Generation Of Extended Defects Research Articles

Genesis and evolution of extended defects: The role of evolving interface instabilities in cubic SiC

Emerging wide bandgap semiconductor devices such as the ones built with SiC have the potential to revolutionize the power electronics industry through faster switching speeds, lower losses, and higher blocking voltages, which are superior to standard silicon-based devices. The current epitaxial technology enables more controllable and less defective large area substrate growth for the hexagonal polymorph of SiC (4H-SiC) with respect to the cubic counterpart (3C-SiC). However, the cubic polymorph exhibits superior physical properties in comparison to its hexagonal counterpart, such as a narrower bandgap (2.3 eV), possibility to be grown on a silicon substrate, a reduced density of states at the SiC/SiO2 interface, and a higher channel mobility, characteristics that are ideal for its incorporation in metal oxide semiconductor field effect transistors. The most critical issue that hinders the use of 3C-SiC for electronic devices is the high number of defects in bulk and epilayers, respectively. Their origin and evolution are not understood in the literature to date. In this manuscript, we combine ab initio calibrated Kinetic Monte Carlo calculations with transmission electron microscopy characterization to evaluate the evolution of extended defects in 3C-SiC. Our study pinpoints the atomistic mechanisms responsible for extended defect generation and evolution, and establishes that the antiphase boundary is the critical source of other extended defects such as single stacking faults with different symmetries and sequences. This paper showcases that the eventual reduction of these antiphase boundaries is particularly important to achieve good quality crystals, which can then be incorporated in electronic devices.

Defect and Threading Dislocations in Single Crystal Diamond: A Focus on Boron and Nitrogen Codoping

For the fabrication of vertical power electronic components, the use of a freestanding highly boron‐doped single crystal diamond substrate is mandatory, on top of which a thin low‐doped active layer is overgrown. The crystalline quality of this substrate is of prime importance as the high doping level required is also likely to lead to the generation of extended defects in the active layer. Herein, the improvement of the quality of thick highly boron‐doped diamond films grown by chemical vapor deposition (CVD) is investigated. Codoping with a low amount of nitrogen is explored as a way to limit the formation and propagation of extended defects. It is shown that the growth rate and boron concentration are not impacted by the addition of nitrogen with the parameters used in this study. Surface morphology and defect density estimated for each amount of nitrogen are presented; and Raman spectroscopy confirms that nitrogen addition compensates the strain induced by boron incorporation in single crystal diamond. Codoping with nitrogen is thus demonstrated as a useful strategy to obtain high quality boron‐doped diamond substrates potentially suitable for the development of power devices.

Full-Wafer Strain and Relaxation Mapping of Hg1−xCdxTe Multilayer Structures Grown on Cd1−yZnyTe Substrates

State-of-the-art Hg1−xCdxTe multilayer structures grown by molecular beam epitaxy on (211)-oriented Cd1−yZnyTe substrates have been characterized and their strain and relaxation analyzed. Techniques for measuring lattice mismatch, strain, and crystal quality by measuring symmetric and asymmetric diffraction profiles in different azimuths were adapted and performed in combination with dislocation delineation for full-wafer and multilayer characterization. It was found that the degree of lattice mismatch and in turn the strain state of epitaxial multilayers can be made uniform across full wafers in optimized structures. A strong correlation was revealed between the Zn composition of the Cd1−yZnyTe substrates and the crystal quality of the active layers in the multilayer structures. This method can be generalized to optimize multilayer structures to minimize relaxation by the generation of extended defects.

Designing antiphase boundaries by atomic control of heterointerfaces

Extended defects are known to have critical influences in achieving desired material performance. However, the nature of extended defect generation is highly elusive due to the presence of multiple nucleation mechanisms with close energetics. A strategy to design extended defects in a simple and clean way is thus highly desirable to advance the understanding of their role, improve material quality, and serve as a unique playground to discover new phenomena. In this work, we report an approach to create planar extended defects-antiphase boundaries (APB) -with well-defined origins via the combination of advanced growth, atomic-resolved electron microscopy, first-principals calculations, and defect theory. In La2/3Sr1/3MnO3 thin film grown on Sr2RuO4 substrate, APBs in the film naturally nucleate at the step on the substrate/film interface. For a single step, the generated APBs tend to be nearly perpendicular to the interface and propragate toward the film surface. Interestingly, when two steps are close to each other, two corresponding APBs communicate and merge together, forming a unique triangle-shaped defect domain boundary. Such behavior has been ascribed, in general, to the minimization of the surface energy of the APB. Atomic-resolved electron microscopy shows that these APBs have an intriguing antipolar structure phase, thus having the potential as a general recipe to achieve ferroelectric-like domain walls for high-density nonvolatile memory.

Deep-ultraviolet light emission from 4H-AlN/4H-GaN short-period superlattice grown on 4H-SiC(112¯)

AlN/GaN short-period superlattices (SPSLs) were grown on 4H-SiC(112¯0) substrates with a 4H-AlN template layer. The thickness of the GaN layers was controlled to be less than 1 nm to suppress the generation of extended defects. The stacking fault and threading dislocation density were found to be 3 × 105 cm−1 and 6 × 107 cm−2, respectively, which were as low as those for a 4H-AlN single layer. The superlattice replicated the polytype of the underlying 4H-AlN layer (4H-SiC substrate), meaning that a 4H-AlN/4H-GaN SPSL was grown. Room-temperature cathodoluminescence measurements revealed that the 4H-AlN/4H-GaN SPSLs exhibited intense luminescence in the deep ultraviolet region of 4.4–5.0 eV, depending on the GaN layer thickness. The emission wavelengths of the SPSLs fairly agreed with the estimation based on the Kronig-Penney model.

Edge and defect luminescence of powerful ultraviolet InGaN/GaN light-emitting diodes

The spectrum of ultraviolet (UV) InGaN/GaN light-emitting diodes and its dependence on the current flowing through the structure are studied. The intensity of the UV contribution to the integrated diode luminescence increases steadily with increasing density of current flowing through the structure, despite a drop in the emission quantum efficiency. The electroluminescence excitation conditions that allow the fraction of UV emission to be increased to 97% are established. It is shown that the nonuniform generation of extended defects, which penetrate the active region of the light-emitting diodes as the structures degrade upon local current overheating, reduces the integrated emission intensity but does not affect the relative intensity of diode emission in the UV (370 nm) and visible (550 nm) spectral ranges.

TEM study of defects versus growth orientations in heavily boron‐doped diamond

Heavy boron‐doping layer in diamond can be responsible for the generation of extended defects during the growth processes (Blank et al., Diam. Relat. Mater. 17, 1840 (2008) ). As claimed recently (Alegre et al., Appl. Phys. Lett. 105, 173103 (2014) ), boron pair interactions rather than strain‐related misfit seems to be responsible for such dislocation generation. In the present work, electron microscopy observations are used to study the defects induced by heavy boron doping in different growth plane orientations. Facets of pyramidal Hillocks (PHs) and pits provide access to non‐conventional growth orientations where boron atoms incorporation is different during growth. TEM analysis on FIB prepared lamellas confirm that also for those growth orientations, the generation of dislocations occurs within the heavily boron‐doped diamond layers. Stacking faults (SFs) have been also observed by high resolution transmission electron microscopy (HREM). From the invisibility criteria, using weak beam (WB) observation, and , Burgers vectors have been identified. Their generation behavior confirms the mechanism reported by Alegre et al. where local in‐plane strain effects induced at the growing surface of the diamond lattice by the neighboring of several boron atoms cause the generation of such extended defects.

Thermomechanical modelling of high power laser diode degradation

ABSTRACTCatastrophic degradation of high power laser diodes is due to the generation of extended defects during the laser operation. The stress necessary for is induced by temperature gradients generated by local enhancement of the temperature due to non radiative recombination and subsequent laser self absorption. The thermal stresses induced by such temperature gradient are calculated using finite element methods, showing that the yield strength can be surpassed. The thermal conductivity of the laser structure is shown to play a relevant role in the process.

Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure

Effect of post-growth thermal annealing within the temperature range 200 to 430 oC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann 270 oС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 oС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 oС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra.

Correlation Between Oxygen Precipitation and Extended Defects in Czochralski Silicon: Investigation by Means of Scanning Infrared Microscopy

Through an investigation of oxygen precipitation and extended defects in Czochralski silicon (CZ-Si) specimens subjected to different isothermal anneals, by scanning infrared microscopy and preferential etching combined with optical microscopy, the correlation between the sizes of oxygen precipitates and the generation of extended defects is revealed. It is found that extended defects are generated when oxygen precipitates grow. Afterward, the sizes of extended defects increase, while those of oxygen precipitates do not change significantly. For the onset of the generation of extended defects, we define the maximum size of oxygen precipitates as a critical size for the generation of extended defects. Moreover, it is revealed that this critical size decreases for higher annealing temperatures or more oxidizing ambients.

Improving solar grade silicon by controlling extended defect generation and foreign atom defect interactions

Multicrystalline silicon has a high commercial potential for solar cell applications. Extended lattice defects, such as dislocations and grain boundaries, are important as recombination and storage centers for metallic impurities. Their control is essential to obtain high efficiencies of the solar cell. Important parameters for the assessment of the final efficiency of the solar cells are the distribution and structure of the defects and their impact on the lifetime of minority carriers. The current understanding of the nucleation mechanisms of the most important defects during crystal growth will be described. Interaction processes between mobile impurities and extended defects are important for the recombination activity. Measurements of the recombination behavior and the contamination level will be presented. Finally, the ramifications on the solar cell efficiency shall be discussed.

Fast Epitaxial Growth of 4H-SiC and Analysis of Defect Transfer

The transfer and generation of extended defects in 4H-SiC epitaxial growth at a high growth rate are examined. An epilayer with virtually no basal plane dislocations (BPDs) is obtained using 4º off Si-face substrates, although the formation of 3C-polytype inclusions is occasionally observed. The behavior of BPDs and threading screw dislocations (TSDs) in epitaxial growth is also investigated by X-ray topography and transmission electron microscopy, and the propagation of BPDs and conversion and generation of TSDs in the epilayers are discussed.

Study of strain relaxation in CdSe/ZnSe nanostructures

Strain relaxation in CdSe/ZnSe self-assembled quantum dot (QD) and CdZnSe/ZnSe quantum well (QW) structures have been investigated by the photoluminescence (PL) and high-resolution X-ray diffraction methods. The PL spectra of the QW structures revealed that splitting of free exciton line in ZnSe cap layer is influenced significantly by Cd content in QWs that allows to obtain information about strain in quantum-confinement structures. It is shown that information about strain relaxation mechanism and interdiffusion processes can be obtained from the analysis of the PL spectra of free and bound excitons. It is found that strain relaxation in the QD structures occurs mainly via QD formation in spite of generation of extended defects. Significant doping of ZnSe cap layer with Cd is found both in the QW and QD structures. The data obtained from the simulations of X-ray diffraction profiles qualitatively proved the results obtained from the analysis of ZnSe excitonic spectra. The reasons of quantitative discrepancy are discussed.

Electron-beam-induced current and cathodoluminescence characterization of InGaAs strain-balanced multiquantum well photovoltaic cells

In x Ga 1−x As/In y Ga 1−y As strain-balanced quantum well cells (QWCs) have been shown to be beneficial for photovoltaic applications in particular to extend the light absorption edge of a single-junction cell toward the near infrared with a lower reduction of the open-circuit voltage compared to a single band-gap cell. The strain-balancing condition ensures that the multi-quantum well as a whole does not relax. However, if the mismatch between wells and barriers exceeds a critical limit, the structure becomes vulnerable to morphological or compositional fluctuations, which can lead to a local structural breakdown with the generation of extended defects of a completely different nature from misfit dislocations. In this work, we investigated a series of strain-balanced InGaAs QWCs grown on InP for thermophotovoltaic applications by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements. Despite being electrically active, these defects appear to have a minor impact on the dark current of the cells but cause a drop of the photocurrent at relatively low forward bias voltage. The higher carrier collection efficiency revealed both by EBIC and CL at the boundaries of the defects suggests that a notch in the valence band edge limits the collection of holes generated in the MQW and the energy states, induced by the defects inside the energy gap, assist the tunneling of holes through the notch. At zero bias, the overall reduction of the collection efficiency is of the order of a few percent but the rate of recombination of photogenerated carriers increases dramatically with increasing forward-bias voltage as the junction built-in field drops more rapidly where the density of in-gap states is higher.

Dopant Dependent Extended Defect Nucleation and Growth Kinetics in Silicon During 1 Mev Electron Irradiation

ABSTRACTIn this paper results of a study of electron irradiation induced extended defect generation in doped silicon is presented. The irradiations are performed in-situ with 1 MeV electrons in a high voltage transmission electron microscope. Preferential generation of extended defects is observed in certain areas of the sample which can be correlated with well defined dopant concentration levels. The defect growth kinetics is studied as a function of the irradiation temperature and dose and the type and concentration of dopant.After the first irradiation experiment some of the samples received a second electron irradiation, during which shrinkage and even complete annihilation of the previously generated defects can be observed. The observed results are interpreted on the base of point defect reactions.

Dopant and Strain Dependence of Extended Defect Generation in Silicon by 1-MeV Electron Irradiation

ABSTRACTResults are presented of an in-situ study of 1-MeV electron irradiation induced {113}-defect generation in silicon with boron, phosphorus or arsenic doped surface layers. By 1-MeV electron irradiation in a high voltage transmission electron microscope, {113}-defects are dorni-nandy formed in areas with a well defined dopant concentration range which depends on the tjpe of dopant atom, the irradiation temperature and fluence and the sample thickness. The observations for samples with boron and phosphorus concentration profiles are compared with published data on electron irradiation induced ( 113}-defect formation in uniformly doped silicon. In the present paper first results on heavily arsenic doped silicon are presented. It is shown that ihe observed dopant dependence of the {113}-defect generation can be understood on the basis of known point defect reactions. A few irradiation experiments are also performed on a local isolation structure illustrating for the first time the influence of localised tensile and compressive strain fields on {113} -defect nucleation.

Effects of surface oxide on low-temperature epitaxial growth of Si

The Si epitaxial growth temperature using Chemical Vapor Deposition (CVD) has been steadily reduced in the last few years to temperatures as low as 600°C. Blanket and selective Si epitaxy at atmospheric pressure is now achieved at low temperature through purification processes which exclude O2 and H2O impurities from the deposition zone. In addition to maintaining the deposition system ultra-clean, the initial Si growth surface must be free of surface oxide to achieve low defect density in the epitaxial layer. Previous experiments have shown that a slight presence of surface oxide leads to generation of extended defects. Therefore, an effective surface cleaning process is as important as an oxygen-free environment for achieving perfect epitaxy at low temperatures. In this work we use electron microscopy to study the structure of defects nucleated from surface oxide and the mechanism of defect formation under different growth conditions. High-resolution observations are made to establish quantitative linkage between surface cleanliness and defect population.

Effects of Heating Samples on the Extended Defect Generation During Pulsed Electron Beam Annealing of Silicon

ABSTRACTA non destructive SEM observation method has been applied to investigate the extended defects created by pulsed electron beam annealing of arsenic–implanted silicon. The defect study was performed on bevelled samples after irradiation using variable beam fluences for both a 20°C or a 450°C specimen starting temperature. Dislocation generation resulting in subgrain boundaries formation occurs during regrowth of the silicon layer which has been heated up to the melt point or higher. For the rather penetrating electron beam pulse used in this work the subgrain size and their depth extent depend on the beam fluence and the substrate temperature. For 450°C pre‐heated samples, annealing of the arsenic implant is possible without any stable extended defect creation using the 1.0 – 1.2 J.cm−2 fluence range.