Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer © Elsevier Ltd. 2019 Accepted version (Final draft) Grushko, V.; Beliuskina, O.; Mamalis, A.; Lysakovskiy, V.; Mitskevich, E.; Kiriev, A.; Petrosyan, E.; Chaplynskyi, R.; Bezshyyko, O.; Lysenko, O. Grushko, V., Beliuskina, O., Mamalis, A., Lysakovskiy, V., Mitskevich, E., Kiriev, A., Petrosyan, E., Chaplynskyi, R., Bezshyyko, O., & Lysenko, O. (2020). Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer. Applied Radiation and Isotopes, 157, Article 109017. https://doi.org/10.1016/j.apradiso.2019.109017 2020 Journal Pre-proof Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer V. Grushko, O. Beliuskina, A. Mamalis, V. Lysakovskiy, E. Mitskevich, A. Kiriev, E. Petrosyan, R. Chaplynskyi, O. Bezshyyko, O. Lysenko PII: S0969-8043(19)30693-1 DOI: https://doi.org/10.1016/j.apradiso.2019.109017 Reference: ARI 109017 To appear in: Applied Radiation and Isotopes Received Date: 16 June 2019 Revised Date: 26 November 2019 Accepted Date: 3 December 2019 Please cite this article as: Grushko, V., Beliuskina, O., Mamalis, A., Lysakovskiy, V., Mitskevich, E., Kiriev, A., Petrosyan, E., Chaplynskyi, R., Bezshyyko, O., Lysenko, O., Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer, Applied Radiation and Isotopes (2020), doi: https://doi.org/10.1016/j.apradiso.2019.109017. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd. CRediT author statement V Grushko: Conceptualization, Formal analysis, WritingOriginal draft preparation. O Beliuskina: Methodology, WritingReviewing and Editing. A Mamalis: WritingReviewing and Editing. V Lysakovskiy: Resources, Investigation, E Mitskevich: Sofrware, Investigation, A Kiriev: Investigation, Visualization. E Petrosyan: Methodology. R Chaplynskyi: Validation, Formal analysis. O Bezshyyko: Conceptualization, Data Curation. O Lysenko: Supervision. Energy conversion efficiency in betavoltaic cells based on the diamond Schottky diode with a thin drift layer V Grushko1, O Beliuskina2, A Mamalis3, V Lysakovskiy1, E Mitskevich1, A Kiriev1,4, E Petrosyan5, R Chaplynskyi5, O Bezshyyko4and O Lysenko1 1V. Bakul Institute for Superhard Materials, 2, Avtozavodska , Kyiv, 04074, Ukraine 2Department of Physics, University of Jyv¨askyl¨a, Survontie 9, FI 40014, Finland 3Project Center for Nanotechnology and Advanced Engineering (PC-NAE), NCSR “Demokritos”, Athens, 15310, Greece 4T. Shevchenko National University, 64/13, Volodymyrska, Kyiv, 01601, Ukraine 5Institute for Nuclear Research, 47, Nauky Ave, Kyiv, 02000, Ukraine Abstract The HPHT diamond Schottky diode was assembled as a Metal/Intrinsic/pdoped structure betavoltaic cell (BC) with a very thin (1 µm) drift layer and tested under 5 −30 keV electron beam irradiation using a scanning electron microscope (SEM). The effect of the β-radiation energy and the backscattering of electrons on the energy conversion was studied. From the results obtained, it is shown that, the efficiency of the investigated BC increases from 1.01 to 3.75 % with the decrease of β-particle energy from 30 to 5 keV due to an increase of the electron beam absorption in a thin drift layer. Maximum efficiency is achieved when the electron beam energy is close to the average β-decay energy of 3H. The BC maximum output power of the 1.6 µW was obtained at an electron beam energy of 15 keV, that matches the β-decay energy of 63Ni. The total BC conversion efficiency at 15 keV electron-beam energy is about 3%. The calculations indicated that a preferable β-source for the diamond based BCs with a thin (1 µm) drift layer is 63Ni. Keywords: betavoltaic, diamond, energy conversion efficiency, thin drift layer, Schottky diode. Preprint submitted to Applied Radiation and Isotopes December 7, 2019 1. Introduction The concept of using p-i-n and p-i-m (Schottky) junctions using for the betavoltaic energy conversion was proposed in the 1950s (Rappaport (1954); Pfann & Van Roosbroeck (1954)). Recently, numerous betavoltaic cell (BC) devices based on Si, SiC and GaN, were developed (Bao et al. (2012); Zhang et al.5 (2018); Chandrashekhar et al. (2006); Qiao et al. (2008); Eiting et al. (2006)). Note that, the betavoltaic devices are widely used in the harsh radioactive environment, like power plant reactors, spent nuclear fuel storages etc., remote places such as space and mine-like underground, undersea and so on. Moreover, they are most notable for medical devices such as pacemakers.10 Most commonly used radioactive sources for the industrial BCs are the 3H, 63Ni, 90Sr, 90Y, 147Pm, 35S, 33P, 204Tl, 85Kr isotopes (San et al. (2013); Sun et al. (2005); Lu et al. (2011); Yao et al. (2012); Chen et al. (2011); Preiss et al. (1957); Meier et al. (2009); Eiting et al. (2006)). The typical output power of the tritium BC is about 120 nW. In the betavoltaic effect studies with 63Ni15 an induced current of tens of nanoamps to few microamps was achieved (San et al. (2013); Lu et al. (2011); Yao et al. (2012); Chen et al. (2011)). A further development of long lifetime energy sources, based on β-isotopes to increase power and reduce size of BC, is extremely challenging. Limitations of betavoltaic cells with a variety of beta sources had been stud-20 ied extensively over past years. For example, Theirrattanakul & Prelas (2017) reported that the betavoltaic efficiency of nuclear batteries based on silicon carbide betavoltaic cells with 3H, 63Ni, 35S, 147Pm, 90Sr and 90Y thin planar beta sources decreases with increasing beta energies. The 3H source had the highest absolute efficiency at 3.95% using the full spectrum energy distribution model.25 Oh et al. (2012) reported that the calculated conversion efficiencies were limited to a range of 0.013% to 2.02% for SiC-based betavoltaic cell with different beta sources. According to Zhang et al. (2018) the conversion efficiency was in a range of 3.74 −4.58% as the result of the SiC PIN betavoltaic cell simulation. Wu et al. (2011) presented results on theoretical study of silicon betavoltaic30 2 microbattery using 63Ni as beta source with obtained conversion efficiency of 5%. Wu & Zhang (2019) introduced the simulation of conversion efficiency of multilayer BC with silicon p-n junction converters and 63Ni sources. The total conversion efficiency of this BC limited to 3.3%. Murphy et al. (2019) reported that in the case of silicon diodes with three-dimensional features when coupled35 with 147Pm oxide theoretical efficiencies of 2.9 −5.8% can be achieved. More information about the BC reported effectiveness can be found in the review of nuclear batteries by Prelas et al. (2014). The reliability and safety of BC, especially for medical applications, mainly depend on the radiation hardness and the mechanical strength of the semicon-40 ducting material. Due to the high radiation tolerance, the high mechanical strength and chemical inertness diamond is an excellent material for betavoltaic devices. Examples of BCs using diamonds are presented by Delfaure et al. (2016), Bormashov et al. (2015), Tarelkin et al. (2016) and Zhao et al. (2017). It is noted that, the small output power of the diamond BCs limits their45 use. The solution to this problem is to create multilayer power sources. In this case, the output power depends on the geometrical dimensions of the multilayer structure and, therefore, on the BC layer’s thickness. Recently, a multilayer BC based on a diamond Schottky diode with the the drift single layer cell size of 15 µm was presented (Bormashov et al. (2018)). A50 reduction of the thickness of the drift layer and, accordingly, a decrease in the size of a multilayer BC based on a diamond Schottky diode, constitute the best way to increase the power density of such a device. The aim of our research was to study a betavoltaic energy conversion efficiency in a diamond Schottky diode with a drift layer thickness of 1 µm.55 2. Methods and Materials 2.1. The current generation in the diamond Schottky diode The operation principle of the betavoltaic device is based on the electronhole pair generation in a diamond Schottky diode induced by β-particles emitted 3 from radioactive isotopes. The process of electron-hole pairs (EHP) generation60 can be described by the generation function: g(x)∼exp(−αx) (Sachenko et al. (2015)), where, αis the linear electron absorption coefficient and xis the depth in diamond bulk. Note that, only part of the β-particle total kinetic energy is absorbed in the drift layer with thickness of l= 1 µm, see Figure 1(a). It is clearly indicated that, the higher the penetrating power (i.e. longer stopping65 depth) of the β-radiation, the smaller this part will be. The electron-hole pairs induced by β-radiation in a drift layer are directly converted into an electric current of BC. Due to its capability of efficiently converting the ionizing radiation into an electric current, and, furthermore, its high radiation tolerance, thermal conduc-70 tivity, mechanical strength and chemical inertness, diamond was chosen as the main component of the radioactive power source. The diamond has a wide band gap and high donor and acceptor ionization energies. At room temperature, as well as below it, the Fermi level is located close to donor or acceptor levels, depending on the concentration of impurities which is predominant in the crystal75 (Collins (2002)). The electron-hole pairs formed inside the depletion region of the Schottky diode are separated by an internal electric field, thereby forming a radiation-induced current in the BC (Manasse et al. (1976)). The Schottky barrier height defines the maximum battery voltage. This value depends strongly on the band gap structure of the semiconductor (Tung (2014)).80 It is known, that a high concentration of a doping impurity in a semiconductor leads to a thinning of the depletion region and, thereby, reduces the number of the electron-hole pair generated in this region (Bormashov et al. (2015)). Therefore, in diamond Schottky diodes, the depletion region is typically broadened to a value of 5 −10 µm at zero bias by the formation of an epitaxial drift85 layer on the surface of a doped semiconductor, into which a small amount of doping impurity diffuses. It is also known that, the charge collection efficiency, Qof BC may dramatically decrease when a diode is irradiated by high-energy electrons which have higher penetrating power, because of the longer stopping depth (Bormashov et al. (2015)).90 4 2.2. Experimental setup The schematic diagram of a diamond Schottky diode betavoltaic cell is shown in Figure 1 (a). The p+substrate of diamond diode was made from a borondoped single crystal diamond grown by the temperature gradient method at high pressure - high temperature (HPHT) (Novikov et al. (2003)) with the boron95 contents of 1018 cm−3. Subsequently, diamond crystal was cut as {001}planes and mechanically polished up to roughness of 2 nm RMS. The surface quality after polishing was controlled by the Scanning Tunnelling Microscope (STM) technique (Grushko et al. (2014); Lysenko et al. (2010)). Since a large number of structural defects significantly reduces the lifetime of non-equilibrium carriers100 in a semiconductor, worsening, therefore, the efficiency of betavoltaic energy conversion, much attention was paid to the quality of this boron-doped plate. Due to the large number of dislocations, stacking faults and twin boundaries in the {111}growth sectors of the HPHT diamond (Chepugov et al. (2013)), mainly {001}and {311}sectors were used as a substrate for the Schottky junction.105 In order to avoid the formation of the ohmic contact between the metal and the crystal plate and to increase the number of the electron-hole pairs in the Schottky junction region the p−epitaxial drift layer was deposited on the diamond plate by the CVD method (Zhao et al. (2017)). The quality of Schottky junction substrate surface significantly affects the quality of the diamond epi-110 taxial CVD layer. In our case the thickness of the deposited CVD layer did not exceed 1 µm. The presence of non-equilibrium carriers in the drift layer limits the penetration depth of the electric field in the semiconductor and, respectively, the depth of the depletion region (Bormashov et al. (2015)). The thickness of the115 drift layer determines the Schottky barrier height and limits the depth of the depletion region where the induced charge is collected. With an increase of the drift layer thickness from 100 to 500 nm, the height of the Schottky barrier increase from 1 to 1.8 eV (Zhao et al. (2017)). According to Bormashov et al. (2015) and Tarelkin et al. (2016), for a drift layer depth of about 10 µm and a120 zero offset the depth of the depletion region is about 5 µm. 5 Figure 1: (a) The schematic diagram of a diamond Schottky diode betavoltaic cell. The solid marker denotes electrons and the open marker denotes holes, ~ε is the built-in electric field in the drift layer, l and W are the drift layer’s depth and width, respectively. (b) The electric circuit for the betavoltaic energy conversion efficiency measurements. (c) The diamond betavoltaic cell prototype. After the drift layer deposition, an Au layer with a thickness of 10 nm was deposited using the magnetron sputtering method in the Ar atmosphere. The Au layer thickness was controlled by STM. On the backside of the diamond p+plate, the Ti adhesive layer, the Cu conducting layer and, finally, a thin125 Ag layer were deposited by the same method. The sputtering of Ti provided the formation of a TiC interlayer on the diamond surface and the stable ohmic contact of the diamond plate with a Cu/Pt conducting layer. The conversion of β-decay energy into electricity in a diamond Schottky BC was investigated using the Electron Beam Induced Current (EBIC) technique130 (Delfaure et al. (2016)). Figure 1 shows the electric circuit used for the estimation of the total efficiency and the prototype of a diamond Schottky diode BC based on the chip KD917A without cover wafer. The diamond diode p+plate is placed at the bottom of the chip, that creates an ohmic contact with a pair of upper electrodes of the chip. Two bottom electrodes provide a contact with135 the gold layer deposited on the top of the plate. 6 stops far beyond the drift layer (stopping depth 1.5 −2µm, see Figure 7(b)).240 The significant lateral spread of β-particles (lateral projection is about 1.5 µm at the beam energy of 20 keV) leads to losses of β-particles at the edges of the drift layer. A further increase of irradiation energy leads to a further decrease of the area under the intensity curve and, as a result, of the observed value of the BC output power.245 The total efficiency slowly saturates at the beam energy of 5 keV, see Figure 4(b), apparently due to an increase of backscattering (from 6% at 15 keV to 30% at 5 keV) and the beam energy losses (from 0.8% at 15 keV to 6% at 5 keV) of the β-radiation in a thin layer of gold on the surface of the drift layer, see Figure 5).250 0 200 400 600 800 1000 Depth (nm) 0 1 2 3 4 EHP generation intensity (pair/(s·nm)) ×1010 5 keV 10 keV 15 keV 20 keV 25 keV 30 keV Figure 6: EHP generation intensity as a function of the penetration depth of β-particles in the drift layer of BC under different electron beam energies. The decrease of the total conversion efficiency with the beam energy increase, see Figure 4(b), agrees well with the observed small value of the total efficiency for high-energy β-particles (Bormashov et al. (2015)). For example, according to Bormashov et al. (2015), for a mixed 90Sr−90Y source with an average β-particle energy of about 1.1 MeV (penetration depth of about 2 mm and depletion255 region of 5 µm), the measured value of η= 0.004%, due to the fact that only a small part of the β-particles kinetic energy is absorbed in the Schottky junction 13 −150 −100 −50 0 50 100 150 −200 −100 0 Depth (nm) β=0.269 Au Diamond (a) −2000 −1000 0 1000 Lateral axis (nm) −2000 −1000 0 Depth (nm) β=0.065 (b) Figure 7: Trajectories of β-particles in the BC and the backscattered coefficient βat a beam energy of 5 keV (a) and 20 keV (b). Backscatter trajectories are marked in red. depletion region and converted to electricity. Backscattering of β-particles in the diamond material is small (up to several percent), due to a small atomic number of carbon, compared to other semicon-260 ductor materials (for example, Si 15% or GaN 25%) (Bormashov et al. (2015)), which helps to increase the total efficiency coefficient η. 5. Conclusions From the theoretical and experimental studies reported above, the following concluding remarks may be drawn:265 (a) The total conversion efficiency of the diamond Schottky diode with a thin (1 µm) drift layer increases from about 1% to 3.8% at the electron beam energy decrease from 30 keV to 5 keV, due to more efficient β-radiation absorption by a thin drift layer at low beam energies. (b) The influence of β-particle backscattering in the BC metallic coating on270 the total conversion efficiency at low (5 −15 keV) beam energies has to be taken into account. (c) According to the Monte Carlo simulations performed with CASINO software the penetration of β-particles into the drift layer at energies greater than 14 15 keV exceeds 1 µm, which leads to a decrease in the BC output power and275 the efficiency of a β-decay energy conversion, since most of the trajectories of β-particles go beyond the drift layer in which the current of BC is generated. (d) Lateral spread of β-particles at electron beam energies greater than 20 keV exceeds 1.5 µm, which can lead to a significant loss of the output power and efficiency at the edges of the drift layer. This effect can be significant for280 micro betavoltaic cells with a small area of drift layer with W <5µm. (e) The β-radiation energy of 5 keV, at which the maximum energy conversion efficiency of about 3.8 % is reached, is close to the average β-decay energy of 3H. 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Diamond and related materials,73, 15–18. doi:10.1016/j.diamond.2016.11.005. 20 Highlights  The total conversion efficiency of the diamond Schottky diode with a thin (1µm) drift layer increases from about 1 to 3.8 % while the electron beam energy decrease from 30 keV to 5 keV due to the increase in the adsorption of beta particles by a thin drift layer.  The most preferred beta source for the diamond Schottky diode with a thin drift layer is 63Ni.  The β-particles backscattered coefficient and the beam energy loss in the Schottky contact significantly affects the total conversion efficiency of the diamond Schottky diode at low (5 -15 keV) beam energies.  The lateral electron spread exceeds 1.5 µm at beam energies greater than 20 keV, which can lead to a significant loss of the output power and efficiency at the edges of the drift layer. This effect can be significant for micro betavoltaic cells with a small (<25 µm2) area of a drift layer. Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: