Since their proposal by Esaki, superlattices have been observed to have fascinating features such as quantum size effects, negative differential resistance, and sequential resonant tunneling. However, the technology threshold for fabricating superlattices is high, requiring methods like molecular beam epitaxy (MBE) and atomic layer deposition (ALD), among others, even for amorphous materials. Thus, the desirable features from superlattices have not been extensively utilized. It is shown that superlattices of Se and As2Se3 (superlattice‐Se), fabricated using rotational evaporation, exhibit sequential tunneling, a typical superlattice feature. From current–voltage measurements of the superlattice deposited on n‐type Si, oscillations in the characteristics are observed. Using models of reverse‐biased Schottky barriers, the observations are explained as tunneling in sequence from superlattice minibands. The superlattice‐Se also shows carrier blocking, with a resistivity of the order of 1012 Ω cm in dark conditions at room temperature, despite the low resistivity (≈10 Ω cm) of the n‐type Si substrate. When the Si is illuminated, the device shows higher detectivity for weaker signals compared to higher illumination. The ease of fabrication, and the blocking and amplifying capabilities make superlattice‐Se an interesting “add‐on” structure to improve conventional photodetector materials such as Si or Ge, which have issues with dark currents at room temperature.
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.
Heterostructures of dissimilar crystalline materials are key in the development of photoelectronic devices. Such structures rely on matching lattice constants for efficient transport. Nevertheless, an amorphous material, amorphous Se is becoming important in high‐energy photoelectronics. Thus there is a need to understand the heterostructures it forms with conventional crystalline materials. Since the transport properties of such heterostructures will be limited by lattice mismatches, methods to improve transport are necessary. We investigated the effect of electrolysis on the electronic properties of Se based multilayer films deposited on n‐type silicon (Se/n‐Si) and showed that conduction and photoresponse can be changed by electrolysis processing. The Se surface of the Se/n‐Si samples was used as an anode in the electrolysis of NaCl solution. Afterwards, the samples were measured using current–voltage and capacitance–voltage variations in dark conditions and under illumination. Electrolysis processed samples showed higher conduction current and photocurrent, compared to samples not used in electrolysis. This is due to the introduction of Cl into the Se, which affects carrier lifetimes: increasing hole lifetime and reducing electron lifetime. We therefore suggest that electrolysis can be applied to modify the electronic properties of Se/n‐Si heterostructures.
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