N. G. Galkin

Enhancement of the Si p-n diode NIR photoresponse by embedding β-FeSi2 nanocrystallites

By using solid phase epitaxy of thin Fe films and molecular beam epitaxy of Si, a p+-Si/p-Si/β-FeSi2 nanocrystallites/n-Si(111) diode structure was fabricated. Transmission electron microscopy data confirmed a well-defined multilayered structure with embedded nanocrystallites of two typical sizes: 3–4 and 15–20 nm, and almost coherent epitaxy of the nanocrystallites with the Si matrix. The diode at zero bias conditions exhibited a current responsivity of 1.7 mA/W, an external quantum efficiency of about 0.2%, and a specific detectivity of 1.2 × 109 cm × Hz1/2/W at a wavelength of 1300 nm at room temperature. In the avalanche mode, the responsivity reached up to 20 mA/W (2% in terms of efficiency) with a value of avalanche gain equal to 5. The data obtained indicate that embedding of β-FeSi2 nanocrystallites into the depletion region of the Si p-n junction results in expansion of the spectral sensitivity up to 1600 nm and an increase of the photoresponse by more than two orders of magnitude in comparison with a conventional Si p-n junction. Thereby, fabricated structure combines advantage of the silicon photodiode functionality and simplicity with near infrared light detection capability of β-FeSi2.

Room temperature 1.5 μm light-emitting silicon diode with embedded β-FeSi2 nanocrystallites

Light-emitting silicon diode structures with embedded β-FeSi2 nanocrystallites have been fabricated using solid phase epitaxy and a combination of reactive deposition and solid phase epitaxy. Electroluminescence (EL) of the structures was studied over various temperatures and current densities under forward and reverse biases. The structures with nanocrystallites formed by the combined method exhibited EL at temperatures below 70 K only, suggesting the presence of a high concentration of defects—non-radiative centers. High-quality defect-free structures with nanocrystallites formed by solid phase epitaxy revealed intensive room temperature EL in energy range 0.76–1.08 eV at current densities as low as 1 A/cm2.

Characterization of the silicon/β-FeSi2 nanocrystallites heterostructures for the NIR photodetection at low temperature

Using solid phase epitaxy of thin Fe films and molecular beam epitaxy of Si, p-Si/β-FeSi2 nanocrystallites/n-Si(001) diode structure was fabricated. The diode exhibited a current responsivity of 15 mA/W and external quantum efficiency of about 1% at a wavelength of 1300 nm at 120 K without bias and 200 mA/W and 10%, respectively, at −30 V. The device specific detectivity calculated at 120 K in zero bias conditions of 2.1 × 1011 cmHz1/2/W at a wavelength of 1.3 µm is the highest ever reported for Si/β-FeSi2 systems. The Franz–Keldysh effect gives grounds for applying such systems not only for the development of optrons but also for that of electro-optical modulators.

Electronic properties of semiconducting Ca2Si silicide: From bulk to nanostructures by means of first principles calculations

Results of our ab initio calculations have revealed changes in electronic properties in Ca2Si semiconducting silicide when reducing dimensionality from bulk to slabs and, eventually, to nanowires. In the case of the bulk, Ca2Si is found to be a direct band-gap semiconductor with the band-gap value of 0.30, 0.60, and 0.79 eV by using the generalized gradient approximation, the modified Becke–Johnson exchange potential and the screened hybrid functional, respectively. We have also identified that among Ca2Si(001), (010), and (100) surfaces the (100) one has the lowest surface energy. Ca2Si slabs with (010) or (100) surfaces are predicted to be semiconductors, while (001) surface provides metallic properties due to surface states. The role of the surface states in the band-gap variation is also discussed. In the case of Ca2Si nanowires with 〈001〉, 〈010〉, and 〈100〉 axes and different morphologies only the 〈001〉 orientation guarantees semiconducting properties because of absence of {001} facets which induce metallic properties as for the corresponding slab.

An approach to growth of Fe-Si multilayers with controlled composition profile - A way to exchange coupled thin films

The growth, composition and structure of sandwich structures (Fe-rich layer/Si-rich layer/Fe-rich silicide layer) grown on a Si(111) surface were studied by a few complementary microscopic and spectroscopic techniques with high spatial resolution. Intermixing at the Fe/Si and Si/Fe interfaces is demonstrated. Fe-rich layers grown directly on the Si(111) surface are crystalline and have abrupt but rough interfaces at both sides. The succeeding layers are disordered and their interfaces are fuzzy. The distributions of Fe and Si within the layers are laterally non-uniform. The reproducible fabrication of thin non-magnetic silicide spacers of predetermined thickness is demonstrated. Sandwich structures with such spacers exhibit exchange coupling between ferromagnetic Fe-rich layers.

On the way to enhance the optical absorption of a-Si in NIR by embedding Mg2Si thin film

Mg2Si thin film was embedded in amorphous silicon matrix by solid phase epitaxy. The structure and optical properties were investigated by electron energy loss, X-ray photoelectron, Raman, and photo thermal deflection spectroscopy measurements. It was found that in the photon energy range of 0.8–1.7 eV, the light absorption of the structure with magnesium silicide (Mg2Si) film embedded in a-Si(i) matrix is 1.5 times higher than that for the same structure without Mg2Si.

VIS-NIR-SWIR multicolor avalanche photodetector originating from quantum-confined Stark effect in Si/β-FeSi2/Si structure

A Si n-i-p avalanche photodetector with embedded β-FeSi2 nanocrystals was developed. The device showed an ultrabroadband photoresponse from the visible (400 nm) to short-wavelength infrared (1800 nm) ranges. Specific detectivity at zero bias conditions reaches 2 × 109 cmHz1/2/W at 1300 nm and 2 × 108 cmHz1/2/W above 1400 nm at room temperature. Observed quantum-confined Stark effect together with avalanche multiplication resulted in a simultaneous two orders of magnitude increase in the photoresponse and spectral sensitivity expanding to 1800 nm when the device is operated in avalanche mode. The application fields of the proposed photodetector potentially include integrated Si photonics and multicolor photodetection; the quantum-confined Stark effect gives grounds for the development of fast-operated electro-optical modulators.

A room-temperature-operated Si LED with β-FeSi2 nanocrystals in the active layer: μW emission power at 1.5 μm

This article describes the development of an Si-based light-emitting diode with β-FeSi2 nanocrystals embedded in the active layer. Favorable epitaxial conditions allow us to obtain a direct band gap type-I band alignment Si/β-FeSi2 nanocrystals/Si heterostructure with optical transition at a wavelength range of 1500–1550 nm at room temperature. Transmission electron microscopy data reveal strained, defect-free β-FeSi2 nanocrystals of diameter 6 and 25 nm embedded in the Si matrix. Intense electroluminescence was observed at a pumping current density as low as 0.7 A/cm2. The device reached an optical emission power of up to 25 μW at 9 A/cm2 with an external quantum efficiency of 0.009%. Watt–Ampere characteristic linearity suggests that the optical power margin of the light-emitting diode has not been exhausted. Band structure calculations explain the luminescence as being mainly due to radiative recombination in the large β-FeSi2 nanocrystals resulting from the realization of an indirect-to-direct band ga...

Influence of the Si(100)-c(4×12)-Al surface phase on formation and electrical properties of thin iron films

The electrical properties of thin iron films deposited at room temperature (RT) on clean Si(100) and on the prefabricated Si(100)-c(4×12)-Al surface phase (SP) have been studied by means of in situ Hall effect and conductance measurements for iron coverage (0–2.3) nm. It is shown that the Si(100)-c(4×12)-Al SP blocks intermixing of iron and substrate atoms. The conductance and mobility of the majority carriers in this SP are higher than those in Si(100)2×1, within the temperature range from RT to 460 K. For iron coverage below 2 nm, the sample conductance is significantly lower than that for the bare Si(100). The iron film grown on the Si(100)-c(4×12)-Al becomes continuous at a coverage approximately half as thick as for the film grown on the Si(100)2×1 surface.

Formation and optical properties of thin Mg2Ge films on Si(001) substrate

Thin Mg2Ge films were grown using two methods: a co-deposition of Ge and Mg on Si substrate kept at room temperature followed by annealing at 200 °C (solid phase epitaxy – SPE) and reactive deposition epitaxy (RDE) of Ge and Mg on Si at 200 °C. Optical properties of these structures were investigated in the photon energy range of 0.02–6.2 eV. Based on optical functions calculation, it was shown that SPE growth results in formation of a crystalline layer of Mg2Si, which exhibits a strong optical phonon originated from the substrate-film interface. In the case of RDE growth, the amount of Mg2Si is sufficiently lower, but Mg‑Si-Ge compound phonon appears. The estimate of a fundamental indirect transition value in the film is 0.72 eV for SPE growth method and 0.56 eV for RDE due to the ternary compound Mg-Ge-Si at the film-substrate interface.