Real-time Reflection Research Articles

Mechanism of Quantum Dot Formation by Postgrowth Annealing of Wetting Layer

Low density InAs/GaAs quantum dots (QDs) can be formed by postgrowth annealing of an InAs wetting layer thinner than the critical thickness corresponding to conventional transition from two- to three- dimensional (2D–3D) growth modes. Using real-time reflection high-energy-electron diffraction, the time evolution of total QD volume is analyzed. Based on a mean-field theory [Phys. Rev. Lett. 79 (1997) 897], the initially increasing and finally saturating QD volume is simply and well explained by QD nucleation from precursors. It is suggested that precursors start to grow at an exponential rate well before the 2D–3D growth mode transition, and are responsible for the usually observed significant mass transport from a wetting layer to QDs in conventional QD growth.

Morphological evolution of ion-sputtered Pd(001): Temperature effects

We have investigated the kinetic effects on the morphological evolution of an ${\mathrm{Ar}}^{+}$-ion sputtered Pd(001) by in situ, real-time x-ray reflectivity and grazing-incidence small angle x-ray scattering measurements at various substrate temperatures. We find that surface roughness $W$ and its associated growth exponent $\ensuremath{\beta}$ increase with $T$ up to a certain temperature ${T}_{m}$. Beyond ${T}_{m}$, however, they decrease with increasing $T$. For $T<{T}_{m}$, surface roughening and coarsening kinetics are mostly driven by the deposition and diffusion of sputter-induced adatoms as reported in molecular beam epitaxial growth. When $T$ is near ${T}_{m}$, however, vacancies rather than adatoms become the dominant surface species. Above ${T}_{m}$, the surface smoothing via adatom detachment and vacancy diffusion across step edges becomes effective.

The influence of wavelength on phase transformations induced by picosecond and femtosecond laser pulses in GeSb thin films

Cycling between the crystalline and amorphous phases of 25-nm-thick GeSb films induced by single laser pulses of duration of 100fs or 20ps is investigated in the 400–800nm wavelength range. The time evolution of the phase transformations has been studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5nm and also with femtosecond and picosecond pump-probe measurements. Upon picosecond irradiation, three regimes are identified: for wavelengths below ∼550nm and above ∼750nm, the total time to transform between the crystalline and amorphous phases is of the order of 10–24ns while in the intermediate wavelength range of 600–750nm, the transformation time is only ∼650ps. Upon 100fs irradiation, the transformation times are observed to decrease with increasing wavelength with the shortest times of ∼5ns for crystallization and ∼10ns for amorphization, both occurring at 800nm. This behavior is discussed in terms of how the wavelength-dependent refractive index of the phases involved influences the initial supercooling of the molten volume and the subsequent resolidification scenario.

Psychophysical Evaluation of In-Situ Ultrasound Visualization

We present a novel psychophysical method for evaluating ultrasonography based on Real-Time Tomographic Reflection (RTTR), in comparison to Conventional Ultrasound (CUS). The method measures the user's perception of the location of an ultrasound-imaged target independently from assessing the action employed to reach it. Three experiments were conducted with the Sonic Flashlight (SF), an RTTR device, and CUS. The first two experiments determined subjects' perception of target location with a triangulation-by-pointing task. Depth perception with the SF was comparable to direct vision, while CUS caused considerable underestimation of target depth. Binocular depth information in the SF was shown to significantly contribute to its superiority. The third experiment tested subjects in an ultrasound-guided needle insertion task. Because the SF provides visualization of the target at its actual location, subjects performed insertions faster and more accurately by using the SF rather than CUS. Furthermore, the trajectory analysis showed that insertions with the SF generally went directly to the target along the desired path, while CUS often led to a large deviation from the correct path consistent with the observed underestimation of target depth. These findings lend great promise to the use of RTTR-based imaging in clinical practice and provide precise means of assessing efficacy.

Optical evidence for a self-propagating molten buried layer in germanium films upon nanosecond laser irradiation

This work describes the phase transitions occurring at the film-substrate interface of amorphous germanium films upon nanosecond laser-pulse-induced melting of the surface. Films with thickness ranging from 50 to 130 nm deposited on glass substrates were studied. Real-time reflectivity measurements with subnanosecond time resolution performed both at the air-film and film-substrate interfaces were used to obtain both surface and in-depth information of the process. In the thicker films (⩾80nm), the enthalpy released upon solidification of a shallow molten surface layer induces a thin buried liquid layer that self-propagates in-depth towards the film-substrate interface. This buried liquid layer propagates with a threshold velocity of 16±1m∕s and causes, eventually, melting at the film-substrate interface. In the thinnest film (50 nm) there is no evidence of the formation of the buried layer. The presence of the self-propagating buried layer for films thicker than 80 nm at low and intermediate laser fluences is discussed in terms of the thermal gradient in the primary melt front and the heat released upon solidification.

C-Mode Real-time Tomographic Reflection for a Matrix Array Ultrasound Sonic Flashlight 1

Real-time tomographic reflection (RTTR) permits in situ visualization of tomographic images so that natural hand-eye coordination can be used directly during invasive procedures. The method uses a half-silvered mirror to merge the visual outer surface of the patient with a simultaneous scan of the patient's interior without requiring a head-mounted display or tracking. A viewpoint-independent virtual image is reflected precisely into its actual location. When applied to ultrasound, we call the resulting RTTR device the sonic flashlight. We previously implemented the sonic flashlight using conventional two-dimensional ultrasound scanners that produce B-mode slices. Real-time three-dimensional (RT3D) ultrasound scanners recently have been developed that permit RTTR to be applied to slices with other orientations, including C-mode (parallel to the face of the transducer). Such slice orientation may offer advantages for image-guided intervention. Using a prototype scanner developed at Duke University (Durham, NC) with a matrix array that electronically steers an ultrasound beam at high speed in 3D, we implemented a sonic flashlight capable of displaying C-mode images in situ in real time. We present the first images from the C-mode sonic flashlight, showing bones in the hand and the cardiac ventricles. The extension of RTTR to matrix array RT3D ultrasound offers the ability to visualize in situ slices other than the conventional B-mode slice, including C-mode slices parallel to the face of the transducer. This orientation may provide a broader target, facilitating certain interventional procedures. Future work is discussed, including display of slices with arbitrary orientation and use of a holographic optical element instead of a mirror.

Real-time X-ray study of roughness scaling in the initial growth of epitaxial BaTiO 3/LaNiO 3 superlattices

The evolution of surface and interface roughness during heteroepitaxial growth of strained BaTiO 3/LaNiO 3(BTO/LNO) superlattices on SrTiO 3 (0 0 1) substrates was investigated using real-time X-ray reflectivity measurements in situ with synchrotron radiation. The roughness scaling of the growth front and interface of superlattices with modulation length below (2 and 6 nm) and beyond (20 nm) the critical thickness was studied against the bilayer number. The fitted results of in situ specular X-ray reflectivity curves reveal a two-dimensional growth of BTO and LNO sublayers on the SrTiO 3 substrate for superlattices with a modulation length below the critical thickness. Moreover, a larger root-mean-square roughness of BTO/LNO interface was obtained in the superlattice with modulation length beyond the critical thickness indicating lattice relaxation in the superlattice structure. Fitted results of in situ specular X-ray reflectivity curves provide the first evidence for power-law scaling of the root-mean-square roughness of an interface and a surface in the superlattice structure. Observation of such a roughness scaling behavior indicates that strain plays an important role in the determination of microstructure in the growth of epitaxial BTO/LNO superlattices.

Femtosecond laser-induced decomposition in triazenepolymer thin films

The damage induced by ultrashort, 130 fs, near-infrared, 800 nm, Ti:sapphire laser pulses in 1 μm thick triazenepolymer films on glass substrates has been investigated. Real-time reflectivity measurements with a ps-resolution streak camera and a ns-resolution photodiode set-up have been performed to study in situ the structural transformation dynamics upon single-pulse excitation with laser fluences above the threshold of permanent damage. Scanning force microscopy has been used to probe ex situ the corresponding surface topography of the ablated spots. Modulated lateral force microscopy (M-LFM) has been applied to observe alterations of the local friction properties within and around the irradiated areas.

Real time in situ diagnostics of PVD growth using synchrotron radiation

For specific applications of hard coatings, the microstructure (or nanostructure) has to be tailored for optimum performance. This requires information about the mechanisms of growth and the connection between the microstructure and the deposition parameters. In order to obtain this knowledge, in situ real time growth studies of magnetron-sputtered thin films were carried out. A growth chamber, equipped with two magnetrons and Kapton windows for X-ray diffraction and reflectivity, was mounted on a six-circle goniometer at a synchrotron beam line at ESRF in Grenoble. As an example, X-ray diffraction measurements were carried out in situ during growth of TiN to follow the development of the microstructure. Recrystallization was identified as the mechanism which controlled the development of texture. The driving force for these texture changes arose from minimalization of the sum of the surface energy and the strain energy of the individual grains. As another example, the heteroepitaxial growth of TiN on MgO(001) was studied. Bragg–Brentano as well as grazing-incidence in-plane wide angle scattering was used to establish the pseudomorphic growth of TiN to the underlying MgO(001). Using real-time specular X-ray reflectivity, layer-by-layer growth was observed, with the surface roughening decreasing with an increase in the deposition temperature. The growth of nanocrystalline Au was also investigated. Among the results we found that changes in the (111) fiber texture arose from grain rotations.

Phase change dynamics in a polymer thin film upon femtosecond and picosecond laser irradiation

The influence of the pulse duration on the laser-induced changes in a thin triazenepolymer film on a glass substrate has been investigated for single, near-infrared (800 nm) Ti:sapphire laser pulses with durations ranging from 130 fs up to 2.6 ps. Post-irradiation optical microscopy has been used to quantitatively determine the damage threshold fluence. The latter decreases from ∼800 mJ/cm 2 for a 2.6 ps laser pulse to ∼500 mJ/cm 2 for a pulse duration of 130 fs. In situ real-time reflectivity (RTR) measurements have been performed using a ps-resolution streak camera set-up to study the transformation dynamics upon excitation with single pulses of duration of 130 fs and fluences close to the damage threshold. Very different reflectivity transients have been observed above and below the damage threshold fluence. Above the damage threshold, an extremely complicated behaviour with oscillations of up to 100% in the transient reflectivity has been observed. Below the damage threshold, the transient reflectivity decreases by as much as 70% within 1 ns with a subsequent recovery to the initial level occurring on the ms timescale. No apparent damage could be detected by optical microscopy under these irradiation conditions. Furthermore, within the 395–410 mJ/cm 2 fluence range, the transient reflectivity increases by ∼10%. The analysis of these results indicates that the observed transformations are thermal in nature, in contrast to the known photochemical decomposition of this triazenepolymer under UV irradiation.

Amorphization induced by subnanosecond laser pulses in phase-change optical recording media.

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.

Kinetic roughening of ion-sputtered Pd(001) surface: beyond the Kuramoto-Sivashinsky model.

We investigate the kinetic roughening of Ar+ ion-sputtered Pd(001) surface both experimentally and theoretically. In situ real-time x-ray reflectivity and in situ scanning tunneling microscopy show that nanoscale adatom islands form and grow with increasing sputter time t. Surface roughness W(t) and lateral correlation length xi(t) follow the scaling laws W(t) approximately t(beta) and xi(t) approximately t(1/z) with the exponents beta approximately 0.20 and 1/z approximately 0.20, for an ion beam energy epsilon=0.5 keV, which is inconsistent with the prediction of the Kuramoto-Sivashinsky (KS) model. We thereby extend the KS model by applying the coarse-grained continuum approach of the Sigmund theory to the order of O(inverted Delta(4),h(2)), where h is the surface height, and derive a new term of the form inverted Delta(2)(inverted Delta h)(2) which plays a decisive role in describing the observed morphological evolution of the sputtered surface.

Stacking of InAs/InP(001) quantum wires studied by in situ stress measurements: Role of inhomogeneous stress fields

Size and spatial distribution homogeneity of nanostructures is greatly improved by making stacks of nanostructures separated by thin spacers. In this work, we present in situ and in real time stress measurements and reflection high-energy electron diffraction observations and ex situ transmission electron microscopy (TEM) characterization of stacked layers of InAs quantum wires (QWRs) separated by InP spacer layers, d(InP), of thickness between 3 and 20 nm. For d(InP)<20 nm, the amount of InAs involved in the created QWR from the second stack layer on, exceeds that provided by the In cell. Our results suggest that in those cases InAs three dimensional islands formation starts at the P/As switching and lasts during further InAs deposition. We propose an explanation for this process that is strongly supported on TEM observations. The results obtained in this work imply that concepts like the existence of a critical thickness for two- to three-dimensional growth mode transition should be revised in correlated QWR stacks of layers.

Influence of pulse duration on the amorphization of GeSb thin films under ultrashort laser pulses

Laser-induced amorphization of crystalline, 25-nm-thick, Sb-rich GeSb films has been studied for pump pulse durations in the range from 100 fs up to 6 ns. The dynamics of the phase change has been investigated using real-time reflectivity measurements with picosecond time resolution performed with a streak camera. For pulses in the femtosecond regime, the time required to complete the transformation is of the order of ∼10–15 ns. When the pulse duration is increased to the 1.5–20 ps range, the transformation time decreases to values as short as 400 ps, while for nanosecond laser pulses amorphization is not achievable. This behavior is discussed in terms of the heat flow dynamics of the system and the influence of pulse duration on the initial supercooling and nucleation rate.

Rewritable phase-change optical recording in Ge2Sb2Te5 films induced by picosecond laser pulses

The phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements with subnanosecond resolution. Evidence was found that the thermal diffusivity of the substrate plays a crucial role in determining the ability of the films to crystallize and amorphize. A film/substrate configuration with optimized heat flow conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced. In this system, we achieved reversible phase transformations with large optical contrast (>20%) using single laser pulses with a duration of 30 ps within well-defined fluence windows. The amorphization (writing) process is completed within less than 1 ns, whereas crystallization (erasing) needs approximately 13 ns to be completed.

Ｓｉ（００１）とＳｉ（１１１）表面の極薄酸化膜形成過程の比較

Real-time reflection high energy electron diffraction combined with Auger electron spectroscopy (RHEED-AES) was applied to investigate the oxide growth kinetics during initial oxidation on Si (001) 2 × 1 and Si (111) 7 × 7 surfaces. The correlation between the amount of adsorbed oxygen atoms obtained from O KLL Auger electron intensity and the unoxidized surface area estimated from RHEED spot intensities due to 2 × 1 or 7 × 7 structure revealed that adsorbed oxygen was able to migrate on the Si (111) 7 × 7 surface during Langmuir-type adsorption at 580°C, leading to nucleation and lateral growth of oxide as in the oxidation mode of two-dimensional oxide island growth appearing at higher temperatures, while growth of oxide during Langmuir-type adsorption on the Si (001) 2 × 1 surface at 576°C progressed at the random sites where O2 adsorption took place.

Reactivity of halogens on a Si(111) surface studied by surface differential reflectivity

The adsorption of Br on a $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface at room temperature and the isothermal desorption of silicon bromides from a Br-terminated Si(111) rest atom surface at about 900 K have been investigated by means of in situ real-time surface differential reflectivity (SDR) spectroscopy in order to elucidate the fundamental processes of Br-etching. The present results are compared with those of previous SDR studies on chlorine adsorption and chloride desorption. In the adsorption process, both the sticking probability on adatom dangling bonds and the breaking probability of adatom back bonds are larger than those for Cl adsorption. In the desorption process, the activation energy for bromide desorption (1.8 eV) is smaller than that for chloride desorption. The experimental results can be well interpreted in terms of larger repulsive interaction between Br adsorbates. The activation energy for the formation of the dimer-adatom-stacking fault structure (3.5 eV) is larger than that for the surface after chloride desorption.

Time resolved dynamics of rapid melting and resolidification of Sb thin films under ns and ps laser pulse irradiation

Real time reflectivity measurements with ps time resolution at the wavelength λ=514.5 nm have been used to analyze the rapid melting and solidification dynamics of Sb thin films on glass substrates irradiated with a visible laser pulse (λ=583 nm) of duration either 30 ps or 2 ns. Significant differences in the solidification dynamics have been obtained for the three different film thickness investigated (25, 50, and 200 nm) as well as between ps and ns laser pulse excitation. Bulk solidification is the dominant mechanism observed at high laser fluence in the thinner films, no matter the pulse duration used, while interfacial solidification is the only mechanism observed in the thicker film for both pulse durations. These results have been interpreted in terms of the differences of the effective thermal conductivity of the films. The comparison of the behavior of these films with the case of Sb-rich GeSb ones makes it clear that relatively minor changes in the film composition can lead to big changes in the solidification dynamics under similar heat flow conditions that are beneficial for the application of such films in phase change optical recording with ultrashort pulses. The optical properties of molten phase of Sb and of the solid material at the melting temperature have been also been determined at 514.5 nm.

A RHEED Study of MBE Growth of ZnSe onGaAs(111)A-(2 × 2)

ZnSe epilayers have been grown under various Se/Zn atomic flux ratios in the range of 0.22–2.45 at a substrate temperature of 350°C on Zn pre-exposed GaAs (111) A surfaces. Real time reflection high energy electron diffraction (RHEED) observations have shown a transition from a two-dimensional (2D) to a three-dimensional (3D) growth mode. The transition time depends directly upon the growth rate. A detailed discussion is presented to explore the cause of this change in the growth mode.

Isothermal desorption process of Cl-covered Si(1 1 1) studied by surface differential reflectivity spectroscopy

The isothermal desorption of silicon chlorides from a chlorine-saturated Si(1 1 1) rest atom surface at about 900 K has been investigated by means of in situ real-time surface differential reflectivity spectroscopy. The spectral features at 1.8 eV (A) and 2.5 eV (B) arise mainly from missing adatom dangling bonds and missing adatom back bonds of the dimer–adatom–stacking-fault (DAS) structure, respectively. The feature A decays twice as fast as B does, which indicates that A also originates from the dangling bonds on the rest atom surface. The decay of A, which is considered to represent the desorption of silicon dichlorides, follows first-order kinetics, and the activation energy for this process is 2.3 eV. The decay of B, which is considered to represent the formation of the DAS structure, also follows first-order kinetics and the activation energy is 2.8 eV. The mechanisms of the desorption of silicon dichlorides and the formation of the DAS structure are discussed on the basis of the experimental results.