D. R. Young

Electron trapping in SiO2 at 295 and 77 °K

The electron trapping behavior of SiO2 has been measured as a function of thickness at 295 and 77 °K. The devices used were metal‐oxide‐semiconductor devices with the SiO2 grown thermally. The results indicate bulk traps are dominant at 295 °K and traps associated with the Si‐SiO2 interface are dominant at 77 °K. The effect of processing conditions was also studied and the optimum conditions are different for the two temperatures used for the measurements. These observations have been verified using a photo I‐V technique. The generation of donor states in the SiO2 near the Si‐SiO2 interface was observed as a result of the electron current through the SiO2.

The effects of water on oxide and interface trapped charge generation in thermal SiO2 films

Water was diffused into very dry thermal SiO2 films under conditions such that the penetration of water related electron trapping centers was of the order of the oxide thickness. In both dry oxides and water diffused oxides, production of negative bulk oxide charge Qot and positive interface charge Qit by an avalanche‐injected electron flux was observed. The efficiencies of both processes were enhanced by water indiffusion. Analysis of the kinetics of charge generation indicated that production of trapped electron centers (Qot ) was required for subsequent production of interface states and charge (Qit ). Models for both processes are discussed. We suggest that inelastic collisions of conduction electrons with the trapped electron centers releases mobile hydrogen atoms or excitons. The mobile species migrate to the Si–SiO2 interface and form states and fixed charge.

Hydrogen migration under avalanche injection of electrons in Si metal‐oxide‐semiconductor capacitors

Aluminum/silicon dioxide/silicon capacitors in which the oxide has been grown thermally under ultra‐dry (≲1 ppm H2O) conditions and subsequently treated by low temperature water diffusion have been characterized electrically and chemically. Avalanche injection of electrons has been observed to produce the complex charging behavior previously observed in similar systems, which includes electron trapping and interface positive charge generation. Secondary ion mass spectrometry depth profiling of these structures has shown that electron injection also results in hydrogen transport. This is the first direct observation of hydrogen redistribution under the influence of an electron current. We demonstrate a linear relationship between injected charge fluence and areal density of hydrogen localized at the SiO2/Si interface. These results indicate that hydrogen release correlates with interface state generation, but not with bulk oxide trapping.

Effects of avalanche injection of electrons into silicon dioxide—generation of fast and slow interface states

In the process of avalanche injection of electrons into silicon dioxide, besides electron trapping in the bulk of the oxide, there are slow and fast interface states generated. The slow states are donors and positively charged when empty. Together with positive charge in the interface states, they compensate the negative bulk charge to give the turn‐around effect. The final C‐V curve is due to a complex sum of different charge components. The slow states can be discharged when heated under +5 V or higher biases at 160 °C or above. The final charge state is only semipermanent. Fast interface states are also annealed in the process, and the anneal is enhanced by a positive bias. Bulk trapped electrons are minimally perturbed by the anneal. It is postulated that the slow states may communicate with silicon through the fast interface states in a thermally activated process. In order to study bulk electron trapping, avalanche injection should be carried out at elevated temperatures.

Electron trapping in electron‐beam irradiated SiO2

In addition to the well‐known positive space charge, electron irradiation of MOS capacitors with 25‐keV electrons is shown to introduce additional uncharged electron traps into the oxide layer. These traps persist after most of the positively charged defects have been removed by the usual low‐temperature (? °C) anneals. Their presence after this anneal is determined by injecting hot electrons into the oxide where they are captured by existing defects. The effective trap densities increase with increasing electron fluence and are reduced by forming‐gas anneals at temperatures in excess of 500 °C. Observed electron‐capture cross sections are between 10−15 and 10−18 cm2. The residual radiation damage in oxides exposed to 10−4 C cm−2 of 25‐keV electrons and subsequently annealed at 400 °C results in an additional neutral density of 5×1011 traps cm−2 with cross sections distributed over the above range. Electron‐trapping cross sections and effective trap densities associated with this damage are found to be id...

Electron trapping by radiation-induced charge in MOS devices

Electron trapping by positive charge centers induced in MOS structures by exposure to x rays has been studied by the avalanche injection technique. Five electron traps with cross sections ranging from 10−13 to 10−19 cm2 have been measured. Two of these account for the majority of positive charge in the SiO2 and the remaining three are associated with positive or neutral centers in the oxide.

Polarization Reversal in the Barium Titanate Hysteresis Loop

In the presence of an opposing field the polarization of tetragonal BaTiO3 reverses itself. Devonshires phenomenological treatment shows that in a sufficiently large opposing field the original direction of polarization becomes unstable. The values of commonly observed coercive fields are much smaller than this, and the polarization actually reverses while the initial state is still metastable. The observed coercive field and the shape of the audio‐frequency hysteresis loop can, however, be explained in terms of the field dependence of the switching rate that was measured by Merz in pulse experiments. Measurements by Drougard, Funk, and Young, of the small signal conductivity, taken while traversing the hysteresis loop, are also explained in terms of Merzs rate. The absence of an appreciable inductive component in the small signal conductivity measurements indicates that variations in the applied field produce corresponding variations in the switching current with a time lag which is at most a few times...

Charge transport and trapping phenomena in off‐stoichiometric silicon dioxide films

The electrical characteristics of off‐stoichiometric silicon dioxide films have been investigated. The off‐stoichiometric oxide films studied had an excess atomic silicon (Si) content in the range of 1%–6%. Raman spectroscopy and photoconductivity measurements indicate that the excess Si is present as amorphous Si islands or small crystallites embedded in silicon dioxide (SiO2) forming a two‐phase material. These films differ in structure from previously reported films where dual dielectric layers of stoichiometric SiO2 and Si‐rich SiO2 with ≥13% excess atomic Si were used. These dual dielectric films were observed to produce electron injection from contacting electrodes via the Si‐rich SiO2 layer into the SiO2 at lower average electric fields. This injection mechanism was believed to be due to localized electric field enhancement near the SiO2–Si‐rich SiO2 interface caused by the curvature of the tiny Si islands in the SiO2 matrix. The current versus voltage characteristics of the off‐stoichiometric oxid...

Reduction of electron trapping in silicon dioxide by high‐temperature nitrogen anneal

Electron trapping in silicon dioxide is reduced by nitrogen annealing in a furnace system with very low moisture content and special cooling techniques. The flatband voltage shift resulting from injected hot electrons is significantly reduced by the annealing together with the interface states generated. The trapped charge is laterally nonuniform when the oxide is not annealed. From photo I‐V measurement, the density of electrons trapped in the bulk of the oxide is reduced by this treatment. The distribution of the trapped charge changes from a uniform distribution in the bulk for a 1‐h anneal to a distorted U shape distribution, with a larger build‐up on the aluminum‐silicon dioxide side for a 17‐h anneal. The density of trapped charge close to the silicon‐silicon dioxide interface is reduced and thus the smaller flatband voltage shift. With the reduction in bulk change, interface state charge beomes important.

Radiation damage in silicon dioxide films exposed to reactive ion etching

The enhanced electron‐trapping characteristics and the location of this trapped charge in an SiO2 layer exposed to either CF4, O2, or Ar plasmas in a reactive ion etching (RIE) system are reported. Capacitance‐voltage (C‐V) and photocurrent‐voltage (photo I‐V) techniques were used to monitor charge trapping and location after the samples were incorporated in metal‐oxide‐semiconductor (MOS) capacitors. Using a CF4 plasma which etches SiO2, trapping sites caused by penetrating radiation were observed. These traps were removed by annealing at temperatures ?600 °C for 30 min in N2 prior to metallization. These bulk SiO2 trapping sites showed no strong dependence on whether the samples were placed on the anode or cathode in the RIE chamber, implying no preferred directionality in the photons which are believed to generate them. With an O2 or an Ar plasma which does not etch SiO2, an additional trapping layer within about 100 A of the exposed SiO2 surface caused by penetration of energetic ions (?400 eV) was de...