IR transmittance of large-sized free-standing transparent diamond films prepared by MWPCVD

NEW CARBON MATERIALS Volume 23, Issue 3, March 2008 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2008, 23(3): 245–249.

RESEARCH PAPER

IR transmittance of large-sized free-standing transparent diamond films prepared by MWPCVD LI Bo1, HAN Bai2, LU Xian-yi1, LI Hong-dong1, WANG Jian-bo1, JIN Zeng-sun1* 1

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

2

College of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150040, China

Abstract: Large-sized free-standing transparent diamond films of 50 mm diameter and 300 μm thickness were prepared by microwave plasma chemical vapor deposition (MWPCVD). The growth rate of the diamond film was only 1-2 μm/h when the diamond film was grown at a methane concentration of 2%, and the infrared (IR) transmittance reached 70% in the range of 500-4000cm-1 after the film was polished on both sides. A high growth rate of 7-8 μm/h was achieved for the film grown at a methane concentration of 4%. The thickness of the film was 260 μm after it was polished on both sides and its IR transmittance in the range of 500-4000 cm-1 reached about 60%. Meanwhile, the IR transmittance was almost the same in the central and fringe regions. These results imply a promising application of large-sized thick diamond films in IR windows. Key Words: Microwave PCVD; Transparent free-standing diamond film; IR transmittance

1

Introduction

Table.1 Deposition parameters of transparent diamond films

Diamond films are the best candidate for IR windows used in hostile environments owing to their excellent optical and other properties[1-3]. Limited by their size and cost, natural and high pressure and high temperature diamonds are unsuitable to be applied as IR windows, and therefore, large-sized diamond films prepared by CVD techniques are used as IR windows. Up to now, several authors[4-9] have studied the IR properties of diamond films systematically. When large-sized free-standing diamond thick films are used as IR windows, we must consider not only their growth rate, but also the uniformity of their optical quality[10] besides their IR transmittance. Normally, an increase of growth rate leads to a decrease of optical quality[11-13]. MWPCVD is known to be one of the best methods to prepare high-quality diamond films[14]. In this study, the free-standing diamond thick films with high IR transmittance and relatively high growth rate were prepared in MWPCVD system by increasing the methane concentration and the microwave power as well as adding a small amount of oxygen into the system.

2

Experimental

The experiments were carried out in a 5kw ASTex 5250 MWPCVD system. H2, CH4, and O2 were employed as precursors and Mirror-polished n(100) Si wafers, 50mm in diameter, were adopted as substrates. The recipe is listed in Table 1.

Deposition parameters Gas flow rates qv /mL·sec-1

Total gas pressure

p /kPa

Values Hydrogen

500

Methane

10-40

Oxygen

2 16

Microwave power P / W

5000

Substrate temperature t /°C

800

The quality of the diamond films was characterized by Raman spectra (Renishaw inVia, U.K., excitation wavelength of 514.5nm). The growth characteristics of their cross-sections were observed by a scanning electron microscope (SEM, Saimadzu SSX-550, Japan), and the IR transmittance of unpolished and polished films was measured by Fourier transform infrared spectra (FTIR, Bruke IFS66 V, Germany).

3 3.1

Results and discussion Growth rate and quality of the diamond films

Fig.1 shows the photos of free-standing diamond thick films after the Si substrates were removed by a mixture of HF and HNO3 (volume ratio 1:1). Fig.2 shows the samples’ growth rate as a function of methane concentration after 10 h of deposition under other

Received date: 25 December 2007; Revised date: 5 August 2008 *Corresponding author. E-mail: diamond@ jlu.edu.cn Copyright©2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LI Bo et al. / New Carbon Materials, 2008, 23(3): 245–249

Fig.1 Photos of the transparent free-standing diamond films: (a) unpolished film with diameter of 50mm, (b) film cut and then polished on both sides

Fig.2 Growth rate as a function of methane concentration after 10 h of deposition

with increasing methane concentration. In the Raman spectra of the films grown at the methane concentrations of 2% and 3%, only a sharp diamond characteristic peak at 1332cm-1 is found, whereas the characteristic peaks of the non-diamond phase are not found, indicating that the quality of the films is high. Meanwhile, the FWHM of the two diamond characteristic peaks is 5.1cm-1 and 6.7cm-1, respectively, at methane concentrations of 2% and 3%, which suggests that the films prepared under these conditions have excellent crystallinity[15]. When the methane concentration reaches 4%, a weak band of non-diamond carbon phase centered at 1550cm-1 appears in the Raman spectrum besides the sharp diamond characteristic peak. Inon-diamond/Idiamond and FWHM increase with increasing methane concentration, which indicates that the quality and crystallinity of the films decrease[15, 16]. 3.2

IR transmittance of the diamond thick films

Fig.4 shows the IR transmittance of the free-standing diamond thick film (about 300Pm in thickness) grown at a methane concentration of 2% before and after polish. The IR transmittance of the film before polish is very low, which can be ascribed to the surface scattering by the rough surface of the polycrystalline diamond films, and the IR transmittance reaches about 70% in the range of 500-4000 cm-1 after polish from both sides. The growth rate, however, is only 1-2 Pm, and the preparation of a diamond thick film of 300 Pm requires several hundred hours.

Fig.3 Raman spectra of the samples grown at different methane concentrations after 10 h of deposition. The inset shows the FWHM dependence of the methane concentration.

wise identical conditions as mentioned above. It can be seen from Fig.1 that the growth rate of the diamond films increases with increasing methane concentration under the conditions investigated in this study. The Raman spectra of the samples grown at different methane concentrations after 10 h of deposition are shown in Fig.3, which indicate that the quality of the films decreases

The quality of the film grown at a methane concentration of 4% is not as good as that of the films grown at 2% methane concentration, but the growth rate of the former is considerably faster than that of the latter. Therefore, we studied the IR transmittance of the former film in detail. Two samples of 10 mm×10 mm were cut from the central and fringe region of the D50 mm free-standing diamond film grown at 4% methane concentration, and were named as sample A (central region) and sample B (fringe region) after polish (both 290 μm in thickness). Sample A was then further polished from the nucleation side and its thickness changed to 260 μm, which was named A’. Sample B was further polished from the growth

LI Bo et al. / New Carbon Materials, 2008, 23(3): 245–249

Fig.4 IR transmittance of the film grown at a methane concentration

Fig.5 IR transmittance of sample A, B, A’ (A polished from nuclea-

of 2% before and after polishing from the growth side

tion side), and B’ (B polished from growth side)

Fig.6 SEM images of the cross-section of the free-standing diamond film grown at a methane concentration of 4% and (a) unpolished, (b) polished from growth side, and (c) polished from the nucleation side

side and its thickness also changed to 260 μm, which was named B’. The IR transmittance of samples A, B, A’, and B’ are shown in Fig.5. It can be seen that the IR transmittance of sample A (central region) and sample B (fringe region) are almost the same, which indicates that the optical quality of the D 50 mm large-scale diamond film is uniform. The IR transmittance of sample B’ is almost the same as that of samples A and B. The IR transmittance of sample A’ reaches about 60%, which is however, apparently larger than that of sample A. Fig.6 shows the SEM images of the cross-section of the diamond films grown at 4% methane concentration. It can be seen from Fig.6(a) that the grains are small and the density of the grain boundaries is large during the initial stage of the nucleation. A columnar growth mode can be seen after the thickness of the film reaches a certain level, and the grains become larger in size and better in crystallinity with increasing thickness. In Fig.6(b), it can be seen that the small grains and the large density of grain boundaries remain in the nucleation side of sample B’ (polished from the growth side). In Fig.6(c), however, only large columnar grains can be seen in sample A’ (polished from the nucleation side). Therefore, a certain thickness of their nucleation side should be polished away to further improve the IR transmittance of the free-standing diamond thick films. In order to further investigate the relationship between the quality and the thickness of the diamond film, Raman spectra

measurements were carried out on its cross-section (the spot size is 1 μm, and the interval of every two adjacent measurement positions is about 60 μm along the growth direction), and are presented in Fig.7. Non-diamond carbon peak centered at 1550cm-1 is observed in the Raman spectrum of the film’s nucleation side, and the FWHM of the diamond characteristic peak is high. When the thickness reaches 60 μm, only the diamond characteristic peak can be found, and its FWHM is small, and when the thickness increases further, the FWHM does not change obviously. The above results further explain

Fig.7 Raman spectra of different positions on the cross-section of the film grown at a methane concentration of 4%. The inset shows the FWHM of the Raman spectra obtained at different positions.

LI Bo et al. / New Carbon Materials, 2008, 23(3): 245–249

that polishing away the region with small grains near the

diamond films[J]. Journal of Materials Research, 1990, 5(4):

film’s nucleation side can improve the IR transmittance of the

811-815.

diamond film.

4

Conclusions

Transparent free-standing diamond film, 50 mm in diameter and 300 μm in thickness, was prepared by MWPCVD. (1) The IR transmittance of large-sized transparent diamond thick film grown at a methane concentration of 2% is very high (about 70%), but the growth rate is very low (only 1-2 Pm/h). (2) The IR transmittance of large-sized transparent diamond thick film grown at a methane concentration of 4% is only about 60%, but the growth rate is high (7-8 μm/h). (3) The region with small grains near the nucleation side of the diamond film largely affected the film’s IR transmittance, and polishing it away could further improve the IR transmittance of the diamond film. (4) The IR transmittance of the D 50 mm large-sized transparent diamond film is the same at both the central region and the fringe region. Thus, it can be deduced that the D 50 mm large-sized transparent diamond film prepared under the conditions in this investigation has uniform optical quality.

References

[6] McNamara K M, Williams B E, Gleason K K, et al. Identification of defects and impurities in chemical-vapor-deposited diamond through infrared spectroscopy[J].

Journal of Applied

Physics, 1994, 76(4): 2466-2472. [7] Kiflawi I, Bruley J, The nitrogen aggregation sequence and the formation of voidites in diamond[J]. Diamond and Related Materials, 2000, 9(1): 87-93. [8] Paolo Dore, Alessandro Nucara, Daniele Cannavo, et al. Infrared properties of chemical-vapor deposition polycrystalline diamond windows[J]. Applied

Optics, 1998, 37(8): 5731-5736.

[9] Yin Z, Akkerman Z, Yang B X, et al. Optical properties and microstructure of CVD diamond films. [J] Diamond and Related Materials, 1997, 6(1): 153-158. [10] LU Xian-yi, JIN Zeng-sun, YANG Guang-liang. Studies for quality and thickness uniformity of diamond thick film synthesized by the EA-CVD method[J]. New Carbon Materials, 2005, 20(3): 270-273. [11] Regel L L, Wilcox W R.. Diamond film deposition by chemical vapor transport[J]. Acta Astronautica, 2001, 48(2-3): 129-144. [12] Goodwin D G, Butler J E. Handbook of industrial diamonds and diamond films[M]. New York: Marcel Dekker Ltd, 1996. [13] Gicquel A, Hassouni K, Silva F, et al. CVD diamond films: from growth to applications[J]. Current Applied Physics 2001, 1: 479-496. [14] May P W. Diamond thin films: a 21st-century material[J].

[1] Klein C A. Diamond windows and domes: flexural strength and

Philosophical Transactions of the Royal Society of London

thermal shock[J]. Diamond and Related Materials, 2002, 11(2):

Series A-Mathematical Physical and Engineering Sciences,

218-227.

2000, 358(1): 473-495.

[2] MAN Wei-dong, WANG Jian-hua, WANG Chuan-xin, et al.

[15] Stuart S A, Prawer S, Weiser P S. Growth-sector dependence of

The properties, production and applications of diamond thin

fine structure in the first-order Raman diamond line from large

films[J]. New Carbon Materials, 2002, 17(1): 62-70.

isolated chemical-vapor-deposited diamond crystals[J]. Applied

[3] MAN Wei-dong. Diamond thin film[J]. New Carbon Materials, 2002, 17(2): 77.

Physics Letters, 1993, 62(11): 1227-1229. [16] Steven Prawer, Alon Hoffman, Sue-Anne Stuart, et al.

[4] ZHONG Guo-fang, SHEN Fa-zheng, TANG Wei-zhong, et al.

Correlation between crystalline perfection and film purity for

Preparation of high quality transparent chemical vapor deposi-

chemically vapor deposited diamond thin films grown on fused

tion diamond films by a DC arc plasma jet method[J]. Diamond

quartz substrates[J]. Journal of Applied Physics, 1991, 69(9):

and Related Materials, 2000, 9(9-10): 1678-1681.

6625-6631.

[5] BI Xiang-xin. Optical properties of chemical vapor deposited