Exciton Localization Effect Research Articles

Effects of strain-compensated AlGaN/InGaN superlattice barriers on the optical properties of InGaN light-emitting diodes

In this article, metalorganic chemical vapor deposition (MOCVD)-grown InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with Al0.03Ga0.97N and Al0.03Ga0.97N/In0.01Ga0.99N superlattices-barrier layers on c-plane sapphire were studied for the influence of the strain-compensated barrier on the optical properties of the LEDs. High-resolution X-ray diffraction (HRXRD) analysis shows that the LEDs with a strain-compensated superlattice barrier (SC-SLB) have better interface quality than those using AlGaN. This difference in quality may result from the alleviation of strain relaxation in superlattice layers to improve the crystalline perfection of the epitaxial structures. It was also found that the degree of the exciton localization effect rises considerably as InGaN grows directly on the AlGaN barrier layers. However, the increase in the strength of the polarization fields within the MQWs (as evaluated from bias-dependent photoluminescence (PL) measurement) could reduce the radiative efficiency of the LEDs and shift their PL peaks toward long wavelengths. With suitable control of crystalline quality and the reduced quantum-confined Stark effect in the MQWs, the SC-SLB LEDs operating at 150-mA-current show a 22.3% increase in light output power as compared to their conventional counterparts.

Temperature insensitivity of the threshold current density in exciton-dominated wide energy gap quantum wire lasers

We present simulations on temperature dependence of the threshold current density (J_th) in InGaN-AlGaN and ZnCdSe-ZnMgSSe quantum wire lasers having exciton transitions. We find that J_th in a quantum wire laser is insensitive to temperature variation when exciton binding energies are in the range of 30 to 50 meV. This behavior is similar to quantum dot lasers having free carrier transitions. We show that the temperature dependence of the threshold current density is greatly reduced in lasers having exciton transitions in comparison to the ones having free carrier transitions. The reduced dependence of J_th on the temperature is attributed to the inherent confinement of electron-hole pairs within the exciton radius along the wire axis, whereas the quantum wire cross section provides confinement of carriers in the plane perpendicular to the wire axis. The large exciton binding energies give rise to high exciton density of states and narrow spectral line width, thus confining the carriers into a pseudoquantum dot-like structure. The effect of exciton localization due to the randomness in quantum wire fabrication, causing exciton inhomogeneous line broadening, is also discussed.

Competing exciton localization effects due to disorder and shallow defects in semiconductor alloys

We demonstrate that excitons in semiconductor alloys are subject to competing localization effects due to disorder (random potential fluctuations) and shallow point defects (impurities). The relative importance of these effects varies with alloy chemical composition, impurity activation energy as well as temperature. We evaluate this effect quantitatively for MgxZn1−xO:Al (0⩽x⩽0.058) and find that exciton localization at low (2 K) and high (300 K) temperatures is dominated by shallow donor impurities and alloy disorder, respectively.

10.1007/s11453-008-2011-z

Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev’s model and that, in the active region of the structures under study, the dispersion σ of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy E U, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that σ and E U, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.

The Effect of Exciton Localization on the Photoluminescence Properties of In0.01Al0.11Ga0.88N

not Available.

Growth study of thin indium nitride layers on InP (1 0 0) by Auger electron spectroscopy and photoluminescence

This article investigates the growth of InN layers on (100) InP in ultra-high vacuum using a glow discharge source (GDS). Auger electron spectroscopy (AES) was used to understand the different steps of the nitridation process with the analysis of In-MNN, N-KLL and P-LMM transitions. A modeling of Auger signals using a stacked layers model allows us to confirm that four monolayers of indium nitride are created on (100) InP.InN layers grown on (100) InP were studied optically by photoluminescence (PL) spectroscopy versus the excitation power and the sample temperature (10–300K). Results show broad spectral band energy close to the lowest reported InN bandgap.The temperature dependence of the PL peak energy showed a S-shaped behavior (decrease–increase–decrease). The results suggest that the InN-related emission is significantly affected by the change in carrier dynamics with increasing temperature: the effect can be due to the large exciton localization effects.

Exciton localization effect in Mn-implanted GaN by photoluminescence measurements

We investigated the temperature-dependent and excitation power-dependent photoluminescence spectra of Mn-implanted (Ga,Mn)N samples with five Mn-implantation doses. The near-band-energy emission was observed and was attributed to the Mn-related exciton transition, which exhibits localized exciton behaviour resulting from alloy potential fluctuations and demonstrates a special temperature-dependence characteristic of alloy disorder. In terms of the integrated photoluminescence intensity as a function of temperature, the activation energy of the localized exciton was obtained. All these results show strong dependence on the Mn concentration of (Ga,Mn)N epilayers.

Time Resolved Magnetophotoluminescence of Biased GaAs/AlGaAs Double Quantum Well Structure

Time resolved photoluminescence of double quantum well structure was investigated versus electric and magnetic fields applied across the sample. The emission due to direct excitons (electron and hole are localized within the same quantum well) decays fast at the nanosecond timescale, whereas the recombination kinetics of indirect excitons is much slower and spreads over microseconds. The time evolution of indirect exciton emission is shown to be altered by application of either electric or magnetic field. This reflects the non-trivial effects of exciton localization which leads to the nonexponential decays of the indirect exciton emission.

Photoluminescence of localized excitons in InGan quantum dots

Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev's model and that, in the active region of the structures under study, the dispersion {sigma} of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy E{sub U}, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that {sigma} and E{sub U}, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.

Characterization of interface fluctuations and emission mechanisms in InGaN/AlGaN multiple quantum wells

The InGaN/AlGaN multiple- quantum- well heterostructures were fabricated by metal-organic chemical vapor deposition system. Samples were grown with different indium and aluminum content during the growth of InGaN well layers and AlGaN barrier layers. The exciton-localization effect on the performance of the samples is investigated by means of temperature-dependent photoluminescence measurements. The dependence of optical characteristics on temperature is consistent with recombination mechanisms involving band-tail states. For sample with higher indium content in the well layer, the localization effect is enhanced; in contrast, the effect is weakened for sample with higher aluminum content in the barrier layer.

Correlation between luminescence properties of AlxGa1−xAs∕GaAs single quantum wells and barrier composition fluctuation

The luminescence mechanism at low temperatures in AlxGa1−xAs∕GaAs single quantum wells grown by molecular-beam epitaxy with different aluminum concentrations in the barrier has been studied in detail using the photoluminescence spectroscopy (PL) as function of temperature (8K⩽T⩽90K) combined with the excitation intensity. The asymmetry presented by the PL spectra at the low-energy side, the blueshift behavior of the PL peak energy, and the PL line broadening with increasing temperature are explained through the exciton localization in confinement potential fluctuations. The exciton localization effects on the PL spectra are progressively reinforced with the increase of the Al concentration in the barrier constituent material. The PL peak energy dependence on temperature has been fitted through the expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)] adapted to systems with potential fluctuations, by subtracting the term σE2∕kBT, where σE is the standard deviation of the potential fluctuations. It was verified that σE increases systematically with the Al concentration in the barrier, according to the AlGaAs alloy compositional disorder theory.

Relaxation dynamics of bimodally distributed CdSe quantum dots

We study the dynamical behavior of excitons in a bimodal distribution of $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}\mathrm{Se}$ quantum dots by intensity dependent, temperature dependent, and time resolved photoluminescence. The effect of exciton localization is investigated, both experimentally and theoretically by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe a dominant exciton emission from high energy dots (QDs1) and weaker emission from low energy dots (QDs2) at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and at low excitation levels. At high excitation densities a CdSe QD-precursor state becomes visible at the high energy side of the QDs1 emission. Temperature dependent photoluminescence studies reveal a thermally activated exciton transfer from QDs1 to QDs2 resulting in an enhanced QDs2 emission above $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Time resolved photoluminescence measurements allow us to estimate the characteristic radiative and nonradiative decay rates as well as the trapping rate from the QD-precursor layer. The experimentally observed photoluminescence is reasonably reproduced using a coupled rate equation model.

Effects of nonradiative centers on localized excitons in InGaN quantum well structures

The authors report the effects of nonradiative recombination on the properties of spatially localized excitons in InGaN quantum well structures studied using a microphotoluminescence (PL) technique. Sharp PL lines (linewidth of less than 1meV) are clearly obtained by combining the PL and nanolithographic techniques. The PL originates from localized excitons induced by quantum-dot-like local potential minima where indium is accumulated. A systematic study with various kinds of samples reveals that suppressing the density of the nonradiative centers is crucially important in terms of observing the exciton localization effects rather than increasing the effects of indium accumulation.

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.

The effect of exciton localization on the optical and electrical properties of undoped and Si-doped AlxGa1−xN

The properties of undoped and Si-doped AlxGa1−xN layers grown by metalorganic vapour-phase epitaxy have been investigated by photoluminescence and Hall effect measurements. The variable temperature photoluminescence properties of the layers were typical of AlxGa1−xN, with the temperature dependence of the peak energy showing the often-observed S-shape. The low and variable temperature photoluminescence properties of undoped material could be well described using alloy potential fluctuation theory. However, this was not the case for moderately Si-doped material, for which much larger S-shape behaviour, and thus larger exciton localization, was observed for samples with x > 0.3. The temperature dependence of the PL peak intensity of the Si-doped AlxGa1−xN layers, as well as their Hall activation energies, were then studied in order to obtain possible origins for the large increase in the exciton localization energy. It is tentatively proposed that the increase in the exciton localization could be related to the Γ9 → Γ7 valence band crossover, which occurs at roughly the same composition (0.2 < x < 0.3).

Exciton localization in AlGaN alloys

Deep ultraviolet (UV) photoluminescence emission spectroscopy has been employed to study the exciton localization effect in AlGaN alloys. The temperature dependence of the exciton emission peak energy in AlxGa1−xN alloys (0⩽x⩽1) was measured from 10to800K and fitted by the Varshni equation. Deviations of the measured data from the Varshni equation at low temperatures directly provide the exciton localization energies, ELoc. It was found that ELoc increases with x for x⩽0.7, and decreases with x for x⩾0.8. Our experimental results revealed that for AlGaN alloys, ELoc obtained by the above method has simple linear relations with the localized exciton thermal activation energy and the emission linewidth, thereby established three parallel methods for directly measuring the exciton localization energies in AlGaN alloys. The consequence of strong carrier and exciton localization in AlGaN alloys on the applications of nitride deep UV optoelectronic devices is also discussed.

Optical Properties of Highly Strained GaInAs/GaAs QuantumWells Grown by Sb Assistance

Optical properties of highly strained GaInAs/GaAs quantum wells (QWs)grown by molecular beam epitaxy with Sb assistance areinvestigated. The samples grown by Sb incorporation and Sbpre-deposition methods display high room-temperature photoluminescence(PL) intensity at extended long wavelength. This result is explained bythe surfactant effects of Sb during the growth of GaInAs/GaAs QWsystems. An abnormal S-shaped temperature dependence of the PL peakposition is found in the In0.42Ga0.58As/GaAs triple QWssample grown with Sb pre-deposition. By investigating the transmissionelectron microscope images and time-resolved PL spectra, it isfound that the S-shaped temperature dependence of the PL peak positionoriginates from the exciton localization effect brought by the Sb-richclusters on the QW interface.

Effect of strain relaxation and exciton localization on performance of 350-nm AlInGaN quaternary light-emitting diodes

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs.

Recombination kinetics of Te isoelectronic centers in ZnSTe

The recombination kinetics of Te isoelectronic centers in ZnS1−xTex (0.0065⩽x⩽0.85) alloys is studied by time-resolved photoluminescence (TRPL) at low temperature. The measured radiative recombination lifetimes of different Te bound exciton states are quite different, varying from a few nanoseconds to tens of nanosecond. As the bound exciton state evolves from a single Te impurity (Te1) to larger Te clusters (Ten,n=2,3,4), the recombination lifetime increases. It reaches maximum (∼40ns) for the Te4 bound states at x=0.155. The increase of the exciton lifetime is attributed to the increasing exciton localization effect caused by larger localization potential. In the large Te composition range (x>0.155), the exciton recombination lifetime decreases monotonically with Te composition. It is mainly due to the hybridization between the Te localized states and the host valence band states. The composition dependences of the exciton binding energy and the photoluminescence (PL) line width show the similar tendency that further support the localization picture obtained from the TRPL measurement.

Time-resolved photoluminescence of Te isoelectronic center in ZnS

The recombination dynamics of localized exciton in ZnS1-xTex ternary alloys has been investigated by time-resolved photoluminescence (PL) in a large Te concentration range from 0005 to 085. It is found that the radiativ e recombination lifetimes of different Te isoelectronic centers show a significa nt difference, varying from a few nanoseconds to tens of nanosecond. The lifetim e reaches a maximum of ～ 40 ns in the sample of x=015. The present result s could be understood in terms of the exciton localization effect. When the Te c oncentration is small, the Te isoelectronic centers evolve gradually from a sing le Te impurity (Te1) to the Te clusters (Ten). The exciton localization is enhanced, resulting in the increase of exciton recombination lifetime. When t he Te concentration is further increased, the Te clusters (Ten) becom e lar ge enough to hybridize the Te localized states and the host valence band states. Therefore, the excitons bound to Te isoelectronic centers become more or less d elocalized, resulting in a shorter lifetime. Furthermore, the concentration depe ndence of the exciton binding energy and the PL intensity variation with tempera ture have been mearsured. The results further confirm the above conclusion.