BN/ZnO heterojunction diodes with apparently giant ideality factors

Until now, a common feature of many wide band gap heterojunction diodes is an unexplained large ideality factor $n>2$⁠. In this context we investigate the diode characteristics of heterojunction diodes consisting of a crystalline semiconductor material such as ZnO covered with a thin semiconducting film of amorphous or disordered material. As thin disordered film we use $sp2$-bonded turbostratic boron nitride. These heterojunctions exhibit a pronounced rectifying behavior, low saturation current, and low parasitic currents. Moreover, we observe an apparently giant ideality factor reaching values of $n>100$⁠. As a consequence, the turn-on voltage is around 5–10 V and the $I-V$ curves can be measured for bias voltages between ±80 V without reaching saturation or electrical breakdown. We present a quantitative model for the unusual diode characteristics of these metal-amorphous semiconductor-semiconductor diodes. We demonstrate that the $I-V$ characteristics of the heterojunctions are well described by a serial arrangement of an ideal Schottky diode, a Frenkel–Poole type resistance, and an Ohmic contact resistance, emulating a $p-n$- or Schottky diode characteristic with giant ideality factor. We propose that heterojunctions exhibiting apparently large ideality factors $n⪢2$ may possess an interfacial disordered or amorphous layer with Frenkel–Poole conduction properties.