Optical and electrical characteristics of plasma enhanced chemical vapor deposition boron carbonitride thin films derived from N-trimethylborazine precursor

Optical and electrical characteristics of plasma enhanced chemical vapor deposition boron carbonitride thin films derived from N-trimethylborazine precursor

Thin Solid Films 558 (2014) 112–117 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Optica...

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Thin Solid Films 558 (2014) 112–117

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Optical and electrical characteristics of plasma enhanced chemical vapor deposition boron carbonitride thin films derived from N-trimethylborazine precursor Veronica S. Sulyaeva a,⁎, Marina L. Kosinova a, Yurii M. Rumyantsev a, Fedor A. Kuznetsov a, Valerii G. Kesler b, Viktor V. Kirienko c a b c

Department of Functional Materials Chemistry, Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia Laboratory of Physical Principles for Integrated Microelectronics, Rzhanov Institute of Semiconductor Physics SB RAS, Novosibirsk 630090, Russia Laboratory of Nonequilibrium Semiconductors Systems, Rzhanov Institute of Semiconductor Physics SB RAS, Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 22 June 2013 Received in revised form 25 February 2014 Accepted 28 February 2014 Available online 11 March 2014 Keywords: N-trimethylborazine Boron carbonitride Thin films Plasma-enhanced chemical vapor deposition Optical properties Dielectric constants

a b s t r a c t Thin BCxNy films have been obtained by plasma enhanced chemical vapor deposition using N-trimethylborazine as a precursor. The films were deposited on Si(100) and fused silica substrates. The grown films were characterized by ellipsometry, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, spectrophotometry, capacitance–voltage and current–voltage measurements. The deposition parameters, such as substrate temperature (373–973 K) and gas phase composition were varied. Low temperature BCxNy films were found to be high optical transparent layers in the range of 300–2000 nm, the transmittance as high as 93% has been achieved. BCxNy layers are dielectrics with dielectric constant k = 2.2–8.9 depending on the synthesis conditions. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Boron carbonitride (BCxNy) material attracts intensive attention mainly because its electronic structure can be tailored by changing its composition. Depending on composition and structure the BCxNy material exhibits different properties. The hexagonal phase of BCxNy is a perspective material with variable band gap energy. Authors of [1–3] have noted that the evolution of optical band gap (Eg) goes in accordance with the change of film composition and bonding state, Eg is increased with nitrogen content increasing. Several groups investigated the microhardness of the BCxNy films [4–6]. Now h-BCxNy is known as hard material with a hardness value of up to 40 GPa [7]. Investigation of dielectric properties of BCxNy allowed considering it as the low-k material. It has been reported that the dielectric constant of the film depends on carbon concentration and part of crystallinity [8,9]. Much effort has been devoted to the synthesis of the ternary BCxNy films with various compositions using chemical vapor deposition (CVD) [10–15]. Typically mixtures of toxic and explosive substances (B2H6, CH4, NH3) are used for film synthesis. Usage of single-source precursors containing all elements being necessary to synthesize BCxNy greatly facilitates growth process. Such precursors are element ⁎ Corresponding author. Tel.: +7 383 330 6646; fax: +7 383 330 9489. E-mail address: [email protected] (V.S. Sulyaeva).

http://dx.doi.org/10.1016/j.tsf.2014.02.082 0040-6090/© 2014 Elsevier B.V. All rights reserved.

organic volatile compounds like alkylamine borane complexes R3 N·BH3 and alkylborazines B3 N 3H3 R3 . These precursors contain fragments with ready chemical bonds \B\N\C\. The films of BCxNy, h-BN and a mixture of h-BN and B4C were deposited by plasma enhanced chemical vapor deposition (PECVD) from (CH3)3N·BH3 [16]. However, much less attention has been focused on the possibility to prepare BCxNy layers from carbon containing borazine derivatives. There are few works where N-trimethylborazine (TMB) [17–19] and borazine derivatives [20] were used to produce BCxNy films. We have developed CVD processes with two different types of boroncontaining precursors: alkylamine boranes R3N·BH3 and borazine derivatives N3B3H3R3, where R = CH3, C2H5, to study the effect of design molecules and B:C:N atomic ratios in the precursors on properties of BCxNy films. Use of methyl or ethyl radicals allowed changing the amount of carbon in the deposited films. This work continues and supplements our previous studies concerning the preparation of BCxNy films by PECVD from trimethylamine borane (CH3)3N·BH3 and its mixtures with He, H2, NH3 [16], by LPCVD from triethylamine borane (C2H5)3N·BH3 and its mixtures with N2, NH3 [7], by PECVD from N-trimethylborazine (CH3)3N3B3H3 with N2 [21], and N-triethylborazine (C2H5)3N3B3H3 with He, NH3 [22]. The aim of this investigation was to explore BC xN y thin film synthesis by PECVD, using mixtures composed of Ntrimethylborazine and hydrogen or ammonia, and to study the dependence of the growth rate, refractive index, types of chemical

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bonds, optical and electrical properties of films depending on the growth conditions. 2. Experimental details 2.1. PECVD synthesis BCxNy films were grown by plasma enhanced chemical vapor deposition, using mixtures of TMB and hydrogen or ammonia. The experimental setup is schematically shown in Fig. 1. It consists of a quartz reactor, which has discharge and growth zones. The reactor is equipped with a precursor gas and an additional gas inlets, a vacuum gauge and RF coils. The deposition was done at a constant RF power of 40 W (40.68 MHz). The reaction chamber was evacuated by rotary pump up to 0.67 Pa before the film deposition. A glass source, containing the precursor was connected to the reaction chamber. The vapor pressure of the precursor was constant and PTMB = 1.33 Pa in all experiments. Pressure of additional gases was equal to 0.67 Pa. The deposition was carried out at the working pressure of 2.67 Pa. The hydrogen or ammonia gases passed through the discharge zone, and mixing with Ntrimethylborazine, appeared at the growth zone which was heated by the resistance furnace. In order to study synthesized film characteristics, the deposition temperature was varied from 373 to 973 K, with the other process parameters being fixed. Silicon wafers with orientation (100) and fused silica were used as substrates that were placed on a holder at the center of growth zone. A liquid nitrogen trap was inserted between the reactor and the rotary pump to avoid any contamination caused by oil back streaming and to condense the reaction products. 2.2. Characterization techniques The thickness and refractive index of BCxNy films were determined by ellipsometry (LEF-3M ellipsometer) at the wavelength of 632.8 nm. The measurements were carried out at seven angles. The surface microstructure and the elemental composition of the films were observed by scanning electron microscope (SEM) JEOL JSM 6700 F equipped with an EX-23000BU analyzer for element composition determination by X-ray energy dispersive spectroscopy (EDX). Fourier transform infrared (FTIR) spectra of the films were recorded using SCIMITAR FTS 2000 spectrometer in the range 300–4000 cm−1. 32 scans and the aperture equal 4 at achievable resolution 2 cm−1 were used during the measurements. In each case the spectrum of a substrate was subtracted from that of a sample. All FTIR spectra were normalized to thickness of the appropriate film. The X-ray photoelectron spectra (XPS) were obtained by a MAC-2 (Riber) analyzer using non-monochromatic Al Kα radiation (1486.6 eV) with the power of 300 W, and X-ray beam diameter of ~ 5 mm. The energy resolution of the instrument was chosen to be 0.7 eV, so as to have sufficiently small broadening of natural core level

Fig. 1. Schematic representation of plasma enhanced chemical vapor deposition system for BCxNy film deposition.

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lines at a reasonable signal–noise ratio. Under these conditions the observed full width at half maximum of the Au 4f7/2 line was 1.31 eV. The binding energy scale was calibrated in reference to the Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, assuring the accuracy of ± 0.1 eV in any peak energy position determination. Since BCxNy are dielectric films, the photoelectron energy drift, due to charging effects, was taken into account in reference to the position of C 1s (284.6 eV) line generated by adventitious carbon on the sample surface when inserted into the vacuum chamber. The component of adventitious carbon was derived from complex carbon peak structure by means of deconvolution. To decompose the overlapped XPS peaks, we used mixed Gaussian and Lorentzian line shape functions with the parameters of the peaks measured on standard sample h-BN and B4C. Optical transmittance of the deposited films was examined, using spectrophotometry (Scanning Spectrophotometer UV-3101PC Shimadzu) in the range 190–3200 nm (resolution 2 nm). For electrical measurements (C–V and I–V characteristics), the metal–insulator–silicon structures were fabricated using the BCxNy films as the insulator. The Al electrodes have a typical area of 4.9 × 10−3 cm2. One contact was directly on the Al electrode and the other contact was on the back of the silicon substrate with the aluminium contact. The C–V characteristics were recorded at f = 1 MHz and T = 293 K.

3. Results and discussion In this investigation, we studied the influence of two growth parameters (deposition temperature and gas phase composition) on deposition rate of the BCxNy films, their chemical composition, surface morphology, optical and electrical properties. The BCxNy film thickness was 100–300 nm. Growth rate decreased from 24 to 2 nm/min with increasing temperature of synthesis from 373 to 973 K (Fig. 2). The variation of the growth rate with deposition temperature for both mixtures of TMB with H2 and NH3 are very close.

3.1. Surface morphology The surface morphology of the films shown in Fig. 3 was observed by scanning electron microscopy. Fig. 3a and b shows the morphology of the BCxNy films synthesized at 973 K with different additional gases. There are nanoparticles on the surface of high temperature films (nanoparticle size up to 20 nm), while the low temperature films had smooth featureless surfaces.

Fig. 2. Refractive index and growth rate of BCxNy films synthesized from mixtures of Ntrimethylborazine with H2 and NH3 as a function of deposition temperature.

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a

b

Fig. 4. Fourier transform infrared spectra of BCxNy films prepared at different deposition temperatures from TMB:H2 = 2:1 (a) and TMB:NH3 = 2:1 (b) initial mixtures. Fig. 3. Morphology of BCxNy films prepared from TMB:H2 = 2:1 (a) and TMB:NH3 = 2:1 (b) initial mixtures at deposition temperature of 973 K.

3.2. FTIR analysis The bond types in the deposited films at various deposition temperatures were identified using FTIR-spectroscopy (Fig. 4a and b for TMB + H2 and TMB + NH3 initial mixtures, respectively). IR absorption of the investigated series of BCxNy films varies dramatically with the synthesis temperature. It should be emphasized that all FTIR spectra have one main band at approximately 1380 cm−1. FTIR spectra of the films deposited at high temperature (973 K) correspond to h-BN spectrum, which contains two bands centered at 780 and 1380 cm−1 respective to out-of-plane B\N\B and in-plane B\N vibrations [23]. High temperature films deposited from both TMB + H 2 and TMB + NH3 mixtures have the shoulder at 1100 cm−1 which corresponds to B\C bond in boron carbide [24]. At lower temperatures of synthesis (Tdep b 773 K) intensity of the main band significantly reduces and it shifts to the region of higher wave numbers. The vibration of B\N bonds in cyclic borazine compounds [25,26] is responsible for the absorption at 1400–1415 cm−1. It probably revealed the presence of ring structure from initial molecule of N-trimethylborazine in the deposited films. Also low temperature films contain stretching and strain vibrations of B\H, C\H and N\H bonds. Thus the infrared spectra show that the deposited films contain boron and nitrogen, possibly bonded in a ring structure, and also contain hydrogen bonded to the boron, carbon and nitrogen. They, therefore, are hydrogenated films

described by the empirical formula BCxNy:H. Similarly to our research, authors of [5] reported that the detected hydrogen concentration depended on the substrate temperature during the deposition. FTIRspectra of BCN layers synthesized at low temperatures showed absorptions of N\H, C\H and B\H bonds. By elastic recoil detection analysis, the relative content of hydrogen increased from 8 to 35 at.%, when decreasing the temperature of synthesis from 1073 to 323 K [27].

3.3. EDX analysis The investigation of elemental composition by EDX shows that the main elements of the films are boron, nitrogen, carbon and oxygen as impurity. The content of these elements changes inefficiently with deposition temperature. Because a small amount of oxygen was detected in the films, their composition should be correctly represented as BC xNyO z where x = 0.7–1.1, y = 1.3–1.7, z = 0.1–0.3 for films synthesized from TMB + H2 mixture and x = 0.5–1.1, y = 1.2–2.0, z = 0.1–0.4 for films synthesized from TMB and ammonia. Unfortunately, the presence of hydrogen in low-temperature films distorts the EDX data, because the limitation of this method is impossibility of determination of the hydrogen presence. In a previous study we used Ntriethylborazine with ammonia as initial mixture [22], the character of the elemental composition change was the same, but carbon content at low temperatures is slightly higher due to more carbon content in precursor.

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3.4. XPS analysis

3.5. Optical properties

Characterization of atomic bonding in BCxNy films was performed by using XPS. To confine surface impurities adsorbed during air handling the samples were introduced into an ultra-high vacuum chamber in order to investigate the chemical composition immediately after preparation. Fig. 5 shows X-ray photoelectron spectra of B 1s, C 1s and N 1s, together with their deconvolution into several overlapping components. A minimal number of spectral components were selected on the base of results of standard sample investigation [28], spectrometer resolution and literature data. The above deconvolution is the most realistic but not the only possible one, which may occur in the process. That is based on fact that, to receive different results at different possible input data, the existence of different environments around the boron atom has been studied earlier by near-edge X-ray absorption fine structure spectroscopy [14,29]. The B 1s spectra have features at 187.0, 189.5, 190.5 and 192.1 eV that correspond to B\B (in B) [30], B\C (in BCx) [31], B\N [32] and B\O [27] bonds, accordingly. Thе latter signal is more intensive in case of the initial mixture TMB + H2 due to the high content of B\O bonds. The main component of C 1s with the binding energy 284.6 eV attributed to graphitic carbon due to adventitious carbon contamination on the sample surface [33]. Other features at 282.7, 285.9 and 286.9, 288.3 can be associated to C\B [33], C\N [34,35], C\O [36] bonds. The N 1s spectra exhibit an intensive peak at 398.3 eV that can be attributed to N\B bonds those the same as for h-BN [27]. The observed peaks at 400.0 and 401.6 can be ascribed to N\C [35] and N\O [34] bonds, correspondingly. The ratio between areas of N 1s and B 1s peaks is independent of carbon and oxygen content and allows us to calculate the nitrogen and boron concentration in the film. For the film synthesized by using the mixture of TMB with NH3 this ratio was equal to 0.93. Hence, the composition of investigated film is close to boron nitride. This phase also dominated in the films synthesized from the mixture of TMB with H2, but in this case, the nitrogen to boron concentration ratio was equal to 0.65. The use of H2 results in the relative increasing of B 1s, N 1s and C 1s components with higher binding energies, typical for bonding of these elements with the oxygen. There are small components in B 1s and C 1s spectra corresponding to B\C bonding, however, the portion of BCx phase does not exceed 5%.

Refractive index for the BCxNy films deposited from TMB + H2 and TMB + NH3 mixtures increased from 1.5 to 2.0 with deposition temperature increasing from 373 to 973 K (Fig. 2), which is the result of composition and structure changing. We associate the composition changes with formation H-containing bonds in the films at the lower substrate temperatures (Tdep b 773 K), that was confirmed by FTIR data. Structure changes supposedly are due to the formation of amorphous structure at the low temperature of synthesis. Therefore, deposited low-temperature films are hydrogenated films of BCxNy:H composition with more friable structure and lesser refractive index. The same low values of refractive index n = 1.5 [11], 1.22–1.52 [37] were obtained for inductively coupled radio frequency PACVD BCN layers at substrate temperature 333–393 K and n = 1.3–1.6 for PECVD hydrogenated boron carbonitride BxCyNz:H films synthesized at room temperature [38]. To study the optical transparency of BCxNy films, transmittance measurements were carried out in the range of 190–2000 nm for samples deposited on fused silica substrates. Fig. 6 shows the transmittance curves of the films synthesized at different temperatures. In the region from 300 to 2000 nm the low temperature films are more transparent (transmittance up to 93%) than high temperature ones. The significant impairment of transmittance of BCxNy films prepared from TMB + H2 were observed only at deposition temperature of 973 K. Also high transmission (90%) in the UV–visible region was achieved by using TMB as precursor [11,37]. 3.6. Electrical properties The capacitance–voltage and current–voltage measurements showed that both low temperature and high temperature BC xN y films are dielectrics: relative dielectric constant k = 2.2–8.9, resistivity ρ = 109–1011 Ω∙cm, electric breakdown field Ebr = 105–106 V/cm. The value of relative dielectric constant decreased monotonically with the increasing of growth temperature (Fig. 7). Any strong dependence of resistivity and electric breakdown field on the investigated growth conditions was not revealed. Similar results were obtained for PECVD boron carbonitride films with k = 5.2, ρ = 1011–1012 Ω∙cm [39]. Contrary to our research, it was estimated that relative dielectric constant

Fig. 5. X-ray photoelectron spectra of the BCxNy films synthesized at 673 K from TMB + H2 and TMB + NH3 gas mixtures.

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a

a

b

b

Fig. 6. Optical transmittance spectra for BCxNy films prepared at different deposition temperatures from TMB:H2 = 2:1 (a) and TMB:NH3 = 2:1 (b) initial mixtures.

Fig. 7. Dependence of relative dielectric constant, k (a) and resistivity, ρ (b) of BCxNy films from growth temperature for N-trimethylborazine with H2 and N-trimethylborazine with NH3 initial mixtures.

value varied in the range from 2.1 to 5.0, and decreased with growth temperature decreasing and with the increasing carbon content in the films [40–42].

increasing of the synthesis temperature. The dielectric constant was estimated to be as low as 2.2 for the BCN film grown at 973 K.

4. Conclusions

Acknowledgments

The deposition of hexagonal BCxNy layers opens a wide range of possible applications where high transparency products are used nowadays. There is not much information available on optical properties of BCxNy films and therefore some basic research is necessary on this topic. A plasma activated CVD procedure for the BCxNy films was developed by using TMB, and ammonia or hydrogen as precursors. Amorphous and nanocrystalline BCxNy films were synthesized by the PECVD technique on Si (100) and fused silica wafers at the deposition temperature region from 373 to 973 K at constant RF plasma power. Low temperature boron carbonitride films showed high optical transparency in the range of 400–2000 nm (transmittance up to 93%). It can be concluded that high transparency is accompanied by high hydrogen content. Transparency of films deposited at temperatures over 773 K falls much, that is connected with change of the film composition: reduction of hydrogen-containing bonds and emergence of an additional phase — amorphous carbon that was shown in our previous data for the BCxNy films synthesized from mixture of N-trimethylborazine with nitrogen [21]. And the authors of [43] have also shown that increasing carbon content in the films leads to deterioration of transparency. The refractive index of the BCxNy films increased from 1.5 to 2.0 with the

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