(Invited) Atomic-Order Thermal Nitridation of Si, Si1-xGex and Ge by NH3
Authors: Murota Junichi , Sakuraba Masao , Tillack Bernd 
Published: February 2020

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 Si1-xGex (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 (nN) 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 nN increases and the Si-hydride coverage decreases with increasing NH3 exposure time. The Si-hydride becomes hardly observable when nN 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, nN 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 nN 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, Si1-xGex (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 Si0.6Ge0.4 (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 Si0.5Ge0.5 (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 Si0.3Ge0.7 (100) dominantly form a Si3N4 structure which stably remains even during heat treatment in H2at 400 °C [10].
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