Monica Brinza

Time-of-flight measurements of carrier drift mobilities in polymorphous silicon

Abstract Time-of-flight transient photoconductivity measurements in polymorphous silicon samples reveal a 10-fold increase of the room temperature hole drift mobility up to 1.5×10−2 cm2 V−1 s−1 when the deposition total gas pressure is raised from 133 to 232 Pa. An accompanying increase of the electron mobility is much more modest.

Electronic density of states in amorphous selenium

Steady-state and transient photoconductivity methods are used to investigate the electronic density of states in evaporated layers of amorphous selenium. From the temperature dependence of the steady-state photocurrent and, independently, from an analysis of the post-transit currents of time-of-flight transients, energy levels in the gap at 1.43 ± 0.02 eV and 0.40 ± 0.02 eV above the valence band have been determined for the occupied state of the negative-U centres. An absorption band around 1.50 eV is seen in the spectral distribution of the photocurrent. The distribution of tail states may—to first approximation—be described by a steep exponential with a characteristic width of meV at the valence band and a more steeply declining functional of similar width at the conduction band.

Time-of-flight photocurrents in expanding-thermal-plasma-deposited a-Si:H

Abstract Time-of-flight (TOF) photocurrent investigations of hydrogenated amorphous silicon (a-Si:H) layers produced by means of an expanding thermal plasma (ETP) reveal a hole drift mobility that is, for depositions near 450 ° C , consistenly one order of magnitude higher than the corresponding mobility in standard PECVD samples. Electron drift mobilities and μτ products do not show such differences. The electronic density of states contains a prominent band of deep traps and the materials show evidence for non-amorphous inclusions.

Steady-state photoconductivity in amorphous selenium glasses

Steady-state photoconductivity measurements are carried out for bulk and thin-film amorphous selenium (a-Se) samples in the temperature range between 190 and 340 K. The temperature and light-intensity dependences of the photoconductivity reveal the presence of both mono- and bimolecular recombination regimes. The current activation energies measured in the two regions point to energy levels in the gap for the recombination centres at 0.36 ± 0.06 and 1.35 ± 0.10 eV above the valence band mobility edge. These values put a-Se in line with the other chalcogenide semiconducting glasses that exhibit negative-U behaviour.

Analysis of electron time-of-flight photocurrent data from a-Se

Electron time-of-flight transient photocurrents from amorphous selenium (a-Se) films were examined over the range of temperatures and applied electric fields in order to deduce a consistent model for the distribution of localized states in the a-Se conduction band tail. Superimposed on an exponential tail with characteristic energy of 20meV, a Gaussian defect band around E−Ec=0.3eV controls the field-independent drift mobility, and a broad distribution of deeper electron traps is responsible for the significant emission currents that are observed for several decades of time after the transit time.

Anomalous DC dark conductivity behaviour in a-Se films

Thin films of amorphous selenium have been prepared by thermal evaporation. DC conductivity measurements were carried out on these films in the temperature range between 208 and 322 K. Above room temperature, the dark conductivity is thermally activated with activation energy Eσ = 1.05 ± 0.08 eV. For temperatures below 285 K, an increase in the dark current is observed, which is interpreted in terms of a shift of the Fermi level that makes more states available for a hopping process.

Structure of the band tails in amorphous selenium

Transient photocurrent measurements on evaporated a-Se layers indicate the presence of two sets of discrete traps in the band tail region. The shallower traps, at EV+0.20 eV and EC−0.28 eV, are found to be electrically neutral, while the deeper ones at EV+0.38 eV and EC−0.53 eV are related to the charged negative-U centres of a-Se. The density of the discrete traps is of the same order of magnitude as the disorder-induced background density in the valence and conduction band tails, preventing the characterization of the a-Se tail-state densities by a simple functional form.

Defect levels and charge carrier photogeneration in amorphous selenium layers

Time-of-flight post-transit data that suggested positions in the band gap atEv þ 0:4 eV andEc � 0:55 eV for the thermally accessible levels of the intrinsic negative-U centers of a-Se have been re-examined. The introduction into the Onsager formalism of a distribution of initial carrier separations in the photogeneration process, together with random internal field fluctuations, and of the ensuing processes of bimolecular recombination and deep trapping lead to a good description of experimentally observed photogeneration efficiency in a-Se.

Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities

Exponential distributions of tail states have been able, within the framework of a multiple-trapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.

Re-examination of the post-transit photocurrents in expanding-thermal-plasma-grown a-Si:H

The interpretation of post-transit photocurrents in a time-of-flight experiment, in terms of the underlying density of localized gap states in the sample, is questioned for the case of previously examined hydrogenated amorphous silicon cells prepared by the expanding thermal plasma technique. Part of the observed current is not generated by re-emission of trapped photo-generated charge and should, therefore, not be used for density-of-states calculations.