(Invited) Characterization of Individual Si/SiO2 Interface Traps: Direct Observation of Single Pb0 Centers by the Charge Pumping (CP) Method and Correction of the Conventional CP Theory

   It is considered that interface traps will cause various reliability and variability problems in future nanoscale MOS devices, such as fluctuations in threshold voltage due to the variation in the number of interface traps involved, and the random position of the traps. Therefore, characterization of individual traps will be considered to become increasingly important.

   Charge pumping (CP) technique is known to be a highly-precise method for evaluating the density of interface traps, therefore, the method has been widely used for the last 30 years. The CP current I_CP can be simply described by I_CP=fqN, where f is the gate pulse frequency, q is the electron charge, N is the total number of traps contributing to the CP current. Therefore, the conventional belief is that the density of interface traps N_it can be derived from N_it=I_CP/(fqA_G), where A_G is gate area of MOSFETs, and the CP current per trap should be fq. Actually, observation of single interface traps has been demonstrated by the CP method.

   We made systematic measurements of CP current from single Si/SiO₂ interface traps, and successfully observed for the first time that their current range is 0<I_CP≤2fq, but not a fixed value of fq. In order to judge if CP characteristics are due to single interface traps, we need to separate the CP characteristics into components from individual traps. For this purpose, we utilized some effective procedures, i.e., observation of the differences in threshold voltage in the local area where each trap is located, the dependences of CP characteristics upon on-time and off-time of the gate pulse. From the on-time and off-time dependences, we can observe electron capture processes and electron emission (i.e., hole capture) processes in individual traps, respectively.

   Using these procedures, we confirmed if CP characteristics are due to single interface traps. We measured seventy samples, and among them, twenty-seven samples showed single trap properties. Thus, we concluded that CP current from single Si/SiO₂ interface traps is indeed in the range of 0<I_CP≤2fq, but not a fixed value of fq, and the conventional CP theory is basically wrong. Moreover, using the procedures, we demonstrated fundamental trap-counting by the CP method. The current range 0<I_CP≤2fq can be expected from the nature of the P_b0 centers, therefore, the obtained results mean that we succeeded to directly and electrically observe single P_b0 centers, for the first time.