials. It cannot be dissolved in common acid and alkali solutions however it is soluble in molten alkalis and nitrides. The oxidation (1200℃) and graphitization (1500℃) temperatures of cBN are much greater than those of diamond (600℃ and 1400℃). Thermo-gravimetric methods enable to measure the temperature-dependent oxidation of diamond, graphite, hBN and cBN in the range of 30 to 1300℃. At temperatures of 600 to 700℃, diamond and graphite, respectively, start to lose their weight. The weight reduction accelerates at 800℃ and rapidly vanishes at 900 - 1000℃. On the other hand, hBN and cBN do not show weight loss up to temperature of 1100℃. However at ranging from 1100 to 1200℃ these materials gain about 10% weight. These properties along with electronic properties discussed above make particularly cBN suitable for design and fabrication of electronic devices operating in harsh environments comprising high temperature and highly radiative environments. [4, 7]



    1. Doping of cubic boron nitride



Table 1 Electrical and electronic properties of cBN.


From table 1, cBN can be a n-type or p-type semiconducting material which depended to the doping atom. Different atoms are used to improve the conductivity, for n-type, carbon(BC), oxygen (NO), nitrogen vacancy(VN), SI and S. For p-type, there are Be and Mg. Be and Mg atoms could substitute B with small formation energy after calculation. The highest mobility is found to be 825 cm2 v-1s-1 as Si doped while 1 cm2 v-1s-1 for S doped. [19-27]



  1. Synthesis of Cubic Boron Nitride

    1. Cubic boron nitride synthesized by a high pressure high temperature method

High pressure high temperature (HPHT) methods have been used in syntheses of both diamond and cubic boron nitride since 1950s. These methods are commercialized and represent billion dollar industry. The synthesis of cBN is based on phase conversion of hexagonal BN to cubic boron nitride by application of high pressure and high temperatures. These conversion parameters are usually reduced by assistance a metal catalysts. The initial phase, hexagonal BN, is prepared by chemical vapor deposition. However output product of cBN is in forms cBN powders or micrometers size crystallites. These powders are used as very effective abrasives at grinding of hard ferrous materials. The powders can be molded with assistance of metal binders to various shapes to produce cutting tools. However this method of production of cBN materials cannot satisfy requirements for electronic and even mechanical applications. Therefore emerging technology of cBN synthesis in forms of cBN films may surpass the obstacles hampering wider cBN applications particularly in electronic and optoelectronic and sensing applications. [8]


    1. Synthesis of cubic boron nitride by low pressure methods

The second approach in cBN synthesis is based on low pressure methods which yield thin films and represents two groups of deposition techniques, i.e., physical vapor deposition (PVD) and plasma enhance chemical vapor deposition (PECVD). The PVD methods are in illustrated in Fig… reactive magnetron sputtering in Fig…a and recoil dynamic mixing in Fig. b are physical vapor deposition techniques. The third schematic of electron cyclotron resonance microwave plasma CVD belongs to the group abbreviated PECVD. Both the groups include many deposition techniques with similar characteristic feature in each group. PECVD can be distinguished based on the fundamental process occurring in deposition chamber. PECVD are such techniques where material for growing a solid phase is supplied in a form of molecular non-condensable gases. The non-condensable gases are converted to condensable radicals when electric power is simultaneously supplied to the reactor. Then reactive and condensable radical are adsorbed on the surface and form via exchange reaction a required material, while other product of reactions, volatile gases are removed from the reactor via pumping process. In the case of PVD material is supplied from a solid source (Fig.3a) or from ion beam. Combination of ion beam sputtering as illustrated in Fig. 3b can be used too. [9-12]




Fig 3 (a) PVD reactive sputtering technique, (b) PVD reactive ion beam mixing; (c) Plasma enhanced chemical etching (PECVD)


However, the most important condition for growing cBN by low pressure method is the requirement of energetic ions. Since the cBN phase can be prepared only with assistance of energetic ions, the substrates have to be negatively biased unless ion beams are used. The bias voltage depends on the deposition method. In PVD techniques bias voltage is typically from 50 to 100 V and even more. However high bias voltage induce highly energetic ions inducing compressive stress that accumulates by ion bombardment. The accumulated stress can disturb the mechanical stability of the film. As a result the film delaminates. The solution is to lower ion bombardment. This possible particularly in PECVD technique where the cBN films are synthesized in synergetic assistance of inherent chemical reaction and reduced ion bombardment. The bias voltage can be reduced down to 20 V while plasma potential can be very low (about ~1 V) which gives effective bias ~21V. The corresponding energy 21 eV is insufficient to cause atomic displacement and thus stress produced is much lower. In addition to this intrinsic stress associated with microstructure very important is thermal stress. The thermal stress results form miss match of substrates particularly in thermal expansion coefficient. It arises especially for substrate-film material with different expansion coefficient and high deposition temperature. Upon cooling the substrate, stress then has to increase. Therefore the selection of substrates with similar expansion coefficients is vital. However mismatch in surface lattice parameter and surface energies are also reason for stress and even formation of undesired interfacial phases that can contribute to the stress formation and even mechanical instability of the cBN films. Since cBN basically has no counterpart materials with similar properties particularly surface energy and lattice parameters synthesis of cBN film on most substrates proceed via amorphous and tubbostratic boron nitride (aBN and tBN) phases. Edges of tBN and curved (0002) basal planes serve as nucleation sites for cBN growth. These interfacial precursor layers comprise boron dangling bonds which are highly reactive; they form with water oxiboron hydrides which are very week in bonding, and such cBN films immediately or with a time delay delaminate. Solution is in elimination of highly defective interfacial aBN and tBN layers or/and passivation of dangling bonds. Especially carbon atoms are very effective in pasivation of boron dangling bonds.

Many substrates including silicon, aluminum nitride, silicon carbide boron carbide and many metals have been tested for growing cBN. In all cases interfacial layer are formed and large stresses are built up. The most suitable substrate material is diamond which matches the most closely all the parameters discussed above.

Since in PVD methods, fairly large ion energy is used at cBN synthesis the cBN growth via subsurface reactions and any point of deposition hexagonal/amorphous phase is found on the surface of cBN films. This is illustrated in Fig. 4a by high resolution transmission electron microscopic (HRTEM) image. Cubic BN was deposited by a reactive magnetron sputtering and immediately coated by a gold (Au) film to resolve clearly the surface interfacing layers. The layer during further growth process aBN/hBN is however converted to cBN phase while a new surface aBN/hBN layer is continuously formed. On the contrary, PECVD process is surface growth process. The ion energy is consumed to open the surface bonds for incorporation of reactive radicals into the cBN structures. Therefore surface cBN grown by PECVD process is void of hexagonal and amorphous phases as it is evidenced in HRTEM image in Fig. 4b.



Fig. 4(a) High resolution transmission electron microscopic (HRTEM) image of cubic BN deposited by a PVD technique (reactive magnetron sputtering) shows amorphous layer on the top of cBN; (b) HRTEM image illustrate that cBN is void of amorphous and hexagonal phases.



    1. Synthesis of cubic boron nitride by Plasma enhanced chemical vapor deposition (PECVD)

Plasma Enhanced Chemical Vapor Deposition (PECVD) may involve different methods of supplying electric energy. For example, direct current (DC) electric power can be supplied using a DC jet system, which provides high power density and high plasma density. However the deposition area is constrained to a small size (~1 cm2). The deposition systems that employ radio-frequency power sources are easier scalable, and they operate with relatively high plasma density. Many CVD reactors are assisted by microwave powers with frequencies of 2.45 GHz and in combination with external magnetic field to obtain resonance mode. The resonance mode is in place when electron oscillations induced by microwave electric field matches the frequency of cyclotron motion of electrons due to the external magnetic field. The electrons are thus trapped on long paths along which intensively ionize molecules in gas phase. In the resonance mode, the energy absorbed in plasma is maximal, and plasma can be on even at pressure to pressure down to 10-5 Torr. Typical operational pressure however is about 10-3 Torr.

The growth environment is plasma which is induced in complex mixtures of molecular gases such as N2, HN3, B2H6, BH3NH3, H2, trimethyl borazol, etc.

Along ion energy and composition of gas mixture, temperature is also the crucial parameter though many articles discuss the temperature as unimportant deposition parameters. It can be demonstrated that interfacial fusing the cBN film with diamond substrate at high temperatures (800 -1000 oC) is responsible for outstanding adhesion and growing very thick films. Lowering the substrate temperature down to 600 oC causes voids at the substrate–cBN film interface. Further reduction temperature to ~400 oC results in partial delamination of the film with thickness of ~2 µm. On the other hand the higher temperature leads to the formation of larger cBN crystallites and faceted surfaces.

High negative bias voltage applied to cBN depositions is needed to promote chemical bonding in cBN structures, open bonds for further incorporation of cBN constituents and finally to provide selective etching of non-diamond phases and stabilizing sp3-growing surface. However these functions are contradicted with the requirement of lowering the bias voltage (ion energy) which is needed for decreasing the film stress. Indeed thicker films can only be grown at lower bias voltages. The bias voltage cBN can be lowered (still supporting cBN growth) can be provided by supplying fluorinated species from which particularly fluorine radicals are very effective for the selective etching of hexagonal phases.

Molecular precursor boron trifluoride (BF3) in plasma form reactive fluorinated radicals including BFx and atomic plasma constituents that can be in various excited and ionized states. Fluorine radicals and “fluorine chemistry” assist to lowering the bases voltage which results in reducing the film stress down to neighborhood of 1 to 2 GPa, and thus growing thicker cBN films with a very good mechanical stability. [13-16]



  1. Experimental
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