PL Peak Energy Research Articles

Type-II CdS/ZnSe core/shell heterostructures: UV–vis absorption, photoluminescence and Raman scattering studies

Abstract This work presents the fabrication of type-II CdS/ZnSe core/shell heterostructures based on the wet chemical method in octadecene (ODE) solvent. The fabrication processes were monitored by spectrometric techniques of UV–vis absorption, photoluminescence (PL) and Raman scattering (RS). We point out that CdS cores are necessary to be grown at high temperatures of 290–310 °C to improve their crystal quality. Meanwhile, the ZnSe-shell growth is needed to be carried out at low-enough temperatures, and the injection speed of the precursors CdS cores, and Zn2+ and Se2− ions into the reaction solvent is as swift as possible. These conditions are necessary to restrict the dissolution of CdS cores during the ZnSe shell growth, and the formation of the other structure types, such as CdS/CdSe or CdS/(Cd,Zn)–(S,Se)/ZnSe. We also suggest typical signatures to identify the successful fabrication of type-II CdS/ZnSe core/shell heterostructures, which are based on the tail features of UV–vis absorption spectra, the linear dependence of the PL-peak energy on a cubic root of the excitation power, and long lifetimes of emission peaks. Besides these signatures, the study of phonon-vibration spectra is also necessary. Having studied the dependences of emission energy on excitation energy, we found an optimal thickness of the ZnSe shell coated on CdS cores. Apart from this thickness, the spatial separation of the carriers between the CdS core and the ZnSe shell will be significantly declined.

Improved thermal stability and narrowed line width of photoluminescence from InGaN nanorod by ytterbium doping

AbstractNanorod of in situ Yb‐doped InGaN and undoped InGaN have been grown on (0001) sapphire substrates by plasma assisted molecular beam epitaxy (MBE). Selected regions on Yb‐doped InGaN sample show single dominant near band edge emission (NBE) in green, yellow or orange color due to the variation of In content. Temperature dependent PL peak energy of InGaN nanorod shows the characteristic S ‐shaped behavior indicating the presents of strong exciton localization energy in undoped InGaN nanorod. The exciton localization energy reduced significantly after incorporating Yb into InGaN, giving rise to damping of the S‐shape profile amplitude and narrowing of the PL line width from ∼20 meV to ∼12 meV at 11 K. It is proposed that the improved PL thermal stability and the PL line width in Yb‐doped InGaN nanorod is affected by the Yb gettering effect. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Characterization of Carrier Localization, Carrier Transportation and Carrier Recombination in Blue-Emitting InGaN/GaN MQWs

Carrier localization, transportation and recombination in blue-emitting InGaN/GaN multiple quantum wells were analyzed using temperature-dependent photoluminescence spectroscopy, confocal laser scanning microscopy and time-resolved photoluminescence (TRPL). The temperature-dependent shift of PL intensity was fitted with Arrhenius equation and explained using two non-radiative channels, which are related with thermal activation of carriers from different confining potentials. The S-shaped shift of PL peak energy and inverse-S-shaped shift of PL full width at half maximum were explained with carrier localization and carrier transportation. The TRPL spectra taken at several different places from bright region to dark region in the confocal microscopic image showed that the fast decay life-time τ1 increases with decreasing PL intensity, indicating a higher carrier transportation rate at bright region, while the slow decay life-time τ2 decreases with decreasing PL intensity, indicating a higher probability of non-radiative recombination at dark region.

Correlation between optical properties surface morphology of porous silicon electrodeposited by Fe3+ ion

In this paper, we analyze the photoluminescence spectra (PL) of porous silicon (PS) layer which is elaborated by electrochemical etching and passivated by Fe3+ ions (PSF) via current density, electro-deposition and temperature measurements. We observe unusual surface morphology of PSF surface and anomalous emission behavior. The PSF surface shows regular distribution of cracks, leaving isolated regions or “platelets” of nearly uniform thickness. These cracks become more pronounced for high current densities. The temperature dependence of the PL peak energy (EPL) presents anomalous behaviors, i.e., the PL peak energy shows a successive red/blue/redshift (S-shaped behavior) with increasing temperature that we attribute to the existence of strong potential fluctuations induced by the electrochemical etching of PS layers. A competition process between localized and delocalized excitons is used to discuss these PL properties. In this case, the potential confinement plays a key role on the enhancement of PL intensity in PSF. To explain the temperature dependence of the PL intensity, we have proposed a recombination model based on the tunneling and dissociation of excitons.

Anomalous temperature-dependent photoluminescence peak energy in InAlN alloys

InAlN has been studied by means of temperature-dependent time-integrated photoluminescence and time-resolved photoluminescence. The variation of PL peak energy did not follow the behavior predicted by Varshni formula, and a faster redshift with increasing temperature was observed. We used a model that took account of the thermal activation and thermal transfer of localized excitons to describe and explain the observed behavior. A good fitting to the experiment result is obtained. We believe the anomalous temperature dependence of PL peak energy shift can be attributed to the temperature-dependent redistribution of localized excitons induced by thermal activation and thermal transfer in the strongly localized states. V-shaped defects are thought to be a major factor causing the strong localized states in our In0.153Al0.847N sample.

Optical properties of diamond-like carbon thin films deposited by DC-PECVD

Diamond-like carbon (DLC) thin films were deposited at different substrate temperatures using methane gas in DC-PECVD at 1x10-1 Torr. From the lighttransmission using UV-VIS spectroscopy it was found that the optical transition had changed from allowed indirect transition to forbidden directtransition as the substrate temperature increased. The photoluminescence study has shown that the recombination centre occurred at 1.738 eV. Theobservation in the present work shows that the Urbach energy decreases with increasing substrate temperature provides evidence that such films have anarrowed band tail. This decrease in Urbach energy is due to a more ordered structure with lower hydrogen concentration and larger cluster by theincrease in the degree of cross-linking of the C-C network. Distorted cluster which contribute to the broadening of the band tail may also contribute to thePL peak width. As the substrate temperature increases, the cluster size in the tetrahedral hydrogenated amorphous carbon (ta-C:H) increases, resulting insmaller band gap and the structure is more ordered as the decrease in Urbach energy, give the decrease in PL peak energy.

(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

InGaN/GaN MQWs grown on c-plane sapphire substrate show high internal quantum efficiency, regardless of their high threading dislocation density (108 cm-2~ 1010 cm-2) that is introduced by lattice mismatch. This high-IQE phenomenon is attributed to bandgap energy fluctuation and carrier localization in the QW layer [1].In the present work, temperature-dependent PL, confocal microscopy, and nanometer-scale time-resolved photoluminescence (TRPL) spectroscopy were employed to analyze the carrier localization, carrier transportation and carrier recombination processes in blue-emitting InGaN/GaN MQWs.The temperature-dependent PL spectra were taken from 10 K to 300 K. The decreased spectrum-integrated PL intensity with increasing temperature was observed, which was fitted with second-order Arrhenius equation, as shown in Fig. 1. The fitting indicated two non-radiative channels: the one with high activation energy (dominated at high temperature) corresponded to thermal activation of carriers out of the strongly localized states, while the one with low activation energy (dominated at low temperature) corresponded to thermal activation of carriers out of the weakly localized states. The S-shaped shift of PL peak energy and the inverse S-shaped shift of PL FWHM were also observed, as shown in Fig. 2 and Fig. 3, which were explained with carrier localization and carrier dynamics.The confocal microscope image collecting near bandedge emission showed inhomogeneous PL intensity distribution, as shown in Fig. 4. Previous analysis [2] has shown that the bright region had smaller bandgap energy than dark region did. Nanometer-scale TRPL spectra were taken at different places in the confocal microscope image. As shown in Fig. 5, TRPL spectra taken at the center of bright region and the center of dark region show single exponential decay, which are characterized by a single lifetime and corresponds to carrier localization in strongly localized state and weakly localized state, respectively. Comparing with bright region, dark region has shorter lifetime, indicating a higher probability of nonradiative recombination and carrier transportation from dark region to bright region. TRPL spectrum taken at the boundary of bright region, however, shows double exponential decay, as shown in Fig. 5, which is characterized by two lifetimes. The short lifetime corresponds to carrier transportation from weakly localized state to strongly localized states, while the long lifetime corresponds to carrier recombination at local strongly localized state.

(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

Carrier localization, transportation and recombination in blue-emitting InGaN/GaN multiple quantum wells were analyzed using temperature-dependent photoluminescence spectroscopy, confocal laser scanning microscopy and time-resolved photoluminescence (TRPL). The temperature-dependent shift of PL intensity was fitted with Arrhenius equation and explained using two non-radiative channels, which are related with thermal activation of carriers from different confining potentials. The S-shaped shift of PL peak energy and inverse-S-shaped shift of PL full width at half maximum were explained with carrier localization and carrier transportation. The TRPL spectra taken at several different places from bright region to dark region in the confocal microscopic image showed that the fast decay lifetime τ1 increases with decreasing PL intensity, indicating a higher carrier transportation rate at bright region, while the slow decay lifetime τ2 decreases with decreasing PL intensity, indicating a higher probability of non-radiative recombination at dark region.

Epitaxial two-dimensional nitrogen atomic sheet in GaAs

We have grown an epitaxial two-dimensional nitrogen (N) atomic sheet in GaAs by using the site-controlled N δ-doping technique. We observed a change of the electronic states in N δ-doped GaAs from the isolated impurity centers to the delocalized impurity band at 1.49 eV with increasing N-doping density. According to the excitation-power- and temperature-dependent photoluminescence (PL) spectra, the emission related to localized levels below the impurity band edge was dominant at low excitation power and temperature, whereas the effects of the localized levels can be neglected by increasing the excitation power and temperature. Furthermore, a clear Landau shift of the PL-peak energy was observed at several Tesla in the Faraday configuration, in contrast to the case in the impurity limit.

Effect of boron incorporation on the structural and photoluminescence properties of highly-strained InxGa1-xAs/GaAs multiple quantum wells

In this research, 5-period highly-strained BInGaAs/GaAs multiple quantum wells (MQWs) have been successfully grown at 480-510ºC by LP-MOCVD. Room-temperature photoluminescence (RT-PL) measurements of BInGaAs/GaAs MQWs showed the peak wavelength as long as 1.17 μm with full-width at half maximum (FWHM) of only 29.5 meV. In addition, a slight blue-shift (∼18 meV) of PL peak energy of InxGa1-xAs/GaAs MQWs was observed after boron incorporation. It has been found boron incorporation (<2%) was mainly dominated by a negative effect even though it can effectively reduce the compressive strain, i.e., it degraded the structural properties and PL efficiency of MQWs containing relatively less indium. Only at the lowest temperature growth (i.e., indium content x > 40%), the positive effect of boron incorporation prevailed, i.e., boron incorporation completely suppressed the thickness undulation and lead to the improvement of PL properties.

Evolution of superclusters and delocalized states in GaAs1−xNx

The evolution of individual nitrogen cluster bound states into an extended state infinite supercluster in dilute GaAs${}_{1\ensuremath{-}x}$N${}_{x}$ was probed through temperature and intensity-dependent, time-resolved and magnetophotoluminescence (PL) measurements. Samples with compositions less than 0.23$%$ N exhibit PL behavior that is consistent with emission from the extended states of the conduction band. Near a composition of 0.23$%$ N, a discontinuity develops between the extended state PL peak energy and the photoluminescence excitation absorption edge. The existence of dual localized/delocalized state behavior near this composition signals the formation of an N supercluster just below the conduction band edge. The infinite supercluster is fully developed by 0.32$%$ N.

Influence of growth temperature and deposition duration on the structure, surface morphology and optical properties of InN/YSZ (1 0 0)

InN films with the wurtzite structure have been grown directly on YSZ (100) substrate by the RF-magnetron sputtering technique. Strongly (002) oriented films with smooth surfaces (0.7–2.9nm surface roughness depending on substrate temperature), were grown within 30min. Films deposited for 60min developed three-dimensional (3D) pyramidal islands on top of their surfaces, which diminished the residual elastic strain. The optical absorption edge and PL peak energy around 1.7eV were found to redshift with increasing film thickness and substrate temperature.

Analysis of confinement potential fluctuation and band-gap renormalization effects on excitonic transition in GaAs/AlGaAs multiquantum wells grown on (1 0 0) and (3 1 1)A GaAs surfaces

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/AlxGa1−xAs quantum wells grown on [100] and [311]A GaAs substrates is evaluated. The results clearly demonstrate the coexistence of the band-tail states filling related to potential fluctuations and the band-gap renormalization caused by an increase in the density of photogenerated carriers during the photoluminescence (PL) experiments. Both phenomena have strong influence on temperature dependence of the PL-peak energy (EPL(T)). As the photon density increases, the EPL can shift to either higher or lower energies, depending on the sample temperature. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density. A simple band-tail model with a Gaussian-like distribution of the density of state was used to describe the competition between the band-tail filling and the BGR effects on EPL(T).

Low‐temperature photoluminescence studies of In‐rich InAlN nanocolumns

AbstractHigh‐quality Inx Al1–xN (0.71 ≤ xIn ≤ 1.00) nanocolumns (NCs) have been grown on Si(111) substrates by rf‐plasma‐assisted molecular‐beam epitaxy (rf‐MBE). Low‐temperature photoluminescence (LT‐PL) spectra of various In‐rich InAlN NCs were measured at 4 K and single peak PL emissions were observed in the wavelength region from 0.89 µm to 1.79 µm. Temperature‐dependent PL spectra of In0.92Al0.08N NCs were studied and the so‐called “S‐shape” (decrease–increase–decrease) PL peak energy shift was observed with increasing temperature. This shift indicates the carrier localization induced by the In segregation effect and is different from the anomalous blue shift frequently observed in InN films and nanowires with high residual carrier concentra‐ tions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the light emission of Si nanocrystals embedded in SiO2

Abstract We present results on the light emission of Si nanocrystals embedded into a SiO2 matrix obtained via ion implantation. As measurement temperature increases, the main photoluminescence peak shows a shift which is consistent with recombination involving confined electronic states of the nanoparticles around 150 – 170 K when the emission shows a sudden blueshift. After that, the usual energy redshift as temperature increases follows again. It was showed before that silicon nanoparticles light emission present a coexistence of localized states (surface or interface, that dominate the emission spectra at room temperature) and confined states (quantum confinement, dominating the low temperature range). In this work we investigate whether this anomaly in the nanoparticle PL peak energy as a function of temperature might be related to some of these localized states.

Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3)A substrate

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.

Investigation of optical and compositional properties of thin SiN x:H films with an enhanced growth rate by high frequency PECVD method

Hydrogenated thin silicon nitride (SiN x:H) films were deposited by high frequency plasma enhanced chemical vapor deposition techniques at various NH 3 and SiH 4 gas flow ratios [R = NH 3/(SiH 4 + NH 3)], where the flow rate of NH 3 was varied by keeping the constant flow (150 sccm) of SiH 4. The deposition rate of the films was found to be 7.1, 7.3, 9 and 11 Å/s for the variation of R as 0.5, 0.67, 0.75 and 0.83, respectively. The films were optically and compositionally characterized by reflectance, photoluminescence, infrared absorption and X-ray photoelectron spectroscopy. The films were amorphous in nature and the refractive indices of the films were varied between 2.46 and 1.90 by changing the gas flow ratio during the deposition. The PL peak energy was increased and the linear band tails become broad with the increase in R. The incorporation of nitrogen takes place with the increase in R.

Carrier-concentration dependent photoluminescence of InAsN films grown by RF-MBE

The observation of bandgap reduction in InAsN by optical absorption is often somewhat obscure due to the band-filling effect caused by the high electron concentration associated with N incorporation. In this report, the detailed photoluminescence properties of 1.6-μm-thick InAsN films grown on semi-insulating GaAs(001) substrates by RF-MBE are investigated in relation with the carrier concentration. The PL peak energy at 10K clearly exhibits a redshift with increase in N content up to 1.84%, whereas it tends rapidly to a blueshift for N contents above 2.39%. The redshift is consistent with the bandgap reduction caused by the N incorporation, whereas the blueshift is attributed to the band-filling effect caused by the highly concentrated degenerate electrons. Moreover, it is found that the temperature dependent PL and the excitation power dependent PL are largely affected by carrier concentration and the potential fluctuations due to the non-uniform incorporation of N into the host. The bowing parameter c estimated from fitting with the quadratic curve, (1−x)EG,InAs+xEG,InN-c(1−x)x, is 2.2eV.

Formation and photoluminescence of Si quantum dots in SiO 2/Si 3N 4 hybrid matrix for all-Si tandem solar cells

In this work, Si quantum dots in SiO 2/Si 3N 4 hybrid matrix on quartz substrates were synthesized by magnetron sputtering of alternating silicon rich oxide and Si 3N 4 layers followed by different post-deposition anneals. XRD results indicate that the average dimension of the Si nanocrystals varies from 1.6 to 5.2 nm. The size and crystallization of the Si nanocrystals are dependent on a number of factors, including the annealing method, the SRO thickness and the Si 3N 4 barrier thickness, as evidenced in XRD and Raman measurements. In particular, thicker Si 3N 4 barrier layers seem to be able to suppress the growth of Si nanocrystals more effectively. PL measurements suggest the apparent bandgap of the samples investigated in this work is in the range 1.12–1.67 eV, which demonstrates the effect of quantum confinement. More interestingly, analysis of the PL data using the modified EMA equations clearly suggests that the PL peak energy not only depends on the size of the nanocrystals but also gets affected by other details in nanocrystal growth. A tentative core–shell model is constructed to illustrate our explanation. These findings offer a preliminary understanding of the nanocrystal growth and radiative recombination processes in this newly synthesized material for photovoltaic applications.

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10 10 cm −2 at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH 3 flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy ( V In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.