Template:WikipediaMolecular Beam Epitaxy (MBE) is a material fabrication process used to produce high-purity nano-scale materials. A material is grown through interactions between a substrate and one or more beams of atoms or molecules incident upon the substrate's surface. MBE exhibits many advantages over similar thin film deposition processes: significantly improved purity, arbitrarily sharp deposition resolution, and operation at low temperatures. Template:MECH370
Introduction
- Image of atomic deposition
System Description
- Insert Image of MBE layout.
The main MBE apparatus is composed of two major sections: the substrate chamber and the effusion cells. All sections must be evacuated to low pressures to ensure that the molecular beam condition is achieved. This condition requires that the free path of a particle be greater than the dimensions of the substrate chamber. For a typical MBE set up, a pressure of 10-4 Torr is sufficient, but pressures on the order of 10-11 Torr are commonly used.
Effusion Cells
Within the effusion cells, the system can be considered contained when the shutters are closed. Thus, the state of the system can be approximated using thermodynamics. The phase behaviour of the source material is important. Gibb's phase rules states that the number of control variables for a system are is related to the system properties as:
F = N +2 – P
Where F is the number of degrees of freedom, N is the number of chemical components, and P is the number of phases present. Within the effusion cell, two phases are present, and there is only one chemical component. The number of degrees of freedom of the system is one, indicating that the system pressure is dependent upon the temperature. This relationship yields an Arrhenius-type relationship, with:
P(T) ~ exp(dH / kT)
Where dH is the latent heat of vaporisation of the material, T is the material temperature, and k is Boltzmann's constant. The pressure of the vapour can be controlled by varying the temperature of the solid phase, which in turn is controlled by variations in crucible heating. Because the beam intensity within the substrate chamber depends on the effusion cell pressures, the amount of material reaching the surface can be controlled by varying the source temperature.
The shutters at the end of the collimation tunnel enable an operator to terminate the material flux from each cell separately. The shutter closing time is significantly smaller than a single layer deposition time. This means that the composition of the final material can be controlled at the monolayer level.
Substrate Stage
A schematic of a basic substrate chamber is shown >. The effusion cells are attached to the chamber and oriented so as to direct the beam upon the substrate holder. The substrate holder is heated with a resistive element, as in the effusion cells. Upon the substrate holder is placed a material upon which the beams are to interact and the source material is to be deposited. Common substrate materials are silicon, GaAs, and sapphire, but other substrates can be used. The choice of substrate is very important, and has been shown to effect the thin film microstructure. The substrate stage can be rotated during deposition to increase the surface homogeneity.
A RHEED (reflection high energy electron diffraction) monitoring system is implemented in modern MBE equipment. The RHEED system is used to monitor the growth surface during deposition. High energy electrons are incident upon the surface, producing a diffraction pattern upon the RHEED screen. The diffraction pattern can be analyzed to determine surface characteristics, like the material structure, as well as composition. The intensity of the diffraction pattern has been found to vary predictably with the surface layer formation, and can be used to determine when a monolayer has been fully formed. This information can be used to control the effusion cell shutters to produce an atomically flat surface.
Each component of the MBE apparatus must have a number of special properties. Due to the high vacuum, all components must be de-gassed before system operation. The materials chosen must have very low rates of gas evolution to prevent their vapour from being deposited on the substrate and contaminating the sample. This property is of particular importance for materials chosen for use in the resistive heating elements, as the increased surface temperature can lead to significant increases in gas evolution. Boron nitride is used for the crucibles and substrate stage, and graphite is commonly used for the heating elements. Similarly, all moving parts of the apparatus must operate oil-free, to avoid evaporation and sample contamination.
Sample Interaction
- Discussion of a sample material produced via MBE and what happens? (may be too inorganic chemistry-oriented)
Efficiency Considerations
- degass cleaning? time...?
- effect of chemistry-related changes on outcome (Al on GaAs thing, N stuff, etc.)
- Temperature considerations
Applications
Semiconductors
- Sharp doping profiles
Heterojunctions
- Applications in photovoltaic cells
Optoelectronics
- Lasers! LEDs! Digital Cameras! DVDs! (Check out HD stuff (BluRay, etc.))