DC REACTIVELY SPUTTERED ZINC-SULFIDE THIN FILMS ON SILICON
Abstract
Zinc sulfide (ZnS) thin films, typically 5000 (ANGSTROM) thick, were DC reactively sputtered onto (100) silicon substrates. The films were deposited in a custom vacuum sputtering system using a 4" diameter pure Zn target and an Ar/H(,2)S sputtering gas. The discharge voltage was 2000V and the current density was 0.12 mA/cm('2) at a pressure of 25 mTorr. The difference in the thermal expansion coefficients of the ZnS and Si caused the films to crack when they were annealed. Heating the substrates during deposition significantly reduced the cracking problem. X-ray diffraction indicated that the films were polycrystalline with both (111) and (100) orientations present. A substrate temperature of 200(DEGREES)C was found to maximize the (100) orientation. Further improvements were made in the orientation by sputtering at a rate of 400(ANGSTROM)/hr initially for 30 minutes and then increasing the rate to 1800(ANGSTROM)/hr. Bias sputtering and sputter etching prior to deposition were both found to decrease the tendency toward (100) orientation. Refractive index measurements by ellipsometry showed a 10% variation across the sample, indicative of non-uniform composition or density. Capacitance measurements were used to determine the relative dielectric constant of the ZnS film to be 4.5 as compared to the bulk value of 8.3. Conductance values taken simultaneously with the capacitance measurements were quite large and increased with frequency while the DC resistivities of the films were at least 10('7) (OMEGA)-cm, thus indicating a highly defect laden film. Doping of the ZnS by diffusion of aluminum from an evaporated layer at 500(DEGREES)C for from 1 to 24 hours was unsuccessful at reducing the resistivity below 10('7) (OMEGA)-cm. This was most likely due to compensation by the large number of crystal defects in the films.
Degree
Ph.D.
Subject Area
Electrical engineering
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