Nonradiative Recombination Channels Research Articles

Carrier and phonon dynamics of wurtzite InN nanorods

Time-resolved second harmonic generation is utilized to investigate the photoexcited carrier and phonon dynamics in wurtzite InN nanorods. The exponential decay response in the pump-induced second-harmonic generation curve reveals the carrier relaxation process via radiative and nonradiative recombination channels. Meanwhile, the generation and detection of the coherent A1(LO) phonon can reveal not only the phonon dynamics but also the crystalline quality in InN nanorods. It is found that the nonradiative structural defects play a dominant role in the transport and optical properties of InN nanorods.

Optically active Er3+ centers in Si/Si1 − x Ge x structures under optical pumping

The Si/Si1 − xGex:Er/Si heterostructures, which are of interest for laser applications, have been investigated. The types of optically active Er3+ centers making a dominant contribution to the photoluminescence response of the structures studied are analyzed and their relationship with the parameters of the Si1 − xGex:Er heterolayer and the post-growth annealing conditions is shown. On the basis of the PL kinetic analysis of the structures with an isolated type of optically active Er3+ centers, it is concluded that population inversion of Er3+ ion states can be obtained under optical pumping and the effect of nonradiative recombination channels in Si1 − xGex:Er heterolayers on the excitation efficiency of Er centers and the conditions for population inversion is shown.

Optical properties of blue light-emitting diodes in the InGaN/GaN system at high current densities

The current-voltage and brightness-voltage characteristics and the electroluminescence spectra of blue InGaN/GaN-based light-emitting diodes are studied to clarify the cause of the decrease in the emission efficiency at high current densities and high temperatures. It is found that the linear increase in the emission intensity with increasing injection current changes into a sublinear increase, resulting in a decrease in efficiency as the observed photon energy shifts from the mobility edge. The emission intensity decreases with increasing temperature when the photon energy approaches the mobility edge; this results in the reduction in efficiency on overheating. With increasing temperature, the peak of the electroluminescence spectrum shifts to lower photon energies because of the narrowing of the band gap. The results are interpreted taking into account the fact that the density-of-states tails in InGaN are filled not only via trapping of free charge carriers, but also via tunneling transitions into the tail states. The decrease in the emission efficiency at high currents is attributed to the suppression of tunneling injection and the enhancement of losses via the nonradiative recombination channel “under” the quantum well.

MBE growth of Ge/Si quantum dots upon low-energy pulsed ion irradiation

Photoluminescence (PL) and Raman spectra of Ge/Si quantum dot (QD) structures grown using conventional molecular beam epitaxy, MBE, (sample M) and pulsed ion beam-assisted MBE (sample MI) were measured before and after vacuum annealing and hydrogen passivation. Two broad bands are observed in the PL spectra, one ascribed to the QDs and another one to radiation defects. Sample MI exhibits a higher overall PL intensity. The post-growth vacuum annealing partially suppresses the defect-related emission, enhances the QD-related PL in sample M, but almost does not affect the latter emission in sample MI. After the atomic hydrogen treatment that passivates the defect-related radiative and nonradiative recombination centers, a high-energy shift by 20–25 meV of the QD PL peak in sample MI appears when compared to sample M. The analysis shows that the shift is mainly due to a reduction of the QD size, which occurs in the case of the ion-assisted MBE. We conclude that the radiation defects created at the QD growth temperatures do not introduce nonradiative recombination channels, on the contrary to the defects caused by the nonequilibrium character of the MBE growth. In situ high-temperature ion bombardment during the QD growth pushes the epitaxial layer system towards equilibrium, thus enhancing the QD PL intensity.

Further insight into the temperature quenching of photoluminescence from InAs∕GaAs self-assembled quantum dots

The possibility of controlling the photoluminescence (PL) intensity and its temperature dependence by means of in-growth and postgrowth technological procedures has been demonstrated for InAs∕GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). The improvement of the QD emission at room temperature (RT), achieved due to a treatment with tetrachloromethane used during the growth, is explained by the reduction of the point defect concentration in the capping layer. It is shown that the PL quenching at RT appears again if the samples are irradiated with protons, above a certain dose. These findings are accounted for by the variations in the quasi-Fermi level position of the minority carriers, which are related to the concentration of trapping centers in the GaAs matrix and have been calculated using a photocarrier statistical model including both radiative and nonradiative recombination channels. By taking into consideration the temperature dependent distribution of the majority and minority carriers between the QDs, embedding QW and GaAs barriers, our calculated results for the PL intensity reproduce very well the experimentally observed trends.

A mechanism of the anti-Stokes luminescence of a dye-sensitized silver halide emulsion

A two-photon stepwise mechanism of the low-temperature anti-Stokes luminescence of a sensitized AgBrI emulsion, caused by photoexcitation of dye aggregates, is proved experimentally. According to this mechanism, the photoexcitation energy is transferred from the dye to the AgHal microcrystal through silver cluster centers. It is found that the luminescence with the maximum at λ ≈ 750 nm is caused by the recombination of an electron localized at the silver cluster with a free hole in the valence band of the AgHal microcrystal. The migration of an electron from a silver cluster to a iodine pair center with a captured hole creates a nonradiative recombination channel, which is responsible for the flare buildup of the green anti-Stokes luminescence upon excitation of dye aggregates.

Optical modelling of a Si-based DBR laser source using a nanocrystal Si-sensitized Er-doped silica rib waveguide in the C-band

The availability of reliable silicon-based laser sources is at the basis of the integration of photonic and microelectronic devices on a single chip with consequent development of wavelength division multiplexing telecommunication systems. A high efficiency Si-based laser source with good stability at room temperature would encourage and push the large scale of integration of electronic and photonic devices within a single chip. Several techniques have been proposed for generating light with an internal quantum efficiency some order of magnitude greater than that typical of silicon (10−6) by using either electrical or optical pumping. Among them we mention the improvement of some fabrication process steps, reduction of the channels of non-radiative recombination, quantum confinement, the use of silicon nanocrystals (Si-ncs) incorporated in a silica matrix. This last technique is used in combination with Er3+ doping to generate light emission around 1500 nm in silicon, since Er-doped Si-ncs behave as electron-hole pairs trap, and the presence of Er shifts the emission peak to around 1500 nm. In this paper we have pointed out the optical model of a Si-based DBR laser including a Si-ncs Er-doped SiO2 rib waveguide, working at a wavelength in C-band. In particular, after a brief description of the structural and optical properties of the silicon crystals, we report on the model and design of the Er:Si-nc/SiO2 rib waveguide, of the optical cavity and of the Bragg mirrors.

Advantages of the time-resolved four-wave mixing technique for studies of non-equilibrium carrier dynamics in bulk semiconductors and structures

We demonstrate versatility of the four-wave mixing technique in investigating non-equilibrium carrier dynamics in semiconductor crystals and epitaxial structures. Measurements of the effective diffusion coefficient in the presence of a light-induced space charge field have allowed us to determine the photoconductivity type of photorefractive CdTe and InP crystals doped with deep impurities. Hole mobility values of μh=(96±22)cm2/Vs in CdTe and μh=(152±16)cm2/Vs in InP have been found from the ambipolar diffusion coefficient. Studies of a number of differently grown GaN layers with dislocation density ranging from 106 to 1010cm−2 have revealed a correlation of the longer carrier lifetimes with the smaller dislocation density. Competition between radiative and non-radiative recombination channels has been observed in low dislocation density HVPE-grown bulk-like GaN.

Insights into Microstructural Features Governing Ce3+ Luminescence Efficiency in Sol−Gel Silica Glasses

The radioluminescence (RL) properties of Ce-doped silica glasses prepared by sol−gel method were investigated as a function of the dopant amount in a wide range of concentrations (Ce:Si molar ratios from 1 × 10-5 to 5 × 10-2). The effects of xerogel densification temperature and of a postdensification thermal treatment were also considered. In order to understand the microstructural features governing Ce3+ luminescence efficiency, optical absorption measurements in the ultraviolet, visible, and infrared regions and time-resolved photoluminescence experiments were performed. The complex dependence of RL intensity upon rare-earth (RE) concentration and thermal treatment was attributed to the role of OH vibrations as nonradiative recombination channels, as well as to the formation of RE aggregates in the silica matrix. Specifically, the segregation of CeO2 nanoparticles, in which Ce4+ does not supply any radiative emission, has been revealed by infrared absorption measurements, in agreement with previous Raman, transmission electron microscopy, and X-ray diffraction data. Moreover, an optical absorption band, centered at 2.4 eV and whose intensity increases with the square of cerium concentration, was observed and tentatively assigned to an intervalence electron-transfer transition involving Ce3+−Ce4+ dimers. Postdensification thermal treatments markedly reduce the intensity of this band and increase RL intensity. The relationship between the 2.4 eV band and the RL properties will be outlined. In particular, the increase in RL intensity will be discussed, mainly as a consequence of microscopic modifications leading to an improvement of the charge-transfer efficiency toward emitting centers.

Ion Beam Analysis of Ge/Si Dots Grown on Ultrathin SiO<sub>2</sub> Interlayers

Monolayer (ML) thick Ge deposition on (100) Si substrates by molecular beam epitaxy (MBE) technique using an ultrathin SiO2 interlayer has been studied by ion beam analysis and photoluminescence (PL). The dependence of the Ge layer growth mode on the amount of the deposited Ge and the SiO2 thickness has been investigated. Atomic hydrogen treatment has been performed in order to passivate non-radiative recombination channels and to enhance the PL intensity. We conclude the formation of Ge quantum dots for the sample with the thickest Ge and SiO2 layers (9 Å and 1 ML, respectively).

Optical and structural study of Ge/Si quantum dots on Si(1 0 0) surface covered with a thin silicon oxide layer

The formation of Ge quantum dots (QDs) grown on an ultrathin interlayer of SiO 2 on top of a Si(1 0 0) substrate was investigated, as a function of the thicknesses of the SiO 2 interlayer (0.5, 0.75 or 1 monolayer (ML)) and the Ge layer (0.3, 0.6 or 0.9 nm). The structural characterization was performed by Rutherford backscattering spectroscopy (RBS). Photoluminescence (PL) studies were done to characterize the optical behavior of all samples. Hydrogen treatment was performed in order to passivate non-radiative recombination channels, thus enhancing the PL intensity. The results suggest the formation of Ge nanoislands (quantum dots, QDs), for the sample with 1 ML of SiO 2 and 0.9 nm of Ge, and exclude their formation for samples with lower SiO 2 and Ge layer thicknesses. We also observe an influence of the SiO 2 interlayer thickness in the QDs formation.

Temperature dependence of carrier lifetimes in InN

AbstractTime‐resolved pump–probe transmission measurements were used to determine the temperature dependence of carrier lifetime for InN epilayers with unintentionally doped levels from 1018 to 1019 cm–3. The observed decay time at 20 K is well explained by a dominating radiative interband recombination, while at room temperature it is attributed to a defect related nonradiative recombination channel. The temperature dependence of the radiative lifetime is deduced from the measurements of both differential transmission decay time and PL intensity. For the best quality sample, we find the radiative lifetime increases proportionally to T3/2, as theory predicts when a k‐selection rule holds, which suggests that the radiative band‐to‐band transition accounts for the observed infrared photoluminescence over the entire temperature. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiative and nonradiative recombination processes in InN films grown by metal organic chemical vapor deposition

The 800meV photoluminescence band in indium nitride is excited under pulsed excitation conditions and is investigated as a function of temperature and time. Our results are consistent with a composite photoluminescence feature composed of two overlapping bands separated by an ∼10meV splitting, with populations described by a thermal equilibrium model. Efficient nonradiative recombination channels rule both the temperature dependence of the time-integrated photoluminescence spectra and the recombination dynamics. At 10K, the radiative recombination time is of the order of 300ns, while the nonradiative recombination time, which is ruled by activation energy of 8meV, is about 100ps.

Ultrahigh gain and non-radiative recombination channels in 1.5 µm range metamorphic InAs–InGaAs quantum dot lasers on GaAs substrates

Metamorphic InAs–InGaAs quantum dot (QD) lasers grown on GaAs substrates by molecular beam epitaxy have been studied. The lasing wavelength of about 1.5 µm and the threshold current density of 1650 A cm−2 for a 2 mm cavity laser diode have been observed. The characteristic temperature of 66 K has been shown in the temperature range from 30 to 80 °C. The analysis of spontaneous emission efficiency corroborates the high contribution of non-radiative recombination in the total recombination current, even at high current densities. The possibility of achieving modal gain as high as 190 cm−1 has been demonstrated.

Photoluminescence probing of non-radiative channels in hydrogenated In(Ga)As/GaAs quantum dots

We report here on the efficient use of hydrogenation as a tool to investigate the intrinsic properties of non-radiative recombination channels in a low-pressure metal-organic vapor phase epitaxy grown quantum dots (QDs) array emitting around 1.3 μm. It is shown that hydrogenation reduces by a factor of two the flow of charge carriers to non-radiative channels in QDs related to the presence of impurities and further suppresses completely the non-radiative recombinations originating from the stacking faults and dangling bonds. Most importantly, hydrogenation improves the room temperature photoluminescence intensity of the ≈1.3 μm emitting In(Ga)As/GaAs QDs by a factor close to ten.

Electroluminescence efficiency of silicon diodes

The intrinsic electroluminescence (EL) of a silicon light-emitting diode with a forward-biased p-n junction is studied. The substantial enhancement of the integrated EL intensity observed with the temperature increasing from the liquid-nitrogen to room-temperature level, which is paralleled by an increase in the EL decay time when the current through the diode is terminated, indicates thermal suppression of the nonradiative recombination channel associated with deep traps. A simple model developed by us for the radiative processes occurring in a p-n junction offers an interpretation for all the experimental data obtained. It is shown that the internal quantum efficiency of the EL may reach a level of a few percent under optimal doping of the diode p and n regions.

Time-resolved photoluminescence study of excitons inα-PTCDA as a function of temperature

~Received 17 January 2003; revised manuscript received 19 May 2003; published 1 October 2003! In the present work, we analyze the radiative and nonradiative recombination channels in a-PTCDA ~3,4,9,10-perylene tetracarboxylic dianhydride! by applying time-resolved photoluminescence ~PL! techniques in the 50 ns range between temperatures of T510 K and T5300 K. The obtained PL spectra are decomposed into exponentially decaying components which can be assigned to different types of excitons, including Frenkel excitons, three types of self-trapped excitons, and two different high-energy PL satellites. From the temperature dependence of the PL spectra, we gain some insight into the influence of nonradiative processes on the decay time of the different PL bands. At room temperature, PL from Frenkel excitons and oppositely charged ionic pairs is quenched due to nonradiative recombination. In contrast to this behavior, the decay of a PL band assigned to excimers shows only a weak temperature dependence, so that it becomes the dominating radiative recombination mechanism at temperatures above T5200 K.

Photoluminescence fatigue and related degradation in thin-film photovoltaics

We observe that junction photoluminescence (PL) intensity excited by a constant power laser beam in polycrystalline CdTe/CdS solar cells gradually decreases, which is similar to the PL fatigue in chalcogenide glasses. As a function of time, it was studied at different laser beam powers and temperatures for contact-free and metallized regions. We were able to discriminate between the fatigue per se and a concomitant short-time PL intensity drop due to the laser heating. The fatigue shows substantial variations between different spots on the sample. It is more profound at higher temperatures and laser beam powers where its value can be as large as 80% in 2 h. At low temperatures and beam powers, it saturates rather quickly not exceeding 10% of the initial PL intensity. We attribute the observed phenomenon to defect creation by the light-generated electrons and holes. The defects provide additional nonradiative recombination channels thus decreasing the PL. Simultaneously, this negative feedback makes the defect-generation rate slow down, so that the PL fatigue saturates. We propose a simple analytical model that fits the data.

Temperature quenching of exciton luminescence intensity in ZnO/(Mg,Zn)O multiple quantum wells

The temperature-dependent behavior of excitonic photoluminescence observed in ZnO/MgZnO multiple quantum wells (MQWs) in the temperature range of 5–300 K is described. In a ZnO/Mg0.27Zn0.73O MQW grown by laser molecular-beam epitaxy, the luminescence was dominated by localized exciton (LE) emission throughout the whole temperature range studied. Luminescence of free excitons (FEs) was not observed. A simple rate equation is used to describe the quenching of LE emission. The activation energy for LE luminescence quenching is of the order of the localization energy of excitons, suggesting that the thermionic emission of the LEs out of the localization potentials leads to nonradiative recombination. In a ZnO/Mg0.12Zn0.88O MQW having lower barriers, the luminescence was dominated by LE emissions at low temperatures, while the FE transition was dominating emissions at temperatures above 175 K. A rate equation assuming one nonradiative recombination channel is used to describe the quenching of the transitions observed. The activation energy for LE luminescence quenching deduced in this sample is also of the order of the localization energy of excitons. The temperature dependences of FE emission intensities are also discussed by using a simple rate equation in which a thermal release effect of LEs toward FEs is taken into account.

Time-resolved differential reflectivity as a probe of on-resonance exciton dynamics in quantum wells

We investigate the use of time-resolved differential reflectivity for the study of the ultrafast and recombination dynamics of resonantly excited quantum-well excitons. We illustrate the technique with extensive time-resolved measurements using femtosecond laser pulses on a 4-nm ${\mathrm{In}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}|\mathrm{GaAs}$ quantum well. Simple considerations applicable to quantum-well systems allow us to relate a generic change in reflectivity to a change in transition oscillator strength, which in turn is proportional to the exciton density (population inversion) created by an intense pump beam. Having as a starting point the temporal evolution of a two-level system under resonant excitation, we interpret differential reflectivity measurements in terms of dynamical processes that range from polarization dephasing (hundreds of femtoseconds) to radiative and nonradiative recombination (nanoseconds). These various dynamical processes span over four orders of magnitude in the time domain and may be extracted from a single time-resolved differential reflectivity experiment. We also find that the observed temporal decays are quite sensitive to the initial (and cold) photogenerated population. We show that a power dependence of the differential reflectivity signal can be utilized to obtain estimates of the exciton saturation density as well as to study the role of exciton-exciton scattering and thermalization on the total recombination time. Under low-power excitation conditions, we relate the differential reflectivity decay to the total exciton recombination lifetime. Lifetime measurements as a function of temperature confirm this assignment as they show an initial and quasilinear rise at low temperatures $(T<70\mathrm{K})$ followed by a decrease at higher temperatures owing to the dominance of nonradiative recombination channels.