Buildup Of Interface States Research Articles

Behavior of the Si/SiO2 interface observed by Fowler-Nordheim tunneling

Thin-oxide (40–50 Å) metal oxide semiconductor (MOS) structures are shown to exhibit, prior to large levels of electron tunnel injection, the near-ideal behavior predicted for a uniform trapezoidal barrier with thick-oxide properties. The oscillatory field dependence due to electron-wave interference at the Si/SiO2 interface indicates an abrupt, one-monolayer barrier transition (∼2.5 Å) consistent with earlier work. After tunnel injection of 1017 –5×1018 electrons/cm2, the barrier undergoes significant degradation leading to enhanced tunneling conductance, with reproducible behavior observed among different samples. This effect is consistent with the generation of positive states in the region of the oxide near the Si/SiO2 interface (<20 Å), where the tunneling electrons emerge into the oxide conduction band. Densities of positive-charge and interface-state buildup are also observed from capacitance-voltage (C-V) measurements and are found to be consistent with the observed tunneling dependence on positive-state generation. We suggest that the interface-state buildup may be identical to the positive-state generation observed by tunneling. The generated oxide states are shown to anneal slowly at room temperature, and more rapidly at 100 °C. Comparisons are made between wet, wet/annealed, and dry oxidation processes, with wet oxides exhibiting the largest densities of state generation, and dry oxides the smallest. The results are consistent with other work on the effect of water-related defects in oxides, and with x-ray photoelectron spectroscopy results, showing that breaking of strained Si-O-Si bonds is responsible for state generation during stress. The tunneling data also indicate that, prior to stress, very few positive states reside in the oxide (?1010 cm−2), even though appreciable interface-state desities are observed from C-V data. We therefore conclude that these initial interface states must be associated primarily with the monolayer transition layer and disorder in the underlying silicon, and are not distributed into the oxide near the interface.

Annealing of MOS Capacitors with Implications for Test Procedures to Determine Radiation Hardness

The short- and long-term anneal of radiation-induced flatband voltage shift in both Al- and Si-gate MOS capacitors following pulsed e-beam irradiation is examined. The short-term anneal was measured and found to be similar for both Al- and Si-gate capacitors. Experiments were performed to investigate the dependence of the long-term anneal on oxide thickness, temperature, and bias. The Al-gate capacitors exhibited a low interface-state buildup following irradiation and had a long-term annealing curve with a simple ln(t) dependence, while the Sigate capacitors studied had a more complicated annealing curve due to the buildup of a larger number of interface states and lateral nonuniformities (LNUs). Thinning the gate oxide was found to dramatically reduce the effects of interface states and LNUs on the long-term anneal. In capacitors where the effects of interface states and LNUs are small the long term annealing was found to be only weakly temperature dependent over the range -60 to 100°C, and removal of bias following irradiation had little effect. On the other hand, the long-term annealing of capacitors with a significant buildup of radiation-induced interface states and LNUs exhibited a strong temperature and post-radiation bias dependence. Some implications of the experimental results for the proper design of test procedures for determining radiation susceptibility are discussed.

Interface-State Generation in Radiation-Hard Oxides

Results of several experiments are presented to demonstrate that there is a time-dependent buildup of interface states in hardened dry oxides following pulsed LINAC irradiation and to establish the field and dose dependencies of the interface-state buildup in these dry oxides. These results suggest that the same mechanisms are responsible for the interface-state buildup in both wet-grown and dry-grown oxide capacitors, the major difference being that the magnitude of the buildup in a hardened dry oxide is considerably smaller than that in a hardened wet oxide. For several lots of wet-grown and dry-grown oxide capacitors no increase in interface states was observed following exposure to a neutron fluence of ~1013 n/cm2.

Trapping Effects in Irradiated and Avalanche-Injected MOS Capacitors

Avalanche-injection of holes and electrons into nonirradiated and irradiated MOS capacitors, respectively, were used to study hole traps in the SiO2. The trapping parameters for holes, and for electrons in the presence of trapped holes, were obtained in the range 10-14 - 10-13 cm2 for oxide thicknesses in the range 200 - 1000A. A dominant bulk specie is determined to tail off from the Si/SiO2 interface with a characteristic distance of 150-200A for dry oxide and approximately 400A for wet oxide. The electron-injection is shown to be an effective probe of the trapped-hole distribution in the SiO2 after irradiation. The effect of electron compensation of trapped holes during irradiation had been included in the trapping kinetics. C-V shifts and interface state build-up near mid- band after irradiation were found, from irradiation experiments, to follow the same linear dependence on the integrated electron and hole flux crossing the Si/SiO2 interface.

Field- and Time-Dependent Radiation Effects at the SiO2/Si Interface of Hardened MOS Capacitors

The field and temperature dependence of the interface-state density as a function of time following pulsed e-beam irradiation, and the dose dependence of the interface-state density following steady state Co60 irradiation were examined in MOS capacitors with both hardened dry and wet (pyrogenic) gate oxides. From the results of the pulsed e-beam experiment, we show that in the wet oxide the electric field affects the time scale for the buildup of interface states as well as the final or saturation value of interface states at late times (~105 s), but that in the dry oxide there is no marked field dependence. For the wet oxide, we observed that temperature affects only the time scale for the buildup of interface states. From total-dose Co60 measurements, we report a power law dependence on dose, D0.65, for both wet and dry oxide capacitors. The buildup of interface states in the wet-oxide capacitors is considerably larger than in the dry.

Comparison of interface-state buildup in MOS capacitors subjected to penetrating and nonpenetrating radiation

Metal-SiO2-Si capacitors under positive bias were subjected to penetrating Co60 γ and nonpenetrating 10.2-eV uv radiation. High-frequency 1-MHz and quasistatic C-V curves were recorded before and after irradiation, and interface-state densities over the Si band gap were calculated from these curves by using the Terman, Berglund, and Castagne techniques. The nature and magnitude of the buildup of interface states were similar for both types of irradiation, demonstrating that the buildup is not related to the direct interaction of radiation at the interface or a structural modification of the SiO2 layer, but to the production of electron-hole pairs in the oxide and the subsequent transport of holes to the oxide-semiconductor interface. The buildup of interface states under irradiation saturated at ∼5×105 rad (SiO2).

Hole Transport and Recovery Characteristics of SiO2 Gate Insulators

The transient response, or flat-band voltage recovery, in a number of pedigreed SiO2 gate insulator MOS capacitors following exposure to a pulsed 13-MeV electron beam was studied as a function of time, temperature, and applied bias. A quantitative comparison of the response characteristics of the different oxides is made. It is found generally that the response consists of two stages. The first (early time) stage in most cases encompasses most of the recovery and is dominated by hole transport through the insulator film to the interface. This hole transport recovery stage in all the oxides studied is well described by the stochastic model of hopping transport based on a continuous time random walk used previously in the analysis of hole transport in SiO2. It is further shown that all the main features of the hole transport are consistent with a model of the holes moving via polaron hopping between localized sites in the SiO2. The second or long-term stage of recovery commences after cessation of the hole transport in the oxide and is found to vary significantly in its importance and time of onset among the various oxides. This stage involves a radiation-induced buildup of interface states and possibly, in some cases, an annealing of a trapped-hole distribution near the SiO2/Si interface.

Dependence of Interface-State Buildup on Hole Generation and Transport in Irradiated MOS Capacitors

Dependence of Interface-State Buildup on Hole Generation and Transport in Irradiated MOS Capacitors

Radiation-Induced Perturbations of the Electrical Properties of the Silicon-Silicon Dioxide Interface

Interface-state densities and MOS transistor characteristics dependent upon such states have been studied as a function of radiation dose and type. Special MOS devices possessing doped silicon-dioxide layers as well as undoped control devices have been utilized. Infrared absorption measurements were performed on silicon-dioxide samples before and after exposure to radiation, as well as for doped and undoped samples. A model based on structural modifications of the silicon-dioxide-films is proposed for the build-up of interface-states resulting from exposure to radiation. It has been found that with the proper doping of the silicon-dioxide films, the build-up of such states can be reduced. Using such doped gate-dielectrics, planar semiconductor devices much less sensitive to radiation have been fabricated.