This paper describes the overview of special issues on recent progress on optical functional semiconductors based on group 14 elements. Currently, silicon (Si) is mainly used in solar cells and optical sensors that are indispensable in the IOT society. In order to advance the performance of these devices, it is essential to improve the materials that compose the devices. New compounds of group 14 elements and metals are attracting attention as new optical functional materials. The recent research on the compounds was summarized.

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Barium disilicide (BaSi2) has attractive features for thin-film solar cell applications. Differently from CuGaInSe2 and CdTe thin-film solar cells, BaSi2 is composed of only safe, stable, and earth-abundant elements. In this article, present status of BaSi2 solar cells research ranging from defect studies using photoluminescence, electron paramagnetic resonance to passivation of defects by atomic hydrogen are presented. We put emphasis on the importance of device structure using electron transport and hole transport layers. BaSi2 solar cells will become market leaders if the energy conversion efficiency of BaSi2 solar cells exceeds 20%.

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We demonstrate optical properties of GeSn wires fabricated by local liquid phase crystallization (LLPC). The LLPC-GeSn wires have Sn fraction beyond the solid solubility in Ge and 5% tensile strain originated from the difference in thermal expansion coefficient of GeSn and quartz substrates. They show more than 50 times higher photoluminescence intensity than that of Ge crystal. We also show the electroluminescence of the PIN diode and the characteristics of PN photodiode for near infrared light of 1.55 and 2 μm wavelength.

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Semiconducting magnesium silicide, Mg2Si, is attracting attentions as a novel environment-friendly semiconductor applicable for the high-sensitive and fast-response short wavelength infrared (SWIR) detector, because its indirect energy gap of 0.61 eV corresponds to the cutoff wavelength approximately 2 μm. The abundant constituent elements of Mg2Si are suitable for mass consumption. Bulk single crystal growth and substrate preparation of Mg2Si, and development of Mg2Si pn-junction photodiode (PD) are reviewed in this paper. The good photoresponse of Mg2Si -PDs below 2.1 μm confirms the potential of Mg2Si as an environment-friendly SWIR photodetector.

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We introduce an example of group IV semiconductor evaluation results by laser Raman spectroscopy. Group IV semiconductors are attractive materials for electric integrated circuits, wearable devices, and sensor devices in the societies of the Internet of Things. However, the group IV semiconductor device process is gradually becoming too complex (miniaturization, low temperature, and so on) to achieve high-performance devices. Laser Raman spectroscopy is a powerful process evaluation technique that features the following advantages: speed, high-spatial resolution, and non-destructive measurements. Since such physical properties as stress and crystal quality are found in the group IV semiconductors of phonon energies (Raman spectra), they can be evaluated by measuring the Raman spectra. In this paper, we report on large area measurements and evaluate the strain distribution in local regions by adding various original improvements to a laser Raman spectrometer.

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This article reviews the electronic states and the optical properties of silicides with unique arrangements of Si atoms, ternary Si clathrates K8Ga8Si38 and barium disilicide BaSi2. K8Ga8Si38 is a type-I clathrate, in which Si/Ga atoms form a cage framework that consists of face-shared (Si/Ga)20 dodecahedra and (Si/ Ga)24 tetrakaidecahedra and where K atoms are located inside the polyhedra as guests. K8Ga8Si38 is an indirect gap semiconductor with a band gap of 1.2 eV. BaSi2 crystallizes in the BaSi2-type structure, where Si atoms form Si4 tetrahedra. BaSi2, an indirect band gap semiconductor with a band gap ranging from 1.1 to 1.3 eV, is attracting industry attentions as a thin-film solar cell material. We discuss why BaSi2 has a large optical absorption coefficient near the absorption edge despite its indirect band gap semiconductor.

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Previous research on distance sensors using the self-coupling effect of semiconductor lasers used current- spouting type LD driver circuits. As a result, the triangular current wave injected into the LD has poor linearity, requiring multiple triangular waves for measurement, which is time-consuming. We improved the triangular wave current’s linearity with a newly created current-absorbing LD drive circuit. We also proposed a novel signal processing method using a self-coupled signal obtained with a single triangular wave and experimentally confirmed that measurements performed with a single triangular wave have the same accuracy as conventional measurements using multiple pulses.

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