H. R. Philipp

Theory of conduction in ZnO varistors

A theory is presented which quantitatively accounts for the important features of conduction in ZnO‐based metal‐oxide varistors. This theory has no adjustable parameters. Using the known values of the ZnO band gap, donor concentration n0, and low‐voltage varistor leakage‐current activation energy, we predict a varistor breakdown voltage of ?3.2 V/grain boundary for n0=1017 carriers cm−3 and T=300 °K. This compares well with measurements on a single grain‐grain junction. The highly nonlinear varistor conduction derives from electron tunneling ’’triggered’’ by hole creation in the ZnO when the conduction band in the grain interior drops below the top of the valence band at the grain interface. The theory predicts coefficients of nonlinearity α=d (lnI)/d (lnV) as high as 50, or even 100.

The physics of metal oxide varistors

This paper outlines our present understanding of the conduction mechanisms and physical processes relevant to the performance of ZnO−based ceramic varistors. Varistor behavior is determined by the gross ceramic microstructure of the device as well as by the localized conduction processes which occur between grains. We show that the qualitative features of the highly nonlinear conductivity are largely independent of the details of varistor composition or processing but rather appear to be a general effect engendered by a microstructure of conducting grains surrounded by thin insulating oxide barriers. Evidence is presented from a variety of sources that this intergranular layer is ∼100 A in thickness resulting in grain−to−grain fields of F∼106 V/cm. The conduction mechanism at breakdown is consistent with a Fowler−Nordheim tunneling process obeying a current−density−vs−field relation given by J∝exp(−γ/F), where γ is a constant. At somewhat lower fields (prebreakdown region) the conduction process follows a...

Optical and bonding model for non-crystalline SiOx and SiOxNy materials

Optical data for non-crystalline SiOx materials are presented and analyzed for the energy region 1 to 26 eV. The results indicate that amorphous substances of all intermediate compositions between Si and SiO2 can be formed and that these materials are not simple mixtures of particles of Si and SiO2 but rather the two atom species are blended on an atomic scale. The basic units of this structure are Si tetrahedra (perhaps highly distorted) of the type Si(SiyO4−y) in which the distribution of atoms for all y = 0 to 4 is statistical for any given atom ratio. Further it is found that the optical properties of these layers are determined by the presence and grouping of SiSi and SiO bonds and that clusters of like bonds of the dimension of a Si(Si4) or Si(O4) tetrahedra have optical properties comparable to those exhibited by amorphous silicon or quartz respectively “in bulk”.

ac properties of metal‐oxide varistors

Measurements of the small‐signal ac response of ZnO‐based ceramic varistors are reported for frequencies f in the range 30<f<108 Hz and temperatures T in the range −200<T<300 °C. The high effective varistor dielectric constant results from the device microstructure with no detectable electrode contribution. The varistor leakage resistance ρp drops rapidly with frequency, a crude approximation being ρp∼1/ω. A broadened relaxation peak in the dissipation factor D=tanδ is observed at 3×105 Hz at room temperature. The peak has an exponential temperature dependence with an activation energy 0.36 eV.

Electroluminescence in ZnO varistors: Evidence for hole contributions to the breakdown mechanism

The previously postulated production of holes during the electrical breakdown of varistors has been directly verified. In some compositions band‐gap (3.2 eV) luminescence from the recombination of these holes with free electrons has been observed. The intensity of this luminescence is greater in varistor compositions exhibiting higher nonlinearity coefficients. It is also proportional to the square of the current which implies hole creation by impact ionization in the depletion region near the grain boundaries. This study used varistors of relatively simple chemical composition.

Influence of Oxide Layers on the Determination of the Optical Properties of Silicon

Experimental reflectance data for silicon are corrected for the presence of a surface oxide layer and analyzed using the Kramers‐Kronig relation to yield a set of optical constants for oxide‐free silicon. For etched silicon samples exposed to the atmosphere for ∼ 1 h, this layer is 12–15 A thick and has the approximate composition SiO. Absorption data are also given for epitaxial silicon on spinel and used as an aid in the analysis.

The infrared optical properties of SiO2 and SiO2 layers on silicon

The complex index of refraction N=n−ik for amorphous SiO2 is derived in the energy range 0.03–1.0 eV by Kramers‐Kronig analysis of reflectance data. The results are used to compute the transmission, reflectance, and absorptivity of thin layers of SiO2 on Si substrates in the vicinity of the 0.14‐eV (9 μ) lattice absorption band of SiO2. The dependence of these quantities on the layer thickness and angle of incidence is discussed.

Optical absorption of some polymers in the region 240–170 nm

Absorption coefficients for some technologically important polymer materials are given in the wavelength range ∼240–170 nm. Absorption coefficients at 193 nm for these polymers show a wide range of values from ∼2×102 cm−1 for polytetrafluoroethylene (Teflon) to ∼4×105 cm−1 for polyimide. The general nature of the optical properties of polymers in the vacuum ultraviolet is also discussed.

High‐frequency and high‐current studies of metal oxide varistors

High‐frequency, 106≲f≲2×109 Hz, impedance measurements on ZnO‐based ceramic varistors are described. Our studies indicate that the ZnO grain resistivity ρg is less than 1 Ω cm for GE‐MOVR varistors at room temperature. The high‐frequency measurements are compared with estimates of ρg from high‐current (J≳103 A/cm2) data and the difference observed is explained in terms of the varistor ceramic microstructure.

Theory of polymer ablation

A new formula is presented for the etch depth l per pulse of an excimer laser of fluence F. Incremental ablation is defined as the etch depth per pulse after many pulses. We show that l is proportional to F, rather than ln(F).