Kenichiro Takakura

Room Temperature 1.6 µm Electroluminescence from a Si-Based Light Emitting Diode with β-FeSi2 Active Region

Room temperature electroluminescence (EL) was obtained for the first time from a Si-based light emitting diode with semiconducting silicide (β-FeSi2) active region. The peak wavelength was 1.6 µm and it is from β-FeSi2 balls embedded in a Si p-n junction. An a-axis oriented β-FeSi2 layer was grown on n+-(001) Si by reaction deposition epitaxy (RDE), then p- and p+-Si layers were grown by molecular beam epitaxy (MBE). The β-FeSi2 aggregated into balls with about 100 nm diameter in this process. The EL intensity increased superlinearly with increase of the injected current density, and reasonable EL was obtained at current density above 10 A/cm2, though the photoluminescence (PL) was difficult to be detected at room temperature.

Investigation of the energy band structure of orthorhombic BaSi2 by optical and electrical measurements and theoretical calculations

Optical and electrical properties of polycrystalline orthorhombic BaSi2 prepared by arc melting in Ar atmosphere were investigated. The optical absorption spectra measured at room temperature showed that indirect and direct absorption edges were 1.15 and 1.25 eV, respectively. The activation energy estimated from temperature dependence of the resistivity was 1.10 eV. These results agreed well with a calculated band structure of the orthorhombic BaSi2 by first principles calculation using density functional theory.

Influence of Si growth temperature for embedding β-FeSi2 and resultant strain in β-FeSi2 on light emission from p-Si/β-FeSi2 particles/n-Si light-emitting diodes

We have fabricated Si/β-FeSi2 particles/Si structures by reactive deposition epitaxy for β-FeSi2 and molecular-beam epitaxy (MBE) for Si. It was found that the photoluminescence (PL) intensity of β-FeSi2 strongly depended on MBE-Si growth temperature for embedding β-FeSi2 in Si. Room temperature 1.6 μm electroluminescence was realized from a Si-based light-emitting diode by embedding the β-FeSi2 active region with Si grown at 500 °C. Observation with transmission electron microscopy revealed that the a axis of β-FeSi2 from which PL was observed was about 9% longer than that of β-FeSi2 without PL or of bulk β-FeSi2.

Direct Growth of [100]-Oriented High-Quality β-FeSi2 Films on Si(001) Substrates by Molecular Beam Epitaxy

We have directly grown [100]-oriented high-quality β-FeSi2 films on Si(001) substrates with a β-FeSi2 template by molecular beam epitaxy(MBE) at 470°C. It was found that the crystalline quality of the as-grown β-FeSi2 film was as good as that of the film grown by the multilayer method as far as X-ray diffraction intensity was concerned. However, the electrical property of the as-grown β-FeSi2 film was very poor. The crystalline quality and the electrical property were much more quickly improved by 900°C annealing compared to the films grown by the multilayer technique. The [100] orientation of MBE-grown β-FeSi2 film was preserved even for a 1-µm-thick film.

Dependence of photoluminescence from β-FeSi2 and induced deep levels in Si on the size of β-FeSi2 balls embedded in Si crystals

We fabricated single-crystalline β-FeSi2 balls with various sizes embedded in Si crystals by reactive deposition epitaxy (RDE) and following molecular beam epitaxy (MBE). The photoluminescence (PL) depended on the size of the β-FeSi2 ball. We observed clear 1.54 μm PL from the sample with approximately 100-nm diameter β-FeSi2 balls, but not from the sample with the balls of approximately 150 nm in diameter or bigger. It was found from deep level transient spectroscopy (DLTS) measurements that absence of the 1.54-μm PL emission was attributed to the existence of deep levels in the Si crystals around the β-FeSi2 balls.

Control of the Conduction Type of Nondoped High Mobility β-FeSi2 Films Grown from Si/Fe Multilayers by Change of Si/Fe Ratios

Highly [100]-oriented nondoped β-FeSi2 continuous films were grown on Si (001) substrates from Si/Fe multilayers with different Si/Fe ratios (1.6–2.0). It was found that the conduction type of the grown layer changed from p-type to n-type when the deposited Si/Fe ratio was increased, suggesting that the conduction type of nondoped β-FeSi2 is governed by stoichiometry. Annealing of the samples at 900°C for up to 14 h decreased the carrier density and increased the mobility, but did not change the conduction type. A p-type sample gave the highest hole mobility ever reported; 13000 cm2/Vs at 50 K.

Investigation of direct and indirect band gaps of [100]-oriented nearly strain-free β-FeSi2 films grown by molecular-beam epitaxy

Optical absorption (OA) spectra of [100]-oriented, nearly strain-free β-FeSi2 continuous films grown by molecular-beam epitaxy were investigated. Although the film is a multidomain epilayer, the OA spectra of the annealed film were fitted well by the sum of the contributions from both direct and indirect absorption transitions for a wide range of temperatures from 77 to 300 K. The indirect absorption edge of 0.78 eV obtained at 77 K agreed with the effective band gap energy obtained from temperature dependence of the carrier density.

Growth of Continuous and Highly (100)-Oriented β-FeSi 2 Films on Si(001) from Si/Fe Multilayers with SiO 2 Capping and Templates

We fabricated continuous and highly (100)-oriented β-FeSi2 films on Si(001) by annealing Si/Fe multilayers. An annealing temperature above 800°C was necessary to obtain high-quality β-FeSi2, but the β-FeSi2 aggregated into islands due to the annealing. Such aggregation was prevented by a SiO2 capping layer deposited on the top of the Si/Fe multilayers, but the obtained β-FeSi2 film showed a polycrystalline nature. Highly (100)-oriented continuous β-FeSi2 film was obtained using Si/Fe multilayers on a (100)-oriented β-FeSi2 template, together with a SiO2 capping layer.

Fabrication of p-Si/β-FeSi2 balls/n-si structures by MBE and their electrical and optical properties

We report on the formation technique of single-crystalline β-FeSi 2 balls (< 100 nm) embedded in a Si p-n junction region by Si molecular beam epitaxy (MBE). β-FeSi 2 films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of β-FeSi 2 aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance-voltage (C-V) characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded β-FeSi 2 balls with an increase of embedded β-FeSi 2 quantity.

Donor and Acceptor Levels in Undoped β-FeSi2 Films Grown on Si (001) Substrates

Donor and acceptor levels of undoped n- and p-type β-FeSi2 films were investigated in terms of temperature dependence of carrier density. β-FeSi2 films were formed from Si/Fe multilayers on Si (001) substrates. Two kinds of donor (ED=0.075 eV and 0.21 eV) and acceptor levels (EA=0.10 eV and 0.19 eV) were observed. The density of these levels was found to be about 1020 cm-3, suggesting that they originate from defects rather than impurities included in the 4N-Fe source. The energy gap of the β-FeSi2 film was determined to be about 0.80 eV. This value corresponded to the reported value of electroluminescence peak wavelength (1.6 µm) of the p-Si/β-FeSi2 balls/n-Si(001) diode we reported recently, and is slightly smaller than the reported energy gap estimated from the absorption edge.