There are three basic thin film growth modes: Volmer-Weber (island), Frank and van der Merwe (layer), and Stranski-Krastanov (layer followed by island). Examples of these growth modes and variations thereof are presented and the theories governing the initial and later stages of growth in each case are reviewed.
For many dielectric and metallic thin films particularly those condensed by physical vapor deposition, a columnar structure can be disclosed by using microfractographical methods with the high resolution electron microscope. From a survey of experimental work and computer simulations of nucleation and growth of thin films, a simple model with the only assumptions of self-shadowing and immediate sticking of impinging particles (i.e. no relaxation or surface diffusion of adatoms or admolecules) is deduced which describes columnar growth adequately. With the same model nodular structures can also be obtained from computer simulations which are in good agreement with the experimental observations made with the scanning electron microscope. Hence, columnar and nodular growth can be concluded to have the same physical origin.
This paper describes the generalities of molecular beam epitaxy (MBE); it is intended for those interested rather than involved in MBE. MBE has been applied to the growth of thin films of a variety of materials: III-V semiconductors, II-VI's, IV-VI's, silicon and germanium, metals and insulators. The early workl on MBE, and the majority of the effort since then,2,3 has been on the growth of GaAs and other III V's, thus this paper will concentrate on that area.
Laser chemical vapor deposition (LCVD) uses a focused laser beam to locally heat the substrate and drive the CVD reaction. LCVD, therefore, shares the advantages of other laser processing techniques including spatial resolution and control and rapid heating and cooling rates. Characteristics of several types of LCVD films and experiments in optical monitoring of LCVD rates using both pulsed and cw laser are presented.
In this presentation, the concepts of bulk glass formation are discussed in light of their possible application to the production of glassy thin films for optical applications. Specifically, the systems Zr02-Si02 and Zr02-Mg0 were explored using electron beam coevap-oration for the film production. The films so produced were characterized using x-ray dif-fraction and scanning electron microscopy. It was found that while coevaporation of a traditional bulk glass former (Si02) to Zr02 was more effective in producing an amorphous mixture coating, other dopants (Mg0) could also achieve the same end if used in higher con-centrations. Key words: Ceramic coatings; Glassy; MgO; Si02; Thin films; and Zr02.
A basic principle of thin-film interdiffusion phenomenon is described. An emphasis is placed on the type of microstructural modifications occurring in diffusion zones during the interdiffusion processes. These structural changes are discussed in relation to intermetallic compound formation, diffusion-induced high stresses, and diffusion-induced grain boundary migration.
This study was to investigate the chemical composition of typical laser mirror coatings to locate and identify absorbing impurities in ThF4 thin films. Analysis by scanning Auger Electron Spectroscopy, X-Ray Photoelectron Spectroscopy and Secondary Ion Mass Spectroscopy showed an abnormally high oxygen content in the films which indicates the presence of water and may eliminate ThF4 as a potential coating material for HF (2.75 micron) laser systems.
A method for compact storage of the appearance of films and surfaces is presented which allows comparisons prior to and following coating and laser damage testing. Observation of substrates coated with ZnS and ThF4 films indicates that for silica substrates the predominate failure mode is cracking and delamination, while for silicon substrates, burns, cracks, and bubble-formation is found. Scratches and dust appear to act as initiators of damage. For the coated silicon substrates, over 90% of the small-scale damage sites were found to have an observable defect (>5 μm) prior to damage testing. Elimination of scratches, voids, dust, film stress, and film defects would substantially increase the damage threshold for large spot cw-radiation applications. There also appears to be 1 μm or small sized defects which would then determine the damage threshold.
A review of the photoemissive materials has been presented. The quantum efficiency of conventional positive electron affinity photoemitters is lower than that of negative electron affinity photoemitters. Transferred electron photoemitters may satisfy the demand for long wavelength photoemitters.
The feasibility of creating anti-reflective silica by ion implanting buried, high refractive index layers below the surface of the glass has been investigated. The implantation of Al and Ti appears to increase significantly the index of the buried layers, although not enough to allow anti-reflective glass to be fabricated; excessive absorption in the layers was also encountered. Additional processing steps are suggested which may overcome these problems.
The transmittance of interference coatings depends both on the wavelength and on the angle of incidence. The wavelength dependence can be used to encode the spatial frequency content of a transparency and to obtain a pseudocolored image. The dependence on the angle of incidence can be used in quasi-monochromatic light to perform spatial frequency filtering in the object space. Analysis, experimental evidence and applications are presented.
Ohmic contacts to semiconductors are usually formed between a metal and a highly doped semiconductor layer. A number of techniques are used to produce such layers: alloying of multilayer contact-metal structures containing a dopant, growth of epitaxial layers, shallow diffusion, and shallow ion implantation. All of these methods require the application of elevated temperatures, usually done by furnace annealing. The trends towards very large scale integration (VLSI) and very high speed integrated circuits (VHSIC) have put more stringent requirements on structural and chemical uniformity, on dimensional accuracy, and on specific contact resistance of Ohmic contacts than can be met easily and routinely with standard methods. Substantial improvements of Ohmic-contact properties have been achieved during the past years by employing novel processing techniques such as molecular beam epitaxy, laser or electron-beam annealing.
Hydrogenated alloys of a-GaAs have been prepared by r.f. sputtering in a mixture of argon and hydrogen at several partial pressures, substrate temperatures and r.f. powers. We report the results of a systematic investigation of the microstructural properties of these films by SEM and TEM, and attempt a correlation with the deposition parameters and other electronic properties.
Amorphous Ge-SisH alloys have been prepared by r.f. sputtering in an argon-hydrogen atmosphere of varying hydrogen partial pressure and substrate temperature. The correlated data of the vibrational properties as measured by IR absorption and Raman scattering on these samples reveal several hydrogenic and lattice modes. The hydrogenic modes which grow with increasing hydrogenation can be identified with various Ge (Si)-H type molecular modes. The absorption due to Si-H modes appears stronger than that due to Ge-H modes, indicating a pre-ferential H attachment to Si atoms. Dehydrogenation by annealing at successively high temperatures leads to selective reduction/disappearance of certain modes assigned to Ge (Si)-H vibrations. Also, the observed variation (0.1 to 0.7) in the depolarisation ratio for the Raman spectra of asdeposited samples become stabilised at 0.5 with annealing. These data are interpreted in terms of the film growth process and associated structural inhomogeneities and H- bonding configurations.