Resolution X-ray Reciprocal Space Mapping Research Articles

Liquid Phase Epitaxy growth of Tm3+-doped CaF2 thin-films based on LiF solvent

Single-crystalline Tm3+-doped calcium fluoride (Tm3+:CaF2) films were grown onto (100) oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy method using LiF as a solvent. The growth was performed at the temperatures of 853–865 °C. Crack-free transparent films with a high doping concentration of ∼6 at.% and a thickness up to ∼100 μm were obtained. The film structure and morphology were characterized by confocal microscopy, X-ray diffraction and scanning electron microscopy. High resolution X-ray diffraction and reciprocal space mapping confirmed the high crystallinity and the low mosaicity of the films. Energy-Dispersive X-ray spectroscopy revealed a uniform distribution of Tm3+ ions in the films. The Tm3+:CaF2 films exhibit a broadband emission at ∼2 μm and are of interest for waveguide lasers. The use of CaF2–LiF binary system is also promising for fabrication of other rare-earth-doped CaF2 layers.

Optimization of MBE Growth Conditions of In0.52Al0.48As Waveguide Layers for InGaAs/InAlAs/InP Quantum Cascade Lasers.

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In0.52Al0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In0.53Ga0.47As/In0.52Al0.48As active region, e.g., growth temperature of Tg = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In0.52Al0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In0.52Al0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In0.52Al0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to Tg = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.

Epitaxial strain adaptation in chemically disordered FeRh thin films

Strain and strain adaption mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaption mechanism in a thin film model system as function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g. epitaxial boundary condition. It is shown by means of high resolution X-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a =0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaption behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study.

Terahertz excitation spectra of GaAsBi alloys

Terahertz excitation spectroscopy was used for the determination of energy separation between the main (Γ) and subsidiary (L and X) conduction band valleys of GaAs1−xBix. The samples used in this study were 1 µm–1.5 µm thick bismide layers grown by Molecular Beam Epitaxy on GaAs substrates. They contained up to 8% of bismuth as determined by high resolution x-ray diffraction (HR-XRD) and reciprocal space mapping (RSM), taking into account the layer relaxation. It was found that both subsidiary conduction band valleys at L and X points of the Brillouin zone move away from the conduction band minimum at rates of 18 meV/%Bi and 25 meV/%Bi, respectively, with increasing Bi content in the alloy.

Linearly polarized photoluminescence of anisotropically strained c-plane GaN layers on stripe-shaped cavity-engineered sapphire substrate

Anisotropic in-plane strain and resultant linearly polarized photoluminescence (PL) of c-plane GaN layers were realized by using a stripe-shaped cavity-engineered sapphire substrate (SCES). High resolution X-ray reciprocal space mapping measurements revealed that the GaN layers on the SCES were under significant anisotropic in-plane strain of −0.0140% and −0.1351% along the directions perpendicular and parallel to the stripe pattern, respectively. The anisotropic in-plane strain in the GaN layers was attributed to the anisotropic strain relaxation due to the anisotropic arrangement of cavity-incorporated membranes. Linearly polarized PL behavior such as the observed angle-dependent shift in PL peak position and intensity comparable with the calculated value based on k·p perturbation theory. It was found that the polarized PL behavior was attributed to the modification of valence band structures induced by anisotropic in-plane strain in the GaN layers on the SCES.

In-line monitoring of strain distribution using high resolution X-ray Reciprocal space mapping into 20 nm SiGe pMOS

Together with the downscaling comes the introduction of new materials, such as strained SiGe, in order to boost the transistor performances. For an effective deployment it requires the implementation of methods capable to monitor the strain directly in the process lines. High Resolution X-Ray Diffraction has played a critical role in industry for thin films metrology purpose and have the potential to extend its capability to the strain characterization in the transistors. The latest industrial instruments provide fast and automatic measurement capabilities. This enables the use of Reciprocal Space Mapping (RSM), a key feature for strain characterization, as a monitoring method. To do so, automatic data extraction of RSMs is of course mandatory, so preliminary work performed on RSM treatment will be introduced in this paper. So far these studies have been carried out on nanostructures representative of transistor technologies and reveal innovative and conclusive results.

Optical properties associated with strain relaxations in thick InGaN epitaxial films.

Structural and optical properties of thick InGaN layers with strain and composition inhomogeneities are investigated. High resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis reveal that the In composition inhomogeneity is accompanied by strain relaxations during the growth of thick InGaN layers. According to the structural analysis, the commonly observed double photoluminescence (PL) peaks have been confirmed to be associated with the strain relaxation in thick InGaN films. Temperature-dependent PL measurements further indicate that the relaxed phase in InGaN films exhibits better emission efficiency than the strained phase. Recombination dynamics reveal that the carrier localization effect is more pronounced in the relaxed phase due to the compositional pulling effect. The correlations between emission efficiency and localization effect in thick InGaN films are discussed.

Effects of molecular beam epitaxy growth conditions on composition and optical properties of InxGa1−xBiyAs1−y

We describe the growth conditions of InxGa1−xBiyAs1−y (lattice-mismatched and matched) on InP substrates by molecular beam epitaxy and the resulting properties. Due to their anomalously narrow bandgaps and the presence of bismuth, these materials are promising for optoelectronics and thermoelectrics. Low growth temperature and moderate As/Bi beam equivalent pressure ratios are beneficial for Bi incorporation, in good qualitative agreement with GaBiyAs1−y on GaAs. Up to 6.75% bismuth is incorporated. High resolution x-ray diffraction and reciprocal space mapping show that InxGa1−xBiyAs1−y samples exhibit good crystalline quality and zero relaxation. The band gap is reduced in agreement with theoretical predictions. Lattice-matched samples have been produced with lattice mismatch ≤0.21%.

Growth and Characterization of GaN/InGaN Multiple Quantum Wells on Nanoscale Epitaxial Lateral Overgrown Layers

GaN/InGaN multiple quantum wells (MQWs) were fabricated on nanoscale epitaxial lateral overgrown (NELO) GaN layers (type N) by metal−organic chemical vapor deposition using a SiO2 layer with nanometer scale windows as the growth mask. Transmission electron microscopy (TEM) results clearly demonstrate coherent growth of GaN in the window regions while lateral growth is observed over the SiO2 layer. Based on the TEM and atomic force microscopy measurements, we observed substantial reduction in the threading dislocation density for the type N GaN films. Experimental results on electroluminescence (EL) measurement indicate substantial improvement in the EL intensity as well as a 15 nm blue shift in the EL peak wavelength. High resolution X-ray diffraction and reciprocal space mapping characterizations clearly indicate significant reduction in strain generation in the MQWs grown on NELO GaN layers compared to the control samples (type C). Such reduction in strain generation in the MQW gives rise to the reduction in the quantum-confined Stark effect. This is also consistent with the observed blue-shift in the EL peak. Detailed analyses of the optoelectronic properties of the devices indicate significant improvements in the internal quantum efficiency, ηi, and the extraction efficiency, ηe, by as much as 55.8 and 57.3%, respectively, compared to the type C devices. The improvement in ηi is attributed to reductions in both the nonradiative recombination centers and the quantum-confined Stark effect in the type N devices and the improvement in the extraction efficiency is attributed to the texturing of the GaN layer due to the incorporation of the SiO2 layer with nanometer scale windows.

Growth of ZnSe(1−x)Tex epilayers by isothermal closed space sublimation

ZnSe ( 1 − x ) Te x (x∼0.06) epilayers were grown on GaAs(001) substrates at 350 °C by isothermal closed space sublimation (ICSS) technique. The epitaxial growth was performed in low-pressure helium atmosphere (∼0.1 mbar) by sequential exposures of the substrate to vapors of a solid solution of selenium-tellurium and elemental zinc. The use of a mixed source is proposed in order to regulate the partial vapor pressure of the constituents by composition. Strain and composition of the ZnSe(1−x)Tex epilayers were extracted from high resolution x-ray reciprocal space mapping. Structural investigations show a reasonably good crystalline quality of the epilayers. Good reproducibility of composition and control of thickness were obtained although atomic layer epitaxy regimen was not achieved. A growth rate of 1.3 monolayers/cycle was ascribed to multilayer adsorption and the existence of an efficient transport of SeTe in graphite under thermodynamic conditions of ICSS. Both Raman and photoluminescence characterizations suggest the existence of random alloy epilayers with larger composition disorder in the mesoscopic scale than those obtained by molecular beam epitaxy.

Photoluminescence of high energy Ar+-irradiated SrTiO3 single crystal

Abstract We report the structural and photoluminescence characteristics of a high energy Ar + -irradiated SrTiO 3 single crystal. High resolution X-ray reciprocal space mapping, cross-section scanning electron microscopy and photoluminescence measurements suggest that thick amorphous and oxygen-deficient SrTiO 3 layers are formed after irradiation. Due to the highly accelerated Ar + (∼100 keV), electrical conductivity of the oxygen-deficient SrTiO 3 layer seems to be highly reduced, which results in the absence of 440 nm (blue-light) emission at 300 K, and broad and weak 380 nm emission at 20 K.

Growth of dilute nitride alloys of GaInSb lattice-matched to GaSb

The growth of dilute nitride alloys of GaInSb by plasma-assisted molecular beam epitaxy is reported. The principle of lattice matching GaInNSb alloys to GaSb(0 0 1) substrates is demonstrated. High resolution X-ray diffraction rocking curves and reciprocal space maps indicate the high crystalline quality of the GaInNSb layers and illustrate a lattice match to GaSb with nitrogen and indium incorporations of 1.8% and 8.4%, respectively. The rms roughness of a nominally lattice matched GaInNSb/GaSb(0 0 1) layer was determined from atomic force microscopy to be ∼ 1.8 nm .

X-ray scattering techniques for assessment of III–V wafer bonding

X-ray scattering techniques were employed to study the processing and materials issues that are important for III–V semiconductor wafer bonding. To demonstrate the feasibility of creating wafer-bonded III–V on insulator structures, InP and GaAs wafers were bonded to GaAs wafers using silicon nitride (SiN) intermediate layers. A key process parameter is the SiN intermediate layer that is deposited by either sputtering or by plasma-enhanced chemical vapour deposition. We demonstrate that x-ray reflectivity is uniquely able to determine thin film density as well as surface roughness of the nitride; both of which are critical for successful bonding and layer transfer. The strain in the InP layer after transfer to a GaAs substrate was assessed using high resolution x-ray reciprocal space mapping. Annealing the bonded pairs at low temperatures (⩽300°C) produced a strong bond with no strain in the InP layer. After subsequent higher temperature (500°C) annealing, approximately 0.1% tensile strain appeared in the InP layer as well as a reduction in the crystalline quality. X-ray topography confirmed that this strain was accompanied by the introduction of dislocations. Topographic measurements of other samples also showed the presence of void-induced defects. X-ray scattering techniques proved to be very valuable in the evaluation of wafer-bonded structures.

Nearly strain-free AlGaN on (0 0 0 1) sapphire: X-ray measurements and a new crystallographic growth model

Two micrometer thick Al x Ga 1− x N layers with 0< x<0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0 0 0 1) substrates. For Al concentrations 0.18< x<0.25 the layers are found to be nearly strain-free as determined by high resolution X-ray reciprocal space mapping around the (0 0 0 2), (2 0−2 4), and (2 0−2 0) Bragg reflections in conventional and grazing incidence geometry, respectively. The in-plane lattice parameter a of layers grown in this composition regime coincides with that of ( 2/ 3) a (sapphire). Their rotational and tilting disorder shows a minimum as compared to layers grown outside this regime. (GaN/Al x Ga 1− x N) multi-quantum well structures on top of such buffer layers are fully pseudomorphic having lowest interface disorder and best surface morphology as evaluated by specular and diffuse X-ray reflectivity measurements. The findings are explained by the assumption of a 2D coincidence site lattice for the epitaxial growth of AlGaN on sapphire. The coincidence site lattice has hexagonal symmetry with the lattice parameter three times a (A 0.22Ga 0.78N) equals two times a (sapphire).

Dislocation Arrangement in a Thick LEO GaN Film on Sapphire

Diffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16µm thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [110] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (100). The third set of dislocations threads laterally through the film along the [100] bar axis with 1/3&lt;110&gt;-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5º.

Dislocation Arrangement in a Thick LEO GaN Film on Sapphire

AbstractDiffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16[.proportional]m thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [11 20] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (1 100). The third set of dislocations threads laterally through the film along the [1 100] bar axis with 1/3&lt;11 20&gt;-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5°.

Shear strains in dry etched GaAs/AlAs wires studied by high resolution x-ray reciprocal space mapping

We have fabricated GaAs/AlAs quantum wires and quantum dots by means of molecular beam epitaxy, electron beam lithography, and subsequent reactive ion etching using SiCl4 and O2. The nominal periods are 300 nm and 350 nm for both wire and dot samples. High resolution x-ray reciprocal space maps of the 350 nm samples exhibit not only satellites corresponding to a periodicity of 350 nm but also additional satellites corresponding to a period of three times 350 nm, whereas there are no such extra peaks in the maps of the 300 nm samples. These secondary satellites are shown to be associated with a discretization effect in electron beam writing. Moreover, we found, that the shear strain in the wires has a distinct influence on the intensities of these weak extra satellites. Hence, they provide a sensitive means for the assessment of shear strains in elastically relaxed quantum wires.

High-resolution x-ray diffraction from multilayered self-assembled Ge dots

We have studied multilayers of self-assembled Ge-rich dots embedded in silicon grown by molecular-beam epitaxy using high resolution x-ray reciprocal space mapping and reflectivity. The Si spacer thicknesses between the dot arrays were in the range of 10--40 nm, the typical dot size was about 150 nm for the diameter and 7 nm for the height. The measured reciprocal space maps were simulated using statistical kinematical x-ray-diffraction theory, and a good agreement between experimental and simulated data has been achieved. From the measurements, the in-plane strain in the dot lattice was determined. We derived the degree of the vertical correlation of the dot positions (``stacking'') and a lateral ordering of the dots in a square array with main axes parallel to the 〈100〉 directions, with an array lattice constant of about 500 nm.

High resolution x-ray diffraction studies of short-period CdTe/MnTe superlattices

We have investigated the elastic properties of epitaxial MnTe layers using triple axis high resolution x-ray diffraction and reciprocal space mapping. A series CdTe/MnTe superlattices (SLs) grown by molecular beam epitaxy and nearly strain compensated, were deposited on [001] Cd1−xZnxTe substrates. In order to obtain the MnTe content of these SLs without an a priori knowledge of the elastic properties of cubic MnTe, annealing experiments were performed to interdiffuse the individual layers into a mixed Cd1−xMnx Te alloy layer. For a precise analysis of the data, it was found to be important to determine the in-plane strain of the superlattice layers using reciprocal space maps around symmetric and asymmetric reciprocal lattice points. The value for the Poisson ratio of zinc-blende MnTe was determined to be ν=C11/2C12=0.77±0.15.

High Resolution X-Ray Reciprocal Space Mapping of Wavy Layers

ABSTRACTWe apply the technique of High Resolution X-ray Reciprocal Space Mapping (HRRSM) to the study of wavy layers in InGaAs multilayer thin films on InP substrates. By accurately measuring the positions of the layer Bragg peaks in reciprocal space we obtain measurements of the compositions and residual coherency strains in the layers. We discuss the contributions to the diffuse scatter around the Bragg peaks from factors such as lattice tilts and interface roughness. By modelling the shapes of the diffraction profiles we obtain measurements of mosaic block size both perpendicular and parallel to the multilayer/substrate interface. We conclude that the wavy interface morphology arises predominantly from layer thickness variations rather than lattice tilts.