Atomic Hydrogen Assisted Growth of Si-Ge Heterostructures on (001) Si
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Institutefor MicrostructuralSciences, NationalResearch Council Canada, Ottawa, K1A OR6, CANADA
ABSTRACT A study of the interface chemical and physical abruptness of Si-Ge heterostructures grown on (001) Si by molecular beam epitaxy under atomic hydrogen exposure is reported. Atomic hydrogen (AH) was produced by the dissociation of molecular hydrogen interacting with a hot tungsten filament. Secondary-ion mass spectroscopy (SIMS) of structures made of alternating Ge (0.5 nm)/Si (40 nm) layers demonstrated that AH can effectively suppress Ge surface segregation. The segregation length was reduced from 1.5 nm to about 0.5 nm in films grown at a hydrogen partial pressure of -5 x 10-3 Pa and cell temperature of 2140 'C with an estimated cracking efficiency of -5%. However, the high hydrogen background pressure had detrimental effects on the physical sharpness of the interfaces. This was evidenced by comparing the interface quality of Si/Ge atomic layer superlattices grown with and without AH exposure. X-ray reflectivity and Raman spectroscopy revealed a significant increase of the interface roughness, although the periodic character and the good crystallinity of the structures were preserved. INTRODUCTION There is currently enormous interest in the growth of strained Si/SiI xGex heterostructures. These offer enhanced electrical characteristics in bipolar devices1 and are promising structures in optoelectronics. 2 In all these applications it is crucial that the interfaces between Si and Si I-xGex be of high quality. For example, interface roughness at the atomic scale or chemical intermixing may affect the carrier transport properties at the hetero-junctions. Surface segregation is another phenomenon that may have detrimental effects on the electronic properties of a hetero-junction. This phenomenon is common in molecular beam epitaxy (MBE) where impurities that lower the surface free energy have a tendency to float at the surface rather than incorporate into the growing film. In Si-based MBE this is the case for most n-type dopants. In Si/SilxGex heteroepitaxy, the Ge atoms also tend to segregate at the surface resulting in a trailing edge in the Ge concentration profile after Ge deposition has ceased. In a first approximation, the Ge concentration profile N(z) at a Sil-xGex/Si interface may be written N(z) = (N(O)/l) exp-(z/e) The where t is the segregation length and N(0) is the initial Ge surface coverage (per unit area). 3 value of e is a function of the growth conditions and is typically of the order of 1-2 nm. It is possible to reduce Ge surface segregation by exposing the surface during growth to another atomic species that has a stronger tendency to segregate. Impurities such has Sb, Ga, Sn and Bi act as surfactants and can suppress Ge surface segregation. However, the introduction of such impurities in a high vacuum system may be difficult or even undesirable, as it may adversely affect other processes. Ge surface segregation is significantly inhibited in techniques that use gases as precursors, such as gas s
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