Trap-controlled space-charge-limited conduction in amorphous As _x Te_1−x thin films with ovonic threshold switching

Abstract

Ovonic threshold switching (OTS) in chalcogenide materials has attracted considerable interest for application in electronic devices to suppress leakage current in cross-point array structures. Although OTS appears to originate from an electronic process, the exact mechanism of OTS remains unclear with respect to sub-threshold conduction and threshold switching. In this study, we demonstrated that the sub-threshold conduction characteristics is affected by the structures of the exponentially distributed trap states in amorphous chalcogenide materials. Using binary AsxTe1−x thin films, we investigated the effects of trap distribution on OTS characteristics, analyzed the band structures, and developed a trap-controlled space-charge-limited conduction model.

Effect of Nitrogen Doping on Threshold Voltage in Amorphous Ga2Te3 for Application of Selector Devices

“…A schematic illustration of the energy band diagram with the trap density profile is shown in Figure , which represents the situation when V = V TFL. [ 28,38,41 ] The of the a‐Ga 2 Te 3 films is observed to decrease with the increasing N doping concentration when the Urbach tails extend deeper into the bandgap, which is reflected by the increasing , as shown in Figure 1c. Furthermore, the deep trap density increases with the increasing V TFL in N‐doped a‐Ga 2 Te 3, and Figure 4 reflects these results.…”

“…The optical Urbach energy ( parameter representing the steepness of the exponentially distributed Urbach tail of an amorphous semiconductor is related to the degree of disorder and the trap density. [ 27,28 ] The value of can be estimated as α = c 0 exp( hν /), where c 0 is an adjustable constant and the remaining symbols are as defined earlier. The plots of ln α versus hν are linear with slopes of 1/, as shown in Figure 1b.…”

“…[ 14,15 ] Furthermore, the decreased can be attributed to the increased , which creates an Urbach tail extending from the lower part of the conduction band into the deep energy gap. [ 28,30,31 ] Figure 1d presents the absorption coefficient spectra of the N‐doped Ga 2 Te 3 thin films, and the inset shows the magnified region of the spectra affected by the defect states. As shown in Figure 1d, the a‐Ga 2 Te 3 thin films with high N doping concentrations have large absorption coefficients because of the presence of defect states, indicating a high density of deep trap states.…”

“…The conduction of subthreshold region in N‐doped a‐Ga 2 Te 3 is interpreted based on the trap‐controlled SCLC. [ 20,28 ] To fit a trap‐controlled SCLC fitting, double‐logarithmic current density–voltage ( J–V ) plots are split into three regions with slopes 1, 2, and l + 1. Then, the intersection of the lines with slopes of 1 and 2 is space charge limited voltage ( V SCL ) and the intersection of the lines with slopes of 2 and l + 1 is trap‐filled limited voltage ( V TFL ) in double‐logarithmic J–V.…”

“…[ 42 ] V th is related to the difference between the Fermi energy level at V TFL and the conduction band. [ 28 ] The Fermi energy level is located above relatively because of the increased deep trap density ( V TFL ) and conduction band falls down relatively because of the increased Urbach energy when a‐Ga 2 Te 3 is doped with N. Therefore, V th is reduced with the increasing N doping concentration. [ 10,17 ] Correspondingly, the trap states and density play an important role as to the V th of the threshold switching devices with two transition voltages, especially a‐Ga 2 Te 3 devices, which is explained based on the trap‐controlled SCLC mechanism.…”