SUBJECT

Describe the most common thin film growth modes and discuss ways of identifying the growth mode by diffraction or scanning probe techniques.

 

There are many methodologies for growing thin films, and the most popular one among them is a vacuum evaporation method. Theoretically, the growing conditions of the thin films are decided by only two parameters in this method, the evaporating speed and the temperature of the substrate. Therefore, the vacuum evaporation method has been taken importantly. Growing some materials on inhomogeneous substrate by the vacuum evaporation method (or by other methods) is called hetero growth. The hetero growth shows some interesting growth mode, due to the combination of the substrate materials and the growing materials. I will describe some of the growth modes of the thin films below

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There are three types of growth modes in the thin film. Those are monolayer overgrowth, nuclear growth, and the growth mode that has both features of the former two.

 

The monolayer overgrowth mode is described in Fig.1. The evaporated atoms (or molecules) cover the substrate uniformly by one atom thickness and the thin film grows layer by layer. This growth mode is chosen when the substrate material and the growth material have good amicability and the surface tension is comparatively not strong. This growth mode is called “Frank-ban der Merwe mode” (FvdM mode).

Fig.1

 

The nuclear growth mode is described in Fig.2. In this growth mode, the evaporated atoms that arrived on the substrate calm down into a thermal equilibrium state. However, they move around on it because of the thermal vibration, they repeat the surface diffusion and with some probability they re-evaporate.

When the re-evaporation is not intense, the atoms, which arrive on the substrate in succession, form 2 dimensional gases on it, and they condense into a “nucley” with high density. High density of nucleys leads to touch and combine each other, and they grow to an island. This is called island state. When it further grows it becomes a channel state a hole state, and finally makes a uniform thin film as described in the picture below. This growth mode is called Volmer-Weber mode (VM mode). The difference between VM mode and FvdM mode is that, VM mode doesn’t grow layer by layer. It means that this surface can have a second layer before the first layer is filled and the nucleys grow like a water drop on the oil film, with a comparatively thick island. This mode is chosen with high surface tension of the growth material and the low amicability between the substrate materials and the growth materials.

Fig.2

 

In addition, there is a third mode that has both features of the FvdM mode and the VM mode. In this mode, the film primarily grows like FvdM mode and covers the substrate uniformly. Secondary it grows like VM mode on it. This growth mode is called Stranski-Krastanov mode (SK mode). It is mainly seen on the metallic film on semiconductor material substrates.

 

Those three growth modes are described as below.

As I wrote before, it depends on the combination of the growth material and the substrate material as to which growths mode is chosen.

 

There are many ways to identify each growth mode. STM, which is widely used in the surface physics field, visualizes the morphology of the surface clearly. By using this probe the growth mode is easily identified. For example, Fig.3 is N on Cu(100) substrate. As seen on this picture, the growth mode is VW mode.

  Fig.3 N on Cu(100): STM picture taken by KOMORI GROUP

By using LEED, we can identify the surface state from the diffraction pattern. For example I am observing on a N adsorbed Cu(100) surface by LEED. In that surface, there are grids on the LEED pattern with a small amount of adsorbing N (<0.3ML), and streaks are seen on it with a higher amount of adsorption. In this way we can approximately estimate the amount of N on this surface.

 

It is possible to use similar methodologies to identify the growth mode of the thin film.

 

STM an LEED, that I mentioned are generally used to study the surface statically. In the case of the FvdM mode, not only these static methodologies are used but also there are efficient ways to study the system dynamically such as RHEED.

RHEED is attached next to the sample holder and irradiates electron beams skimming on the surface with energies of ~20kV. It is possible to observe the growth of the atomic formation of the topmost layer of the surface in real time.

On RHEED observation, it is possible to see the change of the intensity that corresponds to the creation of a new layer, and that allows us to count the layers by growing the thin film at the same time. As a result, this methodology improves the control of the thin film growth, and promotes further understanding of the mechanism of the thin film physics.

 

To study on this field, there are also many other spectroscopic probes, such as XPS, UPS, AES and many real space observation methods such as AFM. The choices are diverse.

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　  　Thank you.

 

Reference：

「表面科学入門」　小間篤、八木克道、塚田捷、青野正和　編著　丸善

「薄膜・微粒子の構造と物性」　日本金属学会偏　丸善