Yasunobu Nashimoto

Si donor neutralization in high‐purity GaAs

The effects of hydrogen plasma exposure on the concentration of donors in high‐purity lightly Si‐doped molecular beam epitaxial GaAs have been investigated by photothermal ionization spectroscopy, low‐temperature photoluminescence, capacitance‐voltage, and Hall‐effect measurements. Photothermal ionization measurements show that in addition to Si donors S and Ge donors are present in the original high‐purity samples. After hydrogenation, the Si donor concentration is significantly reduced with a corresponding increase in mobility. Low‐temperature photoluminescence also showed a decrease in the full width at half‐maximum of the neutral donor bound exciton line indicating that the total impurity concentration is reduced. These results provide spectroscopic evidence to support the neutralization of Si donors confirming earlier results of the effects of hydrogen plasma exposure in GaAs.

Substrate-related kink effects with a strong light-sensitivity in AlGaAs/InGaAs PHEMT

The direct observation of substrate potential (V/sub sub/) changes is experimentally reported, for the first time, on three kinds of kink effects (abrupt increases of drain current) with a strong light-sensitivity in an AlGaAs/InGaAs Pseudomorphic HEMT (PHEMT). The dependence of V/sub sub/ on the kink effects are measured by using a side-gate electrode. The first kink effect is observed with a V/sub sub/ increase from negative to positive value with light. In the dark, with this kink effect eliminated, the second and third kink effects are observed with V/sub sub/ changes at high and low V/sub D/s, respectively. In the second and third kinks, the V/sub sub/s have positive values in all the V/sub D/ region, while there exist two V/sub D/ regions in which logarithmic values of V/sub sub/ remain constant (V/sub sub/ plateaus). Both V/sub sub/s start to increase again near the kink V/sub D/s with different dependences on V/sub G/ reduction by applying further V/sub D/. The mechanisms of the kink effects are discussed based on the behavior of V/sub sub/ and other experimental results, such as photo-sensitivity of the side-gating effect, observation of electron emission in transient waveform, irradiation with a monochromatic light source, and a current DLTS using a sidegate electrode. This work suggests that observation of V/sub sub/ is one of the useful ways to clarify the mechanism of the kink effect.

Kink effect related to the self-side-gating effect in GaAs MESFET's

A kink effect, an abrupt increase in drain current at high drain voltages, was observed in GaAs MESFETs with an Al/sub 0.2/Ga/sub 0.8/As/GaAs heterostructure buffer layer. In these MESFETs, impact ionization occurs at the drain side along the channel current path at high drain voltages. On the other hand, a side-gating effect occurs when a negative voltage applied to the gate pad of the MESFET (self-side-gating effect). From measurements of the substrate potential, we conclude that hole accumulation generated by the impact ionization at the channel-side GaAs/Al/sub 0.2/Ga/sub 0.8/As interface cancels the drain current reduction that arises from the self-side-gating effect. This gives rise to the kink effect we observe. >

Photoemissions related to the kink effect in GaAs metal‐semiconductor field‐effect transistors with an Al0.2Ga0.8As/GaAs buffer layer

Photoemissions related to the ‘‘kink’’ effect are observed in GaAs metal‐semiconductor field‐effect transistors with an Al0.2Ga0.8As/GaAs heterostructure buffer layer. In drain voltage (VDS) regions lower than the kink VDS, the photoemission mainly results from the bremsstrahlung radiation because the energy spectrum is broad between 1.1 and 1.8 eV. Near the kink VDS, photoemission results from the direct recombination process of holes generated by the impact ionization because it has a peak near at 1.4 eV. In addition, at VDS’s higher than the kink VDS, the photo intensity, as well as the drain current, saturates. It is explained by the equilibrium of the hole generation/recombination, the accumulation, and the hole current flow into the substrate.

Charge-trapping defects in Cat-CVD silicon nitride films

We show that Cat-CVD silicon nitride films contain more than 10 19 cm -3 nitrogen-bonded Si dangling bonds, similarly to the case for conventional CVD films. However, the charge-trapping behavior of the Cat-CVD films is found to be quite different, in spite of the same origin for the dominant defects. The difference is ascribed to a non-uniform distribution of defects that is strongly depleted near the surface in Cat-CVD films.