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https://doi.org/10.29013/ESR-22-1.2-40-45
Esbergenov Daryabay Muratbaevich, Sayfillo Saidovich Nasriddinov, Scientific Research Institute of Semiconductor Physics and Microelectronics at the National University of Uzbekistan named after Mirzo Ulugbek, Tashkent, Uzbekistan E-mail: edaryabay@gmail.com
KINETICS OF FORMATION OF COMPLEX DEFECTS IN SILICON DOPED WITH ZINC AND NICKEL
Abstract. The paper presents the results of Deep Level Transient Capacitive Spectroscopy (DLTS) and Raman spectroscopy of silicon doped with zinc and nickel. (DLTS) reveals two electrical levels of substitutional zinc in silicon at EV+0.28 eV and EV+0.56 eV. A number of additional DLTS peaks are observed after hydrogenation of the samples. Total passivation of zinc-related deep levels with nickel was observed. After the introduction of nickel in zinc-doped silicon, DLTS spectra show two new levels with energies EV + 0.54 eV and EV + 0.60 eV. The results of Raman spectroscopy showed that the maximum of the peak at about 530 cm-1 indicates the presence of a nickel impurity in the composition of the ZnO compound.
Keywords: DLTS, Raman spectra, hydrogenation, Zn, Ni, passivation, complex defects.
Introduction Experimental procedure
Interest in the behavior ofzinc impurities in mono- As a starting material, dislocation-free silicon
crystalline silicon is due to the prospects ofusing it as of the KDB (silicon-donor-bor) brand 40 ^ 100
an alloying additive as an alternative to expensive gold Ohm • cm, grown by the Czochralski method, was
in obtaining new structures of fast-acting impurities. used. For the study, silicon samples were made with
Silicon doping with zinc impurity atoms is widely a size of 1 x 5 x 10 mm3. Silicon doped with zinc
used to control the lifetime of charge carriers, thereby and nickel was carried out sequentially. First, the
increasing the speed of semiconductor devices [1]. initial crystals were preliminarily doped with zinc
The electrophysical properties of Zn in Si were by thermal diffusion in the temperature range T =
widely studied in [2-4]. It is known that during the =1000-1250 °C for t = 0.5-5 hours, and then nickel
quenching process after diffusion annealing, reactions was diffused into the same samples by sputtering
of point defects with doped impurities can occur, or at 900-1100 °C for t =0,5-1 hr. Rapid cooling was
the presence of undesirable impurities can further performed for both cases.
complicate the behavior of Zn in Si. Such undesirable To measure DLTS, Schottky barriers were fab-
impurities include O, C, H, Ni, Fe, Cr and others. ricated by vacuum deposition of Al on p-Si. Before
In this regard, the purpose of this work is to study making a Schottky contact, the samples were subject-
the effect of Ni atoms in Si doped with Zn by the ed to liquid chemical etching (LCT) in a mixture of
method of deep level nonstationary spectroscopy HF: HNO3: CH3COOH in a ratio of3: 5: 3 (CP-4).
(DLTS) and Raman spectroscopy. Ohmic contacts were obtained by sputtering Au on
the back of the samples. DLTS scanning was carried out in the temperature range from 77 K to 350 K.
The analysis of the chemical composition and crystal structure of silicon doped with zinc and nickel was carried out using Raman spectroscopy on an In-Via Raman Spectrometer manufactured by Renishaw, UK, with excitation by RL785 Class 3B Laser lines, with a radiation wavelength of 785 nm in the range frequencies 100-1500 cm-1. In the course of measurements, a diffraction grating with a period of 1200 lines/mm was used, and a standard Renishaw CCD Camera detector was used as a recording device.
Results and discussion
After diffusion of Zn, some samples were subjected to low temperature treatment (LTT) at T = =500-600 °C for t = 20-30 min. For such samples, Schottky contacts were obtained after the second heat treatment. The DLTS spectra after diffusion of Zn and subsequent LTT in Si are shown in (Fig. 1).
As can be seen from (Fig. 1), the two main peaks observed at T = 140 K (peak c) and T = 300 K (peak E) are in good agreement with the double acceptor behavior of Zn in Si [1; 5].
Peaks B and D were found to be located close to the bottom of the conduction band. Their concentration increases with increasing hydrogen concentration, and therefore they are most likely related to defects associated with hydrogen. It is likely that when cleaning silicon by chemical etching before sputter-
Table 1. - Parameters characterizing defects in Si after diffusion of Zn and Ni
Peak T , K max7 Et, eV 2 o, cm2 DL
A 95 0.18 1 • 10-15 ZnB
B 120 0.22 5 • 10-14 ZnH
C 140 0.28 4.8 • 10-14 Zn0/-
D 168 0.31 3 • 10-15 ZnH2
X1 284 0.54 2.1 • 10-14 ZnNi0/-
E 300 0.56 1 • 10-14 Zn0/-
X2 305 0.60 5 • 10-15 ZnNi0/-
From (fig. 1) that after annealing the samples at levels is observed in silicon. Moreover, levels A, D T = 500-600 °C, a decrease in the concentration of and E are annealed faster than the other levels.
ing aluminum to obtain a Schottky barrier, hydrogen atoms can remain on the silicon surface [6].
A
I D
100 150 200 250 300 T, K
Figure 1. DLTS spectra for p-type Al-Si <Zn> samples. T = 1100 °C, t = 3 hours.
1 - spectrum of p-Si <Zn> samples without LCT; 2 - spectrum of p-Si <Zn> samples with LCT; 3 - spectrum of samples after additional LTT T = 500-600 °C t = 30 min
The method proposed in [7] for analyzing the microscopic structure of defects showed that deep levels EV+0.22 and EV+0.31 can be attributed to the ZnH and ZnH2 complexes, respectively [8; 9]. These levels were also discovered by the authors of [10], but the structure of these complexes was not clear.
In [8], it is reported that the A level is a ZnB complex, which is formed upon cooling. Table 1 shows the main parameters of deep levels in silicon doped with zinc and nickel.
As is known, Ni is a rapidly diffusing impurity in Si [11; 12], and at T = 600 K its solubility reaches N = 3 x 1014 cm -3. After the diffusion of nickel atoms in the samples, the p-Si <Zn> peak that was associated with zinc atoms (peaks A, B, C, D, E) is not observed in the DLTS spectrum of measurements (Fig. 2). This is explained by the process of passivation of
the levels of zinc atoms in silicon after doped with nickel atoms.
From (Fig. 2) that after the introduction of Ni into Si doped with Zn, the DLTS spectra exhibit two small new peaks X1 and X2 with energies EV + 0.54 and EV + 0.60, respectively, while in [10] these levels were not recorded in DLTS measurements.
Figure 2. DLTS spectra of p-Si <Zn> and p-Si <Zn, Ni>. 1 - spectrum after low-temperature annealing at T = 600 °C for p-Si <Zn> samples, 2 - spectrum after doping with nickel atoms, 3 - spectrum after thermal annealing at T = 130 °C in p-Si <Zn, Ni> samples
Raman spectroscopy is a versatile method for detecting the inclusion ofdopants, defects, and lattice dis-
order in the host lattice [13]. In (Fig. 3) shows the Raman spectra ofsamples Si <Zn>, Si <Ni>, Si <Zn, Ni>.
Raman shift (cm ) Figure 3 A. Raman spectrum of samples Si <Zn>, Si <Ni>, Si <Zn, Ni>
All the spectra obtained show bands corresponding to vibrational modes characteristic of crystalline ZnO, NiO, and silicon. Nonpolar pho-non modes of ZnO are located at frequencies of 430 cm-1, modes with maximum intensity at 240 and 940-1000 cm-1 are related to second-order vibrations of ZnO, which correspond to the data of [14]. Bands with maxima at about 146, 306, 520, 620,
980 cm-1, associated with monocrystalline silicon [15]. The 430 cm-1 mode depends on the presence and concentration of impurities and defects in the ZnO crystal lattice [16] and indicates that the resulting films have a hexagonal crystal structure of the wurtzite type, which is most characteristic of Zn O. Oxygen bond vibrations have a dominant effect on the 430 cm-1 mode.
Figure 3 B. Raman spectrum of samples Si <Zn>, Si <Ni>, Si <Zn, Ni> (increased peak in the range of 940-1000 cm-1)
The Raman spectrum of the NiO compound produces peaks around 516, 997, 1172 cm-1. The authors of[17, 18] investigated the spectrum of Raman light scattering at room temperature for bulk NiO consists of several bands: a one-magnon (1M) band at ~ ~ 34 cm -1; five vibrational bands - one-phonon (1P) TO at 440 cm-1 and LO at 560 cm-1, two-phonon (2P) 2TO at 740 cm-1, TO + LO at 925 cm-1 and 2LO at 1100 cm-1 modes; one-magnon (M) band ~ 40 cm-1. The peak observed at 516 cm-1 in the Raman spectrum of the present study can be attributed to the 1TO + 1M mode. The peaks at 997 cm-1, 1172 cm-1 can be assigned to the TO + LO, 2LO NiO modes. The shifts of the Raman peaks in comparison with
bulk NiO can be explained by the combined action of the phonon confinement effect (e.i., the weakening of the rule for choosing k = 0 and scattering by phonons existing in the region bounded by 2n/d, where d is the size of the crystal around the center of the first band Brillouin) and defect-induced Raman scattering of light. On the whole, the position and relative intensity of the phonon bands are consistent with the literature data for microcrystalline or nano-structured NiOx [19].
The presence of a vibrational mode with a maximum at about 530 cm-1 indicates the presence of a nickel impurity in the composition of the ZnO compound [20]. However, monocrystalline silicon
used as the main grating masks this mode with an eigenmode (about 521 cm1). It is also seen from the spectra that the simultaneous doping in silicon with Zn and Ni impurities leads to a decrease in the concentration of thermal and technological defects such as O, C, H. If we pay attention to the enlarged (Fig. 3) in the range of 940-1000 cm1, it can be seen that the peak increased upon joint doping of Ni and Zn. Taking into account the vibrations of ZnO at 980 cm-1 and NiO at 997 cm-1, we can conclude that the increase in the peak is associated with the vibration of the ZnO and NiO modes in the silicon lattice.
Conclusions
The behavior of nickel and zinc impurities in silicon has been studied by the methods of capacitive spectroscopy and structural analysis. It was revealed
that complex defects in ZnH and ZnH2 are formed during the manufacture of Schottky contacts, which is inevitable in most cases. Analyzing the Raman scattering spectra, it can be concluded that deep levels X1 and X2 are associated with complex Zn --Ni defects. The results of the analysis of the obtained DLTS spectra showed that upon successive doping of silicon with zinc and nickel, deep levels associated with electrically active zinc atoms are passivated by nickel atoms. It is shown that the levels of ZnNi 0/- and ZnNi 0/- with lower concentrations are observed upon the introduction of a nickel impurity in silicon doped with zinc.
The authors express their deep gratitude to the Tashkent State Technical University, Department of Digital Electronics and Microelectronics, for useful discussions and comments on the work.
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