Density of localized electronic states in a-Se from electron time-of-flight photocurrent measurements

Abstract

Density of states in tritiated amorphous silicon obtained with the constant photocurrent method J. Appl. Phys. 98, 093705 (2005); 10.1063/1.2123374Density of states in the gap of amorphous semiconductors determined from modulated photocurrent measurements in the recombination regime Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging ͑relaxation͒, and alloying with As and doping with Cl. The distribution of localized states ͑DOS͒ in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ϳ0.30 eV below E c with an amplitude ϳ10 17 eV −1 cm −3 , a second small peak ͑or shoulder͒ at 0.45-0.50 eV below E c with an amplitude 10 14 -10 15 eV −1 cm −3 , and deep states with an integral concentration 10 11 -10 14 cm −3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.

Effect of repeated x‐ray exposure on the resolution of amorphous selenium based x‐ray imagers

“…11 The hole detrapping time is assumed as dh Ϸ 15 min ͑using the location of deep traps in the mobility gap͒ 25 and N 0 Ϸ 2 ϫ 10 12 cm −3 for both holes and electrons, using a deep trapping capture coefficient C t Ϸ 10 −8 cm 3 s −1 . 26 It is apparent that there is a good agreement between the proposed model and the experimental data 1.5ϫ 10 12 cm −3 , and 2 R, respectively. The fitted values of f r are 0.5 and 0.25 for F 0 =10 V/ m and F 0 =5 V/ m, respectively.…”

Photocurrent measurements in chlorine‐doped amorphous selenium

“…The break in the 67 ppm TPC traces to a steeper decay around t ≈ 10 −5 s signals carrier trapping into a further set of defects states. In fact, the presence of an important set of deep traps in a-Se has been demonstrated by early xerographic residual potential measurements [21], as well as more recent photocurrent simulations [20] and measurements [8].…”

“…For chalcogenide semiconductors, and aSe in particular, both the multiple-trapping transport model and the exponential plus defects density-of-states model have been validated in the past [9,20]. Consequently, the ∼t −1 decay can be attributed to the influence of carrier trapping into the various defects, while the slower initial current decrease in the 67 ppm Cl-doped sample reflects the multiple-trapping in the exponential background density upon suppression of the dihydral-angle and D − defect densities.…”

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

“…The experimental TOF data used in this work have been extracted primarily from two main sources: (a) works carried out by the authors and reported previously in [41][42][43] and (b) TOF measurements carried out at Xerox by Pfister and Scher and reported in [44]. The two sets of samples are labelled S and X respectively.…”