Optical and electrical properties of boron doped diamond thin conductive films deposited on fused silica glass substrates
Graphical abstract
Introduction
Diamond films have outstanding physical properties: mechanical, electrochemical, electronic, thermal and biological. Optically diamond is transparent from the ultraviolet, visible to far infrared region, resulting in many possible technological applications including optical coatings [1], [2], optoelectronic switching devices [3], wide-band IR optical windows or high temperature and chemical corrosive operations [4]. Diamond is a wide bandgap semiconductor with Eg = 5.45 eV but when doped with boron its becomes p-type semiconducting material with outstanding electrochemical properties [5], [6]. Boron-doped diamond (BDD) films are a great electrode material that have a wide electrochemical window from −1.25 to +2.3 V in aqueous electrolytes compared to standard hydrogen electrode (SHE) [7], [8], high anodic stability [9], chemical stability in harsh environments [10], [11], and biocompatibility [12], [13]. These remarkable properties make BDD useful for many applications e.g. electrochemical sensing [14], [15], [16], biosensing [17], [18], electrocatalysis [19], [20], [21], and wastewater treatment [22], [23], [24].
Chemical vapor deposition (CVD) technology allows for the production of high-quality diamond thin films. The two most common techniques to synthesise BDDs are hot-filament CVD (HF-CVD) [25] and microwave plasma-assisted CVD (MW PA CVD) in-situ doped with boron precursors [26]. The boron dopant density, achieved by using MW PA CVD, ranges from 1016 to 1021 atoms cm−3 and p-type semiconducting materials transform to semimetal for the dopant density at 2 × 1020 [25]. The various applications require that structural and morphological properties be optimized: smoother surfaces, optical transparency or electrical conductivity. Growth parameters such as gas mixture, temperature, pressure and boron dopant density influence not only the morphology and structure (sp3/sp2 ratio) [27], [28], but also electrical and electronic properties or optical transparency. Boron dopant density has significant influence on all of these properties which has been confirmed by literature. The average grain size on the diamond film decreases up to 10 times with increasing boron density and was investigated by Liao el al. [29]. Boron introduces re-nucleation which result in the creation of smaller crystalline on primary higher diamond crystals. Recently, Lu et al. [30] has shown a direct visualization of boron dopant distribution and has concluded that boron dopants clearly demonstrate the presence in the diamond lattice and an enrichment of these dopants within twin boundaries and defect centres.
Several papers report optical properties of BDD films deposited on silicon substrates. Gupta et al. [31], [32], [33] applied spectroscopic ellipsometry (SE) to investigate BDD films with varying boron density. Hu el al. [34] and Gupta et al. [33] show ellipsometric angles Ψ, Δ and dielectric constants in a range of 200 up to 1230 nm (1.0/5.5 eV). Zimmer et al. [35] investigated heavily boron-doped nanocrystalline diamond films using spectroscopic ellipsometry. NCD film was deposited on Si wafers at the mean dopant level [B]/[C] of 6500 ppm and with the complex index of refraction calculated from the Lorentz model in the VIS-NIR range (up to 950 nm). Next, Gajewski et al. [36] investigated the optical parameters, namely, photocurrent and optical absorption coefficient in undoped and low-doped nanocrystalline diamond films deposited on monocrystalline silicon. The results of spectrally resolved photocurrent and photothermal deflection spectroscopy in the low energy range, between 0.5 and 1.0 eV, confirmed that boron as well as sp2 carbon phases in the grain boundaries govern the optical-absorption process.
A few reports can be found that focus on boron doped diamond optical transparent electrodes. Stotter et al. [37], [38] enhanced boron doped diamonds electrodes to spectroelectrochemistry studies in the UV wavelength region showing good transparency at 50–60%. Mermoux et al. [39] used confocal Raman imaging to study OTEs for samples with a thickness of 380 μm. Remes et al. [40] investigated the optical properties of undoped NCD film on fused silica by using photothermal deflection spectroscopy, calorimetry and dual beam photocurrent spectroscopy. Kromka et al. [41] investigated the impact of low-temperature MW CVD process on optical properties of nanocrystalline diamond films (NCD) on silicon and quartz substrates; the films displayed a transmittance of ca. 70% and a high refractive index of 2.34 at 550 nm of wavelength. Potocky et al. [42] showed a refractive index of 2.2–2.4 (@550 nm) on quartz substrate for growth temperature below 400 °C. The optical properties of diamond layers strongly depend on deposition temperature [43], [44]. In addition, various boron doping has a crucial impact not only on electronic properties but also on optical properties. A boron dopant introduces an acceptor level located at 0.38 eV from the top of the valence band [45], [46] shifting both absorption edge and refractive index.
In our previous works we reported seeding in different suspensions (water/DMSO) for obtaining diamond films [47] or growth of BDD on fused silica [MMS] but only for one doping level. Scorsone et al. proposed seeding in a solution composed of polyvinyl alcohol (PVA) and detonation nanodiamond (DND) particles but it was studied just on Si wafers. PVA was chosen due to its ability to form thin films as well as its high viscosity and high solubility in water and surfactants [48], [49].
To the best of our knowledge, there is still a lack of information about optical and electronic properties of boron-doped diamond deposited on fused silica substrates and their boron concentration dependence. These parameters are critical for developing the integrated optical sensors [50], [51], transparent electronics [52] and optoelectrochemical biosensing devices [53], [54].
In the present study we applied a novel two step pre-treatment procedure of fused silica substrates to achieve high seeding density and BDD film homogeneity. First, we used pre-treatment of fused silica substrates in hydrogen plasma. Then, substrates were seeded by spin-coating with PVA mixed with diamond slurry, the latter based on diethyl sulfoxide (DMSO) and diamond nanoparticles. The main novelty of the paper is the study of optical and electrical properties of undoped and boron-doped ([B]/[C]ppm ratio between 1000 and 10000) diamond films in two molar ratios of CH4-H2 mixture (1% and 4%). Our motivation to present this investigation has been derived from the optimization growth parameters process of the chemical vapour deposition to achieve BDD film with enhanced optical and electrical properties for the purposes of fibre optical coating and opto-electro active electrode for energy conversion, optical sensors and spectroelectrochemistry. Thin boron-doped films were deposited by MW PA CVD method on fused silica substrates. Micro-Raman spectroscopy was used to examine molecular structure of the BDD films (sp3/sp2 band ratio). Optical properties, thickness and roughness in VIS-NIR wavelength range were investigated by means of ex situ spectroscopic ellipsometry (SE). Moreover, the optical band-gap energy Eg was obtained using Tauc’s plot. Electrical properties were investigated 4-point probe method.
Section snippets
Fused silica glass pre-treatment
The mirror polished fused silica glass slides were used as a substrate for experiments (10 × 10 mm; 1 mm thick). Before treatment, the fused silica slides were cleaned for 5 min in an ultrasonic bath containing acetone, rinsed in 2-isopropanol and then dried.
The substrates then undergo the hydrogenation in the plasma. The process was performed in microwave H2 plasma at 1300 W for 60 min. During the process the total flow of gas reached 300 sccm and the pressure was kept at a level of 50 Torr.
Surface properties and film composition
The PVA-DMSO-DND slurry contact angle measurements were investigated to study the influence of hydrogenation on the surface wetting of fused glass substrates. Water-based wetting studies has been presented as the reference. The hydrogen modification of fused silica glass with microwave plasma treatment leads to strongly changed contact angles in comparison to the bare glass surfaces (see Fig. 1).
Both the H2O and the PVA-based DND slurry results in a similar level of contact angles recorded at
Conclusion
In summary, we have investigated the electrical and optical properties of BDD films deposited at fused silica substrate in microwave plasma assisted CVD. The influence of boron doping level and methane admixture was principally studied. Raman studies reveal that as [B]/[C] ratio increases, the FWHM of diamond band get broadened up to ca. 40 cm−1 or 70 cm−1 for 1 and 4% CH4 grown samples respectively. Since the diamond structure became more distorted, the relative sp3/sp2 band ratio of diamond
Acknowledgements
This work was supported by the Polish National Science Centre (NCN) under the Grants No. 2014/14/M/ST5/00715 and 2015/17/D/ST5/02571. The DS funds of Faculty of Electronics, Telecommunications and Informatics of the Gdansk University of Technology are also acknowledged. The authors acknowledge Alexander Tools (Gdynia, Poland) for their technical support. The AFM measurements were done at the Faculty of Microsystems Electronics and Photonics by the courtesy of Prof. Teodor Gotszalk. R.
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