(Invited) Atomic-Order Thermal Nitridation of Si, Si1-xGex and Ge by NH3

Atomically controlled processing has become indis-pensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration.  Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures which will lead to nanometer scale Si devices as well as Si-based quantum devices. Introduction of N atomic layer at the interface between the high-k dielectric and the channel has been employed to suppress the interface trap formation [4].  Additionally, the insertion of N atomic layer within the channel may be beneficial for high mobility channel formation [5,3].   In this work, we describe atomic-order thermal nitridation of Si_1-xGe_x (100) (x=0 - 1) Si(100) in an NH₃environment in a low temperature range of 300-650°C using an ultraclean low-pressure CVD system and furthermore the thermal stability of nitride surface are discussed.

It has been found that silicon nitride films are formed even at 300-650°C on Si(100) [6]. At 500°C or higher, N initially increases and tends to saturate to a certain value (N atomic amount (n_N) of about 2.7 x 10¹⁵cm^-2; 4 atomic layers).  At 400°C or lower, on the H-terminated Si surface after wet cleaning, the n_N increases and the Si-hydride coverage decreases with increasing NH₃ exposure time. The Si-hydride becomes hardly observable when n_N reaches the surface Si atomic amount (6.8 x 10¹⁴ cm^-2).  On the H-free Si surface after preheating in Ar at 650°C, n_N increases up to 2 x 10¹⁴ cm^-2 (Fig.1) with the appearance of the Si-hydride instantly after NH₃ exposure at 300 and 400°C, indicating that NH₃ molecules dissociatively are adsorbed on the Si dangling bonds. Further ultrathin nitridation is accompanied by a decrease of the Si-hydride coverage. It is found that n_N is well described by Langmuir-type physical adsorption and reaction of NH₃ on the Si surface [3] based on a fitting assuming that the NH₃ adsorption equilibrium constant with flash heating is the same as that without flash heating [7] (Fig.2).

For 30 min exposure of NH₃ at partial pressure of 550 Pa and 400°C, Si_1-xGe_x (100) and Ge (100) are also nitrided at atomic level [8-9].  After Ar or H₂ heat treatment at 400-700 °C under 60 Pa, N atomic amount on the atomic-order nitrided Si (100) and Si_0.6Ge_0.4 (100) is scarcely changed. On the atomic-order nitrided Ge (100), N atomic amount tends to decrease with increasing the heat-treatment temperature and even by H₂ purging at the cooling period after the nitridation.  On the nitrided Si_0.5Ge_0.5 (100), amount of the nitrided Si atoms increases for the higher heat-treatment temperature, although the amount of the nitrided Ge atoms decreases (Fig.3). It is suggested that  N atoms bound to Ge atoms tend to be transferred to Si atoms at temperatures above 400 °C. It is confirmed by angle-resolved XPS that preferential nitridation of Si atoms at surface over Ge atoms induces Ge segregation beneath the surface nitrided layer. Additionally, N atoms for the nitrided Si_0.3Ge_0.7 (100) dominantly form a Si₃N₄ structure which stably remains even during heat treatment in H₂at 400 °C [10].

References

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[3] J. Murota et al, Jpn. J. Appl.Phys., 45, 6767 (2006).

[4] J. Huang et al., Appl. Phys. Lett., 88, 143506 (2006).

[5] Y. Jeong et al., Mat. Sci. Semicond. Process., 8, 121 (2005).

[6] T. Watanabe et al., J. Electrochem. Soc., 145, 4252  (1998).

[7] T. Watanabe et al., Jpn. J. Appl. Phys., 38, 515(1999).

[8] N. Akiyama et al., Appl. Surf. Sci., 254, 6021 (2008).

[9] N. Akiyama et al., Thin Solid Films 517, 219 (2008).

[10] T. Kawashima et al. Thin Solid Films 520, 3392 (2012).