Hole trap distribution on 2 MeV electron irradiated high-k dielectrics

Abstract

The effects of 2 MeV electron irradiation on the electrical properties of high-k dielectric based metal–insulator–semiconductor capacitors have been studied. Samples consist of 5.9 nm-thick films of aluminum oxide and hafnium oxide deposited by atomic layer deposition on silicon substrates. Deep-level transient spectroscopy (DLTS) and admittance measurements reveal that electron irradiation modifies the defect density of both surface states at the dielectric–semiconductor interface and border traps existing inside the dielectric. The experimental results indicate that irradiation has a double effect. The incident electrons transfer their energy and generate additional surface states, leading to a degradation of the interface. On the other hand, irradiation generates electron–hole pairs inside the dielectric. Some of the holes are trapped by border traps located inside the dielectric at locations close to the interface. As a result, border traps capturing holes are neutralized and become inactive after irradiation. Moreover, interface state profiles, as measured by DLTS, are affected by the presence of border traps and yield overestimated interface state densities. Admittance spectroscopy is used to distinguish among border traps and interface traps. A detailed study of the conductance signal as a function of voltage, temperature and frequency for samples with different irradiation doses (nonirradiated, 2.5, 25, and 250 Mrad) is presented here. The influence of the irradiation dose has been analyzed in order to compare the defect distribution before and after irradiation.