A combined approach to fabricating Si nanocrystals with high photoluminescence intensity

This work demonstrates that by combining three methods with different mechanisms to enhance the photoluminescence (PL) intensity of Si nanocrystals embedded in SiO₂ (or Si-nc:SiO₂), a promising material for developing Si light sources, a very high PL intensity can be achieved. A 30-layered sample of Si-nc:SiO₂/SiO₂ was prepared by alternatively evaporating SiO and SiO₂ onto a Si(1 0 0) substrate followed by thermal annealing at 1100 °C. This multilayered sample possessed a fairly high PL efficiency of 14% as measured by Greenham's method, which was 44 times that of a single-layered one for the same amount of excess Si content. Based on this multilayered sample, treatments of CeF₃ doping and hydrogen passivation were subsequently applied, and a high PL intensity which was 167 times that of a single-layered one for the same amount of excess Si content was achieved.     

Room temperature photoluminescence mechanism of SiOx film after annealing at different temperatures

SiO_x films were deposited on Si(1 0 0) substrates by evaporation of SiO powder. The samples were annealed from room-temperature (RT) to 1100 °C. After the samples were cooled down to RT, photoluminescence (PL) spectra from these samples were measured. It was found that when the annealing temperature Ta is not higher than 1000 °C, there is a PL centered at 620 nm, and with Ta increasing the intensity increases at first and then decreases when Ta is higher than 500 °C. When Ta is no less than 1000 °C another PL peak located at 720 nm appears. Combined with Raman and XRD spectra, we confirm that the latter PL is from Si nanocrystals that start to form when Ta is higher than 1000 °C. PL spectra for Ta<900 °C were discussed in detail and was attributed to defects in the matrix rather than from Si clusters.     

Modulation of the photoluminescence of Si quantum dots by means of CO2 laser pre-annealing

Si quantum dots (QDs) embedded in SiO₂ can be normally prepared by thermal annealing of SiO_x (x < 2) thin film at 1100 °C in an inert gas atmosphere. In this work, the SiO_x thin film was firstly subjected to a rapid irradiation of CO₂ laser in a dot by dot scanning mode, a process termed as pre-annealing, and then thermally annealed at 1100 °C for 1 h as usual. The photoluminescence (PL) intensity of Si QD was found to be enhanced after such pre-annealing treatment. This PL enhancement is not due to the additional thermal budget offered by laser for phase separation, but attributed to the production of extra nucleation sites for Si dots within SiO_x by laser irradiation, which facilitates the formation of extra Si QDs during the subsequent thermal annealing.     

Luminescence of ZnO nanocrystals capped with an organic dye

ZnO nanocrystals capped with an organic dye Rhodamine 6G (Rh6G) were investigated by photoluminescence (PL) and cathodoluminescence (CL) techniques. PL and CL spectra showed a remarkable decrease in visible emission intensity after ZnO nanocrystals were capped with Rh6G, indicating that dangling bonds and defect states existing at the surface of ZnO nanocrystals were significantly passivated. Rh6G on the ZnO surface exhibited a monomer-like emission, and the intensity and the position of the emission were dependent on the dye concentration.     

Room-temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical properties

ZnO thin film with strong orientation (0 0 2) and smooth surface morphology was electrosynthesized on ITO-coated glass substrate at room temperature under pulsed voltage. Photoluminescence (PL) shows two obvious peaks: violet band and strong green band. The former is due to the free-excitonic transition and the latter is believed to arise from the single ionized oxygen vacancy (V_O⁺). Raman scattering reveals that the 580 cm⁻¹ mode and the shoulder peak mode at 550 cm⁻¹ originate from the N-related local vibration mode (LVM) and E₁ (LO) mode, respectively.     

Surfactant effect on the upconversion emission and decay time of ZrO2:Yb-Er nanocrystals

Luminescent properties of doped ZrO₂:Er³⁺ and codoped ZrO₂:Yb³⁺-Er³⁺ nanocrystals with average size ∼54 nm were analyzed as a function of non-ionic surfactant (Pluronic F-127) concentration. Surfactant and non-surfactant samples were prepared by the sol-gel micelle process with hydrothermal aging and annealed at 1000 °C for 5 h. The introduction of the surfactant reduces the presence of impurities such as OH and CO₂ on both samples, and increments the tetragonal phase for codoped nanocrystals. It induces an increment larger than 90% and 70% for doped and codoped, respectively, for an optimum molar ratio of 0.0082. The observed enlargement of fluorescence decay time is partly the result of the nanosize effect but is dominated by the reduction of impurities attached on the nanocrystalline surface.     

Auger and photoluminescence analysis of ZnO nanowires grown on AlN thin film

ZnO nanowires were grown on AlN thin film deposited on the glass substrates using a physical vapor deposition method in a conventional tube furnace without introducing any catalysts. The temperature of the substrates was maintained between 500 and 600 °C during the growth process. The typical average diameters of the obtained nanowires on substrate at 600 and 500 °C were about 57 and 22 nm respectively with several micrometers in length. X-ray diffraction and Auger spectroscopy results showed Al diffused from AlN thin film into the ZnO nanowires for the sample grown at 600 °C. Photoluminescence of the nanowires exhibits appearance of two emission bands, one related to ultraviolet emission with a strong peak at 380-382 nm, and the other related to deep level emission with a weak peak at 503-505 nm. The ultraviolet peak of the nanowires grown at 500 °C was blue shifted by 2 nm compared to those grown at 600 °C. This shift could be attributed to surface effect.     

Photoluminescence and absorption in sol-gel-derived ZnO films

Highly c-axis-oriented ZnO films were obtained on corning glass substrate by sol-gel technique. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption in the absorption (UV) spectra are closely related to the post-annealing treatment. The difference between PL peak position and the absorption edge, designated as Stokes shift, is found to decrease with the increase of annealing temperature. The minimum Stokes shift is about 150 meV. The decrease of Stokes shift is attributed to the decrease in carrier concentration in ZnO film with annealing. X-ray diffraction, surface morphology and refractive index results indicate an improvement in crystalline quality with annealing. Annealed films also exhibit a green emission centered at ∼520 nm with activation energy of 0.89 eV. The green emission is attributed to the electron transition from the bottom of the conduction band to the antisite oxygen O_Zn defect levels.     

Photoluminescence properties of trace amounts of Pr and Tb in yttria-stabilized zirconia

We have investigated the photoluminescence (PL) properties of trace amounts of Pr and Tb in single-crystal samples of yttria-stabilized zironia (YSZ), and found that Pr of the order of 10⁻⁶ mass% and Tb of the order of 10⁻⁵ mass% in YSZ can be detected by the PL spectroscopy. The PL spectra of the YSZ samples for the 280 nm excitation were comprised of several peaks and a broad emission. The peaks were attributed to transitions of Pr³⁺ and Tb³⁺ in the YSZ samples, whereas the broad emission seemed to be attributed to the yttria-associated oxygen vacancies. The peak intensities corresponded to the amounts of Pr and Tb in the YSZ samples, the amounts of which were analyzed by glow discharge mass spectrometry. In the PL excitation spectra, but not in the photoabsorption spectra, small peaks at 376 and 381 nm were observed, and were attributed to the transitions of Tb³⁺ in the YSZ samples. The results of the PL excitation spectra corresponding to the Pr³⁺ line emissions suggest that the charge transfer occurs between the YSZ and Pr ion in it. The trace amounts of these lanthanoids in YSZ would disturb the decay process of the photoinduced electrons to the yttria-associated oxygen vacancies.     

Electroluminescence of Si Nanocrystal-Doped SiO2

We perform a comparative st udy on the electroluminescence （EL） and photoluminescence （PL） of Si nanocrystaldoped SiO2 （nc-Si：SiO2） and SiO2, and clarify whether the contribution from Si nanocrystals in the EL of nc-Si：SiO2 truly exists. The results unambiguously indicate the presence of EL of Si nanocrystals. The difference of peak positions between the EL and PL spectra are discussed. It is found that the normal method of passivation to enhance the PL of Si nanocrystals is not equally effective for the EL, hence new methods need to be explored to promote the EL of Si nanocrystals.[第一段]     

Inter-particle spin-polarized tunneling in arrays of magnetite nanocrystals

Inter-particle spin-polarized tunneling was measured in an organically capped magnetite nanocrystal (NC) array deposited between 30 nm spaced gold electrodes. Magnetoresistance (MR) measurements performed around the blocking temperature (T_b) of the magnetic moments of the particles in the array, which was relatively high (220 K), yielded negative MR values of the order of 10-25% under moderate magnetic fields of several kOe. The field dependence of the MR followed closely the square of the film's magnetization and its voltage dependence indicated maximal spin polarization around the Fermi level. These findings suggested that the measured MR is the result of spin-polarized tunneling between individual magnetite NCs acting as superparamagnetic spin polarizers.     

Influence of pH value on the quality of sol-gel derived ZnO films

In sol-gel derived ZnO films the pH value of the sol plays an important role in controlling their properties. In this study, the influence of adding monoethanolamine to zinc acetate solution in methanol on structural, surface morphology and optical properties has been investigated. Addition of monoethanolamine to zinc acetate solution transforms the nature of the sol from acidic to alkaline by changing the pH value from 6.4 to 10.6. The investigations indicate that high quality ZnO films are obtained by using sol having monoethanolamine to zinc acetate ratio of 1:1 and pH value of 10.6.     

Donor-acceptor pair luminescence in nitrogen-doped ZnO films grown on lattice-matched ScAlMgO4 (0001) substrates

We have grown nitrogen-doped ZnO (ZnO:N) films by laser molecular-beam epitaxy. The use of lattice-matched ScAlMgO₄ substrates prevented the degradation of crystallinity induced by the nitrogen incorporation to the films. Despite this improvement, we have not obtained ZnO:N films which showed p-type conductivity. We studied the optical properties of these ZnO:N films. Donor-acceptor pair (DAP) luminescence was observed. The results indicate the formation of an acceptor state. The energy position of the DAP luminescence is lower than that reported by Look et al. [Appl. Phys. Lett. 81 (2002) 1830]. The DAP luminescence band shifts to lower energy with increasing nitrogen concentration. A photoluminescence recombination possibly due to the free-electron-to-acceptor (FA) transition was observed at temperatures higher than 40 K. The acceptor ionization energy was estimated from the energy position of the FA luminescence to be 266 meV.     

Microstructure and multiphoton luminescence of Au nanocrystals prepared by using glancing deposition method

Au crystal columns embedded in SiO₂ with an average length of 480 nm and diameter of 30 nm were prepared by radio frequency co-sputtering technique with glancing angle. The photoluminescence (PL) of the Au-SiO₂ crystal column film exhibited polarization characteristic. With an increase of the laser power, the slope ∂ log(PL intensity)/∂ log(laser power) changed from 2 to 3, which indicated that the PL of Au-SiO₂ crystal columns were induced by two- and three-photon absorption, respectively.     

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 10^19-20 cm⁻³. Current-voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A⁰X) emission and a donor-acceptor pair (DAP) emission due to doped nitrogen, respectively.     

Structure, morphology and properties of Fe-doped ZnO films prepared by facing-target magnetron sputtering system

We have prepared a series of ZnO films with various concentrations of Fe dopant by using facing-target magnetron sputtering system and investigated their structure, morphology, optical properties and magnetic properties by means of the X-ray diffraction, Raman spectrometer, X-ray photoelectron spectroscopy, scanning electron microscope, UV-vis spectrophotometer, spectrofluorophotometer and vibrated sample magnetometer, respectively. The results showed Fe was in 2+ valence state in film. With increasing the concentration of Fe dopant the crystal quality deteriorated gradually and an increasing compressive stress occurred in ZnO films. All the grains grew with a columnar form along the [0 0 2] direction and the average size of grains decreased monotonically as the concentration of Fe dopant increased. The calculated results indicated that the band gap of films decreased with the increment of the concentration of Fe dopant, which resulted in the red-shift of violet emission peak. Moreover, the intensity of violet emission peak increased with the increment of Fe concentration because of the interface trap existing in depletion regions located at the ZnO grains boundaries. When the concentration of Fe dopant was smaller than 0.028, the films exhibited paramagnetic properties while the films displayed weak ferromagnetic properties when the concentration of Fe dopant increased to 0.041.     

Photoluminescence studies from silicon nanocrystals embedded in spin on glass thin films

Photoluminescence (PL) from silicon nanocrystals (Si-nc) prepared from pulverised porous silicon and embedded in undoped (SOG) or phosphorus-doped spin-on-glass (SOD) solutions was studied. Effects of rapid thermal annealing on the PL was also investigated. A strong room temperature PL signal was observed at 710 nm due to the recombination of electron-hole pairs in Si-nc and the PL maximum shifts to the blue region as the phosphorus concentration in the spin on glass increases. However, the rapid thermal annealing process (30 s, 900°C) quenches the PL response. These results suggest that for Si-nc/SOG (SOD) the surface termination is efficient but high phosphorus doping of Si-nc is detrimental to the PL.     

Rapid thermal annealing induced changes on the contact of Ni/Au to N-doped ZnO

N-doped p-type ZnO (p ∼ 10¹⁸cm^-3) was grown on sapphire(0 0 0 1) substrate by metal-organic chemical vapor deposition method. Ni/Au metal was evaporated on the ZnO film to form contacts. As-deposited contacts were rectifying while ohmic behavior was achieved after thermally annealing the contacts in nitrogen environment. Specific contact resistance was determined by circular transmission line method and a minimum specific contact resistance of 8 × 10⁻⁴ Ω cm² was obtained for the sample annealed at 650 °C for 30 s. However, Hall effect measurements indicate that, as the rapid thermal annealing temperature increased up to 550 °C or higher the samples’ conductive type have changed from p-type to n-type, which may be due to the instability nature of the present-day p-type N-doped ZnO or the dissociation of ZnO caused by annealing process in N₂ ambient. Evolution of the sample's electric characteristics and the increment of metal/semiconductor interface states induced by rapid thermal annealing process are supposed to be responsible for the improvement of electrical properties of Au/Ni/ZnO.     

As-doped p-type ZnO films grown on SiO2/Si by radio frequency magnetron sputtering

p-Type ZnO:As films with a hole concentration of 10¹⁶-10¹⁷ cm⁻³ and a mobility of 1.32-6.08 cm²/V s have been deposited on SiO₂/Si substrates by magnetron sputtering. XRD, SEM, Hall measurements are used to investigate the structural and electrical properties of the films. A p-n homojunction comprising an undoped ZnO layer and a ZnO:As layer exhibits a typical rectifying behavior. Our study demonstrates a simple method to fabricate reproducible p-type ZnO film on the SiO₂/Si substrate for the development of ZnO-based optoelectronic devices on Si-based substrates.     

Magnetic field induced metal-insulator transition in semiconductors

Metal-insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. Using Thomas-Fermi screening function and a screened coulomb potential in the Hamiltonian, a two-parameter varitional calculation leads to metallization. The correlation effect among the electrons is considered through an effectivemass that depends on the spatial seperation between impurities. Results are provided for a many valley semiconductor (Si) and two single valley semiconductors (GaAs and CdTe). Our results show that in a magnetic field the critical concentration at which metallization occurs increases. The correlation effects bring out one order change in the critical concentration values, in intense magnetic fields.