Lateral conductivity of p -type doped Si/Ge island structures
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NSIONAL SYSTEMS
Lateral Conductivity of p-Type Doped Si/Ge Island Structures V. A. Gergel’a^^, T. M. Burbaevb, V. A. Kurbatovb^^^, A. O. Pogosovb, M. Rzaevb, N. N. Sibel’dinb, I. M. Shchelevaa^, and M. N. Yakupova aInstitute
of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia ^e-mail: [email protected] ^^e-mail: [email protected], [email protected] bLebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia ^^^e-mail: [email protected] Submitted November 9, 2006; accepted for publication November 23, 2006
Abstract—Numerical calculations of the conductance in structures with doping modulated along the currentflow direction are carried out taking into account band offsets at the interfaces between high- and low-resistivity regions. It is found that such structures exhibit S-shaped current–voltage characteristics; in the limiting case, there should be a negative-conductance region, with the abruptness of the heterojunction between the narrowand wide-gap sections of the structure and the doping level being the critically important parameters in the theory. p-type Si/Si1 – xGex island structures with different sizes of islands and different band offsets were grown by molecular-beam epitaxy. Theoretical results are compared with the data on lateral conductance of the grown structures. PACS numbers: 73.40.Lq, 73.61.Cw, 73.63.Bd DOI: 10.1134/S1063782607070081
1. INTRODUCTION Nanoscale structures in which the doping level is modulated along the current-flow direction are expected to possess conductance characteristics, making them attractive for practical applications. It has been shown [1, 2] that the introduction of alternating low-resistivity regions into the channel of a field-effect transistor should lead to an increase in the average velocity of the charge carriers in the channel due to efficient cooling of the electron gas within the low-resistivity insertions. The above situation was numerically simulated in [3] in the context of the quasi-hydrodynamic model. The analysis given there is applicable, in particular, to n-type Si/Si1 – xGex island structures, where the conductivity of the islands is considerably higher than the conductivity of the matrix and the conduction-band offsets at the heterointerfaces may be disregarded at room temperature. For the same reason, implementation of such a structure requires that the Si region and the Si1 – xGex islands be directly and separately doped, which is closely related to the issue of the effect of doping on the structure morphology; the latter is studied only partially for the case of high impurity contents, comparable to the amount of the main material [4]. It looks equally worthwhile to consider high-field drift in heavily-doped structures of another kind, i.e., in nanoscale heterostructures with fairly large variations of the conductivity originating from band discontinuities in a situation where the constancy of the Fermi
level implies a partial transfer of mobile carriers from wide-gap to narrow-gap
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