Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.
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D.J., S.L. and K.A.D. acknowledge financial support from the Knut and Alice Wallenberg Foundation (KAW), the Swedish Research Council (VR) and the Nanometer Structure Consortium at Lund University (nmC@LU). F.P. and S.H. acknowledge support from ERC Grant 279342: InSituNANO. We acknowledge A. Ellis for technical support.
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Extended data figures and tables
a, Dark-field images recorded using three spots in the diffraction pattern showing how WZ and the two variants of ZB (ZB1 and ZB2) are distinguished in the 〈110〉 direction. The right panel shows a bright-field (BF) image for comparison. b, Diffraction pattern with the three spots indicated. These are post-growth images recorded in a JEOL 3000 TEM. c, Image recorded during growth, also in the 〈110〉 direction, showing the dark-field contrast and the measured angle of the truncated corner. d, A schematic showing the hexagonal cross-section as seen from the side (beam direction) and from above. Seen from the side, the droplet contact angle ϕ and corner-facet angle θ are shown. Seen from above, the electron beam direction is shown.
a, Step-flow kinetics measured for the WZ nanowire shown in Supplementary Video 1 and Fig. 1a. b, Length of a WZ nanowire versus the number of step-flow events. The average gradient of the graph confirms that each step is 0.33 nm in height, that is, a WZ(0001) bilayer. The red line shows the calculated length, assuming that each new bilayer (red squares) adds 0.33 nm in length. The black data are measured lengths, with error bars defined by the standard deviation of each subset (length measured in 3–4 different movie frames for each data point). c, Growth rate versus AsH3 pressure, estimated by measuring the increase in the length of the nanowire shown in Fig. 2 during growth intervals at different AsH3 pressures, and by fitting a linear function to the length versus time plot. Error bars are defined by the standard error of the fits. The growth temperature was 550 °C and TMGa pressure was constant at 3.5 × 10−8 Torr.
Extended Data Figure 3 Droplet angle at the transition does not change with Au composition in the droplet.
a–c, During growth, the crystal structure of a nanowire was switched back and forth several times by changing the AsH3 pressure. Data shows the measured angles and absolute droplet volumes at which the switch from WZ to ZB (red) and ZB to WZ (blue) occurred (a, b). During this time the wire also grew radially (c). a, Switch angles versus nanowire diameter. Shading indicates the range of observed angles at which WZ switches to ZB (red) and ZB to WZ (blue). Some hysteresis in switching is visible, perhaps because the droplet angle continues to change in the time before the switched layer grows. The data are scattered, but there is no strong dependence of angle on diameter (especially for the blue data points). For example, ZB switches to WZ at angles between 123° and 132° at both large and small diameter, and WZ switches to ZB at angles between 127° and 136°. b, Switch angle versus inferred composition, as calculated from the measured h/d ratio assuming that the amount of Au does not change. The data are scattered, but there is no strong dependence of angle on composition. For example, for the first few data points, for which the nanowire had a diameter of 30 mn, ZB switched to WZ for a Ga fraction of less than 60%. For subsequent data points, for which the nanowire had a diameter of 40 nm, the switch did not occur until the composition had a Ga fraction of about 75%. The fact that droplet angles are similar despite the change in diameter suggests that droplet geometry controls the switch, whereas droplet volume and composition do not appear to be important.
Dark-field conditions, speeded up 60×, field of view 100 nm, nanowire diameter 55 nm; scale bar 10 nm. T = 550 °C, AsH3 = 1 × 10−5 Torr, TMGa = 3.5 × 10−8 Torr. Sharp corners are visible with slow step flow. Steps originate from both sides of the nanowire. The adjacent graph shows the step position as a function of time. (AVI 13985 kb)
The video starts with a large droplet, which decreases in size as the AsH3 pressure is increased. For the first ¾ of the movie h/d is large enough that ZB grows, showing truncated corners that slowly fill in, then jump back again as the step flows. Four layers of ZB can be seen growing in this way. The droplet shrinks continuously and after the fourth layer, growth continues in the WZ structure with no further corner truncation. The graph shows h/d and ϕ, as well as the AsH3 pressure and the time of each facet truncation, marked with an arrow. Dark-field conditions, speeded up 30×, field of view 100 nm, nanowire diameter 30 nm; scale bar 10 nm; T=550 °C, AsH3 = 1.4 × 10−5 Torr, TMGa = *2.0 × 10−8 Torr. (AVI 9481 kb)
Bright-field conditions, speeded up 60×, field of view 120 nm, nanowire diameter 35 nm. The initial AsH3 = 1.1 × 10-5 Torr drops to 1 × 10-9 Torr; TMGa = 2 × 10-8 Torr, T = 550 °C. Scale bar 10 nm. Inset is a graph showing pressure and droplet volume from each frame. (AVI 18648 kb)
The video starts with WZ growth at low h/d ratio. Layer growth appears as slightly brighter contrast at the growth interface (although it is indistinct due to the imaging conditions). Then AsH3 is reduced for a short time to trigger ZB growth. The first ZB layer that forms appears as a bright line 7 s after the start of the video. As it appears, a truncated facet forms which cuts down into the WZ below. Three ZB layers grow in this way, at 7, 10 and 11 s. The growth rate increases because AsH3 has already been increased again during this time. Eventually growth transitions back to WZ. The segment of WZ is not clearly visible in the images until 19 s into the video, when the imaging conditions are changed and it shows as a brighter band above the ZB, which now appears as a darker band. Speeded up 30×, field of view 90 nm, nanowire diameter 30 nm. Initially, AsH3 = 1.4 × 10−5 Torr, then lowered to 1 × 10−7 Torr then returned to 1.4 × 10−5; TMGa = 2 × 10−8 Torr, T = 550 °C. Scale bar 10 nm. (AVI 5347 kb)
Dark-field conditions. The first nanowire is shown speeded up 6×, field of view 100 nm, nanowire diameter ~50 nm. AsH3 = (1.3–1.4) × 10−5 Torr; TMGa = 2 × 10−8 Torr, T = 500 °C. Five layers grow. The corner truncation is more visible on the left hand side, but its size changes simultaneously on both sides. The second wire is shown speeded up 6×, field of view 100 nm, nanowire diameter ~50 nm. AsH3 = 1.5 × 10−5 Torr; TMGa = 2 × 10−7 Torr, T = 550 °C. Two layers grow. The contrast is weaker, but a symmetrical oscillation is visible on both sides. Scale bars 10 nm. (AVI 15575 kb)
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Jacobsson, D., Panciera, F., Tersoff, J. et al. Interface dynamics and crystal phase switching in GaAs nanowires. Nature 531, 317–322 (2016). https://doi.org/10.1038/nature17148