Abstract
Nanocrystalline diamond (NCD) films are promising materials for wide-spread applications due to their outstanding characteristics of chemical, physical, and highly smooth surface. Our present work aimed at the fabrication of high performance diamond-based UV detector. NCD films were prepared by microwave plasma enhanced chemical vapor deposition process, and then Au interdigital electrodes were deposited onto the surface of the as-grown NCD film by sputtering technique. Annealing procedures were conducted at various temperatures to obtain Ohmic contact of NCD/Au structure. The surface morphology, microstructure, and wettablity of the NCD films were analyzed by scanning electron microscopy, atomic forced microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and water contact angle measurement, respectively. The electrical property and photoconductivity of the fabricated devices were tested for UV detection application. It was found that the NCD films possessed high sp3 fraction of 68.6%, low surface roughness of 9.6 nm, and good hydrophobicity, as deposited under working pressure of 40 Torr. Also, the NCD/Au structure annealed at 500°C exhibited a good Ohmic contact characteristic, high detection efficiency, and fast response to UV irradiation in air ambient. The proposed study indeed demonstrates prospective applications of NCD films in UV detector, photocatalyst, solar cell, and so on.
1. Introduction
Along with the development of electronics engineering and nanotechnologies, ultraviolet (UV) sensors have attained increasing attention in which their applications have flourished over the past decade, including energy, defense, space-to-space communications, medical treatment, food processing, and water treatment. Till now, several wide-band-gap materials like GaN, Si, AlN, and ZnO compounds have been investigated for the uses in UV sensors [1–4]. Aiming at the aforementioned practical applications, however, many researches have been devoted to achieve highly efficient and highly stable operations of the device in harsh environment, leading to the quest for good performance materials and functional structures [5–7].
Among the semiconductor materials, diamonds possess unrivalled characteristics such as good hardness, wear resistivity, high thermal conductivity, high optical transparency, and chemical inertness [8–12]. As nanoscale structures, nanodiamonds are considered to possess a wide band-gap (~5.5 eV) and large breakdown electric field (~10 MV/cm), resulting in low leakage current [13]. To date, the synthesis techniques of nanodiamond materials have been developed for decades [14–17]. Owing to the above advantages, nanodiamond materials are indeed a promising candidate for UV detection applications. The diamond-based devices with homojunction (including p-n, p-i-m, or p-i-n types), however, seem to be challenged by inadequate dopant concentration in diamond growth processes and a required low resistivity doping layer which tends to exclude voltage loss and Joule heat [18]. In addition, orientation-selective growth of doped diamond films is usually needed for the production of these devices [19, 20], resulting in a high cost and thus hindering the feasibility of their applications. Therefore, we present here the fabrication of nanocrystalline diamond (NCD) thin film based UV detectors using metal-semiconductor-metal structure, aiming at higher production rate and higher quality of the films deposition for improving the efficiency and repeatability/reproducibility of the devices. It is of importance to note here that though a postannealing procedure is found to be necessary for good junctions of metal electrodes and diamond films, the profound effect of that procedure on the characteristics of the diamond films as well as the whole device is not understood well yet.
So far, the growth of NCD films on foreign substrates is usually performed by chemical vapor deposition (CVD) equipment using H2/CH4 or Ar/H2/CH4 gas mixtures [15, 21]. The microwave plasma enhanced chemical vapor deposition (MPECVD) technique is well-known to increase the plasma density and secondary nucleation rate during the diamond growth process and, consequently, ensures high deposition rate and high density of the as-grown diamond films. In the present work, NCD thin films were grown onto silicon substrates by the MPECVD equipment at fixed H2/CH4 concentration ratio and under various working pressures. Subsequently, Au interdigital electrodes (IDE) were coated onto the as-prepared NCD films using nanolithography and sputtering techniques, followed by annealing procedures in vacuum ambient at various temperatures in order to obtain an Ohmic contact for further photodetector investigations. The as-constructed NCD-based UV detector was characterized through either photoresponse or repeatability tests.
2. Experimental Procedures
In this work, the fabrication procedures of diamond-based UV sensor were divided into two stages. In the first stage, the polished single crystal Si wafer (n-type) with (1 0 0) crystalline orientation and 15 × 15 × 1 mm size was used as substrate for NCD deposition. The Si substrates were successively cleaned in methanol and acetone solutions for 10 min, respectively, and then were dried by nitrogen. Prior to the CVD growth process, the substrates were nucleated for 15 min in a suspension of ultradispersed diamond nanoparticles having size of 5 nm in ethanol, followed by a thorough rinse in double distilled water. The plasma was induced under microwave power of 700 W without external heating source, CH4/H2 gas concentration ratio of 8%, and gas flow rate of 800 sccm (standard cubic centimeter per minute). The total pressures were controlled at 20, 40, 60, and 80 Torr, respectively. The deposition time in all of experiments was 2 h. During the CVD deposition process, the plasma condition was in situ monitored using an optical electron spectroscopy (OES). The characteristics of the prepared diamond films were analyzed by field emission scanning electron microscopy (FE-SEM, Libra-200 FE), high resolution transmission electron microscopy (HR-TEM, Philips Tecnai F30), atomic force microscopy (AFM, Veeco Nanoscope 3100), Raman spectroscopy (Renishaw, inVia), and X-ray photoelectron spectroscopy (XPS).
In the second stage, the nanomask was patterned onto surface of the as-prepared diamond films by a nanolithography procedure with positive photoresistor. Au was then deposited onto the as-patterned diamond films at room temperature employing sputtering equipment [22]. Detailed sputtering parameters were as follows: RF power of 50 W, working pressure of 3 mTorr, Ar flow rate of 20 sccm, and processing time of 8 min, respectively. The remaining photoresistor was thoroughly removed by rinsing in acetone for 15 min. Finally, in order to obtain the Ohmic contact for UV detector applications, the diamond/Au structured films were thermally treated in vacuum ambient for 8 min at various temperatures (300°C~500°C) using rapid thermal annealing (RTA) equipment. The properties of Au interdigitated electrode pattern with thickness of ~80 nm on NCD film will be discussed in following parts. All performance tests of the fabricated diamond-based UV detectors were conducted in measurement system equipped with a mercury arc lamp (26 mW/cm2 of power, 365 nm of wavelength) and Keithley 2400 as UV light source and nanoamperometer, respectively.
3. Results and Discussion
Figure 1 shows the surface morphologies for the diamond films grown on Si substrate at microwave power of 700 W, CH4/H2 concentration ratio of 8%, and deposition pressures of 20, 40, 60, and 80 Torr, respectively. For the diamond films grown under 20 Torr, SEM image reveals a discontinuous morphology of the as-grown diamond film which consists of discrete grains with island-like shape as shown in Figure 1(a). Figures 1(b)–1(d) indicate that the increase of working pressure drastically changed the morphology of diamond films and resulted in the aggregation of diamond grains. As shown in Figure 1(b), the film grown under 40 Torr possesses highly uniform diamond grains having size smaller than 50 nm with cauliflower-like shape. This suggests a good performance of our prior pretreatment procedure which has benefit in high diamond nucleation density on the surface of Si substrate the surface of the bare Si substrates. Higher deposition pressure is found to cause the nonuniformity in diamond grains size observed from the formation of cluster aggregation, obviously leading to a higher surface roughness of the as-grown films. As shown in Figure 1(d), the surface morphology of the film grown at 80 Torr reveals relatively large grain cluster having submicron scale size (~100–200 nm). The formation of the larger grain cluster with gradual increase in deposition pressure can be interpreted through the emission of Hα and the formation of diamond growth species in the gas phase. At a fixed CH4/H2 concentration ratio, the plasma density increases as a function of the deposition pressure because of the shortening in mean free path of radicals owing to the shortening in mean free path of radicals, and thus induces a higher concentration of Hα emission. In plasma CVD diamond growth, the Hα species have twofold benefits. First, the Hα reacts with the carbon gaseous precursors in plasma surrounding and then produces CHx species and C2 dimers for the growth of sp3 cluster and secondary nucleation [22]. Second, the Hα species induce the hydrogen-etching of the produced nondiamond phase which mainly consisted in grain boundaries and lead to the formation of larger grain size. This also can be observed through the appearance of the small sized diamond grains around the big cluster as shown in the inset of Figure 1(d). In addition, the higher plasma density can significantly increase the plasma temperature as well as the kinetic energy of the radicals in the gas phase, eventually enhancing the deposition rate and leading to the formation of the grain cluster in the diamond films. Therefore, the low deposition pressure may cause insufficiency of diamond growth species as well as slow growth rate, inducing discontinuous film morphology as shown in Figure 1(a). It should be noted here, however, that the exact alteration in grain size of the as-prepared diamond films cannot be definitely investigated by SEM technique due to the limitation in resolution; therefore, further investigation in the nanostructure is discussed below.
(a)
(b)
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The Raman spectra for the diamond films grown under working pressures of 40, 60, and 80 Torr are shown in Figure 2. The spectra for all samples show the typical characteristic peak of NCD films. Under 40 Torr, the diamond peak at 1332 cm−1 (sp3-bonded carbon phase) is weak and seems to be overlapped by the disordered band (D band, sp2-bonded carbon phase) at around 1350−1, indicating a high fraction of grain boundaries of the films with nanometered size of diamond crystallite size. As the working pressure increased, the position of G-band at ~1580 cm−1 shifted to the lower Raman shift side demonstrate the increase of disordering in sp2 structure, This can be explained as result of the aforementioned hydrogen-etching. Also, the characteristic peak of diamond gradually sharpens and increases in intensity, indicating the increase of diamond crystallite size. The increase of plasma temperature as a function of working pressure can be investigated through the peaks of C–H bonds (at ~1140 cm−1) in the grain boundaries and the surface of as-grown NCD films. The relation of plasma temperature and the hydrogen incorporation in diamond films which forms C–H bonds has been reported previously [15].
Figure 3 shows the surface roughness and contact angle values with water for the as-grown diamond films under various working pressures of 40, 60, and 80 Torr, respectively. AFM image for the NCD film grown at 40 Torr exhibits a high smooth surface morphology with very low surface roughness of 9.6 nm (root-mean-square, rms) as shown in Figure 3(a). This smooth surface morphology of the as-deposited NCD film indeed improve NCD/Au junction for the further constructed UV detectors. As the deposition pressure increased from 40 to 80 Torr, the surface roughness was increased to 21.6 nm (5 μm × 5 μm of scanned area). These results are in good agreement with our shown SEM images and prove the trends in the change of clustered degree and grain size of the diamond films. The hydrophobic property for the grown NCD films on Si substrate was investigated by contact angle measurements as shown in Figures 3(d)–3(f). It can be determined that all the as-prepared films possess highly hydrophobic surface, in which the contact angles with water are 95° above. In addition, further increase in deposition pressure is found to improve the wettability. This can be explained by considering surface energy and the microstructure of the films. The larger diamond grain and higher surface roughness of NCD films are caused by the higher working pressure, which not only can robustly enhance the hydrophobic property but also can increase the internal stress of the films, as the typical characteristics of sp3 phase structure. The observed hydrophobic NCD films also confirm their prospect in further applications such as protective layer, bioengineering, and solar cell.
(a) 40 Torr
(b) 60 Torr
(c) 80 Torr
(d) 40 Torr
(e) 60 Torr
(f) 80 Torr
The details in nanostructures for the NCD films grown under the working pressure of 40 Torr were investigated by TEM and XPS as shown in Figure 4. Plan-view TEM image shows that the diamond films consisted of nanosized nanocrystallite, which uniformly disperse in disorder phase matrix as shown in Figure 4(a). High-resolution TEM reveals the obtained diamond grain possesses 30 nm in crystallite size with round shape in geometry as shown in Figure 4(b). The above data exhibit both our ideal prior diamond nucleation enhancement stage and high plasma density which significantly assist the secondary nucleation during diamond growth. The crystalline structure of the NCD film can be studied by the selected area electron diffraction (SAED) pattern as shown in Figure 4(d), indicating that the films have good polycrystalline structure. Figure 4(c) shows the C-1s peak of XPS spectra of the NCD film grown at working pressure of 40 Torr. Owing to overlap phenomena of the peaks of sp2 and sp3 bonds (at binding energy of 284.5 eV and 285.3 eV, resp.), the spectra were curve-fitted using the Gaussian method in the literature [23]. It is calculated that the film consists of a high sp3 phase fraction of 68.6%.
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Figure 5 shows the photos of the patterned Au IDE electrodes on the surface of the NCD films grown at a working pressure of 40 Torr. SEM image shows that Au films were successfully deposited onto the NCD films, having thickness of 100 nm as shown in the inset of Figure 5(b). The fabricated Au electrodes have finger-pattern with 30 μm in width and 150 μm in interspacing. Figures 5(c)-5(d) show the captured photo and optical microscope image for the NCD/Au structure films after RTA procedure in vacuum at 500°C for 8 min. It is observed that the high annealing temperature has no effect on the surface morphologies of the structure films.
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The electric characteristics of the as-constructed NCD-based UV detector were investigated by I-V curve measurement as shown in Figure 6. All the measurements were conducted at room temperature with bias voltage ranging from −5 V to 5 V. The originated NCD/Au structure exhibits a poor junction characteristic with unstable resistance. Annealed at 300°C and 400°C, the measured I-V curves of the device also show a nonlinear behavior. Interestingly, the sheet resistances of annealed NCD/Au structure seemed to be higher than the original one. We suggest that this can be elucidated by further studies in the formation of carbide structure and Au : H bonds during the annealing process. From the I-V curves of the device annealed at 500°C, the observed linear characteristics indicate that a high-quality Ohmic contact has been obtained at the junction area of NCD and Au with relatively low Ohmic resistance. However, it is not unreasonable to notice here that the thermal treatment may lead to graphitization or disorderization of sp3 structure which hampers the photovoltaic efficiency of the NCD as absorbance layer. Our above data indeed propose a fabrication technique for obtaining a good performance of Ohmic junction of NCD and metal electrodes which is feasible for not only UV photodetectors but also future applications in solar cell industry.
The photoresponse efficiency of the as-constructed UV detector was investigated in dark currents and photocurrents measured from its repeatability/reproducibility tests using repeatedly turning UV light on and off. All the tests were carried out at room temperature and 5 V of bias voltage, as shown in Figure 7. It is found that the detector has quite a low and stable dark current of ~0.2 mA, even for long measuring time of 60 s, This indicates a low resistance of the whole UV detector and thus eliminate thermal stress in NCD/Au junction which caused by Joule heat. As irradiated by UV light, the current increased sharply and endured during exposure time of 60 s. It can be seen that the response and decay corresponding to the turning on and off of the UV irradiation are fast, in which the rise and decay time were 0.8 s and 1.2 s, respectively. The measured results also showed a significant deference of the dark currents and photocurrents as two orders of magnitude. Indeed, the photoresponse and stable operation of the device prove the feasibility in UV sensor applications.
4. Conclusion
In conclusion, a complete fabrication method of nanodiamond-based UV detector device with high performance is proposed. The NCD films grown at a working pressure of 40 Torr exhibited high sp3 fraction, low surface roughness, and good hydrophobicity. The Au electrodes deposited onto surface of NCD film show Ohmic contact characteristics after 8 min of rapid thermal annealing procedure at 500°C in vacuum ambient. The fabricated diamond photodetector achieves high detection efficiency and fast response to UV irradiation in air ambient. The proposed method is indeed cost effective and feasible to large scale production. In addition, this method also can be applied to other photovoltaic-based applications such as solar cell, photocatalyst, and X-ray detection.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work was financially supported by the main research projects of the National Science Council of Republic of China under Grant nos. NSC 103-2622-E-027-005-CC2 and NSC 101-2622-E-027-003-CC2, respectively.