An extensive effort was made to identify specific locations of crystal defects in epitaxial CdTe that might contribute to degraded light-conversion efficiency. Electroluminescence (EL) mapping and the creation of surface etch pits through chemical treatment were combined in attempts to identify specific structural defects for subsequent TEM examination. Observations of these specimens revealed only surface etch pits, without any visible indication of extended defects near their base. While chemical etch pits could be helpful for precisely locating extended defects that intersect with the treated surface, this study concluded that surface roughness surrounding etch pits would likely mitigate against their usefulness.
Defect locations in GaAs solar-cell devices were identified using combinations of EL, photoluminescence, and Raman scattering, and then studied more closely using TEM. Observations showed that device degradation was invariably associated with a cluster of extended defects, rather than a single defect, as previously assumed. AC-STEM observations revealed that individual defects within each cluster consisted primarily of intrinsic stacking faults terminated by 30° and 90° partial dislocations, although other defect structures were also identified. Lomer dislocations were identified near locations where two lines of strain contrast intersected in a large cluster, and a comparatively shallow cluster, largely constrained to the GaAs emitter layer, contained 60° perfect dislocations associated with localized strain contrast.
In another study, misfit dislocations at II-VI/III-V heterovalent interfaces were investigated and characterized using AC-STEM. Misfit strain at ZnTe/GaAs interfaces, which have relatively high lattice mismatch (7.38%), was relieved primarily through Lomer dislocations, while ZnTe/InP interfaces, with only 3.85% lattice mismatch, were relaxed by a mixture of 60° perfect dislocations, 30° partial dislocations, and Lomer dislocations. These results were consistent with the previous findings that misfit strain was relaxed primarily through 60° perfect dislocations that had either dissociated into partial dislocations or interacted to form Lomer dislocations as the amount of misfit strain increased.
Metasurface-Based Optoelectronic Devices for Polarization Detection and Ultrafast Optical Modulation
Optical chirality is essential for generation, manipulation and detection of circularly polarized light (CPL), thus finds many applications in quantum computing, communication, spectroscopy, biomedical diagnosis, imaging and sensing. Compared to natural chiral materials, chiral metamaterials and metasurfaces enable much stronger chirality on subwavelength scale; therefore, they are ideal for device miniaturization and system integration. However, they are usually associated with low performance due to limited fabrication tolerance and high dissipation mainly caused by plasmonic materials. Here, a bio-inspired submicron-thick chiral metamaterial structure was designed and demonstrated experimentally with high contrast (extinction ratio >35) detection of CPL with different handedness and high efficiency (>80%) of the overall device. Furthermore, integration of left- and right-handed CPL detection units with nanograting linear polarization filters enabled full-Stokes polarimetry of arbitrarily input polarization states with high accuracy and very low insertion loss, all on a submillimeter single chip. These unprecedented highly efficient and high extinction ratio devices pave the way for on-chip polarimetric measurements.
All-optical modulation is widely used for optical interconnects, communication, information processing, and ultrafast spectroscopy. Yet, there’s deficiency of ultrafast, compact and energy-efficient solutions all in one device. Here, all-optical modulation of light in the near- and mid-infrared regimes were experimentally demonstrated based on a graphene-integrated plasmonic nanoantenna array. The remarkable feature of the device design is its simultaneous near-field enhancement for pump and probe (signal) beams, owing to the localized surface plasmon resonance excitation, while preserving the ultrafast photocarrier relaxation in graphene. Hence, a distinct modulation at 1560nm with record-low pump fluence (<8μJ/cm^2) was reported with ~1ps response time. Besides, relying on broadband interaction of graphene with incident light, a first-time demonstration of graphene-based all-optical modulation in mid-infrared spectral region (6-7μm) was reported based on the above double-enhancement design concept. Relying on the tunability of metasurface design, the proposed device can be used for ultrafast optical modulation from near-infrared to terahertz regime.
A bowing model is developed for the bandgap and band offsets of the quinary alloy GaInAsSbBi and its quaternary constituents InAsSbBi and GaAsSbBi. The band anticrossing interaction due to the highly mismatched Bi atoms is incorporated into the relevant bowing terms. An algorithm is developed for the design of mid infrared GaInAsSbBi
quantum wells, with three degrees freedom to independently tune transition energy, in plane strain, and band edge offsets for desired electron and hole confinement.
The physical characteristics of the fundamental absorption edge of the relevant III-V binaries GaAs, GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed that describes the key physical characteristics of the absorption edge, including the bandgap energy, the Urbach tail, and the absorption coefficient at the bandgap.
The quantum efficiency and recombination lifetimes of bulk InAs0.911Sb0.089 grown by molecular beam epitaxy is investigated using excitation and temperature dependent steady state photoluminescence. The Shockley-Read-Hall, radiative, and Auger recombination lifetimes are determined.
The ability to spatially resolve the degree to which extended defects impact carrier diffusion lengths and lifetimes is important for determining upper limits for defect densities in semiconductor devices. We show that a new spatially and temporally resolved photoluminescence (PL) imaging technique can be used to accurately extract carrier lifetime values in the immediate vicinity of dark-line defects in CdTe/MgCdTe double heterostructures. A series of PL images captured during the decay process show that extended defects with a density of 1.4 × 105 cm-2 deplete photogenerated charge carriers from the surrounding semiconductor material on a nanosecond time scale. The technique makes it possible to elucidate the interplay between nonradiative carrier recombination and carrier diffusion and reveals that they both combine to degrade the PL intensity over a fractional area that is much larger than the physical size of the defects. Carrier lifetimes are correctly determined from numerical simulations of the decay behavior by taking these two effects into account. Our study demonstrates that it is crucial to measure and account for the influence of local defects in the measurement of carrier lifetime and diffusion, which are key transport parameters for the design and modeling of advanced solar-cell and light-emitting devices.
A CdZnTe/MgCdTe double-heterostructure (DH) consisting of a 3 μm thick Cd0.9946 Zn0.0054Te middle layer that is lattice-matched to an InSb substrate has been grown using molecular beam epitaxy. A long carrier lifetime of 3.4 × 102 ns has been demonstrated at room temperature, which is approximately three times as long as that of a CdTe/MgCdTe DH with identical layer thickness. This substantial improvement is due to the reduction in misfit dislocation density in the CdZnTe alloy. In contrast, a CdTe/MgCdTe DH with 3 μm thick CdTe layer grown on an InSb substrate exhibits a strain relaxation of ∼30%, which leads to a wider x-ray diffraction peak, a weaker integrated photoluminescence intensity, and a shorter minority carrier lifetime of 1.0 × 102 ns. These findings indicate that CdZnTe lattice-matched to InSb has great potential as applied to high-efficiency solar cells as well as virtual substrates for high-performance large-area HgCdTe focal plane arrays.
Atomic resolution transmission electron microscopy is performed to examine the strain distribution in an InAs/InAs1-xSbx superlattice grown on a (100)-GaSb substrate. The strain profiles reveal that the thickness of tensile regions in the superlattice is significantly lower than expected, with a corresponding increase in thickness of the compressive regions. Furthermore, significant grading is observed within the tensile regions of the strain profile, indicating Sb intermixing from the InAsSb growth surface. The results signify an effective reduction in the InAs layer thickness due to the anion (As-Sb) exchange process at the InAs-on-InAsSb interface. (C) 2013 AIP Publishing LLC.
This paper reports the molecular beam epitaxial growth and characterization of high-reflectivity and broad-bandwidth distributed Bragg reflectors (DBRs) made of ZnTe/GaSb quarter-wavelength (lambda/4) layers for optoelectronic applications in the midwave infrared spectral range (2-5 mu m). A series of ZnTe/GaSb DBRs has been successfully grown on GaSb (001) substrates using molecular beam epitaxy (MBE). During the MBE growth, a temperature ramp was applied to the initial growth of GaSb layers on ZnTe to protect the ZnTe underneath from damage due to thermal evaporation. Post-growth characterization using high-resolution x-ray diffraction, atomic force microscopy, and transmission electron microscopy reveals smooth surface morphology, low defect density, and coherent interfaces. Reflectance spectroscopy results show that a DBR sample of seven lambda/4 pairs has a peak reflectance as high as 99.0% centered at 2.56 mu m with a bandwidth of 517 nm.
