(110)-Surface Strained-Channel MOSFETs

Improvement of semiconductor device performances have been brought about by innovations in device structures such as SOI, FinFET and gate-all around FET. The FinFET devices saw a remarkable success owing to their high hole mobility. FinFET devices attain this preferable feature by having the (110)-surface channel. Recently, considerable progress has been made in the nanosheet transistor technology. Owing to the gate-all-around structure and the added dimension, the nanosheet transistor technology has an enormous potential for realization of the electronic devices with a significantly high performance. An issue for the nanosheet transistor is the degradation of the hole mobility. This problem stems from the following factors. First, currently developed nanosheet transistors are characterized as (100)-channel devices which have inferior character regarding the hole mobility. Secondly, for fabrication of nanosheet transistors, crystal growth of multiply stacked Si/SiGe layers is an essential process, which involves deterioration of the crystalline quality. The degradation of the crystalline quality is severer in the case of crystal growths on the (110)-surface than those on the (100)-surface. Therefore, the use of the (110)-wafer is challenging and methods to grow Si/SiGe multilayers on (110)-wafers with high quality is to be developed. The authors have been involved in the crystal growth and characterizations of Si/SiGe heterostructures on Si(110) wafers [1-4]. In these studies, a significant enhancement of the hole mobility in the strained Si layer was demonstrated. The results indicate the effect of the lattice strain on the energy band structure can overcome the adverse effects of the crystalline defects. In addition, characteristic features of the crystalline defects generated in the (110)-oriented heterostructures have been revealed. Along with the knowledge on the defect generation and the formation process of the surface morphology, the effect of strain is also a focus of this talk. We discuss first the effect of strain on the valence band structure of Si. It is shown that the effect of strain in reducing the hole cyclotron effective mass is more significant when the surface is (110) compared to the case where the surface is (100). Interestingly, the strain dependences of the hole cyclotron effective mass differ in these two cases. The computational investigation indicates that the lattice strain combined with the (110)-oriented surface is an effective method to increase the hole mobility. Next, a formation mechanism of the crystalline defects in SiGe grown on the (110) wafer is discussed. The TEM observations clearly show that the dominant crystalline defect induced by strain is a twin structure having the {111} twin boundary. The reason for the twin generation is understood in the framework of a dislocation theory. The shear stress acting on a (111) plane drives the atomic layer to form a stacking fault and/or a twin structure. The strength of the shear stress is higher in the case of the (110)-oriented SiGe film compared to the case of the (100) counterpart, which explains the small critical layer thickness of the SiGe on a Si(110) wafer. Finally, the formation process of the surface morphology during the growths of SiGe on Si(110) wafers is discussed. Even at the initial stages of the SiGe, difficulty in obtaining a flat surface is recognized [4]. Based on currently obtained experimental results, we discuss the effect of a surface inclination on the surface morphology. [1] K. Arimoto et al ., Jpn. J. Appl. Phys. 59, SGGK06 (2020). [2] D. Namiuchi et al ., Materials Science in Semiconductor Processing 113, 105052 (2020). [3] K. Arimoto et al ., ECS Transactions 98 (5), 277 (2020). [4] S. Saito et al ., Materials Science in Semiconductor Processing 113, 105042 (2020).