Real-time Reflection High-energy Electron Diffraction Research Articles

Modified energetics and growth kinetics on H-terminated GaAs (110)

Atomic hydrogen modification of the surface energy of GaAs (110) epilayers, grown at high temperatures from molecular beams of Ga and As4, has been investigated by friction force microscopy (FFM). The reduction of the friction force observed with longer exposures to the H beam has been correlated with the lowering of the surface energy originated by the progressive de-relaxation of the GaAs (110) surface occurring upon H chemisorption. Our results indicate that the H-terminated GaAs (110) epilayers are more stable than the As-stabilized ones, with the minimum surface energy value of 31 meV∕Å(2) measured for the fully hydrogenated surface. A significant reduction of the Ga diffusion length on the H-terminated surface irrespective of H coverage has been calculated from the FFM data, consistent with the layer-by-layer growth mode and the greater As incorporation coefficient determined from real-time reflection high-energy electron diffraction studies. Arsenic incorporation through direct dissociative chemisorption of single As4 molecules mediated by H on the GaAs (110) surface has been proposed as the most likely explanation for the changes in surface kinetics observed.

Ge growth over thin SiO 2 by UHV–CVD for MOSFET applications

The growth of germanium by ultra high vacuum–chemical vapour deposition (UHV–CVD) on a thin SiO 2 layer (0.6 nm) formed on Si(001) was investigated by real-time reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was observed that the growth of Ge at 600 °C on a 2 monolayers thick SiO 2 (0.6 nm) film remains perfectly coherent with the Si (001) substrate. The mechanism of reduction of SiO 2 is also discussed for higher Ge deposition temperatures. It was found to involve both Si from the substrate and Ge from the gas phase.

Epitaxial growth of BaTiO 3 thin films at a low temperature under 300 °C with temperature-controlled BaTiO 3 buffer layer

The epitaxial growth of BaTiO 3 (BTO) thin films on LaAlO 3 (0 0 1) substrates by laser molecular beam epitaxy was studied using real-time reflection-high-energy-electron-diffraction (RHEED). Atomic force microscopy (AFM) was employed to investigate the surface topography. When the BTO thin films were directly grown at 320 °C with a thickness of above two monolayers (ML), the RHEED patterns vanished rapidly, indicating the failure of crystallinity. However, the growth of eight ML BTO layers can be controlled by decreasing temperature from 600 °C step-by-step to 300 °C. The evolution of diffraction streaks revealed a scaling behavior with temperature and film thickness, demonstrating the evolution of the quasi-two-dimensional BTO islands growth and distribution. Furthermore, subsequently deposited 30 ML BTO thin films with a good crystallinity were obtained on the eight ML BTO buffer layers at a low temperature of 280 °C. It is found that the nucleation centers formed during decreasing temperature play a key role in determining the subsequent crystallization of BTO thin films at a lower temperature.

Real-time RHEED investigation of indium segregation in InGaAs layers grown on vicinal GaAs(001) substrates

The surface segregation of indium atoms was investigated in situ and in real time by reflection high-energy electron diffraction (RHEED) during molecular-beam epitaxy of InGaAs layers on misoriented GaAs(001) substrates. The strong damping of the RHEED oscillations was quantitatively related to the strength of the segregation process that was found to decrease with increasing miscut angle.

Hyperthermal ion beam system optimized for studying the effects of kinetic energy on thin-film growth

A hyperthermal and low-energy ion beam (10–1000 eV) optimized for studying morphological trends in epitaxial metal thin films as a function of atomic kinetic energy has been built and characterized. The ion beam line produces metal and inert gas ions and is specially designed to produce up to 2.9 μA of highly collimated ions with single amu mass resolution while precisely controlling the ion’s energy, achieving a ΔE/E∼0.1. Energy resolution can be enhanced further at the expense of flux. Varying the focal length of the final electrostatic lens allows the flux density to be adjusted from 10 to 500 nA/mm2. The beam line has been coupled to an ultra-high-vacuum deposition chamber with a versatile sample manipulator, an electron beam deposition source, residual gas analysis, and real-time reflection high-energy electron diffraction (RHEED). Once prepared, the sample can be moved in situ to perform Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The high fluxes with narrow energy distributions this apparatus produces allows the poorly understood hyperthermal energy regime to be probed with RHEED, AES, and STM. The atomic kinetic energy can be varied to measure effects on nuclei densities, growth mode, and surface morphology. STM images of copper films deposited under a variety of conditions illustrate the diverse range of possible results.

Temperature dependence of the transition from two-dimensional to three-dimensional growth of Ge on (001)Si studied by reflection high-energy electron diffraction

Germanium islands have been grown on Si(001) by solid-source molecular-beam epitaxy at temperatures between 325 and 900 °C. The formation of metastable {105} faceted clusters and macroislands was investigated by real-time reflection high-energy electron diffraction. Up to 600 °C, the two-dimensional (2D)-three-dimensional (3D) growth transition through the formation of hut clusters appears at a temperature-independent critical thickness, while the coalescence to macroislands is thermally activated. An activation barrier was also found for the 2D–3D growth transition directly to macroislands at growth temperatures above 600 °C. The crossing of the two competing transitions at 600 °C determines the two growth regimes.

Real-Time Pseudodielectric Function of Low-Temperature-Grown GaAs

ABSTRACTWe present the real-time pseudodielectric function <ε(E)> of low-temperature-grown GaAs (LT-GaAs) thin films during the growth as a function of growth temperature Tg and thickness. We obtained accurate measurements of the real-time <εc(E)> by using in situspectroscopic ellipsometry (SE) in conjunction with active feedback control of the substrate temperature using diffuse reflectance spectroscopy. We show that for epitaxial LT-GaAs layers, the peak in the imaginary pseudodielectric function <ε2(E)> decreases in amplitude and sharpness systematically with decreasing Tg. We also revealed an abrupt change in <εc(E)> near the critical epitaxial thickness hepi, the value of which decreases with decreasing Tg. Above hepi, the LT-GaAs grows polycrystalline (amorphous) above (below) Tg ∼ 190°C. We also simultaneously monitored the surface roughness and crystallinity by using real-time reflection high-energy electron diffraction (RHEED). These results represent progress in obtaining real-time control over the composition and morphology of LT-GaAs

Smoothening transition during initial epitaxial growth of Mo on sapphire

The initial epitaxial growth of Mo (111) on Al2O3 (0001) substrates has been studied systematically as a function of growth temperature and rate, using real-time reflection high energy electron diffraction (RHEED) and in situ scanning probe microscopy. The evolution of RHEED specular intensity during Mo growth exhibits a distinct peak at Mo coverage of ∼10 Å, where the Mo surface is the smoothest. The observed Mo coverage at the specular peak is independent of growth temperature and rate, indicating that it is energetically stable. However, our findings reveal that surface kinetics plays a key role in determining the observed behavior. At each growth temperature, the highest specular intensity occurs at an optimum growth rate, at which the Mo surface is the smoothest. The temperature dependence of the optimum rate has an Arrhenius form indicating that the observed behavior is an activated process.

Molecular beam epitaxial growth of ZnSe on GaAs substrates: influence of precursors on interface quality

The initial growth of thin ZnSe layers, grown on GaAs(0 0 1) using molecular beam epitaxy (MBE), has been studied using real-time reflection high-energy electron diffraction (RHEED), as well as X-ray double-crystal rocking curves (DCRC), reflectivity difference spectroscopy (RDS) and transmission electron microscopy (TEM). A noticeable difference in interface quality and growth was observed depending upon the initial precursor. Exposure to Se flux before growth led initially to three-dimensional growth, whereas exposure to Zn flux or no precursor led immediately to two-dimensional growth. X-ray DCRC and RDS confirmed that the overall sample quality was better with two-dimensional growth, and TEM observations were consistent with previous studies which have shown that initial three-dimensional growth is related to the formation of Ga2Se3 at the interface, leading to high densities of stacking faults and edge dislocations.

Growth, structure, and magnetic properties of γ-Fe2O3 epitaxial films on MgO

Single-crystal epitaxial thin films of γ-Fe2O3(001) have been grown on MgO(001) using oxygen-plasma-assisted molecular beam epitaxy. The structure and magnetic properties of these films have been characterized by a variety of techniques, including reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy and x-ray photoelectron/Auger electron diffraction (XPD/AED), vibrating sample magnetometry, and ferromagnetic resonance. Real-time RHEED reveals that the film growth occurs in a layer-by-layer fashion. The γ-Fe2O3(001) film surface exhibits a (1×1) LEED pattern. The growth of γ-Fe2O3 films at 450 °C is accompanied by significant Mg outdiffusion. AED of Mg KLL Auger emission reveals that Mg substitutionally incorporates in the γ-Fe2O3 lattice, occupying the octahedral sites. Magnetic moments are ∼2300 G and ∼4500 G for γ-Fe2O3 films grown at 250 °C and 450 °C, respectively. The high magnetic moment for the films grown at 450 °C could be attributed to the high degree of structural order of the films and Mg substitution at octahedral sites.

Synthesis of epitaxial films of Fe3O4 and α-Fe2O3 with various low-index orientations by oxygen-plasma-assisted molecular beam epitaxy

Epitaxial thin films of pure-phase Fe3O4(110), Fe3O4(111), α-Fe2O3(112̄0), and α-Fe2O3(11̄02) have been grown on MgO(110), α-Al2O3(0001), α-Al2O3(112̄0), and α-Al2O3(11̄02) substrates, respectively, by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. Characterization of the crystal structures, chemical states, and epitaxial relationships was carried out using a variety of techniques, including in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, x-ray photoelectron spectroscopy/diffraction, and ex situ x-ray reflectivity and diffraction. Real-time RHEED reveals that Fe3O4 growth on MgO appears in a step-flow fashion, whereas the growth of Fe3O4(111) on α-Al2O3(0001) occurs initially by island formation, and then island coalescence. However, the growth of α-Fe2O3 on α-Al2O3 appears to follow an intermediate growth mode. The formation of pure-phase films is controlled largely by oxygen partial pressure, plasma power, and growth rate, but appears to be independent of growth temperature, at least from 250 to 550 °C. The present study demonstrates that selective growth of pure-phase iron oxides with various low-index orientations can be achieved by controlling the growth conditions and selecting suitable substrates.

Atomic-scale controlled incorporation of ultrahigh-density Si doping sheets in GaAs

Delta-doped GaAs: Si with doping densities up to 4 × 1014 cm−2 has been grown by molecular beam epitaxy (MBE) at a substrate temperature of 590°C. To promote an ordered incorporation and thus avoid clustering of Si atoms, vicinal GaAs(001) surfaces 2° misoriented towards (111)Ga were used and Si was supplied in pulses. As evidenced by real-time reflection high-energy electron diffraction (RHEED) measurements an ordered incorporation of Si atoms on Ga sites along the step edges takes place. Although the ordered (3 × 2) structure degrades at high coverages, unusual high sheet carrier concentrations are obtained by pulsed delta-doping for doping concentrations > 1013 cm−2, as revealed by Hall measurements. The surface conditions during GaAs overgrowth have a strong influence on the free electron concentration, too. Raman scattering by local vibration modes and secondary ion mass spectrometry (SIMS) measurements are used to show that this is related to segregation effects as well as to a modification of the site occupancy.

Atomic-scale controlled incorporation of ultrahigh-density Si doping sheets in GaAs

Delta-doped GaAs: Si with doping densities up to 4 × 10 14 cm −2 has been grown by molecular beam epitaxy (MBE) at a substrate temperature of 590°C. To promote an ordered incorporation and thus avoid clustering of Si atoms, vicinal GaAs(001) surfaces 2° misoriented towards (111)Ga were used and Si was supplied in pulses. As evidenced by real-time reflection high-energy electron diffraction (RHEED) measurements an ordered incorporation of Si atoms on Ga sites along the step edges takes place. Although the ordered (3 × 2) structure degrades at high coverages, unusual high sheet carrier concentrations are obtained by pulsed delta-doping for doping concentrations > 10 13 cm −2, as revealed by Hall measurements. The surface conditions during GaAs overgrowth have a strong influence on the free electron concentration, too. Raman scattering by local vibration modes and secondary ion mass spectrometry (SIMS) measurements are used to show that this is related to segregation effects as well as to a modification of the site occupancy.

Wire-like incorporation of dopant atoms during MBE growth on vicinal GaAs(001) surfaces

The ordered incorporation of dopant atoms by combining lattice step growth on vicinal GaAs(001) surfaces and Si delta(δ)-doping has been studied by real-time reflection high-energy electron diffraction (RHEED) measurements. For Si deposition on 2° toward the (111)Ga plane misoriented surfaces and 0.4° toward the (1 1 1)As plane misoriented surfaces it is shown that the Si atoms arrange themselves preferentially along the step edges in a (3 × 2) structure consisting of an ordered array of Si dimers. For Si concentrations not exceeding substantially the amount expected to be attached at the step edges the GaAs growth can be continued at reduced substrate temperature without adverse effects on the growth front. By pulsed δ-doping an unusual high concentration of Si atoms can be incorporated as donors on Ga sites.

Structural transition in epitaxial Co-Cr superlattices.

Using a real-time reflection high-energy electron diffraction analysis technique, we show that the epitaxial growth of Co-Cr superlattices incorporates coherent close-packed Cr layers up to a thickness of 6 A\r{}. Beyond this thickness there is an abrupt structural transition to bcc (110) Cr in the classic Nishiyama-Wasserman and Kurdjumov-Sachs orientations. Magnetization and magnetoresistance results indicate that interlayer coupling is very weak, which we attribute to the altered crystal structure in the Cr layers.

Surface roughening of Ge(001) during 200 eV Xe ion bombardment and Ge molecular beam epitaxy

The kinetics of surface roughening of Ge(001) during 200 eV Xe ion bombardment and during Ge molecular beam epitaxy (MBE) are studied by real-time reflection high-energy electron diffraction. In both cases, initially smooth surfaces reach a steady state roughness which depends on temperature and incident ion or adatom flux. The data are analyzed in terms of a phenomenological model in which beam-induced roughening competes with beam-induced smoothening, and in which the defect creation rate and surface diffusivity are fitting parameters. For comparable fluxes, the temperature dependences for the net roughening induced by ions and adatoms are strikingly similar, implying a similarity in the surface diffusivities of vacancies and adatoms. For the case of ion-induced roughening, approximately one surface defect (in units of displaced surface atoms) is created per ion which is consistent with calculations assuming that a large fraction of atomic displacements recombine without producing surface defects at these ion energies.

Multiple reflection high-energy electron diffraction beam intensity measurement system

A video-based analysis system for reflection high-energy electron diffraction (RHEED) is described which simultaneously measures the intensities and profiles of multiple diffraction beams. This system is used to record real-time RHEED intensity oscillations for layer-by-layer epitaxial growth. Fast Fourier transform analysis of the oscillation data is used to directly determine the growth rate and to accurately obtain phase information about the oscillations. This system is demonstrated and compared to other methods of recording RHEED oscillation data.

Argon Ion Bombardment During Molecular Beam Epitaxy of Ge (001)

ABSTRACTUsing in situ, real-time reflection high energy electron diffraction (RHEED), we have measured the evolution of Ge (001) surface morphology during simultaneous molecular beam epitaxy and Ar ion beam bombardment. Surprisingly, low-energy Ar ions during growth tend to smoothen the surface. Bombardment by the ion beam without growth roughens the surface, but the surface can be reversibly smoothened by restoring the growth beam. We have measured the effect of such “ion beam growth smoothening” above and below the critical temperature for intrinsic growth roughening. At all measured growth temperatures the surface initially smoothens, but below the critical roughening temperature the final surface morphology is rough whereas above this temperature the final morphology is smooth.