Shiro Hara

Essential Change in Crystal Qualities of GaN Films by Controlling Lattice Polarity in Molecular Beam Epitaxy

GaN heteroepitaxial growth on sapphire (0001) substrates was carried out by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). A Ga-polarity growth was achieved by using an AlN high-temperature buffer layer. The epilayer polarity was characterized directly by coaxial impact collision ion scattering spectra (CAICISS). It was found that the properties of the GaN films showing Ga-face polarity, including their structural and electrical properties, were dramatically improved compared to those of films with N-face polarity. This important conclusion is considered to be a breakthrough in the realization of high-quality III-nitride films by MBE for device applications.

Characterization of Polarity of Wurtzite GaN Film Grown by Molecular Beam Epitaxy Using NH3

Effects of the initial nitridation of a sapphire(0001) substrate by NH3 on the polarity of GaN{0001} film have been investigated by coaxial impact collision ion scattering spectroscopy. The polarity of ammonia-molecular beam epitaxial (MBE) film grown on the substrate nitrided using NH3 is assigned as (0001). The effect of the initial nitridation of the substrate by NH3 is found to be contrary to that by nitrogen plasma, where the GaN film grown on the nitrided substrate shows the polarity of (0001). The polarity of GaN film grown by rf plasma-assisted MBE on the substrate which is nitrided using NH3 is also (0001). These findings suggest the possibility of polarity control of the grown GaN film by choosing the N source for initial nitridation of the substrate.

Self-limiting growth on the β-SiC(001) surface

Abstract Atomic-layer control of cubic SiC(001) was investigated with gas reactions on the surface. Self-limiting growth at about one atomic monolayer at 1050°C is observed both for Si 2 H 6 exposure onto a carbon-terminated surface and for C 2 H 2 exposure onto a silicon-covered surface. This suggests that atomic layer epitaxy is probable for the surface.

A MOSFET Fabrication Using a Maskless Lithography System in Clean-Localized Environment of Minimal Fab

A novel semiconductor manufacturing system minimal fab for customized semiconductor devices and micro electro mechanical systems on a half-inch wafer is developed. A distinct feature of the minimal fab is its clean-localized system eliminating a need of cleanroom environment. The clean level of the system is estimated by measuring separately each clean-localized component of the system including a minimal shipping case, wafer carrier, wafer load-lock system, and machine process chamber using a particle counter. The clean level of the system is analyzed to be in the ISO class 4, which is in the same class of a super clean room. In order to confirm the cleanliness needed for device fabrications, we employ a lithography system of the minimal fab consisting of a minimal coater, a minimal maskless exposure system using digital light processing technique, and a minimal developer hybridizing with conventional fab equipment to fabricate a metaloxide-semiconductor (MOS) field-effect-transistors (FET). The minimum gate-length of the MOSFET is realized at 1 μm with a good transistor characteristic. The measured interface states density of 2.33 × 1010 cm-2 indicates that the whole processes during the fabrication are in the level of a sufficiently low contamination.

Analyses on Cleanroom-Free Performance and Transistor Manufacturing Cycle Time of Minimal Fab

In this paper, we introduce our developed clean-localized system for a cleanroom-free semiconductor manufacturing where a wafer is air-tightly transferred between the carrier and the machine via a load port and process in a clean chamber. The system has been applied to “minimal fab” specially designed to process a half-inch wafer for low-cost and low-volume device productions. To confirm the localized clean-performance of our system, we have measured the clean levels in process chamber of a machine and the wafer transfer system. Both are resulting in ISO class 4, while the clean level of the circumstance is in ISO class 9. In order to estimate the system performance in the issue of electronic device properties, we fabricate a traditional MOSFET using minimal fab for the entire process. The measured density of interface states (Dit) of the MOSFET was 7.7×1010 cm-2 and the off-leak current was 4×10-12 A. These are sufficiently low to confirm the acceptable particle contamination level of the system which has less impact on the device characteristics. Due to a compact size of the minimal machine, the wafer transfer distance between processes is minimized. The process efficiency of the minimal fab in terms of wafer transfer time and wafer waiting time is also studied.

Solid State Reaction of Mo on Cubic and Hexagonal SiC

Mo/3C-SiC and Mo/6H-SiC interfaces have been investigated by Auger electron spectroscopy, Rutherford backscattering spectroscopy, X-ray diffraction, and transmission electron microscopy. High temperature annealing at 1200°C for 1 hour caused a reaction at the interfaces, resulting in forming a Mo2C/Mo5Si3/SiC multilayer. We have found that SiC poly-typism (3C or 6H) and Mo deposition process (evaporation or sputter deposition) make no influence in forming the multilayer. The diffusion mechanism at the Mo/SiC interface will be discussed.

High-Quality InGaN Films Grown on Ga-Polarity GaN by Plasma-Assisted Molecular-Beam Epitaxy

High-quality InGaN films were successfully grown on a Ga-polarity GaN underlayer by plasma-assisted molecular-beam epitaxy (rf-MBE) with good reproducibility. X-ray diffraction (XRD) results showed that there was no phase separation of In with the In mole fraction up to 0.36. Intense photoluminescence emissions from the InGaN films were obtained. Clear evidence was obtained for the difference in the quality between InGaN films grown on the Ga-polarity and those grown on N-polarity GaN buffer layers, in which the Ga-polarity GaN buffer is preferred. Our results provide a basis for fabricating high-quality InGaN/(Al, Ga)N heterostructures for optical and electronic device applications by rf-MBE.

High-Power Pulsed Magnetron Sputtering Glow Plasma in Argon Gas and Pulsed Ion Extraction

Plasma-ion processing enhances the functionality of a film, and as such, metal plasma sources are indispensable in film preparation. A magnetron sputtering glow plasma is generated by a pulsed power source in a process called a high-power pulsed magnetron sputtering glow plasma. Metal species are sputtered by energetic argon ions and are ionized. Ions are extracted from the plasma when a substrate holder electrode (SHE) is immersed in the plasma. This process has been referred to as plasma-based ion implantation and deposition (PBII&D). This paper deals with the electrical and optical characteristics of a pulsed magnetron sputtering glow plasma in which pulsed ion extraction is carried out by the PBII&D method. Work is presented showing that the voltage and current characteristics can be represented as a series connection of voltage source, current-limiting resistor, and plasma impedance. As a result, the characteristics are normalized by the peak current and the maximum power consumed in the plasma where a circuit-matching condition is satisfied. However, when the temporary behavior of the current changes by over 20 A/μs under the experimental conditions, a circuit inductance originating from the connecting wire in the circuit can significantly influence the electrical characteristics. As a result, the peak current and the maximum consumed power cannot be used to normalize the electrical characteristics. When an inductive component is considered, the electrical characteristics obtained experimentally are curve fitted to the calculated values. Ion extraction from the glow plasma was successfully observed. This suggests that the PBII&D method can be employed in the case of a high-power pulsed glow plasma. The waveform of the extracted ion current is seen to have a sharp peak at the initial stage of voltage application to the SHE, followed by a stationary state. This confirms that the SHE is immersed in the plasma. The plasma density is on the order of 1017 m-3 and is estimated by the recovery characteristics of the voltage applied to the SHE. The ion density of the glow plasma is found to be proportional to the extracted stationary ion current at the end of the pulse applied to the SHE. The temporary behavior of the optical emission spectrum of the glow plasma confirms that sputtered titanium species are ionized to a singly ionized state and that their appearance is delayed from the appearance of argon ions.

Optimization of GaN Growth with Ga-Polarity by Referring to Surface Reconstruction Reflection High-Energy Electron Diffraction Patterns

GaN films with Ga-polarity on (0001) sapphire substrates grown by plasma-assisted molecular beam epitaxy were investigated. The optimization of the growth conditions was performed referring to reflection high-energy electron diffraction reconstruction patterns during the cooling processes. Three kinds of surface reconstruction patterns, named (5×5), (1×2) and (2×2), were observed during the cooling processes. Structural, optical and electrical properties of the GaN films, identified by different reconstruction patterns, were characterized to determine the optimal growth conditions. It was found that high-quality films can be obtained when the films show the (1×2) pattern. Under this condition, we obtained an electron mobility at room temperature of as high as 567 cm2/Vs without using a GaN template.

Electronic structure of the 3C–SiC(001)2×1 surface studied with angle-resolved photoelectron spectroscopy

Electronic structure of the 3C-SiC(001)2 x 1 surface studied with angle-resolved photoelectron spectroscopy