Substrate Dislocations Research Articles

Molecular properties of PTCDA on graphene grown on a rectangular symmetry substrate

The chemical modulation associated with moiré patterns, arising at the interface of metal-supported 2D material systems, affects the interaction between molecules and 2D materials on surfaces. Since the crystallography of the support influences the interfacial chemistry of the moiré modulation, this parameter could also play a role in the graphene-molecule interaction, although studies using non-hexagonal metal supports are needed to investigate this effect. It is a key issue since graphene appears combined with organic films in most technological advances related to this material. Here, we have characterized the properties of PTCDA molecules on graphene grown on Rh(110) substrates, which exhibit a rectangular atomic packing, using scanning tunneling microscopy and spectroscopy. The results showed that PTCDA molecules are arranged on the surface into a herringbone structure exhibiting a long-range ordering, which grows continuously across substrate atomic steps edge and dislocations. The quasi-1D moiré patterns of the Gr/Rh(110) surfaces are found to provide an inert chemical landscape for the molecular arrangement. Bias voltage-dependent imaging of the orbital structure of PTCDA molecules and differential conductance spectra back up a weak molecule–substrate interaction scheme. Finally, the α-polymorph of bulk crystal PTCDA has been determined as the favored stacking configuration for bilayer molecules on Gr/Rh(110).

Investigation into the Influence of Substrate Dislocations in 4H-SiC on the Subsequent Epitaxy and Resultant Device Performance

As the demand for SiC based devices continues to grow, there is an aggressive push to drive down sources of yield loss and device failures. One route to mitigate this issue is via connection of device yield loss to defects in the bare substrate material. Here, high throughput X-ray topography in conjunction with KOH etching and photoluminescence imaging were utilized to detect and classify various substrate dislocations. Subsequent epitaxial layers, grown on the substrates from various vendors, were analyzed using a confocal microscope to determine which substrate defects propagated into the epi. Data indicates that there is a good correlation between substrate BPDs nucleating into epitaxial stacking faults. This is supported by data from hundreds of different boules demonstrating that there is a relationship between the number of BPDs in the substrate and the number of partials and stacking faults detected in the resultant epitaxial layer. BPDs are not the only substrate dislocation nucleation source as the data also suggests other parallel sources are involved. Thousands of MOSFET and Diode devices were analyzed to understand the influence that substrate defects, propagated to the epitaxial layers, have on device leakage and yield loss. It was determined that MOSFET devices are more sensitive to the stacking faults that nucleate from the substrate dislocations compared to Diodes. While most of the stacking faults and other dislocations that occur in the epi layers do not independently result in device failures or leakage, an increase in the density of these defects within a die, can lead to increased leakage and decreased device performance. The inherent variability of these relationships requires the analysis of very large numbers of wafers to see the clear trends. This work uses a combination of X-ray and photoluminescence detection techniques to trace dislocations in substrates that nucleate into epitaxy layers and result in device leakage and failure over hundreds of wafers and thousands of devices. This correlation between device characteristics and crystal dislocations provides necessary information in determining which defects are most necessary to reduce or eliminate to improve device yield and performance.

(Invited) Detection of Dislocations Using X-Ray Diffraction Imaging (Topography) KOH Etching and Their Evolution after Epitaxial Growth in 4H-SiC

Silicon Carbide power devices are seeing a tremendous increase in demand and adoption that had been forecast at least a decade back. This is fueled to a great degree by the adoption of electric cars and power electronics needed both inside and supporting the charging infrastructure. Along with the automotive industry comes clear spotlight on costs, quality and efficiency. This in turn results in intense pressure and scrutiny to improve yields and reduce defects in every part of the manufacturing chain. Even with all the recent advances in substrate and epitaxial quality, a substantial portion of device failures still come from extended defects in the epitaxial layers, which are typically nucleated from dislocations in the substrates. The reality is that every SiC wafer is scanned multiple times; before epi, after epi, inline during fabrication, along with each device receiving extensive stress tests to ensure quality by design [1].Killer extended defects (KD) in the epitaxial layers are well understood [1]. With newer epi processes getting close to 0.1 cm-2 – 0.2 cm-2, the focus shifts to other propagated or nucleated extended defects in the epitaxial layers. The current production substrate scans do not reveal enough data on the underlying dislocations to provide a comprehensive picture of the formation of these extended defects. Techniques like molten KOH-etching and X-ray diffraction imaging (XRDI), also known as X-ray topography (XRT) can provide an insight into the substrate dislocations. The use of XRT has recently accelerated [2-8] to study the dislocations, albeit in limited R&D settings. This is due to the large amount of time needed for both Synchrotron X-ray scans [2,3,4,5] and dedicated development systems [6,7,8]. In this work, we analyze SiC wafers with an XRT system with high throughput, designed for compatibility with SiC production environments, while comparing dislocation detection to molten KOH etching and statistically tracking the nucleation / propagation of extended defects in grown epitaxial layers.Each type of scan of a SiC wafer whether of the surface, photoluminescence, XRT, Etch Pits, etc. gives a different view of the defects in the wafer. This often provides complementary information, which has to be interpreted and analyzed in an integrated way to understand the complete picture. SiC wafers from different vendors were scanned using XRT along with automated classification of all the dislocations detected. Some of the wafers were etched by molten KOH to correlate these two views. Further, many of the wafers were used to grow epitaxial layers and the extended defects detected were correlated back to the detected dislocations. As this is a probabilistic process, and not a 1:1 conversion, a framework was developed to analyze these nucleation and propagation processes statistically over a large dataset.A review of all these results will be presented. The applicability of such a production tool will be discussed along with directions where such data can be used and have the maximum impact.

Ultra‐High Vacuum Deposition of Pyrene Molecules on Metal Surfaces

Interfacial properties of organic adsorbates featuring aromatic π‐orbitals on metal surfaces play an important role for organic electronics and spintronics. Pyrene is a flat aromatic molecule with a size between ultimately small benzene and extended graphene segments. The deposition of pyrene molecules onto clean and reactive surfaces with a sub‐monolayer coverage under ultra‐high vacuum (UHV) conditions is challenging, since pyrene is a solid with a high vapor pressure. Here, a sublimation procedure under UHV and image pyrene adlayers on in situ prepared Au(111) and Fe/W(110) substrates by means of low‐temperature scanning tunneling microscopy is presented. For Au(111), the molecule–surface interaction is weak as indicated by the specific herringbone reconstruction of the Au(111) surface that is visible through the self‐assembled pyrene adlayer. Pyrene desorption due to weak intermolecular interaction self‐limits the growth to one monolayer (ML). On the more reactive 2–4 ML thick Fe films on W(110), the molecular order of the pyrene adlayer sensitively depends on the Fe thickness‐dependent dislocation pattern at the substrate surface. Irregular arrangements occur for 1 ML Fe and near substrate dislocations for 2–4 ML Fe. Self‐assembled ordered arrays form predominantly for 2 ML Fe, where the dislocation pattern leaves sufficiently large unperturbed areas between the dislocation lines.

Influences of MOS Device Characteristic under Different Oxygen-Dose Participations in the Silicon Substrate

A silicon substrate is the starting point of producing the semiconductor component, so that the quality of semiconductor substrate is very important during the VLSI fabrication. In this paper, we will evaluate the influence of MOS device characteristics under different oxygen impurities in silicon substrates. In the course of silicon substrate pulling process by Czochralski method, the defect and impurity will be existed; the oxygen atom will be induced substrate dislocations and affected the substrate quality. In this work, different oxygen doses will be used in wafer to study the impacts on MOS CV curve characteristic, interface trap charge characteristic, ID-VDS curve, ID-VGS curve, and threshold voltage behaviors of MOS devices.

Self‐organized defect control during GaN homoepitaxial growth on nanostructured substrates

AbstractThe implementation of the control over defect formation in GaN films grown by Hydride Vapor‐Phase Epitaxy on free‐standing GaN substrates with nanostructured bulk of the material is reported. For the first time, a mechanism is described which is responsible for collective interaction of structural defects, and allows for excluding substrate dislocations from the sources of threading defects in homoepitaxially grown GaN films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Finite element method simulation of interface evolution during epitaxial growth

Epitaxial Lateral Overgrowth (ELO) is a method of epitaxial growth on a partially masked substrate. It can be a promising method for photovoltaic applications due to a possibility of producing thin and high quality silicon substrates. Since the mask prevents propagation of the substrate dislocations to the laterally overgrown parts of the ELO layer they are characterized by a lower dislocation density than the substrate. It means that it is possible to fabricate good quality solar cells on a poor quality Si substrate. The main goal of the research is to obtain a higher growth rate in the lateral direction than in the direction normal to the substrate. The epilayer growth kinetics depends on many technological factors, basically the growth temperature, the cooling rate, the solvent and the mask filling factor. For this reason the best way to achieve the goal is a computational analysis of the epitaxial layer growth process. This work presents a two-dimensional computational study of such a process of growth for different technological conditions. The computational model is based on the assumption of pure diffusion control growth.

Influence of Substrate Dislocation on Passivation of Pure Iron in pH 8.4 Borate Buffer Solution

The influence of substrate dislocations on the passivation behavior of pure iron in a pH 8.4 borate buffer solution was investigated. Cold-rolling was employed to induce dislocation in the annealed substrate. The dislocation density on the cold-rolled substrate depended on the reduction ratio (Red) and was maximum at . The electric charge consumed in the passivation and the passivity-maintaining current increased with an increase in the dislocation density. Electrochemical impedance spectroscopy revealed that the dislocation exposed to the iron surface increased the donor density in the passive film and reduced the charge-transfer resistance for the film aging. It was also demonstrated by radio frequency–glow discharge optical emission spectroscopy that a thick passive film formed on the cold-rolled iron compared with that on annealed iron. These results suggest that substrate dislocations enhance the reactions at the substrate/film interface, resulting in the formation of a highly defective passive film.

Effect of Cold Rolling on Passive Film on Pure Iron in pH 8.4 Borate Buffer Solution

The influence of substrate dislocations on passivation behavior of a pure iron in pH 8.4 borate buffer solution was investigated. Cold rolling was employed to induce dislocation in the annealed substrate. The dislocation density on the cold rolled substrate depended on the reduction ratio (Red) and was maximum at Red = 60%. The electric charge consumed in the passivation and the passivity-maintaining current increased with increase in the dislocation density. EIS revealed that the dislocation exposed to the iron surface increased donor density in the passive film and reduced charge transfer resistance for the film aging. It was also demonstrated by rf-GDOES that a thick passive film formed on the cold rolled iron compared with that on annealed iron. These results suggest that substrate dislocations enhance the reactions at the substrate/film interface, resulting in the formation of the highly defective passive film.

Determination of transistor infant failure probability in InGaP/GaAs heterojunction bipolar technology

A novel circuit for measuring the infant mortality rate in InGaP/GaAs HBT Technology is presented. The circuit allows reliability stressing to be performed on as many as 100,000 transistors per wafer and is necessary in order to predict the infant circuit failure rate in circuits with >500 transistors without costly burn-in screens. This new circuit allows for the rapid identification of failed transistors and subsequent failure analysis to allow for process improvements. A variation of the same circuit has also been used to estimate the activation energy, E a, of the infant failure mechanism. Rough estimates of E a indicate that the infant failure mechanism is ∼0.5 eV, and that there may be two distinct failure mechanisms responsible for infant failures. Process defects have not been found on the vast majority of failed transistors, and there is good correlation between the substrate dislocation density and the infant failure rate. We have concluded that substrate dislocations are the leading cause of infant mortality in our HBT process.

Optimization of the metamorphic growth of GaAs for long wavelength VCSELs

In this paper, we present the study of the growth conditions of metamorphic GaAs on InP(0 0 1) substrate using gas source molecular beam epitaxy (GSMBE). We examined the influence of various growth parameters, such as growth temperature and Group V pressure, and determined optimized conditions to produce low roughness (11.6 Å rms) GaAs. We also demonstrate the existence of a critical thickness limit under which dislocations are generated, or propagate, in the substrate. We experimented different types of post-growth cooling rates, annealing and observed their influence on the localization of the substrate dislocations. Altogether, these experiments indicate that the metamorphic growth of GaAs on InP is a valid approach for long wavelength vertical cavity surface emitting lasers.

Mask-Induced Strain in Gaas Layers Grown by Liquid Phase Epitaxial Lateral Overgrowth

ABSTRACTX-ray diffraction has been used to study the mask-induced strain in GaAs layers grown by the liquid phase epitaxial lateral overgrowth (ELO) on (100) GaAs substrates. SiO2 mask has been investigated to produce seeding areas for the ELO growth. It has been found that SiO2 attracts the ELO layers, which leads to their pronounced bending towards the oxide film and to the appearance of vertical strain in laterally grown parts of ELO. When SiO2 is removed by selective etching this strain disappears. We show evidence that the bending of ELO layers is reduced when the density of substrate dislocations is increased. This is explained as being due to enhancement of the initial vertical growth rate by dislocations supplying steps on the upper surface of ELO.

Relaxation of low mismatched semiconducting layers by misfit dislocations : models and observations

the very first stages of the relaxation process of low mismatched systems GeSi/Si, Si/Si(As), Ge(B)/Ge and specially GaAs/Ge, are considered. X-ray topography observations are performed and allow the measurement of the critical layer thickness.Misfit dislocations first form by glide of grown-in substrate dislocations. They elongate not only in the interface but also in the neutral plane. The observed hairpin configuration reveals that plastic deformation of the substrate is involved in relaxation. GeSi/Si films metastability can be explained directly by kinetic arguments. In the case of GaAs/Ge, in addition to the kinetics of the nucleation and development of dislocations, the misfit evolution with temperature has to be taken into account. A possible mechanism for the emission of new misfit dislocations, whereby relaxation continues, can be related to stacking faults.

Study of RTS noise and excess currents in lattice-mismatched InP/InGaAs/InP photodetector arrays

Random telegraph signal (RTS) or burst noise and excess current have been investigated in arrays of infrared photodetectors ( λ = 1.7 μm) prepared on lattice-mismatched InP/InGaAs/InP hetero-structures. RTS noise is observed in about 1% of devices of the wafer. At reverse bias RTS is always related to distinct excess currents (EC) which in certain cases exhibit a tunnelling behavior. Burst noise at forward biases is not necessarily correlated with the observation of distinguishable excess currents. RTS amplitudes and excess currents are thermally activated. At reverse bias their activation energies are in the 0.15–0.35 eV range. The pulse width distribution of a two level RTS is obtained by using an original digital data processing method. RTS mean pulse width strongly varies with temperature (activation energies in the 0.22–1.30 eV range) and also depends on applied bias. The results suggest that RTS noise is due to excess current which flows through a crystalline defect and is modulated by an action of a small lattice defect. Two alternative RTS models are discussed to explain the data. Finally the influence of substrate dislocations and lattice-mismatched defects on the presence of excess noise is discussed.

Mechanism of Thermal Interaction of In with GaAs

General behavior of In/GaAs couple heat-treated at 570°C for 2 hours was studied with secondary-ion-mass spectrometry, scanning electron microscopy, Rutherford backscattering spectroscopy and Nomarski microscopy. It is shown that, besides the well-known InGaAs crystallites which epitaxially grow upon dissolution of the substrate, In interacts with the substrate dislocations to form In(Ga)As dendrites. The driving force for this process is presumably excess arsenic reported to be present in the vicinity of the individual dislocations. PACS numbers: 73.40.Ns

Dislocations Nucleating Growth in Liquid Phase Epitaxy of Gallium Arsenide

AbstractDislocations in GaAs are analysed by X-ray topography in order to relate their nature to their efficiency in generating growth steps. The analysis is mainly based on comparing double crystal back reflection images of the defects to calculated misorientations of the crystal surface which are caused at the outcrop. All types of dislocations are equally effective provided their Burgers vector has a component normal to the growth interface. Concentric growth step patterns, which develop in varying numbers on different samples are not caused by substrate dislocations.

On the extension of substrate dislocations into heteroepitaxial deposits

Abstract It has been argued that threading dislocations cannot be introduced into heteroepitaxial deposits by the ‘copying’ of emergent substrate dislocations because the Burgers vector of the dislocation must change at the interface. In this Letter it is shown that the extension of an emergent substrate dislocation into the deposit requires the creation of an interfacial dislocation. These three dislocations will meet at an interfacial node where the sum of the Burgers vectors is zero as required.

Scanning tunneling microscopy study of the structure of sulfur [2(3)1/2×2(3)1/2] R 30° overlayer on rhenium (0001)

The structure of the (2(3)1/2×2(3)1/2) R30° sulfur overlayer chemisorbed on rhenium(0001) has been studied with scanning tunneling microscopy (STM). The ordered sulfur overlayer was prepared in ultrahigh vacuum and then transferred through air to an STM operating at a vacuum of 10−7 Torr. The (2(3)1/2×2(3)1/2) R30° sulfur overlayer passivates the rhenium substrate in air. STM images show the atomic structure of the overlayer unit cell to be a hexagonal ring of six sulfur atoms. Defect structures in the two-dimensional lattice of sulfur hexagons, overlayer domain boundaries and substrate dislocations were also observed.

Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area

To investigate the effect of growth area on interface dislocation density in strained-layer epitaxy, we have fabricated 2-μm-high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500-, 7000-, and 8250-Å-thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large-area samples, were then deposited by molecular-beam epitaxy. For the 3500-Å layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface-dislocation density was found to vary linearly with mesa width and to extrapolate to a zero linear interface-dislocation density for a mesa width of zero. This behavior excludes dislocation multiplication or the nucleation of surface half-loops as operative nucleation sources for misfit dislocations in these layers. Only nucleation sources that scale with area (termed fixed sources) are active. In specimens with lower substrate dislocation densities, the density of interface dislocations still varies linearly with mesa size, but the slope becomes independent of substrate dislocation density, indicating that surface inhomogeneities now act as the dominant source for misfit dislocations. Thus, in 3500-Å-thick overlayers, substrate dislocations and substrate inhomogeneities are the active fixed nucleation sources. Since only fixed nucleation sources are active, a single strained layer will dramatically reduce the threading dislocation density in the epilayer. For the 7000-Å layers, we observe a superlinear increase in linear interface-dislocation density with mesa size for mesas greater than 200 μm, indicating that dislocation multiplication occurs in large mesas. For mesas less than 200 μm in width, linear interface-dislocation density decreases linearly with mesa size, but extrapolates to a nonzero linear interface-dislocation density for a mesa size of zero. This nonzero extrapolation suggests an additional active source which generates a dislocation density that cannot be decreased to zero by decreasing the mesa size. Cathodoluminescence (CL) images using radiative recombination indicate that the additional source is nucleation from the mesa edges. Despite a doubling in epilayer thickness from 3500 to 7000 Å, the linear interface-dislocation density for mesas 100 μm in width is still very low, approximately 1500 cm−1. The 8250-Å layers possess interface-dislocation densities too high to be accurately determined with CL. However, increases in CL intensity as mesa width is reduced indicate that the interface-dislocation density is decreasing and that growth on small areas produces higher-quality layers than growth on large areas. Our investigations show that different sources for misfit dislocations become active at different epilayer strain levels. The critical thickness depends on which type of nucleation source becomes activated first; therefore, different critical thicknesses can be observed depending on which kind of source is present in a specimen.

Transmission electron microscopy for the determination of the microstructure of thin films and interfaces

The optics and construction of a transmission electron microscope are reviewed, along with electron diffraction and its relation to image contrast. Three general areas of thin film analysis by transmission electron microscopy are discussed: geometric determinations, involving variations in the thickness of thin films, substrate dislocations and grain size; microstructure determination by electron diffraction, using ionized cluster beam deposited aluminum; analysis of an interfacial reaction by a combination of diffraction and X-ray fluorescence, using Inconel 718 and a lithia-silica glass ceramic. Future trends in the use of transmission electron microscopy for the analysis of thin films are discussed.