JOURNAL OF APPLIED PHYSICS 115, 173109 (2014)
Near-infrared photoactive Cu3BiS3 thin films by co-evaporation Banavoth Murali, M. Madhuri, and S. B. Krupanidhia) Materials Research Centre, Indian Institute of Science, Bangalore-560012, India
(Received 19 March 2014; accepted 26 April 2014; published online 7 May 2014) Semiconducting Cu3BiS3 (CBS) thin films were deposited by co-evaporation of Cu, Bi elemental metallic precursors, with in situ sulphurisation, using a quartz effusion cell. Cu3BiS3 thin films were structurally characterized by XRD and FE-SEM. The chemical bonding of the ions was examined by XPS. As deposited films were demonstrated for metal–semiconductor–metal near IR photodectection under lamp and laser illuminations. The photo current amplified to three orders and two orders of magnitude upon the IR lamp and 60 m W cm2 1064 nm IR laser illuminations, respectively. Larger grains, made up of nano needle bunches aided the transport of carriers. Transport properties were explained based on the trap assisted space charge conduction mechanism. Steady state detector parameters like responsivity varied from 1.04 AW1 at 60 m Wcm2 to 0.22 AW1 at 20 m Wcm2. Detector sensitivity of 295 was found to be promising and further could be tuned for better responsivity and efficiency in utilization of near infra-red C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4875495] photodetector. V INTRODUCTION
EXPERIMENTAL
Near-infrared (NIR) photo detectors (PDs) are prominent in numerous technologically potential applications, such as in night visions, optical communications, medical diagnostics, and neurosciences.1 An optical window of 0.91.7 eV is suitable for the NIR PDs. Commercially, Cu(In,Ga)Se2 (CIGS),2 CuIn1xAlxSe2 (CIAS),2 and HgCdTe3 are available as the current PDs. However, the availability of tellurium and indium limited the production on a large scale of which to 4% of current global electricity demand4 for CdTe and 1% for CIGS. In turn, the alternatives for the photo applications play a crucial long-term role in global energy production. Synthesis of Cu3BiS3 involve the solution methods5–11 and combining the chemical bath deposition12 and sputtering process13,14 besides Cu3BiS3 thin films have a forbidden band gap of about 1.4 eV, an optimum for photo energy conversion. Co-evaporated thin films show good structural and optical properties.15 Tuning the optical and electrical properties, Cu3BiS3 thin films serve the role of an absorber in photovoltaic and photodetector devices.16 Cu3BiS3 is advantageous over the commercially available NIR PDs in terms of material availability and toxicity. Abundance of bismuth makes its utilization in pharmaceuticals, as a replacement for lead in ammunition, solders, and other materials where toxicity is a concern. Hence, the utilization of bismuth-containing compounds as a replacement to CIGS or CdTe would enhance the energy production of thin film photovoltaic technologies. Recently, Cu3BiS3 films have been deposited by co-evaporation;15,17,18 nonetheless, the infra-red photodectection was not reported. We have reported the photo detection of Cu3BiS3 nanostructures recently,19,20 and herein, we report the thin film based metal-semiconductormetal based photodetector. Physical vapor deposition methods for Cu3BiS3 will facilitate in meliorating the technological applications in the forefront of large-scale manufacturing.
Soda lime glass substrates were chemically cleaned by dipping in 5% hydrochloric acid to dissolve any of the surface contamination, followed by boiling in De-ionized water, acetone, and isopropanol for 15 min at 150 C, respectively. Molybdenum was dc-sputtered to get 500 nm thin films for serving as the back contact of the NIR PD. Cu3BiS3 thin film deposition involved the co-evaporation of the Cu, Bi elemental precursors on a molybdenum coated soda lime glass (SLG) substrate, using a cluster system constituted by an evaporation chamber connected to a vacuum system which allows working at pressures of about 106 Torr, two tungsten boats (used to evaporate Bi and Cu, respectively), a quartz effusion cell to evaporate sulfur and a thickness monitor (INFICON SQC-310 C) with a quartz crystal as sensor, used to measure the evaporated elements flux. The substrate temperature was controlled with a programmable PID controller (TOHO TTM-P4). The desired stoichiometric chemical composition of the Cu3BiS3 films was obtained by controlling the substrate temperature and the evaporated mass of the precursor species. The 400 lm diameter circular contact pads were deposited onto the film on both the Cu3BiS3 and Mo surfaces by thermal evaporation of Ni/Al (5 nm/300nm) metals and followed by thermal annealing at 200 C, for proper adhesion of the contacts.
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EXPERIMENTAL CHARACTERIZATIONS
X-ray diffraction (XRD) pattern was recorded on a Bruker D8 Advance X-ray powder diffractometer, using ˚ ) radiation graphite monochromatized Cu Ka (k ¼ 1.54059 A operated at 40 kV and 40 mA. Morphological and compositional investigations were carried out on a scanning electron microscope (SEM) (FEI FESEM equipped with EDAX (energy-dispersive X-ray)). X-ray photoelectron spectroscopy measurements were carried out with an ESCA-3 Mark II Spectrometer (VG Scientific Ltd.) using Mg Ka radiation (1253.6 eV) as the excitation source. Ionic binding energies of all other XPS peaks were corrected in reference to
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C 2014 AIP Publishing LLC V
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