Cui et al. Nanoscale Res Lett 2011, 6:59 http://www.nanoscalereslett.com/content/6/1/59 N A N O E X P R E S S Open Access Formation of Nanopits in Si Capping Layers on SiGe Quantum Dots Jian Cui, Jian Hui Lin, Yue Qin Wu, Yong Liang Fan, Zhenyang Zhong, Xin Ju Yang, Zui Min Jiang* Abstract In-situ annealing at a high temperature of 640°C was performed for a low temperature grown Si capping layer, which was grown at 300°C on SiGe self-assembled quantum dots with a thickness of 50 nm. Square nanopits, with a depth of about 8 nm and boundaries along 〈110〉, are formed in the Si capping layer after annealing. Cross- sectional transmission electron microscopy observation shows that each nanopit is located right over one dot with one to one correspondence. The detailed migration of Si atoms for the nanopit formation is revealed by in-situ annealing at a low temperature of 540°C. The final well-defined profiles of the nanopits indicate that both strain energy and surface energy play roles during the nanopit formation, and the nanopits are stable at 640°C. A subsequent growth of Ge on the nanopit-patterned surface results in the formation of SiGe quantum dot molecules around the nanopits. Introduction Heteroepitaxy has been a powerful method to fabricate functional quantum structures, e. g. quantum wells [1], quantum dots (QDs) [2] and quantum rings (QRs) [3,4]. On the one hand, strain is the most important factor affecting the formation of nanostructures [1-3] and even their capping layers in heteroepitaxy [5]. The evolution of the strain can result in a variety of nanostructures, such as QRs [3]. On the other hand, strain induced by heteroepitaxy has been a very prominent improvement in technology to increase carrier mobility [6]. Recently, strained Si channel induced by SiGe QDs has been pro- posed to enhance hole mobility in field effect transistors [7]. Thus strain, together with its distribution and evolu- tion is a key to understand the growth mechanism of the quantum structures and realize the desired structures. Nanopits are interesting for their use as template to achieve positioning growth of QDs, [8-10] QRs [11] and QD molecules [12]. In 1998, Deng and Krishnamurthy [12] fabricated nanopits by depositing carbon impurity in Si matrix, in which SiGe QD molecules were grown around each nanopit. In recent years, nanopits fabri- cated by electron beam lithography, [8,13] holography lithography [9] and nanosphere lithography [10,11] have * Correspondence: zmjiang@fudan.edu.cn State Key Laboratory of Surface Physics, Fudan University, 200433, Shanghai, People’s Republic of China. been used to fabricated ordered SiGe QDs or QRs. Recently, nanopits were also observed in III–V material system by self-assembling based on droplet epitaxy growth [14]. QD molecules are promising candidates as building blocks for the quantum computing [15-17]. It is highly desired to grow QD molecules on semiconductors for the possible applications in quantum computing. By self-assembly, in III–V system, GaAs [18], and InAs [19,20] QD pairs were grown by droplet epitaxy. In SiGe system, though the QD molecules has been observed by introducing carbon impurities [12], the growth of defect-free QD molecules needs to be further explored. In this paper, a strain governed process, as well as related nanostructures in Si/Ge system are reported. Square nanopits, with a depth of about 8 nm and boundaries along 〈110〉, were formed in the low tem- perature grown Si capping layer by thermal annealing at 640°C. The formation mechanism is proposed based on strain energy relaxation and surface energy minimiza- tion. On the nanopit-patterned surface, by depositing Ge SiGe QD molecules were fabricated. Experimental procedure The sample growth was carried out in a molecular beam epitaxy (Riber Eva-32) system with two electron beam evaporators of Ge and Si sources. P-type Si(001) sub- strates with a resistivity of 1–10 Ω cm were used to © 2010 Cui et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.