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The technologies of fabrication of thin film phosphors based on gallium nitride using rf-magnetron sputtering are developed and the structural properties of these films are studied. Luminescence and electron spin resonance (ESR) spectra of GaN and GaN-Mn thin films are obtained. A correlation between cathodo-luminescence intensity and conductivity of GaN films is found. The nature of emission centers in GaN and GaN-Mn thin films is discussed and a mechanism of luminescence in these films is proposed.
This paper deals with some results of research in SLS performed in the excimer laser laboratory of TRINITI research institute, Russia, where different types of excimer lasers have been developed and manufactured. The research used a new simple SLS approach based on single-axis (i.e., cylinder) projection optics. The method employs a long single melting line extended many centimeters in length. The line is formed by projection through a single slit in a bulk metal mask. Some aspects of the efficiency, potential, and technical challenge of the method are discussed. This method is particularly useful with low pulse energy and high frequency excimer lasers, and one of the most efficient ways of providing directionally crystallized Si films over a large area. Several types of excimer lasers were tested for the SLS technique. It was found that among various parameters, pulse duration is a more important one, e.g., an increase in pulse duration from 25 to 150 ns results in enlargement of lateral growth distance by about three times.
Polycrystalline samples of Pb(Cd1/2W1/2)O3 and Pb(Mo1/2W1/2)O3 were synthesized by a high-temperature solid-state reaction technique. Preliminary crystal structure and microstructure of the compounds at room temperature were studied using X-ray diffraction (XRD) technique and Scanning electron microscopy (SEM), respectively. The dielectric permittivity (⊏) and losstangent (tan ⊏) of the compounds were obtained both as a function of frequency(103-104Hz) atroom temperature and temperature (30-3200C) at 10 kHz. Both the ac and dc conductivity have been studied over a wide range of temperature. The current– voltage (I-V) characteristics of the compound studied at different temperatures reveal that the Pb(Cd1/2W1/2)O3 also has excellent varistor behavior. A PTCR effect and NTCR effect was exhibited by Pb(Cd1/2W1/2)O3 and Pb(Cd1/2Mo1/2)O3 respectively.
The main requirements to electron field emission cathodes are their efficiency, stability and uniformity. In this work we combined the properties of porous silicon layers and diamond-like carbon (DLC) film to obtain emission cathodes with improved parameters. The layered structures of porous silicon and DLC film were formed both on flat n-Si surface and silicon tips created by chemical etching. The conditions of the anodic and stain etching of silicon in HF containing solution under the illumination have been widely changed. The influence of thin (≤10nm) DLC film coating of the porous silicon layer on electron emission has been investigated. The parameters of emission efficiency such as field enhancement coefficient, effective emission areas and threshold voltages have been estimated from current-voltage dependencies to compare and characterize different layered structures. The improvement of the emission efficiency of silicon tip arrays with porous layers coated with thin DLC film has been observed. These silicon-based structures are promising for flat panel display applications.
This paper will report the methods to deposit polycrystalline or amorphous chromium oxide (Cr2O3) thin films, which have potential applications in optoelectronics such as components for flat panel display devices. An atmospheric cold wall chemical vapor deposition (CVD) system was used to thermally decompose sublimed chromium acetylacetonate(Cr(C5H7O2)3) precursor, which gave Cr2O3 films. Oxygen concentration in gas phase is the key factor determining chromia film structure. In oxygen containing CVD system, the deposited films were polycrystalline Cr2O3, with (006), and (110) orientations. In inert, O2 free environment, the grown films were amorphous chromium oxide, as identified by XPS and XRD results. Based on depth profiling AES results, even in an oxygen free ambient, the elemental Cr/O ratio in films still remained stoichiometry(Cr:O = 2:3). Oxygen atoms might originate from acetylacetonyl (C5H7O2) ligands in the Cr(C5H7O2)3precursor.
Low temperature silicon dioxide depositions have been carried out by plasma enhanced chemical vapor deposition (PECVD) using TMS as the Si precursor at 100-200°C at the pressure of 2-8 Torr. An RF power of 40 W and a TMS:O2 gas flow rate ratio of 1:500 without inert gas dilution were used in the depositions. It was found that the current-voltage (I-V) characteristics of as-deposited oxide films improved as the substrate temperature increased or deposition pressure decreased. Oxide films deposited at 2-3 Torr exhibited typical Fowler-Nordheim (F-N) tunneling characteristics and breakdown voltages greater than 8 MV/cm. The best capacitance-voltage (C-V) characteristics, giving a small flat band voltage shift, a small amount of positive oxide charge, a small hysteresis in bi-directional C-V sweep, and a low interface trap density, were obtained at 3 Torr. Post-deposition annealing in forming gas at the deposition temperature was performed and proved to be an effective approach for improving the electrical properties of the deposited oxide films without compromising the low temperature aspect of the process. By annealing at 200°C, the F-N tunneling barrier height increased by as much as 0.6 eV, the flat-band voltage and the hysteresis in C-V sweep were reduced by 0.74 V and 0.08 V, respectively. In addition, hydrogen was found to play a key role in the annealing treatment and its mechanisms were discussed.
This paper presents a one-dimensional numerical simulation of the charge carrier transport and photogeneration within a p-i-n (a-Si:H) homojunction and a p(a-SiC:H)/i (a-Si:H)/n (a-SiC:H) heterojunction with weakly-doped n-layers. A good matching between the simulated J-V characteristics and the corresponding experimental curves has been achieved for both configurations. By analysing the simulated band diagrams, electric field
distributions, the electron and hole current densities, and the free carrier population profiles we conclude that in short-circuit mode the carrier transport is different in the homojunction and heterojunction due to band offsets.
We report new lateral grain growth mechanism by XeCl excimer laser annealing on a-Si film with pre-patterned Al layer. 2000Å-thick Al pattern on 800Å-thick PECVD a-Si film successfully reflects the incident laser beam and results in temperature gradient during the recrystallization process. The TEM images show that grain growth near the boundary between the liquid and the solid region exhibits a different mechanism compared with the conventional ELA. About 1.5 m-long lateral grain has been successfully obtained by single laser irradiation
A poly-Si TFT with single grain boundary in the channel has been fabricated by the proposed excimer laser annealing. An excellent device characteristics such as mobility more than 250cm2/Vsec, high On/Off current ratio of 6.3×106 and low threshold voltage less than 1 V has been obtained. The experimental results show that the mobility, threshold voltage and sub-threshold slope of proposed TFTs are superior to those of the conventional TFTs.
A new double laser recrystallization technique that can produce lateral grains of tens of micrometers is presented. A nanosecond laser (excimer or Nd:YLF laser) and a pulse modulated Ar+ laser are used in the experiment. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer laser fluence, the Ar+ laser power and the pulse duration. This process has wide process window and is insensitive to both the excimer laser fluence and the Ar+ laser power fluctuations. Preheating and melting of the a-Si film with the Ar+ laser before firing the excimer laser is found to be necessary for inducting lateral grain growth. The transient excimer laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The solidification dynamics of the process is probed by high spatial and temporal resolution laser flash photography. A lateral solidification velocity of about 10 m/s is observed.
Nickel Induced Lateral Crystallization (NILC) and Pulsed Rapid Thermal Annealing (PRTA) have been used to study new low temperature and high quality poly-silicon (poly-Si) films and thin film transistors (TFTs). The growth rate of poly-Si films has been found to greatly increase from 0.025μm/minute to 1.07μm/minute, and the drain current and performance of TFTs have increased by around 75%. The new poly-Si technology has good potential to apply in high performance, large area, fast throughput, low cost and even low temperature device applications.
It is known that the direct contact between Al and a-Si enhances the crystallization of a-Si film. But the poly-Si films crystallized by the direct contact of Al metal film suffer the problems of rough surface and pores. In our study, we utilized the vapor from AlCl3 instead of Al metal film. The crystallization was enhanced by annealing a-Si films with AlCl3 that the crystallization was completed in 5h at 540. And the surface was as smooth as that of the a-Si film. The Al incorporation into the poly-Si film took place, but the content was below the detection limit of AES.
In this work we present the results of a study on the uniformity of ZnO thin films produced by spray pyrolysis. The properties of the thin films depend essentially on the carrier gas pressure and gas flow used. The best films for optoelectronic applications were obtained with a carrier gas pressure of 2 bar and solution flow of 37 ml/min. The velocity of the nozzle affects essentially the uniformity of the ZnO thin films. However this important characteristic of the large area thin films is independent of the nature (doped and undoped) of the thin film and exhibits a high dependence on the variation of the temperature along the substrate.
Lead iodide purified by zone refining and repeated sublimation was used for growing Polycrystalline films by physical vapor deposition. Palladium film was deposited as rear contact onto glass and alumina substrates 2.5 × 2.5 cm2 in size. Onto it, lead iodide polycrystalline films were grown by sublimation at 390 °C and 5 × 10−5 mm Hg, substrate temperatures of about 200 °C and deposition times of about 10 days. Film thickness was measured by X-ray transmission at 59.5 keV giving values from 35 to 50 μm (5%). Optical and atomic force microscopy were performed to the films giving an average grain size of (80±20) μm. Low temperature photoluminescence was performed and peak position and broadness confirmed the high purity of starting materials. Films were characterized by X-ray diffraction, giving an [ΣI (0 0 l)] / [ΣI (h k l)] relation of 0.8 that indicates a strong growth preferred orientation along c axis. Front palladium thermal deposition contacts and acrylic encapsulation were done and apparent resistivity (2 × 1014 Ω. cm) and current density (7 pA/cm2 (30 V)) were obtained. X-ray film response was checked by irradiating with 241Am and an X-ray beam. Finally, film and detector characterizations were correlated with starting material, deposition parameters and previous results for the same and alternative materials like mercuric iodide.
In this work we will discuss the feasibility of sputtered-Si as a precursor for low-temperature p-Si films. We used DC sputtering to deposit thin Si films (30-100nm) that were subsequently crystallized by excimer-laser annealing using XeCl (308nm) irradiation. The as-deposited films were sputtered with different gases including Ar, He and Ar-He mixtures. As expected, He sputtering led to less dense films that Ar. However, the plasma voltage (during deposition) was also lower in these cases. It was found that mixing of Ar and He gases within an appropriate ratio range produced sputtered films with good optical properties, at much lower plasma voltages that Ar alone. The lower voltage application could be beneficial from the point of view of reducing microarcing and allowing application of higher DC power levels (to improve deposition rate). Polysilicon TFTs fabricated with the optimum Ar-He ratio (>5% Ar in He) demonstrate similar performance between pure-Ar and Ar-He sputtered p-Si TFTs.
XeCl excimer laser was irradiated on metal induced laterally crystallized (MILC) polycrystalline silicon (poly-Si) film in order to eliminate the intra-grain defects of MILC poly-Si film which incorporated 2 μm wide metal induced crystallized (MIC) poly-Si line pattern. On the irradiation of the laser beams, different melt and recrystallization phenomena were observed in the MILC and the MIC poly-Si region due to the Ni content difference in each film. The transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) measurements indicated that the melting temperature of the poly-Si film decreased as the Ni content increased. With the laser irradiation energy density of 370 mJ/cm2, 2 μm long defect-free poly-Si grain was successfully grown in the MILC poly-Si due to the melting temperature variation at the MILC-MIC poly-Si boundary.
Conventional MEMS devices are based on silicon micro-machining and their maximum size is limited by the wafer. In contrast, we are exploring micro-machining for large area applications on substrates such as glass using polymeric materials. Our research is focused on the photopolymer SU-8, and we apply the MEMS fabrication technology to large area image sensors and displays. There are many challenges concerning the materials and processes, since large area compatibility is essential and integration with large area electronics may be required. The adhesion of SU-8 to the underlying layers as well as stress in the SU-8 are important issues and surface treatments have been investigated. Two applications of SU-8 MEMS are discussed to illustrate large area applications:. First, in the fabrication of an X-ray imager, high aspect ratio SU-8 walls form a micro-patterned phosphor screen to increase the image resolution. Second, a similar approach of patterning SU-8 into arrays of micro-cells is applied to an electrophoretic display.
Poly-Si Schottky-barrier thin-film transistors (SB-TFTs) were fabricated and characterized. In this study, SB-TFTs were first fabricated by using a conventional sidewall spacer to isolate the gate and S/D regions during salicidation. However, it was found that these SB-TFTs depict very poor on/off current ratio (<103) as well as severe GIDL (gate-induced drain leakage)-like leakage current. To overcome these shortcomings, a novel SB-TFT structure is also fabricated in this study to improve the device performance. The new device consists of a field-induced-drain region (FID), which is an offset drain region controlled by a metal field-plate lying on top of the passivation oxide. The FID region is sandwiched between the silicided drain and the active channel region. Carrier types and the conductivity of the transistor are controlled by the metal field-plate. Since the metal field plate is formed simultaneously with the regular metal patterning, no additional processing steps are required. Our results show that the new device can significantly improve the on/off current ratio to over 106 for both p- and n-channel operations, while effectively eliminating the GIDL-like leakage.
Oxidation of emitter surfaces can be a serious problem for Mo field emitter arrays. We studied the oxidation and related changes in the electronic properties of Mo thin films as a function of annealing temperature. Experiments were done on Mo thin films prepared on Si and sodalime glass substrates. These films were thermally oxidized and characterized using a variety of techniques including x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TPD) methods. For films oxidized below 400°C, partial oxidation was observed, with MoO3(110) being the principal oxide phase. However, at a temperature of 500°C and above, oxidation of the film was complete. Electrical characteristics of the films undergo a rapid transition from semiconductive to highly insulating at temperatures between 475 to 500°C. Temperature programmed desorption spectra showed that the oxides are stable at elevated temperature with only a principal O2 desorption peak at approximately 786°C.
Polysilicon-on-insulator singly-addressable arrays, consisting of double-gated field emission cells, were fabricated and tested. The field-emission tips were formed by a subtractive technique, using 2.5 µm thick polysilicon stripes on an insulating substrate. The tip structure was oxidized for dielectric isolation and coated with a 0.4 µm polysilicon layer as a first gate electrode. The polysilicon layer was then subsequently oxidized to provide a second isolation layer for separation from a 0.1 µm gold film, deposited as a second gate electrode. Finally, the 1.5 µm aperture was formed, combining wet etching of the silicon dioxide and dry etching of the polysilicon layers. The matrix allows addressing electrically any emission cell at the intersection of a cathode column and an extracting gate line. An independent voltage can be applied to the second gate during operation to focus the electron beam of an operating tip.
Low temperature process technologies for high performance polycrystalline silicon (poly-Si) thin-film transistors (TFTs) are discussed based on the investigations of pulsed laser crystallization, plasma treatment of poly-Si films, and SiO2/Si interface formation. Although highdensity (∼1018 cm−3) trap states localized at grain boundaries are introduced to the poly-Si films by laser crystallization, they are efficiently decreased to the order of 1016 cm−3 by following hydrogen plasma treatment. It is also shown that high quality SiO2/Si interfaces with the density of interface trap states (Dit) in the order of 1010 cm−2eV−1 are achieved using electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD). By applying these low temperature process technologies to the fabrication process, high performance poly-Si TFTs with high n-channel mobility μn) of 187 cm2V−1s−1, low threshold voltage (Vth) of 1.97 V and small subthreshold swing (S) of 210 mV/dec. were obtained. These results indicate that the development of low temperature process technologies that can control trap states is the key to the next generation high performance poly-Si TFTs.