Kinetic-energy induced smoothening and delay of epitaxial breakdown in pulsed-laser deposition

TLDR: In this paper, the effect of kinetic energy of depositing species from flux pulsing during pulsed-laser deposition (PLD) on surface morphology evolution of Ge(001) homoepitaxy at low temperature was isolated.

Abstract: We have isolated the effect of kinetic energy of depositing species from the effect of flux pulsing during pulsed-laser deposition (PLD) on surface morphology evolution of Ge(001) homoepitaxy at low temperature $(100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$. Using a dual molecular beam epitaxy (MBE) PLD chamber, we compare morphology evolution from three different growth methods under identical experimental conditions except for the differing nature of the depositing flux: (a) PLD with average kinetic energy $300\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (PLD-KE); (b) PLD with suppressed kinetic energy comparable to thermal evaporation energy (PLD-TH); and (c) MBE. The thicknesses at which epitaxial breakdown occurs are ranked in the order $\mathrm{PLD}\text{\ensuremath{-}}\mathrm{KE}g\mathrm{MBE}g\mathrm{PLD}\text{\ensuremath{-}}\mathrm{TH}$; additionally, the surface is smoother in PLD-KE than in MBE. The surface roughness of the films grown by PLD-TH cannot be compared due to the early epitaxial breakdown. These results demonstrate convincingly that kinetic energy is more important than flux pulsing in the enhancement of epitaxial growth, i.e., the reduction in roughness and the delay of epitaxial breakdown.

Figures

Fig. 4. (a) RHEED specular intensity variations during PLD-KE with 1 Hz (dashed line) and 2 Hz (solid line) of repetition rate while keeping instantaneous flux the same in both cases. Time axes are adjusted such that intensity minima and maxima for both cases are aligned at the same positions on abscissa. (b) First 20 seconds of RHEED intensity variation of PLD-KE with 1 Hz from (a). Modulations from individual laser pulses are visible.

Fig. 2. (Color online) AFM images of films grown at 100 °C by (a) MBE, (b) PLD-KE, and (c) PLD-TH, and (d) RHEED pattern taken from surface shown in (c). Scan size and vertical scale of (a) – (c) are 0.25 x 0.25 μm2 and 5 nm, respectively. Thickness of films in (a) – (c) is shown in the left bottom corner of each image.

Table I. Epitaxial thickness of PLD-KE, PLD-TH, and MBE at 100 °C and 150 °C. In the case of PLD-KE, the thickest samples – 270 nm at 100 °C and 410 nm at 150 °C – are still fully epitaxial.