Non-exponential Photoluminescence Decay Research Articles

Impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN quantum wells

We report on a combined theoretical and experimental study of the impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN multi-quantum well systems. The presence of carrier localization effects in this system was demonstrated by experimental observations, such as the “S-shape” temperature dependence of the photoluminescence (PL) peak energy, and non-exponential PL decay curves that varied across the PL spectra at 10 K. A three-dimensional modified continuum model, coupled with a self-consistent Hartree scheme, was employed to gain insight into the electronic and optical properties of the experimentally studied c-plane GaN/AlGaN quantum wells. This model confirmed the existence of strong hole localization arising from the combined effects of the built-in polarization field along the growth direction and the alloy fluctuations at the quantum well/barrier interface. However, for electrons these localization effects are less pronounced in comparison to the holes. Furthermore, our calculations show that the attractive Coulomb interaction between electron and hole results in exciton localization. This behavior is in contrast to the picture of independently localized electrons and holes, often used to explain the radiative recombination process in c-plane InGaN/GaN quantum well systems.

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.

Time-resolved spectroscopy of the ensembled photoluminescence of nitrogen- and boron/nitrogen-doped carbon dots.

Carbon dots (CDs) have potential applications in various fields such as energy, catalysis, and bioimaging due to their strong and tuneable photoluminescence (PL), low toxicity, and robust chemical inertness. Although several PL mechanisms have been proposed, the origin of PL in CDs is still in debate because of the ensembled nature of the heterogeneous luminophores present in the CDs. To unravel the origin of PL in CDs, we performed time-resolved spectroscopy on two types of CDs: nitrogen-doped (N-CD) and boron-nitrogen co-doped (BN-CD). The PL decays were fitted by stretched exponential functions to estimate the distribution of the decay kinetics in the CDs, which have different PL lifetime distributions. Both CDs displayed main, blue emission decaying in 15 ns, which originates from the dominant molecular state. The analysis of the non-exponential PL decay using stretched exponential fits revealed that the functional surface luminophores are of less variety but of more environmental heterogeneity and have much lower populations in BN-CD than in N-CD.

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Effect on SiGe Light Emission

The development of a light emitter compatible with Si based complementary metal-oxide- semiconductor (CMOS) circuit technology and fast optical interconnects is important for the new generations of microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures (NSs) with light emission in the important optical communication wavelength range of 1.3 – 1.55 μm are compatible with conventional CMOS processes. However, the predicted and experimentally confirmed long carrier radiative lifetimes in Si and Si/Si1-xGex NSs impede the demonstration of efficient light-emitting devices and lasers. Thus, engineering of Si/Si1-xGex heterostructures with controlled composition and interface abruptness is critical in producing the desired fast and efficient photoluminescence (PL) peaked at around 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one dimension (corresponding to the case of quantum wells), two dimensions (quantum wires), and three dimensions (quantum dots). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic decay time as much as 1000 times shorter than that observed in conventional Si/SiGe NSs. The experimentally observed non-exponential PL decay in Si/SiGe NSs is explained as being due to variations of the distances separating electrons and holes at the Si/SiGe heterointerface. The results demonstrate that an abrupt Si/SiGe heterointerface reduces the carrier radiative recombination lifetime and increases the PL quantum efficiency making these SiGe NSs promising candidates for applications in CMOS compatible light-emitting devices.

Si/SiGe Interfaces in Three-, Two-, and One-Dimensional Nanostructures and Their Influence on SiGe Light Emission

The development of a light emitter compatible with Si based complementary metal-oxide- semiconductor (CMOS) circuit technology and fast optical interconnects is important for the new generations of microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures (NSs) with light emission in the important optical communication wavelength of 1.3 – 1.55 μm are compatible with conventional CMOS processes. However, the predicted and experimentally confirmed long carrier radiative lifetimes in Si and Si/Si1-xGex NSs impede the demonstration of efficient light-emitting devices and lasers. Thus, engineering of Si/Si1-xGex heterostructures with controlled composition and interface abruptness is critical in producing the desired fast and efficient photoluminescence (PL) peaked around 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one dimension (corresponding to the case of quantum wells), two dimensions (quantum wires), and three dimensions (quantum dots). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic decay time as much as 1000 times shorter than that observed in conventional Si/SiGe NSs. The experimentally observed non-exponential PL decay in Si/SiGe NSs is explained as being due to variations of the distances separating electrons and holes at the Si/SiGe heterointerface. The results demonstrate that an abrupt Si/SiGe heterointerface reduces the carrier radiative recombination lifetime and increases the PL quantum efficiency making these SiGe NSs promising candidates for applications in CMOS compatible light-emitting devices.

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Impact on SiGe Light Emission

Fast optical interconnects together with an associated light emitter that are both compatible with conventional Si-based complementary metal-oxide- semiconductor (CMOS) integrated circuit technology is an unavoidable requirement for the next-generation microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures, which can emit light at wavelengths within the important optical communication wavelength range of 1.3 – 1.55 μm, are already compatible with standard CMOS practices. However, the expected long carrier radiative lifetimes observed to date in Si and Si/Si1-xGex nanostructures have prevented the attainment of efficient light-emitting devices including the desired lasers. Thus, the engineering of Si/Si1-xGex heterostructures having a controlled composition and sharp interfaces is crucial for producing the requisite fast and efficient photoluminescence (PL) at energies in the range 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe nanostructures and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in three dimensions (corresponding to the case of quantum dots), two dimensions (corresponding to quantum wires), and one dimension (corresponding to quantum wells). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for nanostructure confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex nanostructures incorporated into a Si0.6Ge0.4 wavy superlattice structure display PL of high intensity while exhibiting a characteristic decay time that is up to 1000 times shorter than that found in conventional Si/SiGe nanostructures. The non-exponential PL decay found experimentally in Si/SiGe nanostructures can be interpreted as resulting from variations in the separation distance between electrons and holes at the Si/SiGe heterointerface. The results demonstrate that a sharp Si/SiGe heterointerface acts to reduce the carrier radiative recombination lifetime and increase the PL quantum

A luminescence spectroscopy study of SrI2:Nd3+ single crystals

The paper presents the results of a study on the luminescence of SrI2:Nd3+ single crystals grown by the vertical Bridgman method. The photoluminescence (PL) spectra of SrI2:Nd crystals show characteristic lines corresponding to transitions in trivalent Nd3+ ions, the most intense line at ca. 1070nm is due to the radiative F3/24→I11/24 transitions. Efficient PL excitation in a crystal transparency band occurs due to optical 4f−4f transitions from the ground 4I9/2 state, or the charge transfer transitions I−→Nd3+. The paper discussed two new PL emission bands at 2.8 and 3.8eV, associated with the lattice defects formed in SrI2 crystal at introducing Nd3+ impurity ions; two intense narrow PL excitation bands at 5.52 and 5.31eV, originating from free and defect-bound excitons, respectively; an efficient channel of exciton energy transfer between the host lattice and defects. We calculated the H(k) functions of distribution of the elementary relaxations over the reaction rate constants and explained on this basis the nonexponential PL decay kinetics in SrI2:Nd3+ crystal.

Type-II behavior in wurtzite InP/InAs/InP core-multishell nanowires

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires (CMNs) having a wurtzite crystal structure by using photoluminescence (PL) and PL excitation (PLE) spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the “model-solid theory” of Van de Walle [Phys. Rev. B 39, 1871 (1989)] supports type-II band lineup if the InAs layer in the wurtzite CMNs is assumed to sustain compressive strain in all directions.

Dynamical study of the radiative recombination processes in GaN/AlGaN QWs

The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation-doped GaN/Al0.07Ga0.93N multiple-quantum-well (MQW) structures were studied by means of photoluminescence (PL) and time-resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320–420 ps at 2 K for all doping levels. For the undoped and low doped samples (3 × 1018 cm−3), which have less free electrons in the QWs, a non-exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm−3 to 1020 cm−3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. The internal polarization-induced fields of the medium and highly-doped samples are partly screened by the electrons originating from the doping in the barriers. The emission peaks in time delayed PL spectra of these samples exhibit almost no shift as time evolves. Only the PL peak of the undoped and low-doped samples shows a redshift with time delay, related to the photogenerated carriers [1]. The decay time for the undoped sample shows non-exponential behaviour typical for localized excitons in III-N QWs. The same behaviour of decay time as a function of emission energy has been reported for InGaN QWs [2].

Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures: influenceof Al composition and Si doping

A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiplequantum well (MQW) nanostructures has been reported. We have investigated the effect ofSi doping and Al content on PL spectra and PL decay time of these structures. Thetemperature dependence of radiative as well as non-radiative lifetimes have beenevaluated between 2 K and room temperature for different Si doping. We found thatradiative recombination at higher temperatures even up to RT is stronger in thedoped sample, compared to the undoped one. Hole localization in GaN/AlGaNMQWs with different compositions of Al is demonstrated via PL transient decaytimes and LO phonon coupling. It is found that there is an increasing of thedecay time at the PL peak emission with increasing Al composition. For theundoped sample, a non-exponential PL decay behaviour at 2 K is attributed tolocalized exciton recombination. A slight upshift in QWs PL peak with increasingAl composition is observed, which is counteracted by the expected rise of theinternal QW electric field with increasing Al. The localization energies have beenevaluated by studying the variation of the QW emission versus temperature and wefound out that the localization energy increases with increasing Al composition.

Intrinsic photophysics of semiconductor nanocrystals in dielectric media: Formation of surface states

The photoluminescence of CdSe/ZnS nanocrystals in toluene at room temperature shows inhomogeneous emission and excitation spectra. This is accompanied by a non-exponential photoluminescence decay on time scales between 1 and 80 ns. The inhomogeneous behaviour is due to a remaining size distribution, which can be partly photoselected by a combination of excitation and emission wavelengths, which reveals the contribution of at least 4 different electronic states. The identification of these states became possible by the application of a decay time distribution analysis of the photoluminescence decay. In addition to the size-related CdSe exciton emission, two trap or surface states with decay components close to 10 and 80 ns have been identified. The 80 ns component is shifted to the red by about 100 meV from the exciton peak, whereas the 10 ns component cannot be discriminated spectrally from the exciton band.

Photoluminescence study of MBE grown InGaN with intentional indium segregation

Proper control of MBE growth conditions has yielded an In0.13Ga0.87N thin film sample with emission con- sistent with In-segregation. The photoluminescence (PL) from this epilayer showed multiple emission components. Moreover, temperature and power dependent studies of the PL demonstrated that two of the components were excitonic in nature and consistent with indium phase separation. At 15 K, time resolved PL showed a non-exponential PL decay that was well fitted with the stretched exponential solution ex- pected for disordered systems. Consistent with the assumed carrier hopping mechanism of this model, the effective lifetime, τ, and the stretched exponential parameter, β, decrease with increasing emission energy. Finally, room temperature micro-PL using a confocal microscope showed spatial clustering of low energy emission.

Recombination dynamics of localized excitons in InGaN quantum dots

Indium-rich fluctuations in ultrathin InGaN layers act at low temperatures as a dense ensemble of quantum dots (QD). This leads to a complex potential landscape with localization sites of widely varying depth for excitons. We report on investigations of the recombination mechanisms of excitons localized in InGaN∕GaN QD structures by time-resolved and spatially resolved photoluminescence (PL) measurements. The structures were grown by metal-organic chemical-vapor deposition on Si (111) substrates. Sharp lines originating from single QDs could be observed. Their PL decays show monoexponential behavior. Similar transition energies have different time constants. Thus, the well-known nonexponential PL decay of the QD ensemble is assigned to the summation of monoexponential decays originating from individual QDs with different exciton lifetimes.

Quantum confinement and disorder in porous silicon: Effects on the optical and transport properties

The striking optical properties of porous silicon (PS) show a twofold aspect typical of an ordered and a disordered material, respectively. Raman, electron microscopy, and resonant photoluminescence studies indicate that the light emission originates from crystalline regions. On the contrary, several features, like the non-exponential decay of photoluminescence (PL), the broad emission spectrum, the photoluminescence fatigue under light exposure etc. are typical of a disordered material and reminiscent of similar effects found e.g. in amorphous semiconductors. These two apparently conflicting aspects have for a long time hindered the understanding of the basic light emission mechanism. In this paper we report new optical data showing that disorder in porous silicon leads to strong carrier localisation. Light emission in PS is suggested to occur through transitions involving localized states.

Carrier localization in porous silicon investigated by time-resolved luminescence analysis

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give a similar evolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressive suppression of the nonradiative recombination channel. A simple model permits one to link the changes of the PL decay shape to the QE evolution. The nonexponential PL decay shape is interpreted as being due to a distribution of nonradiative recombination rates, the value of the nonradiative recombination rate being limited by a tunneling effect. This highly simplified model explains the origin of the nonexponential decay shape, its modification and gives a good description of the QE evolution as a function of temperature, oxidation level, or porosity.

A QUANTUM-SPONGE MODEL FOR POROUS SILICON

We propose a “quantum-sponge” model for porous silicon. This model exhibits energy-gap widening and nonexponential decay of photoluminescence describable by the stretched exponential function. These properties are in good agreement with those observed for porous silicon. We suggest that the fractal character of its wave functions is the origin of the nonexponential decay of photoluminescence.

Low-temperature fatigue of photoluminescence in aged porous silicon

We present a photoluminescence (PL) fatigue study at liquid helium temperatures in mesoporous and microporous silicon (PSi) layers impregnated in air. In contrast to PL measurements at room temperature, at low temperatures no recovery of the PL is observed. The results show that the infrared (IR) and slow-luminescent visible (S) band display different behaviors under prolonged illumination. In the S band, similar dependencies in both kinds of specimens are observed with much stronger fatigue strength on the high energy side, revealing that this band is possibly composed of two different kinds of luminescent species. This idea is further supported by the measured change of the luminescence intensity and of the non-exponential PL decay at different photon energies. By analogy with the well-known photo-induced effect in CdSSe microcrystallites, photo-ionization may be responsible for the PL fatigue.

Dynamics of localized exciton recombination and migration in GaAs multiple quantum wells

A theory on the dynamics of localized exciton recombination and their intralayer migration in GaAs quantum wells is developed. Non-exponential photoluminescence decay is successfully interpreted by exciton recombination and its influence on the quasi-equilibrium localized exciton population. The recombination rate and the effective downmigration rate can be obtained.

Mechanisms of visible photoluminescence in porous silicon.

We propose a model for porous silicon, an irregular structure obtained by removing some silicon atoms randomly from a perfect silicon crystal. It is shown by using two-dimensional clusters with a tight-binding model that this model exhibits the energy-gap widening and the nonexponential decay of the photoluminescence (PL), which are in good agreement with the observed properties of porous silicon. In this model, the energy-gap widening is due to the localization of eigenstates caused by the randomness of the structure. The distributions in both size and position of the localized eigenstates, i.e., a statistical effect, yield nonexponential PL decay which is describable by the stretched exponential function. A dynamical effect due to electron and hole hopping between the eigenstates explains the observed strong energy dependence of the PL lifetimes.