Band-edge Photoluminescence Intensity Research Articles

Strain-size features of bare ZnSe:Cu and ZnSe:Cu@ZnS core/shell quantum dots from extensive X-ray diffraction analysis, and their photoluminescence properties

ZnSe:Cu d-dots and ZnSe:Cu@ZnS core/shell quantum dots (QDs) were synthesized under different synthesis regimes of Cu-dopant content, microwave irradiation time (MWIRT) and over-coating of ZnSe:Cu by ZnS shell in order to optimize their structural and photoluminescence (PL) properties. The morphological and microstructural characterizations broadly done by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), revealed the spherical shape, synthesis-condition tunable and almost uniform size distribution of bare and core/shell QDs. Extended X-ray diffraction analysis (XDA) was performed by employing both Williamson-Hall (W–H) and Williamson-Smallman (W–S) approaches in order to validate these methods and obtain lattice strain (ε), dislocation density (δ) and particle size. The results indicated that all the QDs had cubic zinc-blend structure with an average crystallite size of ∼1.5 nm, and shift of peak positions shift toward higher angles for core/shells compared with bare QDs. W–S and W–H's outputs for bare and core/shell QDs showed similar trends, i.e. the optimal Cu-content of 0.1 mol% with higher crystalline quality. In addition, bare and core/shell QDs were investigated by Ultraviolet–Visible and PL spectroscopy. The blue-shift of band gap for the synthesized ZnSe QDs (∼3.5 eV) in comparison with bulk counterpart (∼2.7 eV) confirmed the formation of nano-sized particles. It was found that these QDs exhibited PL emission in the range of 423–559 nm. After changing the experimental variables during the experiment, the samples with optimal PL were identified. In reasonable relation to the microstructural aspects, ZnSe:Cu(1.5%) under MWIRT = 6 min has the highest band-edge PL intensity, which was associated with increased particle size, reduced lattice strain and higher atomic order raised from MWIR. Among the core/shell QDs, ZnSe:Cu(1.5%)@ZnS under MWIR indicated the highest band-edge emission intensity, which besides the microstructural parameters, may be a result of the morphological change from spherical to nanorod as certified by TEM. In addition, ZnSe@ZnS under MWIR showed the highest trap-defect emission intensity. In brief, due to the strong experimental condition-dependence of studied features, which is very important in tuning the properties, the studied bare and core/shells can be promising agents in green-blue luminescent photometric-biological applications.

High quality of N-polar GaN film fabricated by layer transfer technology using high selective material removal rate of CMP

A new method has been proposed for layer transfer technology to fabricate N-polar GaN film, which is extremely promising to be applied in the preparation of related devices. In our study, after wafer bonding and original Si substrate removal process, the N-polar GaN was exposed by chemical mechanical polishing (CMP) within a short time due to the high selective material removal rate (MRR) ratio of about 35:1 between N-polar AlxGa1-xN (0.33≤x≤1, 1400 nm/min) and N-polar GaN (40 nm/min). The high selective MRR can be attributed to the different corrosion resistance. Hexagonal hillock structure and corrosion pits were appeared quickly on the AlN surface when immersing the N-polar AlN and N-polar GaN in the same CMP slurry, but was not observed on the GaN surface. Meanwhile, the quality of the fabricated N-polar GaN film has been characterized by atomic force microscope (AFM) and Photoluminescence (PL) spectroscopy. Comparing with the N-polar GaN processed through inductive coupled plasma (ICP) etching, the N-polar GaN fabricated by CMP has smaller surface roughness (Rq=0.307 nm) and stronger band-edge PL intensity indicating fewer surface damage. These results can provide guidance on the fabrication of N-polar GaN related devices.

Unified mechanism of the surface Fermi level pinning in III-As nanowires

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary AlxGa1−xAs (0 ≤ x ≤ 0.45) and GaxIn1−xAs (0 ≤ x ≤ 1) alloys is pinned at the same position of 4.8 ± 0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the AlxGa1−xAs and GaxIn1−xAs nanowires leads to the accumulation of an excess of arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.

Mesoporous GaN Made by Selective Area Sublimation for Efficient Light Emission on Si Substrate

Selective area sublimation of thin (≤0.3 μm) GaN‐on‐Si epitaxial layers through nanoholes of an incomplete monolayer of SixNy results in the formation of mesoporous GaN. This morphologic change from a two‐dimensional flat layer to a nanostructured material presents the unexpected property of a huge increase (more than three orders of magnitude) of the GaN band edge photoluminescence intensity. Furthermore, the integrated photoluminescence intensity of such mesoporous GaN layers is comparable to a high quality 3.5 μm‐thick GaN‐on‐sapphire epitaxial layer or a 350 μm‐thick GaN substrate. Different possible mechanisms are discussed to explain the origin of the strong improvement of the optical properties.

Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption Versus Intercalation

Effects of electrochemical charging of quantum dots (QDs) have been reported in the past, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this presentation, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, it was observed that while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e−, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. In this talk, the nature of charging mechanisms will be discussed using the results from combined photoluminescence studies, radiative lifetime studies, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics and other devices.

Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption versus Intercalation.

Effects of electrochemical charging of quantum dots (QDs) have been reported previously, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this work, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e-, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. The nature of charging mechanisms is discussed using the results of combined photoluminescence, radiative lifetime, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics, and other devices.

Effects of Annealing on GaAs/GaAsSbN/GaAs Core-Multi-shell Nanowires.

The effects of ex-situ annealing in a N2 ambient on the properties of GaAs/GaAsSbN/GaAs core-multi-shell nanowires on Si (111) substrate grown by self-catalyzed molecular beam epitaxy (MBE) are reported. As-grown nanowires exhibit band edge emission at ~0.99 eV with a shoulder peak at ~0.85 eV, identified to arise from band tail states. A large red shift of 7 cm−1 and broadened Raman spectra of as-grown nanowires compared to that of non-nitride nanowires confirmed phonon localization at N-induced localized defects. On annealing nanowires to 750 °C, there was no change in the planar defects in the nanowire with respect to the as-grown nanowire; however, vanishing of the photoluminescence (PL) peak corresponding to band tail states along with enhanced band edge PL intensity, recovery of the Raman shift and increase in the Schottky barrier height from 0.1 to 0.4 eV clearly point to the efficient annihilation of point defects in these GaAsSbN nanowires. A significant reduction in the temperature-induced energy shift in the annealed nanowires is attributed to annihilation of band tail states and weak temperature dependence of N-related localized states. The observation of room temperature PL signal in the 1.3 μm region shows that the strategy of adding small amounts of N to GaAsSb is a promising route to realization of efficient nanoscale light emitters with reduced temperature sensitivity in the telecommunication wavelength region.

A study of charge transfer mechanism and optical properties of Au–CdS core–shell nanocrystals

The enhancement in band edge photoluminescence intensity and Purcell factor associated with red shift in PL features is observed in chemically prepared Au–CdS core–shell nanocrystals. The emission rate is increased by a factor 1.5 in case of Au–CdS core–shell nanocrystals compared to bare CdS nanocrystals, leading to the higher quantum yield. The magnitude of enhancement depends on the area of direct contact between Au and CdS nanocrystals, and hence on the interaction strength. These findings are understood on the basis of charge transfer and mixing of electron energy levels.

Analysis of Nonradiative Carrier Recombination Processes in InN Films by Mid-infrared Spectroscopy

We investigate the reduction in the efficiency of band-edge radiative recombination in InN by two carrier recombination processes via mid-gap states: radiative recombination via deep states and nonradiative recombination (NR). Because of the small band-gap energy value and the existence of the surface electron accumulation layer, the carrier transition processes via deep states cannot be observed easily. We address this problem by using mid-infrared photoluminescence (PL) measurements, and observe an emission peak around 0.32 eV at room temperature, which we interpret as being caused by transition processes via deep-defect states. Since this emission is weaker than the band-edge emission, the dominant carrier recombination process is concluded to be NR by phonon emission. The NR rate is known to be determined by the NR defect density, carrier transport processes to NR defects, and thermal activation processes of carriers. Carrier transport and capture processes by NR defects are investigated using p-type samples for various carrier mobility values. It is concluded that the NR rate is highly affected by the carrier transport, and that the candidates for the NR defect species are point defects and complexes of acceptor nature. We have also observed the correlation between the thermal conductivity and the band-edge PL intensity. As a result, we have found that the NR rate is highly affected by the carrier transport and thermal activation processes in InN.

On photoluminescence and photoreflectance of 1-eV GaInNAs-on-GaAs epilayers

The effects of growth temperature (410 and 470°C) and subsequent rapid thermal annealing (RTA) on luminescence performance of lattice-matched 1-eV GaInNAs-on-GaAs epilayers, grown by molecular beam epitaxy under constant fluxes, has been studied experimentally by 9-K photoluminescence (PL) and 300-K photoreflectance (PR) spectroscopy. The near band edge PL has been found to noticeably decrease and red-shift as the growth temperature (Tgr) is reduced from 470 to 410°C. This red-shift is the consequence of a reduction in the alloy band gap with decreasing Tgr, as confirmed by PR. An N-related broad PL band, whose intensity remarkably enhances with increasing Tgr, has been detected at longer wavelengths in the 9-K PL's. Post-growth RTA (1min at 800°C) significantly enhanced the near band edge PL but it did so at the expense of a PL blue-shift: the more the PL enhancement and blue-shift the lower the Tgr. This annealing-induced blue-shift is due to In–N bonds formation, whose magnitude enhanced as the Tgr is decreased. The broad PL band also enhanced upon the RTA, but the ratio between its intensity and the intensity of the near band edge PL reduced, especially for the sample grown at 410°C. 7-MeV electron irradiation to the dose of around 1015cm−2 applied prior to annealing considerably reduced the broad PL band upon annealing.

Photoluminescence study of ZnO nanostructures grown on silicon by MOCVD

Abstract ZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.

Photoluminescence Properties of Self-Assembled Monolayers of CdSe and CdSe/ZnS Quantum Dots

The photoluminescence (PL) properties of self-assembled monolayers (SAMs) of CdSe and CdSe/ZnS quantum dots (QDs) were investigated. From measurements of the temperature dependence of PL spectra, it is demonstrated that the thiol group, which is a constituent of the reagent for formation of the SAM, degrades the PL properties during the SAM formation process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is attributed to QD surface modification with a Cd(OH)2 layer. On the other hand, the defect-related PL band is not observed at all, even at 10 K, in the SAMs of CdSe/ZnS core/shell QDs. This fact indicates that the ZnS shell layer prevents degradation of the PL properties during the SAM process.

Cl Doping Effect by Thermal Treatment with KCl for ZnO Single Crystals

ABSTRACTWe investigated the optical and electrical properties of ZnO single crystals coated with KCl by examining the photoluminescence (PL) at 9 K and the temperature dependence of the carrier concentration. The band-edge PL intensity of the KCl-coated sample was much larger than that of the uncoated sample. Moreover, a substantial increase in Hall electrons was observed in the coated sample. X-ray photoelectron spectroscopy revealed bonding between chlorine and zinc atoms in the coated sample. Therefore, coating the surface of ZnO single crystals with KCl enhances the donor concentration and improves the surface state.

Unusual Band-Edge Photoluminescence Intensity Emitted by Cu-Diffused Silicon Crystals

Band-edge photoluminescence (BDE-PL) intensities of Czochralski (Cz) and float-zone (FZ)-grown silicon crystals diffused with Cu were observed from room temperature (RT) to 4.2 K. In contrast to the behavior of ordinary transition metals, unexpected enhancements of the BDE-PL intensities of both Cz and FZ samples were observed at RT in an appropriate Cu concentration range. For the observation temperature of 77 K, however, while the BDE-PL intensity of the Cz sample increased following the same trend as that observed at RT, the intensity of the FZ sample abruptly decreased given the same Cu concentration range as that of the Cz sample. The marked difference in the BDE-PL intensities between Cz and FZ samples was also observed with the change in the diffusion temperature of Cu. These unusual BDE-PL intensities were well explained on the basis of two competitive actions of Cu: the intensity enhancement due to surface passivation by Cu and the intensity reduction caused by a deep-level Cu center formed in the bulk.

Layer-by-Layer Assembly of Colloidal CdS and ZnS−CdS Quantum Dots and Improvement of Their Photoluminescence Properties

Thin films of colloidal CdS and ZnS−CdS alloy quantum dots (QDs) were fabricated by layer-by-layer (LBL) self-assembly to synthesize blue luminescent QD thin films. The assembly of negatively charged colloidal QDs and positively charged poly(diallyldimethylammonium chloride) results in QD/polymer multilayers. The relative intensity of the band-edge photoluminescence (PL) to that of the defect-related PL was decreased during the LBL assembly process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH 10, the band-edge PL intensity was recovered. The PL decay profiles before and after the dipping treatment were considerably different. From this fact, we conclude that the enhancement of the band-edge PL intensity originates from the reduction of nonradiative recombination processes at the QD surface.

Structural and luminescent properties of Er-doped ZnO films grown by metalorganic chemical vapor deposition

Structural and luminescent properties of Er-doped ZnO (ZnO:Er) grown by metalorganic chemical vapor deposition were investigated. In comparison to the undoped ZnO, the c-axis crystallographic orientation was deteriorated and the photoluminescence (PL) intensity of the band-edge became lower in the ZnO:Er. The weak band-edge PL in the ZnO:Er was due to the nonradiative recombination centers induced by the Er-doping processes. The band-edge PL intensity of ZnO:Er was drastically increased by the temperature annealing at 800°C in O2 ambient. The annealed ZnO:Er showed clear 1.54μm PL originating from the I13∕24→I15∕24 transitions of the Er3+ ions. These PL properties showed that the local arrangement of the Er ions was changed by the thermal annealing, resulting in a transformation from Er-related nonradiative centers in the as-grown sample to luminescent centers for 1.54μm PL in the annealed one.

Si doping of metal-organic chemical vapor deposition grown gallium nitride using ditertiarybutyl silane metal-organic source

Liquid Si ditertiarybutyl silane (DTBSi) metal-organic source was used as the Si dopant source for the growth of n-type GaN by metal-organic chemical vapor deposition (MOCVD) for the first time to replace the conventional gaseous Si sources like silane SiH 4 [K. Pakula, R. Bozek, J.M. Baranowski, J. Jasinski, Z. Liliental-Weber, J. Crystal Growth 267 (2004) 1] and disilane Si 2H 6 [L.B. Rowland, K. Doverspike, D.K. Gaskill, Appl. Phys. Lett. 66 (1995) 1495]. Electrical, structural, optical, and surface properties of the samples doped by DTBSi as well as an undoped control sample are determined by Hall, high resolution X-ray diffraction (HRXRD), photoluminescence (PL), and atomic force microscopy (AFM) measurements respectively. A constant doping efficiency for GaN is obtained with carrier concentration up to 10 18 cm −3. The typical HRXRD full-width at half-maximum values of symmetric (0 0 2) and asymmetric (1 0 2) planes are 284 and 482 arcsec, respectively. The near band edge PL intensity is found to be increased proportional to the doping concentration. Dark spot density is also determined from AFM measurement.

Comparison of ZnO Dry Etching in High Density Inductively Coupled CH4/H2 and C2H6/H2-Based Chemistries

CH4/H2 and C2H6/H2-based discharges are attractive for dry etching of single crystal ZnO because of their non-corrosive nature. We show that substitution of C2H6 for CH4 increases the ZnO etch rate by approximately a factor of 2 both with and without any inert gas additive. The C2H6/H2/Ar mixture provides a strong enhancement over pure Ar sputtering, in sharp contrast to the case of CH4/H2/Ar.The threshold ion energy for initiating etching is 42.4 eV for C2H6/H2/Ar and 59.8eV for CH4/H2/Ar. The etched surface morphologies were smooth, independent of the chemistry and the Zn/O ratio in the near-surface region was unchanged within experimental error after etching with both chemistries. The plasma etching improved the band-edge photoluminescence intensity and suppressed the deep level emission from the bulk ZnO under our conditions, due possibly to removal of surface contamination layer.

Comparison of CH 4/H 2 and C 2H 6/H 2 inductively coupled plasma etching of ZnO

CH 4/H 2-based discharges are attractive for dry etching of single crystal ZnO because of their non-corrosive nature. We show that substitution of C 2H 6 for CH 4 increases the ZnO etch rate by approximately a factor of 2 both with and without any inert gas additive. The C 2H 6/H 2/Ar mixture provides a strong enhancement over pure Ar sputtering, in sharp contrast to the case of CH 4/H 2/Ar. The threshold ion energy for initiating etching is 42.4 eV for C 2H 6/H 2/Ar and 59.8 eV for CH 4/H 2/Ar. The etched surface morphologies were smooth, independent of the chemistry and the Zn/O ratio in the near-surface region was unchanged within experimental error after etching with both chemistries. The plasma etching improved the band-edge photoluminescence intensity and suppressed the deep level emission from the bulk ZnO under our conditions, due possibly to removal of surface contamination layer.

Dry etching of bulk single-crystal ZnO in CH4/H2-based plasma chemistries

The effect of inert gas additive (He, Ar, Xe) to CH4/H2 discharges for dry etching of single crystal ZnO was examined. The etch rates were higher with Ar or Xe addition, compared to He but in all cases the CH4/H2-based mixtures showed little or no enhancement over pure physical sputtering under the same conditions. The etched surface morphologies were smooth, independent of the inert gas additive species and the Zn/O ratio in the near-surface region decreases as the mass number of the additive species increases, suggesting preferential sputtering of O. The plasma etching improved the band-edge photoluminescence intensity from the ZnO for the range of ion energies used here (290–355eV), due possibly to removal of surface contamination layer.