Measurement of Low Concentration Nitrogen in Czochralski Silicon by Infrared Absorption Spectroscopy
Authors: Inoue Naohisa , Okuda Shuichi , Kawamata Shuichi 
Published: February 2020

Introduction Nitrogen plays an important role in suppressing the grown-in defects in single crystalline silicon. Nitrogen in silicon mainly forms interstitial Ni-Ni pairs. In low concentration CZ silicon, for example in the 1013 cm-3 range, however, nitrogen mainly forms Ni-Oi pair. The concentration of nitrogen in silicon ([N]) is measured by infrared absorption spectroscopy (IR) of local vibration modes of NN pair [1]. Therefore IR measurement of [N] in the 1013 cm-3 range is impossible and far infrared electronic transition absorption of shallow thermal donors due to NO is used [2]. To solve this problem, we have been looking for the IR absorption of NO pairs [3]. Here we show the new absorption we found, tentative identification and trial to measure [N] using these absorption lines.

Experimental Samples were cut from the nitrogen doped CZ (NCZ) silicon crystals. They were polished on both sides for IR measurement. Samples were annealed at 450-1100 oC for 2 hours [4]. IR measurement was done at room temperature with wavenumber resolution of 2 cm-1. The differential absorption spectra using the nitrogen-free sample as the reference were obtained.

New absorption and its thermal behavior As we have already reported, there were many new absorption lines in the annealed samples. They were located at 714, 720, 736, 855, 945, 973, 989, 1002, 1045, 1065 cm-1 and so on [3]. The annealing temperature dependence of their intensity was examined in detail and compared to that of the NN group. The thermal behavior of NN, NNO and NNO2 had been interpreted as the quasi-equilibrium reaction of the oxygen attaching and detaching among these complexes [4]. Absorption lines located at 855, 973, 1002, 1065 cm-1 showed maxima at 600 oC and weak above that, similar to NNO2 behavior. This suggested that similar quasi-equilibrium reaction occurred in NO group also at the same temperature. The 973 and 1002 cm-1 lines saturated for high [N], similar to STD, in contrast to NNO2 increased linearly with [N] [6]. Thus the 1002 cm-1 line was assigned as NOO2i [6]. Absorption lines located at 720 and 736 cm-1 showed maxima at 650 oC and were gradually weakened above that, similar to NNO behavior. Absorption lines located at 945 and 989 cm-1 showed minima at 600 oC and strong around 700-800 oC, similar to NN behavior.　Therefore it is likely that lines at 720 and 736 cm-1 correspond to NOO and lines at 945 and 989 cm-1 to NO.

Concentration measurement of NO group In the high concentration range, nitrogen has three configurations. Nitrogen concentration is obtained as the sum of [NN], [NNO] and [NNO2]. The dipole moments of the three configurations are not equal. They were determined both theoretically and experimentally. As a result, nitrogen concentration is obtained as the weighted sum of absorption coefficients of three NN based configurations, multiplied by the conversion coefficient from absorption coefficient α to concentration, k; k(αNN+1.5×αNNO+0.5×αNNO2) [1]. The weight is reciprocal ratio of the dipole moment. As the quasi equilibrium holds, the sum of [NN] [NNO] and [NNO2] should be constant and was already confirmed in many cases. Fig. 1 shows that in this case, the sum was nearly constant under 800 oC and confirms that [N] can be obtained in this way. In case of NO group, the same procedure is thought to be possible. Dipole moment of NO group was calculated to be about 1/2 of that of NN group [6], because there is only one N in NO group. The weighted sum of absorbance of NO, NOO and NOO2 candidates using the same weight as those of NN group is shown also in the figure. It suggested the similar tendency, though difficulty in determining the weak and overlapping absorption was observed. Conversion coefficient for NO group may also be 1/2 of that for NN group. Roughly speaking, the absorbance of NO group in this sample is about 1/10 of NN group. Therefore [N] in NO group is about 1/20 of that in NN group. It depends on the sample.
[1] N. Inoue, K. Shingu and K. Masumoto, Semiconductor Silicon 2002, 875 (2002). [2] H. Ono and M. Horikawa, Jpn. J. Appl. Phys. 42, L261 (2003). [3] N. Inoue, M. Nakatsu and H. Ono, Physica B, 376-377, 101 (2006). [4] K. Tanahashi and H. Yamada-Kaneta, Jpn. J. Appl. Phys. 42, L223 (2003). [5] H. Ch. Alt and H. E. Wagner, J. Appl. Phys., 106, 103511 (2009). [6] N. Inoue, M. Nakatsu, H. Ono and Y. Inoue, Materials Sci. Eng., B, 134, 202 (2006).

Figure 1