Increased conductance of individual self-assembled GeSi quantum dots by inter-dot coupling studied by conductive atomic
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NANO EXPRESS
Open Access
Increased conductance of individual selfassembled GeSi quantum dots by inter-dot coupling studied by conductive atomic force microscopy Yifei Zhang, Fengfeng Ye, Jianhui Lin, Zuimin Jiang and Xinju Yang*
Abstract The conductive properties of individual self-assembled GeSi quantum dots (QDs) are investigated by conductive atomic force microscopy on single-layer (SL) and bi-layer (BL) GeSi QDs with different dot densities at room temperature. By comparing their average currents, it is found that the BL and high-density QDs are more conductive than the SL and low-density QDs with similar sizes, respectively, indicating the existence of both vertical and lateral couplings between GeSi QDs at room temperature. On the other hand, the average current of the BL QDs increases much faster with the bias voltage than that of the SL QDs does. Our results suggest that the QDs’ conductive properties can be greatly regulated by the coupling effects and bias voltages, which are valuable for potential applications. Keywords: Conductance, Conductive atomic force microscopy, GeSi quantum dots, Coupling
Background Self-assembled semiconductor quantum dots (QDs) have been intensively studied over past decades due to their great importance for both fundamental physics and device applications [1-3]. As the efficiency of single-layer QDs is relatively low, vertically aligned multilayer QDs are often adopted for practical applications [3-6]. By repeating dot layers separated by spacer layers with a few nanometers in thickness, a more homogeneous size distribution could be achieved, simultaneously with novel physical properties induced by coupling [7,8]. The coupling effects between the vertically aligned QDs have been investigated by various macroscopic techniques such as photoluminescence (PL) and admittance spectroscopies [6,8-13], which are found to be strongly dependent on the thickness of the spacer layer. On the other hand, both high-density QDs and QD molecules have attracted a lot of interests for their potential applications [3,14], where the lateral couplings between adjacent QDs significantly modify the QDs’ properties. The * Correspondence: [email protected] State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
lateral coupling effects have also been studied, mainly by macroscopic techniques such as PL spectroscopies [15,16]. Due to the large scattering in QDs’ size, separation, or composition distribution, the quantum properties of coupled QDs obtained by the macroscopic methods would be greatly weakened or eliminated by the averaging effects. Up to now there are only a few microscopic studies performed by STM on InAs [17] and PbSe [18,19] QD clusters recently. In these studies, current–voltage characteristics were found to vary with the dot number in the cluster, indicating the existence of lateral coupling. Thus the coupling effects between individual QDs from a microscopic viewpoint have been scarcely investigated, let alone the modification of the electrical proper
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