Observation of two-level defect system in amorphous Se superlattices

Abstract

Amorphous Se is a well-known photoconductor from its early applications in xerography and ultra-sensitive photodetectors like the High-gain Avalanche Rushing Photoconductor (HARP) device. The established way of fabricating the photoconductor for the HARP is rotational thermal evaporation using multilayers of Se and As2Se3. However, the electronic effects of multilayering have not yet been clarified. In this report, we investigated the multilayer structure as a superlattice of Se and As2Se3 fabricated using rotational evaporation and show that the superlattice structure results in the uniformization of the defect levels in the base materials. We found four energy levels associated with defects in As2Se3 and three levels in amorphous Se. In comparison, the superlattice structure of the two materials shows two clear energy levels at EC,Se − 0.533 eV and EV,Se + 0.269 eV. The resulting two occupied energy levels explain the photoelectronic and transport properties observed in multilayer amorphous Se. This result “reinvents” the multilayer structure as a material with observed quantum effects, which significantly improves the material performance in photodetection.

Spectroscopic ellipsometry of amorphous Se superlattices

“…The results revealed two energy levels at 0.533 eV from the conduction band of Se and 0.267 eV from the valence band of Se. These energy levels were explained as defect level modulation and miniband formation due to the superlattice periodic potential, and sequential tunneling transport from the miniband level [9,10,33]. Optical measurements using spectroscopic ellipsometry reported herein showed transitions from quantized energy levels in the extended states of the superlattice materials.…”

“…The electronic energy structure of the superlattice was evaluated using deep level transient spectroscopy (DLTS) on a HERA DLTS system. The samples were measured in the temperature range from 140 to 300 K, with a filling voltage pulse of 5 V and a time window of 20.48 ms [10]. The resulting capacitance transient was Fourier transformed and the time constants obtained from the resulting Fourier coefficients [32].…”

“…This material combination is also a promising candidate for ultrasensitive flat-panel x-ray imagers [6][7][8]. Recent work revealed that multi-layer films of Se and As 2 Se 3 fabricated using rotational evaporation exhibit electrical properties associated with superlattice structures, altering the band structures of the base materials and resulting in sequential tunneling based transport phenomena observed as oscillations in the current-voltage (I-V) characteristics [9,10]. It is therefore expected that an optical study should also reveal characteristics emanating from the superlattice structure.…”