Boron doped amorphous carbon thin films grown by r.f. PECVD under different partial pressure

Boron doped amorphous carbon thin films grown by r.f. PECVD under different partial pressure

Diamond & Related Materials 14 (2005) 1799 – 1804 www.elsevier.com/locate/diamond Boron doped amorphous carbon thin films grown by r.f. PECVD under d...

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Diamond & Related Materials 14 (2005) 1799 – 1804 www.elsevier.com/locate/diamond

Boron doped amorphous carbon thin films grown by r.f. PECVD under different partial pressure J. Podder *, M. Rusop, T. Soga, T. Jimbo Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya 466-8555, Showa-ku, Gokiso-cho, Japan Available online 29 August 2005

Abstract Boron doped hydrogenated amorphous carbon (a-C) thin films have been deposited by r.f.-plasma CVD with a frequency of 13.56 MHz at room temperature using pure methane as a precursor of carbon source mixed with hydrogen (H2) as a carrier gas. The films were prepared by varying the r.f. power, different flow rates of CH4, and partial pressure of mixed gas (CH4/H2) using solid boron as a target. The thickness, structural, bonding and optical properties of the as-deposited films were studied by Alpha step surface profiler, Raman, FT-IR, XPS and UV – visible spectroscopy. It was found that changing the deposition pressure in presence of solid boron dopant in the r.f. PECVD process has a profound effect on the properties of the deposited films, as evidenced from their Raman scattering and optical results. The grown p-C: B films were found very smooth and thickness in the range of 240 to 360 nm for 1 h deposition. Films deposited at lower pressure appear brownish color whereas those deposited at higher pressure appear pale yellowish. The as-deposited film is found to be dominated by sp2 rather than sp3, which might be due to the formation of small crystallites. The optical band gap is found to be reduced from 2.60¨1.58 eV as the partial pressure of CH4/H2 gas is reduced. D 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon film; r.f. plasma CVD; Raman spectroscopy; Optical properties

1. Introduction The study of hydrogenated amorphous carbon (a-C: H) or tetrahedral hydrogenated amorphous carbon (ta-C: H) thin films has given much attention currently among the photovoltaic researchers because of numerous superior qualities of these materials such as micro-hardness, low friction, semi-conducting behavior, good optical and optoelectronic properties [1– 4]. Thin films of a-C: H are not having a dominant crystalline lattice structure but rather an amorphous phase consisting of a mixture of considerable amount of trigonal sp2 and tetrahedral sp3 bonds with an involvement of k and j electrons [5]. In amorphous carbon, the k electronic states are less spaced in energy than the r˙ electronic states, which are entirely responsible for the * Corresponding author. Permanent address: Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh. Tel.: +88 2 9665613; fax: +88 2 8613046. E-mail address: [email protected] (J. Podder). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.07.020

physical properties of a-C: H in the visible-near UV photon energy range. Undoped amorphous carbon (a-C) is reported to exist as weakly p-typed in nature with complexes structure and high density of intrinsic defects [6], which restricts its efficiency and also the main barrier to its application in various electronic devices. Amorphous carbon shows a wide optical band gap in the range of 5.5 to 0.25 eV range. In recent years, an extensive research has been done on the doping of these a-C films with donors like nitrogen or phosphorous and acceptors like boron or iodine [7]. However, the control of doping can modify the electronic properties, particularly the energy gap states and conductivity in the semiconductor materials by controlling the ratio of sp3 / sp2 carbon. Boron doped hydrogenated amorphous carbon (a-C: H) films using methane and tri-methyl boron (TMB) were studied with varying the r.f. power [8]. The concentration of boron in the film was not measured but it was expected that the boron concentration is increased at higher r.f. power and improves the photoconductivity. P-type carbon films were prepared

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J. Podder et al. / Diamond & Related Materials 14 (2005) 1799 – 1804

by PLD (Pulse laser deposition) from graphite with amorphous boron doping [9,10]. Target was prepared by mixing the powder of pure graphite with varying amount of boron powder (1% to 20% weight) and compressed into pellets. The boron doping significantly increases the Eg by increase of sp3 but also improved the g of photovoltaic solar cells behavior, and suggesting saturation of boron content nearly about 1.75 wt.%. Boron sources like di-boron trioxide (B2O3), tri-methyl boron (TMB), etc. in gaseous form are very toxic and a health hazard. The chemical reactivity of boron depends on its form; generally, high purity crystalline boron is fairly stable and less reactive than gaseous and amorphous sources. However the solid boron source in crystalline form has not yet been used so far our knowledge is concerned. The crystalline borons are an extremely hard, usually jet-black to silver-gray color, brittle and lustrous. In addition, the crystalline form is oxidized only very slowly, even at higher temperatures, where as the amorphous powder is oxidized slowly in air at room temperature and ignites spontaneously at high temperatures. The melting point of boron is 2097 -C, its boiling/ sublimation point is at 2550 -C, the specific gravity of crystalline boron is 2.34. High purity crystalline boron (99.9999%) is obtained by vapor phase reduction of boron tri chloride or boron tribromide with hydrogen on electrically heated filaments. Hydrogenated amorphous carbon (a-C: H) films have been prepared using a variety of different techniques such as ion-beam deposition, sputtering and r.f. PECVD. Among these different techniques, the r.f. PECVD process has been most widely used due to its simplicity and ability to deposit a-C: H films with a wide range of different properties. With an aim of finding easy, reproducible, low cost and cheap materials, as well as environmentally friendly production processes for solar cells devices, in this paper, an attempt has been made to grow boron doped a-C: H films by r.f. plasma-enhanced chemical vapor deposition (r.f.-CVD) using solid boron as target for the first time with a frequency of 13.56 MHz at room temperature over a wide range of partial pressure of CH4/H2 and r.f. power (see Table 1). The advantage of using solid boron over gaseous and amorphous boron sources is expected to minimize the B radicals that generated by r.f. enhanced plasma discharge process due to high purity of solid target. This is also expected to optimize the physical and chemical properties of the films. In this work, we also systematically studied the

effects of partial pressure on the optical properties of the as deposited a-C: H films.

2. Experimental details A parallel-plate r.f. plasma enhanced chemical vapor deposition (PECVD) reactor with a capacitively coupled parallel plate operating at 13.56 MHz was used to deposit aC films on corning glass and n-Si (100) substrates. The amorphous carbon films were deposited under the different controlled deposition parameters viz. r.f. power, gas flow rates and inner partial pressure of CH4/H2 gas. The glass and n-Si substrates having 20  20 mm2 sizes were placed over a metal sieve and pure boron crystalline grains having 3– 7 mm sizes were spread out over the sieve onto the electrode, which was connected to the r.f. oscillator, and kept approximately at room temperature. The r.f. power was varied from 100 to 300 W. The spacing between the electrodes was maintained 49 mm. Before deposition, the glass and n-Si substrates were cleaned ultrasonically with acetone and methanol each for 10 min, respectively. After cleaning they were etched with HF : H2O (1 : 10) for 1 min in order to remove the resistive native oxide formed over the surface and then quickly transferred into the r.f. PECVD chamber. The films deposition was carried out for 1 h. The film thickness, structural and optical characterization were performed by Alpha-step 500 surface profiler, Raman scattering analysis, X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared spectroscopy and UV – visible spectroscopy.

3. Experimental results and discussion 3.1. Raman analyses Raman spectroscopy is widely used for the analysis of structural and phase disorder information in carbon and carbon related materials. In general, two main peaks are seen in the spectra of amorphous carbon films: the G peak and the D peak. The G and D bands are characteristic of a mixture of graphitic and disordered sp2-bonded carbon. The G peak is due to a bond stretching vibration of all pair of sp2 sites in both chain and rings, representing graphite like sp2 while the D peak is due to the breathing vibration of sp2

Table 1 Growth conditions and physical properties of boron doped a-C: H thin films grown by r.f. PECVD at 300 W r.f. power Sample code

CH4 (sccm)

H2 (sccm)

Partial Pressure (Pa)

D-position (cm 1)

FWHM (cm 1)

G-position (cm 1)

FWHM (cm 1)

I (D) / I (G)

Thickness (nm)

Energy (eV)

A B C D E

10 10 10 10 15

50 50 50 50 50

40 20 10 5 5

1390 1336 1339 1342 1338

412 315 313 318 358

1551 1555 1560 1565 1553

160 137 149 162 138

2.05 3.10 3.13 3.29 3.36

360 330 285 240 270

2.60 1.88 1.72 1.58 2.13

J. Podder et al. / Diamond & Related Materials 14 (2005) 1799 – 1804

G-Peak

Raman Intensity (arb. units)

Raman Intensity (arb. units)

D-Peak

(E) (D) (A) (C)

1200

1400

1600

1800

Fig. 1. Raman spectra of boron doped a-C: H thin films deposited from different partial pressure at 200 W. 2

atoms in rings represents disordered sp hybridized C with an amount of sp3 hybridized C. The Raman spectra were fitted with Gaussian and Lorentzian distributions to extract the characteristics of the G and D peak. Figs. 1– 3 show Raman spectra of boron doped and undoped hydrogenated amorphous carbon (a-C: H) films for different r.f. power and partial pressure of CH4/H2 gas. The as deposited films show mostly amorphous nature when the r.f. power is applied 100 W (figure not shown). No individual sharp peak is observed in Fig. 1, except B and C for r.f. power 200 W. Hence it is assumed that C – H bonds such as sp3 C –H and sp2 C – H might merge somewhat into each other. It is reported that the sp2 cluster are embedded in sp3 matrix for a-C: H film [11]. In the Fig. 2, peak appears approximately at about 1550 cm 1 corresponding to the G-band for crystalline graphite, while another peak appears at about 1340 cm 1 corresponding to the D band for polycrystalline graphite. Our results reveal that boron doped a-C: H films are dominated by sp2

Raman Intensity (arb. units)

D-Peak

(A) (C) (B) (E)

(B) (C) (E) (A)

1400

1600

1200

1400

1600

1800

Raman Shift (cm-1) Fig. 3. Raman spectra of boron undoped a-C: H thin films deposited from different partial pressure at 300 W.

rather than sp3 at higher r.f. power and which might be due to the formation of small size of crystallites and clustering of aromatic rings implies the increase of G peak [12]. The increase in I (D) / I (G) observed at lower partial pressure to be an indicative of an increase of sp2 content in the a-C:H films. But for boron-undoped samples; the D band is found much more dominant in most cases (Fig. 3) even the partial pressure is reduced. It means that sp3-C site is more stronger than sp2-C sites. But in case of boron-doped samples, sharp G peak is expected due to the formation of graphitic domains and that is not only for reducing the partial pressure but also for the incorporation of boron into the films. It is true that the partial pressure has an influence on the up shifting of the G peak. But in comparing the Raman spectra of Figs. 2 and 3, we can see the clear evidence and indication of the influence of boron in the as deposited films even the partial pressure is reduced and increasing the r.f. power. Raman spectra of boron doped a-C: H thin films deposited at 300W (Sample-E) with Gaussian fittings are shown in Fig. 4. 3.2. Effect of partial pressure

G-Peak

(D)

1200

G-Peak

(D)

Raman Shift (cm-1)

1000

D-Peak

(B) 1000

1000

1801

1800

Raman Shift (cm-1) Fig. 2. Raman spectra of boron doped a-C: H thin films deposited from different partial pressure at 300 W.

A slight frequency shifting to higher G peak position was found in our sample if the partial pressure of mixed CH4/H2 is reduced gradually from 40 to 5 sccm (Fig. 3.). A little up shifting of the Raman D and G peak positions may be caused by the alignment change of the perpendicular carbon network consisting of sp2 C and growth of graphitic domains [13]. Robertson also pointed out that sp2 C sites tend to be re-oriented with their bonds perpendicular to the plane of the film and their k orbital in the film plane by incident ion species [14]. In support of the previous results, we can conclude that the shifting to higher wave number of G peak position suggesting the relative decrease of sp3 C fraction and increasing of sp2 content in the a-C: H film. For higher partial pressure, sharp D band is observed. This is may be due to higher decomposing rate of methane,

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0.8

3.4. XPS characterization

0.4

0.2

0 1100

1300

1500

Raman Shift

1700

(cm-1)

Fig. 4. Raman spectra (Gaussian fittings) of boron doped a-C: H thin films deposited at 300 W. Sample-E (CH4 = 15 sccm, H2 = 50 sccm, Partial pressure = 5 Pa).

resulting in the high sp3 content. But a combination of low pressure is expected to minimize ion energy loss by collisions in the plasma sheath and increasing the flux energy promoting the formation of graphite like structure. At higher pressures, the deposition rate is increased due to larger hydrocarbon flux reaching the substrate surface. The average thickness of the films was found in the range of 240 to 360 nm under different partial pressure and r.f. power 300 W for 1 h deposition. 3.3. Fourier transform infrared spectroscopy The bonding structure of carbon and hydrogen atom in the as deposited film is obtained by FT-IR spectroscopy measurement. The spectra obtained from FT-IR for as deposited samples are shown in Fig. 5. The band appearing at the wave number in the range of 1050 – 1300 cm 1 corresponds to C –O bond and the band appearing in the range of 1500– 1600 cm 1 corresponds to C_C bond (the sp2 bonding). The C – H bond appearing at two wave number ranges: one at 1340– 1470 cm 1 and the other at 2700 – 3100 cm 1. The band appearing at wave number around 2350 cm 1 represents here only for the C‘C bonding. Hydrogen atom can saturate the C_C bonds, mainly by converting sp2 C sites into sp3 _CH2 and ‘CH sites, rather than by increasing the number of C – C bonds [15]. The band situated in the range of 2950 – 3060 cm 1 is attributed to C –H stretching vibration of sp2-C, while for sp3-C in the range 2850 – 2945 cm 1 range [16]. It is to be noted that all the films exhibit Fcharacteristic C –H related absorption_ in the range 2800 –3000 cm 1. Since the C – H bond signal is found very weak so it is assumed that the attachment of H bond is less. The over all FT-IR spectra for all the samples are not much prominent and the band signal is very weak. So it is not quite satisfactory for quantitative study.

3.5. Energy gap The optical properties of the as-deposited a-C: H films are investigated from the optical transmittance and reflectance spectra of the films in the range of 300– 2000 nm using UV –visible spectroscopy measurements (UV/VIS/ NIR spectrometer—Jasco V570). Tauc relationship [18] was used to evaluate the energy gap (Eg). Fig. 7 shows the plot of (ahr)1/2 as a function of photon energy (hr). The optical gap (Eg) is obtained from the extrapolation of the linear part of the curve at the absorption coefficient a = 0 using the Tauc relation [18,19]. In our experimental results, the

Transmittance (arb.units)

Intensity (a.u)

0.6

The atomic concentrations of C, B and O in the as grown films were determined by XPS measurement (SSX-100 XPS system of Surface Science Instrument) using Al Ka (1486.6eV) radiation as an X-ray source under high vacuum conditions of about 10 10 Torr. The chemical bonding state in the films was analyzed after the 0.5-keV Ar+ ions etching of the film surface for 3 min and displayed in Fig. 6. In the a-C: H film, the C 1s peak is centered at a binding energy of about 293.6 eV. The main signal at around 284.7 eV can be assigned to the C‘C bond. The binding energy located between 284.8 and 285.5 eV is the evidence of the C –H bonds. Our results are found in good agreement with the reported values [17]. The presence of boron is identified by a weak signal at about 200.9 eV. The atomic percentage of boron is calculated about 2.07% (Sample A). Although the intensity of XPS signal for boron is weak (not appeared in the broad spectrum) but the energy peak area was measured by Gaussian distribution function for atomic percent calculation. The binding energy located at 542 eV is observed for oxygen. The small concentration of oxygen is detected in our films, which generally occurs in the voids as sites for oxygen absorption during post deposition exposure to atmospheric air. The atomic percentage of oxygen is found to be in the range of 3% to 7%.

(E)

C=C

C=C

)

C-H (D) C-O

sp3 CH2 Sym sp2 CH2 Sym

(C) (B) )

(A) 700

1100

1500

1900

2300

2700

3100

-1

Wave number (cm ) Fig. 5. FT-IR spectra of boron doped a-C: H thin films deposited from different partial pressure at 300 W.

J. Podder et al. / Diamond & Related Materials 14 (2005) 1799 – 1804

1803

Fig. 6. XPS spectra of Boron doped a-C: H thin films deposited under 10 Pa partial pressure at 300 W (Sample A).

energy band gap gradually reduces from 2.60 to 1.58 eV as the partial pressure is reduced in boron doped a-C: H films. But for a little increase of CH4 concentration, the band gap is found to be increased (Eg = 2.13 eV) even the partial pressure is low. This may be due to the higher decomposition of CH4 thus leading to increase of sp3 content as well as dangling bond (sample E). Due to increase of dangling bonds, there is an increase of small particulates with higher grain boundaries, thus leading to a higher spin density and justifying the decrease in the crystallite size. It means that, at higher hydrogen concentration, it is possible to have higher fraction of tetrahedrally coordinated carbon without becoming constrained. On the other hand, the decreasing amount of hydrogen may be expected to give rise to increase the concentration of the localized electron states in the forbidden gap by increasing the number of free sp3 bonding orbital. The significant reduction of the energy gap 700

(E) 600

(B)

(αhv)1/2

500

(D) (C)

400

(A)

300 200 100 0 1

1.5

2

2.5

3

3.5

4

4.5

Photon Energy (eV) Fig. 7. Optical band gap of boron doped a-C: H thin films deposited from different partial pressure at 300 W.

is expected to be achieving by the distortions of k-bonded pairs. Although it is known that k states determine the gap, but the relationship between the structure, and the optical band gap is quite complicated. It is agreed that the determination of band gap is not accurate using Tauc’s formulation because of the extent of the valence and conduction-band tails in the gap, especially in the case of the films where nanocrystallites are embedded in the amorphous matrix. However, it is expected to give an approximate estimate of the band gap.

4. Conclusions In this paper we mainly utilized Raman, FT-IR, XPS and UV spectroscopy to analyze the bonding structure and optical band gap of as deposited a-C: H films. The grown p-C: B films were found smooth, light brown colored and thickness in the range of 240 to 360 nm. The film thickness is found to increase with increasing r.f. power as well as higher partial pressure. The optical band gap is found to decrease in boron doped a-C: H films at lower partial pressure of CH4/H2 gas, which might be due to the formation of small crystallites in the films. By optimizing the partial pressure we have able to reduce the band gap around 1.6 eV. in boron doped as deposited films. This results show that it is possible to control the optical band gap through changing the partial pressure and flow rate of CH4 gas. Further studies are in progress to control the boron doping and its stability will be reported in future. The study of photovoltaic properties of these boron doped a-C: H films with the dark and illuminated J-V have been performed for finding the possible scope of its application in carbon based solar cell as a low cost material for photovoltaic applications is discussed in another publication.

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Acknowledgements This work is supported by the carbon solar cell project under New Energy and Industrial Technology Development Organization (NEDO).

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