J. Appl. Cryst.の発刊に際して

The III-nitrides include the semiconductors AlN, GaN and InN, which have band gaps spanning the entire UV and visible ranges. Thin films of III-nitrides are used to make UV, violet, blue and green light-emitting diodes and lasers, as well as solar cells, high-electron mobility transistors (HEMTs) and other devices. However, the film growth process gives rise to unusually high strain and high defect densities, which can affect the device performance. X-ray diffraction is a popular, non-destructive technique used to characterize films and device structures, allowing improvements in device efficiencies to be made. It provides information on crystalline lattice parameters (from which strain and composition are determined), misorientation (from which defect types and densities may be deduced), crystallite size and microstrain, wafer bowing, residual stress, alloy ordering, phase separation (if present) along with film thicknesses and superlattice (quantum well) thicknesses, compositions and non-uniformities. These topics are reviewed, along with the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides. A summary of useful values needed in calculations, including elastic constants and lattice parameters, is also given. Such topics are also likely to be relevant to other highly lattice-mismatched wurtzite-structure materials such as heteroepitaxial ZnO and ZnSe.

https://iopscience.iop.org/article/10.1088/0034-4885/72/3/036502/pdf

X-ray diffraction of III-nitrides

Introducing an extremely thin layer of boron nitride between a sapphire substrate and the gallium nitride semiconductor grown on it is shown to facilitate the transfer of the resulting nitride structures to more flexible and affordable substrates. Nitride semiconductors are renowned for their excellent electronic and optical properties and are the materials of choice for many optical devices, including BluRay players. But they have an important practical drawback: they are very particular about the substrates (typically sapphire) on which they can be grown. This has stimulated the search for new ways of transferring such materials from one substrate to another. Here Kobayashi et al. demonstrate, using a gallium nitride-based device, that the addition of an extremely thin layer of hexagonal boron nitride to the initial growth surface facilitates the straightforward mechanical release of the resulting nitride structures, as well as subsequent transfer to any suitable substrate, including metals, glass and transparent plastics. Nitride semiconductors are the materials of choice for a variety of device applications, notably optoelectronics1,2 and high-frequency/high-power electronics3. One important practical goal is to realize such devices on large, flexible and affordable substrates, on which direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques—such as laser lift-off4,5—have been investigated to enable the transfer of nitride devices from one substrate to another, but existing methods still have some important disadvantages. Here we demonstrate that hexagonal boron nitride (h-BN) can form a release layer that enables the mechanical transfer of gallium nitride (GaN)-based device structures onto foreign substrates. The h-BN layer serves two purposes: it acts as a buffer layer for the growth of high-quality GaN-based semiconductors, and provides a shear plane that makes it straightforward to release the resulting devices. We illustrate the potential versatility of this approach by using h-BN-buffered sapphire substrates to grow an AlGaN/GaN heterostructure with electron mobility of 1,100 cm2 V−1 s−1, an InGaN/GaN multiple-quantum-well structure, and a multiple-quantum-well light-emitting diode. These device structures, ranging in area from five millimetres square to two centimetres square, are then mechanically released from the sapphire substrates and successfully transferred onto other substrates.

Layered boron nitride as a release layer for mechanical transfer of GaN-based devices

The mosaicity of GaN layers grown by metalorganic vapor phase epitaxy, on (0001) sapphire and exhibiting different grain diameters is studied using high-resolution x-ray diffraction. The coherence lengths, the tilt, and the twist of the mosaic structure are determined utilizing data taken in different x-ray scattering geometries. The results of different models, which were applied, are then compared and discussed. The dislocation densities, obtained from the x-ray data, are compared with the results of plan-view transmission electron microscopy and atomic force microscopy.

Microstructure of heteroepitaxial GaN revealed by x-ray diffraction

High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in (001)-oriented GaN epitaxial layers. For this, (00l) and (hkl) Bragg reflections with h or k nonzero were studied, the latter one measured in skew symmetric diffraction geometry. The defect analysis was applied to a variety of GaN layers grown by molecular-beam epitaxy under very different conditions. The outcome is a fundamental correlation between the densities of edge- and screw-type dislocations.

X-ray diffraction analysis of the defect structure in epitaxial GaN

Important structural characteristics (correlation lengths of columnar crystallites, dislocation densities, angles of rotational disorder) of hexagonal GaN grown by metallorganic chemical vapour deposition on c-plane sapphire are determined by transmission electron microscopy and triple-axis X-ray diffractometry. GaN films exhibit an edge dislocation density in the range of 1011 cm−2, a tilt and twist angle of 0.1° and 1.3° and a columnar structure with a lateral and vertical correlation length of 150 and 1000 nm respectively. The determination of correlation lengths and dislocation densites from X-ray patterns was undertaken using two independent evaluation methods which are discussed in detail. It is also shown that triple-axis X-ray diffractometry is a highly suitable technique for the separation of different kinds of structural defects such as edge and screw dislocations that lead to a characteristic broadening of symmetric and asymmetric Bragg reflections. The correlation lengths and dislocat...

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

A series of elements are tested for partial substitution in MnBi to form stable compounds suitable for magnetooptic memory applications. While MnBi1−xTex forms a stable NiAs-structure with higher c/a-ratio than the low temperature phase of pure MnBi, new stable compounds with low c/a-ratio are identified in the systems Mn1−xCuxBi, Mn1−xZnxBi, and Mn1−xTixBi. They have crystal structures similar to MnBi0.9Sb0.1 which has a less complicated NiAs-superstructure than the high-temperature phase of pure MnBi. The structural parameters of these compounds are evaluated by least squares refinements of X-ray powder diffraction data.



Zur Herstellung stabiler magnetooptischer Speicherschichten wird eine Reihe von Elementen in MnBi eingebaut. Durch den Zusatz von Te wird die Tieftemperaturphase (NiAs-Struktur) stabilisiert, wahrend durch den Zusatz von Cu, Zn oder Ti Verbindungen der Form Mn1−xTxBi (T = Cu, Zn, Ti) mit der Struktur von MnBi0,9Sb0,1 entstehen. Diese besitzt Ahnlichkeiten mit der Hochtemperaturphase von Mn Bi, jedoch mit einer weniger komplizierten NiAs-Uberstruktur als diese. Die Strukturparameter der neuen Verbindungen werden durch least-squares-Verfeinerungen von Pulver-Rontgenbeugungsdaten ermittelt.

Properties of MnBi compounds partially substituted with Cu, Zn, Ti, Sb, and Te. I. Formation of mixed phases and crystal structures

AlN films, grown by metalorganic chemical vapour deposition on c-plane sapphire, were investigated by high-resolution X-ray diffraction. In Ω-scans very narrow peaks superimposed on a broader diffraction signal were found. The occurrence of very narrow rocking curves is explained in terms of coherent X-ray scattering. This phenomenon is caused by a small rotational out-of-plane disorder resulting in a resolution-limited component of the rocking curve. The tilt (out-of-plane disorder) of individual crystallites is caused by a rotation with small angles around an axis lying parallel to the substrate surface. The diffuse component of the rocking curve was used for the structural characterization of the epitaxial films and a detailed determination of structural imperfection by X-ray diffraction could be obtained by measuring asymmetric reflections. The structural properties of the AlN films are confirmed by atomic force microscopy and transmission electron microscopy.

Coherent X‐Ray Scattering Phenomenon in Highly Disordered Epitaxial AlN Films

The transformation rates of the metastable quenched high-temperature phase of MnBi to the low-temperature phase are tested as a function of the concentration of the substituents Cu, Zn, Ti, and Sb at different temperatures. In all cases a reduction of the transformation rate is observed which is accompanied by the appearance of a second phase in the films described in part I of this paper. These phases are antiferromagnetic or ferrimagnetic with low Curie temperatures. From their portion the strong decrease of the Faraday rotation with increasing substituent percentage is explainable. It may be accepted that the remaining magnetic portion in the film is pure MnBi. The stabilization can be supposed as a clamping effect at the boundaries between the similar and oriented crystal lattices of the quenched high-temperature phase and the substituted phase.



Die Phasenumwandlungsgeschwindigkeit der metastabilen abgeschreckten Hochtemperaturphase von MnBi zur Tieftemperaturphase wird in Abhangigkeit von der Konzentration der in MnBi substituierbaren Elemente Cu, Zn, Ti und Sb bei verschiedenen Temperaturen gemessen. In allen Fallen tritt eine Verlangsamung der Umwandlungsgeschwindigkeit auf, die vom Auftreten einer zweiten Phase in den Schichten begleitet wird. Es handelt sich dabei um die im Teil I dieser Arbeit beschriebenen substituierten Phasen, die oberhalb der Raumtemperatur keine Faradaydrehung besitzen. Die starke Abnahme der Faradaydrehung mit zunehmender Konzentration des Substituenten kann unmittelbar aus dem Anteil dieser Phasen an der gesamten Schicht erklart werden. Es ist anzunehmen, das die verbleibenden Anteile mit Faradaydrehung reines MnBi sind. Die stabilisierende Wirkung der Substituenten sollte daher ein Hafteffekt an den Korngrenzen zwischen den nahezu gleichen und gleich orientierten Gittern der abgeschreckten Hochtemperaturphase und der stabilen substituierten Phase sein.

Properties of MnBi compounds partially substituted with Cu, Zn, Ti, Sb, and Te. II. Stability and magnetooptic properties of thin films

Atomic force microscopy and x-ray scattering are applied to describe changes in the morphology of Si( 100) surfaces due to wet chemical processing. With atomic force microscopy, the rms roughness o in dependence on the lateral scan length XO is measured. Additionally, two lateral order parameters are determined, the lateral correlation length L and Hurst parameter h. With x-ray scattering, the corresponding parameters are determined. X-ray reflectometry provides the rms roughness o of the silicon surface as well as the silicon density p. X-ray diffuse or off-specular scattering gives information on the lateral order of surfaces and interfaces and allows to recalculate the fractal parameters L and h of the silicon surfaces investigated. For the results of o, L and h, atomic force microscopy and x-ray scattering give different quantities. This is attributed to the differing vertical and lateral resolution of the methods applied. Whereas x-ray scattering provides only one set of values for o, L and h, atomic force microscopy can yield different sets of values dependent on the lateral scan length X0 chosen in the atomic force microscopy measurements.

H. Göbel

Papers

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Properties of MnBi compounds partially substituted with Cu, Zn, Ti, Sb, and Te. I. Formation of mixed phases and crystal structures

Coherent X‐Ray Scattering Phenomenon in Highly Disordered Epitaxial AlN Films

Properties of MnBi compounds partially substituted with Cu, Zn, Ti, Sb, and Te. II. Stability and magnetooptic properties of thin films

Comparative studies of fractal parameters of Si(100) surfaces measured by X-ray scattering and atomic force microscopy