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Section 2. Physics
https://doi.org/10.29013/ESR-21-7.8-6-8
Nasriddinov S. S., Esbergenov Daryabay Muratbaevich, Institute of Semiconductor Physics and Microelectronics of the National University of Uzbekistan, Tashkent, Uzbekistan
E-mail: edaryabay@gmail.com
NICKEL-RELATED COMPLEX DEFECTS IN SILICON
Abstract. Electrically active nickel-related deep levels in n- and p-type silicon are investigated using the method of Deep-Level Transient Spectroscopy (DLTS). Two deep levels with energies Ev + 0.17 eV and Ec - 0.42 eV are observed in n- and p-type silicon diffusion-doped with nickel. It was revealed that wet chemical etching in silicon doped with nickel leads to the formation of various complexes related to hydrogen, with levels at Ec - 0.18 eV, Ec - 0.54 eV, Ev + 0.26 eV, and Ev + 0.55 eV.
Keywords: defect complex, DLTS, silicon, transition metals, hydrogen.
1. Introduction
Significant efforts are currently being made to develop gettering and passivation methods to remove impurities from the active region of a silicon device. To study the effect of these methods, it is necessary to know the diffusion and electrical properties of impurities inside a silicon crystal [1; 2]. Thus, the study of transition metal impurities such as nickel, cobalt, copper is becoming more and more important for semiconductor technology.
As is known, nickel has the highest diffusion coefficient and solubility, even at room temperature it easily diffuses into the crystal lattice [3]. Diffusion of nickel in silicon leads to the creation of deep levels. For many years, scientists have studied the energy levels of nickel in silicon, trying to understand the nature of various defects. Until now, using the Hall and DLTS methods, two levels of nickel in silicon havebeenobserved: a donorlevelat EV + (0.15 ^ 0.18) eV and an acceptor level at EC -(0.38 ^ 0.47) eV. In
addition, a third double acceptor level of nickel was found at EC -(0.07;0.08) eV [4].
Impurities of transition metals, in particular Ni in Si, have been studied for a long time, and the properties of isolated atoms have also been studied quite well. But, at the same time, there is insufficient information on the nature of nickel-hydrogen compounds (Ni - H), in which H acts as a passivator, and until now the study of complex Ni - H defects in Si has been considered only in two works [4; 5]. Therefore, in this work, the task is set to fill in the missing amount of information on the identification of defects associated with nickel and their interaction with hydrogen in silicon by DLTS and structural analysis.
2. Experimental technique
For the study, we prepared substrates in the form of n- and p-type silicon plates, grown by the Czochralski method, 10 x 5 mm in size and 2 mm thick. The Si samples were doped with Ni impuri-
NICKEL-RELATED COMPLEX DEFECTS IN SILICON
ties by the thermal diffusion method at temperatures from 800 to 1150 °C for 2-5 h.
Before making a Schottky contact, the samples were wet chemically etched in a mixture of HF : HNO3 : CH3COOH in a ratio of 3 : 5 : 3 (CP-4). It is known that etching in CP-4 occurs with the participation of hydrogen in a thin layer under the surface of the sample [6].
After etching and making contacts, the samples were annealed for 20 min. at a temperature of 120 °C. At this temperature, hydrogen diffuses into the sample.
3. Results and discussions The results of the measurements of the DLTS spectra of n-Si and p-Si samples in different states are shown in (Fig. 1 and 2).
Figure 1. DLTS spectra of n-type Si samples. 1 - initial sample n-Si <P>; 2 - doped with nickel n-Si <Ni>; 3 - after the introduction of hydrogen n-Si <Ni-H>
The spectrum of the initial n-Si samples (Fig. 1, curve 1) shows one peak A at 167 K with an ionization energy EC - 0.31 eV which is associated with technical uncontrolled impurities. Such a low level was also recorded in [7] with an energy of EC - 0.30 eV at 165 K. After diffusion doping of Ni in n-Si <Ni> samples (Fig. 1, curve 2), a deep Ni acceptor level with an ionization energy of EC - 0.42 eV (peak B) at 240 K.
Upon the introduction of hydrogen, two additional levels were observed in the n-Si <Ni-H> samples: EC - 0.18 eV at a temperature of 98 K (peak C) and Ec -0.54 eV at 273 K (peak D). Structural analysis showed that the EC - 0.18 eV and EC - 0.54 eV deep levels refer to complex defects NiH and NiH2, respectively, which is confirmed by work [4]. Also, in works [4, 8] the level EC - 0.18 was investigated, but the author of [8] identifies it as a thermal defect.
The decrease in the concentration of Ni levels in the n-Si <Ni-H> samples is explained by the process
ofpassivation of defects to hydrogens. Complete hydrogen passivation of the EC - 0.31 eV level is also observed [9; 10].
DLTS spectra of p-type Si samples are shown in (Fig. 2). As can be seen from the figure, the initial p-Si samples (Fig. 2, curve 1) create a DL with an ionization energy EV + 0.43 eV at 230 K (peak F) related to technical uncontrolled impurities. Deep donor level of Ni is observed in p-Si <Ni> samples upon diffusion with an ionization energy EV + 0.17 eV at 80 K (Fig. 2, curve 1, peak G). And in the p-Si <Ni-H> samples after the introducing of hydrogen, the spectrum shows 2 new levels, with energies EV + 0.26 eV at 155 K (peak H) and EV + 0.55 eV at 280 K (peak J). The results of the structural analysis and comparison of the results of other authors showed that the H and J peaks are associated with complex Ni - H defects, which also agrees with the results of [4; 5]. These levels are formed in passivation by hydrogen atoms as in p-type silicon.
Figure 2. DLTS spectra of p-type Si samples. 1 - initial sample p-Si <B>; 2 - nickel-doped p-Si <Ni>; 3 - after the introducing of hydrogen p-Si <Ni-H>
4. Conclusions
Thus, it was found that diffusion doping of Ni in Si leads to the formation of two deep levels with energies EV + 0.17 eV and EC - 0.42 eV. It was revealed that after the intentional introduction of hydrogen, four additional DLTS peaks are observed in the samples. Analysis of the spectra showed that the pres-
ence of hydrogen atoms in silicon passivates the nickel levels, which leads to the formation ofvarious Ni - H complexes. It is shown that defects of the Ni -H type are localized in the band gap of silicon with energies EC - 0.18 and EV + 0.26 eV. In addition, it was found that the EV + 0.55 and EC - 0.54 levels of the NiH2 complex belong to the same defect level.
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