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The formation of intermediate layers in covered Ge/Si heterostructures with low-temperature quantum dots: a study using high-resolution transmission electron microscopy and Raman spectroscopy
摘要： The method of software analysis of high-resolution TEM images using the peak pairs algorithm in combination with Raman spectroscopy was employed to study lattice deformations in Ge/Si(001) structures with low-temperature Ge quantum dots. It was found that the stresses do not spread in a thick Si layer above quantum dots, but completely relax via the formation of a thin boundary layer of mixed composition. However, intermixing of Ge and Si is absent beneath the Ge layer in samples with a Ge coverage of 10 ?A. Besides intermixing was not observed at all, both beneath and above the Ge layer, in samples with a Ge coverage of 6 ?A or less. This may be due to the predominance of Ge di?usion into the Si matrix from the {105} facets of Ge huts, not from the Ge wetting layer, at low temperatures of the Ge/Si structure deposition. The critical thickness of Si coverage at which the intense stress-induced di?usion takes place is determined to lie in the range from 5 to 8 nm.
关键词： lattice strain,Ge/Si quantum dots,peak-pairs analysis,HR TEM,Raman scattering,strain relaxation
更新于2025-09-19 17:13:59
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Size effect on optical performance of blue light-emitting diodes
摘要： In this paper, size effects on optical performance of blue light-emitting diodes (LEDs) are investigated. The essential physical mechanism is studied by fabricating LEDs with various sizes of the active area and testing optical characteristics. It is found that micro-LEDs have better light extracting efficiency and thermal dissipation compared with broad-area LEDs, which is likely due to the small ratio of perimeter and active area. Furthermore, micro-LEDs are more beneficial for displays due to the stable wavelength under the low pulse width modulation (PWM) current density.
关键词： size dependence,micro-LED,strain relaxation,light extract efficiency
更新于2025-09-19 17:13:59
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Mechanism of improved luminescence intensity of Ultraviolet Light Emitting Diodes (UV-LEDs) under thermal and chemical treatments
摘要： In this work, the influences of thermal annealing and chemical passivation on the optical and electrical properties of ultraviolet light-emitting-diode (UV-LED) were investigated. The electroluminescence (EL) intensities of the LEDs under KOH treatment and thermal annealing increased by 48% and 81%, respectively compared to as-fabricated LED under current level of 10 mA. Cathodoluminescence (CL) mapping of UV-LEDs confirmed no variation of the density of the non-radiative recombination centers after surface treatments, and no obvious change in surface morphology was identified due to lacking of energy for surface atom migration. However, Raman spectroscopy indicates a relaxation of compressive strains inside the thin film after both thermal and chemical treatments, and conductive atomic force microscopy (c-AFM) also illustrated reduced leakage current after KOH passivation, which are responsible for the improved luminescence properties of UV-LEDs.
关键词： surface treatment,UV-LED,electroluminescence,strain relaxation
更新于2025-09-12 10:27:22
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Strain-Related Degradation of GaN-Based Blue Laser Diodes
摘要： Degradation of GaN-based laser diodes (LDs) is studied using Electroluminescence (EL) and Transmission Electron Microscopy (TEM). The slope efficiency decrease is observed in addition to threshold current increase. Further studies on the optical loss and microstructures of the LDs indicate that the reduction of the slope efficiency is attributed to the increase of optical loss (from 14.2 cm-1 to 24.2 cm-1) and decrease of injection current efficiency induced by the defect generation around the waveguide and cladding layers. Injection current induced blue-shift of the spontaneous EL peak reduced after > 3000 hrs operation, which is ascribed to the compensation of Quantum-Confined Stark Effect (QCSE) caused by partial strain relaxation near the active region.
关键词： Spectroscopy,Degradation,Strain relaxation,Semiconductor lasers,Linewidth
更新于2025-09-12 10:27:22
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Spontaneous Enhancement of the Stable Power Conversion Efficiency in Perovskite Solar Cells
摘要： The power conversion efficiency (PCE) of lead-halide perovskite solar cells (PSCs) is reported to increase over a period of days after their fabrication while they are stored in dark. Thus far, effects underlying this spontaneous enhancement are not understood. This work investigates the phenomenon for a variety of multi-cation-halide PSCs with different perovskite compositions and architectures. The observations reveal that spontaneous enhancement is not restricted to specific charge-transport layers or perovskite compositions. The highest PCE observed in this study is the enhanced stable PCE of 19% (increased by 4% absolute). An increased open-circuit voltage is the primary contributor to the improved efficiency. Using time-resolved photoluminescence measurements, initially-present low-energy states are identified that disappear over a storage period of a few days. Furthermore, trap states probed by thermally stimulated current technique exist in pristine PSCs and strikingly decrease for stored devices. In addition, ideality factor approaches unity and X-ray diffraction analyses show a lattice strain relaxation over the same period of time. These observations indicate that spontaneous enhancement of the PSCs is based on a reduction in trap-assisted non-radiative recombination possibly due to strain relaxation. Considering the demonstrated generality of spontaneous enhancement for different compositions of multi-cation-halide PSCs, our results highlight the importance of determining absolute PCE increase initiated by spontaneous enhancement for developing high-efficiency PSCs.
关键词： spontaneous enhancement,perovskite solar cells,power conversion efficiency,trap states,strain relaxation
更新于2025-09-11 14:15:04
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Formation of uniaxial strained Ge via control of dislocation alignment in Si/Ge heterostructures
摘要： Uniaxially strained Ge/SiGe heterostructures are fabricated by selective ion implantation technique, where dislocation alignments are highly controlled by the local defect introduction. Firstly, ion-implantation-defects are selectively induced into a Ge substrate, followed by the growth of a SiGe buffer layer. As a result, the SiGe on the implanted region is largely strain-relaxed due to the defects acting as dislocation sources. In contrast, it is demonstrated that anisotropic strain relaxation takes place in the SiGe on the unimplanted region, leading to the uniaxial strained SiGe. A strained Ge layer is pseudomorphically grown on the SiGe buffer and the same strain states are observed for the Ge layer. It is found that mis?t dislocations generated at the interface between the SiGe layer and the Ge substrate are aligned along only one direction. These one-directional dislocations are an origin of the uniaxial strain relaxation. Moreover, effects of ion-implantation stripe-pattern widths on the strain states are investigated. With the implanted line width increasing, the anisotropy of the strain in the unimplanted region is enhanced. From these results, it can be said that this technique opens a route to engineer dislocation alignments and anisotropic strain in semiconductor hetero structures toward high performance novel devices.
关键词： dislocation alignment,uniaxial strained Ge,Si/Ge heterostructures,selective ion implantation,anisotropic strain relaxation
更新于2025-09-10 09:29:36
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Modeling charge density in AlGaN/AlN/InGaN/GaN-based double heterostructures including InGaN layer strain relaxation
摘要： An analytical model is presented to calculate the two-dimensional electron gas (2DEG) density and barrier height of bare surface AlGaN/AlN/InGaN/GaN double heterostructures, which use InGaN as the conducting layer. The basic model is derived from electrostatic analysis of the di?erent material interfaces. The e?ect of strain relaxation in the InGaN layer is also incorporated here. Further, the impact of a two-dimensional hole gas at the InGaN/GaN interface, formed when the InGaN layer thickness is high, has been considered. The presented results are seen to agree with the available experimental results. Thus, this model can be a useful tool in the design and modeling of InGaN-based III-nitride heterostructures.
关键词： two-dimensional hole gas,AlGaN/InGaN/GaN heterostructure,strain relaxation,two-dimensional electron gas
更新于2025-09-09 09:28:46
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Enhanced Luminescence from InGaN/GaN Nano-disk in a wire Array Caused by Surface Potential Modulation during Wet Treatment
摘要： Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum (FWHM), strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.
关键词： exciton binding energy,photoluminescence,QCSE,nano-disk,surface potential,strain relaxation,InGaN/GaN,quantum confinement
更新于2025-09-04 15:30:14
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Local Strain and Crystalline Defects in GaN/AlGaN/GaN(0001) Heterostructures Induced by Compositionally Graded AlGaN Buried Layers
摘要： Plastic strain relaxation in epitaxial layers is one of the crucial factors that limit the performance of III-nitride-based heterostructures. In this work, we report on strain relaxation and crystalline defects in heterostructures consisting of compositionally graded AlGaN epitaxial layers tensile-strained between a GaN-buffer and a GaN-cap. We demonstrate the effects of Al concentration and the shape of the concentration-depth profile in the buried graded layers on the accumulated elastic strain energy and how this influences the critical thickness for crack generation or fracture. It is shown that this fracture leads to the formation of partially relaxed regions with their degree of strain relaxation directly related to the density of cracks. Nevertheless, even though the in-plane coherency between the AlGaN layer and the GaN-buffer is broken, the in-plane coherency within the AlGaN layer is preserved for all regions. Furthermore, the tensile strain released in the buried graded AlGaN layers is consistent with compressive strain induced in the GaN-cap layers. Finally, the localized stress and the densities of threading dislocations are correlated with the features of the resulting fractured heterostructures. These results are important towards the control of complex plastic strain relaxation and further facilitate the growth of high quality compositionally graded AlGaN-based devices.
关键词： X-ray diffraction,cracks,graded layers,epitaxy,strain relaxation,AlGaN
更新于2025-09-04 15:30:14