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 sp(2)-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. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3212987]