All-optical wavelength converters for optical switching applications

We present the latest results about all-optical wavelength conversion obtained within the European Union—funded RACE ATMOS project. In particular, solutions based on carrier depletion in Distributed Brag Reflector lasers have reached penalty-free polarization-insensitive operation at up to 2.5 Gbit/s, while solutions based on Semiconductor Optical Amplifiers (SOAS) under cross-gain modulation have been demonstrated at more than 20 Gbit/s. The study of cross-phase modulation in SOAs has also been performed, leading to the monolithic integration of SOAs within an interferometer, according to structures proposed by the project. Penalty-free operation at up to 10 Gbit/s as well as signal reshaping have been obtained with such devices. Besides the study of components, wavelength converters have been used in several demonstrators assessing their maturity and compatibility with photonic-switching applications.     

Thermal donors in silicon: Consistent interpretation of Hall-effect and capacitance transient spectroscopy

Thermal donors are generated in oxygen-rich silicon by an anneal at 450 °C. The energy-levels of these donors are investigated by capacitance transient spectroscopy (DLTS) and the Hall effect. A level at 125 meV below the conduction band observed with DLTS exhibits a strong dependence on the electric field (Poole-Frenkel effect). Evaluation with a novel model for the Poole-Frenkel effect leads to a corrected zero-field energy of 200 meV for this level. The Hall effect indicates two thermal donors at 57 and 120 meV below the conduction band; both centers cannot be observed with our capacitance-DLTS method due to the Poole-Frenkel energy reduction by 70 meV±10 meV. The 45 meV current-DLTS level [1,2] is shown to correlate with the 120 meV Hall level.     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.     

Investigation of impurity levels inn-type indium selenide by means of Hall effect and deep level transient spectroscopy

We report the results of Hall effect and deep-level-transient-spectroscopy measurements onn-type indium selenide samples cleaved from non-intentionally doped material and from chlorine, tin, gallium sulphide or oxygen doped ingots. Some unidentified shallow and deep levels are found. Impurity levels attributed to Cl (310 meV) and to Sn (44 and 120 meV) are also reported. Anomalous degeneracy of electrons at low temperature is discussed.     

Interstitial iron and iron-acceptor pairs in silicon

Deep levels in iron-dopedp-type silicon are investigated by means of Deep Level Transient Spectroscopy (DLTS) and the Hall effect. Pairs of Fe with the acceptors B, Al, and Ga are observed at 0.1, 0.19, and 0.24 eV above the valence band edgeE _v. For interstitial iron, (Fe _i ), a level energy ofE _v+0.39+-0.02 eV is obtained with DLTS after correction with a measured temperature dependence of the capture cross section. The Hall effect yieldsE _v+0.37 eV for Fe_i. The annealing behavior of levels related to Fe is investigated up to 160 °C. Iron participates in at least four different types of impurity states: Fe _i , Fe-acceptor pairs, precipitations (formed above 120 °C) and an additional electrically inactive state, which is formed at room temperature.     

Deep traps in GaAs under hydrostatic pressure

The influence of hydrostatic pressure on the energy levels of eight different deep traps in GaAs is determined by means of a transient capacitance technique. Hydrostatic pressure coefficients are shown to be decisive parameters in order to distinguish isoenergetic traps of different origin. For the EL6-, the M3- and the E3-traps (E_c?0.31eV), strongly different pressure coefficients are found. The HK1- and HK2- hole traps exhibit only small pressure coefficients relative to the valence band edge.     

Iron-related deep levels in silicon

Deep levels in iron-doped p-Si are investigated by means of capacitance transient spectroscopy. Five Fe-related defects are observed in suitably prepared samples. Rapid quenching produces interstitial Fe_i and FeB, which give rise to levels at E_v + 0.43 eVand 0.10 eV, respectively. Three further levels at E_v + 0.33 eV, E_v + 0.40 eV and E_v + 0.52 eV are caused by slower quenching rates. The concentration of E_v + 0.33 eV in slowly cooled Czochralski-grown Si strongly exceeds the concentration in similarly treated float-zone Si.     

Deep levels related to transition metals in Si under hydrostatic pressure

The influence of hydrostatic pressure up to 5×10⁸ Pa on deep levels related to transition metal impurities in silicon is determined by means of an isothermal capacitance method. Under pressure, donor levels of isolated Fe, V, Ti, and Mn shift towards the valence band in contrast to earlier results for deep chalcogen donors. This behavior is contrary to what is expected by considering only effects of hybridization. Quantitative differences between Fe, Ti, V, and, on the other hand, Mn suggest a different microscopic structure of these defects. The Fe-acceptor pairs FeB, FeAl, and FeGa move towards the valence band with a rate comparable to that of the ₁ conduction band. The thermal capture coefficients of isolated Fe, V, and Ti are found to be pressure independent up to 5×10⁸ Pa.On leave from Sony Corp., Research Center, Yokohama, Japan