Kinetic-energy induced smoothening and delay of epitaxial breakdown in pulsed-laser deposition
TL;DR: 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 (3)
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.
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TL;DR: Detailed growth kinetics results are discussed, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness.
Abstract: Pulsed-laser deposition (PLD) is one of the most promising techniques for the formation of complex-oxide heterostructures, superlattices, and well controlled interfaces. The first part of this paper presents a review of several useful modifications of the process, including methods inspired by combinatorial approaches. We then discuss detailed growth kinetics results, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness. Time-resolved surface x-ray diffraction measurements show that crystallization and the majority of interlayer mass transport occur on timescales that are comparable to those of the plume/substrate interaction, providing direct experimental evidence that a growth regime exists in which non-thermal processes dominate PLD. This understanding shows how kinetic growth manipulation can bring PLD closer to ideal LBL than any other growth method available today.
267 citations
TL;DR: In this article, a comparison of growth morphology evolution for pulsed laser deposition and thermal deposition in the same dual-use chamber under identical thermal, background, and surface preparation conditions, and varying the kinetic energy by varying the laser fluence or using an inert background gas was made.
Abstract: This paper reviews our recent studies of the fundamentals of growth morphology evolution in Pulsed Laser Deposition in two prototypical growth modes: metal-on-insulator island growth and semiconductor homoepitaxy. By comparing morphology evolution for pulsed laser deposition and thermal deposition in the same dual-use chamber under identical thermal, background, and surface preparation conditions, and varying the kinetic energy by varying the laser fluence or using an inert background gas, we have isolated the effect of kinetic energy from that of flux pulsing in determining the differences between morphology evolution in these growth methods. In each growth mode analytical growth models and Kinetic Monte Carlo simulations for thermal deposition, modified to include kinetic energy effects, are successful at explaining much of what we observe experimentally.
80 citations
TL;DR: In this paper, the authors studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles.
Abstract: We studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. The kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. We could show that although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. We attribute this to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length.
59 citations
TL;DR: In this paper, Nanosecond pulsed laser deposition (PLD) has been used to grow nanoparticle films of Au on Si and sapphire substrates, and the equivalent solid density thickness was measured with a quartz crystal monitor and the ion flux was measured using a time-of-flight Langmuir probe.
49 citations
TL;DR: In this paper, the formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) is studied in terms of configuration, volume, and types of QDMs.
Abstract: The formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) are studied in terms of configuration, volume, and types of QDMs. QDMs are formed around self-assembled GaAs nanoscale island induced by adapting a hybrid growth approach combining droplet homoepitaxy and Stranski–Krastanov mode. In distinction from our previous results [Lee et al., Appl. Phys. Lett. 89, 202101 (2006)], hexa-QDMs are fabricated without the formation of background QDs, which can be due to a combinational effects of enhanced intermixing of Ga and In atoms, enhanced surface diffusion (high mobility) of adatoms, and higher In desorption rate due to the higher thermal energy provided during the fabrication of QDMs. In addition, a detailed evolution mechanism from bi-QDMs (two QDs per each GaAs island) to hexa-QDMs (six QDs per island) is proposed based on atom diffusion, material transfer, and equilibrium dimension (saturation) of QDs. Under a fixed InAs coverage, depending on postannealing process after ...
27 citations
References
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TL;DR: In this article, a review of low-temperature molecular-beam epitaxy in semiconductors is presented, with a focus on limited thickness epitaxy (LTE), the regime where crystalline growth over an epitaxial thickness hepi is followed by a transition to amorphous deposition.
Abstract: Low‐temperature molecular‐beam epitaxy (MBE) in semiconductors is reviewed, with a focus on limited thickness epitaxy (LTE), the regime where crystalline growth over an epitaxial thickness hepi is followed by a transition to amorphous deposition. The goal is to summarize the main results on this phenomenon, make the connection to other results on low‐temperature MBE, and present the large body of unpublished data on hepi. Since the problem is still not fully understood, all available data that have a bearing on the understanding of the effect are outlined. The scientific questions and practical problems that have driven interest in low‐temperature growth are outlined, and the phenomenon of LTE and the dependence of hepi on the growth conditions are described. The LTE effect is apparently general, but Si(100) is the model system for which most data are available. Breakdown of epitaxy follows a universal curve that is inconsistent with continuous nucleation of the amorphous phase, implying that growth is truly thickness dependent. The epitaxial thickness is thermally activated in substrate temperature T as hepi=h0 exp(−Eact/kBT), with h0 following a weak ln(R) or R1/4 dependence on deposition rate R. hepi is also strongly influenced by lattice mismatch strain, residual H in the ultrahigh vacuum, and annealing during growth interrupts. Possible mechanisms for LTE are discussed, with particular emphasis on the roles played by H and kinetic roughening, and the key experiments distinguishing these mechanisms are described. Finally, an attempt is made to draw up the best current picture of the phenomenon. It is concluded that roughening provides the fundamental limit to epitaxy at low temperature, but with H contamination playing an important part in controlling surface diffusion: outstanding problems include the rate dependence and the details of the roughening behavior.
178 citations
TL;DR: In this article, the authors show that film microstructure is one of the main factors, determining long-term photoluminescence (PL) properties, and that films with different porosity exhibit different peak energies, integral intensities and time-dependent evolutions.
Abstract: Pulsed laser ablation in an inert gas has been used to fabricate films containing silicon nanocrystals. We show that film microstructure is one of the main factors, determining long-term photoluminescence (PL) properties. Films with different porosity were found to exhibit PL signals with quite different peak energies, integral intensities and time-dependent evolutions. The distinction of these PL properties is attributed to the different efficiency of surface chemistry interactions between Si nanocrystallites and the ambient atmosphere for films having different porosities. Oxygen-related defects and other mechanisms are discussed to explain the PL properties of the films.
75 citations
TL;DR: In this paper, the photoluminescence (PL) of silicon nanocrystals due to the quantum confinement effect (QCE) and interface states was investigated, and the peak position and intensity of the PL band at 1.8-2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing.
Abstract: We have investigated the different mechanisms of photoluminescence (PL) of silicon nanocrystals due to the quantum confinement effect (QCE) and interface states. Si nanocrystals were formed by pulsed-laser deposition in inert argon and reactive oxygen gas. The collisions between the ejected species greatly influence the morphology of the Si nanocrystals and cause a transition from a film structure to a porous cauliflowerlike structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The oxygen content of the Si nanocrystals increases with increasing O2 ambient pressure, and nearly SiO2 stoichiometry is obtained when the O2 pressure is higher than 100 mTorr. Broad PL spectra are observed from Si nanocrystals. The peak position and intensity of the PL band at 1.8–2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing. The PL band at 2.55 eV shows vibronic structures with periodic spacing of 97±9 meV, while no peak shift is found...
70 citations
IBM1
TL;DR: The results of this study suggest that two-dimensional layer growth can be induced by the combined use of atomic oxygen and growth conditions, such as low deposition rate, low oxygen partial pressure (2 mTorr), that produce low supersaturation at the growth front.
Abstract: The basic growth mode of a thin epitaxial cuprate film (200 A\r{}) on a given substrate depends sensitively on the balance between various thermodynamic and kinetic factors related to the high-${\mathit{T}}_{\mathit{c}}$ phase formation and the surface microstructure at the growth front of the deposited film. Under the standard optimized growth conditions for high-quality epitaxial films, the deposition of a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ film on an atomically smooth (110) ${\mathrm{SrTiO}}_{3}$ substrate, for example, is characterized by a strong damping in the reflection high-energy electron diffraction (RHEED) oscillation suggesting a predominant island growth mode. We have demonstrated that with an atomic oxygen and the technique of RHEED-controlled growth interruption it is possible to minimize surface roughness and to fabricate unit-cell smooth ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films over a large area (\ensuremath{\sim}0.5 cm\ifmmode\times\else\texttimes\fi{}1 cm). The results of this study suggest that two-dimensional layer growth can be induced by the combined use of atomic oxygen and growth conditions, such as low deposition rate, low oxygen partial pressure (2 mTorr), that produce low supersaturation at the growth front.
64 citations
TL;DR: In this paper, pulsed-laser deposition (PLD) was used to grow ultrathin Fe films on Cu(111) whose instantaneous deposition rate is about six orders of magnitude larger than that of the conventional molecular-beam epitaxy based on thermal deposition (TD).
Abstract: Ultrathin Fe films on Cu(111) have been grown by pulsed-laser deposition (PLD) whose instantaneous deposition rate is about six orders of magnitude larger than that of the conventional molecular-beam epitaxy based on thermal deposition (TD). Compared to the TD prepared Fe/Cu(111) films, the PLD films have a significantly improved layer-by-layer morphology and a substantially enhanced stability of the fcc phase as characterized by scanning tunneling microscopy and electron-diffraction techniques. The magnetic properties of the PLD films, investigated by magneto-optical Kerr effect, also show remarkably different behavior as compared to the TD films. At low thickness (3 ML), while the TD films are characterized by a low net moment and a short-range ferromagnetic order, the PLD films show a high net moment with a true long-range ferromagnetic order. Above 3 ML both PLD and TD films undergo a magnetic transition though with apparently different structural origin. We discuss these results based on the different growth and structure between the PLD and TD grown Fe films.
62 citations