Comparison of Growth Morphology in Ge (001) Homoepitaxy Using Pulsed Laser Deposition and MBE
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Comparison of Growth Morphology in Ge (001) Homoepitaxy Using Pulsed Laser Deposition and MBE John P. Leonard, Byungha Shin, James W. McCamy, Michael J. Aziz Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. ABSTRACT Differences in the homoepitaxy of Ge(001) are explored using a dual MBE/PLD deposition system. With identical substrate preparation, temperature calibration, background pressure and analysis, the system provides a unique comparison of the processes arising only from kinetic differences in the flux and at the surface. All films show mounded growth. At substrate temperatures below 200ºC, PLD films are smoother than MBE films, whereas they are similar at higher temperatures. INTRODUCTION Molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) are generally viewed as techniques capable of producing high-quality epitaxial films. Although both processes typically involve vapor phase deposition of a monomer flux onto a substrate, there are important differences associated with the generation and characteristics of the beam. In traditional MBE the beam is produced by thermal evaporation of a liquid or solid source, yielding a steady state flux with low energy typically 1 eV or less. In PLD a pulsed laser beam is focused onto a target from which, through complex interactions involving melting and plasma formation, a plume of material is ejected. Under typical conditions, the plume contains a partially ionized population of monomers, with a broad distribution of energy ranging from ten to a few hundred eV, as shown in Figure 1b. Deposition of material on a substrate placed in or near this plume occurs in microsecond pulses of high instantaneous flux, often orders of magnitude higher than with steady-state MBE. (b)
(a) µs
Normalized flux
106 PLD
1
MBE 0.1
0 Time (sec)
Figure 1. a) Schematic comparison of deposition rates in PLD and MBE. b) Typical characteristics of flux in PLD of germanium, adapted from Franghiadakis et al. [1].
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Significant experimental and theoretical work over the last few decades has provided a good understanding of steady-state MBE growth. Successful kinetic models involve the interplay of the competing diffusional processes for roughening and smoothening. In the case of PLD, the additional complexities of a broad energy distribution and transient effects associated with the pulsed flux have limited the current understanding. Several ‘enhanced’ MBE techniques have recently been developed which suggest that an energetic and pulsed flux can have important effects on epitaxy: 1) Interrupted (or pulsed) MBE which uses a modulation of the flux to control the supersaturation of monomers on a surface, thereby changing island nucleation rates to produce smoother films [2]. 2) Ion-assisted MBE that incorporates low-energy (30-100 eV) ion bombardment during deposition to affect island nucleation, impurity pinning, vacancy generation and subsurface recoil implantation [3]. 3) Ultrahigh vacuum (UHV) sputtering to produce an energet
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