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Fabrication of diamond-coated germanium ATR prisms for IR-spectroscopy
Vibrational Spectroscopy 84 (2016) 67–73
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Fabrication of diamond-coated germanium ATR prisms for IR-spectroscopy$ O. Babchenko, H. Kozak, T. Izak, J. Stuchlik, Z. Remes, B. Rezek, A. Kromka* Institute of Physics of the ASCR, v.v.i., Cukrovarnicka 10, Praha 6, 16253, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 September 2015 Received in revised form 28 February 2016 Accepted 1 March 2016 Available online 3 March 2016
Monocrystalline germanium is considered a highly useful material for infrared optical applications due to its relatively low price, low toxicity and high optical transparency in the infrared spectral range (5500– 870 cm1). However, it is limited by its low chemical, mechanical, and thermal stability. The thin film diamond coating can provide a mechanical protection of a Ge surface against scratching, chemical protection against solutions, and make a well-defined hydrophilic/hydrophobic surface. Here, we compare low temperature (250–400 C) diamond growth on Ge substrates and attenuated total reflection prisms, using a chemical vapor deposition in linear antenna microwave plasma and focused microwave plasma reactors. We show that by employing a thin amorphous silicon interlayer (20–30 nm), a homogeneous diamond coating with no degradation of the Ge surface is achieved. The infrared spectroscopy of proteins adsorbed on the diamond coated Ge prism demonstrates usability and advantages in comparison with bare Si or Ge prisms and even with the diamond-coated Si prism. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Diamond Low temperature growth Linear antenna microwave plasma Germanium SEM FTIR
1. Introduction Due to the characteristic transmission range and opacity in the visible range of spectrum, monocrystalline germanium is well suited to infrared (IR) spectroscopy [1,2]. However, Ge possesses several drawbacks for optical element fabrication. For example, its low chemical and mechanical resistance (it can be easily scratched), high reflectivity due to the high index of refraction or the need for complicated chemical grafting are some of the most common limiting factors [3]. Therefore, there is demand for a suitable protection and antireflection layer for Ge-based optical elements which will extend their lifetime and expand their uses in the identifying of chemical groups. In the ideal case, the protective layer should have additional functions such as controllable wettability, covalent binding of molecules, electrical conductivity, etc. In this context, a diamond coating is highly attractive as a protective and functional layer for IR-spectroscopy. Synthetic diamond films belong to a class of modern functional materials which combine excellent properties in one platform; more specifically, chemical and mechanical stability, broad optical transparency, adjustable semiconducting properties, carbon
$ Selected paper from 8th International Conference on Advanced Vibrational Spectroscopy, 12–17 July 2015, Vienna, Austria. * Corresponding author. E-mail address: [email protected] (A. Kromka).
http://dx.doi.org/10.1016/j.vibspec.2016.03.002 0924-2031/ ã 2016 Elsevier B.V. All rights reserved.
surface chemistry, and many others [4–7]. In addition, recent microbiological studies have demonstrated the bactericidal character of diamond surface terminated with specific functional groups [8,9]. Our earlier studies have already demonstrated a promising application of diamond-coated gold mirror substrates for fabrication of novel optical elements [10]. In the work of Remes et al. [11], gold or aluminum mirrors coated with thin diamond film have been shown to be advantageous for grazing angle reflectance (GAR) Fourier-transform infrared (FTIR) spectroscopy. The sensitivity of GAR-FTIR to proteins adsorbed from fetal bovine serum (FBS) was further enhanced when the gold mirrors were coated with nanoporous diamond films [12]. Thus, functional groups in ultra-thin molecular layers (<5 nm) were easily detectable. Similarly, the high sensitivity of the diamond-coated silicon attenuated total reflection (ATR) prism as an optical element for IR spectroscopy of functionalized diamond nanoparticles was recently reported [13]. However, the diamond/Si prism could not be used for FTIR-measurements in the region below 1500 cm1 due to the limited transparency of silicon ATR prisms (optical path is tens of cm). In this region the optical absorption spectra of Si are dominated by the multiphonon absorption processes related to the multiple of the single phonon vibration at a Raman band at 520 cm1 [14]. On the other hand, this is a key finger-print region for many biomolecules. Hence, germanium with a Raman band at 300 cm1 would be more appropriate for collecting IR absorbance spectra below 1500 cm1 in the ATR regime.
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The chemical vapor deposition (CVD) of diamond-based protective and functional coatings on Ge is not a trivial task due to several limitations. Ge has a low melting temperature (940 C) which is close to the diamond growth conditions (850–900 C). Ge has also a high mismatch in the thermal expansion coefficient to diamond, exhibits high out-diffusion at elevated temperatures, and has a low chemical resistance to hydrogen plasma. Here, we investigate the diamond growth on planar Ge substrates at temperatures as low as 250 C. We show that the microwave plasma CVD process in a linear antenna arrangement with specific deposition parameters allows the formation of a fully closed diamond layer on the Ge ATR prism with large 3D geometry (i.e., 72 mm in length, 10 mm in width and 6 mm in height). The functionality of the diamond-coated Ge prism is demonstrated by FTIR-measurements of proteins adsorbed on the oxidized diamond surface. 2. Experimental 2.1. Planar substrates Diamond thin films were deposited on polished one side single crystal germanium substrates (100) purchased from MTI Corporation (USA). Deposition of diamond films was performed by the microwave (MW) plasma assisted CVD process using either focused [15] or a linear antenna arrangement [16]. The protective amorphous silicon interlayer (a-Si with thickness of 20–30 nm) was prepared by silane-rich radiofrequency plasma at room temperature. Prior to the diamond growth, bare or a-Si coated Ge substrates were ultrasonically treated in the water-based nanodiamond powder suspension. This treatment resulted in the high diamond seed density (up to 1011 cm2) required for uniform diamond film growth [17,18]. For the focused MW plasma arrangement, the deposition was done in a gas mixture of methane and hydrogen (1% CH4 in H2) at 2000 Pa and MW power of 1100 W for 18 h at the substrate temperature of 400 C. For the linear antenna MW plasma arrangement, the process conditions were as follows: the used gas mixture was a methane/carbon dioxide/ hydrogen gas system (2.5% CH4 + 10% CO2 in H2), pressure 10 Pa and MW power 2 1200 or 2 1700 W employing two antennas. The total deposition time was 20 h and the substrate temperature was 250 or 360 C (depending on power). After the deposition process, the samples were analyzed by scanning electron microscopy (SEM) using an e_LiNE system
(Raith) and Raman spectroscopy using an inVia Reflex Raman microscope (Renishaw) with the laser excitation wavelength of 442 nm. Surface topography was investigated by atomic force microscopy (AFM) in a tapping mode (AFM Microscope Dimension 3100, Veeco). The set-point ratio (ratio of approached vs. free cantilever oscillation amplitude) was 60 5%. Silicon AFM cantilevers were used with a typical tip radius of 10 nm and a resonance frequency of 230 kHz. Surface roughness was calculated from the area of 2 2 mm2. 2.2. ATR prisms Beside the planar Ge substrates, diamond films were also deposited on Ge ATR prisms. For demonstration of the spectroscopic advantages in FTIR-measurements of the diamond-coated Ge prism, the diamond-coated Si ATR prism was used with the same geometry (i.e., 72 mm in length, 10 mm in width and 6 mm in height). ATR prisms were prepared in a similar way to planar substrates as described above. The technology of Si ATR prism was presented in our previous study [13]. After ultrasonic seeding of substrates, the diamond film was deposited in the linear MW CVD (0.5% CH4 in H2 at 710 C). The Ge ATR prism was first coated by a thin a-Si (20–30 nm) as the protective layer, and then ultrasonically seeded with diamond nanoparticles. The diamond deposition was done in a linear antenna plasma (gas mixture 2.5% CH4 + 10% CO2 in H2, pressure 10 Pa, MW power 2 1700 W and temperature 360 C). Prior to FTIR-analysis, the diamond-coated ATR prisms were oxidized in a radio frequency oxygen plasma (45 W, 50 Pa, 1 min, low pressure FEMTO plasma system from Diener electronic GmbH & Co. KG) to achieve hydrophilic surface properties (wetting angle <10 ). Next, proteins were adsorbed on the oxidized diamond surface from 15% fetal bovine serum (FBS) solution (PAA) in McCoy’s 5A medium with stable glutamine without phenolred (BioConcept) [19]. A 200 ml of FBS solution was dropped on the diamond surface, left for 10 min, rinsed in deionized water and dried using a nitrogen blow. Infrared spectra of proteins adsorbed on the diamond-coated Ge or Si prisms were measured with the spectral resolution of 4 cm1 by Nicolet Nexus FTIR spectrometer using a KBr beamsplitter, a liquid nitrogen cooled MCT detector and the standard IRsource using the Specac’s GatewayTM 6-reflection horizontal attenuated total reflection accessory in a nitrogen atmosphere
Fig. 1. SEM image of surface morphology (a) and Raman spectrum (b) of the bare Ge substrate after the diamond growth at 400 C by a focused microwave plasma CVD process.
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[13]. Advanced ATR correction was performed on all measured spectra using the Omnic software. 3. Results 3.1. Diamond growth on Ge substrates The surface morphology of samples after the diamond growth on bare Ge substrates by a focused MW plasma is shown in Fig. 1a. During the CVD, the substrate temperature was as low as 400 C. However, instead of diamond crystals, pyramidal-like pits in size from a few up to a hundred nanometers were observed. Only local areas revealed the deposition of clustered-like features but no diamond fingerprint was observed by Raman measurements (Fig. 1b). It should be noted that the employed standard CVD process parameters resulted in a successful diamond growth on the reference Si substrates and glass [15]. The surface morphology of the samples after the CVD process in a linear antenna MW plasma is shown in Fig. 2. After CVD at the substrate temperature of 360 C, the Ge substrate reveals pit formations and a molten-like surface (Fig. 2a). Decreasing the substrate temperature (250 C) suppressed the presence of molten-like features (Fig. 2c). The density and size of pit-like features also decreased; nevertheless, the surface was still damaged. In both these cases, on some areas of the samples, the early stage of crystal faceting was observed. Raman measurements on samples processed in a linear antenna MW plasma system confirmed the presence of the diamond phase for both deposition temperatures (Fig. 2b and d). The Raman spectrum of the sample processed at 360 C shows a sharp peak at 1330 cm1, which is attributed to the diamond
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characteristic band [20]. Additionally, two broad bands centered at 1320 and 1590 cm1 are assigned to the D (disordered) and G (graphitic) band of sp2 carbon phases (Fig. 2b) [20,21]. For the sample processed at 250 C, the spectrum is dominated by the diamond band, while the D and G bands are not evident (Fig. 2d). AFM measurements collected from the area 1 1 mm2 confirmed a decrease in RMS surface roughness from 41 nm to 27 nm for the process temperatures 360 and 250 C, respectively. The non-uniform diamond coating and persistence of etch pits on the Ge substrate show that a lower substrate temperature is not alone a sufficient parameter for successful growth of diamond on Ge in spite of using a low-energetic and distant linear antenna plasma for the CVD process. Therefore, we applied a thin amorphous silicon (a-Si) layer for Ge protection. A low thickness (20–30 nm) of a-Si is important to keep high enough optical transparency of the ATR prism in the IR-range. The diamond deposition on a-Si coated Ge substrates was done at 400 C in a focused MW plasma and at 360 C in a linear antenna MW plasma, respectively. These deposition temperatures were chosen as a technological compromise minimizing the thermal loading of the Ge substrate and keeping reasonable diamond growth rates. Fig. 3 shows the surface morphology and the Raman spectra of diamond films grown on a-Si/Ge substrates. Faceted crystals with sizes of 150200 nm were formed by a focused MW plasma (Fig. 3a). The porous-like conformation of these crystals is shown on the cross-section SEM image (Fig. 3a–inset). The Raman spectrum confirmed the diamond character of crystals and the sp2 carbon phases (D and G band) were also pronounced (Fig. 3b). A homogeneous and fully closed diamond film consisting of clustered-like crystals was formed by the linear antenna MW
Fig. 2. SEM images of surface morphology of bare Ge substrates after the diamond growth at 360 C (a) and 250 C (c) using the linear antenna MW plasma assisted CVD process and corresponding Raman spectra of grown films (b, d) with the diamond band at 1330 cm1, disordered band centered at 1320 cm1 and graphitic band centered at 1590 cm1.
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Fig. 3. SEM images of diamond films grown on a-Si coated Ge substrates using focused (a, b) and linear antenna (c, d) MW plasma CVD processes and corresponding Raman spectra of grown films (b, d) with the diamond band at 1330 cm1, disordered band centered at 1340 cm1 and graphitic band centered at 1590 cm1.
Fig. 4. IR absorbance spectra of FBS proteins adsorbed on the diamond-coated Si (blue line) and Ge (red line) ATR prisms. The band and peaks in the spectra are assigned to: OH stretching vibrations and stretched amino group NH2 vibrations at 3200–3400 cm1, overtone of amide II band at 3100 cm1, methyl CH2 and CH3 stretching vibrations at 3000–2800 cm1, amide I band from the C¼O stretching vibrations at 1655 cm1, amide II band related to NH bending and C-N stretching vibrations at 1540 cm1, protein side-chain COO- close to 1400 cm1, and amide III band related to CN stretching vibration and NH bending vibration near 1300 cm1. The band centered at 1050 cm1 is discussed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
plasma (Fig. 3c). The Raman spectrum is dominated by the diamond peak with lower amount of sp2 carbon phases (Fig. 3d) compared to the focused plasma. The RMS surface roughness of these diamond films was as low as 12 nm.
3.2. IR-measurements using diamond-coated Si and Ge ATR prisms Fig. 4 shows the IR absorbance spectra of the proteins adsorbed from fetal bovine serum solution on the diamond-coated Ge and Si
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ATR prisms for comparison. In both IR spectra, the broad band at 3400-3200 cm1 is related to hydroxyl OH stretching vibrations of adsorbed water overlapping the stretched amino group NH2 vibrations [22]. The band at 3100 cm1 is assigned to the overtone of the amide II band. Bands near 3000 cm1 are related to CH2 and CH3 stretching vibrations. The band at 1655 cm1 is the amide I band, which results from the C¼O stretching vibrations. The amide II band peak at 1540 cm1 is related to N-H bending and C-N stretching vibrations. However, the spectrum should be more complex due to possible overlapping of water with NH stretching. Next, for the diamond-coated germanium ATR prism, well resolvable spectra were collected also at wavenumbers below 1500 cm1. The observed band close to 1400 cm1 results from protein side-chain COO–. The amide III band near 1300 cm1 is related to C-N stretching vibration and N-H bending vibration. The intensive band centered at 1050 cm1 is discussed later. It is
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obvious that the diamond-coated Ge prism is more advantageous than the Si prism due to a wider range of IR-spectra. 4. Discussion As was stated before, the growth of a diamond thin film on germanium is not a simple task. The standard focused MW plasma system led to etching of Ge and formation of large pits at even a low temperature (400 C) process. The etching of germanium is primarily attributed to highly aggressive growth conditions (high concentration of atomic hydrogen and high temperature) [23]. In the focused plasma system, a thermal load of Ge cannot be avoided due to the plasma ball, which is inherently located in close vicinity to the Ge surface (approx. 1 mm). The linear antenna MW plasma system, also known as the surface wave plasma, appears more suitable for the diamond growth at low temperatures [24].
Fig. 5. Illustrative comparison of principal differences between bare and diamond-coated ATR prisms.
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Previously, it has been successfully used for the diamond growth on thermally [10] or chemically [25] unstable substrates. In contrast to the focused plasma system, here, the substrate material is located at far distance (6–7 cm) from the hot plasma region. At this relatively large distance, the plasma becomes “cold” in the sample vicinity and the temperature of electrons is lower than 2 eV [16,25,26]. Nevertheless, in spite of the deposition temperature as low as 250 C and the distance from the hot plasma region (7 cm), a partial damage of planar bare germanium substrate was still observed (Fig. 2a and c). For the samples processed in the linear antenna MW system, the Raman spectra revealed the presence of a diamond phase (Fig. 2d), which is a positive indication compared to the focused plasma system (Fig. 1b). Thus, a low substrate temperature is alone not sufficient for a successful growth of diamond on germanium (at least for microwave plasma in hydrogen-rich gas mixture). Previously, Rego et al. investigated the ability of an interlayer (amorphous Si, SiO2, or Si3N4) to protect planar germanium substrates from the diamond CVD [23]. They found that thick (1 mm) carbide-forming protective coatings are able to withstand the growth process. However, due to the high temperature process (700 C), their diamond layers delaminated from the Ge substrate. On the other hand, using thinner protective coatings did not preserve the Ge surface from its melting. Miller et al. presented a two-step process which consisted of substrate micro-pattering followed by the deposition of an interlayer. This process significantly improved the adhesion of diamond to the substrate [27]. A disadvantage was that the micro-patterning step resulted in the surface roughening, which is an unfavorable feature for any optical uses. In our experiments, we used an ultra-thin (20–30 nm) amorphous Si coating, which suppressed the chemical reactivity of Ge for both deposition systems. Neither film delamination nor melting of Ge was observed. This feature is attributed to using low temperature (less than 400 C) in both the deposition systems. However, the situation concerning the uniformity of diamond coatings from two deposition systems is different. As is evident from the SEM cross-section image (Fig. 3a inset), the layer grown in a focused MW plasma system, although relatively thick (>600 nm), is porous. This deteriorates its optical properties, primarily due to scattering, and its mechanical stability. In contrast to this, the diamond film grown in the linear antenna MW system (Fig. 3c) is dense and homogeneous even at observed lower thickness (around 250 nm). The linear antenna MW plasma system thus provided suitable results on diamond growth on Ge with the possibility of large area coating. Therefore, ATR prisms for IR spectroscopy were covered by diamond in a linear antenna MW plasma system. The IR-spectra reflected fingerprints from water band overlapping the amino groups (3400–3200 cm1), amide II overtone (3100 cm-1), CH2, CH3 stretching (below 3000 cm1) and C¼O stretching vibrations (1650 cm1) and ending with the amide II band (1540 cm1) for the diamond-coated Si and Ge ATR prism, respectively. It should be noted that, while resolved IR-spectra on Ge and Si ATR prism are of similar quality, an evident cut of the wavenumber for Si prism observed at 1500 cm1 (Fig. 4) limits recognition of chemical bonds at lower wavenumbers. In contrast to the Si limitation, the diamond-coated Ge prism is optically transparent down to 870 cm1 allowing the detection of C-N stretching and N-H bending vibrations (amide III, band at 1300 cm1). The IR-spectrum reveals also an intensive band centered at 1050 cm1 with a right shoulder. The attribution of this band is not simple. In the first assumption, it seems to be a fingerprint of COC asymmetric stretching vibrations (1100– 1040 cm1) [3]. Due to the silicon interlayer, the band can also indicate a presence of Si O C (1000–1110 cm1) or Si OSi
(1030–1090) vibrations [28]. Unfortunately, all three assignments (CO C, Si O C or Si O Si) are known as very strong in intensity and they can exhibit non-linear enhancement. The effect of organic contamination of the IR-setup (lenses and mirrors) should be mentioned too, which can give a false signal in this region. Therefore, background subtraction has to be done carefully, while it can represent a serious problem for a proper interpretation of IR-measurements. It is still a challenge to diminish all above mentioned potential effects. To highlight, the diamond thin film mechanically and chemically protects the soft surface of the ATR prism as well as being a functional surface. Fig. 5 schematically illustrates principal differences between bare and diamond-coated prisms. First, prisms were exposed to an oxygen plasma to achieve a hydrophilic surface. For better optical visualization of the investigated material, we used a liquid solution of atomically-functionalized diamond nanoparticles (f 5 nm). A droplet of the solution was spread on both prisms by a drop technique. After drying, the prisms reveal significantly different surfaces. In the case of oxygen treated bare prism, only a poor and inhomogeneous spreading of diamond nanoparticles was achieved, as observed by optical microscopy. It is known that attenuated total reflection is disturbed by any surface inhomogeneity resulting in an IR-spectrum of low resolution. In contrast to the bare prism, the oxygen treated diamond-coated prism reveals the homogeneous spreading of the investigated material on micro-scale as confirmed by AFM measurements. This observation is attributed to the unique properties of the diamond surface stability and keeping its hydrophilic character. A homogeneous spreading of liquid results in good optical contact of the investigated material with the diamond-coated ATR prism. So, the infrared beam can reflect well the interface via total internal reflection. We propose the same functional principle for the detection of an ultra-thin protein layer, which is hardly detectable on a rough diamond surface by optical microscopy or AFM measurements. We optically observed a homogeneous spreading of phosphate-buffered saline (PBS) on the oxygen terminated diamond-coated ATR prism. Due to the small thickness of the adsorbed PBS layer, we were not able to evaluate a FBS spreading homogeneity by AFM measurements. The AFM signal was dominated by the relatively high surface roughness of diamond coating (12 nm). Nevertheless, based on our earlier AFM measurements on atomically flat diamond surfaces (i.e., monocrystalline diamond), the protein layer thickness is assumed to be approximately 3 nm [19]. Taking into account the observed IR-absorbance spectra (Fig. 4), we can state that the IR-measurements clearly proved the applicability of diamondcoated ATR prisms for the detection of such ultra-thin protein layers. The diamond film adhered well to the germanium prism and no delamination was observed even after multiple cleaning cycles (>30x). The good adhesion is partially assigned to the applied silicon interlayer, low deposition temperature (360 C) and small diamond thickness. For re-using the prism we employed the cleaning procedure which consisted of a mechanical cleaning by a cotton stick, followed by low temperature plasma hydrogenation (reset of the surface to the as-grown state) and plasmatic oxidation for recovering the hydrophilic character of the surface. 5. Conclusions We have demonstrated the progress in the diamond growth on planar germanium, which represents a chemically and thermally sensitive substrate. The deposition process in a focused plasma system at a temperature of 400 C resulted in etching of the Ge surface. Even at deposition temperature as low as 250 C in a linear antenna system, the Ge substrates revealed melting-like features,
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and diamond growth was not efficient enough. Applying 20–30 nm of thin amorphous silicon as the protecting layer stabilized the Ge substrate and resulted in a homogeneous diamond coating with good quality. In contrast to a focused microwave plasma, the linear antenna plasma was shown as more suitable for coating of nonplanar and large substrates such as silicon or germanium ATR prisms. It should be noted that the diamond-coated ATR prisms differ principally from the commercially available single reflection monocrystalline diamond prisms by multiple reflections (7x vs. 1x), larger functional area for interaction of the evanescence wave with the investigated material (700 mm2 vs. 2 mm2), lower cost. The absorbance spectra exhibited good enough resolution and sensitivity to an ultra-thin protein layer. Owing to the wide spectral window offered by the Ge optical element in comparison to silicon, IR-analysis was performed in the range of 4000–870 cm1. Additional perspective is the combination of the diamond-coated germanium ATR prism with real time electrical measurements employing surface conductivity of diamond.
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Acknowledgements
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The technical support of K. Hruska and J. Cermak is gratefully appreciated. This work was financially supported by the GACR research project 15-01687S (AK, HK).
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Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Fabrication of diamond-coated germanium ATR prisms for IR-spectroscopy$ O. Babchenko, H. Kozak, T. Izak, J. Stuchlik, Z. Remes, B. Rezek, A. Kromka* Institute of Physics of the ASCR, v.v.i., Cukrovarnicka 10, Praha 6, 16253, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 September 2015 Received in revised form 28 February 2016 Accepted 1 March 2016 Available online 3 March 2016
Monocrystalline germanium is considered a highly useful material for infrared optical applications due to its relatively low price, low toxicity and high optical transparency in the infrared spectral range (5500– 870 cm1). However, it is limited by its low chemical, mechanical, and thermal stability. The thin film diamond coating can provide a mechanical protection of a Ge surface against scratching, chemical protection against solutions, and make a well-defined hydrophilic/hydrophobic surface. Here, we compare low temperature (250–400 C) diamond growth on Ge substrates and attenuated total reflection prisms, using a chemical vapor deposition in linear antenna microwave plasma and focused microwave plasma reactors. We show that by employing a thin amorphous silicon interlayer (20–30 nm), a homogeneous diamond coating with no degradation of the Ge surface is achieved. The infrared spectroscopy of proteins adsorbed on the diamond coated Ge prism demonstrates usability and advantages in comparison with bare Si or Ge prisms and even with the diamond-coated Si prism. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Diamond Low temperature growth Linear antenna microwave plasma Germanium SEM FTIR
1. Introduction Due to the characteristic transmission range and opacity in the visible range of spectrum, monocrystalline germanium is well suited to infrared (IR) spectroscopy [1,2]. However, Ge possesses several drawbacks for optical element fabrication. For example, its low chemical and mechanical resistance (it can be easily scratched), high reflectivity due to the high index of refraction or the need for complicated chemical grafting are some of the most common limiting factors [3]. Therefore, there is demand for a suitable protection and antireflection layer for Ge-based optical elements which will extend their lifetime and expand their uses in the identifying of chemical groups. In the ideal case, the protective layer should have additional functions such as controllable wettability, covalent binding of molecules, electrical conductivity, etc. In this context, a diamond coating is highly attractive as a protective and functional layer for IR-spectroscopy. Synthetic diamond films belong to a class of modern functional materials which combine excellent properties in one platform; more specifically, chemical and mechanical stability, broad optical transparency, adjustable semiconducting properties, carbon
$ Selected paper from 8th International Conference on Advanced Vibrational Spectroscopy, 12–17 July 2015, Vienna, Austria. * Corresponding author. E-mail address: [email protected] (A. Kromka).
http://dx.doi.org/10.1016/j.vibspec.2016.03.002 0924-2031/ ã 2016 Elsevier B.V. All rights reserved.
surface chemistry, and many others [4–7]. In addition, recent microbiological studies have demonstrated the bactericidal character of diamond surface terminated with specific functional groups [8,9]. Our earlier studies have already demonstrated a promising application of diamond-coated gold mirror substrates for fabrication of novel optical elements [10]. In the work of Remes et al. [11], gold or aluminum mirrors coated with thin diamond film have been shown to be advantageous for grazing angle reflectance (GAR) Fourier-transform infrared (FTIR) spectroscopy. The sensitivity of GAR-FTIR to proteins adsorbed from fetal bovine serum (FBS) was further enhanced when the gold mirrors were coated with nanoporous diamond films [12]. Thus, functional groups in ultra-thin molecular layers (<5 nm) were easily detectable. Similarly, the high sensitivity of the diamond-coated silicon attenuated total reflection (ATR) prism as an optical element for IR spectroscopy of functionalized diamond nanoparticles was recently reported [13]. However, the diamond/Si prism could not be used for FTIR-measurements in the region below 1500 cm1 due to the limited transparency of silicon ATR prisms (optical path is tens of cm). In this region the optical absorption spectra of Si are dominated by the multiphonon absorption processes related to the multiple of the single phonon vibration at a Raman band at 520 cm1 [14]. On the other hand, this is a key finger-print region for many biomolecules. Hence, germanium with a Raman band at 300 cm1 would be more appropriate for collecting IR absorbance spectra below 1500 cm1 in the ATR regime.
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The chemical vapor deposition (CVD) of diamond-based protective and functional coatings on Ge is not a trivial task due to several limitations. Ge has a low melting temperature (940 C) which is close to the diamond growth conditions (850–900 C). Ge has also a high mismatch in the thermal expansion coefficient to diamond, exhibits high out-diffusion at elevated temperatures, and has a low chemical resistance to hydrogen plasma. Here, we investigate the diamond growth on planar Ge substrates at temperatures as low as 250 C. We show that the microwave plasma CVD process in a linear antenna arrangement with specific deposition parameters allows the formation of a fully closed diamond layer on the Ge ATR prism with large 3D geometry (i.e., 72 mm in length, 10 mm in width and 6 mm in height). The functionality of the diamond-coated Ge prism is demonstrated by FTIR-measurements of proteins adsorbed on the oxidized diamond surface. 2. Experimental 2.1. Planar substrates Diamond thin films were deposited on polished one side single crystal germanium substrates (100) purchased from MTI Corporation (USA). Deposition of diamond films was performed by the microwave (MW) plasma assisted CVD process using either focused [15] or a linear antenna arrangement [16]. The protective amorphous silicon interlayer (a-Si with thickness of 20–30 nm) was prepared by silane-rich radiofrequency plasma at room temperature. Prior to the diamond growth, bare or a-Si coated Ge substrates were ultrasonically treated in the water-based nanodiamond powder suspension. This treatment resulted in the high diamond seed density (up to 1011 cm2) required for uniform diamond film growth [17,18]. For the focused MW plasma arrangement, the deposition was done in a gas mixture of methane and hydrogen (1% CH4 in H2) at 2000 Pa and MW power of 1100 W for 18 h at the substrate temperature of 400 C. For the linear antenna MW plasma arrangement, the process conditions were as follows: the used gas mixture was a methane/carbon dioxide/ hydrogen gas system (2.5% CH4 + 10% CO2 in H2), pressure 10 Pa and MW power 2 1200 or 2 1700 W employing two antennas. The total deposition time was 20 h and the substrate temperature was 250 or 360 C (depending on power). After the deposition process, the samples were analyzed by scanning electron microscopy (SEM) using an e_LiNE system
(Raith) and Raman spectroscopy using an inVia Reflex Raman microscope (Renishaw) with the laser excitation wavelength of 442 nm. Surface topography was investigated by atomic force microscopy (AFM) in a tapping mode (AFM Microscope Dimension 3100, Veeco). The set-point ratio (ratio of approached vs. free cantilever oscillation amplitude) was 60 5%. Silicon AFM cantilevers were used with a typical tip radius of 10 nm and a resonance frequency of 230 kHz. Surface roughness was calculated from the area of 2 2 mm2. 2.2. ATR prisms Beside the planar Ge substrates, diamond films were also deposited on Ge ATR prisms. For demonstration of the spectroscopic advantages in FTIR-measurements of the diamond-coated Ge prism, the diamond-coated Si ATR prism was used with the same geometry (i.e., 72 mm in length, 10 mm in width and 6 mm in height). ATR prisms were prepared in a similar way to planar substrates as described above. The technology of Si ATR prism was presented in our previous study [13]. After ultrasonic seeding of substrates, the diamond film was deposited in the linear MW CVD (0.5% CH4 in H2 at 710 C). The Ge ATR prism was first coated by a thin a-Si (20–30 nm) as the protective layer, and then ultrasonically seeded with diamond nanoparticles. The diamond deposition was done in a linear antenna plasma (gas mixture 2.5% CH4 + 10% CO2 in H2, pressure 10 Pa, MW power 2 1700 W and temperature 360 C). Prior to FTIR-analysis, the diamond-coated ATR prisms were oxidized in a radio frequency oxygen plasma (45 W, 50 Pa, 1 min, low pressure FEMTO plasma system from Diener electronic GmbH & Co. KG) to achieve hydrophilic surface properties (wetting angle <10 ). Next, proteins were adsorbed on the oxidized diamond surface from 15% fetal bovine serum (FBS) solution (PAA) in McCoy’s 5A medium with stable glutamine without phenolred (BioConcept) [19]. A 200 ml of FBS solution was dropped on the diamond surface, left for 10 min, rinsed in deionized water and dried using a nitrogen blow. Infrared spectra of proteins adsorbed on the diamond-coated Ge or Si prisms were measured with the spectral resolution of 4 cm1 by Nicolet Nexus FTIR spectrometer using a KBr beamsplitter, a liquid nitrogen cooled MCT detector and the standard IRsource using the Specac’s GatewayTM 6-reflection horizontal attenuated total reflection accessory in a nitrogen atmosphere
Fig. 1. SEM image of surface morphology (a) and Raman spectrum (b) of the bare Ge substrate after the diamond growth at 400 C by a focused microwave plasma CVD process.
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[13]. Advanced ATR correction was performed on all measured spectra using the Omnic software. 3. Results 3.1. Diamond growth on Ge substrates The surface morphology of samples after the diamond growth on bare Ge substrates by a focused MW plasma is shown in Fig. 1a. During the CVD, the substrate temperature was as low as 400 C. However, instead of diamond crystals, pyramidal-like pits in size from a few up to a hundred nanometers were observed. Only local areas revealed the deposition of clustered-like features but no diamond fingerprint was observed by Raman measurements (Fig. 1b). It should be noted that the employed standard CVD process parameters resulted in a successful diamond growth on the reference Si substrates and glass [15]. The surface morphology of the samples after the CVD process in a linear antenna MW plasma is shown in Fig. 2. After CVD at the substrate temperature of 360 C, the Ge substrate reveals pit formations and a molten-like surface (Fig. 2a). Decreasing the substrate temperature (250 C) suppressed the presence of molten-like features (Fig. 2c). The density and size of pit-like features also decreased; nevertheless, the surface was still damaged. In both these cases, on some areas of the samples, the early stage of crystal faceting was observed. Raman measurements on samples processed in a linear antenna MW plasma system confirmed the presence of the diamond phase for both deposition temperatures (Fig. 2b and d). The Raman spectrum of the sample processed at 360 C shows a sharp peak at 1330 cm1, which is attributed to the diamond
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characteristic band [20]. Additionally, two broad bands centered at 1320 and 1590 cm1 are assigned to the D (disordered) and G (graphitic) band of sp2 carbon phases (Fig. 2b) [20,21]. For the sample processed at 250 C, the spectrum is dominated by the diamond band, while the D and G bands are not evident (Fig. 2d). AFM measurements collected from the area 1 1 mm2 confirmed a decrease in RMS surface roughness from 41 nm to 27 nm for the process temperatures 360 and 250 C, respectively. The non-uniform diamond coating and persistence of etch pits on the Ge substrate show that a lower substrate temperature is not alone a sufficient parameter for successful growth of diamond on Ge in spite of using a low-energetic and distant linear antenna plasma for the CVD process. Therefore, we applied a thin amorphous silicon (a-Si) layer for Ge protection. A low thickness (20–30 nm) of a-Si is important to keep high enough optical transparency of the ATR prism in the IR-range. The diamond deposition on a-Si coated Ge substrates was done at 400 C in a focused MW plasma and at 360 C in a linear antenna MW plasma, respectively. These deposition temperatures were chosen as a technological compromise minimizing the thermal loading of the Ge substrate and keeping reasonable diamond growth rates. Fig. 3 shows the surface morphology and the Raman spectra of diamond films grown on a-Si/Ge substrates. Faceted crystals with sizes of 150200 nm were formed by a focused MW plasma (Fig. 3a). The porous-like conformation of these crystals is shown on the cross-section SEM image (Fig. 3a–inset). The Raman spectrum confirmed the diamond character of crystals and the sp2 carbon phases (D and G band) were also pronounced (Fig. 3b). A homogeneous and fully closed diamond film consisting of clustered-like crystals was formed by the linear antenna MW
Fig. 2. SEM images of surface morphology of bare Ge substrates after the diamond growth at 360 C (a) and 250 C (c) using the linear antenna MW plasma assisted CVD process and corresponding Raman spectra of grown films (b, d) with the diamond band at 1330 cm1, disordered band centered at 1320 cm1 and graphitic band centered at 1590 cm1.
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Fig. 3. SEM images of diamond films grown on a-Si coated Ge substrates using focused (a, b) and linear antenna (c, d) MW plasma CVD processes and corresponding Raman spectra of grown films (b, d) with the diamond band at 1330 cm1, disordered band centered at 1340 cm1 and graphitic band centered at 1590 cm1.
Fig. 4. IR absorbance spectra of FBS proteins adsorbed on the diamond-coated Si (blue line) and Ge (red line) ATR prisms. The band and peaks in the spectra are assigned to: OH stretching vibrations and stretched amino group NH2 vibrations at 3200–3400 cm1, overtone of amide II band at 3100 cm1, methyl CH2 and CH3 stretching vibrations at 3000–2800 cm1, amide I band from the C¼O stretching vibrations at 1655 cm1, amide II band related to NH bending and C-N stretching vibrations at 1540 cm1, protein side-chain COO- close to 1400 cm1, and amide III band related to CN stretching vibration and NH bending vibration near 1300 cm1. The band centered at 1050 cm1 is discussed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
plasma (Fig. 3c). The Raman spectrum is dominated by the diamond peak with lower amount of sp2 carbon phases (Fig. 3d) compared to the focused plasma. The RMS surface roughness of these diamond films was as low as 12 nm.
3.2. IR-measurements using diamond-coated Si and Ge ATR prisms Fig. 4 shows the IR absorbance spectra of the proteins adsorbed from fetal bovine serum solution on the diamond-coated Ge and Si
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ATR prisms for comparison. In both IR spectra, the broad band at 3400-3200 cm1 is related to hydroxyl OH stretching vibrations of adsorbed water overlapping the stretched amino group NH2 vibrations [22]. The band at 3100 cm1 is assigned to the overtone of the amide II band. Bands near 3000 cm1 are related to CH2 and CH3 stretching vibrations. The band at 1655 cm1 is the amide I band, which results from the C¼O stretching vibrations. The amide II band peak at 1540 cm1 is related to N-H bending and C-N stretching vibrations. However, the spectrum should be more complex due to possible overlapping of water with NH stretching. Next, for the diamond-coated germanium ATR prism, well resolvable spectra were collected also at wavenumbers below 1500 cm1. The observed band close to 1400 cm1 results from protein side-chain COO–. The amide III band near 1300 cm1 is related to C-N stretching vibration and N-H bending vibration. The intensive band centered at 1050 cm1 is discussed later. It is
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obvious that the diamond-coated Ge prism is more advantageous than the Si prism due to a wider range of IR-spectra. 4. Discussion As was stated before, the growth of a diamond thin film on germanium is not a simple task. The standard focused MW plasma system led to etching of Ge and formation of large pits at even a low temperature (400 C) process. The etching of germanium is primarily attributed to highly aggressive growth conditions (high concentration of atomic hydrogen and high temperature) [23]. In the focused plasma system, a thermal load of Ge cannot be avoided due to the plasma ball, which is inherently located in close vicinity to the Ge surface (approx. 1 mm). The linear antenna MW plasma system, also known as the surface wave plasma, appears more suitable for the diamond growth at low temperatures [24].
Fig. 5. Illustrative comparison of principal differences between bare and diamond-coated ATR prisms.
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Previously, it has been successfully used for the diamond growth on thermally [10] or chemically [25] unstable substrates. In contrast to the focused plasma system, here, the substrate material is located at far distance (6–7 cm) from the hot plasma region. At this relatively large distance, the plasma becomes “cold” in the sample vicinity and the temperature of electrons is lower than 2 eV [16,25,26]. Nevertheless, in spite of the deposition temperature as low as 250 C and the distance from the hot plasma region (7 cm), a partial damage of planar bare germanium substrate was still observed (Fig. 2a and c). For the samples processed in the linear antenna MW system, the Raman spectra revealed the presence of a diamond phase (Fig. 2d), which is a positive indication compared to the focused plasma system (Fig. 1b). Thus, a low substrate temperature is alone not sufficient for a successful growth of diamond on germanium (at least for microwave plasma in hydrogen-rich gas mixture). Previously, Rego et al. investigated the ability of an interlayer (amorphous Si, SiO2, or Si3N4) to protect planar germanium substrates from the diamond CVD [23]. They found that thick (1 mm) carbide-forming protective coatings are able to withstand the growth process. However, due to the high temperature process (700 C), their diamond layers delaminated from the Ge substrate. On the other hand, using thinner protective coatings did not preserve the Ge surface from its melting. Miller et al. presented a two-step process which consisted of substrate micro-pattering followed by the deposition of an interlayer. This process significantly improved the adhesion of diamond to the substrate [27]. A disadvantage was that the micro-patterning step resulted in the surface roughening, which is an unfavorable feature for any optical uses. In our experiments, we used an ultra-thin (20–30 nm) amorphous Si coating, which suppressed the chemical reactivity of Ge for both deposition systems. Neither film delamination nor melting of Ge was observed. This feature is attributed to using low temperature (less than 400 C) in both the deposition systems. However, the situation concerning the uniformity of diamond coatings from two deposition systems is different. As is evident from the SEM cross-section image (Fig. 3a inset), the layer grown in a focused MW plasma system, although relatively thick (>600 nm), is porous. This deteriorates its optical properties, primarily due to scattering, and its mechanical stability. In contrast to this, the diamond film grown in the linear antenna MW system (Fig. 3c) is dense and homogeneous even at observed lower thickness (around 250 nm). The linear antenna MW plasma system thus provided suitable results on diamond growth on Ge with the possibility of large area coating. Therefore, ATR prisms for IR spectroscopy were covered by diamond in a linear antenna MW plasma system. The IR-spectra reflected fingerprints from water band overlapping the amino groups (3400–3200 cm1), amide II overtone (3100 cm-1), CH2, CH3 stretching (below 3000 cm1) and C¼O stretching vibrations (1650 cm1) and ending with the amide II band (1540 cm1) for the diamond-coated Si and Ge ATR prism, respectively. It should be noted that, while resolved IR-spectra on Ge and Si ATR prism are of similar quality, an evident cut of the wavenumber for Si prism observed at 1500 cm1 (Fig. 4) limits recognition of chemical bonds at lower wavenumbers. In contrast to the Si limitation, the diamond-coated Ge prism is optically transparent down to 870 cm1 allowing the detection of C-N stretching and N-H bending vibrations (amide III, band at 1300 cm1). The IR-spectrum reveals also an intensive band centered at 1050 cm1 with a right shoulder. The attribution of this band is not simple. In the first assumption, it seems to be a fingerprint of COC asymmetric stretching vibrations (1100– 1040 cm1) [3]. Due to the silicon interlayer, the band can also indicate a presence of Si O C (1000–1110 cm1) or Si OSi
(1030–1090) vibrations [28]. Unfortunately, all three assignments (CO C, Si O C or Si O Si) are known as very strong in intensity and they can exhibit non-linear enhancement. The effect of organic contamination of the IR-setup (lenses and mirrors) should be mentioned too, which can give a false signal in this region. Therefore, background subtraction has to be done carefully, while it can represent a serious problem for a proper interpretation of IR-measurements. It is still a challenge to diminish all above mentioned potential effects. To highlight, the diamond thin film mechanically and chemically protects the soft surface of the ATR prism as well as being a functional surface. Fig. 5 schematically illustrates principal differences between bare and diamond-coated prisms. First, prisms were exposed to an oxygen plasma to achieve a hydrophilic surface. For better optical visualization of the investigated material, we used a liquid solution of atomically-functionalized diamond nanoparticles (f 5 nm). A droplet of the solution was spread on both prisms by a drop technique. After drying, the prisms reveal significantly different surfaces. In the case of oxygen treated bare prism, only a poor and inhomogeneous spreading of diamond nanoparticles was achieved, as observed by optical microscopy. It is known that attenuated total reflection is disturbed by any surface inhomogeneity resulting in an IR-spectrum of low resolution. In contrast to the bare prism, the oxygen treated diamond-coated prism reveals the homogeneous spreading of the investigated material on micro-scale as confirmed by AFM measurements. This observation is attributed to the unique properties of the diamond surface stability and keeping its hydrophilic character. A homogeneous spreading of liquid results in good optical contact of the investigated material with the diamond-coated ATR prism. So, the infrared beam can reflect well the interface via total internal reflection. We propose the same functional principle for the detection of an ultra-thin protein layer, which is hardly detectable on a rough diamond surface by optical microscopy or AFM measurements. We optically observed a homogeneous spreading of phosphate-buffered saline (PBS) on the oxygen terminated diamond-coated ATR prism. Due to the small thickness of the adsorbed PBS layer, we were not able to evaluate a FBS spreading homogeneity by AFM measurements. The AFM signal was dominated by the relatively high surface roughness of diamond coating (12 nm). Nevertheless, based on our earlier AFM measurements on atomically flat diamond surfaces (i.e., monocrystalline diamond), the protein layer thickness is assumed to be approximately 3 nm [19]. Taking into account the observed IR-absorbance spectra (Fig. 4), we can state that the IR-measurements clearly proved the applicability of diamondcoated ATR prisms for the detection of such ultra-thin protein layers. The diamond film adhered well to the germanium prism and no delamination was observed even after multiple cleaning cycles (>30x). The good adhesion is partially assigned to the applied silicon interlayer, low deposition temperature (360 C) and small diamond thickness. For re-using the prism we employed the cleaning procedure which consisted of a mechanical cleaning by a cotton stick, followed by low temperature plasma hydrogenation (reset of the surface to the as-grown state) and plasmatic oxidation for recovering the hydrophilic character of the surface. 5. Conclusions We have demonstrated the progress in the diamond growth on planar germanium, which represents a chemically and thermally sensitive substrate. The deposition process in a focused plasma system at a temperature of 400 C resulted in etching of the Ge surface. Even at deposition temperature as low as 250 C in a linear antenna system, the Ge substrates revealed melting-like features,
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and diamond growth was not efficient enough. Applying 20–30 nm of thin amorphous silicon as the protecting layer stabilized the Ge substrate and resulted in a homogeneous diamond coating with good quality. In contrast to a focused microwave plasma, the linear antenna plasma was shown as more suitable for coating of nonplanar and large substrates such as silicon or germanium ATR prisms. It should be noted that the diamond-coated ATR prisms differ principally from the commercially available single reflection monocrystalline diamond prisms by multiple reflections (7x vs. 1x), larger functional area for interaction of the evanescence wave with the investigated material (700 mm2 vs. 2 mm2), lower cost. The absorbance spectra exhibited good enough resolution and sensitivity to an ultra-thin protein layer. Owing to the wide spectral window offered by the Ge optical element in comparison to silicon, IR-analysis was performed in the range of 4000–870 cm1. Additional perspective is the combination of the diamond-coated germanium ATR prism with real time electrical measurements employing surface conductivity of diamond.
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Acknowledgements
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The technical support of K. Hruska and J. Cermak is gratefully appreciated. This work was financially supported by the GACR research project 15-01687S (AK, HK).
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