Effects of supersaturation on the crystal structure of gold seeded III–V nanowires 1 Jonas Johansson, 2 Lisa S. Karlsson, 1 Kimberly A. Dick, 1 Jessica Eriksson, 1 Brent A. Wacaser, 1 Knut Deppert, and 1 Lars Samuelson 1 Solid State Physics and the Nanometer Structure Consortium, Lund University, Sweden 2 National Centre for High Resolution Electron Microscopy (nCHREM) / Polymer & Materials Chemistry, Lund University, Sweden

Outline Introduction Experimental Nanowire growth results Nucleation model Conclusions

Introduction High density of planar defects in III-V nanowires with zinc blende structure. This can be tuned by growth conditions.* ** Can we radically change the crystalline properties of gold seeded III–V nanowires by growth at different conditions: –Pulsed growth –Altered seed particle Can we find growth conditions that let us grow pure zinc blende or wurtzite wires? Can we also find a sharper condition for wurtzite formation in zinc blende nanowires than what has recently been proposed?*** *J Johansson et al, Nat Mater, (2006): GaP at 440, 470, and 500°C **H Joyce et al, Nano Lett, (2007): GaAs at °C ***F Glas et al, Phys Rev Lett, (2007): ΔG WZ < ΔG ZB

Stacking sequences Zinc blende: ABC… Wurtzite: AB… fault plane (twinning) fault plane (wurtzite formation) zinc blende

Experimental Growth of GaP(111)B nanowires on GaP(111)B substrates in MOVPE at 470°C. Growth seeded by aerosol fabricated gold particles, 40 nm diameter. Precursors: TMG and PH 3. In contaminated or clean (In free) susceptor. Pulsed growth or continuous growth. Pulsed growth: –TMG: 10 s ON, 1 min OFF –20 pulses –PH 3 always ON HR TEM for structural characterization

Nanowire growth results Pulsed growth In background Continuous growth In background Continuous growth In free background Extended zinc blende (ZB) segments Average: 20 ML Maximum: 200 ML Short ZB segments (lamellar twinning) Average: 3.5 ML Wurtzite (WZ) segments Average: 3.5 ML Maximum: 21 ML

Composition of the seed alloy particle EDS measurements of the Ga and In concentrations in the gold alloy particle Two different growth terminations Cooling down in –H 2 : Ga concentration during growth, C –PH 3 + H 2 : Ga concentration at eqiulibrium, C eq Conclusion: higher supersaturation during In free growth –In background:C/C eq ≈ 1–3 –In free:C/C eq ≈ ~10 In backgroundIn free background H2H2 PH 3 H2H2 In (at-%)20 – 30 0 – 5 Ga (at-%)0020 – 260 Detection limit ≈ 3 at-% (0 means less than 3 at-%)

Schematics of pulsing

Growth at different supersaturations Pulsed growth In background Continuous growth In background Continuous growth In free background Extended zinc blende (ZB) segments Short ZB segments (lamellar twinning) Extended wurtzite (WZ) segments lowSupersaturation ( , C/C eq )high

Nucleation description Two assumptions for the nucleation model: –(i) Layer-by-layer growth, monocenter nucleation at wire edge –(ii) Poissonian nucleation Justifications –(i) Perfect atomic planes in the wires (no grain boundaries), very low P solubility in Au –(ii) = 4.2 ML

2D nucleation model 0 for ordinary plane  t for fault plane Step energy

Finding the nucleation barrier for ordinary and fault plane nucleation Nucleation barrier r ΔG(r) ΔG* r*

Possibility for wurtzite formation? Fault plane nucleation is favoured:* Can happen if Γ f < Γ –Inner step energies same: γ i f = γ i –Edge step energies differ: γ e f < γ e Rewrite ΔG-inequality: –Not much data… If step energies follow surface energies, for ZnS f ≈ 0.6–0.7 (WZ σ av = 0.57 J/m 2, ZB σ av = 0.86 J/m 2 )** Is this relation sufficient for the wurtzite structure to form? **H Zhang et al, J Phys Chem B, (2003)*F Glas et al, PRL, (2007)

Nucleation probabilities Nucleation rates: Fault plane nucleation probability

p f versus  (Edge step energy ratio)

Poissonian nucleation Can we relate the fault plane nucleation probability, p f, to segment thicknesses? Poissonian nucleation: the (geometric) probability distribution of nucleating exactly k fault planes, that is a k ML thick WZ segment, before an ordinary nucleus forms The average WZ segment thickness (the average value of k)

Interpretation of growth results s wz = 3.5, p f = 0.78 s ZB = 3.5, p f = 0.22 s ZB = 20, p f = 0.05 Pulsing Continuous In-free

Condition for WZ formation revisited Fault plane nucleation is favoured when Does this mean that WZ formation will occur? This is a matter of definition… By combining the equations for p f and s wz, we can pose a stronger condition for WZ formation:

Conclusions Pulsed growth – low supersaturation –zinc blende favoured Growth at high supersaturation –wurtzite favoured Qualitative nucleation model –planar defect density along wire –zinc blende / wurtzite occurence MOVPE of gold seeded GaP B oriented nanowires: