Schottky diodes interfacial layers

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

An ozone treatment (10 minutes at room temperature) of the HF-etched SiC surface before the metallization step was introduced as a very convenient processing step to produce Schottky diode gas sensors with an increased stability and reproducibility. The use of spectroscopic ellipsometry analysis and also photoelectron spectroscopy using synchrotron radiation showed that an oxide, 1-nm in thickness, was formed by the ozone exposure [74, 75]. The oxide was also found to be close to stochiometric SiO in composition. This thin oxide increased the stability of the SiC Schottky diodes considerably, without the need for any further interfacial layer such as Ta or TaSi which have been frequently used. Schottky diodes employing a porous Pt gate electrode and the ozone-produced interfacial layer have been successfully operated in both diesel exhausts and flue gases [76, 77]. 

Tobias et al. investigated Pt-gate 6H-SiC Schottky diodes with an interfacial layer of 1-nm Ta or 10-nm TaSq [78]. Both n-type and p-type diodes showed a gas response to hydrogen at 400-600°C. It was postulated that the gas response to hydrogen observed in the forward direction is mainly due to a change in the resistance of the gate contact. 

Another problem with Schottky diodes is that at high temperatures, the metal contact can anneal to the semiconductor, forming a silicide in the case of silicon and SiC [72, 80-83]. This can destroy the diode characteristic of the device, thus producing an unstable sensor. Use of an interfacial insulating layer, such as the oxide layer already mentioned, can prevent this from occurring. 

Also Tobias et al. reported, as already described, resistivity changes to be the main reason for the hydrogen response observed in the forward direction of the /-V characteristic of Schottky diodes with a Pt gate and an interfacial layer of TaSi [78]. 

Due to the technological importance of metal-insulator-semiconductor (MIS) devices, understanding of the nature of their electrical characteristics such as current-voltage (1-V) and tunnel magnetoresistance (TMR) is of great interest. Unless intentionally fabricated, a silicon Schottky diode possesses a thin interfacial oxide layer between the metal and the semiconductor. Additionally, a density of interface states is always generated at the boundary between the semiconductor and insulator. 

If simple band models are assumed for an-T and the contacts, materials like the noble metals with a workfunction of 5.3eV (Au) or 5.6 eV (Pt) lead to ohmic contacts whereas materials with low workfunctions as A1 (4.28 eV), Mg (3.66 eV), or Ca (2.87 eV) form Schottky barriers. The rectification ratio I(- -U)/I(-U) was determined for endcapped a-6T in a LED device to be 240 for Ca, 7 for Mg, and 40 for A1 [330]. This shows that the work function is not the only factor influencing the Schottky barrier height, but that also trap states or an interfacial layer due to reactions between metal and thiophene may play a role. The influence of interface layers on Schottky barriers is also shown for In [331] and eutectic Ga,In [332] on p-doped dodecathiophene. For other Schottky diodes, compare [250] and references therein. 

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