Papers by Author: Tzanimir Arguirov

Abstract: The heteroepitaxial growth of GeSn and Ge crystals on Si substrates are investigated for Si-based photonic applications. Light Emitting Diodes with emission wavelengths from 2,100 to 1,550 nm could be demonstrated with active intrinsic GeSn light emitting layers between Ge barriers. A clear shift of the direct band gap toward the infrared beyond 2 μm is measured. Emission intensity is increased compared to Ge Light Emitting Diodes. Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices are demonstrated. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Pérot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1,660 nm and 1,700 nm.

Abstract: We report on 0.93 eV luminescence observed in multicrystalline silicon. The spectral line is close to the well known D3 one, but its properties are different. The new feature shows a remarkable intensity at room temperature, exceeding the intensity of the band to band radiative transition. Moreover, it appears as a single line in the entire temperature range 10-300K, in contrast to the D3, which is usually accompanied by D4. Cathodoluminescence (CL) and electron beam induced current (EBIC) micrographs revealed that the centers causing 0.93 eV emission are irregularly distributed along certain grain boundaries. Electron backscattering diffraction examination showed that the 0.93 eV luminescence appears at grain boundaries characterized by a lattice rotation around a <344> axis. The EBIC contrast at those irregularities indicates strong total recombination. Based on an analysis of the temperature dependence of the CL intensity and the EBIC contrast we obtained an activation energy of about 120 meV.

Abstract: We present an overview on generation of direct gap photo- and electroluminescence in Ge bulk wafers, Ge thin films deposited on Si, and Ge p-i-n diodes prepared on Si substrates. We analyzed the emission in a spectral range from 0.45 eV to 0.95 eV, covering the radiation caused by direct gap transitions, the indirect one, and also the luminescence related to transition on dislocations. The temperature and excitation level strongly influence the intensities of direct and indirect photoluminescence in bulk samples. As it could be expected, high temperature and excitation favour the generation of direct gap luminescence. Intrinsic bulk Ge shows a quadratic dependence of the direct gap luminescence on the excitation and a sub-quadratic one for the indirect. The photoluminescence spectra taken from intrinsic Ge on Si layers show features related to dislocations. There are two spectral regions associated with dislocation recombination. At room temperature one is at around 0.45 eV and the other at 0.72 eV. We found strong direct gap radiation from the Ge p-i-n diodes with intrinsic, highly dislocated active area (dislocation density of about 10⁸-10¹⁰ cm^-2). There is a threshold current density of 8 kA/cm², at which the direct band luminescence becomes a super-quadratic. The dependence of the radiation intensity on the excitation is governed by a power law with exponent of 1.7 before reaching that threshold and 4.5 after exceeding it. Above the threshold the dislocation radiation shows similar dependence on the excitation as the direct band luminescence.

Abstract: Incorporation of optical components into microelectronic devices will significantly improve their performance. Absence of effective Si-based light emitter hampers such integration. In the present work light emitting Si diodes, fabricated by dopant (boron or phosphorous) implantation and annealing are investigated. Different implantation doses and annealing temperatures were employed. The efficiency of the electroluminescence (EL), obtained from such structures was measured and correlated with the fabrication process parameters. As previously reported, the EL of band-to-band radiative transition in Si is strongly influenced, by the dopant implantation dose, i.e. higher doses usually enhance EL. Our results suggest that the effect is mainly related to the increase of minority carrier lifetime in the substrate. Distinct measurements showed that the higher implantation doses lead longer carrier lifetimes in the samples. The correlation between lifetime and the EL efficiency could be satisfactory explained in the frame of a classical model, considering the carrier-injection dependence of the rates of the three main recombination mechanisms in silicon, i.e. multi-phonon, radiative and Auger recombination. We suppose that the increase in the implantation dose improves minority carrier lifetime due to the gettering of impurity atoms from the substrate material to the highly doped emitter region.

Abstract: The investigation of regular dislocation networks (DN) formed by direct wafer bonding suggests that the D1 and D2 peaks of dislocation-related luminescence (DRL) in silicon is linked to screw dislocations, whereas edge dislocations are responsible for D3 and D4 DRL peaks. Non-radiative recombination activity in DN could be attributed to edge dislocations and could be related to enhanced ability of these dislocations to getter impurity atoms. Obtained relation of DRL intensity with the density of screw dislocations suggests existence of the optimum twist angle for the wafer-bonding geometry for which the DRL intensity has a maximum. The dependence of DRL intensity on the spacing between screw dislocations has the maximum at about 7 nm. Reported radiative and non-radiative recombination properties of DN present substantial interest not only for possible LED applications in all-Si photonics but also for photovoltaics, since DNs represent a model system for grain boundaries controlling carrier lifetime in microcrystalline-Si material.

Abstract: Germanium is an attractive model system for studying the crystallization mechanism and optimization of the growth processes in photovoltaics. In comparison to Si it has a lower melting point and that is why its usage is cost effective. The main aim of our work was to verify the similarities in the growth related defect formation between Ge and Si. We apply standard Si characterization methods to poly and VGF-grown n-type Ge. Room temperature and 80 K EBIC measurements were done to reveal the defect structure. Photoluminescence spectra were used to characterize the optical properties as for instance the Ge band-to-band or defect originated transitions. Additionally, photoluminescence and cathodoluminescence maps were preformed to reveal the defect distribution/activity, too, by using the direct Ge band-to-band transition.

Abstract: Silicon and germanium films epitaxially grown on metal oxide buffer layers on Si(111) substrates are characterized by different X-ray techniques, transmission electron microscopy and Raman spectroscopy. Pr2O3 and Y2O3 or a combination of both is used as buffer material. X-ray pole figure measurements and grazing incident X-ray diffraction prove that epi-semiconductor layers can be grown single crystalline with exactly the same in-plane orientation as the Si(111) substrate. Epi-Ge layers show a small fraction (less than 0.5 vol. %) of so-called type B rotation twin regions located near the oxide-Ge interface. The main structural defects for both epi materials are micro twin lamellas lying in {111} planes 70° inclined to the wafer surface that may reach through the whole layer from the oxide interface to the surface. Furthermore, TEM confirms the existence of stacking faults and threading dislocations. X-ray grazing incident diffraction and Raman measurements show that epi-Ge layers on Pr2O3 buffer are nearly fully relaxed, while epi-Si layers on Y2O3/Pr2O3 double buffer are compressive strained depending on their own thickness and the thickness of the underlying Y2O3 layer. It is demonstrated that the epi-layer quality can be improved by post-deposition annealing procedures.

Abstract: Electrical and structural properties of thin-film photovoltaic (PV) material fabricated using Crystal Silicon on Glass (CSG) technology was investigated applying photoluminescence (PL) and Raman spectroscopy (RS). The obtained results and their correlation with the PV properties of the cells prepared from the same material showed that PL is applicable for in-line characterization of the material before the electrical contact fabrication processes. The results obtained using RS gave useful information on crystallization grade of the material during the fabrication process.

Abstract: We report on the optical and mechanical properties of Si3N4 inclusions, formed in the upper part of mc-Si blocks during the crystallization process. Those inclusions usually appear as crystalline hexagonal tubes or rods. Here we show that in many cases the Si3N4 inclusions contain crystalline Si in their core. The presence of the Si phase in the centre was proven by means of cathodoluminescence spectroscopy and imaging, electron beam induced current measurements and Raman spectroscopy. The crystalline Si3N4 phase was identified as β-Si3N4. Residual stress was revealed at the particles. While the stress is compressive in the Si material surrounding the Si3N4 particles tensile stress is found in the Si core. We assume that the stress is formed during cool down of the Si block and is a consequence of the larger thermal expansion coefficient of Si in comparison to that of β-Si3N4. Iron assisted nitridation of Si at temperatures below 1400 °C is considered a possible mechanism of Si3N4 formation.