PECVD Synthesis of Hexagonal Boron Nitride ... - Springer Link

0 downloads 0 Views 2MB Size Report
carbon compounds, such as MoS2,WS2, and h BN, can be prepared in the form of layered nanostructures. [1, 2]. Such nanomaterials are of practical importance.
ISSN 00201685, Inorganic Materials, 2015, Vol. 51, No. 11, pp. 1097–1103. © Pleiades Publishing, Ltd., 2015. Original Russian Text © I.S. Merenkov, M.L. Kosinova, E.N. Ermakova, E.A. Maksimovskii, Yu.M. Rumyantsev, 2015, published in Neorganicheskie Materialy, 2015, Vol. 51, No. 11, pp. 1183–1189.

PECVD Synthesis of Hexagonal Boron Nitride Nanowalls from a Borazine + Ammonia Mixture I. S. Merenkov, M. L. Kosinova, E. N. Ermakova, E. A. Maksimovskii, and Yu. M. Rumyantsev Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, pr. Akademika Lavrent’eva 3, Novosibirsk, 630090 Russia email: [email protected] Received February 5, 2015

Abstract—Hexagonal boron nitride nanowalls have been grown by plasmaenhanced chemical vapor depo sition (PECVD) from a mixture of borazine (B3N3H6) and ammonia. As the deposition temperature increases from 100 to 700°C, the structure of the films changes from amorphous to nanocrystalline, made up of threedimensional nanowalls normal to the substrate. The ability to produce nanowalls depends on film growth conditions. We have examined the effect of synthesis temperature on the elemental composition and surface morphology of the films. The structure of the nanowalls has been determined by transmission electron microscopy, and the presence of a transition layer between the hBN film and Si(100) substrate has been dem onstrated. The lowest temperature at which nanowalls can be grown by PECVD is 300°C. The films have high transmission in a wide spectral range (350–3200 nm). Their parameters suggest that the nanostructures in question can find application in microelectronics, optics, and catalysis. DOI: 10.1134/S0020168515100118

INTRODUCTION Research effort was until recently concentrated on the synthesis of zero and onedimensional (0D and 1D) carbon nanostructures, such as fullerenes and nanotubes. With the discovery of graphene, two dimensional (2D) nanostructures have attracted researchers' attention. According to recent work, non carbon compounds, such as MoS2,WS2, and hBN, can be prepared in the form of layered nanostructures [1, 2]. Such nanomaterials are of practical importance for a variety of applications, for example, in electron field emitters [3] or for storage of various gases [4]. Of particular interest among such materials is boron nitride (BN) because it is structurally similar to car bon. BN is an isoelectronic and isostructural analog of carbon and has an sp2 (orthorhombic and hexagonal (hBN) polymorphs) or sp3 (cubic (cBN) and wurtz ite polymorphs) bond configuration. The best studied polymorphs are hBN and cBN. hBN layered crys tals possess a number of physicochemical properties similar to those of graphite but surpass it in mechani cal strength, thermal stability, and chemical inertness [5–7]. In addition to hBN monolayers, similar to car bon monolayers reported by Wu et al. [8], boron nitride nanowalls (BNNWs) have recently been pre pared [9–12]. They are described as 2D structures that are aligned vertically on the surface of a substrate and have platelike morphology. Even though BNNWs have been first produced very recently, they have already been shown to possess a number of functional charac teristics, such as cathodoluminescence and superhy drophobicity, which suggest their potential applica tions [10].

Previous attempts to produce BNNWs employed both physical vapor deposition (PVD) [11] and chem ical vapor deposition (CVD). The starting materials used in CVD processes were mixtures of aggressive and explosive gases, such as BF3 + N2 [9] and BCl3 + NH3 [12]. It is worth pointing out that the preparation of BN from boron halides requires high temperatures and corrosionresistant apparatus. The lowest temper ature at which a crystalline hBN film can be grown from a BF3 + N2 + H2 mixture is 800°C, even when a microwave plasma is used [9]. These difficulties can be overcome by using borazine (B3N3H6) as a single source precursor and activating the starting gas mix ture with an rf plasma. In this paper, we report borazine synthesis and purification, borazine decomposition conditions under which BNNW growth on silicon substrates is possible, and some physicochemical and functional properties of the synthesized materials. EXPERIMENTAL Borazine synthesis. Borazine was prepared using a modified process described by Volkov et al. [13], through mechanochemical activation of solid reac tants. To raise the borazine yield, lithium tetrahydri doborate was used instead of its sodium analog. The method takes advantage of the reaction between lith ium tetrahydridoborate (LiBH4) and ammonium chloride (NH4Cl):

1097

3LiBH 4 + 3NH 4Cl → B3N 3H 6 + 3LiCl + 9H 2.

1098

MERENKOV et al.

NH4Cl + LiBH4

N2(l) N2(l)

B3N3H6 Fig. 1. Schematic of the apparatus for mechanochemical synthesis of borazine.

The reaction is accompanied by the evolution of a large amount of hydrogen gas. In the first step, weighed amounts of dry solid LiBH4 and NH4Cl and 10 mm diameter steel balls were loaded into a 500 cm3 stainlesssteel cylindrical reactor. The reactor was evacuated and placed on horizontal rollers rotated by an electric motor, which in turn rotated the reactor, containing the balls and reactants. The rotation rate was 60 Hz and the mechanical activation time was 1.5–2 h. In the second step of synthesis, the reactor was placed in a vertical tube furnace and connected to a vacuum system (Fig. 1). The reactor was then heated at a variable rate to a temperature of 230°C over a period of 1 h and held at this temperature for 1 h. In the second step, the pressure in the system was main tained in the range 60.8–90.2 kPa. The borazine con densed in a liquidnitrogencooled glass trap and was then purified by recondensation in vacuum. The prod uct yield was  24%. Growth and characterization of boron nitride films. Boron nitride films were produced by plasma enhanced chemical vapor deposition (PECVD) from a borazine + ammonia gas mixture. The process was run in a tunneltype quartz reactor (Fig. 2). The growth zone was heated by a cylindrical resistance furnace to the synthesis temperature, which was varied from 100 to 700°C. The furnace temperature was maintained by a Termodat 14E2 temperature controller. The sub strate temperature was monitored with a Chromel– Alumel thermocouple. RF discharges were initiated by an UVCh66 generator (40.68 MHz frequency). The plasma power was measured by a VEGA SX200 SWR meter. The system was pumped by an Alcatel PASCAL 2015SD rough pump. The pressure was measured by a Meradat VT12ST2 gage. A liquidnitrogencooled trap was placed between the reactor and pump to pre vent oil vapor diffusion from the pump to the reactor

and capture gaseous reaction products. Precursor vapor was delivered to the reactor through an inlet located between the plasma generation zone and the substrate heating zone. Films were grown on single crystal Si(100) silicon and quartz glass substrates, which were precleaned by a standard chemical proce dure. The precursor was enclosed in a cylinder ther mostated at 0°C. Before experiments, the borazine containing cylinder was frozen at a temperature of ⎯196°C, and the gases that resulted from borazine decomposition during storage were pumped away from the cylinder. An additional gas was delivered to the reactor through a system of control and shutoff valves. Typical borazine and additional gas partial pressures in the system were 0.6 Pa. The plasma power was 50 W and the total pressure in the reactor was 1.5 Pa. The layers thus grown were characterized by a number of advanced techniques. The thickness and refractive index of the films were determined by ellip sometry (LEF3M ellipsometer) at seven angles of incidence of light, ϕ, in the range from 50° to 80°. The light source used was a He–Ne laser (632.8nm line). The nature of chemical bonds between atoms in the films was assessed by Fourier transform IR spectros copy. Fourier transform IR absorption spectra were measured in the range 200–4000 cm–1 on a SCIMI TAR FTS 2000 doublebeam spectrophotometer. The surface microstructure and elemental composition of the films were analyzed by scanning electron micros copy on a JEOL JSM6700 equipped with an EX23000BU energy dispersive spectrometer system. Transmission spectra of the film/quartz glass struc tures were measured on a Shimadzu 3101PC spectro photometer in the wavelength range from 190 to 3200 nm. A crosssectional image and electron dif fraction pattern of the film/Si(100) structure were INORGANIC MATERIALS

Vol. 51

No. 11

2015

PECVD SYNTHESIS OF HEXAGONAL BORON NITRIDE NANOWALLS Induction coil

1099

Furnace

NH3/He

Substrate Holder

To vacuum pump

Borazinecontaining cylinder

Fig. 2. Schematic of the PECVD reactor.

obtained on a Tecnai G2 F20 STwin TMP scanning analytical transmission electron microscope (TEM). RESULTS AND DISCUSSION We studied the effect of synthesis temperature on the growth rate, elemental and phase compositions, and surface morphology of the films. The growth rate of the films was 10 to 22 nm/min, depending on exper imental conditions. The films ranged in thickness from 30 to 400 nm. Electronmicroscopic examination of the film sur face showed that the films could be divided into two groups according to their surface morphology. The films of one type formed at temperatures below 200°C and had a smooth, uniform surface, with spherical grains 80 nm in average size. Figure 3a shows a typical image of the surface of these films. The films of the other type were obtained in the temperature range 300–700°C. Their surface microstructure differs dras tically from that in Fig. 3a and is formed by socalled nanowalls, which extend along the substrate surface over 50–200 nm, range in width from 15 to 50 nm, and are uniformly distributed over the sample surface (Fig. 3b). Using highresolution TEM, we examined the structure of the films grown at a temperature of 600°С. Their micrographs are presented in Fig. 4. The silicon substrate is in the bottom part of the images, and the protective platinum layer grown in the TEM specimen preparation process is in the top part. The BN film is sandwiched between the Si and Pt. The bright and darkfield crosssectional images in Figs. 4a and 4b demonstrate that the film consists of vertical structures elongated in the film growth direction and perpendic ular to the substrate. The structures, in turn, consist of elongated, anisometric nanocrystallites 2–4 nm in INORGANIC MATERIALS

Vol. 51

No. 11

2015

(а)

100 nm

(b)

100 nm

Fig. 3. Surface images of hBN films grown at (a) 100 and (b) 600°C from a borazine + ammonia mixture at p(B 3N 3H6) : p(NH3) = 1 : 1.

1100

MERENKOV et al.

(а)

100 nm

(c)

(b)

50 nm

(d)

5 nm

Fig. 4. (a) Brightfield, (b) darkfield, and (c) highresolution crosssectional images and (d) selectedarea electron diffraction pattern of an hBN film grown at 600°C from a borazine + ammonia mixture at p(B3N3H6) : p(NH3) = 1 : 1. The scale bars rep resent (a) 100, (b) 50, and (c) 5 nm.

100 90 80 At. percent

70 60 50

N

40

B

30 20 10 0

O 100

200 300 400 500 600 Synthesis temperature, °С

700

Fig. 5. Elemental concentrations vs. synthesis temperature for hBN films grown from a borazine + ammonia mixture at p(B3N3H6) : p(NH3) = 1 : 1.

crosssectional size. The crystallites have a layered structure with an interlayer spacing of 0.32 nm (Fig. 4c), which corresponds to hexagonal boron nitride (sp. gr. P63/mmc) [14]. The crystallites are sep arated by an amorphous material. The film and Si sub strate are separated by an amorphous transition layer 4–6 nm in thickness. Selectivearea electron diffraction data demon strate that the BN film has broadened diffraction rings (halos) in reciprocal space (Fig. 4d), which indicates the presence of hBN nanocrystals in the samples. The observed diffraction spots correspond to reflections from the silicon substrate. The elemental composition of the boron nitride films was determined by energy dispersive Xray spec trometry (Fig. 5). The films consist predominantly of boron and nitrogen, with small amounts of oxygen (within 6.8 at %) and hydrogen. The boron/nitrogen ratio in the films varies little with synthesis tempera ture and is about 1 : 1. There is a small tendency for the INORGANIC MATERIALS

Vol. 51

No. 11

2015

PECVD SYNTHESIS OF HEXAGONAL BORON NITRIDE NANOWALLS

1101 Si

Intensity

B

H O

N Film

0

Transition layer

500

1000

Substrate

1500

Etching time, s Fig. 6. Secondary ion mass spectrometry data for hBN films grown at 400°C from a borazine + ammonia mixture at p(B3N 3H6) : p(NH3) = 1 : 1.

nitrogen concentration in the films to increase and for the oxygen concentration to decrease with increasing synthesis temperature (Fig. 5). Secondary ion mass spectrometry data demon strate that the elemental concentrations are constant across the films (Fig. 6). There is a transition layer in which the boron and nitrogen concentrations decrease and the silicon concentration increases. The oxygen concentration passes through a maximum in the tran sition layer, which is probably due to the presence of a SiOх layer on the silicon substrate. It is worth pointing out that the lines corresponding to the boron and nitrogen concentrations are similar in behavior. This gives grounds to assume that the boron and nitrogen atoms are not directly coordinated to each other and that the character of these bonds remains unchanged throughout the thickness of the film. According to Fourier transform IR spectroscopy results (Fig. 7), the spectra of the films contain fully separated bands of inplane boron and nitrogen vibra tions (B–N) (ν|| = 1370–1400 cm–1), whose fre quency and shape depend primarily on the distortion of the basal planes, and a band due to outofplane lat tice vibrations (B–N–B) (ν⊥ = 790–817 cm–1), whose parameters are determined predominantly by the stacking sequence of the layers [15]. The absorp tion band around 3450 cm–1 corresponds to the N–H hydrogencontaining bonds. The intensity of this absorption band decreases with increasing deposition temperature. In the spectra of the hightemperature INORGANIC MATERIALS

Vol. 51

No. 11

2015

films (t > 500°C), this band was not detected. Absorp tion bands arising from hydrogencontaining bonds with nitrogen were also observed in IR spectra when borazine + nitrogen mixtures were used [16]. In addi tion, the spectra showed a weaker absorption band near 2450 cm–1, which corresponded to vibrations of B–H hydrogencontaining bonds. The concurrent presence of two types of bonds with hydrogen atoms is typical of rf plasma synthesis processes [17]. Note that possible precursors include other boroncontaining compounds, for example, boron halides mixed with ammonia [18]. In such cases, the absorption bands of the N–H and B–H bonds have roughly equal intensi ties, in contrast to the spectra of films produced using borazine + nitrogen mixtures, where the band at 3450 cm–1 is much stronger than that at 2450 cm–1. At the same time, samples obtained through the thermal decomposition of borazine + ammonia mixtures have only hydrogencontaining bonds of nitrogen [19]. Thus, the nature and amount of hydrogencontaining bonds in films depend on the composition of the start ing gas mixture, synthesis temperature, and the way in which the precursor is decomposed. With increasing synthesis temperature, the relative integrated intensity of the absorption band at 1380 cm–1 increases and its width decreases. The integrated intensity of the absorption band at 780 cm–1 also increases. The present spectra have sharper and stron ger absorption bands than do the spectra reported pre viously for hBN films grown through borazine ther

1102

MERENKOV et al. α

ν(B–N)

δ(B–N–B)

ν(N–H) 700°С 600°С 500°С 400°С 300°С 200°С 100°С

500

1000

1500

2000 2500 Wavenumber, cm–1

3000

3500

4000

Fig. 7. Fourier transform IR spectra of hBN films grown from a borazine + ammonia mixture at p(B3N3H6) : p(NH3) = 1 : 1.

the UV spectral region at wavelengths under 250 nm (Fig. 8). Their refractive index evaluated from ellip sometry data is 1.64 ± 0.04 over the entire range of syn thesis temperatures examined.

According to optical characterization results, the boron nitride films containing nanowalls are highly transparent, with >85% transmission in the wave length range 250–3200 nm and an absorption peak in

CONCLUSIONS

Transmission, %

molysis [19]. This may be the consequence of the higher degree of structural order in the hBN films grown by PECVD and containing hexagonal boron nitride nanocrystals.

100 90 80 70 60 50 40 30 20 10

Quartz glass BN

0

500

1000 1500 2000 2500 3000 3500 Wavelength, nm

Fig. 8. Transmission spectrum of an hBN film grown at 600°C from a borazine + ammonia mixture at p(B3N3H6) : p(NH3) = 1 : 1.

Vertically aligned hexagonal boron nitride layers (nanowalls) have been grown by plasmaenhanced chemical vapor deposition from a borazine + ammo nia gas mixture. Conditions have been identified under which amorphous and nanocrystalline hBN films grow. According to scanning and highresolution transmission electron microscopy data, the nanocrys talline layers consist of elongated structures normal to the substrate, with an amorphous component. The nanowalls consist of anisometric, elongated hBN crystallites 2–4 nm in size. The elemental composi tion of the films, which have a B : N ratio of 1 : 1, is essentially independent of synthesis temperature under the conditions of this study. The films have high transmission (>85%) in the visible and infrared spec tral regions. ACKNOWLEDGMENTS This work was supported in part by the Presidium of the Russian Academy of Sciences, program no. 35, project no. 1. INORGANIC MATERIALS

Vol. 51

No. 11

2015

PECVD SYNTHESIS OF HEXAGONAL BORON NITRIDE NANOWALLS

REFERENCES 1. Coleman, J.N., Lotya, M., O'Neill, A., Bergin, S.D., King, P.J., Khan, U., Young, K., Gaucher, A., De, S., Smith, R.J., Shvets, I.V., Arora, S.K., Stanton, G., Kim, H.Y., Lee, K., Kim, G.T., Duesberg, G.S., Hallam, T., Boland, J.J., Wang, J.J., Donegan, J.F., Grunlan, J.C., Moriarty, G., Shmeliov, A., Nicholls, R.J., Perkins, J.M., Grieveson, E.M., Theuwissen, K., McComb, D.W., Nellist, P.D., Nicolosi, V.,, Two dimensional nanosheets produced by liquid exfoliation of layered materials, Science, 2011, vol. 331, no. 6017, pp. 568–571. 2. Britnell, L., Gorbachev, R.V., Jalil, R., Belle, B.D., Schedin, F., Mishchenko, A., Georgiou, T., Katsnel son, M.I., Eaves, L., Morozov, S.V., Peres, N.M.R., Leist, J., Geim, A.K., Novoselov, K.S., and Ponomar enko, L.A., Fieldeffect tunneling transistor based on vertical graphene heterostructures, Science, 2012, vol. 335, no. 6071, pp. 947–950. 3. Wu, B.Y., Yang, B., Han, G., Zong, B., Ni, H., Luo, P., and Chong, T., Fabrication of a class of nanostructured materials using carbon nanowalls as the templates, Adv. Funct. Mater., 2002, vol. 12, no. 8, pp. 489–494. 4. Dillon, A.C., Jones, K.M., Bekkedahl, T.A., and Kiang, C.H., Storage of hydrogen in singlewalled car bon nanotubes, Nature, 1997, vol. 386, pp. 377–390. 5. Ouyang, T., Chen, Y., Xie, Y., Yang, K., and Bao, Z., Thermal transport in hexagonal boron nitride nanorib bons, Nanotechnology, 2010, vol. 21, paper 245 701. 6. Paul, T.K., Bhattacharya, P., and Bose, D.N., Charac terization of pulsed laser deposited boron nitride thin films on InP, Appl. Phys. Lett., 1990, vol. 56, no. 26, pp. 2648–2650. 7. Li, C., Bando, Y., Zhi, C., Huang, Y., and Golberg, D., Thicknessdependent bending modulus of hexagonal boron nitride nanosheets, Nanotechnology, 2009, vol. 20, paper 385 707. 8. Wu, Y., Yang, B., Zong, B., and Sun, H., Carbon nanowalls and related materials, J. Mater. Chem., 2004, vol. 14, pp. 469–477. 9. Yu, J., Qin, L., Hao, Y., Kuang, S., Bai, X., Chong, Y.M., Zhang, W., and Wang, E., Vertically aligned boron nitride nanosheets: chemical vapor synthesis, ultravio let light emission, and superhydrophobicity, ACS Nano, 2010, vol. 4, no. 1, pp. 414–422.

INORGANIC MATERIALS

Vol. 51

No. 11

2015

1103

10. Golberg Shtansky, D., Pakdel, A., Golberg, D., and Bando, Y., Nonwetting and optical properties of BN nanosheet films, Surf. Innov., 2012, vol. 1, no. 1, pp. 32–39. 11. BenMoussa, B., D’Haen, J., Borschel, C., Barjon, J., Soltani, A., Mortet, V., Ronning, C., D’Olieslaeger, M., Boyen, H.G., and Haenen, K., Hexagonal boron nitride nanowalls: physical vapour deposition, 2D/3D morphology and spectroscopic analysis, J. Phys. D: Appl. Phys., 2012, vol. 45, no. 13, paper 135 302. 12. Zhang, C., Hao, X., Wu, Y., and Du, M., Synthesis of vertically aligned boron nitride nanosheets using CVD method, Mater. Res. Bull., 2012, vol. 47, no. 9, pp. 2277–2281. 13. Volkov, V.V., Bagryantsev, G.I., and Myakishev, K.G., Borazine synthesis via reaction between sodium boro hydride and ammonium chloride, Zh. Neorg. Khim., 1970, vol. 11, no. 15, pp. 2902–2906. 14. Wyckoff, R.W.G., Crystal Structures, New York: Inter science, 1963, 2nd ed., vol. 1, pp. 85–137. 15. Geick, R., Perry, C.H., and Ruppecht, G., Normal modes in hexagonal boron nitride, Phys. Rev., 1966, vol. 146, no. 2, pp. 543–545. 16. Smirnova, T.P., Jakovkina, L.V., Jashkin, I.L., Sysoeva, N.P., and Amosov, J.I., Boron nitride films prepared by remote plasmaenhanced chemical vapour deposition from borazine (B3N3H6), Thin Solid Films, 1994, vol. 237, nos. 1–2, pp. 32–37. 17. Akkerman, Z.L., Kosinova, M.L., Fainer, N.I., Rum jantsev, Y.M., and Sysoeva, N.P., Chemical stability of hydrogencontaining boron nitride films obtained by plasma enhanced chemical vapour deposition, Thin Solid Films, 1995, vol. 260, no. 2, pp. 156–160. 18. Kim, H.S., Choi, I.H., and Baik, Y.J., Characteristics of carbon incorporated BN films deposited by radio fre quency PACVD, Surf. Coat. Technol., 2000, vols. 133– 134, pp. 473–477. 19. Ye, Y., Graupner, U., and Krüger, R., Hexagonal boron nitride from a borazine precursor for coating of SiBNC fibers using a continuous atmospheric pressure CVD process, Chem. Vap. Deposition, 2011, vol. 17, nos. 7–9, pp. 221–227.

Translated by O. Tsarev