Development of atomically-controlled oxidation techniques for ultra-thin gate oxide films

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We have clarified the effects of hydrogen and impurities on the initial oxidation processes of Si surfaces and the structural relaxation in SiOィイD22ィエD2 films using high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS).The principal conclusions are shown below.(1) On a H-terminated Si (100) surface, it was found that Si-H bonds enhance the relaxation of Si-O-Si bonding structures by changing the bond angle. Moreover, Si-H bonds on the oxidized surface are more stable by the O adsorption on their two Si-Si back bonds. That is to say, the binding energy of the Si-H bonds on and in the oxide film is larger than that on Si surfaces.(2) In the case of the oxidation of pィイD1+ィエD1-Si (100) surfaces, the impurity concentration influences the relaxation of SiOィイD22ィエD2 structures. B atoms enhance the Si-O-Si structural relaxation. This phenomenon may be caused not by the exchange between Si atoms and B atoms but by the electronic effect such as Fermi level.(3) The strain accompanying the oxidation on H-terminated Si (111) surfaces is relaxed by enlarging the Si-O-Si bond angle. The relaxation is remarkably observed at an oxide thickness of 1ML, where the amount of Si-H species with two or three O atoms in the back bonds increases. As a result, surface Si atoms with a Si-H bond are lifted up.(4) On the pィイD1+ィエD1-Si (100) surface, specific structures formed by surface-segregated B atoms are observed and it was found out that there are two kind of structures. B atoms stabilize an O adsorption site, which is unstable on a Si surface with a low impurity concentration. This finding strongly suggests that the O adsorption site is controllable by changing the surface strain with the B concentration.