STF Growth

15.2.1 Experimental and Phenomenological

Directing a collimated beam of vapor at a substrate in vacuum produces a STF. A schematic of the growth system is shown in Fig. 15.1. The basic components of the system are: vapor source, substrate holder with means of rotation, and crystal monitor to monitor the rate of deposition. Of course, the components are all enclosed in a vacuum chamber. The vapor may be produced either by evaporation or sputtering. If the vapor beam is produced by evaporation, either resistive heating or heating by electron beam may be used.

In order for columns to form, the substrate must be held at a temperature T is less than 0.3Tm, where Tm is the melting point of the material, thereby limiting adatom surface diffusion. An additional requirement is that the vapor pressure be kept sufficiently low during deposition. At low vapor pressure, the mean free path of molecules within the vapor will be larger than the source to substrate distance, leading to a well-defined beam direction. With these conditions met, at normal incidence, the film grows with the so-called match stick morphology. The film is composed of tightly packed columns with rounded ends. The conditions can be summarised by the structural zone model of thin films developed by Movchan and Demchishin (1969), extended for sputtering by Thornton (1977), and later refined by Messier et al. (1984). In the structural zone model, the locus of points in the plot of sputtering potential and temperature leading to column formation is referred to as the M zone.

With large oblique angles of incidence of the vapor beam, self-shadowing by spontaneously nucleated clusters on the substrate, combined with the low surface diffusion, lead to the formation of well separated columns. The nanocolumns grow at an angle from the surface, producing a columnar thin film (CTF). Column diameter ranges from 10 nm to 100 nm. The geometry is shown in Fig. 15.2.

Vapor Flux

Vapor Flux

Fig. 15.2. Geometry for the formation of a columnar thin film

Fig. 15.2. Geometry for the formation of a columnar thin film

Two characteristics of the columns can be cited in general: they have flattened non-circular cross-sections, and the angle at which they are inclined to the surface of the substrate, is greater than the vapor deposition angle, %v. It should be cautioned that many investigators measure the tilt of the column relative to the normal to the substrate rather than the plane of the substrate.

The tilt angle, is often related to%v through the so-called tangent rule. It is written as

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