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P. S. Smertenko , D. A. Grynko , N. M. Osipyonok , O. P. Dimitriev* , and A. A. Pud 1
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Carbon fiber as a flexible quasi-ohmic contact to cadmium sulfide micro- and nanocrystals
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Phys. Status Solidi A 210, No. 9, 1851–1855 (2013) / DOI 10.1002/pssa.201228805
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V. Lashkaryov Institute of Semiconductor Physics, pr. Nauki 45, Kyiv 03028, Ukraine Institute of Bioorganic Chemistry and Petrochemistry, 50 Kharkivske Shose, Kyiv 02160, Ukraine
Received 30 November 2012, revised 27 April 2013, accepted 3 May 2013 Published online 11 June 2013 Keywords CdS, nanowires, ohmic contacts, pyrolytic carbon fiber * Corresponding
author: e-mail
[email protected], Phone/Fax: þ38-044-525530
A conductive pyrolytic carbon fiber (CF) has been found to serve as an alternative material to metal electrodes, since it forms an Ohmic contact to CdS crystals. The methods of preparation of polycrystalline layers and nanocrystalline arrays of CdS are described that allow formation of an ohmic or quasi-ohmic contact to CF. It is shown that the ohmic contact between the CF and polycrystalline CdS layer is stable for at least several months and its exploitation characteristics are not worse than the indium contact. Advantages of the CF electrode, such as thermostability to extremely high temperatures and low cost are discussed.
CdS nanowire arrays grown on a carbon fiber.
ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Cadmium sulfide (CdS) semiconductor has been used as photoconducting, photochemical, photovoltaic, etc., material in layers or components of the respective devices for many years [1–12]. One of the important requirements in the above electronic devices is formation of the ohmic contact between a conventional metal electrode and the CdS layer [13–16]. Indium or silver as well as aluminum are normally used as materials that are able to form such a contact to CdS [17, 18]. These contacts are made by thermal vacuum evaporation or by deposition of specially prepared metal particles suspended in organic solvents and stabilized by surface-active compounds. The above processes involve additional complex, expensive, and time-consumed technological operations, which require special equipment. Moreover, the metal contacts are not always inexpensive and robust; besides, the frequently used indium is not thermostable, since its melting point is rather low (156.6 8C). That is why novel cheap and robust
materials are needed to apply as ohmic electrodes for CdS. For example, recent developments in this field have showed that sputtered titanium nitride forms an ohmic contact to n-type CdS [19]. A new interest how to make an ohmic contact to CdS material has arisen due to the formation of CdS nanocrystals and nanowires [20–22]. In this respect, it is necessary to look for new materials and approaches to obtain the ohmic contacts to CdS with approved properties and parameters. In the present work, an alternative ohmic contact to CdS crystals based on a commercial carbon fiber (CF) is proposed. Since CF is a flexible and thermostable material it can be used as a separate contact and as a flexible conducting support for growth of CdS crystals. This allows one to vary the shape of the sample easily. An additional advantage of CF is its high thermostability which gives a new opportunity to exploit the respective devices at high temperatures up to 1000 8C. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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2 Experimental 2.1 Synthesis of CdS nanocrystals on CF A bundle of CFs was separated from the commercial carbon cloth (LU-3) produced by carbonization of polyacrylonitrile cloth. CdS powder of the chemical grade (purity 99.998%) has been used for sample preparation. Growth of CdS nanowire crystals was performed using a vapor–solid (VS) condensation technique [22]. In our method, no Au or other preliminary deposited metal islands were used as nucleation centers/seeds for the nanocrystal growth and CdS nanocystals were grown directly on the CF substrate. Synthesis has been performed using a vacuum chamber (VUP-5, Ukraine) and a high-temperature reactor inside the vacuum chamber, additionally equipped with an original control system of evaporation and condensation processes. CdS powder was loaded into the oven (T1, Fig. 1) and heated up to 650–750 8C with the basic pressure in the chamber of 105 Torr. The CFs were displaced at some distance from the CdS source and were under the influence of a separate heater (T2, Fig. 1) to create a controlled temperature gradient region with a quasi-closed condition for growth of CdS crystals from the vapor phase. Position of CFs with respect to the CdS source was varied (denoted as different substrates in Fig. 1) to provide the required temperature difference with respect to the CdS source (lower by 50–120 8C depending on the substrate position) and required growth conditions, respectively. The pressure dropped to (5–8) 104 Torr during the CdS growth. The growth duration was 10–40 min followed by cooling down the formed samples directly in the same oven under vacuum conditions. In order to clarify the effect of synthesis conditions on electrical properties of CdS nanocrystals, two types of samples have been prepared, namely, the low-temperature (LT) samples with the temperature of 594 8C in the nucleation zone and the high-temperature (HT) samples with the temperature of 633 8C in the nucleation zone, respectively. 2.2 Growth of polycrystalline CdS layers and formation of the contact to CF The basic method for preparation of polycrystalline layers of CdS was screen printing [23–25]. For this method a CdS paste was prepared as follows. The CdS powder was ground within a ball mill by a chalcedony drum. To prevent coagulation of the particles distilled water was used. The obtained paste was dried at temperature of 190–200 8E in the inert-gas flux. After the drying the powder was annealed at temperature of 700 8E.
Figure 2 Illustration of the CdS sample with two embedded CFs.
Under these conditions, the grain size was obtained to be smaller than 1.5 mm. The CdS powder and CdCl2 were mixed with propylglycole as the organic binder in the weight proportion of 9:1:3 and passed through a special device to obtain a homogeneous mass. The paste was then deposited onto a cleaned quartz substrate and annealed at 690 8C for 90 min under the inert atmosphere. Before use, CF was first annealed under an inert atmosphere. The contact of the CF and the polycrystalline layer of CdS was provided either by a direct deposition of the CdS paste onto the CF support or by mechanical pressing of the CF to the wet CdS paste deposited onto the substrate followed by annealing of the assembly in both cases at 580– 600 8C for 15–30 min under the inert atmosphere (Fig. 2). A standard automated tester 14 TKS-100 (Russia) was used for quasi-DC measurements of current–voltage (I–V) characteristics of the samples. Voltage was applied to the heterostructure step by step with 150 ms duration of the each step and 90 ms measurement time, respectively. I–V characteristics under illumination were measured using white light from a halogen lamp with an intensity of 30 W m2. The ohmic contacts to CF or CdS layer were provided by In clips. An In pad was used as a contact to an ensemble of CdS nanowire crystals. Ohmic behavior has been clarified through the differential approach applied to I–V characteristics [26, 27]. The dimensionless parameter a of the I–V characteristics in the form of aðVÞ ¼
dðlg IÞ V dI ¼ dðlg VÞ I dV
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gives the possibility to indicate the quality of the contact, i.e., a ¼ 1 corresponds to the ohmic contact; when a < 1 there will be a rectifying contact; for a > 1 there will be an injection one.
Figure 1 Scheme of the CdS nanocrystal growth from the gaseous phase under quasiequilibrium conditions. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3 Results and discussion The resistivity of the CF was evaluated by separation of a single thread (having a diameter of 10 mm) from the cloth and measuring its resistance. Thus obtained resistivity was 3.8 103 Vcm. www.pss-a.com
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The quality of the electrical contact between CF and polycrystalline CdS layer prepared by screen printing was studied first (Fig. 2). In order to compare the electrical behavior of a CF/CdS heterojunction with the standard ohmic junction of In/CdS, two indium strips parallel to CF ones have been thermally evaporated in vacuum on the same sample. The distance between the In strips was the same as that between CFs. The length of the electrodes on the CdS surface was also the same. Electrical characteristics of the contacts are compared in Table 1. It can be seen that the dark resistance between the pair of CFs is the same as that between the pair of In electrodes. The resistance under illumination decreased due to the photoconductive properties of the CdS layer (Fig. 3a) and was slightly smaller for the structure with In contacts as compared to CFs. However, Table 1 Resistance of In/CdS/In and CF/CdS/CF structures. measured current geometry
dark resistance resistance of of as-prepared as-prepared samples (kV) samples under illumination (kV)
In/CdS/In 500 CF/CdS/CF 500
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resistance of the aged samples (4 months) under illumination (kV) 39 9
a 4-month aging of the samples showed that the resistance under illumination increases by a factor of 6 for the structure with In contacts, whereas it was almost unchanged for the structure with CFs (Table 1). This result can be explained by a slow diffusion of In atoms into the bulk of the CdS layer that dope and change the properties of the material in the vicinity of the heterojunction. Scission and diffusion of CF fragments into CdS seems to be hardly possible and CF/CdS heterojunction therefore demonstrates higher stability. Comparison of I–V characteristics of the CF/CdS/CF and In/CdS/In structures in the extended region of applied biases showed that for In electrodes a ¼ 1 within the whole voltage range both in the dark and under illumination (Fig. 3d and e), whereas for CF electrodes a can slightly vary, being a ¼ 1 at small bias (up to 0.04 V), then decreasing to a ¼ 0.9 within the voltage region from 0.04 to 0.9 V in the dark and from 0.04 to 0.5 V under illumination, then it increases (Fig. 3b and c). Finally, an asymmetric CF/CdS/In structure also showed practically the same electrical behavior (Fig. 4a–e). But at a negative potential on the In electrode the In contact is slightly injecting (a ¼ 1 1.1), while at the opposite bias the CF contact is slightly rectifying (a ¼ 0.9 1.1). This indicates that the barrier heights of CF/CdS and In/CdS are very close, but the CF/CdS one is slightly higher, because
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Figure 3 Comparison of (a) current–voltage characteristics of CF/CdS/CF (squares) and In/CdS/In (circles) structure in the dark (filled symbols) and under illumination (open symbols) and their differential images: (b) CF/CdS/CF in the dark, (c) CF/CdS/CF under illumination, (d) In/CdS/In in the dark, (e) In/CdS/In under illumination. www.pss-a.com
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Figure 4 Comparison of (a) current–voltage characteristics of CF/CdS/In structure in the dark (filled symbols) and under illumination (open symbols) with the positive potential on CF (squares) and In (circles) and their differential images: (b) injection from In in the dark, (c) injection from In under illumination, (d) injection from CF in the dark, (e) injection from CF under illumination. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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for the negative potential on the In contact the I–V characteristics show 1 < a < 1.1 (Fig. 4a–e). The growth of needle-like crystals of CdS was observed at the surface of the pyrolitic CF (Fig. 5). No special conditions such as the gold seed formation was undertaken for this synthesis and the growth mechanism will be discussed in a subsequent publication. The growth was observed both at the CF surface and in the interior part of the fiber (Fig. 5a). The needle-like crystals of CdS had a typical width of 300–700 nm and a height up to 10 mm (Fig. 5b). The other geometrical parameters of the sample were as follows: diameter of a single carbon thread was 5–10 mm, the distance between the threads was of the same order of magnitude, i.e., 5–10 mm, the CdS layer thickness was determined by the crystal length and was up to 10 mm, the crystal density was several CdS nanocrystals per square micrometer. The CdS nanowire crystals had a sharpened end instead of a drop-like tip observed normally for nanocrystals grown via the vapor–liquid–solid method using gold islands as nucleation centers. Electrical conductivity across the ensemble of the crystals whose length was about 10 mm appeared to be relatively high. I–V curves of the samples with the In top electrode showed that an ohmic behavior takes place at low applied biases up to 1 V (Fig. 6a–e) for the LT sample and up to 0.1 V for the HT sample (Fig. 7a–e). The resistance of the ensemble was 2 105 V in the dark and it decreased by a factor of 2 under the light conditions for the LT sample (Fig. 6a). For the HT sample, the resistance was an order higher, i.e., 3 106 V in the dark and decreased by a factor of 1.5 under the light conditions, respectively (Fig. 7a). Thus, the synthesis conditions somewhat affect electrical behavior of the samples. At higher applied bias the electrical characteristics of the samples become more complicated. For the LT sample (Fig. 6), the injection from the In contact is not restricted (Fig. 6b and c) and one can see two current jumps with a ¼ 3, a ¼ 4.5 and a ¼ 2.1, a ¼ 4.0 in the dark and under illumination, respectively. As shown in Ref. [15] such jumps point to the presence of some states within the bandgap of CdS; in our case the calculated energy of these states is E1 ¼ 0.36 0.026 eV (from the bottom of the conduction band) and their concentration N1 ¼ 1.68 (0.17) 1014 cm3 [15]. The injection from the CF
Figure 5 CdS nanocrystals grown at the surface of carbon pyrolitic fiber: (a) general view; (b) surface of a separate carbon thread from the middle of the fiber. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 6 (a) Current–voltage characteristics of the heterostructure CF/CdS nanocrystals/In (the LT sample) measured in the dark (filled symbols) and under illumination (open symbols) with the positive potential on CF (squares) and In (circles) and their differential images: (b) injection from In in the dark, (c) injection from In under under illumination, (d) injection from CF in the dark, (e) injection from CF under illumination.
contact (Fig. 6d and e) is weaker and does not give the current jump. For the HT sample (Fig. 7b and c), there is a current jump with a ¼ 3 followed by the region of bimolecular recombination (a ¼ 1.5) [15, 16] for injection from the In contact. In this case the states in the bandgap of CdS lie at E2 ¼ 0.41 0.026 eV from the conduction band and their concentration N2 ¼ 2.7 (0.27) 1013 cm3. For the opposite bias direction (Fig. 7d and e) there is a region of monomolecular recombination (a ¼ 2). In this case the predominance of only one type of charge carriers takes place. The difference in the electrical behavior of the HT and LT samples can be explained by the effect of processing temperature on the stoichiometry of the crystals. As is known, sulfur is a more volatile component than cadmium, therefore, CdS crystal formation will take place with a deficit of the sulfur component. This suggestion was confirmed by the analysis of the crystal composition by scanning electron microscopy combined with a WD/ED microanalyzer that allowed us to distinguish different elements via X-ray spectroscopy; this showed that the Cd component indeed prevails over S by several percents. The higher the temperature of the crystal formation, the larger the deficit of the sulfur component should be expected. This conclusion www.pss-a.com
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Figure 7 (a) Current–voltage characteristics of the heterostructure CF/CdS nanocrystals/In (the HT sample) measured in the dark (filled symbols) and under illumination (open symbols) with the positive potential on CF (squares) and In (circles) and their differential images: (b) injection from In in the dark, (c) injection from In under illumination, (d) injection from CF in the dark, (e) injection from CF under illumination.
is consistent with the better photoconductive response of the LT samples as compared with the HT ones (Figs. 6a and 7a). 4 Conclusions CF material has been probed as an electrode material to CdS polycrystalline layers and nanocrystalline arrays and almost ohmic behavior of the CF/CdS heterojunction, with 0.9 < a < 1.1 in the wide range of applied biases was demonstrated. In the case of CF/ nanocrystalline CdS the ohmic behavior is restricted to a narrower region of applied biases, however, this restriction is assumed to be due to the lack of stoichiometry of CdS crystals. It has been shown that the CF creates the ohmic contact to polycrystalline CdS layers that is not worse than the indium contact, and its noticeable ageing was not observed within several months at least. Moreover, there are further advantages of the CF electrodes as compared to the indium electrode, because these can work at extremely high temperatures, where the indium contact melts, and CF is also much cheaper as compared to indium. www.pss-a.com
Acknowledgements This publication is based on work supported by Award No. UKE2-7035-KV-11 of the U.S. Civilian Research & Development Foundation (CRDF). The work was also supported by the State Agency for Science, Innovation and Information of Ukraine.
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