We fabricated n-channel enhancementhnversion mode GaAs-MISFETs with nm-thin gate insulator, which is formed by oxidizing and nitriding GaAs surface. They have sharper pinch-off than GaAs-MOSFETs. Hysteresis is hardly observed. They showed transconductances of 50 m S / m (Vth -0 V) or 20 mSlmm (Vth 1V) depending on recess-etching time. They are 10 -100 times higher than those of previously reported devices.

SUMMARYTest fabrication of n-channel p-channel enhancement/inversion mode GaAs-MISFET with a GaAs oxy-nitrided gate insulation film formed by nitriding after oxidation of the GaAs surface was performed. In comparison with MOSFET with only oxidation, the pinch-off and hysteresis characteristics are improved in MISFET. The confirmed values of the transconductance are 62 mS/mm (V th ~ 0 V), 41 mS/mm (V th ~ 1 V), and 14 mS/mm [V th = V flat (= 1.1 V)] for the n-channel type. These values are 10 to 100 times those reported previously in GaAs-MOSFET. The result realized for the p-channel type MISFET is about 9 mS/mm.

We have reported that the oxi-nitridation of GaAs forms an insulator–semiconductor interface without deteriorating the crystallographic order of GaAs, and is applicable to the fabrication of compound semiconductor devices with metal–insulator–semiconductor (MIS) gates. A problem with oxi-nitridation is that nitrogen plasma ashes and thins the photoresist and hence restricts the possible nitrogen processing times even if a long nitridation is desirable for a better interface quality. To counteract this restriction, we developed a new processing technique utilizing an 0.3-µm-thick Al layer as a mask for selective etching, oxi-nitridation and lift-off. A high transconductance (185 mS/mm) and sharp pinch-off were obtained by a long (8 h) nitridation.

In 2001, the authors demonstrated performance of GaAs MOS-diodes with nm-thin directly oxidated layers fromed by W & ozone process 111. The thin oxide layers are effective in suppressing gate leakage current. However, a MOSFET based on it has hysteresis in current-voltage curves, and a dip in transconductance at a gate voltage around the flatband voltage 121. H. Ikoma et al. showed good influence of nitridation upon oxidated GaAs surfaces [3]. We also demonstrated in 2002 excellent influences of oxi-nitridation (nitrogen plasma treatment after UV & ozone oxidation) of (100) GaAs surfaces, results of which were characterized from view points of 0 -S interface structure (observed by E M ) , photoluminescence and electrical performances of MIS diodes 14, 51. The process gives a very flat interface between a fm insulating GaON layer and a GaAs with very littlecrystallographic disorder. The nitridation, applied to an oxidated GaAs surface, improves the photoluminescence intensity, decreases a leakage current, and improves C-V characteristics. Figure 1 shows C-V curves of MIS diodes with nm-thin 8 hrs oxidated layer (a) and 8 hrs-nitridated-after-8 hrsoxidation layer (b), respectively. The nitridation improves Schottky barrier height from 0.5 eV to 1.1 eV, and decreases the capacitance in the forward voltage region.We applied this process to forming the gate insulating f h s of GaAs-MISFETs and obtained excellent results which demonstrate reproducibility of the above experiment. We used n W = 3E17Icc. t = 400 nm)/S.LGaAs epitaxial wafer. After gate recess-etching with a gate electrode pattern, oxidation by the W & ozone for 4 hours and 0 to 2 hours nitridation by helicon plasma system at an RF power of 50 W were applied before forming A1 gate electrode. The gate length is about 1 p n (designed).Thickness of the insulator, measured with a monitor wafer is ahout 8 nm. Figure 2 shows drain currents vs. drain voltage of simply 4 hr-oxidated gate MOSFET (a) and the 4 hr-oxi-2 hr-nitridated gate MISFET (b), for drain voltage up and down conditions. The gate insulator formed only the oxidation (a) shows large hysteresis, while that formed by oxi-nitridation (b) shows no hysteresis, and better pinch-off. Figure 3 shows gate voltage dependence of transconductances of the MOSF'ET, 4 hr-oxi-1 hr-nitridated, and 4 hr-oxi-2 hr-nitridated MISFETs. Both of the transconductance and the pinchi-off performances are drastically improved by the nitridation process. The maximum transconductance (110 mS/mm) is obtained at a gate voltage of 1.1 V. Which suggests that the MIS junction has very little defects and

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