Performed Photoluminescence Measurements Research Articles

Telecom C-band photon emission from (In,Ga)As quantum dots generated by filling nanoholes in In0.52Al0.48As layers

We present the fabrication of strain-free quantum dots in the In0.53Ga0.47As/In0.52Al0.48As-system lattice matched to InP, as future sources for single and entangled photons for long-haul fiber-based quantum communication in the optical C-band. We achieved these quantum dots by local droplet etching via InAl droplets in an In0.52Al0.48As layer and subsequent filling of the holes with In0.53Ga0.47As. Here, we present detailed investigations of the hole morphologies measured by atomic force microscopy. Statistical analysis of a set of nanoholes reveals a high degree of symmetry for nearly half of them when etched at optimized temperatures. Overgrowth with 50–150 nm In0.52Al0.48As increases their diameter and elongates the holes along the [01̄1]-direction. By systematically scanning the parameter space, we were able to fill the holes with In0.53Ga0.47As, and by capping the filled holes and performing photoluminescence measurements, we observe photoluminescence emission in the O-band up into the C-band depending on the filling height of the nanoholes.

100 Mev Ag and 25 Kev He Induced Defects in 4H-SiC

We demonstrate the formation of radiative defects, known as color centres, in 100 MeV Ag and 25 keV He irradiated semi-insulating 4H-SiC using low-temperature (~77 K) photoluminescence (PL). In PL, the luminescence resulting from radiative recombination of photo-excited carriers (electrons and holes) is studied. In our work, a 266 nm deep UV excitation source is utilized for performing PL measurements. In the range, 850-1150 nm, the photoluminescent peaks ascribed to silicon vacancies (V Si) and unknown defect-related bands (UD3 and UD4) are observed after both ion irradiations. It has also been observed that till an optimum irradiation fluence there is an enhancement of photoluminescence from the ion irradiation-induced defects but further increase in irradiation fluence results in accumulation of photo-absorbing defects, providing pathways for non-radiative processes. In addition, ion irradiation has been shown to suppress the undesirable luminescence in the range 300-600 nm caused by the intrinsic defects in as-grown 4H-SiC.

Detection of Cu2+ ions in aqueous solution via emission quenching of colloidal EuPO4 ultrasmall nanoparticles

EuPO4 and Eu3+-activated REPO4 (RE = La, Dy, Er) ultrasmall fluorescent nanoparticles (NPs) with an average diameter of 2.1 nm were prepared via colloidal synthesis and tested for sensing Cu2+ and other heavy-metal ions. The pure monoclinic monazite crystal structure (space group P121/n1) of the synthesized particles was confirmed using X-ray diffraction measurements, and transmission electron microscopy images showed round particles with a narrow particle-size distribution. The NPs exhibited intense red emission, which is characteristic of the f-f electronic transition of Eu3+, and the quenching of their emission in the presence of heavy-metal ions was revealed by dispersing colloidal particles in a TRIS buffer and then performing photoluminescence measurements. Strong quenching of the emission (at a rate of 0.195 μM−1) was observed upon the addition of Cu2+ ions over the concentration range of 0–10 μM with a limit of detection of 60 nM for Cu2+. The recovery of nearly 90% of the original emission intensity of the probe was achieved via the addition of ethylendiaminetetraacetic acid and was possible in five quenching/recovery cycles.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Ultraviolet optical excitation of near infrared emission of Yb-doped crystalline aluminum oxynitride thin films

Yb3+ ions hold promises for high power emission in the near infrared (NIR). Yet, relevant matrices, comprising mediators to excite Yb3+, have to be found and the optical mechanisms have to be studied in detail. The purpose of this study is to report on the optical excitation and emission mechanisms of NIR photoluminescence (PL) of Yb-doped crystalline aluminum oxynitride thin films prepared at room temperature using reactive magnetron sputtering. Crystal structure and chemical composition are analyzed by transmission electron microscope and Rutherford backscattering spectrometry, respectively. Photoluminescence spectroscopies are used to investigate the excitation and emission mechanisms. NIR emission at 985 nm is obtained under indirect optical excitation using the 325 nm line of a He-Cd laser, the excitation mechanism is explored by photoluminescence excitation measurement (PLE), and the fine structure of the emitted energy levels is investigated by performing PL measurements at low temperature (LTPL). PLE shows that the host defects play the role of mediators to transfer the excitation energy to Yb ions. This offers different possibilities for the development of multiple excitation channels for Yb3+. Stark splitting of the energy levels of the 2F5/2 and 2F7/2 transitions is evidenced using LTPL in the 78 to 295 K range. Electronic transitions are ascribed to experimental emission lines based on good agreement with theoretical values. Moreover, the activation energies for PL thermal quenching are determined and correspond to the energy difference between highest energy quenched lines and thermally activated “hotlines.”

Effect of Tb3+ doping on the photocatalytic activity of TiO2 coatings formed by plasma electrolytic oxidation of titanium

Tb doped TiO2 coatings are formed by plasma electrolytic oxidation (PEO) of titanium in water solution of 10 g/L Na3PO4⋅12H2O with addition of Tb4O7 powder. Surface morphology, chemical, and phase composition of obtained PEO coatings are investigated utilizing scanning electron microscopy equipped with energy dispersive x-ray spectroscopy and X-ray diffraction. Uniformly distributed Ti, O, P, and Tb are identified as the main constituents of the coatings. Diffraction patterns of obtained coatings show well pronounced reflections inherent to anatase phase of TiO2. Performed photoluminescence measurements indicate that incorporated Tb is in trivalent oxidation state. Photoluminescence emission spectra feature two overlapping regions: the first region is related to band of TiO2 host and the second region which features several emission bands is ascribed to f-f transitions of Tb3+ from excited level 5D4 to lower levels 7FJ (J = 3,4,5, and 6). The photocatalytic activity (PA) of Tb3+ doped TiO2 coatings is probed by measuring the photodegradation of methyl orange under simulated sunlight conditions, suggesting that the main factor affecting PA is concentration of Tb3+ ions incorporated into coatings. Tb3+ doped TiO2 coatings formed for short time have better PA than pure TiO2 coatings formed under the same conditions; the highest PA is observed for the coating formed after 1 min of PEO process. Presence of Tb3+ ions into TiO2 coatings in small concentration improves PA through effective suppression of electron/hole recombination.

Optical study of the band structure of wurtzite GaP nanowires

We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ8C conduction band edge.

Influence of hydrostatic pressure on the built-in electric field in ZnO/ZnMgO quantum wells

We used high hydrostatic pressure to perform photoluminescence measurements on polar ZnO/ZnMgO quantum well structures. Our structure oriented along the c-direction (polar direction) was grown by plasma-assisted molecular beam epitaxy on a-plane sapphire. Due to the intrinsic electric field, which exists in polar wurtzite structure at ambient pressure, we observed a red shift of the emission related to the quantum-confined Stark effect. In the high hydrostatic pressure experiment, we observed a strong decrease of the quantum well pressure coefficients with increased thickness of the quantum wells. Generally, a narrower quantum well gave a higher pressure coefficient, closer to the band-gap pressure coefficient of bulk material 20 meV/GPa for ZnO, while for wider quantum wells it is much lower. We observed a pressure coefficient of 19.4 meV/GPa for a 1.5 nm quantum well, while for an 8 nm quantum well the pressure coefficient was equal to 8.9 meV/GPa only. This is explained by taking into account the pressure-induced increase of the strain in our structure. The strain was calculated taking in to account that in-plane strain is not equal (due to fact that we used a-plane sapphire as a substrate) and the potential distribution in the structure was calculated self-consistently. The pressure induced increase of the built-in electric field is the same for all thicknesses of quantum wells, but becomes more pronounced for thicker quantum wells due to the quantum confined Stark effect lowering the pressure coefficients.

1D and 2D arrays of coupled photonic crystal cavities with a site-controlled quantum wire light source

We investigated experimentally 1D and 2D arrays of coupled L3 photonic crystal cavities. The optical modes of the coupled cavity arrays are fed by a site-controlled quantum wire light source. By performing photoluminescence measurements and relying on near-field calculation of the cavitiy modes, we evidence optical coupling between the cavities as well as supermode delocalization. In particular, for small cavity separations, fabrication induced disorder effects are shown to be negligible compared to optical coupling between cavities.

Spatially and energetically resolved optical mapping of self-aligned InAs quantum dots

We perform photoluminescence measurements on self-assembled InAs quantum dots which are self-aligned by a deliberately induced underlying misfit dislocation network. Scanning micro photoluminescence measurements allow for a spatial mapping of the structures and reveal different emission energies for quantum dots aligned along [1 1 0] and [−1 1 0] crystal directions.

Electrical and optical properties of self-assembled InAs quantum dots in InP studied by space-charge spectroscopy and photoluminescence

In this paper, we report on a detailed investigation using junction space-charge techniques and photoluminescence of InAs quantum dots embedded in an InP matrix. From measurements of thermal emission rates we have determined the ground-state energy of electrons and holes bound to the InAs dots. In contrast to other dot systems the holes are found to be more strongly confined than the electrons. Corresponding optical emission rates have been measured for holes and the photoionization is found to be well described by a photothermal excitation process similar to what has previously been observed for deep impurities. Furthermore, we have performed photoluminescence measurements revealing excited hole states with energies in good agreement with theory.

Photoluminescence from Si/Si1−xGex single quantum wells in high magnetic fields: localized and free exciton recombination

We have performed photoluminescence measurements on three Si/Si0.76Ge0.24 single quantum wells with well widths of 2.5, 4.8 and 8.5 nm in magnetic fields up to B=20 T. For the excitation and detection of the photoluminescence a large core (0.6 mm) glass fiber was used, which provides sufficient signal intensities at moderate excitation power densities. The magnetic field leads to a luminescence peak splitting and rather large diamagnetic shifts in the wider quantum wells. The increase of the diamagnetic shift with well width indicates a strong reduction of the exciton binding energy, which is consistent with the previously proposed type II band alignment in these quantum wells, and we interpret the peak splitting in terms of localized and free exciton recombination.

FeP precipitates in hydride-vapor phase epitaxially grown InP:Fe

Fe-doped InP was grown by hydride-vapor phase epitaxy. Doping levels up to 8×1018 cm−3 were determined by secondary ion mass spectrometry. Additionally performed photoluminescence measurements revealed a homogeneous distribution of electrically active Fe atoms. From microstructural investigations by analytical transmission electron microscopy, spherical-shaped precipitates were detected in plan-view samples. These precipitates with diameters up to 13 nm are homogeneously arranged in the epilayer. For conglomerates of precipitates a distinct enrichment with Fe and P was measured by a comparative energy dispersive x-ray analysis. The lattice plane distances of the precipitates were deduced from the electron diffraction patterns and from high-resolution electron micrographs. A comparison with calculated values for different Fe–P alloys indicates that the precipitates consist mainly of orthorhombic FeP.

Depth profiles of strain in In0.10Ga0.90As/GaAs multiquantum well structures obtained by variable-pump wavelength photoluminescence

We have performed photoluminescence measurements probing the strain in a set of In0.10Ga0.90As/GaAs multiquantum well (MQW) samples. By using different continuous wave pump laser wavelengths we obtain a depth profile of strain in individual quantum wells within the MQW structure. We also present evidence for the existence of an equilibrium strain between In0.10Ga0.90As quantum well layers and GaAs barrier layers for thick strain-relaxed MQW structures.

Elastic strains in CdTe-GaAs heterostructures grown by metalorganic chemical vapor deposition

The elastic response associated with the lattice mismatch in (100)CdTe∥(100)GaAs heterostructures was investigated by performing photoluminescence measurements as a function of CdTe layer thicknesses. The heterostructures were grown by metalorganic chemical vapor deposition. Estimates of strains, stresses, and lattice constants were obtained from shifts in near-band-edge photoluminescence features. Biaxial compressive strains are present in CdTe layers thinner than 1 μm. The magnitudes of the strains are larger than those expected from equilibrium models and from transmission electron microscopy results. With increasing CdTe layer thicknesses above 0.1 μm biaxial tensile strains affect the GaAs surfaces.

Photoluminescence investigation of the band around 1.41 eV in heated n-GaAs samples

This paper deals with the nature and origin of the ∼1.41-eV emission band observed in the photoluminescence (PL) spectra of n-GaAs samples subjected to diverse treatments. An investigation of the defect center responsible for this emission is carried by performing PL measurements on several samples (n substrates and n/n+ epilayers) subjected to two types of heat treatments (’’preepigrowth’’ and ’’open tube’’) at different temperatures and under varied conditions of surface protection. In light of the different experimental results we propose that the defect under consideration is a complex center formed by the association of arsenic vacancies and Si atoms (SiAs). Also, the existence of this defect in undoped epilayers grown on heavily silicon-doped substrates by the LPE technique, is tentatively explained as due to the contribution of the substrate during the epiprocess.