Silicon preparation techniques for nucleation and growth studies of zinc sulfide deposited by atomic layer epitaxy.

Abstract

Zinc sulfide with a direct bandgap of 3.6 eV is a potential candidate as blue-light emitting diodes and lasers. Initial growth of ZnS on Si(100) substrates by atomic layer epitaxy (ALE), a deposition technique in which film growth ideally proceeds in a 2-dimensional, layer-by-layer manner, has been investigated. The interaction between the first layer of atoms of the film and the substrate surface initiates film growth and affects the resulting structure. Work has focused on the effects of surface composition, (particularly on the role of sulfur) on the initial growth of ZnS on Si(100), and thus the chemical composition must be well controlled and characterized. Three methods have been used to process Si(100) substrates. The first was a wet chemical clean with either HF or H₂O passivation followed by a low temperature (700-800°C) anneal in UHV. The second processing method was ion sputter cleaning with a post-sputter anneal at 800-900°C. The third technique irradiated substrates held in UHV with a beam from a KrF excimer laser. Initial layers of ZnS (from Zn and H₂S) were then deposited onto processed substrates. Samples were characterized by in-situ angle resolved x-ray photoelectron spectroscopy (ARXPS) to determine the chemical composition of the surface and also the coverage and thickness of contamination and film layers. The main impurities on the surface were oxygen and carbon. The first two processing techniques had difficulty in either eliminating those impurities or caused additional contamination. Elimination of the impurities was achieved using excimer laser irradiation with a pre-dose of reactive gas. The substrate surface could also be chemically modified in a controlled manner using excimer laser irradiation. Deposition studies of initial sulfur and zinc layers onto the processed substrates determined the temperature during ALE growth should be held at 250-310°C. Uniform coverage of both sulfur and zinc was difficult to obtain, but experiments indicated sulfur adhesion improved with the presence of oxygen, and zinc adhesion improved when oxide or sulfide layers were present on the surface.