Reciprocal Space Mapping Analysis Research Articles

Crystallographic and band structure analysis of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 thin film grown on β-Ga2O3 substrate via mist CVD

We successfully achieved the growth of a stacked layer composed of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 on a (010) β-Ga2O3 substrate using mist chemical vapor deposition (CVD). X-ray diffraction and reciprocal space mapping analyses were conducted, elucidating that the multilayer structure of the β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 thin film exhibited excellent crystallinity and coherent growth. Scanning transmission electron microscopy further revealed a continuous atomic arrangement at the heterointerface of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3. Furthermore, the bandgap values of β-(AlxGa1−x)2O3 and β-(InyGa1−y)2O3 thin films were determined to be 5.21 and 4.62 eV, respectively, through electron energy-loss spectroscopy. Notably, a slight broadening was observed in the bandgap transition at the interface of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3. Energy dispersive x-ray spectroscopy analysis indicated that this phenomenon could be attributed to the diffusion of In into the β-(AlxGa1−x)2O3 thin film layer. These results support mist CVD as a promising growth technique for developing β-Ga2O3-based heterojunction devices.

X-ray reciprocal space mapping analysis of ferromagnetic GdN films grown by pulsed laser epitaxy

Epitaxial thin films of ferromagnetic rare-earth nitride, GdN, were synthesized using pulsed laser deposition (PLD) on (001) MgO, pseudo-cubic (001) YAlO3, and (001) TiN buffered (001) MgO substrates. X-ray high-resolution reciprocal space mappings confirmed the epitaxial relationship between GdN and the substrates. The use of a TiN buffer layer changed the growth direction of the GdN films from (001) to (111). The ferromagnetic behavior of the films was characterized, and it was found that the magnetic easy axis could be tuned according to the crystal growth direction. These results suggest that PLD is a viable method for synthesizing epitaxial GdN films with tunable magnetic properties. The ability to control the crystal growth direction and magnetic easy axis of GdN films could be useful for developing spintronic devices.

Thickness-dependent Evolution of the Ferroelectric Domain in Ultrathin BiFeO3 Films below 10 nm

The scaling behavior of the ferroelectric domain in thick BiFeO3 has been reported to follow Kittel’s law both theoretically and experimentally. The law collapses at a certain thickness, known as the critical thickness. However, little experimental work focused on the ferroelectric domain evolution behavior in ultrathin BiFeO3 films below 10 nm. In this work, the BiFeO3 films with various thicknesses were prepared and observed with piezoresponse force microscopy (PFM) and the periodicity of the domain was extracted by a two-dimensional fast Fourier transform (2D-FFT). The reciprocal space mapping (RSM) analysis is consistent with PFM results demonstrating the 71° domain of the BiFeO3 films. It is confirmed that the critical thickness in BiFeO3 films is below 5 nm and the domain size decreases with decreasing thickness in accordance with Kittel’s law and a scaling exponent of 0.41 down to that thickness.

The role of the substrate on the structure of reactive sputtered Co3O4: From polycrystalline to highly oriented films

We report a systematic study of the substrate's influence on the structure of Co3O4 thin films grown by direct current reactive magnetron sputtering. Three different substrates have been simultaneously loaded to the deposition chamber and held at 620 K during growth: amorphous fused silica (a-SiO2), LaAlO3 (001), and α-Al2O3 (0001). Samples were characterized using atomic force microscopy, Raman spectroscopy, and high-resolution X-ray using synchrotron radiation (HR-XRD). Raman spectra showed bands corresponding to modes of Co3O4. The θ-2θ (HR-XRD) scans confirmed only the presence of the cubic spinel Co3O4 phase with its texture reliant on the sample substrate used. Rocking curves indicate that α-Al2O3 favors (111) Co3O4 film orientation (smaller mosaic spread). High-resolution 3-D Reciprocal Space Map analysis covering both (0006) α-Al2O3 and the (222) Co3O4 reflections have shown out-of-plane strain (0.1905%) and presented an in-plane isotropic intensity distribution of the film reciprocal lattice point, indicating no preferred mosaic direction. The substrate epitaxy governs the structure and morphology of the three samples, going from a rather flat, but polycrystalline textured Co3O4 film on a-SiO2, to a flat highly oriented strained film on α-Al2O3 and a rough highly textured and strained film on LaAlO3.

Room-temperature ferroelectric-like behavior in anatase TiO2 epitaxial films prepared by chemical solution deposition

Ferroelectrics of simple oxides have been of great interest due to they are easy to integrate with silicon semiconductors. The FE hysteresis loop was measured for the first time in pure anatase-phase titanium dioxide is reported, demonstrating that the ferroelectric-like behavior in a-TiO2 epitaxial thin films (7.5 nm to 228.8 nm) prepared by chemical solution deposition method on (001) SrTiO3 based substrates. Synchrotron based X-ray diffraction and reciprocal space mapping analysis prove that we have prepared high-quality epitaxial films. According to the results of the FE hysteresis loop, Pr can reach approximately 0.42 μC/cm2 in a-TiO2 film with a thickness of 14.5 nm. Furthermore, the PFM shows that the ferroelectric-like behavior is exhibited in a-TiO2 film of 7.5 nm and that the PFM polarization switch can be maintained for more than 100 min. All of these seem to indicate the presence of ferroelectric behavior in a-TiO2 films. We recognize that the origin of ferroelectric-like behavior is due to the distortion induced by strain and vacancy. However, since the polarization value is not very large and the PFM decays, the electrochemical effect has a non-negligible influence on the intrinsic ferroelectric performance.

Intervalley energy separation in the conduction band of InAs1−x Bi x determined by terahertz emission spectroscopy

InAsBi layers with different bismuth content were grown on InAs substrates by solid source MBE. The amount of bismuth incorporated in the layers was estimated using X-ray diffraction measurements. The relaxation degree of the grown crystalline layers was evaluated using reciprocal space map analysis. The intervalley energy separation in the conduction band of InAsBi was studied by Terahertz Excitation Spectroscopy. It has been found that this separation slightly decreases with increasing Bi content. In the studied samples with Bi content varying from 2.7% to 4.5% the Γ-L separation shifts down to about 0.9 eV.

Effect of Annealing Conditions on Recovery of Lattice Damage in a High-Energy-Implanted 4H-SiC Superjunction PIN Diode

A high energy ion implantation system has been recently developed at the Tandem Van de Graaff facility at Brookhaven National Laboratory with tunable energy to 150 MeV capable of multi-step, deep implantation in 4H-SiC wafers with dopant atoms, such as B, P, Al, and N. Medium and high voltage devices with deep junctions can be fabricated using this system. Lattice strain introduced by the implantation process needs to be recovered and dopant atoms activated by appropriate annealing process as the device performance is strongly associated with the extent of recovery of the lattice. Using Synchrotron X-ray Rocking Curve Topography (SXRCT) and Reciprocal Space Mapping (RSM), the strain induced by high energy implantation of Al and N in 4H-SiC in different patterns are measured and mapped. It is observed that the strain levels correlate with the total fluence levels. PiN diodes fabricated on these implanted wafers were then annealed at temperatures ranging from 1700 °C to 2000 °C for 60 min. The SXRCT and RSM analysis of the annealed samples suggests that for the same annealing duration, higher temperature leads to better lattice recovery.

Au(001) Thin Films: Impact of Structure and Mosaicity on the Oxygen Reduction Reaction in Alkaline Medium

Au(001) thin films with thickness varying from ca. 25 to 83 nm were deposited on MgO(001) substrates using pulsed laser deposition. Epitaxial growth of (001) Au on the (001) MgO single-crystal substrate was observed with a Au(001)[010]//[010](001)MgO cube-on-cube epitaxial relationship for substrate temperature ≤200 °C. Detailed X-ray reciprocal space mapping (RSM) analysis and atomic force microscopy (AFM) imaging of films with different thicknesses were conducted. There is a nonmonotonic dependence of both the in-plane and out-of-plane lattice parameters with the films’ thickness, while the lateral coherence length increases with the thickness. The maximum tensile strain measured was 0.3% for 65 nm thick Au(001) film. Cyclic voltammetry (CV) in acidic and alkaline media and Pb underpotential deposition (Pb UPD) measurements were used as fingerprints to assess the structure of the surface. All films exhibited the characteristic response of Au(001) single-crystal surfaces, although with some differences in the potential region between 0.7 and 1.2 V vs reversible hydrogen electrode (RHE), which corresponds to the adsorption and partial discharge of OH– anions at the electrode surface. Since epitaxial Au(001) thin films do not lend themselves to steady-state voltammetry with rotating disks or microelectrodes due to their square shape and the insulating nature of MgO substrates, the electrocatalytic activity of the films for the oxygen reduction reaction (ORR) in the alkaline electrolyte was assessed by sampled current voltammetry. It was shown that both the onset potential (Eonset) and the half-wave potential (E1/2) for the ORR do not show any dependence on strain but rather become less negative (more active) with an increase of the lateral coherence length. These data are rationalized in the context of recent density functional theory (DFT) calculations and the mechanisms governing the ORR in alkaline electrolytes.

First experimental demonstration and analysis of electrical transport characteristics of a GaN-based HEMT with a relaxed InGaN channel

In this paper, we demonstrate the first GaN/InGaN high electron mobility transistor with a partially relaxed InGaN channel, realized by utilizing a porous GaN buffer obtained by the selective and controlled electrochemical etch of GaN. According to reciprocal space map analysis, ∼70% of InGaN relaxation relative to GaN was achieved by this process. With the transfer length method and extracted charge profile, the relaxed InGaN channel demonstrates ∼9.6% two-dimensional electron gas mobility enhancement with respect to the strained InGaN channel which is consistent with a reduced effective mass predicted by theoretical analysis based on Vegard’s law. On comparing the output characteristics of devices with relaxed and strained InGaN channel, the relaxed channel devices show a 10% improvement of on-resistance while maintaining similar saturation current. A non-uniform charge distribution was found in the relaxed InGaN channel, which can be attributed to the strain relaxation fabrication process.

Dependence of the V/III Ratio on Indium Incorporation in InGaN Films Grown by Metalorganic Vapour Phase Epitaxy.

InGaN epitaxial layers were grown on c-plane sapphire substrates using the metalorganic vapour phase epitaxy (MOVPE) system at 760 °C. By varying the total flow rate of group-III sources (TMI+TEG) with a fixed molar ratio of group-III sources [TMI/(TMI+TEG)], the influence of V/III ratio were investigated from 4500 to 20000. The grown N-polar InGaN layers were investigated by atomic force microscopy and it is found that the surface roughness decreases with increasing the V/III ratios. High resolution X-ray diffraction analyses show that the phase separation decreases with increasing the V/III ratios. Photoluminescence measurements reveal that the peak position of the band-edge emission shifted toward the shorter wavelength with increasing the V/III ratios. Reciprocal space mapping (RSM) analyses were carried out on InGaN films. At low V/III ratio, the phase separation can be detected in InGaN films.

Substrate-imposed strain engineering of multiferroic properties in BCZT/LCMO bilayer heterostructures

Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer epitaxial heterostructures were deposited on LaAlO3 (LAO), (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and SrTiO3 (STO) single-crystalline substrates by using pulsed laser deposition, to investigate the substrate-imposed strain engineering of multiferroic properties. The epitaxial growth and the strain state of the as-deposited bilayer heterostructures were determined by X-ray reciprocal space mapping analysis. Results showed that the substrate-imposed strain engineering could turn the multiferroic properties by modulating the polarization switching and domain wall motion of the bilayer heterostructures. The optimal dielectric, ferroelectric and piezoelectric performances were obtained in the bilayer heterostructure deposited on STO substrate while an improved saturated magnetization was achieved in the bilayer heterostructure deposited on LAO substrate. The maximum magnetoelectric coupling was obtained in the bilayer heterostructure deposited on STO substrate with αE31 value of 61.2 mV/cm·Oe.

Effect of C-doped GaN film thickness on the structural and electrical properties of AlGaN/GaN-based high electron mobility transistors

High-resistivity carbon doped GaN (C-GaN) is highly desirable for high electron mobility transistors (HEMTs) application to reduce the leakage current at high electric fields. Herein, we investigate the structural and electrical properties of AlGaN/GaN-based HEMTs with different thicknesses of the underlying C-GaN layer. Reciprocal space mapping analysis indicates more lattice defects within GaN layer of the AlGaN/GaN heterostructure with thicker underlying C-GaN layer. Finite element method simulations reveal that the stress distribution over the AlGaN/GaN heterostructure thickness can be tuned by increasing the underlying C-GaN layer thickness from 2 to 7 μm. Consequently, a significant decrease of two-dimensional electrons gas mobility from 1188 to 653 cm2 V−1 s−1 measured by Hall effect method is observed at a higher underlying C-GaN layer thickness (7 μm). The change of electrical properties for HEMT structures with different underlying C-GaN thicknesses is not only affected by structural quality of the epilayers but also by thermal stress over HEMT structure thickness. Our results pave the avenue for improving the electrical properties of HEMT with a suitable thickness of underlying C-GaN layer.

Effect of TMAl flow rate on AlInGaN/GaN heterostructures grown by MOCVD

In this work, lattice-matched AlInGaN/GaN epilayers are grown on c-plane Al2O3 substrate by metal organic chemical vapor deposition technique. The structural and optical properties of AlInGaN/GaN epilayers have been studied and compared. TMIn and TMAl flow is believed to have a direct impact on the quality of AlInGaN epilayer. By varying In and Al composition in AlInGaN epilayer, dislocations (V-pit density) have been found to decrease, indicating lower threading dislocation density as confirmed by HRXRD and AFM. Reciprocal space mapping analysis has confirmed that the growth of AlInGaN on GaN buffer is a fully lattice matched growth. AlInGaN exhibits two emissions originating from In(Ga)N clusters and the AlInGaN random matrix is confirmed by photoluminescence. In the Raman spectra of AlInGaN, A1 (LO) mode at 751 cm−1 and In(Ga)N clustering A1(LO) mode at 680 cm−1 is observed. These findings open the new advancement in the field of opto-electronic applications.

Embedded AlN/GaN multi-layer for enhanced crystal quality and surface morphology of semi-polar (11-22) GaN on m-plane sapphire

We demonstrate high quality semi-polar (11-22) gallium nitride thin film grown on m-plane sapphire substrate with the insertion of AlN/GaN multi-layer via MOCVD. The influence of three different number of multi-layers AlN/GaN pairs on the crystal quality and surface morphology of semi-polar (11-22) gallium nitride thin film is investigated. The surface morphology analysis strongly suggests that increasing the number of AlN/GaN pairs from 20 to 60 suppresses the arrowhead-like and undulated features. The increase of AlN/GaN pairs also enhanced the surface quality, with the root mean square roughness improving from 16.24 nm to 6.08 nm. The abruptness of the interface between the AlN/GaN pairs was seen to improve significantly upon reaching the 40th pair where a continuous thin layer was clearly observed for each pair. The crystal quality was also observed to be enhanced at higher number of AlN/GaN pairs, where the on- and off-axis X-ray rocking curve showed significant reduction in the full width at half maximum of at least ~10% and 20%. Finally, x-ray reciprocal space mapping analysis further confirms the enhancement of the crystal quality as the diffuse scattering streak was suppressed, which may indicate a significant reduction of the defect density.

Influence of TMIn flow rate on structural and optical quality of AlInGaN/GaN epilayers grown by MOCVD

In this paper, nearly lattice-matched AlInGaN/GaN epilayers grown on sapphire substrate by metal organic chemical vapor deposition have been studied. The structural and optical properties of these epilayers have been investigated and compared. The TMIn flow is believed to have a direct influence on the quality of the AlInGaN epilayer. By increasing indium composition in AlInGaN epilayer, dislocations have been found to decrease, indicating lower threading dislocation density confirmed by HRXRD. Reciprocal space mapping analysis has confirmed that the growth of AlInGaN on GaN buffer is a fully strained growth. Room temperature photoluminescence from AlInGaN exhibits two emissions originating from In(Ga)N clusters and the AlInGaN random matrix, respectively. Upon increasing the TMIn flow, red shift has been observed in the photoluminescence peak confirming the decrease in bandgap of AlInGaN epilayers. In the Raman spectra, the mode at 751 cm−1 is attributed to the A1 (LO) mode of AlInGaN. The mode at 632 cm−1 is attributed to In(Ga)N clustering and assigned as A1 (LO) mode. V-shaped defect pits with different density and size have been observed on the surface of AlInGaN epilayers.

Effect of strain in sputtered AlN buffer layers on the growth of GaN by molecular beam epitaxy

The strain dynamic of thin film AlN is investigated before and after the deposition of a GaN epitaxial layer using plasma assisted molecular beam epitaxy. X-ray diffraction ω/2θ-scan and asymmetric reciprocal space mapping analysis show that the deposition of GaN alters the strain state of the underlying AlN template. The in-plane lattice constant of the AlN is found to increase upon growth of GaN, giving rise to a more relaxed GaN epitaxial layer. Hence, the subsequent GaN epitaxial thin film possesses better structural quality especially with lower screw dislocation density and flat surface morphology which is evidenced by the X-ray diffraction ω-scan, room temperature photoluminescence, and atomic force microscopy analysis. Such relaxation of AlN upon GaN deposition is only observed for relatively thin AlN templates with thicknesses of 20 nm–30 nm; this effect is negligible for AlN with thickness of 50 nm and above. As the thicker AlN templates already themselves relax before the GaN deposition, the localized strain fields around the misfit dislocations prohibit further change of lattice parameters.

Plasma assisted molecular beam epitaxy growth and effect of varying buffer thickness on the formation of ultra-thin In0.17Al0.83N/GaN heterostructure on Si(111)

This work reports on the detailed plasma-assisted molecular beam epitaxy (PAMBE) growth of ultra-thin In0.17Al0.83N/GaN heterostructures on Si(111) substrate with three different buffer thickness (600 nm, 400 nm, and 200 nm). Growth through critical optimization of growth conditions is followed by the investigation of impact of varying buffer thickness on the formation of ultra-thin 1.5 nm, In0.17Al0.83N–1.25 nm, GaN–1.5 nm, In0.17Al0.83N heterostructure, in terms of threading dislocation (TD) density. Analysis reveals a drastic reduction of TD density from the order 1010 cm−2 to 108 cm−2 with increasing buffer thickness resulting smooth ultra-thin active region for thick buffer structure. Increasing strain with decreasing buffer thickness is studied through reciprocal space mapping analysis. Surface morphology through atomic force microscopy analysis also supports our study by observing an increase of pits and root mean square value (0.89 nm, 1.2 nm, and 1.45 nm) with decreasing buffer thickness which are resulted due to the internal strain and TDs.

Properties of an AlGaN/AlN distributed-Bragg-reflector structure

An AlGaN/AlN distributed-Bragg-reflector (DBR) structure with a high Al content was grown by using plasma-assisted molecular beam epitaxy (PA-MBE). The properties of the sample were characterized by using the transmission electron microscopy, high-resolution X-ray diffraction, atomic force microscopy, and reflectivity spectrum measurements. The reciprocal space mapping analysis indicated that the strain in the AlGaN layers was partially relaxed. The morphology of the DBR exhibited a surface covered by grains (average size of about 130 nm), and the surface roughness was about 2 nm. The spectral measurements showed that the DBR structure presented a peak reflectivity of 68.8% at the center wavelength of 247 nm, which indicated that this DBR structure could work in the deep solar-blind UV region with acceptable reflectivity. However, the optical properties of the DBR structure were deteriorated by the fluctuation of the Al composition, non-uniformity of the layer thickness, the blurry, rough interface in the DBR structure, and so on.

Analysis of reciprocal space maps of GaN(0001) films grown by molecular beam epitaxy

The reciprocal space map of a heteroepitaxial single-crystal GaN(0001) film on 6H-SiC(0001) is analyzed. The film contains a high density of threading dislocations which intersect the film parallel to the surface normal. The strain field associated with these dislocations is expected to broaden all reciprocal lattice points to discs perpendicular to the dislocation lines,i.e.parallel to the surface. Experimentally, however, the reflection is observed to be broadened also perpendicular to the surface and is rotated towards the surface normal. Using Monte Carlo simulations, it is shown that both of these features are a natural consequence of the presence of misfit dislocations at the film/substrate interface.

Stress of Homogeneous and Graded Epitaxial Thin Films Studied by Full-Shape Analysis of High Resolution Reciprocal Space Maps

The Bragg peak position of a homogeneous solid solution epitaxial film is directly related to the solid solution concentration, film strain and, consequently, residual stress. The peak shape contains information about defects present in the sample. In the case of compositionally graded epitaxial films the situation is more complex since instead of a single Bragg peak there is a continuous diffracted intensity distribution which can be measured by means of recording high resolution reciprocal space maps. We analyse the thin film residual stress based not only on peak positions, but taking into account the defect-induced peak shape as well. Consideration of the peak shape enables the determination of the stress depth profile in the case of graded films and to imporves the accuracy in the case of homogeneous films.