NANO LETTERS
Synthesis and Strain Relaxation of Ge-Core/Si-Shell Nanowire Arrays
2008 Vol. 8, No. 11 4081-4086
Irene A. Goldthorpe,*,† Ann F. Marshall,‡ and Paul C. McIntyre†,‡ Materials Science and Engineering and Geballe Laboratory for AdVanced Materials, Stanford UniVersity, Stanford, California 94305 Received August 7, 2008; Revised Manuscript Received September 11, 2008
ABSTRACT Analogous to planar heteroepitaxy, misfit dislocation formation and stress-driven surface roughening can relax coherency strains in misfitting core-shell nanowires. The effects of coaxial dimensions on strain relaxation in aligned arrays of Ge-core/Si-shell nanowires are analyzed quantitatively by transmission electron microscopy and synchrotron X-ray diffraction. Relating these results to reported continuum elasticity models for coaxial nanowire heterostructures provides valuable insights into the observed interplay of roughening and dislocation-mediated strain relaxation.
Radial Ge/Si nanowire heterostructures can significantly improve the electronic and optoelectronic performance of nanowire devices and enable new device architectures. Because of the large surface-area-to-volume ratio of a nanowire, carrier scattering and recombination at the surface of a single-element semiconductor nanowire degrade carrier lifetime. For suitable choices of materials and dimensions, band offsets in a core-shell nanowire may confine carriers to the core, away from surface states, thus increasing the effective conductance. Ge-Si band alignment is such that the confinement potential is greater for holes in the core of a Ge-core/Si-shell structure than for electrons or holes in the core of a Si-core/Ge-shell structure.1 Additionally, the higher intrinsic carrier mobilities of Ge make it the more suitable material for the carrier channel, and using Si as the shell material would simplify chemical passivation of the structure because SiO2 is a much more stable and higher quality surface passivation than are Ge oxides.2 Field-effect transistors (FETs) incorporating Ge-core/Si-shell nanowire channels have been reported in the literature to have significantly better performance metrics than FETs based on single-element Ge or Si nanowires.3 A further advantageous property of epitaxial core-shell nanowires is that the core is sufficiently thin to accommodate a portion of the lattice misfit strain between silicon and germanium. Because the strain is shared between the relatively compliant core and the shell, core-shell nanowires may be able to accommodate by elastic deformation larger misfit strains than are possible in thin films on planar bulk * Corresponding author. E-mail:
[email protected]. † Materials Science and Engineering. ‡ Geballe Laboratory for Advanced Materials. 10.1021/nl802408y CCC: $40.75 Published on Web 10/28/2008
2008 American Chemical Society
substrates. The ability to tailor large strains through the coreto-shell radius ratio could potentially allow for the engineering of electronic properties such as band gap and carrier mobilities.4,5 However, only under some very limiting conditions are core-shell nanowires predicted to be able to coherently accommodate the full 4% lattice mismatch between Ge and Si.6-9 Thus, there are generally defects in the structure, which tend to compromise device performance and reduce the magnitude of residual strain. Although Ge/ Si core-shell nanowires have received much research attention,3,10-13 there has been little experimental characterization of the strain and strain-induced defects in these structures. In this paper, we first outline a process for synthesizing Ge-core/Si-shell nanowires by chemical vapor deposition (CVD) and then discuss the detailed characterization of the strain-relaxation mechanisms that occur in these structures, including measurement of the residual strain by synchrotron X-ray scattering. To be able to measure strain along particular wire directions with X-ray diffraction, alignment of the nanowires is necessary. Thus, the Ge nanowire cores were grown epitaxially from Si substrates, and uniformly sized catalysts were used so that all of the nanowire cores were of similar diameter. A HF-acidified gold colloid solution was first deposited onto a Si(111) substrate.14 Nanowires were then grown in a cold-wall CVD reactor using germane as the source gas. The germane partial pressure was maintained at 0.6 Torr in a hydrogen carrier gas at a total pressure of 30 Torr. It is desirable for the Ge nanowire cores to be untapered, so a two-step temperature profile was used:15 epitaxial nanowire nucleation was initiated at 370 °C for 110 s, and this was followed by uniform diameter elongation at 300 °C. The Ge nanowire diameters were 20 nm and larger
Figure 1. SEM images of a Ge nanowire array with Si deposited at (a) 550 °C, PSiH4 ) 0.35 Torr, (b) 440 °C, PSiH4 ) 4.5 Torr, and (c) 690 °C, PSiH4 ) 0.1 Torr.
and predominantly -oriented with six {112} side facets. The majority of the wires grew vertically from the substrate surface, with the remainder being aligned along one of the three directions which are tilted 70.5° from the substrate normal. In previously reported methods of Ge-core/Si-shell nanowire synthesis, the Si shell is deposited immediately after Ge nanowire growth.10,11 In this work, it was found that the Au remaining at the tip of the Ge nanowires after their growth presents an obstacle to the subsequent CVD of the Si shells, which is an issue not previously addressed in the literature. A conformal Si shell can be achieved at deposition temperatures between 450 and 600 °C and silane partial pressures between 0.3 and 8 Torr. These conditions, however, are similar to those used for Au-catalyzed vapor-liquid-solid (VLS) growth of Si nanowires,16 so in addition to the deposition of Si shells, Si nanowires grow from the Ge nanowire tips (Figure 1a). Using lower deposition temperatures (
