Carrier Escape Research Articles

Self-Heating Effect on the Two-State Lasing Behaviors in 1.3-µm InAs–GaAs Quantum-Dot Lasers

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-µm self-assembled InAs–GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light–current (L–I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.

A mesoporous silica nanoparticle with charge-convertible pore walls for efficientintracellular protein delivery

We report a smart mesoporous silica nanoparticle (MSN) with a pore surface designed toundergo charge conversion in intracellular endosomal condition. The surface of mesoporesin the silica nanoparticles was engineered to have pH-hydrolyzable citraconicamide. Solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared(FT-IR) spectroscopy, and Brunauer–Emmett–Teller (BET) analyses confirmed thesuccessful modification of the pore surfaces. MSNs (MSN–Cit) with citraconicamide functionality on the pore surfaces exhibited a negative zeta potential (−10 mV) at pH 7.4 because of the presence of carboxylate end groups. At cellular endosomal pH (∼5.0), MSN–Cit have a positive zeta potential (16 mV) indicating the dramatic chargeconversion from negative to positive by hydrolysis of surface citraconic amide. Cytochromec (Cyt c) of positive charges could be incorporated into the pores of MSN–Cit byelectrostatic interactions. The release of Cyt c can be controlled by adjusting the pH ofthe release media. At pH 7.4, the Cyt c release was retarded, whereas, at pH 5.0,MSN–Cit facilitated the release of Cyt c. The released Cyt c maintained theenzymatic activity of native Cyt c. Hemolytic activity of MSN–Cit over red blood cells(RBCs) was more pronounced at pH 5.0 than at pH 7.0, indicating the capability ofintracellular endosomal escape of MSN carriers. Confocal laser scanning microscopy(CLSM) studies showed that MSN–Cit effectively released Cyt c in endosomalcompartments after uptake by cancer cells. The MSN developed in this work may serveas efficient intracellular carriers of many cell-impermeable therapeutic proteins.

High-Power and High-Efficiency InGaN-Based Light Emitters

In this paper, we report on the latest advancements in improving AlGaInN-based visible-light-emitting-diode (LED) efficiency in epitaxy, chip, and package designs. We investigate the fundamental origin of the typical high current ¿droop¿ of efficiency observed in such LEDs. We show that this effect is most likely not caused by incomplete carrier injection or carrier escape but that it is rather a fundamental material property of InGaN/GaN-heterostructure-based light emitters. The droop can be reduced in improved epitaxial LED active-layer designs. We show how this can be achieved by lowering InGaN volume carrier density in multiple quantum wells (MQWs) and thick InGaN layers. Improved epitaxial MQW structures are then combined with a new advanced chip concept. It is optimized for high efficiency at high current operation and arbitrary scalability and can be manufactured at low cost. This is accomplished by improving light-extraction efficiency, homogenizing the emission pattern, reducing forward voltage, and lowering thermal resistance. The improved high current efficiency can be fully exploited by mounting the chip in the highly versatile new OSLON SSL package. It features very stable package materials, a small footprint, and an electrically isolated design decoupling electrical and thermal contacts.

10.1007/s11453-008-3009-2

The results of time-resolved photoluminescence studies of heterostructures containing monolayer arrays of InAs/GaAs quantum dots are presented. A two-component time dependence of intensity of photoluminescence from the ground state of quantum dots, with characteristic times of the slow component up to hundreds of nanoseconds and those of rapid one several nanoseconds, is studied. It is shown that the slow component is determined by the transport of nonequilibrium charge carriers between the quantum dots. At low temperatures, the time of the slow component is determined by tunneling, and at high temperatures by thermal escape of nonequilibrium charge carriers. The ratio of the contributions of tunneling and thermal escape is determined by the degree of isolation of quantum dots. A theoretical model is constructed that describes the effect of the dynamics of carrier transport on the emergence and decay of the slow component of photoluminescence.

Electronic energy levels and carrier dynamics in InAs/InGaAs dots-in-a-well structure investigated by optical spectroscopy

We investigate the electronic energy levels and carrier dynamics in InAs/InxGa1−xAs dots-in-a-well (DWELL) structure by comprehensive spectroscopic characterization over a temperature range from 10 to 300 K. The integrated photoluminescence (PL) intensity is observed to increase with raising temperature up to 100 K. Through combining the PL and PL excitation (PLE) analysis, we provide direct evidence that this anomalous temperature dependence of the PL spectrum is due to the carrier trapping in InGaAs quantum well at low temperature. A rate equation model is adopted to quantitatively describe the thermal escape and capture processes of carriers in the DWELL system. The origin of thermal activation energies for quantum dot PL quenching at higher temperatures is discussed referring to the electronic energy levels revealed by PLE spectra.

Role of photovoltaic effects on characterizing emission properties of InGaN/GaN light emitting diodes

Strong photovoltaic effects on photoluminescence (PL) spectra in InGaN/GaN blue light emitting diodes were investigated. Due to severe carrier escape from quantum wells, significant photovoltaic effects occur in PL measurement in open-circuit condition, which strongly affect the PL peak position and intensity. We reveal that proper correlation between electroluminescence and PL peak positions cannot be obtained without proper consideration of the photovoltaic effects. By changing sample temperature and the PL excitation power, the generated photovoltage varies in the range of 2.0 to 2.6 V. We show that in the open-circuit condition, which is the usual case, the determination of radiative efficiency by measuring the PL intensity ratio of low-and high-temperature cannot be accurate, and the excitation intensity dependent PL cannot be solely intrinsic either. Both the absorption of incident laser and the carrier escape from the quantum well are bias-sensitive. By a simple and straightforward method, we determined that 51% of photogenerated carriers escaped in short-circuit condition.

Effect of the Wetting Layer on the Output Power of a Double Tunneling-Injection Quantum-Dot Laser

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To suppress bipolar population and hence electron–hole recombination outside quantum dots (QDs), tunneling-injection of electrons and holes into QDs from two separate quantum wells was proposed earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-injection (DTI) laser. In the Stranski–Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron–hole recombination in the WL, even in a DTI structure. In this work, the light–current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron–hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation. </para>

Effects of carrier escape and capture processes on quantum well solar cells: a theoretical investigation

A theoretical model is proposed to study the effects of carrier escape and capture processes on the photocurrent of quantum well solar cells (QWSCs). The results show that solar cells with very deep quantum wells (QWs) will suffer from extremely slow escape processes and their photocurrent can be inferior to their bulk counterparts. The results suggest that only when the escape time is at least two-order of magnitude smaller than the carrier lifetime of QWs, solar cells will benefit from QW structures. The optimal band gap energies of QW materials for achieving the maximum photocurrent are also calculated and discussed.

Photoluminescence of self-assembled InAs/GaAs quantum dots excited by ultraintensive femtosecond laser

This paper presents the ultrahigh excitation intensity-dependent photoluminescence (UEIPL) spectra of self-assembled InAs/GaAs quantum dots (QDs) excited by femtosecond laser with power intensity up to 450 kW/cm2. Upon ultraintensive excitation, many emission features from QDs and underneath wetting layer (WL) have been observed. As a result of band filling, two emission features D1 and D3 due to the ground (n=1) and second excited state (n=3) transitions, and an emission band from GaAs barrier layer state transition have been distinguished at the experimental temperature of 77K. In addition, an emission feature DD has been identified as the superposition of n=2 (first excited state) in QD and defect state transitions. Moreover, both heavy-hole (HH) and light-hole (LH) states in WL, which are usually optically inactive in emission type of experiment, have been derived from the sample subjected to ultraintensive laser excitation. The ultraintensive excitation generates ultrahigh transient carrier density in QDs and WL. As a result of very strong carrier-carrier and carrier-phonon scattering, all emission peaks shift toward lower photon energy with the increasing excitation power due to lattice heating effect. Further investigations also show that WL could effectively assist in the thermally escape of carriers in QDs; it acts as a channel to facilitate the thermal redistribution of excitons in QDs as well. The UEIPL measurement seems to be a valid alternative to photoreflectance measurements for the investigation of QD and WL system.

Device modeling for split-off band detectors

An approach to develop room temperature detectors is to use transitions between the light/heavy hole bands and the split-off hole band to produce enhanced response at high temperature. Results are presented on a theoretical model to predict the response in these split-off detectors. The model calculates the dark and illuminated currents from the photoabsorption, carrier escape, and transport, explaining the experimental response. The variation in dark current, responsivity, and D∗ with the detector parameters is presented.

Voltage-controlled motional narrowing in a semiconductor quantum dot

We demonstrate the control with a dc voltage of the environment-induced decoherence in a semiconductor quantum dot (QD) embedded in a gated field-effect device. The electrical control of the spectral diffusion dynamics governing the QD decoherence induces various effects, and in particular a narrowing of the QD emission spectrum on increasing the electric field applied to the structure. We develop a model in the framework of the pre-Gaussian noise theory that provides a quantitative interpretation of our data as a function of gate voltage. The standard phenomenology of motional narrowing described in nuclear magnetic resonance is successfully reached by hastening the carrier escape from the traps around the QD through tunneling under reverse bias voltage. Our study paves the way to a protection of zero-dimensional electronic states from outside coupling through a voltage-controlled motional narrowing effect.

Measurement and analysis of optical gain spectra in 1.6 to 1.8 μm InAs/InP (100) quantum-dot amplifiers

Small signal modal gain measurements have been performed on two-section ridge waveguide InAs/InP (100) quantum-dot amplifiers that we have fabricated with a peak gain wavelength around 1.70 μm. The amplifier structure is suitable for monolithic active-passive integration, and the wavelength region and wide gain bandwidth are of interest for integrated devices in biophotonic applications. A 65 nm blue shift of the peak wavelength in the gain spectrum has been observed with an increase in injection current density from 1,000 to 3,000 A/cm2. The quantum-dot amplifier gain spectra have been analyzed using a quantum-dot rate-equation model that considers only the carrier dynamics. The comparison between measured and simulated spectra shows that two effects in the quantum-dot material introduce this large blue shift in the gain spectrum. The first effect is the carrier concentration dependent state filling with carriers of the bound excited and ground states in the dots. The second effect is the decrease in carrier escape time from the dots to the wetting layer with decreasing dot size.

Temperature-Dependent Photoluminescence of CdSe-Core CdS/CdZnS/ZnS-Multishell Quantum Dots

The photoluminescence (PL) spectra of CdSe-core CdS/CdZnS/ZnS-multishell quantum dots (QDs) were studied to understand the radiative and nonradiative relaxation processes in the temperature range from 80 to 360 K. The mechanism of temperature-dependent nonradiative relaxation processes in the CdSe QDs with changing the shell structures was found to evolve from thermal activation of carrier trapping by surface defects/traps in CdSe core QDs to the multiple longitudinal-optical (LO) phonon-assisted thermal escape of carriers in the core/shell QDs. An increase in PL intensity with increasing temperature was clearly observed in the core/shell QDs with a thick CdS monoshell and a CdS/ZnCdS/ZnS multishell. The PL enhancement was considered to come from delocalization of charge carriers localized at the CdSe/CdS interface with the potential depth of ∼30 meV. The experimental results indicated that the improvement of PL quantum efficiency in CdSe-core CdS/CdZnS/ZnS-multishell QDs could be understood in terms of the reduction of nonradiative recombination centers at the interfaces and on the surface of the multishell, as well as the confinement of electrons and holes into the QDs by an outer ZnS shell.

The role of wetting layer states on the emission efficiency of InAs/InGaAs metamorphicquantum dot nanostructures

We report on a photoluminescence and photoreflectance study of metamorphicInAs/InGaAs quantum dot strain-engineered structures with and without additionalInAlAs barriers intended to limit the carrier escape from the embedded quantum dots.From: (1) the substantial correspondence of the activation energies for thermal quenching ofphotoluminescence and the differences between wetting layer and quantum dot transitionenergies and (2) the unique capability of photoreflectance of assessing the confinednature of the escape states, we confidently identify the wetting layer states as thefinal ones of the process of carrier thermal escape from quantum dots, whichis responsible for the photoluminescence quenching. Consistently, by studyingstructures with additional InAlAs barriers, we show that a significant reduction ofthe photoluminescence quenching can be obtained by the increase of the energyseparation between wetting layers and quantum dot states that results from theinsertion of enhanced barriers. These results provide useful indications on the lightemission quenching in metamorphic quantum dot strain-engineered structures;such indications allow us to obtain light emission at room temperature in the1.55 µm range and beyond by quantum dot nanostructures grown on GaAs substrates.

Temperature‐dependent droop of electroluminescence efficiency in blue (In,Ga)N quantum‐well diodes

AbstractTemperature‐dependent electroluminescence (EL) intensity of a blue (In,Ga)N single quantum‐well diode has been extensively investigated as a function of current between 2 nA and 10 mA at 20‐300 K. At 300 K the EL external quantum efficiency (ηex) shows a maximum at a current density of ∼1 A/cm2 and droops under higher injection. However, the current density for the ηex maximum significantly decreases with decreasing temperature, for example, to ∼5 × 10–4 A/cm2 at 20 K, with improved maximum EL efficiency due to reduced nonradiative recombination. This low‐temperature enhancement of the ηex droop at lower injection indicates that the droop phenomena is not involved with high‐density carrier effects like Auger nonradiative recombination, but rather linked with the carrier escape under high forward bias conditions due to the presence of internal piezo‐fields in (In,Ga)N quantum‐well heterostructures. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Intradot dynamics of InAs quantum dot based electroabsorbers

The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots’ ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively.

The fast recovery dynamics of a quantum dot semiconductor optical amplifier

We consider a rate equation model of a quantum dot semiconductor optical amplifier that takes into account carrier capture, escape, and Pauli blocking processes. We evaluate possible differences between phonon-assisted or Auger processes being dominant for recovery. An analytical solution which corresponds to phonon-assisted interaction is then used to accurately fit experimental recovery curves and allows an estimation of both the carrier capture and escape rates.

On resonant optical excitation and carrier escape in GaInN/GaN quantum wells

Recently, photoluminescence studies using resonant optical excitation in GaInN layers have been used to investigate the physical origin of efficiency droop in GaInN/GaN light-emitting diodes. In these studies, it has been assumed that in the case of resonant excitation, where electron-hole pairs are generated in the GaInN layers only, carrier transport effects play no role. We report that in contrast to this assumption, carrier escape from quantum wells does take place and shows strong dependence upon the duration of excitation and bias conditions. We also discuss the time scales required to reach steady-state conditions under pulsed optical excitation.

Thermal Activation of Carriers in InGaAs/InAs/GaAs Quantum Dots

InAs self-assembled quantum-dot (QD) structures with overgrowth InGaAs strain-reducing layers (SRLs) grown by using two di erent growth modes with molecular-beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) were fabricated on GaAs substrates in a solid-source MBE system and their optical properties were investigated by characterizing the temperature-dependent photoluminescence (PL). The activation energies for thermal quenching were derived from tting the temperature-dependent PL data with the assumption of three thermal quenching processes, including escape of carriers from the SRL to the GaAs barrier. The highest activation energy for strong thermal quenching was shown to be comparable to the energy di erence between the PL peak of the ground state of the QDs and that of the SRL, indicating that the escape of electron-hole pairs from QDs to the SRL is probably the dominant process for quenching in the high-temperature region.

Thermal peculiarity of AlAs-capped InAs quantum dots in a GaAs matrix

GaAs and AlAs thin capping layers as well as postgrowth rapid thermal annealing (RTA) were applied to InAs quantum dots (QDs) grown by molecular beam epitaxy to study the tunability of optical properties of QDs by photoluminescence (PL) methods. The PL of AlAs-capped QDs shows double-peak structure, as opposed to GaAs-capped QDs, which is due to the formation of two families of QDs in the AlAs-capped sample confirmed by the power dependent PL measurements. The PL peak of the GaAs-capped samples subjected to RTA showed blueshift and narrowing with an increase in RTA temperature. This is the result of thermally enhanced In–Ga intermixing. More complex changes in the PL spectrum of AlAs-capped QDs during the RTA procedure were found and explained by the different In compositions in two branches of QDs. The features observed in the temperature dependences of PL peak energy of GaAs- and AlAs-capped samples were interpreted in terms of thermal escape of carriers from smaller QDs with further redistribution between larger QDs and different InAs content in two families of QDs.