Walter C. Johnson

High‐field transport in SiO2 on silicon induced by corona charging of the unmetallized surface

A study has been made of high‐field electronic transport in thermally grown SiO2 in which the injection and transport of carriers were induced by charging the exposed surface of the insulator with ions extracted from a corona discharge. This technique totally avoids destructive breakdown through weak spots in the insulator. Auxiliary developments included a comparison technique for measuring the steady‐state potential difference across the unmetallized insulator and a p‐n junction method for determining the sign of the principal charge carrier in the oxide. Using a corona discharge in dry air, breakdown fields of approximately 6.5 and 13.5 MV/cm were obtained for charging with positive and negative ions, respectively. Electrons were identified as the current carriers for both polarities of applied field. Measurements of the discharge of the samples, made with a Kelvin probe, yield results that are consistent with Fowler‐Nordheim tunneling of electrons from the silicon into the oxide, with an electron effe...

Relationship between trapped holes and interface states in MOS capacitors

Metal–silicon‐dioxide–silicon capacitors with dry‐grown oxides, when stressed at fields of 7.1–7.5 MV/cm with field plate positive at 90 K, showed buildup of trapped holes. Interface states appeared only after the samples were warmed. The number of interface states generated in one hour at either 20 or 66 °C was linearly proportional to the number of trapped holes, and the number of states generated in the central 0.7‐eV portion of the Si bandgap during one year’s storage at room temperature was essentially equal to the original number of trapped holes. These results provide quantitative evidence of a cause and effect relationship between trapped holes and interface states in the Si‐SiO2 system.

Radiation-Induced Trivalent Silicon Defect Buildup at the Si-SiO2 Interface in MOS Structures

Electron spin resonance and capacitance versus voltage measurements demonstrate approximately a one-to-one correspondence between the density of radiation-induced trivalent silicon defects at the (111) Si-SiO2 interface and the density of radiation induced electronic interface states.

Relationship between x‐ray‐produced holes and interface states in metal‐oxide‐semiconductor capacitors

Metal‐silicon dioxide‐silicon capacitors were irradiated with soft x rays to 40 krads (SiO2) with different biases (−3 to +12 V) at 83 K. The number of interface states generated after samples were warmed was found to bear a linear relation with the flatband voltage shift measured before warm up. A one‐to‐one relationship was observed between the number of interface states generated in the central 0.7‐eV portion of the Si band gap during a one year storage at room temperature and the number of holes that moved to the Si–SiO2 interface during warm up. Annihilation of the x‐ray‐generated holes by photoinjection of electrons before warm up prevented the generation of interface states. Both experiments support a cause‐and‐effect relationship between holes in the oxide and interface states generated in the metal‐oxide semiconductor structure.

Determination of the sign of carrier transported across SiO2 films on Si

A technique has been developed for determination of the sign of charge carrier transported across an insulating film on a semiconductor substrate, utilizing the charge‐carrier separation properties of a shallow p‐n junction diffused into the semiconductor. For thermally grown SiO2, unmetallized and contacted by a corona discharge in dry air, electrons are found to carry the current for both polarities of surface potential. Also demonstrated is electron‐hole pair production in the Si by electrons entering from the oxide.

Interface state generation in the Si‐SiO2 system by photoinjecting electrons from an Al field plate

We observe the generation of interface states in the Si‐SiO2 system when electrons are internally photoinjected from an Al field plate and are swept through the oxide by a moderate electric field. Interface states appear immediately at 90 K. The rate of interface state generation caused by photoinjected electrons depends on the oxide thickness and the magnitude of the bias field. The generated interface state density is found to increase as Nss = AQs, where Q is the injected charge and s is generally in the range 0.5–0.7. The generation does not saturate after passage of a total charge of 0.35 C/cm2.

Measurement of the steady−state potential difference across a thin insulating film in a corona discharge

A comparison method has been developed for measurement of the potential difference across a thin insulating film under steady−state charging conditions in a corona discharge. The method is based on the dynamics of charged particles (ions or electrons) when drifting under collision−dominated conditions: their trajectories follow, to a close approximation, the electrostatic field lines. The method is simple, accurate, and reliable, and it is particularly valuable where the surface potential, following cessation of charging, decays too rapidly to permit use of the conventional Kelvin probe techniques. The method has been applied successfully to the study of the electrical behavior of thermally grown SiO2 films on Si and thermoplastic materials.

Mechanisms of Charge Buildup in MOS Insulators

The radiation sensitivity of MOS devices has been recognized to be the result of a charge buildup caused by the sweepout of electrons and the trapping of holes following hole-electron pair production by ionizing radiation. Holes have been shown to have a finite, although small, mobility in thermally grown SiO2. Trapping of holes takes place near the Si-SiO2 interface, possibly by oxygen vacancies in the oxide. Normal thermally grown SiO2 possesses only small concentrations of electron traps. Electron traps have been shown to be generated, however, in the oxide by ion implantation, by irradiation with nonpenetrating electrons, and by exposure of the surface of the oxide to negative ions from a corona discharge. Although Na+ and Li+ ions have been shown to be mobile at room temperature in SiO2, contamination can be kept to levels where ionic charge buildup is negligible. The role of contaminants in the formation of hole traps, however, remains to be determined.

Electron trapping in aluminum−implanted silicon dioxide films on silicon

Photoelectric and MOS capacitance−voltage techniques were used to study electron trapping in 1400−A dry−grown silicon dioxide films that were implanted with aluminum ions at 20 keV to a fluence of 1014 cm−2. The photoinjection of electrons into the implanted oxides resulted in the buildup of negative space charge. Data analysis indicated that all injected electrons had been trapped and that the observed transient responses of the photocurrent and flatband voltage were governed by the electric−field dependence of the photoinjected current. The negative space charge could be annealed either optically by photons of energy exceeding 4 eV or thermally at 350 °C. A 600 °C anneal for 30 min considerably reduced the concentration of electron traps. From this it is concluded that a substantial fraction of the traps were associated with displacement damage created by the ion implantation.

Temperature and field dependence of the generation of interface states in the Si‐SiO2 system after high‐field stress

We have studied the time dependence of the generation of interface states in the Si‐SiO2 system after metal‐oxide‐semiconductor capacitors were stressed by a 7.4‐MV/cm gate positive electric field for 8 h at 90 K. The interface state density increased logarithmically with time for temperatures between 180 and 300 K with fields between +2 and −2 MV/cm. An analysis based upon a distribution of formation energies is proposed to explain the logarithmic time dependence. We find that there are two peaks in this distribution. One peak at 0.7 eV is field independent while the other at 0.9 eV disappears when a zero or negative bias is applied.