Role of oxygen at screw dislocations in GaN

Comprehensive analysis of current leakage at individual screw and mixed threading dislocations in freestanding GaN substrates

The electrical characteristics of Schottky contacts on individual threading dislocations (TDs) with a screw-component in GaN substrates and the structures of these TDs were investigated to assess the effects of such defects on reverse leakage currents. Micrometer-scale platinum/GaN Schottky contacts were selectively fabricated on screw- and mixed-TD-related etch pits classified based on the pit size. Current-voltage (I-V) data acquired using conductive atomic force microscopy showed that very few of the screw TDs generated anomalously large reverse leakage currents. An analysis of the temperature dependence of the I-V characteristics established that the leakage current conduction mechanisms for the leaky screw TDs differed from those for the other screw and mixed TDs. Specifically, anomalous current leakage was generated by Poole-Frenkel emission and trap-assisted tunneling via distinctive trap states together with Fowler-Nordheim tunneling, with the mechanism changing according to variations in temperature and applied voltage. The leaky TDs were identified as Burgers vector b = 1c closed-core screw TDs having a helical morphology similar to that of other screw TDs generating small leakage currents. Based on the results, we proposed that the atomic-scale modification of the dislocation core structure related to interactions with point defects via dislocation climbing caused different leakage characteristics of the TDs.

Ablation threshold of GaN films for ultrashort laser pulses and the role of threading dislocations as damage precursors

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm² and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

The impact of dislocations on AlGaN/GaN Schottky diodes and on gate failure of high electron mobility transistors

GaN epitaxially grown on Si is a material for power electronics that intrinsically shows a high density of dislocations. We show by Conductive Atomic Force Microscopy (C-AFM) and Defect Selective Etching that even for materials with similar total dislocation densities substantially different subsets of dislocations with screw component act as current leakage paths within the AlGaN barrier under forward bias. Potential reasons are discussed and it will be directly shown by an innovative experiment that current voltage forward characteristics of AlGaN/GaN Schottky diodes shift to lower absolute voltages when such dislocations are present within the device. A local lowering of the Schottky barrier height around conductive dislocations is identified and impurity segregation is assumed as responsible root cause. While dislocation related leakage current under low reverse bias could not be resolved, breakdown of AlGaN/GaN Schottky diodes under high reverse bias correlates well with observed conductive dislocations as measured by C-AFM. If such dislocations are located near the drain side of the gate edge, failure of the gate in terms of breakdown or formation of percolation paths is observed for AlGaN/GaN high electron mobility transistors.

Three dimensional localization of unintentional oxygen impurities in gallium nitride

Further development of gallium nitride (GaN) based optoelectronic devices requires in-depth understanding of the defects present in GaN grown on a sapphire substrate. In this work, we present three dimensional secondary ion mass spectrometry (SIMS) detection of oxygen. Distribution of these impurities is not homogeneous and the vast majority of oxygen atoms are agglomerated along pillar-shaped structures. Defect-selective etching and scanning electron microscopy imaging complement SIMS results and reveal that oxygen is predominantly present along the cores of screw and mixed dislocations, which proves their high tendency to be decorated by oxygen. A negligible amount of oxygen can be found within the bulk of the material and along the edge dislocations.

Measurement of Diffusion and Segregation in Semiconductor Quantum Dots and Quantum Wells by Transmission Electron Microscopy: A Guide

Strategies are discussed to distinguish interdiffusion and segregation and to measure key parameters such as diffusivities and segregation lengths in semiconductor quantum dots and quantum wells by electron microscopy methods. Spectroscopic methods are usually necessary when the materials systems are complex while imaging methods may suffice for binary or simple ternary compounds where atomic intermixing is restricted to one type of sub-lattice. The emphasis on methodology should assist microscopists in evaluating and quantifying signals from electron micrographs and related spectroscopic data. Examples presented include CdS/ZnS core/shell particles and SiGe, InGaAs and InGaN quantum wells.

Lattice-plane bending angle modulation of Mg-doped GaN homoepitaxial layer observed by X-ray diffraction topography

We have studied the lattice-plane modulation of Mg-doped GaN homoepitaxial layers by X-ray diffraction topography.

Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications

The detrimental influence of oxygen on the performance and reliability of V/III nitride based devices is well known. However, the influence of oxygen on the nature of the incorporation of other co-dopants, such as rare earth ions, has been largely overlooked in GaN. Here, we report the first comprehensive study of the critical role that oxygen has on Eu in GaN, as well as atomic scale observation of diffusion and local concentration of both atoms in the crystal lattice. We find that oxygen plays an integral role in the location, stability, and local defect structure around the Eu ions that were doped into the GaN host. Although the availability of oxygen is essential for these properties, it renders the material incompatible with GaN-based devices. However, the utilization of the normally occurring oxygen in GaN is promoted through structural manipulation, reducing its concentration by 2 orders of magnitude, while maintaining both the material quality and the favorable optical properties of the Eu ions. These findings open the way for full integration of RE dopants for optoelectronic functionalities in the existing GaN platform.

Full-scale characterization of UVLED Al(x)Ga(1-x)N nanowires via advanced electron microscopy

III-Nitride semiconductor heterostructures continue to attract a great deal of attention due to the wide range of wavelengths at which they can emit light, and the subsequent desire to employ them in optoelectronic applications. Recently, a new type of pn-junction which relies on polarization-induced doping has shown promise for use as an ultraviolet light emitting diode (UVLED); nanowire growth of this device has been successfully demonstrated. However, as these devices are still in their infancy, in order to more fully understand their physical and electronic properties, they require a multitude of characterization techniques. Specifically, the present contribution will discuss the application of advanced scanning transmission electron microscopy (STEM) to AlxGa1-xN UVLED nanowires. In addition to structural data, chemical and electronic properties will also be probed through various spectroscopy techniques, with the focus remaining on practically applying the knowledge gained via STEM to the growth procedures in order to optimize device peformance.

Investigating the optical properties of dislocations by scanning transmission electron microscopy

The scanning transmission electron microscope (STEM) allows collection of a number of simultaneous signals, such as cathodoluminescence (CL), transmitted electron intensity and spectroscopic information from individual localized defects. This review traces the development of CL and atomic resolution imaging from their early inception through to the possibilities that exist today for achieving a true atomic-scale understanding of the optical properties of individual dislocations cores. This review is dedicated to Professor David Holt, a pioneer in this field.

Subtleties in ADF imaging and spatially resolved EELS: A case study of low-angle twist boundaries in SrTiO3

A screw dislocation network at the low-angle SrTiO3/Nb:SrTiO3 twist grain boundary has been analyzed by annular dark field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). The cores of one set of dislocations running parallel to the beam direction appear dark in the ADF STEM images. EELS on the dislocation core reveals a reduced Sr/Ti ratio compared to the bulk suggesting Sr-deficient cores. The second set of dislocations, orthogonal to the latter, is imaged by its strain field using low-angle annular dark field (LAADF) imaging. Multislice image simulations suggest channeling of the electron probe on the atomic columns for small tilts, theta < 1 degree, where the Sr columns act as beam guides. Only for larger tilts is the channeling effect strongly reduced and the fringe contrast approaches the value predicted by a purely incoherent imaging model. Ti-L(2,3) EELS across the dislocation core shows an asymmetry between the EELS and the ADF signal which cannot be explained by the geometry or beam broadening. This asymmetry might be explained by an effective nonlocal potential representing inelastic scattering in EELS.

Atomic scale defect analysis in the scanning transmission electron microscope

Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope provide the ability to investigate the structure-composition-property relationship at individual defects on the atomic scale. In this article, the main principles behind the techniques will be described. The application of these methods to the analysis of individual dislocations in GaN will also be discussed. In this case, the atomic scale methods indicate that many of the structural and electronic properties of dislocations are modified by the presence of impurities, such as oxygen.

Atomic and electronic structure of mixed and partial dislocations in GaN

Here we present a detailed study of mixed dislocations in GaN, in which the complexities of the atomic arrangement in the cores have been imaged directly for the first time using an aberration corrected scanning transmission electron microscope. In addition to being present as a full-core structure, the mixed dislocation is observed to dissociate into partial dislocations separated by a stacking fault only a few unit cells in length. The generation of this stacking fault appears to be impurity driven and its presence is consistent with theoretical predictions for dislocation dissociation in materials with hexagonal crystal symmetry.