Diffusional kinetics of SiGe dimers on Si(100) using atom-tracking scanning tunneling microscopy

Ultrafast Dynamics Revealed with Time-Resolved Scanning Tunneling Microscopy: A Review

A scanning tunneling microscope (STM) capable of performing pump-probe spectroscopy integrates unmatched atomic-scale resolution with high temporal resolution. In recent years, the union of electronic, terahertz, or visible/near-infrared pulses with STM has contributed to our understanding of the atomic-scale processes that happen between milliseconds and attoseconds. This time-resolved STM (TR-STM) technique is evolving into an unparalleled approach for exploring the ultrafast nuclear, electronic, or spin dynamics of molecules, low-dimensional structures, and material surfaces. Here, we review the recent advancements in TR-STM; survey its application in measuring the dynamics of three distinct systems, nucleus, electron, and spin; and report the studies on these transient processes in a series of materials. Besides the discussion on state-of-the-art techniques, we also highlight several emerging research topics about the ultrafast processes in nanoscale objects where we anticipate that the TR-STM can help broaden our knowledge.

Toward precise site-controlling of self-assembled Ge quantum dots on Si microdisks

A feasible route is developed toward precise site-controlling of quantum dots (QDs) at the microdisk periphery, where most microdisk cavity modes are located. The preferential growth of self-assembled Ge QDs at the periphery of Si microdisks is discovered. Moreover, both the height and linear density of Ge QDs can be controlled by tuning the amount of deposited Ge and the microdisk size. The inherent mechanisms of these unique features are discussed, taking into account both the growth kinetics and thermodynamics. By growing Ge on the innovative Si microdisks with small protrusions at the disk periphery, the positioning of Ge QDs at the periphery can be exactly predetermined. Such a precise site-controlling of Ge QDs at the periphery enables the location of the QD right at the field antinodes of the cavity mode of the Si microdisk, thereby achieving spatial matching between QD and cavity mode. These results open a promising door to realize the semiconductor QD-microdisk systems with both spectral and spatial matching between QDs and microdisk cavity modes, which will be the promising candidates for exploring the fundamental features of cavity quantum electrodynamics and the innovative optoelectronic devices based on strong light-matter interaction.

Structural properties of templated Ge quantum dot arrays: impact of growth and pre-pattern parameters

In this study we analyze the impact of process and growth parameters on the structural properties of germanium (Ge) quantum dot (QD) arrays. The arrays were deposited by molecular-beam epitaxy on pre-patterned silicon (Si) substrates. Periodic arrays of pits with diameters between 120 and 20 nm and pitches ranging from 200 nm down to 40 nm were etched into the substrate prior to growth. The structural perfection of the two-dimensional QD arrays was evaluated based on SEM images. The impact of two processing steps on the directed self-assembly of Ge QD arrays is investigated. First, a thin Si buffer layer grown on a pre-patterned substrate reshapes the pre-pattern pits and determines the nucleation and initial shape of the QDs. Subsequently, the deposition parameters of the Ge define the overall shape and uniformity of the QDs. In particular, the growth temperature and the deposition rate are relevant and need to be optimized according to the design of the pre-pattern. Applying this knowledge, we are able to fabricate regular arrays of pyramid shaped QDs with dot densities up to 7.2 × 10¹⁰ cm^-2.

Adsorption and dynamics of group IV, V atoms and molecular oxygen on semiconductor group IV (0 0 1) surfaces

In this review we address (1) the co-adsorption of group V (As, Sb, Bi) atoms and molecular oxygen on the Si(0 0 1) surface and (2) the adsorption and dynamics of Sb, Bi, Si and Ge ad-dimers on the Si(0 0 1) and Ge(0 0 1) surfaces. The adsorption and diffusion processes of group IV and V atoms on the (0 0 1) surfaces of group IV semiconductor surfaces have been studied using multi-configuration self-consistent field methods and density functional theory calculations. Results obtained by various types of first-principle total energy calculations are mutually compared and discussed. Our results demonstrate the capability of these quantum chemistry methods to provide relevant and reliable information on the interaction between adsorbate and semiconductor surfaces.

Optimal growth of Ge-rich dots on Si(001) substrates with hexagonal packed pit patterns

This paper reports a three-step method to fabricate hexagonal ordered Ge dots on Si with controllable size and spacing. After the introduction of a thin Si dioxide layer on the Si substrate, porous alumina turns out to be a good candidate for pattern transferring, which is rapid and simple to implement. A density-temperature relation for Ge dots has been discovered in this work; the Arrhenius relation with a slope of 0.33 is proved to be applicable for predicting the optimal temperature for a certain density of patterns. Different widths of pits are also studied to discover the dependence of the dot distribution on the pit morphology. The optimal pit width for ordered Ge dots is around 30 nm, while four aligned Ge dots can be achieved in a 70 nm pit. Extremely high Ge content (>0.92) in capped Ge dots is discovered by Raman characterization because the high density of pits leads to a low enough optimal temperature of 430 °C. The photoluminescence spectra of the capped dots also prove the high purity and quality of the Ge dots.

Dimer motion on a periodic substrate: spontaneous symmetry breaking and absolute negative mobility

We consider two coupled particles moving along a periodic substrate potential with negligible inertia effects (overdamped limit). Even when the particles are identical and the substrate spatially symmetric, a sinusoidal external driving of appropriate amplitude and frequency may lead to spontaneous symmetry breaking in the form of a permanent directed motion of the dimer. Thermal noise restores ergodicity and thus zero net velocity, but entails arbitrarily fast diffusion of the dimer for sufficiently weak noise. Moreover, upon application of a static bias force, the dimer exhibits a motion opposite to that force (absolute negative mobility). The key requirement for all these effects is a nonconvex interaction potential of the two particles.

Flexible drift-compensation system for precise 3D force mapping in severe drift environments

The acquisition of dense 3D data sets is of great importance, but also a challenge for scanning probe microscopy (SPM). Thermal drift often induces severe distortions in the data, which usually constrains the acquisition of dense data sets to experiments under ultra-high vacuum and low-temperature conditions. Atom tracking is an elegant approach to compensate for thermal drift and to position the microscope tip with highest precision. Here, we present a flexible drift compensation system which can easily be connected to existing SPM hardware. Furthermore, we describe a 3D data acquisition and position correction protocol, which is capable of handling large and non-linear drift as typically present in room temperature measurements. This protocol is based on atom-tracking for precise positioning of the tip and we are able to acquire dense 3D data sets over several hours at room temperature. The performance of the protocol is demonstrated by presenting 3D data taken on a CaCO(3)(10 ̅14) surface with the data density being as large as 85×85×500 pixel.

Particle diode: rectification of interacting Brownian ratchets

Transport of Brownian particles interacting with each other via the Morse potential is investigated in the presence of an ac driving force applied locally at one end of the chain. By using numerical simulations, we find that the system can behave as a particle diode for both overdamped and underdamped cases. For low frequencies, the transport from the free end to the ac acting end is prohibited, while the transport from the ac acting end to the free end is permitted. However, the polarity of the particle diode will reverse for medium frequencies. There exists an optimal value of the well depth of the interaction potential at which the average velocity takes its maximum. The average velocity υ decreases monotonically with the system size N by a power law υ ∝ N(-1).

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650 °C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.

Ge diffusion at the Si(100) surface

Using scanning tunneling microscopy movies, we directly observe individual embedded Ge atoms to be mobile within the Si(100)-(2x1)-Ge surface at temperatures as low as 90 degrees C. We demonstrate that Ge atoms move by exchange diffusion with (1) adsorbed monomers and (2) individual constituent atoms of adsorbed dimers. Our observations are consistent with recent density-functional theory calculations, which give the atomistic pathways and energetic barriers for both exchange mechanisms. We find that neither adsorbed monomers nor dimers can diffuse more than a few nanometers between exchange events, illustrating how Ge diffusion and intermixing are intimately coupled at the nanoscale on the Si(100) surface.

Interaction-controlled Brownian motion in a tilted periodic potential

The drift and diffusion of a few interacting, overdamped Brownian particles in a tilted periodic potential are studied analytically and numerically. Both quantities exhibit a complex multipeaked structure as a function of the equilibrium interparticle separation. Upon variation of the interaction strength, both drift and diffusion may exhibit a nonmonotonic, resonancelike behavior.

Kinetically suppressed ostwald ripening of Ge/Si(100) hut clusters

Low area density Ge/Si(100) hut cluster ensembles are stable during days-long growth temperature anneals. Real-time scanning tunneling microscopy shows that all islands grow slowly at a decreasing rate throughout the anneal. Island growth depletes the Ge supersaturation that, in turn, reduces the island growth rate. A mean-field facet nucleation and growth model quantitatively predicts the observed growth rate. It shows that Ostwald ripening is kinetically suppressed for Ge supersaturations high enough to support a critical nucleus size less than the smallest facet.

Dynamics of a dimer in a symmetric potential: ratchet effect generated by an internal degree of freedom

The one-dimensional dynamics of a dimer consisting of two harmonically coupled components is considered. The mutual distance between the dimer components plays the role of an internal degree of freedom. Both components are in contact with the same heat bath and are coupled to a spatially periodic, symmetric potential, whose amplitude is modulated periodically in time and whose coupling strength is different for the two components. In the absence of any external bias, a ratchet effect (directed transport) arises generically unless the mutual coupling of the dimer components tends to zero or infinity. In other words, the ratchet effect is generated by the internal degree of freedom. An accurate analytical approximation for the dimer's velocity and diffusion coefficient is obtained. The velocity of the system is maximized by adding an optimal amount of noise and by tuning the driving frequency to an optimal value. Furthermore, there exists an optimal coupling strength at which the velocity is the largest.

Active drift compensation applied to nanorod manipulation with an atomic force microscope

We have developed a simple algorithm to overcome the problem of thermal drift in an atomic force microscope (AFM) operating under ambient conditions. Using our method, we demonstrate that the AFM tip remains above a 5-nm-high and 50-nm-long CdSe nanorod for more than 90 min despite the thermal drift present (6 nm/min). We have applied our drift compensation technique to the AFM manipulation of CdSe colloidal nanorods lying horizontally on a highly oriented pyrolytic graphite surface. Since we have precise control over the position of the AFM tip relative to the nanorod, we can choose to either translate or rotate the rod by changing the location of the tip-rod interaction point.

Roughening rates of strained-layer instabilities

We study the evolution of the morphology of Si0.75Ge0.25 strained layers using a wide range of deposition times, 60<tau<2400 s, at 600 degrees C on laser textured substrates with miscuts theta<15 degrees off Si(001). Ripple-shaped morphologies form spontaneously on miscuts along the 110 directions. At the shortest deposition times, roughening is suppressed as predicted by a linear stability analysis that uses previously measured values for the mass transport rate on the surface. The measured time constant of the roughening is approximately 80 s, a factor of 4 larger than predicted by theory.

Hydrogen desorption kinetics from the Si(1−x)Gex(100)-(2×1) surface

We study the influence of germanium atoms upon molecular hydrogen desorption energetics using density functional cluster calculations. A three-dimer cluster is used to model the Si((1-x))Ge(x)(100)-(2x1) surface. The relative stabilities of the various monohydride and clean surface configurations are computed. We also compute the energy barriers for desorption from silicon, germanium, and mixed dimers with various neighboring configurations of silicon and germanium atoms. Our results indicate that there are two desorption channels from mixed dimers, one with an energy barrier close to that for desorption from germanium dimers and one with an energy barrier close to that for desorption from silicon dimers. Coupled with the preferential formation of mixed dimers over silicon or germanium dimers on the surface, our results suggest that the low barrier mixed dimer channel plays an important role in hydrogen desorption from silicon-germanium surfaces. A simple kinetics model is used to show that reasonable thermal desorption spectra result from incorporating this channel into the mechanism for hydrogen desorption. Our results help to resolve the discrepancy between the surface germanium coverage found from thermal desorption spectra analysis, and the results of composition measurements using photoemission experiments. We also find from our cluster calculations that germanium dimers exert little influence upon the hydrogen desorption barriers of neighboring silicon or germanium dimers. However, a relatively larger effect upon the desorption barrier is observed in our calculations when germanium atoms are present in the second layer.

Changing the diffusion mechanism of Ge-Si dimers on Si(001) using an electric field

We change the diffusion mechanism of adsorbed Ge-Si dimers on Si(001) using the electric field of a scanning tunneling microscope tip. By comparing the measured field dependence with first-principles calculations we conclude that, in negative field, i.e., when electrons are attracted towards the vacuum, the dimer diffuses as a unit, rotating as it translates, whereas, in positive field the dimer bond is substantially stretched at the transition state as it slides along the substrate. Furthermore, the active mechanism in positive fields facilitates intermixing of Ge in the Si lattice, whereas intermixing is suppressed in negative fields.

Surface segregation of Ge at SiGe(001) by concerted exchange pathways

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.

Unique dynamic appearance of a Ge-Si ad-dimer on Si(001)

We carry out a comparative study of the energetics and dynamics of Si-Si, Ge-Ge, and Ge-Si ad-dimers on top of a dimer row in the Si(001) surface, using first-principles calculations. The dynamic appearance of a Ge-Si dimer is distinctively different from that of a Si-Si or Ge-Ge dimer, providing a unique way for its identification by scanning tunneling microscopy (STM). Its "rocking" motion, observed in STM, actually reflects a 180 degrees rotation of the dimer, involving a piecewise-rotation mechanism. The calculated energy barrier of 0.74 eV is in good agreement with the experimental value of 0.82 eV.