Hole Escape Research Articles

Investigation of Trion Emission in CdSe/CdSeS Core/Crown Nanoplatelets.

Quasi-two-dimensional CdSe nanoplatelets (NPLs) exhibit promising potential for optoelectronic device applications due to their unique optical properties, particularly trion emission. However, the origin of the trion emission in CdSe NPLs remains unclear. In this study, the steady-state optical properties of CdSe NPLs with different CdSeS crown widths have been investigated. At low temperature, the trion emission intensity decreases with the increase of the CdSeS crown width. Using n-butylamine to remove cadmium oleate from NPLs, it is confirmed that holes in CdSe NPLs are captured by cadmium vacancies, leading to a charge imbalance and trion emission. At room temperature, thermal energy (26 meV) facilitates the escape of holes, eliminating the trion emission and shortening the fluorescence lifetime with the increase in CdSeS crown width. This work clarifies the origin of trion emission in CdSe NPLs and offers insights into the design of optoelectronic devices based on trion emission.

Optical and Electronic Properties of Symmetric InAs/(In,Al,Ga)As/InP Quantum Dots Formed by Ripening in Molecular Beam Epitaxy: A Potential System for Broad-Range Single-Photon Telecom Emitters

We present a detailed experimental optical study supported by theoretical modeling of $\mathrm{In}\mathrm{As}$ quantum dots (QDs) embedded in an $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier lattice matched to $\mathrm{In}\mathrm{P}$(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4$--$2.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$, essential for many near-infrared photonic applications. We find that, in contrast to the $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\ifmmode\pm\else\textpm\fi{}0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier to be a primary PL quenching mechanism in these QDs.

On the Current Dependence of the Injection Efficiency and the Relative Contribution of the Escape Rate and Internal Optical Loss to Saturation of the Power–Current Characteristics of High-Power Pulsed Lasers (λ = 1.06 μm)

The results of numerical simulation of the current dependence of the efficiency of injection into the active area of a laser based on separate-confinement double heterostructures are reported. The feature of carrier transport through isotype N–n heterojunctions at the interface between the waveguide and active areas is demonstrated. Using the classic dependences of the Drude–Lorentz theory, the electron (σe) and hole (σp) scattering cross sections for a GaAs waveguide are estimated. Using the obtained values of σe = 1.05 × 10–18 cm2 and σp = 1.55 × 10–19 cm2 and the current dependences of the injection efficiency, the primary cause for confinement of the pulse power of the semiconductor lasers is determined. It is established that the internal optical loss is a minor fraction of the loss and the decisive contribution to saturation of the power–current (P–I) characteristics is made by the escape of holes to the waveguide.

Large barrier InAs quantum dots with efficient room temperature photon emission at telecom wavelengths

The effect that barrier material has on the temperature dependence of photoluminescence of InAs/AlGaInAs/InP quantum dots in the telecom C-band (∼1550 nm) is reported. Increasing Al content in the barrier material, (AlxGa1-x)0.53In0.47As, creates less temperature sensitivity and enhances the high temperature relative quantum yield. Three samples, with x = 0.51, 0.75, and 1, have room temperature relative quantum yield values of 2.0%, 3.7%, and 41.0%, respectively, when compared to low temperature values. Determination of thermal activation energies shows that the loss of relative quantum yield is due to thermal escape of holes from the quantum dots to the barrier. More aluminum-rich barriers require higher temperatures to depopulate the ground state of the quantum dots, which leads to better high temperature emission. These results can guide future designs of telecom C-band quantum dot devices.

In-Situ Annealing and Hydrogen Irradiation of Defect-Enhanced Germanium Quantum Dot Light Sources on Silicon

While light-emitting nanostructures composed of group-IV materials fulfil the mandatory compatibility with CMOS-fabrication methods, factors such as the structural stability of the nanostructures upon thermal annealing, and the ensuing photoluminescence (PL) emission properties, are of key relevance. In addition, the possibility of improving the PL efficiency by suitable post-growth treatments, such as hydrogen irradiation, is important too. We address these issues for self-assembled Ge quantum dots (QDs) that are co-implanted with Ge ions during their epitaxial growth. The presence of defects introduced by the impinging Ge ions results in pronounced PL-emission at telecom wavelengths up to room temperature (RT) and above. This approach allows us to overcome the severe limitations of light generation in the indirect-band-gap group-IV materials. By performing in-situ annealing, we demonstrate a high PL-stability of the defect-enhanced QD (DEQD) system against thermal treatment up to 600 °C for at least 2 h, even though the Ge QDs are structurally affected by Si/Ge intermixing via bulk diffusion. The latter, in turn, allows for emission tuning of the DEQDs over the entire telecom wavelength range from 1.3 µm to 1.55 µm. Two quenching mechanisms for light-emission are discussed; first, luminescence quenching at high PL recording temperatures, associated with the thermal escape of holes to the surrounding wetting layer; and second, annealing-induced PL-quenching at annealing temperatures >650 °C, which is associated with a migration of the defect complex out of the QD. We show that low-energy ex-situ proton irradiation into the Si matrix further improves the light emission properties of the DEQDs, whereas proton irradiation-related optically active G-centers do not affect the room temperature luminescence properties of DEQDs.

Mapping hole hopping escape routes in proteins

A recently proposed oxidative damage protection mechanism in proteins relies on hole hopping escape routes formed by redox-active amino acids. We present a computational tool to identify the dominant charge hopping pathways through these residues based on the mean residence times of the transferring charge along these hopping pathways. The residence times are estimated by combining a kinetic model with well-known rate expressions for the charge-transfer steps in the pathways. We identify the most rapid hole hopping escape routes in cytochrome P450 monooxygenase, cytochrome c peroxidase, and benzylsuccinate synthase (BSS). This theoretical analysis supports the existence of hole hopping chains as a mechanism capable of providing hole escape from protein catalytic sites on biologically relevant timescales. Furthermore, we find that pathways involving the [4Fe4S] cluster as the terminal hole acceptor in BSS are accessible on the millisecond timescale, suggesting a potential protective role of redox-active cofactors for preventing protein oxidative damage.

Effect of Thermal Annealing on Absorption and Hole Escape Processes in Type II GaSb/GaAs Quantum Dots: Implications for Solar Cell Design

We present a theoretical study of the rapid thermal annealing process and strain on the electronic, optical, and charge dynamical properties of type II GaSb/GaAs quantum dots (QDs). Our theoretical results show a blueshift and the enhancement of interband absorption spectrum because of inter-diffusion processes and the annealed samples. We have identified that increased annealing temperature makes the hole escape times from such QD shorter while the effect of strain on the hole confinement potential becomes weaker. At the same time, in both structures, unstrained and strained QDs, the intra-band absorption in the valence band (IVBA) is redshifted, and its magnitude is decreased after annealing when compared with the as-grown sample. To explain those effects we discuss how the electron–hole wave function overlap varies with thermal treatment and strain and discussed those effects on radiative and thermal processes. Our results explain and are in agreement with the recent experimental observations on similar structures. We have identified that holes could thermally escape out of the type II GaSb/GaAs QD before they are recombined and will contribute to the enhancement in absorption of QD solar cells. The emission and tunneling times of holes were estimated to be $ 10^{-12}$ to $10^{-14}$ s about one order of magnitude faster than the escape times because of radiative recombination processes, $ 10^{-11}$ to $10^{-13}$ s.

InP-Based Waveguide-Integrated Photodiodes With InGaAs/GaAsSb Type-II Quantum Wells and 10-GHz Bandwidth at 2 $ \mathbf {\mu }$m Wavelength

Waveguide-integrated photodiodes with InGaAs/GaAsSb type-II quantum well absorption regions designed to absorb light at 2 $ \mu$ m are presented. A novel dual-integrated waveguide-depletion layer was used to maximize quantum efficiency in photodiodes designed with thin absorbers for high-speed optical response. Low dark currents (1 nA at $-$ 1 V) and an internal responsivity of 0.84 A/W along with a bandwidth above 10 GHz and an open eye diagram at 10 Gb/s have been demonstrated at 2 $ \mu$ m. The high-speed carrier dynamics within InGaAs/GaAsSb type-II quantum wells are explored for the first time and suggest that the transit time of the photodiode is limited by light hole escape times in the quantum wells.

Hole capture and emission dynamics of type-II GaSb/GaAs quantum ring solar cells

The capture cross-section, intersubband optical cross-section and non-radiative emission rates related to localized hole states are obtained for p-i-n solar cells containing GaSb/GaAs quantum rings embedded within the i-region of the device. The technique developed uses the intraband photoemission current to probe the charge state of the nanostructures during two-color excitation. Analysis of the excitation power dependence revealed a non-radiative hole capture lifetime of 12 ns under low excitation conditions, with high injection leading to the saturation of the hole occupancy within the quantum-rings. The decay characteristics of the optical hole emission current has also been exploited to determine the spectral and temperature dependence of the radiative and non-radiative hole escape mechanisms from the quantum-rings.

Temperature-dependent radiative and non-radiative dynamics of photo-excited carriers in extremely high-density and small InGaN nanodisks fabricated by neutral-beam etching using bio-nano-templates

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process.

Recoiling supermassive black hole escape velocities from dark matter haloes

We simulate recoiling black hole trajectories from $z=20$ to $z=0$ in dark matter halos, quantifying how parameter choices affect escape velocities. These choices include the strength of dynamical friction, the presence of stars and gas, the accelerating expansion of the universe (Hubble acceleration), host halo accretion and motion, and seed black hole mass. $\Lambda$CDM halo accretion increases escape velocities by up to 0.6 dex and significantly shortens return timescales compared to non-accreting cases. Other parameters change orbit damping rates but have subdominant effects on escape velocities; dynamical friction is weak at halo escape velocities, even for extreme parameter values. We present formulae for black hole escape velocities as a function of host halo mass and redshift. Finally, we discuss how these findings affect black hole mass assembly as well as minimum stellar and halo masses necessary to retain supermassive black holes.

Photoinduced mid-infrared intraband light absorption and photoconductivity in Ge/Si quantum dots

Photoinduced absorption and photoconductivity are studied in undoped and low-doped Ge/Si quantum dot structures in mid-infrared spectral range under interband optical excitation. Steady-state absorption and photoconductivity spectra demonstrate distinctive features at photon energy of approximately 300meV related to hole escape from the bound state of the dot to continuum in Si matrix. Dynamics of photoinduced absorption relaxation is studied under conditions of pulsed laser excitation of nonequilibrium electron–hole pairs. The relaxation of the hole concentration inside the quantum dot is found to be a two-stage process related to spatially direct and indirect recombination. Redistribution of the intensities of these two processes with temperature change from 77K to 300K is observed.

Photoinduced absorption and photoconductivity of Ge/Si quantum dots in mid-infrared range under interband excitation

Photoinduced absorption and lateral photoconductivity of Ge/Si quantum dots with different doping levels are studied under interband optical excitation. The obtained spectra of absorption and photoconductivity are in good agreement. Observed photoconductivity is attributed to hole escape from ground and excited states while interlevel transitions do not impact the photoconductivity signal. Temperature dependence of photoinduced photoconductivity was measured. The decrease of photoresponse with the temperature increase is attributed to the hole recapture into the quantum dots.

Carrier extraction from GaSb quantum rings in GaAs solar cells using direct laser excitation

The authors report on the analysis of the hole escape mechanisms from type-II GaSb quantum rings (QRs) embedded within the active region of a GaAs single junction solar cell. When the solar cell is excited by using a 1064 nm infrared laser with excitation energy lower than the bandgap of the GaAs matrix, photogenerated electron-hole pairs are created directly within the GaSb QRs. The QR photocurrent exhibits a linear dependence on the excitation intensity over several decades. The thermal activation energy was found to be weakly dependent on the incident light level and increased by only a few meV over several orders of excitation intensity. The magnitude of the relative absorption in the author's QRs when directly probed by using a 1064 nm laser with an incident power density of approximate to 2.6 W cm(-2) is found to be approximate to 1.4 x 10(-4) per layer. The thermal escape rate of the holes was calculated and found to be approximate to 10(11) to 10(12) s(-1), which is much faster than the radiative recombination rate 10(9) s(-1). This behaviour is promising for concentrator solar cell development and has the potential to increase solar cell efficiency under a strong solar concentration.

Exciton extraction in nanocrystal solar cells

The efficiency of exciton extraction from nanocrystals is defined as the ratio of the number of extracted excitons to the number of absorbed photons. This efficiency is studied here based on a phenomenological model parameterized by recent experimental results in CdSe nanocrystals. Several possibilities of interplay among all possible electronic transition rates (Auger recombination, impact ionization, radiative recombination and electron–hole escape) are explored showing the possibility of obtaining efficiencies above 100% when multiple exciton generation processes are taken into account.

Experimental investigation and numerical modelling of photocurrent oscillations in lattice matched Ga1−xIn x N y As1−y/GaAs quantum well p-i-n photodiodes

Photocurrent oscillations, observed at low temperatures in lattice-matched Ga1−x In x N y As1−y /GaAs multiple quantum well (MQW) p-i-n samples, are investigated as a function of applied bias and excitation wavelength and are modelled with the aid of semiconductor simulation software. The oscillations appear only at low temperatures and have the highest amplitude when the optical excitation energy is in resonance with the GaInNAs bandgap. They are explained in terms of electron accumulation and the formation of high-field domains in the GaInNAs QWs as a result of the disparity between the photoexcited electron and hole escape rates from the QWs. The application of the external bias results in the motion of the high-field domain towards the anode where the excess charge dissipates from the well adjacent to anode via tunnelling.

Surface photovoltage properties of self-assembled CdSe quantum dots arrays fabricated by slow evaporation approach

Densely packed and highly ordered CdSe quantum dots (QDs) arrays were prepared through a slow evaporation route; to study the photo-generated charges property of ordered CdSe QDs arrays, illumination induced charge separation in the CdSe QDs arrays were investigated through surface photovoltage (SPV) technology based on lock in amplifier. The significantly enhanced SPV signal of CdSe QDs arrays after post-annealing treatment could be attributed to the greatly increased escape of holes from CdSe QDs surface, as well as the decreased the inter nanocrystal distance. Moreover, much faster excess holes dominates in the frequency range of 30 Hz and 200 Hz, which results in the positive charging of the CdSe QDs arrays near the surface. Our results give a guide for its practical photoelectric applications based on nanocrystals.

Carrier dynamics and photoluminescence quenching mechanism of strained InGaSb/AlGaSb quantum wells

GaSb based quantum wells (QWs) show promising optical properties in near-infrared spectral range. In this paper, we present photoluminescence (PL) spectroscopies of InxGa1−xSb/AlyGa1−ySb QWs and discuss the possible thermal quenching and non-radiative carrier recombination mechanisms of the QW structures. The In and Al concentrations as well as the QW thicknesses were precisely determined with the X-ray diffraction measurements. Temperature dependent time-integrated and time-resolved PL spectroscopies resulted in the thermal activation energies of ∼45 meV, and the overall self-consistent calculation of the band parameters based on the measured physical values confirmed that the activation energies are due to the hole escape from the QW to the barriers. The relation of the present single carrier escape mechanism with the other escape mechanisms reported with other material systems was discussed based on the estimated band offset. The relation of the present thermal hole escape to the Auger recombination was also discussed.

Analysis of the modified optical properties and band structure of GaAs1−xSbx-capped InAs/GaAs quantum dots

The origin of the modified optical properties of InAs/GaAs quantum dots (QD) capped with a thin GaAs1−xSbx layer is analyzed in terms of the band structure. To do so, the size, shape, and composition of the QDs and capping layer are determined through cross-sectional scanning tunnelling microscopy and used as input parameters in an 8 × 8 k·p model. As the Sb content is increased, there are two competing effects determining carrier confinement and the oscillator strength: the increased QD height and reduced strain on one side and the reduced QD-capping layer valence band offset on the other. Nevertheless, the observed evolution of the photoluminescence (PL) intensity with Sb cannot be explained in terms of the oscillator strength between ground states, which decreases dramatically for Sb > 16%, where the band alignment becomes type II with the hole wavefunction localized outside the QD in the capping layer. Contrary to this behaviour, the PL intensity in the type II QDs is similar (at 15 K) or even larger (at room temperature) than in the type I Sb-free reference QDs. This indicates that the PL efficiency is dominated by carrier dynamics, which is altered by the presence of the GaAsSb capping layer. In particular, the presence of Sb leads to an enhanced PL thermal stability. From the comparison between the activation energies for thermal quenching of the PL and the modelled band structure, the main carrier escape mechanisms are suggested. In standard GaAs-capped QDs, escape of both electrons and holes to the GaAs barrier is the main PL quenching mechanism. For small-moderate Sb (<16%) for which the type I band alignment is kept, electrons escape to the GaAs barrier and holes escape to the GaAsSb capping layer, where redistribution and retraping processes can take place. For Sb contents above 16% (type-II region), holes remain in the GaAsSb layer and the escape of electrons from the QD to the GaAs barrier is most likely the dominant PL quenching mechanism. This means that electrons and holes behave dynamically as uncorrelated pairs in both the type-I and type-II structures.

Near Infrared InAs/GaAsSb Quantum Dot Light Emitting Diodes

A series of light-emitting diodes (LEDs) with active layers based on InAs quantum dots (QDs) covered by GaAsSb capping layers is presented. Varying the Sb content in the capping layer from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{2}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}28\%$</tex></formula> , room temperature electroluminescence (EL) from 1.15 to 1.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> is obtained. The external efficiency of the devices, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\eta_{\rm ext}$</tex></formula> , increases as the Sb is increased up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> and then decreases for higher Sb contents, consistently with the reported increase of QD height with the Sb content up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> and the band alignment transition from type I to type II above <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> Sb. An analysis of the EL and photocurrent spectra shows that the emission from type I LEDs originates from the recombination between electrons and holes confined in the QDs. On the other hand, the EL from the type II devices is the combination of two different processes. First, recombination between electrons confined in the QDs and holes at the capping layer. Second, a type I-like recombination of electrons from the QDs and holes residing in extended levels of the quantum well composed by the capping layer and the QDs. The mechanisms responsible for the thermal quenching of the EL are also studied. Escape of holes from the QD to the capping layer is identified as the dominant mechanism for the type I devices, whereas in type II structures it is the escape of electrons from QD excited levels to the barrier which dominates.