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We developed a molecular beam epitaxy technique to suppress Be diffusion by incorporating In in the AlGaAs epilayer. Diffusion coefficients of Be-doped In y (Al0.1Ga0.9)1-y As (p: 7×1019 cm-3) grown at 600°C were reduced from 1×10-14 cm2/s to 2×10-15 cm2/s when the InAs mole fraction y was increased from 0 to 0.07, indicating that compressive stress in the epilayer caused by incorporating In plays an important role in suppressing Be diffusion. We fabricated a heterojunction bipolar transistor with a 100-nm-thick p+-In0.055(Al x Ga1-x )0.945As base layer (x: 0 to 0.1), and obtained a DC current gain of 27.

In this paper, we report the first demonstration of microwave power performance of InAlAs/InGaAs double heterojunction bipolar transistors (DHBTs) obtained at an extremely low operating voltage of 1.5 V. In order to obtain a high output power (P out) at a low operating voltage, we used DHBTs rather than single heterojunction bipolar transistors (SHBTs) and thus reduced the offset voltage (V CE,offset). We obtained a much lower V CE,offset of 50 mV for the DHBT than that of 300 mV for a SHBT. At a low operation voltage of 1.5 V, the DHBT exhibited a P out of 20.6 dBm with a power added efficiency (η add) of 36.6% and a power gain (G a) of 8.5 dB biased for class-B operation at 1.9 GHz. The high-speed performance of the DHBT are a unity cutoff frequency (f T) of 76 GHz and a maximum oscillation frequency (f max) of 157 GHz. We also studied the reliability of DHBTs by conducting a 1000-hour accelerated life test. The InGaAs HBTs had a lifetime of 2×106 h at a junction temperature of 125°C with an activation energy of 0.95 eV.

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A bit patterning process using the ferromagnetic (FM)-antiferromagnetic (AF) transition in [001]-oriented L10 FePt1-xRhx films was investigated. First, the composition (x) dependence of magnetization (Ms) and magnetocrystalline anisotropy (Ku) of the films was studied. The films with 0 ≤ x ≤ 0.32 showed ferromagnetism (800 ≤ Ms ≤ 1100 emu/cm3) with a high magnetocrystalline anisotropy (1.7 × 107 ≤ Ku ≤ 7.0 × 107 erg/cm3), and the films with 0.34 ≤ x ≤ 0.40 showed antiferromagnetism. The composition of x = 0.34 was the threshold of the FM-AF transition. Next, the FM-AF transition was applied for bit patterning. Atomic diffusion was used for modifying the composition. Only the magnetic phase of the area whose composition crossed the threshold changed abruptly to the FM phase. FM dots with sizes (D) of 20 ≤ D ≤ 1000 nm were observed in the AF matrix by magnetic force microscopy. The dots with D > 100 nm formed multidomain structures, and the dots with D ≤ 100 nm formed a single-domain structure.