Emission probability of hot electrons from silicon into silicon dioxide

摘要

An experimental method is described for directly measuring the probability of electron emission from the silicon substrate into the SiO2layer after the electron has fallen through a certain potential drop in traversing the depletion layer and reached the Sihyphen;SiO2interface. The method is based on optically induced hothyphen;electron injection in polysiliconhyphen;SiO2hyphen;silicon fieldhyphen;effecthyphen;transistor structures of reentrant geometry. The emission probability was studied as a function of substrate doping profile, substrate voltage, gate voltage, and lattice temperature. It was found that the hot electrons could be emitted by tunneling as well as by surmounting the Schottkyhyphen;lowered barrier. Overhyphen;thehyphen;barrier emission dominates at large substrate voltages, where the emission probability is high, and tunnel emission becomes appreciable and may even dominate at small substrate voltages where the emission probability is low. A simple model was developed based on the assumption that only those hot electrons lucky enough to escape collision with optical phonons were emitted. Using this model, we found that the expressionP=Athinsp;exp(minus;d/lgr;) described very well the dependence of the emission probability on doping profile, substrate voltage, and gate voltage. HereA=2.9 is a constant, lgr; is the opticalhyphen;phononhyphen;electron collision mean free path,dis the distance from the Sihyphen;SiO2interface where the potential energy is equal to the rsquo;rsquo;correctedrsquo;rsquo; barrier of (3.1 eVminus;bgr;EOX1/2minus;agr;EOX2/3ox), bgr;EOX1/2is the Schottky lowering of the barrier, and agr;EOX2/3is a rsquo;rsquo;barrierhyphen;loweringrsquo;rsquo; term introduced to account for the probability of tunneling. The temperature dependence of the collision mean free path was found to follow the theoretical relationship lgr;=lgr;othinsp;tanh(ER/2kbT), with lgr;o=108 Aring; andER=0.63 eV. This model is useful for evaluating potential hothyphen;electronhyphen;related instability problems in IGFET and similar structures.