Nonradiative Recombination Processes Research Articles

Highly-Doped, Highly-Strained Germanium and Schottky Electroluminescent Diodes

Achieving a monolithic germanium laser source represents a major challenge for silicon photonics. Two key parameters have an impact on the laser performances including threshold, output power, reliability... The first key parameter is the germanium doping. Modeling indicates that an active electron carrier concentration of 3 - 4 x 1019 cm-3 is requested to obtain a robust gain. We will show that such doping level can be achieved for germanium grown on silicon using a co-doping method followed by thermal anneal. The thermal annealing is necessary to minimize the non-radiative recombination processes. The second key parameter is the tensile strain present in germanium. The achievement of high or even record tensile strain values in small volumes is not sufficient for lasing. The homogeneity of the strain field within the germanium volume needs to be taken into account. We will show how it is possible to maximize the tensile strain homogeneity either for straight waveguides or germanium microdisks strained by a silicon nitride stressor. The trade-off between homogeneity and maximum strain value allows one to achieve a more robust modal gain. We will finally discuss the transfer of these concepts for electrical injection.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

Paths for the Non-radiative Recombination Occurring in CdS:CdO/Si Multi-Interface Nanoheterostructure Array

A CdS:CdO/Si multi-interface nanoheterostructure array (CdS:CdO/Si-NPA) is prepared by a chemical bath deposition method, and three emission bands are observed in the as-grown CdS:CdO film. By measuring its temperature-dependent photoluminescence (PL) spectrum, the variation trends of the peak energies and intensities with temperature for the three bands are obtained. Based on the theoretical analyses and fitting results, the non-radiative recombination processes corresponding to the PL quenching for the three emission bands are attributed to the thermally activated transition between heavy-hole and light-hole levels (at low temperature) and the thermal escape due to the scattering from longitudinal optical phonons (at high temperature), the transition from acceptor levels to surface states, and the transition related to surface defect states, respectively. The clarification of the non-radiative recombination processes in CdS:CdO/Si-NPA might provide useful information for promoting the performance of optoelectronic devices based on CdS/Si nanoheterostructures.

Investigation of combinatorial coevaporated thin film Cu2ZnSnS4. I. Temperature effect, crystalline phases, morphology, and photoluminescence

Cu2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu2ZnSnS4, Cu2SnS3, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu2ZnSnS4 on non-radiative recombination processes. Implications for processing and performance of Cu2ZnSnS4 solar cells are discussed.

Nonradiative recombination mechanisms in InGaN/GaN-based light-emitting diodes investigated by temperature-dependent measurements

Two kinds of InGaN-based light-emitting diodes (LEDs) are investigated to understand the nonradiative carrier recombination processes. Various temperature-dependent measurements such as external quantum efficiency, current-voltage, and electroluminescence spectra are utilized from 50 to 300 K. Based on these experimental results, we analyze the dominant nonradiative recombination mechanism for each LED device. We also analyze the effect of the dominant nonradiative recombination mechanism on the efficiency droop. On the basis of correlation between the efficiency droop and nonradiative recombination mechanisms, we discuss an approach to reducing the efficiency droop for each LED device.

Bandgap Engineering of ZnO by Alloying with MgO

During last decades, the band gap modulation of ZnO films has been achieved by alloying it with much wider band gap semiconductor material such as MgO (7.8eV). However, there exists a miscibility gap in the ZnO - MgO binary system due to the structural difference and the large lattice mismatch between ZnO (hexagonal wurtzite, 3.25Å) and MgO (cubic rock salt, 4.22Å). Mg+2 ions can replace ~ 40% of Zn+2 ions without changing the structure of ZnO and beyond that phase segregation between hexagonal ZnO and cubic MgO has been observed in Zn1-xMgxO alloy. We deposited ZnMgO thin films with sintered ceramic MgZnO targets having Mg concentration up to 25% on sapphire substrates by pulsed laser deposition (PLD) technique. X-ray diffraction measurements on these films revealed formation of single phase MgZnO with high c-axis orientation for all Mg concentrations. No peaks corresponding to segregation of MgO in these films was observed. Optical transmission studies performed with UV-VIS spectrometer showed ~80% transmittance in the visible region in all films with sharp absorption edge which shifted towards blue up to 3.72 eV. Photoluminescence (PL) measurements carried out at room temperature showed distinct PL peaks in the UV and visible region for ZnMgO. The peak occurring in the UV region can be attributed to band to band emission whereas the peak at visible region could be attributed to oxygen related defects. A blue shift in the PL peak position of the band to band emission was observed as the Mg concentration was increased. Interestingly, with increase in Mg concentration the band to band transition emission peak intensity decreased systematically and disappeared for Zn0.75Mg0.25O. This behavior indicates that non-radiative recombination process is dominating with increase in Mg concentration. The resonant Raman studies indicated presence of several higher order longitudinal optical phonon modes. Further studies are underway.

Temperature-Dependent Photoluminescence of Silicon Nanoporous Pillar Array

Silicon nanoporous pillar array (Si-NPA) is a micron-nanometer hierarchical structure which might be used as functional substrates for constructing optoelectronic nanodevices. This makes understanding the photoluminescence (PL) from Si-NPA important. We measure the PL of Si-NPA in the range of 11–300 K. By analyzing the evolution of the peak energy and intensity with temperature, the ultraviolet, blue, orange and red PL bands from Si-NPA are attributed to the radiative recombination through the deep-levels in silicon oxide, oxygen-related defect states in silicon nanocrystallites (nc-Si), band-to-band transition within nc-Si, and surface/interface states of nc-Si or between nc-Si and SiOx, respectively. At least two non-radiative recombination processes, which are activated at different temperature ranges, are proposed for the PL intensity variation with temperature. These results might provide strong foundations for designing and constructing optoelectronic devices based on silicon nanostructures.

Synthesis and optical properties of Eu3+ doped ZnO nanoparticles used for white light emitting diodes.

ZnO nanoparticles doped with trivalent europium ions (Eu3+) were synthesized by the hydrothermal method. The structural properties of the samples were investigated by the X-ray diffraction (XRD). The results indicated that Eu3+ was incorporated into the lattice of ZnO. Photoluminescence (PL) measurements of ZnO:Eu3+ showed a strong red luminescence emission. Specially, the red emission can be obtained even under the nonresonant excitation of 320 nm, and it is explained by an energy transfer mechanism in which the energy is transferred from ZnO matrix to Eu3+. These results indicate that the ZnO:Eu3+ is the attractive candidate phosphor for the application in phosphor-converted light-emitting diodes (pc-LEDs) as red phosphor. The intensity of Eu3+ PL decreased with the increasing Eu3+ concentration and the concentration quenching mechanism was presented based on non-radiative recombination processes in ZnO and the energy back-transfer from the excited state of Eu3+ to the ZnO host. Moreover, the samples were synthesized under low temperature condition.

Invited Presentation: Prolonged Spontaneous Emission and Dephasing of Excitons in Air-Bridged Swcnts

Carbon nanotubes have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices. To date, the majority of experiments reveal however low quantum efficiencies and strong dependencies on the environment and chemical processing. This is very detrimental for optoelectronic device applications dominant non-radiative optical recombination processes lead to strongly reduced optical emission rates of nanoscale light sources. Likewise, non-radiative recombination leads to reduced light-to-current conversion efficiencies in photodetectors and solar cells.The question arises to what extent previous experiments have so far probed for the true intrinsic properties of the exciton dynamics. In particular, theorists predicted about a decade ago spontaneous emission (SE) lifetimes of excitons of about 10-100 ns while most measurements found orders of magnitude faster lifetimes varying from 20 to 200 ps. In earlier work we have demonstrated how polymer capping of SWCNTs can be used to strongly reduce detrimental environmental interaction effects such as spectral diffusion and blinking [1]. Although these samples showed higher exciton emission efficiencies, the corresponding SE lifetime remained with 30 ps much faster than theoretical predictions, implying that they are still dominated by nonradiative exciton recombination.To overcome these limitations we have combined sophisticated growth of ultra-clean SWCNTs bridging an air gap over pillar posts with time-resolved photoluminescence measurements of individual SWCNTs over 14 orders of magnitude. In this talk we will discuss our recent finding of the onset of a remarkable new regime of intrinsic exciton photophysics with prolonged SE times up to 18 ns, about two orders of magnitude longer than reported before [2]. While these intrinsic optical lifetimes had been predicted by theorists about a decade ago, they had never been reported due to the masking by extrinsic effects of defects and unintentional doping.Measurements of exciton emission linewidth versus pump power demonstrated that air-bridged SWCNTs have significantly narrower linewidths than polymer-embedded SWNTs. The later display a rather large linewidth up to 10 meV, while the exciton linewidth of SWCNTs bridging an air gap reaches down to 220 μeV at low pump powers, if they are grown for short times of only 2 minutes. It appears that our best air-bridged samples are only at the onset of this intrinsic photophysics regime. Nanosecond long SE lifetimes have very recently also independently confirmed by Hofmann et al. [3], who also report even narrower spectral linewidth down to 40 meV. Future work has thus much room for improvements to reach deeper into the intrinsic regime and explore the intrinsic limits.With these supposedly ultraclean air-bridged samples we also carried out first experiments on exciton dephasing times for individual SWCNTs in the time-domain. The interferometric measurements demonstrated exciton dephasing times up to 2 ps, about four-fold longer than in previous ensemble studies (500 fs). The ultra-narrow linewidth and prolonged exciton dephasing times suggest that the acoustic phonon coupling giving rise to exciton dephasing is significantly weaker than previously believed, which is promising news for device applications, and raises the question about the ultimate limit for SE lifetimes and exciton dephasing in the intrinsic regime.

Influence of barrier thickness on the structural and optical properties of InGaN/GaN multiple quantum wells

The structural and optical properties of InGaN/GaN multiple quantum wells (MQWs) with different barrier thicknesses are studied by means of high resolution X-ray diffraction (HRXRD), a cross-sectional transmission electron microscope (TEM), and temperature-dependent photoluminescence (PL) measurements. HRXRD and cross-sectional TEM measurements show that the interfaces between wells and barriers are abrupt and the entire MQW region has good periodicity for all three samples. As the barrier thickness is increased, the temperature of the turning point from blueshift to redshift of the S-shaped temperature-dependent PL peak energy increases monotonously, which indicates that the localization potentials due to In-rich clusters is deeper. From the Arrhenius plot of the normalized integrated PL intensity, it is found that there are two kinds of nonradiative recombination processes accounting for the thermal quenching of photoluminescence, and the corresponding activation energy (or the localization potential) increases with the increase of the barrier thickness. The dependence on barrier thickness is attributed to the redistribution of In-rich clusters during the growth of barrier layers, i.e., clusters with lower In contents aggregate into clusters with higher In contents.

Chemical doping-induced changes in optical properties of single-walled carbon nanotubes

We studied the changes in the optical properties of single-walled carbon nanotubes (SWCNTs) induced by chemical doping. An enhancement in the photoluminescence (PL) of the SWCNTs was observed after doping with reducing agents, whereas a reduction in the PL intensity and the appearance of fast exciton decay resulted from doping with oxidizing agents. Comparisons of the measured PL and transient absorption obtained with the two different types of dopants demonstrated that the PL properties of SWCNTs are strongly linked to the non-radiative exciton recombination processes. Chemical doping strongly modulates the exciton dynamics in SWCNTs.

A direct measurement of the electronic structure of Si nanocrystals and its effect on optoelectronic properties

Since reports that silicon nanocrystals (Si-NCs) can exhibit direct transition emission, the silicon laser field is at a juncture where the importance of this discovery needs to be evaluated. Most theoretical models predicted a monotonic increase in the bandgap and experimental information currently available on the electronic structure at the Γ valley of these promising materials is circumstantial as it is obtained from emission measurements where competing non-radiative relaxation and recombination processes only provide an incomplete picture of the electronic structure of Si-NCs. Optical absorption, the most immediate probe of the electronic structure beyond the band-edges, showing the evolution of the Γ valley states with nanocrystal size has not been measured. Here, we show such measurements, performed with high dynamic range, allowing us to observe directly the effect of crystal size on the Γ valley splitting far above the band-edges. We show that the splitting is 100 s of meV more pronounced than predicted by pseudo potential calculations and Luttinger-Kohn model. We also show that ultrafast red-shifting emission can be observed in plasma enhanced chemical vapor deposition prepared Si-NCs.

Blinking in Photoluminescence of InGaN Devices is Caused by Slow Beating of THz Vibrations of the Quantum Well

The photoluminescence from III-V wide band-gap semiconductors as InGaN is characterized by localized large intensity fluctuations, known as blinking, that, despite decades of research, is not yet completely understood. In structures where there is a three-dimensional confinement, as for example semiconductors nanocrystals, the phenomena is supposed to be related to temporary quenching due to highly efficient non-radiative recombination processes (for example, Auger). Nevertheless, in typical InGaN devices, the band structure is an infinitely wide quantum well, so the understanding of the blinking phenomenon remains elusive. We present experimental data and a model that suggests that the discussed optical fluctuations are a general phenomena caused by the slow beating between THz thermal vibrations of the Quantum Well. These minuscule displacements are occurring naturally all over the device, the displacements along the growth direction induce a modulation of the matrix elements that drives the optical emission process; this have measurable effect on the device photo-luminescence. In presence of impurities or gradient of concentration, the vibrations have locally slight frequency differences on adjacent domains, this give rise to a band of beats, and we observe the lower frequency tail of this band.

Interface recombination velocity measurement in SiO2/Si

The photothermal technique has been used in its orthogonal configuration in order to determine the interface recombination velocity between SiO2 ultra-thin film and Si substrate. This investigation has been performed by studying the variation of the photothermal signal according to the square root modulation frequency of the pump light beam. A general one-dimensional theoretical model taking into consideration the nonradiative recombination process has been developed. The interface recombination velocity has been evaluated by fitting the experimental curves of the phase and normalized amplitude of the photo-thermal signal with the corresponding theoretical ones.

Excitonic recombination dynamics in non-polar GaN/AlGaN quantum wells

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8 K–300 K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells.

Optical detection of nonradiative recombination centers in AlGaN quantum wells for deep UV region

AbstractBy superposing a chopped below‐gap excitation (BGE) light on a cw above‐gap excitation and observing the intensity change of photoluminescence (PL) due to the BGE, we clarified a distribution of nonradiative recombination (NRR) centers in AlGaN multi‐quantum well (MQW) structures for the wavelength region of deep ultraviolet (UV). The decrease in band‐edge PL peak intensity at 10 K exemplified the presence of a pair of NRR centers in AlGaN well layers whose transition energy corresponds to that of the BGE. The BGE power dependence of the normalized PL intensity showed a quenching saturation due to trap‐filling effect of electrons in one of the centers which were activated by the BGE. The BGE energy dependence of the normalized PL intensity revealed that the optical activation energy of the dominant NRR process via these trap states in the AlGaN MQW is around 1.17 eV. The purely optical scheme of detection enables us to obtain clues for identifying defect levels in DUV region and eliminating them during growth process without any preparation of electrodes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recombination dynamics and internal quantum efficiency in InGaN

AbstractRecombination dynamics and internal quantum efficiency (IQE) of green luminescent InGaN nanowires with different crystalline qualities have been studied by means photoluminescence (PL) and time‐resolved PL spectroscopy. Temperature‐ and excitation‐power‐density‐dependent PL spectroscopy enabled to evaluate the IQE as a function of excitation power density. The shape of the efficiency curves at low temperature strongly depended on the magnitude of nonradiative recombination processes. This leads to the misestimation of the IQE in the lower quality nanowire. In addition, the PL decay curves were well described by a double exponential function both at 6 and 300 K. The PL decay time of the faster component was affected by nonradiative recombination processes even at low temperature. This indicated that the radiative recombination lifetime cannot be estimated from the PL decay time in the lower quality nanowire. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-frequency noise in diagnostics of power blue InGaN/GaN LEDs

Low-frequency noise measurements, combined with the conventional techniques for the study of InGaN/GaN-based LEDs, make it possible to separate the contribution of conductive paths associated with the extended defect system (EDS) and point defects (PD) to non-radiative recombination processes. These measurements also can reveal physical mechanisms leading to the unpredictable failure of LEDs, such as non-uniform current distribution, local overheating, and presence of local InGaN regions with a reduced band gap width Eg.

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.

Processing condition dependence of time-resolved photoluminescence in thiophene/phenylene co-oligomer microcavities

The fluorescence lifetimes of thiophene/phenylene co-oligomer crystalline films were measured before and after microcavity fabrication processes. We found that the lifetime of the microcavity part was much shorter than that of the nonprocessed part in a previously reported fabrication process. It is suggested that some parts of the fabrication process induce damage to the cavity parts, leading to increased nonradiative recombination processes. As a result, the fluorescence yield decreases and lifetime shortening occurs. We show that the lifetime shortening originates from the metal removal dry etching process, but does not originate from the dry etching process of the organic layers itself. This indicates that the O2-dry etching process is an effective fabrication method for organic laser cavities.