6533b827fe1ef96bd128596a

RESEARCH PRODUCT

Carrier confinement in Ge/Si quantum dots grown with an intermediate ultrathin oxide layer

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We present computational results for strain effects on charge carrier confinement in Ge${}_{x}$Si${}_{1\ensuremath{-}x}$ quantum dots (QDs) grown on an oxidized Si surface. The strain and free carrier probability density distributions are obtained using the continuum elasticity theory and the effective-mass approximation implemented by a finite-element modeling scheme. Using realistic parameters and conditions for hemisphere and pyramid QDs, it is pointed out that an uncapped hemisphere dot deposited on the Si surface with an intermediate ultrathin oxide layer offers advantageous electron-hole separation distances with respect to a square-based pyramid grown directly on Si. The enhanced separation is associated with a larger electron localization depth in the Si substrate for uncapped hemisphere dots. Thus, for dot diameters smaller than 15--20 nm and surface density of the dots (${n}_{\text{QD}}$) ranging from about 10${}^{10}$ to 10${}^{12}$ cm${}^{\ensuremath{-}2}$, the localization depth may be enhanced from about 8 nm for a pyramid to 38 nm for a hemisphere dot. We find that the effect in a hemisphere dot is very sensitive to the dot density and size, whereas the localization depth is not significantly affected by the variation of the Ge fraction $x$ in Ge${}_{x}$Si${}_{1\ensuremath{-}x}$ and the aspect ratio of the dot. We also calculate the effect of the fixed oxide charge (${Q}_{\text{ox}}$) with densities ranging from 10${}^{\ensuremath{-}9}$ to 10${}^{\ensuremath{-}7}$ C/cm${}^{2}$ for 10-$\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}$cm $p$-type Si wafers on the carrier confinement. Although the confinement potential can be strongly perturbed by the charge at ${n}_{\text{QD}}$ less than $\ensuremath{\approx}$$4\ifmmode\times\else\texttimes\fi{}10$${}^{11}$ cm${}^{\ensuremath{-}2}$, it is not very sensitive to the value of ${Q}_{\text{ox}}$ at higher ${n}_{\text{QD}}$. Since, to our knowledge, there are no data on carrier confinement for Ge QDs deposited on oxidized Si surfaces, these results might be applicable to functional devices utilizing separated electrons and holes such as photovoltaic devices, spin transistors, and quantum computing components. The use of hemisphere QDs placed on oxidized Si rather than pyramid dots grown on bare Si may help to confine charge carriers deeper inside the Ge/Si heterostructure in order to reduce the influence of surfaces and interfaces on transport properties of the structures.