Mohammed L. Benkhedir

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.

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.

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.

Photoconductivity in Materials Research

Photoconductivity is the incremental change in the electrical conductivity of a semiconductor or insulator upon illumination. The behavior of photoconductivity with photon energy, light intensity and temperature, and its time evolution and frequency dependence, can reveal a great deal about carrier generation, transport and recombination processes. Many of these processes now have a sound theoretical basis and so it is possible, with due caution, to use photoconductivity as a diagnostic tool in the study of new electronic materials and devices. This chapter describes the main steady-state and transient photoconductivity techniques applied in the investigation of semiconductors whose performance is limited by the presence of localized electronic states. These materials tend to be disordered , and possess low carrier mobilities and short free-carrier lifetimes in comparison with crystalline silicon. They are often prepared as thin films , and are of interest for large-area applications, for example in solar cells, display backplane transistors, photoemissive devices such as organic light-emitting diodes (OLEDs) and medical imagers. However, examples of where these techniques have been useful in the study of defective crystalline semiconductors are also given. The approach followed here is by way of an introduction to the techniques, the physics supporting them, and their applications, it being understood that readers requiring more detailed information will consult the references provided.