Materials Science Forum Vols. 600-603

Abstract: It is demonstrated that Si ion implantation is useful to fabricate GaN/AlGaN/GaN HEMTs with extremely low gate leakage current and low source resistance without any recess etching process. The source/drain regions were formed using Si ion implantation into undoped GaN/AlGaN/GaN on sapphire substrate. Using ion implantation into source/drain regions with energies of 30 and 80 keV, the performances were significantly improved. On-resistance reduced from 9.9 to 3.5 Ω·mm. Saturation drain current and maximum transconductance increased from 300 to 560 mA/mm and from 75 to 160 mS/mm, respectively.

Abstract: The current collapse of normally-off mode AlGaN/GaN/AlGaN double heterojunction field effect transistors was investigated in comparison with the normal AlGaN/GaN heterojunction filed effect transistors.

Abstract: We investigated the current collapse characteristics of the fabricated MIS-HEMT with the SiO2, SiN and high-k gate insulator. TiO2 was employed as the high-k material. We found the significant drain current change in the switching characteristic when the insulator changes. The SiN MIS-HEMT showed good switching characteristic. On the other hand, the MIS-HEMTs with oxide insulator film showed large drain current reduction. We considered that the degradation of the switching characteristic is due to the current collapse.

Abstract: The effect of ohmic contact location on the buffer leakage current of AlGaN/GaN heterostructure was investigated and the AlGaN/GaN HEMT employing the proposed ohmic contact pattern was fabricated. We have fabricated 3 different types of ohmic patterns; type A - both contacts are on the etched GaN buffer layer, type B - one is on the etched GaN buffer layer and the other is on the unetched GaN cap layer and type C - both contacts are on the unetched GaN cap layer. Our experimental results showed that the ohmic contact on GaN buffer increased the buffer leakage current due to the lateral diffusion of ohmic metals. The proposed AlGaN/GaN HEMT successfully decreased the leakage current and did not affect the forward drain current and the transconductance.

Abstract: In this work, the electrical properties of Pt/GaN Schottky contacts were studied. The temperature dependence of the barrier height and ideality factor, and the low experimental value of the Richardson’s constant, were discussed considering the formation of an inhomogenous Schottky barrier. Local current-voltage measurements on Pt/GaN contact, performed with a conductive atomic force microscope, demonstrated a Gaussian distribution of the local barrier height values and allowed to monitor the degree of inhomogeneity of the barrier. The presence of defects, terminating on the bare GaN surface, was correlated with the electrical behavior of the inhomogeneous barrier.

Abstract: The effect of AlGaN surface traps on breakdown voltage VB and drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs) were investigated using experimental measurement and numerical simulation. The drain current transient due to surface traps was systematically measured and analyzed, and the activation energy of a surface trap was evaluated as approximately 0.7 eV. Results from the device simulation of VB in HEMTs were in good agreement with the experimental results when assuming surface traps. The results indicate that surface traps increase VB, and induce a crucial current collapse.

Abstract: We fabricated diamond MISFETs on polycrystalline films using alumina gate insulator. A hole accumulation layer has been utilized as hole current channel. The hydrogen-termination was achieved by remote hydrogen plasma. The sheet resistance strongly depends on the substrate temperature during hydrogen-termination process. The polycrystalline diamond MISFETs showed high drain current density of -650 mA/mm and cut-off frequency of 42 GHz. These values are higher than those of single crystal diamond FETs ever reported.

Abstract: Multiple P or S hot ion implantation to diamond substrates was performed at 800°C. Optical absorption spectra indicated that instantaneous annealing during hot ion implantation occurs. Temperature dependence of resistance demonstrated that a P as-implanted sample using a homoepitaxial diamond film substrate emerges a weak doping effect. Also on S implantation, a presence of a weak doping effect was observed in an as-implanted sample, but it was suggested that the dopant is not S itself but S and defect complex. However, post-implantation annealing resulted in high resistance of the samples and missing of such weak doping effects.

Abstract: Multiple-energy nitrogen ions (energies:1 to 100 keV and a net concentration:2.24 x 1020 cm-3) are implanted into ZnO bulk single crystals grown by the hydrothermal method. Rutherford backscattering-channeling studies show the presence of displaced Zn atoms (Zni) of about 4 % in as-implanted samples. An A-emission band related to the interstitial oxygen (Oi) is observed at 580 nm in 600 oC-annealed samples, and a new emission appears at 515 nm in 800 oC-annealed samples. It is proposed that the new emission band consists of the superposition of the green band (~525 nm) observed in unimplanted ZnO and the residual A-emission band. In 800 oC-annealed samples, a band to acceptor transition at 3.26 eV is also observed in addition to a donor to acceptor pair transition, suggesting that nitrogen acceptor is located at about 180 meV above the valence band. A thermally stimulated current peak, P1 (165 meV), which has been attributed to a native point defect, observed in unimplanted samples almost disappears in nitrogen-implanted samples annealed at 800 oC.