Plan-view Transmission Electron Microscopy Research Articles

Origin and evolution of threading dislocation in CdZnTe(0 0 1)/GaAs(0 0 1) epilayer grown by close spaced sublimation

The origin and evolution of threading dislocations (TDs) in CdZnTe(0 0 1)/GaAs(0 0 1) epilayer grown by close spaced sublimation (CSS) were experimentally investigated. The dislocation formed at the initial growth stage was observed and analyzed by the plan-view (scanning) transmission electron microscopy (STEM). The thickness-dependent migration and recombination of TDs along the growth direction were revealed by the dislocation etch pit. The origin of TDs is mainly related to the mismatch of growth domains. Strong lateral migration of dislocation in film can be explained by epitaxial lateral overgrowth (ELO) process. The blocking effect on TD movement by growth pit which is related to antiphase boundary (APB) is the primary cause of epilayer quality deterioration.

Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition

Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (>10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec (0002) and 304 arc-sec (202¯1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 × 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δα=αAlN−αSiC=1.13×10−6 °C−1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.

Short-Range Scattering Sources of Two-Dimensional Hole Gases in Undoped Ge/Gesi Heterostructures

Germanium has recently gained attention due to its potential for spintronic applications [1]. While most prior studies were done on modulation-doped Ge/GeSi heterostructures, there have been few reports on the mobility-limiting mechanisms in undoped structures. In this work, we investigate the electrostatics and magneto-transport of two-dimensional hole gases in undoped Ge/Ge1-xSix heterostructures with different Si fractions. The quantum transport data were correlated with material properties, such as dislocation densities and oxygen concentrations. The undoped Ge/Ge1-xSix heterostructures (x = 0.18, 0.28, 0.36) were grown by chemical vapor deposition. The growth details can be found in our prior work [2]. Transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS), and reciprocal space mapping (RSM) were used to characterize the layer thicknesses, the dislocation densities, the strain distributions, and the impurity concentrations in the Ge/GeSi heterostructures. Enhancement-mode Hall bar devices were fabricated for low-temperature (4 K and 0.3 K) mobility measurements [2]. The hole density was modulated by gating. By increasing the Si fraction of the GeSi barrier, the saturation density increases due to a larger valence band offset [2,3]. For each device, the mobility increases with density, showing the dominance of carrier screening, while the mobility decreases with the Si fraction. For the power dependence of mobility versus density (μ=nα), α is 0.20 ~ 0.43 for all devices in the density regime of 8 x 1010 to 4 x 1011 cm-2. This indicates that the remote impurity scattering at the oxide interface, (α =1.5) is unlikely the dominant scattering source. Planar-view TEM were performed to characterize threading and misfit dislocations. For the Ge/GexSi1-x heterostructures with x = 0.18 and x = 0.28, threading dislocations were ~ 4.9 x 108 cm-2 and ~ 7.7 x 108 cm-2, respectively. For all Ge/GeSi heterostructures, the thickness of the strained Ge quantum well is ~ 20 nm, slightly larger than the critical thicknesses. Therefore, misfit dislocations are expected to impact the mobility as the crystal is fully or partially relaxed. Oxygen impurities could also be important scattering sources. The oxygen level increases with the Si fraction of the GeSi barrier (Fig. 1(a)), which could be attributed to a stronger bonding between Si-O over Ge-O [4]. The expected mobility limited by ionized oxygen would be ~100 times lower than the experimental data, which indicates that structural defects and strain inhomogeneity induced by neutral oxygen precipitates could be the more dominant scattering sources than charged impurities [5]. Finally, Dingle-ratio analysis was performed to characterize the mobility mechanisms [6]. The Dingle ratios are small (5 ~ 14) (Fig. 1(b)), indicating that the short-range (large-angle) scattering dominates. As the Si fraction increases, the Dingle ratio decreases, showing that the short-range scattering is stronger. This result suggests that both the oxygen impurities and the dislocations are the most important scattering sources. In summary, the scattering mechanisms in undoped Ge/GeSi heterostructures were investigated. Weak density-mobility dependence and small Dingle ratios suggest that the dominant scattering sources are dislocations and oxygen impurities. Acknowledgements This work at National Taiwan University has been support by the Ministry of Science and Technology (107-2622-8-002-018- and 107-2112-M-002-014-). This work at Sandia National Laboratories has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences, user facility. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. Figure 1

MoTe2 Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering.

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe2) phase, metallic monoclinic (1T') MoTe2 phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T' MoTe2 homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe2 grow out of a polycrystalline 1T'-MoTe2 matrix. We further demonstrate the operation of MoTe2 FETs made on these in situ-grown lateral homojunctions with 1T' contacts. The use of a 1T' phase as electrodes in MoTe2 FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T' electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T' interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe2 homojunction FETs for use in memory and logic circuity applications.

Microstructure and influence of buffer layer on threading dislocations in (0 0 0 1) AlN/sapphire grown by hydride vapor phase epitaxy

The microstructures of AlN films grown by high-temperature hydride vapor phase epitaxy (HT-HVPE) on (0 0 0 1) sapphires with three different kind of buffer layers (BLs) were investigated by plan-view and cross-sectional transmission electron microscopy (TEM). The buffer layers were grown by HVPE at 1350 °C, MOCVD 1050 °C and HVPE at 1050 °C. The microstructures and in-plane misorientation of buffer layers are related to the crystal quality of the AlN films and dislocation evolution. The misorientation angles and average grain sizes of buffer layers were analyzed quantitatively using Moire fringe method. The buffer layer grown at 1350 °C by HVPE was recognized to be single crystalline, but buffer layers with mosaic-structure are formed at 1050 °C either by MOCVD or HVPE. High growth temperature of 1350 °C significantly reduced the in-plane misorientation of the buffer layer and the dislocation density was greatly reduced. AlN films on high-temperature growth buffer layers (∼1350 °C) have the lowest treading dislocation density ∼3 × 108 cm−2 determined using a combination of X-ray rocking curve measurement and plan-view transmission electron microscopy.

Three-dimensional structure of antiphase domains in GaP on Si(0 0 1)

Antiphase domains are three-dimensional crystal defects commonly arising at the interface of III–V semiconductors and Si. While control over their formation has been achieved, the geometry of the antiphase domain itself that is separated from the mainphase of the crystal by the so-called antiphase boundary, has not yet been fully understood. In this work, we first investigate the interface between GaP and Si itself by cross-sectional scanning tunneling microscopy (XSTM) to reveal possible intermixing within an 8 monolayers wide region. Furthermore, we present an extensive analysis combining transmission electron microscopy and XSTM to elucidate the shape of antiphase domains in GaP. To create a true-to-scale, three-dimensional model of an antiphase domain, firstly, plan-view transmission electron microscopy images are drawn on. Subsequently, the progression of many antiphase boundaries through the GaP crystal as viewed from the (1 1 0) and (1 0) cleavage planes is analyzed all the way down to the atomic level by means of XSTM. This enables a detailed analysis of the shape and physical dimensions of the antiphase domains. A typical measured extension in growth directions is found to be a maximum of 60 nm and the maximum measured extension of the base plane in [ 1 0] and [1 1 0] directions is about 160 nm and 50 nm, respectively. They appear as pyramids with anisotropic base planes whose side facets kink many times.

Optimization of In0.6Ga0.4As/InAs electron barrier for In0.74Ga0.26As detectors grown by molecular beam epitaxy

The effects of thickness and period number of In0.6Ga0.4As/InAs superlattice electron barrier on the performances of InP-based metamorphic In0.74Ga0.26As detectors have been investigated. The samples were grown by molecular beam epitaxy and characterized by high resolution X-ray diffraction reciprocal space maps, photoluminescence, atomic force microscope, plan view transmission electron microscopy and the device dark current measurements. Results show that the dark currents are reduced by insertion of an electron barrier with moderate thickness and period number in the absorption layer, but a superlattice electron barrier structure with excessive thickness and period number can lead to the deterioration of the material quality and device performances.

New Relaxation Mechanism Enabling High-Quality, Laterally Grown Ge on Si

We demonstrate a novel growth technique, metal-catalyzed, lateral epitaxial growth, to grow Ge films laterally on Si with reduced threading dislocation densities (TDD). In contrast to traditional planar film growth on Si, this technique starts with a small crystal nucleation at a specific position on Si followed by Ge lateral film growth. In plan-view and cross-sectional transmission electron microscopy micrographs, high-density threading dislocations were found to be present in the initial growth areas, while the lateral overgrowth areas demonstrated substantially reduced TDD or even defect-free areas. Furthermore, the X-ray diffraction results showed that the films were fully relaxed and mostly pure Ge. Full relaxation and low TDD Ge films are two surprising results given that the growth occurred at a low temperature (375 °C), which cannot be understood with the current misfit dislocation nucleation and glide model of film relaxation. For these reasons, we suggest the presence of a new relaxation mechan...

Plan-view transmission electron microscopy specimen preparation for atomic layer materials using a focused ion beam approach

Using the focused ion beam (FIB) to prepare plan-view transmission electron microscopy (TEM) specimens is beneficial for obtaining structural information of two-dimensional atomic layer materials, such as graphene and molybdenum disulfide (MoS2) nanosheets supported on substrates. The scanning electron microscopy (SEM) image in a dual-beam FIB-SEM can accurately locate an area of interest for specimen preparation. Besides, FIB specimen preparation avoids damages and hydrocarbon contamination that are usually produced in other preparation methods, in which chemical etching and polymer adhesion layers are used. In order to reduce harmful ion-beam bombardment and re-deposition on the thin atomic layers during FIB specimen preparation, we develop a method to protect the atomic layers by making a “microcapsule” to insulate the sample surface. The method is applied respectively to prepare plan-view TEM specimens of a graphene sheet with multiple adlayers and MoS2 atomic layers. Useful electron diffraction results can be obtained from these specimens for understanding the interlayer orientation relationships in the two materials. Auger electron spectroscopy analysis further confirms that the sample surface is free from contamination under the sufficient protection given by the proposed method.

Transformation of the defect structure of InGaAs and InAlAs metamorphic buffer layers depending on indium concentration

A study of samples grown by molecular beam epitaxy using the technology of InxAl1-xAs or InxGa1-xAs metamorphic buffer layer formation on GaAs (001) substrate was performed by transmission electron microscopy. The 1 μm thick buffer layers with square-root dependence of In content on buffer layer thickness were studied in plan-view and cross-section geometries. The results demonstrate cascade step-wise relaxation of misfit strain along buffer layer thickness with the reduction of dislocation density to less than 10−6 cm−2 at the subsurface region. An inhomogeneous distribution of dislocations observed on plan-view TEM images is explained by overlap of split-level dislocation networks with different periods.

Synthesis and microstructural characterization of nanoscale multilayer TiAlN/TaN coatings deposited by DC magnetron sputtering

(Ti,Al)N coatings have become the most used option for the protection of different industrial tools due to their excellent properties like high hardness and low wear rate, which is maintained until temperatures of 700 °C are achieved by the formation of a dense and stable layer of Al2O3. Additionally, TaN coatings have proved to be an appropriate alternative to conventional coatings with great potential in different applications, due to their high chemical stability and corrosion resistance. These aspects motivated the combination of both coatings in a multilayer structure at the nanometric scale. In this work, novel coatings of (Ti,Al)N/TaN were deposited on AISI H13 hot work tool steel by a self-manufactured DC magnetron sputtering vacuum chamber without target shutter. In order to obtain coatings with different decreasing periods, four substrate rotation speeds were used: 0.2 rpm, 0.5 rpm, 1.0 rpm, and 3.0 rpm. In addition to the multilayer coatings, the monolayers of (Ti,Al)N and TaN were deposited for comparison purposes. Using AFM, it was possible to observe that both the roughness and the grain size decreased from 17 to 13 nm and from 165 to 137 nm, respectively, as the substrate rotation speed increased. The monolayer coatings (Ti,Al)N and TaN exhibited an fcc crystal structure with growth directions (111) and (200), respectively. On the other hand, for the multilayer coating (Ti,Al)N/TaN, a change in the preferential direction corresponding to the TaN from [200] to [111] was identified. In the SEM images of the multilayers, a microstructure of columnar growth was observed, which is densified with an increased rotation speed of the substrates, and the multilayers are no longer observed for the highest speeds and the smallest periods. Using plan-view TEM images and selected area EDS, it was possible to suggest the formation of a nanoscale-structured multilayer with a well-defined interface between TiAlN and TaN. Regarding the residual stresses of the coatings, it was possible to observe that with the increase in the substrate rotation speed during the deposition, the stresses decreased progressively, reaching values up to − 1.51 GPa, significantly lower than both TiAlN (− 7.09 GPa) and TaN (− 8.66 GPa) monolayer coatings.

Grain boundary segregation and intermetallic precipitation in coarsening resistant nanocrystalline aluminum alloys

In-spite of all of the unique properties of nanocrystalline materials, they are notorious when it comes to their susceptibility to thermally induced grain coarsening, thus imposing an upper limit to their application temperature. In this study, we demonstrate a coupled Monte Carlo-molecular dynamics simulation-guided experimental approach of improving the resistance to thermally induced grain coarsening in light-weight nanocrystalline Al-Mg alloys. The structure, grain boundary segregation of Mg, and extent of grain coarsening of the Al-Mg alloys were characterized using plan view and cross-sectional scanning transmission electron microscopy and atom probe tomography. Coarsening resistance is attributed to a combination of thermodynamic stabilization of grain boundaries by controlled Mg segregation, and kinetic stabilization through pinning of the boundaries with nanoscale intermetallic precipitates. Thus, we highlight the opportunities in extending the upper limit of application temperature for nanocrystalline alloys by using a complementary thermodynamic and kinetic stabilization approach.

Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging

Taking advantage of electron diffraction based measurements, in a scanning electron microscope, can deliver non-destructive and quantitative information on extended defects in semiconductor thin films. In this work, we have studied a (11-22) semi-polar GaN thin film overgrown on regularly arrayed GaN micro-rod array templates grown by metal organic vapour phase epitaxy. We were able to optimise the diffraction conditions to image and quantify basal plane stacking faults (BSFs) and threading dislocations (TDs) using electron channelling contrast imaging (ECCI). Clusters of BSFs and TDs were observed with the same periodicity as the underlying micro-rod array template. The average BSF and TD densities were estimated to be ≈4 × 104 cm−1 and ≈5 × 108 cm−2, respectively. The contrast seen for BSFs in ECCI is similar to that observed for plan-view transmission electron microscopy images, with the only difference being the former acquiring the backscattered electrons and the latter collecting the transmitted electrons. Our present work shows the capability of ECCI for quantifying extended defects in semi-polar nitrides and represents a real step forward for optimising the growth conditions in these materials.

Atomistic Structure of Low-Resistance Si/GaAs Heterointerfaces Fabricated By Surface-Activated Bonding at Room Temperature

Tandem solar cells consisting of silicon (Si) and III-V compounds are one of the promising candidates for next-generation terrestrial photovoltaic systems, that can surpass the efficiency milestone of 30% for non-concentrating solar cells without using expensive Ge or GaAs substrates. Recently, the surface-activated bonding (SAB) at room temperature (RT), in which surfaces of substrates are activated before bonding by creating dangling bonds via the removal of contaminants under an energetic particle bombardment in high vacuum, is applied to form Si/GaAs heterointerfaces for hybrid triple-junction cells with a high conversion efficiency above 26% [1]. Even though the interface resistance (~100 Wcm2) is low enough for solar cells, it is still higher than the ideal one at defect-free heterointerfaces (~10-4 Wcm2). Accordingly, a comprehensive knowledge of the electrical property at the heterointerfaces depending on their atomistic structure is indispensable to establish the fabrication processes of high-efficiency tandem cells by optimizing the interface structure. In the present work, p-Si/n-GaAs heterointerfaces were fabricated at RT under a SAB condition [1], with the substrates of B-doped (100) p-Si (with a carrier concentration of 2x1014 cm-3) and Si-doped (100) n-GaAs (2x1016 cm-3) which was 5o off from (100) toward [011]. A part of them were then annealed at 673 K for 1 min. Their structural properties were determined by plane-view transmission electron microscopy (TEM) (Fig. 1), as well as cross-sectional scanning TEM (STEM). As-bonded heterointerfaces included an As-deficient crystalline GaAs layer less than about 1 nm thick and an amorphous Si layer about 3 nm thick [2]. Dimples were introduced on the whole GaAs surface at the bonding heterointerface (Fig. 1d), presumably via the introduction of As vacancies during the surface activation process, and they disappeared after 673 K annealing. The density in the amorphous Si layer was slightly lower than in the conventional amorphous Si, presumably due to the introduction of Si vacancies during the surface activation process, and it was increased by 673 K annealing. Those structural changes would result in the reduction of the resistance at the bonded Si/GaAs heterointerfaces [3]. On the other hand, dislocations in GaAs cropping out the heterointerfaces induce large strains at the interfaces (indicated with the arrows in Figs. 1a and 1b), and they would increase the interface resistance even after annealing. Therefore, the resistance should be reduced by suppressing those interface defects, via the optimization of SAB conditions with a defect-free GaAs substrate. In order to confirm the hypothesis, the electronic property of the defects was examined by cathodoluminescence (CL) spectroscopy in a transmission electron microscope. The relationship of the defects and the interface resistance will be discussed in the conference.

Large Area Growth of MoS2 By Chemical Vapour Deposition

Transition Metal Dichalcogenindes (TMD) have considerable potential for applications spanning electronics, sensors and optoelectronics due to the wide ranging electronic and optical properties which are displayed by this class of 2D layered materials [1]. Research is focused on issues such as: large area growth [2, 3], stable approaches to doping [4] and achieving required values of specific contact resistivity [5]. We are contributing to the research effort by investigating the structural, optical and electronic properties of crystalline molybdenum disulfide (MoS2) grown by chemical vapour deposition (CVD) in a commercial 300mm atomic layer deposition reactor. In this work we report on the properties of monolayer and multilayer MoS2 growth at 550oC using Mo(CO)6 and H2S precursors on a number of different substrates, including SiO2, sapphire and amorphous alumina. This work focuses on the topology, Raman response and electronic properties of the CVD grown MoS2 thin films. Figure 1 (a) shows the Raman spectrum for an as-grown 10nm MoS2 on a SiO2/Si substrate. MoS2 can be determined by the peaks at approximately 385cm-1 (E1 2g) and 410cm-1 (A1g). There are two other peak characteristics of MoS2 which cannot be seen due to back-scattering at 286cm-1 (E1g), and lack of sensitivity below 200cm-1 at 32cm-1 (E2 2g )[6]. Additional peaks are detected at wavenumbers between 2000-3000cm-1, seen in Fig. 1 (b). Further investigation is needed to establish if these peaks are from photoluminescence, contaminants within the structure or purely a surface effect. Fig. 1 (c) shows an AFM image of an annealed 1.2nm MoS2 sample on a SiO2/Si substrate. The blue marks indicate areas on the material that are above a height of 0.6nm. Further results will be presented as a function of the number of MoS2 monolayers using Conductive AFM (C-AFM) and Kelvin Probe analysis. XTEM images seen in Fig. 1 (d), (e) show MoS2 grown on SiO2/Si and sapphire substrates respectively. They show that polycrystalline and layered MoS2 is formed at the growth temperature of 550oC, with no subsequent post growth annealing. There is no interfacial layer formed at the MoS2/SiO2 interface, but an amorphous interfacial layer of ~0.5nm is observed between MoS2 and sapphire, which is still being investigated. Plan view TEM analysis (not shown) confirms aligned MoS2 with grain sizes (over a local area of around 100 nm x 100nm) in the range 5nm to 20nm. The carrier concentration, carrier type and carrier mobility were studied with Hall measurements carried out at room temperature using a Van der Pauw structure (1cm x 1 cm). Excellent ohmic behavior is achieved on MoS2 (nominally 10nm) deposited on both sapphire and a-Al2O3/sapphire substrates. Table 1 provides a summary of the Hall analysis, showing that the non-intentionally doped MoS2 grown by CVD is n-type with very low carrier concentrations on the order of ~1014cm-3, electron mobility in the range 3.3-16.7cm2/V.s. Mobility values up to ~ 15 cm2/Vs for a grain size in the 10nm to 60nm range, is an interesting result, as in the work of K. Kang et al., [3], the monolayer grain size is around 1 mm with an associated electron mobility of 30 cm2/V. These results suggests that grain boundary defects in 2D MoS2 may not be the main factor limiting carrier mobility, as is typically the case in polycrystalline 3D semiconductors (see for example [7]). In addition, the unintentional n type doping in the CVD grown MoS2 is low, with values around 1-3x1014 cm-3. This low value of unintentional doping provides a useful baseline in-situ for doping studies with elements such as Nb [8] and Re [9].

Catastrophic Optical Bulk Damage – A New Failure Mode in High-Power InGaAs-AlGaAs Strained Quantum Well Lasers

ABSTRACTHigh-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers with emission wavelengths of 910 − 980 nm are extensively used in various fiber lasers and amplifiers for both telecom and defense applications. In particular, underseas and satellite communication systems require stringent reliability from these lasers. Since these lasers predominantly fail by catastrophic and sudden degradation due to COD, it is crucial especially for space satellite applications to investigate reliability, failure modes, and degradation mechanisms of these lasers. Catastrophic optical mirror damage (COMD) was known to be the only failure mode until our group reported a new failure mode in MM and SM InGaAs-AlGaAs strained QW lasers in 2009 and 2016, respectively. Our group reported that bulk failure due to catastrophic optical bulk damage (COBD) has become the dominant failure mode of both SM and MM lasers. Since there have been limited reports on COBD compared to COMD, the intent of this paper is to introduce our studies on COBD that have spanned the last decade. We investigated reliability, failure modes, and degradation processes in SM and MM InGaAs-AlGaAs strained QW lasers by performing short-term step-stress tests and long-term accelerated life-tests as well as failure mode analyses using various nondestructive and destructive micro-analytical techniques including electron beam induced current (EBIC), time-resolved electroluminescence (EL), deep level transient spectroscopy (DLTS), focused ion beam (FIB), and high-resolution TEM. EBIC and EL techniques were employed to study dark line defects generated in degraded lasers stressed under different test conditions. Time-resolved EL techniques were employed to study initiation and progressions of dark spots and dark lines in real time as lasers were aged. DLTS techniques were employed to study electron traps in both pristine and degraded lasers. Lastly, FIB and high-resolution TEM were employed to prepare cross sectional and plan view TEM specimens to study DLD areas in post-aged lasers. We also report our current understanding on degradation mechanisms responsible for COBD in both SM and MM lasers.

Epitaxial growth of high quality InP on Si substrates: The role of InAs/InP quantum dots as effective dislocation filters

Monolithic integration of InP on a Si platform ideally facilitates on-chip light sources in silicon photonic applications. In addition to the well-developed hybrid bonding techniques, the direct epitaxy method is spawning as a more strategic and potentially cost-effective approach to monolithically integrate InP-based telecom lasers. To minimize the unwanted defects within the InP crystal, we explore multiple InAs/InP quantum dots as dislocation filters. The high quality InP buffer is thus obtained, and the dislocation filtering effects of the quantum dots are directly examined via both plan-view and cross-sectional transmission electron microscopy, along with room-temperature photoluminescence. The defect density on the InP surface was reduced to 3 × 108/cm2, providing an improved optical property of active photonic devices on Si substrates. This work offers a novel solution to advance large-scale integration of InP on Si, which is beneficial to silicon-based long-wavelength lasers in telecommunications.

THERMAL OXIDATION EFFECT ON ELECTRICAL PROPERTIES OF MOS CAPACITORS WITH EMBEDDED Ge NANOCRYSTALS

A 2D layer of spherical, crystalline Ge nanodots embedded in a SiO 2 layer was formed by low pressure chemical vapor deposition combined with furnace oxidation and rapid thermal annealing. The samples were characterized structurally by using transmission electron microscopy in plan-view and cross-section geometries. It was found that the formation of highdensity Ge dots took place due to oxidation induced by the Ge segregation. Electrical properties were controlled by measuring C–V and I–V characteristics after the formation of MOS capacitors in different oxidation conditions and the ambient medium. A strong evidence of the charge storage effect on the crystalline Ge-nanodot layer was demonstrated by the hysteresis behavior of the high-frequency C–V curves. It is shown that dry oxidation followed by its reduction increases the hysteresis value compared to wet oxidation conditions. This hysteresis behavior is discussed taking into account the decrease in the Ge concentration and a possible effect of low temperature GeO evaporation is followed by wet oxidation.

Ferroelectricity in Hf0.5Zr0.5O2 Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations.

Because of their full compatibility with the modern Si-based technology, the HfO2-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional Hf0.5Zr0.5O2-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine Hf0.5Zr0.5O2 films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric Hf0.5Zr0.5O2 layer during the wake-up process under an electrical stress.

Homoepitaxial Growth of InP on Electrochemical Etched Porous InP Surface

Single crystal indium phosphide (InP) layers were epitaxially grown on porous (001) InP surfaces with controlled porosities. Changes in the underlying porous layer structure, and hence its stability, during epitaxial growth at elevated temperatures were observed. The same change in the porous structure was achieved by only annealing under the same growth conditions. A 100%-layer transfer rate is demonstrated for annealed dual porous InP layers. Plan-view transmission electron microscopy (TEM) images of the epitaxial layer show that the layer is continuous over the porous underlayer and had a dislocation density lower than 3 × 106 cm−2. The high crystallinity of the epitaxy layer was verified by high resolution X-ray diffraction (HRXRD) rocking curve scans. These high quality epitaxial layers grown on porous InP show promise for active layers in thin, flexible high-performance devices, which are readily transferred from the growth substrate by fracture at the epitaxial-porous layer interface.