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Summer Research Fellowship Programme of India's Science Academies

Self-powered, high response photodetector based on germanium diselenide

G. Vadivazhagan

RKM Vivekananda College, Chennai 600004

Dr. K. Jeganathan

Co-ordinator, Centre for Nanoscience and Nanotechnology, Bharathidasan University, Trichy 620024

Abstract

The photocurrent conversions at the two-dimensional (2D) TMDC materials are important for achieving high performance, self-powered optoelectronic devices such as photodetectors. In this work, we design and fabricate a self-powered germanium diselenide thin-film based high-performance photodetectors. Our synthesized GeSe2 exhibits a direct bandgap of 2.78 eV and performs well as a self-powered detector with excellent photoresponsivity of 320 mA/W and response time of 323 mSec. The self-powered photodetector with superior visible light absorbing and carrier transport behavior is may expected to open-up the door for the future GeSe2 based optoelectronic devices.

Keywords: germanium diselenide, TMDC material, self-powered photodetector.

INTRODUCTION

Photodetectors play a critical role in current and emerging applications in biotechnology, telecommunications, medical applications, atmospheric studies and optoelectronic applications. The Photodetectors are purely based on the semiconductor-based photon detection technology. In this report, a brief outline of the fundamental physical processes behind the photodetectors are presented and I fabricated a self-powered ultra-fast photodetector using 2D germanium diselenide.

Working of Photodetector

Photodetector is a device which converts incident photon into electrical signals and it is also known to be photo-sensor. Generally, when a photon of ample energy strikes the semiconducting sensing surface area of photodetector, it generates electron-hole pair. The energy of incident photons causes the electrons to transfer from valence band to conduction band of the sensing material which make them as a free electron for the current conduction and produces electrical signal.

11.png
    Fig:1 Symbol of photodiode

    Photodiode Operation Modes

    A photodetector can be operated in one of the two modes,

    ➤ Photovoltaic mode

    ➤ Photoconductive mode

    Photovoltaic mode: Here in, a photon of sufficient energy can be absorbed by the detector material to excite an electron from the valence band to the conduction band. The excited electron may be observed through its contribution to the current. In other words, no external voltage is applied to the photodetector under photovoltaic mode. It works in the self-powered mode.

    Photoconductive mode: In photoconductive mode, an external bias voltage is applied to the photodetector. Applying a bias voltage increases the width of depletion region and reduces the junction capacitance which results in increased response speed.

    Types of Photodetectors

    ➱  P-N junction-based photodetectors

    ➱ P-I-N photodetectors

    ➱  APD Avalanche photodetectors

    ➱  SP Schottky photodetectors

    ➱ M-S-M metal-semiconductor-metal photodetectors

    Performance Parameters of Photodetector

    The performance of a photodetector can be characterized by the following, some parameters. These parameters can explain the working of the photodetector device.

    Spectral response: The response of a photodetector as a function of photon frequency. The spectral response of a detector is given by the manner in which the output signal of the detector varies with the change in the wavelength of the incident radiation.

    Quantum efficiency: The "quantum efficiency" (Q.E.) is the ratio of the number of carriers generated by the photodetector, to the number of photons of a given energy incident on the photodetector.

    Ƞ=(no.of generated carriers )/(no.of incident photons)

    Responsivity:

    Responsivity of a detector is given as the ratio of the generated photocurrent (I) to the amount of optical power (Po) incident on the detector. The unit of responsivity is A/W.

    R=I/Po

    GERMANIUM DISELENIDE(GeSe2)

    2D Transition metal dichalcogenides (TMDC) materials attained widespread research interest in the electrical, optoelectrical and electrochemical applications owing to its phenomenal physical and chemical properties. TMDC having the general structure of MX2 where M is transition metal and X is the chalcogenide (S, Se, Te). In terms of photo detecting application, the group-IV TMDC attained intense research interest owing to its tunable bandgap, high on/off ratios, photosensitivity and noticeable carrier mobility. Among the TMDC in group-IV, germanium diselenide (GeSe2) is the only material that has direct bandgaps, and possessing closely placed direct and indirect bandgaps, which makes it a potential material for photovoltaic and photodetecting applications. Importantly, the GeSe2 exhibits a monoclinic molecular structure and lower symmetry in the structure may induce novel physical properties such as anisotropy giving rise to a new degree of freedom for modulating the optical and electronic properties. Importantly, GeSe2 has a high optical absorption in the visible region spectrum and a wide direct bandgap of 2.78eV, because of that wide bandgap the photodetector device covers the visible and near infrared region in the EM spectrum.

    Capture.JPG
      Fig: 2 Crystal structure of GeSe2

      GeSe2: A Superlative Candidate for Self-Powered Photodetector

      By literature survey, previously 2D-GeSe2 material of photodetector result is reported only for the ultra-violet region detection. Now, we detecting the visible region of the electromagnetic spectrum by the 2D self-powered GeSe2 high photo-responsive photodetector. By the reviewing of the literature survey, we discovered some salient features of GeSe2 and that are listed below,

      ✵  The elemental components of GeSe2 are comparatively earth-abundant and low-toxic in nature, which makes it superior to traditional CdTe and CIGS.

      ✵  Both experimental investigation and theoretical simulation demonstrate that GeSe2 has a suitable band gap of 2.7 - 2.9 eV, lying within the optimal bandgap value.

      ✵  It also has a high optical absorption coefficient exceeding 104 cm-1 in visible region, which enable full light absorption within one micrometer thick layer.

      ✵ GeSe2, as a typical binary IV-VI group semiconducting compound, crystallizes in layered structures with few and weak dangling bonds between the layers, which makes it possess high chemical stability and low surface defect density confirmed by theoretical and experimental study.

      ✵  GeSe2 adopts a low melting point of 670 ºC, which represents favorable feature for growing high quality films at relatively low temperature.

      Germanium diselenide, crystallizes at room temperature into stable, orthorhombic structures. The unit cell of GeSe2 consists of eight atoms with unit cell parameters as follows: a = 4.38 Å, b = 3.82 Å, c = 10.79 Å.

      The light can induce various kinds of novel physical phenomena in GeSe2, making it attractive for applications in high-speed optical switching devices.

      SYNTHESIS AND FABRICATION OF SELF-POWERED GeSe2 PHOTODETECTOR

      In this work, fabricating a self-powered photodetector which can able to works on zero bias condition along with superior photoresponsivity. For the purpose, I choose germanium diselenide as the sensing material and the reasons for choosing GeSe2 for self-powered photodetector is already discussed above.

      Experiment Details

      We used Germanium micro crystals of (111) orientation with purity of 99.99% as Ge source. We used 250nm SiO2/p-Si substrate as the substrate to synthesis and fabricate the GeSe2 device. We followed two step approach to fabricate the GeSe2 and the schematic of device fabrication is shown in figure 3.1,

      1. E-beam deposition of Ge thin film

      2. Selenization of Ge thin film

      Capture1.JPG
        Fig 3 : a) E-beam deposition of Ge thin film deposited on the SiO2/Si substrate, b) After the selenization, the GeSe2 thin film deposited on the SiO2/Si substrate and c) Ag metal contact coated on the GeSe2 thin film.

        E-beam deposition of Ge

        The thin film deposition of Ge was carried out using HHV Smart Coat 3.0 electron-beam evaporation unit (in fig4). To achieve the high pure thin film of Ge we carried out the deposition under the high vacuum condition of 1X10-5 mBar. We optimized the parameters to attain the 10nm thick Ge film on the SiO2 substrate. The deposition of Ge is graphically depicted in figure (3a).

        Selenization of Ge thin film

        Ge deposited SiO2 substrate has been selenized using chemical vapour deposition (CVD) (figure 5). The Ge deposited substrate is placed in the center of zone-2 of reactor chamber and the selenium powder is placed in the zone-1. The Ge substrate temperature is maintained to be 700 ºC and the temperature of zone-1 is maintained to be 350 ºC, we used argon : hydrogen (10:1) 20SCCM as a carrier gas. When the growth temperature at zone 1 produces the Se vapors and the carrier gas transfers the Se towards the Ge substrate and subsequent selenization will takes place. As the result of selenization at the elevated temperature leads to the formation strong covalent bonding in between Ge and Se, results in GeSe2 formation.

        Capture2.PNG
          Fig: 4 Image of the electron-beam evaporation unit.
          Capture3.JPG
            Fig: 5 a) and b) shows the closed and opened image of the 2-zone furnace respectively.

            Metal Ohmic Contact Deposition

            To analyses the device performance we need metal contact, we provided silver metal contact using the boron nitride pads in the E-beam evaporation approach with thickness of 250nm. The schematic illustration of device fabrication is shown in figure 3c. Actual fabricated device is shown in inset of figure 6

            Capture4.JPG
              Fig : 6 Digital photograph of as fabricated GeSe2 photodetector.

              RESULT AND DISCUSSION

              Morphological Analysis

              The morphological analysis of the synthesized GeSe2 has been carried out using the Carl Zeiss Sigma field emission scanning electron microscope (FESEM). The morphological image of the synthesized GeSe2 is shown in figure (7a) and the thin film structure of the GeSe2 is confirmed by the figure (7b). The morphological analysis evidences the continuous uniform formation of GeSe2 with the average thickness of 10nm. In addition to that, we carried out energy dispersive X-ray spectroscopy (EDS) using the Oxford instruments Inca X-acta and the EDS spectrum is shown in figure (7c-f). The EDS analysis evidences the absence of other elemental peaks other that Ge and Se. The ratio of Ge and Se is found to be 1:2 and well in match with earlier reports.

              Capture5.JPG
                Fig: 7 a-b) The FESEM of the GeSe2 thin film surface,c-e) EDS mapping results of GeSe2 f) EDS spectrum of GeSe2

                Raman Spectroscopy

                Raman spectroscopy investigations has been carried out to evidences the formation of GeSe2. Raman spectroscopy was evaluated using In Via Raman spectrometer at the excitation wavelength of 532nm and shown in figure (8). The pristine Ge film exhibit the classic Ge Raman vibration at 302cm-1. The Raman spectra of GeSe2 possesses the characteristic vibrations of 189.7 and 152 cm-1 along with three weak peaks at 176.5, 217.6, and 265.05 cm-1 and they are attributes A1 to the symmetric stretching modes of corner sharing (GeSe2) and (GeSe2) tetrahedral and the A1C corresponds to the edge-sharing of (GeSe2) and (GaSe2) tetrahedral. The peak (v1) located at ~ 265.05 cm-1 is related to present of Se-Se pairs.

                Capture6.JPG
                  Fig: 8 Raman spectra of the bare Ge and GeSe2

                  Photoluminescent Spectroscopy

                  Photoluminescence (PL) characterization was an effective and convenient tool to characterize the band gap and the defect properties of semiconductor material. The PL peak for GeSe2 is shown in figure (9). In GeSe2, the band edge emission take place at 435.2 nm and the band gap is calculated to be 2.78eV. The PL and Raman spectroscopy analysis of GeSe2 results are well in match with the earlier reports.

                  Capture7.JPG
                    Fig: 9 PL spectrum of GeSe2 thin film.

                    Current-Voltage Analysis

                    For analyzing the I-V characteristics for our sample it needs a metal contact, so we deposited a silver metal using boron nitride mask. The active area of the material is 50μm×5μm. When a metal and semiconductor are brought into contact, there are two types of junctions formed depending on the work function of the semiconductor.

                    1. Schottky junction - φmetal > φsemiconductor

                    Schottky contact show rectifying behavior, due to the development of a built- in potential, barrier potential.

                    2.  Ohmic junction - φmetal < φsemiconductor

                    Ohmic contacts is a non-rectifying electrical junction, having relatively low resistance and a very low potential barrier, allowing the free flow of carriers across the metal-semiconductor junction in both directions. In our device structure we are creating metal-semiconductor-metal (MSM) junction, however with same amount of Ag on the both contact leads to the formation of similar Schottky barrier at both metal junctions. In addition to that, in 2D materials the effect of the Schottky barrier as the result of MSM junction is negligible owing to its intrinsic charge transfer property. Therefore, the GeSe2 device photo detection performance is purely belonging to the materials only not as the result of MSM junction. Ohmic contacts are indispensable in electronic circuit it does not limit the flow of charge carriers and offers the unhindered current flow in both direction between metal and semiconductor.

                    Current-Voltage (I-V) analysis has been carried out using the Keithley source meter 4285 and we used AM 1.5G equipped 100W Xenon lamp (Newport instruments) as the light source. The incident photon intensity has been maintained to be 100mW/cm2. The current voltage (I-V) characteristics of GeSe2 thin film was measured at room temperature is shown in the Figure11 the device exhibits a strong photon detecting characteristic of photodetector. The existence of Ohmic contact between GeSe2 –Ag was confirmed using I-V measurements.

                    Capture8.JPG
                      Fig: 10 a) shows the Ag-GeSe2 ohmic contact and b) shows the electron-hole transportation takes place in the Ag-GeSe2 ohmic contact.

                      The I-V measurement were carried out under 1 sun illumination, were 1 sun is typically defined as nominal full sunlight intensity on a bright clear day on earth, which measures 100mW/cm2. Due to the visible bandwidth absorption and the charge carrier properties of GeSe2, it allows the GeSe2 self-powered photodetector to work both in visible light and dark mode. Most important figure of merit in photodetectors is photoresponsivity, it is the measure of photocurrent generated in the detector over the incident optical power. The photoresponse of GeSe2 device is calculated as 320 mA/W. When GeSe2 device is illuminated the photocurrent rose to a high value and such an increase in current is due to the lack of charge recombination mechanism. When the illumination is off the current returns to its low value.

                      Capture9.JPG
                        Fig: 11 log plot of I-V characteristics of the GeSe2 photodetector under dark and illumination.

                        Self Powered Photodetection

                        A good photodetector should satisfy the following requirements

                        ✯  Under dark condition, the device offers high impedance to the carriers. Due to the absence of striking photon, the electron-hole pair is not generated and, the current conduction is very minimum (ie. dark current). Therefore, the device state is known to be off state.

                        ✯  Under the illuminated condition, the device offers low impedance the carriers can flow without any barriers. The photons strike the semiconducting material and excite the electron from valence band to conduction band, therefore the current conduction takes place in maximum amount. The device is in the ON state.

                        From I-V plot, we observed a measurable shift in the GeSe2 performance between dark and illumination condition. Therefore, we measured carried our photoresponce nature of the GeSe2 under zero-applied bias condition. As expected, the devices display stable on-off performance more than 1 minute as displayed in figure 12a. We carried out zero-applied bias I-V under dark, illumination and chopped illumination as shown in figure 12b. The chopped illumination results are well in match with the dark and light condition and evidences the stable high response nature of the GeSe2.

                        Capture10_1.JPG
                          Fig:12 a) Graph shows the continuous ON/OFF switching of the device for 10 sec.
                          Capture11.JPG
                            Fig: 12 b) Graph Shows the continuous ON/OFF switching of the GeSe2 photodetector for 70 sec.

                            Photoresponsivity is an important tool to analyze the efficiency of the device and it calculated with formula as mentioned above. The photoresponsivity has been found to be 320mA/W and it is the highest of the reported in GeSe2 photodetector under self-powered condition.

                            Response and Recovery Time

                            The important feature of photodetector is response time and decay (recovery) time. Response time is the time needed for photodetector to reach 90% of final output from 10%. From the figure (13 a) the response time period for the GeSe2 photodetector device is 323 msec. The recovery time has been found to be 285mS for our GeSe2.

                            Capture12.JPG
                              Fig:13 a) Response time and b) recovery time period of the GeSe2 photodetector under self-powered condition.

                              CONCLUSION

                               ➤  In summary of my work, we successfully fabricated the 2D GeSe2 self-powered thin film (TMDC) based Photodetector. We fabricate GeSe2 on the SiO2/p-Si substrate, by the two-step approach of E-beam deposition of Ge thin film and Selenization of Ge thin film. The presence of GeSe2 over the SiO2/p-Si substrate was confirmed using FESEM, Raman spectroscopy, EDS techniques. The bandgap of the GeSe2 was 2.78eV, it was calculated by the Photoluminescence spectroscopy analysis of the GeSe2.

                              ➤ In addition to that, photodection performance has been evaluated for the fabricated GeSe2 device under 1 sun illumination. The fabricated GeSe2 exhibits usperior photoresponsivity of 320mA/W.

                              ➤ Further the response and recovery time of the GeSe2 photodetector is 323msec and 285msec respectively.

                              ACKNOWLEDGEMENTS

                              First and foremost, I would like to pay, my warm thankfulness to my Guide, Dr. K Jeganathan, Professor and Coordinator, Centre for Nanoscience and Nanotechnology, Department of Physics, Bharathidasan University, for his vital support and assistance. The freedom he given to me that made it possible to complete this internship in very good manner. I would like to thank, Mr. G Paulraj (Research Scholar) for guiding and helping me whenever I needed and throughout this research internship, whose reminders and constant motivation encouraged me to pursue this research work. I would like to pay my thankfulness to Mr. N. Anbarsan for his valuable advises, guidance and all other helps. I am very thankful to Mr. A. Gunasekaran, Mr. M. Mukilan, Dr. S. Sadhashivam, Mrs. T. S. Sheena, Mr. S. Gopalakrishnan, Miss Sowndarya for all the care and support they shown on me and for all their support and interactions, their credible ideas have been great contribution in the completion of the internship, and I am very grateful to those research scholars of CNST. They treated me as a younger brother. Heart full of thanks to all of them for helping me in this internship. I also pay my thanks to my college Professor Dr. K. Elankumaran, so many thanks for his guidance and great support. I express my thanks to Indian Academy of Sciences for selecting me for IASs-INSA-NISA summer research fellowship 2019 and supported for me financially and also given a great opportunity for me to work in a research environment.

                              Finally, I would like to express my sincere thanks to my dear parents, and my family members and friends, without whom I was nothing. They not only assisted me financially but also extended their support morally and emotionally. This work would not have been possible without them.

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