Perfect alignment of self-organized Ge islands on pre-grown Si stripe mesas

Self-organized Ge islands grown on patterned Si(001) substrates have been investigated. Selective epitaxial growth (SEG) of Si is carried out with gas-source molecular beam epitaxy to form Si stripe mesas followed by subsequent Ge island growth. Self-aligned Ge islands with regular spacing are formed on the 〈110〉-oriented ridges of the Si mesas. The regular spacing is driven by the repulsive interaction between the neighbor islands through the substrates. A mono-modal distribution of the islands has been observed on the ridges of the Si mesas. The spatial confinement as well as the preferential nucleation is believed to be the mechanism of this alignment of the self-organized Ge islands. PACS: 81.15Hi; 85.40Ux; 81.10Aj; 68.55Jk Self-organized islands have attracted considerable attention due to the fact that self-organization is able to realize nanostructures without using fine lithography and free of processinduced defects or damages, which are frequently seen in the samples defined by lithography and reactive ion etching. The potential device applications of self-organized nanostructures can be found in previous review papers [1]. The size distribution of self-organized islands has been extensively studied because the size uniformity of islands is a crucial concern for optoelectronic applications [2–6]. The spatial distribution of the islands is equally important in order to exploit computational and signal processing applications, for example, the quantum-dot cellular automata (QCA) [7, 8]. For some applications, only one-dimensional ordering of island arrays is needed. However, it is still a challenge to overcome the problem of random site distribution of the islands in order to accomplish ordered island arrays. Facing the challenge, many efforts have been devoted in order to control the spatial distribution of self-organized Ge islands. These efforts include the growth of Ge islands on ∗Corresponding author. the tilted substrates with the surface steps. The islands were somehow aligned along the step edges [9]. Relaxed template with misfit dislocations [10] and the stacking growth of multilayer of islands [11, 12] were also utilized for controlling the location of self-organized islands. Even though some progresses have been made, the problem to control the ordering and position of the islands still remains. Recently, the formation of one-dimensional arrays of Ge islands along the edges of the pre-grown Si stripe mesas has been reported [13]. In this alternating approach, by using SEG on the substrates with lithographic patterns, the stripe mesas were formed as a template for the subsequent Ge island growth. However, the islands also formed across the plateaus of the mesas, making the control of the island arrays difficult. In this paper, we report the perfectly aligned selforganized Ge islands on the pre-grown Si stripe mesas to form one-dimensional island arrays after fully reducing the top plateaus of the stripe mesas. The average spacing between two islands increases with the decrease of the base width of the Si mesas. A mono-modal distribution of the islands is observed on the ridges of the Si mesas. The spatial confinement effect on the mono-modal distribution will be discussed.