RHEED from stepped surfaces and its relation to RHEED intensity oscillations observed during MBE

Multiple scattering theory is used to calculate the intensities of reflection high energy electron diffraction from periodic arrays of surface steps. The intensities are found to depend strongly on the direction of the incident beam azimuth. When the incident beam azimuth is parallel to the step edges, both the specular and diffracted beam intensities are diminished with respect to the intensities from a flat surface. When the incident beam azimuth is perpendicular to the edges, the intensities of all the beams are of the same order of magnitude as for a flat surface but some of the peak heights are oscillatory functions of the number of atoms in the topmost layer. These peak intensity oscillations are very similar to the intensity oscillations observed during molecular beam epitaxial film growth.     

Origin of azimuthal effect of RHEED intensity oscillations observed during MBE

On the basis of calculated RHEED intensities from stepped surfaces, the or igin of the azimuthal dependence of RHEED intensity oscillations observed during Si MBE growth on Si(001) is discussed. By combining the azimuthal dependence of RHEED intensity variations from stepped surface with a model growth mechanism, we obtain the result that in a certain incident beam azimuth one period of the oscillations corresponds to a biatomic height and to a monatomic height in other azimuths. The result is quite consistent with a recently reported experiment.     

Computer modeling of morphological evolution and RHEED intensity oscillations of thin-film growth by laser molecular beam epitaxy

The morphological evolution of the unit cell by unit cell layer growth of thin films by laser molecular beam epitaxy (laser MBE) is simulated by the Monte Carlo (MC) method. As a more realistic simulation, the effect of the surface terrace on reflection high-energy electron diffraction (RHEED) intensity oscillations is considered in our model. The calculation shows that the surface terrace effect has an obvious influence on the intensity oscillation shapes. Moreover, we consider the growth units impinging on the surface batch by batch rather than one by one. It is found that the surface morphologies are different, with different numbers of units impinging by a laser pulse on the surface. Furthermore, the surface roughness increases with a decrease in substrate temperature and with an increase in the unit impinging rate (which is determined by the energy of each laser pulse). The validity of the growth model is demonstrated by comparing our MC-simulated RHEED intensity oscillations with those observed in laser MBE experiments.     

RHEED intensity oscillations during epitaxial growth of fcc Fe on Cu(001)

Up to 14 strong intensity oscillations of the specular beam in reflection high-energy electron diffraction (RHEED) have been observed during the epitaxial growth of face-centered (fcc) Fe on Cu(001) in the temperature range between 333–370 K. The amplitudes of these oscillations are strong even in the initial stages of growth. This provides evidence that the hetero-epitaxial growth of fcc Fe on Cu(001) proceeds predominantly layer-by-layer-like in that temperature regime. For growth at 300 K, initial Fe-bilayer-island formation followed by predominant layer-by-layer growth above 2 monolayers (ML) of Fe is observed. 3D growth prevails at ˜ 100 K.     

On the interpretation of the RHEED intensity oscillations during the growth of vicinal faces

A quantitative treatment of the crystallization kinetics in MBE growth of vicinal surfaces results the relation l ²=2D _sK between the surface diffusion coefficient D _s, the time for a monolayer deposition and the interstep distance l at which the RHEED intensity oscillations disappear. The correction factor K depends on the size and the energy of the two-dimensional critical nucleus and it is estimated to be smaller than 10^–2. The currently used interpretation of the RHEED intensity oscillations ignores the correction factor K and, therefore, the calculated values of D _s are several orders of magnitude smaller than its real values. The surface transport during the time of growth interruption is discussed in connection with the tendency to three dimensional growth at every second interface (where a deposition of the material with strong intermolecular bonds starts) of a small period superlattice.     

Combined study of RHEED spot profiles and intensity oscillations during MBE growth of Ge on Ge(111)

The problem to define particular points in the growth sequence and to obtain quantitative data on the step-terrace structure of the growing surface is addressed in an analysis of RHEED spot profiles and intensity oscillations recorded simultaneously during MBE growth of Ge on Ge(111).     

An investigation of MBE growth of (Zn, Mg)Se using RHEED oscillations

We have used reflection high-energy electron diffraction (RHEED) oscillations to measure the growth rates of Zn_1−x.Mg_xSe and ZnSe as functions of substrate temperature, II/VI flux ratio, and electron-beam filament current. Although the ZnSe and Zn_1−xMg_xSe growth rates vary with these growth parameters, the growth rate difference attributable to the addition of magnesium does not depend on them, if growth is done with a selenium excess. Thus, unlike the other common components of II/VI materials (Zn, Cd, S, Se and Te), the sticking coefficient of magnesium is independent of substrate temperature and II/VI flux ratio. Electron-beam-induced desorption of magnesium is also negligible. For growth with a selenium excess, the composition of Zn_1−xMg_xSe films determined by RHEED oscillations agrees well with X-ray fluorescence measurements. However, if growth is done without an excess of selenium, then a simple composition calculation using the Zn_1−xMg_xSe and ZnSe growth rates indicates a magnesium content which is lower than the true magnesium content. This occurs because of a decrease in the sticking coefficient of zinc induced by the presence of magnesium.     

RHEED studies of Si(100) surface structures induced by Ga evaporation

Si(100) surface structures induced by Ga molecular beam deposition in an ultra-high vacuum have been investigated using a reflection high-energy electron diffraction system (RHEED). It has been found that the Ga evaporation of submonolayer thickness on a clean Si(100) 2 × 1 surface produces surface structures of Si(100) 3 ×2, 5 × 2, 2 ×2 and 8 × 1 sequentially in the temperature range from 350 to 680° C. The RHEED patterns and a two-dimensional phase diagram including five superstructures are presented.     

Frequency multiplication of microwave radiation in a semiconductor superlattice by electrons capable to perform Bloch oscillations

We report the observation of frequency multiplication of microwave radiation in a GaAs/AlAs semiconductor superlattice at room temperature. We observed, for a fundamental frequency of 9 GHz, second and third harmonic generation. We associate the harmonic generation with a nonlinear current-voltage characteristic that is determined by Bloch oscillations of electrons propagating along the superlattice axis. Our results suggest for the frequency multiplication an upper limit in the tetrahertz frequency range.     

The application of RHEED intensity effects to interrupted growth and interface formation during MBE growth of GaAs/(Al,Ga)As structures

There is a diffraction-induced phase effect in the RHEED intensity oscillation technique used in MBE, whereby intensity maxima only correspond to monolayer completion for very restricted conditions. In particular, the angle of incidence of the primary beam is extremely critical. The effect occurs because the total intensity at the measured position of the specular beam is always derived from at least two different diffraction processes, which do not have the same phase relation to monolayer formation. It can be accomodated either by a systematic series of measurements to establish an empirical relationship between incidence angle and phase, or by Fourier transform techniques. Unless full account is taken of this purely diffraction-induced effect, very misleading results can be obtained for the time constants of the recovery period following cessation of growth and this is illustrated for GaAs. The effect also has important implications for the growth-interrupt technique. In addition, it is shown that for heterojunction formation in the GaAs/(Al, Ga)As system, adatom (Ga and Al) migration lengths are of greater importance than the position in the monolayer at which the composition is changed, and that RHEED can provide only limited information on the interface structure.     

Transmission intensity oscillations in GaAs/AlAs superlattices induced by photogenerated charged transport

Two superlattice p-i-n diodes connected in series to a constant dc bias, i.e., in the symmetric self-electro-optic effect device (S-SEED) configuration, exhibit pronounced oscillations of their transmission intensities immediately after optical pulse excitation of one of the diodes. The temporal variations directly reflect the transmission-voltage characteristics due to successive Stark ladder resonances, and they depend strongly on the amount of the illumination power and on the pulse width. The observed oscillatory behavior is explained by changes of the voltage balance between the two diodes induced by photogenerated charge transport.     

In-situ monitoring of interface formation using the phase shift of reflection high-energy electron diffraction intensity oscillations

A novel phase shift of Reflection High-Energy Electron Diffraction (RHEED) intensity oscillations during heterointerface formation allows direct monitoring of segragation at GaAs/Al _x Ga_1–x As interfaces. The effect should be applicable to other materials systems as it is due to the reconfiguration of the surface structure at the heterointerface.     

Investigation of the structure of the 2×4 reconstructions of GaAs (001) by analysis of RHEED intensity data

Rocking curves are computed for electrons incident in turn in the [010], [110] and [¯110] azimuths for 512 different models of the reconstruction. Each model corresponds to a different arrangement of twisted tilted dimers. For each incident beam azimuth many of these distinct models give identical rocking curves. This is explained by the possible values of the structure factor for scattering by surface atoms. In particular it is shown that the incident electrons are insensitive to atomic displacements which are parallel to the incident beam azimuth. Preliminary models are presented for which the agreement with experiment is best for all three azimuths. The effects of tuning the displacements within the dimers are considered and finally the intensities of the non-specular beams are briefly discussed.     

交变极性聚焦磁场抑制Corkscrew运动的机理

分析了对强流相对论电子束采用交变极性聚焦磁场聚焦存在稳定解的条件,提出了采用交变极性聚焦磁场抑制Corkscrew运动的机理.数值计算结果表明,在采用交变周期为1m、峰值磁场设为0.2T的情况下,对于能量为12MeV的电子束,当聚焦布局长度为2.8m时,束质心位移可从5mm（始端）被抑制至近似0 (终端);对于能量为20MeV的电子束,当聚焦布局长度为4.5m时,束质心位移可从5mm（始端）被抑制至近似0(终端)。     

Passive and active interior noise control of box structures using the structural intensity method

This paper presents passive and active vibro-acoustic noise control methods for attenuating the interior noise level in box structures which can be an analogy of cabins of vehicle and aircraft. The structural intensity (SI) approach is adopted to identify the predominant vibration panels and interior noise sources for box structures. In the study, the finite element method is used to determine the structural vibration and structural intensity in the box surfaces. According to structural intensity vectors plot and structural intensity stream lines presentation, the possible effective control positions where the dampers may be attached and the active control forces may act to reduce vibration and interior noise, are identified. From the study, it can be demonstrated that the structural intensity approach and stream line presentation are possible methods for identifying the vibro-acoustic interior noise source and predominant panels which may be modified to reduce the interior noise level. The structural intensity methodology, passive and active noise control results can be extended to the further study of the vibration and interior noise control of actual cabins of vehicles and aircraft.     

Surface structures of K on Mo(1 1 0) surface investigated by RHEED

Phase transitions of K on Mo(1 1 0) have been studied by RHEED technique. As Ba and Cs structures on the bcc(1 1 0) surface, surface structures of K were hexagonal from RT to 250 °C for θ > 0.9 ML. The hexagonal structure successively expanded from α to γ structure with Nishiyama-Wassermamm (N-W) orientation relationship. The nearest neighbor spacing in the α structure at RT was 4.50 Å, which is very closed to the atomic distance of K in metal, and stretched to 5.14 Å in the γ structure at T = 200 °C. At temperatures greater than T = 250 °C, the γ structure oriented in N-W and Kurdjumov-Sachs (K-S) relationships at the same time and stayed up to the temperature of 450 °C. These two orientations of γ structure also appeared in all temperature range for 0.4 < θ < 0.9 ML.     

Computation of direct intensity and medium intensity by using the discrete ordinates method

On the basis of the discrete ordinate scheme with (an) infinitely small weight(s), an easy-to-use and comprehensive method, named multi-rays method, is developed to calculate total, direct and medium intensities in arbitrarily specified directions. In doing this, for each of the specified directions, three identical discrete directions with infinitely small weights are employed to represent the three intensities. The new method is verified with two standard test problems, and is used to compute the intensities in two anisotropically scattering problems.     

Quantum interferometry by using delayed phase-locked ultrashort pulses: implementation of quantum gate operation in molecules

It will be shown that by using two phase-locked delayed ultrashort pulses of different durations one can selectively populate odd or even vibrational levels of an excited state of a molecule. One can control the enhancement or damping in a selective vibrational level by controlling the laser parameters e.g. delay, phase difference, temporal width and carrier frequency of two ultradhort pulses. Conversely by reading the population distribution In vibrational levels of an excited state of a molecule one can study the amplitude and the phase of the wavepacket generated by the interference of two wavepackets excited by two ultrashort pulses i.e. by quantum interferometry. It will be examined whether the information stored in a molecule by quantum computation can be retrieved efficiently by using quantum interferometry.