Radiative Recombination Lifetime Research Articles

Influence of Radiative and Non-Radiative Recombination Lifetimes and Feedback Strength on the States and Relative Intensity Noise of Laser Diode

A systematic treatment of the influence of the optical feedback (OFB) and the ratio of the radiative and the non-radiative recombination lifetime (τr/τnr) on the relative intensity noise (RIN) and the system dynamics of a semiconductor laser (SL) is presented. We found that the ratio τr/τnr causes a significant change in the intensity of noise, states, and route to chaos of such a system. Laser transit from a continuous wave (CW) to a periodic oscillation (PO) becomes faster in terms of the OFB strength by increasing the ratio τr/τnr. The route to chaos was identified in three distinct operating regions, namely, PO, period doubling (PD), and sub-harmonics (SH), which are dependent on the ratio τr/τnr, and injection current. In the route to chaos regime, the ratio τr/τnr triggers a slight shift in the frequency with reference to the frequency of the solitary laser. At upmost levels of the current, the highest value of the ratio τr/τnr stabilizes the laser and stimulates it to operate in CW or PO. In the strong OFB region, when the ratio τr/τnr increases, the chaotic operation changed to CW or PO operation and RIN is suppressed close to the quantum noise level.

Structural and optical properties of cubic GaN on U-grooved Si (100)

Cubic GaN epitaxy on large-area U-grooved silicon (100) dies is demonstrated by metalorganic chemical vapor deposition, and its structural and optical properties are reported. Scanning electron, atomic force, and transmission electron microscopy studies reveal that cubic GaN shows no discernible threading dislocations and a low stacking fault density of 3.27 ± 0.18 × 104 cm−1. Temperature-dependent photoluminescence studies reveal as-grown cubic GaN band edge emission internal quantum efficiency as 25.6% ± 0.9%. Selective etching of the low-temperature AlN buffer layer, SiO2 sidewalls, and hexagonal-phase GaN is demonstrated, which increases the cubic GaN band edge emission internal quantum efficiency to 31.6% ± 0.8%. This increase is attributed to the decrease in the radiative recombination lifetime via the removal of defective hexagonal-phase GaN. Overall, cubic GaN on U-grooved silicon with high structural and optical quality is reported, promising its suitability for next-generation devices.

AlGaN Microfins as Nonpolar UV Emitters Probed by Time-Resolved Cathodoluminescence

Despite the continuous technological progress and recent commercialization of UV light emitting diodes for sterilization and disinfection applications, the performance of solid-state UV emitters still lags far behind that of their InGaN-based counterparts, which emit in the visible blue–green spectrum. Both fundamental physical aspects and material quality restrictions have been discussed as origin of this striking difference. In this study, GaN/AlGaN core–shell microfins with a-plane sidewalls are proposed as a model system to demonstrate the efficiency potential of ultra-low-defect nonpolar UV emitters. Such structures are manufactured by bottom-up selective-area metalorganic vapor phase epitaxy of GaN microfin cores and further overgrowth of the AlGaN active region, employing a compositionally graded GaN/AlGaN short-period superlattice for strain management. Using this approach, mostly crack-free AlGaN quantum wells emitting around 321 nm could be realized with a threading dislocation density of as low as (6 ± 3) × 107 cm–2. The carrier dynamics within the nonpolar AlGaN structures are studied by time-resolved cathodoluminescence spectroscopy, revealing fast radiative recombination lifetimes down to 0.6 ns and distinct signatures of carrier localization at low temperatures. Delayed carrier emission from localized states within the cladding is proposed to be a cause for anomalously high effective carrier lifetimes observed at temperatures below 80 K. By analyzing the characteristic temperature dependence of radiative and nonradiative recombination processes and fitting the lifetime data with an appropriate model across the whole temperature range, a room-temperature internal quantum efficiency of (40 ± 6) % is estimated for the studied sample. The findings corroborate the interest in nonpolar AlGaN structures as highly efficient UV emitters.

Recombination rate analysis in long minority carrier lifetime mid-wave infrared InGaAs/InAsSb superlattices

Gallium is incorporated into the strain-balanced In(Ga)As/InAsSb superlattice system to achieve the same mid-wave infrared cutoff tunability as conventional Ga-free InAs/InAsSb type-II superlattices, but with an additional degree of design freedom to enable optimization of absorption and transport properties. Time-resolved photoluminescence measurements of InGaAs/InAsSb superlattice characterization- and doped device structures are reported from 77 to 300 K and compared to InAs/InAsSb. The low-injection photoluminescence decay yields the minority carrier lifetime, which is analyzed with a recombination rate model, enabling the determination of the temperature-dependent Shockley–Read–Hall, radiative, and Auger recombination lifetimes and extraction of defect energy levels and capture cross section defect concentration products. The Shockley–Read–Hall-limited lifetime of undoped InGaAs/InAsSb is marginally reduced from 2.3 to 1.4 μs due to the inclusion of Ga; however, given that Ga improves the vertical hole mobility by a factor of >10×, a diffusion-limited InGaAs/InAsSb superlattice nBn could expect a lower bound of 2.5× improvement in diffusion length with significant impact on photodetector quantum efficiency and radiation hardness. At temperatures below 120 K, the doped device structures are Shockley–Read–Hall limited at 0.5 μs, which shows promise for detector applications.

Photoluminescence efficiency of zincblende InGaN/GaN quantum wells

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite, crystal phase has the potential to improve efficiency. However, optimization of the emission efficiency of these heterostructures is still required to compete with more conventional alternatives. Photoluminescence time decays were used to assess how the quantum well width and number of quantum wells affect the recombination rates, and temperature dependent photoluminescence was used to determine the factors affecting recombination efficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increase weakly with well width, consistent with a change in the exciton binding energy. The relative efficiency at room temperature was found to increase by a factor of five when the number of wells was increased from one to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2.5 to 7.5 nm. These results indicate that thermionic emission is the most important process reducing efficiency at temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombination on well width means that increasing well thickness is an effective way of suppressing thermionic emission and thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in the wurtzite phase.

Colloidal PbS Quantum Dots for Visible-to-Near-Infrared Optical Internet of Things

The emergence of optical Internet of Things (optical-IoT) for sixth-generation (6G) networks has been envisaged to relieve the bandwidth congestion in the conventional radio frequency (RF) channel, and to support the ever-increasing number of smart devices. Among the plethora of device innovations deemed essential for fortifying the development, herein we report on the visible-to-near-infrared color-conversion luminescent-dyes based on lead sulphide quantum dots (PbS QDs), so as to achieve an eye-safe high-speed optical link. The solution-processed PbS QDs exhibited strong absorption in the visible range, radiative recombination lifetime of 6.4 $\mu$ s, as well as high photoluminescence quantum yield of up to 88%. Our proof-of-principle demonstration based on an orthogonal frequency-division multiplexing (OFDM) modulation scheme established an infrared data transmission of 0.27 Mbit/s, readily supporting an indoor optical-IoT system, and shed light on the possibility for PbS-integrated transceivers in supporting remote access control of multiple nodes. We further envisaged that our investigations could find applications in future development of solution-processable PbS-integrated luminescent fibers, concentrators, and waveguides for high-speed optical receivers.

Exciton radiative recombination time in a group III-V/II-VI core/shell quantum dot: Simultaneous effects of pressure and temperature

Excitonic properties of a compound semiconductor from group III-V and group II-VI materials are studied applying external perturbations. The simultaneous effects of pressure and temperature on exciton energies in an InP/ZnS quantum dot (core/shell) are found taking into account the ratio of radius of core/shell dot. The results bring out that the radiative lifetime enhances monotonically when the ratio of radius of core to shell quantum dot increases and it reaches a saturation value when Rc/Rs → 1 and the radiative recombination lifetime enhances upto 9 ns. The results can be applied for tailoring the excitonic optical transitions in nano-optical devices.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers.

We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.

Correlation between the internal quantum efficiency and photoluminescence lifetime of the near-band-edge emission in a ZnO single crystal grown by the hydrothermal method

The internal quantum efficiency (IQE) and photoluminescence lifetime of the near-band-edge emission in a ZnO single crystal grown by the hydrothermal method were measured by the omnidirectional photoluminescence and time-resolved photoluminescence spectroscopy, respectively. The IQE showed a monotonic increase when the cw photo pumping density exceeds W cm−2, while a constant IQE was observed below that. By using the data set of IQE and measured under pulsed excitation conditions, radiative and nonradiative recombination lifetimes were separately quantified. Since a significant increase was observed for the nonradiative recombination lifetime, the origin of the IQE increase was revealed to be dominated by the saturation of nonradiative recombination centers.

Solution-Processed Epitaxial Growth of Arbitrary Surface Nanopatterns on Hybrid Perovskite Monocrystalline Thin Films.

Semiconductor surface patterning at the nanometer scale is crucial for high-performance optical, electronic, and photovoltaic devices. To date, surface nanostructures on organic-inorganic single-crystal perovskites have been achieved mainly through destructive methods such as electron-beam lithography and focused ion beam milling. Here, we present a solution-based epitaxial growth method for creating nanopatterns on the surface of perovskite monocrystalline thin films. We show that high-quality monocrystalline arbitrary nanopatterns can form in solution with a low-cost simple setup. We also demonstrate controllable photoluminescence from nanopatterned perovskite surfaces by adjusting the nanopattern parameters. A seven-fold enhancement in photoluminescence intensity and a three-time reduction of the surface radiative recombination lifetime are observed at room temperature for nanopatterned MAPbBr3 monocrystalline thin films. Our findings are promising for the cost-effective fabrication of monocrystalline perovskite on-chip electronic and photonic circuits down to the nanometer scale with finely tunable optoelectronic properties.

Tunable Exciton Radiative Recombination Lifetime in Twisted Bilayer Molybdenum Disulfide

The ability to control exciton radiative recombination dynamics is a promising element for expanding applications of atomically thin two-dimensional (2D) materials. Here, we show that twist angle can be used to control the interlayer indirect exciton dynamics of transition-metal dichalcogenide bilayers by changing the indirect band gaps in the two layers. Exciton radiative recombination lifetimes of twisted MoS2 bilayers are monitored by fluorescence lifetime imaging microscopy (FLIM) technology. Interestingly, interlayer twists cause only a weak change of direct excitons, but greatly modify the indirect exciton recombination channel due to the repulsive steric effects. Thus, the indirect exciton radiative recombination lifetime first increases from 5.2 to 27.0 ns with twist angle (θ) changing from 0 to 30° (more than 5-fold), and then decreases to 8.3 ns with θ further increasing to 60°. Our work paves a new route for engineering exciton radiative recombination dynamics of 2D materials, which can improve the future applications of optoelectronic devices based on 2D materials.

Highly Efficient Charge Transfer between Perovskite Nanocrystals and g‐C3N4 Nanosheets

Benefiting from the bandgap engineering implemented on two different structures, heterostructures have significantly extended the applications of semiconductors. Thus, understanding the energy/charge transfer of heterostructures is very important for achieving high‐performance devices. Herein, heterostructures with potential applications in solar harvesting are constructed by CsPb(BrxI1−x)3 perovskite nanocrystals (PNCs) emitting at 640 nm and g‐C3N4 nanosheets (CN) with fluorescence covering from 450 to 550 nm. The fluorescence of CsPb(BrxI1−x)3 PNCs is substantially quenched by CN. Moreover, the photodegradation of CsPb(BrxI1−x)3 is remarkably hastened when interacting with the CN, indicating efficient charge transfer in the heterostructures, which is further verified by time‐resolved photoluminescence (PL) and temperature‐dependent PL results. The charge transfer lifetime is much shorter than the radiative recombination lifetime and defect trapping lifetime, ensuring quick charge transfer from PNCs to CN before radiative recombination and defect trapping, resulting in high charge transfer efficiency up to 98.16%. This work sheds light on heterostructures based on PNCs, which will facilitate their future application in photocatalysts, light‐emitting diodes, and photodetectors.

Area and thickness dependence of Auger recombination in nanoplatelets.

The ability to control both the thickness and the lateral dimensions of colloidal nanoplatelets offers a test-bed for area and thickness dependent properties in 2D materials. An important example is Auger recombination, which is typically the dominant process by which multiexcitons decay in nanoplatelets. Herein, we uncover fundamental properties of biexciton decay in nanoplatelets by comparing the Auger recombination lifetimes based on interacting and noninteracting formalisms with measurements based on transient absorption spectroscopy. Specifically, we report that electron-hole correlations in the initial biexcitonic state must be included in order to obtain Auger recombination lifetimes in agreement with experimental measurements and that Auger recombination lifetimes depend nearly linearly on the lateral area and somewhat more strongly on the thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi-2D materials.

AlGaN Deep-Ultraviolet Light-Emitting Diodes with Localized Surface Plasmon Resonance by a High-Density Array of 40 nm Al Nanoparticles.

We present a remarkable improvement in the efficiency of AlGaN deep-ultraviolet light-emitting diodes (LEDs) enabled by the coupling of localized surface plasmon resonance (LSPR) mediated by a high-density array of Al nanoparticles (NPs). The Al NPs with an average diameter of ∼40 nm were uniformly distributed near the Al0.43Ga0.57N/Al0.50Ga0.50N multiple quantum well active region for coupling 285 nm emission by block copolymer lithography. The internal quantum efficiency is enhanced by 57.7% because of the decreased radiative recombination lifetime by the LSPR. As a consequence, the AlGaN LEDs with an array of Al NPs show 33.3% enhanced electroluminescence with comparable electrical properties to those of reference LEDs without Al NPs.

Annihilation mechanism of excitons in a MoS2 monolayer through direct Förster-type energy transfer and multistep diffusion

Atomically thin MoS2 layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of MoS2 monolayer are extremely important in determining the strength of light matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in MoS2 monolayer using the transient absorption spectroscopy. We rigorously demonstrate that the F\"orster-type resonance energy transfer is the main annihilation mechanism of A- and B-excitons while multistep diffusion process is responsible for C-exciton annihilation, which is supported by critical scientific evidence.

Multiplexed Single-Photon Source Based on Multiple Quantum Dots Embedded within a Single Nanowire.

Photonics-based quantum information technologies require efficient, high emission rate sources of single photons. Position-controlled quantum dots embedded within a broadband nanowire waveguide provide a fully scalable route to fabricating highly efficient single-photon sources. However, emission rates for single-photon devices are limited by radiative recombination lifetimes. Here, we demonstrate a multiplexed single-photon source based on a multidot nanowire. Using epitaxially grown nanowires, we incorporate multiple energy-tuned dots, each optimally positioned within the nanowire waveguide, providing single photons with high efficiency. This linear scaling of the single-photon emission rate with number of emitters is demonstrated using a five-dot nanowire with an average multiphoton emission probability of <4% when excited at saturation. This represents the first ever demonstration of multiple single-photon emitters deterministically incorporated in a single photonic device and is a major step toward achieving GHz single-photon emission rates from a scalable multi-quantum-dot system.

Direct-bandgap emission from hexagonal Ge and SiGe alloys.

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal1 of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades2-6. Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III-V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.

Critical Role of Organic Spacers for Bright 2D Layered Perovskites Light-Emitting Diodes.

Light‐emitting diodes (LEDs) made with quasi‐2D/3D and layered perovskites have undergone an unprecedented surge as their external quantum efficiency (EQE) is rapidly approaching other lighting technologies. Manipulating the charge recombination pathway in semiconductors is highly desirable for improving the device performance. This study reports high‐performance layered perovskites LEDs with benzyl ring as spacer where radiative recombination lifetime is longer, compared with much shorter alkyl chain spacer yields. Based on detailed optical and X‐ray absorption spectroscopy measurements, direct signature of charges localization is observed near the band edge in exchange with the shallow traps in benzyl organics containing layered perovskites. As a result, it boosts the photoluminescence intensity by 7.4 times compared to that made with the alkyl organics. As a demonstration, a bright LED made with the benzyl organics with current efficiency of 23.46 ± 1.52 cd A−1 is shown when the device emits at a high brightness of 6.6 ± 0.93 × 104 cd m−2. The average EQE is 9.2% ± 1.43%, two orders of magnitude higher than the device made with alkyl organics. The study suggests that the choices of organic spacers provide a path toward the manipulation of charge recombination, essential for efficient optoelectronic device fabrications.

Steady-state recombination lifetimes in polar InGaN/GaN quantum wells by time-resolved photoluminescence

Both the steady-state radiative and nonradiative recombination lifetimes of excess carriers in polar InGaN/GaN quantum wells are extracted by a novel technique. Instead of the conventional ultrashort-pulsed laser, a modulated CW laser is utilized as the excitation source in time-resolved photoluminescence (PL) experiments to obtain the steady-state lifetimes. By analyzing the dependence of the PL lifetime and the integrated PL intensity on the injected power density simultaneously, both the radiative and Shockley–Read–Hall (SRH) recombination lifetimes can be determined accurately. The proposed method is applied to study the radiative and nonradiative processes in polar InGaN/GaN quantum wells with various well widths. The results suggest that both the SRH lifetime of holes and the radiative recombination lifetime increase drastically with increasing well width, while the SRH lifetime of electrons exhibits a weaker dependence on well width.