Masaharu Miyamoto

High-quality SiO2 film formation by highly concentrated ozone gas at below 600°C

Highly concentrated (>93 vol %) ozone (O3) gas was used to oxidize silicon for obtaining high-quality SiO2 film at low temperature. Compared to O2 oxidation, more than 500 °C lower temperature oxidation (i.e., from 830 to 330 °C) has been enabled for achieving the same SiO2 growth rate. A 6 nm SiO2 film, for example, could be grown at 600 °C within 3 min at 900 Pa O3 atmosphere. The temperature dependence of the oxidation rate is relatively low, giving an activation energy for the parabolic rate constant of 0.32 eV. Furthermore, a 400 °C grown SiO2 film was found to have satisfactory electrical properties with a small interface trap density (5×1010 cm−2/eV) and large breakdown field (14 MV/cm).

Reactive oxygen beam generation system using pulsed laser evaporation of highly concentrated solid ozone

A reactive oxygen beam generation system is described for the formation of high-quality and high-precision films. This system utilizes pulsed laser evaporation of highly concentrated solidified ozone (O3). The equipment for safely generating and handling a large amount of high-purity liquid and solid O3 was also developed for this purpose. The beam is characterized by its high concentration of oxygen atoms in an excited state [O(1D)], constant flux per laser shot (4×1017 molecules cm−2 shot−1), appropriate level of kinetic energy (KE) for enhancing the surface reaction (mean KE of 0.4 eV, maximum KE of 2 eV) and small angular spread (6°). These characteristics enabled us to precisely control the SiO2 film thickness by the number of laser shots, and achieve an enhanced Si oxidation rate and new local oxidation process.

Field Emission of Spin-Polarized Electrons Extracted from Photoexcited GaAs Tip

A pyramidal-shaped GaAs (tip-GaAs) photocathode for a polarized electron source (PES) was developed to improve beam brightness and negative electron affinity (NEA) lifetime by field emission. The emission mechanism also enables the photocathode to extract electrons from the positive electron affinity (PEA) surface into vacuum, and alleviates the NEA lifetime problem. The measured electrical characteristics of tip-GaAs and its polarization exhibited distinctive field-emission behavior. The polarization of the electron beam extracted from tip-GaAs was 20–38% under irradiation with circularly polarized light of 700–860 nm, and the peak polarization was 37.4±1.4% at a wavelength of 731 nm. These experimental results indicate that spin-polarized electrons can be extracted from the conduction band into vacuum by a field-emission mechanism. This, in turn, shows that this type of photocathode has the prospect of generating a low-emittance spin-polarized electron beam.

Enhanced oxidation of silicon using a collimated hyperthermal ozone beam

Silicon was oxidized by a collimated hyperthermal ozone beam produced by pulsed-laser ablation of solid ozone to increase the controllability of the silicon dioxide film thickness and to achieve low-temperature oxidation. The oxidation rate could be accurately controlled by the number of laser shots to which the number of supplied ozone molecules was proportional. Ozone molecules with a translational energy of around 1 eV obtained by laser ablation produced an initially rapid oxidized region with no temperature dependence in which a 0.6 nm silicon dioxide film could be synthesized at room temperature with only 200 laser shots. Higher-efficiency oxidation was also achieved in comparison with that by using a spray of ozone with thermal energy.

Hyperthermal O3 Beam Produced by Laser Ablation of Solid-Ozone Film

In order to obtain a highly concentrated hyperthermal ozone beam for more effective Si oxidation, we performed laser ablation of solid-ozone. A KrF pulse laser was irradiated onto solidified ozone on a sapphire substrate cooled to 30 to 60 K using a cryocooler. A mixture of ozone, molecular oxygen and atomic oxygen was detected using a time-of-flight method through a quadrupole mass filter. The velocity distribution of ablated ozone molecules was almost the same as the distribution at thermal equilibrium. An ozone beam with a fitted temperature of 2500 K and maximum translational energy of 3 eV was obtained under optimum laser conditions.

Application of 100% ozone gas process to rapid low-temperature oxidation

A 100% ozone oxidation process has been applied for the first time to rapid low-temperature oxidation of silicon to fabricate device quality SiO/sub 2/ films. A new quartz cold wall-type furnace equipped with a halogen lamp heater was built for safe and efficient handling of 100% ozone gas supplied from a pure ozone generator which we have developed. The working pressure of the furnace was typically 900 Pa, which resulted in the growth of 4 nm SiO/sub 2/ film within 4 minutes at 400/spl deg/C. A very low excitation energy observed for the SiO/sub 2/ film growth indicates that the actual reaction species in the process are active oxygen radicals generated from the thermal decomposition of ozone molecules at the sample surface. The electrical properties of ozone-oxidized SiO/sub 2/ films were evaluated by measuring the C-V and I-V characteristics of the MIS structure with Al electrodes. The films fabricated at 400-600/spl deg/C with thickness of 5-11 nm all show properties matching the device quality, i.e. low interface state density (<5/spl times/10/sup 10/ cm/sup -2/eV/sup -1/) and high breakdown voltage (>13 MVcm/sup -1/).

Enhanced Silicon Oxidation by a Hyperthermal Beam Obtained from Laser Evaporation of Solid Ozone

A new oxidation method for gate oxide applications using a collimated neutral hyperthermal ozone beam produced by pulsed laser evaporation of solidified ozone is demonstrated. This method enables ozone molecules, together with oxygen molecules and oxygen atoms, in an electronically excited state, O( 1 D), with a translational energy of around 1 eV to be intermittently supplied to silicon just after each laser shot, so that the silicon dioxide (SiO 2 ) film thickness can be accurately controlled by the number of laser shots. The effect of ozone having this translational energy was clearly apparent from the temperature-independent initially high oxidation rate, which enabled the oxidation to he shorter and with a lower temperature process. Compared to oxidation by the conventional method using thermal ozone gas, higher efficiency was achieved in the sense that a thicker SiO 2 film could be obtained with a smaller number of ozone molecules as long as the oxidized area was localized. Further enhanced oxidation was enabled by using a hyperthermal beam rich in O( 1 D) that had been obtained at a high laser fluence irradiation, demonstrating the high reactivity of O( 1 D) compared to the ground-state oxygen atom, O( 3 P).

RECENT PES PHOTOCATHODE R&D AT NAGOYA UNIVERSITY

The strained-layer superlattice structures have been exhibiting the most promising performance as a photocathode for the polarized electron source (PES). In our experiments, the GaAs-GaAsP photocathode achieved maximum polarization of 92f6% with quantum efficiency of 0.5%, while the InGaAsAlGaAs photocathode provided higher quantum efficiency (0.7%) with lower polarization (77f5%). Criteria for achieving high spin polarization and high quantum efficiency using superlattice photocathodes were clarified by employing the spin-resolved quantum efficiency spectra. However, it seems that major problems still remained for the PES R&D are to improve (1) beam emittance and (2) NEA lifetime under gun operations for high peak current and high average current, respectively. In order to overcome these problems simultaneously, we started a development of a new type photocathode using field emission mechanism. First, we tried to use a pyramidal shape GaAs (tipGaAs). Using the tip-GaAs, electrons can be emitted from a small area at the top of pyramid, and thus the beam emittance is expected to be small. This emission mechanism also enables to extract electrons from the poor NEA or small PEA surface into vacuum, and it helps to relax the NEA lifetime problem. Preliminary results were already obtained.

Hyperthermal beam for oxidation and nitridation produced by laser evaporation of mixed O3/N2O cryogenic film

We have performed KrF pulsed-laser evaporation of a cryogenic film consisting of a mixture of ozone (O3) and nitrous oxide (N2O) to obtain an energetically reactive beam for oxidation as well as nitrogen (N) incorporation. The ejection of hyperthermal nitric oxide (NO) molecules as well as hyperthermal oxygen (O) atoms and N2O molecules was observed at a relatively high laser fluence. The generation of additional NO molecules is explained by the chemical reaction between O atoms in the electronically excited state, which are generated from photodissociation of O3, and surrounding N2O molecules during the evaporation process. By applying this beam to Si, we have achieved an extremely high oxidation rate and 4–7 at.% N incorporation even at room temperature.