K. Koughia

The influence of deposition conditions and alloying on the electronic properties of amorphous selenium

Electronic properties of a-Se as a function of the source (boat) temperature and as a function of As (up to 0.7%) and Cl (up to 40 wt ppm) concentrations have been experimentally studied by carrying out conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity measurements that provide accurate determinations of the drift mobility and the deep trapping time (lifetime). No variation in electron and hole lifetimes and mobilities for pure a-Se was observed with the source temperature, that is, no dependence was observed either on the deposition rate or on the vapor composition. The addition of As reduces the hole lifetime but does not change the hole mobility. At the same time, As addition increases the electron lifetime while reducing the electron mobility. The electron range μτ, however, increases with the As content, which means that the overall concentration of deep electron traps must be substantially reduced by the addition of As. Cl addition in the ppm range increases the hole lifetime but reduces the electron lifetime. The drift mobility of both carriers remains the same. We interpret the results in terms of a shallow-trap-controlled charge transport in which deep traps are due to potential under-and overcoordinated charged defects that can exist in the structure.

Optical and Photoluminescence Properties of Erbium-Doped Chalcogenide Glasses (GeGaS:Er)

We have examined the optical and photoluminescence (PL) properties of Er³⁺-doped GeGaS glasses of near-stoichiometric composition Ge₂₈Ga_6.2S_65.3:Er_0.5. We have also used powdered samples of various mean sizes (L) to examine the dependence of the 1.54 -mum PL emission spectrum and the PL decay time on the average sample size. Optical absorption spectra of Er³⁺ ions arising from transitions between different energy manifolds, such as 4 I₁₅ _/2 -⁴ I_13/2,⁴ I₁₅ _/2 -⁴ I₁₁ _/2 , etc., have been used to extract Omega₂, Omega₄, and Omega₆ values using the Judd-Ofelt analysis and a Judd-Ofelt radiative lifetime T_JO = 2.6 ms for the ⁴ I₁₃ _/ ₂ -⁴ I₁₅ _/ ₂ transition. The PL emission spectra and the decay time have been found to depend on the mean sample size. The spectra are broader and the decay times are longer for larger sample sizes, due to photon trapping occurring in the sample. The extrapolated decay time to zero particle size yields a decay time that matches the Judd-Ofelt radiative lifetime almost perfectly, and confirms the argument that the true PL lifetime needs to be measured in fine powders to avoid reabsorption effects. We have estimated the maximum emission cross section as 15.5 X 10^-21 cm².

Photoluminescence measurements of Er-doped chalcogenide glasses

We have investigated the photoluminescence (PL) emission spectra, photoluminescence lifetimes, and relative photoluminescence intensities of various Er-doped chalcogenide glass alloys, designed for possible use in optical waveguide amplifier applications at the telecommunications wavelength of 1550 nm. Bulk samples were prepared using melt-quenching techniques, and the samples were doped with varying amounts of Er2S3 or ErCl3, in the range of 0.3−3 at. %. Using a 980 nm pump beam, we measured the lifetime of the 1550 nm (4I13/2 to 4I15/2) luminescence and PL spectra. For two of the alloys, the luminescence efficiency as a function of erbium concentration was observed by scaling the luminescence intensity by the weight of each sample. For GaGeS, the luminescence lifetime exhibited quenching at concentrations of erbium greater than 1 at. %, but for GaGeSe, this behavior was not observed for concentrations as high as 3 at. %. However, the photoluminescence efficiency measurements for both glasses showed quen...

Dependence of charge-carrier ranges in stabilized a-Se on preparation conditions and alloying

This work was motivated by the renewed interest in stabilized a-Se because of its recent applications as a photoconductor in X-ray image detectors. Stabilized amorphous selenium thick films were prepared from differently alloyed materials, to investigate the effects of As and Cl addition on the electron and hole ranges (drift mobility–lifetime product). A set of samples was prepared at different substrate temperatures during a-Se deposition from each alloy, to study the effects of substrate heating on the electronic transport. Additionally, some samples were thermally annealed and the effects on the charge transport were studied. The electron and hole mobilities were measured by the time-of-flight technique. The corresponding lifetimes were examined by the interrupted-field time-of-flight technique.

Quadrature frequency resolved spectroscopy (QFRS) of radiative transitions of Er3+ and Nd3+ ions in chalcogenide glasses (ChGs)

Wideband quadrature frequency resolved spectroscopy (QFRS) of photoluminescence (PL) lifetime distributions from 2 ns to 160 s is shown to be very effective in elucidating the characteristic features of radiative transitions of Er3+ ions in GeGaSe and GeGaS chalcogenides glasses (ChGs). Undoped GeGaSe ChGs show triple-peak lifetime distributions of which two short-lifetimes are associated with singlet-triplet excitons and longest-lifetime, ~20 s, with radiative tunnelling (RT) of distant-pairs (DPs). Er-doped GeGaSe and GeGaS ChGs exhibit a double-peak lifetime distribution, consisting of a peak at ~3.3 and ~5.3 ms, respectively, a characteristic of the Er3+ luminescence centre and another peak at ~20 s, similar to that of undoped GeGaSe ChGs. It is shown that the QFRS can separate and analyse two mixed radiative transitions of Nd3+ ions, 4F3/2V4I15/2 and 4F5/2,2H9/2V4I15/2 in GaLaS ChGs. From the QFRS results we can experimentally extract the branching ratio ?J and lifetime ? ? 77 (?s for 4 lasing transitions 4F3/2?4IJ(J = 9/2, 11/2, 13/2, 15/2) of Nd3+ ions in GaLaS ChGs, in particular, the weakest transition 4F3/2?4I15/2.

Thermal Stability and Optical Activity of Erbium Doped Chalcogenide Glasses for Photonics

The glass transition and crystallization temperatures (T g , T c ), heat capacity, thermal stability and glass uniformity of GeSGa, GeSeGa, Ge(SeTe)Ga chalcogenide glasses doped with Er3+ by the addition of Er2S3 have been investigated by conventional differential scanning calorimetry (DSC) and Temperature-Modulated DSC (TMDSC). While some of the glasses have two crystallization peaks, these glasses were nonetheless optically actively and uniform. Essential optical properties have been evaluated, such as the photoluminescence (PL) intensity and lifetime as a function of the glass composition. We present typical results to emphasize some of the important characteristics of these systems and discuss trends within a glass system; and also highlight differences between glass systems.