Preferential Positioning, Stability, and Segregation of Dopants in Hexagonal Si Nanowires

Abstract

We studied the physics of common p- and n-type dopants in hexagonal-diamond Si, a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions, by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed that (i) as observed in recent experiments, at larger diameters (beyond the quantum confinement regime) p-type dopants prefer the hexagonal-diamond phase with respect to the cubic one as a consequence of the stronger degree of three-fold coordination of the former, while n-type dopants are at a first approximation indifferent to the polytype of the host lattice; (ii) in ultrathin nanowires, because of the lower symmetry with respect to bulk systems and the greater freedom of structural relaxation, the order is reversed and both types of dopant slightly favor substitution at cubic lattice sites; (iii) the difference in formation energies leads, particularly in thicker nanowires, to larger concentration differences in different polytypes, which can be relevant for cubic-hexagonal homojunctions; (iv) ultrasmall diameters exhibit, regardless of the crystal phase, a pronounced surface segregation tendency for p-type dopants. Overall these findings shed light on the role of crystal phase in the doping mechanism at the nanoscale and could have a great potential in view of the recent experimental works on group IV nanowires polytypes.

Extrinsic Doping in Group IV Hexagonal-Diamond-Type Crystals

“…By applying DFT based methods, we investigate the formation energy and the local crystal symmetry distortion for group III (B, Al, Ga), group IV (Si, Ge, C) and group V (N, P, As) dopants in Ge and C crystals, both for the 2H and 3C phase. We compare the obtained trends with results previously obtained for Si [37]. Our main findings can be summarized as follows: i) in diamond-type crystals the bulk sites symmetry (T d ) is preserved by doping while in hexagonal crystals the impurity site moves towards a higher (T d ) or lower (C 3v ) symmetry configuration dependently on the valence of the dopant atoms; ii) for Si and Ge, group III impurities can be more easily introduced in the hexagonal-diamond phase -whose local C 3v symmetry better accommodate the three-fold coordination of the impurity -while ntype impurities do not reveal any marked phase preference; iii) for C, both n and p dopants are more stable in the hexagonal-diamond structure than in the the cubic one, but this tendency is much more pronounced for n-type impurities.…”

Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase

“…Much less attention has been dedicated to the behaviour of dopants in 2H-Si-NWs. In a recent study 153 we performed ab initio DFT calculations to describe the properties of p-type and n-type dopants located at the innermost sites in 2H-Si-NWs, comparing our results with those obtained for 3C-Si-NWs. All the surface dangling bonds were saturated by H atoms.…”

“…[140][141][142][143][144][145][146] In particular, Si-NWs showing the hexagonal-diamond (2H) phase have been the object of intense study because of their potential for adding new functionalities to 3C Si-NWs. The electronic, optical and transport properties of 2H-Si-NWs [147][148][149][150][151][152][153] have been under scrutiny, highlighting how this phase can offer a further degree of property modulation with respect to the versatility of 3C-Si-NWs. For example, it has been shown that 2H-Si-NWs are characterized by a more pronounced QC effect than 3C-Si-NWs and by a more enhanced optical absorption in the visible region.…”

“…As a rst step, in order to investigate the structural stability of an impurity in one phase with respect to the other, we performed calculations of the FE, within the scheme reported in ref. 153. Results from these calculations are reported in Table 4.…”

“…This nding is in contrast to what was found in the case of middle and large diameter NWs. 153 This differing behaviour can be related to the different structural reorganization around the impurity that one can observe in ultrathin NWs. In the bulk system and large diameter NWs, the host crystal, due to its symmetry, may favor or not favor the stability of a given impurity; in the case of a small NW the major degree of geometrical relaxation is an ideal chemical environment for all types of dopant.…”