Direct imaging of quantum wires nucleated at diatomic steps

Abstract

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place preferentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2790483͔Atomic steps located at the surfaces of substrates play a central role in controlling the growth mechanism of a wide range of materials. The effect of such steps on the growth process, the morphology, the stress and strain distributions, and the functionality of the materials has been extensively investigated. [1][2][3][4][5][6][7][8][9][10][11] In particular, these steps are thought to constitute a heterogeneous source of nucleation for the formation of nano-objects and structural defects at the initial stages of the growth of many semiconductor nanostructures. 12,13 It is critically important to identify the nucleation sources for individual nano-objects such as quantum dots and wires because they constitute the fundamental blocks of future nanoelectronics and nanophotonics devices. 14 The ability to control the nucleation of these nano-objects will contribute significantly to the development of reliable single-photon sources for applications in quantum information technology. [15][16][17] Nanowires constitute a special case of self-assembled nano-objects. 18 Self-assembled nano-objects can be formed spontaneously via the Stranski-Krastanov growth mode for certain semiconductor materials when a few monolayers are epitaxially deposited on a lattice-mismatched substrate. 19,20 Strain is widely accepted to be the driving force for the nucleation of these nano-objects, but as yet there has been no direct experimental evidence for the role of steps, and their associated stress enhancement, in the self-organized growth of nanowire arrays. 21,22 In this letter we show the direct imaging, by aberrationcorrected Z-contrast scanning transmission electron microscopy ͑STEM͒ and atomic force microscopy, that, in the presence of misfit strain, nanowires preferentially nucleate on the upper terrace of a diatomic step. With finite element elasticity calculations, we demonstrate that the driving force is the stress relaxation at the upper terrace of a diatomic step, which explains the observed nucleation site of semiconductor nanowires.The nanowires investigated consist of InAs͑P͒ selfassembled quantum wires grown by solid source molecular beam epitaxy ͑MBE͒ on InP ͑001͒ substrates. The lattice mismatch between InAs and InP is 3.2%. The control of the relaxation process of these nanostructures has recently been followed with high precision by in situ accumulated stress measurements from the initial phases of the self-assembly process. 23 The process of format...

Exploring semiconductor quantum dots and wires by high resolution electron microscopy

“…This explanation is supported by the analysis of high-resolution Z-contrast images of nanowires located just on InAs x P 1-x /InP (001) interfacial steps. This analysis shows that InAs-rich regions are situated on the upper terrace of interfacial diatomic steps, in good agreement with AFM observations and strain calculations [20]. In order to produce efficient materials from InAs x P 1-x /InP nanowires, it is important to control their size, shape, composition, distribution and density.…”

“…The application of these approaches in combination with other analytical methods has allowed us to successfully predict the nucleation sites of stacked quantum wires for different growth conditions [18][19], learning about the nucleation mechanism of these nanostructures [20] and the defect nucleation mechanism in quantum dots [21]. Good correlation has been found in these nano-wires between the strain maps obtained directly from the strain analysis of HRTEM images and the strain calculated by finite element methods using the compositional maps determined by HAADF-STEM and electron energy-loss spectroscopy (EELS) [22].…”

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems

“…Atomic force microscopy examination of the nanowires deposited on an InP substrate showed the tendency for the nanowires to be located near step edges. While many of the steps were wavy, some wires nucleated near straight segments of steps, and these gave a clear sharp image in cross-section Z-contrast images, as shown in figure 14 (Molina et al 2007). In this 1 1 0 projection, the cubic zinc-blende structure projects as dumbbells with the In column towards the left of the figure and the P or As x P 1−x column towards the right.…”

“…(b) Higher magnification view showing the atomically abrupt transition from the InP to the InAs x P 1−x nanowire, with an interface step present under the nanowire. Reproduced from Molina et al (2007).…”

High resolution STEM of quantum dots and quantum wires

“…Similarly, direct HAADF STEM imaging of self-assembled InAs/InP QWRs, combined with AFM and finite element elasticity calculations were used by Molina et al (2007a) to investigate the role of monolayer steps at the surface of the substrate on the nucleation of QWRs. Preferential nucleation of self-assembled QDs at monolayer terrace edges of the substrate had previously been observed (Leonard et al, 1994b;Xu et al, 2005).…”