PL Peak Energy Research Articles

Evidence of hot carrier extraction in metal halide perovskite solar cells

AbstractThe presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl)3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. This results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

Asymmetric and symmetric interfaces in type II MWIR InAs/GaSb superlattices

We report on the comprehensive investigations of InAs/GaSb type-II superlattice (SL) with asymmetric and symmetric interfaces (IFs) formed using a migration-enhanced epitaxy (MEE) and an interrupted growth. The best structural and optical properties were observed for strain-balanced SL with asymmetric IFs grown using MEE at the bottom IF and continuous growth at the upper IF. The analysis of IF-related lines in Raman spectra of investigated SLs allowed for the evaluation of the quality of the InSb-like and GaAs-like interfaces. The thermal dependency of band gap energy was established from photoluminescence measurements according to the Varshni formula. The activation energies E1 and E2 for the dual-channel non-radiative recombination process were calculated. Linear characteristics of PL intensity vs. excitation power density were fitted using the relation IPL ∼ Eexck with different k. This allowed for the indication of dominant recombination mechanisms: radiative with a share of Auger process (k ≈ 0.8), pure radiative (k ≈ 1.0), and radiative with a slight share of Shockley–Read–Hall (k ≈ 1.1). Different behaviours of PL peak energy depending on the increase of excitation power density were observed: it was either blue-shifted, red-shifted, and/or a combination of both.

Radiative emission mechanism analysis of green InGaN/GaN light-emitting diodes with the Si-doped graded short-period superlattice

The optical features of green InGaN/GaN light-emitting diodes (LEDs) with a graded Si-doped short-period InGaN/GaN superlattice (GSL) were examined by photoluminescence spectroscopy (PL). The power dependency of the PL peak energy and the linewidth (FWHM) at low temperatures showed that the Coulomb screening effects of Quantum confined stark effect and localized state filling are dominant processes for low and high excitation power, respectively. An increase in temperature activated the non-radiative centers at low excitation power. The abnormal temperature dependence of the peak energy indicated that the carrier dynamic is changed by the temperature owing to the spatial fluctuations of the In content and subsequent change in the state of carrier localization in InGaN/GaN MQWs. The localization effects were analyzed in the structure by fitting the temperature-dependent emission energy and utilizing a band-tailed model. The emission wavelength of the green InGaN/GaN LED with the GSL was 555 nm at 25 K and was red-shifted with increasing temperature up to 115 K. Then, it becomes blue-shift with further increases in the ambient temperature up to 260 K at low excitation power. The internal quantum efficiency was associated with the conversion from non-radiative to the radiative mechanism and is related to the carriers escaping from the localized state. The measured strain using the high-resolution x-ray rocking curve indicates that the strain of the structure is reduced because of the GSL layer. The strained structure shows a strong localization, and high efficiency. In addition, low power and high thermal energy are needed to enhance the localization effect and efficacy.

Modulated Photoluminescence Mapping of Long-Wavelength Infrared InAs / GaSb Type-II Superlattice: In-Plane Optoelectronic Uniformity

In-plane uniformity of narrow-gap semiconductor $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ type-II superlattice (T2SL) wafer is a crucial yet hard-to-evaluate prerequisite for high-performance long-wavelength infrared optoelectronic device applications of, e.g., focal-plane-array (FPA) photodetectors. In this work, we report a modulated photoluminescence-mapping (PL-mapping) study of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ T2SL in long-wavelength infrared range with a spatial resolution of a typical FPA-pixel scale. Spatial distributions are analyzed of PL-peak energy, linewidth, and integral intensity, which indicate a high in-plane uniformity of effective band gap but a considerable fluctuation of radiative recombination. The in-plane distributions of effective carrier lifetime and Shockley-Read-Hall defect concentration are evaluated, with the aid of a model that takes into account the pumping power dependence of the PL integral intensity. The results reveal a considerable in-plane nonuniformity of the optoelectronic response that may restrict the performance of the derivative FPA photodetector, and indicate the modulated PL mapping of a good pathway particularly for uniformity analysis of long-wavelength infrared FPA semiconductors.

Study on Inter Band and Inter Sub-Band Optical Transitions With Varying InAs/InGaAs Sub-Monolayer Quantum Dot Heterostructure Stacks Grown by Molecular Beam Epitaxy

Multiple stacking of sub-monolayer (SML) quantum dot (QD) heterostructure exhibits high optical quality and is seen in devices like lasers diodes, photodetectors, etc. In this study, we have investigated the optical and material characterization of InAs/InGaAs SML quantum dot (QD) heterostructure with multiple stacking layers (nSML) on GaAs substrates. The experimentally calculated PL emission energies were found to be 1.19, 1.13, 1.11 and 1.12 eV for 4, 6, 8 and 10 QD stacks at 19 K, excitation power of 1.1 kW/cm2 (25 mW) respectively. A feature of increased strain with increasing nSML was verified experimentally by high-resolution X-ray diffraction (HRXRD) and Raman measurements as well. The experimental PL peak energy data were then validated with nextnano++ simulations based on Schrodinger – Poisson device solver. The hydrostatic and biaxial strain components were computed to correlate the experimental and simulation data. Hence with these enunciated understandings, we conclude that an ideal choice on the number of SML stacks that can be grown on a GaAs substrate was found to be 6 stacks, helpful to realize and fabricate QD based infrared photodetectors (QDIPs) devices in long wavelength regime.

Effects of nitrogen incorporation and thermal annealing on the optical and spin properties of GaPN dilute nitride alloys

Here, we report on the structural, optical and magneto-optical properties of as-grown and thermal annealed GaPN thin films containing different N concentrations grown by Solid-Source Molecular Beam Epitaxy on GaP substrates. A wide range of characterization techniques were used to investigate the effect of N incorporation and thermal annealing effect of GaPN/GaP epilayers for a N content between 0.5% and 3%. Our results have shown that as we increase the N content the PL peak energy presents a red shift which is explained by a band gap reduction. However, important effects of exciton localization on optical properties were also evidenced from cw and time resolved PL (PLRT) and magneto-PL. Our results have demonstrated that as the concentration of N is increased, exciton localization effect is enhanced. A substantial improvement of the optical quality and spin properties of the epilayers has been observed after thermal annealing which are significantly more important for samples with higher nitrogen contents.

High photoluminescence quantum yields generated from N-Si-O bonding states in amorphous silicon oxynitride films.

We investigated the high absolute photoluminescence quantum yields (PL QYs) from tunable luminescent amorphous silicon oxynitride (a-SiNxOy) films. The PL QY of 8.38 percent has been achieved at PL peak energy of 2.55 eV in a-SiNxOy systems, which is higher than those of reported nanocrystal-Si embedded silicon nitride films. The existence of N-Si-O bonding states was confirmed by performing FTIR, XPS and EPR measurements. The PL QY is proportional to the concentration of Nx defects, indicating the dominant contribution of luminescent N-Si-O bonding states in radiative recombination processes. Particularly, we precisely monitored the ns-PL lifetimes evolution profile versus detected emission wavelengths, and further verified that the N-Si-O bonding states are responsible for highly efficient PL.

Effect of disorders on the optical properties of excitons in InAsP/InP quantum wells investigated by magneto-photoluminescence spectroscopy

Disorders acquainted in semiconductor quantum structures can change the crystal potential and thus play an important role in the performance of optoelectronic devices. Such disorders are critical in the performance of InAsP/InP quantum well devices, which are useful for telecommunication applications. These disorders originate due to the difference in adsorption and the sticking coefficient of arsenic and phosphorus. In our previous paper [J. Appl. Phys.119, 095708 (2016)JAPIAU0021-897910.1063/1.4943031], we reported the impact of carrier localizations on the performance of InAsP/InP quantum well detectors. In this work, the magnetic field is used to investigate exciton properties in InAsP/InP quantum wells. Line shape analysis of magneto-photoluminescence (magneto-PL) distinguishes the role of disorders on the excitonic properties that are useful for the identification of sample quality. Furthermore, the blue shift in PL peak energy with an applied magnetic field is used to estimate the effective mass of excitons in InAsP/InP quantum wells. The estimated effective mass of excitons confined in the quantum well is significantly higher than that of the corresponding bulk value, which is explained by considering the non-parabolicity of the conduction band. The non-parabolicity parameter is also estimated and used for obtaining the electron dispersion E-k curve for InAsP/InP quantum wells.

Revealing mechanisms of PL properties at high and low temperature regimes in CdSe/ZnS core/shell quantum dots

Here, we report on the strong temperature dependence of PL properties in CdSe/ZnS core/shell quantum dots, within a broad temperature range of 80–500 K. Our results demonstrate different PL quenching mechanisms in high and low temperature ranges. In the low temperature range of below 320 K, PL quenching is proved to be dominated by thermally activated carrier escape. While in the high temperature range of above 320 K, PL undergoes additional quenching because of surface trapping by the thermally created surface defects, which is calculated and proved by the rapidly decreasing PL lifetime. Our calculating result proves that the model of thermally activated carrier escape is also responsible for PL quenching in the high temperature range. However, the red-shifted PL peak energy and the broadened FWHM with increasing temperature follow the same mechanisms in the whole temperature range. The former is due to the temperature-dependent band-gap shrinkage and the latter is due to the exciton scattering with acoustic and longitudinal-optical phonons.

Effect of piezoelectric field on type II transition in InAlAs/InP (311) alloys with different substrate polarity

A ternary Indium-Aliminium-Arsenic (InAlAs) is grown, by metal organic chemical vapor deposition (MOCVD), on a substrate InP (311) with polarity A and B, doped with Fer (Fe) atomic. Their optical properties have been studied by photoluminescence spectroscopy (PL) in the whole temperature range from 10 to 300 K. At low temperature region, power dependent analysis of the PL intensity reveals a conventional power law with an exponent n close to one, in agreement with the type II nature of the emission. A red shift of the type II transition has been observed from the sample with polarity A to B. The temperature-dependent PL measurements exhibit abnormal behaviors such as an S-shape in PL-peak energy, and an inverted N-shape in the full width at half maximum (FWHM). These atypical behaviors are linked to the exciton localization phenomenon, which it is more pronounced in the structure with polarity A of substrate orientation. Room temperature-PL spectra covers the both emissions of the 1.3 and the 1.55 μm for (311) A and (311) B InP oriented substrates, respectively. Our results indicate that the In0.513Al0.487As/InP (311) A/B is expected to be advantageous for their application in optical telecommunication.

Anomalous temperature dependence of photoluminescence spectra from InAs/GaAs quantum dots grown by formation–dissolution–regrowth method**Project supported by the National Basic Research Program of China (Grant No. 2013CB632104) and the National Key Research and Development Program of China (Grant No. 2016YFB0402302).

Two kinds of InAs/GaAs quantum dot (QD) structures are grown by molecular beam epitaxy in formation–dissolution–regrowth method with different in-situ annealing and regrowth processes. The densities and sizes of quantum dots are different for the two samples. The variation tendencies of PL peak energy, integrated intensity, and full width at half maximum versus temperature for the two samples are analyzed, respectively. We find the anomalous temperature dependence of the InAs/GaAs quantum dots and compare it with other previous reports. We propose a new energy band model to explain the phenomenon. We obtain the activation energy of the carrier through the linear fitting of the Arrhenius curve in a high temperature range. It is found that the GaAs barrier layer is the major quenching channel if there is no defect in the material. Otherwise, the defects become the major quenching channel when some defects exist around the QDs.

Bandgap tuning in GaAs1−xSbx axial nanowires grown by Ga-assisted molecular beam epitaxy

In this work we present a comprehensive study on the effects of Sb incorporation on the composition modulation, structural and optical properties of self-assisted axial GaAs1−xSbx nanowires of 2–6 μm in length grown on (111) Si substrate by molecular beam epitaxy. The Sb composition in the GaAs1−xSbx axial nanowire (NW) was varied from 2.8–16 at.%, as determined from energy dispersive x-ray spectroscopy. Lower Sb composition leads to thinner nanowires and inhomogeneous Sb composition distribution radially with a depleted Sb surface region inducing weak type-II optical emission, the presence of an additional peak at higher Bragg angle in the x-ray diffraction spectra and an electric-field-induced strong Raman LO mode. Higher Sb composition of 16 at.% leads to a more uniform Sb compositional distribution radially leading to type-I optical transitions exhibiting the lowest PL peak energy occurring at 1.13 eV. In addition, the high quality of these nanowires exhibiting pure zinc blende crystal structure, largely free of any planar defects, is borne out by high resolution transmission electron microscopy and selected area diffraction patterns. The shift and broadening of the Raman LO and TO modes reveal evidence of increased Sb incorporation in the nanowires. Significant improvement in optical characteristics was achieved by the incorporation of a Al0.2Ga0.8As passivating shell. The results are very promising and reveal the potential to further red shift the optical emission wavelength by fine tuning of the fluxes during growth.

Thermal conductivity of suspended single crystal CH3NH3PbI3 platelets at room temperature.

Recently, organic-inorganic lead halide perovskites have gained great attention for their breakthrough in photovoltaic and optoelectronics. However, their thermal transport properties that affect the device lifetime and stability are still rarely explored. In this work, the thermal conductivity properties of single crystal CH3NH3PbI3 platelets grown by chemical vapor deposition are studied via non-contact micro-photoluminescence (PL) spectroscopy. We developed a measurement methodology and derived expressions suitable for the thermal conductivity extraction for micro-sized perovskites. The room temperature thermal conductivity of ∼0.14 ± 0.02 W m-1 K-1 is extracted from the dependence of the PL peak energy on the excitation laser power. On changing the film thickness from 80 to 400 nm, the thermal conductivity does not show noticeable variations, indicating the minimal substrate effects due to the advantage of the suspended configuration. The ultra-low thermal conductivity of perovskites, especially thin films, suggests their promising applications for thermal isolation, such as thermal insulation and thermo-electricity.

Carrier localization in In0.21Ga0.79As/GaAs multiple quantum wells: A modified Pässler model for the S-shaped temperature dependence of photoluminescence energy

The optical properties of In0.21Ga0.79As/GaAs MQWs, with triple unequal layer thickness NW (3 nm), MW (6 nm) and WW (9 nm) grown on (001) and (113) GaAs substrates, is studied by using continuous wave photoluminescence (cw-PL) spectroscopy. A comparative study has been performed to demonstrate the influence of electric field and QW thickness on the exciton localization. An S-shaped form in temperature-dependent PL peak energy has been observed in polar middle QW (MW (113)) but not seen in non-polar ones (MW (001)). This behavior is linked to carrier localization in triangular potential and polarity. We have observed also this atypical evolution in non-polar wide QW (WW (001)) but not in non-polar middle QW (MW (001)), which is attributed to potential fluctuation in larger ones. With the aid of modified Pässler model for including the effect of localized states, we can persuasively reproduce the S-shaped temperature dependence of PL band gap energy and contribute to the estimated value of exciton localization energy. The values of σ are obtained from adjustment of experimental points, which indicate the degree of localization in QW layer.

Size-controlled nc-Si:H/a-SiC:H quantum dots superlattice and its application to hydrogenated amorphous silicon solar cells

Boron doped nc-Si:H/a-SiC:H quantum dot superlattice (QDSL) has a great potential to improve thin film silicon solar cells performance for high conductivity, wide band gap and anti-reflection effect. In this study p-type nc-Si:H/a-SiC:H QDSL has been fabricated by in situ grown method without subsequent annealing treatment. High resolution transmission electron microscopy and PL peak energy shift indicate that this method provides the possibility to precisely control the size of silicon quantum dots and the passivation at well/barrier interface. By optimizing structural characteristics and interface passivation, high perpendicular conductivity and strong anti-reflection effect was simultaneously obtained in QDSL films. An initial efficiency of 10.5% was achieved for n-i-p type hydrogenated amorphous silicon solar cell, which may guide further efforts arising the structure engineering of nc-Si:H/a-SiC:H QDSL for high efficient solar cells.

Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2[Formula: see text]off miscut angle toward the [110] direction (2[Formula: see text]F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Pässler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Localized state exciton model investigation of B-content effect on optical properties of BGaAs/GaAs epilayers grown by MOCVD

In this work, BxGa1−xAs/GaAs epilayers with three different boron compositions were elaborated by Metal Organic Chemical Vapor Deposition (MOCVD) on GaAs (001) substrate. The boron fraction has been estimated using High resolution X-ray diffraction (HRXRD) spectroscopy. Using the photoluminescence PL spectroscopy, the emission energy as well as the PL intensity was carried out as functions of temperature in the range 10–300 K. The temperature dependence of the PL peak energy has shown an S-shaped behavior. It is a result of the competition process between localized and delocalized states induced by the boron clusters. We have shown that the localization effects increase by increasing boron composition and then drops for high compositions (up to 8%). Qualitative and quantitative interpretations based on the Localized States Exciton (LSE) parameters evolution in order to explain the unusual photoluminescence behavior of the two luminescence keys. At high temperatures, the model can be approximated to the band-tail-state emission. The LSE model succeeds in accurately reproducing of the atypical PL peak emission and normalized intensity of BGaAs/GaAs epilayers.

Characteristics of InGaAsBi with various lattice mismatches on InP substrate

To develop bismuth-containing infrared optoelectronic devices, InGaAsBi/InP films with different lattice mismatches have been investigated. The lattice mismatch was tailored by changing the Bi content in conjunction with the In content simultaneously. X-ray diffraction analysis revealed that alloy lattice constants have been extended positively by incorporation of Bi into the crystal lattice. Electrical and optical characteristics were investigated by Hall-effect, optical absorption and photoluminescence measurements. A bandgap shrinking of about 56.4 meV/Bi% was deduced by X-ray diffraction and optical absorption measurements. From the excitation dependent photoluminescence measurement at 10 K, the donor-acceptor pair emissions were inferred for samples containing moderate and high levels of Bi. The temperature dependence of the PL peak energy is as small as 0.06 meV/K in In0.5Ga0.5As0.987Bi0.013, which is fairly low compared with that of In0.5Ga0.5As.

Fabrication of CdSe nanocrystals using porous anodic alumina and their optical properties

In this paper, Porous anodic alumina (PAA) template with highly ordered nanopores structure was synthesized on aluminum foils by two step anodization process. PAA template has hexagonal pores with average size between 30 and 180nm. L-cysteine (L-Cys) functionalized cadmium selenide nanocrystals (CdSe NCs) were successfully embedded inside PAA layers by simple immersion in aqueous solution. The effect of pore diameter enlargement on the microstructure of CdSe NCs/PAA films was systematically studied by FE-SEM, XRD, EDX, Raman, UV–VIS absorbance and PL analysis. FE-SEM microscopy was used to investigate the surface morphology of PAA templates before and after CdSe NCs deposition. XRD investigation demonstrates that CdSe NCs into PAA templates were cubic in nature with zinc-blende structure. Raman measurements exhibit the characteristic modes of CdSe on the PAA layers as well as the films crystallinity as function of widening pores diameter. Optical properties of deposited CdSe NCs on PAA templates have been investigated using optical absorption and PL techniques. Photoluminescence spectroscopy has been used to determine the bandgap energy and the average size of CdSe NCs deposited on PAA layer. This method involves fitting the experimental spectra, using a model based on quantum confinement of electrons in CdSe nanocrystals having spherical and cylindrical forms (Quantum Dots (QDs) and Quantum Wires (QWs)). This model allows correlation between the PL spectra and the microstructure of the CdSe/PAA. Both photoluminescence and optical absorption show that the PL peak energy and the optical absorption edge of CdSe NCs/PAA exhibit similar behavior with changes in nanostructure size. The spectral behaviors of optical absorption and PL are consistent with a quantum confinement model throughout the sizes and shapes of the CdSe nanocrystals of the luminescent films. The effective bandgap energies determined from the PL peaks position are in good agreement with those estimated from the optical absorbance spectra.Investigations have shown that optical properties of CdSe/PAA nanostructure were influenced by the pores sizes of PAA. It was observed an increase in CdSe NCs size from 2.22 to 2.56nm when the average pores diameter of PAA increases from 30 to 180nm. This finding indicates an enhancement in PL intensity and a red-shift in PL emission peaks from 2.35 to 2.14eV. By applying the quantum confinement model, we demonstrated that the redshift of PL peak is attributed to the change of CdSe NCs size with the pores diameter and that their spectral behaviors are related to the shape and the size distribution of the nanoparticles.

Piezoelectric field effect on the optical properties of In0.21Ga0.79As/GaAs (113) MQW

Photoluminescence study PL has been performed for the In0.21Ga0.79As multiple quantum wells MQW grown by molecular beam epitaxy MBE on (001) and (113) A GaAs substrates. The electronic structure was obtained by solving the Schrödinger equation, including piezoelectric field and strain effect on the conduction and valence bands of the unequal QWs. We critically review the explanation of S-shape in temperature dependence of PL peak energy for polar Middle In0.21Ga0.79As QW at intermediate temperatures. This abnormal behavior is merely linked to the impact of carrier localization and polarization-induced electric fields in optical properties. A significant blue shift of 18 meV for polar and a negligible shift for non-polar In0.21Ga0.79As/GaAs Middle QW has been observed. In order to follow the evolution of the PL peak energies for each QW in both samples versus temperature, three theoretical models (Varshni, Vïna and Pässler) have been reported. A comparison between theoretical and experimental data demonstrates that the Pässler model is the most accurate fit despite none of the classical models can replicate the excitonic PL energy evolution at cryogenic temperature for Middle QW in the structure grown on (113).