Shunsuke Ikeda

InP/InGaAs Composite Metal–Oxide–Semiconductor Field-Effect Transistors with Regrown Source and Al2O3 Gate Dielectric Exhibiting Maximum Drain Current Exceeding 1.3 mA/µm

We have realized InP/InGaAs composite-channel metal–oxide–semiconductor field-effect transistors with both selectively regrown n+-InGaAs source/drain regions and Al2O3 as a gate dielectric. A 100-nm-long channel was fabricated by laterally buried regrowth in a channel undercut by metalorganic vapor phase epitaxy. The carrier density of the regrown layer was 2.9×1019 cm-3. A drain current Id of 1.34 mA/µm was achieved at a drain voltage Vd of 1 V and a gate voltage Vg of 3 V. A transconductance gm of 817 µS/µm at Vd = 0.65 V was also observed at the same time. The improvement in the subthreshold slope can be explained by the decrease in dielectric/semiconductor interface trap density.

High drain current (>2A/mm) InGaAs channel MOSFET at V D =0.5V with shrinkage of channel length by InP anisotropic etching

In this paper, we report InGaAs channel MOSFETs with an InP source contact. InP source contact enables the suppression of carrier starvation and the easy shrinkage of the channel length by anisotropic etching. In fabricated 50-nm InGaAs channel MOSFETs, ID = 2.4A/mm at VD=0.5V were observed. On the other hand, degradations of Vth and SS by the short channel effect were also observed. Thinner channels will be required in order to suppress this effect.

Submicron InP/InGaAs Composite-Channel Metal?Oxide?Semiconductor Field-Effect Transistor with Selectively Regrown n+-Source

We have demonstrated an InP/InGaAs composite-channel metal–oxide–semiconductor field-effect transistor with a selectively regrown n+-InGaAs source/drain formed by metal organic vapor-phase epitaxy. A 150-nm-long channel was fabricated using a dummy gate and by laterally buried regrowth in the channel undercut. The gate stack was formed after regrowth by replacing the dummy gate. The carrier density of the regrown layer was 4.9×1019 cm-3. The maximum drain current at a drain voltage Vd = 1 V and a gate voltage Vg = 3 V was 0.93 mA/µm and the maximum transconductance was 0.53 mS/µm at Vd = 0.65 V.

Investigation of the Tail of a Fe Plasma Plume Passing Through Solenoidal Magnetic Field for a Laser Ion Source

To control the plasma flux of a laser ion source for a desired beam profile, we investigated the influence of a quasi-static magnetic field generated by a solenoidal coil on the tail of an Fe ablation plasma. We observed that the magnetic field converged the outer plasma and almost did not affect the inner plasma when the plasma drifted through the field region. The convergence points of the outer plasma depended on the longitudinal velocity. The results indicate that diamagnetic current was induced in the outer plasma, and therefore, the outer plasma received the converging force and the magnetic field in the inner region was excluded by the plasma intrusion. The results also mean that we have to consider the magnetic effect on the transverse distribution of plasma flux to minimize the emittance growth and obtain desirable beam optics.

Creation of mixed beam from alloy target and couple of pure targets with laser.

To create mixed species ion beam with laser pulses, we investigated charge state distributions of plasma formed from both Al-Fe alloy targets and pure Al and Fe targets placed close together. With two targets, we observed that the two kinds of atoms were mixed when the interval of two laser pulses was large enough (40 μs). On the other hand, when the interval was 0.0 μs, we observed fewer Fe ions and they did not mix well with the Al ions. The two species were mixed well in the plasma from the alloy target. Furthermore, we observed that specific charge states of Fe ions increased. From the results, it was determined that we can use two pure targets to mix two species whose difference of the drift velocity is large. On the other hand, we must use an alloy target when the drift velocities of the species are close.

Contribution of material’s surface layer on charge state distribution in laser ablation plasma

To generate laser ablation plasma, a pulse laser is focused onto a solid target making a crater on the surface. However, not all the evaporated material is efficiently converted to hot plasma. Some portion of the evaporated material could be turned to low temperature plasma or just vapor. To investigate the mechanism, we prepared an aluminum target coated by thin carbon layers. Then, we measured the ablation plasma properties with different carbon thicknesses on the aluminum plate. The results showed that C(6+) ions were generated only from the surface layer. The deep layers (over 250 nm from the surface) did not provide high charge state ions. On the other hand, low charge state ions were mainly produced by the deeper layers of the target. Atoms deeper than 1000 nm did not contribute to the ablation plasma formation.

Analyses of the plasma generated by laser irradiation on sputtered target for determination of the thickness used for plasma generation

In Brookhaven National Laboratory, laser ion source has been developed to provide heavy ion beams by using plasma generation with 1064 nm Nd:YAG laser irradiation onto solid targets. The laser energy is transferred to the target material and creates a crater on the surface. However, only the partial material can be turned into plasma state and the other portion is considered to be just vaporized. Since heat propagation in the target material requires more than typical laser irradiation period, which is typically several ns, only the certain depth of the layers may contribute to form the plasma. As a result, the depth is more than 500 nm because the base material Al ions were detected. On the other hand, the result of comparing each carbon thickness case suggests that the surface carbon layer is not contributed to generate plasma.

Multiple species beam production on laser ion source for electron beam ion source in Brookhaven National Laboratory.

Extracted ion beams from the test laser ion source (LIS) were transported through a test beam transport line which is almost identical to the actual primary beam transport in the current electron beam ion source apparatus. The tested species were C, Al, Si, Cr, Fe, Cu, Ag, Ta, and Au. The all measured beam currents fulfilled the requirements. However, in the case of light mass ions, the recorded emittance shapes have larger aberrations and the RMS values are higher than 0.06 π mm mrad, which is the design goal. Since we have margin to enhance the beam current, if we then allow some beam losses at the injection point, the number of the single charged ions within the acceptance can be supplied. For heaver ions like Ag, Ta, and Au, the LIS showed very good performance.

Behavior of moving plasma in solenoidal magnetic field in a laser ion source

In a laser ion source, a solenoidal magnetic field is useful to guide the plasma and to control the extracted beam current. However, the behavior of the plasma drifting in the magnetic field has not been well understood. Therefore, to investigate the behavior, we measured the plasma ion current and the total charge within a single pulse in the solenoid by changing the distance from the entrance of the solenoid to a detector. We observed that the decrease of the total charge along the distance became smaller as the magnetic field became larger and then the charge became almost constant with a certain magnetic flux density. The results indicate that the transverse spreading speed of the plasma decreased with increasing the field and the plasma was confined transversely with the magnetic flux density. We found that the reason of the confinement was not magnetization of ions but an influence induced by electrons.

Proton beam production by a laser ion source with Hydride target

We studied proton beam production from a laser ion source using hydrogen rich target materials. In general, gas based species are not suitable for laser ion sources since formation of a dense laser target is difficult. In order to achieve reliable operation, we tested hydride targets using a sub nanosecond Q-switched Nd-YAG laser, which may help suppress target material consumption. We detected enough yields of protons from a titanium hydride target without degradation of beam current during the experiment. The combination of a sub nanosecond laser and compressed hydride target may provide stable proton beam.