Japanese Journal of Applied Physics

Electrical properties of 16 keV, focused-ion-beam (FIB) (beam diameter: 1 µm, current density: 50 mA/cm²) boron-implanted silicon layers have been investigated as a function of beam scan speed and ion dose, and compared with those obtained by conventional implantation (current density: 0.4 µA/cm²). High electrical activation of the FIB implanted layers is obtained by annealing below 800°C as a result of the increase in amorphous zones created in the implanted layers. Amorphous zone overlapping is assumed to occur at FIB implantation doses of 3–4×10¹⁵ ions/cm² from the results of electrical activation and the carrier profile of implanted regions annealed at low temperature, if beam scan speed is lowered to about 10^-2 cm/s.

ITO films, both before and after annealing, were characterized with various methods. The changes in film characteristics are discussed. RIE etching mechanisms for the unannealed ITO films in three reactive gases (CF₂Cl₂, CF₃Cl, and CF₄) and two diluent gases (N₂ and Ar) were delineated by analyzing and comparing the surface ESCA data with the process results. For undiluted gases, indium is the major component left on the surface, and each gas has its own etch preference for different components. The addition of a diluent into the plasma may change the surface composition as well as the etching mechanisms. To increase the whole film etch rate, each component has to be etched off effectively.

We succeeded in fabricating L1₀-ordered MnAl films with a high perpendicular magnetic anisotropy energy of 10⁷ erg/cm³ and a small average film roughness of 0.4 nm by using a molten Mn–Al sputtering alloyed target and optimizing the substrate temperature. In addition, we investigated the tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) with the prepared L1₀-ordered MnAl electrode. The TMR effect was observed at RT in an MTJ with a very thin Co₅₀Fe₅₀ layer inserted into the MnAl electrode and MgO tunneling barrier interface. This is the first observation of the TMR effect in MTJs with an L1₀-ordered MnAl electrode.

We present the fabrication of a novel double-quantum-well (DQW) structure, in which the upper electron layer is supplied via modulation doping while the lower one is fully induced through the field effect from an n ⁺-GaAs back gate. Low-temperature transport measurements demonstrate that two-dimensional electron gases with equally high mobilities are successfully formed in the lower as well as in the upper QWs. By this approach, the electron density in the lower layer can be controlled over a wide range with a small back-gate bias, and hence the electron-density distribution in the DQW can be tuned arbitrarily by using a front gate in conjunction with the back gate.

Second harmonic generation has been observed in electrically poled tellurite glasses with 80TeO₂·10Li₂O·10Nb₂O₅ and 70TeO₂·15Li₂O·15Nb₂O₅ compositions. The poling was carried out at 200 to 300°C for 30 min under applied dc field of 4 to 5 kV. To explain the mechanism generating the second-order nonlinearity in the poled tellurite glasses, it is proposed that strong local polarization is caused by migration of lithium ions and/or periodic orientation of lone pairs of electrons in the tellurium ions.

Studies of field-effect control of the high mobility electrons in MBE-grown selectively doped GaAs/n-Al _x Ga_1-x As heterojunctions are described. Successful fabrication of a new field-effect transistor, called a high electron mobility transistor (HEMT), with extremely high-speed microwave capabilities is reported.