by Dave Norris
Prologue – Importance of surface science.
I have an exciting story to tell. It starts with surfaces, but as we will see end up as being the most important technological advancement in recent years. I talk about the semiconductor quantum dot and its importance in all technology which utilise light emitting screens – such as those of mobile phones, computers, tablets, TV’s other visual display products. I will also go into a further story about transistor devices made from semiconductors and for all of these, you start with the basic surface of a wafer.
Before starting to describe the importance of surface science, I should ask the question – What is a surface? To me it is a complex question because I can see that an object sitting on a table surrounded by air is characterised by its surface as this is what is clearly visible to us. The surface is the outer ‘skin’ of the object and because it is situated in air then it is constantly adsorbing and desorbing oxygen, carbon dioxide and nitrogen molecules which bombard the outer surface or ’skin’. If the atoms of the object are reactive i.e. they can chemically bond with the air molecules then they will do so.
Technically when we talk about a surface we consider an ideal surface which is usually crystalline such that the atoms line up as a plane of atoms and is essentially flat with few imperfections at the atomic sites. Then we can do some experiments to see how an external gas of individual atoms attach to the atoms of the surface.
There are three ways in which atoms grow on a surface. We shall explore the first two ways now. Later we will examine a third way which is a little more complex but very interesting.
If we spray individual atoms that are the same atom-type as the surface then they grow in a layer by layer manner.
If we spray atoms which are not the same onto the surface then something changes and they don’t grow in a layered manner. Instead, tiny islands or protrusions grow.
You may say – What is the difference? Atoms are just atoms and they grow to form crystals with shapes which resemble the nature of their structure. Blue copper sulphate, you may remember from high school, grows to form lots of little cubes. Well, big cubes if they have grown to be seen by the naked eye.
When a different atom-type grows onto a surface, there is evidence to show that islands form on the surface and do so in a way that they have a characteristic shape of island and a characteristic density or spacing across the surface. People analyse these shapes of islands and how many islands appear in a certain area of the surface to get information about the properties of the two materials that are examined.
What is interesting is that we are only considering the first few monolayers of growth here, not the case where a thicker deposited layer is grown on the exposed surface.
So we have identified the extremes, similar (or same) atoms of spray and surface grow in a layer by layer manner. Atoms in the spray which differ from the surface form a new surface composed of islands. These are the 2 cases we can confirm now.
There is a special case which we will come to, which is what happens when we grow a mixture of atoms onto the surface. Say the surface is composed of atom A and the spray consists of atom A but with a small proportion of atom B mixed into it. What happens then?
I will tell you this in due course.