Exciton Emission Peak Research Articles

Influence of temperature ramp on the materials properties of GaSb grown on ZnTe using molecular beam epitaxy

This paper reports high-quality GaSb grown on ZnTe using molecular beam epitaxy with a temperature ramp during growth, and investigates the influence of the temperature ramp on material properties. During growth, in situ reflection-high-energy electron diffraction shows rapid and smooth transition from ZnTe surface reconstruction to GaSb surface reconstruction. Post-growth structural characterization using x-ray diffraction and transmission electron microscopy reveals smooth interface morphology and low defect density. Strong photoluminescence emission is observed up to 200 K. The sample grown with a temperature ramp from 360 to 470 °C at a rate of 33 °C/min showed the narrowest bound exciton emission peak with a full width at half maximum of 15 meV.

Formation of a single In(Ga)As/GaAs quantum dot embedded in a site-controlled GaAs nanowire by metalorganic chemical vapor deposition for application to single photon sources

ABSTRACTWe report the formation and optical properties of site-controlled InAs/GaAs quantum dots (QDs) embedded in GaAs nanowires (NWs) by selective metalorganic chemical vapor deposition for application to single photon sources. InAs/GaAs QD-in-NWs with various InAs thicknesses are realized on patterned GaAs(111)B substrates in the form of InAs/GaAs heterostructures and identified by structural analyses using scanning transmission electron microscopy and photoluminescence characterization. Sharp excitonic emission peaks at 10 K from single QD-in-NWs with the narrowest exciton linewidth of 87 μeV are observed. Light emission from the single QD-in-NW shows photon antibunching which evidences single photon emission from high-quality QD-in-NWs.

Green route synthesis of high quality CdSe quantum dots for applications in light emitting devices

Investigation was made on light emitting diodes fabricated using CdSe quantum dots. CdSe quantum dots were synthesized chemically using olive oil as the capping agent, instead of toxic phosphine. Room temperature photoluminescence investigation showed sharp 1st excitonic emission peak at 568nm. Bi-layer organic/inorganic (P3HT/CdSe) hybrid light emitting devices were fabricated by solution process. The electroluminescence study showed low turn on voltage (∼2.2V) .The EL peak intensity was found to increase by increasing the operating current.

Temperature Dependent Photoluminescence of ZnO Nanorods Synthesized by Hydrothermal Method

Temperature dependent photoluminescence of ZnO nanorods synthesized by hydrothermal method is studied. According to fifteen photoluminescent curves which were measured from 78 K to 288 K with an interval of 15 K, peak energy of exciton emissions, integral intensity of exciton emission peaks and integral intensity of deep-level emission peaks as a function of temperature were studied. The experimental data were fitted by Bose-Einstein relation and thermal activation function. By fitting, some important parameters were obtained and compared, such as the Einstein temperature for the excitons, the thermal activation energy of excitons or deep-level defects etc.

Catalyst free self‐organized grown high‐quality GaN nanorods

AbstractHighly ordered GaN nanorods were grown self‐organized and without catalyst on r‐plane sapphire by molecular beam epitaxy while the AlN nucleation sites for the nanorods were provided by a nitridation process using a metal‐organic vapor‐phase epitaxy system. The growth window for the nanorod formation was analysed in detail and turned out to be very sensitive with respect to the growth temperature. The nanorods are symmetrically tilted with an inclination angle of 62° between the substrate and the nanorods. The mirror axis is the c‐direction of the compact GaN layer surrounding the roots of the nanocolumns. Methods for the control of the nanorod density and the suppression of one nanorod growth direction are presented. The results indicate a diffusion based growth mechanism. Transmission electron microscopy studies and high resolution X‐ray diffraction (HRXRD) polar plots reveal the epitaxial relationship between substrate, compact GaN layer and nanorods. The nanorods grow in c‐direction and the side facets are m‐planes. Transmission electron microscopic and optical analysis of the nanorods reveal the good structural and optical properties, respectively. A full width at half maximum (FWHM) of 1.2 meV of the donor‐bound exciton emission was measured for both the ensemble and single free‐standing nanorods. Successful n‐ and Mg‐doping of nanorods was verified by a strong increase of the micro‐photoluminescence intensity of the respective donor‐bound and acceptor‐bound exciton emission peak in comparison to an undoped sample. magnified imageThe background shows a secondary electron microscope image (angle of 10° between sample surface and electron beam) of the nanorods. Superimposed to the SEM image, a typical micro‐photoluminescence spectrum of the excitonic emission of an ensemble at 25 K is presented. The donor‐bound, acceptor‐bound and free exciton lines are labelled with D0X, A0X and FX, respectively.

Surface localized exciton emission from undoped SnO2 nanocrystal films

We report the UV photoluminescence properties of SnO2 nanocrystalline films. A free exciton decay centered at 3.7 eV and a strong surface localized exciton emission peak at 3.3 eV have been observed at room temperature. The peak energy of the surface localized exciton emission exhibits a redshift with increasing temperature and a blueshift with increasing excitation intensity. The surface localized exciton emission is considered to originate from the radiative recombination of exciton within the surface region of SnO2 nanocrystals. The surface defects and local disorder are believed to be responsible for the formation of band tail states at the conduction band and potential well within the band tails.

Temperature-dependent photoluminescence of GaN grown on β-Si3N4/Si (111) by plasma-assisted MBE

Abstract Photoluminescence (PL) of high quality GaN epitaxial layer grown on β-Si 3 N 4 /Si (1 1 1) substrate using nitridation–annealing–nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5–300 K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an ‘S-like’ shape of free exciton (FX) emission peaks. Distant shallow donor–acceptor pair (DAP) line peak at approximately 3.285 eV was also observed at 5 K, followed by LO replica sidebands separated by 91 meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be ∼27.8±0.7 meV from the temperature-dependent PL studies. Low carrier concentrations were observed ∼4.5±2×10 17 cm −3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2 eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy.

Aqueous Phase Synthesized CdSe Nanoparticles with Well-Defined Numbers of Constituent Atoms

We report aqueous phase synthesized semiconductor nanoparticles with well-defined numbers of constituent atoms. Aqueous phase synthesis provides many advantages over organic phase synthesis for producing such high-quality semiconductor nanoparticles. We synthesized CdSe nanoparticles with excellent colloidal and optical stabilities directly in aqueous solution at room temperature and then identified them as selectively grown (CdSe)33 and (CdSe)34 magic-sized clusters. These clusters displayed extremely sharp excitonic absorption and emission peaks because of their practically monodispersed size distribution. Their X-ray diffraction pattern and Raman spectral features were considerably different from the corresponding pattern and features for typical crystalline CdSe nanoparticles. Growth of our magic-sized clusters was very slow and proceeded via the formation of different sizes of progressively larger CdSe nanoparticle intermediates with time. Our results demonstrated that aqueous phase synthetic routes could be successfully adopted for producing high-quality semiconductor nanoparticles.

Photoluminescence and electrical transport characteristics of ZnO nanorods grown by vapor-solid technique

Hexagonal shaped radial and quasialigned arrays of ZnO nanorods with diameter of about 40–60 nm have been deposited on p-Si (100) substrates by vapor-solid method using Zn as the source material. x-ray diffraction, field emission scanning electron microscopy, temperature dependent-photoluminescence, and impedance spectroscopy have been used to characterize the structural, optical and electrical transport properties of the grown nanostructures. At room temperature, a strong free excitonic emission peak at 3.311 eV with very weak defect emissions is observed. At low temperatures, near-band-edge steady-state photoluminescence spectra of ZnO nanorods are dominated by neutral-donor-bound-exciton (D0X) transitions with corresponding transverse and longitudinal optical phonon replicas. The impedance spectra as a function of bias voltage and temperature have been studied in detail. The differences in characteristics of p-Si/ZnO/Al and Al/ZnO/Al devices are discussed by using one and two RC equivalent circuits. A comparative study of the impedance spectra for bulk ZnO and ZnO nanostructures is presented. The activation energy of ZnO nanorods is found to be 0.08 eV, which is slightly greater than the reported bulk value.

Growth of biepitaxial zinc oxide thin films on silicon (100) using yttria-stabilized zirconia buffer layer

In this work, an approach for integrating zinc oxide thin films with Si(100) substrates using an epitaxial tetragonal yttria-stabilized zirconia buffer layer is reported. Selected area electron diffraction measurements revealed the following epitaxial relationship: [110]YSZ∥[100]Si and (001)YSZ∥(001)Si. X-ray diffraction studies demonstrated that subsequent growth of the zinc oxide thin film on the yttria-stabilized zirconia buffer layer occurred with the following epitaxial relationship: (0002)ZnO∥(001)YSZ. The full width at half maximum value for the (0002) peak of zinc oxide was small (∼0.16°), which indicated good crystalline quality. Transmission electron microscopy revealed that the zinc oxide thin film grew epitaxially on an yttria-stabilized zirconia buffer layer in two different orientations, where one orientation was rotated by 30° from the other. The orientation relationship in this case was [101¯0]ZnO∥[100]YSZ or [21¯1¯0]ZnO∥[100]YSZ and (0002)ZnO∥(001)YSZ. The biepitaxial growth of the zinc oxide thin film has been explained in the framework of domain matching epitaxy. Optical emission measurements showed a strong excitonic emission peak from the zinc oxide thin film at ∼377 nm. Minimal green band emission in the photoluminescence spectrum indicated that the concentration of point defects was low. Integration of epitaxial zinc oxide thin films with Si(100) substrates is an important step toward developing practical applications of zinc oxide in a variety of optoelectronic devices.

Fabrication, Characterization, and Strong Exciton Emission of Multilayer ZnTe Nanowire Superstructures

In this paper, multilayer superstructures of single-crystalline ZnTe nanowire films have been prepared through a new growth process: well-aligned ZnTe nanowires congregate into nanowire film. The growth process takes place discontinuously and consequently produces many layers of aligned ZnTe nanowire superstructures. These interesting findings are apparently different from the conventional vapor−liquid− solid (VLS) process but following a new multiple nucleation growth model. The intriguing growth mode of multilayer superstructures of aligned ZnTe nanowires may enrich our understanding of the growth mechanism of nanowires. The photoluminescence spectrum of the obtained multilayer ZnTe NWs superstructures exhibits strong free exciton emission peak and relatively weak Ia line, indicating that the layers are of high quality, which may have potential applications in nanodevices.

Low-Temperature Growth and Characterization of Single Crystalline ZnO Nanorod Arrays Using a Catalyst-Free Inductively Coupled Plasma-Metal Organic Chemical Vapor Deposition

Vertically aligned ZnO nanorod arrays have been synthesized on c-plane sapphires at a low temperature of 400 degrees C using catalyst-free inductively coupled plasma (ICP) metal organic chemical vapor deposition (MOCVD) technique by varying the ICP powers. Diameters of the ZnO nanorods changed from 200 nm to 400 nm as the ICP power increased from 200 to 400 Watt. TEM and XRD investigations indicated that the ZnO nanorod arrays grown at ICP powers above 200 Watt had a homogeneous in-plane alignment and single crystalline nature. PL study at room temperature (RT) and 6 K confirmed that the ZnO nanorod arrays in the present study are of high optical quality as well as good crystalline quality, showing only exciton-related emission peaks without any trace of defect-related deep level emissions in visible range. The blueshift of exciton emission peak in RTPL spectra was also found as rod diameter decreased and it is deduced that this shift in emission energy may be due to the surface resonance effect resulted from the increased surface-to-volume ratio, based on the observation and behavior of the surface exciton (SX) emission in the high-resolution 6 K PL spectra.

Temperature Dependence of Photoluminescence from Single and Ensemble InAs/GaAs Quantum Dots

We investigate the temperature dependence of photoluminescence from single and ensemble InAs/GaAs quantum dots systematically. As temperature increases, the exciton emission peak for single quantum dot shows broadening and redshift. For ensemble quantum dots, however, the exciton emission peak shows narrowing and fast redshift. We use a simple steady-state rate equation model to simulate the experimental data of photoluminescence spectra. It is confirmed that carrier-phonon scattering gives the broadening of the exciton emission peak in single quantum dots while the effects of carrier thermal escape and retrapping play an important role in the narrowing and fast redshift of the exciton emission peak in ensemble quantum dots.

Photoluminescence and surface photovoltage spectroscopy characterization of Zn1−x−yBexMgySe mixed crystals

This paper presents an optical characterization of three Bridgman-grown Zn1−x−yBexMgySe mixed crystals in the near-band-edge interband transitions using temperature-dependent photoluminescence (PL) in the temperature range of 10–300 K and surface photovoltage spectroscopy (SPS) at room temperature. PL spectra at low temperatures of the investigated samples consist of an excitonic line, an edge emission due to radiative recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. The anomalous S-shape temperature dependence of the exciton emission peak for Zn1−x−yBexMgySe crystals with high Mg content (y=0.26) can be explained as due to localization of excited carriers caused by statistical fluctuations of local composition. The peak positions of the excitonic emission lines in PL spectra correspond quite well to the energies of the fundamental transitions determined from SPS measurements. The parameters that describe the temperature dependence of the transition energy and broadening parameter of the band-edge excitonic emission are evaluated and discussed.

Hydrothermal growth and characterization of ZnO thin film on sapphire (0001) substrate with p-GaN buffer layer

Monocrystalline ZnO thin films on p-GaN/sapphire (0001) substrate were grown by single step hydrothermal technique at 90 °C. ZnO thin films were grown in aqueous solution of zinc nitrate and ammonium hydroxide. The X-ray diffraction, scanning electron microscopy and room temperature photoluminescence analysis were carried out to characterize structural, morphological and optical properties of the films. The thin films revealed single crystalline nature and wurtzite symmetry. The films were c-axis oriented and have honeycomb like pitted surface morphology. The epitaxial relationship between ZnO film and p-GaN buffer layer was observed to be (0001)[112¯0] ZnO||(0001)[112¯0] GaN. Sharp luminescent peak centered at 376 nm due excitonic emission and broad deep level emission peak were obtained from room temperature photoluminescence measurement of the ZnO thin film.

Self-separated freestanding GaN using a NH4Cl interlayer

Thick GaN films were grown on void buffer layer by hydride vapor phase epitaxy. The void-buffer layers were consisted of NH4Cl, GaN dots, and low-temperature (LT) GaN buffer layer. Instead of GaN, NH4Cl was easily synthesized in NH3 and HCl atmospheres by simply lowering the growth temperature to 500°C, and LT GaN buffer growth was followed during increasing substrate temperature to 600°C. The LT GaN buffer acted as a protecting layer against evaporation of the NH4Cl and a seeding layer for the high temperature (HT) GaN. The NH4Cl layer between a sapphire substrate with GaN dots and the LT GaN buffer were fully evaporated during the HT GaN growth at 1040°C. Many voids were formed at interface caused by evaporation of the NH4Cl layer, which strongly assisted self-separation of thick HT GaN during cooldown after the growth resulting in a 200μm thick freestanding (FS) GaN. The FS GaN showed smooth surface morphology and absence of any crack. The a-axis and c-axis lattice constants of FS GaN were 3.189 and 5.185Å, respectively, which well agrees with those of strain-free bulk GaN. The observed donor-bound exciton emission peak at 3.4718eV agreed with the peak position of bulk GaN. All these features indicate that the obtained FS GaN through the self-separation process is nearly strain-free.

Morphology and crystal quality investigation of hydrothermally synthesized ZnO micro-rods

Micro-structural and room and low temperature photoluminescence response of undoped one-dimensional ZnO were investigated. ZnO rods of different morphology and size were synthesized by controlling growth parameters through hydrothermal technique. The phase and microstructure analysis were carried out by X-ray diffraction and scanning electron microscopy. The room and low temperature photoluminescence spectra of the samples were studied. Near band edge sharp exciton emission peaks and broad defect-related peaks were observed. The ratio of band edge emission to deep level emission was controlled by tuning the initial concentration, pH and reaction time period. Optimal growth condition for growth of micro-rods with improved crystal quality was obtained with initial Zn 2+ concentration of 0.5 M, at reaction temperature of 120 °C, pH of 9.9 and in a reaction time period of 6 h.

A different chemical route to synthesize ZnO nanoparticles

A new reaction was developed to synthesize ZnO nanoparticles with nearly uniform, spherical morphology ranging from 25 to 50 nm via directly oxidizing zinc powders with iodine in ethanol. Iodine plays key roles in the nucleation of ZnO nanoparticles and it is favorable for the growth of ZnO nanoparticles gradually and avoids the agglomeration to some extent. Moreover, this process is well repeatable and easy controlled. The morphology evolution of the ZnO nanoparticles at different reaction time was monitored through transmission electron microscope (TEM). The room temperature photoluminescence (PL) spectrum displays strong ultraviolet emission peaks and relative weak near band-edge free excitonic emission peak.

Synthesis and luminescence properties of water-dispersible ZnSe nanocrystals

Water-dispersible ZnSe nanocrystals were synthesized by a green and simple route. Growth of the nanocrystals was performed under water-bath temperature, using selenium powder and zinc acetate as precursors, upon addition of mercaptoacetic acid as a stabilizer. Products were characterized by X-ray photoelectron spectra (XPS), X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), absorption and fluorescence spectra. Photoluminescence (PL) properties of ZnSe nanocrystals in evolution are discussed. PL is the contribution of both excitonic and defect emission. The excitonic emission peak can be tuned by changing the refluxing time and pH value. Mercaptoacetic acid is available in controlling the ratio of excitonic emission to defect emission. All desired properties of nanocrystals prepared here imply the possibility of high quality ZnSe nanocrystals developed under a mild condition.

Optical investigation of exciton localization in AlxGa1−xN

The optical properties of AlxGa1−xN epilayers with x ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the AlxGa1−xN epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the AlxGa1−xN layers. The localization parameter σ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of 52meV at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with x indicating that the degree of localization again increases with x. In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas.