High-resolution Double-crystal X-ray Research Articles

Strained InAs single quantum wells embedded in a Ga0.47In0.53As matrix

Strained InAs single quantum wells (SQWs) embedded in a Ga0.47In0.53As matrix are grown on InP substrates by a modified solid-source molecular beam epitaxy. The series of samples with well between 2 and 11 monolayers (ML) is characterized by high-resolution double crystal x-ray diffraction and photoluminescence (PL) spectroscopy. The excellent agreement obtained between simulated and experimental x-ray rocking curves demonstrates the coherence of the samples. A PL linewidth as narrow as 7 meV is measured at 6 K for the SQW with a thickness of 5 ML. This is the best result reported so far for InAs QWs grown on InP. In addition, luminescence is observed up to room temperature for all samples.

X-ray diffraction study of corrugated semiconductor surfaces, quantum wires and quantum ☐es

We show that high-resolution double-crystal X-ray diffraction is not only sensitive to the periodic structural modulation normal to the crystal surface as it occurs in semiconductor superlatices, but is under appropriate experimental conditions also very sensitive to the periodic modulation parallel to the crystal surface (surface grating). This fact allow us to study the structural properties, geometrical parameters and strain state, of quantum wires and quantum ☐es. We present experimental results on AlGaAs/GaAs and InAs/GaAs quantum wires and show that the quantum wire period and quantum wire width can be determined very precisely. In addition, we found a partial elastic strain relaxation normal to the quantum wires, resulting in an orthorhombic lattice deformation.

Structural analysis of high-Tc epitaxial GdBa2Cu3O7-x superconducting thin films

High-Tc superconducting epitaxial thin films of GdBa2Cu3O7-x, (GBCO), grown in situ by DC magnetron sputtering onto substrates of single-crystal (001) oriented LaAlO3, SrTiO3 and yttrium-stabilized ZrO2, have been studied by high resolution double crystal X-ray diffractometry, X-ray white beam Laue topography and scanning electron microscopy. The lattice mismatch between film and substrate in the (001) direction was determined to be 3.0% for the film on the LaAlO3 substrate, while the mismatch in the (110) direction was estimated to be 2.9% from extremely asymmetric X-ray rocking curves exploiting the tunability of synchrotron radiation. The data show that the unit cell of the GBCO film is greater than that of bulk GBCO materials, implying that the film is totally relaxed. It is proven that when grown on an yttrium-stabilized ZrO2 substrate the a and b axes of the unit cell of the film are rotated by 45 degrees with respect to the a axis of the substrate. Scanning electron microscopy observations reveal that there are many micrograins less than one micron in size on the surface of the films, confirming the results of double-crystal topography.

Structural and optical properties of (100) InAs single-monolayer quantum wells in bulklike GaAs grown by molecular-beam epitaxy.

A single (100) monolayer InAs in GaAs is grown by conventional molecular-beam epitaxy. The structural properties of these highly strained single-monolayer quantum-well samples are investigated with high-resolution double-crystal x-ray diffractometry, double-crystal x-ray reflection topography, and transmission electron microscopy. The calculation of the x-ray-diffraction patterns within the framework of the dynamical diffraction theory provides the precise determination of the strain state and the thickness of the layer even in the submonolayer region. This approach allows us to prove the superior crystalline quality of the samples. In particular, no misfit dislocations are generated at the highly strained InAs/GaAs heterointerface as observed by x-ray topography. These results are confirmed by the high-resolution lattice image of the heterointerface, indicating the excellent homogeneity and flatness of the InAs lattice plane.In addition, we present evidence for a strong impact of the strain on the incorporation of cations in the crystal lattice during epitaxial growth. This structural information on an atomic scale is necessary for a correct interpretation of photoluminescence (PL) and photoluminescence excitation (PLE) spectra. We correlate the results of our structural investigation with a simple analysis of the PL linewidth and the blue shift (Stokes shift) of the heavy-hole exciton resonance in the PLE spectrum with respect to the electron heavy-hole transition in the PL spectrum. Both the PL linewidth and the Stokes shift are explained in terms of a fluctuation of the well width of 1 monolayer and a lateral island size of about 10 nm, in excellent agreement with the lattice image of the structure. The important result of our investigation is that a single monolayer of InAs yields a pronounced redshift of the quantum-well luminescence as compared with that of bulklike GaAs. Additionally, the PLE spectrum provides direct experimental evidence for a weak confinement of the light-hole excitons in the InAs plane. This result clearly indicates a type-I band alignment for both heavy holes and light holes, as predicted by a simple envelope-function calculation. Finally, the samples exhibit a surprisingly high luminescence efficiency, resulting from a very efficient trapping of the photoexcited carriers into the InAs-monolayer quantum well with trapping times of the order of only 20 ps.

Monolayer resolution by means of x-ray interference in semiconductor heterostructures

We show that the interference of x-ray wave fields in semiconductor heterostructures can be used to detect ultrathin layers having 1 monolayer thickness. A detailed theoretical and experimental investigation on Si/Six Ge1−x heterostructures is presented. The interference effect is studied experimentally by using a high-resolution double-crystal x-ray diffractometer. The diffraction patterns are recorded in symmetrical as well as asymmetrical Bragg geometries and are analyzed by using the dynamical x-ray diffraction theory for distorted crystals.

Optical and structural properties of GaAs grown on (100) Si by molecular-beam epitaxy

GaAs epitaxial layers (0.5 μm<d<4 μm) were grown by molecular-beam epitaxy (MBE) on (100) Si substrates (2° off in 〈011〉 direction) by using the two-step growth mode. The structural properties of the grown films (lattice constants, misorientation, crystal quality) have been measured by high-resolution double-crystal x-ray diffraction. The GaAs layers are under biaxial tensile strain at temperatures below the growth temperature because of the difference in thermal expansion coefficients of GaAs and Si. With the use of temperature-dependent photoluminescence and excitation spectroscopy, the band edges as well as the dominant near-band-edge recombination processes have been determined. Excitonic resonances are clearly observed in the excitation spectra. The strain splitting between the heavy- and light-hole valence bands amounts to 13 meV at 4.5 K. The dominant low-temperature luminescence line at 1.489 eV is assigned to exciton-related recombination processes. In addition to the band-to-carbon acceptor (e,A0C) recombination at 1.470 eV, a defect-related (d,A0C) recombination process is identified at 1.411 eV for a sample temperature of 4.5 K. The defect concentration is higher near the GaAs to Si interface region and decreases with increasing GaAs layer thickness. With increasing temperature (80–300 K) the valence-band splitting diminishes and reaches a value of 9 meV at room temperature, in good agreement with the strain values determined by x-ray diffraction.

High-resolution double-crystal x-ray diffraction for improved assessment of modulated semiconductor structures

High-resolution double-crystal x-ray diffraction for improved assessment of modulated semiconductor structures

High-resolution double-crystal X-ray diffraction for improved assessment of modulated semiconductor structures

The structural parameters of MBE grown modulated semiconductor structures composed of AlAs/GaAs and Ga 0.47In 0.53As/Al 0.48In 0.52In 0.52As are investigated by high-resolution doublecrystal X-ray diffraction. A semi-kinematical approximation of the dynamical X-ray diffraction theory is used to analyze the observed diffraction curve. Comparison of experimental and theoretical data yields information on strain distribution, variation of the period, chemical composition homogeneity, and quality of the heterointerfaces. Interface broadening and lattice period variation less than one lattice constant can be detected.

Design, construction and test results of a slitless, high-power, high-resolution double crystal X-ray monochromator for synchrotron radiation

The X-ray monochromator that has recently been implemented on the Anglo-Dutch X-ray beamline for EXAFS and XANES experiments on the Daresbury Synchrotron Radiation Source consists of two bent crystals and an ellipsoidal mirror. By bending the initially flat silicon crystals the energy resolution is 1–5 eV over the energy range of 3–14 keV. The mirror is placed behind the crystals and ensures horizontal and vertical focussing of the monochromatic beam on the sample. Because of the high thermal load of the beam (100 W maximum) the first crystal is water cooled.