Free Exciton Transitions Research Articles

Growth, properties and sensor applications of low temperature grown ZnO nanorods

ZnO nanorods with the average diameter of about 60–300 nm have been grown by the low temperature aqueous chemical growth technique. The impact of growth time and reactant concentration on the morphology on ZnO structures was tested by scanning electron microscopy and photoluminescence spectroscopy. High optical quality of obtained ZnO nanorod films was evidenced by dominating free exciton transitions. The reduction of surface to volume ratio by increasing the diameter of ZnO nanorods resulted in a linear increase of excitonic luminescence intensity with respect to the defect related emission. Excellent photoluminescence properties and high surface area of ZnO nanorods can be used for the detection of organic contaminants dissolved in water. The quenching of both excitonic and defect luminescence was observed upon exposure of ZnO nanorod films to various organic contaminants.

Fabrication and optical properties of ZnO/ZnMgO multiple quantum wells on m-sapphire substrates

The ZnO/Zn0.85Mg0.15O multiple quantum wells(MQWs)are fabricated on m-Al2O3 substrates by plasma-assisted molecular beam epitaxy (P-MBE) using a ZnMgO buffer layers. The reflection high-energy electron diffraction (RHEED) images indicate that the MQWs are of two-dimensional growth .The temperature dependent photoluminescence (PL) of the MQW also shows the quantum confine effect even at room temperature. The PL peak of 3nm MQW is 3.405 eV at 290 K.The PL spectrum in ZnO/Zn0.85Mg0.15O MQW is dominated by localized exciton emission at low temperatures, while the free exciton transition gradually dominates the spectrum at higher temperatures up to room temperature. The exciton binding energy in the 3 nm ZnO/Zn0.85Mg0.15O MQW is about 73 meV.

Effects of Surface Recombination on Exciton Dynamics in GaN Nanorods

We have investigated the optical properties of GaN nanorods with different diameters by micro-photoluminescence (PL) and time-resolved PL measurements grown on (111) Si substrates by using radio-frequency plasma-assisted molecular-beam epitaxy. In order to confirm the formation of the GaN nanorods and their physical dimension, we employed high resolution scanning electron microscopy. As the diameters of the nanorods decreased, the donor bound exciton (D 0 X) emission, which is mainly observed at low temperature for samples with larger diameters, rapidly quenched compared with the free exciton (FX). The activation energy of the FX for GaN nanorods with different diameters was also evaluated. The decay times are a few tens of ps for both the D 0 X and the FX transitions. The overall decay times decrease linearly with decreasing average diameter of the nanorods. Time-resolved PL studies show that surface recombination is dominant in determining the excitonic dynamics.

Self-assembled flower-like nanostructures of InN and GaN grown by plasma-assisted molecular beam epitaxy

Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be ∼28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.

Vertically aligned Mn-doped zinc oxide nanorods by hybrid wet chemical route

Mn-doped zinc oxide (Mn:ZnO) nanorods were synthesized by incorporating manganese in aligned ZnO nanorods. For this, Mn was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing. The nanorods were preferentially oriented in (0 0 2) direction as indicated by the XRD measurement. Optical band gap was seen to decrease with increasing amount of Mn incorporation. XPS studies indicated that incorporated Mn was in Mn 2+ and Mn 4+ states. Mn 2+ atomic concentration was found to be larger than Mn 4+ concentration in all the samples. The Raman spectra of the Mn:ZnO nanorods indicated the presence of the characteristic peak at ∼438 cm −1 for high frequency branch of E 2 mode of ZnO. The PL peak at ∼376 nm (∼3.29 eV) was ascribed to the band edge luminescence while the peak at ∼394 nm (∼3.15 eV) was assigned to the donor bound exciton (D oX) and free exciton transition related to Mn 2+ states.

Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures

The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y 4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime.

Vapor condensation growth and evolution mechanism of ZnO nanorod flower structures

AbstractZnO flower‐like nanostructures were grown on Ge (100) substrate, by a modified chemical vapor condensation technique of zinc acetate dihydrate at 300 °C, without using any catalyst. These self‐organized three‐dimensional nanostructures were composed of hierarchical arrangement of ZnO nanorods of diameter ∼50 nm around a common nucleus and were distributed uniformly over the entire substrate surface. Evolution study of these structures indicates that the growth begins with a two‐dimensional planar arrangement of 〈0001〉‐oriented ZnO nanorods. With increasing growth time, the expanding adjacent two‐dimensional growth fronts approach each other, followed by which, the formation of three‐dimensional flower‐like structures evolve. Surface diffusion mechanism seems to play an important role in forming these nanostructures, which has been discussed in detail. Elaborate electron microscopic (SEM, TEM) techniques have been used to investigate the growth characteristics of the flower structures. The photoluminescence measurements showed pure free excitonic transition centered at about 3.249 eV with full width at half‐maximum of about 141 meV at 300 K, which blue shifted to 3.361 eV at 10 K with corresponding half width of 7 meV with no defect‐related bandgap peak due to relatively low growth temperature. The optical emission area was imaged through a cathodoluminescence technique.magnified image Scanning electron micrograph of a typical ZnO nanorod flower structure grown at 300 °C on Ge (100).

Structure and luminescence of sol-gel synthesized anatase nanoparticles

Two samples of mesoporous anatase nanoparticles, prepared by the sol-gel method, were characterised by Cs-corrected high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD) and Raman spectroscopy. Statistical evaluation of TEM data showed an average diameter of these crystallites of 8.8 nm and 11.1 nm, respectively. Optical spectroscopy by cathodoluminescence (CL) in a scanning electron microscope (SEM) showed free exciton transitions related to the direct and the indirect band gap of anatase TiO2. From the analysis of the excited states of the free excitons an exciton binding energy of 10 meV and a Bohr radius of 2.35 nm is obtained. The small Bohr radius could explain the absence of quantum confinement in the particles presented in this study.

Temperature behavior of electron-acceptor transitions and oxygen vacancy recombinations in ZnO thin films

The present study focuses on the photoluminescence (PL) properties of ZnO films prepared by the reaction of water and zinc. Temperature-dependent PL characteristics of the ZnO films have been investigated in the range from 15 to 260 K. The PL spectrum at 15 K is dominated by neutral donor-bound exciton (D0X) emissions. The emission line at about 3.304 eV can be due to the transition of free electrons to neutral acceptor states (eA0). It is suggested that the ultraviolet emission at room temperature can be attributed to the incorporation of eA0 and free exciton (FX) transitions, rather than sole FX. According to the temperature behavior of green band, it is further confirmed that the green emission is mainly related to the singly ionized oxygen vacancies.

Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy

The band-edge property and built-in electric fields of two different Al(x)Ga(1-x)N/GaN(AlGaN/GaN) heterostructures (HSs) with and without an additional AlGaN inserted layer were studied by thermoreflectance (TR), photoluminescence (PL), and contactless electroreflectance (CER) techniques. The PL spectra characterize the band-edge luminescence property of GaN. Free exciton transitions of AlGaN and GaN were probed experimentally by TR. Prominent Franz-Keldysh oscillations (FKOs) of GaN and the opposite FKO phase of AlGaN were simultaneously detected by the additional AlGaN inserted sample with CER owing to the enhancement effect of built-in electric fields of GaN and AlGaN with the same polarity direction. Optoelectronics properties of the two HSs were characterized by the experimental analyses.

Temperature-Dependent Excitonic Luminescence in ZnO Thin Film Grown by Metal Organic Chemical Vapor Deposition

In this study, the excitonic luminescence behaviors of ZnO thin films in the temperature range of 10–300 K were investigated. The photoluminescence (PL) spectrum exhibits the bound-exciton emission and the donor-acceptor recombination accompanying its multiphonon replicas at low temperatures. The observed features exhibit redshift with an increase in temperature, and the temperature dependence of the peak position was analyzed by the Varshni empirical expression. The study showed the dominant presence of bound excitonic transition below 160 K or free excitonic transition at higher temperatures for the observed PL spectrum. The free exciton emission can be observed up to room temperature. The Debye temperature of ZnO was evaluated by taking into account the elastic constants of ZnO and utilized as a parameter in the Varshni empirical expression giving an accurate description of the free exciton emission behavior up to room temperature.

Fine Structure of the Localized Emission from GaInNAs Layers Studied by Micro-Photoluminescence

R. Kudrawieca,∗, M. Latkowska, G. Sek, J. Misiewicz, J. Ibanez, M. Henini and M. Hopkinson Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Institut Jaume Almera, Consell Superior d’Investigacions Cientifiques, 08028 Barcelona, Spain School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom Department of Electronic and Electrical Engineering, University of Sheffield, S3 3JD Sheffield, United Kingdom GaInNAs bulk-like layers (≈ 20% In and ≈ 3% N) grown on GaAs substrate with various crystallographic orientations have been studied by micro-photoluminescence at low temperatures for a broad range of excitation conditions. In addition to photoluminescence peaks, which are associated with heavyand light-hole free exciton recombination, a band of sharp lines was observed below the fundamental free exciton transition at low excitation. It shows that the localized emission which is typical of this alloy at low temperatures is composed of individual narrow photoluminescence lines which are associated with the recombination of single excitons. They can be localized on various local potential minima including those originating from the alloy content fluctuations and/or deep acceptor(donor)-like complexes.

Synthesis of ZnO Nanoparticles by Solvothermal Method and Their Ammonia Sensing Properties

ZnO nanoparticles were prepared by a simple solvothermal route by using a ethylene glycol (EG)-ethanol (E) solvent system. The shape and size of ZnO nanoparticles were tuned by changing the volume composition of the solvents and the synthesis temperature. Phase purity was confirmed by X-ray diffraction (XRD) and crystal size was determined by transmission electron microscopy (TEM). Ellipsoidal and spherical nanoparticles were obtained by varying the volume ratio of the solvents. Mostly spherical nanoparticles with average diameter of 4 nm was synthesized with higher volume fraction of EG at a lower synthesis temperature. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed for the smallest nanoparticle size that is ascribed to the quantum confinement effect. The ammonia gas sensing characteristics of the smallest diameter nanoparticles showed better sensitivity at a relatively lower temperature than that of large diameter particles.

Hydrothermal synthesis and optical properties of ZnO single-crystal hexagonal microtubes

ZnO microtubes were successfully synthesized by employing zinc acetate dihydrate as the starting material under hydrothermal condition. The ZnO microtubes were characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that the as-prepared ZnO microtubes were hexagonal and single crystal in nature and grew on Si (1 1 1) substrates along the [0 0 0 2] direction, which have diameters of about 3 μm, lengths of 10–20 μm, and wall thickness of 80 nm. Optical properties of the ZnO microtubes were investigated by temperature-dependent photoluminescence (PL) spectroscopy. The PL spectrum at 82.2 K shows a strong UV emission from the near band-edge with a negligible visible emission from deep level, and donor bound exciton (D 0X), free exciton (FX) and its phonon replica were observed in the UV-region. The PL spectra were dominated by the FX emission throughout the whole temperature range, and the energies of FX transitions and D 0X transitions are close to that of the bulk values of ZnO.

A comprehensive study of temperature-dependent reflectance and photoluminescence of Zn1−xMnxO thin films grown on c-Al2O3

We present a systematic study of temperature-dependent reflectance (R) and photoluminescence (PL) measurements on ZnMnO films grown by plasma-assisted molecular beam epitaxy. For the first time, the three free-exciton transitions FXA (Γ7c-Γ7vu), FXB (Γ7c-Γ9v), and FXC (Γ7c-Γ7vl) and the longitudinal-optical phonon replicas of FXB and FXC of ZnMnO films have been clearly observed in the R spectra. The parameters describing the activation energy and the temperature dependence of the transition energy and broadening have been extracted by fitting the experimental R and PL spectra. The spectral data of ZnMnO films not only show the deterioration of crystalline quality with increasing Mn composition fraction but also indicate the Mn clustering caused by Mn atom segregation.

Excimer laser-induced crystallization of CdSe thin films

We have investigated ArF (λ=193 nm) excimer laser-induced crystallization of amorphous CdSe semiconductor thin films. The crystallization has been monitored by a related photoluminescence emission in the free-exciton and defect-band transition regions. For different irradiation conditions, we have observed formation of nanorods, up to 2 μm long, as well as the formation of arrays of CdSe nanobeads with a narrow size distribution and characteristic dimensions corresponding to λ/2 and λ/8. The successful crystallization has also been confirmed by confocal Raman spectroscopy.

Oxygen deficiency effects on recombination lifetime and photoluminescence characteristics of ZnO thin films; correlation with crystal structure

The photoluminescence (PL), recombination lifetime (RL), and X-ray diffraction (XRD) spectra of the samples grown at various O2 fractions of 0.290 (Zn-rich), 0.585 (moderate), and 0.836 (O-rich) over the total pressures in the growth chamber were investigated. XRD measurements revealed that all the films show highly preferred (0002) orientation. The PL measurements exhibit different dominant emissions in the ultraviolet (UV), violet and blue regions for Zn-rich, moderate and O-rich samples, respectively. Well-known green emission and high intensity of free exciton (FX) transition has been observed in Zn-rich sample after the sample is annealed at vacuum probably due to the oxygen deficiencies. Annealing the moderate sample gives rise to the UV emission at energy of 3.263 eV similar to the observed PL emission spectrum for the Zn-rich thin film. O-rich thin film exhibits a 338 meV acceptor level above the valance band maximum, most probably related to zinc vacancy (VZn). Free exciton RL measurements result in 568.23, 397.65, and 797.46 ps for Zn-rich, moderate, and O-rich thin films, respectively. A good correlation was found between crystallite size and the lifetime values.

Splitting of valance subbands in the wurtzite c-plane InGaN/GaN quantum well structure

Peak splitting in the low temperature photoluminescence (PL) spectra of c-plane InGaN/GaN single quantum well samples was observed. For the k∥c configuration, the splitting peaks show a variation in relative intensity as the excitation power is tuned. For the k⊥c configuration, a strong polarization dependence of the luminescence distribution and intensity was spotted. The PL spectra was analyzed with a calculation model based on the k⋅p effective mass theory, and the splitting peaks were identified as free-exciton transitions between the conduction subband C1 and two groups of valence subbands, the {HH1,LH1} and the {HH2,LH2,CH1}, respectively.

Refractive indices and elasto-optic coefficients of GaN studied by optical waveguiding

Asymmetric GaN planar waveguides were grown on (0001) sapphire by molecular beam epitaxy or by metal-organic chemical vapor deposition. The GaN chromatic dispersion is measured in these samples by the prism coupling method at several discrete wavelengths over a large spectral domain (458–1550 nm). Two-term Sellmeier equations are adequate to fit the ordinary no and extraordinary ne refractive index curves as a function of wavelength. GaN waveguides were also grown on AlxGa1−xN and AlN cladding layers, and the GaN refractive indices are further measured from these samples. On the other hand, the biaxial stress state of each sample is determined by measuring the A, B, and C free exciton transition energies by low temperature (10 K) reflectivity measurements. The refractive indices versus in-plane stress σ1 reveal linear variations in the studied range and the slopes ∂n/∂σ1 are determined. It is found that these variations are more than seven times larger for the ordinary index than for the extraordinary index depending on the optical wavelength. We report here for the first time the measurement of the elasto-optic coefficients (p11+p12) and p13 of GaN in the visible wavelength range.

Negative spin polarization in AlGaAs photoluminescence

AbstractOptical orientation of carrier spins has been studied in pure direct‐gap AlxGa1–xAs alloys (x up to 15%). It is shown that optical excitation with circularly‐polarized light with quanta energy in a narrow range (less than 1.6 meV) just above the free exciton transition results in the appearance of negative circular polarization of near‐band edge photoluminescence (PL), which indicates a reversal of the spin orientation direction. The PL polarization degree becomes negative on all the excitonic transitions in the spectrum, free as well as bound. Carrier spin kinetics observed using the Hanle technique shows insignificant modification under the resonant excitation. The resonant switching of PL polarization seen in the excitation spectrum points to a change in the dominating carrier generation channel from heavy‐hole to light‐hole. The change is attributed to the influence of mechanical stress between the ternary epitaxial film and the substrate of GaAs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)