We succeeded in growing highly Fe-doped GaN films by solid-source molecular beam epitaxy using an electron-cyclotron-resonance microwave nitrogen plasma. The substrate temperature was in the range of 380–520 °C. The samples were analyzed by x-ray diffraction and transmission electron microscopy, and showed hexagonal (wurtzite) or cubic (zincblende) structure or a mixture of both phases. The Fe concentration was on the order of 1019 cm−3 and extended x-ray absorption fine structure data show that the Fe is substituting the Ga in GaN. The magnetization measurements as a function of temperature reveal ferromagnetic properties below 100 K for the sample grown at the lowest temperature.

We conducted an experimental study of back-side-illuminated InGaAs photodiodes grown on GaAs and sensitive in the short-wave infrared up to 2.4 μm. Standard metamorphic InGaAs or IR-transparent InAlAs buffers were grown by molecular-beam epitaxy. We studied dark current and photocurrent as a function of buffer thickness, buffer material, and temperature. A saturation of the dark current with buffer thickness was not observed. The maximum resistance area product was ∼10 Ω cm2 at 295 K. The dark current above 200 K was dominated by generation–recombination current. A pronounced dependence of the photocurrent on the buffer thickness was observed. The peak external quantum efficiency was 46% (at 1.6 μm) without antireflective coating.

InAs/Al 0.2 Ga 0.8 Sb quantum well structures of high quality were grown by molecular beam epitaxy. The structural and electrical quality was characterized by x-ray diffractometry, Raman spectroscopy and Hall transport measurements. When an optimized buffer layer was used on a GaAs substrate, electron mobilities of 28 000 cm 2 V −1 s −1 at 300 K and 400 000 cm 2 V −1 s −1 at 20 K were routinely achieved for undoped structures. These excellent transport properties were utilized in a sensitive magnetoresistive sensor.

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A local structural transition in heavily Fe-doped GaN films related to the magnetic properties has been revealed by fluorescence x-ray absorption fine structure (XAFS) analysis. The structural transition is explained (or considered to be induced) by the change in the degree of hybridization between Fe 3d and N 2p states, which can be evaluated by x-ray absorption near edge structure spectra. The XAFS analysis indicates that the present diluted magnetic semiconductor based on GaN can be fabricated by electron cyclotron resonance microwave plasma-assisted low-temperature molecular-beam epitaxy.