Defect filtering for thermal expansion induced dislocations in III-V lasers on silicon

Epitaxially integrated III-V semiconductor lasers for silicon photonics have the potential to dramatically transform information networks, but currently, dislocations limit performance and reliability even in defect tolerant InAs quantum dot (QD) based lasers. Despite being below critical thickness, QD layers in these devices contain previously unexplained misfit dislocations, which facilitate non-radiative recombination. We demonstrate here that these misfit dislocations form during post-growth cooldown due to the combined effects of (1) thermal-expansion mismatch between the III-V layers and silicon and (2) precipitate and alloy hardening in the active region. By incorporating an additional sub-critical thickness, indium-alloyed misfit dislocation trapping layer, we leverage these mechanical hardening effects to our advantage, successfully displacing 95% of misfit dislocations from the QD layer in model structures. Unlike conventional dislocation mitigation strategies, the trapping layer reduces neither the number of threading dislocations nor the number of misfit dislocations. It simply shifts the position of misfit dislocations away from the QD layer, reducing the defects' impact on luminescence. In full lasers, adding a misfit dislocation trapping layer both above and below the QD active region displaces misfit dislocations and substantially improves performance: we measure a twofold reduction in lasing threshold currents and a greater than threefold increase in output power. Our results suggest that devices employing both traditional threading dislocation reduction techniques and optimized misfit dislocation trapping layers may finally lead to fully integrated, commercially viable silicon-based photonic integrated circuits.Comment: 9 pages, 6 figure

Defect characterization of InAs/InGaAs quantum dot p-i-n photodetector grown on GaAs-on-V-grooved-Si substrate

The performance of semiconductor devices on silicon can be severely degraded by the presence of dislocations incurred during heteroepitaxial growth. Here, the physics of the defect mechanisms, characterization of epitaxial structures, and device properties of waveguide photodetectors (PDs) epitaxially grown on (001) Si are presented. A special GaAs-on-V-grooved-Si template was prepared by combining the aspect ratio trapping effects, superlattice cyclic, and strain-balancing layer stacks. A high quality of buffer structure was characterized by atomic force microscopy (AFM) and electron channeling contrast imaging (ECCI) results. An ultralow dark current density of 3.5 × 10–7A/cm2 at 300 K was measured under −1 V. That is 40× smaller than the best reported value of epitaxially grown InAs/GaAs quantum dot photodetector structure on GaP/Si substrate. Low frequency noise spectroscopy was used to characterize the generation and recombination related deep levels. A trap with an activation energy of 0.4 eV was identified, which is near the middle bandgap. With low frequency noise spectroscopy along with the current–voltage and capacitance–voltage characterizations, the recombination lifetime of 27 μs and trap density of 5.4 × 1012 cm–3 were estimated

1.3 μm submilliamp threshold quantum dot micro-lasers on Si

As a promising integration platform, silicon photonics need on-chip laser sources that dramatically improve capability, while trimming size and power dissipation in a cost-effective way for volume manufacturability. Currently, direct heteroepitaxial growth of III–V laser structures on Si using quantum dots as the active region is a vibrant field of research, with the potential to demonstrate low-cost, high-yield, long-lifetime, and high-temperature devices. Ongoing work is being conducted to reduce the power consumption, maximize the operating temperature, and switch from miscut Si substrates toward the so-called exact (001) Si substrates that are standard in microelectronics fabrication. Here, we demonstrate record-small electrically pumped micro-lasers epitaxially grown on industry standard (001) silicon substrates. Continuous-wave lasing up to 100°C was demonstrated at 1.3 μm communication wavelength. A submilliamp threshold of 0.6 mA was achieved for a micro-laser with a radius of 5 μm. The thresholds and footprints are orders of magnitude smaller than those previously reported lasers epitaxially grown on Si

Physical Origin of the Optical Degradation of InAs Quantum Dot Lasers

We present an extensive analysis of the physical mechanisms responsible for the degradation of 1.3-μm InAs quantum dot lasers epitaxially grown on Si, for application in silicon photonics. For the first time, we characterize the degradation of the devices by combined electro-optical measurements, electroluminescence spectra, and current-voltage analysis. We demonstrate the following original results: when submitted to a current step-stress experiment: 1) QD lasers show a measurable increase in threshold current, which is correlated to a decrease in slope efficiency; 2) the degradation process is stronger, when devices are stressed at current higher than 200 mA, i.e., in the stress regime, where both ground-state and excited-state emission are present; and 3) in the same range of stress currents, an increase in the defect-related current components is also detected, along with a slight decrease in the series resistance. Based on the experimental evidence collected within this paper, the degradation of QD lasers is ascribed to a recombination-enhanced defect reaction (REDR) process, activated by the escape of electrons out of the quantum dots

Novel mid-infrared materials and devices grown on InP: From metamorphic lasers to self-assembled nanocomposites

Laser diodes (LDs) emitting in the mid-infrared (mid-IR) spectral region (λ= 2 – 3 gm) are important for applications including molecular spectroscopy and gas detection. Quantum cascade lasers on InP have reached λ=3.0 μm continuous wave (CW) lasing at room temperature (RT), while type-I InAs quantum well (QW) LDs have reached λ= 2.4 μm. However, due to extremely high strain in the active regions for both technologies, demonstration of CW RT lasing at 2.4 – 3.0 μm remains difficult for InP-based lasers. A metamorphic InAsxP1-x graded buffer on InP can perform multiple functions in addressing this challenge, as it not only increases the critical thickness of InAs QWs to enable longer wavelength emission, but also functions as graded-index bottom cladding for optical confinement. In the first part of my thesis, I demonstrate InP-based metamorphic type-I QW LDs that take advantage of such multi-functional metamorphic buffers to achieve lasing at λ= 2.63 µm. The metamorphic LDs were grown on n-InP (001) substrates by solid source molecular beam epitaxy (MBE). We first grew Si-doped n-InAsxP1-x metamorphic graded to ensure a low threading dislocation density below 3 x 106/cm 2. For the active region, we utilize a strain-balanced InAs/In 0.54Ga0.46As multi-quantum well (MQW) with low net strain relative to the relaxed n-InAs0.5P0.5 waveguide. After growing the active region sandwiched by InAso.5Po.5 waveguides, a low-index, highly lattice-mismatched p- Al0.5Ga0.5As layer was deposited for the top cladding. Simulation results on fundamental transverse electric field mode show that the asymmetric cladding structure confines light at the center of the MQW with optical confinement factor of 4.14 %. High-resolution x-ray diffraction measurement revealed that the InAs MQW is fully strained, while the n-InAsxP1-x buffers and p-Al0.5Ga0.5As cladding are fully relaxed. The strong satellite peaks from the InAs MQW indicate high crystalline quality, and transmission electron microscopy (TEM) confirms that strain balancing was successful in avoiding misfit dislocation formation in the MQW region. More importantly, the growth of the p-Al0.5Ga0.5As top cladding did not generate TDs penetrating back to the active region, as expected from a highly lattice-mismatched interface where relaxation is dominated by sessile edge dislocations. We fabricated and tested 10 µm ridge-waveguide LDs, observing pulsed mode lasing up to 250 K at λ = 2.63 µm. The threshold current density at 77 K was 160 A/cm2 and increased to 4 kA/cm 2 at 250 K. Relatively low characteristic temperatures of 46-68 K were extracted, indicating possible carrier losses at elevated temperatures. We believe that further optimization in device design will enable lasing at room temperature and above. In the second part of my thesis, I show self-assembled growth of highly tensile-strained Ge nanostructures, coherently embedded in an InAlAs matrix (i.e. Ge/InAlAs nanocomposite) by using spontaneous phase separation. Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Ge is a very intriguing material for strain engineered nanocomposites, because strain can dramatically enhance its electrical and optical properties. Spurred by theoretical work showing that tensile strain can convert Ge from an indirect-gap to a direct-gap semiconductor, recent research has focused on applying large biaxial or uniaxial tensions using a range of approaches. For example, epitaxial growth of Ge thin films on template layers with larger lattice constant (e.g. InGaAs or GeSn) has enabled biaxial tensile strain up to 2.33%. Top-down fabrication techniques have also been used to fabricate Ge in biaxial or uniaxial tension using structures such as nanomembranes, bridges, and suspended nanowires. Here, I employ spontaneous phase separation during MBE growth as a fundamentally new approach to forming highly tensile-strained Ge/InAlAs nanocomposite. While the mutual immiscibility of Ge with III-V materials provides the driving force for phase separation, changes in growth kinetics enable significant control over nanostructure morphology, from nanowires to nanosheets. TEM reveals a high density of single-crystalline Ge nanostructures coherently embedded in InAIAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the Ge nanostructures. I demonstrate that the strain in the Ge nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering and the largest biaxial tension realized in Ge to date; the cross-over from indirect to direct is predicted at ~2% biaxial tension. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this novel nanocomposite material. We believe that the group-IV/III-V nanocomposites demonstrated here constitute a new materials platform for investigation of basic aspects of phase-separated growth, as well as offering the ability to create ultra-high strain states and properties that are otherwise inaccessible through conventional growth and processing

Microarray analysis of embryo-derived bovine pluripotent cells: The vulnerable state of bovine embryonic stem cells

<div><p>Although there are many studies about pluripotent stem cells, little is known about pluripotent pathways and the difficulties of maintaining the pluripotency of bovine cells <i>in vitro</i>. Here, we investigated differently expressed genes (DEG) in bovine embryo-derived stem-like cells (eSLCs) from various origins to validate their distinct characteristics of pluripotency and differentiation. We identified core pluripotency markers and additional markers which were not determined as pluripotency markers yet in bovine eSLCs. Using the KEGG database, TGFβ, WNT, and LIF signaling were related to the maintenance of pluripotency. In contrast, some DEGs related to the LIF pathway were down-regulated, suggesting that reactivation of the pathway may be required for the establishment of true bovine embryonic stem cells (ESCs). Interestingly, oncogenes were co-down-regulated, while tumor suppressor genes were co-up-regulated in eSLCs, implying that this pattern may induce abnormal teratomas. These data analyses of signaling pathways provide essential information on authentic ESCs in addition to providing evidence for pluripotency in bovine eSLCs.</p></div