Carrier Thermal Escape Research Articles

Structural, Optical and Electrical Properties of Self-Assembled InAs Quantum Dots Based p–i–n Devices Grown on GaAs Substrate by Molecular Beam Epitaxy for Telecommunication Applications

This work aims to investigate the structural, electrical, and optical properties of InAs quantum dots (QDs) grown by Molecular Beam Epitaxy on GaAs substrates. As-made samples were thoroughly characterized using different techniques, including Atomic Force Microscopy (AFM), X-ray diffraction (XRD), and highresolution X-ray diffraction (HRXRD). The patterns of HRXRD revealed an excellent crystallinity of the nanostructure with a maximum diameter of 25 nm as demonstrated by AFM images. The photoluminescence (PL) spectra showed two distinct bands centered at 835 and 1210 nm, and the intensity of these wavelengths increased with decreasing temperature. A redshift accompanied by a decrease in the FWHM as a function of temperature was observed as a consequence of the thermal escape of carriers. The Ideality factor (n), built-in potential energy, and series resistance at different temperatures were also determined from current-voltage characteristics curves.

Effect of as flux rate during growth interruption on the performances of InAs/InGaAsP/InP quantum dots and their lasers grown by metal-organic chemical vapor deposition

We report the effect of As flux rate (FR) during growth interruption (GI) after quantum dot (QD) deposition on the performances of InAs/InGaAsP/InP QDs and their lasers, grown by metal–organic chemical vapor deposition. The active region consists of seven layers of self-assembled InAs QDs separated by InGaAsP lattice-matched to InP. Room-temperature photoluminescence (PL) measurements show that both the PL peak intensity and emission wavelength of the active region increase with As FR. Temperature-dependent PLs reveal that the activation energies of the QDs from the ground state to higher energy level or barrier become greater with the increase of As FR, indicating less carrier thermal escape from the QDs to their barriers. These above observations can be attributed to the promoted migration and suppressed desorption of surface In atoms, leading to enlarged QDs with deepened energy levels and thus enhanced carrier quantum confinement effect. Moreover, a 6-μm-wide and 3-mm-long ridge-waveguide QD laser with the greatest As FR shows a lowest threshold current of 477 mA under pulse mode at room temperature, corresponding to a threshold current density of 379 A/cm2 per QD layer. Meanwhile, the laser exhibits the highest characteristic temperatures T0 and T1 of 331 K and 775 K, respectively, in the temperature range between 20 °C and 60 °C.

Effects of thermal emission and re-trapping of photo-injected carriers on the optical transitions of InAs/GaAs quantum dots

Photoluminescence (PL) measurements are presented for self-assembled InAs/GaAs quantum dots (QDs). Under a very low excitation density, the 8K-PL spectrum of the QD sample shows two well-defined sub-bands. The PL sub-bands are clarified as emissions from the ground state (GS) and the first excited state (FES) of the dots. Different from that of the FES transition, a sigmoidal temperature-dependent variation is observed from the integrated PL intensity (IPLI) of the GS transition. This behavior is related to the carrier exchange between the excited states and the GS of the dots. A simple rate equation model, which takes into account the effects of the thermal escape and re-trapping of photo-injected carriers, is proposed to describe the sigmoidal variation of the IPLI. A good agreement between the model simulation and the experimental results is obtained and therefore supports the argument for the carrier exchange between the excited states and the ground states.

Temperature-dependent photoluminescence of InAs quantum dots Grown on Silicon

This paper presents detailed studies on the temperature dependent photoluminescence (PL) of self-assembled InAs/GaAs quantum dots grown on planar on-axis Si (001) substrates. Through the analyze, we have a better understanding of carrier transport processes into InAs/GaAs quantum dots system. Before 160K, the carrier transport from wetting layer to excited states (ES) and ground states (GS) is domination. As the temperature keeps rising, carrier thermal escape from GS to ES is domination.

InAs/InGaAs quantum dots confined by InAlAs barriers for enhanced room temperature light emission: Photoelectric properties and deep levels

InAs/InGaAs heterostructures with quantum dots (QDs) have been studied for quite some time for light-emitting diodes operating from the near to the far infrared range. However, the room temperature QD emission is rather low, thus it is needed to look for improved structure designs allowing efficient enhancement of luminescence. In this work, we study the modification of photo- and thermo-electrical properties of InAs/In0.15Ga0.85As QD heterostructure, when introducing wide-bandgap In0.15Al0.85As confining barriers (CBs) at one or both sides of the QD layer. The structures demonstrate interband QD photoluminescence (PL) peaked at 1.2 μm measured from room temperature down to 10 K. The introduction of CBs allows to enhance the PL intensity by more than four orders of magnitude. The reason is a strong decrease of the thermal escape of both charge carriers confined in QDs and wetting layer, leading to a highly increased radiative recombination. The changes in optical transitions, involving quantum confinement states and defect-related levels, are studied by photocurrent (PC) measurements, also showing the expectable quenching of PC in the structures with CBs. The existing deep levels of defects are determined by thermally stimulated current spectroscopy. Having the same defect spectrum in all the studied structures, an increase in the defect density was detected near CBs at the QD layer. At low temperatures, defect traps in vicinity of the QDs layer caused the Coulomb screening of conductivity channel, that is studied by kinetics measurements in view of the CB introduction. The PC decrement under the constant illumination is theoretically explained by the screening. We confidently show that, despite of a slight increase in defects and PL blueshift in the QD structure with In0.15Al0.85As CBs, they can serve as improved active elements for energy-efficient QD lasers, single-photon emitters and optical amplifiers.

Photovoltaic Response and Charge Redistribution Processes in GaAs/AlGaAs Multiple‐Quantum Wells Structure

The impact of radiative and non‐radiative processes on the photovoltaic response of GaAs/AlGaAs multiple‐quantum wells is investigated by temperature, excitation power, and chopping‐frequency‐dependent surface photovoltage (SPV) measurements. It is shown that a systematic study of SPV amplitude along with phase spectra can provide important information on thermal escape, carrier localization, and spatial redistribution of charge carriers. Characteristic features in SPV‐phase spectra related to quantum well (QW) transitions and their variation under an external perturbation are explained by the electron–electron interaction. It is found that a many‐body interaction inside the QW is responsible for the capture of charges at the heterointerfaces, which builds up an interface electric field and hinders the drift/diffusion of charges. Such an effect becomes dominant under higher excitation powers, causing a sub‐linear enhancement of photovoltage amplitude and an increase in phase delay as a function of photon flux. From the chopping‐frequency‐dependent SPV measurements, it is concluded that carriers during the drift in barrier layers are repeatedly captured by point defects and other QWs, which enhances the effective time response of the photovoltage signal. Results obtained in this study are beneficial in the engineering of quantum structures for high‐efficiency photovoltaics and non‐invasive characterization of electro‐optical processes.

Improved optical properties of InAs submonolayer quantum dots in GaAsSb/InGaAs double-well structure

In this study, multistacked InAs submonolayer (SML) quantum dots (QDs) were sandwiched in an InGaAs/GaAsSb dot-in-a-double-well (DDwell) structure to enhance the crystal quality and optical properties of QDs. The photoluminescence (PL) intensity of the InAs SML QDs with the DDwell structure was 5.5 times higher than that of conventional InAs/GaAs SML QDs because of the reduced number of nonradiative recombination centers and the enhanced carrier hole confinement. The PL results of the DDwell structure exhibit two peaks that represent the carrier overflow from SML QDs to InGaAs quantum wells (QWs) and hence the radiative recombination in InGaAs QWs because of the shallow carrier confinement of SML QDs. Among the compared samples, the DDwell structure exhibited the highest activation energy of 101.8 meV. Furthermore, the carrier thermal escape was suppressed in these InAs SML QDs. High-resolution transmission electron microscopy revealed that the microstructures of the InAs SML QDs demonstrated larger dots for the DDwell structure, thus verifying that the emission wavelength elongated in the PL measurement. These improved optical properties of the InAs SML QDs with the DDwell structure were attributable to the improved crystal quality because of the use of Sb surfactants and additional volume for carrier recombination provided by the InGaAs quantum well. The DDwell structure can thus be applied in optoelectronic devices to obtain advanced performance.

High photostability and luminescent efficiency of quantum dots: ultrathin epitaxial Al self-passivation layer with a homogeneous ligand

Semiconductor quantum dots (QDs) have aroused extensive interest in various applications because of their excellent optical and electronic properties. However, long-term photostability of the CdSe QDs has always posed application challenges. This study discusses the high-quality QDs synthesized using an uncomplicated coating method. The ultrathin epitaxial Al self-passivation layer is coated with a homogeneous ligand to maintain high luminescent efficiency and photostability of the QDs. The time-dependent intensity responses of the CdSe/CdS, CdSe/CdS/Al, and CdSe/CdS/CdS/Al QDs were verified under continuous irradiation using a single-particle one-watt blue light-emitting diode (LED). The CdSe/CdS photoluminescence (PL) intensity rapidly decreased under the blue LED illumination, whereas that of the CdSe/CdS/CdS/Al QDs remained almost unchanged for 300 min. The PL spectra of the three samples in 10–300 K were verified by comparing the temperature-dependent full-width at half-maximum of their emission energies and luminescent intensities. The mechanism of the temperature-dependent non-radiative relaxation of CdSe QDs coated with CdS layers of different thicknesses evolved from the thermal activation of carrier trapping by surface defects/traps in the CdSe QDs to the thermal escape of carriers in the core/shell QDs, assisted by multiple longitudinal–optical phonons. The luminescence mechanism was almost independent of the Al epitaxial growth layer.

Impact of localization phenomenon and temperature on the photoluminescence spectra of GaSbBi alloys and GaSbBi/GaAs quantum dots

In this work, the low temperature PL spectrum of GaSbBi ternary alloys and GaSbBi/GaAs Quantum Dots (QDs) are investigated using two different mathematical models. The first model based on the Gaussian distribution of the localized states takes into consideration the carrier dynamics associated with bulk and QD systems through the inclusion of different carrier dependent parameters such as carrier generation rate, recapture, thermal escape, radiative and non-radiative recombination times and localization depth to compute and reproduce the temperature dependent of the luminescence keys. The k dot p method realized using a 10 band Hamiltonian is used to calculate the carrier effective masses from the temperature dependent band diagram which serves as an input to reproduce the PL spectra. Two separate expressions of intrinsic carrier concentration for bulk and QD used in this model help to achieve a close agreement between the theoretically calculated results and experimental data. This work imparts a deep level understanding of the behaviour of carriers and optoelectronic parameters in III-V-Bi structures.

In situ measurement of temperature dependent picosecond resolved carrier dynamics in near infrared (NIR) sensitive device on action.

The carrier dynamics study of emerging near infrared (NIR) absorbing materials is an essential need to develop device technology toward enhanced NIR light harvesting. In this study, we have documented the design of an indigenously developed time correlated single photoncounting (TCSPC) system working in the NIR (900 nm-1700 nm) spectral region. The system is compatible to study transient photoluminescence of device samples under tunable bias voltages. The liquid nitrogen cooling and electrical heating of the sample chamber provides additional flexibility of temperature dependent study starting from -196 °C to 400 °C. As a model system to study, we have chosen a multilayer InAs/InGaAs/GaAs/AlGaAs dot in the dual well device sample as the thin film quantum dot heterostructures are of huge relevance in various NIR harvesting devices. We have investigated the detail carrier dynamics of the device sample using the transient photoluminescence upon varying temperature (80 K-300 K), varying emission energy and different bias voltages (0 V-15 V). The critical temperature (160 K) and critical bias (12 V) of achieving longest excited state lifetime has been mechanistically explained using various competing photophysical phenomena such as hole diffusion, energy relaxation, etc. The emission wavelength dependent study at below and above critical temperature further provides an insight into the dominance of carrier capture and thermal escape at the two different temperature zones. Along with the detail understanding of the carrier dynamics, the results can be helpful to get an idea of the electrical stability of the device and the operability temperature as well. The reasonable good resolution of the NIR TCSPC system and considerable good results ensure the future application of the same for other devices also.

Effects of front InGaP layer thickness on solar cell characteristics in InP/InGaP quantum dot solar cells

To investigate the impact of the front InGaP layer thickness on solar cell characteristics in InGaP-based InP quantum dot (QD) solar cells, we prepared InP/InGaP QD solar cells with a front i-InGaP layer and compared their performance with those of a device without i-InGaP layer and InGaP solar cell. Even though the QD solar cells show open-circuit voltage (Voc) reduction compared to the InGaP solar cell, the device with thin i-InGaP layer shows slightly higher Voc compared to the device with no i-InGaP layer. This result is explained by the fact that the saturation current in dark current characteristics decreases in the device with thin i-InGaP layer. This likely originates from the suppressed thermal carrier escape from QDs. Our findings reveal that the i-InGaP layer in the QDs significantly affects the photovoltaic performance. For further improvement of photovoltaic performance of such devices, optimization of the i-InGaP layer thickness is necessary.

Reduced carrier trapping in CdSe/ZnS/CdSe heterostructure quantum dots inferred from temperature dependent spectral studies

We report temperature dependent photoluminescence (TDPL) characteristics of CdSe/ZnS core-shell and coupled 0D-2D CdSe/ZnS/CdSe heterostructure quantum dots (HQDs). Anomalous, behavior of temperature dependent PL linewidth seen in core-shell HQDs is ascribed to the relaxation of carriers into interfacial strain related local energy minimum. TDPL characteristics show largest exciton-acoustic phonon and exciton-LO phonon coupling in core-shell HQDs compared to other QDs. These are caused by enhanced deformation potential interaction and enhanced Fröhlich interaction due to the interfacial strain field. Further, relatively lower strength of exciton-acoustic phonon coupling in 0D-2D HQDs is due to relaxation of interfacial strain. Temperature dependence of spectrally integrated PL intensity shows nonradiative relaxation below 150 K due to thermal activation of carriers in surface tarp states while at temperatures above, it is dominated by carrier thermal escape mediated by exciton-LO phonon interaction involving lesser number of phonons in 0D-2D HQDs.

Photoluminescence characteristics of CdSe quantum dots: role of exciton–phonon coupling and defect/trap states

In this paper, we report temperature dependent photoluminescence (PL) characteristics of CdSe colloidal QDs with average diameter ~2.8 nm. Temperature dependence of strongly confined exciton PL peak position, linewidth and intensity were investigated in 30 K to 300 K temperature range. Our studies reveal nearly four times weaker exciton-LO phonon coupling than bulk CdSe crystal. Theoretically, it should be vanishingly small due to near identical electron and hole charge distributions in strongly confined QDs. On the other hand, exciton-acoustic phonon coupling is an order of magnitude larger than its bulk counterpart. Observed finite value of exciton-LO phonon coupling and enhanced exciton-acoustic phonon coupling are due to piezoelectric strain fields. PL intensity exhibits anomalous behavior in the temperature range 100–230 K. This has been explained by thermally activated detrapping of the charge carriers trapped in the potential wells formed at the interface adjoining dislocations/stacking faults developed during the synthesis process. Above 230 K, PL is partially quenched by thermal escape of charge carriers from luminescing exciton state to higher lying nonluminescing states.

Investigation of the thermal photocarrier interaction–recombination in ZnO nanostructures fabricated by the hydrothermal method

The characteristics of the photocarrier transfer process in ZnO nanostructures fabricated on silicon substrates by the hydrothermal method are studied using temperature-dependent photoluminescence (PL) measurements. The luminescence shows distinct features arising from the dynamics of the excited carriers which occur as a consequence of the different size distributions of the ZnO nanostructures. It is found that the temperature-dependent photoluminescence phenomena can be attributed to the carrier-thermalization processes that occur in the ZnO nanostructures. The photogenerated carrier redistribution leads to a blueshift in the emission energy of the radiative transitions in the ZnO nanostructures. The random configuration of the photogenerated carriers in the ZnO nanostructures can be qualitatively understood in terms of the transition enthalpy and the transition entropy in the photocarrier transfer process. The oxygen vacancy induced emission with the trapping of electrons and spontaneous fluctuations of ZnO nanostructures has an influence on the variation of the full width at half maximum (FWHM). The carrier thermal escape and retrapping anomalies affect the temperature-dependence of the photoluminescence phenomena. The activation energy is larger for ZnO nanostructures with larger size distributions and the carrier confinement is better. Thermally-related spectroscopy allowed observation of the photon-phonon interactions associated with ZnO nanostructures with different size distributions.

Interpretation of photovoltaic performance of n-ZnO:Al/ZnS:Cr/p-GaP solar cell

We investigate Cr-doped ZnS (ZnS: Cr) as a potential deep-level intermediate band material for high efficiency solar cells. We study n-ZnO:Al/ZnS:Cr/p-GaP heterojunction cell for the first time, and this paper presents an interpretation of the performance of the solar cell in the framework of intermediate band solar cells. We conclude that the ZnS:Cr used in this work has two characteristic energy levels at 0.88eV and 2.68eV below the conduction band. This material also has a quasi-continuum of energy levels between the former level and the valence band maximum. This quasi-continuum results in thermal carrier escape that limits the open-circuit voltage to the lowest energy gap in ZnS: Cr, ≃0.8V.

Photoluminescence and time-resolved photoluminescence studies of lateral carriers transfer among InAs/GaAs quantum dots

We report on the lateral transfer and thermal escape of carriers in InAs quantum dots (QDs) grown on a GaAs substrate by solid source molecular beam epitaxy by mean of photoluminescence (PL) and time-resolved PL measurements. The temperature-dependent PL spectra are discussed in terms of the inhomogeneous size distribution of the QDs and the carrier tunneling process from small to large QDs. The dependence of the photoluminescence decay time on the emission-wavelength is attributed to lateral carriers’ transfer within QDs with an interdot carrier tunneling time of 910 ps under low excitation conditions.

InAs quantum dots grown by MOCVD in GaAs and metamorphic InGaAs matrixes

We have studied MOCVD-grown structures with InAs layers in GaAs and metamorphic InGaAs matrices. Deposition of 2 ML of InAs in a GaAs or In0.24Ga0.76As matrix results in quantum dots formation with photoluminescence (PL) peaks at 1240 and 1380 nm, respectively. In case of deposition of 2 ML of InAs in a In0.30Ga0.70As metamorphic matrix, formation of a corrugated quantum well emitting at 1450 nm has been revealed. The integrated PL intensity of the metamorphic structures is much higher than that for the GaAs based structure. For the metamorphic structures, the thermal escape of carriers from InAs quantum dots (a quantum well) results in high intensity of the PL line due to InGaAs matrix at room temperature.

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage $V_{{{\rm OC}}}$ in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), $V_{{\rm{OC}}}$ degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), $V_{{\rm{OC}}}$ does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved $V_{{\rm{OC}}}$ of 0.23 V, which is beyond the detailed balance limit of standard solar cells. $V_{{\rm{OC}}}$ dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of $V_{{{\rm OC}}}$ .

High-temperature carrier density and mobility enhancements in AlGaN/GaN HEMT using AlN spacer layer

In this paper, we experimentally studied the effect of AlN spacer layer on optical and electrical properties of AlGaN/GaN high electric mobility transistors (HEMTs) grown by metal organic chemical vapor deposition method. For AlGaN layer in HEMT structure, the Al composition of the sample was determined using x-ray diffraction and photoluminescence. Electrolyte electro-reflectance (EER) measurement not only confirmed the aluminum composition of AlGaN layer, but also determined the electric field strength on the AlGaN layer through the Franz-Keldysh oscillation phenomenon. This result indicated that the electric field on the AlGaN layer could be improved from 430 to 621kV/cm when AlN spacer layer was inserted in HEMT structure, which increased the concentration of two dimensional electron gas (2DEG) and improve the mobility. The temperature dependent Hall results show that both the mobility and the carrier concentration of 2DEG would decrease abruptly causing HEMT loss of function due to phonon scattering and carrier thermal escape when temperature increases above a specific value. Meanwhile, our study also demonstrates using AlN spacer layer could be beneficial to allow the mobility and carrier density of 2DEG sustaining at high temperature region.

Evaluating the effects of nonlinear optical gain and thermal carrier escape on the performance of InGaAs/GaAs self-assembled quantum dot lasers

The present paper reports the effects of nonlinear optical gain and thermal carrier escape on light-current characteristics in self-assembled quantum dot (QD) lasers based on a rate equation model. Comparison of the obtained simulated results with empirical findings showed that it is necessary to consider the effects of thermal carrier escape and nonlinear optical gain in order to evaluate dynamic and static characteristics of self-assembled QD lasers. Moreover, it was revealed that the describing terms of the thermal carrier escape from QDs is not enough in order to describe the effects of temperature on dynamics of carrier escape. Also, it was found that rate of (thermal) carriers stimulation depends directly on the inhomogeneous broadening (IHB). In other words, changes of IHB effects on the dynamics of thermal carrier changes. In addition, the results showed that optimized amount of homogenous broadening (HB) in which the maximum emission occurs in the central mode depended on both IHB and temperature. Comparing two conditions of linear and total optical gain illustrated that output power decreases if linear gain is replaced by total gain; also, lasing emissions broadens and number of lasing modes increases by increase in current. Investigating the effect of carrier’s relaxation lifetime on output power showed that increase of relaxation lifetime increases the threshold current and decreases the output power and slope efficiency. Furthermore, increase in relaxation lifetime caused decline in maximum output power and broadening of lasing spectrum.