Masayuki Iwami

Normally Off n-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation

We have demonstrated n-channel gallium nitride (GaN) MOSFETs using a selective area growth (SAG) technique and ion implantation on a silicon substrate. Both MOSFETs realized good normally off operations. The MOSFET using the SAG technique showed a large drain current of 112 mA/mm, a lower leakage current, and a high field mobility of 113 cm2/V . s, which is, to our knowledge, the best for a GaN MOSFET on a silicon substrate.

Relation between the Radius of Tip Curvature and the Light Emission Efficiency from Scanning Tunneling Microscope

The relation between the efficiency of scanning tunneling microscope light emission (STM-LE) and the radius of tip curvature has been studied using silver tips. The emission efficiency increases with decreasing radius of tip curvature ρ for ρ>80 nm. This result agrees qualitatively with the prediction of a theory that includes the effect of electromagnetic retardation.

The positron trapping efficiency of dislocations in deformed dilute aluminium alloys

Positron-lifetime measurements have been made on deformed Al-0.01 at.% Mg, Al-0.01 at.% Si and Al-0.01 at.% Cu alloys at room temperature. The specific trapping rate mu d of positrons into dislocations depends strongly on alloying elements. Mg and Cu atoms can reduce mu d to a great extent, while Si atoms are not so effective. These results are closely related to the pinning of dislocations by solute atoms.

Change in the positron-trapping efficiency of dislocations in Al on heating after deformation

Positron-lifetime measurements have been made in order to study how the positron trapping efficiency of dislocations, mu d, in Al (99.9999% purity) changes during isochronal annealing after deformation at room temperature. It has been shown that mu d goes on increasing during the anneal, while the lifetime tau d for trapped positrons decreases at first but than becomes a constant, approximately=210 ps. The highest value of mu d obtained was about 4.5*10-4 m2 s-1, much larger than previous values, (0.066-2)*10-4 m2 s-1. After heating at a higher temperature, it has also been shown that a small applied stress can yield a marked decrease in mu d without any significant change in tau d. These results suggest that mu d depends strongly on dislocation configurations. The increase in mu d during the anneal may be due to a development of long straight dislocation-line segments.

Investigation of Surface Pits Originating in Dislocations in AlGaN/GaN Epitaxial Layer Grown on Si Substrate with Buffer Layer

An AlGaN/GaN heterostructural layer with a crack-free smooth surface was grown on multiple buffer layers formed on a Si(111) substrate. On the AlGaN surface, pit arrays forming a network structure were observed by atomic force microscopy (AFM). In order to clarify the origin of these pit arrays, the AlGaN/GaN layer was investigated using transmission electron microscopy (TEM). As a result, similar network structures of threading edge dislocations in the AlGaN/GaN layer were observed by plan-view TEM. It was thus confirmed that the surface pit arrays observed by AFM represent the surface termination of edge dislocations formed at the small-angle boundaries of slightly misoriented crystal domains.

Vacancy-type dislocation loops observed at fairly high temperatures in nearly perfect Al crystals

Vacancy-type dislocation loops appeared at 230°C even during a slow-cooling process from 300°C in nearly perfect aluminum crystals when examined by synchrotron radiation topography with white radiation. The temperature of the specimen was cyclicly varied between 300 and 230°C at a cooling rate of 2000°C/h and a heating rate of 1000°C/h. Almost all of these vacancy loops appeared at the same place at each repetition of the cooling process, and some of the loops were formed along the dislocation lines observed in the preceding stage. Formation of these loops cannot be explained by homogeneous nucleation theory. Therefore, it is concluded that the vacancy loops grown at fairly high temperatures were nucleated heterogeneously, and the nucleation site appears to be inclusions in the specimen. Almost all vacancy loops that remained in a nearly perfect aluminum crystal are formed by heterogeneous nucleation during the cooling process.

Positron Trapping Rate into Dislocations in Aluminum

The experimental determination has been made on the specific trapping rate µ d of a positron into dislocations in deformed aluminum (99.999% purity) at room temperature. A good linear relationship between the trapping rate and the dislocation density yields a value of (1.7±0.2)×10 -4 m 2 s -1 for µ d . A comparison with published values suggests that impurities may reduce the trapping rate even in trace amounts.

Indistinct Defect Images in Topographs of Nearly Perfect Aluminum Crystals Just Prior to Appearance of Dislocation Loops

Vacancy generation and annihilation mechanisms by the growth of two types of dislocation loops, the interstitial and the vacancy type, in nearly perfect aluminum single crystals at fairly high temperatures were investigated by synchrotron radiation topography. Topographs were continuously taken with white beam X-ray at an elevated temperature. Just prior to the appearance of the loops, an indistinct image topograph, in which not only defect images but also fringes were unclear, was taken for both types of loops in spite of a short exposure time (<3 s). This phenomenon was interpreted as being due to the scattering of the diffracted X-ray beam by very small clusters of interstitial atoms or vacancies which are invisible on the X-ray topograph. Moreover, some of them grow into interstitial or vacancy type dislocation loops as revealed by X-ray topography.

Application of High-Resolution Film for Lithography to Synchrotron X-Ray Topography

A high-resolution film for lithography is applied to a detector for synchrotron radiation topography, instead of a nuclear plate. The film shows much better resolution than that of the plate although exposure time an about 500 times longer is required. The size distribution of interstitial loops grown as vacancy sources in a nearly perfect aluminum crystal after a temperature rise is examined from the white beam topograph.

Effects of impurities on the positron trapping efficiency of dislocations in deformed aluminium

Positron lifetime and Doppler-broadening measurements have been made on deformed aluminium ( approximately=99.95- approximately=99.9999% purity) and Al-0.01 at.% Si alloy. It has been found that impurities even in trace amounts can reduce the positron trapping efficiency of dislocations.