Electron trapping and detrapping characteristics of arsenic-implanted SiO₂ layers

Abstract

The electron trapping and detrapping properties of As+-implanted thermally grown SiO2 layers incorporated into metal-silicon dioxide-silicon (MOS) structures have been studied. The samples were charged using avalanche injection from the silicon substrate or internal photoemission from either interface. The charge state of the MOS devices was investigated using capacitance-vs-voltage (C-V) and photocurrent-vs-voltage (photo I-V) measurements. After a 1×1013 As+/cm2 implant and a 1000 °C anneal in N2 for 30 min, a dominant (in terms of trapping probability) trapping center with a capture cross section of (1-2) ×10-15 cm2 is found, together with various other centers with cross sections of (1-4) ×10-17, (2-5) ×10-16, and (0.4-1.6) ×10-14 cm2, respectively. The sum of the trap densities for the various centers with cross sections in the range 10-15-10-16 cm2 is equal to 0.7-1 times the ion fluence, independent of ion energy (10-100 keV), avalanche injection current (5×10-11-5×10-10 A), and oxide thickness (490-1430 Å). The dominant trap density is proportional to the ion fluence over our experimental range (3×10 12-1×1014 As+/cm2). Increasing the anneal temperature from 600 to 1100°C gradually decreases the electron trapping rate, without extensively changing the dominant-trap parameters. The negative-charge distribution centroid (measured with respect to the Al-SiO 2 interface) is shown to be proportional to the ion energy but does not depend on trapped charge density (4×1011-1×10 13 cm-2), ion fluence (3×1012- 1×1014 cm-2), injection mechanism (avalanche injection or internal photoemission), oxide thickness (490-1430 Å), anneal temperature (600-1100-°C), or ambient temperature (83-295 K). The charge centroids were found to be in excellent agreement with the experimentally determined (SIMS) ion distribution centroids, but systematically larger (up to 40%) than the values predicted by the theory of Lindhard, Scharff and Schiøtt (LSS). It is shown that optical excitation removes the trapped negative charge from the trapping centers in a homogeneous way (i.e., with constant charge centroid). From the detrapping experiments, an effective photoionization cross section is determined as a function of photon energy with an absorption threshold of 3.3 eV. This is corroborated by a model which assumes that the electron trapping is related to the implanted arsenic ions, incorporated into the SiO2-network at oxygen-sites. Isothermal detrapping experiments (in the range 100-250°C) and thermally stimulated current measurements are reported. Both experiments can be modeled by assuming a Gaussian trap distribution around a median thermal trap depth E m=1.2-1.3 eV with a standard deviation ΔE=0.2 eV.