Solid State Phenomena ISSN: 1662-9779, Vol. 242, pp 312-315 doi:10.4028/www.scientific.net/SSP.242.312 © 2016 Trans Tech Publications, Switzerland Submitted: 2015-05-21 Accepted: 2015-05-22 Online: 2015-10-23 Characterization of Si convertors of beta-radiation in the scanning electron microscope M.A. Polikarpov1,a and E.B. Yakimov2,3,b * 1 National Research Center Kurchatov Institute, Moscow, 123182 Russia 2 Institute of Microelectronics Technology RAS, 142432, Chernogolovka, IPTM, Russia 3 National University of Science and Technology MISiS, Moscow, Russia a email@example.com, firstname.lastname@example.org Keywords: Beta-voltaics, SEM, simulation Abstract. The approach for imitation of beta radiation using the e-beam of scanning electron microscope (SEM) for semiconductor energy converter testing is proposed. It is based on the Monte-Carlo simulation of depth-dose dependence for beta-particles and a determination of collection probability from the EBIC measurements of collection efficiency dependence on beam energy. Experiments with the 63Ni radiation source confirm that such approach allows to predict the efficiency of semiconductor structures for radiation energy conversion to electric power. Introduction In beta voltaic batteries energy of nuclear radiation is directly converted to electric power using a semiconductor convertor. The principle of such converters is similar to that of solar cells. The electron-hole pairs are created by beta particles emitted from radioactive isotopes, then they diffuse to a collector and are separated by the built-in electric field of a collector (depletion region of the Schottky barrier or p-n junction) to generate electric current. If a Si based structure is used as the convertor, the MEMS technology can be applied for fabricating a beta voltaic microbattery compatible with IC technology. Thus, a battery can be integrated on the same chip with other micro/nano devices or components, i.e., “battery-on-chip” can be realized. 63Ni isotope is probably the most suitable for such applications because Ni can be used simultaneously as a circuit element, e.g. for interconnections. Radioactive isotopes are rather expensive, therefore for a successful design and optimization of effective converters methods for their testing before radioactive isotope deposition should be developed. A good idea is to use for this purpose e-beam of SEM, which could imitate the beta radiation. This idea was realized in some works (for example see ) and beam energy of 17 keV corresponding to the mean energy of electrons emitted by the 63Ni isotope was usually used for the beta radiation imitation. However, elastic and inelastic scattering and absorption inside the 63Ni film modify significantly the spectrum of electrons emitted from the film. Depth-dose functions describing the depth dependence of excess carrier generation rate in Si, SiC and GaN for beta particles emitted from 63Ni films of different thickness were calculated in [2,3] by the Monte-Carlo program taking into account an isotropic emission of radiation, the full beta energy spectrum for the 63Ni isotope and inelastic and elastic scattering processes in a Ni film. It was shown that a decay of these functions with a depth is close to exponential one while the depth-dose functions for monoenergetic electron beam perpendicular to the surface are rather well described by the Gaussian functions [4,5]. For this reason it is impossible to imitate the beta particle depth-dose function using one or a few e-beam energies and other approaches for beta-radiation imitation using SEM should be developed. In the present paper one of such approaches based on the calculated depth-dose functions and on then experimentally determined collected probability by the EBIC investigations of particular semiconductor structures is discussed. The main features of such approach are illustrated by the results of Si and SiC based structure investigations. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.scientific.net. (#103369937, California Institute of Technology, Pasadena, USA-12/11/17,10:49:45) Solid State Phenomena Vol. 242 313 Numerical simulation h(z) A spectrum of electrons living the 63Ni film differs essentially from that emitted by the 63Ni isotope due to scattering and absorption in the film . However, for the induced current simulation it is more useful to calculate the depth-dose function for beta particles emitted from 63Ni film because they are emitted in a wide angle range. Such calculations for Si, SiC and GaN were carried out in [2,3] by the Monte-Carlo method. It was shown that the normalized depth-dose function depends on the Ni film thickness and is saturated at thicknesses exceeding 4-5 µm. For the illustration the normalized depth-dose functions in Si obtained by the Monte-Carlo simulation for thin (10 nm) and thick (3000 nm) 63Ni films are shown in Fig. 1. The depth-dose functions calculated for for e-beam perpendicular to a surface with energy of 17.1 eV, which is the average energy of beta-particles for 63Ni, and 40 keV, which is the maximum energy for the most of SEMs, are presented in the same Figure for a comparison. It is seen that a distribution of excess charge carrier generation rate under the beta radiation is indeed qualitatively different from those for a monoenergetic electron beam. Therefore it is impossible to simulate the beta radiation with a SEM e-beam by a corresponding choice of one or a few beam 1 energies and other approaches should be 4 developed. As shown in [7,8], the induced current in semiconductor structures can be 2 calculated as a convolution of depth-dose function with the collection probability, i.e. 0.1 the probability that a minority carrier generated at a depth z below the surface will 1 3 be collected by a collector. Thus, if the depth-dose function for beta radiation is 0.01 simulated by the Monte-Carlo method to 0 2 4 6 8 10 12 14 calculate the induced current value the z, µm collection probability for the structure under study should be determined. The collection Fig. 1. Normalized depth-dose functions in Si probability for the particular structure calculated for beta-particles emitted from 63Ni depends on the parameters of this structure film with a thickness of 10 nm (1) and 3000 nm only. The approach proposed in the recent (2). The depth-dose functions for e-beam paper is based on the determination of perpendicular to a surface with energy of 17.1 and collection probability by fitting the collected 40 keV are shown by curves 3 and 4, respectively. current dependence on beam energy measured in the Electron Beam Induced Current (EBIC) mode. A few ways can be used to obtain the collection probability from the collected current dependence on beam energy. For the simple structures such as the Schottky barriers or p-n junctions the main parameters of the structure under study such as the depletion region width, the metal thickness, the diffusion length, the junction depth, the surface recombination velocity and the diffusion length values in n- and p-regions for p-n junctions can be obtained by fitting the experimental dependence of collected current Ic in the EBIC mode on beam energy Eb. Then the collection probability can be calculated using these parameters by the numerical solution of homogeneous diffusion equation . The similar procedure can be used for the structures with the Schottky barriers. Such approach is illustrated in Fig. 2, where the normalized Ic(Eb) dependence measured on Si p-i-n diode with a very thin passivation layer and the p-n junction depth of 70 nm is fitted with calculated one. The corresponding collection probability calculated using the parameters obtained by fitting the Ic(Eb) dependence is shown in Fig. 3. For more complex structures the collection probability can be calculated from the measured collection efficiency dependence on beam energy by the methods proposed in  or . Then the induced current can be easily 314 Gettering and Defect Engineering in Semiconductor Technology XVI calculated for any depth-dose function. To obtain other parameters of semiconductor energy convertor e-beam current of SEM should be adjusted to obtain the collected in the EBIC mode current equal to the calculated value of induced current. Using such value of e-beam current other parameters of semiconductor convertor necessary for the prediction of its efficiency, such as the open-circuit voltage and the fill factor can be obtained. Ic/(IbEb) 250 200 150 100 50 0 0 5 10 15 20 Eb, keV Experimental verification To check the approach proposed the current induced by the 63Ni radioactive source with an activity of 1.3 mCi/cm2 in the Si p-i-n diode with an area of 8 mm2 presented in Figs. 2 and 3 and in a SiC Schottky diode with an area of 4 mm2 were measured. Simultaneously the induced current values are calculated using the 1.0 depth-dose dependence calculated by the 0.8 Monte-Carlo method for this radiation source and the collection probabilities 0.6 obtained by fitting the corresponding Ic(Eb) dependences. Experimental induced current 0.4 densities measured using the 63Ni radioactive source are 21 nA/cm2 and 0.2 7 nA/cm2 for Si and SiC structures, 0.0 respectively. The corresponding calculated 0 2 4 6 8 10 values are 20 and 7.2 nA/cm2, respectively. z, µm Taking into account that the exact thickness of radioactive film is unknown, the precision Fig. 3. Calculated collection probability for a Si of calculation of converter parameters can p-i-n diode. be discussed as rather good. The corresponding open-circuit voltage and maximum power of converters were estimated as 390 mV and 1.5 nW/cm2, respectively, for the SiC detector and 90 mV and 0.85 nW/cm2 for the Si detector. Collection probability Fig. 2. Experimental (symbols) and simulated (line) normalized collected current dependences on beam energy for Si p-i-n diode. Summary Thus, the approach for imitation of beta radiation using the e-beam of SEM for the prediction of efficiency of semiconductor structure for the conversion of radiation energy to electric power is proposed. The experimental verification of approach proposed was carried out and its reliability was confirmed. The work was partially supported by the Russian Foundation for Basic Research (grant 14-29-04056). Solid State Phenomena Vol. 242 315 References  M.V.S. Chandrashekhar, C.I. Thomas, H. Li, M.G. Spencer, A. 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