Masahiro Daibo

Examination of relationship between resistivity and photocurrent induced magnetic field in silicon wafers using laser SQUID

Using the laser SQUID method, we examined the relationship between silicon wafer resistivity and magnetic field. By irradiating the wafer with a 3.2 W high-power laser beam, we were able to generate a measurable magnetic field perpendicular to the wafer, even for the wafers without p-n junctions. For samples having resistivities of 130 /spl Omega//spl middot/cm or less, we observed magnetic field distributions in the form of concentric circles. Histogram analysis showed that as resistivity decreases, mean value of magnetic field and standard deviation increase. For p-type wafers having resistivities of up to 300 /spl Omega//spl middot/cm, both mean value of magnetic field and standard deviation vary as a power law with respect to resistivity. However, for samples having resistivities of 300 /spl Omega//spl middot/cm or higher, the relationship was saturated, and determining resistivity was difficult. Since the laser SQUID method can be used to measure resistivity without electrical contact, it works well even for wafers with surface oxide films, and does not cause contamination.

Ultraviolet Laser SQUID Microscope for GaN Blue Light Emitting Diode Testing

We carried out non-contacting measurements of photocurrent distributions in GaN blue light emitting diode (LED) chips using our newly developed ultraviolet (UV) laser SQUID microscope. The UV light generates the photocurrent, and then the photocurrent induces small magnetic fields around the chip. An off-axis arranged HTS-SQUID magnetometer is employed to detect a vector magnetic field whose typical amplitude is several hundred femto-tesla. Generally, it is difficult to obtain Ohmic contacts for p-type GaN because of the low hole concentration in the p-type epitaxial layer and the lack of any available metal with a higher work function compared with the p-type GaN. Therefore, a traditional probecontacted electrical test is difficult to conduct for wide band gap semiconductors without an adequately annealed electrode. Using the UV-laser SQUID microscope, the photocurrent can be measured without any electrical contact. We show the photocurrent vector map which was reconstructed from measured magnetic fields data. We also demonstrate how we found the position of a defect of the electrical short circuits in the LED chip.

Vector-Potential Transformer With a Superconducting Secondary Coil and Superconducting Magnetic Shield

We created a complex coiled coil (vector-potential coil) to generate a curl-free vector potential. The device comprises a long flexible solenoid wound around a hollow nonmagnetic nonconducting cylinder; the entire coil has a coiled-coil structure. A straight superconducting BiSrCaCuO (BSCCO) wire was placed in the hollow cylinder at 77 K to serve as a secondary coil of a vector-potential transformer. We detected a nonmagnetic voltage induced between the ends of the secondary coil. The open-circuit voltage that was detected was the same as that of a normal conductor, i.e., a copper wire, which was also used as the secondary coil. Moreover, we replaced the hollow cylinder with a hollow superconducting BiPbSrCaCuO cylinder, which provides an ideal magnetic shielding. Even with the secondary coil inside the hollow BiPbSrCaCuO magnetic shield, the voltage induced across both BSCCO and copper secondary coils was identical to that measured when using the hollow nonmagnetic nonconducting cylinder.