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Formation of ZnO luminescent films on SiN films for light source of high-resolution optical microscope
Abstract
We fabricated ZnO/SiN films for use as a light source of a high-resolution optical microscope and characterized the properties of the films, and demonstrated images obtained with the microscope using the fabricated ZnO/SiN films. A 100-nm-thick ZnO film deposited on a SiN film showed a much higher CL intensity than the SiN film, and it was enhanced by high-temperature annealing of the ZnO film. Electron beam excitation assisted optical microscope images of gold particles of 200nm diameter taken using the ZnO/SiN film and SiN indicated that the ZnO/SiN films can provide a higher signal-to-noise (S/N) ratio and a higher frame rate than the SiN film. It was shown that the dynamic observation of living cells becomes possible using the high-resolution optical microscope with a bright light source. Moreover, this fact promises that such optical microscope can contribute to progress in the medical and biological fields. (C) 2014 The Japan Society of Applied Physics
... It is possible that this EL originates from the ZnS host, but it is reasonable to think that the emission around 380 nm corresponds to the excitonic emission of ZnO and the visible emission originates from defects in the ZnO. 42) In the EL spectrum, if ZnO was excited by electron impacts, higher voltage was necessary because a ZnO thin film cold cathode, which has the same structure as a typical high field EL device (ZnS/Ta 2 O 5 /ITO), requires an applied voltage higher than 150 V and an anode bias of over 1000 V. 43) It is suggested that carrier recombination occurs in the ZnO layer, so the ZnTe/ZnO layer acts as an inorganic LED. ...
A high field electroluminescent device having two light emitting mechanisms—direct impact excitation and carrier recombination—was fabricated in order to improve the luminous efficiency. For the carrier recombination process, a TPD/Alq3 stacked layer and a ZnTe/ZnO stacked layer were used. Although the emission intensity from carrier recombination is poor, emission due to carrier recombination and impact excitation was observed from a single device. In this device, an electron has two roles: one is impact excitation and the other is recombination with a hole. It is shown that one electron gives two or more photons from a DC electroluminescent device.
... We have succeeded to develop a Zn 2 SiO 4 film of 20 nm thickness by annealing a ZnO film of 50 nm thickness sputtered on a Si 3 N 4 substrate at 1000 °C in nitrogen gas 33,34 . The luminescence of the light emitted by Zn 2 SiO 4 is 20 times brighter luminescence than that of an as-deposited ZnO film even though the thickness is 20 nm, which is 5 times thinner than that of an as-deposited ZnO 35 Fig. 3(b). The full width at half maximum (FWHM) of the intensity profile is 107 nm, and the SNR is 10. ...
Optical microscopes are effective tools for cellular function analysis because biological cells can be observed non-destructively and non-invasively in the living state in either water or atmosphere condition. Label-free optical imaging technique such as phase-contrast microscopy has been analysed many cellular functions, and it is essential technology for bioscience field. However, the diffraction limit of light makes it is difficult to image nano-structures in a label-free living cell, for example the endoplasmic reticulum, the Golgi body and the localization of proteins. Here we demonstrate the dynamic imaging of a label-free cell with high spatial resolution by using an electron beam excitation-assisted optical (EXA) microscope. We observed the dynamic movement of the nucleus and nano-scale granules in living cells with better than 100 nm spatial resolution and a signal-to-noise ratio (SNR) around 10. Our results contribute to the development of cellular function analysis and open up new bioscience applications.
... To observe the ultrafine structures and physiological functions of living cells at high spatial resolution without labeling, we have proposed an electron-beam excitation assisted (EXA) optical microscope, in which an electron beam focused on a luminescent thin film excites a nanometric light source near the specimen [11][12][13]. EXA microscopy is very similar to SNOM in that a nanometric light source is produced with a small aperture, except that in EXA microscopy, the light source is produced by the electron beam focused on the luminescent film. The light source can reach nanoscale size because the electron beam can be focused in a region with a size of a few nanometers. ...
We fabricated a bright and thin Zn<sub>2</sub>SiO<sub>4</sub> luminescent film to serve as a nanometric light source for high-spatial-resolution optical microscopy based on electron beam excitation. The Zn<sub>2</sub>SiO<sub>4</sub> luminescent thin film was fabricated by annealing a ZnO film on a Si<sub>3</sub>N<sub>4</sub> substrate at 1000 °C in N<sub>2</sub>. The annealed film emitted bright cathodoluminescence compared with the as-deposited film. The film is promising for nano-imaging with electron beam excitation-assisted optical microscopy. We evaluated the spatial resolution of a microscope developed using this Zn<sub>2</sub>SiO<sub>4</sub> luminescent thin film. This is the first report of the investigation and application of ZnO/Si<sub>3</sub>N<sub>4</sub> annealed at a high temperature (1000 °C). The fabricated Zn<sub>2</sub>SiO<sub>4</sub> film is expected to enable high-frame-rate dynamic observation with ultra-high resolution using our electron beam excitation-assisted optical microscopy.
The luminescent properties of ZnO thin films have been developed to form a nanometric light source for an electron beam-excitation assisted optical microscope. Spatial uniformity of the electron-induced luminescence is important when using this luminescent ZnO film-based light source. In this work, ZnO thin film surfaces have been flattened using broad ion beam polishing with Ar+ions at a grazing angle of incidence. Atomic force microscopy and electron microscopy confirmed transformation of needle-like ZnO surface structures into scale-like morphologies during ion irradiation. The root-square-mean roughness, corresponding to the height distribution decreased dramatically from 32 to 8 nm after 5 keV Ar+ion irradiation for 60 min. X-ray photoelectron spectroscopy and Monte Carlo simulations revealed changes in the chemical state and lattice defects during ion irradiation. Positive shifts in the binding energies of the Zn 2p and O 1s spectra suggested preferential Zn atom sputtering and chemical composition change from oxygen-deficient to stoichiometric ZnO. The ultraviolet region luminescent intensity improved after ion irradiation for 60 min. Valence band electron redistribution was considered to influence the ZnO thin film’s luminescent properties. We propose that homogeneous luminescence over a specified area with flat surfaces can be accomplished by repeated short-duration ion irradiation.
We fabricated flat and homogeneous Al 2 O 3 /ZnO/Al 2 O 3 heterostructure luminescent layers by atomic layer deposition (ALD) to serve as a nanometer-scaled light source for high-spatial-resolution optical microscopy based on electron beam excitation (EXA). A smooth surface was obtained by inserting an Al 2 O 3 buffer layer and an Al 2 O 3 barrier layer resulting in brighter and more uniform cathodoluminescence (CL) compared with that from a directly deposited ZnO layer. The root mean square (rms) value determined by atomic force microscope drastically decreased from 2.4 nm (for typical ZnO film) to 0.5 nm (for the six-layer pairs of the Al 2 O 3 /ZnO/Al 2 O 3 heterostructure). The CL brightness increased by two times of that in the Al 2 O 3 /ZnO/Al 2 O 3 heterostructure due to a waveguide effect. However, the increase in the number of the layer pairs from one to six reduced the CL brightness by half. The CL emission variability was about 30% improved that is supposed to enable high-resolution using Al 2 O 3 /ZnO/Al 2 O 3 luminescent layers for an EXA microscope.
Phosphorus-doped ZnO nanorods were successfully prepared by hydrothermal method using ammonium dihydrogen phosphate as the dopant source. The effects of different Zn sources, zinc nitrate concentration, reaction duration, ammonium dihydrogen phosphate concentration, and annealing temperature on the morphologies and phosphorus content of ZnO nanorods were investigated. The doping mechanism of phosphorus-doped ZnO nanorods has been discussed. Detailed photoluminescence studies of phosphorus-doped ZnO revealed characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission at 3.347 eV, and donor-to-acceptor pair emission at 3.232 eV, free-electron to neutral-acceptor emission at 3.312 eV. This means that stable acceptor levels with a binding energy of about 126 meV were formed by phosphorus doping.
We fabricated ZnO thin films for use as a light source of a high resolution optical microscope and characterized the properties of the films. The properties of the fabricated ZnO films were evaluated as the light source of the microscope. The cathodoluminescence intensity of the 100 nm thick ZnO film deposited on a Si 3 N 4 film was enhanced significantly by high temperature annealing in N 2 . The microscope images of 200 nm diameter gold particles taken using as-deposited and annealed ZnO film indicated that annealed ZnO films can provide a higher signal-to-noise ratio and a higher frame rate than the asdeposited ZnO film.
A feasible alternative to current energy-saving light sources is environmentally friendly new-generation cathodoluminescent light sources (CLSs) based on luminescence produced by electrons emitted from the field emission cathode. Because of the lack of available optimally designed general-purpose lamps with field emission cathodes, the development of an efficient prototype CLS potentially mass-produced at a low cost is currently the top priority.
Silicon-rich silicon nitride (SiNX) films have attracted enormous interests due to their promising luminescence properties and well compatibility with current CMOS technique. In this short review, the fabrication process of SiNX was addressed as well as their chemical composition and structure. The optical properties and future applications of the light emitting devices (LEDs) were also discussed. By analyzing the carrier conduction and combination mechanisms, electroluminescence (EL) intensity of LEDs was greatly improved through the following approaches: passivation of the interfacial states between SiN X films and Si substrates through NH3 plasma pre-treatment and post-annealing process; the balance of carrier injection via SiO2 electron accelerating layer; increase of the carrier-injection efficiency and light extraction efficiency and the enhancement of radiative recombination efficiency by surface plasmon.
The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
This study examined the origin of visible luminescence from ZnO layers deposited on p -Si substrates by various growth methods using temperature dependent photoluminescence measurements. The deep level emissions of ZnO layers are found to be strongly dependent on the growth conditions and growth methods used. For the samples grown by sputtering, the visible emission consisted of violet, green, and orange-red regions, which corresponded to zinc interstitial ( Zn <sub>i</sub>) , oxygen vacancy (V<sub> O </sub>) , and oxygen interstitial ( O <sub>i</sub>) defect levels, respectively. In contrast, the deep level emissions of metal organic chemical vapor deposition grown samples consisted of blue and green emissions and blue and orange-red emissions at low and high oxygen flow rates, respectively. The ZnO nanorods synthesized by thermal evaporation showed a dominant deep level emission at the green region, which is associated with oxygen vacancies (V<sub> O </sub>) .
Lattice-matched (Δa/a=0.09%) ScAlMgO <sub> 4 </sub>(0001) substrates were employed to grow single crystalline quality ZnO films by laser molecular-beam epitaxy. Extremely smooth surface represented by atomically flat terraces and half unit cell (0.26 nm) high steps and extremely small orientation fluctuations both in-plane (≪0.02°) and out-of-plane (≪0.01°) are achieved. The films have high mobility (∼100 cm <sup> 2 </sup> /V s ) together with low residual carrier concentration (∼10<sup>15</sup> cm <sup> -3 </sup>). Excellent optical properties, including a clear doublet of A and B exciton peaks in absorption spectra, were also observed. These features could not be simultaneously achieved for ZnO films grown on sapphire(0001) having a large lattice mismatch (Δa/a=18%). © 1999 American Institute of Physics.
Thin films of ZnO have been deposited on glass and silicon substrates by the pulsed laser deposition technique employing a KrF laser (λ=248 nm). The influence of the deposition parameters, such as substrate temperature, oxygen pressure, and laser fluence on the properties of the grown films, has been studied. All the films grown over a rather wide range of deposition conditions were found to be optically transparent, electrically conductive, and c‐axis oriented, with the full width at half‐maximum (FWHM) of the (002) x‐ray reflection line being very often less than 0.25°. Under optimized laser fluence and oxygen pressure conditions, highly c‐axis oriented films having a FWHM value less than 0.15° and optical transmittance around 85% in the visible region of the spectrum have been grown at a substrate temperature of only 350 °C. These are among the best properties yet reported for ZnO films grown by any technique at such a low temperature. © 1994 American Institute of Physics.
Zinc-oxide (ZnO) nanoparticles (NPs) fabricated by ion implantation combined with thermal oxidation were installed into vacuum fluorescent displays. The cathode-ray induced luminescence observed was strongly dependent on sample oxidation temperature. Raising the temperature form 650 to 800 ° C , the relative intensity of the defect band around 480 nm to that of the free-exciton recombination at 380 nm increased. With increasing accelerating voltage, the free-exciton peak of ZnO NPs shifted to a longer wavelength. This was ascribed to a temperature increase due to cathode ray irradiation because the shift was only observed during continuous wave mode operation and not during pulsed mode operation. As the oxidation temperature was further raised to 900 ° C strong green band emission around 520 nm became the only luminescence band. This was due to the formation of the Zn <sub>2</sub> SiO <sub>4</sub> phase from the reaction between ZnO NPs and the SiO <sub>2</sub> substrate, which was confirmed by x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption spectroscopy.
We have developed a high-resolution fluorescence microscopy method based on high-accuracy localization of photoswitchable fluorophores. In each imaging cycle, only a fraction of the fluorophores were turned on, allowing their positions to be determined with nanometer accuracy. The fluorophore positions obtained from a series of imaging cycles were used to reconstruct the overall image. We demonstrated an imaging resolution of 20 nm. This technique can, in principle, reach molecular-scale resolution.
We introduce a method for optically imaging intracellular proteins at nanometer spatial resolution. Numerous sparse subsets
of photoactivatable fluorescent protein molecules were activated, localized (to ∼2 to 25 nanometers), and then bleached. The
aggregate position information from all subsets was then assembled into a superresolution image. We used this method—termed
photoactivated localization microscopy—to image specific target proteins in thin sections of lysosomes and mitochondria; in
fixed whole cells, we imaged vinculin at focal adhesions, actin within a lamellipodium, and the distribution of the retroviral
protein Gag at the plasma membrane.
Heteroepitaxial ZnO films were grown on (111) surface of yttria stabilized zirconia (YSZ) and (0001) surface of sapphire by PLD method, using KrF eximer laser (248nm) in an ultra-high-vacuum chamber. ZnO grown on YSZ (111) at the substrate temperature of 800"C had an epitaxial relationship at the ZnO/YSZ interface of ZnO [1120J//YSZ [110]. Hexagonalshaped grains were observed whose surfaces were atomically flat. The grain size of ZnO increased and the Hall mobility rose toward 1400nm and 75cnr/Vs. respectively as film thickness increased from 10 nm to 800 nm.
Cerium ions were implanted into amorphous silicon nitride thin films,
and the cathodoluminescence (CL) of the thin films was investigated. The
Ce ions were distributed within a depth of around 20 nm from the surface
of the amorphous silicon nitride, and they formed three kinds of cations
in the thin films. By implanting the Ce ions, the CL of the films was
distributed around a wavelength of 405 nm. The CL increase of more than
2.2 times compared to silicon nitride thin films was observed at 25-kV
electron beam excitation. This increase was significantly enhanced by
adopting a high-temperature annealing process after the cerium-ion
implantation.
We analyzed light intensity distributions in a subwavelength fluorescent film, which was excited by a focused electron beam. We have developed an analyzing method using Monte Carlo simulation and the finite-difference time-domain (FDTD) method. Electron scattering and trajectories were calculated by Monte Carlo simulation. Propagation and scattering of light excited with the electrons was calculated by FDTD method. A nanometric light spot was formed on the fluorescent film surface and its light intensity and its full width at half maximum (FWHM) were evaluated. We discuss the intensity and the FWHM dependence on the thickness of the fluorescent thin film and the acceleration voltage of an incident electron beam.
Electrophoretic deposition of the phosphor particles was employed to prepare an anode plate for the field emitter display. The process was analyzed by depositing magnesium hydroxide with phosphor particles via electrolysis of Mg(NO3)(2) . 6H(2)O in isopropyl alcohol. It was found that the concentration of MgNO3+ and OH- ions from the water present in the coating bath play important roles in deposition of phosphors onto the substrate. The deposition rate and the deposited amounts of phosphors are optimized for maximum brightness under low-voltage excitation. Also, the effects of MgO as a binder on luminance are discussed. In ZnO:Zn phosphors, the brightness of 400 cd/m(2) can be obtained at 500 V and 30 mu A.
High-quality ZnO films were successfully prepared on Si wafers by low-pressure MO-CVD using zinc acetylacetonate (Zn(C5H7O2)2) and oxygen. The c-axis oriented ZnO films were grown on p-type Si wafers at temperature of 520 °C with ZnO buffers layers deposited by RF sputtering. Although, the ZnO layer deposited by sputtering has a poor c-axis orientation, the films prepared by MO-CVD on the ZnO buffer layer shows a sharp X-ray diffraction peak at 34.4° corresponding to the (0002) of hexagonal ZnO. Room temperature photoluminescence spectrum of the all film exhibits a strong peak consisted of near-band edges emission at 378 nm. Current–voltage characteristics of the ZnO(n)/Si(p) heterojunction exhibits non-linear and rectifying characteristics with a small current leakage in the reverse direction. A dark-blue light was clearly observed around the periphery of the top Al electrode by applying forward bias voltages.
Electrophoretic deposition of the phosphor particles was employed to prepare an anode plate for the field emitter display. The process was analyzed by depositing magnesium hydroxide with phosphor particles via electrolysis of Mg(NO 3 ) 2 .6H 2 O in isopropyl alcohol. It was found that the concentration of MgNO 3 + and OH - ions from the water present in the coating bath play important roles in deposition of phosphors onto the substrate. The deposition rate and the deposited amounts of phosphors are optimized for maximum brightness under low-voltage excitation. Also, the effects of MgO as a binder on luminance are discussed. In ZnO:Zn phosphors, the brightness of 400 cd/m 2 can be obtained at 500 V and 30 μA.
ZnO thin films were grown on Si(111) substrates by employing an epitaxial ZnS thin film as a buffer layer. The structural and luminescent properties of the ZnO thin films have been investigated in view of the application to opto-electronic devices due to near-ultraviolet emission by exciton the binding energy of which is about 60meV. When the epitaxial ZnS buffer layer was grown on the Si(111) substrate at a substrate temperature of 200°C by electron beam evaporation, the epitaxial ZnO film was successfully grown on the ZnS/Si(111) layer with the orientation of (0002), [112̄0]ZnO∥(111), [11̄0]ZnS∥(111), [11̄0]Si(111) at a substrate temperature of 400°C. An excitonic emission with a peak at 3.35eV at 20K was successfully obtained by exciting at 325nm of He–Cd laser.
ZnO epitaxial thin films have been grown on sapphire(0001) substrates by molecular beam epitaxy using elemental zinc and oxygen supplied by an RF radical source. Despite the large lattice mismatch between ZnO and the underlying sapphire substrate, ZnO layers with (0002) rocking curve half-widths of ∼12arcsec have been grown. X-ray reciprocal lattice scans along the [0001] direction show strong Pendellösung fringes indicating the presence of an extremely flat interface and surface as was confirmed by atomic force microscopy experiments. X-ray pole figure measurements indicate that the a-axis of the epilayer was rotated with respect to the a-axis of the substrate by 30°. Preliminary photoluminescence measurements indicate predominant near-bandedge emission.
The atomic force microscopy (AFM), the X-ray diffraction with glancing input angle (GXRD) and the cathodoluminescence (CL) spectra of a ZnO film on Si, annealed at different temperatures, were measured. The results showed that, the crystal quality of the film improved with increasing the annealing temperature, while the hexagonal phase of the ZnO film transformed into a kind of mixture phase including a hexagonal and a trigonal phase, after annealing at 800 °C for 1 h. The CL spectrum also shows the intrinsic emission bands of ZnO and Zn2SiO4, in which the ZnO is the main source of the spectrum. Increasing the temperature continuously up to 950 °C changed the main source of sample's luminescence from the emission of ZnO to the emission of zinc silicate. This indicates a creation of new ternary compound Zn2SiO4 in the film.
Tentative symmetry assignments of p valence bands and s conduction bands can be made for ZnO and CdS on the basis of a tight-binding model. The six-fold degenerate p-bands are split in hexagonal crystals into a four-fold degenerate and a two-fold degenerate band. The fourfold degeneracy is split by spin-orbit coupling. The polarization of recombination radiation depends upon which band the hole belongs to, and is almost independent of the recombination mechanism. The polarization of the edge emission (the series of equally spaced emission lines) should be strongly temperature-dependent. Quantitative agreement is obtained between the predictions of this band model for CdS and the edge-emission polarization experiments of Dutton (J. Phys. Chem. Solids6, 101 (1958)). It is shown that the spectra of edge-emission cannot be reasonably explained without the introduction of impurities or surfaces to absorb crystal momentum. In CdS, recombination from a shallow trap seems necessary to explain the large coupling to the lattice apparent in the observed emission spectrum. The edge-emission spectrum should approximate a Poisson distribution for recombination from a trap. The mean number of emitted phonons is a measure of the radius of the trapped carrier-wave function.
Highly-oriented zinc oxide (ZnO) films were grown on quartz glass substrates by radio frequency magnetron sputtering method. The temperature dependence of the photoluminescence spectra of the ZnO films annealed in argon, argon mixed with 5% hydrogen (H2/Ar) and oxygen ambient, respectively, was investigated from −190 to 600 °C. Results shown that UV light emission was greatly enhanced by annealing the as-grown ZnO film in H2/Ar ambient. Meanwhile, strong visible light emission was observed from the ZnO film annealed in oxygen ambient, and intense emissions in both UV and visible region were obtained from the ZnO films annealed in argon ambient. The UV emission from the ZnO films showed a high thermal stability that can be clearly observed up to 400 °C. The effect of the annealing ambient and the photoluminescence temperature dependence are discussed with the relations to the structural defects. © 2002 American Institute of Physics.
Zinc oxide (ZnO) films were synthesized by thermal oxidation of metallic zinc films in air. The influence of annealing temperatures ranging from 320 to 1000 °C on the structural and optical properties of ZnO films is investigated systematically using x-ray diffraction and room temperature photoluminescence (PL). The films show a polycrystalline hexagonal wurtzite structure without preferred orientation. Room temperature PL spectra of the ZnO films display two emission bands, predominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is observed that the ZnO film annealed at 410 °C exhibits the strongest UV emission intensity and narrowest full width at half maximum (81 meV) among the temperature ranges studied. The excellent UV emission from the film annealed at 410 °C is attributed to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects at such low temperature. The visible emission consists of two components in the green and yellow range, and they show different temperature dependent behavior from UV emission. Their possible origins are discussed. © 2003 American Institute of Physics.
The mechanism of ultraviolet (UV) and green emission of ZnO thin films deposited on (001) sapphire substrates by pulsed laser deposition was investigated by using postannealing treatment at various annealing temperatures after deposition. Structural, electrical, and optical properties of ZnO films have been also observed. As the postannealing temperature increased, the intensity of UV (380 nm) peak and the carrier concentration were decreased while the intensity of the visible (about 490–530 nm) peak and the resistivity were increased. The role of oxygen in ZnO thin film during the annealing process was important to the change of optical properties. The mechanism of the luminescence suggested that UV luminescence of ZnO thin film was related to the transition from near band edge to valence band, and green luminescence of ZnO thin film was caused by the transition from deep donor level to valence band due to oxygen vacancies. The activation energy derived by using the variation of green emission intensity was 1.19 eV. © 2004 American Institute of Physics.
Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle. © 2001 American Institute of Physics.
The photoluminescence (PL) spectra of the undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV (UV) and 2.38 eV (green). The variation of these peak intensities and that of the I–V properties of the ZnO/Si heterojunctions were investigated at different annealing temperatures and atmospheres. The defect levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital. It is concluded that the green emission corresponds to the local level composed by oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi. © 2001 American Institute of Physics.
Neutral-donor-bound-exciton transitions have been observed in ZnO. The
isolated neutral donors are made up of defect pair complexes. The
neutral-donor nature of these pair complexes was determined from
magnetic-field measurements and from two-electron transitions. Excited
states of the neutral-donor bound excitons were observed in the form of
rotator states analogous to rotational states of the H2
molecule.
Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.
The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.
An exact derivation of the Scherrer equation is given for particles of spherical shape, values of the constant for half-value breadth and for integral breadth being obtained. Various approximation methods which have been used are compared with the exact calculation. The tangent plane approximation of v. Laue is shown to be quite satisfactory, but some doubt is cast on the use of approximation functions. It is suggested that the calculation for the ellipsoidal particle based on the tangent plane approximation will provide a satisfactory basis for future work.
ZnO thin films on (1 0 0) p-type silicon substrates have been deposited by pulsed laser deposition technique. In order to investigate the effect of post-annealing treatment with oxygen on the optical property of ZnO thin films, films have been annealed at various substrate temperatures after deposition. After post-annealing treatment in the oxygen ambient, the stoichiometry of ZnO film has been observed to be improved which results in increasing ultra-violet (UV) emission intensity of photoluminescence (PL).
ZnO thin films on fused quartz substrates were prepared by a glycol-based Pechini method. The structural and optical properties were charac-terized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance spectrum, and photoluminescence (PL) spectrum. A red emission around 700 nm was found in PL spectrum, and its peak intensity gained a strong enhancement (∼140%) while annealing temper-ature increased from 700 • C to 800 • C. The red emission was ascribed to the possible high defect density in boundary layers of nanocrystalline grains.
The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements. At liquid helium temperature in undoped crystals the neutral donor bound excitons dominate in the PL spectrum. Two electron satellite transitions (TES) of the donor bound excitons allow to determine the donor binding energies ranging from 46 to 73 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. The Al, Ga and In donor bound exciton recombinations are identified based on doping and diffusion experiments and using secondary ion mass spectroscopy. We give a special focus on the recombination around 3.333 eV, i.e. about 50 meV below the free exciton transition. From temperature dependent measurements one obtains a small thermal activation energy for the quenching of the luminescence of 10 ± 2 meV despite the large localization energy of 50 meV. Spatially resolved CL measurements show that the 3.333 eV lines are particularly strong at crystal irregularities and occur only at certain spots hence are not homogeneously distributed within the crystal contrary to the bound exciton recombinations. We attribute them to excitons bound to structural defects (Y-line defect) very common in II–VI semiconductors. For the bound exciton lines which seem to be correlated with Li and Na doping we offer a different interpretation. Li and Na do not introduce any shallow acceptor level in ZnO which otherwise should show up in donor–acceptor pair recombinations. Nitrogen creates a shallow acceptor level in ZnO. Donor–acceptor pair recombination with the 165 meV deep N-acceptor is found in nitrogen doped and implanted ZnO samples, respectively. In the best undoped samples excited rotational states of the donor bound excitons can be seen in low temperature PL measurements. At higher temperatures we also see the appearance of the excitons bound to the B-valence band, which are approximately 4.7 meV higher in energy. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Near‐field optical‐scanning (NFOS) microscopy or ‘‘optical stethoscopy’’ provides images with resolution in the 20‐nm range, i.e., a very small fraction of an optical wavelength. Scan images of metal films with fine structures presented in this paper convincingly demonstrate this resolution capability. Design of an NFOS microscope with tunnel distance regulation, its theoretical background, application potential, and limitations are discussed.
We explore the interrelationships between the green 510 nm emission, the free‐carrier concentration, and the paramagnetic oxygen‐vacancy density in commercial ZnO phosphors by combining photoluminescence, optical‐absorption, and electron‐paramagnetic‐resonance spectroscopies. We find that the green emission intensity is strongly influenced by free‐carrier depletion at the particle surface, particularly for small particles and/or low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in ZnO; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect. © 1996 American Institute of Physics.
We report the effects of the growth ambient on photoluminescence (PL) emission properties of ZnO films grown on Si (100) by rf magnetron sputtering. Upon increasing the O <sub>2</sub>/ Ar + O <sub>2</sub> ratio in the growing ambient, the visible emission in the room-temperature PL spectra was drastically suppressed without sacrificing the band-edge emission intensity in the ultraviolet region. This tendency is estimated to be due to the reduction of the oxygen vacancies and zinc interstitials in the film induced by the improvement of the film stoichiometry with respect to high oxygen content, indicating that the visible emission in ZnO originates from oxygen vacancy or zinc interstitial related defects. The violet emission peaked at about 401 nm (3.09 eV) was observed in the low-temperature PL spectra of the ZnO films grown under oxygen-rich conditions. This emission band was assigned to the electron transition from the bottom of the conduction band to the Zn vacancy level, positioned approximately 3.06 eV below the conduction band edge. © 2003 American Institute of Physics.
Subwave length‐resolution optical image recording is demonstrated by moving an extremely narrow aperture along a test object equipped with fine‐line structures. Details of 25‐nm size can be recognized using 488‐nm radiation. The result indicates a resolving power of at least λ/20 which is to be compared with the values of λ/2.3 obtainable in conventional optical microscopy.
Low resistivity Ga-doped ZnO films were prepared on a glass substrate by ion plating with direct current arc discharge. Thickness dependent changes in the electrical properties of the films are reported, focusing on the thin films of less than 100 nm thickness. Structural analyses showed that the thinnest film of 30 nm thickness consists of well-oriented columnar grains normal to the substrate, and the resistivity was as low as 4.4×10<sup>-4</sup> Ω cm . The changes in lattice strain and c -axis fluctuation with the growth of grains are also shown to be associated with the electrical properties.
High-quality c-axis-oriented single-crystal ZnO films have been successfully grown on the (0 0 0 2) sapphire substrate by the low-pressure metal organic chemical vapor deposition technique. The effect of doping and annealing on the optical and structural properties has been investigated by means of X-ray diffraction (XRD), photoluminescence (PL) spectrum and atomic force microscopy (AFM). Annealing at high temperature was found to enhance the intensity of the (0 0 0 2) XRD peak and decrease the c-axis oriented lattice constant. However, the (0 0 0 2) XRD peak for the N-doped sample shifted to a low degree due to tensile stress possibly caused by nitrogen doping. The green–yellow band emission was observed in the room temperature PL spectrum of the undoped sample while the blue band emission emerged in the PL spectrum of the N-doped one. Low-temperature PL spectrum of the ZnO films was dominated by a sharp bound exciton line. Possible causes to the above differences will be given and discussed.
The luminescence and structural properties of silicon-rich nitride thin films were comprehensively examined by X-ray absorption spectroscopy. The near-edge X-ray absorption fine structures demonstrated the formation of silicon clusters in the silicon-rich nitride films. In addition, atomic resolved images of transmission electron microscope evidenced constitution of ultra-small (2–5 nm) silicon clusters. The X-ray absorption near-edge fine structures surveyed in photoluminescence yield and total electron yield configurations revealed that the luminescence are mainly coming from the Si–N bonding sites for lower degree of silicon-rich films. For higher silicon-rich samples, the Si–Si bonding sites increasingly contribute to the band edge absorption in the photoluminescence yield near-edge fine structures. These results strongly suggested that the transitions from the gap states localized in the surface of embedded silicon clusters and quantum confinement states within silicon clusters might contribute to the luminescence. The contribution weightiness of quantum confinement states increase while increasing the silicon-richness. It provides a reasonable explanation for the prolonged debates on the luminescence origins.
The electronic properties of ZnO have traditionally been explained by invoking intrinsic defects. In particular, the frequently observed unintentional n-type conductivity has often been attributed to oxygen vacancies. We report first-principles calculations showing that the oxygen vacancy V-O is not a shallow donor, but has a deep epsilon(2+/0) level at similar to 1.0 eV below the conduction band. The negative-U behavior that causes the 1+charge state to be unstable is associated with large local lattice relaxations. We present a detailed configuration coordinate diagram, which allows us to provide a detailed interpretation of recently reported ODEPR (optically detected electron paramagnetic resonance) measurements [L. S. Vlasenko and G. D. Watkins, Phys. Rev. B 71, 125210 (2005)]. (c) 2005 American Institute of Physics.
Highly oriented polycrystalline ZnO films were deposited on Si substrate by rf reactive sputtering technique. X-ray diffraction (XRD), atomic force microscope (AFM) and the refractive index were employed to analyze the influence of the post-annealing treatment on the structural properties of ZnO thin films. It has been found that the grain size of ZnO thin films increases with increasing the annealing temperature, the shift of the diffraction peak position from its normal powder value was observed. AFM analysis shows that the surface roughness of ZnO films is very low at temperature between 250 and 600 °C. The packing density investigation shows ZnO films can obtain high packing densities (above 0.973) in the annealing temperature rang from 450 to 600 °C.
We tried to control preferred orientation of ZnOx films deposited by radio frequency (RF) magnetron sputtering, and to make the growth mechanisms clear. Zinc oxide has tetrahedral coordinates caused by sp3 hybridized orbits, and the (0001) plane has the lowest surface free energy. Therefore, the film grows with strong (0001) preferred orientation even on glass. Considering the formation of tetrahedral coordination in the vapor phase and the deposition rate, however, we succeeded to control the preferred orientation of ZnOx films on glass. The (112̄0) textured film was obtained under sputtering gas composition which deteriorates the formation of tetrahedral coordination in the vapor phase and the high deposition rate. Formation of each texture is strongly related to the formation of tetrahedral coordination in the vapor phase and on the substrate during sputtering. Therefore, (112̄0) textured film had higher carrier concentration than that of the (0001) textured film caused by existing excess Zn atoms. Moreover, the growth mechanism with considering the density of surface energy, and the applications of the control for the epitaxial growth, are discussed.
We propose electron beam excitation assisted optical microscope, and demonstrated its resolution higher than 50 nm. In the microscope, a light source in a few nanometers size is excited by focused electron beam in a luminescent film. The microscope makes it possible to observe dynamic behavior of living biological specimens in various surroundings, such as air or liquids. Scan speed of the nanometric light source is faster than that in conventional near-field scanning optical microscopes. The microscope enables to observe optical constants such as absorption, refractive index, polarization, and their dynamic behavior on a nanometric scale. The microscope opens new microscopy applications in nano-technology and nano-science.
We propose a new type of scanning fluorescence microscope capable of resolving 35 nm in the far field. We overcome the diffraction resolution limit by employing stimulated emission to inhibit the fluorescence process in the outer regions of the excitation point-spread function. In contrast to near-field scanning optical microscopy, this method can produce three-dimensional images of translucent specimens.
Contrary to the well known diffraction limit, the fluorescence microscope is in principle capable of unlimited resolution. The necessary elements are spatially structured illumination light and a nonlinear dependence of the fluorescence emission rate on the illumination intensity. As an example of this concept, this article experimentally demonstrates saturated structured-illumination microscopy, a recently proposed method in which the nonlinearity arises from saturation of the excited state. This method can be used in a simple, wide-field (nonscanning) microscope, uses only a single, inexpensive laser, and requires no unusual photophysical properties of the fluorophore. The practical resolving power is determined by the signal-to-noise ratio, which in turn is limited by photobleaching. Experimental results show that a 2D point resolution of <50 nm is possible on sufficiently bright and photostable samples.
• super resolution
• moiré
• resolution extension
• saturation