Asymmetric Reciprocal Space Mapping Research Articles

The effect of ammonia partial pressure on the growth of semipolar (11–22) InGaN/GaN MQWs and LED structures

This paper presents the effect of ammonia partial pressure at a relatively low fixed V/III (∼765) growth atmosphere on the growth of multi-quantum well (MQW) structures and the light emitting diode (LED) emission wavelength. At fixed V/III ratio, ammonia partial pressure together with the consequent group III partial pressures are shown to influence the growth of InGaN/GaN MQWs in term of growth rate and the indium incorporation for the quantum well. It is also observed that the growth of InGaN quantum well (QW) behaves differently as compared to GaN quantum barrier (QB) at low V/III ambient. For the growth of InGaN QW, the data collected shows the existence of threshold point for ammonia partial pressure of ∼300 Pa. In the regime below the threshold value, the decomposition of InGaN due to severe dissociation of In-N bonds caused the growth rate to be extremely low. As the ammonia partial pressure increases above the threshold value, the growth rate of InGaN QW increases sharply together with an improvement in indium incorporation, as proven by the XRD and (11–24) asymmetric reciprocal space mapping (RSM) results. The XRD result is consistent with the atomic force microscopy (AFM) results. As the partial pressures increases, the RMS and peak-to-valley roughness increases which is credited to the increment in indium incorporation. Furthermore, “needle-like” structures can be observed over all samples when the MQWs were grown with limited ammonia partial pressure. During the growth for the LED structure with thin layers of InGaN/GaN, the partial pressure of ammonia became a dominating factor in the formation of well-abrupt layer interfaces. Through the XRD and room-temperature photoluminescence (RTPL) results, it is shown that abrupt layers can be formed at higher ammonia partial pressures, while indium clusters tend to form at a lower partial pressure, which are most probably due to the insufficient N concentration, while the electroluminescence (EL) results were consistent as well. Therefore, it is once again confirmed that the performance of the InGaN/GaN LED is highly governed by the partial pressures of the ammonia.

Influence of an ultra-thin buffer layer on the growth and properties of pseudomorphic GaAsBi layers

A series of 100 nm-thick pseudomorphic GaAsBi layers with the Bi content varying from 0.97 to 11.2% have been grown by molecular beam epitaxy (MBE) on the semi-insulating GaAs(100) substrates buffered with an ultra-thin up to 20 nm thick GaAs layer. The main attention in this work was focused on the investigation of relaxation in the Bi induced compressively-strained GaAsBi layers containing a various content of Bi. The lattice parameters of GaAs- Bi compound and the Bi concentration have been evaluated from high resolution X-ray diffraction measurements. The relaxation values of GaAsBi layers ranging from 0.4 to 3.5% were obtained analyzing the symmetric and asymmetric reciprocal space maps of (004) and (115) planes, respectively. Also, the complex study was performed to clarify the relaxation effect on structural, morphological and optical properties of bismide layers. Optical measurements revealed a significant reduction of the energy band gap from 1.34 to 0.92 eV for the layers containing 0.97–8.6% of Bi in the GaAs lattice.

Robust perpendicular magnetic anisotropy in off-axis sputtered europium iron garnet (EuIG) thin films

The rare earth iron garnet (REIG), Eu3Fe5O12 (EuIG) has emerged as a promising material for ferrimagnetic insulator based spintronics. Earlier studies have shown that perpendicular magnetic anisotropy (PMA) in EuIG and other REIGs can be achieved by magnetostriction and misfit strain at the film-substrate interface. Typically, the strain relaxes significantly with thickness, limiting the thickness of films that can be grown using this strategy. Here we show PMA that does not reduce substantially with thickness in epitaxial EuIG films grown by off-axis sputtering; The PMA remains robust in films > 100 nm thick. Epitaxially induced strain with minimal relaxation is confirmed using high resolution symmetric and asymmetric x-ray diffraction reciprocal space maps.

Full reciprocal-space mapping up to 2000 K under controlled atmosphere: the multipurpose QMAX furnace.

A furnace that covers the temperature range from room temperature up to 2000 K has been designed, built and implemented on the D2AM beamline at the ESRF. The QMAX furnace is devoted to the full exploration of the reciprocal hemispace located above the sample surface. It is well suited for symmetric and asymmetric 3D reciprocal space mapping. Owing to the hemispherical design of the furnace, 3D grazing-incidence small- and wide-angle scattering and diffraction measurements are possible. Inert and reactive experiments can be performed at atmospheric pressure under controlled gas flux. It is demonstrated that the QMAX furnace allows monitoring of structural phase transitions as well as microstructural evolution at the nanoscale, such as self-organization processes, crystal growth and strain relaxation. A time-resolved in situ oxidation experiment illustrates the capability to probe the high-temperature reactivity of materials.

Investigation of the Structural and Optical Properties of Compositionally V‐Graded Strained InxGa1–xN Layers

Compositionally graded InxGa1–xN‐based materials have been receiving more attention recently due to both their novel structure and intrinsic properties. However, high‐quality material with a well‐controlled and optimized grade to high In composition remains challenging to grow. Herein, the growth and characterization of continuous 2D films of compositionally graded InxGa1–xN are investigated using molecular beam epitaxy (MBE) on (0001) GaN templates at 575 °C. Each film is formed by compositionally grading the growth from GaN to InxGa1–xN and back to GaN, symmetrically, such that the bandgap and the composition of Ga follow a V‐shaped pattern. Here x has been chosen to be ≈0.20 and 0.22. These compositions are confirmed through high‐resolution X‐ray diffraction simulations. Furthermore, asymmetric reciprocal space mapping reveals complete coherence with the GaN substrate. Finally, photoluminescence is measured at low temperature to demonstrate the luminescent properties of the films.

Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films.

Pd(111) thin films, ∼245 nm thick, are deposited on Al2O3(0001) substrates at ≈0.5Tm, where Tm is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired in situ from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd. Double-axis x-ray diffraction (XRD) ω-2θ scans show only the set of Pd 111 peaks from the film. In triple-axis high-resolution XRD, the full width at half maximum intensity Γω of the Pd 111 ω-rocking curve is 630 arc sec. XRD 111 pole figure obtained from the sample revealed six peaks 60°-apart at a tilt angles corresponding to Pd 111 reflections. XRD ϕ scans show six 60°-rotated 111 peaks of Pd at the same ϕ angles for 11[Formula: see text]3 of Al2O3 based on which the epitaxial crystallographic relationships between the film and the substrate are determined as [Formula: see text]ǁ[Formula: see text] with two in-plane orientations of [Formula: see text]ǁ[Formula: see text] and [Formula: see text]ǁ[Formula: see text]. Using triple axis symmetric and asymmetric reciprocal space maps, interplanar spacings of out-of-plane (111) and in-plane (11[Formula: see text]) are found to be 0.2242 ± 0.0003 and 0.1591 ± 0.0003 nm, respectively. These values are 0.18% lower than 0.2246 nm for (111) and the same, within the measurement uncertainties, as 0.1588 nm for (11[Formula: see text]) calculated from the bulk Pd lattice parameter, suggesting a small out-of-plane compressive strain and an in-plane tensile strain related to the thermal strain upon cooling the sample from the deposition temperature to room temperature. High-resolution cross-sectional transmission electron microscopy coupled with energy dispersive x-ray spectra obtained from the Pd(111)/Al2O3(0001) samples indicate that the Pd-Al2O3 interfaces are essentially atomically abrupt and dislocation-free. These results demonstrate the growth of epitaxial Pd thin films with (111) out-of-plane orientation with low mosaicity on Al2O3(0001).

Circular Transmission Line Measurement (CTLM) Studies on Epitaxial Layers of AlGaN

Al0.23Ga0.77N/GaN and Al0.27Ga0.73N/GaN epitaxial layers were grown on sapphire substrates by metal organic chemical vapour deposition (MOCVD). The in-plane strain (εxx) values were found to be -2.4 ×10-4 and -2.8 ×10-4 for Al0.23Ga0.77N and Al0.27Ga0.73N layers using asymmetric reciprocal space mapping (RSM). Surface morphology and emission properties of the samples were analysed using atomic force microscopy (AFM) and photoluminescence spectroscopy (PL) respectively. Circular Transmission Line Measurement (CTLM) technique was adopted to find the variation in contact resistance (Rc), transfer length (LT) and specific contact resistance (ρc) between metal contacts (Ti/Al/Ni/Au: 200/1000/200/1000 Å) and AlxGa1-xN layers.A minimum specific contact resistance, ρc of about 7.81x10-6 Ωcm2 was obtained for Ti/Al/Ni/Au contact on the Al0.23Ga0.77N layer. The device characteristics were correlated with the structural and surface quality of AlxGa1-xN layers.

Effects of post-deposition annealing on the structure and magnetization of PLD grown yttrium iron garnet films

We report on the recrystallization of 200 nm thick as-grown Yttrium Iron Garnet ( Y3.4Fe4.6O12) films on the (111) face of gadolinium gallium garnet single crystals by post-deposition annealing. Epitaxial conversion of the as-grown microcrystalline yttrium iron garnet films was seen after annealing at 800 °C for more than 30 min both in ambient oxygen and in air. The as-grown oxygen annealed samples at 800 °C for 60 min crystallize epitaxially and show excellent figure-of-merit for saturation magnetization (MS = 3.3 μB/f.u., comparable to the bulk value) and coercivity (HC ∼ 1.1 Oe). The ambient air annealing at 800 °C with a very slow rate of cooling (2 °C/min) results in a double layer structure with a thicker unstrained epitaxial top layer having the MS and HC of 2.9 μB/f.u. and 0.12 Oe, respectively. The symmetric and asymmetric reciprocal space maps of both the samples reveal a locking of the in-plane lattice of the film to the in-plane lattice of the substrate, indicating a pseudomorphic growth. The residual stress calculated by the sin2ψ technique is compressive in nature. The lower layer in the air annealed sample is highly strained, whereas the top layer has negligible compressive stress.

Temperature dependence of the interband critical points of bulk Ge and strained Ge on Si

Epitaxial Ge layers on a Si substrate experience a tensile biaxial stress due to the difference between the thermal expansion coefficients of the Ge epilayer and the Si substrate, which can be measured using asymmetric X-ray diffraction reciprocal space maps. This stress depends on temperature and affects the band structure, interband critical points, and optical spectra. This manuscripts reports careful measurements of the temperature dependence of the dielectric function and the interband critical point parameters of bulk Ge and Ge epilayers on Si using spectroscopic ellipsometry from 80 to 780K and from 0.8 to 6.5eV. The authors find a temperature-dependent redshift of the E1 and E1+Δ1 critical points in Ge on Si (relative to bulk Ge). This redshift can be described well with a model based on thermal expansion coefficients, continuum elasticity theory, and the deformation potential theory for interband transitions. The interband transitions leading to E0′ and E2 critical points have lower symmetry and therefore are not affected by the stress.

Growth and characterization of epitaxial Zr(0001) thin films on Al2O3(0001)

The authors report the growth of epitaxial Zr(0 0 0 1) thin films on Al2O3(0 0 0 1) substrates at a temperature of 700 °C via dc magnetron sputtering in an ultrahigh vacuum deposition system equipped with facilities for chemical vapor deposition, low-energy electron diffraction, and Auger electron spectroscopy. Zr layers with a nominal thickness of ∼220 nm are deposited at a rate of ∼0.06 nm/s in 10 mTorr Ar atmosphere. In situ Auger electron spectra of the as-deposited film surface reveal the presence of a Zr peak at 145 eV and Hf peak at 172 eV, the latter due to the presence of Hf impurities in the Zr sputter target. In situ low-energy electron diffraction patterns acquired from the Zr sample show sixfold symmetric spots with an in-plane lattice spacing of 0.31 ± 0.02 nm, characteristic of Zr(0 0 0 1)–(1 × 1) surface. Cross-sectional transmission electron microscopy images reveal columnar growth and the formation of a crystalline, 22 ± 8 nm thick, interfacial layer. Energy dispersive x-ray spectra obtained from this region reveal the presence of both Zr and Al. The authors attribute the formation of this interfacial layer to plasma-induced substrate decomposition during sputtering followed by interdiffusion of Al and Zr at the film–substrate interface. ω-2θ x-ray diffraction data show that the Zr layers are single-phase with hexagonal close-packed structure. Using high-resolution symmetric as well as asymmetric reciprocal space maps, the authors determined that the film is fully relaxed with in-plane and out-of-plane orientation lattice parameters of 0.324 and 0.516 nm, respectively, and identified epitaxial orientation relationships as Zr(0 0 0 4) ‖ Al2O3(0 0 0 12) and Zr(101¯0) ‖ Al2O3(112¯0).

Structural, surface potential and optical studies of AlGaN based double heterostructures irradiated by 120 MeV Si9+ swift heavy ions

The Al0.33Ga0.77N/Al0.14Ga0.86N based double heterostructure was irradiated using Si9+ ion at room temperature (RT) and liquid nitrogen temperature (LNT) with four dissimilar ion fluence. The effect of Si9+ ion irradiation in dislocation densities and in-plane strain of GaN layer were discussed. The in-plane strain values of AlxGa1-xN layers were calculated from asymmetric reciprocal space mapping (RSM). The surface modification and the variation in phase shift on Al0.33Ga0.77N surfaces due to the irradiation were measured by Electrostatic Force Microscopy (EFM). The capacitance of the tip-sample system was determined from EFM. The band edge emissions of heterostructures were measured by the room temperature phototluminescence (PL) and the shift in the Al0.14Ga0.86N active layer emission peaks towards the low energy side at low fluence ion irradiation has been noted.

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.

Enhancement of two dimensional electron gas concentrations due to Si3N4 passivation on Al0.3Ga0.7N/GaN heterostructure: strain and interface capacitance analysis

Enhancement of two dimensional electron gas (2DEG) concentrations at Al0.3Ga0.7N/GaN hetero interface after a-Si3N4 (SiN) passivation has been investigated from non-destructive High Resolution X-ray Diffraction (HRXRD) analysis, depletion depth and capacitance-voltage (C-V) profile measurement. The crystalline quality and strained in-plane lattice parameters of Al0.3Ga0.7N and GaN were evaluated from double axis (002) symmetric (ω-2θ) diffraction scan and double axis (105) asymmetric reciprocal space mapping (DA RSM) which revealed that the tensile strain of the Al0.3Ga0.7N layer increased by 15.6% after SiN passivation. In accordance with the predictions from theoretical solution of Schrödinger-Poisson’s equations, both electrochemical capacitance voltage (ECV) depletion depth profile and C-V characteristics analyses were performed which implied effective 9.5% increase in 2DEG carrier density after passivation. The enhancement of polarization charges results from increased tensile strain in the Al0.3Ga0.7N layer and also due to the decreased surface states at the interface of SiN/Al0.3Ga0.7N layer, effectively improving the carrier confinement at the interface.

Thickness-dependent charge ordering state in Bi0.4Sr0.28Ca0.32MnO3 thin films under the anisotropic strain

We have achieved clear charge ordering (CO) state in Ca doped Bi0.4Sr0.6−xCaxMnO3 (BSCMO) films on (110)-orientated SrTiO3. The BSCMO film's CO transition temperature (TCO) was tuned to room temperature (RT) by optimum doping Ca (x = 0.32). Asymmetric x-ray reciprocal space mapping was used to determine the structure evolution of the films with thickness, in detail, under the anisotropic stress in films. We found that the CO phase only appears in thicker films, accompanied by an unique distortion in structure, in which twin-domains structure occurs with the [110] axis slightly tilting toward [1–10] and [−110] directions. The results demonstrate the effects of anisotropic stress on the structure and electronic phases in BSCMO films as thickness increases. Our works are valuable for allowing a RT study in the CO film and further operation of the devices for a practical use.

Plasma‐assisted molecular beam epitaxy of strain‐compensated a‐plane InGaN/AlGaN superlattices

AbstractStrain‐compensated InGaN/AlGaN structures can enable the growth of thick layers of InGaN epitaxial films far beyond the critical thickness for InGaN grown pseudomorphically to GaN. In this paper, we demonstrate the epitaxial growth of high‐quality strain‐compensated a‐plane In0.12Ga0.88N/Al0.19Ga0.81N superlattices up to 5 times thicker than the critical thickness on free‐standing a‐plane GaN substrates by plasma‐assisted molecular beam epitaxy (PA‐MBE). The superlattices consist of 50 to 200 periods of 10 nm thick In0.12Ga0.88N and 6 nm thick Al0.19Ga0.81N layers. The structures are characterized using a double crystal X‐ray diffractometer, asymmetric reciprocal space mapping, and atomic force microscopy. We use X‐ray diffraction to determine the strain, composition, degree of relaxation, and superlattice period of our samples. The structural characteristics of periodic structures containing from 50 to 200 periods are compared to single layer, uncompensated In0.12Ga0.88N films. A 100 period structure exhibited only 15% relaxation compared to 69% relaxation for the bulk In0.12Ga0.88N film grown with the same total InGaN thickness but without strain‐compensating layers. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exchange bias in strained SrRuO3 thin films

Recently, it was found that the ferromagnetic SrRuO3 when combined with another ferromagnet in thin film form gives rise to exchange bias (EB) effect. However, we observed EB in single, strained, SrRuO3 thin films grown on diamagnetic LaAlO3 (100) substrates. It displays the training effect, which essentially confirms EB. The temperature dependence of the EB reveals the blocking temperature to be around ∼75 K. The strength of the exchange bias decreases with the increase in thickness of the film. We observe tensile strain in the out of plane direction. Further, the presence of in-plane compressive strain is observed through asymmetric reciprocal space mapping. Finally, we find a direct link between strain and EB. The evolution of strain with thickness matches well with the nature of scaled EB. It has been shown earlier by first principle calculations that this strain can induce EB in thin films.

Structural and Carrier Dynamics of GaN and AlGaN-Based Double Heterostructures in the UV Region

Aluminum gallium nitride-based double heterostructures with two different active layer widths have been grown on GaN templates by metalorganic chemical vapor deposition. Crystalline quality has been investigated using high-resolution x-ray diffraction analysis, and screw, edge, as well as total dislocation densities in the GaN epilayer have been calculated. The dislocation density of GaN has been found to be on the order of 108 cm−2. The nominal Al composition and in-plane strain exx for the AlGaN layer grown on the GaN layer have been measured by asymmetric reciprocal-space mapping. Surface properties and cross-sectional views of the samples have been analyzed using atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM), respectively. Room-temperature time-resolved photoluminescence and photoluminescence measurements have been performed on Al0.18Ga0.82N/Al0.45Ga0.55N double heterostructures and the GaN template. The interface recombination velocity (S) of AlGaN-based double heterostructures has been calculated using carrier decay time measurement, increasing from 8.7 × 103 cm/s to 13.4 × 103 cm/s with varying active layer thickness.

Lattice tilt, concentration, and relaxation degree of partly relaxed InGaAs/GaAs structures

AbstractThe series of samples is investigated to verify the validity of the scattering theory within the layers of different relaxation degree. The samples composed of In0.06Ga0.94As layer of different thicknesses on GaAs [001] substrates were grown using MBE technique. The symmetric and asymmetric reciprocal space maps (RSM) were measured and simulated for the samples with the fully coherent layer, in the vicinity of the critical thickness of relaxation, and with the fully relaxed layer. The crystallographic layer miscuts, indium concentrations, the relaxation degrees, and density of dislocations have been precisely evaluated.

Concentration and relaxation depth profiles of InxGa1−xAs/GaAs and GaAs1−xPx/GaAs graded epitaxial films studied by x-ray diffraction

A method is proposed to determine the concentration and relaxation depth profiles in graded epitaxial films from x-ray reciprocal space maps (RSMs). Various approximations in the kinematical x-ray diffraction from epitaxial films with the misfit dislocation density depth profile are developed. We show that a symmetric and an asymmetric RSM, or two asymmetric RSMs, contain enough information to obtain the concentration, relaxation, and lattice tilt depth profiles without any additional assumptions. The proposed approach is applied to In${}_{x}$Ga${}_{1\ensuremath{-}x}$As/GaAs and GaAs${}_{1\ensuremath{-}x}$P${}_{x}$/GaAs epitaxial graded films. The reconstructed concentration and dislocation density depth profiles are found to be in an agreement with the ones expected from the growth conditions.

High-resolution x-ray diffraction investigation of relaxation and dislocations in SiGe layers grown on (001), (011), and (111) Si substrates

This work presents a detailed characterization, using high-resolution x-ray diffraction, of multilayered Si1-xGex heterostructures grown on (001), (011), and (111) Si substrates by reduced pressure chemical vapor deposition. Reciprocal space mapping has been used to determine both the strain and Ge concentration depth profiles within each layer of the heterostructures after initially determining the crystallographic tilt of all the layers. Both symmetric and asymmetric reciprocal space maps were measured on each sample, and the evaluation was performed simultaneously for the whole data set. The ratio of misfit to threading dislocation densities has been estimated for each individual layer based on an analysis of diffuse x-ray scattering from the defects.