Change in the Charge and Defect Impurity State of Silicon for Solar-Power Engineering under the Effect of a Magnetic Field

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1 ISSN 13-7, Semiconductors, 1, Vol., No., pp Pleiades Publishing, Ltd., 1. Original Russian Text V.A. Makara, L.P. Steblenko, O.A. Korotchenkov, A.B. Nadtochiy, D.V. Kalinichenko, A.N. Kuryliuk, Yu.L. Kobzar, A.N. Krit, S.N. Naumenko, 1, published in Fizika i Tekhnika Poluprovodnikov, 1, Vol., No., pp SURFACES, INTERFACES, AND THIN FILMS Change in the Charge and Defect Impurity State of Silicon for Solar-Power Engineering under the Effect of a Magnetic Field V. A. Makara a, L. P. Steblenko a, O. A. Korotchenkov a, A. B. Nadtochiy a, D. V. Kalinichenko a, A. N. Kuryliuk a, Yu. L. Kobzar a^, A. N. Krit b, and S. N. Naumenko a a Taras Shevchenko Kyiv National University (Faculty of Physics), Kyiv, 11 Ukraine ^ ukaka@ukr.net b Scientific Investigation Center Physicochemical materials, Taras Shevchenko Kyiv National University, and the National Academy of Sciences of Ukraine, Kyiv, 11 Ukraine Submitted July, 13; accepted for publication September 3, 13 Abstract The effect of a weak permanent magnetic field on the structure and charge state of silicon for solar-power engineering is investigated. It is revealed that magnetostimulated changes in the defect impurity state and surface potential have a reversible character. DOI: 1.113/S INTRODUCTION In studying magnetostimulated changes in the structure and physical characteristics of silicon used for solar-power engineering (solar-si, or s-si), there are significant gaps in knowledge. Meanwhile, these investigations are relevant and expedient because devices fabricated on the basis of s-si crystals quite frequently operate under extreme conditions including under the action of magnetic fields. Recently, the effect of magnetic fields including weak ones (with an induction of B 1 T) on various physical characteristics of low-magnetic materials with different types of chemical bonds [1 13] has been widely studied. The proportion of specific scientific investigations on covalent crystals, in particular, s- Si crystals is relatively small. The fact that problems concerning the effect of a magnetic field on the properties and real structure of nonmagnetic s-si semiconductor crystals are practically unstudied makes their consideration appropriate and relevant. In [13], we investigated the effect of a permanent magnetic field (B =.17 T) on the kinetics of a decrease in the photoconductivity in s-si crystals and the character of variation in the physical characteristics of s-si depending on the time elapsed after carrying out magnetic treatment. To explain the obtained results, a hypothesis regarding the effect of charged impurity centers adsorbed and gettered by a magnetoactivated s-si surface on the character of a decrease in photoconductivity was stated. This study is devoted to verification of the hypotheses stated in [13]. The purpose of this study is to reveal changes in the defect impurity composition of surface layers and in the charge state of the surface of silicon crystals used for solar-power engineering under the effect of a weak permanent magnetic field.. EXPERIMENTAL In the study, we investigated s-si crystals grown by the Czochralski method and boron-doped to the resistivity ρ = 5 Ω cm. Magnetic treatment (MT) was carried out by storing the s-si samples in a permanent magnetic field with an induction of B =.17 T for a specific amount of time (t M = 7 or 1 day). The impurity composition of the control samples, i.e., the samples, which were not exposed to the magnetic effect, and also the samples, which were subjected to magnetic action, was studied using X-ray spectral analysis. The analysis was carried out by means of a setup combining an X-ray spectrum analyzer and a scanning electron microscope. Investigation of charge state of the surface was carried out by means of the photovoltage-relaxation technique. These investigations were carried out using an automated installation combining the contactless signaldetection method and the possibility of mapping the signal over the sample surface. The details of the experiment are described in [1, 15]. The photovoltage was generated using a 7-mW red laser LED (wavelength of 5 nm) focused into a light spot of ~1 μm in diameter. In addition, the photovoltage signal was scanned over the sample surface with a step of 1 μm. This technique enables us to study the electric-potential distribution over the s-si surface before magnetic treatment. 7

2 CHANGE IN THE CHARGE AND DEFECT IMPURITY STATE OF SILICON EXPERIMENTAL RESULTS, DISCUSSION, AND CONCLUSION In [13], we proposed a hypothesis according to which the magnetostimulated changes in the kinetics of the photoconductivity decrease can be related to the occurrence of a microscopic recombination barrier formed by impurities, which were adsorbed and gettered by the magnetoactivated surface. This hypothesis was confirmed by investigations of the distribution of the surface electric potential carried out in this study. These investigations are testimony to the fact that an extension of the electric-potential area (Fig. 1) is observed on the surface of s-si samples after the magnetic effect in comparison with the control samples, which is possibly related to the spatial dissipation of the electric potential. In this case, as can be seen from the comparison of Figs. 1b and 1c, an increase in the MT duration resulted in an increase in the area of the surface electric-potential distribution. As follows from the maps of photovoltage distribution U (Figs. 1a, 1b, and 1c), the MT results also in a double increase in its value instead of only in extension of the surface-potential area. An increase in the distribution area and the value of the surface potential, in our opinion, is the manifestation of the process of adsorption and gettering of various impurities including electrically charged impurities by the surface activated in the magnetic field. In our opinion, the magnetostimulated redistribution of the impurity concentration due to adsorption and gettering in the surface s-si layers also causes an increase in the surface electric potential. Additional investigations showed that a variation in the electric-potential-distribution area was observed in the s-si crystals also after magnetic-field action by another type, in particular, after the action of a lowfrequency (1 Hz) modulated magnetic field instead of only after magnetic treatment in a permanent magnetic field. Contrary to the permanent magnetic field, it is only the photovoltage-signal distribution that varied in the case of modulated magnetic-field action, with its magnitude remaining costant. Our investigations established that the distribution and magnitude of the surface potential altered during MT gradually return to values typical for the control samples (Figs. 1d and 1e) after a certain time after MT termination. The last fact points to the reversibility of the structural changes caused by the magnetic effect. The investigations carried out in the study involving the method of scanning-electron microscopy and X-ray spectral analysis showed that, as a result of the magnetic-field action in the surface layer of the crystals under investigation with a thickness of ~ μm, the number of carbon atoms increases ~. times in average, the number of oxygen atoms increases ~1. times, and a certain decrease in the number of basic matrix elements, i.e., silicon atoms is also observed. The increase in the carbon concentration after MT can be related to the magnetostimulated strengthening of the interdefect reactions. According to [1], interstitial silicon atoms can interact with carbon displacing carbon, which is located at crystal-lattice sites, to an interstitial position by the Watkins reaction Si i + C s Si s + C i (the subscript i corresponds to interstitial atoms, and the subscript s, to atoms at the sites). An increase in the oxygen concentration revealed here in the surface s-si layer after magnetic action can occur due to the magnetostimulated breaking of chemical bonds in nanoclusters of structural defects, such as, for example, oxide precipitates. The interdefect reactions stimulated by the magnetic field can result in the following: due to mutual interaction between carbon atoms and their interaction with oxygen and silicon atoms, carbonaceous and silicon carbonaceous oxygen complexes can be formed, which, according to [1], have electrical activity. The charged impurity adsorbed and gettered by the magneto-active surface result in an increase in the microscopic recombination barrier ϕ. In essence, an increase in the area of the electricpotential distribution observed in this study and the increase in the potential amplitude confirms the assumption about the magnetostimulated increase in the surface-potential-barrier height stated in [13] and explains an increase in the long-term component of the photoconductivity relaxation time τ found in [13]. When investigating the surface potential, we determined also the time of nonequilibrium-carrier recombination through the potential barrier generated by charged impurities instead of only the character of distribution of the photovoltage signal over the surface and its value. It was established that an increase in the distribution area and the photovoltage-signal value stimulated by magnetic action well correlates with the increase in the carrier-recombination time τ r. In the investigation of the electric potential, it was established that the value of τ r after magnetic treatment of the s-si samples increases ~(.5) times as compared with the s-si samples without magnetic treatment. It is typical that the variation in τ r agrees well with the variation in the value of the long-term component τ established in [13] upon investigation of photoconductivity relaxation. According to [13], the value of the long-term components τ also increases almost twice right after completion of MT in comparison with similar parameters for the s-si samples, which were not subjected SEMICONDUCTORS Vol. No. 1

3 7 MAKARA et al (a) (b) (c) (e) (d) Fig. 1. Surface distribution of the photovoltage value in s-si crystals: (a) before MT, (b) after MT during t M = days and (c) 1 days, (d) after 1 days after MT, and (е) 15 days after MT. to MT. It was experimentally established that the area of photovoltage-signal distribution over the surface, the value of the potential, and also such parameters as τ r and τ changed as a result of MT gradually return after a certain time after MT completion to the initial values inherent to the s-si crystals, which were not subjected to the magnetic action. The reversible character of the variation in the investigated characteristics testifies to gradual relaxation of the microscopic recombination barrier and SEMICONDUCTORS Vol. No. 1

4 CHANGE IN THE CHARGE AND DEFECT IMPURITY STATE OF SILICON 75 Intensity, arb. units Time, days ~ Fig.. Dependence of the ESR-signal intensity on the time elapsed after MT of silicon crystals for solar-power engineering. indicates the instability of the charge state of centers responsible for the behavior of the long-term component τ of the photoconductivity relaxation investigated in [13]. In our opinion, it is the oxygen and carbon atoms and also ions of impurities of alkaline metals and aluminum gettered from the material bulk by the magnetoactivated surface after MT which can act as the mentioned centers. It is quite probable that with increasing time, after the end of MT, the specified centers participate in interdefect reactions stimulated by the magnetic field, for example, with oxygen, carbon, and hydroxyl groups adsorbed by the magnetoactivated surface from the surrounding atmosphere due to which their charge state is neutralized. The latter results in the fact that the parameters τ r and τ changed as a result of the magnetic action come back in time to the initial values. The assumption about the effect of processes of adsorption of certain chemical elements, in particular, oxygen and carbon by a magnetoactivated surface on the kinetics of surface charge-state change and photovoltage relaxation was confirmed by us when investigating the quantitative content of the specified elements in the s-si surface layers. This investigation, which was carried out using X-ray spectral analysis, showed that the oxygen content in the s-si surface layers increased from.33 to.7% 1 days after the end of MT. This points to the fact that oxygen adsorption gradually increases in time. The carbon concentration increases from 7.73 to.57% 1 days after MT. The result obtained with the help of the X-ray spectral method evidencing the gradual increase in the oxygen and carbon concentration after MT well agrees with the results obtained in additional investigations carried out using electron-spin resonance (ESR) (Fig. ). From Fig., it can be seen that the intensity of the ESR-spectrum line with a g factor of.55, which corresponds to broken bonds, gradually increases after MT. It should be noted that, upon the breakage of silicon oxygen bonds Si O Si which are dominant in silicon, a radical in the form of SiO arises on the surface of the natural-oxide film, which is always present in silicon, alongside with the radical Si, which is not observed directly (by the ESR method) through the degeneration of its basic orbital state. According to [17], this radical interacts with gas molecules (O, CO) adsorbed on the SiO surface forming new radicals (SiOO O, SiOC O ), the ESR spectrum of which can be investigated and, thus, allows one to judge the state of the surface before the adsorption of gases. It is improbable that an increase in the ESR signal intensity from the line with the g factor.55 observed under our experimental conditions after a certain time after the end of magnetic treatment evidences the following. The magnetic effect results in the occurrence of several processes, namely, the breakage of Si O Si bonds and also the formation of radicals of the form Si O instead of only radicals of the Si type. The interaction of adsorbed CO molecules with the radical Si is carried out with charge transfer according to the scheme Si + CO = Si + CO, and resonance arises already at paramagnetic ions CO. The radicals Si O form new radicals SiOOO and SiOC O due to adsorption. It is improbable that the transformation of radicals effects the character of the dependence of the ESR-signal intensity on the duration of time elapsed after carrying out MT (Fig. ). The data of X-ray spectral analysis point to the fact that the effect of the increasing number of oxygen and carbon atoms in the surface layers observed right after the performance of MT relaxed in time (5 days after the end of MT), i.e., the number of oxygen and carbon atoms gradually returned to values typical of the initial s-si samples. The last fact correlates with the gradual decrease in the ESR signal intensity (see Fig. ). In summary, it is possible to state that the MT of s-si crystals causes long-term evolution of the impurity composition in its surface layers. REFERENCES 1. V. I. Al shits, E. V. Darinskaya, T. M. Perekalina, and A. A. Urusovskaya, Sov. Phys. Solid State 9, 5 (197).. Ya. B. Zel dovich, A. L. Buchachenko, and E. L. Frankevich, Sov. Phys. Usp. 31, 35 (19). 3. V. M. Maslovskii and S. N. Postnikov, in Proceedings of the th Scientific Technical Seminar on Processing by Pulse Magnetic Field (Methods and Technology) (Gor kii, Sofiya, 199), p. 5. SEMICONDUCTORS Vol. No. 1

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