CaF2/Si/CaF2double-barrier resonant-tunnelling diodes on Si substrates

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Abstract
CaF2/Si/CaF2 double-barrier resonant-tunnelling diodes have been fabricated by various processes with molecular beam epitaxy in SiO2 windows patterned on Si(111) substrates. Two types of I-V characteristics with negative differential resistance (NDR) were observed in the temperature range from 75 K up to about 200 K. The one is very stable in forward and backward voltage sweeps, and, in contrast to earlier reports, it does not show any hysteresis or trapping effects. Typical diode parameters were: peak voltage V-p = 0.7 V, peak-to-valley current ratio P / V = 1.7 and peak current density I-p = 2 mA cm(-2) at 105 K. The second is less stable and the NDR appeared only during forward bias sweeps, possibly due to local trapping effects. But it has larger P / V ratios, the typical parameters being: V-p = 1.2 V, P / V = 5 and I-p = 11 mA cm(-2) at 77 K. The observed peak current densities of both NDR types are about five orders of magnitude higher than those reported previously for diodes with the similar structure. We assume that an inhomogeneous, Si quantum-well thickness with the current flow localized in small regions of the diode area causes the observed variations in peak voltages, multiple NDR regions, peak spreading and exponentially rising background currents.