Alvin M. Goodman

A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors

A method is presented whereby short minority carrier diffusion lengths in semiconductors may be determined by measuring the variation of surface photovoltage as a function of optical absorption coefficient. The method does not depend upon the specific form of the relationship between the surface photovoltage and the density of the excess minority carriers injected at the bulk edge of the surface space charge region. Only capacitive contacts to the sample are needed for the measurement. The method has been used to determine the minority carrier diffusion length in both n and p type gallium arsenide.

Optical interference method for the approximate determination of refractive index and thickness of a transparent layer

Optical interference fringe measurements of the thickness of transparent layers can be rapid, accurate, and nondestructive. If the refractive index n of the layer being measured is known, it may be combined directly with interference fringe information to yield the layer thickness t. If, however, n is unknown, the measurement procedure necessarily becomes more complicated. In this paper, a new and simpler optical interference method is presented for the approximate determination of both n and t of a transparent layer on a transparent substrate. The required experimental information is obtained from a single spectrophotometric recording of either the reflectance or transmittance of the layer and its substrate. The theory of the method is presented, and an application of the method to measurements of layers of SIPOS (Semi-Insulating POlycrystalline Silicon) is described. The method requires that the layer being measured must be uniformly deposited on a flat substrate, and it must neither absorb nor scatter the light passing through it. The major approximation inherent in the method is that both the layer and the substrate are assumed to be nondispersive over the wavelength region of interest.

Wetting of thin layers of SiO2 by water

We have measured the contact angle θ of water on silicon and on very thin layers of silicon dioxide grown on silicon. The silicon is hydrophobic and θ is near 90°. Oxides thicker than 30 A are hydrophilic and θ is near 0°. For intermediate thicknesses, θ varies smoothly between these limits. Our results show that the interaction energy between water and the solid surface depends strongly on the oxide thickness. Consideration of different possible interactions leads us to conclude that this is due to corresponding changes in the structure or composition of the oxide surface.

Improved COMFETs with fast switching speed and high-current capability

Conventional vertical power MOSFETs are limited at high voltages (>500V) by the appreciable resistance of their epitaxial drain region. In a new MOS-gate controlled device called a COMFET (or an IGR), this limitation is overcome by modulating the conductivity of the resistive drain region, thereby reducing the on-resistance of the device by a factor of at least 10. However, the device previously described is slow in turn-off, having a fall time in the range 8 to 40 µs. The purpose of our present work has been to reduce the fall time significantly and to increase the latching current level of the COMFET, while retaining its desirable features. By modification of the epitaxial structure and addition of recombination centers, we have achieved fall times as low as 0.1 µs and latching currents as high as 50 A, while retaining on-resistance values < 0.2 ohms for a 0.09 cm2chip area. The techniques used for the introduction of recombination centers include electron, gamma-ray, and neutron irradiation, as well as heavy metal doping. For a series of COMFETs (with forward blocking voltage capabilities of 400-600V), the fall time can be reduced by more than one order of magnitude with a penalty of less than a 20% increase in on-resistance.

Low‐energy ion‐scattering spectrometry (ISS) of the SiO2/Si interface

The interface between Si and thermally grown SiO2 has been examined in detail by low‐energy 4He+ ion scattering. The oxide composition is shown to be stoichiometric to within 15–20 A of the interface. ISS depth profiles clearly establish a region of excess Si that extends from the interface into the oxide for 15–20 A, corresponding to four or five molecular layers. The amount of excess Si is calculated from the experimental data as 1.4×1014 atoms/cm2 per monomolecular layer (about 20% excess).

Photoemission of Electrons from Metals into Silicon Dioxide

Metal‐insulator contacts have been prepared by evaporating partially transparent metal electrodes onto a thermally grown silicon dioxide layer. The metals employed were Mg, Al, Ag, Cr, Cu, Au, Ni, Pd, and Pt. Photoemission of electrons from the metal electrode into the oxide conduction band has been observed in each case. The photoelectric threshold energy φT for this process depends primarily upon the metal employed and is consistent with the relation φT=φM−χ, where φM is the vacuum value of the metal work function and χ, the electron affinity of the oxide, is 1.0 eV.

Evaporated Metallic Contacts to Conducting Cadmium Sulfide Single Crystals

Vapor deposited metallic contacts (area =1 mm2) have been made to oriented conducting cadmium sulfide crystals. The metals employed were Al, Ag, Cu, Pd, Au, and Pt. The electrical properties of these contacts have been studied by measurements of: (1) differential capacitance as a function of bias voltage, (2) current‐voltage characteristics, and (3) spectral variation of photoemission from the metal into the cadmium sulfide. These measurements have been analyzed to yield values of Δφ (work function of the metal with respect to the cadmium sulfide at the contact) that are reproducible from contact to contact for a given metal and different for different metals. The relationship between Δφ, φm (work function of the metal in vacuum), and χ (semiconductor electron affinity), Δφ=φm−χ, is confirmed consistent with χ=4.0 volts for cadmium sulfide.

PHOTOEMISSION OF ELECTRONS AND HOLES INTO SILICON NITRIDE

The photoemission of both electrons and holes from degenerate silicon into thin (160–270 A) layers of silicon nitride has been observed. The threshold energies for these processes are found to be 3.17 ± 0.1 eV for electrons and 3.06 ± 0.1 eV for holes. In addition, the photoemission of electrons from aluminum into silicon nitride has been observed; the threshold energy in this case is 2.11 ± 0.1 eV. An approximate electron energy band diagram for silicon nitride based on these values is presented.

An investigation of the silicon&#8212;Sapphire interface using the MIS capacitance method

Measurements of the MIS capacitance as a function of voltage were carried out on aluminum-sapphire-silicon structures. The results were used to determine the doping of the silicon and the interface-state density and flat-band charge density at the silicon-sapphire interface. The density of states function Ds(cm-2eV-1) is found to be qualitatively similar to that reported for the Si-SiO2interface but is larger in magnitude by a factor of 5-10.

Silicon‐wafer‐surface damage revealed by surface photovoltage measurements

Anomalous results of surface photovoltage (SPV) measurements on Si wafers are shown to be associated with a damaged region beneath the illuminated surface of the wafer being measured. The anomaly is a concave‐upward curvature of the I0(α−1) plot with an r2 value, derived from linear regression analysis, less than the normally observed minimum value (∼0.98). Removal of the damaged region by an appropriate etching procedure allows subsequent SPV measurements whose results are substantially free of the previously observed anomaly. The qualitative character of the anomaly can be reproduced by a simple theoretical model in which only one effect of the damage is considered; this effect is a diminished quantum efficiency for hole‐electron pair generation by photon absorption in the damaged region. The results suggest the use of SPV measurements as a test procedure for revealing the presence of surface damage in Si wafers.