Investigation of stacking faults in MOVPE-grown zincblende GaN by XRD and TEM

X-ray diffraction and bright-field transmission electron microscopy are used to investigate the distribution and density of {111}-type stacking faults (SFs) present in a heteroepitaxial zincblende GaN epilayer with high phase purity, grown on a 3C-SiC/Si (001) substrate by metalorganic vapour-phase epitaxy. It is found that the 4° miscut towards the [110] direction of the substrate, that prevents the formation of undesirable antiphase domains, has a profound effect on the relative densities of SFs occurring on the different {111} planes. The two orientations of SFs in the [-110] zone, where the SF inclination angle with the GaN/SiC interface is altered by the 4° miscut, show a significant difference in density, with the steeper (111) SFs being more numerous than the shallower (-1-11) SFs by a factor of ~ 5 at 380 nm from the GaN/SiC interface. In contrast, the two orientations of SFs in the [110] zone, which is unaffected by the miscut, have densities comparable with the (-1-11) SFs in the [-110] zone. A simple model, simulating the propagation and annihilation of SFs in zincblende GaN epilayers, reproduces the presence of local SF bunches observed in TEM data. The model also verifies that a difference in the starting density at the GaN/SiC interface of the two orientations of intersecting {111} SFs in the same zone reduces the efficiency of SF annihilation. Hence, (111) SFs have a higher density compared with SFs on the other three {111} planes, due to their preferential formation at the GaN/SiC interface caused by the miscut.the Ministry of Education, Youth and Sports of the Czech Republi

Investigation of wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlxGa1−xN nucleation layers

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an Al0.95Ga0.05N nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an Al0.29Ga0.71N nucleation layer, wurtzite inclusions tend to form at the GaN/ Al0.29Ga0.71N interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morpholog

Polarity determination of crystal defects in zincblende GaN by aberration-corrected electron microscopy

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions. I. INTRODUCTIO

Ti Alloyed α -Ga 2 O 3: Route towards Wide Band Gap Engineering

The suitability of Ti as a band gap modifier for α-Ga2O3 was investigated, taking advantage of the isostructural α phases and high band gap difference between Ti2O3 and Ga2O3. Films of (Ti,Ga)2O3 were synthesized by atomic layer deposition on sapphire substrates, and characterized to determine how crystallinity and band gap vary with composition for this alloy. We report the deposition of high quality α-(TixGa1−x)2O3 films with x = 3.7%. For greater compositions the crystalline quality of the films degrades rapidly, where the corundum phase is maintained in films up to x = 5.3%, and films containing greater Ti fractions being amorphous. Over the range of achieved corundum phase films, that is 0% ≤ x ≤ 5.3%, the band gap energy varies by ∼270 meV. The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a modification in band gap, shows promising prospects for band gap engineering and the development of wavelength specific solar-blind photodetectors based on α-Ga2O3

Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors

We investigate the impact of a fluorine plasma treatment used to obtain enhancement-mode operation on the structure and chemistry at the nanometer and atomic scales of an InAlN/GaN field effect transistor. The fluorine plasma treatment is successful in that enhancement mode operation is achieved with a +2.8 V threshold voltage. However, the InAlN barrier layers are observed to have been damaged by the fluorine treatment with their thickness being reduced by up to 50%. The treatment also led to oxygen incorporation within the InAlN barrier layers. Furthermore, even in the as-grown structure, Ga was unintentionally incorporated during the growth of the InAlN barrier. The impact of both the reduced barrier thickness and the incorporated Ga within the barrier on the transistor properties has been evaluated theoretically and compared to the experimentally determined two-dimensional electron gas density and threshold voltage of the transistor. For devices without fluorine treatment, the two-dimensional electron gas density is better predicted if the quaternary nature of the barrier is taken into account. For the fluorine treated device, not only the changes to the barrier layer thickness and composition, but also the fluorine doping needs to be considered to predict device performance. These studies reveal the factors influencing the performance of these specific transistor structures and highlight the strengths of the applied nanoscale characterisation techniques in revealing information relevant to device performance.</jats:p

Ni/Au contacts to corundum α-Ga2O3

The structural, chemical and electrical properties of Ni/Au contacts to atomic layer deposited α-Ga2O3 were investigated. Ni forms a Schottky contact with α-Ga2O3, irrespectively of the post-annealing temperature. No sign of metal oxidation was observed at the metal-semiconductor interface (unlike what is observed for other metals like Ti), and instead, the metallurgical processes of the Ni-Au bilayer dominate the electrical properties. It is found that 400-450oC is the optimal annealing temperature, which allows for metal diffusion to heal gaps at the metal/semiconductor interface, but is not sufficient for Ni and Au to significantly interdiffuse and form an alloy with compositional inhomogeneities

Analysis of structural and optical properties of semipolar (Al, Ga, In)N by high-resolution X-ray diffraction and polarization-resolved transmission spectroscopy

Semipolare AlGaN und AlInN-Schichten sind vielversprechende Materialien für die Herstellung hocheffizienter Laser- und Leuchtdioden im sichtbaren und ultravioletten Spektralbereich. Durch die reduzierte Symmetrie von epitaktisch hergestellten (Al, Ga, In)N Heterostrukturen in semipolarer Orientierung im Vergleich zu Heterostrukturen mit polarer Orientierung, verändern sich deren strukturelle und optische Eigenschaften. Bei semipolaren (Al, Ga, In)N Heterostrukturen führen anisotrope Dehnungen und Scherungen zu einer komplexen triklinen Deformation des normalerweise hexagonalen Kristallgitters. Um diese zu analysieren, wurde ein Röntgenverfahren erarbeitet mit dem die Gitterparameter, die Zusammensetzung und der Dehnungs- sowie Verspannungszustand sehr genau bestimmt werden können. Die Genauigkeit dieser Methode liegt in der Größenordnung von wenigen 10^-4 Å für Gitterparameter und unter 0,2% (absoluter Fehler) für chemische Zusammensetzungen. Damit ist die Methode deutlich genauer als die meisten bisher in der Literatur vorgestellten Ansätze und erlaubt selbst sehr kleine Abweichungen der Gitterkonstanten durch Dehnungen zu bestimmen. Das Rötngenverfahren wurde verwendet um eine Serie von (11-22) planaren AlGaN-Schichten mit verschiedener Zusammensetzung, die auf Saphir-Substraten gewachsen wurden, zu charakterisieren. Es zeigte sich, dass die Schichten zum Teil stark anisotropen Dehnungen und Verspannungen innerhalb der Grenzfläche zum Saphir-Substrat unterliegen, die sehr von der Zusammensetzung und den Herstellungsbedingungen der jeweiligen Schicht abhängen. Als dominante Ursachen für die Verspannungen wurden die thermische Fehlanpassung der vorliegenden Materialien, sowie die Bildung von Kristalldefekten während des Wachstums identifiziert. Letztere ist auf eine stark anisotrope Oberflächenkinetik bei der Koaleszenz von Inseln zurück zuführen und ist die Hauptursache für die starke Abhängigkeit von den Wachstumsbedingungen. Diese defektinduzierte Verspannung verursacht Diffusionsprozesse auf den AlGaN-Oberflächen, die zu einer leicht welligen Morphologie führen. Unter Berücksichtigung der Dehnungen und Zusammensetzungen wurden die optischen Eigenschaften von GaN, AlGaN und AlInN experimentell und theoretisch untersucht. Dabei wurden Bowing-Parameter von (0,58 ± 0,07)eV in AlGaN und (4,2 ± 0,2)eV in AlInN bestimmt. Des Weiteren wurden die Verschiebung der Valenzsubbänder zueinander in der Bandstruktur und die Polarisation der dominanten Übergänge zwischen oberstem Valenzband und Leitungsband analysiert. In AlGaN wie auch in AlInN wurde ein Polarisationswechsel des obersten Valenzbandes beobachtet, der eng mit der Änderung der Anordnung der Valenzsubbänder in der Bandstruktur verbunden ist. In AlN und aluminiumreichen Schichten hat das oberste Valenzband pz-artigen Orbitalcharakter und die darunterliegenden Subbänder pxpy-artigen Charakter. In sehr galliumreichen AlGaN-Schichten beziehungsweise indiumreichen AlInN-Schichten ist die Reihenfolge umgekehrt, d.h. die Zustände des untersten Bandes sind jeweils pz-polarisiert. An Hand des beobachteten Polarisationswechsels war es möglich das System der Gruppe-III-Nitridhalbleiter vollständig in zwei Bereiche der Lichtpolarisation – parallel und senkrecht zur c-Achse – zu unterteilen. Der Wechsel zwischen beiden Bereichen findet in AlGaN bei einem Aluminiumgehalt von (11 ± 2)% und in AlInN bei einem Aluminiumgehalt von etwa 53% statt. Die Bereiche für AlInGaN ergeben sich dann durch lineare Näherung zwischen diesen beiden Punkten.Semipolar AlGaN und AlInN layers are promising materials for the fabrication of highly efficient laser and light-emitting diodes in the visible and ultraviolet spectral range. Due to the lower symmetry of epitaxially grown (Al, Ga, In)N heterostructures with semipolar crystal orientation compared to heterostructures with polar orientation, their structural and optical properties change. In semipolar (Al, Ga, In)N heterostructures, anisotropic strain and shear strain lead to a complex triclinic distortion of the natural hexagonal crystal lattice. In this work a procedure was developed to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. The precision of this method is in the order of a few 10^-4 Å for lattice parameters and 0.2% (absolute error) for chemical compositions. Thus, the method is significantly more accurate than most previously presented approaches in literature and allows the determination of very small deviations of the lattice constants due to strain. The X-ray method was used to characterise a series of (11-22) AlGaN layers with different aluminium contents which were grown directly on (10-10) m-plane sapphire substrates. Highly anisotropic strains and stresses at the interfaces between AlGaN layers and the sapphire substrate were measured. These depend very strongly on the composition and the growth conditions of the respective layers. Thermal mismatch of AlGaN and sapphire and the formation of crystal defects during growth have been identified as the origin of anisotropic strain. The formation of crystal defects is caused by a highly anisotropic surface kinetics during coalescence of islands and is the main cause for the strong dependence on the growth conditions. This defect-induced strain causes diffusion processes on the AlGaN surfaces leading to surface undulations. The optical properties of GaN, AlGaN and AlInN were studied experimentally and theoretically under consideration of the strain state and the composition. Bowing parameters were determined to be (0.58 ± 0.07)eV in AlGaN and (4.2 ± 0.2)eV in AlInN. Furthermore, the energy splitting of the valence sub-bands and the polarization of the dominant transition between topmost valence band and conduction band were analyzed. In AlGaN as well as in AlInN a change in polarization was observed which is closely connected with the change in the sequence of valence sub-bands. In AlN and aluminium-rich layers, the topmost valence band has pz-like orbital character and the underlying sub-bands have pxpy -like character. In very gallium-rich AlGaN layers and indium-rich AlInN-layers this order is reversed, i.e. the states of the lowest sub-band are pz-polarized. On the basis of the observed polarization changes, it is possible to completely divide the group of III-nitride semiconductors according to their orbital (polarization) character. The change between the states occurs in case of AlGaN at an aluminium content of (11 ± 2)% and in AlInN at an aluminium content of approximately 53%. Then the regions of polarization for AlInGaN can be obtained by linear interpolation between these two points