Segregation of In to dislocations in InGaN

Cathodoluminescence Spectroscopy in Graded InxGa1-xN

InGaN materials are widely used in optoelectronic devices due to their excellent optical properties. Since the emission wavelength of the full-composition-graded In_xGa_1-xN films perfectly matches the solar spectrum, providing a full-spectrum response, this makes them suitable for the manufacturing of high-efficiency optoelectronic devices. It is extremely important to study the optical properties of materials, but there are very few studies of the luminescence of full-composition-graded In_xGa_1-xN ternary alloy. In this work, the optical properties of full-composition-graded In_xGa_1-xN films are studied by cathodoluminescence (CL). The CL spectra with multiple luminescence peaks in the range of 365-1000 nm were acquired in the cross-sectional and plan-view directions. The CL spectroscopy studies were carried out inside and outside of microplates formed under the indium droplets on the InGaN surface, which found that the intensity of the light emission peaks inside and outside of microplates differed significantly. Additionally, the paired defects structure is studied by using the spectroscopic method. A detailed CL spectroscopy study paves the way for the growth and device optimization of high-quality, full-composition-graded In_xGa_1-xN ternary alloy materials.

Atomic Diffusion of Indium through Threading Dislocations in InGaN Quantum Wells

The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.

Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires

The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm² was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

Dopant Segregation Inside and Outside Dislocation Cores in Perovskite BaSnO3 and Reconstruction of the Local Atomic and Electronic Structures

Distinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO₃ with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.

The heterogeneous nucleation of threading dislocations on partial dislocations in III-nitride epilayers

III-nitride compound semiconductors are breakthrough materials regarding device applications. However, their heterostructures suffer from very high threading dislocation (TD) densities that impair several aspects of their performance. The physical mechanisms leading to TD nucleation in these materials are still not fully elucidated. An overlooked but apparently important mechanism is their heterogeneous nucleation on domains of basal stacking faults (BSFs). Based on experimental observations by transmission electron microscopy, we present a concise model of this phenomenon occurring in III-nitride alloy heterostructures. Such domains comprise overlapping intrinsic I₁ BSFs with parallel translation vectors. Overlapping of two BSFs annihilates most of the local elastic strain of their delimiting partial dislocations. What remains combines to yield partial dislocations that are always of screw character. As a result, TD nucleation becomes geometrically necessary, as well as energetically favorable, due to the coexistence of crystallographically equivalent prismatic facets surrounding the BSF domain. The presented model explains all observed BSF domain morphologies, and constitutes a physical mechanism that provides insight regarding dislocation nucleation in wurtzite-structured alloy epilayers.

560 nm InGaN micro-LEDs on low-defect-density and scalable (20-21) semipolar GaN on patterned sapphire substrates

We demonstrate InGaN-based semipolar 560 nm micro-light-emitting diodes with 2.5% EQE on high-quality and low-defect-density (20-21) GaN templates grown on scalable and low-cost sapphire substrates. Through transmission electron microscopy observations, we discuss how the management of misfit dislocations and their confinement in areas away from the active light-emitting region is necessary for improving device performance. We also discuss how the patterning of semipolar GaN on sapphire influences material properties in terms of surface roughness and undesired faceting in addition to indium segregation at the proximity of defected areas.

Fast Diffusion and Segregation along Threading Dislocations in Semiconductor Heterostructures

Heterogeneous integration of semiconductors combines the functionality of different materials, enabling technologies such as III-V lasers and solar cells on silicon and GaN LEDs on sapphire. However, threading dislocations generated during the epitaxy of these dissimilar materials remain a key obstacle to the success of this approach due to reduced device efficiencies and reliability. Strategies to alleviate this and understand charge carrier recombination at threading dislocations now need an accurate description of the structure of threading dislocations in semiconductor heterostructures. We show that the composition around threading dislocations in technologically important InGaAs/GaAs/Ge/Si heterostructures are indeed different from that of the matrix. Site-specific atom probe tomography enabled by electron channeling contrast imaging reveals this at individual dislocations. We present evidence for the simultaneous fast diffusion of germanium and indium up and down a dislocation, respectively, leading to unique compositional profiles. We also detect the formation of clusters of metastable composition at the interface between Ge and GaAs, driven by intermixing in these two nearly immiscible materials. Together, our results have important implications for the properties of dislocations and interfaces in semiconductors and provide new tools for their study.

Evolution of lateral V-defects on InGaN/GaN on Si(111) during PAMBE: the role of strain on defect kinetics

In this article, a unique correlation has been established between the defect kinetics of III-nitride adatoms and strain during plasma assisted molecular beam epitaxial (PAMBE) growth of InGaN/GaN heterostructures on silicon(111) for the first time.

Dislocations in AlGaN: Core Structure, Atom Segregation, and Optical Properties

We conducted a comprehensive investigation of dislocations in Al_0.46Ga_0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In_0.09Ga_0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.

Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

Reduction of Polarization Field Strength in Fully Strained c-Plane InGaN/(In)GaN Multiple Quantum Wells Grown by MOCVD

The polarization fields in c-plane InGaN/(In)GaN multiple quantum well (MQW) structures grown on sapphire substrate by metal-organic chemical vapor deposition are investigated in this paper. The indium composition in the quantum wells varies from 14.8 to 26.5% for different samples. The photoluminescence wavelengths are calculated theoretically by fully considering the related effects and compared with the measured wavelengths. It is found that when the indium content is lower than 17.3%, the measured wavelengths agree well with the theoretical values. However, when the indium content is higher than 17.3%, the measured ones are much shorter than the calculation results. This discrepancy is attributed to the reduced polarization field in the MQWs. For the MQWs with lower indium content, 100% theoretical polarization can be maintained, while, when the indium content is higher, the polarization field decreases significantly. The polarization field can be weakened down to 23% of the theoretical value when the indium content is 26.5%. Strain relaxation is excluded as the origin of the polarization reduction because there is no sign of lattice relaxation in the structures, judging by the X-ray diffraction reciprocal space mapping. The possible causes of the polarization reduction are discussed.

Direct imaging of boron segregation at dislocations in B:diamond heteroepitaxial films

A thin film of heavily B-doped diamond has been grown epitaxially by microwave plasma chemical vapor deposition on an undoped diamond layer, on top of a Ir/YSZ/Si(001) substrate stack, to study the boron segregation and boron environment at the dislocations present in the film. The density and nature of the dislocations were investigated by conventional and weak-beam dark-field transmission electron microscopy techniques, revealing the presence of two types of dislocations: edge and mixed-type 45° dislocations. The presence and distribution of B in the sample was studied using annular dark-field scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy. Using these techniques, a segregation of B at the dislocations in the film is evidenced, which is shown to be intermittent along the dislocation. A single edge-type dislocation was selected to study the distribution of the boron surrounding the dislocation core. By imaging this defect at atomic resolution, the boron is revealed to segregate towards the tensile strain field surrounding the edge-type dislocations. An investigation of the fine structure of the B-K edge at the dislocation core shows that the boron is partially substitutionally incorporated into the diamond lattice and partially present in a lower coordination (sp(2)-like hybridization).