1. Surfaces and Multilayers &
Prof.P. Ravindran,
Department of Physics, Central University of Tamil Nadu, India
&
Center for Materials Science and Nanotechnology, University of Oslo, Norway

2. Cross section of a thin Film solar cell

3. Solar Cell:Thin Films vs. Crystalline Si

4. Surface Science ansatz and its Experimental techniques

5. Surface: Introduction

6. Basic Processes at Surfaces

7. How steps and kinks are formed ?
Low Miller index surface – no structural complexities BUT less interesting for catalysis
Cutting Low Miller index surface at certain angle gives a step surface ((211), (221), (533) etc)
(211)
(221)
(533)
Cutting a step surface at certain angle gives a kinked surface (for e.g. (532), (10,8,7))
(532)
Steps/kinks also called defects – sometimes defect are intentional, especially in heterogeneous catalysis [Zambelli and Ertl, Science 273, 1688 (1996)]

8. Surface Structure: High Miller index surfaces

9.  Surfaces:
The surface of a semiconductor is the typical starting
places for growth. There are microscopic (atomic
displacements) and macroscopic structure (steps, ledges,
etc.) on a surface.
 Atomically:
Surface could reconstruct to minimize the surface
energy. For example, a ball and stick model of Si (100)
surface

10. Atomic Structure: relaxation and reconstruction

11. Relaxation and Reconstruction

13. Surface Reconstruction
From or
Si (100)
surface
unreconstructed
reconstructed

14. Surface Morphology  Macroscopically:
Surface can contain terraces, steps, and defects
(vacancy).
Surface Morphology
step
adatom
kink
cluster 
vacancy h L0
terrace
Average step spacing (L0) is related to miscut of the
crystal (miscut angle ).
 = tan(h/L0) ~ h/L0;
For  = 0.1o, monolayer step of Si 100 surface h = a/4 =
0.543 nm/4 => L0 ~ 77.8 nm
For miscut of Si 100 surface by 0.5o, => L0 ~ 77.8/5 nm
~15.6 nm

15. Steps in surfaces Si (100) surface Two types of steps
 Steps is far from perfect! Steps and terraces form because low index planes, such as 100 plane, are more stable than higher index planes.
Si (100) surface
Two types of steps
(a) Straight low energy steps SA
(b) Rough high energy step SB SB SA
From Prof. Max Lagally, MSE, Wisconsin Madison ;

16. Surface Chemistry  Surface layer of compound semiconductor must be
further characterized by their atomic composition.
For example, the (100) and (111) surface of GaAs
contain either all Ga atoms (“A” face; Ga terminated
surface) or all As atoms (“B” face; As terminated surface).
 Atoms on a surface:
An atom on a solid surface sees an array of binding sites
or potential wells formed by substrate atoms. Adatoms
can hop from one well to another (surface diffusion) and
can escape from the well to vacuum (desorption).
 Representation of the kinetics of the surface diffusion
and desorption:

17. Surface Structures

18. Surface Coordination
Coordination (# of first nearest neighbors of an atom)
Surface Coordination (# of first nearest neighbors of top layer atom)
Surfaces has different local coordination, generally
Highest for low Miller index fcc surfaces (111), (100), (110) etc.
Lower of high Miller index fcc surfaces (211), (221), (533) etc.
Lowest for kinked surface (532), (10,8,7)
Low Miller index surfaces can have different coordination (for e.g. 9 – 7 for above stated one)
Surface local coordination plays important role in studying adsorption [Vollmer et. al., Catalysis Letters, 77, 97 (2001)]

19. Surface Energy Surface energy is the most important concept that runs
through much of thin film science.
Surface energy: the energy required to create a surface. (+)
Typically, metals have high surface energy while oxides
have low surface energy.
Deposition of films with high surface
energy on substrate with low surface energy
or vice versa tends to form different film
morphology
E.g. Organics usually have low surface!
Waxing the car for water to form droplets!
(manipulating surface energies)
substrate
substrate
High surface
energy materials
Low surface
energy materials

20. Surface Energy: Two equivalent Definitions

21. Surface Energy: Anisotropy

22. Surface Energy & Broken Bonds
Crystals tend to facet in order to expose those surfaces
with lowest surface energy.
What is surface energy?
Break a solid  two new surfaces
With broken bonds  surface energy is related to the bond
energy and the number of broken bonds on the surface.
(related to the binding energy of the material)
If the broken bonds on the surface is reconstructed or bond to other atoms  surface energy changed

23. Estimation of Surface Energy